We report on the performance and durability of four Pt-based cathode catalysts in a microfluidic H-2/O-2 fuel cell: commercial unsupported Pt and Pt3Co as well as in-house acid-treated Pt3Co (Pt3Co-at )and Pt3Co/Mo. Commercial Pt3Co was used as the starting material for both Pt3Co-at and Pt3Co/Mo. The composition of the resulting catalysts was confirmed via X-ray photoelectron spectroscopy analysis. In situ cathode studies were performed using an acidic microfluidic H-2/O-2 fuel cell with an analytical platform. The electrolyte flow rate was optimized to minimize the effects of water management such that fuel cell performance is kinetically limited by the oxygen reduction reaction (ORR). In addition, electrolyte concentration was separately varied to determine cathode performance as a function of acidic pH. All four catalysts demonstrated good short-term activity and stability under fuel cell operating conditions in harsh acidic environments, with the Pt3Co/Mo alloy exhibiting the highest activity. Furthermore, both modified catalysts, Pt3Co/Mo and Pt3Co-at, exhibited superior durability compared to commercially available Pt3Co and Pt in the accelerated cathode aging studies performed within the microfluidic fuel cell via potential cycling. In situ impedance analysis of the Pt3Co/Mo cathode revealed enhanced catalyst stability and electrode durability as the cause of the dramatic improvements in long-term performance. (C) 2010 The Electrochemical Society. [DOI: 10.1149/1.3368721] All rights reserved.