Mechanical-Effect descriptor for oxygen reduction reaction and hydrogen evolution reaction on Single-Atomic Ni-Graphene catalysts

Abstract

Mechanical effects on single atomic Ni-based Graphene (Ni-gra) catalysts for oxygen reduction reaction (ORR) and hydrogen evolution reaction (HER) were investigated under implicit aqueous via density functional theory (DFT) calculations. The mechanical strain induced Ni-C4 configurations change between pentahedral and planar quadrilateral. Ni-gra exhibited chemico-physical adsorption of O2, favoring 2e- ORR at 5.5% to 12.0% with overpotential of 0.119 V to 0.248 V and selectivity of 78.9% to ≥ 99.9% in uniaxial strain (2.0% to 8.0% and 12.0%, 0.107 V to 0.455 V, 69.5% to ≥ 99.9%, biaxial strain). The preferred 4e- ORR occurred when O2 was chemosorbed at 3.0% to 4.5% with 0.240 V to 0.293 V overpotential and 72.9% to 94.1% selectivity in uniaxial strain (1.0% to 1.5% and 8.5% to 11.5%, 0.139 V to 0.290 V, 69.7% to ≥ 99.9%, biaxial strain). The presence of C–H-Ni coordination facilitated HER due to stronger H-attractiveness at −4.0% to 2.5% in uniaxial strain (-4.0% to 0.5%, biaxial strain). The overpotentials of ORR (2e-, 4e-) and HER were highly correlated with the adsorption energies of O2 and H (ΔEO2* and ΔEH*). Based on geometric and electronic analysis, the Fermi level was found to be the optimum descriptor to monitor the variations in ΔEO2*, ΔEH* and overpotentials under mechanical strain. This study reveals that mechanical engineering is an efficient approach to design catalysts for ORR and HER.