Sustainable ammonia synthesis through electrochemical dinitrogen activation using an Ag2VO2PO4 catalyst

Abstract

Ammonia (NH₃) is the second most produced chemical commodity with a worldwide production of 235 million tonnes in 2019, by virtue of its importance in fertilizer production, energy storage and transportation, and in the production of industrial chemicals. The most frequent method of NH₃ production in large plants (1000 to 1500 t day⁻¹) is the Haber–Bosch process, which has drawbacks of high greenhouse gas emissions (2.16 tonnes CO₂ per tonne of NH₃) and high energy consumption (over 30 GJ per tonne of NH₃) due to high pressure and temperature conditions. For sustainable NH₃ production, we require alternative green routes, wherein the electrochemical route holds huge potential due to reduced energy consumption and plant costs, higher selectivity, lower temperatures and pressures, and small to medium scale utilization of NH₃. However, there are a number of challenges faced during the same viz. low production rates due to difficult N₂ activation and reduced faradaic efficiency due to competing side reactions in aqueous electrolytes. Therefore, the most crucial aspect of electrochemical ammonia production technology is the design of an electrocatalyst which could activate the strong NN triple bond and effectively suppress the competing hydrogen evolution reaction (HER). In addition, the true NH₃ yield estimation is of major concern due to the presence of possible N-contaminants, which may possibly lead to false estimation or overestimation of NH₃. In this context, we have synthesized an Ag₂VO₂PO₄ electrocatalyst with rice-grain like morphology via an energy efficient and less time consuming sonochemical method to carry out low temperature NH₃ synthesis in an alkaline electrolyte. The choice of Ag metal and an alkaline environment effectually suppresses the HER and the bimetallic phosphate materials (Ag and V metals) induce high activity during nitrogen reduction, while rigorous analysis for tracing/elimination of N-labile and reducible species is considered for true NH₃ production and assessment.