Year

2016

Degree Name

Doctor of Philosophy

Department

Intelligent Polymer Research Institute

Abstract

With the rapid development of portable electronic devices, increasing interest is being aroused in flexible energy storage units with high energy and power density. The electrodes used in such energy systems should combine superior electrochemical properties with high mechanical flexibility. Graphene based free-standing thin films or papers are promising binder-free flexible electrodes. The main drawback of graphene paper electrode is the deterioration of electrochemical performance caused by the restacking of graphene sheets during the fabrication process. Fabricating 3D structured graphene macro-assemblies can address this issue, but compromise mechanical strength and low areal capacitance.

This thesis considers the development of non-stacked graphene paper with good flexibility, in which the graphene sheets are highly separated. The flexible porous structured graphene paper was prepared from graphene cryogel, which was formed by freeze-drying a solution containing chemically reduced graphene and graphene oxide (CRG/GO). At a current density of 2000 mA g-1, it exhibited a discharge capacity higher than 400 mAh g-1, in sharp contrast to that 229 mAh g-1 at 50 mA g-1 delivered by conventional graphene papers. The CRG/GO ratio in the precursor solution determined the mechanical properties of this cryogel paper. It can achieve a Young’s modulus nearly 9 times greater than an equivalent paper made from pure GO when the CRG/GO mass ratio was 2:1.

With an improved strategy, by directly freeze-drying graphene wet gel obtained from filtration, graphene paper with continuous pores was prepared without mechanical pressing. This porous graphene (PG) paper showed high discharge capacity and excellent rate capability in lithium ion batteries. The discharge capacity can reach 420 mAh g-1 at a current density of 2000 mA g-1. A flexible all-solid-state supercapacitor based on such PG papers exhibited a specific capacitance of 137 F g-1 at 1 A g-1.

Due to low areal loading mass, graphene paper with porous structure delivers a low areal capacitance. We developed a free-standing chemically reduced graphene (CRG)-polypyrrole (PPy) hybrid paper with enhanced areal capacitance, via electropolymerization of pyrrole on a paper-like graphene gel. A high areal mass loading of 2.7 mg cm-2 can be obtained. It afforded a greatly enhanced areal capacitance of 440 mF cm-2 at 0.5 A g-1, in sharp contrast to that 186 mF cm-2 from flexible graphene paper. It is also much higher than the previously reported 151~198.5 mF cm-2 for flexible polypyrrole-graphene papers/films.

The most frequently used graphene preparation approach, exfoliation of graphite oxide, involves a time-consuming procedure and the resultant defected graphene structure. As an alternative, graphene obtained via surfactant assisted liquid phase exfoliation from expanded graphite produces less defects in the graphene and avoids involving complicated post-treatment. In this work, graphene obtained from direct exfoliation of expanded graphite was used to fabricate flexible film electrodes incorporated with polypyrrole fibre (PPyF). PPy fibre was synthesized in the presence of the surfactant that served as dopant and template. Such composite film delivered a capacitance of up to 161 F g-1 at 0.5 A g-1, and 80 % of this value can be retained even at a higher current density of 8 A g-1.

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