Composite for electrochemical catalyst and method for preparing the same

Abstract

To provide a method for preparing a composite material, particularly a method for preparing a composite material useful as a catalyst in electrolytic hydrogen evolution reaction and / or oxygen evolution reaction and / or urea oxidation assisted water electrolysis.SOLUTION: A method for producing a composite material, the method comprising the steps of: (i) electrochemical deposition on a substrate from a deposition solution comprising a nickel (II) salt and graphene oxide; Obtaining a nickel-reduced graphene oxide composite material comprising nickel dispersed on reduced graphene oxide, the composite material being deposited on a substrate; (iii) after step (i), placing the substrate on which the composite material is deposited in an alkaline solution together with a counter electrode; (iv) after step (iii), partially electrochemically oxidizing nickel to obtain a partially oxidized nickel-reduced graphene oxide composite material comprising partially oxidized nickel dispersed on reduced graphene oxide.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a method for preparing a composite material, particularly a composite material useful as a catalyst in water electrolysis. [Background technology]

[0002] Hydrogen is considered the fuel of the future, but its widespread use is fraught with problems. In addition to transportation and storage, the main problem is the production of hydrogen, as pure hydrogen does not exist in molecular form.

[0003] Hydrogen produced by water electrolysis is the most attractive potential fuel for hydrogen fuel cell-based systems. Hydrogen produced in this manner is highly pure, but it is the most expensive of all types of hydrogen currently produced. This is due to the low efficiency of electrochemical catalysts for hydrogen production, which are selected based on their price-efficiency ratio. Platinum is the most efficient catalyst for producing hydrogen, but its high price and limited reserves limit its availability for industrial hydrogen production. Alkaline electrolysis is the primary method of hydrogen production, and nickel-based materials are the most common type of catalyst used due to their low cost compared to platinum and platinum group metal-based catalysts.

[0004] A problem with nickel-based catalysts is the gradual decline in activity due to the formation of nickel hydride during prolonged electrolysis. This leads to the poisoning of Ni active sites and reduces the activity of the catalyst. As one approach to address this issue, Bouzek et al. (Phys. Chem. Chem. Phys., 2015, 17, 26864-26874) described a Ni electrode modified with reduced graphene oxide (rGO), prepared by electrolytic deposition of graphene oxide on a Ni foam substrate. Hydrogen atoms adsorbed on the Ni foam can spill over onto the rGO. Thus, the rGO acts as a hydrogen atom trap, continuously cleaning the Ni active sites on the foam and providing an alternative pathway for hydrogen production. The rGO-modified Ni foam cathode achieved a current of 223 mA·cm at a cell voltage of 1.85 V. -2 The current density was shown.

[0005] The second problem is that HER in alkaline media is significantly slower than in acidic media. This is due to the slow rate of water dissociation, the first step in the HER mechanism, in alkaline solutions. A high overpotential is required to initiate catalysis in alkaline media. As one approach to address this problem, Markovic et al. (Nature Materials 11, 550-557 (2012)) proposed the use of Ni 2+ described the formation of an oxidized surface phase of , which accelerates the dissociation of water in alkaline media.

[0006] Many approaches have been made to the preparation of materials for electrolytic hydrogen production under alkaline electrolysis conditions.

[0007] CN105576216A describes the preparation of an α-nickel sulfide / graphene composite and its application as an electrochemical hydrogen evolution catalyst. The composite is prepared by hydrothermal synthesis and deposition onto a glassy carbon electrode. The catalytic activity of the demonstrated composite is up to 40 mA / cm at -1.3 V vs. saturated calomel electrode (SCE, recalculated from the corresponding literature). 2 reaches.

[0008] CN108588754A describes the preparation of nickel molybdate / graphene composites for electrochemical catalysis by hydrothermal synthesis. The catalytic activity of the composites demonstrated was up to 80 mA / cm at -1.3 V vs. SCE. 2 reaches.

[0009] CN109898093B describes the preparation of rGO / CoWO4 / Co3O4 supported nickel foam composite hydrogen evolution electrodes by hydrothermal synthesis. The catalytic activity of the electrode demonstrated was 100 mA / cm at -1.2 V vs. SCE. 2 reaches.

[0010] CN109876833A describes the preparation of a nickel oxide-supported sulfur- and phosphorus-doped graphene composite electrochemical catalyst by hydrothermal synthesis. The catalytic activity of the composite demonstrated was 10 mA / cm at -1.6 V vs. SCE. 2 It was less than.

[0011] CN106087002A describes the preparation of three-dimensional Ni / rGO composites by hypergravity electrolytic deposition on nickel foam in a strong gravity field of 350 g for 10-100 minutes. The catalytic activity of the composites demonstrated was up to 100 mA / cm at -1.2 to -1.3 V vs. SCE. 2 reaches.

[0012] CN110876946A describes the preparation of a hydrogen-evolving composite material consisting of MoS2-rGO-NiO on nickel foam. The preparation is carried out by multiple application of a solution onto the nickel foam along with heat treatment. The catalytic activity demonstrated is up to 35 mA / cm at -1.2 to -1.3 V vs. SCE. 2 reached.

[0013] CN106967986B describes the preparation of a Ni(OH)2 / Ni / reduced graphene oxide (rGO) composite hydrogen evolution electrode. The preparation involves pretreatment of a nickel foam substrate, preparation of a Ni / rGO composite on the nickel foam substrate by ultra-gravity electrolytic deposition for 60 minutes, and preparation of the Ni(OH)2 / Ni / rGO composite by hydrothermal synthesis for 1-12 hours. The Ni(OH)2 / Ni / rGO composite thus obtained has a tertiary structure: the primary structure comprises the nickel foam substrate, the secondary structure comprises graphene sheets carrying nickel nanoparticles, and the tertiary structure comprises Ni(OH)2 nanosheets. The catalytic activity of the electrode demonstrated was up to 120 mA / cm at -1.18 V vs. the reversible hydrogen electrode (RHE). 2 reached.

[0014] One of the main problems with the above-mentioned methods is the complex synthesis requiring multiple demanding preparative steps that are difficult to scale for industrial applications.

[0015] This problem is particularly acute when powders are obtained during synthesis, which then must be mechanically fixed to the electrodes used in the electrolytic process, and the synthesis of the material itself can take tens of minutes to hours.

[0016] Another factor contributing to the efficiency of an electrolyzer (an electrolytic cell for hydrogen and / or oxygen production) is the performance of the anode where the oxygen evolution reaction (OER) occurs. The OER at the anode is coupled to the HER at the cathode, and the slow kinetics of the OER result in high overpotentials, significantly affecting the overall energy efficiency of the electrolyzer. Precious metal catalysts such as iridium and ruthenium have been investigated as benchmark anode catalysts, but they are expensive and naturally scarce. Non-precious metal-based catalysts (NPMCs), such as non-precious transition metal oxides / (oxy)hydroxides, metal-free carbon materials, and hybrid composites of non-precious metals and carbon, have also been investigated, but these materials often exhibit poor activity and stability.

[0017] In some technologies, such as urea oxidation-assisted water electrolysis, other substances (e.g., urea) are added to the electrolyte, which can be oxidized at the anode at a potential lower than that corresponding to the decomposition of water. This allows electrolysis to occur at a lower potential while producing H2 at the cathode, reducing the required energy input. Due to their low cost, easy structural control, good compatibility, and easy active phase formation, research has been conducted into the use of Ni-based anode catalysts for urea oxidation-assisted water electrolysis. However, monometallic Ni-based catalysts have low intrinsic activity, low stability, and poor poisoning resistance.

[0018] Therefore, finding a method to rapidly synthesize efficient catalysts and electrodes for alkaline water electrolysis and urea oxidation-assisted water electrolysis is of great importance for hydrogen-based technologies.

[0019] The present invention has been made in view of the above points. Summary of the Invention

[0020] The present inventors have developed a method for preparing a composite material useful as a catalyst for hydrogen evolution reactions and / or oxygen evolution reactions, and / or urea oxidation-assisted water electrolysis, the material itself, a method for electrolytic hydrogen production and / or electrolytic oxygen production under alkaline electrolysis conditions using the composite material or an electrode thereof, and a method for urea oxidation under urea oxidation-assisted water electrolysis conditions using the composite material or an electrode thereof.

[0021] In its broadest aspect, the composite material of the present invention comprises partially oxidized nickel particles dispersed on reduced graphene oxide flakes.

[0022] The composite material has a unique microstructure obtained by electrochemical deposition onto a substrate from a solution containing nickel salt and graphene oxide, followed by partial electrochemical oxidation of the deposited material.

[0023] The composite material can be used for industrial hydrogen production in industrial alkaline electrolysis processes, implemented in hydrogen generators of different capacities (e.g., for laboratory applications), or used in combination with photovoltaic cells to produce hydrogen from renewable energy sources.

[0024] The composite material can be used as a catalyst for electrolytic hydrogen production under alkaline water electrolysis conditions.The composite material can be used as a catalyst for electrolytic oxygen production under alkaline water electrolysis conditions.The composite material can be used as a catalyst for electrolytic urea oxidation under urea oxidation-assisted alkaline electrolysis conditions.

[0025] In particular, the present invention allows for the rapid synthesis of catalytic cathode and / or anode materials based on a composite of partially oxidized nickel particles dispersed on reduced graphene oxide.

[0026] When used at the cathode, the composite material of the present invention exhibits high catalytic activity for electrolytic hydrogen production under alkaline water electrolysis conditions (e.g., up to 500 mA cm at −0.35 V vs. RHE at room temperature). -2 of hydrogen evolution current).

[0027] When used as an anode, the composite material of the present invention exhibits high catalytic activity for electrolytic oxygen production under alkaline water electrolysis conditions (e.g., up to 200 mA cm at 1.6 V vs. RHE). -2 The oxygen evolution current is shown.

[0028] When used as an anode, the composite material of the present invention exhibits high catalytic activity for electrolytic urea oxidation under urea oxidation-assisted water electrolysis conditions (e.g., up to 325 mA cm at 1.9 V vs. RHE). -2 urea oxidation current).

[0029] High activity is exhibited even when the substrate (on which the composite is deposited) contains no nickel or at most trace amounts of nickel. Thus, the electrodes of the present invention (i.e., the combination of the composite and the substrate) exhibit higher catalytic activity per unit mass of nickel compared to conventional electrodes based on nickel foam.

[0030] In the method of the present invention, the composite material is obtained directly on the electrode material and can be directly transferred to an electrolysis cell (electrolyzer) for hydrogen production.

[0031] Synthesis is Ni 2+ This is done by electrochemical deposition from a solution containing a source of graphene oxide and dispersed graphene oxide, followed by partial electrochemical oxidation of the nickel surface.

[0032] Thus, in a first aspect of the present invention there is provided a method for preparing a composite material, said method comprising the steps of: (i) electrochemically depositing a material onto a substrate from a deposition solution comprising a nickel(II) salt and graphene oxide to obtain a nickel-reduced graphene oxide composite material comprising nickel dispersed on reduced graphene oxide, said composite material being deposited on the substrate; (ii) after step (i), placing the substrate on which the nickel-reduced graphene oxide composite material has been deposited together with a counter electrode in an alkaline solution; and (iii) after step (ii), partially electrochemically oxidizing the nickel to obtain a partially oxidized nickel-reduced graphene oxide composite comprising partially oxidized nickel dispersed on reduced graphene oxide, the composite being deposited on the substrate.

[0033] According to this method, nickel and graphene oxide are electrochemically deposited directly on a substrate in step (i), making it possible to obtain a catalytic composite directly on an electrode (i.e., the substrate material is preferably one that can be used as an electrode, i.e., is preferably electrically conductive). Therefore, according to the present invention, the step of transferring the catalytic composite to an electrode material can be eliminated. Furthermore, unlike known methods that require several tens of minutes to several hours to prepare a material, in this method, the total duration of the electrochemical deposition in step (i) and the partial electrochemical oxidation in step (iii) can be as short as 120 seconds, e.g., less than 10 minutes but more than 2 minutes, thereby significantly shortening the preparation of a catalytic electrode.

[0034] In step (i), Ni 2+ and graphene oxide is deposited on the substrate and reduced during electrochemical deposition. Thus, the composite material obtained in step (iii) comprises nickel, e.g., in the form of particles, dispersed on reduced graphene oxide (rGO), e.g., in the form of flakes.

[0035] Without wishing to be bound by theory, it is believed that when the composite material is used as a catalyst at the cathode during the hydrogen evolution reaction (HER), H atoms adsorbed on Ni active sites due to water dissociation at the interface between the oxidized and non-oxidized portions of the Ni surface during alkaline water electrolysis may spill over onto reduced graphene oxide (rGO), which acts as a hydrogen atom trap. This provides the free Ni active sites necessary for HER to proceed. Furthermore, the spillover of H atoms from the Ni active sites to rGO also provides an additional pathway for hydrogen generation. This contributes to the overall production of hydrogen and thus increases the efficiency of the catalyst. As described herein, the composite material obtained from step (i) may be referred to as Ni@rGO.

[0036] Without wishing to be bound by theory, it is believed that when the composite material is used as a catalyst in the anode during the oxygen evolution reaction (OER) and / or urea oxidation-assisted water electrolysis, the rGO component functions as an efficient current collector and a highly stable support for partially oxidized nickel particles. In some embodiments, the substrate comprises nickel or titanium. In some embodiments, the substrate contains no nickel, or at most trace amounts of nickel. This reduces the overall nickel content in the electrode (i.e., the combination of the composite material and the substrate in such embodiments), allowing cheaper alternatives to nickel, such as titanium, to be used as the substrate.

[0037] The surface Ni of Ni@rGO is partially electrochemically oxidized during step (iii), which results in Ni 2+ and / or Ni 3+ The surface Ni is only partially electrochemically oxidized. 2+ The surface phase may be in the form of nickel hydroxide or nickel(II) oxide. The nickel hydroxide may be selected from α-nickel hydroxide or β-nickel hydroxide. Ni 3+ The surface phase may be in the form of nickel(III) oxide or nickel oxyhydroxide.

[0038] Without wishing to be bound by theory, when the composite material is used as a catalyst in the cathode during alkaline water electrolysis, Ni|Ni 2+ and / or Ni|Ni 3+ It is believed that water dissociation occurs at the surface phase interface. H is adsorbed on metallic Ni, while OH - Ni from water 2+ and / or Ni 3+ It migrates to the surface phase site and is then released back into the solution. 2+ and / or Ni 3+ The surface phase is OH -Since the water dissociation barrier is reduced (lowered) according to the Bronsted-Polanyi relationship, which asserts an approximately linear relationship between the reaction enthalpy and the kinetic barrier, in this particular case, the OH bond to the oxidized portion of the Ni surface. - Due to the stronger bond, the reaction enthalpy is more exothermic, which reduces the water dissociation barrier and makes the reaction faster. As described herein, the surface-modified composite material obtained in step (iii) can be called ox-Ni@rGO.

[0039] Without wishing to be bound by theory, it is believed that when the composite material is used as a catalyst at the anode during alkaline water electrolysis, oxidation of the catalyst surface (i.e., partial oxidation of the nickel particles) promotes the decomposition of water molecules and hydroxide ions, resulting in increased production of O molecules.

[0040] In this way, a composite is obtained that exhibits improved electrochemical catalytic activity for hydrogen and / or oxygen evolution compared to catalyst deposition without the presence of graphene oxide or without the presence of a Ni oxidized surface phase.

[0041] The present inventors have discovered that a method comprising the combination of steps (i) and (iii) unexpectedly provides a composite containing a lower mass percent (wt.%) of nickel relative to the total composite mass compared to known Ni-based catalysts, while providing comparable or higher catalytic activity at the cathode and anode (e.g., up to 500 mA cm at −0.35 V vs. RHE). -2 of hydrogen evolution current, up to 200 mA cm at 1.7 V vs. RHE -2 and oxygen evolution current up to 325 mA cm at 1.9 V vs. RHE -2 We found that the urea oxidation current was 1000 kJ / s.

[0042] In some embodiments, in the composite material obtained in step (iii), nickel is present in an amount of 20 to 80 wt % based on the total weight of the composite material.

[0043] In some embodiments, the concentration of the nickel(II) salt in the deposition solution is between 0.01 and 3 mol dm -3 and optionally, the concentration of nickel(II) salt in the deposition solution is about 0.125 mol dm -3 is.

[0044] In some embodiments, the concentration of graphene oxide in the deposition solution is between 0.01 and 2 g dm -3 and optionally, the concentration of graphene oxide in the deposition solution is about 0.13 g dm -3 is.

[0045] In some embodiments, the electrochemical deposition of step (i) is carried out under galvanostatic conditions.

[0046] In some embodiments, the electrochemical deposition in step (i) is performed at a current of 50 to 1000 mA cm -2 and optionally, the electrochemical deposition of step (i) is carried out at a constant current density selected from the range of about 500 mA cm -2 The current is applied at a constant current density of .

[0047] In some embodiments, the electrochemical deposition of step (i) is carried out under potentiostatic conditions.

[0048] In some embodiments, the electrochemical deposition in step (i) is carried out at a constant potential selected from the range of 2.5 to 6.0 V, and optionally, the electrochemical deposition in step (i) is carried out at a constant potential of about 4 V.

[0049] In some embodiments, the electrochemical deposition of step (i) is carried out for 5 to 500 seconds, and optionally, the electrochemical deposition of step (i) is carried out for about 90 seconds.

[0050] In some embodiments, the partial electrochemical oxidation of step (iii) is carried out for 5 to 2000 seconds, and optionally, the electrochemical oxidation of step (iii) is carried out for about 30 seconds.

[0051] In some embodiments, the partial electrochemical oxidation of step (iii) is carried out at a voltage corresponding to 0 V to 2 V vs. the reversible hydrogen electrode, and optionally, the partial electrochemical oxidation of step (iii) is carried out at a voltage corresponding to about 1.0 V vs. the reversible hydrogen electrode.

[0052] In some embodiments, the method further comprises the steps of: (iv) after step (iii), removing the composite material from the substrate to obtain a free composite material. and, optionally, (v) after step (iv), grinding the loose composite material to obtain a powder composite material.

[0053] In a second aspect of the present invention there is provided a composite material obtained or obtainable by the method of the first aspect of the present invention. Accordingly, in a second aspect of the present invention there is provided a composite material obtained or obtainable by a method for preparing a composite material, the method comprising: (i) electrochemically depositing a material onto a substrate from a deposition solution comprising a nickel(II) salt and graphene oxide to obtain a nickel-reduced graphene oxide composite material comprising nickel dispersed on reduced graphene oxide, said composite material being deposited on the substrate; (ii) after step (i), placing the substrate on which the nickel-reduced graphene oxide composite material has been deposited together with a counter electrode in an alkaline solution; and (iii) after step (ii), partially electrochemically oxidizing the nickel to obtain a partially oxidized nickel-reduced graphene oxide composite comprising partially oxidized nickel dispersed on reduced graphene oxide, said composite being deposited on a substrate.

[0054] The composite material comprises partially oxidized nickel particles dispersed on reduced graphene oxide flakes.

[0055] The nickel is present in particulate form, i.e., in the form of discrete deposition locations rather than in a continuous layer.

[0056] Reduced graphene oxide exists in the form of flakes, ie particles of small thickness (1-10 layers) and significantly larger diameter.

[0057] In some embodiments, nickel is present in the composite in an amount of 20 to 80 weight percent based on the total weight of the composite.

[0058] In a third aspect of the present invention, there is provided an electrode comprising a composite material obtained or obtainable by the method of the first aspect of the present invention, or an electrode comprising the composite material of the second aspect of the present invention. The electrode thus suitably comprises a composite material comprising partially oxidized nickel particles dispersed on reduced graphene oxide deposited on a substrate which is electrically conductive.

[0059] The electrode may be used as a cathode for electrolytic hydrogen production under alkaline electrolysis conditions. The electrode may be used as an anode for electrolytic oxygen production under alkaline electrolysis conditions. The electrode may be used as an anode for electrolytic urea oxidation under urea oxidation-assisted alkaline electrolysis conditions.

[0060] To enable this, the substrate material is preferably electrically conductive.

[0061] In some embodiments, the substrate comprises nickel or titanium.

[0062] In some embodiments, the substrate contains no nickel or at most trace amounts of nickel.

[0063] In some embodiments, nickel is present in the electrode in an amount of 20 to 80 weight percent based on the total weight of the composite.

[0064] In a fourth aspect of the present invention, there is provided a method for electrolytic hydrogen production under alkaline electrolysis conditions, the method comprising the steps of: (A) constructing a system including an alkaline aqueous solution, a first electrode comprising a composite material according to the second aspect of the present invention, a second electrode, and an ion-permeable diaphragm disposed between the first electrode and the second electrode; and (B) applying a current between the first electrode and the second electrode to generate hydrogen at the first electrode;

[0065] In some embodiments, the first electrode is an electrode according to the third aspect of the invention. The first electrode may be a cathode.

[0066] In some embodiments, the method further comprises the steps of: (C) Periodically partially electrochemically oxidizing the first electrode during step (B), optionally with step (C) occurring at intervals of 50-5400 seconds and / or each occurrence of step (C) occurring for 0.5-100 seconds, to maintain the catalytic activity of ox-Ni@rGO during electrolytic hydrogen generation under alkaline electrolysis conditions. The oxidation step serves to "replenish" the oxidation level of the partially oxidized nickel on the rGO.

[0067] In a fifth aspect of the present invention, there is provided a method for electrolytic oxygen production under alkaline electrolysis conditions, the method comprising the steps of: (A) constructing a system including an alkaline aqueous solution, a first electrode, a second electrode including the composite material according to the second aspect of the present invention, and an ion-permeable diaphragm disposed between the first electrode and the second electrode; (B) applying a current between the first electrode and the second electrode to generate oxygen at the second electrode.

[0068] In some embodiments, the second electrode is an electrode according to the third aspect of the invention. The second electrode may be an anode.

[0069] In some embodiments, the method further comprises the steps of: (C) Periodically partially electrochemically oxidizing the second electrode during step (B), optionally with step (C) occurring at intervals of 50-5400 seconds and / or each occurrence of step (C) occurring for 0.5-100 seconds, to maintain the catalytic activity of ox-Ni@rGO during electrolytic oxygen generation under alkaline electrolysis conditions. The oxidation step serves to "replenish" the oxidation level of the partially oxidized nickel on the rGO.

[0070] In a sixth aspect of the present invention, there is provided a method for electrolytic urea oxidation (i.e., preferably to produce nitrogen and carbon dioxide) under urea oxidation-assisted water electrolysis conditions, the method comprising the steps of: (A) constructing a system including a solution containing an alkaline aqueous solution and urea, a first electrode, a second electrode including a composite material according to the second aspect of the present invention, and an ion-permeable diaphragm disposed between the first electrode and the second electrode; (B) applying a current between the first electrode and the second electrode to oxidize urea at the second electrode (i.e., preferably to produce nitrogen and carbon dioxide).

[0071] In some embodiments, the method further comprises the steps of: (C) Periodically partially electrochemically oxidizing the second electrode during step (B), optionally with step (C) occurring at intervals of 50-5400 seconds and / or each occurrence of step (C) occurring for 0.5-100 seconds, to maintain the catalytic activity of ox-Ni@rGO during urea oxidation under alkaline electrolysis conditions. The oxidation step serves to "replenish" the oxidation level of the partially oxidized nickel on the rGO.

[0072] The present invention includes any combination of the described embodiments and preferred features except where expressly not permitted or explicitly avoided. [Brief explanation of the drawings]

[0073] Next, embodiments and experiments illustrating the principles of the present invention will be described with reference to the accompanying drawings. [Figure 1A] Figure 1A shows scanning electron microscope (SEM) images of Ni@rGO composites deposited at different current densities with a graphene oxide concentration of 0.13 g / dm3 in the deposition solution. [Figure 1B] Figure 1B shows the hydrogen evolution curve corresponding to the Ni@rGO composite compared to that of an electrode prepared using a deposition solution without graphene oxide (i.e., pure nickel was deposited). [Figure 2A] Figure 2A shows SEM images of Ni@rGO composites deposited with different concentrations of dispersed graphene oxide at a current of 200 mA·cm−2. [Figure 2B] Figure 2B is the hydrogen evolution curve corresponding to the Ni@rGO composite shown in Figure 2A. [Figure 3A] Figure 3A shows the transmission electron microscopy (TEM) results of Ni@rGO composite deposited at a current of 200 mA·cm−2. [Figure 3B] Figure 3B shows the results of electron diffraction analysis of the Ni@rGO composite deposited at a current of 200 mA·cm−2. [Figure 4] Figure 4 shows the hydrogen evolution current in electrochemically deposited nickel and Ni@rGO composites before ("pure Ni", Ni@rGO) and after electrochemical oxidation ("Ni after oxidation treatment", ox-Ni@rGO). [Figure 5] Figure 5 shows an SEM image of a single flake of the ox-Ni@rGO composite, which shows partially oxidized nickel particles dispersed on a single reduced graphene oxide flake. [Figure 6] Figure 6 shows the periodic oxidation of the ox-Ni@rGO composite during HER with the aim of maintaining high catalytic activity for hydrogen evolution. [Figure 7] Figure 7 shows the oxygen evolution current in nickel (ox-Ni) and ox-Ni@rGO after oxidation treatment. [Figure 8]Figure 8 shows the urea oxidation current in nickel (ox-Ni) and ox-Ni@rGO after oxidation treatment. DETAILED DESCRIPTION OF THE INVENTION

[0074] Aspects and embodiments of the present invention will now be described with reference to the accompanying drawings. Further aspects and embodiments will be apparent to those skilled in the art. All documents mentioned herein are incorporated by reference.

[0075] (Preparation of the complex - method) A first aspect of the present invention provides a method for preparing a composite material, the method comprising the steps of: (i) electrochemically depositing a material onto a substrate from a deposition solution comprising a nickel(II) salt and graphene oxide to obtain a nickel-reduced graphene oxide composite material comprising nickel dispersed on reduced graphene oxide, said composite material being deposited on the substrate; (ii) after step (i), placing the substrate on which the nickel-reduced graphene oxide composite material has been deposited together with a counter electrode in an alkaline solution; and (iii) after step (ii), partially electrochemically oxidizing the nickel to obtain a partially oxidized nickel-reduced graphene oxide composite comprising partially oxidized nickel dispersed on reduced graphene oxide, and depositing said composite on a substrate.

[0076] The inventors have confirmed that composite materials obtained or obtainable by such methods have a lower mass % of nickel compared to known Ni-based catalysts, but exhibit comparable or higher catalytic activity (e.g., up to 500 mA cm at −0.35 V vs. RHE). -2 of hydrogen evolution current, up to 200 mA cm at 1.7 V vs. RHE -2 and oxygen evolution current up to 325 mA cm at 1.9 V vs. RHE -2 It was established that the urea oxidation current was

[0077] The first step of the method is step (i) electrochemical deposition, in which nickel and graphene oxide in a deposition solution of step (i) are electrochemically deposited onto a substrate to form a composite material comprising nickel dispersed on reduced graphene oxide.

[0078] The nickel is believed to be present in particulate form, ie, in the form of discrete deposition locations rather than in a continuous layer.

[0079] The reduced graphene oxide (note that the graphene oxide in the deposition solution is naturally reduced during deposition, so the deposited species is reduced graphene oxide, not graphene oxide) is believed to be in the form of flakes, i.e., particles with small thickness (1-10 layers) and significantly larger diameter. The term "flake" is widely used and understood in the field of 2D materials.

[0080] Step (i) - Deposition The solution of step (i), as described herein, may be known as a deposition solution. The deposition solution comprises a nickel(II) salt.

[0081] In some embodiments, the concentration of the nickel(II) salt in the solution in step (i) is 3 mol dm -3 For example, 2 mol dm -3 Below, 1mol dm -3 Below, 0.5mol·dm -3 Below, 0.4mol·dm -3 Below, 0.3mol·dm -3 or less, or 0.2 mol dm -3 The following is the result.

[0082] In some embodiments, the concentration of the nickel(II) salt in the solution in step (i) is 0.01 mol dm -3 More than, for example, 0.05 mol dm -3 More than 0.06mol dm -3 More than 0.07mol dm -3 More than 0.08mol dm -3 More than 0.09 mol dm-3 or more, or 0.1 mol dm -3 That's all.

[0083] The concentration of the nickel(II) salt in the solution in step (i) may be an amount selected from a range having upper and lower limits selected from the values ​​shown above. For example, in some embodiments, the concentration of the nickel(II) salt in the solution in step (i) is 0.01 to 3 mol dm -3 and optionally, the concentration of nickel(II) salt in the deposition solution is about 0.125 mol dm -3 is.

[0084] In a preferred embodiment, the concentration of the nickel(II) salt in the solution in step (i) is 0.1 to 1 mol dm -3 , preferably 0.1 to 0.2 mol dm -3 For example, about 0.125 mol dm -3 is.

[0085] In some embodiments, the nickel(II) salt in the solution of step (i) is selected from nickel sulfate (NiSO), nickel carbonate (NiCO), nickel nitrate (Ni(NO), nickel chloride (NiCl), and nickel phosphate (Ni(PO).

[0086] In a preferred embodiment, the nickel (II) salt in the solution of step (i) is nickel sulfate (NiSO2).

[0087] The deposition solution of step (i) also includes graphene oxide dispersed in the solution.

[0088] In some embodiments, the concentration of graphene oxide in the solution in step (i) is 2 g dm -3 For example, 1 g dm -3 Less than 0.5g dm -3 Below, 0.4g·dm -3 Below, 0.3g dm -3 or less than 0.2g·dm -3 The following is the result.

[0089] In some embodiments, the concentration of graphene oxide in the solution in step (i) is 0.01 g dm -3 For example, 0.05 g dm -3 More than 0.06g dm -3 More than 0.07g dm -3 More than 0.08g dm -3 More than 0.09g dm -3 or more, or 0.1 g·dm + That's all.

[0090] The concentration of graphene oxide in the solution in step (i) may be an amount selected from a range having upper and lower limits selected from the values ​​shown above. For example, in some embodiments, the concentration of graphene oxide in the solution in step (i) is 0.01 to 2 g dm -3 and optionally, the concentration of graphene oxide in the solution of step (i) is about 0.13 g dm -3 is.

[0091] In some embodiments, the concentration of graphene oxide in the solution in step (i) is between 0.01 and 1.7 g dm -3 and preferably 0.05 to 1.5 g dm -3 , more preferably 0.1 to 1.3 g dm -3 For example, about 0.13 g dm -3 is.

[0092] Without wishing to be bound by theory, it is believed that at higher concentrations, aggregation of graphene oxide in the deposition solution may occur, reducing the deposition efficiency of the flakes.

[0093] In some embodiments, the solution of step (i) further comprises an acid. Acidic conditions are preferred in the solution of step (i) because at higher pH, graphene oxide may precipitate.

[0094] In some embodiments, the concentration of the acid in the solution in step (i) is 0.1 to 1 mol dm -3and preferably 0.1 to 0.5 mol dm -3 , more preferably 0.1 to 0.3 mol dm -3 For example, approximately 0.2 mol dm -3 is.

[0095] In some embodiments, the pH of the deposition solution in step (i) is less than 7.0 and greater than or equal to 3.5.

[0096] In some embodiments, the pH of the deposition solution in step (i) is between 3.5 and 6.5, preferably between 4.0 and 6.5, more preferably between 4.5 and 6.5, for example a value of about 5.0.

[0097] In a preferred embodiment, the solution in step (i) comprises boric acid (H3BO3).

[0098] In some embodiments, the solution of step (i) comprises an ammonium salt. The ammonium salt contributes to the conductivity of the deposition solution, controls the pH, and provides Ni from the nickel(II) salt. 2+ Stabilizes ions.

[0099] In some embodiments, the concentration of the ammonium salt in the solution in step (i) is 0.1 to 1 mol dm -3 , preferably 0.2 to 0.8 mol dm -3 , more preferably 0.4 to 0.6 mol dm -3 For example, about 0.5 mol dm -3 is.

[0100] In a preferred embodiment, the solution in step (i) comprises ammonium chloride (NH4Cl).

[0101] In step (i), nickel and reduced graphene oxide are electrochemically deposited on a substrate.

[0102] Once deposition is complete, with appropriate selection of substrate material, it can be directly used as an electrode for electrolytic hydrogen production under alkaline electrolysis conditions.

[0103] The electrode may be used as a cathode for electrolytic hydrogen production under alkaline electrolysis conditions.

[0104] The electrode may be used as an anode for electrolytic oxygen production under alkaline electrolysis conditions.

[0105] The electrode may be used as an anode for urea oxidation under urea oxidation assisted alkaline electrolysis conditions.

[0106] To enable this, the substrate material is preferably electrically conductive.

[0107] In some embodiments, the substrate in step (i) comprises nickel or titanium.

[0108] In a preferred embodiment, the substrate in step (i) comprises titanium.

[0109] In some embodiments, the substrate in step (i) is a titanium network. In some embodiments, the substrate in step (i) is a titanium mesh. In some embodiments, the substrate in step (i) is a titanium grid.

[0110] In some embodiments, the substrate contains no nickel or only trace amounts of nickel at most, for example, at most 10 wt% nickel, at most 5 wt% nickel, at most 2 wt% nickel, at most 1 wt% nickel, or at most 0.5 wt% nickel.

[0111] In some embodiments, the substrate in step (i) comprises nickel.

[0112] In some embodiments, the substrate in step (i) is a nickel network. In some embodiments, the substrate in step (i) is a nickel mesh. In some embodiments, the substrate in step (i) is a nickel grid. In some embodiments, the substrate in step (i) is a nickel foam. In some embodiments, the substrate in step (i) is a perforated nickel plate.

[0113] In some embodiments, the thickness of the substrate is between 50 and 400 μm, and optionally the thickness of the substrate is between 100 and 150 μm.

[0114] In step (i), Ni present in the deposition solution 2+ and graphene oxide are electrochemically deposited on the substrate. During the electrochemical deposition, Ni 2+ is reduced to metallic Ni, and the graphene oxide is reduced to reduced graphene oxide on the substrate.

[0115] Electrochemical deposition may be carried out in a system in which the substrate on which deposition is to occur is connected to the negative pole of a DC voltage source and a counter electrode, such as a nickel wire, is connected to the positive wire of the DC voltage source. Electrochemical deposition is preferably carried out at a current density calculated based on the geometric surface of the substrate on which deposition is to occur.

[0116] In a preferred embodiment, the electrochemical deposition of step (i) is carried out under galvanostatic conditions.

[0117] In some embodiments, the electrochemical deposition in step (i) is performed at a current of 50 to 1000 mA cm -2 , preferably 150 to 800 mA cm -2 , more preferably 250 to 700 mA cm -2 , for example, about 500 mA cm -2 The current is applied at a constant current density selected within the range of

[0118] In some embodiments, the electrochemical deposition of step (i) is carried out under potentiostatic conditions.

[0119] In some embodiments, the electrochemical deposition in step (i) is carried out at a constant potential selected within the range of 2.5 to 6.0V, preferably 2.5 to 5.0V, more preferably 2.5 to 4.5V, for example about 4.0V.

[0120] In some embodiments, the electrochemical deposition of step (i) is carried out for 5 to 500 seconds, and optionally, the electrochemical deposition of step (i) is carried out for about 90 seconds.

[0121] In some embodiments, the electrochemical deposition in step (i) is carried out for 10 to 300 seconds, preferably 50 to 200 seconds, more preferably 60 to 150 seconds, for example, about 90 seconds.

[0122] Following electrochemical deposition, a composite material containing nickel dispersed on reduced graphene oxide (Ni@rGO) is obtained on the substrate.

[0123] In a preferred embodiment, the nickel in the composite material obtained in step (i) is in the form of particles with nanometer dimensions (for example particles with an average diameter of 10 nm to 50 nm as measured by SEM).

[0124] The nickel in the composite material obtained in step (i) may form a nickel lattice.

[0125] In some embodiments, the nickel lattice is in the form of a face-centered cubic lattice.

[0126] In some embodiments, step (i) further comprises washing the resulting composite material deposited on the substrate with distilled water.

[0127] In some embodiments, step (i) is carried out at a temperature selected within the range of 25 to 70°C.

[0128] Preferably, step (i) is carried out under normal gravity conditions, e.g., standard gravity. For example, in some embodiments, step (i) is carried out in a gravity field of 0.8 to 1.5 g.

[0129] (Step (ii) - Soaking) The second step of the method is performed after step (i) and involves placing the substrate obtained in step (i) on which the nickel-reduced graphene oxide composite material has been deposited, together with a counter electrode, in an alkaline solution. Thus, the electrochemical oxidation in step (iii) is performed in an alkaline solution. The alkaline solution in step (ii) may also be suitable for alkaline water electrolysis. The alkaline solution in step (ii) may also be suitable for urea oxidation-assisted alkaline water electrolysis after adding urea to the alkaline solution. Alternatively, in some embodiments, the alkaline solution in step (ii) may contain urea, and thus may also be suitable for urea oxidation-assisted alkaline water electrolysis. Thus, the composite material obtained in step (iii) does not need to be transferred to a separate solution for use as a catalyst or catalytic electrode for electrolytic hydrogen production under alkaline electrolysis conditions. In this way, the method of the present invention provides a more efficient method for obtaining and using electrodes for alkaline electrolysis.

[0130] In some embodiments, the concentration of the alkaline solution in step (ii) is 0.1 to 6 mol dm -3 , preferably 0.3 to 3 mol dm -3 , more preferably 0.5 to 1.5 mol dm -3 , for example, about 1 mol dm -3 is.

[0131] The pH of the alkaline solution in step (ii) is greater than 7.0 and equal to or less than 14.0.

[0132] In some embodiments, the pH of the alkaline solution in step (ii) is between 8.0 and 14.0, preferably between 9.0 and 14.0, more preferably between 10.0 and 14.0, for example, a value of about 14.0.

[0133] In some embodiments, the alkaline solution in step (ii) is selected from aqueous potassium hydroxide (KOH), aqueous sodium hydroxide (NaOH), aqueous lithium hydroxide (LiOH), and a combination of any two or three thereof.

[0134] In a preferred embodiment, the alkaline solution in step (ii) is an aqueous potassium hydroxide solution (KOH).

[0135] In some embodiments, the counter electrode in step (ii) is a Ni-based electrode.

[0136] A reference electrode may be used to measure the working electrode potential, and thus the reference electrode may be used to measure the potential of the half-cell of the substrate on which the nickel-reduced graphene oxide composite material obtained in step (i) is deposited.

[0137] In some embodiments, the reference electrode in step (ii) is selected from a reversible hydrogen electrode and a saturated calomel electrode.

[0138] In a preferred embodiment, the reference electrode in step (ii) is a reversible hydrogen electrode.

[0139] (Step (iii)-oxidation) The third step (iii) of the method is carried out after step (ii). In step (iii), the nickel in the composite material is electrochemically oxidized in some areas to form Ni 2+ and / or Ni 3+ The surface oxide phase is formed at the surface of the Ni. Other areas of the surface Ni remain unoxidized. Thus, the surface Ni is only partially electrochemically oxidized.

[0140] The partial electrochemical oxidation of step (iii) can be carried out in the alkaline solution of step (ii) as described above.

[0141] The electrochemical oxidation is selective to nickel because the electrochemical reduction of graphene oxide in step (i) is irreversible, and the substrate also generally remains unoxidized because it is covered by the composite material obtained in step (i) following electrochemical deposition.

[0142] Ni 2+ The surface phase may be in the form of nickel hydroxide or nickel(II) oxide. The nickel hydroxide may be selected from α-nickel hydroxide and β-nickel hydroxide. Ni 3+ The surface phase may be in the form of nickel oxyhydroxide or nickel(III) oxide.

[0143] In some embodiments, the partial electrochemical oxidation of step (iii) is carried out for 5 to 2000 seconds, and optionally, the partial electrochemical oxidation of step (iii) is carried out for 30 seconds.

[0144] In some embodiments, the partial electrochemical oxidation in step (iii) is carried out for 10 to 1000 seconds, preferably 15 to 100 seconds, more preferably 20 to 50 seconds, for example, about 30 seconds.

[0145] In some embodiments, the total time for electrochemical deposition in step (i) and partial electrochemical oxidation in step (iii) may be in the range of 50 to 600 seconds, preferably 100 to 300 seconds, for example about 120 seconds.

[0146] In some embodiments, the partial electrochemical oxidation in step (iii) is carried out at a voltage of 0 V to 2 V versus a reference electrode.

[0147] For example, the partial electrochemical oxidation of step (iii) may be carried out at a voltage corresponding to 0 V to 2 V versus the reversible hydrogen electrode, and optionally, the partial electrochemical oxidation of step (iii) is carried out at a voltage corresponding to about 1.0 V versus the reversible hydrogen electrode.

[0148] That is, the voltage may be selected depending on the type of reference electrode. Because many different reference electrodes can be envisioned and various "absolute" voltages may be appropriate accordingly, it may be useful to discuss voltages in terms of the effective potential difference if the counter electrode were a reversible hydrogen electrode. This does not imply that the reversible hydrogen electrode is necessarily present in embodiments of the present invention, but merely provides a useful reference point for evaluating voltages. Thus, voltages may be discussed herein as "equivalent to" the voltage relative to the reversible hydrogen electrode. Potentials may be recalculated from any other reference electrode to values ​​relative to the reversible hydrogen electrode. Other reference electrodes may include a saturated calomel electrode.

[0149] In some embodiments, the partial electrochemical oxidation in step (iii) is carried out at a voltage of 0.3 V to 1.5 V, preferably 0.5 V to 1.2 V, more preferably 0.6 V to 1.1 V, for example, about 1.0 V, versus a reference electrode.

[0150] For example, the partial electrochemical oxidation in step (iii) is carried out at a voltage corresponding to 0.3 V to 1.5 V, preferably 0.5 V to 1.2 V, more preferably 0.6 V to 1.1 V, for example, about 1.0 V, versus the reversible hydrogen electrode.

[0151] In some embodiments, the partial electrochemical oxidation of step (iii) is carried out at a voltage between 0 V and 2 V versus a reversible hydrogen electrode.

[0152] In some embodiments, the partial electrochemical oxidation in step (iii) is carried out at a voltage of 0.3 V to 1.5 V, preferably 0.6 V to 1.2 V, more preferably 0.8 V to 1.1 V, for example about 1.0 V, versus the reversible hydrogen electrode.

[0153] In some embodiments, the method comprises the steps of: (i) 0.125 mol dm -3 of nickel sulfate (NiSO4), 0.2 mol dm -3 of boric acid (H3BO3), 0.5 mol dm -3ammonium chloride (NH4Cl) and graphene oxide under constant current conditions for 90 seconds to obtain a nickel-reduced graphene oxide composite material comprising nickel dispersed on the reduced graphene oxide, said composite material being deposited on the titanium network; (ii) after step (i), placing the titanium network on which the nickel-reduced graphene oxide composite is deposited together with a counter electrode in an alkaline solution; (iii) after step (ii), partially electrochemically oxidizing the nickel for 30 seconds at a voltage corresponding to 1.0 V vs. a reversible hydrogen electrode to obtain a partially oxidized nickel-reduced graphene oxide composite comprising partially oxidized nickel dispersed on reduced graphene oxide, and depositing the composite on a substrate.

[0154] Following partial electrochemical oxidation, a composite material comprising partially oxidized nickel dispersed on reduced graphene oxide (ox-Ni@rGO) is obtained, which is then deposited onto a substrate.

[0155] In some embodiments, the nickel obtained in step (iii) is present in the composite in an amount of 20 to 80 wt % based on the total weight of the composite.

[0156] In some embodiments, the amount of nickel present in the composite material obtained in step (iii) is 80% by weight or less, e.g., 70% by weight or less, 60% by weight or less, or 50% by weight or less, based on the total weight of the composite material. In some embodiments, the amount of nickel present in the composite material obtained in step (iii) is 20% by weight or more, 30% by weight or more, or 40% by weight or more, based on the total weight of the composite material.

[0157] The amount of nickel present in the composite material obtained in step (iii) may be an amount selected from a range having upper and lower limits selected from those values ​​described above. For example, in some embodiments, the nickel obtained in step (iii) is present in the composite material in an amount of 20 to 80 wt % based on the total weight of the composite material.

[0158] (Preparation of Powder Composite) The method of the present invention may further provide for the preparation of a powder composite material.

[0159] In some embodiments, the method further comprises the steps of: (iv) after step (iii), removing the composite material from the substrate to obtain a free composite material; and, optionally (v) after step (iv), grinding the loose composite material to obtain a powder composite material.

[0160] The fourth step (iv) is carried out after step (iii) and involves removing (e.g. scraping) the layer of composite material obtained in step (iii) from the substrate to obtain a free composite material, i.e. a composite material separated from the substrate.

[0161] The fifth step (v), which occurs after step (iv), involves comminuting the loose composite material (i.e., reducing it to particles, for example, by grinding, crushing, cutting, vibrating, etc.) to obtain a powdered composite material. Any suitable grinding or mechanical comminution technique or procedure may be used to obtain the powder. For example, grinding with a mortar and pestle may be used to obtain the powdered composite material.

[0162] The powder composite may be fixed onto different substrates to form electrical contacts, allowing the catalytic material to be used in a variety of different electrolysis cells (electrolyzers) for hydrogen production.

[0163] The powder composite material obtained from such an embodiment of the present invention may be suitable for anion exchange membrane water electrolysis.

[0164] For example, powder composite materials resulting from such embodiments of the present invention may be suitable for use in electrolytic hydrogen production under alkaline proton exchange membrane (PEM) electrolysis conditions.

[0165] (composite material) A second aspect of the present invention provides a composite material obtained or obtainable by the method of the first aspect of the present invention. Accordingly, in the second aspect of the present invention there is provided a composite material obtained or obtainable by a method for preparing a composite material, said method comprising the steps of: (i) electrochemically depositing a material onto a substrate from a deposition solution comprising a nickel(II) salt and graphene oxide to obtain a nickel-reduced graphene oxide composite material comprising nickel dispersed on reduced graphene oxide, said composite material being deposited on the substrate; (ii) after step (i), placing the substrate on which the nickel-reduced graphene oxide composite has been deposited together with a counter electrode in an alkaline solution; (iii) after step (ii), partially electrochemically oxidizing the nickel to obtain a partially oxidized nickel-reduced graphene oxide composite comprising partially oxidized nickel dispersed on reduced graphene oxide, and depositing said composite on a substrate.

[0166] Thus, the composite material has a unique microstructure obtained by the electrochemical deposition in step (i) and the partial electrochemical oxidation in step (iii).

[0167] The composite material comprises partially oxidized nickel particles dispersed on reduced graphene oxide flakes.

[0168] The nickel is in the form of particles, ie, not a continuous layer, but in discrete locations in the deposit, which can be considered as "islands" of partially oxidized nickel.

[0169] The partially oxidized nickel particles may include nickel nanoparticles, which may comprise metallic Ni and Ni 2+and / or Ni 3+ and at least one surface phase comprising:

[0170] The partially oxidized nickel particles may include nickel nanoparticles, which may be composed of metallic Ni and Ni 2+ and / or Ni 3+ and a plurality of surface phases including:

[0171] The partially oxidized nickel particles may comprise nickel nanoparticles, which comprise metallic Ni and at least one surface phase selected from Ni(OH), NiOOH, NiO, and any combination of two or three thereof, and optionally, the Ni(OH) is selected from α-Ni(OH) and β-Ni(OH).

[0172] The partially oxidized nickel particles may include nickel nanoparticles, which comprise metallic Ni and a plurality of surface phases selected from Ni(OH), NiOOH, NiO, and any combination of two or three thereof, and optionally, the Ni(OH) is selected from α-Ni(OH) and β-Ni(OH).

[0173] The partially oxidized nickel particles have at least one Ni-Ni 2+ and / or Ni-Ni 3+ Without wishing to be bound by theory, the cathode during alkaline water electrolysis may include a surface phase interface. 2+ and / or Ni|Ni 3+ It is believed that water dissociation occurs at the surface phase interface. Without wishing to be bound by theory, at the anode during alkaline water electrolysis, Ni|Ni 2+ and / or Ni|Ni 3+ Hydroxide ions (OH - ) is thought to dissociate.

[0174] The nickel nanoparticles can have a wide particle size distribution, for example, in some embodiments, the nickel nanoparticles have a particle size distribution of 5 to 500 nm.

[0175] In some embodiments, the nickel nanoparticles have a particle size distribution of 50-450 nm, optionally, the nickel nanoparticles have a particle size distribution of 100-400 nm, and optionally, the nickel nanoparticles have a particle size distribution of 200-300 nm.

[0176] In some embodiments, the maximum diameter of each nickel particle or island is 100 nm or less, e.g., 90 nm or less, 80 nm or less, 70 nm or less, 60 nm or less, 50 nm or less, 40 nm or less, 30 nm or less, 20 nm or less, or 10 nm or less.

[0177] Without wishing to be bound by theory, it is believed that smaller sized nickel particles provide a higher surface area to volume ratio, thereby increasing the number of active sites per unit mass.

[0178] Reduced graphene oxide is in the form of flakes, ie particles of small thickness (1-10 layers) and significantly larger diameter.

[0179] In some embodiments, each flake of reduced graphene oxide has a diameter of 1 to 50 μm.

[0180] In some embodiments, each flake of reduced graphene oxide has a diameter of 1 to 25 μm.

[0181] In some embodiments, each flake of reduced graphene oxide has a diameter of 1 to 10 μm.

[0182] The composite may be formed of a monolayer comprising partially oxidized nickel particles dispersed on reduced graphene oxide flakes.

[0183] The composite material can be formed from a first layer comprising reduced graphene oxide flakes and a second layer comprising partially oxidized nickel particles dispersed on the first layer.

[0184] The composite material may include partially oxidized nickel particles dispersed on reduced graphene oxide sheets.

[0185] The composite material may include a foam layer.

[0186] In some embodiments, nickel is present in the composite in an amount of 20 to 80 weight percent based on the total weight of the composite.

[0187] In some embodiments, the amount of nickel present in the composite is 80% by weight or less, eg, 70% by weight or less, 60% by weight or less, or 50% by weight or less, based on the total weight of the composite.

[0188] In some embodiments, the amount of nickel present in the composite is 20% or more, 30% or more, or 40% or more by weight, based on the total weight of the composite.

[0189] The amount of nickel present in the composite can be an amount selected from a range having upper and lower limits selected from those indicated above. For example, in some embodiments, nickel is present in the composite in an amount of 20 to 80 wt. % based on the total weight of the composite.

[0190] In some embodiments, the thickness of the composite material is between 5 and 30 μm.

[0191] In some embodiments, the thickness of the composite is between 10 and 25 nm, and optionally, the thickness of the composite is between 15 and 20 nm.

[0192] (electrode) A third aspect of the present invention provides an electrode comprising a composite material obtained or obtainable by the method of the first aspect of the invention, or an electrode comprising the composite material of the second aspect of the invention. The electrode thus suitably comprises a composite material comprising partially oxidized nickel particles dispersed on reduced graphene oxide deposited on a substrate which is electrically conductive.

[0193] The electrode may be used as an electrode for electrolytic hydrogen production under alkaline electrolysis conditions.

[0194] The electrode may be used as a cathode for electrolytic hydrogen production under alkaline electrolysis conditions.

[0195] The electrode may be used as an anode for electrolytic oxygen production under alkaline electrolysis conditions.The electrode may be used as an anode for electrolytic urea oxidation under urea oxidation-assisted alkaline electrolysis conditions.

[0196] To enable this, the substrate material is preferably electrically conductive.

[0197] In some embodiments, the substrate comprises nickel or titanium.

[0198] In a preferred embodiment, the substrate comprises titanium.

[0199] In some embodiments, the substrate is a titanium network, hi some embodiments, the substrate is a titanium mesh, hi some embodiments, the substrate is a titanium grid.

[0200] In some embodiments, the substrate contains no nickel or at most trace amounts of nickel, for example, it may be at most 10% nickel by weight, at most 5% nickel by weight, at most 2% nickel by weight, at most 1% nickel by weight, or at most 0.5% nickel by weight.

[0201] In some embodiments, the substrate is a nickel network. In some embodiments, the substrate is a nickel mesh. In some embodiments, the substrate is a nickel grid. In some embodiments, the substrate is a nickel foam. In some embodiments, the substrate is a nickel perforated plate.

[0202] In some embodiments, the thickness of the substrate is 50 to 400 μm, and optionally, the thickness of the substrate is 100 to 150 μm.

[0203] In some embodiments, the nickel in the electrode may be present in an amount of 20 to 80 wt % based on the total weight of the composite material.

[0204] In some embodiments, the amount of nickel present in the electrode is 80% by weight or less, e.g., 70% by weight or less, 60% by weight or less, or 50% by weight or less, based on the total weight of the composite. In some embodiments, the amount of nickel present in the electrode is 20% by weight or more, 30% by weight or more, or 40% by weight or more, based on the total weight of the composite.

[0205] The amount of nickel present in the electrode can be an amount selected from a range having upper and lower limits selected from the values ​​described above. For example, in some embodiments, nickel is present in the composite material obtained in step (iii) in an amount of 20 to 80 wt % based on the total weight of the composite material.

[0206] In some embodiments, the electrode comprises a substrate at least partially coated with a foam layer of the composite material.

[0207] In some embodiments, the electrode comprises a substrate at least partially coated with multiple foam layers of the composite material.

[0208] In some embodiments, the electrode comprises a substrate substantially coated with a foam layer of the composite material.

[0209] In some embodiments, the electrode may include a substrate substantially coated with multiple foam layers of the composite material.

[0210] (Electrolytic hydrogen production method) A fourth aspect of the present invention provides a method for electrolytic hydrogen production under alkaline electrolysis conditions, the method comprising the steps of: (A) constructing a system including an alkaline aqueous solution, a first electrode comprising the composite material according to the second aspect of the present invention, a second electrode, and an ion-permeable diaphragm disposed between the first electrode and the second electrode; (B) applying a current between the first electrode and the second electrode to generate hydrogen at the first electrode;

[0211] (Process (A)-Setting) The first step (A) involves establishing the system in an alkaline solution, so that electrolytic hydrogen production is carried out under alkaline conditions.

[0212] The aqueous alkaline solution in step (A) may be the same as the aqueous alkaline solution in step (ii) in the method of the first aspect of the present invention.

[0213] In some embodiments, the concentration of the alkaline aqueous solution in step (A) is 0.1 to 5 mol dm -3 , preferably 0.2 to 2 mol dm -3 , more preferably 0.5 to 1.5 mol dm -3 , for example, about 1 mol dm -3 is.

[0214] The pH of the alkaline aqueous solution in step (A) is greater than 7.0 and less than or equal to 14.0.

[0215] In some embodiments, the pH of the alkaline aqueous solution in step (A) is 8.0 to 14.0, preferably 9.0 to 14.0, more preferably 10.0 to 14.0, for example, about 14.0.

[0216] The first electrode comprises a composite material according to the second aspect of the present invention.

[0217] In some embodiments, the first electrode is an electrode according to the third aspect of the invention. The first electrode may be a cathode.

[0218] The second electrode functions as a counter electrode to the first electrode and may be an anode.

[0219] In some embodiments, the second electrode comprises a composite material according to the second aspect of the invention or is an electrode according to the third aspect of the invention.

[0220] In a preferred embodiment, the second electrode comprises nickel.

[0221] In a preferred embodiment, the second electrode is nickel foam.

[0222] The system of step (A) also includes an ion-permeable diaphragm disposed between the first electrode and the second electrode.

[0223] An ion-permeable membrane separates the first and second electrodes and also separates the product gases generated at the first and second electrodes, preventing undesirable side reactions from occurring.

[0224] An ion-permeable membrane allows the passage / transfer of ions between the first and second electrodes, and is therefore porous and selective to ions.

[0225] In some embodiments, the ion-permeable membrane comprises nickel oxide or Zirfon®.

[0226] As described herein, the system of step (A) may also be referred to as an electrolytic cell (electrolyzer) for producing hydrogen.

[0227] (Step (B) - Hydrogen Production) The second step (B) is carried out after the first step (A).

[0228] Step (B) involves applying an electric current between the first electrode and the second electrode, and then hydrogen is produced at the first electrode.

[0229] Step (B) may be carried out in a system in which the first electrode is connected to the negative pole of a power source and the second electrode is connected to the positive pole of the power source. The power source may be a DC voltage source.

[0230] Without wishing to be bound by theory, it is believed that during step (B), the hydrogen evolution reaction (HER) occurs at the first electrode, while the oxygen evolution reaction (OER) occurs at the second electrode.

[0231] At the first electrode, the HER mechanism starts with the Volmer step (1), in which HO in the alkaline solution dissociates to form OH. - ions and generate intermediate species H (Hads) that adsorb onto the composite material on the first electrode surface: H2O+e ─ +*→2Hads+OH ─ (1) where * denotes unoccupied active sites on the surface of the composite, such as free Ni active sites.

[0232] The composite material also contains Ni 2+ and / or Ni 3+ The H generated in (1) is the Ni 2+ and / or Ni 3+ Without wishing to be bound by theory, it is believed that during step (B), dissociation of water (1) results in Ni|Ni 2+ and / or Ni|Ni 3+ This is thought to occur at the surface phase interface. H is adsorbed onto metallic Ni, while OH from HO - Ni 2+ Ni migrates to the surface phase site. 2+ and / or Ni 3+Since the surface phase provides a strong bond for OH, the water dissociation barrier is reduced (lowered) according to the Bronsted-Polanyi relationship, thus increasing the efficiency of the water dissociation step (1).

[0233] The intermediate species Hads are then subsequently removed from the surface of the composite via either the Tafel reaction (2) or the Heyrovsky reaction (3). Hads+Hads→H2+2* (2) H2O+Hads+e - →H2+OH - +* (3) where * denotes unoccupied active sites on the surface of the composite, such as free Ni active sites.

[0234] Thus, hydrogen is produced at the first electrode during step (B). The overall reaction at the first electrode is represented by equation (4): 2H2O+2e - →H2+OH - (4)

[0235] Without wishing to be bound by theory, it is believed that H atoms adsorbed on Ni active sites during water dissociation during alkaline water electrolysis may spill over onto reduced graphene oxide (rGO), which acts as an H atom trap. This provides the free Ni active sites required for the hydrogen evolution reaction (HER) to proceed. Furthermore, H atom spillover from Ni active sites to rGO can also provide an additional pathway for hydrogen production (in addition to (2) and (3)). This contributes to the overall production of hydrogen and therefore increases the efficiency of the catalyst.

[0236] In parallel, OH in the alkaline aqueous solution generated at the first electrode (1) - Ions can diffuse across the ion-permeable membrane to the second electrode. The overall OER mechanism at the second electrode is believed to proceed according to equation (5): 2OH - →H2O+1 / 2O2+2e - (5)

[0237] Thus, oxygen can be produced at the second electrode.

[0238] As such, the overall equation for the electrolytic cell can be expressed as equation (6). H2O→H2+1 / 2O2(6)

[0239] In some embodiments, step (B) is carried out at a temperature selected from 50 to 90°C, preferably 60 to 80°C, more preferably 70 to 80°C, for example, at about 75°C.

[0240] It is understood in the art that the theoretical minimum energy for the hydrogen evolution reaction (i.e., the process represented by equation (4) above) is 1.23 V in a two-electrode system (i.e., a system including a cathode and an anode), or 0 V versus the reversible hydrogen electrode. However, experimentally, additional energy (known as the cell overpotential) is required due to the polarization overpotential of the cathode and anode and the internal resistance of the electrolyte. Thus, the experimental minimum energy for the hydrogen evolution reaction is more negative (indicating a loss of energy). Therefore, catalysts are used to reduce this energy loss.

[0241] The first electrode comprises a composite material according to the second aspect of the present invention.

[0242] The composite material of the second embodiment may be used as a catalyst for hydrogen production under alkaline electrolysis conditions. Without wishing to be bound by theory, the use of the composite material may reduce energy losses in the first electrode during step (B) due to a synergistic effect of the unique microstructure produced by the method of the present invention.

[0243] In addition to the first and second electrodes, a reference electrode may be used to measure the working electrode potential. Thus, electrolytic hydrogen production can be carried out using a three-electrode system. The reference electrode can be used to measure the half-cell potential at the first electrode.

[0244] For example, in some embodiments, step (B) is carried out at a voltage of −4.00 V to 0.00 V, preferably −3.00 V to 0.00 V, more preferably −1.00 V to 0.00 V, and even more preferably −0.40 V to 0.00 V relative to the reference electrode.

[0245] In some embodiments, step (B) is carried out at a voltage corresponding to −4.00 V to 0.00 V, preferably −3.00 V to 0.00 V, more preferably −1.00 V to 0.00 V, and even more preferably −0.40 V to 0.00 V, relative to a reference hydrogen electrode.

[0246] That is, the voltage may be selected depending on the type of reference electrode. Because many different reference electrodes can be envisioned, which may result in a variety of suitable "absolute" potentials, it may be useful to discuss the voltage in terms of the effective potential difference when the reference electrode is a reversible hydrogen electrode. This does not imply that such a reversible hydrogen electrode is necessarily present in embodiments of the present invention, but merely a useful reference point against which voltages can be evaluated. Thus, voltages are sometimes discussed herein as being "equivalent" to the voltage relative to the reversible hydrogen electrode. Potentials may be converted from other reference electrodes to potentials relative to the reversible hydrogen electrode. Other reference electrodes may include a saturated calomel electrode.

[0247] Of course, in some embodiments, the reference electrode may be a reversible hydrogen electrode.

[0248] In some embodiments, step (B) is carried out at a voltage of −4.00 V to 0.00 V, preferably −3.00 V to 0.00 V, more preferably −1.00 V to 0.00 V, and even more preferably −0.40 V to 0.00 V versus the reversible hydrogen electrode.

[0249] Additionally, the use of a composite material in the first electrode can result in a higher hydrogen evolution current for a fixed voltage.

[0250] For example, in some embodiments, the composite material exhibits a current density of up to 100 mA cm at a voltage corresponding to −0.35 V vs. the reversible hydrogen electrode during step (B). -2 , e.g., up to 200 mA cm -2 , or up to 300mA·cm -2 , or up to 400mA·cm -2 , or up to 500mA·cm -2 The hydrogen evolution current is shown.

[0251] In some embodiments, the composite material has a current density of up to 100 mA cm at −0.35 V vs. the reversible hydrogen electrode during step (B). -2 , e.g., up to 200 mA cm -2 , or up to 300mA·cm -2 , or up to 400mA·cm -2 , or up to 500mA·cm -2 The hydrogen evolution current is shown.

[0252] Thus, the composite material of the present invention can exhibit high catalytic activity for electrolytic hydrogen production under alkaline water electrolysis conditions, even when the substrate on which the composite material is deposited contains no nickel, or at most only trace amounts of nickel.

[0253] In some embodiments, step (B) is carried out at a pressure of 1 to 30 bar.

[0254] In some embodiments, the method further comprises the step (C) of: (C) partially electrochemically oxidizing the first electrode, and periodically performing step (C) during the performance of step (B); Optionally, step (C) is performed periodically at intervals of 50 to 5400 seconds, and / or each performance of step (C) is performed for 0.5 to 100 seconds.

[0255] Therefore, step (C) is an optional step that can be periodically performed during the performance of step (B). The partial oxidation of step (C) may be performed in a manner similar to the partial electrochemical oxidation of step (iii) in the method of the first aspect of the present invention. Therefore, during step (C), the Ni on the composite material 2+ and / or Ni 3+ The surface layer can be periodically renewed.

[0256] After each execution of step (C), the hydrogen evolution current may be returned to its initial value (i.e., the value in step (B)).

[0257] This allows the catalytic activity of ox-Ni@rGO to be maintained during electrolytic hydrogen generation under alkaline electrolysis conditions. The oxidation process acts to "replenish" the oxidation level of the partially oxidized nickel on rGO.

[0258] In some embodiments, the partial electrochemical oxidation in step (C) is carried out at a voltage of 0 V to 2 V relative to a reference electrode.

[0259] For example, the partial electrochemical oxidation in step (C) may be carried out at a voltage corresponding to 0 V to 2 V versus the reversible hydrogen electrode.

[0260] In some embodiments, the partial electrochemical oxidation in step (C) is carried out at a voltage of 0.3 V to 1.5 V, preferably 0.5 V to 1.2 V, more preferably 0.6 V to 1.1 V, for example, about 1 V, relative to a reference electrode.

[0261] For example, the partial oxidation in step (C) may be carried out at a voltage corresponding to 0.3 V to 1.5 V, preferably 0.5 V to 1.2 V, more preferably 0.6 V to 1.1 V, for example, about 1 V, relative to the reversible hydrogen electrode.

[0262] In some embodiments, the partial electrochemical oxidation of step (C) may be carried out at a voltage of 0 V to 2 V versus a reversible hydrogen electrode.

[0263] In some embodiments, the partial electrochemical oxidation in step (C) may be carried out at a voltage of 0.3 V to 1.5 V, preferably 0.5 V to 1.2 V, more preferably 0.6 V to 1.1 V, for example about 1 V, versus the reversible hydrogen electrode.

[0264] In some embodiments, step (C) is carried out periodically at intervals of 50 to 5400 seconds.

[0265] In some embodiments, step (C) is carried out periodically at intervals of 100 to 5000 seconds.

[0266] Preferably, in some embodiments, step (C) is carried out periodically at intervals of 150 to 1000 seconds, more preferably 200 to 500 seconds, for example, about 240 seconds.

[0267] In some embodiments, each performance of step (C) is performed for 0.5 to 100 seconds.

[0268] Preferably, in some embodiments, each performance of step (C) is carried out for 1 to 50 seconds, more preferably 5 to 25 seconds, for example, about 10 seconds.

[0269] Furthermore, the overpotential of the hydrogen evolution reaction can be reduced by 0.2 V compared to the composite containing only Ni.

[0270] The voltage stability of the first electrode can be evaluated by carrying out step (B) at a constant current for a long period of time and measuring the voltage decrease rate, that is, the negative voltage change per unit time.

[0271] Generally, high surface area Ni electrodes have a current capacity of 100 mA cm -2 At a constant current of 1000 mV, the potential decline rate exceeds -500 mV / month.

[0272] In some embodiments, the first electrode has a current of 100 mA cm -2At a constant current of 0.1 V, the voltage drop rate is less than -500 mV / month, for example, less than -400 mV / month, or less than -300 mV / month, or less than -200 mV / month, or less than -100 mV / month.

[0273] In a preferred embodiment, the first electrode is -2 At a constant current of 100 mV / month, the potential decline rate is less than -100 mV / month, more preferably less than -50 mV / month, for example, less than -30 mV / month.

[0274] Thus, the composite material can provide a first electrode with improved voltage stability.

[0275] (Electrolytic oxygen production method) A fifth aspect of the present invention provides a method for electrolytic oxygen production under alkaline electrolysis conditions, the method comprising the steps of: (A) constructing a system comprising an aqueous alkaline solution, a first electrode, a second electrode comprising a composite material according to the second aspect of the present invention, and an ion-permeable diaphragm disposed between the first electrode and the second electrode; and (B) applying a current between the first electrode and the second electrode to generate oxygen at the second electrode.

[0276] In some embodiments, the second electrode is an electrode according to the third aspect of the invention. The second electrode may be an anode.

[0277] (Process (A)-Construction) The first step (A) involves establishing the system in an alkaline solution, so that electrolytic oxygen production is carried out under alkaline conditions.

[0278] The aqueous alkaline solution in step (A) may be the same solution as the alkaline solution in step (ii) in the method of the first aspect of the present invention.

[0279] In some embodiments, the concentration of the alkaline aqueous solution in step (A) is 0.1 to 5 mol dm -3 , preferably 0.2 to 2 mol dm-3 , more preferably 0.5 to 1.5 mol dm -3 , for example, about 1 mol dm -3 is.

[0280] The pH of the alkaline aqueous solution in step (A) is greater than 7.0 and equal to or less than 14.0.

[0281] In some embodiments, the pH of the alkaline aqueous solution in step (A) is 8.0 to 14.0, preferably 9.0 to 14, more preferably 10.0 to 14.0, for example, a value of about 14.0.

[0282] The second electrode comprises a composite material according to the second aspect of the invention.

[0283] In some embodiments, the second electrode is an electrode according to the third aspect of the invention. The second electrode may be an anode.

[0284] The first electrode functions as a counter electrode to the second electrode, and may be a cathode.

[0285] In a preferred embodiment, the first electrode comprises a composite material according to the second aspect of the invention or is an electrode according to the third aspect of the invention.

[0286] In a preferred embodiment, the first electrode comprises nickel.

[0287] In some embodiments, the first electrode is nickel foam. In some embodiments, the first electrode is nickel mesh. In some embodiments, the first electrode is a nickel perforated plate.

[0288] The system of step (A) also includes an ion-permeable diaphragm disposed between the first electrode and the second electrode.

[0289] An ion-permeable membrane separates the first and second electrodes and also separates the product gases produced at the first and second electrodes, preventing undesirable side reactions from occurring.

[0290] An ion-permeable membrane allows the passage / transfer of ions between the first and second electrodes, and is therefore porous and selective to ions.

[0291] In some embodiments, the ion-permeable membrane comprises Zirfon®.

[0292] As described herein, the system of step (A) may also be referred to as an electrolytic cell for producing oxygen.

[0293] (Step (B) - Production of Oxygen) The second step (B) according to the fifth aspect of the present invention is the same as the step (B) according to the fourth aspect of the present invention. The second step (B) is carried out after the first step (A).

[0294] Step (B) involves applying a current between the first electrode and the second electrode, and then oxygen is produced at the second electrode.

[0295] OH in alkaline aqueous solution - ions, OH generated at the first electrode (1) - Ions can diffuse through the ion-permeable membrane to the second electrode. The overall OER mechanism at the second electrode is believed to proceed according to equation (5): 2OH - →H2O+1 / 2O2+2e - (5)

[0296] Thus, oxygen can be produced at the second electrode.

[0297] Without wishing to be bound by theory, it is believed that the high OER activity is the result of the combined effect of rGO acting as a conductive and stable support for the partially oxidized nickel particles, and the interfacial reaction between the metallic nickel and nickel oxide phases, which facilitates the decomposition of water and hydroxide ions and their recombination into O. As such, the overall reaction equation for the electrolytic cell can be expressed as Equation (6): H2O→H2+1 / 2O2(6)

[0298] In some embodiments, step (B) is carried out at a temperature selected from 50 to 90°C, preferably 60 to 80°C, more preferably 70 to 80°C, for example, at about 75°C.

[0299] It is understood in the art that the theoretical minimum energy for the oxygen evolution reaction (i.e., the process represented by equation (5) above) is 1.23 V in a two-electrode system (i.e., a system including a cathode and an anode), or 0 V vs. the reversible hydrogen electrode. However, experimentally, additional energy (known as the cell overpotential) is required due to the polarization overpotentials of the cathode and anode, as well as the internal resistance of the electrolyte. Thus, the experimental minimum energy for the oxygen evolution reaction is more negative (indicating a loss of energy). Therefore, catalysts are used to reduce this energy loss.

[0300] The second electrode comprises a composite material according to the second aspect of the present invention.

[0301] The composite material of the second embodiment can be used as a catalyst for oxygen production under alkaline electrolysis conditions. Without wishing to be bound by theory, the use of the composite material may result in reduced energy loss at the second electrode during step (B) due to a synergistic effect resulting from the unique microstructure produced by the method of the present invention.

[0302] In addition to the first and second electrodes, a reference electrode may be used to measure the working electrode potential. Thus, electrolytic oxygen production can be performed using a three-electrode system. The reference electrode can be used to measure the half-cell potential at the second electrode.

[0303] For example, in some embodiments, step (B) is carried out at a voltage of 1.00 V to 4.00 V, preferably 1.00 V to 3.00 V, more preferably 1.00 V to 2.50 V, and even more preferably 1.00 V to 2.00 V relative to the reference electrode.

[0304] In some embodiments, step (B) is carried out at a voltage corresponding to 1.23 V to 4.00 V, preferably 1.23 V to 3.00 V, more preferably 1.23 V to 2.50 V, and even more preferably 1.23 V to 2.00 V versus a reference hydrogen electrode.

[0305] That is, the voltage may be selected depending on the type of reference electrode. Many different reference electrodes can be envisioned, and each will have its own appropriate "absolute" voltage. For this reason, it may be useful to discuss voltages in terms of the effective potential difference if the reference electrode were a reversible hydrogen electrode. This does not imply that a reversible hydrogen electrode is necessarily present in embodiments of the present invention; it is merely used as a convenient reference point for evaluating voltages. Thus, voltages may be discussed herein in terms of voltages "equivalent to" the voltage relative to the reversible hydrogen electrode. Potentials may be converted from any other reference electrode to values ​​relative to the reversible hydrogen electrode. Other reference electrodes may include a saturated calomel electrode.

[0306] Of course, in some embodiments, the reference electrode may be a reversible hydrogen electrode.

[0307] In some embodiments, step (B) is carried out at a voltage of 1.23 V to 4.00 V, preferably 1.23 V to 3.00 V, more preferably 1.23 V to 2.50 V, and even more preferably 1.23 V to 2.00 V versus the reversible hydrogen electrode.

[0308] Furthermore, the use of a composite material in the second electrode can result in a higher oxygen evolution current for a fixed voltage.

[0309] For example, in some embodiments, the composite material exhibits a current density of up to 50 mA cm at a voltage corresponding to 1.6 V vs. the reversible hydrogen electrode during step (B). -2 , e.g., up to 100 mA cm -2 , or up to 150mA·cm -2 , or up to 200mA·cm -2 The oxygen evolution current is shown.

[0310] In some embodiments, the composite material has a current density of up to 50 mA cm at 1.6 V vs. the reversible hydrogen electrode during step (B). -2 , e.g., up to 100 mA cm -2 , or up to 150mA·cm -2 , or up to 200mA·cm -2 The oxygen evolution current is shown.

[0311] Thus, the composite material of the present invention can exhibit high catalytic activity for electrolytic oxygen production under alkaline water electrolysis conditions, even when the substrate on which the composite material is deposited contains no nickel, or at most only trace amounts of nickel.

[0312] In some embodiments, the method may further comprise the step (C) of: (C) partially electrochemically oxidizing the second electrode, which step (C) is carried out periodically during the performance of step (B); Optionally, step (C) is performed periodically at intervals of 50 to 5400 seconds, and / or each performance of step (C) is performed for 0.5 to 100 seconds.

[0313] Thus, step (C) is an optional step that may be carried out periodically during the performance of step (B). The partial oxidation of step (C) may be carried out in a manner similar to the partial electrochemical oxidation of step (iii) of the method according to the first aspect of the present invention. Thus, during step (C), the Ni on the composite material may be 2+ and / or Ni 3+ The surface layer can be periodically renewed.

[0314] After each execution of step (C), the oxygen evolution current may be returned to its initial value (i.e., the value in step (B)).

[0315] This allows the catalytic activity of ox-Ni@rGO to be maintained during electrolytic oxygen generation under alkaline electrolysis conditions. The oxidation process acts to "replenish" the oxidation level of the partially oxidized nickel on rGO.

[0316] In some embodiments, the partial electrochemical oxidation in step (C) is carried out at a voltage of 0 V to 2 V relative to a reference electrode.

[0317] For example, the partial electrochemical oxidation in step (C) may be carried out at a voltage corresponding to a voltage of 0 V to 2 V versus the reversible hydrogen electrode.

[0318] In some embodiments, the partial electrochemical oxidation in step (C) is carried out at a voltage of 0.3 V to 1.5 V, preferably 0.5 V to 1.2 V, more preferably 0.6 V to 1.1 V, for example, about 1 V, relative to a reference electrode.

[0319] For example, the partial oxidation in step (C) may be carried out at a voltage corresponding to 0.3 V to 1.5 V, preferably 0.5 V to 1.2 V, more preferably 0.6 V to 1.1 V, for example, about 1 V, relative to the reversible hydrogen electrode.

[0320] In some embodiments, the partial electrochemical oxidation of step (C) may be carried out at a voltage of 0 V to 2 V versus a reversible hydrogen electrode.

[0321] In some embodiments, the partial electrochemical oxidation in step (C) may be carried out at a voltage of 0.3 V to 1.5 V, preferably 0.5 V to 1.2 V, more preferably 0.6 V to 1.1 V, for example about 1 V, versus the reversible hydrogen electrode.

[0322] In some embodiments, step (C) is carried out periodically at intervals of 50 to 5400 seconds.

[0323] In some embodiments, step (C) is carried out periodically at intervals of 100 to 5000 seconds.

[0324] Preferably, in some embodiments, step (C) is carried out periodically at intervals of 150 to 1000 seconds, more preferably 200 to 500 seconds, for example, about 240 seconds.

[0325] In some embodiments, each performance of step (C) is carried out for 0.5 to 100 seconds.

[0326] Preferably, in some embodiments, each run of step (C) is carried out for 1 to 50 seconds, more preferably 5 to 25 seconds, for example, about 10 seconds.

[0327] (Urea oxidation assisted water electrolysis - method) The alkaline aqueous solution in step (A) may also contain urea, which allows electrolytic urea oxidation to occur under urea oxidation-assisted water electrolysis conditions.

[0328] Thus, a sixth aspect of the present invention provides a method for electrolytic urea oxidation (i.e., preferably to produce nitrogen and carbon dioxide) under urea oxidation-assisted water electrolysis conditions, the method comprising the steps of: (A) constructing a system comprising a solution comprising an alkaline aqueous solution and urea, a first electrode, a second electrode comprising a composite material according to the second aspect of the present invention, and an ion-permeable diaphragm disposed between the first electrode and the second electrode; and (B) applying a current between the first electrode and the second electrode to oxidize urea at the second electrode (i.e., preferably to produce nitrogen and carbon dioxide).

[0329] Thus, in some embodiments, the present invention provides a method for producing electrolytic nitrogen and carbon dioxide under urea oxidation assisted water electrolysis conditions, the method comprising the steps of: (A) constructing a system comprising a solution comprising an alkaline aqueous solution and urea, a first electrode, a second electrode comprising a composite material according to the second aspect of the present invention, and an ion-permeable diaphragm disposed between the first electrode and the second electrode; and (B) applying a current between the first electrode and the second electrode to produce nitrogen and carbon dioxide at the second electrode.

[0330] In some embodiments, the second electrode is an electrode according to the third aspect of the invention. The second electrode may be an anode.

[0331] (Process (A)-Construction) The first step (A) involves establishing the system in an alkaline aqueous solution, so that the electrolytic urea oxidation is carried out under alkaline conditions.

[0332] In some embodiments, the concentration of the alkaline aqueous solution in step (A) is 0.1 to 6 mol dm -3 , preferably 0.3 to 3 mol dm -3 , more preferably 0.5 to 1.5 mol dm -3 , for example, about 1 mol dm -3 is.

[0333] The pH of the alkaline solution in step (A) is greater than 7.0 and equal to or less than 14.0.

[0334] In some embodiments, the pH of the alkaline aqueous solution in step (A) is 8.0 to 14.0, preferably 9.0 to 14.0, more preferably 10.0 to 14.0, for example, about 14.0.

[0335] In some embodiments, the aqueous alkaline solution in step (A) is selected from aqueous potassium hydroxide (KOH), aqueous sodium hydroxide (NaOH), aqueous lithium hydroxide (LiOH), and a combination of any two or three thereof.

[0336] In a preferred embodiment, the alkaline solution in step (A) is potassium hydroxide solution (KOH) aq.

[0337] In some embodiments, the concentration of urea in step (A) is 0.5 to 10 mol dm -3 , preferably 1 to 5 mol dm -3 , more preferably 2 to 4 mol dm -3 , for example, about 3 mol dm -3 is.

[0338] In a preferred embodiment, the solution in step (A) comprises an aqueous potassium hydroxide solution (KOH) and urea, and the concentration of the aqueous potassium hydroxide solution (KOH) is about 1 mol dm -3 and the concentration of urea is about 3 mol dm -3 is.

[0339] The first electrode functions as a counter electrode to the second electrode, and may be a cathode.

[0340] In a preferred embodiment, the first electrode is an electrode according to the third aspect of the present invention.

[0341] In a preferred embodiment, the first electrode comprises nickel.

[0342] In some embodiments, the first electrode is nickel foam. In some embodiments, the first electrode is nickel mesh. In some embodiments, the first electrode is a nickel perforated plate. The system of step (A) also includes an ion-permeable diaphragm disposed between the first electrode and the second electrode.

[0343] An ion-permeable membrane separates the first and second electrodes and also separates the product gases generated at the first and second electrodes, preventing undesirable side reactions from occurring.

[0344] An ion-permeable membrane allows the passage / transfer of ions between the first and second electrodes, and is therefore porous and selective to ions.

[0345] In some embodiments, the ion-permeable membrane comprises nickel oxide or Zirfon®.

[0346] As described herein, the system of step (A) may also be referred to as an electrolytic cell (electrolyzer) for urea oxidation.

[0347] (Step (B)-Urea oxidation) The second step (B) is carried out after the first step (A).

[0348] Step (B) involves applying a current between the first electrode and the second electrode. Then, urea is oxidized at the second electrode. That is, preferably, nitrogen and carbon dioxide can be produced at the second electrode. Oxygen can also be produced at the second electrode.

[0349] Step (B) may be carried out in a system in which the first electrode is connected to the negative pole of a power source and the second electrode is connected to the positive pole of the power source. The power source may be a DC voltage source.

[0350] Without wishing to be bound by theory, it is believed that during step (B), the hydrogen evolution reaction (HER) occurs at the first electrode, while urea oxidation occurs at the second electrode.

[0351] At the first electrode, the overall HER mechanism is expressed as: 2H2O+2e - →H2+ 2OH - (7) In parallel, urea and OH in the alkaline aqueous solution generated at the first electrode (7) - Ions can diffuse through the ion-permeable membrane to the second electrode.

[0352] The overall mechanism of urea oxidation-assisted water electrolysis at the second electrode is believed to proceed according to the following equation (8): CO(NH2)2+6OH - →N2+5H2O+CO2+6e - (8) Thus, nitrogen and carbon dioxide can be produced at the second electrode.

[0353] Without wishing to be bound by theory, it is believed that the unique microstructure of the composite electrode facilitates the urea oxidation reaction, which is further enhanced by the reduced graphene oxide acting as an efficient current collector.

[0354] Therefore, the overall reaction of the electrolytic cell can be expressed as equation (9): CO(NH2)2+H2O→N2+3H2+CO2(9)

[0355] Furthermore, the OH in the alkaline aqueous solution generated at the first electrode - Ions can diffuse through the ion-permeable membrane to the second electrode, where OER can also occur, and thus oxygen can also be produced at the second electrode.

[0356] In some embodiments, step (B) is carried out at a temperature selected from 50 to 90°C, preferably 60 to 80°C, more preferably 70 to 80°C, for example, at about 75°C.

[0357] It is understood in the art that the theoretical minimum energy for urea oxidation-assisted water electrolysis (i.e., the process represented by Equation (9) above) is 0.37 V vs. the reversible hydrogen electrode (RHE) in a two-electrode system (i.e., a system including a cathode and an anode). However, experimentally, additional energy (known as cell overpotential) is required due to the polarization overpotentials of the cathode and anode and the internal resistance of the electrolyte. Therefore, the experimental minimum energy for urea oxidation-assisted water electrolysis is more negative (indicating a loss of energy), and catalysts are used to reduce this energy loss.

[0358] The second electrode comprises a composite material according to the second aspect of the present invention.

[0359] The composite material of the second embodiment can be used as a catalyst for urea oxidation under urea oxidation-assisted water electrolysis conditions. Without wishing to be bound by theory, the use of the composite material may result in reduced energy loss at the second electrode during step (B) due to a synergistic effect resulting from the unique microstructure produced by the method of the present invention.

[0360] In addition to the first and second electrodes, a reference electrode may be used to measure the working electrode potential. Thus, a three-electrode system may be used to perform urea oxidation. The reference electrode may be used to measure the half-cell potential at the second electrode.

[0361] For example, in some embodiments, step (B) is carried out at a voltage of 0.00 V to 4.00 V, preferably 0.00 V to 3.00 V, more preferably 0.00 V to 2.50 V, and even more preferably 0.00 V to 2.00 V relative to the reference electrode.

[0362] In some embodiments, step (B) is carried out at a voltage corresponding to 1.00 V to 4.00 V, preferably 1.00 V to 2.50 V, more preferably 1.00 V to 2.30 V, and even more preferably 1.00 V to 1.90 V vs. the reversible hydrogen electrode.

[0363] That is, the voltage may be selected depending on the type of reference electrode. Because many different reference electrodes can be envisioned and, therefore, a variety of "absolute potentials" are applicable, it may be useful to discuss voltages in terms of the effective potential difference when the reference electrode is a reversible hydrogen electrode. This does not imply that a reversible hydrogen electrode is necessarily present in embodiments of the present invention, but merely provides a useful reference point for evaluating voltages. Thus, voltages may be discussed herein in terms of voltages "equivalent to" the voltage relative to the reversible hydrogen electrode. Potentials may be converted from any other reference electrode to potentials relative to the reversible hydrogen electrode. Other reference electrodes may include a saturated calomel electrode.

[0364] Of course, in some embodiments, the reference electrode may be a reversible hydrogen electrode.

[0365] In some embodiments, step (B) is carried out at a voltage of 1.00 V to 4.00 V, preferably 1.00 V to 2.50 V, more preferably 1.00 V to 2.30 V, and even more preferably 1.00 V to 1.90 V versus the reversible hydrogen electrode.

[0366] Furthermore, the use of a composite material in the second electrode can result in a higher urea oxidation current for a fixed voltage. For example, the first electrode can result in a higher nitrogen and carbon dioxide current for a fixed voltage. The use of a composite material in the second electrode can also result in a higher oxygen evolution current for a fixed voltage.

[0367] For example, in some embodiments, the composite material exhibits a current density of up to 100 mA cm at a voltage corresponding to 1.9 V vs. the reversible hydrogen electrode during step (B). -2 , e.g., up to 150 mA cm -2 , or up to 200mA·cm -2 , or up to 300mA·cm -2 , or up to 325mA·cm -2 urea oxidation current.

[0368] In some embodiments, the composite material has a current density of up to 100 mA cm at 1.9 V vs. the reversible hydrogen electrode during step (B). -2 , e.g., up to 150 mA cm -2 , or up to 200mA·cm -2 , or up to 300mA·cm -2 , or up to 325mA·cm -2 urea oxidation current.

[0369] Thus, the composite material of the present invention can exhibit high catalytic activity for electrolytic urea oxidation under urea oxidation-assisted water electrolysis conditions, even when the substrate on which the composite material is deposited contains no nickel or at most trace amounts of nickel.

[0370] In some embodiments, the method further comprises the steps of: (C) a step of partially electrochemically oxidizing the second electrode, wherein step (C) is performed periodically during the performance of step (B); Optionally, step (C) is performed periodically at intervals of 50 to 5400 seconds, and / or each performance of step (C) is performed for 0.5 to 100 seconds.

[0371] Thus, step (C) is an optional step that may be carried out periodically during the performance of step (B). The partial oxidation of step (C) may be carried out in a manner similar to the partial electrochemical oxidation of step (iii) of the method of the first aspect of the present invention. Thus, during step (C), the Ni on the composite material may be 2+ and / or Ni 3+ The surface layer can be periodically renewed.

[0372] After each run of step (C), the urea oxidation current can return to its initial value (i.e., the value in step (B)). This allows the catalytic activity of ox-Ni@rGO to be maintained under urea oxidation-assisted water electrolysis conditions. The oxidation step acts to "support" the oxidation level of the partially oxidized nickel on rGO.

[0373] In some embodiments, the partial electrochemical oxidation in step (C) is carried out at a voltage of 0 V to 2 V relative to a reference electrode.

[0374] For example, the partial electrochemical oxidation in step (C) may be carried out at a voltage corresponding to a voltage of 0 V to 2 V versus the reversible hydrogen electrode.

[0375] In some embodiments, the partial electrochemical oxidation in step (C) is carried out at a voltage of 0.3 V to 1.5 V, preferably 0.5 V to 1.2 V, more preferably 0.6 V to 1.1 V, for example, about 1 V, relative to a reference electrode.

[0376] For example, the partial oxidation in step (C) can be carried out at a voltage corresponding to 0.3 V to 1.5 V, preferably 0.5 V to 1.2 V, more preferably 0.6 V to 1.1 V, for example, about 1 V, relative to the reversible hydrogen electrode.

[0377] In some embodiments, the partial electrochemical oxidation of step (C) may be carried out at a voltage of 0 V to 2 V versus a reversible hydrogen electrode.

[0378] In some embodiments, the partial electrochemical oxidation in step (C) is carried out at a voltage of 0.3 V to 1.5 V, preferably 0.5 V to 1.2 V, more preferably 0.6 V to 1.1 V, for example about 1 V, versus the reversible hydrogen electrode.

[0379] In some embodiments, step (C) is carried out periodically at intervals of 50 to 5400 seconds.

[0380] In some embodiments, step (C) is carried out periodically at intervals of 100 to 5000 seconds.

[0381] Preferably, in some embodiments, step (C) is carried out periodically at intervals of 150 to 1000 seconds, more preferably 200 to 500 seconds, for example, about 240 seconds.

[0382] In some embodiments, each performance of step (C) is performed for 0.5 to 100 seconds.

[0383] Preferably, in some embodiments, each performance of step (C) is carried out for 1 to 50 seconds, more preferably 5 to 25 seconds, for example, about 10 seconds.

[0384] The features disclosed in the foregoing description, or in the following claims, or in the accompanying drawings, in their specific form, or means for performing a disclosed function, or method or process for obtaining a disclosed result, may be utilized to realize the invention in various of its forms, either separately or in any combination of such features, as appropriate.

[0385] While the present invention has been described in conjunction with the above exemplary embodiments, many equivalent modifications and variations will be apparent to those skilled in the art given this disclosure. Accordingly, the exemplary embodiments of the present invention described above are considered to be illustrative and not limiting. Various changes can be made to the described embodiments without departing from the spirit and scope of the invention.

[0386] For the avoidance of doubt, any theoretical explanations provided herein are provided for the purpose of improving the understanding of the reader, and the inventors do not wish to be bound by any of these theoretical explanations. Any section headings used herein are for organizational purposes only and should not be construed as limiting the subject matter described.

[0387] Throughout this specification, including the claims which follow, unless the context requires otherwise, the words "comprise" and "include," and "comprising" and "including," will be understood to mean the inclusion of a stated integer or step or group of integers or steps, but not to the exclusion of any other integer or step or group of integers or steps.

[0388] It should be noted that, as used in this specification and the appended claims, the singular forms "a," "an," and "the" include plural referents unless the context clearly dictates otherwise. Ranges may be expressed herein as from "about" one particular value and / or to "about" another particular value. When such a range is expressed, another embodiment includes from the one particular value and / or to the other particular value. Similarly, when values ​​are expressed as approximations, by use of the antecedent "about," it will be understood that the particular value itself also constitutes a separate embodiment. With respect to numerical values, the term "about" is arbitrary and means, for example, + / - 10%. [Example]

[0389] (General Experimental Information) (Where to get reagents) Unless otherwise stated, reagents were obtained commercially from Sigma Aldrich.

[0390] (General methodological notes) Electrochemical deposition was performed under galvanostatic conditions calculated based on the geometric surface of the substrate on which the deposition was performed. Deposition was performed in a two-electrode electrochemical cell with a spiral Ni anode symmetrically arranged around a titanium mesh cathode. After deposition, the electrodes were washed with deionized water and transferred to a three-electrode electrochemical cell for oxidation and HER, OER, and urea oxidation-assisted water electrolysis measurements.

[0391] Before performing HER, OER, and urea oxidation-assisted water electrolysis, the deposited composite material was oxidized at 1.0 V (relative to RHE). The electrolyte resistance was corrected by hardware setup. Measurements of HER, OER, and urea oxidation-assisted water electrolysis were performed at 10 mV s -1 The measurements were carried out using cyclic voltammetry at a sweep rate of 0.05V.

[0392] Electrochemical measurements were performed in a single-chamber, three-electrode electrochemical cell using an IVIUM Vertex One potentiostat. A saturated calomel electrode (SCE) was used as the reference electrode, and the potential was converted to the RHE standard. A 3 × 3 cm Ni foam (Goodfellow Cambridge Limited, England) was used as the counter electrode. A 1 mol dm -3 A KOH solution (Sigma Aldrich) was used.

[0393] Transmission electron microscopy (TEM) images and electron diffraction patterns were obtained using a HRTEM JEOL 2100F field emission analytical electron microscope equipped with a spherical aberration corrector and operated at 200 kV. Scanning electron microscopy (SEM) images were obtained using a Phenom ProX electron microscope (Phenom, The Netherlands) equipped with an EDX detector.

[0394] (Effect of reduced graphene oxide) First, to demonstrate the beneficial effect of including graphene oxide in the deposition solution, Ni@rGO composites were prepared according to step (i) of the method of the present invention. A comparative composite made using a deposition solution that did not contain graphene oxide (thus depositing "pure nickel") was also prepared.

[0395] Example 1a The synthesis of Ni@rGO composite was carried out according to step (i) of the method of the present invention, i.e., 0.2 mol dm -3 Boric acid (H3BO3), 0.5mol·dm -3 Ammonium chloride (NH4Cl), and 0.125 mol dm -3 Contains nickel sulfate (NiSO4), 0.13 g dm -3 Graphene oxide was electrochemically deposited directly onto a titanium grid from a deposition solution containing graphene oxide dispersed at a concentration of .

[0396] The deposition was performed in a system in which the titanium mesh (substrate) was connected to the negative electrode of a DC voltage source and the nickel wire was connected to the positive electrode, at a current of 50 mA cm.-2 The deposition was carried out for 90 seconds under constant current conditions.

[0397] Example 2a 200mA·cm -2 Example 2a was prepared similarly to Example 1a, except that the deposition was carried out under galvanostatic conditions.

[0398] Example 3a 500mA·cm -2 Example 3a was prepared similarly to Example 1a, except that the deposition was carried out under galvanostatic conditions.

[0399] (Comparative example 1b) Comparative Example 1b was prepared similarly to Example 1a, except that no graphene oxide was present in the deposition solution.

[0400] (Comparative Example 2b) 200mA·cm -2 Comparative Example 2b was prepared similarly to Comparative Example 1b, except that the deposition was carried out under constant current conditions.

[0401] (Comparative example 3b) 500mA·cm -2 Comparative Example 3b was prepared similarly to Comparative Example 1b, except that the deposition was carried out under constant current conditions of 0.1 V.

[0402] (result) After deposition, the resulting electrodes of Examples 1a, 2a, and 3a, and Comparative Examples 1b, 2b, and 3b were washed with distilled water and transferred to a three-electrode system for evaluation of electrode kinetics. -3 A potassium hydroxide solution of 5 mV·s was used. -1The polarization rate was measured in steady-state mode at 1000 kJ / s. In the case of the Ni@rGO composite, regardless of the deposition current, the resulting deposit was a composite of reduced graphene oxide and nickel in a two-dimensional form formed by the presence of reduced graphene oxide, with nickel particles dispersed on the reduced graphene oxide sheets (Figure 1A). Compared to pure nickel deposits obtained under the same deposition conditions but from the same deposition solution without dispersed graphene oxide, the activity of the Ni@rGO composite was higher (approximately 50%) in all cases tested (Figure 1B).

[0403] (Effect of graphene oxide concentration) To investigate the effect of varying the concentration of graphene oxide in the deposition solution, we followed step (i) of the method of the present invention and used 0.13 g dm -3 , 0.4g·dm -3 , and 1.3 g·dm -3 Three Ni@rGO composites were prepared using a deposition solution containing graphene oxide at a concentration of .

[0404] Example 4 The synthesis of the three Ni@rGO composites was carried out using 0.2 mol dm -3 of boric acid (H3BO3), 0.5 mol dm -3 of ammonium chloride (NH4Cl), 0.125 mol dm -3 of nickel sulfate (NiSO4), each containing 0.13 g dm -3 , 0.4g·dm -3 , and 1.3 g·dm -3 The deposition was carried out by direct electrochemical deposition onto titanium grids from solutions containing graphene oxide at concentrations of 200 mA cm. -2 The deposition was carried out under constant current conditions of 0.1 V and for 90 seconds.

[0405] After deposition, the resulting electrode was washed with distilled water and transferred to a three-electrode system for evaluation of the electrode kinetics. -3 A potassium hydroxide solution of 5 mV·s was used. -1The polarization rate was 0.13 g dm , and the graphene oxide concentration was found to partially affect the morphology of the resulting Ni@rGO composite precipitates (Figure 2A). The lowest graphene oxide concentration (0.13 g dm ) -3 The highest catalytic activity was obtained for the Ni@rGO composite deposited from a solution with rGO (Figure 2B). This is likely due to the fact that under higher concentration conditions, aggregation of graphene oxide in the deposition solution can occur, resulting in a decrease in flake deposition efficiency.

[0406] Without wishing to be bound by theory, the Ni@rGO composites of Examples 1a, 2a, 3a, and 4 exhibit improved catalytic activity for hydrogen evolution compared to the pure nickel composites of Comparative Examples 1b, 2b, and 3b due to the overflow of adsorbed hydrogen from nickel to the reduced graphene oxide.

[0407] This effect is therefore demonstrated using a wide range of graphene oxide concentrations.

[0408] (Transmission electron spectroscopy (Ni@rGO)) Example 5 Based on the transmission electron microscopy results (Figure 3A) for Example 4, it was confirmed that the nickel particles in the Ni@rGO composite had nanometer dimensions. Electron diffraction analysis (Figure 3B) showed that the nickel lattice had a face-centered cubic lattice morphology. The nickel particles were deposited on the reduced graphene oxide layer, and their structure was also clearly observed.

[0409] (Preparation of nickel oxide-reduced graphene composite (ox-Ni@rGO)) To demonstrate the effectiveness of the present invention, a (partially) oxidized nickel-reduced graphene oxide composite (ox-Ni@rGO) was prepared according to the method of the present invention. A comparative composite containing only nickel was also prepared (prepared using a deposition solution without graphene oxide, as described above). The hydrogen evolution currents of the electrochemically deposited nickel and Ni@rGO composites were measured before and after the oxidation treatment.

[0410] Example 6 The synthesis of Ni@rGO composite was carried out using 0.2 mol dm -3 of boric acid (H3BO3), 0.5 mol dm -3 of ammonium chloride (NH4Cl), 0.125 mol dm -3 of nickel sulfate (NiSO4), 0.13 g dm -3 The deposition was performed by direct electrochemical deposition onto a titanium grid from a solution containing graphene oxide at a concentration of 500 mA cm. -2 The deposition was performed at a constant current of 0.05 V for 90 seconds. In a similar manner, pure nickel was deposited on a titanium grid from the same deposition solution, except that graphene oxide was not present.

[0411] After deposition, the electrode was washed with distilled water and transferred to a three-electrode system for evaluation of the electrode kinetics. -3 The measurement was performed at room temperature with a voltage of 5 mV·s -1 The polarization rate was measured in a steady-state mode at 1000 kJ / cm². The hydrogen evolution rate was first measured on an unoxidized electrode. As already shown in Examples 1a-3a and Comparative Examples 1b-3b, Ni@rGO exhibits higher catalytic activity than pure Ni.

[0412] A new electrode was then prepared and filled with the same solution (1 mol dm -3 The electrochemical oxidation was performed in a potassium hydroxide solution (pH 7.0) at a potential of 1.0 V (based on RHE) for 30 seconds, and the hydrogen evolution rate was measured. In this way, the ox-Ni@rGO composite was obtained, as well as the "oxidized Ni composite." As shown in Figure 4, after the oxidation treatment, the activity of the pure nickel electrode improved, and the activity of the ox-Ni@rGO composite also improved. In the latter case, the current obtained was up to 500 mA cm at -0.35 V vs. the reversible hydrogen electrode. -2 is.

[0413] Therefore, the ox-Ni@rGO composite exhibits improved catalytic activity in cathodes for HER applications over pure nickel, nickel oxide, and Ni@rGO composites. Without wishing to be bound by theory, the increased activity of the ox-Ni@rGO composite is the result of a combination of hydrogen overflow and accelerated water dissociation by the formed surface phase of nickel oxide. The surprising synergistic effect caused by the unique microstructure produced by the method of the present invention was unexpected.

[0414] (Scanning electron microscope observation (ox-Ni@rGO)) Example 7 Scanning electron microscopy results for Example 7 (Figure 5) show an SEM image of a single flake of the ox-Ni@rGO composite, which shows partially oxidized nickel particles dispersed on a single reduced graphene oxide flake.

[0415] (Further oxidation treatment of nickel oxide-reduced graphene oxide composite (ox-Ni@rGO)) Example 8 To demonstrate that the activity of the ox-Ni@rGO composite can be maintained by periodic oxidation treatment within the scope of the present invention, the ox-Ni@rGO composite was prepared according to Example 6 and then subjected to alkaline electrolytic hydrogen generation conditions at a constant potential of −0.4 V vs. RHE. Every 240 s, the ox-Ni@rGO composite was subjected to a short oxidation treatment at 1.0 V vs. RHE for 10 s, after which the hydrogen evolution current returned to its initial value (Figure 6).

[0416] The increase in catalytic activity of ox-Ni@rGO over time coincides with each oxidation treatment, as shown in Figure 6. Therefore, this additional oxidation treatment allows the composite to maintain its high activity.

[0417] Use of nickel oxide-reduced graphene oxide composite (ox-Ni@rGO) for OER Example 9 To demonstrate that the ox-Ni@rGO composite can also effectively function as an anode catalyst for OER, an electrode was prepared using a titanium grid according to the method described in Example 6. Meanwhile, 0.2 mol dm -3 of boric acid (H3BO3), 0.5 mol dm -3 of ammonium chloride (NH4Cl), 0.125 mol dm -3 of nickel sulfate (NiSO4), 0.13 g dm -3 The deposition was performed from a solution containing graphene oxide at a concentration of 500 mA cm. -2 The deposition was performed at a constant current of 0.05 V for 90 seconds. In a similar manner, pure nickel was deposited on a titanium grid from the same deposition solution, except that graphene oxide was not present.

[0418] After deposition, the electrode was washed with distilled water and transferred to a three-electrode system for evaluation of the electrode kinetics. Then, before measuring the rate of OER, the same solution (1 mol dm -3 The electrode was electrochemically oxidized by holding it in a potassium hydroxide solution at a potential of 1.0 V (relative to RHE) for 30 seconds, yielding the ox-Ni@rGO composite and the similarly obtained composite, “oxidized Ni” (ox-Ni).

[0419] The ox-Ni and ox-Ni@rGO composites thus obtained were treated with 1 mol dm -3 The OER was tested in a potassium hydroxide solution at 5 mV s at room temperature. -1 The polarization rate was 1.0 V vs. the reversible hydrogen electrode (REHE) in a steady-state mode. As shown in Figure 7, the activity of the ox-Ni@rGO composite electrode for OER is higher compared to that of the ox-Ni composite. In the former case, the obtained current was up to 200 mA cm at 1.7 V vs. the reversible hydrogen electrode. -2 is.

[0420] Thus, the ox-Ni@rGO composite exhibits improved catalytic activity in the anode for OER compared with nickel oxide composite.

[0421] Use of nickel oxide-reduced graphene oxide composite (ox-Ni@rGO) in urea oxidation-assisted water electrolysis Example 10 To demonstrate that the ox-Ni@rGO composites can also effectively function as anode catalysts for the electroproduction of nitrogen and carbon dioxide under urea oxidation-assisted water electrolysis conditions, ox-Ni and ox-Ni@rGO were prepared according to Example 9.

[0422] The obtained ox-Ni and ox-Ni@rGO composites were dissolved in 3 mol dm -3 of urea in 1 mol dm -3 The urea oxidation-assisted alkaline water electrolysis was tested in a potassium hydroxide solution at room temperature. -1 The polarization rate was 0.01, and the polarization rate was 0.01. As can be seen in Figure 8, the activity of the ox-Ni@rGO composite electrode is higher than that of the ox-Ni composite electrode.

[0423] Thus, the ox-Ni@rGO composite exhibits improved catalytic activity as an anode for electrolytic urea oxidation under urea oxidation-assisted water electrolysis conditions compared with nickel oxide composites.

Claims

1. 1. A method for preparing a composite material, comprising the steps of: (i) electrochemically depositing a material onto a substrate from a deposition solution comprising a nickel(II) salt and graphene oxide, thereby obtaining a nickel-reduced graphene oxide composite material comprising nickel dispersed on reduced graphene oxide, said composite material being deposited on said substrate; (ii) after step (i), placing the substrate bearing the nickel-reduced graphene oxide composite material together with a counter electrode in an alkaline solution; and (iii) after step (ii), partially electrochemically oxidizing the nickel to obtain a partially oxidized nickel-reduced graphene oxide composite comprising partially oxidized nickel dispersed on reduced graphene oxide, the composite being deposited on the substrate.

2. The method of claim 1 , wherein the substrate comprises nickel or titanium.

3. The method of claim 1 , wherein the substrate is nickel-free or contains at most trace amounts of nickel.

4. 4. The method according to any one of claims 1 to 3, wherein nickel is present in the composite material obtained in step (iii) in an amount of from 20 to 80 wt% by weight based on the total weight of the composite material.

5. The concentration of the nickel (II) salt in the deposition solution is 0.01 to 3 mol dm －３ and optionally, the concentration of the nickel(II) salt in the deposition solution is about 0.125 mol dm －３ The method according to any one of claims 1 to 4, wherein

6. The concentration of graphene oxide in the deposition solution is 0.01 to 2 g dm －３ and optionally, the concentration of graphene oxide in the deposition solution is about 0.13 g dm －３ The method according to any one of claims 1 to 5, wherein

7. The electrochemical deposition of step (i) is carried out under constant current conditions, and optionally, the electrochemical deposition of step (i) is carried out under constant current conditions of 50 to 1000 mA cm －２ and optionally, the electrochemical deposition of step (i) is carried out at a constant current density selected within the range of about 500 mA cm －２ The method according to any one of claims 1 to 6, wherein the method is carried out at a constant current density of

8. 8. The method of any one of claims 1 to 7, wherein the electrochemical deposition of step (i) is carried out for 5 to 500 seconds, optionally the electrochemical deposition of step (i) is carried out for about 90 seconds.

9. 9. The method of any one of claims 1 to 8, wherein the partial electrochemical oxidation of step (iii) is carried out for 5 to 2000 seconds, optionally wherein the electrochemical oxidation of step (iii) is carried out for about 30 seconds.

10. 10. The method of any one of claims 1 to 9, wherein the partial electrochemical oxidation of step (iii) is carried out at a voltage corresponding to between 0 V and 2 V vs. a reversible hydrogen electrode, and optionally, the partial electrochemical oxidation of step (iii) is carried out at a voltage corresponding to about 1.0 V vs. a reversible hydrogen electrode.

11. The method according to any one of claims 1 to 10, further comprising the steps of: (iv) after step (iii), removing the composite material from the substrate to obtain a free composite material; and optionally (v) after step (iv), grinding the loose composite material to obtain a powder composite material.

12. A composite material obtained or obtainable by the method according to any one of claims 1 to 11.

13. 1. A method for producing hydrogen electrolytically under alkaline electrolysis conditions, comprising the steps of: (A) constructing a system including an alkaline aqueous solution, a first electrode comprising the composite material of claim 12, a second electrode, and an ion-permeable diaphragm disposed between the first electrode and the second electrode; and (B) applying a current between the first electrode and the second electrode to generate hydrogen at the first electrode; Optionally, the method further comprises the steps of: (C) partially electrochemically oxidizing the first electrode, wherein step (C) is performed periodically during the performance of step (B); Optionally, step (C) is performed periodically at intervals of from 50 to 5400 seconds, and / or each performance of step (C) is performed for from 0.5 to 100 seconds.

14. 1. A method for electrolytic oxygen production under alkaline electrolysis conditions, comprising the steps of: (A) constructing a system comprising an aqueous alkaline solution, a first electrode, a second electrode comprising the composite material of claim 12, and an ion-permeable diaphragm disposed between the first electrode and the second electrode; and (B) applying a current between the first electrode and the second electrode to generate oxygen at the second electrode.

15. 1. A method for electrolytic urea oxidation under urea oxidation-assisted water electrolysis conditions, comprising the steps of: (A) constructing a system comprising an aqueous alkaline solution and urea-containing solution, a first electrode, a second electrode comprising the composite material of claim 12, and an ion-permeable membrane disposed between the first electrode and the second electrode; and (B) applying a current between the first electrode and the second electrode to oxidize urea at the second electrode.

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