The Catalysis of the Oxygen Evolution Reaction by Iron Impurities in Thin Film Nickel Oxide Electrodes

Abstract

The effects of iron coprecipitated into thin film nickel oxide electrodes or introduced in the 25 weight percent KOH electrolyte were studied by a combination of galvanostatic and open-circuit decay methods. Effects on voltammetric behavior and on the oxygen overpotential were observed in thin film electrodes with as little as 0.01% iron present. The overpotential was further lowered by 1% iron to the extent that the nickel oxide charge storage reaction was adversely affected. Higher concentrations of iron coprecipitated into a composite iron/nickel hydrous oxide dramatically catalyzed the oxygen evolution reaction by lowering the Tafel slope from about 70 mV/decade with no iron present to about 25 mV/decade with 10-50% iron. This composite oxide shows promise as an anodic electrocatalyst for alkaline electrolysis to produce hydrogen; the oxygen overpotential on thin film electrodes was over 200 mV lower than the overpotential with nickel oxide, itself, when polarized at 80 mA/cm 2. This work is part of a larger effort to understand the effects of various chemical additives and impurities on the performance of nickel oxide I charge storage electrodes. The specific interest here is on the effect of iron impurities which can poison the nickel oxide active material particularly in nickel-iron batteries (1) but also in other nickel batteries (2). Since iron has a lower oxygen overpotential than nickel (3), iron is thought to exert its effect by catalyzing the oxygen evolution reaction which is a parasitic side reaction during charge. The present work contributes some quantitative measurements of the effect of iron concentration on the oxygen evolution overpotential and the discharge capacity of thin film nickel oxide electrodes,The work here was originally directed towards battery applications. Thus, the electrolyte, the electrode preparation, and the electrochemical procedures used were chosen to be most relevant to sintered nickel oxide battery electrodes. However, a potentially more important application of this work is to oxygen evolving anodes for the electrolytic production of hydrogen. Some predict that future use of hydrogen as a fuel may dwarf the present market for hydrogen in the fertilizer, chemical, and metallurgical industries (4, 5). Electrolysis is an attractive process for hydrogen production when cheap electrical power is available. The present work with composite oxides of iron and nickel impacts on a major source of inefficiency in commercial electrolyzers, the oxygen overpotential at the nickel anodes customarily used (6). We have previously reported exceptionally strong catalytic properties for the evolution of oxygen and the possible utilization of these properties for alkaline electrolysis (7). Subsequently, another report has confirmed the exceptional catalytic properties of these materials (8). The present paper presents a more detailed picture of the oxygen evolution kinetics on composite hydrous oxides of iron and nickel with a view towards electrolysis applications. ExperimentalElectrolyt...

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