Na and Ti share roles

Abstract

Synthesizing ammonia electrochemically from N₂ could help to decarbonize production of this important chemical and potential energy carrier. In recent years, a Li-mediated process has been shown to be effective for ammonia synthesis. In this approach, it is generally believed that when a voltage is applied, Li(I) is reduced to Li metal at the cathode, reacting with N₂ to form nitrides; the nitrides react with a proton source in the electrolyte, making ammonia and Li(I) salts, which, in principle, allows the process to start again. A downside of this approach is that Li — being the species that is electrochemically reduced and that binds and reduces N₂ — must play multiple roles, making optimization challenging. Now, Karthish Manthiram and colleagues at the California Institute of Technology report a cascade system comprising two different metals — Na and Ti — which share the required roles to electrochemically produce ammonia.

The team use an electrolyte comprising a Na(I) salt, naphthalene and Ti(IV) tetraisopropoxide in an ethereal solvent. The researchers propose that, in the key electrochemical step, Na metal plates out on the cathode before reacting with naphthalene to form Na(I) naphthalenide, which then reduces the Ti(IV) species to Ti(II). This Ti(II) species is thought to act as the site for N₂ binding and reduction to ammonia. Manthiram and colleagues report reaction rates up to 475 nmol cm^–2 s^–1 and a Faradaic efficiency of 24%, which are competitive with other, Li-based, systems. In addition to the increased scope for optimization due to the separation of roles across different components, the approach also has the benefit of not relying on Li, which is in increasing demand.