Electrolyte Rebalancing Method for Capacity Recovery in Iron-Chromium Redox Flow Batteries
Iron-chromium redox flow batteries (ICRFBs) often experience capacity decay due to hydrogen evolution and side reactions during operation. These reactions can lead to an imbalance in the valence states of the active materials in the electrolytes, causing a decline in battery performance. To address this issue, researchers have proposed various electrolyte rebalancing methods to restore battery capacity.
1. Electrolyte Rebalancing Methods
The core of electrolyte rebalancing is to restore the charge balance of the electrolytes by electrochemical reactions. This involves reducing Fe³⁺ in the positive electrolyte to Fe²⁺ while replenishing hydrogen ions in the negative electrolyte. Here are several typical rebalancing methods:
1.1 Chlorine Generation and Hydrogen Ion Replenishment
A common rebalancing method utilizes a rebalancing cell to oxidize Cl⁻ at the anode to generate chlorine gas while reducing Fe³⁺ to Fe²⁺ at the cathode. The high concentration of hydrogen ions generated at the anode diffuses into the cathode solution, restoring the balance of the negative electrolyte. This method employs a PLC control system to efficiently replenish acid in the anode solution and uses an alkaline solution tank to absorb the generated chlorine gas, ensuring environmentally friendly operation.
1.2 Hydrogen Reduction Method
Another approach involves using hydrogen gas to reduce Fe³⁺ in the positive electrolyte to Fe²⁺. NASA's team developed a hydrogen-iron rebalancing cell, using hydrogen as fuel and iron chloride solution as the oxidant, separated by an ion-exchange membrane. This method achieves high hydrogen utilization rates (up to 100%) at low hydrogen concentrations (≤5%) and has significant potential for industrial application.
1.3 Electrolyte Purification
Impurities such as copper and nickel in the electrolyte can promote hydrogen evolution reactions, leading to capacity decay. Electrolyte purification techniques can remove these impurities, thereby reducing hydrogen evolution reactions.
2. Rebalancing Apparatus
To implement the above rebalancing methods, researchers have developed various rebalancing devices. For example, a typical rebalancing apparatus includes an anode tank, a cathode tank, a concentrated hydrochloric acid storage tank, an alkaline solution tank, a rebalancing cell, and a PLC control system. These components are connected through pipes and pumps to form a circulating loop for electrolyte rebalancing.
3. Advantages and Applications
These rebalancing methods and devices offer several advantages:
Efficient Capacity Recovery: By restoring the charge balance through electrochemical reactions, these methods can effectively reduce capacity decay.
Environmentally Friendly Operation: By using alkaline solutions to absorb chlorine gas, harmful gas emissions are minimized.
Cost Savings: The chemicals used in the rebalancing process are the same as those used in the electrolytes of iron-chromium redox flow batteries, avoiding the need for additional reducing agents and catalysts, thus reducing costs.
4. Future Outlook
Electrolyte rebalancing technology provides an effective solution for capacity recovery in iron-chromium redox flow batteries. With continuous optimization of the technology and cost reduction, these methods are expected to find widespread application in large-scale energy storage systems.
