Oxide Derived Copper for Electrochemical Reduction of CO2 to C2+ Products

The electrochemical reduction of carbon dioxide (CO2) on copper electrode derived from cupric oxide (CuO), named oxide derived copper (ODCu), was studied thoroughly in the potential range of −1.0 V to −1.5 V versus RHE. The CuO nanoparticles were prepared by the hydrothermal method. The ODCu electrode was used for carbon dioxide reduction and the results revealed that this electrode is highly selective for C2+ products with enhanced current density at significantly less overpotential. This catalyst shifts the selectivity towards C2+ products with the highest Faradaic efficiency up to 58% at −0.95 V. In addition, C2 product formation at the lowest onset potential of −0.1 V is achieved with the proposed catalyst. X-ray diffraction and scanning electron microscopy revealed the reduction of CuO to Cu (111) nanoparticles during the CO2 RR. The intrinsic property of the synthesized catalyst and its surface reduction are suggested to induce sites or edges for facilitating the dimerization and coupling of intermediates to ethanol and ethylene.


Introduction
The anthropogenic carbon emissions and rise in global temperatures has increased the demand for energy production. Renewable energy sources are continuing to progress for greater advantages, but long-term energy storage remains a challenge that must be addressed to meet the global energy demand. In this regard, CO 2 reduction not only meets the requirement of energy storage but also helps in mitigating the anthropogenic carbon dioxide emission. Energy storage in chemical bonds has several advantages as compared to battery storage in terms of high energy density, transportability, and enhanced safety [1][2][3][4][5][6][7]. When coupled with carbon capturing technology, the carbon dioxide reduction offers a solution for balancing the natural carbon cycle. The electrochemical reduction of carbon dioxide involves multiple electron transfer steps depending upon the electrocatalysts. The energy barrier to CO 2 reduction is lowered by an electrocatalyst by a mechanism involving stabilization of the intermediates in the multistep electrochemical reduction process. Among all the CO 2 reduction products; ethene, ethanol and propanol have higher volumetric energy density and commercial values. Various electrodes have been used for this purpose, especially Cu, which is capable of catalyzing the electrochemical reduction of CO 2 to highly reduced products (any product requiring the transfer of >6 electrons in an aqueous solution) at near-neutral pH in high yields [8][9][10][11][12]. 2CO 2 + 8H 2 O + 12e → C 2 H 4 + 12OH − 2CO 2 + 9H 2 O + 12e → C 2 H 5 OH + 12OH − C 2 products are commercially more important as compared to C1 products and can act as building units for the synthesis of oxygenates, polymers and long chain hydrocarbon fuels. However, until now, the reported highest Faradaic efficiency of the C 2 products is lower as compared to C 1 products (close to 100%) [12][13][14][15]. This may be due to the complex reaction pathways involved in C 2 products formation as a much higher kinetic barrier of the C-C coupling step may be responsible for decreasing energy efficiency. Generally, a higher potential is required for the reduction of compounds containing C=O than those containing C-H and C-OH groups. The CO 2 RR process involves three main steps. The first step is the adsorption and bonding of CO 2 with surface atoms of the catalyst. The second step is the activation and then the reduction of CO 2 involving electron/proton transfer processes. The third step is the desorption of the products from the catalyst surface. Copper has a unique catalytic property of producing hydrocarbons and alcohols with promising Faradaic efficiency [15][16][17][18][19][20]. Copper is arguably the best-known electrocatalyst for electrochemical CO 2 reduction as it is capable of electrochemically converting CO 2 into more than 30 different products including hydrocarbons and alcohols. Cu metal increases the CO dimerization process and hence facilitates the formation of C 2 products. Product distribution mainly depends upon the surface geometry and morphology of Cu surfaces, as well as on applied potential and electrolytes. Oxide derived copper catalysts have enhanced selectivity for C 2 products. Their selectivity depends on the oxidation state and surface morphology of copper. It is believed that the reduction of copper oxide catalyst can induce undercoordinated sites, grain boundaries and rough surfaces as catalytically active sites. In recent years, many investigations have led to the development of many efficient metal-based electrocatalysts for CO 2 RR [20][21][22][23]. Nano catalysts such as nanoparticles, nanowires, nanotubes and nanofoams have shown improved selectivity and efficiency over bulk materials [20][21][22][23][24][25][26]. Nano catalysts derived from copper oxides have shown better selectivity towards ethylene and ethanol (C 2 products) at lower overpotentials. Despite these remarkable results of oxide derived copper nanoparticles for CO 2 RR, the mechanism is still unclear. It is believed that grain boundaries may be the catalytic active sites induced by the oxygen derived copper species. The other reason could be the higher current densities on the defect sites that increase the local pH, which may alter the reaction pathway in favor of ethanol and ethylene. It is generally accepted that Cu oxides are completely reduced at a very negative potential (−1.0 V vs. RHE) and the reaction only occurs on metallic copper. Copper catalysts with different morphologies can be obtained from various synthetic methods such as annealing, electrodeposition, the hydrothermal method, chemical treatment, and colloidal synthesis [15][16][17][18][19][20][21][22]. The current study presents the performance of hydrothermally produced surfactant coated CuO nanoparticles towards CO 2 RR. CuO NPs in the presence of triton were synthesized and the resulting catalytic property towards CO 2 reduction was evaluated using gas chromatography and NMR spectroscopy. The effects of electrolyte, crystal orientation and nanoparticles size were examined on the product formation of ethene, ethanol and propanol.

Preparation of CuO Nanoparticles
CuO nanoparticles were synthesized by the reported co-precipitation method [24]. Firstly, 0.5 M solution of copper acetate (Cu(CH 3 COO) 2 ) was prepared. Then, 2 mL of the Triton X-100 was added into it drop wise followed by stirring for 40 min. Then 0.5 M solution of ammonium carbonate ((NH 4 ) 2 CO 3 ) was added in the solution drop wise and stirred up to 40 min at 60 • C. In order to maintain the pH of the solution between pH 10-11, 1 M solution of NaOH was added drop wise followed by stirring of the solution up to 1.5 h at 90 • C. The precipitate was then centrifuged at 13,000× g rpm and washed several times with deionized water and ethanol. CuO nanoparticles were then dried at 80 • C over night and followed by grinding the precipitates into fine powder.

Preparation of Glassy Carbon Plate/Electrodes
Prior to experiments, a glassy carbon electrode was polished with alumina powder and then washed with distilled water, followed by drying under a stream of N 2 . The glassy carbon electrode with a geometric area of 0.07 cm 2 was used for cyclic voltammetric experiments. The GCE with a geometric area of 1 cm 2 was used for bulk electrolysis.

Instruments for SEM and XRD
Scanning electron microscopic images were obtained using ZEISS EVO 40 (Oberkochen, Germany). For analysis of the morphology of nanoparticles, the XRD technique was employed using Bruker D8 ADVANCED power diffractometer (Bruker, Germany) with source Cu Kα radiation (λ = 0.154 nm).

Electrochemical Instrumentation and Procedures
All electrochemical experiments were carried out at room temperature in a threeelectrode system using a CHI700D electrochemical workstation (CHI Instruments, Austin, TX, USA). The CuO derived Cu coated carbon plate electrode was using as a working electrode (cathode), Pt plate as counter electrode and Ag/AgCl (3M KCl) as the reference electrode. The potential was converted to RHE scale using the formula: E (vs. RHE) = E (vs. Ag/AgCl) + 0.190 V + 0.0586 V × pH.
Bulk electrolysis was performed in a two-compartment gas tight H-shaped cell under a CO 2 atmosphere with the cathodic and anodic compartments separated by a porous glass frit. Prior to electrolysis, CO 2 gas of high purity was used to saturate the solution for almost 20 min in a H type cell, which was tightly sealed with a rubber stopper.

Analysis of the Electrolysis Products
Gaseous product was identified by Gas Chromatography (GC) in the headspace of an H-cell working electrode of a bulk electrolysis cathodic compartment. Calibration curves for C 2 H 4 and H 2 were built by injecting known concentrations of pure C 2 H 4 and H 2 . The standard calibration plot was constructed by plotting the area of the peak against the amount of gases injected. Each gaseous product was identified by the retention time and quantified by the relevant calibration curve. The liquid products were characterized by 1H NMR spectroscopy. For the NMR analysis, a liquid sample was prepared by mixing 500 µL of electrolyzed solution with 100 µL D2O and 100 µL of DMSO/H 2 O (1/1000) v/v) used as an internal standard.

Preparation of H 2 and C 2 H 4 Calibration Curves
The steps involved in the construction of standard calibration curves of the gaseous products H 2 and C 2 H 4 are as follows

1.
Electrolyte solution and head space were saturated with pure CO 2 into the same H-cell that was used for bulk electrolysis;

2.
Standard gases of C 2 H 4 and H 2 of known amounts were injected into the electrolyte solution and head space of the same volume; 3.
The mixture of gases of the same volume was then injected to GC to obtain the area of the peak corresponding to H 2 and C 2 H 4 ; 4.
Range of different amounts of C 2 H 4 and H 2 were tested and repeated the experiments to obtain the calibration plots of C 2 H 4 and H 2; 5.
Calibration curves of C 2 H 4 and H 2 were obtained by plotting the concentration of gases against the peak intensity of the respective gases.

Faradaic Efficiency
Faradic efficiency of gaseous and liquid products was calculated from the following equation: where z is the number of electrons required per molecule to obtain the respective product, F is the Faraday constant, n is the no. of moles of each product (i.e., obtained from the calibration plot) and Q is the total charge (C) consumed during bulk electrolysis. Overpotential was calculated from the following equation: where E is the applied potential, η is the overpotential, and E 0 is the equilibrium potential for the reduction of CO 2 to Ethylene and Ethanol.

Physical Characterization
The oxidation state of CuO was reduced during CO 2 reduction as evidenced from XRD. Cu 111 ions are believed to be an active species for reducing CO 2 to C 2 compounds [27,28]. It was observed that the surface of CuO, apparently in blue, reduced and converted to brick brown Cu NPs particles during ERC. SEM images were taken to show the surface morphology of CuO and Cu NPs. The SEM image ( Figure 1a) shows a uniform coating of thick and dense copper oxide nanoparticles. After electrolysis, the SEM image ( Figure 1b) displays a granule like Cu nanoparticles surface morphology with a size approximately in the range of 20-78 nm. The SEM image after CO 2 reduction indicates the defect sites on the catalyst surface along with spherical Cu particles. In XRD (Figure 2), only Cu 0 peaks can be seen in the XRD pattern after electrolysis. These results specify the complete reduction of CuO during electrocatalysis [27][28][29].

Cyclic Voltammetry
The electrochemical behavior of the Cu/CuO catalyst was firstly investigated by cyclic voltammetry in N2 and CO2 saturated 5 M KHCO3 electrolyte using a three electrode system, a glassy carbon electrode as a working electrode, Ag/AgCl as a reference electrode and Pt as a counter electrode at a scan rate of 100 mV/s. Cyclic voltammetric behavior of the Cu/CuO catalyst in N2 atmosphere is different than that of the Cu/CuO catalyst in CO2 atmosphere, as shown in Figure 3. The higher current in N2 atmosphere can be attributed to the hydrogen evolution reaction. However, the rapid increase of current in CO2 atmosphere as compared to N2 suggests that the CO2 reduction is catalytically more favorable than HER [30].

Cyclic Voltammetry
The electrochemical behavior of the Cu/CuO catalyst was firstly investigated by cyclic voltammetry in N 2 and CO 2 saturated 5 M KHCO 3 electrolyte using a three electrode system, a glassy carbon electrode as a working electrode, Ag/AgCl as a reference electrode and Pt as a counter electrode at a scan rate of 100 mV/s. Cyclic voltammetric behavior of the Cu/CuO catalyst in N 2 atmosphere is different than that of the Cu/CuO catalyst in CO 2 atmosphere, as shown in Figure 3. The higher current in N 2 atmosphere can be attributed to the hydrogen evolution reaction. However, the rapid increase of current in CO 2 atmosphere as compared to N 2 suggests that the CO 2 reduction is catalytically more favorable than HER [30]. In the first scan cyclic voltammogram, a reduction peak was observed around 0.6 V after purging with N2 and CO2, followed by the successive two oxidation signals appearing at 0.52 V and 0.65 V during the reverse scan. These two oxidation peaks also appeared in the successive scan. The first oxidative peak corresponds to the oxidation of In the first scan cyclic voltammogram, a reduction peak was observed around 0.6 V after purging with N 2 and CO 2 , followed by the successive two oxidation signals appearing at 0.52 V and 0.65 V during the reverse scan. These two oxidation peaks also appeared in the successive scan. The first oxidative peak corresponds to the oxidation of Cu to Cu(I) and the second oxidation peak may be due to the conversion of Cu(I) to CuO as reported by the previous investigators [31,32]. During the second scan of cyclic voltammograms two reductive peaks were observed at 0.30 V and 0.069 V, respectively, which can be due to the conversion of CuO to Cu(I) and the transition of Cu(I) to Cu. Moreover, the disappearance of peak I in the subsequent scan can be attributed to the combination of reduction of the CuO to Cu + and Cu at more negative potentials. Therefore, we suggest that the reduction peak observed in the 1st scan is the reduction of CuO to Cu. The catalyst attained a more stable structure as most of the CuO was already converted to Cu + and Cu. Therefore, after the 1st scan, all the subsequent scans had a similar pattern of reduction peaks. The trend of the catalyst in N 2 and CO 2 saturated KHCO 3 is almost similar; however, the shifting of peaks and their intensification can be attributed to the CO 2 reduction on the electrode surface.
The electrochemical activity of the CuO and reduced Cu was evaluated by EIS. As shown in Figure 4, reduced copper exhibits a better charge transfer rate as the reduced copper nanoparticles have a high electrochemical surface area which offers more active sites.

Bulk Electrolysis
Bulk electrolysis was carried out in a sealed H-cell to investigate the CO2RR performance of the Cu derived CuO catalyst. Ethene, Ethanol and H2 were the dominant reduction products accompanied by propanol and formic acid under applied potential. The formation of ethanol and ethene occurs at 0.10 V vs. RHE with a lower F.E but still on the lowest overpotential. This confirms that the surface morphology of the catalyst plays a significant role in C2 product formation. At the highest negative potential, the Faradic efficiency of C2+ is significantly high as the values of F.E of ethylene, ethanol and propanol are 20%, 33% and 4% respectively at −0.95 V vs. RHE. So, overall, the F.E of C2+ products is 57%, which is remarkably high as compared to the 14% Faradic efficiency of C1 product i.e., formic acid and 29% Faradic efficiency of H2. Statistically, CuO derived

Bulk Electrolysis
Bulk electrolysis was carried out in a sealed H-cell to investigate the CO 2 RR performance of the Cu derived CuO catalyst. Ethene, Ethanol and H 2 were the dominant reduction products accompanied by propanol and formic acid under applied potential. The formation of ethanol and ethene occurs at 0.10 V vs. RHE with a lower F.E but still on the lowest overpotential. This confirms that the surface morphology of the catalyst plays a significant role in C 2 product formation. At the highest negative potential, the Faradic efficiency of C 2+ is significantly high as the values of F.E of ethylene, ethanol and propanol are 20%, 33% and 4% respectively at −0.95 V vs. RHE. So, overall, the F.E of C 2+ products is 57%, which is remarkably high as compared to the 14% Faradic efficiency of C1 product i.e., formic acid and 29% Faradic efficiency of H 2 . Statistically, CuO derived Cu plays a significant role in C 2 product formation. This may be due to the origin of the structural transformation of the oxides of Cu to Cu during CO 2 reduction as confirmed by XRD and SEM results. Hence, the transformation induces a defective site in the catalytic surface which is believed to play a crucial role in C 2 products formation even at the lowest overpotential. CO 2 reduction was performed using the CuO derived Cu catalyst at a fixed potential between −0.1 V to −1.5 V vs. RHE. Figure 5 shows the plot of current vs. time during CO 2 reduction at the potential of −0.35 V and −0.25 V. A reduction peak observed during the initial phase of the curve represents the reduction of CuO to Cu. A similar trend was observed in all amperograms at all applied potentials. This feature is consistent with the XRD result.

Effect of Electrolyte
Alkali-metal ions favor CO2 adsorption and are preferentially used in CO2RR. These cations stabilize the surface intermediates by creating a field effect or by interacting with the adsorbed species. Moreover, the overpotential of the catalyst decreases with the increase of cationic size. In addition, the current density also increases with the increase of cationic size, hence favors more C2 products as compared to C1. In our study, Cu 111 in KHCO3 showed more Faradic efficiency for C2 products as compared to Cu 111 in Na-HCO3. This study confirms that multivalent cations used as supporting electrolytes influence the rate of CO2RR. The possible explanation may be due to the preferential hydrolysis or the greater capacity for specific adsorption [32][33][34][35][36].
Product distribution of CO2 reduction mainly depends upon the type of crystal facets. CO2 adsorption depends upon the surface orientation of crystals as its variation affects the Lewis acidity and polarizability of CO2. Facets which decrease activation energy and increase the adsorption of CO2 are more feasible for CO2. In our study, XRD results reveal that the formation of Cu (111) from CuO favors the C2 products over C1, suggesting the dimerization of CO that leads to the production of ethylene and alcohols [36][37][38][39].
A catalytic approach towards the formation of products with different Faradic efficiencies at various applied potentials (from −0.10 to −1.5 V) was studied thoroughly as shown in Figure 6A. The Cu 111 derived from CuO delivered a remarkable result with excellent F.E of hydrocarbons at the lowest overpotential for the catalytic selectivity towards the formation of C2+ products over C1. These results show that C2+ product is the dominant product, and that the selectivity is strongly dependent on the applied potential, current density and the catalytic activity of the electrode. At an applied potential of −0.20 V, C2 products are formed with F.E of 8% ethylene and 15% ethanol. Thus, the intrinsic property of catalyst favors multi-carbon products formation even at the lowest overpotential. During CO2RR, the FE of H2 was found in the range of 20% to 36%. Another im-

Effect of Electrolyte
Alkali-metal ions favor CO 2 adsorption and are preferentially used in CO 2 RR. These cations stabilize the surface intermediates by creating a field effect or by interacting with the adsorbed species. Moreover, the overpotential of the catalyst decreases with the increase of cationic size. In addition, the current density also increases with the increase of cationic size, hence favors more C 2 products as compared to C 1 . In our study, Cu 111 in KHCO 3 showed more Faradic efficiency for C 2 products as compared to Cu 111 in NaHCO 3 . This study confirms that multivalent cations used as supporting electrolytes influence the rate of CO 2 RR. The possible explanation may be due to the preferential hydrolysis or the greater capacity for specific adsorption [32][33][34][35][36].
Product distribution of CO 2 reduction mainly depends upon the type of crystal facets. CO 2 adsorption depends upon the surface orientation of crystals as its variation affects the Lewis acidity and polarizability of CO 2 . Facets which decrease activation energy and increase the adsorption of CO 2 are more feasible for CO 2 . In our study, XRD results reveal that the formation of Cu (111) from CuO favors the C 2 products over C 1, suggesting the dimerization of CO that leads to the production of ethylene and alcohols [36][37][38][39].
A catalytic approach towards the formation of products with different Faradic efficiencies at various applied potentials (from −0.10 to −1.5 V) was studied thoroughly as shown in Figure 6A. The Cu 111 derived from CuO delivered a remarkable result with excellent F.E of hydrocarbons at the lowest overpotential for the catalytic selectivity towards the formation of C 2+ products over C 1 . These results show that C 2+ product is the dominant product, and that the selectivity is strongly dependent on the applied potential, current density and the catalytic activity of the electrode. At an applied potential of −0.20 V, C 2 products are formed with F.E of 8% ethylene and 15% ethanol. Thus, the intrinsic property of catalyst favors multi-carbon products formation even at the lowest overpotential. During CO 2 RR, the FE of H 2 was found in the range of 20% to 36%. Another impressive result at the lowest overpotential is the formation of n-propanol with 4% Faradic efficiency. The total current density versus maximum C2+ alcohol Faradaic efficiency is presented in Figure 6B. The current densities ranged from 10 to 70 mA/cm 2 depending on the applied potential. The optimal potential for C2+ products (ethylene ethanol and propanol) was −0.95 V versus RHE with a peak FE of 57%. Beyond this potential, a decrease in C2 FE is observed, whereas the FE of H2 increases. This may be due to increased current density which produces larger bubbles inducing cracks in the catalyst surface and thus exposes the carbon plate surface. This exposed carbon surface facilitates hydrogen production and thus a decrease in the Faradic efficiency of ethylene is observed.
To probe the durability of the CuO catalyst, electrolysis was carried out by chronopotentiometry at −0.80 V. The current density was stable at about −30 mA cm −2 over 5 h as shown in Figure 6C. A comparison of the reported Faradaic efficiency of C2 products on various copper surfaces with our catalyst can be seen in Table 1. The listed data reveal that the surface morphology of the catalyst affects the overpotential and selectivity of the CO2RR product. The Faradaic efficiency of ODCu for the C2+ product formation is higher than that in the reported literature [38][39][40][41][42][43][44][45][46] cited in Table 1. Moreover, one of the remarkable results of our designed catalyst is its onset potential of −0.1 V for the C2 products formation, which is the lowest overpotential amongst the reported The total current density versus maximum C 2+ alcohol Faradaic efficiency is presented in Figure 6B. The current densities ranged from 10 to 70 mA/cm 2 depending on the applied potential. The optimal potential for C 2+ products (ethylene ethanol and propanol) was −0.95 V versus RHE with a peak FE of 57%. Beyond this potential, a decrease in C 2 FE is observed, whereas the FE of H 2 increases. This may be due to increased current density which produces larger bubbles inducing cracks in the catalyst surface and thus exposes the carbon plate surface. This exposed carbon surface facilitates hydrogen production and thus a decrease in the Faradic efficiency of ethylene is observed.
To probe the durability of the CuO catalyst, electrolysis was carried out by chronopotentiometry at −0.80 V. The current density was stable at about −30 mA cm −2 over 5 h as shown in Figure 6C. A comparison of the reported Faradaic efficiency of C 2 products on various copper surfaces with our catalyst can be seen in Table 1. The listed data reveal that the surface morphology of the catalyst affects the overpotential and selectivity of the CO2RR product. The Faradaic efficiency of ODCu for the C 2+ product formation is higher than that in the reported literature [38][39][40][41][42][43][44][45][46] cited in Table 1. Moreover, one of the remarkable results of our designed catalyst is its onset potential of −0.1 V for the C 2 products formation, which is the lowest overpotential amongst the reported works. Another figure of merit of our catalyst is its higher current density at lower overpotentials.

Origin of Selectivity
To obtain mechanistic insights about the kinetics of RDS of the reaction on the catalyst, a Tafel plot analysis was carried out. This analysis is applicable only when the reaction is kinetically controlled [4][5][6][7][8]. Interestingly, we attained the C 2 products at a lower overpotential owing to the excellent intrinsic property of the catalyst. The corresponding Tafel slopes for C 2 H 4 and C 2 H 5 OH are shown in Figure 7. The plot of C 2 H 4 production exhibited a lower Tafel slope of 124 mV dec −1 than that of EtOH (150 mV dec −1 ). Both plots are close to the value of 118 mV dec −1 , which indicates that C 2 H 4 and EtOH involve a common intermediate with a rate determining single electron transfer to CO 2 . For ethanol and ethylene, Tafel plots are linear at a lower potentials range which is mainly a kinetically controlled region while the higher potential region is controlled by the mass transportation of ions and diffusion of gases. Due to this combined effect, a steeper slope is obtained at a higher potential region. During CO 2 reduction, CuO reduces to Cu, hence stepped edges and grain boundaries are expected to be formed on the catalytic surface. In the literature, temperature-controlled desorption studies and density functional theory calculations show that CO adsorbs more strongly on Cu edges and grains as compared to the flat surfaces.
Step edges and grain boundaries are suggested to have highly active sites that favor C-C coupling, as they offer lower energy barriers for the formation of key intermediates (CO, CHO etc.) [1][2][3][4][5][40][41][42][43][44][45][46][47][48]. CO2 electroreduction on a copper surface results in many C1 and multi-c products including CH4, CO, HCOOH, C2H4, C2H5OH etc. In the CO2RR process, g ally one electron transfer to CO2 is considered as the rate determining step, i.e., th mation of *CO2 •− . Cu can convert CO2 to methane, ethylene and ethanol with high daic efficiency owing to a strong bonding propensity of Cu with *CO2 •− interme which reduces CO further into higher order hydrocarbons. Complex processes inv in CO2 reduction to C2+ products with several possible pathways are still the fo substantial theoretical study. Density functional theory (DFT) calculations using the (computational hydrogen electrode) model propose that a potential-limiting step f hydrocarbon's formation is the protonation of CO* to CHO* on the surface of Cu [ number of pathways have been proposed that suggest the formation of interm hydroxyl-methylidyne and the dimerization of two CO* molecules [50]. Curre vancement in computational studies further helps in understanding the mechanism suggests that *CHO or *COH intermediate formation from the hydrogenation of *C play a pivotal role in determining the favorable products such as C2H4, CH4, and C on a Cu (111) surface [51]. This kinetically challenging step is due to the high e barrier required to drive the reaction. Ethanol and ethylene as a multi-carbon pr require coupling of C-C between the adsorbed intermediates on the surface, so t merization steps favor multi-carbon products. Recent theoretical calculations on t (111) surfaces suggest that the C-C coupling strongly depends on the degree of COa hydrogenation and that energy barrier of coupling decreases with the increase hydrogenation of surface bounded CO, which tends to favor C2 product formatio cording to the Tafel plot, the reduction of CO2 is the rate determining step, so th sorbed intermediate CO2 − must be stable enough to persist until the availability of intermediates for coupling. Copper nanocrystals may stabilize C1 and C2 intermed allowing them to trimerize to a C3 compound (n propanol). The reaction mechanis volves various steps and the formation of several products [3] as shown in Figure 8  CO 2 electroreduction on a copper surface results in many C 1 and multi-carbon products including CH 4 , CO, HCOOH, C 2 H 4 , C 2 H 5 OH etc. In the CO2RR process, generally one electron transfer to CO 2 is considered as the rate determining step, i.e., the formation of *CO 2 •− . Cu can convert CO 2 to methane, ethylene and ethanol with high Faradaic efficiency owing to a strong bonding propensity of Cu with *CO 2 •− intermediate, which reduces CO further into higher order hydrocarbons. Complex processes involved in CO 2 reduction to C 2+ products with several possible pathways are still the focus of substantial theoretical study. Density functional theory (DFT) calculations using the CHE (computational hydrogen electrode) model propose that a potential-limiting step for the hydrocarbon's formation is the protonation of CO* to CHO* on the surface of Cu [49]. A number of pathways have been proposed that suggest the formation of intermediate hydroxyl-methylidyne and the dimerization of two CO* molecules [50]. Current advancement in computational studies further helps in understanding the mechanism and suggests that *CHO or *COH intermediate formation from the hydrogenation of *CO also play a pivotal role in determining the favorable products such as C 2 H 4 , CH 4 , and CH 3 OH on a Cu (111) surface [51]. This kinetically challenging step is due to the high energy barrier required to drive the reaction. Ethanol and ethylene as a multi-carbon product require coupling of C-C between the adsorbed intermediates on the surface, so the dimerization steps favor multi-carbon products. Recent theoretical calculations on the Cu (111) surfaces suggest that the C-C coupling strongly depends on the degree of CO ads and hydrogenation and that energy barrier of coupling decreases with the increase of the hydrogenation of surface bounded CO, which tends to favor C 2 product formation. According to the Tafel plot, the reduction of CO 2 is the rate determining step, so that adsorbed intermediate CO 2 − must be stable enough to persist until the availability of other intermediates for coupling. Copper nanocrystals may stabilize C 1 and C 2 intermediates, allowing them to trimerize to a C 3 compound (n propanol). The reaction mechanism involves various steps and the formation of several products [3] as shown in Figure 8.

Conclusions
In summary, CuO derived Cu was prepared, which demonstrated an efficient electrocatalytic performance for carbon dioxide reduction in aqueous media. The oxide derived Cu reduced to metallic copper during CO2RR induced defective sites and that transformation facilitated C2 products. C2+ products were obtained at a higher FE mainly consisting of ethylene, ethanol and propanol at −0.95 V. The catalyst was found to exhibit long term stability with a current density of almost −30 mAcm −2 . The remarkable finding of this catalyst is its onset potential of −0.1 V for C2 product which is quite a low overpotential for C2 products formation. The intrinsic property of the catalyst and catalyst surface reduction may induce sites or edges that facilitate the dimerization and coupling of intermediates to ethanol and ethylene. Further research is still required to ensure the mechanistic pathway. In this regard, DFT calculations can be helpful to determine the formation of possible intermediates on the surface of the catalyst and the possible interaction of the intermediates on the catalyst surface that stabilizes the C2H3O. It can also be possible through DFT calculations to calculate the energy barrier required for the formation of CO2 − and for the key step involved in the C-C coupling leading to the formation of C2 products at the lowest overpotential. There is an ongoing debate about the nature of C-C coupling and whether it is a chemical step or an electrochemical step that involves the transfer of H + /e for the formation of C2 products at lower overpotentials.

Conclusions
In summary, CuO derived Cu was prepared, which demonstrated an efficient electrocatalytic performance for carbon dioxide reduction in aqueous media. The oxide derived Cu reduced to metallic copper during CO 2 RR induced defective sites and that transformation facilitated C 2 products. C 2+ products were obtained at a higher FE mainly consisting of ethylene, ethanol and propanol at −0.95 V. The catalyst was found to exhibit long term stability with a current density of almost −30 mAcm −2 . The remarkable finding of this catalyst is its onset potential of −0.1 V for C 2 product which is quite a low overpotential for C 2 products formation. The intrinsic property of the catalyst and catalyst surface reduction may induce sites or edges that facilitate the dimerization and coupling of intermediates to ethanol and ethylene. Further research is still required to ensure the mechanistic pathway. In this regard, DFT calculations can be helpful to determine the formation of possible intermediates on the surface of the catalyst and the possible interaction of the intermediates on the catalyst surface that stabilizes the C 2 H 3 O. It can also be possible through DFT calculations to calculate the energy barrier required for the formation of CO 2 − and for the key step involved in the C-C coupling leading to the formation of C 2 products at the lowest overpotential. There is an ongoing debate about the nature of C-C coupling and whether it is a chemical step or an electrochemical step that involves the transfer of H + /e for the formation of C 2 products at lower overpotentials.  Acknowledgments: Iltaf Shah graciously acknowledges the generous support of the UAE University (UPAR 12S091). Afzal Shah acknowledges the support of Quaid-i-Azam University and the Higher Education Commission of Pakistan.

Conflicts of Interest:
The authors declare no conflict of interest.