In Situ Raman Study of Nickel Oxide and Gold-Supported Nickel Oxide Catalysts for the Electrochemical Evolution of Oxygen

An in situ Raman spectroscopic investigation has been carried out to identify the composition of the active phase present on the surface of nickel electrodes used for the electrochemical evolution of oxygen. The electrolyte in all cases was 0.1 M KOH. A freshly polished Ni electrode oxidized upon immersion in the electrolyte and at potentials approaching the evolution of oxygen developed a layer of γ-NiOOH. Electrochemical cycling of this film transformed it into β-NiOOH, which was observed to be three times more active than γ-NiOOH. The higher activity of β-NiOOH is attributed to an unidentified Ni oxide formed at a potential above 0.52 V (vs Hg/HgO reference). We have also observed that a submonolayer of Ni oxide deposited on Au exhibits a turnover frequency (TOF) for oxygen evolution that is an order of magnitude higher than that for a freshly prepared γ-NiOOH surface and more than 2-fold higher than that for a β-NiOOH surface. By contrast, a similar film deposited on Pd exhibits a TOF that is similar to that of bulk γ-NiOOH. It is proposed that the high activity of submonolayer deposits of Ni oxide on Au is due to charge transfer from the oxide to the highly electronegative Au, leading to the possible formation of a mixed Ni/Au surface oxide

Electrochemical Reduction of Carbon Dioxide to Ethane Using Nanostructured Cu₂O‑Derived Copper Catalyst and Palladium(II) Chloride

A method to facilitate the electrochemical reduction of carbon dioxide (CO₂) to ethane (C₂H₆) was developed. The electrolyte used was aqueous 0.1 M KHCO₃. Chronoamperometry, scanning electron microscopy, X-ray photoelectron spectroscopy, X-ray diffraction, online gas chromatography, and nuclear magnetic resonance spectroscopy were used to characterize the electrochemical system and products formed. Carbon dioxide reduction using a Cu₂O-derived copper working electrode gave ethylene (C₂H₄) and ethanol as main C₂ products, with optimized faradic efficiencies (FE) of 32.1 and 16.4% at −1.0 V vs RHE. The active catalysts were ∼500 nm-sized crystalline Cu⁰ particles, which were formed via the reduction of the Cu₂O precursor during the initial phase of the CO₂ reduction reaction. When palladium­(II) chloride was added to the electrolyte, C₂H₆ formation could be achieved with a significant FE of 30.1% at the said potential. The production of C₂H₄ was, on the other hand, suppressed to a FE of 3.4%. The alternate use of Pd⁰, PdO, or Pd–Al₂O₃ dopants did not afford the same conversion efficiency. Extensive mechanistic studies demonstrate that C₂H₄ was first produced from CO₂ reduction at the Cu⁰ sites, followed by hydrogenation to C₂H₆ with the assistance of adsorbed PdCl_<i>x</i>. Interestingly, we discover that both Cu and PdCl_<i>x</i> sites are necessary for the efficient reduction of C₂H₄ to C₂H₆. The PdCl₂ was “consumed” during the reaction, and a hypothesis for how it contributes to the reduction of CO₂ to ethane is proposed

Tuning the Selectivity of Carbon Dioxide Electroreduction toward Ethanol on Oxide-Derived Cu_<i>x</i>Zn Catalysts

The electrochemical reduction of carbon dioxide (CO₂) to ethanol (C₂H₅OH) and ethylene (C₂H₄) using renewable electricity is a viable method for the production of these commercially vital chemicals. Copper (Cu) and its oxides are by far the most effective electrocatalysts for this purpose. However, the formation of ethanol using these catalysts is generally less favored in comparison to that of ethylene. In this work, we demonstrate that the selectivity of CO₂ reduction toward ethanol could be tuned by introducing a cocatalyst to generate an in situ source of mobile CO reactant. Cu-based oxides with different amounts of Zn dopants (Cu, Cu₁₀Zn, Cu₄Zn, and Cu₂Zn) were prepared and used as catalysts under ambient pressure in aqueous 0.1 M KHCO₃ electrolyte. By varying the amount of Zn in the bimetallic catalysts, we found that the selectivity of ethanol versus ethylene production, defined by the ratio of their Faradaic efficiencies (FE_ethanol/FE_ethylene), could be tuned by a factor of up to ∼12.5. Ethanol formation was maximized on Cu₄Zn at −1.05 V vs RHE, with a remarkable Faradaic efficiency and current density of 29.1% and −8.2 mA/cm², respectively. The Cu₄Zn catalyst was also catalytically stable for the production of ethanol for at least 5 h. The importance of Zn as a CO-producing site was demonstrated by performing CO₂ reduction on Cu–Ni and Cu–Ag bimetallic catalysts. Operando Raman spectroscopy revealed that the as-deposited Cu-based oxide films were reduced to the metallic state during CO₂ reduction, after which only signals belonging to CO adsorbed on Cu sites were recorded. This showed that the reduction of CO₂ probably occurred on metallic sites rather than on metal oxides. A two-site mechanism to rationalize the selective reduction of CO₂ to ethanol is proposed and discussed

Surfactant-Enhanced Formation of Ethylene from Carbon Monoxide Electroreduction on Copper Catalysts

Surface functionalization has been found to be promising for enhancing the electrochemical CO2 reduction reaction (CO2RR) to C2+ products on copper catalysts, typically by suppressing the parasitic hydrogen evolution reaction and increasing the local concentration of CO2 at the electrode. Expanding upon this approach, we developed surface-functionalized catalysts for CO reduction (CORR) to C2+ products with high activity and selectivity. Using an oxide-derived copper (OD-Cu) catalyst coated with tetrabutylammonium cations (TBA+), CO was reduced at −0.65 V vs RHE to C2+ products with a Faradaic efficiency (FE) of 78% (jC2+ = –765 mA cm–2), of which the FEethylene was 40%. In contrast, unmodified OD-Cu catalysts achieved lower FEs of C2+ products (60%) and ethylene (27%). Our mechanistic study to explore the above product distribution, which involved performing CORR at different CO partial pressures, in situ Raman spectroscopy, and the use of other surfactants, reveals that the enhanced selectivity of C2+ products in the presence of TBA+, especially ethylene, is attributed to heightened CORR activity and increased ethylene production rather than HER suppression. Optimization of the mass loading of the TBA+-coated Cu catalysts and applied potentials enabled a jC2+ exceeding −1 A cm–2 (jethylene = −694 mA cm–2)

Continuous Production of Ethylene from Carbon Dioxide and Water Using Intermittent Sunlight

The large-scale deployment of efficient artificial photosynthesis systems to convert carbon dioxide (CO₂) into carbon-based fuels and chemical feedstocks holds great promise as a way to ensure a carbon neutral cycle. While catalysts have been developed for the pertinent half-reaction of CO₂ reduction to C₂ molecules, an integrated system for this purpose has never been designed and built. In this work, we demonstrate an energetically efficient formation of ethylene directly from CO₂ and water (H₂O) using solar energy at room temperature and pressure. A two-electrode cell (electrolyzer) was designed, and cell parameters such as electrolyte and voltage were optimized. Oxide-derived copper (Cu) and iridium oxide (IrO_<i>x</i>) were used as electrocatalysts respectively in the cathode and anode. Coupling this electrolyzer with silicon solar panels under laboratory 1 sun illumination (100 mW/cm²), we show that CO₂ could be facilely reduced to ethylene with a faradaic efficiency of 31.9%, partial current density of 6.5 mA/cm², and a solar-to-ethylene energy efficiency of 1.5%. When liquid fuels such as ethanol and <i>n</i>-propanol were included, the total solar-to-fuel efficiency was 2.9%. These outstanding figures-of-merits are the state-of-the-art. We also introduced insoluble chelating agents in the electrolyte to capture contaminants such as dissolved iridium ions, and thus significantly improved the longevity of the electrolyzer. Compared to previously reported solar-to-fuel setups which were only tested under simulated sunlight, our system, when coupled with a rechargeable battery, could run and produce ethylene continuously using only intermittent natural sunlight

<i>In Situ</i> Raman Spectroscopy of Copper and Copper Oxide Surfaces during Electrochemical Oxygen Evolution Reaction: Identification of Cu^III Oxides as Catalytically Active Species

Scanning electron microscopy, X-ray diffraction, cyclic voltammetry, chronoamperometry, <i>in situ</i> Raman spectroscopy, and X-ray absorption near-edge structure spectroscopy (XANES) were used to investigate the electrochemical oxygen evolution reaction (OER) on Cu, Cu₂O, Cu­(OH)₂, and CuO catalysts. Aqueous 0.1 M KOH was used as the electrolyte. All four catalysts were oxidized or converted to CuO and Cu­(OH)₂ during a slow anodic sweep of cyclic voltammetry and exhibited similar activities for the OER. A Raman peak at 603 cm^–1 appeared for all the four samples at OER-relevant potentials, ≥1.62 V vs RHE. This peak was identified as the Cu–O stretching vibration band of a Cu^III oxide, a metastable species whose existence is dependent on the applied potential. Since this frequency matches well with that from a NaCu^IIIO₂ standard, we suggest that the chemical composition of the Cu^III oxide is CuO₂^–-like. The four catalysts, in stark contrast, did not oxidize the same way during direct chronoamperometry measurements at 1.7 V vs RHE. Cu^III oxide was observed only on the CuO and Cu­(OH)₂ electrodes. Interestingly, these two electrodes catalyzed the OER ∼10 times more efficiently than the Cu and Cu₂O catalysts. By correlating the intensity of the Raman band of Cu^III oxide and the extent of the OER activity, we propose that Cu^III species provides catalytically active sites for the electrochemical water oxidation. The formation of Cu^III oxides on CuO films during OER was also corroborated by <i>in situ</i> XANES measurements of the Cu K-edge. The catalytic role of Cu^III oxide in the O₂ evolution reaction is proposed and discussed

Enhanced Catalysis of the Electrochemical Oxygen Evolution Reaction by Iron(III) Ions Adsorbed on Amorphous Cobalt Oxide

The oxygen evolution reaction (OER) is the bottleneck in the efficient production of hydrogen gas fuel via the electrochemical splitting of water. In this work, we present and elucidate the workings of an OER catalytic system which consists of cobalt oxide (CoO_<i>x</i>) with adsorbed Fe³⁺ ions. The CoO_<i>x</i> was electrodeposited onto glassy-carbon-disk electrodes, while Fe³⁺ was added to the 1 M KOH electrolyte. Linear sweep voltammetry and chronopotentiometry were used to assess the system’s OER activity. The addition of Fe³⁺ significantly lowered the average overpotential (η) required by the cobalt oxide catalyst to produce 10 mA/cm² O₂ current from 378 to 309 mV. The Tafel slope of the CoO_<i>x</i> + Fe³⁺ catalyst also decreased from 59.5 (pure CoO_<i>x</i>) to 27.6 mV/dec, and its stability lasted ∼20 h for 10 mA/cm² O₂ evolution. Cyclic voltammetry showed that oxidation of the deposited CoO_<i>x</i>, from Co²⁺ to Co³⁺ occurred at a more positive potential when Fe³⁺ was added to the electrolyte. This could be attributed to interactions between the Co and Fe atoms. Comprehensive X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectroscopy were conducted. The in situ XANES spectra of Co sites in the CoO_<i>x</i>, CoO_<i>x</i> + Fe³⁺, and control Fe₄₈Co₅₂O_<i>x</i> catalysts were similar during the OER, which indicates that the improved OER performance of the CoO_<i>x</i> + Fe³⁺ catalyst could not be directly correlated to changes in the Co sites. The XANES spectra of Fe indicated that Fe³⁺ adsorbed on CoO_<i>x</i> did not further oxidize under OER conditions. However, Fe’s coordination number was notably reduced from 6 in pure FeO_<i>x</i> to 3.7 when it was adsorbed on CoO_<i>x</i>. No change in the Fe–O bond lengths/strengths was found. The nature and mechanistic role of Fe adsorbed on CoO_<i>x</i> are discussed. We propose that Fe sites with oxygen vacancies are responsible for the improved OER activity of CoO_<i>x</i> + Fe³⁺ catalyst

Mechanistic Insights into the Enhanced Activity and Stability of Agglomerated Cu Nanocrystals for the Electrochemical Reduction of Carbon Dioxide to <i>n</i>‑Propanol

The reduction of carbon dioxide (CO₂) to <i>n-</i>propanol (CH₃CH₂CH₂OH) using renewable electricity is a potentially sustainable route to the production of this valuable engine fuel. In this study, we report that agglomerates of ∼15 nm sized copper nanocrystals exhibited unprecedented catalytic activity for this electrochemical reaction in aqueous 0.1 M KHCO₃. The onset potential for the formation of <i>n-</i>propanol was 200–300 mV more positive than for an electropolished Cu surface or Cu⁰ nanoparticles. At −0.95 V (vs RHE), <i>n-</i>propanol was formed on the Cu nanocrystals with a high current density (<i>j</i>_{<i>n</i>‑propanol}) of −1.74 mA/cm², which is ∼25× larger than that found on Cu⁰ nanoparticles at the same applied potential. The Cu nanocrystals were also catalytically stable for at least 6 h, and only 14% deactivation was observed after 12 h of CO₂ reduction. Mechanistic studies suggest that <i>n-</i>propanol could be formed through the C–C coupling of carbon monoxide and ethylene precursors. The enhanced activity of the Cu nanocrystals toward <i>n-</i>propanol formation was correlated to their surface population of defect sites

Ruthenium–Tungsten Composite Catalyst for the Efficient and Contamination-Resistant Electrochemical Evolution of Hydrogen

A new catalyst, prepared by a simple physical mixing of ruthenium (Ru) and tungsten (W) powders, has been discovered to interact synergistically to enhance the electrochemical hydrogen evolution reaction (HER). In an aqueous 0.5 M H₂SO₄ electrolyte, this catalyst, which contained a miniscule loading of 2–5 nm sized Ru nanoparticles (5.6 μg Ru per cm² of geometric surface area of the working electrode), required an overpotential of only 85 mV to drive 10 mA/cm² of H₂ evolution. Interestingly, our catalyst also exhibited good immunity against deactivation during HER from ionic contaminants, such as Cu²⁺ (over 24 h). We unravel the mechanism of synergy between W and Ru for catalyzing H₂ evolution using Cu underpotential deposition, photoelectron spectroscopy, and density functional theory (DFT) calculations. We found a decrease in the d-band and an increase in the electron work function of Ru in the mixed composite, which made it bind to H more weakly (more Pt-like). The H-adsorption energy on Ru deposited on W was found, by DFT, to be very close to that of Pt(111), explaining the improved HER activity

Catalytic Activities of Sulfur Atoms in Amorphous Molybdenum Sulfide for the Electrochemical Hydrogen Evolution Reaction

The catalytic activities of sulfur sites in amorphous MoS_<i>x</i> for the electrochemical hydrogen evolution reaction (HER) was investigated in aqueous 0.5 M H₂SO₄ electrolyte. Using X-ray photoelectron spectroscopy and linear sweep voltammetry, we found the turnover frequency for H₂ production to increase linearly with the percentage of S atoms with higher electron binding energies. These S atoms could be apical S^2– and/or bridging S₂^2–. To distinguish the catalytic performances of these two types of atoms, we turn to quantum chemical simulations using density functional theory. The apical S^2– atoms were found to adsorb H weakly with a Gibbs free energy for atomic H adsorption (Δ<i>G</i>_H) in excess of +1 eV, and were thus ruled out as reaction sites for HER. <i>In situ</i> Raman spectroscopy of the model [Mo₃S₁₃]^2– cluster further demonstrate the higher catalytic reactivity of the bridging S₂^2– over terminal S₂^2– (which have lower electron binding energy) for proton reduction