Poor oxygen evolution reaction (OER) catalysis limits the efficiency of H2 production from water electrolysis and photoelectrolysis routes to large-scale energy storage. Despite nearly a century of research, the factors governing the activity of OER catalysts are not well understood. In this Perspective, we discuss recent advances in understanding the OER in alkaline media for earth-abundant, first-row, transition-metal oxides and (oxy)­hydroxides. We argue that the most-relevant structures for study are thermodynamically stable (oxy)­hydroxides and not crystalline oxides. We discuss thin-film electrochemical microbalance techniques to accurately quantify intrinsic activity and in situ conductivity measurements to identify materials limited by electronic transport. We highlight the dramatic effect that Fe cationsadded either intentionally or unintentionally from ubiquitous electrolyte impuritieshave on the activity of common OER catalysts. We find new activity trends across the first-row transition metals, opposite of the established ones, and propose a new view of OER on mixed-metal (oxy)­hydroxides that illustrates possible design principles and applications.

Heterogeneous electrocatalysts for the oxygen evolution reaction (OER) are complicated materials with dynamic structures. They can exhibit potential-induced phase transitions, potential-dependent electronic properties, variable oxidation and protonation states, and disordered local/surface phases. These properties make understanding the OER, and ultimately designing higher efficiency catalysts, challenging. We report a series of procedures and measurement techniques that we have adopted or developed to assess each of the above challenges in understanding materials for the OER. These include the targeted synthesis of hydrated oxyhydroxide phases, the assessment and elimination of electrolyte impurities, the use of a quartz crystal microbalance to monitor film loading and dissolution, and the use of an in situ conductivity measurement to understand the flow of electrons from the catalyst active sites to the conductive support electrode. We end with a recipe for the synthesis and characterization of a "standard" Ni(Fe)O x H y catalyst that can be performed in any laboratory with a basic electrochemical setup and used as a quantitative comparison to aid the development of new OER catalyst systems.

One practical metric for electrocatalyst performance is current per geometric area at a given applied overpotential. An obvious route to increase performance is to increase the catalyst mass loading -as long as the intrinsic performance (i.e. specific activity or turnover frequency) of the catalyst is independent of loading, and other electrical, ionic, or mass-transfer resistances are not severe. Here we report the geometric and intrinsic oxygen evolution reaction (OER) activities of Ni(Fe)OOH films, the fastest known water oxidation catalyst in basic media, as a function of mass loading from 0 to ~100 µg cm -2 . We discuss practices for measuring and reporting intrinsic activities, highlighting experimental conditions where the film activity on a per-metal-cation basis can be accurately measured and where capacitance measurements of electrochemically active surface area fail. We find that the electrochemical reversibility of the (nominally) Ni 2+/3+ redox wave correlates with the apparent intrinsic activity as a function of loading. We report a pulsed-electrodeposition method that dramatically improves the catalyst reversibility and performance at high loading compared to continuous electrodeposition, which we attribute to improved connectivity in the micro/nanostructure and better composition control.Pulse electrodeposited films are shown to have geometric performance similar to a number of advanced composite electrocatalyst structures, and to maintain effective per-metal turnover frequencies of > 0.4 s -1 at 300 mV overpotential even for loadings of ~100 µg cm -2 .

Fe-doped Ni (oxy)hydroxide, Ni(Fe)O x H y , is one of the most-active oxygen-evolution-reaction (OER) catalysts in alkaline conditions, while Fe-free NiO x H y is a poor OER catalyst. One approach to better understand the role of Fe, and enable the design of catalysts with higher activities, is to find other cations that behave similarly and compare the common chemical features between them. Here we evaluate the effects of La, Mn, Ce, and Ti incorporation on the OER activity and redox behavior of NiO x H y in rigorously Fe-free alkaline solution using cyclic voltammetry and electrochemical quartz-crystal microgravimetry. We use X-ray photoelectron spectroscopy and time-of-flight secondary-ionmass spectrometry to confirm that measurements are free from relevant levels of trace Fe contamination. We find that only Ce leads to increased activity in NiO x H y (about a factor of 10 enhancement), but this effect is transient, likely due to phase separation. We further find no clear correlation between activity and the nominal Ni 2+/3+ redox potential, suggesting that the "oxidizing" power of the Ni is not directly correlated with the OER activity. These findings suggest a uniqueness to Fe and are consistent with it being the active site in Ni(Fe)O x H y .

Ni−Fe (oxy)hydroxides, Ni (1−z) Fe z O x H y , are among the fastest-known water oxidation catalysts in alkaline media on a per-cation basis. At current densities relevant for electrolysis (e.g., >0.5 A/cm −2 ), mass and electron transport through catalyst films with high mass loading are critical and depend substantially on the extended and intermediate catalyst architecture. Here we use X-ray pair distribution function (PDF) analysis to determine the intermediate nanostructures of electrodeposited Ni (1−z) Fe z O x H y films. We report the effects of electrodeposition technique (pulsed versus continuous), electrochemical cycling, and Fe content on the structure of the catalyst film. The PDF patterns for Ni (1−z) Fe z O x H y films are best simulated by model structures consisting of brucite-like β-Ni(OH) 2 fragments 1 to 3 layers in thickness. Only the oxidation state of the film significantly affects the intralayer scattering behavior (i.e., metal−oxygen bond distance). The interlayer interactions, however, are affected by Fe content and deposition conditions. The domain size of many of the systems are similar, extending to ∼5 nm, which are best modeled by sheets containing upward of ∼250 metal cations. Smaller domains were found for films deposited through a larger number of electrochemical cathodic current pulses. Films can be cycled between as-deposited, oxidized, and reduced states, with minimal loss of intrasheet coherence, indicating a degree of structural stability. We estimate heterogeneity in the domain structures by modeling the PDF data to linear combinations of oxyhydroxide fragments with different sizes and numbers of layers.