Abstract

Electrocatalytic reduction of CO2 into value added chemicals or fuels is a promising technique towards a carbon-neutral chemical process. The electrochemical reduction of CO2 is a complicated process involving multiple protons coupled electron transfer, theoretically resulting in a variety of products (e.g. CO, HCOOH, CH4, 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 metal-based materials for electrochemical reduction of CO2, and found only copper-based metals exhibit selectivity towards formation of hydrocarbons and oxygenates at fairly high efficiencies while most others favor production of carbon monoxide or formate. Here we present the development of carbon materials as an alternative to Cu for efficient and high-rate electro-reduction of CO2 into hydrocarbons and oxygenates. We will discuss the key structural and electronic factors that govern the selectivity of carbon catalysts towards production of CO, CH4 and C2 products (e.g. C2H4 and C2H5OH). Three categories of carbon catalysts were developed based on the primary products of CO, CH4 and C2H4 in our group. The first carbon catalyst featuring the metal-nitrogen-carbon structure exclusively catalyzes CO2 electro-reduction into CO. The second catalyst called functionalized carbon nanostructure can selectively reduce CO2 into CH4 with Faradaic efficiency up to 90% while the third one namely doped carbon nanostructure (e.g. N-doped graphene quantum dots) can yield C≥2 products with a Faradaic efficiency up to 70%. Both carbon nanostructure can achieve partial current density at the scale of 100 mA cm-2 for target product at fairly low overpotentials. This study provides in-depth insights into developing high-performance carbon-based catalysts for electrochemical reduction of CO2.

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