| Resumo: |
The sun is one of the major sustainable world energy sources and in Portugal it is probably the major one [Tr2005]. There are presently three technologies that take advantage of the sun energy: solar panels, photovoltaic (PV) cells and solar power concentrators. However, these technologies have a limited ability to store energy; solar power concentrators use huge tanks to store heated salts that can be transformed into electricity whenever necessary [Co2008]. On the other hand, PV cells convert sunlight directly into electricity, and due to the variability of daily solar radiation an effective method to store energy for later dispatch is therefore needed [Tr2005]. One of the most attractive methods for energy storage is the conversion of energy to chemical fuels [Tr2005; US2005]. The challenge in solar fuel technology is to produce chemical fuels directly from sunlight in a robust, cost-effective way.\nHydrogen has the potential to be a sustainable, carbon-neutral fuel if produced from renewable sources and namely from sunlight [WETO2007]. This may be achieved with water photoelectrolysis, a process in which a semiconductor material absorbs solar energy and uses it to drive water reduction (2H2O + 2e- --> H2 + 2OH-) and oxidation (2H2O --> O2 + 4e- + 4H+). The photoelectrode materials should meet several criteria [KrLi2008]: a) strong (visible) light absorption, b) high chemical stability in the dark and under illumination, c) suitable band edge positions to enable reduction/oxidation of water, d) efficient charge transport in the semiconductor, e) low overpotentials for the reduction/oxidation reactions, and f) low cost.\nThe water-split phenomenon was first reported in the 70s in a pioneer-work by Fujishima and Honda [FuHo1972]. However, only recently the water-split begun to be investigated with a renewable interest; indeed, the number of publications has jumped after 2005. Presently photoelectrolytic devices use a semiconductor to promote water photo-oxidatio  |
Resumo
The sun is one of the major sustainable world energy sources and in Portugal it is probably the major one [Tr2005]. There are presently three technologies that take advantage of the sun energy: solar panels, photovoltaic (PV) cells and solar power concentrators. However, these technologies have a limited ability to store energy; solar power concentrators use huge tanks to store heated salts that can be transformed into electricity whenever necessary [Co2008]. On the other hand, PV cells convert sunlight directly into electricity, and due to the variability of daily solar radiation an effective method to store energy for later dispatch is therefore needed [Tr2005]. One of the most attractive methods for energy storage is the conversion of energy to chemical fuels [Tr2005; US2005]. The challenge in solar fuel technology is to produce chemical fuels directly from sunlight in a robust, cost-effective way.\nHydrogen has the potential to be a sustainable, carbon-neutral fuel if produced from renewable sources and namely from sunlight [WETO2007]. This may be achieved with water photoelectrolysis, a process in which a semiconductor material absorbs solar energy and uses it to drive water reduction (2H2O + 2e- --> H2 + 2OH-) and oxidation (2H2O --> O2 + 4e- + 4H+). The photoelectrode materials should meet several criteria [KrLi2008]: a) strong (visible) light absorption, b) high chemical stability in the dark and under illumination, c) suitable band edge positions to enable reduction/oxidation of water, d) efficient charge transport in the semiconductor, e) low overpotentials for the reduction/oxidation reactions, and f) low cost.\nThe water-split phenomenon was first reported in the 70s in a pioneer-work by Fujishima and Honda [FuHo1972]. However, only recently the water-split begun to be investigated with a renewable interest; indeed, the number of publications has jumped after 2005. Presently photoelectrolytic devices use a semiconductor to promote water photo-oxidation, whereas the reduction is normally obtained with the help of an external energy source, such as a PV cell, in an arrangement know as tandem cell [KrLi2008]. The energy efficiency in water-split has been improved tremendously in the past few years; presently the best performing system has 18 % hydrogen conversion efficiency [PeDi2007]. However, this device is made of an expensive highly efficient PV cell and should have a limited lifetime. During the 70s and 80s, many semiconductors were identified and characterized, but none of them was suitable for conducting alone water-split under solar radiation with attractive efficiencies. However, with the nanomaterials advent there is a new hope and progresses are being made in this field [KrLi2008].\nThe present project targets the development of a test bench for photoelectrochemical cells, the development of a new photoanode and the unsteady state modeling and simulation of a photoelectrochemical cell. The project goes beyond the state of the art in the following aspects: (i) development of a nanowire and nanotube arrays of hematite (-Fe2O3) that targets the simultaneous oxidation and reduction of water (blue-shift is observed, with upward shift of the reduction band edge); and (ii) the unsteady-state modeling and simulation of a photoelectrochemical cell which has never been reported despite the great benefits that one such model will certainly bring. |