Engineering of solid state ionic devices

Abstract

Solid state ionic devices such as high performance batteries, fuel and electrolysis cells, electrochromic devices, chemical sensors, thermoelectric converters or photogalvanic solar cells are of tremendous practical interest in view of our energy and environmental needs. The challenges are the achievement of higher energy and power densities, longer lifetimes, cheaper materials, lower cost, improved sensitivity and higher stability. The engineering of new devices is based on the better fundamental understanding of materials for galvanic cells and their interaction in order to approach solutions more systematically than in the past. The fundamental aspects of the generation of voltages and electrical currents are compiled and analysed in view of the materials requirements. Conflicts exist in forming chemically stable interfaces of functionally different electrolyte and electrode materials, achieving simultaneously high energy and power densities in view of low conductivities of chemically stable materials, fast chemical diffusion in electrodes which should have a wide range of non-stoichiometry for delivering and absorbing the mobile ionic species, practical problems of using less expensive polycrystalline materials which have high intergranular resistances and finally reaching both ionic and electronic equilibria at the electrolyte - electrode interfaces at low temperatures. The engineering of new or improved solid state ionic devices is commonly based on individual materials considerations and their interaction in galvanic cells. Simultaneously high ionic conductivity and chemical stability may be reached by designing structures of poly-ions of the non-conducting components with the conducting species in-between. The chemical stability may be based on kinetic restrictions for sufficiently long periods of time of operation of the devices. Electrodes should not be made of metallic conductors but of electronic semi-conductors with fast enhancement of the diffusion of ions by internal electrical fields. Device considerations are based on the development of single element arrangements (SEAs) which incorporate the electrodes into the electrolyte in the case of fuel and electrolysis cells. The electronic conductivity is generated by the applied gas partial pressures or the applied voltage. The same simplification may be applied for electrochromic systems which consist of a single active layer instead of the conventional three galvanic cell materials. A new design of active chemical sensors probing the environment by the magnitude of the applied voltage or current may overcome the limitations of cross sensitivities and interfacial reactions which allows the simultaneous detection of several species by a single galvanic cell.