Phosphoric Acid Diethylmethylammonium Trifluoromethanesulfonate-Based Electrolytes for Nonhumidified Intermediate...

The present study reports a new series of electrolytes for nonhumidified intermediate temperature fuel cells (IT-FCs). This series of new mixed electrolytes, composed of phosphoric acid (PA) and diethylmethylammonium trifluoromethanesulfonate ([dema][TfO]), was designed as nonhumidified IT-FC electrolytes. The mixed electrolytes show a higher thermal stability than pure PA, which is dehydrated at ITs. The thermal stability of the mixed electrolytes could be explained by the interaction between the triflate group in [dema][TfO] and PA, as indicated by Fourier transform infrared and proton nuclear magnetic resonance (¹H NMR) spectroscopies. On the other hand, the ionic conductivity and proton transference number of the mixed electrolytes were similar to those of the pure PA. However, the oxygen reduction reaction (ORR) activity of a platinum catalyst is significantly enhanced in the mixed electrolytes, which was due to the several orders of magnitude increase in oxygen solubility by the addition of [dema][TfO] to PA. Specifically, for the equimolar fraction mixed electrolyte, the diffusion coefficient and the solubility of oxygen were ca. 1.47 × 10^-5 cm² s^-1 and ca. 1.28 mmol dm^-3 at 150 °C, respectively. The addition of [dema][TfO] to PA could significantly enhance the ORR activity. Therefore, the PA_[dema][TfO] mixed electrolyte can be one of the solutions to develop nonhumidified intermediate FC electrolytes.