Photoelectrochemistry Essay Research Paper PhotoelectrochemistryProducing Hydrogen from

Photoelectrochemistry Essay, Research Paper

PhotoelectrochemistryProducing Hydrogen from Solar Power I. Introduction A. Producing fuel with solar power B. The problems C. The solution II. Process of Splitting Water A. Electrolysis B. Disadvantages of electrolysis III. Using Solar Power to Produce Electrical Energy A. The process IV. The Problems with Solar Power A. Compatibility of semiconductors B. Energy range C. The expense V. The Solution A. Single device to perform task 1. Eliminates the loss of energy B. More efficient semiconductors developed VI. Summary VII. Conclusion It s the ultimate in clean energy: Generate fuel from water using onlythe power of sunlight, and when the fuel burns, it gives off nothing but water. As outlandish as it sounds, the dream was accomplished decades ago byusing solar energy to split water into its components, oxygen and hydrogen–apowerful fuel that can be used to run everything from power plants to cars. But as a commercial proposition, the process has been a nonstarter becauseit s so inefficient and expensive. The two steps involved–generatingelectricity from sunlight and using it to split water–normally take place inseparate devices, and energy is lost in between. Now, researchers at the National Renewable Energy Laboratory(NREL), have come up with a single device that accomplishes both tasks andhas set a world record in efficiency for converting photons1, found insunlight, to fuel (4). This new solar powered water splitter, built by NRELchemists John Turner and Oscar Khaselev, converts about 12.5% of theenergy in sunlight to gaseous fuel–nearly double the previous recordachieved by a conventional two step process (10). Process of Splitting Water Splitting water to create gaseous hydrogen and oxygen is quite simple. First, stick a pair of metal electrodes into water. Then, apply an electriccurrent to them, and the oxygen gas attracts to one electrode and hydrogengas to the other. The process, known as electrolysis, is commonly used toproduce pure hydrogen for making everything from food oils to computerchips (6). But it s expensive and requires fossil fuels to generate theelectricity that powers the process. So energy researchers have long dreamedof using solar energy to drive the electrolysis. Using Solar Power The basic principle of generating electricity from sunlight is, again,well known. When photons from sunlight strike normally static electrons insome semiconductor materials, they push the electrons into a higher energylevel, allowing them to roam about. Left behind are electron holes, orvacancies, that act like positive charges that can also move through thematerial. Additional semiconductor layers on either side of the absorbinglayer then channel the electrons and holes in opposite directions, creating anelectric current that can perform work or be stored in a battery (1). Butunfortunately, combining this so called photovoltaic2 effect with electrolysisin a single device isn t simple. The Problems with Solar Power First, there s a compatibility problem. Solar cells must be placed inwater in order to split it into hydrogen and oxygen, but semiconductors thatare efficient light absorbers are often unstable in water. Also, a watermolecule splits into hydrogen and oxygen atoms only if each atom absorbselectrical charges that have very precise, and different, amounts of energy. Inconventional electrolysis, the metal electrodes carry electrical charges with awide energy range, allowing those with just the right amount of energy to

catalyze the split, but semiconductors are more finicky and charges in thesematerials can exist only at well-defined energy levels(10). Unfortunately, theonly semiconductor materials known to produce electrical charges at just theright levels to generate both hydrogen and oxygen are very poor absorbers ofsunlight. Looking at a Solution To overcome these problems, Turner and Khaselev constructed a single device that contains two different semiconductor materials. One, made fromgallium indium phosphide, which absorbs ultraviolet and visible light andproduces mobile electrons with the right energy to produce hydrogen. Theother, made from gallium arsenide, that absorbs infared light and producesholes with the right amount of energy to produce oxygen. Gallium indiumphosphide is stable in water, so it can be used directly as an electrode. Gallium arsenide, however, is unstable in water, but in order to counter thisproblem the material is shielded by a special epoxy coating, and the holes aredriven to a separate platinum electrode (4). Although the new device appears to be efficient and stable, it is estimated that the cost of producing hydrogen in bulk would be three timesthat of the cheapest method, in which hydrogen atoms are stripped fromnatural gas by super heated steam(1 and 6). Scientists are currently trying toengineer cheaper semiconductors to perform the water-splitting reaction. Ifthey succeed, the energy of the future may finally find its way to the present. In summary of this report, the ability to use solar energy to split water,may revolutionize the world as we know it. It is very likely to serve as anunlimited fuel, one that is harmless to our environment, to power ourautomobiles, power plants, and possibly even our homes. It may also prove to be useful in the production of the vast materials that are already made fromhydrogen. This new technology, certainly appears likely to be of greatbenefit to us in the future, if only we are able to work out the problems ofexpense in production, as well as the inefficiency of the process as a whole. Nevertheless, the chances of solving these problems seem very good, asscientists continue diligently researching and developing the processesinvolved, and it s very likely that changes may be seen, very soon, in ourlives as a result of this technology. References 1. A. Heller, Science 223, 1141 (1984)2. J.R. Bolton, Solar Energy 57, 37 (1996)3. J.R. Bolton, Nature 316, 495 (1985)4. Robert F. Service, Science 280, 382 (1998)5. Author N/A, Science 234, 353 (1996)6. Hydrogen, The World Book Encyclopedia, 1996 ed. vol. 9, pp. 467-4687. Photon, The World Book Encyclopedia, 1996 ed. vol. 15, pg. 4308. Solar Energy, The World Book Encyclopedia, 1996 ed. vol. 18, pp. 576-579 9. Merriam Webster s Collegiate Dictionary, Tenth Edition, 1993 ed. Thomas E. Stanley publishing.10. O. Khaselev and J.A. Turner, Science 280, 425-427 (1998) Bibliography Bolton, J.R., Polarization dependence of two-photon absorption in GaAs/Ga(sub1-x)Al(sub x)As heterostructures Solar Energy, vol. 57, 1996) pg. 37 Bolton, J.R., It takes two to tangle. Nature, vol. 316, (1985) pg. 495 Heller, A., Power from Photons. Science, vol. 223, (1998) pg. 1141 Hydrogen. The World Book Encycolpedia, vol. 9, (1996) pp. 467-468 Khaselev, O. and Turner, J.A., Study on Solar Power. Science, vol. 280 (1998) pp. 425-427 Merriam Webster s Collegiate Dictionary, Tenth Edition, (1993) Photon. The World Book Encyclopedia, (computer reference) vol. 18, (1996) pg. 430 Service, Robert F., Renewable energy technologies in the Carribiean. Science, vol. 280, (1998) pg. 382 Solar Energy. The World Book Encyclopedia, vol. 15, (1996) pp576-579 _____, Science, vol. 316 (1996) pg. 353


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