Structural engineering of bimetal-organic framework via a direct etching method and conversion to phosphide for electrochemical capacitors

Abstract Developing next-generation advanced electrode materials by tailoring the structural properties may provide a feasible pathway to enhance the electrochemical functions of energy storage devices. In this study, the rational strategy to construct hollow spheres of zinc cobalt phosphide through etching of zinc cobalt-glycolate followed by phosphorization method is presented. This method involves the fabrication of non-uniform zinc-cobalt glycolate solid spheres, which consists of an unstable inner layer compared to the chemically robust outer surface. The inner layer was then etched by utilizing the services of ammonium hydroxide without destroying the original size and shape. For comparison, the resultant hollow spheres were then subjected to sulfurization and calcination process to obtain zinc cobalt sulfide and zinc cobalt oxide respectively. Benefiting from the obtained porous structure and higher electrical conductivity of the zinc cobalt phosphide was exhibited remarkable electrochemical properties. Besides, the fabricated zinc cobalt phosphide//activated carbon asymmetric capacitor produced an energy/power density of 45.7 Wh kg−1/900 W kg−1 with outstanding cycling stability of 99.3% after 5000 cycles. In comparison, the assembled zinc cobalt sulfide//activated carbon and zinc cobalt oxide//activated carbon asymmetric capacitors delivered an energy/power density of 38.4 Wh kg−1/925 W kg−1, 31.4 Wh kg−1//940 W kg−1 with a surprisingly low cycle life of 75.7% and 53.1% respectively. This insight obtained from this comprehensive work offers a new perspective for the synthesis of exceptional electrode materials for supercapacitors.