Synthesis and characterization of Ruthenium based composite materials for supercapacitors

Liu, Xiaorong (2010) Synthesis and characterization of Ruthenium based composite materials for supercapacitors. PhD thesis, Memorial University of Newfoundland.

[English] PDF - Accepted Version Available under License - The author retains copyright ownership and moral rights in this thesis. Neither the thesis nor substantial extracts from it may be printed or otherwise reproduced without the author's permission. Download (13MB)

Abstract

Ruthenium-based materials, including pure hydrous Ru oxide and composites of Ru oxide/carbon fabric (CF), nanostructured Ru oxide/multiwall carbon nanotube (MWCNT), thin film Ru oxide/CF and binary Mn-Ru oxide/CF have been synthesized using various methods. Their properties were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), thermogravimetric analysis (TGA), cyclic voltammetry, impedance spectroscopy and constant current discharging. The purpose of the work was to improve the performances of Ru oxide supercapacitors and enhance the utilization of Ru in the composites. -- Hydrous Ru oxide was prepared by a modified sol-gel method. The factors influencing the performances of Ru oxide supercapacitors, such as annealing temperature, electrode preparation method, electrode loading, separator, and stability, have been investigated in detail. XRD patterns show that the hydrous Ru oxide had an amorphous structure, while a crystalline structure was formed at annealing temperatures over 200°C. The best specific capacitance for hydrous Ru oxide was 716±24 F/g at an annealing temperature of 110°C. Hydrous Ru oxide electrodes prepared by coating Ru oxide on the surface of carbon fibre paper with 5% Nafion binder showed the best performance. Specific capacitances (ca. 700 F/g) did not change significantly with increasing electrode loading (up to 51 mg/cm²) due to the good proton conductivity of the Nafion binder and the high mechanical stability of the electrodes. The best power density for supercapacitors reached 130.5 kW/kg for half discharge, while the best energy densities were 40.5 W h/kg and 31.2 W h/kg for the operating voltages of 1.3 V and 1.0 V, respectively. Their lifetimes were over 120000 cycles. -- In order to improve the utilization of Ru, Ru oxide/CF composites were prepared by an impregnation method. Specific capacitances of the composites increased with Ru oxide loadings. However, the specific capacitance for the Ru oxide component decreased with increasing loading. An average specific capacitance of 1085 F/g based on the Ru oxide component was obtained for a 9.2% Ru oxide/CF supercapacitor. The high specific capacitance was ascribed to synergistic effects between functional groups on the CF surface and the Ru oxide component with high surface area. -- Nanostructured Ru oxide/MWCNT, thin film Ru oxide/CF, thin film Mn oxide/CF, and thin film Mn-Ru oxide/CF composites were synthesized by a spontaneous reduction method. The sizes of Ru oxide particles for the nanostructured Ru oxide/MWCNT composites were less than 3 nm. Thin film structures were observed by SEM for all of the CF composites. The utilization of Ru in these composites was enhanced due to the thin film structure and nanostructured Ru oxide particles, which allows electrolytes to access the bulk Ru oxide. An average specific capacitance of 704±62 F/g for the hydrous Ru oxide component was obtained for a 25.1% hydrous Ru oxide/MWCNT composite. A maximum average specific capacitance of 824±152 F/g based on the Ru oxide component was obtained for a 7.4% Ru oxide/CF composite with a film structure. For an 11.6% Mn oxide/CF composite, the specific capacitance for the Mn oxide component in 2 M LiOH was over 1000 F/g, however, it quickly decreased with cycling. The stability of Mn-Ru oxide/CF composites was sharply improved due to the introduction of Ru. -- A few types of hybrid supercapacitors were tested by cyclic voltammetry and constant current discharging. The operating voltages for hybrid supercapacitors of Ru oxide/CF or Mn oxide/CF with the CF reached 2.0 V in 2 M KNO₃ electrolyte, and the maximum energy density was ca. 18 Wh/kg. A Ru oxide//Pd/C supercapacitor exhibited a maximum energy density of 41.3 W h/kg. A hybrid supercapacitor with anthraquinone modified CF (the negative electrode) and Ru oxide (the positive electrode) electrodes requires 76.3% less Ru oxide relative to a symmetric Ru oxide supercapacitor to provide a similar energy. -- An approach for quickly predicting the energy and power from impedance spectroscopy is proposed. The usable energy (E=1/2 Cre (f)V² ) is related to the real capacitance (Cre(f )) and operating voltage (V), while power (P=2fE) depends on E and the frequency (f). The predicted results at low frequencies greatly agreed with those from constant current discharging.

Actions (login required)

View Item