Borotungstic acid - Polyacrylamide solid electrolytes for electrochemical capacitors with H3PO4 plasticizer

Proton-conducting polymer electrolytes consisting of borotungstic acid (BWA) and polyacrylamide (PAM) were developed. The addition of BWA to PAM progressively improved the electrolyte conductivity. The as-fabricated BWA-PAM electrolyte exhibited a high ionic conductivity of ca. 27 mS cm−1, but suffered from low shelf life due to poor water retention. With the modification of 10 wt% phosphoric acid (H3PO4), a longer shelf life and improved ionic conductivity of ca. 30 mS cm−1 were achieved. A ternary electrolyte composition of 75BWA-PAM + 10% H3PO4 was identified to have a high conductivity and good shelf life. At BWA content greater than 75 wt%, the addition of H3PO4 led to an undesirable phase separation within the electrolyte. The ternary 75BWA-PAM+ 10% H3PO4 electrolyte was used to assemble solid cells with CNT-graphite electrodes. The cells exhibited 11.2 mF cm−2 at 50 mV s−1 and high rate performance at 1 V s-1. The cells also retained ≥90% of their initial capacitance and maintained ca. −85° phase angle after 10,000 charging-discharging cycles at 6 A g−1.