Autonomy in robotic systems is limited by the lack of high energy density energy storage technologies. At the current rate of battery energy density increase, \~7% per year, it will take eleven years before robots can operate for twice as long. Overcoming this slow rate of energy density increase requires new battery architectures and multi-functional materials that vary from the classic two electrode layered design used in all current batteries.

Biology provides multiple compelling examples of multifunctional interconnected systems that work together to increase system-level performance. Animal circulatory systems, for example, use blood to transport oxygen and nutrients while simultaneously regulating temperature, removing waste, and helping to fight infection. Recent work has shown that synthetic electrolytic vascular systems could increase the energy density of a soft robot by simultaneously acting as an actuator and energy storage fluid. The vascular system had a two-electrolyte flow-cell configuration which, like all engineered electrochemical energy storage systems to date, separated the anode and cathode materials.

One of the most effective ways to increase the energy density of a battery is to use oxygen in the air as the cathode. This approach works because the oxygen does not need to be carried by the robot. Animals use the same approach, where lungs saturate blood with oxygen so that it can be delivered to cells for oxygen reduction during cellular respiration.

In this work, we realize an electrolyte that can increase the energy density of synthetic vascular systems by using artificial hemoglobin to store high oxygen concentrations for oxygen cathodes. Perfluorocarbon emulsions were used as artificial hemoglobin to increase the dissolved oxygen concentration in the aqueous electrolyte. To create perfluorochemical emulsions, perfluorooctane was added to water in a ratio of 1:4 (v/v). Perfluorooctanic acid (0.8 % w/v) was used as a surfactant to stabilize the emulsions. The mixture was stirred vigorously at 600 rpm for two hours until a homogenous solution resulted. An air inlet was then connected to the solution and the mixture was purged with air for an additional hour while stirring continued. For ionic conductivity, 0.1 M NaCl was then added to the solution. Emulsion images indicate that the average droplet size was 250 micrometers.

Perfluorocarbons are effective for delivering oxygen because they oversaturated water. For example, the equilibrium saturation for oxygen in salt water at 21°C is 7.16 mg/L, while our emulsion achieved a dissolved oxygen concentration of 9.66 mg/L. An identical electrolyte without the addition of perfluorochemicals was only able to reach 8.16 mg/L of dissolved oxygen after being purged with air overnight.

Cyclic voltammograms of the two electrolytes demonstrated that increased oxygen content is critical for oxygen reduction to occur in a system with fully submerged electrodes. For both samples, cyclic voltammogram scans were recorded with a fully submerged platinum-carbon working electrode at a scan rate of 5 mV/s. In the aqueous system without the perfluorochemical emulsions no peaks emerged, signifying no reaction. For the sample with perfluorocarbon emulsions, the reduction peak current density occurred at 1.35 V vs. SHE which is indicative of the oxygen reduction reaction in acidic media.

This work demonstrates that the high oxygen solubility of perfluorocarbon emulsions can enable oxygen reduction in a submerged electrode. This is an important step towards achieving synthetic vascular systems with increased energy density.


Perfluorocarbon Emulsions for High Energy Density Synthetic Vascular Systems

Division / Sponsors: 
Aerospace Division

Paper Proceedings: https://asmedigitalcollection.asme.org/SMASIS/SMASIS2020/volume/84027

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