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Cite this: Phys. Chem. Chem. Phys.,2024, 26, 10711

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First-principles studies of enhanced oxygen
reduction reactions on graphene- andnitrogen-doped graphene-coatedplatinum surfaces†
Ho Ngoc Nam, *ab Quan Manh Phung, cd Pongpol Choeichom,aYusuke Yamauchibef and Nagahiro Saito *ag
Developing innovative platinum-based electrocatalysts and enhancing their eﬃciency are crucial foradvancing high-performance fuel cell technology. In this study, we employed DFT calculations toprovide a theoretical basis for interpreting the impact of graphene coatings on various Pt surfaces on
oxygen reduction reaction (ORR) catalytic activity, which are currently applied as protective layers inexperiments. We comprehensively assess the geometric and electronic properties of Pt(100), Pt(110), andPt(111) surfaces in comparison to their graphene-coated counterparts, revealing diﬀerent adsorption
behaviors of O2 across these surfaces. The ORR mechanisms on diﬀerent Pt surfaces show distinct ratedetermining steps, with Pt(111) showing the highest ORR activity, followed by Pt(110) and Pt(100).
Graphene coatings play a key role in enhancing charge transfer from the surface, resulting inmodifications of O2 adsorption. Despite influencing ORR kinetics, these graphene-coated surfaces
demonstrate competitive catalytic activity compared to their bare counterparts. Notably, Pt(111) witha graphene coating exhibits the lowest activation energy among graphene-coated surfaces. Our

Received 20th January 2024,
Accepted 11th March 2024
DOI: 10.1039/d4cp00269e

calculations also suggest that the ORR can occur directly on non-defective Pt@graphene surfaces rather
than being restricted to exposed Pt centers due to point defects on graphene. Furthermore, ourwork highlights the potential of nitrogen doping onto the Pt(111)@C surface to further enhance ORR
activity. This finding positions nitrogen-doped Pt@C as a promising electrocatalyst for advancing

rsc.li/pccp

electrochemical technologies.

1. Introduction
Global warming and the massive decline in fossil fuels in recent
years have further proven the importance for humanity toaInstitute of Materials Innovation, Institutes of Innovation for Future Society,
Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8601, Japan.
E-mail: honam@sp.material.nagoya-u.ac.jp, hiro@sp.material.nagoya-u.ac.jpbDepartment of Materials Process Engineering, Graduate School of Engineering,
Nagoya University, Nagoya 464-8603, Japanc
Department of Chemistry, Graduate School of Science, Nagoya University,
Furo-cho, Chikusa-ku, Nagoya 464-8602, Japand
Institute of Transformative Bio-Molecules (WPI-ITbM), Nagoya University,
Furo-cho, Chikusa-ku, Nagoya 464-8601, JapaneAustralian Institute for Bioengineering and Nanotechnology (AIBN),
The University of Queensland, Brisbane, QLD 4072, AustraliafDepartment of Chemical and Biomolecular Engineering, Yonsei University,
Seoul 03722, South Korea
g
Department of Chemical Systems Engineering, Graduate School of Engineering,
Nagoya University, Nagoya 464-8603, Japan† Electronic supplementary information (ESI) available. See DOI: https://doi.org/
10.1039/d4cp00269e

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transition towards the use of green, clean, and sustainableenergy resources.1 Advanced energy conversion technologies(e.g., solar photovoltaics, thermoelectrics, geothermal power,etc.)2–9 have made a significant contribution to the common
effort to produce environmentally sustainable alternative
energy sources. Notably, fuel cells have emerged as a promisingtechnology for generating electricity through reactions involving oxygen and hydrogen.10–12 Despite these advancements, a
key challenge persists in the development of efficient anddurable cathode catalysts for fuel cells. Specifically, the slow
kinetics of the oxygen reduction reaction (ORR) occurring in
the cathode are a primary bottleneck, resulting in a large
proportion of potential loss.13 Therefore, research on highperformance catalysts that can accelerate the ORR is highly
required.
Despite its high cost and scarcity, platinum (Pt) remains
popular as one of the most dynamic ORR electrocatalysts in commercialized proton exchange membrane fuel cells (PEMFCs).13,14
Nevertheless, this type of material has certain drawbacks that
limit its performance. According to Sabatier’s principle,15 thePhys. Chem. Chem. Phys., 2024, 26, 10711–10722 | 10711

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interaction between the catalyst and the reactant should neither
be too weak nor too strong. The pronouncedly strong bond
between Pt and oxygen could thereby inhibit the eﬃcient
progress of the ORR activity and slow down the overall
reaction. For example, it has been reported that alloying
transition metals (e.g., Fe, Co, Ni, or Cu) with Pt has thepotential not only to reduce material costs but also to help
improve the ORR performance of the catalyst.16–18 Thismight be due to the electronic structure of Pt-based alloys
being different from that of pure Pt, resulting in significant
changes in how reactants and catalysts bond together andthereby improving ORR activity. Alternatively, materials derived
from metal–organic frameworks (MOFs), offering large specificsurface areas and different pore sizes,19–21 as well as single-atom
catalysts with tunable electronic structures,22–28 are promising for
further exploration of ORR electrocatalysis.
Moreover, catalytic durability is the next challenge for
PEMFCs. Operating in harsh environments like high temperatures, strong acidic media, or high-potential regions could leadto a significant degradation of Pt catalysts over time.29 Other
adverse factors, such as particle coalescence, dissolution, andpoisoning, could even make this degradation worse.30,31 As aresult, the catalytic activity and overall performance of fuel cells
significantly decrease. In previous studies,32,33 MgO and SnO2
used as coatings have shown eﬀectiveness in improving the
durability and stability of Pt-based catalysts. Unfortunately,
these coatings also hindered ORR activity on the catalyst surface due to the poor conductivity and blocking of the active
sites.34 Therefore, enhancing both the catalytic performanceand durability of Pt catalysts is crucial.
Graphene is a well-known material for its outstanding
electrical conductivity, high surface area, and chemical stability.35
It can form specific solid-state ligand bonds to augment
electron transfer with metals.36 Recently, several studies haveapplied graphene as an eﬀective coating for Pt-based catalysts.37–40 For instance, it was found that Pt-based nanoparticles
with graphene encapsulation fabricated via the solution plasma
process improved not only the durability but also the efficient
ORR activity of the catalysts.34,41 This suggests the great
potential of graphene coatings for catalytic applications. Moreover, it was found that nitrogen-doped graphene could be
beneficial in enhancing the ORR catalytic performance compared to pure graphene.42 However, the properties of thesegraphene coatings on Pt surfaces and how they influence the
ORR activity of the system have not been clear yet. Previous
theoretical studies mainly investigated the ORR mechanism
on bare Pt surfaces with various models (e.g., clusters43–45 andslabs46–48) using different exchange–correlation functionals(i.e., PBE,43 PBE-D3,45 SCAN-rVV10,49 PW91,48 B3LYP,50 andPZ51), making comparisons and analysis of the related properties difficult. Furthermore, some studies on the ORR mechanism on graphene-supported Pt nanoparticles52,53 or single Pt
atom-supported graphene54,55 have also been done. However,not many studies related to the graphene coating on Pt surfaces
have been reported.49 It should be noted that bare Pt surfaces
with different energies might have different effects on the10712 | Phys. Chem. Chem. Phys., 2024, 26, 10711–10722

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geometric and electronic structures of the graphene coatings,
leading to significant changes in the ORR activity of catalysts.
Therefore, elucidating the role of graphene coatings in the ORR
performance is necessary.
In this work, we systematically investigated the mechanismof the oxygen reduction reaction on graphene-coated Pt surfaces. First, a slab model of low-index Pt facets is constructed,
and ORR activity on these bare surfaces is investigated. After
that, we discuss the changes in the geometric and electronic
structure of systems when graphene is coated. The eﬀect of thegraphene coating on ORR activity is then revealed via thecalculation of charge density and overpotential. Finally, theinfluence of nitrogen doping on the graphene coating is also
discussed, suggesting its potential to reduce the system’s overpotential as compared to the pristine graphene layer.

2. Computational methods
and models
Our DFT calculations are performed using the projector-augmented wave56 (PAW) potential as implemented in the QuantumESPRESSO package.57 For the PAW potentials, the PSLIBRARY
version 1.0.0 was adopted.58 The wave functions and augmentation charge density in terms of a plane-wave basis set wereexpanded with kinetic energy cut-oﬀs of 50 and 400 Ry, respectively. The spin polarization was taken into account in thecalculations. A Gaussian smearing of 0.02 Ry was also added totreat the Fermi surface. For the exchange–correlation, a generalized gradient approximation (GGA) functional in the form ofPerdew–Ernzerhof–Burke59 (PBE) functional was employed alongwith Grimme’s van der Waals (vdW) correction60 (D3) to describe
the weak dispersion interactions between the adsorbates and thesurfaces. It should be noted that the use of different functionals
possibly enhances the accuracy of the results but is computationally demanding, which is therefore not considered in this current
work. Instead, only the PBE-D3 functional was used throughout
the calculations to systematically and comprehensively evaluate
how the graphene coating affects ORR activity.
To analyze the stability of pristine Pt facets, we estimatedtheir surface formation energy as follows:g¼
1  totalatom
 nEbulk
E
2A slab(1)

atomwhere A, Etotalslab , Ebulk , and n are the surface area, the total energyof the surface slab, the bulk energy per atom, and the numberof atoms in the slab, respectively. Besides, since the binding
energy of adsorbates on surfaces is relative to the electronic
structure of the surface itself, we estimated the d- and p-band
center model,61,62 which was proposed by Hammer and Nørskov to discuss their role in surface interactions:Ð þ1nd;p ðeÞede
ed;p ¼ Ð1(2)
þ1
1 nd;p ðeÞde

where nd,p(e) and e are the density and energy of d- or p-states,
respectively. A graphene-coated platinum (Pt@C) model was

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then constructed from a Pt slab with five layers of atoms in thez-direction and a graphene layer covering the top side of theslab. A vacuum region of 20 Å was inserted between neighboring surfaces to eliminate undesired coulombic interactions
between the model and its self-images caused by periodic
boundary conditions. Except for the two bottom layers thatwere fixed, the remaining atoms in the system were allowed torelax until the residual forces were less than 0.002 Ry bohr1with a 4  4  1 k-point grid. Here, we consider three slab
models representing the three low-index facets, includingPt(100)@C, Pt(110)@C, and Pt(111)@C. Specifically, thePt(100)@C system generated by interfacing a 2  5 unit cellpﬃﬃﬃ
of Pt(100) with a 2  3 3 unit cell of graphene; the Pt(110)@C
system constructed by interfacing a 2  5 unit cell of Pt(110)pﬃﬃﬃ
with a 2  4 3 unit cell of graphene; and the Pt(111)@C systemcreated by interfacing a 3  3 unit cell of Pt(111) with a 3  3unit cell of graphene. The choice of diﬀerent cell sizes forsurfaces is to ensure the lattice mismatch between the Pt
surfaces and graphene layers is minimal. These models correspond to a low adsorption coverage of O2. We note that thegraphene sheet can have many diﬀerent orientations on themetal surfaces. In this study, we only adopted orientations thatlead to a moderate interaction between graphene and Pt
surfaces, which is believed to increase the adsorption energy
between graphene and O2.
The adsorption energy of an intermediate is defined by:
Eads = Esystem  (Ecatalyst + Eadsorbate)(3)

where Esystem, Ecatalyst, and Eadsorbate are the total energy ofcombined systems, isolated surface systems, and isolated
adsorbates, respectively. To consider the catalytic activity for
ORR processes, the Gibbs free energy of individual adsorbatesat pressure P = 1 atm is investigated by adding up theiradsorption energy along with corrections from zero-point
energy (EZPE) and entropy (S) at T = 298.15 K as follows
GðT; PÞ ¼ Eelec þ EZPE þðT

Cp dT  TSðT; PÞ

(4)

0
N hn
P
i
is the zeroi 2
point energy with ni being the vibrational frequency of N modes

where Eelec is the electronic energy, EZPE ¼

obtained from the finite displacement method in an atomic
simulation environment (ASE),63 and Cp is the heat capacity
involving translational, rotational, vibrational, electronic parts,
and a kB term. It should be noted that for ideal gas cases (e.g.,
H2 and H2O), translational and rotational degrees of freedom
were taken into account. For adsorbates, however, all degrees of
freedom of the adsorbate are treated harmonically since theyoften have no real translational or rotational degrees of freedom. The associative ORR mechanisms are then investigated
via the four-electron pathway in an acidic medium (i.e., pH = 0)
with intermediates including:64* + O2 - *O2 (DG0)(5)*O2 + H+ + e - *OOH (DG1)(6)

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*OOH + H+ + e - *OH + *OH (DG2)(7)



*OH + *OH + H + e - *OH + H2O (DG3)(8)*OH + H+ + e - H2O (DG4)(9)+

where * represents the active site of catalysts. We note that onsome surfaces, the hydrogenation of *O2 can directly lead to *O
and *OH via transient *OOH.65 It is well-known that the highspin ground state of the O2 molecule is poorly described in DFT
calculations,66 so we therefore derived its free energy via therelation to H2O(l) and H2(g) as GO2 (eV) = 2GH2O  2GH2 + 4 
1.23. The change in Gibbs free energy (in eV) of intermediates
DGi can be calculated using
DG0 = G*O2  G*  2GH2O + 2GH2  4.92

(10)

1
DG1 ¼ GOOH  GO2  GH2
2

(11)

1
DG2 ¼ GOHþOH  GOOH  GH2
2

(12)

1
DG3 ¼ GOH þ GH2 O  GOHþOH  GH2
2

(13)

1
DG4 ¼ GH2 O þ G  GOH  GH2
2(14)

Finally, the overpotential (ZORR, in V) could be estimated using
Nørskov’s model with a lower overpotential implying better
ORR performance.66
ZORR = 1.23  UL

(15)

where UL is the limiting potential, which corresponds to theminimum free energy change among reaction steps
UL = min(DG1, DG2, DG3, DG4)

(16)

Finally, it is important to note that solvent eﬀects were notconsidered. However, previous research by Iyemperumal and
Deskins67 found that on the Pt(111) surface, solvent eﬀects(especially for small species investigated for the ORR such as
O2, O, and OH) are minimal. For example, the solvation energyof O2 on Pt(111) is only 0.01 eV. Additionally, in a recent related
work on the ORR performance of a single-layer Pt–graphenehybrid,49 it was found that the use of the implicit solvation
model could affect the ORR performance of hybrid Pt–graphene,
however, the energy difference is not significant.

3. Results and discussion
3.1. Geometric and electronic structures of bare Pt and Pt@C
surfaces
First, we examine the structures and the corresponding surface
energies of bare Pt facets, as illustrated in Fig. 1. Notably,Pt(111) emerges as the most stable facet, having a surface
energy of 0.120 eV Å2 owing to its larger planar density.
In contrast, Pt(110) and Pt(100) exhibit significantly higher
surface energies (0.174 and 0.162 eV Å2, respectively),

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Fig. 1 The calculated surface energy corresponds to three diﬀerent lowindex Pt facets, including Pt(100), Pt(110), and Pt(111).

reflecting their lower planar density and higher number ofdangling bonds. The less stable Pt(110) and Pt(100) surfaces are
thereby ready to form additional bonds with adsorbates tostabilize their structures. As a result, the geometry of graphene
coatings on Pt(100) and Pt(110) could be diﬀerent from that onPt(111). Because of the interaction of Pt(100) and Pt(110)
surfaces with the graphene top layer [see Fig. 2(a and b)], theirbonds exhibit considerable strength with an average distance ofapproximately 2.1–2.2 Å, indicative of some covalent characteras shown by the electron localization function calculations(Fig. S1, ESI†). Remarkably, the graphene layer undergoes
distortion from its planarity. This reduces the stress due tolattice misfit between graphene and the metal surfaces. The
maximum buckling in the carbon layer reaches around 0.33 Å
on Pt(100) and Pt(110), while the C–C bond distance ranges
from 1.467 to 1.570 Å. In contrast, Pt(111) exhibits higher
surface stability, resulting in the physisorption of graphene[Fig. 2(c)]. The Pt(111)–graphene distance is 2.7 Å, which is in

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fair agreement with the distance estimated in previous work by
Wang et al.68 (i.e., 3.3 Å). The small mismatch between thePt(111) unit cell and graphene (1.08%) minimizes the distortion, allowing graphene to maintain planarity, albeit with anincreased C–C bond distance from 1.424 Å (in isolated graphene) to 1.556 Å. In fact, the adsorption of graphene on
pﬃﬃﬃ pﬃﬃﬃ

Pt(111), in several Moiré patterns G=Ptð111Þ- 3  3 R30
 pﬃﬃﬃﬃﬃ
pﬃﬃﬃ pﬃﬃﬃ
(this work), G=Ptð111Þ- 7  7 R19:1 , and G=Ptð111Þ- 2 13
pﬃﬃﬃﬃﬃ

2 13ÞR13:9 , has been experimentally shown to be very weak.69,70
Moreover, we also look at the electronic structure of thesesurfaces. The density of states (DOS) along with the partial
density of states (pDOS) is depicted in Fig. 3. According to thed-band center theory proposed by Nørskov et al.,61,62,71 theadsorption energy of the adsorbate is related to the relative
energy between the d-states and the Fermi level. Simply put, thehigher the d-states, the more empty the anti-bonding states,
and the stronger the adsorption. The calculated energy of thed-band center (ed) stands at 2.637, 2.570, and 2.729 eV forPt(100), Pt(110), and Pt(111), respectively. The estimated ed ofPt(111) closely aligns with values previously estimated by
Hammer et al.72 The highest d-band center of Pt(110) indicatesthat this surface should have the strongest adsorption, whereas
the adsorption ability of Pt(111) should be the weakest (seeTable 1 for the case of O2). This tendency matches well thetrend of the surface energy above, i.e., the least stable surface
has the highest adsorption energy.
As graphene is coated on Pt surfaces, the d-band center
slightly shifts to values of 2.712, 2.675, and 2.781 eV forPt(100)@C, Pt(110)@C, and Pt(111)@C, respectively [seeFig. 3(b)]. This indicates that, if O2 could penetrate the graphene layer to interact with the Pt surface (see ref. 73), theanticipated interaction would be weaker than that observed on
the pristine Pt surface. However, since adsorbates directly

Fig. 2 The side and top views of graphene-coated platinum models used for calculations include (a) Pt(100)@C, (b) Pt(110)@C, and (c) Pt(111)@C. The
purple and grey balls represent C and Pt atoms, respectively. The distortion of graphene surfaces due to Pt–C covalent bonds induces two different sp2and sp3-like hybridizations of carbon.

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Pt(110)@C surface adsorbs stronger than Pt(111)@C. Thisobservation can be easily understood as the adsorption of O2
not only depends on the interaction between O2 and graphene
but also on the interaction between the (mobile) graphene layer
and the top layer of Pt. Indeed, upon O2 adsorption, thedistance between graphene and the Pt surfaces significantly
changes (vide infra). Therefore, solely looking at the p-band
center will not give the correct trend of O2 adsorption on thesegraphene-coated surfaces. Furthermore, due to the buckling ofgraphene layers on Pt(110) and Pt(100), the electronic structureof graphene is expected to change, in which the hybridizationof some carbon atoms deviates from a perfect sp2 towards sp3.
Such a change in hybridization has been linked to the ability ofgraphene to bind to O2. The change of hybridization is also
associated with the formation of additional Pt–C bonds (Fig. S1,
ESI†). We therefore referred to these carbons as ‘‘sp3-like’’, as
their character should lie between sp2 and sp3.
To conclude, our data clearly show that, whereas the d-band
center of Pt is a good indicator of the adsorption strength on
the bare Pt surfaces, neither it nor the p-band center ofgraphene can be used to explain the adsorption trend of O2
on the Pt@C surfaces.

Fig. 3 Total DOSs and partial DOSs of (a) bare Pt and (b) Pt@C surfaces.
The Fermi level (eF, black dashed line) is found at 0 eV. Positions of the C pband center (ep, orange vertical line), Pt p-band center (ep, green verticalline), and Pt d-band center (ed, blue vertical line) are marked forcomparison.

interact with the graphene surface in Pt@C, the specific location of d-band centers is less relevant. Indeed, the adsorption of
O2 on Pt(100)@C is the strongest (Table 1), whereas the d-band
center of Pt in Pt(100)@C is not the highest. Similarly, thep-band centers of Pt in Pt@C (1.234, 0.987, and 1.270 eV
for Pt(100)@C, Pt(110)@C, and Pt(111)@C, respectively) also
do not correlate well with the adsorption energy tendency.
We instead look at the carbon p-band center as an alternativedescriptor for the adsorption on non-metallic surfaces.
The calculated p-band center of C is at 3.030, 3.138, and2.954 eV for Pt(100)@C, Pt(110)@C, and Pt(111)@C, respectively. It is then assumed that Pt(110)@C has weaker adsorption
than the other surfaces. However, the results of adsorptionenergy (see Table 1) invalidate this assumption, e.g., the

3.2.

O2 adsorption on Pt and Pt@C surfaces

In Table 1, we present the adsorption energy of O2 on both baregraphene, Pt surfaces, and graphene-coated Pt surfaces, alongwith the corresponding bond distances between O, Pt, and C
atoms. The stable adsorption configurations are visually illustrated in Fig. 4.
On graphene, the adsorption is primarily physisorption,
characterized by a modest adsorption energy of only 0.12 eV
and a relatively long distance between O2 and graphene(B3.4 Å). Our calculated adsorption energy is in excellent
agreement with a recent experimental value of 0.15 eV76 as
well as a diﬀusion Monte Carlo result (0.14 eV).77 The O2
molecule is found to be parallel to the surface and maintains its
triplet spin state. The O–O equilibrium bond distance is 1.23 Å,
being close to the bond length of 1.21 Å in its isolated state. The
resulting charge transfer between O2 and graphene remains
minimal, amounting to less than 0.1 electrons [see also
Fig. 5(c)]. It should be noted that while the O2 singlet can bechemisorbed on graphene (with a C–O bond length of B1.5 Å),
it is highly unstable. The adsorption energy of a singlet-O2 ongraphene was estimated to be highly positive (B1.8 eV).75

Table 1 Adsorption energy (in eV) of O2 on diﬀerent surfaces; bond distances between O, Pt, and C atoms (in Å); and the amount of charge transferredfrom the surface to O2 DQ

Surface

Eads (eV)dX–O (Å) [X = Pt, C]

dO–O (Å)

DQ

Pt(100)@C
Pt(110)@C
Pt(111)@CPt(100)Pt(110)Pt(111)Graphene

1.02
0.59
0.32
1.22
1.52
0.70 (0.72,a 0.84,b 1.11b)
0.12 (0.13,c 0.15d)

1.476
1.480
1.477
1.983
1.968
2.036
3.380

1.486
1.490
1.489
1.370
1.368
1.358
1.230

0.95
0.91
0.97
0.49
0.53
0.48
0.09

a

Ref. 51. b Ref. 49. c Ref. 74. d Ref. 75.

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Fig. 4 Stable configurations of O2 adsorption on (a) graphene, (b) Pt(100), (c) Pt(110), (d) Pt(111), (e) Pt(100)@C, (f) Pt(110)@C, and (g) Pt(111)@C surfaces
from top and side views. The red, purple, and grey balls represent O, C, and Pt atoms, respectively.

Fig. 5 (a) Interaction between C(sp3)–O(sp3), (b) Pt(dz2)–O(sp3) andPt(dxz)–O(sp3). (c) Charge transfers between O2 and three surfaces:graphene, Pt(111), and Pt(111)@C. Color code: green: gaining electron
region, blue: losing electron region (isosurface value of 0.002 e bohr3).
The red, purple, and grey balls represent O, C, and Pt atoms, respectively.

On the bare Pt surfaces, O2 is strongly chemisorbed, with anadsorption energy of 0.7 eV to 1.5 eV. In all cases, O2 is parallelto the surfaces and is characterized as a closed-shell singlet
species. As previously discussed, the Pt(111) surface, being themost stable among the three bare surfaces, displays the weakest
adsorption. The calculated value of 0.7 eV is in good agreement with previously reported values ranging from 0.72 eV to1.11 eV.49,51 Notably, O2 is found to bridge two adjacent Pt
atoms, also referred to as the top–bridge–top (t–b–t) site.78 At thissite, the distance between O2 and the surface measures approximately B2.0 Å, with the O–O bond distance elongated to B1.36 Å.
A charge transfer from Pt(111) to the p* orbital of O2 is estimatedat 0.5 electrons [see Fig. 5(b)]. This observation clearly indicatesthat the character of the adsorbed O2 molecule is somewhere
between that of an isolated O2 and a superoxo O2 species.
In comparison to the Pt(111) surface, the adsorption behavior of O2 on Pt(110) and Pt(100) is quite similar. The distance10716 | Phys. Chem. Chem. Phys., 2024, 26, 10711–10722

between O2 and the surfaces is slightly shorter, ranging from1.97 to 1.98 Å, and the O–O bond exhibits a slight weakening
with a bond distance of 1.37 Å. Remarkably, the binding of O2
on Pt(110) is significantly stronger, manifesting as an adsorption energy of 1.5 eV. On this surface, O2 could be adsorbed
along the [11% 0] or [001] direction, referred to as top–shortbridge–top (t–sb–t) or top–long-bridge–top (t–lb–t) configurations, respectively. The latter configuration is found to be morestable than the former by 0.2 eV. It is important to note that aclean Pt(110) surface may undergo reconstruction, forming a
missing-row type structure.79–81 Such structural changes facilitate dissociative chemisorption of O2,82 though this aspect is
not considered in this work. Also, a recent Raman spectroscopy
experiment showed that the oxygen reduction reaction (ORR)
on the three Pt facets under acidic conditions most likely
proceeds through molecular oxygen instead of atomic
oxygen.83 On the unreconstructed Pt(100) surface, O2 adsorbs
at the bridge site (t–b–t configuration) with an adsorptionenergy of 1.22 eV, closely matching the results obtained in
the work of Duan and Wang48 employing the PW91 functional(1.10 eV).
The adsorption of O2 on graphene-coated surfaces is also
summarized in Table 1 and Fig. 4(e–g). The important findingis that, in contrast to the bare graphene surface, O2 chemisorbs
on graphene-coated Pt surfaces with adsorption energies ranging from 0.3 eV to 1.0 eV. These negative values indicate
the potential of Pt@C as a promising candidate for the ORR.
Various configurations of O2 on the three Pt@C were investigated, revealing a preference for the parallel orientation to thegraphene layer in a top–top (t–t) configuration. The distance
between carbon and oxygen is B1.48 Å, while the O–O bondlength increases to around 1.49 Å. The C–O bond distanceclosely mirrors the values typically found in epoxides, whereas
the O–O bond length resembles that of peroxide. As comparedto the bare Pt surfaces, the strength of the O–O bond on Pt@C
is clearly weaker. This difference can be attributed to theenhanced directional nature of the C(sp3)–O(sp3) bond, as
illustrated in Fig. 5(a). On the other hand, the Pt–O bondmanifests as either Pt(dz2)–O(sp3) (displaying less directionality)
or Pt(dxz)–O(sp3) (exhibiting more directionality).
Upon binding with O2, two carbon atoms change theirhybridization to sp3, leading to a further distortion of theThis journal is © the Owner Societies 2024

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graphene layer and inducing additional sp3 carbons. The C–O
covalent bonds not only alter the structure of the graphene
layer but also influence the interaction between the graphene
layer and Pt. For instance, while the interaction between Pt(111)and graphene is diffusive, the introduction of O2 adsorption is
responsible for the formation of more covalent bonds between
C-sp3 and Pt. In terms of the electronic structure, the adsorbed
O2 molecule can be characterized as a superoxide O2 species,
as we found that 0.9–1.0 electrons were transferred from thesurface to O2 [Fig. 5(c)]. More interestingly, the charge transfer
process involves not only the graphene layer but also the firstand second layers of the Pt surface.

Paper
3.3.

ORR mechanism on Pt and Pt@C surfaces

The associative ORR mechanisms on six surfaces includingPt(100), Pt(110), Pt(111), and their corresponding graphenecoated counterparts, Pt(100)@C, Pt(110)@C, Pt(111)@C aresummarized in Fig. 6. Following the well-established computational protocol for the ORR, we only focus on four intermediates along the reaction: *O2, *O–*OH, *OH–*OH, and *OH.
Specifically, *O2 is the starting active species of the ORR on
all surfaces. We noted that OOH+, as discussed in a recent
work of Gueskine et al.,84 is not a viable species in aqueous
acidic media. They calculated that the proton exchange reaction O2 + H3O+ - OOH+ + H2O is endothermic by 2.65 eV.

Fig. 6 (a) Free energy diagram and structures of ORR intermediates on Pt(100)@C (green steps line), Pt(110)@C (blue steps line), and Pt(111)@C (orange
steps line) at URHE = 0 V, where URHE is the potential of the electrode relative to the RHE. The red, purple, grey, and white balls represent O, C, Pt, andH atoms. Free energy of the corresponding bare Pt surfaces (grey steps line) is inserted for comparison. (b) Free energy profile of the ORR on thegraphene-coated surfaces at URHE = 0 V (in black), URHE = 1.23 V (in red), and limiting potential UL (in blue).

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Therefore, OOH+ should not be the starting active species, as its
concentration is negligible. They also pointed out that numerous researchers have fallen into this trap by picking up OOH+ as
the starting active species of ORR simulations.
Although OOH+ is not the starting active species, the formation of *OOH on the surface is still plausible. This intermediate could be directly formed from the first hydrogenationof *O2. Thus, we also calculated the *OOH species on all
surfaces (see Fig. S2, ESI†). On all surfaces, the first hydrogenation of *O2 can yield either *OOH or *O–*OH. Interestingly,*O–*OH is systematically more stable than *OOH by B2.0 eV
on Pt(100) and Pt(100)@C; B1.0 eV on Pt(110) and Pt(110)@C;
and B1.4 eV on Pt(111) and Pt(111)@C. On the Pt(111) surface,a similar value of 1.51 eV was also found by Duan and Wang.85
On a carbon alloy catalyst, Chai et al.86 also reported theinstability of *OOH. Given the significant stability of *O–*OH,
it is likely that once *OOH is formed, it is rapidly decomposed
into *O–*OH. Indeed, on the Pt(111) surface, the activation energy for the *OOH dissociation reaction is negligible(0.05 eV).85
Let us first discuss the reactivity of the three bare Pt
surfaces. Here, we focus only on the associative mechanism
involving the *O–*OH intermediate. The mechanism with the*OOH species is discussed in the ESI.† In general, all surfaces
possess similar ORR mechanisms, summarized as O2 +4(H+ + e) - *O2 + 4(H+ + e) - *O + *OH + 3(H+ + e) *OH + *OH + 2(H+ + e) - H2O + *OH + (H+ + e) - 2H2O.
Each surface exhibits different gaps between the reaction
intermediates, resulting in different rate-determining steps(RDSs). At the electrode potential URHE (versus the reversiblehydrogen electrode (RHE)) of zero, the free energy profiles are
all downhill along the reaction pathway, i.e., all reduction steps
are exergonic. For example, on Pt(111), the reduction freeenergies are 1.74, 0.71, 0.92, and 1.04 eV (Fig. S2, ESI†).
The gaps between the intermediates can then be adjusted bychanging the working potential. Specifically, the Gibbs freeenergy of the intermediates shifts by an amount of neURHE,
where n represents the number of electrons. On Pt(111), at thelimiting potential UL = 0.71 V, the reduction free energies
change to 1.03, 0, 0.21, and 0.33 eV (Fig. S3, ESI†). Beyond
this electrode potential, one of the reduction steps (the second
reduction step *O–*OH + (H+ + e) - *OH–*OH) becomesendergonic. When the electrode potential URHE is increased to1.23 V, this reaction step has a reaction free energy of 0.52 eV,
which corresponds to the RDS. This result is in line with thestudy of Duan and Wang,48 in which the authors found thesame RDS with an activation energy of 0.79 eV.
Hereafter, we will focus our discussion on the ORR at
an electrode potential URHE of 1.23 V. For the Pt(110) surface,
the RDS is the last reduction (*OH hydrogenation). The result
agrees with the calculations reported by Dong et al.,83 where
they estimated an activation free energy of 1.18 eV. In our study,
we estimated a slightly lower activation free energy of 0.96 eV.
For the Pt(100) surface, the RDS is related to the third reduction
step (*OH–*OH + (H+ + e) - *OH + H2O), which has anactivation free energy of 0.98 eV. The overpotential can then be

10718 | Phys. Chem. Chem. Phys., 2024, 26, 10711–10722

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calculated as the activation free energy of the RDS divided by eor the difference between 1.23 V and the limiting potential (seeeqn (15)). Therefore, the overpotential for the ORR on Pt(100),
Pt(110), and Pt(111) surfaces is estimated to be 0.98, 0.96, and0.52 V, respectively. This suggests a decreasing ORR activity
trend for Pt surfaces in the order of (111) 4 (110) 4 (100).
Notably, this result is in good agreement with both priorexperimental findings87 and recent results reported by
Dong et al.83 utilizing Raman spectroscopy. In the same work,
the authors also performed a similar computational study.
According to their presented data, the RDS on all surfaces isinterpreted as the last hydrogenation step: *OH + (H+ + e) - *+ H2O. The different RDSs between our and their data areunclear to us but probably come from the fact that (i) asignificantly smaller model was employed in their models,therefore accounting for higher adsorption coverage, and(ii) different configurations were used. For instance, the higher
adsorption coverage leads to unfavored chemisorption, e.g., theadsorption free energy on Pt(100) is positive.
The ORR mechanisms on Pt(100)@C, Pt(110)@C, andPt(111)@C surfaces are depicted in Fig. 6. Similar to the Ptbare surfaces, the associated mechanism involves the sameintermediates. To the best of our knowledge, only one theoretical study has examined the ORR mechanism of a graphenecoated Pt surface.49 In their work, Choi et al.49 found stability ina hybrid system of graphene and a monolayer of the Pt(100)-likesurface, which is active for the ORR. Their identified RDS was
the third hydrogenation step (*OH–*OH + (H+ + e) - *OH +H2O). As we employed a Pt(100) surface with several layers,
we found the RDS to be the last hydrogenation, namely *OH +(H+ + e) - H2O. The activation free energy for this stepis estimated to be 1.03 eV, which is slightly higher than theactivation free energy for the RDS on bare Pt(100) (i.e., 0.98 eV).
This result suggests that Pt(100)@C should exhibit ORR reactivity comparable to Pt(100). Our results also nicely corroborate
the experimental observations of Abdelhafiz et al.88 regarding
the ORR activity of thin graphene–Pt hybrid catalysts.
Although having a smaller aﬃnity towards O2 compared to
the Pt(100)@C surface, the ORR mechanism on the Pt(110)@C
surface does not deviate much from Pt(100)@C. Most importantly, the Pt(110)@C surface also identifies the last hydrogenation reaction as the RDS, with an activation free energy ofapproximately 0.82 eV. Therefore, we predict that the ORR
reactivity of Pt(110)@C should be slightly better than that ofPt(100)@C. In contrast, the ORR mechanism on Pt(111)@Cturns out to deviate the most from that on Pt(111) as well as theother graphene-coated surfaces. First, the adsorption freeenergy is significantly smaller. Second, we found two competing RDSs, the second and last hydrogenation steps, with pretty
similar activation energies, 0.78 eV and 0.71 eV. Based on theactivation free energy of the RDS, we predict the overpotential
values of Pt(100)@C, Pt(110)@C, and Pt(111)@C to be 1.03,
0.82, and 0.78 V. Thus, the ORR reactivity of graphene-coatedsurfaces can be ordered as Pt(111)@C 4 Pt(110)@C 4
Pt(100)@C. This order parallels the order found on the baresurfaces.

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Among the six surfaces studied, the bare Pt(111) surface
turns out to have the smallest overpotential (i.e., 0.53 V), closely
followed by Pt(111)@C (i.e., 0.78 V) and Pt(110)@C (i.e., 0.82 V).
In contrast, the Pt(100)@C surface displays significantly larger
overpotential values (approximately 1.0 V). Although the results
suggest that Pt(111)@C possesses slightly lower catalytic
reactivity compared to the pristine Pt(111) surface, our findings
highlight the dual role of graphene. On the one hand, grapheneenhances the durability and stability of the catalyst by serving
as a physical barrier, preventing Pt aggregation.88–90 On theother hand, graphene-coated Pt surfaces can still catalyze the
ORR with a competing catalytic reactivity compared to thepristine Pt surface. Notably, introducing defects in graphene,
leading to Pt exposure to O2, can further enhance the activity of
Pt@C.90
3.4. ORR mechanism on the N-doped graphene-coated Pt
surface
Considering the higher reactivity observed in the Pt(111)@C
surface, coupled with the established eﬀectiveness of nitrogendoped carbon materials as metal-free catalysts for the oxygen
reduction reaction (ORR),91 we briefly discuss the ORR mechanism on a nitrogen-doped Pt(111)@C surface (also referred toas graphitic-N defects),92 denoted as Pt(111)@C/N. Accordingly,
the doping of nitrogen on the graphene coating is anticipated
to exhibit better ORR activity due to (i) a higher concentrationof protons, which can facilitate four-electron reduction, and(ii) enhanced O2 adsorption capabilities.93
The calculated ORR mechanism on Pt(111)@C/N, in comparison with the Pt(111)@C surface, is illustrated in Fig. 7. First,
we investigated diﬀerent locations of nitrogen on graphene.
We found that nitrogen is preferably located on the hollow site

Paperof the top Pt layer, thus preserving its sp2 character. Consequently,
the three carbon atoms binding with this nitrogen (denoted as CN)
reside on the top sites of the Pt layer. Upon bonding with Pt, thehybridization of these carbons changes to sp3. Here, we only
explored the adsorption of O2 in the vicinity of the nitrogen and
CN atoms, aiming at probing the impact of nitrogen. Most
importantly, we found that O2 neither adsorbs on nitrogen nor
these three CN atoms due to a lack of dangling bonds. Thisbehavior is diﬀerent from the adsorption of O2 on nitrogen-dopedcarbon materials, in which the active sites are CN.94 On thisPt(111)@C/N surface, the most favorable adsorption configuration
is observed when O2 adopts an end-on (or Pauling) configurationon the carbon atoms bonding to CN. Similar to the behavior
observed for O2 on Pt(111)@C, O2 on Pt(111)@C/N can becharacterized as a superoxide O2, as approximately 0.8 electrons
are transferred from the surface to O2 [see Fig. 8(a)]. The slightly
reduced charge transfer on Pt(111)@C/N compared to Pt(111)@C(i.e., 0.98 electrons) probably comes from the end-on adsorption
configuration. The adsorption energy on Pt(111)@C/N is around0.3 eV, which is approximate to the adsorption energy on
Pt(111)@C.
The major distinction between the ORR on Pt(111)@C/N andPt(111)@C lies in the hydrogenation product of *O2. As mentioned above, the ORR mechanism on Pt(111)@C is expected toproceed via the *O–*OH intermediate, as it is significantly morestable than the *OOH intermediate. On Pt(111)@C/N, only
*OOH is formed, a feature attributed to the inability of N and
CN to provide dangling bonds to oxygen. As a result, the ORR
mechanism on Pt(111)@C/N takes a distinct pathway involving*OOH, *O–H2O, and *OH–H2O intermediates [Fig. 7(b)].
The second hydrogenation from *OOH to *O–H2O is similar
to findings observed in an iron-embedded N-doped graphene

Fig. 7 Comparison of the ORR mechanism on (a) Pt(111)@C and (b) Pt(111)@C/N surfaces at five intermediates (ii–vi). The green, red, purple, grey, andwhite balls represent N, O, C, Pt, and H atoms, respectively.

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PCCP
demonstrated the potential of graphene-encapsulated Pt-based
nanoparticle catalysts for the ORR.34,41

Conflicts of interest

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There are no conflicts to declare.

Acknowledgements
Fig. 8 (a) Charge transfers between O2 and the Pt(111)@C/N surface.
Color code: green: gaining electron region, blue: losing electron region(isosurface value of 0.002 e bohr3). (b) Free energy profile of the ORR on
the Pt(111), Pt(111)@C, and Pt(111)@C/N surfaces at URHE = 0 V. (c) Free
energy profile of the ORR on the Pt(111)@C/N surface at URHE = 0 V(in black), URHE = 1.23 V (in red), and limiting potential UL (in blue).

catalyst.95 The third hydrogenation *O–H2O + (H+ + e) *OH–H2O turns out to be the RDS, with an activation freeenergy of 0.73 eV. Finally, the remaining *OH species undergoes hydrogenation, leading to the release of water moleculesfrom the surface. The activation free energy of 0.73 eV is slightly
smaller than that of the Pt(111)@C surface (i.e., 0.78 eV),suggesting a small enhancement in ORR catalytic reactivity
on the N-doped surface [Fig. 8(b and c)]. These results affirm
the potential of the as prepared N-doped graphene-encapsulated
PtFe alloy core–shell catalyst in our lab previously.34,41

4. Conclusions
In summary, we performed first-principles calculations to gain
insight into the geometric and electronic properties of bare Pt
surfaces, graphene- and N-doped graphene-coated Pt surfaces,
as well as the mechanism of oxygen reduction reactions onthese surfaces. We show that O2 is strongly adsorbed on thebare Pt surfaces, whereas the adsorption on graphene-coatedsurfaces is slightly weaker. We then found that diﬀerent bare Pt
surfaces behaved diﬀerently in ORR catalytic activities, with themost eﬃcient facets ordered as Pt(111) 4 Pt(110) 4 Pt(100).
On the other hand, due to having a lower aﬃnity towards O2,
the graphene-coated surfaces display diﬀerent rate-determining
steps. By considering only the activation energies of the ratedetermining steps, the catalytic activity follows the order ofPt(111)@C 4 Pt(110)@C 4 Pt(100)@C. Estimated overpotentials indicated comparable ORR catalytic activity between thegraphene-coated surfaces and bare Pt surfaces. Our calculations also highlight the dual role of the graphene coating.
On the one hand, it improves the stability of the catalyst, forinstance, by inhibiting the aggregation of Pt nanoparticles. On
the other hand, it can actively participate in the reaction
mechanism. Finally, we also pointed out that the introductionof nitrogen on the Pt(111)@C surface could potentially enhance
the ORR activity by reducing the overpotential. The activityof Pt(111)@C and Pt(111)@C/N, together with the fact thatthey possess good stability owing to the graphene layer, have10720 | Phys. Chem. Chem. Phys., 2024, 26, 10711–10722

This research was supported by the project JPNP20003, commissioned by the New Energy and Industrial Technology Development Organization (NEDO), Japan.

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