Dealloyed Core-Shell Fuel Cell Electrocatalysts

Abstract

We review our recent work on dealloyed nanoparticle electrocatalysts and address their synthesis, structural characterization and surface catalytic performance in low-temperature Polymer Electrolyte Membrane fuel cells (PEMFCs). The active form of the catalyst is obtained by voltammetric dealloying of non-noble metal rich Pt alloy precursors. In the dealloying process, the less noble precursor component, here Cu, is selectively removed from the surface of the precursor alloy particles and hence a Pt enriched particle shell is formed. Single fuel cell tests showed that, when used on the cathode of PEMFCs, dealloyed Pt catalysts show reactivities for the oxygen reduction reaction (ORR) which are up to 6 times higher than those of conventional pure Pt fuel cell catalysts. Similarly, the stability of dealloyed nanoparticle catalysts is superior to that of pure Pt particles. X-ray based structural and compositional studies suggested a core—shell particle structure as the active form of the catalyst consisting of a Pt enriched particle shell surrounding a Pt alloy core. At the present time, this catalyst system constitutes one of the most active fuel cell catalyst system reported in the literature. I. THE CONCEPT OF DEALLOYED NANOPARTICLE CATALYSTS Despite much recent focus on the development of advanced Li ion batteries for use as power source for short-range inner city transportation 255 Bereitgestellt von | Technische Universitat Berlin Angemeldet Heruntergeladen am | 14.04.15 16:12 Vol. 25. No. 4. 2009 Dealloyed Core-Shell Fuel Cell Eleclrocatalysts applications, low temperature fuel cells continue to be the solution of choice for medium and long range transportation technologies based on their gravimetric power density as well as the gravimetric energy density of commonly used fuels. Wider use of low-temperature fuel cell technology is hampered by performance, cost, and durability issues associated with materials and components of a single fuel cell. Figure 1 displays a cross section of the layered structure of a low temperature PEMFC showing the anode (left) and cathode (right) gas diffusion layers (GDLs), which sandwich the anode and cathode catalyst layers and the proton exchange membrane. Figure 1 also schematically shows the molecular as well as electrical pathways of hydrogen fuel molecules, oxygen molecules, protons as well as of electrons. The overall performance of a PEMFC in terms of its practical cell voltage is limited by kinetic, ohmic, and mass transport processes for low, medium and high current densities, respectively. Of these, the kinetic surface catalytic reactions cause the most severe fuel cell voltage losses. * Anode fl( / Cathode ·>' '..·· Electrical | O2+*H* + 4e -> 2 H2O Energy Fig. 1: Cross sectional SEM micrograph through a membrane electrode assembly sandwiched between gas diffusion layers. Reaction processes at anode and cathode, mass and charge flows are indicated (from ref) In particular, the electrocatalytic Oxygen Reduction Reaction (ORR) at the cathode according to O2 + 4H + +4e->2H 2 O E°=+1.23 V/RHE (1) represents the key challenge to improved PEMFCs. In acidic media, Pt catalysts offer the highest catalytic activities which made first unsupported, later high surface area carbon-supported Pt particles the ORR catalyst of 256 Bereitgestellt von | Technische Universitat Berlin Angemeldet Heruntergeladen am | 14.04.15 16:12 Peter Strasser Reviews in Chemical Engineering choice· . Figure 2 illustrates the associative (via O2H) as well as the dissociative (via O) pathways of reaction (1) from molecular oxygen to water. Key for the catalysis is the chemisorption energy of the adsorbed oxygenated intermediates, such as Pt-O2H, Pt-O, and Pt-OH. On pure Pt, atomic oxygen is bonded too strongly and requires considerable overpotentials to react to Pt-OH. As a result of this, Pt is covered by (hydr)oxide adsorbates near the equilibrium potential of reaction (1). There is a consensus within the fuel cell catalysis community that a moderate reduction of the Pt-O chemisorption energy would result in significant ORR activity increases. H2O2 Hydrogen peroxide 0=0 Oxygen adsorption Oxygenated OH intemediates