Monitoring the Spontaneous Growth of Nickel-Iron Layered Double Hydroxides during Alkaline Oxygen Evolution at Industrial Conditions
Authors: Ehlers Johan Christian , Chatzichristodoulou Christodoulos , Larrazábal Gastón 
Published: December 2024

The imperative shift towards a sustainable energy infrastructure necessitates cost-effective production of green hydrogen, wherein enhancing the electrolysers' efficiency stands essential. Among the possible levers for reducing internal losses is the oxygen evolution reaction (OER), which, due to its complex four-electron reaction mechanism, causes a substantial overpotential. In alkaline water electrolysis, where nickel electrodes are conventionally employed, small additions of iron to the nickel surface can reduce the OER overpotential tremendously.1 However, while many studies have developed catalysts based on this elemental composition with exceptional OER activity, their long-term stability under harsh industrial conditions often remains untested, which might lead to false conclusions regarding the catalyst's commercial potential.2

The active structure of these catalysts has been identified as nickel-iron layered double hydroxides (NiFe-LDHs),3 and research has logically strived towards optimisation of their synthesis to minimise the OER overpotential. In this context, structural parameters such as the iron content, vacancies and defects, intercalated anions and edge-to-plane site ratio are often in focus.4 The chemical environment of the electrolyser is nonetheless very different from the environment where many optimised NiFe-LDHs are formed, and despite any robust kinetic stabilisation of the initial electrocatalyst, the electrolytic environment and the operational conditions are expected to shape the structure during the electrolyser's lifetime.5 Meanwhile, with iron present in the electrolyte, NiFe-LDHs are known to grow spontaneously during alkaline OER on unactivated nickel anodes, offering an alternative and potentially more scalable and cost-effective route to obtain an efficient OER catalyst.6

In this study, we aim to leverage the spontaneous formation of NiFe-LDH catalysts via electrolyte-mediated pathways to produce highly active alkaline OER anodes that can endure the chemical environment of industrial electrolysers. To this end, we employ in situ Raman spectroscopy's exceptional ability to distinguish metal oxides in aqueous environments alongside electrochemical impedance spectroscopy, XPS and FEG-SEM analysis, and we monitor the evolution of various NiFe catalysts under industrially relevant conditions (80oC, 8M KOH, 600 mA/cm2). By rationalising the interplay between catalyst structure and the electrolytic environment, this research strives to elucidate whether NiFe-LDH catalysts synthesised in situ can outperform their ex situ counterparts in long-term stability, offering crucial insights for advancing sustainable hydrogen production.
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