**3.1 Mechanistic insight of ECO2RR**

Thermodynamically, CO2 is a quite stable molecule (bond dissociation enthalpy of C=O is ~750 KJ mol−1), so high energy is required for its activation. Moreover, the highest oxidation state of CO2, causes problems for its selective reduction [34]. A catalyst in this regard is an alternative that offers reactive sites for its selective and rapid transformations. In this chapter, the authors are focusing specially on NC-based electrocatalyst for ECO2RR. Generally, ECO2RR involves several proton/ electron transfers processes that take place at the cathode (catalyst). This process is considered to have three major stages. First, CO2 is absorbed on the catalytic surface and its binding strength depends on the structure and composition of the NC as well as the nature of the electrolyte. Once it is absorbed, an electron is transferred

**Figure 3.**

*Reaction pathways leading to the formation of formate, CO, and C–H products are highlighted. Adapted from [36]. Abbreviation: RDS, rate-determining step.*

to the CO2 molecule that produces surface bound CO2 −. Intermediate as depicted in **Figure 3.** This step is known as a rate-limiting step because it requires large reconstitution energy to convert linear CO2 into twisted form, that is, CO2 .−. For this reason, it requires extra potential (overpotential of −1.91 V) for electrochemical CO2 conversion, even if it is thermodynamically feasible. After the formation of CO2 . **−** its reactivity on the catalytic surface determines resulting product in ECO2RR. In principle, an optimal binding of the intermediate on the surface of the electrode is required for the rapid electron transfer process, thereby, increasing the selectivity and kinetics of the conversion. The reason for this is, a very strong affinity with intermediates will poison the surface of the electrode, while weak interaction will disrupt the electron transfer process. Sn, In and Pb, for example, represent week interaction with CO2 − **.** intermediate, therefore, further reduction leads to production of HCOO<sup>−</sup> as resulting product [35]. In comparison, CO2 − **.** on the surface of Ag, Au and Zn is reduced to COOH\* which can be further reduced into CO\* [35]. However, CO\* has week affinity towards these metal ions, and thus, gaseous CO is generated. Interestingly, Cu, which has been extensively studied by Hori et al., shows optimal binding with CO\* and uniquely reduces CO2 in many products including alcohol and hydrocarbons [35]. Whereas, metals, such as Pt and Ni, have a strong affinity for CO\*, which prevents further reduction, and for this reason, these metals favor EH2ER over ECO2RR [35]. Therefore, optimal interaction between metal-based electrodes and surface bound intermediates has a significant importance in product selection and/or reaction rate.

Furthermore, it was realized that surface properties of electrocatalyst, such as, surface area, roughness, composition, and morphological design have profound influence on efficiency, selectivity and durability of electrodes in electrochemical reactions [37–38]. It is a general understanding that the higher surface area provides good economy of the active center on the surface relative to the bulk, and thus, accelerates CO2 reduction. Similarly, Cu shows optimal coordination with CO, however, changes in surface structure, such as roughness, may deviate from their normal behavior. For example, Jiang et al. showed that the high population of under-coordinated sites on the rough surface of the Cu leads to the formation of oxygen-containing compounds and hydrocarbons compared to CO due to enhanced interaction with CO\* intermediates [38]. However, it is still challenging to adjust the optimal binding energy for intermediates to increase selectivity/reactivity towards ECO2RR, because of the large number of intermediates and many possible intricate pathways involved. Until now, many bulk metal-based electrodes have been investigated from both a material and structural point of view; however,

**197**

**Table 1.**

**C1**

**C2**

Ethylene Ethylene glycol

**C3**

*Colloidal Nanocrystal-Based Electrocatalysts for Combating Environmental Problems…*

they are still facing many hurdles, such as: 1) high overpotential of the existing electrodes, 2) obtaining a mixture of products due to poor product selection of the catalyst, 3) deactivation of metal-based electrodes in short periods, 4) high cost of metal-based electrodes such as Ag, Au, Pt etc. discourages their use economically, 5) slow kinetics, or low activity. Beside there is always a competitive reaction EH2ER to the ECO2RR based on the thermodynamics. Rapid electron transfer in first step of electrochemical CO2 reduction may be an effective strategy to suppress EH2ER, which, in turn, accelerates ECO2RR. In **Table 1**, the authors have summarized electrochemical Eqs. (1–16) of CO2R in some valuable products such as CH4, CH3OH, HCOOH, etc., with their respective equilibrium potentials in aqueous medium (pH 6.8) at 1 atm. and 25°C with respect to standard hydrogen electrode (SHE) [39, 40]. The equilibrium potential of the ECO2RR, as provided, corresponds to the

Recent studies have shown that nanometer-sized (1–100 nm) electrocatalyst is not only capable of reducing overpotential, but also shows an improvement in current density for CO2 conversion. Regardless of the metal, the electronic structure of the catalysts at nanoscale is a key player in determining their efficiency, selectivity and durability for ECO2RR. Several electronic factors have been determined, such as finite size effects, and the location of the d band center that can tune the

**Products Thermodynamic half-cell equations E(V) Hydrogen 2H+ + 2e− → H2 0.000**

2CO2 + 12H+

**Oxygen O2 + 4H+ + 4e− → 2H2O 1.23**

*Thermodynamic electrochemical half-cell equations of CO2R products, along with their relative standard* 

2CO2 + 10H+

, CHO\*

, etc., on the nanoparticle

+ 8e− **→** CH4 + H2O 0.17

+ 2e− **→** CO + H2O −0.10

+ 6e− **→** CH3OH + H2O 0.03

+ 2e− **→** HCOOH + H2O −0.02

+ 10e− **→** CH3CHO + 3H2O 0.05

+ 8e− **→** CH3COOH + 2H2O −0.26

+ 12e− **→** C2H5OH + 3H2O 0.09

+ 6e− **→** C2H2O2 + 2H2O −0.16

+ 8e− **→** C2H4O2 + 2H2O −0.03

+ 16e− **→** CH3COCH3 + 5H2O −0.14

+ 16e− **→** C3H6O + 5H2O 0.11

+ 16e− **→** C3H6O + 5H2O 0.14

+ 18e− **→** C3H7OH + 5H2O 0.21

+ 14e− **→** C3H6O2 + 4H2O 0.46

0.08 0.20

+ 12e− **→** C2H4 + 4H2O

+ 10e− **→** C2H6O2 + 2H2O

*DOI: http://dx.doi.org/10.5772/intechopen.95338*

equilibrium potential of the EH2ER.

**3.2 C-NCs-based heterogonous catalyst for ECO2RR**

binding strength of intermediates, such as CO\*

Methane CO2 + 8H+

Carbon-mono oxide CO2 + 2H+

Methanol CO2 + 6H+

Formic acid CO2 + 2H+

Acetaldehyde 2CO2 + 10H+

Acetate 2CO2 + 8H+

Ethanol 2CO2 + 12H+

Glyoxal 2CO2 + 6H+

Glycoaldehyde 2CO2 + 8H+

Acetone 3CO2 + 16H+

Allyl alcohol 3CO2 + 16H+

Propionaldehyde 3CO2 + 16H+

1-Propanol 3CO2 + 18H+

Hydroxyacetone 3CO2 + 14H+

*redox potential (vs SHE in volt), or E(V) at pH 6.8 [39, 40].*

*Colloidal Nanocrystal-Based Electrocatalysts for Combating Environmental Problems… DOI: http://dx.doi.org/10.5772/intechopen.95338*

they are still facing many hurdles, such as: 1) high overpotential of the existing electrodes, 2) obtaining a mixture of products due to poor product selection of the catalyst, 3) deactivation of metal-based electrodes in short periods, 4) high cost of metal-based electrodes such as Ag, Au, Pt etc. discourages their use economically, 5) slow kinetics, or low activity. Beside there is always a competitive reaction EH2ER to the ECO2RR based on the thermodynamics. Rapid electron transfer in first step of electrochemical CO2 reduction may be an effective strategy to suppress EH2ER, which, in turn, accelerates ECO2RR. In **Table 1**, the authors have summarized electrochemical Eqs. (1–16) of CO2R in some valuable products such as CH4, CH3OH, HCOOH, etc., with their respective equilibrium potentials in aqueous medium (pH 6.8) at 1 atm. and 25°C with respect to standard hydrogen electrode (SHE) [39, 40]. The equilibrium potential of the ECO2RR, as provided, corresponds to the equilibrium potential of the EH2ER.
