Optimization of Eutectic Electrolytes for Enhanced Redox Flow Battery Performance

The drive for energy independence and reduction of carbon footprints has propelled the shift from fossil fuels to renewable energy sources like wind and photovoltaic technologies. However, the intermittent nature of these energy sources poses significant challenges for their integration into the electrical grid. To address this, energy storage systems such as redox flow batteries (RFBs) have emerged as promising solutions due to their modular design, scalability, and flexible operation. RFBs, particularly those employing aqueous electrolytes, have been widely studied, with vanadium-based systems leading in maturity and commercialization. However, the narrow electrochemical windows and low solubility of vanadium species limit their energy and power densities. Organic non-aqueous electrolytes offer wider electrochemical windows and tunable solvation capacities compared to aqueous electrolytes. However, issues such as poor conductivity, high volatility, flammability, and toxicity have hindered their widespread adoption. Ionic liquids present an alternative with large electrochemical windows and reduced flammability but suffer from high viscosity, low ionic conductivity, and high preparation costs. Deep eutectic solvents (DESs) have emerged as attractive, environmentally benign, and economical alternatives. DESs, typically formed from hydrogen bond acceptors (HBAs) like quaternary ammonium salts and hydrogen bond donors (HBDs) such as alcohols and amides, exhibit high salt concentrations, suppressed volatility, wide electrochemical stability, and high solvent strength. Identifying optimal DES compositions for RFBs requires extensive screening due to the vast number of potential HBA, HBD, and co-solvent combinations. Traditional experimental approaches are time-consuming and labor-intensive. High throughput experimental methods facilitate rapid screening of multiple compositions, enabling the identification of promising electrolytes with desirable properties. To this end, new study published in Electrochimica Acta and conducted by Dr. William Dean, Dr. Miguel Muñoz and Professor Burcu Gurkan from the Case Western Reserve University alongside Dr. Juran Noh, Dr. Yangang Liang, Dr. Wei Wang from the Pacific Northwest National Laboratory developed an HTE approach to assess eutectic solvents based on choline chloride (ChCl) as the HBA and ethylene glycol (EG) and aniline (AN) as HBDs. Co-solvents like water, acetonitrile (ACN), and dimethyl sulfoxide (DMSO) were evaluated for their ability to tune the viscosity, conductivity, and solubility of redox-active species such as methyl viologen dichloride hydrate (MV) and 2,1,3-benzothiadiazole (BTZ).

The authors prepared eutectic solvents by mixing ChCl with respective HBDs in specific molar ratios. Co-solvents were added to these mixtures, and the resulting solutions were characterized for their viscosity, conductivity, electrochemical stability, solubility, and diffusivity of redox-active species. Viscosity measurements were performed using a high-throughput viscometer, while conductivity was assessed through impedance spectroscopy. Electrochemical properties were evaluated using cyclic voltammetry, and solubility limits were determined via quantitative 1H NMR. They found that the inclusion of co-solvents generally reduced the viscosity of the eutectic mixtures. ACN was the most effective in lowering viscosity, followed by DMSO and water. They also showed the conductivity measurements that the addition of co-solvents improved the ionic conductivity, with the extent of improvement dependent on the specific HBA-HBD-co-solvent combination. For instance, the ChCl:EG mixtures with ACN exhibited higher conductivities than those with water or DMSO.

Moreover, the solubility of MV and BTZ in the eutectic mixtures varied with the type and amount of co-solvent. Water addition resulted in the highest MV solubility, particularly in ChCl:AN:H2O systems. Conversely, BTZ exhibited higher solubility in ChCl:AN systems, with slight improvements observed with ACN and DMSO co-solvents. The solubility trends were consistent with the polarity of the co-solvents, with more polar solvents like water enhancing the solubility of ionic species like MV. According to the authors, the electrochemical stability window of the eutectic mixtures was influenced by the choice of HBD and co-solvent. Mixtures with AN as the HBD generally exhibited wider ESWs compared to those with EG, due to the stronger hydrogen bonding and intermolecular interactions involving AN. The addition of co-solvents like DMSO further enhanced the ESW, particularly in ChCl:EG mixtures. In contrast, water and ACN additions tended to reduce the ESW.  Diffusion coefficients of MV in the eutectic mixtures were determined using steady-state microelectrode measurements. The results indicated that the diffusion coefficients were generally lower in eutectic mixtures compared to aqueous solutions, reflecting the higher viscosities and stronger hydrogen bonding networks in DESs. Among the co-solvents, ACN significantly enhanced the diffusivity of MV, consistent with its effectiveness in reducing viscosity.  The redox potentials of MV and BTZ were evaluated in the eutectic mixtures. MV exhibited two distinct redox processes, with half-wave potentials influenced by the co-solvent and HBD composition. The addition of water led to cathodic shifts in the redox potentials, while ACN and DMSO caused more anodic shifts. The redox behavior of BTZ was found to be irreversible in all studied systems, highlighting the challenges in achieving stable redox processes for this compound in eutectic mixtures.

In conclusion, the study by Professor Burcu Gurkan and colleagues demonstrated the potential of high throughput experimental screening for identifying optimal eutectic electrolytes with co-solvents for redox flow batteries. The findings revealed that the choice of HBD and co-solvent significantly influences the viscosity, conductivity, solubility, electrochemical stability, and redox behavior of the eutectic mixtures. Among the studied systems, ChCl:EG:ACN mixtures emerged as promising candidates for RFBs utilizing the MV redox couple, offering a balance of high conductivity, low viscosity, and desirable electrochemical properties. The insights gained from this study provide a foundation for the rational design of next-generation electrolytes for energy storage applications.