Fe is a critical component of record-activity Ni/Fe (oxy)hydroxide (Ni(Fe)OH) oxygen evolution reaction (OER) catalysts, yet its precise role remains unclear. We report evidence for different types of Fe species within Ni(Fe)OH- those that are rapidly incorporated into the Ni oxyhydroxide from Fe cations in solution (and that are likely at edges or defects) and are responsible for the enhanced OER activity, and those substituting for bulk Ni that modulate the observed Ni voltammetry. These results suggest that the exceptional OER activity of Ni(Fe)OH does not depend on Fe in the bulk or on average electrochemical properties of the Ni cations measured by voltammetry, and instead emphasize the role of the local structure.

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.

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Iron-doped, nickel oxyhydroxide (Ni­(Fe)­OOH) is one of the best catalysts for the oxygen evolution reaction (OER) under alkaline conditions. Due to Ni­(Fe)­OOH’s layered structure, electrolyte species are able to easily intercalate between the octahedrally coordinated sheets. Electrolyte cations have long been considered inert spectator ions during electrocatalysis, but electrolytes that penetrate into the catalyst may play a major role in the reaction process. In a joint theoretical and experimental study, we report the role of electrolyte counterions (K+, Na+, Mg2+, and Ca2+) on Ni­(Fe)­OOH catalytic activity in alkaline media. We show that electrolytes containing alkali metal cations (Na+ and K+) yield dramatically lower overpotentials than those with alkaline earth cations (Mg2+ and Ca2+). K+ and Na+ lower the overpotential because they have an optimal acidity and size that allows them to not bind too strongly or alter the stability of reaction intermediates. These two features required for intercalated cation species provide insight into selecting appropriate electrolytes for layered catalyst materials, and enable understanding the role(s) of electrolytes in the OER mechanism.

We show that a new terpyridine ligand comprising a directly-connected methyldisulfide group (tpySSMe) can be used to prepare a modular series of metal bis(terpyidine) complexes, [M(tpySSMe)2](PF6)2 (M = Fe, Co,...