MMTA alumnus, Tiesheng Wang (王铁胜), has found a simple way to fabricate a family of regular 3-D carbon-based materials by soaking solutions of metal salts with supramolecular sponges then baking them at around 800 degrees centigrade in an inert atmosphere (a.k.a. carbonization). Together with his collaborators, they have prototyped a fast-charging lithium-ion battery anode with competitively high capacity and stability using one of the 3-D carbon-based materials.
The study was recently published in JACS (Journal of the American Chemical Society) titled “Bottom-up Formation of Carbon-Based Structures with Multilevel Hierarchy from MOF–Guest Polyhedra”. The news was later released in EurekAlert! titled “Scientists Discover How a Pinch of Salt Can Improve Battery Performance”.
According to Tiesheng, many of the salts working for the process contain minor metals. In the paper, Tiesheng used Mo-based and W-based salts to trigger the 3-D carbon-based nanostructure formation.
Carbon-based material, including graphite, graphene and carbon nanotube, is a family of the most versatile materials in nature and is widely used in energy storage, catalysis and electronics because of its electrical conductivity and chemical and thermal stability. 3D carbon-based nanostructures with multiple levels of hierarchy can not only retain useful physical properties like good electronic conductivity, but can also have unique properties. These 3D carbon-based materials can exhibit improved wettability (to facilitate ion infiltration), high strength per unit weight, and directional pathways for fluid transport. It is, however, very challenging to make carbon-based multilevel hierarchical structures, particularly via simple chemical routes, yet these structures would be useful if such materials are to be made in large quantities for industry.
When the salt-incorporated supramolecular sponge is exposed to very high temperature, the organic part of the sponge will be carbonized (e.g. pyrolysis) and is sometimes interacting strongly with the salt. Such interaction allows the carbonized material to self-organize into a 3-D assembly of carbon-based fibres. The formation of individual fibre can be similar to that for carbon nanotube growth.
The supramolecular sponges are called metal-organic frameworks (MOFs), which have been developed since early 1990s. MOFs have periodically arranged metal centres interconnected by organic ligands. Structurally, they are similar to zeolites. Minor metals, such as Mg, Sc, Ti, Co, Zr and Hf, can be used as the metal centres to build the MOFs. Zr, in particular, is extensively investigated due to the outstanding chemical stability of many Zr-based MOFs. Recently, many of the MOFs can be produced cheaply. The retention of high surface area after MOF carbonisation makes them interesting as electrode materials for batteries. However, so far most of the carbonising MOFs have preserved the structure of the initial particles like that of dense carbon foam. By adding salts to these MOFs and carbonising them, a series of 3-D carbon-based materials with multiple levels of hierarchy can be produced.
Apart from the fascinating metamorphosis, Tiesheng has discovered that the process can also be a simple and workable method to functionalize the carbon with other desired elements. Though carbon is electrically conductive and stable, it is often too inert to be used as a catalyst for chemical reactions. However, by effectively doping the carbon with other elements such as minor metals, active sites for catalysis can be established, while the carbon matrix can transfer the electrons, which is very useful for electrocatalysis, such as hydrogen production from water.
Since there are thousands of MOFs and metal-containing salts available, the discovery and its following work on understanding the formation mechanisms, will enable the design of future carbon-based functional materials, not only for energy storage but also for catalysis, and sensing.
Tiesheng worked as an intern for the MMTA in March 2014. He is now a final year PhD candidate at the Department of Materials Science and Metallurgy, University of Cambridge (Darwin College). He is supervised by Dr R. Vasant Kumar and Dr Stoyan K. Smoukov and works closely with Professor Anthony K. Cheetham, FRS. Tiesheng is developing materials inside nanopores and multifunctional materials with interpenetrating structures that can benefit sensing, energy storage, and catalysis.
Adding salts to the MOF sponges can trigger metamorphosis and create regular 3-D carbon-based nanostructures. (Figure credit: Tiesheng Wang)
Electron microscopy images for the fabricated 3-D carbon-based nanostructures using a Mo-containing salt and Cu-based MOF. (Figure credit: Tiesheng Wang)