CONSIDERATIONS FOR AN INNOVATIVE HIGH TEMPERATURE BATTERY IN POWERPLANT APPLICATIONS

A novel high temperature battery based on the concept of the solid oxide fuel cell (SOFC) is presented. Due to the use of cheap iron- and calcium-based storage materials providing a high theoretical capacity of roughly 1300 Wh/kg the battery could be used to optimize the part load properties and the long term durability of conventional power plants. The elevated working temperature of 800 °C makes it applicable where high quality heat is available and needed. In this paper an economical consideration leads to general design recommendations for this battery which operates in test mode with current densities of 150 mA/cm2 and approximately one hour charging/discharging time at cell voltages between 0.7-1.2 V

A first prototype of high-temperature rechargeable oxide batteries (ROB) with iron-based storage material

The present investigation describes the results of a kinetic study of porous storage material for a novel high-temperature rechargeable oxide battery (ROB). The new planar battery design consists of a regenerative solid oxide cell and a storage redox unit with a stagnant hydrogen/steam atmosphere and embedded porous Fe-based material which has to provide high oxygen-ion storage capacity, good reaction kinetics and long-term stability.During long-term exposure in the alternating redox atmosphere at 800 °C, the structure of the storage material shows degradation effects like structural coarsening and outward iron diffusion, thus making the storage element incapable of storing the required amount of oxygen during continuous operation of the rechargeable battery. The porous Fe/Fe-oxide storage material is therefore supported by inert or reactive oxides. Addition of inert oxides (e.g. ZrO2) in sufficient amount reduces the microstructural degradation, however results in a substantial decrease in storage capacity. Among the added oxides forming mixed oxides with Fe in the relevant oxygen partial pressure range of ~ 10-18 – 10-20 bar, the most promising results were obtained with additions of MgO and CaO. During the oxidation step these oxides form mixed oxides with Fe oxide which in turn change composition during the reduction step. In this way a framework is obtained which reduces sintering and outward Fe migration in the storage component.In addition to these studies, supplementary research of the iron-based storage material was carried out aiming at microstructural optimization of porosity and powder morphology. Also, more feasible manufacturing methods such as tape casting and extrusion were successfully implemented for production of storage elements. The storage materials with the best results regarding capacity, efficiency, and lifetime were used during pilot battery testing to examine material behavior under real operating conditions, showing ~ 200 full charge-discharge cycles of up to 70 minutes each with a current density of 150 mA/cm2

Entwicklung von Hochtemperatur-Energiespeichern auf Basis von Eisen-Eisenoxidkeramiken in Verbindung mit einem oxid-keramischen Brennstoffzellen-Elektrolyseursystem

Ein neuartiger aufladbarer Hochtemperatur-Energiespeicher, der in Verbindung mit einem oxidkeramischen Brennstoffzellen-Elektrolyseursystem arbeitet, wird vorgestellt. Die elektrochemische Energie wird dabei in Eisen-Eisenoxid gespeichert, das sich auf der Brennstoffseite befindet. Im Unterschied zur klassischen oxidkeramischen Brennstoffzelle liegt hier eine stehende H2-/H2O-Atmosphäre zur definierten Einstellung eines Sauerstoffpartialdrucks an. Beim Laden wird das Eisenoxid reduziert, die Brennstoffzelle arbeitet als Elektrolyseur. Beim Entladen wird das Eisen oxidiert, die Brennstoffzelle erzeugt Elektrizität. Die über Foliengießen oder Extrusion hergestellten Metalloxidspeicher müssen sowohl gute reaktionskinetische Eigenschaften als auch eine große Sauerstoff-speicherkapazität aufweisen.Zur Vermeidung der Degradation (Verlust an Speicherkapazität und -kinetik) des Speichers werden verschiedene Stoffkombinationen aus „inerten“ Oxiden (8YSZ, Al2O3, ZrO2) oder „reaktiven“ Oxiden (Mn2O3, Y2O3, CeO2, TiO2, MgO, CaO) mit dem Eisenoxidspeicher auf ihr Oxidations-/Reduktionsverhalten, ihr Sinterverhalten und andere mikrostrukturelle Alterungserscheinungen untersucht. Erste Ergebnisse zum Einfluss der Mikrostruktur und Porosität sowie deren Einfluss auf die Leistungsdichte des Hochtemperatur-Energiespeichers werden dargestellt. Weiter werden die für den anvisierten Betrieb als stationärer Energiespeicher benötigten Betriebsparameter aus Ergebnissen von Stack-Tests abgeleitet

Development of storage materials for high-temperature rechargeable oxide batteries

A high-temperature rechargeable oxide battery (ROB) comprises a regenerative solid oxide cell (SOC) and oxygen-ion storage that consists of a porous Fe2O3 base redox material. This material possesses good redox kinetics, a high oxygen-ion storage capacity, and an acceptable long-term stability. Yet, observations demonstrate degradation effects such as particle coarsening and an outward diffusion of iron leading to layer formation during operation of the ROB at 800 °C.To clarify the influence of the material composition on degradation, various oxides were added as a stabilizing scaffold for the Fe2O3 base material. Pressed samples of the binary mixtures were sintered in air at 900 °C and subsequently redox-treated up to 20 times under conditions that simulate those present in an actual ROB (800 °C, Ar–2%H2 or Ar–7%H2O–2%H2). Afterwards, the degradation properties were analyzed by laser microscopy and the phase composition was measured using X-ray diffraction. Results indicate that the addition of yttria-stabilized zirconia (8YSZ) or pure zirconia (ZrO2) can suppress structural degradation thus maintaining reaction kinetics. In contrast, the use of yttria (Y2O3) does not significantly mitigate degradation phenomena. Consequently, storage components consisting of 8YSZ and Fe2O3 were employed in an ROB test, resulting in more than 200 cycles with a current density of 150 mA/cm2 and cycle durations of up to 70 min

Power-to-Storage - The use of an Anode-Supported Solid Oxide Fuel Cell as a High-Temperature Battery

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.</jats:p