Low-Cost Nanomaterials for High-Performance Polymer Electrolyte Fuel Cells (PEMFCs) - Low-cost Nanomaterials: Toward Greener and More Efficient Energy Applications - page 375

catalyst layer [ 124 ], the MEA has been obtained by pressing the two electrodes
uniaxially with the Nafion 115 membrane in between at 110 C at a pressure of
1 ton for 4 min. Hormone-modified MEAs were fabricated by introducing different
amounts of IAA in the catalyst layer by dissolving them in the catalyst ink (both in
the cathode and in the anode). Proportions (by weight) of IAA with respect to the
weight of Pt in the catalyst layer include W Pt :W IAA : 1:0, 3:1, 2:1, 1:1, 2:3 and 1:2.
The MEAs were used to form single fuel cells by passing humidified H 2 (80 %
RH) on one electrode and humidified O 2 on the other at 0.2 slpm through ser-
pentine flow fields. The fuel cells were conditioned at 0.2 V for 30 min and
polarization measurements were carried out at 60 C.
Figure 12 a shows the superimposed steady-state polarization plots of H 2 /O 2
fuel cells (at 60 C) containing Nafion 115 membrane as the polymer electrolyte
with different hormones in the catalyst layer. While the maximum performance
reported for the system built with state-of-the-art technology is 600 mW cm -2 , the
performance obtained with the fuel cell design and testing procedures employed in
the present study is 250 mW cm -2 , which is used as the benchmark (denoted as
the 'reference system'). An interesting enhancement in the fuel cell performance
(by 100 mW cm -2 ) is observed for the MEA containing IAA in the catalyst layer
compared to the reference system. On the other hand, the rest of the hormones
exhibit only poor performance compared to the reference system with the cyto-
kinins (BAP and kinetin) showing inferior performance compared to that with the
auxin, IBA. While the pH of the composite dispersions and the electrochemical
stability window of the corresponding membranes are almost similar, only IAA is
capable of enhancing the performance of the PEFC. Although specific reasons for
this discrimination could not be clearly identified, a probable reason could be the
difference in chemical stability of the hormones in the presence of the reactive
intermediates produced during the fuel cell reactions. Also, in this context, special
mention is to be made of the auxin, IAA, a powerful plant hormone capable of
stimulating a number of functions at in vivo concentrations as low as 10 -8 M
[ 125 ]. There is an astonishing correspondence of its physiological activity with the
critical functional aspects of the catalyst layer [ 126 ]. More specifically, IAA is
known to trigger proton pumps across plasma membranes resulting in the acidi-
fication of protoplasts to effect cell elongation [ 127 ], reduce molecular oxygen to
superoxide radical ion and disintegrate H 2 O 2 to a hydroxyl radical to create oxi-
dative stress [ 128 ] and impart better permeability of ions through the cell mem-
branes [ 129 ]. It should be noted that the disintegration of the H 2 O 2 intermediate is
the rate determining factor in the two-step O 2 reduction mechanism at fuel cell
cathodes. Nevertheless, the indole derivative, IAA, is quite different in its reac-
tivity compared to N-heterocycles such as imidazole, pyrazole and benzimidazole,
deployed frequently as electrolytes in PEMs due to the presence of both proton
donor and proton acceptor nitrogen atoms [ 129 ].