title: Engineering Hierarchical Functionalized-Nanowire and Nanotube Networks as Self-supported Electrodes for Electrochemical Applications: Sensors, Water Splitting, and Supercapacitors
Creators: name: Amin, Khaled
Publisher: name: UNSPECIFIED
Year: 2023

description: To meet the pressing need for green and sustainable energy sources, many researchers are engaged in the field of electrochemical energy conversion and storage. Likewise, the development of efficient portable electrochemical sensors and biosensors has emerged as a key player in disease diagnosis. In this context, the performance of such electrochemical devices critically depends on the intrinsic properties and structure of the applied electrode materials. Conventional electrode designs typically rely on bulk materials of microscale dimensions that are loaded on supporting electrodes such as foams, meshes, and 3D-printed substrates, and thus often require the use of binders. However, this fabrication approach limits the performance and reduces the mechanical stability of such electrodes, necessitating the exploration of advanced nanostructured architectures and alternative design approaches to achieve enhanced efficiency and meet diverse application requirements. In this thesis, the development and characterization of advanced electrode materials for electrochemical applications including nonenzymatic glucose sensing, supercapacitors, and the oxygen evolution reaction (OER) are presented. The fabrication of hierarchical 3D structures in core-shell form as self-supported electrodes offers numerous benefits for electrochemical applications. In particular, this work introduces a new strategy for the synthesis of 3D nanowire and nanotube networks decorated with various LDH nanosheets. This design approach utilizes feasible and simple synthetic routes, including template-assisted synthesis, electroless plating, and electrodeposition. The prepared 3D architecture, with its unique "2D on 1D" structure, exhibits synergistic effects through combining the advantages of the individual components at multiple levels of architecture: i) the 1D core networks provide continuous pathways for electron transfer, while the 2D nanosheets as the shell enlarge the surface area and boost the electrocatalytic activity, ii) these interconnected 3D networks provide both mechanical robustness and high porosity for efficient mass transport, and iii) the direct growth of catalytical active LDH over the interconnected nanotubes or nanowires accelerates the charge transfer due to the high electric conductivity of the core and the intimate junction between the active material and the substrate. In this thesis, a detailed introduction on the topic, theoretical background, and a wider literature survey are presented in Chapter I. Three peer-reviewed articles reporting the main results of this work were published and are presented in this cumulative thesis. Briefly, two electrode designs were developed and systematically characterized. The first design is based on a Ni nanowires network (Ni-NWN) (Chapter II) whilst the second is built on a Ni nanotube network (Ni-NTNW) (Chapter III). Both systems were decorated with different LDH nanosheets. The as-synthesized active electrodes were investigated for the following three applications. For the water splitting process, the focus was on engineering active sites enriched NiCo LDH/Ni-NWN and NiFe LDH/Ni-NWN electrocatalysts for the OER, which is the bottleneck step in water oxidation. Herein, LDH-decorated nanowire networks were investigated as efficient OER electrocatalysts. Transmission electron microscopy (TEM) images confirmed the conformal growth of LDH sheets over the entire wires, while X-ray photoelectron spectroscopy revealed the presence of a metal hydroxide shell. The NiFe LDH/Ni-NWN electrocatalyst demonstrated remarkable performance with a low overpotential of 222 mV to achieve a current density of 10 mA cm-2 and a small Tafel slope of 42 mV dec-1, suggesting favorable OER kinetics. The 3D architecture, featuring a "nanosheet on nanowire" configuration, not only secured mechanical stability but also promoted ion diffusion through the porous network and facilitated the charge transfer through the metallic Ni-NWN. These factors ensured the regeneration of active species during the OER, resulting in excellent stability over 50 hours. For the hybrid supercapacitor (HSC) application, the NiCo-LDH@Ni-NTNW electrode was investigated. The proposed ultrathin electrode (20 µm) overcomes the shortcomings of classical electrode designs that rely on macroscopic support materials. Increasing the mass loading of the active material is a typical tactic to enhance the volumetric energy density. However, this approach results in reduced porosity, morphology distortions, and demolition of portions of the active material and dead mass, thus reducing the electrical contact. In contrast, the micro- and nano-structuring of a hierarchical electrode combines the highly porous Ni-NTNW with the large interface of the LDH nanosheets. In addition to the achieved high rate-capability and cycling stability, the electrode demonstrated a remarkable volumetric capacity of 126.4 C cm-3. Furthermore, when assembled with activated carbon (AC) into an HSC, it delivered a high energy density of 14.7 mWh cm-3, outperforming state-of-the-art high volumetric energy density supercapacitors. For glucose electrooxidation, Ni(OH)2@Ni-NTNW and NiCo-LDH@Ni-NTNW were explored as binder-free electrodes that represent a hierarchical paradigm to overcome the inherent limitations of LDH materials. Electroless plating was employed as a simple and scalable method to fabricate the Ni-NTNW substrate, followed by an optimized electrodeposition step to grow LDH nanosheets over the Ni-NTNW. Scanning electron microscopy (SEM) micrographs, energy dispersive X-ray analysis (EDX), and X-ray diffraction (XRD) measurements confirmed the deposition of conformal NiCo-LDH nanosheets on the inner and outer surfaces of the Ni-NTNW. The NiCo-LDH reinforced network exhibited superior activity compared to the Ni(OH)2 modification. The optimized catalyst layer demonstrated excellent sensitivity of 4.6 mA mM-1 cm-2, low detection limit of 0.2 mM, wide linear range of 0.03 to 2.5 mM glucose, high selectivity, and stability. To summarize, this PhD thesis emphasizes the significance of tailored electrode architectures and nano-structuring strategies in improving the performance of electrochemical devices. The research outcomes contribute to the development of an emerging design paradigm that can be extended to other customized electrocatalysts for widespread applications.