Electrochemical Synthesis of Ammonia Using Origami-like Molybdenum Carbide Nanoflakes As Nitrogen Reduction Reaction Catalyst

Abstract

Ammonia plays a crucial role in feeding the majority of today’s population and its production has increased from a few thousand tons per year in 1908 to above 100 million tons per year today, revolutionizing the fertilizer industry thanks to the Haber-Bosch (HB) process. However, the HB process is highly energy intensive consuming about 1.4 % of the fossil energy generated worldwide and releasing 1.87 tons of the greenhouse gas CO2 into the environment per ton of ammonia synthesized. Hence, finding alternative and energy efficient ways to synthesize ammonia under milder conditions is important from more than one perspective. The challenge associated with NH3 synthesis from its constituent nitrogen and hydrogen gases is breaking the strong N≡N bond (947 kJmol-1). While heterogeneous catalytic synthesis needs high temperature and pressure operation, electrochemical synthesis routes offer a milder approach. However, electrochemical synthesis of ammonia under different temperatures, utilizing different electrolytes and catalysts has not yet reliably produced ammonia at viable rates and efficiencies. We report a Mo2C cathode catalyst for ammonia synthesis that has demonstrated synthesis rates on the order of 2x10-12 molcm-2s-1 for more than 24 hours at 30 °C using Nafion 212 as the electrolyte and Pt/C as the anode. The β-Mo2C has a hexagonal lattice structure with an origami-like morphology containing numerous kinks which may improve its electrocatalytic activity. The maximum synthesis rate was obtained at -0.5 V, however, the faradaic efficiency decreased with increasing over-potential. This catalyst offers a promising alternative for producing ammonia especially with electrolytes that conduct protons under dry conditions. This presentation further covers the utility of Nafion as an electrolyte for electrochemical ammonia synthesis.