Article
pubs.acs.org/JPCC

Direct Observation of Active Nickel Oxide Cluster in Nickel−Borate
Electrocatalyst for Water Oxidation by In Situ O K‑Edge X‑ray
Absorption Spectroscopy
Masaaki Yoshida,*,†,‡ Yosuke Mitsutomi,† Takehiro Mineo,† Masanari Nagasaka,§ Hayato Yuzawa,§
Nobuhiro Kosugi,§ and Hiroshi Kondoh††

Department of Chemistry, Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama 223-8522, Japan
Cooperative Research Fellow, Catalysis Research Center, Hokkaido University, Kita21, Nishi10, Kita-ku, Sapporo, Hokkaido
001-0021, Japan§
Institute for Molecular Science, 38 Nishigo-Naka, Myodaiji, Okazaki 444-8585, Japan‡

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S Supporting Information*

ABSTRACT: In situ O K-edge X-ray absorption ﬁne structure (XAFS)
spectroscopy was applied to investigate the electronic and structuralchange in the nickel−borate (Ni−Bi) electrocatalyst during the oxygen
evolution reaction (OER). An absorption peak was observed around528.7 eV at 1.0 V versus Ag/AgCl in a potassium borate aqueous
solution, which relates with the formation of nanoscale order domains ofedge-sharing NiO6 octahedra in the Ni−Bi electrocatalyst. XAFS spectra
were measured with variation of the electrode potential from 0.3 up to
1.0 V. The measured absorption peaks suggest that the quantity of NiO6octahedra increased in correlation with the OER current; however, when
the potential was changed downward, the XAFS absorption peakassigned to NiO6 octahedra remained constant, even at the electrodepotential for no OER current. This diﬀerence implies that the wateroxidation catalysis proceeds at the domain edge of NiO6 octahedra. The XAFS technique provides the ﬁrst successful direct
probing of the active species in the Ni−Bi electrocatalyst during electrochemical reaction.

1. INTRODUCTION
Water splitting using solar energy is a promising approach to
produce renewable hydrogen gas toward a sustainable
society.1−7 Various types of photoelectrodes and photovoltaics
for water splitting have been developed with high solar-tohydrogen eﬃciencies of over several percent.8−12 Recently, aperovskite photovoltaic attained a solar-to-hydrogen eﬃciencyof 12.3% using hydrogen and oxygen evolution electrocatalysts.8 For the realization of eﬃcient photovoltaic andphotoelectrochemical systems for water splitting, oxygen
evolution catalysts have often been used to catalyze wateroxidation due to their lower activation energies. Therefore, thedevelopment of eﬃcient catalysts for the oxygen evolution
reaction (OER) has attracted considerable attention due to thepotential toward highly eﬃcient energy conversion.
Nocera and colleagues reported that nickel−borate electrodeposited from a dilute Ni2+ solution in a borate-buﬀered
electrolyte (Ni−Bi) functions as an eﬃcient OER electrocatalyst.13−15 The Ni−Bi OER catalyst is unique because it iscomposed of inexpensive and earth-abundant materials, and it
operates safely with high activity.13−15 As similar materials,
cobalt−phosphate (Co−Pi) and cobalt−borate (Co−Bi)
complexes have also been reported as eﬃcient electrocatalysts
for OER in phosphate and borate electrolytes, respectively.16−19© 2015 American Chemical Society

These OER electrocatalysts can enhance the activities for water
splitting, when the electrocatalysts are deposited on asemiconductor photoelectrode or photovoltaic surfaces suchas Si, Fe2O3, and BiVO4.9,11,20−23
Mechanistic studies that employ various techniques havebeen aimed toward determining the origin of eﬃcient oxygen
evolution for Ni−Bi electrocatalysts or similar materials (Co−Pi, Co−Bi).24−35 An electron paramagnetic resonance (EPR)
spectroscopy study revealed the presence of Co4+ species in a
Co−Pi electrocatalyst in a frozen electrolysis solution.35 In
addition, Ni K-edge X-ray absorption ﬁne structure (XAFS)
analysis demonstrated that the average valence state of thenickel species in a Ni−Bi electrocatalyst changes from +2.0 to+3.6 in response to an applied positive voltage.24 It was
concluded that oxidation of the nickel species is due to thegeneration of bis-oxo/hydroxo-bridged nickel centers organized
into sheets of edge-sharing NiO6 octahedra within ordereddomains 2 to 3 nm in diameter by extended X-ray absorption
ﬁne structure (EXAFS) analysis.24 The nanoscale order
structures of the Co−Pi and Co−Bi electrocatalysts were
Received: June 25, 2015
Revised: July 31, 2015
Published: July 31, 2015
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The Journal of Physical Chemistry C

Figure 1. Schematic illustration of optical geometry for in situ O K-edge XAFS measurements with the transmission mode at the BL3U beamline. A
liquid layer of several hundreds of nanometers was sandwiched between the SiC and Au/Cr/SiC membranes by setting Teﬂon and Au spacers (100μm thickness) to the window frames of the membranes in an in house-built electrochemical XAFS cell with Pt mesh counter and Ag/AgCl (saturatedKCl) reference electrodes. The potential of the Au/Cr/SiC working electrode was controlled using a potentiostat connected to the Au spacer with aconducting wire.

potentiostat. A Ni−Bi electrocatalyst was deposited on the Au
thin ﬁlm electrodes in 0.1 M K2B4O7·4H2O (potassium borate,
K−Bi, 99.0%, Showa Chemical) aqueous solution containing
0.4 mM Ni(NO3)2 (99.9%, Wako Pure Chemical Industries)
with bubbling He, according to a previous report.24 Milli-Q
water (total organic carbon <5 ppb, resistivity >18 MΩ·cm)
was used for all experiments. The electrochemical activity of theNi−Bi electrocatalyst for the OER was tested in K−Bi aqueous
solution with bubbling He by measurement of the current−
voltage characteristics. The surface morphology, chemical state,
and ﬁlm thickness of each prepared sample were observed using
scanning electron microscopy (SEM; FEI, Inspect S50),
energy-dispersive X-ray spectroscopy (EDX; EDAX, Genesis),and confocal laser scanning microscopy (CLSM; Keyence, VK9710), respectively.
2.2. In situ O K-Edge XAFS. Electrochemical soft X-ray
XAFS measurements were performed with the transmission
mode at the BL3U beamline of the UVSOR Synchrotron, in
accordance with previous work.40,41 The radiation was
monochromatized with a varied-line-spacing plane-grating
monochromator (VLS-PGM). The intensities of the incident
X-rays were monitored using a gold mesh under ultrahighvacuum in front of the sample (I0) and a photodiode (IRD
AXUV100) under a helium atmosphere after the sample (I1).
The XAFS data were collected as transmission spectra with theLambert−Beer formula ln(I0/I1). The photon energy wascalibrated according to the O 1s−π* peak (530.80 eV)42 for O2
gas mixed in a buﬀer gas with He gas. The O K-edge XAFS
spectrum of Milli-Q water was measured as a reference sample
for sample sandwiched between two SiC membranes (100 nm
thickness, 2 × 2 mm2, NTT AT). On the contrary, the O Kedge XAFS spectra for the Ni(OH)2, NiO, β−NiOOH, and K−
Bi reference samples were obtained by measuring the sample
drain current under ultrahigh vacuum. For the investigation ofelectrochemical reactions, we sandwiched a liquid layer of aseveral hundreds of nanometers between the SiC and Au/Cr/
SiC membranes by setting Teﬂon and Au spacers (100 μm
thickness) to the window frames of the membranes in an inhouse-built electrochemical XAFS cell with a Pt mesh counterand Ag/AgCl (saturated KCl) reference electrodes (Figure 1).

investigated by X-ray pair distribution function (PDF)
analysis32,33 and a combination study using infrared spectroscopy (IR), atomic force microscopy (AFM), and X-ray grazing
incidence diﬀraction (GID),34 from which it was determinedthat nanoscale CoO6 clusters can assemble into stacked
superstructures with highly porous morphology. It is suggested
that the porous electrocatalyst ﬁlm enables proton-coupledelectron hopping to proceed in the electrocatalyst, which canincrease the OER sites with growth of the ﬁlm thickness.28,30
Therefore, the unique characteristics of these electrocatalysts
are currently of particular interest as a material for eﬃcientOER.
XAFS spectroscopy is a powerful tool to investigate the local
electronic states for OER catalysts.24,25,36,37 The in situ Ni Kedge XAFS technique using hard X-rays was applied to the Ni−
Bi electrocatalyst; however, it is diﬃcult to quantitatively
discuss the active species in the OER electrode potential withthis technique because the Ni K-edge XAFS spectra become asuperposition of each spectrum for Ni2+ and Ni3.6+ species in
the Ni−Bi electrocatalyst and Ni2+ cations in the electrolyte
solution.24,36 On the contrary, O K-edge XAFS spectroscopyusing soft X-rays has been useful to directly observe the oxygen
species in transition-metal oxides at the liquid/solid interface.38,39 Therefore, in this study, the Ni−Bi electrocatalyst wasinvestigated using in situ O K-edge XAFS measurements with
control of the potential. An attempt was made to directly detect
the active oxygen species in the Ni−Bi electrocatalyst anddiscuss the relationship between the amount of active species
and the OER activity.

2. EXPERIMENTAL SECTION
2.1. Sample Preparation. Au thin ﬁlms (20 nm) were
deposited on Si substrates (10 × 10 mm2) with an evaporated
Cr layer (5 nm thick) and SiC (150 nm thick, 2 × 2 mm2, NTTAT) or Si3N4 (500 nm thickness, 2 × 2 mm2, CORNES
Technologies) windows (Au/Cr/SiC or Au/Cr/Si3N4, respectively) for use as working electrodes for electrochemical
reactions. An in-house-built electrochemical cell was usedwith a Pt mesh counter electrode and a Ag/AgCl (saturatedKCl) reference electrode. The potential was controlled using a
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next Discussion). Figure 2b,c shows SEM images of the Au/Cr/
SiC substrate before and after Ni−Bi electrodeposition andconﬁrms that the Ni species was deposited on the substrate as athin ﬁlm. SEM-EDX analysis conﬁrmed that the observed thinﬁlm was composed of Ni−Bi electrocatalyst as well as theprevious work.13 The ﬁlm thickness was estimated to be ca. 300
nm by the CLSM method (Figure S2).
3.2. In situ O K-Edge XAFS. Figure 3a shows an in situ O
K-edge XAFS spectrum measured for the electrodeposition

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The electrolyte thickness was adjusted by changing the outerhelium gas pressure of the SiC and Au/Cr/SiC membranes,
according to the previous works.40,41 The potential of the Au/
Cr/SiC working electrode was controlled using a potentiostat
connected to the Au spacer with a conducting wire. The Ni−Bi
electrocatalyst was deposited on the Au thin ﬁlm electrode in
0.1 M K−Bi aqueous solution containing 0.4 mM Ni(NO3)2
with bubbling He. The Ni−Bi electrocatalyst was then anodized
at 1.0 V for 2 h in 0.5 M K−Bi aqueous solution with bubbling
He by exchanging the solutions with a tubing pump system. In
situ O K-edge XAFS spectra were measured for the Ni−Bi
electrocatalyst before and after anodization while maintaining
the electrode potential at 1.0 V. The potential dependence of
the XAFS spectra was tested in 0.1 M K−Bi aqueous solution
with bubbling He by exchanging solutions with the tubing
pumping system during current monitoring for the OER withthe potentiostat. The XAFS spectra were taken with scanning
time of 20 min at cumulated number of 3 times, which resultsin each electrode potential being applied for 1 h.

3. RESULTS AND DISCUSSION
3.1. Electrochemical Properties and Surface Morphologies. The current during electrodeposition of the Ni−Bi
electrocatalyst increased for 2 h, as shown in Figure S1a, whichindicates growth of the Ni−Bi thin ﬁlm and the evolution ofoxygen gas not only at the Ni−Bi surface but also from the Ni−
Bi interior, as previously reported.13−15,24,28,30 The anodization
reaction of the Ni−Bi electrocatalyst was monitored, as shownin Figure S1b, and the current continued to increase for 2 h as
in the previous work.24 Figure 2a shows the electrochemical

Figure 3. (a) In situ O K-edge XAFS spectrum for Ni−B ielectrocatalyst under electrochemical control at 1.0 V in 0.1 M K−Bi
aqueous solution containing 0.40 mM Ni(NO3)2. XAFS spectra for
H2O, β−NiOOH, NiO, Ni(OH)2, K−Bi, and O2 gas are shown forreference. (b) In situ O K-edge XAFS spectra (i) before and (ii) after
anodization for 2 h at 1.0 V in 0.5 M K−Bi aqueous solution.

reaction of Ni−Bi thin ﬁlm at 1.0 V versus Ag/AgCl in a 0.1 M
K−Bi aqueous solution containing 0.4 mM Ni(NO3)2, together
with those for H2O, β-NiOOH, NiO, Ni(OH)2, K−Bi, and O2
gas as references that relate with the Ni−Bi catalysis. A peakassociated with an oxygen species was observed at ca. 528.7 eV,
which was similar to that in the β-NiOOH reference. To
evaluate the electrode potential dependence of the Ni−Bi
electrocatalyst, we changed the electrolyte solution to 0.5 M
K−Bi aqueous solution without nickel ions, and we conducted
XAFS measurements while changing the applied electrodepotential. The peak at ca. 528.7 eV disappeared at the lower
potential (0.5 V) and reappeared at the higher potential (1.0 V)
accompanied by the OER activity, as shown in Figure 4a. The
same behavior was observed even in 0.05 M K−Bi aqueous
solution (Figure 4b). Previous Ni K-edge EXAFS measurements24 suggested that the Ni−Bi electrocatalyst before
anodization forms μ-oxo/hydroxo nickel centers organized
into higher order domains of edge-sharing NiO6 octahedra with
β-NiOOH structure at higher potential. Thus, in the present
study, the formation of NiO6 octahedra domains can bemonitored by the direct observation of oxygen species in the
Ni−Bi thin ﬁlm. The NiO6 absorption was observed as a single
peak at 528.7 eV, which originates from transition to hybridized
Ni(3d)−O(2p) states with eg orbital symmetry and the lowspin d7 (Ni3+) electron conﬁguration.24 It should be noted thatthe small peak around 530.8 eV in Ni−Bi is attributed to thestrong 1s-π* resonance absorption of residual oxygen gas.42 No
detection of the Ni(OH)2 peak at lower potential in the O Kedge spectra may be mostly because the electronic state ofoxygen species of Ni(OH)2 structure in the Ni−Bi electrocatalyst is not uniform due to hydration or protonation on the
Ni−Bi ﬁlm.

Figure 2. (a) Current−voltage characteristics for (i) bare (black) and(ii,iii) Ni−Bi-modiﬁed (nonanodized; red and anodized; blue) Au thinﬁlms in a 0.1 M K−Bi aqueous electrolyte. SEM images of the Au thinﬁlm (b) before and (c) after the electrodeposition of Ni−Bi.

OER activities of the prepared samples, where the eﬃciency of
the Ni−Bi-deposited substrate was higher than that of the baresubstrate; therefore, the Ni−Bi thin ﬁlm functions as an OER
electrocatalyst. In addition, the current of the anodized sample
was higher than that of the nonanodized sample, whichindicates that anodization of the Ni−Bi electrocatalyst enhances
the OER activity, in agreement with the previous report.24 Itshould be noted that the oxidation currents for nonanodized
and anodized samples were both observed around 0.7 V
because the nickel species in the nonanodized or anodized Ni−
Bi electrocatalyst was oxidized from Ni(OH)2 (Ni2+) to βNiOOH (Ni3+) or to γ-NiOOH (Ni3.6+), respectively (See the19281

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Figure 4. Potential dependence of in situ O K-edge XAFS spectra by changing the electrode potential at (i) 1.0, (ii) 0.5, and (iii) 1.0 V versus Ag/
AgCl in (a) 0.5 M and (b) 0.05 M K−Bi aqueous electrolyte. The peak at ca. 528.7 eV measured at 1.0 V disappeared at the lower potential (0.5 V)
and then reappeared at the higher potential (1.0 V) accompanied by OER activity.

Figure 5. (a) Upward potential dependence of in situ O K-edge XAFS spectra for the Ni−Bi electrocatalyst in 0.1 M K−Bi aqueous solution from 0.3
to 1.0 V. (b) Relationship between XAFS peak area and current for the OER with respect to the electrode potential.

(Ni3.6+). On the contrary, the ﬁrst peak positions in the O Kedge XAFS spectra for transition-metal oxides are empiricallyrelated with the cation valences to some extent.44,45 Therefore,
the local nickel valence bound by oxide oxygen species in the
Ni−Bi electrocatalyst was tentatively estimated as ca. 4+ using
the linear ﬁtting of ﬁrst peak positions of reference compounds
against the cation valences, as shown in Figure S4, whichsuggests that the highly oxidized Ni species such as NiO2 areformed during catalytic water oxidation. It should be noted that
the O K-edge XAFS spectra are sensitive to chemical bonding(molecular orbitals), valence, spin state, and so on.38,39,43−46
Further investigation is required to evaluate the cation valencequantitatively.
3.3. Electrode Potential Dependence of XAFS Spectra
and Reaction Model. The upward potential dependence of
the Ni−Bi electrocatalyst on the Au thin ﬁlm was evaluated.
Figure 5a shows O K-edge XAFS spectra for this sample
measured under various controlled potentials. As the potential
was changed from 0.3 to 1.0 V, the intensity of the peak for the
NiO6 octahedra increased from 0.6 V and remained constantover 0.8 V, which indicates that the nickel species was oxidized
from Ni2+ species with the Ni(OH)2 structure to NiO6octahedra on the basis of previous works.24,36 The increase in

Anodization of the Ni−Bi electrocatalyst caused the NiO6octahedra peak to increase, as shown in Figure 3b. Bediako et
al. reported that the inactive Ni−Bi species with β-NiOOH
structure (Ni3+) changed to the active Ni−Bi species with γNiOOH structure (Ni3.6+), which resulted in the relaxation of
Jahn−Teller distortion with nonequivalent Ni−O and Ni−Ni
distances.24 We also veriﬁed the existence of γ-NiOOH
structure for our Ni−Bi sample by in situ Ni K-edge XAFS
measurements as well as the previous work,24 as shown in
Figure S3. Both NiOOH structures have two kinds of oxygen
species derived from hydroxide (OH) and oxide (O) at an
diﬀerent OH/O ratio (e.g., β−NiOOH = Ni3+(O)1(OH)1, γ−
NiOOH = Ni3.6+(O)1.6(OH)0.4). It is known that the 1s corelevel of oxygen species of hydroxide (OH) is shifted to higher
binding energy with respect to that of oxygen species of oxide(O) in transition-metal oxyhydroxides, which results in theoxide oxygen peak being observed at a lower energy than thehydroxide oxygen peak in the O K-edge XAFS spectra.43 Thus,in the present work, the ﬁrst peaks in the O K-edge XAFS
spectra are likely attributed to the oxide oxygen species in theβ- and γ-NiOOH structures, and the increase of the XAFS peakis most likely due to an increase in the oxide oxygen species by
the structural change from β-NiOOH (Ni3+) to γ-NiOOH
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Figure 6. (a) Downward potential dependence of in situ O K-edge XAFS spectra for the Ni−Bi electrocatalyst in 0.1 M K−Bi aqueous solution from
0.9 to 0.5 V. (b) XAFS peak areas and (c) currents for the OER with respect to the electrode potential.

Figure 7. Schematic model for nickel oxide nanoclusters in the Ni−Bi electrocatalyst during OER catalysis in K−Bi aqueous solution with theelectrode potential swept upward and downward. It should be noted that Ni(OH)2 has an octahedral structure similar to NiO6.26,48

6a). The NiO6 XAFS peak at 528.7 eV was observed at higher
potential as well as the result of upward potential dependence;
however, the peak was observed even at 0.6 V, which indicatesthat the generated NiO6 octahedra were not easily reduced to
the Ni2+ species. The relationship between the electrodepotential and XAFS peak area suggests that the downward
potential dependence is diﬀerent from that of the upward
potential dependence, as shown in Figure 6b. The result
indicates that the nickel oxide nanocluster was gradually
reduced from the domain edge because the oxidation andreduction of domain edge is likely to proceed by lower
activation energy than those of domain inside; however, thecurrents for OER activity were the same for the upward anddownward sweeps (Figure 6c), which implies that the interiorof the NiO6 octahedra structure does not function as an OER
site. Figure 7 shows a schematic model for water oxidation
during upward and downward potential sweeps. The upward

the edge of XAFS peak area for NiO6 octahedra was consistent
with the onset of OER activity (Figure 5b), which indicates thatthe high activity of the Ni−Bi thin ﬁlm for the OER is derived
from the formation of NiO6 octahedra domains. On thecontrary, the XAFS peak remains constant at higher electrodepotentials (≥0.8 V) because the nickel oxide nanoclusters werecompletely oxidized to NiO6 octahedra. The increase in the
OER current even in the suppression region of peak growing in
XAFS spectra suggests that the reaction rate for OER catalysis
at higher potentials is probably restricted according to the Tafel
equation, relating the rate of an electrochemical reaction to theoverpotential as V = a + b log I, where V is the potential(overpotential), I is the current density, and a and b are theTafel equation constants.47
To compare the diﬀerence according to the direction ofpotential sweep, we measured the downward potential
dependence of XAFS spectra under potential control (Figure
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potential change results in oxidation of the Ni2+ species with
Ni(OH)2 structure48 to the Ni3.6+ species with NiO6 octahedra
structure accompanied by OER activity. In contrast, thedownward potential change gradually reduces the Ni3.6+ species
to Ni2+ from the domain edge and the OER activity issuppressed when the outer NiO6 octahedra domain is reduced.
On the basis of these results, water oxidation is likely toproceed at the domain edge of NiO6 octahedra.
Reaction pathways for OER catalysis with Ni−Bi, Co−Bi, and
Co−Pi electrocatalysts were previously proposed.24−35 Bediako
et al. elucidated that the OER over the Ni−Bi electrocatalyst in
borate-buﬀered electrolyte entails the reversible dissociation of
borate anions from the domain edge of the NiO6 octahedra
structure and a two-electron and three-proton equilibrium,
followed by a chemical turnover-limiting step.26 Our present
study spectroscopically conﬁrmed the report of Bediako et al. inthat the active species functions at the domain edge of the NiO6octahedra structure. Thus, an increase in the number of NiO6
domains is necessary to achieve an eﬃcient OER catalyst.
The octahedra structures of transition-metal oxides (MnO6,
CoO6, and NiO6) are generally considered to exhibit high OER
activity;13−19,49−52 however, the direct spectroscopic detectionof active octahedral species had not been achieved around the
OER electrode potential. The XAFS technique used here has
provided the ﬁrst successful direct probing of the active speciesof Ni−Bi electrocatalyst during electrochemical reaction. This
probing technique can be applied to many other OER
electrocatalysts composed of transition-metal oxides such as
Co−P i , 16−19 Co−B i , 17,18 nickel-glycine catalyst, 51 andMnOx49,50 to reveal the relationship between the active species
and OER activity. The knowledge obtained by this study will be
useful to develop more eﬃcient water-splitting systems.

AUTHOR INFORMATION

Corresponding Author*Tel: +81-45-566-1592. Fax: +81-45-566-1697. E-mail:yoshida@chem.keio.ac.jp.
Notes

The authors declare no competing ﬁnancial interest.

■

ACKNOWLEDGMENTS
This work using soft X-ray was performed at BL3U in the
UVSOR Synchrotron of the Institute for Molecular Science(2014_206, 2013_603), and that using hard X-ray was
performed at the Photon Factory (2014G539) and SPring-8(2015A1138 and 2014A1058). This work was supported by
Grants-in-Aid (nos. 22850015, 24750134, and 26248010) fromthe Japan Society for the Promotion of Science (JSPS), the
Keio Gijuku Academic Development Fund, Keio Kogaku-kai
Fund, and the Cooperative Research Program of the Catalysis
Research Center of Hokkaido University (14A1004 and
15B1002). We thank K. Nitta and T. Ina in JASRI and T.
Iida, M. Kawamura, M. Sasai, T. Yomogida, and N. Gon in Keio
University for conducting the XAFS measurements.

■

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4. CONCLUSIONSIn summary, in situ O K-edge XAFS spectra were measured fora nickel−borate OER electrocatalyst. The XAFS peak was
observed at 528.7 eV by changing the electrode potential foreach sample, which demonstrates that higher order domains of
the edge-sharing NiO6 octahedra structure are generated at
higher potentials. The upward potential dependence of the
XAFS spectra resulted in an increase in the intensity of theabsorption peak for the NiO6 octahedra with the OER current.
In contrast, the downward potential dependence of the XAFS
spectra revealed that the NiO6 peak remains even if the OER
current is suppressed. The diﬀerent behavior between the
upward and downward potential sweeps indicates that wateroxidation catalysis is likely to proceed at the domain edge of theNiO6 octahedra. This is the ﬁrst successful direct observation ofactive nickel oxide species reported for OER electrocatalysts.

■

ArticleASSOCIATED CONTENT

S Supporting Information*

The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.jpcc.5b06102.
Experimental procedures of in situ Ni K-edge XAFS, timecourse of current during electrodeposition and anodic
oxidation, estimation of ﬁlm thickness, in situ Ni K-edge
XAFS spectra, and estimation of cation valence. (PDF)
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DOI: 10.1021/acs.jpcc.5b06102
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