Electronic Structure Modulation and Electrochemically Induced Reconstruction of Ni-Doped Fe Oxyhydroxides for Overall Water Splitting

Abstract

Structurally analogous metal oxyhydroxides such as NiFeOOH and CoFeOOH lie at the pinnacle of the oxygen evolution reaction (OER) volcano plot for alkaline water electrolysis. However, stabilization of a single FeOOH phase is inherently challenging because of the small thermodynamic energy differences among FeOOH polymorphs and the phase selection being governed by crystallization kinetics. Herein, we report a heterovalent Ni doping strategy to stabilize β-FeOOH structural morphology and enhance the local electric field, thereby tuning its electronic conductivity to promote OER activity. Moreover, Ni doping alters the interfacial structure in an optimal OH– ionic environment by lowering the reaction energy barriers at Fe active sites with a significantly lower adsorption free energy of 3.56 eV for the *OOH intermediate. Under cathodic polarization, NFO24/CFC undergoes in situ reconstruction, transforming bulky α-Fe2O3 NCs into hydroxylated rich FeOOH-like thin nanosheets. This structural evolution generates abundant active sites and facilitates charge-transfer kinetics, thereby accounting for its superior hydrogen evolution reaction (HER) activity. Consequently, NFO6/CFC delivers a high anodic current density with an OER overpotential of 355 mV, while NFO24/CFC exhibits superior HER performance with an overpotential of 272 mV at a current density of 10 mA cm–2 in alkaline media. Furthermore, the two-electrode assembly using NFO6/CFC∥NFO24/CFC operates at a cell voltage of 1.87 V to drive overall water splitting and demonstrates long-term stability of 24 h at 10 mA cm–2 in alkaline conditions.