Gaseous Nitrogen Oxides Catholyte for Rechargeable Redox Flow Batteries.

There is a strong interest in finding highly soluble redox compounds to improve the energy density of redox flow batteries (RFBs). However, the performance of electrolytes is often negatively influenced by high solute concentration. Herein, we designed a high-potential (0.5 V vs. Ag/Ag+ ) catholyte for RFBs, where the charged and discharged species are both gaseous nitrogen oxides (NOx ). These species can be liberated from the liquid electrolyte and stored in a separate gas container, allowing scale-up of storage capacity without increasing the concentration and volume of the electrolyte. The oxidation of NO in the presence of NO3 - affords N2 O3 , and the reduction of N2 O3 regenerates NO and NO3 - , together affording the electrochemical reaction: NO3 - +3 NO⇌2 N2 O3 +e- with a low mass/charge ratio of 152 grams per mole of stored electron. A proof-of-concept NOx symmetric H-cell shows 200 stable cycles over 400 hours with >97 % Coulombic efficiency and negligible capacity decay.