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

Abstract

Manganese phosphate (Mn₃(PO₄)₂ hexagonal micro-rods and (Mn₃(PO₄)₂ with different graphene foam (GF) mass loading up to 150 mg were prepared by facile hydrothermal method. The characterization of the as-prepared samples proved the successful synthesis of Mn₃(PO₄)₂ hexagonal micro-rods and Mn₃(PO₄)₂/GF composites. It was observed that the specific capacitance of Mn₃(PO₄)₂/GF composites with different GF mass loading increases with mass loading up to 100 mg, and then decreases with increasing mass loading up to 150 mg. The specific capacitance of Mn₃(PO₄)₂/100 mg GF electrode was calculated to be 270 F g⁻¹ as compared to 41 F g⁻¹ of the pristine sample at a current density of 0.5 A g⁻¹ in a three-electrode cell configuration using 6 M KOH. Furthermore, the electrochemical performance of the Mn₃(PO₄)₂/100 mg GF electrode was evaluated in a two-electrode asymmetric cell device where Mn₃(PO₄)₂/100 mg GF electrode was used as a positive electrode and activated carbon (AC) from coconut shell as a negative electrode. AC//Mn₃(PO₄)₂/100 mg GF asymmetric cell device was tested within the potential window of 0.0–1.4 V, and showed excellent cycling stability with 96% capacitance retention over 10,000 galvanostatic charge-discharge cycles at a current density of 2 A g⁻¹.

Introduction

Electrochemical capacitors or supercapacitors (SCs), because of their high power density, low fabrication cost, and high stability have attracted a lot of research attention as result of increasing needs of energy storage devices with different properties [1], [2], [3], [4]. SCs are widely used in numerous areas as power supply, hybrid-electric vehicles (HEVs) and electronic devices. Generally, based on charge-discharge mechanism, SCs can be categorized into two types of capacitors: electrochemical double-layer capacitors (EDLC) and pseudocapacitors [5]. EDLC consist of high specific surface area materials, like carbon nano-materials and store energy in the electrical double layers. Pseudocapacitors materials show several oxidation states and store energy not only in electric double layers, but also in reversible redox reactions occurring on or near the electrode surface, like conductive polymers and transition metal oxides/hydroxides [5], [6], [7], [8]. Therefore, in terms of comparison, pseudocapacitor exhibits better capacitive behavior with high specific capacitance [9].

Furthermore, metal oxides including RuO₂, MnO₂, NiO, and Co₃O₄ exhibit high specific capacitance [10], [11]. Thus metal oxide/carbon composite which can affect the electrochemical performance of supercapacitors have been intensively investigated as electrode materials for supercapacitors [12], [13], [14]. Manganese oxide (MnO₂), which is promising pseudocapacitive electrode material for supercapacitors has become an interesting electrode material due to its excellent theoretical capacitance (1370 F g⁻¹), low-cost and is eco-friendly cathode material [15], [16]. However, in supercapacitor applications, it is hard to reach this theoretical value because of poor electrical conductivity (10⁻⁵–10⁻⁶ S cm⁻¹) and slow ion transfer rate which limit the specific capacitance and charge/discharge ability of the electrode [17], [18], [19], [20], [21]. On the other hand, metal phosphates or manganese phosphates have been extensively explored in high-performance solar cells, lithium batteries and rarely in SCs [22], [23], [24]. In addition, cobalt phosphate has been used as a bright positive electrode for rechargeable ion batteries, catalysis, ion exchangers and magnetic materials due to layers of interconnected Co_x and PO₄ and outstanding electronic or magnetic properties [25], [26]. Li et al. synthesized amorphous colloidal sphere structure of metal phosphate and they have observed that this structure can significantly enrich the pool of available materials in the fields of catalysis and lithium-ion battery electrodes [22]. Massa et al. reported a new orthophosphate Mn₃(PO₄)₂ where (MnO₅) polyhedra and (MnO₆) octahedra share common edges to form chains alternating along the (1 1 0) and (1 1 0) directions [27]. Ma et al. studied electrochemical performance of manganese phosphate with nanosheets structure in a three-electrode cell configuration, but using alkaline and neutral electrolytes and they reported a high specific capacitance of 203 F g⁻¹ and 194 F g⁻¹ at a current density of 0.5 A g⁻¹ using 1 M Na₂SO₄ and 2 M KOH electrolytes respectively [28]. Generally, synthesis of manganese phosphates with different morphologies always remains very attractive due to their unique advantages, such as abundant active sites for reactions with fast interfacial transport of charge carriers by decreasing the diffusion path length through the structure, and also because phosphate has strong PO covalent bonds which makes Mn₃(PO₄)₂ structure chemically very stable. In supercapacitor applications, a short diffusion path length of charge carriers and chemically stable structure of the electrode are very important. Furthermore, despite previously published results, the studies concerning manganese phosphates electrodes for supercapacitor applications are lacking in the literature.

Herein, we report a facile hydrothermal route to synthesis Mn₃(PO₄)₂ hexagonal micro-rods using manganese acetate (C₄H₆MnO₄) and ammonium phosphate (N₂H₉PO₄) as precursors. In addition, Mn₃(PO₄)₂/graphene foam (GF) composites with different GF mass loading were also synthesized by a hydrothermal process to improve on the surface electrical conductivity of Mn₃(PO₄)₂ to enhance the electrochemical performance of the electrode. Our choice of the hydrothermal technique is due to the fact that it facilitates the fabrication of most complex material(s) with a desired physio-chemical properties, and it also offers several advantages over the other conventional processes like energy saving, simplicity, cost effectiveness, high purity and crystalline materials, pollution free (since the reaction is carried out in a closed system), higher rate of reaction, better morphology control, and lower temperature of operation in the presence of an appropriate solvent, etc. The electrochemical performance of as-prepared Mn₃(PO₄)₂ hexagonal micro-rods and Mn₃(PO₄)₂/GF composites was evaluated in a three-electrode cell configuration using 6 M KOH. The specific capacitance of Mn₃(PO₄)₂ electrode was obtained as 41 F g⁻¹ and that of Mn₃(PO₄)₂/100 mg GF electrode which showed the highest value as 270 F g⁻¹ at a current density of 0.5 A g⁻¹. The electrochemical performance of the Mn₃(PO₄)₂/100 mg GF electrode was also evaluated in two-electrode asymmetric cell device where Mn₃(PO₄)₂/100 mg GF electrode served as a positive electrode and activated carbon (AC) from coconut shell as a negative electrode. AC//Mn₃(PO₄)₂/100 mg GF asymmetric cell device showed excellent cycling stability with 96% capacitance retention over 10,000 galvanostatic charge-discharge cycles at a current density of 2 A g⁻¹.