Enhancing organic cathodes of aqueous zinc-ion batteries via nitro group modification

Organic compounds present promising options for sustainable zinc battery electrodes. Nevertheless, the electrochemical properties of current organic electrodes still lag behind those of their inorganic counterparts. In this study, nitro groups were incorporated into pyrene-4, 5, 9, 10-tetraone (PTO), resulting in an elevated discharge voltage due to their strong electron-withdrawing capabilities. Additionally, a novel electrochemical conversion of nitro to azo groups was observed in aqueous electrolytes. This transformation can be leveraged to enhance cycling stability, especially at low current densities. The electrochemical process of nitro-PTO during discharge comprises three distinct steps. Initially, two stages of H⁺/Zn²⁺ coordination to the carbonyl groups led to a high capacity of ∼284 mA h g^-1 above 0.80 V-significantly higher than that of PTO. Further discharge irreversibly transformed -NO₂ groups into N[double bond, length as m-dash]N bonds, resulting in exceptionally high specific capacities of approximately 695 mA h g^-1 and 905 mA h g^-1 for PTO decorated with single and double -NO₂ groups, respectively. As -NO₂ was continuously reduced to N[double bond, length as m-dash]N, the resultant azo-conjugated PTO (PTO-Azo) demonstrated reversible H⁺/Zn²⁺ co-storage and release during subsequent charge/discharge cycles, with improved capacity retention and higher kinetics. This work, therefore, elucidates the impact of nitro group chemistry on the electrochemical performance of carbonyl-rich organic electrodes.