JP7713685B2 - Hydrogen and/or ammonia production apparatus and method - Google Patents

Description

The present invention relates to an apparatus for producing hydrogen and/or ammonia from urea and water, and a method for producing hydrogen and/or ammonia.

Hydrogen is useful as fuel for fuel cells and as a raw material for e-fuels. Ammonia is a hydrogen carrier and a raw material for chemical fertilizers, and is useful for removing harmful substances such as NOx through selective catalytic reduction (SCR). Urea, on the other hand, has a high energy density, can be safely transported and stored, and can be easily obtained from factories and domestic wastewater, so it is attracting attention as a raw material for producing hydrogen and ammonia.

For example, Patent Document 1 proposes a method of electrolytic hydrolysis of urea using a Teflon (registered trademark) membrane or a polypropylene membrane as a separator (in other words, a diaphragm or separator). This method makes it possible to produce ammonia from urea.

Patent No. 5932764

In recent years, the efficiency of electricity generation (specifically, energy conversion efficiency) has increased through solar power generation and other sources, and the use of renewable energy is expected to expand. Therefore, there is a demand for the development of new methods and equipment for producing ammonia and hydrogen with low energy, for example, using electricity (specifically, electrolytic reactions).

The present invention has been made in consideration of these problems, and aims to provide a hydrogen and/or ammonia production device and production method that can electrochemically generate hydrogen and/or ammonia from urea and water.

One aspect of the present invention is a hydrogen and/or ammonia production device (1) comprising an electrochemical cell (4) having an electrode assembly (2) and an electrolyte (3),
the electrode body is composed of a laminate in which a cathode (21), a diaphragm (22), and an anode (23) are laminated in this order,
At least the anode is in contact with urea;
the electrolyte is an alkaline aqueous solution, and at least one of the anode and the cathode is in contact with the electrolyte;
In the hydrogen/ammonia production apparatus, the diaphragm is composed of an ion exchange membrane.

Another aspect of the present invention is a method for producing hydrogen and/or ammonia, comprising the steps of:
A cathode (21), a diaphragm (22) made of an ion exchange membrane, and an anode (23) are sequentially stacked to form an electrode body (2);
the anode electrode of the electrode assembly is brought into contact with urea, and at least one of the anode electrode and the cathode electrode is brought into contact with an electrolyte solution composed of an alkaline aqueous solution;
The present invention relates to a method for producing hydrogen/ammonia, in which hydrogen and/or ammonia is produced by applying a voltage between the cathode and the anode.

The manufacturing apparatus has the above configuration, and the diaphragm is made of an ion exchange membrane. Therefore, the reaction factors can be directly sent from the cathode to the anode or from the anode to the cathode through the ion exchange membrane. The reaction factors are specifically ^OH- and H ⁺ . Then, hydrogen is produced at the cathode and ammonia is produced at the anode. In this way, hydrogen and/or ammonia can be efficiently produced.

In the above manufacturing method, the anode of the electrode assembly is brought into contact with urea, and at least one of the anode and cathode of the electrode assembly is brought into contact with an electrolyte consisting of an alkaline aqueous solution. Then, a voltage is applied between the cathode and the anode. By applying the voltage, a reactant can be directly transferred from the cathode to the anode or from the anode to the cathode. As a result, hydrogen is produced at the cathode and ammonia is produced at the anode. In this manner, hydrogen and/or ammonia can be efficiently produced.

As described above, according to the above aspects, it is possible to provide a hydrogen/ammonia production apparatus and a hydrogen/ammonia production method capable of producing hydrogen and/or ammonia from urea and water.
In addition, the symbols in parentheses described in the claims and the means for solving the problems indicate a correspondence with the specific means described in the embodiments described below, and do not limit the technical scope of the present invention.

FIG. 2 is a schematic diagram of a hydrogen/ammonia production device having a diaphragm made of a cation exchange membrane and having an electrolytic solution on the anode electrode side in the first embodiment. FIG. 2 is a schematic diagram of a hydrogen/ammonia production device according to the first embodiment, which has a diaphragm made of an anion exchange membrane and has an electrolytic solution on the cathode side. FIG. 1 is a schematic diagram of a hydrogen/ammonia production device having an electrolytic solution on the anode electrode side and the cathode electrode side in a first embodiment. FIG. 2 is a schematic diagram of a hydrogen/ammonia production apparatus in Experimental Example 1. 1 is a graph showing the relationship between the material of the anode and the current density when a urea water electrolysis reaction was carried out under the condition of an applied voltage of 2.0 V in Experimental Example 1. 4 is a graph showing the relationship between the material of the anode electrode, the Faraday efficiency, and the oxygen evolution rate of the anode electrode in Experimental Example 1. 11 is a graph showing the relationship between elapsed time and current density, and the relationship between elapsed time and Faraday efficiency in Experimental Example 1. 1 is a graph showing the relationship between the material of the anode and the current density when a urea water electrolysis reaction was carried out under the condition of an applied voltage of 1.5 V in Experimental Example 1. 1 is a graph showing the relationship between KOH concentration and urea conversion rate in Experimental Example 2. FIG. 1 is a schematic diagram of a hydrogen/ammonia production apparatus in Experimental Example 3. 13 is a graph showing changes in impedance over time in Experimental Example 3. 1 is a graph showing current density and ammonia production amount in Experimental Example 4. 13 is a graph showing the relationship between elapsed time and current density, and the relationship between elapsed time and applied voltage in Experimental Example 5. 13 is a graph showing the relationship between applied voltage and current density in Experimental Example 5. FIG. 13 is a schematic diagram of a hydrogen/ammonia production apparatus in Experimental Example 6. 13 is a graph showing the relationship between elapsed time and current density in Experimental Example 6. 13 is a graph showing the Faraday efficiency and hydrogen production rate in Experimental Example 6. 13 is a graph showing the Faraday efficiency and generation rate of nitrogen in Experimental Example 6. 13 is a graph showing the Faraday efficiency and production rate of oxygen in Experimental Example 6.

(Embodiment 1)
An embodiment of a hydrogen and/or ammonia production apparatus and production method will be described with reference to Figures 1 to 3. As illustrated in Figures 1 to 3, a hydrogen/ammonia production apparatus 1 is composed of an electrochemical cell 4. The electrochemical cell 4 has an electrode body 2 and an electrolyte 3.

The electrode body 2 is composed of a laminate in which a cathode 21, a diaphragm 22, and an anode 23 are stacked in order. In the electrode body 2, the cathode 21, the diaphragm 22, and the anode 23 are integrally formed. Another layer may be formed between the cathode 21 and the diaphragm 22, and between the anode 23 and the diaphragm 22.

In the hydrogen/ammonia production device 1, the anode electrode 23 comes into contact with urea. At least one of the anode electrode 23 and the cathode electrode 21 comes into contact with the electrolytic solution 3. That is, the electrolytic solution 3 may come into contact with the anode electrode 23, the electrolytic solution 3 may come into contact with the cathode electrode 21, or the electrolytic solution 3 may come into contact with both the cathode electrode 21 and the anode electrode 23. The electrolytic solution 3 is composed of an alkaline aqueous solution. The diaphragm 22 is composed of an ion exchange membrane, and the ion exchange membrane is a cation exchange membrane or an anion exchange membrane. Thus, in the hydrogen/ammonia production device 1, there are variations in the combination of the type of ion exchange membrane and the electrode with which the electrolytic solution comes into contact.

Urea may be supplied as a solid or as an aqueous solution (i.e., urea water). In the hydrogen/ammonia production device 1, urea is supplied to the anode electrode 23 side. When solid urea is supplied, the anode electrode 23 side can be made into a dry environment (see FIG. 2), which makes it easier to recover ammonia generated on the anode electrode 23 side. As shown in FIG. 2, when the anode electrode 23 side is made into a dry environment, an electrochemical cell 4 is formed by contacting the cathode electrode 21 with the electrolyte 3. The contact between the cathode electrode 21 and the electrolyte 3 is achieved, for example, by immersing the cathode electrode 21 in the electrolyte 3. Note that a dry environment means an environment in which no liquid such as the electrolyte 3 or urea water exists, but it is acceptable for the electrolyte 3 to contain moisture contained in the air.

Preferably, the cathode 21 is in contact with the electrolyte 3, which is an alkaline aqueous solution (see Figures 2 and 3). In this case, hydrogen is produced from the water in the alkaline aqueous solution.

When an aqueous solution of urea is supplied to the anode electrode 23 side, an electrolyte solution 3 containing urea can be supplied to the anode electrode 23 side (see Figures 1 and 3). In this case, the cathode electrode 21 side can be made into a dry environment as shown in Figure 1. This makes it easier to collect hydrogen generated on the cathode electrode 21 side. It is also expected to prevent the generated hydrogen gas from forming bubbles in the liquid, covering the electrode surface and inhibiting the reaction. Also, by introducing an inert gas such as Ar gas from the outside, hydrogen can be easily extracted from the dry environment on the cathode electrode 21 side to the outside.

Also, both the anode electrode 23 and the cathode electrode 21 may be brought into contact with the electrolyte solution 3 containing urea (see FIG. 3). In other words, the anode electrode 23 and the cathode electrode 21 may be immersed in the electrolyte solution 3 containing urea.

An alkaline aqueous solution (i.e., a basic aqueous solution) is used as the electrolytic solution 3. An aqueous solution of an alkali metal hydroxide and/or an alkaline earth metal hydroxide can be used as the electrolytic solution 3. From the viewpoint of being able to adjust the pH to 14 or more, the alkaline aqueous solution is preferably a KOH aqueous solution. A higher pH of the alkaline aqueous solution has the effect of promoting the oxidation of urea, increasing the current density, and improving the rate of hydrogen production.
When the anode 23 is brought into contact with the electrolyte 3, an alkaline aqueous solution containing urea can be used as described above.

The pH of the electrolyte 3 at 25°C is preferably 8 or higher. In this case, the oxidation of urea is sufficiently promoted, the current density increases, and the rate of hydrogen production also improves. From the viewpoint of further improving this effect, the pH of the electrolyte 3 at 25°C is more preferably 12 or higher, and even more preferably 14 or higher.

At least one metal selected from the group consisting of Fe, Co, Ru, Rh, Ni, Ir, Pt, and Cu can be used as the anode electrode 23. At least one metal selected from the group consisting of Pt, Ir, Pd, Ru, and Ni can be used as the cathode electrode 21. By changing the type of metal constituting the anode electrode 23, it is possible to vary the products generated by the electrolysis of urea and urea water, thereby generating products other than ammonia, and to increase the amount of ammonia generated. From the viewpoint of increasing the amount of ammonia generated and the amount of hydrogen generated, it is preferable that the anode electrode 23 is an electrode composed of a titanium mesh and a deposit of the above metal formed on the titanium mesh. From the viewpoint of facilitating hydrogen generation, it is preferable that the cathode electrode 21 is a composite of a conductive agent composed of a carbon material such as Ketjen black and the above metal.

From the viewpoint of further improving the amount of ammonia produced from the anode electrode 23, it is preferable to use Ru and/or Ni as the anode electrode 23, and it is more preferable to use Ni. Also, from the viewpoint of sufficiently producing hydrogen from the cathode electrode 21, it is preferable to use at least one selected from the group consisting of Rh, Co, and Fe as the anode electrode 23, it is more preferable to use Co and/or Fe, and it is even more preferable to use Fe.

The anode 23 preferably contains a base metal. In this case, the anode 23 can be formed without using a precious metal, which allows for reduced manufacturing costs. In the hydrogen/ammonia production device 1, hydrogen can be produced in the anode 23 without using a precious metal in the anode 23. Furthermore, when the anode 23 contains a base metal, manufacturing costs can be reduced. Furthermore, in this case, the type and amount of product can be controlled by selecting the base metal species.

The diaphragm 22 is made of an ion exchange membrane. The ion exchange membrane may be a cation exchange membrane or an anion exchange membrane.

As shown in FIG. 1, when the diaphragm 22 is made of a cation exchange membrane, when a voltage is applied between the anode 23 and the cathode 21, H ⁺ as a reaction factor is sent from the anode 23 to the cathode 21 through the cation exchange membrane. At this time, the reactions of the following formulas <2.1>, <2.2>, <2.3>, and <2.4> occur on the anode 23 side. That is, NH _x species such as ammonia, NCO species, N ₂ , CO ₂ , and the like are generated by co-electrolysis of urea and water. As shown in formula <2.1>, the generation reaction proceeds when a reaction of urea and water to generate hydrogen ions (i.e., protons) is triggered, so it is important that the reaction of formula <2.1> starts. When a cation exchange membrane is used as the ion exchange membrane, hydrogen ions as a reaction factor are sent from the anode 23 to the cathode 21 by the cation exchange membrane, so the trigger reaction proceeds. It is believed that the subsequent ammonia production reaction and hydrogen production reaction proceed in this manner. Furthermore, oxygen is produced on the anode electrode 23 side by the reaction of formula <2.5>. Then, electrons and protons are produced on the anode electrode 23 side. The electrons move to the cathode electrode 21 side through an external circuit, and the protons move to the cathode electrode 21 side through a cation exchange membrane. At the cathode electrode 21, the electrons and protons react to produce hydrogen as shown in formula <2.6>. In formulas <2.1> to <2.6>, SHE represents the standard hydrogen electrode, and the electrode potential E ⁰ represents a converted value at the standard hydrogen electrode.

As shown in FIG. 2, when the diaphragm 22 is made of an anion exchange membrane, when a voltage is applied between the anode 23 and the cathode 21, the reaction factor OH ^- is sent from the cathode 21 to the anode 23 through the anion exchange membrane. At this time, the reactions of the following formulas <3.1>, <3.2>, and <3.3> occur on the anode 23 side. The reaction of <3.3> further causes the reactions of <3.3.1> to <3.3.4>. That is, on the anode 23 side, nitrogen, carbon dioxide, etc. are generated by the reaction of OH ^- with urea, and ammonia is generated from urea. As shown in formula <3.1>, the reaction of urea with hydroxide ions is a trigger to proceed with the production reaction, so it is important that the reaction of formula <3.1> starts. When an anion exchange membrane is used as the ion exchange membrane, the reaction factor hydroxide ions are sent from the cathode 21 to the anode 23 by the anion exchange membrane, and the trigger reaction proceeds. It is believed that the subsequent ammonia production reaction and hydrogen production reaction proceed in this manner. Electrons are produced on the anode electrode 23 side. The electrons move to the cathode electrode 21 side through an external circuit. Then, at the cathode electrode 21, water reacts with the electrons to produce hydrogen, as shown in formula <3.4>. In formulas <3.1> to <3.4>, SHE stands for standard hydrogen electrode, and the electrode potential E ⁰ stands for a converted value at the standard hydrogen electrode.

As shown in the above reaction formula, in the hydrogen/ammonia production device 1, hydrogen is produced on the cathode 21 side, and ammonia is produced on the anode 23 side. In the production device 1, it is preferable that the anode 23 side and the cathode 21 side are separated by the electrode body 2. In this case, it is possible to prevent the hydrogen generated on the cathode 21 side and the ammonia generated on the anode 23 side from being mixed together. This makes it easier to recover the hydrogen and ammonia separately.

Specifically, in the hydrogen/ammonia production device 1, a cathode reaction chamber is formed on the cathode electrode 21 side of the electrode body 2, and an anode reaction chamber is formed on the anode electrode 23 side, and the cathode reaction chamber and anode reaction chamber can be separated by the electrode body 2. This makes it possible to separately recover the hydrogen produced in the cathode reaction chamber and the ammonia produced in the anode reaction chamber.

The ion exchange membrane is preferably an anion exchange membrane. If the ion exchange membrane is a cation exchange membrane, ammonium ions produced from the electrolytic reaction of the urea water are ion-exchanged in the cation exchange membrane, increasing the electrical resistance of the cation exchange membrane. As a result, the cation exchange membrane is damaged and loses its function over time. As a result, ammonia and hydrogen cannot be produced sufficiently. In contrast, if the ion exchange membrane is an anion exchange membrane, ammonium ions are not ion-exchanged, and the electrical resistance does not increase. As a result, ammonia and hydrogen can be produced continuously, thereby increasing the amount produced.

The anion exchange membrane is preferably composed of a polymer containing at least one ligand selected from the group consisting of imidazolium ligands, pyridinium ligands, and phosphonium ligands. More specifically, the anion exchange membrane is composed of a polymer having a ligand portion and a skeleton portion chemically bonded to the ligand portion, and the ligand portion is preferably composed of at least one functional group or a salt thereof selected from the group consisting of an imidazolium group, a pyridinium group, and a phosphonium group. From the viewpoint of the stability and base strength of the functional group, the ligand is preferably an imidazolium ligand. In other words, the ligand portion is preferably composed of an imidazolium group or a salt thereof. The skeleton portion is composed of, for example, a styrene-based resin such as polystyrene or a styrene-divinylbenzene copolymer; an acrylic resin such as polyhydroxymethacrylate; polyvinyl alcohol, etc.

In the hydrogen/ammonia production device 1 of this embodiment, the diaphragm 22 is composed of an ion exchange membrane. Therefore, it is possible to directly send a reaction factor from the cathode 21 to the anode 23 or from the anode 23 to the cathode 21 through the ion exchange membrane. This makes it possible to control the local pH at the reaction point of the electrode. Specifically, the reactions of the above formulas <2.1> and <3.1> can induce a local increase in proton concentration at the cathode 21 and a local increase in OH ⁻ concentration at the anode 23. Therefore, it becomes possible to cause a reaction that is not dependent on the pH of the electrolyte. More specifically, for example, when an anion exchange membrane is used as the ion exchange membrane, OH ⁻ is generated at the anode 23, so that an environment in which the pH is locally increased is formed on the surface of the anode 23. Therefore, even if the pH of the electrolyte is near neutral (for example, pH is 8 to 12), the ammonia and hydrogen production reaction proceeds, making it possible to produce hydrogen and ammonia.
In this manner, according to the hydrogen/ammonia production apparatus 1 of this embodiment, hydrogen is produced at the cathode 21, and ammonia is produced at the anode 23. In this manner, at least one of hydrogen and ammonia, or both, can be efficiently produced.

Next, a method for producing hydrogen and/or ammonia will be described. This method can be easily realized by the above-mentioned production apparatus, but the production method disclosed herein is not limited to the method using the above-mentioned production apparatus. The hydrogen/ammonia production method is performed by an electrode formation process, an assembly process, and an application process.

In the electrode formation process, the cathode 21, the diaphragm 22, and the anode 23 are sequentially stacked. This forms the electrode body 2, which is composed of a laminate of the cathode 21, the diaphragm 22, and the anode 23. The cathode 21, the diaphragm 22, and the anode 23 are as described above.

In the assembly process, the anode 23 of the electrode assembly 2 is brought into contact with urea, and at least one of the anode 23 and the cathode 21 is brought into contact with the electrolyte 3 composed of an alkaline aqueous solution. In this way, an electrochemical cell 4 is constructed by the electrode assembly 2 and the electrolyte 3. The urea and the electrolyte 3 are as described above.

In the application process, a voltage is applied between the cathode 21 and the anode 23. This causes hydrogen to be produced from the cathode 21 and ammonia to be produced from the anode 23.

In the manufacturing method of this embodiment, the anode 23 of the electrode body 2 is brought into contact with urea, and at least one of the anode 23 and the cathode 21 of the electrode body 2 is brought into contact with the electrolyte 3 composed of an alkaline aqueous solution. Then, a voltage is applied between the cathode 21 and the anode 23. By applying the voltage, a reaction factor can be directly sent from the cathode 21 to the anode 23 or from the anode 23 to the cathode 21. This makes it possible to control the local pH at the reaction point of the electrode. In this way, the same effect as that of the manufacturing device described above can be achieved. Then, hydrogen is generated at the cathode 21, and ammonia is generated at the anode 23. In this way, at least one of hydrogen and ammonia, or both, can be efficiently generated.

(Experimental Example 1)
This example is an example in which hydrogen and ammonia are produced by a hydrogen/ammonia production apparatus 1 using an anion exchange membrane. In this example, the influence of changing the material of the anode electrode 23 in the hydrogen/ammonia production apparatus 1 is examined. Note that, among the symbols used in Experimental Example 1 and subsequent examples, the same symbols as those used in the previously described embodiments represent the same components, etc. as those in the previously described embodiments, unless otherwise specified.

In this example, a hydrogen/ammonia production device 1 was constructed as shown in FIG. 4. Specifically, first, AdBlue (a registered trademark of the German Association of the Automotive Industry (i.e., VDA) 5BIB), a urea solution manufactured by Mitsui Chemicals, was prepared. The urea concentration of this urea solution was 32.5 wt%. 1 M of KOH was dissolved in this urea solution to prepare an alkaline aqueous solution containing urea (i.e., electrolyte 3). The pH of the alkaline aqueous solution was 14 or higher. Next, 30 mL of electrolyte 3 was placed in a one-chamber container 40, and the electrode body 2, in which the anode 23, the diaphragm 22, and the cathode 21 were integrally formed, was immersed in the electrolyte 3.

The cathode 21 was an electrode made of a mixture of Ketjen black and Pt. The anode 23 was an electrode in which Fe, Co, Ru, Rh, or Ni was deposited on a titanium mesh. That is, the anode 23 was made of an Fe/Ti catalyst, a Co/Ti catalyst, a Ru/Ti catalyst, a Rh/Ti catalyst, or a Ni/Ti catalyst. The diaphragm 22 was made of Sustainion (registered trademark) X-37, an anion exchange membrane manufactured by Dioxide Materials. In the hydrogen/ammonia production device 1 of this example, an electrochemical cell 4 is constructed from an electrode body 2 and an electrolyte 3 in which the electrode body 2 is immersed.

The cathode electrode 21 and the anode electrode 23 are electrically connected to an electrochemical measurement device 5 (specifically, a potentiostat/galvanostat). Specifically, the anode electrode 23 is connected to the electrochemical measurement device 5 as a sample electrode (the sample electrode is also called a working electrode), and the cathode electrode 21 is connected to the electrochemical measurement device 5 as a counter electrode.

As shown in FIG. 4, a tube 611 made of polytetrafluoroethylene (i.e., PTFE) is inserted into the electrolyte 3 in the container 40, and this tube is connected to a gas inlet 61 formed at the top of the container 40 in the vertical direction. In addition, a gas outlet 62 is formed at the top of the container 40, and the gas outlet 62 is connected to a sample measurement section of the gas chromatography. "GC" in FIG. 4 stands for gas chromatography.

Next, while stirring the electrolyte 3 using a magnetic stirrer, He was blown into the container 40 from the gas inlet 61 to replace the air in the container 40 with He. The He blowing conditions were 0.1 MPa and 20 mL/min.

The open circuit voltage was measured while continuing the gas replacement with He for 60 minutes or more. Next, the voltage applied between the anode 23 and the cathode 21 was controlled to 2.0 V, and constant voltage electrolysis was performed for 2 hours to evaluate the urea water electrolysis activity. The evaluation was performed by measuring the average current density, the faradaic efficiency of oxygen (i.e., FE(O ₂ )), and the oxygen production rate (i.e., r(O ₂ )). The results are shown in Figs. 5 to 7. Fig. 7 shows the change over time in the current density and the current efficiency of hydrogen when an electrode in which Ni is deposited on a titanium mesh is used as the anode 23.

In the hydrogen/ammonia production device 1 of this example, the reactions of the above formulas <3.1> to <3.4> are considered to occur. That is, in the anode electrode 23, N ₂ and CO ₂ are generated by co-electrolysis of urea and water according to formula <3.1>. In addition, the generation reaction of O ₂ proceeds by water electrolysis according to formula <3.2>. Then, the electrons generated in the anode electrode 23 move to the cathode electrode 21 through the external circuit of the electrochemical cell 4, and the water and the electrons react in the cathode electrode 21 according to formula <3.4> to cause the H ₂ generation reaction. The electrolysis was performed at a constant voltage as described above. For example, when Pt/KB is used as the cathode electrode 21, it is generally known that it shows high activity in the H ₂ generation reaction and has a small overvoltage even in a high current density region. Therefore, in this example, the experiment was performed by regarding the voltage between the two electrodes as the anode potential.

In this example, as described above, the electrode catalyst activity was evaluated by the average current density, the production rate of products such as _O2 , and the product selectivity (specifically, the Faraday efficiency). Assuming that all _N2 production reactions proceed according to formula <3.1>, and assuming that the Faraday efficiency is a 6-electron reaction, the Faraday efficiency FE was calculated based on formula (I). In formula (I), Y represents the product yield (unit: mol), n represents the number of reaction electrons (unit: -), and Q represents the amount of current charge (unit: C).
FE (%) = 100 x Y x n x F/Q ... (I)

If the electrolysis reaction of urea water as shown in the above formula <3.1> progresses, _N2 and _CO2 should be generated in amounts equal to urea (i.e., ( _NH2 ) _2CO ), but since a strongly basic electrolyte 3 is used in this example, CO2 dissolves in the aqueous solution (i.e., electrolyte 3). Therefore, the _CO2 detected by gas chromatography is only a part of the _CO2 generated. Therefore, the selectivity of the N2 generation reaction was regarded as the selectivity of the urea water electrolysis reaction, and the catalytic activity was evaluated by comparing it with the selectivity of the _O2 generation reaction by water electrolysis. In other words, as shown in Figure 6, the hydrogen generation rate, ammonia generation rate, hydrogen selectivity, and ammonia selectivity can be evaluated by investigating the O2 generation rate and selectivity.

As can be seen from FIG. 5 and FIG. 6, when the Fe/Ti catalyst was used for the anode 23, a current density of nearly 50 A cm ⁻² was observed. In this case, the Faraday efficiency of O ₂ exceeded 80%, and the production rate was close to 700 μmol ^{h −1} , so that the production of hydrogen can be promoted mainly by electrolysis. On the other hand, the production of ammonia can be suppressed. In other words, when the Fe/Ti catalyst is used, hydrogen can be produced preferentially. Co/Ti catalyst and Rh/Ti catalyst are equivalent to this.

As can be seen from Figures 5 and 6, when Ni/Ti catalyst was used for the anode 23, a current density of nearly 150 Acm ^-2 was observed. On the other hand, the Faraday efficiency of _O2 was 0, and no _O2 was produced. Ammonia production can be promoted along with hydrogen production by electrolysis. Ru/Ti catalyst is similar to this.

7, when the Ni/Ti catalyst was used for the anode 23, a high current density exceeding 100 mA cm ^-2 was observed even after 2 hours. Moreover, hydrogen was generated at the cathode 21 side at a generation rate of 5 mmol h ^-1 and a current efficiency of 100%, and highly efficient hydrogen generation by urea water electrolysis proceeded.
In addition, gas chromatographic analysis revealed that _N2 and a small amount of _CO2 were generated in the gas phase on the anode electrode 23 side. As described above, it is believed that most of the generated _CO2 was dissolved in the alkaline aqueous solution (i.e., the electrolyte 3). In addition, when the outlet gas was analyzed by gas chromatographic analysis, almost no harmful compounds such as _NO2 or NO were observed.

Figure 8 shows the average current density measurement results when electrolysis of urea water was performed by performing the same experiment as above, except that Ir/KB, Ru/KB, Rh/KB, Pt/KB, Ni/KB, Co/KB, Fe/KB, Fe/Ti, Pt/Ti, Ru/Ti, Rh/Ti, Ni/Ti, and Ti-mesh were used as the anode 23, and the voltage applied between the anode 23 and the cathode 21 was 1.5 V. Note that Ir/KB refers to an electrode in which iridium is deposited on Ketjen Black, and Fe/Ti refers to an electrode in which iron is deposited on a titanium mesh. The same applies to the other electrodes.

As can be seen from FIG. 8, when various transition metals and precious metals were used for the anode 23, the Ni/Ti catalyst showed extremely high activity in the electrolytic reforming reaction of urea water. Therefore, it can be said that it is preferable to use a Ni/Ti electrode for the anode 23 in order to produce ammonia and hydrogen from urea.

(Experimental Example 2)
In this example, the effect of the KOH concentration in the electrolyte on the urea conversion rate was investigated. First, a hydrogen/ammonia production device 1 similar to that in Experimental Example 1 was fabricated. Ni/Ti was used as the anode 23. Under room temperature (25° C.) conditions, the KOH concentration in the electrolyte was set to 0, 1 M, or 3 M, and a voltage of 1.5 V, 1.8 V, or 2 V was applied between the cathode and anode for 2 hours, and the urea conversion rate was measured and calculated. The results are shown in FIG. 9.

The urea conversion rate was calculated based on the following formula (I).
Urea conversion rate = amount of urea used in the reaction / total amount of urea (I)
The total substance amount of urea is calculated from the urea concentration of the urea water (32.5 wt % in this example).
The "amount of urea used in the reaction" is calculated from the amount of current and the Faraday efficiency of _O2 (i.e., FE) according to the following formula (II).
Amount of urea used = amount of electricity / 96485 x (1-FE of _O2 ) / 6
This urea conversion rate is an apparent conversion rate calculated on the assumption that the remaining amount of electricity, obtained by subtracting the amount of electricity for oxygen generation from the amount of electricity passed, is entirely used for the electrolysis of urea, that is, the six-electron reaction in the above formula <3.1> has proceeded.

As can be seen from FIG. 9, in the hydrogen/ammonia production device 1 of this example, the urea water electrolysis reaction proceeds sufficiently even in an electrolyte with a low KOH concentration of 1M. In other words, even in a low-concentration KOH aqueous solution, ammonia is produced from the anode 23 and hydrogen is produced from the cathode 21 through the urea water electrolysis reaction. This is thought to be because, by using an anion exchange membrane as the diaphragm 22, the pH increases locally even when the KOH concentration is low, accelerating the urea water electrolysis reaction.

(Experimental Example 3)
This example is an example in which hydrogen and ammonia are produced by a hydrogen/ammonia production device 1 using a cation exchange membrane. In this example, a hydrogen/ammonia production device 1 shown in FIG. 10 was constructed.

First, an alkaline aqueous solution containing urea (i.e., electrolyte 3) was prepared in the same manner as in Experimental Example 1. Next, an electrode body 2 was prepared in which an anode 23, a diaphragm 22, and a cathode 21 were integrally formed, and the electrode body 2 was inserted into a container 40 (specifically, a first container 40) to divide the space inside the container 40 into two. More specifically, the diaphragm 22 of the electrode body 2 divided the inside of the container 40. The anode 23 was made of Pt-Black__, and the cathode 21 was made of Pt. Note that Pt-Black is a black powder of platinum. The diaphragm 22 was made of Nafion (registered trademark) 117, a cation exchange membrane manufactured by Chemours.

Electrolyte solution 3 (specifically, first electrolyte solution 3) was injected into the anode electrode 23 side of the container 40. Meanwhile, the cathode electrode 21 in the container 40 is exposed to a dry environment. A tube 611 made of polytetrafluoroethylene (i.e., PTFE) is inserted into the electrolyte solution 3, and this tube 611 is connected to a gas inlet 61 (specifically, first gas inlet 61) formed at the top of the container 40 in the vertical direction.

A first gas outlet 621 is formed in the upper part of the container 40 on the anode electrode 23 side, and the first gas outlet 621 is connected to a sample measurement section of the gas chromatography. On the other hand, a second gas inlet 63 is provided on the cathode electrode 21 side in the container 40. In the hydrogen/ammonia production apparatus 1 of this example, Ar gas is introduced from the second gas inlet 63 to send out hydrogen generated on the cathode electrode 21 side.

A second gas outlet 622 is formed in the upper part of the container 40 on the cathode electrode 21 side, and the second gas outlet 622 is connected to the sample measurement section of the gas chromatography. Note that the gas chromatography is not shown in FIG. 10.

10, the diaphragm 22 extends from the lower portion in the vertical direction V to the outside of the container 40, and is immersed in a second electrolytic solution 39 in a second container 49 provided outside the container 40. The second electrolytic solution 39 is a 1N _{H2SO4 aqueous} solution. Furthermore, an Ag/AgCl electrode 25 serving as a reference electrode is inserted into the _{H2SO4 aqueous} solution.

The cathode 21, anode 23, and Ag/AgCl electrode 25 are electrically connected to an electrochemical measurement device 5 (i.e., potentiostat/galvanostat). Specifically, the anode 23 is connected to the electrochemical measurement device 5 as a sample electrode (the sample electrode is also called a working electrode), the cathode 21 is connected to the electrochemical measurement device 5 as a counter electrode, and the Ag/AgCl electrode 25 is connected to the electrochemical measurement device 5 as a reference electrode. In the hydrogen/ammonia production device 1 of this example, a three-electrode electrochemical cell 4 is constructed.

Next, while stirring the electrolyte 3 using a magnetic stirrer, He was blown into the container 40 from the first gas inlet 61 to replace the air in the container 40 with He. The conditions for blowing He were the same as in Experimental Example 1.

The open circuit voltage was measured while continuing the gas replacement with He for 60 minutes or more. Next, the voltage applied between the anode 23 and the cathode 21 was controlled to 1.4 V, and constant voltage electrolysis was performed for 2 hours to allow the urea water electrolysis reaction to proceed. At this time, the impedance was measured using the electrochemical measurement device 5. The change in impedance over time is shown in Figure 11.

In this example, it was confirmed that HNCO was produced at the anode electrode 23 side by the urea water electrolysis reaction at room temperature by the SPE electrolysis method. Furthermore, according to formula <3.3>, it can be said that ammonia was produced at the anode electrode 23 by the urea water electrolysis reaction. Furthermore, according to formula <2.5>, it can be said that hydrogen was produced at the cathode electrode 21 by the urea water electrolysis reaction.

As can be seen from FIG. 11, in this example, the electrical resistance increases over time.
This is believed to be because NH ₄ ⁺ in the urea aqueous solution is trapped in the cation exchange membrane by ion exchange. In other words, if a cation exchange membrane is used for the diaphragm 22, the membrane may deteriorate over time. Therefore, an anion exchange membrane is preferable as the diaphragm 22.

(Experimental Example 4)
This example is an example of directly detecting the concentration of ammonia produced by the hydrogen/ammonia production apparatus 1. First, a hydrogen/ammonia production apparatus 1 similar to that of Experimental Example 1 was constructed using an electrode in which Ni was deposited on a titanium mesh as the anode electrode 23 (see FIG. 4).

Urea water electrolysis was carried out by performing constant voltage electrolysis for 2 hours using this hydrogen/ammonia production device 1. The method and conditions for urea water electrolysis were the same as those in Experimental Example 1, except that the voltage applied between the electrodes was 1.8 V and the KOH concentration of the electrolyte was 3 M.

After 2 hours of constant voltage electrolysis, 30 mL of the electrolyte was collected as a measurement sample, and the ammonia concentration in the measurement sample was measured using a Toko Chemical Industry Co., Ltd. ammonia electrode "Ti9001". The concentration was measured using a calibration curve based on an ammonia standard solution. The average current density results and ammonia concentration results for this example are shown in Figure 12.

12, in this example, the current density was high, the urea water electrolysis proceeded efficiently, and ammonia was also produced in sufficient amounts. Therefore, it is understood that ammonia was produced with high efficiency.
Thus, in this example, the generation of ammonia was confirmed by the ammonia electrode.

(Experimental Example 5)
This example is an example for evaluating the dependency of the urea water electrolysis reaction on the applied voltage. First, a hydrogen/ammonia production device 1 similar to that of Experimental Example 1 was constructed using an electrode in which Ni was deposited on a titanium mesh as the anode electrode 23 (see FIG. 4).

Using this hydrogen/ammonia production device 1, urea water electrolysis was carried out in the same manner as in Experimental Example 1. The method and conditions for urea water electrolysis were the same as in Example 1, except that the voltage applied between the electrodes was changed. Specifically, in this example, the voltage was increased from 1.3 V to 1.6 V in increments of 0.1 V every 30 minutes. In other words, after urea water electrolysis was carried out for 30 minutes at 1.3 V, the voltage was increased in increments of 0.1 V up to 1.6 V, and urea water electrolysis was carried out for 30 minutes at each voltage. The relationship between the elapsed time, the current density, and the applied voltage in this experiment is shown in FIG. 13. As shown in FIG. 13, a 5-minute period of no voltage application was provided between urea water electrolysis at each voltage. The relationship between the applied voltage and the current density in this example is shown as a solid line graph in FIG. 14.

In addition, for comparison with the above-mentioned urea water electrolysis, a similar experiment (specifically, water electrolysis) was conducted using an electrolyte solution without added urea. The results (i.e., the relationship between applied voltage and current density) are shown as a dashed line graph in Figure 14.

As can be seen from Figure 14, the current density is higher at a lower potential in an electrolyte with urea added than in an electrolyte without urea added, and electrolysis occurs. In other words, since urea water electrolysis occurs at a low potential, ammonia and hydrogen can be produced with low energy by using urea water as a raw material.

(Experimental Example 6)
In this example, an anion exchange membrane is used as the diaphragm 22, and hydrogen and ammonia are produced by a hydrogen/ammonia production device 1 in which one of the electrodes is exposed to a dry environment. The production of hydrogen and ammonia was performed by evaluating the decomposition activity of urea.

In this example, the hydrogen/ammonia production device 1 shown in FIG. 15 was constructed. Specifically, first, an alkaline aqueous solution containing urea (i.e., electrolyte 3) was prepared in the same manner as in Experimental Example 1. In addition, an electrode body 2 in which an anode electrode 23, a diaphragm 22, and a cathode electrode 21 were integrally formed was inserted into a container 40 whose outer wall was made of PTFE, and the space inside the container 40 was divided into two. In other words, the inside of the container 40 was divided by the diaphragm 22 of the electrode body 2. The diaphragm 22 was the same anion exchange membrane as in Experimental Example 1. In addition, the anode electrode 23 was made of Ni/Ti, and the cathode electrode 21 was made of Pt/KB. The anode electrode 23 and the cathode electrode 21 were each electrically connected to a power source 55.

As shown in FIG. 15, the electrolyte 3 is poured into the anode electrode 23 side of the container 40, and the cathode electrode 21 in the container 40 is exposed to a dry environment. The electrolyte 3 used was the same as in Experimental Example 1. In the following explanation, the side of the container 40 facing the anode electrode 23 and filled with the electrolyte 3 is referred to as the anode chamber 231 (i.e., the left chamber in FIG. 15), and the side facing the cathode electrode 21 and exposed to a dry environment is referred to as the cathode chamber 211 (i.e., the right chamber in FIG. 15).

A tube 611 was inserted into the anode chamber 231, and He was passed through the tube 611 to the electrolytic solution 3. A first gas outlet 621 was also formed in the anode chamber 231, and a multi-way valve (specifically, a six-way valve) and a membrane flowmeter (not shown) connected to a gas chromatograph were connected to the first gas outlet 621 in sequence. On the other hand, a second gas inlet 63 and a second gas outlet 622 were provided in the cathode chamber 211, and an inert gas (specifically, Ar) was passed through the second gas inlet 63 and the second gas outlet 622 into the cathode chamber 211.

The anode 23 is electrically connected to the working electrode terminal and potential measurement terminal of an electrochemical measurement device (specifically, a potentiostat/galvanostat) by gold wires, and the cathode 21 is electrically connected to the counter electrode terminal and reference electrode terminal of the electrochemical measurement device by gold wires. Note that the electrochemical measurement device is not shown in Figure 15.

After sufficient inert gas was blown into the cathode chamber 211 to replace the air with the inert gas, the applied voltage was controlled to 2.0 V and constant voltage electrolysis was performed for 2 hours in the same manner as in Experimental Example 1 to evaluate the urea water electrolysis activity. The evaluation was performed by measuring the Faraday efficiencies of hydrogen, nitrogen, and oxygen (i.e., FE(H ₂ ), FE(N ₂ ), FE(O ₂ )) and the production rates of hydrogen, nitrogen, and oxygen (i.e., r(H ₂ ), r(N ₂ ), r(O ₂ )). The results are shown in Figures 17 to 19. Also, the change in current density over time in this example is shown in Figure 16.

17, the Faraday efficiency FE( _H2 ) of hydrogen at the cathode 21 is 100%, and the hydrogen production rate r( _H2 ) is also fast. From this, it is understood that hydrogen is produced efficiently and sufficiently from the cathode 21 in the hydrogen/ammonia production apparatus 1 of this embodiment.
Furthermore, as can be seen from FIG. 18, the current efficiency of nitrogen production is low at about 30%, and it is found that nitrogen compounds other than nitrogen and ammonia are produced in the reaction solution.
Furthermore, as can be seen from FIG. 19, the current efficiency of oxygen generation is low at about 6%, and it is understood that urea is efficiently oxidized.

In this way, it can be seen that even if an anion exchange membrane is used as the ion exchange membrane and the cathode side is in a dry environment, hydrogen and ammonia can be produced by urea water electrolysis.

The present invention is not limited to the above-described embodiments, and can be applied to various embodiments without departing from the spirit of the present invention.

Reference Signs List 1 Hydrogen/ammonia production device 2 Electrode body 21 Cathode 22 Diaphragm 23 Anode 3 Electrolyte 4 Electrochemical cell

Claims (11)

A hydrogen and/or ammonia production device (1) comprising an electrochemical cell (4) having an electrode body (2) and an electrolyte (3),
the electrode body is composed of a laminate in which a cathode (21), a diaphragm (22), and an anode (23) are laminated in this order,
At least the anode is in contact with urea;
the electrolyte is an alkaline aqueous solution, and at least one of the anode and the cathode is in contact with the electrolyte;
A hydrogen/ammonia production apparatus, wherein the diaphragm is composed of an ion exchange membrane. The hydrogen/ammonia production device according to claim 1, wherein the ion exchange membrane is an anion exchange membrane. The hydrogen/ammonia production device according to claim 1 or 2, wherein the anode side and the cathode side in the electrochemical cell are separated by the electrode body. The hydrogen/ammonia production device according to any one of claims 1 to 3, wherein the electrode body is immersed in the electrolyte solution containing the urea. The hydrogen/ammonia production device according to any one of claims 1 to 3, wherein the anode and cathode of the electrode body that are not in contact with the electrolyte are exposed to a dry environment in which no liquid is present. The hydrogen/ammonia production device according to any one of claims 1 to 5, wherein the anode electrode contains a base metal. 1. A method for producing hydrogen and/or ammonia, comprising the steps of:
A cathode (21), a diaphragm (22) made of an ion exchange membrane, and an anode (23) are sequentially stacked to form an electrode body (2);
the anode electrode of the electrode assembly is brought into contact with urea, and at least one of the anode electrode and the cathode electrode is brought into contact with an electrolyte solution composed of an alkaline aqueous solution;
A method for producing hydrogen/ammonia, comprising applying a voltage between the cathode and the anode to produce hydrogen and/or ammonia. The method for producing hydrogen/ammonia according to claim 7, wherein the ion exchange membrane is an anion exchange membrane. The method for producing hydrogen/ammonia according to claim 7 or 8, wherein the electrode body is immersed in the electrolyte solution containing the urea. The method for producing hydrogen/ammonia according to claim 7 or 8, wherein the anode and cathode of the electrode body that are not in contact with the electrolyte are exposed to a dry environment in which no liquid is present. The method for producing hydrogen/ammonia according to any one of claims 7 to 10, wherein the anode electrode contains a base metal.