Interfacial Interaction between FeOOH and Ni-Fe LDH to Modulate the Local Electronic Structure for Enhanced OER Electrocatalysis

Toward the pursuit of high-performance Ni2+/Co2+/Fe3+-relevant oxygen evolution reaction (OER) electrocatalysts, the modulation of local electronic structure of the active metal sites provides the fundamental motif, which could be achieved either through direct modifications of local chemical environment or interfacial interaction with a second metal substrate which possesses high electronegativity (typically noble metal Au). Herein, we report that the local electronic structure of Ni-Fe layered double hydroxide (LDH) could be favorably modulated through strong interfacial interactions with FeOOH nanoparticles (NPs). The biphasic and multiscale composites FeOOH/LDH demonstrated an increasingly pronounced synergy effect for OER catalysis when the average size of FeOOH NPs decreases from 18.0 to 2.0 nm. Particularly, the composite with average size of FeOOH NPs of 2.0 nm exhibited an overpotential of 174 mV at 10 mA cm(-2) and a tafel slope of 27 mV dec(-1) in 1.0 M KOH, outmatching all the noble and non-noble OER catalysts reported so far; it also operates smoothly in various stability tests. A mechanistic study based on XANES and EXAFS analysis, d.c. voltammetry and large amplitude Fourier Transformed a.c. voltammetry proved the presence of high-oxidation-state Fe((3+delta)+)sites with relatively short Fe(3+delta)+-O bond from the highly unsaturated ultrafine FeOOH NPs which could reform the local electronic structure and favorably manipulate the electronic oxidation and thus electrocatalytic behaviors of the Ni2+ species in the Ni-Fe LDH, hence leading to the easy formation, excellent OER activity, and extraordinary structural and catalytic stability. Our work puts an emphasis on the role of the solid-solid interfacial chemistry between a Ni-Fe LDH and a non-noble-metal component in engineering the local electronic structure of the active metal sites, which successfully pushed forward the catalytic activity of the well-studied Ni-Fe LDH far beyond its current limit in OER catalysis and opened up an avenue for rational design of OER electrocatalysts.