This project investigates the role of surface stress and composition on cathode catalyst degradation in Proton Exchange Membrane (PEM) fuel cells. Loss of cathode catalyst activity occurs by four main mechanisms: (a) Particle detachment from carbon support, (b) Agglomeration and sintering, (c) Ostwald ripening, and (d) Pt dissolution at cathode and precipitation in the membrane. Ostwald ripening and Pt precipitation in the membrane depend on Pt ion concentration (Pt2+ and/or Pt4+) in aqueous electrolyte and/or ionomer. Ostwald ripening involves coupled transport of Pt2+/Pt4+ ions through aqueous/ionomer medium and electrons through carbon support. Agglomeration and sintering involve coupled transport of Pt2+/Pt4+ through aqueous electrolyte/ionomer medium and electron transport through direct particle contact. In mechanism (d), Pt precipitation occurs by a reaction of Pt2+/Pt4+ and H2. Thus, three mechanisms - agglomeration/sintering, Ostwald ripening and precipitation of Pt in the membrane depend upon Pt2+/Pt4+ ion concentration. All factors which increase Pt2+/Pt4+ concentration will increase degradation kinetics.
Intellectual Merits: Many factors determine Pt2+/Pt4+ concentration, some materials related and some related to operating conditions. This proposed work addresses fundamental materials-related properties which determine Pt2+/Pt4+ concentration such as the thermodynamics of alloy systems and surface stress. Fundamental thermodynamic parameter of interest is the partial molar enthalpy of Pt alloy formation. The role of stress is also of profound significance. First, it is known that greater tendency for growth of smaller particles is the surface energy effect, which essentially is the effect of pressure on chemical potential. The greater the magnitude of surface compression, the greater is the chemical potential and degradation kinetics. In pure Pt catalysts, only particle size determines this stress. However, in core-shell catalysts comprising Pt shell and an alloy or non-noble metal core, additional coherency stresses exist. By suitable choices of lattice parameters and interfacial structure, the chemical potential can be reduced thereby reducing Pt2+ concentration and thus reducing degradation kinetics. This proposes to investigate a) the effect of Pt2+ /Pt4+ concentration and temperature on the kinetics of pure Pt, Pt alloy, and core-shell catalysts; and b) the role of stress and alloy composition on the chemical potential of Pt using electrochemical techniques. Broader Impacts: The results of this research should provide a scientific basis for the synthesis of degradation-resistant cathodes. The proposed methodology is general and applicable to essentially all electrochemical devices which require the use of nanosize materials in electrodes and the presence of aqueous/ionic medium. The University of Utah has a strong commitment to undergraduate and graduate education and in enhancing the involvement of socially under-represented groups. One undergraduate student and one graduate student will participate in this research.
