Band 4, Heft 1-6

Electrochemical double layer capacitors (EDLCs) and hybrid devices (HDs) become more and more popular solutions in various green energy technologies, in particular, in hybrid transport and wind power stations. After many years of research in EDLC and in HD technology we have developed some approaches and methods aimed at improving the performance of those devices. The results for EDLCs with the lowest inner resistance and highest power density, as well the results for HDs with the largest energy density as compared with the best similar competing devices are presented.

Methanol has been proposed frequently as an energy carrier in recent years. High storage capacity, easy manageability and similarity to existing fuels make it an interesting option for energy storage. However, the usage of methanol is constrained by its low boiling point and its toxicity and the energy balance of a methanol economy is disputable. Net energy ration and renewability factor for different scenarios of a methanol economy have been analyzed, considering hydrogen production, methanol synthesis and transport and repowering. It appears that the overall efficiency of methanol based energy storage is rather low, but still can be sustainable due to a high renewability factors.

The self-discharge behavior of organic electrodes and symmetric devices for sustainable energy storage, composed of electrodes containing a thin layer of polypyrrole coated onto a high surface area cellulose matrix, has been studied for the first time using different electrode sizes and electrolytes. Experimental data from open circuit measurements of the individual electrode potentials of charged symmetrical two-electrode energy storage devices as a function of time were evaluated based on three different self-discharge models. This evaluation clearly showed that the self-discharge process of the positive electrode is governed by a previously undetected activation-controlled faradaic reaction while the self-discharge of the negative electrode is due to diffusion controlled oxidation involving oxygen dissolved in the electrolyte. Potentiostatic three-electrode measurements and spectroelectrochemical experiments also showed that protons as well as maleimide were released from positively polarized polypyrrole electrodes. These new findings clearly show that the self-discharge of the cells originate from two different types of reactions on the positive and negative electrodes and that the main contribution to the self-discharge of the cells comes from an activation controlled reaction involving the positive electrode. These results provide an improved understanding of polypyrrole based devices and also yield new possibilities for the development of stable conducting polymer system for energy storage applications.

The transition to a sustainable electricity supply from renewable energy sources (RES) imposes major technical and economic challenges upon market players and the legislator. In particular the rapid growth of volatile wind power and photovoltaic generation requires a high level of flexibility of the entire electricity system, therefore major investments in infrastructures are needed to maintain system stability. This raises the important question about the role that central large-scale energy storage and/or small-scale distributed storage (“energy storage at home”) are going to play in the energy transition. Economic analyses show that the importance of energy storage is going to be rather limited in the medium term. Especially competing options like intelligent grid extension and flexible operation of power plants are expected to remain favourable. Nonetheless additional storage capacities are required if the share of RES substantially exceeds 50% in the long term. Due to the fundamental significance of energy storages, R&D considers a broad variety of types each suitable for a specific class of application.