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upon the dimensions of CNTs used and their mode of preparation. Lie et al. in yet
another report have prepared a three-phase composite membrane by sandwiching a
Pt-CNT-Nafion membrane between two pure Nafion membrane phases [ 88 ]. This
has improved the performance of Nafion membranes significantly with respect to
both mechanical stability and water management perspectives. Thus undoubtedly,
the addition of CNTs results in both improved mechanical stability and chemical
inertness while the Pt particles supported on it helps to produce water by reacting
with H 2 and O 2 that penetrates the membrane and thereby keeping the membrane
wet under dry conditions. However, critical lifetime data about Pt dissolution and
carbon corrosion are necessary to comment on the benefits of these composite
materials with respect to robustness and durability.
7 Role of Functionalization of Carbon Nanotubes
All the above reports are primarily intended to use CNTs as mechanical stability
boosters or reinforcing phases albeit in small amounts, for Nafion in order to
withstand the high processing conditions of stack fabrication and to prolong
membrane life. As a result, the CNT-Nafion composite membranes have shown
conductivity values similar or less than that of pure Nafion membranes, especially
if pristine single or multiwalled CNTs are used for composite fabrication. How-
ever, CNTs can be functionalized on the sidewalls with desired groups through
careful chemical process, which can be used in a constructive manner to enhance
the proton conductivity of Nafion membranes. The main advantages of using
functionalized CNTs for composite polymer electrolytes are (i) reduction in
electronic conductivity: since all types of CNTs are known to have electronic
conductivity and functionlization can effectively reduce the electronic conduc-
tivity. (ii) Tuning the interfacial structure: it is always desirable to attach mole-
cules structurally analogous to that of the polymer backbone to ensure uniform
properties in the composite. (iii) Better dispersion and adhesion: agglomeration
and phase segregation of CNTs at isolated regions could be avoided using
appropriate functionalization. (iv) Easy processability: since CNTs can be made
soluble either in aqueous or organic solvents using different types of chemical
functionalization approaches, these composites can be easily processed in the form
of films of uniform thickness and properties. However, the choice of functionali-
zation is critical since some functional methods could on the other hand enhance
carrier density to finally yield better electronic conductivity while some others can
damage the morphology [ 91 , 92 ].
The choice of functionalization of CNTs for a composite electrolyte with
Nafion is obvious. Since sulphonic acid groups on the side chains are responsible
for the conductivity of PFSA membranes like Nafion via domain formation, CNTs
are to be functionalized with sulphonic acid groups as any increase in their con-
centration is expected to improve the proton transport. We have utilized this
concept
and
functionalized
both
single-walled
and
multiwalled
CNTs
with
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