ELECTRONIC STRUCTURE MODULATION OF UNUSUAL PHASE OF METAL NANOMATERIALS FOR CATALYSIS

Description

TECHNICAL FIELD

The present invention relates to a heterostructured electrocatalyst for example particularly, but not exclusively, a heterostructured electrocatalyst for carbon dioxide reduction reaction; and a preparation method and use of the heterostructured electrocatalyst.

BACKGROUND OF THE INVENTION

Various methods for facilitating carbon dioxide (CO₂) fixation and fossil resource consumption reduction have been developed for tackling the increasing atmospheric greenhouse gas emissions. Among those methods, the electrocatalytic CO₂ reduction reaction (CO₂RR) is considered as one of the ustainable approaches to generate value-added chemicals and fuels to promote carbon-neutral energy cycle. For example, carbon monoxide (CO) converted from CO₂ may be used as essential feedstock to produce high-value chemicals through Fischer-Tropsch reaction.

In this regard, various noble metal nanomaterials such as gold (Au), silver (Ag) and palladium (Pd) have been used for electrocatalytic CO₂RR toward CO production. However, because of the weak adsorption of CO₂ molecules on nobel metal surfaces, it is believed that it remains challenging to achieve highly efficient and selective CO₂ electroduction to a single product, particularly in a broad potential window.

The present invention thus seeks to eliminate or at least mitigate such shortcomings by providing a new or otherwise improved electrocatalyst for CO₂RR.

SUMMARY OF THE INVENTION

In a first aspect of the present invention, there is provided a heterostructured electrocatalyst for carbon dioxide reduction reaction comprising a lanthanide oxide nanomaterial deposited on a gold-containing nanosupport.

In an optional embodiment, the gold-containing nanosupport is physically distinguishable from the lanthanide oxide nanomaterial.

It is optional that structure of the lanthanide oxide nanomaterial is different from that of the gold-containing nanosupport.

Optionally, the lanthanide oxide nanomaterial comprises a nanoparticle of cerium oxide.

In an optional embodiment, oxidation state of the gold in the gold-containing nanosupport is different from that of the lanthanide in the lanthanide oxide nanomaterial.

Optionally, the oxidation state of the gold is 0 and the oxidation state of the lanthanide is +3 or +4.

It is optional that the gold-containing nanosupport has a hetero-crystal phase of 4H/fcc.

It is optional that the lanthanide oxide nanomaterial has a homo-crystal phase of fcc.

In an optional embodiment, the gold-containing nanosupport is a 4H/fcc gold nanorod.

In an optional embodiment, the lanthanide oxide nanomaterial is a fcc cerium oxide nanoparticle

Optionally, the 4H/fcc gold nanorod is partially covered by the fcc cerium oxide nanoparticles to provide a metal-oxide interface as reactive sites for electrocatalytic carbon dioxide reduction reaction

It is optional that the gold-containing nanosupport has a deposit of about 3 nm to about 10 nm of fcc cerium oxide nanoparticles.

Optionally, the lanthanide oxide nanomaterial include cerium (IV) oxide nanoparticles and cerium (III) oxide nanoparticles at a ratio of about 2:1.

It is optional that atomic ratio of gold:cerium is about 3:1 to about 5:1.

Optionally, the 4H/fcc gold nanorod has a diameter of about 12 nm to about 25 nm and a length of about 400 nm to about 900 nm.

In a second aspect of the present invention, there is provided a method of preparing the heterostructured electrocatalyst in accordance with the first aspect, comprising the steps of: a) providing a reaction mixture including a hetero-crystal phase gold-containing nanosupport, a lanthanide precursor and a first reducing agent; b) heating the reaction mixture at a temperature of about 100° C. for about 5 mins; and c) isolating the electrocatalyst from the reaction mixture.

In an optional embodiment, the gold-containing nanosupport is suspended in ethanol.

Optionally, the gold-containing nanosupport comprises a 4H/fcc gold nanorod formed from the steps of: providing a closed reaction mixture including gold (III) chloride hydrate, n-heptane, a surfactant including oleylamine, and a second reducing agent including N-ethylcyclohexylamine; heating the closed reaction mixture at about 68° C. for about 48 h to form crude 4H/fcc gold nanorod; isolating the crude 4H/fcc gold nanorod from the closed reaction mixture; purifying the isolated 4H/fcc gold nanorod by successively washing the isolated 4H/fcc gold nanorod with cyclohexane, a cyclohexane/ethanol mixture (1:1, v/v), and ethanol; and resuspending the washed 4H/fcc gold nanorod in ethanol to obtain an ethanol suspension of the 4H/fcc gold nanorod.

It is optional that the lanthanide precursor comprises cerium nitrate hexahydrate, and the first reducing agent comprises hexamethylenetetramine.

Optionally, the lanthanide precursor and the first reducing agent have a concentration ratio of 1:1.

In an optional embodiment, the lanthanide precursor includes a concentration of 1 mg/mL to 3 mg/mL.

In an optional embodiment, the first reducing agent includes a concentration of 1 mg/mL to 3 mg/mL.

In a third aspect of the present invention, there is provided an electrode for carbon dioxide reduction reaction comprising an electrocatalytically active mixture including the heterostructured electrocatalyst in accordance with the first aspect provided on a conductive substrate.

In an optional embodiment, the conductive substrate comprises glassy carbon.

Optionally, the electrocatalytically active mixture further includes carbon black and tetrafluoroethylene-perfluoro-3,6-dioxa-4-methyl-7-octenesulfonic acid copolymer.

It is optional that the electrode has a working area of about 0.5 cm².

Optionally, the electrode has a catalyst loading of about 400 μg cm⁻².

BRIEF DESCRIPTION OF DRAWINGS

The patent or application file contains at least one drawing executed in color. Copies of this patent or patent application publication with color drawing(s) will be provided by the Office upon request and payment of the necessary fee.

The invention will now be more particularly described, by way of example only, with reference to the accompanying drawings, in which:

FIG. 1 shows a schematic illustration of synthesis of Au—CeO_x-3 nm in accordance with an embodiment of the present invention. Step (a) shows the synthesis of 4H/fcc Au nanorods and step (b) shows the overgrowth of CeO_x nanostructures on the surface of 4H/fcc Au nanorods;

FIG. 2A shows the scanning electron microscope (SEM) image of 4H/fcc Au nanorods, with a magnification scale of 200 nm;

FIG. 2B shows the transmission electron microscope (TEM) image of 4H/fcc Au nanorods, with a magnification scale of 50 nm;

FIG. 2C shows the aberration-corrected HAADF-STEM image of a representative Au nanorod;

FIG. 2D shows the fast Fourier transform (FFT) patterns of selected area (d) in FIG. 2C, indicating the characteristic diffraction patterns of fcc phase;

FIG. 2E shows the fast Fourier transform (FFT) patterns of selected area (e) in FIG. 2C, indicating the characteristic diffraction patterns of 4H phase;

FIG. 3 shows the typical XRD pattern of as-synthesized 4H/fcc Au nanorods. The XRD pattern was collected in the θ/2θ mode;

FIG. 4A shows the SEM image of as-synthesized Au—CeO_x-3 nm heteronanostructures, with a magnification scale of 100 nm;

FIG. 4B shows the TEM image of the as-synthesized Au—CeO_x-3 nm heteronanostructures, with a magnification scale of 50 nm;

FIG. 4C shows the high-magnification aberration-corrected HAADF-STEM images of representative Au—CeO_x-3 nm heteronanostructures;

FIG. 4D shows the high-resolution aberration-corrected HAADF-STEM images of representative Au—CeO_x-3 nm heteronanostructures;

FIG. 4E shows the FFT patterns of selected area (e) in FIG. 4D, indicating the characteristic diffraction patterns of fcc phase;

FIG. 4F shows the FFT patterns of selected area (f) in FIG. 4D, indicating the characteristic diffraction patterns of 4H phase;

FIG. 4G shows the zoom-in HAADF-STEM image of corresponding region (g) marked in FIG. 4D for the CeO_x nanostructures;

FIG. 4H shows the zoom-in HAADF-STEM image of corresponding region (h) marked in FIG. 4D for the CeO_x nanostructures;

FIG. 4I shows the HAADF-STEM image of a representative Au—CeO_x-3 nm heteronanostructure;

FIG. 4J shows the corresponding STEM-EDS elemental mapping (Au) of the representative Au—CeO_x-3 nm heteronanostructure of FIG. 4I;

FIG. 4K shows the corresponding STEM-EDS elemental mapping (Ce) of the representative Au—CeO_x-3 nm heteronanostructure of FIG. 4I;

FIG. 4L shows the corresponding STEM-EDS elemental mapping (O) of the representative Au—CeO_x-3 nm heteronanostructure of FIG. 4I;

FIG. 4M shows the corresponding STEM-EDS elemental mapping (Au+Ce) of the representative Au—CeO_x-3 nm heteronanostructure of FIG. 4I;

FIG. 5A shows the HAADF-STEM image of an individual Au—CeO_x-3 nm heteronanostructure. The white line indicates the position for EDS line-scanning;

FIG. 5B shows the corresponding EDS line-scanning profile across the heteronanostructure indicated by the white line in FIG. 5A;

FIG. 6 shows the EDS spectra of as-synthesized Au—CeO_x-3 nm;

FIG. 7A shows the SEM image of as-synthesized Au—CeO_x-10 nm, with a magnification scale of 200 nm;

FIG. 7B shows the TEM image of as-synthesized Au—CeO_x-10 nm, with a magnification scale of 50 nm;

FIG. 8 shows the EDS spectra of as-synthesized Au—CeO_x-10 nm;

FIG. 9 shows the typical XRD pattern of as-synthesized Au—CeO_x-3 nm. The XRD pattern was collected in the θ/2θ mode;

FIG. 10 shows the typical XRD pattern of as-synthesized Au—CeO_x-10 nm. The XRD pattern was collected in the θ/2θ mode;

FIG. 11 shows the TEM image of as-synthesized CeO_x nanoparticles as the control sample;

FIG. 12 shows the typical XRD pattern of as-synthesized CeO_x nanoparticles. The XRD pattern was collected in the θ/2θ mode;

FIG. 13 shows the XPS spectra of Au 4f for Au—CeO_x-3 nm and Au nanorods;

FIG. 14 shows the XPS spectra of Au 4f for Au—CeO_x-10 nm;

FIG. 15 shows the XPS spectra of Ce 3d for Au—CeO_x-3 nm heteronanostructures;

FIG. 16A shows the XPS spectra of Ce 3d for Au—CeO_x-10 nm heteronanostructures;

FIG. 16B shows the XPS spectra of Ce 3d for CeO_x nanoparticles;

FIG. 17A shows the XPS spectra of O 1s for Au—CeO_x-3 nm heteronanostructures;

FIG. 17B shows the XPS spectra of O 1s for Au—CeO_x-10 nm heteronanostructures;

FIG. 17C shows the XPS spectra of O 1s for CeO_x nanoparticles;

FIG. 18 shows the normalized XANES spectra of Au-L₃ edge of Au—CeO_x-3 nm, Au nanorods and Au foil;

FIG. 19 shows the Fourier transforms of Au-L₃ edge EXAFS spectra of Au—CeO_x-3 nm, Au nanorods and Au foil;

FIG. 20A shows the k²-weighted EXAFS fitting results at Au L₃-edge for Au—CeO_x-3 nm;

FIG. 20B shows the k²-weighted EXAFS fitting results at Au L₃-edge for Au nanorods;

FIG. 21A shows the fitting result of EXAFS spectra at Au L₃-edge for Au—CeO_x-3 nm in K space;

FIG. 21B shows the fitting result of EXAFS spectra at Au L₃-edge for 4H/fcc Au nanorods in K space;

FIG. 22A shows the fitting result of EXAFS spectra at Au L₃-edge for Au foil in K space;

FIG. 22B shows the k²-weighted EXAFS fitting results at Au L₃-edge for Au foil in R space;

FIG. 23 is a table summarizing the Au L₃-edge EXAFS fitting results of Au—CeO_x-3 nm, pristine Au nanorods and standard Au foil;

FIG. 24A shows the wavelet transforms from the EXAFS spectra of Au—CeO_x-3 nm;

FIG. 24B shows the wavelet transforms from the EXAFS spectra of Au nanorods;

FIG. 25 shows the LSV plots of Au—CeO_x-3 nm, Au—CeO_x-10 nm and pristine Au nanorods in CO₂-saturated 0.5 M KHCO₃ aqueous solution;

FIG. 26A shows the Faradaic Efficiency (FE) for CO production on Au—CeO_x-3 nm, Au—CeO_x-10 nm and Au nanorods under different potentials;

FIG. 26B shows the FE of HCOO⁻ product generated by Au—CeO_x-3 nm, Au—CeO_x-10 nm and pristine Au nanorods at different potentials when conducting CO₂RR in H-type cell;

FIG. 27 shows the FE of H₂ product generated by Au—CeO_x-3 nm, Au—CeO_x-10 nm and pristine Au nanorods at different potentials when conducting CO₂RR in H-type cell;

FIG. 28 shows the partial current density for CO production on Au—CeO_x-3 nm, Au—CeO_x-10 nm and Au nanorods under different potentials;

FIG. 29A shows the cyclic voltammetry (CV) plots of Au—CeO_x-3 nm in the electrolyte of 0.5 M KHCO₃ aqueous solution, with the scan rate being 50 mV s⁻¹;

FIG. 29B shows the cyclic voltammetry (CV) plots of Au nanorods in the electrolyte of 0.5 M KHCO₃ aqueous solution, with the scan rate being 50 mV s⁻¹;

FIG. 30 shows the Tafel plots of Au—CeO_x-3 nm, Au—CeO_x-10 nm and Au nanorods;

FIG. 31 shows the long-term durability test of Au—CeO_x-3 nm toward CO₂ electroreduction at −0.5 V (vs RHE) for 12 h;

FIG. 32A shows the TEM image of Au—CeO_x-3 nm after the long-term durability test in H-type cell corresponding to FIG. 31, with a magnification scale of 500 nm;

FIG. 32B shows the TEM image of Au—CeO_x-3 nm after the long-term durability test in H-type cell corresponding to FIG. 31, with a magnification scale of 20 nm;

FIG. 32C shows the SAED pattern of Au—CeO_x-3 nm after the long-term durability test in H-type cell corresponding to FIG. 31;

FIG. 33A shows the current densities of CeO_x nanoparticles when conducting CO₂RR in H-type cell at different potentials;

FIG. 33B shows the FE of H₂ and CO products generated by CeO_x nanoparticles when conducting CO₂RR in H-type cell at different potentials;

FIG. 34A shows the in-situ ATR-IR spectra of electrochemical CO₂ reduction on Au—CeO_x-3 nm in CO₂-saturated 0.5 M KHCO₃ aqueous solution, ranging from −0.05 to −1.20 V (vs RHE) at an interval of 50 mV. The IR reference was taken at −0.05 V (vs RHE);

FIG. 34B shows the in-situ ATR-IR spectra of electrochemical CO₂ reduction on Au nanorods in CO₂-saturated 0.5 M KHCO₃ aqueous solution, ranging from −0.05 to −1.20 V (vs RHE) at an interval of 50 mV. The IR reference was taken at −0.05 V (vs RHE);

FIG. 35 shows the in-situ ATR-IR spectra of CO₂RR on Au—CeO_x-3 nm in an Ar-saturated 0.5 M KHCO₃ aqueous solution, ranging from −0.05 to −1.20 V (vs RHE) at an interval of 50 mV. The IR reference was taken at −0.05 V (vs RHE);

FIG. 36A shows the DFT calculations of CO₂RR on Au—CeO_x-3 nm and 4H/fcc Au nanorods: vertical view of electron density difference of Au—Ce₃O₇ with an isosurface value of 0.002 e/bohr³ (yellow regions indicate gaining electrons, while cyan regions represent losing electrons);

FIG. 36B shows the DFT calculations of CO₂RR on Au—CeO_x-3 nm and 4H/fcc Au nanorods: side view of electron density difference of Au—Ce₃O₇ with an isosurface value of 0.002 e/bohr³ (yellow regions indicate gaining electrons, while cyan regions represent losing electrons);

FIG. 37 shows the projected density of states of Au nanorods;

FIG. 38 shows the projected density of states of Au—Ce₃O₇;

FIG. 39 shows the calculated free energy diagram of Au—Ce₃O₇ and Au nanorods; and

FIG. 40 shows the schematic illustration of the overgrowth of CeO_x on 4H/fcc Au nanorods to enhance electrocatalytic CO₂RR towards CO production.

DETAILED DESCRIPTION OF OPTIONAL EMBODIMENT

As used herein, the forms “a”, “an”, and “the” are intended to include the singular and plural forms unless the context clearly indicates otherwise.

The words “example” or “exemplary” used in this invention are intended to serve as an example, instance, or illustration. Any aspect or design described in this disclosure as “exemplary” is not necessarily to be construed as preferred or advantageous over other aspects or designs. Rather, use of the words “example” or “exemplary” is intended to present concepts in a concrete fashion. As used in this application, the term “or” is intended to mean an inclusive “or” rather than an exclusive “or”. That is, unless specified otherwise, or clear from context, “X employs A or B” is intended to mean any of the natural inclusive permutations. That is, if X employs A; X employs B; or X employs both A and B, then “X employs A or B” is satisfied under any of the foregoing instances.

As used herein, the phrase “about” is intended to refer to a value that is slightly deviated from the value stated herein. Examples have been described throughout the present disclosure.

Without wishing to be bound by theory, the inventors have, through their own research, trials, and experiments, devised that electronic structure modulation of metal nanomaterials may be effective for improving the electrocatalytic carbon dioxide reduction reaction (CO₂RR) performance. In particular, it is believed that by constructing a metal-oxide interface, such as by introducing a lanthanide oxide (LnO_x) to the metal, the resultant metal-oxide interface may feature superior active and highly selective sites for the electrocatalytic CO₂RR. Thus, it is also believed that the present invention may provide an effective strategy for rational electronic structure regulation of the metal nanomaterials to enhance the electrocatalytic performance thereof.

In a first aspect of the present invention, there is provided a heterostructured electrocatalyst for carbon dioxide reduction reaction comprising a lanthanide oxide nanomaterial deposited on a gold-containing nanosupport. As used herein, the term “nanosupport” generally refers to a nanostructured material that acts as a physical support for another material, in particular, for active compounds such as catalysts, which might thereby enhance the surface area of the active compounds for chemical reaction. For example, in some embodiments of the present invention, the lanthanide oxide nanomaterial may form an interface with the gold-containing nanosupport and the interface may reduce the energy barrier of the rate-limiting (rate-determining) step, such as the formation of *COOH species, upon the CO₂RR process, and therefore the CO₂RR process may be optimized.

In some embodiments, the gold-containing nanosupport may be physically distinguishable from the lanthanide oxide nanomaterial. For example, the gold-containing nanosupport may have different structures, dimensions, size, density, mass, hardness, ductility, conductivity, etc. as compared with the lanthanide oxide nanomaterial. In an embodiment, the gold-containing nanosupport may be a zero-dimensional, one-dimensional, two-dimensional, or three-dimensional nanomaterial. Examples of zero-dimensional nanomaterials may include nanoparticles, quantum dots, fullerenes and the like. Examples of one-dimensional nanomaterials may include nanowires, nanorods, nanotubes and the like. Examples of two-dimensional nanomaterials may include graphene, nanofilms, nanocoatings and the like. Examples of three-dimensional nanomaterials may include nanocomposites, nanoporous materials, nanoprism and the like. In some particular embodiments, the gold-containing nanosupport may be a two-dimensional nanomaterial such as a gold-containing nanorod.

In some embodiments, the oxidation state of the gold-containing nanosupport may be different from that of the lanthanide in the lanthanide oxide nanomaterial. In some particular embodiments, the oxidation state of the gold may be 0 (i.e., in its metallic state) and the oxidation state of the lanthanide may be +3 and/or +4. Examples of such a lanthanide may include cerium, praseodymium, lanthanum, etc.

The lanthanide oxide nanomaterial may be a nanomaterial with the zero-dimension, one-dimension, two-dimension, or three-dimension as described herein. In some embodiments, the structure of the lanthanide oxide nanomaterial may be different from that of the gold-containing nanosupport. For example, in an embodiment where the gold-containing nanosupport may be a two-dimensional nanomaterial, such as a nanorod, the lanthanide oxide nanomaterial may have both a different structure and a different dimension compared to the gold-containing nanosupport, such as a nanoparticle of the lanthanide oxide. In an alternative embodiment, the gold-containing nanosupport and the lanthanide oxide nanomaterial may have the same dimension but different structures. For example, the gold-containing nanosupport may be a nanorod and the lanthanide oxide nanomaterial may be a nanotube.

In some embodiments, the gold-containing nanosupport may have a hetero-crystal phase (i.e., unconventional phase) of 4H/face-centered cubic (4H/fcc) whereas the lanthanide oxide nanomaterial may have a homo-crystal phase of fcc. It is believed that unconventional phase metal nanomaterials may have unique electronic structures, and such electronic structures may be tuned by electronic transfer between the metal and the lanthanide oxide, as it is also believed that the lanthanide oxide may have unique electronic structures, abundant oxygen vacancies as well as special redox properties, thereby enhancing the electrocatalytic CO₂RR performance towards practical applications. Detailed electrocatalytic reaction mechanism of the heterostructured electrocatalyst will be discussed in the later part of the present disclosure.

In some preferred embodiments, the gold-containing nanosupport may be a 4H/fcc gold nanorod whereas the lanthanide oxide nanomaterial may be a fcc cerium oxide (CeO_x) nanoparticle. The gold nanorods may act as seeds for which the cerium oxide may grow thereon (i.e. for the overgrowth of cerium oxide). In particular, the 4H/fcc gold nanorod may be partially covered by the fcc cerium oxide nanoparticles to provide a metal-oxide interface such as a gold-cerium oxide (Au—CeO_x) interface as reactive sites for electrocatalytic carbon dioxide reduction reaction. As used herein, the subscript x of CeO_x generally denotes that the numbers of oxygen atom in CeO_x vary in accordance with the oxidation state(s) of the cerium. In other words, there are more than one type of cerium oxide exist in the lanthanide oxide nanomaterial. In some particular embodiments, the lanthanide oxide nanomaterial may include cerium (IV) oxide nanoparticles and cerium (III) oxide nanoparticles at a ratio of about 2:1. In some other embodiments, the heterostructured electrocatalyst may also have atomic ratio of gold:cerium of about 3:1 to about 5:1 such as about 83.9:16.1 to about 75.2:24.8.

The gold-containing nanosupport such as the 4H/fcc gold nanorod may have a deposit of about 3 nm (e.g., from 2.90 nm . . . 2.91 nm . . . 2.95 nm . . . 2.99 nm, 3 nm, 3.01 nm . . . 3.03 nm . . . to 3.10 nm) to about 10 nm (e.g., from 9.90 nm . . . 9.92 nm . . . 9.99 nm, 10.00 nm, 10.01 nm . . . 10.04 nm to 10.10 nm) of fcc cerium oxide nanoparticles. In an embodiment, the 4H/fcc gold nanorod may have a deposit of about 3 nm of the fcc cerium oxide nanoparticles. In another embodiment, the 4H/fcc gold nanorod may have a deposit of about 10 nm of the fcc cerium oxide nanoparticles.

In some other embodiments, the 4H/fcc gold nanorod may have a diameter of about 12 nm (e.g., from 11.9 nm . . . 11.95 nm . . . to 11.99 nm, 12.00 nm, 12.01 nm . . . 12.05 nm . . . to 12.10 nm) to about 25 nm (e.g., from 24.9 nm . . . 24.95 nm . . . to 24.99 nm, 25.00 nm, 25.01 nm . . . 25.05 nm . . . to 25.10 nm) and a length of about 400 nm (e.g., from 399.0 nm . . . 399.5 nm . . . 400.0 nm . . . 400.2 nm . . . 400.5 nm to 401 nm) to about 900 nm (e.g., from 899.0 nm . . . 899.5 nm . . . 900.0 nm . . . 900.2 nm . . . 900.5 nm to 901.0 nm).

The method of preparing the heterostructured electrocatalyst as disclosed herein is now described. The method may comprise the steps of: a) providing a reaction mixture including a hetero-crystal phase gold-containing nanosupport, a lanthanide precursor and a first reducing agent; b) heating the reaction mixture at a temperature of about 100° C. for about 5 mins; and c) isolating the electrocatalyst from the reaction mixture.

In some embodiments, the hetero-crystal phase gold-containing nanosupport may be suspended in ethanol. For example, in an embodiment where the hetero-crystal phase gold-containing nanosupport comprise a 4H/fcc gold nanorod, it may be formed from the following steps. First, a closed reaction mixture including gold (III) chloride hydrate, n-heptane, a surfactant including oleylamine, and a second reducing agent including N-ethylcyclohexylamine may be provided by sealing a glass vial containing a homogeneous reaction mixture of the gold (III) chloride hydrate, the n-heptane, the surfactant, and the second reducing agent with a sealing agent such as a parafilm. Then, the closed reaction mixture may be heated, such as in an oil bath, at about 68° C. for about 48 h to form crude 4H/fcc gold nanorod. Next, the crude 4H/fcc gold nanorod may be isolated from the closed reaction mixture by way of precipitation and/or centrifugation (e.g., at about 4000 rpm to about 4500 rpm for about 5 mins). After that, the isolated 4H/fcc gold nanorod may be purified by successively washing it with cyclohexane, a cyclohexane/ethanol mixture (1:1, v/v), and ethanol. Finally, the washed 4H/fcc gold nanorod in ethanol may be suspended in ethanol to obtain an ethanol suspension of the 4H/fcc gold nanorod.

It is believed that the lanthanide precursor and the first reducing agent may be selected in accordance with practical needs. For example, in an embodiment where the electrocatalyst is a heteronanostructure of Au—CeO_x comprising the 4H/fcc gold nanorod and the fcc CeO_x nanoparticle as described herein, the lanthanide precursor may comprise cerium nitrate hexahydrate, and the first reducing agent may comprise hexamethylenetetramine.

In some embodiments, the hetero-crystal phase gold-containing nanosupport, the lanthanide precursor, and the first reducing agent in the reaction mixture may have a concentration ratio of about 1:1.67-5:1.67-5, such as 1:1.67:1.67, 1:2:2, 1:2.5:2.5, 1:3:3, 1:4:4, 1:4.5:4.5, 1:5:5 and the like. For example, in an embodiment where the concentration of the hetero-crystal phase gold-containing nanosupport, the lanthanide precursor, and the first reducing agent are 600 ppm, 1000 ppm, and 1000 ppm, respectively, their concentration ratio is 1:1.67:1.67. In some other embodiments, the lanthanide precursor and the first reducing agent may have a concentration ratio of 1:1. For example, in some embodiments where the lanthanide precursor may include a concentration of 1 mg/mL to 3 mg/mL, the first reducing agent may also include a concentration of 1 mg/mL to 3 mg/mL. As a specific embodiment, when the lanthanide precursor such as cerium nitrate hexahydrate has a concentration of 1 mg/mL, the first reducing agent such as hexamethylenetetramine may also have a concentration of 1 mg/mL upon conducting the synthesis.

It is also believed that by varying the concentrations of the lanthanide precursor and the first reducing agent, the thickness of the lanthanide oxide deposited on the gold-containing nanosupport would be varied/adjusted accordingly. For example, in an embodiment where the concentration of the lanthanide precursor such as cerium nitrate hexahydrate is 1 mg/mL and the concentration of the first reducing agent such as hexamethylenetetramine is 1 mg/mL, the resultant CeO_x nanoparticles deposited on the gold-containing nanosupport may have a thickness of about 3 nm. In another embodiment, where the concentrations of the lanthanide precursor and the first reducing agent are each 3 mg/mL, the resultant CeO_x nanoparticles deposited on the gold-containing nanosupport may have a thickness of about 10 nm.

In step b), the reaction mixture may be heated in an oil bath at, for example 100° C. for 5 mins to obtain the crude product of the electrocatalyst, such as, in an embodiment, crude Au—CeO_x. Then, the crude product may be, in step c), isolated from the reaction mixture by centrifugation (e.g., at about 4000 rpm to about 4500 rpm for about 5 mins), followed by purifying the isolated product by washing it with suitable solvent such as ethanol for, e.g., three times. Optionally or additionally, the purified product may be resuspended in ethanol for storage.

Another aspect of the present invention is an electrode for carbon dioxide reduction reaction comprising an electrocatalytically active mixture including the heterostructured electrocatalyst as described herein provided on a conductive substrate. The conductive substrate may be selected from one or more of the followings according to practical needs: glassy carbon, indium doped oxide glass (ITO/glass), fluorine-doped tin oxide glass (FTO/Glass), indium doped tin oxide polyethylene terephthalate (ITO/PET), etc. In an embodiment, the conductive substrate may comprise glassy carbon. The glassy carbon may have an area of about 0.5 cm² (e.g., from 0.48 cm² . . . 0.482 cm² . . . 0.485 cm² . . . 0.49 cm² . . . 0.493 cm² . . . 0.499 cm², 0.5 cm² . . . 0.501 cm² . . . 0.505 cm² to about 0.51 cm²). In other words, the electrode in this embodiment may have a working area of about 0.5 cm².

The conductive substrate, in particular, may have a catalyst loading of about 400 μg cm⁻² (e.g., from 399.1 μg cm⁻² . . . 399.5 μg cm⁻² . . . 399.8 μg cm⁻² . . . 400 μg cm⁻² . . . 400.2 μg cm⁻² . . . 400.6 μg cm⁻² . . . to 401 μg cm⁻²). The electrocatalyst may be loaded/deposited onto the conductive substrate along with other components of the electrocatalytic active mixture. For example, in an embodiment, in addition to the electrocatalyst as descried herein, the electrocatalytic active mixture may further include carbon black and tetrafluoroethylene-perfluoro-3,6-dioxa-4-methyl-7-octenesulfonic acid copolymer (i.e., Nafion). This mixture may be formed by mixing the electrocatalyst (e.g., about 950 μg) with carbon black (e.g., 50 μg) in ethanol, followed by Nafion solution (e.g. 5 wt %). Optionally or additionally, the mixture may be sonicated over an ice-bath to obtain a homogeneous electrocatalytically active mixture. Then, this homogeneous mixture may be dropped on the surface of a glass carbon electrode, followed by drying under ambient conditions to obtain a glass carbon electrode with the electrocatalyst as described herein.

In some embodiments, the electrode as described herein may be applied to an electrochemical cell, such as a gas-tight two-chamber H-type cell separated by an ion exchange membrane (e.g., Nafion 212) for CO₂RR. In particular, the electrode as described herein may be used as a working electrode (cathode), and may be used along with a reference of Ag/AgCl (saturated KCl) and a counter electrode (anode) of platinum foil in the presence of an electrolyte of, such as, 0.5 M KHCO₃ aqueous solution. In some embodiments, it is found that a highly selective electrocatalytic CO production with Faradaic efficiency (FE) of at least about 97% in a wide potential range of about −0.5 V to about −0.8 V (vs. RHE) may be achieved. In some other embodiments, it is also found that the CO partial current density of such electrode may reach about 9.7 mA cm⁻² at about −0.6 V (vs. RHE). Detailed CO₂RR performance of the electrode comprising the electrocatalyst as described herein will be discussed in the later part of the present disclosure.

Hereinafter, the present invention is described more specifically by way of examples, but the present invention is not limited thereto.

EXAMPLES

Materials

Gold (III) chloride hydrate (HAuCl₄·3H₂O, ≥99.9%) and oleylamine (OAm, technical grade, 70%) were purchased from Sigma-Aldrich. Cerium nitrate hexahydrate (Ce(NO₃)₃·6H₂O, 99.9%), hexamethylenetetramine (HMT, 99%), N-ethylcyclohexylamine (99%), cyclohexane (99%), n-heptane (99%), ethanol (absolute, ≥99.9%) and isopropanol (99%) were purchased from Aladdin. All the chemicals and reagents were used as received without any further purification.

Characterization and Methods

Structural Characterization

The scanning electron microscopy (SEM) images were acquired on a QUATTRO S SEM operated at 20 kV. The transmission electron microscopy (TEM) images were obtained on a Philips Technai 12 operated at 120 kV. The high-angle annular dark-field scanning TEM (HAADF-STEM) images and the energy dispersive X-ray spectroscopy (EDS) results were collected on a FEI-Titan Cubed Themis G2 300 operated at 300 kV. The powder X-ray diffraction (XRD) patterns were obtained on the Rigakur SmartLab X-ray diffractometer equipped with Cu Kα X-ray source at the wavelength of 1.5406 Å. The X-ray photoelectron spectroscopy (XPS) spectra were recorded on an ESCALAB 250Xi (Thermal Fisher Scientific, US) spectrometer and corrected using C 1s (284.8 eV) as the standard peak. The X-ray absorption spectroscopy (XAS) measurements was conducted in a transmission mode at beamline X-ray absorption fine structure for catalysis (XAFCA) of Singapore Synchrotron Light Source operated at 700 MeV with the beam current of 150 mA. The XAS-related data processing was performed using Athena and Artemis software packages. The concentration of catalysts (based on Au) were analyzed on the inductively coupled plasma-optical emission spectroscopy (ICP-OES, Optima 8000 spectrometer). The liquid products of CO₂RR were measured and analyzed by a ¹H nuclear magnetic resonance spectroscopy (NMR 300 MHz, Bruker AVANCE III BBO Probe).

Preparation of Working Electrodes

Typically, 950 μg of catalysts (based on Au element) and 50 μg of carbon black (Vulcan XC-72R) were dispersed into 1 mL of ethanol, followed by the addition of 10 μL of Nafion solution (5 wt %). Then the resultant solution was sonicated in an ice-water bath for 30 mins to obtain homogeneous catalyst ink. Subsequently, 200 μL of catalyst ink were dropped on the surface of glassy carbon electrode (GCE). The area of working electrode was 0.5 cm² and the loading amount of catalysts (based on Au element) was 400 μg cm⁻². The obtained working electrodes were dried under ambient conditions before further test.

Electrochemical CO₂RR Measurements

All electrochemical CO₂RR tests were performed in a gas-tight two-chamber H-type cell separated by an ion exchange membrane (Nafion 212). The GCE loaded with electrocatalysts was used as the working electrode. The Ag/AgCl (saturated KCl) electrode and platinum foil were used as reference electrode and counter electrode, respectively. The electrochemical measurements were conducted using a CHI 760E workstation. All potentials in this work were converted to reversible hydrogen electrode (RHE) scale according to to the equation of E (vs RHE)=E (vs Ag/AgCl)+0.197 V+0.0591×pH, where no iR compensation was used.

In the typical electrocatalytic CO₂RR test, 8 mL of 0.5 M KHCO₃ aqueous solution were added to the anode and cathode chambers, respectively. After that, the electrolyte in cathode chamber was first saturated by purging CO₂ gas for 15 mins. During the electrocatalytic test, the CO₂ gas was continuously bubbled into the cathode chamber with a flow rate of 30 standard cubic centimeters per minute (sccm), and the electrolyte in cathode chamber was kept stirring at 300 rpm. For CO₂RR product analysis, all gaseous products were routed directly and quantitatively analyzed by an on-line gas chromatography (GC, Agilent 7890B). Each collection of GC spectrum takes around 13.5 mins. After acquiring three GC spectra under every applied potential, the obtained liquid products were taken out and further analyzed by NMR. Typically, 600 μL of the obtained electrolyte were added to the NMR tube, and then 30 μL of the mixture of D₂O and 16.7 ppm (m/m) dimethyl sulfoxide (DMSO) were also added as the internal standard. The 1H NMR test was performed with water suppression using a pre-saturation approach.

In-situ Infrared Spectroscopic Study

The in-situ ATR-IR measurement was conducted on a Nicolet iS50 IR spectrometer based on the absorption mode equipped with a Mercury-Cadmium-Telluride (MCT) detector cooled by liquid nitrogen. The working electrode for ATR-IR test was prepared by dropping the catalyst ink onto an Au-coated Si hemispherical prism (20 nm in diameter, MTI Corporation). The procedure for preparation of Au-coated Si prism and the instrumental setup is based upon reported work. The loading amount of 4H/fcc Au—CeO_x and 4H/fcc Au nanorods was 250 μg (based on Au element). The ATR-IR spectra were collected at a resolution of 8 cm⁻¹. The scan rate is 5 mV/s with a time of expose of 10 s per spectrum.

Theoretical Calculations

DFT calculations were performed with the Perdew-Burke-Ernzerhof (PBE) functional in the generalized gradient approximation (GGA) using the Vienna ab initio simulation package (VASP). The standard pseudopotential was used to describe the valence electron configurations. The plane-wave basis cutoff energy was set as 400 eV and a 2×3×1 Gamma centered k-mesh was used to sample the first Brillouin zone. To match with the experiments, the constructed 4H/fcc Au models are consisted of two different atomic arrangements with 6-layer thick. The accuracy of electronic self-consistency was set to be 10⁻⁵ eV between the two electronic steps. Because of the strongly corrected electrons, a Hubbard on-site Coulomb potential was set for Ce to U-J=4.5 eV, as referred to the previous works. The structures were fully relaxed until the forces on each atom converged to 0.01 eV/Å. Additionally, DFT-D3 (BJ) corrections were included to consider the van der Waals interactions.

Example 1A

Synthesis of 4H/fcc Au Nanorods

The 4H/fcc Au nanorods were synthesized through wet-chemical method (i.e. hydrothermal method). In a typical synthesis, 15 mg of HAuCl₄·3H₂O were dissolved into 10 mL of a mixture of oleylamine (Oam) and n-heptane (v/v=1/1) in a glass vial by vortexing. Then 200 μL of N-ethylcyclohexylamine was added to the above solution. The glass vial was sealed with parafilm and further heated at 68° C. for 48 h. After that, 10 mL of cyclohexane was added to the glass vial and the obtained dark reddish brown solution was sonicated for 5 mins. The final product was collected by centrifugation at 4,000 rpm for 5 mins and washed with cyclohexane for three times. The obtained 4H/fcc Au nanorods were re-dispersed into 5 mL of cyclohexane for further use.

Example 1B

Synthesis of Au/CeO_x-3 nm Heteronanostructures

The as-obtained 4H/fcc Au nanorods were used as seeds for the overgrowth of CeO_x. In particular, cerium nitrate hexahydrate (Ce(NO₃)₃·6H₂O) was used as a metal precursor in the presence of hexamethylenetetramine (HMT) for synthesizing the Au—CeO_x heteronanostructures, as illustrated in FIG. 1.

Specifically, 3 mL of as-synthesized 4H/fcc Au nanorods (0.2 mg/mL) were centrifuged and washed with cyclohexane to remove the excess OAm molecules on the surface. In order to enable the good suspension of 4H/fcc Au nanorods in ethanol, 4H/fcc Au nanorods were firstly washed with the mixture of cyclohexane and ethanol (v/v=1/1), followed by the pure ethanol. After that, 4H/fcc Au nanorods were re-dispersed into 1 mL of ethanol in a glass vial. Then 1 mL of Ce(NO₃)₃·6H₂O solution (1 mg/mL in ethanol) and 1 mL of HMT solution (1 mg/mL in ethanol) were quickly injected into the glass vial, and the resultant solution was placed in an oil bath at 100° C. for 5 mins. To collect the final products, 10 mL of ethanol were added into the glass vial and the obtained solution was sonicated intensely for 5 mins. The final product was collected by centrifugation at 4,500 rpm for 5 mins and washed with ethanol for three times. The obtained Au—CeO_x-3 nm heteronanostructures were re-dispersed into 5 mL of ethanol for further use.

Example 1C

Synthesis of Au/CeO_x-10 nm Heteronanostructures

Typically, the synthesis of Au—CeO_x-10 nm heteronanostructures was similar to that of Au—CeO_x-3 nm with a slight modification. The synthesized 4H/fcc Au nanorods were successively washed with cyclohexane, the mixture of cyclohexane and ethanol (v/v=1/1) and pure ethanol, and then re-dispersed into 1 mL of ethanol in a glass vial. After that, 1 mL of Ce(NO₃)₃·6H₂O solution (3 mg/mL in ethanol) and 1 mL of HMT solution (3 mg/mL in ethanol) were quickly injected into the glass vial and the obtained solution was placed in an oil bath at 100° C. for 5 mins. The final product was collected by centrifugation at 4,500 rpm for 5 mins and also washed with ethanol for three times. The obtained Au—CeO_x-10 nm heteronanostructures were re-dispersed into 5 mL of ethanol for further use.

Example 1D

Synthesis of CeO_x Nanoparticles

The synthesis of CeO_x nanoparticles was similar to the growth of CeO_x nanostructures on 4H/fcc Au nanorods, but without adding the Au nanorods as seeds. Typically, 2 mL of Ce(NO₃)₃·6H₂O solution (1 mg/mL in ethanol) were added to a glass vial. Then 2 mL of HMT solution (1 mg/mL in ethanol) were quickly injected into the glass vial, and the resultant solution was placed in an oil bath at 100° C. for 5 mins. To collect the final products, 10 mL of ethanol were added to the glass vial and the obtained solution was sonicated intensely for 5 mins. The final product was acquired by centrifugation at 8,000 rpm for 5 mins and washed with ethanol for three times.

Example 2

Structure Characterization

The scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images show that Au nanorods with high purity have been synthesized (FIGS. 2A and 2B). The as-synthesized Au nanorods possess the diameters of 12-25 nm and lengths of 40 A and 0-900 nm. The spherical aberration-corrected high-angle annular dark-field scanning TEM (HAADF-STEM) image of a representative Au nanorod and the fast Fourier transform (FFT) patterns of selected areas demonstrate the characteristic electron diffraction patterns of fcc and 4H phases (FIGS. 2C to 2E), suggesting the heterophase nanostructure of Au nanorods. In addition, the 4H/fcc heterophase of Au nanorods was also identified by the X-ray diffraction (XRD) patterns (FIG. 3), in which two sets of diffraction peaks can be assigned to the 4H and fcc phases, respectively.

In FIGS. 4A and 4B, the SEM and TEM images show that the resultant Au—CeO_x-3 nm maintains the nanorod morphology, and the surface of Au nanorods becomes rough after CeO_x growth. As shown in FIG. 4C, the CeO_x nanostructures with a thickness of about 3 nm are deposited on the surface of Au nanorods.

The HAADF-STEM was applied to determine the structure of as-prepared Au—CeO_x-3 nm. As show in FIG. 4D, it is noted that the surface of 4H/fcc Au nanorods was not fully covered by CeO_x nanostructures and the exposed Au surfaces can be clearly observed. The Au core of Au—CeO_x-3 nm maintains the 4H/fcc heterophase with characteristic “ABCB” and “ABC” stacking sequences along the close-packed directions, which is further confirmed by the selected-area FFT patterns (FIGS. 4E and 4F). As for the crystal structure of CeO_x nanostructures, the lattice spacings of 0.31 and 0.27 nm in the enlarged edge-area of HAADF-STEM image can be attributed to the {111} and {020} facets of fcc phase (FIG. 4G). Meanwhile, the corresponding FFT pattern matches well with the electron-diffraction pattern of fcc phase (FIG. 4H), suggesting the fcc phase of CeO_x nanostructures as well. The typical HAADF-STEM image of Au—CeO_x-3 nm and the corresponding energy dispersive X-ray spectroscopy (EDS) elemental mapping and the linear scanning profile further identify the dense growth of CeO_x nanostructures on the Au surface (FIGS. 4I to 4L, FIGS. 5A and 5B, and FIG. 6).

It is believed that through controlling the amount of Ce(NO₃)₃·6H₂O and HMT in the reaction system, thicker CeO_x nanostructures with a thickness of about 10 nm can be grown on 4H/fcc Au nanorods (denoted as Au—CeO_x-10 nm, FIGS. 7A and 7B and FIG. 8). As shown in the FIGS. 9 and 10, the XRD patterns of Au—CeO_x-3 nm and Au—CeO_x-10 nm mainly show the typical peaks of 4H/fcc Au nanorods, indicating that the 4H/fcc heterophase of Au is well preserved. Due to the small size and low crystallinity of CeO_x, two peaks assigned to (111) and (311) facets of CeO_x structures can be observed, suggesting the successful overgrowth of CeO_x nanostructures. Moreover, free-standing CeO_x nanostructures with fcc phase were also synthesized using the same method without adding Au nanorods, and TEM image and XRD pattern of the as-synthesized CeO_x nanoparticles are shown in FIGS. 11 and 12, respectively.

The X-ray photoelectron spectroscopy (XPS) was used to investigate the electronic structure evolution of heterophase 4H/fcc Au nanorods during the overgrowth of CeO_x nanostructures. As shown in FIG. 13, the Au element in Au—CeO_x-3 nm remains the metallic state, similar to that of Au nanorods. Compared with the Au nanorods, Au 4f peaks of Au—CeO_x-3 nm shift to higher binding energy by 0.2 eV, which can be attributed to the strong metal-oxide interactions and the electronic transfer between Au support and CeO_x nanostructures. Meanwhile, the Au element in Au—CeO_x-10 nm also adopts the metallic state (FIG. 14).

Considering the spin-orbit splitting of Ce 3d_5/2 and Ce 3d_3/2, the Ce 3d spectrum of Au—CeO_x-3 nm can be deconvoluted into ten peaks, six of which are assigned to Ce(IV) state and the other four are attributed to Ce(III) state (FIG. 15). Importantly, the ratio of Ce(IV) to Ce(III) is calculated to be about 2/1 by comparing the integrated peak areas, suggesting the existence of oxygen vacancies in the CeO_x nanostructures of Au—CeO_x-3 nm. Similarly, the deconvoluted Ce 3d spectra of Au—CeO_x-10 nm and CeO_x nanoparticles also show ten peaks, referring to Ce(IV) or Ce(III) (FIGS. 16A and 16B).

As shown in FIG. 17A, the O 1s spectrum of Au—CeO_x-3 nm exhibits two peaks centered at 529.3 and 531.4 eV, which are assigned to the lattice oxygen and oxygen vacancies in CeO_x, respectively. In parallel, two main peaks corresponding to the lattice oxygen (529.1 eV) and oxygen vacancies (531.3 eV) can also be observed in the O 1s spectrum of Au—CeO_x-10 nm (FIG. 17B). Moreover, the O 1s spectrum of CeO_x nanoparticles exhibits three peaks centered at 529.3, 531.2 and 532.7 eV (FIG. 17C), which correspond to the lattice oxygen and oxygen vacancies in CeO_x as well as the surface-chemisorbed oxygen species, respectively.

The X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectroscopies were combined to analyze the electronic structure and atomic coordination environment of Au—CeO_x-3 nm. As shown in the normalized Au L₃-edge XANES spectra (FIG. 18), Au—CeO_x-3 nm exhibits the similar white line intensity and rising edge energy to that of Au nanorods and Au foil, indicating the metallic state of Au element in Au—CeO_x-3 nm. The Fourier transforms of EXAFS spectra of Au L₃-edge were conducted to investigate the atomic coordination environment of Au in Au—CeO_x-3 nm (FIG. 19). The Au foil shows two characteristic peaks at 2.48 and 2.93 Å, corresponding to the Au—Au scattering paths. The similar Au—Au scattering paths were also found in Au—CeO_x-3 nm and Au nanorods, suggesting the existence of Au—Au bonds in both samples. Based on the fitting results, the Au—CeO_x-3 nm exhibits the similar average Au—Au bond distance to that of Au nanorods or Au foil (FIGS. 20A and 20B, FIGS. 21A and 21B, FIGS. 22A and 22B and FIG. 23). Notably, Au—CeO_x-3 nm displays a higher average coordination number (CN) of 10.4 than that of original Au nanorods (9.7), which could be attributed to the strong metal-oxide interactions between the Au support and CeO_x nanostructures. As shown in FIGS. 24A and 24B, the wavelet transform (WT) of Au L₃-edge EXAFS spectra of Au—CeO_x-3 nm and Au nanorods show the similar patterns, indicating the well preservation of 4H/fcc Au structure after CeO_x overgrowth.

Example 3

Electrocatalytic CO₂RR Performance

The electrochemical CO₂RR measurements of Au—CeO_x-3 nm, Au—CeO_x-10 nm and 4H/fcc Au nanorods were performed in the H-type cell with CO₂-saturated 0.5 M KHCO₃ aqueous solution as the electrolyte. The gaseous products were characterized through the on-line gas chromatography (GC), while the liquid products were analyzed by 1H nuclear magnetic resonance (NMR) spectroscopy. As shown in FIG. 25, the linear sweep voltammetry (LSV) curves show that the current densities of three catalysts follow the order of Au—CeO_x-3 nm>Au nanorods>Au—CeO_x-10 nm, suggesting that Au—CeO_x-3 nm possesses the highest catalytic activity. Remarkably, at the potential of −0.8 V (vs reversible hydrogen electrode (RHE)), the Au—CeO_x-3 nm delivers the current density of 17.2 mA cm⁻², which is almost 2.8 and 4.7 times that of Au nanorods and Au—CeO_x-10 nm, respectively.

Three Au-based electrocatalysts exhibit the similar product distribution for CO₂RR, where CO is the dominant product along with trace amount of formate (FIGS. 26A and 26B). Both Au—CeO_x-3 nm and Au—CeO_x-10 nm exhibit the much higher selectivity for CO production in a wide range of applied potentials than that of pristine Au nanorods. Significantly, Au—CeO_x-3 nm demonstrates CO selectivity over 97.1% in a broad potential window from −0.5 to −0.8 V (vs RHE). At −0.6 and −0.7 V (vs RHE), Au—CeO_x-3 nm achieves the highest FE_CO near 100%, which suggests the almost entire suppression of the competing hydrogen evolution reaction (FIG. 27). Meanwhile, Au—CeO_x-10 nm also exhibits good selectivity for CO production with FE over 90.8% in the potential range of −0.5 to −0.8 V (vs RHE). At −0.6 V (vs RHE), Au—CeO_x-10 nm displays the highest FE_CO of 97.7%. In contrast, pristine Au nanorods only show the highest FE_CO of 86.3% at −0.8 V (vs RHE). Moreover, Au—CeO_x-3 nm displays much larger geometric CO partial current density (J_CO) than that of Au—CeO_x-10 nm and Au nanorods in the whole potential range (FIG. 28). In particular, Au—CeO_x-3 nm delivers the geometric J_CO of 15.3 mA cm⁻² at −0.8 V (vs RHE), which is almost 5.1 and 3.1 times that of Au—CeO_x-10 nm and Au nanorods, respectively.

The electrochemically active surface area (ECSA) of catalysts was investigated by cyclic voltammetry. The cyclic voltammograms (CV) curves of Au—CeO_x-3 nm and pristine Au nanorods were measured. As shown in FIGS. 29A and 29B, Au—CeO_x-3 nm and Au nanorods exhibit similar ECSA values of 0.0072 and 0.0086 cm⁻² for electrocatalytic CO₂RR.

The Tafel slope analysis was also performed to study the reaction kinetics of CO₂ electroreduction (FIG. 30). Notably, Au—CeO_x-3 nm demonstrates much smaller Tafel slope of 74.1 mV dec⁻¹ than that of Au—CeO_x-10 nm (90.9 mV dec⁻¹) and Au nanorods (89.5 mV dec⁻¹). This indicates that three Au-based electrocatalysts show the similar reaction pathway, i.e., the formation of CO₂^•− through a fast one-electron transfer process and the successive combination of CO₂^•− with a proton to produce *COOH. In addition, the smallest Tafel slope of Au—CeO_x-3 nm reveals the fastest reaction kinetics for CO₂RR.

Moreover, the long-term durability test of Au—CeO_x-3 nm was conducted at −0.5 V (vs RHE) for CO₂RR. As shown in FIG. 31, the FE_CO of Au—CeO_x-3 nm is well maintained over 95.9% and the current density exhibits no obvious decrease after 12 h electrolysis. Meanwhile, TEM characterizations identify that the morphology and structure of Au—CeO_x-3 nm are well preserved (FIGS. 32A to 32C), suggesting the excellent catalytic stability of Au—CeO_x-3 nm toward CO₂RR.

In addition, the electrocatalytic CO₂RR performance of free-standing CeO_x nanostructures was also measured in the H-type cell under the same experimental condition as that of the aforementioned three Au-based electrocatalysts. As shown in FIGS. 33A and 33B, CeO_x nanoparticles deliver low current densities in the whole range of applied potentials, indicating the poor electrocatalytic activity. Simultaneously, it was observed that the dominant product is H₂, together with a very low selectivity for CO production over the entire potential window. These results reveal that the active sites in Au—CeO_x-3 nm, which can effectively promote the conversion of CO₂ to CO, mainly locate in the Au part and/or Au—CeO_x interface.

Example 4

CO₂RR Mechanism

In-situ ATR-IR spectroscopy was employed, with the catalysts deposited on an Au-coated Si prism as the working electrode, to have a better understanding to the CO₂RR mechanism on Au—CeO_x-3 nm. As shown in FIGS. 34A and 34B, the in-situ ATR-IR spectra of Au—CeO_x-3 nm and 4H/fcc Au nanorods were collected at the applied potential from −0.05 to −1.20 V (vs RHE) in CO₂-saturated 0.5 M KHCO₃ aqueous solution. Multiple absorbance bands are observed between 2400 and 1300 cm-1. The negative bands centered at 2343 cm⁻¹ correspond to the consumption of CO₂ in the solution. It typically appears at −0.45 V (vs RHE) and becomes more pronounced with further decreasing the potential. The signal of *CO is absent on both catalysts within the applied potential range of CO₂ reduction, which is believed to be mainly arisen from the quick desorption of *CO due to its weak binding strength on Au. It is noteworthy that the ATR-IR spectra of Au—CeO_x-3 nm feature a weak peak at 1369 cm⁻¹ belonging to HCO₃⁻ (aq) in the vicinity of electrode surface. In particular, the intensity of HCO₃⁻ band gradually increased from −0.05 to −0.4 V (vs RHE), then decreased at higher overpotentials. In addition, a strong peak at 1400 cm⁻¹ assigned to carbonate anions in solution became dominant from −0.6 to −1.2 V (vs RHE). The accumulation of CO₃²⁻ aq) at high overpotentials indicates the increasing local pH near the electrode surface. However, no HCO₃⁻ (aq) was detected on 4H/fcc Au nanorods, with only the CO₃²⁻ (aq) peaks centered at 1403 cm⁻¹, suggesting a higher HCO₃⁻ concentration near the Au—CeO_x-3 nm surface.

It is believed that a higher HCO₃ concentration is beneficial for CO₂ generation via the equilibrium with bicarbonate anions, which will promote the subsequent CO₂ reduction process. This is further validated by the observation that the peak intensity of the consumed CO₂ near the electrode is much stronger on Au—CeO_x-3 nm than that on Au nanorods. It is noted that the signal of HCO₃⁻ was also detected in Ar-saturated 0.5 M KHCO₃ aqueous solution on Au—CeO_x-3 nm (FIG. 35). The reason for the higher concentration of HCO₃⁻ near Au—CeO_x-3 nm surface may result from the abundant oxygen vacancies in CeO_x nanostructures, which could enhance the adsorption of OH⁻ and alleviate the increase of local pH during CO₂RR.

Example 5

Theoretical Calculations

To further unveil the modulation effect of CeO_x on 4H/fcc Au nanorods in the electrocatalytic CO₂RR, DFT calculations have been performed. In order to imitate the oxidation state of Ce in experiments, the model of 4H/fcc Au—Ce₃O₇ was established to describe the Au—CeO_x interface. FIGS. 36A and 36B illustrate the structure and electron density difference of the 4H/fcc Au—Ce₃O₇ interface. It can be found that after depositing Ce₃O₇ on the top of Au, Au substrate would lose electrons and the O atoms had the tendency to gain additional electrons.

To provide more quantitative information, Bader charge analysis was conducted, revealing a charge transfer of 0.68 e from Au substrate to Ce₃O₇ cluster. Furthermore, projected density of states was plotted to reveal the influence of Ce₃O₇ on the CO₂RR. For the 4H/fcc Au nanorods, Au-5d orbitals exhibit a peak near −2.3 e V with the d-band center located at −3.26 eV (FIG. 37). In comparison, 4H/fcc Au—Ce₃O₇ shows more complicated electronic structures (FIG. 38). Firstly, the electron loss causes a slight downshift of the Au-5d band center to −3.38 eV, which might potentially suppress the electroactivity. But the newly formed unoccupied Ce-4f orbitals play more critical role, which could help to enhance the hybridization and bonding of the adsorbates.

Specifically, the adsorption of *COOH and *CO is facilitated at the Au—CeO_x interface. As shown in FIG. 39, the energy barrier of CO₂→*COOH step decreases, while the energy barrier of *CO→CO step increases. Notably, the formation of *COOH species via protonation is the rate-limiting step in the energy diagram, with the highest energy barrier during the entire CO₂RR process. In specific, compared with the bare 4H/fcc Au substrate, 4H/fcc Au—Ce₃O₇ reduces the highest energy barrier from 1.50 to 0.68 eV, thereby the overall CO₂RR process is optimized. Based on both experimental and theoretical studies, the plausible mechanism of CeO_x overgrowth on 4H/fcc Au nanorods to enhance the electrocatalytic CO₂RR has been schematically illustrated in FIG. 40.

The invention has been given by way of example only, and various other modifications of and/or alterations to the described embodiment may be made by persons skilled in the art without departing from the scope of the invention as specified in the appended claims.