Advanced Hydroxyl Salt Based Electrocatalysts for Efficient Overall Water Splitting

The growing consumption of fossil fuels and related environmental pollution is prompting huge research on a clean, safe and sustainable alternative energy source. Among various solutions, electro-driven water splitting is believed to be one of the promising and appealing strategies to achieve the conversion of electric energy into chemical energy [1]. Because of the high cost, scarcity and unsatisfied stability of the noble metal (Pt, Pb, Ru etc), transition metal based catalysts, such as metal alloy, metal carbides/nitride, metal chalcogenides, metal phosphate, metal oxides, hydroxides attracted lots of attention [2]; however, hydroxyl salts, another promising alternatives received much less attention till now. In most cases, hydroxyl salts were used as a kind of precursor or intermediate product while people underestimate the function of hydroxyl ions within the electrocatalytic system utilized.
Here we synthesized a series of hydroxyl salts, including metal phosphate hydroxides, metal carbonate hydroxides and metal sulfate hydroxides and metal nitrate hydroxides. It is discovered that the differences of constitution and morphology have a great influence on the performance of the basic salt. Among these four kinds of hydroxyl salts, metal phosphate hydroxides and metal carbonate hydroxides have a good bifunctional electrocatalytic activity for oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) after proper modification in alkane solution. Then taking metal phosphate hydroxides {Co₂(PO₄)OH} as an example, we discussed the effects of hydroxyl and phosphate on the OER performance. The overpotential of Co₃(PO₄)₂, Co(OH)₂ and Co₂(PO4)OH are 275 mV, 220 mV and 275 mV at 10 mA/cm², respectively, while the corresponding Tafel slope is 138 mv/decade, 98 mv/decade and 78 mv/decade. In this case, we can conclude that hydroxyl improves the Tafel slope while phosphate have a negative impact on both overpotential and Tafel slope. All these findings may strengthen the understanding of hydroxyl salt and explore other kinds of hydroxyl salt as highly active electrocatalysts towards efficient overall water splitting.