Metal oxide composite structures, methods of making the structures, and systems comprising the structures

Abstract

The present disclosure provides for composite structures including metal oxide catalysts (e.g., a CeOx catalyst with metallic nanoparticles or metal atoms) and systems including the composite structure. Also disclosed herein are methods of making metal oxide composite structures. The composite structures can be used for conducting thermocatalytic reactions (e.g., CO2 hydrogenation), electrocatalytic reactions (e.g., water electrolysis), and the like.

Description

METAL OXIDE COMPOSITE STRUCTURES, METHODS OF MAKING THE STRUCTURES, AND SYSTEMS COMPRISING THE STRUCTURESCROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of: US Provisional Patent Application No. 63 / 632,668 filed on April 11 , 2024, which is incorporated herein by reference in its entirety.STATEMENT ON FUNDING PROVIDED BY THE U.S. GOVERNMENT

[0002] This invention was made with Government support under contract DE-SC00234430 awarded by the Department of Energy. The Government has certain rights in the invention.BACKGROUND

[0003] The water electrolysis process involves two redox half-reactions: the formation of H2occurs at the cathode via the hydrogen evolution reaction (HER), and the production of O2via the oxygen evolution reaction (OER) at the anode. Catalysts are needed to overcome kinetic barriers and reduce overpotentials, but this is challenging. Technology is needed to address these challenges.SUMMARY

[0004] The present disclosure provides for composite structures including metal oxide catalysts (e.g., a CeOxcatalyst with metallic nanoparticles or metal atoms) and systems including the composite structure. Also disclosed herein are methods of making metal oxide composite structures.

[0005] The present disclosure provides for a composite structure, comprising: a support; at least one layer of CeOxcoated onto a surface of the support; and a plurality of metallic nanoparticles, a plurality of metal atoms, or a combination thereof deposited onto the CeOxlayer.

[0006] The present disclosure provides for a system, comprising: a first electrode; a second electrode, in electrical communication with the first electrode; and at least one electrolyte in contact with at least one of the first electrode or second electrode; wherein the first electrode comprises at least one layer of CeOxcoated onto a surface of the first electrode and a plurality of metallic nanoparticles, a plurality of metal atoms, or a combination thereof deposited onto the CeOxlayer.

[0007] A method for producing a catalyst, comprising: coating a support with CeOxnanosheets to form a CeOxlayer on the support; and depositing a plurality of metallic nanoparticles, a plurality of metal atoms, or a combination thereof onto the CeOxlayer.BRIEF DESCRIPTION OF THE DRAWINGS

[0008] Many aspects of the present disclosure can be better understood with reference to the following drawings. The components in the drawings are not necessarily to scale, emphasis instead being placed upon clearly illustrating the principles of the present disclosure. Moreover, in the drawings, like reference numerals designate corresponding parts throughout the several views.

[0009] Figure 1 is a schematic illustration of preparing Pt / CeOx / C catalyst.

[0010] Figures 2A-2D illustrate the morphology and structure characterizations of CeOxNSs. Fig. 2A is a TEM image and Fig. 2B is a HAADF-STEM image of Ce2O3NSs. Inset of Fig. 2B is a FFT pattern from the whole image. Fig. 2C is TEM image of vertically aligned CeOxNSs. Fig. 2D is a HAADF-STEM and EELS mapping of the CeOx / C support.

[0011] Figures 3A and 3F are HAADF-STEM images and Figures 3B-3E illustrate EELS mapping of Pt / CeOx / C (3 nm Pt) catalyst.

[0012] Figure 4A-4G illustrate alkaline HER performance, in-situ XAS, and DRIFT results of different catalysts. Figures 4A and 4B illustrate LSV curves. Figure 4C(c) ECSA-normalized LSVs. (d) ECSA-normalized current at overpotential of 100 and 150 mV. In-situ Pt L3-edge (e) XANES and (f) EXAFS spectra in the R space on Pt / C and Pt / CeOx / C (3 nm Pt) catalysts at -0.1 V vs. RHE. (g) CO adsorption DRIFTS in the vCo region characterizing Pt / C and Pt / CeOx / C (3 nm Pt) catalysts after CO adsorption.

[0013] Figure 5A illustrates the geometric configuration of the optimized Ptio / Ce203(100) model system. Green, red, grey spheres represent Ce, O, and Pt atoms, respectively. Figure 5B illustrates the charge distribution using Bader charge analysis. Blue and red color represents net positive and negative oxidation states, respectively. Figure 5C illustrates the 18 unique H binding sites on Ptio / Ce203(100). Large grey spheres represent Pt atoms while the small spheres indicate H atom binding sites. Blue and red color represent weaker and stronger H binding, respectively, compared to that on Pt (111). Figure 5D illustrates the top and Figure 5E illustrates the side view of the charge redistribution upon Ptw binding on Ce203(100). Yellow and blue isosurfaces represent charge accumulation and depletion, respectively.

[0014] Figures 6A-6B illustrates XPS spectra of (Figure 6A) as-synthesized CeOxNSs and (Figure 6B) Pt / CeOx / C (3 nm Pt) catalysts.

[0015] Figures 7A-7C illustrate the morphology characterizations of different sizes of Pt NPs and Pt / CeOx / C catalysts. Figures 7A-7C illustrate TEM images of 3 nm, 5 nm, and 7 nm of Pt NPs, respectively. Figure 7D illustrates a TEM image of Pt / CeOx / C (3 nm Pt) catalyst after heat treatment.

[0016] Figures 8A-8F illustrates the morphology characterizations of different sizes of Pt NPs loaded onto carbon and CeOxNSs supported carbon. Figures 8A-8C illustrate TEM images of 3 nm, 5 nm, and 7 nm of Pt NPs loaded onto carbon, respectively (after 200 °C treatment). Figures 8A-8F illustrate TEM images of 3 nm, 5 nm, and 7 nm of Pt NPs loaded onto CeOxNS supported carbon, respectively (after 200 °C treatment).

[0017] Figures 9A and 9N illustrates tafel slope plots of (Figure 9A) Pt / C and (Figure 9B) Pt / CeOx / C catalysts with different Pt NPs sizes.

[0018] Figure 10 illustrates the H+adsorption and stripping over all catalysts in 0.01 M HCIO4 solution.

[0019] Figure 11 illustrates stability tests at constant current density of 100 mA cm2.

[0020] Figure 12 illustrate the EXAFS spectra of Pt l_3-edge and corresponding fitting plots in R space for the Pt / CeOx / C catalyst under different conditions. k1k2k3-weighted EXAFS fitting range for Pt l_3-edge: 3.0 < k < 11.0 A1, 1.0 < R < 3.3 A.

[0021] Figure 13 illustrate the EXAFS spectra of Pt l_3-edge and corresponding fitting plots in R space for the Pt / C catalyst under different conditions. k1k2k3-weighted EXAFS fitting range for Pt L3-edge: 3.0 < k < 11 .0 A1, 1 .0 < R < 3.3 A.

[0022] Figure 14A illustrates one unit cell of the bulk cubic Ce2O3(Ia3) structure. Figure 14B illustrates spin distribution. The blue isosurface represents the spin density on Ce3+.

[0023] Figure 15A illustrates the top and side view of the Ce2O3(100) surface structure as a model substrate with 5.26 A thickness, and Figure 15B illustrates the spin distribution. Blue and pink isosurfaces represent spin up and down, respectively.

[0024] Figures 16A and 16B illustrate atomic configuration of Pti0 / Ce2O3(100) with (Figure 16A) a 2-layered Ptio and (Figure 16B) a triangular pyramidal Ptio cluster.

[0025] Figure 17 illustrates reduction free energy of O on Ptio cluster at 0 V vs RHE.

[0026] Figures 18A (temperature) and 18B (potential energy) illustrate ab-initio molecular dynamics at 298 K for 10 ps.

[0027] Figures 19A-19C illustrate H binding sites considered in this study. Figure 19A 9 on top (1-fold) sites, Figure 19B 17 bridge (2-fold) sites, and Figure 19C 9 hollow (3-fold) sites.

[0028] Figures 20A and 20B illustrate atomic configurations and reaction free energy for water dissociation on (Figure 20A) Pt(111) and (Figure 20B) Pti0 / Ce2O3(100) surfaces.

[0029] Figure 21 illustrates HAADF-STEM image of AI2O3coated by CeOx. The CeOxpresents a thin shell on the surface of AI2O3particles. This material was annealed in air at 200 °C for 12 h to remove the ligands on the surface of CeOx.

[0030] Figure 22 illustrates a TEM image of 2-3 nm Pd NPs loaded onto CeOxNS supported carbon.

[0031] Figure 23 a TEM image of 14 nm Fe NPs loaded onto CeOxNS supported carbonDETAILED DESCRIPTION

[0032] This disclosure is not limited to particular embodiments described, and as such may, of course, vary. The terminology used herein serves the purpose of describing particular embodiments only, and is not intended to be limiting, since the scope of the present disclosure will be limited only by the appended claims.

[0033] Where a range of values is provided, each intervening value, to the tenth of the unit of the lower limit unless the context clearly dictates otherwise, between the upper and lower limit of that range and any other stated or intervening value in that stated range, is encompassed within the disclosure. The upper and lower limits of these smaller ranges may independently be included in the smaller ranges and are also encompassed within the disclosure, subject to any specifically excluded limit in the stated range. Where the stated range includes one or both of the limits, ranges excluding either or both of those included limits are also included in the disclosure.

[0034] Embodiments of the present disclosure will employ, unless otherwise indicated, techniques of chemistry, inorganic chemistry, synthetic chemistry, and the like, which are within the skill of the art. Such techniques are explained fully in the literature.

[0035] The following examples are put forth so as to provide those of ordinary skill in the art with a complete disclosure and description of how to perform the methods and use the compositions and compounds disclosed and claimed herein. Efforts have been made to ensure accuracy with respect to numbers (e.g., amounts, temperature, efc.), but some errors and deviations should be accounted for. Unless indicated otherwise, parts are parts by weight, temperature is in °C, and pressure is at or near atmospheric. Standard temperature and pressure are defined as 20 °C and 1 atmosphere.

[0036] It should be noted that ratios, concentrations, amounts, and other numerical data may be expressed herein in a range format. It is to be understood that such a range format is used for convenience and brevity, and thus, should be interpreted in a flexible manner to include not only the numerical values explicitly recited as the limits of the range, but also to include all the individual numerical values or sub-ranges encompassed within that range as if each numerical value and sub-range is explicitly recited. To illustrate, a concentration range of “about 0.1 percent to about 5 percent” should be interpreted to include not only the explicitly recited concentration of about 0.1 weight percent to about 5 weight percent but also include individual concentrations (e.g., 1 percent, 2 percent, 3 percent, and 4 percent) and the subranges (e.g., 0.5 percent, 1.1 percent, 2.2 percent, 3.3 percent, and 4.4 percent) within the indicated range. The term “about” can include traditional rounding according to significant figures of the numerical value. In addition, the phrase “about ‘x’ to ‘y’” includes “about ‘x’ to about ‘y’”.

[0037] Furthermore, the terms “about”, “approximate”, “at or about”, and “substantially” as used herein mean that the amount or value in question can be the exact value or a value that provides equivalent results or effects as recited in the claims or taught herein. That is, it is understood that amounts, sizes, formulations, parameters, and other quantities and characteristics are not and need not be exact but may be approximate and / or larger or smaller, as desired, reflecting tolerances, conversion factors, rounding off, measurement error and the like, and other factors known to those of skill in the art such that equivalent results or effects are obtained. In some circumstances, the value that provides equivalent results or effects cannot be reasonably determined. In such cases, it is generally understood, as used herein, that “about” and “at or about” mean the nominal value indicated ±10% variation unless otherwise indicated or inferred. In general, an amount, size, formulation, parameter or other quantity or characteristic is “about,” “approximate,” or “at or about” whether or not expressly stated to be such. It is understood that where “about,” “approximate,” or “at or about” is used before a quantitative value, the parameter also includes the specific quantitative value itself, unless specifically stated otherwise.

[0038] Before the embodiments of the present disclosure are described in detail, it is to be understood that, unless otherwise indicated, the present disclosure is not limited to particular materials, reagents, reaction materials, manufacturing processes, dimensions, frequency ranges, applications, or the like, as such can vary. It is also to be understood that the terminology used herein is for purposes of describing particular embodiments only and is not intended to be limiting. It is also possible in the present disclosure that steps can be executed in different sequence, where this is logically possible. It is also possible that the embodiments of the present disclosure can be applied to additional embodiments involving measurementsbeyond the examples described herein, which are not intended to be limiting. It is furthermore possible that the embodiments of the present disclosure can be combined or integrated with other measurement techniques beyond the examples described herein, which are not intended to be limiting.

[0039] It should be noted that, as used in the specification and the appended claims, the singular forms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to “a support” includes a plurality of supports. In this specification and in the claims that follow, reference will be made to a number of terms that shall be defined to have the following meanings unless a contrary intention is apparent.

[0040] Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this disclosure belongs. Although any methods and materials similar or equivalent to those described herein can also be used in the practice or testing of the present disclosure, the preferred methods and materials are now described.General Discussion

[0041] The present disclosure provides for composite structures including metal oxide catalysts (e.g., a CeOxcatalyst with metallic nanoparticles or metal atoms) and systems including the composite structure. The composite structures can be used for conducting thermocatalytic reactions (e.g., CO2hydrogenation), electrocatalytic reactions (e.g., water electrolysis), and the like. Also disclosed herein are methods of making metal oxide composite structures. The presence of metallic nanoparticles or metal atoms and CeOxin the structure can create metal-metal oxide interfaces. The creation of metal-metal oxide interfaces is one approach to fine-tuning a catalyst’s properties through strong interfacial interactions. Developing interfaces between metals (e.g., metallic particles) and metal oxides such as CeOxcan improve surface energetics within thermocatalytic or electrocatalytic reactions. These interfaces can improve overall reaction activity and cost-efficiency.

[0042] Composite Metal Oxide Structure

[0043] In one aspect, disclosed herein is a composite structure comprising a support, at least one layer of CeOxcoated onto a surface of the support, and a plurality of metallic nanoparticles a plurality of metal atoms, or a combination thereof deposited onto the CeOxlayer. CeOxis used to refer to a material that includes a heterogeneous mixture of Ce ions, including Ce3+(Ce2O3) and Ce4+(CeO2).

[0044] In an aspect, the support can be a carbon support, a non-carbon support, or a combination thereof. Carbon support materials can include glassy carbon, graphite, carbon nanotubes, graphene sheets, or boron-doped diamond. Non-carbon support materials caninclude silica, metal oxides (e.g., aluminum oxide or zirconium dioxide), carbides, nitrides, or combinations thereof.

[0045] The composite material can be used as a catalyst for conducting thermocatalytic reactions or electrocatalytic reactions. Examples of such thermocatalytic reactions include, but are not limited to, CO2hydrogenation or methane decomposition. Examples of such electrocatalytic reactions include, but are not limited to, water electrolysis, hydrogen oxidation, hydrogen evolution, chlorine evolution, CO2reduction, CO reduction, or small molecule (e.g., methanol, formic acid) oxidation. In one aspect, the support can be selected based on the type of reaction the catalyst is to be used for. For example, a carbon-based support may be selected when using the composite material for conducting electrocatalytic reactions. As another example, a non-carbon support may be selected when using the composite material for conducting thermocatalytic reactions.

[0046] The layer of CeOxcoating of the composite structure can have a thickness of about 1 nm to about 2 nm, about 1 nm to about 1 .8 nm, about 1 nm to about 1 .6 nm, about 1 .2 nm to about 2 nm, about 1.4 nm to about 2 nm, or about 1.4 nm to about 1.6 nm. The primary component of the CeOxlayer can be Ce2O3, for example the CeOxlayer can be over 50% Ce2O3. In one aspect, only a single layer of CeOxis coated onto the surface of the support.

[0047] In one aspect, the CeOxlayer can be comprised of a plurality of CeOxnanosheets. In one aspect, the CeOxnanosheets used can have a length of about 10 nm to about 150 nm, about 10 nm to about 100 nm, or about 10 nm to about 50 nm. In one aspect, the CeOxnanosheets used can have a width of about 10 nm to about 150 nm, about 10 nm to about 100 nm, or about 10 nm to about 50 nm. In one aspect, the nanosheets length is the same as the width. In other aspect, the nanosheets have different lengths and widths. The same size nanosheets can be used to deposit onto the support, or nanosheets of varying sizes can be used. The nanosheets can have a thickness of about 1 nm to about 2 nm, about 1 nm to about 1 .8 nm, about 1 nm to about 1 .6 nm, about 1 .2 nm to about 2 nm, about 1 .4 nm to about 2 nm, or about 1 .4 nm to about 1 .6 nm. Methods for producing CeOxnanosheets are provided in the Examples.

[0048] The metallic nanoparticles or metal atoms can be deposited or loaded on the support with the CeOxcoating. The metal-metal oxide interface produced upon depositing metallic nanoparticles or metal atoms onto the CeOxlayer can improve the performance of the catalyst. In one aspect, the metal atoms can include iron, cobalt, nickel, copper, ruthenium, rhodium, silver, platinum, gold, palladium, or a combination thereof. In another aspect, the metallic nanoparticles can include metallic nanoparticles such as iron nanoparticles, cobalt nanoparticles, nickel nanoparticles, copper nanoparticles, ruthenium nanoparticles, rhodiumnanoparticles, silver nanoparticles, platinum nanoparticles, gold nanoparticles, palladium nanoparticles, or a combination thereof. Metallic nanoparticles can be deposited onto the CeOxlayer at a weight percentage of metallic nanoparticle to CeOxlayer of about 1 % to about 70%, about 1 % to about 50%, about 1 % to about 30%, about 1 % to about 10%, about 1 % to about 5%, about 2% to about 10%, or about 2% to about 5%.

[0049] The size of the metallic nanoparticles can be precisely selected. In one aspect, the size of the nanoparticles is uniform. In another aspect, the nanoparticles can vary in size. The metallic nanoparticles can have diameters of about 1 nm to about 100 nm about 1 nm to about 75 nm, about 1 nm to about 50 nm, about 1 nm to about 25 nm, about 1 nm to about 10 nm, or about 1 nm to about 7 nm. The nanoparticles can have the same shape or vary in shape. The nanoparticle shapes can include spheres, rods, cubes, stars, triangles, hexagons, prisms, needles, clusters, and the like. Examples of methods for producing metallic nanoparticles are provided in the Examples.

[0050] Method for Producing a Composite Metal Oxide Structure

[0051] Also disclosed herein is a method for producing a composite structure such as a metal oxide catalyst, comprising coating a support with CeOxnanosheets to form a CeOxlayer on the support and depositing a plurality of metallic nanoparticles, a plurality of metal atoms, or a combination thereof onto the CeOxlayer. The method can further include annealing the support after coating it with CeOxnanosheets and prior to depositing the metallic nanoparticles, metal atoms, or a combination thereof. The CeOxnanosheet-coated support can be annealed in air at a temperature of about 150 °C to about 250 °C, about 175 °C to about 225 °C, or about 200 °C. Annealing the nanosheets can produce a relatively uniform layer of CeOxon the support.

[0052] The support can be a carbon or a non-carbon support. Carbon support materials can include glassy carbon, graphite, carbon nanotubes, graphene sheets, or boron-doped diamond. Non-carbon support materials can include silica, metal oxides (e.g., aluminum oxide or zirconium dioxide), carbides, nitrides, or combinations thereof.

[0053] In one aspect, the metal atoms deposited onto the CeOxlayer can include iron, cobalt, nickel, copper, ruthenium, rhodium, silver, platinum, gold, palladium, or a combination thereof. In another aspect, the metallic nanoparticles deposited onto the CeOxlayer can include iron nanoparticles, cobalt nanoparticles, nickel nanoparticles, copper nanoparticles, ruthenium nanoparticles, rhodium nanoparticles, silver nanoparticles, platinum nanoparticles, gold nanoparticles, palladium nanoparticles, or a combination thereof. In another aspect, the metallic particles deposited onto the CeOxlayer can include a combination of metal atoms and metal nanoparticles. Metallic nanoparticles can be deposited onto the CeOxlayer at a weightpercentage of metallic nanoparticle to CeOxlayer of about 1 % to about 70%, about 1 % to about 50%, about 1 % to about 30%, about 1 % to about 10%, about 1% to about 5%, about 2% to about 10%, or about 2% to about 5%.

[0054] The size of the metallic nanoparticles can be precisely selected, if desired. In one aspect, the size of the nanoparticles is uniform. In another aspect, the nanoparticles can vary in size. The metallic nanoparticles can have diameters of about 1 nm to about 100 nm about1 nm to about 75 nm, about 1 nm to about 50 nm, about 1 nm to about 25 nm, about 1 nm to about 10 nm, or about 1 nm to about 7 nm. The nanoparticles can have the same shape or vary in shape. The nanoparticle shapes can include spheres, rods, cubes, stars, triangles, hexagons, prisms, needles, clusters, and the like. Examples of methods for producing metallic nanoparticles are provided in the Examples.

[0055] In one aspect, enough CeOxnanosheets are deposited onto a surface of the support to entirely cover the surface or substantially (e.g., about 90% or more, about 95% or more, about 98% or more, about 99% or more) the entire surface. In another aspect, enough CeOxnanosheets are deposited onto a surface of the support so that, once annealed, a relatively uniform layer of CeOxis formed on the surface of the support. The layer of CeOx(with an annealing step or without) can have a thickness of about 1 nm to about 2 nm, about 1 nm to about 1 .8 nm, about 1 nm to about 1 .6 nm, about 1 .2 nm to about 2 nm, about 1 .4 nm to about2 nm, or about 1.4 nm to about 1.6 nm. The primary component of the CeOxlayer can be Ce2C>3, for example the CeOxlayer can be over 50% Ce2C>3. The CeOxnanosheets used can have dimensions (length and width) of about 10 nm to about 150 nm, about 10 nm to about 100 nm, or about 10 nm to about 50 nm. In one aspect, the nanosheets length is the same as the width. In other aspect, the nanosheets have different lengths and widths. The same size nanosheets can be used to deposit onto the support, or nanosheets of varying sizes can be used. The nanosheets can have a thickness of about 1 nm to about 2 nm, about 1 nm to about 1 .8 nm, about 1 nm to about 1 .6 nm, about 1 .2 nm to about 2 nm, about 1 .4 nm to about 2 nm, or about 1 .4 nm to about 1 .6 nm. Examples of methods for producing CeOxnanosheets are provided in the Examples.

[0056] Systems Comprising a Composite Metal Oxide Structure

[0057] Also disclosed herein is a system comprising a first electrode; a second electrode, in electrical communication with the first electrode; and at least one electrolyte in contact with at least one of the first electrode or second electrode. In a further aspect, the electrolyte is in contact with both the first and second electrode. The first electrode can include at least one layer of CeOxcoated onto a surface of the first electrode and a plurality of metallic nanoparticles, a plurality of metal atoms, or a combination thereof deposited onto the CeOxlayer. The first electrode and the second electrode can be carbon electrodes. In another aspect, only the first electrode is a carbon electrode. Examples of carbon materials used to produce the electrodes include glassy carbon, graphite, carbon nanotubes, graphene sheets, or boron-doped diamond. In one aspect, this system is used for conducting electrocatalytic reactions, such as water electrolysis, hydrogen oxidation, oxygen reduction reaction, hydrogen evolution, CO2reduction, CO reduction, or small molecule (e.g., methanol, formic acid) oxidation.

[0058] In one aspect, the layer of CeOxcan be coated onto a surface of the first electrode that is in contact or is configured to be in contact with an electrolyte. In one aspect, the CeOxlayer can be comprised of a plurality of CeOxnanosheets. The nanosheets can be a single layer thick. The layer of CeOxcan have a thickness of about 1 nm to about 2 nm, about 1 nm to about 1 .8 nm, about 1 nm to about 1 .6 nm, about 1 .2 nm to about 2 nm, about 1 .4 nm to about 2 nm, or about 1.4 nm to about 1.6 nm. The primary component of the CeOxlayer can be Ce2O3, for example the CeOxlayer can be over 50% Ce2C>3. In one aspect, only a single layer of CeOxis coated onto the surface of the support.

[0059] The metallic nanoparticles, metal atoms, or combination thereof can be deposited or loaded on the first electrode after forming the CeOxcoating. The metal-metal oxide interface produced upon depositing metallic particles onto the CeOxlayer can improve the performance of the catalyst. In one aspect, the metal atoms can include iron, cobalt, nickel, copper, ruthenium, rhodium, silver, platinum, gold, palladium, or a combination thereof. In another aspect, the metallic nanoparticles can include iron nanoparticles, cobalt nanoparticles, nickel nanoparticles, copper nanoparticles, ruthenium nanoparticles, rhodium nanoparticles, silver nanoparticles, platinum nanoparticles, gold nanoparticles, palladium nanoparticles, or a combination thereof. In another aspect, the metallic particles can include a combination of metal atoms and metal nanoparticles. Metallic nanoparticles can be deposited onto the CeOxlayer at a weight percentage of metallic nanoparticle to CeOxlayer of about 1 % to about 70% , about 1% to about 50%, about 1% to about 30%, about 1% to about 10%, about 1% to about 5%, about 2% to about 10%, or about 2% to about 5%.

[0060] In one aspect, the metallic nanoparticles of the first electrode can have a greater specific activity than an equivalent system that does not comprise any layers of CeOx. Specific activity is a measure of activity per catalytic site, i.e. , activity normalized over surface area of active materials. In another aspect, the first electrode can have a smaller decay rate or deactivation rate than an electrode in an equivalent system that does not comprise any layers of CeOx. Catalyst or electrode deactivation is a measure of the loss over time of activity or selectivity.

[0061] The size of the metallic nanoparticles can be precisely selected. In one aspect, the size of the nanoparticles is uniform. In another aspect, the nanoparticles can vary in size. The metallic nanoparticles can have diameters of about 1 nm to about 100 nm about 1 nm to about 75 nm, about 1 nm to about 50 nm, about 1 nm to about 25 nm, about 1 nm to about 10 nm, or about 1 nm to about 7 nm. The nanoparticles can have the same shape or vary in shape. The nanoparticle shapes can include spheres, rods, cubes, stars, triangles, hexagons, prisms, needles, clusters, and the like. Examples of methods for producing metallic nanoparticles are provided in the Examples.

[0062] The electrolyte of the system can be a liquid electrolyte or a solid electrolyte. Liquid electrolytes can include a liquid alkaline solution produced using a base such as sodium hydroxide or potassium hydroxide. Solid electrolytes can include a polymer electrolyte (e.g., polymer electrolyte membrane), or an alkaline electrolyte (e.g., alkaline exchange membrane). In one aspect, the electrolyte is a solid alkaline exchange membrane.

[0063] While embodiments of the present disclosure are described in connection with the Examples and the corresponding text and figures, there is no intent to limit the disclosure to the embodiments in these descriptions. On the contrary, the intent is to cover all alternatives, modifications, and equivalents included within the spirit and scope of embodiments of the present disclosure.EXAMPLESEXAMPLE 1 - Pt-CeOx Interface for the Electrocatalytic Hydrogen Evolution ReactionIntroduction

[0064] Deep decarbonization of the energy sector needs green hydrogen (H2) from water electrolysis to complement renewable electricity.111The water electrolysis process involves two redox half-reactions: the formation of H2 occurs at the cathode via the hydrogen evolution reaction (HER), and the production of O2 via the oxygen evolution reaction (OER) at the anode. Both reactions require catalysts, typically precious metals, to overcome kinetic barriers and reduce overpotentials to achieve a reasonable reaction rate.12 31This challenge is pivotal for the widespread adoption of water electrolysis at a larger scale. An exciting advance in water electrolysis technologies in recent years is the development of alkaline exchange membrane electrolysis, which allows for the use of a diluted alkaline electrolyte, which is a major departure from the concentrated base solution used in conventional alkaline electrolyzers and the acidic environment utilized in proton exchange membrane electrolyzers.141Such a benign electrolyte enables utilization of earth abundant and less expensive OER catalysts (e.g., FeNiOOH and Co-based catalysts).1481However, the HER becomes kinetically sluggish inalkaline electrolytes. Despite significant research efforts aimed at finding alternative, nonprecious metal HER catalysts, including Ni-, Co-, Mo-, and W-based alloys, sulfides, nitrides, and phosphides as well as porphyrin and corrole complexes,

[0924] Pt-based materials remain the most effective HER catalysts in an alkaline environment.

[0025]

[0065] While low proton concentration and the necessary water dissociation step are considered two factors limiting the HER kinetics in an alkaline electrolyte, prior investigation onto the HER activity on various monometallic surfaces revealed that it can be correlated with the calculated hydrogen binding energy (HBE), which determines why Pt outperforms other monometallic surfaces for HER.

[0026] Moreover, theoretical studies indicated that the HBE on Pt (111) is slightly stronger than the “ideal” value for the favorable HER kinetics.

[0027] This issue may become more pronounced when utilizing small Pt nanoparticles, a more practical form for catalysts, as they possess a greater number of low-coordination sites with higher HBE values.

[0066] Described herein is that construction of metal-metal oxide interfaces offers a viable approach to modulate Pt surface energetics for the HER in an alkaline environment. As illustrated in Figure 1 , we load Pt nanoparticles (NPs) onto a Vulcan carbon that is precoated with ultrathin CeOxnanosheets (NSs). The ultrathin CeOxNSs (predominantly Ce2O3) is believed to serve as electron sinks, and due to their atomic-thin nature (comprising 2-4 atomic layers) and the short electron transport pathway across them, the electric conductivity of the electrocatalyst remains uncompromised. Our DFT calculations confirm the electron transfer from Pt to CeOxwhen they are in direct contact, and consequently a decreased HBE, which also aligns with experimental characterizations. The formation of Pt-CeOxinterfaces leads to a 2-to-3-fold improvement in both the specific activity and the mass activity of Pt.Results and Discussion

[0067] Catalyst Synthesis and Structural Characterization

[0068] The CeOxNSs were prepared through the decomposition of cerium (III) acetate in benzyl ether, using oleylamine and oleic acid as surfactants (Methods). In a previous study, we employed a similar colloidal synthesis to make 1 -dimensional nanorods assembled by atomically precise ceria clusters, when the sole surfactant of oleic acid was used.

[0031] By modifying the synthesis with additional surfactant of oleylamine, we obtained 2-dimensional NSs instead of nanorods. This change in morphology is likely attributed to the strong coordinating influence of oleylamine, which stabilizes the basal plane of the NSs, leading to their preferential two-dimensional growth. The transmission electron microscopy (TEM) image, displayed in Figure 2A, exhibits a cubic-like morphology of as-synthesized CeOxwith a lateral length of approximately 53 nm. The TEM image in Figure 2C provides a side-view ofCeOxbased on their vertical assemblies, confirming the ultra-thin nature of the CeOxNSs (~1 .2 nm). The aberration-corrected scanning transmission electron microscopy (STEM) high- angle annular dark field (HAADF) image of CeOxand its fast Fourier transformation (FFT) pattern indicate that the cubic lattice of Ce2O3 with characteristic {004} fringe (Figure 2B).

[3234]

[0069] After loading the CeOxNSs onto Vulcan carbon (CeOx / C), we annealed the sample in air at 200 °C to remove the organic surfactant. This annealing process is commonly employed to clean the surfactant of nanoparticles for catalysis.135 361As revealed by the HAADF-STEM images and the electron energy loss spectroscopy (EELS) elemental mapping (Figure 2D), the carbon support surface is uniformly coated by the CeOxNSs, forming a thin layer of CeOxover carbon. Additionally, the analysis conducted using X-ray photoelectron spectroscopy (XPS) indicated that a portion of the Ce2O3underwent oxidation upon annealing in air. This is evident from the decrease in the Ce3+ratio from 81 .1 % to 55.2% during the annealing process (Figure 6). It is important to note that while Ce2O3 remains the primary component in our catalyst, the use of CeOxhere specifically denotes partial oxidation of Ce.

[0070] In parallel, the Pt NPs were synthesized by reducing platinum acetylacetonate in 1- Octadecene. Formaldehyde was added as a mild reduction agent during the synthesis, while oleylamine served as a surfactant. The size of Pt NPs can be controlled by adjusting the concentrations of metal salt and oleylamine. As displayed in TEM images in Figure 7A-C, the as-synthesized Pt NPs are highly uniform and can be precisely controlled in size, with diameters of around 3, 5, and 7 nm.

[0071] To build the interface between Pt and CeOx, we followed a process graphically portrayed in Figure 1 . Generally, Pt NPs of varying sizes were loaded onto the CeOx / C as well as pure C as control catalysts, denoting as Pt / CeOx / C and Pt / C, respectively. A subsequent heat treatment in air was utilized to remove the ligands encapsulating the Pt NPs and facilitate the direct contact of Pt and CeOx. As shown in the TEM image (Figure 7D), Pt NPs are well- dispersed within Pt / CeOx / C. The size and morphology of Pt within Pt / CeOx / C are also well maintained after the thermal annealing (Figure 8). Furthermore, the HAADF-STEM and EELS mapping images of Pt / CeOx / C in Figure 3 confirms the dispersity of Pt NPs, the well-preserved CeOxthin layer, and the physical attachment between Pt and CeOx.

[0072] Electrochemical Performance and Spectroscopic Analysis

[0073] We evaluated the HER performances of the Pt / CeOx / C and Pt / C samples using a typical three-electrode system with a carbon rod counter electrode, a Hg / HgO reference electrode, and a catalyst-loaded glassy-carbon rotating disk electrode (RDE) as working electrode. As shown in Figure 4A and 4B, the linear sweep voltammetry (LSV) curve of bare CeOx / C, obtained in H2-saturated 1 M KOH, exhibits negligible current density even at a verynegative potential. This indicates that the electrocatalytic activity of the HER for Pt / CeOx / C is exclusively attributed to the Pt NPs. Furthermore, it is observed that 3 nm Pt NPs present a significantly lower overpotential compared to 5 nm and 7 nm Pt NPs for both Pt / CeOx / C and Pt / C catalysts. This could be ascribed to the larger surface area of the smaller nanoparticles. Once coupled with CeOx, the Pt / CeOx / C catalysts demonstrate HER activities superior to the Pt / C counterparts, regardless of Pt size, while their Pt mass loadings are consistent across different catalysts. In particular, the Pt / CeOx / C catalyst with 3 nm Pt exhibits an overpotential of only 19 mV at 10 mA cm-2. This suggests that the HER catalysis benefits from the formation of the Pt-CeOxinterfaces.

[0074] As summarized in Figure 9 and Table 1 , the Pt / CeOx / C catalyst consistently exhibits lower Tafel slopes and a higher exchange current density ( / 0) compared to Pt / C with the same size of Pt. The lowest T afel slope value of 50 mV dec-1and the highest jo of 4.1 mA cm-2are obtained on Pt / CeOx / C with 3 nm Pt. Moreover, to assess the intrinsic activity of Pt, we obtained the electrochemical active surface area (ECSA) of Pt by analyzing the hydrogen underpotential deposition / stripping current area in the cyclic voltammetry (CVs) curves measured in a ^-saturated 0.1 M HCIO4, and the results are tabulated in Figure 10 and Table 2. It is worth noting that changing supports (CeOx / C and C) has negligible influence on Pt ECSA when the loaded Pt sizes are consistent. After normalizing the current density against ECSA, the Pt / CeOx / C catalysts deliver the higher HER current densities than Pt / C with an enhancement factor of 2 to 3 at different overpotentials (Figure 4C), further confirming the benefit of the Pt-CeOxinteraction. As highlighted in Figure 4D and Tables 1-2, the specific activity of Pt / CeOx / C (3 nm Pt) reaches 2.51 mA cnr2EcsA at an overpotential of 100 mV, higher than Pt / CeOx / C (5 nm Pt) (1 .94 mA cnr2EcsA), Pt / CeOx / C (7 nm Pt) (1 .62 mA cnr2ECsA), and all Pt / C catalysts.

[0075] To assess the stability of the catalysts, the chronopotentiometry (CP) method was employed at a constant current density of 100 mA cm2(Figure 1 1). In the absence of CeOx, the Pt / C (3 nm Pt) catalyst exhibits a rapid increase in overpotential within the first 2 hours, followed by a sustained increase with a high deactivation rate of 129.1 mV h1. In contrast, the Pt / CeOx / C (3 nm Pt) demonstrates an enhanced stability with a decay rate of 28.4 mV IT1. Clearly, in addition to the lower overpotential and the higher activity, the Pt / CeOx / C catalyst is better suited for prolonged electrochemical applications.

[0076] To provide more insight into electronic and atomic structures of our catalysts, in-situ X-ray absorption spectroscopy (XAS) measurements were carried out at -0.1 V vs. reversible hydrogen electrode (RHE) for both Pt / CeOx / C and Pt / C (3 nm Pt) samples. As shown in Figure 4E, the X-ray absorption near edge structure (XANES) results of both samples show broadened resonance peaks than the Pt foil, suggesting the small size of Pt NPs and thecontracted distance of Pt-Pt bonds.137 381Furthermore, the extended X-ray absorption fine structure (EXAFS) results (Figure 4F) reveals that the average coordination number (CN) of Pt-Pt bond for Pt / CeOx / C is 10.1 and 10.2 at -0.1 and -0.15 V vs. RHE, respectively, which are very close to the values for Pt / C under the same condition (10.5 and 10.9 at -0.1 and -0.15 V vs. RHE, respectively) (Table 3). The obtained CN values correspond to Pt NPs with a size of around 3 nm assuming a hemispherical shape, and this finding is consistent with the observation from TEM and STEM images. It is important to point out that Pt / CeOx / C and Pt / C exhibit similar oxidation states under in-situ XAS experiments, both of them being metallic Pt at -0.1 V (Figure 4E and 4F and Figures 12 and 13). This suggests that their different catalytic performances under the HER condition cannot be correlated to in-situ XAS result, probably due to the non-surface sensitive nature of the XAS technique.

[0077] To elucidate the Pt surface properties, CO adsorption diffuse reflection infrared Fourier-transform spectroscopy (DRIFTS) was used to characterize the surface electron densities of Pt / CeOx / C and Pt / C catalysts. The vibrational frequencies of adsorbed CO are sensitive to the back-donation involving d electrons of the metal and IT* orbitals of the CO ligand.139 40]Thus, the frequencies of the vco bands are good indicators of the surface electron densities on the metal sites in heterogeneous catalysts.139 41]The spectrum of the Pt / CeOx / C (3 nm Pt) in Figure 4G, after CO adsorption, shows a vCo band near 2061 cm-1that is assigned to linearly bound CO on Pt sites.[<|J In contrast, the spectrum of Pt / C (3 nm Pt) was characterized by a vCo band with a red-shifted vibrational frequency at 2054 cm-1, suggesting that the Ce2O3 / C supported Pt are more electron-deficient than C supported Pt.

[4243] Furthermore, the CeOx / C supported Pt nanoparticles show a broader CO adsorption band with a full width at half-maximum (fwhm) of 28 cm-1, higher than that on bare C supported Pt nanoparticles (fwhm = 24 cm-1) and a shoulder peak located at approximately 2085 cm-1as only observed on Pt / CeOx / C (3 nm Pt), which supports the presence of Pt-CeOxinterfacial structure. This interfacial phenomenon is further validated by the DFT calculations discussed in the following section.

[0078] DFT Calculations

[0079] Cubic Ce2Os has a corundum-like structure (Figure 14A) containing distorted CeOe octahedra that result in a large unit cell, Ce32C>48. Eight octahedra have six equivalent Ce-0 bonds with a bond length of 2.37 A, while the other 24 distorted octahedra have Ce-0 bond lengths ranging from 2.32 to 2.46 A. Each Ce3+has an unpaired spin with all 32 Ce coupled ferromagnetically (Figure 14B). Based on the experimental structure, we built a two-Ce-atom thick slab for the Ce203(100) surface (Figure 15A) to serve as the substrate for deposition of Pt. In this slab, most unpaired spins are found on surface oxygen, but the coupling betweenthe top and bottom surfaces reduces the net magnetization to a net spin of 6 for the whole system (Figure 15B).

[0080] Although our experiments involve Pt nanoparticles with a size from 3 nm to 7 nm, we used smaller 8-10 atom Pt clusters to keep the system at a practical size for DFT calculations. Watanabe et al. reported exclusive formation of 2-layer Ptncluster for n > 9 on TiO2(1 10), while the global minimum Ptio cluster in vacuum has a 3-layered triangular pyramidal structure.144-461Our calculations show that the 2-layer Pt cluster on Ce203(100) (Figure 16A) is 3 eV lower in energy compared to the pyramidal geometry (Figure 16B). For this Ptio cluster structure, we found that one O climbed up onto a 3-fold site on the Ptio cluster during the structure optimization. Since the oxygen is easily reduced to water at HER working conditions (Figure 17), we removed this O and further minimized the Ptio cluster. The final geometry is shown in Figure 5A. This structure does not exhibit any structural deformations during 10 ps of ab-initio molecular dynamics at 298 K (Figure 18). After binding the Ptio cluster to the Ce2C>3(100) surface, the charges redistribute, leading to an overall spin-singlet state. The Bader charge analysis1471(Figure 5B) predicts that the Ptio cluster lose 2.44 electrons to the substrate, mostly from the 1stlayer as shown in Figure 5D and 5E.

[0081] The HBE on the catalyst surface is considered as a reliable descriptor for the HER.126481For example, DFT calculations have suggested that Pt (11 1) binds hydrogen slightly stronger than the optimal value for the HER.148 491We explored all possible 1-fold on-top, 2- fold bridge, and 3-fold hollow sites for hydrogen adsorption on Ptio clusters (35 binding sites in total), as shown in Figure 19.

[0082] During the geometry minimization, H atoms spontaneously diffuse to nearby more stable sites, leading to 18 unique sites shown in Figure 5C. The color on the H atoms represents the relative H binding energy compared to that on the Pt (11 1) surface (hollow site). We find that the relative binding energies vary from -0.28 to 0.47 eV with on-top sites on the 1stlayer and the hollow sites between the 1stand 2ndlayer being the weakest.

[0083] Clearly, the direct contact with Ce2O3induces a substantial electron transfer across the interface, leading to the weakened HBE found in our DFT calculations. Such decreased hydrogen adsorption is likely the reason for the enhanced HER activity in Pt / CeOx / C observed in experiment. In addition, the Pt / CeOxinterface may also facilitate the water dissociation step in the HER in an alkaline electrolyte. Previous work has reported that the incorporation of an Ni(OH)2domain on a single-crystal Pt surface allowed an improved HER kinetics because these Ni(OH)2domains enable accelerated water dissociation.1501Given the well-established ability of water dissociation on ceria surface in thermal catalysis,151-541we postulated that the Pt / CeOx interface could ease the water dissociation step for the HER, particularly in alkalineworking conditions. Our DFT calculations indicate that the water dissociation energy on Pt / Ce2O3(-0.76 eV) is 1 eV lower than that on Pt (1 11) (Figure 20, where the value is +0.25 eV), thereby supporting this proposed mechanism.Methods

[0084] Chemicals and materials. Platinum(ll) acetylacetonate (98%) was purchased from Thermo Scientific Chemicals. Cerium(lll) acetate hydrate (Ce(Ac)3xH2O, 99.9%) was purchased from STREM Chemicals. 1 -octadecene (ODE, 90%) and benzyl ether (BE, 99%) were purchased from Acros Organics. Oleylamine (OLAM, 70%) and oleic acid (OAC, technical grade 90%) were purchased from Sigma Aldrich. Formaldehyde (37%), Isopropanol (IPA, Certified ACS Plus) and hexanes (Certified ACS) were purchased from Fisher Chemical. 20 wt% of commercial Pt / C, Vulcan carbon (XC-72R) and anion exchange membrane (AEM, FAS-PET-75) were purchased from Fuel Cell Store. All the materials were used without further purification. Electrolyte solutions were prepared using 18.2 MQ cm H2O (Millipore SAS).

[0085] Synthesis of Pt nanoparticles (NPs). Different sizes of Pt NPs were synthesized by modifying the previous reported literatures, which used the formaldehyde as a reducing agent.155561In a typical synthesis, Pt(acac)2(50 mg) was dissolved in 10 mL of ODE and 0.5 mL of OLAM, 8 mL of ODE and 2 mL of OLAM, and 2 mL of ODE and 7 mL of OLAM for preparing 3 nm, 5 nm, and 7 nm Pt NPs, respectively. The mixture was degassed at 90 °C for 1 h under vacuum. Then, 0.5 mL of formaldehyde solution was injected under N2protection. The solution was heated up at 3 °C min1and kept at 180 °C for 2 h before it was cooled down to room temperature. The final mixture was collected by centrifugation at 10,000 rpm after the addition of 45 mL of isopropanol. Then, the Pt NPs were purified twice by centrifugation in 0.5 mL of hexanes and 45 mL of isopropanol. The collected Pt NPs were re-dispersed and stored in hexanes for further use.

[0086] Synthesis of CeOxnanosheets (NSs). In a typical synthesis, cerium (III) acetate (76 mg) was added to the mixture of BE (10 mL), OLAM (10 mL) and OAC (0.27 mL), and the mixture was degassed at 150 °C for 1 h under N2environment. The solution was then refluxed at 290 °C for 4 h before it was cooled down to room temperature. The final mixture was collected by centrifugation at 8,000 rpm after the addition of 45 mL of acetone. Then, the CeOxNSs were purified twice by centrifugation in 5 mL of hexanes and 40 mL of acetone. The collected CeOxNSs were re-dispersed and stored in hexanes for further use.

[0087] Preparation of vertically aligned CeOxassemblies.^ Generally, 20 mg of the as- synthesized CeOxNSs were dispersed in 10 pL of hexanes, which was transferred on the surface of diethylene glycol (DEG) in a Teflon well. The well was then covered with a glass slide to allow the hexanes to evaporate slowly for 2 days. After the hexanes being evaporated,a vertically aligned CeOxNSs assembly was achieved on the surface of DEG, which can be transferred on TEM grid by placing it under the CeOxNSs followed by slowly lifting the TEM grid. The remaining DEG on the TEM grid was dried in vacuum oven for one day.

[0088] Preparation of CeOxNSs supported carbon. In a typical preparation, 50 mg of Vulcan carbon was dispersed in 30 mL of hexanes under sonication for 15 min, and 50 wt.% of the as-prepared CeOxNSs (dispersed in hexanes) were then added dropwise into the carbon black suspension and sonicated for 1 h. The CeOxNSs supported carbon (CeOx / C) was collected by centrifugation at 8,000 rpm and dried overnight. The CeOx / C was then heated at 200 °C for 12 h to remove the ligands on the surface of CeOxNSs, and the resulting CeOx / C powder was stored for further use. Similar strategy was used to form CeOx / AI2O3 using commercial Alumina powder.

[0089] Preparation of different sizes of Pt NPs over CeOxNSs supported carbon. In a typical preparation, 50 mg of CeOx / C powder was dispersed in 30 mL of hexanes under sonication for 15 min, and 20 wt.% of the different sizes of as-prepared Pt NPs (dispersed in hexanes) were then added dropwise into the CeOx / C suspension and sonicated for 1 h. The Pt / CeOx / C catalysts were collected by centrifugation at 8,000 rpm and dried overnight. Pt / CeOx / C catalysts were then heated at 200 °C for 12 h to remove the ligands on the surface of Pt NPs, and the remaining Pt / CeOx / C catalysts were stored for further use. Different sizes of Pt NPs loaded onto the pure Vulcan carbon followed the same aforementioned procedure.

[0090] Electrocatalytic HER measurements. The HER electrocatalytic studies were performed at room temperature with a three-electrode system in H2-saturated 1 .0 M KOH electrolyte. A Biologic Potentiostat (Model VMP3) was utilized, with a 5 mm glassy carbon RDE working electrode, a carbon rod counter electrode, and a Hg / HgO (4.0 M KOH) reference electrode. The electrocatalyst ink was prepared by sonicating the catalysts (concentration 5 mg mL1) in IPA and 10 wt.% of Nation solution. After the glassy carbon electrode was cleaned by polishing with alumina slurry, the working electrode was prepared by spin-coating the desired volume of electrocatalyst ink onto the rotating disk electrode (RDE, Pine), which equaled to 10 pg of Pt (confirmed by ICP-OES measurements). All potentials (E) were reported vs. RHE according to the following equation: 0.926 V (1)

[0091] Here, the potential difference between the Hg / HgO reference electrode and the hydrogen reference electrode in a 1 .0 M KOH solution was calibrated through the open circuit potential test prior to the electrochemical measurement, resulting in a value of 0.926 V.

[0092] The HER catalytic activity was evaluated using LSV at a scan rate of 5 mV s1from 0.0V to -0.2 V vs. RHE, with 85% / R compensation. The overpotential (q) for the HER was calculated with the following equation: r) = E (vs. RHE) - 0 V (2) where E was the potential obtained at a certain current density.

[0093] The stability of the catalyst was evaluated by chronopotentiometry (CP) test at 100 mA cm2(without / R compensation) in H-cell separated with AEM.

[0094] Catalyst characterization. TEM images were obtained on a FEI Tecnai Spirit (120 kV). Inductively coupled plasma optical emission spectrometry (ICP-OES) analyses were conducted on a PerkinElmer Avio-200 ICP optical emission spectrometer to determine the Pt metal loading on the supports. STEM images and EELS measurements were acquired with a Hitachi HD2700C dedicated STEM with a probe Cs corrector at an accelerating voltage of 200 kV. The EELS spectrums were acquired with a dispersion of 0.5 eV / channel and collected using a 5 mm aperture.

[0095] Electrochemical active surface area (ECSA) measurements. The ECSA of each CeOx- supported Pt NPs was determined by hydrogen-underpotential deposition (HUPD) method in solution of 0.1 M HCIO4.[58110 gPt cm2of each catalyst was deposited on the RDE, and the solution was bubbled with N2for 30 mins. The adsorption and stripping of H on Pt were carried out by sweeping the potential from 0.05 ~ 1.20 V (vs. RHE) at 50 mV s1(Figure 10). After integrating the H-stripping peak at 0.05 ~ 0.375 V, a charge consumed for desorption was obtained via dividing the integrated area by the scan rate. Then, the ECSA could be obtained using the specific charge of 0.21 mC cnr2.

[0059]

[0096] DRIFT Measurements. A Nicolet iS50 FTIR spectrometer (Thermo Scientific, USA) with a liquid-nitrogen-cooled HgCdTe (MCT) detector and a spectral resolution of 4 cm-1in the range 1000-4000 cm1for 128 scans was used to collect DRIFTS data characterizing powder samples. The spectrometer was equipped with a DiffusIR™ diffuse reflectance high temperature cell (Pike Technologies) fitted with a ZnSe window. The temperature was controlled by a compact temperature controller and a circulating water-cooling device. The DRIFTS cell was connected into a flow system and filled with approximately 50 mg of the mixture of catalyst and inert KBr powder. The cell was first heated in N2flowing at 30 mL (NTP) min1as the temperature was ramped from room temperature to 100 °C at a rate of 5 °C min-1followed by a dwell of 1 h at 100 °C. Then the cell was cooled to room temperature, and the background spectra were recorded at room temperature in the presence of flowing N2. CO adsorption was conducted by flowing 30 mL (NTP) min-1of CO for 30 min. The CO desorptionspectra were recorded during a N2purge at a flow rate of 30 mL (NTP) min1to remove gasphase CO from the cell for 1 h.

[0097] In-situ XAS. The HER was studied using in situ X-ray absorption spectroscopy (XAS) at beamline 7-BM (QAS, 1012ph s1@ 10 keV) of the National Synchrotron Light Source II (NSLS-II) at Brookhaven National Laboratory (BNL). The in situ experiments were conducted using a custom-made acryl H-type cell.1601The working electrodes (1 *1 cm2) were prepared with the fresh catalyst and 25 pL 5wt.% Nation, resulting in an areal density of 3 mg cm2. 1 M KOH (pH=14) was used as the electrolyte. A reference electrode of Hg / HgO and a Pt wire as the counter electrode were employed. All potentials were converted to the scale of a reversible hydrogen electrode (RHE) using the equation E (vs. RHE) = E (vs. Hg / HgO) + 0.0592 x pH + 0.0983. In prior to the reaction, the catalysts were pretreated by scanning between 0.0 and - 0.1 V vs. RHE at a scan speed of 50 mV s1for 20 cycles. Subsequently, linear sweep voltammetry (LSV) was performed, scanning the cathode from 0.0 to -0.15 V vs. RHE at a rate of 5 mV s1. The HER experiments involved potentials of open circuit potential (OCP), -0.1 , and -0.15 V vs. RHE, which were controlled by a BioLogic potentiostat. The Pt L3 or L2-edge XAS spectra were continuously collected at a fluorescence mode using a passivated implanted planar silicon (PIPS) detector. The acquisition time was adjusted between 6-30 seconds per spectrum to reduce the signal interference caused by bubble formation. Pt foil was used for energy shift calibration and obtaining the passive factor (So2) for extended X-ray absorption fine structure (EXAFS) fittings. It also served as a reference standard for comparing X-ray absorption near-edge structure (XANES) of different catalysts. All data processing was performed using the IFEFFIT package.1611The average size of Pt NPs was estimated based on the coordination number of the Pt-Pt bond, following the method described in Refs.138’621

[0098] DFT calculations. The spin-unrestricted DFT calculations were performed using the Vienna ab initio simulation package (VASP ver. 5.4.5) .163 641Electron exchange and correlation were treated within the generalized gradient approximation (GGA)1651using the PBE functional plus D3(BJ) van der Waals attraction.1661The interaction between the ionic core and the valence electrons was described by the projector-augmented wave (PAW) method.1671The plane-wave basis set was truncated with an energy cutoff of 500 eV. The Brillouin zone was sampled using the 2x2x1 Monkhorst-Pack grid1681for all slab calculations. The energy minimization used convergence criteria for the electronic structure and the atomic geometry as 10~6eV and 0.01 eV A1, respectively. The conventional unit cell of Ce2O3bulk structure (Ia3) was employed with a lattice parameter of 1 1.036 A. The catalytic Ce2O3(100) surface is constructed with a 5.26 A of thickness (two layers) with at least 15 A of vacuum. The whole system was allowed to relax during minimization.Table 1. Measured HER parameters from the catalytic tests.r)@10 mA cm’2-Tafel Slope JoCatalysts (mV) (mV dec1) (mA cm’2)Pt / CeOx / C (3 nm) 19 50 4.1Pt / CeOx / C (5 nm) 42 81 3.1Pt / CeOx / C (7 nm) 65 87 1.9Pt / C (3 nm) 29 74 3.9Pt / C (5 nm) 70 119 2.6Pt / C (7 nm) 100 139 1.9Table 2. Measured ESCA parameters from the HUPD method and the normalized j.CatalystsPt / CeOx / C (3 nm) 8.3 83.0 2.51 4.78Pt / CeOx / C (5 nm) 4.1 40.5 1.94 4.16Pt / CeOx / C (7 nm) 3.0 30.2 1.62 3.92Pt / C (3 nm) 8.1 81.4 1.25 2.26Pt / C (5 nm) 4.2 41.5 0.87 1.77Pt / C (7 nm) 2.9 29.3 0.62 1.36Table 3. Fitting results of the in-situ Pt L3-edge FT-EXAFS spectra for the Pt / CeOx / C and Pt / C catalysts under various conditions.Bond „ EQ shiftConditions Shell CNoo2(A2) R factor length (A) (eV)Pt / CeOx / C -0.1V Pt-Pt 10.1 ±0.2 2.75±0.00 0.0047 6.84 vs RHEPt / CeOx / C -0.15V Pt-Pt 10.2±0.1 2.76±0.00 0.0047 6.90 0.04 vs RHEPt / C -0.1V vs Pt-Pt 10.5±0.3 2.76±0.00 0.0047 6.87RHEPt / C_-0.15V vsRHE Pt-Pt 10.9±0.7 2.75±0.00 0.0050 6.53Notes: CN— average coordination number (normalized to all the absorbers) around the absorbing center atom; o2— mean square variation in path length; R-factor— quality of fitting.a: Ak=3.0-11.0 -1, AR=1.0-3.3 A, k1k2k3-weighted EXAFS fitting.

[0099] Figure 22 illustrates a TEM image of 2-3 nm Pd NPs loaded onto CeOx NS supported carbon.

[0100] Figure 23 illustrates a TEM image of 14 nm Fe NPs loaded onto CeOx NS supported carbonConclusion

[0101] In summary, this study concentrated on examining the distinctive electronic structure of Pt NPs attached to ultra-thin Ce2O3NSs and their impact on the performance of the HER. Our DFT calculations revealed a discernible electron transfer from Pt to the Ce2O3substrates. Consequently, specific on-top and hollow Pt sites, in comparison to the unmodified Pt counterparts, displayed a desirable weakening of HBE, contributing to the improved HER activity. Additionally, the Pt-Ce2O3interface was found to facilitate the water dissociation step, further enhancing the HER kinetics. Experimentally, the Pt / CeOx / C catalyst exhibited enhanced HER activity compared to the Pt / C catalyst, consistent with DFT calculations. This study underscores the potential of manipulating metal-metal oxide interfaces for electrocatalytic reactions. Furthermore, we anticipate the applicability of this strategy to other domains. For instance, our CeOxnanosheets were applied as a coating on a commercial AI2O3powder support, forming a core / shell structure of AI2O3materials with a thin layer of CeOxon the surface (Figure 21). This configuration can support metal NPs for thermal catalysis and extends the versatility of the approach beyond electrocatalysis.

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[0103] It should be emphasized that the above-described embodiments of the present disclosure are merely possible examples of implementations set forth for a clear understanding of the principles of the disclosure. Many variations and modifications may be made to the above-described aspects. All such modifications and variations are intended to be included herein within the scope of this disclosure and protected by the following claims.

Claims

CLAIMSWhat is claimed:

1. A composite structure, comprising: a support; at least one layer of CeOxcoated onto a surface of the support; and a plurality of metallic nanoparticles, a plurality of metal atoms, or a combination thereof deposited onto the CeOxlayer.

2. The composite structure of claim 1 , wherein the layer of CeOxcomprises a plurality of CeOxnanosheets.

3. The composite structure of claim 2, wherein the CeOxnanosheets have a width of about 10 nm to about 150 nm.

4. The composite structure of claim 2 or 3, whereon the CeOxnanosheets have a length of about 10 nm to about 150 nm.

5. The composite structure of any one of claims 1-4, whereon the CeOxnanosheets are about 1 nm to about 2 nm thick.

6. The composite structure of claim 1 , wherein the layer of CeOxis about 1 nm to about 2 nm thick.

7. The composite structure of any one of claims 1-6, wherein metal atoms are deposited onto the CeOxlayer.

8. The composite structure of any one of claims 1-6, wherein metallic nanoparticles are deposited onto the CeOxlayer.

9. The composite structure of claim 8, wherein the metallic nanoparticles comprise iron nanoparticles, cobalt nanoparticles, nickel nanoparticles, copper nanoparticles, ruthenium nanoparticles, rhodium nanoparticles, silver nanoparticles, platinum nanoparticles, gold nanoparticles, palladium nanoparticles, or a combination thereof.

10. The composite structure of claim 8, wherein the metallic nanoparticles are platinum nanoparticles.11 . The composite structure of any one of claims 8-10, wherein the metallic nanoparticleshave diameters of about 1 nm to about 100 nm.

12. The composite structure of any one of claims 8-10, wherein the metallic nanoparticles have diameters of about 1 nm to about 10 nm.

13. The composite structure of any one of claims 8-12, wherein the metallic nanoparticles are deposited onto the CeOxlayer at a weight percentage of about 1% to about 70% metallic nanoparticle to CeOxlayer.

14. The composite structure of any one of claims 8-12, wherein the metallic nanoparticles are deposited onto the CeOxlayer at a weight percentage of about 2% to about 10% metallic nanoparticle to CeOxlayer.

15. The composite structure of any one of claims 8-12, wherein the metallic nanoparticles comprise spheres, rods, cubes, stars, triangles, hexagons, prisms, needles, clusters, or a combination thereof.

16. The composite structure of any one of claims 1-15, wherein the support comprises carbon materials, non-carbon materials or a combination thereof.

17. The composite structure of claim 16, wherein the carbon materials comprise glassy carbon, graphite, carbon nanotubes, graphene sheets, or boron-doped diamond.

18. The composite structure of claim 16, wherein the non-carbon materials comprise silica, metal oxides, carbides, or nitrides.

19. A system, comprising: a first electrode; a second electrode, in electrical communication with the first electrode; and at least one electrolyte in contact with at least one of the first electrode or second electrode; wherein the first electrode comprises at least one layer of CeOxcoated onto a surface of the first electrode and a plurality of metallic nanoparticles, a plurality of metal atoms, or a combination thereof deposited onto the CeOxlayer.

20. The system of claim 19, wherein the layer of CeOxcomprises a plurality of CeOxnanosheets.

21. The system of claim 19 or 20, wherein the first electrode is a carbon electrode.

22. The system of any one of claims 19-21 , wherein the layer of CeOxis from about 1 nm to about 2 nm thick.

23. The system of any one of claims 19-22, wherein metal atoms are deposited onto the CeOxlayer.

24. The system of any one of claims 19-22, wherein metallic nanoparticles are deposited onto the CeOxlayer.

25. The system of claim 24, wherein the metallic nanoparticles comprise iron nanoparticles, cobalt nanoparticles, nickel nanoparticles, copper nanoparticles, ruthenium nanoparticles, rhodium nanoparticles, silver nanoparticles, platinum nanoparticles, gold nanoparticles, palladium nanoparticles, or a combination thereof.

26. The system of claim 24, wherein the metallic nanoparticles are platinum nanoparticles.

27. The system of any one of claims 24-26, wherein the metallic nanoparticles have diameters of about 1 nm to about 100 nm.

28. The system of any one of claims 24-26, wherein the metallic nanoparticles have diameters of about 1 nm to about 10 nm.

29. The system of any one of claims 24-28, wherein the metallic nanoparticles are deposited onto the CeOxlayer at a weight percentage of from about 1% to about 70% metallic nanoparticle to CeOxlayer.

30. The system of any one of claims 24-28, wherein the metallic nanoparticles are deposited onto the CeOxlayer at a weight percentage of about 2% to about 10% metallic nanoparticle to CeOxlayer.31 . The system of any one of claims 24-30, wherein the metallic nanoparticles comprise spheres, rods, cubes, stars, triangles, hexagons, prisms, needles, clusters, or a combination thereof.

32. The system of any one of claims 19-31 , wherein the second electrode is a carbon electrode.

33. The system of any one of claims 19-32, wherein the electrolyte is a solid electrolyte.

34. The system of claim 33, wherein the solid electrolyte is a solid alkaline exchange membrane.

35. The system of any one of claims 19-34, wherein the electrolyte is in contact with both the first electrode and the second electrode.

36. The system of any one of claims 19-35, wherein the metallic nanoparticles have a greater specific activity than an equivalent system that does not comprise a layer of CeOx.

37. The system of any one of claims 19-36, wherein the first electrode has a smaller decay rate than an electrode in an equivalent system that does not comprise a layer of CeOx.

38. A method for producing a catalyst, comprising: coating a support with CeOxnanosheets to form a CeOxlayer on the support; and depositing a plurality of metallic nanoparticles, a plurality of metal atoms, or a combination thereof onto the CeOxlayer.

39. The method of claim 38, further comprising annealing the CeOxnanosheet-coated support prior to depositing the metallic nanoparticles, the metal atoms, or the combination thereof.

40. The method of claim 39 wherein the CeOxnanosheet-coated support is annealed at about 150 °C to about 250 °C in air.

41. The method of any one of claims 38-40, wherein the support comprises carbon materials, non-carbon materials or a combination thereof.

42. The method of claim 41 , wherein the carbon materials comprise glassy carbon, graphite, carbon nanotubes, graphene sheets, or boron-doped diamond.

43. The method of claim 41 , wherein the non-carbon materials comprise silica, metal oxides, carbides, or nitrides.

44. The method of any one of claims 38-43, wherein metal atoms are deposited onto the CeOxlayer.

45. The method of any one of claims 38-43, wherein metallic nanoparticles are depositedonto the CeOxlayer.

46. The method of claim 45, wherein the metallic nanoparticles comprise iron nanoparticles, cobalt nanoparticles, nickel nanoparticles, copper nanoparticles, ruthenium nanoparticles, rhodium nanoparticles, silver nanoparticles, platinum nanoparticles, gold nanoparticles, palladium nanoparticles, or a combination thereof.

47. The method of claim 45, wherein the metallic nanoparticles comprise platinum nanoparticles.

48. The method of any one of claims 45-47, wherein the metallic nanoparticles have diameters of about 1 nm to about 100 nm.

49. The method of any one of claims 45-47, wherein the metallic nanoparticles have diameters of about 1 nm to about 10 nm.

50. The method of any one of claims 45-49, wherein the metallic nanoparticles are deposited onto the CeOxlayer at a weight percentage of about 1% to about 70% metallic nanoparticle to CeOxlayer.

51. The method of any one of claims 45-49, wherein the metallic nanoparticles are deposited onto the CeOxlayer at a weight percentage of about 2% to about 10% metallic nanoparticle to CeOxlayer.

52. The method of any one of claims 45-51 , wherein the metallic nanoparticles comprise spheres, rods, cubes, stars, triangles, hexagons, prisms, needles, clusters, or a combination thereof.

53. The method of any one of claims 38-52, wherein the CeOxlayer is about 1 nm to about 2 nm thick.

54. The method of any one of claims 38-53, wherein the CeOxnanosheets have a width of about 10 nm to about 150 nm.

55. The method of any one of claims 38-53, whereon the CeOxnanosheets have a length of about 10 nm to about 150 nm.

56. The method of any one of claims 38-55, whereon the CeOxnanosheets are about 1 nm to about 2 nm thick.

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