Article pubs.acs.org/IECR
Effects of Amphiphilic Carbonaceous Nanomaterial on the Synthesis of MnO2 and Its Energy Storage Capability as an Electrode Material for Pseudocapacitors Ming-ming Chen,* Xiao-yuan Zhang, Li-qun Wang, and Cheng-yang Wang Key Laboratory for Green Chemical Technology of MOE, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, P. R. China Synergetic Innovation Center of Chemical Science and Engineering, Tianjin 300072, P. R. China ABSTRACT: An amphiphilic carbonaceous material (called CP-A5) derived from coal tar pitch has been introduced to synthesize a hollow MnO2/CP-A5 hybrid on the meso- or microscale in a poly(ethylene glycol) 400−water mixture using a selftemplate method. In the presence of CP-A5, the MnCO3 precursors change from cubes of approximately 450 nm to spheres of approximately 300 nm, and their basic building blocks vary from two-dimensional nanosheets to nanoflakes. The derived two types of hollow MnO2 preserve the morphologies of their own precursors well and inherit the basic building blocks. The electrochemical data indicate that MnO2/CP-A5 hollow spheres exhibit not only higher specific capacitance and lower resistance, including the intrinsic resistance of the electrode, Faradaic reaction resistance, and ion-diffusion/-transport resistance, but also better electrochemical stability than MnO2 hollow cubes because of their reduced particle size, induced morphology, and modified electrode/electrolyte interface.
1. INTRODUCTION As an intermediate between traditional capacitors and batteries, supercapacitors have attracted much attention because of their high power density and relatively high energy density.1−3 According to the charge-storage mechanism, supercapacitors are generally classified as electrical double-layer capacitors (EDLCs) or pseudocapacitors. EDLCs utilize the electrical charge at the electrode−electrolyte interface,4−6 usually using porous carbon as the active material, whereas pseudocapacitors are based on the fast and reversible Faraday reactions for charge storage, usually using transition-metal oxides or conducting polymers as the electrode material. In general, pseudocapacitors exhibit a relatively high specific capacitance7−9 and hence have gained more attention. Among the various transition-metal oxides, RuO2 is considered the most promising material because of its high specific capacitance (760 F g −1 ) and excellent cycle stability.10,11 However, its cost is too high for commercialization. Toward this end, significant efforts have been made to develop less expensive candidates such as MnO2,8,12 NiO,13 V2O5,14 and MoO315 as substitutes for RuO2. Among these candidates, MnO2 has received special attention because of its high theoretical specific capacitance (1370 F g−1), environmental compatibility, and low cost.16 However, MnO2 powder suffers from limited electric conductivity 17 and poor capacitance retention upon repeated cycling. The short cycle life is mainly caused by mechanical issues and active material dissolution during electrochemical cycling.7,18 A composite incorporating nano-MnO2 with a carbonaceous backbone is thought to be an ideal approach to optimize the electronic conductivity, chemical/mechanical stabilities, and flexibility of MnO2 electrodes for pseudocapacitors. Recently, carbon nanofibers,19 graphene oxide,20 graphitic carbon spheres,21 and carbon nanotubes22 have been reported as carbonaceous © XXXX American Chemical Society
matrixes for MnO2 deposition. These so-called carbonaceous “beds” are usually much larger than the nanoscale MnO2 particles, and these compositions are assembled on the macroscopic level. Inspired by interesting molecular dynamics simulations concerning the effects of both the nanoscale geometry on fluid behaviors and the interface chemistry on phase-transfer behaviors, 23−26 in this work, we selected a nanoscale amphiphilic carbonaceous material (called CP-A5) derived from coal tar pitch (CP) as the carbonaceous candidate and investigated a MnO2/CP-A5 hybrid on the micro- and mesoscopic scales, including the morphology, microstructure, and resulting electrochemical performances. Particular attention was paid to elucidation of the role of nano-CP-A5 in guiding the crystallization of MnCO3, which is the precursor of MnO2, and the synergistic effect of nano-CP-A5 and poly(ethylene glycol) 400 (PEG 400) on the morphology and size of the hybrid. The possible synthesis mechanism and the effect of the amphiphilic CP-A5 on the electrochemical performances of MnO2 are discussed in detail.
2. EXPERIMENTAL SECTION 2.1. Chemicals and Material Synthesis. CP was provided by Tianjin Tie-zhong Coal-Chemical Co. (Tianjin, China). All other chemicals and solvents were purchased from Tianjin Chemical Reagent Co. and were of analytical grade, including MnSO4·H2O, NaHCO3, KMnO4, PEG 400, hydrochloric acid (HCl, 36 wt %), and ethanol. Received: February 27, 2014 Revised: May 29, 2014 Accepted: June 18, 2014
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dx.doi.org/10.1021/ie500850v | Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Industrial & Engineering Chemistry Research
Article
CP-A5 was synthesized based on the procedure described by Wang et al.27 The spherical MnCO3/CP-A5 material was prepared in a CP-A5−PEG 400−H2O ternary mixture following the method of a previous work,28 using directly precipitated MnSO4·H2O and NaHCO3 as the raw materials. First, 100 mg of CP-A5 was well-dispersed in 40 mL of deionized water in a beaker under ultrasound. Then, MnSO4· H2O (169 mg) and PEG 400 (70 mL) were added to the beaker in succession under stirring. After complete dispersion, 30 mL of NaHCO3 (840 mg) aqueous solution was poured into the above mixture, and the resulting mixture was stored for 7 h at room temperature. The resultant precipitates were washed with water and ethanol alternately until the supernatant became clear and colorless. Subsequently, the synthesized MnCO3/CP-A5 spheres continued to be oxidized with a 0.032 M KMnO4 aqueous solution to obtain a (MnCO3 core−MnO2 shell)/CP-A5 composite. The acid-washing process was conducted by dispersing the above composite in 0.1 M HCl aqueous solution under stirring for 12 h. Finally, MnO2/CP-A5 hollow spheres were obtained by centrifugation, washing, and drying in air. For comparison, the same procedures were performed without CP-A5 to obtain MnCO3 cubes and MnO2 hollow cubes. 2.2. Instrumental Analysis. X-ray diffraction (XRD) patterns were obtained on a Rigaku D/max 2500 v/PC system using Cu Kα radiation (λ = 1.5406 Å) and a scanning rate of 5 dr−1. The morphologies of the materials were examined by field-emission scanning electron microscopy (FESEM, Nano430) and high-resolution transmission electron microscopy (HRTEM, Philips Tecnai G2 F20). Nitrogen physical adsorption/desorption measurements were performed on a Tristar-3000 instrument at 77 K. The specific surface area and pore size distribution of the samples were calculated according to the Brunauer−Emmett−Teller (BET) and Barrett−Joyner− Halenda (BJH) models, respectively. Thermogravimetric analysis (TGA) was conducted using a TA-50 instrument from room temperature to 1000 °C at a heating rate of 10 °C min−1 in an air atmosphere. Fourier transform infrared (FTIR) spectroscopy measurements were performed on a Nicolet 560 spectrometer using KBr pellets. To exploit the potential application for supercapacitors, the samples were prepared into electrodes as follows: hollow MnO2 (75 wt %), carbon black (20 wt %), and polytetrafluoroethylene (PTFE, 5 wt %) were dispersed in absolute ethanol to obtain a mixed slurry. Then, the slurry was painted onto a stainless steel mesh, which was then subjected to uniaxial pressing and drying at 80 °C. The electrochemical performances of the hollow MnO2 materials were measured using galvanostatic charge/ discharge measurements, cyclic voltammetry (CV), and electrochemical impedance spectroscopy (EIS) on an electrochemical workstation (PARSTAT 2273, Princeton Applied Research). All electrochemical measurements were performed in 1 M Na2SO4 electrolyte within a three-electrode system, in which platinum foil and a Hg/Hg2SO4 electrode (K2SO4saturated) were employed as the counter electrode and reference electrode, respectively. EIS tests were performed in the frequency range between 100 kHz and 10 mHz with a perturbation amplitude of 10 mV versus the open-circuit potential. In addition, the specific capacitances (Cg, F g−1) were calculated from the CV curves and the discharge curves, respectively, according to the equations
Cg = Cg =
∫ i(u) du mEv It Em
(1) (2)
where m is the mass of MnO2 or MnO2/CP-A5 (g), v is the potential scan rate (mV s−1), i(u) is the voltammetric current (A), E is the potential window of the CV or discharge curves excluding the initial drop (V), I is the applied current (A), and t is the discharge time (s).
3. RESULTS AND DISCUSSION Coal tar pitch (CP) is a mixture of polycyclic aromatic hydrocarbons. After oxidation by a concentrated nitric/sulfuric acidic mixture, a nanoscale amphiphilic carbonaceous material called CP-A5 is formed.27 As illustrated in Figure 1, CP-A5
Figure 1. Chemical structure of CP-A5.
contains abundant hydrophilic functional groups, such as −COOH, −NO2, and −SO3H. The material is readily dispersed in distilled water to form a nanoscale negatively charged colloid. 3.1. Characterization. As shown in Figure 2, MnCO3 and MnCO3/CP-A5 are rhombohedral (JCPDS card no. 44-1472); the two resultant MnO2 samples, MnO2 and MnO2/CP-A5, are amorphous.16 With the addition of CP-A5, the peaks of MnCO3/CP-A5 are broadened to some extent, and a degree of distortion exists in MnO2/CP-A5. This result indicates that the CP-A5 additive reduces the crystallization of MnCO3, and hence, the derived MnO2/CP-A5 composite is likely to be greatly disordered. Notably, the FESEM images in Figure 3 show that MnCO3 appears as
