Construction of ultrathin NiCo(HPO4)2·3H2O nanosheet with desirable inner boundary for hybrid supercapacitor

The considerably theoretical electrochemical performance makes NiCo(HPO4)2·3H2O a potential electrode material for hybrid supercapacitor (HSC). Nevertheless, its practical application is impeded by the limited preparation process and undesirable structure, causing its incomplete utilization and unsatisfactory electrochemical performance. Herein, we rationally design a freestanding HSC electrode based on NiCo(HPO4)2·3H2O with the inner boundary to achieve its high utilization. The construction of an inner boundary can increase the ion transport channels and active sites during electrochemical processes. The freestanding NiCo(HPO4)2·3H2O-based electrode in three-electrode system delivers high specific capacitance (3005.1 mF cm−2 at 1 mV s−1) and favorable rate capability (2795 mF cm−2 at 10 mV s−1). When evaluated as a cathode and matched with the activated carbon anode, the asymmetrical HSC exhibits a specific capacitance of 2547.3 mF cm−2 (10 mV s−1) and desirable cycling stability after 4000 cycles (87 % capacitance retention).