Two-dimensional metal–organic TMTAP monolayers as a promising class of monoatomic electrocatalysts for oxygen reduction, oxygen evolution, and hydrogen evolution reactions

Abstract

Regenerative fuel cells, water splitting, and metal-air batteries all require high-performance electrocatalysts to drive the oxygen reduction reaction (ORR), oxygen evolution reaction (OER), and hydrogen evolution reaction (HER). Single-atom catalysts (SACs) have drawn considerable attention due to their superior catalytic properties. In this work, we evaluate 30 transition metals anchored to tetraazaporphyrin (TAP) monolayers as SACs for ORR, OER, and HER using density functional theory (DFT). Among the candidates operating via the 4e⁻ pathway, Zn-TAP (ORR overpotential: 0.43 V) and Rh-TAP (OER overpotential: 0.58 V) exhibit top-tier performance. Fe-TAP also shows good ORR activity (0.58 V). For the 2e⁻ pathway, Ag-TAP delivers overpotentials of 0.36 V for ORR, making it promising for H₂O₂ synthesis. In the 2e⁻ OER, the Cr, Mn, Fe, Co, and Zn systems show very low overpotentials (down to 0.17 V), albeit with poor 2e⁻ ORR selectivity. Regarding HER, Cd- and Os-TAP are identified as promising candidates, with near-ideal hydrogen adsorption free energies of −0.08 and −0.12 eV, respectively. Further mechanistic insights into O₂ activation are provided through analyses of spin density, charge density difference, and electronic structure. We note that this study is based on the computational hydrogen electrode (CHE) model under idealized acidic conditions (pH = 0) without explicit or implicit solvent effects. Therefore, the calculated overpotentials and free-energy profiles represent a thermodynamic screening framework, and extension to neutral/alkaline or experimental environments requires additional consideration of solvation and pH effects. Overall, this work highlights the potential of TMTAP SACs and offers guidance for the rational design of such catalysts.