As Precious as Platinum: Iron Nitride for Electrocatalytic Oxidation of Liquid Ammonia

The electrolysis of ammonia (NH₃), a potential carrier for hydrogen fuel, has only been studied in detail in systems employing expensive, noble metal anodes such as platinum, ruthenium, and iridium. For NH₃ to serve as a practical hydrogen storage medium, the electrolysis process must be energy efficient, scalable, and inexpensive. Clearly, alternatives to precious metals would greatly reduce costs if the performance of less expensive, more abundant metals rivaled those of their expensive counterparts. In this regard, no metal is less expensive than iron. Iron exhibits complex anodic behavior in liquid ammonia (NH₃(l)), with a high sensitivity to trace amounts of dissolved water, and a tendency to corrosively dissolve with appropriate applied bias. However, with sufficient applied overpotential in distilled NH₃(l), an iron nitride film forms in situ that is resistant to dissolution. On this in situ-modified surface, dinitrogen evolution out-performs anodic dissolution with an efficiency of over 95%. Amazingly, the onset potential for dinitrogen evolution in NH₃(l) on this in situ-modified iron surface is almost identical to what is measured on a platinum electrode.