Redox‐Additive‐Enhanced High Capacitance Supercapacitors Based on Co₂P₂O₇ Nanosheets

Abstract

Cobalt pyrophosphate (Co 2 P 2 O 7 ) has emerged as an attractive material due to its high specific energy and redox behavior of cobalt, however, problems associated with its poor specific capacitance and cyclic stability have prevented its realization. Here, the authors circumvent these problems by hydrothermally synthesizing layered Co 2 P 2 O 7 nanosheets (lateral size ≈300 nm with average thickness ≈15 nm) and demonstrate significant improvements in the specific capacitance of Co 2 P 2 O 7 nanosheets by the addition of a redox additive (K 3 Fe(CN) 6 ) into KOH aqueous electrolyte. Without the additive, Co 2 P 2 O 7 nanosheets show specific capacitance of 286 F g −1 at 1 A g −1 current density. However, by introducing 0.1 m redox additive to the electrolyte the specific capacitance of Co 2 P 2 O 7 nanosheets increased more than twofolds (580 F g −1 at 1 A g −1 current density), which is due to the improvement of redox reactions at the electrode/electrolyte interface and the enhanced ionic conductivity of electrolyte. Furthermore, with the redox additive, Co 2 P 2 O 7 nanosheets show an excellent cyclic stability (96% retention of its initial capacitance) and coulombic efficiency (99% retention) up to 5000 cycles at high current density 10 A g −1 .