Electrochemical Ammonia Synthesis: The Energy Efficiency Challenge
Yuanyuan Zhou, Xianbiao Fu, Ib Chorkendorff, Jens K. Nørskov
Abstract
High Resolution Image Download MS PowerPoint Slide 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 N 2 reduction electrocatalysts at ambient conditions is that the thermal elemental processes, N 2 dissociation and NH 3 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 N 2 at these conditions but they all bind NH 3 too strongly. Strategies to circumvent this problem are discussed, as are the other requirements for a good N 2 reduction electrocatalyst.