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The applied pyrolysis temperature was found to strongly affect composition, structure, and oxidation behavior of pure and iron oxide nanoparticle (NP)-loaded carbon materials originating from hydrothermal carbonization (HTC) of cellulose. A strong loss of functional groups during pyrolysis at temperatures beyond 300 °C of the HTC-derived hydrochars was observed, resulting in an increase of the carbon content up to 95 wt% for the carbon materials pyrolyzed at 800 °C and an increase of the specific surface area with a maximum of 520 m 2 g –1 at a pyrolysis temperature of 600 °C. Devolatilization mainly took place in the range from 300 to 500 °C, releasing light pyrolysis gases such as CO, CO 2 , H 2 O and larger oxygen-containing molecules up to C 11 . The presence of iron oxide NPs lowered the specific surface areas by about 200 m 2 g –1 and resulted in the formation of mesopores. For the iron oxide-containing composites pyrolyzed up to 500 °C, the oxidation temperature was decreased by about 100 °C, indicating tight contact between the iron oxide NPs and the carbon matrix. For higher pyrolysis temperatures, this catalytic effect of iron oxide on carbon oxidation vanished due to carbothermal reduction to iron and iron carbide, which, however, catalyzed the graphitization of the carbon matrix. Thus, the well-controlled two-step synthesis based on a biomass-derived precursor yielded stably embedded iron NPs in a corrosion-resistant graphitic carbon matrix.

A novel high-temperature energy storage system based on an SOFC is presented (Power-to-Storage). The energy is stored as a metal/metal oxide which is part of the fuel side. However, in contrast to a classical SOFC, the fuel side is kept under stagnant hydrogen/water vapor. By using the cell as an electrolyzer (SOEC), the surplus electricity (from renewable energy sources) is used to charge the system by reducing a metal oxide. Vice versa, if energy is needed the system works as an SOFC thereby oxidizing the metal (discharging the battery). First results from storage material development and stack testing are presented.

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