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In the electrochemical reduction of CO2 on a Cu foil electrode in a neutral solution, the electrode suffers a decline in its catalytic activity for the reduction of CO2 to hydrocarbons. This is caused by the deposition of poisonous compounds such as graphitic carbon. To settle this problem, we have developed an electrolysis system in which CO2 is reduced at the three-phase (gas|liquid|solid) interface on a Cu-mesh electrode. In the present study, a Cu-mesh electrode is modified beforehand by copper(I) halides, and CO2 is reduced at a constant potential with these modified electrodes in an acidic solution of potassium halide. The Faradaic efficiency for C2H4 is considerably increased and reversely that for H2 is decreased by confining a copper(I) halide. In the case of CuBr, the conversion of CO2 to C2H4 and the hydrogen evolution are observed with Faradaic efficiencies of about 80% and 9%, respectively. Such a contribution of copper(I) halide to the CO2 reduction is related to its reversible combination with CO and C2H4. At the three-phase interface, CO2(g) is first reduced to CO(g). This gas is ready to adsorb to copper(I) halide with its π-bond perpendicular to the surface, and the CO is subjected to electron injection from the electrode to be reduced to the methylene radical. The coupling of methylene radicals results in the formation of C2H4 and this product is stabilized by adsorbing to copper(I) halide with its π-bond parallel to the surface.

Mn II –glycine complex in slightly alkaline solution is oxidized at a less positive potential (+0.25 to +0.6 V vs. Ag ∣ AgCl) where free Mn 2+ ion is not oxidizable, leading to the formation of a thin uniform film containing Mn III oxide. The film prepared by continuous cyclic scans in the potential region between +0.2 and +0.6 V exhibited an electron spin resonance (ESR) peak attributed to Mn II (H 2 O) 6 , but this resonance peak disappeared when the lowest potential was shifted to 0 V. From the electrochemical quartz crystal microbalance (EQCM) measurements, the film deposited anodically can be represented as Mn 2 III O 6 [Mn II (H 2 O) 6 ] 3 which corresponds to a structure where Mn II in Mn 5 O 6 (Mn 2 III Mn 3 II O 6 ) is not bound to the oxygen atom in the Mn III O network but is coordinated with water molecules. The Mn II (H 2 O) 6 in the film was suggested to be released during the reduction of the oxide matrix and oxidized to form the Mn III O bond at more positive potentials than +0.6 V.

In the long-term electrolysis of CO 2 with a metal electrode, it is very important to maintain the catalytic activity of the electrode. The Faradaic efficiency for the reduction of CO 2 on an Ag foil electrode is known to decrease rapidly several tens of minutes after the start of electrolysis, owing to the deposition of graphitic carbon, adsorbed organic intermediate or oxide/hydroxide acting as a catalytic poison. In order to settle this problem, the electrochemical reduction of CO 2 on a net Ag electrode was here performed at the three-phase (gas ∣ solid ∣ liquid) interface where CO 2 was supplied directly from the gas phase and a high concentration of CO 2 was sustainable in the course of electrolysis. The conversion of CO 2 (initial volume 219 cm 3 ) with this electrode (11.8 cm 2 ) was found to reach 60% at an electrolysis time of 5 h with the selective formation of CO. Furthermore, the addition of AgNO 3 to the electrolyte resulted in a 100% conversion of CO 2 . This was brought about by in situ formation of the silver crystal face providing a favorable reaction site for the CO 2 reduction, and the Faradaic efficiency of H 2 was kept below 8% during the electrolysis.

There are many studies on the electrochemical reduction of CO 2 on a Cu foil electrode. However, one of most serious problems in a practical use of this process is that this electrode loses its catalytic activity for the reduction of CO 2 to hydrocarbons owing to the deposition of poisoning species in a short time after the start of electrolysis. In the present study, the reduction of CO 2 has been performed at the three-phase (gas | liquid | solid) (GLS) interface on a Cu net electrode. In this electrode system, CO 2 is supplied to the electrode surface from the gas phase, and a high concentration of CO 2 can be maintained even in acidic solution. The poisoning process was extremely suppressed at the GLS-Cu electrode in a long-term electrolysis, and the main products obtained with this electrode were CO, C 2 H 4 and CH 4 .

A new electrochemical preparation of a homogeneous film of cupric oxide has been proposed, which is achieved on an electrode by polarizing to a more positive potential than 0.5 V versus Ag ∣ AgCl in an alkaline solution containing Cu 2+ and glycine. The Cu(II)–glycine complex is first electrooxidized to CuOH 2+ and then this ion is reduced to copper oxide by the redox reaction with the oxidation of water. The constituent of the copper oxide was determined to be CuO from the EPMA and XRD results. FE-SEM observation of this oxide indicates that the leaf-shaped crystals spread vertically on the electrode surface. The redox behavior of a CuO-deposited ITO electrode in a solution with Fe(CN) 6 4− was quite similar to that on a naked ITO electrode, suggesting a relatively high conductive nature of the cupric oxide prepared from solution.

Ion transport during the redox switching of a Prussian blue (PB) | polyaniline (PAn) bilayer electrode has been investigated by means of an electrochemical quartz crystal microbalance (EQCM) and in situ Fourier transform infrared (FTIR) reflection spectroscopy. The movement of ions in the potential cycling of the bilayer electrode is affected considerably by the thickness ratio of PB to PAn and the electrode potential. It is at the bilayer electrode with the medium thickness ratio (2.5–3.25) of PB (165 nm) to PAn that cation (K+) and anion (Cl−) move simultaneously in opposite directions. On the positive scan of such an electrode, K+ ions are first ejected from PB to the PAn layer into which Cl− ions are concurrently taken from solution to maintain the charge balance. The oxidation of PAn at more positive potential leads to the expulsion of K+ ions to solution and the simultaneous incorporation of Cl− ions.

Electrochemical oxidation of diglycine (G 2 ), triglycine (G 3 ) and tetraglycine (G 4 ) has been studied in an alkaline medium by means of in situ FTIR spectroscopic and electrochemical quartz crystal microbalance (EQCM) methods. The adsorption of the terminal carboxyl group of these peptides occurred around +0.3 V versus Ag ∣ AgCl, and the oxidative decomposition was observed from ca. +1.0 V. A striking difference in the oxidation of amino acid and peptides was that the former reaction continued to proceed with increasing potential but the oxidation of peptides was rather restrained at high positive potentials. The major reason for such constraint was that the glycine or glycil–glycine generated in the anodic oxidation of peptides was in the acid form and its adsorption and following oxidation on the electrode became considerably difficult. Formation of the acid form of glycine or glycil–glycine was attributed to the lowering of pH which occurred because oxygen evolution occurs in further positive polarization. The EQCM data indicated that the adsorption of glycil-peptide occurred from ca. +0.2 V, the adsorbed carboxyl group was decomposed oxidatively at ca. +1.1 V and a part of the remaining adsorbents was detached simultaneously from the electrode with the evolution of oxygen in further positive polarization.

The redox reaction of freshly prepared and well cycled Prussian blue (PB) films has been studied by means of an electrochemical quartz crystal microbalance (EQCM) and in situ Fourier transform infrared (FTIR) reflection spectroscopy. In the first reduction, insoluble PB (Fe 3+ 4 [Fe II (CN) 6 ] 3 ·6H 2 O) was found to be converted irreversibly to Everitt’s salt (ES, K 2 Fe 2+ [Fe II (CN) 6 ]) by incorporating 6 mol of potassium ions, and 6 mol of water molecules coordinated to iron ions are released simultaneously. The change in molar mass estimated from this reaction was the gain of 17.7 g mol −1 per mol electrons, which was close to the observed value. The reoxidation of ES results in the formation of soluble PB (KFe 3+ [Fe II (CN) 6 ]) and the concurrent release of potassium ions, and water molecules are introduced to interstitial positions in the PB lattice. The reversible change between soluble PB and ES takes place in further repeated potential cycling.

The electrochemical oxidation of cyclic amino acids such as histidine and tyrosine has been studied in a strongly basic solution by means of an in-situ FTIR spectroscopic method. The terminal COO – group of fully unprotonated anions was adsorbed on a platinum electrode from +0.4 V versus Ag ∣ AgCl ∣ sat KCl, and the concentration of the adsorbent increased with increasing potential. However, the adsorption strength of the terminal carboxyl was weakened at +1.0 or +1.1 V for histidine or tyrosine, respectively. In the oxidation of tyrosine, the benzene ring started to be oriented in parallel to the electrode surface at +0.3 V and the flat orientation reached completion at +0.6 V. Histidine and tyrosine were both oxidized from +1.1 V in a NaOH solution of pH 13.

The ion-transport properties of the tungsten trioxide (WO 3 )-, WO 3 ∣ polyaniline (PAn)-, and WO 3 ∣ PAn ∣ polyvinylsulfate (PVS)-confined electrodes have been studied by means of the in situ Fourier transform infrared (FTIR) spectroscopic method. In highly acidic solution (pH=0), W VI O 3 was reduced to H x W V O 3 by the reaction with H 3 O + and the generated H 2 O molecules were incorporated in the film, but in solutions of pH≧2, the reduction of W VI O 3 was attained by the reaction with H 2 O and the concentration of H 2 O decreased in the film. The charge-compensating reaction for the reduction of WO 3 ∣ PAn films in solutions of pH≧2 depended on the electrode potential. In the potential range between +0.2 and −0.2 V versus Ag ∣ AgCl, the reduction of the positively charged PAn was achieved by expelling anions without the accompanying injection of cations into WO 3 . In the potential range between −0.3 and −0.8 V, however, WO 3 and the charged PAn were simultaneously reduced by protons. PVS remained doped at the WO 3 ∣ PAn ∣ PVS-confined electrode during potential cycling, and the charge-compensation was accomplished by involving cations.

The electrochemical oxidation of glycine and lysine in strongly alkaline solution has been studied, and the nature of the adsorbed species disclosed by means of an in situ FTIR spectroscopic method. The oxidation of water was strongly suppressed by the adsorption of the amino acids and the subsequent oxidative decomposition. The adsorption of amino acid onto the electrode was achieved through the terminal COO− group of fully unprotonated anions from +0.4 V vs. Ag|AgCl, and the concentration of the adsorbent attained the maximum at +0.9 V. Beyond this potential, the adsorption strength was weakened, and the oxidative decomposition to CO2, NH3, acids, etc., occurred from +1.1 V.

Potential oscillations of a stainless steel electrode anodized galvanostatically have been observed in a mixture of chromic and sulfuric acids. The potential oscillated regularly between 1.6 V vs. SCE (lower) and 2.1 V (upper) with a certain frequency. The potential values were affected neither by the composition of chromic and sulfuric acids nor the magnitude of applied current density although the potential frequency was very sensitive to these parameters. This phenomenon is interpreted in terms of the repeated formation and disappearance of the saturated layer of dichromate ions on the electrode surface. Such a scheme is consistent with the in situ FTIR spectroscopic result that the concentration of Cr2O72− ions on the electrode surface increased with time during the polarization at the lower potential but the change in concentration of Cr2O72− ions was negligible at the higher potential.

Various Fe(II) complexes have been incorporated into Prussian blue (PB)|polyaniline (PAn)-modified electrodes, and their spectroelectrochemical properties been investigated using in situ and ex situ FTIR methods. It is shown that large anionic complexes once incorporated in the PAn matrix are not dedoped during the potential cycling and the charge balance is maintained by dedoping or incorporating electrolyte cations. This electrode system was applied to the electrocatalytic reduction of CO 2 in aqueous solution, and the reduction products were identified by taking in situ FTIR spectra during the anodic stripping. At potentials higher than 0 V, the IR bands associated with the loss of carboxylic acid at 1362 cm −1 and the gain of CO 2 at 2343 cm −1 were simultaneously observed, indicating that the CO 2 was derived from the reoxidation of carboxylic acid. It is therefore confirmed that CO 2 can be reduced to organic species including carboxylic acid on the PB|PAn-modified electrode with anionic Fe(II) complexes in aqueous solution, with an indication that the existence of the anionic metal complex is essential to such mediated reduction of CO 2 .

The electrochemical properties of the Pt electrodes modified with Prussian blue (PB), polyaniline (PAn) and inner PB|outer PAn films have been examined using in situ Fourier transform infrared spectroscopy. At the PB-modified electrode, two strong peaks were observed at 2040 and 2110cm−1 owing to the bridging CN ligand in the PB structure. Three absorption bands appeared at 2780, 1015, and 982 cm−1 by stepping the potential to more positive than +0.2V vs. Ag|AgCl|l M KCl, which were assigned to various vibrational modes of water molecules coordinated to the high-spin Fe3+ ion of PB. It is suggested that doping or undoping of potassium ions during the polarization of the Pt|PB electrode is accompanied by expulsion or introduction of water molecules respectively. At the PB|PAn-modified electrode, the absorption spectrum of PAn in the vibrational region was searcely affected by the underlying PB. The spectral intensity of the PAn on Pt|PB was much larger than that of the PAn on Pt, which is brought about by the K+ ions being more effective as the charge-compensating ions in the PB|PAn film.

The poly(vinyl chloride) (PVC) in laminated PVC + polypyrrol (PPy) films has been converted to an electrically conducting structure by photodehydrochlorination and subsequent electrochemical doping. The maximum conductivity (6 × 10 −1 S cm −1 ) of the PVC film was obtained by potentiostatic oxidation at 0.3 V vs. SCE in HCl solution. The reflection spectra of the PVC film showed a new band around 600 to 800 nm after the electrochemical doping, indicating the formation of a charge-transfer complex. The maximum conductivity of the PVC film was reached with the highest concentration of the charge transfer complex, polyene + … X − , formed between the photogenerated polyenes and dopant anions.