Influence of ammonia on the conductivity of Nafion membranes

The effect of NH ₃ and NH ₄ ⁺ poisoning on the conductivity of Nafion membranes was investigated via electrochemical impedance spectroscopy. The conductivities of membranes prepared with different NH ₄ ⁺ compositions were measured in deionized water at room temperature and compared to those at 80 °C in a gas phase for various relative humidities. The liquid-phase conductivity decreased linearly with an increase in the NH ₄ ⁺ composition in the membrane (_{y N ^{H4 +}}), with that of the NH ₄ ⁺-form having a conductivity 25% that of the H ⁺-form. The gas-phase conductivity of the NH ₄ ⁺-form, on the other hand, declined by 66-98% relative to the H ⁺-form depending on humidity. The conductivities of fresh membranes in the presence of gas-phase NH ₃ at different humidities were also studied. The conductivity decreased with time-on-stream and reached the same conductivity at a given humidity regardless of the NH ₃ concentration, but the time to reach steady-state varied with NH ₃ concentration. The _{y N ^{H4 +}} at steady-state conductivity was equivalent for all the NH ₃ concentrations studied. The kinetics of conductivity decrease was slower at higher humidities. The humidity and _{y N ^{H4 +}} appear to have a concerted effect on the conductivity. The quantitative conductivity data under practical fuel cell conditions should be useful for future fuel cell modeling.