A Research On The Performance Of All-iron Redox Flow Battery

As a new technology able to store large quantities of energy, redox flow batteries provide some advantages, such as fast response time, large energy capacity, high overall energy efficiency and safety. The key parts of redox flow batteries are the active material stored in the electrolyte and the electrochemical cell itself. Both the oxidised and reduced species must be soluble with their redox potential as far apart as possible and the electrode reactions must be reversible. Electrolytes must be chemically stable and easy to be prepared at high concentrations and the cost of electrolytes must be reasonable.The standard electrode potential of Fe(Ⅲ)/Fe(II) is0.77V. The ligands are capable of shifting the formal potential of Fe(Ⅲ)/Fe(II) and increasing the reversibility. An all-iron redox flow cell therefore appears feasible in which the ligands with different formal potentials would be used in the electrolytes and avoids cross-contamination.The comparison of cyclic voltammograms recorded in Fe2(SO4)3, Fe(Ⅲ)-TEA and Fe(phen)3SO4solution at a graphite electrode and a graphite felt electrode respectively demonstrates that using TEA and phen to complex with the Fe(Ⅲ)/Fe(Ⅱ) couple results in significant shifts in the redox potential to-1.05V and0.83V respectively. And the electrode processes for Fe(Ⅲ)-TEA and Fe(Ⅱ)-phen are close to reversible reaction with fast kinetics. These results indicate that Fe(Ⅲ)-TEA and Fe(Ⅱ)-phen could be used in redox flow battery.In addition, the comparison of cyclic voltammograms for Fe(phen)3SO4solution with different pH values indicates that the Fe(phen)3SO4solution with pH value at about0.1exists higher reversibility.From the charge-discharge tests, it is demonstrated that all-iron redox flow battery is technically feasible. The open circuit cell voltage is around1.9V for Fe(phen)3SO4/Fe(Ⅲ)-TEA system and1.6V for FeSO4/Fe(Ⅲ)-TEA system, which indicates that the all-iron redox flow battery offers the possibility of higher cell power. The efficiency and discharge voltage of the cell with Fe(Ⅱ)-phen/Fe(Ⅲ)-TEA is higher than the cell with FeSO4/Fe(Ⅲ)-TEA.Due to the advantage of Fe(phen)3SO4/Fe(Ⅲ)-TEA system, it is necessary to do some further work on it. It is demonstrated that long storage time and Fe(Ⅲ)-phen complex generated during the charging process would make the equilibration move to the Fe(phen)3SO4direction and increase the energy efficiency from28.8%to44.8%. And the Fe(phen)3SO4/Fe(III)-TEA system applying Nafion117is better than that applying Nafion112in discharge capacity, energy output, current efficiency and energy efficiency. In addition, results obtained from a comparison of the charge-discharge performance of the Fe(phen)3SO4/Fe(III)-TEA system employing a variety of currents demonstrate that charging with high current such as30mA and discharging with low current such as10mA produce a little effect on the efficiency and discharge voltage but shorten the duration of charging. So this method can be applied for the quick charge in the redox flow battery.