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

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 binding strength of intermediates, such as CO\* , CHO\* , etc., on the nanoparticle


#### **Table 1.**

*Colloids - Types, Preparation and Applications*

to the CO2 molecule that produces surface bound CO2

represent week interaction with CO2

*[36]. Abbreviation: RDS, rate-determining step.*

[35]. However, CO\*

Hori et al., shows optimal binding with CO\*

the surface of Ag, Au and Zn is reduced to COOH\*

importance in product selection and/or reaction rate.

leads to production of HCOO<sup>−</sup>

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

−

gaseous CO is generated. Interestingly, Cu, which has been extensively studied by

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

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,

this reason, it requires extra potential (overpotential of −1.91 V) for electrochemical CO2 conversion, even if it is thermodynamically feasible. After the formation

*Reaction pathways leading to the formation of formate, CO, and C–H products are highlighted. Adapted from* 

**−** 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,

−. Intermediate as depicted

**.** intermediate, therefore, further reduction

and uniquely reduces CO2 in many

which can be further reduced

as resulting product [35]. In comparison, CO2

has week affinity towards these metal ions, and thus,

.−. For

− **.** on

**196**

of CO2 .

**Figure 3.**

into CO\*

*Thermodynamic electrochemical half-cell equations of CO2R products, along with their relative standard redox potential (vs SHE in volt), or E(V) at pH 6.8 [39, 40].*

(NP) surface and thus, alter the reactivity of NP-based electrocatalysts [41–43]. In addition, the geometric effect is an important factor that is crucial in stabilizing the specific intermediate during the reduction process, which leads to an increase in selectivity [44, 45]. When using C-NC-based electrocatalysts, electronic and geometric factors can be adapted to increase selectivity, stability and activity using several approaches, such as size adjustment, shape modification, compositional control, surface functionalization and reaction conditions. As discussed earlier, the nucleation and growth process can be optimized by changing the reaction conditions (thermodynamic and kinetic of the reaction) to achieve the desired C-NCs with well-defined composition, and morphology. Therefore, C-NCs-based catalytic systems have been found to be highly promising to lead the catalytic field that can achieve higher selectivity and efficiency than existing systems. Here, our focus will be mainly on to understand how properties like size, shape, morphology, composition and surface functionalization of nanocrystal affect efficiency and durability of C-NCs-based electrocatalysts for ECO2RR.
