Overall Water‐Splitting Electrocatalysts Based on 2D CoNi‐Metal‐Organic Frameworks and Its Derivative

Efficient electrocatalysts composed of non‐precious elements are central to the development of overall water‐splitting system. Herein, it is demonstrated that 2D CoNi‐metal‐organic framework (CoNi‐MOF) array grown directly on a Cu substrate is a highly efficient and stable electrocatalyst for oxygen evolution reaction with a low overpotential (265 mV at 10 mA cm−2) in alkaline medium. With the combination of in situ conducting atomic force microscopy and two‐point conductivity measurements, it is revealed that the electron transfer is facilitated along the Z‐axis better than the other directions, which can lead to an enhanced catalytic activity. In addition, a simple annealing treatment converts this 2D CoNi‐MOF to a hybrid nanoplate array of metallic nitrides on an amorphous carbon network, which catalyzes the hydrogen evolution reaction with a compatible overpotential of 120 mV (at 10 mA cm−2) and excellent stability. The overall water splitting using these 2D CoNi‐MOF‐based catalysts shows a 99% Faradaic efficiency at 1.64 V, suggesting a promising pathway forward for the commercial realization of complete water‐splitting systems. 2D bimetallic CoNi‐metal‐organic framework (CoNi(1:1)‐MOF) with the (22¯0) crystalline facets exposed catalyzes oxygen evolution at a low overpotential of 265 mV. CoNiN@C prepared using CoNi‐MOF as a precursor shows comparable hydrogen evolution rate in alkaline solution. Using CoNi(1:1)‐MOF and CoNiN@C, overall water splitting is achieved with 99.0% (hydrogen evolution reaction) and 98.9% (oxygen evolution reaction) Faradaic efficiencies.