Facile Synthesis of Mn0.68Bi0.32OCl Mix-Crystals and Its Supercapacitive Behavior

Mn0.68Bi0.32OCl mix-crystals for supercapacitor are successfully synthesized by a thermosolid method, and are characterized by scanning electron microscopy, X-ray diffraction, Brunauer–Emmett–Teller surface area measurements and thermogravimetry and differential scanning calorimetry, respectively. The supercapacitive properties of Mn0.68Bi0.32OCl mix-crystals in 1 M Na2SO4 aqueous solutions are investigated by cyclic voltammetry and galvanostatic charge/discharge technique, and the specific capacitance of Mn0.68Bi0.32OCl is about 650 F g−1 at the current density of 3 A g−1, owing to the high power density and the higher surface area, good conductivity, and high stability of Mn0.68Bi0.32OCl mix-crystals.


Introduction
Supercapacitor as a next generation of promising electrochemical energy storage device has the advantages of fast charge and discharge speed, long service life, high power density, high safety and good cycle stability. Based on the charge storage mechanism, supercapacitors can be divided into double-layer capacitors, pseudo capacitors and hybrid supercapacitors. The two-layer capacitor consists of activated carbon electrode material, while the pseudocapacitive supercapacitors include metal oxides and polymers. The Mn and Bi oxides are widely used in the preparation of supercapacitors due to their low cost, environmental friendliness and simple preparation process, and the Mn 3 O 4 [1], MnO [2], MnOx [3], Mn 2 O 3 [4], Bi 2 O 3 [5][6][7][8], BiOCl [9], (Cu)BiOCl [10], and BiOBr [11] have been applied to supercapacitors.
BiOCl as a semiconductor material has been widely used in the field of photocatalysis [12,13] due to its special layered structure, which is composed of [Bi 2 O 2 ] 2+ layer and double-layer halogen ion layer staggered along the [001] direction, and its synthesis mainly includes hydrothermal/ solvothermal method [14,15], template method [16], ultrasonic method [17], solid-phase method [18] and sol-gel method [19]. The band gap of BiOCl is 3.6 eV, the ion doping was a effective for improving the catalytic performance of BiOCl [20], the different molar ratio manganesedoped BiOCl composites (Mn/BiOCl) were prepared using Bi(NO 3 ) 3 and Mn(CH 3 COO) 2 as precursors by the hydrothermal method [21], Cao et al. synthesized the Mn 3+ doping BiOCl in hydrochloric acid and potassium chloride solution using Bi(NO 3 ) 3 and MnCl 2 (molar ratio of 1:0.015) as the precursors by hydrothermal method [22]. In addition, the Bi(Mn)OCl could be synthesized using hydrothermal method by adjusting the pH of MnCl 2 -BiCl 3 -HCl solution with ammonia water [23].
In present work, the Mn 0.68 Bi 0.32 OCl mix-crystals were synthesized in one step using Bi(NO 3 ) 3 and MnCl 2 (molar ratio of 1:1) as precursors by a thermosolid method, and was characterized by spectroscopy and electrochemistry, and its supercapacitor behavior was also studied.

Materials
Sixty percentage polyvinylidene fluoride (PVDF) water emulsion was provided by Shanghai San-Ai-Fu new material Ltd (Shanghai, China). N,N-Dimethylformamide, Na 2 SO 4 , Bi(NO 3 ) 3 ·5H 2 O and MnCl 2 ·4H 2 O were purchased from Shanghai Analytical Chemicals Company. All chemicals were of analytical grade without further purification. Graphite was used as active material carrier because of its low price, good adsorb ability and electrochemical stability. The graphite working electrode was prepared by the following method: a cylindrical graphite with diameter of 14.0 mm was inserted into the polypropylene-random (PPR) plastic pipe with inner diameter of 14.00 mm, the surface of graphite electrode was polished to the mirror using sandpaper of 1000 and 7000 mesh, respectively, then further cleaned with polishing cloth, and the other end was connected with a copper column. The prepared graphite electrode was sonicated with deionized water and ethanol for 30 min, respectively, and then dried for standby.

Synthesis of Mn 0.68 Bi 0.32 OCl Mix-Crystals
Bi(NO 3 ) 3 ·5H 2 O of 0.01 M and MnCl 2 ·4H 2 O of 0.01 M were dissolved in 0.01 M HNO 3 of 30 ml in a porcelain crucible, heated in a 100 °C oven for 24 h to make it nearly dry, and then the porcelain crucible was transferred into a muffle furnace, heated from room temperature to 500 °C for 0.5 h, kept at 500 °C for 4 h, continuously heated from 500 to 600 °C for 0.5 h, kept at 600 °C for 1 h, and finally cooled to room temperature for use.

Material Characterization
The nitrogen adsorption and desorption experiments of Mn 0.68 Bi 0.32 OCl mix-crystals were carried out at 77 K using SA3100 surface area and pore size analyzers (Beckman Coulter, Inc. USA). The morphologies of Mn 0.68 Bi 0.32 OCl were examined by scanning electron microscope (SEM) (QUANTA FEG 450, USA), equipped with an EDAX OCTANE PRO energy dispersive spectrometer (EDS) (FEI, USA). The thermogravimetry (TG) and differential scanning calorimetry (DSC) in air were performed using a NETZSCH STA 449F3 simultaneous thermal analyzer (German). X-ray diffraction (XRD) analysis was carried out on the Mn 0.68 Bi 0.32 OCl mix-crystals with a Switzerland ARL X'TRA X-ray diffractometer rotating anode with Cu-K α radiation source (λ = 0.1540562 nm).

Electrochemical Characterization
To investigate the supercapacitive behavior of the mix-crystals, the active material, carbon black (CB) and PVDF were taken in the weight ratios of 100: 10: 10. Firstly, the mixture of Mn 0.68 Bi 0.32 OCl mix-crystals of 10.0 mg, CB of 1.0 mg and PVDF of 1.0 mg was dispersed in N,N-dimethylformamide of 10.00 ml and sonicated for 30 min for 30 min, then the mixture of 100 µl evenly dispersed on the clean surface of graphite electrode, and finally dried at 100 °C for 1 h. The loading mass of active material was 0.10 mg. Land-CT2001A battery analyzer (Wuhan, China) was used for the charge/discharge test of Mn 0.68 Bi 0.32 OCl mix-crystals. The electrochemical characterization of the prepared capacitive electrodes was also carried out with a CHI660e electrochemical analyzer (CHI, USA) in a two-electrode cell system. The prepared capacitive electrode was used as working electrode, a graphite rod with a diameter of about 1.40 cm and a length of about 0.80 cm was connected with the counter electrode and reference electrode of electrochemical analyzer, and both electrodes were sealed in a PPR plastic pipe filled with 1 M Na 2 SO 4 solution. The length of reference graphite electrode was selected according to the equal electric quantity of charge and discharge curve. The electrochemical impedance spectroscopy (EIS) was measured at the open-circuit voltage over the frequency range of 0.02 to 10 5 Hz with an a.c. amplitude of 5 mV. All electrochemical measurements were carried out at room temperature.

Specific Surface Area of Mn 0.68 Bi 0.32 OCl Mix-Crystals
As shown in Fig. 1, the isotherm of Mn 0.68 Bi 0.32 OCl mixcrystals was classified as type IV with an H3 hysteresis loop. The specific surface area (SSA) was calculated using the Brunauer-Emmett-Teller (BET) method. The pore-size distributions (PSDs) of Mn 0.68 Bi 0.32 OCl mix-crystals were also computed by the Barrett Joyner Halenda (BJH) plots. The peak in PSDs (shown in Fig. 1 inset) was centered at 37.3 and 242.2 nm, the SSA values of Mn 0.68 Bi 0.32 OCl mix-crystals calculated by BET method were 14.6 m 2 g −1 , which was more than 13.28 m 2 g −1 of Mn-doped BiOCl [22], indicting that the Mn 0.68 Bi 0.32 OCl mix-crystals were conducive to the penetration of electrolyte into the surface of electroactive substances.

Micrographs and EDS Spectrum of Mn 0.68 Bi 0.32 OCl Mix-Crystals
The SEM micrographs and the EDS spectrum of Mn 0.68 Bi 0.32 OCl mix-crystals are shown in Figs. 2 and 3, respectively. The SEM micrographs in Fig. 2 showed that the smooth surface, neat edges and layered particles could be ascribed to the Bi (Mn) OCl mix-crystals. The EDS measurements in Fig. 3 further confirmed the chemical composition of products, the EDS spectra and element analysis revealed that the ratio of O:(Mn + Bi):Cl was 1.42:0.73:1, while the ratio of Mn:Bi was 2.09:1. It could be seen from

XRD Pattern of Mn 0.68 Bi 0.32 OCl Mix-Crystals
The XRD pattern of Mn 0.68 Bi 0.32 OCl mix-crystals is shown in Fig. 4

TG and DSC of Mn 0.68 Bi 0.32 OCl Mix-Crystals
The TG and DSC curves of Mn 0.68 Bi 0.32 OCl mix-crystals are shown in Fig. 5 [27] because BiOCl was steady below 600 °C [13], while when the temperature reaches 1000 °C the mass of 79.3% was remained, revealing that Mn 0.68 Bi 0.32 OCl mix-crystals was a higher safe material because the supercapacitor usually worked at room temperature.

Electrochemical Performance of Mn 0.68 Bi 0.32 OCl Mix-Crystals
The electrochemical performance of Mn 0.68 Bi 0.32 OCl mixcrystals was examined with cyclic voltammetry (CV) in Fig. 6 and the galvanostatic charge/discharge in 1 M Na 2 SO 4 aqueous solutions in a fixed voltage window of − 0.50 to 0.50 V vs. graphite in Fig. 7. From Fig. 6a it could be seen that the oxidation current increased with the increase of voltage, and a oxidation peak was found at 2.556 V, which could be attributed to Mn 3+ /Mn 4+ . The electrode of Mn 0.68 Bi 0.32 OCl mix-crystals with rich O and Cl elements of high electronegativity, had high adsorption ability to the Na + ions. Figure 6b indicated that the electrodes in the range of − 0.5 to 0.5 V had the supercapacitor characteristics at low sweep speed. These results shown that the capacitances were mainly from pseudocapacitance characteristics, and the mechanism of Bi(Mn)OCl mix-crystals could be described by the following reactions [9][10][11]28]: The galvanostatic charge/discharge curves are shown in Fig. 7 OCl were evaluated at a current density of 3 A g −1 for 10,000 cycles in Fig. 8. In Fig. 8 the SC values from cycle 1 to 12 at 3 A g −1 increased due to the penetration of electrolyte into electrode active material, and the SC value of the first cycle was 372.0 F g −1 at 3 A g −1 , which was close to the value of 341.7 F g −1 obtained with CHI660e electrochemical analyzer. And from cycle 13 to 10,000 the SC values at 3 A g −1 were about 650 F g −1 with cycle efficiencies (n%) of 94.5-106.0%, indicating that the electrode materials had higher stability.
The EIS analysis was also used to predict the behavior of the capacitive electrode. The Nyquist plot of BE, CBE and WE, fitting EIS of WE and equivalent circuit are shown in Fig. 9. The equivalent circuit consists of Rs related to the electrical resistance of the electrolyte, R ct (charge transfer resistance), CPE (constant phase element), and W (Warburg impedance), the R ct corresponding to the diameter of the semicircle in the plot, were closely related to the surface area and conductivity of the electrode, and the straight sloping line was associated with the ions diffusion. And the Rs values of BE, CBE, and WE in the Nyquist plots were about 0.53, 0.79, 4.64 Ω, respectively, indicated that the Rs values of WE were more than that of BE due to the poor conductivity PVDF and Mn 0.68 Bi 0.32 OCl, which led to the Ohm voltage increase and electrode polarization. The results were consistent with the experimental result in Fig. 7. The R ct were smaller, indicating that the electrolyte ions easily accessed the surface of active material, and the electroactive material on the electrode had stronger depolarization ability and could promote the transfer of ions and electrons. Also, the slope of ions diffusion was steeper, demonstrating that Mn 0.68 Bi 0.32 OCl had faster diffusion of electrolyte ions.

Conclusions
In this paper the Mn 0.68 Bi 0.32 OCl mix-crystals were prepared using Bi(NO 3 ) 3 and MnCl 2 as precursors by thermosolid method. The Mn 0.68 Bi 0.32 OCl mix-crystals of simple preparation and low cost distinctly improved