Porous NiCoP nanosheets as efficient and stable positive electrodes for advanced asymmetric supercapacitors

Abstract

Materials with good capacitance performance at high loading are desirable for application in supercapacitors at the device level. Here, we have reported the synthesis of sheet-like porous NiCoP (S-NiCoP) by controllable phosphorization of a 2-D NiCo hydroxide precursor. In the synthesis, the NiCo hydroxide sheets were firstly prepared through a solvothermal process in large quantity. Under the phosphorization, the PH₃ released by NaH₂PO₂ could react with NiCo in the precursor to form NiCoP. The release of gas by the decomposition of hydroxide could lead to the formation of pores, thus forming 2-D porous NiCoP with a high specific surface area (216.39 m² g⁻¹). S-NiCoP has combined the advantages of porous nanosheets (faster ion transport along the pores) and metal phosphides (TMPs) (good conductivity), and thus is promising for application in supercapacitors. S-NiCoP-300 has shown high capacitance (1206 F g⁻¹ at 1 A g⁻¹) and good rate performance (612 F g⁻¹ at 20 A g⁻¹), much superior to those of the corresponding NiCo hydroxide (566 F g⁻¹), NiP-300 (436 F g⁻¹) and CoP-300 (404 F g⁻¹). Notably, the electrode still exhibits a high capacitance of 1095 F g⁻¹ at a high loading of 13.5 mg cm⁻², ascribed to the ability of easily transferring electrolyte along the pores. In this case, the area capacitance can reach a high value of 14.78 F cm⁻². An asymmetric supercapacitor composed of activated carbon and S-NiCoP could achieve a high specific capacitance, good cycling stability and high energy density. A white LED (3 V) can be lit for more than 2 h by charging two S-NiCoP-2-300//AC ASC devices for about 170 s. The easy synthesis process and good performance makes 2-D NiCoP promising in portable energy systems.