Proton Conductivities Manipulated by the Counter-Anions in 2D Co-Ca Coordination Frameworks

Three new layered Co-Ca phosphonates are reported, namely, [CoCa(notpH(2))(H2O)(2)]XnH(2)O (CoCa-XnH(2)O; X = Cl-, NO3-, PF6-), where notpH(6) is 1,4,7-triazacyclononane-1,4,7-triyltris(methylenephosphonic acid) [C9H18N3(PO3H2)(3)]. The layer topology of these compounds are identical to that of [CoCa(notpH(2))(H2O)(2)]ClO(4)nH(2)O (CoCa-ClO(4)nH(2)O). The most significant difference lies in the counter-anions, which not only have different size and shape but also show different capabilities towards hydrogen bonding. Consequently, the number of lattice water molecules and the hydrogen-bond networks between the layers can be varied depending on the particular counter-anion. AC impedance measurements reveal that the proton conductivities at 25 degrees C and 95?% relative humidity (RH) follow the sequence: CoCa-ClnH(2)O > CoCa-NO(3)nH(2)O > CoCa-PF(6)nH(2)O approximate to CoCa-ClO(4)nH(2)O. Compound CoCa-ClnH(2)O shows the highest proton conductivity of 1.40 x 10(-4) S cm(-1) at 25 degrees C and 95?% RH, which is one order of magnitude higher than those of CoCa-PF(6)nH(2)O (1.39 x 10(-5) S cm(-1)) and CoCa-ClO(4)nH(2)O (1.55 x 10(-5) S cm(-1)).