Engineering Porous Electrodes for Advanced Redox Flow Batteries

The escalating adoption of renewable energy sources underscores the pressing need for efficient and scalable energy storage systems to ensure consistent energy supply and grid stability. Within energy storage technologies, redox flow batteries have been regarded as promising solutions for stationary grid-scale energy storage. However, the path to their widespread commercialization has been restricted due to the high capital costs, and one of the key solutions to cost reduction lies in enhancing the battery performance.

Porous electrodes are crucial components of redox flow battery system, which are closely related to the reactor internal resistance and battery performance. The main focus of this thesis is the development of engineering porous electrodes with a balance between the active surface area and effective electrolyte transport pathways for advanced redox flow batteries. Efforts are devoted to three aspects: i) unveiling the relationship between the morphological properties and electrochemical performance of three distinct carbon-fiber electrodes, including carbon felt, carbon paper, and carbon cloth. Investigating the optimal compression condition for the three electrode in redox flow battery system; ii) developing a dual-layer electrode configuration to simultaneously meet the requirement of high active surface area and low mass transfer resistance; iii) Tailoring the electrode microstructure using non-solvent induced phase separation as well as integrating a micro-flow field into the electrode architecture for improved mass transfer performance.

First, the 3D electrode morphology of the three commercially available carbon-fiber electrodes was characterized through X-ray computed tomography at 0-50% compression ratios. The electrochemical performance was evaluated at the same compression range in a lab-scale full-cell vanadium redox flow battery. It was found that the cloth possessed a bimodal pore size distribution. However, its distinct microstructure and the associated advantageous mass transfer properties were deteriorated significantly at applied high compression (e.g., >30% compression ratios). The optimal trade-off between the pressure drop and the electrochemical performance occurs at the compression ratios of 30%, 20%, and 20% for the felt, paper, and cloth, respectively.

Secondly, the carbon cloth electrode was combined with a sub-layer consisting of carbon paper to assemble a dual-layer electrode configuration. Quantitative analysis of contributions from each type of polarization was investigated under both V²⁺/V³⁺ and VO²⁺/VO₂⁺ redox couples in one-container symmetric cells with flow-through flow fields. The results showed that the proposed strategy was effective to obtain decreased overall kinetic and mass transport resistances. At the current density of 100 mA/cm², a decrease of ~35% and ~17% of the overall cell overpotential was achieved compared to single-layer carbon cloth and paper, respectively.

Finally, the non-solvent induced phase separation technique was used to fabricate tailored non-fibrous porous electrodes. Inspired by the flow field designs in redox flow batteries for low pressure drop and uniform electrolyte distribution, a strategy to imprint micropatterned flow fields directly into the electrode architecture during the phase exchange process was explored. Two micro-patterned designs (i.e., groove and pillar patterns) were selected and the electrochemical measurements were conducted using the Fe²⁺/Fe³⁺ redox couple in a symmetric cell. It was found that the pillar-patterned electrodes combined with an external interdigitated flow field showed the best performance. At an electrolyte velocity of 10 cm/s, the total resistances for kinetic and mass transfer are less than 0.1 Ω·cm² in symmetric iron cell tests, corresponding to a reduction of ~60% when comparing to the activated carbon paper electrode.