Micro‐Tubular Flow Cell Design Utilizing Commercial Hollow Fiber Dialysis Membranes for Size‐Exclusion Based Flow Batteries

Abstract A micro‐tubular flow cell (μTFC) is presented as a novel approach for flow cell design, which differs from previous tubular approaches by its lower cell diameter (3 mm) and half‐cell thicknesses (≤850 μm). Analytical expressions for the ohmic resistances of both micro‐tubular and conventional filter‐press flow cells (cFFC) are derived, which imply that optimized μTFCs can be expected to have an at least two‐fold lower ohmic resistance compared to equally sized cFFCs in the future. The theoretical findings motivate the experimental realization of a first μTFC single‐cell unit to investigate the basic feasibility of this approach. This investigation is enabled by the utilization of a redox‐active polymer in conjunction with a commercially available micro‐tubular dialysis membrane (diameter: 1 mm). Aqueous solutions of poly(2,2,6,6‐tetramethylpiperidin‐4‐yl methacrylate‐ co ‐poly(ethylene glycol)methyl ether methacrylate) (positive half‐cell) and the Zn/Zn 2+ redox couple (negative half‐cell) are utilized as model electrolytes to examine the electrochemical performances of this first μTFC prototype. Furthermore, the common usage of area current densities based on the geometric area of the flow cell is critically discussed. A current density based on the cell volume is suggested as a less ambiguous and, hence, a more reliable parameter for performance evaluation.