Electrocatalyis for PEFCs: Oxygen Reduction on Nanoparticles and Extended Surfaces
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Electrocatalyis for PEFCs: Oxygen Reduction on Nanoparticles and Extended Surfaces Thomas J. Schmidt, Annett Rabis, Bernhard Schwanitz, Günther G. Scherer Electrochemistry Laboratory, Paul Scherrer Institut, CH-5232 Villigen PSI, Switzerland ABSTRACT This work makes an attempt to correlate experimentally observed Tafel slopes from the oxygen reduction reaction in both model rotating disk electrode and polymer electrolyte fuel cell measurements, respectively, with the kinetic description of a coverage dependent currentpotential relationship. It is shown that the potential dependent OHad coverage can be used as a descriptor of potential dependent Tafel slopes, pointing to the validity of underlying TemkinFrumkin adsorption properties in combination with the Butler-Volmer approach. INTRODUCTION Electrocatalysis in Polymer Electrolyte Fuel Cells (PEFCs) is a lively field in PEFC research, due to the fact that especially the cathodic oxygen reduction reaction (ORR) is one of the reasons for efficiency limitations [1]. In addition, the cathode itself is one of the components with major challenges for lifetime, due to corrosion instabilities of both, the catalyst support and the active phase, typically consisting of Pt nanoparticles. One of the approaches to overcome catalyst lifetime limitations is the transition from supported Pt nanoparticles to unsupported extended Pt-surfaces (i.e., Pt-black type catalyst layers). During the last decade, this approach has been mainly driven by 3M with their nanostructured thin-film (NSTF) catalysts, currently representing one of the most feasible catalyst (layer) systems available for PEFCs. [2,3] In this contribution, we will use our own preparation approach of sputtered electrode utilizing Pt particle and layer growth by Magnetron sputtering [4]. Strategically, we combine model studies using rotating-disk electrode (RDE) measurements of layered and nanoparticle Pt catalysts, respectively, and electrochemical PEFC electrode characterization in order to understand kinetic properties of the different systems, also elucidating the effect of low Pt loading limits for cathodes. In addition, we demonstrate the effect of Pt loading on the kinetics of the ORR in both RDE and PEFC environments. In this context, we will confirm recent findings on the existence of the coverage-dependent ORR kinetics, only accessible using low loaded PEFC cathodes [5]. EXPERIMENTAL All electrode preparation conditions can be found in our recent publications [4,6]. In short, various amounts of platinum have been sputtered onto either glassy carbon electrodes coated with a thin layer of Vulcan XC 72 (210 μgcm-2 carbon loading) for RDE measurements (Pt loading: 2, 10, 20, 100 and 200 μgcm-2) or on onto carbon cloth (BASF Fuel Cell, type LT1410W), covered with a microporous carbon layer (MPL, Pt loading: 5, 25, 50, 100 and 500 μgcm-2), respectively. The different loadings have been achieved by calibration of the sputter process using determination of the sputter time, power and voltage, respectively, with the Pt loa
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