Alkali-Induced In Situ Formation of Amorphous NiFe(OH) from a Linear [M(COO)]-Based MOF Template for Overall Electrochemical Water Splitting.

Amorphous and bifunctional electrocatalysts based on 3d transition metals tend to exhibit better performance than their crystalline counterparts and are a promising choice for efficient overall water splitting yet far from being well explored. A 3,6-net metal-organic framework (MOF) of [Ni(bpt)(DMF)(HO)]·1.5DMF (Ni-MOF), based on linear [Ni(COO)] as a node and [1,1'-biphenyl]-3,4',5-tricarboxylic acid (Hbpt) as a linker, was conveniently prepared via a hydrothermal reaction. Benefitting from the wide compatibility of the octahedral coordination geometry in Ni-MOF for different 3d metal ions, the molecular level and controllable metal doping facilitates the production of the desired Ni/Fe bimetallic MOF. A high-concentration alkali solution of 1 M KOH induced the in situ transformation of the MOF as a precursor to new amorphous electrocatalysts of [Ni(OH)(HO)]·HO [-Ni(OH)] and its metal-doped derivatives of -NiFe(OH) and -NiFe(OH). In particular, the costly organic ligand Hbpt was fully dissolved in the alkaline solution and can be recovered for cyclic utilization by subsequent acidification. The obtained amorphous hydroxide was deduced to be loose and defective layers containing both coordinated and lattice water based on combined characterizations of TG, IR, Raman, XPS, and sorption analysis. As opposed to the crystalline counterpart of Ni(OH) with stacked packing layers and an absent lattice water, the abundant catalytic active sites of the amorphous electrocatalyst endow good performance in both oxygen evolution reaction (OER) and hydrogen evolution reaction (HER). The bifunctional -NiFe(OH) coated on nickel foam realizes small overpotentials of 247 and 99 mV for OER and HER, respectively, under a current density of 10 mA cm, which can work with a cell voltage of merely 1.60 V for overall water splitting. This study provides an efficient strategy for widely screening and preparing new functional amorphous materials for electrocatalytic application.