Access Type

Open Access Dissertation

Date of Award

January 2014

Degree Type

Dissertation

Degree Name

Ph.D.

Department

Chemical Engineering and Materials Science

First Advisor

K. Y. S. Ng

Second Advisor

Da Deng

Abstract

Graphene has been widely studied in electrochemical energy storage devices, such as Li-ion batteries, Li-air batteries and supercapacitors, due to its high conductivity, high surface area, mechanical and electrochemical stability. However, the fact that graphene (and other carbon based materials) can only store charges on exposed surface, which severely limits the capacitance. On the other hand, it is well-known transition metal oxides have much higher inherent capacitance than graphene. However, transition metal oxides also have their limitations. Most significantly, the low conductivity confines the efficient storing and releasing process which makes the practical capacitance of the bulk transition metal oxides far away behind their theoretical values. To address this problem, thick transition metal oxide films are explored to reduce the electric charge pathway so that the resistance could be minimized. However, when the thickness is reduced, the transition metal oxide weight ratio in devices is significant compromised which is not desirable.

Chapter 3 and chapter 4 aim to develop a technology that could integrate the high surface area graphene with high intrinsic capacitance MnO2 to achieve a better energy performance without scarifying power performance. In this project we will design and fabricate integrated 1-D and 2-D composites at nanoscale for pseudocapacitive positive electrode in asymmetric aqueous supercapacitors. Various experiments are carried out to optimize the formation conditions, and the fabricated materials will be extensively characterized by spectroscopy and microscopy. The integration of 1-D and 2-D materials at nanoscale into high-order 3-D structures could enhance electrode conductivity, stability, and easy preparation. Different aqueous electrolyte will be evaluated. The improved electrochemical performance in terms of specific energy, specific power, cycling performance are elucidated by both half-cell and full-cell electrochemical characterization.

In chapter 4, a facile method was developed to prepare MnO2/holey graphene oxide (MnO2/HGO) materials based on graphene oxide (GO) flakes for supercapacitor applications. FESEM images show that MnO2 nanorods were formed on the surface of HGO flakes, serving as spacers and preventing the HGO layers from stacking. This provides pathways between the layers for the electrolyte to access the bulk active materials. By introducing the high intrinsic capacitance MnO2 nanorods together with the modified 3-D structure, the capacitance increases to 71.0 F/g compared with 30.0 F/g of GO. More pathways were created by nitric acid etching holes on the surface of the GO. This 3-D MnO2/HGO structure achieves a capacitance of 117.45 F/g, which is 1.65 times higher than that of MnO2/GO composite and 3.9 times higher than that of GO only. BET surface area, XRD and AC impedance were also used to analyze the possible reasons for the enhanced electrochemical performance.

In addition, graphene and transaction metal oxide composites could also be used as Li-air battery electrodes with graphene as support providing reaction site and metal oxides serves as the catalyst which can lower the activation energy of Li2O2 decomposition reaction energy so that the cyclability and the energy efficiency of the battery can be improved. The 2-D graphene and metal oxide composite made for supercapacitor are demonstrated to show improved electrochemical performance in Li-air batteries in chapter 5.

Rechargeable lithium-air batteries offer great promise for transportation and stationary applications due to their high specific energy and energy density compared to all other battery chemistries. Although the theoretical discharge capacity of the Li-air cell is extremely high, the practical capacity is much lower and is always cathode limited. A key for rechargeable systems is the development of an air electrode with a bifunctional catalyst on an electrochemically stable carbon matrix. The use of graphene as a stable catalyst matrix for the air cathode has been studied in this work. A Li-air cell constructed using an air cathode consisting of nano Pt on graphene nanosheets (GNS) has shown promising performance at 80% energy efficiency with an average capacity of 1200 mAh/g and more than 20 cycles without significant loss of total energy efficiency. Replacement of Pt with a nano structured perovskite type bifunctional catalyst resulted in more than 100 cycles with an average capacity of 1200 mAh/g and total energy efficiency of about 70%. Electrochemical impedance spectroscopy data revealed increasing solution and charge transfer (polarization) resistance during cycling, which hindered the cycle life. The increased solution resistance can be attributed to the evaporation and decomposition of carbonate-based electrolyte especially at high charge voltages. Further improvements of the bifunctional catalyst as well as the use of non-carbonate and ionic liquid based electrolytes on the electrochemical performance of Li-air systems are under investigation.

SEI formation at highly ordered pyrolytic graphite (HOPG) surface has been studied with in situ AFM in chapter 6. The morphology and thickness of both the top particle layer of the SEI and the bottom layer of the SEI that was caused by lithium insertion were investigated. The formation mechanism of the SEI was proposed accordingly. Ex situ FESEM and EDS were also used to analyze the composition of the electrode after cycling to confirm the proposed mechanism.

The evolution of lattice constants and abundances of metal (α) and metal hydride (β) phases during hydrogenation process of an AB5 with a CaCu5 crystal structure, and two AB2 alloy with a predominating C14 and a C14/C15 mixed crystal structures were reported. During the hydrogenation of the AB5 alloy, the a/c ratio in the α phase decreases, stabilizes, and then decreases again while that in the β phase decreases and then stabilizes, and the trends in the changes of unit cell volumes are increasing, plateauing, and increasing again in the α phase and increasing followed by plateauing in the β phase. In the C14-predominated AB2 alloy, the a/c ratio in the α phase increases in the beginning and then stabilizes while that in the β phase remains about the same and then increases during the addition of hydrogen; moreover, the unit cell volume in the α phase increases slightly but increases, decreases, and then increases in the β phase. In the C14/C15 mixed AB2 alloy, most of C14 phase is hydrided before the hydrogenation of C15 phase begins. This agrees with the hypotheses that C15 phase is the catalytic phase with a higher plateau pressure compared to C14 phase.

Download

Share

COinS