Yoshio Hori

Electrocatalytic process of CO selectivity in electrochemical reduction of CO2 at metal electrodes in aqueous media

Electrochemical reduction of CO2 at metal electrodes yields CO, HCOO−, CH4, C2H4, and alcohols in aqueous media. Metal electrodes are roughly divided into two groups, CO formation metals (Cu, Au, Ag, Zn, Pd, Ga, Ni, and Pt) and HCOO− ones (Pb, Hg, In, Sn, Cd, Tl). Foreign atom modified electrodes (the coverage is virtually unity) showed variable product selectivity between CO and HCOO−, which depends upon the combination of modifier atom and substrate electrode. Critical investigation of foreign atom modified electrodes derived a series of CO selectivity, as Au > Ag > Cu > Zn > Cd > Sn > In > Pb > Tl > Hg. The electrode potentials of CO2 reduction are well correlated with the heat of fusion of metals and the potential of H2 evolution. The order of CO selectivity agrees roughly with that of the electrode potential of CO2 reduction, and is rationalized in terms of stabilization of intermediate species CO−2 at the electrode surface. CO is produced from stably adsorbed CO−2, and HCOO− is formed from free or weakly adsorbed CO−2.

Formation of hydrocarbons in the electrochemical reduction of carbon dioxide at a copper electrode in aqueous solution

Electroreduction of CO2 at Cu in aqueous inorganic electrolytes was studied by means of voltammetric, coulometric and chronopotentiometric measurements. CO, CH4, C2H4, EtOH and PrnOH are produced at ambient temperatures. Formation of CO predominates at less negative potentials (more positive than –1.2 V vs. NHE); hydrocarbons and alcohols are favourably produced below –1.3 V vs. NHE, where the Faradaic efficiency of CO drops. CO, formed as an intermediate from CO2, is adsorbed on the Cu electrode, interfering with cathodic hydrogen formation. The adsorption strength of CO on Cu is very weak as compared with that on Pt. Adsorbed CO is reduced to Hydrocarbons and alcohols at more negative potentials. The product distribution from CO2 depends strongly upon the electrolytes employed. Formation of C2H4 and alcohols is favoured in KCl, K2SO4, KClO4 and dilute HCO–3 solutions, whereas CH4 is preferentially produced in relatively concentrated HCO–3 and phosphate solutions. The product selectivity depends upon availability of hydrogen or protons on the surface, which is controlled by pH at the electrode. The pH at the electrode is greatly affected by the electrolyte, since OH– is released in the electrode reactions. The production of hydrocarbons and alcohols is discussed in comparison with the mechanism of the Fishcher–Tropsch reaction.

Electrochemical reduction of CO2 on single crystal electrodes of silver Ag(111), Ag(100) and Ag(110)

Abstract Structural effects on the rates of CO 2 reduction were studied on Ag(111), Ag(100) electrodes in 0.1 M KHCO 3 using macroelectrolysis. The partial current density of each reduction product (CO, HCOO − , H 2 ) was measured at various potentials. All the Ag single crystal electrodes mainly gave CO at any potential. The partial current density of CO on the atomatically stepped Ag(110) was remarkably higher than those on the flat Ag(111) and Ag(100),is the case with Pt group metals. The partial current density of HCOO − gave a smaller orientation dependence.

Electrochemical reduction of CO2 at copper single crystal Cu(S)-[n(111)×(111)] and Cu(S)-[n(110)×(100)] electrodes

Electrochemical reduction of CO2 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 KHCO3 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 CO2 reduction was greatly improved. The product distribution of the CO2 reduction varies greatly with the crystal orientation. CO2 reduction at the Cu(110) (=Cu(S)-[2(111)×(111)]) electrode gives a current yield of 20% of CH3COOH; the formation of CH3COOH in CO2 reduction has not been reported previously. The yield of CH4 was very low (6%) at the Cu(110) electrode. The formation of CH4 and CH3COOH 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 CH3COOH and enhances that of CH4. 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 CH4 with a product distribution similar to that of Cu(111) which has the lowest density of dangling bonds.

Adsorption of CO accompanied with simultaneous charge transfer on copper single crystal electrodes related with electrochemical reduction of CO2 to hydrocarbons

CO2 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 KHCO3 at ambient temperature. C2H4 is favorably produced on Cu(100), and CH4 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.

Electrochemical Reduction of Carbon Dioxide at a Platinum Electrode in Acetonitrile‐Water Mixtures

Electrochemical reduction of CO 2 yields (COO - ) 2 as a main product in water free electrolyte. The current efficiency of CO and HCOO - initially increases and then decreases with increase of water concentration in the electrolyte. CO, reduced from CO 2 , is strongly adsorbed on Pt electrode, and prevents further reduction of CO 2 in aqueous media. Adsorbed CO is present on Pt in nonaqueous media as well, but reduction of CO 2 still proceeds. Since the supply of H 2 O is severely limited in nonaqueous media, H 2 evolution is suppressed. CO 2 , diffusing to the electrode, may be readily reduced to CO 2 · - CO 2 · - will react with CO 2 , eventually forming (COO - ) 2 (oxalate). (COO - ) 2 may also be formed by coupling of CO 2 · - With increase of H 2 O concentration, H 2 evolution is enhanced. HCOO - and CO may be produced in the reaction of CO 2 · - and H 2 O.

Voltammograms of the single-crystal electrodes of palladium in aqueous sulfuric acid electrolyte: Pd(S)-[n(111)×(111)] and Pd(S)-[n(100)×(111)]

Abstract A series of Pd(S)-[ n (111)×(111)] and Pd(S)-[ n (100)×(111)] electrodes give voltammograms in 0.5 M H 2 SO 4 , which depend remarkably on the surface structure. Pd surfaces with more than three atomic rows of terrace show redox peaks due to the desorption and adsorption of HSO 4 − /SO 4 2− between 0.25 and 0.33 V (RHE). In the oxide film formation region, Pd(S)-[ n (111)×(111)] electrodes give two anodic peaks at 0.95 and 1.1 V (RHE): the charge of the former peak increases with the increase of the step atom density, whereas that of the latter depends linearly on the terrace atom density. Pd(S)-[ n (100)×(111)] electrodes provide an anodic peak at 0.93 V (RHE). The charge correlates with the terrace width. These electrodes give no peak for which the charge depends on the step density.

Infrared spectroscopy of adsorbed CO and intermediate species in electrochemical reduction of CO2 to hydrocarbons on a Cu electrode

Abstract Voltammetric measurements showed in the previous paper that charge transfer is accompanied with CO adsorption on Cu electrode. Adsorbed CO is present at Cu electrode surface below the potential of the cathodic charge transfer, and not present above the potential, as confirmed by infrared spectroscopy. This article reports significant features of in situ infrared spectra of adsorbed CO in detail, obtained with a Cu polycrystal electrode in aqueous electrolytes at 0 to 2 °C with regard to electrochemical reduction of CO 2 to hydrocarbons. Two infrared absorption peaks were assigned to adsorbed CO. A sharp absorption band appeared at ca . 2080 cm − between −0.8 and −1.0 V vs. nhe in 0.2 M KHCO 3 . Another absorption peak was broad at 2040 cm − between −0.7 and −0.8 V in the same solution. The infrared absorption intensity is closely related with the amount of charge accompanied with CO adsorption. The infrared absorption bands were studied using D 2 O electrolyte solutions; the results agreed well with those obtained in H 2 O electrolyte system. Thus the spectra will not be affected by hydrogen. Infrared absorption bands of reduced CO 2 were measured as well, showing identical spectroscopic features with adsorbed CO. The reduced CO 2 is present at −1.0 V at Cu electrode and not present at −0.4 V. The reduced CO 2 at Cu electrode is thus identified as adsorbed CO.

Electrochemical reduction of carbon dioxide at a series of platinum single crystal electrodes

Electrocatalytic activity in the reduction of CO2 to adsorbed CO was studied systematically on a series of Pt single crystal electrodes using voltammetry. The single crystal electrodes examined were stepped surfaces (Pt(S)-[n(111)×(111)]), Pt(S)-[n(111)×(100)], Pt(S)-[n(100)×(111)]), and kinked step surfaces (Pt(S)-[n(110)×(100)] and Pt(S)-[n(100)×(110)]). Atomically flat surfaces, Pt(111) and Pt(100), show poor activity for CO2 reduction. Introduction of step sites to (111) or (100) surface significantly enhances the electrocatalytic activity in CO2 reduction. The rate increases proportionally with the step atom density. The order of the activity series is obtained for the stepped surfaces: Pt(111)<Pt(100)<Pt(S)-[n(111)×(100)]<Pt(S)-[n(111)×(100)]<Pt(S)-[n(111)×(111)]<Pt(110). The most active site in the stepped surface is derived from the psudo-4-fold bridged site in Pt(S)-[n(111)×(111)]. Kinked step surfaces show higher activity than stepped surfaces: the reduction rate per kink atom is more than twice as high as the value per step atom. Densely packed kink atoms along the step line greatly promote the reduction of CO2.

Adsorption of CO, intermediately formed in electrochemical reduction of CO2, at a copper electrode

Electroreduction of CO2 at Cu electrodes in aqueous electrolytes yields CH4 and C2H4, with adsorbed CO formed as an intermediate species. The state of adsorbed CO on a Cu electrode was studied with voltammetric and chronopotentiometric measurements. Adsorbed CO severely restricts hydrogen formation. During cathodic polarisation, coverage by adsorbed CO of the Cu electrode is ca. 90% in CO-saturated electrolytes. The adsorbed CO is easily desorbed by stirring the electrolyte vigorously as revealed by chronopotentiometric measurements. The chronopotentiometric measurements also confirmed that the surface of the copper electrode is fully covered with adsorbed CO during the electro-reduction of CO2, indicating that the rate-determining step for the reduction of CO2 to hydrocarbons must include a reaction which involves the adsorbed CO. The cross-sectional area of adsorbed CO was estimated to be 39 × 10–16 cm2, and the rate of desorption is also discussed using coulometric measurements reported previously.