Flow Battery Technology

Context: Researchers at IIT-Madras have developed an improved flow battery technology using a new type of organic electrolyte, pyrylium.

About Flow Batteries

• Flow batteries are a type of rechargeable electrochemical batteries in which electrolyte flows through one or more electrochemical cells from one or more tanks. 
• Simply speaking, energy is stored in two liquid electrolytes in separate tanks in flow batteries.
• Mechanism: When we charge, the energy supplied urges electrons from the electron poor side to move to the electron rich side, creating a potential difference. During discharge, the reverse happens i.e., electrons flow from the electron rich side to the electron poor side. 
• Conversion of energy from chemical to electrical energy happens in a cell, which is split into two-halves by a membrane. 
• Electrolytes used: 
  • Vanadium is used as an electrolyte as it is capable of existing in four ionic species – with two, three, four or five (positively charged) protons.
  • Zinc-bromine (ZNBR): they use zinc and bromine ions to store electrical energy.
  • Iron-Chromium electrolytes
  • Proton exchange membrane (PEM): Use a proton-conducting membrane to separate positive cathode and negative anode electrodes. 
  • Organic electrolytes

Advantages of flow batteries

• Long-lasting: Do not show performance degradation for 25-30 years. 
• Modular & scalable: Capable of being sized according to energy storage needs with limited investment. Energy storage capacity can be increased by simply raising the volume of electrolyte tanks. Also, if we want to store more power, one can just add more cells in the cell stack. 
• Low cost: Battery can be constructed using low cost & readily available materials such as thermoplastics and carbon-based materials. 
• Suitable for grid-scale storage: Flow batteries can come handy in large storage applications, especially in maintaining grid stability and can come handy in increasing renewable energy penetration. 
• Slow discharge: Flow batteries can store power for long durations. This is significant for storing seasonal renewable energy such as wind. 
• Safety: Low flammability and low environmental impact.
• Robust: Overcharging & fully discharging does not usually cause permanent damage to the electrode or electrolytes.
• Electrolytes can be used as a heat management strategy for the battery which reduces the need for complex heating or cooling of battery system. 
• Full recycling of electrolytes.

Challenges associated with flow batteries

1. High cost of vanadium
2. Low-energy density
3. Low charge and discharge rates
4. Lower energy efficiency
5. Cross-over: This is a phenomenon when a positive electrolyte travels through the membrane separator and travels into the negative electrolyte. This leads to automatic discharge. 

Improved Flow Battery Technology Developed by IIT-Madras

• Researchers at Indian Institute of Technology Madras have developed a ‘non-aqueous all-organic redox flow battery (NORFB)’ which leads to improved performance by flow batteries.
• Conventional flow batteries employ aqueous (water-based) electrolytes like hydrochloric acid, sulphuric acid and alkali metal hydroxides.
• Water based electrolytes leads to following issues:
  • Water interferes with undergoing electrolysis reducing operating voltage limit and energy density (amount of energy per unit volume or gram).
  • Water based electrolytes corrode battery components.
• Pyrylium electrolytes: To address the situation scientists have been looking for non-aqueous and organic electrolytes. In this respect, researchers at IIT-Madras have developed a new-type of electrolyte using ‘pyrylium salts’ (a class of organic compounds). Pyrylium allows high-voltage operations allowing more flow batteries to store more energy and more current density.