UBC Theses and Dissertations
Modified catalyst layer interfaces for higher utilization and improved operational flexibility of low loading polymer electrolyte fuel cell catalyst layers Daniel, Lius
Featuring low operational temperature and high-power density, polymer electrolyte membrane fuel cells (PEMFCs) have become the most researched and used fuel cell for the emerging automotive applications. To further promote the competitiveness of the fuel cell, improvement in operational flexibility to enable fuel cell to maintain its performance under various conditions is critical. The approach taken here was to modify the membrane electrode assembly (MEA) structure, particularly the interfaces of the cathode catalyst layer. Two interfaces were studied and modified, namely the membrane | cathode catalyst layer (CCL) and the CCL | microporous layer (MPL) interfaces. Firstly, the interface of the membrane and CCL was modified by addition of a thin, dense Pt layer in the membrane subsurface (<250 nm). This Pt layer was physically and electrochemically characterized. The application of this platinized membrane with a loading <20 µgPt/cm⁻² demonstrates a comparable performance to the baseline but improves the performance at low humidity conditions due to a better humidification of the membrane and catalyst layer. The performance benefits are also maintained during a longer humidity cycling test. This new platinized membrane structure also shows reduction in hydrogen crossover (up to 65%) with the loading studied (<80 µgPt/cm⁻²). Secondly, the interface of the CCL and MPL was modified by applying a modified MPL directly on the CCL. A new MEA architecture with a modified MPL consisting of 0.8 mgVC/cm⁻² reduces gaps at this interface, and hence reduces cell contact resistance by 31% and increases the limiting current density by about 10%. The modified MPL with 0.8 mg/cm⁻² Acetylene Black shows the highest performing MEA with ~37% maximum power density gain, while the optimum loading for VC-based MPL is ~0.4 - 0.8 mg/cm² yielding ~27% maximum power density gain at 100% RH. The loading of 0.8 mg/cm⁻² appears to be the threshold to enable the modified MPL to have performance benefits under low humidity. The long-term performance under low and high humidity showed that MEAs with additional modified MPL have a more stable and lower performance drop than for MEAs with a conventional MPL only.
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