Microwave solvothermal synthesis of mixed pine tree seed-like/disc-shaped microstructures of MnO_x (x = 4/3 and 1) with high specific capacitance for electrochemical capacitors

The mixed pine tree seed-like/disc-shaped microstructures of MnO_x (x = 4/3 and 1) were synthesized using a simple microwave-assisted solvothermal synthesis and heat treatment under N₂ atmosphere (heating rate 5 °C/min), without the need for carbon composite. This new mixed morphology showed high specific capacitance, as measured by the cyclic voltammetry (182 F g⁻¹ at 1 mV s⁻¹) and by the constant galvanostatic charge-discharge cycling (195 F g⁻¹ at 0.1 A g⁻¹). During the cycling process the specific capacitance of the material increased, which was caused by the partial electrochemical oxidation of the major phase Mn₃O₄ (x = 4/3) to MnO₂ in a neutral electrolyte of 1 M Na₂SO₄. According to the galvanostatic charge-discharge cycling tests at 1 A g⁻¹, this material also showed good capacitance retention for over 1000 cycles. MnO_x synthesized herein can thus be a good candidate for the electrode material for electrochemical capacitors.