Using tin oxide (SnO 2 ) and niobium-doped tin oxide (Nb-SnO 2 ) as alternative electrocatalyst support materials can effectively solve the issue of carbon corrosion in polymer electrolyte fuel cell (PEFC) cathodes. Here, we systematically explore the effect of support surface area, Pt loading, and Pt nanoparticle size on the electrochemistry of these carbon-free electrocatalysts. Reducing the Pt loading leads to an increase in electrochemical surface area, but the specific activity decreases as previously observed in conventional carbon based electrocatalysts. Removing residual chlorine impurities by replacing the H 2 PtCl 6 nanoparticle precursor with Pt(acac) 2 increases the specific activity. Niobium-doping of the SnO 2 support also results in an increase in specific activity, due to the increased electronic conductivity. Consequently, the oxygen reduction reaction activity of optimized Pt-decorated Nb-SnO 2 is approaching to that of Pt-decorated carbon black, the current state-of-the-art PEFC electrocatalyst. The polymer electrolyte membrane fuel cell (PEFC) is one of the most promising energy conversion technologies for e.g. residential and automotive applications. Pt nanoparticles supported on carbon black are widely used as electrocatalysts.1-6 However, carbon corrosion is an issue under high cathode potential, resulting in detachment of Pt catalyst particles from the carbon support, and leading to a decrease in electrochemical surface area (ECSA) and oxygen reduction reaction (ORR) activity over time. [2][3][4][5][6] 15,[18][19][20] To fundamentally solve this technological issue and improve electrocatalyst durability, alternative carbon-free electrocatalyst support materials such as tin oxide (SnO 2 ) 21-29 and titanium oxide (TiO x ) [30][31][32][33][34] have also been proposed. It has been shown that SnO 2 is a promising alternative platinum support, since it has good electronic conductivity and thermochemical stability under the strongly acidic cathode conditions in a PEFC. 28,[35][36][37] Remarkable retention of ESCA in Pt-decorated SnO 2 electrocatalysts has been demonstrated compared to Pt-decorated Vulcan carbon black, by severe voltage cycling (up to 60,000 cycles) between 0.9 and 1.3 V versus the reversible hydrogen electrode (V RHE ) in electrochemical half-cell durability tests. 28 Recently, the high durability has been confirmed in membrane electrode assemblies (MEAs), during more severe voltage cycling between 1.0 and 1.5 V RHE . 29 These studies have demonstrated the stability of Pt-decorated SnO 2 against the start-stop cycles typical in fuel cell vehicles. However, high ORR activity is also required for PEFC cathode electrocatalyst applications and this has not yet been demonstrated. Relatively low ORR activity has been reported to date compared with state-of-the-art Pt supported on carbon black. 28,36 Most of the ORR values have been measured below 0.9 V RHE for oxide-supported electrocatalysts, while the ORR of conventional Pt-decorated carbon black is usually evaluated at * Electroch...