Catalyst and preparation method thereof

Abstract

Provided herein is a catalyst comprising metal nanoparticles as an active phase disposed on a solid support, and a porous layer comprising amorphous hafnium oxide disposed on the metal nanoparticles. The invention also provides a method for preparing the catalyst and application of the catalyst in an electrode, a proton exchange membrane water electrolyser or a fuel cell. Figure 1 is specified to be disclosed.

Description

Cross - Reference to Related Applications

[0001] This application claims priority to Singapore Patent Application No. 10202260211T, filed on November 25, 2022, the entire content of which is incorporated herein by reference for all purposes. Technical Field

[0002] Various embodiments relate to catalysts and methods for preparing catalysts. Background Art

[0003] Industrial - scale uphill reactions driven by electrocatalysis can produce renewable clean energy, and are expected to become disruptive technologies and change the current global energy market pattern dominated by fossil fuels.

[0004] In particular, hydrogen gas (H2) is widely regarded as a future sustainable fuel because it only produces water at the end of emissions and does not produce greenhouse gases. Due to the increasing energy shortage and extreme climate, the world is accelerating the development of a hydrogen - fuel economy, including building hydrogen - fuel infrastructure, developing hydrogen - powered household appliances, advancing fuel - cell technology, and powering transportation facilities.

[0005] However, the feasibility of the hydrogen - energy economy highly depends on the maturity of large - scale and low - cost hydrogen - production technologies. Currently, approximately 95% of this clean - burning gas is grey or blue hydrogen, produced by steam methane reforming, which requires a large amount of energy from fossil fuels. Only 5% of hydrogen, so - called green hydrogen, achieves zero carbon emissions through water - electrolyzer technology. Unfortunately, the current cost of green hydrogen is more than twice that of grey or blue hydrogen ($5 / kg vs. $1 - 2 / kg), and is higher than that of fossil fuels (diesel, $0.82 / kg).

[0006] The cost of water - electrolyzer technology is mainly determined by the catalyst - coated membrane (40% - 50%), in which platinum metal accounts for the main body of the capital investment. To meet the terawatt - scale clean - energy demand, it is urgent to rationally design noble - metal catalysts to maintain high catalytic activity with the least amount of noble - metal loading. However, the durability of low - loading catalysts is a key factor for their successful industrial application.

[0007] To achieve commercial - scale low - cost renewable hydrogen energy, the academic community has devoted a great deal of effort to developing highly efficient, low - cost, and highly durable hydrogen - evolution electrocatalysts. So far, platinum (Pt) - based catalysts have been regarded as benchmarks for the hydrogen - evolution reaction (HER) due to their optimized hydrogen adsorption energy. Unfortunately, due to the large demand for platinum but scarce reserves, practical price barriers need to be overcome.

[0008] To meet the global energy demand (the total global energy supply reached 18.83 terawatts in 2018), it is necessary to scale up the energy stored in H2 to the terawatt level. Even calculated at a Pt loading of 0.25 mg cm -2 For terawatt-scale hydrogen production, approximately 100 tons of Pt (60% of the annual production) would be required. Therefore, it is necessary to reduce the tonnage requirements of Pt catalysts and increase the intrinsic activity, i.e., the mass activity, rather than the geometric activity, of Pt-based catalysts.

[0009] Extensive related research has developed a variety of highly efficient Pt-based catalysts with high Pt utilization, including needle-like nanostructures, rods, cages, frameworks, cavities, supported monolayer (ML) films, binary (ternary) alloys, core-shell nanoparticles, and even supported single Pt atoms / nanoparticles / (sub)monolayer catalysts. For example, the Pt surface layer of an alloy is 10 times more active than that of a single-metal Pt electrode. However, for the large-scale production of green hydrogen, the Pt loading needs to be further reduced from 0.5 to 1.0 mg / cm² to the microgram or even nanogram level while maintaining the geometric activity.

[0010] Driven by the need to reduce costs while maintaining performance, researchers have made extensive efforts to construct low-metal-loading catalysts, including nanoalloys, shape-controlled nanostructures, supported metal skins, and supported single atoms. However, the nano-catalysts developed to date are not ideal because they are prone to rapid degradation - including Ostwald ripening, leaching, migration, poisoning, etc. - which severely restricts the large-scale application of clean energy technologies but has received little research attention. In particular, when the size of nano-materials is further reduced, the instability problem is exacerbated due to the significantly increased surface energy and decreased cohesive energy. Therefore, such catalysts fall far short of meeting the actual lifetime targets (e.g., 5000 to 8000 hours for light-duty vehicles). As a typical example, the performance decay rate of 3nm-Pt particles is 30 times and 500 times higher than that of 5nm particles and well-defined single-crystalline materials, respectively.

[0011] Therefore, the lifetime issue of Pt-based catalysts remains a concern. Even if the tonnage requirements of Pt-based electrocatalysts can be significantly reduced, the time between catalyst replacements must be managed to ensure their stable operation. Acidic electrolysis is carried out under harsh conditions such as corrosive solutions and high temperatures (80°C). In addition, catalyst atoms are constantly rearranged at the electrolyte-catalyst interface during operation, triggering inevitable migration, aggregation (Ostwald ripening), and dissolution. According to the Gibbs-Thompson theory, this structural transformation is particularly true for supported low-dimensional Pt-based catalyst systems because of their much higher surface energy. Moreover, the migration of Pt nanoparticles is promoted by the mechanical stress generated by the formed H2 bubbles, followed by coalescence and agglomeration, resulting in a loss of the available electrochemically active surface area (ECSA).

[0012] To address the instability challenges, load engineering methods have been developed, such as doping (e.g., S-doped carbon-supported metal nanoparticles), chemical modification (e.g., aniline-stacked graphene), and crystal edge stabilization (e.g., Pt nanoparticles on the MgO crystal edge), to enhance the metal-support interaction, thereby improving the electrochemical stability or thermal durability.

[0013] To solve the stability problems of low-dimensional Pt-based catalyst systems, a large number of interface engineering strategies have emerged, aiming to increase the physical / chemical barriers to deactivation. Interface engineering can effectively adjust the binding strength of the Pt support, further reducing the migration / coalescence of Pt species. In addition, during electrochemical measurements, proton-conducting ionomer confinement methods have been applied, either intentionally or unintentionally. Although ionomers can protect Pt species as binders, they also tend to block active sites, affect local electronic conductivity, and hinder H2-related mass transport.

[0014] Unfortunately, the above strategies are insufficient to prevent the ECSA loss caused by metal electromigration and electrochemically induced atomic migration during long-term stability tests. To evaluate the electrochemical stability, materials with a high loading were tested, but the stability of such catalysts could not be determined because as long as the material is not completely corroded, a thicker film can still exhibit activity even if it is severely degraded.

[0015] In view of the above, there is a need for an improved catalyst and a method for preparing the catalyst to solve or at least alleviate one or more of the above problems. Summary of the Invention

[0016] In a first aspect, various embodiments relate to a catalyst comprising metal nanoparticles as an active phase disposed on a solid support, and a porous layer comprising amorphous hafnium oxide disposed on the metal nanoparticles.

[0017] In a second aspect, various embodiments relate to a method for preparing a catalyst, the method comprising forming metal nanoparticles as an active phase on a solid support, and disposing a porous layer comprising amorphous hafnium oxide on the metal nanoparticles.

[0018] In a third aspect, various embodiments relate to the use of the catalyst according to the first aspect or the catalyst prepared by the method according to the second aspect in an electrode, a proton exchange membrane water electrolyzer, or a fuel cell. Brief Description of the Drawings

[0019] In the accompanying drawings, like reference numerals generally represent like parts in different views. The drawings are not necessarily drawn to scale, but generally focus on illustrating the principles of the various embodiments. In the following description, various embodiments of the present invention will be described with reference to the accompanying drawings.

[0020] Figure 1 is a schematic diagram showing the method 100 for preparing a catalyst disclosed herein. Electron beam deposition 150 can be used to deposit single atoms or nanoclusters on a solid support 101. The single atoms or nanoclusters can coalesce to form metal nanoparticles 103 and exist as the active phase on the solid support 101. While forming the metal nanoparticles 103, and / or after forming the metal nanoparticles 103, atomic layer deposition 152 can be performed on the metal nanoparticles 103. Oxygen precursor 105 (such as H2O) and hafnium precursor 107 (such as [(CH3)2N]4Hf) can be introduced in an alternating sequence for a plurality of cycles. The oxygen precursor and the hafnium precursor can react or transform in the next step 154 to form a porous layer 109 containing amorphous hafnium oxide on the metal nanoparticles 103.

[0021] Figure 2 is a representative atomic resolution high-angle annular dark-field scanning transmission electron microscope (HAADF-STEM) image of as-deposited Pt single atoms / nanoclusters (denoted as Pt 0.1nm ) with an evaporation time of 10 seconds.

[0022] Figure 3A shows an atomic resolution STEM image of monodisperse Pt single atoms / nanoclusters on graphene, presenting the distribution of Pt species evaporated onto the graphene substrate, with the deposition rate increasing with deposition time to form a Pt thickness of 0.005 nm.

[0023] Figure 3B shows an atomic resolution STEM image of monodisperse Pt single atoms / nanoclusters on graphene, presenting the distribution of Pt species evaporated onto the graphene substrate, with the deposition rate increasing with deposition time to form a Pt thickness of 0.01 nm.

[0024] Figure 3C shows an atomic resolution STEM image of monodisperse Pt single atoms / nanoclusters on graphene, presenting the distribution of Pt species evaporated onto the graphene substrate, with the deposition rate increasing with deposition time to form a Pt thickness of 0.02 nm.

[0025] Figure 3DShows the atomic-resolution STEM image of monodispersed Pt single atoms / nanoclusters on graphene, presenting the distribution of Pt substances evaporated onto the graphene substrate, with The deposition rate increases with the deposition time, forming a Pt thickness of 0.1 nm.

[0026] Figure 3E Shows the relationship diagram between the Pt mass loading and the deposition thickness of monodispersed Pt single atoms / nanoclusters on graphene.

[0027] Figure 4A Is the atomic-resolution HAADF-STEM image of m-HfO2@Pt on graphene 0.1nm with the thickness of m-HfO2 being 3 nm.

[0028] Figure 4B Is the atomic-resolution HAADF-STEM image of m-HfO2@Pt on graphene 0.1nm with the thickness of m-HfO2 being 10 nm.

[0029] Figure 5 Is the energy-dispersive X-ray (EDX) elemental mapping of 3 nm m-HfO2@Pt on graphene 0.1nm showing the elemental distributions of Pt, O, Hf, and C.

[0030] Figure 6A Shows the typical Raman spectra of 10 nm m-HfO2@Pt on graphene, including the Raman spectrum of Pt 0.1nm evaporated on graphene 0.1nm (620), and the Raman spectrum of 10 nm m-HfO2@Pt on graphene 0.1nm (610).

[0031] Figure 6B Is for Figure 6A the extracted I D / I G and I 2D / I G ratios in a table, corresponding to Pt 0.1nm evaporated on graphene (G-Pt) and 10 nm m-HfO2@Pt on graphene 0.1nm (G-Pt-HfO2) respectively.

[0032] Figure 7 Is the Pt 4f X-ray photoelectron spectroscopy (XPS) spectrum of graphene-supported 10 nm m-HfO2@Pt 0.1nm .

[0033] Figure 8 Is 10 nm m-HfO2@Pt on graphene0.1nm Cross-sectional STEM image showing that the 10-nm-thick m-HfO2 layer completely covers the 2-3-nm Pt nanoparticles. The scale bar is 5 nm.

[0034] Figure 9A Shows 2-nm m-HfO2@Pt 0.1nm Typical O 1s XPS spectrum.

[0035] Figure 9B Shows 2-nm m-HfO2@Pt 0.1nm Typical Hf 4f XPS spectrum.

[0036] Figure 10 Charge density distribution for the m-HfO2@Pt nanoparticle model. In the example, Pt(1003) is shown together with O(1005) and Hf(1007), and the charge accumulation and depletion regions are distinguished by (1010) and (1020) respectively (contour line: 0.001).

[0037] Figure 11 For graphene-supported m-HfO2@Pt according to the example 0.1nm Schematic diagram of the hydrogen evolution process. The m-HfO2 porous layer is located on the Pt nanoparticles as a covering layer. During use, hydrogen bubbles may be generated on the surface of m-HfO2 rather than on the surface of Pt nanoparticles.

[0038] Figure 12A Shows m-HfO2@Pt 0.1nm Typical polarization curves, where the m-HfO2 thicknesses are 5 nm (1210), 10 nm (1220), and 20 nm (1230) respectively.

[0039] Figure 12B Shows m-HfO2@Pt 0.1nm Typical Tafel plots, where the m-HfO2 thicknesses are 5 nm (1210) and 10 nm (1220) respectively.

[0040] Figure 13A Shows on-chip microfluidic cell device or microelectrochemical device for hydrogen evolution reaction (HER) testing with 10-nm m-HfO2@Pt on graphene 0.1nm as the working electrode (WE). A carbon rod and a leak-free Ag / AgCl are used as the counter electrode (CE) and reference electrode (RE) respectively. Fresh electrolyte is continuously flowed into the microcell using an injection pump.

[0041] Figure 13B Is a photograph of a conventional three-electrode device for hydrogen evolution reaction (HER) testing of 10-nm m-HfO2@Pt on carbon paper 1nm ​

[0042] Figure 14A For 10nm-HfO2@Pt on graphene in 0.5M H2SO4 0.1nm LSV curves of the hydrogen evolution reaction (HER) test from the 1st to the 701st cycle for, measured at 1 cycle, 101 cycles, 201 cycles, 301 cycles, 401 cycles, 501 cycles, 601 cycles and 701 cycles.

[0043] Figure 14B Shows the relationship between overpotential and Tafel slope at 10 mA cm -2 Below.

[0044] Figure 15A Shows the relationship between overpotential and mass loading for the materials disclosed herein and the relationship between overpotential and mass loading for various other Pt-based hydrogen evolution reaction (HER) catalysts.

[0045] Figure 15B Shows the mass activity of the materials disclosed herein and the mass activity of other Pt-based catalysts in 0.5M H2SO4.

[0046] Figure 16 Schematic diagram of a four-electrode electrochemical impedance spectroscopy (EIS) cell, where two platinum foils are the working electrode and the counter electrode respectively, and two platinum wires are the reference electrodes. L, W, and T represent the distance between the reference electrodes, the width, and the thickness of m-HfO2@Pt deposited on the graphene film, respectively.

[0047] Figure 17 Shows the manufacturing process of the device disclosed herein. The typical fabrication of an on-chip device may involve the following five main steps. Step (1), pre-pattern 5nm thick titanium (Ti) / 50nm thick gold (Au) contact pads on a silica / silicon (SiO2 / Si) substrate or a transparent glass substrate using conventional photolithography and electron beam evaporation techniques. Step (2), transfer a monolayer graphene film grown by chemical vapor deposition (CVD) onto the on-chip device by a standard polymethyl methacrylate (PMMA)-assisted method (1701). Step (3), to disconnect adjacent electrodes, use electron beam lithography (EBL) (1702) and O2 plasma (20W, 1 minute) (1703) to remove selected graphene channels. Step (4), well-controlled m-HfO2@Pt is synthesized on graphene by the above method. Step (5), expose the reaction window to a selected area in a 1μm thick PMMA protective film by electron beam lithography (EBL) (1704), ensuring that the reaction occurs only within the target area.

[0048] Figure 18AThe Nyquist plot of the electrochemical impedance spectroscopy (EIS) of the 10 nm m-HfO2 film is shown for studying the HER activity and proton conductivity of graphene-supported m-HfO2@Pt through an on-chip micro-battery system. 0.1nm

[0049] Figure 18B is shown Figure 18A of the equivalent circuit model.

[0050] Figure 19 is shown for the in-situ observation of electrochemically generated bubbles on the surface of 10 nm m-HfO2@Pt 0.5nm Scale bar is 1 μm.

[0051] Figure 20 A photograph of a typical H-cell device for studying amplification and generality is shown, where the anode (carbon rod) and cathode (catalyst) are separated by a Nafion membrane.

[0052] Figure 21A is shown for 10 nm m-HfO2@Pt on carbon paper 0.5nm and Pt 0.5nm (2120) linear sweep voltammetry (LSV) curve.

[0053] Figure 21B is shown Figure 21A for 10 nm m-HfO2@Pt on carbon paper in 0.5nm the Tafel slope.

[0054] Figure 22A is shown for the chronopotentiometric response of 10 nm m-HfO2@Pt on carbon paper in an H-cell at constant current densities of 10 mA cm -2 20 mA cm -2 and 40 mA cm -2 including the curve of Pt 0.5nm at 10 mA cm 0.5nm the curve (2210), 10 nm HfO2@Pt -2 at 10 mA cm 0.5nm the curve (2220), 10 nm HfO2@Pt 2 at 10 mA cm 0.5nm the curve (2230) at 20 mA cm -2 and the curve of 10 nm HfO2@Pt 0.5nm at 40 mA cm -2 (2240).

[0055] Figure 22B is shown for the chronopotentiometric response of 10 nm m-HfO2@Pt on carbon paper at constant current densities of 10 mA cm -2 20 mA cm​-2 and 40 mA cm -2 chronopotentiometry curves of 10 nm m-HfO2@Pt on carbon paper at 1nm .

[0056] Figure 23 shows the real-time thermal stability study of 3 nm m-HfO2@Pt on graphene at 0 min, 10 min, 20 min, 40 min, 50 min, and 60 min. The in-situ imaging of high temperature on the 0.1nm structural stability of m-HfO2@Pt was studied by annular dark field scanning transmission electron microscopy (ADF-STEM). Images were recorded at 500 °C. 0.1nm .

[0057] Figure 24 shows the HAADF-STEM real-time thermal stability study of 3 nm m-HfO2@Pt at initial, 10 min, 20 min, 30 min, 40 min, and 50 min. The in-situ imaging of high temperature on the 0.1nm structural stability of m-HfO2@Pt was studied. Images were recorded at 700 °C. 0.1nm .

[0058] Figure 25 shows an example of an atomic resolution HAADF-STEM image of 3 nm HfO2@Pt on graphene, showing uniform distribution of Pt nanoparticles. The scale bar is 50 nm. 0.1nm .

[0059] Figure 26A shows an atomic resolution HAADF-STEM image of 3 nm m-HfO2@Co on graphene according to an embodiment. The scale bar is 100 nm. 0.1nm .

[0060] Figure 26B shows an atomic resolution HAADF-STEM image of 3 nm m-HfO2@Co on graphene according to an embodiment. The scale bar is 100 nm. 0.1nm .

[0061] Figure 26C shows an atomic resolution HAADF-STEM image of 10 nm m-HfO2@Co on graphene according to an embodiment. The scale bar is 10 nm. 0.1nm .

[0062] Figure 26D shows an atomic resolution HAADF-STEM image of 10 nm m-HfO2@Co on graphene according to an embodiment. The scale bar is 10 nm. 0.1nm .

[0063] Figure 27A shows m-HfO2@Co 0.2nm 's typical polarization curve, where the thickness of m-HfO2 is 5nm HfO2@Co 0.2nm (2710), 10nm HfO2@Co 0.2nm (2720) and 20nm HfO2@Co 0.2nm (2730), demonstrating the OH of m-HfO2 - ionic conductivity.

[0064] Figure 27B shows m-HfO2@Co 0.2nm 's Tafel plot as a function of the thickness of m-HfO2, where the thickness of m-HfO2 is 5nm HfO2@Co 0.2nm (2710), 10nm HfO2@Co 0.2nm (2720) and 20nm HfO2@Co 0.2nm (2730).

[0065] Figure 28 shows Co on carbon paper 0.5nm (2810) and 10nm HfO2@Co on carbon paper 0.5nm (2820) in 1M KOH, at 10mA cm -2 during the oxygen evolution process. Detailed Description of the Invention

[0066] The following detailed description refers to the accompanying drawings, which illustrate, by way of example, specific details and embodiments in which the present invention may be practiced. These embodiments have been described in sufficient detail to enable those skilled in the art to practice the present invention. Other embodiments may be utilized and structural, logical, and electrical changes may be made without departing from the scope of the present invention. The various embodiments are not necessarily mutually exclusive, as some embodiments may be combined with one or more other embodiments to form new embodiments.

[0067] The various embodiments disclosed herein relate to a scalable and precise method for synthesizing ultra-low-loading monodisperse Pt nanoparticles (NPs) on a large-scale substrate, e.g., in the range of tens to hundreds of nanograms per square centimeter, for industrial-scale clean hydrogen production. The method disclosed herein is capable of eliminating structural transformation and extending the lifespan of the loaded Pt NPs.

[0068] This paper discloses a simple and general protection design that uses a rigid porous non-reducing oxide - amorphous HfO2 (m-HfO2) as a capping layer to extend the lifespan of ultra-low-loading electrocatalysts. As a proof of concept, m-HfO2 layers were synthesized to cover sub-monolayer Pt or Co nanoparticles on graphene (denoted as m-HfO2@Pt or m-HfO2@Co). By using amorphous HfO2 (m-HfO2) as the capping layer, spatial confinement is provided for the supported Pt NPs, thereby preventing the electromigration and electrochemical migration of active Pt atoms, and preventing or even eliminating the structural transformation of Pt atoms, thus significantly extending the electrochemical durability.

[0069] In the experiments conducted, a Pt / C catalyst with a Pt loading as low as 81.39 ng cm -2 was obtained. This paper demonstrates that the m-HfO2 layer (with a thickness of 10 nm) can serve as an effective mass transport channel under the Pt active sites and effectively alleviates the blockage of active sites caused by bubbles by separating the bubble formation sites from the Pt active sites. The resulting catalyst exhibits a remarkable mass activity of 122.87 A mg -1 , with an overpotential of 11 mV at 10 mA cm -2 . In addition, m-HfO2 plays a crucial role in eliminating structural transformation and extending the lifespan of Pt-based catalysts, as evidenced by the fact that the specific activity shows no loss after the catalyst is continuously cycled for more than 100 hours. This capping strategy may be applicable to other types of reactions and catalyst systems.

[0070] Combined with analytical tools such as atomic-resolution aberration-corrected high-angle annular dark-field (HAADF) scanning transmission electron microscopy (STEM), in-situ STEM, Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and on-chip micro-cells, it is demonstrated that in these configurations, the multi-hole or porous m-HfO2 capping layer may play the following roles.

[0071] First, the porous m-HfO2 capping layer can firmly anchor highly dispersed metal nanoparticles (2 to 3 nm) on the support. In addition, the porous m-HfO2 capping layer can provide a large number of ion / gas conduction channels to ensure effective mass transfer and effectively separate the bubble nucleation sites from the Pt active sites. Next, the porous m-HfO2 capping layer may have good thermal stability at 500 °C. Last but not least, the porous m-HfO2 capping layer has excellent anti-corrosion properties in acidic, neutral, and alkaline solutions.

[0072] As described above, by preparing m-HfO2@Pt on carbon paper and testing it in an H-cell configuration, the practicality of the capping layer was demonstrated, showing 122.87 A mg -1world record high quality activity (MA), and excellent durability at 10 mA cm -2 and 100 mA cm -2 under both conditions.

[0073] In view of the above, in a first aspect, various embodiments relate to a catalyst comprising metal nanoparticles as an active phase disposed on a solid support, and a porous layer comprising amorphous hafnium oxide disposed on the metal nanoparticles.

[0074] As used herein, the term "catalyst" refers to a substance that causes a change in the reaction rate but is not consumed in the reaction. For example, the catalyst may be a catalyst for hydrogen generation.

[0075] The catalyst comprises metal nanoparticles as an active phase disposed on a solid support. The choice of metal nanoparticles may depend on the type of reaction. In various embodiments, the metal nanoparticles comprise a metal selected from platinum, palladium, cobalt, nickel, iron, iridium, aluminum, and alloys thereof. In some embodiments, the metal nanoparticles comprise platinum or consist of platinum.

[0076] The metal nanoparticles can be of any shape. For example, each metal nanoparticle can be spherical or irregular in shape. In various embodiments, each metal nanoparticle is spherical. Since the shape of the metal nanoparticles is not necessarily regular, such as a perfect sphere, the size of the metal nanoparticles can be characterized by the maximum size, which refers to the maximum size of the metal nanoparticles in any direction. In embodiments where the metal nanoparticles are spherical, the size of the metal nanoparticles can be defined by their average diameter.

[0077] The size of the metal nanoparticles can range from 0.1 nm to 5 nm, such as 1 nm to 5 nm, 2 nm to 5 nm, 3 nm to 5 nm, 0.1 nm to 4 nm, 0.1 nm to 3 nm, 0.1 nm to 2 nm, 1 nm to 4 nm, 1 nm to 3 nm, 2 nm to 4 nm, or 2 nm to 3 nm. In a specific embodiment, the size range of the metal nanoparticles is 2 nm to 3 nm.

[0078] The metal nanoparticles can be uniformly distributed on the surface of the solid support. In other words, the metal nanoparticles can be at least substantially uniformly distributed on the surface of the solid support.

[0079] In various embodiments, the metal nanoparticles are monodisperse in terms of size. Monodispersity is typically characterized by a low coefficient of variation (or variance), which is defined as the quotient of the standard deviation of the size distribution divided by the average particle size. In various embodiments, the metal nanoparticles can be uniformly distributed on the surface of a solid support. At the same time, it can be monodisperse because the sizes of the metal nanoparticles may be substantially the same. The monodispersity of the nanoparticles is usually expressed as a percentage. In various embodiments, the size variation or variance of the metal nanoparticles is less than 10%, such as less than 8%, less than 6%, less than 5%, less than 4%, or less than 3%.

[0080] The surface density of the metal nanoparticles on the solid support can be in the range of 80 ng cm -2 to 900 ng cm -2 . The term "surface density" refers to the mass of the metal nanoparticles per unit surface area on the solid support. In various embodiments, the surface density of the metal nanoparticles on the solid support can be in the range of 80 ng cm -2 to 900 ng cm -2 , such as 100 ng cm -2 to 900 ng cm -2 , 200 ng cm -2 to 900 ng cm -2 , 300 ng cm -2 to 900 ng cm -2 , 400 ng cm -2 to 900 ng cm -2 , 500 ng cm -2 to 900 ng cm -2 , 600 ng cm -2 to 900 ng cm -2 , 700 ng cm -2 to 900 ng cm -2 , 80 ng cm -2 to 800 ng cm -2 , 80 ng cm -2 to 700 ng cm -2 , 80 ng cm -2 to 600 ng cm -2 , 80 ng cm -2 to 500 ng cm -2 , 80 ng cm -2 to 400 ng cm -2 , 80 ng cm -2 to 300 ng cm -2 , 80 ng cm -2 to 200 ng cm-2 、 100 ng / cm -2 to 800 ng / cm -2 、 200 ng / cm -2 to 800 ng / cm -2 、 200 ng / cm -2 to 700 ng / cm -2 or 250 ng / cm -2 to 650 ng / cm -2 。

[0081] As disclosed herein, a porous layer containing hafnium oxide can be provided on the metal nanoparticles. The use of hafnium oxide is advantageous because it is the most stable, especially in an electrochemical environment, while other materials may not be able to withstand the harsh conditions of the electrochemical environment.

[0082] The thickness of the porous layer containing hafnium oxide can be in the range of 5 nm to 10 nm. For example, the thickness of the porous layer containing hafnium oxide can be in the range of 6 nm to 10 nm, 7 nm to 10 nm, 8 nm to 10 nm, 5 nm to 9 nm, 5 nm to 8 nm, 6 nm to 9 nm, or 6 nm to 8 nm.

[0083] The porous layer containing hafnium oxide can be nanoporous. The term "nanoporous" means that the average pore diameter or average pore size of the porous layer can be up to 100 nm. For example, the average pore diameter of the pores in the porous layer can be 1 nm to 100 nm, 10 nm to 100 nm, 20 nm to 100 nm, 30 nm to 100 nm, 50 nm to 100 nm, 60 nm to 100 nm, 70 nm to 100 nm, 1 nm to 90 nm, 1 nm to 80 nm, 1 nm to 70 nm, 1 nm to 60 nm, 1 nm to 50 nm, 1 nm to 40 nm, 10 nm to 90 nm, 20 nm to 80 nm, or 30 nm to 60 nm.

[0084] The solid support can be a carbon-based material. Carbon-based materials are valuable due to their low cost and large specific surface area. For example, the solid support can be selected from the group consisting of graphene and carbon paper. In various embodiments, the solid support is graphene.

[0085] In a second aspect, various embodiments relate to a method for preparing a catalyst, the method comprising forming metal nanoparticles as an active phase on a solid support, and providing a porous layer containing hafnium oxide on the metal nanoparticles.

[0086] Examples of suitable metal nanoparticles and solid supports have been mentioned above.

[0087] In various embodiments, forming the metal nanoparticles includes depositing atoms, nanoclusters, or atoms and nanoclusters of a metal on a solid support using electron beam evaporation.

[0088] Electron beam evaporation can be carried out in the temperature range of 12 °C to 14 °C of the solid support, such as 13 °C to 14 °C or 12 °C to 13 °C, or about 12.5 °C, 12.8 °C, 13 °C, 13.2 °C, or 13.5 °C.

[0089] Electron beam evaporation can be carried out at a pressure ranging from 1x10 -9 to 1x10 -8 Torr, such as 1x10 -9 to 5x10 -9 Torr, 5x10 -9 to 1x10 -8 Torr, or about 5x10 -9 Torr.

[0090] In various embodiments, electron beam evaporation is carried out at a deposition rate for a duration of 10 to 100 seconds. The time period can be adjusted to a suitable duration according to the size of the metal nanoparticles to be formed. Examples of the time period can be in the range of 20 to 100 seconds, 30 to 100 seconds, 40 to 100 seconds, 50 to 100 seconds, 60 to 100 seconds, 70 to 100 seconds, 10 to 90 seconds, 10 to 80 seconds, 10 to 70 seconds, 10 to 60 seconds, 10 to 50 seconds, 10 to 40 seconds, 20 to 80 seconds, 30 to 70 seconds, or 40 to 60 seconds.

[0091] In various embodiments, the method further includes placing the solid support having atoms, nanoclusters, or atoms and nanoclusters of a metal thereon at a temperature of 250 °C or lower to cause the atoms, nanoclusters, or atoms and nanoclusters of a metal to coalesce and form metal nanoparticles on the solid support.

[0092] In various embodiments, providing a porous layer containing amorphous hafnium oxide on the metal nanoparticles is carried out simultaneously with forming the metal nanoparticles on the solid support.

[0093] For example, after electron beam evaporation, the solid support containing atoms, nanoclusters, or atoms and nanoclusters of a metal can be subjected to a temperature of 250 °C or lower, while a porous layer containing amorphous hafnium oxide can be formed and / or deposited on the metal nanoparticles. Thus, when the atoms, nanoclusters, or atoms and nanoclusters of a metal coalesce and form metal nanoparticles on the solid support, a porous layer containing amorphous hafnium oxide can be formed and / or deposited on the metal nanoparticles.

[0094] In various embodiments, depositing a porous layer comprising amorphous hafnium oxide on metal nanoparticles comprises atomic layer deposition of a hafnium precursor and an oxygen precursor in an alternating sequence at a temperature of 250 °C or lower.

[0095] Atomic layer deposition can be carried out at a temperature of, for example, 200 °C or lower, 150 °C or lower, or at a temperature in the range of 150 °C to 250 °C, 180 °C to 250 °C, 200 °C to 250 °C, 150 °C to 220 °C, 150 °C to 200 °C, or 180 °C to 220 °C. In various embodiments, setting a porous layer comprising amorphous hafnium oxide on metal nanoparticles comprises atomic layer deposition of a hafnium precursor and an oxygen precursor in an alternating sequence at a temperature of 250 °C.

[0096] As used herein, the term "precursor" refers to a compound that can be processed or further processed to form a target material. Thus, the terms "hafnium precursor" and "oxygen precursor" can refer to compounds that can be further processed to form hafnium and oxygen, respectively.

[0097] The hafnium precursor can be, for example, [(CH3)2N]4Hf.

[0098] The oxygen precursor can be, for example, water.

[0099] The term "alternating sequence" means repeated exchange. For example, a hafnium precursor can be introduced onto the metal nanoparticles first by atomic layer deposition, followed by introduction of the oxygen precursor. This may constitute one atomic layer deposition cycle. This sequence can be repeated one or more times, depending on the number of atomic layer deposition cycles.

[0100] For example, the number of cycles for which atomic layer deposition can be carried out is in the range of 50 cycles to 100 cycles, such as 60 cycles to 100 cycles, 70 cycles to 100 cycles, 80 cycles to 100 cycles, 50 cycles to 90 cycles, 50 cycles to 80 cycles, 50 cycles to 70 cycles, 60 cycles to 90 cycles, or 70 cycles to 80 cycles.

[0101] When carrying out atomic layer deposition, the exposure times of the hafnium precursor and the oxygen precursor can be independently selected from 0.1 to 0.5 seconds. For example, the exposure time can be in the range of 0.2 to 0.5 seconds, 0.3 to 0.5 seconds, 0.1 to 0.4 seconds, 0.1 to 0.3 seconds, or 0.2 to 0.4 seconds. Alternatively, or in addition to the above, when carrying out atomic layer deposition, the reaction time for waiting for the hafnium precursor and the oxygen precursor to react is 3 to 8 seconds. In other words, the waiting time can be set to an appropriate period to allow the hafnium precursor and the oxygen precursor to react, and this waiting time can be in the range of, for example, 4 to 8 seconds, 5 to 8 seconds, 6 to 8 seconds, 3 to 7 seconds, 3 to 6 seconds, 3 to 5 seconds, 4 to 7 seconds, 4 to 6 seconds, or 5 to 7 seconds.

[0102] In a third aspect, various embodiments relate to the use of the catalyst according to the first aspect or the catalyst prepared by the method according to the second aspect in an electrode, a proton exchange membrane water electrolyzer, or a fuel cell.

[0103] To enable the present invention to be easily understood and put into practice, specific embodiments will now be described by the following non-limiting examples. Examples

[0104] Various embodiments relate to a coverage strategy to extend the stability of an ultra-low-loading metal-based electrocatalyst, and more specifically to an ultra-low-loading supported sub-monolayer metal electrocatalyst for hydrogen evolution in a proton exchange membrane (PEM) water electrolyzer. In one embodiment, the electrocatalyst is a protected sub-monolayer platinum nanoparticle (NP) on a large-scale carbon substrate.

[0105] Embodiments herein relate to a surface coverage strategy to prevent sintering / dissolution of underlying metal nanomaterials. The catalyst according to the embodiments disclosed herein can achieve effective structural stability of metal nanoparticles, excellent corrosion resistance, good thermal stability, good electrochemical accessibility, and large-scale production capacity. Advantageously, in embodiments using Pt, the Pt loading can be significantly reduced, thereby reducing costs. The accelerating development of the global hydrogen fuel market requires reducing the Pt loading from the current 0.5 - 1.0 mg cm -2 to a few μg cm -2 . The methods disclosed herein are capable of reducing the current Pt usage on graphene and carbon paper in PEM water electrolyzers to 81.39 ng cm -2 and 809.07 ng cm -2 , respectively.

[0106] Other advantages may involve an increase in the intrinsic activity, i.e., high-quality activity and turnover frequency (TOF). Specifically, 10 nm m-HfO2@Pt on graphene 0.1nm exhibits a mass activity of 122.87 A mg -1 at an overpotential of 11 mV (10 mA cm -2 ), and achieves a TOF of 1939.412 s -1 at an overpotential of 100 mV; 10 nm m-HfO2@Pt on carbon paper 1nm exhibits a mass activity of 12.36 A mg -1 at an overpotential of 59 mV (specific activity of 10 mA cm -2 ), and achieves a TOF of 25.62 s -1 at an overpotential of 100 mV.

[0107] Further advantages may involve a significant enhancement in the durability of the Pt NP catalyst with ultra-low loading in PEM electrolyzers. For example, after continuously cycling the catalyst for 93 hours at 10 mA cm -2 −2, for 22 hours at 20 mA cm -2 −2, and for 23 hours at 40 mA·cm -2 −2, the 10 nm m-HfO2@Pt on the carbon paper 1nm showed no loss of specific activity. This is because the m-HfO2 coating has good rigidity, excellent thermal stability, high corrosion resistance in all pH solutions, and strong metal-support interaction (SMSI). In addition, due to the large number of pore structures that can serve as proton conduction and H2 transport nanochannels, the porous m-HfO2 coating has good proton conductivity.

[0108] Example: Synthesis and Characterization of m-HfO2@M Nanoparticles

[0109] An atomic layer co-deposition method was developed to fabricate m-HfO2@M nanoparticles ([[]] Figure 1 ) on a large-scale support (up to 3 cm x 3 cm) in an atomically precise manner.

[0110] To ensure a uniform and dense distribution of isolated metal nanomaterials, low-speed electron beam evaporation was used to deposit sub-monolayer single-metal atoms / nanoclusters ([[]] ) at a rate of Figure 2 for 10 seconds.

[0111] On the wafer scale, Pt was used as M. First, at a low temperature of 12 °C, sub-monolayer Pt single atoms / nanoclusters with controllable size were deposited on supports (CVD graphene, carbon paper, titanium mesh) by low-rate electron beam evaporation.

[0112] In the examples, low-speed electron beam evaporation was carried out in a low-speed electron beam evaporation (E-beam) chamber (ATC Orion-8E). The characteristics of this technique are that the deposited amount of Pt can be precisely controlled and the degree of contamination is extremely low. Pt on graphene and Pt on carbon paper 0.1nm were obtained at a rate of 1nm in 10 seconds and 100 seconds, respectively. The low substrate deposition temperature (14 °C) and ultra-high vacuum pressure (5*10 -9 Torr) prevent atomic aggregation and ensure strong adhesion between metal atoms and the support.

[0113] Figures 3A to 3Eshows the uniform distribution of freshly evaporated Pt atoms on graphene at different deposition times. The Pt mass loadings of different samples were determined by mapping Pt atoms in the corresponding STEM images ( Figures 3A to 3D ). Figure 3E shows the average mass loadings at specific Pt thicknesses. The mass loadings show a positive linear correlation with the deposited Pt thickness, which implies a stable and precise deposition rate. STEM counting verified that the average Pt mass loading of Pt 0.1nm was 81.39 ng cm -2 ( Figure 3E ).

[0114] Next, an amorphous HfO2 overlay was deposited onto the loaded Pt atoms / nanoclusters by atomic layer deposition (ALD). During growth, the m-HfO2 thin film grew in a cyclic manner. Two chemical precursors, [(CH3)2N]4Hf and H2O (Strem Chemicals), of the Hf precursor and the O precursor were alternately exposed to the substrate at 250 °C. The deposition temperature was set at 250 °C to avoid aggregation of adjacent Pt NPs.

[0115] The reaction time (waiting time) was set at 5 s to construct a porous HfO2 structure. In all examples of forming amorphous porous HfO2, the Pt nanoclusters loaded on carbon paper or graphene alternately interacted with the two chemical precursors [(CH3)2N]4Hf and H2O, with exposure times of 0.3 s and 0.25 s, respectively, and a waiting time of 5 s.

[0116] Given the self-limiting growth characteristics of the ALD process, the deposition thickness at the nanoscale can be precisely controlled by changing the number of growth cycles. For example, during one ALD cycle, one atomic layer of HfO2 can be deposited on the loaded Pt 0.1nm nanoclusters. By changing the number of cycles, the thickness of the m-HfO2 layer can be controlled at the nanoscale. In the example, after 100 ALD cycles, 10 nm m-HfO2@Pt on graphene 0.1nm and 10 nm m-HfO2@Pt on carbon paper 1nm were obtained.

[0117] This manufacturing method can be extended to protect other metal / alloy nanoparticle systems for various applications. It is worth noting that both electron beam and ALD technologies are capable of achieving scalable and precise synthesis of X m-HfO2@M Y NPs for industrial use.

[0118] Example: Structural Characterization Method

[0119] The morphology and atomic structure of m-HfO2@Pt nanoparticles were characterized by annular dark-field scanning transmission electron microscopy (ADF-STEM) imaging (JEOL ARM200F). The interface between Pt and m-HfO2 will be shown by cross-sectional STEM. Chemical analysis was carried out using a WITEC alpha 200R confocal Raman system with a laser wavelength of 532 nm. X-ray photoelectron spectroscopy (XPS) was used to detect the chemical states and detailed coordination environments of Pt, Hf, C, and O.

[0120] As shown in the high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images ( Figure 4A ), such pre-deposited materials can not only serve as nucleation sites for amorphous HfO2 atomic layer deposition but also easily aggregate and crystallize into small nanoparticles at 250 °C. Figure 4A and Figure 4B depict the structures of the obtained m-HfO2@Pt nanoparticles with the thickness of m-HfO2 being 3 nm and 10 nm, respectively. It is worth noting that nanopores exist in both samples, indicating that reactants and products can easily penetrate the overlayers and reach / depart from the Pt nanoparticles. In addition, as shown in Figure 4A , the well-dispersed Pt nanoparticles are of uniform size, with diameters between 2 - 3 nm. In contrast, no Pt nanoparticles can be seen in Figure 4B , indicating that they are completely embedded beneath the 10 nm m-HfO2. Cross-sectional STEM images further confirm that m-HfO2 completely wraps the underlying Pt nanoparticles and may be firmly anchored to the Pt nanoparticles through covalent bonds at the Pt-HfO2 interface ( Figure 5 ).

[0121] To examine the chemical bonding, Raman and XPS analyses were carried out. The Raman spectra show a sharp increase in the D peak and the emergence of the D’ peak due to the disruption of the lattice symmetry of graphene by structural defects ( Figure 6A and Figure 6B ). In addition, the intensity ratio of I D / I G (I 2D / I G ) increases from 0.0978 to 0.5237 (decreases from 3.8411 to 0.8447), indicating that the as-grown highly crystalline CVD monolayer graphene exhibits a certain degree of disorder after m-HfO2 growth. This may be due to the nucleation and growth of HfO2 on graphene. The XPS spectrum of Pt ( Figure 7 ) shows obvious partial Pt(I) characteristics and metallic Pt(0), with a ratio of approximately 1:2, indicating the presence of Pt-O bonding at the Pt nanoparticle interface ( Figure 8 ). Figure 8The structure of the as-prepared 10 nm m-HfO2@Pt is also depicted. 0.lnm The cross-sectional STEM image in Figure 8 clearly shows that m-HfO2 completely encapsulates the underlying Pt nanoparticles, firmly anchoring the Pt nanoparticles on graphene. In addition, the Pt nanoparticles are of uniform size, with a diameter of 2 - 3 nm.

[0122] The O 1s peak is deconvoluted into three peaks centered at 530.5, 532.1, and 533.6 eV, respectively ( Figure 9A and Figure 9B ). The first peak corresponds to Hf-O, the second peak corresponds to Hf-OH, and the last peak corresponds to Si-O from the SiO2 / Si substrate. Notably, this enriched hydroxyl group indicates that the m-HfO2 overlayer can not only capture protons but also serve as a proton reservoir in the HER reaction. To examine the potential interaction between the nanoparticles and the overlayer, the charge density distribution of the m-HfO2@Pt nanoparticles was further investigated ( Figure 10 ). Although it may not be possible to simulate the real m-HfO2 structure, this is aimed at elucidating the metal (Pt)-support (m-HfO2) interaction.

[0123] To calculate the charge density difference, charge density difference maps are usually used to visually display the charge redistribution in heterostructures. By calculating and analyzing the differential charge density, one can gain in-depth understanding of the movement of charges and the direction of bond polarization during the bonding and electron coupling processes. Figure 10 The definition formula of the differential charge density in

[0124]

[0125] is as follows: where is the charge density of the entire system, Pt is the charge density of the m-HfO2 layer, and ρ

[0126] The interfacial m-HfO2 contributes a large number of electrons to the surface Pt layer, indicating that an electron transfer process occurs at the interface and further strong metal-support interaction takes place.

[0127] Example: Ion Penetration Depth and Ionic Conductivity Measurement

[0128] Due to the presence of a large number of nanopores in the m-HfO2 thin film, reactants and products can transfer to / from the active sites under electrochemical / concentration difference potential ( Figure 11 ). However, in a thicker m-HfO2 layer, the interconnected transport channels may be blocked. Therefore, m-HfO2@Pt with different m-HfO2 thicknesses was prepared on graphene.0.1nm , and the ion penetration depth and product diffusion length were detected using a microbattery device ( Figure 12A ).

[0129] When studying the activity and durability of m-HfO2-coated nanoparticles towards HER and OER, the activity and durability of the prepared catalysts were evaluated at both micro- and macro-scales. At the micro-scale, well-defined graphene-supported m-HfO2@Pt was prepared, and the proton conductivity, HER, and OER activities were tested in Ar-saturated 0.5 M H2SO4 and / or O2-saturated 1 M KOH using a home-made microelectrochemical system with an on-chip microfluidic cell (current resolution of 10 -12 μA). Saturated Ag / AgCl and Hg / HgO were used as reference electrodes in acidic and alkaline solutions, respectively, and a graphite rod was used instead of a Pt foil as the counter electrode to exclude potential Pt deposition on the counter electrode. At the macro-scale, a typical H-cell device was assembled to evaluate the durability of large-scale carbon paper-supported m-HfO2@Pt in a practical working environment. To eliminate carbon redeposition on the working electrode, a Nafion 117 membrane separated the carbon rod serving as the counter electrode from the working electrode and the reference electrode.

[0130] As Figure 12A and Figure 12B shown, the 5 nm and 10 nm m-HfO2-coated materials exhibited similar HER activities, while the 20 nm-coated material showed negligible activity, indicating that the nanochannels may be blocked when the thickness is further increased to 20 nm. To ensure sufficient proton transport and effective protection, a 10 nm m-HfO2 coating was selected for the following electrochemical tests.

[0131] Electrochemical characterization of 10 nm m-HfO2@Pt on graphene was carried out in a home-made three-electrode microelectrochemical system, and electrochemical characterization of 10 nm m-HfO2@Pt on carbon paper was carried out in a conventional three-electrode system (Biologic SVP). 0.1nm 1nm

[0132] A microfluidic cell was designed and used to detect the stability of 10 nm m-HfO2@Pt on graphene ( 0.1nm ). Figure 13A )

[0133] As Figure 13A shown, in the microelectrochemical system, with 10 nm-HfO2@Pt on graphene 0.1nmThe working electrode is [electrode material], the counter electrode is a carbon rod (Ted Pella), the reference electrode is a leak-free Ag / AgCl electrode (EDAQ), and the electrolyte is 0.5 M H2SO4 (Honeywell). In Figure 13B In the traditional three-electrode system of Figure 13B , 10 nm m-HfO2@Pt on carbon paper 1nm is used as the working electrode, a Pt foil is used as the counter electrode, an Ag / AgCl electrode (Metrohm Autolab) is used as the reference electrode, and 0.5 M H2SO4 (Honeywell) is used as the electrolyte ( Figure 13B ). In both cases, the potential of Ag / AgCl was pre-measured relative to the standard hydrogen electrode (EDAQ) in a two-electrode system.

[0134] For 10 nm m-HfO2@Pt on graphene 0.1nm , cyclic linear sweep voltammetry (LSV) exhibits enhanced activity, with a smaller overpotential at 10 mA cm -2 and a smaller Tafel slope during continuous cycling, indicating excellent stability and activity under acidic working conditions ( Figure 14A and Figure 14B ). After 700 cycles, the overpotential required to obtain a current density of 10 mA cm -2 is only 11 mV. Figure 15A and Figure 15B show a comparison of the loading, mass activity, and overpotential of the materials disclosed in this article with state-of-the-art Pt-based materials. Notably, 10 nm m-HfO2@Pt on graphene disclosed in this article 0.1nm simultaneously exhibits the smallest loading (81.39 ng cm -2 , Figures 3A to 3E ), the best mass activity (122.87 A mg -1 ), and the lowest overpotential (11 mV). In addition, 10 nm m-HfO2@Pt on carbon paper 1nm exhibits good performance, with a mass loading of 809.07 ng cm -2 and a mass activity of 12.36 A mg -2 at a current density of 10 mA cm -1 .

[0135] Example: On-Chip Microbattery Fabrication Process

[0136] In addition, on-chip four-electrode cells were fabricated, and the proton conductivity of the m-HfO2 thin film was measured by measuring the ohmic resistance ( Figure 16 ).

[0137] The typical fabrication of the on-chip devices disclosed in this article involves five main steps (Figure 17 ) First, a 5 nm thick Ti / 50 nm thick Au contact pad was pre-patterned on a SiO2 / Si substrate or a transparent glass substrate using conventional photolithography and electron beam evaporation techniques. Next, a CVD-grown single-layer graphene film was transferred onto the on-chip device by a standard PMMA-assisted method. Next, in order to disconnect adjacent electrodes, electron beam lithography (EBL) and O2 plasma (20 W, 1 minute) were used to remove the selected graphene channels. Next, well-controlled m-HfO2@Pt was synthesized on graphene by the above method. Then, reaction windows were exposed in the selected areas by EBL in a 1 μm thick PMMA protective film to ensure that the reaction occurred only in the target areas.

[0138] Before performing electrochemical impedance spectroscopy (EIS) measurements, in order to fill the film with H + ions, the device was immersed in 2 M H2SO4 for 24 hours and then in deionized water for 24 hours. The proton conductivity (σ) can be determined by the formula σ = L / (RWT), where L, W, and T are the distance (2.86 μm), width (10.25 μm), and thickness (10 nm) between the reference electrodes, respectively, and R represents the resistance, which can be obtained from the Nyquist plot ( Figure 18A and Figure 18B ) and the resistance was found to be 13.1 Ω. The results showed that the m-HfO2 film exhibited a high proton conductivity with a value of 2.13 x 10 4 mS cm -1 .

[0139] Example: In-Situ Observation of Bubble Nucleation and Growth

[0140] To study the bubble nucleation sites on 10 nm m-HfO2@Pt on graphene and whether bubble escape would cause perforation of the m-HfO2 film, an inverted optical microscope (60 frames per second, spatial resolution 260 nm pix 0.1nm ) was combined with a transparent on-chip electrochemical microcell device. -1 )

[0141] Figure 19Examples of bubble evolution during linear sweep voltammetry (LSV) and without applied potential are shown. This set of images tracks the temporal evolution of bubbles from nucleation, growth to rupture. A hydrogen bubble with a diameter of approximately 500 nm can be seen on the device within 16.7 milliseconds from frame 696 to frame 670. From frame 970 to frame 973, the further growth of the bubble is mainly governed by Henry's law as the local H2 concentration in the aqueous solution continuously increases under a higher overpotential driving force. After LSV treatment, the microbubbles gradually shrink into nanobubbles with a diameter of approximately 260 nm. The results indicate that the nucleation and growth of H2 bubbles do not lead to the structural destruction of the coating, suggesting that H2 molecules diffuse away from the Pt active sites through m-HfO2 and eventually form bubbles on the surface of m-HfO2. Therefore, the coating can act as an effective mass transport channel and avoid clogging of the active sites due to bubble formation.

[0142] Example: Macrobattery Measurement

[0143] To further evaluate the effectiveness and cost reduction of the coating strategy, commercial carbon paper was used as the substrate. Pt nanoparticles and the m-HfO2 coating were directly deposited on it, and its performance was further evaluated in an H-cell configuration ( Figure 20 ).

[0144] As Figure 21A and Figure 21B shown, 10 nm m-HfO2@Pt on carbon paper 0.5nm shows an overpotential of 59 mV and a Tafel slope of 35 mV dec -2 at 10 mA cm -1 . In addition, 10 nm m-HfO2@Pt on carbon paper 0.5nm exhibits good performance, with a mass loading of 404.54 ng cm -2 and a mass activity of 24.72 A mg -2 at a current density of 10 mA cm -1 .

[0145] In addition, continuous chronoamperometric measurements were carried out on 10 nm m-HfO2@Pt on carbon paper 0.5nm at current densities of 10 mA cm -2 , 20 mA cm -2 and 40 mA cm -2 respectively. The results show that the material exhibits excellent stability during continuous operation for 93 hours at 10 mA cm -2 , 22 hours at 20 mA cm -2 and 23 hours at 40 mA cm -2 ( Figure 22A)。This significantly extended lifespan can be attributed to the inherent properties of the m-HfO2 coating, including good rigidity, excellent thermal stability, high corrosion resistance in all pH solutions, and strong metal-support interaction (SMSI). In contrast, without a protective layer, Pt on carbon paper 0.5nm at 10 mA cm -2 will degrade significantly within 2 hours.

[0146] Continuous constant current measurements were also carried out on 10 nm m-HfO2@Pt on carbon paper 1nm at current densities of 10 mA cm -2 , 20 mA cm -2 and 40 mA cm -2 . As Figure 22B shown, the material showed excellent stability during continuous operation at 10 mA cm -2 for 93 hours, 20 mA cm -2 for 22 hours, and 40 mA cm -2 for 23 hours. This significantly extended lifespan is attributed to the inherent properties of the m-HfO2 coating, including good rigidity, excellent thermal stability, high corrosion resistance in all pH solutions, and strong metal-support interaction (SMSI).

[0147] Example: Thermal Stability

[0148] In-situ high-temperature stability characterization was carried out. To directly observe the behavior of Pt nanoparticles at high temperatures, we conducted in-situ heating experiments in a STEM (JEOL ARM200F) at an operating voltage of 200 kV. The prepared HfO2-based material was transferred onto a MEMS microheater chip (Fusion Thermal E-chips). This microscope can achieve high-angle annular dark-field, annular bright-field, and bright-field STEM imaging with a resolution of less than 0.078 nm.

[0149] To examine the anti-sintering performance, high-resolution STEM images were recorded during aging at 500 °C and 700 °C to obtain atomic-scale information about the possible structural evolution of 3 nm m-HfO2@Pt 0.1nm .

[0150] As Figure 23 shown in the snapshots (taken from 3 nm m-HfO2@Pt 0.1nmIn-situ TEM heating structural evolution movie at 500 °C (acceleration X600), with a complete sequence, available upon request), no structural deformation was observed during heating at 500 °C for 1 hour, i.e., aggregation of Pt nanoparticles or phase transformation of m-HfO2; further heating to 700 °C, although the Pt nanoparticles encapsulated within HfO2 remained intact, some amorphous domains of the HfO2 overlay underwent phase transformation( Figure 24 , taken from 3 nm m-HfO2@Pt 0.1nm In-situ TEM heating structural evolution movie at 700 °C (acceleration X600), with a complete sequence, available upon request). These observations indicate that the m-HfO2@Pt NPs disclosed herein exhibit excellent anti-sintering performance and are promising for use in high-temperature catalysts.

[0151] Conclusion

[0152] In summary, the embodiments disclosed herein construct a metal nanoparticle embedded structure, i.e., amorphous HfO2 (m-HfO2) covers monodisperse metal nanoparticles on a large-scale carbon support (X m-HfO2@M Y ). The m-HfO2 overlay has excellent thermal stability and corrosion resistance in all pH solutions and can be used to firmly anchor monodisperse metal nanoparticles on carbon substrates, graphene, and carbon paper. By adjusting the thickness of m-HfO2 and the size of metal NPs in an atomically precise manner, a super-stable and highly efficient electrocatalyst is achieved with a Pt loading of tens to hundreds of nanograms per square centimeter. The embodiments disclosed herein can effectively address the scarcity, cost, and durability issues of Pt and can further promote the development of the global green H2 economy. The commercial applications of the embodiments disclosed herein may include the cathodes and anodes of PEM water electrolyzers and fuel cells.

[0153] As disclosed herein, a capping strategy for sub-monolayer metal electrocatalysts with ultra-high durability and mass activity is provided. The capping strategy disclosed herein can provide electrocatalysts with ultra-low metal loading.

[0154] To address the challenges of activity and durability of electrocatalysts in water electrolyzers and fuel cells, the embodiments disclosed herein propose using a rigid porous oxide overlay to inhibit the degradation of the underlying metal species. In a specific embodiment, it has been demonstrated to construct a nano-sized metal embedded structure, i.e., amorphous HfO2 (m-HfO2) covers monodisperse metal nanoparticles on a large-scale carbon support. This structure is represented by the formula X m-HfO2@M Y NPs, where M can represent nano-scale metal species such as Pt, Co, Ni, PtCo alloy, and PtNi alloy, and where X and Y are the thickness of m-HfO2 and the amount of deposited metal species, respectively.

[0155] In an exemplary embodiment, the composition being tested is 10 nm m-HfO2@Pt on graphene 0.1nm NP and 10 nm m-HfO2@Pt 1nm NP on carbon paper. It should be understood that the metal is not necessarily limited to Pt and can be any other metal or metal alloy, including Fe, Ni, Pd, Al, Ir, etc. Similarly, the amount of the deposited metal determines the metal loading of the catalyst and, in turn, the mass activity of the catalyst, rather than the durability of the catalyst. It is not limited to 0.1 nm or 1 nm and can be any value as long as the resulting catalyst has the desired mass activity.

[0156] "Comprising" means including but not limited to whatever follows the word "comprising". Thus, the use of the term "comprising" indicates that the listed elements are required or mandatory, but other elements are optional and may or may not be present.

[0157] "Consisting of" includes but not limited to all that follows the phrase "consisting of". Thus, the phrase "consisting of" indicates that the listed elements are required or mandatory and that no other elements are present.

[0158] The inventions described herein by way of example can be practiced appropriately in the absence of any element or combination of elements, limitation or combination of limitations not specifically disclosed herein. Thus, for example, terms such as "comprising", "including", "containing", etc. should be understood broadly and without limitation. In addition, the terms and expressions used herein have been used as descriptive terms and not of limitation, and the use of such terms and expressions is not intended to exclude any equivalents of the features shown and described or portions thereof, but it should be recognized that various modifications are possible within the scope of the invention as claimed. Thus, it should be understood that although the present invention has been specifically disclosed by way of preferred embodiments and optional features, those skilled in the art can make modifications and variations to the specific inventions disclosed herein, and such modifications and variations are considered to be within the scope of the present invention.

[0159] For a given numerical value (such as temperature and time period), "about" means a numerical value within 10% of the specified value.

[0160] The present invention has been described in a broad and general manner herein. Each narrower genus and subgenus grouping that falls within the scope of the general disclosure also forms part of the present invention. This includes the general description of the present invention with provisos or negative limitations, where the provisos or negative limitations exclude any subject matter, whether or not the excluded material is specifically recited herein.

[0161] Other embodiments are within the scope of the following claims and non-limiting examples. Additionally, when features or aspects of the invention are described in terms of a Markush group, those skilled in the art will recognize that the invention is also thereby described in terms of any single member or subgroup of members of the Markush group.

Claims

1. A catalyst, which comprises metal nanoparticles as an active phase disposed on a solid support, and a porous layer containing amorphous hafnium oxide disposed on the metal nanoparticles.

2. The catalyst according to claim 1, wherein the metal nanoparticles comprise a metal selected from platinum, palladium, cobalt, nickel, iron, iridium, aluminum, and alloys thereof.

3. The catalyst according to claim 1 or 2, wherein the size of the metal nanoparticles is in the range of 0.1 nm to 5 nm.

4. The catalyst according to any one of claims 1 to 3, wherein the metal nanoparticles are monodisperse.

5. The catalyst according to any one of claims 1 to 4, wherein the surface density of the metal nanoparticles on the solid support is in the range of 80 ng cm -2 to 900 ng cm -2 .

6. The catalyst according to any one of claims 1 to 5, wherein the thickness of the porous layer containing amorphous hafnium oxide is in the range of 5 nm to 10 nm.

7. The catalyst according to any one of claims 1 to 6, wherein the porous layer containing amorphous hafnium oxide is nanoporous.

8. The catalyst according to any one of claims 1 to 7, wherein the solid support is a carbon-based material.

9. The catalyst according to any one of claims 1 to 8, wherein the solid support is selected from graphene and carbon paper.

10. The catalyst according to any one of claims 1 to 9, wherein the catalyst is a catalyst for hydrogen generation.

11. A method for preparing a catalyst, the method comprising forming metal nanoparticles as an active phase on a solid support, and disposing a porous layer containing amorphous hafnium oxide on the metal nanoparticles.

12. The method according to claim 11, wherein forming the metal nanoparticles comprises depositing atoms, nanoclusters, or atoms and nanoclusters of a metal on the solid support using electron beam evaporation.

13. The method according to claim 12, wherein the electron beam evaporation is carried out at a temperature of the solid support in the range of 12 °C to 14 °C.

14. The method according to claim 12 or 13, wherein the electron beam evaporation is carried out at a pressure of 1 x 10 -9 to 1 x 10 -8 Torr.

15. The method according to any one of claims 12 to 14, wherein the electron beam evaporation is carried out at a deposition rate of and for a duration of 10 to 100 seconds.

16. The method according to any one of claims 12 to 15, further comprising placing the solid support having atoms, nanoclusters, or atoms and nanoclusters of a metal disposed thereon at a temperature of 250 °C or lower to cause the atoms, nanoclusters, or atoms and nanoclusters of a metal to coalesce and form the metal nanoparticles on the solid support.

17. The method according to any one of claims 11 to 16, wherein disposing the porous layer containing amorphous hafnium oxide on the metal nanoparticles is carried out simultaneously with forming the metal nanoparticles on the solid support.

18. The method according to any one of claims 11 to 17, wherein disposing the porous layer containing amorphous hafnium oxide on the metal nanoparticles comprises atomic layer deposition of a hafnium precursor and an oxygen precursor in an alternating sequence at a temperature of 250 °C or lower.

19. The method according to claim 18, wherein the hafnium precursor is [(CH3)2N]4Hf.

20. The method according to claim 18 or 19, wherein the oxygen precursor is water.

21. The method according to any one of claims 18 to 20, wherein the number of cycles of the atomic layer deposition is in the range of 50 cycles to 100 cycles.

22. The method according to any one of claims 18 to 21, wherein during atomic layer deposition, the exposure times of the hafnium precursor and the oxygen precursor are each independently selected from 0.1 to 0.5 seconds, and / or the reaction time for waiting for the reaction of the hafnium precursor and the oxygen precursor is 3 to 8 seconds.

23. Use of the catalyst according to any one of claims 1 to 10 or the catalyst prepared by the method according to any one of claims 11 to 22 in an electrode, a proton exchange membrane water electrolyzer or a fuel cell.

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