Environmental Engineering Reference
In-Depth Information
without any change in performance is essential for rapid progress in this area. This
can be accomplished by the effective distribution of Pt on the supporting material
with a high surface area as well as higher electrical conductivity [
42
]. For this
purpose, a given support material for PEMFC electrodes should fulfil the following
requirements, namely (i) high surface area, (ii) chemical stability under oxidative/
reductive conditions, (iii) mechanical robustness under both open and closed cir-
cuit conditions and (iv) good electrical and thermal conductivity. Many of these
conditions are met exceptionally well with CNTs although CNT is not cheap at
present for large-scale applications. However, there are sufficient indications that
CNT cost is likely to come down with increased production [
57
,
58
].
One of the main problems associated with even commercial electrode formula-
tions is the isolation of carbon particles by the use of Nafion as a binder in the catalyst
layer. The insulating nature of this binder can indeed block Pt particles associated
with carbon from accessing the external circuit due to the lack of electrical network
resulting in the decrease of further Pt utilization. Pt nanoparticles deposited on
CNTs, however, are almost certain to have electrical contact with external circuit
which eliminates these types of problems associated with the presence of a thin
insulating layer of Nafion covering the carbon particles [
59
]. Hence, a judicious use
of CNTs could in fact overcome many of such issues that other forms of carbon-
based electrodes struggle to overcome in power source related applications.
5.3 Carbon Nanotubes as Cathode Support Material
Sluggish kinetics of oxygen reduction reaction (ORR) sustained at the cathode of
MEA typically necessitates the use of higher Pt loading in comparison with that in
the anode. However, in many cases the use of CNTs has been shown to be
profitable in terms of providing a better exchange current density towards ORR
without causing any detrimental mechanical behaviour as a support material for
the electrocatalyst. For example, Yan et al. have carried out pioneering work on
the use of CNT in PEMFC electrodes especially for the cathode to improve Pt
utilization [
59
-
62
]. Their initial study of depositing 4 nm Pt particles on CNTs has
shown improved current and power density in the regions (i.e. activation, ohmic
and mass transport domains), presumably due to the intrinsic properties of CNTs to
increase the oxygen reduction kinetics. This is in accordance with the findings of
Britto et al., where CNT/metal electrodes show higher exchange current density
than that on other metal/C electrodes [
63
]. Enhanced mass transport is also
anticipated to be beneficial in the case of CNT-based electrodes coupled with
reduced ohmic loss, which is acceptable while comparing the electrical conduc-
tivity of CNTs, graphitic powders and other forms of conducting carbon. This has
resulted in an enhanced Pt utilization of 58 % against 34 % of carbon-based
electrodes under favourable conditions. However, water clogging remains as a
critical problem in the cathode which severely restricts the performance of PEMFC
under high humidity conditions [
59
].
Search WWH ::
Custom Search