ABSTRACT The large-scale practical application of fuel cells cannot come true if the high-priced Pt-based electrocatalysts for oxygen reduction reaction (ORR) cannot be replaced by other

efficient, low-cost and stable electrodes. Here, based on density functional theory (DFT), we exploited the potentials of layered SiC sheets as a novel catalyst for ORR. From our DFT

results, it can be predicted that layered SiC sheets exhibit excellent ORR catalytic activity without CO poisoning, while the CO poisoning is the major drawback in conventional Pt-based

catalysts. Furthermore, the layered SiC sheets in alkaline media has better catalytic activity than Pt(111) surface and have potential as a metal-free catalyst for ORR in fuel cells. SIMILAR

OXYGEN REDUCTION REACTION Article 13 July 2020 INTRODUCTION As one of the most promising power sources, fuel cells have received considerable attention due to their high efficiency and low

environmental impact. The sluggish kinetics of the oxygen reduction reaction (ORR) at the cathode is one of the key factors limiting the performance of fuel cells and efficient ORR

electrocatalysts are essential for practical applications of the fuel cells. Pt has conventionally been employed as the cathode catalyst due to its high activity for the ORR1,2. The high

cost, limited supply and poor durability of Pt catalyst however hinders the large-scale commercialization of fuel cells. Hence, during the last few decades, numerous studies have been

devoted to find alternative electrocatalysts for the cathode side of fuel cells, including Pt-based alloys3,4,5,6, Pt-based core-shell/alloy nanoparticles7,8,9, carbon nanotubes-supported

metal particles10,11, graphitic carbon nitride/carbon composite12,13 N/B/P/S doped carbon nanotubes (CNTs) and graphenes14,15,16. Carbon-based catalysts are expected to be the most promising

alternatives to Pt catalysts because C is more abundant and durable, as well as less expensive than Pt. The introduction of N (or B, P, S) atoms into _sp_2-hybridized carbon frameworks of

graphenes or CNTs is generally effective in modifying their electrical properties and chemical activities17,18. Recent studies have confirmed that N, B, P and S doped carbon materials are

hopeful candidates to replace Pt-based catalysts for fuel cells due to their high catalytic activity, long-term stability and excellent CO tolerance14,15,19,20,21,22,23. Theoretical studies

have confirmed that the improved electrocatalytic activity of these materials can be attributed to the changes of electronic structure by doping N, B, P and S into carbon materials24,25.

However, the dopant concentration in present carbon-based catalysts is low (N ~ 4–6 at%, B ~ 0–2.24 at% and 1–2 at% of S)19,20,21, which limits the improvement of the catalytic activity of

carbon-based catalysts. SiC has a unique combination of high saturated carrier mobility, high critical electrical field and high thermal conductivity. SiC with the atom ratio of 1:1 can

provide more active reaction sites than present doped carbon materials to serve as a feasible metal-free ORR electrocatalyst. Herein, we predicted that layered SiC sheets exhibit excellent

ORR catalytic activity and high CO tolerance and may be a very likely candidate for the next generation of low-cost ORR electrocatalysts. RESULTS Firstly, the relative stability between

layered and cubic SiC sheets, which is a function of layer number _N_, is determined by calculating the energy difference Δ_E_ = _E_layered − _E_cubic, where _E_layered and _E_cubic are the

total energies of layered and cubic SiC sheets, respectively. The layered SiC sheets are energetically favorable in comparison to cubic SiC sheets when _N_ < 4, while cubic SiC sheets are

advantaged as _N_ ≥ 4, as shown in Fig. S1 and Table S1 in supporting information. Similar to graphite, layered SiC sheets usually show weak layer-layer interactions. Even further

considering the van der Waals bonding, layered SiC sheets are still energetically more favorable than cubic SiC sheets when _N_ < 4. This is consistent with the recent experimental

results that the thickness of synthesized 2D SiC nanosheets is between 0.5 and 1.5 nm26. We then turn to consider the adsorption and reduction of O2 on a single-layer SiC. The adsorption

energies of adsorbates on layered SiC sheets are determined by _E_ad = _E_ads + _E_SiC − _E_ads/SiC, where _E_ads, _E_SiC and _E_ads/SiC are the total energies of the isolated adsorbate

molecule, the layered SiC sheets and the adsorption systems, respectively. Positive adsorption energies correspond to exothermic adsorption processes, while negative adsorption energies do

endothermic ones. Several possible high-symmetry adsorption sites are considered, including the top, bridge and hollow sites. The most stable adsorption structures and energies of O2 and ORR

intermediates are summarized in Fig. 1 and Table 1. The meta-stable adsorption structures and energies are displayed in Fig. S2 of supporting information. It is found that the most

energetically favorable site for O2 adsorption on the single-layer SiC is characterized by O2 nearly parallel to the SiC bond with adsorption energy of 0.56 eV. The O-O bond is elongated by

0.29 Å due to the electronic charge transfer from SiC to the 2π* orbital of O2, indicating that O2 can be dissociated easily on SiC. The charge transfer from the single-layer SiC to O2

molecule is quantified as 0.34 _e_ according to Hirshfeld population charge distribution. Furthermore, the maximum coverage of O2 on SiC is also explicitly identified. As shown in Table S2

and Fig. S3 of supporting information, only two O2 molecules can be adsorbed on the single-layer SiC with 2 × 2 supercell, corresponding to a coverage 0.5 ML (monolayer) (1 ML is defined as

one O atom per surface atom). The adsorption energy of the first O2 molecules on single-layer SiC is 0.48 eV. The second O2 molecule has two distinct adsorption structures with adsorption

energies of 0.61 and 0.13 eV, respectively. It is found that the energetically more favorable configuration of second O2 forms two chemical bonds with Si atoms. This special adsorption

structure results in the stronger adsorption of the second O2 molecule than the first one. This is because the electronegativity of Si (1.90) is smaller than that of C (2.55) while the Si-O

bond is stronger than the C-O bond. Similar results are also found in 4 × 4 supercell, where the coverage also reaches 0.5 ML. Furthermore, O2 adsorbed as ordered structures in a 4 × 4

supercell is considered, as shown in Fig. S3 of supporting information, the corresponding coverage can reach 0.75 ML. However, the coverage of O2 on Pt(111) surface can reach 1 ML. All the

O2 molecules adsorb on the bridge sites. This is consistent with previous theoretical work about O2 adsorption on Pt(111) surface27,28. Although the coverage of O2 on layered SiC is smaller

than that on Pt(111) surface, the remaining adsorption sites on single-layer SiC is favorable for adsorption of other reactants, which may be beneficial for ORR. The most stable adsorption

site for O atom is the bridge site, which gives rise to a highly stable epoxide-like structure with adsorption energy of 4.18 eV. In addition, H and OH prefer to adsorbing at atop site of Si

atom other than that of C atom with the adsorption energies of 1.32 and 2.80 eV, respectively, which differs from the case in N-doped carbon materials29. Because H and OH tend to adsorb at

positive charged adsorption sites. Note that H2O is stable on atop site of Si atom of the single-layer SiC. In contrast, H2O dissociates to H and OH spontaneously on cubic SiC sheets, which

may prevent ORR on cubic SiC sheets. The reason is that Si and C atoms are _sp_3 hybridized in cubic SiC sheets, while they are _sp_2 in layered ones. The dangling bonds of Si or C atoms at

the surface of cubic SiC sheets are very active. When H is introduced to O2, no matter whether the presence of H2O molecules around, O-O bond dissociates to form O and OH spontaneously, as

shown in Fig. S4 of supporting information. OOH cannot exist as a stable intermediate in the ORR processes. And then, H2O2 cannot be produced based on OOH. Thus, OOH and H2O2 pathways of ORR

on the single-layer SiC are neglected here30,31. For CO adsorption, adsorption energy value changes from −0.08 eV to 0.07 eV after considering the effect of van der Waals bonding. Since the

small adsorption energy value of CO adsorption, layered SiC sheets show excellent CO tolerance. In contrast, the adsorption energy of CO on Pt(111) surface (1.86 eV) is twice as large as

that of O2 (0.84 eV), as shown in Table S3 of supporting information. Therefore, CO blocks the active sites and hinders the ORR on Pt(111) surface32,33. From partial density of states (PDOS)

(Fig. S5 of supporting information), it is found that the hybridization between CO molecule and Si atom is very small, suggesting that the interaction between CO and SiC is little. By

contrast, the interaction between CO and Pt(111) surface is very strong. All the main orbitals of CO hybridize with Pt states. The renowned mechanism of CO-metal interaction, namely, via

donation from CO-5σ to metals and back-donation from metals to CO-2π* orbital, is applicable for CO adsorbed on Pt(111)34,35. This is confirmed by the fact of electron depletion at CO-5σ and

partially occupied CO-2π*, which is empty above the Fermi level in gas CO. Consequently, CO will poison the Pt(111) surface. It is known that ORR mechanisms at cathodes in acidic and

alkaline solutions are different. In an acidic solution, the electrode reaction can be written as O2 + 4H+ + 4e− → 2O + 4H+ + 4e− → O + OH + 3H+ + 3e− → O + H2O + 2H+ + 2e− → OH + H2O + H+ +

e− → 2H2O, while in an alkaline solution that can be expressed as O2 + 2H2O + 4e− → O + 2OH− + H2O + 2e− → 4OH−. Both reaction pathways are considered here. In general, ORR can proceed in

Langmuir-Hinshelwood (LH) or Eley-Rideal (ER) mechanisms. LH mechanism involves all the reacting intermediates on the surface, whereas ER mechanism involves species from the electrolyte

reacting with a surface intermediate. Both mechanisms are also taken into account one by one. The ORR following the LH mechanism is discussed firstly. The ORR mechanism on the single-layer

SiC in LH mechanism under acidic media can be divided into five elemental steps: O2 dissociation, two O atom hydrogenation steps and two OH hydrogenation steps, as shown in Fig. 2. Some

possible ORR pathways with higher barrier energies are shown in Fig. S6 of supporting information. The activation energies for the five elemental steps are 0.29 eV for O2 dissociation, 0.43

and 0.49 eV for first and second OH formation, 1.11 and 1.05 eV for first and second H2O formation, respectively. The corresponding reaction energies are −1.61, −1.13, −1.00, 0.44 and 0.51 

eV. H2O formation is the rate-determining step (RDS) in the O2 dissociation mechanism of ORR in the above process. The Si-H bond broken from the initial state to the transition state

contributes to the most part of the activation energy of H2O formation. As the number of H atoms introduced to the O atom increases, the activation energy increases from 0.49 eV to 1.05 eV

and the reaction energy increases from −1 eV to 0.51 eV, corresponding to the Brønsted-Evans-Polanyi relation between activation energy and reaction energy in the heterogeneous

catalysis36,37,38. Activation energy is a almost linear function of reaction energy and reactions belong to the same class even follow the same relation36,37,38. The decrease of the activity

of O atom is due to the increase of the number of introduced H atoms. ORR on the single-layer SiC in LH mechanism under an alkaline environment can be divided into the two elemental steps:

O2 + H2O → O + 2OH and O + H2O → 2OH, as shown in Fig. 3. Differing from that in acidic environment, both steps are easy to cross with small activation energies (0.11 and 0.23 eV) and

exothermic reaction energies (−2.20 and −0.40 eV). These can be deduced from the special geometrical parameters along reaction paths. O and H atoms only need a little movement from the

initial state to the transition state, which costs little energy. Furthermore, the hydrogen bond between O atom and H atom in H2O molecule also reduce the energy barrier for H atom transfer.

The activation energies for isolated H2O and O2 dissociations are 0.56 and 0.29 eV, both of them are surmountable at room temperature. This is consistent with the small activation energies

for ORR elemental steps in alkaline media. Thus, we predict that ORR in LH mechanism on the single-layer SiC in the alkaline media is more favorable than that in the acidic media. As _N_

increases from 1 to 3, adsorption energies for ORR intermediates, activation and reaction energies for ORR elemental steps in LH mechanism on layered SiC sheets are almost unchanged as shown

in Tables 1 and 2. Thus, layered SiC sheets with different _N_ values should show similar ORR catalytic activities. For a comparison purpose, ORR elemental steps in LH mechanism on Pt(111)

surface are also calculated, as depicted in Fig. S7 and Tables S4 of supporting information. The five barriers in O2 dissociation mechanism are calculated to be 0.85 eV for O2 dissociation,

1.00 and 1.22 eV for two OH formation steps and 0.38 eV for H2O formation. For the OOH association mechanism, it is found that the barriers are 0.61 eV for OOH formation, 0.03 eV for OOH

dissociation, 1.22 eV for OH formation and 0.38 eV for H2O formation. OH formation is RDS for both the O2 dissociation and OOH association mechanisms. This is consistent with the recent

literature30. The RDS of ORR on Pt(111) surface in acidic media is the O atom hydrogenation to OH with activation energy of 1.22 eV, which is similar to that of ORR on layered SiC sheets,

suggesting that layered SiC sheets and Pt(111) surface may exhibit similar activities for ORR in LH mechanism under an acidic environment. In the alkaline media, however, the activation

energies for the two elemental ORR steps in LH mechanism on Pt(111) surface are 0.43 and 0.55 eV, respectively, both of them are larger than that on layered SiC sheets since the bond length

of Pt-Pt (2.77 Å) is longer than that of Si-C (1.79 Å). The energy cost for the longer distance diffusion of H atom on Pt(111) surface is larger than that on layered SiC sheets. In addition,

the elemental step of O + H2O → 2OH, which is the RDS of ORR in LH mechanism on Pt(111) surface in alkaline media, is endothermic by 0.51 eV, while that on layered SiC sheets is exothermic

with reaction energy of nearly −0.40 eV, suggesting that ORR in LH mechanism on layered SiC sheets is more advantaged than that on Pt(111) surface in alkaline environment. Although energy

diagram of ORR in ER mechanism is quite different from that in LH mechanism, both of which show similar results somehow. As shown in Fig. 4(a), ORR in ER mechanism exhibits exothermic

reactivity at electrode potential _U_ = 0 V under an acidic medium. As the pH value increases, the energy of each net coupled proton and electron transfer (CPET) step is shifted due to the

effect of concentration on the free energy of H+. At pH = 1, the reaction energies of five elemental steps calculated are −1.61 eV for O2 dissociation, −1.44 and −1.32 eV for first and

second OH formation, −0.06 and −0.34 eV for first and second H2O formation, respectively. When pH ≥ 4, the reaction energy of first H2O formation changes from exothermic to endothermic,

while other steps remain exothermic. As _U_ increases from 0 V to the ideal electrode potential of 1.23 V, energy levels are shifted for each net CPET step31,39. The reaction energies of the

reactions in the LH mechanism and O2 dissociation in the ER mechanism remain unchanged. This is because no CPET step involves in these steps and the electrode potential only affects the

energy levels of the electrons in the CPET steps31,39. At _U_ = 1.23 V, OH formation in ER mechanism is still exothermic, while two H2O formation steps change to 1.17 and 0.89 eV

endothermically at pH = 1, respectively. Consistent with ORR in LH mechanism, H2O formation is also the RDS of ORR on single SiC layer in ER mechanism at _U_ = 1.23 V. The high concentration

of H2O in electrolyte can improve the rate constant for the backward reaction and may have a negative effect on the entire process. Just as any coin has two sides, the issue being discussed

here is no exception. H2O solution also has some positive effects on the ORR. As shown in Table S5 of supporting information, the adsorption of O2 is enhanced in H2O solution, resulting in

the increase of reactant concentration on catalyst surface and forward reaction rate. A substantially different picture is obtained for ORR in ER mechanism under alkaline environment.

Similar with that under acidic media, pH imports similar effect and shifts up the energy level of every CPET step. As shown in Fig. 4(b), at _U_ = 0 V, adsorbed O2 molecule reaction with H2O

is exothermic with reaction energy of −1.40 eV, while O atom reaction with H2O is endothermic with reaction energy of 0.06 eV at pH = 14. When an ideal electrode potential of 0.40 V is

applied, the energy levels of every CPET step are shifted up by 0.80 eV for every double CPET step. The reaction energies of O2 molecule reaction with H2O and O atom reaction with H2O vary

to −0.60 and 0.86 eV at pH = 14. O2 molecule reaction with H2O is the RDS of ORR in ER mechanism under alkaline environment and the energy barrier increases as electrode potential increases.

ORR in ER mechanism on Pt(111) surface is also calculated. As shown in Fig. S8(a) of supporting information, all elemental steps involved in ORR via ER mechanism on Pt(111) are exothermic

at pH = 1 and _U_ = 0 V, while OH, H2O and OOH formations are 0.69, 0.53 and 0.63 eV endothermic at _U_ = 1.23 V, which are smaller than the barrier of the RDS step (H2O formation) on

single-layer SiC. These results are consistent with that obtained by Nørskov et al, where the reaction energy of OH and H2O formations change form exothermic to endothermic at _U_ = 1.23 V

and the OOH associative mechanism has the lowest barrier and dominates at low oxygen coverges40,41. Fig. S8(b) of supporting information also shows the energy diagram of ORR in ER mechanism

on Pt(111) under alkaline environment. At _U_ = 0 V and pH = 1, the reaction energies of O2 molecule reaction with H2O and O atom reaction with H2O on Pt(111) surface are −1.42 and 0.38 eV,

respectively. At _U_ = 0.40 V and pH = 1, the reaction energies of these two elemental steps vary to −0.62 and 1.18 eV, respectively. O2 molecule reaction with O atom is the RDS and the

barrier are larger than that on single SiC layer, suggesting that layered SiC sheets exhibit better catalytic activity than Pt(111) surface in ER mechanism under alkaline environment. Based

on Eyring's canonical transition state theory, our calculations can be incorporated into a reduced kinetic model that should report qualitative features of the reaction mechanisms at

different applied potentials39,42,43. As displayed in Fig. 5(a), the RDS of ORR in ER mechanism is O2 dissociation at low potential region, while it changes to H2O formation when _U_ >

0.63 V at pH = 1. As shown in Fig. 5(b), O2 reaction with H2O is the RDS of ORR in ER mechanism under alkaline environment and the reaction rate decreases as electrode potential increases.

Comparing Fig. 5 with Fig. S9 of supporting information, we can predict that the catalytic activity of layered SiC sheets is better than that of Pt(111) surface in alkaline media.

Furthermore, due to the higher doped ratio, SiC can provide more ORR active sites than present doped carbon materials and may act as a feasible low-cost metal-free ORR electrocatalyst.

DISCUSSION Since O2 adsorption and dissociation are the key points for ORR on layered SiC sheets, the origin of the interaction between O2 and the single-layer SiC is investigated by

considering their electronic structure changes. Fig. S9 of supporting information illustrates the spin-polarized partial density of states (PDOS) of O-O bond on the single-layer SiC and

Pt(111) surface. Generally, the 5σ, 1π and 2π* orbitals of O2 dominate the adsorption and all of them are broadened due to the interaction with the single-layer SiC and Pt(111) surface. The

antibonding 4σ* state is not involved in the adsorption because its position is far below the Fermi level. The antibonding 2π* orbital of gas O2 is partially filled. After adsorption on the

single-layer SiC and Pt(111) surface, the initial empty spin-down component of 2π* becomes partially occupied, inducing proportional elongation of the O-O bond length and weakening of O-O

bond strength44,45. The electron occupying the antibonding 2π* orbital of adsorbed O2 on the single-layer SiC is 1.55 _e_ compared with 1.19 _e_ on Pt(111) surface and 1.07 _e_ in gas phase

according to integrated DOS. Thus, the activation energy of O2 on the single-layer SiC are smaller than that on Pt(111) surface. In summary, our DFT calculations suggest that novel

metal-free layered SiC sheets with _N_ = 1 ~ 3 can exist stably and possess potential ORR catalytic activity due to two advantages of layered SiC sheets compared to Pt(111) surface: (i) free

from CO poisoning and (ii) lower activation energies for the RDS of ORR on layered SiC sheets in alkaline media than that on Pt(111) surface. In addition, the corresponding electronic

structures are analyzed. The results show that the layered SiC sheets are candidates for practical applications in fuel cells. METHODS All calculations are performed within density

functional theory (DFT) framework as implemented in DMol3 code46,47. The All Electron Relativistic core treat method is implemented for relativistic effects, which explicitly includes all

electrons and introduces some relativistic effects into the core. The generalized gradient approximation (GGA) with Perdew−Burke−Ernzerhof (PBE) functional is employed to describe exchange

and correlation effects48. PBE is the most common density functional in materials and surface science. However, it cannot accurate describe the van der Waals forces49,50,51,52. This is

mainly because the GGA-PBE fails to describe non-local dispersion forces, which are expected to be relevant in layered inorganic compounds and weak adsorption systems. The lack of inclusion

of long range correlation in the local density functional (LDF) calculations prevents the accurate calculation of the van der Waals bonds46. Therefore, a Grimme approach is adopted for

dispersion corrections. Dmol3 uses a double set of numerically tabulated basis functions46. A more precise term would be “double numerical basis” to be contrasted with double zeta basis,

where the radial functions are defined as Slater zeta functions. The basis set can be significantly improved by adding higher angular momentum valence polarization functions and also by core

polarization functions46. Total energy for the double basis is quite uniformly higher than the ones for the extended basis sets. It has shown that the basis set produces errors in

self-consistent eigenvalues and total energy of ≈0.00003 Ha. In this work, the double numerical atomic orbital augmented by a polarization _p_-function (DNP) is chosen as the basis set46.

The accuracy of DNP is comparable to a Gaussian 6-31(d) basis53. The double basis set may be considered as a large basis especially for the larger molecules. Recent theoretical works based

this basis set have shown excellent consistency with experiments54,55. We have also compared our results with that obtained based on triple numerical plus polarization (TNP), as shown in

Tables S6 and S7. It can be found that the adsorption energies based on TNP are slightly larger than that based on DNP, while the activation energies and reaction energies of ORR in LH

mechanism based on both basis sets are almost the same. A smearing of 0.005 Ha (1 Ha = 27.21 eV) to the orbital occupation is applied to achieve accurate electronic convergence. The

spin-unrestricted method is used for all calculations. To ensure high-quality results, the real-space global orbital cutoff radius is chosen as high as 4.6 Å in the computations. The k-point

density is set as 2/3 × 2/3 × 1 for per unit cell. The convergence tests for k-point density are shown in Table S8 of supporting information, which shows that the energy of single SiC layer

is converging when k-point density is larger than 2/3 × 2/3 × 1. The convergence tolerance of energy is 1.0 × 10−5 Ha, maximum force is 0.002 Ha/Å and maximum displacement is 0.005 Å in the

geometry optimization. The transition states for ORR elemental steps are obtained by LST/QST tools in DMol3 code. Frequency calculations are performed to confirm the location of the

transition states. A conductor-like screening model (COSMO) is used to simulate a H2O solvent environment throughout the whole process. All data, except for clearly stated, are obtained

under this method. The COSMO is a continuum model in which the solute molecule forms a cavity within the dielectric continuum of permittivity56. The DMol3/COSMO method has been generalized

to the periodic boundary cases. The deviation of this COSMO approximation from the exact solution is small. For strong dielectrics like H2O, it is less than 1%. The dielectric constant is

set as 78.54 for H2O solvent. Our previously theoretical work, focused on ORR on N doped CNT based on COSMO, is consistent with experiments well29. In addition, some other results also show

that this implicit solvation model is an effective method to describe the solvation30,31. The solvation energies of ORR intermediates on SiC sheets are shown in Table S9 of supporting

information and adsorption energies in a gas phase environment are shown in Table S5 of supporting information, from which we can find that O2 and OH are stabilized on SiC sheets by

solvation. For layered SiC sheets, all simulations are performed in a 3 × 3 supercell except for coverage examination of O2 adsorption, where 2 × 2 and 4 × 4 supercells are used. The minimal

distance between the SiC layers and their mirror images is set as 15 Å, which is sufficiently large to avoid the interaction between them. The Si-C bond length is 1.79 Å. The Pt(111)

surface is modeled by a periodic array of Pt slabs with a vacuum width in excess of 15 Å. The p(3 × 3) unit cells with three layer Pt slabs are used throughout in the ORR calculations. The

bottom two Pt layers are fixed at their bulk positions and the top Pt layer is allowed to relax fully. Tests performed with four and five atomic layers, top two of which are allowed to

relax, do not show significant differences in structural parameters. As shown in Table S10 of supporting information, the adsorption energy variation for adsorbed molecules is smaller than

0.09 eV. In this work, both LH and ER mechanisms are expected to proceed. The LH reaction referrs to the reaction of adsorbed hydrogen atoms with another adsorbate and the ER reaction does

the reaction of a proton from solution interacting with an adsorbate. The complete electrochemical ORR in ER mechanism involves four CPETs to O2 molecule at the cathode31,39. Electron

transfers are coupled with a proton transfer as well. Barriers for electrochemical proton-transfer have been calculated for the reduction of O2 to OOH and OH to H2O on Pt57,58. In both

cases, the proton-transfer reaction barrier calculated is small (0.15 eV to 0.25 eV) at the low potential that elementary steps are exothermic and diminishes with higher applied

voltages57,58. Similarly, as a first approximation, we expect also that barriers for electrochemical proton transfers to adsorbed species will be small and be easily surmountable at room

temperature. As a result, we only calculated reaction energy for ORR in the ER mechanism. Free energies of the ORR intermediates are calculated based on a computational hydrogen electrode

(CHE) model suggested by Nøskov et al40,59. Free energy change (Δ_G_) is determined by Δ_G_ = Δ_E_ + Δ_ZPE_ − _T_Δ_S_ + Δ_G_pH + Δ_G_U, where Δ_E_ is the reaction energy, Δ_ZPE_ is the zero

point energy, _T_ is temperature and Δ_S_ is the change in the entropy. Δ_G_pH and Δ_G_U are the free energy contributions due to variations in H+ concentration and electrode potential, _U_,

respectively. In this work, we consider the contributions of Δ_E_, Δ_G_pH and Δ_G_U to free energy and neglect the effects of other terms60. Effects of other terms will be included in a

forthcoming publication. We assume pH = 0, 1, 3 and 5 for acidic medium and pH = 9, 11 and 14 for alkaline medium. The pH effect is very hard to be considered directly in the electrolyte.

Generally, it can be treated following the method directed by Nørskov et al40. At a pH differing from 0, the free energy of H+ ions can be corrected by the concentration: _G_pH =

−_k_B_T_ln[H+] = pH × _k_B_T_ln10. This pH-dependence effect does not enter the COSMO-approach. Naturally, ORR is a highly complicated process. Undertaking a kinetics analysis using rate

constants derived from first-principles calculations would be an ideal way to determine which pathways are relevant under different conditions. Such a kinetics analysis is unfortunately

difficult, since it would require rigorous double-layer effects, such as intermediate concentrations, surface coverages, or the potential drop within the interface. At here, the potential

dependent rate constants _k_(_U_) are obtained based on Eyring's canonical transition state theory: , where _k_B is Boltzmann constant, _T_ = 298.15 K, _h_ is Planck constant and

Δ_G_(_U_) is the potential-dependent barrier for that process39,42,43. REFERENCES * Gasteiger, H. A., Kocha, S. S., Sompalli, B. & Wagner, F. T. Activity benchmarks and requirements for

Pt, Pt-alloy and non-Pt oxygen reduction catalysts for PEMFCs. Appl. Catal. B 56, 9–35 (2005). Article  CAS  Google Scholar  * Yu, X. & Ye, S. Recent advances in activity and durability

enhancement of Pt/C catalytic cathode in PEMFC part II: degradation mechanism and durability enhancement of carbon supported platinum catalyst. J. Power Sources 172, 145–154 (2007). Article

  ADS  CAS  Google Scholar  * Stamenkovic, V. R. et al. Trends in electrocatalysis on extended and nanoscale Pt-Bimetallic alloy surfaces. Nat. Mater. 6, 241–247 (2007). Article  ADS  CAS 

PubMed  Google Scholar  * Wang, C. et al. Rational development of ternary alloy electrocatalysts. J. Phys. Chem. Lett. 3, 1668–1673 (2012). Article  CAS  PubMed  Google Scholar  * Wang, C.,

Markovic, N. M. & Stamenkovic, V. R. Advanced platinum alloy electrocatalysts for the oxygen reduction reaction. ACS Catal. 2, 891–898 (2012). Article  CAS  Google Scholar  * Greeley, J.

et al. Alloys of platinum and early transition metals as oxygen reduction electrocatalysts. Nat. Chem. 1, 552–556 (2009). Article  CAS  PubMed  Google Scholar  * Strasser, P. et al.

Lattice-strain control of the activity in dealloyed core-shell fuel cell catalysts. Nat. Chem. 2, 454–460 (2010). Article  CAS  PubMed  Google Scholar  * Karan, H. I. et al. Catalytic

activity of platinum monolayer on Iridium and Rhenium alloy nanoparticles for the oxygen reduction reaction. ACS Catal. 2, 817–824 (2012). Article  CAS  Google Scholar  * Srivastava, R.,

Mani, P., Hahn, N. & Strasser, P. Efficient oxygen reduction fuel cell electrocatalysis on voltammetrically dealloyed Pt–Cu–Co nanoparticles. Angew. Chem. Int. Ed. 46, 8988–8991 (2007).

Article  CAS  Google Scholar  * Kongkanand, A., Kuwabata, S., Girishkumar, G. & Kamat, P. Single-wall carbon nanotubes supported platinum nanoparticles with improved electrocatalytic

activity for oxygen reduction reaction. Langmuir 22, 2392–2396 (2006). Article  CAS  PubMed  Google Scholar  * Guo, S. & Sun, S. FePt nanoparticles assembled on graphene as enhanced

catalyst for oxygen reduction reaction. J. Am. Chem. Soc. 134, 2492–2495 (2012). Article  CAS  PubMed  Google Scholar  * Zhang, Y. et al. Nanoporous graphitic-C3N4@carbon metal-free

electrocatalysts for highly efficient oxygen reduction. J. Am. Chem. Soc. 133, 20116–20119 (2011). Article  PubMed  CAS  Google Scholar  * Yang, S., Feng, X., Wang, X. & Müllen, K.

Graphene-based carbon nitride nanosheets as efficient metal-free electrocatalysts for oxygen reduction reactions. Angew. Chem. Int. Ed. 50, 5339–5343 (2011). Article  CAS  Google Scholar  *

Yu, D., Zhang, Q. & Dai, L. Highly efficient metal-free growth of nitrogen-doped single-walled carbon nanotubes on plasma-etched substrates for oxygen reduction. J. Am. Chem. Soc. 132,

15127–15129 (2010). Article  CAS  PubMed  Google Scholar  * Liu, Z. W. et al. Phosphorus-doped graphite layers with high electrocatalytic activity for the O2 reduction in an alkaline medium.

Angew. Chem. Int. Ed. 50, 3257–3261 (2011). Article  CAS  Google Scholar  * Choi, C. H., Park, S. H. & Woo, S. I. Binary and ternary doping of nitrogen, boron and phosphorus into carbon

for enhancing electrochemical oxygen reduction activity. ACS Nano 6, 7084–7091 (2012). Article  CAS  PubMed  Google Scholar  * Cruz-Silva, E. et al. Electronic transport and mechanical

properties of phosphorus- and phosphorus-nitrogen-doped carbon nanotubes. ACS Nano 3, 1913–1921 (2009). Article  CAS  PubMed  Google Scholar  * Zhang, L. & Xia, Z. Mechanisms of oxygen

reduction reaction on nitrogen-doped graphene for fuel cells. J. Phys. Chem. C 115, 11170–11176 (2011). Article  CAS  Google Scholar  * Gong, K., Du, F., Xia, Z., Durstock, M. & Dai, L.

Nitrogen-doped carbon nanotube arrays with high electrocatalytic activity for oxygen reduction. Science 323, 760–764 (2009). Article  ADS  CAS  PubMed  Google Scholar  * Yang, L. et al.

Boron-doped carbon nanotubes as metal-free electrocatalysts for the oxygen reduction reaction. Angew. Chem. Int. Ed. 50, 7132–7135 (2011). Article  CAS  Google Scholar  * Yang, Z. et al.

Sulfur-doped graphene as an efficient metal-free cathode catalyst for oxygen reduction. ACS Nano 6, 205–211 (2011). Article  CAS  PubMed  Google Scholar  * Sun, X. J. et al. A class of high

performance metal-free oxygen reduction electrocatalysts based on cheap carbon blacks. Sci. Rep. 3, 2505 (2013). Article  PubMed  PubMed Central  Google Scholar  * Zhang, Y. et al.

Manageable N-doped graphene for high performance oxygen reduction reaction. Sci. Rep. 3, 2771 (2013). Article  PubMed  PubMed Central  Google Scholar  * Hu, X., Wu, Y., Li, H. & Zhang,

Z. Adsorption and activation of O2 on nitrogen-doped carbon nanotubes. J. Phys. Chem. C 114, 9603–9607 (2010). Article  CAS  Google Scholar  * Ikeda, T. et al. Carbon alloy catalysts: active

sites for oxygen reduction reaction. J. Phys. Chem. C 112, 14706–14709 (2008). Article  CAS  Google Scholar  * Lin, S. S. Light-emitting two-dimensional ultrathin silicon carbide. J. Phys.

Chem. C 116, 3951–3955 (2012). Article  CAS  Google Scholar  * Eichler, A. & Hafner, J. Molecular precursors in the dissociative adsorption of O2 on Pt(111). Phys. Rev. Lett. 79,

4481–4484 (1997). Article  ADS  CAS  Google Scholar  * Eichler, A., Mittendorfer, F. & Hafner, J. Precursor-mediated adsorption of oxygen on the (111) surfaces of platinum-group metals.

Phys. Rev. B 62, 4744–4755 (2000). Article  ADS  CAS  Google Scholar  * Zhang, P., Lian, J. S. & Jiang, Q. Potential dependent and structural selectivity of the oxygen reduction reaction

on nitrogen-doped carbon nanotubes: a density functional theory study. Phys. Chem. Chem. Phys. 14, 11715–11723 (2012). Article  CAS  PubMed  Google Scholar  * Sha, Y., Yu, T. H., Liu, Y.,

Merinov, B. V. & Goddard, W. A. Theoretical study of solvent effects on the platinum-catalyzed oxygen reduction reaction. J. Phys. Chem. Lett. 1, 856–861 (2010). Article  CAS  Google

Scholar  * Keith, J. A., Jerkiewicz, G. & Jacob, T. Theoretical investigations of the oxygen reduction reaction on Pt(111). ChemPhysChem 11, 2779–2794 (2010). Article  CAS  PubMed 

Google Scholar  * Thorum, M. S., Hankett, J. M. & Gewirth, A. A. Poisoning the oxygen reduction reaction on carbon-supported Fe and Cu electrocatalysts: evidence for metal-centered

activity. J. Phys. Chem. Lett. 2, 295–298 (2011). Article  CAS  Google Scholar  * Zhang, P., Chen, X. F., Lian, J. S. & Jiang, Q. Structural selectivity of CO oxidation on Fe/N/C

catalysts. J. Phys. Chem. C 116, 17572–17579 (2012). Article  CAS  Google Scholar  * Blyholder, G. Molecular orbital view of chemisorbed carbon monoxide. J. Phys. Chem. 68, 2772–2777 (1964).

Article  CAS  Google Scholar  * Mazzone, G., Rivalta, I., Russo, N. & Sicilia, E. Interaction of CO with PdAu(111) and PdAu(100) bimetallic surfaces: a theoretical cluster model study.

J. Phys. Chem. C 112, 6073–6081 (2008). Article  CAS  Google Scholar  * Liu, Z. P. & Hu, P. General trends in CO dissociation on transition metal sufaces. J. Chem. Phys. 114, 8244–8247

(2001). Article  ADS  CAS  Google Scholar  * Michaelides, A. et al. Identification of general linear relationships between activation energies and enthalpy changes for dissociation reactions

at sufaces. J. Am. Chem. Soc. 125, 3704–3705 (2003). Article  CAS  PubMed  Google Scholar  * Bligaard, T. et al. The Brønsted–Evans–Polanyi relation and the volcano curve in heterogeneous

catalysis. J. Catal. 224, 206–217 (2004). Article  CAS  Google Scholar  * Keith, J. A. & Jacob, T. Theoretical studies of potential-dependent and competing mechanisms of the

electrocatalytic oxygen reduction reaction on Pt(111). Angew. Chem. Int. Ed. 49, 9521–9525 (2010). Article  CAS  Google Scholar  * Nørskov, J. K. et al. Origin of the overpotential for

oxygen reduction at a fuel-cll cathode. J. Phys. Chem. B 108, 17886–17892 (2004). Article  CAS  Google Scholar  * Lim, D. H. & Wilcox, J. Mechanisms of the oxygen reduction reaction on

defective graphene-supported Pt nanoparticles from first-principles. J. Phys. Chem. C 116, 3653–3660 (2012). Article  CAS  Google Scholar  * Eyring, H. The activated complex in chemical

reactions. J. Chem. Phys. 3, 107–115 (1935). Article  ADS  CAS  Google Scholar  * Gao, W., Keith, J. A., Anton, J. & Jacob, T. Theoretical elucidation of the competitive

electro-oxidation mechanisms of formic acid on Pt(111). J. Am. Chem. Soc. 132, 18377–18385 (2010). Article  PubMed  CAS  Google Scholar  * Wang, C. M., Fan, K. N. & Liu, Z. P. Origin of

oxide sensitivity in gold-based catalysts: a first principle study of CO oxidation over Au supported on monoclinic and tetragonal ZrO2 . J. Am. Chem. Soc. 129, 2642–2647 (2007). Article  CAS

  PubMed  Google Scholar  * Ou, L., Yang, F., Liu, Y. & Chen, S. First-principle study of the adsorption and dissociation of O2 on Pt(111) in acidic media. J. Phys. Chem. C 113,

20657–20665 (2009). Article  CAS  Google Scholar  * Delley, B. An all-electron numerical method for solving the local density functional for polyatomic molecules. J. Chem. Phys. 92, 508–517

(1990). Article  ADS  CAS  Google Scholar  * Delley, B. From molecules to solids with the DMol3 approach. J. Chem. Phys. 113, 7756–7764 (2000). Article  ADS  CAS  Google Scholar  * Perdew,

J. P., Burke, K. & Ernzerhof, M. Generalized gradient approximation made simple. Phys. Rev. Lett. 77, 3865–3868 (1996). ADS  CAS  PubMed  Google Scholar  * Graziano, G., Klimeš, J.,

Fernandez-Alonso, F. & Michaelides, A. Improved description of soft layered materials with van der Waals density functional theory. J. Phys.: Condens. Matter 24, 424216 (2012). ADS 

Google Scholar  * Carrasco, J., Klimeš, J. & Michaelides, A. The role of van der Waals forces in water adsorption on metals. J. Chem. Phys. 138, 024708 (2013). Article  ADS  PubMed  CAS

  Google Scholar  * Björkman, T., Gulans, A., Krasheninnikov, A. V. & Nieminen, R. M. van der Waals bonding in layered compounds from advanced density-functionals first-principles

calculations. Phys. Rev. Lett. 108, 235502 (2012). Article  ADS  PubMed  CAS  Google Scholar  * Schimaka, L. et al. Accurate surface and adsorption energies from many-body perturbation

theory. Nat. Mater. 9, 741–744 (2010). Article  ADS  CAS  Google Scholar  * Liu, P. & Rodriguez, J. A. Catalysts for hydrogen evolution from the [NiFe] hydrogenase to the Ni2P(001)

surface: the importance of ensemble effect. J. Am. Chem. Soc. 127, 14871–14878 (2005). Article  CAS  PubMed  Google Scholar  * Rodriguez, J. A. et al. Activity of CeOx and TiOx nanoparticles

grown on Au(111) in the water-gas shift reaction. Science 318, 1757–1760 (2007). Article  ADS  CAS  PubMed  Google Scholar  * Wu, Z. et al. Structure sensitivity of low-temperature NO

decomposition on Au surfaces. J. Catal. 304, 112–122 (2013). Article  CAS  Google Scholar  * Klamt, A. & Schüürmann, G. COSMO: A new approach to dielectric screening in solvents with

explicit expressions for the screening energy and its gradient. J. Chem. Soc. 2, 799–805 (1993). Google Scholar  * Janik, M. J., Taylor, C. D. & Neurock, M. First-principles analysis of

the initial electroreduction steps of oxygen over Pt(111). J. Electrochem. Soc. 156, B126–B135 (2009). Article  CAS  Google Scholar  * Tripkovic, V., Skúlason, E., Siahrostami, S., Nørskov,

J. K. & Rossmeisl, J. The oxygen reduction reaction mechanism on Pt(111) from density functional theory calculations. Electrochim. Acta 55, 7975–7981 (2010). Article  CAS  Google Scholar

  * Yu, L., Pan, X., Cao, X., Hu, P. & Bao, X. Oxygen reduction reaction mechanism on nitrogen-doped graphene: a density functional theory study. J. Catal. 282, 183–190 (2011). Article 

CAS  Google Scholar  * Roudgar, A., Eikerling, M. & van Santen, R. Ab initio study of oxygen reduction mechanism at Pt4 cluster. Phys. Chem. Chem. Phys. 12, 614–620 (2010). Article  CAS

  PubMed  Google Scholar  Download references ACKNOWLEDGEMENTS This work was financially supported by National Key Basic Research and Development Program (Grant No. 2010CB631001), Natural

Science Foundation of Jiangsu (No. SBK201341900), China Postdoctoral Science Foundation (No. 2013M541611) and the Senior Intellectuals Fund of Jiangsu University (No. 12JDG094 and 13JDG032).

We acknowledge the supports from High Performance Computing Center (HPCC) of Jilin University. AUTHOR INFORMATION AUTHORS AND AFFILIATIONS * Key Laboratory of Automobile Materials, Ministry

of Education, and Department of Materials Science and Engineering, Jilin University, Changchun, 130022, China P. Zhang, B. B. Xiao, X. L. Hou, Y. F. Zhu & Q. Jiang * Institute for

Advanced Materials and School of Materials Science and Engineering, Jiangsu University, Zhenjiang, 212013, China P. Zhang & X. L. Hou Authors * P. Zhang View author publications You can

also search for this author inPubMed Google Scholar * B. B. Xiao View author publications You can also search for this author inPubMed Google Scholar * X. L. Hou View author publications You

can also search for this author inPubMed Google Scholar * Y. F. Zhu View author publications You can also search for this author inPubMed Google Scholar * Q. Jiang View author publications

You can also search for this author inPubMed Google Scholar CONTRIBUTIONS P.Z. conceived the initial idea of this research, performed the computer simulations and wrote the paper. B.B.X.,

X.L.H., Y.F.Z. and Q.J. participated in the discussion and revised the manuscript. Y.F.Z. and Q.J. guided the work. ETHICS DECLARATIONS COMPETING INTERESTS The authors declare no competing

financial interests. ELECTRONIC SUPPLEMENTARY MATERIAL SUPPLEMENTARY INFORMATION Layered SiC Sheets: A Potential Catalyst for Oxygen Reduction Reaction RIGHTS AND PERMISSIONS This work is

licensed under a Creative Commons Attribution 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by/3.0/ Reprints and permissions ABOUT THIS

ARTICLE CITE THIS ARTICLE Zhang, P., Xiao, B., Hou, X. _et al._ Layered SiC Sheets: A Potential Catalyst for Oxygen Reduction Reaction. _Sci Rep_ 4, 3821 (2014).

https://doi.org/10.1038/srep03821 Download citation * Received: 04 July 2013 * Accepted: 03 January 2014 * Published: 22 January 2014 * DOI: https://doi.org/10.1038/srep03821 SHARE THIS

ARTICLE Anyone you share the following link with will be able to read this content: Get shareable link Sorry, a shareable link is not currently available for this article. Copy to clipboard

Provided by the Springer Nature SharedIt content-sharing initiative