Differentiated Modulating the Electronic Structure of NiFe@Ni/Fe-MnOx via Phase Transformation Engineering to Synergy Promote Bifunctional Water Splitting Reactions.

Modulating electronic structure to balance the requirement of both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) is crucial for developing bifunctional catalysts. Herein, phase transformation engineering is utilized to separately regulate catalyst structure, and the designed NiFe@Ni/Fe-MnOOH schottky heterojunction exhibits remarkable bifunctional electrocatalytic activity with low overpotentials of 19 and 230 mV at 10 mA cm for HER and OER in 1M KOH, respectively. Meanwhile, an anion-exchange membrane water electrolyzer employing NiFe@Ni/Fe-MnOOH as electrodes shows low voltages of 1.487/1.953 V at 10/1000 mA cm, and operating over 200 h at 1000 mA cm. Combining theoretical calculations and experiments reveal that phase transformation engineering can differentially regulate the active phases of HER/OER. In the HER, Ni/Fe-MnOOH and metallic NiFe act as the *OH and *H acceptors respectively to accelerates the water dissociation and subsequent Heyrovsky/Tafel step. While in the OER, the significant Jahn-Teller effect of Mn induces the surface reconstruction from Ni/Fe-MnOOH to Ni/Fe-MnO. The formative high value Mn can modify the M-O hybridization and activate the lattice oxygen mechanism, which is pivotal for breaking the restriction of volcanic relationship and reducing OER overpotential. These findings provide valuable design guidelines for high-performance multi-functional electrocatalysts via phase transformation engineering.