Studies on iron (Fe3+/Fe2+)-complex/bromine (Br-2/Br-) redox flow cell in sodium acetate solution

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/Br-2 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/Br-2 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 Br-2/iron-complex cells are technically feasible in redox flow batteries but need further investigation. (c) 2006 The Electrochemical Society