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.

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.

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