Li, Zhuangnan; (2020) Design of graphene-based structures for capacitive energy storage. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Supercapacitors are the promising next-generation energy storage devices that bridge the gap between traditional capacitors and batteries, but still require their electrode material to be further developed. Here, this thesis aims at design and development of the graphene-based porous structures as the supercapacitor electrode for efficient electrochemical energy storage. Step-by-step research is carried out by firstly investigating the effect of graphene-oxide precursors, then enhancing the specific capacitance of a single electrode, and finally increasing the overall performance (in terms of the energy density and power density) at the entire device level. The details of these three main work in the PhD project are as follows: (1) Graphene-based materials are highly desirable for supercapacitors, but vary considerably in reported properties despite being prepared by similar procedures; therefore, a clear route to improve the performance is currently lacking. Here, a direct correlation between the initial oxidation of graphene-oxide precursors and final supercapacitor performance is demonstrated. Building on this significant understanding, the optimized three-dimensional graphene frameworks achieve a superior gravimetric capacitance of 330 F g-1 in an aqueous electrolyte. This extraordinary performance is also validated in various electrolytes at a device level. In a commercially used organic electrolyte, an excellent volumetric energy density of 51 Wh L-1 can be delivered, which significantly outperforms the state-of-the-art commercial carbon-based devices. Furthermore, solid-state supercapacitor with a gel electrolyte shows an impressive capacitance of 285 F g-1 with a rate capability of 79% at 20 A g-1 and capacitance retention of 93% after 20,000 cycles. This study presents a versatile design principle for engineering chemically derived graphene towards diverse applications in energy storage. (2) Graphene-oxide (GO) based porous structures are highly desirable for supercapacitors, as the charge storage and transfer can be enhanced by advancement in the synthesis. Here, this study presents an effective route of, first, synthesis of a three dimensional assembly of GO sheets in a spherical architecture by flash-freezing of GO dispersion, and then development of hierarchical porous graphene networks by facile thermal-shock reduction of GO spheres. Thus, this process leads to a superior gravimetric specific capacitance of ~306 F g−1 at 1.0 A g−1, with a capacitance retention of 93% after 10,000 cycles. The values represent a significant capacitance enhancement by 30–50% compared with the GO powder equivalent, and are among the highest reported for GO-based structures from different chemical reduction routes. Furthermore, a solid-state flexible supercapacitor is fabricated by constructing the porous graphene networks with polymer gel electrolyte, exhibiting an excellent areal specific capacitance of 220 mF cm−2 at 1.0 mA cm−2 with exceptional cyclic stability. The work reveals the synthetic and further processing effects of GO-based materials to enhance their structure-performance relationships for capacitive energy storage. (3) Supercapacitors have shown extraordinary promise for miniaturized electronics and electric vehicles, but are usually limited by electrodes with rather low volumetric performance largely due to the inefficient utilization of pores in charge storage. Herein, this study designs a freestanding graphene laminate film electrode with highly efficient pore-utilization for compact capacitive energy storage. The interlayer spacing of this film can be precisely adjusted, which enables a tunable porosity. By systematically tailoring the pore size for the electrolyte ions, pores are utilized optimally and thereby the volumetric capacitance is maximized. Consequently, the fabricated supercapacitor delivers a record-high stack volumetric energy density of 88.1 Wh L-1 in an ionic liquid electrolyte, representing a critical breakthrough for optimizing the porosity towards compact energy storage. Moreover, the optimized film electrode is assembled into an ionogel-based all-solid-state flexible smart device with multiple optional output and superior stability, demonstrating enormous potential as portable power supply in practical applications.

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