Phospho-oxynitride layer protected cobalt phosphonitride nanowire arrays for high-rate and stable supercapacitors

Construction of a highly efficient and durable electrode plays the key dominant function in determining the performance of supercapacitors. Metal phosphides have been considered as a class of promising candidates to give rise to high energy density and overall performance. However, they suffer from fast capacity decay in alkaline electrolyte, where a surface layer of metal hydroxides with low conductivity is developed rapidly. Herein, we show for the first time the construction of a conductive phospho-oxynitride (PON) surface layer on the self-supported furs-like cobalt phosphonitride (CoPN) nanowire arrays/Ni foam (NF). The rationally constructed PON layer can also protect the cobalt phosphide grains in the polycrystalline structure, which is beneficial to enhance the specific capacity (1.57 mAh/cm² at 2 mA/cm²). It demonstrates an excellent cycle stability, and 52% of the capacity can be maintained at a current density of 25 times higher. The hybrid supercapacitor assembled with an active material mass loading of 46.5 mg/cm² shows a high areal capacitance of 3.81 F/cm² at 5 mA/cm², and at 100 mA/cm², the device can maintain about 55.6% of its capacitance. It can feed a maximum energy density of up to 1.35 mWh/cm², a maximum power density of 77.53 mW/cm², as well as an excellent cycle stability with 80% retention after 5000 cycles. The present study suggests a new avenue in developing new battery-type durable materials for energy storage and conversion systems.