Zeolite – Protic Ionic Liquid Composites: Preparation, Characterization and Evaluation of Ion Conduction Properties

Fuel Cells (FCs) have been recognized as one important device for efficient transformation of chemical energy to electricity. Among the different FC types those of Polymer Electrolyte Membranes (PEMs) have received significant attention. In such FCs, a membrane that ideally enables only proton transport is the barrier between the anode and cathode. Nafion, which is usually used for this purpose, requires relatively low operating temperatures (<100oC) in order to maintain the necessary levels of hydration for proton conduction. However, the low operating temperature has two significant drawbacks: 1. It is not easy to recover the waste heat; and 2. The Pt catalyst of the electrodes is very sensitive to CO poisoning. To address these issues many researchers are trying to develop High Temperature Polymer Electrolyte Membranes (HTPEMs), with operating temperatures of up to 200oC. Unfortunately, the performance of HTPEMs is often limited by the membrane fuel crossover and the loss of hydration which results in the loss of proton conductivity. The incorporation of hydrophilic zeolite nanocrystals in PEs has been suggested as a possible alternative for increasing membrane water content at elevated temperatures and decreasing fuel cross-over. Zeolites conduction is primarily due to cation migration[1], and is significantly enhanced in the presence of water. However, even in that case the maximum conductivity values reported are of the order of ~10-4 S.cm-1 [2]. These values are relatively low indicating that the presence of zeolites is expected to have a negative effect on the overall PEM proton conduction. To improve zeolite conductivity several groups have synthesised and tested acid functionalized mesoporous materials and zeolites[3]. Ionic liquids (ILs) are salts with low melting points that are characterized by their enhanced thermal stability, non-volatility and non-flammability. It has recently been shown that protic ILs can undergo hydrogen oxidation and oxygen reduction reactions in the absence of water molecules, thus they have the potential to be used as proton carriers in Fuel Cells operating under anhydrous conditions[4].   The goals of this work are, initially, to study the encapsulation of a protic IL in zeolite NaY, and to understand its effect on zeolite conduction properties. Such composite materials might become one constituent of a composite membrane for HTPEM fuel cells. H-3-methylimidazolium bis(trifluoromethanesulfonyl) imide (HMITFSI) was selected as the protic IL of this study. Composites with HMITFSI/ Zeolite NaY weight ratios (IL/Z) between 0.03 and 1 were prepared.  N2 physisorption measurements have shown that the micropore surface area gradually decreases with IL loading, indicating that HMITFSI enters the zeolite pores. Furthermore, the water uptake of the zeolite crystals decreased from ~9% at 150oC for zeolite NaY to ~2.5% for the sampe with IL/Z ~1. Analysis of X-ray diffraction data and FT-Raman spectra show the formation of NaTFSI which indicates that the entrapment of HMITFSI is also accompanied by an ion-exchange reaction. The ionic conductivity of the composites was measured using impedance spectroscopy. At dry conditions it increased with temperature and with IL/Z ratio. The maximum conductivity was ~2.5 mS.cm-1 at 150oC for the sample IL/Z=1. This value is about 4 orders of magnitude higher than that of NaY powder and about 1 order of magnitude lower than that of the pure IL. The presence of ~4.2 kPa of water primarily enhanced the conductivity of the samples with low IL/Z values. The Effective Medium Theory was also used to estimate the zeolite/ionic liquid conductivity at the grain scale as inferred from conductivity measurements at the macroscopic scale. The results will be analysed with respect to the possible proton transfer mechanisms and with respect to the potential use of such composites in Fuel Cell devices.