Research Article pubs.acs.org/acscatalysis
Electrochemical Reduction of CO2 Using Copper Single-Crystal Surfaces: Effects of CO* Coverage on the Selective Formation of Ethylene Yun Huang,† Albertus D. Handoko,†,§ Pussana Hirunsit,*,‡ and Boon Siang Yeo*,† †
Department of Chemistry, Faculty of Science, National University of Singapore, 3 Science Drive 3, Singapore 117543 National Nanotechnology Center (NANOTEC), National Science and Technology Development Agency (NSTDA), Pathum Thani 12120, Thailand
‡
S Supporting Information *
ABSTRACT: Copper oxide-derived Cu catalysts are known to exhibit enhanced energetic efficiencies and selectivities towards the reduction of carbon dioxide to commercially vital C2 products such as ethylene (C2H4). However, the cause of this selectivity is not fully understood. In this work, we elucidated a fundamental reason underlying the selectivity of CO2 reduction toward C2 products by studying its reactivity on Cu(100), Cu(111), and Cu(110) single-crystal surfaces. A combination of cyclic and linear sweep voltammetries, chronoamperometry, online gas chromatography, 1H nuclear magnetic resonance spectroscopy, and density functional theory (DFT) calculations was employed for this end. A wide range of electrochemical potentials from −0.28 to −1.25 V versus the reversible hydrogen electrode was investigated. Aqueous 0.1 M KHCO3 was used as the electrolyte. We report here two general trends on Cu2O-derived Cu and Cu single-crystal surfaces: (i) the onset potential for the formation of C2H4 always starts 300−400 mV more negative than the onset potential for CO evolution, and (ii) C2H4 was formed only after a significant amount of CO gas was produced. Among the single-crystal surfaces investigated, Cu(100) required the lowest overpotential to reduce CO2 to C2H4. These observations were rationalized using DFT simulations. Of the three single-crystal surfaces modeled, the dimerization of two CO* molecules on Cu(100) exhibited the lowest energy barrier, and this barrier can be further lowered with higher CO* coverages. The application of our observed experimental trends to other previously reported Cu-based systems strongly suggests that a high surface coverage of CO* is central for the selective formation of C2H4. KEYWORDS: CO2 reduction, copper, single-crystal surfaces, ethylene, electrocatalysis, surface structure, density functional theory calculations −5 mA/cm2 in an aqueous KHCO3 electrolyte.6 Cu(100) and Cu(111) surfaces favored the formation of C2H4 and CH4, respectively, while Cu(110) promoted the production of acetate and acetaldehyde. CO was also shown to be a key intermediate of CO2 reduction.7−10 Recently, Koper and co-workers utilized online electrochemical mass spectrometry (OLEMS) to monitor CO reduction products (CH4 and C2H4) on Cu single-crystal surfaces during slow potentiodynamic scans in pH 7 phosphate buffers.11,12 Their results confirmed that C2H4 formation is favored on Cu(100), with a relatively positive onset potential of −0.4 V versus the RHE (reversible hydrogen electrode). On Cu(111) though, CH4 and C2H4 evolved simultaneously but at a more negative potential of −0.9 V versus the RHE. While highly meritorious, these experimental studies were limited by the number of products detected and/or the narrow potential range in which they were performed. The mechanism behind the favored formation of C2H4 on Cu(100) surfaces has been modeled using density functional theory (DFT) calculations. Koper and co-workers suggested a
1. INTRODUCTION The electroreduction of carbon dioxide (CO2) to hydrocarbons and alcohols using renewable electricity is a promising solution for generating a sustainable supply of carbon-neutral chemical feedstocks and fuels.1 Copper-based electrodes have been the most widely studied electrocatalysts for this purpose.2 Roughened Cu surfaces such as Cu2O-derived Cu are particularly promising in this regard as they have been shown to catalyze reduction of CO2 to ethylene (C2H4) and ethanol with Faradaic efficiencies of >45%.3,4 The effects of their surface morphologies and chemistry on the selectivity of the processes under electrochemical potentials are, however, largely unclear. To gain a deeper understanding of the fundamental processes occurring on these surfaces, it is essential to investigate the effects of surface structures on CO2 reduction using well-defined single-crystal surfaces. Key conclusions derived from these studies can then be used to understand more complex systems such as copper oxidederived Cu surfaces. The electrochemical reduction of CO2 on copper single-crystal surfaces was first conducted by Frese, who observed an increasing level of CH4 production for Cu(100), Cu(110), and Cu(111) electrodes.5 Subsequently, Hori and co-workers performed CO2 reduction using chronopotentiometry at © XXXX American Chemical Society
Received: November 4, 2016 Revised: January 12, 2017
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DOI: 10.1021/acscatal.6b03147 ACS Catal. 2017, 7, 1749−1756
Research Article
ACS Catalysis
Cu(111), and Cu(110) surfaces were calculated using the DFT method (Section S5 of the Supporting Information). The energy barrier for the CO*−COH* coupling on Cu(111) was also investigated. The adsorption energy of n CO molecules on a copper surface can be related to the electronic energy of adsorbed CO* molecules (EnCO+surf), the electronic energy of the bare copper surface (Esurf), and the free energy of the CO molecule [GCO(g)] by the following equation:
pH-independent mechanism in which two CO molecules dimerize alongside a one-electron transfer to become a C2O2− anion.13 The square arrangement of copper atoms on Cu(100) is believed to contribute to the kinetic and electronic stability of the CO dimerization products.14 Nørskov et al. further proposed that CO dimerization has a lower activation barrier on Cu(100) than on Cu(111), probably because of a more exergonic CO* adsorption energy and a correspondingly higher CO* coverage on the former surface.15,16 While theoretical studies have proposed a link between CO* coverage and C2H4 formation, key experimental observations correlating CO* coverage and C2H4 evolution remained absent. Herein, we performed the electrochemical reduction of CO2 on Cu2O-derived Cu catalysts and observed that the onset potentials for the formation of C2H4 and CO along with the plateauing of the partial current density of CO (jCO) are closely related. Such observations were also made with Cu single-crystal surfaces. We propose and demonstrate through our experimental data and DFT calculations that CO* coverage and surface structures are key factors in determining the extent of C2H4 formation.
ΔGad = [EnCO + surf − Esurf + ΔZPE − T ΔS − n × GCO(g)]/n + Gcorr
Several energy corrections, including zero-point energy correction (ΔZPE) and entropic energy correction (TΔS), were taken into account at 298.15 K. A further energy correction term, attributed to the limitation of the Perdew−Burke−Ernzerhof (PBE) exchange-correlation functional in describing the standard formation energy of CO(g) (Gcorr), was set at 0.24 eV.13 A more negative ΔGad indicates a stronger CO* adsorption. Here, we report only the most stable configuration at each CO* coverage (Section S5 of the Supporting Information), as determined by preliminary calculations of various configurations. The coverage of CO* is expressed in monolayer (ML) units, defined as the number of adsorbed CO* molecules (n) divided by the total number of copper surface atoms. Cu(111), Cu(110), and Cu(100) surfaces were constructed with 4 × 4, 4 × 3, and 3 × 3 supercells, respectively. These sizes were chosen so that models with high CO* coverages can be represented with at least 10 CO* adsorbates. The total numbers of copper surface atoms per supercell are 16, 12, and 18 atoms on Cu(111), Cu(110), and Cu(100) surfaces, respectively. Nudged Elastic Band (NEB)18 and Dimer19 methods were employed to locate transition state structures for CO* dimerization and CO*−COH* coupling and to calculate the corresponding nonelectrochemical barrier energies. All transition states were verified by the number of imaginary frequencies, which should always be one.
2. EXPERIMENTAL SECTION 2.1. Preparation and Characterization of Electrodes. Cu2O-derived Cu catalysts were prepared by growing Cu2O films on polished polycrystalline Cu discs (99.99%, 10 mm diameter, Goodfellow) using hydrothermal synthesis (Section S1 of the Supporting Information).4 Cu(100), Cu(111), and Cu(110) single-crystal surfaces (99.99%, 10 mm diameter, MTI Corp.) were prepared via mechanical polishing, electropolishing, and diluted acid rinsing (Section S1 of the Supporting Information).17 Their qualities were checked with X-ray diffraction, cyclic voltammetry, and linear sweep voltammetry (Section S2 of the Supporting Information). 2.2. Electrochemical Reduction of CO2. A two-compartment electrochemical cell that had been described in our earlier work was used for CO2 electrolysis.3 The cathodic compartment (6.4 mL of electrolyte, ∼2 mL of headspace) contained the working electrode (0.385 cm2 exposed geometric area), which was positioned close to a Ag/AgCl reference electrode (saturated KCl, Pine). The anodic compartment (8 mL of electrolyte, ∼2 mL of headspace) accommodated a Pt counter electrode (ALS Japan), which was separated from the cathodic compartment by an anion exchange membrane (Selemion AMV, AGC Asahi Glass). Each CO2 reduction experiment was performed in an aqueous 0.1 M KHCO3 electrolyte (99.5%, Sigma-Aldrich) for 40 min. CO2 (99.999%, Linde Gas) was continuously bubbled into the electrolyte at 20 sccm through a mass flow controller (MC 100SCCM-D, Alicat Scientific). All electrochemical measurements were performed using a Gamry Reference 600 potentiostat at room temperature. The iR drop was compensated using the current interrupt mode. All potentials reported in this work were referenced to the RHE. The CO2-saturated 0.1 M KHCO3 electrolyte had a pH of 6.8. CO2 reduction products were quantified using gas chromatography (GC-7890A, Agilent) and 1H nuclear magnetic resonance spectroscopy (500 MHz, Bruker Avance 500) (Section S3 of the Supporting Information). The Faradaic efficiencies (FEs) and partial current densities (j) of all the detected products are tabulated in Section S4 of the Supporting Information. 2.3. Computational Method. The adsorption energies of CO and energy barriers of CO* dimerization on Cu(100),
3. RESULTS AND DISCUSSION 3.1. Physical and Electrochemical Characterization of the Cu2O-Derived Cu Electrode and Cu Single-Crystal Surfaces. The Cu2O-derived Cu electrode was prepared using our previously published method.4 The orientations of the Cu(100), Cu(111), and Cu(110) single crystals were ascertained by X-ray diffraction and showed only the relevant Bragg peaks (Section S2 of the Supporting Information). The surface qualities of the copper single crystals were checked regularly using cyclic voltammetry (Section S2 of the Supporting Information). The recorded CV profiles were in excellent agreement with those presented in the literature.20,21 Linear sweep voltammetry was also employed to electrochemically characterize the Cu singlecrystal surfaces (Section S2 of the Supporting Information).22,23 In the presence of CO, Cu(111) was found to exhibit the weakest suppression of the H2 evolution reaction (HER), which suggested a possibly small population of CO* on the surface. 3.2. CO2 Reduction Products Formed at Various Electrochemical Potentials on the Cu2O-Derived Cu Electrode and Cu Single-Crystal Surfaces. The Faradaic efficiencies and partial current densities of the products formed during CO2 electroreduction on Cu2O-derived Cu and Cu singlecrystal electrodes are shown in Figures 1 and 2, respectively (Section S4 of the Supporting Information). The main products detected are CO, formate, CH4, C2H4, ethanol, and H2 1750
DOI: 10.1021/acscatal.6b03147 ACS Catal. 2017, 7, 1749−1756
Research Article
ACS Catalysis
Figure 1. (A) Faradaic efficiencies and (B) partial current densities of CO2 reduction products formed on Cu2O-derived Cu. The dashed lines mark the onset potentials for CO and C2H4 formations and show that C2H4 started to evolve at a potential at which jCO plateaued. For better visualization of the relation between CO and C2H4, the standard deviations of the data sets have been omitted from the partial current density plot but included in Section S4 of the Supporting Information. At −0.28 V, the generated H2 could not be quantified because of the detection limit of the thermal conductivity detector of the gas chromatograph.
approached a plateau. These two phenomena were similar to what was found for Cu2O-derived Cu electrodes (section 3.2.1). The formation of C2H4 on Cu(100) at relatively high overpotentials has been attributed by Luo et al. to a high kinetic barrier in forming CO intermediates (from CO2).28 However, our results indicated that CO evolution had occurred 400 mV prior to C2H4 evolution. This suggests that the rate-limiting process for the reduction of CO2 to C2H4 may not be the reduction of CO2 to CO intermediates. The formation of formate also started at −0.7 V. With an increasing overpotential, the production of formate (indicated by its FEformate and jformate) surpassed that of CO at approximately −0.85 V. At potentials negative to −0.85 V, the formation of CH4 began. The high onset potential for CH4 formation is consistent with previous DFT calculations.26,28,29 Peterson et al. had calculated an onset potential of −0.74 V versus the RHE for CH4 formation on Cu(211) [a surface consisting of (111) facets separated by (100) steps6] and suggested the protonation of CHO* as the potential-limiting step.26 Luo et al. also predicted high onset potentials for CH4 formation on Cu(100) (more negative than −0.9 V) and Cu(111) (−1.15 V) but proposed a different reaction path through the thermodynamically and kinetically favored COH* intermediate.28,29 As the overpotential increased, the production of C2H4 rose and peaked around −1.0 V (FEC2H4 of 30.6%, jC2H4 of −1.30 mA/cm2). FECH4 and jCH4 also increased sharply and eventually surpassed that of C2H4 around −1.05 V (FECH4 of 27.7%, FEC2H4 of 21.9%). Other C2 products such as ethanol and acetate were first observed around −0.9 V on Cu(100). At potentials negative to −1.1 V, the partial current densities for all products, apart from H2 and CH4, declined. The enhanced production of H2 and CH4 can be attributed to the highly favored proton-coupled electron transfer steps at high overpotentials.30 jCH4 continued to rise but at a reduced rate, most likely because of mass transport limitation of the CO2 reactant to the cathode. 3.2.3. Cu(111). In general, the product distribution as a function of potential exhibited by Cu(111) was shifted toward more negative potentials (Figure 2C,D). Evolution of CO and C2H4 occurred at −0.6 and −0.9 V, respectively. Similar to
(produced from the competing hydrogen evolution reaction). n-Propanol was also detected in intermediate amounts (maximum FE of 8.2%) on Cu2O-derived Cu, but only in minor quantities [FE of
