Enhanced hydrogen evolution on Mg (OH)2 covered Mg surfaces

Pure Mg was immersed in saturated Mg(OH)2 solution, forming an Mg(OH)2 coated Mg surface. This surface was electrochemically characterised for the hydrogen evolution reaction (HER) by cathodic polarisation (at different pH); and scanning electrochemical microscopy (SECM). The kinetics of HER on this coated surface was then compared with that on pristine (uncoated) Mg, pristine Fe, coated Fe and Mg-Fe alloy surfaces. The kinetics of the HER (inferred from both the electrochemical tests) on the Mg(OH)2 covered surface was significantly higher than that on the pristine Mg surface. The HER kinetics were also found to be the highest in a pH 11 solution, implying that conditions which favour Mg(OH)2 precipitation enhance the HER. A model was proposed relating the enhanced HER upon the Mg(OH)2 covered surface to the enhanced self-dissociation of water, which arises from the disruption of the local ionic equilibrium, as dissolved Mg2+ ions undergo precipitation forming Mg(OH)2 under supersaturated conditions. This result is also in agreement with recent DFT-based calculations by other researchers on hydroxyl covered Mg surfaces (akin to Mg(OH)2 covered Mg surfaces) activating the self-dissociation of water.