Electrocatalytic nitrogen reduction in continuous-flow cell via water oxidation at ambient conditions: Promising for ammonia or diazene?

• Electrocatalytic N 2 reduction in continuous-flow cells via water oxidation. • Hydrogenation reaction occurs at the N 2 -supplied cathode, producing diazene (N 2 H 2 ). • Products are in situ detected via an online mass spectrometer. • Control experiments by supplying Ar to the cathode verified genuine N 2 reduction. • Isotopic labeling experiments (D 2 O and 15 N 2 ) confirmed N 2 H 2 production. Electrochemical nitrogen reduction reaction (eNRR) is recognized as an alternative green approach to the traditional energy-demanding and fossil-based catalytic processes (e.g. Haber Bosch). In this study, we implement eNRR in a proton exchange membrane (PEM) water electrolyzer in which nitrogen (N 2 ) is fed in the cathode. This operation mode has been suggested as a way to overcome mass transfer limitations, however, there is a lack of developed evaluation protocols for appropriate product identification. Herein, we exemplify the spirit of the evaluation protocols for gas phase operation at the device level with a combination of online product analysis and isotopic labeling. Our protocol involves control experiments by replacing the cathodic N 2 feed with (i) inert gas (i.e. Ar) and (ii) isotopic labeled 15 N 2 and by replacing the anodic water feed with isotopic labeled D 2 O. Taking advantage of the gas phase operation in the cathode product analysis is realized with online techniques i.e. quadrupole mass-spectrometer (QMS) and Fourier transform infrared (FTIR) spectrometer. This allows us to verify the production of diazene (N 2 H 2 ) resulted from genuine N 2 reduction, rather than from nitrogen-containing contaminants. Our methodology provides a pathway for how the false positive results can be eliminated in the gas phase study and a platform for follow-up studies using promising or exotic catalysts in the cathode, especially to validate the eNRR products or discover more products.