Towards surface-enlarged diamond materials - creation of surface-enlarged diamond electrodes for electrochemical energy applications
At a Glance
Section titled āAt a Glanceā| Metadata | Details |
|---|---|
| Publication Date | 2016-01-01 |
| Journal | FreiDok plus (UniversitƤtsbibliothek Freiburg) |
| Authors | Fang Gao |
Abstract
Section titled āAbstractāDriven by the demand for energy generation and storage systems in portable devices, electrical vehicles and renewable energy for power grids, the past decades have seen a tremendous amount of research on the topic of energy storage and conversion. Devices such as supercapacitors and fuel cells have been intensively investigated. Since energy is either con-verted or stored at the electrolyte-electrode interface, an important issue for these electrochemical power sources is to enlarge the effective surface area of the working electrode. For this purpose, sp2 carbon based materials, especially activated carbon, have been widely used in both scientific researches and commercially available products. For this kind of materials, two aspects may be improved. One is the small working potential window in aqueous solutions (~ 1 V), which limits the energy storage. Another is the low stability, especially towards oxidation. Boron-doped diamond is an electrode material with extreme stability and a wide window of usable potentials even in aqueous solutions. Together with other unrivaled physical and chemical properties, diamond has attracted extensive electrochemical research since 1990s. However, the typical planar diamond electrode has a low surface area, which limits its applicability for energy-conversion and -storage. There has been successful scientific development of nanostructured diamond with enhanced surface areas, but the state-of-the-art technology remains limited regarding the achievable surface enhancement factor. The fabrication of nanostructured boron-doped diamond almost solely depends on reactive ion etching. This method generates vertical structures with a limited aspect ratio and therefore a maximum achievable surface enhancement factor in the range of 10 - 50, which is insufficient for energy applications. The electrochemical applications of these nanostructured electrodes remain limited to electrochemical sensing. Their application in energy related topics has rarely been reported. This thesis introduces an alternative and scalable bottom-up approach for the fabrication of surface-enlarged diamond electrodes, which is capable of producing significantly higher surface enhancements. The potential of these electrodes is investigated in two exemplary energy applications, which profit from the achieved surface enhancement. This thesis begins with state-of-the-art diamond nanostructuring techniques, i.e. plasma assisted reactive ion etching. The current technology is carefully investigated in terms of limitations and non-ideal effects. Efforts have been made on the theoretic prediction of the wire shapes and achieving maximized surfaces. During this pursuit, the conventional top-down etching technique was found to be insufficient for manufacturing a surface large enough for energy applications. As a result, a new bottom-up approach was developed. In this method, a variety of non-diamond templates have been applied, and diamond coatings were fabricated on them. With this new approach the aspect ratio of diamond nanostructures are no longer limited by the etching isotopy. Also, the surface enlargement of diamond electrodes becomes fully scalable. In the second part, the surface enlarged diamond electrodes are investigated for the exemplary energy storage system of electrochemical capacitors, where the surface enhancement is directly translated to an increased capacitance of the device. In the beginning of this part a discussion on electrolyte properties is included, because for supercapacitors the electrolyte plays an important role. Many important properties, like the potential window, the conductivity, as well as the double layer capacitance are all closely related to the chosen electrolyte. Particularly ionic liquids, a new category of solvent-free electro-lytes have aroused wide attention, because they can effectively widen the potential window for sp2 carbon materials up to 4.5 V. However, the combination of diamond and ionic liquid has not been investigated in depth so far. Therefore, in this part the performances of diamond electrodes in both ionic liquids and aqueous electrolytic solutions were investigated. Following this, real double-layer supercapacitor devices based on diamond are assembled and tested. Finally, to further enhance the areal capacitance, a thin coating of pseudocapacitive materials is coated onto the nanostructured diamond samples. In the final part, diamond-based catalyst systems are investigated as a second possible application of large-surface diamond electrodes, Diamond-supported catalysts can be potentially used in energy related topics such as water splitting and direct-methanol fuel cells, where the high corrosion resistance of diamond plays a crucial role. Both planar diamond and 3D dia-mond nanostructures were used as robust supports for Pt catalyst. The highest catalytic performance so far reported for a Pt-diamond composite in terms of Pt specific area (m2/g) has been achieved by diamond-Pt core shell nanowires. This material also shows stability under harsh working conditions. To summarize, the results from this research provide valuable information for future diamond nanostructuring methods. Also, this work may serve as a pioneering attempt to use diamond as a material for energy conversion and storage. Finally, due to the successful fabrication of diamond nanomaterials with functional coatings, the gate to a group of new diamond-based composite materials is opened.