Highly active and efficient metal-decorated silicon-based nanostructured photoelectrodes for water splitting solar cells

Abstract

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.

La quema de grandes cantidades de combustibles fósiles para satisfacer la demanda energética stá empeorando la contaminación ambiental cada vez más, y la crisis energética se está volviendo más grave. La división de agua mediante fotoelectroquímica (PEC, por sus siglas en inglés) utilizando luz solar es uno de los métodos más prometedores para producir combustible hidrógeno de forma limpia, debido a la estructura simple, el bajo costo de fabricación y el buen rendimiento. En las células fotoelectroquímicas, un fotoelectrodo semiconductor se sumerge en un electrolito y, cuando se ilumina, se puede generar hidrógeno y/u oxígeno en su superficie mediante electrólisis. Para obtener un mejor rendimiento, es extremadamente importante seleccionar semiconductores adecuados para absorber la luz, catalizadores para mejorar el rendimiento, y electrolitos que contienen varios iones. El silicio ha suscitado mucho interés debido a su bajo costo y a sus propiedades eléctricas (banda prohibida de 1.1eV). Sin embargo, debido a su inestabilidad en el electrolito, los catalizadores metálicos a menudo se usan para evitar la degradación de los fotoelectrodos de silicio y para mejorar su actividad en el electrolito. Dado que el grado de protección se puede reducir después de algunos períodos de tiempo, la vida útil de los fotoelectrodos de semiconductores sigue siendo el principal cuello de botella de esta tecnología de división de agua PEC. Además, ajustar el pH o la composición química de los electrolitos, incluidas las especies especiales, podría mejorar la actividad y la estabilidad de las células. En esta tesis, he estudiado el uso de niquel como capa protectora y catalizadora sobre fotoelectrodos de silicio, y hemos analizados su envejecimiento bajo condiciones de funcionamiento reales. También he desarrollado un electrolito avanzado (una mezcla de hidróxido de potasio [KOH] e hidróxido de litio (LiOH), pH 12.5) que muestra una buena actividad y estabilidad para los fotoelectrodos de silicio a base de metal. Además, también diseñamos, fabricamos y testeamos fotocatodos n-3C-SiC/p-Si cubiertos con nanopartículas de metales nobles para la división de agua PEC en KOH, y observamos una mejora en el rendimiento de PEC debido a los efectos de resonancia catalítica y plasmónica de las nanopartículas introducidas.