Electrochemical behavior of CO2 reduction on palladium nanoparticles: Dependence of adsorbed CO on electrode potential

The electrochemical reduction of CO₂ is strongly influenced by both the applied potential and the surface adsorption status of the catalyst. In this work a gas diffusion electrode (GDE) coated with Pd nanoparticles/carbon black (Pd/XC72) was used to study the electrochemical reduction of CO₂. Cyclic voltammetric (CV) analysis of Pd/XC72 between 1.5 V and − 0.6 V (vs. RHE) shows the formation of intermediates and the blocking of hydrogen absorption on the Pd nanoparticles (NPs) under a CO₂ atmosphere. The relationships between the Faradaic efficiency/current density and the applied potential reveal that the onset potential of CO formation is around − 0.4 V. Moreover, the presence of adsorbed CO was confirmed through CV analysis of Pd/XC72 under CO₂ and CO/He atmospheres. This demonstrates that H atoms and CO intermediates co-adsorb on the surface of the Pd NPs at an applied potential of around − 0.4 V. When the applied potential is more negative than − 0.6 V, adsorption of CO intermediates on the surface of the Pd NPs becomes dominant.     

The electrochemical reduction of carbon dioxide (CO2) to methanol in the presence of pyridoxine (vitamin B6)

Experiments aimed at ameliorating carbon dioxide (CO₂) into methanol were explored using pyridoxine, a member of the vitamin B₆ family, to enhance the reduction process. At a platinum electrode, an aqueous solution (pH ≈ 5) of pyridoxine showed a quasi-reversible redox couple with the cathodic peak detected at ca. − 0.55 V vs. Ag/AgCl (3 M KCl) in the presence of CO₂ and argon. An increase in the corresponding cathodic peak current was observed following saturation of the solution with CO₂ using a Pt electrode, but with no detectable reduction current recorded at a glassy carbon electrode for the same system. Confirmation of methanol formation during the pyridoxine-assisted CO₂ reduction was conducted by using gas chromatography analysis of the electrolyzed solutions and faradic yields of ca. 5% were afforded. A combination of the results from the cyclic voltammetry and constant current chronopotentiometry experiments revealed an overpotential of ≤ 200 mV was required. The results indicate a potential utility of pyridoxine as an alternative reagent to the more toxic pyridine during the electrochemical reduction of CO₂.     

Electrochemical and FTIR spectroscopic study of CO2 reduction at a nanostructured Cu/reduced graphene oxide thin film

Here we report a facile approach to synthesize a novel nanostructured thin film comprising Cu nanoparticles (NPs) and reduced graphene oxide (rGO) on a glassy carbon electrode (GCE) via the direct electrochemical reduction of a mixture of cupper and graphene oxide (GO) precursors. The effect of the applied potential on the electrochemical reduction of CO₂ was investigated using linear sweep voltammetric (LSV) and chronoamperometric (CA) techniques. Carbon monoxide and formate were found as the main products based on our GC and HPLC analysis. The electrochemical reduction of CO₂ at the Cu/rGO thin film was further studied using in situ ATR-FTIR spectroscopy to identify the liquid product formed at different applied cathodic potentials. Our experimental measurements have shown that the nanostructured Cu/rGO thin film exhibits an excellent stability and superb catalytic activity for the electrochemical reduction of CO₂ in an aqueous solution with a high current efficiency of 69.4% at − 0.6 V vs. RHE, promising for the efficient electrochemical conversion of CO₂ to valuable products.     

In situ FT-IR and TPD-MS study of carbon monoxide oxidation over a CeO2/Co3O4 catalyst

The forming of surface species during the adsorption of carbon monoxide (CO) and CO/O₂ on a CeO₂/Co₃O₄ catalyst was investigated by in situ Fourier transform infrared (FT-IR) spectroscopy and temperature programmed desorption-mass spectroscopy (TPD-MS). When CO was adsorbed on the CeO₂/Co₃O₄ catalyst, two types of surface species were distinguishable at room temperature: carbonate and bicarbonate. Surface carbonate was adsorbed on the cerium and cobalt, while the surface bicarbonate absorbed on the CeO₂/Co₃O₄ catalyst at 1611, 1391, 1216 and 830 cm⁻¹. Furthermore, the TPD-MS profiles revealed that the CeO₂/Co₃O₄ catalyst showed a greater amount of CO₂ than CO at 373 K. The CO desorption from the CeO₂/Co₃O₄ catalyst with increasing temperature showed that the order of thermal stability was surface bicarbonate < surface carbonate < interface carbonate species. Interestingly, the residual carbonate species could remain on the interface up to 473 K. The results revealed that surface bicarbonate could promote the conversion of CO into CO₂ for CO oxidation below 50 K.     

Bioelectrocatalytic formate oxidation and carbon dioxide reduction at high current density and low overpotential with tungsten-containing formate dehydrogenase and mediators

We show a great possibility of mediated enzymatic bioelectrocatalysis in the formate oxidation and the carbon dioxide (CO₂) reduction at high current densities and low overpotentials. Tungsten-containing formate dehydrogenase (FoDH1) from Methylobacterium extorquens AM1 was used as a catalyst and immobilized on a Ketjen Black-modified electrode. For the formate oxidation, a high limiting current density (j_lim) of ca. 24 mA cm^− 2 was realized with a half wave potential (E_1/2) of only 0.12 V more positive than the formal potential of the formate/CO₂ couple (E°′_CO2) at 30 °C in the presence of methyl viologen (MV^2 +) as a mediator, and j_lim reached ca. 145 mA cm^− 2 at 60 °C. Even when a viologen-functionalized polymer was co-immobilized with FoDH1 on the porous electrode, j_lim of ca. 30 mA cm^− 2 was attained at 60 °C with E_1/2 = E°′_CO2 + 0.13 V. On the other hand, the CO₂ reduction was also realized with j_lim ≈ 15 mA cm^− 2 and E_1/2 = E°′_CO2 − 0.04 V at pH 6.6 and 60 °C in the presence of MV^2 +.     

A membrane electrode assembly for the electrochemical synthesis of hydrocarbons from CO2(g) and H2O(g)

We report the electrochemical reduction of CO₂ into hydrocarbons using a new electrochemical membrane reactor holding a yet unreported membrane electrode assembly comprising a copper mesh cathode and a Ti felt coated with mixed metal oxide (MMO) catalyst anode separated by a proton conductive membrane. CO_2(g) was supplied to the cathodic reduction compartment, whilst humidified N₂ was supplied to the anodic oxidation compartment. The MMO anode produces protons transported across the proton exchange membrane and electrons transported via the external circuit to the copper cathode to reduce CO_2(g). Production rates of methane, propane, propene, iso-butane and n-butane were determined as a function of cell potential at temperatures between 30 and 70 °C and relative humidity between ca. 25% and 75%. Maximum methane concentration and the current efficiency for production of hydrocarbons were 3.29 ppm and 0.12%, respectively. Whilst the observed product spectrum is desirable, such low current efficiencies require systematic optimization of the catalytic membrane system, in particular an improved cathode with an optimum contact between proton conducting membrane, electrode and catalyst is desired.     

Fabrication and electrochemical properties of the Sn–Ni–P alloy rods array electrode for lithium-ion batteries

A new ternary Sn–Ni–P alloy rods array electrode for lithium-ion batteries is synthesized by electrodeposition with a Cu nanorods array structured foil as current collector. The Cu nanorods array foil is fabricated by heat treatment and electrochemical reduction of Cu(OH)₂ nanorods film, which is grown directly on Cu substrate through an oxidation method. The Sn–Ni–P alloy rods array electrode is mainly composed of pure Sn, Ni₃Sn₄ and Ni–P phases. The electrochemical experimental results illustrate that the Sn–Ni–P alloy rods array electrode has high reversible capacity and excellent coulombic efficiency, with an initial discharge capacity and charge capacity of 785.0 mAh g^?1 and 567.8 mAh g^?1, respectively. After the 100th discharge–charge cycling, capacity retention is 94.2% with a value of 534.8 mAh g^?1. The electrode also performs with an excellent rate capacity.     

Use of a carbon nanocage as a catalyst support in polymer electrolyte membrane fuel cells

The corrosion-resistance of a carbon nanocage used as a catalyst support in a polymer electrolyte membrane fuel cell was investigated by measuring CO₂ generation using on-line mass spectrometry at a constant potential of 1.4 V for 30 min. Polarization curves of membrane electrode assemblies containing Pt/carbon nanocage were obtained and used to evaluate performance degradation. The carbon nanocage was found to possess significant resistance to electrochemical corrosion, exhibiting low performance degradation of only about 2.3% after the corrosion test. This high corrosion resistance is attributed both to the strong hydrophobic nature of the surface and the graphitic structure of the carbon nanocage.     

Toward more efficient photochemical CO2 reduction: Use of scCO2 or photogenerated hydrides

Rhenium(I) and ruthenium(II) complexes have been successfully used for photochemical CO₂ reduction to CO or formate. However, a typical turnover frequency for such reactions is <20 h^?1 and the formation of reduced species beyond CO or formate is very limited. In the case of the rhenium(I) bipyridyl tricarbonyl system, the key intermediate has been shown to decay with a first-order dependence on [CO₂] to produce CO, which is the rate-determining step. The limited concentration of dissolved CO₂ in organic solvents results in extremely slow CO₂ reduction. To improve the reaction rate, we prepared new CO₂-soluble rhenium(I) bipyridine complexes bearing fluorinated alkyl ligands and investigated their photophysical properties in CH₃CN and supercritical CO₂. We also investigated the properties of a metal complex with an NAD⁺ model ligand, [Ru(bpy)₂(pbn)]²⁺ (bpy = 2,2′-bipyridine, pbn = 2-(2-pyridyl)-benzo[b]-1,5-naphthyridine), and prepared the corresponding NADH-like complex [Ru(bpy)₂(pbnHH)]²⁺ upon MLCT excitation followed by reductive quenching. This species can be used as a renewable hydride donor. The electrochemical and photochemical properties, and the reactivity of these species toward CO₂ reduction were investigated.     

Electrodeposition of monodispersed metal nanoparticles in a nafion film: Towards highly active nanocatalysts

We have explored a new and facile method for the fabrication of metal nanoparticles on the electrode surface. The approach for fabricating metal nanoparticles was carried out by two steps consisting of ion-exchange in nafion film coated on the electrode and subsequent reduction of metal ions to metallic nanoparticles by electrochemical method. The results of characterization by TEM show that metal nanoparticles were nearly monodispersed in the whole nafion film. The average diameters of Cu, Co and Ni nanoparticles were statistically measured to be 5.1 nm ± 0.2 nm, 4.6 nm ± 0.2 nm and 4.7 nm ± 0.2 nm, respectively. The amount of metal nanoparticles can be readily controlled by the amount of nafion coated on the electrode. By performing the H₂O₂ reduction at the obtained Cu nanoparticles, the high electrocatalytic activity of metal nanoparticles fabricated has been confirmed.     

Efficient bioelectrocatalytic CO2 reduction on gas-diffusion-type biocathode with tungsten-containing formate dehydrogenase

A new gas-diffusion-type biocathode was constructed for carbon dioxide (CO₂) reduction. In this work, tungsten-containing formate dehydrogenase (FoDH1), which is a promising enzyme for interconversion of formate and CO₂, was used as a catalyst and was absorbed on a Ketjen Black (KB)-modified electrode. We used 1,1′-trimethylene-2,2′-bipyridinium dibromide as a mediator, and the hydrophobicity of the FoDH1-absorbed electrode was optimized according to the weight ratio of the polytetrafluoroethylene binder to KB. We achieved cathodic current densities of about 20 mA cm^− 2 under mild and quiescent conditions (neutral pH, atmospheric pressure, and room temperature).     

Screening heterogeneous catalysts for the pyrolysis of lignin

The pyrolytic conversion of pure lignin at 600 °C in flowing helium over five catalysts is described and compared to the control bed material, sand. Product distribution as char, liquid, and gas are described as well as the composition of the liquid and gas fractions. The catalysts examined were HZSM-5, KZSM-5, Al-MCM-41, solid phosphoric acid, and a hydrotreating catalyst, (Co/Mo/Al₂O₃). The sand yielded a liquid phase that was 97% oxygenated aromatics and a gas phase that was CO (18 vol%), CO₂ (16 vol%), and CH₄ (12 vol%). HZSM-5 was the best catalyst for producing a deoxygenated liquid fraction yielded almost equal amounts of simple aromatics (46.7%) and naphthalenic ring compounds (46.2%). The gas phase over this catalyst consisted of CO (22 vol%), CO₂ (14 vol%), H₂ (12 vol%), and CH₄ (10 vol%). The Co/Mo/Al₂O₃ hydrotreating catalyst yielded a liquid consisting of 21% aromatics, 4% naphthalenics, and 75% oxygenated aromatics and a gas phase that was rich in hydrogen: H₂ (18 vol%), CO₂ (16 vol%), CO (12 vol%), and CH₄ (8 vol%).     

Electrochemical CO2 reduction with power generation in SOFCs with Cu-added LSCF–GDC cathode

Solid oxide fuel cell (SOFC) unit was constructed with Ni–GDC (gadolinia-doped ceria) as the anode, YSZ as the electrolyte, and Cu-added La_0.58Sr_0.4Co_0.2Fe_0.8O_3–δ–GDC as the cathode. Electrochemical CO₂ reduction occurs. The CO formation rate, the CO₂ conversion and the generated current density increase with increasing CO₂ concentration and temperature. The CO₂ conversion rate equals exactly the CO formation rate. No carbon deposition occurs. The activation energy is 2.72 kcal mol^?1. The electrochemical CO₂ reduction (dissociation) can have much lower activation barrier than the catalytic one. Simultaneous CO₂ reduction with power generation in SOFCs can be feasible.     

On-line mass spectrometry study of carbon corrosion in polymer electrolyte membrane fuel cells

The electrochemical corrosion of carbon catalyst supports in polymer electrolyte membrane (PEM) fuel cells is investigated by monitoring the generation of CO₂ using an on-line mass spectrometer at a constant potential of 1.4 V. Our results suggest that carbon supports with a high degree of graphitization are more corrosion-resistant, which results in a dramatic improvement of the catalyst durability. We also show that CO₂ measurements performed using on-line mass spectrometry represent a time-effective and reliable method for studying the electrochemical corrosion of carbon supports in PEM fuel cells.     

Ordered mesoporous silicon carbide-derived carbon for high-power supercapacitors

Micro/mesoporous carbon was prepared by chlorination of ordered mesoporous silicon carbide derived from magnesio-thermal reduction of templated carbon-silica precursors. These materials were then used as active materials for electrochemical capacitors and characterized in 1.5 M NEt₄BF₄/AN. The electrodes showed outstanding rate capability (90% of capacity retention at 1 V/s and time constant of 1 s) with high specific areal capacitance (0.5 F/cm² of electrode), that makes such hierarchical porous carbons promising for high power and energy density supercapacitors.     

Co-deposition of arsenic and arsine on Pt,Cu, and Fe electrodes

An electrochemical investigation of arsenic in aqueous solutions was carried out in order to assess the possibility of removing it by reduction from As(III) to its elemental form. Arsine evolution was significant at potentials below −0.650 V_SCE on Cu, and below −0.728 V_SCE on Pt. As(V) could also be removed on Cu, with a larger evolution of arsine. When a potential equal to −0.560 V_SCE was applied to an iron electrode, arsenic deposition took place simultaneously with iron dissolution, and arsine evolution was negligible.     

Electrochemical reduction of CO2 with clathrate hydrate electrolytes and copper foam electrodes

We report on the first use of clathrate hydrates as electrolyte additive in the electrochemical reduction of carbon dioxide. Clathrate hydrates allow the enrichment of significantly larger volumes of gas than liquids can usually dissolve. Electrolyte solutions containing 10%_mass THF with and without CO₂ containing clathrate hydrates were investigated with a copper-foam working electrode. Our results show that at − 1.0 V vs Ag/AgCl the Faradaic efficiency for the production of CO and further reduced carbonaceous products was 80% with clathrates vs 20% with non-clathrate electrolytes of identical chemical composition at nearly equal temperature.     

Modification of carbon electrode in ionic liquid through the reduction of phenyl diazonium salt. Electrochemical evidence in ionic liquid

Electrochemical reduction of the 4-nitrophenyl diazonium salt in ionic liquid media has been investigated at carbon electrode. The ionic liquid chosen for this study was 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, [EMIM][TFSI]. The cyclic voltammetry study demonstrated the possibility of the electrochemical grafting of the nitrophenyl groups onto carbon electrode after the reduction of its corresponding diazonium in ionic liquid. The electrochemical characterization of the modified electrode achieved on ionic liquid displays the presence of the nitrophenyl group at the carbon surface. Moreover, the surface concentration of the attached group obtained in this media was found to be around 1.7 × 10⁻¹⁰ mol cm⁻², this value may suggest the possibility of the formation of monolayer. Furthermore, the characterization of the modified electrode in [EMIM][TFSI] showed the conversion of some NO₂-phenyl groups to NHOH-phenyl. This observation could indicate the presence of surface interaction between the reduced NO₂-phenyl and the ionic liquid cation, thanks to the presence of acidic proton in the ionic liquid cation.     

Highly efficient In–Sn alloy catalysts for electrochemical reduction of CO2 to formate

Gas diffusion electrodes (GDEs), including GDE-In_0.90Sn_0.10, GDE-In_0.47Sn_0.53 and GDE-In_0.22Sn_0.78, were prepared by electrodeposition of In–Sn alloys on carbon fiber paper, and then used to explore the electroreduction of CO₂ to formate in aqueous solution. Compared with commercial indium or Sn foil catalysts, the GDE-In_0.90Sn_0.10 electrode in particular is shown to have excellent catalytic performance towards electroreduction of CO₂ to formate, with a high Faradaic efficiency (~ 92%). More importantly, the catalytic activity of GDE-In_0.90Sn_0.10 remained reasonably stable over a 22-hour period of electrolysis, and a relatively high electrolytic current density (15 mA cm^− 2) was obtained in an aqueous medium, demonstrating its potential for electrochemical reduction of CO₂ to formate.     

High-pressure phase equilibria of {carbon dioxide (CO2) + n-alkyl-imidazolium bis(trifluoromethylsulfonyl)amide} ionic liquids

Ionic liquids (ILs) and carbon dioxide (CO₂) systems have unique phase behavior that has been applied to applications in reactions, extractions, materials, etc. Detailed phase equilibria and modeling are highly desired for their further development. In this work, the (vapor + liquid) equilibrium, (vapor + liquid + liquid) equilibrium, and (liquid + liquid) equilibrium of n-alkyl-3-methyl-imidazolium bis(trifluoromethylsulfonyl)amide ionic liquids with CO₂ were measured at temperatures of (298.15, 323.15, 343.15) K and pressure up to 25 MPa. With a constant anion of bis(trifluoromethylsulfonyl)amide, the n-alkyl chain length on the cation was varied from 1-ethyl-3-methyl-imidazolium ([EMIm][Tf₂N]), 1-hexyl-3-methyl-imidazolium ([HMIm][Tf₂N]), to 1-decyl-3-methyl-imidazolium ([DMIm][Tf₂N]). The effects of the cation on the phase behavior and CO₂ solubility were investigated. The longer alkyl chain lengths increase the CO₂ solubility. The Peng–Robinson equation of state with van der Waals 2-parameter mixing rule with estimated IL critical properties were used to model and correlate the experimental data. The models correlate the (vapor + liquid) equilibrium and (liquid + liquid) equilibrium very well. However, extrapolation of the model to much higher pressures (>30 MPa) can results in the prediction of a mixture critical point which, as of yet, has not been found in the literature.