Hydrogen production on demand by redox-mediated electrocatalysis: A kinetic study

Producing hydrogen from water using a redox mediator on solid electrocatalyst particles in a reactor offers several advantages over classical electrolysis in terms of safety, membrane degradation, purity and flexibility. Herein, vanadium-mediated hydrogen evolution on a commercial and low-cost Mo2C electrocatalyst is studied through the development of a reaction kinetics model. Based on a proposed mechanistic reaction scheme, we established a kinetic rate law dependent on the concentration of V2+, the state-of-charge of the vanadium electrolyte from a vanadium redox flow battery and the amount of available catalytic sites on solid Mo2C. Kinetic experiments in transient conditions reveals a first-order dependence on both the concentration of V2+ and the concentration of catalytic active sites and a power law with an exponential factor of 0.57 was measured on the molar ratio V2+/V3+, i.e. on the electrochemical driving force generated on the Mo2C particles. The kinetic rate law was validated by studying the rate of reaction in steady-state conditions using a specially developed rotating ring-disk device (RRD) methodology. The kinetic model was demonstrated to be a useful tool to predict the hydrogen production via the chemical oxidation of V2+ over Mo2C at low pH ( > 1 M H2SO4). For a perspective, the model was implemented in a semi-batch reactor. The simulations highlight the optimal state-of-charge (SOC) to carry out the reaction in an efficient way for a given demand in hydrogen.