Powerful Energy Storage with Micro-supercapacitors

People have been trying to use supercapacitors to supplement batteries. Although capacitors do store energy, unlike batteries they charge up fast and discharge the energy stored quickly, such as in a camera flash. Common lithium-ion batteries take time to charge up and discharge according to requirement of the load. However, technology is catching up fast and new type of micro-supercapacitors is rivaling commercial supercapacitors in terms of storage capacity and power delivery similar to batteries.

At the Rice University, a team of researchers has developed a solid-state micro-supercapacitor. Although not a battery, this new device stores energy just as commercial supercapacitors can and it releases its stored energy just as a battery does. The specialty of the micro-supercapacitor developed by the researchers is it charges more than 50 times faster than batteries and discharges more slowly than traditional capacitors do.

The manufacturing process for these micro-supercapacitors allows them to be produced in a cost-effective and roll-to-roll method. The researchers used commercial lasers to burn electrode patterns in plastic sheets at room temperatures to form the basic structure of the supercapacitors. Manufacturing commercial supercapacitors involves several lithographic steps, making the process time-consuming and expensive. It also limits the widespread application of supercapacitors.

The new technique makes micro-supercapacitors in minutes, including burning the pattern, adding the electrolytes and packaging the devices. Since all this is done at room temperature, the fabrication process is simple, speedy, and cost-effective. According to the researchers, these micro-supercapacitors offer energy densities rivaling those offered by commercial thin-film batteries, while providing power densities nearly two times in magnitude. Additionally, they outlasted the batteries in terms of life and mechanical stability.

The energy density of micro-supercapacitors comes from laser-induced graphene or LIG. When the research group heated a commercial polyimide plastic sheet with a laser, they found it burnt everything. Only a top layer of carbon, in the form of graphene, was left over. However, this leftover layer was not a flat sheet of hexagonal rings of atoms, but a spongy array of graphene flakes that were attached to the polyimide. The LIG patterns etched into the plastic look like folded hands and the graphene has a huge surface area.

The researchers achieved capacitances of 934 mF per square centimeter, with energy density of 3.2 mW per cubic centimeter. This is at least twice that offered by commercial thin-film lithium batteries. In addition, the devices showed high resilience and mechanical stability even when repeatedly bent more than 10,000 times.

The researchers at Rice used electrodeposition to treat the LIG pattern of spongy graphene with manganese dioxide and ferric oxyhydroxide to turn the resulting composites into positive and negative electrodes. Forming the composites into solid-state micro-supercapacitors did not involve separators, binders, or current collectors. The entire process takes just minutes from burning the patterns, adding the electrolytes, and covering the capacitors.

The manufacturing process developed at the Rice University has great potential for bulk production of small and flexible micro-supercapacitors at room temperatures. The researchers are convinced that such plastic micro-supercapacitors will replace batteries entirely in the future.