Copper Single Crystal Electrodes Research Articles

Fabricating copper and copper/nickel alloy single crystal bead electrodes with a hydrogen–oxygen torch in ambient air

The use of copper single-crystal electrodes (CSCEs) has revealed the crucial role of electrode structure in electrocatalysis and corrosion. CSCEs, although commercially available, can be prepared in-house by melting the end of a Cu wire to form a single crystal bead (SCB) in ambient air. Unlike the conventional torch used for making Pt and Au SCBs, the torch used in this study allows the Cu bead to cool down to room temperature in a nitrogen stream. The resulting Cu bead is subsequently fabricated into a CSCE with a desired orientation. Copper’s propensity for oxidation does not pose a problem for our method as long as the H2 and O2 fluxes to the flame are well-controlled. The bulk oxide in the Cu SCB prepared as above is below the detection limit of our X-ray photoelectron spectrometer. The devised method opens up an avenue toward the study of electrochemistry at well-defined Cu and possibly other non-noble transition metal electrodes. Moreover, this method can also be used to make alloyed SCBs, as illustrated by the successful fabrication of a Cu/Ni (98.5/1.5) bead. The resulting Cu(1 1 1) and Cu98.5Ni1.5(1 1 1) electrodes are examined by cyclic voltammetry and in situ scanning tunneling microscopy in 0.1 M sulfuric acid. The trace amount of Ni in the Cu(1 1 1) electrode affects the potential, but not the structure, of specific adsorption of the (bi)sulfate anion. A (√3 × √7)–HSO4− structure and the Cu(1 1 1) lattice are imaged by STM at positive and negative potentials.

In-Situ XRD during Electrochemical CO Reduction on Cu

Almost all-successful CO2 and CO reduction catalysts to higher chain carbons are based on copper,1 however various facets and pre-treatments of copper have shown to give a wide variety of product selectivity.2, 3 Currently there are many unanswered questions regarding active sites and mechanisms thus fundamental studies are essential for this reaction. This talk will focus on in-situ monitoring the copper-copper oxide crystal structure at a variety of oxidative and reductive potentials. Using the synchrotron facilities at the SLAC national laboratory we were able to achieve an in-depth understand both of how deep the native oxide layer penetrates and how this oxide becomes reduced as we shift towards more cathodic potentials. This talk was also discuss under what conditions there is an amorphous copper oxide layer and what conditions lead to a crystalline copper oxide surface. This will be followed by a discussion of how the copper oxide effects the crystal structure of the metallic copper at the operating conditions. The in-situ work focused on a preferentially oriented <111> thin film, however there were other facets to a lesser extent. This talk will discuss the degree to which these facets are modified during in-situ operating conditions at various potentials. Hori, Y.; Wakebe, H.; Tsukamoto, T.; Koga, O., Electrocatalytic Process of CO Selectivity in Electrochemical Reduction of CO2 at Metal-Electrodes in Aqueous Media. Electrochimica Acta 1994, 39 (11-12), 1833-1839.Li, C. W.; Ciston, J.; Kanan, M. W., Electroreduction of carbon monoxide to liquid fuel on oxide-derived nanocrystalline copper. Nature 2014, 508 (7497), 504-+.Hori, Y.; Takahashi, I.; Koga, O.; Hoshi, N., Electrochemical reduction of carbon dioxide at various series of copper single crystal electrodes. Journal of Molecular Catalysis a-Chemical 2003, 199 (1-2), 39-47.

Structure- and Potential-Dependent Cation Effects on CO Reduction at Copper Single-Crystal Electrodes.

The complexity of the electrocatalytic reduction of CO to CH4 and C2H4 on copper electrodes prevents a straightforward elucidation of the reaction mechanism and the design of new and better catalysts. Although structural and electrolyte effects have been separately studied, there are no reports on structure-sensitive cation effects on the catalyst’s selectivity over a wide potential range. Therefore, we investigated CO reduction on Cu(100), Cu(111), and Cu(polycrystalline) electrodes in 0.1 M alkaline hydroxide electrolytes (LiOH, NaOH, KOH, RbOH, CsOH) between 0 and −1.5 V vs RHE. We used online electrochemical mass spectrometry and high-performance liquid chromatography to determine the product distribution as a function of electrode structure, cation size, and applied potential. First, cation effects are potential dependent, as larger cations increase the selectivity of all electrodes toward ethylene at E > −0.45 V vs RHE, but methane is favored at more negative potentials. Second, cation effects are structure-sensitive, as the onset potential for C2H4 formation depends on the electrode structure and cation size, whereas that for CH4 does not. Fourier Transform infrared spectroscopy (FTIR) and density functional theory help to understand how cations favor ethylene over methane at low overpotentials on Cu(100). The rate-determining step to methane and ethylene formation is CO hydrogenation, which is considerably easier in the presence of alkaline cations for a CO dimer compared to a CO monomer. For Li+ and Na+, the stabilization is such that hydrogenated dimers are observable with FTIR at low overpotentials. Thus, potential-dependent, structure-sensitive cation effects help steer the selectivity toward specific products.

Electrochemical Reduction of Carbon Dioxide on Single-Crystal Copper Membrane

The electrochemical reduction of carbon dioxide (CO2) on copper electrodes has been attracting great interest as an intermittent CO2 conversion system, which can synthesize hydrocarbons at room temperature and under ambient pressure. Many extensive efforts clarified the synthesis mechanism of hydrocarbons in the electrochemical reduction of CO2[1-2]. Recent progress of the selective synthesis utilizing the crystal structure on a copper electrode offers a promising route to produce the desired hydrocarbon product. Although this system converts CO2 to hydrocarbons effectively, it is difficult to construct the manufacturing process because a large-area single crystal electrode is expensive. In practice, the past efforts were almost utilized a fine single crystal electrode, such as bead or rod. In this research, we reports the electrochemical reduction of CO2 on a ”large-area” single crystal copper membrane epitaxially deposited on sapphire and magnesium oxide (MgO) as a support substrate. In the experiments, copper membranes were deposited onto glass, c-plane sapphire, and MgO substrates in argon atmosphere by the radio frequency magnetron sputtering machine. A series of copper membranes were characterized by X-ray diffraction (XRD) analysis to confirm the copper crystal structure. Figure 1 shows the experimental results, where the commercial copper plate (Aldrich, 1.0 mm thick 99.999% purity) and the copper membrane deposited on the glass substrate have three peaks of the crystal structure, Cu(111), Cu(100), and Cu(110). On the other hand, copper membranes deposited on c-plane sapphire, MgO(100) and MgO(110) substrate are highly crystalline with Cu(111), Cu(100), and Cu(110), respectively. This suggests that the copper membranes epitaxially grew along a crystal structure on the single crystal support substrates. The electrochemical reduction of CO2 on the single crystal copper membranes was performed in an electrochemical cell with a three-electrode assembly at room temperature and ambient pressure. Figure 2 shows the time dependence of the methane concentration in the effluent gas on Cu(111) membrane and the commercial copper plate as electrodes, respectively. Only a trace amount of ethylene was observed with the concentration of < 1 ppm. It is noted that the methane emission is strongly dependent on the crystalline of the copper electrode. Moreover, significantly higher CO2 conversion in sodium hydroxide (NaOH) electrolyte (pH=13.8) than that in phosphate buffer (pH=7.2) was observed on Cu(111) membrane. The electrochemical conversion of CO2 to methane reveals to be favored in strong alkali electrolyte. However, methane concentration in this electrolyte decreased with the electrolyte pH drop during electrochemical reduction (pH=13.8→7.4), which is accounted for by the neutralization between CO2 and NaOH. We will also show the electrochemical reduction of CO2 on another copper crystal structure, and the electrochemical mechanism based on dissolved carbon oxide concentration in the electrolyte discussed. Acknowledgement This work is supported by the Sasakawa Scientific Research Grant from The Japan Science Society and the Shiraishi Scientific Research Grant. Reference [1] Y. Hori, I. Takahashi, O. Koga, N. Hoshi, “Electrochemical reduction of carbon dioxide at various series of copper single crystal electrodes”, Journal of Molecular Catalysis A: Chemical, 2003, 199, 39–47. [2] K. J. P. Schouten, E. P. Gallent, and M. T. M. Koper, "Structure Sensitivity of the Electrochemical Reduction of Carbon Monoxide on Copper Single Crystals", ACS Catal., 2013, 3, (6), 1292–1295. Figure 1. XRD profiles of copper electrode in this study. (a)commercial copper plate, (b)copper membrane on glass substrate, (c)copper membrane on c-sapphire substrate, (d)copper membrane on MgO(100) substrate, (e)copper membrane on MgO(110) substrate. Figure 2. Methane concentration curves plotted as function of reaction time for the electrochemical reduction of CO2 on Cu(111) membrane and Cu plate at an electrode potential of -0.9 V vs. RHE. Figure 1

Infrared spectroscopic and voltammetric study of adsorbed CO on stepped surfaces of copper monocrystalline electrodes

Voltammetric and infrared (IR) spectroscopic measurements were carried out to study adsorbed CO on two series of copper single crystal electrodes n(1 1 1)–(1 1 1) and n(1 1 1)–(1 0 0) in 0.1 M KH 2PO 4 + 0.1 M K 2HPO 4 at 0 °C. Reversible voltammetric waves were observed below −0.55 V versus SHE for adsorption of CO which displaces preadsorbed phosphate anions. The electric charge of the redox waves is proportional to the step atom density for both single crystal series. This fact indicates that phosphate anions are specifically adsorbed on the step sites below −0.55 V versus SHE. Voltammetric measurements indicated that (1 1 1) terrace of Cu is covered with adsorbed CO below −0.5 V versus SHE. Nevertheless, no IR absorption band of adsorbed CO is detected from (1 1 1) terrace. Presence of adsorbed CO on (1 1 1) terrace is presumed which is not visible by the potential difference spectroscopy used in the present work. IR spectroscopic measurements showed that CO is reversibly adsorbed with an on-top manner on copper single crystal electrodes of n(1 1 1)–(1 1 1) and n(1 1 1)–(1 0 0) with approximately same wavenumber of C O stretching vibration of 2070 cm −1. The IR band intensity is proportional to the step atom density. Thus CO is adsorbed on (1 1 1) or (1 0 0) steps on the single crystal surfaces. An analysis of the IR band intensity suggested that one CO molecule is adsorbed on every two or more Cu step atom of the monocrystalline surface. The spectroscopic data were compared with those reported for u h v system. The C O stretching wavenumber of adsorbed CO in the electrode–electrolyte system is 30–40 cm −1 lower than those in u h v system.

Electrochemical reduction of carbon dioxide at various series of copper single crystal electrodes

Abstract Electrochemical reduction of carbon dioxide was studied with various series of copper single crystal electrodes in 0.1 M KHCO3 aqueous solution at constant current density 5 mA cm−2; the electrodes employed are Cu(S)-[n (1 0 0)×(1 1 1) ], Cu(S)-[n (1 0 0)×(1 1 0) ], Cu(S)-[n (1 1 1)×(1 0 0) ], Cu(S)-[n (1 1 1)×(1 1 1) ] and Cu(S)-[n (1 1 0)×(1 0 0) ]. The electrodes based on (1 0 0) terrace surface give ethylene as the major product. The ethylene formation is further promoted by the introduction of (1 1 1) or (1 1 0) steps to the (1 0 0) basal plane. The highest C2H4 to CH4 formation ratio amounts to 10 in terms of the current efficiency for the (7 1 1) (=4(1 0 0) − (1 1 1)) surface as compared with the value 0.2 for the (1 1 1) electrode. The n(1 1 1)−(1 1 1) surfaces favor the formations of acetic acid, acetaldehyde and ethyl alcohol with the increase of the (1 1 1) step atom density. CH4 formation at the n(1 1 1)−(1 1 1) electrodes decreases with the increase of the (1 1 1) step atom density. The n(1 1 1)−(1 0 0) surfaces give higher gaseous products; the major product is CH4 with lower fraction of C2+ compounds.

Electrochemical reduction of CO 2 at copper single crystal Cu(S)-[ n(111)×(111)] and Cu(S)-[ n(110)×(100)] electrodes

Electrochemical reduction of CO 2 was studied using two series of single-crystal electrodes, Cu(S)-[ n(111)×(111)] and Cu(S)-[ n(110)×(100)] at a constant current density of 5 mA cm −2 in 0.1 M KHCO 3 aqueous solution. Copper single crystals were grown from 99.999% copper metal in a graphite crucible, and the crystal orientation was determined by the X-ray back reflection method. The surface treatment of the copper single crystal electrodes was studied in detail, and the reproducibility of the CO 2 reduction was greatly improved. The product distribution of the CO 2 reduction varies greatly with the crystal orientation. CO 2 reduction at the Cu(110) (=Cu(S)-[2(111)×(111)]) electrode gives a current yield of 20% of CH 3COOH; the formation of CH 3COOH in CO 2 reduction has not been reported previously. The yield of CH 4 was very low (6%) at the Cu(110) electrode. The formation of CH 4 and CH 3COOH changes significantly with the crystal orientation. A decrease of the step atom density in the Cu(S)-[ n(111)×(111)] series reduces the yield of CH 3COOH and enhances that of CH 4. Introduction of the (100) step to the Cu(110) basal plane, leading to the Cu(S)-[ n(110)×(100)] series with kink sites, diminishes the feature of the Cu(110). The Cu(210) (=Cu(S)-[2(110)×(100)]), which has the highest number of dangling bonds of fcc metals, gives a high yield of CH 4 with a product distribution similar to that of Cu(111) which has the lowest density of dangling bonds.

Selective Formation of C2 Compounds from Electrochemical Reduction of CO2 at a Series of Copper Single Crystal Electrodes

Electrochemical reduction of CO2 was studied using single-crystal electrodes, Cu(111), Cu(100), Cu(S)-[n(100) × (111)], and Cu(S)-[n(100) × (110)] at a constant current density 5 mA cm-2 in 0.1 M KHCO3 aqueous solution. Copper single crystals were prepared from 99.999% copper metal in graphite crucibles by the Bridgeman method. The crystal orientation was determined by the X-ray back reflection method. The Cu(111) electrode yields mainly CH4 from CO2, and the Cu(100) favorably gives C2H4. Introduction of (111) steps to Cu(100) basal plane, leading to Cu(S)-[n(100) × (111)] orientations, significantly promoted C2H4 formation and suppressed CH4 formation. The selectivity ratio C2H4/CH4 on Cu(711) (n = 4) amounted to 14, 2 orders of magnitude higher than that on Cu(111).

Charge displacement adsorption of carbon monoxide on [110] zone copper single crystal electrodes in relation with PZC

CO is adsorbed on Cu electrode with a simultaneous transfer of charge. The apparent charge transfer is derived from displacement of anions specifically adsorbed on the electrode during the adsorption of CO. This paper presents the results of voltammetric measurements of the adsorption of CO on single crystal electrodes of Cu[110] zone in phosphate buffer solution (pH 6.8) at 0°C. The profile of the charge transfer wave depends significantly on the crystal orientation of Cu single crystals. The potential of the charge transfer is well correlated with the work functions and the potential of zero charge of the Cu single crystals. We propose a model that H 2PO 4 − is specifically adsorbed on (100) steps of Cu[110] zone crystal electrodes occupying 3 to 4 atomic sites.

Electrocatalytic oxidation of formaldehyde on copper single crystal electrodes in alkaline solutions

Electrocatalytic oxidation of formaldehyde on Cu single crystal planes was investigated by cyclic voltammetry in aqueous 0.1 M NaOH solutions. The catalytic activity of the Cu(110) surface appeared to be significantly higher than that of Cu(100) and Cu(111) planes. The surface roughness and the dislocation density of Cu single crystal electrodes were shown not to be responsible for a different catalytic activity of a Cu single crystal plane in the formaldehyde oxidation reaction. A more pronounced surface imperfection facilitates a surface passivation process. A different electrocatalytic activity of Cu single crystal planes may be accounted for by a different geometrical arrangement of surface atoms.

Adsorption of CO accompanied with simultaneous charge transfer on copper single crystal electrodes related with electrochemical reduction of CO 2 to hydrocarbons

CO 2 and CO were electrochemically reduced with Cu(100), Cu(110) and Cu(111) electrodes at a constant current density of 5 mA cm −2 in 0.1M KHCO 3 at ambient temperature. C 2H 4 is favorably produced on Cu(100), and CH 4 is predominantly yielded on Cu(111). Cu(110) showed an intermediate product selectivity. Voltammetric measurements in the presence of CO in phosphate buffer solutions gave a pair of sharp charge transfer redox peaks with Cu(100) and Cu(110), not with Cu(111). The charge transfer peaks depend greatly on the anionic species of the electrolyte, and are rationalized by charge displacement due to desorption of specifically adsorbed anions during CO adsorption. Competitive adsorption of anions and CO on Cu single crystal electrode surfaces is discussed on the basis of electrochemical and in situ FTIR spectroscopic measurements.

Oxygen electrosorption on copper single crystal electrodes in sodium hydroxide solution

Oxygen electrosorption on copper single crystal electrodes in sodium hydroxide solution

Oxygen electrosorption on copper single crystal electrodes in sodium hydroxide solution

In a study of the initial stages of anodic oxidation of polycrystaUine copper electrodes in alkaline solutions [1 ], we measured potential sweep vol tammetry curves which show that formation of bulk Cu~O is preceded by electrosorption of oxygen species. The electrosorption occurs in two successive stages, each represented by a current peak, corresponding to a different submonolayer state with a different adsorption energy. We postulated that a priori heterogenei ty of the surface, e.g. exposure of various crystal planes, could be an explanation for the occurrence of multiple peaks. In order to test this hypothesis we have investigated the electrosorption of oxygen on carefully prepared copper single crystal surfaces. Clear evidence is found in our results, that the electrosorption of oxygen is a plane specific reaction. Each peak in the vol tammogram on polycrystaUine copper can be at t r ibuted to exposure of regions with specific adsorption sites as present in majority on different single crystal planes.