If carbon nanotubes (CNTs) are to be used for their lightweight and electrical conduction properties on a large/global scale [1
], is there a scenario that justifies their use, especially given the (typically) large embodied energy requirements for their manufacture? Moreover, in the age of sudden climatic shifts linked to carbon emissions, then where would the carbon come from that would be used to make these CNTs? Additionally, can the related production method from that carbon source contribute to the goal of achieving a positive climate output? To address these questions, we suggest that both plastics and solvents be used as carbon sources for CNT manufacture, and that they can create a positive environmental impact over the lifespan of their application in the aerospace sector. For instance, one of the key attributes of CNTs is being lightweight with a density that is 1/6th that of copper. This mass decrease will result in fuel savings for the automotive and aviation sectors.
Plastic products synthesised from recycled plastics are often of inferior quality compared to freshly manufactured plastics, and are not feasible for the same applications. This results in fresh plastics having a wider range of uses, and therefore, holding greater economic value. Moreover, the recycling process is often thwarted by the inclusion of fillers, pigments, and flame retardants, for example. Additionally, mixed plastics and/or composite plastic products are also challenging or impossible to recycle unless separated. To overcome these problems, one can consider open-loop recycling by making new products other than plastic. The most prominent technique uses thermal pyrolysis to break down the long chain polymer molecules into smaller, less complex molecules [2
] via the application of intense heat [3
]. This is typically carried out in the absence of oxygen to avoid the formation of undesirable carbon oxides, and is done in the presence of a catalyst to increase efficiency, tailor the resulting product, and improve scalability. The product mix is typically composed of oils, chars, and gases that require subsequent refinement and separation, thereby requiring more energy. As such, the primary focus of the field has been to improve synthesis techniques with decreased energy requirements and to create more refined and homogenous products [4
]. To that end, we have developed a novel approach towards the pyrolytic growth of carbon nanotubes by including a dissolution step prior to the high temperature cycle.
CNT growth from plastics is typically achieved using pyrolysis, whereby polymers such as polypropylene (PP) [6
], polyethylene (PE) [7
], polyethylene terephthalate (PET) [8
] and high-density polyethylene (HDPE) [9
] are heated in a solid–gas fluidized bed reactor [10
]. The resultant off-gas traverses via a carrier-gas to a catalyst site to complete the conversion from vapor to the solid CNT product. The pyrolytic growth of CNTs can be improved by the addition of a thermodynamically compatible solvent [11
] such as toluene [14
] to dissolve plastics including polystyrene (PS). The dissolution process confers five benefits over dry plastic pyrolysis. Polymer disentanglement initiates in the solvent [16
], thus increasing the reactive surface area. The polymer begins to decompose [17
], and thus lowers the required energy for subsequent C–H cleavage, thereby facilitating the processing of mixed plastic products. All nonsoluble material crashes out of solution, such as flame retardants and other noncompatible additives, effectively cleaning the plastic prior to CNT growth. Also, once dissolved, mass transport at scale is readily achieved using pumps and pipes; therefore, the liquid injection method is beneficial for large-scale operations. Moreover, mixing plastics in a solvent increases the carbon density, which can lead to increased production capacity of CNTs via chemical recycling of mixed plastics and solvents.
In conducting a life-cycle analysis (LCA), one must consider the lifespan of the material from inception to application, with particular focus on its manufacturing, transformation, use, and disposal [18
]. The complexity of the phenomena involved and the interactions among these steps is a source of uncertainty regarding the real value of the impacts, which is why we can only create “potential” life cycle assessments. Although considered for a long time as an experimental tool, the international standards ISO 14040 and 14044 (revised in 2006) have set the methodological and ethical bases for this type of assessment. In this age of climate uncertainty, it is imperative to adopt protocols and solutions that minimise harm compared with the problems of ‘business as usual’, which these protocols are intended to solve [19
The energy cost associated with the high entropy and exergy of plastics reconstitution is often cited as the single biggest reason to avoid plastics feedstock, especially with respect to CNT growth. Moreover, this reason is used to justify the continued use of virgin hydrocarbons, despite the fact that CNT growth from these sources never accounts for the energy or material cost associate with purification and delivery of the refined feedstocks. Though this is true in terms of energy consumption, the longer-term challenge deals with the supply of virgin materials based on oil extraction before reaching a peak oil scenario. Before that point, it would be prudent to establish and develop the science and technology to use premade plastics, in all the states in which they are found [20
], especially given that these materials will otherwise be strewn about the planet, ending up in our soil and food supply [22
Herein, we report that carbon from plastics can act as a feedstock for carbon nanotube growth by the upcycling of plastic to high-value materials via a chemical process. This can be considered a viable alternative to landfill and incineration. Environmental challenges exist from both liquid and solid hydrocarbons, so we applied toluene and PS as model materials.
2. Materials and Methods
2.1. The Synthesis of Carbon Nanotubes
The growth of multiwalled carbon nanotubes (MWCNTs) was carried out via catalytic chemical vapour deposition (CCVD) in a two-zoned horizontal furnace (Nanotech Innovations SSP-354, Oberlin, OH, USA) liquid injection reactor (LIR), with full details described previously [23
]. In summary, control CNTs were grown by injection of 1 mL (865 mg) anhydrous toluene (98% (C6
) Sigma Aldrich (Gillingham, UK)) at 5 mL/h under a gas flow of 1 L/min using blended carrier gas having 5 vol% hydrogen in argon (BOC, Guildford, UK) into the two-zone horizontal furnace. The first zone, used for vapour formation was set to 225 °C; the second zone used for growth was set to 780 °C. MWCNTs were grown in a 100 cm long quartz tube with diameter of 38 mm (Multi-Lab, Newcastle upon Tyne, UK). All reactions were carried out using a 20 gauge needle.
, with a molecular weight of 6400 (Mn 64,000, Sample#P2444-S, Polymer Source Inc., Dorval, QC, Canada), was added to toluene in concentrations of 1, 2, and 4 wt% (w
) using PS masses of 8.75, 17.5, and 35.00 mg, respectively. All reactions were carried out with a fixed catalyst ratio of ferrocene (5 wt% w
) (98% (C10
Fe) Sigma Aldrich (Gillingham, UK)) with respect to the total reactant from toluene and/or toluene and polystyrene. Prior to each growth, the reactants were thoroughly mixed and degassed for 15 min using bath sonication. All materials were used as received without prior cracking or drying, and handled as described here [24
]. Each concentration of PS and control was grown three times to ensure that the observed trends were valid for each series. No noticeable effect or carbothermal reduction from the aging of the quartz tube was identified [25
] (Figure S1
2.2. The Characterization and Measurement of Material Properties
High resolution transmission electron microscopy (HRTEM) was used to characterise the as-grown samples (Figure 1) using a FEI Talos 200X (FEI, Hillsboro, OR, USA) Transmission Electron Microscope (TEM) in high-resolution TEM mode, operating at 200 kV. The TEM samples were prepared by dipping holey carbon TEM grids into CNT powders. Fast Fourier transforms (FFT) of selected images were obtained to determine the materials’ atomic structures.
Scanning electron microscopy (SEM) using a JEOL 7800F FEG (JEOL, Akishima, Tokyo, Japan) was used to corroborate the presence of MWCNTs (Figure 2). A small fraction of each sample was suspended in 3 mL ethanol, and 100 µL of the suspension was dried on the surface of a clean silicon wafer for imaging. The SEM was used at an operating voltage to 5 kV or below, with a working distance of ca. 10 mm. Diameters were measured using ImageJ [26
A Renishaw inVia Raman microscope (Renishaw plc, Miskin, Pontyclun, UK) using a laser at 633 nm wavelength and 5% beam power was used for data acquisition between 100 cm−1
and 3200 cm−1
Raman shift. The laser beam was focused by maximizing the G-peak intensity to confirm best z-height alignment of the beam between sample and detector. For each CNT sample, a Raman spectrum was acquired in three separate locations. All G/D values for that series were then averaged and reported along with the maximum G/D for that series and plotted in Figure 3. The Raman spectra also show background intensity in the region from 1400 cm−1
to 1475 cm−1
that is due to fluorescence in amorphous carbon [27
]. Line integration of the background intensity between that range was plotted to compare amorphous carbon content (Figure 3C). For each reaction condition, the individual CNT samples were probed in at least three separate locations in order to both overcome variance within a single sample and to create a significant quantity of data for comparative analyses between series.
Thermogravimetric analysis (TGA/DTA) (TA Instruments, Stamford Avenue, Cheshire, UK) [28
] of the CNT samples was used to determine the MWCNT product yield by using ca. 10 mg of sample placed in a platinum pan and heating under active air flow up to 800 °C. The ramp rate was 5 °C/min and hold time was 30 min at 765 °C. The sampling interval was set for 3 s. MWCNT wt% is determined as the complete weight loss after full oxidation at 800 °C in air using TGA [29
] (Equation (1)).
2.3. Preparation of CNTs for Testing Voltage Drop
Due to presence of residual iron catalyst witnessed in the HRTEM images, an acid wash was used to remove excess iron to establish the CNT voltage drop. The oxidising acid wash, using equimolar HNO3
reflux (70 °C, 24 h), stripped away the amorphous carbon in addition to helping clear out the graphitic “onion” layers from the residual catalyst materials that would otherwise hinder iron from being digested [30
]. Note that the acid wash may have inadvertently damaged and etched some of the MWCNTs, thus increasing the measured resistance. However, it was important to prioritise iron removal, given its potential influence in electrical contact measurements.
2.4. Device Preparation and Measurement
Thin films were prepared using the “bucky-paper” [32
] technique to the measure electrical conductivity of acid washed samples. After the acid wash, the CNTs were suspended in isopropanol and CNT films were made using vacuum filtration [33
]. The CNT films were dried at 80 °C for 3 h prior to testing and use.
Electrical resistance values were derived using Ohm’s law based on measured values of voltage drop at constant current in a range of values between 0 to 100 mA. The samples were measured along a 2 cm separation for all samples to ensure consistent path length.
The carbon nanotube cables were comprised of CNT powders firmly packed into the sheath of heat shrink tubing. Copper wire was inserted into the CNT wire ends and compressed to ensure maximum contact with the Cu lead prior to heat shrinking the outer sheath. The Cu leads were then used to crimp into pins and inserted into retail purchased RJ45 connectors (RS Components, Corby, UK) for testing as ethernet cable.
Quantification of the CNT ethernet cable was conducted using iPerf3 (iPerf3 is principally developed by ESnet/Lawrence Berkeley National Laboratory. It is released under a three-clause BSD license). The cable was directly connecting two computers, both running the same Windows 10 OS update, going from a Realtek 8125B 2.5 G LAN adapter (Realtek Semiconductor Corp., Hsinchu, Taiwan) capable for two-way traffic (server) to an Intel Killer E3100X LAN adapter (Intel, Santa Clara, CA, USA) as the client; both were capable of transfer speeds up to 2500 Mbps, ensuring that the CNT cable could perform without being bottlenecked. A standard 10 s/10 run test was performed to both authenticate the data transfer and measure the transfer speeds from the server (uplink) to the client (downlink). The test was repeated for statistical accuracy and the results recorded for further discussion.
2.5. Life Cycle Assessment(LCA) Methodolgy, Assumptions, and Boundary Conditions
An LCA was conducted using Simapro 9.1 with Ecoinvent 3.6 as database using the method “IPCC 2013 GWP 100a (incl. CO2 uptake) V1.00”. In the following LCA, the CO2 emissions (Global Warming Potential) are expressed as CO2 equivalent (CO2 eq.) for the extraction, production, transport, use and end of life phases of CNT and copper wires. They were compared in the use case of electrical wiring in a standard Boeing 747-400 aircraft. This study includes analyses of both the environment impact of the manufacturing of CNT wires compared to Cu wire, and of the effect these wires have over the lifespan of a 747 in terms of CO2 generation from fuel consumption.
Assessing the potential environmental impacts of CNT manufacturing based on life cycle assessment required the following assumptions: the mass of 141 miles [34
] of copper wire is 1519.6 kg (Supplementary information
); copper ore extraction was conducted in Spain; copper wire manufacturing is done in the UK within a 100 km radius of London. The mass of 141 miles of CNT wire was 356.2 kg for the same cross-section as the copper wire [35
]. The raw materials for the CNTs were produced locally (within a 300 km radius of Swansea) and the CNTs were manufactured in the UK (using energy from the average UK electricity mix). The CNTs were produced on a laboratory scale as per the methods described herein; this is because no large scale or industrial process uses the LIR model for comparative purposes at this time, and a full study of a scaled reaction process is beyond the scope of this work.
To study the effective change in CO2 emissions from an aircraft utilising CNTs wires over Cu wires during its use phase required the following assumptions: an aircraft equipped with CNT wire will be 1163.4 kg lighter. The lifespan of a Boeing 747-400 is approximately 100,000 h of flight time with an average speed of 900 km/h. Only the CO2 emissions generated during the use phase of the aircraft were considered in this analysis; accounting for the CO2 emissions resulting from the manufacture of the aircraft is beyond the scope of this work.
Multiwalled carbon nanotubes (MWCNTs) were grown using liquid injection chemical vapour deposition (CVD) at 780 °C using ferrocene catalyst particles to obtain carbon from ferrocene, toluene and polystyrene (PS) at various PS concentrations, i.e., from 1 to 4 wt% (w/w). Samples were characterized using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). MWCNT diameters were found to increase with greater PS concentrations due to carbon from PS incorporating into the MWCNTs. Quality was measured using Raman spectroscopy. The maximum Raman G/D values both increased and the range of mean values narrowed due to the presence of higher quality products, with greater homogeneity at greater PS concentration. This synthesis method improved MWCNT quantity without incurring a measurable loss in quality. Due to the electrical nature of MWCNTs, a MWCNT ethernet cable was produced, which found to have ~99.5 Mbps uplink and downlink speeds, i.e., comparable to those of Cu wires of similar diameter. A life cycle assessment (LCA) of the MWCNT wires made using PS suggested that the electricity powering the CVD furnace represented the largest impact. Moreover, the LCA determined that over the lifespan of a single Boeing 747-400, the use of MWCNTs wires would decrease CO2 production by 21 kTe due to the lightweight nature of MWCNTs which is projected to reduce 14,574 kTe CO2 footprint across the entire fleet of 747-400 aircraft. This projection demonstrates how a plastic circular economy making lightweight energy transmission carbon cables can impact global grand challenges.