Technological development and human mobility depends heavily on fossil-fuel-based energy infrastructure. The byproduct of this is a surplus generation of carbon dioxide (CO₂) which causes major environmental challenges, and addressing this problem is technologically challenging. The mitigation of CO₂ emissions involve separation, capture and sequestration of a significant fraction of the billion tons of CO₂ currently produced worldwide each year. A more promising and direct alternative is a chemically recycling process that converts CO₂ to carbon-neutral fuels or commodity chemicals employing water as the hydrogen source via electrochemical catalysis. This technology not only reduces greenhouse gases but also provides pathways to sustainable energy. The chemical reduction of CO₂ is a complicated process involving multiple proton coupled electron transfer, theoretically resulting in a variety of products (e.g. CO, HCOOH, CH₃OH, C₂H₄ and C₂H₅OH). Therefore the major challenge in CO₂ reduction lies in the manipulation of the selectivity towards a specific product as demanded. However, the study on CO₂ reduction has not substantially advanced primarily because of the lack of fundamental understanding of the reaction mechanism and the challenge of discovering efficient and robust catalysts for the various multi-electron transfer processes. Researchers have screened a wide range of materials for electrochemical reduction of CO₂, including metals, alloys, organometallics, layered materials and carbon nanostructures, only copper (Cu) exhibits selectivity towards formation of multi-carbon hydrocarbons and oxygenates at fairly high efficiencies while most others favor production of carbon monoxide or formate. However, Cu suffers from the poor selectivity and large overpotential in the reactions.

Compared to traditional metal nanoparticle catalysts, sub-nanometer and single atom metal catalysts (atomic catalysts) possess enhanced catalytic activity. Beyond metal-based atomic catalysts, we have recently shown metal free atomic catalysts of nitrogen (N) doped carbon sp² sheets (graphene) as possible CO₂ reduction catalysts. We designed N-incorporated carbon nanostructures (N-doped carbon nanotubes and N-doped graphene) for selective and efficient electro-reduction of CO₂ into CO with high efficiency (~80%) and at low overpotential.^1-3 We further demonstrated that the N-doped carbon materials can be atomically engineered to achieve the yield of high order (C2 and C3) products. when enriching the N-doping at the edge of carbon nanostructures, the N-doped graphene quantum dots (NGQDs, thickness ~ 1nm and diameter ~ 2 nm) exhibit exceptional activity towards formation of C2 products (C₂H₄ and C₂H₅OH) with high Faradaic efficiency of 40%, and the current density is enhanced to the order of magnitude of 100 mA/cm² at this potentialOc.⁴ This is for the first time the metal-free electrocatalyst has been discovered to steer the CO₂reduction to produce C2 hydrocarbons and oxygenantes at a relatively high yield comparable to those obtained with copper nanoparticle-based electrocatalysts.

References

1. J. Wu et al. Achieving highly efficient, selective and stable CO₂ reduction on nitrogen doped carbon nanotubes. ACS nano 9, 5364–5371 (2015).

2. P. P. Sharma et al. Nitrogen-doped carbon nanotube arrays for high-efficiency electrochemical reduction of CO₂. Angewandte Chemie International Edition 54, 13701–13705 (2015).

3. J. Wu et al. Incorporation of nitrogen defects for efficient reduction of CO₂ via two-electron pathway on three-dimensional graphene foam. Nano Letter 16, 466-470 (2016).

4. J. Wu et al. A metal free electrocatalyst for carbon dioxide reduction to multi-carbon hydrocarbons and oxygenates. Nature Communications 7,13869 (2016)