Modelling of Transport Processes inside Rechargeable Oxide Battery

Change of electricity supply infrastructure based on renewables (photovoltaic, wind, biomass, etc.) requires large storage capacities to ensure continuous electricity supply if photovoltaics or wind turbines are not in operation due to weather conditions. At high temperatures of around 800 °C modified planar solid oxide fuel/electrolyzer cells (SOFC/SOEC) can be used as storage devices (Fig. 1). The concept of SOFC/SOEC can be transferred to a so-called rechargeable oxide battery (ROB). Instead of directly fueling the cell with mixtures of hydrogen and steam in the now sealed-off fuel compartment a metallic component is integrated which will be converted to metal oxide during discharging (Fig. 2) and back to metal during charging by reducing the metal oxide (Fig. 3).

An interesting metal / metal oxide system is iron/iron oxide which results in a reasonable open circuit voltage of approximately 1.02 V per cell at 800 °C due to the dissociation pressure of e.g. iron oxide (FeO) in the range of pO₂≈10^-18bar.

Oxidation and reduction rates strongly depend on the exact atmospheric compositions, the surface area, the porosity, and the diffusion velocities in the storage component. Latter will change due to the chemical expansion (change of pore sizes) during charging and discharging of the battery. Also the formation of oxide or metallic layers on top of the storage components can massively influence the reaction kinetics.

Therefore, in this study a 1D and a 2D model are developed to simulate the gas diffusion in the prevailing H₂O/H₂ atmosphere from the cell to the storage component (and back) as well as the gas-solid reaction in the porous iron/iron oxide. The model is implemented using the Navier-Stokes equations and the method of finite elements in Matlab.

The redox process which takes place within the microscopic pores of the metallic storage material can be described by the following equations:

Me + H₂O ⇌ MeO + H₂
2Me + 3H₂O ⇌ Me₂O₃  + 3H₂
3Me + 4H₂O ⇌ Me₃O₄ +4H₂

In addition, the formation of volatile oxides or hydroxides has to be considered which may cause long-term degradation of the storage components:

Me + 2H₂O ⇌ Me(OH)₂ + H₂
MeO + H₂O  ⇌ Me(OH)₂
Me₃O₄ + 2H₂O + H₂ ⇌ 3Me(OH)₂

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 for optimizing both the battery design and the operation conditions.