Surfactant-directed hydrothermal synthesis of 3D niobium phosphate microflowers for enhanced asymmetric supercapacitors

The exhaustion of traditional natural resources and the intermittency of renewable energy sources emphasize the crucial need for advanced technologies for high-efficiency energy conversion and storage. Electroactive materials are essential in improving the performance of these systems, requiring scalable approaches for their design. Here, we report the synthesis of niobium phosphate (NbOPO₄) with variable surface morphologies according to the different surfactant-assisted hydrothermal methods, employing cationic (hexamethylenetetramine; HMTA), anionic (sodium dodecyl sulfate; SDS), and non-ionic (polyvinylpyrrolidone; PVP) surfactants. X-ray diffraction and Raman spectroscopy affirmed the crystalline structure, while FE-SEM analysis demonstrated the notable influence of various surfactants on the morphological characteristics of NbOPO₄ electrodes. Electrochemical tests demonstrated that the NbP-S electrode delivered a notable areal capacitance of 7200 mF/cm² at a current density of 10 mA/cm², and an energy density of 0.203 mWh/cm² at a power density of 2.25 mW/cm². Moreover, the electrode exhibited outstanding cycling stability, maintain 81.80 % of its total capacitance over 12,000 continuous galvanostatic charge-discharge cycles. In an asymmetric pouch supercapacitor device, employing NbP-S as the positive electrode and activated carbon (AC) as the negative electrode, the system attained an areal capacitance of 627.96 mF/cm², an energy density of 0.147 mWh/cm², and a power density of 3.25 mW/cm². These results highlight the versatile potential of surfactant-assisted NbOPO₄ electrodes for diverse supercapacitor applications, offering insights into their role in advancing sustainable energy storage solutions.