Understanding the redox effects of amine and hydroxyl groups of p-aminophenol upon the capacitive performance in KOH and H2SO4 electrolyte

- PAP substance serves as effective redox additive both in KOH and H2SO4 solution.
- Incorporating PAP can drastically improve the capacitance and energy density.
- Two different redox reaction mechanisms exist for KOH and H2SO4 solution.
- The incorporation of PAP into solution is simply operated but highly effective.

As a dual functional redox additive, p-aminophenol (abbr. PAP) simultaneously possessing hydroxyl and amine groups has been incorporated into KOH or H2SO4 electrolyte. It reveals that the PAP concentration and cell configuration play crucial roles in the determination of capacitive behaviors. PAP substance can release two protons and two electrons in KOH solution, whereas one proton and one electron occur in H2SO4 solution. The incorporation of PAP into KOH or H2SO4 solution can markedly elevate the specific capacitance, energy density and both of which remain high cycling stabilities. When designating 15 mmol L− 1 PAP in KOH or H2SO4 solution, the increase folds in capacitance are of 2. 2 and 1.4, respectively, and the correlative energy densities can reach up to 7.24, and 8.44 Wh kg− 1, respectively. What's more, the redox process in KOH solution is controlled by the surface reaction, while the one in H2SO4 solution indicates the semi-infinite diffusion mechanism.