Review of carbon-support-free platinum and non-platinum catalysts for polymer electrolyte fuel cells: will they feature in future vehicles?

Abstract

Polymer electrolyte fuel cells have attracted considerable attention as possible replacements for internal combustion engines (ICEs) in light duty vehicles for journeys typically over 300 miles as well as for medium/heavy duty vehicles. In these vehicle types, carbon black is currently used as the support for platinum (Pt)-based catalysts at the cathode and anode. Carbon black is protected from corrosion during startup/shutdown and unwanted fuel (H₂) starvation by controlling the potential of both electrodes using high-cost system-level measures. Carbon-support-free Pt-based cathode catalysts, which are durable at the high potentials experienced during startup/shutdown due to the reverse current decay mechanism, have therefore been studied extensively over the last two decades. Anodes with impeded oxygen reduction reaction (ORR) activity have also been developed over the last decade to suppress cathode degradation, as the high potential at the cathode is caused by the reduction of contaminated O₂ molecules at the anode. During H₂ starvation, the potential of the anode exceeds that of the cathode, which reverses the cell voltage. Theoretical studies have predicted that binary and nonbinary stoichiometric oxides should be stable under these cathode and anode conditions. This paper focuses on non-carbon supports beyond the typical oxides. Both conductivity and the surface area are crucial in decreasing Pt loading to the platinum group metal (PGM) level used in exhaust gas catalytic converters in conventional gasoline-fueled ICE-powered vehicles. As the conductivity of powders/particles is a particular focus of this article, reports on suboxides and nitrides with metallic characters are covered. Some Pt/non-carbon catalysts exhibit higher ORR activity and durability against startup/shutdown at the cathodes, as well as higher durability against cell reversal at the anodes, compared with conventional carbon-supported Pt or platinum–cobalt (PtCo) catalysts under specific conditions such as high Pt loading or low relative humidity. The origin of these beneficial properties is reviewed. In contrast to these positive results, negative reports of non-carbon supports for the anode and cathode are also highlighted, and the advantages and disadvantages of using non-carbon supports are discussed. Recent improvements in carbon-support-free, non-PGM cathode materials with the use of conductive TiN and associated challenges are also reviewed.