Copper electrode fabricated via pulse electrodeposition: toward high

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Copper electrode fabricated via pulse electrodeposition: toward high methane selectivity and activity for CO2 electroreduction Yan-Ling Qiu, He-Xiang Zhong, Tao-Tao Zhang, Wen-Bin Xu, Xian-Feng Li, and Hua-Min Zhang ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b00571 • Publication Date (Web): 07 Aug 2017 Downloaded from http://pubs.acs.org on August 7, 2017

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Copper

electrode

fabricated

via

pulse

electrodeposition: toward high methane selectivity and activity for CO2 electroreduction Yan-Ling Qiu†, He-Xiang Zhong†,§, Tao-Tao Zhang†, ‡, Wen-Bin Xu†, Xian-Feng Li*,†,§, Hua-Min Zhang*,†,§ †

Division of Energy Storage, Dalian Institute of Chemical Physics, Chinese Academy of

Sciences, Zhongshan Road 457, Dalian 116023, China. ‡

University of Chinese Academy of Sciences, Beijing 100049, China.

§

Collaborative innovation Center of Chemistry for Energy Materials (iChEM), Dalian 116023,

China.

ABSTRACT: Electrochemical reduction of CO2 (ERC) to methane has significant economic benefits and represents one promising solution for energy and environmental sustainability. However, traditional metal electrodes suffer from higher overpotentials, low activities and poor selectivity. In this article, pulse electrodeposition (P-ED) method is employed to prepare copper electrode for ERC. The P-ED method can easily create Cu coatings on carbon paper with much rougher surface and extending surface area, which is highly beneficial for improving their activity and selectivity. As a result, the prepared Cu electrodes exhibits high faradaic efficiency

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(of 85% at -2.8 V) and enhanced partial current density (jCH4=38 mA cm-2) for methane, which is by far the highest value ever reported at room temperature and ambient pressure. The enhanced activity is attributed to the extended reactive areas with rough morphology and the loosened coating structure to assure CO2 access the reaction sites located at the sub-layers of the deposited Cu coatings. The prominent selectivity for CH4 is likely due to the presence of stepped surface which is formed by introduction of Cu (100) step into Cu (111) and Cu (220) terraces during the P-ED processes. The lower resistance to the one-electron transfer to CO2 which is a preequilibrium step prior to the rate-limitation non-electrochemical step is another positive factor to improve the ERC activity for CH4. Furthermore, we surprisingly find that activity and selectivity of the Cu electrode can be easily recovered through continuous CO2 bubbling. This paper provides a facile method to prepare high effective electrodes for electrochemical conversion of CO2.

KEYWORDS: carbon dioxide electroreduction, methane, Faradaic efficiency, electrocatalytic activity, Cu electrode, pulse-electrodeposition 1. INTRODUCTION Reduction of carbon dioxide to organic chemicals is one of the effective ways to utilize CO2 and mitigate CO2 emissions. The electrochemical reduction of carbon dioxide (ERC) by using renewable energy in aqueous solution has potential advantages of high conversion efficiency, low cost as well as environment-friendly and becomes a hot research topic in recent years. Methane, one of the most important ERC products, has attracted great attention in ERC research for its significant economic benefits. Copper is currently the only metal which can catalyze CO2 to hydrocarbons (especially methane) in aqueous solution. However, the traditional


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Cu electrodes suffer from higher overpotentials (onset potential of -0.80 V vs. RHE)1, low activities and poor product selectivity. In addition, the competing hydrogen evolution reaction (HER) under negative potentials on Cu electrodes is very serious and further significantly decreases the product selectivity of ERC. Normally, sufficiently effective active sites are required for Cu electrodes to get high efficiency for hydrocarbons (especially for CH4), where the active sites are significantly dependent on the crystal orientation, particle size, surface morphology of copper as well as reaction conditions like electrolyte and temperature2, 3. Up to now, Cu bulk electrodes with different morphologies and Cu nanoparticles (CuNPs) coated electrodes have been developed to enhance the activity and selectivity toward hydrocarbons2, 4-10. However, the efficiency and selectivity of Cu electrodes toward hydrocarbons are very limited. The highest reported faradaic efficiency to CH4 in aqueous solution is only 80% with CuNPs supported on glass carbon2. Recently, many efforts have been devoted to develop Cu electrodes with nanostructured morphologies (such as nanowire, nanoflower, etc.) to further improve their surface area6, 8, 10-15. However, carbon monoxide and formic acid were mostly obtained by using the prepared Cu based electrodes, and only small amount of hydrocarbons was produced. In general, Cu electrode with smooth surfaces (such as Cu (111)) is more active toward CH415-17, while rough surfaces seem to be beneficial to produce C2H4 due to the moderate exposure of lower-coordinated sites6- 8. However, Cu single crystals with stepped surfaces can also produce CH4 with high yield for their abundant number of dangling bonds18,19. Therefore, rough morphology with specific stepped surfaces is expected to extend reaction area and simultaneously enhance the catalytic activity and selectivity toward CH4 over Cu electrodes. Electrodeposited Cu electrodes have been reported to modulate the ERC product distribution and improve selectivity to hydrocarbons20-22. In addition, etching-based strategy has been proved


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to create high energy facets on nanocrystal surface and significantly increase the ERC activity, which could overcome the thermodynamic limitations of crystal growth habits23, 24. Therefore, techniques combined electrodeposition process with in-situ etching has the potential of increasing the catalytic activity and selectivity of ERC for hydrocarbons. Herein, a pulseelectrodeposition (P-ED) technique is introduced to in situ etch Cu particles on carbon paper. Compared with traditional constant current electrodeposition (CI-ED), the P-ED method can easily create Cu coatings with much rougher surface due to the oxidation process, which is highly beneficial for extending surface areas and changing the surface morphology of the Cu coatings6, 25-27. During the P-ED cycles, the deposited particles are oxidized by the oxidation etching process, and the crystal orientation and particle morphology varies along with the periodically deposition-etching cycles. As a result, the P-ED coatings on carbon paper will differ significantly in morphology and structure from that of the CI-ED coatings. Moreover, some etching pits or steps will appear on the surfaces of the P-ED coatings, and the atoms located on steps and vacancies have very high reaction activities toward interfacial reactions compared with atoms in the metal bulk28. Thus the prepared Cu electrodes are expected to demonstrate very high catalytic activity toward ERC and selectivity of target products. The prepared Cu electrodes are characterized by X-ray diffraction (XRD) analysis, scanning electron microscopy (SEM) to obtain information on catalyst structure, particle size and surface morphology. The linear voltammogram sweep (LSV), cyclic voltammograms (CV) and chronoamperometry technique are employed on the Cu-P-ED electrodes to evaluate their electrocatalytic activities, the surface areas and the Faradaic efficiencies toward CO2 electroreduction. The catalytic stability and performance recovery ability of Cu electrodes are also monitored to evaluate their potential applications.


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2. RESULTS 2.1. Crystal structure of the Cu-P-ED coatings. The preferred orientation of different facets for the Cu-P-ED and Cu-CI-ED coatings on carbon paper was determined by the X-ray diffraction pattern (Figure 1), and was characterized by the texture coefficient (TC) calculated through Formula S1 in supporting information (Table 1). The higher TC value suggests the higher facet preferential degree. The diffraction patterns of both electrodes in Figure 1 exhibit four instinct diffraction peaks at 2θ=43.4°, 50.5°, 74.2° and 90°, which can be indexed to the typical diffraction from (111), (200), (220) and (311) facets of copper according to the standard crystallographic spectrum of Cu (PDF#00-004-0836). The calculated Cu particle size from Cu (111) is about 100 nm for both electrodes. No peaks corresponding to the Cu oxides can be observed, suggesting the following catalytic properties should be originated from the deposited Cu particles. The existence of the diffraction peak for the (002) plane of graphite (26.5°) in Cu-P-ED electrode indicates the incomplete coverage of the carbon paper, which would be highly benefit for mass transport in ERC reaction. The calculated thickness of the Cu coating obtained by PED method under the optimum deposition conditions is ca. 47 nm, about 20% of the thickness of Cu layer prepared by CI-ED method. From this result, we can infer that much thinner Cu coatings can be obtained by the PED method, and much higher catalytic activity might be expected on the uncompacted Cu coatings. In Table 1, the Cu-CI-ED electrode shows preferred orientation for (220) facet, while Cu-P-ED electrode doubles the TC value for (200) facet. The increased Cu (200) facet would be favorable to C2H4 production in ERC reaction29. 2.2. Cu particle growth mechanism during pulse electrodeposition. To investigate growth mechanism of Cu particles during P-ED process, the morphologies of Cu particle with different


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deposition time are recorded by SEM and shown in Figure S1. At 30 s (Figure S1a) and S1d)), the differences in particle number, particle size and their distributions between the Cu-P-ED and Cu-CI-ED electrodes can be observed obviously. The number of deposited Cu particles for Cu-PED electrode is smaller than that for Cu-CI-ED electrode due to the etching process under positive current. As the deposition proceeds to 120 s (Figure S1b) and S1e)), the differences are more prominent. Much more fine particles and large amount of grain boundaries as well as small pits appear on almost each particle of the Cu-P-ED electrode. However, the deposited Cu particles are connected to each other constructing much larger particles with the Cu-CI-ED electrode. When the deposition time reaches 300 s (Figure S1c) and S1f)), Cu particles of the CuP-ED electrode pack together to form a thin layer with loosen structure. Comparatively, the CuCI-ED sample grows into a denser layer with large particles and smooth particle boundaries, which correspond to relatively lower surface area. Although the deposition layer of the Cu-CIED is dense and thicker than the Cu-P-ED layer, its exposure area to the electrolyte and reactant is obviously limited to the surface particles. In comparison, the Cu-P-ED layer is much looser with almost all of the Cu particles accessing to the reactant and electrolyte, which will results in larger reaction interface and provide more active sites during ERC reaction. 2.3. ERC activity and selectivity for hydrocarbons. LSV curves of the Cu-P-ED and Cu-CIED electrodes were collected in CO2 and N2 saturated 0.5 M NaHCO3 solution. The onset potential of the Cu-P-ED electrode in CO2 saturated electrolyte is -1.089 V, which is 156 mV positive than that of the Cu-CI-ED electrode (Figure. 2a), -1.245 V), implying lower overpotential of ERC on the surface of the Cu-P-ED electrode. Moreover, the total current for Cu-P-ED is higher than its counterpart (Figure 2b)), suggesting the enhanced catalytic activity of Cu-P-ED for ERC. Through comparison of the LSV curves under CO2 and N2 atmosphere, we


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can infer that electroreduction of CO2 is concurrently occurred with the competitive hydrogen evolution reaction whether on Cu-P-ED electrode or on Cu-CI-ED electrode. Normally, the drop of overpotential and the increasing catalytic activity of the ERC are correlated with the reaction area and the amount of the catalytic sites6, 14. To verify this correlation, the specific capacitances (SC) of both electrodes are calculated based on the line slope of current density versus scan rate and shown in Figure 2c). The specific capacitance of the Cu-P-ED electrode is 8, 16 times larger than that of the Cu-CI-ED electrodes and polycrystalline Cu (Table S1), indicating the substantial increase of its surface area with sufficient amount of catalytic sites for ERC, which contributes to the positive shift of the onset potential and the activity improvement of the ERC reaction. On the Cu-P-ED and Cu-CI-ED electrodes, the gas products of ERC are CH4, C2H4, H2 and trace amount of CO (FE is 2.9% and 0.4% under -2.3 V and -2.8 V, respectively). Liquid product is HCOOH, which FE is lower than 8% in the potential range of -2 V~ -2.8 V (vs. SCE, Figure S2). The total FE of the five products is in the range of 95% to 110%, and the lower total FEs fall in the relatively positive potentials (E > -2.15 V vs. SCE). Compared with Cu-CI-ED electrode, Cu-P-ED electrode demonstrates higher selectivity toward CH4 (Figure 2d)). The Faradaic efficiency for CH4 on Cu-P-ED electrode increases as the applied potential shifting to negative position, and very high FECH4 values with 79% and 85% at -2.5 V and -2.8 V are obtained, respectively, which corresponds to about 66% and 77% higher than the FECH4 on Cu-CI-ED electrode. In addition, the FE ratios between CH4 and C2H4 on Cu-P-ED electrode are 42 and 65 at -2.5 V and -2.8 V, respectively, also suggesting very high selectivity to CH4 over the Cu-P-ED electrodes. The high selectivity toward CH4 on the Cu-P-ED electrode has outperformed those reported on electrodeposited Cu20,22,23, and electropolished bare Cu in aqueous solutions 30-33,


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even higher than that of Cu nanoparticles on glass carbon (n-Cu/C)34, and

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is the highest

selectivity to CH4 up to date (Table S2). The ERC testing was also carried out on different concentration of NaHCO3 solution and 0.5M KHCO3 to investigate effect of the electrolyte concentration and cation type on the catalytic properties and product selectivity of the Cu-P-ED electrodes. The CH4 selectivity of the Cu-P-ED electrode decreases with the decrease of the NaHCO3 concentration (Figure 3). Under 2.8 V, the FE for CH4 drops 31% and 84% as NaHCO3 concentration decreasing from 0.5 M to 0.3 M and 0.1 M, respectively. In addition, slightly higher FE for CH4 can be obtained in 0.5 M NaHCO3 solution compared with that in 0.5 M KHCO3 solution (Figure S3). From these phenomena, we can infer that high concentration of NaHCO3 solution is beneficial to CH4 production, and this result is consistent with Hori’s report35. 2.4. Correlation between the ERC catalytic activities and selectivity with the ratio of depositing current and etching current. During the pulse-electrodeposition, the deposition rate of copper crystals and particle size as well as their morphologies are directly dependent upon the ratio of the reduction current (or depositing current, Ir) and oxidation current (or etching current, Io), and this dependency is finally reflected in the catalytic activity and the formation rate of the product during ERC. This correlation is expressed in Figure 4 and Figure 5. SEM images in Figure 4 show that all the Cu-P-ED coatings on carbon paper fabricated under different ratio of the reduction current (or depositing current, Ir) and oxidation current (or etching current, Io) consist of particles with size equal or larger than 5 µm. This is much larger than XRD prediction, implying severe aggregation occurring during P-ED process. In addition, obvious morphology differences exist among Cu-CI-ED and Cu-P-ED surfaces. The Cu-CI-ED is composed of a dense layer with large and irregular particles, while the Cu-P-ED presents


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relatively loose structures. As the etching current increasing, uneven steps with different depth appear on the Cu particle surfaces (Figure S4). This indicates the rough feature of the Cu-P-ED, which might contain large amount of low coordinated sites such as steps, edges and defects6. However, the prominent feature of Cu-P-ED surface with (Ir:Io) ratio of 9:4 (Figure 4e) and 4f)) is completely different from the surfaces of Cu-P-ED electrodes with (Ir:Io) ratios of 9:1 to 9:3, which present irregular polyhedron morphology with smooth surfaces and many small particles mixed between large ones (Figure 4e)). In addition, considerable amount of zigzag steps and wrinkles can also be observed (Figure 4f)) which is the major contribution to the larger surface area. The enlarged surface area of Cu-P-ED electrodes was further characterized by the specific capacitances (SCs) shown in Figure 5a) and Table S1. The SCs of the Cu-P-ED electrodes are obviously much higher than that of Cu-CI-ED electrode, implying more surface area exposing during ERC reaction. Especially, the SC of the Cu-P-ED electrodes under (Ir:Io) ratios of 9:2 and 9:4 are 9 and 10 folds about that of the Cu-CI-ED electrodes, respectively, suggesting more surface area exposing during the ERC reaction. The partial current densities of CH4 (jCH4) on CuP-ED electrodes with (Ir:Io) ratios from 9:1 to 9:3 are much higher than that of Cu-CI-ED electrode in Figure 5b). Under -2.8 V, jCH4 reaches 38 mA cm-2 for the Cu-P-ED electrode with (Ir:Io) ratio of 9:2, which is two and four folds higher than that for Cu-CI-ED (Figure 5b)) and bare Cu electrode32. This indicates the significant improvement of catalytic activity for CH4 on Cu-P-ED electrodes. However, jCH4 on the electrode with (Ir:Io) ratio of 9:4 is obvious lower than that on electrodes with other three ratios, and even lower than that on Cu-CI-ED electrode, implying not all of the exposed surface area can effectively reduce CO2 to target products since the low-coordinated active sites are key factors to determine the ERC properties36. The smallest


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jCH4 on the largest SC of the Cu-P-ED electrode obtained under (Ir:Io) ratio of 9:4 verifies the significant effect of surface structure and morphology on the ERC catalytic activities6. As established earlier, the smooth surface consists mostly of low index crystal facets, such as Cu(100) and Cu(111), which are not highly active to ERC reaction10. Furthermore, small particles with surface dominated by low coordinated atoms (i.e. corner) have better electrocatalytic activity for hydrogen evolution9, 36. The FE for CH4 over Cu-P-ED electrodes with (Ir:Io) ratios of 9:1 to 9:3 demonstrates an overall increasing tendency as the applied potential mitigating to more negative position (Figure 5c)), and more than 80% FE for CH4 is obtained for the Cu-P-ED electrodes with (Ir:Io) ratios of 9:2 at -2.8 V. The highest FEs for C2H4 and HCOOH on these Cu-P-ED electrodes are lower than 7% when the potential is lower than -2.5 V (Figure 5d)) and Figure S2), indicating the relative inactive properties to C2H4 and formic acid on these Cu-P-ED electrodes. It is worthy to point out that the coating thickness and density of Cu-P-ED electrodes are dramatically influenced by depositing time and duty ratios, which will eventually affect the surface area and the porosity of the deposited coatings. Therefore, different deposition time and duty ratios were carried out to optimize the catalytic activity of the Cu-P-ED electrodes (Figure S5 and Figure S6). The combined results indicate the Cu-P-ED-60min electrode with duty raio of 40% possesses the optimum selectivity for CH4. 2.5. Stability tests on Cu-P-ED electrode. To examine the practical application prospect of the Cu-P-ED electrodes, the long-term stability test was carried out on Cu-P-ED-60min electrode with (Ir:Io) ratio of 9:2 in CO2 saturated 0.5 M NaHCO3 solution via chronoamperometry technique. The electrolysis potential was -2.3 V (vs. SCE). As shown in Figure 6, CH4 yield demonstrates upward trend during the 1st hour of stability test, and reaches the highest amount in


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about 1.2 h. Then, dramatic decrease could be observed during the following test, implying poor stability of Cu-P-ED electrode. At 2.5 h, only ten percent of the initial CH4 yield is left, i.e., the catalytic activity degradation rate of the Cu-P-ED electrode is over 90%, indicating most of the surface active sites are covered or poisoned during stability test duration. This result is consistent with the stability test of Cu electrode in previous report37. However, the Cu-P-ED electrode demonstrates excellent activity recovery ability after continuous bubbling of CO2 without electrochemical reaction (Figure 6 and Figure S7), suggesting almost all of the catalytic sites on the Cu-P-ED surface can be refreshed. Furthermore, the recovery catalytic activity excludes the possibility of the prominent morphology change of the Cu-P-ED electrode, implying the robust structure of the Cu-P-ED electrodes. 3. DISCUSSION The prominent FE differences among CH4, C2H4 and HCOOH indicate the extremely high selectivity for CH4 on these prepared Cu-P-ED coatings on carbon paper, which could be contributed to the number differences of surface sites and the morphology effect of the electrodes prepared with varied (Ir:Io) ratios, as depicted in Figure 4 and Figure S5. It is worth to note that the results of this study is drastically different from the general trend, in which the roughed surfaces could enhance C2H4 selectivity6,23,38. In contrast, we observed CH4 as the main products. This can be likely attributed to the following two aspects. The first one is the morphology of the Cu-P-ED electrode surfaces. The enlarged morphology features of Cu-P-ED electrode (Figure S4) imply that the pulse electrodeposition processes can induce some steps into the Cu surfaces, which will play an important role in atomic migration and participation in chemical reactions39. In Figure 1 and Table 1, the TC value of Cu (200) facet of the Cu-P-ED electrode in XRD spectrum doubles that of the orientation of Cu (200) facet of the Cu-CI-ED electrode, and the


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TCs of Cu (111) and Cu (220) decreases about 3% and 25%, respectively. Based on the XRD results and the electrocatalytic activities and selectivities for CH4 on the Cu-P-ED electrodes， we speculate that the P-ED process might introduce some Cu (100) or Cu (200) steps to Cu (110) and Cu (111) terraces to obtain stepped surfaces, such as Cu (210) and Cu (533) (Figure S8), which have higher number of dangling bonds of face-centered cubic metals and generate higher CH4 yield as the result18, 19. The low selectivity to C2H4 on Cu-P-ED electrodes (Figure 2d)) indirectly suggests that the increased Cu (200) orientation compared to Cu-CI-ED should be located on steps rather than on Cu (200) terrace16. Furthermore, Luo et. al. reported that the route of CO* to CHO* was favored on rough Cu surfaces relative to smooth surfaces29(such as the low indexed (111) and (100) surfaces on which terrace dominate). The second attribution to the high CH4 selectivity is the high concentration of NaHCO3. CH4 is preferred on rough copper surfaces at high bicarbonate concentrations, while ethylene selectivity is higher when the concentration of the supporting electrolyte is low. This is evidenced by Kas and Varela40,41. Our validating results on Cu electrode (Figure S9 and Figure S10) are also consistent with this statement. Almost no considerable C2H4 and CO are produced in 0.5 M electrolyte solution on Cu electrode, while relatively higher C2H4 and CO are detected in 0.1M electrolyte solution, especially as the potential shifts to more negative position. In this work, we carried out the ERC testing in 0.5 M NaHCO3 aqueous solution which was beneficial for CH4 production (Figure 3 and Figure S3), and this can also partially explain the higher jCH4 and higher selectivity toward CH4 on the rough surfaces of Cu-P-ED electrodes. In order to collect mechanistic evidence regarding the improved catalytic behavior of Cu-PED electrodes toward CH4, the Tafel slope of both electrodes based on jCH4 is measured and the results are shown in Figure 7. In Tafel region, the linear slope for CH4 production is 88 mV


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decade-1 on Cu-P-ED, about one fourth less than 122 mV decade-1 on Cu-CI-ED, indicating faster kinetic reaction of ERC on the Cu-P-ED surfaces. It is generally thought that the ratelimiting step for CH4 generation on copper foils involves a single electron transfer to CO2 which characterizes with 120 mV decade-1as the Tafel slope2. The fact that Tafel slope on Cu-CI-ED electrode is very close to this common value suggesting the catalytic performance of Cu-CI-ED is similar to the bulk Cu. This is consistent with the relatively large particles of the Cu-CI-ED electrode (Figure 4a)). On the other hand, the lower Tafel slope on Cu-P-ED electrode suggests the kinetic reaction resistance falls in the middle of bulk Cu and Cu nanoparticles (59 mV decade-1)2. This reduced Tafel slope on Cu-P-ED compared with Cu-CI-ED is beneficial to driving CH4 current gain with smaller overpotential, and the improved catalytic behavior is obtained as the result. The Cu-P-ED electrode demonstrates dramatic performance degradation during stability test and almost totally recovery performances for catalytic activity to CH4 with continuous CO2 bubbling (Figure 6 and Figure S7). To explore the possible triggering factors, speculations and experimental verifications are provided. One possible factor might be the coverage variation (θCO*) of CO* intermediate during the stability test. Theoretical work reported that the tendency in ERC activity could be altered by a high CO* coverage on the surfaces of transition metals42, and CO* can’t completely occupy the Cu surface at lower potentials relative to ERC reaction, thus leaving active sites for H* adsorption and C-H bond formation43. Figure 8 gives the schematic diagram for the variation of the Cu surface sites during ERC stability test and the following CO2 bubbling processes. During the stability test, the CO* coverage on Cu-P-ED electrode gradually increases in the initial 1 h, the active sites left for H* adsorption and further C-H bond formation will be slowly decreased correspondingly. After about 2 h, the CO*


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coverage and desorption tends to reach equilibrium, and the ERC activity drops to the lowest as the result. When the ERC reaction is terminated and the electrode is continuously bubbled with CO2, the adsorbed CO* on the electrode surface would detach from the electrode through diffusion or replacement by CO2. After about 3~4 h bubbling, almost all of the Cu-P-ED electrode surface sites are released from the CO* desorption, and the electrode recover its activity and selectivity to hydrocarbons, indicating the robust structure of Cu-P-ED in ERC reaction. The phenomenon of the ERC performances are not influenced during the intermittent chronoamperometry processes also support that the CO* occupation could be weakened or eliminated on the Cu surface by continuous CO2 bubbling. Another possible factor influencing the stability of Cu-P-ED electrode might be the poison effect of liquid product species’ adsorption or intermediates bonding on the catalytic sites which cannot diffuse into the bulk solution in time or be converted to the next species to leave the sites. To verify this speculation, different concentrations of formic acid were added into the supporting electrolyte, and the effects on CH4 yields and efficiencies of ERC are listed in Table S3. The HCOOH concentration seems to have fixed effect on the ERC performances once it is adsorbed on the electrode surface, and further increase of the concentration might just keep the dynamic balance between the adsorption and desorption process. Half of CH4 yield and ERC efficiency are decreased by the trace amount of HCOOH. The almost completely recovery of the catalytic activity and selectivity to CH4 of the Cu-P-ED electrodes also implies the HCOOH poison effect on the electrode surface is not fatal, and can also be eliminated by continuous CO2 bubbling as the CO* intermediate. 4. CONCLUSIONS


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This study presents a novel Cu electrode fabrication method to tremendously improve the activity and selectivity for CH4 with CO2 and H2O as reactant and proton sources. The Cu-P-ED electrodes demonstrate an extremely high faradaic efficiency 79% and 85% at -2.5 V and -2.8 V respectively. The partial current density for methane reaches up to 38 mA cm-2, which is the highest value ever reported by far at room temperature and ambient pressure. The high catalytic activity for CO2 electrochemical reduction is due to the more roughened surface morphology of the Cu-P-ED electrodes, which can provide abundant active sites. The prominent catalytic selectivity for the reduction of CO2 to CH4 might be owing to the significantly improved surface roughness with large amount of stepped facets on the Cu-P-ED surface. This study demonstrates the surface morphology of Cu electrodes could significantly influence the catalytic activity and product selectivity for CO2 reduction, making pulse-electrodeposition as an excellent technique for electrode fabrication of electrochemical reduction of CO2. 5. EXPERIMENTAL METHODS 5.1. Preparation of Cu-P-ED and Cu-CI-ED electrodes. Copper coatings with various deposition parameters were prepared on carbon paper (TGP-H-060, Toray Industries, Inc.) by recurrent galvanic pulse technique (Versa STAT 3, Princeton Applied Research). Before deposition, the carbon paper was pretreated in air for 4 h by 550 oC heat treatment to increase its hydrophilicity to the plating electrolyte. Scheme 1 is the schematic diagram of the recurrent galvanic pulse electrodeposition. The plating electrolyte was composed of 0.64 M CuSO4, 1.1 M H2SO4 and 0.684 mM KBr. The depositing process was carried out at 60 oC with stirring rate of 300 rpm. The Cu coatings prepared on carbon paper by pulse-electrodeposition will be referred as Cu-P-ED electrode, and that prepared on carbon paper by constant current-electrodeposition as comparison be referred as Cu-CI-ED electrode. The applied deposition current density (Ir) for


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both electrodes was 90 mA cm-2, and the oxidation current density (Io) for the Cu-P-ED electrode was controlled as the (Ir:Io) ratios variation. During the deposition process, the duration for the Ir, Io and OCV was 300 ms, 100 ms and 400 ms, respectively. The iteration for the Cu-P-ED electrode was 2250 cycles, 3375 cycles, 4500 cycles and 6750 cycles respectively, corresponding to about 30 min, 45 min, 60 min and 90 min, respectively. The Cu-P-ED electrodes under these deposition iterations are designated as Cu-P-ED-30min, Cu-P-ED-45min, Cu-P-ED-60min and Cu-P-ED-90min separately. The saturated calomel electrode (SCE) was used as the reference, and high purity copper bulk was used as the counter electrode to maintain the constant concentration of Cu2+ in the plating electrolyte. To prevent the deposited copper from being oxidized, high purity N2 with 60 ml min-1 flow rate was bubbled through the deposition process. After deposition, the electrode was removed from the electrolyte and washed with ultrapure water thoroughly, and dried with N2 blow. 5.2. Surface area determination. The surface area of Cu electrodes can be estimated by calculating the specific capacitance (SC) of the electrode double layer in 0.1 M HClO4 aqueous solution with N2 saturated. To obtain the SC values, cyclic voltammograms were recorded in the potential range from -0.1 V to -0.3 V (vs. SCE) at different scan rates between 5 and 300 mVs-1. The SC value was then calculated from the line slope of the current density versus scan rate, under the assumption that the faradaic contribution is negligible44. 5.3. ERC onset potential measuring. The electrocatalytic reduction of CO2 on both electrodes was studied by linear sweep voltammetry (LSV) in 0.5 M NaHCO3solution (pH 7.2) saturated with high purity CO2 (99.999%) and N2 to obtain the onset potentials required to reduce CO2. LSV curves were measured in the potential range from -0.8 V~ -2.8 V (vs. SCE) at scan rate of 10 mV s-1. The onset potential was determined from the current density of 0.1 mA cm-2.


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5.4. CO2 electrochemical reduction testing and product detection. All CO2 electrochemical reduction experiments were conducted at atmospheric pressure and room temperature by a potentiostat (EG&G 2273, Princeton Applied Research) in a home-made H-type cell with a piece of proton exchange membrane (Nafion®115, Dupont) separating the two compartment. The cathode compartment contained 180 ml 0.5 M NaHCO3 aqueous solution with CO2 continuously bubbling and approximately 80 ml headspace left. Cu electrode with 3 cm2 effective area as the working electrode and SCE reference electrode were both placed in the cathodic compartment, and platinum foil (99.99%, Tianjin Aida Corp.) counter electrode was positioned in the anodic compartment with 0.1 M H2SO4 as the electrolyte. The electrochemical reduction of carbon dioxide (ERC) was carried out by chronoamperometry technique in the range between -1.7 V and -2.8 V vs. SCE. An electrolysis time of 16 min was applied for each potential. During the test, the cell outlet (consisting of residual CO2 and the gas-phase products) was on-line delivered directly into gas chromatography (GC-2014, Shimadzu) which was equipped with a thermal conductivity detector (TCD), a Porpack-N column and a Molecular Sieve-13X column to analyze gas products with 1.0 ml loop. Liquid products were quantified by ion chromatography (ICS-1100, Dionex) equipped with an AS23 column. The flow rate of the eluent was 1 ml min-1. The faradaic efficiencies (FE) of ERC products were calculated on the basis of the transferred number of electrons for the formation of one molecule of the products from CO2 and H2O, i.e., 8 for CH4, 12 for C2H4, 2 for HCOOH. The evaluability of the detection system and the experimental method are validated by Cu foil (99.8%) and high purity Cu (99.999%) in NaHCO3 and KHCO3 solutions with concentration of 0.5 M and 0.1 M (See supporting information). 5.5. Surface characterization. The crystal structures of the Cu electrodes were characterized using a X-ray diffractometer (X-Pert Pro) equipped with a Cu Kα radiation (λ=0.15418 nm).


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Morphological characteristics were investigated by scanning electron microscope scanning electron microscope (SEM, QUNA TA 200FEG). ■ASSOCIATED CONTENT Supporting Information. Calculation of the texture coefficient (TC) of Cu electrodes, Cation effect on FE of CH4 on Cu-P-ED electrode, the effect of the deposition period, the effect of (Ir:Io) ratios on the FE of HCOOH in CO2 saturated 0.5 M NaHCO3 solution, calibration of our analytical and detection system, comparison of particle growth evolution for Cu-P-ED and CuCI-ED electrodes, the effect of (Ir:Io) ratios on the FE of HCOOH in CO2 saturated 0.5 M NaHCO3 solution, the FE comparison of hydrocarbons on Cu-P-ED electrode in 0.5 M NaHCO3 and 0.5 M KHCO3 solutions with CO2 saturated, the morphology of Cu-P-ED electrodes with different (Ir:Io) ratios, the effect of P-ED time on selectivity of CH4 and C2H4, the effect of the duty ratio of the P-ED cycles on the catalytic activity of Cu-P-ED electrodes toward CH4, current variation on Cu-P-ED electrode via ERC reaction time, rigid sphere models of two stepped surfaces for Cu-P-ED electrodes, XRD pattern of Cu foil (99.8%) used to calibrate our detection system in this study, product selectivity of electrochemical CO2 reduction on Cu electrodes in different electrolyte with CO2 saturated, summary of the roughness factors of Cu-CI-ED electrode and Cu-P-ED electrodes with different (Ir:Io) ratios, summary of CH4 selectivity on Cu electrodes, effect of HCOOH concentration on CH4 yields and faladaic efficiencies during ERC on the Cu-P-ED electrodes. These materials are available free of charge via the Internet at http://pubs.acs.org ■AUTHOR INFORMATION Corresponding Authors


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*Email: [email protected], [email protected]. Author contributions The manuscript was written through contributions of all authors. Yan-Ling Qiu designed the experiments and wrote the paper. Tao-Tao Zhang performed part of the experiments. Yan-Ling Qiu carried out the XRD and SEM imaging analysis. Wen-Bin Xu was responsible for the model simulations. He-Xiang Zhong discussed the results and contributed to the modification of the manuscript. Xian-Feng Li and Hua-Min Zhang guided the work. All authors discussed the results and have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. ■ACKNOWLEDGMENT The authors thank the financial grants from the National Natural Science Foundation of China (No.21577141 and No. 21576255), the Youth Innovation Promotion Association CAS (2015147). ■REFERENCES 1. Schwarz, H. A.; Dodson, R. W. J. Phys. Chem. 1989, 93, 409−414. 2. Manthiram, K.; Beberwyck, Brandon J.; Aivisatos, A. P. J. Am. Chem. Soc. 2014, 136, 13319−13325. 3. Kaneco, S.; Hiei, N.; Xing, Y.; Katsumata, H.; Ohnishi, H.; Suzuki, T.; Ohta, K. J. Solid State Electrochem. 2003, 7, 152−156. 4. Ogura, K.; Yano, H.; Shirai, F. J. Electrochem. Soc. 2003, 150, D163−D168. 5. Durand, W. J.; Peterson, A. A.; Studt, F.; Abild-Pedersen, F.; Nørskov, J. K. Surf. Sci. 2011, 605, 1354-1359.


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6. Tang, W.; Peterson, A. A.; Varela, A. S.; Jovanov, Z. P.; Bech, L.; Durand, W. J.; Dahl, S.; Nørskovb, J. K.; Chorkendorff, I. Phys. Chem. Chem. Phys. 2012, 14, 76–81. 7. Baturina, O. A.; Lu, Q.; Padilla, M. A.; Xin, L.; Li, W.; Serov, A.; Artyushkova, K.; Atanassov, P.; Xu, F.; Epshteyn, A.; Brintlinger, T.; Schuette, M.; Collins, G. E. ACS Catal. 2014, 4, 3682−3695. 8. Kas, R.; Kortlever, R.; Milbrat, A.; Koper, M. T. M.; Mul, G.; Baltrusaitis, J. Phys. Chem. Chem. Phys. 2014, 16, 12194−12201. 9. Reske, R.; Mistry, H.; Behafarid,F.; Cuenya, B. R.; Strasser, P. J. Am. Chem. Soc. 2014, 136, 6978−6986. 10. Xie, J. F.; Huang, Y. X.; Li, W. W.; Song, X. N.; Xiong, L.; Yu, H. Q. Electrochim. Acta 2014, 139, 137−144. 11. Raciti, D.; Livi, K. J.; Wang, C. Nano Lett. 2015, 15, 6829−35. 12. Ma, M.; Djanashvili, K.; Smith, W. A. Phys. Chem. Chem. Phys. 2015, 17, 20861−20867. 13. Qiao, J.; Jiang, P.; Liu, J.; Zhang, J. Electrochem. Commun. 2014, 38, 8−11. 14. Li, C. W.; Kanan, M. W. J. Am. Chem. Soc. 2012, 134, 7231−7234. 15. Bugayong, J. G.; Griffin, G. L. ECS Trans 2013, 58, 81−89. 16. Schouten, K. J. P.; Gallent, E.P.; Koper, M. T. M. ACS Catal. 2013, 3, 1292−1295. 17. Christophe, J.; Doneux, T.; Buess-Herman, C. Electrocatalysis 2012, 3, 139−146. 18. Takahashi, I.; Koga, O.; Hoshi, N.; Hori, Y. J. Electroanal. Chem. 2002, 533, 135−143. 19. Hori, Y.; Takahashi, I.; Koga, O.; Hoshi, N. J. Mol. Catal. A-Chem. 2003, 199, 39−47. 20. Goncalves, M. R.; Gomes, A.; Condeco, J.; Fernandes, R.; Pardal, T.; Sequeira, C. A. C.; Branco, J. B. Energy Convers. Manage. 2010, 51, 30−32. 21. Goncalves, M. R.; Gomes, A.; Condeco, J.; Fernandes, T. R. C.; Pardal, T.; Sequeira, C. A. C.; Branco, J. B. Electrochim. Acta 2013, 102, 388−392. 22. Baturina, O. A.; Lu, Q.; Padilla, M. A.; Xin, L.; Li, W.; Serov, A.; Artyushkova, K.; Atanassov, P.; Xu, F.; Epshteyn, A.; Brintlinger, T.; Schuette, M.; Collins, G. E. ACS Catal. 2014, 4, 3682−3695. 23. Wang, Z., Yang, G., Zhang, Z., Jin, M.; Yin, Y. ACS Nano 2016, 10, 4559−4564. 24. He, Z.; Shen, J.; Ni, Z.; Tang, J.; Song, S.; Chen, J.; Zhao, L. Catal. Commun. 2015, 72, 38−42. 25. Lee J.;Farhangfar, S.; Lee, J.; Cagnon, L.; Scholz, R.; G¨osele, U.; Nielsch, K. Nanotechnology 2008, 19, 365701− 365708.


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26. Xia, F.; Tian, J.; Wang, W.; He, Y. Ceram. Inte. 2016, 42, 13268−13272. 27. Reis, M. A. S.; Fernandes, T. R. C.; Pardal, T.; Rangel, C. M. Mater. Sci. Forum Vols. 2013, 730-732, 239-244. 28. Gerhard, E. In Advances in Soid State Physics 27, Aachen, P. G., Eds. Friedr. Vieweg & Sohn: Braunschwei, 1987; pp 169-174. 29. Luo, W.; Nie, X.; Janik, M. J.; Asthagiri, A. ACS Catal. 2016, 6, 219−229. 30. Cook, R. L.; Macduff, R. C.; Sammells, A. F. J. Electrochem. Soc. 1987, 134, 1873−1874. 31. Hori, Y.; Takahashi, R.; Yoshinami, Y.; Murata, A. J. Phys. Chem. B 1997, 101, 7075−7081. 32. Kuhl, K. P.; Cave, E. R.; Abram, D. N.; Jaramillo, T. F. Energy Environ. Sci. 2012, 5, 7050−7059. 33. Xie, M. S.; Xia, B. Y.; Li, Y.; Yan, Y.; Yang, Y.; Sun, Q.; Chan, S. H.; Fisher, A.; Wang, X., Energy. Environ. Sci. 2016, 9, 1687−1695. 34. Manthiram, K.; Beberwyck, B. J.; Aivisatos, A. P. J. Am. Chem. Soc. 2014, 136, 13319−13325. 35. Hori, Y.; Murata, A.; Takahashi, R. J. Chem. Soc., Farad. Trans. 1 1989, 85, 2309-2326. 36. Zhu, W.; Zhang, Y. J.; Zhang, H.; Lv, H.; Li, Q.; Michalsky, R.; Peterson, A. A.; Sun, S. J. Am. Chem. Soc. 2014, 136, 16132−16135. 37. Hori, Y.; Konishi, H.; Futamura, T.; Murata, A.; Koga, O.; Sakurai, H.; Oguma, K. Electrochim. Acta 2005, 50, 5354−5369. 38. Chen, C. S.; Handoko, A.D.; Wan, J. H.; Ma, L.; Ren, D.; Yeo, B. S. Catal. Sci.Technol. 2015, 5, 161−168. 39. Eilert, A.; Roberts, F. S.; Friebel, D.; Nilsson, A. J. Phys. Chem. Lett.2016, 7, 1466−1470. 40. Kas, R.; Yilmaz, H.; Koper, M. T. M.; Mul, G. ChemElectroChem 2015, 2, 354−358. 41. Varela, A. S.; Kroschel, M.; Reier, T.; Strasser, P. Catal. Today 2016, 260, 8−13. 42. Shi, C.; Hansen, H. A.; Lausche, A. C.; Norskov, J. K. Phys. Chem. Chem. Phys. 2014, 16, 4720−4727. 43. Akhade, S. A.; Luo, W.; Nie, X.; Bernstein, N. J.; Asthagiri, A.; Janik, M. J. Phys. Chem. Chem. Phys. 2014, 16, 20429−20435. 44. Waszczuk, P., Zelenay, P., Sobkowski, J. Electrochim. Acta 1995, 40, 1717−1721.


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Table 1. Facet TC comparison of Cu-CI-ED and Cu-P-ED electrodes Electrode Cu-CI-ED Cu-P-ED

(111) 24.61 23.82

Facet TC (%) (200) (220) 11.61 42.32 23.02 31.75


(311) 21.46 21.41

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Scheme 1. Schematic diagram of the recurrent galvanic pulse electrodeposition on carbon paper.


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Figure 1. XRD patterns of Cu-CI-ED and Cu-P-ED electrodes.


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Figure 2. Comparison of onset potential, catalytic activity, surface area and selectivity for Cu-PED and Cu-CI-ED electrodes. a) LSV polarization curves comparison in CO2 and N2 saturated 0.5 M NaHCO3, scan rate: 10 mV s-1, b) Total current density comparison versus ERC time in CO2 saturated 0.5 M NaHCO3, c) Specific capacitance comparison measured in N2 saturated 0.1 M HClO4 solution, d) The Faradaic efficiencies (FEs) versus potentials of the hydrocarbons for Cu-P-ED and Cu-CI-ED electrodes in CO2 saturated 0.5 M NaHCO3 under room temperature and ambient pressure.


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Figure 3. Concontration effect of NaHCO3 solution on the ERC selectivity to CH4 on Cu-P-ED electrode. Testing condition: The geometry area is 3.0 cm2, the NaHCO3 solution is saturated with CO2, Ir:Io of the Cu-P-ED electrode is 9:2, and the deposition time is 60 min.


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Figure 4. Morphology of the copper coatings fabricated by electrodeposition method under different (Ir:Io) ratios. a) Cu-CI-ED, b) Cu-P-ED (Ir:Io=9:1), c) Cu-P-ED (Ir:Io=9:2), d) Cu-PED (Ir:Io=9:3, e) Cu-P-ED (Ir:Io=9:4)-1, f) Cu-P-ED (Ir:Io=9:4)-2


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Figure 5. Effect of (Ir:Io) ratio on the catalytic activities, surface areas and product selectivity of the Cu-P-ED electrodes. a) Specific capacitance comparison measured in N2 saturated 0.1 M HClO4, b) The partial current density of CH4 versus potentials for Cu-CI-ED and Cu-P-ED under different (Ir:Io) ratios in CO2 saturated 0.5 M NaHCO3 under room temperature and ambient pressure, c) FE for CH4 with different (Ir:Io) ratios, d) FE for C2H4 with different (Ir:Io) ratios.


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Figure 6. Stability test of Cu-P-ED electrode in CO2 saturated 0.5 M NaHCO3 solution at ambient pressure and room temperature. The applied potential is -2.3 V vs. SCE. The geometry surface area of the electrode was 3.0 cm2. CO2 flow rate was 60 ml min-1. From 2.5 h ~ 15 h, the ERC reaction was terminated with CO2 continuously bubbling into the electrolyte.


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Figure 7. Tafel analysis. Potential versus partial current densities of CH4 on Cu-P-ED and CuCI-ED electrodes. Testing electrolyte: CO2 saturated 0.5 M NaHCO3.


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Figure 8. The proposed coverage and desorption processes of the Cu surface sites by the CO* intermediate. The yellow sphere represents Cu active sites on the Cu-P-ED surfaces.


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TOC Graphic


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Copper electrode fabricated via pulse electrodeposition: toward high

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