Electrochemical Nitrate Reduction to Ammonia in a PEM Cell: Maximizing Faradaic Efficiency and Single Pass Conversion
Authors: Bunea Sorin , Clemens Kevin , Urakawa Atsushi 
Published: July 2021

Polymer electrolyte membrane (PEM) electrolysis is considered a promising technology for the renewable hydrogen production from renewable energy, due to the high energy efficiency and robust design of the PEM cells.1 Nevertheless, despite their attractiveness, PEM cells did not find wide implementation for other electrochemical transformations than water splitting. The main reason is the low faradaic efficiencies of reduction reactions at the cathode of PEM cells, due to the competing hydrogen evolution reaction (HER), highly favorable in the acidic electrolyte environment of the PEM cell. We investigated the electrochemical nitrate reduction reaction at the cathode side of a PEM cell, aiming at reaching high faradaic efficiency and selectivity towards a single reaction product, and at the same time, high single pass nitrate conversion.

Nitrates represent one of the most prominent ground water pollutants, and significant efforts are targeted for their removal. Reductive abatement treatment typically yields nitrogen, which is environmentally benign but does not represent an economically attractive reaction product.2 Nitrates can be converted electrochemically to nitrogen (1), but also ammonia (2), which is an important building block in the chemical industry and the main pillar of fertilizer production.3-5 By enabling this transformation in the PEM cell, added-value can be added to an otherwise waste chemical, by making use of renewable electricity that can power PEM cells, which is highly relevant for a future circular economy.

2NO3
− + 12H+ + 10e− → N2 + 6H2O E° = 1.25 V (vs NHE) (1)

NO3
− + 10H+ + 8e− → NH4
+ + 3H2O E° = 0.875 V (vs NHE) (2)

We investigated four different catalysts for their activity, selectivity and faradaic efficiency towards ammonia formation: copper, palladium, rhodium and ruthenium. Of the four, we identified ruthenium as the most active and selective catalyst. We were able to limit the extent of the competing HER, by precisely tuning the supply of reactant and catalyst amount at the anode of the PEM cell. By this, we show the importance of matching the cathode and anode reaction conditions. This approach allowed us to reach 94% faradaic efficiency towards ammonia, one of the highest values reported to date. At the same time, we aimed at maximizing the single pass nitrate conversion in the cell. We found that mass transport limitations play an important role in achieving this goal. Nevertheless, by recirculating the reactant feed through the cell, we obtained 93% single pass nitrate conversion after 8 h of electrolysis at 10 mA/cm2 current density.

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Figure 1