Umwandlung von Lichtenergie in chemische Energie

II. Experimente zu diesem Thema

Abstract

Experiments carried out by the author and his group during the last three years are described. Since the combination of thermochemical and photochemical reactions can lead to a higher efficiency than photochemical reactions alone, I explain the principle of thermochemically charging an accumulator. Finding selective electrodes is not only important for photogalvanic cells as e.g. the iodine/iron system. We have found that the silver/ silverchloride/chlorine system also works only due to selective electrodes. Silverchloride undergoes a photoredox reaction which leads to a very good storage capacity: Ag, AgCl | HCl (1 m) | Cl^-, ½ Cl₂ E° 1.14 Volt. After irradiation of a silvercation exchanged zeolite in a 0.2 m NaCl, 0.05 m HCl electrolyte, we have measured short circuit potentials of 1.1 Volt and currents of e. g. 0.1 mA/cm² at 700 mV between a carbon and a silver electrode. In another experiment we found that methylviologene in a deoxygenated solution of a citric acid buffer can be reduced by irradiating with a mercury or xenon high pressure lamp through a pyrex filter without adding any other component. Heterogeneous photoredox systems, in which the photosensitive part is a semiconductor, seem to be among the most promising systems for converting solar energy into chemical energy. We have studied photochemical properties of vacuum deposited films of sintered and doped iron(III)oxide. From the wavelength dependence of the photopotentials we know that photoactivity of the electrochemical cell starts at the same wavelength (600-650 nm) as the absorption of light and the photoconductivity of vacuum deposited films. Open circuit potentials of about 400 mV have been observed with the visible radiation from a 150 Watt xenon lamp. Another interesting question is whether d-π^* transitions which occur along a well defined intramolecular axis may be used in order to achieve an organized charge redistribution under irradiation. Therefore, we have analyzed d-π^* transitions of this type and give first results obtained by measuring photocurrents of surface modified tin dioxide.