Computational exploration of borophane-supported single transition

Computational exploration of borophane-supported single transition metal atoms as potential oxygen reduction and evolution electrocatalysts Yashpal Singh, Seoin Back, and Yousung Jung* Graduate School of EEWS, Korea Advanced Institute of Science and Technology (KAIST), 291 Daehak-ro 34141, Daejeon

Synthesis of novel monolayer-boron (borophene) is a recent addition to the family of 2D materials. In particular, full surface hydrogenation of triangular borophene (borophane (BH)) to passivate empty p orbital in boron is identified as a new stable 2D material that possesses direction-dependent Dirac cones similar to graphene. By a series of density functional theory (DFT) computations, we investigated the potential of single transition metal atom supported on borophane with a vacancy (TM-BH system) as an efficient ORR/OER electrocatalyst for its applications in renewable energy technologies. In TM-BH systems, the coupling of d-orbitals of TM dopant with the p-orbitals of surrounding boron atoms results in an increase in the density of states near Fermi-level generating an active site to facilitate ORR/OER via an efficient four-electron transfer mechanism. Among the considered TM-BH systems, Fe-BH and RhBH were found to be the promising ORR electrocatalysts with an overpotential (ORR) of 0.43 V and 0.47 V, respectively, whereas, for OER, Rh-BH with 0.24 V has the smallest OER value followed by Co-BH (0.37 V), under the equilibrium electrode potential. These ORR and OER values indicate higher activities than the currently most active ORR (Pt(111) (0.63 V)) and OER (rutile-type RuO2 (0.37 V)) electrocatalysts.

1. INTRODUCTION The demand for the development of the low-cost, highly efficient and pollution-free renewable energy technologies for power generation and storage, such as proton exchange membrane (PEM) fuel cells, water splitting, and metal-air batteries, has grown tremendously. 1–5 The efficiencies of these technologies are highly limited by the sluggish kinetics of oxygen reduction (ORR) and evolution (OER) reactions that require electrocatalysts based on noble metals/alloys (such as Pt, Pt3Ni) and metal oxides (RuO2, MnO2, and IrO2) to boost those reactions.6–10

However, scarcity, high cost and poor stability of above

metals/metal-oxides hamper the large-scale production of

energy systems based on these

electrocatalysts.11–13 Hence, intensive research efforts have been devoted to the development of efficient, cost-effective and more abundant electrocatalysts to serve the purpose. The strategies toward the quest for optimum ORR catalysts mainly follow the approaches such as, (i) Pt-alloys or Pt-overlayers to reduce Pt usage, (ii) metal-nitrides/chalcogenides containing non-precious metals, and (iii) p-block element based materials such as (B-,N-,S-,P-) doped graphene and other carbon-based nanomaterials.14–35 However, for an ideal OER electrocatalyst, attempts are being made by synthesizing layered double hydroxide (LDH) (for e.g. Ni-Fe/Co/V-LDH)36–39 and transition-metal oxyhydroxide (CoO(OH), CrO(OH), etc.)40,41. In addition, low-cost perovskite materials with high intrinsic activities have shown OER activity as par with “gold standards” RuO2 and IrO2.42–47 With the success of graphene-based materials, various other two-dimensional (2D) atomic crystals have recently attracted a considerable interest for their possible applications in catalysis.48,49 In recent past, Mannix et al. have synthesized an atomically thin 2D triangular boron sheet (t-borophene) on Ag(111) surface by physical vapour deposition technique.50 In another parallel experimental work, Feng et al. using similar technique have grown and boron sheets with hexagonal holes on Ag(111) surface.51 Such new developments in the sector of 2D structures have opened up an interesting research area to 2

explore novel physical and mechanical properties of borophenes.52–56 The ability of boron to form multicentred bonds due to the presence of electron-deficient orbitals allows borophene to get oxidized easily in the ambient environment but makes it challenging to experimentally realize free-standing 2D borophene.53,54 As a way of stabilizing borophene, passivating it with hydrogen atoms has been proposed which result in the generation of direction-dependent Dirac cones due to the removal of unfilled orbitals near Fermi level.57,58 Moreover, the B-H group in borophane can be viewed as isoelectronic with C, and thus like pristine graphene, the catalytic activity (for e.g. ORR/OER) of pristine borophane is expected to be inert or negligibly small. In contrast to chemical inertness, borophane possess an ultrahigh Femi velocity (3.0 × 106 m/s) that can yield extremely high carrier mobility and can outperform graphene for applications in nanoelectronic devices.57–59 At the same time, the isoelectric character also suggests that, as in graphene, one can also further consider doping the hydrogenated borophene (i.e., borophane (BH)) with main group elements (B, N, S, etc.) or single transition metal atoms for enhanced catalytic activity. Recently, in a series of experiments, Wang and co-workers synthesized Co- and Rh-doped boron clusters in the gas phase.60,61 The planar CoB18 ¯ and quasi-planar RhB18¯ represent a new class of metaldoped boron clusters, suggesting that a metal atom can be doped into the planes of borophene to create metallo-borophene. Inspired by the latter studies, in this paper, we first explore the feasibility of doping BH with a large variety of transition metal (TM) atoms to get transition metal-doped borophane (TM-BH) and report their applications as potentially efficient bi-functional ORR/OER electrocatalyst using firstprinciples calculations.

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2. COMPUTATIONAL MODELS AND METHODS Our calculations were performed using density functional theory (DFT) as implemented in the Vienna Ab-initio Simulation Package (VASP)62,63 with projector-augmented-wave (PAW) method.64 The revised Perdew-Burke-Ernzerhof (RPBE) functional within generalized gradient approximation (GGA) is used to describe exchange-correlation functional effects.65,66 Well known density functionals such as RPBE lag in accounting correct behaviour of van der Waals (vdW) interactions resulting from the dynamical correlations between fluctuating charge distribution, thus they are intrinsically unsuitable to accurately optimize lattice parameters in molecular materials including layered structures.67–69 Moreover, Tripkovic in his recent work shows that the inclusion of vdW interaction reproduces the right experimental trend in the ORR activity for Pt and its alloys.70 Therefore in this work, we account for vdW interactions by using the empirical dispersion correction scheme proposed by Grimme and implemented in VASP as a DFTD3 method with zero damping.71 A plane wave basis set was generated with a kinetic energy cutoff of 500 eV and k-points were sampled using 3  3  1 Monkhorst-Pack mesh.72 To optimize borophane, a periodic supercell with 36 B-H pairs were considered and a vacuum of 20 Å in z-direction was set to avoid unwanted interactions between periodic images. To construct TM-BH, the supercell is optimized by simply replacing a B-H pair with a TM atom. Due to the existence of magnetic TM atom, spin polarization was considered in all the calculations. Further, we perform Bader population analysis to probe charge transfer due to TM dopant in TM-BH. The effect of water solvent environment is considered through a polarized continuum model of Hennig and co-workers as implemented in VASP as VASPsol with the dielectric constant set as 78.4.73 The ORR has been proposed to follow a four-electron (4e) reduction pathway and the elementary reaction steps are given as 74 (OER is the reverse process of ORR)75–77

∗ + O2 + (H + + e− ) ↔ ∗ OOH

(1)

4

and

∗ OOH + (H + + e− ) ↔ ∗ O + H2 O

(2)

∗ O + (H + + e− ) ↔ ∗ OH

(3)

∗ OH + (H + + e− ) ↔ 2H2 O

(4)

where * denotes the active site of the catalyst. The possible dissociative mechanism for ORR requires at least two catalytic sites, to begin with, therefore, not feasible on single atom catalyst like TM-BH systems with inert BH plane (Figure S1.). The change in Gibbs free energy (∆G) of each elementary reaction step is computed using the computational hydrogen electrode (CHE) model of Nørskov and co-workers, where the chemical potential of proton-electron pair is assumed to be equivalent to that of half of the hydrogen gas at standard conditions (μ(H+ + e-) = 0.5μ(H2) at pH=0, 101325 Pa of H2, 298.15 K).77–79 The effect of electrode potential on the chemical potential of an electron can be expressed as ∆GU = –neU, where n is the number of electrons transferred and U is the applied electrode potential. The explicit contribution to ∆G is defined as, ∆G= ∆E + ∆Ezpe – T∆S + ∆GpH + ∆GU,80 where, the reaction energy ∆E is directly obtained from DFT calculations, ∆Ezpe is the change in zero-point energy, T is temperature (298.15K), ∆S is the change in entropy. ∆GpH =2.303kBT *pH represents free energy contribution due to variations in the H+ concentration, where kB is the Boltzmann constant and pH is assumed to be zero for acidic medium. The entropies and the vibrational frequencies of the adsorbed species and molecules in the gas-phase were calculated using harmonic oscillator approximation at 298.15 K. Thus, based on the above reaction steps, the thermodynamic overpotential of the ORR and OER (ORR andOER) for a given electrocatalyst is obtained as:

ηORR = [

max(ΔG1 ,ΔG2 ,ΔG3 ,ΔG4 ) 𝑒−

] V, at U=1.23 V

and, ηOER = [max(−ΔG4 ,−ΔG3 ,− ΔG2 ,−ΔG1 )] V, at U=1.23 V 𝑒−

(5) (6)

5

Figure 1. Top and side views of the optimized structures of borophene (a) with lattice constants a and b given as 1.67 and 2.89 Å respectively, and for borophane (BH) (b) with parameters a and b as 1.94 Å and 2.82 Å, respectively. A total density of states (DOS) of borophene (c) and BH (d) are centered at Fermi energy. where, ΔG1 , ΔG2 , ΔG3 , and ΔG4 , are the Gibbs free energies of the reactions shown in equations (1) to (4), respectively, in the forward direction at equilibrium potential (U=1.23 V).

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Figure 2. (a) Binding energies of various 3d, 4d, and 5d transition metal incorporated in BH matrix (TMBH systems. (b) Reaction free energy corresponding to equation (7) showing stability of TM-BH systems against metal dissolution at pH=0.

3. RESULTS AND DISCUSSIONS 3.1 Structure and Stability of Borophane We consider the anisotropic buckled structure of borophene as synthesized by Mannix et al., where the adjacent rows of boron atoms are arranged in a zigzag fashion.50 The top and side view of an optimized structure of borophene along with its total density of states (DOS) is shown in Figure 1. (a) and (c), where, lattice parameters a and b are given as 1.67 and 2.89 Å, respectively, that agree well with the experiment (a=1.7(±0.1) Å, b=2.9(±0.2) Å.50 In order to construct BH, the optimized borophene is passivated by hydrogen atoms alternately on the opposite side of the sheet along the zigzag direction.57 The top and side view of the fully relaxed BH structure is displayed in Figure 1. (b) with the lattice constants a and b evaluated as 1.94 Å and 2.82 Å, respectively. As pointed out by Xu et al., we also noticed, hydrogenation leads to the stretching of parameter a by 0.27 Å, however, b is decreased by 0.07 Å which is insignificant when compared to the former. To gain insights into the catalytic nature of BH, we compare its DOS (Figure 1. (d)) with that of borophene (Figure 1. (c)). We find that most of the states in borophene near Fermi level disappeared due to hydrogenation.57 Therefore, like graphene which is a zero-overlap

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Figure 3. (a) Top and side views of a typical optimized single transition metal-doped borophane (TM-BH) where dTM-B1 and dTM-B2 are the bond lengths along and perpendicular to the zigzag directions, respectively. (b) The partial density of states (PDOS) of Fe-, Rh-, and Co-BH systems. The vertical dashed lines represent the Fermi level rescaled to zero. Black and blue lines represent the d-orbital of TM and p-orbital of B atoms, respectively.

semimetal with a very low density of states at the Fermi level, BH is expected to behave as a catalytically inert system (see Figure S1. in supporting information (SI)). Further, we observed that all the ORR/OER intermediates including H atom move away from the BH surface during geometry optimization and bind non-covalently to the surface, suggesting that the surface won’t change as the ORR/OER reaction proceeds.

3.2 Transition Metal-doped Borophane (TM-BH): Structure and Stability Analysis The practical use of an electrocatalyst depends upon its stability in realistic catalytic conditions of the fuel cells. The main issue with single atom catalysts is the dissolution of metal atoms due to its weak bonding with the surrounding non-metal atoms.81,82 In this work, we consider several 3d, 4d and 5d TM atoms (Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Y, Zr, Nb, Ru, Rh, Pd, Ag, Hf, Ta, W, Re, Os, Ir, Pt, and Au) as a dopant in the BH matrix. The stability of transition metal-doped borophane (TM-BH ) systems can be judged by the values of their binding energy (Eb) defined as Eb = ETM-BH – ( 𝜇TM +EBHv), where ETMBH,

EBHv are total energies of TM-BH system, and BH with vacancy, respectively. 𝜇TM is the chemical 8

Figure 4. The linear scaling relationship among the Gibbs free binding energies of the ORR/OER intermediates (*OOH (∆G*OOH), *O (∆G*O), and *OH (∆G*OH)). The red line represents the universal scaling relation [76,85] and the green triangles represent the positions of an ideal ORR catalyst.

potential of TM atom evaluated considering their most stable bulk crystal. Since 𝜇TM is referenced with respect to the bulk metal, negative values of Eb (Figure 2. (a)) represent higher tendency to form TM-BH complex rather than the cohesion of TM atoms and vice versa. It is clearly seen that except for Ag other 3d- and 4d- TM atoms show favorable doping in BH matrix with high stability against agglomeration. However, 5d TM-BH systems are prone to metal agglomeration (with Hf-BH and Ta-BH as exceptions) probably due to the larger size of 5d TM atoms. In addition, an estimate of the stability of metal centres against dissolution due to proton attack to the active site can be drawn by evaluating the reaction free energy (ΔGdiss ) of the equation,83 TM − BH + nH + → nH − BH + TM n+ ,

(7)

where n denotes the number of electrons transferred (or oxidation state for TM atom) and nH-BH represents the BH with TM vacancy occupied by n hydrogen atoms. The positive and negative values of

9

ΔGdiss provide an estimate of the stability and instability of the TM-BH systems against dissolution at pH=0. In order to simplify computation, we split equation (7) into two half-reactions as,

and,

TM − BH + n(H + + e− ) → nH − BH + TM

(8)

TM → ne− + TM n+

(9)

with ΔGa (evaluated using DFT) and ΔGb (obtained from the values of standard dissolution potential (Udiss))84 as the reaction free energies, respectively, such that, ΔGdiss = ΔGa + ΔGb . We plot the ΔGdiss values (In Figure 2. (b)) for all the considered TM-BH systems showing several (Co-, Ni-, Cu-, Zr-, Nb-, Ru-, Rh-, Pd-, Ag-, Re-, Ir-, Pt-, and Au-BH) systems as stable against dissolution at 0 pH. Moreover, the minimum pH for which the Gibbs free energy of Eq. 7 is zero can be evaluated using pHmin = −ΔGdiss /(n ∗ 0.0591) (Table S1.) Thus, based on the values of pHmin, Fe-BH is found to be stable against dissolution at pH=0.63. Here it is important to point out that the trend in the stability against metal centre dissolution is inverse to that of the binding energies (with some exceptions). Most of the 3d TM-BH systems are sensitive to proton attack despite being strongly bonded in BH matrix which is opposite of 5d systems. Further, most 4d TM-BH systems show optimum stabilization against both agglomeration and dissolution.

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Figure 5. Colored counter plots representing (a) ORR, and (b) OER activity volcanos for TM-BH systems showing the thermodynamic overpotentials ORR and OER as a function of Gibbs binding free energies of reaction intermediates, respectively. The color bars values of ORR and OER are in units of V vs RHE.

In Figure 3. (a), we show the top and side views of a typical optimized structure of TM-BH system with dTM-B1 and dTM-B2 as the bond lengths along and perpendicular to the zigzag directions (for bond lengths see Table S2. in SI). To understand how doped metal atoms affected the electronic structure of the system, we first compute projected DOS (PDOS) (Figure 3. (b)). Notably, we observe an increase in the DOS at Fermi level compared to pristine BH due to hybridization of d-orbitals of TM atom with porbitals of surrounding boron atoms (for PDOS of other TM-BH systems see Figure S2. of SI). Secondly, 11

Figure 6. The computed reaction free energy diagram of (a) ORR on Fe-BH and Rh-BH, (b) OER on CoBH and Rh-BH at the equilibrium potential (U=1.23 V). The numbers shown are corresponding to the potential determining steps that determine thermodynamic overpotential of the respective electrocatalysts.

we perform Bader population analysis that showed non-uniform charge distribution due to the dopant TM atom (Bader charges on TM atoms are shown in Table S2.).

3.3 Thermodynamics of ORR and OER on TM-BH systems The intrinsic catalytic activity of an ORR/OER electrocatalyst was found to be highly correlated with the optimal binding strength of reaction intermediates (*O, *OOH, *OH).76,85 The Gibbs free binding energies of these intermediates are calculated using following relation,76,85 ∆G∗OOH = G∗OOH – G(∗) − [ 2 GH2 O –

3 G ] 2 H2

∆G∗O = G∗O – G(∗) − [ GH2 O – GH2 ] and,

∆G∗OH = G∗OH – G(∗) − [GH2 O –

1 G ] 2 H2

(8) (9) (10)

where G(∗), G∗OOH , G∗O , and, G∗OH are the ground state free energies of clean TM-BH surface and TMBH with adsorbed *OOH, *O, and *OH species, respectively. GH2 O and GH2 are the free energies of H2O and H2 molecules calculated using DFT. It has been observed that ∆G∗OOH and ∆G∗OH for various metal (111) facets and oxide surfaces follow a universal scaling relation of ∆G∗OOH = ∆G∗OH + 3.20, regardless 12

Figure 7. Top and side view of the optimized structures of various reaction intermediates on Rh-doped borophane (Rh-BH). All lengths are given in the units of Å.

of the binding site.76,85 In order to determine a similar scaling relation, we plot ∆G∗OOH against ∆G∗OH for all the considered TM-BH systems as shown in Figure 4. Our regression analysis yields a strong linear relationship between ∆G∗OOH and ∆G∗OH (with a coefficient of determination, R2=0.96) as ∆G∗OOH = 0.80∆G∗OH + 3.11 with the slope less than unity. Smaller slope compared to the universal scaling relation (shown in red dashed line in Figure 4.) indicates a shift in the relative binding strengths of *OOH and *OH on the surfaces of TM-BH systems probably due to the differences in the covalent character of the TM atoms embedded in the BH matrix.86 Moreover, smaller slope brings certain TM-BH systems like Rh-BH, Fe-BH, Ru-BH closer towards the region of the ideal catalyst (shown by the top green triangle in Figure 4.). In order to identify the most the efficient electrocatalyst for ORR and OER processes, we evaluate and compare ORR and OER values given by equations (5) and (6), respectively for all the TM-BH systems. Numerically, ORR (OER) is a function of four variables (ΔG1 , ΔG2 , ΔG3 , and, ΔG4 ) that can be

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reduced to two independent variables on applying constraints obtained from the scaling relation and ΔG1 + ΔG2 + ΔG3 + ΔG4 = 4.92 eV (standard Gibbs energy of formation of water from 2H2 and O2)76,78 such that:

and

ΔG1 = 4.92 − (0.80ΔG∗OH + 3.11)

(11)

ΔG2 = (0.80ΔG∗OH + 3.11) − ΔG∗O

(12)

ΔG3 = ΔG∗O − ΔG∗OH

(13)

ΔG4 = ΔG∗OH

(14)

Thus, knowing only two descriptors ΔG∗OH and (ΔG∗O − ΔG∗OH ) are sufficient for the calculation of ORR (OER).87 In Figure 5 (a), we display the two-dimensional volcano plot showing the ORR activity trends through ORR as a function of descriptors ΔG∗OH , and (ΔG∗O − ΔG∗OH ) as the independent descriptors. The blue part of the plot shows the region of highest activity with ORR = 0.23 V under the optimum condition (ΔG1 = ΔG4 = 1.0 eV). Based on the above activity volcano, Fe-BH (ORR = 0.43 V) is found to be the most efficient electrocatalyst followed by Rh-BH (0.47 V) with fourth (*OH desorption) and first (*OOH formation) protonation steps as the potential determining step (PDS), respectively (Figure 6 (a)). The ORR values of Fe- and Rh-BH are highly competitive and noticeably smaller than Pt (111) (~0.63 V)70, graphene/carbon nanofiber functionalized with TM atoms (Fe-N-carbon nanofiber 0.54 V)88,89 and Fe-doped 2D hexaaminobenzene-based coordination polymers (2D-HAB-CPs) (0.52 V)87. It is important to mention here that we observe non-covalent adsorption of O2 molecule on Fe-BH and Rh-BH with binding energies -0.13 eV and -0.10 eV, respectively. Since O2 adsorption is not an essential requirement for ORR, we expect the first electron transfer to take place in the outer Helmholtz plane (ET-OHP mechanism) similar to what is observed by Choi et al. on N-doped graphene.90 The catalytic activity of OER can be maximum when ΔG∗O lies in the middle between the ΔG∗OOH and ΔG∗OH values such that ΔG3 = ΔG2 . Systems like Rh-BH and Co-BH closely satisfy the above criterion along with their ΔG∗O values lie close to the ideal catalyst (green triangle) as can be seen from the scaling graph of ΔG∗O 𝑣𝑠 ΔG∗OH (see Figure 4.). Further, the two-dimensional volcano plot (Figure 5 14

(b)) that peaks at ORR = 0.19 V in the optimum condition (ΔG4 = ΔG3 = ΔG2 = 1.42 eV) show Rh-BH and Co-BH as the most active OER catalysts with ORR values as 0.24 V and 0.37 V, respectively. Free energy diagram of Rh-BH and Co-BH as displayed in Figure 6 (b) at U=1.23 V show *O and *OOH formations as the PDS with largest free energy change of 0.24 eV and 0.37 eV, respectively. Interestingly, 2D-HAB-CPs doped with same TM atoms (Rh- and Co-) have been identified as the best OER electrocatalyst with ORR values as 0.32 V and 0.41 V, respectively.87 A similar trend in the OER activity has also been observed on TM doped graphitic material with Rh followed by Co dopants as the best OER catalysts.88 Moreover, OER value of Rh-BH is found to be even lower than currently best rutile-type RuO2 (0.37 V) and IrO2 (0.52 V) based on DFT studies.77 Considering Rh-BH as the representative system, we display top and side views of the optimized structures of various reaction intermediates in Figure 7. It is to be noted here that Rh-BH with ORR and OER values as 0.47 V and 0.24 V can be single-handedly used as a bi-functional ORR/OER electrocatalyst which is important for the development of unitized regenerative fuel cells and water electrolyzer systems.91,92 In order to negate the possibility of interaction between H on BH with the ORR/OER intermediates, we evaluate energy required to remove single H atom from the BH surface using 𝐸vac = 𝐸BHvac − (𝐸BH − 𝜇H ), where 𝜇H is the chemical potential of H atom, 𝐸BHvac , and, 𝐸BH are the DFT energies of BH with and without H vacancy. The vacancy formation energy is obtained as 0.50 eV which is positive and higher than the thermodynamic barriers of our best Fe-, Rh-, and Co-BH systems.

4

CONCLUSIONS

By means of spin-polarized DFT computations, we investigated borophane (BH) with a vacancy as a support material for single transition metal atom (TM-BH systems) as a potential ORR/OER electrocatalyst. In our screening, we considered 3d (Mn, Fe, Co, Ni, Cu), 4d (Y, Zr, Nb, Ru, Rh, Pd), and 5d (Hf, Ta, W, Re, Os, Ir, Pt, Au) TM atoms as the dopant in TM-BH systems. The most important findings are summarized in the following points:

15

(i)

Like graphene, pristine BH is catalytically inert due to its weak binding towards ORR/OER reaction intermediates.

(ii)

Based on the results of binding energies, we found 3d and 4d TM atoms show stronger binding to the vacancy in BH matrix as compared to 5d TM atoms, whereas, reaction free energies for metal centre dissolution yield superior stability for late TM atoms at 0 pH.

(iii)

There exists a strong linear scaling between the binding free energies of *OOH and *OH species with a slope less than unity probably due to the variation in the covalent character of the late TM atom supported on BH matrix that further pushes several TM-BH systems closer to the ideal catalyst region.

(iv)

According to the volcano plot of ORR, the better electrocatalyst is found to be Fe-BH followed by Rh-BH with an overpotential (ORR) of 0.43 V and 0.47 V, respectively, indeed smaller than Pt (111) (0.63 V), various non-metal (0.38 V and 0.44 V for B- and N-doped graphene nanoribbon) and metal-doped (0.54 V for Fe-N-doped carbon nanofiber) carbon nanomaterials in literature.

(v)

At the top of the OER volcano plot, Rh-BH with an overpotential (OER) of 0.24 V is found to be the most active electrocatalyst considered here which is followed by Co-BH (0.37 V), comparable to or smaller than existing rutile-type RuO2 (0.37 V) and IrO2 (0.52 V) based on DFT studies. It is to be noted that Rh-BH can itself catalyze both ORR/OER reactions with sufficiently low overpotential.

Our DFT computations highlight a promising new family of single atom catalysts based on the 2D material for their possible applications as efficient ORR/OER electrocatalysts. We hope that our work on transition metal supported on borophane could persuade for further theoretical investigations as well as for experimental verifications.

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Supporting Information (Word Style “TE_Supporting_Information”). A listing of the contents of each file supplied as Supporting Information should be included. For instructions on what should be included in the Supporting Information as well as how to prepare this material for publication, refer to the journal’s Instructions for Authors. AUTHOR INFORMATION Corresponding Author *Prof. Yousung Jung, Graduate School of EEWS, Korea Advanced Institute of Science and Technology, 305-335, Daejeon, South Korea.

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