The Contributions of Catalyst Layer Resistance on Overall Overpotentials in Anion Exchange Membrane Water Electrolyzers
Authors: Volk Emily K. , Padgett Elliot , Kwon Stephanie , Alia Shaun M 
Published: December 2024

Anion exchange membrane water electrolyzers (AEMWE) combine the benefits of traditional liquid alkaline (LAWE) and proton exchange membrane water electrolyzers (PEMWE), producing H2 at high current densities while allowing the use of low-cost, low-criticality PGM-free materials for catalysts and hardware. Advances in AEMWE performance have risen rapidly in recent decades, and research on AEMWEs has increased dramatically.1 However, despite significant component-level advances in catalysts and membranes,2,3 gaps remain to achieve performance parity with PEMWEs and the relative overpotentials of AEMWE remain high. Strategies are thus needed to understand, diagnose, and quantify the origins of overpotentials in AEMWEs and help bridge the gap between AEMWE and PEMWE performance.

By utilizing Tafel analysis and electrochemical impedance spectroscopy, voltage breakdown analyses can be performed to decouple the origins of high overpotentials in electrolyzers. While such methods are used frequently in the literature to determine kinetic and Ohmic losses, voltage breakdown analyses are rarely performed beyond these contributions, even though other losses can often account for 100s of mVs (20-30% of non-thermodynamic losses). Recent works have demonstrated that catalyst layer resistances can be quantified in electrolyzers using transmission line theory for porous electrodes.4 To the best of our knowledge, however, this method has not yet been adapted to AEMWEs. Furthermore, differences in cell operation and performance (e.g., use of supporting electrolyte, high hydrogen evolution reaction overpotentials) leads to differences in the expected nature of catalyst layer resistances for these two technologies. There is therefore a need for AEMWE-specific methods to diagnose and quantify catalyst layer resistance.

In this work, a method for determining catalyst layer resistance from transmission line theory for porous electrodes4 is adapted to AEMWE and applied to four anode catalyst materials tested in single-cell MEAs. Changes to the catalyst layer thickness, ionomer content, and electrolyte concentration reveal trends about the origins of catalyst layer resistances in AEMWE cells. Investigations of the anode transport layer show that this component introduces significant impedance, which thus must be considered for AEM diagnostics and performance optimization. Furthermore, the results suggest that discontinuities in electronic and ionic transport networks in the catalyst layer may explain discrepancies between idealized kinetic results from rotating disc electrode testing and those observed for MEAs. These results then inform the development of a model to understand and differentiate contributions from anode and cathode catalyst layers in AEMWEs. These results will ultimately allow for the design of more efficient catalyst layers, reducing overpotentials and facilitating AEM deployment commercially.

[1] Volk, E. K. et al., EES Catalysis
2024, [2] Chen, N. et al., Trends Chem.
2022, [3] Plevová, M. et al., J. Power Sources
2021, [4] Padgett, E. et al., J. Electrochem. Soc.
2023