Enhancing Electrochemical Hydrogen Evolution Performance of CoMoO₄-Based Microrod Arrays in Neutral Media through Alkaline Activation

We present that activation of CoMoO₄-based microrod arrays in KOH (1.0 M, 2 h) allows us to significantly improve their electrochemical hydrogen evolution performance in phosphate buffer solution (1.0 M, pH = 7.1). The activation mechanism originates from the conversion of the surface layer of CoMoO₄ to Co(OH)₂ nanosheets, together with the release of Mo₃O₁₀^2- ions into the activation solution. Our experimental and calculated results suggest that the Co(OH)₂ nanosheets on the surface of the CoMoO₄-based microrod arrays show the ability to improve water molecule disassociation and stabilize the catalytic activity of the two-component catalysts by decreasing their overpotentials in the hydrogen evolution reaction. When extending this strategy to activate P-doped CoMoO₄ with a low hydrogen absorption free energy, we report the synthesis of a new class of superior neutral electrochemical hydrogen evolution catalysts of P-doped CoMoO₄-Co(OH)₂ microrod arrays. We show that a low overpotential of about 30 mV (obtained from bulk electrolysis) is required to deliver a current density of 10 mA cm^-2 in the neutral media. By making use of our catalyst and NiFe double hydroxide as cathodic and anodic electrodes, respectively, we fabricated a two-electrode electrolysis device for neutral overall water splitting. Our results showed a low cell voltage of 1.78 V (obtained from bulk electrolysis) that is needed for delivering a current density of about 10 mA cm^-2 in the neutral electrolyte, even outperforming the state-of-the-art catalyst combination of Pt/C∥RuO₂ in terms of catalytic activity and stability. These findings suggest that our strategy may be utilized as a facile but useful strategy toward the activation of molybdate catalysts to improve their HER performance in both basic and neutral media.