Low-Temperature Electrochemical Ammonia Synthesis with Nanoscale Fe-Ni Bimetallic Catalysts

The ammonia production market in the U.S. is $20 billion in size, with ammonia being the primary chemical input to agricultural fertilizer.  Currently, ammonia synthesis occurs through the Haber-Bosch process, which uses a bulk iron catalyst to reduce nitrogen and hydrogen gas at high temperature and pressure.  The source of hydrogen primarily comes from steam reforming of natural gas, where methane combines with water to produce hydrogen, as well as carbon monoxide and carbon dioxide.  As a result, the ammonia production industry is responsible for approximately 320 million tons of greenhouse gas emissions per year, putting the ammonia fertilizer industry as the second-largest contributor of greenhouse gas emissions in the U.S.  While the invention of the Haber-Bosch process has been the primary reason for agricultural expansion and prosperity over the past 100 years, particularly in developed and industrialized nations, there remains an enormous opportunity to improve upon the Haber-Bosch process both in terms of the energy used and the greenhouse gases produced.

Our work has focused on the development of a non-precious bimetallic FeNi nanoparticle material for the alkaline electrochemical reduction of nitrogen to ammonia.  FeNi nanoparticles are synthesized using an aqueous-based solution synthesis technique, where first iron core nanoparticles are reduced from an argon-bubbled solution of ferrous sulfate heptahydrate and phosphonate ligand stabilizer.  The nickel shell is then added through electroless deposition when a solution of nickel chloride hexahydrate and polyvinylpyrrolidone is added.  Through the difference in the standard reduction potentials of Fe²⁺ and Ni²⁺ (E for Fe²⁺ = -0.44 V and E for Ni²⁺= -0.25 V), electrons are transferred from Fe in the core nanoparticle to nickel in solution at the surface of the nanoparticles, and nickel plates down onto the iron nanoparticle surface.  The size and morphology of the FeNi nanoparticles are varied by changing the molar ratio of phosphonate stabilizer to iron and the molar ratio of nickel to iron.  Alloy versions of the nanoparticles, made through a one-pot synthesis technique are used as comparisons to the core-shell morphology.

Ammonia synthesis is evaluated through a series of potentiostatic experiments, where the effect of the applied potential on ammonia production rate is investigated.  A gas-tight electrochemical cell is used for all experiments and an in-line acidic acid trap is used to capture offgased ammonia.  Due to the alkaline electrolyte and high pH environment, all ammonia is expected to be in the NH₃ form of the NH₃/NH₄⁺ pair, and constant nitrogen bubbling during electrochemistry experiments causes the ammonia to flow out of the electrochemical cell and become dissolved as NH₄⁺in the aqueous acid trap.  Ammonia concentrations are quantified using a commercially-available assay, and mass concentrations are correlated to the current measured over time to determine ammonia production and efficiency as a function of applied potential. 

In this talk, nanoparticle synthesis and characterization will be discussed, as well as electrochemical and ammonia production results for specific FeNi nanoparticle catalysts.