Ni-based electrocatalysts using the cyclic voltammetry method

Journal Pre-proof Investigation of CO2 electrochemical reduction to syngas on Zn/Ni-based electrocatalysts using the cyclic voltammetry method Mohammadali Beheshti, Saeid Kakooei, Mokhtar Che Ismail, Shohreh Shahrestani PII:

S0013-4686(20)30368-6

DOI:

https://doi.org/10.1016/j.electacta.2020.135976

Reference:

EA 135976

To appear in:

Electrochimica Acta

Received Date: 28 November 2019 Revised Date:

24 February 2020

Accepted Date: 25 February 2020

Please cite this article as: M. Beheshti, S. Kakooei, M.C. Ismail, S. Shahrestani, Investigation of CO2 electrochemical reduction to syngas on Zn/Ni-based electrocatalysts using the cyclic voltammetry method, Electrochimica Acta (2020), doi: https://doi.org/10.1016/j.electacta.2020.135976. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier Ltd.

Investigation of CO2 electrochemical reduction to Syngas on Zn/Ni-based electrocatalysts using the cyclic voltammetry method Mohammadali Beheshti*, Saeid Kakooei*, Mokhtar Che Ismail, Shohreh Shahrestani Centre for Corrosion Research, Mechanical Engineering Department, Universiti Teknologi PETRONAS, Bandar Seri Iskandar mohammadali_g03438 @utp.edu.my [email protected] Abstract The electrochemical CO2 reduction reaction (CO2RR) has been studied on various electrocatalysts including Zn, Ni, Zn0.65-Ni0.35, and Zn0.7-Ni0.2-Co0.1, in 0.1 M of KCl cathodic solution by the cyclic voltammetry (CV) method. The CV measurements for the CO2RR showed that the Zn-Ni bimetallic electrocatalyst has a current density of 8.4 mA.cm-2 and overpotential of -130 mV vs. Ag/AgCl. The gas chromatography results presented that the Zn-Ni bimetallic electrocatalyst produced an appropriate amount and ratio of CO (55%) and H2(45%) among other electrocatalysts in this study. SEM and EDX results demonstrated that after 48 hours of testing for the CO2RR, the Zn-Ni electrocatalyst had the lowest coke formation among the other electrocatalysts. The microstructure of electrocatalyst for the CO2RR plays a key role, so in this respect, the cluster microstructure of the Zn-Ni electrocatalyst has a more suitable performance than the Zn-Ni-Co electrocatalyst with spherical microstructure. Since the cluster microstructure of Zn-Ni provides more catalyst activation points for the electrocatalyst and less binding energy with intermediate molecules for CO2RR, therefore it is still effective for CO2RR after 48 h, with low coke formation and high efficiency. Keywords: Zn/Ni based Electrocatalysts; Electrochemical CO2 Reduction Reaction; Catalyst Activity and Stability; Coke formation; cyclic voltammetry.

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1. INTRODUCTION Climate change caused by greenhouse gases (CO2) is a serious threat to the health and safety of the human community [1 -4]. The carbon capture and conversion to other fuels have 23

been considered as the most acceptable way to prevent the increase of CO2 in the atmosphere. However, the high cost of available technology for capturing, storing and converting carbon dioxide prevents its practical implementation [5]. Recycling carbon dioxide and turning it into fuel or valuable chemicals are the current challenges faced by scientists in the field of catalysts. Among the many reduction methods, the electrochemical reduction has unique benefits [6 - 13]. 7891011

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Recent work has shown that the CO2RR to value-added chemical results in a lower efficiency that is affected by the binding energy of the CO intermediate molecule. For example, Au and Ag catalysts reduce CO2 to CO more preferably, because these electrodes need less binding energy to form the intermediate CO-molecules and it can be released from the surface without further reduction. As a result, the production of carbon products at these levels is very low [18]. Electrocatalysts including Pt, Ni, Fe, Ga, and Ti had been used for hydrogen reduction. In these metals, CO is not produced as the major carbonaceous product [14 - 17]. The hydrogen ,15

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evolution reaction in these metals is in total more than the CO2RR. Another class of metals including gold, silver, and zinc converts CO2 to CO with a suitable current efficiency [18]. In recent years, nanostructures of Au or Ag electrocatalysts for the electrochemical CO2RR to CO with faradaic efficiency more than 90% and current density above 30 mAcm-2 have been reported [19 - 22]. At the same time, Zn serves as a metal catalyst to reduce the reaction of CO2 to ,20,

21,

CO, which is a non-noble, abundant and cost-effective alternative to gold and silver [23]. There have been several reports of nanostructured zinc catalysts including nanoscale, dendritic and hexagonal [24- 26]. Quan et al. have investigated Zn foil and Zn nanoscale as electrocatalyst for 25

the electrochemical CO2RR at the NaHCO3 and NaCl cathodic electrolytes. They showed that the nanoscale catalysts at NaCl solution have the best performance in terms of faradaic efficiency and current density >90% and 6 mA.cm-2, respectively, at -1.6 V by linear sweep voltammetry method [24]. Rosen et al. have investigated Zn dendrite and Zn balks electrocatalysts for CO2RR in 0.5 M NaHCO3 electrolyte. They reported Zn dendrite catalyst has a CO current density of 4 mA.cm2- at the potential of -1.14 V vs. RHE and CO faradaic efficiency of about 80% [25]. By changing the surface orientation or morphology of the zinc electrocatalysts, the high product selectivity and faradaic efficiency can be achieved for reducing CO2 to CO. 2

Recent studies demonstrated that morphological or microstructural changes in metals play an important role in improving CO2RR [27]. The zinc metal surface is easily oxidized even if exposed to air or immersed in aqueous solutions. Therefore, conditions should be controlled to prevent zinc oxidation [27]. Nguyen et al. have proposed porous nanostructured Zn electrocatalysts which were fabricated of ZnO reduction for the electrochemical CO2RR. With using this electrocatalyst, they achieved 78.5% of faradaic efficiency for CO2production at -0.95 V vs RHE in KHCO3 cathodic solution [27]. Keerthiga and Chetty have proposed a modified Zn-Cu electrocatalyst for the electrochemical CO2RR towards methane, ethane, and hydrogen products. They deposited Zn on the Cu substrate with low and high-concentration deposits, and the results were compared with bare copper and zinc catalysts. They demonstrated that Zn-Cu with a high concentration of deposit had greater conversion efficiency, also, the maximum faradaic efficiency of methane was the order Cu-Zn (52%)>Cu (23%)> Zn (7%). Furthermore, the hydrogen formation efficiency for Cu-Zn and Cu were 8% and 68%, respectively [28]. According to the progress of electrocatalysts for CO2RR in recent years, there are still challenges in this field due to their high reaction potential (great over potential), deactivation of electrocatalysts (coke formation and reaction with solutions), unacceptable product selectivity and slow ion transfer (low exchange current density) for large-scale industrial applications. Bimetallic materials are a desirable set of materials to improve the activity and selectivity of the electrochemical CO2RR [29]. According to Table S1, Zinc and cobalt are low-cost materials and have acceptable faradaic efficiency as electrocatalyst for CO production and nickel is also cost-effective and has suitable faradaic efficiency for H2 production [15, 23]. Therefore, in this study to produce syngas (CO+H2), the bimetallic material is selected of both groups of electrocatalysts to produce CO and H2 products. For the CO2RR toward H2 and CO, other electrocatalysts are either expensive or inefficient. This study aims to investigate the performance of the Zn, Ni, Zn-Ni and Zn-Ni-Co electrocatalysts for the electrochemical CO2RR in terms of the current density, coke formation and product selectivity for commercial applications. Scanning electron microscopy coupled to energy dispersive analysis (SEM/EDX), Gas Chromatography (GC) and Cyclic Voltammetry (CV) have been used as characterization techniques.

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EXPERIMENTAL 1.1.Materials The Zn (1.8×3.5 cm2), Ni (1Ø×6.2h cm2 -rod), Zn-Ni (2.5×2.5 cm2) and Zn-Ni-Co (2.5×2.5 cm2) electrodes were used as the electrocatalysts for the electrochemical CO2RR. The substrate used for the Zn-Ni and Zn-Ni-Co coatings were low carbon steel that were deposited by chronopotentiometry method. For the deposition of Zn-Ni on the carbon steel substrate, zinc chloride (ZnCl2), nickel chloride hexahydrate (NiCl2.6H2O) and ammonium chloride (NH4Cl) were used for the bath solution preparation (electrolyte) and 25% ammonia solution was for the bath pH adjustment, as can be seen in Table S2. The solution was prepared using distilled water. The chronopotentiometry method was applied for the electrodeposition of the Zn-Ni coating on the low carbon steel substrate. The electrodeposition parameters for Zn-Ni coating include a current density of 10 mA.cm-2, pH value of 5, at temperature 25 °C with a deposition time of 60 min [30-31]. For the deposition of Zn-Ni-Co on the carbon steel substrate, zinc chloride (ZnCl2), nickel chloride hexahydrate (NiCl2.6H2O), ammonium chloride (NH4Cl) and cobalt sulfate (CoSO4) were used for the bath solution (electrolyte) preparation, as can be seen in Table S3. The solution was prepared using distilled water. The chronopotentiometry method was applied for the electrodeposition of the Zn-Ni-Co coating on the low carbon steel substrate. The electrodeposition parameters for the Zn-Ni-Co coating include a pH value of 5, current density of 40mA.cm-2, at temperature 25 °C and deposition time of 20 min. 1.2.Electrochemical cell setup for CO2RR For electrochemical cell setup, 0.1 M KCl (with pH 6.98, after saturation with CO2, it decreased to 4.1) and 0.1 M H2SO4 (with pH 1.3) were prepared as cathodic and anodic solutions, respectively. The electrochemical cell used in this study has two chambers (H-shaped) that are connected by the Nafion 117 membrane as it is shown in Fig. 1. CO2 is injected into the cathodic solution for reduction. In this paper, stainless steel 316 and Ag/AgCl are the counter and reference electrodes, respectively. In this method, the electrocatalyst, Ag/AgCl reference electrodes (RE) and CO2 saturated cathodic electrolyte (0.1 M KCl) are in the cathodic section, where the electrochemical CO2RR has occurred. On another side, the counter electrode (CE) and the anodic electrolyte (0.1 M H2SO4) are subjected to oxidation in the anodic section. Therefore, 4

it prospects that SYNGAS released in the cathodic side and oxygen released on the anodic side. Three electrodes (RE, CE and WE) were connected to the Autolab potentiostat device to investigate current and potential data. b)

a)

Fig. 1. a) Schematic and b) image of electrochemical cell setup for CO2RR including Zn-Ni bimetallic electrocatalyst, stainless steel 316 counter electrode, 0.1 M KCl as a cathodic solution, H2SO4 anodic solution and Ag/AgCl reference electrode of H-shaped electrochemical cell. 1.3.Electrochemical analysis for CO2RR At this step, electrolysis of the electrochemical cell for the CO2RR using the potentiostat AutoLAB device was investigated. The electrochemical cell setup for CO2RR is shown in Fig. 1. Cyclic voltammetry (CV) measurement has been applied to investigate the current density, potential, and overpotential of reactions. The potential range was used in a negative range of potential (with negative current, I < 0 mA) to prevent any oxidation in the cathodic part as can be seen in Fig. S1. This method provides different potential values that can be investigated for various electrocatalysts in terms of over potential and current density. 1.4.Gas chromatography (GC) of the produced gas from CO2RR The gases produced during the electrochemical CO2RR by cyclic voltammetry method at potential ranges -1.1 V to -1.7 V, -0.9 V to -1.6 V, -0.6 V to -1.2 V and -0.8 V to -1.8 V for Zn, Ni, Zn-Ni and Zn-Ni-Co, respectively, were analyzed by the gas chromatography device (SHIMADZU GC-2010 Pro) with two detectors. The H2 and CO products were detected by TCD with Argon gas carrier and molecular sieve 5A column. The CO2 and other hydrocarbons products were detected by TCD with Helium gas carrier and pro packed Q column.

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1.5.SEM and EDX analysis of electrocatalysts The images of the electrocatalysts were obtained and analyzed by scanning electron microscopy (PhenomPro X) equipped with EDX before and after the CO2RR. Information on particle size distribution, substrate deposition, and elemental compositions was obtained. 2. RESULTS AND DISCUSSION 2.1.Electrochemical study upon electrocatalyst for CO2RR Fig. 2 shows electrochemical voltammograms obtained with the different electrocatalysts including Zn metal, Ni metal, Zn-Ni bimetallic and Zn-Ni-Co multi-metallic. As seen, they give different values in the current density and potential ranges. The results demonstrated that the maximum current density values for the CO2RR occur at more negative potential than -1.58 V, 1.61 V, -0.92 V and -1.14 V for the electrocatalysts of zinc, nickel, zinc-nickel, and zinc-nickelcobalt, respectively. As a result, zinc-nickel bimetallic electrocatalyst can convert carbon dioxide to value-added chemicals at low potential ranges that conclude in energy savings, longer life and durability for electrocatalysts, counter electrodes, anodic, and cathodic solutions and membranes. On the other hand, at a more positive potential range (with positive current, I > 0 mA), oxidation occurs on the electrocatalyst which is related to the decomposition of electrocatalysts in solution. Therefore, for SEM/EDX and GC analysis the potential range has been restricted to the region providing negative currents (I < 0 mA), as can be seen in Fig. S1. The extra energy due to thermodynamic barriers is well-known as overpotential. The thermodynamic standard potential for CO2RR to CO at pH of 6.98 (0.1 M KCl) is -0.56 V vs RHE (-0.79 V vs Ag/AgCl). From Fig. 2 the overpotentials of the Zn, Ni, Zn-Ni and Zn-Ni-Co electrocatalysts for CO2RR were obtained by subtracting the thermodynamic value to the potential value corresponding to the highest current density. The values obtained are -790 mV, 820 mV, - 130 mV and -350 mV, respectively. As shown in Fig. 3, the Zn-Ni electrocatalyst has a higher current density in response to the CO2RR at the lower potential range and it also provides more sensitivity to potential changes among other electrocatalysts in this study. With a slight potential changed from -0.6 V to -0.8 V, the current density suddenly changed from +8 mA/Cm2 to -8 mA/Cm2 while this was a very slow and gradual change for the Zn and Ni electrocatalysts. It should also be noted that the voltammogram variation of Zn-Ni-Co is similar to Zn-Ni bimetallic, but this change occurs at more negative potential and less intensity of current density. Therefore, the Zn-Ni electrocatalyst 6

can be a more suitable electrode for the electrochemical CO2RR, because it has the appropriate ranges of the current density and potential, which makes it suitable from an energetic point of view. The results demonstrate that bimetallic electrocatalyst can be considered as a desirable set of materials to improve the current efficiency of the electrochemical CO2RR. Not only do they supply different binding positions for the intermediate reactions but also with the different compounds they can perform different reactions during the electrochemical CO2RR [29]. This indicates that Zn-Ni and Zn-Ni-Co coatings have a high affinity and therefore tend to interact more with the CO2 molecules and their intermediates than the bare metals of zinc and nickel.

Fig. 2. Cyclic voltammograms of electrochemical processes for CO2RR with scan rate 0.05 V.s1 , SS316 counter electrode, 0.1M KCl solution on the a) Zn, b)Ni, c)Zn-Ni, and d) Zn-Ni-Co electrocatalysts Fig. 4 shows the current density in terms of the time of electrolysis for the various electrocatalysts during CO2RR. The results indicate that the stability of Zn-Ni and Zn-Ni-Co electrocatalysis is better than the Zn and Ni electrodes, which proves the formation of large 7

amount of non-desirable carbon compounds on the Zn and Ni electrocatalysts and its effect on the current density over time. 10 Current density (mA/cm2)

8 6

Zn

Ni

Zn-Ni

Zn-Ni-Co

4 2 0

-2 -4 -6 -8

-10 -0.6

-0.8

-1

-1.2 -1.5 Potential (V)

-1.8

-2

-2.5

Fig. 3. Results concluded of cyclic voltammogram for CO2RR by various electrocatalysts reaction with scan rate 0.05 V. s-1, SS316 counter electrode, and 0.1M KCl solution

Current density (mA/cm2)

-9.00 -8.50 -8.00 -7.50 -7.00 -6.50 -6.00 -5.50

Zn

Ni

Zn-Ni

Zn-Ni-Co

-5.00 5

15

25 35 45 Time (h) Fig. 4. Results concluded of the current density in terms of the time for the different electrocatalysts by CV method at potential -1.5 V for CO2RR in 0.1 M KCl cathodic solution 2.2.Gas Chromatography study upon electrocatalysts for CO2RR According to the CV results (as seen in Fig. 2) in terms of current density and potential for the CO2RR, the potential ranges for Zn, Ni, Zn-Ni and Zn-Ni-Co electrocatalysts were optimized 8

at -1.2 V to -3.0 V, -1.0 V to -3.0 V, -0.7 V to -2.8 V and -0.8 V to -2.8 V, respectively, to prevent oxidation of electrocatalysts. Therefore, GC experiments and SEM / EDX analysis were performed on the produced gases and electrocatalysts, after 48 h CO2RR by CV method at these ranges of potentials. Fig. 5 shows the gas chromatography results in terms of the percentage of the gas selectivity and intensity for the different types of electrocatalysts for the CO2RR. The Zn, Ni, ZnNi and Zn-Ni-Co electrocatalysts have selectivity for H2-CO with 34%-66%, 83%-17%, 45%55% and 31%-69%, respectively. According to Fig. 5-a, among four types of electrocatalysts, Zn-Ni is an acceptable electrocatalyst to convert CO2 to SYNGAS (CO+H2), while Zn and Ni are electrocatalysts of CO and H2, respectively. Because of its low selectivity, Zn-Ni-Co is an inadequate electrocatalyst to produce Syngas. Fig. 5-b presents the results of the gas chromatography in terms of the intensity of gas selection for the different types of electrocatalysts for CO2RR. As seen in Fig. 5-b, among the 4 types of electrocatalysts, the Zn-Ni bimetallic electrocatalyst has the best activity and selectivity properties for CO2RR, while the Zn electrocatalyst has a poor activity for the CO + H2 production, the Ni metal has a poor activity for the CO production, and the Zn-Ni-Co has a poor activity for the H2 production. Therefore, the Zn-Ni electrocatalyst is suitable for the CO2RR related to another three types of electrocatalysts in terms of the selectivity and activity for CO and H2. Non-noble metals (Zn, Ni, and Co) are affected by coke formation issue of the CO2RR [32]. Some active points of Zn and Ni substrates are inactivated and a significant portion of the current of CO2 reaction is converted to coke and prevents CO production. Therefore, non-noble single metals such as zinc and nickel are not suitable for CO2RR because they have strong bonds with non-desirable carbon compounds that prevent the intermediates from being converted to products. The development of coke-resistant electrocatalysts is an essential challenge for CO2RR. The selectivity and intensity of CO and H2 products for zinc-nickel and zinc-nickel-cobalt are based on the morphology of their microstructures. The performance of the CO2RR on the Zn-Ni electrocatalyst with the cluster-like microstructure in terms of product (CO+H2) selectivity and intensity, as well as coke formation, is more suitable than the Zn-Ni-Co electrocatalyst with the spherical-like microstructure. The reason can be attributed that the spherical microstructure of Zn-Ni-Co has a greater affinity for coke formation compared by Zn-Ni with the cluster microstructure that can prevent the electrocatalyst from the further conversion of CO2 to CO and release of the 9

intermediate molecules. Also, electrocatalyst with the cluster microstructure increases the active electrocatalyst area, which causes less binding energy for CO-intermediated molecules and

90 80 70 60 50 40 30 20 10 0

0.3

83

a)

H2 percent

66

69

CO percent

55 31

Ni

H2 amount

0.1

17

Zn

CO amount

0.2

45 34

b)

Gas selectivity intensity×10

Gas selectivity percent (%)

prevents the deposition of the compounds with high carbon content.

0.0

Zn-Ni Zn-Ni-Co

Zn

Ni

Zn-Ni

Zn-Ni-Co

Fig. 5. Gas chromatography results for CO2RR by CV method on the various electrocatalysts in terms of CO and H2 gas selectivity a) percent and b) intensity Fig. 6 and Fig. 7 show the faradaic efficiency in terms of the potential for carbon monoxide and hydrogen products from CO2RR by the CV method. As seen in Fig. 6 and Fig. 7, Zinc and nickel electrodes as electrocatalysts have a poor performance in both CO and H2 productions in response to CO2RR. On the other hand, Zn-Ni and Zn-Ni-Co electrocatalysts have better efficiency in H2 and CO production in response to CO2RR than single metals of Zn and Ni. However, the Zn-Ni-Co electrocatalyst operates at a high potential range, which is inappropriate for energy consumption. Therefore, the bimetallic Zn-Ni electrode is more suitable as an electrocatalyst in producing the concurrent reaction of H2 and CO in response to the CO2RR in aqueous solution due to its high efficiency in the low potential range among other electrocatalysts in this study. The maximum faradaic efficiency obtained at the lower potential for Zn-Ni can be attributed to the optimum binding strength between the intermediate product and the electrocatalyst, thus the intermediate species on the surface of the electrode are likely to desorb as CO products rather than remain and couple with other intermediates to form higher carbon-containing compounds.

10

60 Zn

Ni

Zn-Ni

Zn-Ni-Co

50 CO FE%

40 30 20 10 0 -10 -0.5

-0.8

-1.1

-1.4 -1.7 Potential (V)

-2

-2.3

-2.6

Fig. 6. Effect of faradaic efficiency in terms of potential for CO production from CO2RR on different electrocatalysts by CV method with scan rate 0.05 V.s-1, SS316 counter electrode, and 0.1 M KCl cathodic solution 75 Zn

Ni

Zn-Ni

Zn-Ni-Co

65

H2 FE%

55 45 35 25 15 5 -5 -0.5

-0.8

-1.1

-1.4 -1.7 -2 -2.3 -2.6 Potential (V) Fig. 7. Effect of faradaic efficiency in terms of potential for H2 production from CO2RR on different electrocatalysts by CV method with scan rate 0.05 V. s-1, SS316 counter electrode, and 0.1 M KCl cathodic solution 2.3.SEM and EDX analysis upon electrocatalyst for CO2RR To understand the effects of the reactions on the electrocatalysts to reduce CO2, therefore, the surface morphology, composition, and structure of the electrodes were evaluated before and 11

after 48 h of the CO2RR. Fig. 8 shows SEM images of the Zn, Ni, Zn-Ni and Zn-Ni-Co electrocatalysts before and after the 48 hours for the CO2RR by CV method at the limiting potential range for reduction (as seen in Fig. S1) in 0.1 M KCl cathodic solution. Prior to the reaction (Fig. 8-a), the zinc electrode was precisely white-silver with flat grain morphologies, which after being reacted for 48 hours (Fig. S4 and Fig. 8-b), turned into gray-black with the fibrous microstructure that was widely deposited on the zinc electrode. Also, the nickel electrode was precisely silver in color with the typical shape of nickel crystallites (Fig. S6 and Fig. 8-c), whereby, after being reacted for 48 hours, turned into black in the exposed area as seen in Fig. 8d (coke formation with blocks microstructure). As can be obvious in Fig. S8 and Fig. 8 (e, f), the black zinc-nickel electrode had a cluster microstructure that showed any changes in color and coke growth after CO2RR for 48 h. In the end, the zinc-nickel-cobalt electrode was precisely black, however, after being reacted for 48 hours, it turned into a black-brown as can be seen in Fig. S10. Fig. 8 (g, h) demonstrates that CO2RR after 48h on Zn-Ni-Co with spherical microstructure, coke has been deposited on some of the electrocatalyst surface areas. It can be found that the Zn and Ni balk metals have strong binding energy to the intermediate molecules which cause CO2RR to occur at high value of potential and therefore the carbon compounds deposit on the electrocatalyst. (a)

(d)

(c)

(b)

(f)

(e)

12

(g)

(h)

Fig. 8. SEM microstructure images of (a, b) Zn, (c, d) Ni, (e, f) Zn-Ni and (g, h) Zn-Ni-Co before and after CO2RR, respectively, by CV method in 0.1 M KCl cathodic solution. According to EDX analysis, as shown in Fig. 9, carbon was strongly deposited on the zinc and nickel electrodes and slightly deposited on the Zn-Ni and Zn-Ni-Co electrocatalysts in the exposed area after CO2RR for 48 h of testing. The formation of coke on the electrocatalyst

100 90 80 70 60 50 40 30 20 10 0

Carbon deposited % electrocatalyst efficiency after 48h (%)

69.61

70

66.00 50.67

69.38

60 50 40 30

25.20 12.00

23.89 9.92

20

Efficiency (%)

Carbon deposited (%)

can affect the catalyst activity points and harm the performance and lifetime of the catalyst.

10 0

Zn

Ni

Zn-Ni Electrocatalysts

Zn-Ni-Co

Fig. 9. Carbon deposited based on EDX analysis and efficiency for different electrocatalysts after 48 of CO2RR by CV method with scan rate 0.05 V. s-1, SS316 counter electrode, and 0.1 M KCl cathodic solution. Fig. 9 shows the efficiency and coke formation of the four types of electrocatalysts after 48 hours of CO2RR by the CV method. As can be seen in Fig. 9, coke formation on the Zn-Ni-Co electrocatalyst is higher than that of the Zn-Ni electrocatalyst, indicating that the catalyst activity sites are more covered with carbon, and harms the catalyst performance. These results can be concluded that the electrocatalyst performances are significantly affected by the electrocatalyst microstructures and morphologies as well as compositions of alloys for CO2RR. Therefore, Zn13

Ni electrocatalyst, among other electrocatalysts in this study, is more appropriate as electrocatalyst for CO2RR to Syngas (CO+H2). It has also been concluded that the ratio of the electrocatalyst coating components and microstructure for the electrochemical CO2RR with acceptable gas selectivity and intensity as well as the coke formation and electrocatalyst activity is crucial. However, the presence of some randomly distributed short fibers in the SEM image implies that, for the Zn-Ni bimetallic electrocatalyst, a little amount of carbon deposit could still form on this catalyst after 48 hours of the reaction time as shown in the EDX analysis in Fig. S8. The formation of heavy carbonaceous deposits is an intrinsic property of the reaction. So, these large species can be regarded as a highly undesirable factor that leads to the catalyst deactivation. CONCLUSIONS The electrochemical CO2RR on the various electrocatalysts including Zn, Ni, Zn-Ni, and Zn-Ni-Co were performed in 0.1 M KCl cathodic and 0.1 M H2SO4 anodic solutions by CV method. Cyclic voltammograms were observed for CO2RR in the different electrocatalysts. It was noted that Zn-Ni bimetallic had the best performance in terms of the current density, over potential, and potential range. The best potential range for the CO2RR by the Zn-Ni electrocatalyst in 0.1 M KCl cathodic solution was -0.6 V to -1.0 V, while for Zn, Ni and Zn-NiCo were -1.2 V to -1.8 V, -1.2 V to -2.2 V and -0.8 V to -1.8 V, respectively. The result of gas chromatography was suitable for the Zn-Ni electrocatalyst compared to other electrocatalysts in terms of the selectivity and intensity of H2 and CO production. SEM images and EDX results showed that the Zn-Ni electrocatalyst had good morphology and low coke formation for the electrochemical CO2RR after 48 hours of testing. The Coke deposition on the Zn and Ni electrocatalysts after 48 h of CO2RR was high, while it was low for the Zn-Ni and Zn-Ni-Co electrocatalysts. In the end, alloys compositions, as well as the microstructures and morphologies of electrocatalysts, have great influence on the electrocatalyst performance for CO2RR, therefore the Zn-Ni electrocatalyst with cluster microstructure and 0.35 wt% of Zn and 0.65 wt% of nickel for composition had the best performance for CO2RR among other materials in this study. ACKNOWLEDGMENT This research is financially supported by YUTP-FUNDAMENTAL RESEARCH GRANT (YUTP-FRG) (015LC0-165) and Universiti Teknologi Petronas.

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Highlights
The electrochemical CO2RR has been studied with Zn, Ni, Zn0.65Ni0.35 and Zn0.7-Ni0.2Co0.1.
The maximum syngas efficiency reaches 85.7% at Zn0.65-Ni0.35 electrocatalyst with current density 8mAcm-2, overpotential -130 mV and potential -0.8V vs. Ag/AgCl reference electrode.
Very good selectivity for CO (55%) and H2 (45%) and high stability (48 hours) was achieved with Zn-Ni.
Zn-Ni electrocatalyst possesses 2.4-fold larger electrochemical active surface area (ECSA) compared with Zn-Ni-Co electrocatalyst.

Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

we declare there is not any financial interests/personal relationships which may be considered as potential competing interests.