H ₂ O ₂ Synthesis with Molecular Electrocatalysts Enables Substantial Current Densities in a Flow Cell Using Gas Diffusion Electrodes

The development of catalysts for the two-electron oxygen reduction reaction (ORR) toward H₂O₂ generation has gained substantial interest, as well as studying their activity in advanced electrochemical cell configurations. However, molecular ORR catalysts are seldom studied outside of the idealized environment of a rotating disk electrode (RDE) setup. Consequently, the catalytic currents reported for all molecular catalysts are limited by the mass transport of oxygen, and their properties in industrially relevant reactor configurations have remained unexplored. To assess their full potential, we report herein the application of a molecular, copper-based ORR catalyst, Cu-(tmpa) (tmpa = tris-(2-pyridylmethyl)-amine), in a cell configuration with gas diffusion electrodes (GDEs) to enhance mass transfer and thereby catalytic currents. We have identified the factors that control the catalytic current, such as buffer and catalyst concentration and GDE composition, using a readily assembled GDE cell, and we demonstrate that Cu-(tmpa) can generate H₂O₂ over multiple hours in a GDE flow cell. Under these conditions, a Faradaic efficiency of 50% can be maintained and H₂O₂ can be generated at a rate of 0.11 mmol cm^-2 h^-1 at a current density of -20 mA/cm². The current density is increased by almost 10 times in a GDE configuration compared to a conventional RDE setup, and the selectivity trends of the GDE system differ from the results obtained in an RDE setup, as a higher current density generates a higher selectivity toward H₂O₂ in a GDE setup. Overall, this work shows that the full potential of molecular catalysts can be assessed upon their implementation in a GDE setup and that their well-defined active sites can contribute to the emerging field of H₂O₂-generating catalysts.