Mark Hendriks

Ph.D. thesis

Summary

Conventional capacitors use a dielectric medium that becomes polarised in the high frequency regime and are commonly used in ac and dc applications. In contrast, supercapacitors make use of an electrolyte in which space-charge polarisation takes place at low frequencies (<1 Hz). The electrical behaviour of supercapacitors can be found between that of conventional capacitors and batteries. The space-charge layer can store an enormous amount of charge and since the space charge layer is formed at the complete interface between electrolyte and electrode, a large electrode surface area can be used to increase the capacitance. The work described in this thesis deals with the preparation of supercapacitors from metal/yttria-stabilised zirconia (YSZ) composites and their capacitive properties.

This thesis is broadly divided in two parts. The first part concerns the capacitive behaviour of YSZ and the defect structure of the space-charge layer. In the second part the capacitive behaviour of metal/YSZ composites with varying microstructure is described.

A general model for describing the (differential) capacitance in YSZ was derived on a thermodynamic basis. This model is based on the assumption that a random distribution of all oxygen vacancies is present over the entire oxygen sub-lattice. The model gives a qualitative description of the double-layer capacitance as a function of yttria concentration, temperature and bias potential. Based on the differential capacitance measurements, a defect structure for the near-surface layer of YSZ was proposed. On the assumption that only a part of the oxygen sub-lattice is accessible for mobile oxygen vacancies, thus limiting the maximum attainable oxygen vacancy fraction in the double-layer, an improved model for describing the differential capacity in YSZ was obtained. The near-surface dielectric constant of YSZ was used as a fit parameter and its value appeared to result in permittivity values lower than reported in literature for the bulk YSZ. According to the developed model, the double-layer capacitance is proportional to e_r^½, while for conventional dielectric materials that operate on the basis of ionic and electronic polarisation the capacitance is proportional to e_r. Elevated temperatures (>400°C) are required for the formation of the double-layer capacitance, since at room temperature the mobility of the oxygen ions is insufficient. At high temperatures the capacitance is determined by the oxygen vacancy concentration in the double layer, while at room temperature the capacitance is determined by the crystal structure.

For composites with a randomly dispersed metal phase the employed preparation techniques did not result in significant differences in the room temperature dielectric properties. However, the homogeneity of the positional distribution of the metal phase decreased with increasing metal concentration. The capacitance of the composites increased with concentration of the metal phase. The component of the electrode surface area perpendicular to the electric field strength contributed to the enhancement of the capacity. However, the increase in capacity was merely attributed to an effective decrease in the distance between the electrodes. The normalised percolation theory was applied for fitting the data. For composites with a randomly distributed metal phase a percolation threshold close to the value of 31.2 vol% for random dual-phase composites with overlapping particles of equal size and shape was obtained. Comparable values for the enhancement in capacitance could not be verified experimentally. Most likely this is due to a very inhomogeneously distributed metal phase near the theoretical percolation threshold, resulting in conducting paths in the compacts.

At elevated temperatures, the entire YSZ/electrode interface contributes in the formation of the double-layer capacity. By creating large metallic networks that are connected with the electrodes, large capacitances up to 4.3 kF/m² were obtained. To decrease the required noble metal concentration, platinum was electrolytically deposited at the electrodes in a porous YSZ compact. A large electrode surface area was created in this manner. However, at this temperature no pure double-layer capacitance was obtained and other diffusion and/or surface exchange processes occurred at or in the vicinity of the electrodes, which was attributed to the presence of pores. The largest electrode surface area was obtained in dense compacts consisting of a YSZ layer sandwiched between two layers of Pt/YSZ composites. At a Pt concentration above the percolation threshold, large three-dimensional structures were formed which resulted in large double-layer capacitances at high temperatures.