CROSS REFERENCE

[0001 ] The present application claims priority to Australian provisional application number 2020902843, filed 1 1 August 2020, which is incorporated herein by reference in its entirety.

FIELD OF THE INVENTION

[0002] The invention relates to catalysts for the production of hydrogen peroxide, and to methods and apparatus for the use of such catalysts. However, it will be appreciated that the invention is not limited to this particular field of use.

BACKGROUND

[0003] The following discussion of the prior art is provided to place the invention in an appropriate technical context and enable the advantages of it to be more fully understood. It should be appreciated, however, that any discussion of the prior art throughout the specification should not be considered as an express or implied admission that such prior art is widely known or forms part of the common general knowledge in the field.

[0004] Hydrogen peroxide (H2O2) is an important chemical commodity that has been widely used as an environmentally benign oxidant and a potential energy carrier in various applications, including wastewater treatment, disinfection, chemical synthesis, paper/pulp bleaching, semiconductor cleaning and fuel cells. The global demand of H2O2 is growing rapidly, reaching US$4.0 billion in 2017 and is expected to further increase to around US$5.5 billion by 2023. Currently, over 95% of H2O2 is produced in a concentrated form using the anthraquinone process.

[0005] This centralized production requires a huge infrastructure investment and high energy inputs, as well as posing safety concerns during the product distribution. Specific drawbacks of the anthraquinone process include:

(1 ) High cost: the centralized production of H ₂ C>2 via anthraquinone process requires large infrastructure investment and high energy input, which cannot be readily implemented in remote areas or developing countries;

(2) Hazards in handling the product: the highly corrosive and unstable H2O2 produced in concentrated form (70%) poses safety concerns in storage and transport;

(3) Energy intensive process: the complicated hydrogenation/oxidation processes of anthraquinone derivatives during the reaction makes this method super energy intensive, which results in a low energy efficiency as well as high overall cost;

(4) Pollution: the organic by-products generated during the reaction process makes this approach not eco-friendly; (5) Difficulties in distribution: the highly concentrated H2O2 (70%) makes its longdistance transportation difficult.

[0006] In fact, in many applications, only dilute H2O2 (for example, around 0.3%) is required, indicating the concentrated peroxide agent needs to be diluted before usage and making the centralised production of highly concentrated peroxide rather wasteful.

[0007] Direct synthesis of hydrogen peroxide from the combination of H ₂ and O2 gases is a well-known alternative to the traditional anthraquinone method. However, the explosion risks involved from the mixture of hydrogen and oxygen gases make this method impractical for large scale applications.

[0008] Thus, it is highly desirable to develop a clean and energy-efficient approach to generate H2O2 on-site, making the production of hydrogen peroxide close to the point of consumption at a low cost, which is especially suitable for developing countries and remote area applications.

[0009] Recently, the electrochemical reduction of oxygen via a selective two-electron transfer pathway has attracted extensive research interest as a promising alternative to the anthraquinone process. Electrochemical oxygen reduction provides a feasible way to synthesize hydrogen peroxide per demand, requiring the clean electrons, O2 and water as the only reactants. The oxygen reduction reaction (ORR) method will enable the decentralized production of H2O2 on demand under ambient reaction conditions without any hazardous by-products. Furthermore, if the ORR process is integrated with renewable electricity supplies (e.g. generation by photovoltaic cells or wind turbines), the H2O2 generated can be regarded as a renewable chemical.

[00010] More recently, there has been a surge of interest in developing metal-free carbon catalysts for the electro-synthesis of H2O2 via O2 reduction. The electronic structure of these carbon materials can be easily tuned by heteroatom-doping or defect-engineering, thus endowing them high activity and selectivity towards H2O2 production in alkaline electrolytes. However, the production of H2O2 in alkaline media is somewhat constrained by the following limitations:

(i) the H2O2 (or HC>2 ^_ at a pH value above 1 1 .6) tends to readily decompose at basic conditions;

(ii) chelating agents (e.g. ethylenediamine tetraacetic acid) are required to prevent the tramp-metal-ion-induced H2O2 decomposition, thus resulting in the increased operation costs;

(iii) devices (such as fuel cells) based on hydroxide-conducting polymeric electrolytes exhibit low membrane stability, poor water management and low hydrogen oxidation activity. Therefore, it is more desirable to generate H2O2 in acids via ORR. [0001 1] Currently, the state-of-the-art electrocatalysts for H2O2 production in acid are still restricted to precious-metal-based (e.g. Pt-Hg and Pd-Hg) alloys or amalgams, for which high cost and scarcity have severely hindered their commercial viability. Mesoporous carbon with nitrogen dopants or a defective structure has emerged as a promising class of catalysts for H2O2 electrosynthesis in acids, owing to their high H2O2 selectivity and low cost. Nevertheless, large amounts of overpotential (>400 mV) are normally associated with these materials to achieve high productivity and selectivity, which has rendered the production process energy inefficient.

[00012] Hence, the development of highly selective and active catalysts with low cost for electrolytic H2O2 production in acids remains highly sought after.

[00013] It is an object of the present invention to overcome or ameliorate one or more the disadvantages of the prior art, or at least to provide a useful alternative.

SUMMARY

[00014] The inventors of the present application have surprisingly developed an effective method to stabilize metal-N active species as well as maintain their high selectivity towards H2O2 production under corrosive reaction environments, such as in the presence of strong acids. The present inventors have focussed on an approach of forming single-atom catalysts (SACs) on a carbon substrate to stabilize N-coordinated transition-metal centers (metal-Nx). Without wishing to be bound by theory, it is thought that the conductive nature of a carbon substrate enables maximal exposure of the active metal-N _x sites. The inventors have also surprisingly found that epoxide modification of the substrate provides improved selectivity of catalysts for peroxide formation.

[00015] In a first aspect of the invention there is provided a catalyst comprising a conductive carbon substrate doped with nitrogen and a transition-metal, wherein the carbon substrate comprises an epoxy group.

[00016] The following options may be used in conjunction with the first aspect, either individually or in any combination.

[00017] In certain embodiments, the catalyst comprises a conductive carbon substrate doped with nitrogen and a transition-metal, and the carbon substrate is adapted to comprise epoxy functionality.

[00018] In certain embodiments, the catalyst is a heterogeneous catalyst.

[00019] In certain embodiments the nitrogen is coordinated to the transition-metal. It may form, for example, a metal-N _x group, wherein x is an integer from 2 to 6, preferably 4.

[00020] The skilled person will understand that any suitably conductive carbon material may be used for the substrate. In certain embodiments the conductive carbon substrate is selected from the group consisting of: carbon nanotubes, vertical graphene, metal-organic framework (MOF)-derived carbon, carbon fibre paper, carbon felt, and combinations thereof.

[00021] In certain specific embodiments the conductive carbon substrate comprises carbon nanotubes. The carbon nanotubes may have a diameter of from about 5 nm to about 100 nm, or from about 10 nm to about 30 nm. They may, for example, have an average diameter of about 5, 10, 20, 25, 30, 40, 50, 60, 70, 80, 90, or 100 nm.

[00022] In certain embodiments the conductive carbon substrate comprises vertical graphene. The graphene may have a mean aspect ratio of at least about 20, or at least about 50, 100, 200, 500, 1000, 2000, 5000, 10 ⁴ , or 10 ⁵ . It may be from about 20 to about 10 ⁶ , or from about 10 ² to 10 ⁶ , 10 ³ to 10 ⁶ , 10 ⁴ to 10 ⁶ , 10 ⁵ to 10 ⁶ , 20 to 10 ⁵ , 20 to 10 ⁴ , 20 to 10 ³ , 20 to 10 ² , 10 ² to 10 ³ , 10 ³ to 10 ⁴ , or 10 ⁴ to 10 ⁵ . It may be for example about 20, 30, 40, 50, 100, 200, 500, 10 ³ , 5 x 10 ³ , 10 ⁴ , 5 x 10 ⁴ , 10 ⁵ , 5 x 10 ⁵ , or 10 ⁶ . The aspect ratio may be defined as the ratio of the minimum non-thickness dimension to the average thickness. The graphene may be non-uniform in shape, but on average may have non-thickness dimension at least 20 times greater than its average thickness. The graphene may have an average thickness of less than about 50 nm, or less than about 20, 10, 5, 2 or 1 nm. It may be from about 0.5 nm to about 50 nm, or from about 1 to 50, 2 to 50, 5 to 50, 10 to 50, 20 to 50, 0.5 to 20, 0.5 to 10, 0.5 to 5, 0.5 to 2, 2 to 5, 5 to 10, or 10 to 20 nm. It may be for example about 0.5, 1 , 2, 5, 10, 20 or 50 nm. The graphene may comprise particles formed from a number of sheets of laminar material. The average number of individual sheets in each particle may be 1 or may be greater than about 1 , or greater than about 2,

3, 4, 5, 6, 7, 8, 9, 10, 15, 20 or 50 sheets. It may be from about 1 sheet to about 100 sheets, or from about 1 to 50, 1 to 20, 1 to 10, 1 to 5, 5 to 10, 5 to 100, 10 to 100, 20 to

100, 50 to 100, 5 to 10, 10 to 20, or 20 to 50 sheets. It may be for example about 1 , 2, 3,

4, 5, 6, 7, 8, 9, 10, 15, 20, 25, 30, 40, 50, 60, 70, 80, 90, or 100 sheets.

[00023] In certain embodiments the conductive carbon substrate is mesoporous. It may have an average pore diameter of about 2 nm to about 50 nm. The average pore diameter may be, for example, about 2, 5, 10, 20, 30, 40, or 50 nm.

[00024] In certain embodiments the transition metal is selected from the group consisting of: cobalt, nickel, iron, and combinations thereof. In certain embodiments it is a metal selected from IUPAC Group 8, IUPAC Group 9, and IUPAC Group 10. It may be, for example, selected from IUPAC Group 9.

[00025] In certain specific embodiments the transition metal is cobalt.

[00026] In certain embodiments the transition metal concentration in the catalyst is from about 0.01 wt.% to about 30 wt.%, or it may be from about 0.01 wt.% to about 15 wt.%, about 0.1 wt.% to about 15 wt.%, about 0.1 wt.% to about 10 wt.%, about 0.1 wt.% to about 5 wt.%, or about 0.1 wt.% to about 1 wt.%. It may be present, for example, at about 0.01 , 0.02, 0.05, 0.1 , 0.2, 0.5, 1 , 2, 5, 10, 15, 20, or 30 wt.%

[00027] In certain embodiments the nitrogen concentration in the catalyst is from about 0. 1 at.% to about 30 at.%, or it may be from about 0.1 at.% to about 15 at.%, about 0.1 at.% to about 10 at.%, about 0.1 at.% to about 5 at.%, or about 0.1 at.% to about 1 at.%. It may be present, for example, at about 0.1 , 0.2, 0.5, 1 , 2, 5, 10, 15, 20, or 30 at.%. In some embodiments, the concentration values above may refer to wt.%.

[00028] In certain embodiments the C-O-C (i.e. epoxy) oxygen concentration in the catalyst is from about 0.01 at.% to about 30 at.%, or it may be from about 0.01 at.% to about 15 at.%, about 0.1 at.% to about 15 at.%, about 0.1 at.% to about 10 at.%, about 0.1 at.% to about 5 at.%, or about 0.1 at.% to about 1 at.%. It may be present, for example, at about 0.01 , 0.02, 0.05, 0.1 , 0.2, 0.5, 1 , 2, 5, 10, 15, 20, or 30 at.%. In some embodiments, the concentration values above may refer to wt.%.

[00029] In certain embodiments the epoxy group is the predominant oxygen functionality. In particular embodiments the ratio of epoxy groups to non-epoxy oxygen containing groups is from about 1 :3 to about 100:1 , or from about 1 :2 to about 50:1 , about 1 :1 to about 50:1 , about 1 :1 to about 10:1 , about 1 :1 to about 5:1 , about 1 :1 to about 2.5:1 , about 1 :1 to about 2:1 , or about 1 :1 to about 1 .5:1 . It may be, for example, about 1 :3, 1 :2, 1 :1 , 2:1 , 3:1 , 4:1 , 5:1 , 10:1 , 20:1 , 50:1 , or 100:1 . In particular embodiments, the percentage of epoxy groups with respect to the total number of oxygen containing groups may be greater than or equal to about 25 at.%, or it may be greater than or equal to about 20 at.%, 30 at.%, 40 at.%, 50 at.%, 60 at.%, 70 at.%, 80 at.%, 90 at.%, or 95 at.%. In some embodiments, the percentage values above may refer to wt.%.

[00030] In certain specific embodiments, the carbon support is a carbon nanotube or vertical graphene or a combination of said structures. The metal may be any suitable metal that provides a suitable catalytic effect, with cobalt particularly preferred. The metal can be loaded onto the carbon support in any amount, with relatively high loadings (15% or greater) preferred to obtain optimum catalytic activity per mass of catalyst.

[00031] In a specific embodiment, there is provided a catalyst for preparing hydrogen peroxide comprising an Co/N co-doped carbon nanotube and an epoxy group as the predominant oxygen functional species. In particular embodiments the ratio of epoxy groups to non-epoxy oxygen containing groups is from about 1 :3 to about 100:1 , or from about 1 :2 to about 50:1 , about 1 :1 to about 50:1 , about 1 :1 to about 10:1 , about 1 :1 to about 5:1 , about 1 :1 to about 2.5:1 , about 1 :1 to about 2:1 , or about 1 :1 to about 1 .5:1 . It may be, for example, about 1 :3, 1 :2, 1 :1 , 2:1 , 3:1 , 4:1 , 5:1 , 10:1 , 20:1 , 50:1 , or 100:1 . In particular embodiments, the percentage of epoxy groups with respect to the total number of oxygen containing groups may be greater than or equal to about 25 at.%, or it may be greater than or equal to about 20 at.%, 30 at.%, 40 at.%, 50 at.%, 60 at.%, 70 at.%, 80 at.%, 90 at.%, or 95 at.%. In some embodiments, the percentage values above may refer to wt.%.

[00032] In a specific embodiment, there is provided a catalyst for preparing hydrogen peroxide comprising an Co/N co-doped vertical graphene and an epoxy group as the predominant oxygen functional species. In particular embodiments the ratio of epoxy groups to non-epoxy oxygen containing groups is from about 1 :3 to about 100:1 , or from about 1 :2 to about 50:1 , about 1 :1 to about 50:1 , about 1 :1 to about 10:1 , about 1 :1 to about 5:1 , about 1 :1 to about 2.5:1 , about 1 :1 to about 2:1 , or about 1 :1 to about 1 .5:1 . It may be, for example, about 1 :3, 1 :2, 1 :1 , 2:1 , 3:1 , 4:1 , 5:1 , 10:1 , 20:1 , 50:1 , or 100:1 . In particular embodiments, the percentage of epoxy groups with respect to the total number of oxygen containing groups may be greater than or equal to about 25 at.%, or it may be greater than or equal to about 20 at.%, 30 at.%, 40 at.%, 50 at.%, 60 at.%, 70 at.%, 80 at.%, 90 at.%, or 95 at.%. In some embodiments, the percentage values above may refer to wt.%.

[00033] According to a second aspect of the invention there is provided a method of producing a catalyst comprising a conductive carbon substrate doped with nitrogen and a transition metal, said method comprising the following steps: heating a mixture comprising a carbon containing species, a nitrogen containing species and a transition metal containing species under such conditions to thereby produce the catalyst, and oxidising the conductive carbon substrate to introduce additional oxygen functionality to the conductive carbon substrate, wherein the additional oxygen functionality comprises one or more epoxy groups.

[00034] The following options may be used in conjunction with the second aspect, either individually or in any combination.

[00035] In one embodiment, the additional oxygen functionality is predominantly epoxy groups. In particular embodiments the ratio of epoxy groups to non-epoxy oxygen containing groups after the oxidising step is from about 1 :3 to about 100:1 , or from about 1 :2 to about 50:1 , about 1 :1 to about 50:1 , about 1 :1 to about 10:1 , about 1 :1 to about 5:1 , about 1 :1 to about 2.5:1 , about 1 :1 to about 2:1 , or about 1 :1 to about 1 .5:1 . It may be, for example, about 1 :3, 1 :2, 1 :1 , 2:1 , 3:1 , 4:1 , 5:1 , 10:1 , 20:1 , 50:1 , or 100:1 . In particular embodiments, the percentage of epoxy groups with respect to the total number of oxygen containing groups after the oxidising step may be greater than or equal to about 25 at.%, or it may be greater than or equal to about 20 at.%, 30 at.%, 40 at.%, 50 at.%, 60 at.%, 70 at.%, 80 at.%, 90 at.%, or 95 at.%. In some embodiments, the percentage values above may refer to wt.%. [00036] The catalyst, conductive carbon substrate, and transition metal may be as hereinbefore described with respect to the first aspect.

[00037] In certain embodiments the transition metal containing species is selected from the group consisting of nitrate, chloride or acetate salts of cobalt, iron or nickel, or a zeolitic imidazolate framework (e.g. ZIF-8).

[00038] In certain embodiments, the carbon containing species and/or nitrogen containing species comprises dicyanamide, aniline or a zeolitic imidazolate framework (e.g. ZIF-8).

[00039] In certain embodiments the method further comprises a step of reducing the conductive carbon substrate to reduce non-epoxy oxygen functionality. In particular embodiments the oxidising and/or reducing are conducted electrochemically. A person of skill in the art will understand that the oxidation and/or reduction steps may alternatively be a chemical oxidation and/or reduction, i.e. using a chemical oxidant and/or reductant rather than an electrochemical process.

[00040] In specific embodiments the ratio of epoxy groups to non-epoxy oxygen containing groups of the conductive carbon substrate is higher after the oxidising and reducing steps than prior to the oxidising and reducing steps. The ratio of epoxy groups to non-epoxy oxygen containing groups may increase by from about 10 % to about 500 % as a result of the oxidisation and reduction steps, or it may increase by from about 10% to about 400%, about 10% to about 200%, about 10% to about 100%, or about 50% to about 200%. It may, for example, increase by about 10, 20, 50, 100, 200, or 500%.

[00041] In certain embodiments, the heating is a pyrolysis. The pyrolysis may be performed at a temperature of from about 500 °C to about 1000 °C, or about 600 °C to about 1000°C, or about 700 °C to about 900°C. It may be at a temperature of about 500, 600, 700, 800, 900, or 1000°C. The heating may be performed for a period of from about 1 hour to about 8 hours, or from about 1 hour to about 6 hours, about 1 hour to about 5 hours, about 3 hours to about 6 hours, or about 3 hours to about 4 hours. It may be performed for a period of about 1 , 2, 3, 3.1 , 3.2, 3.3, 3.4, 3.5, 3.6, 3.7, 3.8, 4, 5, or 6 hours.

[00042] In another embodiment, there is provided a method of preparing a catalyst according to the first aspect, the method comprising the steps of:

(i) oxidising a carbon support comprising a doped metal to produce an oxidised carbon support comprising epoxy oxygen groups and at least one other oxygen species; and

(ii) reducing the oxidised carbon support to produce a catalyst comprising substantially epoxy species. [00043] Typically, oxidising the carbon support comprising a doped metal may produce an oxidised carbon support comprising epoxy oxygen groups and other oxygen groups including ketonic oxygen. Reducing takes place to convert and/or modify non-epoxy oxygen groups, preferably such that the ketonic oxygen species are minimised (i.e., substantially reduced or substantially eliminated in total number) and epoxy oxygen groups are maximised (i.e., substantially increased in total number, or are the predominant oxygencontaining groups present).

[00044] The oxidising and reducing may be conducted electrochemically.

[00045] In certain specific embodiments the carbon support may be a carbon nanotube or vertical graphene. The metal may be a transition metal, such as cobalt.

[00046] In a specific embodiment, there is provided a method of preparing a catalyst for preparing hydrogen peroxide comprising an Co/N co-doped carbon nanotube and an epoxy group as the sole or predominant oxygen functional species comprising the steps of:

(i) oxidising a Co/N co-doped carbon nanotube to produce an oxidised Co/N codoped carbon nanotube comprising at least epoxy and other oxygen species (such as ketonic species);

(ii) reducing the oxidised Co/N co-doped carbon nanotube to produce a Co/N co-doped carbon nanotube catalyst comprising substantially only or predominantly epoxy species (and optionally substantially free from other oxygen species, in particular, substantially free from ketonic species).

[00047] In a specific embodiment, there is provided a method of preparing a catalyst for preparing hydrogen peroxide comprising an Co/N co-doped vertical graphene and an epoxy group as the sole or predominant oxygen functional species comprising the steps of:

(i) oxidising a Co/N co-doped vertical graphene to produce an oxidised Co/N co-doped vertical graphene comprising at least epoxy and other oxygen species (such as ketonic species)

(ii) reducing the oxidised Co/N co-doped vertical graphene to produce a Co/N co-doped vertical graphene catalyst comprising substantially only or predominantly epoxy species (and optionally substantially free from other oxygen species, in particular, substantially free from ketonic species).

[00048] The method according to the second aspect may produce the catalyst according to the first aspect. The catalyst according to the first aspect may be produced using the method according to the second aspect.

[00049] In a third aspect of the invention there is provided a catalyst produced according to the method of the second aspect. [00050] In a fourth aspect of the invention there is provided use of the catalyst according to the first or third aspect as a cathode catalyst.

[00051] In a fifth aspect of the invention there is provided use of the catalyst of any according to the first or third aspect in the production of hydrogen peroxide or the production of Fenton’s reagent.

[00052] In a sixth aspect of the invention there is provided an electrolyser for producing hydrogen peroxide from oxygen and water, the electrolyser comprising a cathode which comprises a catalyst layer comprising the catalyst according to the first or third aspect.

[00053] The following options may be used in conjunction with the sixth aspect, either individually or in any combination.

[00054] In certain embodiments the cathode is on an electrolyte facing side and the electrolyser further comprises a hydrophobic gas permeable layer on an oxygen input side.

[00055] In certain specific embodiments the hydrophobic gas permeable layer is formed from polytetrafluoroethylene, such as a polytetrafluoroethylene coating layer or membrane, or is formed from an aggregate of polytetrafluoroethylene nanoparticles.

[00056] In specific embodiments the catalyst layer and hydrophobic gas permeable layer are (immediately) adjacent each other.

[00057] In alternative embodiments the catalyst layer and hydrophobic gas permeable layer are present in conjunction with a mechanical support layer, such as carbon paper or a perforated metal substrate. In such embodiments, the catalyst layer may comprise vertical graphene and the mechanical support layer may comprise a graphene growth support.

[00058] In a specific embodiment, there is provided an electrolyser for producing hydrogen peroxide from oxygen and water, the electrolyser comprising a cathode which comprises a catalyst layer comprising a catalyst according to the first or third aspect on an electrolyte facing side and a hydrophobic gas permeable layer on an oxygen input side.

[00059] In certain embodiments the catalyst layer and hydrophobic gas permeable layer are (immediately) adjacent each other.

[00060] In certain embodiments the catalyst layer and hydrophobic gas permeable layer are present in conjunction with a mechanical support layer, such as carbon paper or a perforated metal substrate.

[00061] In certain embodiments the catalyst layer comprises vertical graphene and the support layer is a graphene growth support. [00062] In a seventh aspect of the invention there is provided a method of synthesising hydrogen peroxide comprising providing oxygen in an acidic or neutral aqueous media to an electrolyser according to the sixth aspect under reducing conditions.

[00063] The following options may be used in conjunction with the seventh aspect, either individually or in any combination.

[00064] In certain embodiments the electrolyser produces at least about 100 mg L ^-1 h’ ¹ , or at about least 5000 mg L ^-1 h ^-1 hydrogen peroxide. It may produce hydrogen peroxide at a rate of from about 100 to about 50000 mg L ^-1 h’ ¹ , or from about 200 to about 50000, about 500 to about 50000, about 1000 to about 50000, or about 10000 to about 50000 mg L ^-1 h ^-1 . It may produce hydrogen peroxide at a rate of, for example, about 100, 200, 500, 1000, 2000, 5000, 10000, 20000, or 50000 mg L’ ¹ h ¹ .

[00065] In certain embodiments the acidic or neutral media may have a pH of from about 0 to about 8, or about 1 to about 7, about 2 to about 7, about 3 to about 7. It may, for example, be about 0, 1 , 2, 3, 4, 5, 6, or 7, or 8.

[00066] In certain embodiments the electrolyser produces hydrogen peroxide at a concentration of least about 100 mg L ^-1 (0.01 wt.%), or at least about 2000 mg L ^-1 (0.2 wt.%), or at least about 5000 mg L ^-1 (0.5 wt.%), or at least about 30000 mg L ^-1 (3 wt.%). It may produce hydrogen peroxide at a concentration of from about 100 to about 50000 mg L’ ¹ , or from about 200 to about 50000, about 500 to about 50000, about 1000 to about 50000, or about 10000 to about 50000 mg L ^-1 . It may produce hydrogen peroxide at a concentration of, for example, about 100, 200, 500, 1000, 2000, 5000, 10000, 20000, or 50000 mg L ¹ .

[00067] In an eighth aspect of the invention there is provided a method of synthesising Fenton’s reagent comprising providing a source of Fe ²⁺ and oxygen in an acidic or neutral aqueous media to an electrolyser according to the sixth aspect under reducing conditions.

[00068] In a ninth aspect of the invention there is provided a method for improving the selectivity of a catalyst for hydrogen peroxide production, said catalyst comprising a conductive carbon substrate doped with nitrogen and a transition-metal, the method comprising the following steps: oxidising the conductive carbon substrate to introduce additional oxygen functionality to the conductive carbon substrate, wherein the additional oxygen functionality includes one or more epoxy groups; and reducing the conductive carbon substrate to reduce non-epoxy oxygen functionality. [00069] The following options may be used in conjunction with the ninth aspect, either individually or in any combination.

[00070] In certain embodiments the oxidising step promotes the production of surface 02 and/or 04 groups and/or suppresses the production of surface 01 groups, wherein 01 groups are oxygen species having an average binding energy of 531.2±0.2 eV, 02 groups are oxygen species having an average binding energy of 532.3±0.2 eV, 03 groups are oxygen species having an average binding energy of 533.3±0.2 eV, and 04 groups are oxygen species having an average binding energy of 534.2±0.2 eV.

[00071] In certain embodiments the reducing step suppresses the production of surface 03 and/or 04 groups.

[00072] In certain embodiments, overall the concentration of 02 groups is higher than without the oxidising and reducing treatment steps.

[00073] The method of the ninth aspect may be a component of the method according to the second aspect. The method of the second aspect may incorporate the method according to the ninth aspect.

[00074] In a tenth aspect of the invention there is provided a membrane comprising the catalyst according to the first or third aspect.

[00075] In certain embodiments the membrane comprises a perfluorosulfonate resin. A person of skill in the art will appreciate that other suitable membrane materials may be used.

[00076] The membrane may be used in the electrolyser according to the sixth aspect. The electrolyser of the sixth aspect may comprise the membrane according to the tenth aspect.

[00077] In an eleventh aspect of the invention there is provided an electrolyser for producing hydrogen peroxide from oxygen and water, the electrolyser comprising the membrane according to the tenth aspect.

Definitions

[00078] In describing and claiming 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.

[00079] 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. [00080] Unless the context clearly requires otherwise, throughout the description and the claims, the terms “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”. For example, a composition, mixture, process or method that comprises a list of elements is not necessarily limited to only those elements but may include other elements not expressly listed or inherent to such composition, mixture, process or method.

[00081] The transitional phrase "consisting of” excludes any element, step, or ingredient not specified. If in the claim, such would close the claim to the inclusion of materials other than those recited except for impurities ordinarily associated therewith. When the phrase "consisting of" 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.

[00082] The transitional phrase "consisting essentially of" is used to define a composition, process or method that includes materials, steps, features, components, or elements, in addition to those literally disclosed, provided that these additional materials, steps, features, components, or elements do not materially affect the basic and novel characteristic(s) of the claimed invention. The term "consisting essentially of" occupies a middle ground between "comprising" and "consisting of".

[00083] Where applicants have defined an invention or a portion thereof with an open-ended term such as "comprising", it should be readily understood that (unless otherwise stated) the description should be interpreted to also describe such an invention using the terms "consisting essentially of" or "consisting of." In other words, with respect to the terms “comprising”, “consisting of”, and “consisting essentially of”, where one of these three terms is 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”.

[00084] 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”. 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”.

[00085] The terms “predominantly”, “predominant”, and “substantially” as used herein shall mean comprising more than 50% by weight, unless otherwise indicated. [00086] As used herein, with reference to numbers in a range of numerals, the terms "about," "approximately" and "substantially" are understood to refer to the range of - 10% to +10% of the referenced number, preferably -5% to +5% of the referenced number, more preferably -1 % to + 1 % of the referenced number, most preferably -0.1 % to +0.1 % of the referenced number. Moreover, with reference to numerical ranges, these terms should be construed as providing support for a claim directed to any number or subset of numbers in that range. For example, a disclosure of from 1 to 10 should be construed as supporting a range of from 1 to 8, from 3 to 7, from 1 to 9, from 3.6 to 4.6, from 3.5 to 9.9, from 8 to 10, and so forth.

[00087] 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.

[00088] As used herein, the term “conductive carbon substrate” means an electrically conductive substrate that predominantly comprises carbon. In certain embodiments it may consist essentially of carbon optionally with some minor non-carbon impurities, wherein the minor impurities are less than about 2%, 1%, or 0.5% by weight of the total weight of the substrate. In certain embodiments the substrate may have a conductivity of greater than or equal to about 10 ² S/m, or greater than or equal to about 10 ³ S/m, 10 ⁴ S/m, or 10 ⁵ S/m.

[00089] As used herein, the term “epoxy group” refers broadly to any C-O-C group, including where the two carbons adjacent the oxygen are directly bonded to each other, and where the two carbons adjacent the oxygen are not directly bonded to each other (like ether groups). In certain embodiments, the term “epoxy group” means a C-O-C group where the two carbons adjacent the oxygen are directly bonded to each other.

[00090] As used herein, the term “epoxy functionality” with respect to the carbon substrate refers to the number of epoxy groups of the substrate. For example, “the substrate having increased epoxy functionality”, means an increase in the number of epoxy groups of the substrate.

[00091] As used herein, the term “oxygen functionality” with respect to the carbon substrate refers to the number of oxygen-containing groups of the substrate (whether epoxy groups or not). For example, “the substrate having increased oxygen functionality”, means an increase in the number of oxygen-containing groups of the substrate. Oxygen containing groups include C-O-C groups, carbonyl groups, and other groups having at least one oxygen atom. [00092] As used herein, the term “non-epoxy oxygen functionality” with respect to the carbon substrate refers to the number of non-epoxy oxygen-containing groups of the substrate. That is, the number of oxygen-containing groups that are not epoxy groups. For example, “the substrate having increased non-epoxy oxygen functionality”, means an increase in the number of non-epoxy oxygen-containing groups of the substrate. Non- epoxy oxygen-containing groups include, for example, ketonic oxygen groups and other non C-O-C groups having at least one oxygen atom.

[00093] As used herein, the terms “carbon containing species”, “nitrogen containing species” and “transition metal containing species” respectively mean a material comprising at least one carbon atom, a material comprising at least one nitrogen atom, and a material comprising at least one transition metal atom, respectively. In certain embodiments the “carbon containing species” and “nitrogen containing species” may be the same material. In certain embodiments the “nitrogen containing species” and “transition metal containing species” may be the same material. In certain embodiments the “carbon containing species” and “transition metal containing species” may be the same material. In certain embodiments the “carbon containing species”, “nitrogen containing species” and “transition metal containing species” are the same material.

Abbreviations

[00094] Carbon fiber paper (CFP); carbon nanotubes (CNTs); chemical vapor deposition (CVD); cobalt/nitrogen doped carbon nanotubes (CoN@CNTs); cobalt/nitrogen doped vertical graphenes (CoN@VGs); cobalt(ll) phthalocyanine (CoPc); energy dispersive X-ray spectroscopy (EDS); dimethyl sulfoxide (DMSO); electrochemical activation (EA); electrochemically activated cobalt/nitrogen doped carbon nanotubes (EA- CoN@CNTs); electrochemical oxidation (EO); electrochemically oxidised cobalt/nitrogen doped carbon nanotubes (EO-CoN@CNTs) electrochemical reduction (ER); electrochemical treatment (ET); electron energy loss spectroscopy (EELS); extended X- ray absorption fine structure (EXAFS); faradaic efficiency (FE); Fourier transformed (FT); Fourier-transform infrared spectroscopy (FTIR); High-angle annular dark field scanning transmission electron microscopy (HAADF-STEM); hydrogen peroxide (H2O2); H2O2 treatment (HT); H2O2 -treated cobalt/nitrogen doped carbon nanotubes (HT-CoN@CNTs); H2O2 -treated and electrochemically activated cobalt/nitrogen doped carbon nanotubes HE-CoN@CNTs); Inductively coupled plasma mass spectroscopy (ICP-MS); iron/nitrogen doped carbon nanotubes (FeN@CNTs); metal-organic framework (MOF); N-coordinated transition-metal centers (metal-N _x ); near edge X-ray absorption fine structure (NEXAFS); nickel/nitrogen doped carbon nanotubes (NiN@CNTs); nitrogen-doped graphene (NG); oxygen reduction reaction (ORR); partial electron yield (PEY); Perdew, Burke, and Ernzerhof exchange-correlation functional within a generalized gradient approximation (GGA-PBE); projector augmented wave method (PAW); reversible hydrogen electrode (RHE); rotating ring disk electrode (RRDE); scanning electron microscopy (SEM); singleatom catalysts (SACs); Transmission electron microscopy (TEM); van der Waals (vdW); vertical graphenes (VGs); X-ray absorption spectroscopy (XAS); X-ray absorption nearedge structure (XANES); X-ray photon spectroscopy (XPS); zeolitic imidazolate framework (ZIF).

DESCRIPTION OF THE DRAWINGS

[00095] Figure 1 shows a microstructural analysis of an example CoN@CNTs catalyst: a) SEM images of CoN@CNTs. Inset is the high-resolution SEM image of a selected area of CoN@CNTs. b) TEM image of CoN@CNTs showing a bamboo-like structure of the carbon nanotubes. c) HAADF-STEM image (top-left) and corresponding EDS maps of CoN@CNTs for O (top-right), N (bottom-left), and Co (bottom-right). d) HAADF-STEM image of the CoN@CNTs showing many Co atoms (circled) well- dispersed in the carbon layers. e) EELS analysis of selected area in (d) showing the signals of Co. f) XPS elemental survey of CoN@CNTs.

[00096] Figure 2 shows oxygen reduction performance of an example CoN@CNTs catalyst: a) RRDE voltammograms of CoN@CNTs and NG at 1600 rpm in an Os-saturated 0.1 M HCIO4 electrolyte with disc current and ring current. b) RRDE voltammograms of fresh and aged CoN@CNTs at 1600 rpm in an O2- saturated 0.1 M HCIO4 electrolyte with disc current and ring current. All potentials are recorded without iR correction. c) XPS elemental survey of CoN@CNTs and aged CoN@CNTs. d) XPS O1 s spectra of CoN@CNTs and aged CoN@CNTs.

[00097] Figure 3 shows electrochemical activation (EA) of an example CoN@CNTs catalyst:

(a-c) XPS O1 s spectra of: a) aged CoN@CNTs, b) EO-CoN@CNTs and c) EA- CoN@CNTs. d) Background-corrected FTIR spectra of CoN@CNTs before and after ETs. e) RRDE voltammograms of CoN@CNTs after EA (including EO and then ER) at 1600 rpm in an Os-saturated 0.1 M HCIO4 electrolyte with disc current and ring current. f) Calculated H2O2 selectivity on CoN@CNTs, aged CoN@CNTs and EA- CoN@CNTs based on the RRDE measurements. g) H2O2 production amount (determined via the potassium permanganate titration) as a function of time on the EA-CoN@CNT s. Current (thick curve) and concentration (dashed o) behaviour with time for the electrochemical H2O2 production is shown.

[00098] Figure 4 shows ORR activities of an example HE-CoN@CNTs catalyst: a) RRDE measurements of HE-CoN@CNTs for ORR in 0.1 M HCIO4 solution purged with O2 and air. b) Calculated H2O2 selectivity on HE-CoN@CNTs in O2- and air- saturated 0.1 M HCIO4 based on the RRDE measurements.

[00099] Figure 5 shows the Co K-edge (a) XANES spectra and (b) FT-EXAFS spectra of a CoN@CNTs composite, CoPc and Co foil; and (c) FT-EXAFS curve-fitting analysis of the CoN@CNTs composite.

[000100] Figure 6 shows calculated H2O2 selectivity on CoN@CNTs based on the RRDE measurements.

[000101 ] Figure 7 shows TEM images of: (a) FeN@CNTs and (d) NiN@CNTs composites. HAADF-STEM images of: (b) FeN@CNTs and (e) NiN@CNTs, showing the isolated distribution of Fe and Ni single atoms (bright dots). HAADF-STEM images (top-left) and corresponding EDS maps of: (c) FeN@CNTs and (f) NiN@CNTs for C (top-right), N (bottom-left), Fe and Ni (bottom-right).

[000102] Figure 8 shows ORR polarization curves of CoN@CNTs before and after adding 5 mM SCN ions into the 0.1 M HCIO4 electrolyte.

[000103] Figure 9 shows chronoamperometry performed using the RRDE system with the glassy carbon disk and Pt ring held at 0.55 and 1.2 V vs. RHE, respectively, in 0.1 M HCIO4.

[000104] Figure 10 shows polarization curves of the electrochemical oxidation applied on CoN@CNTs. The Pt ring was held at 0.2 V during the oxidation process to detect the possible O2 evolved from the oxidation process.

[000105] Figure 11 shows polarization curves of the reduction process applied on the CoN@CNTs. The ring was held at 1 .2 V during the reduction process to detect the possible H2O2 formed from the ORR reduction process. [000106] Figure 12 shows: (a) the ratios of different oxygen species within the total amount of oxygen in aged CoN@CNTs, EO-CoN@CNTs and EA-CoN@CNTs powders; and (b) the atomic percentages of different oxygen functional groups within the aged CoN@CNTs, EOCoN@CNTs and EA-CoN@CNTs powders. All these results were obtained by XPS O 1 s measurements and analysis.

[000107] Figure 13 shows: (a) H2O2 selectivity obtained from both RRDE measurements and chemical titration for the electro-activated CoN@CNTs in an O2- saturated 0.1 M HCIO4 electrolyte; and (b) /-t curves obtained from the H2O2 bulk production on the electro-activated carbon fibre paper electrode loaded with EA-CoN@CNTs under different operation potentials for the chemical titration.

[000108] Figure 14 shows: (a) XPS O1 s spectra of the aged CoN@CNTs and HTCoN@CNTs, showing an apparent emergence of epoxy groups on CoN@CNTs after H2O2 treatment; and (b) background-corrected FTIR spectrum of HT-CoN@CNTs.

[000109] Figure 15 shows: (a) calculated H2O2 selectivity of HT-CoN@CNTs and aged CoN@CNTs from the RRDE measurements; and (b) RRDE voltammograms of the aged CoN@CNTs before and after H2O2 treatment (HT) at 1600 rpm in an 02-saturated 0.1 M HCIO4 electrolyte with disc current and ring current.

[0001 10] Figure 16 shows: (a) XPS O 1 s spectrum of HE-CoN@CNTs, showing an apparent emergence of epoxy groups on CoN@CNTs after both H2O2 and electrochemical treatment; (b) XPS N 1 s spectra of the aged CoN@CNTs and HE-CoN@CNTs; (c) XPS Co 2p spectra of the aged CoN@CNTs and HE-CoN@CNTs; and (d) background-corrected FTIR spectrum of HE-CoN@CNTs.

[0001 11 ] Figure 17 shows: (a) XPS elemental survey of the HE-CoN@CNTs sample before and after 12-hour testing session for C>2 reduction in 0.1 M HCIO4; and high resolution XPS (b) Co 2p spectra, (c) O 1 s and (d) N 1 s of the HE-CoN@CNTs sample before and after 12-hour testing session for O2 reduction in 0.1 M HCIO4.

[0001 12] Figure 18 shows background-corrected FTIR spectrum of the HE- CoN@CNTs sample after 12-hour testing session for O2 reduction in 0.1 M HCIO4, showing clear emergence of epoxy groups.

[0001 13] Figure 19 shows an SEM image of an example 3D structured membrane incorporating the catalyst.

[0001 14] Figure 20 shows a schematic diagram of an example electrochemical cell design incorporating the membrane. [0001 15] Figure 21 shows a schematic diagram of an example electrolyser items 4 & 6 (from Figure 20), with flow channel and gas mass transport channel for the electrochemical cell.

[0001 16] Figure 22 shows the electrical performance of an example catalyst in an example electrochemical cell. A 100 cm ² cell can produce 0.45 wt.% neutral H2O2 solution.

DETAILED DESCRIPTION

[0001 17] The first general step in the process of catalyst preparation may involve the synthesis of a metal and N doped carbon catalyst. The metal may be any transition metal, but in certain embodiments is cobalt. The carbon may be in the form of any high surface area conductive carbon. In certain embodiments the carbon may be in the form of carbon nanotubes and/or vertically formed graphene sheets.

[0001 18] In certain specific embodiments, the catalyst may comprise cobalt/nitrogen doped graphene, for example cobalt/nitrogen doped vertical graphenes (CoN@VGs). These may be prepared by any suitable means, which in general may involve the synthesis of the carbon substrate from precursors and dopants.

[0001 19] For instance, where carbon nanotubes (CNTs) are the support material or substrate, the high surface area cobalt/nitrogen doped carbon nanotubes (CoN@CNTs) may be formed from the pyrolysis of the carbon and metal precursors in Argon, followed by an acid leach to remove metallic debris.

[000120] Where vertical graphenes (VGs) are used as the support material or substrate, these may be prepared by a plasma-assisted chemical vapor deposition (CVD) method. The method may use commercially available carbon fiber paper as a substrate upon which vertical graphene may be grown through a CVD process.

[000121 ] The Co and N dopants may subsequently be introduced onto the vertical graphene electrode via an electro-polymerization process in an aqueous solution containing both aniline and nitric acid, followed by immersing the polyaniline-coated vertical graphene electrode into the K ₃ [Co(CN)6] solution. Then the as-obtained electrode may, for example, be annealed in a nitrogen atmosphere to afford the Co and N co-doped vertical graphene electrode.

[000122] Thus, the catalysts disclosed herein can be prepared by at least two distinct approaches, namely, i) the incorporation of the metal and nitrogen dopants during the stage of formation of the carbon substrate (as per CoN@CNTs), or ii) by first forming the carbon substrate and then subsequently loading that with metal atoms and nitrogen sites postproduction (as per CoN@VGs). Those skilled in the art will appreciate that other approaches are possible, for example loading CNTs post-production with metal and nitrogen or modifying the CVD process to incorporate metal and nitrogen dopants into VGs during the formation of the VG structure.

[000123] The high surface area metal/N doped catalyst, such as a CoN@CNT or CoN@VGs may be subjected to a hitherto unknown sequential electrochemical oxidation/reduction process which further modifies the high surface CoN catalyst to produce a highly specifically epoxy functionalised catalyst, that is a catalyst in which C-O-C groups are present. This catalyst has unique structural properties that make it useful in hydrogen peroxide production.

[000124] In the C-O-C catalyst disclosed herein, oxygen may be tethered to the carbon surface via two separate (but most usually adjacent) carbon atoms in the CoN@CNT or CoN@VG. This is distinct from a ketonic link, O=C, where the oxygen is tethered to the CoN@CNT or CoN@VG surface via a single carbon, or other oxygenated forms where an oxygen has a single point of attachment to the catalyst (C-O-). The presence of a C-O-C epoxy oxygen has been found to be particularly advantageous in the ability of the catalyst to promote peroxide formation. Further, the electrochemical oxidation/reduction process disclosed herein may reduce or at least strongly suppress the formation of ketonic oxygen, which the present inventors have surprisingly found can be particularly deleterious to the production of hydrogen peroxide.

[000125] Turning specifically to the two-step electrochemical oxidation/reduction process disclosed herein, this process involves in turn a sequential electrochemical oxidation and then a reduction. The first step may be oxidation in a suitable medium, such as perchlorate, using, for example, an anodic linear sweep voltammetric scan to produce highly oxygenated CoN@CNTs or CoN@VGs. The oxygen is present in a variety of forms including the desirable epoxy form, undesirable ketonic form, and other forms. However, the subsequent reduction process may lead to an enhancement of the ratio of epoxy carbons by elimination of non-epoxy oxygen forms, or by conversion of non-epoxy oxygen forms to epoxy forms. In addition, the reduction step may supress ketonic oxygen on the surface of the catalyst. Without being bound by theory, the inventors of the present invention postulate that this two-step oxidation reduction process may exploit the fact that epoxy groups are more thermodynamically stable than either ester or carboxyl groups.

[000126] The resultant catalyst was found to be advantageously employable in the generation of hydrogen peroxides. Through a series of experiments, it was found that functionalizing the Co and N co-doped carbon catalysts with substantially only epoxy groups could exclusively enhance the H2O2 selectivity (-100%) during the electrochemical O2 reduction with a low overpotential (nearly zero). In contrast, the appearance of ketonic oxygen (C=O) may lead to the suppression of the production of H2O2. [000127] This very high selectivity and low cost of the catalysts disclosed herein may lend itself to many different approached bases around the safe and inexpensive production of H2O2.

[000128] Bearing in mind the particular advantages of the CoN@CNTs or CoN@VGs catalysis, the present inventors have devised a suitable electrochemical cell to best exploit the particular properties of the catalyst.

[000129] The basic principal of catalytic hydrogen peroxide production requires air or oxygen and water to come in contact with the supported catalyst at the reaction cathode. One challenge to be overcome is that the inherent nature of the catalyst, being, for example, an epoxy modified CoN@CNTs or CoN@VG, which is hydrophilic, whereas the air or gaseous oxygen is hydrophobic. The present inventors have found that the hydrophobic layer can be advantageously prepared by, for example, applying a spray of teflon nanoparticles to a support such as carbon fibre paper, and then applying to that the catalyst layer. That arrangement provides good access to the epoxy modified CoN@CNTs or CoN@VG by both the hydrophilic (aqueous) and hydrophobic (gaseous) sides.

[000130] Without being bound by theory, the electrochemical production of H2O2 is thought to proceed via the cathodic reduction of O2 (ORR), which can either produce the desired H2O2 via a 2-electron pathway or H ₂ O via an undesired 4-electron pathway. Herein, we focus on the 2-electron pathway as the H2O2 is the desired electrochemical product.

[000131 ] The 2-electron-pathway ORR under acid conditions (or conditions where the pH is less than 1 1 .6) may proceed according to the following reaction:

O ₂ + 2H ⁺ + 2e- H2O2 (Uo ^02/H202 = 0.7 V vs. RHE).

This reaction can be followed by either a further 2-electron reduction process:

H2O2 + 2H ⁺ + 2e- 2H ₂ O (t/ ₀ ^H2O2/H2 ° = 1 .76 V vs. RHE) or a chemical disproportionation process:

2H2O2 2H ₂ O + O ₂ .

[000132] In alkaline conditions (having pH > 1 1.6), HO2' may be formed via the following two electron pathway ORR:

O ₂ + H ₂ O + 2e- HO ₂ - +OH- ( Uo ^02/H02 ~ = 0.74 V vs. RHE)

The reaction can also be followed by either a further reduction to OH-:

HO ₂ - + H ₂ O + 2e- 3OH- (Uo ^H0220H ~ = 0.86 V vs. RHE) or a chemical decomposition process:

2HO ₂ - 2OH- + O ₂ . [000133] The inventors of the present invention have surprising found that the catalysts according to the present invention are capable of selectively producing hydrogen peroxide under acidic or neutral conditions, optionally wherein the pH is less than about 7.

[000134] The catalyst according to the present invention may be incorporated into an electrolyser device. The device may have an Os/air inlet on its cathode side, a current collector perforated to allow gas movement to the cathode itself (formed from, for example, a teflon nanoparticle gas facing side and a CoN@CNT s or CoN@VGs aqueous facing side), a channel for flow of electrolyte and egress of hydrogen peroxide, an ion exchange membrane, a channel for electrolyte flow, an anode and current collector, and an endpiece to seal the unit.

[000135] It can be seen, for example, that if the H2O2 production is conducted in acidic media, clean and cost-effective Fenton’s reagent, which is important for the remediation of drinking water (including that contaminated with heavy metals) can readily be made by simply adding Fe ²⁺ into the product of the electrolyser of the invention, i.e. the H2O2- containing electrolyte.

[000136] Furthermore, in the current system, a H2O2 concentration of -1200 ppm can readily be accumulated in acidic media within 30 min at 0 V, satisfying most applications including an electro-Fenton process for water treatment at low cost. An electro-Fenton process in water treatment typically would require about 10 ppm of H2O2 or more.

[000137] The electrocatalysts according to certain embodiments generally may be inexpensive to manufacture (as they typically do not contain precious metals) and may require only low energy input and may be highly active, with low overpotentials and/or a high selectivity (up to around 100% in certain embodiments). The H2O2 produced may thus be produced in a manner which can be economical and may be eco-friendly, especially when renewable electricity is employed as an energy supply for electrolysers using the catalysts.

[000138] In certain embodiments the method and electrocatalysts disclosed herein are free from toxic elements (such as mercury in a Pt-Hg catalyst). Moreover, the H2O2 production approach used herein may require clean electrons, oxygen and water as the only reactants, thereby making the H2O2 production method potentially eco-friendly, particularly compared with the traditional anthraquinone method.

[000139] The methods and devices of the present invention may be suitable for both small and large-scale operation. In certain embodiments the simple H2O2 production method disclosed herein may require only a potentiostat as the main equipment, enabling the production of H2O2 in both a small and large scale. In a small scale (or even a portable device), on-site production of hydrogen peroxide may be achieved at a desirable rate using the inventive method in remote areas and developing countries, where centralized production is not feasible. In a large scale, clean electricity can be employed to generate H2O2, making it a renewable chemical which traditional methods cannot achieve.

[000140] The use of vertical graphene is considered particularly advantageous in preparing such electrolysers as it can be grown directly onto conductive substrates, i.e. current collectors. This approach means that the functionalised catalyst on substrate can be used in the electrolysers with very little intermediate processing. Moreover, such vertical graphenes can readily be grown on large scale conductive substrates.

EXAMPLES

[00074] Embodiments will now be described with reference to the following nonlimiting Examples.

Synthesis and characterizations of CoN@CNTs, FeN@CNTs and NiN@CNTs catalysts.

[000141 ] The CoN@CNTs composite was prepared by pyrolyzing the carbon and metal precursors in Ar, followed by an acid leaching to remove the accessible metallic debris (see details in the Methods).

[000142] To characterize the typical structural morphologies of CoN@CNTs, scanning electron microscopy (SEM) and transition electron microscopy (TEM) were employed. The SEM images (Fig. 1a) show that numerous carbon nanotubes were formed after pyrolysis. Besides that, a bamboo-like feature of the nanotube structure was revealed by the TEM images (Fig. 1 b), showing diameters ranging from 50 to 200 nm. The energy dispersive X- ray spectroscopy (EDS) elemental mapping of a selected area on the CoN@CNTs composite (Fig. 1 c) exhibited uniform distributions of the Co, N, O and C elements across the tubular structure, demonstrating the successful incorporation of Co, N and O into the carbon substrate.

[000143] To identify the status of the Co species in CoN@CNTs, high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) measurements were conducted. As shown in Fig. 1d, the bright dots corresponding to Co atoms were homogenously dispersed throughout the CNTs. Electron energy loss spectroscopy (EELS) analysis (Fig. 1e) of an area within Fig. 1e further revealed the existence of isolated Co atoms in the carbon matrix.

[000144] An X-ray photon spectroscopy (XPS) elemental survey (Fig. 1f) also confirmed the existence of Co, N, O and C on the CoN@CNTs. To further reveal the nature of the isolated Co atoms in CoN@CNTs, X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) measurements (Fig. 5) were performed. Figure 5 shows that the measured and calculated spectra were well matched for the CoN@CNTs sample. Without being bound by theory, the weak signals at ~2.3 to 2.7 A in the Co K-edge EXAFS of CoN@CNTs may be ascribed to the Co-Co scattering path originating from a metallic feature in the Co nanoparticles wrapped at the closed end of carbon nanotubes. Compared with the strong Co-N scattering feature (at ~1 .5 A), the weak signals of Co-Co bond suggest a very rare appearance of metallic cobalt nanoparticles in the CoN@CNTs samples, as evidenced from TEM imaging.

[000145] Again, without being bound by theory, the higher white line intensity and position compared with CoO indicates the possible existence of positively charged Co with an oxidation state in CoN@CNTs. Furthermore, the slightly lower absorption edge position than that of cobalt(ll) phthalocyanine (CoPc) suggests the valence state of atomic Co may be between 0 and 2 ⁺ . The nearly absent appearance of a pre-edge feature (arisen from the forbidden 1 s-to-3d transition) at -7709 eV in the CoN@CNTs indicates a symmetric coordination environment of Co, e.g. C0-N4. Notably, the Fourier transformed (FT) Co K- edge EXAFS spectra exhibit the signal of light scattering nearest neighbours at -1.5 A, corresponding to the Co-N/C scattering pair and further corroborating the existence of tetrahedrally coordinated Co (e.g. C0-N4) as revealed by fitting analysis. The weak signals originating from the Co-Co scattering path in a metallic feature might be ascribed to the rare appearance of Co nanoparticles as discussed above.

[000146] FeN@CNTs and NiN@CNTs composites were prepared via a similar method to the CoN@CNTs and exhibited an analogous nanotube structure (Fig. 7)

Synthesis and characterizations of CoN@VGs catalysts.

[000147] The CoN@VGs electrodes were fabricated through a three-step synthesis process: Firstly, polyaniline was coated onto vertical graphene via an electro-polymerization process. Then Co was adsorbed and reduced onto the polyaniline-coated vertical graphene by surface imine-group reduction. Finally, the as-obtained electrode was pyrolyzed under a nitrogen atmosphere to form the Co and N doped vertical graphene electrode.

Electrochemical measurements and investigations of active sites.

[000148] The electrocatalytic performances and efficiency of CoN@CNTs for H2O2 production via ORR were examined in 02-saturated 0.1 M HCIO4 using a rotating ring disk electrode (RRDE) setup. The Pt ring electrode was held at 1 .2 V to quantify the amount of H2O2 produced on the disk electrode (see calculation details in the Methods). Fig. 2a shows the polarization curve obtained on CoN@CNTs, with the oxygen reduction current measured on the disk electrode (solid lines) and the H2O2 oxidation current measured on the Pt ring electrode (dashed lines). It is apparent that the CoN@CNTs composite exhibits decent catalytic activity towards the electrolytic production of H2O2 in acid, showing obvious H2O2 oxidation currents at a potential range below 0.7 V. [000149] Notably, it appears that the majority of electrons are consumed by a 2- electron pathway on the CoN@CNTs, showing a high H2O2 selectivity (~ 80% from 0.5 V to 0.7 V, Fig. 6), as determined by RRDE measurements.

[000150] Furthermore, in the absence of Co species, nitrogen-doped graphene (NG) with a similar amount of N dopants to the CoN@CNTs composite exhibits negligible H2O2 production within the whole potential range tested herein (Fig. 2a). Without being bound by theory, this suggests the prerequisite role of C0-N4 in enabling the efficient H2O2 generation via ORR. The same conclusion can also be obtained with a poisoning experiment, where a significantly receded disk was detected after the C0-N4 centers were poisoned by thiocyanate ions (SCN-) (details can be found in Fig. 8). Fig. 8 shows ORR polarization curves of CoN@CNTs before and after adding 5 mM SCN ions into the 0.1 M HCIO4 electrolyte. During the whole process, the solution was purged with O2 continuously. Due to the huge oxidation current of SCN- ions reflected by the Pt ring, the peroxide current was not detected.

[000151 ] Chronoamperometry was performed using the RRDE system with the glassy carbon disk and Pt ring held at 0.55 and 1.2 V vs. RHE, respectively, in 0.1 M HCIO4 (Fig. 9). The peroxide current on the Pt ring (j _rin g) was corrected by the collection efficiency. The H2O2 selectivity (solid curve) was calculated from the RRDE measurements. The ORR activity (reflected by jdisk) on the CoN@CNTs was maintained constant over a 3 hour period. Notably, during the three-hour H2O2 production session, the selectivity was maintained at -78% with negligible drop, showing a decent stability on the CoN@CNTs under these static operation conditions.

[000152] However, despite its high catalytic performance, after a long-term (~ 30 days) exposure to air, the selectivity of the CoN@CNTs composite towards H2O2 production fell dramatically. Fig. 2b compares the RRDE curves obtained with freshly prepared CoN@CNTs and CoN@CNTs that had been exposed to air for one month (named as aged CoN@CNTs). It can be seen from Fig. 2b that /ring on the aged CoN@CNTs drops significantly meanwhile the /disk remains almost unchanged (compared with CoN@CNTs), leading to the H2O2 selectivity <30% over the whole potential range.

[000153] Without being bound by theory, this phenomenon may indicate the occurrence of an undesired oxidation process on the CoN@CNTs composite during the air exposure that has adversely affected its H2O2 productivity. XPS was utilized to reveal the chemical changes between the CoN@CNTs and aged CoN@CNTs composites. As displayed in Fig. 2c, the only noticeable change between these two samples was the increment of O concentration, which rises from 3.50 at.% in CoN@CNTs to 5.17 at.% in aged CoN@CNTs. [000154] More details could be seen in the high-resolution XPS O 1 s spectra (Fig. 2d) of the two composites, which showed the presence of four oxygen components: (i) ketonic oxygen (C=O, 01 , 531.2±0.2 eV), (ii) oxygen atoms in epoxy (C-O-C) or hydroxyl groups and carbonyl oxygen in ester groups (02, 532.3±0.2 eV), (iii) the C-O-C oxygen in ester groups (03, 533.3±0.2 eV), and (iv) oxygen atoms in carboxyl groups (04, 534.2±0.2 eV).

[000155] Among all the four O functionalities detected within the composite, only the ketonic groups (01 ) exhibited a noticeable increase, of which the concentration had increased from merely 0.84 at.% in the fresh sample to 2.64 at.% in the aged sample (Table 1). This observation suggests that ketonic groups may be responsible for the receded H2O2 productivity on the aged CoN@CNTs.

Table 1 : Summarization of the content of different O groups in the different CoN@CNTs samples based on the XPS measurement. The content of 02 could be ascribed to the epoxy O. The ratio means the percentage of a certain O group in the total O content, and the atom% reflects the amount of a certain O group in the whole materials.

[000156] Electrochemical treatment (ET) was selected here as an approach to in situ re-construct the surface oxygen functionalities on the carbon-based materials. Nevertheless, by simply performing electrochemical reduction (ER) it is difficult to remove oxygen functional groups (such as ketonic O) that are thermodynamically more stable than the carboxyl groups. Compared with the ER process, the inventors of the present application postulated that an electrochemical oxidation (EO) treatment may be more effective in rebuilding the surface oxygen functional groups on carbon-based materials. Thus, herein, a two-step electrochemical process combing both EO and ER was adopted to treat the aged CoN@CNTs to modify the O functionalities.

[000157] Firstly, the aged CoN@CNTs was electrochemically oxidized in a 0.1 M HCIO4 solution (EO-CoN@CNTs) by an anodic linear sweep voltammetric scan (details can be seen in Fig. 10). Specifically, the aged CoN@CNTs was electrochemically oxidized by conducting an anodic linear scan voltammetry (from 1.2 to 2.4 V vs. RHE) in the 0.1 M HCIO4 solution, during which an anodic peak appeared at ~2 V and no oxygen evolution was detected at the Pt ring, indicating that the anodic peak was related to a surface oxidation process on CoN@CNTs rather than oxygen evolution.

[000158] Based on the post-reaction XPS measurements (Fig. 3b), the EO treatment provided the aged CoN@CNTs composite a higher O content (12.66 at.%) owing to the highly positive potential applied that may have caused carbon oxidation. Specifically, the EO-CoN@CNTs exhibited a higher ratio of 02 and 04, while that of ketonic O (01 ) decreased, indicating the electrochemical generation of epoxy and carboxyl groups on the CoN@CNTs and a possible conversion of C=O into other O species during this treatment.

[000159] Then, the EO-CoN@CNTs was subjected to an ER treatment to afford the electrochemically activated CoN@CNTs (EA-CoN@CNTs, Fig. 11 ). Specifically, the aged CoN@CNTs was electrochemically reduced through performing a cathodic linear scan voltammetry (from 0.6 to -0.6 V vs. RHE) in a 0.1 M HCIO4 solution, during which a cathodic peak appeared at ~-0.1 V and no H2O2 was detected at the Pt ring, suggesting the cathodic peak may be related to a surface reduction process on CoN@CNTs.

[000160] From the XPS O 1 s spectrum (Fig. 3c) of the EA-CoN@CNTs, it appears that after the electrochemical reduction process, the ratios of 03 and 04 both dropped dramatically (from 33.2 and 24.7% to 15.9 and 6.9% respectively) while that of the 02 became dominant (55.8%) compared with those of EO-CoN@CNTs (Fig. 12). The significantly reduced amount of 03 and 04 may be correlated to the reduction of some O functional groups (such as ester and carboxyl) under a cathodic potential. Without being bound by theory, it is thought that epoxy groups are thermodynamically more stable than ester/carboxyl groups, and therefore more difficult to be removed by ER.

[000161 ] Accordingly, compared with the aged CoN@CNTs, the exclusively increased 02 species in the EA-CoN@CNTs may be attributed to the emergence of epoxy groups on the catalysts. Fourier-transform infrared spectroscopy (FTIR) measurements (Fig. 3d) further confirmed the appearance of these epoxy functionalities on the EA-CoN@CNTs, showing a strong and broad absorption band (alkoxy or epoxy C-O) at a region from 1000 to 1200 cm' ¹ . [000162] Moreover, no leaching of Co in the electrolyte was detected by the inductively coupled plasma mass spectroscopy (ICP-MS) measurement during the ETs, indicating the chemically stable property of the Co sites.

[000163] To test the effectiveness of the electrochemical treatments, electrolytic H2O2 production over the EA-CoN@CNTs was also evaluated in 02-sat. 0.1 M HCIO4 solution. Surprisingly, EA-CoN@CNTs exhibited a superior H2O2 productivity that was even higher than the freshly prepared CoN@CNTs. Specifically, the onset potential of EA-CoN@CNTs at the ring and disk coincided at ~0.7 V (Fig. 3e), which is the thermodynamic onset potential of the genuine 2-electron pathway of ORR in acid for H2O2 production.

[000164] As the overpotential increased, most of the /disk recorded on EA- CoN@CNTs can be accounted for by the production of H2O2. The selectivity of H2O2 production was well above 90% within a region between 0.3 and 0.6 V (Fig. 3f) based on the RRDE measurement, indicating a nearly complete 2-electron ORR process on the EA- CoN@CNTs. Also, at a potential of 0.3 V, the /' on the disk (0.125 cm ² ) achieved 3 mA cm- ² , which is close to the theoretical mass transport limit for the 2-electron reduction of oxygen.

[000165] To investigate the Faradaic efficiency (FE) and stability of EA-CoN@CNTs during bulk electrolysis, the electrocatalytic H2O2 production and accumulation through ORR was carried out in a H-cell setup separated by a Nation membrane with continuous O2 bubbling. The FE was verified for the purpose of a more intuitive demonstration employing permanganate titration (average of three repetitions) within a range of potentials from 0.45 to 0.65 V. Of note, the titration results (Fig. 13) exhibited a FE as high as 95%, which coincides with the RRDE results. The long-term stability performed via a chronoamperometric measurement at 0.45 V for 12 h (Fig. 3g) yielded a current of -10 mA, during which the concentration of accumulated H2O2 also increased continuously.

[000166] Without being bound by theory, from the electrochemical results shown above, the inventors of the present application postulate that the synergistic effect between C0-N4 centers and their adjacent C-O-C groups may play an advocating role in reducing oxygen exclusively to H2O2, which makes the EA-CoN@CNTs among the most effective catalyst for electrolytic H2O2 production in acids.

[000167] C-O-C groups on carbon can also be introduced through chemical approaches and H2O2 treatment (HT) was found to be an effective method. Hence, to further understand the critical role of epoxy functionalized C0-N4 centers in promoting the 2-electron ORR for H2O2 production, an aged CoN@CNTs sample was chemically treated in 0.1 M HCIO4 solution containing 5 wt% H2O2 at 70°C for 2 hours.

[000168] A dominant presence of epoxy groups was detected by both XPS (Fig. 14a) and FTIR (Fig. 14b) on the H2O2 -treated CoN@CNTs (HT-CoN@CNTs), accompanied by a significantly reduced intensity of the ketonic group, suggesting the effectiveness of this treatment. The HT-CoN@CNTs exhibited a much higher H2O2 selectivity (Fig. 15a) as well as productivity (Fig. 15b) than the aged CoN@CNTs, echoing the findings from the electrochemical treatments.

[000169] Thus, to further enhance the H2O2 productivity of the CoN@CNT s composite, both H2O2 treatment (HT) and electrochemical activation (EA) methods were applied together (see details in the Methods) on the aged CoN@CNTs sample to obtain the HE- CoN@CNTs. A high content of O (12.3 at.%) and the highest ratio of epoxy groups (71 .3%) among all samples prepared were obtained (Fig. 16a and Table 1 ), accompanied by a significantly increased ratio of pyridonic N (Fig. 16b). The HE-CoN@CNTs exhibits, to the best of our knowledge, a record-breaking catalytic performance towards H2O2 production in acid. Compared to both CoN@CNTs and aged CoN@CNTs, the HECoN@CNTs appears to exhibit a significantly higher ratio of N component located at -399. 3 eV (namely N*), corroborating well the potential formation of more pyridonic N that might be converted from the pyridinic N near the HE-generated epoxy groups. Co 2p spectrum of HECoN@CNTs also revealed a slightly positive shift (-0.4 eV) of binding energy of Co-N peak, suggesting an interaction between the HE-generated epoxy groups and Co-N _x species via a possible electron-withdrawing effect.

[000170] In the FTIR spectrum of HE-CoN@CNTs (Fig. 16d), the much receded intensity of a peak at -3500 cm ^-1 compared to HT-CoN@CNTs (Fig. 14b) indicated some oxygen functional groups (e.g. C-OOH, C-OH) may be more easily removed by an electrochemical reduction method than the epoxy oxygen, and that these functionalities may not have any correlation with the enhanced H2O2 selectivity.

[000171 ] Shown in Fig. 4a and 4b, with the onset potential at 0.7 V, the HE- CoN@CNTs composite was capable of maintaining a nearly 100% selectivity of H2O2 production within a wide potential range from 0.3 to 0.6 V (> 95%), exceeding the catalytic performances of those benchmarks and state-of-the-art catalysts, including precious metals, their alloys and recently reported carbon-based materials, in terms of both overpotential and selectivity.

[000172] Post-reaction characterizations (XPS and FTIR, Fig. 17 and 18) showed negligible changes on the HE-CoN@CNTs after 12-hour reaction session, evidencing the fact that not only the Co species, but also the epoxy groups appear to be stable during ORR. In addition, the catalytic activity and H2O2 selectivity was kept almost unchanged on the HE- CoN@CNTs sample even after a long-term aging period in air. Notably, even when the reaction was conducted using air bubbling (Fig. 4b), the H2O2 selectivity was still maintained at >80% within the whole voltage range, demonstrating the treated catalyst’s potential to be applied in practical applications. Material and Methods

[000173] All reaction reagents and chemicals were obtained and used in their as- received form without any further purification. Dicyandiamide, Co(N03)2-6H ₂ 0, Ni(NO3)2‘6H ₂ O, Fe(NO3)3-9H ₂ O, Nation solution (5 wt%) and cobalt(ll) phthalocyanine (CoPc) were obtained from Sigma-Aldrich. Deionized water was obtained through the water purification system (Milli-Q water) in the lab.

Synthesis and surface treatments of CoN@CNTs

[000174] Typically, to prepare the CoN@CNTs catalyst, 3.5 mmol Co(N03)2-6H ₂ 0, 35 mmol dicyandiamide and 2 mL ethanol were mixed in an agate mortar, followed by continuous grinding until the mixture formed a uniform pink paste. The as-obtained mixture was placed into a crucible boat and heated to 800°C with a ramping rate of 3°C min ^-1 for 3.5 hour under an Ar atmosphere. The impurities and undesirable nanoparticles outside the carbon nanotubes were removed through an acid leaching process in 0.5 M H ₂ SC>4 at 90°C for 4 hours, followed by a repeated filtering and washing process using deionized water.

[000175] The FeN@CNTs and NiN@CNTs were prepared using a similar pyrolysis process by changing the type of metal nitrate in the precursors.

[000176] The effect of carbonization time on H ₂ O ₂ productivity was investigated in detail, and it was found that CoN@CNTs pyrolyzed at 800°C for 3.5 h yielded the highest ORR activity towards H ₂ O ₂ production.

[000177] The EA-CoN@CNTs were obtained by oxidizing the CoN@CNTs electrochemically through conducting an anodic linear sweep voltametric scan from 1 .2 to 2.4 V vs. RHE, followed by an electrochemical reduction process via a cathodic linear sweep voltametric scan from 0.6 to -0.6 V vs. RHE. To prepare the HE-CoN@CNTs, the CoN@CNTs sample was treated through a combined H ₂ O ₂ treatment and electrochemical activation process. Hot alkaline treatment was conducted by heating the aged CoN@CNTs sample in 6 M KOH solution with an autoclave reactor under 180°C for 12 h.

Physicochemical Characterizations

[000178] Transmission electron microscopy (TEM) was carried out on a Phillips/CM 200 microscope operated at an accelerating voltage of 200kV. Scanning electron microscopy (SEM) was conducted on a JEOL 7001 F operated at 5 kV. FTIR measurements were conducted on a PerkinElmer FTIR Spectrometer. X-ray photoelectron spectroscopy (XPS) measurements were carried out on a Thermo ESCALAB250Xi X-ray photoelectron spectrometer using Cu Ka X-rays as the excitation source with a voltage of 12.5 kV and power of 250 W. High angle annular dark field-scanning transmission electron microscopy (HAADF-STEM) and energy dispersive X-ray (EDX) mapping were obtained on a spherical aberration corrected transmission electron microscope (FEI Titan G2 80-200) which was operated at 200 kV. Inductively coupled plasma mass spectroscopy (ICP-MS) was carried out using a PerkinElmer quadrapole Nexion instrument. Co K-edge X-ray absorption spectroscopy (XAS) measurements were performed at the 10-ID-B beamline of the Advanced Photon Source (APS), Argonne National Laboratory (ANL). Data was collected using a fluorescence geometry and scanned from 200 eV below the Co K-edge (7709 eV) to ~ 1000 eV past the edge. Data reduction and subsequent modelling efforts were performed using the Demeter software package. Modelling results used a Co-(N ₃ C2)2 cluster to modelling Co-N and Co-C backscattering paths. An S02 value of 0.776 was used for all models and obtained from modelling a reference Co foil. O K-edge and Co L-edge NEXAFS measurements were performed at the SXR beamline of the Australian Synchrotron. Powders were pressed onto Ir foil and mounted onto metallic sample holders for measurements under partial electron yield (PEY) mode. O K-edge measurements were performed from 520 eV to 580 eV while Co L-edge measurements were performed from 770 to 810 eV and internally calibrated using MnO and Co foil reference samples respectively. All data processing, energy calibrations, and normalization was performed using the program QANT.

Electrochemical measurements

[000179] The electrochemical tests were all performed in 0.1 M HCIO4 aqueous solution within a three-electrode system at room temperature on a computer-controlled potentiostat (CH Instrument, CHI 760E). The oxygen reduction activity and selectivity were investigated by polarization curves and rotating ring-disk electrode (RRDE) measurements in oxygen -saturated electrolyte with a scan rate of 5 mV s ^-1 . A glassy carbon electrode loaded with catalyst was used as the working electrode. A graphite rod and a saturated calomel electrode were used as counter and reference electrode respectively. To prepare the working electrode loaded with the catalyst, typically, 5 mg as-prepared catalyst and 25 pL Nation solution (Sigma Aldrich, 5 wt %) were dispersed in 1 mL ethanol aqueous solution (50 %) to form a homogeneous ink with the help of a sonication process for 30 min. Then, 6.5 pL of the ink was drop casted onto the surface of a polished glassy carbon electrode and dried under room temperature in atmosphere. The final loading of the catalysts on the working electrode was 0.25 mg-crm ² . All of the obtained potentials were calibrated to a reversible hydrogen electrode (RHE, ERHE = ESCE + 0.2415 + 0.059 x pH). No correction of the system resistance was employed. The Tafel slope was calculated using the Tafel equation: q = b log (J/jd) (q, b, j and jo represent the overpotential, Tafel slope, current density and exchange current density respectively).

[000180] H2O2 selectivity of the catalysts was calculated from the current of both disc and ring electrodes using the following equation: H ₂ O ₂ (%) = 200 x lr/N/(ld+lr/N) where Id is disk current, Ir is ring current, and N is current collection efficiency of the Pt ring. N was determined to be 0.32 in the present system after calibration using the reversible [Fe(CN) ₆ ] ^{4 /3-} redox couple (+0.36 vs. SHE). A potential of 1 .2 V vs. RHE was applied on the Pt ring of the working electrode at a speed of 1 ,600 r.p.m. during the entire testing process.

[000181 ] Faradaic efficiency and electrocatalytic production of H ₂ O ₂ were measured in a two-compartment cell with a Nation membrane as a separator. Both the cathode compartment (75 mL) and anode compartment were filled with 0.1 M HCIO4 aqueous solution. Oxygen was continuously purged into the cathode (working) compartment under vigorous stirring. A graphite rod and a saturated calomel electrode were used as a counter and reference electrode respectively. A hydrophobic carbon fiber paper (CFP) loaded with as-prepared CoN@CNTs catalyst (0.5 mg cm' ² ) was used as the working electrode.

[000182] To quantify the amount of H ₂ O ₂ produced, an independent test not based on electrochemical methods was used: permanganate titration. Typically, samples with a volume of 1 mL collected at certain time intervals were diluted into 10 mL and titrated with 0.005 M KMnC aqueous solution. The concentration of H ₂ O ₂ produced was calculated according to the following equation:

CH2O2 = 5CKMHO4 X VKMHO4 + 2VH2O2 where CH202 is the H ₂ O ₂ concentration (mol L ^-1 ), CKMH M is the precise concentration of KMnC solution (mol L ^-1 ), VKMH M is the volume of KMnC solution consumed during titration (mL), and V 2O2 is the volume of H ₂ O ₂ solution.

Membrane

[000183] Disclosed herein is a novel membrane design to optimise the catalysts to reduce Air or O ₂ into H ₂ O ₂ and produce a neutral H ₂ O ₂ solution without any additional chemicals. A thermoplastic perfluorosulfonate ion-exchange resin was hot-pressed or extruded under 200 to 300 °C with solid templates (including, for example, dissolvable and/or indissoluble metal salts (such as sodium salts, potassium salts, etc.) and/or silica- based material with size from 100nm to 100 pm) into a sheet or filament form. In one example, 200g thermoplastic resin was mixed with 10 pm size SiO ₂ beads in a volume ratio of 1 :1 as a solid template for the membrane formation.

[000184] Thereafter the composition was further hot-pressed, or 3D printed into a membrane with a thickness of from 50 pm to 5mm. The hot-press temperature used was from about 200 to 260 °C. The 3D printing method incorporated printing under 300 °C on a 200 °C hotbed. After removing the template, the membrane was treated by base hydrolysis in a solution of 15 wt% KOH, 35 wt% dimethyl sulfoxide (DMSO), and 50 wt% deionised water at 80 °C for 24 - 72 hours, and subsequently dipped into a 5 w% aqueous H ₂ O ₂ solution at 80 °C for 24 - 72 hours, and then dipped into a 1 M sulfuric acid solution at 80 °C for 24 - 72 hours.

[000185] The catalysts or catalysts on the conductive substrate (carbon or metal fibre, cloth, felt) were coated on each side or pressed into the membrane to form a 3D structured porous membrane electrode with a thickness of from 60 pm to 15mm. An example membrane is shown in Fig. 19.

Electrolyser

[000186] The membrane-based catalysts were used in a novel designed electrochemical cell to produce a neutral H2O2 solution with a concentration of from about 0.01 wt.% to about 1 wt.%. The electrochemical cell assembly consisted of cell frames, an anode, a cathode, and a membrane. In certain embodiments the anode, cathode and membrane are hot-pressed together. Alternatively, in other certain embodiments the anode, cathode and membrane are not hot-pressed together.

[000187] A schematic depiction of an example electrochemical cell (electrolyser) is shown in Fig. 20. The cell comprises an anode current collector 1 with a liquid/gas flow channel, and an anode catalyst layer 2 (which can be, for example, a commercial catalyst with or without a conductive substrate) that together form the anode part of the cell.

[000188] The cell further comprises a 3D structured membrane 3 housed within a frame 4 which has a flow channel for the membrane 3. The cell additionally comprises a cathode gas layer 5 (which may comprise the catalyst according to the invention, or a commercial catalyst (such as carbon black, active carbon, etc.) with or without a conductive substrate) and a cathode current collector 6 with a gas chamber and dispersers inside. The frame 4, cathode gas layer 5, and cathode current collector 6 together form the cathode side gas/liquid mix system of the cell. The anode current collector 1 , frame 4, and cathode current collector 6 may, for example, be made, for example, of stainless steel, titanium or other suitable metals.

[000189] This novel design combining the cell and membrane-based electrode provides a new concept and a solution to mix gases and liquids and deliver them to the surface of the catalyst. This design, compared with conventional membrane electrodes, may provide more 3-phase interface (solid-liquid-gas (catalysts-water-air)) in the reaction system to facilitate H2O2 generation efficiently and effectively, optionally even in pure water without adding any other chemical or electrolyte. Specifically, the gas pathway may deliver gas more efficiently without disturbing the liquid. The liquid pathway does not block the gas pathway and effectively flushes the produced H2O2 out of the system. The working temperature of the example cell was between 0 and 80 °C. [000190] For optimising the performance, the membrane with catalyst is incorporated into the designed electrochemical cell with a specially designed cell frame 4 and cathode current collector 6. Fig. 21 shows a diagram of a suitable example design for these components.

[000191 ] The cell frame 4, comprises an empty space 7 (shadow) for placing the 3D membrane and/or porous material and also allowing the liquid to flow. The cathode current collector 6, comprises a gas disperser window 8 containing a porous material. The size of the gas disperser window 8 may be, for example, from about 0.5 to about 2 cm, and preferably about 1 cm. The distance between adjacent gas disperser windows 8 in the array shown in Fig. 21 may be from about 0.1 to about 1 cm, and preferably about 0.5 cm.

[000192] Fig. 22 shows the electrical performance of an example catalyst according to the invention in the example electrochemical cell electrolyser. A 100 cm ² cell was able to produce a 0.45 wt.% neutral H2O2 solution.

[000193] The skilled person will appreciate that this cell design can be used for the catalysts of the present invention in the production of hydrogen peroxide, but could also be useful for the production of other products, in particular for systems and methods requiring the interaction of liquid and gaseous materials with a solid catalyst.

[000194] Disclosed herein are the following embodiments:

1 . A catalyst comprising a carbon support, a doped metal and an epoxy group.

2. The catalyst according to embodiment 1 wherein the epoxy group is the predominant or sole oxygen functionality.

3. The catalyst according to embodiment 1 or embodiment 2 wherein the carbon support is a carbon nanotube.

4. The catalyst according to embodiment 1 or embodiment 2 wherein the carbon support is vertical graphene.

5 The catalyst according to any preceding embodiment wherein the carbon support is N doped.

6. The catalyst according to any one of the preceding embodiments wherein the metal is a transition metal.

7. The catalyst according to any one of the preceding embodiments wherein the metal is cobalt.

8. The catalyst for preparing hydrogen peroxide comprising an Co/N co-doped carbon nanotube and an epoxy group as the predominant oxygen functional species. 9. The catalyst for preparing hydrogen peroxide comprising an Co/N co-doped vertical graphene and an epoxy group as the predominant oxygen functional species.

10. A method of preparing a catalyst according to anyone of the preceding embodiments comprising the steps of :

(i) oxidising a carbon support comprising a doped metal to produce an oxidised carbon support comprising epoxy oxygen groups and at least one other oxygen species

(ii) reducing the oxidised carbon support to produce a catalyst comprising substantially only epoxy species.

11 . The method according to embodiment 10 wherein oxidising and reducing are conducted electrochemically.

12. The method according to embodiment 10 or 11 wherein the carbon support is a carbon nanotube or vertical graphene.

13. The method according to any one of embodiments 10 to 12 wherein the carbon support is N doped.

14. The method according to any one of embodiments 10 to 13 wherein the doped metal is a transition metal.

15. The method according to any one of embodiments 10 to 14 wherein the doped metal is cobalt.

16. A method of preparing a catalyst for preparing hydrogen peroxide comprising an Co/N co-doped carbon nanotube and an epoxy group as the sole oxygen functional species comprising the steps of:

(i) oxidising a Co/N co-doped carbon nanotube to produce an oxidised Co/N co-doped carbon nanotube comprising at least epoxy and ketonic species

(ii) reducing the oxidised Co/N co-doped carbon nanotube to produce a Co/N co-doped carbon nanotube catalyst comprising epoxy species and substantially no ketonic species.

17. A method of preparing a catalyst for preparing hydrogen peroxide comprising an Co/N co-doped vertical graphene and an epoxy group as the sole oxygen functional species comprising the steps of : (i) oxidising a Co/N co-doped vertical graphene to produce an oxidised Co/N co-doped vertical graphene comprising at least epoxy and ketonic species

(ii) reducing the oxidised Co/N co-doped vertical graphene to produce a Co/N co-doped vertical graphene catalyst comprising epoxy species and substantially no ketonic species.

18. An electrolyser for producing hydrogen peroxide from oxygen and water, the electrolyser comprising a cathode which comprises a catalyst layer comprising a catalyst according to any one of embodiments 1 to 9 on an electrolyte facing side and a hydrophobic gas permeable layer on an oxygen input side.

19. An electrolyser according to embodiment 18 wherein the hydrophobic gas permeable layer on an oxygen input side is formed from a polytetrafluoroethylene permeable material, such as a polytetrafluoroethylene coating layer or membrane or an aggregate of polytetrafluoroethylene nanoparticles.

20. An electrolyser according to embodiment 18 or 19 wherein the catalyst layer and hydrophobic gas permeable layer are immediately adjacent each other.

21 . An electrolyser according to embodiment 18 or 19 wherein the catalyst layer and hydrophobic gas permeable layer are present in conjunction with a mechanical support layer, such as carbon paper or a perforated metal substrate.

22. An electrolyser according to embodiment 21 wherein the catalyst layer comprises vertical graphene and the support layer is the graphene growth support.

23. A method of synthesising hydrogen peroxide comprising providing oxygen in an acidic or neutral aqueous media to an electrolyser according to any one of embodiments 18 to 22 under reducing conditions.

24. A method according to embodiment 23 wherein the cell produces at least 2000 mg L ^-1 h ^-1 , or at least 5000 mg L ^-1 h ^-1 hydrogen peroxide.

25. A method according to embodiment 23 or 24 wherein the cell produces hydrogen peroxide at a concentration of least 2000 mg L ^-1 (0.2%) or at least 5000 mg L ^-1 (0.5%) or at least 30000 mg L ^-1 (3%).

26. A method of synthesising Fenton’s reagent comprising providing a source of Fe ²⁺ and oxygen in an acidic or neutral aqueous media to an electrolyser according to any one of embodiments 18 to 22 under reducing conditions.

[000195] Although the invention has been 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. In particular, features of any one of the various described examples may be provided in any combination in any of the other described examples. Various modifications and alterations to this invention will become apparent to those skilled in the art without departing from the scope and spirit of this invention. It should be understood that this invention is not intended to be unduly limited by the illustrative embodiments and examples set forth herein and that such examples and embodiments are presented by way of example only with the scope of the invention intended to be limited only by the claims set forth herein as follows.