Flow Battery Driven by a Difference in Solvation Toward the Effective Use of Diverse Forms of Energy
Authors: Matsui Yohei , Maeda Yuki , Kawase Makoto , Suzuki Takahiro , Tsushima Shohji 
Published: September 2023

Further utilization of renewable and waste energy is a crucial factor for the decarbonization of energy systems. We have recently proposed a concept of an electrochemical cell using a difference in solvation [1]. Figure 1 shows a flow battery derived from the concept, called solvation difference flow battery (SDFB). Although the anolyte and catholyte employ the same redox species, the ratio in the binary solvent is different. In this situation, the difference in the solvation states of the redox couple between the anolyte and catholyte generates the electromotive force. As shown in Fig.1, ferro-/ferricyanide ions (Fe(CN)6
4-/3-) are employed as the redox species in both anolyte and catholyte. The electrolyte solvent is composed of dimethyl sulfoxide (DMSO) and water. The anolyte is water-lean and the catholyte is water-rich, respectively. After discharging, the SDFB can be charged by transferring the water from the water-rich side (spent catholyte) to the water-lean side (spent anolyte), which means that the transfer of water regenerates the spent anolyte as the fresh catholyte, and vice versa. It is noteworthy that transferring the solvent can be performed by utilizing diverse forms of energy, such as thermal and mechanical energy as an energy input for the charge.
We previously reported that the combination of DMSO and water allows a large cell voltage [1] owing to the significant difference in the electron acceptability between the solvents [2, 3]. However, the cell performance and its rate-determining processes have not been fully understood. In this work, we investigated the current-voltage (IV) characteristic of the DMSO/water-based SDFB using the prototype flow cell.
For the electrolyte, tetraethyl ammonium salts of Fe(CN)6
4-/3- and tetraethyl ammonium bromide as redox species and a supporting electrolyte, respectively, were dissolved in DMSO and pure water was added to the solution to adjust the molar ratio of DMSO and water. The molar ratios of water in the anolyte and catholyte were kept at 0.1 and 0.8, respectively. The carbon paper electrodes were employed for both anode and cathode of the flow cell. The anion exchange membrane (AEM) was used as a separator. Therefore, bromide ion works as a carrier ion in this flow cell. As a result, the maximum power density of 4.5 W m-2 was observed for the prototype SDFB.
We also conducted Electrochemical impedance spectroscopy (EIS) for the flow cell applying a symmetric electrolyte system to investigate the crucial factor that limits the power density. Both water-lean and water-rich electrolytes were prepared for the EIS to investigate the effect of the solvent composition. Moreover, in addition to the AEM used in the investigation of IV characteristics, porous filter was also used as the flow cell's separator to clarify the total cell resistance factor. The Nyquist plots represented the large cell resistance when the AEM and water-lean electrolyte was applied. On the other hand, the cell resistance was small when the porous filter was utilized as the separator, regardless of the molar ratio of water in the electrolyte. The Nyquist plots also indicated that the reaction resistance was relatively small.
Therefore, these results indicate that the large AEM resistance influences the total cell resistance in the water-lean electrolyte (anolyte). These results imply that it should be a practical approach to develop an alternative SDFB electrolyte that can avoid the significant AEM resistance for increasing the power density. From the viewpoint of the cell assembly, the exploration of alternative separators that exhibits both high ion conductivity in the water-lean electrolyte and low permeability of solvent molecules should drastically improve the power density of the DMSO/water-based SDFB.
References
[1] Matsui, Y., et al., Sci Rep 12, 3739 (2022).
[2] Mayer, U., et al. Monatsh. Chem. 106, 1235–1257 (1975).
[3] Gritzner, G., et al., J. Electroanal. Chem. Interfacial Electrochem. 72, 177–185 (1976).
Figure 1 Schematic of the charge-discharge cycle of the DMSO/water-based SDFB

Figure 1