**3. Results and discussions**

## **3.1 Zn-Ni electrodeposition**

The cathodic protection (CP) graph at various temperatures for the Zn-Ni deposition is displayed in **Figure 6**. The graph of the potential in terms of time for Zn-Ni coating depositions at 25°C, 40°C, 60°C, and 70°C were seen throughout

#### **Figure 6.**

*Deposition potential of Zn-Ni coatings in terms of time by chronopotentiometry method at various bath solution temperatures.*

the electrodeposition process. A decrease (more positive) in CP was detected over time with increasing temperature. The CP in chronopotentiometry was related to the ion's concentration becoming reduced at the substrate surface in response to the utilized current.

The standard potential Eo (V) for Ni and Zn is −0.25V and 0.76 V (vs. SHE), respectively [90]. The CPs seen in the deposition were nearest to Eo (V) of the reactants that were converted to its metal. Therefore, the outcomes on the decrease in CP towards a further positive amount over time for electrodepositions at high temperature (60°C and 70°C) demonstrated the CP's deposition was shifting nearer to Eo (V) of Ni reduction, favoring the reaction of Ni-ion reduction to Ni-solid on the substrate. This opinion was more confirmed by EDX outcomes (**Figure 7**). The rise in Ni amount was assigned to a decrease in CP (more positive) over time through the electrodeposition reaction. This supposition is reported by Velichenko et al. [91], who stated the decrease in CP with rising Ni-ion concentration in the bath solution resulting in an improvement in Ni deposition. Qiao et al. [92] detected similar findings of reducing CP with increasing deposited Ni amount in the surface deposition with rising temperature.

#### **3.2 Linear polarization resistance testing for Zn-Ni deposits**

Zn and Ni amounts in the coatings have a considerable effect on the corrosion properties of Zn-Ni deposits. As revealed by Baldwin et al. [93] and Conde et al. [21], the lowest corrosion rate is obtained when the Ni amount is between 12 wt.% to 15 wt.% in the coating. Zn being a lower noble metal plays as an anode that sacrifices in relative to the substrate under a standard situation. Zn is extra favored compared with a nobler metal for instance Ni to be developed into coatings regarding its further sacrificial behavior. But adding more noble elements to Zn improves the corrosion resistance of Zn. By adding Ni to Zn, the rate of sacrificing of coating for the substrate is lower compared to bare Zn. Ni act to hinder or reduce the dissolution rate of Zn. But, when the Ni amount in the coating enhancements to more than 30%, the sacrificial performance decreases, and the coating turn nobler compared to the substrate (Steel). At this stage, the corrosion rate is entirely according to the coating characteristics. As seen in **Figure 8**, with rising temperatures, the corrosion rate increases. This shows that adding Ni to Zn no more enhances corrosion resistance. The coating turns nobler than the substrate and the existence of cracks that are detected causing a rise in the corrosion rate. As the bath solution temperature increases, hydrogen reduction occurs around the working electrode,

**Figure 7.** *EDX analysis results in terms of Zn and Ni contents (wt%) vs. bath solution temperature (°C).*

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

**Figure 8.** *LPR corrosion rate measurements taken for Zn-Ni alloy coatings vs. uncoated carbon steel for hourly for 24 h.*

which creates bubbles form on the surface that prevents the deposition. On the other hand, hydrogen penetrates the coating and makes internal stress. The cracks and disruptions (as shown in **Figure 9**) in the coatings increase speed the corrosion rate of the substrate. These clarify the important variation in the corrosion rate for coatings deposited at 25°C and 40°C, 60°C, and 70°C.

The ratio of Zn and Ni for deposits formed at 25° C is in the optimal range of Ni amount from 12 to 15 wt.%. Therefore, the sacrificial performance of Zn is retained relative to the adding of the Ni, and this makes the steel substrate with decreasing corrosion rate as Zn acts as an anode. By adding 12–15 wt.% of Ni, the dissolution rate of Zn slows down, and the corrosion rate reduces. The cracks and defects in the

#### **Figure 9.**

*SEM images for electrodeposition of Zn-Ni alloy coatings at temperature of (a) 25°C, (b) 40°C, (c) 60°C, and (d) 70°C of bath solution.*

deposits do not substantially influence the corrosion properties of the metal layers, as further anodic Zn causes preferential corrosion.

### **3.3 SEM and EDX analysis for Zn-Ni deposits**

As the bath electrolyte temperature raises, the ion mobility in the electrolyte rises. Hence, the coatings can be smoother. However, the SEM results displayed in **Figure 9** indicate that microcracks are detected in all deposited coatings at various temperatures. The micro-cracks intensity with rising the bath solution temperature is considered to be 25°C < 40°C < 60°C < 70°C. The microcracks formation can depend on the internal stress created and the evolution of hydrogen during the deposition. As the temperature increased, the evolution of hydrogen happened.

Enhancement of inner stress through deposition can be attributed to a lot of reasons. Alfantazi et al. [94] revealed the existence of microcracks in Zn-Ni coatings when the Ni amount in the coating increased. Qiao et al. [92] and Rehim et al. [95] reported the micro-cracks in the Zn-Ni coatings deposited in the acidic bath solution were attributed to H2 embrittlement via the H2 evolution. A rise in the hydrogen release was observed with the outputs of a rise in the hydrogen CD with the temperature rises. This H2 reduction reaction causes H2 atoms to penetrate the coated layer, straining the crystal lattice, and causing high-stress internal cracks.

## **3.4 Investigation of Zn-Ni bimetallic electrocatalysts for CO2RR**

To realize the impacts of the catalysts for the CO2RR, the composition, morphology, and structure of the catalysts were investigated. **Figure 10** and **Figure 11** display EDX results and SEM images of the Zn-Ni with various compositions after the 48 h for the CO2RR by cyclic voltammetry with scan rate 0.05 V. s−1, graphite counter electrode, 0.1 M KCl cathodic, and 0.1 M H2SO4 anodic solutions. According to EDX analysis, as shown in **Figure 10**, carbon with ~28–30 wt.% was deposited on the Zn85%-Ni15% electrocatalyst after 48 h of testing. The microstructure of Zn0.85 - Ni0.15 is a block-like morphology in which carbon is almost uniformly distributed in the substrate due to CO2 reduction. As can be seen in **Figure 11(a)**, some electrocatalytic regions are carbon-covered, preventing CO2 reduction over time. Therefore, for further consideration of this electrocatalyst, gas chromatography of produced gases (the produced gases were collected with the gas bag) has been investigated.

**Figure 10.** *EDX results of C content (wt.%) in terms of Zn-Ni compositions after 48 h of electrochemical CO2RR.*

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

#### **Figure 11.**

*SEM images of Zn-Ni electrocatalysts after 48 h of CO2RR on (a) Zn85%- Ni15%, (b) Zn65%-Ni35%, (c) Zn35%-Ni65%, and (d) Zn20%-Ni80% electrocatalysts.*

According to EDX analysis, as shown in **Figure 10**, carbon with ~10 wt.% was deposited on the Zn65%-Ni35% electrocatalysts after 48 h of testing. As shown in **Figure 11(b)**, the microstructure of the Zn65%-Ni35% electrocatalyst is a cluster-like morphology where coke formation is minimized by the reaction of CO2 with this microstructure after 48 h. With comparing Znx-Ni1-x electrocatalysts, with decreasing Zn amount in Znx-Ni1-x coatings from 85 wt.% to 65 wt.% of Zn, coke formation upon Zn-Ni electrocatalysts decreases. Furthermore, the electrocatalyst microstructures have changed from block-like to cluster-like with decreasing Zn content from 85 wt.% to 65 wt.%. Therefore, the activity and efficiency of electrocatalysts increase with decreasing Zn content from 85 wt.% to 65 wt.% in Zn-Ni electrocatalysts. By further reducing the amount of Zn until ~33 wt.%, coke formation upon Zn-Ni electrocatalyst increases.

Furthermore, as seen in **Figure 11c**, the microstructure of the Zn35%-Ni65% electrocatalyst is semi-spherical, where carbon was deposited between the semi-spherical grains with needle-like microstructure. This high coke formation is due to changes in the microstructure and electrocatalytic activity due to the interaction between Zn and Ni with the ions present in the solution. By further reducing the amount of Zn up to 20 wt.% in the Zn-Ni coating, the coke formation (after 48 h of CO2RR) on the electrocatalyst decreased. The microstructure of 20%Zn-80%Ni is a glossy spherical morphology where carbon is grown with a dark semi-spherical morphology about 22 wt.%.

#### **Figure 12.**

*Gas chromatography results for CO2RR by various electrocatalysts in terms of CO and H2 gas selectivity by cyclic voltammetry method.*


**Table 2.**

*Zn-Ni Electrocatalysts performance with different compositions and electrodeposition parameters for CO2RR.*

Due to the results of gas chromatography, as shown in **Figure 12**, the Zn85%- Ni15%, Zn65%-Ni35%, Zn35%-Ni65%, and Zn20%-Ni80% electrocatalysts have 63%, 55%, 25%, and 30% selectivity for CO and 37%, 45%, 75%, and 70% selectivity for H2 products, respectively. Also, according to **Table 2**, the total efficiency for CO2RR after 48 h of testing is 53%, 66%, 31%, and 57%, respectively. The Zn65%-Ni35% electrocatalyst is appropriate in terms of morphology, stability, coke formation, product selectivity, and intensity of the electrochemical CO2RR. The coke formation on the catalysts can influence the activity spots of the catalyst and have a negative impact on the efficiency and life cycle of the catalyst. Consequently, the chemical compositions, microstructure, and morphology of catalysts have a crucial role for the CO2RR to produce gases with satisfactory ratio, desired product, least-coke formation, and suitable efficiency, activity, and stability.

### **4. Conclusions**

The lower corrosion rate of coatings deposited at 25°C is mainly related to the role of nickel in zinc-nickel alloy and a higher corrosion rate at higher temperatures *Investigation of Zn/Ni-Based Electrocatalysts for Electrochemical Conversion of CO2 to SYNGAS DOI: http://dx.doi.org/10.5772/intechopen.95626*

of 40°C, 60°C, and 70°C are related to the lower barrier properties such as uniformity, compactness and cracks in the alloy. Zinc-nickel alloy coatings with the highest corrosion resistance, within required composition of 12–15%, dense and compact morphology, better uniformity with less crack is achieved with coatings deposited at 25°C. CO2RR on Znx-Ni1-x electrocatalysts in 0.1 M KCl as the cathodic solution and 0.1 M H2SO4 as the anodic solution using cyclic voltammetry method demonstrated that the Zn65%-Ni35% electrode had the best performance for the CO2RR with regarding the minimum coke formation (<10%) and optimal faradaic efficiencies of CO and H2 (FECO = 55% and FEH2 = 45%). The coke formation on the catalysts can influence the activity spots of the catalyst and have a negative impact on the efficiency and life cycle of the catalyst. Consequently, the chemical compositions, microstructure, and morphology of catalysts have a crucial role for the CO2RR to produce gases with satisfactory ratio, desired product, least-coke formation, and suitable efficiency, activity, and stability. Therefore, the Zn65%-Ni35% electrocatalyst with cluster microstructure had the best performance for CO2RR among other electrocatalysts in this study.
