Electrodeposited Transition Metal Oxide and Selenide Thin Films for Supercapacitor Zhenjun Qi A thesis in fulfilment of the requirements for the degree of Doctor of Philosophy School of Materials Science & Engineering Faculty of Science University of New South Wales September 2020 PLEASE TYPE THE UNIVERSITY OF NEW SOUTH WALES Thesis/Dissertation Sheet Surname or Family name: QI First name: ZHENJUN Other name/s: Abbreviation for degree as given in the University calendar: Doctor of philosophy School: School of Materials Science and Engineering Faculty: Faculty of Science Title: Electrodeposited Transition Metal Oxide and Selenide Thin Films for Supercapacitor Abstract 350 words maximum: (PLEASE TYPE) As electrode materials of supercapacitors, transition metal oxide and selenide can exhibit superior specific capacitance and energy density, owing to their rich valence states for reversible faradaic reactions. In this dissertation, the simple and fast electrochemical deposition method has been implied to directly deposit transition metal oxide and selenide on suitable substrates to form thin films, which effectively avoids the use of polymer binders and conductive additives, thereby reducing the loss of electrode capacity. Moreover, the morphology, structure and composition of thin films can be controlled for better performance by simply adjusting the electrodeposition parameters, such as temperature, current, potential, and the composition of electrolyte. The main aspects of this research are as follows: (i) A temperature-controlled electrodeposition method was designed and utilized to Mn3O4 and Mn3O4/MnOOH nanocomposite thin films on FTO substrates. The performance of Mn3O4 based electrode was improved due to the optimized morphology and the synergistic effect between Mn3O4 and MnOOH. (ii) Mn3O4/MnOOH thin film with nanorods morphology was obtained by introducing Pt atoms during the electrodeposition and exhibited a superior electrochemical behaviour. (iii) In potentiostatic method, the influence of HCl content in electrolyte and temperature on the morphology and structure of cobalt selenide thin films was systematically analysed. The CoSe film with 3D porous network structure prepared with the addition of 20 μl HCl at 5 °C showed an excellent capacitive storage capacity. (iv) Various nickel selenides, including Ni3Se4, Ni3Se2, and NiSe, were synthesized on FTO substrates by potentiostatic deposition with different potentials. The performance of nickel selenide was further optimized by using carbon foam, nickel foam and copper foam as substrates. (v) Cobalt nickel bimetallic selenide (CoxNi1-xSe) thin films were synthesised on nickel foams in electrolytes with various ratios of Co2+/Ni2+. When the ratio was 7:3, the prepared electrode showed the best electrochemical performance. This work provides a novel temperature-controlled electrodeposition method for improving the performance of Mn3O4 based electrodes and demonstrated the great potential of electrodeposited transition metal selenide thin films for supercapacitor applications. Declaration relating to disposition of project thesis/dissertation I hereby grant to the University of New South Wales or its agents the right to archive and to make available my thesis or dissertation in whole or in part in the University libraries in all forms of media, now or here after known, subject to the provisions of the Copyright Act 1968. I retain all property rights, such as patent rights. I also retain the right to use in future works (such as articles or books) all or part of this thesis or dissertation. I also authorise University Microfilms to use the 350 word abstract of my thesis in Dissertation Abstracts International (this is applicable to doctoral theses only). …………………………………… ……………………………………..……………… ……….……………………...…….… Signature Witness Signature Date The University recognises that there may be exceptional circumstances requiring restrictions on copying or conditions on use. Requests for restriction for a period of up to 2 years must be made in writing. Requests for a longer period of restriction may be considered in exceptional circumstances and require the approval of the Dean of Graduate Research. FOR OFFICE USE ONLY Date of completion of requirements for Award: THIS SHEET IS TO BE GLUED TO THE INSIDE FRONT COVER OF THE THESIS ORIGINALITY STATEMENT COPYRIGHT STATEMENT AUTHENTICITY STATEMENT i Inclusion of Publications Statement ii ACKNOWLEDGEMENTS First of all, I would like to extend my deep gratitude to my wonderful supervisors, Prof. Sean Li and A/Prof. Dewei Chu for the support and trust to conduct my research projects. Their assistance and guidance has been immeasurable throughout this project. I gratefully appreciate our group members: Dr. Lin Xi, Dr. Tao Wan, Mr. Bo Qu, Mr. Shuangyue Wang, Mr. Wei Zhang and Mr. Yang Liu. They have been kind, encouraging, supportive and always willing to assist. It is my pleasure to work with them. I want to thank lab managers working at the School of Materials Science and Engineering, who always patiently and earnestly guide me to use various instruments: Mrs. Soo Wong Chong, Mr. Bill Joe, and Dr. George Yang. Many thanks go to UNSW Mark Wainwright Analytical Centre staffs: Dr. Yu Wang. Mr. Yin Yao, and Mr. Sean Lim for their assistance in XRD, SEM and TEM characterization respectively. The authors of all the cited papers in this dissertation must be thanked for the valuable research achievements they shared. .I would like to sincerely appreciate my parents Zidi Yu and Fa Qi, my sister Tianrong Qi and my brother Lock Qi for their love and support during these years. Finally, I would like to extend my sincere and significant thanks to my wife Yumao. We have known each other for over 15 years and been married for over 4 years. I cannot forget her forever love and her sacrifice for accompanying and supporting me through these tough four years. iii List of Figures Figure 2-1 (a) Share of net electricity generation, world; (b) share of net electricity generation from renewables, world [10]. ............................................................. 8 Figure 2-2 (a)Helmholtz model; (b)Gouy–Chapman model; (c)Stern model [30]. .......................................................................................................................... 14 Figure 2-3 Mechanisms of pseudocapacitance [34]. ......................................... 16 Figure 2-4 (a) CV curves of an AC/CNTs compound electrode at different scan −1 rates; (b) GCD curves at a current density of 0.5 A g [45]. ............................. 18 Figure 2-5 Crystal structures of α-, β-, γ-, δ-, and λ-MnO2 [72]. ......................... 25 Figure 2-6 (a) Co3O4 nanowires [116]; (b) NiO nanorods [119]; (c) NiO hollow spheres [134]; (d) Co3O4 hollow spheres [126]; (e) 3D highly nanoporous CuO [135]; and (f) 3D-nanonet hollow structured Co3O4 [135]. ................................. 28 Figure 2-7 XRD patterns of Mn3O4 nanoparticles with H2O2: (a) 0 M; (b) 1 M; (c) 2 M; (d) 3 M. ...................................................................................................... 32 Figure 2-8 SEM images of the Mn3O4 nano-octahedrons for different reaction times: (a) 1.5 h; (b) 2 h; (c) 3 h; (c) 8 h. ............................................................ 33 Figure 2-9 Schematic illustration of cathodic electrodeposition of Mn3O4 film; (I) electrochemical step; (II) chemical step; (a–f) mechanism of the formation and growth of the nanorods [118]. ........................................................................... 35 Figure 2-10 (a) Nyquist plots; (b) CV curves; (c) GCD curves; (d) capacitance retention at different bending angles; (e) cycle life; (f) lighting an light-emitting diode [142]. ....................................................................................................... 36 Figure 2-11 SEM images of (a) bare stainless-steel mesh; (b–d) Mn3O4-stacked nanosheets; (e) TEM; (f) HRTEM images of Mn3O4 nanosheets [143]. ............ 37 Figure 2-12 AFM images of Mn3O4 thin films at various temperatures [17]. ..... 39 iv Figure 2-13 SEM images of Co0.85Se with (a) nanotube [153]; (b) petal-like [55]; (c) nanowire [154]; (d) nanosheet structures [155]. ........................................... 41 Figure 3-1 Schematic illustration of electrochemical synthesis setup [167]. ..... 47 Figure 3-2 Schematic diagram of diffraction on a set of planes . ...................... 49 Figure 3-3 A photo of CM200 TEM. .................................................................. 50 Figure 3-4 A photo of FEI Nova Nano SEM 450. .............................................. 51 Figure 4-1 (a) CV curve obtained with FTO substrate at 25 ℃, and (b) potential change during the electrodeposition process for 30 mins. ................................ 57 Figure 4-2 XRD patterns of Mn3O4 thin films electrodeposited under different temperature methods. ....................................................................................... 58 Figure 4-3 XRD patterns of Mn3O4/MnOOH thin films electrodeposited under different temperature method. ........................................................................... 59 Figure 4-4 SEM images of Mn3O4 thin films electrodeposited at 65 ℃ for 30 mins (a, b), heating from 25 ℃ to 65 ℃ with a rate of 2 ℃ min-1 (c, d) and 25 ℃ for 15 mins and 65 ℃ for 15 mins (e, f). ...................................................................... 61 Figure 4-5 SEM images of Mn3O4/MnOOH nanocomposite thin films electrodeposited at 25 ℃ for 30