Uniform and porous Mn-doped Co₃O₄ microspheres: Solvothermal synthesis and their superior supercapacitor performances

Abstract

Uniform and porous Mn-doped Co₃O₄ microspheres (Mn@Co₃O₄ MSs) assembled with many nanoparticles (NPs) were prepared through an initially solvothermal reaction and subsequent annealing treatment at 550 ^oC in air. These Mn@Co₃O₄ MSs had an average diameter of about 9 μm and possessed a BET specific surface area of 70.4 m²/g. The pore diameter was mainly centered at 12.3 nm and the mean pore size was measured to be 15 nm. When the Mn@Co₃O₄ MSs were used as electrode material for supercapacitors, the electrochemical performances were assessed in 2 M KOH aqueous solution using a typical three-electrode configuration. Such Mn@Co₃O₄-MSs-modified electrode exhibited a highly specific capacitance of 773 F/g at 1 A/g, 62.7% rate capability at 16 A/g, and 73.9% capacitance retention of its original value after 5000 cycles at 5 A/g. The excellently electrochemical behaviors indicate that such Mn-doped Co₃O₄ MSs can be used as a superior electrode material for advanced supercapacitors in the future. The current synthesis strategy is facile and can be further employed to prepare other electrode materials with outstanding electrochemical performances.

Introduction

In order to resolve the increasingly environmental problems and severe energy crises, supercapacitors (SCs), an environmentally friendly energy storage device, have received great interest in the past decade owing to the highly specific capacitance, fast charge-discharge capability, and long-term cycling durability as well [[1], [2], [3], [4]]. As a major class of electrode materials for supercapacitors, transition metal oxides (TMOs), play an essential role for their rich and reversible Faradaic redox reactions [[5], [6], [7], [8]]. Among them, Co₃O₄ is regarded as a promising material for SC applications in that such material exhibits highly theoretical capacitance of 3560 F/g [9,10]. Though it is a well-studied TMOs-based electrode material, many difficulties always exist in achieving its theoretical capacitance in the practical SCs application, due to its small available surface area, unsatisfactory stability, and poorly electrical conductivity [[11], [12], [13], [14], [15]]. Hence, it is very significant to develop Co₃O₄-based electrode materials with excellently electrochemical performances. For a given type of electrode material, it is well known that its electrochemical performance may be deeply influenced by the ionic diffusion rate, electrical conductivity, as well as the specific surface area. These issues are closely related to the composition, morphology, and surface texture of the electrode material [16,17]. The synthesis of mesoporous and micro-/nano-structural materials is a possible approach to produce high specific capacitance [18,19]. Mesoporous materials usually have large active surface areas and short diffusion pathways for electrons/ions. The contact of electrode material and electrolyte can be efficiently improved, which may lead to the enhancement of electrochemical performances more or less [[20], [21], [22]].

Much attention has been paid to the preparation of mesoporous Co₃O₄ nanomaterials for supercapacitive electrode so far. For example, mesoporous Co₃O₄ nanocubes with a specific surface area of 25.8 m²/g were prepared at 120 ^oC for 12 h with a subsequent calcination procedure, and these nanocubes displayed a specific capacitance of 350 F/g at 0.2 A/g [18]. Porous Co₃O₄ nanoflakes with a specific surface area of 125 m²/g could be prepared in an eggshell membrane supported system for 15 min, such Co₃O₄ nanoflakes exhibited a capacitance of 448 F/g with a good cycling performance in 2000 cycles (63% capacitance retention) at 5 A/g [23]. Porously ultralayered Co₃O₄ was prepared at 120 ^oC for 24 h, and it possessed a specific capacitance of 604 F/g at 4 A/g as well as 98.5% capacitance retention during 2000 cycles at 16 A/g [24]. Tremella-like NiO@Co₃O₄@MnO₂ could possess a specific capacitance of 792.5 F/g [25]. In their work, NiO precursor was firstly prepared by a hydrothermal method and then combinated with Co₃O₄ through a secondary hydrothermal process, later on, KMnO₄ solution was then introduced, the precursor was calcined in air to obtain the final NiO@Co₃O₄@MnO₂ composite. Despite many progresses have been made, it still remains a challenge to prepare mesoporous Co₃O₄ nanostructures because the current routes involve in tedious procedures, long processing time, and some extremely experimental conditions. So, it is necessary and urgent to hunt for a facile, effective, and cheap method to synthesize mesoporous Co₃O₄ nanostructures with very large specific surface areas.

The methods including reduction of material size to nanoscale [26], combination with other electrode material to form composite [27], and ion doping [28], are considered to be efficient to obtain porous Co₃O₄ with desirable electrochemical performance in SCs field, and many studies have been demonstrated to address these issues. Among them, ion doping in a mild synthesis condition is promising because this method can effectively avoid the complex procedures. There are many reports related to this research. Typically, the 5% Cd-doped Co₃O₄ nanosheets electrode exhibited a specific capacitance of 737 F/g at 1 A/g, while the undoped Co₃O₄ electrode only delivered 508.5 F/g [29]. Ambare et al. reported that Co₃O₄ with 0.6% Ru doping exhibited 628.3 F/g at 1 mV/s, and the value was higher than those of Co₃O₄ (338.6 F/g) and RuO₂ (35.0 F/g) [30]. The BET surface area and specific capacitance of sulphur-doped Co₃O₄ nanowires grown on carbon cloth were 47.8 m²/g and 0.55 F/cm² (at 10 mV/s), respectively, which were both larger than those of undoped Co₃O₄ with 35.6 m²/g and 0.05 F/cm²) [31]. These examples indicate that ion doping is effective to improve the electrochemical performances of electrode materials.

In present work, Mn-doped Co₃O₄ porous microspheres with large specific surface area and excellently electrochemical performance were prepared via a simple template-free strategy, which was involved in the solvothermal synthesis in ethylene glycol solvent and a post annealing treatment in air. The Mn-doped Co₃O₄ MSs were uniform in diameter and assembled with many NPs, exhibiting a BET specific surface area of 70.4 m²/g and pore volume of 0.264 cm³/g, respectively. The Mn@Co₃O₄-MSs-modified electrode possessed a highly specific capacitance of 773 F/g at 1 A/g, superior rate capability (62.7% capacitance retention at 16 A/g), and outstanding cycling durability (73.9% capacitance preservation) during 5000 cycles in KOH aqueous electrolyte. The good electrochemical performances may be attributed to the large specific surface area as well as the doping of Mn. The porous Mn@Co₃O₄ MSs can be served as a candidate for electrodes material applicable to energy storage device.