A Strategy for Enhancement of Power Density in Fe-Based Solvation Difference Flow Battery by Using Solvent that Generates a Large Potential Shift of Ferrocyanide/Ferricyanide.

Effective utilization of thermal energy is attracting attention for the realization of a carbon-neutral society. To convert thermal energy into electrical energy, we recently proposed a solvation difference flow battery (SDFB), a new type of thermally regenerative flow battery, whose electrolyte can be regenerated using thermal energy. The power density of SDFB, however, is relatively low compared with that of the other thermally regenerative flow batteries, and the strategy of the electrolyte design for SDFB to improve the power density has not been clear so far. In this study, we focused on the redox potential of ferrocyanide/ferricyanide in six different solvents (water, ethanol, acetone, 1-butanol, acetonitrile, and dimethyl sulfoxide (DMSO)), where the largest potential difference was recorded between water and DMSO. Density functional theory (DFT) calculations indicated that DMSO had a stronger interaction with ferricyanide than with ferrocyanide, unlike the other solvents, which is the origin of a large potential shift in DMSO. The cell voltage of the water-DMSO SDFB was 1.0 V, which is approximately five times higher than that of the water-acetone SDFB we reported previously. We also investigated the electrochemical behavior of the electrolyte of water-DMSO SDFB and revealed that the electrochemical reaction is quasi-reversible, but the solution and separator resistance were high due to the high viscosity and low conductivity of the water-DMSO solution. As a strategy to decrease the high resistance, we discovered that a porous membrane as the separator was suitable for water-DMSO SDFB, and a maximum power density of 92 W m-2 was achieved by decreasing the separator resistance. The maximum power density of the water-DMSO SDFB is more than twice that of the water-acetone SDFB, which we previously reported. In addition, the power density of the water-DMSO SDFB is moderate or high compared to that of the conventional thermally regenerative batteries. This work succeeded in the enhancement of the power density of the SDFB and provides effective insight into the electrolyte design for the improvement of the performance of thermally regenerative flow batteries, including the SDFB.