(106a) Synthesis of PVA Based Membranes Modified by Benzimidazole Group for Direct Methanol Fuel Cells

Abstract

In
recent years, there has been a great effort to look for alternate membrane
material and Polybenzimidazole (PBI) membranes have been synthesized where the
conducting moiety of this polymer is benzimidazolium groups. The monomers used
to synthesize these polymers are bis conjugated amines and diacids are
expensive. In this work, we have synthesized grafted polymers of PVA as
membrane materials where the grafts are either Poly Acrylic Acid, Poly maleic
anhydride (PMA) or Poly Methacrylic acid (PMAA) and these materials were denoted
by PVA-g-PAA, PVA-g-PMA, and PVA-g-PMAA. We then synthesized C1, C3 & C4
compounds (formula given at the bottom of Table S1) having bis conjugated
amines whereas compound C2 having only one conjugated amine group. One of the
steps in synthesizing C1 compound is nitration of phenyl ring, where there is a
possibility of formation of two isomers out of which only one of them has
strictly bis conjugated amines and can be polymerized with Isophthalic acid
giving the formation of PBI. The other byproduct formed is an impurity and is
minimized by carrying out the nitration step at low temperature but this cannot
be completely removed from the product which is a mixture. Table S1 presents
the summary of our experimental results on characterization of membrane, In the
C1 to C4 compounds and in some cases (membranes 9, 10, 11), Sulfocuccinic acid were
added to the casting polymer solution as additional crosslinking agent in order
to achieve higher conductivity. We added the polymer, thus formed, to the
casting solution, we first compared the results of the Fuel cell formed by
adding C1 (C3 and C4) compound alone and heating at 180°C for some time. Table
S1 has two sets of results, one on characterization (columns 1, 2, 3,4,5,6, and
7) of membranes while the other on Fuel cell (columns 8 to 11). For the latter
columns 10 and 11 gave results on characterization on Fuel cell while columns 8
and 9 gave results when the Fuel cell was run on control mode for higher
efficiency.

            Experiments
on membrane pore sizes are given in column 1 which lies between 8.88 to 9.52nm
and they all are ultra filtration type. The values of the pore size are mostly
governed by the nature of the graft chains, e.g. membranes 1,3,4,7 and 12
consist of PAA grafts and were found to have similar pore sizes. Similarly
membranes 2,5,8,9 were of the similar pore size because the PVA have the same
grafts of PMA. The next important thing is to determine the wall charges of
these membranes and they were estimated by experimentally determining the
rejection of Chromic (VI) acid solution through these membranes. Subsequently,
a space charge model was proposed for the flow of the solution through their
pores and a semi analytical solution was derived for the overall problem of
separation of chromic acid solution. We observed that the high surface charge
was because of the benzimidazolium groups present on the surface and they
ranged from minimum of 465 to maximum of 525mV. The thermal degradation (TGA)
experiments were carried out which showed that, through the modification of the
polymer by benzimidazole groups, lead to stable membrane and the degradation
temperature varies from 150°C for 10% weight loss and for approximately 80%
weight reduction, a minimum of 300°C to 700°C.

            The
twelve membranes reported in Table S1 were first characterized completely in
terms of pore size, wall charge, conductivity, IEC and thermal degradation. In
fuel cell applications, higher temperatures give large current density and it
is observed that some of the membranes (rows 1, 2, 5, 6, 7, 8 and 12) could
easily be operated upto 150°C without the membrane degrading. However in our
case, we insisted that the membrane of direct methanol fuel cell were operated
at 90°C for which most of the fuel cell results for Nafion 117 are available.
The catalysts used on the anode and cathode side were Pt-Ru and Pt loaded on
carbon respectively, which were synthesized by following procedure given in
literature. We find that in each case, the power density and the current
density were higher and for some of our results (membranes 1,6,12) were atleast
two to three times higher than that of Nafion 117 as seen in column 10 and 11.

            It
was recognized that the membrane material has hydrophilic PVA matrix and hydrophobic
benzimidazolium groups at the surface formed by reaction of the C1 to C4
compounds with grafts (see Fig.S-1). When the reaction occurs (confirmed by IR
and NMR), the hydrophobic benzimidazolium are formed which tends to separate as
confirmed by the SEM results. In view of this we assumed that the pores of the
membrane would have a thin hydrophobic layer with positive charge walls. This
is confirmed by the contact angle measurements where the minimum contact angle
is 51° and varied upto 89°. Even though most of the polymer matrix is PVA with
its grafts, when the ions move through the pores (giving rise to current in
Fuel cell), the pore walls get highly charged (as measured by column 2 of Table
S1). Some of the membranes have Sulfo succinic acid SSA as part of the matrix
(e.g. the membrane 9 to 11) which would tend to give sulfonate groups inside
the PVA matrix as well as at the phase boundaries, this way giving rise to
bipolar nature of the membrane (benzimidazolium group being strictly cationic
in nature). In addition to this, some of the compounds such as C1 and C2
compounds have bonded sulfonate groups which would give the positive and
negative charge separated by the molecular distance, once again producing a
bipolar nature to the membrane. In remainder of the cases the charges would be
induced on the phase boundaries as seen in Fig.S-1.  The conductivity observed
in our membranes is very high and could be attributed to the special bipolar
structure of our polymers.

Table S1. Fuel
cell and Membrane Characterization