GWU team demonstrates one-pot process for optimized synthesis of controlled CNTs from CO2; coupling cement and C2CNT
Researchers at George Washington University led by Dr. Stuart Licht (earlier post) have developed a new process that transforms CO2 into a controlled selection of nanotubes (CNTs) via molten electrolysis; they call the process C2CNT (CO2 into carbon nanotubes). This synthesis consumes only CO2 and electricity, and is constrained only by the cost of electricity.
Controlling the electrolysis parameters opens up a wide portfolio of CNT morphologies, including hollow or solid, thick- or thin-walled and doped CNTs. Molten carbonate electrosynthesized boron-doped CNTs exhibit high electrical conductivity. The process is described in a paper published in the Journal of CO2 Utilization. In a second paper in that journal, the team reports on the uses of C2CNT to retrofit cement plants. Per ton CO2 avoided, the C2CNT cement plant consumes $50 electricity, emits no CO2, and produces $100 cement and ∼$60,000 of CNTs.
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.
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)
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)
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