Method for removing nitrogen compounds

Abstract

The present invention relates to a method for removing nitrogen compounds, comprising hydrolyzing in an aqueous medium a urea derivative of general formula I: (R 1 , R 2 )N-C(=X)-N(R 3 , R 4 ) [wherein X represents NH, NR 5 , or S, and R 1 , R 2 , R 3 , R 4 , and R 5 may be the same or different and have the meanings indicated in claim 1], or a polymer of a compound of formula I, in at least one electrolysis cell comprising an anode containing a metal, where "metal" means one or more metals, one or more compounds of a metal, or a mixture of metal compounds, or combinations thereof, and a metal cathode. This method also includes obtaining nitrogen as a result of oxidation of the nitrogen compound at the anode and obtaining hydrogen as a result of reduction of water at the cathode, provided that when the anode is made of platinum, the cathode is not made of platinum.

Description

Technical Field

[0001] The present invention belongs to the field of removal and utilization of nitrogen compounds for obtaining clean energy sources such as hydrogen.

Background Art

[0002] Nitrogen compounds are excessively present in the air, water, soil, and food, and thus are risk factors for public health. For example, nitrates and nitrites are harmful to both humans and the environment, and it is required to reduce their presence. In particular, their use in energy production has attracted great attention.

[0003] In addition, hydrogen is known as a future energy source because it has a higher energy value and is unlimited compared to fossil fuels (gas, coal, and oil). Furthermore, its conversion to energy produces only water vapor and is thus completely clean. Currently, there is only one feasible electrolysis method for obtaining hydrogen, which is the electrolysis of water, but due to the low production efficiency of this element, its commercial application is limited. As a result, 96% of hydrogen is obtained from fossil fuels, causing serious pollution resulting from its extraction.

[0004] Despite being a research field that many researchers are working on, there is no technology that can convert urea derivatives such as thiourea and guanidine by electrolysis to produce hydrogen in a single step. In addition to increasing hydrogen resources, this process could also denitrify water from rivers, water currents, and wastewater treatment plants of municipal sewage.

[0005] Currently, the nitrate content of these waters before discharge is regulated, but since the available denitrification technologies are expensive and inefficient, the development of a technology that can directly convert urea derivatives into valuable products before they are naturally hydrolyzed into ammonia and toxic nitrogen derivatives could be a major technical advantage.

[0006] Urea derivatives have appeared in the literature as electrolytes in some processes, as materials for electrode manufacturing, or as sulfur sources. For example, the use of thiourea in the manufacture of electrodes is described in Mixed MoS2 / MoO3 Nanostructures for Hydrogen Evolution Reaction; Aftab, Umair; Ansari, Hamza Majeed; (...); Ibupoto, Zafar Hussain, 2021-04-01, Journal of Nanoscience and Nanotechnology 21 (4), pp.2500-2510.

[0007] The scientific paper Abdulkadir Levent, Ertugrul Keskin, Yavuz Yardim and Zuehre Sentuerk; Electrooxidation of thiourea and its square-wave voltammetric determination using pencil graphite electrode; Article in Reviews in Analytical Chemistry, April 2011; DOI:10.1515 / revac.2011.005 aims to detect these compounds in the micromolar concentration range and discloses the electrochemical properties of thiourea in the pH range of 2.0 to 12.0 using cyclic voltammetry and square-wave voltammetry.

[0008] The electrode system used in this paper included a PG (pencil lead graphite) electrode, an Ag / AgCl / 3M NaCl working electrode, and a reference electrode (Model RE-1, BAS, USA), as well as platinum as the counter electrode. Thiourea was oxidized on the pencil lead graphite electrode. Furthermore, its applicability to wastewater treatment was tested. However, according to this paper, the electrolysis of thiourea was not carried out.

[0009] On the one hand, Japanese Unexamined Patent Application Publication No. 2-163392 (A) "Method for Manufacturing Electrodes", published in 1990, relates to a method for manufacturing electrodes with low hydrogen surge and excellent durability by forming a sulfur-containing Ni coating layer on a substrate at a specific designated pH. The substrate is, for example, an iron substrate, and a Ni bath with pH ≤ 2 containing Ni salts such as NiCl2 and soluble sulfur compounds such as thiourea is used. Electrodes with a low hydrogen surge of about 100 - 150 mV, which are useful as electrodes for electrolysis on an industrial scale such as electrolysis of water, can be obtained.

[0010] International Publication No. 2020 / 213648 (A1) pamphlet describes the use of thiourea for the purpose of realizing electrolysis of water, but this relates to obtaining oxygen (ORR). The catalyst in this document contains Ni atoms, a condensate of thiourea and formaldehyde, and porous carbon.

[0011] The scientific literature "Effect of thiourea on the hydrogen yield in electrolysis", Yazici, B.; Arslan, G.; Erbil, M.; Zor, S.; International Journal of Hydrogen Energy (1998), 23(10), 867 - 872, relates to the effect of the use of thiourea on electrolysis for obtaining hydrogen. Therefore, the effect of thiourea in 1.0 M Na2SO4 + x mM TU (x = 0, 5, 15, 50) at different pH (2 - 8) has been analyzed in electrolysis using Pt as the electrode for both the anode and the cathode.

[0012] Regarding other urea derivatives such as guanidine, the document of European Patent Application Publication No. 3054034 (A1) "amine-containing electrolyte for electrochemical devices" relates to a method for enhancing the reliability of electrochemical devices operating in an alkaline medium by avoiding deterioration of stainless-steel parts and anionic membranes. Guanidine is part of an aqueous solution of the electrolyte.

[0013] There are also documents related to driving a solid oxide fuel cell (SOFC) that reacts guanidine with water to produce at least hydrogen or ammonia and generates electrical energy by the oxidation of hydrogen.

[0014] One such document is International Publication No. 2008 / 115662 (A2) pamphlet, carbon dioxide sequestering fuel synthesis system and use thereof, but this is not related to the electrolysis of guanidine itself.

[0015] Another such document is International Publication No. 2012 / 123380 pamphlet, which is also related to energy generation from water and a cyanoguanidine-containing composition, but not related to the electrolysis of guanidine itself.

[0016] Finally, International Publication No. 2017 / 125610 (A1) pamphlet is related to guanidine as a possible component of an electrolyte, but does not disclose the electrolysis of guanidine.

[0017] Both electrodes of the anode and the cathode need to meet several functions as follows in order to be useful in the electrolysis of urea derivatives to obtain hydrogen. - It must catalyze the decomposition reaction of the urea derivative. - It must generate bubbles without blocking the pores of the active material. The "active material" is generally understood to be a substance used on the surface of the electrode, such as a metal that catalyzes the redox reaction (which becomes the "active metal" in this case). - The target chemical species must be passed to the catalyst center. - It must have conductivity such that excessive resistance loss does not occur. - It must be chemically stable even when in contact with a basic or neutral electrolyte (such as KOH, etc.). - It must be as inexpensive, efficient, and lightweight as possible.

[0018] All of these preconditions make it difficult to design and manufacture electrodes that are problem-free in all of them.

Prior Art Documents

Patent Documents

[0019]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Patent Document 5

Patent Document 6

Non-Patent Documents

[0020]

Non-Patent Document 1

Non-Patent Document 2

Non-Patent Document 3

Summary of the Invention

Problems to be Solved by the Invention

[0021] For this reason, an object of the present invention is to remove a nitrogen compound by its electrolysis accompanied by the production of hydrogen and nitrogen.

Means for Solving the Problems

[0022] Hydrogen and nitrogen are obtained directly or indirectly from urea derivatives such as thiourea or guanidine by electrochemical treatment of the waste liquid. That is, the waste liquid may be used directly, or a synthetically produced compound, or a waste liquid obtained and chemically treated before the hydrogen (electrolysis) and nitrogen production processes may be used.

[0023] According to the present invention, the inventors can produce hydrogen and nitrogen using the same energy source (urea derivative) at a much more competitive price than fossil fuels, which will be restricted in the EU in the coming years. Similarly, since raw materials such as thiourea and guanidine are materials that can be purchased anywhere without restriction, the present invention can avoid the major problems of the distribution and storage of different energy sources that cause high piping costs and transportation costs, and apply this technology to combustion engines or fuel cells.

[0024] The present invention relates to a method for removing nitrogen compounds, the method being of the general formula I: (R 1 ,R 2 )N-C(=X)-N(R 3 ,R 4 ) [wherein, X means NH, NR 5 , or S, R 1 , R 2 , R 3 , R 4 , and R 5 may be the same or different: - hydrogen, - an alkyl group having 1 to 6 carbon atoms, which may be linear or branched, and may be unsubstituted or substituted with one or more heteroatoms, functional groups, and / or saturated or unsaturated alkyl groups, - an unsaturated alkyl group having at least one double bond or triple bond, having 2 to 6 carbon atoms, which may be linear or branched, and may be unsubstituted or substituted with one or more heteroatoms, functional groups, and / or saturated or unsaturated alkyl groups, - cycloalkyl which is unsubstituted or substituted with one or more heteroatoms, functional groups, and / or saturated or unsaturated alkyl groups, - aryl which is unsubstituted or substituted with one or more heteroatoms, functional groups, and / or saturated or unsaturated alkyl groups, means] a urea derivative, or a polymer of a compound of formula I, in an aqueous medium, - an anode comprising a metal, where "metal" is - one or more metals, - one or more compounds of a metal, or a mixture of metal compounds, - or a combination of the above, an anode, and - a metal cathode in at least one electrolytic cell, hydrolyze, and obtain nitrogen as a result of oxidation of a nitrogen compound at the anode and hydrogen as a result of reduction of water at the cathode, provided that when one electrode, for example the anode, is made of platinum, the other electrode, the cathode, is not made of platinum.

[0025] The substituents of radical R 1 R 2 R 3 R 4 and R 5 can be one or more heteroatoms, functional groups, and / or saturated or unsaturated alkyl groups.

[0026] The heteroatoms can be oxygen, nitrogen, sulfur, halogen, phosphorus.

[0027] The substituted functional groups can be hydroxy, alkoxy, ether, ester, acid (carboxyl), amino, cyano, amide, nitro, carbonyl, carbamoyl.

[0028] The saturated or unsaturated alkyl groups that can be substituents can have a variable number of carbon atoms, preferably 1 to 5 carbon atoms.

[0029] The expression "aqueous medium" means that the medium includes water alone, or alcohol, or water mixed with alcohol, or water mixed with ketone, or a combination of these three components. The alcohol may be an alcohol having 1 to 6 carbon atoms, such as methanol, ethane, propanol, isopropanol, butanol, isobutanol, pentanol, or hexanol. The ketone may be, for example, acetone, butanone, or pentanone. The aqueous medium may be acidic, basic, or neutral.

Brief Description of Drawings

[0030]

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Mode for Carrying Out the Invention

[0031] According to certain embodiments, when a nickel compound is used at the anode, in addition to the reduction of water at the cathode with hydrogen generation during electrolysis, the oxidation of the nitrogen compound occurs at the anode, and at the same time, on the same electrode, the oxidation reaction from Ni(OH)2 to NiOOH occurs.

[0032] The urea derivative is preferably selected from thiourea and guanidine.

[0033] The electrolysis can be carried out under the following conditions. - Concentration of the urea derivative: It can be any value, for example, from 0.01 to 100 mol / l. - Voltage: It can be any value, for example, from 1 to 100 volts. - Current: It can be any value, for example, from 0.01 to 1 ampere.

[0034] The main function of the anode is to catalyze the oxidation of the nitrogen compound.

[0035] The anode can be any metal or can contain any metal. The term "metal" includes the following options: - Metal - Metal compound - Or a mixture of metals - A mixture of metal compounds, Or a combination of the above, including, for example, nickel hydroxide, nickel oxyhydroxide, platinum, platinum oxide, or a mixture thereof.

[0036] According to certain embodiments, the anode is a mixture such as nickel and platinum, or contains such a mixture.

[0037] The "metal" component as defined for the anode can be one or more active metals or their compounds that enable the electrochemical conversion of nitrogen compounds. The active metals can be, for example, nickel, palladium, cobalt, iron, copper, platinum, iridium, ruthenium, rhodium, titanium, or any lanthanide such as europium, terbium, and alloys or combinations thereof, and may particularly be nickel.

[0038] According to certain embodiments, the anode is made of nickel hydroxide, or nickel oxyhydroxide, or both. For the sake of simplicity, in either case it is represented as "nickel in oxide form" or "oxidized / oxyhydroxide nickel".

[0039] The term "nickel in oxide form" means, unless otherwise specified, either nickel hydroxide, nickel oxyhydroxide, or a mixture of both.

[0040] According to further specific embodiments, the anode can be a material containing nickel in oxide form and platinum, more specifically nickel hydroxide / platinum [Ni(OH)2 / Pt] or nickel oxyhydroxide / platinum [NiOOH / Pt] for electrolysis.

[0041] The symbol " / " in this specification is equivalent to ",", meaning that the materials or substances mentioned are present and are connected by this symbol. Further, it also means the order of addition or deposition of a certain material to the previously mentioned material. As an example, the NiOOH / Pt / C allotrope means that Pt is deposited or mixed with NiOOH, and the C allotrope is deposited or mixed with the material obtained from the previous mixture.

[0042] The percentages shown in this specification mean weight percentages unless otherwise stated.

[0043] According to further specific embodiments, the anode is - nickel hydroxide Ni(OH)2 / platinum (Pt) in any ratio of Ni to Pt, or - Nickel oxyhydroxide (NiOOH / platinum (Pt)) in any ratio of Ni to Pt, containing a metal formed by , for example, the Ni to Pt ratio is in the range of 75 - 95% Ni to 5 - 25% Pt, more preferably 80 - 90% Ni to 10 - 20% Pt.

[0044] The anode may contain other components and, in addition to the metals that are necessarily present, it may include a semiconductor material and / or a photovoltaic material.

[0045] The anode may include a semiconductor material such as a carbon allotrope, for example, graphite, graphene, nanotubes, or fullerenes.

[0046] When the anode includes a combination of a metal / semiconductor material, the ratio of the metal to the semiconductor material may be any ratio, preferably in the range where the metal is 6 - 99% with respect to the semiconductor material and the semiconductor material is 1 - 94% with respect to the metal.

[0047] According to a further specific embodiment, the anode is made of or includes a semiconductor such as (oxidized / nickel oxyhydroxide) / C allotropes. In this case, the ratio of (oxidized / nickel oxyhydroxide) in the anode to the weight of the semiconductor may be any ratio, in particular, for example, 2 - 95% (oxidized / nickel oxyhydroxide) with respect to the weight of the semiconductor.

[0048] Another specific option is an embodiment where the anode contains a metal that includes Ni(OH)2 / Pt or NiOOH / Pt in any ratio between the metal components (Ni(OH)2 and Pt, or NiOOH and Pt), which may be deposited or mixed, for example, on any carbon allotrope, more specifically, graphite or graphene, nanotubes, or fullerene semiconductor materials.

[0049] According to certain embodiments, the anode comprises a mixture of a metal and a semiconductor material, such as a Ni(OH)2 / Pt / C allotrope or a NiOOH / Pt / C allotrope, in any ratio between the metal and the semiconductor component, preferably with the metal in the range of 6 to 99% relative to the carbon allotrope and the carbon allotrope in the range of 1 to 94% relative to the metal.

[0050] The anode may further comprise a photoactive material, which is understood as a substance capable of causing electrolysis by applying light of any wavelength thereto. Examples of photoactive materials are metals or metal oxides such as TiO2, WO3, ZnO, CdS, Fe2O3, and SnO2.

[0051] According to a further particular embodiment, the anode comprises a material formed by a mixture of a metal and a photoactive material in any ratio of the metal to the photoactive material and of the photoactive material to the metal.

[0052] According to certain embodiments, the anode is made of or comprises a metal / photoactive material, such as (nickel oxide / hydroxide) / TiO2, i.e., (nickel oxide / hydroxide) / photoactive material. In this case, the ratio of the metal (such as nickel oxide / hydroxide) in the anode can be any ratio, particularly, for example, 5 to 98% relative to the weight of the photoactive material.

[0053] According to certain embodiments, the anode is made of or comprises (nickel oxide / hydroxide) / photoactive material / semiconductor, exemplified by (nickel oxide / hydroxide) / TiO2 / C allotrope. In this case, the ratio of the nickel oxide / hydroxide metal in the anode can be any ratio, particularly, for example, 5 to 98% by weight relative to the total of the semiconductor and the photoactive material.

[0054] Yet another preferred alternative is an embodiment in which the anode comprises Ni(OH)2 / Pt or NiOOH / Pt metal deposited or mixed within a range of 1 to 94% metal ratio with respect to a photoactive material on a photoactive material such as TiO2, WO3, ZnO, CdS, Fe2O3, and SnO2, particularly TiO2. "Between the metal components" means between Ni(OH)2 and Pt, or between NiOOH and Pt.

[0055] According to a further specific embodiment, the anode comprises Ni(OH)2 / Pt / TiO2 or NiOOH / Pt / TiO2 deposited or mixed on a semiconductor material, preferably any carbon allotropes such as graphite, graphene, nanotubes, or fullerenes.

[0056] When TiO2 is included, Ni(OH)2 or NiOOH, Pt, and TiO2 are mixed and aggregated into aggregates having a particle size analysis value of about micrometers or less.

[0057] According to a further specific embodiment, the anode comprises a mixture of Ni(OH)2 or NiOOH and Pt and TiO2 in any ratio therebetween, for example, a mixture of Ni(OH)2 or NiOOH and Pt and TiO2 at 5 to 50% with respect to the photoactive material, and a mixture of Ni(OH)2 / Pt / TiO2 or NiOOH / Pt / TiO2 at 50 to 95% TiO2 with respect to the metal.

[0058] According to a further specific embodiment, the anode comprises a mixture of Ni(OH)2 / Pt / TiO2 / any form of carbon allotropes or NiOOH / Pt / TiO2 / any form of carbon allotropes in any ratio between Ni and Pt, preferably Ni(OH)2 or NiOOH (5 to 97%), Pt (1 to 93%), TiO2 (1 to 93%), and carbon (1 to 93%). In the context of the present invention, "nickel" should be understood to be elemental metal, nickel oxide, nickel oxyhydroxide, or a mixture of nickel oxide and nickel oxyhydroxide.

[0059] A further preferred alternative is that the anode is - Ni(OH)2 / Pt / photoelectroactive material / semiconductor material in any ratio between Ni and Pt, or - NiOOH / Pt / photoelectroactive material / semiconductor material in any ratio between Ni and Pt, in an embodiment comprising, preferably with the metal component ratio as follows: Ni of Ni(OH)2 or NiOOH, Pt is 7-98% metal relative to the other components, 1-93% photoelectroactive material and (1-93%) semiconductor material compared to the metal.

[0060] A further alternative is an embodiment in which the anode comprises Ni(OH)2 / Pt or NiOOH / Pt in any ratio between Ni and Pt, deposited or mixed on a photoelectroactive material such as TiO2, WO3, ZnO, CdS, Fe2O3, and SnO2, especially TiO2, and subsequently deposited or mixed on a semiconductor material, more specifically graphite, graphene, nanotubes, or fullerenes.

[0061] The anode of the present invention may have any shape, such as layered or cylindrical.

[0062] According to a particular embodiment, the anode comprises a nickel compound deposited on porous titanium, preferably nickel hydroxide deposited on porous titanium.

[0063] To obtain the anode, i.e., nickel hydroxide deposited on a porous titanium current collector, conventional methods such as application using an airbrush gun and tape casting can be used.

[0064] According to a particular embodiment, the first process for obtaining the anode comprises deposition of nickel hydroxide on a porous titanium current collector by airbrush.

[0065] According to certain embodiments, the second process for obtaining an anode includes the deposition of nickel hydroxide onto a porous titanium current collector by tape casting, which is a technique that enables the production of stable and reproducible electrodes.

[0066] The cathode may include any metal or mixture of metals. Preferred examples of metals are cobalt, copper, iridium, iron, nickel, platinum, palladium, ruthenium, rhodium, or any lanthanide such as europium or terbium, and mixtures and alloys thereof, and more preferably platinum (Pt) for example.

[0067] Any cathode according to the present invention may be combined with any anode according to the present invention.

[0068] Another specific option is an embodiment in which the cathode includes any metal or mixture of metals in any ratio supported or mixed with a semiconductor material. Examples of metals are cobalt, copper, iridium, iron, nickel, platinum, palladium, ruthenium, rhodium, or any lanthanide such as europium or terbium, and mixtures and alloys thereof, deposited or mixed with a semiconductor material such as any allotrope of carbon, preferably graphite, graphene, nanotubes, and fullerenes. According to certain embodiments, the metal is Pt supported or mixed with a semiconductor material, preferably an allotrope of carbon, preferably graphite, graphene, nanotubes, and fullerenes.

[0069] Yet another option is an embodiment in which the cathode includes any metal or mixture of metals in any ratio deposited or mixed with a photoactive material. Preferred examples of metals are cobalt, copper, iridium, iron, nickel, platinum, palladium, ruthenium, rhodium, or any lanthanide such as europium or terbium, and mixtures and alloys thereof, supported or mixed with a photoactive material, and preferably Pt deposited or mixed with a photoactive material such as TiO2, WO3, ZnO, CdS, Fe2O3, and SnO2, preferably TiO2.

[0070] Yet another alternative is an embodiment where the cathode comprises any metal such as cobalt, copper, iridium, iron, nickel, platinum, palladium, ruthenium, rhodium, or any lanthanide such as europium or terbium, and mixtures and alloys thereof, any metals or mixtures of metals in any ratio, which may be deposited and mixed on a semiconductor material, specifically any form of graphite or any form of graphene, and subsequently supported or mixed with a photoactive material such as TiO2, WO3, ZnO, CdS, Fe2O3, and SnO2.

[0071] According to a particular embodiment, the cathode is made of platinum.

[0072] The present invention further relates to an electrolysis cell or a set of cells comprising one or more anodes such as the anode defined above.

[0073] The cell or set of cells may also contain one or more cathodes, an electrolyte, a separator, and various catalysts such as a base exemplified by KOH for increasing the reaction rate, a chromophore derived from ruthenium or iridium, or an organic compound such as thiourea, at these different concentrations depending on the cell size and the volume and concentration of the electrolyte.

[0074] The electrolysis cell may be composed of any material commonly used in this technology, but may be associated with particularities that may arise in certain embodiments. For example, in the case of a cell where the electrodes use a photoactive material, a mixture or single material that transmits light in any range of the electromagnetic spectrum, specifically quartz, is used.

[0075] According to a particular embodiment, the present invention also relates to an electrolysis cell for a solid polymer fuel cell, comprising an anode as defined above.

[0076] The shapes of both the anode and the cathode may be any geometric or amorphous structure, specifically, they may be layered, rectangular, or cylindrical.

[0077] The absorption of the nitrogen compound is carried out at the anode. Therefore, the conductive component of the anode is one or more active metals that enable the electrochemical conversion of the nitrogen compound.

[0078] For passive elements such as conductive agents, binders, and current collectors, commercially available ones can be used.

[0079] When electrolysis is carried out in a basic medium, the basic medium may be generated using compounds such as metal hydroxides, carbonates, phosphates, hydrogen phosphates, etc. In this case, the metal may be any alkaline chemical element (e.g., K2HPO4, KH2PO4), alkaline earth metal, transition metal, lanthanide, actinide, p-block metal, or non-metal.

[0080] According to a specific embodiment, the basic medium is generated using KOH.

[0081] The electrolyte used may be a derivative of urea according to the shown formula I, for example, thiourea, thiourea polymer, or a mixture of any of these, guanidine, guanidine polymer, etc. Any of these or a mixture of these may be used in any other alternative of the present invention.

[0082] Such an electrolyte may be in a dissolved state, liquid state, or solid state.

[0083] Such an electrolyte may act directly or as a catalyst in the production of hydrogen and nitrogen. For example, thiourea may be an electrolyte from which hydrogen and nitrogen are obtained, but it may also be mixed with another urea derivative in a small proportion to promote the electrooxidation of the latter, in which case it is acting as a catalyst.

[0084] Any chromophore, more specifically ruthenium derivatives and iridium derivatives, may be used as a reaction catalyst in this reaction.

[0085] Preferably, the electrolysis cell includes a platinum cathode and an anode of a nickel compound. More preferably, the nickel compound is deposited on porous titanium. According to a particular embodiment, an electrolysis cell for carrying out the electrolysis of a urea derivative in a basic medium is - a nickel hydroxide cathode deposited on porous titanium, - an anode connected to a Pt cathode by a conductive agent, - an aqueous KOH solution as a base, and includes.

[0086] According to a further particular embodiment, the cell is an electrode of the following materials - cathode: platinum sheet, - anode: a material containing a mixture of Ni(OH)2 / Pt / photoelectroactive material or NiOOH / Pt / photoelectroactive material in any ratio between Ni and Pt, and contains.

[0087] According to a further particular embodiment, the cell is an electrode of the following materials - cathode: platinum sheet, - anode: a material containing a mixture of Ni(OH)2 / Pt / photoelectroactive material / any form of carbon allotrope or NiOOH / Pt / photoelectroactive material / any form of carbon allotrope in any ratio between Ni and Pt, and contains.

[0088] The cell may have electrodes divided into two compartments.

[0089] Also, it may be a cell including three or more electrodes.

[0090] According to certain embodiments, during the application of current to the electrochemical cell, the urea derivatives contained in the solution of potassium hydroxide (KOH), preferably thiourea or guanidine, are oxidized at the anode consisting of a porous nickel hydroxide electrode (positive electrode), and at the same time, at the same electrode, the oxidation reaction from Ni(OH)2 to NiOOH occurs, which functions as a catalyst for the decomposition of the urea derivative. At the negative electrode consisting of a platinum mesh, the alkaline reduction reaction of water occurs to produce hydrogen.

[0091] The present invention demonstrates that guanidine and thiourea can serve as alternatives to urea due to their similar nitrogen-based pollutant removal performance and better energy consumption values. Therefore, they exhibit advantages compared to the known current technologies. The present invention enables the improvement of the cathode for the purpose of avoiding interference caused by the presence of nitrogen compounds that may cause side reactions.

[0092] The advantages of using thiourea or guanidine as nitrogen compounds are that when they are added to water molecules containing 2 hydrogen atoms, with each containing 4 or 5 hydrogen atoms respectively, the electrolysis process is more efficient than the electrolysis of water with only 2 hydrogen atoms (used in other fuel cells), the energy consumption is reduced, and the hydrogen production amount increases.

[0093] Nitrogen compounds such as thiourea or guanidine are oxidized at the anode, while at the cathode, the reduction of water (e.g., alkaline reduction) occurs.

[0094] Reaction of thiourea in a specific case of alkaline electrolysis: (1) Anode: CS(NH2)2(aq)+6OH - →N2(g)+5H2O+CO(aq)+S(aq)+6e - (2) Cathode: 6H2O+6e - →3H2(g)+6OH - (3) Overall reaction: CS(NH2)2(aq)+H2O→N2(g)+3H2+CO(aq)+S(aq) Global reaction in the case of guanidine TIFF2025523397000001.tif11112

Example

[0095] The test of the anode material most suitable for electrooxidation was carried out in an electrochemical cell with a three - electrode configuration, and the hydrogen production test in the second part was carried out in a complete cell using the selected electrode and different urea solutions and derivatives for comparison.

[0096] The characteristics of nickel electrode preparations (NiOOH) with different preparation modes were investigated, and their electrochemical behaviors were investigated in a half - cell and a complete cell under electrolysis operating conditions.

[0097] The preparation of nickel hydroxide preparations was mainly carried out by two procedures: ink casting and application of ink by an airbrush gun, and the latter method was selected as the most appropriate.

[0098] In addition, thermal and electrical stability tests were carried out on solutions containing guanidine and thiourea, where the operating conditions of the electrolysis process were activated.

[0099] These preparations are solutions / suspensions with an active substance (e.g., nickel hydroxide), and are mixed with an aqueous solvent, a conductive agent, and a binder that impart rheological conditions to enable their handling in the mixture and subsequent dispersion onto a metal support, thereby enabling application to the anode.

[0100] Regarding the electrochemical performance of the NiOOH anode, experiments were first carried out in a three - electrode electrochemical cell and then in a complete cell simulating an electrolyzer, and analysis of hydrogen production, pollutant removal, and energy consumption was performed.

[0101] Preparation of the anode Examples of nickel hydroxide preparations were prepared according to different methods to obtain electrodes with homogeneous dispersion and stabilization of the material on a porous titanium metal support.

[0102] Tape casting The preparation of nickel hydroxide preparations by tape casting is based on the preparation of a suspension of the target active material, i.e., nickel hydroxide, in a medium composed of a solvent such as N-methyl-2-pyrrolidine (NMP), a conductive agent such as carbon black or acetylene black, and a binder such as polyvinylidene fluoride (PVDF). This suspension is cast onto the surface of a smooth support such as porous titanium, and the suspension is leveled with a blade to obtain a film with a controlled width and thickness, for example, a width of 2 - 3 cm and a thickness of 50 - 100 μm. Then, when the solvent evaporates, the particles solidify to obtain a mechanically compliant component that can be used as an electrode. The tape casting process was applied to the preparation of electrodes using a COATMASTER Mod 510, Erichsen, Germany.

[0103] Samples of nickel hydroxide preparations by tape casting (with the appropriate consistency for the desired application and a filling mass of about 10 - 15 mg Ni(OH)2 / cm 2 of electrodes (composition: nickel hydroxide, carbon black, and PVDF in a ratio of 7:1:2)) were prepared in the form of an ink as follows. After weighing all the components, they were poured into a 250 mL beaker, and while vigorously stirring the mixture with a glass rod, the solvent N-methyl-2-pyrrolidine (NMP) was gradually added. The final goal is to obtain an appropriate pulp.

[0104] The binder content (nickel hydroxide, carbon black, and PVDF in ratios of 8:1:1, 6:2:2, 7:2:1, and 7:1:2) was essentially varied to examine various compositions. The most performance - optimized ink was prepared by dispersing a mixture containing 1.6 g of nickel hydroxide, 0.2 g of carbon black, and 0.5 g of PVDF in 10 mL of N - methyl - 2 - pyrrolidine (NMP) using a homogenizer - disperser, and all reagents were sourced from Alfa Aesar. Subsequently, they were dried under vacuum at 110 °C, whereby the solvent evaporated, leaving behind a pure substance composed of nickel hydroxide and carbon black.

[0105] The electrodes obtained by this procedure exhibited consistency suitable for the desired application and a filling mass of approximately 10 - 15 mg Ni(OH)2 / cm 2 The appearance of the electrodes is shown in Figure 3.

[0106] Thermal stability test of the electrolyte To investigate the stability of solutions containing nitrogen contaminants, a thermal test was designed where different samples, a 0.33 M thiourea solution in 5 M potassium hydroxide, a 0.33 M guanidine nitrate solution in 5 M potassium hydroxide, and a 0.33 M urea solution in 5 M potassium hydroxide, were subjected to various temperatures in the presence and absence of Ni(OH)2 electrodes prepared by tape casting. Glass containers and a thermostatic bath were used. The samples were subjected to different temperatures of 40 °C, 60 °C, and 80 °C over a certain time (1 hour, 5 hours, and 10 hours), and a visual inspection of the electrolyte was performed.

[0107] The thermal stability characteristics of the described nitrogen derivatives were evaluated by two methods: the stability of the component solutions at a concentration of 0.33 M in a thermostatic container and the stability when two electrodes connected to each other but with no current applied were added to the same configuration.

[0108] Heating was carried out stepwise (40~60~80 °C). When the temperature reached that level, it was maintained for a certain period of time, and the possible color change of the solution or the appearance of precipitation was observed. Table 1 shows the test scheme and the results for the case without electrodes and the case where the electrodes were immersed and connected to each other after 5 hours at 80 °C.

[0109]

Table 1

[0110] These results indicate that only the temperature rise does not cause significant changes in the solution (however, in the case of thiourea, the color becomes darker at 80 °C). However, when the electrodes are introduced and short-circuited, the appearance of suspended solids is observed, and this presence is more clearly observed in the thiourea solution. Also, in the guanidine solution, a certain degree of suspended particles is observed.

[0111] Characteristic evaluation of electrolyte reactions in half-cells To examine the electrochemical processes of various prepared anodes, cyclic voltammetry experiments were conducted. The procedure consists of applying an overpotential from an initial voltage, known as the potential window that must cover the region where the target reaction occurs, to the limiting potential to the working electrode (WE), also referred to as the test electrode, compared to the reference electrode (RE), and then performing a scan in the opposite direction. At the same time, the signal intensity obtained at the working electrode, in this case, the signal intensity corresponding to the electrons released (oxidation reaction) and the electrons that need to be consumed at the counter electrode (CE), also known as the auxiliary electrode, where the reduction reaction occurs, is recorded, and all are carried out at a determined scan rate.

[0112] By this procedure, a triangular potential-time curve and another curve corresponding to the current intensity measured in response to the application of overpotential are created, as seen in the left and right images of Figure 2, respectively. Regarding the latter interpretation, the application of a voltage difference to the working electrode promotes the potential at the solution-electrode interface to exceed the thermodynamic potential of the redox reaction that can occur depending on the chemical species contained therein. Essentially, these reactions are promoted by reducing their activation energy. This is clearly seen in the current-voltage curve of Figure 2, which is called cyclic voltammetry. As the potential moves towards a negative potential (left side), the region where the current intensity measured in response to the application of voltage modifies the activation energy of the redox reaction. When this section of the curve is reached, the current becomes large with a negative value, indicating that a reduction reaction is occurring at the working electrode. This phenomenon intensifies as the potential shifts further to a more negative value. This is because the activation energy of the reaction decreases more and more, thus increasing the reaction rate and causing a rapid increase in current. This results in the formation of the starting part of the peak facing the direction of negative current.

[0113] However, at this point, as a result of the high speed, the oxidizing species, which is the reagent causing the reduction reaction, begins to be depleted near the solution-electrode interface. When the speed is moderate, the chemical species available on the electrode surface exist by gradually replacing the reacted chemical species. However, when the speed increases, the reaction chemical species cannot access them at the same rate as they are disappearing. Therefore, after reaching the maximum, the current rapidly decreases, resulting in the peak shape shown in Figure 2.

[0114] Subsequently, as the potential scan approaches the same region but is a voltage scan at a positive value, the same behavior is observed for the reduced form of the reactant that proceeds to oxidation, and the current signal now appears as a positive value in the form of a waveform. The combination of the oxidation peak and the reduction peak is known as the redox reaction peak and can be associated with the redox reaction of interest. By considering the shape of these peaks, the redox phenomenon occurring in the system can be interpreted. This is because, in particular, it represents the current density and the separation between the oxidation peak and the reduction peak whose magnitude indicates the electrochemical reversibility and the reaction rate of these chemical species.

[0115] In the anode characteristic evaluation, a commercially available glass cell with a three-electrode configuration was used. The working electrode was a porous titanium sheet (grade 2 >99% Titanium Metals UK Limited (TML)) with an effective area of 2 - 5 cm, with the sides and back of the electrode controlled by Teflon coating and hot melt adhesive. 2 Hg / HgO (1M NaOH) was used as the reference electrode, and a platinum mesh with a very large area (12.5 cm 2 ) was used as the counter electrode. Since the distance between the working electrode and the reference electrode was about 2 cm, a certain degree of error was assumed in the measured values due to the influence of resistance loss.

[0116] To conduct experiments at different temperatures, a thermostatic bath that could be operated in the range of 25°C to 80°C was used.

[0117] Results For the electrochemical property test, electrodes were prepared by tape casting.

[0118] To examine the electrochemical performance of the electrodes, cyclic voltammetry tests were conducted in a three-electrode cell. 75 mL of an aqueous electrolyte solution (concentration to be shown later), urea, thiourea, and guanidine were introduced into the cell as appropriate, and the temperature was controlled using a thermostatic bath. The experiments were carried out at different temperatures, and the best results were obtained at room temperature.

[0119] Next, in the presence and absence of 0.33 M urea, thiourea, and guanidine nitrate, cyclic voltammetry was performed in a 5 M KOH solution at 0.0 to 0.8 V Hg / Hgo to investigate the catalytic activity of the nickel hydroxide material (thiourea, guanidine).

[0120] Urea test at 25 °C To verify the experimental system and obtain a reference value for comparison, experiments on the electrochemical properties of urea solution were conducted under room temperature conditions.

[0121] Figure 5 shows the cyclic voltammetry of the solution in 5 M KOH base in the presence and absence of urea.

[0122] In the system without urea (Figure 5a), peaks for both the oxidation of Ni(OH)₂ to NiOOH and its reduction at potentials of 0.52 and 0.20 V Hg / Hgo can be clearly observed. The faster the scan rate, the higher the current density and the greater the separation between the peaks. These reactions are well-known in alkaline media and correspond to the following equations. JPEG2025523397000003.jpg9104

[0123] When the scan is extended to a slightly more positive potential value near 0.70 V Hg / Hgo an increase in current associated with the generation of oxygen due to water decomposition is observed, which proceeds according to the following equation in an alkaline medium. 2OH - →1 / 2O 2(g) +H2O (l) +2e - (2)

[0124] When water molecules in the solution are decomposed, a reaction accompanied by hydrogen generation should occur at the counter electrode (CE) (Equation 3), which was confirmed by the observation of bubbling on the platinum mesh used as the counter electrode during the experiment. Furthermore, although not shown in the voltammogram, recording the voltage at the counter electrode revealed that it was stable at a value close to -1.15 V Hg / Hgo was observed. 6H2O (l) + 6e - → 3H 2(g) + 6OH - (3)

[0125] Using these potential values for both oxygen evolution and hydrogen generation, in an electrolysis cell with a platinum electrode and nickel hydroxide used for the negative and positive electrodes respectively in a 5M KOH solution in the absence of urea, it can be estimated that the cell voltage can be predicted to correspond to the difference obtained by subtracting the negative electrode component from the reaction at the positive electrode, i.e., approximately 1.85V. The overall reaction is as described below, and hydrogen twice that of oxygen is generated. Overall reaction: 2H2O (l) → 2H 2(g) + O 2(g) (4)

[0126] When a solution containing urea was used, an oxidation peak to NiOOH could not be clearly observed. The shape of the voltammetry indicates a direct transition to urea decomposition without the formation of NiOOH. However, the oxidation from Ni(OH)2 to NiOOH is due to the catalytic property of decomposing urea, which is a process accompanied by strong bubbling at the electrode that can be attributed to the generation of nitrogen accompanied by the reduction of water at the counter electrode that produces hydrogen when the scan is extended to a slightly more positive potential value near 0.55V Hg / Hgo and is the same process as that without using urea.

[0127] Therefore, during the anodic scan, urea contained in the solution is oxidized according to the reaction detailed below. CO(NH2) 2(ac) + 6OH - → N 2(g) + 5H2O (l) + CO 2(g) + 6e - (5)

[0128] Next, during the cathodic scan, loops related to the nucleation process associated with the electrodeposition of a certain chemical species can usually be observed. Furthermore, the reduction from NiOOH to Ni(OH)2 is observed.

[0129] Similarly to the above, the voltage at the auxiliary electrode was recorded, and stable reduction of water was observed at a value close to -1.20 V. Thus, together with the urea decomposition potential around 0.50 V, the voltage of the following global reaction can be estimated to be 1.70 V. Hg / Hgo near which the stable reduction of water was observed. Thus, together with the urea decomposition potential around 0.50 V, the voltage of the following global reaction can be estimated to be 1.70 V. Hg / Hgo near which the stable reduction of water was observed. Thus, together with the urea decomposition potential around 0.50 V, the voltage of the following global reaction can be estimated to be 1.70 V. Overall reaction: CO(NH2) 2(ac) + H2O (l) → N 2(g) + 3H 2(g) + CO 2(g) (6)

[0130] These voltage values are indicative because they essentially represent the minimum potential that needs to be transferred for electrolysis to occur. In practice, due to the occurrence of overvoltage, the potential difference required for electrolysis may be larger, which is particularly important when gases are evolved or substances are deposited on the electrodes.

[0131] Testing of Urea Derivatives The overall reaction with urea is as follows. Overall reaction: CO(NH2) 2(ac) + H2O (l) → N 2(g) + 3H 2(g) + CO 2(g) (6)

[0132] After verifying the experimental setup and test conditions with urea solution, the electrochemical behavior of thiourea and guanidine was evaluated. The latter was prepared in the form of its nitrate because it had low solubility as a pure salt. Different scan rates were evaluated, and the cyclic voltammetry of these solutions at 10 mV / s is summarized in Figure 6. The voltammetry of the solution in the absence of nitrogen-based compounds based on 5M KOH is also included as a blank.

[0133] In all configurations using urea derivatives, the same behavior as the sample using the test solution of urea, the reduction peak of NiOOH, and 0.55 V Hg / HgOThe absence of an oxidation peak that presumably overlaps with the decomposition reaction of the compound in the region exceeding the value of

[0134] For thiourea and guanidine, differences from urea were observed, and their NiOOH reduction potentials were about 0.30 V Hg / HgO compared to urea (0.25 V Hg / HgO ) and were larger on the positive side than those observed with urea. Practically, this suggests that the polarization of the electrode for the reduction reaction from NiOOH to Ni(OH)2 needs to reach that potential at a lower cathode value, resulting in a more energy-efficient process.

[0135] Tests of the reagents urea, thiourea, and guanidine at different temperatures To examine the electrolysis of urea solutions and their derivatives, cyclic voltammetry was performed at a scan rate of 10 mV / s using a three-electrode cell (same configuration as above). In each test, new electrolytes and electrodes were used to ensure that the possible degradation effects of the materials and / or media did not affect subsequent tests. Tests were conducted at four temperatures: 25 °C, 40 °C, 60 °C, and 80 °C. Figure 7 shows the voltammetry using solutions of urea, thiourea, and guanidine.

[0136] Generally speaking, these results indicate that higher temperatures than ambient temperature conditions are less favorable for the progress of electrochemical reactions. This is inferred from the low current density obtained with increasing temperature and is particularly prominent at 80 °C, where the current density decreases for all electrolytes.

[0137] In the case of guanidine, no significant difference was observed between 40 °C and 60 °C, and the voltammogram at room temperature showed the highest intensity. In the case of thiourea, instability was observed with increasing temperature.

[0138] Properties of the electrolyte and behavior of the electrodes in a complete cell The experiment was conducted in a complete cell mimicking an electrolysis device. The cell had electrodes located on both sides, a platinum mesh for hydrogen generation in the cathode compartment, and an electrode prepared with nickel hydroxide in the anode compartment. A urea solution, or thiourea, or guanidine (at the same concentration as indicated by voltammetry) was introduced into the cell through a supply tank.

[0139] Also, this was compared with the electrolysis of water.

[0140] The procedure consisted of applying a current to the system formed by two electrodes using a potentiostat and recording the voltage change. The gas generated in each compartment was quantified in volume, collected, and subjected to gas chromatographic analysis using different detectors, namely thermal conductivity (TCD) and mass (MS) at specific times.

[0141] In addition, the applied load to the system was recorded and correlated with the volume of hydrogen generated, thereby calculating the efficiency of the system in hydrogen production.

Number

[0142] In this procedure, hydrogen is generated at the cathode, and the corresponding gas is generated at the anode depending on the solution. Oxygen is generated with KOH, and with urea, thiourea, or guanidine, the main pathway should mainly result in nitrogen generation. Therefore, the following formula is proposed to estimate the decomposition of contaminants.

Number

[0143] This parameter is expressed in the following unit. [Number]

[0144] For the application of this experiment, electrodes with the same composition as above (NiOH / NiOOH for the anode and platinum for the cathode) were used. This time, however, the effective area of 5 cm of the electrodes, whose edges and back surfaces were controlled by Teflon coating and hot melt adhesive, 2 was used. A platinum mesh with a much larger area (12.5 cm 2 ) than the working electrode was used as the cathode. Since the distance between the electrodes was about 10 cm, a certain degree of error in the measured values was assumed due to the influence of resistance loss.

[0145] To conduct full cell experiments with different urea, thiourea, and guanidine solutions, a cell with a solution volume of 90 mL described in the section "Properties of Electrolytes and Behavior of Electrodes in a Full Cell" was used. Using this, first, a constant current test was conducted at a current intensity of 2 mA and room temperature (20 - 25 °C), recording both the cell voltage and the applied charge amount, as well as the gas generation amount. Subsequently, a test was conducted to monitor the hydrogen generation volume and correlate it with the applied charge amount.

[0146] At this current intensity, thiourea and guanidine yield better results than urea.

[0147] Polarization test First, for the purpose of measuring the overvoltage of the reaction under reference conditions, a polarization test was conducted on a 5M KOH solution in the presence and absence of a urea solution, which was used as a nitrogenous base for reference. Here, after applying a certain constant current for a short time, the resulting voltage was recorded. The results are shown in Figure 8. From this, the cell voltage can be linearly correlated with the applied current, and the current intensity applicable to a complete cell designed to simulate an electrolysis device can be measured.

[0148] The potential values measured in these experiments are higher compared to the standard electrolysis potential. This is because the half-reactions occurring in the electrolysis cell are not spontaneous, and thus, to initiate the reaction, at least a potential difference exceeding the standard potential of the cell needs to be applied. However, when electrolysis of substances in an aqueous solution is carried out, reactions competing with the main reaction can occur at both electrodes, making it difficult to predict the operating voltage. Which reaction occurs depends on various factors such as the concentration of the dissolved substance or the physical properties of the electrodes. Generally, the potential difference required for electrolysis can be much higher than the standard potential due to the generation of a surge voltage, which is particularly important when gases are evolved or substances are deposited on the electrodes. As a result, it can be established that the cell potentials obtained in these experiments are appropriate for solutions containing thiourea and guanidine.

[0149] Based on these results, a current of 2 mA was selected for the full cell test.

[0150] Full cell assay In this experiment, 90 mL of the electrolyte solution was used at a current intensity of 2 mA. As the anode, an electrode with a filling mass of the active material [Ni(OH)2] of 3.5 - 4.5 mgMA / cm 2 of 5 cm 2 was used. A new electrode was used for each test. The duration of the test was determined by the generation of the minimum volume of the average anode gas in the cell and was at least 1 hour. Figure 10 shows the voltage changes for three compounds, all examined at a concentration of 0.33 M (0.33 M in 5 M KOH respectively), and their comparison with 5 M KOH corresponding to an alkaline electrolysis device. The results show that the system using a potassium hydroxide electrolyte exhibits the highest voltage, while guanidine and thiourea, where the oxidation reaction at a positive potential lower than the generation of oxygen is dominant, operate at a lower voltage. This is consistent with the results of the electrochemical experiments conducted at the three-electrode cell level.

[0151] Figure 10 shows the potential-time curves, and Table 4 summarizes the average voltage values, hydrogen generation amounts at the cathode, energy consumption amounts, and efficiencies for the four solutions investigated during the experiment. The test was simplified considering only hydrogen generation, the applied load, and the consumed energy.

[0152]

Table 2

[0153] The energy analysis was performed at the stable state of the system. Specifically, the measurements of hydrogen generation amount and energy consumption amount were carried out from 100 minutes to 150 minutes after the start of operation.

[0154] Therefore, for the comparative analysis of energy consumption, 50 minutes after the first stable moment in each experiment was adopted. These results show that when comparing the case where nitrogen derivatives are absent and the case where they are present, when the solution contains nitrogen derivatives, hydrogen is generated at a very low voltage of about 1.42 - 1.45 V, while in 5M KOH, a value of 1.63 V is obtained. Under these conditions, when approximately the same amount of hydrogen is generated, there is a difference in the energy consumption per kilogram of hydrogen generated, indicating that the electrolysis using solutions containing nitrogen derivatives, especially those based on guanidine and thiourea, proceeds with a very low energy consumption.

Claims

1. A method for removing a nitrogen compound, comprising: General formula I: (R １ , R ２ )N-C(=X)-N(R ３ , R ４ ) [wherein, X represents NH, NR ５ , or S, and R １ 、R ２ 、R ３ 、R ４ 、and R ５ may be the same or different: - hydrogen, - an alkyl group having 1 to 6 carbon atoms, which may be linear or branched, may be unsubstituted or substituted with one or more heteroatoms, functional groups, and / or saturated or unsaturated alkyl groups, - an unsaturated alkyl group having at least one double bond or triple bond, having 2 to 6 carbon atoms, which may be linear or branched, may be unsubstituted or substituted with one or more heteroatoms, functional groups, and / or saturated or unsaturated alkyl groups, - cycloalkyl, unsubstituted or substituted with one or more heteroatoms, functional groups, and / or saturated or unsaturated alkyl groups, - aryl, unsubstituted or substituted with one or more heteroatoms, functional groups, and / or saturated or unsaturated alkyl groups, ] a urea derivative of or a polymer of a compound of formula I in an aqueous medium, - an anode containing a metal, where "metal" means - one or more metals, - one or more compounds of a metal, or a mixture of metal compounds, - or a combination thereof, an anode, and - a metal cathode in at least one electrolysis cell, hydrolyzing, and obtaining nitrogen as a result of oxidation of the nitrogen compound at the anode and obtaining hydrogen as a result of reduction of water at the cathode, characterized in that when the anode is made of platinum, the cathode is not made of platinum.

2. The method according to claim 1, wherein the aqueous medium is a basic medium prepared using potassium hydroxide.

3. The method according to claim 1, wherein the electrolysis cell comprises at least one platinum cathode.

4. The method according to any one of claims 1 to 3, wherein the electrolysis cell comprises at least one anode made of nickel or a nickel compound, or the anode is made of nickel or a nickel compound.

5. The method according to claim 4, wherein the nickel compound is nickel hydroxide.

6. The method according to claim 5, wherein the nickel hydroxide is oxidized at the anode to form nickel oxyhydroxide.

7. The method according to any one of claims 1 to 6, wherein the nickel compound is deposited on porous titanium.

8. The method according to claim 1, wherein the anode is made of a nickel compound deposited on porous titanium.

9. The method according to any one of claims 1 to 8, wherein hydrogen is obtained as a result of the reduction of water at the cathode.

10. The method according to any one of claims 1 to 9, wherein the urea derivative is guanidine.

11. The method according to any one of claims 1 to 10, wherein the urea derivative is thiourea.

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