A core-shell catalyst, methods of making and uses thereof

Abstract

The present disclosure relates to core-shell particles, such as core-shell particles comprising a core comprising TiOx; and a shell comprising iridium, methods of preparing core-shell particles, and uses thereof, such as a catalyst in an oxygen evolution reaction (OER).

Description

A CORE-SHELL CATALYST, METHODS OF MAKING AND USES THEREOFCROSS-REFERENCE TO RELATED APPLICATIONS

[0001] The present application claims the benefit of priority from U.S. provisional application no. 63 / 634,555 filed on April 16, 2024, the contents of which are incorporated herein by reference in their entirety.FIELD

[0002] The present disclosure relates to core-shell particles, such as core-shell particles comprising a core comprising TiOx; and a shell comprising iridium, methods of preparing core-shell particles, and uses thereof.BACKGROUND

[0003] With clear understanding that human activity is the primary cause of global warming, many countries are actively seeking ways to reduce greenhouse gases, particularly carbon dioxide, a significant contributor to climate change. One promising route for the decarbonization of modem processes — such as fertilizer production, heavy-duty transport, steel making, and chemical / fuels refining — is utilizing proton exchange membrane water electrolysis (PEMWE). This method offers an environmentally friendly method to generate pure hydrogen (Pham et al., 2021; Shiva Kumar & Himabindu, 2019; Minke et al., 2021). The production of hydrogen through PEMWE utilizing renewable energy sources, commonly referred to as green hydrogen, offers an environmentally conscious and sustainable alternative to the conventional method of hydrogen production via natural gas reforming, which produces ‘gray hydrogen’ along with CO2 emissions (Aj ano vic et al., 2022). Despite its promise, the scale-up of PEMWEs still faces challenges, primarily attributed to the anodic oxygen evolution reaction (OER) in acidic media which is inherently sluggish and involves a four-electron transfer mechanism (Kim et al., 2017; Yu et al., 2018; Hubert et al., 2020; Li et al., 2021a). Accordingly, achieving significant current densities for acidic OER necessitates high applied potentials. The combination of high potential with a low pH environment at the anode leads to harsh and severely corrosive reaction conditions, resulting in the corrosion of carbon materials and rapid dissolution of most metals into the electrolyte, in other words, a limited availability of stable catalyst and electrode materials (Shi et al., 2022; He et al., 2021; Shi et al., 2021; Park et al., 2022).

[0004] Iridium (Ir) and Ir-based materials are widely recognized as state-of-the-art acidic OER catalysts due to their high activity and moderate stability under harsh acidicOER conditions. However, the scarcity and high cost of Ir currently inhibit the long-term prospect of broader industrialization and deployment of PEMWEs (Chang et al., 2023; Alia et al., 2019; Abbott et al., 2016; Lettenmeier et al., 2016; Nde et al., 2023). Recent efforts have made significant progress toward developing catalysts for acidic OER to overcome the activity, stability, and scarcity issues associated with Ir. The various approaches that have been explored include incorporating additional elements into an Ir-based alloy or mixed-metal oxide structure (Chen et al., 2022; Audichon et al., 2015; Li et al., 2021b; Zaman et al., 2019; Yan et al., 2018), combining catalyst particles with novel support materials (Zhang et al., 2017; Yu et al., 2023), or modulating the surface area or surface structure (and by extension adsorbate binding energies) through nano-structuring of the catalyst particles (He et al., 2021; Meng et al., 2019; Lu et al., 2021).

[0005] Nevertheless, many efforts prioritize improving the activity of the catalyst rather than the stability, the latter being one of the main obstacles in the practical application of PEMWEs. In addition, to enable and ensure the widespread commercialization of PEMWEs, the total platinum group metal (PGM) content is desirably reduced to 0.125mgpGM cm’2or below as outlined by the United States Department of Energy (US DOE) for PEMWEs in “Technical Targets for Proton Exchange Membrane Electrolysis”. This goal underscores the need for catalysts possessing not only minimal Ir content but also demonstrating prolonged stability and high mass-normalized activity to be economically viable. However, few studies have reported catalysts that display stable activity for more than 100 h (Fan et al., 2023; Cheng et al., 2019; Gou et al., 2022; Yu et al., 2020), falling short of the benchmarks necessary for commercial viability (Pham et al., 2021 ; Park & Lee, 2023; Bensmann & Hanke-Rauschenbach, 2022; Marangio et al., 2009), or exceeding the PGM content limits specified by the US DOE (Yu et al., 2020; Jang et al., 2019).

[0006] The background herein is included solely to explain the context of the disclosure. This is not to be taken as an admission that any of the material referred to was published, known, or part of the common general knowledge as of the priority date.SUMMARY

[0007] The present disclosure includes a core-shell particle, comprising: a core comprising TiOx; and a shell comprising iridium. In an embodiment, the core-shell particle is a core-shell nanoparticle. In an embodiment, the core has an average diameter in the range of from about 70 nm to about 150 nm. In another embodiment, the shell has an average thicknessin the range of from about 8 nm to about 13 nm. In another embodiment, the core-shell particle consists of: the core comprising TiOx; and the shell comprising iridium. In another embodiment, the TiOxcomprises TiCh. In another embodiment, the TiOxcomprises anatase phase TiCh. In another embodiment, the iridium comprises metallic iridium. In another embodiment, the iridium comprises crystalline metallic iridium.

[0008] The present disclosure also includes a method of preparing a core-shell particle comprising: a core comprising TiOx; and a shell comprising iridium, the method comprising: contacting a particle comprising TiOxwith an iridium salt and a reducing agent to prepare a particle comprising iridium and TiOx; and annealing the particle comprising iridium and TiOxin a reducing atmosphere to prepare the core-shell particle.

[0009] In an embodiment, the reducing agent comprises urea. In another embodiment, the iridium salt comprises an iridium(III) chloride and / or a hydrate thereof. In another embodiment, the iridium(III) chloride is an IrCh hydrate. In another embodiment, the contacting comprises heating the particle comprising TiOxin an aqueous environment that comprises the iridium salt and the reducing agent. In another embodiment, the method further comprises drying the particle comprising iridium and TiOxprior to annealing. In another embodiment, the annealing is of particles comprising iridium and TiOxin the form of a powder. In another embodiment, the reducing atmosphere comprises hydrogen and an inert gas. In another embodiment, the reducing atmosphere is Th / balanced N2. In another embodiment, the hydrogen is present in an amount of from about 3 vol% to about 6 vol%, based on the total volume of the atmosphere. In another embodiment, the annealing is at a temperature of about 500°C. In another embodiment, the TiOxparticles are prepared by a method comprising polyol thermolysis. In another embodiment, the polyol thermolysis comprises use of a titanium dioxide precursor that is titanyl sulfate hydrate. In another embodiment, the polyol is ethylene glycol. In an embodiment, the particle comprising TiOxis a nanoparticle comprising TiOx.

[0010] The present disclosure also includes a core-shell particle prepared by a method of preparing a core-shell particle as described herein.

[0011] The present disclosure also includes a use of a core-shell particle comprising a core comprising TiOxand a shell comprising iridium as described herein or prepared by a method of preparing a core-shell particle as described herein as a catalyst for an oxygen evolution reaction (OER). In an embodiment, the OER is acidic OER. In an embodiment, the OER is in a process comprising proton exchange membrane water electrolysis (PEMWE).

[0012] The present disclosure also includes an electrode comprising a core-shell particle comprising a core comprising TiOxand a shell comprising iridium as described herein or prepared by a method of preparing a core-shell particle as described herein. In an embodiment, the electrode comprises a catalyst-coated electrode. In an embodiment, the catalyst is present in a loading amount of about 0. 1 mgircm2.

[0013] The present disclosure also includes a proton exchange membrane water electrolysis (PEMWE) cell comprising a core-shell particle comprising a core comprising TiOxand a shell comprising iridium as described herein or prepared by a method of preparing a core-shell particle as described herein or an electrode comprising such a core-shell nanoparticle as described herein.

[0014] The present disclosure also relates to a core-shell structured catalyst, and in particular, to a core-shell IrA.TiCh structure as an electrocatalyst, methods of making and uses thereof. Accordingly, the present disclosure provides a core-shell structured electrocatalyst which is composed of a TiCh nanoparticle core and an Ir shell. The method of making this Ir r / TiCh core-shell structure comprises synthesizing theTiCh nanoparticles, coating Ir on TiCh, and annealing the Ir-coated TiCh, wherein the Ir coating is a continuous layer rather than some Ir nanoparticles. Such an electrocatalyst exhibited a high electrocatalytic stability and activity during an oxygen evolution reaction. Accordingly, in an aspect of the present disclosure, the core-shell catalyst comprises a titanium dioxide core and an iridium shell coated on the core, wherein the core has a diameter ranging from about 70nm to about 150nm, and the shell has a thickness ranging from about 8nm to about 13nm. In some embodiments, the titanium dioxide is a titanium dioxide nanoparticle. In some embodiments, the iridium has a crystalline structure. In some embodiments, the iridium is a non-nanoparticle iridium coating. According to another aspect of the present disclosure, the method of preparing the core-shell catalyst initially involves synthesizing a titanium dioxide nanoparticle using titanium(IV) oxysulfate, sulfuric acid, ethylene glycol, and sodium hydroxide with a desired reaction time and reaction temperature. The iridium was then coated on the titanium dioxide nanoparticle using the titanium nanoparticle, iridium(III) chloride hydrate, urea, and water with a desired reaction time andreaction temperature. After that, the iridium-coated titanium dioxide was annealed with a desired time and temperature in a nitrogen atmosphere having about 3 vol% hydrogen to about 6 vol% hydrogen. In some embodiments, the reaction time of synthesizing titanium dioxide nanoparticle ranges from about 2 hours to about 4 hours. In some embodiments, the reaction temperature of synthesizing titanium dioxide nanoparticle ranges from about 100 °C to about 160 °C. In some embodiments, the reaction time of coating iridium ranges from about 15 hours to about 24 hours. In some embodiments, the reaction temperature of coating iridium ranges from about 60 °C to about 100 °C. In some embodiments, the time of annealing ranges from about 1 hour to about 3 hours. In some embodiments, the temperature of annealing ranges from about 400 °C to about 600 °C. According to another aspect of the present disclosure, the iridiumtitanium dioxide core-shell structured catalyst can be used for oxygen evolution reaction, hydrogen evolution reaction, or oxygen reduction reaction.

[0015] Other features and advantages of the present disclosure will become apparent from the following detailed description. It should be understood, however, that the detailed description and the specific examples, while indicating embodiments of the disclosure, are given by way of illustration only and the scope of the claims should not be limited by these embodiments, but should be given the broadest interpretation consistent with the description as a whole.BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Certain embodiments of the disclosure will now be described in greater detail with reference to the attached drawings in which:

[0017] FIG. 1 shows calibration of Ag / AgCl reference electrode in 0.5 M H2SO4 (upper) and an expanded view of the calibration (lower). The electrolyte was saturated with H2 for more than 30 min. Flame-annealed Pt wire (short), flame-annealed Pt wire (long), and Ag / AgCl served as the working electrode, the counter electrode, and the reference electrode, respectively. The calibration potential range was -0.4 - 0.4 V vs. Ag / AgCl, and the scan rate was 10 mV s'1.

[0018] FIG. 2 shows a schematic image of an exemplary preparation of a Core-shell Ir@TiO2 catalyst that may be useful, for example, for oxygen evolution reaction in acidic media (acidic OER) according to an embodiment of the present disclosure.

[0019] FIG. 3 shows X-ray diffraction (XRD) patterns of the synthesized TiCh nanoparticle (NP) support (core) material synthesized using a modified polyol method, with the triangles indicatingTiCh anatase phase (#21-1272) peaks.

[0020] FIG. 4 shows a high-angle dark-field scanning transmission electron microscopy (STEM-HAADF) image of Amorphous Ir / TiOx(upper left image), energy-dispersive X-ray (EDX) mapping images of Ir&Ti (upper right image), Ir (middle left image), and Ti (middle right image), and EDX spectra (lower image), All scale bars correspond to 50 nm.

[0021] FIG. 5 shows a STEM-HAADF image of Mixed IrTiCh (upper left image), EDX mapping images of Ir&Ti (upper right image), Ir (middle left image), and Ti (middle right image), and EDX spectra (lower image). All scale bars correspond to 100 nm.

[0022] FIG. 6 shows a transmission electron microscopy (TEM) image of the prepared core-shell Ir@TiO2 (upper left image; with inset showing selected area electron diffraction (SAED) patterns), a HAADF-STEM image (upper right image), and EDX mapping of Mix (Ir&Ti; middle left image), Ir (middle right image), Ti (lower left image), and O (lower right image). All scale bars correspond to 50 nm.

[0023] FIG. 7 shows a STEM-HAADF image of Core-shell frt.TiCh with ling-profiling (upper left image), EDX mapping images of Ir&Ti (upper right image), Ir (middle left image), Ti (middle right image), and O (lower left image), and EDX line spectra depending on the position (lower right image), All scale bars correspond to 50 nm.

[0024] FIG. 8 shows in-situ environmental TEM as a function of temperature (from left to right: 100, 200, 300, 400, 500 and 600 °C) with a ramping rate of 5 °C s'1. The atmosphere of the annealing condition was 5% Rebalanced Ar. All scale bars correspond to 20 nm.

[0025] FIG. 9 shows a bar graph of each component atomic ratio (from top to bottom at each temperature: iridium, titanium and oxygen) based on the EDX-equipped in-situ environmental TEM at temperatures of, from left to right: 100, 200, 300, 400, 500 and 600 °C.

[0026] FIG. 10 shows illustrative diagrams depicting the transition process from disordered (upper diagram) to ordered (core-shell; lower diagram) states in Core-shell Ir@TiO2.

[0027] FIG. 11 shows XRD paterns of the synthesized samples (Core-shell Ir@TiO2 (top), Mixed IrTiCh (second from top), and Amorphous Ir / TiOx(second from botom)), and commercial IrOx(bottom). The bar indicator below represents anatase TiC (PDF #21- 1272) and metallic Ir (indicated with *, PDF #46-1044).

[0028] FIGs. 12-14 show X-ray photoelectron spectroscopy (XPS) spectra of the synthesized samples (in each, top: Core-shell Ir@TiO2, middle: Mixed IrTiCh, and bottom: Amorphous Ir / TiOx). FIG. 12 shows O Is spectra, FIG. 13 shows Ti 2p spectra, and FIG. 14 shows Ir 4f spectra

[0029] FIG. 15 shows XPS survey spectra of the synthesized catalysts with atomic composition table (top: Core-shell Ir@TiO2, middle: Mixed IrTiCh. and bottom: Amorphous Ir / Ti-Ox).

[0030] FIGs. 16-17 show comparison with a benchmark catalyst IrOx. FIG. 16 shows a linear sweep voltammetry (LSV) curve, and FIG. 17 shows a Tafel plot.

[0031] FIG. 18 shows LSV curves with a scan rate of 10 mV s'1for Core-shell Ir@TiO2, Mixed IrTiCh, Amorphous Ir / TiOx, commercial IrOxand the bare electrode.

[0032] FIG. 19 shows overpotential comparison of the prepared catalysts (Core-shell Ir@TiCh, Mixed IrTiCh, and Amorphous Ir / TiOx) and commercial IrOxat 10 mA cm’2. The test was conducted at the potential range of 1.0 - 2.0 V vs. RHE with a scan rate of 10 mV s'1in N2-purged 0.5 M H2SO4 solution.

[0033] FIG. 20 shows acidic OER activity of Core-shell Ir@TiCh as a function of Ir loading (0. 1 mgir, 0.2 mgiror 0.4 mgir). The test was conducted at the potential range of 1.0 - 2.0 V vs. RHE with a scan rate of 10 mV s'1in N2-purged 0.5 M H2SO4 solution.

[0034] FIG. 21 shows double layer capacitance of the Core-shell Ir@TiCh, Mixed IrTiCh, Amorphous Ir / TiOx, commercial IrOxand the bare electrode from different cyclic voltammetry (CV) scan rates (0.88, 2.85, 3.32, 10.9 or 25.0 mF cm’2).

[0035] FIGs. 22-26 show cyclic voltammetry curves for the double layer capacitance in the non-Faradaic region of open circuit potential (OCP)±0.025 V with scan rates of 1, 2, 5, 10, 20, and 50 mV s’1: Bare electrode (FIG. 22), IrOx(FIG. 23), Amorphous Ir / TiOx(FIG. 24), Mixed IrTiCh (FIG. 25), and Core-shell Ir@TiO2(FIG. 26).

[0036] FIG. 27 shows electrochemical surface area (ECSA) comparison of, from left to right: commercial IrOx, Amorphous Ir / TiOx, Mixed IrTiCh and Core-shell Ir@TiCh.

[0037] FIG. 28 shows ECSA-normalized catalytic activity of commercial IrOx, Amorphous Ir / TiOx, Mixed IrTiCh and Core-shell Ir@TiCh. The test was conducted at the potential range of 1.0 - 2.0 V vs. RHE with a scan rate of 10 mV s’1in N2-purged 0.5 M H2SO4 solution.

[0038] FIG. 29 shows Tafel slopes of commercial IrOx, Amorphous Ir / TiOx, Mixed IrTiCh and Core-shell Ir@TiCh with a low scan rate of 1 mV s’1to avoid the capacitive current.

[0039] FIG. 30 shows acidic OER activity of Core-shell Ir@TiO2 as a function of the heat treatment temperature (300, 400, 500 or 600 °C). A catalyst loading amount of 0.03 mgir cm'2was used for the test.

[0040] FIG. 31 shows extreme chronoamperometry of commercial IrOx, Amorphous Ir / TiOx, Mixed IrTiCh, Core-shell Ir@TiO2 and the bare electrode at 2.0 V vs. RHE for Ih

[0041] FIG. 32 shows chronoamperometry test results for Core-shell Ir@TiO2 as a function of Ir loading (0.1 mgir, 0.2 mgir or 0.4 mgir) at 2.0 V vs. RHE for 1.0 h in N2-purged 0.5 M H2SO4 solution.

[0042] FIG. 33 shows results from a sequential chronopotentiometry (CP) test using Coreshell Ir@TiO2 at the current densities of 200, 350, and 500 mA cm'2. The dark grey line is the smoothed value of each recorded potential.

[0043] FIG. 34 shows S-number calculated by ratio of the amount of generated oxygen to the amount of Ir dissolution measured by inductively coupled plasma optical emission spectroscopy (ICP-OES) after extreme chronoamperometry at 2.0 V vs. RHE for Ih for, from left to right: commercial IrOx, Amorphous Ir / TiOx, Mixed IrTiCh and Core-shell Ir@TiO2.

[0044] FIG. 35 shows Ir dissolution amount measured by ICP-OES after chronoamperometry test at 2.0 V vs. RHE for 1 h for, from left to right: commercial IrOx, Amorphous Ir / TiOx, Mixed IrTiO2 and Core-shell Ir@TiO2.

[0045] FIG. 36 shows long-term chronopotentiometry (CP) of Core-shell Ir@TiO2 at 10 mA cm'2for 600 h.

[0046] FIG. 37 shows a comparison graph of acidic OER performance in terms of catalytic activity and durability gathered from various reported literature (10 mA cm'2, 0.5 M H2SO4) in comparison to Core-shell Ir@TiO2 after 600 h.

[0047] FIG. 38 shows LS V curves with a scan rate of 10 mV s'1for Core-shell Ir@TiO2 after CP test for 40 h and 600 h, inset: electrochemical impedance spectroscopy (EIS) Nyquist plot.

[0048] FIG. 39 shows changes in electrochemical active surface area (ECSA) as a function of duration time of the CP test for Core-shell Ir@TiO2 at 10 mA cm'2in 0.5 M H2SO4 solution.

[0049] FIG. 40 shows a TEM image (upper image), a HAADF-STEM image (middle left image), and EDX mapping for Mix (Ir&Ti; middle right image), Ir (bottom leftimage), and O (bottom right image) of Core-shell Ir@TiO2 after the durability test for 600 h. All scale bars correspond to 50 nm.

[0050] FIG. 41 shows the TEM image of Core-shell Ir@TiO2 with Nation ionomer after durability test for 600 h of FIG. 40 with an inset showing SAED patterns.

[0051] FIG. 42 shows XRD patterns of Core-shell Ir@TiO2 after the CP test at 10 mA cm2for 600 h in 0.5 M H2SO4.

[0052] FIG. 43 shows HR-TEM images (upper) and EDX-mapping of oxygen of Core-shell Ir@TiO2 (lower) before (left) and after (right) the 600 h chronopotentiometry test at 10 mA cm'2. Arrows in lower images indicate the thickness of the outermost layer of Core-shell Ir@TiO2.

[0053] FIG. 44 shows HR-TEM images of Core-shell Ir@TiO2 before (upper) and after (lower) the 600 h chronopotentiometry test at 10 mA cm2, showing inverse fast Fourier transform (FFT) and d-spacing of the selected area. The steps of lattice spacing were measured by: cropping the selected area of interest, proposing a FFT analysis, masking a crystalline lattice, reproducing the inversed-FFT, and analysis of the plot profile.

[0054] FIGs. 45-46 show XPS spectra of Core-shell Irr / TiCh after the CP test at 10 mA cm'2for 600 h in 0.5 M H2SO4; FIG. 45 shows the O Is spectrum and FIG. 46 shows the Ir 4f spectrum.

[0055] FIG. 47 shows scanning electron microscopy (SEM) images of Core-shell Ir@TiO2 before (left) and after (right) chronopotentiometry test at 10 mA cm'2for 600 h. Scale bars correspond to 500 pm (upper), 50 pm (middle) and 5 pm (lower).DETAILED DESCRIPTIONI, Definitions

[0056] Unless otherwise indicated, the definitions and embodiments described in this and other sections are intended to be applicable to all embodiments and aspects of the present disclosure herein described for which they are suitable as would be understood by a person skilled in the art. It is also to be understood that the terminology used herein is for the purpose of describing particular aspects only and is not intended to be limiting.

[0057] In understanding the scope of the present disclosure, the term “comprising” and its derivatives, as used herein, are intended to be open ended terms that specify the presence of the stated features, elements, components, groups, integers, and / or steps, but do not exclude the presence of other unstated features, elements, components, groups, integers and / or steps. Theforegoing also applies to words having similar meanings such as the terms, “including”, “having” and their derivatives. The term “consisting” and its derivatives, as used herein, are intended to be closed terms that specify the presence of the stated features, elements, components, groups, integers, and / or steps, but exclude the presence of other unstated features, elements, components, groups, integers and / or steps. The term “consisting essentially of’, as used herein, is intended to specify the presence of the stated features, elements, components, groups, integers, and / or steps as well as those that do not materially affect the basic and novel characteristic(s) of features, elements, components, groups, integers, and / or steps.

[0058] Terms of degree such as “substantially”, “about” and “approximately” as used herein mean a reasonable amount of deviation of the modified term such that the end result is not significantly changed. These terms of degree should be construed as including a deviation of at least ±5% of the modified term if this deviation would not negate the meaning of the word it modifies. In addition, all ranges given herein include the end of the ranges and also any intermediate range points, whether explicitly stated or not.

[0059] As used in this disclosure, the singular forms “a”, “an” and “the” include plural references unless the content clearly dictates otherwise.

[0060] In embodiments comprising an “additional” or “second” component, the second component as used herein is different from the other components or first component. A “third” component is different from the other, first, and second components, and further enumerated or “additional” components are similarly different.

[0061] The term “and / or” as used herein means that the listed items are present, or used, individually or in combination. In effect, this term means that “at least one of’ or “one or more” of the listed items is used or present. The abbreviation, “e.g.” is derived from the Latin exempli gratia and is used herein to indicate a non-limiting example. Thus, the abbreviation “e.g.” is synonymous with the term “for example.” The word “or” is intended to include “and” unless the context clearly indicates otherwise.

[0062] It will be understood that any component defined herein as being included may be explicitly excluded by way of proviso or negative limitation, such as any specific compounds or method steps, whether implicitly or explicitly defined herein.

[0063] The term “halide” as used herein refers to a suitable halogen anion.

[0064] The term “suitable” as used herein means that the selection of the particular compound, material and / or conditions would depend on the specific manipulation to be performed, and / or the identity of the compound(s) and / or material(s) to be transformed, but the selection would be well within the skill of a person skilled in the art. All method steps described herein are to be conducted under conditions sufficient to provide the product shown.

[0065] The term “nanoparticle” as used herein, for example, in respect to a core-shell nanoparticle and / or a nanoparticle comprising TiOxand the like may refer to a particle in which all three dimensions are in the nanometer range. However, the term “nanoparticle” as used herein, for example, in respect to a core-shell nanoparticle and / or a nanoparticle comprising TiOxand the like may also include a particle in which one or more dimensions are outside of that range. For example, in some embodiments, the methods of preparing a core-shell nanoparticle of the present application may prepare core-shell particles that are completely in the form of particles in which all three dimensions are in the nanometer range but in alternative embodiments, the particles prepared by such methods may include minor amounts of particles having one or more dimensions outside of that range; e.g., such methods may prepare particles that consist essentially of or consist of particles in which all three dimensions are in the nanometer range. The person skilled in the art would readily appreciate that this may similarly apply to other embodiments of the present application, as appropriate. For example, the coating of a catalyst-coated electrode of the present disclosure may comprise particles that consist essentially of or consist of particles in which all three dimensions are in the nanometer range. The person skilled in the art having regard to the present disclosure would also readily appreciate that the term “nanometer range” as used herein may refer any suitable dimension(s) within the full nanometer range (i.e., from about 1 nm to less than 1000 nm) and is not limited to particles within the “nanoscale range”. The term “nanoscale range” as used herein refers to a dimension of from about 1 nm to about 100 nm. In an embodiment, the core-shell particles are “round-like in shape”; e.g., at least partially, optionally substantially spherical.II. Core-shell particles, methods for their preparation and uses thereof

[0066] The present disclosure includes a core-shell particle, comprising: a core comprising TiOx; and a shell comprising iridium.

[0067] In an embodiment, the core-shell particle is a core-shell nanoparticle.

[0068] In an embodiment, the core has an average diameter in the range of from about 50 nm to about 200 nm. In another embodiment, the core has an average diameter in the range of from about 70 nm to about 150 nm. In another embodiment, the core has an average diameter in the range of from about 82 nm to about 113 nm or about 80 nm to about 115 nm.

[0069] The average thickness of the shell can be any suitable thickness. For example, it will be appreciated by a person skilled in the art that in certain embodiments, such as uses of such a core-shell particle in proton exchange membrane water electrolysis (PEMWE), a suitable thickness of the shell is such that it provides suitable stability without increasing the iridium content to an undesirable level. It will also be appreciated by a person skilled in the art that the shells comprising iridium in the core-shell particles of the present disclosure are a continuous or at least substantially continuous layer comprising the iridium and are not, for example, in the form of discrete particles (e.g., nanoparticles) forming the layer. In an embodiment, the shell has an average thickness in the range of from about 8 nm to about 13 nm. In another embodiment, the shell has an average thickness in the range of from about 10 nm to about 12 nm. In another embodiment, the shell has an average thickness of about 11 nm.

[0070] In an embodiment, the core-shell particle consists essentially of: the core comprising TiOx; and the shell comprising iridium.

[0071] In an embodiment, the core-shell particle consists of: the core comprising TiOx; and the shell comprising iridium.

[0072] The core comprising TiOxcan comprise TiOxthat is in any suitable form or mixture thereof. In an embodiment, the TiOxcomprises TiCh. The TiCh can comprise, consist essentially of or consist of any suitable form of TiCh or mixtures thereof. In an embodiment, the TiCh comprises amorphous TiCh. In another embodiment, the TiCh comprises a crystalline form of TiCh. In another embodiment, the TiCh comprises a mixture of amorphous TiCh and crystalline TiCh. In an embodiment, the crystalline TiCh comprises anatase phase TiCh, rutile phase TiCh, brookite phase TiCh or mixtures thereof. In an embodiment, the TiCh comprises anatase phase TiCh. In another embodiment, the TiCh comprises anatase phase TiCh. In an embodiment, the X-ray diffraction (XRD) pattern of the core-shell particles shows peaks at about 25.3°, 37.8°, 38.6°, 48.0°, 53.9°, 55.0°, 62.7°, 70.3°, and 75.0° 20.

[0073] The shell comprising iridium can comprise iridium that is in any suitable form or mixture thereof. In an embodiment, the iridium comprises metallic iridium. In another embodiment, the iridium comprises crystalline metallic iridium. In an embodiment, the X- ray diffraction (XRD) pattern of the core-shell particles shows peaks at about 40.7°, 47.3°, 63.1°, 83.4°, and 88.0° 20.

[0074] The present disclosure also includes a method of preparing a core-shell particle comprising: a core comprising TiOx; and a shell comprising iridium, the method comprising: contacting a particle comprising TiOxwith an iridium salt and a reducing agent to prepare a particle comprising iridium and TiOx; and annealing the particle comprising iridium and TiOxin a reducing atmosphere to prepare the core-shell particle.

[0075] The contacting of the particle comprising the TiOxwith an iridium salt and a reducing agent can comprise any suitable conditions that could be readily selected by a person skilled in the art having regard to the present disclosure. For example, in some embodiments, the reducing agent comprises urea. However, the person skilled in the art would appreciate that other suitable reducing agents could be used and could readily select a suitable reducing agent having regard to the present disclosure. Similarly, in some embodiments, the iridium salt comprises an IrCh hydrate. However, the person skilled in the art would appreciate that other suitable iridium salts could be used and could readily select a suitable iridium salt having regard to the present disclosure. In an embodiment, the iridium salt comprises an iridium(III) halide and / or a hydrate thereof. In another embodiment, the iridium salt comprises an iridium(III) chloride and / or a hydrate thereof. In another embodiment, the iridium salt comprises IrCIs and / or a hydrate thereof. However, in an alternative embodiment, the iridium salt is devoid of a halide counteranion such as the chloride. In an embodiment, the contacting comprises heating the particle comprising TiOxin an aqueous environment that comprises the iridium salt and the reducing agent. In an embodiment, the heating is at a temperature in the range of from about 60 °C to about 100 °C. In another embodiment, the heating is at a temperature of about 80 °C. In an embodiment, the heating is for a time of about 15 hours to about 24 hours. In another embodiment, the heating is for a time of about 20 hours.

[0076] In an embodiment, the method further comprises drying the particle comprising iridium and TiOxprior to annealing. In an embodiment, the drying comprises heating. However, the person skilled in the art would appreciate that other suitable means and / or methods for drying could be used and could readily select a suitable means and / or method for drying having regard to the present disclosure.

[0077] In an embodiment, the annealing is of particles comprising iridium and TiOxin the form of a powder. In an embodiment, the powder is prepared by a method comprising grinding the dried particles comprising iridium and TiOx. However, the person skilled in the art would appreciate that other suitable means and / or methods could be used and could readily select a suitable means and / or method having regard to the present disclosure.

[0078] The annealing of the particle comprising iridium and TiOxin a reducing atmosphere to prepare the core-shell particle can comprise any suitable conditions that could be readily selected by a person skilled in the art having regard to the present disclosure. For example, in some embodiments, the reducing atmosphere comprises Rebalanced N2. However, the person skilled in the art would appreciate that other suitable reducing atmospheres could be used and could readily select a suitable reducing atmosphere having regard to the present disclosure. In an embodiment, the reducing atmosphere comprises hydrogen and an inert gas. In an embodiment, the inert gas is argon, nitrogen or combinations thereof. In another embodiment, the inert gas is nitrogen. In an embodiment, the hydrogen is present in an amount of from about 3 vol% to about 6 vol%, based on the total volume of the atmosphere. In another embodiment, the hydrogen is present in an amount of about 3 vol%, based on the total volume of the atmosphere. In an embodiment, the annealing is at a temperature in the range of about 400 °C to about 600 °C. In another embodiment, the annealing is at a temperature in the range of about 450 °C to about 550 °C. In an embodiment, the annealing is at a temperature of about 500 °C. In an embodiment, the annealing is for a time of about 1 hour to about 3 hours. In an embodiment, the annealing is for a time of about 2 hours.

[0079] The particle comprising TiOxcan be any suitable particle comprising TiOx, the selection of which could be readily made by a person skilled in the art having regard to the present disclosure. In an embodiment, the particle comprising TiOxis a nanoparticle comprising TiOx. In some embodiments, the method comprises use of TiOxparticles (e.g., TiOxnanoparticles) from a suitable commercial source. In an embodiment, the particle comprising TiOxis prepared by a method comprising polyol thermolysis. However, the person skilled inthe art would appreciate that other suitable methods could be used and could readily select a suitable method having regard to the present disclosure. In an embodiment, polyol thermolysis comprises use of a titanium dioxide precursor that is titanyl sulfate hydrate. However, the person skilled in the art would appreciate that other suitable titanium dioxide precursors could be used and could readily select a suitable titanium dioxide precursor having regard to the present disclosure. In an embodiment, the titanium dioxide precursor comprises (or is derived from) a suitable titanium halide (e.g., titanium(IV) chloride), a suitable titanium alkoxide (e.g., titanium(IV) isopropoxide or titanium(IV) butoxide), a titanyl sulfate and / or a hydrate thereof, or combinations thereof. A person skilled in the art would also appreciate that the conditions for preparing the particle comprising TiOxmay depend, for example, on the identity of the titanium dioxide precursor and / or the polyol and could readily select suitable conditions. In an embodiment, the method comprises dissolving a titanyl sulfate hydrate in a polyol in the presence of a suitable acid (e.g., sulfuric acid) under suitable conditions (e.g., while heating at a suitable first temperature such as a temperature of about 60 °C) then adding a suitable base (e.g., sodium hydroxide) under suitable conditions (e.g., if the method comprises heating at a suitable first temperature, cooling to a suitable second temperature e.g., a temperature of about 4 °C to about 40 °C or room temperature prior to addition of the base) until a desired pH (e.g., a pH of 11) is achieved then heating at a suitable third temperature (e.g., a temperature in a range of from about 100 °C to about 160 °C or about 125 °C) for a suitable time (e.g., a time of from about 2 hours to about 4 hours or about 3 hours) followed by cooling to a suitable fourth temperature e.g., a temperature of about 4 °C to about 40 °C or room temperature). In an embodiment, the polyol is ethylene glycol. However, the person skilled in the art would appreciate that other suitable polyols could be used and could readily select a suitable polyol having regard to the present disclosure.

[0080] In an embodiment, the method further comprises drying the particle comprising TiOxprior to contacting with the iridium salt and the reducing agent. In an embodiment, the drying comprises heating. However, the person skilled in the art would appreciate that other suitable means and / or methods for drying could be used and could readily select a suitable means and / or method for drying having regard to the present disclosure.

[0081] In an embodiment, the contacting with the iridium salt and the reducing agent is of particles comprising TiOxin the form of a powder. In an embodiment, the powder is prepared by a method comprising grinding a dried particle comprising TiOx. However, the person skilled inthe art would appreciate that other suitable means and / or methods could be used and could readily select a suitable means and / or method having regard to the present disclosure.

[0082] The present disclosure also includes a core-shell particle prepared by a method of preparing a core-shell particle as described herein.

[0083] The present disclosure also includes a use of a core-shell particle comprising a core comprising TiOxand a shell comprising iridium as described herein or prepared by a method of preparing a core-shell particle as described herein as a catalyst for an oxygen evolution reaction (OER). In an embodiment, the OER is acidic OER. In an embodiment, the OER is in a process comprising proton exchange membrane water electrolysis (PEMWE).

[0084] The present disclosure also includes an electrode comprising a core-shell particle comprising a core comprising TiOxand a shell comprising iridium as described herein or prepared by a method of preparing a core-shell particle as described herein. In an embodiment, the electrode comprises a catalyst-coated electrode. In another embodiment, the catalyst is present in a loading amount of about 0. 1 mgircm2

[0085] The present disclosure also includes a proton exchange membrane water electrolysis (PEMWE) cell comprising a core-shell particle comprising a core comprising TiOxand a shell comprising iridium as described herein or prepared by a method of preparing a core-shell particle as described herein or an electrode comprising such a core-shell particle as described herein.III. Additional Embodiments

[0086] The present disclosure also provides a core-shell catalyst, comprising: a titanium dioxide core; and an iridium shell coated on the core; wherein the core has a diameter ranging from about 70nm to about 150nm, and wherein the shell has a thickness ranging from about 8nm to about 13nm. In an embodiment, the titanium dioxide is a titanium dioxide nanoparticle. In another embodiment, the iridium has a crystalline structure. In another embodiment, the iridium is anon-nanoparticle iridium coating. The present disclosure also provides a method of preparing such a core-shell catalyst, comprising: synthesizing a titanium dioxide nanoparticle using titanium(IV) oxysulfate, sulfuric acid, ethylene glycol, and sodium hydroxide with a desired reaction time and reaction temperature; coating iridium on the titanium dioxide nanoparticle using the titanium nanoparticle, iridium(III) chloride hydrate, urea, and water with a desired reaction time and reaction temperature; and annealing the iridium-coated titanium dioxide with a desired time and temperature in a nitrogen atmosphere having about 3 vol%hydrogen to about 6% vol% hydrogen. In an embodiment, the reaction time of synthesizing titanium dioxide nanoparticle ranges from about 2 hours to about 4 hours. In another embodiment, the reaction temperature of synthesizing titanium dioxide nanoparticle ranges from about 100 °C to about 160 °C. In another embodiment, the reaction time of coating iridium ranges from about 15 hours to about 24 hours. In an embodiment, the reaction temperature of coating iridium ranges from about 60 °C to about 100 °C. In another embodiment, the time of annealing ranges from about 1 hour to about 3 hours. In another embodiment, the temperature of annealing ranges from about 400 °C to about 600 °C. The present disclosure also provides such a core-shell catalyst or a core-shell catalyst prepared by such a method for oxygen evolution reaction, hydrogen evolution reaction, or oxygen reduction reaction.

[0087] The following non-limiting examples are illustrative of the present disclosure:EXAMPLESI. Methods

[0088] Preparation ofTiC NPs for supporting metal: TiO2NPs were synthesized with a modified polyol method reported previously (Isaifan et al., 2017). In a typical preparation, 1.38 g of TiOSO4xH2O + H2SO4 (Thermo Fisher Scientific) was dissolved in 200 mL of ethylene glycol (99.8%, Sigma- Aldrich) at 60 °C under constant stirring using a three-neck flask in a heating mantle to make a transparent solution. After cooling the solution down to room temperature, 1.0 M NaOH (97%, Sigma-Aldrich) was added to the solution until pH 11 was achieved as indicated by an electronic pH meter. Subsequently, the solution was heated and refluxed at 125 °C (polyol method) for 3 h under constant stirring in the heating mantle. After cooling the solution to room temperature, the slurry was collected by centrifugation and thoroughly washed using Type I Ultrapure water (> 18.2 M -cm, Millipore) several times. The slurry was then collected via vacuum fdtration and dried overnight at 60 °C in an oven to provide a powdery sample. Finally, the synthesized powder was ground vigorously for 20 to 30 minutes using an agate mortar and pestle, resulting in a fine powder of TiO2NPs.

[0089] Preparation oflr-coated Ti€h NP: Urea reduction was used for the synthesis of Ir-coated TiO2NP (Gomez-Cortes et al., 2009). First, 313.5 mg of IrC13-xH2O (99.9%, Sigma- Aldrich) and 631 mg of Urea (Sigma- Aldrich) were added to 25 mL of Type I Ultrapure water (> 18.2 M -cm, Millipore). When the salt was fully dissolved, 250 mg of TiO2NPs prepared by the polyol method was added to the solution. The solution was thensealed and stirred in an oil bath at 80 °C for 20 h. During the stirring, the pH value of the solution changed from pH 1 - 2 to pH 7 - 8 after urea reduction. After the process, centrifugation and filtration were carried out repeatedly to remove any unreacted species. Subsequently, the gel-like sample was dried in an oven at 60 °C for 24 h. Using an agate mortar and pestle, the sample was ground to a fine powder and then annealed in a 3% H2 / balanced N2 atmosphere at 500 °C for 2 h in a tube furnace (which may be referred to herein as Core-shell Ir@TiO2). Meanwhile, the sample annealed in an N2 atmosphere under the same conditions may be referred to herein as Mixed IrTiCh. The non-annealed sample may be referred to herein as Amorphous Ir / TiOx. All synthesized catalysts contained 20 - 30 wt% Ir content, confirmed with inductively coupled plasma optical emission spectroscopy (ICP-OES; Varian Inc., Vista Pro CCD).

[0090] Physical characterization of synthesized catalysts: The morphology and structure of the synthesized samples were confirmed by transmission electron microscopy (TEM; Thermo Scientific, Talos F200X & Talos L 120C) and in-situ environmental TEM (Protochips) located at the Canadian Centre for Electron Microscopy (CCEM). Particle sizes were measured by ImageJ software. X-ray diffraction measurements (XRD; Cu Ka, Z = E5406 A) were conducted to probe the crystalline phase stmcture(s) of the synthesized materials at McMaster’s Analytical X-ray Diffraction Facility (MAX). For the near-surface compositions of the synthesized samples, X-ray photoelectron spectroscopy (XPS; Kratos, AXIS Supra, Al Ka radiation, hv = 1486.6 eV) was carried out at Surface Science Western (SSW). The fitting for chemical components was conducted after C Is (285.0 eV) calibration using CasaXPS software.

[0091] Electrochemical characterization of synthesized catalysts: The electrochemical catalytic performance of all synthesized materials was conducted using a catalyst-coated electrode (CCE) (Park et al., 2021). For comparison, commercial IrOxcatalysts were also purchased from Tanaka Precious Metals for electrochemical evaluation. Catalyst inks containing the synthesized (or commercial) catalyst, isopropyl alcohol (IP A, Sigma- Aldrich), and Nafion resin solution (5 wt%, Sigma-Aldrich) were prepared by ultrasonication in an ice bath for more than 20 min. The weight ratio between the ionomer and catalyst (I / C ratio) was 20%. The well-dispersed catalyst ink was coated onto platinized titanium felt (Fuel Cell Store) using an airbrush spray gun until a loading amount of 0.1 mgir cm'2was achieved based on the Ir mass fraction obtained from ICP-OES measurement. A CCE-connected electrode holder (exposed geometric surface area: 0.5 cm2), graphite rod,and Ag / AgCl served as working electrode, counter electrode, and reference electrode, respectively. The Ag / AgCl reference electrode was calibrated in ^-saturated 0.5 M H2SO4 versus a home-made reversible hydrogen electrode (RHE). Potentials herein are presented in terms of RHE according to the following relationship: E (vs. RHE) = E (vs. Ag / AgCl) + 0.256 (in 0.5 M H2SO4, FIG. 1). To confirm the catalytic performance of the prepared catalysts, linear sweep voltammetry (LSV) was evaluated at the potential range of 1.0 - 2.0 V vs. RHE with a scan rate of 10 mV s'1in N2-purged 0.5 M H2SO4 solution using the BioLogic EC-Lab software. Electrochemical impedance spectroscopy (EIS) was conducted at open circuit potential (OCP) at a frequency range from 2 MHz to 2 kHz. Based on EIS results, the high-frequency resistance (HFR) was collected to compensate for electrolyte resistance. For the Tafel plot, a low scan rate of 1 mV s'1was carried out to minimize the effect of capacitive current. Double-layer capacitance was calculated using cyclic voltammetry (CV) at various scan rates from 1 to 50 mV s'1to serve as an approximation for the electrochemically active surface area. To ascertain the rate of Ir dissolution under highly stressful acidic OER conditions, chronoamperometry (CA) was conducted at an extreme potential of 2.0 V vs. RHE for 1 h. Subsequently, the electrolyte was collected to quantitatively verify the extent of Ir dissolution using ICP-OES. A long-term stability test was conducted using chronopotentiometry (CP) at 10 mA cm'2for 600 h. The electrolyte was replaced every ca. 100 h to minimize the influence of changing H2SO4 concentration and pH fluctuations caused by evaporation or reactant consumption during the CP test.II. Results and Discussion

[0092] Referring to the exemplary synthesis 10 shown schematically in FIG. 2, Coreshell Ir@TiO2 particles were synthesized through a polyol method 14, a urea reduction 16, and an annealing process 18. The crystal structure of the as-prepared TiCh NPs, serving as the support material for Ir deposition, was identified as the anatase phase by XRD (FIG. 3). The combination of the Ir precursor with TiCh NP, coupled with the use of urea for surface coating, yielded an Ir / TiCh composite exhibiting an amorphous phase (Amorphous Ir / TiOx, FIG. 4). The EDX mapping images of the Amorphous Ir / TiOxin FIG. 4 show that Ir and TiO2 are in a disordered state; in other words, Ir and TiOxare randomly distributed throughout the entire particle, without any indication of order or crystallinity. Annealing the Amorphous Ir / TiOxin a reducing atmosphere (3% H2 / N2 balance) proved crucial in forming the final core-shell structured catalyst for acidic OER (Core-shell Ir@TiO2). It isnoteworthy that the core-shell structure did not emerge in a catalyst annealed at the same temperature of 500 °C in a pure N2 atmosphere (Mixed IrTiCh, FIG. 5). While not wishing to be limited by theory, under high-temperature conditions, hydrogen may facilitate the migration of Ir metal species from the amalgamated state of amorphous Ir / TiOxto the outer shell. The rationale behind this phenomenon is discussed in greater detail below. The morphology of Core-shell Ir@TiC>2 was characterized by TEM equipped with EDX (FIG. 6). High-angle annular dark field scanning transmission electron microscopy (HAADF - STEM) images revealed that most of the synthesized Core-shell Ir@TiO2 NP were roundlike in shape (FIG. 6; upper right image). Labels (1) and (2) in the upper images in FIG. 6 indicate smaller particles agglomerated onto a big main particle (FIG. 7 shows other TEM images of Core-shell Ir@TiO2). The size range of the TiCh core was between 70 - 150 nm, with an average core diameter of 97.2 ± 15.7 nm. The thickness of the Ir-shell of Core-shell Ir@TiC>2 was measured to be around 11 nm. The length and thickness were measured by ImageJ software. As shown in the middle and lower images of FIG. 6, the inner core (yellow in color images) and outer shell (blue in color images) represent Ti and Ir, respectively. EDX line profding was also in line with the characteristics of a core-shell structure (FIG. 7, lower right image). Specifically, a relatively higher and lower atomic fraction of Ir was detected in the exterior regions (shell) and interior (core) of the particle, respectively.

[0093] To investigate the mechanism by which the core-shell structure of this catalyst was formed, in-situ environmental TEM was performed in an H2 / Ar-mixed atmosphere while steadily increasing the sample temperature (FIG. 8). It was postulated that the difference in the use between 3% H2 / N2 balance for the catalyst synthesis and 5% H2 / Ar balance for the TEM analysis below 600 °C would have a negligible impact on the catalyst structure. While the temperature ramping rate for the in situ environmental TEM was quite high (5 °C s'1) compared to the ramping rate used in the thermal treatment of Core-shell Ir@TiO2 (5 °C min'1), the formation of an outer Ir shell was confirmed. Prior to the start of the temperature ramp, Ir and TiCh were in a random, disordered amorphous state (Amorphous Ir / TiOx) (FIG. 4, upper right image). As the temperature was increased under the H2 / Ar atmosphere, the oxygen weight ratio decreased (FIG. 9) and an Ir-rich shell was formed through surface segregation by diffusion of Ir from the bulk of the particle towards the exterior regions. The core-shell structure was fully formed at 500 °C (see FIG. 8 and schematic in FIG. 10). In addition, it was observed that the core-shell structure seemed to revert to a mixed state above a temperature of 500 °C (FIG. 8). Similar sequential processeswere reported by in-situ observation in previous studies (Gan et al., 2016; Chi et al., 2015; Dai et al., 2017), where core-shell structured nanoparticles were synthesized at a specific annealing temperature followed by structural collapse at a higher temperature.

[0094] To confirm the crystallinity of the prepared catalysts, XRD measurement was carried out alongside commercial IrOxfor comparison (FIG. 11). The observed peaks of the annealed samples (Core-shell Irrt.TiCh. Mixed IrTiCh) matched well with those of metallic Ir and anatase TiCh. Amorphous Ir / TiOxdisplayed only weak intensity anatase TiCh peaks at 25.3°, 48.0°, and 62.7°, corresponding to (101), (200), and (204) facets, respectively (Park et al., 2018). Along with TiCh peaks, the annealed samples showed four main metallic Ir peaks at 40.7°, 47.3°, 63.1°, and 83.4°, corresponding to (111), (200), (220), and (311) crystal planes, respectively (Chen et al., 2022). Core-shell Irrt.TiCh had an additional metallic Ir peak at 88.0°, corresponding to the (222) facet. Furthermore, as shown in the inset of the upper left image in FIG. 6, the SAED patterns of Core-shell Ir@TiCh were measured to be 1.84 A, 1.34 A, and 1.12 A, consistent with the anatase TiCh (200) and (220) planes and metallic Ir (311) planes detected by XRD, respectively. Ir oxide peaks were not detected by XRD, which is in accordance with the observation of no overlap between Ir and O in the shell of the core-shell structure from STEM-EDX mapping (middle left and bottom right images in FIG. 6).

[0095] The near-surface chemical compositions of the synthesized catalysts were analyzed by XPS (FIGs. 12-14, XPS survey spectra in FIG. 15). The annealing processes resulted in a slight increase in the relative iridium content, transitioning from Amorphous Ir / TiOx(1.41 at.%) to Mixed IrTiCh (3.07 at.%), and ultimately to Core-shell Ir@TiCh (4.19 at.%) as indicated by the XPS survey spectra (FIG. 15). While not wishing to be limited by theory, this increase can be attributed to the removal of nitrogen at around 400 eV, which likely originates from the unreacted urea. Therefore, the annealing process played a crucial role in purifying both Mixed IrTiCh and Core-shell Ir@TiCh of impurities remaining from synthesis.

[0096] To confirm the oxide transition on the near-surface of the prepared catalysts before and after the annealing process, we focused on O Is spectra (FIG. 12). In general, the O Is XPS spectra are deconvoluted into four peaks, which are metal-oxygen (M-O) peaks, oxygen vacancies / defects (Ov) peaks, surface hydroxide (-OH) peaks, and peaks corresponding to physisorbed / chemisorbed surface water (Park et al., 2021; Yu et al, 2020; Wu et al., 2017). However, Ovpeaks were omitted in this study due to controversy surrounding their interpretation, stemming from reports suggesting that Ovis rapidly oxidized under naturalconditions (Idriss, 2021; Frankcombe et al., 2023). As shown in FIG. 12 (hydroxide peaks indicated by *), the hydroxide peak in the O Is spectrum was notably decreased going from Amorphous Ir / TiOxto both Mixed IrTiCh and Core-shell Ir@TiO2, while not wishing to be limited by theory, suggesting that amorphous Ir hydroxides present in Amorphous Ir / TiOxwere reduced to metallic Ir° after the annealing process. This is corroborated by the presence of metallic Ir° peaks in the XRD pattern of the annealed samples (Mixed IrTiCh, Core-shell Ir@TiO2) and the Ir° content indicated by deconvolution of the Ir 4f spectrum (FIG. 14). Since most of the Ir existed as a metallic state in both Mixed IrTiCh and Core-shell Ir@TiO2, the peak of M-0 in the O Is spectrum was primarily associated with TiCh, showing the increasing area of M-0 and Ti4+following the order of Amorphous Ir / TiOx, Mixed IrTiO2, and Core-shell Ir@TiO2 (see the areas indicated with ** for M-0 in FIG. 12 and the areas indicated with * for Ti4+in FIG. 13, respectively). In FIG. 13, the formation of Ti3+in Ti 2p caused the peak shift of M-0 in O Is, particularly observed in Core-shell Ir@TiO2, which has a high amount of Ti3+(Jackman et al., 2015). As shown in FIG. 14, there were only Ir3and Ir4+peaks in Amorphous Ir / TiOx. However, the annealing process served to deoxidize Ir hydroxides while fostering their chemical integration with TiO2. Consequently, the combination of the heavy atomic mass of Ir and light TiO2 increased the binding energy (B.E) of Ti4+(Kim et al., 2021; Yoo et al., 2017). Therefore, the observed peak shift of Ti4+appears to correlate with the metallic state of Ir.

[0097] The OER activity of the prepared catalysts was evaluated in N2-saturated 0.5 M H2SO4 using a three-electrode system, with commercial IrOxalso tested as a benchmark. Prior to evaluating the performance of the synthesized catalyst, a comparison was conducted with other literature using the same benchmark IrOx(obtained from the same supplier of TKK) from a previous report (Tan et al., 2019), ensuring consistency and reliability (FIG. 16 and FIG. 17). We followed the identical experimental condition as previously performed by the report as follows. Catalyst ink: 5 mg catalyst, 3.75 mL IP A, 1.25 mL Milli-Q water, 20 pL Nafion 5 wt.% ionomer.; 10 pL drop cast on glassy carbon rotating disk electrode (GC RDE) (geometric area: 0.196 cm2) for working electrode; Pt wire for counter electrode; Ag / AgCl for reference electrode. The linear sweep voltammetry (LSV) test was conducted in 02-saturated 0.5 M H2SO4 from 0 - 1.6 V vs. RHE at 1600 rpm using a scan rate of 10 mV s'1. For iR-compensation (85%), high-frequency resistance was measured at 100 kHz with 20 mV amplitude at open-circuit potential. While variations in onset potential were noted owing to RE calibration differences (FIG. 16), the consistency in the slopes of the Tafel plot (FIG. 17) while not wishing to be limited by theory, suggests that both thebenchmark catalysts and the reported one operate via the same OER mechanism. Then, LSV measurements were collected and normalized by Ir loading amount as shown in FIG. 18. Among the synthesized catalysts and commercial IrOx, Core-shell Ir@TiO2 demonstrated a lower overpotential (rpo) of 305 mV at an OER mass activity of 100 A g'1(equivalent to a current density of 10 mA cm'2) than that of IrOx(T|IO = 338 mV), Mixed IrTiO2 (rpo = 674 mV), and Amorphous Ir / TiOx(T|IO = 612 mV) (FIG. 19), with all electrodes uniformly prepared to possess an identical Ir loading of 0.1 mgircm'2, suggesting that Core-shell Ir@TiO2 enhanced the catalytic OER activity in acidic condition.

[0098] To further understand the OER performance of Core-shell Ir@TiO2 as a function of Ir loading, the catalytic activity with different electrode loadings of Ir was evaluated (FIG. 20). Core-shell Ir@TiO2 with a loading amount of 0. 1 mgir cm2showed the highest mass-normalized activity at all potentials in the range investigated. This is especially prominent at the potential of 1.53 V versus RHE (corresponding to an overpotential of 300 mV) at which the 0.1 mgir cm2electrode significantly outperformed the 0.2 and 0.4 mgir cm2electrodes, achieving an Ir mass- normalized activity of 58 A g1compared to 32 A g1(0.2 mgir cm2) and 26 A g1(0.4 mgircm2). These results demonstrate that tuning the Ir loading has a significant impact on the measured acidic OER performance of the Core-shell Ir@TiO2 catalyst, whereby at electrode loadings greater than 0.1 mgircm2. lower performance was measured, while not wishing to be limited by theory, likely due to mass transport limitations leading to reduced catalyst utilization.

[0099] To further probe the OER performance of Core-shell Ir@TiO2, the catalytic activity was evaluated by several routes. Particularly, while measuring the electrochemical active surface area (ECSA) is a recommended method to compare intrinsic catalytic activities, finding reliable routes for ECSA estimation is challenging in the case of supported Ir-based catalysts. For example, hydrogen underpotential deposition (Hupd) or CO underpotential deposit! on / stripping (COupd) only applies to metallic Ir and not to Ir oxides (Alia et al, 2016; Arminio-Ravelo et al., 2020; Woods, 1974; Cherevko et al., 2016). Though mercury underpotential deposition (Hgupd) is one route to estimate the ESCA of both Ir metal and Ir oxides (Alia et al., 2016; Duran et al., 2021), the use of mercury in many jurisdictions is forbidden by regulations. To the best of our knowledge, double-layer capacitance (Cdi) is currently the only way to approximate the ECSA of different metal oxide catalysts and compare them relative to each other in the same electrolyte. Though Hupd and COupd are applicable towards Core-shell Ir@TiO2 for the estimation of ECSA due to the metallic Ir shell, for the sakeof consistency in comparisons of our synthesized materials, we have chosen to rely on the doublelayer capacitance as a proxy for the ECSA. As shown in FIG. 21, Core-shell Ir@TiO2 exhibited a significantly higher Cai of 25.0 mF cm'2compared to commercial IrOx(10.9 mF cm'2), Amorphous Ir / TiOx(3.32 mF cm'2), and Mixed IrTiCh (2.85 mF cm'2) based on CV at different scan rates (FIGs. 22-26). Considering that the bare electrode showed almost negligible capacitance of 0.88 mF cm'2, the measured Cai values could mainly be attributable to the coated catalyst layer. While not wishing to be limited by theory, the elevated Cai signifies that Coreshell Ir@TiO2 may offer a larger ECSA (FIG. 27), facilitating the exposure of additional active sites, which is essential for high acidic OER activity. In addition, ECSA-normalized LSV curves were also in accordance with the mass activity trend of Core-shell Ir@TiO2 > commercial IrOx» Amorphous Ir / TiOx> Mixed IrTiCh (FIG. 28), while not wishing to be limited by theory, suggesting that the Ir-rich surface on TiCh NP boosts the acidic OER activity.

[0100] Further investigations were conducted to ascertain the origin of the significantly higher catalytic activity displayed by Core-shell Ir@TiO2 compared to Amorphous Ir / TiOxand Mixed IrTiO2. It was initially hypothesized that the catalytic activity would be unaffected by the presence or absence of metallic iridium or variations in the crystallinity of the catalyst. This assumption was based on the observation that both commercial IrOxand Amorphous Ir / TiOxexhibited an amorphous phase in the XRD analysis (FIG. 11). However, only commercial IrOxdemonstrated high acidic OER activity. Moreover, despite the presence of the metallic Ir peak (Ir°) in XPS results in both Core-shell Ir@TiO2 and Mixed IrTiO2 (FIG. 14), Mixed IrTiO2 showed minimal OER activity. This observation indicated neither the crystallinity nor the presence of metallic Ir in the catalyst were crucial factors for OER activity.

[0101] The focus turned to the Tafel plots of two active catalysts: Core-shell Ir@TiO2 and commercial IrOx. Remarkably, both catalysts demonstrated closely aligned Tafel slopes (FIG. 29), each hovering around the range of 56 - 60 mV dec'1. This strongly implies a shared mechanism governing the OER across both materials, underscoring their comparable efficacy in facilitating the acidic OER. Various theoretical tools, including Bockris and Reddy’s equation (Eq. 1) and the Tafel slope equation derived from the Butler-Volmer equation (Eq. 2), as well as a comprehensive assessment of mechanistic scenarios (Table 1) (Guidelli et al., 2014; Reier et al., 2017; Bockris et al., 1956; Damjanovic et al., 1966) were leveraged to further analyze and interpret the acidic OER activity of these materials. / tzs a = - 1- nrft Eq. 1 v dE 2.303 X RT- = - IEq. 23 (logy) aFL

[0102] In equation 1, a represents the transfer coefficient, nsdenotes the number of electrons released before the rate-determining step (rds), v indicates the frequency of the rds reaction occurrence, nrsignifies the number of electrons involved in the rds, and is a symmetric factor typically assumed to be 0.5.Table 1. The cases of the expected reaction mechanism for OER with transfer coefficient and the calculated slope based on variables of Bockris and Reddy’s equation.Case 2 Electrochemical oxide path nsv nrP a ^^V dec^?2-1) M* + H2O -> M*- 0H + H++ e“ 0 1 1 0.5 0.5 118.32-2) M* - OH M - OH 1 1 0 0.5 1 59.22-3) M- OH -> M*- 0 + H++ e~ 1 1 1 0.5 1.5 39.42-4) 2M*- O -> 2M* + O t 4 1 0 0.5 4 14.8Case 3 Electrochemical metal peroxide path nsv nra3-1) M* + H20 M'- OH + H++ e 0 1 1 0.5 0.5 118.33-2) 2M'- 0H M'- O + M' + H2O 2 1 0 0.5 2 29.63-3) M*- O + H2O -> M*- OOH + H++ e~ 2 1 1 0.5 2.5 23.73-4) 2M*- 00H -> M*- 0 + M* + H2O + O2t 4 1 0 0.5 4 14.8

[0103] Despite uncertainties regarding its precise mechanistic implications in complex multielectron reactions, the theoretical Tafel slope of Case 2-2 (a = 1, 59.2 mV dec'1), among the various OER paths enumerated in Table 1, closely resembles that of both commercial IrOx and Core-shell Ir@TiO2. Such a value of the Tafel slope has conventionally been linked to a rapid electrochemical equilibrium reaction followed by a non-faradaic, chemically rate-determining step, as previously reported in iridium-based acidic OER catalysts (Hu et al., 2004; Diaz-Morales et al., 2016; Oh et al, 2015). Consequently, both Core-shell Ir@TiO2 and commercial IrOx, allegedly following the electrochemical oxide path, were active toward the acidic OER.

[0104] Structural differences between Core-shell Ir@TiO2 and commercial IrOx(active in OER) and Amorphous Ir / TiOx-and Mixed IrTiO2 (inactive in OER) may be another possible reason behind the observed significant activity differences between thesematerials towards the acidic OER. However, from the Ir mass-normalized activity (FIG. 18) and ECSA-normalized activity plots (FIG. 28), it is evident that the presence of Ir in both Mixed IrTiCh and Amorphous Ir / TiOxhas minimal impact on their acidic OER activity. Notably, both catalysts share a disordered structure of Ir and TiO2 (upper right images in FIG. 4 and FIG. 5). On the other hand, Ir in Core-shell Ir@TiO2 was clustered on the TiO2 NP as the shell structure, and commercial IrOxis composed of the sole component of Ir (oxides). Furthermore, Core-shell Ir@TiO2 formation was exclusively observed at the annealing temperature of 500 °C during in situ environmental TEM analysis (FIG. 8), coupled with its superior activity compared to other materials annealed at varying temperatures (FIG. 30). Therefore, this distinction highlights the pivotal significance of non-mixed Ir with TiCh in enabling efficient catalysis for acidic OER.

[0105] Notably, the enduring stability of a catalyst stands as a crucial factor in evaluating long term acidic OER performance, and is essential for its viability in industrial applications. As shown in FIG. 31, we carried out chronoamperometry (CA) at a high applied potential of 2.0 V vs. RHE (an / / ^-corrected potential of around 1.75 V vs. RHE) to evaluate the dissolution resistance of the synthesized catalysts under aggressive acidic OER conditions more reminiscent of the anodic operating conditions in practical PEMWEs. At this electrode potential, Core-shell Ir@TiO2 exhibited a stable current density of ca. 200 mA cm'2, which is highest among our prepared catalysts, with measured current densities increasing with increasing Ir loading amount (FIG. 32). To further evaluate the performance of Core-shell Ir@TiO2, chronopotentiometry (CP) measurements at current densities ranging from 200 to 500 mA cm2were conducted (FIG. 33). At an applied current density of 200 mA cm2, the potential initially increased slightly from 1.9 to 1.95 V versus RHE during the first hour. These values are consistent with the CA results, whereby the current density of ca. 200 mA cm2was recorded at 2.0 V versus RHE. Notably, the electrode potential did not significantly change at higher current densities, stabilizing -2.08 and 2.2 V versus RHE at the current densities of 350 and 500 mA cm2respectively. This stability under elevated current densities highlights the durability of the synthesized Core-shell Ir@TiC>2, reinforcing its potential as a durable catalyst for acidic OER applications. An important factor in this context is the possible dissolution of the active material under high applied anodic potential and acidic conditions. Most metals cannot tolerate such conditions; thus, metal dissolution occurs. Therefore, a lower amount of dissolved Ir in the electrolyte after the CA test compared to other prepared catalysts would indicate the ability of Core-shell Ir@TiO2 to retain its active material even under severe conditions. FIG. 34 displays the stability number (S-number), which is defined by the ratio of the amount of evolved oxygen (n02) to the amount of dissolved Ir (n / r) (Geiger et al., 2018). All S-numbers were calculated with the assumption of 100% Faradaic efficiency towards the acidic OER. Ir dissolution was quantitatively measured using ICP-OES (FIG. 35). Core-shell Ir@TiO2 demonstrated a higher S-number (3.34*106) than that of commercial IrOx(1.02*106), Mixed IrTiCh (I.27*105), and Amorphous Ir / TiOx(I.72*103). Furthermore, Core-shell Ir@TiO2 exhibited the lowest concentration of dissolved Ir ions in the electrolyte, measured at 0.054 ppm, in comparison to commercial IrOx(0.068 ppm), Amorphous Ir / TiOx(0.73 ppm) and Mixed IrTiCh (0.074 ppm). These values correspond to Ir dissolution rates of 0.11%, 0.14%, 1.46%, and 0.15% per hour (@ 2.0 V vs. RHE), respectively. As the same amount of Ir was coated on each electrode substrate, this result demonstrated the superior dissolution resistance of Core-shell Ir@TiO2. The total Ir dissolution rate of Core-shell Ir@TiC>2 was 0.1% after a 40 h CP test at 10 mA cm2(corresponding to 0.0025% per hour).

[0106] For a longer-term durability test, chronopotentiometry (CP) was performed on Core-shell Ir@TiO2 at a current density of 10 mA cm'2for 600 h (FIG. 36). During the test, the potential exhibited a near imperceptible increase of 0.1 mV per hour, which aligns with the superior dissolution resistance of Ir. Though Core-shell Ir@TiC>2 displays a relatively higher overpotential at a current density of 10 mA cm'2compared to previously reported catalysts, its stability far surpasses these previously reported catalysts including Ir-based catalysts for acidic OER (FIG. 37, Table 2). (Li et al., 2021b; Yan et al., 2018; Fan et al., 2023a; Cheng et al., 2019; Gou et al., 2022; Xu et al, 2021; Yu et al., 2020; Wang et al., 2023; Li et al., 2018; Tackett et al., 2018; Islam et al., 2020; Gou et al., 2019; Yang et al., 2016; Seitz et al.; Tian et al., 2020; Fan et al., 2023b; Feng et al., 2019)Table 2. Comparison table of acidic OER catalysts with overpotential and duration time of chronopotentiometry from several recent literature reports.

[0107] The reason for the moderate overpotential is that we employed the trade-off strategy between active but unstable Ir and inactive but stable TiCh in acidic OER. Notably, as shown in FIG. 38, the activity of Core-shell IrT / TiCh increased after 40 h and 600 h of the CP test. This is because the catalyst and Nafion ionomer became increasingly more exposed and accessible to the electrolyte during the test, leading to a greater number of active sites as well as a net increase in ionic conductivity (Zaman et al., 2019; Seitz et al.; Lebedev et al., 2017). The decrease in HFR shown in the inset of FIG. 38 supported this hypothesis. The increase in ECSA shown in FIG. 39 also supports this hypothesis. Furthermore, TEM was performed on Core-shell Ir@TiO2 to investigate morphological changes following the long-term CP test for 600 h. As shown in FIG. 40, the core-shell morphology and structure of Core-shell Ir@TiO2 is retained and seems largely unchanged despite enduring a 600 h CP test under harsh acidic OER conditions. Further, the SAED patterns in FIG. 41 indicates that there was no substantial oxidation of Ir as a result of the 600 hCP test, consistent with the results of XRD, TEM, and XPS characterization performed after the 600 h CP test (FIGs. 42-46). The bulk morphological stability of Core-shell Ir@TiO2 was further confirmed by SEM (FIG. 47). To evaluate thecrystalline and morphological stability of Core-shell Ir@TiO2, various post-analyses, including XRD (FIG. 42), HR-TEM (FIGs. 43-44), XPS (FIGs. 45-46), and SEM (FIG 47) after the 600 h CP test at 10 mA cm'2were conducted. As shown in the XRD patterns in FIG. 42, diffraction peaks corresponding to metallic Ir and anatase TiCh were still detected without any shift or change compared to the XRD peaks of as-prepared Core-shell Ir@TiO2 before the 600 h CP test. Additionally, HR-TEM was utilized to confirm changes in the crystallinity of Core-shell Ir@TiO2 (FIG. 43, upper images). A clear interface between the Ir Shell and TiCh Core was observed. The d-spacing results for metallic Ir, including the (1 1 1) and (2 0 0) planes, and anatase TiCh, including the (1 0 1) and (0 0 4) planes, were measured in the Ir Shell and TiCh Core, respectively. Furthermore, metallic Ir was still observed in the Ir Shell (Ir (1 1 1); 0.22 nm, and Ir (2 0 0); 0.19 nm) after the 600 h CP test. Meanwhile, the d-spacing for IrCh was not detected in HR-TEM, consistent with the XRD results. Moreover, STEM-EDX mapping shows no overlap between Ir and O in the shell of the core-shell structure (FIG. 43, lower images). As shown in the XPS results (FIG. 45), the intensity of O Is corresponding to near- surface water is stronger than that of the as-prepared Core-Shell Ir@TiO2, while not wishing to be limited by theory, likely due to the physical adsorption of water residue onto the catalyst surface after the 600 h CP test. However, the metallic Ir near the catalyst surface retained its chemical composition without peak shift, intensity change, or further oxidation (FIG. 46). Ultimately, the synthesized Core-shell Ir@TiO2 shows significant crystalline and morphological stability under prolonged acidic OER reaction conditions.

[0108] While the present disclosure has been described with reference to examples, it is to be understood that the scope of the claims should not be limited by the embodiments set forth in the examples, but should be given the broadest interpretation consistent with the description as a whole.

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Claims

CLAIMS:

1. A core-shell particle, comprising: a core comprising TiOx; and a shell comprising iridium.

2. The core-shell particle of claim 1, that is a core-shell nanoparticle.

3. The core-shell particle of claim 1 or 2, wherein the core has an average diameter in the range of from about 70 nm to about 150 nm.

4. The core-shell particle of any one of claims 1 to 3, wherein the shell has an average thickness in the range of from about 8 nm to about 13 nm.

5. The core-shell particle of any one of claims 1 to 4, wherein the core-shell particle consists of: the core comprising TiOx; and the shell comprising iridium.

6. The core-shell particle of any one of claims 1 to 5, wherein the TiOxcomprises TiCh.

7. The core-shell particle of any one of claims 1 to 6, wherein the TiOxcomprises anatase phase TiCh.

8. The core-shell particle of any one of claims 1 to 7, wherein the iridium comprises metallic iridium.

9. The core-shell particle of claim 8, wherein the iridium comprises crystalline metallic iridium.

10. A method of preparing a core-shell particle comprising: a core comprising TiOx; and a shell comprising iridium, the method comprising:contacting a particle comprising TiOxwith an iridium salt and a reducing agent to prepare a particle comprising iridium and TiOx; and annealing the particle comprising iridium and TiOxin a reducing atmosphere to prepare the core-shell particle.

11. The method of claim 10, wherein the reducing agent comprises urea.

12. The method of claim 10 or 11, wherein the iridium salt comprises an iridium(III) chloride and / or a hydrate thereof.

13. The method of claim 12, wherein the iridium(III) chloride is an IrCh hydrate.

14. The method of any one of claims 10 to 13, wherein the contacting comprises heating the particle comprising TiOxin an aqueous environment that comprises the iridium salt and the reducing agent.

15. The method of any one of claims 10 to 14, wherein the method further comprises drying the particle comprising iridium and TiOxprior to annealing.

16. The method of any one of claims 10 to 15, wherein the annealing is of particles comprising iridium and TiOxin the form of a powder.

17. The method of any one of claims 10 to 16, wherein the reducing atmosphere comprises hydrogen and an inert gas.

18. The method of claim 17, wherein the reducing atmosphere is fh / balanced N2.

19. The method of claim 17 or 18, wherein the hydrogen is present in an amount of from about 3 vol% to about 6 vol%, based on the total volume of the atmosphere.

20. The method of any one of claims 10 to 19, wherein the annealing is at a temperature of about 500°C.

21. The method of any one of claims 10 to 20, wherein the TiOxparticles are prepared by a method comprising polyol thermolysis.

22. The method of claim 21, wherein the polyol thermolysis comprises use of a titanium dioxide precursor that is titanyl sulfate hydrate.

23. The method of claim 21 or 22, wherein the polyol is ethylene glycol.

24. The method of any one of claims 10 to 23, wherein the particle comprising TiOxis a nanoparticle comprising TiOx.

25. A core-shell nanoparticle prepared by a method of any one of claims 10 to 24.

26. A use of a core-shell particle of any one of claims 1 to 9 or 25 as a catalyst for an oxygen evolution reaction (OER).

27. The use of claim 26, wherein the OER is acidic OER.

28. The use of claim 26 or 27, wherein the OER is in a process comprising proton exchange membrane water electrolysis (PEMWE).

29. An electrode comprising a core-shell particle of any one of claims 1 to 9 or 25.

30. The electrode of claim 29, wherein the electrode comprises a catalyst-coated electrode.

31. The electrode of claim 30, wherein the catalyst is present in a loading amount of about 0.1 mgir cm232. A proton exchange membrane water electrolysis (PEMWE) cell comprising a core-shell particle of any one of claims 1 to 9 or 25 or an electrode of any one of claims 29 to 31.

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