Water flooding behavior in flow cells for ammonia production via electrocatalytic nitrogen reduction.

The green production of ammonia, in an electrochemical flow cell under ambient conditions, is a promising way to replace the energy-intensive Haber-Bosch process. In the operation of this flow cell with an alkaline electrolyte, water is produced at the anode but also required as an essential reactant at the cathode for nitrogen reduction. Hence, water from the anode is expected to diffuse through the membrane to the cathode to compensate for the water needed for nitrogen reduction. Excessive water permeation, however, tends to increase the possibility of water flooding, which would not only create a large barrier for nitrogen delivery and availability, but also lead to severe hydrogen evolution as side reaction, and thus significantly lower the ammonia production rate and Faradaic efficiency. In this work, the water flooding phenomenon in flow cells for ammonia production via electrocatalytic nitrogen reduction is verified via the visualization approach and the electrochemical cell performance. In addition, the effects of the nitrogen flow rate, applied current density, and membrane thickness on the water crossover flux and ammonia production rate are comprehensively studied. The underlying mechanism of water transport through the membrane, including diffusion and electro-osmotic drag, is precisely examined and specified to provide more insight on water flooding behavior in the flow cell.