The Origin of Catalytic Activity of Nickel Phosphate for Oxygen Evolution in Alkaline Solution and its Further Enhancement by Iron Substitution

Nickel(II) phosphate (NPO) can be an effective catalyst for oxygen evolution reaction (OER) in alkaline solution. Unlike other transition metal phosphates, the OER is preceded by a prominent reversible redox reaction. The current study is aimed at understanding the origin of NPO-catalyzed OER, in particular the role of the pre-OER redox reaction. We detected the progressive formation of Ni(OH)(2)/NiOOH on the NPO surface during potential cycling in the anodic region similar to previous observations from Ni oxides, and found that it is associated with the change in the Ni/phosphate stoichiometry from 1.5 to 1.0, resulting in the loss of phosphate to the alkaline solution. Despite the conversion, NPO still has a good and sustained OER performance due to the activity similarity between NiPO4 (and possibly NPO) and NiOOH (the activated form of Ni oxide and hydroxide catalysts). NPO is however an easier catalyst to use than Ni oxides and hydroxides, which require extensive electrode conditioning to reach a stable performance. We also mixed NPO with Fe in view of recent findings that Ni-Fe double hydroxides are more active than simple Ni oxides in oxygen catalysis. The experimental results corroborate the positive outcome of this modification.