CO Coverage Research Articles

Sequential *CO management via controlling in situ reconstruction for efficient industrial-current-density CO2-to-C2+ electroreduction

Sequentially managing the coverage and dimerization of *CO on the Cu catalysts is desirable for industrial-current-density CO2 reduction (CO2R) to C2+, which required the multiscale design of the surface atom/architecture. However, the oriented design is colossally difficult and even no longer valid due to unpredictable reconstruction. Here, we leverage the synchronous leaching of ligand molecules to manipulate the seeding-growth process during CO2R reconstruction and construct Cu arrays with favorable (100) facets. The gradient diffusion in the reconstructed array guarantees a higher *CO coverage, which can continuously supply the reactant to match its high-rate consumption for high partial current density for C2+. Sequentially, the lower energy barriers of *CO dimerization on the (100) facets contribute to the high selectivity of C2+. Profiting from this sequential *CO management, the reconstructed Cu array delivers an industrial-relevant FEC2+ of 86.1% and an FEC2H4 of 60.8% at 700 mA cm-2. Profoundly, the atomic-molecular scale delineation for the evolution of catalysts and reaction intermediates during CO2R can undoubtedly facilitate various electrocatalytic reactions.

Ag decorated-Cu2O catalysts with enhanced selectivity for CO2 electroreduction toward C2+ products

Copper-based electrocatalysts have been widely used to generate C2+ products via the electrocatalytic carbon dioxide reduction reaction (CO2RR) because the catalysts possess unique properties for C-C coupling. Tandem catalysis is an attractive concept for C2+ production, enhancing selectivity and activity by combining CO-generating catalysts with Cu-based catalysts. Herein, the ability of Ag decoration to improve the selectivity of the Cu2O catalysts for C2+ production was investigated. The Ag/Cu2O catalyst with 10 wt% Ag content on the Cu2O surface (10 Ag/Cu2O) exhibited high Faradaic efficiency (FE) of 48% for C2+ products at –0.99 V (vs. RHE). Moreover, the partial current density of C2+ products on the 10 Ag/Cu2O was improved by 33% compared to the bare Cu2O catalyst. The intermediate (CO) generated on the Ag surface migrates by diffusion and/or sequential desorption-re-adsorption from Ag to Cu at the Ag-Cu interface of the Ag/Cu2O, increasing CO coverage on the Cu surface. The increase in CO coverage on the Cu surface of the Ag/Cu2O catalysts after decoration with Ag effectively reduces the hydrogen evolution reaction (HER), enhancing C-C coupling to generate C2+ products on the Ag/Cu2O. In addition, density functional theory (DFT) calculation results also revealed that Ag provides more CO2 activation sites, increasing CO coverage and reducing hydrogen adsorption energy, thus suppressing HER and improving the selectivity of CO2RR toward C2+ production.

Promoting CO2 Electroreduction to Multi‐Carbon Products by Hydrophobicity‐Induced Electro‐Kinetic Retardation

AbstractAdvancing the performance of the Cu‐catalyzed electrochemical CO2 reduction reaction (CO2RR) is crucial for its practical applications. Still, the wettable pristine Cu surface often suffers from low exposure to CO2, reducing the Faradaic efficiencies (FEs) and current densities for multi‐carbon (C2+) products. Recent studies have proposed that increasing surface availability for CO2 by cation‐exchange ionomers can enhance the C2+ product formation rates. However, due to the rapid formation and consumption of *CO, such promotion in reaction kinetics can shorten the residence of *CO whose adsorption determines C2+ selectivity, and thus the resulting C2+ FEs remain low. Herein, we discover that the electro‐kinetic retardation caused by the strong hydrophobicity of quaternary ammonium group‐functionalized polynorbornene ionomers can greatly prolong the *CO residence on Cu. This unconventional electro‐kinetic effect is demonstrated by the increased Tafel slopes and the decreased sensitivity of *CO coverage change to potentials. As a result, the strongly hydrophobic Cu electrodes exhibit C2+ Faradaic efficiencies of ≈90 % at a partial current density of 223 mA cm−2, more than twice of bare or hydrophilic Cu surfaces.

Promoting CO2 Electroreduction to Multi-Carbon Products by Hydrophobicity-Induced Electro-Kinetic Retardation.

Advancing the performance of the Cu-catalyzed electrochemical CO2 reduction reaction (CO2 RR) is crucial for its practical applications. Still, the wettable pristine Cu surface often suffers from low exposure to CO2 , reducing the Faradaic efficiencies (FEs) and current densities for multi-carbon (C2+ ) products. Recent studies have proposed that increasing surface availability for CO2 by cation-exchange ionomers can enhance the C2+ product formation rates. However, due to the rapid formation and consumption of *CO, such promotion in reaction kinetics can shorten the residence of *CO whose adsorption determines C2+ selectivity, and thus the resulting C2+ FEs remain low. Herein, we discover that the electro-kinetic retardation caused by the strong hydrophobicity of quaternary ammonium group-functionalized polynorbornene ionomers can greatly prolong the *CO residence on Cu. This unconventional electro-kinetic effect is demonstrated by the increased Tafel slopes and the decreased sensitivity of *CO coverage change to potentials. As a result, the strongly hydrophobic Cu electrodes exhibit C2+ Faradaic efficiencies of ≈90 % at a partial current density of 223 mA cm-2 , more than twice of bare or hydrophilic Cu surfaces.

Influence of Cations on HCOOH and CO Formation during CO2 Reduction on a PdMLPt(111) Electrode.

Understanding the role of cations in the electrochemical CO2 reduction (CO2RR) process is of fundamental importance for practical application. In this work, we investigate how cations influence HCOOH and CO formation on PdMLPt(111) in pH 3 electrolytes. While only (a small amount of adsorbed) CO forms on PdMLPt(111) in the absence of metal cations, the onset potential of HCOOH and CO decreases with increasing cation concentrations. The cation effect is stronger on HCOOH formation than that on CO formation on PdMLPt(111). Density functional theory simulations indicate that cations facilitate both hydride formation and CO2 activation by polarizing the electronic density at the surface and stabilizing *CO2-. Although the upshift of the metal work function caused by high coverage of adsorbates limits hydride formation, the cation-induced electric field counterbalances this effect in the case of *H species, sustaining HCOOH production at mild negative potentials. Instead, at the high *CO coverages observed at very negative potentials, surface hydrides do not form, preventing the HCOOH route both in the absence and presence of cations. Our results open the way for a consistent evaluation of cationic electrolyte effects on both activity and selectivity in CO2RR on Pd-Pt catalysts.

Acidic Media Impedes Tandem Catalysis Reaction Pathways in Electrochemical CO2 Reduction.

Electrochemical CO2 reduction (CO2 R) in acidic media with Cu-based catalysts tends to suffer from lowered selectivity towards multicarbon products. This could in principle be mitigated using tandem catalysis, whereby the *CO coverage on Cu is increased by introducing a CO generating catalyst (e.g. Ag) in close proximity. Although this has seen significant success in neutral/alkaline media, here we report that such a strategy becomes impeded in acidic electrolyte. This was investigated through the co-reduction of 13 CO2 /12 CO mixtures using a series of Cu and CuAg catalysts. These experiments provide strong evidence for the occurrence of tandem catalysis in neutral media and its curtailment under acidic conditions. Density functional theory simulations suggest that the presence of H3 O+ weakens the *CO binding energy of Cu, preventing effective utilization of tandem-supplied CO. Our findings also provide other unanticipated insights into the tandem catalysis reaction pathway and important design considerations for effective CO2 R in acidic media.

Acidic Media Impedes Tandem Catalysis Reaction Pathways in Electrochemical CO2 Reduction

AbstractElectrochemical CO2 reduction (CO2R) in acidic media with Cu‐based catalysts tends to suffer from lowered selectivity towards multicarbon products. This could in principle be mitigated using tandem catalysis, whereby the *CO coverage on Cu is increased by introducing a CO generating catalyst (e.g. Ag) in close proximity. Although this has seen significant success in neutral/alkaline media, here we report that such a strategy becomes impeded in acidic electrolyte. This was investigated through the co‐reduction of 13CO2/12CO mixtures using a series of Cu and CuAg catalysts. These experiments provide strong evidence for the occurrence of tandem catalysis in neutral media and its curtailment under acidic conditions. Density functional theory simulations suggest that the presence of H3O+ weakens the *CO binding energy of Cu, preventing effective utilization of tandem‐supplied CO. Our findings also provide other unanticipated insights into the tandem catalysis reaction pathway and important design considerations for effective CO2R in acidic media.

Vibrational frequencies of CO bound to all three low-index cerium oxide surfaces: A consistent theoretical description of vacancy-induced changes using density functional theory.

The facet-dependent adsorption of CO on oxidized and reduced CeO2 single crystal surfaces is reviewed, with emphasis on the effect of CO coverage and the ability of state-of-the-art quantum-mechanical methods to provide reliable energies and an accurate description of the IR vibrational frequency of CO. Comparison with detailed, high-resolution experimental infrared reflection absorption spectroscopy data obtained for single crystal samples allows the assignment of the different CO vibrational bands observed on all three low-index ceria surfaces. Good agreement is achieved with the hybrid density functional theory approach with the HSE06 functional and with saturation coverage. It is shown that CO is very sensitive to the structure of cerium oxide surfaces and to the presence of oxygen vacancies. The combined theoretical-experimental approach offers new opportunities for a better characterization of ceria nanoparticles and for unraveling changes occurring during reactions involving CO at higher pressures.

Analyzing characteristic and modeling of high-temperature proton exchange membrane fuel cells with CO poisoning effect

High-temperature proton exchange membrane fuel cells (HT-PEMFC) have strong resistance to CO poisoning. However, the published CO poisoning models for HT-PEMFC are based on the finite element analysis method, which are difficult to use for the performance prediction and development of system control strategies due to a large amount of calculation. In the beginning, a semi-empirical model of HT-PEMFC is deduced based on the analysis of the CO poisoning characteristics. Then the key parameters of ohmic impedance and concentration polarization advance correction factor are obtained by fitting the polarization curves, dissociation and adsorption activation energy of H2 and CO are obtained by the calculation, respectively. The slope of the straight line segment in the ohmic polarization interval of the polarization curve is linearly related to ln[H2/CO]. The change of limiting current density is the main factor for the advance of concentration polarization. The fitted ohmic impedance varies with temperature and CO concentration, but changes little above 175 °C. CO coverage depends primarily on dissociation and adsorption activation energy of CO. Subsequently, the proposed model is validated with experimental data and shows high similarity. The model can be used for studying the systematic control strategy development and performance monitoring of HT-PEMFC.

Solvent Effect on Electrochemical CO2 Reduction Reaction on Nanostructured Copper Electrodes.

The electrochemical reduction of CO2 (CO2RR) is a sustainable alternative for producing fuels and chemicals, although the production of highly desired hydrocarbons is still a challenge due to the higher overpotential requirement in combination with the competitive hydrogen evolution reaction (HER). Tailoring the electrolyte composition is a possible strategy to favor the CO2RR over the HER. In this work we studied the solvent effect on the CO2RR on a nanostructured Cu electrode in acetonitrile solvent with different amounts of water. Similar to what has been observed for aqueous media, our online gas chromatography results showed that CO2RR in acetonitrile solvent is also structure-dependent, since nanocube-covered copper (CuNC) was the only surface (in comparison to polycrystalline Cu) capable of producing a detectable amount of ethylene (10% FE), provided there is enough water present in the electrolyte (>500 mM). In situ Fourier Transform Infrared (FTIR) spectroscopy showed that in acetonitrile solvent the presence of CO2 strongly inhibits HER by driving away water from the interface. CO is by far the main product of CO2RR in acetonitrile (>85% Faradaic efficiency), but adsorbed CO is not detected. This suggests that in acetonitrile media CO adsorption is inhibited compared to aqueous media. Remarkably, the addition of water to acetonitrile has little quantitative and almost no qualitative effect on the activity and selectivity of the CO2RR. This indicates that water is not strongly involved in the rate-determining step of the CO2RR in acetonitrile. Only at the highest water concentrations and at the CuNC surface, the CO coverage becomes high enough that a small amount of C2+ product is formed.

Boosting C2+ production from photoelectrochemical CO2 reduction on gallium doped Cu2O

Photoelectrochemical CO2 reduction into C2+ products not only realizes the efficient conversion and utilization of CO2, but also provides a new way to synthesize value added hydrocarbons. However, it remains a grand challenge for highly selective synthesis of C2+ product due to the complexity of CO2 reduction and the coexistence of competitive reaction on the electrode surface. Herein we report a gallium doped Cu2O catalyst (Ga/Cu2O) prepared by a typical electrochemical deposition method via oriented growth on the surface of Cu mesh with easily controllable Ga contents. As the self-supporting electrode, it exhibits a highly efficient and selective CO2 reduction into C2+ products with Faradaic efficiencies of C2+, CH3CH2OH, and CH3CH2CH2OH as high as 20%, 6.5% and 6.64%, respectively at the potential of −1.8 V vs reversible hydrogen electrode (RHE). Experimental results show that Ga doping generated more oxygen vacancies via partial substitution in the lattice of Cu2O, resulting in rapid separation of the generated electron-hole pairs and regulating the electronic structure of catalyst surface and promotion of CO2 adsorption and activation. As a result, a higher *CO coverage and better C-C coupling probability on the surface of Ga/Cu2O were reached compared to those of unmodified Cu2O, which are responsible for the enhancement of the selectivity of CO2RR into C2+ products.

Boosting C–C coupling to multicarbon products via high-pressure CO electroreduction

Electrochemical CO reduction reaction (CORR) provides a promising approach for producing valuable multicarbon products (C2+), while the low solubility of CO in aqueous solution and high energy barrier of C–C coupling as well as the competing hydrogen evolution reaction (HER) largely limit the efficiency for C2+ production in CORR. Here we report an overturn on the Faradaic efficiency of CORR from being HER-dominant to C2+ formation-dominant over a wide potential window, accompanied by a significant activity enhancement over a Moss-like Cu catalyst via pressuring CO. With the CO pressure rising from 1 to 40 atm, the C2+ Faradaic efficiency and partial current density remarkably increase from 22.8% and 18.9 mA cm−2 to 89.7% and 116.7 mA cm−2, respectively. Experimental and theoretical investigations reveal that high pressure-induced high CO coverage on metallic Cu surface weakens the Cu–C bond via reducing electron transfer from Cu to adsorbed CO and restrains hydrogen adsorption, which significantly facilitates the C–C coupling while suppressing HER on the predominant Cu(111) surface, thereby boosting the CO electroreduction to C2+ activity.

Surface morphology of Fe3C catalyst under different CO coverage from DFT and thermodynamics

High-coverage CO adsorption on seven surfaces of Fe3C catalyst has been systematically studied by density functional theory and thermodynamics. Results show that CO molecules prefer to bind on surface iron atoms of Fe3C catalyst, and the saturation coverage ranges from 0.5 to 0.833 monolayers (ML) on different surfaces. The equilibrium phase diagrams of CO adsorption on different Fe3C surfaces at different CO coverage are plotted, and the stable CO concentration on each surface of Fe3C at any CO partial pressure and temperature could be easily identified. CO adsorption/activation on Fe3C catalyst via an acceptance-donation mechanism can be explained by the projected density of states (PDOS) and charge density difference plots. Interestingly, stabilities of the Fe3C surfaces can be modified by adsorbed CO, and eventually the morphologies of Fe3C nanoparticle can also be changed. Our results provide essential information for selective synthesis of Fe3C catalyst with specific structures experimentally.

Tolerance of Silicon Oxide‐Coated Pt/C Catalyst Toward CO and H2S Contamination in Hydrogen for Proton Exchange Membrane Fuel Cells

Platinum on graphitized low surface area carbon (Pt/C) is coated with a silicon oxide thin film and is employed as anode catalyst to manipulate the tolerance of proton exchange membrane fuel cells toward carbon monoxide and hydrogen sulfide contamination. The SiO2 coating, prepared by successive hydrolysis of 3‐aminopropyl‐triethoxisilane and tetraethoxysilane, forms clusters in proximity to Pt in sizes comparable to the catalyst particles, leaving most of the carbon surfaces free. The performance with and without CO is investigated in situ at relative humidities (RH) of 100%, 70%, and 40%. When operated with neat hydrogen, SiO2‐Pt/C shows marginally better performance owing to an improved protonic conduction due to the water retaining hydrophilic SiO2. Upon operation with CO‐contaminated fuel, the SiO2‐Pt/C performs worse than that of Pt/C particularly at high RH. CO stripping measurements reveal an increase in CO oxidation potential for the SiO2‐Pt/C, suggesting an increased CO coverage and consequently higher anode overpotentials during operation with CO‐contaminated fuel. Upon operation with H2S in the fuel, the SiO2 coating extends the lifetime until the cell voltage broke down, which is attributed to the enhanced water retention due to SiO2 and the solubility of sulfuric species.

Ambient-Pressure X-ray Photoelectron Spectroscopy Study of Acetic Acid Thermal Decomposition on Pd(111)

Acetic acid on Pd(111) is an excellent model system to study the effect of water on thermal decomposition of fatty acids and other oxygenates. Acetic acid is a simple molecule that has both methyl and acid groups, where its decomposition involves many of the key bond-breaking steps involved in more complex chemical decompositions. Here, we use ambient-pressure X-ray photoelectron spectroscopy and mass spectrometry supported by density functional theory (DFT) calculations of the core-level binding energies to study the effect of co-adsorbed water on the decomposition of acetic acid. The addition of water to acetic acid results in an increased acetic acid/acetate coverage and a decreased CO coverage. Moreover, when water is co-dosed with acetic acid, the gas phase composition from the reaction shows an approximately 30% increase in the CO2/CO ratio, suggesting a shift toward CO2 production in the presence of water. These observations are supported by previous DFT calculations, which show that the presence of co-adsorbed water lowers the barriers for O–H bond breaking but increases the barriers for C–O bond breakage, leading to increased CO2 production.

Pressure dependence and mechanism of Mn promotion of silica-supported Co catalyst in the Fischer-Tropsch reaction

The mechanism of Mn promotion of a silica-supported Co catalyst in the Fischer-Tropsch reaction has been studied at varying pressures up to 20 bar. IR spectroscopy in combination with DFT calculations suggest adsorbed CO is activated by reaction with an oxygen vacancy in the MnO, which covers the Co surface. This leads to a higher activity, higher CHx coverage and thus higher C5+ and lower CH4 selectivity. Increasing the pressure magnifies the selectivity differences. However, above around 4 bar, the effect of Mn on the selectivities is reversed and the C5+ selectivity is decreased by Mn addition. This is tentatively attributed to Mn promoting the C-O bond dissociation but not the chain growth. Formed monomers have to migrate to stepped sites for chain growth on the Co surface. Whilst this is migration is not impeded by co-adsorbates at low pressure, migration could be hindered by especially the high CO coverage at high pressure.

Understanding the restructuring and degradation of oxide-derived copper during electrochemical CO2 reduction

Renewable energy produced through electrochemical CO2 reduction is promising for mitigating the energy crisis and environmental issues. Notably, oxide-derived copper (OD-Cu) has attracted considerable attention owing to its unique ability to catalyze CO2 electroreduction to >2e− products, especially C2+ products, which are the key feedstocks for the chemical process. However, Cu under operating conditions usually leads to structure reconstruction and degradation, which hinders understanding the origin of its activity. Understanding the restructuring and degradation of OD-Cu catalyst to form C2+ products is critical for understanding the active sites and, thus for designing effective and durable catalysts for CO2 reduction. This paper describes that Cu defect sites are easily dissolved during electrochemical CO2 reduction, and the presence of defect sites is strongly related to the catalytic performance of C2+ products. Here, the C2+ products Faradaic efficiency (FE) of OD-Cu decreased from 39.4% at 1 h to 26.0% at 5 h at −1.5 V vs. RHE, along with a decrease in low-coordinated defect sites. The low-coordinated defect sites facilitate CO adsorption and help to increase the CO coverage, thus promoting CO–CO coupling. The decrease in the number of defect sites and activity degradation is circumvented by promoting the Cu redeposition process. The C2+ products FE of OD-Cu can be maintained at ca. 47.2% without an obvious decrease in CO2-saturated 0.1 M KHCO3 with 1 mM Cu2+. The results of this study help in understanding the activity of OD-Cu and reveal its deactivation mechanism. This knowledge will aid in designing highly active and stable catalysts for CO2 reduction.

Pressure dependence in aqueous-based electrochemical CO2 reduction

Electrochemical CO2 reduction (CO2R) is an approach to closing the carbon cycle for chemical synthesis. To date, the field has focused on the electrolysis of ambient pressure CO2. However, industrial CO2 is pressurized—in capture, transport and storage—and is often in dissolved form. Here, we find that pressurization to 50 bar steers CO2R pathways toward formate, something seen across widely-employed CO2R catalysts. By developing operando methods compatible with high pressures, including quantitative operando Raman spectroscopy, we link the high formate selectivity to increased CO2 coverage on the cathode surface. The interplay of theory and experiments validates the mechanism, and guides us to functionalize the surface of a Cu cathode with a proton-resistant layer to further the pressure-mediated selectivity effect. This work illustrates the value of industrial CO2 sources as the starting feedstock for sustainable chemical synthesis.

Constrained C2 adsorbate orientation enables CO-to-acetate electroreduction.

The carbon dioxide and carbon monoxide electroreduction reactions, when powered using low-carbon electricity, offer pathways to the decarbonization of chemical manufacture1,2. Copper (Cu) is relied on today for carbon-carbon coupling, in which it produces mixtures of more than ten C2+ chemicals3-6: a long-standing challenge lies in achieving selectivity to a single principal C2+ product7-9. Acetate is one such C2 compound on the path to the large but fossil-derived acetic acid market. Here we pursued dispersing a low concentration of Cu atoms in a host metal to favour the stabilization of ketenes10-chemical intermediates that are bound in monodentate fashion to the electrocatalyst. We synthesize Cu-in-Ag dilute (about 1 atomic per cent of Cu) alloy materials that we find to be highly selective for acetate electrosynthesis from CO at high *CO coverage, implemented at 10 atm pressure. Operando X-ray absorption spectroscopy indicates in situ-generated Cu clusters consisting of <4 atoms as active sites. We report a 12:1 ratio, an order of magnitude increase compared to the best previous reports, in the selectivity for acetate relative to all other products observed from the carbon monoxide electroreduction reaction. Combining catalyst design and reactor engineering, we achieve a CO-to-acetate Faradaic efficiency of 91% and report a Faradaic efficiency of 85% with an 820-h operating time. High selectivity benefits energy efficiency and downstream separation across all carbon-based electrochemical transformations, highlighting the importance of maximizing the Faradaic efficiency towards a single C2+ product11.

Identifying the Structure of Supported Metal Catalysts Using Vibrational Fingerprints from Ab Initio Nanoscale Models.

Identifying active sites of supported noble metal nanocatalysts remains challenging, since their size and shape undergo changes depending on the support, temperature, and gas mixture composition. Herein, the anharmonic infrared spectrum of adsorbed CO is simulated using density functional theory (DFT) to gain insight into the nature of Pd nanoparticles (NPs) supported on ceria. The authors systematically determine how the simulated infrared spectra are affected by CO coverage, NP size (0.5-1.5nm), NP morphology (octahedral, icosahedral), and metal-support contact angle, by exploring a diversity of realistic models inspired by ab initio molecular dynamics. The simulated spectra are then used as a spectroscopic fingerprint to characterize nanoparticles in a real catalyst, by comparison with in-situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) experiments. Truncated octahedral NPs with an acute Pd-ceria angle reproduce most of the measurements. In particular, the authors isolate features characteristic of CO adsorbed at the metal-support interface appearing at low frequencies, both seen in simulation and experiment. This work illustrates the strong need for realistic models to provide a robust description of the active sites, especially at the interface of supported metal nanocatalysts, which can be highly dynamic and evolve considerably during reaction.