Towards Conformally Grafted Thin Film Ionomers for High Performance Polymer Electrolyte Fuel Cells

Abstract

The development of the next generation of polymer electrolyte fuel cells has become a major focus in the decarbonization of heavy-duty transportation, such as public transport, land, aviation, and maritime freight1. Specifically, there is a need for improved catalyst layers with lower amounts of expensive platinum group metals2, as well as improved mass transport characteristics at high current densities, fluorine-free ionomers, and increased durability3. Achieving an optimal electrode microstructure is crucial in this process, as it influences the mass transport of reactant gases and products2. The chemistry and morphology of the ionomer film determine proton availability and affect oxygen transport and overall catalyst utilization. Thus, it is critical to maintain a delicate balance between the transport of reactants, products, protons, and electrons to ensure optimal cell performance. Traditional catalyst layers are fabricated using catalyst ink dispersions which limits the control over the triple-phase boundary microstructure4. This is problematic as poor distribution of the ionomer has been shown to have a negative impact on fuel cell performance5. It has been observed that some catalytic sites are inundated with ionomer, leading to a high oxygen transport resistance, while other uncoated sites may experience a lack of protons. Moreover, research has shown that the catalyst's active sites can be poisoned by the adsorption of sulfonic acid moieties present in commonly used ionomers. Motivated by these challenges, and the negative environmental impact of conventional fluorinated ionomers, it is essential to develop alternative ionomer chemistries and coating techniques6.To address these challenges, our research focuses on developing innovative synthetic methods that leverage both electrochemical and chemical principles to enable local control of ionomer coating in the catalyst layer. Our interest in these methods originates from their scalability and ability to control the morphology of coatings (e.g. thickness, conformality, porosity and polymer length). We employ grafting techniques based on radical mechanisms to covalently graft conformal ionomer thin films onto conventional Pt/C catalysts to produce improved triple phase boundaries. A vast array of monomers with different kinds of functionalities (e.g. amines, vinylics, diazo-compounds, carboxylates) can be used for grafting, including fluorine-free systems7. In this presentation, I will first describe the synthetic procedure to produce ionomer thin films. Second, we perform ex-situ characterization including electron microscopy, porosimetry and wettability measurements. Furthermore, we study the ionomeric nature of the films by its ion exchange capacity. Subsequently, we screen the most promising samples in single cell tests and compare them against a state-of-the-art benchmark. By combining potentiostatic and impedance-based techniques, we seek to gain insight into the in-operando characteristics of these new electrode materials. Through this comprehensive set of experiments, we aim to elucidate morphology – property – performance relationships for these ionomer thin films to further guide optimization of catalyst microstructures. Acknowledgement: This publication is part of the project 3D-FCoat (3-Dimensional structures for Fuel Cells through advanced Coating techniques) of the research program LIFT which is (partly) financed by the Dutch Research Council (NWO) and Nedstack Fuel Cell Technology B.V.