Lifting the Lids on Reference Electrodes in PEMWE: Previous Approaches and Current Application

Abstract

The authors of this contribution have undertaken a comprehensive review of reference electrodes (REs) used in PEM Water Electrolysis (PEMWE) research. The importance of REs cannot be understated as they are instrumental in determining the potential within the system and in separating the two electrodes. Examining the electrodes' characteristics yields insights into their individual performance and can e.g. help to assess new catalyst layer designs, their interplay with the adjacent porous transport layer or understand the complex and multi-faceted mechanisms of degradation1.Previous research on REs has generated a significant amount of information and detailed data for each reference electrode setup. However, this information is currently distributed across various publications. As a result, the present review compresses this information into a single, comprehensive resource. The review covers the classification of various reference electrodes, their advantages and disadvantages, and the process of implementing them. The purpose of the work is to provide guidance to researchers in the field, enabling them to effectively tailor the reference electrode to meet the specific demands and contexts of their research.Furthermore, two different RE approaches have been selected for experimental studies. The first one is a salt bridge reference electrode (SBRE), which probes to the proton potential of the catalyst layer at the interface to the porous transport layer (PTL). It has been used for analysis of half-cell kinetics. At current densities beyond 0.1 A cm-2, a trade-off between the needed ionic connectivity of the PTL and additional losses limited the application2. A systematic study is carried out to improve the PTL impregnation with ionomer resulting in almost no performance decline. The improved setup is used to study the effect of temperature on the half-cell kinetics in a range from 80 to 30 °C. Corresponding polarization curves are presented in Figure 1, with UA-REa including the kinetic overpotential from oxygen evolution reaction and electric losses at the anode, UC-REa containing all remaining losses from the protonic losses in the anode ionomer towards the cathode bipolar plate.The second type is a platinum wire micro-reference electrode based on the concept of a dynamic hydrogen electrode3. The wire is in contact with the membrane outside the active CL area in an isolated compartment of humidified hydrogen. A total of 119 individual tests are carried out to assess the RE reproducibility. The results of the half-cell potential with HFR measurements, and kinetic analysis lead to a discussion on the prevalence of misalignment in solid state systems and the benefits of using EIS to diagnose it. Finally, the robustness and the limitations of the method are discussed through a set of variations of influencing parameters.References(1) Kuhnert, E.; Hacker, V.; Bodner, M. A Review of Accelerated Stress Tests for Enhancing MEA Durability in PEM Water Electrolysis Cells. International Journal of Energy Research 2023, 2023, 1–23. DOI: 10.1155/2023/3183108.(2) Bühre, L. V.; Bullerdiek, S.; Trinke, P.; Bensmann, B.; Deutsch, A.-L.; Behrens, P.; Hanke-Rauschenbach, R. Application and Analysis of a Salt Bridge Reference Electrode Setup for PEM Water Electrolysis: Towards an Extended Voltage Loss Break Down. J. Electrochem. Soc. 2022. DOI: 10.1149/1945-7111/ac9ee1.(3) Bühre, L. V.; McLeod, A. J.; Trinke, P.; Bensmann, B.; Herrera, O. E., Mérida, W.; Hanke-Rauschenbach, R in preparation Figure 1