Electrolyte acidification from anode reactions during lithium mediated ammonia synthesis

Li-mediated electrochemical ammonia synthesis (LiMEAS), a potential alternative to conventional thermochemical synthesis, is enabled by non-aqueous electrolytes with precisely controlled proton activity. However, the effects of proton generating anode reactions, such as hydrogen or electrolyte oxidation, is unknown but crucially important for enabling a steady-state LiMEAS without the need of sacrificial proton sources. By employing cyclic voltammetry on a platinum electrode, we demonstrate that protons are generated not only by hydrogen oxidation but also electrolyte oxidation, which has the consequence of a continuous acidification of the electrolyte over the course of a LiMEAS experiment. In addition, the cyclic voltammograms reveal that the generated protons show reactivity towards electrolyte components, which would disrupt the replenishment of the added proton source. We therefore suggest to design new electrolytes that also contain proton acceptors, ultimately resulting in a buffered electrolyte with a stable proton activity.

Automated summary

Li-mediated electrochemical ammonia synthesis (LiMEAS) is a potential alternative to conventional thermochemical synthesis. By conducting experiments on a platinum electrode, it is found that protons are generated not only by hydrogen oxidation but also by electrolyte oxidation, resulting in continuous acidification of the electrolyte. The generated protons also react with electrolyte components, hindering the replenishment of the proton source. It is suggested to design new electrolytes that contain proton acceptors to achieve a stable buffered electrolyte with proton activity.