Nanotubes for carbon captures

This project consisted of a number of interdisciplinary and speculative research activities that collectively were focused on the goal of determining the feasibility of developing a small scale carbon capture system (CCS) based on the adsorption properties of chemically functionalised carbon nanotubes (CNT). Carbon nanotubes (CNT) are cylinders of pure carbon with diameters on the order of some nm and lengths that can range from 100nm to mm. They exist both as single-walled CNT where the wall of the cylinder is 1 atom thick and as multi-walled nanotubes where a number of cylinders are nested inside each other. The bulk material is extremely lightweight and highly porous and due to its high surface area is very suitable for gas storage applications. CNT also possess a high thermal and electrical conductivity and exhibit a rich chemistry. These properties all make them very promising materials for efficiently and selectively adsorbing and desorbing CO2. The worldwide production of CNT has increased dramatically in the past couple of years and the price is falling rapidly, making the large scale application of bulk quantities of CNT feasible. The activities addressed within this Feasibility Account include the synthesis and characterisation (including toxicological studies) of new CNT material with a high selectivity and affinity for CO2 adsorption as well as potential for the development of selective gas sensors, the modelling and design of a small scale CCS taking into account extensive feedback from public consultation and a life cycle analysis to determine the economic and environmental feasibility of the development of such a system.

Functionalization schemes were successfully developed for carbon (CNT) with emphasis being placed on the application of multiwalled nanotubes that are readily available at a reasonable cost.

A major problem was encountered with the porosity of the functionalised material. After wet chemical treatment the functionalised CNT material was significantly less porous than the pristine material and the surface pore structure became blocked due to efficient carbonate/carbamate formation. The uptake of CO2 in the initially modified material was therefore disappointing and not competitive with the best available materials. An approach was found that improved the porosity of the CNT material but this was achieved late in the project and, due to technical problems with the CO2 uptake measurement equipment it was only possible to obtain preliminary data at the very end of the project. The results showed that the material was a factor of three times better than than the original functionalised CNT material for low partial pressures of CO2.The porous CNT-PEI material was found to besignificantly better than activated carbon, vastly superior to EDA-functionalised material and is competitive with other carbon capture materials such as functionalised mesoporous silicon.

The CNT-EDA material, although unsuitable for carbon capture was found to give excellent results when dispersing CNT within polyamide matrices.

The functionalised CNT material could be heated to temperatures of a few hundred degrees centigrade by passing a small current through and would thus be suitable for more extensive electrical swing studies. It was shown that CO2 could be removed in this way thus regenerating the material.

Electrical transport measurements showed a strong response to the presence of CO2 indicating the potential of the functionalised materials for CO2 sensing.

The toxicology of the various functionalised CNT materials and the pristine material was tested. The functionalised materials did not show any significant cytotoxicity, in contrast to the as-purchased non-purified MWNT material that did show a significant toxic effect. The reasons for the toxicity of the non-purified material are currently unclear but may be due to the presence of residual metal catalyst particles. The functionalisation process thus rendered the multiwalled CNT material safe from teh toxicological viewpoint.

A design for a domestic CO2 air capture device was prepared using parameters from the best available materials. The design was shown to be feasible, allowing captured CO2 to be concentrated in multiple cycles. The non-CNT materials considered in the simulation are not practical for the construction of a real device due to the high heat of adsorption of CO2 in these materials. Since it was possible to show at the very end of the project that functionalised CNT materials are competitive in terms of CO2 uptake, the greater ease of desorbing the captured CO2 using electrical swing desorption powered by e.g. photovoltaic cells would be extremely advantageous for such a device.

• A life cycle analysis was carried out. With the current CO2 uptake from the functionalised CNT such a device is not feasible. It would need a factor of2-3 higher adsorption rate (using porous functionalised material) to offset the costs of producing the CNT within a period of 1 year, before any net CO2 reduction could be achieved. Such an improvement is thought to be achievable in the near future with further improvements in the morphology of the CNT material. This calculation does not take into consideration the carbon cost of transporting the harvested CO2.

• Public engagement activities were organised to gauge the public knowledge of carbon capture technology and public acceptance of a potential domestic CO2 air capture device. The domestic device was regarded as expensive and unfeasible but there was considerable support for a similar larger scale device at community level to be situated e.g. along motorways.



In summary, the functionalised CNT were shown to be very suitable, non-toxic, materials for further development of carbon capture schemes for gaseous sources with low concentrations of CO2. The domestic carbon capture unit was treated with scepticism by the general public and it would appear more feasible to develop larger scale community units or concentrate on first developing units suitable for capture from relatively low concentration sources of CO2 such as gas power stations.