Development of alternative energy sources has focused on two different types of energy sources that can be used to power devices such as robots and automobiles. Batteries have emerged as the leading energy storage device while energy harvesters such as solar and wind have been utilized through devices such as solar cells and wind turbines.

This column provides updates on new research in both sectors. For example, a previous TLT article¹ described the development of an aqueous lithium-ion battery that can overcome problems that have been encountered with the organic electrolyte used in conventional lithium-ion batteries. This organic electrolyte has been found to be the source of flammability, moisture sensitivity and toxicity.

One of the challenges in improving the effectiveness of wind turbines is to identify the proper location where farms should be situated. A previous TLT article² discussed a modeling study conducted to determine where wind farms should be placed geographically to maximize electricity generation. Specific locations were evaluated based on wind speeds found at specific dates in the past.

James Pikul, assistant professor in the department of mechanical engineering and applied mechanics at the University of Pennsylvania in Philadelphia, says, “The primary approach in advancing battery technologies has been the development of new material chemistries and architecture. These have made it possible to commercialize electric vehicles, which would not have been possible 10 years ago. But batteries do not store enough energy to be the power source for newly designed robots. Modern robots only last tens of minutes on battery power, and adding more batteries just adds weight and makes it harder for the robot to move, in effect reducing its efficiency. Another issue is that batteries are limited because they cannot be recharged when used in remote areas.”

Pikul comments on energy harvesters, “The various harvester options available can all provide large amounts of energy but are limited by only being able to provide power under specific situations, such as needing sunlight in the case of solar harvesters and needing wind to drive turbines. Another limitation is that energy harvesters can only provide low power densities below 10 milliwatts per square centimeter. This places them several orders of magnitude lower than a battery.”

An ideal compromise between these two energy sources is to develop a device that can act as a battery without the added weight and also extract energy from its environment without limitation. Such a device has now been produced and evaluated.

Metal-air scavenger
Pikul and his colleagues have developed a device known as a metal-air scavenger (MAS) that combines the positive characteristics of batteries and energy harvesters. The device generates energy by harvesting metal present in the environment.

Pikul says, “Our approach was to take advantage of the energy present in chemical bonds. In particular, metals are a source that can generate higher energy than natural sugars (such as glucose) or hydrocarbons (petroleum).”

Figure 1 shows a MAS powering an electric vehicle, which was used to demonstrate this new technology. The MAS had an anode, cathode and electrolyte like a battery. The cathode was a polytetrafluoroethylene coated platinum/carbon cloth connected to a hydrogel electrolyte that was prepared with either three-millimeter-thick polyacrylamide or poly(vinyl alcohol). These polymers were saturated with either six molar potassium hydroxide or one molar hydrochloric acid aqueous solutions.


Figure 1. A MAS that combines the benefits of a battery, and an energy harvester powers an electric vehicle by extracting energy from metal in the environment. Figure courtesy of the University of Pennsylvania.

Pikul says, “The metal surface is used as the anode, which completes the battery circuit by making electrical contact with the electric vehicle.”

As the electric vehicle moves across a metal surface, the metal is oxidized at the anode releasing electrons that produce hydroxide ions at the cathode. The hydroxide ions react with metal at the anode to form a metal oxide byproduct. Pikul indicates that the net effect is the MAS generates electricity by taking advantage of the Gibbs free energy difference between the metal surface and oxygen through the two electrochemical redox reactions occurring at the electrodes.

Pikul says, “The MAS extracts energy from the top 100-300 microns of metal on the surface. The amount extracted depends on the metal and is not likely to negatively affect the integrity of the metal structure.”

The researchers measured the electrochemical performance of a MAS on aluminum and pure zinc sheets with alkaline hydrogel electrolytes and on stainless steel with acidic hydrogels. The alloys used were 6601 Aluminum and 316 Stainless Steel.

In testing, the MAS extracted 94% and 79% of the potential energy capacities on clean aluminum and zinc. The researchers estimate that the energy density generated by the MAS was 13 times that of lithium-ion batteries, and the energy was extracted at 10 times higher power density than that of the best energy harvesters. The energy densities of MAS and metal-air batteries were compared through the use of a Ragone plot, and the former was found to exhibit twice the energy density of the latter.

The researchers found that the two different hydrogel materials caused differences in performance. Pikul says, “Surface morphology changes as the MAS moves across the metal, leading to changes in roughness. In testing done with a vehicle moving in a circular path across an aluminum surface, we found that the speed increased after the first lap due to improved electrochemical kinetics even though the coefficient of friction likely increased as the aluminum oxide layer was created.”

One factor that has to be better understood is how much water the MAS needs to carry when moving across metal surfaces. Pikul says, “With some metals such as zinc, no net water loss occurs, but with aluminum, water needs to be added to enable the electrochemical process to continue. We need to better understand this issue along with looking to upgrade MAS performance by evaluating new electrolytes.”

Additional information can be found in a recent article³ or by contacting Pikul at pikul@seas.upenn.edu.

REFERENCES
1. Canter, N. (2020), “Aqueous lithium-ion battery,” TLT, 76 (3), pp. 12-13.
2. Canter, N. (2020), “Where to locate wind farms?” TLT, 76 (2), pp. 16-17.
3. Wang, M., Joshi, U. and Pikul, J. (2020), “Powering Electrons by Scavenging Energy from External Metals,” ACS Energy Letters, 5 (3), pp. 758-765.