Composite membranes for high temperature and low relative humidity operation in proton exchange membrane fuel cells

Proton Exchange Membrane Fuel Cells (PEMFCs) are in the forefront among other fuel cell technologies and significant efforts in the fundamental areas of research have enabled rapid advances in the PEMFC technology development. PEMFCs provide the highest power density and specific power among all the other fuel cell types and hence have use in portable devices, transportation and stationary power generation and cogeneration applications. PEM fuel cells based on perfluorinated membrane electrolytes operate in the temperature range between 60 and 80°C while elevating the operating temperature provides improved carbon monoxide tolerance, faster electrode kinetics and simpler thermal management. However, high temperature results in dehydration of the polymer electrolyte leading to increased membrane resistance and degradation of the membrane-electrode interface.; Addition of an inorganic material like SiO2, TiO2 or other metal oxide can alter and improve the physical and chemical properties of a polymer electrolyte (such as NafionRTM) and enable high temperature and low relative humidity PEMFC operation. This thesis investigates fuel cell testing of NafionRTM/metal oxide composite membranes at elevated temperatures (115--130°C) and relative humidity as low as 60%. Fuel Cell test results demonstrate significantly improved performance of the composite membranes over the unmodified membranes under these conditions and with up to 500 ppm CO in the hydrogen fuel stream. Thin film Nafion RTM/metal oxide composites exhibit low hydrogen crossover and sustain potential oscillations at low relative humidities compared to the unmodified membranes. Physical and chemical characteristics of the composite membranes are studied in detail to comprehend the reasons for the superior performance.; A model is put forward for the beneficial effects of metal oxide particles on the fuel cell operation. It is suggested that improved cell behavior is not related to water retention properties of the metal oxide component, but rather the effect of metal oxide particle on the temperature dependent structure of the polymer matrix. In particular, an understanding of the interfacial chemistry that occurs between the metal oxide particles and the Nafion RTM membrane is emphasized, along with a consideration of how this interface affects elevated temperature fuel cell dynamics.