Energy Storage for Robotics

Modern robots lack the multifunctional, interconnected systems found in living organisms and, consequently, exhibit reduced efficiency and autonomy. Energy storage systems are among the most visible limitations to robot autonomy, but the basic design of battery cells has undergone relatively few changes since the late 1800’s, despite the dramatic advances in chemistry and material processing. In addition, emerging energy storage applications are placing increased demands on the mechanical, thermal, and chemical properties of batteries, as well as requiring improved energy storage capabilities. We seek to create new classes of energy storage devices with a focus towards robotics applications by realizing new designs that take advantage of modern robotic capabilities and increased autonomy.

Powering electronics by scavenging energy from external metals

In this work, we show that semi-solid hydrogel electrolytes with oxygen reduction cathodes, a device we call a metal-air scavenger (MAS), can electrochemically extract energy from external metals to achieve high energy and power density, combining the benefits of batteries and energy harvesters. The MAS facilitates electrochemical reactions that occur in metal-air batteries, but the anode and cathode are both external to the MAS. We show that, when stationary, a MAS can extract 159 mAh/cm2, 87 mAh/cm2, and 179 mAh/cm2 from aluminum, zinc, and stainless-steel surfaces at up to 130, 81, and 25 mW/cm2 power densities. A principal advantage of the MAS is that it can continue to power a device as it travels across a metal surface, so that the total mass and volume of metal oxidized are many multiples of the mass and volume of the device. This property of the MAS breaks energy storage scaling laws by allowing small robots and electronics to extract energy from large volumes of energy dense material without having to carry the material on-board. Finally, we demonstrate the utility of a traveling MAS by powering a 5 x 3 x 1 cm electric vehicle on an aluminum surface with a 2 x 3 x 0.2 cm MAS. When storing excess electrolyte, a traveling MAS can achieve 3,082 Wh/kgMAS energy densities on aluminum surfaces, which is more than 2 times larger than the best aluminum-air batteries fabricated to date, and 12 times greater than commercial lithium-ion batteries (243 Wh/kg).

Computer-free autonomous decision making based on environmental cues would provide exciting alternatives to classic control systems for robots and smart materials. Although this functionality has been studied in microswimmers and active colloids where energy in the surrounding liquid is prevalent, there are no devices that can provide sufficient power from environmental chemicals to move and steer larger scale robots and vehicles in dry environments. This work overcomes this limitation with an Environmentally Controlled Voltage Source (ECVS, similar to the MAS above) that, when directly attached to electric motors on a vehicle, can increase the energy available to the vehicle and provide computer-free autonomous navigation towards chemical fuels in the environment and away from hazards. The ECVS uses electrochemistry to extract power from the chemical fuels and the vehicle avoids hazards that reduce the output voltage or electrochemical kinetics. Two ECVSs can also be arranged in series or parallel to perform logical functions based on the chemicals in contact with the ECVSs. This work presents a new method to simultaneously steer and power vehicles and robots without computers by directly responding to a wide variety of chemical fields in their environment using electrochemistry.

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Figure 1. Autonomous navigation. (A) An illustration of an ant that avoids hazards and follows food to gain energy. The photo is modified from “Jerdon’s jumping ant with prey” by vipin baliga, licensed under CC BY-NC-SA 2.0. (B) A schematic of a synthetic analog consisting of a vehicle that navigates along a metal fuel source while avoiding hazards. (C) Sequential images of a reactive agent vehicle following a metal fuel path without computers.

Synthetic multifunctional circulatory systems

In this project, we presented a synthetic, energy-dense circulatory system embedded in an untethered, aquatic soft robot. Modeled after redox flow batteries, this vascular system combines the functions of hydraulic force transmission, actuation, and energy storage into a single integrated design that geometrically increases the energy density of the robot to enable long duration operation. The fabrication techniques and compliant materials used in its construction allow the vascular system to be shaped into complex form factors that continuously deform with robot movement. The complete robotic fish has a system energy density of 53 J g-1, a 4X gain over the same fish with only lithium ion batteries, and can swim for long durations (max theoretical operating time = 36.7 hours) at 1.56 body lengths per minute, up stream. This use of electrochemical energy storage in hydraulic fluids could facilitate increased energy density, autonomy, efficiency, and multifunctionality in future robot designs.

Figure 2: A lionfish inspired robot powered by a multifunctional zinc-iodide redox flow battery. (A) Renderings of the robot with the liquid catholyte in the tail fin (red) and dorsal/pectoral fins (yellow) highlighted. (B) Schematic of the zinc-iodide redox flow battery. (C) The assembled robot swimming underwater via tail fin actuation.

Figure 3: Synthetic vascular system schematic and swimming demonstration. (A) A block diagram showing the configuration of the pumping, control, and electronics components of the robot’s vascular system. (B) One half of the disassembled robot, showing how the pumps and control hardware are housed internally. (C) A peristaltic pump, configured to resemble a heart, transporting catholyte from the dorsal fins to the pectoral fins. (D) Untethered swimming demonstration in a salt water tank.


[3] Min Wang, Yue Gao, James H. Pikul, “Computer‐Free Autonomous Navigation and Power Generation Using Electro‐Chemotaxis.” Adv. Intell. Syst. 2000255, 2021.

[2] Min Wang, Unnati Joshi, James H. Pikul, “Powering electronics by scavenging energy from external metals” ACS Energy Letters, vol. 5, no.3, pp. 758-765, 2020.

[1] Cameron A. Aubin, Snehashis Choudhury, Rhiannon Jerch, Lynden A. Archer, James H. Pikul, Robert F. Shepherd, “Electrolytic Vascular Systems for Energy Dense Robots”, Nature, 1, June, 2019.