π⋯π interaction directed 2D FeNi-LDH nanosheets from 2D Hofmann-MOFs for the oxygen evolution reaction

Abstract

Layered double hydroxides (LDHs) have attracted much attention for the oxygen evolution reaction (OER) in electrochemical water splitting. In this work, we use a two-dimensional (2D) MOF transformation strategy to synthesize FeNi layered double hydroxides (FeNi LDHs). 2D FeNi Hofmann MOFs ([Fe(L)₂Ni(CN)₄]) with a similar structure but different ligands (L = py, ISOQ) are synthesized as precursors. Accordingly, hydrolysis transformation assisted by NaBH₄ leads to the growth of 2D FeNi-LDH nanosheets from 2D Hofmann-MOFs. During hydrolysis, OH⁻ ions preferentially attack the coordination bond of [Fe(L)₂Ni(CN)₄], which allows FeNi-LDHs to form quickly on the surface of 2D MOFs. This rapid conversion process can effectively avoid the self-accumulation of LDHs. The hydrogen bubbles produced by NaBH₄ can also inhibit the self-stacking of LDHs, which expose ample active species for the OER. Meanwhile, the synergistic effect between Fe and Ni contributes to the outstanding OER performance of FeNi-py-LDH, which shows an overpotential of 238 mV at 10 mA cm⁻², a Tafel slope of 22 mV dec⁻¹ and an excellent stability for 15 h. This work provides a universal synthesis method for ultra-thin FeNi-LDHs, which avoids the self-stacking of nanosheets and offers new insights into the OER.