Synthesis of polyaniline nanofibers/graphene composites with high electrochemical performance.
Graphene is a new carbon material, which is composed of two-dimensional monoatomic carbon atoms and SP2. Because of its extraordinary electrical and thermal conductivity, excellent mechanical strength and large specific surface area, it has attracted great attention of researchers at home and abroad. Graphene has been explored and applied to electronic and energy storage devices, sensors, transparent conductive electrodes, supramolecular assembly and nanocomposites [8]. However, rGO has a low capacitance (1000) because it is easy to aggregate or stack.
On the other hand, polyaniline, as a typical conductive polymer, has attracted much attention because of its simple synthesis, good environmental stability and adjustable conductivity. Conductive materials with nanostructures can not only improve the inherent properties of materials, but also open up new application fields due to nano-effect. Many achievements have been made in the synthesis of polyaniline nanostructures. As an electrode material for supercapacitors, PANI has a high pseudo-capacitance, and its capacitance can even be as high as 3 407 F/g[ 10]. However, when the PANI chain degenerates due to repeated expansion and contraction after repeated charging and discharging, its capacitance loss is large. Carbon materials have high conductivity and stable electrochemical properties. In order to improve the electrochemical capacity and stability of PANI, PANI was compounded with carbon materials with nanostructures in order to obtain supercapacitor electrode materials with high capacity and stability [1 1].
As a new carbon material, the combination of graphene and polyaniline has attracted great attention [12]. However, PANI/GO composite electrode composed of GO and PANI synthesized by Hummers method had to reduce GO because of its low conductivity. Although the addition of chemical reducing agent reduced part of GO and improved the conductivity, it also passivated PANI[ 13] to some extent. In addition, the elimination of reducing agent will pollute the environment to a certain extent, so it is still a difficult problem to develop a simple and environmentally friendly route to prepare PANI/rGO composites as electrodes of supercapacitors.
Based on the above analysis, firstly, PANI and GO were dispersed and assembled, and PANI/rGO composites were prepared by hydrothermal reaction, a green and environmentally friendly reduction method, in order to obtain high-performance electrode materials for supercapacitors.
1 experimental part
1. 1 raw materials
Aniline (AR, Sinopharm Group) is used after vacuum distillation; Graphene oxide (homemade); Ammonium persulfate (APS, AR, Hunan Huihong reagent); Oxalic acid (OX, AR, Tianjin Yongda Chemical Reagent); Cetyltrimethyl ammonium bromide (CTAB, AR, Tianjin Guangfu Institute of Fine Chemical Industry).
Preparation of 1.2 Polyaniline
PANIF was prepared according to our previous method [14], and the preparation process was as follows: after adding 250 mL deionized water into a three-necked flask, sequentially adding 1.82 g CTAB, 0.63 g oxalic acid and 0.9 mL aniline, and stirring in a 12℃ water bath for 8 hours; Subsequently, 20 mL of ammonium persulfate aqueous solution containing aniline was added to the above solution at one time, and the reaction was kept for 7 hours under the same conditions. The prepared sample was washed with a large amount of deionized water until the filtrate was neutral, and then dried in vacuum at 30℃ for 24 h to prepare1.3go.
GO was prepared by Hummers method, and the specific process was as follows: adding 65438 00 g natural flake graphite (325 mesh) into a dry 2 000 mL three-necked flask (ice water bath), adding 5 g sodium nitrate solid, adding 220 mL concentrated sulfuric acid while stirring, and adding 30 g potassium permanganate for 65438±020min while stirring. Then move the three-necked flask to a water bath at 35℃ and stir for 65438±080min, then drop 460ml of deionized water into the flask, raise the temperature of the water bath to 95℃ and keep stirring at 95℃ for 60min, then quickly drop 720ml of deionized water into the flask,10min and then add 80ml of hydrogen peroxide. 10 minutes later, filter while it is hot. Transfer the drained filter cake to a beaker, add about 800 mL of hot water and 200 mL of concentrated hydrochloric acid, filter it while it is hot, and then wash it with a lot of deionized water until it is neutral. The obtained product was centrifuged at 5 000 r/min and stirred for 65438±00min to obtain graphene oxide solution.
Preparation of 1.4 Polyaniline /rGO Composite
Mix a certain amount of PANIF solution with a certain amount of 6.8 mg/mL GO solution in a certain proportion, so that the total volume of the mixed solution is 30 mL, and the final concentration of GO in the mixed solution is 0.5 mg/ mL. After magnetic stirring 10 min, the mixed solution was transferred to a reaction kettle with 50 mL PTFE lining for hydrothermal reaction, and kept at 180℃ for 3 hours; After the reaction kettle is naturally cooled to room temperature, take it out, wash it with deionized water until the lotion is colorless, and vacuum dry it at 60℃ for 24 hours. The mass ratios of PANIF and GO prepared according to the above steps are 5 10 and 15 respectively, and they are named PAGO5, PAGO 10 and PAGO 15 respectively, and the corresponding mass of PANIF is
1.5 instruments and characteristics
The morphology of the samples was analyzed by the S4800 field emission scanning electron microscope (SEM) of Hitachi, Japan. The samples were mixed with KBr and tabletted, and then analyzed by Nicolet 5700 Fourier infrared spectrometer. XRD analysis uses the x-ray diffractometer of Siemens company in Germany. The electrochemical performance was tested by Shanghai Brilliance CHI660c Electrochemical Workstation.
Electrode preparation and electrochemical performance test: the active substance (PANIF or PANIF/rGO), acetylene black and PTFE were mixed according to the mass ratio of 85∶ 10∶5 to form emulsion, which was evenly coated on the stainless steel current collector, tabletted under the pressure of 10 MPa, and dried to obtain the working electrode. Saturated calomel electrode (SCE) is used as reference electrode in electrochemical performance test. A three-electrode test system with Pt as counter electrode and 1 M H2SO4 as electrolyte has a potential window of-0.2 ~ 0.8V. 。
According to the charge-discharge curve and the formula (1)[ 15], calculate the specific capacitance:
Cs=i？ t？ Vm。 ( 1)
Where: I stands for current, a; ? T represents discharge time, s; ? Represents a potential window. M represents the mass of the active substance, g.
2 Results and discussion
2. 1 morphological characterization
Figure 1 is the SEM image of PANIF and PAGO 10. Low power SEM (figure 1(a)) shows that PANIF is a large-area nanofiber network. The high magnification diagram 1(b) clearly shows that the 3D nanofiber network structure contains many cross-linking points. After the hydrothermal reaction of the mixed solution of PANIF and PAGO 10, it can be seen from the low-power SEM (Figure 1(c)) that PAGO 10 composites have a cross-linked pore structure. After increasing the observation multiple (Figure 1(d) and Figure 1(e)), it can be found that there are two cases of rGO and PANIF*** in the sample; The high magnification 1(d) clearly shows that rGO and PANIF are closely combined, and the folded rGO-covered PANIF can be observed because of the few layers. As can be seen from the figure 1, a large area of PANIF and PANIF/rGO composites which are uniformly dispersed with each other have been successfully synthesized.
2.2FTIR analysis
Fig. 2 shows FTIR images of PANIF, GO and PAGO 10. Curve A in Figure 2 is in 1 58 1 cm- 1, 1 500 cm- 1, 1 305 cm- 1. The peak at 829 cm- 1 is the characteristic peak of PANI, which corresponds to the stretching vibration of C=C double bond in quinone structure, C-N stretching vibration peak of C=C double bond in benzene ring, C=N stretching vibration of * * yoke aromatic ring and C-H out-of-plane bending vibration of para-disubstituted benzene respectively. In fig. 2, curve b is the infrared spectrum of GO. The peaks of 1 700 cm- 1 correspond to the O-H and C=O bond vibration in -COOH respectively, and the absorption peaks in the range of1550 ~1050 cm-1represent the C-O vibration in COH/ COC [ Comparing the spectra of GO and PANIF, it can be found that the characteristic peaks of GO in PAGO 10 are not obvious, but all the characteristic peaks of PANI appear. This result is attributed to the low content of GO and the formation of rGO after the hydrothermal reaction of GO, which also shows that the hydrothermal reaction has no great influence on the quality of PANI.
2.4 electrochemical performance analysis
Fig. 4 shows the CV curves of the samples, in which fig. 4(a) shows the CV curves of different samples at the scanning rate of1mv/s. It can be seen that all four samples have obvious redox peaks, which are attributed to the doping/undoping transition of PANI, indicating that PANIF and its composites show excellent Faraday pseudocapacitance characteristics. Fig. 4(b) shows the CV of PAGO 10 at different scanning rates. As can be seen from the figure, the specific capacitance of PAGO 10 electrode increases steadily with the decrease of scanning rate. When the scanning rate is 1 mV/s, the specific capacitance of PAGO 10 electrode is 521.2f/g. 。
Fig. 5 shows the charge-discharge curves and AC impedance diagrams of PANI, PAGO5, PAGO 10 and PAGO 15. Fig. 5(a) shows the discharge curves of the samples at the current density of 1 A/g, from which it can be seen that all four samples have obvious redox platforms, which is consistent with the results in the above CV analysis. According to the charge-discharge curve, with the help of the formula, the specific capacitance of four samples at different current densities is calculated, and the result is shown in Figure 5(b). Obviously, at the same current density, the specific capacitance of PAGO 10 is the largest, and when the current density is 1 A/g, the specific capacitance is 517 f/g. This result shows that the electrochemical performance of PAGO 10 is obviously better than that of PANI/ graphene microspheres and 3D PANI/ G. The specific capacitance is 26 1 and 495 F/g)[ 1] only 35 f/g. At the current density of 10 A/g, the specific capacitance of PAGO 10 is still around 356 F/g, which indicates that PAGO/kloc-0. Due to the synergistic effect of rGO and PANIF, the specific capacitance and rate performance of the composites are greatly improved. The rGO connected between PANIF provides a highly conductive path for electron transfer during charging and discharging. At the same time, PANIF closely connected with rGO effectively prevented the agglomeration of graphene during hydrothermal reduction, and increased the electrode/electrolyte contact area, thus improving the utilization rate and capacity of PANIF. In order to understand the electron transfer characteristics and ion diffusion path of the prepared materials more clearly, the AC impedance of the samples was tested, and the Nyquist diagrams of four samples are given in fig. 5(c). As can be seen from fig. 5(c), in the high frequency region and the low frequency region respectively.
The results show that the addition of rGO improves the conductivity of electrode materials. In the low frequency region, the linear shape reflects that the electrochemical process of the sample is controlled by diffusion, and PAGO5 shows the largest linear slope, indicating that its capacitance behavior is closest to the ideal capacitance, that is, the frequency response characteristics are the best, which is also due to the addition of rGO to improve the conductivity of the material and the unique microstructure of the composite.
The occurrence of redox reaction leads to a very high pseudocapacitance of PANIF, but the cyclic stability of PANIF is poor due to the repeated expansion and contraction of polymer chain during high current charging and discharging, which limits its practical application. Therefore, the cyclic stability of ANIF and PAGO 10 is analyzed. Fig. 6 shows that PAGO 10 is charged and discharged 100 times at a current density of 5 A/g, and the capacitance retention rate is 77%, while the PANIF electrode without rGO is charged and discharged 1000 times at a current density of 2 A/g, and the capacitance retention rate is only 54.3%, indicating that the PANIF cycle stability is poor. In addition, the close connection of PANIF/rGO formed by the addition of RGO reduces the expansion and contraction of PANI chain during charging and discharging, making the chain segment not easy to fall off or break, so PAGO 10 has excellent cycle stability.
3 Conclusion
PANIF/rGO composite electrode materials were prepared by self-assembly and hydrothermal reaction. In addition, when the mass ratio of PANIF to GO is 10∶ 1, the electrochemical performance of the composite is the best. When the current density is 1 and 10 A/g, the specific capacitance is 5 17 and 356 F/g, respectively.
Development and application of new environmental protection refractories for cement kiln.
1 overview
With the rapid popularization of new dry cement production technology in China, China's cement industry has developed rapidly. In 20 12 years, the total output of cement reached 21800,000 tons, accounting for about 55% of the world's total output. In 1960s and 1970s, magnesia-chrome refractories were widely used in the new dry-process cement kiln firing zone [1] because of their good kiln skin hanging and chemical corrosion resistance of cement clinker, and achieved good application results. However, during the use of magnesia-chrome brick, the Cr2O3 component in the brick combines with alkali and sulfur in kiln gas and kiln materials to form toxic Cr6+ compounds. [In addition, sulfur brought by raw fuel, alkali and sulfur will form another water-soluble toxic carcinogen of Cr6+: R2(Cr, S)O4. When the cement kiln is in normal operation, some Cr6+ compounds in the magnesia-chrome brick lining escape with kiln gas and dust, and fall into the factory area and surrounding environment, causing air pollution in the factory area. The other part remains in the demolished waste bricks, which will cause groundwater pollution when it meets water; The more direct harm is that Cr+6 in kiln gas and brick dust will cause poison to the field personnel when the cement kiln is broken and overhauled. According to experts' argument, Cr+ corrodes the skin, making people prone to osteoporosis and causing cancer. Therefore, magnesia-chrome refractories as cement kiln lining will cause long-term pollution and public hazards to the environment and human beings.
Developed industrial countries have a series of supporting specifications in water source, environment and sanitation, among which Germany has taken precautions against cement plants. Chromium pollution? The most common regulations are also strictly enforced. See Table 1 for details:
China promulgated the national standard GB3838-88 in April 1988, which clearly stipulated the content of Cr6+ in surface water, as shown in Table 2:
This makes the environmental protection cost of using magnesia-chrome bricks as cement kiln lining increase in cement enterprises, especially the treatment cost of used magnesia-chrome bricks is very expensive. Therefore, chrome-free refractories for cement kilns are an inevitable development trend.
Development of New Environmental Protection Refractory for Sintering Zone of Cement Kiln
2. 1 development ideas
At present, the chrome-free environmental protection refractories used in the firing zone of cement rotary kiln mainly include magnesia dolomite brick and magnesia-alumina spinel brick. Magnesium dolomite brick has good chemical compatibility with cement clinker and excellent hanging performance of kiln skin, but its thermal shock resistance and hydration resistance are poor. Magnesia-alumina spinel brick has good thermal shock resistance and erosion resistance, but the kiln skin is poor [3, 4]. The second generation of new environmental protection refractory-a new environmental protection refractory made by introducing hercynite into magnesia brick, has good structural toughness, strong corrosion resistance to alkali salt and cement clinker, and good kiln skin hanging performance, which can effectively prolong the service life of fired zone. It is a new generation of chrome-free refractory suitable for cement kiln firing zone in China. However, the key of this product is the synthesis of hercynite raw materials, the addition amount, the addition method and the influence of related technological conditions on the product performance.
2.2 Testing and research
2.2. Synthesis of1hercynite. Hercynite is a rare mineral in nature, and its chemical molecular formula is FeAl2O4, which contains 58.66% A 12O3 and 4 1.34% FeO. The hercynite has a cubic structure, with divalent cations occupying tetrahedral positions and trivalent cations filled in the face-centered cube composed of oxygen ions. Its theoretical density is 4.39 g/cm3 and Mohs hardness is 7.5. To form hercynite, it is necessary to ensure that ferrous oxide (FeO or FeOn) is in its stable state. Only in the region where FeO can exist stably can the compound formed with Al2O3 be guaranteed to be FeO? Al2O3 spinel, but under the condition outside the region where FeO exists stably, the product obtained by the reaction of iron oxide with Al2O3 is almost not FeO. Al2O3 spinel, but it can be a solid solution containing a large amount or mainly Fe2O3-Al2O3 [5]. The phase diagram of FeOn- Al2O3 is shown in figure 1:
In order to obtain high-quality synthetic hercynite, we specially invited well-known European refractory experts to give professional technical guidance. After a lot of experiments, we have mastered the key technology of sintering synthesis of hercynite, which has laid a good foundation for the production of new environmentally friendly refractories that have reached the international level. During production, FeO and Al2O3 are evenly mixed in a certain proportion and pressed into blanks. FeO？ Preparation of FeO by high temperature roasting in stable atmosphere? Sintered hercynite with Al2O3 spinel content greater than 97%. The product diffraction is shown in Figure 2:
2.2.2 Characteristics of raw materials and products ① Selection of raw materials. According to our production experience, combined with the requirements for refractories in the firing zone of cement kiln, we selected high-quality magnesia and synthetic spinel as raw materials, and added special additives to strengthen the performance of the products, and developed and produced the second generation of chrome-free magnesia spinel brick, a new type of environmental protection refractory. The physical and chemical indexes of raw materials used are shown in Table 3. ② Performance of the product. Crushing raw materials to the required granularity, adopting four-stage ingredients, and then vigorously mixing, grinding, high-pressure forming and high-temperature firing. The microstructure of the product is shown in Figure 3, and the physical and chemical indexes of the product are compared with similar foreign products in Table 4.
2.2.3 Influence of hercynite on the properties of products ① Influence of the addition of hercynite on the compressive strength of products. As can be seen from Figure 4, with the increase of hercynite, the compressive strength of the product first increases and then decreases, which is due to the mutual solubility of hercynite and magnesium oxide. When the addition of hercynite is 10%, the strength of the product reaches the maximum. (2) The effect of the addition form of hercynite on the thermal shock resistance of the product. From Table 5 of the experimental results, it can be seen that the thermal shock resistance of products added with hercynite in the form of particles is relatively better than that of products added with hercynite in the form of fine powder.
2.3 Product performance
2.3. 1 microstructure has good toughness and thermal shock stability. During the firing and use of the new environmentally friendly refractories, Fe2+ ions diffuse into the surrounding magnesium oxide matrix, and at the same time, part of Mg2+ ions diffuse into the hercynite particles, which react with the remaining alumina after the decomposition of hercynite to generate magnesia-alumina spinel. This activation effect leads to the formation of a large number of microcracks in the product during firing or use. It is important that the decomposition process of hercynite and the mutual diffusion of Fe2+ ions and Mg2+ ions continue at high temperature, so that MgO-Feal can be obtained.
A large number of micro-cracks will be formed in the whole process of refractory used at high temperature. The existence of these micro-cracks is beneficial to buffer thermal stress and improve the structural flexibility and thermal shock stability of products.
2.3.2 High strength. From the microstructure of the product, it can be seen that the hercynite and high-purity magnesium oxide in the product are mutually soluble, the structure is very uniform and dense, and the grains are well developed. The particles are connected with the matrix through the intergranular spinel, which obviously improves the density and high-temperature strength of the brick.
2.3.3 Good kiln skin adhesion performance. In the course of use, Fe2O3 and Al2O3 in the product can easily react with CaO in cement clinker to generate low-melting-point minerals such as C2F and C4AF, which have certain viscosity and can firmly adhere to the hot surface of new environmentally-friendly refractories to form a stable kiln skin. We directly combine the new environmental protection refractory and magnesia-chrome brick into 40mm? 40mm？ 60mm test block, pressed with 90% cement raw meal +5% pulverized coal +5%K2SO4? 30? 10mm round cake, put the round cake between two sample blocks, heat it in an electric furnace, raise the temperature to 1500℃, keep the temperature for 3 hours, and measure its flexural strength after cooling. The two are basically the same. It can be seen that the new environmental protection refractory has excellent performance of sticking kiln skin.
2.4 Application of products
Since 20 12 new environmental protection refractory was successfully developed and put on the market, it has passed through more than 20 large cement enterprises such as Hebei Luquan Quzhai Cement Company, Ningxia Yinghai Tianchen Cement Company, Inner Mongolia Hadatu Cement Company, Shaanxi Bai Yao Cement Group, North Cement Group, Henan Vivian Dawson Cement Company, Xinjiang Tianji Cement Company, Anyang Hu Bo Cement Company, etc.
3 Conclusion