Carbon is a remarkable engineering material. The diamond structure made from simple carbon atoms is one of the hardest known substances—perfect for grinding high strength steels. A structurally allotropic form of carbon, graphite, can be used as a dry lubricant, preventing friction between those same hardened steel surfaces. Graphite conducts electricity, can be used as a filter material, and is the lead in a pencil.
Graphite forms in two-dimensional layers that can store ions between them and release those ions in a process called intercalation. This is the basis of the graphite anode of a lithium ion battery. The lithium ions lodge between the graphite layers of the anode during charging, and are released into the electrolyte and return to the cathode during discharging.
The two-dimensional layer structure of graphite is limited, however, in the amount of lithium that can be stored by intercalation. The layers also limit the ability of the electrolyte to infiltrate the layers to deliver and remove lithium ions. More efficient and powerful lithium ion batteries might be possible with an anode made from three-dimensional carbon structures. This is one of the motivations for a recent investigation jointly undertaken by researchers at Queen Mary University of London, University of Cambridge, and the Max Planck Institute for Solid State Research.
|The addition of a "guest" salt to a metal organic framework (MOF) compound results in retention of the complex structure when the MOF is carbonized with high heat. (Image source: Reprinted with permission from J Am. Chem. Soc. 140,19,6130-6136; Copyright 2018 American Chemical Society)|
The research is reported in the Journal of the American Chemical Society (JACS) and is also described in a press release from Queen Mary University of London. The researchers started with a metal organic framework (MOF). MOFs are compounds that consist of clusters of metal ions that can form, along with organic polymer chains, into highly porous, three-dimensional, sponge-like structures. MOFs are useful in applications such as gas separators, catalysts, and sensors. But, according to the Queen Mary University press release, researchers looked at the potential for carbonization of intricate MOF structures to form a 3D sponge-like carbon structure.
Unfortunately, simply subjecting an MOF material to high heat results in a solid lump of carbon with little of the original porous structure remaining intact. But the researchers discovered that adding a “guest” salt to the MOF structure and then baking it at 800°C resulted in a transformation into a carbon structure that retained the intricate fibers, struts, and pillars of the original MOF material. The researchers termed the structures “nano-diatoms” because of the similarity in appearance of their structure to the diatom microorganisms found in oceans and waterways.
The potential for the discovery is significant. “This work pushed the use of MOFs to a new level,” stated Dr. R Vasant Kumar, a collaborator in the study from the University of Cambridge. “The strategy for structuring carbon materials could be important not only for energy storage, but also in energy conversion and sensing,” he added, as reported in the press release.
The research is still in its early stages, and improved anodes for lithium ion batteries is a primary interest. The authors of the study demonstrated that the use of the nano-diatom anode in a lithium ion battery enabled faster charging and higher capacity. It is a promising starting point. As lead author Tiesheng Wang from the University of Cambridge noted, “Potentially, we could design nano-diatoms with desired structures and active sites incorporated in the carbon, as there are thousands of MOFs and salts for us to select.”
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Senior Editor Kevin Clemens has been writing about energy, automotive, and transportation topics for more than 30 years. He has masters degrees in Materials Engineering and Environmental Education and a doctorate degree in Mechanical Engineering, specializing in aerodynamics. He has set several world land speed records on electric motorcycles that he built in his workshop.
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