Investigating the performance and stability of zeolitic imidazolate framework-67 electrode in alkaline redox electrolytes for energy storage applications

The zeolitic imidazolate framework-67 (ZIF-67) is an attractive material as electrodes for energy storage applications due to its scalability, and high surface area. Although, its low electrical conductivity limits the specific capacitance and rate capabilities, properties which can be significantly increased when redox electrolytes are employed. Furthermore, the successful application of ZIF-67 electrodes depends on their cyclic stability. However, this material undergoes decomposition in alkaline electrolyte, which is the most commonly employed electrolyte in previously published studies, but this effect has not been taken into account. In this study the capacitance, efficiencies and stability of ZIF-67 electrodes was studied in conventional electrolyte (KOH) and with the addition of three different redox species: p-phenylenediamine (PPD), potassium hexacyanoferrate (II), and potassium iodide. Our results reveal that the addition of ferrocyanide and halides promote the greatest increases in capacitance, but their high oxidation potentials limit capacitance retention to 42% (10,000 cycles). With a lower formal potential, the use of PPD allows retentions of more than 80%. To the best of our knowledge, this is the first study that demonstrates the impact of formal potentials in the performance of ZIF-67 electrodes, highlighting the importance of a judicious selection of electrolytes to comprise this electrode.