The fundamental studies of polybenzimidazole/phosphoric acid polymer electrolyte for fuel cells.

摘要

The operation of the polymer electrolyte membrane fuel cell (PEMFC) at high temperatures (>120°C, preferably >150°C) is desirable because of the enhanced catalyst activity, reduced poisoning effect of fuel impurities, simplification of the system, and easy thermal compatibility.; After reviewing the recent development of high temperature polymer electrolyte membranes, and properties and applications of PBI membrane in PEMFC and other electrochemical systems, this work focuses on understanding fundamental properties of acid doped PBI membranes.; Water uptake and proton conductivity of phosphoric acid doped PBI membrane studies are reported as a function of temperature, relative humidity and acid doping level. Membranes directly cast from PBI/TFA/acid solution show the presence of TFA at low acid doping levels (x ≤ 2), which is replaced by H3PO4 progressively with increasing acid doping level. At low doping level, water molecules form hydrogen bonds not only with phosphoric acid species but also with PBI and TFA. At high doping levels, water molecules interact mainly with excess phosphoric acid in the amorphous phase of the membranes. The dependence of the conductivity on temperature can be interpreted with an Arrhenius equation. The excess phosphoric acid molecules interact with PBI mainly by hydrogen bonding. Proton transfer in this system occurs along different paths under different doping levels, RHs, and temperatures.; In order to explore strategies and understand the rationales for approaches to acid doped PBI membranes having both high conductivity and high acid-retention capability, several acids have been investigated. The proton conductivity, chemical and thermal stability, and acid-retention capability have been investigated for all membranes. The proton conductivity of acid, related to its acidity, chemical structure and melting point, gives an upper limit of that of the acid doped PBI membrane. All membranes exhibit lower conductivity than phosphoric acid doped PBI membrane, which is still the best choice for high temperature PEM. A hydrophobic acid dopant, such as DPPA, gives a better dopant-retention capability. Phosphate esters have low thermal stability and hydrolyze at low temperatures in the presence of water while phosphonate esters exhibit much better thermal and chemical stability.