Spatial structure regulation towards armor-clad five-membered pyrroline nitroxides catholyte for long-life aqueous organic redox flow batteries

Five-membered pyrroline nitroxides with high-potential is fascinating as catholyte for aqueous organic redox flow batteries (AORFBs), however, it suffers from a primary deficiency of insufficient stability due to ring-opening side reaction. Herein we report a spatial structure regulation strategy by host-guest chemistry, encapsulating 3-carbamoyl-2,2,5,5-tetramethylpyrroline-1-oxyl (CPL) into hydrosoluble cyclodextrins (CDs) with an inclusion structure of N-O. head towards cavity bottom, to boost the solubility and cyclability of pyrroline nitroxides significantly. The armor-clad CPL (CPL subset of HP-beta-CD) catholyte in 0.05-0.5 M presents a battery capacity fade rate as low as 0.002 %/cycle (0.233 %/day) compared to the sole CPL in 0.05 M (0.039 %/cycle or 5.23 %/day) over 500 cycles in assembled AORFBs. The optimized reclining spatial structure with N-O. head towards CD cavity bottom effectively inhibits the attack of Lewis base species on the hydrogen abstraction site in pyrroline ring, and thus avoids the ring-opening side reaction of pyrroline nitroxides.