Quantification of surface protonic conduction in porous oxides

Abstract

The present work provides a fundamental and methodological framework for parameterisation of the thermodynamics and kinetics involved in surface protonic conduction of porous ionic oxides. A novel nomenclature for defect surface species is introduced. The surface protonic conductances have been estimated where various proton migration routes were considered. A brick layer model is developed that connects surface conductance with macroscopic sample conductivity via microstructural parameters. This allows quantitative discrimination between types of adsorption and surface transport in the different water layers on oxide surfaces, supported by water vapour partial pressure dependencies and enthalpies of adsorption and conductivity. The enthalpies for protonic surface mobility for oxides decrease systematically with lower temperatures (higher relative humidity) as the water layers grow in coverage and thickness and protons go from jumping between surface oxide ions to jumping between loosely bonded adsorbed hydroxide ions and water molecules. The findings and models developed in this work contribute to the understanding and control of surface protonics in porous oxides, which is believed to play an important role in electrochemical and photoelectrochemical cells, humidity sensors, and heterogeneous catalysis.