Study of coupled transport and its effect on different electrochemical systems: implications in high temperature energy storage batteries and proton exchange membrane fuel cells

Coupled transport is studied on two electrochemical systems: Na-ZnCl2 batteries and Proton Exchange Membrane Fuel Cells (PEMFC). The energy storage system of interest here is based on sodium P"-alumina solid electrolyte (BASE): Na/BASE/ZnCl2. BASE is an excellent Na+ conductor with a very high conductivity at 300oC. Its high Na+ ion conductivity and high stability are the principal reasons for its application in electrochemical storage systems. A novel vapor phase process was invented facilitating the fabrication of high strength and moisture/CO2 resistant BASE. A two-phase composite of alumina+YSZ is formed by sintering and exposed to Na2O vapor, keeping the activity of Na2O lower than that in NaAlO2. This prevents the formation of hygroscopic NaAlO2 at the grain boundaries. A thin layer of P"-alumina is formed on the surface upon exposure. Further reaction occurs by transporting Na+ ions through the formed P"-alumina and a parallel transport of O - ions through YSZ. This occurs by a coupled transport of Na through P"-alumina and O - ions through YSZ, thus expediting the process. The second electrochemical system of interest is PEMFC. The degradation mechanism of catalysts is studied using inexpensive copper particles. The mechanism of 2+ growth involves a coupled transport of Cu2+ through the aqueous medium and an electron transport through the direct particle-to-particle contact. Effect of applied stress on coarsening of platinum was also investigated. Two platinum wires/foils were immersed in a PtCl4+DMSO (Dimethyl sulfoxide) solution. A tensile load was applied to one wire/foil and the other one was left load-free. The wire/foil subjected to a tensile load became cathodic with respect to the unstressed wire/foil. Thus, under a tensile stress, the chemical potential of Pt decreases. This result suggests design strategies for core-shell catalysts used in PEMFCs: stable core-shell catalysts for PEMFC with Pt shell should be designed such that the shell is under a tensile stress. Specific surface energies (y) of f.c.c. metals as a function of shape and size were investigated using the broken-bond model. y decreased with increasing particle size and approached asymptotic values beyond an equivalent diameter of ~5 nm. Octahedralshaped particles were the most stable.