Modelling of Processes inside a Rechargeable Oxide Battery
Authors: Erfurt Viktoria , Braun Waldemar , Spatschek Robert , Singheiser Lorenz 
Published: February 2020

Renewables requires large storage capacities to ensure continuous electricity supply if photovoltaics or wind turbines are not in operation due to weather conditions. For this purpose, amongst other, solid oxide cells (SOC) are used. At operating conditions at 800 °C they can run in two modes: In fuel cell mode (figure 1) a fuel (hydrogen or methane) is burned to generate electricity in the time of electricity undersupply. During electricity oversupply the surplus electricity can be used to produce hydrogen by running the cell in the electrolyzer mode (figure 2). However, storage and transport of hydrogen is cost intensive and controversial regarding safety issues. High security standards have to be fulfilled which increases additionally running cost of SOCs. Therefore, this study focuses on a new concept of running a SOC without external hydrogen supply. Thus, in particular SOFCs can also be used as stationary large scale batteries, so called rechargeable oxide batteries (ROB). In this case the fuel compartment is sealed-off and filled with an atmosphere of hydrogen and steam. In this atmosphere a metallic component, usually an iron based material, will be converted to a metal oxide during discharging (figure 3) and back to metal during charging by reducing the metal oxide (figure 4). Thus, the iron acts as a storage volume in a certain oxygen partial pressure (pO2) environment to achieve a high as possible current density.
A special requirement for this setup is the pO2 has to be held in the range of 10-18 to 10-21 bar in the fuel compartment since as anode built-in Ni-cermet could oxidize above pO2 = 10-16 bar and lead to a breakdown of the cell performance. Additionally, the drop of pO2 between the air and fuel side has a direct influence of the transport of oxygen ions through the electrolyte. A beneficial metal/metal oxide system is iron/iron oxide (Fe/FeO) which results in an open circuit voltage of approximately 1.02 V per cell at 800 °C due to the dissociation pressure of FeO in the range of pO2≈10-18 bar. Oxidation and reduction processes strongly depend on the exact atmospheric compositions, the surface area, the porosity, and the diffusion velocities in the storage component. The latter will change due to the reduction/oxidation (change of pore sizes) during charging and discharging of the battery. Also the formation of dense metallic layers on top of the storage components can massively influence the reaction kinetics regarding a higher cycling number.
To analyze the processes inside of an ROB, a 1D and a 2D model are developed to simulate the gas transport in the prevailing H2O/H2 atmosphere from the anode surface to the storage component (and reverse) as well as the gas-solid reaction inside the porous Fe/FeO oxide micro structure. The model is implemented by using the Navier-Stokes equations and the method of finite elements using Matlab. The degradation process reduces the performance of the battery along with its recyclability and lifetime. Therefore the degradation should be held as low as possible. Thus, the model contributes to identifying the critical processes in the battery and will allow further optimization considering the development of the battery design and within the performance enhancement.

Figure 1