Carbon Dioxide Electroreduction at Highly Porous Nitrogen and Sulfur Co-Doped Iron-Containing Heterogeneous Carbon Gel

Out of the transition metals capable of electrochemical carbon dioxide reduction, Pd is interesting as it can convert carbon dioxide electrochemically into formate or carbon monoxide depending on the applied potential. In fact, it is capable of producing formate at the most positive known potentials that are close to zero overpotentials although at an unfortunately low activity and at the cost of deactivation by carbon monoxide poisoning. One aim is to improve the activity and stability of Pd-based electrocatalysts towards formate production in low overpotentials. As Pd is a critical raw material, we also wish to decrease the amount of Pd required while maintaining high carbon dioxide electroreduction capability. These goals can be achieved by nanostructuring and supporting the Pd catalyst. Here, we have employed a simple wet impregnation synthesis approach to prepare small nanoparticles and nanowires of Pd supported on single walled carbon nanotubes and tested the optimum loading of Pd to obtain high formate yield with improved activity and stability. Reactive sites can be created on the carbon support by subjecting it to ozonation prior to supporting the metal, which may help certain interesting nanostructures, such as nanowires, to grow. Additionally, the oxygen functional groups on the carbon surface are expected to affect the wettability of the electrode which is important for achieving an efficient carbon dioxide electroreduction and a longer-term stability of the reaction. Therefore, we also studied the effects of ozonation of the carbon supports on the electrochemical reduction of carbon dioxide into both formate and syngas (mixture of hydrogen and carbon monoxide) on Pd. Carbon atoms inevitably participate in hydrogen evolution reaction and, thus, in syngas production on Pd-supported catalysts at higher overpotentials. Our results show that ozonation greatly enhances the activity of the catalyst material and improves its stability when applying low overpotentials for formate formation in comparison to the pristine carbon support. The current density on Pd supported ozone treated carbon nanotube material remains stable over 4h of carbon dioxide electrolysis at an applied potential of -0.45 V (vs. RHE) while Pd on pristine carbon support deactivates during the initial 30 min of the experiment. Longer electrolysis times do reveal slow changes in product distribution although activity on ozone-treated single walled carbon nanotube-supported catalyst is excellent. Additionally, the different support materials cause interesting changes in product selectivity upon applying higher overpotentials for the production of syngas. Pd supported on pristine nanotubes produces syngas with carbon monoxide-to-hydrogen ratios of 0.72 and 1.38 at applied potentials of -0.85 V (vs. RHE) and -0.95 V (vs. RHE), respectively, while ozone treated material produces less than 10% of carbon monoxide. Through physico-chemical characterizations of the materials we aim at understanding the observed changes in electrochemical reduction of carbon dioxide on carbon supported Pd nanostructures.

Constructing FeN4/graphitic nitrogen atomic interface for high-efficiency electrochemical CO2 reduction over a broad potential window

Quantifying Electrocatalytic Reduction of CO2 on Twin Boundaries

The development of an energy efficient carbon dioxide (CO2) electroreduction process could simultaneously curb anthropogenic CO2 emissions and provide a sustainable pathway for fuel generation. If these electroreactors employed CO2 feedstocks from major emission sources (e.g., thermal power plants) and excess electricity from intermittent locally available renewable sources (e.g., solar, wind), then chemicals of economic value could be generated in a carbon neutral fashion. To this end, the research community has devoted significant effort to developing catalysts that exhibit energy efficient and selective reduction of CO2 1,2. Carbon monoxide (CO) electrosynthesis is perhaps the simplest and, to date the most extensively studied, carbon dioxide transformation. Gold (Au) is a particularly attractive catalyst for this reaction due to its reasonable selectivity, > 80%, and activity, > 3mA/cm2, at moderate applied potentials (-0.4 V to -0.7 V vs RHE)3,4. To further improve performance, Au has been extensively studied in many forms including oxide-derived Au5, nanoparticles of various sizes6, and ultrathin nanowires7. However, there has been less focus on role of the substrate material on the performance of the electrocatalysts, an interaction that has been shown to be relevant in several other electrochemical reactions (i.e., oxygen reduction reaction8, hydrogen evolution reaction9). Substrates improve electrochemical performance by aiding in the dispersion, stabilization, and utilization of the active electrocatalyst sites, especially for smaller nanoparticles that lack stability and are susceptible to aggregation. Prior work by Ma et al. demonstrated that silver nanoparticles supported on titania produced a two-fold increase in CO partial current density over its carbon substrate counterpart, resulting in comparable results to unsupported silver nanoparticles with only 40% of the metal loading10. We compare metal oxides (i.e. TiO2, Ag2O, CuO) and carbonaceous materials as substrates due to their differing physical and electrical properties. Most metal oxide substrates appear highly susceptible to the competing hydrogen evolution reaction resulting in low faradaic efficiency for CO and other CO2 reduction products. By comparison, carbonaceous supporting materials produced large yields of CO at medium overpotentials but primarily H2at both very high and very low overpotentials (Figure 1), resulting in a parabolic profile of faradaic efficiency versus working electrode potential. Supported electrocatalysts were evaluated in both a 3-electrode cell for the purpose of quantitative analytical measurements and in a flow reactor for an engineering assessment of performance and durability. Acknowledgments We gratefully acknowledge the financial support of the Massachusetts Institute of Technology Energy Initiative and the Kuwait – MIT Center for Natural Resources and the Environment. The assistance of Dr. Kyler Carroll, Mr. Jarrod Milshtein, and Mr. John Barton is much appreciated.

Electroreduction of carbon dioxide into higher-energy liquid fuels and chemicals is a promising but challenging renewable energy conversion technology. Among the electrocatalysts screened so far for carbon dioxide reduction, which includes metals, alloys, organometallics, layered materials and carbon nanostructures, only copper exhibits selectivity towards formation of hydrocarbons and multi-carbon oxygenates at fairly high efficiencies, whereas most others favour production of carbon monoxide or formate. Here we report that nanometre-size N-doped graphene quantum dots (NGQDs) catalyse the electrochemical reduction of carbon dioxide into multi-carbon hydrocarbons and oxygenates at high Faradaic efficiencies, high current densities and low overpotentials. The NGQDs show a high total Faradaic efficiency of carbon dioxide reduction of up to 90%, with selectivity for ethylene and ethanol conversions reaching 45%. The C2 and C3 product distribution and production rate for NGQD-catalysed carbon dioxide reduction is comparable to those obtained with copper nanoparticle-based electrocatalysts.

On the Conversion Efficiency of CO2 Electroreduction on Gold

Using the rotating ring (platinum)—disk (glassy carbon) electrode methodology, electrocatalytic activity of the microstructured copper centers (imbedded within the polyvinylpyrrolidone polymer matrix and deposited onto the glassy carbon disk electrode) has been monitored during electroreduction of carbon dioxide both in acid (HClO4) and neutral (KHCO3) media as well as diagnosed (at Pt ring) with respect to formation of the electroactive products. Combination of the stripping-type and rotating ring-disk voltammetric approaches has led to the observation that, regardless the overlapping reduction phenomena, the reduction of carbon dioxide at copper catalyst is, indeed, operative and coexists with hydrogen evolution reaction. Using the fundamental concepts of surface electrochemistry and analytical voltammetry, the reaction products (thrown onto the platinum ring electrode) could be considered and identified as adsorbates (on Pt) under conditions of the stripping-type oxidation experiment. Judging from the potentials at which the stripping voltammetric peaks appear in neutral CO2-saturated KHCO3 (pH 6.8), formic acid or carbon monoxide seem to be the most likely reaction products or intermediates. The proposed methodology also permits correlation between the CO2 electroreduction products and the potentials applied to the disk electrode. By performing the comparative stripping-type voltammetric experiments in acid medium (HClO4 at pH 1) with the adsorbates of formic acid, ethanol and acetaldehyde (on Pt ring), it can be rationalized that, although C2H5OH or CH3CHO are very likely CO2-reduction electroactive products, formation of some HCOOH, CH3OH or even CO cannot be excluded.

In situ embedding Co9S8 into nitrogen and sulfur codoped hollow porous carbon as a bifunctional electrocatalyst for oxygen reduction and hydrogen evolution reactions

The electroreduction of carbon dioxide offers a promising avenue to produce valuable fuels and chemicals using greenhouse gas carbon dioxide as the carbon feedstock. Because industrial carbon dioxide point sources often contain numerous contaminants, such as nitrogen oxides, understanding the potential impact of contaminants on carbon dioxide electrolysis is crucial for practical applications. Herein, we investigate the impact of various nitrogen oxides, including nitric oxide, nitrogen dioxide, and nitrous oxide, on carbon dioxide electroreduction on three model electrocatalysts (i.e., copper, silver, and tin). We demonstrate that the presence of nitrogen oxides (up to 0.83%) in the carbon dioxide feed leads to a considerable Faradaic efficiency loss in carbon dioxide electroreduction, which is caused by the preferential electroreduction of nitrogen oxides over carbon dioxide. The primary products of nitrogen oxides electroreduction include nitrous oxide, nitrogen, hydroxylamine, and ammonia. Despite the loss in Faradaic efficiency, the electrocatalysts exhibit similar carbon dioxide reduction performances once a pure carbon dioxide feed is restored, indicating a negligible long-term impact of nitrogen oxides on the catalytic properties of the model catalysts.

Well-dispersed pyrite-type RuS2 nanocrystals anchored on porous nitrogen and sulfur co-doped hollow carbon spheres for enhanced alkaline hydrogen evolution

Selectivity of CO2, carbonic acid and bicarbonate electroreduction over Iron-porphyrin catalyst: A DFT study

Spin-Enhanced C–C Coupling in CO ₂ Electroreduction with Oxide-Derived Copper

Many strategies have been proposed for carbon dioxide (CO2) reduction aiming to close the carbon cycle and compensate anthropogenic CO2 emissions. Electrochemical CO2 reduction has several advantages that it can be carried out under ambient conditions with water as the only additional feedstock, and electricity generated from renewable sources can be employed to achieve a fully sustainable route [1]. Copper (Cu) has unique features for electrochemically converting CO2 to a range of chemicals including hydrocarbons [2], but its selectivity for specific products is low. Especially, at a low or intermediate overpotential, the hydrogen evolution reaction (HER) dominates the overall process on polycrystalline Cu in aqueous solutions [3]. However, the selectivity of Cu can be tuned by introducing a secondary metal. Tin-modified nanostructured Cu (CuSn) electrocatalysts have been developed for electrochemically converting CO2 to carbon monoxide (CO). The doped Sn atoms in the topmost layer alloy with Cu, form a CuSn shell, and efficiently suppress the HER. By optimizing the Sn content in the CuSn layer and the nanostructure, the faradaic efficiency for CO reaches > 90 % at -0.7 V vs. RHE, and remains above 85 % in a wide potential range from -0.7 to -1.0 V [4]. However, the excellent selectivity for CO can be only observed at CuSn dendrites. When the CuSn catalyst has a particulate morphology, the selectivity for CO becomes much worse, even though the surface composition has the optimal Cu/Sn ratio. The CuSn particles are active for the HER at a low overpotential. Although the HER can be efficiently suppressed to a faradaic efficiency of less than 15 % at a high overpotential, the major products for CO2 reduction are both CO and formate (COOH-) [5]. This work highlights the importance of nanostructure and surface composition of CuSn catalysts for active and selective CO2 reduction. Electrodes consisting of a three-dimensional porous architecture are prepared for a large active surface per unit geometric area. The CO production achieves a high partial current density of 4.9 mA·cm-2 at -0.8 V [4]. [1] Centi, G., et al. Energy Environ. Sci. 2013, 6, 1711-1731 [2] Qiao, J., et al., Chem. Soc. Rev. 2014, 43, 631-675 [3] Li, C., et al., J. Am. Chem. Soc. 2012, 134, 7231-4 [4] Zeng, J., et al., submitted [5] Ju, W., et al., in preparation

The reduction of carbon dioxide is a very inert process that requires breaking the double C=O bond in the stable CO2 molecule. There is a need for conversion of carbon dioxide, a potent greenhouse gas and a contributor to the global climate change, to useful carbon-based fuels or useful chemicals. Our contribution addresses the low-temperature CO2-conversion processes based on electrocatalytic and photoelectrochemical approaches. In the realistic electrolysis cells, the reduction of CO2 (at cathode or photocathode) would have to be accompanied by water oxidation (at anode or photoanode). Our interest is in the PEM electrolyzers utilizing the proton exchange polymer electrolyte membranes (as separators between anode and cathode compartments) allowing electrolysis at low pH’s and, in principle, capable of yielding higher current densities than alkaline electrolyzers. Nevertheless, under such conditions, the parasitic hydrogen evolution reaction is likely to become predominant and complicate formation of any CO2-reduction products. Our preliminary results with the copper or ruthenium containing tungsten oxide nanorods as catalytic materials are promising with respect to the investigations in acid media. Recent advances (including our studies) in photoelectrochemical water splitting are based on semiconducting photoanodes (typically n-type semiconducting metal oxides) combined either with a metallic cathode (e.g. Pt) or with a p-type semiconductor as a photocathode should be mentioned here in a context of the development of the CO2-electrolyzers exploring photoelectrocatalytic properties and operating under illumination. Among representative examples of metal oxides with n-type semiconducting and promising photoelectrocatalytic properties, tungsten oxide (WO3) should be mentioned. The WO3-based semiconductors are highly stable in acid media, and they are characterized by an energy band gap of 2.5-2.7 eV thus allowing absorption of a reasonable fraction of the solar spectrum up to ca. 500 nm. It comes from our research that, upon illumination, the onset potential for photooxidation of water (in acid medium) is as low as 0.45 V (vs. RHE) what is of practical importance to the energy efficient photoelectrolysis. Further improvement of performance of photoactive metal oxides, e.g. through doping with certain anions or oxo-species will be discussed. Utilization of such systems in the CO2-electrolyzers would require, however, development of practical cathodes (CO2-reduction) operating in acid media that will be addressed during presentation. In our work, the solar energy will be absorbed by a suitable semiconductor to drive water oxidation to oxygen and protons, whereas the reduction of CO2 will be achieved electrochemically with use of a proper (selective) catalyst. By utilizing the aqueous acidic medium, no external hydrogen is required for the CO2-reduction process because the H+ ions not only exist at sufficiently large population but they are generated in situ during the anodic reaction. Coupling of the two processes in a single unit leading to one-step photoelectrochemical/electrocatalytic approach should be possible (e.g. with use of a tandem cell) but, in practice, physical separation of the two reactions in photoanodic (photoelectrochemical water oxidation) and cathodic (carbon dioxide electroreduction) compartments is necessary to increase efficiency and to limit charge recombination. Conversion of CO2 to CO (accompanied by H2-evolution) is an attractive target because syngas (CO + H2) can be utilized for manufacturing of many useful chemicals including synthetic fuels via the Fischer-Tropsch type process.

The reduction of carbon dioxide is a very inert process that requires breaking the double C=O bond in the stable CO2 molecule. There is a need for conversion of carbon dioxide, a potent greenhouse gas and a contributor to the global climate change, to useful carbon-based fuels or useful chemicals. Our contribution addresses the low-temperature CO2-conversion processes based on electrocatalytic and photoelectrochemical approaches. In the realistic electrolysis cells, the reduction of CO2 (at cathode or photocathode) would have to be accompanied by water oxidation (at anode or photoanode).Our interest is in the PEM electrolyzers utilizing the proton exchange polymer electrolyte membranes (as separators between anode and cathode compartments) allowing electrolysis at low pH’s and, in principle, capable of yielding higher current densities than alkaline electrolyzers. Nevertheless, under such conditions, the parasitic hydrogen evolution reaction is likely to become predominant and complicate formation of any CO2-reduction products. Our preliminary results with the copper or ruthenium containing tungsten oxide nanorods as catalytic materials are promising with respect to the investigations in acid media.Recent advances (including our studies) in photoelectrochemical water splitting are based on semiconducting photoanodes (typically n-type semiconducting metal oxides) combined either with a metallic cathode (e.g. Pt) or with a p-type semiconductor as a photocathode should be mentioned here in a context of the development of the CO2-electrolyzers exploring photoelectrocatalytic properties and operating under illumination. Among representative examples of metal oxides with n-type semiconducting and promising photoelectrocatalytic properties, tungsten oxide (WO3) should be mentioned. The WO3-based semiconductors are highly stable in acid media, and they are characterized by an energy band gap of 2.5-2.7 eV thus allowing absorption of a reasonable fraction of the solar spectrum up to ca. 500 nm. It comes from our research that, upon illumination, the onset potential for photooxidation of water (in acid medium) is as low as 0.45 V (vs. RHE) what is of practical importance to the energy efficient photoelectrolysis. Further improvement of performance of photoactive metal oxides, e.g. through doping with certain anions or oxo-species will be discussed. Utilization of such systems in the CO2-electrolyzers would require, however, development of practical cathodes (CO2-reduction) operating in acid media that will be addressed during presentation. In our work, the solar energy will be absorbed by a suitable semiconductor to drive water oxidation to oxygen and protons, whereas the reduction of CO2 will be achieved electrochemically with use of a proper (selective) catalyst. By utilizing the aqueous acidic medium, no external hydrogen is required for the CO2-reduction process because the H+ ions not only exist at sufficiently large population but they are generated in situ during the anodic reaction. Coupling of the two processes in a single unit leading to one-step photoelectrochemical/electrocatalytic approach should be possible (e.g. with use of a tandem cell) but, in practice, physical separation of the two reactions in photoanodic (photoelectrochemical water oxidation) and cathodic (carbon dioxide electroreduction) compartments is necessary to increase efficiency and to limit charge recombination. Conversion of CO2 to CO (accompanied by H2-evolution) is an attractive target because syngas (CO + H2) can be utilized for manufacturing of many useful chemicals including synthetic fuels via the Fischer-Tropsch type process.