Unravelling the effect of Cl^- on alkaline saline water electrooxidation on NiFe (oxy)hydroxides

One of the major problems for seawater electrolysis is chloride corrosion arising from the chlorine oxidation reaction (ClOR). Although researchers have endeavored to improve the OER activity (overpotentials below 490 mV) to avoid the ClOR, an OER deactivation effect of chloride anions still looms large even before the ClOR becomes a serious issue, and its mechanism remains unclear. Herein using a simple Fourier transform ac voltammetry (FTacV) method, we unveil a hidden fact about the detrimental effect of Cl- over an exemplar catalyst NiFe (oxy)hydroxide: it increases the formation potential of Ni(IV) thus lowering the OER performance. This is borne out by theoretical calculations, which reveal changes in the electronic environment of Ni sites induced by replacing the bridge OH- species with Cl- and the resulting difficulty to form high-valence Ni(IV). The consequence is a large increase in energy barrier of the OH* deprotonation step that lowers the OER activity. The poison effect of Cl- can be restrained by adding carbonate ions, which restores the facile formation of Ni(IV) and stabilizes the electrode for over 140-h saline water oxidation reaction at the current density of 100 mA cm(-2). This work helps to guide the design of active and durable seawater-splitting electrocatalysts.