UC Berkeley

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.