Impact of Porosity on the Charge Transport in Mesoporous Oxides and Composite Materials

Abstract

Porous materials are an ideal model system to study the influence of the surface on the electrical and protonic transport properties due to their large surface-to-volume ratio and their grains in the nanometer size range. The defect chemistry of nanostructured oxide ceramics with a high density of interfaces, such as grain boundaries and free surfaces, is not comparable with that of the respective bulk materials. This is due to space charge layers (SCLs) at the interfaces, which cause changes to the local defect chemistry, and thus affect the electrical transport properties. The objective of this dissertation was to get a better understanding of the impact of the (surface) defect chemistry on the electrical and protonic transport properties of (meso-)porous zirconia-based oxide materials, in particular of yttria-stabilized zirconia (YSZ) and CeO2- and TiO2-coated YSZ. Accordingly, porous thin films of YSZ with a high surface area were prepared using two different routes. Pulsed laser deposition (PLD) has been used to produce porous oxide thin films with a random pore structure, while an evaporation-induced self-assembly (EISA) process has been used to deposit mesoporous oxides with a regular pore arrangement. The pores were uniformly coated by means of atomic layer deposition (ALD) with varying thicknesses of oxide materials, such as CeO2 and TiO2, to prepare composite materials with tailored structural and electrical properties. The thin films were structurally characterized by several analytical techniques, and electrochemical characterization was done by electrochemical impedance spectroscopy (EIS) measurements varying temperature, oxygen partial pressure or relative humidity. The thesis is divided into three parts. In chapter 1, a detailed description of the transport mechanisms of the charge carriers in metal oxides is presented. In addition, this chapter gives an overview about the defect chemistry of the metal oxides investigated in this work as well as the description of space charge layers at interfaces. The second chapter summarizes the scientific results of this doctoral thesis, which have been published in three research articles. The transport properties of porous PLD-derived YSZ thin films were investigated using EIS. Under dry conditions, the pristine YSZ thin film exhibits only dominant oxygen ion conductivity. However, a significant increase in total conductivity with increasing relative humidity is observed. Here, the additional protonic contribution arises from the adsorption of water molecules at the material surface. While an amorphous titania coating layer results in a decrease of the protonic conductivity compared to pristine YSZ, the crystalline TiO2 layer increases the protonic conductivity contribution again. The crystallinity of the titania layer can be adjusted by varying the thickness, which significantly influences the transport properties of the composites. EIS measurements of mesoporous sol–gel-derived YSZ thin films reveal no effect of pore size on the total electrical conductivity. The composites with thin ceria coatings exhibit mixed ionic/electronic conductivity, but a dominant electronic contribution at low temperatures. In addition, investigations show that the electronic contribution significantly decreases with increasing CeO2 layer thickness. Surface engineering enables to modify certain structural and electrochemical properties by active oxide layer coating, which tailors the transport mechanisms (electronic, ionic and protonic conductivity) in the nanostructured composites. A short summary of the results, final comments on the prospective scientific challenges, and suggestions for further work conclude the thesis.