The reactions in redox flow batteries are not usually limited by electron transfer. Hence, the power density and efficiency are largely determined by mass transport and ohmic limitations. Consequently, electrode structure and flow field designs are important in optimizing efficiency. Therefore, assessment of various types of flow fields in combination with porous electrodes has been the subject of simulations and analysis by several developers of redox flow batteries. [1-3] The studies show the impact of flow field design on cell performance, simulated effects of channel design, and pressure drops across the electrode when used in combination with porous carbon paper. These studies highlight the importance of such analysis for the development of the redox flow batteries as a viable energy storage solution. Our focus on improving the performance of the aqueous organic redox flow battery has led us to qualify modified graphite felt structures in combination with flow-through, interdigitated flow fields and columnar flow fields. To this end, we have characterized the cell performance using highly reversible redox couples such as ferrocyanide/ferricyanide. Specifically, the measurements include the current-voltage curves under potentiostatic conditions at various flow rates to determine the limiting current, and electrochemical impedance spectroscopy over the range of 10 mHz to 10 kHz to determine mass transport parameters and surface area of the electrode.

We have observed that the flow-through flow field configuration exhibited the lowest mass-transport resistance, highest limiting current, and the lowest electrical contact resistance with the electrode (Figure 1). However, the flow-through designs entail high pressure drop across the electrode compared to the columnar and interdigitated designs. For a given electrode structure, the diffusion layer thickness at the surface of the electrode was dependent on the flow configurations. The porosity and surface area of the electrode structures was modified by incorporation of nano-structured carbon coatings. An optimal design combines an electrode with the highest available surface area, low pressure drop, and minimal diffusion layer thickness. The rational design of such an optimal combination of electrode and flow properties is rendered possible by the measurements and mathematical analysis carried out in this study.

Acknowledgement

The Authors acknowledge the financial support for this research from ARPA-E Open-FOA program (DE-AR0000337), the University of Southern California, and the Loker Hydrocarbon Research Institute.

1. C. R. Dennison, E. Agar, B. Akuzum, and E. C. Kumbur, J. Electrochem. Soc. , 163, A5163–A5169 (2016) http://jes.ecsdl.org/content/163/1/A5163.abstract.

2. M. Skyllas-Kazacos, M. H. Chakrabarti, S. a. Hajimolana, F. S. Mjalli, and M. Saleem, J. Electrochem. Soc., 158, R55 (2011) http://jes.ecsdl.org/cgi/doi/10.1149/1.3599565.

3. C. Yin, Y. Gao, S. Guo, and H. Tang, Energy, 74, 886–895 (2014).