Redox-flow batteries, which are using organic compounds, aqueous solution, and new metal complexes, have been actively investigated. Non -metal redox-flow batteries are strongly expected by the increasing in needs for low-cost, safe, and trusted energy storage applications. Both of redox active materials and cell systems of redox-flow batteries have made rapid and great progress in recent years. Redox-flow batteries are investigated frequently, and are now in a transition stage.

 Recently, redox-flow batteries using organic compounds and metal complexes (or inorganic materials) as active materials were investigated. In such cases, redox potentials and efficiencies of electron transfer of each active materials on the electrode surface will be quite important as with the stability of the active materials. If we can control the redox potentials of organic/inorganic active materials flexibly, we will be able to receive suitable systems.  

   The electrode modified with self-assembled monolayers having electrochemical active sites have been often adopted as mediator for controlling the redox reaction of other electro active species in the electrolyte solution. In our previous studies, ferrocene derivatives attached on the gold electrode worked as suitable mediator for the reduction of Fe(III) ion in solution (1). On the other hand, same ferrocene derivatives on the electrode did not work as the mediator of the K₄Fe(CN)₆ ion at all. Although redox potentials of both Fe(III) and K₄Fe(CN)₆ were ca. +0.4 V (vs. SCE) in the acidic solution (1M HClO₄), ferrocene attached electrode only mediated for Fe(III) oxidation. Electrochemical properties of quinone/ hydroquinone modified electrodes were also reported previously (2) (3). In the case of quinone-modified electrode, the redox potential of quinone species was controlled stably by changing solution pH. We newly investigate suitable mediator attached on the electrode /substrate surfaces for constructing all organic-aqueous redox-flow battery.  

References

(1) Y. Sato, H. Itoigawa and K. Uosaki, "Unidirectional Electron Transfer at Self-Assembled Monolayers of 11-Ferrocenyl-1-undecanethiol on Gold", Bull. Chem. Soc. Jpn., 66, 1032 - 1037 (1993).

(2) Y. Sato, M. Fujita, F. Mizutani, and K. Uosaki, "Electrochemical Properties of 2-Mercaptohydroquinone Monolayer on Gold Electrode. Effect of Solution pH, Adsorption Time and Concentration of Modifying Solution", J. Electroanal. Chem., 409, 145 -154 (1996).

(3) S. Ye, A. Yashiro, Y. Sato, and K. Uosaki, “Electrochemical in situ FT-IRRAS Studies of Self-Assembled Monolayer of 2-(11- Mercaptoundecyl)hydroquinone”, J. Chem. Soc., Faraday Transaction, 92, 3813 - 3821 (1996).