Coordination Environment Tuned Mo Single-Atom Catalysts as High-Efficiency Electrocatalysts for Hydrogen Evolution Reaction

Abstract

Unconventional coordination environments of Mo-O₂C_xN_y for Mo single atoms (SAs) on carbon substrates of increasing N-doping levels were created to explore pronounced effects of coordination environments on electrocatalytic efficiency of hydrogen evolution reactions (HER). The HER performances of Mo-₂C_xN_y single atom catalysts in 1.0 M KOH run in the improving order of Mo-O₂N₂, Mo-O₂N₁C₁, and Mo-O₂C₂, exhibiting η₁₀ of 98, 71, and 61 mV, η₅₀₀ of 340, 248, and 200 mV, Tafel slopes of 95.8, 39.4, and 33.8 mV dec^-1, and current density decays of 9, 6, and 6% after 50 hour operations at an initial current density of 100 mA cm^-2, respectively. Substituting C with N in the coordination environment results in inferior HER catalytic efficiency and stability. Density functional theory calculations reveal that replacing carbon with nitrogen for coordination with the Mo SA on a carbon substrate of a higher level N-doping shifts the d-band center of Mo more negatively from the Fermi level, thereby increasing the hydrogen adsorption energy and thus slowing the hydrogen desorption kinetics, giving consequent inferior HER activities. For Mo SAs, the coordination environment of Mo-O₂C₂ is demonstrated to be much better suited for catalyzation of HER than that of conventional Mo-N_xC_y coordination.

Keywords: Mo single atom catalyst, coordination environment, Electronic structure, hydrogen evolution reaction, electrocatalyst