Measurement Techniques for the Study of Thin Film Heterogeneous Water Oxidation Electrocatalysts

Abstract

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.

Hydration‐Effect‐Promoting Ni–Fe Oxyhydroxide Catalysts for Neutral Water Oxidation

“…The ECSA‐normalized current density of the HEP catalyst is fully 1.3‐times and 1.8‐times higher than those of reference Ni–Fe catalyst and HEL Ni–Fe–Ba catalyst, respectively, at 1.7 V versus RHE (Figure S9, Table S3, Supporting Information). The activation energy (Figure d) and turnover frequency (TOF, Figure S10 and Table S4, Supporting Information) confirm the same trend: the HEP catalyst exhibits the lowest activation energy of 64 kJ mol −1 and the highest TOF of 0.3 s −1 .…”

Selectively Etching Vanadium Oxide to Modulate Surface Vacancies of Unary Metal–Based Electrocatalysts for High‐Performance Water Oxidation

“…It is observed that η onset for Co 0.5 (V 0.5 ) is less than 250 mV, while the average overpotential required to achieve 10 mA cm −2 catalytic current density (η 10 ) is ≈300 mV (the best electrocatalyst in our case can achieve this at η 10 = 282 mV) ( Figure S13, Supporting Information), and the average current density that can be attained at an overpotential of 350 mV (J 350 ) is ≈42 mA cm −2 (54 mA cm −2 for the best electrocatalyst in our case). To make the comparison reasonable, electrocatalysts with carbon additives (e.g., carbon nanotubes and graphene) or porous substrates (e.g., Ni foam and carbon fiber paper) have been excluded from this list, owing to their "overestimated" geometric catalytic current density, as suggested by Stevens et al [19] Unexpectedly, Co 0.5 (V 0.5 ) is found to exhibit a significantly higher OER activity than most electrocatalysts in the list (Table S2, Supporting Information). Surprisingly, all the Co x (V 1−x ) samples, even without vanadium participation (i.e., Co 1 (V 0 )), are found to significantly outperform RuO 2 for OER (Figure 4a), demonstrating the advantage of this synthetic approach to obtain highly efficient electrocatalysts.…”

MoFe‐Codoped Ni3S2/Ni(OH)2 Nanosheets with Large Sample Size toward High‐Performance Oxygen Evolution

“…The electrocatalytic kinetics was evaluated by the charge transfer property determined from the Nyquist plots . From the semicircles diameter in the low‐frequency zone, the charge transfer resistance ( R ct ) of the composites is shown in Figure d.…”