Environmental Engineering Reference
In-Depth Information
change in water content is of severe concern as it affects the integrity and dura-
bility of MEAs. Typical thickness values of membranes range 50-120 microns as
indicated in the names (Nafion 112 and 115 etc.). A possible method to reduce the
cost of PEM is reducing the thickness, which might also help in decreasing the
membrane resistance thereby improving the performance of PEMFC. However,
reduction in thickness could lead to increased hydrogen and methanol permeability
coupled with reduced mechanical stability.
Efforts directed on improving the proton conductivity of these membranes
include various approaches to mainly increase the water content by incorporating
hygroscopic inorganic and organic additives such as SiO 2 , ZrO 2 , TiO 2 , zirconium
phosphate and zeolites which can keep the membrane humid at high temperatures
during the operation [ 83 - 87 ]. However, most of these composite membranes show
higher conductivity than Nafion only at higher temperatures, their base value being
less than that of Nafion. More significantly, long-term operation of these mem-
branes in MEAs faces severe limitations due to the agglomeration of these par-
ticles during operation resulting in extensive degradation in performance with
time. Also as these membranes do not have any cross-linking, these dispersed
particles often reduce the mechanical stability of the composite membranes [ 85 ].
Solution cast membranes made up of commercial Nafion solutions (also reinforced
by fillers) and porous PTFE matrix help to some extent, in increasing the
mechanical strength and structural integrity, finally enabling the use of reduced
thickness of the membrane without any change in performance. However, the
proton conductivity of the resulting membranes is very poor to limit their power
density [ 87 ].
Even though CNTs have been used earlier to fabricate polymer composites with
increased mechanical stability, applications as an additive for composite membrane
electrolyte have not been tried mainly due to the fear of electrical short circuiting
[ 88 - 90 ]. The addition of CNTs can be expected to give increased mechanical
robustness and integrity as it is well known for its highest Young's modulus. Liu
et al. studied the impact of CNTs on the electrolytic behaviour of Nafion
membranes upon reinforcement to observe several improved features [ 88 - 90 ].
Fabrication of composite membranes of CNTs and Nafion at 1:99 wt% after ball-
milling and solution casting show similar performance to pure Nafion membrane in
terms of proton conductivity, but with significantly less dimensional change for the
case of CNT-reinforced composite membranes. Similarly, Thomassin et al. have
used melt extrusion to incorporate the CNTs on to Nafion membrane to observe
reduced methanol permeability to about 60 % along with an unusual increase in the
Young's modulus up to 140-160 % in comparison with that of commercial Nafion
membranes [ 90 ].
In all the above applications, one has to naturally consider the effect of elec-
tronic conductivity of CNTs. Certain type of functionalization indeed enhances the
electronic conductivity and it is always important to consider the risk of electrical
short-circuiting despite the use of very low amounts of CNTs (\0.1 %). Proper
dispersion is essential for ensuring uniform behaviour and the normally reported
value of percolation threshold for CNTs in Nafion is around 5-11 % depending
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