This new "hard" carbon, revealed through advanced spectroscopy techniques, significantly outperforms commercial hard carbon and graphite. It offers a storage capacity of over 700 milliampere-hours (mAh) per gram, nearly double that of graphite. "Hard carbon can be produced by combustion in this case because as you burn away the carbon of rice hulls, you create a shell of silica around the remaining carbon and it bakes it like a pie," explained Richard Laine, a University of Michigan professor and corresponding author of the study published in *Advanced Sustainable Systems*.
Previously, hard carbon was thought to require heating biomass to extreme temperatures of around 1200 C in an oxygen-free environment. The discovery that combustion can produce hard carbon opens a pathway to leveraging agricultural waste for battery materials.
Rice hulls, typically discarded in landfills, offer an untapped domestic resource. In the U.S. alone, about 20 billion pounds of rice are grown annually, creating significant potential for scaling up the process. Moreover, burning rice hulls for electricity, such as at Wadham Energy in California, generates 200,000 megawatt-hours annually while remaining carbon-neutral. "The CO2 released while burning rice hulls comes from the same CO2 the rice plant took up from the atmosphere during photosynthesis, making the electricity produced green and carbon neutral," Laine added.
The process involves partially removing silica, which constitutes about 90% of rice hull ash, leaving behind 60%-70% carbon. While initially believed to be amorphous, the material contains nanoscale graphite islands within an amorphous carbon matrix, giving rise to the superior "hard" carbon structure.
In battery applications, the nanoporous structure of rice hull ash hard carbon enhances lithium storage capacity, providing a critical advantage for energy-dense batteries. This could help meet the growing demand for batteries in electric vehicles and renewable energy storage while lowering costs and reducing environmental impact.
The team, with collaborators from Karlsruhe Institute of Technology in Germany and supported by the National Science Foundation and Mercedes-Benz Research and Development North America, is working to bring this innovation to market. Patent protection has been filed through U-M Innovation Partnerships, and the researchers are seeking industry partners.
Related Links
Michigan Center for Materials Characterization
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