ELECTROCHEMICAL CELL AND PROCESS FOR SYNTHESIS OF AMMONIA

The present invention relates to electrochemical cells, particularly electrochemical cells for synthesis of ammonia 5 N¾ . The present invention also relates to processes for synthesis of ammonia N¾.

Known approaches to the requirement for synthesis of ammonia include: 10 (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 15 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. 334-20 335 (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 25 [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/j cat .2000.2877

[5] Lan, R. , Irvine, J. T . S . , and Tao, S. (2013) . Synthesis of 30 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. The present invention seeks to provide alternative methods and apparatus for the synthesis of ammonia from water and nitrogen N₂. 5

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

The above, and further, objects, characteristics and 10 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 15 provided by 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.

20

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

25 wide distribution of inlet steam.

The second volume 2 is a cathode gas electrode. Nitrogen gas N2 26 is introduced into this gas electrode, on a surface of the porous electrode 24 away from the nitride conductor 20. 30

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 5 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 N³-0

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

5 (water) to produce ammonia:

2 N³- + 3 H₂0 => 2 NH₃ + 3 O²-

Ammonia is accordingly produced from nitrogen gas and steam.0 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

5 anode reaction returns electrons to the DC power supply and generates oxygen into the gas anode electrode:

2 O²- => 02 + 4 e-. 0 According to a feature of the present invention, an electrically insulating porous material 4, preferably an aerogel material, is placed between the porous anode 28 and the nitride conductor 20. The electrically insulating porous material may be bonded to the porous anode 28 or may simply be immersed in the nitride conductor. Oxide ions 0 - diffuse through the electrically insulating porous material 4 to the anode 28 under the influence of the potential gradient between the anode 28 and the cathode 24. Water molecules do 5 not tend to diffuse through the electrically insulating porous material due to their neutral charge and therefore lack of driving force under the influence of the potential gradient between the anode 28 and the cathode 24. 0 The electrically insulating porous material 4 shields the porous anode from the water molecules (steam) within the nitride conductor. In the absence of such an electrically insulating porous material shield, the following unwanted reduction would take place at the anode and would contaminate

5 the evolved ammonia N¾ with hydrogen ¾ as well as providing a competing side reaction thereby reducing the efficiency:

3 H₂0 ^Λ 3/2 0₂ + 6 H⁺ + 6 e- 0 Ammonia gas trapped in enclosure 6 is 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

5 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 to0 buy and store hydrogen ¾, resulting in a much simpler system design .

A particular feature of the present invention is the electrically insulating porous material 4 which acts as an electrode protector, preventing steam from reaching the anode. The electrically insulating porous material should be an electrically insulating structure which allows the passage of oxide ions 0^2~ under the influence of the applied 5 potential gradient, but not the diffusion of water molecules ¾0.

The electrically insulating porous material may be a sol-gel derived silica aerogel. The texture and porosity of a such an aerogel may be controlled to provide desired properties.

10 The resulting silica can exist as a relatively dense, microporous xerogel or it can be synthesized as a non-dense, mesoporous aerogel. The three dimensional structure and porosity properties of silica aerogel impart certain advantages in the present application. Surface area can be

15 high (>800 m²/g), average pore size is in the mesoporous regime (2-50 nm) , and the pores exist as an interconnected network.

While the present invention has been described with 20 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. 25

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 30 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 5 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 10 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) ·

15 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.

20 Although the anode is described as a gas electrode arranged for collection of a gaseous by-product, such arrangement may not ne 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

25 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. 30