Tuning of operational conditions and by using pre-treatment of electrodes can lead to the increase in performance of Vanadium redox flow cell (VRFC) as evident through our published article [1]. This work reports the tuning of operational conditions of a VRFC having impregnated graphite bipolar plate with interdigitated flow-field as compared to conventional flow-field. The schematic is shown in figure 1.

The as-received Fumatech FAP-450 anion exchange membrane (AEMs) were used without any treatment. All membranes were cleaned in 1M H₂SO₄ solution at 80 ^oC until all vanadium stains were removed and stored in deionized water before being used again. The electrolyte consisted of a commercially available 4 M sulfuric acid-based vanadium solution (1.6 M V³⁺/V⁴⁺ 50:50 M ratio, GfE Metalle und Materialien GmbH, Germany) and it was used without further treatment or purification. Graphite felt electrode (GFE) of thickness 4.6 mm was used with compression ratio of 25%. GFE were boiled in 1M H₂SO₄ for 12 hours and then left for another 12 hours inside the same solution. Then, GFE are washed with DI water and dried at 100 ^oC in oven. Afterwards, GFE are thermally treated at 390 ^oC for 12 hrs inside the furnace injected by air and then cooled down in the furnace upto room temperature. These arrangements allowed to reach higher current density at lower flowrate than used in published article [1].

Charge discharge cycling was done at 40ml/min using current densities of 120mA/cm², 150mA/cm², 170mA/cm², 190 mA/cm² and 210 mA/cm². Results of efficiencies (%), energy density (Wh/L) and capacity (mAh) are shown in attached figure 2,3 and 4 respectively. Electrochemical impedance spectroscopy (EIS) is carried out to measure the contribution of Area Specific Resistance at 40 ml/min which is 1.36 Ωcm² for interdigitated flow-field. EIS results are further fitted using equivalent circuits to determine the contribution of each component.

As per the previous article [1], in the absence of flowfield, it was observed that increase in flowrate is feasible to increase the energy efficiency. But, current work highlighted the significance of using flowfields as an efficient way is to decrease the pressure drop to avoid continous increase in flowrate. This approach helps to reach higher current densities at much lower requirement of pumping flowrate.

Energy efficiency start decreasing significantly with increment in current density. Only voltaic efficiency is effected whereas columbic efficiency remain around 98% at all current densities. More research efforts are required to optimise the treatment of electrodes and also flowrate distribution to get higher efficiency at higher current density.

Reference:

[1] R. Monteiro, J. Leirós, M. Boaventura, and A. Mendes, "Insights into all-vanadium redox flow battery: A case study on components and operational conditions," Electrochimica Acta, vol. 267, pp. 80-93, 2018.