WO2022067377A1

WO2022067377A1 - Catalyst and electrolyser for the synthesis of hydrogen peroxide

Info

Publication number: WO2022067377A1
Application number: PCT/AU2021/051112
Authority: WO
Inventors: Yuan Chen; Li Wei; Chang Liu
Original assignee: The University Of Sydney
Priority date: 2020-09-29
Filing date: 2021-09-23
Publication date: 2022-04-07

Abstract

The present invention provides a series of novel, low-cost catalysts formed by loading transition metal incorporated phthalocyanine on conductive carbon substrates. The novel catalysts have high activity and selectivity in ORR towards H2O2 production in both acidic and neutral conditions. Notably, such activity is superior to existing catalysts. The invention further resides in a novel self-powered electrolyser, which uses one proton exchange membrane and polymer-based solid electrolyte. The electrolyser design works in acidic or neutral conditions, avoiding the use of high cost and unstable anion exchange membranes and the self-decomposition of H2O2 in alkaline conditions.

Description

CATALYST AND ELECTROLYSER FOR THE SYNTHESIS OF HYDROGEN PEROXIDE

Related Application

[001] This application claims priority to Australian provisional patent application 2020903503, filed 29 September 2020. The content of AU’ 503 is incorporated by reference herein in its entirety.

Field of the Invention

[002] The present invention relates to M-phthalocyanine/C compounds, per se, wherein M is a transition metal selected from Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu and Zn. In alternative forms, the invention relates to the M-phthalocyanine/C compounds for use in the synthesis of hydrogen peroxide. A further alternative form of the invention relates to a two-electrode electrolyser, per se, and more specifically, the electrolyser for use in the synthesis of hydrogen peroxide, preferably using the M-phthalocyanine/C compounds as catalysts.

[003] Although the present invention will be described hereinafter with reference to its preferred embodiment, it will be appreciated by those of skill in the art that the spirit and scope of the invention may be embodied in many other forms.

Background of the Invention

[004] Any discussion of the prior art throughout the specification should in no way be considered as an admission that such prior art is widely known or forms part of common general knowledge in the field.

[005] The present invention is set against a background in which hydrogen peroxide (H2O2) is an essential chemical with a global production capacity of over 5.5 MT in 2015. H2O2, as an environmentally friendly oxidising agent, has a wide range of applications in disinfection control, chemical synthesis, pulping and textile bleaching, and wastewater treatment.

[006] Currently, over 95% of H2O2 is produced by the energy and material intensive hydrogenation-oxidation cycle of anthraquinone, which also requires palladium-based precious metal catalysts and additional transportation and distribution of hazardous high concentration H2O2. A small amount of H2O2 is currently produced by decentralised methods such as electrochemical two-electron oxygen reduction reaction (ORR) using different types of electrolysers. Decentralised methods are attractive because they can provide H2O2 on-demand, with better safety.

[007] However, all current electrochemical synthesis methods have essential limitations including catalysts with high cost, low activity and selectivity, high cost and low-performance ion exchange membranes, and low H2O2 production yield and purity. [008] A representative survey of the prior art is provided below and summarised with reference to Figure 1(a) to Figure 1(f), and for comparative purposes with the present invention, Table 1.

[009] The Dow-Huron process¹² is shown in Figure 1(a). H2O2 is produced by the oxygen reduction reaction on a cathode, which consists of graphite chips coated with a mixture of carbon black and Teflon. This process can produce 2wt.% H2O2H1 1 M sodium hydroxide at an efficiency of about 67%, which is suitable for pulp bleaching. This method has been commercialised and exhibits competitiveness in the mature anthraquinone redox process, which produces over 95% global H2O2. However, the strong alkaline electrolyte (1 M sodium hydroxide) used in this process is problematic. The strong alkaline electrolyte causes spontaneous decomposition of H2O2 and requires CO2-free gases because CO2 would react with the electrolyte to form detrimental carbonate salts. The strong alkaline electrolyte also corrodes carbon black electrocatalysts and graphite rods.

[010] The Electro-Fenton process^[3-8] is shown in Figure 1(b). This process is a modified version of the Huron-Dow process. It produces H2O2by the oxygen reduction reaction on a carbon-based cathode in a low pH electrolyte (~3). This process is suitable for wastewater treatment facility where the process can be integrated with the existing facility. However, the process cannot directly produce H2O2/H2O solutions.

[Oil] A microbial electrochemical cell^[9-14] is shown in Figure 1(c). Such a device uses carbon materials-based cathode for H2O2 production. It can produce an H2O2 solution with a concentration of up to ~1%. The anode is modified with microbes. This configuration can produce electrical energy during operation. However, microbes on the anode usually exhibit low catalytic activity.

[012] A proton-exchange membrane electrolyser is embodiment in two different configurations. The scheme of the first setup is shown in Figure 7(<7 .^{[15 18]} H2O2 is produced on the cathode by the oxygen reduction reaction. Hydrogen oxidation reaction or oxygen evolution reaction is proceeded on the anode to provide protons. ^[19] A different version is displayed in Figure le, in which H2O2 is produced on the anode by water oxidation, while the cathode can produce H2 by hydrogen evolution reaction. ^[20-22] However, both versions are difficult to maintain a high H2O2 selectivity for the water oxidation reaction. Further, the water oxidation pathway requires the high electrical potential to drive the electrochemical reaction (2H2O — H2O2+ 2H⁺ + 2e“; Eo = 1.76 V vs. RHE).^[23’ ^24]

[013] A dual-membrane electrolyser with a solid electrolyte is represented in Figure 1(f). Recently, a dual membrane electrolyser with a solid electrolyte has been reported, which uses both cationic and anionic exchange membranes (CEM and AEM).^[1] As shown in Figure If, a solid electrolyte (functionalised styrene-divinylbenzene copolymer microspheres) is used to separate the cathode and anode.

[014] The electrolyser uses oxidised carbon black to produce H2O2on the cathode via the oxygen reduction reaction. The proton and HO2’ anions, which are produced across the membranes, recombine to form H2O2 in the solid electrolyte. It was reported that this electrolyser could produce up to 20wt.% H2O2 water solution. However, the high alkalinity in the CEM side can cause HO2’ decomposition. The cost of the AEMs is also very high. The transport of large HO2’ anions through the AEMs is also too slow to engender widespread commercial interest.

[015] Representative patent literature includes US 2007/0275160, to the Board of Regents The University of Texas System (CNF electrodes prepared by pyrolysis of iron(II) phthalocyanine); US 2011/0034325, to Acta SpA (phthalocyanine-transition metal complexes for catalytic ORR, characterised by low peroxide generation); US 2011/0244357, to Swift Enterprises Ltd (an electrocatalyst composition comprising carbon black and a catalyst comprised of a macrocycle and a metal); US 2013/0029234, to Samsung Electronics Co Ltd (a porous carbonaceous composite comprising a carbon nanotube and a hetero -element); and US 2017/0023508, to King Abdulaziz University (thin film humidity sensors comprising nickel-phthalocyanine-fullerenes).

[016] It is an object of the present invention to overcome or ameliorate at least one of the disadvantages of the prior art, or to provide a useful alternative.

[017] It is an object of an especially preferred form of the invention to provide an alternative means for commercial scale H2O2 production.

[018] To this end, the present Inventors have developed a series of novel, low-cost catalysts formed by loading transition metal incorporated phthalocyanine on conductive carbon substrates. The novel catalysts have high activity and selectivity in ORR towards H2O2 production in both acidic and neutral conditions. Preferably, such activity is superior to existing catalysts.

[019] Based on such catalysts, the invention further resides in a novel self-powered electrolyser, which uses one proton exchange membrane and polymer-based solid electrolyte. The electrolyser design works in acidic or neutral conditions, avoiding the use of high cost and unstable anion exchange membranes and the self-decomposition of tkChin alkaline conditions.

[020] Although the invention will be described with reference to specific examples it will be appreciated by those skilled in the art that the invention may be embodied in many other forms.

Definitions

[021] In describing and defining the present invention, the following terminology will be used in accordance with the definitions set out below. It is also to be understood that the terminology used herein is for the purpose of describing particular embodiments of the invention only and is not intended to be limiting. Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one having ordinary skill in the art to which the invention pertains.

[022] Unless the context clearly requires otherwise, throughout the description and the claims, the words “comprise”, “comprising” and the like are to be construed in an inclusive sense as opposed to an exclusive or exhaustive sense; that is to say, in the sense of “including, but not limited to”.

[023] As used herein, the phrase “consisting of’ excludes any element, step, or ingredient not specified in the claim. When the phrase “consists of’ (or variations thereof) appears in a clause of the body of a claim, rather than immediately following the preamble, it limits only the element set forth in that clause; other elements are not excluded from the claim as a whole. As used herein, the phrase “consisting essentially of’ limits the scope of a claim to the specified elements or method steps, plus those that do not materially affect the basis and novel characteristic(s) of the claimed subject matter.

[024] With respect to the terms “comprising”, “consisting of’ and “consisting essentially of’, where one of these three terms are used herein, the presently disclosed and claimed subject matter may include the use of either of the other two terms. Thus, in some embodiments not otherwise explicitly recited, any instance of “comprising” may be replaced by “consisting of’ or, alternatively, by “consisting essentially of’. [025] Other than in the operating examples, or where otherwise indicated, all numbers expressing quantities of ingredients or reaction conditions used herein are to be understood as modified in all instances by the term “about”, having regard to normal tolerances in the art. The examples are not intended to limit the scope of the invention. In what follows, or where otherwise indicated, “%” will mean “weight %”, “ratio” will mean “weight ratio” and “parts” will mean “weight parts”.

[026] The term “substantially” as used herein shall mean comprising more than 50%, where relevant, unless otherwise indicated.

[027] The recitation of a numerical range using endpoints includes all numbers subsumed within that range (e.g., 1 to 5 includes 1, 1.5, 2, 2.75, 3, 3.80, 4, 5, etc.).

[028] The terms “preferred” and “preferably” refer to embodiments of the invention that may afford certain benefits, under certain circumstances. However, other embodiments may also be preferred, under the same or other circumstances.

Furthermore, the recitation of one or more preferred embodiments does not imply that other embodiments are not useful and is not intended to exclude other embodiments from the scope of the invention.

[029] It must also be noted that, as used in the specification and the appended claims, the singular forms “a”, “an” and “the” include plural referents unless the context clearly dictates otherwise.

[030] The person skilled in the art would appreciate that the embodiments described herein are exemplary only and that the electrical characteristics of the present application may be configured in a variety of alternative arrangements without departing from the spirit or the scope of the invention.

[031] Although example embodiments of the disclosed technology are explained in detail herein, it is to be understood that other embodiments are contemplated. Accordingly, it is not intended that the disclosed technology be limited in its scope to the details of construction and arrangement of components set forth in the following description or illustrated in the drawings. The disclosed technology is capable of other embodiments and of being practised or carried out in various ways.

Summary of the Invention

[032] According to a first aspect of the present invention there is provided a compound of the general formula MPc/C, wherein

[033] Pc is phthalocyanine; [034] M is a d transition metal selected from fourth-row, group IIIB to IIB elements coordinated to the phthalocyanine to form the MPc moiety; and

[035] C is a sp²-hybridised conductive carbon substrate, the conductive carbon substrate being electrochemically associated with the MPc moiety.

[036] In an embodiment, M is selected from the group consisting of Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu and Zn. Preferably M is Co. Alternatively, M is Ni.

[037] In an embodiment, the MPc moiety is represented by the general formula (1):

(1); M = Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn

[038] In an embodiment, C is selected from the group consisting of multi-walled carbon nanotubes (MWCNT), graphene, graphene oxide (GO), reduced graphene oxide (rGO), or carbon black (CB). Preferably, C is multi-walled carbon nanotubes (MWCNT). Alternatively, C is carbon black (CB).

[039] In an embodiment, the compound of the general formula MPc/C has between about 0.1 and about 0.5wt.% M content. In other embodiments, the compound of the general formula MPc/C has about 0.15, 0.2, 0.25, 0.3, 0.35, 0.4, 0.45 or 0.5wt.% M content. Preferably, the compound of the general formula MPc/C has about 0.2wt.% M content.

[040] According to a second aspect of the present invention there is provided a compound of the general formula MPc/C, wherein Pc is phthalocyanine; M is a transition metal selected from fourth-row, group IIIB to IIB elements coordinated to the phthalocyanine to form the MPc moiety; and C is a sp²-hybridised conductive carbon substrate, the conductive carbon substrate being electrochemically associated with the MPc moiety, for use as a catalyst in the synthesis of hydrogen peroxide.

[041] According to a third aspect of the present invention there is provided a compound of the general formula MPc/C, wherein Pc is phthalocyanine; M is a transition metal selected from fourth-row, group IIIB to IIB elements coordinated to the phthalocyanine to form the MPc moiety; and C is a sp²-hybridised conductive carbon substrate, the conductive carbon substrate being electrochemically associated with the MPc moiety, when used as a catalyst in the synthesis of hydrogen peroxide.

[042] According to a fourth aspect of the present invention there is provided a compound of the general formula MPc/C, wherein Pc is phthalocyanine; M is a transition metal selected from fourth-row, group IIIB to IIB elements coordinated to the phthalocyanine to form the MPc moiety; and C is a sp²-hybridised conductive carbon substrate, the conductive carbon substrate being electrochemically associated with the MPc moiety, restricted in use as a catalyst in the synthesis of hydrogen peroxide.

[043] In an embodiment of the second, third and/or fourth aspects, M is selected from the group consisting of Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu and Zn. Preferably M is Co. Alternatively, M is Ni.

[044] In an embodiment of the second, third and/or fourth aspects, the MPc moiety is represented by the general formula (1):

(1); M = Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn

[045] In an embodiment of the second, third and/or fourth aspects, C is selected from the group consisting of multi-walled carbon nanotubes (MWCNT), graphene, graphene oxide (GO), reduced graphene oxide (rGO), or carbon black (CB). Preferably, C is multi-walled carbon nanotubes (MWCNT). Alternatively, C is carbon black (CB).

[046] In an embodiment of the second, third and/or fourth aspects, the compound of the general formula MPc/C has between about 0.1 and about 0.5wt.% M content. In other embodiments, the compound of the general formula MPc/C has about 0.15, 0.2, 0.25, 0.3, 0.35, 0.4, 0.45 or 0.5wt.% M content. Preferably, the compound of the general formula MPc/C has about 0.2wt.% M content.

[047] According to a fifth aspect of the present invention there is provided a method of synthesising a compound of the general formula MPc/C, wherein Pc is phthalocyanine; M is a transition metal selected from fourth-row, group IIIB to IIB elements coordinated to the phthalocyanine to form the MPc moiety; and C is a sp²-hybridised conductive carbon substrate, the conductive carbon substrate being electrochemically associated with the MPc moiety, the method comprising the steps of:

[048] sonicating over a predetermined period one part by weight MPc with between about 50 and about 200 parts by weight of C in an appropriate solvent; and [049] removing the solvent.

[050] In an embodiment, M is selected from the group consisting of Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu and Zn. Preferably M is Co. Alternatively, M is Ni.

[051] In an embodiment, the MPc moiety is represented by the general formula (1):

(1); M = Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn

[052] In an embodiment, C is selected from the group consisting of multi-walled carbon nanotubes (MWCNT), graphene, graphene oxide (GO), reduced graphene oxide (rGO), or carbon black (CB). Preferably, C is multi-walled carbon nanotubes (MWCNT). Alternatively, C is carbon black (CB).

[053] In an embodiment, the compound of the general formula MPc/C has between about 0.1 and about 0.5wt.% M content. In other embodiments, the compound of the general formula MPc/C has about 0.15, 0.2, 0.25, 0.3, 0.35, 0.4, 0.45 or 0.5wt.% M content. Preferably, the compound of the general formula MPc/C has about 0.2wt.% M content.

[054] In an embodiment, one part by weight MPc is sonicated with about 100 parts by weight of C.

[055] In an embodiment, the solvent is a polar aprotic solvent. Preferably, the solvent is selected from DMF, DMA, CH3CN, DMSO, NMP and mixtures thereof. More preferably, the solvent is DMF.

[056] In an embodiment, the predetermined period is about 60 minutes. In other embodiments, the predetermined period may be about 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90, 95, 100, 105, 110, 115 or 120 minutes.

[057] In an embodiment, MPc is cobalt phthalocyanine; C is multi-walled carbon nanotubes (MWCNT) in a weight ratio of about 100 parts per part of MPc.

[058] In an embodiment, the method further comprises the step of: after sonication and prior to solvent removal, further stirring at room temperature for about 12 h. In other embodiments, further stirring at room temperature is for about 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, or 24 h.

[059] In an embodiment, the step of removing to solvent is performed by heating under vacuum at about 100 mbar. In other embodiments, the step of removing to solvent is performed by heating under vacuum at about 10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 110, 120, 130, 140, 150, 160, 170, 180, 190 or 200 mbar.

[060] In an embodiment, after removing the solvent, the resultant solid is dried under vacuum at a temperature of about 60 °C for a period of about 12 h. In other embodiments, the temperature is about 20, 25, 30, 35, 4, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90, 95 or 100 °C. In other embodiments, the period is about 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, or 24 h.

[061] According to a sixth aspect of the present invention there is provided a compound of the general formula MPc/C, wherein Pc is phthalocyanine; M is a transition metal selected from fourth-row, group IIIB to IIB elements coordinated to the phthalocyanine to form the MPc moiety; and C is a sp²-hybridised conductive carbon substrate, the conductive carbon substrate being electrochemically associated with the MPc moiety, when synthesised by a method defined according to the fifth aspect of the invention.

[062] In an embodiment, M is selected from the group consisting of Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu and Zn. Preferably M is Co. Alternatively, M is Ni.

[063] In an embodiment, the MPc moiety is represented by the general formula (1):

(1); M = Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn

[064] In an embodiment, C is selected from the group consisting of multi-walled carbon nanotubes (MWCNT), graphene, graphene oxide (GO), reduced graphene oxide (rGO), or carbon black (CB). Preferably, C is multi-walled carbon nanotubes (MWCNT). Alternatively, C is carbon black (CB).

[065] In an embodiment, the compound of the general formula MPc/C has between about 0.1 and about 0.5wt.% M content. In other embodiments, the compound of the general formula MPc/C has about 0.15, 0.2, 0.25, 0.3, 0.35, 0.4, 0.45 or 0.5wt.% M content. Preferably, the compound of the general formula MPc/C has about 0.2wt.% M content.

[066] According to a seventh aspect of the invention there is provided a method for the synthesis of hydrogen peroxide, the method comprising an oxygen reduction reaction catalysed by a compound of the general formula MPc/C, wherein Pc is phthalocyanine; M is a transition metal selected from fourth-row, group IIIB to IIB elements coordinated to the phthalocyanine to form the MPc moiety; and C is a sp²-hybridised conductive carbon substrate, the conductive carbon substrate being electrochemically associated with the MPc moiety.

[067] In an embodiment, M is selected from the group consisting of Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu and Zn. Preferably M is Co. Alternatively, M is Ni.

[068] In an embodiment, the MPc moiety is represented by the general formula (1):

(1); M = Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn

[069] In an embodiment, C is selected from the group consisting of multi-walled carbon nanotubes (MWCNT), graphene, graphene oxide (GO), reduced graphene oxide (rGO), or carbon black (CB). Preferably, C is multi-walled carbon nanotubes (MWCNT). Alternatively, C is carbon black (CB).

[070] In an embodiment, the compound of the general formula MPc/C has between about 0.1 and about 0.5wt.% M content. In other embodiments, the compound of the general formula MPc/C has about 0.15, 0.2, 0.25, 0.3, 0.35, 0.4, 0.45 or 0.5wt.% M content. Preferably, the compound of the general formula MPc/C has about 0.2wt.% M content. [071] In an embodiment, MPc is cobalt phthalocyanine; C is multi-walled carbon nanotubes (MWCNT) in a weight ratio of about 100 parts per part of MPc. In other embodiments, the weight ratio is about 10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 110, 120, 130, 140, 150, 160, 170, 180, 190 or 200 parts per part MPc.

[072] In an embodiment, the MPc/C compound is present in an amount of about 1 mg/mL. In other embodiments, the MPc/C compound is present in an amount of about 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1.0, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9 or 2.0 mg/mL.

[073] In an embodiment, the MPc/C catalyst exhibits a selectivity of >70% toward hydrogen peroxide under acidic or neutral conditions. In other embodiments, the MPc/C catalyst exhibits a selectivity of about 70, 75, 80, 85, 90, 95 or 100% toward hydrogen peroxide under acidic or neutral conditions.

[074] According to an eighth aspect of the invention there is provided a two-electrode electrolyser comprising:

[075] a cathode for receiving a catalyst for catalysing an oxygen reduction reaction and a hydrogen oxidation reaction;

[076] an anode;

[077] a proton-exchange membrane; and

[078] a polymer-based solid electrolyte.

[079] In an embodiment, the catalyst is compound of the general formula MPc/C, wherein Pc is phthalocyanine; M is a transition metal selected from fourth-row, group IIIB to IIB elements coordinated to the phthalocyanine to form the MPc moiety; and C is a sp²-hybridised conductive carbon substrate.

[080] In an embodiment, M is selected from the group consisting of Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu and Zn. Preferably M is Co. Alternatively, M is Ni.

[081] In an embodiment, the MPc moiety is represented by the general formula (1):

(1); M = Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn [082] In an embodiment, C is selected from the group consisting of multi-walled carbon nanotubes (MWCNT), graphene, graphene oxide (GO), reduced graphene oxide (rGO), or carbon black (CB). Preferably, C is multi-walled carbon nanotubes (MWCNT). Alternatively, C is carbon black (CB).

[083] In an embodiment, the compound of the general formula MPc/C has between about 0.1 and about 0.5wt.% M content. In other embodiments, the compound of the general formula MPc/C has about 0.15, 0.2, 0.25, 0.3, 0.35, 0.4, 0.45 or 0.5wt.% M content. Preferably, the compound of the general formula MPc/C has about 0.2wt.% M content.

[084] In an embodiment, the anode is -20% Pt/C, 0.5 mg/cm² In other embodiments, the loading is about 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1.0, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9 or 2.0 mg/cm².

[085] In an embodiment, catalyst is mass loaded to the cathode at about 0.5 mg/cm². In other embodiments, the loading is about 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1.0, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9 or 2.0 mg/cm².

[086] In an embodiment, the proton exchange membrane is Nafion 212 (or similar), at a thickness of about 50 microns. In other embodiments, the thickness is about 10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 110, 120, 130, 140, 150, 160, 170, 180, 190 or 200 microns.

[087] In an embodiment, the electrolyser has a space between cathode and anode filled with polystyrene-divinylbenzene spherical macroporous polymer beads (CT275, Purolite, average diameter of 400 um to 550 um) as a solid electrolyte, or similar.

[088] In an embodiment, the cathode is adapted to receive oxygen at a flow rate of about 50 seem at about 1 atm. In other embodiments, the cathode is adapted to receive oxygen at a flow rate of about 10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 110, 120, 130, 140, 150, 160, 170, 180, 190 or 200 seem.

[089] In an embodiment, the anode is adapted to receive humidified hydrogen at a flow rate of about 50 seem at about 1 atm. In other embodiments, the anode is adapted to receive humidified hydrogen at a flow rate of about 10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 110, 120, 130, 140, 150, 160, 170, 180, 190 or 200 seem.

Brief Description of the Drawings

[090] A preferred embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawings in which: [091] Figure 7 is a schematic illustration of prior art electrochemical H2O2 production units as described in the section headed “Background of the Invention”, specifically: Figure 1(a) The Huron-Dow process; Figure 1(b) electro-Fenton process; Figure 1(c) microbial electrosynthesis; Proton-exchange membrane (PEM) electrolysers; Figure 1(d) oxygen reduction reaction; Figure 1(e) water oxidation; and Figure 1(f) Dualmembrane electrolyser with solid electrolyte (adapted from Reference^[1]).

[092] Figure 2 is a schematic drawing of a one-membrane electrolyser with a solid electrolyte. (1) and (6) represent the gas diffusion layer; (2) is the anode catalyst (20% Pt/C, 0.5 mg/cm²); (3) is the proton exchange membrane (Nafion 212, 50 //m thick); (4) is the polymer bead filler; and (5) is the cathode catalyst (CoPc/C, 0.5 mg/cm²). The electrolyser is described more particularly in the above description of the eighth aspect of the invention.

[093] Figure 3 is a photograph of a lab-scale two-electrode electrolyser, shown in situ with the other working componentry of the experimental set-up.

[094] Figure 4 is a close-up photograph of a lab-scale two-electrode electrolyser.

[095] Figure 5(a) shows Rotating Ring-Disk Electrode (RRDE) polarisation curves and Figure 5(b) shows calculated H2O2 selectivity of CoPc/CNT catalyst in 0.1 M acetic buffer electrolyte (pH 3.6).

[096] Figure 6(a) shows Rotating Ring-Disk Electrode (RRDE) polarisation curves and Figure 6(b) shows calculated H2O2 selectivity of CoPc/CNT catalyst in 0.1 M potassium phosphate buffer electrolyte (pH 7.4).

Detailed Description of a Preferred Embodiment

[097] In order to address one or more of the limitations of the prior art identified above, the present Inventors have proposed a series of novel, low-cost catalysts formed by loading transition metal incorporated phthalocyanine on conductive carbon substrates. The novel catalysts have high activity and selectivity in ORR towards H2O2 production in both acidic and neutral conditions. Notably, such activity is superior to existing catalysts.

[098] Based on such catalysts, the invention further resides in a novel self-powered electrolyser, which uses one proton exchange membrane and polymer-based solid electrolyte. The electrolyser design works in acidic or neutral conditions, avoiding the use of high cost and unstable anion exchange membranes and the self-decomposition of H2O2H1 alkaline conditions. (1) Catalyst synthesis method

[099] The catalysts were synthesised using a 3d-transition metal (fourth-row, group IIIB to IIB elements, including Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, or Zn) incorporated phthalocyanine (C32H18N8) molecule as a metal precursor, and an sp²-hybridised carbon material, such as single-walled carbon nanotube (SWCNT), multi-walled carbon nanotube (MWCNT), graphene, graphene oxide (GO), reduced graphene oxide (rGO), or carbon black (CB) as a conductive substrate.

[0100] The catalysts were denoted as MPc/C, where M stands for the metal centre in C32H16N8; Pc refers to C32H16N8; C refers to carbon material. Cobalt incorporated phthalocyanine on MWCNT substrate is used as an example to explain the catalyst synthesis process.

[0101] In a typical synthesis, about 0.1 g of cobalt phthalocyanine (C0C32H16N8; structure below) was dissolved in 100 mL N,N-dimethylformamide (DMF), followed by adding 10 g of MWCNT. The mixture was bath- sonicated for 60 min and further stirred at room temperature for 12 h. Afterward, the DMF solvent was removed by heating under vacuum (100 mbar), and the solvent was recovered by a cold trap for recycle. The solid mixture was recovered and further dried in a vacuum over at 60 °C for 12 hours to obtain the catalyst (denoted as CoPc/C). The Co metal loading in the CoPc/C catalyst, as determined by inductively coupled plasma atomic emission spectroscopy, is about 0.2 ± 0.02wt.%.

M = Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn

(2) Catalyst performance tests

[0102] The catalytic performance of the CoPc/C catalyst for H2O2 production was first evaluated using the three-electrode configuration on a rotary ring-disk electrode (RRDE, E6R2, glassy carbon disk with OD=5.5 mm, Pt ring with OD=8.5 mm and ID=6.5 mm, Pine Research Inc). Two types of liquid electrolytes were used, including oxygen saturated neutral (pH 7.4, 0.1 M phosphate -buffered saline, PBS) and acidic (pH 3.6, 0.1 M acetic buffer, ABS). All tests were carried out on an electrochemical workstation (CHI760, CHI Instrument). A pre-calibrated Ag/AgCl electrode (3 M KC1 filling, Basi, MF-2056) and a graphite electrode (Pine, AFCTR3B) were used as the reference and counter electrode, respectively. The catalyst ink was prepared by dispersing 1 mg of CoPc/C catalyst in 1 mL solution containing 975 pL of water/isopropanol solution (9/1 v/v) and 25 pL of 5% wt/v Nafion 117 solution by bath sonication for 30 min. The ink was dropped cast on the glassy carbon disk rotating at 200 rpm to reach a uniform catalyst coverage at a mass loading of 0.01 mg/cm²

[0103] The RRDE loaded with the catalyst was cycled with 50 cyclic voltammetry (CV) scans between 0.1 to 1 V versus a reversible hydrogen electrode (RHE) at a scan rate of 50 mV/s in Ar-saturated electrolytes. Afterward, linear sweep voltammetry (LSV) polarization curves were recorded at 2 mV/s without iR-compensation for background correction.

[0104] Electrochemical H2O2 synthesis via oxygen reduction reaction was then performed by switching the electrolyte to the O2 saturation electrolyte, and the LSV curves on the disk were recorded at a scan rate of 2 mV/s without iR-compensation. Meanwhile, the ring current was collected by biasing the electrode at 1.2 V vs. RHE. The currents were corrected with the background currents obtained in Ar-saturated electrolytes and used for the catalyst activity calculations.

(3) Two-electrode electrolyser

[0105] The inventive two-electrode electrolyser has two electrodes loaded with catalysts for an oxygen reduction reaction and a hydrogen oxidation reaction, one proton-exchange membrane, and a polymer-based solid electrolyte, illustrated in Figure 2.

[0106] The CoPc/C catalyst ink was deposited on a gas diffusion layer electrode (GDE, deposition area = 10 cm², W1S1010, CeTech) with a mass loading of 0.5 mg/cm² as the cathode. The counter electrode (anode) was prepared by loading a commercial Pt/C (20wt.% Pt on Vulcan XC-72R carbon black) catalyst on another GDE with the similar mass loading at 0.5 mg/cm² After drying under ambient condition, the GDE deposited with the Pt/C catalyst was hot-pressed against a piece of Nafion 212 membrane (~50 um thick, cleaned and activated with H2O2 and H2SO4 using the standard treatment method) at 130 °C under a pressure of 6.8 atm (100) psi for 15 min.

[0107] The space between the cathode and anode (about 10 mL) was filled with polystyrene-divinylbenzene spherical macroporous polymer beads (CT275, Purolite, average diameter of 400 um to 550 um) as a solid electrolyte. 0.1 M acetic buffer or deionized water was injected into the electrolyser at a flow rate of 2 mL/min to create an acidic or neutral environment.

[0108] The flow also carried out hydrogen peroxide produced out of the electrolyser. Pure oxygen is delivered to the cathode side at a flow rate of 50 seem at 1 atm. Humidified hydrogen gas was delivered to the anode at a flow rate of 50 seem at 1 atm. Representative photographs of an assembled device are shown in Figure 3 and Figure 4. [0109] The H2O2 concentration carried out from the electrolyser was quantified by a colorimetric method using Ce(SO4)2. The ultraviolet-visible light absorption spectra of H2O2 solutions were recorded on a UV-vis spectrometer (UV-3600, Shimadzu).

(4) Catalytic performance

[0110] The linear polarisation curves of the CoPc/C catalyst loaded on RRDE were collected in oxygen-saturated electrolytes.

[0111] Figure 5(a) displays test results obtained in acidic 0.1 M ABS electrolyte. The selectivity toward H2O2 was also calculated and displayed in Figure 5(b).

[0112] The catalyst exhibits a small onset overpotential of about 20 mV and it can maintain a selectivity toward H2O2 higher than 90% in a wide potential window tested (0.7-0.1 V vs. RHE). The catalyst mass activity (based on the Co mass) can reach 750 and 4000 A/g at 0.6 (100 mV overpotential) and 0.5 V (200 mV overpotential), respectively.

[0113] Figure 6(a) displays test results obtained in neutral 0.1 M PBS electrolyte. The catalyst exhibits a small onset overpotential of about 5 mV and it can maintain a selectivity toward H2O2 higher than 90% in a wide potential window tested (0.7-0.1 V vs. RHE). The catalyst mass activity (based on the Co mass) can reach 2300 and 9700 A/g at 0.6 (100 mV overpotential) and 0.5 V (200 mV overpotential), respectively. The CoPc/C catalyst can maintain a selectivity toward H2O2 higher than 70% in the potential window from 0.7 to 0.1 V vs. RHE as shown in Figure 6(b).

[0114] In comparison, MWCNT without the CoPc/C catalyst, as shown in Figure 5(a) and Figure 6(a), exhibits negligible catalytic activity.

[0115] It should be noted that although the selectivity in the neutral PBS electrolyte is lower than that in the acidic ABS electrolyte, their H2O2 specific current densities - the dashed line in Figure 5(a) and Figure 6(a) are about the same, which indicate that the catalyst has similar catalytic activities.

Table 1. H2O2 production performance comparison of recently reported catalysts

[0116] Significantly, the inventors have compared the activity of the inventive CoPc/C catalyst with some recently reported catalysts. The relativities appear significant and show high commercial potential; see, Table 1, above.

[0117] The inventive catalyst and electrolyser for decentralised H2O2 production have the following practical and commercial advantages over existing catalysts and processes:

[0118] Catalyst: (1) The inventive catalyst exhibits outstanding H2O2 production performance in acidic and neutral electrolytes. It delivers a mass activity up to 4000 A/g at a small overpotential of 200 mV. The high activity is expected to improve the device performance significantly; (2) The cost of the inventive catalyst is negligible. Using the exemplified CoPc/CNT catalyst as an example, the cost of CNT and cobalt phthalocyanine is about $100 and $13,000 per kg, resulting in a catalyst cost of $410/kg (most of the weight comes from CNT). Besides, the inventive process does not involve the use of high-temperature thermal annealing or strong acid washing. The solvent used can be recycled, affording an environmentally friendly and cost-efficient process. At the proposed catalyst loading of 0.5 mg/cm², 1 kg catalyst is sufficient to prepare 200 m² electrodes. In comparison, the cost of existing catalysts is much higher; they use precious metals, such as Pd or Au. The state-of-the-art single Co atom or Pt decorated carbon catalysts, are usually synthesised using high- temperature thermal annealing of expensive metal-organic framework precursors (ZiF-8, >$10,000/kg) at a 30% carbon material yield.

[0119] Electrolyser: (1) Low energy cost. This device is preferably embodied in a fuelcell configuration and exhibits an open-circuit potential of ~0.7 V. Compared to other electrolysers that require significant external electricity input, and the inventive electrolyser uses H2 at the anode side to generate H⁺ ions, which can generate electricity to drive the controlling unit and necessary electromechanical components, for example, pumps and valves during operation, reducing the operation cost. (2) Long term stability. The inventive design avoids the use of expensive and unstable anion exchange membranes (AEM), which significantly extend the lifetime of the device. (3) H2O2 can be produced in either acidic electrolytes or pure water, making it applicable f or various fUCL-consuming industrial applications. Importantly, this avoids the self - decomposition of H2O2, which commonly happens in alkaline electrolytes.

[0120] Finally, both the inventive catalyst and electrolyser demonstrate impressive overall process safety. H2O2 is a strong oxidant and potentially explosive. The proposed new process can produce an H2O2 solution at a suitable concentration for direct consumption in various applications, which eliminates safety hazards in transportation and handling of high concentration H2O2 (for example, 100% or commonly used 30- 70% solutions).

Industrial Applicability

[0121] The inventive catalysts and electrolyser have potential utility in commercially significant fields such as wastewater treatment and disinfection. The production of acidic or pure H2O2 aqueous solutions at tunable concentrations is especially amenable to such applications.

[0122] The present invention is a novel decentralised H2O2 production method that can produce H2O2 at different concentrations (e.g., l-3wt.%) f or various applications, including wastewater treatment and disinfection.

Example 1.

[0123] Wastewater treatment. Organic contaminants are often removed in wastewater treatment facilities by the Fenton reaction, where Fe²⁺ is used as a catalyst to promote the formation of hydroxyl radicals, a powerful oxidant f or the decomposition of contaminants. The wastewater is usually acidified to pH=3-4. Hence the H2O2 solution produced from the proposed device by using an acidic electrolyte can be directly injected into the wastewater for oxidative decomposition of the organic contaminants. A decentralised H2O2 production unit can be set up in a wastewater treatment plant. This can eliminate safety hazards related to the transportation and storage of high concentration H2O2.

Example 2.

[0124] Disinfection. Low concentration (~2-3wt.%) H2O2-water solutions are commonly used as a disinfection agent for medical or surface disinfection. The H2O2 production unit can be installed in hospitals to produce an H2O2 solution at this concentration by using neutral electrolyte or deionised water (after osmosis and ultrafiltration) for various disinfection requirements Considering the recent outbreak of COVID-19, such a device may have a high demand by various medical organisations. References

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[4] Sheng, Y., et al., Electrogeneration of hydrogen peroxide on a novel highly effective acetylene black-PTFE cathode with PTFEfilm. Electrochimica Acta, 2011. 56(24): p.8651-8656.

[5] Pajootan, E., M. Arami, and M. Rahimdokht, Discoloration of wastewater in a continuous electro-Fenton process using modified graphite electrode with multi-walled carbon nanotubes/surfactant. Separation and Purification Technology, 2014. 130: p.34-44.

[6] Yu, X., et al., A novel dual gas diffusion electrodes system for efficient hydrogen peroxide generation used in electro-Fenton. Chemical Engineering Journal, 2015. 263: p.92-100.

[7] Panizza, M. and M.A. Oturan, Degradation of Alizarin Red by electro-Fenton process using a graphite-felt cathode. Electrochimica Acta, 2011. 56(20): p.7084-7087.

[8] Zhou, L., et al., Chemically modified graphite felt as an efficient cathode in electro- Fenton for p -nitrophenol degradation. Electrochimica Acta, 2014. 140: p.376-383.

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[10] Li, N., et al., A novel carbon black graphite hybrid air-cathode for efficient hydrogen peroxide production in bioelectrochemical systems. Journal of Power Sources, 201 6. 306: p.495-502.

[11] Siami, S., et al., Process optimization and effect of thermal, alkaline, H2O2 oxidation and combination pretreatment of sewage sludge on solubilization and anaerobic digestion. BMC biotechnology, 2020. 20(1): p.21.

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[15] Otsuka, K. and I. Yamanaka, One step synthesis of hydrogen peroxide through fuel cell reaction. Electrochimica Acta, 1990. 35(2): p.319-322.

[16] Yamanaka, I., et al., Direct and Continuous Production of Hydrogen Peroxide with 93% Selectivity Using a Fuel-Cell System. Angewandte Chemie International Edition, 2003. 42(31): p.3653-3655.

[17] Yamanaka, I. and T. Murayama, Neutral H2O2 Synthesis by Electrolysis of Water and O2. Angewandte Chemie International Edition, 2008. 47(10): p.1900-1902.

[18] Murayama, T. and I. Yamanaka, Electrosynthesis of Neutral H2O2 Solution from 02 and Water at a Mixed Carbon Cathode Using an Exposed Solid -Polymer-Electrolyte Electrolysis Cell. The Journal of Physical Chemistry C, 2011. 115(13): p.5792-5799.

[19] Li, W., et al., Production of Hydrogen Peroxide for Drinking Water Treatment in a Proton Exchange Membrane Electrolyser at Near-Neutral pH. Journal of The Electrochemical Society, 2020. 167(4): p.044502.

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Claims

- 22 -THE CLAIMS DEFINING THE INVENTION ARE AS FOLLOWS

1. A compound of the general formula MPc/C, wherein

Pc is phthalocyanine;

M is a d transition metal selected from fourth-row, group IIIB to IIB elements coordinated to the phthalocyanine to form the MPc moiety; and

C is a sp²-hybridised conductive carbon substrate, the conductive carbon substrate being electrochemically associated with the MPc moiety.

2. A compound according to claim 1, wherein M is selected from the group consisting of Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu and Zn.

3. A compound according to claim 2, wherein M is Co.

4. A compound according to claim 1, wherein C is selected from the group consisting of multi-walled carbon nanotubes (MWCNT), graphene, graphene oxide (GO), reduced graphene oxide (rGO), or carbon black (CB).

5. A compound according to claim 4, wherein C is multi-walled carbon nanotubes (MWCNT).

6. A compound according to claim 1, wherein M is Co; and wherein C is multiwalled carbon nanotubes (MWCNT).

7. A compound according to any one of the preceding claims, having between about 0.1 and about 0.5wt.% M content.

8. A compound according to claim 7, having between about 0.2wt.% M content.

9. A compound of the general formula MPc/C, wherein Pc is phthalocyanine; M is a transition metal selected from fourth-row, group IIIB to IIB elements coordinated to the phthalocyanine to form the MPc moiety; and C is a sp²- hybridised conductive carbon substrate, the conductive carbon substrate being electrochemically associated with the MPc moiety, for use as a catalyst in the synthesis of hydrogen peroxide.

10. A method of synthesising a compound of the general formula MPc/C, wherein

Pc is phthalocyanine;

M is a d transition metal selected from fourth-row, group IIIB to

IIB elements coordinated to the phthalocyanine to form the MPc moiety; and

C is a sp²-hybridised conductive carbon substrate, the conductive carbon substrate being electrochemically associated with the MPc moiety, the method comprising the steps of: sonicating over a predetermined period one part by weight MPc with between about 50 and about 200 parts by weight of C in an appropriate solvent; and removing the solvent.

11. A method according to claim 10, wherein one part by weight MPc is sonicated with about 100 parts by weight of C.

12. A method according to claim 10, wherein the solvent is a polar aprotic solvent.

13. A method according to claim 11, wherein the solvent is selected from DMF, DMA, CH3CN, DMSO, NMP and mixtures thereof.

14. A method according to claim 11, wherein the solvent is DMF.

15. A method according to claim 10, wherein the predetermined period is about 60 minutes.

16. A method according to claim 10, wherein MPc is cobalt phthalocyanine; C is multi-walled carbon nanotubes (MWCNT) in a weight ratio of about 100 parts per part of MPc.

17. A method according to any one of claims 10 to 16, further comprising the step of: after sonication and prior to solvent removal, further stirring at room temperature for about 12 h.

18. A method according to any one of claims 10 to 17, wherein the step of removing to solvent is performed by heating under vacuum at about 100 mbar.

19. A method according to any one of claims 10 to 18, wherein after removing the solvent, the resultant solid is dried under vacuum at about 60 °C for about 12 h.

20. A compound of the general formula MPc/C, wherein:

Pc is phthalocyanine;

M is a 3d transition metal selected from fourth-row, group IIIB to IIB elements coordinated to the phthalocyanine to form the MPc moiety; and

C is a sp²-hybridised conductive carbon substrate, the conductive carbon substrate being electrochemically associated with the MPc moiety, when synthesised by a method defined according to any one of claims 10 to 19.

21. A method for the synthesis of hydrogen peroxide, the method comprising an oxygen reduction reaction catalysed by a compound of the general formula MPc/C, wherein:

Pc is phthalocyanine;

22. A method according to claim 21, wherein MPc is cobalt phthalocyanine; C is multi-walled carbon nanotubes (MWCNT) in a weight ratio of about 100 parts per part of MPc.

23. A method according to claim 21 or claim 22, wherein the MPc/C compound is present in an amount of about 1 mg/mL.

24. A method according to any one of claims 21 to 23, wherein the MPc/C catalyst exhibits a selectivity of >70% toward hydrogen peroxide under acidic or neutral - 25 - conditions.

25. A two-electrode electrolyser comprising: a cathode for receiving a catalyst for catalysing an oxygen reduction reaction and a hydrogen oxidation reaction; an anode; a proton-exchange membrane; and a polymer-based solid electrolyte.

26. A two-electrode electrolyser according to claim 25, wherein the catalyst is compound of the general formula MPc/C, wherein Pc is phthalocyanine; M is a transition metal selected from fourth-row, group IIIB to IIB elements coordinated to the phthalocyanine to form the MPc moiety; and C is a sp²- hybridised conductive carbon substrate.

27. A two-electrode electrolyser according to claim 25 or claim 26, wherein the anode is -20% Pt/C, 0.5 mg/cm²

28. A two-electrode electrolyser according to any one of claims 25 to 27, wherein the catalyst is mass loaded to the cathode at about 0.5 mg/cm².

29. A two-electrode electrolyser according to any one of claims 25 to 28, wherein the proton exchange membrane is Nafion 212 (or similar), at a thickness of about 50 microns.

30. A two-electrode electrolyser according to any one of claims 25 to 29, having a space between cathode and anode filled with polystyrene-divinylbenzene spherical macroporous polymer beads (CT275, Purolite, average diameter of 400 um to 550 um) as a solid electrolyte, or similar.

31. A two-electrode electrolyser according to any one of claims 25 to 30, wherein the cathode is adapted to receive oxygen at a flow rate of about 50 seem at about 1 atm; and wherein the anode is adapted to receive humidified hydrogen at a flow rate of about 50 seem at about 1 atm.