(19e) Novel Oxygen Reduction Catalyst Development in a pH-Flexible Microfluidic Platform

The continued development of fuel cell technologies, especially for portable and automotive applications, is vital to furthering strategies aimed at addressing current global energy challenges. For widespread market penetration, significant and simultaneous reductions in fuel cell system costs and enhancements in durability are required. Improving oxygen reduction reaction (ORR) catalysis at the fuel cell cathode remains a key challenge. Sluggish kinetics and high overpotentials associated with the ORR hamper both energetic efficiency and peak power production. Furthermore, the high cost and limited availability of platinum (Pt) necessitates the development of alternative catalysts which either reduce or eliminate Pt content. Alloying Pt with transition metals (i.e., M = Co, Ni, Fe) not only reduces catalyst costs by lowering Pt loading but can also increase the ORR activity over pure Pt in acidic media. However, Pt-M alloys may exhibit instability and lose catalytic activity due to transition metal leaching limiting their employment in acidic fuel cell systems. For the development of durable Pt-M alloys, an improved understanding of the local operating environment at the fuel cell cathode is required. Alternatively, with the advent of novel anion-exchange membranes, alkaline fuel cells have gained renewed interest. Operating fuel cells in alkaline media, as opposed to acidic media, is advantageous as enhanced fuel oxidation and oxygen reduction kinetics improve fuel cell energetic efficiency. Furthermore, inexpensive non-Pt catalysts may be used without significant performance reductions. Thus, the development of electrodes, especially cathodes, for use in alkaline media has been the subject of increased focus. To probe the performance and durability of novel cathode catalysts, we use our microfluidic H2/O2 fuel cell with a flowing aqueous electrolyte stream as a catalyst / electrode characterization tool. For analytical investigations, the convecting stream enables autonomous control of electrolyte parameters (i.e., pH, composition), facilitates the removal and downstream analysis of reaction by-products and allows for in-situ monitoring of individual electrode characteristics via an external electrode. Thus, this microfluidic platform enables facilitates detailed in-situ electrochemical analysis of individual catalysts / electrodes within the construct of an operating fuel cell. Here, we investigate the performance and durability of novel cathode catalyst materials over a wide range of fuel cell operating conditions using this analytical platform. We have modified commercially-available Pt3Co catalysts to develop more robust ORR catalysts for acidic fuel cell applications. In particular, we found that a Pt-Co-Mo alloy exhibits enhanced activity and long-term stability compared to commercial Pt3Co and Pt. Furthermore we have studied Ag/C and Cu-triazole catalysts as cheaper alternatives to Pt/C cathodes for alkaline fuel cell applications. Presently, we are further investigating the underlying physical mechanisms that would explain the enhanced activity and stability of some of these ORR catalysts.