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Making a Better Solid-State Supercapacitor

A $450,000 grant from Office of Naval Research will help a College of Engineering faculty member develop an all-solid-state mediator supercapacitor for naval applications. Dr. Xiangyang Zhou, associate professor in the College’s Department of Mechanical and Aerospace Engineering, will receive the grant over three years, with the aim of transitioning the technology to the U.S. Navy and Marine Corps. through collaborations with industrial partners.

The mediator supercapacitors perform like extremely powerful, high-capacity rechargeable batteries. They can accept and deliver charges faster than batteries, and can tolerate many more charge and discharge cycles than rechargeable batteries. Capable of providing peak power and power conditioning, they have shown the potential to be used as standalone energy-storage devices. They’re used in military power systems, electric vehicles, hybrid electric vehicles, uninterruptible power supplies and mobile devices. Existing commercial supercapacitors are expensive to produce, pose certain dangers and have limited low specific energy at a high specific power.

The goal of Zhou’s work is two-fold: to develop supercapacitors with increased specific energy while maintaining a high specific power and to reduce the dangers associated with current supercapacitors.

Specific Energy

Specific energy is the energy the battery can deliver per mass, and is defined as battery capacity per weight – for example, watt-hours per kilogram. Products that need to run for a long time need to be optimized for high specific energy. For example, a cell phone manufacturer, when sourcing batteries for use in cell-phones, will want to know how much energy can be stored in the battery per battery weight. If you have a cell phone with a 5 gram battery, can that battery be used for 10 hours? 5 hours? Zhou aims to develop a supercapacitor with increased specific energy while keeping a high specific power.

A Safer Supercapacitor

The second obstacle Zhou seeks to overcome is the danger inherent in current supercapacitors. Electrolytes are the most important components of a supercapacitor, and they may be liquid or solid-state. Most current supercapacitors use liquid electrolytes, which are flammable. Recall the Samsung 7 phone whose batteries were igniting; this was the result of the flammability of its liquid electrolytes. In order to resolve the safety issue associated with liquid electrolytes, Zhou’s supercapacitor is completely solid-state.

Balancing the Two

However, with something gained comes something lost. Liquid electrolytes, such as acid and alkaline, outperform solid-state electrolytes in many situations. Generally speaking, a solid-state electrolyte is less conductive than a liquid electrolyte. In addition, the fluidity of a solid-state electrolyte is much lower than that of a liquid electrolyte. Lower conductivity and lower fluidity result in a supercapacitor with specific energy of the supercapacitor.

Zhou and his team face an additional obstacle in designing supercapacitors for naval applications. At lower temperatures, the conductivity of a supercapacitor’s electrolytes decreases. In freezing temperatures, in fact, a supercapacitor may not function at all. And the temperatures in some parts of a submarine can sink very low. In addition, these vessels travel to the north and south poles, so their supercapacitors must function at below-freezing temperatures.

Fortunately, solid-state supercapacitors do have an advantage over non-solid-state supercapacitors in cold temperatures. Liquid electrolytes will become solid at very low temperatures, decreasing their conductivity. In Zhou’s supercapacitors, the electrolyte is already solid, so there is no decrease in conductivity due to change in temperature.

Still, Zhou and his team must find a way to make a safe, high-specific-energy supercapacitor that will function well in low temperatures. They are experimenting with synthesizing new materials, including polymer electrolytes and new mediator materials. When a mediator (an added chemical) is dispersed into a solid-state electrolyte, all of its molecules can be charged because they are surrounded by ions in the solid-state electrolyte. The electrolyte becomes what is known as “mediator-doped polymer electrolyte.”

Zhou and his team have already determined the best polymer electrolyte with high conductivity at room temperature, and are using that as a base. They are working to convert it using a mediator, which changes the property of the polymer electrolyte – in particular, increasing its ionic conductivity. This will help mitigate the loss of conductivity in lower temperatures, and increase the supercapacitor’s specific energy. Zhou expects that the supercapacitor he and his team develop will be used with the mediator in combination with polymer electrolyte to obtain the highest specific energy.

Using Nanostructured Carbon Materials

Zhou is also investigating another way of combining his mediator approach and a nano material approach that his partner at the Naval lab is exploring. Zhou and his team will explore the optimal way to assemble the polymer electrolyte with a fast redox-pair mediator – a mediator made of a pair of oxidizing and reducing agents – into nanostructured carbon-based materials.

Carbon is a desirable material to use in supercapacitors, because it is highly conductive, has a large surface area, is stable in high temperatures and resists corrosion. It also is a low-cost material in this application. The nano material approach has great potential to increase the specific energy. The hope is that, by combining these approaches, Zhou and the Navy can achieve a supercapacitor with the highest specific energy.

Analyzing What Works and Why

Zhou and his team will then examine the solid-state polymer electrolyte, the fast redox-pair mediator and the carbon materials, down to the atomic level. They’ll probe the composition, structure and stability of the materials, as well as components’ interactions at the molecular level. To do so, Zhou and his team will use high-tech scanning and imaging procedures, including x-ray adsorption spectroscopy, x-ray photoelectron spectroscopy, X-ray diffraction and scanning electron microscopy combined with energy dispersive spectroscopy.

Finally, Zhou and his team will fabricate supercapacitor prototypes, with a focus on Navy and Marine Corps. needs and collaboration with industrial partners.

The proposal was titled “Development, Study, and Prototyping of an All-Solid-State Mediator Supercapacitor for Naval Applications.