Optimization of Ionomer Content in Membrane Electrode Assemblies and Its Impact on the Performance in Anion Exchange Membrane Fuel Cells

Abstract

Anion exchange membrane fuel cells (AEMFCs) have recently attracted significant attention as low-cost alternative fuel cells to traditional proton exchange membrane fuel cells due to the possible use of platinum-group metal-free electrocatalysts [1]. Over the past decade, new materials dedicated to the alkaline medium, such as anion exchange membranes (AEMs) and anion exchange ionomers (AEIs), have been developed and studied in AEMFCs [2, 3]. However, only a few AEMs and AEIs are commercially available, and there are not ready to use catalyst coated membranes (CCMs) and/or gas diffusion electrodes (GDEs) with the wished AEMs or AEIs.In order to manufacture CCMs and/or GDEs on the basis of commercial materials, catalyst inks need to be prepared before testing them in AEMFC. It is well known that the composition of catalytic ink and the way to deposit it can influence the interaction between solvent, ionomer and catalyst particles during solvent evaporation and thus on the final structure and morphology of the catalyst layer. However, there are only a few papers dealing with catalyst layer compositions and structures for AEMFCs [4, 5], probably due to the recent development of alkaline fuel cells and new dedicated materials such as AEM and AEI.Since the ionomer/catalyst particle interface plays a crucial role in electrochemical reactions, it is essential to understand the impact of ionomer content on AEMFC performance as well as on water management. For this purpose, catalytic inks were prepared with different amounts of ionomer, ranging from 13 to 33 % in ratio. During this work, Pt / C (40 % in wt) catalyst as well as Aemion® membranes and ionomers were used. Different CCMs and GDEs were manufactured at 60 °C using a commercial ultrasonic spray coating bench. The morphology of the catalyst layers was characterized by scanning electron microscopy, and the thickness of the deposition was measured by a profilometer. Before testing in AEMFC, all prepared samples and membranes were converted to OH- form for 48 h in KOH 3M (the solution was changed every 12h). The performance of the prepared CCMs and GDEs was studied in a home-made AEMFC bench after an activation step. The results shown in Fig.1 highlight that: (i) the ionomer content in the catalyst layers affects the performance of the fuel cell, regardless of the coated support (membrane or GDL), (ii) concerning CCMs-based MEAs, the lower the ionomer content, the better the performance via the polarization curve, (iii) CCMs and GDEs-based MEAs do not behave similarly, (iv) GDEs-based MEAs show high OCV and high voltage for a given current density in comparison with CCMs-based MEAs, (v) concerning GDEs-based MEAs, the variation of the ionomer content in catalyst layer affects less the OCV value than the water management, and (vi) the water management of GDEs-based MEAs seems depend on the relative humidity of both gases and ionomer content in catalyst layers. This work is still under investigation. We will attempt to understand the relationship between the membrane/ionomer under different relative humidity and gas flow rates.[1] H. A. Firouzjaie and W. E. Mustain, “Catalytic Advantages, Challenges, and Priorities in Alkaline Membrane Fuel Cells,” ACS Catal., pp. 225–234, 2019, doi: 10.1021/acscatal.9b03892.[2] J. R. Varcoe et al., “Anion-exchange membranes in electrochemical energy systems †,” 2014, doi: 10.1039/c4ee01303d.[3] N. Chen and Y. M. Lee, “Anion exchange polyelectrolytes for membranes and ionomers,” Prog. Polym. Sci., vol. 113, p. 101345, Feb. 2021, doi: 10.1016/j.progpolymsci.2020.101345.[4] J. Hyun et al., “Tailoring catalyst layer structures for anion exchange membrane fuel cells by controlling the size of ionomer aggreates in dispersion,” Chem. Eng. J., vol. 427, no. August 2021, 2022, doi: 10.1016/j.cej.2021.131737.[5] P. Santori, A. Mondal, D. Dekel, and F. Jaouen, “The critical importance of ionomers on the electrochemical activity of platinum and platinum-free catalysts for anion-exchange membrane fuel cells,” R. Soc. Chem., vol. 2020, no. 7, pp. 3300–3307, doi: 10.1039/d0se00483aï. Figure 1