(702f) 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 (<100^oC) 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 200^oC. 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 150^oC
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 150^oC
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.