Effect of the Nanostructured Zn/Cu Electrocatalyst Morphology on the Electrochemical Reduction of CO2 to Value-Added Chemicals

nanomaterials

Article Effect of the Nanostructured Zn/Cu Electrocatalyst Morphology on the Electrochemical Reduction of CO2 to Value-Added Chemicals

Piriya Pinthong 1, Phongsathon Klongklaew 1, Piyasan Praserthdam 1 and Joongjai Panpranot 1,2,*

1 Center of Excellence on Catalysis and Catalytic Reaction Engineering, Department of Chemical Engineering, Faculty of Engineering, Chulalongkorn University, Bangkok 10330, Thailand; [email protected] (P.P.); [email protected] (P.K.); [email protected] (P.P.) 2 Bio-Circular-Green-economy Technology & Engineering Center, BCGeTEC, Department of Chemical Engineering, Faculty of Engineering, Chulalongkorn University, Bangkok 10330, Thailand * Correspondence: [email protected]

Abstract: Zn/Cu electrocatalysts were synthesized by the electrodeposition method with various bath compositions and deposition times. X-ray diffraction results conﬁrmed the presence of (101) and (002) lattice structures for all the deposited Zn nanoparticles. However, a bulky (hexagonal) structure with particle size in the range of 1–10 µm was obtained from a high-Zn-concentration bath, whereas a fern-like dendritic structure was produced using a low Zn concentration. A larger particle size of Zn dendrites could also be obtained when Cu2+ ions were added to the high-Zn-concentration bath. The

catalysts were tested in the electrochemical reduction of CO2 (CO2RR) using an H-cell type reactor under ambient conditions. Despite the different sizes/shapes, the CO2RR products obtained on the nanostructured Zn catalysts depended largely on their morphologies. All the dendritic structures led Citation: Pinthong, P.; Klongklaew, to high CO production rates, while the bulky Zn structure produced formate as the major product, P.; Praserthdam, P.; Panpranot, J. Effect of the Nanostructured Zn/Cu with limited amounts of gaseous CO and H2. The highest CO/H2 production rate ratio of 4.7 and Electrocatalyst Morphology on the a stable CO production rate of 3.55 µmol/min were obtained over the dendritic structure of the

Electrochemical Reduction of CO2 to Zn/Cu–Na200 catalyst at −1.6 V vs. Ag/AgCl during 4 h CO2RR. The dissolution and re-deposition Value-Added Chemicals. of Zn nanoparticles occurred but did not affect the activity and selectivity in the CO2RR of the Nanomaterials 2021, 11, 1671. https:// electrodeposited Zn catalysts. The present results show the possibilities to enhance the activity and doi.org/10.3390/nano11071671 to control the selectivity of CO2RR products on nanostructured Zn catalysts.

Academic Editor: Alexandru Keywords: Zn dendrite; bulky Zn; Zn/Cu electrode; electrodeposition; CO2 reduction; electrocatalysis Mihai Grumezescu

Received: 24 May 2021 Accepted: 18 June 2021 1. Introduction Published: 25 June 2021 Removal of CO2, the principal greenhouse gas, from the atmosphere is critical in order

Publisher’s Note: MDPI stays neutral to avoid climate disasters caused by global warming. While CO2 capture and storage face with regard to jurisdictional claims in geological risks such as leakage and earthquakes, the sequestration and use of CO2 as a published maps and institutional afﬁl- carbon feedstock for chemicals, fuels, and other derivative materials are an alternative iations. feasible and economic pathway to recycle CO2 into various useful resources [1]. Among the different CO2 conversion routes, electrochemical reduction of CO2 (CO2RR) using electricity from renewable energies is one of the best currently available technologies that can meet the energy demand for CO2 reduction by low-carbon and low-cost electricity. A variety of carbon-containing products, such as formate, carbon monoxide, methane, Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. and alcohol, can be derived from CO2RR in aqueous electrolytes [1–4]. Carbon monoxide This article is an open access article (CO) is an interesting product of this process because it can be used as a reactant for the distributed under the terms and Fischer–Tropsch process to produce hydrocarbon fuels and chemicals [5,6]. Furthermore, it conditions of the Creative Commons can be simply separated from liquid electrolytes. Attribution (CC BY) license (https:// There are various kinds of electrocatalysts that promote CO production in the elec- creativecommons.org/licenses/by/ trochemical reduction of CO2, including both noble metals such as Au and Ag [7–9] and 4.0/). non-noble metals such as Cu, Sn, and Zn [10–13]. Noble metals are expensive, limiting

Nanomaterials 2021, 11, 1671. https://doi.org/10.3390/nano11071671 https://www.mdpi.com/journal/nanomaterials Nanomaterials 2021, 11, 1671 2 of 12

their uses in large-scale commercial applications; thus, non-noble metals are preferred. Typically, a metal foil or plate is used as a cathode (working electrode) in the electrochemical reduction of CO2, but this exhibits low efficiency and selectivity toward CO production [14–16]. Using catalysts with a high surface area is a promising way to improve catalytic activity. Nanostructured materials have been employed in several works, such as nanocoral silver [17] and nanoporous ZnO [18], due to the large surface area, and a CO Faradaic efficiency above 90% can be achieved. For Zn-based catalysts, the performance of CO2RR to CO has been reported to be improved using nanostructured Zn electrocatalysts prepared by the electrodeposition method. The electrodeposited Zn catalysts, however, possessed different structures, such as a dendritic structure [19], a hexagonal structure [20], or a foam structure [21]. However, it is quite difficult to make a fair comparison among these studies to determine which morphologies of the Zn nanostructure would provide the best performance. A number of research works have also paid attention to the development of bimetallic catalysts, because higher performances can be obtained by modifying the structure and morphology of the catalysts [22–24]. Gue et al. reported that a AgZn bimetallic thin film can promote the CO2RR to CO with a Faradaic efficiency of 84.2% [24] due to the synergistic effect of Zn and Ag. In addition, the Zn structure was further modified by mixing Cu and Zn [21]. Zn/Cu alloy catalysts exhibit higher CO selectivity than pure Cu or pure Zn [25]. In this work, Zn/Cu electrocatalysts were prepared by electrodeposition of Zn on Cu foil (Zn/Cu) and electrodeposition of Zn and Cu on Cu foil (ZnCu/Cu) in an electrodeposition solution consisting of NaCl (Zn/Cu-Na, ZnCu/Cu-Na) or HCl (Zn/Cu-H, ZnCu/Cu-H) with different deposition times. These catalysts were evaluated in the electrochemical reduction of CO2 to higher-value chemicals. The morphology, surface composition, and crystalline structure of these catalysts were investigated by scanning electron microscopy–energy dispersive X-ray spectroscopy (SEM-EDX) and X-ray diffraction (XRD).

2. Experimental Section 2.1. Electrode Preparation (Electrodeposition of Zn/Cu Electrodes) Cu and Zn electrodes were prepared by cutting commercial Cu (0.1 mm thick, 99.9999%) and Zn (0.1 mm thick, 99.994%) foil, from Alfa Aesar® (Ward Hill, MA, USA), to a size of 10 × 25 mm2. The metal electrodes were physically polished with 800G sandpaper to remove the oxide surface and were then rinsed with deionized water before drying with nitrogen. Nanostructured Zn/Cu and ZnCu/Cu electrodes with different morphologies were prepared by electrodeposition of ZnCl in either a NaCl or a HCl bath. A platinum rod (length 76 mm, diameter 2 mm) from Metrohm® (Herisau, Switzerland) was used as the counter electrode (anode). A polished Cu electrode was used as a cathode in a solution containing 0.05 M ZnCl2 (Ajax Finechem Pty Ltd., New south whales, Australia) and 0.05 M NaCl (Sigma-Aldrich, St. Louis, MO, USA). Electric current was applied at 20 mA/cm2 for 60 and 200 s to obtain Zn/Cu–Na60 and Zn/Cu–Na200, respectively. Fur- thermore, the effect of copper ions on the electrodeposition was investigated by adding CuCl2 to the solution. A solution consisting of 0.05 M ZnCl2, 0.05 M NaCl, and 1.5 mM CuCl2 was employed under similar electrodeposition conditions to obtain ZnCu/Cu–Na60 and ZnCu/Cu–Na200. Under acidic conditions (HCl bath), electrodeposition was performed in a solution 2 consisting of 0.2 M ZnCl2 and 1.5 M HCl. Electric current was applied at 0.3 A/cm for 60 and 200 s to obtain Zn/Cu–H60 and Zn/Cu–H200, respectively. The effect of Cu ions was also studied using a solution consisting of 0.2 M ZnCl2, 1.5 M HCl, and 6 mM CuCl2 to obtain ZnCu/Cu–H60 and ZnCu/Cu–H200. The obtained electrodes were rinsed with deionized water and dried with nitrogen. NanomaterialsNanomaterials 20212021,, 1111,, 1671x FOR PEER REVIEW 33 of 1212

2.2. Characterizatio Characterizationn of Electrodes The crystallinecrystalline structurestructure of of electrocatalyst electrocatalyst samples samples was was analyzed analyzed using using a SIEMENSa SIEMENS D D 5000 X-ray diffractometer (Munich, Germany) with a CuKα radiation source (λ = 5000 X-ray diffractometer (Munich, Germany) with a CuKα radiation source (λ = 0.154439 nm) and0.154439 nickel nm) ﬁltered and nickel in the 2filteredθ degree in rangethe 2θ of degree 20◦–80 range◦ (scan of rate 20°–80° = 0.5 (scan s/step). rate The = 0.5 morphol- s/step). ogyThe andmorphology surface composition and surface ofcomposition the electrocatalysts of the electrocatalysts were analyzed were by scanninganalyzed electronby scan- microscopy–energyning electron microscopy–ene dispersivergy X-ray dispersive spectroscopy X-ray spectroscopy (SEM-EDX) (SEM-EDX) using a Hitachi using model a Hi- S-3400Ntachi model (Tokyo, S-3400N Japan) (Tokyo, scanning Japan) electron scanning microscope. electron microscope.

2.3. Electrochemical Reduction of CO2

All experimentsexperiments onon the the electrochemical electrochemical reduction reduction of COof CO2 were2 were performed performed in an in H-type an H- celltype at cell room at room temperature temperature and atmospheric and atmospheric pressure, pressure, as shown as shown in the schematicin the schematic diagram dia- in Figuregram in1. TheFigure cathodic 1. The and cathodic anodic and parts anodic were separatedparts were by separated a Nafion ®by117 a Nafion membrane® 117 (Sigma- mem- Aldrich,brane (Sigma-Aldrich, St. Louis, MO, St. USA). Louis, The MO, experiments USA). The wereexperiments carried outwere in carried a three-electrode out in a three- cell ® system.electrode A cell silver/silver system. A chloridesilver/silver (Ag/AgCl) chloride electrode (Ag/AgCl) from electrode Metrohm from(Herisau, Metrohm Switzer-® (Heri- land)sau, Switzerland) was used as was the used reference as the electrode. reference electrode. The counter The electrode counter electrode was a platinum was a plati- foil (0.1num mm foil thick,(0.1 mm 99.997%) thick, from99.997%) Alfa from Aesar Alfa (Ward Aesar Hill, (Ward MA, USA).Hill, MA, The USA). working The electrodes working (Znelectrodes foil, Cu (Zn foil, foil, and Cu thefoil, electrodeposited and the electrodep electrodes)osited electrodes) were immersed were immersed in an electrolyte in an elec- 2 solutiontrolyte solution with a geometricalwith a geometrical area of area 1 cm of. The1 cm catholyte2. The catholyte and anolyte and anolyte were 20 were mL 20 of mL 0.1 Mof KHCO0.1 M KHCO3 solution.3 solution. The electrolyte The electrolyte was saturated was saturated with 100with mL/min 100 mL/min of CO of2 gasCO2 for gas 30 for min 30 with a CO flow rate of 20 mL/min during the reaction. The reduction potential was min with a2 CO2 flow rate of 20 mL/min during the reaction. The reduction potential was controlled by a potentiostatpotentiostat duringduring aa reactionreaction timetime ofof 7070 min.min. TheThe gaseousgaseous productsproducts werewere analyzed by an online gasgas chromatographychromatography systemsystem withwith aa thermalthermal conductivityconductivity detectordetector (TCD).(TCD). TheThe liquid-phase liquid-phase products products were were identified identified and and quantified quantified using using the NMRthe NMR technique. tech- Thenique. electrocatalysts The electrocatalysts with outstanding with outstanding performances performances were further were investigated further investigated at different at − − reductiondifferent reduction potentials potentials from 1.4 from to 2.0 −1.4 V vs.to − Ag/AgCl,2.0 V vs. Ag/AgCl, and a stability and testa stability was performed test was at the appropriate potential for 4 h. performed at the appropriate potential for 4 h.

Figure 1. Schematic electrochemical CO2 reduction setup. Figure 1. Schematic electrochemical CO2 reduction setup.

3. Results and Discussion 3.1. Electrocatalys Electrocatalystt Characterization The Zn/Cu and and ZnCu/Cu ZnCu/Cu electrocatalysts electrocatalysts in in this this study study were were prepared prepared by by electrodep- electrode- positionosition techniques techniques under under different different conditions conditions in inorder order to toobtain obtain different different morphologies morphologies of ofthe the deposited deposited metal. metal. The The method method allows allows successful successful deposition deposition of of 2D 2D and and 3D metalmetal

Nanomaterials 2021, 11, 1671 4 of 12

nanostructures on the substrates. Grain sizes of the deposited metals in the nanometer range were obtained by selecting electrodeposition variables (e.g., bath composition, pH, temperature, current density) such that nucleation of new grains was favored rather than growth of existing grains [26]. SEM images of the electrocatalyst samples are shown in Figure2. A rock-like surface with some streaks was observed on the mechanically polished Zn and Cu foil (Figure2a,b). After electrodeposition, a fern-like dendritic structure of Zn nanoparticles was obtained on Zn/Cu–Na60 and Zn/Cu–Na200, as shown in Figure2c,d, respectively. The high deposition rate of Zn for 0.05 M ZnCl2 and the lower-concentration region of Zn ions at the electrocatalyst surface forced the dendritic structures to grow outward toward the higher-concentration region of Zn ions [19]. The deposition time (60 and 200 s) did not affect the morphology of Zn/Cu–Na, as the morphologies of Zn/Cu–Na60 and Zn/Cu–Na200 were similar. When Cu ions were added, the dendritic structures were not much altered, as seen in Figure2e for ZnCu/Cu–Na60 and Figure2f for ZnCu/Cu– Na200, probably due to the low concentration of CuCl2 (0.0015 M) in the deposition bath. In contrast, bulky (hexagonal) structures with average particle sizes of 1–4 and 4–10 µm were obtained on Zn/Cu–H60 and Zn/Cu–H200, respectively (Figure2g,h). It is suggested that a bulky structure is formed because of the high ZnCl2 concentration (0.2 M) used in the deposition bath [19]. Increasing the deposition time resulted in an increased average particle size of Zn/Cu–H. Moreover, when Cu ions were simultaneously added to the HCl bath with Zn ions, another form of dendritic structure was observed, as shown in Figure2i,j for ZnCu/Cu–H60 and ZnCu/Cu–H200, respectively. Although the deposition time did not have much impact on the dendritic morphology of ZnCu/Cu–H, adding Cu ions to the solution changed the morphology from a bulky structure to a dendritic structure as the concentration of Zn ions near the electrocatalyst surface was diluted by the presence of Cu ions. Cu ions can compete with Zn ions for deposition on a Cu foil. The deposited Cu can also accelerate the formation of hydrogen gas during the electrodeposition process. In addition, Cu has a higher hydrogen bond strength than Zn [21]. The hydrogen gas that is produced during the electrodeposition process also competes with Zn ions. It is also noticed that the particle size of the dendritic structure of ZnCu/Cu–H is bigger than that of ZnCu/Cu–Na due to the higher concentration of ZnCl2 in the deposition bath. The surface compositions of the deposited electrodes are shown in Table1. At a low Zn ion concentration in the NaCl bath, although increasing the deposition time did not affect the morphology of the fern-like dendritic structure being formed, the percentages of Zn increased from 75% to 93% as the deposition time increased from 60 to 200 s. Adding Cu did not have much impact on the amount of Zn being deposited under the conditions used (60 s, NaCl bath). Conversely, at a high Zn ion concentration in the HCl bath, there was little effect of the deposition time, whereas a lower amount of Zn was obtained when ZnCl2 and CuCl2 were simultaneously deposited. Nanomaterials 2021, 11, x FOR PEER REVIEW 4 of 12

nanostructures on the substrates. Grain sizes of the deposited metals in the nanometer range were obtained by selecting electrodeposition variables (e.g., bath composition, pH, temperature, current density) such that nucleation of new grains was favored rather than growth of existing grains [26]. SEM images of the electrocatalyst samples are shown in Figure 2. A rock-like surface with some streaks was observed on the mechanically polished Zn and Cu foil (Figure 2a,b). After electrodeposition, a fern-like dendritic structure of Zn nanoparticles was obtained on Zn/Cu–Na60 and Zn/Cu–Na200, as shown in Figure 2c,d, respectively. The high deposition rate of Zn for 0.05 M ZnCl2 and the lower-concentration region of Zn ions at the electrocatalyst surface forced the dendritic structures to grow outward toward the higher-concentration region of Zn ions [19]. The deposition time (60 and 200 s) did not affect the morphology of Zn/Cu–Na, as the morphologies of Zn/Cu–Na60 and Zn/Cu–Na200 were similar. When Cu ions were added, the dendritic structures were not much altered, as seen in Figure 2e for ZnCu/Cu–Na60 and Figure 2f for ZnCu/Cu–Na200, probably due to the low concentration of CuCl2 (0.0015 M) in the deposition bath. In contrast, bulky (hexagonal) structures with average particle sizes of 1– 4 and 4–10 μm were obtained on Zn/Cu–H60 and Zn/Cu–H200, respectively (Figure 2g,h). It is suggested that a bulky structure is formed because of the high ZnCl2 concentration (0.2 M) used in the deposition bath [19]. Increasing the deposition time resulted in an increased average particle size of Zn/Cu–H. Moreover, when Cu ions were simultaneously added to the HCl bath with Zn ions, another form of dendritic structure was observed, as shown in Figure 2i,j for ZnCu/Cu–H60 and ZnCu/Cu–H200, respectively. Although the deposition time did not have much impact on the dendritic morphology of ZnCu/Cu–H, adding Cu ions to the solution changed the morphology from a bulky structure to a dendritic structure as the concentration of Zn ions near the electrocatalyst surface was diluted by the presence of Cu ions. Cu ions can compete with Zn ions for deposition on a Cu foil. The deposited Cu can also accelerate the formation of hydrogen gas during the electrodeposition process. In addition, Cu has a higher hydrogen bond strength than Zn [21]. The hydrogen gas that is produced during the electrodeposition process also competes with Zn ions. It is also noticed that the particle size of the dendritic structure of ZnCu/Cu– H is bigger than that of ZnCu/Cu–Na due to the higher concentration of ZnCl2 in the deposition bath. The surface compositions of the deposited electrodes are shown in Table 1. At a low Zn ion concentration in the NaCl bath, although increasing the deposition time did not affect the morphology of the fern-like dendritic structure being formed, the percentages of Zn increased from 75% to 93% as the deposition time increased from 60 to 200 s. Adding Cu did not have much impact on the amount of Zn being deposited under the conditions Nanomaterials 2021, 11, 1671 used (60 s, NaCl bath). Conversely, at a high Zn ion concentration in the HCl bath, there 5 of 12 was little effect of the deposition time, whereas a lower amount of Zn was obtained when ZnCl2 and CuCl2 were simultaneously deposited.

Nanomaterials 2021, 11, x FOR PEER REVIEW 5 of 12 20.0 μm (a) (b)

(g) (h) (i) (j) Figure 2. SEM images of (a) Zn foil, (b) Cu foil, (c) Zn/Cu–Na60, (d) Zn/Cu–Na200, (e) ZnCu/Cu–Na60, (f) ZnCu/Cu– FigureNa200, 2. (SEMg) Zn/Cu–H60, images of ((ah)) ZnZn/Cu–H200, foil, (b) Cu ( foil,i) ZnCu/Cu–H60, (c) Zn/Cu–Na60, and ((dj)) ZnCu/Cu–H200. Zn/Cu–Na200, (e) ZnCu/Cu–Na60, (f) ZnCu/Cu– Na200, (g) Zn/Cu–H60, (h) Zn/Cu–H200, (i) ZnCu/Cu–H60, and (j) ZnCu/Cu–H200. Table 1. Percentage by weight of deposited metals on the electrocatalysts. Table 1. Percentage by weight of deposited metals on the electrocatalysts. Percentage by Weight Electrocatalyst Zn (%)Percentage by Weight Cu (%) Electrocatalyst Zn/Cu–Na60 Zn74.9 (%) Cu 25.1 (%) Zn/Cu–Na60Zn/Cu–Na200 74.992.9 25.1 7.1 Zn/Cu–Na200ZnCu/Cu–Na60 92.972.5 7.1 27.5 ZnCu/Cu–Na60ZnCu/Cu–Na200 72.593.1 27.5 6.9 ZnCu/Cu–Na200Zn/Cu–H60 93.196.0 6.9 4.0 Zn/Cu–H60 96.0 4.0 Zn/Cu–H200Zn/Cu–H200 98.198.1 1.9 1.9 ZnCu/Cu–H60ZnCu/Cu–H60 86.186.1 13.9 13.9 ZnCu/Cu–H200ZnCu/Cu–H200 88.888.8 11.2 11.2

TheThe XRD XRD results results of of all all the the electrocatalysts electrocatalysts are are shown shown in in Figure Figure3. 3. The The XRD XRD patterns patterns ofof the the deposited deposited electrocatalysts electrocatalysts matchedmatched perfectlyperfectly with metallic Zn Zn and and Cu Cu according according to tothe the JCPDS JCPDS database database (Cu: (Cu: JCPDS JCPDS 04-0836; 04-0836; Zn: Zn: JCPDS JCPDS 00-004-0831). 00-004-0831). Except Except for the for Zn the foil, Zn a foil,strong a strong characteristic characteristic peak peakcorresponding corresponding to the to Cu(200) the Cu(200) facet was facet apparent was apparent at a 2θ at degree a 2θ degreeof 50.1° of [25]. 50.1 The◦ [25 intensity]. The intensity of the Cu(200) of the Cu(200)peak decreased peak decreased after deposition after deposition of Zn nanostruc- of Zn nanostructurestures on the Cu on foil. the CuThe foil. diffraction The diffraction peaks at peaks 36.3° atand 36.3 43.3°◦ and (2θ 43.3) confirmed◦ (2θ) conﬁrmed that Zn that with Zna (101) with aand (101) a (002) and alattice (002) structure lattice structure was proper was properlyly deposited deposited on the onsurface the surface of the Cu of the foil. CuThe foil. crystal The crystalplanes planes(100) and (100) (102) and of (102) Zn became of Zn became more visible more visiblefor Zn/Cu-–H200, for Zn/Cu—H200, in which inthe which highest the amount highest of amount Zn was of deposited Zn was depositedon the substrate. on the For substrate. the higher For Zn the concentration higher Zn concentrationin the HCl bath, in the additional HCl bath, peaks additional corresponding peaks corresponding to the Cu4Zn to phase the Cu were4Zn phase detected were on detectedboth ZnCu/Cu–H60 on both ZnCu/Cu–H60 and ZnCu/Cu–H200 and ZnCu/Cu–H200 [21], indicating [21 the], indicating formation the of CuZn formation alloy of by CuZnsimultaneous alloy by simultaneousdeposition of depositionCu and Zn ofions. Cu Such and Znresult ions. was Such not result observed was under not observed low-Zn- underconcentration low-Zn-concentration conditions (i.e., conditions ZnCu/Cu–Na60 (i.e., ZnCu/Cu–Na60 and ZnCu/Cu–Na200), and ZnCu/Cu–Na200), and the addition and of Cu2+ during electrodeposition neither changed the morphologies nor changed the structure of Zn being deposited.

Nanomaterials 2021, 11, 1671 6 of 12

2+ Nanomaterials 2021, 11, x FORthe PEER addition REVIEW of Cu during electrodeposition neither changed the morphologies nor 6 of 12 changed the structure of Zn being deposited.

Figure 3. XRD patternsFigure of 3. (a)XRD Zn/Cu–Na60, patterns of (a)(b) Zn/Cu–Na60,Zn/Cu–Na200, (b) (c) Zn/Cu–Na200, ZnCu/Cu–Na60, (c) (d) ZnCu/Cu–Na60, ZnCu/Cu–Na200, (d) (e) ZnCu/Cu– Zn/Cu–H60, (f) Zn/Cu–H200, (g)Na200, ZnCu/Cu–H60, (e) Zn/Cu–H60, and (h) (f) ZnCu/Cu–H200. Zn/Cu–H200, (g) ZnCu/Cu–H60, and (h) ZnCu/Cu–H200.

3.2. Electrocatalytic3.2. Electrocatalytic Performances Performances in CO2RR in CO2RR

All the depositedAll the deposited electrodes electrodes were tested were in tested the electrochemical in the electrochemical reduction reduction of CO2 at of a CO2 at a potentialpotential of −1.6 Vof vs. −1.6 Ag/AgCl V vs. Ag/AgCl for 70 min. for 70 The min. production The production rates for rates both for gaseous both andgaseous and liquid productsliquid products and CO Faradaic and CO efﬁciency Faradaic areefficiency shown inare Table shown2 and in FigureTable 42, respectively.and Figure 4, respec- Both Zntively. foil and Both Cu foilZn foil can and produce Cu foil CO, can formate, produce and CO, n-propanol formate, and as then-propanol products as from the products CO2RR, whilefrom CO H22gasRR, waswhile produced H2 gas was by theproduced hydrogen by the evolution hydrog reaction.en evolution Typically, reaction. a Cu Typically, foil exhibitsa Cu higher foil exhibits activity higher than a Znactivity foil because than a Zn its resistancefoil because is lower,its resistance resulting is inlower, a faster resulting in electrona transfer faster electron rate [27]. transfer Cu also rate has [27]. a lower Cu theoreticalalso has a lower limiting theoretical potential limiting for formate potential for productionformate than Zn production [28]. Furthermore, than Zn [28]. the higher Furthermor bindinge, the energy higher of the binding Cu–CO energy bond leadsof the Cu–CO to a higherbond production leads to a rate higher of n-propanol production on rate a Cu of foiln-propanol than a Zn on foil a Cu [29 foil]. than a Zn foil [29].

− Table 2. CatalyticTable 2. performancesCatalytic performances of electrocatalysts of electrocat withalysts different with depositiondifferent deposition times at a times reaction at a potential reaction ofpotential1.6 V of vs. −1.6 V vs. Ag/AgClAg/AgCl for 70 min. for 70 min.

Production RateProduction (µmol/min) Rate (μmol/min) CO/HCO/H2 2 ProductionAverage RateElectrochemical Average Electrochemical Charge Electrocatalyst Electrocatalyst CO H2 Formate n-PropanolProduction Rate RatioCharge PassingPassing per per Minute Minute (C/min) CO H2 Formate n-Propanol Zn foil 0.87 0.21 0.07 0.04 Ratio 4.1 (C/min) 0.15 Zn foilCu foil 0.87 0.96 0.21 2.21 0.07 0.48 0.04 0.15 4.1 0.4 0.15 0.45 Cu foilZn/Cu–Na60 0.962.93 2.21 0.92 0.48 0.13 0.15 0.06 0.4 3.2 0.45 0.39 Zn/Cu–Na60Zn/Cu–Na200 2.93 3.61 0.92 0.77 0.13 0.09 0.06 0.06 3.2 4.7 0.39 0.42 Zn/Cu–Na200ZnCu/Cu–Na60 3.61 3.28 0.77 0.93 0.09 0.05 0.06 0.04 4.7 3.5 0.42 0.40 ZnCu/Cu–Na60ZnCu/Cu–Na200 3.28 4.72 0.93 1.29 0.05 0.06 0.04 0.03 3.5 3.7 0.40 0.56 ZnCu/Cu–Na200Zn/Cu–H60 4.720.65 1.29 0.45 0.06 0.36 0.03 - 3.7 1.5 0.56 0.13 Zn/Cu–H60Zn/Cu–H200 0.651.01 0.45 0.54 0.36 0.36 - - 1.5 1.8 0.13 0.21 Zn/Cu–H200ZnCu/Cu–H60 1.01 3.21 0.54 0.89 0.36 0.13 - 0.02 1.8 3.6 0.21 0.41 ZnCu/Cu–H60ZnCu/Cu–H200 3.21 4.14 0.89 1.63 0.13 0.16 0.02 0.02 3.6 2.5 0.41 0.63 ZnCu/Cu–H200 4.14 1.63 0.16 0.02 2.5 0.63

Nanomaterials 2021, 11, 1671 7 of 12 Nanomaterials 2021, 11, x FOR PEER REVIEW 7 of 12

FigureFigure 4. 4. COCO Faradaic Faradaic efficiency efﬁciency measure measure at at −−1.61.6 V V vs. vs. Ag/AgCl. Ag/AgCl.

AllAll the the electrodeposited electrodeposited Zn Zn catalysts catalysts with with a dendritic a dendritic structure structure (prepared (prepared under under both lowboth and low high and Zn high ion Zn concentrations) ion concentrations) exhibit exhibiteded much higher much CO higher production CO production rates (~3–4.7 rates μ(~3–4.7mole/min)µmole/min) comparedcompared to the Zn to foil the (0.9 Zn foilμmole/min) (0.9 µmole/min) due to duethe higher to the highersurface surface area. Amongarea. Among them, the them, CO the production CO production rate increa ratesed increased with increasing with increasing Zn deposited. Zn deposited. The CO/H The2 productionCO/H2 production rate ratio rateranged ratio between ranged 2.5 between and 4.7, 2.5 which and 4.7, was which not much was notdifferent much from different that offrom the thatZn foil of the (4.1). Zn It foil has (4.1). been It suggested has been suggested that the dendritic that the dendriticstructure structurecontains a contains higher densitya higher of density stepped of sites stepped that sitescan suppress that can suppresshydrogen hydrogen evolution evolution [19]. Among [19]. the Among various the electrocatalysts,various electrocatalysts, the highest the CO/H highest2 production CO/H2 production rate of 4.7 and rate a ofCO 4.7 Faradaic and a COefficiency Faradaic of 83.2%efficiency were of obtained 83.2% were on obtainedZn/Cu–Na200 on Zn/Cu–Na200 as it maintained as it maintainedthe fern-like the dendrite fern-like structure dendrite withstructure a high with amount a high of amountZn. Furthermore, of Zn. Furthermore, the CO Faradaic the CO efficiency Faradaic compared efficiency with compared previ- ouswith works previous is shown works in Table is shown 3. The in CO Table Faradaic3. The efficiency CO Faradaic of the efficiency dendritic of Zn the structure dendritic in thisZn work structure was incomparable this work to was those comparable reported in to the those literature reported and in was the relatively literature high and com- was paringrelatively to the high non-dendritic comparing to structure the non-dendritic in a bicarbonate structure electrolyte. in a bicarbonate The results electrolyte. emphasize The results emphasize the effect of Zn morphology on the activity of Zn electrocatalysts. The the effect of Zn morphology on the activity of Zn electrocatalysts. The CO:H2 ratio was relativelyCO:H2 ratio high was compared relatively to high other compared Zn dendrit toe other electrodes, Zn dendrite as reported electrodes, in the as literature reported [19,30].in the literatureThe production [19,30]. rates The of production formate (0.07–0.16 rates of formateμmole/min) (0.07–0.16 and n-propanolµmole/min) (0.02–0.06 and n- μpropanolmole/min) (0.02–0.06 over theµ Znmole/min) foil and the over Zn the dendrit Zn foile andelectrocatalysts the Zn dendrite were electrocatalysts essentially similar, were despiteessentially the different similar, despite conditions the differentof the electrodeposition conditions of the bath electrodeposition used. It should also bath be used. noted It thatshould the low also coverage be noted of that Zn theparticles low coverage (as found of in Zn Zn/Cu–Na60 particles (as and found ZnCu/Cu–Na60) in Zn/Cu–Na60 and and ZnCu/Cu–Na60) and the presence of Cu Zn alloy (as found in ZnCu/Cu–H60 and the presence of Cu4Zn alloy (as found in ZnCu/Cu–H64 0 and ZnCu/Cu–H200) did not have ZnCu/Cu–H200) did not have much influence on the selectivities of CO2RR products of the much influence on the selectivities of CO2RR products of the Zn-based electrocatalysts. Zn-based electrocatalysts. Cu4Zn alloy can promote the formation of ethylene and ethanol, Cu4Zn alloy can promote the formation of ethylene and ethanol, as reported by Ren et al. as reported by Ren et al. [27]. However, it was probably not the dominant phase in this [27]. However, it was probably not the dominant phase in this study. Although the for- study. Although the formation of other liquid hydrocarbon products such as methane and mation of other liquid hydrocarbon products such as methane and methanol has been methanol has been reported on dendritic Zn [30], a Ag foam substrate with some certain reported on dendritic Zn [30], a Ag foam substrate with some certain crystalline structures crystalline structures and much higher over-potential is necessary in order to strongly bind and much higher over-potential is necessary in order to strongly bind CO* on the surface. CO* on the surface. Unlike the dendritic structure Zn, the production rate of formate was drastically en- Unlike the dendritic structure Zn, the production rate of formate was drastically enhanced by more than five-fold on the electrodeposited Zn catalysts with bulky morphol- hanced by more than five-fold on the electrodeposited Zn catalysts with bulky morphology, ogy, such as Zn/Cu–H60 and Zn/Cu–H200, compared to the Zn foil. However, there was such as Zn/Cu–H60 and Zn/Cu–H200, compared to the Zn foil. However, there was little little effect of the particle size of bulky Zn electrocatalysts (in the range of 1–10 μm) on the effect of the particle size of bulky Zn electrocatalysts (in the range of 1–10 µm) on the CO2RR activity and selectivity of the products. It is likely that the hexagonal close-pack CO2RR activity and selectivity of the products. It is likely that the hexagonal close-pack structure of bulky Zn promotes formate production. According to a DFT study on various

Nanomaterials 2021, 11, x FOR PEER REVIEW 8 of 12

metals by Yoo et al., both COOH* (carboxyl) and HCOO* are presented as the intermediates of CO2RR, but the selective reduction of CO2 to formate (HCOOH) is more likely to occur via the HCOO* intermediate than the COOH* intermediate, which further reduces to CO and H2 [28]. The present results suggest that the pathway through HCOO* to formate is dominant on a bulky zinc structure, as depicted in the proposed scheme in Figure Nanomaterials 2021, 11, 1671 8 of 12 5. Recently, cation vacancies (VZn) in the ZnS structure have been proposed to be new active sites on the ZnS surface that can stabilize HCOO* via O bridging between the intermediate and zinc ion vacancies VZn and hence is more efficient for the production of formate [31]. Fromstructure these of findings, bulky Zn it promotes is speculated formate that production. the bulky Accordingzinc structure to a contains DFT study a on various high amountmetals of cation by Yoo vacancies, et al., both leading COOH* to th (carboxyl)e high production and HCOO* rate are of presentedformate, with as the lim- intermediates ited formationofCO of 2CORR, and but H the2. The selective absence reduction of n-propanol of CO2 toon formate bulky Zn (HCOOH) could be is related more likelyto to occur the high coveragevia the of HCOO* Zn species intermediate on the surf thanace. the As COOH* revealed intermediate, by the EDX which results, further Zn/Cu– reduces to CO H60 and Zn/Cu–H200and H2 [28 ].showed The present the lowest results percen suggesttages that of Cu the on pathway the substrate through (2–4 HCOO* wt%). to formate Typically, theis dominantformation onof n-propanol a bulky zinc occu structure,rs via C–C as depictedcoupling inof theadsorbed-CO proposed schemeand ad- in Figure5. sorbed-methaneRecently, intermediates cation vacancies on the (VCuZn surface) in the [32]. ZnS structure have been proposed to be new active sites on the ZnS surface that can stabilize HCOO* via O bridging between the intermediate Table 3. Comparison of COand Faradaic zinc ion efficiency vacancies on VdifferentZn and Zn hence electrocatalysts is more efﬁcient in CO2 forRR. the production of formate [31]. From these ﬁndings, it is speculated that the bulky zinc structure contains a high amount of Potential CO Faradaic Efficiency Zn Electrocatalyst Electrolyte Ref. cation vacancies, leading(vs. to Ag/AgCl) the high production rate(%) of formate, with limited formation of Zn/Cu–Na200 CO0.1 M and KHCO H2.3 The absence of n-propanol−1.60 on bulky Zn83.2 could be relatedThis to thework high coverage of ZnCu/Cu–Na200 Zn0.1 speciesM KHCO on3 the surface. As−1.60 revealed by the EDX81.5 results, Zn/Cu–H60This work and Zn/Cu–H200 Zn dendrite showed0.5 M NaHCO the lowest3 percentages−1.72 of Cu on the substrate79 (2–4 wt%). Typically,[33] the formation of Reduced nanoporous ZnO n-propanol0.25 M K2SO4occurs via C–C coupling−1.66 of adsorbed-CO92 and adsorbed-methane[18] intermediates Dendrite PD–Zn/Ag foam on0.1 theM KHCO Cu surface3 [32]. −1.80 76.4 [30] Nano–Zn 0.5 M NaHCO3 −1.47 57 [14]

Figure 5. Proposed reaction pathways for CO2RR over dendritic and bulky structures of Zn/Cu Figure 5. Proposed reaction pathways for CO2RR over dendritic and bulky structures of Zn/Cu catalysts. catalysts.

Table 3. Comparison of CO Faradaic efﬁciency on different Zn electrocatalysts in CO2RR. To further investigate the electrochemical behavior of the Zn/Cu–Na200 catalyst in CO2RR, linear sweep voltammetry (LSV) wasPotential performed. AccordingCO Faradaic to the Efﬁciency LSV results in Zn Electrocatalyst Electrolyte Ref. Figure 6, the Zn/Cu–Na200 catalyst was(vs. tested Ag/AgCl) in CO2RR at different(%) over-potentials of

Zn/Cu–Na200−1.4, −1.8, and −2.0 0.1 V M vs. KHCO Ag/AgCl.3 The activity−1.60 and selectivity of the Zn/Cu–Na200 83.2 catalyst This work ZnCu/Cu–Na200are given in Table 0.1 M4. KHCOAt −1.43 V vs. Ag/AgCl,−1.60 the CO formation rate 81.5was fairly low and This work Zn dendritesurprisingly formate 0.5 M NaHCOwas not3 detected. Thermo−1.72dynamically, the electron 79 requirement for [33] − Reduced nanoporousthe half-cell ZnO reaction 0.25 M for K 2n-propanolSO4 formation1.66 is 18 electrons, much higher 92 than CO and [18] Dendrite PD–Zn/Ag foam 0.1 M KHCO −1.80 76.4 [30] formate production, which requires3 only 2 electrons [33]. The present results suggest that Nano–Zn 0.5 M NaHCO3 −1.47 57 [14] the production of formate may occur via a different pathway than the formation of CO and n-propanol on the Zn electrocatalysts. Increasing the over-potential from −1.4 to −1.6 To further investigate the electrochemical behavior of the Zn/Cu–Na200 catalyst in resulted in a higher CO/H2 production rate, whereas a further increase in the potential CO RR, linear sweep voltammetry (LSV) was performed. According to the LSV results beyond −1.6 V vs.2 Ag/AgCl led to lower CO/H2 production. Generally, the electron trans- in Figure6, the Zn/Cu–Na200 catalyst was tested in CO RR at different over-potentials fer rate at higher potential is faster, whereas the CO2 transfer rate remains2 unchanged. It of −1.4, −1.8, and −2.0 V vs. Ag/AgCl. The activity and selectivity of the Zn/Cu– Na200 catalyst are given in Table4. At −1.4 V vs. Ag/AgCl, the CO formation rate was fairly low and surprisingly formate was not detected. Thermodynamically, the electron requirement for the half-cell reaction for n-propanol formation is 18 electrons, much higher than CO and formate production, which requires only 2 electrons [33]. The present

results suggest that the production of formate may occur via a different pathway than the formation of CO and n-propanol on the Zn electrocatalysts. Increasing the over- potential from −1.4 to −1.6 resulted in a higher CO/H2 production rate, whereas a further increase in the potential beyond −1.6 V vs. Ag/AgCl led to lower CO/H2 production. Generally, the electron transfer rate at higher potential is faster, whereas the CO2 transfer Nanomaterials 2021, 11, 1671 9 of 12

Nanomaterials 2021, 11, x FOR PEER REVIEW 9 of 12

rate remains unchanged. It has been suggested that this mass transfer limitation causes H2 formation [34]. The production rates of gaseous products monotonically increased as the over-potentialhas been suggested increased that fromthis mass−1.4 transfer to −2.0 Vlimitation vs. Ag/AgCl, causes with H2 formation only a slight [34]. increase The pro- in liquidduction products. rates of gaseous products monotonically increased as the over-potential increased from −1.4 to −2.0 V vs. Ag/AgCl, with only a slight increase in liquid products.

Figure 6. LSV results of the Zn/Cu–Na200 catalyst. catalyst.

Table 4. Performance of Zn/Cu–Na200 in CO2RR at various potentials for 70 min. Table 4. Performance of Zn/Cu–Na200 in CO2RR at various potentials for 70 min.

Potential (V) vs. Production Rate (μmol/min) CO/H2 Production Entry µ Potential (V) vs. Production2 Rate ( mol/min) Entry Ag/AgCl CO H Formate n-Propanol CO/H2 ProductionRate Ratio 1 −1.4Ag/AgCl 1.08 CO0.48 H 2 Formate- n-Propanol0.02 Rate Ratio2.3 2 −1.6 3.61 0.77 0.09 0.06 4.7 1 −1.4 1.08 0.48 - 0.02 2.3 3 −1.8 6.98 2.95 0.10 0.06 2.4 2 −1.6 3.61 0.77 0.09 0.06 4.7 4 −2.0 7.97 10.18 0.19 0.06 0.8 3 −1.8 6.98 2.95 0.10 0.06 2.4 4 −2.0 7.97 10.18 0.19 0.06 0.8 The stability test results of Zn/Cu–Na200 are shown in Figure 7. The formation rate of COThe was stability comparatively test results stable of Zn/Cu–Na200 around 3.55 μmol/min are shown throughout in Figure7 the. The 4 h formation reaction time. rate ofThe CO morphologies was comparatively of the electrocatalysts stable around 3.55 beforeµmol/min and after throughout the stability the test 4 h reactionare shown time. in TheFigure morphologies 8. It is clearly of evident the electrocatalysts that the partic beforele size and of the after Zn the dendrite stability increased test are shownfrom 0.2– in Figure0.5 to 0.5–1.28. It is μ clearlym, as shown evident in Figure that the 8a,b, particle respectively, size of indicating the Zn dendrite that dissolution increased and from re- 0.2–0.5deposition to 0.5–1.2 of Zn µnanoparticlesm, as shown occurs in Figure during8a,b, respectively,CO2RR [19]. Nevertheless, indicating that the dissolution percentage and of Zn remained the same at around 93 wt%. In addition, it was confirmed that the particle re-deposition of Zn nanoparticles occurs during CO2RR [19]. Nevertheless, the percentage ofsize Zn or remained shape of thethe samedendritic at around structure 93 wt%. of Zn In did addition, not affect it was the conﬁrmed activity and that selectivity the particle of sizethe CO or shape2RR of ofthe the electrodeposited dendritic structure Zn catalysts. of Zn did not affect the activity and selectivity of the CO2RR of the electrodeposited Zn catalysts.

Nanomaterials 2021, 11, 1671 10 of 12 Nanomaterials 2021, 11, x FOR PEER REVIEW 10 of 12 Nanomaterials 2021, 11, x FOR PEER REVIEW 10 of 12

FigureFigure 7. 7. CatalyticCatalytic performance performance of of Zn/Cu–Na200 Zn/Cu–Na200 at at −1.6−1.6 V Vvs. vs. Ag/AgCl Ag/AgCl for for4 h. 4 h. Figure 7. Catalytic performance of Zn/Cu–Na200 at −1.6 V vs. Ag/AgCl for 4 h.

Figure 8. SEM images of Zn/Cu–Na200 (a) before and (b) after the stability test. FigureFigure 8. 8.SEM SEM images images of of Zn/Cu–Na200 Zn/Cu–Na200 ((aa)) beforebefore andand ((bb)) afterafter thethe stabilitystability test.test. 4. Conclusions In this study, Zn/Cu and ZnCu/Cu electrocatalysts were prepared via the electrodeposition method with various bath compositions and deposition times. A low Zn concen-

Nanomaterials 2021, 11, 1671 11 of 12

tration (e.g., 0.05 M ZnCl2 in NaCl solution or 0.2 M ZnCl2 in HCl solution in the presence of Cu2+) enabled mass transfer limited the growth of well-deﬁned Zn dendrite structures. All the Zn dendrites exhibited higher activities in CO2RR than the Zn foil. The highest CO/H2 production rate (4.7) was obtained over Zn/Cu–Na200 having a fern-like dendritic structure and a relatively high amount of Zn being deposited. However, at the same over-potential, the activities and product selectivities of the Zn dendrites were essentially similar, regardless of the particle size/shape of the dendritic structure. On the contrary, the bulky Zn structure with an average particle size of 1–10 µm obtained from the high- Zn-concentration bath strongly favored the production of formate with limited amounts of gaseous CO and H2 products. The dissolution and re-deposition of Zn nanoparticles during the 4 h of reaction led to a larger particle size of the Zn dendritic structure but did not affect the activity and selectivity of the CO2RR of the electrodeposited Zn catalysts.

Author Contributions: Conceptualization, P.P. (Piriya Pinthong), P.K. and J.P.; formal analysis, P.P. (Piriya Pinthong) and P.K.; funding acquisition, J.P.; methodology, P.P. (Piriya Pinthong) and P.K.; project administration, J.P.; resources, P.P. (Piyasan Praserthdam); supervision, P.P. (Piyasan Praserthdam); validation, P.P. (Piyasan Praserthdam) and J.P.; writing—original draft, P.P. (Piriya Pinthong); writing—review and editing, J.P. All authors have read and agreed to the published version of the manuscript. Funding: Financial support from the Rachadapisek Sompote Endowment Fund for the Postdoctoral Fellowship, Chulalongkorn University for P.P., the Research Team Promotional Grant from the National Research Council of Thailand (NRCT) for J.P., and the Malaysia-Thailand Joint Authority (MTJA) Research Cess Fund are gratefully acknowledged. Conﬂicts of Interest: Piriya Pinthong, Phongsathon Klongklaew, Piyasan Praserthdam, and Joongjai Panpranot declare that they have no conﬂict of interest.

References

1. Choi, J.; Kim, M.J.; Ahn, S.H.; Choi, I.; Jang, J.H.; Ham, Y.S.; Kim, J.J.; Kim, S.-K. Electrochemical CO2 reduction to CO on dendritic Ag–Cu electrocatalysts prepared by electrodeposition. Chem. Eng. J. 2016, 299, 37–44. [CrossRef] 2. Krause, R.; Reinisch, D.; Reller, C.; Eckert, H.; Hartmann, D.; Taroata, D.; Wiesner-Fleischer, K.; Bulan, A.; Lueken, A.; Schmid, G. Industrial application aspects of the electrochemical reduction of CO2 to CO in aqueous electrolyte. Chem. Ing. Tech. 2020, 92, 53–61. [CrossRef] 3. Abdinejad, M.; Dao, C.; Deng, B.; Dinic, F.; Voznyy, O.; Zhang, X.-A.; Kraatz, H.-B. Electrocatalytic reduction of CO2 to CH4 and CO in aqueous solution using pyridine-porphyrins immobilized onto carbon nanotubes. ACS Sustain. Chem. Eng. 2020, 8, 9549–9557. [CrossRef] 4. de Brito, J.F.; Irikura, K.; Terzi, C.M.; Nakagaki, S.; Zanoni, M.V.B. The great performance of TiO2 nanotubes electrodes modiﬁed by copper (II) porphyrin in the reduction of carbon dioxide to alcohol. J. CO2 Util. 2020, 41, 101261. [CrossRef] 5. Weststrate, C.; Van De Loosdrecht, J.; Niemantsverdriet, J. Spectroscopic insights into cobalt-catalyzed Fischer-Tropsch synthesis: A review of the carbon monoxide interaction with single crystalline surfaces of cobalt. J. Catal. 2016, 342, 1–16. [CrossRef] 6. Khodakov, A.Y.; Chu, W.; Fongarland, P. Advances in the development of novel cobalt Fischer−Tropsch catalysts for synthesis of long-chain hydrocarbons and clean fuels. Chem. Rev. 2007, 107, 1692–1744. [CrossRef][PubMed] 7. Hori, Y. Electrochemical CO2 Reduction on Metal Electrodes. In Modern Aspects of Electrochemistry; Vayenas, C.G., White, R.E., Gamboa-Aldeco, M.E., Eds.; Springer: New York, NY, USA, 2008; pp. 89–189. 8. Guo, W.; Shim, K.; Ngome, F.O.O.; Moon, Y.H.; Choi, S.-Y.; Kim, Y.-T. Highly active coral-like porous silver for electrochemical reduction of CO2 to CO. J. CO2 Util. 2020, 41, 101242. [CrossRef] 9. Sarno, M.; Ponticorvo, E.; Piotto, S.; Nardiello, A.M.; De Pasquale, S.; Funicello, N. AuAg/ZnO nanocatalyst for CO2 valorization and H2 and CO electrochemical production. J. CO2 Util. 2020, 39, 101179. [CrossRef] 10. Nguyen, D.L.T.; Jee, M.S.; Won, D.H.; Jung, H.; Oh, H.-S.; Min, B.K.; Hwang, Y.J. Selective CO2 Reduction on Zinc Electrocatalyst: The Effect of Zinc Oxidation State Induced by Pretreatment Environment. ACS Sustain. Chem. Eng. 2017, 5, 11377–11386. [CrossRef] 11. Jiang, K.; Huang, Y.; Zeng, G.; Toma, F.M.; Goddard, W.A., III; Bell, A.T. Effects of surface roughness on the electrochemical reduction of CO2 over Cu. ACS Energy Lett. 2020, 5, 1206–1214. [CrossRef] 12. Cheng, F.; Zhang, X.; Mu, K.; Ma, X.; Jiao, M.; Wang, Z.; Limpachanangkul, P.; Chalermsinsuwan, B.; Gao, Y.; Li, Y. Recent Progress of Sn-Based Derivative Catalysts for Electrochemical Reduction of CO2. Energy Technol. 2021, 9, 2000799. [CrossRef] 13. Xiao, J.; Gao, M.-R.; Liu, S.; Luo, J.-L. Hexagonal Zn nanoplates enclosed by Zn (100) and Zn (002) facets for highly selective CO2 electroreduction to CO. ACS Appl. Mater. Interfaces 2020, 12, 31431–31438. [CrossRef] Nanomaterials 2021, 11, 1671 12 of 12

14. Quan, F.; Zhong, D.; Song, H.; Jia, F.; Zhang, L. A highly efficient zinc catalyst for selective electroreduction of carbon dioxide in aqueous NaCl solution. J. Mater. Chem. A 2015, 3, 16409–16413. [CrossRef] 15. Wu, J.; Risalvato, F.G.; Ke, F.-S.; Pellechia, P.; Zhou, X.-D. Electrochemical reduction of carbon dioxide I. Effects of the electrolyte on the selectivity and activity with Sn electrode. J. Electrochem. Soc. 2012, 159, F353. [CrossRef] 16. Lv, W.; Zhang, R.; Gao, P.; Lei, L. Studies on the faradaic efficiency for electrochemical reduction of carbon dioxide to formate on tin electrode. J. Power Sources 2014, 253, 276–281. [CrossRef] 17. Hsieh, Y.-C.; Senanayake, S.D.; Zhang, Y.; Xu, W.; Polyansky, D.E. Effect of chloride anions on the synthesis and enhanced catalytic activity of silver nanocoral electrodes for CO2 electroreduction. ACS Catal. 2015, 5, 5349–5356. [CrossRef] 18. Jiang, X.; Cai, F.; Gao, D.; Dong, J.; Miao, S.; Wang, G.; Bao, X. Electrocatalytic reduction of carbon dioxide over reduced nanoporous zinc oxide. Electrochem. Commun. 2016, 68, 67–70. [CrossRef] 19. Rosen, J.; Hutchings, G.S.; Lu, Q.; Forest, R.V.; Moore, A.; Jiao, F. Electrodeposited Zn Dendrites with Enhanced CO Selectivity for Electrocatalytic CO2 Reduction. ACS Catal. 2015, 5, 4586–4591. [CrossRef] 20. Won da, H.; Shin, H.; Koh, J.; Chung, J.; Lee, H.S.; Kim, H.; Woo, S.I. Highly Efficient, Selective, and Stable CO2 Electroreduction on a Hexagonal Zn Catalyst. Angew. Chem. Int. Ed. Engl. 2016, 55, 9297–9300. [CrossRef][PubMed] 21. Moreno-Garcia, P.; Schlegel, N.; Zanetti, A.; Cedeno Lopez, A.; Galvez-Vazquez, M.J.; Dutta, A.; Rahaman, M.; Broekmann, P. Se- lective Electrochemical Reduction of CO2 to CO on Zn-Based Foams Produced by Cu(2+) and Template-Assisted Electrodeposition. ACS Appl. Mater. Interfaces 2018, 10, 31355–31365. [CrossRef] 22. Zhu, S.; Wang, Q.; Qin, X.; Gu, M.; Tao, R.; Lee, B.P.; Zhang, L.; Yao, Y.; Li, T.; Shao, M. Tuning structural and compositional effects in Pd–Au nanowires for highly selective and active CO2 electrochemical reduction reaction. Adv. Energy Mater. 2018, 8, 1802238. [CrossRef] 23. Kim, J.-H.; Woo, H.; Yun, S.-W.; Jung, H.-W.; Back, S.; Jung, Y.; Kim, Y.-T. Highly active and selective Au thin layer on Cu polycrystalline surface prepared by galvanic displacement for the electrochemical reduction of CO2 to CO. Appl. Catal. B Environ. 2017, 213, 211–215. [CrossRef] 24. Guo, W.; Shim, K.; Kim, Y.-T. Ag layer deposited on Zn by physical vapor deposition with enhanced CO selectivity for electrochemical CO2 reduction. Appl. Surf. Sci. 2020, 526, 146651. [CrossRef] 25. Keerthiga, G.; Chetty, R. Electrochemical Reduction of Carbon Dioxide on Zinc-Modified Copper Electrodes. J. Electrochem. Soc. 2017, 164, H164–H169. [CrossRef] 26. Clark, D.; Wood, D.; Erb, U. Industrial applications of electrodeposited nanocrystals. Nanostruct. Mater. 1997, 9, 755–758. [CrossRef] 27. Ren, D.; Ang, B.S.-H.; Yeo, B.S. Tuning the selectivity of carbon dioxide electroreduction toward ethanol on oxide-derived Cu x Zn catalysts. ACS Catal. 2016, 6, 8239–8247. [CrossRef] 28. Yoo, J.S.; Christensen, R.; Vegge, T.; Norskov, J.K.; Studt, F. Theoretical Insight into the Trends that Guide the Electrochemical Reduction of Carbon Dioxide to Formic Acid. ChemSusChem 2016, 9, 358–363. [CrossRef][PubMed] 29. Kuhl, K.P.; Hatsukade, T.; Cave, E.R.; Abram, D.N.; Kibsgaard, J.; Jaramillo, T.F. Electrocatalytic conversion of carbon dioxide to methane and methanol on transition metal surfaces. J. Am. Chem. Soc. 2014, 136, 14107–14113. [CrossRef][PubMed] 30. Low, Q.H.; Loo, N.W.X.; Calle-Vallejo, F.; Yeo, B.S. Enhanced Electroreduction of Carbon Dioxide to Methanol Using Zinc Dendrites Pulse-Deposited on Silver Foam. Angew. Chem. Int. Ed. 2019, 58, 2256–2260. [CrossRef][PubMed] 31. Pang, H.; Meng, X.; Li, P.; Chang, K.; Zhou, W.; Wang, X.; Zhang, X.; Jevasuwan, W.; Fukata, N.; Wang, D. Cation vacancy-initiated CO2 photoreduction over ZnS for efficient formate production. ACS Energy Lett. 2019, 4, 1387–1393. [CrossRef] 32. Ren, D.; Wong, N.T.; Handoko, A.D.; Huang, Y.; Yeo, B.S. Mechanistic Insights into the Enhanced Activity and Stability of Agglomerated Cu Nanocrystals for the Electrochemical Reduction of Carbon Dioxide to n-Propanol. J. Phys. Chem. Lett. 2016, 7, 20–24. [CrossRef][PubMed] 33. Kuhl, K.P.; Cave, E.R.; Abram, D.N.; Jaramillo, T.F. New insights into the electrochemical reduction of carbon dioxide on metallic copper surfaces. Energy Environ. Sci. 2012, 5, 7050–7059. [CrossRef] 34. Hatsukade, T.; Kuhl, K.P.; Cave, E.R.; Abram, D.N.; Jaramillo, T.F. Insights into the electrocatalytic reduction of CO2 on metallic silver surfaces. Phys. Chem. Chem. Phys. 2014, 16, 13814–13819. [CrossRef][PubMed]