The Role of the Ionomer in the Catalyst Layer of Anion Exchange Membrane Water Electrolyzers
Authors: Volk Emily K. , Clauser Arielle L , Kreider Melissa E , Chan Ai-Lin , Sugar Josh D , Kwon Stephanie , Alia Shaun M 
Published: December 2024

Anion exchange membrane water electrolyzers (AEMWE) have gained recent attention as a promising low temperature H2O electrolysis technology that combines the benefits of commercial liquid alkaline and proton exchange membrane water electrolyzers (PEMWE), while alleviating concerns over capital cost and materials criticality associated with these existing technologies.1,2 While AEMWEs have achieved significant performance advances in recent decades, overpotentials remain high relative to PEMWE counterparts,2 requiring AEMWE-specific catalyst layer design strategies to further advance this technology. Catalyst layers conventionally consist of catalyst particles which are co-deposited with an ion-conducting polymer or ionomer that is of the same chemistry as the ion-exchange polyelectrolyte phase.3 In PEMWEs, which are operated in pure water, this ionomer serves as the sole ion (H+) transport network through the catalyst layer. The ionomer further serves roles as a binding agent, adhering catalyst particles to the electrode, as well as an ink rheology agent.4 In contrast, AEMWEs often operate in dilute supporting electrolyte, raising questions about the role of the ionomer in AEMWE systems.

In this work, we assess the specific roles of ionomers in supporting electrolyte fed AEMWEs using NiFe2O4, identified in our previous work as a strong candidate for a commercial alkaline oxygen evolution reaction (OER) catalyst, and PiperION TP85 from Versogen, a common commercially available AEM. Catalyst layers were constructed at variable ionomer contents (0 – 30 wt%) to investigate the role of the ionomer phase in supporting electrolyte-fed AEMWEs. Specifically, the role of the ionomer in catalyst structure and quality, catalyst adhesion, and ion transport were investigated via electrochemical and physical characterization of pristine and tested catalyst layers. Ex-situ scanning electron microscopy with energy dispersive x-ray spectroscopy (SEM-EDS), x-ray diffraction (XRD), and x-ray photoelectron spectroscopy (XPS) measurements were employed to provide insights into catalyst and ionomer loss mechanisms over 200 h durability tests at different ionomer contents. Voltage breakdown analyses were used to further investigate the effects of ionomer content on cell overpotentials. Electrochemical impedance spectroscopy (EIS) and Tafel analyses were utilized to decouple thermodynamic, Ohmic, kinetic, and catalyst layer resistance contributions to overpotentials for each ionomer content.

The results show that intermediate ionomer contents (5-10 wt%) provide the highest cell current densities. Intermediate ionomer contents were found to a achieve a balance of catalyst adhesion, catalyst layer porosity, and catalyst layer homogeneity, leading to improved site accessibility and decreased cell overpotentials. Testing with Nafion (a non-anion conducting polymer) further demonstrated that the ionomer is not required for ion conduction in AEMWE catalyst layers under supporting-electrolyte operation and therefore that ionomers do not need to match the chemistry of the anion exchange membrane. Instead, we find that, for initial performance, the ionomer is crucial for catalyst layer structure and quality, and for long-term stability, the ionomer is crucial to provide catalyst adhesion for long-term durability.

[1] IRENA Green Hydrogen Cost Reduction - Scaling up Electrolyzers to Meet the 1.5C Climate Goal
2020, [2] Alia, S. et al., Electrochem. Soc. Interface
2021, [3] Volk, E. K. et al., EES Catalysis
2024, [4] Khandavalli, S. et al., ACS Appl. Mater. Interfaces
2019