The vanadium redox flow battery (VRFB) is one of the most mature and commercially available electrochemical technologies for large-scale energy storage applications. The VRFB has unique advantages, such as separation of power and energy capacity, long lifetime (>20 years), stable performance under deep discharge cycling, few safety issues and easy recyclability. Despite these benefits, practical VRFB operation suffers from electrolyte imbalance, which is primarily due to the transfer of water and vanadium ions through the ion-exchange membranes. This can cause a cumulative capacity loss if the electrolytes are not rebalanced. In commercial systems, periodic complete or partial remixing of electrolyte is performed using a by-pass line. However, frequent mixing impacts the usable energy and requires extra hardware. To address this problem, research has focused on developing new membranes with higher selectivity and minimal crossover. In contrast, this study presents two alternative concepts to minimize capacity fade that would be of great practical benefit and are easy to implement: (1) introducing a hydraulic shunt between the electrolyte tanks and (2) having stacks containing both anion and cation exchange membranes. It will be shown that the hydraulic shunt is effective in passively resolving the continuous capacity loss without detrimentally influencing the energy efficiency. Similarly, the combination of anion and cation exchange membranes reduced the net electrolyte flux, reducing capacity loss. Both approaches work efficiently and passively to reduce capacity fade during operation of a flow battery system.

Progress in renewable energy production has directed interest in advanced developments of energy storage systems. The all-vanadium redox flow battery (VRFB) is one of the attractive technologies for large scale energy storage due to its design versatility and scalability, longevity, good round-trip efficiencies, stable capacity and safety. Despite these advantages, the deployment of the vanadium battery has been limited due to vanadium and cell material costs, as well as supply issues. Improving stack power density can lower the cost per kW power output and therefore, intensive research and development is currently ongoing to improve cell performance by increasing electrode activity, reducing cell resistance, improving membrane selectivity and ionic conductivity, etc. In order to evaluate the cell performance arising from this intensive R&D, numerous physical, electrochemical and chemical techniques are employed, which are mostly carried out ex situ, particularly on cell characterizations. However, this approach is unable to provide in-depth insights into the changes within the cell during operation. Therefore, in situ diagnostic tools have been developed to acquire information relating to the design, operating parameters and cell materials during VRFB operation. This paper reviews in situ diagnostic tools used to realize an in-depth insight into the VRFBs. A systematic review of the previous research in the field is presented with the advantages and limitations of each technique being discussed, along with the recommendations to guide researchers to identify the most appropriate technique for specific investigations.

This study presents the application of pyrolyzed spent coffee beans as a potential electrode material to replace commercial bipolar graphite plate in vanadium redox flow batteries (VRB). The results indicate that the biochar obtained from spent coffee beans shows relatively good electrochemical charge transfer kinetics of vanadium redox reactions as well as generates higher energy and voltage efficiency in a static cell test when compared to TF6 bipolar graphite plate. Additionally, the biochar was activated via steam at various activation times to increase its surface area, and their effect on the kinetics of the electrochemical reactions was investigated. The activated carbon did not exhibit any improvement neither in electron transfer kinetics nor in the battery efficiency, despite their increased surface area. The performed studies demonstrate that the biochar obtained from spent coffee beans can be a low-cost electrode material for VRB with improved performance characteristics.