10.2 Flow battery chemistry and design

Flow batteries are a unique energy storage solution that separates power and energy capacity. They use liquid electrolytes stored in external tanks, circulated through a cell stack for charging and discharging. This design allows for flexible scaling and long-term storage.

Various flow battery chemistries exist, each with pros and cons. Vanadium redox flow batteries offer high efficiency but are costly, while zinc-bromine systems provide high energy density but face corrosion issues. Understanding these differences is key to choosing the right system for specific applications.

Redox Flow Battery Components

Electrolyte Storage and Separation

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• Electrolyte tanks store anolyte and catholyte solutions separately
  • Anolyte contains electroactive species that undergo oxidation at the anode
  • Catholyte contains electroactive species that undergo reduction at the cathode
• Membrane separates the anolyte and catholyte compartments
  • Allows selective passage of ions to maintain charge balance
  • Prevents mixing of the anolyte and catholyte solutions (cross-contamination)

Electrolyte Circulation

• Pumps circulate the anolyte and catholyte solutions through the cell stack
  • Ensures a continuous supply of electroactive species to the electrodes
  • Helps maintain a uniform concentration of electroactive species in the electrolyte
• Circulation system includes pipes, valves, and flow controllers
  • Regulates the flow rate of the electrolyte solutions
  • Enables control over the power output and efficiency of the battery

Redox Flow Battery Types

Vanadium Redox Flow Battery (VRFB)

• Uses vanadium ions in different oxidation states as electroactive species
  • V(II)/V(III) redox couple in the anolyte
  • V(IV)/V(V) redox couple in the catholyte
• Advantages include high energy efficiency, long cycle life, and low self-discharge
• Challenges include high cost of vanadium and limited energy density

Zinc-Bromine Flow Battery (ZBFB)

• Uses zinc and bromine as electroactive species
  • Zinc is plated on the anode during charging and dissolved during discharging
  • Bromine is stored as a complex in the catholyte and reduced to bromide during discharging
• Advantages include high energy density, low cost, and abundant materials
• Challenges include corrosive nature of bromine and formation of zinc dendrites

Redox Flow Battery Characteristics

Energy Storage Mechanism

• Redox flow batteries store energy in the form of chemical potential in the electrolyte solutions
  • Energy is converted between electrical and chemical forms during charging and discharging
• Electroactive species undergo reversible redox reactions at the electrodes
  • Oxidation occurs at the anode, releasing electrons
  • Reduction occurs at the cathode, accepting electrons

Decoupled Power and Energy Capacity

• Power and energy capacity are decoupled in redox flow batteries
  • Power is determined by the size of the cell stack (electrode area and number of cells)
  • Energy capacity is determined by the volume and concentration of the electrolyte solutions
• Decoupling allows independent scaling of power and energy
  • Power can be increased by adding more cells to the stack
  • Energy capacity can be increased by using larger electrolyte tanks or higher concentrations

Scalability and Flexibility

• Redox flow batteries are highly scalable
  • Can be easily scaled up to MW/MWh levels by increasing the number of cell stacks and electrolyte volume
  • Modular design allows for customization based on specific power and energy requirements
• Offer flexibility in operation and application
  • Can be used for both short-term (power quality, frequency regulation) and long-term (energy arbitrage, renewable integration) energy storage
  • Can be rapidly charged and discharged without significant degradation