Preliminary tests of electrochemical nitrogen conversion into ammonia in aqueous electrolyte

The electrochemical route for ammonia production using nitrogen and water as feedstocks represents an environmental-friendly alternative to the highly polluting Haber-Bosh process. The biggest challenge of nitrogen reduction reaction (NRR) in aqueous electrolytes is the optimization of the system to reduce the selectivity of the process towards hydrogen evolution reaction (HER). As well as catalyst and electrolyte design, the optimization of the solid-liquid-gas interface can play a fundamental role in the increase of Faradaic efficiency (FE) and yield to ammonia. Such an effect can be obtained in a flow cell in which the gas phase transportation to the catalyst layer is guaranteed and facilitated by a gas diffusion electrode (GDE). Regarding catalyst selection, metallic bismuth stands out both for its promising theoretical ability to stabilize the adsorbate intermediates of ammonia production reaction and for its low activity towards HER. Recent experiments with that catalyst have reached 66% FE and 200 mmol g-1 h-1 yield in an H-cell type, using potassium sulfate as electrolyte. Having this in mind, our study aims at replicating that bismuth synthesis and to test its performances in a less studied flow cell system, equipped with a GDE on which the catalyst is immobilized through air-brushing technique. Since the preliminary experiments results were not in agreement with the literature data, a commercial molybdenum disulfide has been tested in the same cell, under similar conditions. Promising results have been obtained by replacing potassium sulfate electrolyte with lithium perchlorate, which can promote molybdenum disulfide NRR activity thanks to the modification of the crystalline structure after Li+ ions intercalation. This interplay between catalyst and electrolyte made it possible to obtain around 5% FE and 85 μmol g-1 h-1 yield at -0.6 V vs. RHE. This study has still room for improvement by optimizing catalyst ink formulation and loading onto the electrode, the electrolyte concentration and pH, the potential applied, and the gas and liquid flow rate.