The chemistry and performance evaluation of a Zn/Br2 redox flow battery.

  • Supervisors: Dr Leonard Berlouis, Dr John Reglinski and Dr Mark Spicer (University of Strathclyde), Mr Alan Davidson (Edinburgh Napier University).
  • Sponsor Company: Lotte Chemical Corporation, Korea
  • PhD Student: Declan Bryans

It is the stated aim of the Scottish Government to generate the equivalent of 100% of Scotland’s electrical energy demand (~40 TWh) through renewable sources by 2020.  It is thought that this will primarily be achieved by the deployment of wind turbines (on-shore and off-shore), biomass and tidal generation. Although in 2012 power generation from renewables was reasonably high at 35% in Scotland, and 8% for the UK, further large increases in the share of generation from variable sources will require substantial innovation including the use of energy storage. Redox Flow Batteries (RFBs) are one such energy storage technology, best suited to medium and large scale stationary applications.

Our research is concerned with the zinc-bromine RFB, where zinc is deposited at the negative electrode during charging, while bromide ions are oxidized to bromine at the positive electrode according to:

Zn2+ + 2e    Zn            and        2Br    Br2 + 2e                           ECell =1.82V

During discharge, the reverse reactions occur. In the discharged battery the positive electrolyte is an aqueous solution of bromide and quaternary ammonium complexing agents. As the battery is charged bromine is formed (in actual fact an equilibrium of less highly oxidised bromine species such as Br3 and Br5 is established), the bromine is trapped by the quaternary ammonium agents. This prevents the battery from self-discharge by stopping bromine species from crossing the cell and being reduced at the negative electrode. However, the bromine-quaternary ammonium complex precipitates as an oily organic phase within the aqueous electrolyte. This precipitation means organic films, detrimental to the battery performance, can form on the electrode and that the electrolyte requires careful management to ensure the reactants are available when required.

The project will investigate the use of new complexing agents to enhance the ability of the positive electrolyte to contain and store the bromine, thus increasing energy density and reducing self-discharge.  What is important here is that the kinetics of discharge of the bromine species must not be compromised.  The nature of the electrode surface species formed during charging, at various current densities, will be examined using spectroscopy and electroanalytical techniques. 

As well as material support, Lotte Chemical Corporation of South Korea will provide a 25 kW/50 kWh Zn-bromine system, together with a 25 kW power conversion system and a Wind DC power conversion unit which will allow the integration of the battery module to a wind turbine test facility.