Environmental Engineering Reference
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phosphoric acid-doped PBI as a proton-conducting membrane. Moreover, they
found that phosphoric acid in PBI plays a dual function, where it acts as a proton
conductor and more significantly as a proton-conducting medium [ 130 - 134 ].
Phosphoric acid-doped PBI membranes represent one of the emerging proton-
conducting membranes for the electrolyte applications in fuel cells that would
operate in the range of 120-200 C. Working at higher temperatures is extremely
critical to prevent CO poisoning of Pt electrocatalyst (either present inadvertently
in the hydrogen feed from a reformer or generated as an intermediate during
oxidation of fuels like methanol and ethanol) apart from the improved kinetics and
higher efficiency. Further, PBI membranes do not rely on water for its proton
conductivity thus simplifying the balance of plant components greatly. Proton
transport in H 3 PO 4 -doped PBI is mainly through two modes; the rapid exchange of
proton via hydrogen bonds between phosphate, N-heterocycles of PBI (Grotthus
type) and through the self-diffusion of phosphate ions. While the chemisorbed 2 %
phosphoric acid alone cannot provide proton conductivity, higher phosphoric acid
content due to physorption results in mechanically poor membranes and lack
durability under fuel cell operating conditions [ 135 , 136 ]. Hence, an optimum
level of physisorbed phosphoric acid should be maintained to keep the membrane
integrity intact.
Even though the H 3 PO 4 -doped PBI membrane holds advantages, like increased
reaction kinetics, higher CO tolerance, easy water management and lesser balance
of plant components but still there are several challenges. For example, the proton
conductivity of H 3 PO 4 -doped PBI membranes is normally in the range of
0.01 S cm -1 which is one order of magnitude lesser than that of Nafion-based
PEMs at room temperature. Moreover, the durability is also a serious issue in
phosphoric acid-doped PBI membranes mainly due to the leaching out of physi-
cally adsorbed phosphoric acid that reduces the proton conductivity of the mem-
brane thereby reducing the performance of the fuel cell stack. In addition, the
mechanical strength of these membranes gets reduced by the adsorption of
phosphoric acid due to the swelling of polymer matrix. On the other hand, lower
phosphoric acid uptake results in poor proton conductivity. Attempts to increase
the proton conductivity of PBI membranes include the addition of inorganic and
organic fillers containing phosphate molecules resulted in proton conductivity
improvement but hampered the mechanical stability [ 137 - 141 ]. On the other hand,
the cross linkers added to improve the mechanical stability resulted in a sacrifice in
proton conductivity [ 142 - 144 ].
Apart from the proton-conducting polymer electrolyte membranes, there are also
other ions, such as Li + -, Na + - and hydroxyl-conducting PEMs that are being
developed due to many advantages associated with solid PEMs over their liquid
counterparts such as easy handling, removal of leakage problems, improved safety
and flexibility. For example, crystalline complexes formed between alkali metal
salts and poly(ethylene oxide) are capable of demonstrating significant Li +
conductivity which highlight possible applications in LIB electrolytes. Various Li
ion-conducting PEMs including LiClO 4 -polyacrylonitrile and poly(vinylidene
fluoride) have been reported [ 145 - 148 ]. Solid-state polymer-silicate nanocomposite
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