Composition-Dependent Dynamics of Nickel-Iron-Cobalt Metal Catalysts for Oxygen Evolution Reaction
Authors: Rojas Mendoza Milenia , Schröder Johanna , Hannagan Ryan T. , Jaramillo Thomas F. , Burke Stevens Michaela 
Published: August 2024

Water electrolyzers, such as proton exchange membrane water electrolyzers (PEM) and anion exchange membrane water electrolyzers (AEM) are important technologies for sustainable electrochemical energy conversion. The oxygen evolution reaction (OER) at the anode is a pivotal half-reaction in these systems. Despite substantial progress over the years, OER kinetics remain a major source of inefficiency in these devices, necessitating the development of stable catalysts. Currently, precious metal-based catalysts dominate the landscape of OER catalysts for these devices. As non-precious alternatives, nickel-iron (Ni/Fe) catalysts have emerged as particularly promising in alkaline conditions. However, they exhibit dynamic behavior during electrocatalysis—for instance, oxidizing to an active oxyhydroxide phase during the OER. This dynamic nature makes it challenging to establish clear structure-activity-stability relationships. Incorporating cobalt (Co) into the catalyst composition has been found to influence the activity and material conversion of mixed Ni-Fe-Co oxides and (oxy)hydroxides. Methods for rationally activating and stabilizing Ni-Fe-based catalysts via electrochemical and chemical (e.g. composition) routes are necessary for enhancing scalability and accessibility in hydrogen production devices.

Herein, our goal is to understand and exploit activation protocols to enhance the OER performance of as-synthesized nickel-iron-cobalt metal thin films, addressing three primary objectives: synthesizing and characterizing homogeneous metal catalysts, investigating the impact of different potential ranges and scan rates on catalyst oxidation and surface conversion from the metal, and assessing how the addition of cobalt influences conversion rates and current density of the catalyst. We use cyclic voltammetry to facilitate and disentangle conversion (integrated reductive peak for Ni and/or Co2+ → 3+ transition) and OER performance (current density (mA cm-2) at η = 350 mV). For all compositions tested of (Ni80Fe20)100-xCox, x = 0 - 5 % we observe that the most effective conversion requires both redox and oxygen evolution. To further investigate the relationship between conversion and performance catalysts were tested in varying electrolytes such as 1M KOH, 0.1M KOH, and an ionomer overlayer to understand how these impacted conversion.

Subsequently, post-analysis techniques such as X-ray photoelectron spectroscopy (XPS) and inductively coupled plasma mass spectrometry optical emission spectroscopy (ICP-OES) were conducted to gain insights into the surface composition and elemental distribution of the catalysts following potential cycling. Additionally, electrochemical mass spectrometry (EC-MS) was employed to quantify the faradaic efficiency of oxygen, providing an understanding of at what point the catalyst begins converting and how much of the OER current is directed towards OER versus metal conversion.

This project advances the fundamental understanding of NiFeCo catalyst conversion, shedding light on the critical role of catalyst composition and investigating the conditions that foster optimal conversion efficiency. The use of physical vapor deposition for thin film catalysts offers distinct advantages, making deposition and scaling processes facile. This capability, coupled with the gained insights into conversion dynamics, sets the stage for in situ optimization of the surface to achieve the active oxyhydroxide phase with the highest performance for AEM water electrolysis.