Resolving Potential-Dependent Degradation of Electrodeposited Ni(OH)2 Catalysts in Alkaline Oxygen Evolution Reaction (OER): In Situ XANES Studies

Abstract

The activation and degradation mechanism of Ni-LDH catalysts electrodeposited on low and high-density carbon papers for the oxygen evolution reaction (OER) in alkaline media was investigated using in situ X-ray absorption near-edge structure (XANES) spectroscopy coupled with CV cycles in a potential range of 0–0.9 V (vs Hg/HgO), which allowed proposing two-stage degradation mechanisms with respect to cyclic voltammetry (CV) cycles. The electrodeposited α-Ni(OH)2 is firstly transformed to γ-NiOOH as an active phase in OER. In the reducible potential region, however, γ-NiOOH was partially reduced to β-Ni(OH)2, isolating the rest, which is the first stage of degradation. In the following anodic potential region, β-Ni(OH)2 is readily converted to β-NiOOH, which is mostly reversible, but only a small portion of β-NiOOH is overcharged to unstable γ-NiOOH, being responsible for the second stage degradation. It was noted that Ni(OH)2 catalysts electrodeposited on a low-density carbon paper (Ni-LC) underwent a severe degradation in the first stage, losing at least 56.9 % current density at 0.65 V (vs Hg/HgO), followed by a steady degradation in the second stage, while the use of a high-density carbon substrate (Ni−HC) effectively improved redox stability, maintaining a minimal loss of overpotential less than 5%, particularly with the second stage degradation being negligible even under potential changes, providing an important insight into designing durable and active Ni-LDH catalysts for the OER.