STUDY OF COMPOSITE STRUCTURAL SUPERCAPACITORS BASED ON MODIFIED CARBON FIBER ELECTRODES

Composite structural supercapacitors (CSSs), which provide both mechanical load-bearing capability and electrochemical energy storage capacity, have been developed rapidly in the past two decades. By serving as both structural elements and energy storage units in a single engineering structure, CSSs have the potential to reduce the volume and mass of overall systems. Despite the potential benefits of CSSs for numerous engineering applications, current development efforts have fallen short of expectations, and significant advancements are needed to meet the demanding requirements of CSSs.

Initially, this thesis introduces a new type of CSS, denoted as 1:1 ACC-CSS, composing of flexible energy storage devices and a structural unit made of carbon fiber reinforced polymer (CFRP). The energy storage devices utilize KOH activated carbon cloth (ACC) as the electrode and Poly(vinyl alcohol) (PVA)-KOH gel electrolyte. 1:1 ACC-CSS exhibits promising mechanical properties with a flexural strength of 230 MPa, a flexural modulus of 21 GPa, and a shear strength of 8.75 MPa. Additionally, 1:1 ACC-CSS demonstrates good electrochemical properties, including a specific capacitance of 88 mF×g^-1, energy density of 9.9 mWh×kg^-1 and power density of 445.5 mW×kg^-1. It also demonstrates the superior stability of 1:1 ACC-CSS's electrochemical performance under various external static and dynamic loads through in-situ mechano-electrochemical tests. Furthermore, even after mechanical failure, 1:1 ACC-CSS is still able to maintain its electrochemical performance, which ensures the safety and reliability of the structure.

To enhance the electrochemical capacity of CSS, the thesis proposes a battery-like asymmetric structure for CSS by replacing the ACC electrode with a carbon cloth (CC) electrode coated by Ni-Co-layer double hydroxide, resulting in a new CSS, called NiCo@CC-CSS. This new structure exhibits significantly improved electrochemical performance, with a specific capacity of 610 mF×g^-1, an energy density of 191 mWh×kg^-1 and a power density of 1508 mW×kg^-1. However, the flexural strength of NiCo@CC-CSS remains similar to that of ACC-CSS, at 270 MPa, because the CFRP component that primarily provides mechanical strength remains unchanged. In-situ mechano-electrochemical tests demonstrate the stable electrochemical behavior of NiCo@CC-CSS under various external forces. Furthermore, the thesis systematically studies the electrochemical cycling performance of NiCo@CC-CSS and analyzes the mechanism of capacity degradation.

To improve the mechanical properties, a new composite laying mode is proposed. Activated carbon nanotubes (ACNTs) are introduced on the surface of carbon fibers to further improve the electrochemical performance. PVA-ionic liquid (IL) gel electrolyte is applied to replace PVA-KOH to expand the potential window. The resulting CSS, called ACNT-CSS, exhibits excellent mechanical (flexural strength: 505 MPa) and electrochemical properties (specific capacitance: 1,430 mF×g^-1, energy density: 1,488 mWh×kg^-1, power density: 7,650 mW×kg^-1). In addition to in-situ mechano-electrochemical tests, the thesis also studies the effect of drop-weight impact on the electrochemical performance of ACNT-CSS. The results demonstrate that ACNT-CSS can still retain some electrochemical capacity in a non-perforated state after impact, providing assurance of the system's reliability. Furthermore, the stable electrochemical behavior of ACNT-CSS under high temperatures and high bending forces confirms its safety.

In addition to the evaluation of the electrochemical and mechanical properties of CSSs, this thesis also proposes a new sensor-free structural health monitoring method, which allows the assessment of the structural health of CSSs solely through electrochemical performance tests. It is found that the changes in ESR values under external dynamic loads is closely related to the ripples on the GCD curves of CSS, and the ESR values decrease linearly with increasing external forces.

The series of improvements in the mechanical and electrochemical properties of the CSSs are very significant and demonstrate the effectiveness of the research and optimization efforts. The increase in flexural strength from 230 MPa of 1:1 ACC-CSS to 505 MPa of ACNT-CSS indicates a significant improvement in the mechanical stability of the structure, making it more suitable for load-bearing applications. The increase in specific capacitance from 88 mF×g^-1 to 1,430 mF×g^-1 and energy density from 9.9 mWh×kg^-1 to 1,488 mWh×kg^-1 demonstrate that the electrochemical properties of the CSS have also been significantly enhanced, making it a more effective energy storage material. Overall, this thesis successfully demonstrates the design and fabrication of load bearing/energy storage CSSs and presents a comprehensive systematic study compared to other reports, providing guidance for the future development and research of CSSs.

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