On the Mass Transport in Tubular Vanadium Redox Flow Batteries

Efficient mass transport is critical for tubular flow battery performance and for its eventual scale‐up; yet the influence of design parameters like electrode fiber filling density, internal membrane volume, and electrode structure remains largely unexplored. Herein, a tubular all‐vanadium flow battery with a fibrous 4 mg cm ⁻¹ electrode filling density and a 0.238 cm internal diameter (ID) membrane is fed with dilute vanadium electrolyte at varying flow rates, and its mass transfer coefficient is calculated. The filling densities are increased to 8 and 16 mg cm ⁻¹ , where the 4 mg cm ⁻¹ shows the highest mass transfer coefficient, 2.64 × 10 ⁻⁶ cm s ⁻¹ . Decreasing the internal membrane diameter to 0.144 and 0.116 cm while restraining the 4 mg cm ⁻¹ filling density reveals the 0.238 cm diameter results in the largest mass transfer coefficient. Comparing a solid electrode to the 4 mg cm ⁻¹ , 0.238 cm ID electrode, the solid electrode reports the highest mass transfer coefficient, 2.56 × 10 ⁻⁵ cm s ⁻¹ . These findings surprisingly demonstrate that fibrous electrodes, despite their higher total surface area, do not inherently lead to improved mass transfer performance in tubular flow batteries, rather, it is the amount of available surface area and the uniformity in electrolyte flow, seen in the solid graphite electrode, that is critical for mass transfer.