P-containing coated carbons from phosphonium ionic liquids as catalyst supports for fuel cell applications

Abstract

Research on heteroatom doping of carbon materials for advanced electrochemical devices such as fuel cells became prominent over the last decade to improve longevity and performance, with phosphorus-containing, nitrogen-free carbons seldom investigated. Here, we show that precursors coated with P-containing carbon, as supports for noble metal catalysts, enhance the performance and durability of cathodes in fuel cell operation. Simple wet coating of a series of carbon substrates with ionic liquids and subsequent pyrolysis at 400 °C produced carbon materials with homogeneously distributed phosphorus atoms. This process is applicable to a variety of different carbons, yielding P-content up to 3.0 wt% with PO₃⁻ and PO₄⁻ like species as confirmed by XPS-measurements. Fuel cell test systems revealed not only superior performance but also enhanced durability. The results show that small amounts of surface phosphorus on carbon supports have a positive impact on key characteristics including reduced ohmic and cathode transfer resistances. In addition, durability and ORR activity were improved with neither the morphology nor the geometry being significantly affected. We explain our observation regarding different modifications of the structure and surface of carbon materials as being due to new and favourable active sites provided by the bond length, atomic radius, and electronegativity of the introduced phosphorus species. The novelty of this approach is the formation of PO₄⁻ like species from ionic liquids at relatively low pyrolysis temperatures, which promotes improved ORR activity. Ionic liquids can serve as precursors for heteroatom-containing carbon-based materials with improved properties, opening a new avenue for the fabrication of devices in which carbon-based materials are utilized.