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Carbon-based nanomaterials are among some of the most sought-after nanomaterials for many new high-tech applications, as well as for innovating the already-present bulk carbon-based materials in existing applications. However, for many of the carbon nanomaterials, efforts still need to be made into making their production more viable from a large-scale and greener perspective.
In recent years, to adopt a greener approach - and to obtain raw materials that would not otherwise be there - scientists are starting to look at converting the toxic industrial gases produced in a range of industrial processes into carbon-based nanomaterials, such as carbon nanotubes and carbon nanofibers.
The removal of carbon-based gases has been around for many years and is what is commonly known as carbon capture. There has been a big drive in this area to reduce the emissions of industrial and manufacturing processes to slow global warming and reduce the carbon footprint that companies are imposing on the world.
While capturing the carbon-based industrial gases is nothing new, a new approach has emerged that goes one step further than just capturing the gases. Discoveries from academic and commercial settings are enabling the production of carbon nanotubes from industrial gases. This approach is helping to reduce carbon emissions and providing a way of producing carbon nanotubes on a large scale. It is a developing area that is following in the footsteps of using industrial gases to make carbonates and other types of nanoparticles.
The C2CNT Process
The carbon to carbon nanotube process (C2CNT) emerged out of academia. While there are a lot of different academic developments that arise, this one has been significant enough to make it to the semi-finals of the Carbon XPrize. The scientists behind the project state that the process can create tailored carbon nanotubes, including those with special shapes and specific conductive properties.
In terms of the process itself, an electrolyzer chamber is used on the hot flue gas outlets within a typical power plant setup and is regulated using an oxygen feedback loop. The synthetic process is tuned by changing the amount of oxygen gas that is fed back into the flue system, as high oxide concentrations produce tangled nanotubes. In contrast, a low oxide concentration in the flue gas produces straight nanotubes.
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An electrolysis reaction produces the nanotubes. This reaction occurs when the effluent gas leaving the processing system passes through a molten carbonate layer within the electrolyzer chamber. The dissolved gas is dissociated at one of the electrodes and nickel nucleation sites within the chamber act as a catalyst to initiate the growth of the dissolved carbon into carbon nanotubes.
Commercial Methods of Converting Toxic Industrial Gases
Aside from promising developments at the academic level, there are already existing solutions being implemented within the industry to convert toxic industrial gases into usable carbon nanotubes. One example of a company who is producing results is Clean Carbon Technologies from South Africa.
Like the academic developments, the commercial efforts produce carbon nanotubes from industrial waste gases via trapping it and converting it before it escapes into the atmosphere. It is a commercially feasible method as the raw material is an ever-present waste effluent that is produced by many industrial processes. It is not only a low-cost way of obtaining raw materials, but it is also a greener method than many other commercial approaches.
Clean Carbon Technologies uses a specialist reactor that is retrofitted into power plants near the source of the gaseous emissions. The effluent gases are captured within these reaction chambers, and a catalyst is regularly fed into the reactor to convert the carbon-based gases into single and multi-walled carbon nanotubes. The synthesis process is a modified, larger-scale form of chemical vapor deposition (CVD) that vaporizes the carbon in the gases to build the carbon nanotubes, using the catalyst as support.
Once the reaction has finished, the carbon nanotubes are collected in cylindrical containers. From this raw form, the carbon nanotubes are collected and purified, and from the full life cycle of gas to purification, the company uses the purified nanotubes to create carbon nanotubes aggregates, CNT polymer resins, CNT yarn, and CNT sheets.
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One example of the future potential of this area includes the announcement of a recent industrial-academic partnership between Rice University (USA) and Shell.
The research involved capturing methane emissions, turning them into carbon nanotubes, and then using the carbon nanotubes to make carbon nanofibers. The study simultaneously harnesses the hydrogen by-products given off in the processes to offset the energy costs of making the nanotubes. The US Department of Energy has funded the project and aims to produce another way of establishing a large-scale and green commercial production method for carbon nanotubes.
Zeroing in on the commercial realities, the discoveries and processes that have been developed to convert industrial gases into usable carbon nanotube products have great potential for both the carbon nanotube industry and the end-user sectors.
Carbon nanotubes are usually produced by nanofabrication methods such as laser ablation, electric arc discharge, and CVD. While these methods are ideal for producing high-quality nanotubes on a small scale, they struggle to present them in sufficient quantities, and many processes adapted for large scale can produce nanotubes of varying quality.
Therefore, not only do these capture and synthesis methods offer a way of reducing the world’s carbon footprint, they also provide a technique of producing high-quality carbon nanotubes (which can be made into carbon nanofibers) that can be used in everyday applications such as consumer electronics.
The Future of Capturing Toxic Industrial Gases
Capturing toxic industrial gases and converting them into carbon nanotubes could offer a way of expanding the current application base for nanotubes, as the potential is there for high volume and high-quality production.
These methods do not rely on the average raw material (where there is a risk of the raw material becoming scarce) and instead rely on our waste.
Given the amount of industrial and manufacturing processes that are prevalent in this day and age, there will be ‘raw material’ industrial gases for many years to come―and that is another point that many should consider when looking at the long-term feasibility of these methods.
References and Further Reading
Williams, M. (2019) Turning natural gas into carbon nanotubes cuts energy use, carbon dioxide emissions. [Online] Rice University. Available at: https://news2.rice.edu/2019/01/28/turning-natural-gas-into-carbon-nanotubes-cuts-energy-use-carbon-dioxide-emissions-2/ (Accessed on 30 June 2020).
Berger, M. (2017) Transforming greenhouse gas CO2 into carbon nanotube. [Online] Nanowerk. Available at: https://www.nanowerk.com/spotlight/spotid=46170.php (Accessed on 30 June 2020).
Licht, S. (2017) Carbon dioxide to carbon nanotube scale-up. Cornell University. https://arxiv.org/ftp/arxiv/papers/1710/1710.07246.pdf
Clean Carbon Technologies. Welcome to the world of Clean Carbon Technologies. [Online] Available at: https://cleancarbontech.co.za/pages/about-us/ (Accessed on 30 June 2020).
Clean Carbon Technologies. Our Carbon capture Device. [Online] Available at: https://cleancarbontech.co.za/pages/blank/blog-grid/ (Accessed on 30 June 2020).