Topotactically Transformed Polygonal Mesopores on Ternary Layered Double Hydroxides Exposing Under-Coordinated Metal Centers for Accelerated Water Dissociation.

Layered double hydroxides (LDHs) have been recognized as potent electrocatalysts for oxygen evolution reaction (OER), but are lacking in hydrogen evolution reaction (HER) activities due to the sluggish kinetics of water dissociation in alkaline medium. Herein, aiming to simultaneously bolster the HER and OER kinetics, a metal-organic framework (MOF) mediated topotactic transformation tactic is deployed to fabricate holey ternary CoFeNi LDHs on nickel foam, exposing polygonal mesopores with atomistic edge steps and lattice defects. The optimized catalyst requires only an external voltage of 1.49 V to afford the water splitting current density of 10 mA cm-2 apart from the superb electrolytic stability, far surpassing the benchmark Pt/C||RuO2 couple. More importantly, mechanistic investigations utilizing advanced spectroscopies in conjunction with density function theory (DFT) understandings unravel while the synergetic effect among under-coordinated metal centers lowers the energy barrier of water dissociation, Fe-doping enables further modulating the d-band density of states (DOS) of Co and Ni in favor of intermediates binding, thereby promoting the intrinsic HER activity. Operando Raman studies reveal negligible structural change of the LDHs during the HER process, whereas for OER the active sites can quickly turn into oxyhydroxides in the presence of lattice defects and under-coordinated metal centers.