Thesis
Nanostructured phenol-formaldehyde based carbons as electrodes for lithium/oxygen batteries
- Creator
- Rights statement
- Awarding institution
- University of Strathclyde
- Date of award
- 2012
- Thesis identifier
- T13154
- Qualification Level
- Qualification Name
- Department, School or Faculty
- Abstract
- As environmental concerns regarding CO2 emissions continue to increase and renewable technologies begin to emerge, electrochemical energy storage devices have proven to be crucial for successful implementation of these new energy strategies. The Lithium/air (Li/air) rechargeable battery represents by far one of the most competitive energy storage devices to date. High energy densities which are about 5 - 10 times that of Lithium-ion (Li-ion) batteries are achievable with these devices. However, due to the problems associated with electrode clogging with discharge product, considerable research has been focused on optimizing the porous structure of the electrode. Studies performed in this work revealed high discharge capacities for Li/air batteries that utilised carbon-based electrodes with high pore volumes. However the number of discharge/charge cycles was relatively low and still require much improvement. The porous properties of the carbon electrodes were dependent on component ratios and synthesis methods of the carbon precursors (phenolformaldehyde gels). Varying levels of porosity in the corresponding carbons were obtained with different carbonisation temperatures. Subsequent physical activation considerably increased porous properties of the carbons. Sol-gel polymerisation was used in the synthesis of low cost phenol-formaldehyde gels as carbon precursors for electrode preparation. By varying various synthesis parameters such as phenol/catalyst ratio (P/C), phenol/formaldehyde ratio (P/F), solvent exchange liquid, and drying methods, maximised pore volumes for these gels were obtained. The most desirable gel reactant ratios were a P/C ratio of 8 and a P/F ratio of 0.4. Furthermore, freeze drying with t-butanol showed the best porous properties in the phenol-formaldehyde (PF) gel. The corresponding gel possessed total pore and mesopore volumes of ̃1.16 and 1.15 cm3/g respectively. An investigation of the effect of carbonisation temperatures on the aforementioned gel revealed 1050 oC as the optimum. Further enhancement of porous structure was achieved by physical activation. Both O2 and CO2 activation were carried out. The highest total pore and mesopore volumes of activated carbons obtained in this work were approximately 2.01 and 1.87 cm3/g, corresponding to a CO2 activated carbon. Surface characterisation of both O2 and CO2 activated carbons showed similar levels of surface functionality. Electrochemical measurements showed that discharge capacity increased with mesopore volume, with the highest discharge capacity of 1852 mAh/g corresponding to the highest mesopore volume of 1.87 cm3/g. Furthermore the discharge capacities normalised to the mesopore volumes of the carbons in the electrodes, showed an increase with increasing average pore size. Power capabilities of the batteries were tested by varying discharge rates where it was found that the discharge capacity decreased with increasing discharge rates
- Resource Type
- DOI
- Date Created
- 2012
- Former identifier
- 947311
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