High proton conductivity at low and moderate temperature in a simple family of Prussian blue analogs, divalent transition metal hexacyanocobaltates (III)

Proton conductivity behavior was studied in a family of hexacyanocobaltates with divalent transition metals (II), HCCMs. The HCCMs, with molecular formula M₃[Co(CN)₆]₂∙xH₂O (where M = Ni, Co, Fe, Mn and Cd), had cubic crystal structures and similar cell parameters. The number of water molecules per formula unit (x) present in each HCCMs was determined from thermogravimetric analysis data. Differences in conductivity values were evaluated by running dielectric impedance experiments. The values of the permittivity and conductivity real and imaginary parts were obtained for each material. The actual conductivity part was analyzed as a temperature and frequency function. Mobilities, diffusivities, and ion charge densities were derived from the electrode polarization model that appropriately fits the loss tangent curves. The measurement conditions for all the samples were relative humidity of 99% and temperature ranging from 25 to 105 °C. The conductivity values obtained for the HCCMs varied from 10⁻⁴ to 10⁻² S cm⁻¹. At low temperatures, proton conductivity values for the nickel hexacyanocobaltate (HCCNi) stood out (from 10 ^-3 and 10⁻² S cm⁻¹, at 25 and 45 °C, respectively), followed by Fe, Cd, Co and Mn. In addition to the results stated above, activation energies were determined using the Arrhenius model, where the obtained values were below 21.1 kJ mol⁻¹. The proton transport activation energies suggest that the transport through the HCCM porous framework was achieved by the Grotthuss mechanism. The diffusivity in the porous framework increased with temperature for all the samples except for HCCNi, following the trend D_HCCFe > D_HCCMn> D_HCCCo> D_HCCCd. The variability observed between the samples could be related to the ion-binding energies (E_b). These results indicate that hexacyanocobaltates can be useful as mixed matrix membrane (MMM) fillers, providing excellent conductivity and diffusivity when the medium contains a sufficient amount of ionic components depending on the involved transition metal.