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Boron-doped manganese dioxide for supercapacitors† Hong Zhong Chi,* Yuwei Li, Yingxu Xin and Haiying Qin

Received 17th July 2014, Accepted 15th September 2014 DOI: 10.1039/c4cc05457a

The addition of boron as a dopant during the reaction between carbon fiber and permanganate led to significant enhancement of the growthrate and formation of the porous framework. The doped MnO2 was superior to the pristine sample as electrode materials for supercapacitors in terms of the specific capacitance and rate capability.

The increasing worldwide interest in manganese oxides1–4 as electrode materials for supercapacitors is based on the fact that MnOx-based supercapacitors will ultimately serve as a safety and low cost alternative to current commercial organic electric double layer capacitors and RuO2-based acid systems. Manganese dioxide5–8 is believed to be a very promising electrode material due to its high theoretical specific capacitance (1370 F g1), good environmental compatibility, and abundance in raw materials. However, as a semi-conductor material, the poor electronic conductivity must be considered when MnO2 is used as an electrode material. It may cause a decrease in specific capacitance of supercapacitors during high-rate charge–discharge. There are two possible explanations for the decrease. The first one is the low acquisition of the cations at the interface between the electrolyte and the manganese oxide matrix. An ideal morphology should have high specific surface area, a large number of exposed active sites and highly hydrophilic properties, leading to an intimate contact with an electrolyte. The second one can be attributed to the low diffusion coefficient of ions, i.e., protons or alkaline metal ions, in the manganese oxide matrix, which means the pseudocapacitive reactions occur only near the metal oxide surface and the under lying bulk materials remain as dead volume even though there is a high mass loading, resulting in significant reduction in specific capacitance.7 Many attempts have been made to overcome the drawbacks, such as morphology and crystalline structure controlling fabrication of manganese oxide9,10 and doping of heterogeneous elements into the manganese College of Materials and Environmental Engineering, Hangzhou Dianzi University, Hangzhou 310018, China. E-mail: [email protected]; Fax: +86 571 87713538; Tel: +86 571 86878609 † Electronic supplementary information (ESI) available: Experimental details and supplementary figures. See DOI: 10.1039/c4cc05457a

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oxide matrix. Some kinds of dopants such as metals,11,12 metal ions13,14 and even composites15 are doped into the MnO2 matrix to improve the capacitance and reversibility in cyclic voltammetry. The doped MnO2 materials generally show better performance in magnetic, electrical and electrochemical properties, owing to an improved electrical conductivity of MnO2 through the doping of the additives.16 To the best of our knowledge, there are a few reports on boron-doped manganese oxides as electrode materials for supercapacitors. Here, boron has been chosen as a dopant and the boron-doped MnO2 with three-dimensional structure was prepared by the chemical reaction between KMnO4 and carbon fiber (see details in the ESI†). The effects of boron doping on the morphology, conductivity and electrochemical performance of MnO2 have been investigated. When carbon fibers were placed in the solution of potassium permanganate, MnO2 crystal seeds were deposited on the surface of carbon fiber through the direct reduction of permanganate at the very beginning. The reaction is described as follow: 4KMnO4 + 3C + H2O - 4MnO2 + K2CO3 + 2KHCO3


Then the crystallization process continued through a complex disproportionation reaction between Mn7+ and Mn3+. As a result, the MnO2/carbon fiber composite electrode was formed. The undoped MnO2 film (Fig. 1a) consisted of some nano-size clusters, which are interconnected or aggregated with each other. While the boron-doped electrode, as can be seen in Fig. 1b, was a porous framework formed by randomly interlaced nanosheets. The most significant features of such nanosheets are their exceptionally high specific surface area and large number of exposed surface active sites,17,18 indicating that such morphology provides large surface areas for the pseudocapacitive reactions and facilitates the contact between the electrochemical active material and cations of the electrolyte. Fig. 2 shows the typical XRD patterns for the carbon fiber and the as-prepared MnO2 electrode. The pattern for the carbon fiber shows sharp diffraction peaks at 25.21, 43.51, 63.81 and 76.91. It is interesting to note, however, that the diffraction peaks

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Fig. 3 (a) Charge–discharge curves at a current density of 1 A g1; (b) CV curves at a scan rate of 50 mV s1 and (d) the specific capacitances for the undoped and boron-doped electrodes in a 0.5 M Na2SO4 aqueous electrolyte; (c) CV curves for boron-doped electrodes in a 0.5 M Na2SO4 aqueous electrolyte at scan rates of 2, 5, 10, 20, 50, and 100 mV s1. Fig. 1 SEM images of (a) the undoped and (b) the boron-doped MnO2 film chemically deposited on the carbon fibers through reduction of permanganate at 100 1C for 2 hours.

Table 1 Values of active material mass, specific capacitance and coulomb efficiency of undoped and boron-doped electrodes

Parameter Electrode

Specific capacitancea Coulomb efficiencyb Mass (mg) (F g1) (%)

Undoped 0.162 Boron-doped 0.249

206.4 269.0

a At a scan rate of 50 mV s1. 1 A g1.

Fig. 2

XRD patterns for carbon fiber, undoped and boron-doped electrode.

appeared obviously in the XRD patterns for the MnO2 electrodes are mainly associated with the carbon material except for a broad peak at around 12.71. Similar phenomena have been reported elsewhere19 and the birnessite-type MnO2 (JCPDS 42-1317), with the standard values of 12.51, 25.31, 42.61 and 63.31, has been demonstrated (Fig. S1, ESI†). The diffraction intensities of MnO2 are so weak, owing to a lower content in weight or poor crystallization, that the diffraction peaks at 25.31, 42.61 and 63.31 are all overlapped by the strong diffraction of the carbon material. Compared with that for the un doped electrode, the diffraction peak at 12.71 becomes more obvious for the boron-doped MnO2 due to a higher loading level. The capacitive characteristics of the as-prepared electrodes were obtained by galvanostatic charge–discharge tests and cyclic voltammetry measurements. Fig. 3a exhibits the charge–discharge profiles for the as-prepared electrodes at 1.0 A g1. The discharge curves and the corresponding charge curves are almost symmetrical

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91.8 96.1

At a charge–discharge current density of

over the whole potential region without any obvious internal resistance drop during the tests, indicating high coulomb efficiency (see Table 1) and ideal capacitive behavior. Similarly, the CV curves, take the sweep rate of 50 mV s1 as an example (Fig. 3b), display relatively rectangular mirror images with respect to the zero current line in the range between 0 and 1.0 V (vs. SCE), suggesting that the electrodes are charged and discharged in an ideal pseudocapacitive behavior over the complete voltammetric cycle. According to the CV curve area (Fig. S2b, ESI†), the contribution of the carbon fiber substrate to the capacitance of the as-prepared electrodes is negligible. It should be noted that the voltammetric current for the boron-doped electrode is about 175% higher than that provided by the undoped electrode. This trend may result from a higher specific capacitance or more active material. The specific capacitance of the electrodes can be calculated by using the following equation and listed in Table 1: C¼

1 mvðVc  V a Þ

ð Vc Iv dV



where C is the specific capacitance (F g1), m is the mass of the active materials in the electrodes (g), v is the potential scan rate (mV s1), (Vc  Va) is the sweep potential range during discharge (V) and Iv denotes the voltammetric current (A).

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Scheme 1

The deposition process of MnO2 in the presence of boron ions.

The amount of MnO2 deposited on the carbon fiber is 0.236 mg for the boron-doped electrode and 0.151 mg for the electrode obtained through the same procedure but without boron addition, respectively (Table 1). In fact, the addition of boron as a dopant leading to significant growth-rate enhancement is also found by other researchers.20,21 Due to the electron deficient character of B3+, the concentration of hydroxyl groups in the boron-doped MnO2 surface increases. These OH may act as ‘‘hunters’’ through hydrogen bonding as reported in the literature22 to capture the MnO2 crystal seeds formed via the disproportionation reaction between Mn7+ and Mn3+. On the other hand, when the boron atoms incorporate into the crystal lattice, the difference between the ionic radius of boron, manganese and oxygen readily gives rise to the occurrence of the crystal lattice distortion and dislocation. The lattice defects could reduce the interfacial energy, and thus accelerate the growth-rate. A schematic illustration of the deposition process is shown in Scheme 1. The rate capability of the boron-doped electrodes was estimated by cyclic voltammetry measurements at various scan rates, and the results are presented in Fig. 3c. The cyclic voltammograms are symmetric rectangular graphs at low scan rates, indicating an ideally capacitive behavior. As the scan rate increases, the voltammetric currents increase proportionally, and the CV profiles deviate gradually from rectangularity owing to polarizations. According to eqn (2) and the CV curves (Fig. S3, ESI†), the specific capacitances of the as-prepared electrodes at various scan rates are calculated and drawn in Fig. 3d. Both the electrodes show decreases in specific capacitance with an increase in the scan rate. But nearly 58.7% of the initial capacitance value (at 2 mV s1) still remains for the boron-doped electrode at a scan rate of 500 mV s1 while only 37.1% level remains for the undoped electrode, indicating that the doped MnO2 electrode possesses a better rate capability. Considering the results in Table 1 and Fig. 3d, the boron doping increases not only the specific capacitance but also the rate capability of the manganese dioxide electrode materials. It is generally believed that the poor capacitive behavior at a higher scan rate is due to the change of ions available near the surface of the electrode23 and low diffusion coefficient of ions in the manganese oxide matrix.7 Wang et al. have found that the surface wettability of their electrode to electrolyte was enhanced after boron-doping.24,25 This is important for supercapacitor applications, because high wettability should provide the electrochemically active material an intimate contact with the electrolyte. As shown in Fig. 1b, the boron-doped MnO2 film

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is a kind of porous network structure, which not only gives sufficient electrochemical active sites participating in the Faradaic reaction, but also possesses good adsorption of electrolytes, just like an ‘‘ion buffering reservoirs’’, for a redox reaction between the III and IV oxidation states of Mn which is accompanied by an ion exchange reaction between the cations and protons. On the other hand, researchers have found that the electrochemical properties of electrode materials can be improved by boron doping26,27 owing to the enhancement of electrical conductivity. Thus, we supposed that the increase of conductivity of manganese dioxide may play an important role in the improvement of electrochemical performance (Fig. S4, ESI†). In summary, the boron-doped MnO2 deposited tightly onto the surface of carbon fibers via the chemical reaction between carbon fiber and potassium permanganate in the presence of boric acid. The porous and hydrophilic film, formed by randomly interlaced nanosheets, can provide large surface areas for the pseudocapacitive reactions and facilitates the contact between the electrochemical active material and the electrolyte. By contrast with the electrode obtained through the same procedure but without boron addition, the boron-doped electrode has a higher loading level of active material, larger specific capacitance and better rate capacity. These results therefore imply that the doping of boron into manganese dioxide was a very promising way to improve and modify the morphology characteristics with significant enhancement of electrochemical performance and the boron-doped electrodes possess great potential for application in energy storage devices. The authors thank the Innovation Foundation of Hangzhou Dianzi University (No. KYS205612012) and the Zhejiang Provincial Natural Science Foundation of China (No. LR14B060002) for financial support.

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13352 | Chem. Commun., 2014, 50, 13349--13352

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Boron-doped manganese dioxide for supercapacitors.

The addition of boron as a dopant during the reaction between carbon fiber and permanganate led to significant enhancement of the growth-rate and form...
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