Characterization of Plasma-Sprayed Nickel-Based Electrodes for Oxygen and Hydrogen Evolution in Alkaline Media

Abstract

Low-carbon hydrogen is forecast to play an essential role in reducing global greenhouse gas emissions in the future. To establish the wide-scale adoption of low-carbon hydrogen via water electrolysis, reducing the levelized cost of hydrogen through high-performing electrocatalysts is necessary [1]. Alkaline-based water electrolysis has seen a recent resurgence in research and development, due to the growing concern over the reliance on platinum group metals as electrocatalysts in acidic conditions [2]. However, in alkaline water electrolysis, the kinetics of both the oxygen and hydrogen evolution reaction are considered sluggish as opposed to acidic conditions which favour hydrogen formation [3]. This severely limits the electrical efficiency of alkaline water electrolysis and leads to an increased cost of hydrogen. Herein lies a need to develop low-cost, highly active, non-platinum group metal electrocatalysts for alkaline water electrolysis at an industrially relevant scale. Plasma thermal spraying has recently gained attention as a large-scale electrode manufacturing technique. In this process, a catalyst powder is fed into the plasma plume of the plasma torch. Within the high temperature, high velocity plume the powder particles are melted and accelerated towards the substrate, where they collide to form a catalyst coating layer upon impact [4]. This low catalyst loading leads to a reduced capital cost. With versatile feed material and operating conditions, this method offers high tunability and is suitable for fabricating large surface area electrodes [5]. Additionally, this method allows for the rapid screening of materials and surfaces due to its short synthesis time. In this study, the feasibility of plasma-sprayed nickel-based electrocatalysts for alkaline water electrolysis was investigated. A range of nickel-based metals, alloys, and oxides were synthesized via plasma thermal spraying and characterized using scanning electron microscopy (SEM), x-ray diffraction, and x-ray fluorescence. SEM imaging revealed the molten nature of the deposited “splats”, which resulted in rough surfaces with high active surface areas (Figure 1). Electrochemical characterization was carried out using two and three-electrode testing. Activity toward both the oxygen and hydrogen evolution reaction was studied using three-electrode testing. It was found that with the addition of a plasma-sprayed nickel layer, the overpotential towards the hydrogen and oxygen evolution where reduce by 233 and 21 mV at -0.3 and 0.3 A cm -2 , respectively, compared to a benchmark stainless steel electrode. During two-electrode testing, the stability and cell performance of 25 cm -2 electrodes were investigated. The plasma-sprayed electrodes were stable at high operating current densities with minimal voltage decay. These preliminary results suggest plasma-sprayed nickel-based electrodes are viable electrocatalysts for alkaline water electrolysis. With these promising initial results, further surface modifications to increase electrochemical activity and electrical efficiency will be investigated. References [1] International Renewable Energy Agency, “Global hydrogen trade to meet 1.5 C climate goal,” International Renewable Energy Agency, Abu Dhabi, 2022. [2] Z. Chen, D. Xiaoguang, W. Wei, S. Wang and B.-J. Ni, “Recent advances in transition metal-based electrocatalysts for alkaline hydrogen evolution,” Journal of Materials Chemistry A, vol. 7, no. 25, pp. 14971-15005, 2019. [3] F. Razmjooei, T. Liu, D. A. Azevedo, E. Hadjixenophontos, R. Reissner, G. Schiller, S. A. Ansar and K. A. Friedrich, “Improving plasma sprayed Raney-type nickel–molybdenum electrodes towards high-performance hydrogen evolution in alkaline medium,” Nature, vol. 10, no. 1, p. 10948, 2020. [4] J. R. Davis, “Introduction to Thermal Spray Processing,” in Handbook of Thermal Spray Technology , ASM International, 2004, pp. 3-13. [5] J.-E. Kim, K.-K. Bae, C.-S. Park, S.-U. Jeong, K.-H. Baik, J.-W. Kim, K.-S. Kang, K.-B. Lee and Y.-H. Kim, “Electrochemical characterization of Raney nickel electrodes prepared by atmospheric plasma spraying for alkaline water electrolysis,” Journal of Industrial and Engineering Chemistry, vol. 70, pp. 160-168, 2019. Figure 1