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Foeschungszentrum Jülich - Research in Jülich 1_2013

Silicon anode Cathode: porous carbon and catalyst Teflon layer OxygenO2 O2- O2- RTIL electrolyte (room-temperature ionic liquid) Current flow e- Nickel mesh Si4+ 1|2013 Research in Jülich 19 serves are practically inexhaustible. It forms the basis of present computer chips. THE OXYGEN TRICK Just like lithium-air batteries, the theoretically high energy density of sili- con-air batteries is due to a special fea- ture. The function is based on a reac- tion of oxygen, which is not contained in the battery but is taken up from the ambient air during the discharge pro- cess. Since the oxygen is not contained in the battery the mass is much lower than that of a conventional battery. Eichel, head of the new subinstitute of Fundamental Electrochemistry at IEK, is working on the silicon-air battery which consists of nontoxic and environ- mentally compatible components. In his research, he cooperates with the bat- tery’s inventor, Prof. Yair Ein-Eli. Collab- oration across boundaries has long be- come the norm in science. It is, however, a surprise to hear where the Israeli and the German got to know each other. “We met in China,” says Eichel. The reason was a conference that both scientists were attending. Since then Eichel and his team have been exploring the reactions inside the battery which prevent it from providing as much energy as expected theoreti- cally. First of all, the scientists used spe- cial spectroscopic methods to investi- gate processes at the electrode where oxygen is reduced (see also ‘How the new battery works’). This electrode, the cathode, consists of porous carbon and a catalyst which accelerates the con- version of oxygen. So far, the German and Israeli scientists have been using manganese dioxide as a catalyst. Eichel’s team have now discovered that some of this material reacts with the battery electrolyte. This has two un- desirable consequences. Firstly, the activity of the catalyst particles is re- duced. Secondly, they get bigger and thus probably increasingly clog the electrode pores so that less oxygen can pass through. CATALYST UNDER OBSERVATION This is why the Jülich scientists are ex- perimenting with other catalysts. “The aim is to discover an optimum compro- mise between very active but very expen- sive materials and less effective but cheaper catalysts,” explains Eichel. Ac- cording to Eichel, the search was indeed successful but the results have not yet been published. Eichel, who is a physicist, is pleased that they managed to observe the catalyst using spectroscopic methods at all. This is by no means self-evident since the catalyst is finely distributed in the form of tiny particles, some of which are deep inside the cathode pores. In the meantime, the researchers have also made another very surprising discov- ery. “It used to be considered obvious that if the metal-air batteries did not yet function as desired then the cathode was the culprit,” says Eichel. But now the sci- entists from Jülich and Haifa have discov- ered that this is not the case with the sili- con-air battery. They demonstrated that above all processes at the silicon anode currently inhibit the battery discharge. This opens up a completely new approach for improving this innovative energy stor- age technology. :: Dr. Frank Frick RESEARCH AT THE CENTRE | Battery Research The battery is based on the oxidation of elementary silicon into the silicon- oxygen compound silicon dioxide. When the battery discharges, the metal- lic silicon is oxidized to silicon oxide. The resulting electrons flow through a power cable from the silicon anode (left) to a nickel mesh on the cathode (right). This is where molecular oxygen is reduced to oxygen ions. At the same time, silicon ions migrate through an ionic liquid and react with oxy- gen ions at the cathode to form silicon dioxide. The oxygen for this reaction comes from the ambient air. It passes through a mechanically stable Teflon membrane into the battery to the cathode. The cathode consists of porous carbon and a catalyst which accelerates the reactions taking place there. The ionic liquid – an organic salt with a particularly low melting point – is decisive for the battery function. It dissolves the elementary silicon out of the anode and converts it into ions which then migrate to the cathode. How the new battery works

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