nite number of **q** positions where RAXS spectra are recorded, the resonant element peak in the deduced electron density profile is broadened, but the peak positions correctly represent the positions of the associated resonant atoms. The atomic positions determined by this model-independent approach can then be used as constraints in further fits of the RAXS data by conventional modeling, which then provides the full structural parameters, including position, occupancy, and Debye-Waller factor. This procedure can be iterated until a self-consistent structural model is obtained where the positions from model-independent fitting and the deduced electron density profile of the resonant element are in agreement with the total electron density distribution derived from non-resonant CTR fitting.

Because of the strongly increasing X-ray absorption with decreasing photon energy, most *in situ* RAXS studies of electrochemical or solid-liquid systems employ hard X-rays (> 5 keV). Studies at lower photon energy ("tender X-rays") typically have to be performed under vacuum, as even air leads to pronounced absorption. Light elements are therefore difficult to access by *in situ* RAXS. Nevertheless, first *ex situ* RAXS studies of electrochemical materials that studied Bragg reflections at the Cl and S L edges using tender X-rays have recently emerged.¹³⁷ Similar *in situ* studies seem feasible but will require adapting cell designs and approaches from *in situ* X-ray absorption and photoelectron spectroscopy.

### 4.7.2 Resonant anomalous surface X-ray scattering

Even though the theory of RAXS was established by Walker and Specht as early as 1994, its wide application to surfaces and interfaces was only realized one decade later. Park et al.⁸³ employed RAXS in pioneering anomalous X-ray reflectivity studies to resolve two species of outer-sphere adsorbed $Pt(NH_3)_4^{2+}$ above a quartz (100) surface. In that work, the mathematical basics for modeling resonant anomalous surface X-ray scattering (RAXS) data were provided, which laid the foundation for

many follow-up studies. Most *in situ* RAXS measurements focused on the surface sorption chemistry of ions or nanoparticles at different mineral-water interfaces under open circuit conditions. Applications of RAXS in the field of electrochemistry and energy science are still scarce. Nevertheless, this technique is of great interest here as well, as it's element sensitivity can significantly help to unambiguously resolve the location of species of interest at the solid-fluid interface.

In a RAXS experiment, the resonant anomalous scattering at **q** values along the specular and non-specular CTRs (see chapter 4.4.2) is recorded. These data are analyzed using the methodology described in the previous chapter, i.e., by applying Eq. 63 to the structure factors of the CTRs. As an example, data obtained from an *in situ* RAXS study of $Zr_xO_y$ aggregation on muscovite (001) surface in aqueous NaCl solutions is shown in Figure 22.¹³⁸ Here, the evolution of the aggregate on the surface was investigated as a function of ionic strength. The measurement in each solution included recording the specular reflectivity (Figure 22a) at a photon energy of 16 keV (figure 22) and collecting 21 RAXS spectra (example shown in Figure 22b) at different **q** positions along the specular axis by scanning the photon energy through the Zr K-edge (17.998 keV). The modeling of the reflectivity data provided the total electron density profile (solid lines in Figure 22c), while the vertical distribution of the electrons associated with the Zr atoms was obtained from independent fits of the RAXS spectra (filled area in Figure 22c). In combination, these data revealed together, with complementary AFM measurements, a multistep growth process with increasing ionic strength. Here, the primary building units first aggregated into nanosheets, which then grew laterally and, finally, became stacked vertically, resulting in 3D nanoparticles. RAXS played a key role in unraveling this complex mechanism, as it allowed to unambiguously separate the contribution of the Zr atoms from that of other interfacial species in the total energy density distribution. This example illustrates the value of element-sensitive data in determining complex
interface processes on a molecular scale.

### 4.7.3 Determination of electronic structure by RAXS

By combining RAXS with *ab initio* theory calculations, information about the electronic structure can be obtained, for example, on electron transfer and electric orbital hybridization during covalent bond formation. To understand how this works in principle, we first revisit the conventional approach for fitting RASXS spectra described in chapter 4.7.1. Here, the dispersion corrections to the form factor ($f' + if''$) are obtained from experimental XANES data, using the assumption that all resonant atoms are structurally equivalent in terms of geometrical surrounding, symmetry, and electronic state. This constraint works well in many cases, for example, in RASXS studies of systems where the resonant atoms are present as adsorbates above a substrate. However, problems arise if the resonant atoms are present in different environments, e.g., at the interface and in the substrate itself, since those structural non-equivalent atoms of the resonant element should respond differently to the energy scan through the absorption edge. In this case, the resolved distribution of the minority species of the resonant atoms, i.e., the adsorbed atoms at the interface, could be faulty, because the experimental dispersion corrections used in the fits by the structural model are only average values. This limitation can be overcome by using dispersion corrections $f'$ and $f''$ that are calculated by density functional theory (DFT). This method became practically feasible only recently with the advent of advanced self-consistent *ab initio* software for the simulation of X-ray absorption and X-ray resonant scattering spectroscopy,¹³⁹ which allows the simulation of the CTR intensity for a known surface structure (to be resolved by fitting CTR data) at any specified momentum transfer $q$ and X-ray energy $\epsilon_{Ph}$). In these simulations, the structurally nonequivalent resonant atoms can be described by different resonant form factors, depending on their geometrical surroundings and symmetry. In addition, also the polarization dependence of

RAXS spectra can be simulated. As a result, a full picture of the local environment of the resonant absorbing atoms can be obtained, including both geometrical surroundings and electronic parameters. This allows getting insights into changes of the electronic orbitals due to interface processes, such as electron transfer and orbital hybridization. Although the application of this DFT-assisted approach to the electrochemical interface is still in its early stages, some interesting results have recently been published by Soldo-Olivier, Grunder and coworkers.¹⁴⁰,¹⁴¹ In this work, pioneering experimental results on the charge distribution at metal-electrolyte interfaces, specifically platinum and Br-covered copper electrodes, were reported. These studies found that, contrary to common belief, the surface charge is not located directly at the metal surface but spread out over the two outermost atomic planes. Such data can be directly compared to *ab initio* molecular dynamics simulations. RAXS is unique in its ability to probe simultaneously geometric and electronic structures and expected to play an important role in future fundamental studies of electrochemical interfaces.

## 5 Fundamental electrochemical interface structure

Insight into the atomic-scale structure of electrodes in an electrochemical environment is an important prerequisite to the fundamental understanding of reactions at these interfaces. *In situ* surface X-ray diffraction methods have provided detailed structural data on the electrode surface structure. In particular, these methods have revealed surface structural changes as a function of potential as well as differences in the surface structure found under UHV conditions. In this section, we review results on the interface structure in the absence of electrochemical reactions, i.e., in the double layer potential regime. Because many of these studies have already been described in previous reviews,¹⁴²⁻¹⁵³ we keep this section brief. We first discuss *in situ* SXRD studies of structural transitions at metal electrodes, both of the metal itself and of
<figure><img src="image_39.png" /></figure>

Figure 22: *in situ* RASXS measurements of ZrₓOᵧ nanoparticle aggregation on the muscovite (001) surface in NaCl solutions of different ionic strength. (a) CTR profiles, (b) exemplary RASXS spectra collected at q = 0.192 Å⁻¹, and (c) total electron density distribution along the surface normal direction obtained from CTR modeling (solid lines) together with Zr electron density distribution derived from the RASXS modeling (filled area). (d) Lateral and vertical ZrₓOᵧ size obtained from the RASXS modeling and (e) proposed aggregation mechanism as a function of ionic strength. (reprinted from Ref. ¹³⁸. Copyright 2018 American Chemical Society).

adsorbate layers on top of the metal, and then
provide an overview of the interface structure
at non-metallic electrodes, especially oxides.

**5.1 Metal electrode structure**

**5.1.1 Surface reconstruction**

Surface reconstruction is a well-known phenomenon in UHV surface science. It denotes the case where the atoms in the materials' surface layer(s) form a 2D ordered arrangement that deviates from that of the ideally truncated crystal lattice. In the following, we adopt this traditional definition of surface reconstruction. However, we note that in the more recent literature, often a less stringent wording is employed, where any restructuring at the surface of a material, including morphological changes (e.g., surface roughening), the formation of disordered structures, and chemical phase changes (e.g., surface oxidation), is called surface reconstruction.

Under UHV conditions, surface reconstruc-
tions are found on the clean surfaces of cer-

tain metals, e.g., Au, Pt, and Ir, but also can
be induced by adsorbates. They originate from
the different chemical environments of the sur-
face atoms and result in a reduced surface en-
ergy. Typically, the 2D unit cell describing
the periodicity of the reconstructed surface is
larger than that of the unreconstructed sur-
face and has a defined orientation with respect
to the underlying bulk lattice. This results
in additional superstructure rods in between
the CTRs, which are well accessible to sur-
face diffraction measurements. Indeed, GIXRD
studies and surface crystallographic analysis of
CTR data played a central role in determining
the structure of the reconstructions of Au and
Pt at the metal-vacuum interface.<sup>154-158</sup>

Surface reconstructions can also be observed
on Au and Pt single-crystal surfaces in an elec-
trochemical environment, where their stability
usually depends on the potential. ¹⁵⁹ Specifi-
cally, the reconstructed surface is usually only
stable below a critical potential, which is de-
termined by the electrolyte composition. Pos-
itive of this critical potential, a surface phase
transition to an unreconstructed surface oc-
curs. Direct evidence for the presence of sur-
face reconstruction in electrolyte solution was
first provided by *in situ* GIXRD measurements
of Ocko et al. for the case of Au(100) electrodes
in HClO₄ solution.¹⁶⁰ They demonstrated that,
similar to the surface structure found in UHV,
the reconstruction of the Au(100) electrode con-
sists of a densely-packed hexagonal Au sur-
face layer on top of the square substrate.
Upon increasing the potential, the reconstruc-
tion was found to disappear above the poten-
tial of zero charge but recovered upon chang-
ing the potential back to values below -0.3
V<sub>Ag/AgCl</sub>, albeit with some hysteresis. Similar
*in situ* GIXRD were performed by Ocko and
coworkers for Au(111)¹⁶¹,¹⁶² and Au(110)¹⁶³
in acidic and neutral electrolyte solutions, re-
vealing a qualitatively similar potential depen-
dence. Au(111) electrodes exhibit at negative
potentials a (23 × √3) reconstruction, consist-
ing of dislocation stripes. The reconstructed
Au surface layer is 4% uniaxially compressed
along the [100] direction and exhibits the same
local order as in the herringbone reconstruction
found on well-annealed Au(111) single crystals
but has a more disordered arrangement of the
3 differently oriented domains. Upon increas-
ing the potential, the phase transition between
the reconstructed and unreconstructed surface
at the critical potential occurs gradually via
a gradual change in the average separation of
the reconstruction stripes. Furthermore, the
transition potential was found to strongly de-
pend on the anion species in the electrolyte.
This was attributed to the effect of the sur-
face charge on the electrode surface but may
also be caused by the onset of anion chemisorp-
tion, as both are closely correlated and can-
not be easily separated (see Ref.¹⁶⁴,¹⁶⁵ for a
discussion of this still controversially discussed
topic). In the case of Au(110), a mix of a
(1 × 2) and (1 × 3) reconstruction was found
in HClO₄, whereas in alkali halide solutions
only a (1 × 3) reconstruction was observed.¹⁶³
Also here, highly reversible transitions be-
tween reconstructed and unreconstructed sur-
faces occurred. Furthermore, a similar struc-
tural behavior was observed for the Au(111)
and Au(100) reconstruction in sulfuric acid

solution¹⁶⁶ and alkaline electrolyte.¹⁶⁷⁻¹⁶⁹ All
these results were in good agreement with data
obtained by complementary surface-sensitive
methods, such as *in situ* scanning tunneling mi-
croscopy (STM).¹⁷⁰⁻¹⁷²,¹⁷²⁻¹⁷⁵

In contrast to the very detailed studies of re-
constructions of Au single crystals, data on the
reconstruction of other metals is scarce. For
platinum electrodes, only the thermally pre-
pared (1 × 2) reconstruction of Pt(110) was
observed by *in situ* SXRD.176,177 This struc-
ture was remarkably stable over a wide poten-
tial range and found to persist on the surface
even in the presence of adsorbed CO, which un-
der vacuum conditions induces a lifting of the
(1 × 2) phase on Pt(110). The reconstructed
Pt(100) surface does not survive immersion into
the electrolyte according to *in situ* STM obser-
vations, resulting in an unreconstructed surface
covered by 0.25 monolayer (ML) of Pt mono-
layer islands.178 In contrast to Au, potential-
induced transitions from the unreconstructed to
a reconstructed surface were not observed on Pt
electrodes. This is probably related to kinetic
limitations of the required long-range surface
mass transport, because the surface mobility of
Pt is much lower than that of Au. Similar rea-
sons may be responsible for the inability to ob-
serve surface reconstructions on the electrodes
of other high melting point metals, such as Ir.
These limitations do not exist for metals such
as Cu and Ag, which have very high surface
mobilities at room temperature. Although the
clean metal surfaces of Cu and Ag do not recon-
struct, surface reconstructions can be induced
by adsorbates. Such reconstructions were ob-
served by *in situ* STM and AFM on all three
low-index Cu surfaces in the presence of ad-
sorbed anions, hydrogen, and hydroxide.179–185
However, *in situ* SXRD studies of these systems
have not been reported up to now.

In addition to the steady-state surface struc-
ture, also the dynamic behavior of surface re-
constructions was assessed by *in situ* X-ray
scattering techniques. The kinetics of the struc-
tural transitions between reconstructed and un-
reconstructed surfaces were already addressed
in the early studies of Au(111) by Wang et al,
where the (23 × √3) ↔ (1 × 1) transition was
studied during potential sweeps and steps by
monitoring the intensity at selected reciprocal
space positions along the CTRs and (23 × √3)
superstructure rods.¹⁶¹,¹⁶² Lucas and cowork-
ers extended such *in situ* SXRD studies to el-
evated temperatures and found faster kinetics
of the surface phase transitions, which could
be attributed to the faster surface mass trans-
port.¹⁸⁶,¹⁸⁷ You and coworkers pioneered the
use of coherent surface scattering techniques to
study equilibrium fluctuations on Au(100) elec-
trode surfaces.¹⁸⁸,¹⁸⁹ Using *in situ* X-ray pho-
ton correlation spectroscopy (XPCS), they in-
vestigated fluctuations between reconstructed
and unreconstructed areas on the partly recon-
structed surface. These studies showed that the
electrochemical environment leads to increased
dynamics as compared to Au(100) surfaces un-
der UHV conditions and that the dynamics in-
crease further in the presence of halides, as ex-
pected on the basis of studies by *in situ* scan-
ning probe microscopy.

5.1.2 Surface relaxation

Even in the absence of surface reconstruction,
the symmetry break at the surface of a crystal
leads to changes in the bond distances. This
surface relaxation results in deviations of the
spacings between the topmost atomic layers as
compared to the bulk spacing, which are typ-
ically on the order of a few percent or lower.
Specular and non-specular CTRS are highly
sensitive to such changes in the layer spacings
(see Figure 17). Surface relaxations are signifi-
cantly affected by the presence of chemisorbed
species,¹⁹⁰ which provides an indirect probe of
the latter, even for very weak X-ray scatterers
such as hydrogen. On the other hand, the effect
of surface relaxation as well as increased surface
vibrations (i.e., Debye-Waller factors) has to be
taken into account in any quantitative surface
crystallographic analysis.

Surface relaxation at a metal electrode was
first reported by Wang et al., who found on
the (23 × √3) reconstructed Au(111) a 3.3% re-
laxation of the topmost layer whereas on the
unreconstructed, halide-covered surface no sur-
face relaxation was observed.¹⁶² More detailed

studies of the potential-dependent surface re-
laxation were reported by the group of Ross,
focusing on Pt electrodes. For Pt(100), an ex-
pansion of the top layer spacing was observed
when the potential was shifted 400 mV neg-
atively, from the region of hydroxide adsorp-
tion to a potential where the surface was cov-
ered by a hydrogen monolayer.¹⁹¹ Similar re-
sults were obtained for (1 × 2) reconstructed
Pt(110), revealing a complex relaxation of the
topmost 4 Pt layers and an additional expan-
sion in the presence of both hydrogen and hy-
droxide adsorbates.¹⁷⁶ An outward surface re-
laxation was also induced by adsorbed CO¹⁹² or
halides¹⁹³ on Pt(111) electrodes. The amount
of vertical relaxation can depend strongly on
the local adsorption geometry. For example,
the atoms in the surface layer of Pt(111) cov-
ered by a (√(19) × √19) adlayer of CO exhibit
a very large expansion of 0.28 Å under near-top-
site CO molecules, but only 0.04 Å under near-
bridge-site CO.¹⁹⁴ Similar local relaxation ef-
fects were also observed on halide-covered Cu
electrodes¹⁹⁵,¹⁹⁶ and will be discussed in more
detail in the next chapter. Surface relaxation
effects were also observed in SXRD studies of
bimetallic electrode surfaces¹⁹⁷⁻²⁰⁰ and in non-
aqueous electrolytes.²⁰¹ Furthermore, a close
correlation between the vertical surface relax-
ation and the surface stress was found, which
could be rationalized by their dependence on
the metal's electronic structure.²⁰²,²⁰³

5.2 Adsorbates at metal elec-
trodes

Adsorption of electrolyte species at the elec-
trode surface, such as anions, cations, neu-
tral chemisorbed species, and solvent molecules,
is a ubiquitous phenomenon in electrochemi-
cal systems. These adsorbates can either di-
rectly participate in electrode reactions or be
present as spectator species that influence the
reactions. Clarifying the precise arrangement
of these species at the interface is thus of cen-
tral importance for understanding the electro-
chemical reactivity. Together with *in situ* scan-
ning probe microscopy, *in situ* surface X-ray
scattering methods have played a central role
in such data on the adlayer structure. First
attempts at investigating adsorption phenom-
ena by *in situ* XRD were made by Fleischmann
and coworkers, who employed a laboratory X-
ray anode for studies of potential-dependent
changes in the scattering of polycrystalline and
thin film electrodes.²⁰⁴⁻²⁰⁶ However, unambigu-
ous surface structural data was only obtained
in studies by Melroy, Toney, and coworkers,
where synchrotron radiation was used for *in situ*
GIXRD studies of metal monolayers formed
by underpotential deposition (UPD) on single
crystal electrodes.²⁰⁷⁻²¹⁰ Indeed, these studies
represent the very first direct determination
of the atomic-scale structure at an electrode-
electrolyte interface. Many similar studies of
adlayer structure have been performed since
then, revealing a complex, potential-dependent
surface phase behavior of the adsorbed species.
In the following chapter, a short overview is
given, focusing on the results from SXRD mea-
surements. For a more detailed discussion of
adlayer structures at electrode surfaces, we re-
fer to Ref. ¹⁴⁸,¹⁴⁹,²¹¹.

5.2.1 Anion adlayers

*In situ* SXRD studies of anion adlayers fo-
cused on halides adsorbed on coinage met-
als and platinum electrodes.¹⁴⁵,¹⁴⁸ Here, vari-
ous ordered adlayers were found positive of a
critical potential. The structure of these ad-
layers depends on the in-plane order of the
substrate and the coverage, which in electro-
chemical systems is determined by the poten-
tial. On the (111) surface of Au, Ag, and Cu,
incommensurate structures were dominant, in
which the halide packing density continuously
increased with increasing potential (see Figure
16c-e).⁹⁶,¹⁴⁵,²¹²⁻²¹⁶ At the most positive poten-
tials, prior to the onset of halide-induced metal
dissolution, Au(111) and Ag(111) are covered
by close-packed 2D incommensurate adlayers.
In the case of iodine, this 2D adlayer is pre-
ceded by an uniaxially-incommensurate $c(p \times
\sqrt{3})$ phase and, on Ag(111), a commensurate
$(\sqrt{3} \times \sqrt{3})$R30°.¹⁴⁵ On Cu(111), GIXS mea-
surements were only performed for Cl, finding a
2D incommensurate adlayer.²¹⁵,²¹⁶ On Pt(111)

disordered adlayers or commensurate structures
with larger unit cells were observed, usually
with more than one adsorbate in the super-
structures unit cell. Examples are a (3 × 3)
and (√7 × √7)R19° superstructure of iodide on
Pt(111)¹⁴⁵,¹⁹³ Apart from GIXS studies, also *in
situ* CTR studies were performed for Cl and
Br on Au(111), which provided similar adlayer
coverages as the in-plane studies and a vertical
spacing between the halide and the Au surface
layer of 2.4 Å, indicating chemisorption of these
species.¹⁶²

On (100) and (110) surfaces, predominantly
commensurate structures are observed, reflect-
ing the higher energetic corrugation of the (100)
as compared to the (111) lattice. Incommensu-
rate adlayers were only found on Au(100) elec-
trodes, namely a (√2 × p)R45° structure, which
undergoes a 2D phase transition to a commen-
surate, quasi-hexagonal (√2 × 2√2)R45° ad-
layer,¹⁴⁵,¹⁴⁸,²¹⁷ and for iodide coadsorbed with
alkali cations on Au(110).²¹⁸ The most com-
monly observed structure on (100) surfaces is a
c(2 × 2) adlayer with a coverage of 0.5 ML (Fig-
ure 23a), which has been observed for Cl and Br
on Ag(100) and Cu(100).¹⁴⁶,¹⁹⁵,¹⁹⁶,²¹⁹⁻²²⁴ This
particularly simple type of adlayer is highly
suited for in-depth studies of anion adsorp-
tion. Ocko et al. studied by *in situ* GIXS
the potential-dependent transition between the
disordered and the c(2 × 2) ordered Br ad-
layer on Ag(100).²¹⁹,²²⁰ They found that the
formation of the c(2 × 2) phase occurred in
a second-order phase transition at a coverage
of 0.35 ML as determined from chronocoulo-
metric measurements, in good agreements with
simulations.²²⁵,²²⁶ Detailed *in situ* CTR mea-
surements were performed for Cl and Br on
Cu(100), revealing the detailed adsorption ge-
ometry.¹⁹⁵,¹⁹⁶,²²¹⁻²²⁴ As in UHV,²²⁷ the halide
ions adsorb in fourfold-hollow sites. However,
in-depth SXRD studies and corresponding DFT
calculations revealed subtle but distinct differ-
ences to adsorption from the gas phase, caused
by the adjacent electrolyte.¹⁹⁵,¹⁹⁶,²²¹,²²³,²²⁸,²²⁹
Specifically, the Cl-Cu bond length is notice-
ably increased, the spacing between the two
topmost Cu layers is contracted, and the buck-
ling of the Cu atoms in the second layer is
reversed in the presence of the electrolyte as compared to UHV. On the basis of DFT calculations, these effects could be assigned to the influence of water and cations in the outer Helmholtz plane of the electrochemical double layer.

In a similar way, also the influence of “specifically adsorbing” cations that interact more strongly with the adlayer has been assessed. On Ag(100) and Cu(100) surfaces that are fully covered by a $c(2 \times 2)$ halide adlayer a partial coverage of coadsorbed (hydrated) $K^+$ and $Cs^+$ cations was found by SXRD (Figure 23b).$^{223,228-230}$ These coadsorbed cations were proposed to be located in fourfold coordinated sites with respect to the halide anions underneath. They were bound to those via bridging hydration water and gradually desorbed towards more positive potentials (Figure 23b,c). In an intermediate coverage range, the anion and cation mutually promoted their adsorption on the electrode surface.$^{228}$ This cooperative adsorption manifests in an enhanced Br adsorption as compared to electrolytes with non-specifically adsorbing $Li^+$ cations and a corresponding maximum in $Cs^+$ coverage near the potential of the 2D phase transition between disordered and $c(2 \times 2)$ adlayer. Recently, also adsorbed $Cs^+$ on the $(1 \times 2)$ reconstructed Pt(110) was reported.$^{231}$ These measurements illustrate that also the outer parts of the electrochemical double layer are accessible to *in situ* SXRD studies, making it possible to probe the subtle interplay of the molecular interactions in this part of the interface region. Furthermore, Nakamura et al. demonstrated for the same system that the dynamics of adsorption processes can be studied on sub-millisecond time scales by fast time-resolved SXRD measurements, allowing to detect short-lived transient states.$^{229}$ Based on these experiments they proposed that the $Cs^+$ ions were in a first step accumulated in the outer Helmholtz plane, followed by a slight vertical relaxation towards the surface (Figure 23c). These time-resolved studies required averaging over $\approx 10^5$ potential cycles and thus require highly reversible systems. However, with the increasing availability of 4th generation synchrotron sources and

XFELs, also dynamic studies of less reversible processes may get into reach. Finally, also the electronic interface structure of halide-covered electrodes has been recently probed by pioneering studies of Grunder et al., using resonant surface X-ray diffraction.$^{141,232}$ Together with *ab initio* calculations, these measurements allowed to determine the charge distribution at $c(2 \times 2)$ - Br covered Cu(100) electrode. They showed that the bonding of the chemisorbed adsorbate to the metal leads to a rearrangement of the charge and a shift of the surface dipole moment into the metal electrode.$^{141}$

<figure><img src="image_40.png" /></figure>

Figure 23: (a) Structural arrangement in the $c(2 \times 2)$ halide adlayer on (100)-oriented fcc metal surfaces. (b) Influence of alkali cations on the Br adlayer coverage on Ag(100) (reprinted from Ref.$^{228}$ with the permission of the APS). (c) Temporal changes of the adlayer structure in Cs-containing solution during a potential step from -0.1 to -0.6 V, derived from *in situ* XRR measurements with 100 µs time resolution (Reprinted from Ref.$^{229}$. Copyright 2014 American Chemical Society).

### 5.2.2 Metal adlayers

The formation of metal adlayers via underpotential deposition (UPD) is an extensively studied phenomenon in interfacial electrochemistry.$^{233,234}$ *In situ* surface X-ray scattering technique have contributed greatly in resolving the atomic-scale structure of these adlayers. From the initial *in situ* GIXS studies
on, UPD adlayers of heavy metals, such as Pb,<sup>177,193,207-210,235,236</sup> Tl,<sup>236-243</sup> and Bi,<sup>244-247</sup> were studied on the (111) and (100) surfaces of Ag, Au, and Pt. For Pb and Tl on Ag(111) and Au(111), hexagonal close-packed adlayers with 2D incommensurate structures were found that were continuously compressed with decreasing potential.<sup>207-210,236</sup> This electrocompression effect is similar as found for halide adlayers, but much smaller in magnitude. For Tl on Au(111) in alkaline electrolyte, in addition, an incommensurate aligned-hexagonal phase was found at more positive potentials.<sup>238,239</sup> Tl UPD layers on Au(100) and Ag(100) form ($p \times \sqrt{2}$), which compress uniaxially.<sup>240,241</sup> A particularly rich structural phase behavior was observed for Bi UPD layers. On Au(111), first a (2 × 2) and then a uniaxially incommensurate ($p \times \sqrt{3}$) structure forms with decreasing potential.<sup>245,246</sup> The latter structure was also observed for Bi on Ag(111).<sup>244</sup> On Au(100), a sequence of even three ordered adlayer phases is observed: a (3 × 3)R45° structure consisting of square Bi quadrometers, a ($c\sqrt{2} \times 3\sqrt{2}$) structure, where the Bi adatoms are arranged in zigzag chains, and a quasi-hexagonal close-packed $c(p \times 2)$ adlayer, which is uniaxially incommensurate and compressible.<sup>247</sup> In contrast, UPD layers of these metals on Pt electrodes had commensurate structures or were disordered. Pb forms a (3 × √3) structure on Pt(111), whereas it is disordered on Pt(100) and Pt(110).<sup>235</sup> Bromide anions have a strong influence on the Pb adlayer, inducing on Pt(100) a (2 × 2) ordering<sup>248,249</sup> and on Pt(111) a (2 × 2) structure, which was attributed to a PbPt surface alloy.<sup>250</sup> SXRD studies of irreversibly adsorbed Bi found a disordered adlayer with a coverage of ≈1/3 ML on Pt(111)<sup>251</sup> and a (2 × 2) structure on Pt(100).<sup>252</sup>

The arguably best-studied system is the UPD of copper. In this case not merely a metal monolayer is formed, rather Cu coadsorbs with specifically adsorbing anions. A key system here is Cu UPD on Au(111) in sulfuric acid solution where a with decreasing potential first a (√3 × √3)R30° superstructure is formed, which in a second surface phase transition changes to a full Cu monolayer with adsorbed sulfate on

top. The arrangement in the (√3 × √3)R30° phase was clarified in detailed surface crystallographic studies and shown to consist of a Cu honeycomb lattice with 2/3 ML coverage, in which 1/3 ML of sulfate ions were adsorbed in the honeycomb centers, bound via 3 of their oxygen atoms.<sup>253,254</sup> An identical behavior was found for the Cu UPD adlayer in sulfuric acid on Pt(111).<sup>255</sup> The influence of the anion on the adlayer structure increases for more strongly coadsorbing ions, such as halides. For Cu UPD on Pt(111) in Cl or Br containing solution stoichiometric bilayers with a hexagonal incommensurate structure are formed, in which the 2D Cu and the halide lattices were in registry with each other, but not with the substrate.<sup>256-258</sup> This type of arrangement, which was determined using anomalous surface scattering, resembles that found in the (111) plane of CuCl and CuBr salts, respectively. Cu UPD on Pt(100) was found to occur always in form of a pseudomorphic (1 × 1) layer, but the potential range and formation kinetics significantly increased in the bromide-containing solution.<sup>249,259,260</sup>

The mutual stabilization of anions and cations on metal electrodes is not restricted to Cu UPD, but has been observed in a number of systems. Particularly insightful are *in situ* X-ray surface scattering studies of Wang and coworkers that reported the presence of salt-like adlayers of halides coadsorbed with Tl or Pb on Au(111) electrodes in acidic electrolyte.<sup>242,243,261</sup> These systematic studies reported a complex potential-dependent sequence of differently ordered adlayer structures. With increasing potential, the stoichiometry changes stepwise from a purely metallic adlayer to a pure halide layer. In an intermediate potential range, various salt-like phases are found, including phases with a square 1:1 arrangement of halide and metal. The halide is either coadsorbed with the metal species on the Au surface or resides on top of the adsorbed metal layer. In both cases, the ad-metal is stabilized on the electrode surface at much more positive potentials as in halide-free electrolyte. For Pb in Br-containing solution, the transition between the different mixed adlayer phases was found to
be slow, resulting in the long-term coexistence
of those phases.²⁶¹

Among the other UPD systems studied by *in situ* SXRD methods, the most interesting cases are Ag and Pd UPD. Ag UPD on Au and Pt is unusual in that respect that not only monolayer but also bilayer phases are formed prior to the onset of bulk Ag deposition. According to SXRD measurements on Pt(111), the initially formed Ag monolayer is pseudomorphic, despite the large lattice mismatch to the Pt substrate, whereas the bilayer adapts an expanded hexagonal lattice to partly release the associated surface stress.²⁶²,²⁶³ This commensurate to incommensurate transition is accompanied by a substantial decrease of the Ag-Pt spacing and an expansion of the vertical spacing between the two Ag layers as compared to bulk Ag, which was attributed to charge transfer from Ag to Pt and results in enhanced adsorption of sulfate anions and CO on the Ag top layer. Pd UPD is of interest for electrocatalysis and hydrogen storage (see section). It was studied on Pt(111) and Pt(100) by the Markovic group, who reported a pseudomorphic growth behavior.¹⁹⁸,²⁶⁴,²⁶⁵

### 5.2.3 Molecular adsorption

Adsorbed molecular species play a central role
in electrochemical reactions, e.g. as intermediates, inhibitors, or accelerators. Although these
molecular adsorbates often only contain low-Z elements (C, O, H) and thus scatter X-rays
only weakly, their in-plane order often could
be detected successfully. The reason for this
is the local relaxation of the substrate lattice,
induced by the chemisorbing molecule. This relaxation leads to a periodic modulation of the
(strongly scattering) atoms in the substrate's
surface layer(s) with the same in-plane periodicity as that of the adlayer structure, which can
provide a strong and often dominant contribution to the diffraction signal.¹⁹⁴

The by far most studied molecular adsorbate
is CO, owing to its importance as a catalyst
poison and reaction intermediate. Especially
CO on Pt(111) is a key model system in interfacial electrochemistry and has been studied

in detail by *in situ* surface diffraction.¹⁹⁴,²⁶⁶⁻²⁷⁰ *In situ* STM studies showed that CO adlayers formed by adsorption from CO-saturated solution adapt at potentials near the onset of hydrogen evolution a (*p*2 × 2) structure with a coverage of 0.75 ML, which undergoes a transition to a (√19×√19)R23.4° adlayer with 13 CO in the unit cell (13/19 ML coverage) upon increasing the potential into the "pre-oxidation" range.²⁷¹,²⁷² This adlayer phase behavior was confirmed by *in situ* SXRD measurements, which allowed not only to clarify the structure but also to study the adlayer kinetics during potential sweeps.¹⁹⁴,²⁶⁷⁻²⁷⁰,²⁷³ These studies revealed that the stability of the two phases and the surface phase transition depend strongly on the CO partial pressure in the solution as well as on the anion species in the electrolyte. Furthermore, temperature-dependent SXRD studies of the *p*(2 × 2) ↔ (√19 × √19)R23.4° transition showed that the ordering of the (*p*2×2) CO adlayer has a maximum near room temperature and that (√19 × √19)R23.4° ordering increases with temperature.¹⁴⁹ The (*p*2 × 2) CO adlayer was also studied by *in situ* resonant anomalous surface X-ray scattering, from which the Pt-C binding distances were determined.²⁷⁴

On Pt(100) electrodes, CO induces surface relaxation, but no ordered CO adlayers were observed in SXRD studies,²⁷⁵ in contrast to reports by *in situ* STM.²⁷⁶,²⁷⁷ No ordered CO adlayers were also found on Au(111), Au(100), and Au(110) electrodes, however, in alkaline electrolyte the presence of CO in the electrolyte stabilized the surface reconstruction in these systems over a wide potential regime.¹⁶⁹,²⁷⁸ In addition, bimetallic systems of Pt and other metals have been investigated, owning to the interesting electrocatalytic properties of Pt alloys. Lucas et al. studied the interaction of CO with metal UPD layers adsorbed on Pt single-crystal surfaces. Cu and Pb adlayers on Pt(111) and Pt(100) were found to be displaced by CO, whereas the (*c*2 × 2) Bi UPD layer on Pt(100) was unaffected.¹⁹⁷,²⁷⁹ Pd monolayers on Pt(111) exhibited a reversible vertical contraction of the Pd-Pt spacing upon CO adsorption and transformed into a Pd oxide phase in the CO oxidation region.²⁸⁰ Ag mono- and bilayers
on Pt(111) were found to adsorb and oxidize
CO, contrary to Ag(111) electrodes, which was
attributed to Pt-induced changes in the elec-
tronic structure.²⁶³

A second type of molecular adsorbates that
are of great importance in fundamental elec-
trochemistry and electrocatalysis are oxygen
species, such as OH, water, and H₃O⁺. For
Pt(111) in HClO₄ solution, Kondo et al. re-
ported a continuous increase in the coverage
of oxygen species and a concomitant relax-
ation of the spacing of the topmost Pt layer
in the entire double-layer regime.²⁸¹ In accor-
dance with results for Pt(111) oxidation in the
gas phase and DFT calculations, they reported
a p(2 × 2) phase in their *in situ* SXRD studies
and assigned it to a mixed layer of OHads and
H₂O. Furthermore, they observed in potential-
dependent measurements of the non-specular
CTR a distinct two-step process. This behav-
ior was attributed to random adsorption in the
broad region, followed by the formation of a
transient p(2 × 2) adsorbate phase and a sub-
sequent phase transition to a p(1 × 1) adlayer
at the sharp butterfly peak. In addition, SXRD
studies by Liu et al. found the formation of a
buckled top Pt layer in the butterfly region,²⁸²
in good agreement with DFT predictions.²⁸³ In
both SXRD studies, the CTR data was modeled
with the oxygen atoms in fcc or hcp hollow sites,
on the basis of results from theory and oxygen
adsorption in the gas-phase. Nakamura et al.
reported a pronounced cation effect on OH ad-
sorption on Pt(111) in alkaline electrolytes.²⁸⁴
Whereas Li⁺ ions were found to stabilize OHad,
no such stabilization was observed in the case
of Cs⁺ ions. A similar cation-induced stabiliza-
tion of OHad was also reported for Ag(111) in
an alkaline solution.²⁸⁵ Furthermore, *in situ* X-
ray reflectivity studies of Ru(0001) in H₂SO₄
solution revealed that the (bi)sulfate adlayer is
partially replaced by adsorbed oxygen species
between 0.6 and 1 V vs. reversible hydrogen
electrode (RHE).

The structure of water near metal electrodes
has been a topic of some controversy. Toney
et al. performed detailed CTR measurements
of Ag(111) in NaF solution.²⁸⁶,²⁸⁷ According
to their analysis, the orientation of the water

dipoles flips upon reversal of the electrode's polarity and the density of the interfacial water layer significantly exceeds that of bulk water. While the water layering and orientation reversal are undisputed, the strongly enhanced water density was not found in other X-ray surface scattering studies and molecular dynamics (MD) simulations of water at metal surfaces. For Ag(100) in NaF electrolyte²⁸⁸ and Au(111) in halide solutions,¹⁶² the CTRs were well described with water layers that corresponded to the bulk water density. MD simulations as well as more recent AIMD simulations of water at Ag, Hg, and Pt electrodes also found water layers with bulk-like density at the interface.²⁸⁹⁻²⁹² Furthermore, the interfacial water at multilayer graphene on SiC was studied by SXRD.²⁹³,²⁹⁴ Although the electron density contrast between water and the solid is more favorable here than in the case of metals, the intrinsic heterogeneity of these electrodes leads to additional difficulties in the data analysis. Zhou et al. studied this interface at open circuit conditions and reported a slight hydrophobic gap between graphene and the first water layer, as predicted by AIMD simulations.²⁹³ However, the water-graphene interactions depended strongly on the number of epitaxial graphene layers and defects. Hayashi et al. performed potential-depend measurements and proposed an ice-like water bilayer at the interface without adsorbed ions in the double layer.²⁹⁴

More recently, these studies of the near-
surface structure of the electrolyte near the
solid surface were extended to non-aqueous sys-
tems, in particular ionic liquids (ILs). First,
in situ XRR studies of these types of inter-
faces were performed for ILs at a sapphire sin-
gle crystal surface and revealed strong stratifi-
cation into alternating cationic and anionic lay-
ers, with the layering decaying into the bulk on
length scales of 1-2 nm.²⁹⁵ Studies of electro-
chemical interfaces to Au(111)²⁹⁶,²⁹⁷, graphene
on SiC,²⁹⁸,²⁹⁹, SrTiO₃,³⁰⁰, boron doped di-
mond,³⁰¹, and Si.³⁰² These studies found simi-
lar interface layering, as well as a reversal of the
anion and cation layers with potential. In some
cases, substantial crowding, i.e., accumulation
of more than a single layer of one of the ion
species at the electrode surface, was observed at potentials far away from the pzc.³⁰⁰,³⁰³ Very recent studies of mixtures of ILs with carbonate solvents reported at positive potentials several nanometer thick layers, in which the anion concentration was 3 to 5 times higher than that of the cations.³⁰² Furthermore, very slow interfacial dynamics on time scales from 10 to 1000 s were reported.²⁹⁷⁻²⁹⁹,³⁰⁴

Overall, these studies illustrate the unique capabilities of X-ray surface scattering methods for probing not only species that are directly chemisorbed at the electrode surface, but also the molecular arrangement within the outer parts of the electrochemical double layer. However, precise measurements are challenging, and clarification of the detailed structure of the electrolyte near the electrode and its dependence on the potential will require further efforts.

## 5.3 Interface structure of oxides and minerals

Metal oxides have received increasing attention in many research fields, including catalysis, gas sensing, and contaminant removal. In the context of electrocatalysis, oxides have become of great interest in recent years, in particular as catalysts for oxygen evolution and reduction. Furthermore, oxide films and oxide-electrolyte interfaces are of great interest in electrochemical materials science and corrosion, especially if they exhibit sufficiently high electric conductivity. Most studies of oxide interface structure have been driven by questions coming from geochemical and environmental science and focus on common minerals, such as calcite, barite, quartz, corundum, hematite, rutile, and muscovite. Although these are, with the exception of hematite, good electric insulators and the state of their interfaces is, thus, controlled by the solution chemistry rather than the potential, the results obtained here are still relevant for electrochemical systems. First, general insight on oxide-electrolyte interactions is obtained that can be transferred to oxidic electrode materials. Second, (ultra)thin films of such oxides or related species may form during electrochemical reactions, e.g., due to passiva-

tion, and affect the latter. We, therefore, included this work in this and the following chapter.

Oxides and other minerals typically have multiple energetically stable crystallographic planes of different terminations that would expose a rich combination of different surface oxygen groups. The latter are potential binding sites and are involved in a range of interfacial processes. For example, they may facilitate the surface complexation of metal ions, modulate the H-bonding network of interfacial water, and induce the aggregation of small molecules to form nanoparticles at the surface. Similar to metals, oxide surfaces may reconstruct as a function of potential and electrolyte conditions. Obtaining insight into the atomic-scale oxide-electrolyte interface structure is thus of substantial importance for an improved understanding of oxide surface chemistry. In this chapter, we will discuss *in situ* SXRD results on the surface structure of oxides and minerals in aqueous solutions.

### 5.3.1 Surface termination and surface reconstruction

Because of the more complex bulk structure, also the surface and interface structure of oxides is much more complex than those of metals, as shown in extensive studies of clean oxide surfaces under UHV conditions.³⁰⁵,³⁰⁶ Different crystal facets on oxides and minerals can have very different stability. For oxides in contact with the vacuum, a key aspect is surface polarity. Surfaces that are charge-neutral (nonpolar surfaces), i.e., carry identical anionic and cationic charges in the top layer, are typically thermodynamically stable. In contrast, surfaces that carry an excess charge (polar surfaces) are unstable. Here, stabilization is achieved either by reconstruction of the surface or adsorbates, in particular the replacement of surface oxygen species by OH. The latter stabilization mechanism is easily accessible in aqueous solutions. Furthermore, in electrochemical systems, the surface charge and, thus, the degree of surface hydroxylation can be varied by the potential.
Most oxide interfaces found in natural (aqueous) environments correspond to low-index crystallographic facets. High-index surfaces can usually only be obtained through specific synthesis procedures and are typically very reactive. Although the latter is often desired in electrocatalysis, such high-index surfaces are unstable and tend to undergo structural reconstruction. In addition, different surface terminations may coexist on surfaces with the same crystallographic orientation, depending on the preparation conditions. For example, an r-cut hematite (1102) surface tends to terminate with a stoichiometric crystallographic plane (Figure 24a) upon annealing at 500°C,³⁰⁷ whereas preparation via a chemical mechanical polishing procedure resulted in a partial transformation of this stoichiometric surface termination into a so-called half-layer termination.³⁰⁸. Considering that in all oxide crystal lattices multiple atomic layers with different surface symmetries exist, different surface termination patterns are likely to occur in other oxide materials as well.

On well-prepared oxide single-crystal surfaces in ultrahigh vacuum, often ordered surface reconstructions are observed. Whether similar reconstructions exist also in an electrochemical environment is largely not clear, because of the challenges to prepare oxide electrode surfaces of similar quality. A first *in situ* GIXD study on this topic was recently performed by Grumelli et al. for the case of the (100) surface of magnetite.³⁰⁹ This surface exhibits a characteristic (√2 × √2)R45 deg reconstruction, in which the presence of subsurface Fe vacancies and interstitials leads to a modified structure where the top four atomic layers exclusively contain Fe³⁺ cations.³¹⁰,³¹¹ By exposure to pure water vapor, this reconstruction is quickly lifted.³¹¹ Nevertheless, the work by Grumelli et al. revealed that the (√2 × √2)R45 deg structure remained at least partly stable in 0.1 M NaOH, if the potential is kept above 0.8 V vs RHE. Only at more negative potentials, where the magnetite surface starts to be reduced, irreversible removal of the (√2 × √2)R45 deg superstructure rods occurred. Considering that the reconstruction corresponds to an oxidized state of the surface, these results indicate that sufficiently

oxidizing conditions are required to maintain the reconstruction. Similar may be true for other reconstructed oxide surfaces in an aqueous environment.

### 5.3.2 Surface relaxation

Similarly as in the case of metals, the atomic layers at oxide surfaces tend to relax at surfaces and interfaces, i.e., the bonds to neighboring atoms expand or contract as compared to the bulk lattice. This relaxation depends on the oxidation state of surface species and the protonation of the surface and, thus, on the solution pH. The associated displacements from the bulk lattice positions are in the sub-angstrom range and can be easily resolved by surface X-ray diffraction measurements. Schlegel et al. applied X-ray reflectivity to probe the structure of quartz (100) and (101) surfaces in water.³¹² The vertical relaxation of terminal oxygen was found to be as large as 0.4 Å, and the relaxation observed for atoms in deeper layers were decreasing quickly within a slab of 14 Å. A similar surface relaxation was observed on barite (001) and (210) surfaces, which showed a larger displacement of 0.4 Å for the outermost SO₄ ions.³¹³ In addition, the second and third SO₄ ion group was significantly rotated with respect to the surface normal direction by 8° and 19°, respectively. The relaxation of Ba ions is negligible (≈0.07 Å), and the relaxation extended to a much lower depth (≈3 Å) much smaller than that observed on quartz.³¹² The solution chemistry can influence surface relaxation as well. Zhang et al. compared the relaxation of the rutile (110) surface in deionized water with that in Rb⁺ solution at pH 12.³¹⁴ Interestingly, the surface Ti atoms are displaced from their bulk positions by up to 0.05 Å when exposed to deionized water, whereas this relaxation is largely suppressed in Rb⁺ solution. Lee et al. studied by X-ray reflectivity the surface relaxation of the muscovite (001) surface in solutions containing different alkali ions, including Li⁺, Na⁺, K⁺, Rb⁺ and Cs⁺.³¹⁵ They reported systematic trends in the vertical atomic displacements within a region up to 40 Å from the surface. These involved a general
<0.1 Å shift of the interlayer K⁺ cations toward the interface and an expansion of the tetrahedral-octahedral-tetrahedral layers, except for the top layer in contact with the solution. This cation-dependent relaxation was found for the distortion of the top tetrahedral sheet as well as its tilting angle.

## 5.4 Adsorption on oxides and minerals

### 5.4.1 General aspects

*In situ* surface X-ray scattering methods have played a central role in determining the binding geometry of adsorbed species on oxide surfaces, which is considerably more complex than in the case of metal surfaces. On oxides, surface functional groups, namely under-coordinated oxygen species, are reactive surface sites responsible for a variety of interface reactions, including adsorption of surface complex species. In an aqueous solution, metal atoms at oxide surfaces are coordinated with aqueous O species (OH⁻, H₂O, H₃O⁺). Those groups are under-saturated, i.e., containing unoccupied orbitals that enable surface complexation reactions. In addition, lattice O atoms may be under-coordinated as well due to the presence of broken metal-O bonds at the surface, e.g., at surface defects. Consequently, surface complex species can be bound to different types of surface functional groups, giving rise to numerous binding configurations. Traditionally, surface binding is categorized into two large groups, inner-sphere (IS) and outer-sphere (OS) binding mode, which correspond to species in the inner and outer Helmholtz layer in electrochemical nomenclature.

In IS binding mode, the adsorbate forms at least one covalent bond with a surface O group, resulting in a comparably short distance of the complex from the surface. The binding configuration is then further distinguished by the number of formed covalent bonds to the surface. Common configurations are monodentate (MoD), bidentate (BD), tridentate (TD), and tetradentate (TeD) binding corresponding to adsorption via one, two, three, and four cova-

lent bonds, respectively. Apart from the number of covalent bonds, the origin of the surface O species that are involved in the surface complexation reaction can be different. One distinguishes mononuclear (MN), binuclear (BN), and trinuclear (TN) surface complex species, depending on whether the complexing oxygen groups belong to the same structural motif in the oxide lattice, i.e., are coordinated to the same metal cation, or belong to two or three structural motifs, respectively. Furthermore, involved O atoms can share the edge or face of the same structural motif (called edge-sharing and face-sharing, respectively) or, on polynuclear complexes, binding O atoms can be located at the outer corners of the structural motifs (corner-sharing). In combination, these classifications are commonly used to describe the adsorption geometry of species that bind covalently to oxide surfaces.

In the OS binding mode, the adsorbate is surrounded by a complete coordination shell of oxygen species (OH⁻, H₂O, H₃O⁺) from the solution, and the complex species adsorbs through electrostatic attraction or hydrogen bonding. Accordingly, this binding mode is structurally less defined and the binding strength is sensitive to the surface net charge. Furthermore, OS complex species are located further away from the oxide surface and are described by a broader density distribution along the surface normal direction. In some high-resolution X-ray reflectivity studies,³¹⁶ it was found that OS species could be further classified into two sub-types according to the distance of the species from the substrate surface, namely adsorbed OS species and extended OS species. Adsorbed OS species are located directly above the substrate surface with one or two water layers confined between the surface complex species and the oxide surface. In contrast, the extended OS species is located further away, typically >1 nm from the surface, and exhibits a broader density distribution.

### 5.4.2 Metal cation adsorption

Cation complexation on oxide and mineral surfaces has been extensively addressed by *in situ*
SXRD methods. Here, we briefly summarize those studies, starting with calcite (CaCO₃), the most stable polymorph of calcium carbonate. Calcite is ubiquitous in natural water and has a significant impact on the environment due to its ability to buffer pH and scavenge trace metals. In SXRD studies, both As(IV)³¹⁷ and Pb(II)³¹⁸ were found to associate with the calcite (104) surface in an IS mode, where the cation adsorbate substitutes the structural Ca atoms (also called incorporation process). The bound Pb species were vertically displaced by 0.2 Å compared to the unrelaxed Ca positions, which was related to the larger size of Pb. In contrast, U(VI), which has a similar size as Pb(II), was found to not bind to calcite(104) through isomorphic substitution but via the formation of covalent bonds with O groups originating from surface carbonate groups.³¹⁹ Similar studies on barite(BaSO₄) showed both Pb(II)³²⁰ and Sr(II)³²¹ to be able to incorporate into the barite surface structure through substitution. Interestingly, in the case of Pb(II) on barite(001), these incorporated species were found to coexist with IS binding species and, at the highest Pb concentrations (200 µM), OS species.

Hematite (α-Fe₂O₃) is a common natural mineral as well as a widely studied material for electrochemical water oxidation.³²⁵ It has a corundum-type structure and a hexagonal unit cell with two-thirds of the sites filled with Fe atoms. As mentioned in section 5.3.1, the r-cut hematite (1102) surface can be prepared with different types of surface terminations (Figure 24a), containing a mixture of different surface O groups. The adsorption of different metal cations on r-cut hematite has been studied using *in situ* X-ray reflectivity and X-ray standing wave (XSW) measurements. The associated binding structure was found to depend on the type of studied cation. Se(IV) was found to form a surface complex species via a bidentate binuclear binding.³²⁶ XSW studies of arsenate adsorption on r-cut hematite suggested a bidentate bound As species, bridging two terminal O groups.³²⁷ In subsequent RAXS measurements, evidence of additional OS-bound species was also found (Figure 24b).³²² In detailed sur-

face crystallographic studies, Qiu et al. investigated the Pb(II) binding mechanism on r-cut hematite (1102), treated by different procedures to prepare different types of surface termination.³²³,³²⁸ The best fit of the CTRs was obtained by a structural model in which Pb(II) is bound in bidentate binuclear edge-sharing sites. On a half-layer terminated hematite surface two types of such sites were shown to be occupied (Figure 24c,d). In contrast, on a stoichiometrically terminated surface Pb(II) selectively binds to only a single type of these sites whereas the other site was not occupied. The latter was attributed to the structural mismatch between the binding species and the surface sites. The same group also investigated Sb(V) adsorption on a half-layer terminated hematite surface.³²⁴ Here, the SXRD results indicated a tridentate binding structure that bridged two terminal O groups and another type of O group that is coordinated with two lattice Fe atoms (Figure 24e).

A key oxide system that has been studied extensively in the past is titanium dioxide, especially in the rutile structure.³²⁹ Surface complexation of metal ions on the rutile (110) surface has been widely studied using surface X-ray diffraction. Rutile (110) has a well-defined surface structure with two alternating rows of oxygen atoms, one composed of terminal O groups binding to one Ti atom (¹O) and one in which the O atoms are coordinated by two adjacent Ti atoms (¹¹O). This configuration enables multiple binding modes, such as bidentate binuclear corner-sharing and tetradentate trinuclear corner-sharing/edge-sharing binding geometries. A comprehensive study of metal binding occurring at rutile (110) surface in contact with Rb⁺, Sr²⁺, Zn²⁺ and Y³⁺ solutions was conducted by Zhang et al. using a range of different techniques, including X-ray reflectivity and X-ray standing wave measurements, *ab initio* calculations, molecular dynamics simulations, and classical macroscopic surface sorption experiments.³³⁰ The molecular structure derived from the X-ray data suggests that all cations are bound to rutile (110) via a tetradentate binding geometry which bridges four adjacent surface functional O groups, except for
<caption>a</caption>

Fe₂O₃ (1102)

stoichometric termination

half-layer termination

<figure><img src="image_41.png" /><figcaption>b</figcaption></figure>

<figure><img src="image_42.png" /><figcaption>c</figcaption></figure>

<figure><img src="image_43.png" /><figcaption>d</figcaption></figure>

<figure><img src="image_44.png" /><figcaption>e</figcaption></figure>

<figure><img src="image_45.png" /></figure>

<figure><img src="image_46.png" /></figure>

<figure><img src="image_47.png" /></figure>

<figure><img src="image_48.png" /></figure>

Figure 24: Adsorption of different species on the r-cut hematite (1102) surface. (a) Molecular structure of the hematite (1102) surface with stoichiometric and half-layer termination. (b) Si-multaneous IS and OS binding geometry of As(V) on the stoichiometrically terminated surface (reprinted from Ref. ³²². Copyright 2008 Elsevier). Binding structures of Pb(II) on (c,d) half-layer terminated hematite (1102) at two different surface sites (reprinted from Ref. ³²³. Copyright 2018 American Chemical Society). (e) Binding structure of Sb(V) on half-layer termination hematite (1102) (adapted from Ref. ³²⁴. Copyright 2018 American Chemical Society)
Zn²⁺, which dominantly adsorbs in a bidentate mode above two ¹¹O and, to a smaller extent, ¹O groups (Figure 25a). Similar results were reported by Predota et al., who studied the binding geometry of different ions (Rb⁺, Na⁺, Sr²⁺, Zn²⁺, Cl⁻) by molecular dynamics simulations, X-ray standing waves and CTR measurements.³³¹ Also here, a bidentate geometry of Zn²⁺ was found, as opposed to the other cations which displayed a tetradentate binding configuration. The difference in the binding geometry observed for Zn²⁺ compared to the other cations was attributed to the difference in ionic radii of the studied cations (Figure 25b). Interestingly, a more recent work by Zhang et al. proposed Zn²⁺ to be bound in addition in a monodentate mode to a single ¹¹O group.³³² This study also showed that the pH (pH 8 and 6) has no effect on the adsorption geometry but favors the bidentate surface species at pH 6. The same group also studied the binding geometry of Rb⁺ on rutile (110) in an alkaline solution at pH 12 and found the same binding mode as in earlier reports, i.e., a tetradentate binding geometry.³³⁴ Using RAXS, the adsorption heights of Rb⁺ (3.72 ± 0.02 Å) and Sr²⁺ (3.05 ± 0.16 Å) relative to the first Ti layer was determined and identified as the average height of surface species with both bidentate and tetradentate binding modes.³³³ Such a multi-site adsorption mechanism was not proposed in those studies mentioned above, and further work is necessary to reach a consensus on the metal binding mechanism at rutile (110) under different conditions.

Muscovite has been widely used as a model substrate for studying interfacial chemistry using surface X-ray diffraction techniques since it has a cleavable (001) surface that is atomically flat. The muscovite (001) surface has a fixed structural charge (0.34 C/m²), independent of solution pH, and thus differs from the oxides described above, which carry a surface charge that depends on the protonation of surface functional groups and thus on solution pH. X-ray reflectivity studies on cation sorption on muscovite (001) were first performed by Park et al., who compared the binding geometry of Rb⁺ and Sr²⁺ cations in pH 5.5 solution.³³⁴ The

structural modeling of these data suggested simultaneous IS and OS binding for Sr²⁺, but only IS binding of Rb⁺. However, in a later study also for Rb⁺ an extended OS species at a distance of 7 Å was reported and attributed to a diffuse ion profile with a large vertical mobility.³¹⁵ In addition, the IS Sr²⁺ species was found to disappear upon decreasing the pH to 3.5.³³⁵ Schlegel et al. investigated by XRR the adsorption geometry of K⁺, Cs⁺, Ca²⁺ and Ba²⁺.³³⁶ The derived electron density profiles revealed two solution layers adjacent to the muscovite surface, corresponding to IS surface complex species with a partial hydration shell and completely hydrated OS species. In a systematic *in situ* XRR study of the sorption behavior of alkali ions (Li⁺, Na⁺, K⁺, Rb⁺, Sr²⁺) on muscovite (001), it was found that the speciation of adsorbed ion could be understood by the difference of the associated hydration energy of the alkali ions³¹⁵. It was proposed that metal ions of smaller hydration energy (K⁺, Rb⁺, Sr²⁺) tend to form IS species in which the hydration layer is partially replaced by surface oxygen, whereas for metal ions of larger hydration energy (Li⁺, Na⁺) OS-binding geometry with an intact hydration shell energetically more favorable. Similar studies of divalent cations (Cu²⁺, Zn²⁺, Sr²⁺, Hg²⁺, Pb²⁺) indicated a more complex adsorption geometry.³¹⁶ Here, the proposed binding configuration consists of an IS species, an adsorbed OS surface complex species close to the surface, and an extended OS complex species further away (Figure 25c). RAXS modeling allowed to determine the relative proportions of those three surface complexes species (Figure 25d). The results could be explained by the energy balance of cation hydration, interfacial hydration, and electrostatic attraction. The introduction of competing ions can alter the surface complexation speciation in different ways. For example, the introduction of NaCl background electrolyte does not alter the binding speciation of Y(III), which can be described by one IS species, one adsorbed OS species, and another extended OS species, but rather suppressed the surface overcharging.³³⁷. The influence of fulvic acid (FA) on the adsorption
<figure><img src="image_49.png" /><figcaption>Figure 25: Systematic trends in cation sorption on oxides. (a) binding geometry of different cations on the rutile (110) surface, as obtained from SRXD measurements and MD simulations, and (b) corresponding dependence of the adsorbates' height over the TiO surface plane on the ion radius. (reprinted from Ref. ³³⁰. Copyright 2004 American Chemical Society). (c) Schematic illustration of three surface binding species on muscovite (001), including IS species, adsorbed OS species, and extended OS species, and (d) relationship among measured heights of these complexes of divalent metal cations adsorbed at the muscovite (001)-water interface vs the effective ionic radius. (reprinted from Ref. ³¹⁶. Copyright 2004 American Chemical Society).</figcaption></figure>
of Ba²⁺³³⁸, Hg²⁺³³⁹ and Sr²⁺³⁴⁰ on mucovite (001) was investigated by Lee et al. by RAXS. FA was found to decrease the IS/OS ratio of Sr²⁺ surface species, suggesting the competitive adsorption of FA for IS binding sites. In the case of Ba²⁺, the binding geometry, dominated by IS mode, is not affected by the presence of FA, if Ba²⁺ is premixed with FA before exposure to the mucovite surface. However, if mucovite is first exposed to FA first and then to Ba²⁺ solution, the IS Ba²⁺ species was found to be more broadly distributed and present also in the FA layer, suggesting an enhanced uptake of Ba²⁺ in the presence of FA. In contrast, in Hg solution premixed with FA the uptake of Hg²⁺ was enhanced and exhibited an additional broader distribution. The difference in competitive adsorption of Ba²⁺ and Hg²⁺ with FA may reflect the intrinsic difference in the binding strength of these cations on the mucovite (001) surface. In addition, surface-induced nanoparticle aggregation and accumulation, another surface-induced reaction on a charged oxide mineral surfaces, was commonly observed for tetravalent cations, such as Zr⁴⁺¹³⁸,³⁴¹ and numerous actinide ions (Pu⁴⁺, Th⁴⁺)³⁴²⁻³⁴⁴. It was proposed that nanoparticle aggregation starts with the oligomerization of simple structural monomers to form species of larger polymer structure, such as dimers, trimers, and tetramers, which then form nanoparticles. This nanoparticle aggregation mechanism was attributed to the presence of surface charges, as it should not occur in a pure solution system according to specification predicted from the thermodynamic model.

Surface complexation of ions represents an important surface process on all mineral-fluid interfaces. The described studies by *in situ* SXRD methods showed the surface binding geometry to be determined by a range of different factors, such as cation hydration energy, surface charge, solution chemistry, and availability/reactivity of surface binding sites. Many of these factors are coupled and interrelated to each other, and thus defy up to now a uniform thermodynamic description of the heterogeneous surface adsorption process over a wide range of external conditions. Deeper insights

into these complex issues will require systematic studies by a combination of different structure-sensitive and computational methods such as in the work of Zhang.³³⁰ These complexation phenomena are also of high relevance in the context of electrocatalysis on oxide surfaces, where adsorbed species may significantly alter the catalyst activity. A well-known example of this is the strongly activating influence of small amounts of adsorbed Fe ion impurities on the oxygen evolution reaction on transition metal catalysts.³⁴⁵ Future *in situ* SXRD studies may provide a structural basis for explaining this important phenomenon.

### 5.4.3 Interfacial water structures at oxide surface

The importance of interfacial water at the oxide surface has been realized for decades. *In situ* X-ray reflectivity studies allowed its structural characterization, showing that the interfacial water ordering depends on the surface chemistry of the studied substrate. As for other liquid interfaces, interfacial water forms a layered structure, consisting of one or two near-surface layers with strong structural ordering and subsequent quick decay of the layering within a few nanometers. The layered water structure typically has weak lateral structural order, but well-defined vertical positions. It manifests in the scattering along the surface-normal direction, as described in section 2.3.5, and thus can be probed by XRR. Pioneering XRR studies of the water structure on mucovite (001) surface were performed by Cheng et al.³⁴⁶ In that work, the proposed water layering included a near-surface oscillatory region with a layer spacing matching the size of the water molecule and a more broadly distributed extended structure.

For some substrates, a water layering structure is not observed. Instead, adsorbed water species either directly bind to surface cations to complete their coordination shell or form H-bonds to surface oxygen groups, resulting in both lateral and vertical ordering. For example, the derived positions of bound water molecules on both barite (001) and (210) surfaces reacting with pure water are consistent with the saturation
tion of broken Ba-O bonds at the surface without additional water layering structures.³¹³ Recent CTR studies on barite (001) surface gave a more detailed picture of the water structure, revealing four distinct water species in addition to those required to complete the broken Ba-O bonds.³⁴⁷ It should be noted that in the latter study, not pure water but a solution containing a mixture of different ions was used, including Ba²⁺, Na⁺, and SO₄²⁻. Therefore, the observed difference may reflect an effect of the solution chemistry on interfacial water speciation. Different from barite, a single-layer water structure was found above quartz (100) and (101) surfaces³¹² and on brushite (010).³⁴⁸ From modeling of the reflectivity data and two non-specular CTRs, it was deduced that the deduced water layer only exhibits ordering in the vertical direction but no lateral ordering. Interestingly, the adsorbed water species on the rutile (110) surface exhibits both vertical and lateral ordering at three distinct sites, according to CTR measurements by Zhang et al.³¹⁴ Similarly, on calcite (104) surface the derived water structure has two interfacial water species at distinct heights of 2.3(1) Å and 3.5(2) Å without extended layering structure.³⁴⁹ The derived water structure is consistent to that reported later in an X-ray reflectivity experiment³⁵⁰. A more extended water structure including two sites of adsorbed water and an additional layering was reported on the hematite (110) surface (Figure 26a,b).³⁵¹ The water structure on hematite (110) was compared to that on the (1102) surfaces reported earlier by the same group (Figure 26c),³⁵² showing differences in the vibrational amplitudes or positional disorder of the adsorbed water molecules and in the average spacing of near-surface layered water. In that earlier work, Catalano et al. also studied the difference of the water structure on the (1102) plane of two isostructural oxides (hematite and corundum). The obtained water structure is similar on the studied oxide surfaces, except for a less pronounced structural ordering on the hematite surface (Figure 26d). These studies highlight the importance of surface composition and structure on the induced interfacial water network. Subtle differences observed

in the interfacial water structure may reflect different oxygen exchange rates from surface functional groups or the structural match of water adsorption sites on the terminated surface. Furthermore, Rao et al. studied by various *in situ* techniques the interface structure of (100), (101), and (110) RuO₂ surfaces in the context of electrochemical water splitting (see chapter 6.2.3).³⁵³⁻³⁵⁶

The interfacial water structure has an important influence on the surface complexation of foreign ions. A strong hydrogen bonding network will hamper the inner-sphere binding of ions, which requires breaking the hydrogen bonding network. For a charged surface, such as muscovite, the orientation of water dipoles affects the water polarization properties, which in turn can influence the transport of ions, biomolecular self-assembly, and surface chemical reactions. However, because H atoms are almost invisible to X-rays, a full picture of the water network at the surface is hard to achieve by XRD methods alone.

## 5.5 Liquid-liquid interfaces

Interfaces between immiscible liquids are particularly difficult to access by surface-sensitive probes. X-ray scattering techniques can provide unique insights into these interfaces, especially into the average structural arrangement along the surface-normal direction, which can be directly determined by XRR. Because of the three-phase boundary between the two liquids and the containment vessel induces curvature (see Figure 27a, inset), large sample diameters of several centimeters are required. Thus, the X-ray beam is strongly absorbed and the measurements are only possible with beams of high brilliance and high photon energies > 20 keV. Furthermore, specialized liquid surface diffractometers are required that allow to tilt the beam down on the liquid interface.¹²,²⁹

The most well-known type of liquid-liquid interface in electrochemistry is the boundary between a liquid metal and a liquid electrolyte solution. This type of interface has historically been of great importance for developing a fundamental understanding of the electrochemical
<figure><img src="image_50.png" /><figcaption>Figure 26: (a) Structural model and total electron density distribution of the hematite (110)-water interface and (b) corresponding molecular structure in a top view showing two specific adsorbed water species (Reprinted from Ref. ³⁵¹. Copyright 2009 by Elsevier). (c) Structural model and total electron density distribution of the hematite (11̄02)-water interface. (d) Comparison of water structures on the hematite and corundum (11̄02) surfaces (Reprinted from Ref. ³⁵². Copyright 2007 by Elsevier).</figcaption></figure>
double layer structure, adsorption phenomena, and electroanalytical methods, but has recently received new interest because of applications in electrochemical materials synthesis,³⁵⁷ materials processing,³⁵⁸ and electrocatalysis.³⁵⁹ The structure of liquid metal-electrolyte interfaces has first been determined by Elsen et al. for the case of liquid Hg, the most prominent liquid metal electrode, in NaF solution.³⁶⁰ *In situ* XRR studies indicate a low surface roughness and a shallow peak at $q_z = 2.15$ Å⁻¹, where the reflectivity increases above the Fresnel reflectivity (Figure 27a,b). This peak has been found before in studies of the Hg-vapor interface and results from an atomic layering in the Hg near the interface that exponentially decays into the bulk.¹³ The corresponding electron density profile and interface structure is illustrated in Figure 27c. This type of structure was found over the entire double-layer potential range. Variations in the profile with potential could be explained by a combination of surface roughness, induced by electrocapillary effects, and changes in the intrinsic interface profile due to the electronic polarization at the liquid metal surface.³⁶⁰ Later temperature-dependent studies allowed to separate these contributions in more detail and confirmed this picture.³⁶¹

Surface layering has been shown to be a general property of liquid Hg and other liquid metals³⁶²,³⁶³ and was also observed in other electrolytes, including electrolytes that contain chemisorbing species.³⁶⁴⁻³⁶⁸ In the latter studies, also insight into the adlayer structure could be obtained. Specifically, measurements in solutions containing Pb and halide ions revealed the formation of salt-like layers at the interface,³⁶⁵,³⁶⁸ similar to those observed on solid electrodes.²⁴²,²⁴³,²⁶¹ As an example, the electron density profile and interface structure of Hg in PbBr₂-containing NaF solution is shown in Figure 27d, where the formation of a defined layer of crystalline c-axis oriented PbFBr with a thickness of one unit cell was observed.³⁶⁵

A second class of liquid phase boundaries that have been studied by *in situ* XRR are interfaces between immiscible electrolyte solutions. Seminal studies of such interfaces were performed by Schlossman and coworkers, who

investigated the ion distribution at the interface between water and nitrobenzene containing a common tetrabutylammonium cation.³⁶⁹,³⁷⁰ They found clear deviations from the predictions of the classic Gouy-Chapman theory, but good agreement with MD simulations. In these initial experiments, the measurements were performed under open circuit conditions and the ion distribution was controlled via the concentration. Later studies analyzed the distribution as a function of potential and arrived at similar conclusions.³⁷¹⁻³⁷³ Together with DFT calculations, the obtained results revealed the role of ion-ion correlations and ion-solvent interactions in the structure of these interfaces. More recently, these studies were extended to interfaces between aqueous electrolytes and ionic liquids (IL).³⁷⁴,³⁷⁵ Here, anion/cation multilayers were observed in the IL's near-surface region, an effect known also for the free surfaces of ILs and their interfaces to solids. The polarity of these layers was inverted upon changing the sign of the applied potential. Finally, also phospholipid monolayers at liquid-liquid interfaces were studied at variable potentials as a model for biomembranes and shown to be highly stable in the range of typical membrane potentials.³⁷⁶

## 6 Electrocatalysis

Driven by seminal results from model surfaces science which established activity descriptors of metallic surfaces for numerous reactions in the framework of the Sabatier principle³⁷⁷,³⁷⁸, the development of commercial electrocatalysts follows a ‘catalyst-by-design’ approach, in which both catalysts activity and stability on the long-term are tailored by their crystallographic structure and chemistry³⁷⁹. In that framework, *operando* X-ray scattering methods are powerful tools for monitoring relationships between structure, activity, and stability in electrochemical environments. As detailed in the following, *operando* X-ray diffraction allows not only the identification of the active phase present in the system, which often differs from the as-prepared or post-reaction states observed in *ex situ* studies, but also enables tracking of the structural
<figure><img src="image_51.png" /></figure>

Figure 27: (a) X-ray reflectivity $R(q_z)$ of the interface between liquid Hg and 0.01 M NaF solution at 0.28 V above the pzc. For comparison, the Fresnel reflectivity of an ideally sharp interface and the reflectivity of an interface with a simple Gaussian density profile are shown. The inset presents a scheme of the experimental geometry. (b) Corresponding Fresnel-normalized reflectivity, showing the quasi-Bragg peak at $q_z = 2.15 \ \AA^{-1}$. (Adapted from Ref. $^{360}$). Copyright 2010 American Physical Society. (c,d) Electron density profile and schematic presentation of (c) the Hg/NaF interface structure (Reprinted from Ref. $^{366}$). Copyright 2016 Royal Chemical Society) and (d) the PbFBr adlayer formed at this interface in Br containing electrolyte (Adapted from Ref. $^{365}$). Copyright 2013 National Academy of Sciences).
evolution during catalyst operation. This holds true for both single crystal model catalysts and nanoparticulate electrodes.

# 6.1 Hydrogen oxidation, evolution, and absorption

## 6.1.1 Hydrogen oxidation and evolution

Hydrogen oxidation (HOR) and hydrogen evolution (HER) are central electrochemical reactions and are commonly investigated due to their relevance to energy technologies, such as electrolysis and fuel cells. Platinum is typically used as an efficient catalyst for both HOR and HER, because of its unrivaled electrocatalytic performance. However, the high price and scarcity of Pt push research to find less noble alternative materials. Furthermore, whereas Pt is an excellent HER/HOR catalyst in acidic media, its activity in an alkaline solution is lower and depends on the cation species in the electrolyte. Tanaka et al. investigated this cation effect by operando SXRD studies of Pt(110) in Li⁺ and Cs⁺ containing solution.²³¹ They found that the (1×2) surface reconstruction remained stable during the HER, but that the interface structure on the solution side could substantially rearrange, dependent on the cation species. In Li⁺ containing solution, the dense interfacial water layer was maintained, resulting in high activity, whereas the Cs⁺ adsorbed in a bilayer on the Pt surface, which inhibited the HER. Chattot et al. systematically studied the structure of Pt nanoparticulate catalyst during HER in alkaline electrolyte⁶⁰. Besides the change of lattice parameter due to the H<sub>UPD</sub> before the onset of HER, no structural differences were observed at potentials in the HER regime.

Whereas the internal structure of platinum catalysts does not significantly change under HOR/HER reaction conditions, this is not the case for many non-noble catalysts. For the latter, reaction-induced phase transformations have been studied by operando XRD. In the case of Nickel based selenides, Zhai et al. showed that the original NiSe₂ cubic phase is rapidly transformed into the hexagonal NiSe

phase in the first several hours of HER (Figure 28).³⁸⁰ The formed NiSe phase has an enhanced catalytic activity and is the active phase under HER conditions. Theoretical calculations based on this observation suggested that the charge transfer from Se to Ni site during this phase transition leads to an increase in conductivity, shifting up the d-band center and overall enhancement of activity.

Multimodal operando characterization employing XRD was also used to determine the phase transformations of a phosphide-based bi-functional catalyst (CoP) for hydrogen/oxygen evolution.³⁸¹ Two catalysts, pristine and Fe-doped CoP, were studied. Upon polarization in an alkaline electrolyte, Fe-doped CoP retained its structure under HER conditions, whereas undoped CoP transformed first to a mixture of hydroxides and subsequently to an amorphous Co metal phase. This two-step transformation was found to be detrimental to catalytic activity.

Electroreduction of Iridium and Ruthenium oxides under HER conditions was thoroughly studied by XRD and SXRD on polycrystalline thermally prepared RuO₂ and IrO₂<sup>382,383</sup> and on ultrathin RuO₂(110)/Ru(0001) and IrO₂(110)/TiO₂(110) films<sup>384–386</sup> respectively. According to Pourbaix diagrams, both metals should first reduce to a hydroxide phase and then further reduce to their metallic phase under HER conditions<sup>387</sup>. The first step was confirmed in the basic environment (0.1M NaOH) by an early *in situ* SXRD study, where the formation of hydroxide on a RuO₂(110) single crystal surface was clearly observed<sup>388</sup>. In acidic conditions, (0.5M H₂SO₄) the situation is not as obvious and polycrystalline RuO₂ is neither transformed to the corresponding hydroxide nor to the metallic phase. Instead, it undergoes lattice expansion at HER conditions, which was attributed to proton incorporation<sup>383</sup>. However, further experiments on ultrathin RuO₂ films provided clear evidence of a full transformation from Ru<sup>4+</sup> oxide to Ru<sup>0</sup> via transformation of lattice oxygen to OH⁻ and water<sup>384</sup>. Even though IrO₂ should behave in a similar manner, it was shown to be very stable under reductive potentials, at least down to
<figure><img src="image_52.png" /><figcaption>Figure 28: (a) Chronoamperometric curve of the NiSe₂ at -0.18 V for 6 h and the corresponding *operando* XRD patterns ($\lambda = 0.495\,47(1)\AA$). (b–g) Enlarged characteristic peaks of the NiSe₂ and NiSe phases and the corresponding intensity variations (Reprinted from Ref. ³⁸⁰. Copyright 2020 American Chemical Society.</figcaption></figure>
cathodic potentials of -1.2V vs. RHE. Studies using IrO₂(110) ultrathin layers found no evidence of swelling or loss of crystallinity under these conditions.³⁸⁶ Such extraordinary stability of both polycrystalline RuO₂ and IrO₂(110) can be traced to a hindered kinetics of the reduction reaction. Such *operando* diffraction experiments play a prominent role in assessing the structural transformation of electrodes during HER and provide important insight into degradation mechanisms.

### 6.1.2 Electrochemical hydrogen loading of Pd

Beyond its excellent electrocatalytic properties for various fuel cell devices³⁸⁹, the peculiar ability of Pd to absorb large volumetric quantities of hydrogen in the form of hydrides (PdHₓ) enables its application in numerous other hydrogen technologies, such as hydrogen purification, storage, and detection³⁹⁰. In all cases, a detailed understanding of the physico-chemical parameters ruling the onset and amount of hydrogen insertion into Pd is pivotal to the further development of these applications. In the gas phase, it has been established for a long time that at low hydrogen partial pressure, H₂ molecules dissociate on the Pd surface and occupy the surface, subsurface, and octahedral interstitial lattice sites of the bulk.³⁹¹⁻³⁹⁴ As the hydrogen partial pressure is increased, the hydrogen content first increases in the Pd metal phase, referred as PdHₓ-α. Below a critical temperature, a certain gas pressure can be reached at which the hydrogen chemical potential in the PdHₓ-α phase equals the one of a PdHₓ-β phase, thus triggering the phase transition. This phase transition manifests as a plateau in the pressure-composition isotherm. As the phase transition continues to the PdHₓ-β phase, further increase in hydrogen partial pressure increases the hydrogen content in the PdHₓ-β phase. Importantly, the existence of a microscopic energy barrier for the phase transition results in a hysteresis, and the phase transition from PdHₓ-β to PdHₓ-α occurs at lower hydrogen partial pressure.

Because this phase transition consists in

a sharp lattice expansion of the Pd lattice, the latter is easily monitored by *in situ* and *operando* XRD techniques. Numerous studies investigated the hydrogen pressure-composition isotherms in the gas phase³⁹⁵⁻³⁹⁸, where the hydrogen partial pressure controls the hydrogen chemical potential. Electrochemistry, however, represents a convenient and straightforward alternative to such an approach, as the chemical potential of hydrogen can be controlled by mild polarization of a Pd electrode. Thermodynamically, only about 100 mV polarization should induce hydrogen content in Pd equivalent to that obtained with pressures in the GPa range³⁹⁹,⁴⁰⁰ at room temperature. The versatility of electrochemical hydrogen insertion opens an avenue to numerous applications for Pd, (including the highly controversial case of cold fusion⁴⁰¹), but requires a better understanding of the enabled fast insertion dynamics in nanoparticulate Pd under an electrochemical environment, which makes adapted characterization methods necessary.

The electrochemical loading of Pd foil cathodes with deuterium and hydrogen in LiOH (H₂O) or water LiOD (D₂O) electrolytes, respectively, has been investigated *in situ* via energy dispersive X-ray diffraction by Felici *et al.*⁴⁰² and Knies *et al.*⁴⁰³ The authors estimated the Pd loadings from its lattice parameter, measured by XRD, and the relationship between composition and lattice parameter at 77 K⁴⁰⁴, which they extrapolated to room temperature. They demonstrated that this approach was more accurate than other *in situ* methods, such as the electrical resistance ratio measurement technique⁴⁰⁵,⁴⁰⁶. Moreover, they confirmed the possibility to reach a H:Pd ratio close to 1 at room temperature by electrochemical loading. However, the nature and temporal resolution (> 5 min) of the energy-dispersive X-ray diffraction method employed did not allow to obtain mechanistic insights on the hydride formation. X-ray diffraction was also used by Jisrawi *et al.* to assign particular cathodic or anodic peaks in the adsorption and desorption of hydrogen to specific layers in thin film multilayered electrodes (Pd, Pd-capped Nb and Pd/Nb)⁴⁰⁷. These authors re-
ported an anisotropic lattice expansion along
the *c*-axis, which was interpreted as a con-
sequence of a film-substrate interaction. Us-
ing *in situ* surface X-ray diffraction, Ball and
co-workers followed the morphology of electro-
chemically deposited Pd films on Pt(001) elec-
trode surface in the presence of CO¹⁹⁸. They re-
ported the formation of larger Pd islands follow-
ing a pseudomorphic Stranski–Krastanov (SK)
growth. The effect of adsorbed CO was found to
enhance hydrogen permeation into the Pd film
only at low electrode potential below the H<sub>UPD</sub>
region. Similar SXRD studies were performed
by Lebouin et al., who investigated the hydro-
gen insertion into a 3 ML thick electrodeposited
Pd film on Pt(111)⁴⁰⁸. The results showed over-
all good reversibility of the insertion-desorption
processes in the thin film. However, a high re-
sistance to hydration of the two first atomic
Pd layers closest to the Pt substrates was de-
duced from observed changes of the correspond-
ing interlayer distances. The strong effect of the
Pt(111) substrate thus explains the low H:Pd
ratio that can be achieved in such films up to
14 MLs⁴⁰⁹.

More recently, Benck et al.⁴⁰⁰ used high-
energy powder X-ray diffraction to investigate
the electrochemical insertion of hydrogen into
Pd foils and thin films via aqueous liquid,
solid polymer and solid ceramic electrolytes.
In their seminal contribution, the authors em-
ployed three different electrochemical cells for
*operando* XRD, and provided important new
insights on the H:Pd ratios possibly achieved
in the different systems investigated. As shown
in Figure 29a-b, they could reach a temporal
resolution below 10 min for powder XRD pat-
tern collection. However, the quality of the sig-
nal was highly dependent on the nature of the
electrochemical cell. Importantly, using a col-
lection of data from past literature, they pro-
posed an updated calibration of the PdH<sub>x</sub>–β lat-
tice parameter (a) as a function of H:Pd ratio
(c) at 25 °C and 1 atm (Figure 29c), namely
a[Å] = 3.9321 + 0.179 · c ± 0.0034. Employ-
ing this calibration, Benck et al. showed that a
H:Pd ratio in Pd thin films of up to 0.96 ± 0.02
could be obtained in room temperature aqueous
0.05 M H<sub>2</sub>SO<sub>4</sub> at a cathode potential of -1.5 V

vs. RHE. As shown in Figure 29d, faster insertion rates and shorter stabilization times were observed for thin films compared to thicker foils. Also, the liquid electrolyte was found to possibly produce higher H:Pd ratios than polymeric or ceramic electrolytes. In general, achieving a higher H:Pd ratio was found to be limited by competition with the hydrogen evolution reaction (HER) and electrochemical damage due to significant volume changes in the Pd layer during hydrogen absorption.

Using the recently upgraded Extremely Bril-
liant Source of the European Synchrotron Radi-
ation Facility (ESRF-EBS, Grenoble, France),
Chattot et al. investigated the mechanism of
electrochemical hydrogen absorption in com-
mercial carbon-supported Pd nanoparticles
(Pd/C) in alkaline aqueous electrolyte (0.1
M NaOH)⁶⁰. The unprecedented temporal
resolution (333 ms) and signal-to-noise ratio
achieved during these operando high energy
powder X-ray diffraction experiments enabled
the continuous recording of diffraction patterns
during cyclic voltammograms and potential
steps experiments (Figure 29e-f). This allowed
capturing the transient core-shell mechanism
of the PdHₓ phase transition and showed that
it involved supersaturated and undersaturated
hydrogenated phases, in agreement with pre-
vious theoretical predictions and observations
from the gas phase³⁹⁴,³⁹⁶,³⁹⁷,⁴¹⁰,⁴¹¹. Finally, the
onset and end potentials for hydride formation
in various PdM alloys was found to depend on
the second M metal in studies by Lee et al.⁴¹²

## 6.2 Oxygen reduction and evolution

The oxygen reduction reaction (ORR) and the
oxygen evolution reaction (OER) are undoubt-
edly the most studied electrocatalytic reactions
in recent decades. These reactions have large
overpotentials, which contribute significantly to
the limited efficiency of electrolysers and fuel
cells. To improve the kinetics, the elemen-
tary reaction mechanisms have to be under-
stood, which requires knowledge of the atomic-
scale interface structure of the electrocatalyst
under reaction conditions. Furthermore, ORR
<figure><img src="image_53.png" /><figcaption>Figure 29: Understanding electrochemically-induced Pd hydride formation with operando XRD. Studies were performed on Pd thin-film cathode mounted in (a) an aqueous electrolyte cell and (b) a Nafion electrolyte cell. Shown are (top) representative XRD pattern and (bottom) the evolution during electrochemical H insertion. (c) Calibration of PdHₓ lattice parameter vs H/Pd ratio and (d) PdHₓ composition as a function of the cathode potential for Pd thin film in (left) Nafion and (right) aqueous electrolyte (Adapted from Ref. 400. Copyright 2019 American Chemical Society). (e) 3D representation and corresponding 2D projection of WAXS patterns intensities recorded during Pd cyclic voltammetry experiment in N₂-purged 0.1 M NaOH electrolyte at room temperature. (f) (top) Change of the lattice constants during the PdHₓ α ↔ β phase transition in the steady state (square symbols interpolated by black lines), measured after 60 s hold at various potentials, and during a cyclic voltammogram at 5 mV s⁻¹ (circles) and (bottom) evolution of the coherent domain sizes as a function of time for the transition from the PdHₓ α to the β phase and back during the cyclic voltammogram (adapted from Ref. 60. Copyright 2021 American Chemical Society).</figcaption></figure>
and, especially, OER occur at very positive potentials, i.e., under highly oxidizing conditions. This poses significant challenges for catalyst stability, which is a main issue for technological applications. Knowledge-based approaches to improve stability require insights into the precise mechanisms of electrocatalyst degradation and dissolution under ORR/OER conditions. *Operando* X-ray scattering methods played an important role in addressing these issues.

### 6.2.1 Platinum group metal based single crystals

In recent years, Pt electrochemical oxidation has received renewed interest, triggered by the relevance of this process for the stability of Pt-based oxygen reduction catalysts. Detailed studies by state-of-the-art *in situ* X-ray scattering techniques and *ab initio* theory provided an atomistic picture of the surface during oxidation and new insight into the elementary mechanisms of oxidation/reduction and its detailed connection to Pt dissolution. The basic understanding of the surface structure at potentials relevant to oxygen reduction and evolution for Pt(111) surface is depicted in Fig. 30, mostly determined by a combination of SXRD techniques with electrochemical measurements and DFT.<sup>413</sup> The relevant potential range for ORR and OER starts in the “butterfly” region, 0.60 to 0.95 V in the case of $HClO_4$, which consists of two parts: the sharp peaks at 0.80 V and the broader region leading up to them. The broad regions were associated with OH and $H_2O$ adsorption into a transient disordered $p(2 \times 2)$ adlayer, while the sharp peaks are attributed to a phase transition into a $p(1 \times 1)$ phase.<sup>281</sup> These transitions are accompanied by Pt surface buckling.<sup>282</sup> In the case of $H_2SO_4$, the phase transition in the “butterfly” region is from disordered bidentate (bi)sulfate to an ordered ($\sqrt{3} \times \sqrt{7}$) overlayer of tridentate (bi)sulfate with coadsorbed water.<sup>281</sup> Most of this information has been found by careful analysis of CTR profiles acquired during *in situ* measurements, providing the exact position of substrate Pt atoms as well as adsorbate molecules.

The anodic peak at ≈1.06 V is traditionally

assigned to the further oxidation of $OH_{ads}$ to $O_{ads}$. However, first *in situ* SXRD measurements on the structure of the oxidized Pt(111) surface by You et al. showed that place exchange, in which a Pt surface atom exchanges with an oxygen species, occurs in the potential region of this peak.<sup>147,414</sup> In addition, early *in situ* RASXS experiments by the same group verified the oxidation of Pt surface atoms in this process.<sup>415,416</sup> The place exchange process is structurally reversible on Pt(111), i.e., cycling returns the Pt atoms to their original state, as evidenced by SXRD measurements. Thus, the voltammogram is stable for many cycles at typical sweep rates. The main structural motif responsible for this rather unusual behavior is the particular location of extracted (place-exchanged) atom ($Pt_{ex}$) on Pt(111) surface. For low $Pt_{ex}$ coverages, this atom is located exactly above the vacancy, stabilized by three oxygen atoms adsorbed in close vicinity.<sup>417</sup> This stabilization allows $Pt_{ex}$ atoms to return to the original position in the Pt lattice upon the subsequent reduction. This unusually stable atomic arrangement is also responsible for the overall stability of Pt(111) surface against dissolution, as shown by combined *operando* HESXRD and inductively coupled plasma mass spectrometry (ICP-MS) studies by Fuchs et al.<sup>98</sup> Furthermore, similar SXRD measurements performed in $O_2$ containing electrolyte did not show any major influence of $O_2$ presence on the place exchange process.<sup>418</sup>

Time-resolved studies of Pt(111) oxidation indicated a complex kinetics of Pt surface extraction with a fast initial process and a subsequent slower logarithmic growth law.<sup>99,417,418</sup> In addition, recent studies where SXRD data were obtained during voltamograms at sweep rates up to 1000 mV/s indicated a very fast $Pt_{ex}$ formation whereas the corresponding charge transfer in the $O_{ads}$ peak was much slower.<sup>100</sup> This behavior suggests that the extraction is driven by the potential rather than the $O_{ads}$ coverage, indicating fundamental differences of electrochemical oxidation as compared to oxidation in the gas phase. HESXRD studies employing very slow sweep rates to thermodynamically stabilize the surface structures showed
<figure><img src="image_54.png" /><figcaption>Figure 30: Typical cyclic voltammograms of Pt(111) in 0.1 M HClO₄ together with a schematic illustration of the atomic-scale surface structures in the different potential regions (Reproduced from Ref. 413. Copyright 2017 by Elsevier).</figcaption></figure>
that increasing the potential beyond the place
exchange onset (>1.2 V) leads to the forma-
tion of a mixed oxide structure, where about
20% of atoms remain on the stable place ex-
changed sites, 30% occupy regular sites away
from the original vacancy, and the remaining
50% are incorporated in the rather disordered
oxide. ¹⁰¹ This severe restructuring of the sur-
face leads to less stable arrangements, where
Pt atoms are more prone to dissolution and sur-
face migration during the reduction. The struc-
tural reordering of these less stable atoms in-
duced by oxidation/reduction cycles was stud-
ied in detail by *in situ* GISAXS. ¹⁰⁷,⁴¹⁹ This ap-
proach allows quantitative characterization of
the nanoscale surface morphology as a func-
tion of the upper potential limit and the num-
ber of cycles. The results indicate a continu-
ous increase in the height of the nanoscale is-
lands and an increasingly homogeneous island
distribution with an increasing number of cy-
cles, whereas the characteristic distances be-
tween the islands predominantly depend on the
potential limit.

According to the combined *in situ* HESXRD and ICPMS studies, oxidation of the Pt(100) surface exhibits a strikingly different behavior.⁹⁸,⁹⁹ In this case, cyclic voltammetry shows irreversible behavior from the very onset of the place exchange process and the surface degrades much faster at the potentials relevant to the oxygen reduction reaction. This rapid degradation is linked to the different place exchange mechanisms, where the oxide grows in rows along the main lattice directions and the O-stabilized Pt<sub>ex</sub> are shifted by 1/2 of the unit cell distance from the corresponding vacancies. These rows cannot accommodate all the extracted atoms which leads to unstable Pt species on the surface and enhanced anodic dissolution (Figure 31). At potentials of ≥ 1.2 V, the coverage of this stripe-like oxide saturates and the amount of anodic dissolution starts to decrease again.⁹⁹ In parallel, a more disordered, second Pt oxide phase is observed that linearly increase in coverage with increasing oxidation potential and can be linked to the cathodic dissolution during oxide reduction.

The structure of bimetallic surfaces, and

its relation to ORR catalysis, have also
been thoroughly investigated by SXRD
on model single crystal surfaces in the
last decade. The studies on most rel-
evant systems, such as Ni/Pt(111),<sup>420,421</sup>
Pt<sub>3</sub>Co(111),<sup>422</sup>,Cu<sub>3</sub>Pt(111),<sup>423</sup> Zn/Pt(111),<sup>424</sup>,
and Gd/Pt(111),<sup>425</sup> paint a similar picture. In-
dependently of the initial surface composition
and structure of the crystal, a Pt-skin evolves
rapidly upon contact with the electrolyte. This
skin is then the active surface for ORR and its
electronic properties, which are significantly af-
fected by the underlying alloy layer, determine
the overall electrocatalyst activity.<sup>420,423,425</sup> The
structure of the surface further changes upon
cycling to oxidative potentials, similar to that
of the Pt(111) surface, but in these cases, the
transformation is mainly characterized by the
surface roughening and the dissolution of the
less noble metal. The thickness of the resulting
skin depends strongly on the upper potential
limit and the number of cycles.<sup>423,425</sup> Further-
more, the out-of-plane relaxation of the skin
layer's uppermost atomic layer also changes
with its thickness and the most active surfaces
show compressive strain in this direction.<sup>423,425</sup>
This effect can be explained by the weakening
of surface OH bonding, which affects not only
the electrocatalytic performance but also the
stability of the surface through suppression of
the oxide formation.<sup>420</sup>

6.2.2 Platinum group metal based nanoparticles

Platinum group metal (PGM)-based catalysts
are extensively used in the most developed
PEMFCs and water electrolyzer systems for hy-
drogen conversion and production. However,
considering the price and scarcity of such ma-
terials (Pt, Pd, Ir, etc.), the viability of PEMFC
systems relies on dramatic constraints in terms
of PGM catalysts performance and stability.
For both PEMFCs and electrolyzers, ensur-
ing efficient and durable oxygen electrocataly-
sis is the current major bottleneck notably be-
cause of the poor reaction kinetics and highly
corrosive environments. In this context, tun-
ing the nanocatalysts structure, size, morphol-
<figure><img src="image_55.png" /></figure>

Figure 31: Atomic structure of the Pt(100) surface during electrochemical oxidation. (a) Two CTRs of Pt(100), obtained by *in situ* HESXRD measurements (68 keV) at different potentials. The best fit (coloured lines) and the CTR fits for the smooth surface at 0.95 V (grey line) are shown. (b) Corresponding electron density profile along the surface normal, obtained from the quantitative modeling of the CTRs. Bulk Pt lattice positions are indicated by dashed vertical lines. (c,d) Atomic arrangement of the extracted Pt atoms. (e) Potential-dependent coverage of extracted Pt surface atoms on oxidized Pt(100) and Pt(111) (Adapted with permission from Ref. ⁹⁸. Copyright 2020 Nature-Springer).
ogy, and chemistry are the main parameters
to optimize their initial performance and long-
term stability. As reviewed in the following,
*operando* X-ray diffraction methods are pow-
erful in monitoring such parameters successful
implementation at the device level, also with
their evolution during operation. Due to the
powder nature of practical catalyst layers, pow-
der X-ray diffraction-based techniques (WAXS,
SAXS, PDF) are particularly suited.

A convenient metric of activity for PGM-
based catalyst materials is the metal mass-
normalized current density associated with a
given reaction reported at application-relevant
electrode potential. The mass activity can be
seen as the product of the catalyst electrochem-
ically active specific surface area (ECSA, in m²
g⁻¹*P*<sub>PGM</sub>) and the surface-normalized current den-
sity (SA, in A m⁻²). At fixed chemical com-
position, the ECSA is mostly a consequence of
the catalyst particle size and morphology, which
should aim at maximizing the atomic surface-
to-volume ratio. Complementarily, at fixed par-
ticle size, the SA is mostly controlled by the
surface fine structure and chemistry. As sum-
marized in Figure 11, all those pivotal charac-
teristics are more or less directly accessible from
*operando* powder X-ray diffraction.

Implementing and maintaining the smallest
particle size possible during operation is prob-
ably the most straightforward but still chal-
lenging approach to ensuring the long-term
performance of heterogeneous catalyst materi-
als. In fact, if the smallest nanoparticles fea-
ture the highest ECSA, they are particularly
prone to dissolution in the environment of oxy-
gen electrocatalysis⁹⁸,⁴²⁶⁻⁴³⁵ favoring the Os-
wald electrochemical ripening and redeposition
processes⁴³⁶⁻⁴³⁹. In addition, corrosion of the
carbon or metal oxide supports is known to fa-
vor nanoparticle motion, aggregation and coa-
lescence⁴⁴⁰. In other words, nanoparticles’ size
increase is symptomatic of various degradation
mechanisms, with a direct detrimental impact
on the cell performance. Using anomalous small
angle X-ray scattering (ASAXS) on commercial
carbon-supported Pt nanoparticles mounted in
an electrochemical liquid cell, Haubold et al.
demonstrated during the late 1990ies the pos-

sibility of investigating Pt nanoparticle size
distribution evolution *in situ* ⁴⁴¹,⁴⁴². By com-
bining *in situ* ASAXS and XANES, the au-
thors showed the apparent size of oxidized Pt
nanoparticle increases due to the formation of
a 1 nm thick oxide shell at high electrode po-
tential (1.1 V vs. Ag/AgCl compared to 0.25
V vs. Ag/AgCl in 1 M H₂SO₄). Complemen-
tarily, the method was shown to be powerful
in separating the scattering of the nanoparti-
cles from the porous carbon background scat-
tering even at low Pt loadings (5 wt.%)⁴⁴³. If
ASAXS allows the obtention of strong scattered
signal from Pt, this is to the detriment of tem-
poral resolution since each measurement must
be performed at two distinct X-ray energies. In
2008, Smith et al. achieved high quality *in situ*
SAXS of carbon-supported Pt nanoparticle in
0.1 M HClO₄ with 30 s temporal resolution⁴⁴⁴.
This seminal work revealed the incomplete size
dynamics reversibility during electrode cycling
(Figure 32.a). The continuous size increase of
the Pt nanoparticles even at the reduced state
was interpreted as the first *in situ* observation
of the Ostwald ripening process. From this
point, ASAXS was extensively used (possibly
in combination with XAS)⁴⁴⁵ to monitor the
particle size distribution evolution during ac-
celerated stress test procedures (electrode po-
tential cycling). In a liquid electrochemical cell,
Gilbert et al. first evidenced the strong correla-
tion existing between the maximum surface ox-
ide coverage produced by different potential cy-
cle profiles and the resulting particle size growth
during ageing²⁰. However, while constant po-
tential holds at 1.0 or 1.1 V vs. RHE resulted in
only minimal changes in Pt nanoparticles size,
cycling to these same upper potential limits in
a high-frequency fashion was shown to trigger
significant particle growth. The fundamental
role of Pt oxide formation and subsequent re-
duction in Pt dissolution and redeposition pro-
cesses according to the place-exchange mecha-
nism was further evidenced by means of on-line
ICPMS⁴³⁴. The complementary effects of po-
tential cycle profile²⁰, upper potential limit⁴⁴⁶,
particles’ chemistry⁴⁴⁷, particles’ initial size dis-
tribution⁴⁴⁸ or catalyst layer thickness⁴⁴⁹ on the
particles’ size evolution were also investigated
with *in situ* SAXS.

If the nanoparticle size distribution derived from SAXS measurements meets the intuitive definition of particle size, such as those observed in the transmission electron microscope (TEM) images, another interesting parameter is the coherent domain size possibly obtained by WAXS. Contrary to the SAXS-derived size which is independent of the particle's crystallinity, WAXS only captures the characteristic size of the crystalline domains from the Bragg peaks broadening. One of the main advantages of the WAXS technique at high X-ray energy considering *in situ* studies is its ability to provide signal from the catalyst at high diffraction angle virtually free from the cell environment and background, with higher temporal resolution compared to SAXS. Contrary to the previously discussed SAXS observations, Sasaki et al.⁴⁵⁰ showed the formation of the oxide shell during the potential step from 0.41 V vs. RHE to 1.51 V vs. RHE leads to a decrease of the coherent domain size of the Pt nanoparticles from 2.6 nm to 2.1 nm, thus demonstrating the amorphous nature of the as-formed Pt oxide layer (Figure 32.b). By combining WAXS and time-resolved quick scan X-ray absorption fine structure (QXAFS), Sekizawa et al., captured the key structural transformations kinetics of Pt nanoparticles in PEMFC during fast potential steps⁴⁵²,⁴⁵³. These authors evidenced the Pt oxidation process being composed of a quick Pt-O bond formation followed by subsequent slower subsurface events, such as the formation of subsurface oxygen leading to Pt crystal disordering⁴⁵². Interestingly, the kinetics trends were found inversed during oxides reduction, with fast removal of subsurface oxygen and slow desorption of oxygenated species. Also using WAXS, Martens et al. later evidenced the scale factor (i.e. the intensity of scattered signal) a powerful descriptor of Pt oxidation in close relation with the particles' size but with better sensitivity⁴⁵⁴. Beyond the oxidation-dependent size dynamics, WAXS has also been extensively used to investigate the long-term stability of Pt-based catalysts in the environment of electrochemical cells. Taking the nanoparticles' coherent domain size increase as an indicator of cat-

alyst degradation, the influence of the catalyst support material chemistry such as doped metal oxides⁴⁵⁵⁻⁴⁵⁷ or carbon functionalization⁴⁵⁸,⁴⁵⁹, the initial particles 'size distribution'⁴⁴⁸,⁴⁴⁹, the liquid vs. solid electrolyte⁴⁶⁰, the upper potential limit during accelerated cycling⁵⁴, or the post-mortem cyclic voltammetry experiment⁴⁶¹ on the material stability were investigated by means of *in situ* WAXS.

The PDF can also provide insights on the size of the nanoparticles, by probing the longest atomic pair distribution distances. Compared to the ASAXS and WAXS approaches, the PDF combines the advantages of not being dependent on particle crystallinity nor the periodicity of their distribution on the support. *in situ* monitoring of the growth of Pt/C and various PtCo/C catalysts in PEMFC was successfully performed by Redmond et al, and the largest PtCo/C sample was found more resistant to particle growth among the different samples⁴⁶².

Finally, because the different X-ray scattering techniques can provide insights into the nanoparticles size evolution with some subtle differences in the definition of size, their comparison can be of high interest. For example, it was proposed by Martens et al. to compare the sizes obtained during simultaneous WAXS and SAXS *in situ* measurements. Because SAXS does not distinguish between aggregated (polycrystalline) or coalesced (single crystalline) nanoparticles but WAXS does, the distinct evolutions of the SAXS and WAXS sizes allowed the authors to decorrelate catalyst aggregation, ripening, and coalescence processes inside operating PEMFC⁴⁵¹ (Figure 32.c). Contrary to previous reports which used only one of these techniques, nanoparticle aggregation, and coalescence sequential steps during ageing were revealed as major causes for particles' size increase, likely more significant than the Oswald ripening process.

Nanostructured, or shape-controlled nanocatalysts are used in various fields of electrocatalysis as they feature highly desirable catalytic properties⁴⁶³. However, the as-implemented preferential-shape is subject to changes during catalysis long-term operation. Post mortem, *ex situ* TEM imaging is by far the most
<figure><img src="image_56.png" /><figcaption>Figure 32: Monitoring nanoparticles’ size evolution during oxidation or long-term stability test using *in situ* (a) SAXS (Reprinted from Ref. 444. Copyright 2008 American Chemical Society), (b) WAXS (Reprinted from Ref. 450. Copyright 2016 American Chemical Society), and (c) combined SAXS and WAXS (Reprinted with permission from Ref. 451. Copyright 2012 Elsevier).</figcaption></figure>
powerful and widely used method to moni-
tor such changes (albeit the development of
ETEM)⁴⁶⁴⁻⁴⁶⁹, but X-ray scattering methods
can also provide valuable *operando* insights.
While tracking nanoparticles' size from WAXS
can be performed using simple anisotropic de-
scription of the particles (spherical shape), re-
finements of X-ray scattering data can also be
performed by introducing anisotropy. Schmies
et al. used this approach to better describe
the anisotropic ellipsoidal shape of Pt nanocrys-
tallites synthetized on various carbon or ox-
ide support materials, and could correlate *in
situ* the shape of the particles with their resis-
tance to oxidation and dissolution⁴⁵⁷. A more
versatile description of the particle shape is
possible with SAXS. Using a model of hollow
nanoparticles to fit SAXS data, Asset et al.
could monitor *in situ* the shape evolution of
hollow PtNi/C nanoparticles during mild and
harsh accelerated stress tests⁵⁴. The distinct
evolution of the inner and outer diameters of
the PtNi shell captured the partial collapse of
the hollow structure, triggered by enhanced re-
structuring during the harsher test (upper po-
tential limit of 1.1 V vs. RHE compared to 1.0
V vs. RHE). Following this study, Gommes de-
veloped a versatile stochastic model of dense or
hollow nanoparticles to fit SAXS data, which
was demonstrated to be powerful in capturing
the morphological changes that occur during
accelerated stress testing of hollow PtNi/C cat-
alyst in operating PEMFC¹⁰⁶.

The main advantage of X-ray scattering tech-
niques is undoubtedly their ability to probe
the nanoparticles' structure and (to some ex-
tent) chemistry, which controls their intrin-
sic catalytic properties. A particularly conve-
nient and well-established structural descrip-
tor of Pt-based catalysts activity is the lat-
tice strain<sup>463,470,471</sup>, easily accessible from XRD
during *operando* measurements. Using WAXS,
the lattice strain dynamics of Pt nanopar-
ticles during oxidation were largely investi-
gated<sup>60,450,452,454,457,472</sup>, and it was generally ob-
served that the Pt lattice expands during ad-
sorption of oxygenated species, prior to the for-
mation of Pt oxides. Consequently, the lattice
strain was recognized as a parameter sensitive

to the adsorption of electroactive species, pro-
viding fundamental insights on adsorbate sur-
face coverage. Reggrading Pt-based nanoalloys,
the direct application of Vegard's law from lat-
tice constant measurements during catalyst op-
eration can inform on possible dealloying. Such
an approach can be performed either in the re-
ciprocal space by analysis of the Bragg peaks
or in the real space from PDF analysis.

Asset et al. used the Rietveld refinement of *operando* WAXS patterns to track the progressive losses of both global and local strain in hollow PtNi/C catalysts caused by Ni leaching and restructuring during accelerated stress tests⁵⁴. More recently, Hornberger used the same approach to investigate the stability of quaternary RhMo-doped PtNi/C octahedra nanocrystals⁴⁷⁴. The presence of Rh and Mo was found to enhance structural integrity of the particles by suppressing the migration of Pt surface atoms. In general, it can be stated that the reciprocal space analysis has been surprisingly underused in the field for *in situ* and *operando* studies of PGM materials in PEMFC than the real-space analysis (PDF). In fact, PDF analysis has been extensively used notably by Petkov, Chuan-Jian Zhong and coworkers. In 2018, these authors used high energy XRD coupled with PDF analysis to investigate the structural evolution of carbon-supported PdSn/C and PdNiCu/C nanoalloys of various compositions in an X-ray transparent PEMFC during electrochemical cycling⁵⁶. This study nicely showcases the power of *operando* XRD technique, as combined *operando* insights on the sample-dependent crystal phase transitions (between orthorhombic, hexagonal and face-centered cubic PdSn) and strain relaxation in direct comparison with electrochemical performance in PEMFC were obtained (Figure 33.a-b). Whereas similar information could have been probably reached from Rietveld refinement of the WAXS patterns, Petkov later further exploited the capabilities of the PDF on trimetallic PtNiCo/C PEMFC cathode catalysts⁴⁷³. Using 'large box' models of various PtNiCo nanoparticles' type-structures (sandwich, janus, intermixed or core-shell) to fit the PDF data, it was possible to resolve the var-
<figure><img src="image_57.png" /><figcaption>Figure 33: Probing the atomic scale structure of nanocatalysts during operation in PEM fuel cell by means of high energy X-ray diffraction coupled with PDF analysis. (Top) representative PDF curves and (bottom) color maps of the phase content of (a) Pd₄₃Sn₆₇ and (b) Pd₇₈Sn₂₂ alloy NPs vs time of electrochemical cycling; c) Evolution of the atomic-level, chemical composition, and apparent ORR activity for different nanoalloy catalysts deposited on the cathode of an operating PEMFC. Panels a-b are adapted with permission from Ref⁵⁶. Copyright 2018 Elsevier. Panel c) is adapted with permission from Ref⁴⁷³. Copyright 2019 Royal Society of Chemistry.</figcaption></figure>
ious particles' compositional profiles reached *in situ* during the segregation of the various metals during catalyst acceleration stress test (Figure 33.c). A similar approach was previously used for *post mortem*, ex situ analysis of PtNi/C catalysts⁴⁴⁷. More recently, Chuan-Jian Zhong's group applied this PDF approach to the investigation of PtFe nanowires⁴⁷⁵ and PtPdCu/C⁴⁷⁶ catalysts. Especially, in the case of PtPdCu/C, the authors introduced the concept of alloying-realloying, by probing the absence of pronounced segregation (maintaining the disordered homogeneous alloy) even after extended electrode cycling for this class of multimetallic materials. Very recently, Altan et al. used in situ electrochemical BCDI to probe the potential-dependent strain evolution of a single Pt nanoparticle in a liquid electrolyte. Their study directly maps the adsorbate-induced surface train field initiating at edge and corner sites and propagating toward the facets, thus providing important insights for the design of strain-engineered catalysts⁴⁷⁷.

### 6.2.3 Ir and Ru oxide electrodes

Iridium and Ruthenium-based oxides are the best-performing materials for the oxygen evolution reaction in an acidic environment, and currently, there are no substitutes. In this respect, these materials have got substantial attention in recent decades, and our understanding of the activity/stability relationships improved considerably³⁸⁷. While $RuO_2$ is one of the most active of the known oxides in an acidic environment, stability is an issue. Contrary, $IrO_2$ is much less active but has better stability. Several studies concluded that the stability and activity of these materials are coupled: more active materials are typically less stable and vice-versa⁴²⁹,⁴⁷⁸⁻⁴⁸⁰. This spurred research into the structural aging of the catalysts using *operando* techniques, in which SXRD and XRD play a prominent role. The atomistic picture of the degradation process has been elucidated for both anodic and cathodic corrosion. The anodic corrosion was studied on a model $IrO_2(110)-RuO_2(110)/Ru(0001)$ surface by Weber et al. using SXRD and XRR

techniques⁴⁸¹. The anodic corrosion for this system starts already at 1.48V vs. SHE via pitting through the $IrO_2(110)$ layer. Once the pits reach the $RuO_2(110)/Ru(0001)$ interface, the corrosion is accelerated due to the low stability of both $RuO_2(110)$ and $Ru(0001)$. In addition, evolving $O_2$ disrupts the cohesion of $IrO_2(110)$ leading to further degradation by lifting some of the weakly bonded $IrO_2(110)$ flakes. The residual $IrO_2(110)$ seems to remain stable up to 1.98V vs. SHE. To better understand the stability of $IrO_2(110)$, the thin film was also deposited directly on the $TiO_2(110)$ substrate to circumnavigate the problem of preferential $RuO_2(110)$ corrosion. Such $IrO_2(110)-TiO_2(110)$ films were studied in *operando* by high energy SXRD and XRR, and separately with supercritical fluid chromatography ICPMS⁴⁸². The results unambiguously showed that such film is very stable (at least for 26 hrs) during galvanostatic hold in the OER range (50 mA/cm⁻²) and that indeed the instability of $RuO_2(110)$ is the main degradation pathway in $IrO_2(110)-RuO_2(110)/Ru(0001)$ films (Figure 34 a). Interestingly, the dissolution rate seems to depend on the operation conditions: under steady-state conditions the film is more stable than under dynamic conditions (variable currents), pointing to the importance of catalyst design when a water electrolyzer is coupled with intermittent sources of energy. The SXRD and XRR measurements were also performed at lower X-ray energies (21.5 keV vs. 67.15 keV) for the same system⁴⁸³. In the studies at 21.5 keV, the researchers observed X-ray induced delamination of $IrO_2(110)$ film due to the detoration of the $IrO_2(110)-TiO_2(110)$ interface (Figure 34 b). This serves as a reminder that damage induced by X-ray radiation can severely affect the outcome of the experiment and suitable condition for measurements have to be found prior to the experiment. The activity-stability relationship of high surface area $IrO_2$ at OER conditions was studied by Povia et al. using *operando* ASAXS and XAS techniques, probing both oxidation state and morphology changes⁴⁸⁴. In agreement with previous results, the lower oxidation extent of as-synthesized sample ($IrO_2-AS$)
correlated well with higher mass- and surface-specific activity when compared with the high-temperature treated sample (IrO₂-HT). On the other hand that was accompanied by lower stability. ASAXS measurement reveal that the IrO₂-AS sample is composed of less stable configurations of disk-shaped particles assembled in a 2D structure, causing the change of the ASAXS signal (Figure 34 c). In contrast, ASAXS studies of IrO₂-HT showed that the sample stays stable during the aging test, disclosing excellent mechanical stability. This work is an important step to understand the relative contributions of different degradation mechanism, in this case, morphological stability. Rao et al. studied the nature of the active site and the influence of their coordination on the activity of (100), (101) and (110) RuO₂ surfaces by surface scattering techniques³⁵³⁻³⁵⁶. Based on a combination of SXRD and DFT, they proposed specific adsorption sites of OOH species at different potentials in 0.1 M HClO₄ and elucidated the rate-determining step for each surface. However, some skepticism about the analysis of the SXRD data in this work renders the current interpretation open³⁸⁷.

### 6.2.4 Fe, Co, and Ni oxide electrodes

First-row transition-metal oxides and (oxy)-hydroxydes containing Fe, Co, or Ni have been identified as promising OER catalysts in alkaline to neutral electrolytes, since they are earth-abundant, scalable, highly efficient, and stable without a serious degradation of catalytic performance.⁴⁸⁵⁻⁴⁸⁹ Accordingly, extensive efforts have been devoted to study the OER mechanism on these catalysts. Because these material systems exhibit a rich potential and pH dependent structural phase behavior,⁴⁹⁰ structural transformations under reaction conditions are likely. The atomic-scale composition and structure of the OER active phase at the catalysts' surface and the nature of the active sites is still controversially discussed and the subject of much current research. There is increasing evidence from operando scattering and spectroscopy studies that the as-prepared oxide materials (often denoted "pre-catalyst") is

<figure><img src="image_58.png" /></figure>

Figure 34: (a) Comparison of experimental (black hollow circles) and fitted (red lines) XRR data at OCP and after 26 h of galvanostatic hold at 50 mA cm⁻². Film thickness as derived by the fitting of the XRR data and applied current density as a function of polarization time (Adapted with permission from Ref. ⁴⁸². Copyright 2021 American Chemical Society). (b) Analysis of the IrO₂(110) Bragg reflections at (h, k, l) = (0, 3.88, 2.01) and (0, 3.88, 4.02): integrated intensities of the specified reflections as a function of the applied potential and the reflection positions and FWHM values. Adapted with permission from Ref. ⁴⁸³. Copyright 2022 American Chemical Society. (c) BOL, in situ ASAXS curves obtained (at 1.0 V vs. RHE) on the IrOx-AS and IrO₂-HT catalysts, diameter's log-normal distribution, the corresponding variation of the oxide disks' average diameter (\<D\>) and thickness (\<L\>) (Reproduced from Ref. ⁴⁸⁴. Copyright 2019 by the Royal Society of Chemistry).
structurally transformed in a near-surface region due to chemical interaction with the electrolyte or the OER itself.⁴⁹¹,⁴⁹² In contrast to the 2D-ordered surface reconstructions described in section 5, the phases formed by this restructuring are often amorphous or highly disordered.

A key example for the dynamic restructuring of iron-group metal oxides under OER conditions are Co oxides, which are the most active single-metal oxides of this class of materials. Especially $Co_3O_4$ has been addressed in several *operando* X-ray scattering studies, which allows comparing the merits of the different approaches. Bergmann et al. studied polycrystalline $Co_3O_4$ films in a neutral electrolyte by powder diffraction under grazing incidence angles.⁴⁹¹ Because of the employed thin layer cell, these studies were limited to potentials in the double layer range up to the onset of the OER. At the most positive potential, a broadening of the $Co_3O_4$ Bragg peaks was observed, which indicated a decrease in the average crystal size by ≈ 1 nm, followed by a reversal of his broadening in diffractograms recorded *ex situ* after the OER (Figure 35a,b). Based on these changes and corresponding variations in complementary X-ray adsorption spectroscopy data, the authors concluded that the OER induces a reversible and highly dynamical structural transition of the near-surface region, where an amorphous Co (oxy)hydroxide skin layer without long-range ordering is formed. In parallel to this work, similar grazing incidence powder diffraction studies in neutral and alkaline electrolytes were performed by Tung et al., who employed as samples cubic $Co_3O_4$ nanoparticles with (100) surfaces, which were coated by an ≈ 1 nm thin epitaxial layer of CoO with (100) orientation and were supported on a fluorine-doped tin oxide substrate (Figure 35c,d).⁴⁹³ At the onset of the OER, new peaks that were assigned to β-CoOOH and, at somewhat higher potentials, a further α-CoOOH phase was observed. Although the ultrathin CoO surface layer could not be detected by XRD, it was proposed that it was reversibly transformed into CoOOH under OER conditions. The authors did not analyze the coherent crystal size,

but the large intensity of the CoOOH peaks suggests that the average thickness of these phases clearly exceeds that of the original CoO layer. These measurements were performed up to currents of 30 mAcm⁻², which required high overpotentials, because of the high ohmic drop within the thin layer geometry. Similar *in situ* XRD measurements were performed by Wang et al. on calcinated $Co_3O_4$,⁴⁹⁴ but in this case, no new crystal phases were detected.

More recently, *operando* X-ray diffraction studies of the oxide's structural transformation were performed using thin epitaxial Co oxide films with well-defined orientation on metal single crystals ((Figure 35g)).⁵³,⁴⁹⁵ The use of such model systems has several advantages. First, the roughness of these highly planar samples is small, i.e., the ECSA is close to the geometric electrode area, and the oxide film has a defined surface orientation. Second, all oxide crystallites have the same macroscopic lattice orientation. For this reason, their Bragg peaks are located at identical locations in reciprocal space, rather than on the surface of a sphere as in powder diffraction. This provides a gain in scattered X-ray intensity of several orders of magnitude, which allows structural studies with ≤ 1 s time resolution (as compared to ≈ 1 min in the powder diffraction studies⁴⁹³). Furthermore, the defined reciprocal space location allows easy separation of the oxide peaks from the isotropic background scattering of the electrolyte and the sample environment. The measurements therefore can tolerate the higher background, which are concomitant with the use of transmission cells as in Figure 8b. Such cells enable studies under extensive gas evolution conditions, i.e., deep into the OER regime, as well as simultaneous high-quality measurements of current density and XRD scattering, which allows direct correlation of the electrochemical and X-ray data.

Reikowski et al. applied this approach to *operando* studies of well-defined epitaxial $Co_3O_4(111)$ and CoOOH(001) thin film model catalysts in 0.1 M NaOH under OER conditions.⁵³ There, changes in the position and width of one of the oxide's Bragg peaks were continuously imaged with a 2D detector as il-
<figure><img src="image_59.png" /></figure>

Figure 35: *Operando* X-ray scattering studies of Co₃O₄ OER catalysts. (a) Powder diffraction data of polycrystalline Co₃O₄ and (b) corresponding coherence length at potentials in the pre-OER range and at 1.62 V, above the onset of the OER, showing a reversible decrease in Co₃O₄ crystal size (Reproduced from Ref. 491. Copyright 2015 Springer Nature). (c,d) GIXRD studies of cubic (100)-terminated Co₃O₄ nanoparticles that are coated by an ultrathin CoO layer in 0.5 M KOH. Shown are (c) a TEM image of the nanoparticle structure (scale bar 10 nm) and the scattering geometry as well as (d) the diffraction data, which reveals the emergence of CoOOH phases at the onset of the OER (Reproduced from Ref. 493. Copyright 2015 Springer Nature). (e-i) *Operando* SXRD studies of epitaxial Co oxide thin films. (e) Scattering geometry and 2D detector images of the Co₃O₄(440) Bragg peak at 1.00 and 1.65 V vs. RHE. (f) CVs and (h) corresponding vertical grain size change of electrodeposited Co₃O₄(111) (red) and CoOOH(001) films (blue) on Au(111) in 0.1 M NaOH, showing the formation of a skin layer on Co₃O₄ (indicated by pink shading). (g) AFM image of the Co₃O₄(111) film (Reproduced from Ref. 53. Copyright 2019 American Chemical Society). (i) Correlation between average skin layer thickness and OER activity, obtained on 5 different types of epitaxial Co₃O₄(111) films. (Reproduced from Ref. 495. Copyright 2022 American Chemical Society).
lustrated in Figure 35e. These changes provide data on changes in lattice strain and average coherent width of the oxide film's grains, with changes in vertical and lateral direction corresponding to structural changes normal to and within the surface plane. By this method, the oxide structure could be monitored during cyclic voltammograms into the OER regime (Figure 35f). The obtained data indicate a profoundly different behavior of the two types of catalysts (Figure 35h). The CoOOH samples were perfectly stable during potential cycles into the OER regime. In contrast, the studies of Co₃O₄ films confirmed the reversible formation of (sub-)nanometer thick skin layers with an amorphous structure, reported by Bergmann et al.,⁴⁹¹ and showed that it is accompanied by a contraction of the remaining Co₃O₄ lattice. However, the higher time resolution allowed to verify that this transformation of the oxide surface already started at a potential well before the OER onset, indicating that the skin layer forms because of the intrinsic Co redox chemistry rather than a product related to the OER process, and demonstrated a fast reversible back-transformation in the cathodic potential sweep. Furthermore, the employed transmission cell allowed to extend these studies to high OER current densities (Figure 35h, right panel), revealing a continuous thickening of the surface skin layer.

Employing the same strategy, Wiegmann et al. recently performed more systematic studies of a variety of epitaxial Co₃O₄(111) films, prepared by electrodeposition or physical vapor deposition on different single crystal substrates.⁴⁹⁵ Despite the identical oxide lattice structure, the deposit morphology and the extent of skin layer formation differed on these samples. By correlating the simultaneously obtained structural and electrochemical data, a clear scaling of the OER activity with the effective skin layer thickness could be shown. This indicates that the entire skin layer is a 3D OER-active region.

Similar structural transformations were also found in X-ray diffraction studies of other Fe, Co, and Ni-containing oxides. Hsu et al. performed *in situ* powder diffraction studies of fer-

rite nanocubes (MFe₂O₄ with M = Fe, Co, Ni, Zn) in alkaline electrolyte.⁴⁹⁶ On the highly active CoFe₂O₄ and NiFe₂O₄ catalysts the formation of new phases was observed near the onset of the OER, which were assigned to oxyhydroxides of Co and Ni, respectively. In contrast, no additional phases were found for Fe₃O₄ and ZnFe₂O₄ up to potentials high in the OER range, together with a lower OER activity. These observations suggest that the formed oxyhydroxide phases are active phases in the OER. Furthermore, similar XRD measurements found a structural phase transformation and high OER current densities in CoCr₂O₄ but not in ZnCo₂O₄, indicating that phase transformations involving the divalent Co²⁺ ions have an important influence on the OER activity. The same group also studied Co and CoFe phosphides and found first conversion into crystalline Co(OH)₂ upon immersion into the KOH electrolyte, followed by a gradual transformation into an amorphous state near the onset of the OER.³⁸¹

Strong evidence for dynamic restructuring of the near-surface region into an amorphous interphase has been also found for perovskites, another important class of OER catalyst in alkaline and neutral media.⁴⁹⁷⁻⁵⁰² In many cases, this restructuring results in thin transition metal oxide layers, which constitute the actual catalyst. As an example, Wan et al. reported the irreversible amorphization of a 2.3 nm thick region at the surface of SrIrO₃ films during cycling into the OER range, using *ex situ* XRR, CTR measurements, and complementary X-ray absorption spectroscopy.⁵⁰² Detailed *operando* X-ray scattering studies of these structural transitions are still missing, however.

Studies of single crystalline model electrocatalysts of Fe, Co, and Ni oxides are largely lacking. Many of these oxides have a limited electric conductivity, which impedes the use of large bulk crystals. A notable exemption is magnetite(Fe₃O₄), which is sufficiently metallic for electrochemical studies. Although the OER activity of magnetite is moderate, it exhibits the inverted spinel crystal structure found also in other more active phases. A first *operando*
SXRD study of the OER on magnetite was performed by Grumelli et al., who investigated UHV-prepared Fe₃O₄(100) by GIXRD.³⁰⁹ Interestingly, the (√2 × √2)R45 deg reconstruction of this surface was found to be moderately stable at potentials deep in the OER regime, showing that this system is highly suitable for studying the OER reaction at a highly defined transition metal surface.

## 6.3 Other electrocatalytic reactions

### 6.3.1 Electrochemical reduction of CO and CO₂

The electrochemistry of the carbon monoxide reduction reaction (CORR) and the carbon dioxide reduction reaction (CO₂RR) has become a subject of great current interest, as it may enable direct electrochemical formation of valuable chemical compounds by a one-step electrolysis process.⁵⁰³,⁵⁰⁴ Most of the research in this area focuses on copper and copper alloys, which have the unique capability to reduce CO₂ to multi-carbon products. A key challenge here is to identify parameters that steer the reaction toward specific products. It was found that the reactivity and selectivity of CO₂RR strongly depend on the catalyst material, on the interface structure and composition, and on the catalyst's nanoscale morphology.⁵⁰⁵

Particularly strongly debated has been the question of whether, despite the very negative potentials, Cu oxides and subsurface oxygen species are present on the electrode under reduction conditions and whether these influence the catalytic reactions. *In situ* GIXRD studies of CORR on polycrystalline Cu electrodes in CO-saturated 0.1 M KOH indicated a full reduction of Cu₂O surface oxide at potentials below 0.3 V vs RHE as well as evidence for the formation of Cu(100) facets.⁵⁰⁶ Similar studies in 0.1 M KHCO₃ provided very similar results and showed that Cu faceting occurs only in the presence of CO₂ but not in Ar-saturated solution.⁵⁰⁷ Furthermore, the ratio of the Cu(100) to the Cu(111) Bragg peak increased with more negative potentials, indi-

cating an increasing tendency towards faceting. *Operando* powder XRD studies with a laboratory source were used to study Cu oxide and hydroxide on carbon paper during CO₂RR.⁵⁰⁸ For all samples full reduction to metallic Cu was observed, albeit with different kinetics. Furthermore, the Cu samples derived from Cu hydroxides exhibited considerable tensile strain with corresponding increases in CO₂RR reactivity, whereas those derived from CuO were strain-free.

In contrast to studies under stationary reaction conditions, recent time-resolved *operando* XRD and XAS measurements of reduced Cu₂O nanocubes under pulsed CO₂RR conditions by Timoshenko et al. found clear evidence for transient oxide species and correlations of its presence with the catalyst's selectivity.⁵⁰⁹ At the anodic potential of the pulse a sub-nanometer thick region of the Cu nanoparticle was converted into a Cu₂O surface oxide on timescales of ≈ 10 s. This oxide phase was removed within 1-2 s after the cathodic potential was applied. The strain in the Cu₂O lattice was found to evolve on similar time scales and to be anisotropic, whereas the Cu lattice strain changed more rapidly to a steady-state value. Comparison with electrocatalytic measurements indicated an enhanced selectivity towards ethanol formation in the regime of cathodic pulses of intermediate length, which was attributed to an optimal balance between oxidized and reduced Cu surface species and a high fraction of Cu(II) species.

Very recently, first *operando* SXRD results of the Cu surface structure under CO₂RR conditions were reported for the case of Cu(100) in CO₂-saturated 0.1 M KHCO₃.⁵¹⁰ Here, CTR measurements were combined with *operando* STM and Raman spectroscopy studies, showing the formation of CO-induced Cu nanoclusters below -0.2 V vs. RHE. These clusters were stable down to -1.1 V and decayed into Cu adatoms upon increasing the potential back into the double layer range. These results indicate that irreversible formation of low-coordinated Cu surface species even occurs, if the Cu oxidation range is avoided.

In addition, CO₂RR on CuAg bimetallic
catalysts was studied, as those have an enhanced selectivity towards multi-carbon products. *Operando* GIXRD studies of Ag-coated Cu nanowires reported that the Cu Bragg peaks irreversibly broadened for CO₂RR below -0.55 V vs. RHE for materials containing 32% Ag and attributed this change to alloy formation.⁵¹¹ As Cu and Ag are immiscible in the bulk, it was proposed that intermixing occurs during the reduction of Cu surface oxide, as the latter results in a highly defective Cu phase. Studies of CuAg metallic foams found that oxidation of these catalysts enhances the selectivity towards ethanol and propanol formation.⁵¹² Here, *operando* XRD studies indicated a full reduction of the Cu oxide at -0.2 V vs RHE, showing that the activation of the catalyst before the onset of the CO₂RR.

X-ray scattering studies of CO₂RR catalysts other than Cu are rarer. Medina et al. employed XRR to investigate the structure of bis-muth thin film electrodes during CO₂ electroreduction in electrolytes containing ionic liquids.⁵¹³,⁵¹⁴ They found that these electrodes originally correspond to a mixture of Bi and Bi oxide, but are fully reduced to metallic Bi under CO₂RR conditions. Furthermore, extensive surface restructuring was observed at a potential near the onset of CO₂ reduction in an imidazolium-based electrolyte that promotes CO formation, but not in electrolytes containing electrochemically inactive tetrabutylammonium cations. Thus, surface restructuring processes seem to play in general a critical role in steering the catalyst selectivity under these highly reducing conditions.

### 6.3.2 Nitrogen reduction

The electrochemical N₂ reduction reaction (NRR) has recently attracted significant attention as a mild and viable alternative to ammonia synthesis through Haber-Bosh process. Among others, Li-mediated, non-aqueous NRR systems seem to be a promising route. In this process, Li electrodeposition is followed by a reaction with N₂ forming a Li₃N intermediate, which subsequently reacts with a proton source to form NH₃. Using *in situ* XRD, Gao et

al. clarified the role of good electrocatalysts for NRR. They showed that catalysts with higher Li affinity, in this case Au plated carbon paper, ease the first step of Li reduction and therefore speeds up the overall process.⁵¹⁵ The role of the solid electrolyte interface (SEI) was investigated by *in situ* neutron reflectometry by Blair et al.⁵¹⁶. They demonstrated that the SEI layer consists mainly of carbonaceous electrolyte degradation products. Current-cycled NRR allows to limit the growth of the SEI, which has positive effects on the NRR rate. While this report uses a neutron technique, the X-ray equivalent could also provide the same level of information, and may be a valid approach to assess the SEI growth and composition in future studies. In the case of a 1 T–MoS₂–Ni catalyst, Li has a different role. *In situ* XRD revealed intercalation of Li⁺ ions in 0.1 M LiClO₄ at –0.3 V, which results in structural distortions, lowering the energy barrier for N₂ fixation and higher NRR activity.⁵¹⁷

Other materials for NRR have been also investigated by *in situ* X-ray techniques. Covalent organic frameworks (COF) show an interesting behavior, where the as synthesized material is less active than the electrochemically activated equivalent.⁵¹⁸ The amorphization of the structure, as revealed by *in situ* XRD and Raman spectroscopy, has a positive effect on the accessibility of active sites as well as a strong affinity to N₂. A low overpotential during the NRR was found to provide the optimal configuration with high NRR activity.

## 6.4 Electrocatalytic devices

Conditions inside commercial electrochemical reactors diverge from typical laboratory cells in terms of temperatures, pressures, gas vs. liquid phases, flowing vs. static electrolytes, pH, fouling, reagent purity, and more. These factors influence the kinetics, thermodynamics, and mass transport of reactions inside the cell. In many cases, such as CO₂ reduction, where a diverse array of product distributions are possible, the reactions that occur inside the cell are a strong function of coupled mass transport and electrokinetics conditions. Therefore, studying
complex multistep processes such as PEMFC
catalyst aging, benefits from probing actual de-
vices, instead of laboratory model systems.

**6.4.1 Proton exchange membrane fuel cells**

Platforms for *operando* X-ray scattering mea-
surements on complete PEMFC devices have
been developed at several synchrotrons. The
first-generation cells repurpose sample environ-
ments from early spectroscopy efforts, modify-
ing commercial cell designs by thinning down
flow field plates, and operating in transmission
mode.⁴⁶² This strategy allows *operando* scatter-
ing to be combined with simultaneous imaging,
fluorescence,⁵¹⁹,⁵²⁰ absorption spectroscopy,
and other complementary techniques.⁴⁵³ In
transmission mode, the thin sample minimizes
parallax distortion⁵²¹ in the scattering signal
while providing a relatively constant back-
ground and a wide angular range. These ad-
vantages allow for high-quality *operando* pair
distribution function analysis of catalyst ag-
ing inside complete fuel cell membrane elec-
trode assemblies, as pioneered by the Petkov
group.⁴⁷³,⁴⁷⁵,⁵²² Studies across a diverse family
of alloy catalysts reveal how strain and local
disorder in the alloy change due to corrosion
over the life cycle of the device⁴⁷⁶, as well as
reveal the microstructural evolution, e.g., of the
crystallite size⁴⁶².

Unfortunately, transmission geometry designs
that accumulate X-ray damage in a single spot,
do not allow probing of many heterogeneous lo-
cations inside the device, and provide a very
low signal-to-background ratio signal, necessi-
tating a sparse number of slow measurements
per sample. Grazing incidence designs offer
a solution to these problems without compro-
mising the device performance⁵⁷, but require a
high-energy, microfocused beam, and preclude
spectroscopy or high angular resolution diffrac-
tion. Grazing incidence geometry allows cor-
relative XRD and SAXS computed tomogra-
phy (XRD/SAXS-CT) experiments with suffi-
ciently high spatial resolution, which can reveal
interesting heterogeneities during PEMFC op-
eration. Using this combination Martens et al.

showed that the catalyst degradation in the cell
is more severe under the land of the flowfield
and it is correlated with the increased water
content in the membrane in these areas⁴⁶⁰. The
same cell was later used to decouple the cata-
lyst degradation mechanisms in the operating
fuel cell by combined WAXS/SAXS measure-
ments⁴⁵¹. This has been achieved by comparing
the NP size measured by both techniques. From
the WAXS data the size of coherent domains,
i.e., the individual crystallites, was determined
and the SAXS data provided the size of aggre-
gated particles, given that aggregation occurs
during the aging of the catalyst. Furthermore,
Chattot et al. also characterized in *operando*
the anomalous aging of the Pt catalyst after
accelerated stress test (AST)⁴⁶¹. Interestingly,
the catalyst fully degrades only when the po-
tential is swept below the AST’s low potential
limit, which removes the surface oxygen from
oxophylic sites and causes sintering of the cata-
lyst particles and increase of the electrochemi-
cal surface area. These findings are only obtain-
able by performing *operando* measurements in
industrially relevant, high-performance devices.

**6.4.2 Solid oxide fuel cells**

The major challenge of solid oxide fuel cell (SOFC) engineering is their high operating temperature, 500°C to 1000°C, which is necessary for oxide ion conduction. This difficulty is even more acute for *operando* cell design<sup>59</sup>, and therefore relatively few studies have investigated microstructure and phase transitions of the ceramic and metallic composites used in the anode, cathode, and solid electrolyte *in situ* with X-ray scattering.

SOFCs are relatively durable and typically
operated under steady-state conditions when
fed with hydrogen. This complicates *operando*
aging studies, particularly those requiring syn-
chrotrons, since the dynamics are very slow and
require hundreds to thousands of hours for sub-
stantial performance loss. The Hardy group
has run several long-term investigations into the
degradation of cathode microstructure, show-
ing that the occupancy of different sites in the
ceramic lattice is altered over time by diffu-
sion.⁵²³,⁵²⁴ Specifically, XRD is suited for tracking changes in strain, cation migration, and depletion of O atoms, which are well understood to be linked to conductivity loss, interdiffusion/poisoning, and other deleterious reactions.⁵²⁵

Startup/shutdown phenomena and side reactions with fuel gases and impurities⁵²⁶ can occur much faster. Korjus et al. investigated the *in situ* activation of the metallic Ni anode catalyst from NiO.⁵²⁷ Obvious strain effects on both the Ni and the ceria-based support appeared as a function of operating potential and fuel pressure, indicating considerable structural sensitivity to redox conditions of the conductive oxides. Li and coworkers used high resolution *operando* XRD-CT in a microtubule SOFC to image the stability of the ceramic/metallic phases during thermal cycling. The homogeneous strain of the Ni catalyst is observed throughout the whole device, which is correlated with the quality of gas distribution and local ionic/electronic conductivity.⁵²⁸

### 6.4.3 Electrolysers

The clear need for *operando* investigations of materials functioning at high current densities during electrolysis led recently to the development of an electrochemical XRD cell able to achieve high performance in membrane electrode assembly (MEA) configuration⁵⁸. This cell has been used to study anion exchange membrane-based CO₂ electrolyzers⁵²⁹,⁵³⁰. In those studies, XRD was used together with reaction product analysis by gas and liquid chromatography, to quantify the amount of electrolyte and salt in the gas diffusion layer (GDL) during the electrolyzer operation. Both GDL flooding and carbonate precipitation phenomena play important role in the performance of the device, because they block the access of the CO₂ feed to the electrode, leading to switching between CO₂RR and HER. It was found that the carbonate salt from the anode side transfers through the membrane to the cathode causing salt precipitation and electrolyte buildup, shifting the reaction to HER. HER accelerates H₂O transport through the membrane, dissolv-

ing the blocking salt deposits and allowing CO₂ to reach the cathode, which switches the reaction back to CO₂RR. The cycle then repeats itself⁵²⁹. The key parameter is the solubility of the carbonate salt, where the key to circumventing salt precipitation is to use highly soluble alkali cation salts as the anolyte (e.g. CsHCO₃) along with an optimal salt concentration between 0.01 and 0.1 M⁵³⁰.

## 7 Electrochemical phase formation

The formation of materials by electrochemical growth or structural changes in a material due to electrochemical dissolution or phase change processes are key application areas of scattering techniques. Such processes play an important role in the context of (electro-)chemical material synthesis, electrodeposition, etching, and corrosion. XRD has been employed for a long time as a standard method for the *ex situ* characterization of the resulting materials. However, also the processes itself can be investigated in *operando*, often on the atomic scale, which can provide detailed insights into the growth mechanisms and kinetics.

### 7.1 Electrodeposition and dissolution

Electrodeposition and electrochemical etching are widely used in industry, e.g., for electroplating, electrorefinement, and electropolishing. Applications range from the production of functional coatings to chip metallization in the microelectronics industry and the preparation of nanomaterials. The latter ones require to improve our understanding of the underlying processes, which currently is still largely phenomenological, on the atomic and nanometer scale. As will be shown in the following, *in situ* and *operando* XRD methods are well suited tools for this.
### 7.1.1 Homoepitaxial electrodeposition

The conceptually most simple type of growth process is the growth of a material on a single crystal substrate of the same material. Such homoepitaxial growth has been studied extensively in the gas phase.⁵³¹,⁵³² Here, studies that monitor the scattering of electrons, ions, or X-rays as a function of deposition time have played an important part in determining the temporal evolution of the surface morphology during deposition. In particular, these methods allow to identify the kinetic growth modes, which are determined by the flux of the deposited atoms to the surface, the intralayer transport on atomically flat terraces of the crystal surface, the interlayer transport across steps between neighboring terraces, and the probability for island nucleation on the terraces (Figure 36a). If the flux is sufficiently low and the surface diffusion of the deposited adatoms high, all deposited atoms are rapidly transported to existing step edges, where they are incorporated into the crystal. During this step-flow growth the surface morphology is maintained and, thus, the scattered intensity at reciprocal positions along the specular and non-specular CTRs does not change with time. Upon increasing the flux, the surface concentration of deposited adatoms increases until the nucleation threshold for the formation of monolayer islands on the terraces is reached. If adatoms can easily descend from these islands, island growth and subsequent coalescence to a new layer occurs, on which then the next layer islands nucleate and grow (layer-by-layer growth), that is, the occupation of the topmost layer oscillates between zero and a full monolayer. The partially filled top layer results in destructive interference of the scattered beam, which manifests particularly clearly at reciprocal space positions that are halfway between two Bragg peaks along a CTR ("anti-Bragg position"). Consequently, the scattered intensity varies periodically in the form of so-called growth oscillations, where each intensity maximum corresponds to a newly deposited layer. At very high flux or if the Ehrlich-Schwoebel barrier for the diffusion of adatoms across steps is high, new nuclei are formed on

existing islands before they coalesce to a closed layer. This results in a 3D growth mode, in which multilayer islands are formed. This results in a gradual roughening of the growing surface, leading to a continuous decay in the scattered intensity along the CTRs.

In the gas phase, such studies are comparatively simple, because the deposition process can be easily switched on and off. However, this is not the case in homoepitaxial electrodeposition, where either electrochemical deposition or dissolution occurs. These processes proceed even at the equilibrium potential, making it difficult to start the growth on a defined surface. For these reasons, most studies of electrodeposition have focused on heteroepitaxial growth. Furthermore, the flux of deposited species and the deposition potential, which influences the interface structure and the dynamic behavior of adsorbed species, cannot be easily controlled independently due to the mass transport in the electrolyte: Under potentiostatic conditions, the deposition current will vary with time; under galvanostatic conditions, changes in the deposition potential will occur. To circumvent these problems, first *operando* XRD studies of homoepitaxial electrodeposition used low concentrations of the deposited ions and performed the studies under diffusion-limited conditions. Krug et al. employed this strategy to study Au electrodeposition on Au(100) in Cl-containing electrolyte and showed that the kinetic growth behavior depends strongly on potential Figure 36b,c).⁵²,⁵³³ At the most positive potentials, no changes in the scattered intensity with time were found, indicating step flow growth. Subsequently, the potential was step-wise changed from this range to a more negative value. This resulted first in the characteristic growth oscillations of layer-by-layer growth and, at even lower potentials, rapid intensity decay due to step-flow growth. As the Au flux to the surface was constant, these transitions indicate a strong increase in the Au surface mobility with increasing potential, which is in good agreement with results from scanning probe microscopy.⁵³⁵,⁵³⁶ Interestingly, the growth reverted to layer-by-layer mode at even more negative potentials, where the Au(100) surface reconstructs, which
<figure><img src="image_60.png" /><figcaption>Figure 36: (a) Schematic illustration of the three kinetic growth modes. (b) Time-dependent scattered intensity during homoepitaxial Au deposition on Au(100) in 0.1 M HCl + 0.5 mM HAuCl₄, indicating a crossover from step-flow (yellow) to layer-by-layer (orange) to 3D growth (red) with decreasing deposition potential, and (c) the regions of different growth modes as a function of potential and deposition rate (Adapted from Ref. 533). Copyright 2006 American Physical Society). (d) Selected GISAXS data and (e) corresponding intensity distributions as a function of in-plane momentum transfer, showing the temporal evolution of mounds during 3D deposition on Au(100). The inset shows the change of the peak position with deposition time (Adapted from Ref. 534). Copyright 2014 American Physical Society).</figcaption></figure>

may be caused by a higher Au adatom mobility on the hexagonally ordered top layer of the reconstructed surface as well as quasi-collective transport mechanisms.⁵³⁷

In a subsequent study such step experiments were performed systematically at different positions along the specular CTR, which allowed measuring the latter with sub-100 ms time resolution.⁵³⁸ Fits of these CTRs in the initial stages of growth showed that deposition at 0.35 V vs Ag/AgCl is well described by nucleation and growth of Au monolayer islands, whereas at -0.2 V first the hexagonal reconstruction is formed, followed by layer-by-layer growth. The formation of the Au(100) reconstruction was found to occur two orders of magnitude faster than in the absence of Au deposition, which can be attributed to the enhanced surface mass transport under growth conditions. A similar influence of Au electrodeposition on surface reconstruction was found in SXRD studies of Au(111).⁵³⁹ Here, the compression of the Au surface layer did not saturate at a (23 × √3) superstructure, but linearly increased with potential. This behavior resembles the electro-compression effect found for UPD metal adlayers and can be explained by lower kinetic limitations for surface restructuring under growth conditions.

The evolution of the Au(100) surface morphology during 3D growth was studied by Ruge et al. via *operando* GISAXS.⁵³⁴ The authors monitored peaks in the small angle regime, which result from the scattering by nanoscale mounds that are formed on the surface by the electrodeposition process (Figure 36d,e). With increasing deposition time, the peak intensity increased and the peak position shifted to smaller angles, indicating vertical growth and lateral coarsening of the mounds, respectively. The coarsening followed a well-defined scaling law with an exponent that shifted from 1/4 to 1/3 in the potential range of -0.1 to 0.2 V. In the context of 3D growth theory,⁵³² this behavior may be explained by a crossover from mounds without defined slope to mounds with faceted side walls.

Analogous SXRD studies were performed for Cu electrodeposition, which is a process of
tremendous importance, especially because of its applications in the microelectronics industry. These measurements were performed in an Cl-containing electrolyte at growth rates up to 38 ML per minute. Interestingly, the growth behavior changed from 3D to layer-by-layer to step-flow growth with increasingly positive potential, i.e., in the opposite order as in the case of unreconstructed Au(100).<sup>216,540,541</sup> This surprising effect was attributed to the influence of adsorbed Cl, specifically the formation of an ordered $c(2 \times 2)$ Cl adlayer, on the Cu surface mobility. *Vice versa*, the Cu deposition process was found to significantly slow down the kinetics of Cl adlayer ordering.<sup>541</sup> Furthermore, the measurements indicated an oscillatory strain in the surface layer, which was tentatively attributed to surface stress induced by island step edges and Cu adatoms. Furthermore, the influence of polyethylene glycol (PEG), which is an important additive in the Cu damascene plating process, was studied by the same group.<sup>542</sup> CTR and superstructure rod measurements revealed that the polymer interacts more strongly with the partially $c(2 \times 2)$ covered surface than with the close-packed Cl adlayer. However, the growth-inhibiting properties of the PEG-Au complex are more pronounced in the latter regime according to time-resolved SXRD studies. In addition, the presence of PEG in the solution shifted the crossover from layer-by-layer to 3D growth to significantly more negative potentials.

### 7.1.2 Electrochemical and chemical dissolution

We first discuss dissolution at the crystalline surface of a simple elemental metal, which is the inverse process to homoepitaxial deposition. This process was studied for the case of Au(111) and Au(100) electrodissolution in Cl-containing electrolyte using the same methodology as in the electrodeposition experiments above.<sup>543,544</sup> That is, the dissolution mode was analyzed by time-resolved measurements of the X-ray intensity near anti-Bragg positions and simultaneous measurements of the electrochemical current density. Because the dissolution process

is rather irreversible, these studies could only be performed as single-shot experiments. Nevertheless, a time resolution of down to 2 ms could be obtained, which allowed studying the etching of the surface at technologically relevant rates of up to 20 ML/s. During the dissolution of Au(100), the characteristic intensity oscillations of a layer-by-layer mechanism were observed.<sup>544</sup> However, at the most positive potentials, the oscillating intensity rapidly decayed, indicating an increasing surface roughening at higher dissolution rates. Studies of Au(111) at lower dissolution rates ($\le 20$ ML/min) found a transition from step-flow to layer-by-layer dissolution prior to surface passivation via gold oxide formation.<sup>543</sup> From a quantitative analysis involving both the X-ray intensity transients as well as the current transients the dissolution behavior could be shown to be described by a smooth multilayer growth model. Here, progressive vacancy island nucleation occurs in the whole active dissolution regime with next-layer nucleation occurring at rather low critical top layer coverages between 0.29 and 0.44 ML.

Furthermore, detailed studies were performed on the dissolution of alloys, in particular dealloying. Here, the less noble component dissolves selectively, leaving behind the matrix of the more noble metal. Pioneering work in this area was performed by Renner et al., who studied this process by *in situ* SXRD and anomalous GIXRD on alloy single crystals.<sup>545-553</sup> Studies of (111)-oriented $Cu_3Au$, an important model system for dealloying, in sulfuric acid solution revealed in the first stage of this process the formation of a 2-3 ML thick passivation layer with depleted Cu content (less than 40 %).<sup>545,548</sup> Interestingly, the stacking sequence of the atomic planes in this passivation layer was inverted relative to the fcc stacking sequence in the underlying $Cu_3Au$ substrate (Figure 37. In the later stages of the dealloying process, the passivation layer transformed into a thicker layer consisting of Au-rich islands. This transition heralds the onset of the nanoporous Au formation and confirms predictions by Monte-Carlo simulations.<sup>554</sup> Subsequent systematic studies of the effect of halides on $Cu_3Au$(111) dealloying,<sup>546,549,550</sup> found for Cl and Br very similar
behavior as in pure H₂SO₄ solution, apart from a negative shift in the potential of the transition between passivation layer and island formation. These results suggest that the transition is not thermodynamically driven but a kinetic effect, caused by the (potential-dependent) rate of surface diffusion, which is higher in the presence of halides. In iodide-containing solution, no stable passive film was observed. Instead, precipitation of an epitaxial CuI(111) film was found, which partly inhibited the dealloying, resulting in the formation of a bimodal pore size distribution in the resulting Au layer. In addition, functionalization on Cu₃Au(111) electrodes by thiols resulted in an inhibition of the dealloying.⁵⁵²,⁵⁵³ Other Cu alloy electrodes exhibit a similar dissolution behavior, i.e., the formation of an epitaxial metallic passivation layer composed of islands of the noble alloy component that suppresses massive Cu dissolution. On Cu₃Au(100), a layer of islands with (111) facets was observed.⁵⁴⁷ Dealloying of (111) and (100) oriented Cu₃Pd lead to the formation of epitaxial passivation layers of Pd on both surfaces.⁵⁴⁷,⁵⁵¹

<figure><img src="image_61.png" /><figcaption>Figure 37: Dealloying of Cu₃Au(111) electrodes. (a) Reciprocal space map showing the Bragg reflections from the passivation layer (red) and the Cu₃Au substrate (blue). (b) Schematic illustration of the electrode structure in the passivated state. (c) CV of the dealloying process (Adapted from Ref. ⁵⁴⁵. Copyright 2006 Springer Nature).</figcaption></figure>

More recently, dealloying has become of interest for the formation of nanoporous metals, especially nanoporous gold. A prototypical sys-

tem here is the dealloying of AuAg in nitric acid solution under open circuit conditions. This process was first addressed by *in situ* XRD in a study of polycrystalline Au₃₀Ag₇₀ dealloying by Van Petegem et al..⁵⁵⁵ They observed the evolution of a broad component in the Bragg peaks over several hours and revealed a decrease in the coherent grain size by a factor of four and a parallel build-up of tensile stress in the residual Au lattice. More detailed *in situ* SAXS and XRD studies by Toney and coworkers at different acid concentrations, performed using AuAg thin film samples to achieve more uniform dealloying, revealed several characteristic stages of this process.⁵⁵⁶ After an initial fast dealloying, in which a random distribution of pores and compressively strained Au ligaments were formed, coarsening of the morphology was observed. In the latter, first a bicontinuous morphology with quasi-periodic character emerged, which then coarsens in a self-similar manner. Furthermore, both of these studies deduced the presence of directional strain anisotropies in the Au ligaments. More recent studies investigated the influence of temperature on the dealloying process by *in situ* SAXS⁵⁵⁷,⁵⁵⁸ and determined the structural changes in isolated AuAg nanocrystals that were induced by 10 s of dealloying by *ex situ* BCDI.⁵⁵⁹ The latter work provided detailed strain distributions within the nanoparticle that showed the formation of nanopores near the surface induced compressive strain 60-80 nm deep beneath, but weak tensile strain deeper within the particle. This coexistence of differently strained areas is in agreement with previous averaging XRD studies⁵⁵⁵,⁵⁶⁰ and demonstrates the potential of coherent imaging techniques in resolving structural transformations in complex materials.

In addition, the chemical dissolution of oxides has been addressed. The surface dissolution of orthoclase (001) has been studied using X-ray reflectivity by Teng et al., who looked at the dissolution process of orthoclase (001) at two extreme pH conditions (pH 1.1 and pH 12.9).⁵⁶¹ It was found the dissolution process in an acidic solution proceeded nonstoichiometrically and is accompanied by terrace roughening. In an alkaline solution, the dissolution is mostly stoi-
chiometric and occurs via a step-flow mechanism. The dissolution mechanism at two different pH was thought to be controlled by surface reactions at distinct reactive sites. Fenter et al. studied the same system and identified two dominant reactive sites, namely bridging oxygen between Al and Si sites (site A) and bridging oxygen between Si surface sites (site B).⁵⁶² Those two reactive sites are responsible for dissolution occurring at pH>0.5 and pH <0.5, respectively. The terrace roughening at pH 1.1 observed by Teng et al. may correspond to the surface reaction occurring dominantly at site B with a smaller portion of surface dissolution occurring at site A. In contrast, the step flow motion at pH 12.9 should be related to surface reactions occurring solely at site B. Besides solution pH, the crystal orientation can also play a role in the dissolution kinetics on orthoclase surfaces.⁵⁶³ In addition, NaCl background electrolyte was found to enhance the surface dissolution of orthoclase.⁵⁶⁴ This effect was assigned to a higher terrace reactivity in the presence of NaCl that leads to more 3D dissolution, resulting in faster dissolution rates.

### 7.1.3 Heteroepitaxial electrodeposition

The evolution of the deposit's lattice structure and morphology during electrochemical nucleation and growth on a substrate of a different material is a complex process, which has been studied extensively by a variety of *in situ* methods.⁵⁶⁵ Commonly, a well-defined relationship between the orientations of deposit and substrate lattice exist, which is denoted as epitaxial growth and depends on the mismatch of the in-plane lattice geometries at the interface. A specific case is pseudomorphic growth, where the deposit adopts the in-plane lattice of the substrate, but this is usually only observed in systems with very small lattice mismatch. The interplay of interface energies and strain in the deposit results in structural transitions with increasing thickness of the deposit that can be probed well by XRD methods.

*In situ* X-ray scattering studies of heteroepitaxial electrodeposition originally focused on metal-on-metal deposition in aqueous elec-

trolytes. In particular, the crossover from UPD (see section 5.2.2 to multilayer growth was addressed. Early SXRD studies of Pb electrodeposits on Ag(111) found a disruption of the Pb UPD layer at coverages ≥5 ML and the formation of (111)-textured but randomly oriented islands.²⁰⁹ In contrast, bulk electrodeposition of Tl on Ag(100) and Au(100) proceeded epitaxially on top of the Tl UPD bilayer and monolayer, respectively, in form of well-defined 3D islands of (0001) hcp Tl.²⁴⁰ The stability of the UPD layers underneath the bulk deposit was explained by the low interface energies, resulting from the similar (quasi-)hexagonal in-plane lattices. Several *in situ* SXRD studies investigated the electrodeposition of Pd on single crystal substrates, because of its interest in electrocatalysis and hydrogen storage. Pd growth on Pt(111)²⁶⁵ and Pt(100)¹⁹⁸ was found to proceed via a Stranski–Krastanov growth mode, in which first a pseudomorphic Pd monolayer formed, followed by the growth of 3D islands. For Pd electrodeposition on Au(111), the first two layers are deposited via a non-ideal layer-by-layer pseudomorphic growth, on which subsequently relaxed 3D islands form.⁵⁶⁶ The important influence of interface energy and deposit strain was illustrated in SXRD studies of Cu bulk electrodeposition on Au(100)⁵⁶⁷,⁵⁶⁸ and Ag(100).⁵⁶⁹ In both cases, the large lattice mismatch enforces an initial layer-by-layer growth in form of body-centered cubic (bcc) Cu up to 10 ML. However, at larger film thickness, this bcc-like structure becomes unstable, resulting in a buckling transition. The resulting phase exhibits a uniaxial periodic modulation, in which local orthorhombic regions are formed, reminiscent of martensitic distortions in Fe alloys.

The electrodeposition of thin magnetic films, a topic of substantial interest for industry, has been studied likewise on defined single crystal substrates. Co and Fe electrodeposition on Cu(100) was investigated using simultaneous *in situ* XRD and magnetooptical Kerr effect (MOKE) measurements.⁵⁷⁰,⁵⁷¹ For Co, the growth of a pseudomorphic fcc Co deposit was found up to 15 ML, together with the formation of an in-plane relaxed component, which
was attributed to Stranski-Krastanov growth of 3D islands. In the case of Fe, a transition from pseudomorphic fcc Fe to a bcc Fe(110) film was observed at coverages ≥7 ML. In this transition, the fcc lattice was reoriented. In parallel, the MOKE measurements indicated a reorientation of the easy axis of magnetization from out-of-plane to in-plane. *In situ* SXRD and STM studies of electrodeposited Co layers on Au(111) indicated that the growth started with the formation of an hcp Co(001) bilayer, followed by a smooth layer-by-layer growth.⁵⁷² Interestingly, the presence of the Co film was found to restructure the (23 × √3) reconstructed top Au layer into an isotropically compressed arrangement, demonstrating the unique capability of XRD methods to access such deeply buried metal/metal interfaces. Subsequent studies demonstrated the epitaxial electrodeposition of Pd(111) capping layers onto Co/Au(111) and showed that the in-plane strain in the Pd film upon H absorption was coupled into the underlying Co film.⁵⁷³

Only a few *in situ* XRD studies have addressed epitaxial electrodeposition on semiconductors. The main problem here is the strong photoelectrochemical effects induced by the X-ray beam.⁵⁷⁴,⁵⁷⁵ Zegenhagen and coworkers performed a series of *in situ* SXRD and X-ray standing wave measurements of noble metal deposition on Si and GaAs electrodes from aqueous electrolytes.⁵⁷⁴,⁵⁷⁶⁻⁵⁷⁹ Characteristic for these systems is a 3D cluster growth of the deposit. Studies of the electroless deposition of Au and Pb on H-terminated n-Si(111) indicated wetting of the semiconductor substrate by Au at sub-monolayer coverages, followed by growth of epitaxially aligned (111)-oriented clusters.⁵⁷⁶,⁵⁷⁷ A similar epitaxy was found for electrodeposited Au films on this surface by *ex situ* XRD.⁵⁸⁰ Cu electrodeposition on H-terminated n-Si(111) proceeded via the growth of Cu clusters, in which the (111), (110), and (100) planes were oriented parallel to the Si(111) surface (with the first two orientations being dominant) but only the Cu(110) clusters had defined in-plane orientations relative to the substrate lattice.⁵⁷⁴,⁵⁷⁵ For Cu electrodeposits on UHV-prepared GaAs(100) a splitting

of the Cu diffraction pattern into 8 Bragg peaks was found, indicating an unusual epitaxial relationship, in which the Cu(100) planes were rotated by ≈5° and tilted by ≈9° with respect to the substrate lattice.⁵⁷⁸,⁵⁷⁹ In addition, diffuse intensity streaks extending from the Cu(022) peak were observed, indicating that the sides of the Cu clusters were terminated by (111) facets.

Non-aqueous electrolytes and, in particular, room-temperature ionic liquids (ILs) allow to deposit a wider range of materials and have accordingly received much attention. SXRD studies of Zn electrodeposition from ILs found in the initial stages formation of epitaxial Zn and Au₁₂Zn₈₈ films.⁵⁸¹ More extensive deposition, studied on polycrystalline Au films by powder diffraction under grazing incidence angles, resulted in stronger alloying with the parallel formation of Zn and several Zn intermetallic phases. XRR studies of Si electrodeposition in ionic liquids reported the formation of an intermediate polymer-like Si chloride layer, which subsequently was further reduced to the Si deposit.⁵⁸² Of particular interest in this field are studies of alkali and second row metal electrodeposition as anode processes in advanced batteries, as performed e.g. by Renner et al. for Li⁵⁸³ and Ebenshade for Mg⁵⁸⁴ (see also chapter 8).

Electrodeposits of non-metallic materials have been frequently characterized using *ex situ* XRD methods, but *in situ* studies of the corresponding electrodeposition processes themselves are still scarce. A challenge for these studies may be the more complex growth conditions and, especially, the higher temperatures often required for these studies. A rare example was published by Ingham et al., who studied ZnO electrodeposition on Au(111) under conditions where growth was limited by Zn ion diffusion.⁵⁸⁵ The XRD measurements showed that a (002) texture develops in the nucleation stage, followed by anisotropic growth of ZnO nanorods along this axis.

Apart from wide-angle scattering studies of the deposit crystal structure and epitaxial orientation, also the nanoscale evolution of the morphology during nucleation and growth have been studied by *in situ* SAXS. Such studies are well established for studies of chemical and
physical deposition processes<sup>78</sup> and have also been applied to electrodeposition.<sup>586-591</sup> Ustarroz et al. employed SAXS to determine the evolution of the nanoparticle size during Ag nucleation and growth on HOPG and found good agreement with the growth behavior deduced from electrochemical measurements.<sup>586</sup> Combination of *in situ* SAXs and *ex situ* electron tomography allowed to monitor the electrochemical growth of dendritic aggregates of Pt, showing that the deposition potential influenced the aggregation and coalescence of the primary nanoclusters.<sup>587</sup> In similar SAXS and WAXS measurements, lateral and orientational ordering during Pt electrodeposition in 3D ordered lipid templates was studied.<sup>588,592</sup> SAXS is not only sensitive to the deposited particles, but can also provide insight into the surrounding electrolyte, as indicated by detailed studies of Hammons et al. on Pb and Zn nanoparticle growth in deep eutectic solvents.<sup>589,590</sup> For Pb, mesoscale anisotropic scattering was observed in SAXS measurements at different incidence angles, which was attributed to strong potential-dependent perturbation of the solution structure around the Pb particles. The latter was described in a simplified model by a layered structure, similar to that found for ionic liquids at planar interfaces (see section 5.2.3), that extended several 10 nm into the solution. The evolution of the porous growth morphology of electrodeposited MnO<sub>2</sub> was followed by *in situ* SAXS in studies by Dupont and coworkers, revealing a gradual change in pore size with deposition time.<sup>593,594</sup> In principle, also GISAXS should be highly suitable for such nanoscale studies, but up to now has been employed only for *ex situ* characterization of electrodeposits.<sup>595</sup>

All the SXRD studies above provided information about the average film structure over macroscopic surface areas, owing to the large footprint of the beam on the sample surface. However, this problem can be overcome at modern synchrotron beamlines, which typically allow the focusing of the beam to micrometer or even sub-micrometer dimensions. Such microdiffraction studies can be performed by employing focused high photon energy beams in

transmission geometry, as illustrated schematically in Figure 18b. This allows to map structural heterogeneities of the deposit, as has been shown in *in situ* studies of electrodeposited Co<sup>102</sup> and Bi films.<sup>103</sup> Similar as in low-energy electron diffraction, this transmission surface diffraction method provides at each point on the sample 2D detector images that show the in-plane reciprocal space geometry over a wide $q_\parallel$ range (Figure 38a). The sensitivity of this method is sufficient to resolve single atomic layers, e.g., the Au(111) surface reconstruction or the Bi UPB superstructures. By performing such measurements at different locations on the sample and automated quantitative analysis of the diffraction peaks, the local variation of characteristic structural properties of the local deposit can be investigated. This is illustrated in Figure 38b where maps of the local thickness, in-plane grain size, and strain in the Co electrodeposit as well as induced strain in the Au substrate are presented, revealing clear correlations of these properties.<sup>102</sup> Similar correlations were also found in analog measurements of Bi electrodeposition within a microfluidic cell.<sup>103</sup> Such locally resolved measurements will benefit strongly from the emerging fourth-generation synchrotron sources. However, the very high local X-ray intensity in such experiments will lead to increased problems with beam damage effects.

Finally, *in situ* X-ray scattering techniques also offer unique capabilities to study electrodeposition at liquid-liquid interfaces. Such deposition processes have received renewed interest, because they offer a substrate that is free of defects and strain, enable growth from both sides of the interface, and thus open up opportunities for the electrodeposition of a wide range of materials, including semiconductors.<sup>357</sup> Only hard X-rays allow to probe the atomic-scale structural studies of these deposition systems directly. Several studies investigated electrochemical growth at liquid metal electrodes by *in situ* XRR and GIXRD. Specifically, the nucleation and growth Pb halide films on liquid Hg were studied in detail.<sup>95,365,366,368</sup> It was shown that ordered salt-like adlayers, formed initially on the Hg surface (see section 5.5), pro-
<figure><img src="image_62.png" /></figure>

Figure 38: Structure of electrodeposited Co this
films on Au(111), studied by HESXRD at 40
keV in transmission geometry. (a) Difference in
the scattered X-ray intensity, obtained by sub-
tracting the 2D detector images recorded prior
and after Co deposition. Sixteen Au CTRs and
12 Co CTRs are visible, with peaks that emerge
after deposition depicted in red and peaks that
disappear in blue, respectively. (b) Maps show-
ing the micrometer-scale variation of the de-
posit thickness, lateral grain size, and strain in
Co deposit and Au substrate, derived from a
quantitative analysis of the Bragg peaks in de-
tector images recorded at each point of the map.
(Adapted from ref. ¹⁰². Copyright 2014 Ameri-
can Chemical Society).

vide a template for the subsequent growth of a
strongly textured bulk deposit. Detailed stud-
ies of PbBrF growth indicated with increasing
overpotential a crossover from the slow growth
of a small number of large crystals to the forma-
tion of a compact film with a saturation thick-
ness of 25 nm.⁹⁵ In the context of studies of the
electrochemical liquid-liquid-solid deposition of
Ge on Hg and HgIn alloys, *in situ* XRR was
used to determine the influence of the inter-
face structure on the growth process.³⁶⁷,⁵⁹⁶ *In
situ* X-ray scattering techniques have also been
applied to study growth at interfaces between
other immiscible liquids, e.g., between aque-
ous and non-aqueous solutions,⁵⁹⁷⁻⁶⁰¹ but cor-
responding studies of electrodeposition in such
systems have not been reported yet.

7.2 Passivation and corrosion

The ability of *in situ* X-ray scattering meth-
ods to obtain insight into buried interfaces has
proven to be highly valuable in corrosion sci-
ence, especially for the structural investigation
of passive layers and for monitoring the struc-
tural evolution of the surface morphology dur-
ing metal dissolution. This section will focus on
the more classical subjects of corrosion science,
such as Cu, iron-group metals, and valve met-
als. Related studies on electrocatalyst degrada-
tion and noble metal dissolution can be found
in sections 6 and 7.1.2, respectively.

Detailed *in situ* SXRD studies of passiva-
tion processes, performed on single crystal elec-
trodes, showed that passive film in many cases
grows epitaxially on the underlying metal and
typically consists of nanoscale islands sepa-
rated by grain boundaries. Toney et al. re-
ported such epitaxial films, consisting of Fe<sub>3</sub>O<sub>4</sub>
on Fe(001) and Fe(110) electrodes and showed
them to exhibit significant numbers of cation
vacancies and interstitials.<sup>602,603</sup> SXRD stud-
ies of Ni(111) passivation in sulfuric acid solu-
tion indicated the presence of an epitaxial crys-
talline NiO(111) inner layer and a more porous
and probably amorphous outer layer of Ni hy-
droxide, supporting the duplex model of passive
film growth.<sup>604,605</sup> The thickness of the crys-
talline inner layer was found to increase lin-
early with potential from 1.0 to 2.0 nm. A very similar passive film structure was observed on Ni(111) electrodes in pH 14 KOH solution.⁶⁰⁶ Studies of thin epitaxial Co(001) films in 0.1 M NaOH indicated a stepwise oxidation mechanism, starting with the rapid growth of an ultrathin Co(OH)₂ layer, followed by the slower formation of 3D Co(OH)₂ crystallites at grain boundaries in the Co film.⁶⁰⁷ On Cu(111) in pH 4.5 solution, phases with the same cuprite structure and orientation as the air-grown native oxide film were found, but exhibited strong differences in growth behavior and strain.⁶⁰⁸ In particular, the experiments revealed the formation of an initial monolayer oxide phase, which was also found in independent *in situ* STM measurements.¹⁸⁴ In general, *in situ* SXRD and STM results on the initial stages of passivation in the above systems have been in excellent agreement and found to provide highly complementary information on the underlying atomic-scale mechanisms.⁶⁰⁹

In addition to these model systems, *in situ* X-ray scattering methods have also been used to study passivation and corrosion in more realistic systems, typically on polycrystalline materials. Early studies of thin Cu films on Si by *in situ* XRR and diffuse scattering found in borate buffer solution the formation of ≈ 2.5 nm thick Cu₂O layers after anodic oxidation, whereas in bicarbonate solution evidence for pitting corrosion was reported.⁶¹⁰⁻⁶¹² Powder diffraction studies under grazing incidence conditions of the anodic oxidation of lead in sulfuric acidic electrolytes, an important process in Pb acid batteries, allowed to identify several crystalline products, such as Pb sulfate, PbO₂, and tribasic Pb sulfate hydrates.⁶¹³

Several studies addressed the structure and evolution of passive films on steel, a key topic in corrosion science. Sinha and coworkers determined by anomalous XRR the passive film on type 430 stainless steel in pH 8.4 borate buffer.⁶¹⁴ In the regime of passivity, the film was shown to consist of an inner layer consisting of insoluble Cr₂O₃, which acted as a barrier to oxygen anion incorporation, and a Fe/Cr oxide outer layer. In the transpassive regime, the entire film consisted of a mixed Fe/Cr oxide.

Ingham and coworkers performed systematic *in situ* SAXS and WAXS studies of the formation of protective scales during the corrosion of carbon steels in CO₂-saturated brine at temperatures up to 90° C, a problem of practical interest in oil and gas pipelines.⁶¹⁵⁻⁶²³ The protective phase was identified as crystalline siderite (FeCO₃). It was shown to form at high supersaturation via a complex reaction, which involved the spreading of a colloidal amorphous precipitate over the metal surface, followed by the formation of pre-nucleation clusters and finally the nucleation and growth of the crystalline phase. This process was strongly affected by the presence of Cr and Mo in the steel, the solution pH and flow conditions, the nature of cations in the solution, the presence of trace amounts of oxygen, and the presence of organic scale inhibitors.

Related to passivation and corrosion is anodization, a technologically important process in which the metal is electrochemically oxidized in a controlled way. Conventional anodization results in films of defined thickness that increases linearly with potential, as also confirmed by *in situ* XRR.⁶²⁴ However, most research in this area focused on the self-organized growth of oxides with well-defined ordered arrays of cylindrical pores, especially the growth of porous anodic aluminum oxides.⁶²⁵ Various groups investigated the structural evolution of the pores during anodic aluminum oxidation by *in situ* SAXS,⁶²⁶,⁶²⁷ GISAXS,⁶²⁸ and grazing incidence transmission small angle X-ray scattering (GTSAXS),⁶²⁹,⁶³⁰ coming to similar conclusions. SAXS measurements can only provide data on the in-plane ordering. In contrast, GISAXS and GTSAXS provide information about the in-plane nanoscale morphology as well as the film structure vertical to the surface, as illustrated in Figure 39a. The in-plane ordering was found to proceed via a power law and could be described by domain structure formation and Ostwald ripening of domains. The pore has a well-defined vertical length, and manifests in oscillations along the *qz* direction at the in-plane peak positions (Figure 39b). The vertical growth of the pores, determined from the periodicity of these oscillations, pro-
ceeds at a constant rate, which depends on the
substrate orientation (Figure 39c). In addition,
in situ GTSAXS has been employed to monitor
the filling of these pores by electrodeposition of
Sn.⁶³¹,⁶³²

<figure><img src="image_63.png" /></figure>

Figure 39: *In situ* GTSAXS measurements of the formation of porous anodic alumina oxides. (a) X-ray scattering geometry. The intensity distributions along *q*<sub>y</sub> and *q*<sub>z</sub> provide information about the in-plane and out-of-plane structure, respectively. (b) Detector images illustrating oxide growth during 2nd anodization in oxalic acid solution. (c) Increase in pore length, obtained from the period of the oscillations along *q*<sub>z</sub> (Reproduced from ref. ⁶²⁹). Copyright 2018 American Chemical Society).

**7.3 Synthesis of electrocatalysts**

Another interesting utilization of *in situ* X-ray
scattering techniques for electrochemistry and
electrocatalysis purposes, beyond the study of
electrode materials during electrochemical op-
eration, is to monitor how the electrode ma-
terials' properties emerge during their fabrica-

tion. This aspect is essential in the ‘electrode-
by-design’ approach largely adopted in the field,
as desirable catalyst structure and chemistry
must be tailored mostly during their synthe-
sis. Early uses of this possibility were made
in the 1980ies to investigate the formation of
Pt-alumina reforming heterogeneous catalysts
in high-temperature chambers under different
gas atmospheres,⁶³³⁻⁶³⁵ where guiding strate-
gies toward the preparation of targeted Pt alloy
phase(s) were established. In electrocatalysis, a
similar approach was used to monitor the for-
mation of binary intermetallic PtM (M=Co, Fe
or Ni) ORR catalysts during thermal annealing.
In the case Pt₃Co particles, Xiong et al. in-
vestigated the impact of a variety of annealing
conditions on the degree of ordering that could
be obtained.⁶³⁶ It was found that an annealing
temperature of 750 °C maximizes the catalyst
ordering (30 %) and so the long-term durabil-
ity of the material in practical PEMFC tests.
Recently, Zeng et al. extended the study to
PtCo, PtFe, and PtNi materials.⁶³⁷ Using pow-
der diffraction, they monitored the structural
evolution during heating/cooling cycles and ex-
tracted the degree of structural ordering (Fig-
ure 40a-c). This allowed to develop rational
annealing processes that maximized the order-
ing degree for the three materials by sequen-
tial alloying-ordering steps. *In situ* WAXS and
SAXS were also used to investigate the sol-gel
formation of FeOx, Fe₃N, and Fe₃C ORR cat-
alysts.⁶³⁸,⁶³⁹ Here, it was found that the tran-
sition from oxides to carbide was strongly size-
dependent, having important consequences on
the resulting catalyst morphology, while the
intermediate nitride form was highly unstable
and difficult to isolate. The combination of *ex
situ* electron microcopy and *in situ* SAXS and
WAXS with PDF analysis also allowed to un-
derstand the formation and growth of hollow
PtNi/C ORR catalysts during a one-pot syn-
thesis, which was found to result from the se-
quential nucleation of NiB nanoparticles, the
galvanic displacement of Ni by Pt, and the
nanoscale Kirkendall effect (Figure 40.e-f).⁶⁴⁰
<figure><img src="image_64.png" /><figcaption>Figure 40: Monitoring nanostructure formation and tailoring for electrocatalysis using *in situ* X-ray scattering. (a) schematic illustration of the synthesis of structurally ordered Pt–M (M= Fe, Co, and Ni) intermetallic compounds, (b) *in situ* HT-XRD results on PtFe structural evolution during heating and subsequent cooling, and (c) calculated ordering degrees of PtFe and PtCo obtained from the *in situ* HT-XRD experiments (Adapted from Ref. 637). Copyright 2022 by Nature -Springer). Time-resolved formation and growth of hollow PtNi/C nanoparticles as revealed by (d) electron microscopy, (e) PDF analysis, and (f) SAXS. (Adapted from Ref. 640). Copyright 2017 American Chemical Society.)</figcaption></figure>
<figure><img src="image_65.png" /><figcaption>Figure 41: *Operando* battery cell designs and X-ray scattering techniques. (a) Schematic of *operando* coin cell with Kapton windows (Adapted from Ref. 641. Copyright 2016 Elsevier). (b) Schematic of *operando* Swagelok cell (Adapted from Ref. 642. Copyright 2020 Wiley). (c) Schematic of *operando* pouch cell with Kapton windows (Adapted from Ref. 643. Copyright 2014 Royal Society of Chemistry). (d) Schematic of cylindrical cell (Adapted from Ref. 644. Copyright 2019 MDPI).</figcaption></figure>
## 8.2 Powder Diffraction

The vast majority of *operando* X-ray scattering studies in batteries utilize powder diffraction. This is due to the wide availability of powder diffraction instruments and familiarity with the principles of this technique, in addition to the structural sensitivity noted above.

The first *in situ* powder diffraction experiments on lithium-ion batteries date back to 1978, when Chianelli and colleagues used a cell with a beryllium window in reflection geometry to study structural transformations in the LixTiS₂ cathode during intercalation and deintercalation of lithium⁶⁵⁷. Subsequently, Dahn and colleagues improved the cell design to allow a better correlation between electrochemistry and powder diffraction: the former measured reactions at the cathode particle surface while the latter is a bulk technique. Their study on LixTiS₂ by using laboratory X-ray diffractometer⁶⁵⁸ and expanded the understanding of structural changes in the cathode material introduced by S. Whittingham. *In situ* X-ray diffraction was extended to other cathode materials, notably LixNiO₂⁶⁵⁸ and LixCoO₂⁶⁴⁵. Particularly the study in LixCoO₂ - the cathode material introduced by J. Goodenough - generated significant interest because LixCoO₂ is the cathode material of most of today's batteries in portable electronics. *In situ* powder diffraction revealed transitions between three distinct phases as *x* was varied between 1 and 0.4, and distinguished between order-disorder transitions and first-order phase transitions.

Commercial sample environments for laboratory instruments are available from several manufacturers, almost exclusively based on a windowed coin cell design. Windows of Be or thin polyimide are needed for X-ray transmission. These windows often produce either electrochemical incompatibility (e.g. Be corrosion), small leaks, or inhomogeneous electrode compression, which lead to poor long-term cycling and voltage gradients across electrodes⁶⁵⁹. With modern 1/2D detectors, the time required for simple measurements (1-3 hrs) is appropriate for slow cycling cells. Despite these limitations, laboratory PXRD is both

common and valuable for probing the mechanism of phase transformations in active materials. Intercalation commonly follows either a solid solution mechanism, where ion insertion is continuously variable in stoichiometry, or a phase-segregated model, where ions are intercalated at fixed stoichiometries, separated by phase transitions. An idealized solid solution mechanism produces only peak shifts in the powder pattern, while phase-segregated transitions produce new peaks or different patterns entirely. Sharp phase transitions create large strain gradients inside active materials, which generally results in accelerated degradation and poor cyclability. Changes in doping, coating, and growth conditions of active materials can produce dramatic changes in the intercalation chemistry and phase transitions.

The extension of *in situ* powder diffraction to synchrotron sources⁶⁶⁰ significantly improved the signal to noise ratio, as well as the temporal, spatial, and angular resolution, as well as penetration depth, which are all useful for *operando* PXRD battery experiments. The increased signal enabled experiments in transmission geometry where the material throughout the thickness contributes to the diffraction signal. Higher flux allows for PXRD measurements down to the ms scale, which permits the same measurements described above, but over cycling rates relevant to battery operation. Spatially resolved PXRD allows for linescan or imaging experiments where lithiation heterogeneity across electrodes can be evaluated.⁶⁶¹ Measurements scanning the plane of the electrode to explore gradients at the mm and cm level require minimal focusing, while focusing towards the 1 µm level allows reactions gradients through the thickness of conventional electrode films to be evaluated. High angular resolution PXRD allows for accurate peak shape analysis.

While most *in situ* powder diffraction focused on transition metal oxide cathodes, which generate a high signal, *in situ* diffraction on graphitic anode⁶⁶², silicon⁶⁶³,⁶⁶⁴ and lithium-sulfur systems⁶⁶⁵ have also been reported. Following this pioneering work, *in situ* powder diffraction is a pillar of today's structural char-