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ePaper created 2013-10-29, 15:50:53 | version 1.25.20
Annual Report 2012 | Forschungszentrum Jülich 19 storage systems and identified a few combinations that are particularly suitable. “We have also integrated storage systems into fuel cell stacks and demonstrated that they work in principle and can be charged and dis- charged several times,” says project coordinator Dr. Norbert H. Menzler from Materials Synthesis and Processing. When the battery is charged (left image) with power from surplus wind energy, for example, the metal oxide is reduced to metal, and the hydrogen oxidizes. The electrical voltage applied externally ‘drives’ the steam that is formed towards the cathode, where it is reduced to hydrogen. The oxygen ions migrate through the electrolyte towards the anode. There, molecular oxygen is formed and released into the air. During discharging (right image), this process is reversed: the battery absorbs oxygen and the oxygen ions migrate towards the fuel gas electrode, where hydrogen is oxidized to water and the oxygen partial pressure in the closed fuel gas chamber increases. This causes the metal to oxidize. When the battery discharges, silicon is oxidized to silicon ions. The released electrons flow from the silicon anode through a power cable to a nickel mesh on the cathode. There, molecular oxygen is reduced to oxygen ions. At the same time, silicon ions migrate through an ionic liquid and react with oxygen ions at the cathode, forming silicon dioxide. The high-temperature battery Jülich scientists from the Institute of Energy and Climate Research are also working on another innovative type of energy storage. It promises similar advantages as the silicon-air battery. The materials are cheap and hazard- free, they have a high storage density, and oxygen is taken from the surround- ing air while the battery discharges and is released again during charging. However, this energy storage medium of average to high capacity is quite distinct from a silicon-air battery. The concept is based on solid oxide fuel cells, which have been developed and fabricated at Forschungszentrum Jülich for many years. In future high-tempera- ture metal-metal oxide batteries, these fuel cells will be used not only in their standard operating mode, in which they directly and efficiently convert chemical energy into electricity. Instead, they will also be operated in reverse mode – referred to as electrolysis mode – in order to convert the excess electric energy that is produced at times by wind turbines into chemical energy. This chemical energy can then be stored by reducing a metal oxide to a metal. When the battery is discharging, the metal in the storage medium reacts with oxygen to form metal oxide again, and the system delivers power in the fuel cell mode. For the charging and discharging cycles to work, the battery must be operated at temperatures of more than 650 °C. The research project MeMO for the development of high-temperature ener- gy storage systems based on metal- metal oxides for short- and mid-term storage of volatile renewable excess energy was launched in September 2012 and is funded by the Federal Ministry of Education and Research (BMBF). Since then, the Jülich scientists have already tested a variety of materials and designs for the metal-metal oxide Silicon anode Cathode: porous carbon and catalyst Teflon layer OxygenO2 O2- O2- RTIL electrolyte (room temperature, ionic liquid) Current flow e- Nickel mesh Si4+ Anode Metal oxide Metal Fuel gas chamber » Discharging Oxygen H2 e- Fuel cell mode O2- O2- H2 O Cathode Electrolyte » Charging – + Electrolysis mode Cathode Metal oxide Metal Fuel gas chamber Oxygen H2 O2- O2- H2 O Anode At the launch of the MeMO project: Thomas Rachel (3rd from left), Parliamentary State Secretary to the Fed- eral Minister of Education and Research, Prof. Harald Bolt, member of the Board of Directors of Forschungszentrum Jülich (2nd from left), as well as Dr. Hans Peter Buchkremer (left) and Dr. Norbert Menzler (right), both from the Institute of Energy and Climate Research. Silicon-air battery High-temperature metal-metal oxide battery Institute Background information