Recent Advances in Graphitic Carbon Nitride Based Electro-Catalysts for CO₂ Reduction Reactions

Abstract

The electrocatalytic carbon dioxide reduction reaction is an effective means of combating the greenhouse effect caused by massive carbon dioxide emissions. Carbon nitride in the graphitic phase (g-C₃N₄) has excellent chemical stability and unique structural properties that allow it to be widely used in energy and materials fields. However, due to its relatively low electrical conductivity, to date, little effort has been made to summarize the application of g-C₃N₄ in the electrocatalytic reduction of CO₂. This review focuses on the synthesis and functionalization of g-C₃N₄ and the recent advances of its application as a catalyst and a catalyst support in the electrocatalytic reduction of CO₂. The modification of g-C₃N₄-based catalysts for enhanced CO₂ reduction is critically reviewed. In addition, opportunities for future research on g-C₃N₄-based catalysts for electrocatalytic CO₂ reduction are discussed.

1. Introduction

Fossil fuels have been used extensively to meet humanity’s primary energy needs since the beginning of the industrial revolution [1,2] and this has led to excessive emissions of carbon dioxide into the air [3], resulting in a high CO₂ concentration of 420 ppm in the air and consequent global warming [4]. Therefore, in recent years, many countries have started to pay more attention to climate change and have introduced measures to mitigate the emission of carbon dioxide into the atmosphere. Among the possible technologies to reduce CO₂ emissions into the air, the conversion of CO₂ into valuable chemicals is considered a viable and promising option [5,6].

To date, a variety of methods have been used to convert CO₂ [7], including biochemical [8,9], photochemical [10,11], thermochemical [12,13] and electrochemical processes. Electrocatalytic reduction of CO₂ has many advantages over other CO₂ conversion technologies [14], such as the following: (1) the reaction can be carried out at room temperature and atmospheric pressure; (2) the reaction can lead to the formation of carbon-based fuels that are conventionally produced based on non-renewable feedstock; and (3) the reduction products can be tuned by adjusting the operating parameters and using different electrocatalysts. These advantages have led to widespread interest in the research on the electrocatalytic reduction of CO₂. However, the low catalytic activity and poor stability of electrocatalysts are the main obstacles [14] limiting the large-scale conversion of CO₂. Therefore, there is still a need to develop novel catalysts for the electrocatalytic reduction of CO₂ that are efficient and durable.

The g-C₃N₄ is a chemically stable polymeric semiconductor composed of different isomers of C₃N₄, which was first synthesized by Melon in 1834 [15]. There are generally five isomers of C₃N₄, namely pseudocubic-C₃N₄ (p-C₃N₄), cubic-C₃N₄ (c-C₃N₄), α-C₃N₄, β-C₃N₄, and graphite-C₃N₄. The first four phases all have an atomic density close to that of diamond and are excellent thermal conductors. The g-C₃N₄ has the smallest relative forbidden band and is the most stable isomer of carbon nitride.

The synthesis of g-C₃N₄ usually involves heating inexpensive nitrogen-rich precursors such as cyanamide, dicyanamide, melamine, thiourea, and urea to remove the amino group. The residue of such a process, a light-yellow powder, is the g-C₃N₄. In general, the g-C₃N₄ is thermally stable at temperatures up to 600 °C, insoluble in organic solvents, and very stable in acids and bases [16,17,18]. The excellent chemical and physical stability of g-C₃N₄ makes it a good choice as a catalyst support in heterogeneous catalysis.

Current research on the application of g-C₃N₄ in heterogeneous catalysis(Figure 1) mainly includes photocatalytic water splitting [6,19,20], photocatalytic carbon dioxide reduction [21,22], pollutant degradation [23,24], photocatalytic ammonia synthesis [25,26], photoelectrochemical catalysis [27,28], electrocatalytic hydrogen evolution [29,30], electrocatalytic oxygen evolution [31,32], electrocatalytic water splitting [33,34], electrocatalytic carbon dioxide reduction, electrocatalytic oxygen reduction [35,36], electrocatalytic organic oxidation [18,37], photothermal hydrogenation of carbon dioxide [38,39], photothermal catalytic hydrogen evolution [40,41], etc. The g-C₃N₄ has shown a good capability in these catalytic reactions.

Due to the high nitrogen content of g-C₃N₄, the Lewis sites and Bronsted sites of the g-C₃N₄ structure enhance the adsorption of CO₂ [42]. In addition, the carbon in g-C₃N₄ has a high affinity for oxygen-binding intermediates (_*OCH_x, O and OH) in the reactions that results in the production of deep reduction products such as methane (CH₄) [43]. This property increases the efficiency of the electrocatalytic reduction of CO₂ and leads to the production of more valuable hydrocarbon products [44]. Bulk g-C₃N₄, as a big sheet of lamellar structure, has a low specific surface area (<10 m²g⁻¹) [45] when used as an electrocatalyst [46] resulting in low electrical conductivity [47] and poor catalytic effect [48] which subsequently limits its use as a catalyst support due to the lack of exposed active sites. Therefore, it is necessary to increase the electrical conductivity, specific surface area, and number of exposed active sites of g-C₃N₄ before it can be used as an excellent electrocatalyst and catalyst support [49].

This review focuses on the recent advances in the electrocatalytic reduction of CO₂ using g-C₃N₄ as a catalyst and/or catalyst support. The properties and role of the g-C₃N₄ in the electrocatalytic reduction of CO₂ are discussed. The synthesis, functionalization, and modification of g-C₃N₄ materials as electrocatalysts for enhanced CO₂ reduction are summarized. Finally, the future opportunities for g-C₃N₄ as a catalyst for electrocatalytic CO₂ reduction are discussed.

1.1. Fundamentals of Electrocatalytic Reduction of CO₂

Carbon dioxide is a stable molecule. The carbon dioxide reduction reaction (CO₂RR) is a commonly used approach to convert CO₂ into valuable chemicals and is classified as a heterogeneous reaction [6,50]. One-carbon products of CO₂RR include carbon monoxide (CO), methane (CH₄), formaldehyde (CH₂O), methanol (CH₃OH), and formic acid (HCOOH), while two-carbon reduction products include ethylene (C₂H₄), ethane (C₂H₆), acetylene (C₂H₂), ethanol (C₂H₆O), and oxalic acid (H₂C₂O₄), among others. The heterogeneous catalytic conversion of CO₂ at the electrode surface consists of three main steps: (i) the electrode material loses its absolute linearity due to the adsorption of the normal linear CO₂ molecule; (ii) the conversion of the C-O bond to a C-H bond by proton-coupled electron transfer; and (iii) the intermediate is desorbed from the catalytic electrode and then diffuses into the electrolyte [51].

Since CO₂ is structurally stable and its carbon-oxygen double bond is energetic and not easily broken, from a thermodynamic point of view, an equilibrium potential must be applied during the CO₂RR reaction to reduce CO₂, as shown in

Table 1. Electrode potentials of selected CO₂ reduction reaction in a 0.1 M solution of KHCO₃ at 1.0 atm and 25 °C.

	Chemical Formula and Molecular Formula	Half-Electrochemical Reaction	Potential versus Reversible Hydrogen Electrode (V vs. RHE)
C1	HCOOH	CO2 + 2H+ + 2e− = HCOOH	−0.651
	CO	CO2 + 2H+ + 2e− = CO + H2O	−0.507
	CH2O	CO2 + 4H+ + 4e− = CH2O + H2O	−0.471
	CH3OH	CO2 + 6H+ + 6e− = CH3OH + H2O	−0.385
	CH4	CO2 + 8H+ + 8e− = CH4 + 2H2O	−0.232
C2	C2H2O4	2CO2 + 2H+ + 2e− = H2C2O4	−0.901
	C2H6	2CO2 + 14H+ +14e− = C2H6 + 4H2O	−0.261
	C2H4	2CO2+ 12H+ + 12e− = CH2CH2 + 4H2O	−0.337
	C2H5OH	2CO2 + 12H+ + 12e− = CH3CH2OH + 3H2O	−0.3172

Carbon, oxygen, and hydrogen atoms are red, yellow, and gray, respectively.

. This is the reversible hydrogen electrode potential applied for the conversion of carbon dioxide into various products by electrocatalysis. However, the actual electrode potential required for the reduction reaction is more negative than the equilibrium potential [14,52]. This is because in the actual reaction, the carbon dioxide molecule is adsorbed onto the catalyst and gains a single electron, leading to the formation of the key intermediate CO₂⁻. In this step, the reaction requires a significant amount of energy to convert the linear carbon dioxide molecule into a bent radical anion, making the actual potential more negative than the standard potential.

In the CO₂RR reaction, different reduction products are formed via different reaction pathways, which can be divided into four categories depending on the reaction intermediates, namely, formaldehyde, carbene, glyoxal, and enol-like intermediates.

Mono-carbon products are usually formed via the formaldehyde pathway (Figure 2a), which involves the synergistic transfer of protons and electrons to CO₂ to form a carboxylic intermediate (COOH⁻). The intermediate then accepts electrons and hydrogen ions to form HCHO. The carbene pathway (Figure 2b) is the most likely reaction pathway when CO₂ is bound to the active site via its carbon atom, which gains electrons and H⁺ and then loses hydroxide to form a carbon monoxide intermediate, which is converted to carbon monoxide (*CO⁻). By gaining electrons and H⁺ again, it loses hydroxyl to form carbene intermediates (CH₂); this is the carbene pathway which is the most likely pathway. Glyoxal and enol-like pathways are common in the formation of multi-carbon (C2–C3) products [53]. The glyoxal pathway (Figure 2c) involves the formation of a coordination bond between the two oxygen atoms of carbon dioxide and a catalytic active site [54,55,56]. The pathway for glyoxal is the formation of a double O coordination bond between carbon dioxide and the catalytic active site, loss of the hydroxyl group to give the formyl radical *HCO after continuous addition of electrons and H⁺, and then dimerization to form the glyoxal intermediate (C₂H₂O₂). C-C bond coupling via C1 or C2 intermediates leads to an aldehyde-free enol intermediate, but there is much uncertainty about the key intermediates involved in C-C bond coupling.

1.2. Unique Properties of g-C₃N₄ as an Electrocatalyst

Surface catalysis and high mass transfer efficiency are the characteristics and advantages of electrocatalytic reactions. The g-C₃N₄ is a layered material with a huge specific surface area and large number of active sites. It has other advantages, such as easy adjustment of the electron distribution at the interface, high acid-base stability, etc. All these advantages suggest that g-C₃N₄ can be used as an electrocatalyst for various applications.

1.2.1. Morphology of g-C₃N₄

The morphology of g-C₃N₄ varies depending on the preparation method and precursors. The nitrogen-rich precursor is thermally polymerized directly to a massive solid g-C₃N₄ with a low specific surface area and porosity. The g-C₃N₄ with high specific surface area and large porosity (up to 830 m²g⁻¹ and 1.25 cm³g⁻¹) [57] was obtained by precursor treatment [58]. In terms of ion/carrier transport, g-C₃N₄ has a distinct advantage. When used in multiphase catalysis, porous g-C₃N₄ can enhance electrocatalysis by providing solution, electrolyte, and gas channels on the exposed surface, thus improving ion transfer and diffusion processes between different substances. Different preparation methods lead to g-C₃N₄ materials with different properties, making them useful as catalytic materials and/or supports for various applications. g-C₃N₄ with a large surface area and uniform pore size can be prepared by using the template method [57], and by modulating the temperature and pyrolysis time [59,60,61]; the specific surface area, pore size, and pore volume of g-C₃N₄ have been varied, allowing the morphology of the catalyst to be adjusted (

Table 2. Specific surface area, pore volume, and pore size of g-C₃N₄ under different precursor types and different synthesis parameters.

Precursors of g-C3N4	Reaction Method	Specific Surface Area (m2g−1)	Pore Volume (mLg−1)	Pore Diameter (nm)	Reference
ethylenediamine (EDA) and carbon tetrachloride (CTC)	Hydrothermal synthesis,100 °C	505	0.55	4.2	[57]
EDA and CTC	Hydrothermal synthesis,130 °C	830	1.25	5.1	[57]
EDA and CTC	Hydrothermal synthesis,150 °C	650	0.89	6.4	[57]
Melamine	470 °C, 2 h, air	6.0	0.02	35.2	[59]
Melamine	500 °C, 2 h, air	41.5	0.14	9.2	[59]
Melamine	520 °C, 2 h, air	173.6	0.77	15.6	[59]
Melamine	540 °C, 2 h, air	0.77	0.94	16.5	[59]
urea	550 °C, 0.5 h, air	52	N/A	N/A	[60]
urea	550 °C, 1 h, air	62	0.30	N/A	[60]
urea	550 °C, 2 h, air	75	0.34	N/A	[60]
urea	550 °C, 4 h, air	288	1.41	N/A	[60]
Water-assisted urea	450 °C, 3 h, air	96	0.72	N/A	[61]
Water-assisted urea	450 °C, 5 h, air	106	0.68	N/A	[61]
Dicyandiamide	550 °C, 2 h, air	10	N/A	N/A	[64]
Melamine	550 °C, 2 h, air	8.6	0.02	N/A	[59]
Thiourea	550 °C, 2 h, air	11	N/A	N/A	[64]

N/A indicates that the data is not mentioned in the reference.

) [62]. The specific surface area and pore size of g-C₃N₄ obtained by thermal polymerization of different precursors at the same temperature of 550 °C and in an air atmosphere for 2 h are also different [59,60,63,64].

1.2.2. Surface Active Sites

Catalytic activity is usually related to the active sites where reactants and intermediates are adsorbed and charge transfer takes place [62]. The pyridinic N atom on the heptazine (Figure 3 has a strong electron accepting capacity, making the g-C₃N₄ surface an active site for initiating electrochemical reactions and enhancing the electrocatalytic properties of g-C₃N₄ [65]. The surface reactivity varies depending on the position of the surface atoms [66]. For the construction of an integrated composite catalyst system, the many active sites of g-C₃N₄ (with floating bonds at the pore edge) may also allow better dispersion and binding with auxiliary catalysts or other coupling materials. In the field of mesoporous or macroporous g-C₃N₄ catalyst heterogeneous catalysis, its large specific surface area and porosity provide a high density of active sites [49,67]. For the eCO₂RR, g-C₃N₄ provides plenty of N-sites, which enhances the material’s ability to bind CO₂ and increases the local concentration of CO₂ around the catalyst [68]. When g-C₃N₄ is used as a catalyst, the state of the active site can be changed by adjusting the nitrogen or carbon elements in the structure [44]. When g-C₃N₄ is used as a support, the active site of the g-C₃N₄ involved in the catalytic reaction changes due to different loading conditions. g-C₃N₄ forms coordination bonds with doped elements as the active site of the catalyst [69,70].

1.2.3. Stability

The g-C₃N₄ is normally produced by pyrolysis of nitrogen-containing precursors such as urea, dicyandiamide, and melamine at 500 °C, resulting in exceptional thermal stability. Thermal stability begins to decrease when the temperature exceeds 600 °C. Thermal decomposition begins at 650 °C and is completely degraded at 700 °C [71]. g-C₃N₄ is not afraid of strong acids and bases; in the preparation of g-C₃N₄ by template method, the template can be removed by washing with strong acid [72,73], and its chemical properties are stable [65,73], so g-C₃N₄ is widely used as a catalyst carrier. Its stable chemical properties make it a promising organic framework for a single atom catalyst. When g-C₃N₄ is used as a catalyst support, its numerous porous channels and active sites can reduce the transfer distance between the catalyst and the electrolyte or solution, accelerate the reaction rate, and protect the catalyst from corrosion for a short period of time.

2. Graphitic Carbon Nitride-Based Catalysts for Electrocatalytic CO₂RR

The g-C₃N₄-based catalysts are divided into three main categories: pristine g-C₃N₄, metal doped g-C₃N₄, and non-metal doping g-C₃N₄. Their applications in the field of electrocatalytic carbon dioxide reduction are reviewed, and the synthesis methods of these catalysts as well as the product selectivity control of g-C₃N₄-based catalysts for electrocatalytic carbon dioxide reduction reactions are described in detail.

2.1. Pristine g-C₃N₄

The g-C₃N₄ with a specific surface area of 10 m²g⁻¹ was obtained by holding dicyandiamide in a covered crucible that was heated to 550 °C at a heating rate of 3 °C per minute and kept isothermal for 4 h. The Faraday efficiency (FE) of pristine g-C₃N₄ electrocatalysis of carbon dioxide to carbon monoxide in a 0.1 M KHCO₃ electrolyte is approximately 5% [70,74,75,76]. Compared to the g-C₃N₄ nanosheet molecules (with a surface area of 235 m²g⁻¹) [77] and bulk g-C₃N₄ (with a surface area of 8 m²g⁻¹) [49] and others obtained by conventional methods, the ultra-thin polarized g-C₃N₄ layer (2D-pg-C₃N₄), which was obtained by hydrothermal stripping (thickness: ~1 nm), has a larger specific surface area of 292.4 m²g⁻¹, which subsequently enhances the adsorption effect of CO₂. The overall foam-like structure promotes the diffusion of CO₂ molecules to the active surface of the catalyst. The ultrathin layered structure of 2D-pg-C₃N₄ enables the faster release of electrons from the polarized Melem subunit, which promotes the CO₂ reduction reaction. At a potential of 1.1 V vs. Ag/AgCl, CO₂RR achieved a total Faraday efficiency of 91%, resulting in the conversion of CO₂ to CO (80%) and formic acid (11%), almost completely blocking the HER process. At a potential of −1.2 V, the current density of 2D-pg-C₃N₄ reached 3.05 mA cm⁻², almost 30 times that of bulk g-C₃N₄. The production of CO also increased by 17.1 times.

Compared with g-C₃N₄ with a complete crystal structure, which can only reduce CO₂ to CO [44,49,76,78], the π-electron leaving domains of the engineered vacancies in the g-C₃N₄ conjugated skeleton and the effect of the vacancies on g-C₃N₄ in electrocatalytic CO₂ reduction reactions have been investigated by Density Flooding Theory (DFT) calculations. A g-C₃N₄ obtained by thermal stripping is close to the theoretically calculated N vacancy (vacancy engineered) and is named DCN. A suitable N vacancy can change the geometry of g-C₃N₄ and adjust its adsorption strength for key intermediates, which not only increases the desorption energy barrier of *CO intermediates but also limits the formation of CO and H₂. This reduces the activation energy barrier of CO₂ reduction to CH₄ and promotes CH₄ formation. The carbon atom in DCN was identified as the active site of the CO₂RR reaction according to DFT calculations. The presence of the N-vacancy changes the tri-coordinating carbon atom around the N vacancy to a di-coordinating carbon atom, making the carbon atom more unsaturated and more prone to combine with the *CO intermediate to form a stronger bond. This makes the desorption of the *CO intermediates more difficult, preventing the formation of CO and allowing further conversion to CH₄ [79]. The nitrogen-rich DCN electrocatalyst showed high activity in the electrocatalytic reduction of CO₂ over the whole potential range (Figure 4b). At a potential of −1.27 V vs. RHE, the Faraday efficiency of CH₄ can reach 44% (Figure 4a), and the current density of CH₄ generation reaches 14.8 mAcm⁻², which are 6.3 and 7 times more effective than the ordinary bulk g-C₃N₄ catalysts, respectively, when reacting in CO₂-saturated 0.5 M KHCO₃, indicating that nitrogen vacancies in carbon nitride can enhance the electrocatalytic reaction.

The modification effect of g-C₃N₄ can be achieved by a special treatment of g-C₃N₄ itself or the precursor compound, which changes the morphology and electronic structure of g-C₃N₄ and increases the electrical conductivity and selectivity of the catalyst for certain products.

Table 3. Electrocatalytic parameters of the pristine g-C₃N₄ catalysts in eCO₂RR.

Electrode	Product	FE	Potential (V vs. RHE)	Electrolyte	Current Density (mAcm−2)	Ref
Bulk g-C3N4	CO	5%	−1.2	0.1 M KHCO3	ca.0	[70]
g-C3N4	CO	ca.8%	−1.1	0.1 M KHCO3	ca.30	[75]
2D-pg-C3N4	CO	80%	−0.6	2 M KHCO3	3.05	[76]
DCN	CH4	44%	−1.27	0.5 M KHCO3	14.8	[44]

shows the electrocatalytic performance of the pristine g-C₃N₄ catalysts for the electrocatalytic CO₂ reduction reaction. The use of the modified g-C₃N₄ as a catalyst support can further increase the efficiency of the electrocatalytic CO₂ reduction reaction.

2.2. Metal Doped g-C₃N₄

g-C₃N₄ is an excellent catalyst support. At present, various applications of metal doped g-C₃N₄ catalysts have been studied in many fields, but less so in CO₂RR. The metal doped g-C₃N₄ catalysts generally have higher electrical conductivity and catalytic activity than non-metal doping g-C₃N₄ catalysts due to the electronic properties of the metal atoms. The metal catalysts were classified into single metal doped g-C₃N₄ catalysts, bimetallic-doped g-C₃N₄ catalysts, and ternary composite catalysts.

2.2.1. Single Metal Doped g-C₃N₄

Single metal doped catalysts are currently one of the most used types of electrocatalysts for the CO₂ reduction reaction. The metal doped catalysts discussed in this article are mainly nanocatalysts. Monometallic catalysts are classified as monatomic catalysts, metal cluster catalysts, and metal nanocatalysts depending on the size of the metal particles.

The active site has the greatest influence on the CO₂RR activity and selectivity of metal catalysts [80]. Single atom catalysts are catalysts in which metals are uniformly and individually loaded as single atoms onto metals, metal oxides, two-dimensional materials, and molecular sieves, using the single atom as the catalytic active center for the catalytic reaction. Studies have integrated and summarized the stabilization mechanism of single atom catalysis on different supports, where MN_x, MS_x, and other stable structures (M is monatomic) with heteroatoms (N, S, P, etc.) are generated [81] on carbon supports. The electronic properties and catalytic performance of a single atom depend on its coordination with the nitrogen and sulfur atoms in the support. The potential advantages of the single atom catalyst on g-C₃N₄ support are as follows: (1) the porous structure of g-C₃N₄ has a large specific surface area, which can improve the loading rate of metal atoms and create more active sites [82]; (2) the delocalization of the π-electron in the conjugated framework of g-C₃N₄ can change the electronic and the catalytic properties of the monatomic center [44]; (3) strictly single atom layers can facilitate the adsorption and diffusion of reactant molecules from either sides of g-C₃N₄ on separate single atoms; (4) g-C₃N₄ is a good model catalyst, which make it easier to identify uniform active sites and predict catalytic performance using chemical theoretical methods; (5) single-atom anchoring can promote or activate the original catalytic activity of two-dimensional materials [83].

Studies have shown that transition metal atoms (Fe, Ti, Ru, V, Cr, Ir, Mn, Co, Ni, Cu, Rh, Sc, Pd, Au, Ag, Pt) can be combined with a single layer of g-C₃N₄ to form a single atom catalyst using first-principles calculations [84,85]. Low temperature embedding Cr and Mn in g-C₃N₄ allows the preparation of promising single atom catalysts. Three single dispersed transition metal atoms (Fe, Co, Ni) have been modified in the structure of g-C₃N₄. In all optimized structures, the transition metal is located in the hexagonal hole of g-C₃N₄ and forms strong coordination bonds with the adjacent mono-atom. Figure 5a–d shows the structural optimization of single atom catalysts of g-C₃N₄ and three transition metals. The strength of the M-N bond formed by metal atoms and g-C₃N₄ varies with the difference between the atomic radius and valence electron number of cobalt and iron. Stable M-C₃N₄ structures are formed by strong M-N bonds [36]. The adsorption process of CO₂ on the catalyst was studied by DFT calculations. It was found that CO₂ exhibits weak physical adsorption on Ni-C₃N₄ and that the adsorption configuration of CO₂ on Co-C₃N₄ and Fe-C₃N₄ is chemical adsorption. In addition to the M-C bond, an M-O bond was also formed between the CO₂ molecules and the metal atoms of the catalyst. During adsorption, large number of electrons are transferred from Co or Fe atoms to CO₂ via M-C and M-O bonds. The density of states (DOS) analysis explains that the different adsorption configurations are caused by the different distribution of the d orbitals after the doping of the transition metal atoms. The differences in the adsorption conditions lead directly to the differences in the initial hydrogenation products. *CO₂ + H⁺ + e⁻ → *COOH is the most favorable first protonation step on Ni-C₃N₄, and *CO₂ + H⁺ + e⁻ → *OCHO is the first step of protonation on Co-C₃N₄ and Fe-C₃N₄. The carbon dioxide reduction reaction on the surface of three catalysts was studied by constructing a thermodynamic reaction network. The results show that the three catalysts can inhibit the formation of H₂, CO, and HCOOH, and the end products tend to be CH₃OH and CH₄. The main rate-determining steps of Ni-C₃N₄ and Fe-C₃N₄ in the process of CH₃OH and CH₄ formation are the steps to form the intermediate *CHO, so their selectivity is limited. However, the rate-determining steps of the process on Co-C₃N₄ are different and show a high methanol selectivity and the lowest initial reaction potential (UL = −0.65 V, as shown in Figure 5e). Therefore, the single atom catalyst based on g-C₃N₄ is advantageous to achieve deep CO₂ reduction at a low electrode potential.

To investigate the effect of the catalysts Co-C₃N₄, Fe-C₃N₄ and Mg-C₃N₄ on the weak CO hybridization (Figure 5f), the reaction pathways of these catalysts were investigated. As shown in Figure 5g, the analytical energy barrier of Mg-C₃N₄ (0.13 eV) is much lower than that of Fe-C₃N₄ (1.37 eV) and Co-C₃N₄ (1.52 eV) [86]. Mg atoms are thought to desorb CO more readily than Fe and Co atoms. CO temperature program desorption (TPD) and in-situ attenuated total reflection infrared (ATR-IR) spectroscopy was used to demonstrate the CO desorption capability of Mg-C₃N₄. In contrast to the large CO desorption peaks on Fe-C₃N₄ and Co-C₃N₄, Mg-C₃N₄ shows no CO desorption peak, indicating that CO can be easily desorbed from the Mg sites (Figure 5h). Similarly, significant changes in current density were observed on Fe-C₃N₄ and Co-C₃N₄ during the electro-response tests in Ar and CO atmospheres. The larger differences in the current density of Fe-C₃N₄ (0.16 mAcm⁻²) and Co-C₃N₄ (0.09 mAcm⁻²) between the cases with CO and with Ar (Figure 5i) indicate the stronger CO adsorption capacity of the Fe and Co sites compared to the Mg sites (0.006 mAcm⁻²). No significant *CO was found on Mg-C₃N₄, although CO was produced in large quantities, indicating that the produced CO was well desorbed (Figure 5i,k). In contrast, the presence of distinct *CO bands on the Fe and Co-C₃N₄ electrodes confirmed the significant adsorption of CO on Fe and CO sites (Figure 5l). The results of the TPD, the electro-response measurement, and the ATR-IR spectra show that CO exhibits weak desorption at the Mg site, which is in good agreement with the theoretical calculation results. The electrocatalytic CO₂ reduction reaction takes place in an H-cell electrolysis cell with a turnover frequency (TOF) of 18,000 h⁻¹ for the Mg-C₃N₄ catalyst and the Faraday efficiency of CO in the KHCO₃ electrolyte reaches ≥ 90%. The electrochemical reduction of CO₂ in the flow cell can achieve a current density of −300 mAcm⁻² and ensure a Faraday efficiency of 90%. The results show that the metal in the s-block can be used for highly efficient electrochemical reduction of CO₂ to produce CO.

Unlike monoatomic catalysts, which focus on the dispersed state of individual metal atoms, monometallic nanoparticle catalysts exhibit a much more diverse state of metal element presence. It has been demonstrated experimentally and through theoretical calculations that the interaction of Au with g-C₃N₄ causes the Au surface to become extremely electron-rich, thereby facilitating the adsorption of the key chemical intermediate *COOH [87]. Similarly improved CO₂RR performance was observed on Ag NPs loaded with g-C₃N₄ (Ag/C₃N₄), which also had electron-rich Ag surfaces [48]. The Ag₂O precursor was incompletely decomposed under hydrothermal conditions to form super-stable oxides and nanosilver and loaded onto g-C₃N₄, and the super-stable oxides in the catalyst improved the binding energy of the *COOH intermediate [48]. The rate-determining step of the electrocatalytic CO₂ reduction reaction is changed from electron transfer to proton transfer due to the strong interaction of the Ag nanoparticles (NPs) with the g-C₃N₄ support in the catalyst via the Ag-N bond. This improves the performance of electrocatalytic CO₂ reduction, and the Faraday efficiency of CO at −0.7 V vs. RHE can reach 94% at low potential. Ag-decorated B-doped g-C₃N₄ catalysts were synthesized by loading Ag NPs on boron-doped g-C₃N₄, and the electrochemical reduction properties of the catalysts were investigated by theoretical calculations and experiments [70]. The DFT calculations show that the Ag-B-g-C₃N₄ catalyst can significantly reduce the adsorption free energy for the formation of the *COOH intermediate. In addition, the electron accumulation at the Ag-B-g-C₃N₄ interface can promote electron transit and increase the electrical conductivity. The simulation results show that the addition of B atoms and Ag NPs can significantly improve the eCO₂RR performance of g-C₃N₄. The electrocatalysts g-C₃N₄, B-g-C₃N₄, and Ag-B-g-C₃N₄ were prepared for comparative experiments and it was proved by XPS, XRD, TEM, and other characterization methods that CO can only be generated by Ag-B-g-C₃N₄, proving that Ag is the only active site. Electrochemical impedance spectroscopy (EIS) analysis showed that the Ag atom has a catalytic effect on electron transport. An Ag-B-g-C₃N₄ catalyst with an average diameter of 4.95 nm has a total current density of 2.08 mAcm² and the Faraday efficiency of CO is 93.2% at a potential of −0.8 V vs. RHE.

A study was carried out by performing extensive DFT calculations on Cu-C₃N₄ model catalysts and comparing their CO₂ reduction potential with Cu(111) surface and standard Cu-NC complexes, showing that Cu-C₃N₄ has better CO₂ reduction activity, lower starting potential, and a significantly higher C2 formation rate compared to Cu-NC (Figure 6a–d) [78].

Cu₂O/CN was obtained by immobilizing Cu₂O nanocubes on the structure of g-C₃N₄ by chemical precipitation. On the one hand, the g-C₃N₄ framework provides an anchor center for the in-situ growth of Cu₂O, which promotes uniform dispersion of Cu₂O and exposes more active sites. On the other hand, g-C₃N₄ shows good CO₂ adsorption and activation capabilities. At the interface between the g-C₃N₄ support and the Cu₂O NPs, the CO₂ is adsorbed onto the g-C₃N₄, generating *CO intermediates. The *CO intermediates generated on the g-C₃N₄ have the possibility of C-C coupling with the C atoms in the intermediates generated on the Cu₂O surface, which improves the active site and increases the electrocatalytic kinetic rate, thus increasing the yield of C₂H₄ [68]. The specific steps are the reduction of incoming CO₂ to CO by combining 2H⁺ and 2e⁻ on g-C₃N₄. The CO formed on g-C₃N₄ can then be transported to the Cu₂O site due to the stronger bonding between CuO and CO and the increased local CO concentration and residence time near the Cu₂O surface due to the high surface coverage of *CO. Further reduction of CO or CO dimers at the active site leads to the formation of C₂H₄, followed by C₂H₄ desorption from Cu₂O/CN. The key step in the transfer of CO from C₃N₄ to Cu₂O was shown by calculations to be due to the synergistic effect of g-C₃N₄ and Cu₂O. At −1.1 V vs. RHE, the Faraday efficiency of the Cu₂O/CN composite on C₂H₄ is 32.2% and the local current density is −4.3 mAcm⁻². At −1.1 V vs. RHE for at least 4 h, the Cu₂O/CN catalyst maintained its stable performance and structure.

Compared to pure CuO nanosheets and spherical CuO particles, g-C₃N₄ plays a role in increasing the specific surface area and exposing active sites in the CuO/g-C₃N₄ catalyst, providing new opportunities for CO₂ adsorption and promoting mass transfer kinetics. In addition, the interaction of pyridine-N and copper oxide contributes to C-C coupling, further enhancing the activity of the CO₂ reduction reaction, with Faraday efficiencies of up to 64.7% for all C2 products below −1.0 V vs. RHE [89]. MnO₂/g-C₃N₄ [90] and ZnO/g-C₃N₄ [90] catalysts can be prepared by a simple pyrolysis method. Possible catalytic routes for the reduction of CO₂ to formates in alkaline media with additional bases such as triethylamine are shown in (Figure 6e). First, CO₂ molecules can combine with water molecules on the surface of g-C₃N₄ to form carbonic acid. CO₂ acts as a Lewis acid and is activated by triethylamine to form amphoteric carbamate intermediates. The amphoteric intermediates formed on the metal surface and the active metal oxides on the other side play a role in the activation, adsorption, and dissociation of hydride molecules. The hydride molecule is transported from the catalyst surface to the active intermediate, the amphoteric carbamate ion (Schiff base), to form carbamate, which undergoes acid–base neutralization in the aqueous medium and is then converted to formic acid. When graphite carbon nitride donates electrons, the electrons reach the surface of metal oxide and help the hydride (-H) dissociate from the surface of the metal oxide, giving it easy access to the electron-deficient carbamate intermediate to form formic acid. For formates, the MnO₂/g-C₃N₄ catalyst has a Faraday efficiency of 65.28%, while the ZnO/g-C₃N₄ catalyst has a Faraday efficiency of 80.99%, which is related to the properties of the metal itself.

Monoatomic metal catalysts are popular with researchers due to their excellent electrochemical properties and have been well studied theoretically, but actual experiments are still rare due to the difficulty of their synthesis. Nanocluster catalysts have a very high surface area and unique surface structural features. The doping of the mono-metal on g-C₃N₄ increases the active sites for the reaction and improves the stability of the nanometallic particles in the catalyst and the selectivity of the products.

Table 4. Electrocatalytic parameters of single metal doped g-C₃N₄ catalysts in eCO₂RR.

Electrode	Product	FE	Potential (V vs. RHE)	Electrolyte	Current Density (mAcm−2)	Ref
Mg-C3N4	CO	90%	−1.178	KHCO3	32	[86]
Ag/g-C3N4	CO	94%	−0.7	1.0 M KHCO3	11.5	[48]
Au/C3N4	CO	90%	−0.45	0.5 M KHCO3	2.56	[87]
Ag/C3N4	CO	92%	−0.9	0.5 M KHCO3	22	[87]
Ag-Decorated B-Doped g-C3N4	CO	93.20%	−0.8	0.5 M KHCO3	2.08	[70]
Fe@C/g-C3N4	CO	88%	−0.38	0.1 M KHCO3	5.5	[91]
ZnO/g-C3N4	formate	80.99%	−0.934	0.5 M KHCO3	ca.33	[90]
Cu2O/CN	C2H4	32.20%	−1.1	0.1 M KHCO3	−4.3	[68]
Cu/C3N4	CO	ca.30%	ca.−1.0	0.1 M KHCO3	8	[78]
MnO2/g-C3N4	formate	65.28%	−0.54	0.5 M KHCO3	ca.5	[88]
C3N4/(Co/Co(OH)2)	formate	N/A	−0.9	0.5 M KHCO3	0.08	[92]

shows the electrocatalytic parameters of the single metal doped g-C₃N₄ catalysts in the electrocatalytic carbon dioxide reduction reaction.

2.2.2. Bimetallic Doped g-C₃N₄

Bimetallic catalysts often outperform monometallic catalysts of the same metal composition due to the synergistic interaction between the different atoms in the bimetallic catalyst [93]. The dynamic structure and chemical changes on the surface of the bimetallic catalyst during inhomogeneous catalytic reactions make the synergistic mechanism more complex [94]. Studying the catalytic reaction process and reaction pathway of bimetallic catalysts and selecting suitable metal elements for g-C₃N₄ modification can improve stability and catalytic activity.

Cu_xRu_yCN was obtained by modifying copper–ruthenium bimetallic compounds on the surface of π-conjugated g-C₃N₄. The Cu_xRu_yCN samples exhibited excellent BET surface area, pore size, and pore volume due to the appropriate Cu and Ru doping ratio, which attracted reaction molecules and provided active sites for enhanced electrocatalytic processes. The Mott–Schottky effect is caused by the formation of metal-semiconductor interfaces between Ru, Cu, and g-C₃N₄ heterojunctions, which significantly increases the efficiency of charge separation and prevents reverse flow from the metal to the semiconductor. The mixed state of CuO and Cu₂O acts as an active center for the adsorption and activation of CO₂, while RuO₂ acts as a center for the enrichment of holes for the synergistic transfer of H protons to promote the reduction of CO₂. In an air or Ar atmosphere, the current density of the reaction decreases to below 0.05 mAcm⁻² when the applied potential is −1.5 V, indicating that the high current density of the Cu_xRu_yCN catalyst in the reaction is related to the flow of CO₂ and its reduction. Moreover, the current density of Cu_xRu_yCN remains constant at an applied potential of −1.4 V vs. Ag/AgCl for at least 2000 s, indicating its high stability in the CO₂ reduction process [95].

The CuSe/g-C₃N₄ catalyst can be obtained by anchoring hexagonal CuSe nanoplates on g-C₃N₄ nanosheets by hydrothermal method [96,97]. The morphologies of the hexagonal CuSe nanoplates before and after anchoring are shown in SEM images (Figure 7a,b). The internal electric field formed between the electron coupling Cu and Se on the electrode is confirmed by the DFT calculation, and the electrons move from the g-C₃N₄ nanosheets to the CuSe nanoplates. The results show that CuSe is the active site of the catalyst and that CO₂ is activated on the surface of the CuSe nanoplates and controlled by the activation process. The CuSe/g-C₃N₄ with 50% CuSe nanoplate content was tested and showed the best catalytic performance. At −1.2 V vs. RHE, its CO Faraday efficiency was 85.28% (Figure 7c,d). The Faraday efficiency is 1.47 times higher than that of pure CuSe nanoplates, which is due to the addition of g-C₃N₄ nanosheets with a planar structure, which provide a larger specific surface area.

The catalyst g-C₃N₄/Cu₂O-FeO was obtained by dissolving iron salt, copper salt, and g-C₃N₄ in a certain ratio in triethylene glycol and reacting in an autoclave for 12 h [98]. High resolution transmission electron microscopy (HRTEM) results showed that the prepared nanocomposites have Cu₂O-FeO mixed metal oxide nanoclusters uniformly distributed on the surface of the g-C₃N₄ nanosheets. The size is about 10 nm (Figure 7e–g). This composite catalyst is used as an electrode for the electrochemical reduction of CO₂ and exhibits high catalytic activity. The total current density is 4.65 mAcm⁻², the overpotential is −0.865 V vs. NHE, and the applied potential is −1.60 V vs. Ag/AgCl. The maximum CO Faraday efficiency is 84.4% (Figure 7h,i). The conversion rate is up to 10,300 h⁻¹ and the selectivity is 96%. This improvement is the result of the close interfacial interaction between g-C₃N₄ and the metal oxide (Cu₂O-FeO), which leads to a larger electrochemically active surface area and oxygen vacancies on the surface.

The C₃N₄/Co(OH)₂/Cu(OH)₂ catalyst is a bimetallic hydroxide catalyst whose synthesis is divided into two steps [92]. The first step consists of the synthesis of Co NPs by reduction of Co²⁺ ions on the surface of C₃N₄ with the strong reducing agent NaBH₄. Co²⁺ ions are hydrolyzed in water to form Co(OH)₂. In the second stages, these Co NPs undergo an electrical exchange process on C₃N₄ where the Co NPs are exchanged for Cu atoms to form Cu(OH)₂ in an aqueous solution. Overall, the synthesis process culminates in the deposition of cobalt and copper hydroxides (C₃N₄/(Co(OH)₂/Cu(OH)₂) on the surface of C₃N₄. By changing the surface morphology during the primary cell replacement process, more active sites and suitable adsorption-matrix interactions can be achieved. The electrocatalytic activity of C₃N₄/(Co(OH)₂/Cu(OH)₂ is more than three times that of C₃N₄/(Co(OH)₂/Cu(OH)₂ due to the synergistic effect of cobalt and copper hydroxide.

Bimetallic doped g-C₃N₄-based catalysts, with g-C₃N₄ material as the catalyst support, are able to stably load bimetals onto the g-C₃N₄ structural skeleton. The bimetals not only create interactions with g-C₃N₄ similar to those between metal and g-C₃N₄ in monometallic catalysts, but also have their unique synergistic effects between the bimetals to further enhance the electrocatalytic effect. Current research in this area is based on nanoparticles. Dual atom catalysts (DACs), obtained by modulating the morphology of metal particles, and exploiting the interaction between the two metals can effectively overcome some of the application limitations of SACs.

Table 5. The electrocatalytic parameters of bimetallic doped g-C₃N₄ catalysts in eCO₂RR.

Electrode	Product	FE	Potential (V vs. RHE)	Electrolyte	Current Density (mAcm−2)	Ref
CuSe/g-C3N4	CO	85.28%	−1.2	0.1 M KHCO3	11	[96]
CuxRuyCN	N/A	N/A	−0.8	0.1 M KHCO3	0.3	[95]
g-C3N4/Cu2O-FeO	CO	84.40%	−0.9829	0.1 M KCl	3.91	[98]
C3N4/(Co(OH)2/Cu(OH)2	formate	N/A	−0.9	0.5 M KHCO3	0.23	[92]

shows the electrocatalytic parameters of the bimetallic doped g-C₃N₄-based catalyst in eCO₂RR.

2.2.3. Ternary Compound Catalyst

A ternary complex catalyst usually consists of three parts. Metal atoms or metal clusters are the key components of the ternary complex catalyst that control the catalytic performance or catalytic activity of the catalyst. g-C₃N₄ provides abundant active sites for ternary complex catalysts. It can also enhance the dispersion and interaction with co-catalysts or other coupling materials for the construction of an integrated composite catalyst system. Graphene, CNT, porous carbon, molybdenum disulfide, and other compounds in the terpolymer catalyst are mainly co-catalysts, which mainly help to increase the specific surface area of the catalyst, create a larger active site, and improve the conductivity.

Mn-C₃N₄/CNT is a monatomic catalyst with Mn-N3 as the active site. This conclusion was reached by analyzing the N1s XPS spectra of Mn-C₃N₄/CNT and C₃N₄/CNT (Figure 8a,b). The results showed that the N atom of C-N-C was the coordination site for the formation of the Mn-NX structure [99]. The quantitative EXAFS curve fit analysis (Figure 8c) was used to calculate the structural parameters of Mn-C₃N₄/CNT. The Mn-NX structure had a coordination number x of about 3.2, indicating that an isolated Mn atom was coordinated cubically by the N atom and the final coordination structure was Mn-N₃. The CO₂ adsorption, activation, and transformation processes on Mn-C₃N₄/CNT were investigated using in situ X-ray absorption spectroscopy and DFT calculations. The three N atoms in the Mn central coordination reduce the free energy barrier for CO₂ to form important intermediates (Figure 8d). At a low overpotential of 0.44V, the Mn-C₃N₄/CNT catalyst showed a CO Faraday efficiency of 98.8% in 0.5M KHCO₃ solution, and the CO partial current density was 14.0 mAcm⁻². The addition of CNT mainly improved the catalyst conductivity and the catalytic effect of CO₂RR.

The ternary compound catalyst CoPPc@g-C₃N₄-CNTs was obtained by polymerizing cobalt phthalocyanine (CoPc) on three-dimensional (3D) g-C₃N₄ nanosheets and carbon nanotubes. The results of the electrocatalytic experiment showed that the CO Faraday efficiency is 95 ± 1.8% at −0.8 V vs. RHE and the conversion frequency is 4.9 ± 0.2 s⁻¹, indicating good long-term stability within 24 h. The hydrothermal synthesis (Figure 8e) of the protonated g-C₃N₄ nanosheets and CNTs improved the immobilization uniformity at high catalyst loading and the interaction between the molecular catalyst and the conductive support compared to similar hybrid electrocatalysts prepared by drop-drying or dip-coating. The electrochemically active surface area was increased, the structure was improved, and the active sites were enriched, resulting in excellent catalytic performance [101].

The Ag-S-C₃N₄/CNT [69] ternary composite catalyst showed exceptional performance in eCO₂RR with a high current density of 21.3 mAcm⁻² at −0.77 V vs. RHE. The highest CO Faraday efficiency in the H-cell is over 90% (Figure 8f,g). When the same catalyst is applied in the flow cell configuration, the current density is shown to exceed 200 mAcm⁻², which is essentially the current density required for industrial CO₂RR. The Faraday efficiency of CO is greater than 80% over a wide range of potentials. DFT and electrochemical methods were used to further investigate the catalytic mechanism of the nanocomposites (Figure 8h). The synergistic effect of Ag NPs, sulfur elements, the C₃N₄ framework, and carbon nanotube supports results in very efficient performance of the eCO₂RR. The outermost catalytic surface consists of Ag NPs, sulfur atoms, and C₃N₄ frameworks, on which CO₂ is directly converted to CO. As a result, electron accumulation at the interface of Ag-S-C₃N₄/CNT and S-C₃N₄/CNT is combined with the excellent charge transport performance of CNT and the properties of the S-material C₃N₄ to improve the electrical properties of Ag-S-C₃N₄/CNT nanocomposites. The Faraday efficiency of the ternary electrocatalyst Au-CDots-C₃N₄ [99] doped with precious metal is as high as 79.8% at a potential of −0.5V, and the current density increases by a fact of 2.8 at −1.0 V (where the Au loading is only 4%). DFT calculations and experimental observations have shown that the synergistic effect of Au NPs, CDots, and C₃N₄ and the adsorption capacity of CDots for H⁺ and CO₂ are the sources of the high activity of Au NPs in CO₂RR. At the same time, the combination of CDots and Au-C₃N₄ can accelerate the rate of charge transfer in the reaction. The charge transfer process in CO₂ reduction was studied using EIS of Au-CDots-C₃N₄ (Figure 8i). It was found that the radius of CDot-C₃N₄ was obviously smaller compared to C₃N₄, and Au-CDot-C₃N₄ was smaller compared to Au-C₃N₄, which was attributed to the enhanced conductivity due to the high charge transfer ability of the CDots. Therefore, the CDots in the Au-CDots-C₃N₄ terpolymer have good adsorption capacity for CO₂ and H⁺ and play a leading role in promoting the formation of CO.

The Cu-g-C₃N₄/MoS₂ [102] ternary composite catalyst of g-C₃N₄, MoS₂ and copper nanoparticles (Cu NPs) showed good electrocatalytic activity in eCO₂RR, and the Faraday efficiencies for methanol and ethanol were 19.7% and 4.8%, respectively. Compared with Cu-g-C₃N₄ and Cu-MoS₂, the EIS results of Cu-g-C₃N₄/MoS₂ composites (Figure 8j) show that the interaction between MoS₂ and g-C₃N₄ enhances the electron and charge transfer on the catalyst surface. The Cu-g-C₃N₄/MoS₂ composites have the lowest resistivity, as indicated by the smallest semicircle radius. The EIS results show that compared to Cu-g-C₃N₄ and Cu-MoS₂, the charge transfer in the Cu-g-C₃N₄/MoS₂ composite is improved after the combination of g-C₃N₄ and MoS₂, which makes a greater contribution to the electrocatalytic activity in CO₂ reduction. Cu-g-C₃N₄/MoS₂ composites have lower ohmic resistance, and thus have better catalytic activity in CO₂ reduction than Cu-g-C₃N₄ and Cu-MoS₂.

The common feature of the terpolymer catalyst is that the catalyst can achieve a high current density in CO₂RR. The addition of a co-catalyst mainly plays a role in improving the conductivity and enlarging the active site, while the combination of different metals and different co-catalysts produces various synergistic effects to achieve the catalytic improvement effect.

Table 6. Electrocatalytic parameters of the ternary composite catalysts on eCO₂RR.

Electrode	Product	FE	Potential (V vs. RHE)	Electrolyte	Current Density (mAcm−2)	Ref.
Mn-C3N4/CNT	CO	98.8%	−0.5	0.5 M KHCO3	14	[100]
CoPPc@g C3N4-CNTs	CO	95%	−0.8	0.5 M KHCO3	21.9	[101]
Ag–S–C3N4/CNT	CO	91.40%	−0.77	0.1 M KHCO3	21.3	[69]
NiCu-C3N4-CNT	CO	ca.90%	−0.8	0.5 M KHCO3	ca.14	[103]
NiMn-C3N4-CNT	CO	ca.90%	−0.8	0.5 M KHCO3	ca.12	[103]
Au-CDots-C3N4	CO	79.80%	−0.5	0.5 M KHCO3	0.29	[99]
Cu-g-C3N4/MoS2	CH3OH	19.70%	−0.67	0.5 M KHCO3	78	[102]

shows the electrocatalytic parameters of the ternary composite catalyst for eCO₂RR.

2.3. Non-Metal Doping g-C₃N₄

Among the current doping techniques, the insertion of heteroatoms can change the electrical structure of g-C₃N₄. Doping of g-C₃N₄ with metals and non-metals is the most common type of elemental doping. Non-metallic elements enter the g-C₃N₄ system more easily than metallic components. For example, the elements O, C, S, N, and F are doped into the g-C₃N₄ system by replacing the elements C, N, and H in the heptazine structural unit. The non-metallic doping strategy is to improve the catalytic performance of the catalysts by increasing the adsorption of carbon dioxide and the selectivity of products.

Figure 9a,b shows the Gibbs free energy conversion diagrams for the reduction of CO₂ to carbon monoxide by g-C₃N₄ and B-g-C₃N₄, respectively. From this, it can be seen that doping with elemental boron reduces the free energy barrier for the reaction to produce CO and improves the eCO₂RR performance of the catalyst. As shown in Figure 9c, the charge transfer resistance of B-g-C₃N₄ is much lower than that of g-C₃N₄. The B atom can effectively enhance the electron transport of g-C₃N₄ [70]. A similar conclusion is drawn from Figure 8h, where sulfur doping lowers the Gibbs free energy barrier of CO conversion [69]. The enhanced intrinsic electrical properties and CO₂ reactivity of the Ag NPs, elemental sulfur, the g-C₃N₄ framework, and CNT support synergistically promote electron transfer and stabilize the reaction intermediates. Figure 9d shows that the g-C₃N₄/CNT doped with sulfur elements has a higher current density and better electrochemical properties at the same electrode potential compared to the g-C₃N₄/CNT undoped with any element.

g-C₃N₄/MWCNT [74] composites can be prepared by the typical thermal polymerization method of multi-walled carbon nanotubes (MWCNTs) and g-C₃N₄ [49]. The addition of multi-walled carbon nanotubes improved the ability of the catalyst to conduct electricity, as well as its total specific surface area and the number of active sites. Analysis of the XPS spectrum of C1s (Figure 9e) showed that the C1 peak at 284.7 eV was classified as a sp2 carbon–carbon double bond, the C1 peak at 288.2 eV as the N=C-N group of the triazine ring in g-C₃N₄, and the C1 peak at 285.9 eV as a C-OH species in MWCNT. Crucially, the additional C1 peak at 287.5 eV is the sp3 C-N covalent bond formed in the g-C₃N₄/MWCNT composite. This bond shows that g-C₃N₄ is not bound to MWCNTs by simple physical adsorption. This is because g-C₃N₄ is co-synthesized into the graphite network of MWCNTs through C-N covalent bonds. The active site of the composite is precisely the C-N covalent bond formed between g-C₃N₄ and MWCNTs, which can selectively reduce CO₂ to CO. The maximum Faraday efficiency of carbon monoxide reaches 60% at a potential of −0.75 V vs. RHE.

Table 7. Electrocatalytic parameters of non-metal doping g-C₃N₄ catalysts on eCO₂RR.

Electrode	Product	FE	Potential (V vs. RHE)	Electrolyte	Current Density (mAcm−2)	Ref
S-C3N4	CO	N/A	−0.77	0.1 M KHCO3	N/A	[69]
C3N4/CNT	CO	N/A	−0.77	0.1 M KHCO3	10	[69]
g-C3N4/MWCNTs	CO	60%	−0.75	0.1 M KHCO3	ca. 0.55	[74]

shows the electrocatalytic parameters of non-metal doping g-C₃N₄ catalyst for CO₂RR.

At present, there is little research on non-metal doping of g-C₃N₄ in the field of electrocatalytic carbon dioxide reduction reaction. The pristine g-C₃N₄ has a low conductivity. Compared with the metal doped g-C₃N₄ catalyst, the non-metallic doping can only slightly improve the conductivity of the g-C₃N₄ catalyst. In addition, the non-metallic doping can form a new C-X coordination bond (X is a non-metallic element) on the surface of g-C₃N₄, increasing the active site of the catalyst and improving the catalytic effect of the catalyst.

3. Method for the Synthesis of g-C₃N₄-Based Catalysts

The g-C₃N₄-based catalysts are prepared by thermal polycondensation [104], thermal decomposition method [105], hydrothermal synthesis [48,70,87], and reduction method [86], which are described in detail below.

3.1. Thermal Polycondensation

Thermal polycondensation [106,107] is a polycondensation reaction of nitrogen-containing triazine ring structure precursors at 400~600 °C. In a specific atmosphere (air, nitrogen, argon, or hydrogen), in a muffle or tube furnace, at a heating rate of 2 to 10 degrees Celsius per minute, in a crucible with a lid to maintain a specific temperature for 2 to 4 h, this method allows for bulk access to g-C₃N₄ [49].

The doping strategy based on this method is to obtain precursor powders by direct mixing and milling of non-metallic or metallic compounds with nitrogen-containing organic compounds, or by reacting aqueous solutions of non-metallic compounds with nitrogen-rich organic compounds and obtaining precursor powders by heating or freeze-drying. The precursor powders obtained from the above steps are heated under a specific atmosphere to obtain non-metal doped g-C₃N₄-based catalysts [69], single metal or bimetallic doped g-C₃N₄-based catalysts [78,86,88,95,103] wherein the precursor powders obtained after mixing the metal salt solution and urea and freeze-drying under an inert atmosphere are prepared by thermal polycondensation to obtain metal oxide-doped g-C₃N₄-based catalysts [90]. Examples of non-metal doping g-C₃N₄-based catalysts are as follows: S-C₃N₄ containing sulfur is obtained by thermal polymerization of thiourea as a precursor [69]. Boron-doped g-C₃N₄ is obtained by direct mixing and milling of boric acid and urea followed by thermal polycondensation [70] or by mixing and dissolving a phosphoric acid solution with urea and freeze-drying the solvent to obtain a bulk precursor [108]. Phosphorus-containing g-C₃N₄ can be obtained by mixing of phosphoric acid and urea followed by thermal polycondensation [109].

3.2. Thermal Decomposition Method

The thermal decomposition process essentially involves the pyrolysis of selected feedstocks at a specific temperature and under a specific atmosphere (N₂, NH₃, Ar, or H₂). The pyrolysis temperature is between 200 °C and 500 °C, depending on the decomposition temperature of the metal salts. The precursor is usually a mixture of carbon skeleton and metal complexes or a precursor containing a sacrificial template. The thermal decomposition method is usually used for the preparation of single atom catalysts, where g-C₃N₄ is used as a support and metal compounds are mixed with it and decomposed by heating under a specific atmosphere to obtain a single atom catalyst with metal monomers anchored to g-C₃N₄ [86,100].

3.3. Hydrothermal Synthesis

The hydrothermal synthesis method is specified by using a high-pressure reactor as the reaction vessel, selecting a suitable solvent and nitrogen-containing reactants (usually ethanol and dicyandiamide), and controlling the reaction by adjusting the reaction temperature (120–200 °C) and pressure and finally obtaining the g-C₃N₄ catalyst. The precursor solutions were mixed with metal salts by adding sodium hydroxide, and the precursor solutions were obtained as g-C₃N₄-based catalysts doped with metal oxides or hydroxides in an autoclave by an alkali-assisted synthesis method [92].

The template method is an advanced method of synthesis using thermal solvents. The addition of various templating agents changes the structure and morphology of the g-C₃N₄ material. Finally, the compounds used as templates in the catalysts were removed with acid to obtain g-C₃N₄-based catalysts with high porosity and high specific surface area (up to 830 m²g⁻¹ and 1.25 cm³g⁻¹) [48,70,87]. This method gives good control of the carbon and nitrogen content of the product.

3.4. Wet Chemical Reduction

The first step in the wet chemical reduction method is to mix the metal salt solution with g-C₃N₄, and the second step is to reduce the metal ions to monoatomic metal uniformly charged on g-C₃N₄. There are two methods of reducing the metal ions. In one method, the metal ions uniformly distributed in the pores of g-C₃N₄ are reduced by dropwise addition of a reducing agent (sodium borohydride, ethylene glycol, etc.) to obtain a monoatomic catalyst; in the other method, the precursor solution is stirred under a hydrogen atmosphere for 4–10 h and the resulting product is collected by centrifugation and washed several times before being annealed under an argon atmosphere. The most important point in the liquid-phase reduction method is that the experiment must be strictly controlled to avoid monoatomic agglomeration [86].

In the preparation of g-C₃N₄-based bimetallic catalysts, the reduction method can be divided into co-reduction, replacement, and sequential reduction methods. Bimetallic nanoparticles with an alloy structure are prepared by the co-reduction method. The g-C₃N₄ is thoroughly mixed with a metal salt solution and then reduced together with a reducing agent [96]. Dissolve 100 mg g-C₃N₄ in 300 mL DI water, stir for 1 h at room temperature, and add 0.59 g sodium citrate and 7.9 mg silver nitrate to the water solution. Slowly add 20 mL of 0.1 M sodium borohydride dropwise to the solution and stir for 8 h, then filter by centrifugation or filtration. The product is purified with DI water and dried overnight in an oven at 60–80 °C [69]. In the replacement method, a metal is first loaded onto the g-C₃N₄ framework and then part of the metal is oxidized by another metal with a higher reduction to obtain a g-C₃N₄-based bimetallic catalyst [92]. This uses a sequential reduction method in which one metal is first loaded onto the g-C₃N₄ framework and then the other metal is reduced to the original single metal catalyst by the addition of a reducing agent. Unlike the replacement method, the sequential reduction method does not consume the metal originally deposited on the g-C₃N₄ framework.

There are suitable synthesis methods for different materials. The advantages and shortcomings of the four synthesis methods are described in

Table 8. Advantages and shortcomings of g-C₃N₄-based catalyst synthesis methods.

Synthesis Method	Advantages and Disadvantages of Catalyst	The Advantages and Shortcomings of the Method
Thermal polycondensation	Low specific catalyst surface area, high temperature resistance, and good stability	Easy synthesis, high yield, low cost, part of the precursor powder must be uniformly dispersed before participating in the reaction, reaction temperature at (400–600 °C)
Thermal decomposition method	Uniform structure, good heat resistance	Simple reaction process, requires specific atmosphere (Air, N2, Ar, H2) and temperature requirements (200–500 °C)
Hydrothermal synthesis	Variety of porous catalysts with regular morphology can be produced according to the characteristics of the template, and good heat resistance	Easy to control synthesis, low yields, long preparation cycles, reaction temperatures between (120–200 °C)
Wet chemical reduction	Homogeneous morphology, easy formation of nanocluster structure through doped metal elements, high electrochemical performance	The reaction takes place at room temperature and the reaction steps are cumbersome.

.

4. Regulation of Reactant Selectivity by g-C₃N₄-Based Catalyst

With g-C₃N₄-based catalysts, electrocatalytic carbon dioxide reduction reactions mainly produce hydrogen, methane, carbon monoxide, formic acid, and ethylene. The g-C₃N₄-based catalysts modulate the selectivity of the reactants mainly by modulating the active sites on the catalysts, and the different energy barriers for adsorption and desorption of the main reaction intermediates result in higher or lower selectivity of the products. In the case of g-C₃N₄-based catalysts doped with Cu metal elements, deeper reactions often occur, producing a variety of two-carbon products such as ethylene, ethanol, and acetic acid. This is due to the properties of copper itself resulting in product selectivity [68,78]. Therefore, the selectivity of the catalyst for the reaction can be modulated by elemental doping and by changing the surface structure. Elemental doping, which is described in detail in the section on catalyst preparation, can be used to modulate the active site in two ways. One is that the element forms a new coordination bond with the carbon or nitrogen in the g-C₃N₄ material, creating a new active site that affects the selectivity of the reaction. The other is that the anchoring of the element in the active site of the g-C₃N₄ material affects the selectivity of the reaction due to the unique properties of the element itself.

Modification of the surface structure involves the modulation of the morphology and structure of the g-C₃N₄-based catalyst itself. Modulation of g-C₃N₄ materials is usually done using the template method. In the hard template method, the g-C₃N₄ precursor is filled with an inorganic templating agent with microscopic pore structure, thermally condensed in situ, and then the hard template is removed with a solvent such as hydrofluoric acid to obtain the modulated g-C₃N₄ catalyst. In the soft template method, a surfactant, ionic liquid, or amphiphilic block polymer is used as a template to condense with the precursor compound of g-C₃N₄, and then a thermal polycondensation reaction is carried out to obtain a porous g-C₃N₄ with the soft template removed. For g-C₃N₄-based catalysts doped with metal atoms, the size of the metal particles can be varied to adjust the selectivity. For metal nanocatalysts, catalysts with metal cluster structures and metal nanocatalysts all have different effects on the reaction [110]. The reaction selectivity of the catalyst can be modified by adjusting the morphology and structure [111]. For bimetallic doped g-C₃N₄-based catalysts, the selectivity of the product can be adjusted by adjusting the composition ratio of the two metals.

5. Summary

g-C₃N₄-based catalysts have a wide range of promising applications in multiphase catalytic reactions, such as photocatalytic degradation, photo/electrocatalytic water splitting, and photo/electrocatalytic carbon dioxide reduction. Moreover, their unique electronic structure, abundant active sites, and high stability make them well-suited for use as electrocatalysts. In this paper, the synthesis of g-C₃N₄-based catalysts is summarized and g-C₃N₄-based catalysts are classified into pristine g-C₃N₄, metal doped g-C₃N₄, and non-metal doping g-C₃N₄. The practical applications of g-C₃N₄-based catalysts in CO₂RR under different doping modes are discussed. In addition, the role of different types of g-C₃N₄-based catalysts in modulating reaction selectivity and synthetic ideas are discussed.

While g-C₃N₄ catalysts can be obtained by simple thermal polycondensation, single atom doped g-C₃N₄-based catalysts are difficult to prepare and most studies on single atom doped g-C₃N₄-based catalysts are still at the stage of theoretical calculations and the experimental part has not been fully explored. Some monometallic nanoparticle catalysts, which are close to the monoatomic catalyst structure, also have very high electron conversion efficiencies and are currently the most studied CO₂RR electrocatalysts, with the disadvantage that the current density is low and the conversion effect of non-precious metals is not as excellent as that of precious metals. For bimetallic doped g-C₃N₄-based catalysts, the interaction between the internal bimetal and g-C₃N₄ is similar to that between the metal and g-C₃N₄ in monometallic catalysts, and there is a unique synergy between the bimetals that further enhances the electrocatalytic effect and improves the product selectivity and Faraday efficiency of the non-precious metal for CO₂RR products. Ternary compound catalysts combine the advantages of the previous catalysts and provide not only higher current densities but also greater product selectivity in the reaction, but the synthesis method is complex and most long-term stability tests are limited to 24 h and further long-term stability studies are needed.

6. Outlook

It is expected that further technical development of the g-C₃N₄-based catalyst will enable large-scale CO₂ reduction applications [112]. The morphology, atomic composition, crystal surface, and defect type of the g-C₃N₄-based catalyst influence the CO₂ reduction. The g-C₃N₄ is a polymeric semiconductor composed of C and N atoms. In coordination designs, N vacancies [113] or other elemental vacancies [114,115,116] can be introduced on the surface of g-C₃N₄, changing the surrounding electronic structure and coordination environment to anchor the metal atom [117]. Alternatively, uniform coordination sites can be designed on the surface of g-C₃N₄ to adsorb stable metal atoms and metal precursors and prevent their agglomeration and migration, resulting in a monatomic catalyst. Other common atoms or groups that have strong interactions with metal atoms, such as O, S, P, -C≡C-, etc., can also be considered as active components of g-C₃N₄-based catalysts. In addition, diatomic catalysts and ternary catalysts, which have higher metal loading, more versatile active sites, and unique active reactions compared to monoatomic catalysts, are also worthy of investigation [118]. At present, research on monoatomic and diatomic catalysts based on g-C₃N₄ is still largely at the stage of theoretical calculations, and experimental synthesis and testing has only just begun. The difficulty and challenge in the preparation of such mono- and diatomic catalysts is to exploit the unique chemical and physical properties of g-C₃N₄ to make the coordination on g-C₃N₄ uniform.

Although there are still significant challenges to overcome, it is widely believed that g-C₃N₄-based catalysts have potential in CO₂RR in the future, especially with advances in synthesis techniques that can translate theory into practical applications.