Electrochemical CO₂ reduction (ECR) offers a promising route to convert greenhouse gases into value-added chemicals such as carbon monoxide (CO), formate, methane, and ethylene. Graphene-based catalysts have emerged as highly tunable materials for this process due to their exceptional conductivity, large surface area, and the ability to incorporate defects and heteroatoms that enhance catalytic activity. The design of graphene catalysts involves strategic modifications, including doping, vacancy creation, and integration with metal nanoparticles, to optimize selectivity and efficiency. This article explores the mechanisms behind graphene-based ECR, contrasts their performance with metal-organic frameworks (MOFs) and molecular catalysts, and evaluates their stability under operational conditions.

Graphene’s intrinsic sp² carbon network is not inherently catalytic for CO₂ reduction, but its electronic structure can be modified to activate CO₂ molecules. Defect engineering plays a crucial role in this process. Nitrogen (N) and sulfur (S) doping introduce charge polarization and create active sites for CO₂ adsorption. N-doped graphene, for example, exhibits improved Faradaic efficiency (FE) for CO production, often exceeding 80%, due to the electron-donating effect of pyridinic and graphitic nitrogen, which lowers the energy barrier for CO₂ activation. Similarly, S doping introduces thiophene-like structures that facilitate proton-coupled electron transfer, enhancing formate selectivity. Vacancies, such as single or double carbon vacancies, further expose edge sites that serve as additional active centers. Studies show that vacancy-rich graphene can achieve FE for CO production above 90% at moderate overpotentials.

The integration of metal nanoparticles (NPs) with graphene significantly enhances catalytic performance by combining the conductive support with highly active metal sites. Copper (Cu) NPs are particularly effective for multi-carbon products like ethylene and ethanol due to their ability to stabilize *CO intermediates, enabling C-C coupling. Silver (Ag) NPs, on the other hand, favor CO production with FE values often surpassing 95% at low overpotentials, attributed to their weak *CO binding energy, which prevents further reduction. Bimetallic systems, such as Cu-Ag alloys on graphene, have demonstrated tunable selectivity, where the ratio of metals dictates the dominant product. The graphene substrate not only disperses the NPs to prevent aggregation but also participates in charge transfer, optimizing the reaction kinetics.

Heteroatom coordination further refines catalytic behavior. Dual-doped graphene, such as N,S-coordinated systems, creates asymmetric charge distributions that stabilize key intermediates. For instance, N,S-co-doped graphene has shown FE for formate production exceeding 70%, as the synergistic effect of both dopants lowers the activation energy for *OCHO intermediate formation. Transition metal single-atom catalysts (SACs) anchored on graphene, such as Fe-N₄ or Ni-N₄ sites, exhibit remarkable activity for methane production, with FE values reaching 60%. The atomic dispersion maximizes metal utilization while the graphene matrix ensures electron delocalization, reducing energy losses.

Mechanistic studies reveal that the reaction pathway on graphene-based catalysts depends on the interplay between electronic structure and intermediate binding. For CO production, the initial step involves CO₂ adsorption on defect or dopant sites, followed by electron transfer to form *COOH, which subsequently dissociates into CO. In contrast, formate production proceeds via a *OCHO intermediate stabilized by S or N sites. Methane formation, typically favored on Cu or Ni SACs, requires multiple proton-electron transfers to *CO intermediates, with the rate-limiting step being *CH₃O hydrogenation. Operando spectroscopy techniques, such as in-situ Raman and X-ray absorption, have provided insights into these pathways, confirming the dynamic nature of active sites under applied potentials.

Faradaic efficiency and stability are critical metrics for practical applications. Graphene-based catalysts often outperform traditional metal foils in FE due to their tailored active sites. However, long-term stability remains a challenge, particularly for systems involving metal NPs, which may undergo dissolution or agglomeration. Strategies such as encapsulating NPs in graphene layers or using covalent functionalization have improved durability, with some catalysts maintaining performance for over 100 hours. In contrast, MOFs and molecular catalysts, while highly selective, suffer from limited conductivity and structural degradation under electrochemical conditions. MOFs, such as ZIF-8 or UiO-66, exhibit high FE for CO but require conductive additives, complicating device integration. Molecular catalysts, like cobalt phthalocyanine, offer precise control over active sites but often degrade under reducing potentials.

Compared to MOFs and molecular systems, graphene-based catalysts strike a balance between activity, selectivity, and stability. Their synthetic versatility allows for systematic optimization, while their robustness makes them suitable for industrial-scale applications. Future research should focus on scaling up production methods, understanding degradation mechanisms, and exploring novel dopant combinations to further improve performance. The integration of computational screening and machine learning could accelerate the discovery of optimal graphene configurations for specific ECR products.

In summary, graphene-based catalysts represent a versatile platform for electrochemical CO₂ reduction, with defect engineering, metal NP integration, and heteroatom coordination serving as powerful tools to tune reactivity. Their superior conductivity and stability position them as leading candidates for sustainable carbon utilization, offering a viable alternative to conventional catalysts in the transition toward a carbon-neutral economy.