Studies on Iron (Fe[sup 3+]∕Fe[sup 2+])-Complex/Bromine (Br[sub 2]∕Br[sup −]) Redox Flow Cell in Sodium Acetate Solution

Abstract

The formal potential of the Fe(III)Fe(II) couple shifts markedly in the negative direction by complexation with ethylenediamine tetraacetate (EDTA), oxalate, and citrate. The potentials of the complexes with EDTA and oxalate are less pH-dependent than with citrate. But, the relatively high pH of around 6.0 is favorable electrochemically due to high corresponding currents. Complexation of Fe(III)Fe(II) couple can provide fast electrode kinetics except for the complex with citrate. But, the solubility of the complex with citrate is up to 0.8 M. Charge-discharge measurements were conducted with the iron-complex/ Br2 redox cells. The results show that performance of the cells with 0.1 M Fe(III)Fe(II) -oxalate or Fe(III)Fe(II) -citrate is relatively poor due to slow kinetics for the Fe(III)Fe(II) -citrate and the unstability of the ferric form for the Fe(III)Fe(II) -oxalate, whereas performance of the iron-citrate/ Br2 cell is improved considerably by increasing concentration of the Fe(III) -citrate complex. Also, energy efficiencies of up to approximately 80 and 70% could be obtained for the cell with 0.1 M Fe(III)Fe(II) -EDTA and 0.8 M Fe(III)Fe(II) -citrate, respectively. The preliminary study shows that novel Br2 /iron-complex cells are technically feasible in redox flow batteries but need further investigation. © 2006 The Electrochemical Society. All rights reserved.