Hydrothermal synthesis of manganese phosphate/graphene foam composite for electrochemical supercapacitor applications

Abstract

Manganese phosphate (Mn(PO) hexagonal micro-rods and (Mn(PO) with different graphene foam (GF) mass loading up to 150mg were prepared by facile hydrothermal method. The characterization of the as-prepared samples proved the successful synthesis of Mn(PO) hexagonal micro-rods and Mn(PO)/GF composites. It was observed that the specific capacitance of Mn(PO)/GF composites with different GF mass loading increases with mass loading up to 100mg, and then decreases with increasing mass loading up to 150mg. The specific capacitance of Mn(PO)/100mg GF electrode was calculated to be 270Fg as compared to 41Fg of the pristine sample at a current density of 0.5Ag in a three-electrode cell configuration using 6M KOH. Furthermore, the electrochemical performance of the Mn(PO)/100mg GF electrode was evaluated in a two-electrode asymmetric cell device where Mn(PO)/100mg GF electrode was used as a positive electrode and activated carbon (AC) from coconut shell as a negative electrode. AC//Mn(PO)/100mg GF asymmetric cell device was tested within the potential window of 0.0-1.4V, and showed excellent cycling stability with 96% capacitance retention over 10,000 galvanostatic charge-discharge cycles at a current density of 2Ag.

Metal Phosphides and Phosphates‐based Electrodes for Electrochemical Supercapacitors

“…There have been considerable attempts to develop metal phosphates for electrochemical energy storage, especially those containing transition metals . Metal phosphates of open structures with different morphologies have always remained attractive, considering their abundant active sites for reactions . The PO covalent bonds in the structures are relatively strong, which makes them chemically stable .…”

“…The combination of different precursors, mineralizers, additives and even solvents, can lead to different phases and morphologies for transition metal phosphates. Therefore, different structures can be prepared via hydrothermal/solvothermal process, including one‐dimensional nanorods/nanowires, two‐dimensional nanosheets/nanoflakes, and even some micro‐sized structures …”

“…The integration of a proper conducting phase, such as graphene, into transition metal phosphates can act as not only a desired charge transfer medium, and also a buffer zone of the volume change of active materials . Nickel phosphates, manganese phosphates, and vanadyl phosphates were all investigated for development of hybrid‐type structures, by coupling with graphene. Some of these studies with manganese phosphates and vanadium phosphates are briefly described below.…”

“…Firstly, the two‐dimensional ultrathin Mn 3 (PO 4 ) 2 ·3H 2 O NSs and graphene NSs provide large surface areas; Secondly, graphene NSs with high conductivity lead to a fast electron/charge transfer; Thirdly, the dense stacking/re‐stacking of graphene NSs is avoided, which enhances the accessible surface area and cycling stability. Similarly, MnPO 4 ·H 2 O nanowire and Mn 3 (PO 4 ) 2 micro‐rods were integrated with graphene, where a proper adhesion of graphene layers on the surface of manganese phosphates increases the electrical conductivity and enhances electrochemical performance.…”

“…[108] The PO covalent bonds in the structures are relatively strong, which makes them chemically stable. [108,109] Attributed to the open structure framework and rich redox behavior, transition metal phosphates hold great promise as electrode materials for electrochemical energy storage devices. [110]…”