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Forschungszentrum Jülich - Annual Report 2012

18 Forschungszentrum Jülich | Annual Report 2012 N ew types of batteries that are more powerful than those available today are the key to the energy supply of the fu- ture. Power from wind or solar energy can only be generated depending on the weather instead of on demand, so that the expansion of renewable energies re- quires options for storing large amounts of energy and rapidly making them avail- able again. The lithium-air battery is a candidate for such energy storage, because theoretically, it can achieve 50 times the energy density of current lithium-ion batteries. “The use of lithium does, however, involve certain difficulties: it reacts violently with atmospheric humidity or water. Furthermore, the metal is in short supply and will soon become more expensive if demand increases sharply,” says Jülich scientist Prof. Rüdi- ger Eichel. This is where he sees major advantages for an alternative: silicon-air batteries. Silicon is obtained from sand and reserves are practically inexhausti- ble. The silicon-air battery consists of nontoxic and environmentally compati- ble components: one pole, the anode, is made of silicon. At the other pole, the cathode, molecular oxygen is ‘reduced’ to oxygen ions, as the experts say (see graphic). The oxygen does not have to be carried along in the battery, but is taken up from the surrounding air dur- ing the discharge process. Such batter- ies would therefore be smaller and more lightweight than conventional bat- teries and could store more energy in little space. In order to fully exploit the potential of this new type of battery, the scien- tists have yet to overcome a number of hurdles. Eichel and his team from Fun- damental Electrochemistry at the Insti- tute of Energy and Climate Research are working closely with the inventors of the silicon-air battery at Technion, the Israel Institute of Technology. The Jülich scien- tists are exploring above all the reac- tions inside the battery which prevent it from providing as much energy during the discharge process as expected the- oretically. They discovered that manga- nese dioxide, which is currently utilized at the cathode as a catalyst, reacts with the liquid electrolyte of the battery. This has two undesirable consequences. Firstly, the activity of the catalyst parti- cles is reduced. Secondly, the particles become bigger, which probably causes them to clog the pores at the electrode, so that less oxygen can pass through. In the meantime, the scientists have dis- covered that tricobalt tetroxide (Co3O4) makes a more effective catalyst. The researchers from Jülich and Haifa also made another, very surprising dis- covery. “It used to be considered obvi- ous that if metal-air batteries did not function as desired then the cathode was the main culprit,” says Eichel. But the scientists demonstrated that in silicon- air batteries, it is primarily processes at the silicon anode that currently inhibit battery discharge. This constitutes an entirely new starting point for improve- ments. Next-Generation Batteries At the moment, lithium-ion batteries are the state of the art for storing as much energy as possible in as little material as possible. Jülich scientists are developing environmentally friendly and robust batteries that have even higher energy densities – and do not require limited raw materials. Silicon in sand and oxygen in the balloon: Prof. Rüdiger Eichel shows symbolically that the raw materials for silicon-air batteries can be found almost anywhere.