Proton Conductivity and Phase Transition in Potassium Hydroxide Monohydrate
E. I. Nikulin* and Yu. M. Baikov Ioffe PhysicalTechnical Institute, Russian Academy of Sciences, Politekhnicheskaya ul. 26, St. Petersburg, 194021 Russia * email: e.nikulin@mail.ioffe.ru Received December 17, 2013
Abstract—The ionic conductivity of a proton conductor, namely, potassium hydroxide monohydrate, has been studied in the temperature range of 200–410 K. It has been established that the temperature dependence of the conductivity has the Arrhenius form with an activation enthalpy of ~0.4 eV. The preexponential fac tors for the intervals above and below room temperatures differ by a factor ~2.5. The anomalous temperature behavior observed in the range of 285–345 K indicates a phase transition with Tc ~ 295 K. The mechanism of proton transport has been discussed. DOI: 10.1134/S1063783414060286
1. INTRODUCTION Hydrogencontaining solid compounds (salts, acids and hydroxides) have been attracting attention for more than half a century as promising proton conducting materials. Inorganic proton conductors meet presently with strong competition with polymer materials in the field of applications, if not as subjects for use in basic research centered on the investigation of charge and mass transport processes. Proton con ductors are unique in this respect, because they occupy an intermediate position between the “con ventional” electronic and ionic conductors. Leaving aside the historic aspect, we note that hydroxides of metals (including the alkali ones) have become sub jects of intense investigation in the recent decade (see review in [1]). It turned out that derivatives of individ ual compounds in the form of solid eutectics and crys talline hydrates exhibit high proton conductivity at temperatures below 370 K (and even below room temperatures). Significantly, they feature electro chemical activity when used in assemblies with cheaper, other than noble metals. KOH monohy drate, the subject of investigation in the present paper, is one of several hydrate compounds in the KOH– H2O system. The phase diagram of this system was studied in detail long ago. But the data on the ionic conductivity of KOH ⋅ H2O presented in [1–3] are the only ones available. It appeared of interest to continue investigation of this proton conductor below the room temperature region.
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