The mechanistic origin of the oxygen evolution reaction on Nickel-Iron electrocatalyst in alkaline water splitting

Abstract

The significant findings were achieved with a thorough exploration of the fundamental reaction and degradation mechanisms of NiFe-based oxygen evolution reaction (OER) electrocatalysts, as well as the development of an active and durable stack AEMWE with a developed NiFe anode. Our group strengthened the claim that Fe is the active site by detecting high-valent Fe (Fe4+). Real-time changing oxidation states of Fe and Ni K-edge were possible using in-situ X-ray absorption spectroscopy (XAS). We concluded that the Fe4+ was detected with the aid of easily reducible adjacent Ni. Even it has great potential as a non-noble metal OER electrocatalyst it has some drawbacks in terms of sustainability. Several papers have addressed the low stability and durability of NiFe in Fe-free 1 M KOH electrolyte. We have deciphered the degradation mechanism as the dissolution of Fe in NiFe. It was systematically found with the online inductively coupled plasma optical emission spectroscopy (ICP-OES) technique. We have developed a unique protection layer, tetraphenylporphyrin (TPP), that can mitigate the dissolution of Fe, enhancing stability. The layer includes a nonpolar area that increases the lifetime of solvated Fe ions inside the double layer. To increase the volumetric and gravimetric energy density of green H2 production, it is necessary to scale up the anion exchange membrane water electrolysis (AEMWE) into a stack. Electrodeposited NiFe-layered double hydroxide was chosen as the anode catalyst after an optimization process of thickness (c.a. 102 μm) control, achieving not only activity and stability but also economic benefits. Fe impurity was added in electrolyte to enhance the rate of Fe re-deposition into the anode, reducing the cell voltage to 1.56 Vcell@1 A cm-2 and showing a mitigated degradation rate of 0.127 mVcell hr-1. Quantification of Fe in the anode after the stability test shows significant maintenance of Fe after the test. Even when compared to the result of the small-sized single-cell AEMWE, it shows the state-of-the-art performance.