Metal‐Free Carbon Materials for CO₂ Electrochemical Reduction

Abstract

The rapid increase of the CO concentration in the Earth's atmosphere has resulted in numerous environmental issues, such as global warming, ocean acidification, melting of the polar ice, rising sea level, and extinction of species. To search for suitable and capable catalytic systems for CO conversion, electrochemical reduction of CO (CO RR) holds great promise. Emerging heterogeneous carbon materials have been considered as promising metal-free electrocatalysts for the CO RR, owing to their abundant natural resources, tailorable porous structures, resistance to acids and bases, high-temperature stability, and environmental friendliness. They exhibit remarkable CO RR properties, including catalytic activity, long durability, and high selectivity. Here, various carbon materials (e.g., carbon fibers, carbon nanotubes, graphene, diamond, nanoporous carbon, and graphene dots) with heteroatom doping (e.g., N, S, and B) that can be used as metal-free catalysts for the CO RR are highlighted. Recent advances regarding the identification of active sites for the CO RR and the pathway of reduction of CO to the final product are comprehensively reviewed. Additionally, the emerging challenges and some perspectives on the development of heteroatom-doped carbon materials as metal-free electrocatalysts for the CO RR are included.

Doping of Carbon Materials for Metal‐Free Electrocatalysis

“…[8] It is the doping-induced charge transfer from carbon atoms to their adjacent nitrogen atoms, changing the chemisorption mode of O 2 and weakening the OO bond for improving the ORR performance of VA-NCNT. [4,5,7] Since then, C-MFCs have been demonstrated to catalyze hydrogen evolution reaction (HER) for the production of clean fuel (H 2 ) from photo-/electrochemical water-splitting, ORR in fuel cells for energy generation/conversion, [10] and oxygen evolution reaction (OER) in metal-air batteries for energy storage, [7] two-electron transfer ORR to generate H 2 O 2 (an energy carrier and green oxidizer), [13] CO 2 reduction reaction (CO 2 RR) for the direct conversion of CO 2 into fuel, [14] N 2 reduction reaction (NRR) for synthesis of NH 3 under ambient environment, [15] and for the renewable energy generation/conversion from water driven by sunlight. [4,5,7] Since then, C-MFCs have been demonstrated to catalyze hydrogen evolution reaction (HER) for the production of clean fuel (H 2 ) from photo-/electrochemical water-splitting, ORR in fuel cells for energy generation/conversion, [10] and oxygen evolution reaction (OER) in metal-air batteries for energy storage, [7] two-electron transfer ORR to generate H 2 O 2 (an energy carrier and green oxidizer), [13] CO 2 reduction reaction (CO 2 RR) for the direct conversion of CO 2 into fuel, [14] N 2 reduction reaction (NRR) for synthesis of NH 3 under ambient environment, [15] and for the renewable energy generation/conversion from water driven by sunlight.…”

Catalytic Mechanisms and Design Principles for Single‐Atom Catalysts in Highly Efficient CO2 Conversion

“…[1,2] At the heart of the CO 2 conversion technology, electrocatalysts are needed to promote a critical reaction, CO 2 reduction reaction (CO 2 RR) that determines the efficiency and selectivity. [5,6] Therefore, CO 2 RR is much more complex than other energy-related electrochemical reactions such as oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), and it is still a great challenge to design and synthesize electrocatalytic materials with higher product selectivity and catalytic activity for CO 2 RR.Among the electrocatalysts including metals oxides and metal-doped carbon materials, single-atom catalysts (SACs) represent an exciting class of catalysts with monodispersed metal catalytic centers and have emerged as the frontier science in both homogeneous and heterogeneous catalysis, including CO 2 conversion. [3] The former involves associated multielectron transfer process and is difficult to accurately control the reaction process by external conditions, [4] while the latter is mainly due to the fact that the equilibrium potentials for most of the CO 2 RR are very close to hydrogen evolution reaction (HER) toward undesirable side-products in aqueous electrolytes, which degrades the electrocatalytic performance during the CO 2 RR process.…”

Carbon-Based Metal-Free Electrocatalysis for Energy Conversion, Energy Storage, and Environmental Protection

“…Rapidly increasing CO 2 concentrations around the globe has resulted in numerous environmental issues, such as global warming, ocean acidification, polar ice melting, sea level rising, and species extinction [204]. CO 2 capture and conversion is considered to be an effective means to minimize atmospheric CO 2 by converting CO 2 into cost-effective carbon-containing fuels [205,206].…”