FULL PAPER © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 120 wileyonlinelibrary.com same material, or the corresponding reac- tions are separated to two electrodes of photoelectrochemical (PEC) cells. Such solar to fuel conversion systems are based on liquid water and despite being heavily investigated, high efficiencies together with high system stability have not been achieved yet. [8,10–12] Thermochemical energy conversion based on high-temperature materials is a third approach and generally employs two step processes with oxygen release (i.e., a reduction reaction) at high temperature and chemical water splitting at lower tem- peratures. [13–17] Interestingly, only very few activities aim at a combination of high temperatures and photoelectrochemical energy storage. In a theoretical paper, a solid-state electrochem- ical cell was suggested for splitting of steam at high tempera- tures by solar radiation. [18] Such a high-temperature cell running on steam may avoid stability problems of photoactive materials in liquid water, can exploit the reduced theoretical energy input required for water splitting at higher temperatures, and does not suffer from problems due to heating in case of high solar energy intensity. However, to the best of the authors’ knowl- edge, experimental realization of solid electrolyte-based (photo-) electrochemical cells for photon-driven steam electrolysis has not been reported yet. The scope of this paper is to experimentally demonstrate the feasibility of a high-temperature solid-state photoelectro- chemical cell (SOPEC). The multilayer cell used consists of a sequence of different oxides, some being active in terms of photovoltaics and some using the voltage to chemically store energy. At operating temperatures between 400 and 500 °C UV light-induced open circuit voltages as high as 900 mV are directly transferred into chemical energy via oxygen pumping from low to high oxygen partial pressure. The experiments show that solid-state photoelectrochemical cells can indeed be realized and that improved cells for water splitting and thus hydrogen production are feasible. 2. The High-Temperature Photoelectrochemical Cell The SOPEC investigated in this study is sketched in Figure 1a. It includes a thin layer of La 0.8 Sr 0.2 CrO 3 (LSCr) deposited on a nominally undoped SrTiO 3 (100) single crystal; a scanning UV-Light-Driven Oxygen Pumping in a High-Temperature Solid Oxide Photoelectrochemical Cell Georg Christoph Brunauer,* Bernhard Rotter, Gregor Walch, Esmaeil Esmaeili, Alexander Karl Opitz, Karl Ponweiser, Johann Summhammer, and Juergen Fleig* A solid-state photoelectrochemical cell is operated between 400 and 500 °C under 365 nm UV light. The cell consists of a photovoltaic part, based on a La 0.8 Sr 0.2 CrO 3 /SrTiO 3 junction, and an electrochemical part including a zir- conia solid electrolyte with a shared (La,Sr)FeO 3 electrode. The photovoltaic cell part leads to open circuit voltages up to 920 mV at 400 °C. Upon UV light, this driving force is used in the electrochemical part of the cell to pump oxygen from low to high partial pressures, i.e., to convert radiation energy to chemical energy. This demonstrates the feasibility of high-temperature photo- electrochemical cells for solar energy storage. The detailed characterization of the different resistance contributions in the system by DC and AC methods reveals the parts of the cell to be optimized for finally achieving high-tempera- ture photoelectrochemical water splitting. DOI: 10.1002/adfm.201503597 G. C. Brunauer, B. Rotter, E. Esmaeili, Prof. K. Ponweiser Institute for Energy Systems and Thermodynamics Getreidemarkt 9 E302, 1060 Vienna, Austria E-mail: georg.brunauer@tuwien.ac.at G. C. Brunauer NOVAPECC GmbH Opernring 19, 1010, Vienna Austria G. Walch, Dr. A. K. Opitz, Prof. J. Fleig Institute of Chemical Technologies and Analytics Getreidemarkt 9 E164-EC, 1060 Vienna, Austria E-mail: juergen.fleig@tuwien.ac.at Prof. J. Summhammer Institute of Atomic and Subatomic Physics Stadionallee 2 E141, 1020 Vienna, Austria 1. Introduction Exploiting solar energy by solar-to-fuel conversion, i.e., by trans- ferring radiation energy to chemical energy, is of extraordinary relevance for future sustainable energy supply. One approach is based on photovoltaic (PV) cells and subsequent use of elec- tricity in electrolysis cells to generate fuels such as hydrogen, CO, or even hydrocarbons. [1–3] Integration of the photovoltaic part into one electrode of an electrochemical (EC) cell is also an option. [4] Alternatively, direct transfer of solar energy to chemical energy is possible in photo(electro)chemical cells. [5–12] By illuminating photochemically active materials such as TiO 2 , water splitting and thus hydrogen production takes place without preceding photovoltaic electricity generation. Either oxygen and hydrogen generation take place at one and the Adv. Funct. Mater. 2016, 26, 120–128 www.afm-journal.de www.MaterialsViews.com