Investigating Cathode Ionomer Content and Assembly Techniques for Anion Exchange Water Electrolysers
Authors: Titheridge Laura , Marshall Aaron Timothy , Soisson Anouk , Roth Christina , Sharma Shailendra Kumar 
Published: December 2024

An ionomer is often used in catalyst layers to a) adhere catalysts to the membrane or transport layer, b) facilitate ionic conductivity for OH- species through catalyst layers, and c) achieve ideal catalyst dispersions in catalyst inks used in spraying methods[1]. The choice of ionomer plays a crucial role in determining ion conductivity, water management, and mechanical stability within the membrane electrode assembly of anion exchange membrane water electrolysers (AEMWEs). Therefore, optimizing ionomer properties is crucial to achieving high-performance AEMWEs with efficient hydroxide transport.

Both low and high ionomer loadings can lead to challenges in charge transport resistance. At low loadings, poor electronic conductivity and reduced contact between the catalyst layer and the membrane surface can occur due to a rough electrode interface[2]. This can result in inefficient charge transfer kinetics and hinder the overall efficiency (Figure 1c). Conversely, high ionomer loadings can lead to increased charge transport resistance, attributed to decreased gas permeability to the catalyst surface. The excessive presence of ionomer within the catalyst layer can block gas-liquid transport pores inside catalyst aggregates, limiting mass transport of reactants to the active sites[2]. This phenomenon can impede gas diffusion and oxygen reduction kinetics, leading to decreased performance and lower efficiency in AEMWEs (Figure 1d). Achieving an optimal ionomer loading balance is therefore essential for promoting efficient charge transfer and gas transport processes (Figure 1e), ultimately enhancing the overall performance and durability of AEMWE devices. The use of a proton conducting ionomer such as Nafion seems counterintuitive for use within AEMWEs, its hydroxide conductivity is comparatively lower, potentially limiting electrolyser performance. However, compared to Nafion, OH- conductive binders (Sustainion and PiperION) typically yield poorer electrodes, with larger catalyst aggregates[3].

Two anion exchange ionomers and one ion exchange ionomer are evaluated across a range of loadings, and two different cathode assembly techniques, Catalyst coated substrates (CCS) and catalyst coated membranes (CCM) (Figure 1a). Since different catalyst surfaces are observed to influence ionomer degradation differently[4], commercially available Pt/C was utilised as a catalyst to focus on ionomer content and fabrication techniques. To investigate the effect of using hydrophobic or hydrophilic ionomers on catalyst layer homogeneity, the content of Nafion, Sustainion, and PiperION ionomers in the catalyst layers was varied (Figure 1b). Electrochemical impedance spectroscopy was used to determine the trade-off between high and low ionomer contents, polarisation curves found the in-situ electrocatalytic performance and highlighted the best performing ionomer and fabrication technique. Scanning electron microscopy was used to highlight the differences in catalyst layer structures and homogeneity.

[1] Volk et al. Nov. 2023, EES. Catal. 2(1). 109-137. DOI: 10.1039/D3EY00193H.

[2] Zhao et al. Feb. 2023, Int. J. Hydrogen Energy. 48(13). 5266-5275. DOI: 10.1016/j.ijhydene.2022.11.057.

[3] Jervis et al. Nov. 2017, J. Electrochem. Soc. 164(14). 1551-1555. DOI: 10.1149/2.0441714jes.

[4] Li et al. Mar. 2019, ACS Appl. Mater. Interfaces. 11(10). 9696–9701. DOI: 10.1021/acsami.9b00711.

Figure 1: Schematic illustrations of: a) Membrane electrode assembly fabrication techniques, catalyst coated membrane and catalyst coated substrate. b) The chemical structures of PiperION A5 by Versogen, Sustainion XA-9 by Dioxide Materials, and Nafion by Liquion. c) Catalyst layer with low ionomer loading. d) Catalyst layer with moderate ionomer loading. e) Catalyst layer with high ionomer loading.

Figure 1