NaBH4 induces a high ratio of Ni3+/Ni2+ boosting OER activity of the NiFe LDH electrocatalyst

Electrochemical water splitting is a promising way to produce hydrogen gas, but the sluggish kinetics of the oxygen evolution reaction (OER) extremely restrict the overall conversion efficiency of water splitting. Transition metal based LDHs (TM LDHs) are one of the most effective non-noble metal OER catalysts and have attracted wide interest, especially the nickel-iron LDH (NiFe LDH). The high valence Ni³⁺ species with a large coordination number play a vital role in OER catalysis. Herein, we report on a surprising discovery that reaction between NiFe LDH and NaBH₄ with multi-hydrides induces vacancy formation around Fe³⁺ and enrichment in Ni³⁺, crucially activating the OER performance. The ratio of Ni³⁺/Ni²⁺ is found to be closely tied to the OER performance, nicely accounting for the leading role of Ni³⁺ ions in octahedral sites in electrocatalysis. Significantly, the NaBH₄ treated NiFe LDH directly on nickel foam (NF), denoted as NaBH₄-NiFe LDH@NF exhibited an outstanding OER performance with an overpotential of only 310 mV at 100 mA cm^-2, and a Tafel slope of 47 mV dec^-1. For the series of TM LDHs we studied with different metal combinations, the high valence metal ion is found to be positively related to OER performance.