(Invited) Electrifying Nitrogen Transformations: Decarbonizing Ammonia Using Plasma Catalysis and Electrochemical Reduction of Nitrate in Wastewaters

Abstract

Nitrogen fixation in fertilizers forms the basis of modern agriculture and mediates global food insecurity. Energy-dense nitrogen-based compounds also represent promising carbon-free alternative fuels when synthesized using sustainable energy and materials. However, conventional thermally-activated nitrogen conversion processes consume substantial amounts of fossil fuels as materials and energy inputs, leading to an unsustainable energy and carbon footprint. Therefore, electron-driven approaches are needed to establish fossil-free nitrogen interconversions. Electrochemical and plasma-activated reactions operate under mild conditions, enabling facile coupling to intermittent renewable energy sources to reduce CO2 emissions and reactive nitrogen pollution, facilitating storage and transportation of renewable energy in nitrogen-based fuels.Non-thermal plasma has been used to synthesize ammonia under mild conditions, but the dearth of fundamental understanding of plasma catalytic reactions handicaps the development of efficient plasma catalytic N2 conversion. Therefore, an in situ FTIR reactor was employed in combination with steady-state flow reactor experiments and plasma kinetic modeling to elucidate the surface reaction mechanisms and plasma-catalyst interactions. Ammonia yield can be influenced by plasma-derived intermediates and their interactions with catalyst surfaces, which lead to different reaction pathways. Furthermore, techno-economic analysis was employed to reveal the threshold efficiency required for a plasma process to become environmentally and economically competitive.Beyond electrification of N2 fixation for ammonia synthesis, existing waste streams containing fixed nitrogen as nitrate may be mined for fertilizer or carbon-free renewable energy storage while avoiding the need for H2 as a reagent, via selective nitrate electroreduction to ammonium. Nitrate electrochemical reduction can eliminate the production of concentrated waste streams while avoiding the addition of reductant or hole scavenger chemicals. However, major challenges for nitrate removal from water via electrochemical conversion involve reducing the use of expensive precious metal electrocatalysts while also improving the reaction activity and selectivity, stability, and mass transport of nitrate to electrocatalyst active sites. Therefore, electrochemical membranes were employed as porous flow-through electrodes to address these challenges based on improved mass transport and altered kinetics under flow conditions within membrane pores.Conductive electrified membranes, fabricated using inexpensive materials and green synthesis methods, showed significantly higher nitrate removal efficiency during electrified filtration compared to flow-by mode. The small pore sizes in the membrane reduced the diffusional boundary layer by several orders of magnitude with respect to flow-by mode, overcoming diffusional mass transport limitations. During electro-filtration, the nitrate conversion activity was easily controlled by tuning the permeate flow rate, regardless of applied potential. These multifunctional electrified membranes for reduction of nitrate in wastewaters have the potential to partially displace carbon-intensive industrially synthesized ammonia while simultaneously accomplishing water decontamination.