Engineering Redox Flow Battery Electrodes with Spatially Varying Porosity Using Non-Solvent-Induced Phase Separation

Redox flow batteries (RFBs) are a promising electrochemical platform for efficiently and reliably delivering electricity to the grid. Within the RFB, porous carbonaceous electrodes facilitate electrochemical reactions and distribute the flowing electrolyte. Tailoring electrode microstructure and surface area can improve RFB performance, lowering costs. Electrodes with spatially varying porosity may increase electrode utilization and provide surface area in reaction-limited zones; however, the efficacy of such designs remains an open area of research. Herein, a non-solvent-induced phase-separation (NIPS) technique that enables the reproducible synthesis of macrovoid-free electrodes with well-defined across-thickness porosity gradients is described. The monotonically varying porosity profile is quantified and the physical properties and surface chemistries of porosity-gradient electrodes are compared with macrovoid-containing electrode, also synthesized by NIPS. Then, the electrochemical and fluid dynamic performance of the porosity-gradient electrodes is evaluated, exploring the effect of changing the direction of the porosity gradient and benchmarking against the macrovoid-containing electrode. Lastly, the performance is examined in a vanadium RFB, finding that the porosity-gradient electrode outperforms the macrovoid electrode, is independent of gradient direction, and performs favorably compared to advanced electrodes in the contemporary literature. It is anticipated that the approach motivates further exploration of microstructurally tailored electrodes in electrochemical systems.