Silicone Oil Emulsions As Oxygen Enriched Flow Battery Catholytes That Enable Fully Submerged Air Cathodes

Abstract

In biology, multifunctional interconnected systems provide increased system-level efficiency. Animal circulatory systems, for example, transport oxygen and nutrients while simultaneously regulating temperature and maintaining homeostasis. Inspired by biology, recent work has shown how multifunctional fluids can increase the total energy density of a robotic fish by both storing electrochemical energy and transmitting mechanical work to hydraulic actuators1. Despite the low energy density of the zinc-iodide electrolytes, this multifunctional approach increased the robot energy density by 4X compared to the same robot with just a lithium-ion battery. New low viscosity fluids that can store large amounts of electrochemical energy would enable even further improvements in energy density and transform the way robots and vehicles are designed.We have created a liquid electrolyte capable of storing large amounts of dissolved oxygen that can be directly reduced with a fully submerged electrode (See Fig. 1A, 1D). This capability mimics biological circulatory systems that store oxygen in hemoglobin suspended in blood, and also takes advantage of the high energy density of redox pairs that reduce oxygen in the cathode, such as metal-air batteries. Our electrolyte is a silicone oil emulsion suspended in 0.5 M KOH (1:4 v/v with Span-60 (sorbitane monostearate) (1.0 % w/v) as a surfactant). Here, the high oxygen solubility of the silicone oil droplets (up to 2247 mg O2/L) acts like hemoglobin and increases the oxygen solubility of the entire emulsion to 30 mg O2/L compared to 6.2 mg O2/L for 0.5 M KOH. The approach of storing oxygen in emulsions was inspired by synthetic blood that can support the respiration needs of animals in emergencies. These emulsions can remain stable for several weeks and, once saturated, can maintain their high dissolved oxygen levels.The heterogeneous nature of this catholyte is essential for electrochemical energy conversion. Within the electrolytic emulsion, droplets of silicone oil are dispersed throughout aqueous potassium hydroxide. The oil phase provides oxygen for the oxygen reduction reaction while the aqueous phase provides ionic conductivity. Thus, oxygen reduction occurs at the three-phase interface between the platinum-carbon electrode, the silicone oil droplets, and the aqueous potassium hydroxide (see Fig. 1C). Importantly, all three interfaces are either solid or liquid.Cyclic voltammograms (CV) comparing the performance of electrolytes with and without silicone oils demonstrated that oxygen can be electrochemically reduced in both systems. For both samples, cyclic voltammogram scans were recorded with a fully submerged platinum-carbon working electrode at a scan rate of 5 mV/s (see Fig. 1E). The aqueous system without the silicone oil emulsions, showed weaker CV peaks and more capacitive current (see Fig. 1D). The sample with the silicone oil emulsions demonstrated a more prominent reduction peak at 0.056 V vs. SHE which is indicative of the oxygen reduction reaction in basic media (see Fig. 1D).Both electrolytes were used in a full cell set-up to demonstrate the discharge performance of a zinc-air flow battery with an underwater air cathode. While both cells had a starting open circuit voltage of ~1V, the cell without silicone oil emulsions showed a steep drop in voltage to below 0.3V after only 43 minutes. The cell with silicone oil emulsions demonstrated significantly higher discharge capacity, discharging for over 10 hours at 50 µA (0.0625 mA/cm2). The increased oxygen content in the emulsion-based catholyte is critical for superior discharge performance.This work presents a new way for storing and transporting oxygen in pumpable electrolytes. The high oxygen solubility of silicone oil emulsions allows high-rate ORR over longer durations than control electrolytes. This electrolyte is a promising candidate for multifunctional power systems and presents new design opportunities for flow batteries by removing the need for the challenging gas-liquid-solid interfaces in conventional ORR cathodes.Reference: C. A. Aubin et al., Nature, 571, 51–57 (2019) https://doi.org/10.1038/s41586-019-1313-1. Figure 1