A Composite Membrane for High-Temperature Operation of Fuel Cells Based on Polymer ElectrolytesEPA Grant Number: U915930
Title: A Composite Membrane for High-Temperature Operation of Fuel Cells Based on Polymer Electrolytes
Investigators: Yang, Christopher
Institution: Princeton University
EPA Project Officer: Jones, Brandon
Project Period: January 1, 2001 through January 1, 2002
Project Amount: $102,000
RFA: STAR Graduate Fellowships (2001) RFA Text | Recipients Lists
Research Category: Fellowship - Other Engineering , Academic Fellowships , Engineering and Environmental Chemistry
Energy conversion has an enormous impact on the quality of our environment, and a major goal of our society should be to provide low-cost, efficient energy conversion with minimal negative environmental impacts. The development of fuel cells is one step towards achieving these goals and reducing emissions of pollutants and emissions that lead to acid rain, photochemical smog, and climate change. Current proton-exchange membrane fuel cells based on perfluorinated sulfonic acid membranes have excellent performance with respect to power density and reliability when operating with pure hydrogen and air. However, in near-term applications, the fuel cell will likely be supplied with a hydrogen-rich gas by chemical conversion of hydrocarbon fuels. In such "reformate" mixtures, CO2, H2O, and trace amounts of CO also are present. The most significant problem occurs because of the trace amounts of carbon monoxide in the fuel stream, which leads to poisoning of the platinum electrocatalyst for hydrogen electro-oxidation. Increasing the temperature of operation is one approach to alleviating some of this poisoning effect by the CO by changing the fractional coverage of the adsorbed species on the platinum surface, thereby increasing the rate of hydrogen oxidation. Increasing operating temperatures from 80°C to approximately 130°C or higher can result in a greater than 10-fold increase in the CO concentration with the same fuel cell performance. The objective of this research project is to improve the water retention characteristics of these polymer membranes to maintain sufficient hydration while allowing for reduced humidification requirements and reduced pressurization.
These fuel cell membranes must be hydrated for optimal proton conductivity of the role of water in promoting membrane acidity and proton mobility. To maintain hydration, the hydrogen and air entering the fuel cell must be humidified to prevent drying of the membrane. Achieving higher temperature operation with these polymer electrolytes can be accomplished by saturating the inlet gas, which, at temperatures greater than 100°C, requires pressurization. However, any increase in operating pressure parasitizes output electrical energy and reduces the net power output and efficiency of the fuel cell. To achieve the stated objective of my research project, I am making composite membranes that incorporate an inorganic hydrophilic material within the polymer structure. This can have the effect of improving water uptake and improving conductivity. In particular, I am focusing on a composite membrane composed of zirconium phosphate and Nafion. I have demonstrated improved performance in a H2/O2 fuel cell as compared with unmodified Nafion at 130°C. Additional work involves characterizing the composite membrane more carefully to understand the interaction of zirconium phosphate with the acidic membrane and the influence of various chemical and physical parameters on water retention and proton conductivity. Furthermore, I have collaborated with a group in Messina, Italy, who have tested these membranes in a direct methanol fuel cell with impressive results. Future work will involve testing other polymer-inorganic composite membranes. Those with increased acidity could improve proton conductivity significantly while improving water uptake and conductivity. These systems operating at elevated temperatures also have benefits with respect to waste heat rejection and/or utilization.