Probing the Surface Area of Nickel Catalyst Materials: The Effect of Oxalate Adsorption on the Electrochemistry of Nickel in Basic Media

INTRODUCTION Nickel and nickel-based materials find widespread applications as secondary battery anodes, supercapacitors, electro-catalysts, photocatalysts, and photovoltaic cell components (e.g., [1]-[3]). In alkaline media, nickel-based electrodes are effective catalysts for the hydrogen and oxygen evolution reactions (HER, OER), by which hydrogen may be renewably produced as fuel or industrial feedstock. However, accurate methods to measure the electrochemically active surface area (ECSA) for nickel-based electrodes have not yet been established [4]. The ECSA directly affects the catalytic activity and electrode capacitance, hence, a reliable method for the measurement of the ESCA is desirable. In this work, a new method to probe the surface area of metallic nickel electrodes utilizing voltammetry will be presented. The results are compared to alternative ECSA measurement techniques in terms of accuracy, precision and practical applications for nickel-based catalyst materials. RESULTS AND DISCUSSION Ni electrodes were mechanically polished using 320 grit SiC paper, 9 and 3 μm polycrystalline diamond pastes and a 50 nm Al2O3 suspension. Electrochemical measurements were collected in 0.1 or 1 M KOH with and without the addition of 0.001 M – 0.1 M K2C2O4. In comparison with a forward voltammetric sweep in an electrolyte containing only KOH, the addition of oxalate anions to the solution has no observed effect on the voltammetry of metallic nickel in the hydrogen evolution reaction (HER) or the Ni/Ni(II) oxidation potential regions (< 1.3 VRHE). This result is similar to those reported for several other organic additives [5]. However, whereas literature evidence shows that at higher potentials many organic additives oxidize at a diffusion-limited rate, oxalate anions do not show the expected oxidation current. Instead, we observe that the Ni(II)/Ni(III) oxidation peak, normally at ~1.45 VRHE, occurs at lower potentials (1.35 – 1.40 VRHE). Further, in the presence of sufficiently high concentrations of oxalate anions, the Ni(II)/Ni(III) oxidation peak is very sharp, with full widths at half maxima (FWHMs) less than 20 mV. The voltammetric peak characteristics, i.e., the peak position, width and area, have been measured at various concentrations of oxalate and at various potential sweep rates. The peak position is very sensitive to the electrode’s surface state. However, the peak width and area are very reproducible. The practical result of the present work is that we now have a sharp voltammetric peak that is well-separated from concurrent electrochemical processes, in particular the OER, which typically onsets near 1.45 - 1.5 VRHE. The electrical charge associated with this oxidation peak is consistent with a process involving a monolayer. The implementation of these results for measuring the ECSA of metallic nickel electrodes has been tested by conducting voltammetry on different electrode preparation regimes, which ranged from a rough polish to a mirror finish. The validity of this characterization method will be discussed and the results will be compared with alternative methods [6]. The extension of this work to characterizing nickel-based catalyst materials will be discussed. ACKNOWLEDGEMENTS This work was supported by National Research Council Canada (NRC), the Natural Sciences and Engineering Research Council of Canada (NSERC) and the University of Ottawa. REFERENCES 1. D. Chade, L. Berlouis, D. Infield, A. Cruden, P. T. Nielsen and T. Mathiesen, Int. J. Hydrogen Energy, 38, 14380 (2013). 2. Y. Miao, L. Ouyang, S. Zhou, L. Xu, Z. Yang, M. Xiao and R. Ouyang, Biosens. Bioelectron., 53, 428 (2014). 3. Y. Wang, S. Gai, N. Niu, F. He and P. Yang, J. Mater. Chem. A, 1, 9083 (2013). 4. D. S. Hall, C. Bock and B. R. MacDougall, J. Electrochem. Soc., 160, F235 (2013). 5. M. Fleischmann, K. Korinek and D. Pletcher, J. Electroanal. Chem. Interfacial Electrochem., 31, 39 (1971). 6. S. Trasatti and O. A. Petrii, J. Electroanal. Chem., 327, 353 (1992).

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