PROTON METAL-ION CONDUCTION IN MONOALKALI SALT FORMS OF ALPHA-ZIRCONIUM PHOSPHATE

Qualitative information about proton-metal ion contributions to the conductivity of a proton-metal ion solid electrolyte (HM) can be obtained by measuring the EMF of the following cell: H-2(g)1, H2O(g)1, Pt/HM/SSPC/Pt, H-2(g)2, H2O(g)2, with P(H-2)1 = P(H-2)2 and P(H2O)1 = P(H2O)2; SSPC is a pure solid state protonic conductor. If protons are the only mobile species of the mixed electrolyte, then the EMF is zero; otherwise an EMF appears across the cell, with the positive pole on the side of the pure protonic conductor: the higher the M transport number, the larger the EMF. Measurements were carried out on hydrogen salt forms of alpha-zirconium phosphate (alpha-ZrHM(PO4)2.nH2O), with M = Li, Na, K and n = 3.5, 5, 0, respectively, by using monohydrogen zirconium phosphate (alpha-Zr(HPO4)2.H2O) as a pure protonic conductor. The experiments were carried out at 25-degrees-C as a function of relative humidity (33% less-than-or-equal-to RH1 = RH2 less-than-or-equal-to 75%). In all cases the EMF increased with increasing relative humidity, while the following EMF sequence was observed at the same relative humidity: HK > HNa > HLi. Therefore, the protonic component of the ionic conduction increases with decreasing relative humidity and ionic radius of M. This interpretation was supported by studying the decrease of the dc conductivity of HM pellets coated with H+/H-2 reversible electrodes under a dc load of 0.5 V.