Carbide-derived carbon (CDC), also known as tunable nanoporous carbon, is the common term for carbon materials derived from carbide precursors, such as binary (e.g. SiC, TiC), or ternary carbides, also known as MAX phases (e.g., Ti2AlC, Ti3SiC2). CDCs have also been derived from polymer-derived ceramics such as Si-O-C or Ti-C, and carbonitrides, such as Si-N-C. CDCs can occur in various structures, ranging from amorphous to crystalline carbon, from sp2- to sp3-bonded, and from highly porous to fully dense. Among others, the following carbon structures have been derived from carbide precursors: micro- and mesoporous carbon, amorphous carbon, carbon nanotubes, onion-like carbon, nanocrystalline diamond, graphene, and graphite. Among carbon materials, microporous CDCs exhibit some of the highest reported specific surface areas (up to more than 3000 m2/g). By varying the type of the precursor and the CDC synthesis conditions, microporous and mesoporous structures with controllable average pore size and pore size distributions can be produced. Depending on the precursor and the synthesis conditions, the average pore size control can be applied at sub-Angstrom accuracy. This ability to precisely tune the size and shapes of pores makes CDCs attractive for selective sorption and storage of liquids and gases (e.g., hydrogen, methane, CO2) and the high electric conductivity and electrochemical stability allows these structures to be effectively implemented in electrical energy storage and capacitive water desalinization.
History
The production of SiCl4 by high temperature reaction of Chlorine gas with Silicon Carbide was first patented in 1918 by Otis Hutchins, with the process further optimized for higher yields in 1956. The solid porous carbon product was initially regarded as a waste byproduct until its properties and potential applications were investigated in more detail in 1959 by Walter Mohun. Research was carried out in the 1960-1980s mostly by Russian scientists on the synthesis of CDC via halogen treatment, while hydrothermal treatment was explored as an alternative route to derive CDCs in the 1990s. Most recently, research activities have centered on optimized CDC synthesis and nanoengineered CDC precursors.
Nomenclature
Historically, various terms have been used for CDC, such as "mineral carbon" or "nanoporous carbon". Later, a more adequate nomenclature introduced by Yury Gogotsi was adopted that clearly denotes the precursor. For example, CDC derived from silicon carbide has been referred to as SiC-CDC, Si-CDC, or SiCDC. Recently, it was recommended to adhere to a unified precursor-CDC-nomenclature to reflect the chemical composition of the precursor (e.g., B4C-CDC, Ti3SiC2-CDC, W2C-CDC).
Synthesis
CDCs have been synthesized using several chemical and physical synthesis methods. Most commonly, dry chlorine treatment is used to selectively etch metal or metalloid atoms from the carbide precursor lattice. The term "chlorine treatment" is to be preferred over chlorination as the chlorinated product, metal chloride, is the discarded byproduct and the carbon itself remains largely unreacted. This method is implemented for commercial production of CDC by Skeleton in Estonia and Carbon-Ukraine. Hydrothermal etching has also been used for synthesis of SiC-CDC which yielded a route for porous carbon films and nanodiamond synthesis.
Chlorine treatment
The most common method for producing porous carbide-derived carbons involves high-temperature etching with halogens, most commonly chlorine gas. The following generic equation describes the reaction of a metal carbide with chlorine gas (M: Si, Ti, V; similar equations can be written for other CDC precursors):
MC (solid) + 2 Cl2 (gas) → MCl4(gas) + C (solid)
Halogen treatment at temperatures between 200 and 1000 °C has been shown to yield mostly disordered porous carbons with a porosity between 50 and ~80 vol% depending on the precursor. Temperatures above 1000 °C result in predominantly graphitic carbon and an observed shrinkage of the material due to graphitization.