Ionic transport in atomic and nanodimensional structures in oxides

Abstract

Oxides with high ionic conductivity represent a class of functional materials with characteristics suitable for electrochemical energy converters, such as fuel cells and batteries. This work addresses effects from changes in crystal structure and of grain boundaries on the ionic transport properties in different oxide materials.

Defects are a prerequisite for transport of charged species in solids, and in this thesis defect chemistry is the theoretical foundation. The first part describes the formation of defects and important aspects for the transport of charged species in the grain interior of oxides. Relevant literature is reviewed in the second part, emphasising various effects of interfaces on transport processes.

The gradual change in the crystal structure of the high temperature proton conductor LaNbO4 through a second order phase transition and its relation to the activation enthalpy of mobility of protons were studied by means of first principles calculations and conductivity measurements. Above the transition temperature, the proton conductivity curve displayed Arrhenius-type behaviour, yielding an activation enthalpy of the mobility of protons of 35 kJ/mol. At the onset of and through the second-order phase transition the conductivity dropped off steeply, followed by a less steep decrease towards lower temperatures. This was interpreted as a gradual increase in the activation enthalpy with increasing degree of distortion of the crystal structure, reaching ~ 57 kJ/mol at 205 °C. The experimentally determined values were in agreement with the computational investigation, where activation enthalpies were determined to 39 and 60 kJ/mol in tetragonal and monoclinic LaNbO4, respectively.

The intrinsic transport properties of the grain boundaries in many ceramics have previously been accounted for by a space-charge model. The model predicts depletion of positive charge carriers and accumulation of negative charge carriers in the space-charge layers adjacent to a positively charged core, which explains why many ionic conductors exhibit high intrinsic grain boundary resistance. Here, we have applied the model to analyse the transport properties of the grain boundaries in BaZrO3, LaNbO4 and Er2Ti2O7 (all acceptor doped).

The pO2-dependencies of the conductivity of acceptor doped BaZrO3 indicated dominating ionic and mixed ionic and p-type electronic conduction for the grain interior under reducing and oxidizing conditions, respectively, while the grain boundaries showed an additional n-type electronic contribution under reducing conditions. Transmission electron microscopy revealed enrichment of Y in the grain boundary region. These findings indicate the existence of space-charge layers in the grain boundaries. Assuming a constant level of acceptors through the space-charge layer (the so-called Mott-Schottky approximation), a space-charge potential of 0.5–0.6 V was obtained at 250 °C in wet oxygen.

An increase in the grain boundary conductivity upon a D to H isotope exchange under isothermal conditions, and a positive pH2O dependency after quenching from different pH2O made us conclude that protons were the major charge carrier in the grain boundaries of acceptor doped LaNbO4. The higher functional pH2O dependency of the conductivity of the grain boundaries as compared to grain interior suggested that protons were relatively more predominating as charge carriers in the grain boundaries than in the grain interiors. This behaviour was rationalised by noting that in the space-charge layers, doubly charged oxygen vacancies exhibit a quadratic decay compared to singly charged protons. Using a Mott-Schottky approximation, a space-charge potential of 0.66 V was obtained at 250 °C for this material.

The electrical properties of ceramics of (Er1-xCax)2Ti2O7-x (x = 0.02 and 0.005) with low and high contents of Si impurities at the grain boundaries were investigated by impedance spectroscopy as a function of pO2, pH2O and pD2O in the temperature range 300 – 800 °C. For the grain interior, oxygen vacancies are the major charge carrier, with protons as a minor contributor at the lower temperatures. In the grain boundaries – as compared to grain interior – the singly charged protons were relatively more predominating than the doubly charged oxygen vacancies, again suggesting that negatively charged space-charge layers govern the electrical properties. In support of this, the samples with a lower content of Si in the grain boundaries exhibited an electrical response essentially similar to those containing more Si and even a secondary Si-rich phase.

The electrical response of Er2Ti2O7 contained an additional impedance – the third in the Nyquist plots – which was suggested to be caused by chemical polarization of the mixed oxide ion and proton conducting grain interiors induced by blocking of oxygen vacancies at the grain boundaries. This was supported by the tear-drop shape of this contribution, the different response on the characteristic frequency when changing either pH2O or the hydrogen isotope in the surrounding gas, and a good agreement between the experimentally and calculated value for the chemical capacitance.

All together, the electrical characterisations which were carried out on BaZrO3, LaNbO4 and Er2Ti2O7 suggest that the grain boundary core-space-charge layer model can be well applied to oxides that conduct protons. The theory of chemical polarization induced by grain boundaries was applied on Er2Ti2O7, which, as we see it, provides a first application of this theory on a mixed oxide ion and protonic conductor.