Proton-conducting solid acids are compounds, such as KHSO4 and CsHSO4, that feature spectacular phase transitions during heating for which the proton conduc­tivity increases by several orders of magnitude [81, 86, 87]. CsHSO4, for example, has a phase-transition temperature of 414 K [81]. Below this temperature, CsHSO4 has a monoclinic structure in which the number of protons is equal to the number of proton sites. Consequently, the hydrogen atoms are localized within rigid hydrogen bonds between SO4 tetrahedra and hence their mobility is low. In the high-tem­perature phase, the SO4 tetrahedra can rotate rather freely between crystallo — graphically identical positions, creating six times as many possible proton sites as there are protons available. As a result, an almost isotropic and dynamic hydrogen­bonding network between the different sulfate groups is created, where all oxygens are involved in hydrogen bonding. In this hydrogen-bonded network, proton dif­fusion is a fast process which occurs through proton jumps between neighbouring SO4 groups, as assisted by rotational motion of these groups.

Chan et al. [88] addressed the question of how the addition of nanoparticles, such as SiO2 and TiO2, impacts on the CsHSO4 phase-transition temperature and proton conductivity. Using QENS the authors showed that nanostructuring has the twin effect of lowering the superprotonic phase-transition temperature and increasing the local diffusional-dynamics in the superprotonic phase. The results are summarized in Fig. 9.17, which shows (a) the QENS spectra of bulk CsHSO4 and nanocomposite CsHSO4 with SiO2 (7 nm), (b) the quasielastic width, and (c) the fraction of mobile protons as derived from the quasielastic intensity as a function of temperature. As can be seen in Fig. 9.17b, the phase-transition temperature for the nanostructured

samples is reduced to a least 360 K. As suggested by Chan et al. [88], this behaviour may be linked to the creation of space-charge layers between the conducting phases and the nanoparticles. The creation of such space-charge layers would lead to an increase of the number of vacant sites for the protons to move to, hence allowing a larger fraction of the protons to become as mobile as they are in the superprotonic phase. Indeed, Fig. 9.17c shows that up to 25 % of the protons are mobile in the nanocomposite sample below the phase-transition temperature of 414 K, whereas no sign of proton mobility can be seen in the bulk sample at those temperatures. The results suggest that nanostructuring may be a rewarding research direction in the pursuit of optimized proton-conductivity.