Unveiling the HER and ORR activity origin of isolated Co sites supported on N-doped carbon

. Electrocatalysis plays very important role in clean energy conversion. In which, developing high active and robust electrocatalysts for the hydrogen evolution reaction (HER) and oxygen reduction reaction (ORR) are highly crucial and challenging. Using density functional theory (DFT) calculations, the Gibbs free energy diagrams and electronic structure of N-graphene, Ir-N4 and Co-N4 are investigated in this work. The results show that Co-N4 have the appropriate H adsorption and lower ORR overpotentials, which gives expectation of high multifunctional electrocatalytic performance. This theoretical study provides vital insights into the enhanced electrocatalytic mechanism of Co sites supported on N-doped carbon.


Introduction
Electrochemical reactions, including hydrogen evolution reaction (HER) and oxygen reduction reaction (ORR) are significantly essential in energy-related applications and future renewable energy technologies.Rational developing the highly active and robust electrocatalysis are crucial and desirable.[1].Among various promising alternatives, single-atom catalysts (SACs) with the isolated active sites have attracted tremendous interest, owing to its unique coordination geometry, strong metalsupport interaction and maximum atomic efficiency [2].The above characteristics also offer ideal configurations for exploring the origin of the activity.Though great efforts have been devoted, the electronic structure of SACs has remained elusive and the electrocatalytic activity origin of SACs are still not clear.Recently, single-atom iridium catalysts [3] and singleatom cobalt on N-doped carbon [4] catalysts has been synthesized and exhibited excellent electrochemical ability for ORR.Meanwhile, single-atom catalysts also show good performance in application of HER [5].Hence, gain deep insights into the electronic structure and catalytic mechanism of these catalysts are vitally important.In this work, we investigate the Gibbs free energy diagrams and electronic structure of pure N-doped graphene (N-graphene), isolated iridium on N-doped carbon (Ir-N4) and isolated cobalt on N-doped carbon (Co-N4).The results show that Co-N4 have the appropriate H adsorption and lower overpotentials for ORR.Meanwhile, unique electronic interactions between dopants of Co and N form near the Fermi level, ensuring fast charge transfer kinetics and guaranteeing excellent electrocatalytic performance.

Computational methods
All calculations here are performed by spin polarized density functional theory (DFT) methods using Vienna Ab initio Simulation Package (VASP) [6].The Projector augmented wave (PAW) method is adopted to describe electron-ion interaction [7].And Perdew-Burke-Ernzerhof (PBE) exchange-correlation functional within a generalized gradient approximation (GGA) is employed, while a 520 eV cut off energy for the plane-wave basis set is used for the valence electrons.The self-consistent filed (SCF) calculations are performed with an energy and force convergence of 10-5 eV and 0.02 eV Å-1, respectively.To avoid the interactions between two adjacent periodic images, the vacuum thickness was set to be 15 Å.All atomic configurations are analyzed using the VESTA code [8].In order to model the Ir-N4 and Co-N4 monolayers, the 6×6 supercell consisting of 72 carbon atoms is used firstly as the substrate.Then, the Ir/Co atoms are anchored via displacing two neighboring carbon atoms, and four carbon atoms bond with metal atom are replaced by N atom, forming the Ir-N4 and Co-N4 catalyst model.For HER, the integral adsorption energy is defined by: ΔEH=E(surface+H)-E(surface)- where the E(surface+H) and E(surface) are the total energy of metal surface with H atoms adsorption and without, respectively.E(H2) is the total energy of hydrogen molecule in the gaseous phase.The Gibbs free energy of H atoms (ΔGH) is an important parameter for the descriptor of HER activity: where ZPE is the zero-point vibrational energy using the harmonic approximation, T is the kelvin temperature, and

Results and discussion.
We perform the density functional theory (DFT) calculation to understand the HER intrinsic activity of Ngraphene, Ir-N4 and Co-N4.Various active sites are considered and we calculate their Gibbs free energy for H (ΔGH), which is widely accepted as the HER activity descriptor that has an optimal value of zero when the HER activity reaches the maximum [11].For N-graphene (Figure 1a), H atoms thermodynamically preferred to adsorb on N sites owing to their lower ΔGH (-1.41 eV), the lower value of ΔGH also suggest the molecular hydrogen is difficult to be released on N-graphene and not active for hydrogen evolution.As shown in Figure 1b, the values of ΔGH on Ir-N4 increased to -0.37 eV getting much closer to optimum value (0 eV), which indicates that doping of Ir greatly assisted the HER performance.Remarkably, the ΔGH on Co-N4 is 0.15 eV (Figure 1c), which is better than N-graphene and Ir-N4.The catalytic activity is also related to the positions of i0 and ΔGH, the closer the position of these values to the peak, the better the catalyst is.As can be seen from Fig. 1d, the Co site on Co-N4 locate near the peak of the volcanic curve indicating Co-N4 is more catalytically active for HER.For oxygen reduction reaction (ORR), the metal active sites will adsorb oxygen molecules in the first step and the negative adsorption energies of O 2 reveal that the above structures can spontaneously activate O 2 molecules (Figure 2a).The Co-N 4 show lower O 2 adsorption than that of N-graphene and Ir-N 4 , demonstrating the better activation ability of O 2 molecules.However, due to the weak adsorption of OOH species, the N-graphene site shows a high overpotential of 1.55 V (Figure 2b).After the introducing of Ir and Co, the overpotential on Ir-N 4 and Co-N 4 structures decline to 1.02 V, and 0.62 V, respectively (Figure 2c and 2d), suggesting that the doping of Ir and Co are beneficial to optimize the adsorption energies of ORR intermediates.Here, the Co-N 4 shows lowest overpotential, confirming its better ORR electrocatalytic performance.In order to gain deep insights into the catalytic mechanism and electronic structure.we further investigate the projected density of states (PDOS) and charge density of different catalyst configurations.Interestingly, the interaction between Co and N result in unique electronic structure for Co-N 4 (Figure 3a), more charge depletion region exists.And as can be observed from the PDOS (Figure 3b,3c and 3d), for Co-N 4 , there are more hybridization effect between Co 3d and N 2p orbitals compared with that of pure N-graphene and Ir-N 4 , which leads to more electron overlaps in the vicinity of Fermi level, demonstrating fast charge transfer kinetics and guaranteeing excellent electrocatalytic performance.Based on the above theoretical analysis, we give expectation of high HER and ORR electrocatalytic performance of Co-N 4 .

Conclusion
In summary, DFT calculations have been carried out to investigate the Gibbs free energy diagrams and electronic structure of N-graphene, Ir-N 4 and Co-N 4 .The results here show that Co-N 4 show appropriate H adsorption (0.15 eV) and lower ORR overpotentials (0.62 V) than that of N-graphene and Ir-N 4 , which shows expectation of high multifunctional electrocatalytic performance.This theoretical study gives vital insights into the enhanced catalytic mechanism of Co-N 4 and also helps to institute the principles for designing high performance SACs.

MATEC
Web of Conferences 363, 01001 (2022) https://doi.org/10.1051/matecconf/202236301001AMME 2022 S is the entropy.According to thermodynamics, large positive values of ΔGH indicate that the hydrogen atom is difficult to bind to the catalytic sites, whereas large negative values of ΔGH indicate that molecular hydrogen is difficult to be released from the catalytic sites [9].The reaction steps considered for the electrochemical reduction of ORR under base condition are generally reported to proceed as follow: O2(g) + * → *O2 *O2 + H2O(l) + e-→ *OOH + OH-*OOH + e-→ *O + OH-*O + H2O(l) + e-→ *OH + OH-OH*+ e-→ * + OHwhere * represents either the catalytic active sites of vacant surface, or intermediate species adsorbed on the active sites.By considering the zero-point energy (ZPE) and entropy corrections, the Gibbs free energy for OER can be calculated with the following equation [10]: ΔG=ΔE+ΔZPE-TΔS (3) where ΔE indicates the adsorption energy difference for each species adsorbed on the catalyst, ΔZPE and ΔS are the zone point energy and entropy difference between the adsorbed state and corresponding free-standing state, respectively.

Figure 1 .
Figure 1.Gibbs free energy diagrams of hydrogen adsorption on different active sites of (a) N-graphene, (b) Ir-N4 and (c) Co-N4.(d) The calculated exchange current (log ( � )) for hydrogen evolution over different configurations as a function of Gibbs free energy for H*.

Figure 2 .
Figure 2. (a) The adsorption energies and corresponding configurations of O2 on N-graphene, Ir-N4 and Co-N4.The calculated Gibbs free energy evolution diagrams for through a 4e − ORR pathway on different active sites of (b) N-graphene, (c) Ir-N4 and (d) Co-N4.

Figure 3 .
Figure 3. (a) Charge density differences of N-graphene, Ir-N4 and Co-N4 configurations.The isosurface levels are set to 0.003 e Å -3 , where charge depletion and accumulation were depicted by cyan and yellow, respectively.Projected density of state (PDOS) analysis of (b) N-graphene, (c) Ir-N4 and (d) Co-N4.The Fermi level is set to be zero and the spin-polarization is considered.