Abstract:

With the excessive exploitation and utilization of conventional fossil fuels such as coal, petroleum, and natural gas, the concentration of carbon dioxide (CO₂) in the atmosphere has increased significantly, leading to serious greenhouse effect. The electrocatalytic conversion of CO₂ to liquid fuels and value-added chemicals is one of ideal strategies, considering the atomic economy and artificial carbon circle. Moreover, this process can be driven by renewable energy (solar, wind, tidal power, etc.), thus achieving efficient clean-energy utilization. Electrocatalytic CO₂ reduction (ECR) can be carried out under ambient conditions, yielding diverse products such as C₁ (carbon monoxide, methane, methanol, formic acid/formate), C₂ (ethane, ethanol, ethylene, acetic acid), and C₂₊ (propyl alcohol, acetone, etc.). However, it faces some challenging problems such as high overpotential on electrodes, the poor selectivity of C₂ and C₂₊ products, the severely competitive hydrogen evolution reaction and the stability in the practice. The rational design and construction of highly active electrocatalysts with low cost, high selectivity, and robust stability are key to these issues. Recently, graphene-based materials have attracted significant attention owing to the following attributes: (1) robust stability in electrochemical environments; (2) tailorable atomic and electronic structures, leading to tuned catalytic activity; (3) adjustable dimensions and hierarchical porous structure, large surface area, and number of active sites; and (4) an excellent conductivity coupled with active, well-defined materials, synergistically enhancing the electrocatalytic activity in the ECR. In this review, recent progress in graphene-based electrocatalysts for ECR is summarized. First, ECR fundamentals, such as reaction routes, products, electrolyzers (e.g., H-cell electrolyzers, flow-cell electrolyzers, and membrane electrode assembly cells), electrolytes (e.g., inorganic electrolytes, organic electrolytes, and solid-state electrolytes), and evaluation parameters of ECR performance (e.g., faradaic efficiency, onset potential, overpotential, current density, Tafel slope, and stability) are briefly introduced. The methods for making graphene-based catalysts for ECR are outlined and discussed in detail, including in situ or post-treatment doping, surface functionalization, microwave-assisted synthesis, chemical vapor deposition, and static self-assembly. The relationships between the graphene structures, including the point/line defects, the surface functional groups (e.g., -COOH, -OH, C-O-C, C＝O, C≡O), heteroatom-doping configurations (e.g., pyridinic N, graphitic N, and pyrrolic N, and oxidized pyridinic N), metal single-atom species (e.g., Fe, Zn, Ni, Cu, Co, Sn, Mo, In, Bi), surface/interface properties, and catalytic performance are highlighted, shedding light on the design principles for efficient yet stable carbon-based catalysts for ECR. Finally, the opportunities and perspectives of graphene-based catalysts for ECR are outlined.

Key words: Electrocatalysis, Carbon dioxide reduction, Graphene, Defect, Modification