electrocatalytic activity of cu electrode in electroreduction of co2

Electrochimica Acta 46 (2001) 3015–3022

Electrocatalytic activity of Cu electrode in electroreductionof CO2

Jaeyoung Lee a,*, Yongsug Tak b,1

a Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, D-14195 Berlin, Germanyb Department of Chemical Engineering, Inha Uni�ersity, 402-751 Inchon, South Korea

Received 20 February 2001; accepted 17 April 2001

Abstract

The electrocatalytic activity of Cu electrode in the electrochemical reduction of carbon dioxide (CO2) wasinvestigated. Electroreduction mechanism of CO2 was studied by the adsorption/desorption behaviors of reactingspecies by using an in-situ electrochemical quartz crystal microbalance. (EQCM) and the surface changes measuredby ex-situ SEM, AES, and XRD analysis. During cathodic reduction of CO2 on Cu, the adsorption of amorphouscarbon was observed. After electrolysis time of 1 h at constant cathodic potential, the poisoning of amorphous carbonresulted in the decrease of the faradaic efficiency for the formation of hydrocarbons such as CH4 and C2H4. On theother hand, the potential modulation method caused the change of the surface structure of copper, i.e. the formationof cuprous oxide (Cu2O). This structural change prevented the adsorption of amorphous graphite and the constantproduction rate of methane was obtained in long-term electrolysis. © 2001 Elsevier Science Ltd. All rights reserved.

Keywords: Electroreduction mechanism; CO2; Electrocatalytic activity; In-situ EQCM

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1. Introduction

CO2 known as global warming gas can be extensivelyreduced by electrochemical method because of twomain reasons: (1) selectivity of the gases or liquidproducts at different cathodic electrode used and (2) theexperimental equipment required is inexpensive becauseneither high vacuum nor high reaction temperatures areneeded. Global warming gas, CO2 have been convertedinto potential energy sources such as CH4, C2H4,HCOOH, CH3OH, and etc. [1,2]. Product distributionof the electroreduction of CO2 strongly depends on theelectrode used. CO, HCOOH, H2 and aqueous prod-ucts were preferentially produced on Ag, Au, Sn, and

Ti electrodes, respectively [3,4]. Particularly, main prod-ucts of the electroreduction of CO2 on Ti, which is oneof hydrogen storing metals [5], were CO and HCOOHin KOH�CH3OH media [6]. Recently, Ishimaru et al.[7] presented that by comparing the production effi-ciency of C2 molecules on pure Cu and Ag metalelectrodes, the higher faradaic efficiencies of C2 com-pounds such as C2H4, C2H5OH, and CH3CHO oncopper-silver alloy were observed as using the pulsedelectroreduction method. Hydrocarbons such as CH4

and C2H4 were mainly obtained on Cu electrode andmany studies have been reported on electroreductionmechanism of CO2 on Cu electrode [8–10]. Hori et al.[3] observed that intermediate species (CO) was ad-sorbed on Cu electrode. Jermann et al. [11] andNogami et al. [12] investigated the progressive degrada-tion of Cu cathode during the electroreduction of car-bon dioxide and they suggested that thesuperimposition of anodic potential hindered the ad-

* Corresponding author. Tel.: +49-30-84135149; fax: +49-30-841351106.

E-mail address: [email protected] (J. Lee).1 ISE member.

0013-4686/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved.

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J. Lee, Y. Tak / Electrochimica Acta 46 (2001) 3015–30223016

sorption of deactivation species, amorphous carbon onthe cathode. In addition, Nogami et al. studied that theselectivity of production was due to both the changes inthe concentration of H+ or adsorbed hydrogen atomsand the formation of a copper oxide thin layer oncathode surface by applying pulsed potential technique[12,13].

In this work, we report that the adsorption/desorp-tion behaviors of CO2, intermediate species (CO), gasproducts, and by-product on Cu cathode are investi-gated by using an in-situ EQCM in the electroreductionof CO2. The structural changes of Cu electrode arecharacterized through ex-situ X-ray diffratogram(XRD), scanning electron microscope (SEM), andauger electron spectroscopy (AES) analytical tech-niques. Electrocatalytic activity of Cu is analyzed bygas chromatograph (GC) measurement and followingtheoretical calculation of the faradaic efficiency of mainproducts.

Electrochemical quartz crystal microbalance(EQCM) can measure the order of nanogram masschanges caused by the adsorption/desorption or deposi-tion/dissolution on the electrode and this mass detectorcan be used to identify the reaction mechanism andkinetics of electrochemical reactions. In the EQCM, theresonant frequency change for mass change has beenpresented by the well-known Sauerbrey equation[14,15],

�f= −2f02 · �m/A(�q�q)1/2, (1)

where f0 is the resonant frequency of the quartz, 9MHz; A represents the electrode area (0.196 cm2); �q isthe shear modules of quartz, 2.947×1011 g cm−1 s−2;�q is the density of quartz, 2.648 g cm−3. Eq. (1) can besimplified as

�f= −Cf · �m, (2)

in which Cf (935.7 Hz/�g) in our case is a constantsolely determined by the properties of the quartzcrystal.

Up to now, the in-situ EQCM technique has beenused for the study of several electrochemical processessuch as mono/multi-layers deposition, dissolution, masstransport in polymeric modified electrodes, and corro-sion process by comparison of parameters like currentor charge with mass changes on the electrode surface[16–22].

2. Experimental

An anion exchange membrane (CMX, Tokuyamacorp.) separated a conventional H-type cell (Fig. 1). ACu electrode was cathodically prepared onto an AT-cut9 MHz Pt quartz crystal (approximately 5 �/�, SeikoInstruments Inc.) in a 0.25 mM CuSO4 (Aldrich)/ace-

tate buffer solution with pH value of 4.5. While BT-cut,CT-cut, DT-cut and NT-cut can be used at constanttemperature (29°C), AT-cut (35.5°) quartz crystal canbe used for a wide temperature range between −20and 75°C without the correlation of frequency due totemperature variation. Carbon plate was used for coun-ter electrode and a saturated calomel electrode (SCE)was used as a reference electrode. 0.1 M KHCO3

prepared with ultrapure water (Millipore, 18.2 M� cm)was used for all experiments. In order to remove thedissolved oxygen and to ensure comparable solutionconditions, high purity CO2 (99.99%) was passedthrough the catholyte to be saturated before each ex-periment. The solution was maintained at low tempera-ture of 5°C to increase the solubility of CO2 gas duringthe electroreduction and had an initial pH of 6.5.

Cyclic voltammetry, constant potential mode, andpotential modulation methods for the understanding ofthe electroreduction mechanism of CO2 on copper elec-trode were performed with a potentiostat and galvanos-tat (EG&G, PAR 273A). Mass change of cathode wasmeasured by sensitive piezoelectric device, in-situEQCM (Seiko EG&G, QCA917). The data were trans-ferred to an IBM compatible PC controlled by a GPIBinterface (M270). Reduction products were analyzed bya gas chromatograph (HP 5890 II plus) equipped witha thermal conductivity detector (TCD). Porapak Qcolumn and Molecular Sieve 5A were used for analyz-ing hydrocarbons (CH4, C2H4) and carbon monoxide,respectively. Morphology and crystal structure of thecopper surface were measured by SEM (HITACHIX-650), XRD (Philips DY 616), and AES (Perkin–Elmer PHI-670).

3. Results and discussion

Fig. 2 shows a typical j/E (a) and �m/E (b) for CO2

reduction during both scans onto Cu working elec-trode. All experiments are carried out after the massreading of the electrode is stabilized in a solution,which takes about 3 min. Prior to the experiments, afew tens of mass (ng) of the Cu electrode is increaseddue to the possible adsorption of H2O.

Cathodic current density linearly increases, as thepotential becomes more negative. However, the masschanges of the cathode have a relatively complicatedprofile. From open circuit potential (OCP) of −0.05 V(vs. SCE) to −1.3 V, no faradaic reaction is observed,but the mass of Cu cathode is shifted into negativedirection by −1.2 V. This result is due to the dissocia-tion of adsorbed water molecule and the adsorption ofhydrogen ion (H+) and the following repulsion of OH−

from the Cu electrode. The mass increase of copperelectrode is observed from −1.2 V, representing thatCO2 reduction initiates with the adsorption of interme-

J. Lee, Y. Tak / Electrochimica Acta 46 (2001) 3015–3022 3017

Fig. 1. Schematic diagram of H-type cell for the electrochemical reduction of CO2.

diate species, CO (reaction 3). The reduction of ad-sorbed proton and hydrogen evolution occur at furthercathodic potential. Below cathodic potential of −1.8V, slower rate of mass change of copper cathode isobtained, in which the main reducing products are CH4

and C2H4 according to reactions 4 and 5.

CO2+2H++2e− +*�COad+H2O (3)

COad+6H++6e− �CH4+H2O+* (4)

2COad+8H++8e− �C2H4+2H2O+2* (5)

where * denotes a vacant surface site and subscript ad

indicates species adsorbed on the surface.After the anodic scan, the slight increase of the

surface mass at initial potential of −0.05 V is ob-served. In following morphological (SEM) and struc-tural analysis (AES), detailed discussion of thisphenomenon is dealt with.

To investigate the reduction mechanism of CO2 byanalyzing the faradaic efficiency of the product, fivedifferent cathodic potentials of −1.5, −1.8, −2.1,−2.4, and −2.7 V are used. Applied electrolysis timeis 5 min. The faradaic efficiency of CH4, C2H4, and COreduced from CO2 at different potentials is shown inFig. 3. First, tiny amount of CO is only detected at−1.5 V and CH4 and C2H4 are formed at furthercathodic potential. The maximum faradaic efficiency ofhydrocarbons such as CH4 and C2H4 is obtained at−2.1 V. At further negative potential of −2.4 V, thefaradaic efficiency of CH4 is decreased, resulting fromthe retardation of the reduction of CO2 due to thestrong adsorption of an amorphous carbon [8] (reaction6).

COad+2H++2e− �Cad(graphite)+H2O (6)


Fig. 2. j/E (solid line) and �m/E (dotted line) of a cyclicvoltammetry with scan rate of 10 mV/s on Cu electrode.Electrolyte is CO2 saturated 0.1 M KHCO3 and the solutiontemperature keeps 5°C.

As mentioned in above Eqs. (3)– (5), CO2 consumesthe 2, 6, and 8 electrons for the formation of onemolecule of CO, CH4, and C2H4, respectively. Thesethree reactions suggest that the mass change of elec-trode depend on the reaction rate.

Considering the current efficiency of CH4 at variouspotentials as shown in Fig. 3, the optimal potential of−2.1 V for the formation of CH4 is chosen for theinvestigation of the electrocatalytic activity of Cu. Fig.4 shows the faradaic efficiency of CH4 formation withelectrolysis time. The adsorption of by-product,graphitic carbon, on Cu electrode results in the decreaseof the electrocalytic activity of copper cathode and thuslow efficiency of 5% is obtained after electrolysis timeof 1 h.

In-situ EQCM measurement is carried out in poten-tiostatic method to investigate the adsorbing effect ofamorphous carbon. Fig. 5a and b show the masschange of Cu cathode and cathodic current profile,respectively when the potential of −2.1 V is applied.Fig. 5a shows that the mass slowly increases and Cusurface mass reaches 517 ng after the electrolysis timeof 10 min. Although this value is 5% of theoretical one,the adsorption of amorphous carbon deactivates theelectrocatalytic activity of Cu for the electroreductionof CO2.

In order to obtain information on the structure andmorphology of the Cu substrate before and after theelectroreduction of CO2, AES and SEM analysis areinvestigated. The comparative study between as-de-posited Cu substrate before the experiments and the Cuelectrode used for the reduction of CO2 at −2.1 V for10 min is analyzed. Fig. 6 shows the AES data for themeasurement of carbon reduced from CO2. The relativehigher amount of carbon formed by electroreduction ofCO2 can be determined by simple comparative study ofauger peak height of as deposited Cu sample. In Fig.6a, the peak assigned to carbon, which is universalcontaminant in AES/XPS analysis, is observed and its

Fig. 3. Faradaic efficiency (%) of CH4, C2H4, and CO atvarious cathodic potential. Applied time is 5 min for eachexperiment.

Fig. 4. Faradaic efficiency (%) of CH4 by applying cathodic potential of −2.1 V at solution temperature of 5°C in 0.1 M KHCO3.


Fig. 5. Mass change (a) onto Cu and cathodic current profile (b) when constant potential of −2.1 V is applied for 10 min.

value is 30% of the peak intensity of substrate Cu. Onthe other hand, the relative peak intensity of carbon inFig. 6b is found to be ten times more than that ofcarbon in Fig. 6a. Thus, this quantitative AES analysisclearly suggests that the mass increase of the copper isdue to the adsorption of the carbon during the cyclicvoltammetry (Fig. 2) and potentiostatic reduction ofCO2 (Fig. 5)

Results of SEM measurement are shown in Fig. 7.Fig. 7a is the surface morphology of the as-depositedCu onto Pt quartz crystal, showing only copper phasewith the average size of 2 �m. To measure the surfacechange of Cu cathode, we apply the constant cathodicpotential of −2.1 V for 10 min. The black spot on theCu surface is observed by using an optical microscopeand SEM analysis of the black part is carried out toinvestigate the morphological study in detail. In Fig.7b, we observe amorphous carbon on the Cu substrate.Figs. 5a, 6b, and 7b show that the activity of the Cuelectrode for the hydrocarbon production is degener-ated by the poisoning of the amorphous phase on thesurface.

Nogami et al. [12,13] previously presented that thefaradaic efficiency of the hydrocarbons on the Cu elec-trode was constantly kept even long term electrolysis byapplying the pulsed electroreduction. Fig. 8a shows theshape of the potential modulation method. The ca-thodic potential of −2.1 V (Ec) for 10 s (tc) and anodicpotential of 0.00 V (Ea) for 5 s (ta) are repetitivelyapplied. Open circuit potential (OCP) is −0.05 V. InFig. 8b, during the anodic pulse period, the burst ofanodic current is first observed and the current reacheszero value. The electrodissolution of Cu and the forma-tion of copper oxide can be assumed in this anodicprocedure.

To prove the above hypothesis, the morphology andstructure of the cathode are analyzed by ex-situ XRDand SEM after applying the potential modulation ex-

periment for 7 h. Fig. 9a shows the XRD data of theas-deposited Cu substrate and the structure of Cucathode is changed after applying potential modulationmethod, as shown in Fig. 9b. Before applying thepotential modulation electrolysis, several dominantcopper phases on the Pt surface is shown in Fig. 9a. Onthe other hand, Fig. 9b shows that Cu (hydro)oxidesare obtained, but we do not find any peak showingcopper substrate. This result is due to the following tworeasons such as the dissolution of copper metal byflowing the small anodic current and the formation ofcopper oxide. Here copper hydroxide phase (unmarked)is a well known intermediate species of the cathodicdeposition of the Cu2O [22,23].

Fig. 10 is the SEM measurement of the sample of theFig. 9b. The size of single copper phase is about 1 �m,

Fig. 6. AES data of Cu electrode (a) before and (b) after theelectroreduction of CO2 by applying −2.1 V for 10 min.


Fig. 7. SEM images of Cu prepared onto Pt quartz crystal (a) before and (b) after CO2 reduction (b). Applied potential is −2.1V and electrolysis time is 10 min.

Fig. 8. Schematic diagram of potential modulation method (a) and current response (b). Cathodic potential (Ec) of −2.1 V for 10s (tc) and anodic potential (Ea) of 0.00 V for 5 s (ta) is applied.


Fig. 9. XRD structure of (a) freshly deposited Cu onto Ptquartz crystal and (b) Cu cathode after the electroreduction ofCO2 by potential modulation method for 7 h. Cathodic poten-tial (Ec) of −2.1 V for 10 s (tc) and anodic potential (Ea) of0.00 V for 5 s (ta) is applied. Square, closed diamond, andreversed triangle indicate the peaks originated from Pt, Cu,and Cu2O, respectively.

Fig. 11. Mass change onto Cu quartz crystal by potentialmodulation method. Cathodic potential (Ec) of −2.1 V for 10s (tc) and anodic potential (Ea) of 0.00 V for 5 s (ta) is applied.

reacts with hydroxide ion (OH−) and chemically pre-cipitated on the copper surface (reaction 7 and 8).Finally, cuprous oxide is formed on Cu substrate bydehydration process (reaction 9) [22,23].

Cu�Cu++e− (7)

Cu++OH− +*�Cu(OH)ad (8)

2Cu(OH)ad�Cu2O+H2O (9)

which is half of the original size of as-deposited Cuphase (see Fig. 7a). This result proves the observationof XRD that Cu is dissolved during anodic pulse timeand cuprous ions are produced. Cuprous ion (Cu+)

Fig. 10. SEM images of Cu cathode by applying potential modulation method for 7 h at solution temperature of 5°C in 0.1 MKHCO3. Cathodic potential (Ec) of −2.1 V for 10 s (tc) and anodic potential (Ea) of 0.00 V for 5 s (ta) is applied.


Fig. 11 shows that the mass of Cu electrode decreasescontinuously, suggesting the electrodissolution of Cu.Structural changes of Cu electrode during pulsed elec-trolysis is explained by the electrochemical dissolution-chemical precipitation mechanism and the dehydrationof Cu(OH)2 to form Cu2O. In other words, this massdecrease is understood by the consideration of chemicaldissolution of the part of Cu(OH)2 and the dissolutionrate of copper is higher than the mass increase due tothe formation of copper oxide. As the cycle is iterated,surface of Cu electrode is changed into copper oxide.

Fig. 12 shows the production rate of CH4, which isthe ratio of volume of CH4 to that of CO2, in twodifferent reduction methods. Fig. 12a shows the resultof the constant potential mode and the deactivation ofCu cathode is observed after the electrolysis time of 3 h.However, the electrocatalytic activity of Cu remainsconstant for 7 h by applying superimposed potentialmethod, as shown in Fig. 12b. This result implies thatCu2O has similar electrocatalytic activity with Cu inelectroreduction of CO2. Detail reduction mechanismon Cu2O is still unclear and thus further study ofelectrodeposition of CO2 on Cu2O cathode should beconsidered, in this respect.

4. Conclusions

CO2 on Cu electrode was electrochemically reducedto CH4 and C2H4 and the reduction mechanism isinvestigated by using in-situ EQCM measurement andex-situ SEM, XRD, and AES analysis. In constantpotential mode, the degradation of the catalytic activityof Cu is observed due to the adsorption of amorphouscarbons on the Cu substrate and thus the efficiency ofhydrocarbons was decreased. Ex-situ SEM and AESproved the adsorption of graphite on the Cu surface.

However, potential modulated electrolysis showed theconstant catalytic activity of Cu cathode led by super-imposed anodic bias. In the SEM analysis, the precipi-tation of the graphite on the cathode is not obtainedand in-situ EQCM and ex-situ XRD data showed thedissolution of Cu and the formation of Cu2O. Cu2Oformed during anodic cycle prevented the poisoning ofcarbon and had similar electrocatalytic activity for CO2

reduction compared to that of Cu electrode.

Acknowledgements

J. Lee gratefully acknowledges Max-Planck-Gesellschaft for the fellowship.

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Fig. 12. Variation of the production rate of CH4 with electrol-ysis time: (a) constant potential of −2.1 V and (b) potentialmodulation method (refer Fig. 8a).

electrocatalytic activity of cu electrode in electroreduction of co2

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