Spontaneous formation of reactive redox radical species at the interface of gas diffusion electrode.

The aqueous interface-rich system has been proposed to act as a trigger and a reservoir for reactive radicals, playing a crucial role in chemical reactions. Although much is known about the redox reactivity of water microdroplets at "droplets-in-gas" interfaces, it remains poorly understood for "bubbles-in-water" interfaces that are created by feeding gas through the porous membrane of the gas diffusion electrode. Here we reveal the spontaneous generation of highly reactive redox radical species detected by using electron paramagnetic resonance under such conditions without applying any bias and loading any catalysts. In combination with ultraviolet-visible spectroscopy, the redox feature has been further verified through several probe molecules. Unexpectedly, introducing crown ether allows to isolate and stabilize both water radical cations and hydrated electrons thus substantially increasing redox reactivity. Our finding suggests a reactive microenvironment at the interface of the gas diffusion electrode owing to the coexistence of oxidative and reductive species.