Role of Protons in Electrochemical Ammonia Synthesis Using Solid-State Electrolytes

Abstract

Electrochemical methods of synthesizing ammonia from nitrogen gas have the potential to replace the energy intensive Haber-Bosch process. In doing so, they offer a CO2-free route to the production of the ever-promising energy carrier. In this study, an effort was made to reveal the relationship between proton involvement in the rate-limiting step of the ammonia synthesis reaction and the overall ammonia synthesis rate, particularly for electrolytic cells using solid-state electrolytes, as no such rule based on the measured parameters of the materials has ever been reported. An empirical atomistic expression was derived to explain the observed correlation between the proton conductivity of the solid-state electrolyte and the ammonia formation rate, by considering the proton excorporation and migration enthalpies. This relationship was determined by examining experimental results from the literature that had been obtained using diverse proton-conducting electrolytes. An almost linear energy relationship was demonstrated for state-of-the-art heterogeneous electrocatalysis.