Conductivity and transport properties of aqueous phosphotungstic and silicotungstic acid electrolytes for room-temperature fuel cells

Conductivity measurements were performed on phosphotungstic and silicotungstic aqueous electrolyte solutions. The conductivity obtained from complex impedance plots had non-Arrhenius dependence on temperature and could be fit by the Vogel-Tamman-Fulcher expression, T-1/2 exp[Q(c)/(T-T-o)]. Conductivities reached 0.257 and 0.32 S cm(-1) at 298 K for the 60 wt % phosphotungstic and silicotungstic solutions, respectively. Low concentration silicotungstic solutions obeyed the Kohlrausch relation allowing for the determination of infinite dilution molar conductivity, 1637 S cm(2) mol(-1), with the anionic species 1/4[SiW12O40](4-) having a molar conductivity of 237 S cm(2) mol(-1). The diffusion coefficient of the anionic species at 303 K was 4.8 x 10(-6) cm(2) s(-1) with a hydrodynamic radius of 5.78 Angstrom. Analysis of the charge transference numbers indicated that the conductivity was predominantly protonic, and suitable for room-temperature fuel cells. (C) 2003 The Electrochemical Society.