Probing the Evolution of Active Sites in MoO₂for Hydrogen Generation in Acidic Medium

MoO2 is an effective host and catalytic material for electrochemical hydrogen evolution reaction in the form of composites, heterostructures, core-shell structures, and doped systems. Using pristine MoO2 as the electrocatalyst is less known due to a lack of knowledge of the active sites and their development during the hydrogen evolution reaction (HER). In this work, we have correlated the active-site evolution on applying a cathodic potential to pristine MoO2 and the corresponding improvement in catalytic activity. The activity enhancement is observed as a progressive reduction of overpotential from 0.62 to 0.42 V (vs RHE) at 1 mA/cm2 during potential cycles accompanied by a tremendous lowering of the Tafel slope (b) from 127 to 72 mV/dec. The decreased b value reveals that the developed active sites hasten the reaction via a change in the rate-determining step from Volmer to Heyrovsky. The changes in the Mo oxidation states and the local coordination environment of Mo-O are monitored by advanced X-ray absorption spectroscopy, unfolding the emergence of active sites in MoO2 through a change of Mo-O coordination from octahedral (Oh) to the coordinatively unsaturated distorted Oh environment with a lowering of the Mo-O coordination number and oxidation states of Mo ions. The key thermodynamic and kinetic parameters, namely, the change in Gibbs free energy for hydrogen adsorption on the catalyst surface and the kinetic activation barrier related to the HER rate-determining step for both activated (MoO2-x) and pristine MoO2, have been simulated and rationalized with the experimental results.