HER Kinetics of Pt and Ni in Alkaline Medium at Elevated Temperatures (25-200 ºC) and Pressures (1-50 bar)

The hydrogen evolution reaction (HER) in alkaline medium is of great importance for the economic production of sustainable hydrogen. Nevertheless, the origin of the sluggish reaction kinetics observed in alkaline compared to acidic environment, remains a point of debate. [1-5] In order to promote understanding, and to extend the discussion at conditions closer to industrial operation, we have undertaken a systematic determination of the HER kinetics on Pt and Ni within a broad range of temperatures (25-200 ⁰C) and pressures (1-50 bar) in alkaline environment using 1 M or 10 M aqueous KOH as electrolyte. At present, the study of HER at high temperatures and pressures (HTP) is limited by the interim stability and lifetime of the available reference electrodes [6, 7]. We have overcome this limitation by employing a novel design for a Palladium hydride (PdH) and a reversible hydrogen (RHE) reference electrodes that enable facile and stable utilization at HTP [8]. A constant electrochemically driven supply of H 2 on the PdH surface ensures preservation of the mixed (α+β)-phase in the PdH layer and thereby stable potential at HTP conditions for prolonged periods. Furthermore, the potential of the PdH reference electrode has been calibrated as a function of temperature and pressure with respect to the reversible hydrogen electrode (RHE). The potential of the PdH electrode in the mixed (α+β)-phase is ~55 mV vs. the RHE at 25 ⁰C, 1 bar, and observed to be independent of temperature and pressure. As shown in Figure 1, the HER kinetics are faster on Pt relative to Ni and become more sluggish on both metal surfaces upon increasing KOH concentration from 1 to 10 M at 25 ⁰C, 1 bar. A more pronounced thermal activation is observed in 10 M versus 1 M KOH though, resulting in a reversal of the pH trend at 100 ⁰C, 1 bar. References Sheng, M. Myint, J. G. Chen, Y. Yan, Energy Environ. Sci. , 6 , 1509–1512 (2013). Ledezma-Yanez, W. D. Z. Wallace, P. Sebastián-Pascual, V. Climent, J. M. Feliu, M. T. M. Koper, Nature Energy , 2 , 17031–17037 (2017). Rossmeisl, K. Chan, E. Skúlason, M. E. Björketun, V. Tripkovic, Catalysis Today , 262 , 36–40 (2016). Subbaraman, D. Tripkovic, D. Strmcnik, K. Chang, M. Uchimura, A. P. Paulikas, V. Stamenkovic, N. M. Markovic, Science , 334 , 1256–1261 (2011). Liu, J. Li, L. Jiao, H. T. T. Doan, Z. Liu, Z. Zhao, Y. Huang, K. M. Abraham, S. Mukerjee, Qingying Jia, J. Am. Chem. Soc ., 141 , 3232−3239 (2019). H. Miles, G. Kissel, P. W. T. Lu, S. Srinivasan, J. Electrochem. Soc. , 123 , 332–336 (1976). J. Appleby, G. Crepy, J. Jacquelin, Int. J. Hydrog. Energy , 3 , 21–37 (1978). P. Leuaa, C. Chatzichristodoulou, J. Electrochem. Soc. , 169 , 054534 (2022). Figure 1 : (a) HER overpotential versus current density (normalized by the electrochemically active surface area) on Pt and Ni at 25 ⁰C, 1 bar, in 1 and 10 M KOH, and (b) temperature evolution of the HER overpotential at 10 mA/cm 2 (normalized by the electrochemically active surface area) for Pt and Ni at 25-100 ⁰C, 1 bar, in 1 and 10 M KOH. Figure 1