Electrodeposited Bismuth Catalyst for Nitrogen-Containing Compounds Electrochemical Reduction

Finding an alternative path to the highly polluting Haber-Bosh process for NH3 production is one of the major challenges of our century. N2 or NO3− electrochemical reduction reaction represents a greener alternative also able to decentralise NH3 production. The crucial step in the field is the synthesis of a catalyst able to make N2 triple bond or NO3− react with protons. Indeed, the competing hydrogen evolution reaction (HER) is favoured due to its lower activation energy. Theoretical calculations have shown that Bi can activate N-containing species to produce NH3 thanks to the interaction of the Bi 6p band with nitrogen 2p orbitals. In particular, bismuth nanocrystals obtained through solvothermal synthesis have reached 66% Faradaic efficiency (FE) and 200 mmol g-1 h-1 productivity in an H-cell type, using potassium sulfate as electrolyte. However, the attempt to replicate this outstanding material, slavishly following the synthesis procedure, did not produce the expected results. Having this in mind, our study aims to synthesize a Bi catalyst with a nano-dimensional structure through electrodeposition and compare it with a commercial metallic Bi. Electrochemical NO3− reduction reaction is carried out both in an H-cell configuration and in a less studied flow cell system, equipped with a gas diffusion electrode (GDE) on which the catalyst is either immobilised through air-brushing technique or in-situ electrodeposited. Flow cell configuration could allow the achievement of even greater NH3 production, due to higher mass transport of the reactive species on the catalyst layer compared to the H-cell. Regarding NO3− reduction in H-cell, electrodeposited Bi shows 9.5% FE and a production rate of 7.15 µmol h−1, while commercial Bi shows 3.26% FE and 2.62 µmol h−1 production rate at the same current density (16 mA cm−2). FESEM images show that electrodeposited Bi has a nanoflower-like structure that could explain the better catalytic activity compared to simple metallic Bi. Such preliminary results open the possibility of scaling up the system into a flow-cell GDE configuration, even if there are many challenges still to overcome, i.e. in-situ electrodeposition of Bi film with the same morphology as the small-scale electrode, long-term stability of the film, and reproducibility.