Preparation of Electrolyte Membrane Composite Composed of Ionic Liquid and Porous Polyimide for Non-Humidified Intermediate-Temperature Fuel Cell

Abstract

Introduction Polymer electrolyte fuel cells (PEFCs) have been used as clean power generation systems for houses and cars because of their high energy conversion efficiency and low emissions. Presently, perfluorosulfonic acid membranes such as Nafion® have been used as electrolyte membranes in PEFCs because of their excellent chemical stability, high ionic conductivity and good mechanical strength. However, this kind of membranes requires water to have high proton conductivity, which limits the operating temperature of PEFCs to be below 100 °C and makes the system very large. For example, this system needs a humidifier. To realize more compact PEFC systems with higher energy conversion efficiency, new electrolytes applicable to intermediate temperatures, above 180 ºC, without humidification are needed. Ionic liquid (ILs) have unique properties such as non-volatility, high thermal stability, wide electrochemical potential window and high ionic conductivity. Therefore, they have been focused as candidate electrolytes for intermediate temperature fuel cells. In this study, we prepared and tested a composite membrane composed of a porous polyimide (PI) matrix with three-dimensionally ordered macroporous (3DOM) structure and a protic IL as a new electrolyte membrane to realize intermediate temperature fuel cells. Experimental Diethylmethylamine-trifluoromethanesulfonic acid (dema-TfO) was synthesized by direct neutralization of the N,N-diethylmethylamine and trifluoromethanesulfonic acid in a 1 : 1 molar ratio followed by vacuum drying at 100 °C for 48 h, as a protic IL to prepare the composite membrane. Poly(dimethylaminoethyl methacrylate) (PDMEMA) was added to dema-TfO in concentration of 50 g L-1 and stirred at 150 ºC. After dissolving PDMEMA in dema-TfO, the solution was cooled to room temperature and N,N,N’,N’-tetra(trifluoromethanesulfonyl)- dodecane-1,12-diamine was added to the solution in concentration of 50 g L-1 to prepare a precursor solution of dema-TfO-gel. 3DOM PI / dema-TfO-gel composite electrolyte membrane was then prepared by immersion of 3DOM PI into the precursor solution under vacuum at room temperature, followed by heating at 80 °C for gelation. Results and discussion The ionic conductivities of 3DOM PI / dema-TfO-gel composite electrolyte membrane, Nafion® and dema-TfO were investigated in a temperature range from 80 to 150 °C under non-humidified condition. Figure 1 shows the Arrhenius plots for three electrolytes. 3DOM PI / dema-TfO-gel composite electrolyte membrane showed three orders of magnitude higher ion conductivity than that of Nafion®. The activation energies of those electrolytes were estimated to be 16.1 kJ mol-1, 17.0 kJ mol-1 and 18.3 kJ mol-1, respectively. The similar activation energies for dema-TfO and 3DOM PI / dema-TfO-gel composite electrolyte membrane indicate that the ionic conduction mechanism of dema-TfO is maintaind in the composite electrolyte membrane. Figure 2 shows I-V and I-W curves of fuel cells using 3DOM PI / dema-TfO-gel composite and Nafion® membranes at 100 ºC under non-humidified condition. The open circuit voltage and maximum power density of fuel cell using the composite electrolyte membrane and Nafion® membrane were 0.658 V, 22.7 mW cm-2 and 0.592 V, 13.7 mW cm-2, respectively. The fuel cell using 3DOM PI / dema-TfO-gel composite electrolyte membrane showed about 1.7 times higher maximum power density than that of the fuel cell using Nafion® membrane. From this result, it can be said that 3DOM PI / dema-TfO-gel composite electrolyte membrane has advantage for fuel cell operation at intermediate temperature under non-humidified conditions. Figure 1