Patent Application
20220411941
The technical field generally relates to catalytic methods for carbon dioxide (CO2) and monoxide (CO) reduction, and more particularly to electrocatalysts composed of metallic material such as Cu doped with Ag or with Ag and Ru, and associated methods of manufacture and use in electrochemical reduction for the production of C3 products.
The electrosynthesis of C3 products from carbon dioxide (CO2) and carbon monoxide (CO) addresses the need for long-term storage of renewable electricity. Unfortunately, present-day performance remains below that needed for practical applications. There is a need for improved techniques and catalyst materials for efficient electrochemical reduction of gases, such as CO and CO2, and related methods and systems of producing chemical compounds, such as C3 compounds.
Various implementations, features and aspects of the technology are described herein and relate to electroreduction electrocatalysts for converting a carbon-containing gas to produce products, such as n-propanol, where the electrocatalysts include a multi-metallic material comprising a primary metal, such as Cu, and a metal dopant, such as Ag and optionally a second metal dopant, selected and distributed to provide structures that promote C2-C1 coupling. The technology also relates to manufacturing of the electrocatalysts and their use for the electroreduction of CO or CO2, for example.
In some implementations, there is provided an electrocatalyst for electroreduction of a carbon-containing gas to produce C3 products, the electrocatalyst comprising a multi-metallic material comprising a primary metal and a metal dopant selected and distributed to provide asymmetric active sites that include neighbouring atoms of the primary metal having distinct electronic structures to promote C2-C1 coupling.
Preferably, the primary metal is Cu, the metal dopant is Ag, and the multi-metallic material is a bimetallic material. The metal dopant can include Ag and a second dopant metal and the multi-metallic material is a trimetallic material.
In some implementations, the carbon-containing gas comprises or is CO, CO2, or a mixture thereof. In some implementations, the C3 product is n-propanol.
In some implementations, multi-metallic material comprises the primary metal doped with the metal dopant using galvanic replacement. The metal dopant can be present in the primary metal in a doping concentration of 2 wt % to 9 wt %, in a doping concentration of 3 wt % to 8 wt %, in a doping concentration of 3wt % to 5 wt %, in a doping concentration of 3.5 wt % to 4.5 wt %, or in a doping concentration of approximately 4 wt %, measured with XPS.
In some implementations, the electrocatalyst is provided in the form of bimetallic or trimetallic nanoparticles. The bimetallic or trimetallic nanoparticles can have an average size between about 20 nm and about 200 nm, or between about 50 nm and about 200 nm, between about 70 nm and about 150 nm, or between about 90 nm and 130 nm, measured based on SEM or TM imaging. The bimetallic or trimetallic nanoparticles can be generally spheroid in shape, determined from SEM or TM imaging.
In some implementations, the electrocatalyst is formed as a deposited catalyst layer on a first side of gas diffusion membrane, wherein the deposited catalyst layer is configured to be in direct contact with an electrolyte and wherein a second opposed side of the gas diffusion membrane is configured to be in direct contact with the carbon-containing gas.
In some implementations, there is provided an electrocatalyst for electroreduction of a carbon-containing gas to produce a C3 product, the electrocatalyst comprising a bimetallic or trimetallic material comprising a copper (Cu) and at least one metal dopant in a doping concentration of 2 wt % to 9 wt %, or 3 wt % to 8 wt %, or 3wt % to 5 wt %, or 3.5 wt % to 4.5 wt % or approximately 4 wt %, measured with XPS. The metal dopant can include or be Ag. The metal dopant can include a primary dopant and a secondary dopant. In some implementations, the primary dopant is Ag and/or the secondary dopant is Ru.
In some implementations, there is provided a method of fabricating an electrocatalyst composed of a primary metal and at least one metal dopant for electroreduction of a carbon containing gas into C3 products, such as n-propanol, comprising galvanic replacement.
The method can include providing a layer of Cu particles on a substrate to provide a coated substrate; immersing the coated substrate in an Ag containing aqueous solution to induce doping and form an Ag-doped multimetallic catalyst material supported by the substrate; and removing the coated substrate from the solution, the coated substrate comprising a layer of the Ag-doped multimetallic catalyst material. In some implementations, the Cu particles comprise Cu nanoparticles. In some implementations, the providing of the layer of Cu particles on the substrate is performed by spray coating to form the coated substrate. In some implementations, the Ag containing aqueous solution is an AgNO3 aqueous solution. In some implementations, a second dopant metal is incorporated to form the multimetallic catalyst material supported by the substrate.
In some implementations, there is provided a process for electrochemical production of a C3 multi-carbon compound from a carbon-containing gas, comprising: contacting the carbon-containing gas and an electrolyte with an electrode comprising the electrocatalyst as defined herein or as manufactured by the method as defined herein, such that the carbon-containing gas contacts the electrocatalyst; applying a voltage to provide a current density to cause the carbon-containing gas contacting the electrocatalyst to be electrochemically converted into the C3 multi-carbon compound; and recovering the C3 multi-carbon compound.
In some implementations, the C3 multi-carbon compound is an alcohol, such as propanol which may be n-propanol. In some implementations, the electrolyte comprises an alkaline compound. The electrolyte can include KOH and/or other alkaline solutions. In some implementations, carbon-containing gas comprises or is CO, CO2 or both.
In some implementations, there is provided a system for CO and/or CO2 electroreduction to produce a multi-carbon compound, comprising: an electrolytic cell configured to receive a liquid electrolyte and CO and/or CO2 gas; an anode; a cathode comprising an electrocatalyst as defined herein or as manufactured by the method as defined herein; and a voltage source to provide a current density to cause the CO and/or CO2 gas contacting the electrocatalyst to be electrochemically converted into the multi-carbon compound.
The Figures describe various aspects and information regarding the technology.
The present description relates to metal catalyst materials to promote the formation of C3 compounds from reactants, such as CO and CO2 gas, in electroreduction conditions as well as related processes for producing the C3 compounds and for manufacturing the catalyst materials. The present description particularly relates to electroreduction multi-metallic catalysts including a primary metal, such as copper (Cu), and one or more dopant metals, such as silver (Ag) and Ruthenium (Ru). The electroreduction catalysts can be composed so as to have asymmetric active sites, providing a structure to interact with two adsorbates to catalyse C2-C1 coupling, thereby promoting formation of C3 compounds.
Electroreduction of C1 feed gas to high-energy-density fuels provides an attractive avenue to the storage of renewable electricity. Much progress has been made to improve selectivity to C1 and C2 products; however, the selectivity to desirable high-energy-density C3 products remains relatively low. It was reasoned that C3 electrosynthesis relies on a higher-order reaction pathway that requires the formation of multiple carbon-carbon (C—C) bonds; and thus pursued a strategy explicitly designed to couple C2 with C2 intermediates. This work developed an approach to construct asymmetric active sites by doping a very low amount of Ag in Cu via galvanic replacement. The asymmetric active site contains two neighbouring copper atoms with distinct electronic structures, which can interact with two adsorbates to catalyse an asymmetric reaction, thereby boosting C2-C1 coupling. This work achieved a notable Faradaic efficiency (FE) of 33±1% with a conversion rate of 4.5±0.1 mA cm−2; and a notable cathodic energy conversion efficiency (EE) of 21%, all for n-propanol. This innovation also represents the first report of CO electroreduction to C3 based on multi-metallic (e.g., bimetallic or trimetallic) catalysts.
The following provides descriptions of some embodiments, implementations, features, aspects, and experimentation that was conducted in the context of the instant technology.
The below description relates to the efficient upgrading of CO to C3 fuel using asymmetric C-C coupling active sites.
The electroreduction of C1 feedgas to high-energy-density fuels provides an attractive avenue to the storage of renewable electricity. Much progress has been made to improve selectivity to C1 and C2 products; however, the selectivity to desirable high-energy-density C3 products remains relatively low. This work reasoned that C3 electrosynthesis relies on a higher-order reaction pathway that requires the formation of multiple carbon-carbon (C-C) bonds; and thus pursue a strategy explicitly designed to couple C2 with C1 intermediates. This work developed an approach wherein neighbouring copper atoms having distinct electronic structures interact with two adsorbates to catalyze an asymmetric reaction. This work achieved an n-propanol Faradaic efficiency (FE) of (33±1) % with a conversion rate of (4.5±0.1) mA cm−2, and an n-propanol cathodic energy conversion efficiency (EEcathodic half-cell) of 21%.
The CO2 electroreduction reaction (CO2RR) to high-energy-density liquid products is an attractive avenue to achieving the storage of renewable energy. In recent years, much progress has been made in CO2RR, but the main products reported have been C1 (CO, CH4, methanol, and formate) and C2 (ethylene, acetate, and ethanol) products. N-propanol, a high-value and high-energy-density C3 product that can be generated via either CO2 or CO electroreduction, has been produced with low-to-moderate Faradaic efficiencies (FEs) in prior reports.
In CO2RR, the formation of multi-carbon products starts with the formation of the CO intermediate, followed by the CO electroreduction process. Recently, significant progress has been made in CO2RR to CO with a FE nearly 100%. To achieve the ultimate goal of high selectivity to high-value-added C3 products from CO2RR, it is of interest to improve significantly the FE for CO electroreduction (CORR) to C3 products.
The formation of C3 products from CORR relies on the sequential formation of two carbon-carbon (C—C) bonds, the main reaction mechanism for C3 formation reported previously. Cu provides excellent C-C coupling and produces multi-carbon chemicals in the electroreduction of CO; however, the selectivity towards C3 products on Cu has remained low. The generation of C3 products from CO requires multiple product/intermediate formation steps, and it is prone to the competing production of a wide variety of chemical products.
Increasing selectivity in the electroreduction of CO to C3 products is thus an important challenge to address in the field of electrocatalysis. Until now, catalysts for CORR have focused on Cu and oxide-derived Cu catalysts, and a number of factors have been found to increase performance: these include engineering the oxidation state of the atoms making up the metal catalyst, as well as grain-boundary effects and the selective formation of desired facets. However, the main products of these Cu and oxide-derived Cu catalysts have been C2 chemicals (ethanol, acetate, and ethylene), and the selectivity to C3 products has saturated in recent manuscripts based on Cu catalysts.
Here this work explored instead a doping strategy involving different metal-doped Cu (M-doped Cu) catalysts in an attempt to increase C3 production in CORR. The low C3 selectivity on Cu catalysts is associated with the low rate of C-C bond formation, including C1-C1 and C1-C2 coupling. Mechanisms underpinning C1-C1 coupling to C2 products have been explored extensively in prior studies; while C1-C2 coupling to C3 products is less explored. Here this work screened the propensity to catalyze C1-C1 and C1-C2 coupling using density functional theory (DFT) based on M-doped Cu catalysts. This work found that, among different M-doped Cu candidates explored, Ag-doped Cu can offer the highest activity for both C1-C1 and C1-C2 coupling, and this work pinpointed a role for the asymmetric C-C coupling active site in this high activity. Specifically, this active site consists of two neighbouring Cu atoms that exhibit different electronic structures: this asymmetry among the neighbours' energetics is determined by the combination of strain and ligand effects arising upon Ag doping. This work then fabricated Ag-doped Cu nanocatalysts via a galvanic replacement approach. The work demonstrated that the synthesized Ag-doped Cu catalyst exhibits higher FEs for n-propanol compared to all previous CORR reports. This leads to superior energy conversion efficiency in the cathodic half-cell (EEcathodic half-cell) for n-propanol.
The nanocatalysts could also include a second dopant metal in addition to Ag to produce a trimetallic catalyst. The second dopant metal can be Ru or another metal suitable for doping along with Ag. The second dopant metal can be incorporated into the material using techniques known in the art; and is selected for compatibility with the Cu and Ag metals as well as the electrocatalytic applications of the catalyst material. The second dopant metal can also be selected to enhance certain properties of the catalyst material and its operation in electrocatalysis applications.
Catalysts that convert CO into C3 chemicals require high activity for both C1-C1 and C1-C2 coupling. With the goal of designing better catalysts for C3 production, this work investigated the energetics of C1-C1 and C1-C2 coupling reactions to illustrate the C2 and C3 formation rates with the aid of DFT (more details of DFT methods and choice of sequential mechanism can be found in Supplementary Information). Several M-doped Cu systems (M=Ag, Au, Ru, Rh, and Pd) were considered because bimetallic catalysts have been shown to tune catalyst performance in other catalytic reactions. CO dimerization is one reaction pathway for C1-C1 coupling, and thus this work used the barrier of CO dimerization to describe the readiness of C1-C1 coupling. Due to the abundance of CO species in CORR, the work used the barrier of OCCO and CO coupling as the indicator for the C1-C2 coupling (e.g.,
This work carried out further theoretical investigations to uncover physical origins of enhancement in C1-C1 and C1-C2 coupling on Ag-doped Cu. The work used a model with one Ag atom doped in a Cu(111) slab with a 3×3 unit cell, which corresponds to about 3% doping concentration for four layers of Cu atoms. By a margin of 0.63 eV, Ag at the surface of Cu(111) is more favourable than in subsurface of Cu(111), inducing us to focus on surface-localized Ag in the ensuing studies.
As the radius of the Ag atom is larger than that of Cu atom, Ag doping produces surface strain. The bond length of Cu—Cu changed from 2.57 Å on the Cu(111) surface to 2.55 Å and 2.48 Å on the Ag-doped Cu surface, resulting in asymmetric compressive strain. The ligand effect caused by Ag doping in Cu also has the potential to affect C1-C1 and C1-C2 coupling. To evaluate the effect of the strain and ligand, a model was built with the same strain of Ag-doped Cu but without Ag substitution by fixing the bond length of Cu surface (denoted Cu with strain). This work calculated the activation energies for C1-C1 and C1-C2 coupling steps on Cu, Cu with strain, and Ag-doped Cu models (Supplementary Table 4). Calculation results show that both compressive strain and ligand effects contribute to enhanced activity for C1-C1 and C1-C2 coupling (
Ag doping in Cu leads to two classes of neighbouring Cu atoms (denoted Cu-a and Cu-b atoms,
This work sought to prepare experimentally Ag-doped Cu catalysts. The work employed a galvanic replacement reaction driven by the difference in the reduction potential of Ag vs. Cu (ref. 46). Firstly, this work deposited a thin layer of commercial Cu nanoparticles with average size of 100 nm on a carbon-based gas diffusion layer (GDL) via spray-coating (
This work then investigated the performance of the Ag-doped Cu GDE in CORR flow cell reactors (
It should be noted that the highest n-propanol FE on both Ag-doped Cu and Cu GDEs were achieved at relatively low potential (−0.46 VRHE), and n-propanol FE decreased when further increasing the potential to −0.56 VRHE (
To study the chemical state of the catalyst during CORR, this work performed operando X-ray absorption spectroscopy (XAS) of Ag-doped Cu GDE under operando CORR conditions at a constant applied potential of −0.46 VRHE. Operando Cu K-edge X-ray absorption near edge structure (XANES) spectra of Ag-doped Cu GDE show that Cu atoms were reduced to Cu0 in the first 10 s during CORR (
To explore further the role of Ag doping in Cu in facilitating C1-C1 and C1-C2 coupling, this work investigated the CORR performance of Ag-doped Cu catalysts with different Ag concentrations. To vary the Ag concentration, this work changed the immersion time of the Cu GDE in AgNO3 solution to 20 min and 2 h, respectively, denoted Cu—Ag-20 min and Cu—Ag-2 h. As in Ag-doped Cu, both Cu—Ag-20 min and Cu—Ag-2 h catalysts also retained the particle size and the morphology of the pristine Cu nanoparticles (
The carbon-based gas diffusion layers suffer from liquid penetration and gas diffusion blockage, termed flooding, over time. To overcome the flooding issue on carbon-based GDE after long operation time, this work fabricated the Ag-doped Cu polytetrafluoroethylene (PTFE) electrode based on a configuration (graphite/carbon nanoparticle/Ag-doped Cu/PTFE electrode) that has been developed. The Ag-doped Cu layer was prepared by immersing a Cu layer in 5 □mol L−1 AgNO3 aqueous solution at 65° C. for 1 h. When a potential of −0.46 VRHE was applied, the FE of n-propanol on the Ag-doped Cu PTFE electrode achieved 33% and operated stably over 200 min of CORR (
This work also compared the CO2RR performance of Ag-doped Cu and pristine Cu to explore whether the asymmetric active sites enhance in this distinct context the C1-C1 and C1-C2 coupling (
This work demonstrates Ag doping in Cu to facilitate C1-C1 and C1-C2 coupling and thus improve the selectivity to C3 products during CORR. DFT results show that the strain and ligand effects due to Ag doping jointly provide an asymmetric C—C coupling active site containing two neighbouring Cu atoms with different electronic structures, and that these are capable of enhancing C1-C1 and C1-C2 coupling. Experimentally, this work achieved a total C2+ FE of about 80% and a record n-propanol FE of (33±1) % with a partial n-propanol current density (4.5±0.1) mA cm−2 on Ag-doped Cu catalyst in CORR. The EEcathodic half-cell for n-propanol also reaches 21% at a low potential of 0.416 VRHE, with a low overpotential of 0.616 V. These findings provide a framework for rational catalyst design for tuning the CORR selectivity towards high-energy-density C3 liquid products, a relevant step in overcoming the bottleneck of the electroproduction of C3 products.
In this work, all the DFT calculations were carried out with a periodic slab model using the Vienna ab initio simulation program (VASP) (https://www.vasp.at/). Detailed theoretical methods are found in the Supplementary Information.
8.5 mg of commercial Cu was dispersed in a mixture of 0.85 mL of methanol and 8.5 □l of 5% Nafion under ultrasonication for 30 min. The suspension was deposited on a carbon-based GDL using spray-coating with a catalyst loading of ≈1 mg cm−2 to prepare the Cu GDE. The prepared Cu GDE was immersed in 5 □mol L−1 AgNO3 aqueous solution at 65° C. for a certain time period to prepare Ag-doped Cu GDE as cathodes. The main goal of the work was to focus on improving the efficiency of the cathodic side of CORR to propanol. Thus, the work used Ni foam (1.6 mm thickness, MTI Corporation) as the oxygen evolution reaction (OER) catalyst in the anode side because it is commercially available and have been showed as a good OER catalyst. Details of chemicals and materials information are found in the Supplementary Information.
Scanning electron microscopy (SEM) images were taken using a Quanta FEG 250 microscope. (Certain commercial equipment, instruments, or materials are identified in this paper and Supplementary Information in order to specify the experimental procedure adequately. Such identification is not intended to imply recommendation or endorsement by the National Institute of Standards and Technology, nor is it intended to imply that the materials or equipment identified are necessarily the best available for the purpose). HAADF-STEM images were taken using an aberration-corrected FEI Titan 80-300 kV TEM/STEM microscope at 300 kV, with a probe convergence angle of 30 mrad and a large inner collection angle of 65 mrad to provide a nominal image solution of 0.7 Å. EELS elemental mapping was collected on aberration-corrected JEOL JEM-ARM200F electron microscope at 200 kV equipped with Gatan GIF quantum energy filters. Structural characterization of cathodes was obtained using XRD (MiniFlex600) with Cu-K□ radiation. The surface compositions of cathodes were determined by XPS (model 5600, Perkin-Elmer) using a monochromatic aluminum X-ray source. Operando XAS measurement were conducted at 9BM beamline at Advanced Photon Source (APS, Argonne national laboratory, IL). Athena and Artemis software included in a standard IFEFFIT package were used to process XAS data. WAXS measurements were carried out in transmission geometry at the CMS beamline of the National Synchrotron Light Source II (NSLS-II), a U.S. Department of Energy (DOE) office of the Science User Facility operated for the DOE Office of Science by Brookhaven National Laboratory. Samples were measured with an imaging detector at a distance of 0.177 m using X-ray wavelength of 0.729 Å. Nika software package was used to sector average the 2D WAXS images. Data plotting was done in Igor Pro (Wavemetrics, Inc., Lake Oswego, Oreg., USA).
All the electrochemical measurements were conducted in flow cell reactor. Electrocatalytic measurements were operated using the three-electrode system at an electrochemical station (AUT50783). In the flow cell reactor, the prepared GDEs, anion exchange membrane, and nickel foam were positioned and clamped together between silicone gaskets and PTFE flow fields. Then 10 mL of electrolyte (1 M KOH aqueous solution) was introduced into the anode chamber between anode and membrane, as well as the cathode chamber between membrane and cathode, respectively. The electrolytes in cathode and anode were circulated by two pumps at the rate of 10 mL min−1. CO gas (Linde, 99.99%) or CO2 gas (Linde, 99.99%) was continuously supplied to gas chamber located at the back side of cathode GDE at the rate of 50 mL min−1. Gas could diffuse into the interface between cathode and electrolyte, thus generating a triple-phase interface between gas, electrode, and electrolyte. The catalytic performance of cathodes was evaluated by performing potentiostatic electrolysis.
All potentials were measured against an Ag/AgCl reference electrode (3 M KCl, BASi). Gas and liquid products were respectively analyzed using gas chromatograph (PerkinElmer Clarus 600) equipped with thermal conductivity and flame ionization detectors, and NMR spectrometer (Agilent DD2 600 MHz) by taking dimethylsulfoxide (DMSO) as an internal standard. All the potentials were converted to values with reference to RHE using: ERHE=EAg/AgCl+0.210 V+0.0591×pH.
ECSA was determined based on the equation: ECSA=RfS, where Rf was roughness factor and S was geometric area of electrode (1 cm−2). Rf=CdI/29 □F cm−2, where Cdl is the double layer capacitance of catalyst and the double-layer capacitance of a smooth Cu surface is assumed to be 29 □F cm−2 (ref. 21). Double layer capacitances of catalysts were determined by measuring cyclic voltammetry with different scan rates (40, 60, 80, 100, 120, and 140 mV s−1, respectively) in the potential ranges between 0.20 VRHE and 0.24 VRHE where no Faradaic process occurred. The cyclic voltammetry (CV) measurement was operated in the same flow cell reactor and 1 M KOH aqueous solution saturated with nitrogen (Linde, 99.998%) was used as the electrolyte. The flow cell reactor was filled with electrolyte prior to the CV measurement and the electrolyte was not circulated during the CV measurement. N2, instead of CO2, was continuously supplied to gas chamber of the cell. By plotting the average current j (j=(ja−jc)/2, where ja and jc are anodic and cathodic current densities, respectively) at 0.22 VRHE against the scan rate, Cdl value was given by the slope.
Electrochemical impedance spectroscopy (EIS) technique was used to measure the ohmic loss between the working and reference electrodes and 70% iR compensation was applied to correct the potentials manually.
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At the low applied potential, the calculated barrier of OC—OCCOH on Cu(111) is 0.88 eV (EOC—OCCOH), higher than of that of OC—OCCO (0.63 eV, EOC—OCCO), indicating that the OC—OCCO is more favorable than OC—OCCOH (Supplementary Table 5).
According to the energetic span, the total barriers of OC—OCCO (E1) and OC—OCCOH (E2) pathways are:
E1=ΔHOC—CO+EOC—OCCO (equation 1)
E2=max(ΔHOC—CO+ΔHOCCOH+EOC—OCCOH+eU,EOC—OCCOH) (equation 2),
where U is the applied potential vs. the computational hydrogen electrode (CHE) (ref. 2). The reaction energy of CO dimerization is 0.65 eV (ΔHOC—CO), and the reaction energy of OCCO hydrogenation is −0.05 eV (ΔHOCCOH) Therefore, E2 decreases with the increase of the applied potential. As the reaction potential showed the highest FEn-propanol in this experiment is −0.46 VRHE (equal to −1.286 VCHE), the total barrier is 0.88 eV, giving the maximum turnover frequency (TOF) of 0.01 s−1.
The total barrier of OC−OCCOH on Ag-doped Cu is 0.76 eV, lower than that on pure Cu, giving a maximum TOF of 0.87 s−1 at reaction applied potential (−1.286 VCHE). Therefore, after considering the proton/electron transfer in C1-C2 coupling, the designed Ag-doped Cu also favors C1-C2 coupling reaction compared to Cu.
1Potentials shown here are without iR compensation.
J.
Phys.
Chem.
C 120,
J.
Phys.
Chem.
Lett. 7,
Proc.
Natl.
Acad.
Sci.
U.S.A. 114, 10560-
ACS
Catal. 1, 7946-
Nat.
Catal. 1, 111-119
Nat.
Catal. 1, 421-428
Nature 508, 504-507
J.
Am.
Chem.
Soc. 137,
ACS
Cent.
Sci. 2, 169-
ACS
Catal. 7,4467-
Nat.
Catal. 1, 748-755
Nat.
Commun. 9, 4614
Nat.
Catal. 1, 946-951
aPotential corrected by ohmic loss.
1Potentials before iR compensation.
2Potentials after iR compensation.
Theoretical Methods. In this work, all the DFT calculations were carried out with a periodic slab model using the Vienna ab initio simulation program (VASP). The generalized gradient approximation (GGA) was used with the Perdew-Burke-Ernzerhof (PBE) exchange-correlation functional. The projector-augmented wave (PAVV) method was utilized to describe the electron-ion interactions, and the cut-off energy for the plane-wave basis set was 450 eV. In order to illustrate the long-range dispersion interactions between the adsorbates and catalysts, this work employed the D3 correction method of Grimme et al. (ref. 10). Brillouin zone integration was accomplished using a ×3×1 Monkhorst-Pack k-point mesh. All the adsorption geometries were optimized using a force-based conjugate gradient algorithm, while transition states (TSs) were located with a constrained minimisation technique. At all intermediate and transition states, one charged layer of water molecules was added to the surface to take the combined field and solvation effects into account. In the CO dimerization, there is no proton or electron transfer, thus the computational hydrogen electrode was not used in this work. For the modelling of Cu(111), the crystal structure was optimized, and Cu(111) was modelled with a periodic four-layer p(4×4) model with the 2 lower layers fixed and 2 upper layers relaxed. Cu(111) was chosen because Cu(111) is more stable relative to Cu(100) (ref. 14), and thus improving the activity of Cu(111) for C3 formation is more significant. In addition, the overall barrier of C1 to C3 product on Cu(100) is higher than that on Cu(111), despite the low barrier of C1-C1 dimerization on Cu(100).
This work started with CO adsorption on Cu(111) surface. The reaction barrier of CO dimerization is calculated to be 0.72 eV, which is similar to the value proposed by Nørskov and co-workers in the same solvent model. The barrier of CO and OCCO coupling is calculated to be 0.63 eV. Given that C2 FEs are always higher than C3 FEs in previous CORR and CO2RR reported, C—C coupling is likely more favorable than concerted C—C—C coupling. Thus, rather than concerted C—C—C coupling, sequential formation of C—C bonds was considered as the main reaction mechanism for the C3 products formation, which is also the most commonly used mechanism in previous reports. To screen the possible metals for doping in Cu, this work substituted one surface copper atom with Ag, Au, Pd, Rh, and Ru, some of which is shown in
To support the geometries proposed, this work calculated the CO adsorption energies on all the possible 16 sites of Ag-doped Cu surface including all of fcc hollow, hcp hollow, bridge, or top sites of Cu-a, Cu-b, and Ag (Cu-a and Cu-b determined by their coordination environment), as shown in Supplementary Table 2. By comparing all the CO adsorption energies on different sites, the configuration with CO adsorbing on hcp site of Cu-b, Cu-b, and Cu-a is the most stable and thus were used for this calculation. Due to the similarity of adsorption schemes and structures of CO, OCCO, and OCCOCO, this work assumed the strongest adsorption sites of CO are also the adsorption sites for OCCO and OCCOCO.
Additionally, this work also tested other functionals including Bayesian error estimation functional (BEEF), Perdew-Burke-Ernzerhof revised for solids (PBEsol), and Perdew-Wang 91 (PW91) for the activation energies of C1-C1 and C1-C2 coupling on Cu and Ag-doped Cu (Supplementary Table 3). Each functional predicts an enhancement of C—C coupling of Ag-doped Cu compared to pure Cu.
The effect of strain is evaluated by using the structures of all transition states and intermediate states on Ag-doped Cu surface and Cu with strain surface. The surface atoms are then fixed, and all the transition states and intermediate states are optimized based on the methods above.
Using the energetic span model, the rate of C2 formation should be rC2˜e−E
Commercial Cu nanopowder (99%) and silver nitrate (AgNO3, 99%) were purchased from Sigma-Aldrich. Potassium hydroxide (KOH) and methanol were purchased from Caledon Laboratory Chemicals. Gas diffusion layer (GDL, Freudenberg H14C9), anion exchange membrane (Fumasep FAB-PK-130) were received from Fuel Cell Store. Ni foam (1.6 mm thickness) was purchased from MTI Corporation. All chemicals were used as received. All aqueous solutions were prepared using deionized water with a resistivity of 18.2 MΩcm−1.
Equilibrium potentials for the half reactions of CO to n-propanol and CO2 to n-propanol were calculated based on the values of the standard molar Gibbs energy of formation at 298.15 K (ref. 35). This work assumed that gases are at 1 atm and liquids are in the pure form.
3CO(g)+12e−+12H+→C3H7OH(l)+2H2O(l) ΔG°=−231.5 kJ mol−1
3CO2(g)+18e−+18H+→C3H7OH(l)+5H2O(l) ΔG°=−171.2 kJ mol−1
Based on
one can get En-propanol-1O=0.20 V versus RHE and En-propanol-2O=0.10 V versus RHE for the reactions of CO to n-propanol and CO2 to n-propanol, respectively. Herein, n is the number of electrons transferred and F is the Faraday constant.
The OER in anode side is one of main contributors to the energy lost, but here this work excluded the effect of the OER and analyzed the cathode performance using cathodic energy conversion efficiency (EEcathodic half-cell), where the overpotential of oxygen evolution is assumed to be 0.
The n-propanol EEcathodic half-cell can be calculated as follows:
where E is applied potential versus RHE, FEn-propanol is the measured Faradaic efficiency of n-propanol in percentage, and En-propanol=0.20 VRHE for CORR or En-propanol=0.10 VRHE for CO2RR. As shown in the equation above, n-propanol EEcathodic half-cell is governed by both FE and overpotential, which are the two important factors in CORR.
For example, at −0.416 VRHE after iR compensation, Ag-doped Cu GDE delivered a n-propanol FE of 33% in CORR. Then the n-propanol EEcathodic half-cell for n-propanol can be calculated as follows:
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In addition, it is noted that any of the particle values disclosed herein can be considered as being ±10% for disclosure purposes. Thus, for example, if a concentration value of 1 g/L is mentioned herein, it should be considered that the range 0.9 to 1.1 g/L is disclosed. It is also noted that one or more features (e.g., values, ranges, pieces of equipment or features thereof, operating conditions, sizes, etc.) disclosed herein can be combined with any other combination of features. For example, if a size of 100 nm is disclosed for a certain nanoparticle herein, it should be noted that the multimetallic (e.g., bimetallic or trimetallic) nanoparticle catalyst material disclosed herein can be, in an optional embodiment, of this size (i.e., 100 nm in addition to within the range of 90 nm to 110 nm as per the ±10% disclosure). Various other combinations of features are also possible and should be considered as being disclosed herein.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/CA2020/051597 | 11/24/2020 | WO |
| Number | Date | Country | |
|---|---|---|---|
| 62939734 | Nov 2019 | US | |
| 63013266 | Apr 2020 | US |
