Wednesday, January 2, 2013
Green Batteries from a Plant
Purpurin which is extracted from madder root, is chemically lithiated for use as an organic cathode in batteries. The material was developed as a less expensive, easier-to-recycle alternative to cobalt oxide cathodes now used in lithium-ion batteries.
Scientists at Rice University and the City College of New York have discovered that the madder plant (Rubia tinctorum) is a good source of purpurin, an organic dye that can be turned into a highly effective, natural cathode for lithium-ion batteries. The plant has been used since ancient times to create dye for fabrics.
The discovery is the subject of a paper ("Lithium storage mechanisms in purpurin based organic lithium ion battery electrodes") appearing in Nature's online, open-access journal Scientific Reports.
According to lead author Arava Leela Mohana Reddy, a research scientist in the Rice lab of materials scientist Pulickel Ajayan, their team's research is creating environmentally friendly batteries that will solve many of the problems associated with the ordinary lithium-ion batteries widely in use today.
"Green batteries are the need of the hour, yet this topic hasn't really been addressed properly," Reddy said. "This is an area that needs immediate attention and sustained thrust, but you cannot discover sustainable technology overnight." He says the focus of the research community is currently still primarily on improving the features of conventional batteries. Issues such as sustainability and recyclability tend to get sidelined.
Though lithium-ion batteries are the standard, Reddy said, rechargeable units cost a lot to produce. "They're not environmentally friendly. They use cathodes of lithium cobalt oxide, which are very expensive. You have to mine the cobalt metal and manufacture the cathodes in a high-temperature environment.
"And then, recycling is a big issue," he said. "In 2010, almost 10 billion lithium-ion batteries had to be recycled, which uses a lot of energy. Extracting cobalt from the batteries is an expensive process." Eliminating cobalt would mean eliminating a hazardous material, allow batteries to be produced at room temperature, and greatly reduce the cost of recycling.
The team first discovered the special properties of purpurin while they were testing various organic molecules for the ability to electrochemically interact with lithium. Purpurin turned out to be the best at binding lithium ions. To add conductivity they added 20 percent carbon, and then built a half-battery cell with a capacity of 90 milliamp hours per gram after 50 charge/discharge cycles. As it turns out, such cathodes can be made at room temperature.
"It's a new mechanism we are proposing with this paper, and the chemistry is really simple," Reddy said. He suggested agricultural waste may be a source of purpurin, as may other suitable molecules, which makes the process even more economical.
But Reddy hopes to formulate completely green batteries. The team is looking for organic molecules suitable for a working prototype of a complete organic battery within a few years. "What we've come up with should lead to much more discussion in the scientific community about green batteries," he said.
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