Solid-hydroxide protonic conductors: Superionic conductivity, phase transitions, isotopic effect, and self-organized microheterogeneity

Abstract

New members of the family of inorganic solid-state protonic conductors are synthesized on the basis of KOH, NaOH, and H2O. The equimolar eutectics KOH/NaOH and KOH/KOH · H2O and the monohydrate KOH · H2O are superprotonic conductors in the temperature ranges 360–458, 360–370, and 320–420 K, respectively, with the conductivities of above 1 mS/cm and activation energies below 0.4 eV. A characteristic feature of the eutectics is the presence of anomalies in the temperature dependences of the conductivity and heat capacity in the range 360 ± 1 K. The isotopic effect of the protonic conductivity for KOH · H2O and for the high-temperature form of KOH/NaOH is found to be 1.40 ± 0.15. The role played by the self-organized microheterogeneity of the KOH/NaOH and KOH/KOH · H2O solid eutectics in the substantial increase in the conductivity and in the change of its pattern as compared to the individual KOH and NaOH is discussed.