Owing to high thermal and chemical stability and good mechanical properties, polybenzimidazole (PBI) doped with phosphoric acid is a very promising material to be used as an electrolyte in the medium-temperature fuel cells. Their use at temperatures below ~160°C is impeded by the leaching of the free H3PO4 from the membrane. In order to overcome this problem one of the possible approaches is the incorporation of inorganic particles capable to stabilize H3PO4 in PBI matrix. Surface-modified particles can be more efficient for this purpose.
In this work we studied the properties of proton-conducting membranes based on PBI and silica particles surface-modified by propylphosphonic groups.
Composite membranes were obtained by casting of polymer solution containing tetraethoxysilane and modified silane ((2-diethylphosphatoethyl)triethoxysilane) with next hydrolysis by HCl. The mass concentration of the dopant was 5 or 10 wt %, and the mole fraction of functional groups on the oxide surface was varied in the range of 0–100 mol % by changing the composition of the precursor mixture. All films were treated by 75% H3PO4.
The resulting membranes have been characterized using transmission and scanning electron microscopy, IR spectroscopy, and impedance spectroscopy. Grafting of functional –PO3H2 groups onto the silica surface leads to a significant increase in the uptake of phosphoric acid by hybrid membranes, the content of which determines the conductivity of these materials. An increase in the number of –PO3H2 groups leads to both an increase in the degree of acid doping and ionic conductivity. The conductivity of the best samples obtained reaches 0.081 S/cm at 160°C. The introduction of acid groups on the dopant surface is a promising approach from the point of view of reducing the amount of phosphoric acid required to maintain a high proton transport rate.
