Zinc-air batteries

Zinc-air batteries use oxygen from the atmosphere rather than the usual metal-dissolved electrolyte. Air reaches the cathode surface, where an active electrocatalyst promotes the reduction of oxygen. Metallic zinc is oxidised on the opposite side of the cell, usually in an alkaline electrolyte.

 

Zinc Air Button Cell

 

Zinc-air button cell Because the cathodic reactant is not packaged within, zinc-air batteries have a higher energy-to-weight ratio than other types, and also since the cathode is very thin, the anode compartment can be packed with more zinc, resulting in a very high energy density.

Size and Weight of Battery Technologies (Ref. Energy Storage Association) Zinc-air batteries are usually small coin or button-type primary batteries, often used in medical applications due to their low toxicity and long life. Larger variants are available, but are limited in power due to limitations in the cathode, and are restricted to applications that require long-term, low rate discharge, such as rail signalling, lighting, and remote communications equipment.

The development of rechargeable zinc-air batteries has been limited due to problems of dendrite formation, cell dehydration, and degradation of the air electrode. Also, bifunctional oxygen catalysts for the reversible cathode reaction have been limited to date. This can be overcome by combining separate oxygen evolving and reducing catalysts, but this can increase weight and cost.

 

Flow Batteries

 

Flow Cell Diagram

Flow batteries, or redox batteries, store the active materials in external tanks in the liquid phase. Electrolyte is circulated through an electrochemical cell stack, and the reactants charged and discharged as required. The essential advantage of flow batteries is their decoupling of power from energy. High power applications may require large area cells or multiples of cell stacks, while energy capacity can be boosted by increasing the volume of electrolyte.

Flow battery Modern flow batteries are generally two electrolyte systems and can be based on various redox couples. Zn-Cl was the first to be used in 1884 by Charles Renard to power his airship, which contained its own chlorine generator. NASA later developed a system based on Fe-Cr, and there have since been a number of commercial projects based around V-V, Zn-Br, polysulphide-Br, Pb-PbO and Zn-Ce.

Flow battery schematic Because of their scalability, flow batteries have a wide power range, from a few kWs to many MWs, and much higher capacity ratings than static batteries. The figure below shows a range of energy storage technologies, with only compressed air storage and pumped hydro having a higher combined power-capacity, and as such, is being considered for large-scale energy storage applications at grid-scale.

Power-Energy Ratings of Energy Storage Technologies In all of the chemistries previously mentioned, concerns exist over the toxicity of some reactants or in the corrosion resistance of cost-effective electrode materials.

 

Powair

 

Zinc Air Flow Battery

The project takes existing knowledge of electrochemical technologies within the consortium, to develop a flow battery that will compete with systems currently under development. Instead of using the traditional two-electrolyte system, Powair’s approach will be to implement a liquid electrolyte for the negative electrode, while providing the positive electrode with a continuous supply of oxygen, via atmospheric air. Eliminating the need for a positive electrolyte effectively doubles the energy density of the system. On discharge, the electrode acts as a fuel cell cathode, whilst charging is achieved by using alternative catalysts that promote the evolution of oxygen.

The zinc half-cell is dissimilar to static zinc-air batteries, as metal-containing liquid electrolyte is pumped through the battery stack during charge, and zinc is deposited onto the electrode surface, typically a planar design.

Zinc-air flow battery representation The development of large zinc-air flow batteries is a major innovation and has many advantages over other technologies, including:

  • High power and energy density
  • The battery contains relatively benign and readily available chemicals when compared to other flow battery variations
  • The high alkali content and flow prevents Zn passivation and reduces dendrite formation
  • Well-known chemistry with fast kinetics o Known additive systems from electroplating industry to control deposit morphology (eg reduce dendrites)
  • Liquid electrolyte means the batter is unlikely to dry out, reducing the need for humidity control for the air electrode
  • Thermal management issues are minimal due to the re-circulated electrolyte and the air electrode
  • Can utilise non-precious metal catalysts with low over-potentials o Equilibrium cell voltage of ~ 1.7 V (and a practical output voltage of ~ 1.3 V)
 

Consortium Websites

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E-on logo Fuma-Tech logo
GreenPower logo DNV KEMA logo
University of Seville logo University of Southampton logo

 

Funded by

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