(446e) Proton Control in Electrochemical Ammonia Synthesis

Electrochemical ammonia synthesis offers a pathway to electrify the production of ammonia, one of the most widely produced industrial chemicals. However, unlike the longstanding Haber-Bosch process, the heterogeneous electrochemical pathway (N₂ + 6H⁺ + 6e^- → 2NH₃) is in its infancy. Achieving a high faradaic efficiency to ammonia by minimizing the competing hydrogen evolution reaction (HER) is still a major challenge. While a number of potential catalytic systems exist in the literature, faradaic efficiencies over 10% are rarely observed. In this talk, we discuss the use of non-aqueous electrolytes as a means to engineer the relative accessibility of nitrogen and protons to the surface in order to increase the faradaic efficiency to ammonia. First, we use the results of density functional theory to frame the problem. Essentially, any material that is able to readily activate dinitrogen will require very reducing conditions to turn the reaction over. At these potentials, HER proceeds rapidly on most surfaces. While it may be possible to find a material that overcomes these problems, another possibility is to engineer the electrolyte to favor nitrogen reduction by reducing the concentration of protons, allowing nitrogen to win a kinetic battle for surface coverage. We demonstrate, through a rigorous protocol and series of control experiments including ¹⁵N isotopic labelling that this can be achieved using an organic, aprotic solvent with alcohols added as proton donors. Electrochemical studies of the electrolyte show that the HER is indeed slowed, although the exact mechanism of ammonia production is uncertain. Electrolyte engineering is a promising path forward as it will be complementary to further developments in ammonia synthesis catalysts.