Electrochemical reduction of carbon dioxide to ethylene with high Faradaic efficiency at a Cu electrode in CsOH/methanol

The electrochemical reduction of CO₂ with a Cu electrode in CsOH/methanol-based electrolyte was investigated. The main products from CO₂ were methane, ethylene, ethane, carbon monoxide and formic acid. A maximum Faradaic efficiency of ethylene was 32.3% at −3.5 V vs. Ag/AgCl saturated KCl. The best methane formation efficiency was 8.3% at −4.0 V. The ethylene/methane current efficiency ratio was in the range 2.9–7.9. In the CsOH/methanol, the efficiency of hydrogen formation, being a competitive reaction against CO₂ reduction, was depressed to below 23%.     

Pulse-mode electrochemical reduction of carbon dioxide using copper and copper oxide electrodes for selective ethylene formation

Although the electrochemical reduction of CO₂ at a copper electrode produces hydrocarbons, the activity for the conversion of CO₂ is significantly reduced after several tens of minutes by the deposition of poisoning species on the electrode. In order to solve the poisoning species problem, the electrochemical reduction of CO₂ was carried out using a copper electrode in the pulse electrolysis mode by anodic as well as cathodic polarization. The anodic polarization intervals suppressed the deposition of the poisoning species on the electrode, and the amount of two hydrocarbons produced, CH₄ and C₂H₆, decreased only slightly even after one hour. By choosing the appropriate anodic potential and time duration the selectivity for C₂H₆ formation was significantly enhanced. The enhancement was found to be due to the copper oxide formed on the copper electrode. The selectivity was further improved when the electrochemical reduction was carried out using a copper oxide electrode. The highest efficiency of about 28% was obtained at −3.15 V.     

Electrochemical reduction of high pressure CO2 at a Cu electrode in cold methanol

The electrochemical reduction of high pressure CO₂ with a Cu electrode in cold methanol was investigated. A high pressure stainless steel vessel, with a divided H-type glass cell, was employed. The main products from CO₂ by the electrochemical reduction were methane, ethylene, carbon monoxide and formic acid. In the electrolysis of high pressure CO₂ at low temperature, the reduction products were formed in the order of carbon monoxide, methane, formic acid and ethylene. The best current efficiency of methane was of 20% at −3.0 V. The maximum partial current density for CO₂ reduction was approximately 15 mA cm⁻². The partial current density ratio of CO₂ reduction and hydrogen evolution, i(CO₂)/i(H₂), was more than 2.6 at potentials more positive than −3.0 V. This work can contribute to the large-scale manufacturing of fuel gases from readily available and inexpensive raw materials, CO₂-saturated methanol from industrial absorbers (the Rectisol process).     

Silver-coated ion exchange membrane electrode applied to electrochemical reduction of carbon dioxide

Silver-coated ion exchange membrane electrodes (solid polymer electrolyte, SPE) were prepared by electroless deposition of silver onto ion exchange membranes. The SPE electrodes were used for carbon dioxide (CO₂) reduction with 0.2 M K₂SO₄ as the electrolyte with a platinum plate (Pt) for the counterelectrode. In an SPE electrode system prepared from a cation exchange membrane (CEM), the surface of the SPE was partly ruptured during CO₂ reduction, and the reaction was rapidly suppressed. SPE electrodes made of an anion exchange membrane (SPE/AEM) sustained reduction of CO₂ to CO for more than 2 h, whereas, the electrode potential shifted negatively during the electrolysis. The reaction is controlled by the diffusion of CO₂ through the metal layer of the SPE electrode at high current density. Ultrasonic radiation, applied to the preparation of SPE/AEM, was effective to improve the electrode properties, enhancing the electrolysis current of CO₂ reduction. Observation by a scanning electron microscope (SEM) showed that the electrode metal layer became more porous by the ultrasonic radiation treatment. The partial current density of CO₂ reduction by SPE/AEM amounted to 60 mA cm⁻², i.e. three times the upper limit of the conventional electrolysis by a plate electrode. Application of SPE device may contribute to an advancement of CO₂ fixation at ambient temperature and pressure.     

Electrochemical conversion of carbon dioxide to methane in aqueous NaHCO3 solution at less than 273 K

The electrochemical reduction of CO₂ on a Cu electrode was investigated in aqueous NaHCO₃ solution, at low temperature. A divided H-type cell was employed, the catholyte was 0.65 mol dm⁻³ NaHCO₃ aqueous solution and the anolyte was 1.1 mol dm⁻³ KHCO₃ aqueous solution. The temperature during the electrolysis of CO₂ was decreased stepwise to 271 K. Methane and formic acid were obtained as the main products. The maximum Faradaic efficiency of methane was 46% at −2.0 V and 271 K. The efficiency of hydrogen formation, a competing reaction of CO₂ reduction, was significantly depressed with decreasing temperature. Based on the results of this work, the proposed electrochemical method appears to be a viable means for removing CO₂ from the atmosphere and converting it into more valuable chemicals. The synthesis of methane by the electrochemical method might be of practical interest for fuel production and the storage of solar energy.     

“Deactivation of copper electrode” in electrochemical reduction of CO2

Metallic Cu electrode can electrochemically reduce CO₂ to CH₄, C₂H₄ and alcohols with high yields as revealed by the present authors. Many workers reported that formation of CH₄ and C₂H₄ rapidly diminishes during electrolysis of CO₂ reduction. This paper shows that such deactivation of Cu electrode is reproduced with electrolyte solutions prepared from reagents used by these workers. Deactivated Cu electrodes recovered the electrocatalytic activity for CO₂ reduction by anodic polarization at −0.05 V versus she in agreement with the previous reports. Features of the deactivation depend greatly on the individual chemical reagents. Purification of the electrolyte solution by preelectrolysis with a Pt black electrode effectively prevents the deactivation of Cu electrode. Anode stripping voltammetry of Cu electrodes, which were deactivated during electrolysis of CO₂ reduction, showed anodic oxidation peaks at ca. −0.1 or −0.56 V versus she. The severer the deactivation of the Cu electrode was, the higher electric charge of the anodic peak was observed. It is presumed that some impurity heavy metal, originally contained in the electrolyte, is deposited on the Cu electrode during the CO₂ reduction, poisoning the electrocatalytic activity. On the basis of the potential of the anodic peaks, Fe²⁺ and Zn²⁺ are assumed to be the major contaminants, which cause the deactivation of the Cu electrode. Deliberate addition of Fe²⁺ or Zn²⁺ to the electrolyte solutions purified by preelectrolysis exactly reproduced the deactivation of a Cu electrode in CO₂ reduction. The amount of the deposited Fe or Zn on the electrode was below the monolayer coverage. Electrothermal atomic absorption spectrometry (etaas) showed that Fe originally contained in the electrolyte solution is effectively removed by the preelectrolysis of the solution. Mechanistic difference is discussed between Fe and Zn in the deterioration of the electrocatalytic property of Cu electrode in the CO₂ reduction. The concentration of the impurity substances originally contained in the chemical reagents as Fe or Zn is estimated to be far below the standard of the impurity levels guaranteed by the manufacturers. Presence of trimethylamine in the electrolyte solution also severely poisons a Cu electrode in the CO₂ reduction. It was concluded that the deactivation of Cu electrode in CO₂ reduction is not caused by adsorption of the products or the intermediates produced in CO₂ reduction.     

Electro-reduction of carbon dioxide to formate on lead electrode in aqueous medium

The electrochemical reduction of carbon dioxide on a lead electrode was studied in aqueous medium. Preliminary investigations carried out by cyclic voltammetry were used to determine the optimized conditions of electrolysis. They revealed that the CO₂ reduction process was enhanced at a pH value of 8.6 for the cathodic solution i.e. when the predominant form of CO₂ was hydrogenocarbonate ion. Long-term electrolysis was carried out using both potentiometry and amperometry methods in a filter-press cell in which the two compartments were separated by a cation-exchange membrane (Nafion^® 423). Formate was detected and quantified by chromatography as the exclusive organic compound produced with a high Faradaic yield (from 65% to 90%). This study also revealed that the operating temperature played a key role in the hydrogenation reaction of carbon dioxide into formate in aqueous medium.     

Electrochemical behavior of cysteine at a CuGeO3 nanowires modified glassy carbon electrode

A CuGeO₃ nanowire modified glassy carbon electrode was fabricated and characterized by scanning electron microscopy. The results of electrochemical impedance spectroscopy reveal that electron transfer through nanowire film is facile compared with that of bare glassy carbon electrode. The modified electrode exhibited a novel electrocatalytic behavior to the electrochemical reactions of l-cysteine in neutral solution, which was not reported previously. Two pairs of semi-reversible electrochemical peaks were observed and assigned to the processes of oxidation/reduction and adsorption/desorption of cysteine at the modified electrode, respectively. The electrochemical response of cysteine is poor in alkaline condition and is enhanced greatly in acidic solution, suggesting that hydrogen ions participate in the electrochemical oxidation process of cysteine. The intensities of two anodic peaks varied linearly with the concentration of cysteine in the range of 1 × 10⁻⁶ to 1 × 10⁻³ mol L⁻¹, which make it possible to sensitive detection of cysteine with the CuGeO₃ nanowire modified electrode. Furthermore, the modified electrode exhibited good reproducibility and stability.     

Electrochemical impedance spectroscopic investigation of CO2 reduction on polyaniline in methanol

The ac response of polyaniline thin films on platinum electrodes was measured at different dc potentials during the CO₂ reduction in methanol/LiClO₄ electrolyte with a small amount of 0.5 M H₂SO₄. The complex capacitance curves were simulated and the data obtained were used to calculate kinetic parameters, based on the assumption that the thermodynamic potential E₀ is in the region of −0.2-−0.1 V versus saturated calomel electrode (SCE). With E₀=−0.2 V versus SCE and β=0.6, a j₀ value of ca. 10⁻⁴ A cm⁻² was found for the electroreduction of CO₂ on the polyaniline electrode.     

Electrochemical characterizations on MnO2 supercapacitors with potassium polyacrylate and potassium polyacrylate-co-polyacrylamide gel polymer electrolytes

MnO₂·nH₂O supercapacitors with potassium polyacrylate (PAAK) and potassium polyacrylate-co-polyacrylamide (PAAK-co-PAAM) gel polymer electrolytes (GPEs) having the weight compositions of polymer:KCl:H₂O = 9%:6.7%:84.3% have been characterized for their electrochemical performance. Compared with the liquid electrolyte (LE) counterpart, the GPE cells exhibit remarkable (50–130%) enhancement in specific capacitance of the oxide electrode, and the extent of the enhancement increases with increasing amount of the carboxylate groups in the polymers as well as with increasing oxide/electrolyte interfacial area. In situ X-ray absorption near-edge structure (XANES) analysis indicates that the oxide electrodes of the GPE cells possess higher Mn-ion valences and are subjected to greater extent of valence variation than that of the LE cell upon charging/discharging over the same potential range. Copolymerization of PAAK with PAAM greatly improves the cycling stability of the MnO₂·nH₂O electrode, and the improvement is attributable to the alkaline nature of the amino groups. Both GPEs exhibit ionic conductivities greater than 1.0 × 10⁻¹ S cm⁻¹ and are promising for high-rate applications.     

925银电极上同步电化学还原NO2-与CO2合成尿素

引言煤炭、石油等化石燃料燃烧的同时会产生大量氮氧化物(NOx)和二氧化碳(CO2),导致地球的生态系统遭受严重破坏[1]。NOx是导致酸雨、光化学烟雾的主要污染物,而CO2是主要的温室气体。目前国内外使用比较广泛的NOx废气处理方法可分为干法和湿法两大类,其中干法可以分为选择性催     

Electrochemical reduction of carbon dioxide in methanol at ambient temperature and pressure

The electrochemical reduction of carbon dioxide was studied in methanol-based supporting electrolytes on various metal electrodes at ambient temperature and pressure. The ionophore of the catholyte was benzalkonium chloride, [RN(CH3)2CH2C6H5]⁺Cl^–, where R=C8–C18, the chain length being distributed around C14. A divided H-type cell was used, the supporting electrolytes were 10–2moldm–3 benzalkonium chloride in double distilled methanol (catholyte) and a 10–1moldm–3 aqueous KHCO3 solution (anolyte). Nine different, high purity (>99.5%) metal electrodes were used: Ti, Fe, Co, Ni, Pt, Ag, Au, Zn and Sn. Carbon monoxide, methane and ethane were the main organic products. Silver, Au, Zn and Sn cathodes allowed for the best faradaic yields of CO production, the maximum amount of CO (71%, 185 mmol) being formed on the Ag electrode. Methane evolved on each of the nine tested electrodes, with current yields in the range from 0.2 to 3.0%. Ethane and ethylene were produced on the nickel electrode, with low faradaic efficiencies, 0.5 and 0.3%, respectively. No dimerization products were detected. This research can contribute to large-scale manufacturing of useful organic products from a readily available and cheap raw material: CO2-saturated methanol from industrial absorbers (the Rectisol process).     

Review on photocatalytic conversion of carbon dioxide to value-added compounds and renewable fuels by graphitic carbon nitride-based photocatalysts

Photocatalytic reduction of CO₂ is known as one of the most promising methods to produce valuable fuels and value-added compounds. To overcome selectivity and efficiency downsides, various photocatalysts have been designed and developed. This review discusses the state-of-the-art in photo-conversion of CO₂ over graphitic carbon nitride (g-C₃N₄)-based composites. The modification strategies to improve photocatalytic activity of g-C₃N₄ were classified into different categories and discussed as structural modifications, elemental doping, copolymerization, fabricating heterojunctions between g-C₃N₄ and other semiconductors, Z-scheme heterojunctions, noble metal/g-C₃N₄ photocatalysts, and design of ternary nanocomposites based on g-C₃N₄. Finally, perspectives and future research works in this field were also outlined.     

Synthesis of PCL/clay masterbatches in supercritical carbon dioxide

Pre-exfoliated nanoclays were prepared through a masterbatch process using supercritical carbon dioxide as solvent and poly(?-caprolactone) as organic matrix. In situ polymerization of ?-caprolactone in the presence of large amount of clay was conducted to obtain these easily dispersible nanoclays, collected as a dry and fine powder after reaction. Dispersion of these pre-exfoliated nanoclays in chlorinated polyethylene was also investigated. All the results confirm the specific advantages of supercritical CO₂ towards conventional solvents for filler modification.     

Electrochemical window of molten LiCl-KCl-CaCl2 and the Ag/Ag reference electrode

The electrochemical window of an LiCl-KCl-CaCl₂ eutectic melt (52.3:11.6:36.1 mol%) was determined by cyclic voltammetry and open-circuit potentiometry at 723-873 K. The reaction at the anodic limit was confirmed to be Cl₂ gas evolution. The reaction at the cathodic limit was found to be a liquid Ca-Li alloy formation on the basis of ICP analysis of the deposits. An Ag⁺/Ag reference electrode separated with a Pyrex membrane showed good stability for more than 1 week. The standard electrode potential of Ag⁺/Ag was determined in the temperature range of 723-823 K by measuring the potential of a silver electrode in different concentrations of Ag⁺ ions.     

Interfacial tension of linear and branched PP in supercritical carbon dioxide

In this study, sessile drops are imaged in a high-pressure and high-temperature view chamber to determine the density and interfacial tension of linear polypropylene (LPP) and branched polypropylene (BPP) melts in supercritical carbon dioxide (CO₂). The pressure-volume-temperature (PVT) data of polyprophylene (PP)-CO₂ is investigated by monitoring the swelling changes of the polymer melt in supercritical CO₂. The density difference between the polymer/CO₂ mixture and the CO₂ is determined by combining the swelling results with the CO₂ solubility information in the polymer melt. Both the Sanchez-Lacombe (SL) and the Simha-Somcynsky (SS) equations-of-state (EOS) are applied to predict the density of the PP-CO₂ mixture, which is then compared to the density data obtained experimentally. The dependence of interfacial tension on the temperature and pressure of PP in supercritical CO₂ is investigated at temperatures from 180 °C to 220 °C and pressures up to 31 MPa. Effects of long-chain branching on the density and interfacial tension of PP-CO₂ mixtures are discussed.     

Methanol synthesis from carbon dioxide at atmospheric pressure over Cu/ZnO catalyst. Role of methoxide species formed on ZnO support

Methanol synthesis from CO₂ and H₂ was carried out over a Cu/ZnO catalyst (Cu/Zn = 3/7) at atmospheric pressure, and the surface species formed were analyzed by diffuse reflectance FT-IR spectroscopy and temperature programmed desorption method. Two types of formate species and zinc methoxide were formed in the course of the reaction. Zinc methoxide was readily hydrolyzed to methanol. H₂O formed through the reverse water gas shift reaction was suggested to be involved in the hydrolysis of zinc methoxide.     

Investigation on electrochemical reduction process of Nb2O5 powder in molten CaCl2 with metallic cavity electrode

The direct electrochemical reduction process of Nb₂O₅ powder was investigated by cyclic voltammetry and constant potential electrolysis with a novel metallic cavity electrode in molten calcium chloride at 850 °C. The products of both constant potential and constant voltage electrolysis were characterized by XRD, SEM and EDX. CaNb₂O₆ was formed upon addition of solid Nb₂O₅ into molten CaCl₂ when CaO was present. During the electrolysis solid Nb₂O₅ was reduced to various niobium oxides of lower oxidation states, including some composite oxides, and then was converted completely to metallic niobium near −0.35 V (vs. Ag/AgCl), which was more positive than the reduction potential of Ca²⁺. Constant potential electrolysis was applied at the potentials near the reduction current peaks derived from the cyclic voltammetry curves, and cell voltages were monitored. The voltage was near 2.4 V when the oxide was metallized at −0.35 V (vs. Ag/AgCl). Nb₂O₅ pellet could be used to prepared metallic niobium at cell voltage 2.4 V in a larger electrolysis bath filled with calcium chloride at 850 °C. The experiment results further demonstrated the direct electrochemical reduction mechanism of Nb₂O₅ powder in a molten system.     

Extraction of nimbin from neem seeds using supercritical CO2 and a supercritical CO2–methanol mixture

Nimbin, a component found in neem seeds, which is reported to have several valuable medicinal properties including: anti-inflammatory, anti-pyretic, anti-fugal, antihistamine and antiseptic was extracted from neem seeds using supercritical CO₂ and CO₂ with a methanol modifier.Nimbin extraction yields using supercritical carbon dioxide were found to be approximately 85% at 308 K, 23 MPa and a CO₂ flow rate of 0.62 cm³/min for a 2-g sample of neem. An optimum extraction pressure appears to exist at ≈23 MPa and 328 K. Although extraction using a methanol modifier did improve the extraction somewhat, methanol was not found to be an effective modifier for extracting nimbin.Dynamic extraction curves were predicted using three empirical models and a theoretical model. The three empirical models were: a Langmuir gas adsorption model, a first order plus dead time (FOPDT) model and a so-called tⁿ cyclone model used to incorporate sigmoidal curves. The parameters in the empirical models were fitted to the experimental data. The Goto et al. [J. Chem. Eng. Jpn. 31 (1998) 171] theoretical model was compared to the experimental results and was found to fit the data well. The theoretical model shows that the extraction yield depends strongly on the solvent flow rate, that is, external mass transfer or equilibrium is the controlling step of this process.