Jury :

• Rabah BOUKHERROUB, DR1, University of Lille, Thesis supervisor
• Saïd SADKI, Professor, University Grenoble Alpes, Rapporteur
• Sorin MELINTE, Professor, Université Catholique de Louvain, Examiner
• Christine TAVIOT GUEHO, Professor, Université Clermond Auvergne Campus Universitaire des Cézeaux TSA 60026 - CS 60026, Rapporteur
• Christophe LETHIEN, Senior Lecturer, IEMN, University of Lille, Examiner
• Sabine  SZUNERITS, Professeur, Université de Lille, CoDirecteur de thèse

Summary:

Supercapacitors, as energy storage devices, have attracted a great deal of attention in our daily lives to bridge the gap between batteries and capacitors. Consequently, the preparation of high-performance composite material electrodes for supercapacitors plays a vital role in their future technological development. In this context, double lamellar hydroxides (HDL) and Ni(OH)2 have been identified as promising electrodes for supercapacitors due to their fast redox reactions and battery-like behaviour.
Chapter 1 gives a brief historical overview, as well as the principles and mechanism of energy storage, the electrode materials used in supercapacitors, and the corresponding characterisation methods.
Chapter 2, after a brief review of the synthesis and use of HDL-based materials as supercapacitor electrodes, describes the preparation of Ni-based HDLs as electrodes for supercapacitors. Firstly, NiFe-based HDLs on Ni foam (NF) coated with reduced graphene oxide (LDF NiFe/rGO/NF) were prepared by an electrochemical deposition method. The HDL NiFe/rGO/NF electrode has a specific capacitance of 585 C g-1 at a current density of 5 A g-1. In addition, a flexible asymmetric supercapacitor was assembled using NiFe HDL/rGO/NF as the cathode and mesoporous carbon (MC) deposited on NF as the anode. The supercapacitor has an energy density of 17.71 Wh kg-1 and a power density of 348.49 W kg-1. In the second part of this chapter, we describe the hydrothermal synthesis of NiAl-based HDLs coated on carbon spheres (CS) supported by Ni foam electrodes (NiAl DHL@CS/NF). The performance of the prepared materials as binderless electrodes in supercapacitors was evaluated. The NiAl DHL@CS/NF electrode material exhibits a surface capacitance of 1042 mC cm-2 at 1 mA cm-2, which is much higher compared to the surface capacitance values of NiFe HDL@CS/NF (705.8 mC cm-2) and NiCr LDHs@CS/NF (814.9 mC cm-2) at 1 mA cm-2. As a result, a hybrid supercapacitor comprising NiAl HDL@CS/NF as the positive electrode and nitrogen-doped reduced graphene (N-rGO)/NF as the negative electrode was assembled. The device exhibits an energy density of 43 μWh cm-2 at a power density of 0.805 mW cm-2, and was applied to operate a wind machine continuously for 32 s. Finally, a composite material based on NiMnCr on a nickel foam substrate coated with carbon spheres (NiMnCr HDL@CS/NF) was prepared using a two-step hydrothermal process. The resulting nanocomposite was investigated as an electrode in a supercapacitor and exhibited a specific capacitance of 569 C g-1 at 3 A g-1 with good stability. In addition, a hybrid supercapacitor was fabricated using NiMnCr HDL@CS/NF as the positive electrode and FeOOH deposited on NF (FeOOH/NF) as the negative electrode. The device has an energy density of 48 Wh kg-1 at a power density of 402.7 W kg-1.
In Chapter 3, binder-free Ni(OH)2@CuO electrodes on a copper foam were synthesised by a two-step process at room temperature. We studied the effect of deposition time (30, 50, 90, 150 and 200 s) on the electrochemical behaviour of the resulting electrodes. Of all the samples, Ni(OH)2@CuO@Cu-150 has the highest surface capacitance of 7063 mC cm-2 at 20 mA cm-2, and was therefore chosen as the positive electrode in a hybrid supercapacitor. Using nitrogen-doped reduced graphene oxide on nickel foam (N-rGO/NF) as the negative electrode, a hybrid supercapacitor was assembled. It exhibits good flexibility, cyclic stability and a high areal energy density of 130.4 μWh cm-2 at a power density of 1.6 mW cm-2.
A general conclusion recalls the main results obtained in this thesis work on the application of Ni-based lamellar double hydroxide composites as electrodes in energy storage devices, and presents some possible prospects in the light of this work (Chapter 4).

Abstract:

Supercapacitors, as energy storage devices, have drawn great attention in our daily life to bridge the gap between batteries and capacitors. Therefore, the preparation of high-performance material electrodes for supercapacitors plays a vital role in the future technological developments. In this context, layered double hydroxides (LDHs) and Ni(OH)2 have been recognized as promising electrodes for supercapacitors, owing to their fast redox reaction and battery-type behaviour.
The Chapter 1 of my PhD work gives a brief historic overview, principles and mechanism of energy storage, electrode materials of supercapacitors and the corresponding characterization methods.
In Chapter 2, after a brief introduction on LDHs and their investigation as electrode materials in supercapacitors, we summarize our results obtained on Ni-based LDHs as electrodes for supercapacitors. Firstly, NiFe LDHs on Ni foam (NF) coated with reduced graphene oxide (NiFe LDHs/rGO/NF) was prepared by electrochemical deposition method. NiFe LDHs/rGO/NF achieved enhanced specific capacity (585 C g-1 at a current density of 5 A g-1). Additionally, a flexible asymmetric supercapacitor was assembled using NiFe LDHs/rGO/NF as the cathode and mesoporous carbon (MC) coated on NF as the anode. The supercapacitor exhibited an energy density of 17.71 Wh kg-1 at a power density of 348.49 W kg-1. Secondly, NiAl LDHs coated on carbon spheres (CS) supported by Ni foam (NiAl LDHs@CS/NF) electrodes were synthesized by a facile hydrothermal method. The performance of the prepared materials as binder-free electrodes in supercapacitors was assessed. The NiAl LDHs@CS/NF electrode achieved the largest areal capacity (1042 mC cm-2), as compared to the areal capacity values attained by NiFe LDHs@CS/NF (705.8 mC cm-2) and NiCr LDHs@CS/NF (814.9 mC cm-2) at 1 mA cm-2. Therefore, a hybrid supercapacitor device comprising NiAl LDHs@CS/NF as the positive electrode and N-doped reduced graphene/NF as the negative electrode was assembled, which attained an energy density of 43 μWh cm-2 at a power density of 0.805 mW cm-2. The hybrid supercapacitor was successfully applied to operate a windmill device continuously for 32 s. Finally, NiMnCr LDHs-carbon spheres modified Ni foam (NiMnCr LDHs@CS/NF) nanocomposite was prepared using a two-step hydrothermal process and exhibited a high specific capacity of 569 C g-1 at 3 A g-1 with good reversibility and stability. Furthermore, a hybrid supercapacitor was fabricated using NiMnCr LDHs@CS/NF as the positive electrode and FeOOH coated on NF (FeOOH/NF) as the negative electrode. The energy storage device reached an energy density of 48 Wh kg-1 at a power density of 402.7 W kg-1.
In Chapter 3, Ni(OH)2@CuO@Cu foam binder-free electrodes were fabricated by a two-step process at room temperature with various deposition times (30, 50, 90, 150 and 200s). Among all the samples, Ni(OH)2@CuO@Cu-150 exhibited the largest areal capacity of 7063 mC cm-2 at 20 mA cm-2, and was therefore chosen as the positive electrode in a hybrid supercapacitor. Using N-doped reduced graphene oxide on nickel foam (N-rGO/NF) as the negative electrode, a hybrid supercapacitor was assembled. It displayed good flexibility, cycling stability and high areal energy density of 130.4 μWh cm-2 at a power density of 1.6 mW cm-2.
In conclusion, all the results obtained in this thesis imply the promising potential application of Ni-based hydroxide composites as energy storage devices and provide valuable highlights to the exploration of new composite materials for supercapacitor electrodes in future works (Chapter 4).