Ionomer Distribution Strategy of Anion Exchange Membrane Fuel Cell Catalyst Layer in Terms of Interaction between Catalyst Slurry Components
Authors: Hyun Jonghyun , Kim Hee-Tak 
Published: July 2022

Anion exchange membrane fuel cells (AEMFCs) have been intensively studied in recent years to replace proton exchange membrane fuel cells (PEMFCs). The acid-to-alkali transitions have the potential to lower overall system costs because it allows the non-precious metal catalysts and inexpensive metal stack hardware. Significant strides have been made in materials science in the last few decades, particularly the development of high IEC-containing anion exchange membrane (AEM) and ionomer (AEI) possessing high OH- conductivity, enabling comparable cell performance to that of PEMFC. In addition, the discovery of high HOR activity of PtRu by oxophilic and/or electronic effects, and reducing ionomer poisoning, provided an opportunity to further advance the cell performance of AEMFC. However, despite the remarkable development of materials, modest AEMFCs still have inadequate power performance (< 0.5 W cm-2) even using the precious catalysts, which stems from a lack of understanding of the catalyst layer (CL) design.

CL consists of a catalyst and an ionomer, where the chemical energy of the fuel is converted into electrical energy in triple-phase-boundaries (TPBs). TPB is an electrochemically active site where catalyst (electrons), ionomer (H+ or OH-), and reactant gases were concurrent. In order to realize a high-performance fuel cell, the TPB should be high enough to utilize the capabilities of the catalyst, and the ideal state would be to maximize the contact area between the ionomer and the catalyst while minimizing the loss of CL porosity. Accordingly, the structure of the CL is one of the key factors that directly affect the performance of the fuel cell and is in great account.

CL is fabricated from a slurry containing a catalyst, ionomer, and dispersing solvent, whose structure is determined by the complex interactions between slurry components. Therefore, fine-tuning of comprehensive interaction (catalyst/ionomer, catalyst/solvent, and ionomer/solvent) is the core technology in CL design, and an in-depth understanding of each interaction is also essential. In this regard, we recently presented the rational design of the CL by controlling AEI size, distribution via a solvent selection of the catalyst slurry. Specifically, the larger the solubility parameter (excluding the hydrogen bonding term) of the organic solvent mixed with water, the smaller the dispersed AEI size, leading to an even distribution of the AEI in the CL. This induces a high electrochemical surface area of the CL, making it possible to achieve high performance AEMFC from the low current region. However, nevertheless, we found that the distribution of AEI in the AEMFC catalyst layer was still not completely uniform through various types of electron microscopy analysis. In particular, when the morphology of the AEMFC CL was compared with the Nafion ionomer (commonly used in PEMFC field) as a reference, AEI caused lower porosity by clogging the pores of Pt/C nanoparticles. For this reason, we found that AEMFCs had lower performance than Nafion-based PEMFCs, despite their high ORR activity under alkaline conditions.

Molecular dynamics (MD) and density functional theory (DFT) simulation analyzes show that AEI has a particularly low interaction with carbon compared to Nafion, and for this reason, we found that AEI aggregates with each other rather than evenly distributed on the Pt/C surface. We used QPC-TMA as AEI in this study and confirmed that commercialized AEI (FAA-3 and XA-9) also form low CL porosity and pore-closing characteristics. This result suggests that low interaction between AEI/carbon is a universal property of current levels of AEI. Therefore, when designing an ionomer to improve the performance of AEMFC, high interaction with carbon should be considered as another important variable.