Colloidal Nanocrystal Films Reveal the Mechanism for Intermediate Temperature Proton Conductivity in Porous Ceramics

Over the past few years, the observation of unexpected but significant proton conductivity in porous, nanocrystalline ceramics has generated substantial scientific interest mirroring the excitement surrounding ionic conduction in other nanostructured or porous materials. Here, we utilize colloidally synthesized ceramic nanocrystals of cerium oxide (CeO2) and titanium oxide (TiO2) to systematically study how grain size, microporosity, and composition influence proton conduction. By measuring the temperature-dependent impedance of porous thin films of these nanocrystals under dry and wet atmospheres, we find that both CeO2 and TiO2 display significant proton conductivity at intermediate temperatures between 100 and 350 degrees C. Furthermore, we investigate the effect of oxygen activity on proton transport, finding that using oxygen as a carrier gas drastically reduced the proton conductivity by up to 60 times. Together, these results suggest that the most likely source of mobile protons in these systems is dissociative adsorption of water at surface oxygen vacancies, with composition, nanocrystal size, and oxide defect equilibria influencing the surface activity toward this reaction and, hence, the proton conductivity.