Theoretical Inspection of M₁/PMA Single-Atom Electrocatalyst: Ultra-High Performance for Water Splitting (HER/OER) and Oxygen Reduction Reactions (OER)

Developing a cost-effective and highly efficient electrocatalyst with superior catalytic activity is crucial for clean and green water splitting, including the hydrogen evolution reaction (HER), the oxygen evolution reaction (OER), and the oxygen reduction reaction (ORR). The single-atom catalyst (SAC) is a breakthrough in industrial catalysis because of the advantages of maximum metal atom utilization, single active sites, strong metal–support interactions, and great potential to accomplish high catalytic performance and selectivity. Herein, we investigate the electrocatalytic performance of a series of SACs supported on a phosphomolybdic acid (PMA) cluster for the HER, OER, and ORR by using first-principles-based calculations. It has been found that the most plausible binding site for the single-metal adatoms is the 4-fold hollow (4H) site over the PMA cluster. Due to the higher stability and catalytic activity of single-metal adatoms, fast electron transfer kinetics is permissible through catalysis. Mainly, Pt1/PMA, Ru1/PMA, V1/PMA, and Ti1/PMA realized decent catalytic performance toward the HER due to nearly ideal (ΔGH* = 0) ΔGH* values via the Volmer–Heyrovsky pathway. The Co1/PMA (0.45 V) and Pt1/PMA (0.49 V) can be active and selective catalysts for the OER with their overpotentials comparable those of to MoC2, IrO2, and RuO2. Among the considered candidates, a non-noble metal Fe1/PMA SAC is a promising electrocatalyst for the ORR with an overpotential of 0.42 V, which is lower than that for the most favorable Pt (0.45 V) catalyst. Furthermore, Pt1/PMA is an auspicious multifunctional electrocatalyst for overall water splitting (−0.02 V for the HER and 0.49 V for the OER) and a metal-air battery (0.79 V for the ORR) catalyst. The current study is further extended to calculate the kinetic potential energy barrier for the excellent catalytic performance of Co1 for the OER and Fe1 for the ORR. The results suggest that the kinetic activation barrier values in all proton-coupled electron transfer steps are in good agreement with the thermodynamic results. It was revealed that the PMA cluster is a promising single-atom support for the HER, OER, and ORR and provides low-cost and highly efficient electrocatalytic activity under normal reaction conditions.