Deposition of Nickel Hydroxide on Pt/C to Improve the Efficiency of the Hydrogen Evolution Reaction in Solid-State Alkaline Water Electrolyzers

There is an ongoing effort to develop alkaline stable anion exchange membranes to be employed as solid-electrolytes in alkaline membrane fuel cells (AMFCs) and solid-state alkaline water electrolyzers so as to take advantage of the distinct and attractive advantages of operation at high pH. These advantages, which are not accessible to acidic proton-exchange membrane fuel cells and water electrolyzers, include the possibility of using stainless steel flow fields and endplates (with the subsequent reduction in costs), more flexibility in the choice of fuels, and the possibility of using cheaper non-platinum-group-metal electrocatalysts for both the hydrogen evolution/oxidation and the oxygen evolution/reduction reactions (1). However alkaline operation has an important drawback. The hydrogen evolution reaction (HER) is much more sluggish because of the low concentration of protons in alkaline media and the lower activity of noble metals for this reaction. Pt electrocatalysts show considerable lower HER activity in alkaline conditions than in acidic conditions. The following mechanism for water electrolysis in alkaline conditions has been proposed: a) Volmer: H2O+e- +* → Had + OH- [1] b1) Tafel: Had + Had → Had + 2OH-+2* [2] b2) Heyrovsky: Had + H2O+e- → H2 + OH-+ * [3] It is generally accepted that the Volmer step is the rate determining step (rds) for the overall reaction. This assumption implies that the presence of a bi-functional catalyst with hydrophilic domains able to stabilize the water in the transition complex will speed up the overall reaction, as observed by several authors for platinum, iridium, ruthenium and nickel electrocatalysts modified by deposition of transition metal hydroxides (i.e. nickel hydroxide) (2, 3). In this work, we investigated the HOR/HER kinetics on Pt/C and Pt/C/Ni(OH)2 bi-functional electrocatalysts. Our objective was to determine the optimum concentration of Ni(OH)2 required for the best performance of the electrocatalyst for the HER under alkaline conditions. The catalysts were prepared by mixing the required amounts of colloidal dispersions of Ni(OH)2 with commercial Pt/C catalyst previously dispersed (using ultrasonication) in water. The RDE measurements were done in 0.1M KOH, at temperatures ranging from 0°C to 30°C to extract useful kinetic information such as exchange current densities and activation energies for each catalyst. Kinetic currents were obtained after IR and mass transport corrections. The HOR/HER kinetic current densities (see Figure 1) were fitted using the Butler–Volmer equation to estimate the exchange current densities for each catalyst at each temperature. Arrhenius behavior was observed. The activation energy was the same for all Pt/C and the bi-functional catalysts, at 37.2 kJ/mol, which was in good agreement with previously published value for Pt (4). The highest exchange current densities were obtained for the catalyst containing 10wt% Ni(OH)2. Higher Ni(OH)2 concentrations resulted in a poorer performance due to the coverage of the Pt active sites; a reduction in ECSA was observed. The bi-functional catalysts were also tested in a solid-state alkaline water electrolyzer operated with ultrapure water. The membrane electrode assemblies were fabricated with a commercial anion exchange membrane The anode catalyst was iridium oxide (2.5 mg/cm2). The optimal bi-functional electrocatalyst outperformed Pt/C by 0.1-0.2 V at relevant current densities. References 1. N. W. Li and M. D. Guiver, Macromolecules, 47, 2175 (2014). 2. N. Danilovic, R. Subbaraman, D. Strmcnik, K. C. Chang, A. P. Paulikas, V. R. Stamenkovic and N. M. Markovic, Angewandte Chemie, 51, 12495 (2012). 3. D. Strmcnik, M. Uchimura, C. Wang, R. Subbaraman, N. Danilovic, D. van der Vliet, A. P. Paulikas, V. R. Stamenkovic and N. M. Markovic, Nature chemistry, 5, 300 (2013). 4. W. Sheng, H. A. Gasteiger and Y. Shao-Horn, Journal of The Electrochemical Society, 157, B1529 (2010). Figure 1