Electrochemical characterization of materials for next generation polymer electrolyte fuel cells

Sammanfattning: Polymer electrolyte fuel cells occupy a key position in implementing the hydrogen economy on a global scale. However, assessments of cost, availability and sustainability of materials currently used to manufacture state-of-the-art fuel cell stacks have given cause for concern. Platinum and platinum-group metals are prohibitively expensive and of low abundance. The benchmark ion-conductive polymer NafionTM and related perfluorosulfonic acid-based polymers are difficult to synthesize and environmentally persistent to extreme degrees. Graphitic and carbon composite bipolar plates are unsuitable for mass production and have low recycling potential. In the compiled works, alternative materials were evaluated both for the acidic and alkaline variations of polymer electrolyte fuel cells. Electrochemical characterization was carried out in single cell tests with a focus on finding the limitations in terms of ohmic, charge transfer and transport resistances in the cell, through polarization and impedance measurements.Carbon coated stainless steel bipolar plates were tested operando in a proton exchange membrane fuel cell (PEMFC) under realistic conditions based on the New European Drive Cycle. Observed trace metal contamination of the MEA was linked to metal dissolution from coating defects caused by manufacturing (Paper I). A theoretical understanding of observed metal dissolution was confirmed experimentally and a concept for preventing metal dissolution was developed for PEMFC bipolar plates (Paper II). The developed concept was extended to uncoated stainless steel bipolar plates and tested successfully for three stainless steel types in operando PEMFC (Paper III)Anion exchange polymers based on poly(arylene piperidinium) (PAP) were tested as both membranes and ionomers in a comparative study with a commercial reference material, showing higher performance and the significance of ionomer-carbon support interactions (Paper IV). PAP-based ionomers with varying ion exchange capacities were studied to optimize electrodes in anion exchange membrane fuel cells (AEMFC). A combination of high ion exchange capacity ionomer on both cathode and anode was best performing, linked to small water transport resistance in cathode and increased kinetic contribution of the anode HOR (Paper V). The effects of modifying the catalyst layer through the introduction of crosslinked PAP particles were studied in operando AEMFCs. A positive impact on charge transfer and diffusion resistances in electrodes containing particles could be observed. (Paper VI).Silver nanoparticles were used as catalyst material in the cathode of previously optimized membrane electrode assemblies in AEMFC. The results showed promising performance compared to platinum electrodes based on monetary and sustainability considerations, but also challenges regarding catalyst stability and detrimental silver-ionomer interactions. (Paper VII).