Morphological Properties and Electrochemical Performance for Compressed Carbon-fiber Electrodes in Redox Flow Batteries

Recognizing the urgent need for further cost reduction to drive deep penetration of redox flow batteries as grid-scale stationary energy storage systems in the global energy mix, it is critical to improve reactor performance to bring down the high capital cost. As one of the main contributors to the overall internal resistance, porous electrodes with properly designed microstructure and optimized physiochemical properties are preferable for boosting electrochemical and fluid dynamic performance. The present study aims to unveil the relationship between electrode morphology and electrochemical performance under varying electrode compression. Three representative, commercially available, carbon-fiber electrodes (i.e., paper, felt, and cloth) with distinct microstructures were selected here, and a comprehensive study was conducted to compare morphology, hydraulic permeability, mechanical behavior, electrochemical properties including decoupled kinetic and mass transfer resistances, and overall battery performance in a lab-scale vanadium redox flow battery at 0-50% compression ratios. The 3D electrode morphology was characterized through X-ray computed tomography and the extracted key microstructure parameters (e.g., surface area and tortuosity) were compared with corresponding electrochemically determined parameters. It was found that the cloth possessed a bimodal pore size distribution due to its distinct woven microstructure and that it was sensitive to the applied compression. The large pores formed at the intersections between fiber bundles collapsed under excessive compression, which greatly reduced the advantageous high permeability and low mass transfer resistances characteristic of the uncompressed cloth. The paper electrodes exhibited a strongly growing compression force accompanied by increased in-plane tortuosity and mass transfer resistance even at low compression (<20%). During battery operation, the cloth maintains a good balance between performance and pressure drop at moderate compression. The optimal trade-off between fluid dynamics and electrochemical performance occurred at the compression ratios of 30%, 20%, and 20% for the felt, paper, and cloth, respectively.