Abstract
In high-temperature polymer electrolyte membranes, phosphoric acid is used as dopant for polybenzimidazole-type membranes to provide the protonic conductivity. In addition, phosphoric acid also serves as proton conductor in the porous electrodes in order to establish the three-phase boundary. In the first part of this chapter a short overview is given on the physico-chemical properties of (aqueous) phosphoric acid. In the second part the focus is on the adsorption of phosphoric acid as a protic electrolyte on polybenzimidazole-type polymers. Although polybenzimidazole-type membranes are routinely doped with phosphoric acid, few studies on the exact nature of the acid inside the membrane have been published. Experimental data from our institute and data compiled from literature indicate that the polymer chain is protonated by the acid and that the anions are bound by coulomb interactions. Additional electrolyte molecules can interact with the polymer chain by formation of H bonds or via H bonds with other H3PO4 molecules. It is demonstrated that the uptake of phosphoric acid can be described by a modified BET isotherm, assuming multilayer-like adsorption. The assumption of free phosphoric acid in the membrane at high doping levels is supported by Raman spectroscopy.
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