Advances in electrocatalytic reduction of CO2 over carbon-based metal-free catalysts

Advances in electrocatalytic reduction of CO2 over carbon-based metal-free catalysts
Converting CO2 into valuable products is more difficult because CO2 reduction is a multi-electronic complex process that requires breaking high bond energies. To overcome this challenge, there are different technologies for converting CO2 into useful fuels/chemicals, including electrochemical, thermochemical, biochemical, radiochemical and photochemical methods. Among them, electrochemical CO2 reduction (eCO2R), which is carried out at neutral pH, atmospheric pressure and room temperature, has unique advantages over other conversion methods. So far, different types of monometallic, bimetallic, metal oxides, metal/carbon complexes and doped carbon materials have been reported as catalysts to catalyze the electrochemical conversion of CO2. However the stability of the catalysts as well as the price of the catalysts for large scale applications are the areas that need to be emphasized in this field.

Carbon-based metal-free catalysts have attracted wide interest due to their large specific surface area and strong electronic interactions between carbon-loaded and doped materials. Carbon-based metal-free electrocatalysts consist of carbon nanofibers, carbon nanotubes, graphene, diamond, carbon nanoribbons, nanoporous carbon, and heteroatom doping of the above mentioned materials, where the heteroatoms are N, S, B, F, P, etc., which are doped to improve the current efficiency and conductivity of the materials.
The allotropes of carbon

01
Metal/metal oxide doped carbon-based electrocatalysts
Usually, combining carbon with metal catalysts can improve the catalytic activity and increase the level of dispersion and the number of active active sites. In the eCO2R process, the selectivity of the products is usually influenced by experimental parameters such as temperature, electrolyte cation and cathode morphology. The current density can be increased by improving the surface area of the cathode. Newly developed carbon nanomaterials (e.g., carbon nanofibers, carbon nanotubes, and graphene) are widely used as cathode materials for eCO2R because of their high chemical resistance, high specific surface area, and moderate conductivity.
Metal electrodes supported by carbon-based materials have the disadvantages of high cost, low current density, low iron content, and high energy required for the mining process, which limits their use in eCO2R large-scale applications. Another major issue is the poor stability of metals on carbon electrodes, which is a problem that needs to be seriously addressed. In order to solve the major problem of covalent bonding of metals to carbon in such materials, the use of thermal methods for metal formation and physical trapping of metals will provide some promising and possible strategies for metal formation. In addition, a better understanding of the mechanism of action of carbon-supported metal electrodes will open new doors for the design of composites with specific compositions. In order to overcome these limitations, researchers must find an emerging approach that uses carbon-based metal-free catalysts that may be doped with some other elements such as N, P, B, etc.

02
Carbon-based metal-free catalysts
Carbon materials that do not contain any metal elements are called carbon-based metal-free electrocatalysts. For eCO2R, metal catalysts such as Ag, Cu, Pd, and Au suffer from a number of problems, including relatively high overpotentials, low selectivity, and poor tolerance to acidic and alkaline environments. To overcome these drawbacks, carbon-based metal-free electrocatalysts have received increasing attention and are considered promising and potential alternatives. Although the eCO2R activity of pure carbon-based catalysts is generally low, doping N-doped heteroatoms such as carbon nanofibers and carbon nanotubes can greatly improve their selective eCO2R efficiency. These N-doped carbon catalysts have properties such as high specific surface area, natural abundance, acid and alkali resistance, and high conductivity for eCO2R. It is generally believed that the activity, selectivity and stability of these catalysts depend on the nature of the carbon material and the doping sites.
1. Carbon nanotubes
Carbon nanotubes are mainly composed of carbon atoms and can be categorized into two types: single-walled carbon nanotubes and multi-walled carbon nanotubes. Their unique electronic and geometrical properties have attracted much attention in selective eCO2R research.
2. Nanoporous carbon materials
Nanoporous carbon materials usually consist of regular structures that are naturally porous. Nanoporous materials are categorized as membrane materials and bulk materials. Two examples of bulk nanoporous materials are activated carbon and zeolite, while membranes are also considered as a type of nanoporous material. These porous solids can also interact with gases and liquids not only on their surfaces but also through their volume making this material excellent.

3. Carbon nanofibers and nanoribbons
Nanofibers have different physical and chemical properties and therefore have different potential applications. They can be generated from natural and synthetic polymer chains connected with covalent bonds. The diameter of the nanofibers depends on the production of the polymer Nanoribbons are often referred to as quasi one-dimensional carbon nanostructures, which are cut from graphene sheets and have a size of about less than 50 nm. Nanofibers and polymers are flexible and have a large surface area and high porosity.
4. Graphene
Graphene is an allotrope of carbon in the form of a monolayer with hexagonal carbon atoms. Reduction of graphene oxide (GO) at high temperatures produces atomically thin sheets of graphene with high electrical conductivity. In addition, synthesizing graphene with ideal size and morphology is essential to regulate density defects. Although satisfactory results have been achieved on graphene-based metal-free catalysts, the precipitation reaction of parasitic hydrogen is promoted under pure graphite during eCO2R. Therefore, the properties of graphene must be altered by heteroatom doping to enhance activity, selectivity and durability.
5. Carbon nitride (C3N4)
C3N4 is a unique and novel material that is suitable for eCO2R on carbon-based metal-free catalysts. So far, five different structures have been theoretically predicted, among which graphitic carbon nitride (g-C3N4) is a more stable allotrope of carbon nitride. g-C3N4 is a two-dimensional structure based on the C-N atomic bonding, with a visible bandgap of 2.7 eV in the sp2 layer in the presence of van der Waals forces. g -C3N4 symbolizes an emerging polymer semiconductor suitable for photocatalysis, H2 generation, water decomposition, photoelectrochemical reduction, and importantly for eCO2R. g – C3N4 has a large number of pyridinium-like nitrogen active sites due to the introduction of a metal-free catalyst, which has attracted a high level of research interest.
6. Diamond
Diamond atoms are arranged in crystal shapes and formed as pure carbon solids. Depending on the nature of the chemical bonding, the pure carbon solid form is mainly derived from different allotropes. Diamond and graphite are the two solid forms of carbon, with graphite bonded as sp2 and diamond bonded as sp3. two-electron reduction occurs in the sp2 bond, while sp3 has the potential for multiple-electron reduction. Diamond has high thermal conductivity and hardness compared to other natural elements. Diamond also has a high natural density, ranging from 3150 to 3530 kg / m3, with pure diamond having a density of 3520 kg / m3.
The article reports in detail on the above carbon-based metal-free catalysts in terms of factors affecting activity, selectivity and stability and research progress. If you are interested in the review, please reply to “Carbon based metal-free catalysts” in the background to get the full-text review in pdf.