Electrochemical Dinitrogen Fixation

Materials that catalyze efficient electrochemical N2 fixation would open up the possibility of using an N2/NH3 cycle to store and utilize energy from diffuse renewable sources. In such a cycle, N2 and H2O would be electrolytically transformed to NH3 and O2 in an electrolyzer powered by a renewable electricity source. With existing technologies, NH3 can be utilized in solid oxide fuel cells or combustion engines to generate electricity or work and regenerate N2 and H2O. In contrast to H2, which has received the most attention for fuel-based renewable energy storage, NH3 is easy to store in solid form by complexation with earth-abundant salts. Despite the attraction of an N2/NH3 cycle, to date no suitable electrocatalysts have been developed for this reaction. Moreover, there is no reliable dataset available that characterizes the activities of common electrode materials under any set of conditions. A principal obstacle to studying electrochemical N2 fixation and progressing towards a useful catalyst has been the lack of a sensitive and rapid method for NH3 quantification. Conventional spectrophotometric methods are cumbersome and prone to false positives from the presence of metal ions or other sample impurities. We have addressed this analytical problem by developing a device that quantifies NH3 liberated from an aqueous solution. This device accurately quantifies NH3 content in solutions with as little as a few hundred ppb of NH3 and reaches a saturation response within minutes. Because NH3 is removed from a solution prior to detection, this method is completely immune to interference from metal ions or other electrolyte impurities that impair spectrophotometric methods. Using this unique analytical tool, we have begun determining the N2 reduction activity of a large collection of metal electrodes in both acidic and alkaline electrolytes. We have also constructed electrolysis cells suitable for evaluating catalysts in gas diffusion electrodes that provide much higher concentrations of N2 at the catalyst surface. The information available from these studies will provide the foundation for the subsequent discovery and development of catalysts that are suitable for use in an electrolytic device.