Reference Electrodes in PEM Water Electrolysis – a Review and Experimental Investigation of Oxygen and Hydrogen Evolution Reaction Kinetics

Abstract

Membrane electrode assemblies (MEA) and specifically the anode catalyst are of particular interest in PEM water electrolysis (WE) research as they are considered key components for the capital expenditure and operating expenditure of a PEMWE system (1). An effective method to understand the operation of the MEA is to apply diagnostic tools during the operation of the electrolysis cell. Previous studies in this regard have been mainly limited to the total cell voltage. However, the individual electrode behavior cannot be investigated in the standard setup. To improve the MEA design, the individual electrode behavior is crucial. This can be captured by using a reference electrode (RE). In this work, a salt bridge is used for the first time as RE concept for polarization curves and electrochemical impedance spectroscopy (EIS). The implementation is shown in figure 1.a. Via a porous transport layer impregnated with Nafion, the reference electrode can access the ionic potential of the catalyst layer under investigation. This isolates the kinetic overvoltage from other cell voltage contributions such as ohmic membrane losses, see figure 1.b. The experiments in this study include polarization curves with conventional materials and operating conditions. In addition, EIS is performed to test the applicability of the measurement method in RE operation. Exemplary results are shown in Figure 1.c. In addition, a previous analysis of RE from fuel and electrolytic cells is presented and a classification by electrode type and positioning is made. The key results are subsequently summarized. First, a distinction is made according to the principle behavior of the electrode or its possible configurations. The principal electrode behavior includes the dynamic hydrogen electrode (DHE), the quasireference electrodes, external REs and other configurations. Concrete concepts follow from this. For example, the DHE can be implemented with two platinum wires. By applying a current in the microampere range, the hydrogen evolution reaction takes place in the presence of water. The platinum wire in hydrogen atmosphere becomes the RE (2). Quasireference electrodes are very commonly used with ionic liquids. They are often metal wires (3). A platinum wire is used in a recent work for recording individual electrochemical impedance spectra of anode and cathode (4). Any type of RE can be used as an external RE, such as the silver-silver chloride electrode. Another possibility is to create a free-standing catalyst strip by laser ablation of the MEA. This strip can be used as a reversible electrode to measure the voltage difference with the active catalyst layer participating in the reaction (5). In addition to the behavior of the electrode, a subdivision is made into the position of the RE. In order to separate the potential of one of the two electrodes from the total potential, the measurement can be made as close as possible to the electrode under investigation. This can be implemented by a direct measurement in the catalyst layer (6) or a special geometry of the active area (7). Alternatively, the potential can be measured at the membrane and a correction of the ohmic membrane losses can be used to infer the electrode potential to be investigated. Here, too, two implementations are possible. On the one hand, the RE can be positioned between two membrane halves (4). On the other hand, the membrane can be contacted outside the active area (8). One requirement chosen for the presented concept is the applicability of the RE independent of the geometry of the MEA. Furthermore, the RE should be insensitive to misalignment of the catalyst layers. The exact alignment of the anodic and cathodic catalyst layers is challenging and even small deviations lead to a shift of the potential in the membrane (8). For the two reasons mentioned above, the concept of salt bridge with external RE is used in the direct approach at the electrode. References The national hydrogen strategy (Juni 2020). M. V. Lauritzen, P. He, A. P. Young, S. Knights, V. Colbow and P. Beattie, Journal of New Materials for Electrochemical Systems, 10(3), 143–145 (2007). G. Inzelt, A. Lewenstam, F. Scholz and F. G. K. Baucke, Editors, Handbook of reference electrodes, Springer, Berlin (2013). A. Hartig-Weiß, M. Bernt, A. Siebel and H. A. Gasteiger, J. Electrochem. Soc., 168(11), 114511 (2021). D. Gerteisen, J Appl Electrochem, 37(12), 1447–1454 (2007). E. Brightman, J. Dodwell, N. van Dijk and G. Hinds, Electrochemistry Communications, 52, 1–4 (2015). A. A. Kulikovsky and P. Berg, J. Electrochem. Soc., 162(8), F843-F848 (2015). G. Li and P. G. Pickup, Electrochimica Acta, 49(24), 4119–4126 (2004). Figure 1