Halide diffusion in polyaniline membranes

The feasibility of application of polyaniline (PAni) as electrolyte in polymer–electrolyte–membrane fuel cells (PEMFC) was investigated. PAni was dissolved in N-methyl pyrrolidone (NMP), cast as emeraldine base membranes (EB) and then doped with halide acids. The proton conductivity was measured according to Hittorf. The chloride ion distribution within the membrane was evaluated using energy-dispersive-X-ray analysis (EDX) and the diffusion coefficient was calculated using EDX, impedance spectroscopy and photometric analysis. The specific resistance was determined using conventional four-point measurement. The proton conductivity of HCl_conc.-doped PAni is approximately half that of the reference material Nafion^®117. The diffusion coefficient for the chloride ion within the membrane was determined to be between 1.86 and 3.4 × 10⁻⁸ cm² s⁻¹. Although halide-doped membranes were found to be proton conducting, slow halide removal during fuel cell operation occurred causing a rapid decline in cell performance. It is, therefore, concluded that in order to prepare suitable PAni membranes for fuel cell applications, it will be necessary to dope PAni with acids in such a way that loss of the dopant can not occur. Doping with solid acids or large molecular acids (in order to reduce their diffusivity) may be a solution to the problem.