Bioelectrochemical Nitrogen Fixation Towards Sustainable Hydroponics

Abstract

Ammonia plays a crucial role as a nitrogen source in agriculture; however, its traditional production method through the Haber-Bosch process is highly energy-intensive and highly carbon-intensive. By combining renewable-powered electrocatalysis with biological nitrogen fixation pathways, bioelectrochemical systems offer promising alternatives to the Haber-Bosch process. In this study, we developed a bioelectrochemical system that combines a nickel-molybdenum (NiMo) based cathode for efficient hydrogen evolution reaction (HER) with the hydrogen-oxidizing diazotroph Xanthobacter autotrophicus (X. autotrophicus), enabling ammonia production under ambient conditions. The NiMo@carbon rod electrode exhibited excellent electrocatalytic activity, reaching a current density of 10 mA cm-2 at an overpotential of –97 mV. During 7-days of stable operation at a constant current of 10 mA, the system produced ammonia with a maximum extracellular concentration of 13 ± 1.0 ppm upon inhibition of glutamine synthetase, demonstrating effective nitrogen fixation. The resulting bioelectrolyte was directly applied in hydroponic seedling cultivation. Seedlings treated with the bioelectrolyte exhibited enhanced shoot elongation (~160 mm after 5 days), surpassing the growth observed in synthetic electrolyte and DI water controls. In our benchtop assay, sterile ammonium-containing electrolyte enhanced shoot elongation of mung bean seedlings. We further validated the approach in a 3 L cultivation system using bacteria-containing bioelectrolyte and in a 27 L recirculating prototype with leafy vegetables. This work establishes new opportunities to develop sustainable, scalable, and decentralized alternatives bioelectrochemical technologies for nitrogen fixation for agriculture in hydroponic systems.