Comparison of Different Ionomers for the Anode Catalyst Layer of Anion Exchange Membrane Water Electrolysis
Authors: Chuluunbandi Khajidkhand , Thiele Simon , Freiberg Anna T.S. 
Published: December 2024

In water electrolysis cells, optimizing the anode electrode where the oxygen evolution reaction takes place is crucial for high performance and durability. The ionomer is an essential part of the catalyst layer (CL). It provides ionic conductivity, enabling ions to move to the catalyst sites where the electrochemical reaction occurs. Additionally, ionomers contribute to tuning the hydrophobicity and hydrophilicity of the CL, which influences water availability at catalyst sites and gas removal. Moreover, ionomers ought to help to maintain optimal pH levels, an important factor for catalyst stability.
In anion exchange membrane water electrolysis (AEMWE), when a supporting electrolyte is used, the ionic conduction takes place not only via the ionomer but also via the supporting electrolyte. Therefore, many studies in AEMWE use NafionTM as an ionomer even though it is a cation exchange ionomer.1, 2 Also, several studies use polytetrafluoroethylene (PTFE) as a binder.3, 4 Thus, in this study, different commercial ionomers/binder are compared for the usage on the anode side of the AEMWE to understand their influence on the performance of the AEMWE cell at different pH feed electrolytes (1M, 0.1M and 0.01M); Hydrophilic cation exchange Nafion™, hydrophilic anion exchange Aemion® and PiperION®, and hydrophobic inert PTFE are used in the Ir-based anode CL. A commercial anion-exchange membrane (PiperION®) and a Pt/C cathode with PiperION® ionomer are used, respectively. CL compositions and morphologies are investigated via thermogravimetric analysis (TGA) and scanning electron microscopy (SEM). Electrochemical characterizations such as polarization curve and constant current hold analysis along with electrochemical impedance spectroscopy (EIS) are performed on a full cell set up to assess the performance, durability, and ionic transport resistance of the anode CL.
We show that anionic ionomers (Aemion® and PiperION®) do not lead to a better performance of the cell at 1M KOH concentration (see figure) and even at lower concentrations such as 0.1M KOH. Nafion™ ionomer incorporated membrane electrode assemblies (MEAs) exhibit better performance than anionic ionomers (Aemion® and PiperION®) and inert binder (PTFE) in 1M KOH measurement. One hypothesis on Nafion’s TM outstanding performance is that Nafion’s TM anionic head groups form an ionic interaction with the cationic head groups of the PiperION membrane and therefore lower the contact resistance between CL and membrane.5 However, in our measurements, all high-frequency resistances of different ionomers/binder incorporated MEAs are in the same range (55-65 mΩ cm-2), which opposes the hypothesis. Based on the overall polarization characteristics, the performance for the different ionomers changes mainly in the kinetic region. As this difference can already be observed at begin of life where ionomer decomposition can be neglected, we assign this difference to an ionomer-catalyst interaction phenomenon (i.e., poisoning effect) or to a structural change in the CL when using different ionomers. Nafion’s ™ swelling in 1M KOH solution is lower than anionic ionomers. Therefore, it might be influencing the performance by having the optimal triple phase boundary and not blocking some of the catalyst particles. However, the PTFE binder also doesn’t swell in the KOH solution, but the performance is worse than for the Nafion™-based anode CL. By a variation of testing parameters (KOH feed molarity, temperature) and ink/CL fabrication we discuss the possible reasons for this phenomenon. In general, the role of ionomers for AEMWE must be reevaluated as the ionic conductivity of anion exchange ionomers becomes irrelevant when supporting electrolyte is in use, and anion exchange ionomers do not follow the same design criteria as for proton exchange ionomers.
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Figure 1