**1.2 CO2 reduction reaction**

Environments change due to greenhouse gases (CO2) is a significant hazard to the protection of human society. The capture and conversion of carbon to the valueadded chemical are attended to be the most agreeable method to prevent the rise of CO2 in the environment as seen in **Figure 1**. But the cost of high technology accessible to capture, store, and convert CO2 stops its functional operation [26]. Recycling CO2 and transforming it into value-added chemicals create challenges for researchers in the area of catalysts. Among the various methods, the electrochemical method has unique advantages [27–29]. Most of the electrochemical reactions can be seen in small to industrial conditions. Besides, if the electricity is required from renewable sources, these sources of energy generate the required electricity, CO2 will not be produced and, therefore, will have a good effect on the worldwide CO2 level [30].

Studies showed that CO was an intermediator and also methane (CH4) or ethylene (C2H4) was generated from HCO\* or COH\* intermediates. Norskov et al. presented details of reaction pathways to produce C2H4 and CH4 from the CO2 reduction reaction at copper catalysts using the Density Functional Theory (DFT) [31–34].

The outcomes demonstrated that the formation of HCO\* was a key step for the reaction. They also compared the carbon dioxide reduction reaction in several transitionmetal electrodes and determined that copper is the most efficient electrode for this case [35]. In the electrolysis of CO2, the anode and cathode were located on separate sides, which were interconnected with a membrane in the middle of them. In the anode, the water oxidized to ion hydrogen (H+ ) and molecular oxygen (O2), while in the cathode, CO2 was reduced to carbon compounds, and hydrogen was reduced [36]. The electrocatalysts for the reduction reaction of CO2 totally divided into a few different classes as seen in **Figure 2**. Metals such as Ni, Pt, Al, Fe, Ti, and Ga were used as the catalysts for H2 production, and CO was not created as the main product [50–53]. The H2 evolution reaction rates by these group metals are commonly greater than that of the CO2RR rate.

Another class of metals of Ag, Au, and Zn convert CO2 to CO with an acceptable efficiency [54]. Catalysts consist of In, Pb, Hg, and Sn convert CO2 to formate as the main product. On these metals, the mechanism of CO2RR to formate is different in which there is no breaking of the C-O bond. Electrodes including W, Cr, and Mo have been reported as inadequate catalysts because of weak selectivity and reduction rate. Copper as a metal catalyst can react to a reduction in CO2 to alcohol and hydrocarbons (C2H4, CH4, CH3OH). However, recent research had shown that the CO2RR to these fuels was made at lower efficiency, which was influenced by the binding-energy of the intermediate species of CO. For example, Ag and Au catalysts can produce CO more rather due to less energy for intermediate carbon monoxide molecules. Since it can be evolved from the surface without more reaction. Therefore, producing higher carbon species at these levels is extremely minimal. However, Cu is a unique catalyst that allows it to produce various carbonaceous products (such as, alcohol and hydrocarbons) with higher activity [54].

Electrodes play a key role in all reactions according to heterogeneous electrochemical reactions, such as CO2RR [55]. The durability and performance of the electrochemical cells are essentially defined by the processes happening at the electrolyte-electrode interface. Overall, electrodes include an electrocatalyst layer as well as a backing layer or substrate that attend multiple acts: firstly, to transport reactant gases, CO2, from the electrolyte to the catalyst layer; secondly, to derive products from the catalyst layer into the membrane/electrolyte; and lastly, electrons connectivity with little resistance [55–58]. Most electrode efficiency, and accordingly electrochemical cell efficiency, requires enhancing all these three processes that greatly relate to the complicated microstructure of the electrodes. Till now, the

#### **Figure 2.**

*A set of three main categories of electrocatalysts for CO2 reduction reaction [36, 37–49].*

### *Investigation of Zn/Ni-Based Electrocatalysts for Electrochemical Conversion of CO2 to SYNGAS DOI: http://dx.doi.org/10.5772/intechopen.95626*

nanoparticles of Ag [59, 60], Sn [61], Au [62], MoO2 [63, 64], Bi [65], MoS2 [62], etc., coated on low carbon steel substrate and Cu2O/TiO2/FTO [66] have been utilized to convert CO2 to CO applying room-temperature ionic liquids (RTILs) as electrocatalysts. Nevertheless, none of these materials enabled the development of CO with a CD of >100 mA/cm2 in CO2RR during controlled potential electrolysis (CPE) tests in combination with any of the utilized RTIL assistant catalysts, which is required to commercially use any of these procedures. In the last decade, the electrochemical CO2RR had been widely considered [67–69]. The reduction reaction products of electrochemical CO2RR on the Cu-based electrodes are hydrocarbons for example C2H4 and CH4 [70–72]. Practical investigations on the electrochemical CO2RR in the base electrodes of copper showed that the exhaust gas contains CO, CH4, C2H4, and primary alcohol that depended on their electrolyte [73, 74]. There were numerous studies of electrochemically CO2 reduction reaction on Cu-based electrodes [37–39].

**Table 1** shows the summarized characterization of electrocatalysts for the CO2RR to various. As shown in **Table 1** and **Figure 3** for SYNGAS (CO + H2) production,



#### *Electrocatalysis and Electrocatalysts for a Cleaner Environment - Fundamentals...*

#### **Table 1.**

*Product distribution for electrochemical CO2 reduction reaction on various electrocatalysts.*

#### **Figure 3.**

*Total published documents for the electrochemical CO2 reduction reaction and specifically convert CO2 to SYNGAS in terms of over time [Scopus data based].*

there is not enough research in this field. Also, for the production of SYNGAS, the Au0.76 –Pd0.24 electrocatalyst has the highest Faraday efficiency (~90%) and CD (~10 mA/cm2 ), which is a high-cost and unsuitable alloy electrode for large-scale use [42]. Other electrocatalysts for SYNGAS production have low FE and/or low CD, as can be seen in **Table 1**. The Ag/Au nanostructure catalysts for electrochemical CO2RR to CO with a FE of further than 90% and a CD greater than 30 mAcm−2 have been stated by researchers [40–43]. Zinc performs as an electrocatalyst for CO2RR to CO, while it is a cost-effective, non-noble, and abundant choice to gold and silver [44].

#### *Investigation of Zn/Ni-Based Electrocatalysts for Electrochemical Conversion of CO2 to SYNGAS DOI: http://dx.doi.org/10.5772/intechopen.95626*

There are also statements of nano-structured Zn catalysts including hexagonal, dendritic, and nanoscale [45–47]. Quan et al. have reported Zn nanoscale and Zn foil as a catalyst for the CO2RR at the NaCl and NaHCO3 electrolytes. They demonstrated that the nano-scale catalyst at NaCl cathodic solution has the greatest proficiency in terms of CD and FE about 6 mA.cm−2 and > 90%, respectively, at a potential of −1.6 V by linear sweep voltammetry (LSV) method [45]. Rosen et al. have studied Zn balks and Zn dendrite catalysts for the electrochemical CO2RR in 0.5 M NaHCO3 cathodic solution. They stated Zn dendrite electrocatalyst has a CD of 4 mA.cm−2 at the potential value of −1.14 V (*vs.* RHE) and FE of 80% [46]. By modifying the surface microstructure, morphology, or orientation of the Zn catalyst, the more FE and product selectivity can be attained for converting CO2 to CO.

Nguyen et al. showed that microstructural or morphological changes in catalysts play a significant role in developing CO2RR [48]. The surface of the Zn catalyst is simply oxidized although immersed in aqueous solutions or exposed to air. Thus, situations should be restricted to avoid zinc oxidation [48]. Nguyen et al. have also reported a porous nanostructure of the Zn catalysts which were prepared of zinc-oxide for the CO2RR. By applying this porous metal, they obtained a faradaic efficiency of 78.5% for CO2RR at a potential value of −0.95 V (*vs* RHE) in the KHCO3 electrolyte [48]. Keerthiga and Chetty have reported a modified zinccopper catalyst for the CO2RR to hydrogen, C2H6, and CH4 products. They coated zinc on the copper with different concentrations of electrolytes, and the outcomes were evaluated with pure Cu and Zn catalysts. They showed that zinc-copper with a high-level concentration of electrolyte had superior performance, also, the FE of CH4 was the order Zn (7%) < Cu (23%) < Cu-Zn (52%). Moreover, the H2 FE for Cu and Cu-Zn were 68% and 8%, respectively [49].

In this way, it has been selected inexpensive materials as electrocatalysts for commercial and industrial applications. Electrocatalysts must be appropriate that could have acceptable efficiency and cheap price for the reforming process. By referring to **Figure 2**, zinc and nickel are affordable materials for carbon monoxide and hydrogen production, respectively. Hence, to produce SYNGAS (CO + H2) in this study, the Zn-Ni bimetallic material is chosen from these two groups of catalysts for CO and H2 products. Other electrocatalysts are either inefficient or expensive. This work aims to investigate the Znx-Ni1-x coatings for the electrochemical CO2 reduction reaction.
