The intrinsic defects exist widely in carbon materials, which are formed by lattice distortion and atom deletion. The geometric configuration of intrinsic carbon defects mainly includes edges, vacancies, holes, topological defects, etc. The zigzag and armchair edges make the carbon materials full of unpaired π electrons, which can enhance the local electron density, accelerate electron transfer and lower the formation energy barriers of key intermediates. It was demonstrated that the mechanical ball milling method has the generality of producing more edge defects in the graphitic carbon skeleton (Dong et al., 2019). Meanwhile, chemical etching is an effective strategy to introduce defects on the surface of carbon materials (Li et al., 2019a). Dong et al. used NH₃ thermal-treatment to adequately remove pyrrolic-N and pyridinic-N atoms and introduce a high density of defects (Figure 1A) (Dong et al., 2020). As a consequence, the obtained carbon materials possessed a Faradaic efficiency (FE) of 95.2% for CO production (Figure 1B), outperforming that of most metal-free CO₂RR electrocatalysts. DFT calculations revealed that the edge pentagonal carbons with the lowest free energy dominated the active sites for CO₂RR. This work provided a facile approach for introducing the intrinsic defects to enhance the electrocatalytic activity of carbon materials.

In addition to ammonia treatment and etching strategies, denitrification is also a common strategy for constructing topological defects. For instance, Wang et al. prepared two types of carbon materials by the high temperature removal of heteroatoms (Wang et al., 2019). They revealed that the CO₂RR performance of carbon-based catalysts was positively correlated with the content of intrinsic carbon defects. They found that the defective porous carbons without heteroatom dopants also show impressive CO₂RR activity. The results of synchrotron and theoretical calculations revealed that the sp ² defects, such as octagonal and pentagonal, are essential for the enhancement of CO₂RR performance (Figures 1C,D).

Daiyan et al. (2018) prepared nitrogen-removed mesoporous carbons by carrying out different heat treatments on nitrogen-doped mesoporous carbon. With the removal of the original nitrogen species, the topologically defective structure of the catalyst was formed. When applied to CO₂ reduction, the metal-free mesoporous carbon catalyst exhibited a CO FE of 80% and a CO partial current density of −2.9 mA cm⁻² at a low overpotential of 0.49 V. The authors demonstrated that the defect sites are the main active sites for CO₂RR. Similarly, Chen et al. prepared carbon-based electrocatalysts with topological defects from silk cocoons for CO₂RR (Chen et al., 2020b). The resulting electrocatalyst was able to achieve a CO FE of −89% and maintained high selectivity during the 10 days test. Theoretical calculations show that topological defects containing pentagonal carbon rings are the main active sites for accelerated electron transfer in CO₂ reduction.

The introduction of topological defects will break the electronic symmetry of the aromatic ring, and the adjacent carbon atoms can be optimized as the active sites for CO₂ reduction. Although the construction of intrinsic defects has been well developed, the accurate and quantitative description of certain intrinsic defects is still in its infancy. Some straightforward and simple protocols to precisely quantify the number of defects is highly demanded (Sebastian et al., 2022).

By introducing a small number of other atoms into the original carbon structure, the electronegativity distribution on the surface of the original carbon structure is changed, i.e., the doping of heteroatoms. The introduction of heteroatoms will optimize the intrinsic electronic structure of carbon materials and greatly improve their electrocatalytic activity. The heteroatoms doped into carbon materials are usually nitrogen (N) (Li et al., 2018; Li et al., 2019b; Ning et al., 2022), phosphorus (P) (Chen et al., 2020a), boron (B) (Du et al., 2022), and sulfur (S) atoms (Wang et al., 2020a).

Among them, nitrogen atom doping is the most widely studied. The N doping can be divided into four configurations due to its different locations: graphitic N, pyridinic N, pyrrolic N and N oxide. In the earlier study of N doped carbon materials, Wu et al. (2015) developed N-doped carbon nanotube (NCNT) arrays as efficient electrocatalysts for CO₂ reduction. The authors claimed that the structural nature and defect density of N in CNTs determined the catalytic activity. These studies demonstrated the vital role of various configurations of N-doped carbon materials in CO₂RR. Wang et al., 2021b fabricated the N-doped porous carbon (NPC) from poly (aniline-co-pyrrole) copolymer with a pyridinic-N content of 2.86 wt% in the presence of salt templets. The optimized catalyst exhibited excellent CO₂RR performance with a CO FE of 95.3%, exceeding the performance of most previous N doped carbon electrocatalysts. The experimental and theoretical calculation revealed that the pyridinic-N species contributed significantly to CO₂RR.

Besides N, other types of heteroatoms doping/codoping have also been studied. Nakata et al. (2014) used a boron-doped diamond (BDD) electrode to produce formaldehyde under ambient conditions in seawater. This approach broke the traditional limit of the low yield of high-order products, and also suppressed the hydrogen generation. The BDD electrodes with p-type surfaces exhibited a high FE (74%) for formaldehyde, which was helpful for practical applications. The superior performance was attributed to the sp ³ -bonded carbon of the BDD. These results have great inspiration to achieve high efficiency and low-cost CO₂ transformation. Li et al. (2016) reported the S-doped and dual S, N-doped polymer-derived carbons could be used as catalysts for CO₂RR. The S, N-doped carbons were obtained by treating the polymer-derived carbon with urea, which showed better catalytic activity towards CO₂ reduction than the single S-doped carbons. Xue et al. (2019) synthesized a P and N coordinated fullerene-like carbons (N, P-FC) via a facile soft template pyrolysis method, which showed an impressive activity for CO production in CO₂RR (Figure 1E).

Highly dispersed single-metal-atom sites in carbon skeleton can significantly improve the electrocatalytic performance of CO₂RR (Wang et al., 2020b; Huang et al., 2022). Single-metal-atom catalysts have received extensive attention due to their high atomic utilization rate and excellent catalytic activity (Zhao et al., 2021). Unlike intrinsically defective or heteroatom-doped active sites, these highly dispersed metal species are generally considered to be directly available as active sites. A large body of research relating to single-metal-atom catalysts has focused on metal-nitrogen-carbon (M-N-C) materials in recent years. MNx constituted the catalytic active sites for the majority of M-N-C materials (Guo et al., 2020), which is similar to the MN₄ sites in the macrocylic N-C complexes. For example, metalloporphyrin is a highly effective homogeneous electrocatalyst for CO₂RR. Thus, MNx sites existing in solid carbon materials are also considered as active sites for CO₂RR.

Research on single-metal-atom catalysts applied to CO₂RR can be traced back to 2015. Varela et al. found that the metal-doped nitrogenated carbon, such as Fe-N-C, Mn-N-C and Fe, Mn-N-C showed high CO₂RR performance (Varela et al., 2015). Undeniably, for M-N-C catalysts, the category of metal elements and the coordination number of the metal and nitrogen atoms are of key importance for the active sites in CO₂RR.

The immobilization of transition metal-nitrogen-carbon complexes has been studied for decades. Various methods and techniques have been generated in this area as well. Meshitsuka et al. are among the earliest researchers who successfully immobilized CoPc and NiPc onto the surface of graphite electrodes (Meshitsuka et al., 1974). In recent years, there has been an increasing number of reports on transition M-N-C complexes as catalysts applied to CO₂RR involving various metal centers and supports. However, it is difficult to control the number or thickness of catalyst layers for M-N-C materials prepared by absorption or immobilization methods, which may result in insufficient or too dense active centers and hinder the diffusion of CO₂. Therefore, many researchers have been exploring other methods to prepare heterogeneous M-N-C catalysts. A metal-organic framework (MOF) is a polymer consisting of metal-organic ligands on an open framework with a coordinated, porous heterogeneous network structure.

Zhao et al. prepared Ni SAs/N-C catalyst containing Ni-N single-atom sites by ionic exchange of MOFs, showing a superior CO₂RR performance with a CO production turnover frequency (TOF) of 5,273 h⁻¹ (Zhao et al., 2017). Ye et al. (2017) synthesized Fe-N-C catalyst by surface functionalization of ZIF-8 with ammonium ferric citrate. The highly exposed Fe-N-C presented an excellent performance for CO₂RR, achieving a CO FE of up to 93% at −0.43 V vs. RHE. Gu et al. (2019) prepared a monodisperse Fe³⁺ single-atom catalyst by controlling the pyrolytic process. The onset potential for CO production was only 80 mV. The partial current density of CO reached 94 mA cm⁻² at an ultra-low overpotential of 340 mV. The authors revealed that the active sites were monodisperse Fe³⁺ particles coordinated with the pyrrolic N on the carbon support using the in-situ synchrotron X-ray absorption spectroscopy. Similarly, Boppella et al. (2022) developed soft-template assisted technology to synthesize pyrrolic N-stabilized single Ni atom electrocatalyst (Figure 1F). The results of the control experiments showed that the synergistic interaction between Ni-Nx and metal free defect sites could effectively promote CO₂RR activity.

Furthermore, Lu et al. (2021) developed highly dispersed indium (In) atoms on an N-doped carbon (In-N-C) as a highly efficient catalyst for formic acid production. Hao et al., 2022 synthesized dual-single-atom catalysts consisting of atomically dispersed CuN₄ and NiN₄ bimetallic sites using electrospun carbon nanofibers. The optimal catalyst exhibited excellent CO FE of 99.6% in a wide potential range (−0.78 to −1.18 V vs. RHE) with a high turnover frequency of 2,870 h⁻¹. The authors revealed that the electronegativity offset of the bimetal atoms induced the strong electron interactions, thereby accelerating the *COOH adsorption and decreasing water dissociation kinetics (Figure 1G). Wang et al. synthesized a Co-N-C catalyst dominated with isolated CoN₄ sites (CoN₄-CNT) (Wang et al., 2022). When employed in CO₂RR, CoN₄-CNT exhibited both excellent selectivity (FE_CO = 99.4%) and activity (j _CO = −24.8 mA cm⁻²) at a low overpotential of 0.49 V in H-type cell, and the FE_CO remained above 90% with increasing current density from 50 to 600 mA cm⁻² in the flow cell.

Single-metal-atom catalysts have become a research hotspot in various catalytic fields, but there are still some doubts about the exact location of their catalytic active center. This is because there are many potential active sites in single-metal-atom catalysts, such as single-metal sites, heteroatoms, intrinsic carbon defects, etc., and it becomes more difficult to quantitatively describe these defects (Qin et al., 2019b). Therefore, more efforts are needed to accurately construct single-metal-atom catalysts with high purity and good homogeneity and to establish a linear relationship between defect density and catalytic activity.

Thanks to the lower formation energy, oxygen vacancies are the most common anion defects in metal oxides. The physicochemical properties of metal oxides are changed by the presence of oxygen vacancies, such as the activation of the electron-deficient carbon atoms in CO₂, thereby promoting CO₂RR (Cao et al., 2022; Wang et al., 2022). For example, Zeng’s group reported a facile H₂ plasma treatment to produce oxygen vacancies-riched ZnO nanosheets (Geng et al., 2018). The obtained catalyst exhibited a CO FE of 83% while maintaining a current density of 16.1 mA cm⁻² at −1.1 V vs. RHE. DFT calculations revealed that the charge density of ZnO near the valence band maximum increased due to the presence of oxygen vacancies, which promoted the activation of CO₂. Li et al. (2020) synthesized oxygen vacancy-rich SnO_x nanosheets supported by carbon foam via solvothermal coupled plasma etching strategy (Figure 2A). The optimized electrode exhibited a high FE (86%) and a high partial current density (30 mA cm⁻²) for formate production (Figure 2B). The oxygen vacancies greatly increased the electrochemical surface area and boosted the adsorption and activation of CO₂. Numerous studies have shown that the presence of oxygen vacancies can improve the efficiency of CO₂RR by enhancing CO₂ adsorption and facilitating its conversion to intermediates (Gao et al., 2017; Wang et al., 2018a; Han et al., 2021; Teh et al., 2021). Consequently, quantitative characterization of oxygen vacancies is thus important to the understanding and application of vacancies-rich catalysts. Lei et al. (2022) found that ZnO nanoparticles with oxygen vacancies can trigger the luminol-H₂O₂ system to emit strong chemiluminescence (CL), and the CL intensity is strongly dependent on the oxygen vacancies of ZnO nanoparticles. Based on this characteristic, the authors proposed one method for quantifying the oxygen defects in ZnO. In addition to oxygen vacancies, sulfur defects also play a vital role in modifying the electronic structure and reducing the reaction energy barrier of CO₂RR (Kang et al., 2019; Cheng et al., 2020). Qin et al. prepared a catalyst of CNTs coated with CdS and found that the S-vacancies generated in the CO₂RR process (Qin et al., 2019a). Interestingly, there was a positive correlation between the content of S vacancies and CO formation. DFT calculations indicated that the presence of S-vacancies changed the electron density of the CdS-CNTs catalyst and reduced the energy barrier of CO production.

In addition to anion vacancies, cation vacancies can also greatly affect the physicochemical properties of metal compounds (Yan et al., 2022). Thanks to their unique electron and orbital distribution, cation vacancies-rich materials also exhibit excellent electrocatalytic performance. For example, Pang et al. (2019) developed a cation vacancy-rich ZnS catalyst, achieving a high selectivity (>85%) of formate in CO₂RR. Both experimental and calculation results have demonstrated that the surface Zn vacancies lower the barrier of CO₂RR and suppress the HER (Figure 2C). Xia’s group reported the synthesis of ZnIn₂S₄ nanosheets with abundant Zn vacancies by ultrasonication strategy (Wang et al., 2021d). The obtained catalyst exhibited a partial current density of 245 mA cm⁻² and a FE of 94% for formate production in a flow cell. The results revealed that the Zn vacancies with large electrochemically active surface area contributed to the formation of formate.

As mentioned above, both cations and anions vacancies can further enhance the performance of CO₂RR. In addition to vacancies, the grain boundaries in polycrystalline materials may also alter the intrinsic physical and chemical properties of the material. For example, Tang et al. (2020) synthesized a series of copper twin boundaries on polished Cu electrodes. The atoms at grain boundaries greatly contribute to methane production. Both the experiment and DFT studies confirmed the twin boundaries facilitate the conversion of absorbed CO* into CH₄. Furthermore, Choi et al., 2019 prepared unique star decahedron Cu nanoparticles (SD-Cu NPs) with twin boundaries and multiple stacking faults, exhibiting a low onset potential for methane formation in CO₂RR. The DFT calculations revealed that the twin boundaries significantly reduced the formation energy of *CHO, thereby boosting the formation of CH₄ at low overpotentials (Figure 2D). Wang et al., 2021c prepared carbon sustained SnO₂-Bi₂O₃ hollow nanofibers as Janus catalyst for formate production. The common boundary of the two components synergistically promotes intrinsic activity. All these suggest that grain boundaries may promote the charge density distribution or change the local electronic environment, thus improving the electrocatalytic performance of metal-based catalysts.

Lattice defects, such as lattice dislocations, expansion and distortion, may significantly alter the physicochemical properties of the metal compounds. The unique electronic structure and enhanced electrical conductivity provided by lattice defects help accelerate charge transport during the CO₂RR, thereby increasing catalytic activity. For instance, Zhang’s group synthesized in situ AgCo surface alloy electrocatalysts by cold H₂-plasma activation method (Zhang et al., 2020). The as-prepared alloy exhibited high FE of ethanol (72.3%) when used as a CO₂RR electrocatalyst. The experimental and DFT results revealed that the distortion of the Ag lattice reduced the energy barrier for the intermediate formation and enhanced the C-C coupling. Zhou et al., 2020 used a supercritical CO₂-assisted strategy to fabricate intralayer [Bi₂O₂]²⁺ structural distortion in BiOCl. Thanks to the abundant intralayer [Bi₂O₂]²⁺ structural distortions in thin nanoplate of BiOCl, the catalyst showed a high selectivity of formate (92%) production in a broad potential range from −0.6 to −0.9 V vs. RHE. This work provides a new route to synthesize two-dimensional materials with crystal distortions for electrocatalytic applications.

Zhang et al. (2019) developed a facile in situ electrochemical transformation strategy to synthesize copper foam supported lattice-dislocated Bi nanowires (Cu foam@BiNW). The obtained Cu foam@BiNW showed an enhanced CO₂RR performance with a FE for formate of 95% at −0.69 V vs. RHE (Figures 2E–G). The high CO₂RR activity is attributed to the lattice dislocations on the twisted Bi nanowires and the porous structure. They also synthesized a defect-rich Bi catalyst from Bi₂S₃ through a one-pot hydrothermal reaction (Zhang et al., 2018). When applied to CO₂RR, the as-prepared catalyst exhibited the maximum FE of 84% for formate production at an overpotential of 670 mV. The authors revealed that the lattice defects associated with the Bi₂S₃-derived Bi might have a positive effect on CO₂RR.