Protonic surface conduction controlled by space charge of intersecting grain boundaries in porous ceramics

Water physisorbed on surfaces in porous ceramics under near-ambient conditions provides a medium for fast protonic diffusion and conduction, but the presence of dual relaxation frequencies in H-1-NMR spectra has remained unexplained. We report two well-separated time constants for surface protonic conduction in the water layers by impedance spectroscopy, and assign them to transport on grain surfaces and to resistance to cross intersects of grain boundaries. The latter can be understood in terms of charge carrier depletion by the positive core charge of the grain boundary intersecting the surface and penetrating the adsorbed water layer. This rationalises the very strong dependency of surface conductance on relative humidity and water layer thickness. It may help design better sensors, electrodes, and electrolytes which utilise surface protonics, as well as better insulators and dielectrics.