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Graphene Electrodes for Hybrids

Atom-thick sheets of carbon make high-energy capacitors.
February 22, 2011

Source: “Graphene-Based Super­capacitor with an Ultrahigh Energy Density”
Bor Z. Jang et al.
Nano Letters 10: 4863-4868

Crumpled sheets: Ordinarily flat pieces of graphene have been coaxed to fold in on themselves to hold more charge.

Results: Using graphene, a form of carbon made of sheets just a single atom thick, researchers have built ultracapacitor electrodes that can store nearly as much energy as the electrodes now used in batteries for hybrid vehicles. The electrodes stored 86 watt-hours per kilogram. That would translate to 21 to 43 watt-hours per kilogram in a complete ultracapacitor, which would weigh more than just the electrode. Nickel-metal hydride batteries for hybrids store between 40 and 100 watt-hours per kilogram.

Why it matters: Automakers typically oversize the batteries in hybrids to make up for the loss of energy storage capacity over time as well as the fact that batteries can’t be discharged completely without damaging them. Conventional ultracapacitors don’t have this problem—they can be charged and discharged tens of thousands of times without losing much storage capacity—but they store just 5 to 10 percent as much energy as nickel-metal hydride batteries. The new high-energy electrodes could allow ultracapacitors to compete with nickel-metal hydride batteries. Although they still store less energy overall, more of their capacity can be used. And since they don’t have to be oversized to compensate for the loss of capacity over time, they could be cheaper than batteries.

Methods: Increasing the surface area of an ultracapacitor electrode boosts storage capacity because more of the ions in a liquid electrolyte are able to reach it. Graphene has high potential surface area, since it is so thin. But the thin sheets tend to stack up, blocking access to their surfaces. The researchers had previously developed a way to make the graphene sheets crumple so that they can’t stack closely, making it easy for the electrolyte to reach its surfaces. In the latest study, they showed that electrodes made with the crumpled graphene performed better than those made with ordinary graphene and activated carbon.

Next steps: The researchers are continuing to refine the shape and dimensions of the graphene to improve performance, and they plan to work with others to develop cheaper electrolytes. They are scaling up the production of their graphene, with the goal of commercializing it within two to three years.

Bacteria Begone

Coating for artificial joints protects against infection

Source: “Dual Functional Polyelectrolyte Multilayer Coatings for Implants: Permanent Microbicidal Base with Controlled Release of Therapeutic Agents”
Paula Hammond et al.
Journal of the American Chemical Society 132(50): 17840-17848

Results: A multipart polymer coating for the surface of a medical implant can prevent bacterial infection over time. Antibiotics released from the surface kill microbes in the short term, and an underlying antimicrobial polymer permanently bound to the surface prevents bacterial colonization over longer periods. When submerged in a solution of strep, a piece of silicon treated with the coating resisted the growth of bacteria as the drugs dissolved and, after they were gone, over a period of two weeks.

Why it matters: Infections resulting from joint-replacement surgeries are rare but can be deadly. When they happen, surgeons must remove the joint and any infected areas and wait six to eight weeks for a course of drug treatment to be completed before a second replacement can be attempted. This complication raises the cost of a joint replacement, which averages $30,000 in the United States, to as much as $150,000. Infections can also occur many years after the initial surgery when bacteria enter the bloodstream, such as during a colonoscopy or a dental procedure. Antimicrobial coatings developed for static implants, such as stents, do not work for artificial joints because a thick coating interferes with their movement.

Methods: Researchers at MIT started with an existing implant whose structural antimicrobial coating pierces bacterial cells that try to land on the surface. They added layers of a biodegradable polymer, an antibiotic, and an anti-inflammatory drug by dipping the implant alternately in solutions of negatively and positively charged polymer and drug molecules. The differences in charge hold the layers together, creating a coating—just tens of nanometers thick—with a high concentration of antimicrobial drugs. The drugs are released as the polymer degrades inside the body. The process could be adapted to add different drugs.

Next steps: The chemical engineers are working with clinicians at Boston’s Veterans Hospital to determine whether this process improves outcomes after joint replacements in small animals.

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