Electrochemical Characterization and Development of Nickel-based Materials as Oxygen Evolution, Hydrogen Evolution, and Urea Oxidation Electrocatalysts

Abstract

In the past decade, energy generation using solar, wind, and hydroelectricity has increased dramatically. Widespread adoption of these renewable sources, however, is bottlenecked by energy conversion and storage devices such as fuel cells and batteries. Hydrogen is a promising energy carrier that can be produced through low temperature alkaline water electrolysis, stored as a gas or liquid, and later converted back to electrical energy when needed. The cost-effectiveness of a hydrogen-based energy storage system, however, is in part limited by the high electrochemical overpotentials needed to drive the hydrogen evolution reaction (HER), oxygen evolution reaction (OER), and/or urea oxidation reaction (UOR). Consequently, development and understanding of advanced electrocatalysts is a critical step in realizing hydrogen energy for grid-scale power applications. In this work, nickel and nickel-transition metal alloys were investigated as HER, OER, and UOR catalysts. Reproducible oxide layers were grown by potential cycling between 0.85 and 1.52 V vs. RHE up to 600 cycles, and the transition between Ni(OH)2 and NiOOH was monitored by cyclic voltammetry throughout. Through voltammograms and a Tafel analysis, it was determined that dissolution of chromium and molybdenum led to the formation of high electrochemical surface area electrodes with increased formation of the γ-NiOOH phase. Alloys with dissimilar Cr:Mo ratios leached significantly more, suggesting an electrode with similarly high Cr and Mo content is more stable in the examined conditions. Dissolution of Cr was verified through the use of x-ray impedance spectroscopy. The equal Cr:Mo concentration alloy and pure Ni developed a primarily β-NiOOH surface, and had 1.8–2.0 times larger TOF values than those containing significant γ-NiOOH. The NiCrMo alloys required smaller overpotentials (54–80 mV) to produce 10 mA cm−2 of OER current, and had comparable Tafel slopes to pure Ni. The findings here indicate a β-NiOOH-developed surface to be more OER-active than a γ-NiOOH-developed surface and suggest certain NiCrMo alloys have promise as OER electrocatalysts. Next, the OER-active nickel-oxyhydroxide (NiOOH) phases were characterized using cyclic voltammetry, impedance spectroscopy, and scanning electron microscopy on pure Ni. Using selective electrochemical cycling from 0.9 VRHE to switching potentials between 1.51 and 1.61 VRHE in 0.5 M KOH at 25 ◦C, it was determined that the γ-NiOOH phase would preferentially form at higher switching potentials. This phenomenon was attributed to induced surface roughening through NiOOH lattice expansion and contraction, thereby enhancing electrochemical surface area (ECSA) and improving the intercalation of cations. The resulting increase in the number of grain boundaries was verified through scanning electron microscopy. Kinetics of the OER were evaluated using Tafel analysis and turnover frequency (TOF); an electrode developed with a switching potential of 1.51 VRHE had TOF values 6–17 times larger than an electrode developed with a switching potential of 1.61 VRHE, indicating improved OER kinetics of the β-NiOOH phase. The results from this study provided evidence for the relative activity of NiOOH phases, and showed that selective electrochemical cycling can be used to control the formation of NiOOH species. The findings in these studies indicate that, while alkaline water electrolysis is a mature technology, there is still significant room for improvement using nickel-based electrodes. Moreover, the techniques developed here may have application in other NiOOH-based systems for examining and improving their electrocatalytic performance.