Electrochemical Ammonia Synthesis on Bimetallic Catalysts via Proton Adsorption Controlled Nitrogen Reduction Reaction

Abstract

Ammonia (NH3) plays a crucial role as a feedstock for the production of fertilizers and chemicals. Recently, NH3 has also been considered as an efficient hydrogen energy carrier because of its high volumetric density of hydrogen and zero carbon content in the chemical structure. The Haber-Bosch process is the most widely used industrial NH3 production method, which converts nitrogen and hydrogen to NH3 at high temperatures around 500 °C and at high pressures up to 200 bar. More serious is that the Haber-Bosch process releases a large amount of carbon dioxide owing to the hydrogen feedstock out of natural gas. As an alternative route for green NH3 production, the electrochemical nitrogen reduction reaction (NRR) under ambient conditions has attracted considerable attention in research area. Such electrochemical reaction involves water adsorption and dissociation, providing proton source for the formation of NH3 instead of hydrogen gas, which offers a promising carbon-neutral strategy. However, the practical manufacture of NH3 using the electrochemical method remains a low selectivity challenge of NRR over a competing hydrogen evolution reaction (HER) which is stem from the favorable adsorption of water on most types of electrocatalysts. In this dissertation, we have developed bimetallic electrocatalysts for achieving the selective NRR towards NH3 production. First, we design a Rh-Mo oxide with strong chemical interaction with nitrogen for efficient NRR in a neutral pH electrolyte. We have also developed Co-Mo carbide-embedded carbon nanofiber composites for NRR. Such material exhibits an improved electrocatalytic performance owing to the assisted protonation step of NRR intermediates with appropriate amount of water from the electrolyte and also proton source provided from the HER on Co. The correlation between the adsorption of proton source and the reaction selectivity has been confirmed by in situ ATR-SEIRAS and isotopic labeling experiments using 15N2 and D2O. This study demonstrates the importance of designing of electrocatalysts and optimizing the operating conditions in achieving a high performance of electrochemical NH3 production.