Electrochemical Reduction of Dinitrogen to Ammonia Using Different Morphologies of Copper As Electro Catalysts

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Abstract

Ammonia is an effective hydrogen storage medium due to ease of transport as liquid, high storage capacity (17.65%) and it can easily be converted to hydrogen by electro-chemical oxidation. Haber-Bosch process is used for the synthesis of ammonia which is energy intensive as it requires high temperature and pressure. It also causes intense carbon emissions as the hydrogen is produced by steam reforming. Alternatively, ammonia can be synthesized electrochemically at ambient conditions from nitrogen and water by employing renewable energy in the presence of an electro catalyst.

The major challenge in electrochemical synthesis of ammonia is low Faradaic efficiency. This issue can be overcome by efficient design of the electrochemical cell and the appropriate choice of the catalyst. The competing reaction along with the nitrogen reduction reaction (N2RR) is the hydrogen evolution reaction (HER) which suppresses the efficiency of the ammonia. So, measures have to be taken to suppress the HER thereby improving the N2RR. Alkaline medium is chosen for the study than acidic medium as acidic medium favors HER more than the alkaline medium and also the transition metal catalyst considered for the study is not stable in acidic medium.

Copper suppresses HER than most other transition metals and, in this study, different morphologies of copper are used as the electrocatalyst. Bulk copper, copper nanoparticles and copper nanowires are electrodeposited on the carbon paper which is the gas diffusion electrode. Platinum is used as the counter electrode and the resulting ammonia synthesis reaction is carried out in the electrochemical cell which is custom designed, and 3D printed. The ammonia formed is quantified using UV-Visible Spectroscopy by adding two separate reagents namely Nessler’s reagent and Indo-Phenol blue. The Faradaic efficiency and the production rate of ammonia and hydrogen is found for the different configurations of the copper. A high Faradaic efficiency greater than 10 % is observed when copper nanoparticles are used as the electro catalyst.

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