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Molten carbonate electrolysis can produce a range of carbon nanomaterials, including graphene, from CO2 at high yield

Researchers from Huazhong University of Science and Technology in China and George Washington University in the US report in a new paper in the ACS journal Accounts of Chemical Research that a range of important carbon nanomaterials can be produced at high yield by molten carbonate electrolysis.

In the Solar Thermal Electrochemical Process (STEP), developed by Professor Stuart Licht and his group at GWU, solar UV–visible energy is focused on a photovoltaic device that generates the electricity to drive the electrolysis, while concurrently the solar thermal energy is focused on a second system to generate heat for the electrolysis cell. The utilization of the full spectrum of sunlight in STEP results in a higher solar energy efficiency than other solar conversion processes.

STEP has been applied to conduct:

  1. CO2-free ammonia synthesis from nitrogen and water with the aid of nano-Fe2O3 in a molten hydroxide electrolyte;

  2. CO2-free production of iron via electrochemical reduction of iron ore in molten carbonate;

  3. CO2 capture and conversion into nanostructured carbon products (carbon nano-onions (earlier post); carbon nano-tube wools (earlier post); carbon nanotubes from flue gas (earlier post)) as well as fuels in molten or mixed molten electrolytes; and

  4. organic electrosynthesis of benzoic acid from benzene without over-oxidizing into CO2.

CO2 transformed into a portfolio of carbon nanomaterials

Source: Prof. Licht


Graphene has a high surface area, high thermal and electrical conductivity, strength, surface tailorability, and high charge carrier conductivity that makes it uniquely suitable for energy storage and electronics.

Now, in a paper in the Journal of CO2 Utilization, Professor Licht and his GWU researchers report producing graphene inexpensively from CO2.

The production of graphene via STEP is accomplished by direct molten carbonate electrolytic splitting of the CO2 to a nano-thin carbon product (carbon nanoplatelets) comprising 25 to 125 graphene layers, and subsequent electrochemical exfoliation of the nanoplatelets to graphene in a carbonate soluble aqueous solution.

The sole products of the carbon dioxide electrolysis are straightforward: high yield carbon nanoplatelets and oxygen. The carbon nanoplatelets provides a thinner starting point than a conventional graphite reactant to facilitate electrochemical exfoliation.

Electrochemical exfoliation of graphene is a process in which intercalated ions between graphite layers are oxidized, forming gases which break apart the interlayer bonds, and release graphene sheets.

Graphene from CO2

Source: Prof. Licht


Professor Licht says that the molten electrolysis of CO2 process is unusual in that it sustains removal of the greenhouse gas not only from concentrated streams, such as industrial flue gases, but also (without the need for preconcentration or purification) from the air (direct air capture).

In addition, the strong graphene bonds of the carbon nanomaterials can permanently (over a geologic timeframe) store the removed CO2, as opposed to fuel products which again releases the CO2 when the fuels are consumed.

This study is intended as a proof of concept demonstration. Future variations of the condition, for example variation of the molten carbon- ate synthesis electrolysis time and use of a binary or ternary carbonate mixture to lower viscosity have a high probability to increase the graphene lateral dimension. Furthermore, to introduce the new synthesis in a logical stepwise manner, although the carbon nanoplatelet component of the synthesis is unconventional, conventional exfoliation conditions were utilized with the principle differences being the use of (i) ultrathin molten carbonate carbon platelets to facilitate graphene exfoliation and (ii) using carbon sourced from CO2electrolysis, rather than using commercial graphite as the exfoliation anode to decrease the carbon footprint of graphene production. As discussed in the study, it is likely that in future variations of this new synthesis the requisite exfoliation voltage can be substantially decreased.

Here, graphene is synthesized to high yield from the greenhouse gas CO2 by (i) electrolysis in a molten carbonate to form carbon platelets at high yield on a galvanized steel cathode, followed by cooling and placing the cathode within a cellulose membrane and (ii) then high yield exfoliation of the platelets as an anode in a carbonate dissolving aqueous ammonium sulfate solution and generating gas between the graphene layers to promote separation of the individual graphene layers. The produced molten carbonate synthesized platelets are nano-thin and promote fewer carbon layers in the product and higher yield than thicker, conventional graphite exfoliation reactions.

Utilization of CO2 as the sole reactant produces graphene as a low carbon footprint product.

—Liu et al.

Resources

  • Xinye Liu, Xirui Wang, Gad Licht, Stuart Licht (2019) “Transformation of the greenhouse gas carbon dioxide to graphene” Journal of CO2 Utilization doi: 10.1016/j.jcou.2019.11.019

  • Jiawen Ren, Ao Yu, Ping Peng, Matthew Lefler, Fang-Fang Li, and Stuart Licht (2019) “Recent Advances in Solar Thermal Electrochemical Process (STEP) for Carbon Neutral Products and High Value Nanocarbons” Acc. Chem. Res. doi: 10.1021/acs.accounts.9b00405

Comments

yoatmon

This may also be a first step towards producing high-power and energy dense graphene capacitors that could last virtually forever. No more long waiting times for a charge -up and goodbye to chemical reactive batteries.

Engineer-Poet

Electrons in graphene caps don't have the energy/weight of chemical bonds, so they won't have the energy density of batteries.  OTOH they have far less in the way of limitations on power density, as all they are moving around is electrons.  This makes them suitable for applications such as energy buffers in hybrid vehicles, reducing the power-density required of batteries or eliminating them entirely.

SJC

nanostructured carbon...
Might make some higher capacity lithium ion anodes.

Wiredsim

I think you are thinking too small. This is one of many recent studies/breakthroughs showing we can use solar energy at high efficiency to produce highly useable / highly valuable carbon material products.

This may be the path towards sustainable atmospheric Co2 regulation through large industrial production of carbon materials. Potentially replacing existing materials such as plastics.

Mega Large carbon based space habitats? Building materials?

Revolutionizing energy storage and production is just a small potential benefit.

Engineer-Poet

The flip side of these things is that it takes a huge amount of energy to produce a given amount of fixed carbon, whether it is fuel or chemicals or CNTs.  Not that it's not good, mind you, but you have to be aware of the constraints.  If your job requires X energy to do one way, and 5X energy to do the "green" way, the environmental impact of even "green" power argues for doing it the non-"green" way.

Then there's fitness for purpose.  If you intend to turn the entire excess of atmospheric CO2 into CNTs, you're going to require literally staggering amounts of energy.  I calculate it takes about 23 MJ (6.4 kWh) to make 1 liter of methanol from CO2 by electrolyzing water and reacting CO2 with the hydrogen.  This comes to about 21 GJ per ton of CO2 consumed.  Using polyanthraquinones to extract CO2 at maybe 2.5 GJ/ton is a far better use of your energy if you are mostly trying to get back down to 350 ppm.

yoatmon

Considering the solar energy impacting our planet per annum, we are presently using approx. 1hrs worth of 365.25 d/a x 24 hrs/d = 8766 hrs or 1:8766 = 0.0001 %.
It really is not a problem of energy availability rather a problem of implementation and willingness.

Engineer-Poet

If you insist on doing it that way, it's a problem of collecting, concentrating and storing very diffuse flows of energy.

If you go out into geosynchronous orbit you have less trouble collecting and concentrating, and the near-24-hr flow (save for some brief interruptions near midnight around the equinoxes) gets rid of most of the storage problem too.  But for some reason, "environmentalists" hate that idea.

yoatmon

Sounds like a good idea but I'm afraid it wouldn't work. For some reason, it seems there is too much gunk and junk floating around our planet. Could it be possible that the greatest problem for humanity is humanity itself?

Engineer-Poet

There's precious little junk in geosync because end-of-life satellites are boosted away.  It would be easy to catch and remove those old birds if desired; they're all at very similar altitudes and planes, so little delta-V required to rendezvous and pick them up.

Now I need to calculate the antenna size required to relay from Earth-Sun L1.  On second thought, I'll do that when I actually need the number.

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