The Role of Individual Surface Species in the Volcano-Type Behavior of HER on Nickel

Traditional design strategies for catalytic materials for HER rely on the volcano plot paradigm, where the metal-Had bond energy is used as a single activity-descriptor. However, the use of metal-Had energetics alone completely fails to predict the behavior of HER in alkaline electrolytes. We have persistently drawn attention to the importance of kinetic synergy (bifunctional nature) of the HER in alkaline electrolytes, where both the beneficial OHad–M and Had–M energetics are necessary for achieving a highly effective catalyst [1-4]. This is especially evident on metal surfaces, decorated with small clusters of Ni(OH)2, where an order of magnitude increase in catalytic activity for the HER can most often be achieved [5]. Furthermore, we have stressed the importance of spectator species in any satisfactory description of most common electrocatalytic reactions, with HER being no exception [6,7]. Nonetheless, both topics are still a subject of many academic discussions up to this day. In this presentation, we will focus on the electrochemical behavior of Ni in alkaline electrolyte. We argue that the experimentally obtained “intrinsic” HER activity values reported in the available literature are misleading possibly due to two main challenges: i) Ni has very complex surface chemistry (because of the metal-hydride formation during the HER and/or the partial coverage by various (hydr)oxide species, even in the HER potential region), and ii) analysis of the material intrinsic properties has most often not been conducted on well-defined systems. By overcoming these two challenges, we will show the role of the individual Ni surface species in the kinetics of the HER, identify the active sites involved in the reaction, and present strategies for controlling and manipulating the electrochemical interface to enhance the efficiency of Ni-based material for HER. Finally, we again highlight the importance of 2 major factors controlling the rate of HER in alkaline solutions on Ni-based catalysts: the availability of active sites on Ni electrode surface [the 1−Θad term] and the energetics of the activated water complex [the ΔG0# (H2O) term]. Through meticulous experimental design, we were able to isolate and examine these variables, revealing their distinct influence on reaction kinetics. We will discuss our findings in the broader context of the volcano plot for HER. References: [1] Stamenkovic, V.R., Strmcnik, D., Lopes, P.P. and Markovic, N.M., Nature Materials, 16 (2017) [2] Subbaraman, R., Tripkovic, D., Strmcnik, D., Chang, K.C., Uchimura, M., Paulikas, A.P., Stamenkovic, V.R., Markovic, N.M., Science, 334 (2011) [3] Subbaraman, R., Tripkovic, D., Chang, K.C., Strmcnik, D., Paulikas, A.P., Hirunsit, P., Chan, M., Greeley, J., Stamenkovic, V.R., Markovic, N.M., Nature Materials, 11 (2012) [4] Strmcnik, D., Lopes, P. P., Genorio, B., Stamenkovic, V. R. & Markovic, N. M., Nano Energy, 29 (2016) [5] Danilovic, N., Subbaraman, R., Strmcnik, D., Chang, K.C., Paulikas, A.P., Stamenkovic, V.R., Markovic, N.M., Angewandte Chemie International Edition, 124 (2012) [6] Stamenkovic, V.R., Fowler, B., Mun, B.S., Wang, G., Ross, P.N., Lucas, C.A., Markovic, N.M., Science, 315 (2007) [7] Strmcnik, D., Uchimura, M., Wang, C., Subbaraman, R., Danilovic, N., Van Der Vliet, D., Paulikas, A.P., Stamenkovic, V.R. and Markovic, N.M., 2013, Nature Chemistry, 5 (2013)