For BHPC synthesis, 10 mmol of 3,4-diaminobenzoic acid in 100 mL glacial acetic acid, with 10 mmol 2-hydroxy-naphthquinone slowly added was stirred for 12 h at 50 °C. BHPC was obtained with yield > 90% after pouring the mixture into cold water and washed. BHPC electrolyte was prepared by dissolving a certain concentration of BHPC in 1 M NaCl, with NaOH adjusting the pH to 13.