Article pubs.acs.org/JPCC
Hydrogen Oxidation and Oxygen Reduction at Platinum in Protic Ionic Liquids Lee Johnson, Andinet Ejigu, Peter Licence, and Darren A. Walsh* School of Chemistry, The University of Nottingham, Nottingham NG7 2RD, U.K. S Supporting Information *
ABSTRACT: H2 oxidation and O2 reduction have been studied as a function of temperature at Pt electrodes in the protic ionic liquid diethylmethylammonium trifluoromethanesulfonate. Hydrodynamic voltammetry showed that the H2 oxidation reaction (HOR) became hindered at positive potentials (>1.0 V). Electrochemical analysis and X-ray photoelectron spectroscopy revealed that this drop in HOR activity was due to the formation of an adsorbed blocking oxide layer, which formed on the Pt surface due to trace H2O oxidation at positive potentials. Electrochemical analysis also revealed that the O2 reduction reaction (ORR) occurred at an appreciable rate only when pre-existing surface oxides were reduced. As the temperature increased, the potential at which the surface oxides were reduced shifted to more positive potentials and the reduction peak narrowed. The net result was significantly higher rates of the ORR at positive potentials at higher temperatures. Finally, even when Pt surfaces were not initially covered with an oxide adlayer, the rate of the ORR increased significantly upon increasing the temperature and some possible reasons for this temperature dependence are discussed.
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°C and in the absence of liquid H2O. Well known examples of this approach include the development of electrolytes based on anhydrous H + -conducting H 3 PO 4 /poly(benzimidazole) blends7,8 and H+-conducting solid acids such as CsHSO4 and CsH2PO4.9 Another approach, which has received a lot of attention in recent years, is the development of fuel cell electrolytes based on H+-conducting, protic ionic liquids (PILs).10,11 PILs do not evaporate readily, even above 100 °C, and Watanabe and co-workers recently demonstrated the operation of a H2 fuel cell containing the PIL diethylmethylammonium trifluoromethanesulfonate, [dema][TfO], as the electrolyte with an open circuit potential (OCP) greater than 1.0 V at 150 °C.10 This high OCP was attributed to fast ORR and HOR kinetics in the PIL. In a separate study, rapid ORR kinetics at Pt-based ORR electrocatalysts in PIL-based electrolytes were attributed to confinement of O2 within the PIL environment.12 Considering that the use of PILs as electrolytes in fuel cells could potentially offer a way toward increasing reaction rates and reducing the precious metal loading to meet performance targets, it is crucially important that electrocatalysis of the ORR and HOR in PILs is well understood. In the case of the ORR and HOR in aqueous electrolytes, the recent literature has shown that a thorough understanding of each reaction is required before one can develop better electrocatalysts13−17 but
INTRODUCTION Polymer electrolyte membrane fuel cells (PEMFCs) are the most promising fuel cell candidates for automotive and portable applications.1 However, despite the use of Pt electrocatalysts to drive the H2 oxidation reaction (HOR) and O2 reduction reaction (ORR) in PEMFCs, slow electrode kinetics, in particular for the ORR, hinder PEMFC performance.2 Recent theoretical and experimental advances in the design and synthesis of novel Pt-based electrocatalysts have led to typical specific power densities of ∼0.5 gPt kW−1 in PEMFCs but this loading is still higher than the target of
