| The environmental impact caused by global energy demand has led to a vast surge in research on clean and renewable energy technologies. The failure of current fuel cell technologies to help mitigate CO2-producing energy sources on a large scale is directly related to their high (>500°C) or low (<200°C) operating temperatures. There are several advantages of the intermediate temperature range; however, it remains unexploited in the realm of fuel cell electrolytes due to a significant lack of materials that are both stable and highly conductive. Novel proton conducting La-phosphate based electrolytes offer the promise of a solution.;In this work, La-phosphate based crystalline and amorphous materials are examined. In polycrystalline LaPO4, previously unrecognized amorphous grain boundary films are shown to act as rapid proton conduction paths. Experiments reveal the amorphous phosphorus-rich grain boundary films to be stable in air at 500°C, with a conductivity upwards of 2.5x10 -3 S/cm; many orders of magnitude higher than the crystalline bulk. The results indicate the possibility of forming stable, high proton-conductivity nano-composites of RE-phosphate glasses and crystalline ceramics for intermediate temperature fuel cells.;On bulk glasses, a combination of structural and electrical analysis of acceptor doped La-metaphosphate has revealed that high concentrations of protons may be successfully incorporated as charge compensating defects. A shallow-trap model for proton conduction is presented whereby protons are locally trapped at aliovalent cation defects and transported via dissociation of OH bonds from trapping centers along phosphate tetrahedra. The conductivity increases two orders of magnitude from the unsubstituted to the 60% substituted glass, reaching a maximum conductivity of ∼10-6 S/cm at 450°C. The activation energy is found to be dependant on cation type, and decreases with increasing cation radius. The average transport distance between proton centers predicted by the complex modulus electrical analysis is on the order of nanometers, which is the same order of magnitude for proton-proton distance calculated from the structural data. The diffusivity of protons is estimated to be between ∼10-11cm2/s and ∼10 -9 cm2/s in the 300-500°C range. The model for proton transport is expected to hold for most glasses where molecular water is not present. |
