Electrocatalytic ammonia synthesis

Concluding from previous works, the key factor for electrocatalytic ammonia synthesis could be the number of TPB sites at the cathode to facilitate the reaction between protons from the electrolyte, N2 from gas phase, and electrons from external power source. Therefore, the big challenge on the electrochemical synthesis of ammonia is the low formation rates thus more efficient catalysts need to be identified. 

It is well known that, in general, BaCeOs oxides have higher protonic conductivity than SrCe03-based oxides. The increase of protonic conductivity in membrane material would be beneficial. A large number of cells with a barium cerate membrane were used in electrocatalytic ammonia synthesis. The results showed that there is no significant improvement between cells with SrCe03-based or BaCeOa.based oxides. It can be concluded that materials with protonic conductivity of 10 —10 S/cm would be adequate for ammonia synthesis. 

Typical operating temperatures of electrocatalytic ammonia synthesis devices via a ceU with a proton-conducting solid oxide electrolyte are around 400—700 °C corresponding to the working temperature of the proton conductor electrolyte. A volcano-shape dependence of ammonia formation rates on temperature is observed in most cases. The formation rate of ammonia increases with temperature until a maximum is reached due to the increase of the proton conductivity. When the temperature is further increased, the ammonia formalion rate then turns downhill, because the decomposition of ammonia becomes prominent. In addition, the water loss from the electrolyte membrane due to high temperatures may decrease proton conductivity of the electrolyte. 

Electrocatalytic ammonia synthesis at normal pressure 10.2.3.1 Electrocatalytic reaction and electrocatalyst... 

Hasnat M A, Karim M R and Machida M (2009), Electrocatalytic ammonia synthesis Role of cathode materials and reactor configuration , Catal Common, 10,1975-1979. 

Catalysts play an important role in overcoming the activation barrier in ammonia synthesis. It is weU known that strong N=N triple bond and the low sticking coefficient of the molecule nitrogen limit the choice of catalyst. However, the mechanism of ammonia formation on an electrocatalyst seems to be different from that of the conventional catalyst. The information about the conventional catalyst in the Haber-Bosch process and the electrocatalyst in the electrocatalytic membrane reactor are described in this section. 

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