MAX phases as an electrocatalyst support material: a DFT study
| dc.contributor.advisor | Rampai, Tokoloho | |
| dc.contributor.advisor | Van Heerden, Tracey | |
| dc.contributor.advisor | Levecque, Pieter | |
| dc.contributor.author | Gertzen, Jonathan | |
| dc.date.accessioned | 2020-03-04T07:54:31Z | |
| dc.date.available | 2020-03-04T07:54:31Z | |
| dc.date.issued | 2019 | |
| dc.date.updated | 2020-03-02T13:41:20Z | |
| dc.description.abstract | The insatiable global demand for energy cannot be sustained by fossil fuels without irreparable damage to the environment. Various alternative energy sources are being investigated to provide renewable clean energy. One promising technology is the hydrogen fuel cell, which uses hydrogen and oxygen to produce electricity. However, the currently used catalyst support material, carbon black, corrodes in the low pH and oxidative environment. Therefore, new catalyst support materials are being sought. A new class of material, called MAX phases, shows potential because some possess a combination of properties of metals and ceramics. Three of them, Ti2AlC, Ti3AlC2, and Ti3SiC2, show good electrical conductivity and oxidation resistance. These MAX phases have been investigated using density functional theory (DFT) in this thesis to determine their properties. The density of states show that they are electrically conductive, with a continuous band over the Fermi level primarily from the Ti d orbital. Calculating the Boltzmann transport properties, yielded electrical resistivity values of 0.460 µΩ m for Ti2AlC, 0.370 µΩ m for Ti3AlC2, and 0.268 µΩ m for Ti3SiC2 at 300 K. Therefore, Ti3SiC2 should be the most electrically conductive of the three. The vacancy formation energy of an A group atom was investigated using a 2 x 2 x 2 supercell. The vacancy formation energies were calculated to be 2.882 eV for Ti2AlC, 2.812 eV for Ti3AlC2, and 2.167 eV for Ti3SiC2. The formation of a vacancy increases the electrical resistivity of the bulk MAX phases. As a catalyst support material, a MAX phase particle will have surfaces present. Due to the layered structure of the MAX phases, multiple terminations of (0 0 0 1) surfaces could be possible, which were investigated. It was shown that terminations where the Ti-C cage structure remained intact produced the lowest cleavage energies. For Ti2AlC, the two low cleavage energy surfaces are Al(Ti) and Ti(C), for Ti3AlC2, Al(Ti2) and Ti2(C), and for Ti3SiC2, Si(Ti2) and Ti2(C). The surfaces with the lowest cleavage energy should be more stable than other surfaces and would therefore be expected to be present on a MAX phase particle. Vacancies were also formed in the surface systems. The surfaces with the vacancy in the surface layer had the lowest vacancy formation energy, with that of Si(Ti2) being positive. The surface slabs generally showed a higher electrical resistivity than the bulk systems, while the formation of a vacancy generally increased the resistivity, in agreement with the bulk vacancy trend. These MAX phases are electrically conductive, however a quantifiable oxidation resistance was not able to be calculated. They do however show signs of being good electrocatalyst support materials. | |
| dc.identifier.apacitation | Gertzen, J. (2019). <i>MAX phases as an electrocatalyst support material: a DFT study</i>. (). ,Engineering and the Built Environment ,Department of Chemical Engineering. Retrieved from http://hdl.handle.net/11427/31459 | en_ZA |
| dc.identifier.chicagocitation | Gertzen, Jonathan. <i>"MAX phases as an electrocatalyst support material: a DFT study."</i> ., ,Engineering and the Built Environment ,Department of Chemical Engineering, 2019. http://hdl.handle.net/11427/31459 | en_ZA |
| dc.identifier.citation | Gertzen, J. 2019. MAX phases as an electrocatalyst support material: a DFT study. . ,Engineering and the Built Environment ,Department of Chemical Engineering. http://hdl.handle.net/11427/31459 | en_ZA |
| dc.identifier.ris | TY - Thesis / Dissertation AU - Gertzen, Jonathan AB - The insatiable global demand for energy cannot be sustained by fossil fuels without irreparable damage to the environment. Various alternative energy sources are being investigated to provide renewable clean energy. One promising technology is the hydrogen fuel cell, which uses hydrogen and oxygen to produce electricity. However, the currently used catalyst support material, carbon black, corrodes in the low pH and oxidative environment. Therefore, new catalyst support materials are being sought. A new class of material, called MAX phases, shows potential because some possess a combination of properties of metals and ceramics. Three of them, Ti2AlC, Ti3AlC2, and Ti3SiC2, show good electrical conductivity and oxidation resistance. These MAX phases have been investigated using density functional theory (DFT) in this thesis to determine their properties. The density of states show that they are electrically conductive, with a continuous band over the Fermi level primarily from the Ti d orbital. Calculating the Boltzmann transport properties, yielded electrical resistivity values of 0.460 µΩ m for Ti2AlC, 0.370 µΩ m for Ti3AlC2, and 0.268 µΩ m for Ti3SiC2 at 300 K. Therefore, Ti3SiC2 should be the most electrically conductive of the three. The vacancy formation energy of an A group atom was investigated using a 2 x 2 x 2 supercell. The vacancy formation energies were calculated to be 2.882 eV for Ti2AlC, 2.812 eV for Ti3AlC2, and 2.167 eV for Ti3SiC2. The formation of a vacancy increases the electrical resistivity of the bulk MAX phases. As a catalyst support material, a MAX phase particle will have surfaces present. Due to the layered structure of the MAX phases, multiple terminations of (0 0 0 1) surfaces could be possible, which were investigated. It was shown that terminations where the Ti-C cage structure remained intact produced the lowest cleavage energies. For Ti2AlC, the two low cleavage energy surfaces are Al(Ti) and Ti(C), for Ti3AlC2, Al(Ti2) and Ti2(C), and for Ti3SiC2, Si(Ti2) and Ti2(C). The surfaces with the lowest cleavage energy should be more stable than other surfaces and would therefore be expected to be present on a MAX phase particle. Vacancies were also formed in the surface systems. The surfaces with the vacancy in the surface layer had the lowest vacancy formation energy, with that of Si(Ti2) being positive. The surface slabs generally showed a higher electrical resistivity than the bulk systems, while the formation of a vacancy generally increased the resistivity, in agreement with the bulk vacancy trend. These MAX phases are electrically conductive, however a quantifiable oxidation resistance was not able to be calculated. They do however show signs of being good electrocatalyst support materials. DA - 2019 DB - OpenUCT DP - University of Cape Town KW - Chemical Engineering LK - https://open.uct.ac.za PY - 2019 T1 - MAX phases as an electrocatalyst support material: a DFT study TI - MAX phases as an electrocatalyst support material: a DFT study UR - http://hdl.handle.net/11427/31459 ER - | en_ZA |
| dc.identifier.uri | http://hdl.handle.net/11427/31459 | |
| dc.identifier.vancouvercitation | Gertzen J. MAX phases as an electrocatalyst support material: a DFT study. []. ,Engineering and the Built Environment ,Department of Chemical Engineering, 2019 [cited yyyy month dd]. Available from: http://hdl.handle.net/11427/31459 | en_ZA |
| dc.language.rfc3066 | eng | |
| dc.publisher.department | Department of Chemical Engineering | |
| dc.publisher.faculty | Faculty of Engineering and the Built Environment | |
| dc.subject | Chemical Engineering | |
| dc.title | MAX phases as an electrocatalyst support material: a DFT study | |
| dc.type | Master Thesis | |
| dc.type.qualificationlevel | Masters | |
| dc.type.qualificationname | MSc |