In situ neutron reflectometry reveals the interfacial microenvironment driving electrochemical ammonia synthesis

Electrified interfaces are critical to the performance of energy systems and often demonstrate substantial complexity under operating conditions. Nanoscale understanding of the interfacial microenvironment, i.e., the solid electrolyte interphase (SEI), in lithium-mediated nitrogen reduction (Li-N2R) is key for realizing efficient ammonia production. Using in situ neutron reflectometry, we found the Li-N2R SEI comprises a thick, diffuse outer layer and a thin, compact inner layer at low current cycling. Increasing current density resulted in a thinner outer layer with a thicker inner layer; sustained current led to LiH formation. Neutron absorption indicated boron uptake in the SEI. Time-resolved tracking of SEI growth with isotope contrasting revealed the proton donor modifies the inner layer, and the solvent modifies the outer layer. Li dendritic growth was observed in the absence of a proton donor. Our results inform Li-based systems and reaction microenvironments, and these methods can be applied broadly to interfacial energy technologies.