Low overpotential NiFe-Layered-Double Hydroxide on Ni foam for OER catalyst and anode in anion exchange membrane electrolyzer

Ni has been implemented as an oxygen catalyst and anode in the emerging anion exchange membrane electrolyzer (AEMEL) due to its characteristics of superior corrosion resistance in alkaline environments and low oxygen overpotential. The aforementioned Ni characteristics enable the oxygen evolution reaction (OER) at the anode side without a need to use critical raw materials. Nevertheless, due to the sluggish OER kinetics, further anode development to increase this kinetics has been directed to a Ni-based alloy. In this contribution, an OER catalyst of NiFe-Layered Double Hydroxide (NiFe-LDH) with low oxygen overpotential was prepared. The study of NiFe-LDH growth was first carried out on Au-coated glass substrates by galvanostatic-mode electrochemical deposition from aqueous electrolyte comprising various Ni(NO3)2 hexahydrate and FeSO4 heptahydrate concentrations. The as-deposited layers show porous lamellae-like morphologies. It was found that the lamellae thickens as the ratio of Fe/(Ni+Fe) salt concentration ratio in the electrolyte increases, with the final effect of reducing the layer porosity size. Grazing Incidence XRD suggests that the as-deposited layers crystallize as Ni-based LDH from their 003 characteristic peaks at 2 ~ 10°. Cyclic voltammetry with iR compensation demonstrates that the addition of Fe reduces the as-deposited layer’s overpotential from 340 mV (pure Ni layer) to 275 mV (NiFe layer with 37 at. % Fe, measured at 10 mA/cm2. From the i-V polarization curve, the Tafel slope of NiFe-LDH with 37 at.% Fe is 50 mV/dec. To prepare an anode for AEMEL application, a catalyst-coated substrate method was adopted. The NiFe-LDH layers with Fe content of 37 at. % were grown on HCl pre-treated Ni foams (95% porosity, 99.5% purity), forming a catalyst on Ni foam porous transport layer (i.e. NiFe-LDH/Ni). The NiFe-LDH/Ni shows overpotential as low as 255 mV. This value is about 20 mV lower than the NiFe-LDH on an Au-coated glass substrate (275 mV). During the five-hour stability test, the potential observed for NiFe-LDH/Ni is much lower than the bare Ni foam. The potential difference is almost 150 mV, which denotes that the NiFe-LDH/Ni promotes the OER, at a much lower potential than the bare Ni foam does. This first result also shows the potential of NiFe-LDH/Ni to be implemented as an anode in the AEMEL.