Supervisors: Dr. Leonard Berlouis, University of Strathclyde and Alan Davidson, Edinburgh Napier University
PhD Student: Declan Bryans, University of Strathclyde
Status: Closed
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 are one such energy storage technology, best suited to medium and large scale stationary applications. Examples of this include; electric energy time shift, the storage of energy when the price is low for sale when the price is high; the reduction of the need to install back-up electricity generation equipment; grid frequency stabilisation; the reduction of peak loads in networks; managing the impacts of distribution level generation and smart technologies. The main research challenges are in maximising the energy density, power output and reliability while maintaining a reasonable system cost.
Our research concerns the zinc-bromine RFB, were 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. The system is configured as in Figure 1. The system is divided into two electrolyte compartments separated by a microporous layer or an ion-exchange membrane. The negative compartment, shown in blue, contains the negative electrolyte, an aqueous solution of zinc ions. The positive compartment, shown in yellow and red, contains the positive electrolyte. In the discharged battery the negative 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 reacting directly with zinc. However, the bromine-quaternary ammonium complex precipitates as an oily organic phase within the aqueous electrolyte (shown in red in Figure 1). 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.