Total organic aqueous redox flow batteries – Numerical Investigations
Redox flow batteries (RFB) are efficient stationary energy storage systems for capturing the peaks of energy acquired from renewable resources. The independent scalability of capacity and power of the system and the high durability are the main advantages towards currently leading storage technologies like Li-Ion batteries. However, high and fluctuating costs of metal-based active materials in commercial RFB, mostly vanadium, have so far prevented a market penetrating success of this technology.
Due to their low cost, non-corrosive and environmentally friendly active materials, organic redox flow batteries can overcome the common RFB drawbacks. This is why safe and abundant materials are very much in the focus of research in order to be able to serve as the forward-looking energy storage of tomorrow.
In order to examine the performance of promising organic material candidates for RFB application, processes on the microscale are of significant importance in addition to experimentally determinable macroscopic quantities like electronic current and potential. Parameter like initial species concentration, diffusion coefficient or the microstructure of the porous electrode also have a considerable influence on capability of the RFB. Therefore, a spatially resolved 3D continuum simulation model is set up to gain insight into the microstructural processes inside the RFB.
The three-dimensional model is parameterized with the known model chemistry of the organic TEMPO/methyl viologen redox pair and accounts for mass transport and the electrochemical reaction. The first version of the model focus on the cathode half-cell of the RFB, including coupled solid and liquid region. Simplified fiber structures with different porosity exposed to the electrolyte flow account for the porous of the electrode. The membrane and the current collector are considered as boundary conditions. For the liquid phase, a periodic inflow and outflow condition is applied. The chemical reaction is implemented using Butler-Volmer type kinetics at the solid-electrolyte interface. Furthermore, the model is assumed to work isothermal comprising a laminar and steady-state flow with constant density and viscosity.
A deeper understanding of the mentioned processes is inevitable to enhance RFB performance and to overcome cell limitations such as structure-dependent transport limitations or fluctuating local overpotentials
Galvanostatic simulations are conducted to investigate the charge and discharge process at different current densities for different concentrations and porosities. The simulations show improved voltage efficiency for higher concentrated electrolyte due to higher electrolyte conductivity and for increasing active surface area.