Surface reconstruction of NiCoP pre-catalysts for bifunctional water splitting in alkaline electrolyte

The emerging earth-abundant metal phosphides are regarded as promising bifunctional water splitting electrocatalysts. However, the real active sites in metal phosphides remain a debate. Herein, the hierarchical flower-like NiCoP microsphere is designed with the outstanding performance toward both hydrogen and oxygen evolution reaction (overpotentials of 238 and 47 mV for oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) at 10 mA cm−2, respectively). The symmetrical overall water splitting devices based on NiCoP only require an ultralow voltage of 1.52 V to reach 10 mA cm−2, which is rarely reported. In-situ Raman and ex-situ TEM/XPS reveal that the surface reconstruction of designed NiCoP pre-catalysts transforms into metallic Ni(Co) for HER and Ni(Co)OOH for OER, respectively. The degradation process of HER during resting is revealed, suggesting the formation of high valence state Ni(Co)(OH)2 and blocking the active sites. We believe that the comprehensive understanding of the surface reconstruction of electrode is crucial in understanding the fundamental mechanism.