Ultrasonic treatment of Co7(PO4)2(HPO4)4 using NMP for supercapacitors and oxygen evolution reaction

Abstract In the alkaline system, the mechanism relationships of the transition metal phosphate materials between pseudocapacitors and OER are analyzed, more active sites, excellent conductivity and enhanced intrinsic activity are important factors in enhancing their OER and capacitance performances. In this study, the capacitance of synthesized Co7(PO4)2(HPO4)4 after ultrasonic treatment with N-methyl-pyrrolidinone (NMP) improved 20.1% (345 F g−1 at 1 A g−1), the overpotential for OER reduced 20 mV (293 mV at 10 mA cm−2). Based on detailed characterization analysis, ultrasonic treatment with NMP does not destroy the phases and the surface phosphate groups of the materials, but reduce the overall particle size, improve the pore volume and create some new interfaces originally located inside the material. These changes bring increased active sites, reduced internal resistance, enhanced charge transfer capability and improved intrinsic activity to the synthesized Co7(PO4)2(HPO4)4, enhancing its OER and capacitance efficiency. Additionally, the ultrasonic treatment can achieve those changes, which is most likely achieved by sacrificing some mesopores with small diameters, it also provides a simple and convenient method for enhancing the performance of other three-dimensional mesoporous materials.