Kinetic Study of Iron Based Storage Materials for the Use in Rechargeable Oxide Batteries (ROB)
Authors: Braun Waldemar , Erfurt Viktoria , Singheiser Lorenz , Thaler Florian 
Published: February 2020

The transition from conventional fossil energy sources to renewable energies, known under the term “Energiewende” (energy turnaround) is in strong focus [1] and public discussion often reduces the term to the sector electricity which covers about 31 % of the actual energy production in Germany [2]. However, Germany’s goal is to balance at least 80 % of gross end energy consumption through renewable energy by 2050 [3].

To achieve these targets, future energy supply will inevitably dominated by new energy converter. These include the most frequent occurrence and the potential easiest access to solar and wind energy [4-6].

For this purpose, we are faced to the fact to develop appropriate energy storage and grid stabilizing technologies, in order to obtain a decentralized and centralized power supply, allowing us to stabilize and compensate for the volatile availability of regenerative power plants.

As a possible solution a new upcoming battery concept combines the function of a conventional solid oxide fuel cell with an iron based storage media, implemented in the fuel gas chamber, to a so called Rechargeable Oxide Battery (ROB) [7-10].

By varying the operation mode in fuel cell or electrolyzer mode, the porous Fe-storage reacts with its surrounding H2/H2O-atmosphere in order to discharge (Fe + H2O à FeO + H2) or charge (FeO + H2 à Fe + H2O) the ROB-system. Therefore, the stagnant H2/H2O-atmosphere acts as oxygen-ion transport mediator to accelerate the oxidation and reduction kinetics by means of a shuttle-mechanism between the electrode side and the storage media. To clarify the influence of the material composition on the redox-kinetic, various oxides were added as a stabilization matrix to the Fe storage media. Therefore, tape-cast samples (Fe, Fe-8YSZ, Fe-ZrO2, Fe-Al2O3) were sintered in air at 900 °C and subsequently redox-treated up to ten times in a furnace, based on top of a microbalance, which is connected to a mass spectrometer to analyze the released H2 under simulated ROB conditions (800 °C, Ar-50%H2 or Ar-9%H2O).

The composition and morphologies of the oxide scales were analyzed by optical metallography (OM), scanning electron microscopy equipped with energy dispersive X-ray spectroscopy (SEM/EDX), and X-ray diffraction (XRD).

In particular, the redox cyclability of the selected storage composition plays an essential role in the charge and discharge behavior of an ROB. In this context, our studies present results of cycling experiments which show the redox-responding behavior of the investigated materials and provide evidence towards the long-term stability for the use in an ROB-Stack.

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Figure 1