AU2016369851B2

AU2016369851B2 - Electrochemical cell and process

Info

Publication number: AU2016369851B2
Application number: AU2016369851A
Authority: AU
Inventors: Timothy Hughes; Ian Wilkinson
Original assignee: Siemens AG
Current assignee: Siemens AG
Priority date: 2015-12-16
Filing date: 2016-11-02
Publication date: 2021-12-02
Anticipated expiration: 2036-11-02
Also published as: EP3390692A1; GB2545444B; GB2545444A; JP6952698B2; WO2017102167A1; AU2016369851A1; GB201522169D0; EP3390692B1; JP2019505663A

Abstract

An electrochemical cell (10) comprising a first volume exposed to respective surfaces of an anode (28) and a cathode (24), has a steam inlet (5) to allow steam into the first volume. A structure (4) comprising a solid catalyst is provided within the first volume.

Description

ELECTROCHEMICAL CELL AND PROCESS

The present invention relates to electrochemical cells, particularly electrochemical cells for synthesis of ammonia NH₃. The present invention also relates to processes for synthesis of ammonia N¾ .

Known approaches to the requirement for synthesis of ammonia include :

(1) Haber Bosch process - pressurization and heating of 2 and ¾ over an iron catalyst; (2) Electrochemical synthesis with a molten salt electrolyte and gas electrodes [1-3]; and

(3) Electrochemical synthesis with a solid electrolyte and eletrocatalytic electrodes [4-6].

[1] Murakami T., T. Nishikiori, T. Nohira, and Y. Ito, "Electrolytic Synthesis of Ammonia in Molten Salts Under Atmospheric Pressure", J. Amer. Chem. Soc. 125 (2) , pp. 334- 335 (2003) .

[2] Murakami T. et al . , "Electrolytic Ammonia Synthesis from Water and Nitrogen Gas in Molten Salt Under Atmospheric Pressure", Electrochim. Acta 50 (27), pp. 5423-5426 (2005).

[3] US Patent 6,881,308 B2

[4] Marnellos, G. , Zisekas,S., and Stoukides,M. (2000). Synthesis of ammonia at atmospheric pressure with the use of solid state proton conductors. J. Catal . 193, 80-88. doi:10.1006/jcat.2000.2877

[5] Lan, R., Irvine, J.T.S., and Tao, S. (2013). Synthesis of ammonia directly from air and water at ambient temperature and pressure. Sci.Rep. 3, 1145. doi : 10.1038/srep01145 [6] Skodra, A., and Stoukides, M. (2009). Electrocatalytic synthesis of ammonia from steam and nitrogen at atmospheric pressure. Solid State Ionics 180, 1332-1336.

[7] Banares-Alcantra et al . "Analysis of Islanded Ammonia- based Energy Storage Systems", University of Oxford, September 2015.

The present invention seeks to provide alternative methods and apparatus for the synthesis of ammonia from water and nitrogen N₂.

Accordingly, the invention provides methods and apparatus as defined in the appended claims.

The above, and further, objects, characteristics and advantages of the present invention will become more apparent from the following description of certain embodiments thereof, in conjunction with the appended claims wherein:

Fig. 1 illustrates an exemplary electrochemical cell as provided by an embodiment of the present invention; and

Fig. 2 shows an expanded 3D grid structure of a catalyst used in an embodiment of the present invention.

The embodiment of the invention shown in Fig. 1 comprises an electrochemical cell 10 with three porously partitioned volumes 1-3.

The first volume 1 contains a nitride conductor 20 such as a molten salt eutectic, for example LiCl/KCl/LisN . In use, steam H₂0 is introduced into this first volume through a steam inlet 5. A steam diffuser 22 may be provided to ensure wide distribution of inlet steam. The second volume 2 is a cathode gas electrode. Nitrogen gas 2 26 is introduced into this gas electrode, on a surface of the porous electrode 24 away from the nitride conductor 20.

The third volume 3 is an anode gas electrode. A porous electrode 28 is in contact with the nitride conductor 20 on one side.

A DC power supply 7 applies a potential difference between the two porous electrodes 24, 28, with the more positive voltage +V being applied to the anode gas electrode 3 and the more negative voltage -V being applied to the cathode gas electrode 2. Typically, the applied potential difference may be in the region of 0.5 V to 2 V.

In use, nitrogen gas is reduced to nitride ions at the gas cathode 2 : N₂ + 6 e^" => 2 N^3~

Within the nitride conductor 20, the nitride ions migrate towards the anode under the influence of the voltage gradient between the anode and the cathode. Within the nitride conductor 19, the nitride ions encounter and react with steam (water) to produce ammonia:

2 N^3" + 3 H₂0 => 2 NH₃ + 3 O^2"

Ammonia is accordingly produced from nitrogen gas and steam. The ammonia diffuses through the nitride conductor 20 to be evolved at the surface of the nitride conductor. An enclosure 6 traps the evolved ammonia gas and allows it to be harvested. The resulting oxide ions migrate towards the anode under the potential gradient between the electrodes. The anode reaction returns electrons to the DC power supply and generates oxygen into the gas anode electrode: 2 O^2" => 0₂ + 4 e^". Ammonia gas diffuses through nitride conductor 20 and is trapped in enclosure 6. It may be dried and cleaned as necessary, and may be stored for later use. The structure of the electrochemical cell of the present invention allows steam ¾0 to be used as the source of hydrogen in the ammonia product, rather than hydrogen ¾ as was commonly the case in conventional methods and apparatus for synthesising ammonia. This enables the electrochemical synthesis of ammonia N¾ without requiring a separate electrolysis stage to generate hydrogen ¾, or the need to buy and store hydrogen ¾, resulting in a much simpler system design .

A particular feature of the present invention is the solid catalyst 4 provided within the first volume 1.

In an embodiment of the invention, the solid catalyst is provided in the form of a 3D grid structure. In another embodiment of the invention, the solid catalyst is provided in the form of a wool structure. Other structures may be used, such as spheres or other shapes made of, or coated with, a catalyst material.

In all of these embodiments, the catalyst is provided on a relatively open structure: a 3D grid, a wool structure, spheres or other shapes and so on. Depending on the material chosen for the catalyst, the structure may be made of the catalyst .

The relatively open structure ensures that fluids can pass through the catalyst structure relatively freely, in close proximity to the catalyst. The structure, such as the 3D grid, wool, spheres, other shapes etc. enables the catalyst to effectively be suspended in the nitride solution in such a way that (a) the steam injected through the inlet structure 22 has a good chance of interacting with the catalyst 4; (b) that the ion movement through the nitride conductor 20 is not significantly impeded by the presence of the catalyst and (c) ammonia N¾ which is produced in the nitride conductor and at the surface of the catalyst can rise upwards to the enclosure 6 for collection.

The catalyst 4 promotes the reaction: 2 N^3" + 3 H₂0 => 2 NH₃ + 3 O^2"

The reaction rate for this reaction is enhanced due to the use of the catalyst. Suitable catalyst materials include known Fe-based catalysts and Ru-based catalysts. A discussion of suitable catalysts is provided in reference [7].

While the present invention has been described with particular reference to the application of ammonia synthesis from steam and nitrogen gas, the electrochemical cell and the synthesis method, of the present invention may be applied to the production of other gaseous products from first and second ionic components.

In general, means are provided for introducing a first source material 5 (in the above example, steam ¾0) into the first volume 1 and means are provided for introducing a second source material 26 (in the above example, nitrogen N₂) to a cathode 24. An electrolyte (in the above example in the form of the nitride conductor) is provided between anode 28 and cathode 24. Voltages +V and -V are applied respectively to the anode and cathode. At the cathode, a first ionic component (in the above example, N^3~) is produced from the second source material. The first ionic component traverses the electrolyte under the influence of the voltage gradient between the anode and the ground electrode, towards the anode. Within the electrolyte, the first ionic component encounters the first source material, and a reaction takes place to generate a product (in the above example, ammonia N¾) and an ionic by-product (in the above example, oxide ions 0^2~) . The ionic by-product continues to traverse the electrolyte under the influence of the voltage gradient between the anode and the ground electrode, towards the anode. On reaching the anode, the ionic by-product gives up its charge and becomes an evolved by-product (in the above example, oxygen 0₂) .

Means should be provided to collect the product and preferably also the evolved by-product. Means may also be provided to collect any by-products generated at the anode or cathode .

Although the anode is described as a gas electrode arranged for collection of a gaseous by-product, such arrangement may not be necessary in electrochemical cells set up to perform a different reaction. In such cases, it may be sufficient to provide a solid cathode, in which case the third volume 3 may be omitted.

Further modifications and variations are possible, within the scope of the invention as defined by the appended claims, as will be apparent to those skilled in the art.

Claims

1. An electrochemical cell (10) comprising a first volume exposed to respective surfaces of an anode (28) and a cathode (24), the electrochemical cell also being provided with a steam inlet (5) to allow steam into the first volume, characterised in that the electrochemical cell further comprises :

- an enclosure (6) to trap a gaseous product produced within the first volume; and - a structure (4) comprising a solid catalyst within the first volume, below the enclosure (6) such that a gaseous product evolved on a surface of the catalyst will be trapped by the enclosure (6), the catalyst having a relatively open structure such that fluids can pass through the catalyst structure in close proximity to the catalyst, whereby the catalyst is effectively suspended in the first volume.

2. An electrochemical cell according to claim 1 wherein the solid catalyst comprises at least one of the following, exposed to the first volume: an Fe-based catalyst; an Ru- based catalyst.

3. An electrochemical cell according to claim 1 wherein the structure (4) comprising a solid catalyst is provided in the form of a 3D grid structure.

4. An electrochemical cell according to claim 1 wherein the structure (4) comprising a solid catalyst is provided in the form of a wool structure.

5. An electrochemical cell according to claim 1 wherein the structure (4) comprising a solid catalyst is provided in the form of spheres or other shapes made of, or coated with, a catalyst material.

6. An electrochemical cell according to any preceding claim wherein the cathode (24) is a gas electrode, comprising a porous cathode and a second volume (2) .

7. An electrochemical cell according to any preceding claim wherein the anode (28) is a gas electrode, comprising a porous anode and a third volume (3) .

8. An electrochemical cell according to any preceding claim wherein the steam inlet (5) is provided with a steam diffuser

(22) .

10. An arrangement for producing a gaseous product from first and second source materials, comprising:

- an electrochemical cell (10) itself comprising: - a first volume exposed to respective surfaces of an anode (28) and a cathode (24),

- an enclosure (6) to trap a gaseous product produced within the first volume; and a structure (4) comprising a solid catalyst within the first volume, below the enclosure (6) such that a gaseous product evolved on a surface of the catalyst will be trapped by the enclosure (6), the catalyst having a relatively open structure such that fluids can pass through the catalyst structure in close proximity to the catalyst, whereby the catalyst is effectively suspended in the first volume;

- means (5, 22) for introducing a first source material into the first volume (1);

- means (2, 24) for introducing a second source material (26) into the first volume (1); and - an electrolyte (20) provided in the first volume.

11. An arrangement according to claim 10, further comprising:

- a power supply (7), arranged to apply a positive voltage +V to the anode (28), and to apply a negative voltage -V to the cathode (24 ) .

12. A method for production of a gaseous product by use of an arrangement according to claim 10, comprising the steps of:

- applying a positive voltage +V to the anode (28);

- applying a negative voltage -V to the cathode (24);

introducing first source material into the first volume

(l) ;

- introducing second source material into a second volume (2) exposed to a porous cathode, said second source material reacting at the cathode to provide a first ionic component in the electrolyte in the first volume (1);

generating the gaseous product by reaction between the first ionic component and the first source material.

13. A method according to claim 12, further comprising the step of collecting (6) the gaseous product produced in the first volume.

14. A method according to claim 12 or claim 13, further comprising the step of collecting a by-product generated at the anode .

15. A method according to any of claims 12-14, wherein:

- the first source material is steam ¾0;

- the first ionic component is nitride ions N^3~;

- the second source material is nitrogen 2; and

- the gaseous product is ammonia N¾ .