Materials Chemistry of Superprotonic Solid Acids

The goal of this research is to advance the science of superprotonic solid acids, which are of increasing technological importance. Solid acid compounds can be described generically as MnHm(AX4)p with M = alkali metal, and A = S, P, or Se, and X = O or H. A superprotonic solid acid is one in which rapid reorientation of AX4 polyanion groups facilitate fast proton conductivity. Typically, this disordered state is encountered at slightly elevated temperatures and the transition to the superprotonic phase is accompanied by 3-4 order of magnitude increase in conductivity. Here we will pursue targeted chemical modifications to known superprotonic conductors to, on the one hand, elucidate the origins of the superprotonic transition and the magnitude of the conductivity in the superprotonic phase, and on the other hand, develop advanced materials that overcome some of the technological barriers facing known superprotonic conductors. We aim to elucidate the governing principles that define (i) the magnitude of the conductivity in particular in the superprotonic phase, (ii) the temperature of the transition, (iii) and the tendency towards dehydration (and deliquescence). With this objective in mind, the work proposed here has several critical components: (1) synthesis and structural characterization of solid acids with controlled substitutions on cation and polyanion sites; (2) measurement of transport properties by impedance spectroscopy and by H  D relaxation experiments; and (3) determination of phase stability by thermogravimetric and calorimetric methods.