A New Class of Carbon Catalysts for Nitrogen Reduction during NH3 Electrosynthesis

Abstract

Developing catalyst to facilitate ammonia synthesis was so challenging that it gave Haber and Bosch the Noble prize in 1918. However, the reaction is very energy intensive involving high temperature, high pressure and use of conventional fossil fuels. Sustainable synthesis of ammonia by renewably reducing nitrogen at ambient conditions without using hydrogen as a reactant is an ever growing field of research now; However in presence of water, when a negative potential is applied to reduce nitrogen, hydrogen evolution is favored. Thus, like CO2 reduction, increasing the selectivity of NRR over HER is a major challenge. We focused on identifying and tuning surface of adsorption (electrocatalyst), environment (electrolyte) to minimize HER as much as possible. Here we present a new type of carbon catalysts with Fe doping, which are known to containing FeN4 sites. We observed that higher the metal doping, lower is difference in current density in N2 and Ar atmosphere (dominant HER). Surprisingly, the ammonia detected was higher for very low metal doping. This stimulated us to study the effect of metal doping content on NRR activity and ammonia production rates. Here we report an ammonia production rate of 3.1 x 10-6 mol cm-2hr-1 at -0.3 V vs RHE with new carbon catalysts (and a FE of 30 % using an alkaline electrolyte 0.1 M KOH under ambient conditions. We also tested our catalyst in acidic electrolyte where the FE reduced due to increased HER in a higher concentrated proton environment.