Silicone Oil Emulsions as Oxygen-Enriched Flow Battery Catholytes for Multifunctional Energy Storage

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 4 times compared to the same robot with just a lithium-ion battery.<br/>Gas transport in aqueous environments is important for many biological and chemical processes; however, the low solubility of gases in water is often performance limiting. When animals require more energy in the form of oxygen, they can breathe in air to extract oxygen from their surroundings. Animal blood contains hemoglobin which allows for high concentrations of oxygen to be dissolved into the bloodstream and then distributed throughout the body to provide power locally to various cells. Combining multifunctional fluidic energy storage with the high energy density of air-rechargeable oxygen reduction chemistry will unlock the potential for ultra-high energy density soft robotic systems.<br/>We present a fluid capable of storing twice as much oxygen as water and demonstrate its use as a catholyte for oxygen reduction in a metal air flow battery. Our catholyte is an emulsion with silicone oil droplets suspended in 0.5M potassium hydroxide (KOH) (1:4 v/v), stabilized by a surfactant (Span-60, 1.0 % w/v). The silicone oil acts like hemoglobin to increase the oxygen solubility of the entire emulsion. The silicone oil droplets have an average diameter of 280 nm and are uniformly dispersed within the aqueous phase. While droplet agglomeration is observed over time, this can be reversed with a simple re-homogenization process. Emulsions can maintain a uniformly dispersed oil phase and remain saturated with oxygen (15 mg/L) for several months.<br/>We can extract energy directly from the oxygen in this fluid with a fully submerged electrode that is not exposed to surrounding air. Our emulsion-based electrolyte shows fast oxygen reduction reaction (ORR) kinetics, demonstrating twice as much diffusion limiting current as a control 0.5M KOH electrolyte in a rotation disk electrode study. Kinetics studies with stationary electrodes showed well-defined oxygen-reduction peaks with five times as much ORR peak current density for an emulsion electrolyte compared to KOH. Once the oxygen in the emulsion has been depleted, emulsions can be re-saturated and thus recharged with an influx of surrounding air.<br/>We demonstrate the electrochemical performance of emulsion air-catholytes in two electrochemical cell configurations, a stationary flow cell and a multifunctional actuator flow cell (MAFC). Both configurations use a zinc anode and an emulsion electrolyte with a platinum carbon cloth catalyst for oxygen reduction. In the flow cell configuration, we achieve 4.6 mW/cm² peak instantaneous power density at 5.6 mA/cm² discharge current density. The MAFC demonstrates the ability for emulsion catholytes to enable multifunctional, flexible power sources for soft robotic systems. This cell uses the emulsion catholyte as a hydraulic working fluid for a McKibben actuator. The MAFC expands and contracts (~20% maximum strain) while energy is being extracted as the cell discharges. Our emulsion catholyte is a fundamental first step toward integrating soft robotic systems with multifunctional, high-energy, air-rechargeable energy storage.<br/> <br/>Reference:<br/>1. C. A. Aubin et al., <i>Nature</i>, <b>571</b>, 51–57 (2019) https://doi.org/10.1038/s41586-019-1313-1.