Design and development of vanadium(IV) electrolyte production system for vanadium redox flow batteries (VRFB) using vanadium(V) pentoxide

An effective energy storage system is critical needed, as seen by the growing demand for renewable energy [1-4]. Vanadium Redox Flow Batteries (VRFBs) are a promising solution for large-scale energy storage due to their high safety, long durability, and operational flexibility [5-12]; nevertheless, the application of these batteries is hindered by the high cost of vanadium electrolytes, which account for 40-50% of the total system expenses [13, 14]. To address this problem, vanadium pentoxide (V₂O₅), a cost-effective and stable vanadium source, has been investigated for electrolyte synthesis [15]. Unfortunately, there are difficulties given that V₂O₅ is poorly soluble in sulfuric acid (H₂SO₄), especially at concentrations needed for operating VRFB [16-19].

Several studies have attempted to improve the solubility of V₂O₅ and optimize the production of vanadium(IV) electrolyte. Rahman and Skyllas-Kazacos (2022) demonstrated that increasing the concentration of H₂SO₄ solvent and decreasing the surrounding temperature enhances the solubility of V₂O₅. However, excessively high acid concentrations and lower temperatures can lead to the precipitation of other vanadium species during VRFB operation [20, 21]. Martin et al. (2020) reported a rapid dissolution technique for V₂O₅ powder using a mixture of vanadium(III) and vanadium(IV) solutions from a commercial electrolyze. In addition, the process involves the reduction reaction of vanadium(V) to vanadium(III) via an electrolyzer. Although this approach has improved its performance, it was evaluated for its potential to produce vanadium electrolyte more cost-effectively [15]. Therefore, further research is required to improve the performance of eletrolyzer, whereas also reducing material cost and process time to produce vanadium electrolyte efficiently.

This study aims to optimize the production of vanadium(IV) electrolyte for VRFBs by enhancing the solubility of V₂O₅ in H₂SO₄ solution and developing a scalable, cost-effective production system. The goal is to achieve a vanadium(IV) concentration of 1.5 M in a 3 M aqueous H₂SO₄ solution, thereby significantly increasing energy storage capacity of VRFBs [6], and to establish an automated system which can consistently produce this electrolyte. The concentration of vanadium(IV) electrolyze will be precisely determined using UV-Visible spectroscopy (UV-Vis), ensuring precision and reliability in the final product [22, 23]. 

The experimental set-up consists of a mixer with a temperature-controlled bath, a vanadium storage tank, and an electrolyzer, as shown in Figure 1. The electrolyzer is specifically designed to convert vanadium(V) solution to vanadium(III) solution, which is subsequently used in the mixer to dissolve vanadium(V) solid to vanadium(IV) solution. The overall process involves the following steps:

1. Trial of the initial concentration’s vanadium(V): V₂O₅ powder is added to a 3 M aqueous H₂SO₄ solution at a concentration of 0.2 M, with the temperature controlled at 10°C. Measure the time required for the complete dissolution of V₂O₅ powder, confirming that no solid particles remain in the solution.
2. Synthesis of vanadium(III): The completely dissolved V₂O₅ solution is transferred to the electrolyzer, where it is converted into a vanadium(III)-containing solution. The concentration of vanadium(III) is then measured via UV-Vis to calculate the amount of water generated during the charging process of the electrolyzer.
3. Mixing of recirculated vanadium(III): The generated vanadium(III) solution is fed back into the mixer. V₂O₅ powder is then added to this solution to produce vanadium(IV) electrolyte, targeting the desired concentration level as 1.5 M.
4. Test of the target concentration’s vanadium(IV): The concentration of produced vanadium(IV) solution will be tested using UV-Vis after the addition of V₂O₅ powder.
5. Validation of energy storage: The performance of the vanadium(IV) electrolyte produced through electrolysis is measured and compared with that of commercially available vanadium(IV) electrolytes.
6. Operation of continuous process production: The continuous system will be placed to the test to provide it operates efficiently and consistently.
   
   This research is expected to result in a significant reduction in the production costs of vanadium(IV) electrolyte. It is anticipated that this development will improve the commercial viability of VRFBs. The results will contribute to the advancement of VRFB technology by improving the scalability and cost-effectiveness of vanadium(IV) electrolyte production, ultimately supporting the broader adoption of renewable energy storage systems.