Oxygen Electrocatalysis on Mixed-Metal Oxides/Oxyhydroxides: From Fundamentals to Membrane Electrolyzer Technology

Abstract

Metrics & MoreArticle Recommendations CONSPECTUS: Catalyzing the oxygen evolution reaction (OER) is important for key energy-storage technologies, particularly water electrolysis and photoelectrolysis for hydrogen fuel production. Under neutral-to-alkaline conditions, first-row transitionmetal oxides/(oxy)hydroxides are the fastest-known OER catalysts and have been the subject of intense study for the past decade. Critical to their high performance is the intentional or accidental addition of Fe to Ni/Co oxides that convert to layered (oxy)hydroxide structures during the OER. Unraveling the role that Fe plays in the catalysis and the molecular identity of the true "active site" has proved challenging, however, due to the dynamics of the host structure and absorbed Fe sites as well as the diversity of local structures in these disordered active phases.In this Account, we highlight our work to understand the role of Fe in Ni/Co (oxy)hydroxide OER catalysts. We first discuss how we characterize the intrinsic activity of the first-row transition-metal (oxy)hydroxide catalysts as thin films by accounting for the contributions of the catalyst-layer thickness (mass loading) and electrical conductivity as well as the underlying substrate's chemical interactions with the catalyst and the presence of Fe species in the electrolyte. We show how Fe-doped Ni/Co (oxy)hydroxides restructure during catalysis, absorb/desorb Fe, and in some cases degrade or regenerate their activity during electrochemical testing. We highlight the relevant techniques and procedures that allowed us to better understand the role of Fe in activating other first-row transition metals for OER. We find several modes of Fe incorporation in Ni/Co (oxy)hydroxides and show how those modes correlate with activity and durability. We also discuss how this understanding informs the incorporation of earthabundant transition-metal OER catalysts in anion-exchange-membrane water electrolyzers (AEMWE) that provide a locally basic anode environment but run on pure water and have advantages over the more-developed proton-exchange-membrane water electrolyzers (PEMWE) that use platinum-group-metal (PGM) catalysts. We outline the key issues of introducing Fe-doped Ni/Co (oxy)hydroxide catalysts at the anode of the AEMWE, such as the oxidative processes triggered by Fe species traveling through the polymer membrane, pH-gradient effects on the catalyst stability, and possibly limited catalyst utilization in the compressed stack configuration. We also suggest possible mitigation strategies for these issues. Finally, we summarize remaining challenges including the long-term stability of Fe-doped Ni/Co (oxy)hydroxides under OER conditions and the lack of accurate models of the dynamic active surface that hinder our understanding of, and thus ability to design, these catalysts.

Synergistic Effect of Bimetallic Sulfide Synthesized by a Simple Solvothermal Method for High-Efficiency Oxygen Evolution Reaction

“…As shown in Figure A, the oxidation peak was caused by transformation of Ni 2+ to Ni 3+ . Also, the oxidation peak of Ni 2+ /Ni 3+ in Fe–Ni-based OER catalysts shifts positive and becomes weak with the increment of Fe element ratio, which could be assigned to the fact that the strong interaction between Fe and Ni significantly affects the average redox performances and electronic structure of Ni. − According to the LSV curve, the NiS nanoparticles need an overpotential of 271 mV to deliver a current density of 10 mA cm –2 , while FeS 2 needs an overpotential of 317 mV to reach the same current density. As for the FeNiS-10 electrode, the overpotential at the current density of 10 mA cm –2 decreases to 242 mV, which may be caused by the synergistic effect of the elements nickel and iron.…”

Purification of Residual Ni and Co Hydroxides from Fe‐Free Alkaline Electrolyte for Electrocatalysis Studies

“…However, slow oxygen-evolution-reaction (OER) kinetics limit electrolyzer performance, and it remains a challenge to discover inexpensive OER catalysts with both high activity and long-term stability. [1][2][3][4] Materials based on abundant first-row transition metals like Fe, Ni, and Co are among the most promising with some already used in commercial alkaline electrolyzers. [5][6][7][8] Nickel (oxy)hydroxide in particular has been studied for decades as a high-activity OER catalyst in alkaline conditions.…”

“…Water electrolysis using renewable electricity is a promising route to produce clean renewable hydrogen. However, slow oxygen‐evolution‐reaction (OER) kinetics limit electrolyzer performance, and it remains a challenge to discover inexpensive OER catalysts with both high activity and long‐term stability [1–4] . Materials based on abundant first‐row transition metals like Fe, Ni, and Co are among the most promising with some already used in commercial alkaline electrolyzers [5–8] …”

Phase transition of SrCo0.9Fe0.1O3 electrocatalysts and their effects on oxygen evolution reaction

“…Development of high‐performance electrocatalysts of oxygen evolution reaction (OER) is essential for various sustainability applications, including water electrolyzer for low‐temperature green hydrogen generation. The sluggish kinetics of OER together with the requirement in durability under harsh operating conditions hinder the widespread applications of water electrolysis at the industrial scale 1–15 . Currently, IrO 2 and, to a lesser extent, RuO 2 are used as OER catalysts in industries and they are also served as the benchmark catalysts 2,16–19 .…”