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Electrical Storage
BMBF
Power-to-Storage 3.5.2017

Possible integration of the novel high-temperature Power-to-Storage / Storage-to-Power battery into the future German energy scenery
© Forschungszentrum Jülich, IEK-1, N. Menzler

MeMO - a novel high-temperature battery

Forschungszentrum Jülich is developing a novel kind ot high-temperature

battery. In this Power-to-Storage / Storage-to-Power concept, surplus

electrical energy is stored by reduction of a metal oxide and regained

by oxidation of the metal. The stacks have been operated for more than

200 redox cycles successfully.

Project status Project completed
Project duration July 2012 until March 2016
Typical system size Energy ~ 0.3 MWh
Typical system size Output ~ 0.15 MW
Volumetric energy density ~ 450 Wh/l
Gravimetric energy density ~ 90 Wh/kg
Volumetric power density ~ 225 W/l
Gravimetric power density ~ 45 W/kg
Efficiency AC/AC ~ 50%
Storage loss < 0.1%
Cycle durability (80% discharge level) 10,000 cycles
Service life of the system (1 cycle/day) > 10 years
Typical discharge time 0,5-2 h
Response time when preparing the energy within seconds (if system is on temperature)
Example application areas centralized buffer of off-shore wind energy, decentralized buffer for on-shore wind energy or solar energy, decentralized private buffer (industry, housing complexes, agricultural industry)

Within the scope of a governmentally funded project Forschungszentrum Jülich is developing a novel kind ot high-temperature battery. In this Power-to-Storage / Storage-to-Power concept, surplus electrical energy is stored by reduction of a metal oxide and regained by oxidation of the metal. A solid oxide fuel cell acts here as a "transport media" for oxygen ions. In times of surplus electricity the fuel cell works in electrolysis mode thus reducing a metal oxide. In times of higher electricity demand the system works in its classical fuel cell mode, oxidizing the metal. The fuel compartment contains a stagnant atmosphere composed of a changing ratio between water vapor and hydrogen.

  • Schematic overview of an ROB repeat unit, including the storage (used materials in brackets). &copy; Forschungszentrum Jülich (IEK-1)
  • Time plot of a battery test with 210 full charging and discharging cycles &copy; Forschungszentrum Jülich (IEK-3)
  • Figure show details of the charging/discharging curves at the beginning of the test, respectively &copy; Forschungszentrum Jülich (IEK-3)
  • Figure show details of the charging/discharging curves at the end of the test, respectively &copy; Forschungszentrum Jülich (IEK-3)
  • Top view of a real interconnect with machined cavities and storage material placed in. &copy; Forschungszentrum Jülich, ZEA-1

Basic research

Goal of the project is the storage of surplus electricity for short and intermediate terms primarily in stationary applications by using a high-temperature battery. The technology is based on the well-known solid oxide fuel cell which can by operated either in electrolysis mode (storing energy) or in fuel cell mode (delivering energy). The basic "electricity storage component" is a metal oxide which can be reduced (during electrolysis mode) or oxidized (during fuel cell mode). Main advantage of this type of battery is its simplicity and easyness of operation. The single system switches easily between both operational modes (charging/discharging). The project is initially basic research oriented and is handled only by institutes of the Forschungszentrum Jülich.

Project results: storage material

The base material for the storage was iron/iron oxide. At the operation conditions of the rechargeable oxide battery iron/iron oxide is the best choice with respect to temperature, oxygen partial pressure and additionally extremely cheap. But a storage composed of pure iron degrades during multiple redox cycling drastically. Degradation effects are coarsening and formation of dense outer layers. This leads finally to reduced storage capacity and varying charging/discharging times. Therefore, as a first solution, an inert second oxidic component was added as kind of a backbone material. As backbone material, zirconia with 8 mol% yttria stabilized (8YSZ) was chosen. 8YSZ is part of the solid oxide cells electrolyte, anode and substrate. The amount of 8YSZ added has to be chosen carefully, not to reduce the storage capacity too much. By doing so the degradation effects could be minimized but not be suppressed completely. As second solution a pair of two reactive oxide materials was chosen. At high oxygen partial pressure a mixed oxide is stable, while at lower pO2 metallic iron and a less iron containing oxide are stable. Again the degradation effects were reduced markedly. A patent is pending for this type of solution.

Besides the storage material development the material itself was characterized intensively with respect to its reduction and oxidation kinetics. These tests gave valuable support in understanding the cycling ability of the material. Those tests were typically done ex-situ in annealing tests. Annealing tests outside the real battery have the advantage of being better controllable and the amount of test variations could be raised.

Battery tests

Parallel to the storage material development first battery tests were performed. The basis for the battery design was the well established Jülich solid oxide fuel cell (SOFC) design. The development of an adequate novel design was outside the project time frame. The Jülich SOFC design is based on planar anode-supported cells which could be operated either in fuel cell or in electrolysis mode. But, in both cases the fuel side is open delivering e.g. hydrogen and producing water vapor thereof. For the use as a rechargeable oxide battery (ROB) the fuel compartment has to be closed and the storage material must be introduced. For this, cavities, perpendicular to the normal flow direction, have been machined into the metallic interconnects.

With these specially designed storage bearing interconnects various battery stacks have been built and tested. Approximately 15 tests have been performed with individual test goals. On the one hand side more system-related tasks were done, e.g. “how to close-up the fuel compartment?”, “which operation parameters a the right ones?” and “which charging/discharging times are possible?”. And, on the other side various different storage materials types, amount per repeating unit and processing routes for the storage have been tested. Figure 3 shows one example of a successful battery test. Here, a two layer battery was operated for 210 full redox cycles. In Figures 3a) and 3b) details of the charging/discharging curves at the beginning and the end of the test are shown. The voltage levels of 900 and 980 mV correspond to the oxidation reactions of Fe to FeO and from FeO to Fe3O4, respectively. The storage should not be oxidized until Fe2O3 as the oxidation voltage of this step is closed to the Ni/NiO conversion voltage. And the oxidation of the metallic nickel from the substrate and the anode should be avoided. The operation temperature of such a battery is 800°C.

Summary

Within the MeMO project, in which a so-called rechargeable oxide battery, based on a high-temperature fuel cell/electrolyzer cell design was developed the following results were obtained:

  • The principle functionality of the battery
  • During various battery tests, operating conditions have been developed and different storage materials have been tested
  • A storage based on pure iron/iron oxide degrades too much which leads to capacity loss and elongated charging/discharging times
  • The addition of an inert backbone material minimizes the degradation effects but does not prevent them
  • Die addition of a second reactive oxide leads to less degradation with minimized microstructural changes in the storage
  • The redox kinetics of the storage is complex with being the oxidation much more faster than the reduction
  • Up to 260 full charging/discharging cycles could be shown
  • The charging/discharging times are in the range of 30-60 minutes
  • The storage efficiency is up to 80%

Concerning the storage microstructure there is still room for optimization and the battery tests have shown that the storage amount per repeating unit is too less. That means that always the battery and it’s kinetics are the rate limiting step. To overcome this limitation a specialized battery design has to be developed.
Additionally to the scientific and technical tasks also economical and socio-economical aspects of such kind of battery was checked. The economics of the ROB are governed by the costs of the storage, the battery design and the costs of competing technologies like redox-flow batteries, electrolyzers and so on. For the socio-economical aspects it turned out that the knowledge about battery storage are less but to a certain extent positive. Detailed analysis could only be possible if the technology shifts from R&D to field testing.

After the project has ended regularly, Forschungszentrum Jülich will work in future with limited extent on the ROB. This is then part of the basic governmental funding. The next goal would be a battery showing more than 1,000 successful redox cycles.

Project-related literature

  • Menzler N. H., Hospach A., Niewolak L., Bram M., Tokariev O., Berger C., Orzessek P., Quadakkers W.J., Fang Q., Buchkremer H.P.: Power-to-storage – the use of an anode-supported solid oxide fuel cell as a high-temperature battery. ECS Transactions 57 (1) (2013), 255-267
  • Berger C. M., Tokariev O., Orzessek P., Hospach A., Menzler N. H., Bram M., Quadakkers W.J., Buchkremer H. P.: Towards the conversion of a solid oxide cell into a high temperature battery. Ceramic Engineering and Science Proceedings 35/7 (2014), 3-12
  • Hospach A., Menzler N. H., Bram M., Buchkremer H. P., Niewolak L., Quadakkers W. J., Zurek J.: Elektrochemisches Speichermaterial und elektrochemische Speichereinrichtung zur Speicherung elektrischer Energie umfassend ein solches Speichermaterial. Patentanmeldungen DE/18.05.13/DEA102013008659 (02.05.2014), PCT/DE2014/000232( 04.11.2015)
  • Berger C. M., Hospach A., Menzler N. H., Guillon O., Bram M.: Reversible oxygen-ion storage for solid oxide cells. ECS Transactions 68 (1), (2015), 3241-3251
  • Berger C. M., Tokariev O., Orzessek P., Hospach A., Fang Q., Bram M., Quadakkers W.J., Menzler N.H., Buchkremer H.P.: Development of storage materials for high-temperature rechargeable oxide batteries. J. Energy Storage 1 (2015), 54-64
  • Niewolak L., Zurek J., Menzler N. H., Grüner D., Quadakkers W. J.: Oxidation and reduction kinetics of iron and iron based alloys used as storage materials in high temperature battery. Materials at High Temperatures Vol. 32, Iss. 1-2 (2015), 81-91
  • Fang Q., Berger, C. M., Menzler N. H., Bram M., Blum L.: Electrochemical characterization of Fe-air rechargeable oxide battery in planar solid oxide cell stacks. Journal of Power Sources 336 (2016), 91-98
  • Berger C. M., Abdelfattah M., Hermann R. P., Braun W., Yazhenskhik E., Sohn Y.J., Menzler N.H., Guillon O., Bram M.: Calcium-Iron Oxide as Energy Storage Medium in Rechargeable Oxide Batteries. Journal of the American Ceramic Society 99 [12] (2016), 4083-4092
  • Yildiz S., Vinke I. C., Eichel R.-A., de Haart L. G. J.: Electrochemical characterization of a high temperature metal/metal oxide battery. 12th Europ. SOFC & SOE Forum, 05.-08.07.2016, Lucerne, Switzerland
Supported by: The Federal Government on the basis of a decision by the German Bundestag

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Research funding

The information system EnArgus provides information on research funding, including on this project (German only).