Mathematical modeling of electric and hydraulic resistances of reconstructed carbon felt electrodes using micro-computed tomography

Abstract

Redox flow batteries of various chemistries are potential electrochemical energy storages for residential accumulation and grid stabilization. Decoupled power and capacity represent the main advantages of the technology. The key component of the battery, the carbon felt electrode, does not participate in the electrochemical reaction directly, but it provides active sites for the electrochemical reaction of electroactive ions. In addition, the electrode contributes to the battery stack polarization through the charge transfer, ohmic and mass transport resistances and increases the hydraulic resistance of the battery stack and resulting pumping losses associated with the electrolyte circulation. In this contribution, a mathematical model was developed to evaluate geometrical parameters and simulate the effective electric conductivity and hydraulic resistance of two commercially available carbon felt electrodes using their microtomography images. These were preprocessed and binarized into a reconstructed computation domain. Geometric descriptors, such as porosity, specific surface area and fiber spatial orientation, were calculated using in-house developed algorithms. The area specific resistance and hydraulic resistance were then estimated and validated against our own experimental data, which were measured for both felts under six different relative compressions. The results of the fiber spatial orientation showed a change in the orientation of the carbon felt fiber with increasing compression rate. As the result of increasing compression, the electrical resistance decreases, whereas the hydraulic resistance increases. Interestingly, due to a change in spatial fiber orientation, Carman-Kozeny constant is also decreasing with increasing compression. The developed model can be further used to optimize the textural properties of 3D fibrous electrodes from hydraulic and ohmic point of view, within the development of flow electrochemical reactors