Hydrogen-Bond Network-Activated O₂ in ChCl-Based Deep Eutectic Solvent Lowers the Overpotential of Oxygen Reduction...

The oxygen reduction reaction (ORR) constitutes a central process in electrochemical energy conversion technologies such as fuel cells and metal-air batteries, yet it is severely limited by sluggish kinetics and large overpotentials. Beyond catalyst design, electrolyte regulation plays a critical role in determining ORR activity. In this study, through a combination of kinetic measurements, spectroscopic analyses, and theoretical calculations, we demonstrate that ORR in the choline chloride (ChCl)-based deep eutectic solvent (DES) Ethaline is thermodynamically more favorable but kinetically more constrained than in ionic liquids. The introduction of O₂ is found to substantially perturb the intrinsic hydrogen-bond network of the DES, while the hydrogen-bonded environment in turn facilitates the activation of O₂ toward reduction. On this basis, we propose a novel "hydrogen-bond network-activated O₂" mechanism for ORR in DESs, which can be classified as a CEC pathway, wherein O₂ undergoes a solvent-induced activation step prior to electron transfer in electrolytes featuring complex hydrogen-bond networks such as Ethaline. This mechanism provides new insights into electrolyte regulation of ORR and opens new avenues for the design of efficient electrocatalytic systems.