Highly active and efficient metal-decorated silicon-based nanostructured photoelectrodes for water splitting solar cells
[eng] With the burning of large amounts of traditional fossil fuels, global environmental pollution is getting worse and worse, and energy crisis is becoming more serious for meeting human’s life demand. In order to solve these problems, it’s imperative to find renewable and clean energy sources. Al...
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| Tipo de recurso: | tesis doctoral |
| Estado: | Versión publicada |
| Fecha de publicación: | 2020 |
| País: | España |
| Institución: | Universidad de Barcelona |
| Repositorio: | Dipòsit Digital de la UB |
| OAI Identifier: | oai:diposit.ub.edu:2445/174198 |
| Acceso en línea: | https://hdl.handle.net/2445/174198 http://hdl.handle.net/10803/670880 |
| Access Level: | acceso abierto |
| Palabra clave: | Catàlisi Fotoelectroquímica Silici Hidrogen com a combustible Electròlisi Catalysis Photoelectrochemistry Electrolysis Silicon Hydrogen as fuel |
| Sumario: | [eng] With the burning of large amounts of traditional fossil fuels, global environmental pollution is getting worse and worse, and energy crisis is becoming more serious for meeting human’s life demand. In order to solve these problems, it’s imperative to find renewable and clean energy sources. Although solar is one of the most abundant renewable energy on earth, it’s difficult to collect and store. As a non-polluting energy, hydrogen is a highly promising candidate to replace fossil fuels. Sunlight can be used to split water into hydrogen, producing chemical energy stored in hydrogen bonds. This easy way of producing clean fuels (hydrogen) has attracted the attention of both industry and academy. Photoelectrochemical (PEC) water splitting is one of the most promising methods to produce hydrogen by utilizing solar energy, due to the simple structure, low fabrication cost and good performance of the prepared cells. In these cells, a semiconductor photoelectrode is immersed in an electrolyte, and when illuminated, hydrogen and/or oxygen can be generated on its surface by electrolysis. To obtain better performance for PEC water splitting devices, it’s extremely significant to select proper semiconductors for absorbing light, catalysts for enhancing the PEC performance and electrolytes containing various ions. Silicon has garnered very much interest as semiconductor photoelectrodes due to its low cost and proper band gap (1.1 eV). However, the electrolyte can oxidize and/or corrode its surface, resulting in a reduction of its performance. Metal catalysts are often used to avoid the degradation of silicon photoelectrodes, and to enhance their activity in the electrolyte. However, the degree of protection can be reduced after some periods of time, and consequently the lifetime of the semiconductor photoelectrodes is still the main bottleneck of this PEC water splitting technology. Besides, tuning the pH of the electrolytes or the chemical composition of the electrolytes including special species could improve the activity and stability of the cells. In this PhD thesis I present a deep study about the ageing mechanisms of Ni layers with different thicknesses as protective and catalytic coatings on n-type Si photoanodes for PEC water splitting in strong alkaline condition. Before and after performed long-time PEC characterizations, we comprehensively analyzed the photoanodes at nano and atomic scales using atomic force microscopy (AFM) and electron microscopy. By investigating the morphology changes and the chemical composition of the photoanodes after long operation times, we find that the ageing mechanisms extremely rely on the thickness of the Ni coating layer. The activity of the 2 nm nickel coated silicon photoanode decays faster than thicker ones due to the formation of a thick interfacial SiOX film and the extensive penetration of potassium impurities into the NiOX layer. Whilst the photoanodes with more than 5 nm Ni coatings show longer stability, and the degradation is due to the formation of holes in the NiOX layer. Then, using 5 nm Ni-based n-Si photoanodes, we analyzed the effect of different alkaline electrolytes for PEC water splitting. Although the photoanodes show lower onset potential at high pH electrolyte, we also developed an advanced electrolyte (a mixture of potassium hydroxide (KOH) and lithium hydroxide (LiOH), pH 12.5) that shows good activity and stability for metal-based silicon photoelectrodes. Furthermore, we also designed, fabricated and tested n-3C-SiC/p-Si photocathodes for PEC water splitting in KOH, and observed an enhancement of PEC performance due to the catalytic and plasmonic resonance effects of the noble metal nanoparticles (NPs) introduced. By tuning the size and shape of Au NPs on the photocathodes, higher saturated photocurrent can be achieved. And Pt NPs coated n-3C-SiC/p-Si photocathodes show lowest onset potential and highest saturated photocurrent for PEC performance. |
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