Exploring Reference Electrodes in Polymer Electrolyte Membrane Water Electrolysis: Analysis of Design Challenges and Review of Characterization Results for Oxygen and Hydrogen Evolution Reaction Kinetics
Authors: Buehre Lena Viviane , Bensmann Boris , Hanke-Rauschenbach Richard 
Published: December 2024

Clean hydrogen from renewable energy sources via proton exchange membrane water electrolyzers (PEMWE) has great potential to decarbonize various sectors and integrate intermittent renewables into the energy mix (1). However, its large-scale deployment faces significant hurdles, mainly due to the high cost of materials like platinum and iridium used as electrocatalysts (2). Improving the characterization of catalyst layers in PEMWE helps to address this, as a better understanding of the efficiency and durability of the catalyst layers drives research advancements and facilitates cost-reduction efforts.

Current characterization tools have limitations in providing realistic insights into catalyst performance in PEMWE. This contribution explores using reference electrodes (REs) to address this gap. REs offer the advantage of measuring the proton potential within the system, which helps to optimize performance and understand degradation mechanisms. However, integrating REs into PEM cells presents challenges, such as the impact of the RE device on the cell or its accuracy (3–5).

This study presents the integration of two reference electrode (RE) setups, a salt bridge RE (SBRE) and a dynamic hydrogen RE (DHE), into a standardized PEMWE test cell. The focus is on overcoming design optimization challenges and mitigating performance impacts. Among these challenges is the influence of ionomer treatment of the porous transport layer (PTL), an integral part of the SBRE design, which affects cell performance. The impregnation of the PTL can result in increased ohmic, mass transport and other losses (5). The Figure below shows the full cell polarization curve for both pristine PTLs and PTLs subject to various degrees of impregnation. A clear pattern can be observed where lower impregnation volumes result in cell performance closer to that of the unimpregnated counterpart. An optimal impregnation volume of 1 µL of a 5 wt% electrolyte solution is identified. This adjustment minimizes the impact on cell performance while ensuring the necessary ionic connectivity is maintained.

Additionally, another challenge lies in electrochemical impedance measurements (EIS) for half-cells. This is addressed by integrating improved EIS settings that strategically avoid high frequencies prone to inductance, enabling accurate HFR measurements in half-cells for subsequent kinetic analyses. In addition to these challenges, further obstacles, such as potential gradient across the salt bridge and RE precision, are also addressed. These challenges are thoroughly analyzed both theoretically and experimentally, and corresponding solutions are presented.

Building on the thorough analysis of the RE setups and the implemented solutions and enhancements, experimental investigation and kinetic analysis are carried out. An overview of the findings is given, which includes OER and HER kinetic parameters, investigates temperature effects on kinetics, and elucidates aspects of degradation. In addition, the application of the Arrhenius equation to analyze the temperature dependence of the exchange current density provides insight into the activation energies for half-cell reactions.

References

U.S. Department of Energy, Hydrogen Production Processes, https://www.energy.gov/eere/fuelcells/hydrogen-production-processes.

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L. V. Bühre, A. J. McLeod, P. Trinke, B. Bensmann, M. Benecke, O. E. Herrera, W. Merida and R. Hanke-Rauschenbach, J. Electrochem.
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A. J. McLeod, L. V. Bühre, B. Bensmann, O. E. Herrera and W. Mérida, Journal of Power Sources,
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L. V. Bühre, S. Bullerdiek, P. Trinke, B. Bensmann, A.-L. Deutsch, P. Behrens and R. Hanke-Rauschenbach, J. Electrochem.
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Figure 1