An electrocatalyst participates in an electron transfer reaction (at an electrode) and facilitates the acceleration of a chemical reaction
[13][14]. Electron transfer reactions are the most important processes at electrochemical interfaces. They are determined by the interaction between the interaction of the reagent with the solvent and the electronic levels of the electrode surface. Both electron transfer and kinetic chemistry must be fast for an efficient electrocatalyst
[15]. Furthermore, an ideal electrocatalyst must present a good thermodynamic correspondence between the redox potential for the electron transfer reaction and the chemical reaction being catalyzed (e.g., reduction of CO
2 to CO)
[13][14].
The effectiveness of the electrochemical reduction of CO
2 is directly linked to the electrocatalyst, making it possible to adjust its activity and selectivity by modifying its structure. To improve electrocatalysts, two engineering protocols are widely employed: (1) increase the number of active sites on an electrode, exposing more active sites per gram through optimization of the catalyst structure; and (2) improve the intrinsic activity of each active site. These strategies are not mutually exclusive and can be combined to achieve significant improvements. Several approaches, such as nanostructures, the use of supports, modeling, alloy formation, and doping with heteroatoms, among others, are used in the manufacture of high-performance catalysts
[16].
The electrocatalyst used and the applied potential electrode have a great influence on the final reduction products
[10][11][17]. Metallic electrodes, such as Cu, Au, and Sn, have been extensively explored in recent decades for CO
2 reduction. The generation of intermediate CO
2 is crucial to the rate of CO
2 reduction in most cases. Therefore, the main function of these electrocatalysts is to stabilize this intermediate to achieve high energy efficiency in reducing CO
2. Metal electrodes can be classified into three groups, depending on the binding tendency of intermediates and final products, as shown in
Figure 1. Group I has difficulty binding to the CO
2 intermediate, resulting in the formation of formate or formic acid by an outer sphere mechanism. Group II, the intermediate *CO obtained, is weakly bound to the metal surface, being easily dissolved and emerging as the predominant product. Group III, represented only by Cu, is capable of binding and converting the *CO intermediate into higher value-added products, such as hydrocarbons and alcohols, through *COH or *CHO intermediates
[18].
Electrocatalysts can be classified into three types, such as metallic, non-metallic, and molecular. The materials used in the preparation of the catalysts are what designate their characteristics. Therefore, metallic catalysts can have a metal in their composition or a combination of two metals, termed monometallic or bimetallic. For non-metallic catalysts, the most commonly used materials are carbon nanofiber, nitrogen-doped carbon, metal-organic structure, known as MOF, and covalent-organic structure, Covalent-Organic Framework (COF). On the other hand, molecular catalysts present in their composition the formation of macrocyclic complexes linked to some metals
[21][22]. The electrocatalyst material involves the conversion of CO
2 into various products, as listed in
Table 1.
Table 1. Main electrocatalysts described in the literature with their respective Faradaic efficiency, stability, and final products.
Electrocatalyst |
Electrolyte |
Main Product |
Faradaic Efficiency |
Stability |
Ref. |
FeF20TPP/CNT-CF/CC |
0.5 M NaHCO3 |
HCOOH |
95% |
50 h |
[23] |
CuSn-4 |
0.5 M KHCO3 |
HCOOH |
93.7% |
- |
[24] |
Ni@HNC |
0.1 M KHCO3 |
CO |
98.7% |
- |
[25] |
Ag-NP |
2 M KOH |
CO |
99.9% |
- |
[26] |
Cu-polyamine |
1 M KOH |
C2H4 |
72% |
3 h |
[27] |
Cu/PTFE |
0.1M KHCO3 |
C2+ |
80% |
24 h |
[28] |
Cu-12 |
0.1M KHCO3 |
C2H4 |
64% |
190 h |
[29] |
Cu/Ni(OH) |
0.5 M NaHCO3 |
CO |
92% |
22 h |
[30] |
4H/fcc Au-MMT |
1.0 M KHCO3 |
HCOOH |
92.3% |
12 h |
[31] |
CNT@mC/Ni |
0.5 M KHCO3 |
CO |
98% |
24 h |
[32] |
Ag75/C |
1 M KOH |
CO, H2, and HCOOH |
90.1% |
30 h |
[33] |
Pd/PdOx |
0.5 M KHCO3 |
CO |
90% |
24 h |
[34] |
ZnO |
1 M KOH |
CO |
91.6% |
18 h |
[35] |
Ni-N2-C |
0.5 M KHCO3 |
HCOOH |
98% |
10 h |
[36] |
Bi-N4 |
0.1 M NaHCO3 |
CO |
97% |
4 h |
[37] |
CuNNs |
5 M NaOH |
C2H4 |
52% |
6 h |
[38] |
Sn |
0.1 M Na2SO4 |
HCOOH |
95% |
10 h |
[39] |
Cu–In |
0.1 M KHCO3 |
CO |
55% |
24 h |
[40] |
CuO–Sn |
0.1 M KHCO3 |
CO |
90% |
14 h |
[41] |
Cu nanowire |
0.1 M KHCO3 |
CO, HCOOH, C2H4 |
17.5% |
5 h |
[42] |
Cu (dendrite) |
[EMIM](BF4)/H2O (85/15 v/v) |
HCOOH |
87% |
8 h |
[43] |
Ag |
0.5 M KHCO3 |
CO |
30–80% |
285 min |
[44] |
Sn |
0.5 M KHCO3 + 2 M KCl |
HCOOH |
70% |
4 h |
[45] |
Cu-based metal–organic porous materials |
0.5 M KHCO3 |
CH3OH, C2H5OH |
56% |
90 min |
[46] |
Cu2O/ZnO |
0.5 M KHCO3 |
CH4 C2H4 |
31.4% |
90 min |
[47] |
Analyzing the work presented in
Table 1, it can be observed that several types of electrocatalysts enable the reduction of CO
2 with high efficiency. Furthermore, it is noted that the most enterprising venture is metallic
[22][48], with copper being used. This factor is related to its characteristic of continuing the reduction of CO
2 into aldehydes, hydrocarbons, and alcohol with high efficiency
[49].
In previous publications regarding CO
2, Hori et al. studied the intermediate products by reducing CO at Cu electrodes, which is subsequently reduced to hydrocarbons and alcohols. CO is also formed on Ni and Pt electrodes and subsequently adsorbed on the electrode. And in this way, the adsorbed CO prevents further reduction of CO
2 in Ni and Pt. These points lead the metal electrodes to be classified into two groups: CO formation metals (Cu, Au, Ag, Zn, Pd, Ga, Ni, and Pt) and HCOO
− (Pb, Hg, In, Sn, Cd, and Tl)
[50][51][52].
In 2019, Zhou and collaborators
[53] highlighted the need to recycle CO
2 in the atmosphere due to the greenhouse effect and the energy crisis. It addresses electrocatalytic CO
2 reduction as a viable solution but highlights the challenge of developing electrocatalysts with high activity, selectivity, and durability for this reaction. Explores recent advances in nanostructures of different dimensions as promising catalysts to accelerate CO
2 conversion, discussing the challenges and prospects for achieving high efficiency in this process.
Moreover, in 2020, Tang and collaborators
[54] discussed the carbon dioxide reduction electrochemistry along transition metals, addressing a complex network of reactions. The research combines experimental observations from the literature with theoretical analysis to explain that not all intermediates in CO
2 reduction are formed by direct protonation steps. A selectivity map for two-electron products (carbon monoxide and formate) on pure metal surfaces is derived, using only the CO and OH binding energies as descriptors. The analysis rationalizes the experimentally observed product distributions in CO
2RE in pure metal systems, highlighting the need for additional descriptors for screening materials in CO
2 reduction and the importance of considering the competition in the elementary steps of the hydrogen evolution reaction.
Johnson and collaborators
[55] highlighted the significant role of carbon dioxide in global warming, with the burning of fossil fuels being the main source of pollution. The capture and reduction of CO
2 for the production of valuable chemicals using renewable energy sources is an important area of research. Catalysts, support structures, and electrolytes are key factors affecting the electrochemistry of CO
2 reduction for the production of value-added chemicals and fuels. The research covers the field of CO
2RE electrocatalysis, focusing on non-precious transition metal-based catalysts and highlighting design, synthesis, characterization, and mechanisms. Advanced catalyst design, including two-dimensional metal carbide and nitride materials, and state-of-the-art in situ/operando spectroelectrochemical techniques are emphasized. The text concludes by highlighting the remaining challenges and perspectives for future research and opportunities.
In 2022, Jiang and collaborators
[56] highlighted the electrochemistry of CO reduction as a promising approach to address the energy crisis and reduce carbon emissions. Cu-based electrocatalysts have been considered to generate hydrocarbons and alcohols in CO
2RE, but face challenges of high initial potential and low selectivity. The research proposes a series of Cu-based single-atom alloy catalysts (SAACs), TM1/Cu (111), designed by isolated doping of transition metal (TM) atoms on the Cu (111) surface. TM1/Cu (111) demonstrated, theoretically, greater stability and efficiency in the hydrogenation of CO
2, avoiding hydrogen evolution. Theoretical calculations suggest that the initial hydrogenation of CO
2 in SAACs would form the *CO intermediate, which could be subsequently hydrogenated to produce methane. The bond angle of adsorbed *CO
2 and the binding energy of *OH were identified as important descriptors of the activity and activation capacity of TM1/Cu (111) in CO
2RE. It is speculated that V/Cu (111) may present the best activity and selectivity among all TM1/Cu doped with 3d-TM (111). The research provides rational guidance for the efficient design of new single-atom catalysts for CO
2RE.
Clark and collaborators
[57] address electrosynthesis, such as CO
2 reduction, which involves the formation of dipolar and polarizable transition states during the rate determination step. Highlights the need for systematic and independent control of surface reactivity and electric field strength to accelerate the discovery of highly active electrocatalysts. The research shows that intermetallic alloys allow independent control over the d-band energetics and work function by varying the alloy composition and the identity of the oxophilic constituent. Identifies intermetallic phases with the potential for greater intrinsic activity in CO reduction compared to conventional Cu-based electrocatalysts. However, it highlights the propensity of these alloys to segregate in the air as a significant challenge for investigating their electrocatalytic activity.