(55e) Plasma-Activated Electrochemical Reduction of Nitrogen to Ammonia

Ammonia is key to nitrogen fertilizer production, which is essential for meeting the agricultural needs of the growing population. It is primarily produced via the Haber Bosch process, which operates at high temperatures and pressures in centralized plants that require continuous hydrogen supply. Thus, the industry accounts for 1-2% of global energy demand and contributes 1% of global carbon dioxide emissions. Electrochemical methods offer promising ways to produce ammonia under ambient conditions and in turnkey, modular reactors that can be employed on-site and are compatible with renewable energy. This approach could also allow ammonia to be used as a carbon-free fuel for energy storage. However, making ammonia electrochemically from N₂ has been limited by the high energy barrier required to activate nitrogen and the competitive hydrogen evolution reaction. Harsh and unsustainable electrolytes have previously been employed to overcome these obstacles.

Nonthermal plasma offers a potential way to address the challenges of electrocatalytic reduction in aqueous electrolytes by pre-exciting the N₂, which lowers the energy barrier for subsequent conversion. To ensure that the unstable species are not quenched in the electrolyte, the excited N₂ is sent through a gas diffusion electrode to the catalyst surface. Electrolyte is circulated in a flow cell reactor, which enables much higher current densities than typical H-cell assemblies. In this custom reactor, we found that combining plasma and electrocatalysis significantly increases the production rate of ammonia, with higher plasma power and electrochemical potential promoting ammonia generation. Different metal catalysts also influence generation rate. Control experiments, in situ analysis, and computation modelling offer further insight into the reaction mechanisms. Overall, the combined plasma-electrochemical process overcomes traditional limitations of electrocatalyst thermodynamic/kinetic tradeoffs, slow mass transfer, and quenching of excited plasma species in aqueous electrolytes.