Electrochemical Ammonia Synthesis: The Energy Efficiency Challenge

We discuss the challenges associated with achieving high energy efficiency in electrochemical ammonia synthesis at near-ambient conditions. The current Li-mediated process has a theoretical maximum energy efficiency of ∼28%, since Li deposition gives rise to a very large effective overpotential. As a starting point toward finding electrocatalysts with lower effective overpotentials, we show that one reason why Li and alkaline earth metals work as N₂ reduction electrocatalysts at ambient conditions is that the thermal elemental processes, N₂ dissociation and NH₃ desorption, are both facile at room temperature for these metals. Many transition metals, which have less negative reduction potentials and thus lower effective overpotentials, can dissociate N₂ at these conditions but they all bind NH₃ too strongly. Strategies to circumvent this problem are discussed, as are the other requirements for a good N₂ reduction electrocatalyst.