Chronopotentiometric Methodology to Characterize the Electrochemically Active Surface Area in PEM Water Electrolysis Cells and Stacks

Proton exchange membrane water electrolysis (PEM-WE) emerges as a rapidly evolving and promising technology in the pursuit of green hydrogen generation from renewable energy sources [1]. As the demand for large-scale implementation grows, efficient cell diagnostics at the stack level become increasingly important. However, due to a non-uniform voltage distribution across the individual cells of a stack, many potential-controlled diagnostic methods are not realistically applicable [2]. Among these methods, cyclic voltammetry (CV) stands out as a commonly employed voltage-based measurement technique, offering valuable insights into the dependence of the oxidation states of catalyst materials on the operating conditions and also in the changes of the electrochemically active surface area (ECSA) [3]. This is essential to gain information and understanding about the degradation modes of the catalyst layer in an application-relevant environment. In the context of stack-level diagnostics, chronopotentiometry (CP) presents a promising alternative to CV. CP involves applying a constant current pulse to the stack and measuring the voltage response of each individual cell. Through the conversion of the change in cell potential into differential capacity values, according to C = dQ/dt = I*dt/dV (1), CP offers comparable insights into the surface chemistry of a catalyst as CV [4]. It has been successfully demonstrated in PEM fuel cells (PEM-FC) [5], allowing the investigation of the ECSA and of shorting/crossover currents at stack level for PEM-FCs [6]. In this study, we extend the application of CP to PEM-WE. Initially validated on 5 cm2 PEM-WE single-cells, the CP approach reveals characteristic features of iridium- and iridium oxide catalysts similar to our standard CV analysis. Figure 1 illustrates a comparative analysis between a CV at a scan rate of 50 mV/s and a CP measured with a constant current density pulse of 5 mA/cm2 for an anode electrode based on an Ir/TiO2 catalyst with an iridium loading of 0.68 mgIr/cm2. The CP measurement data can be converted mathematically in a form that mimics a cyclic voltammogram (orange line in Figure 1), reflecting hydrogen underpotential deposition/adsorption features (<0.3 V vs. RHE) that are in good agreement with those obtained from a CV measurement (grey line). Furthermore, measurements will be shown from a 25 cm2 rainbow stack comprising five cells with four different anode catalysts (IrO2/TiO2, Ir/TiO2, and two variations of a hydrous iridium oxide catalyst with varying iridium contents). Based on these results, potentials and limitations of the CP methodology applied to PEM-WE cells and stacks will be discussed, demonstrating that the CP methodology can be used to characterize individual cells in a PEM-WE stack. Acknowledgements: This work was funded by the German Federal Ministry of Education and Research (BMBF) in the framework of the Iridios project (funding number 03HY129E). The authors gratefully acknowledge Dr. Christian Gebauer (Heraeus Deutschland GmbH & Co. KG) for providing IrOx/TiO2 catalyst materials. Special thanks to Markus Pietsch and Yan-Sheng Li (TUM-TEC) for sharing their knowledge on chronopotentiometry in PEM-FC and for numerous discussions. References: [1] A. Buttler, H. Spliethoff; "Current status of water electrolysis for energy storage, grid balancing and sector coupling via power-to-gas and power-to-liquids: A review"; Renewable and Sustainable Energy Reviews 82 (2018) 2440-2454. [2] T. Génevé, C. Turpin, J. Régnier, O. Rallières, O. Verdu, A. Rakotondrainibe, and K. Lombard, “Voltammetric Methods for Hydrogen Crossover Diagnosis in a PEMFC Stack”, Fuel Cells 17 (2017) 210-216. [3] A. Weiß, A. Siebel, M. Bernt, T.-H. Shen, V. Tileli, and H. A. Gasteiger, „Impact of Intermittent Operation on Lifetime and Performance of a PEM Water Electrolyzer”, J Electrochem Soc 166 (2019) F480-F497. [4] DA Stevens, JR Dahn, “Electrochemical characterization of the active surface in carbon-supported platinum electrocatalysts for PEM fuel cells”, J Electrochem Soc 150 (2003) 770-775. [5] Y. Chatillon, C. Bonnet, F. Lapicque, “Differential capacity plot as a tool for determination of electroactive surface area within a PEMFC stack”, J Appl Electrochem 43 (2013) 1017-1026. [6] S. Torija, L. Prieto-Sanchez, S. J. Ashton, “In-situ electrochemically active surface area evaluation of an open-cathode polymer electrolyte membrane fuel cell stack”, Journal of Power Sources 327 (2016) 543-547 Figure 1