Development of Nanocomposite Polymer Electrolyte Membranes for Higher Temperature PEM Fuel Cells

Jalani, Nikhil H.

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Development of Nanocomposite Polymer Electrolyte Membranes for Higher Temperature PEM Fuel Cells

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Proton exchange membrane (PEM) fuel cells are one of the most promising clean energy technologies under development. The major advantages include electrical efficiencies of up to 55 %, high energy densities (relative to batteries), and low emissions. However, the main obstacles to commercialization of PEM fuel cells are related to the limitations of the proton conducting solid polymer electrolytes such as Nafion. These membranes are expensive, mechanically unfavorable at higher temperatures, and conduct protons only in the presence of water, which limits the fuel cell operating temperature to about 80 C. This in turn, results in low fuel cell performance due to slow electrode kinetics and virtually no CO tolerance. The potential operation of PEM fuel cells at higher temperature (above 100 C) can provide many advantages such as improved kinetics at the surface of electrode, which is especially important in methanol and CO-containing reformate feeds, and efficient heat rejection and water management. Another issue above 100 C is the reduction of electrochemical surface area of the electrodes due to shrinkage of electrolyte (Nafion phase) within the catalyst layers. This research work is thus focused on the development of nanocomposite proton exchange membranes (NCPEMs) which are chemically and mechanically more stable at higher temperatures and electrodes which can result into better fuel cell performance. These are composite materials with inorganic acidic nanoparticles incorporated within a host polymer electrolyte membrane such as Nafion. The target operating fuel cell temperature in this work is above 100 oC with relative humidity around 30 to 40 %. To achieve these targets, both theoretical and experimental investigations were undertaken to systematically develop these NCPEMs. Various experimental techniques, namely, TEOM (Tapered Element Oscillating Microbalance), Impedance Spectroscopy, MEA (membrane electrode assembly) testing, Ion Exchange Capacity, Scanning Electron Microscope (SEM), Optical Electronic Holography (OEH), Thermal Gravimetric Analysis (TGA), and Dynamic Mechanical Analysis (DMA) were employed to characterize the NCPEMs. A thermodynamic model was developed to describe sorption in proton-exchange membranes (PEMs), which can predict the complete sorption isotherm. A comprehensive proton transport model was also developed to describe proton diffusion in Nafion/(ZrO2/SO42-) nanocomposite membranes. The conductivity of the in situ sol-gel prepared Nafion/ (ZrO2/SO42-) nanocomposite membranes was accurately predicted by the model as a function of relative humidity (RH) without any fitted parameters. This transport model developed offers a theoretical framework for understanding the proton transfer in nanocomposite membranes and is an insightful guide in systematically developing high proton-conducting nanocomposite. Nafion-MO2(M = Zr, Si, Ti) nanocomposite membranes were synthesized with the goal to increase the proton conductivity and water retention by the membrane at higher temperatures and lower relative humidity (120 C, 40% RH) and also to improve the thermo-mechanical properties. The results obtained are promising and indicate that this is a potentially useful approach for developing PEMs with desirable properties. Finally, commercially available high temperature PBI (polybenzimidazole)-H3PO4 (phosphoric acid) gel membrane fuel cell was investigated in the temperature range of 160-180 C. This system exhibited very good and stable performance in this temperature range.

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