TRANSITION-METAL-OXIDE-BASED NANOSTRUCTURES AS SUPERCAPACITOR ELECTRODES ZHANG JINTAO NATIONAL UNIVERSITY OF SINGAPORE 2012 TRANSITION-METAL-OXIDE-BASED NANOSTRUCTURES AS SUPERCAPACITOR ELECTRODES ZHANG JINTAO (M. Sci. Shandong University) A THESIS SUBMITTED FOR THE DEGREE OF DOCTOR OF PHILOSOPHY DEPARTMENT OF CHEMICAL & BIOMOLECULAR ENGINEERING NATIONAL UNIVERSITY OF SINGAPORE 2012 Acknowledgement Acknowledgement I would like to convey my deepest appreciation to my supervisors, Associate. Prof. Jiang Jianwen and Prof. Zhao X. S., George for their constant encouragement, invaluable guidance, patience, and encouragement. It is no exaggeration to say that I could not complete the PhD work without their generous help. I would like to express my special thanks to Prof. Zhao for his guidance on the research work and writing scientific papers. I would also like to take this opportunity to acknowledge my oral defense examiners, Dr. Xie, A/P. Hong, and Dr. Lu., thesis examiners, and oral qualification examination committee, for offering inspired suggestion and comments on this thesis. In addition, I want to express my sincerest appreciation to Department of Chemical & Biomolecular Engineering for offering me the chance to study at NUS. Particular acknowledgement goes to Mr. Chia Phai Ann, Dr. Yuan Zeliang, Mr. Mao Ning, and Mr. Liu Chicheng for their kind supports. It’s my pleasure to work with a group of brilliant, warmhearted and lovely labmates, Dr. Chen Yifei, Dr. Anjaiah Nalaparaju, Dr. Lv Lu, Dr. Wang Likui, Dr. Bai Peng, Dr. Lei Zhibin, Dr. Xiong Zhigang, Dr. Lee Fang Yin, Dr. Liu Jiajia, Dr. Tian Xiao Ning, Dr. Zhang Li Li, Dr. Wu Pingping, Mr. Dou Haiqing, Mr. Cai Zhongyu, Mr. Xu Chen, Mr. Zhou Rui, Mr. Yu Yong, Mr. Han Gang, Ms. Ma Jizhen, Ms. Zhao Shanyu, and Dr. Nikolay Christov Christov. I would like to express my heartfelt thanks to my family, particular thanks to my wife. Without their boundless love, encouragement and support, this research could not have been completed successfully. i Table of Contents Table of Contents Acknowledgement . i Table of Contents ii Summary vi Nomenclature . ix List of Tables x List of Figures xi List of publications . xvii CHAPTER INTRODUCTION 1.1 Background . 1.2 Objectives and scope of thesis 1.3 Structure of this thesis . CHAPTER LITERATURE REVIEW . 2.1 Working principles and methods of experimental evaluation of supercapacitors 2.1.1 Energy storage in supercapacitors . 2.1.2 Principles and methods of experimental evaluation 10 2.2 Electrode materials for supercapacitors 17 2.2.1 Carbon materials 17 2.2.2 Conducting polymers . 25 2.2.3 Transition-metal-oxide-based materials . 27 CHAPTER EXPERIMENTAL SECTION 43 3.1 Reagents and apparatus . 43 3.2 Characterization techniques 44 3.2.1 Fourier transform infrared (FT-IR) spectrometer 44 3.2.2 Thermogravimetric analysis (TGA) . 44 3.2.3 Electron microscope . 45 3.2.4 Physical adsorption of nitrogen . 45 ii Table of Contents 3.2.5 Power X-ray diffraction (XRD) . 45 3.2.6 X-ray photoelectron spectroscopy (XPS) 46 3.2.7 Raman spectroscopy 46 CHAPTER SYNTHESIS AND CAPACITIVE PERFORMANCE OF MANGANESE OXIDE NANOSTRUCTURES 47 4.1 Introduction . 47 4.2 Experimental section . 48 4.2.1 Synthesis of MnO2 nanostructures . 48 4.2.2 Material characterization . 48 4.3 Results and Discussion . 49 4.3.1 Synthesis and characterization of hydrous MnO2 nanostructures 49 4.3.2 Capacitive performance of hydrous manganese dioxide nanostructures. 59 4.4 Summary . 66 CHAPTER SYNTHESIS AND CAPACITIVE PERFORMANCE OF MANGANESE OXIDE NANOSHEETS DISPERSED ON FUNCTIONALIZED GRAPHENE SHEETS 68 5.1 Introduction . 68 5.2 Experimental Section 69 5.2.1 Preparation of chemically exfoliated graphene oxide 69 5.2.2 Functionalization of RGO with poly(diallyldimethylammonium chloride) 70 5.2.3 Preparation of a colloidal suspension of manganese dioxide nanosheets 70 5.2.4 Dispersion of manganese dioxide nanosheets on FRGO-p 71 5.2.5 Preparation of Na-typed birnessite 71 5.2.6 Characterization 71 5.3 Results and Discussion . 72 5.4 Summary . 89 CHAPTER A COMPARATIVE STUDY OF MnO2-CARBON COMPOSITE MATERIALS AS SUPERCAPACITOR ELECTRODES . 90 6.1 Introduction . 90 iii Table of Contents 6.2 Experimental Section 92 6.2.1 Preparation of MnO2 clusters and MnO2–carbon composites . 92 6.2.2 Characterization . 92 6.3 Results and discussion 93 6.4 Summary . 109 CHAPTER TEMPLATE SYNTHESIS OF RUTHENIUM OXIDE NANOTUBES AS SUPERCAPACITOR ELECTRODES . 111 7.1 Introduction . 111 7.2 Experimental Section 112 7.2.1 Preparation of manganite nanorods . 112 7.2.2 Preparation of ruthenium oxide nanotubes 113 7.2.3 Preparation of ruthenium oxide nanoparticles . 113 7.2.4 Characterization . 113 7.3 Results and Discussion . 113 7.4 Summary . 129 CHAPTER FABRICATION OF ASYMMETRIC SUPERCAPACITOR WITH GRAPHENE-BASED MATERIALS . 131 8.1 Introduction . 131 8.2 Experimental section . 133 8.2.1 Deposition of RuO2 nanoparticles on RGO sheets 133 8.2.2 Preparation of polyaniline-modified RGO sheets 133 8.3 Characterization 133 8.4 Results and Discussion . 134 8.5 Summary . 147 CHAPTER CONCLUSIONS AND RECOMMENDATIONS . 148 9.1 Conclusions 148 9.2 Recommendations 150 iv Table of Contents REFERENCES . 153 APPENDIX . 170 CHAPTER A1 SYNTHESIS AND CHARACTERIZATION OF RGO–METAL– OXIDE COMPOSITES 170 A1.1 Introduction 170 A1.2 Experimental section 171 A1.3 Results and discussion . 172 A1.4 Summary 181 A1.3 References 181 v Summary Summary Electrode material with desirable properties is the key for realizing highperformance supercapacitors. In the thesis work, transition-metal-oxide-based nanostructures, manganese dioxide nanostructures, ruthenium oxide nanotubes, as well as composite materials consisting of transition metal oxide and reduced graphene oxide (RGO) were prepared, characterized, and evaluated as supercapacitor electrodes. Manganese dioxide (MnO2) nanostructures were synthesized by using a redox reaction method at a mild temperature. The morphologies of MnO2 nanostructures were found to be different in the reaction systems with different pH. With increasing the pH of reaction systems, the morphology of MnO2 nanostructures changed from urchin-like structures to nanobelts. The electrochemical results revealed that the electrocapacitive performance of MnO2 nanostructures depended on their microstructural properties in terms of particle size, surface area, and crystallinity. MnO2 nanostructures with a high surface area obtained in base solution exhibited the superior performance in comparison with other MnO2 nanostructures. A nanotubular ruthenium oxide was prepared by using manganite nanorods as a morphology template. Notably, the template dissolved away completely during the formation of ruthenium oxide nanotubes. A mechanism was proposed to interpret the formation of ruthenium oxide nanotubes. The ruthenium oxide nanotubes exhibited better electrochemical performance than that of ruthenium oxide nanoparticles. The results showed that the unique nanotubular structure and proton-rich electrolyte are essential to achieve the high capacitive performance of ruthenium oxide nanotubes. A composite material consisting of MnO2 nanosheets and functionalized RGO (FRGO-p) sheets was prepared by making full use of the electrostatic interaction between negative charged MnO2 nanosheets and positive charged FRGO-p sheets. The vi Summary composite material (FRGO-p-MnO2) exhibited an enhanced capacitive performance in comparison with pure FRGO-p and Na-typed birnessite sheets. The FRGO-p sheet served as a good support led to effective charge transfer for redox reactions of MnO2. In addition, anchoring of MnO2 nanosheets on FRGO-p sheets prevented the latter from agglomeration, resulting in facile ion transportation pathway for electrolyte to access the surface of active material. Therefore, the superior microstructure of FRGOp-MnO2 composite led to a synergic effect between the two components, which contributed to not only an enhanced specific capacitance but also a good rate capability. The combination of manganese oxide and carbon material is effective to improve the electrochemical performance of electrode materials. For comparison, RGO, carbon nanotube (CNT), and carbon black (Vulcan XC-72) were used to prepare nanocomposites with birnessite-type MnO2 clusters by a room-temperature solution growth method. The synergetic effect between MnO2 and carbon materials resulted in enhanced capacitive performance. Among the three composite materials, RGO and MnO2 composite (RGO-MnO2) performed the best with a specific capacitance as high as 260 F/g at a current density of 0.3 A/g. The unique structure of two-dimensional RGO sheets provided much efficient synergetic effect to minimize the equivalent series resistance (ESR), which led to the enhanced performance in terms of large specific capacitance and better high-rate capability. The results demonstrated a facile approach to incorporate RGO sheets with MnO2 to form a robust composite material as supercapacitor electrode. Fabrication of an asymmetrical suoercapacitor (ASC) was demonstrated by using RGO modified with ruthenium oxide (RGO–RuO2) and polyaniline (RGO–PANi) as positive and negative electrodes, respectively. In comparison with the symmetric counterparts, the ASC yielded a significantly enhanced energy density and a high vii Summary power density. For example, an ASC fabricated with only about 17 wt% Ru loading exhibited an energy density as high as 26.3 Wh/kg and a high power density of 49.8 kW/kg. The energy density was about two times higher than that of symmetrical supercapacitor, RGO–RuO2//RGO–RuO2 (12.4 Wh/kg) and RGO–PANi//RGO–PANi (13.9 Wh/kg). This study demonstrated a facile and efficient way to construct highperformance supercapacitors. viii References Ye, J. S., H. F. Cui, X. Liu, T. M. Lim, W. D. Zhang and F. S. Sheu. Preparation and Characterization of Aligned Carbon Nanotube–Ruthenium Oxide Nanocomposites for Supercapacitors. Small 1(5): pp.560-565. 2005. Yu, P., X. Zhang, D. Wang, L. Wang and Y. Ma. Shape-controlled synthesis of 3D hierarchical MnO2 nanostructures for electrochemical supercapacitors. Cryst. Growth Des. 9(1): pp.528-533. 2009. Yuan, C., L. Chen, B. Gao, L. Su and X. Zhang. Synthesis and utilization of RuO2 ·xH2O nanodots well dispersed on poly(sodium 4-styrene sulfonate) functionalized multi-walled carbon nanotubes for supercapacitors. J. Mater. Chem. 19(2): pp.246-252. 2009a. Yuan, C., X. Zhang, L. Su, B. Gao and L. Shen. Facile synthesis and self-assembly of hierarchical porous NiO nano/micro spherical superstructures for high performance supercapacitors. J. Mater. Chem. 19(32): pp.5772-5777. 2009b. Zhang, H., G. Cao, Z. Wang, Y. Yang, Z. Shi and Z. Gu. Growth of Manganese Oxide Nanoflowers on Vertically-Aligned Carbon Nanotube Arrays for High-Rate Electrochemical Capacitive Energy Storage. Nano Letters 8(9): pp.2664-2668. 2008. Zhang, J., J. Ma, J. Jiang and X. S. Zhao. Synthesis and capacitive properties of carbonaceous sphere@MnO2 rattle-type hollow structures. J. Mater. Res. 25(8): pp.1476-1484. 2010a. Zhang, J., J. Ma, L. L. Zhang, P. Guo, J. Jiang and X. S. Zhao. Template synthesis of tubular ruthenium oxides for supercapacitor applications. J. Phys. Chem. C 114(32): pp.13608-13613. 2010b. Zhang, J. T., W. Chu, J. W. Jiang and X. S. Zhao. Synthesis, characterization and capacitive performance of hydrous manganese dioxide nanostructures. Nanotechnology 22(12): pp.25703. 2011a. Zhang, J. T., J. W. Jiang and X. S. Zhao. Synthesis and Capacitive Properties of Manganese Oxide Nanosheets Dispersed on Functionalized Graphene Sheets. J. Phys. Chem. C 115(14): pp.6448-6454. 2011b. Zhang, J. T., Z. Xiong and X. S. Zhao. Graphene-metal-oxide composites for the degradation of dyes under visible light irradiation. J. Mater. Chem. 21(11): pp.36343640. 2011c. Zhang, K., L. L. Zhang, X. S. Zhao and J. Wu. Graphene/Polyaniline Nanofiber Composites as Supercapacitor Electrodes. Chem. Mater. 22(4): pp.1392-1401. 2010c. Zhang, L. L., S. Li, J. T. Zhang, P. Z. Guo, J. Zheng and X. S. Zhao. Enhancement of Electrochemical Performance of Macroporous Carbon by Surface Coating of Polyanilineb. Chem. Mater. 22(3): pp.1195-1202. 2010d. Zhang, L. L., T. Wei, W. Wang and X. S. Zhao. Manganese oxide-carbon composite as supercapacitor electrode materials. Micro. Meso. Mater. 123(1-3): pp.260-267. 2009a. 168 References Zhang, L. L. and X. S. Zhao. Carbon-based materials as supercapacitor electrodes. Chem. Soc. Rev. 38(9): pp.2520-2531. 2009. Zhang, L. L., R. Zhou and X. S. Zhao. Graphene-based materials as supercapacitor electrodes. J. Mater. Chem. 20(29): pp.5983-5992. 2010e. Zhang, S. W. and G. Z. Chen. Manganese oxide based materials for supercapacitors. Energy Mater. 3(3): pp.186-200. 2008. Zhang, X.-Y., H.-P. Li, X.-L. Cui and Y. Lin. Graphene/TiO2 nanocomposites: synthesis, characterization and application in hydrogen evolution from water photocatalytic splitting. J. Mater. Chem. 20(14): pp.2801-2806. 2010f. Zhang, Y., H. Feng, X. Wu, L. Wang, A. Zhang, T. Xia, H. Dong, X. Li and L. Zhang. Progress of electrochemical capacitor electrode materials: A review. Int. J. Hydrogen Energ. 34(11): pp.4889-4899. 2009b. Zhao, D.-D., S.-J. Bao, W.-J. Zhou and H.-L. Li. Preparation of hexagonal nanoporous nickel hydroxide film and its application for electrochemical capacitor. Electrochem. Commun. 9(5): pp.869-874. 2007a. Zhao, D. D., W. J. Zhou and H. L. Li. Effects of deposition potential and anneal temperature on the hexagonal nanoporous nickel hydroxide films. Chem. Mater. 19(16): pp.3882-3891. 2007b. Zhao, X., H. Tian, M. Zhu, K. Tian, J. J. Wang, F. Kang and R. A. Outlaw. Carbon nanosheets as the electrode material in supercapacitors. J. Power Sources 194(2): pp.1208-1212. 2009. Zheng, J. P. High energy density electrochemical capacitors without consumption of electrolyte. J. Electrochem. Soc. 156(7): pp.A500. 2009. Zheng, J. P., P. J. Cygan and T. R. Jow. Hydrous ruthenium oxide as an electrode material for electrochemical capacitors. J. Electrochem. Soc. 142(8): pp.2699-2703. 1995. Zheng, J. P. and T. R. Jow. A New Charge Storage Mechanism for Electrochemical Capacitors. J. Electrochem. Soc. 142(1): pp.L6-L8. 1995. Zhou, W.-J., J. Zhang, T. Xue, D.-D. Zhao and H.-L. Li. Electrodeposition of ordered mesoporous cobalt hydroxide film from lyotropic liquid crystal media for electrochemical capacitors. J. Mater. Chem. 18(8): pp.905-910. 2008. Zhu, Y., S. Murali, M. D. Stoller, K. J. Ganesh, W. Cai, P. J. Ferreira, A. Pirkle, R. M. Wallace, K. A. Cychosz, M. Thommes, D. Su, E. A. Stach and R. S. Ruoff. CarbonBased Supercapacitors Produced by Activation of Graphene. Science 332(6037): pp.1537-1541. 2011. Zhu, Y., S. Murali, M. D. Stoller, A. Velamakanni, R. D. Piner and R. S. Ruoff. Microwave assisted exfoliation and reduction of graphite oxide for ultracapacitors. Carbon 48(7): pp.2118-2122. 2010. 169 APPENDIX APPENDIX CHAPTER A1 SYNTHESIS AND CHARACTERIZATION OF RGO– METAL–OXIDE COMPOSITES A1.1 Introduction As demonstrated in Chapters and 6, reduced graphene oxide (RGO) offers a good opportunity to prepare composite materials with metal oxides for wide applications due to synergistic effect between the two counterparts.1–4 RGO is generally prepared by chemical oxidation of graphite to exfoliated sheets of graphene oxide (GO), followed by reduction with sodium borohydride or highly toxic hydrazine.5,6 However, RGO dispersion tends to aggregate in aqueous solution because of the loss in surface oxygen–containing groups.6-8 GO can also be reduced to RGO by other means. Recently, RGO was modified with P25 TiO2 using a hydrothermal treatment method. During the hydrothermal treatment process, GO was reduced to RGO. The obtained P25–RGO composite exhibited a good photocatalytic activity towards the degradation of methyl blue.3 UV-assisted photocatalytic reduction of GO was proven to be useful to prepare RGO–semiconductor composites, including RGO– TiO2 and RGO–ZnO.9,10 Cao et al.11 reported the preparation of RGO–CdS nanocomposite by a one-step reaction. In the reaction, GO could be reduced solvothermally to RGO in dimethyl sulfoxide (DMSO) at 180 °C. In the presence of cadmium acetate, DMSO, as a source of sulfur, results in the formation of CdS quantum dots on the surface of RGO. 170 APPENDIX In this Chapter, RGO was modified with tin dioxide (SnO2) and titanium dioxide (TiO2) by a direct redox reaction between GO and SnCl2 or TiCl3 at 90 °C as illustrated in Scheme A1.1. Here, GO was reduced by SnCl2 or TiCl3 to RGO, and in the meantime, the metal ions were oxidized to SnO2 or TiO2 nanoparticles, depositing on the surface of the RGO. The obtained samples are designated as RGO–SnO2 and RGO–TiO2, respectively. The present method offers several advantages, including (1) an extra reducing agent such as toxic hydrazine is not required for the reduction of GO to RGO, (2) the in–situ growth of metal oxides leads to the formation of uniform nanoparticles on individual RGO sheets, and (3) the process can be carried out under mild conditions. Scheme A1.1. A scheme showing the preparation of samples RGO-SnO2 and RGOTiO2. A1.2 Experimental section Sample Preparation and Characterization 171 APPENDIX In the preparation of RGO–TiO2, mL of the GO dispersion (4.8 mg/mL) was diluted in 50 mL of DI water. mL of titanium trichloride (TiCl3, 15 wt.%, Sigma– Aldrich) was added to 50 mL of HCl solution (1 mL 37 wt.% HCl, Sigma–Aldrich). The two solutions were mixed and sonicated for min. The resulting mixture was stirred at 90 °C for h. The same procedure was employed to prepare RGO–SnO2 except that 0.14 g of tin(II) chloride dihydrate (SnCl2·2H2O, 98%, Sigma–Aldrich) was used. The RGO-metal oxide solids were collected by centrifuge, washed several times with DI water, and dried in a vacuum oven at 60 °C. Samples were investigated by using XRD, FESEM, HRTEM, XPS techniques (the details in Chapter 3). A1.3 Results and discussion Figure A1.1 shows the XRD patterns of samples GO, RGO, RGO–SnO2, and RGO–TiO2. For sample GO, the sharp peak (Figure A1.1a) at about 2 = 10.4 ° corresponds to the (002) reflection of stacked GO sheets with an interlayer spacing of 0.86 nm, larger than that of pristine graphite (0.34 nm). This suggests the introduction of oxygen-containing groups on the GO sheets. After chemical reduction of GO, only a broad peak centered at around 24 °, corresponding to an interlayer spacing of about 0.37 nm, is observed on sample RGO (Figure A1.1b), indicating the presence of residual oxygen–containing groups on the RGO sheets. For samples of RGO–SnO2 and RGO–TiO2, no diffraction peak of layered GO can be seen, indicating the absence of layer-stacking regularity after reduction by the cations.4,7 The XRD peaks at about 2 = 26.6 °, 33.9 °, and 51.6 °(Figure A1. 1c) can be indexed to the diffractions of SnO2 (110), (101), and (211) planes (JCPDS, No. 41–1145).12 The reflection peaks at 2 = 25.4 °, 37.9 °, 48.0 °, 54.6 °(Figure A1. 1d) can be indexed to (101), (103, 004, and 112), (200), (105 and 211) crystal planes of anatase phase (JCPDS, No. 21–1272).13,14 172 APPENDIX The broad diffraction peaks for RGO–SnO2 and RGO–TiO2 indicate the small particle sizes and/or poor crystallization of SnO2 and TiO2 particles. Figure A1.1 XRD patterns of (a) graphene oxide, (b) reduced graphene oxide, (c) RGO–SnO2, and (d) RGO–TiO2. The standard cards of SnO2 and TiO2 are also included in the Figure for reference. XPS was employed to characterize the surface chemical compositions and the valence states. Figure A1.2a shows the survey XPS spectra of RGO–SnO2 and RGO– TiO2. The peaks due to Sn and Ti reveal the presence of Sn and Ti species in the composites. The core–level XPS signals of Sn 3d are shown in Figure A1.2b. The binding energies centered at 487.3 eV and 495.7 eV are due to Sn4+, suggesting the formation of SnO2 on the RGO sheets.15–17 Two peaks centered at 458.5 and 464.5 eV can be seen from the Ti 2p spectrum (Figure A1.2c) that are assigned to Ti 2p1/2 and Ti 2p3/2, respectively, in good agreement with the binding energy values of Ti4+ in pure anatase.18 The core–level XPS signals of C 1s shown in Figure A1.2d are deconvoluted into three components. For the GO sample, the main peak centered at about 284.6 eV originates from the graphitic sp2 carbon atoms. The binding energy located at 286.7 eV 173 APPENDIX and 287.9 eV are due to carbon atoms connecting with oxygenate groups, such as C–O and O– C=O.19 The C 1s XPS spectrum of the RGO sample exhibits the same peaks Figure A1.2 (a) XPS survey spectra of RGO–SnO2 and RGO–TiO2. Core–level XPS spectra of (b) Sn 3d, (c) Ti 2p. C 1s XPS spectra of (d) GO, (e) RGO, (f) RGO–SnO2, and (g) RGO–TiO2. to these of the GO sample, but the intensity of the peaks related to oxygenate functionalities is weaker than that of the GO. The small peaks related to oxygenate groups indicates the presence of residual oxygenate groups on the RGO sample. The C 1s XPS signal of RGO–SnO2 in Figure A1.2f shows a main peak at 284.6 eV and other 174 APPENDIX peaks at 286.1 and 288.9 eV due to residually oxygen–containing groups. The weaker peaks due to oxygenate groups suggest a considerable de–oxygenation and the formation of RGO.7 A similar XPS spectrum of C 1s is seen for sample RGO–TiO2. The peak at 286.7 eV indicates the increase in the residual oxygenate groups compared to RGO–SnO2. The XPS results show that most oxygenate groups are removed during the redox reactions. A small amount of residual oxygenate groups on RGO–SnO2 and RGO–TiO2 are believed to be favorable for maintaining a good dispersion of the composite nanosheets. TEM image of the chemically exfoliated GO sheet is shown in Figure A1.3a. The presence of wrinkles and folds on the sheet is the characteristic feature of a single– layer GO sheet. Figure A1.3b shows a smooth graphene sheet decorated with SnO2 nanoparticles, suggesting the uniform deposition of SnO2 nanoparticles on the RGO sheet. A HRTEM image of the RGO–SnO2 sheet (Figure A1.3c) reveals that the whole surface of the RGO sheet decorated with worm–like SnO2 nanoparticles. The lattice– resolved image of RGO–SnO2 (Figure A1.3b) shows a lattice spacing of 0.33 nm, corresponding to the d–spacing of (110) planes of SnO2.17 Energy–dispersive X–ray (EDX) spectrum of RGO–SnO2 (the inset in Figure A1.3b) exhibits the presence of C, O, and Sn elements, further confirming the formation of RGO–SnO2 composite. The TEM image of RGO–TiO2 shown in Figure A1.4a displays a similar morphology to that of RGO–SnO2. It is seen that TiO2 nanoparticles uniformly dispersed on the RGO sheets. The inset is the corresponding EDX spectrum, revealing the presence of C, Ti and O. The high–magnification TEM image shown in Figure A1.4b shows that the TiO2 nanoparticles randomly oriented on the RGO sheets. Figure A1.4c shows the atomic force microcopy (AFM) image of sample RGO–TiO2. The lateral dimension of the RGO–TiO2 sheet is on the order of micrometers. The layered structure with 175 APPENDIX Figure A1.3 (a) TEM image of exfoliated GO sheets. (b) and (c) TEM images of RGO–SnO2. The inset of Figure A1.3b is the EDX spectrum. (d) High–resolution TEM image of SnO2 nanoparticles on a RGO sheet. Figure A1.4 (a) and (b) TEM images of RGO–TiO2. The inset of Figure 4(a) is the EDX spectum. (c) AFM image of RGO–TiO2. (d) An enlarged AFM image of RGO– TiO2. The section line shows that the thickness of RGO–TiO2 was less than 15 nm. 176 APPENDIX wrinkles and folds can be clearly seen from the TEM and AFM images in Figures A1.4a and 4c as indicated by the arrows, showing the characteristic feature of single– layer RGO sheets decorated with nanoparticles.11 The enlarged AFM image shown in Figure A1.4d demonstrates that TiO2 nanoparticles cover the whole surface of the RGO sheet with a thickness of less than 15 nm. The surface of GO is covered with abundant of oxygen–containing groups, such as epoxy, hydroxyl, and carboxylic acid, which are favorable for interacting with metal cations.20-22 In the present case, reactive Sn2+ and Ti3+ cations with a strong reduction ability gradually reduced the oxygen–containing groups of GO. Simultaneously, the oxidation of the metal ions resulted in the formation of metal oxide nanoparticles on the RGO sheets.23 The redox reactions led to the in–situ formation of RGO–SnO2 and RGO–TiO2 nanocomposites (Scheme 1). However, SnO2 and TiO2 nanocrystals formed on the RGO sheets exhibited different morphologies, despite that the particles formed via the same mechanism. At 60 °C, petal–like TiO2 clusters with much free spaces of RGO sheets were observed whereas RGO sheets were covered with the worm–like SnO2 nanoparticles for sample RGO–SnO2 (Figures A1.5). Dai and co– workers24 reported that the size, morphology, and crystallinity of nanocrystals formed on graphene are dependent on the oxidation degree of graphene substrate due to the different interactions of coating species with graphene sheets. In the present work, the growth of SnO2 and TiO2 nanocrystals with different morphologies on the RGO sheets would be attributed to the different reduction abilities and hydrolysis rates of Sn2+ and Ti3+. Sn2+ is extremely easy to interact with oxygen containing groups,25 leading to a uniform growth and dispersion of SnO2 nanocrystals on the RGO sheets. In contrast, the reducing ability of Ti3+ is stronger than that of Sn2+, because the standard electrode potential of TiO2+/Ti3+ (0.10 V vs. NHE) is lower than that of Sn4+/Sn2+ (0.15 V vs. 177 APPENDIX NHE), resulting in the faster reduction of GO. In addition, the lower rate of hydrolysis of Ti3+ led to the preferable growth of TiO2 nanocrystals rather than on the RGO sheets, resulting in the formation of aggregated TiO2 particles and more free spaces of the RGO sheets (indicated by the arrows in Figure A1.5b).26 Figure A1.5 TEM image of RGO-TiO2 (a) and RGO-SnO2 (c) prepared at 60 °C. b) The enlarged TEM image of RGO-TiO2 (b) and RGO-SnO2 (d). Figure A1.6 shows the N2 adsorption–desorption isotherms and the BJH pore size distribution (PSD) curves of samples RGO–SnO2 and RGO–TiO2. A Type IV isotherm is seen from Figures A1.6a and 6b, indicating the presence of mesopores in the composites.27 The BJH pore size distribution curve of sample RGO–SnO2 shown in Figure A1. 6c derived from the desorption branch indeed showed a narrow distribution centered at 3.4 nm. In contrast, the corresponding PSD curve of sample RGO–TiO2 showed two pore size distributions centered at 3.6 nm and 8.6 nm, respectively. The mesopores would be attributed to the interstitial space between the nanoparticles and 178 APPENDIX the interlayers of RGO–SnO2 and RGO–TiO2. The BET surface areas for RGO–TiO2 and RGO–SnO2 were estimated to be 341 m2/g and 241 m2/g, respectively, much larger than that reported previously.26,28 After reduction of GO, RGO sheets are easy to aggregate due to the removal of oxygenate groups, resulting in the loss in specific surface area. The decoration of both sides of RGO with SnO2 or TiO2 nanoparticles is an effective approach to depressing the aggregation of the RGO sheets, leading to the high BET surface area. Figure A1.6 N2 adsorption–desorption isotherms of (a) RGO–SnO2 and (b) RGO– TiO2. (c) The corresponding pore size distributions. 179 APPENDIX On the basis of TGA, the mass ratio of SnO2 and TiO2 in the RGO–SnO2 and RGO– TiO2 composites was about 85 wt.%. Importantly, the composite materials can be processed as free–standing film by using a vacuum filtration method as shown in Figure A1.7. The diameter of the free–standing film is about cm. The free–standing film with different sizes can be fabricated according to need. The thickness is determined by the concentration and volume of the composite suspension. Figure A1.7 Photographs of free-standing RGO–SnO2 (a) and RGO–TiO2 films (b). The electrochemical characterization exhibited that the capacitive performance of RGO-SnO2 and RGO-TiO2 is not good enough (the data was not shown here). Thus, it is not desirable as electrode materials for supercapacitors, although the composite materials exhibited enhanced lithium storage abilities in lithium ion batteries.29-32 It is worthy of note that the RGO-SnO2 and RGO-TiO2 exhibited very interesting photocatalytic properties for degradation of organic dyes under visible light irradiation.33 However, the photocatalytic properties are beyond the scope of this thesis, which are not presented here. 180 APPENDIX A1.4 Summary As demonstrated in this Chapter by using Sn2+ and Ti3+ as examples, chemically exfoliated graphene oxide can be reduced by metal ions to in–situ form composite materials consisting of reduced graphene oxide and SnO2 or TiO2 nanoparticles. The composite materials exhibit a layered structure with well–dispersed nanoparticles on the surface of reduced graphene oxide, which are expected to be used in the promising applications as photocatalysts and electrode materials for lithium ion batteries. A1.3 References I. 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Chem. 2011, 21, 3634. 183 [...]... of carbon-manganese oxide composite materials as supercapacitor electrodes  optimize the energy and power densities by fabricating asymmetric supercapacitor in an aqueous electrolyte The results presented in this thesis may provide simple and effective approaches to preparing transition- metal- oxide- based nanomaterials as supercapacitor electrodes In addition, devicing asymmetric supercapacitors in... capacitive properties of transition- metal- oxide by preparing manganese oxide and ruthenium oxide nanostructures with controllable morphology and structure as supercapacitor electrodes  design nanocomposite materials consisting of transition- metal- oxide and reduced graphene oxide for high-performance supercapacitors  identify the effects of different carbon materials (reduced graphene oxide, carbon nanotube,... and methods of experimental evaluation of supercapacitors 2.1.1 Energy storage in supercapacitors On the basis of electrode materials used, there are generally two types of supercapacitors, namely, electrical double-layer capacitors (EDLCs) with carbon materials as the electrodes and pseudocapacitors with transition metal oxides or conducting polymers as the electrodes (Zhang and Zhao, 2009) Both charge... symmetric supercapacitors RGO-RuO2//RGO-RuO2 with different Ru loadings Figure 8.9 Cyclic voltammograms at different scan rates (a) and charge/discharge curves at a current density of 2.0 A/g (b) for asymmetric supercapacitor xv List of Figures RGO-RuO2//RGO-PANi Specific capacitances of symmetric and asymmetric supercapacitors based on RGO-RuO2 and RGO-PANi electrodes (c) Cycle stability of asymmetric supercapacitor. .. Graphene Oxide Sheets as High-performance Supercapacitor Electrodes J Phys Chem C, 2012, 116, 5420–5426 3 Jintao Zhang, Jianwen Jiang, Hongliang Li, and X S Zhao A highperformance asymmetric supercapacitor fabricated with graphene -based electrodes Energy & Environmental Science, 2011, 4, 4009–4015 Chapter 8 4 Jintao Zhang, Jianwen Jiang, and X S Zhao Synthesis and Capacitive Properties of Manganese Oxide. .. materials for high-performance supercapacitors 1.2 Objectives and scope of thesis To develop high-performance supercapacitors, a couple of fundamental issues, such as low energy density, must be addressed The energy density of commercial supercapacitors based on carbon electrodes is generally less than 10 Wh/kg, much lower than that of batteries While metal oxide or conducting polymer electrodes are available... by configuring smart supercapacitors with appropriate electrode materials Thus, to exploit advanced electrode materials is the key to develop high-performance supercapacitors With these considerations, this thesis aims to design and prepare novel transitionmetal -oxide- based materials with enhanced electrochemical performance in terms of high energy and power densities as well as good cycling stability... 2000; Zhang and Zhao, 2009) The existing commercial and the tz developing supercapacitor devices can be mainly divided into three main types: symmetric supercapacitor, asymmetric supercapacitor, and hybrid supercapacitor Simply, the two electrodes are the same for symmetric supercapacitors and are different for asymmetric and hybrid supercapacitors The conventional three-electrode systems are suitable... X S Zhao Carbonbased materials as supercapacitor electrodes Chem Soc Rev 38(9): pp.2520-2531 2009] Copyright (2009) The Royal Society of Chemistry With respect to pseudocapacitors, the pseudocapacitance is faradic in origin, involving reversible redox reactions of electro-active species at or near the electrode surface Hydrous ruthenium oxide (RuO2•xH2O) is a typical example of metal oxide 8 Chapter... µm tall and less than 1 nm thick (c) A virtual supercapacitor- cell containing carbon nanosheets as the electrode material A rolled sandwiched-pad xi List of Figures forms the supercapacitor The sandwich-pad contains two conductive electrodes as current collectors It has one insulating layer as an ion permeable separator Carbon nanosheets are filled in as electrode material Left corner inset shows the . TRANSITION- METAL- OXIDE- BASED NANOSTRUCTURES AS SUPERCAPACITOR ELECTRODES ZHANG JINTAO NATIONAL UNIVERSITY OF SINGAPORE 2012 TRANSITION- METAL- OXIDE- BASED. supercapacitors. In the thesis work, transition- metal- oxide- based nanostructures, manganese dioxide nanostructures, ruthenium oxide nanotubes, as well as composite materials consisting of transition. materials consisting of transition metal oxide and reduced graphene oxide (RGO) were prepared, characterized, and evaluated as supercapacitor electrodes. Manganese dioxide (MnO 2 ) nanostructures