Modelling of Transport Processes inside Rechargeable Oxide Battery
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.