F-doped NiOOH derived from progressive reconstruction for efficient and durable water oxidation

Abstract

Transition metal fluorides are efficient oxygen evolution reaction (OER) pre-catalysts and rapidly transform into metal (oxy)hydroxides under OER conditions. However, one of the commonest problems of dramatic reconstruction is severe structural collapse, resulting in unsatisfactory OER stability. Moreover, the role of fluorine is not fully elucidated after reconstruction and extensive leaching. Here, we present a progressive reconstruction strategy for converting the rock-like NH₄NiF₃ pre-catalyst into a F-doped NiOOH phase. Various in/ex situ experiments validate that the preferential electro-oxidation of NH₄⁺, similar to a corrosion inhibitor, can slow down the reconstruction of NH₄NiF₃, thus ensuring sufficient time to cope with the crystal structure mismatch during the structural transition to NiOOH. Eventually, the reconstruction-derived F-doped NiOOH exhibits favorable OER performance with a low overpotential of 240 mV at 10 mA cm⁻² and can be operated at 200 mA cm⁻² for more than 300 h. Theoretical calculations further reveal that residual F atoms doped in the NiOOH can well tailor the electronic structure of Ni sites and reduce the energy barrier of the rate-determining step during the OER. Our findings offer unique insights into the transformation mechanism of pre-catalysts during electro-oxidation.