Until recently, CNTs had not been produced at low energies, nor had they been produced in high yield from CO₂. In 2015 we introduced the efficient transformation splitting of CO₂ by electrolysis in molten carbonates to form carbon nanotubes and nanofibers at high yield. The electrolysis occurs at low electrical energy and high coulombic efficiency ($4 Faraday per mole CO₂). For other synthetic approaches in forming CNTs, elements doped into the carbon such as to produce boron doped CNF/CNTs have displayed enhanced physical/chemical properties, i.e. conductivity, oxidation resistance, or superior performances in oxygen reduction compared to pure CNTs.

In 2015 we demonstrated that specific transition metals at the cathode, such as Ni, Co, Cu and Fe, act as nucleation points for high yield carbon nanotube growth in molten carbonates. The addition of Zn, such as occurs in a coating on galvanized steel, further lowers the activation energy for CNT formation.

… This research article focuses on a highly favored route to the synthesis of controlled nanostructures at high rate, high yield, and low cost by molten carbonate electrolysis. Synthetic components (steel cathode, nickel anode and inorganic carbonate electrolyte) are readily available and advantages include (1) that production is limited only by the cost of electrons (electricity) providing a substantial cost reduction compared to conventional CVD and polymer spinning syntheses and that (2) the only reactant is CO₂—transforming this greenhouse gas into a stable, valuable product and providing an economic incentive to remove anthropogenic CO₂ from flue gas or from the atmosphere.

—Ren et al.

Scheme for the electrolytic synthesis of carbon nanostructures from carbon dioxide: (a) the source of CO2 can be either smoke stack concentrations of CO2 or air (without requiring pre-concentration) dissolved in molten carbonate, (b) and (c) isotope controls formation of carbon nanotubes or nanofibers, the more valued CNTs are made from the less expensive natural abundance COCO22. High oxide concentration produce tangled morphologies (d) while low oxide produces straight nanotubes (f). (e) Bright nickel nucleation sites, as identified by EDS, initiate CNT growth. Lower panel: The CNTs and CNFs provide high conductivity and superior carbon composite lightweight structural materials for jets, bridges, wind turbines, and electric vehicle bodies and batteries. Licht (2017). Click to enlarge.

The process uses electrolysis to split CO₂ dissolved in a molten carbonate. The carbon footprint of the energy cost to produce the CNTs from CO₂ is insignificant when nuclear or solar, wind or hydro renewable energy is used to drive the electrolysis, and is only a small fraction of the electricity generated when natural gas is used to drive the electrolysis.

Under the process, oxygen is evolved at a nickel anode, and with control of low levels of transition metal additives, uniform carbon nanotubes are produced at high yield at a steel cathode.

Controlling the electrolysis conditions results in a range of product.

• Solid core carbon nanofibers are formed with ¹³C isotope CO₂, whereas hollow core CNTs are formed with natural abundance CO₂ 99% ¹²C and 1% ¹³C).

• The first doped electro-synthesized carbon nanotubes are produced using added electrolytic LiBO₂ for boron doping, and salts for phosphorous, nitrogen or sulfur CNT doping.

• Electrolytic CaCO₃ produces thin-walled CNTs, while excess electrolytic oxide yields tangled CNTs. Addition of up to 50 mol% Na₂CO₃ to a Li₂CO₃ electrolyte decreases electrolyte costs and improves conditions for intercalation in Na-ion CNT anodes.

• Addition of BaCO₃ increases electrolyte density. Longer electrolysis time leads to proportionally wider diameter CNTs.

C2CNT and cement. Cement production is the largest producer of CO₂ of any current manufacturing process, generating 5–6% of the global anthropogenic emissions of this greenhouse gas. Humans consume more than 1 ton of concrete per person per year—10¹² kg of cement annually—and the cement industry releases 9kg of CO₂ for each 10kg of cement produced, emitting 36 Gt globally annually.

Cement feedstock primarily comprises limestone, calcium carbonate, and, typically, clays with aluminum, iron, magnesium and calcium containing silicates and oxides. A rotary kiln partially melts the mix at 1450 ˚C, facilitating interaction between the oxide and silicates. Pulverized coal is often used as the flame fuel, and fuel is also added to heat and drive the calciner and preheaters. More than half of the CO₂ produced is due to limestone-to-lime dissociation; the remainder is due to the combustion of the fossil fuel.

A challenge to the economics of carbon capture at a cement plant is the relatively higher cost of capture compared to other industrial products. For example, the value per ton of cement produced is an order of magnitude less than the value per ton of steel, and hence the relative (as compared to product) price of carbon capture is substantially greater in the cement compared to the steel industry. A solution to this challenge is accomplished by the co-generation of a valuable product from the captured carbon, which can offset the carbon capture cost. The production of synthetic fuels from carbon dioxide (and/or water) has been widely studied and can be efficient. However, the value of such fuels including syngas and methane and hydrogen is not significantly higher that of the cement product. In comparison, the highest grade industrial carbon nanotubes made from carbon dioxide are valued at $1000 fold higher than such fuels, and provide substantial economic incentive to transform and eliminate the carbon dioxide emissions from cement plants.

Stack emissions from cement power plants contain ~30% carbon dioxide, which is a higher component than in fossil fuel power plant emissions. This higher concentration is due to the contribution of both the carbon dioxide from combustion and the carbon dioxide from the limestone to lime dissociation.

—Licht (2017)

Top: C2CNT Cement: Partial and full oxy-fuel coupling of C2CNT to a cement plant. Bottom: C2CNT Cement wind plant: The full oxy-fuel configuration is shown. The plant does not emit CO2, and over time cement produced absorbs CO2. Hence the process is carbon negative, which compares favorably to the large positive carbon signature of conventional cement plants. Licht et al. Click to enlarge.

When integrated with the C2CNT process, rather than emitting CO₂ to the atmosphere, the cement plant gas enters the C2CNT electrolysis chamber and is converted to carbon nanotubes; the flue gas emission contains no anthropogenic carbon dioxide. Oxygen generated in the C2CNT electrolysis chamber loops back into the cement line improving the combustion efficiency (heat delivered) of the fuel and decreasing plant gas volume decreasing radiative heating and increasing the rate of feedstock processing.

The overall C2CNT cement process is carbon negative, as the cement produced absorbs atmospheric CO₂ over time (rate dependent on the cement mixture and curing conditions).

For an advanced configuration, Dr. Licht suggests that direct electrolysis of CaCO₃—which is highly soluble in L₂CO₃, could eliminate the calcination step.

An upper limit to the electrical cost to drive C2CNT electrolysis is $70 based on Texas wind power costs, but will be lower with fuel expenses when oxy-fuel plant energy improvements are taken account. The current value of a ton of carbon nanotubes is substantially in excess of a ton of cement. Hence a C2CNT cement plant consumes ~$50 of electricity, emits no CO₂, and produces ~$100 of cement and $60,000 of carbon nanotubes per ton of CO₂ avoided. A net profit in excess of over $50,000 per ton of CO₂ avoided provides an incentive to mitigate this greenhouse gas. Even with a significant drop in CNT value with market growth, the C2CNT cement plant provides high marginal cost profits with CO₂ elimination from the plant. This is a powerful economic incentive, rather than economic cost, to mitigate climate change through a carbon negative process.

—Licht (2017)

Resources

• Jiawen Ren, Marcus Johnson, Richa Singhal, Stuart Licht (2017) “Transformation of the greenhouse gas CO₂ by molten electrolysis into a wide controlled selection of carbon nanotubes,” Journal of CO₂ Utilization, Volume 18, Pages 335-344 doi: 10.1016/j.jcou.2017.02.005

• Stuart Licht (2017) “Co-production of cement and carbon nanotubes with a carbon negative footprint,” Journal of CO₂ Utilization, Volume 18, Pages 378-389 doi: 10.1016/j.jcou.2017.02.011