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【分享】太阳能电池－燃料电池研究进展系列专贴【已搜索无重复】
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Nano-Manhattan' 3D solar cells boost efficiency (Update) Unique three-dimensional solar cells that capture nearly all of the light that strikes them could boost the efficiency of photovoltaic (PV) systems while reducing their size, weight and mechanical complexity. The new 3D solar cells capture photons from sunlight using an array of miniature "tower" structures that resemble high-rise buildings in a city street grid. The cells could find near-term applications for powering spacecraft, and by enabling efficiency improvements in photovoltaic coating materials, could also change the way solar cells are designed for a broad range of applications. "Our goal is to harvest every last photon that is available to our cells," said Jud Ready, a senior research engineer in the Electro-Optical Systems Laboratory at the Georgia Tech Research Institute (GTRI). "By capturing more of the light in our 3D structures, we can use much smaller photovoltaic arrays. On a satellite or other spacecraft, that would mean less weight and less space taken up with the PV system." The 3D design was described in the March 2007 issue of the journal JOM, published by The Minerals, Metals and Materials Society. The research has been sponsored by the Air Force Office of Scientific Research, the Air Force Research Laboratory, NewCyte Inc., and Intellectual Property Partners, LLC. A global patent application has been filed for the technology. The GTRI photovoltaic cells trap light between their tower structures, which are about 100 microns tall, 40 microns by 40 microns square, 10 microns apart -- and built from arrays containing millions of vertically-aligned carbon nanotubes. Conventional flat solar cells reflect a significant portion of the light that strikes them, reducing the amount of energy they absorb. Because the tower structures can trap and absorb light received from many different angles, the new cells remain efficient even when the sun is not directly overhead. That could allow them to be used on spacecraft without the mechanical aiming systems that maintain a constant orientation to the sun, reducing weight and complexity – and improving reliability. "The efficiency of our cells increases as the sunlight goes away from perpendicular, so we may not need mechanical arrays to rotate our cells," Ready noted. The ability of the 3D cells to absorb virtually all of the light that strikes them could also enable improvements in the efficiency with which the cells convert the photons they absorb into electrical current. In conventional flat solar cells, the photovoltaic coatings must be thick enough to capture the photons, whose energy then liberates electrons from the photovoltaic materials to create electrical current. However, each mobile electron leaves behind a "hole" in the atomic matrix of the coating. The longer it takes electrons to exit the PV material, the more likely it is that they will recombine with a hole -- reducing the electrical current. Because the 3D cells absorb more of the photons than conventional cells, their coatings can be made thinner, allowing the electrons to exit more quickly, reducing the likelihood that recombination will take place. That boosts the "quantum efficiency" – the rate at which absorbed photons are converted to electrons – of the 3D cells. Fabrication of the cells begins with a silicon wafer, which can also serve as the solar cell’s bottom junction. The researchers first coat the wafer with a thin layer of iron using a photolithography process that can create a wide variety of patterns. The patterned wafer is then placed into a furnace heated to 780 degrees Celsius. Hydrocarbon gases are then flowed into furnace, where the carbon and hydrogen separate. In a process known as chemical vapor deposition, the carbon grows arrays of multi-walled carbon nanotubes atop the iron patterns. Once the carbon nanotube towers have been grown, the researchers use a process known as molecular beam epitaxy to coat them with cadmium telluride (CdTe) and cadmium sulfide (CdS) which serve as the p-type and n-type photovoltaic layers. Atop that, a thin coating of indium tin oxide, a clear conducting material, is added to serve as the cell’s top electrode. In the finished cells, the carbon nanotube arrays serve both as support for the 3D arrays and as a conductor connecting the photovoltaic materials to the silicon wafer. The researchers chose to make their prototypes cells from the cadmium materials because they were familiar with them from other research. However, a broad range of other photovoltaic materials could also be used, and selecting the best material for specific applications will be a goal of future research. Ready also wants to study the optimal heights and spacing for the towers, and to determine the trade-offs between spacing and the angle at which the light hits the structures. The new cells face several hurdles before they can be commercially produced. Testing must verify their ability to survive launch and operation in space, for instance. And production techniques will have to scaled up from the current two-inch laboratory prototypes. "We have demonstrated that we can extract electrons using this approach," Ready said. "Now we need to get a good baseline to see where we compare to existing materials, how to optimize this and what’s needed to advance this technology." Intellectual Property Partners of Atlanta holds the rights to the 3D solar cell design and is seeking partners to commercialize the technology. Another commercialization path is being followed by an Ohio company, NewCyte, which is partnering with GTRI to use the 3D approach for terrestrial solar cells. The Air Force Office of Scientific Research has awarded the company a Small Business Technology Transfer (STTR) grant to develop the technology. "NewCyte has patent pending, low cost technology for depositing semiconductor layers directly on individual fullerenes," explained Dennis J. Flood, NewCyte’s president and CTO. "We are using our technology to grow the same semiconductor layers on the carbon nanotube towers that GTRI has already demonstrated. Our goal is to achieve performance and cost levels that will make solar cells using the GTRI 3D cell structure competitive in the broader terrestrial solar cell market." On the Net: http://www-stage.gatech.edu/news-room/flash/CNTpv.html Source: Georgia Institute of Technology [ Last edited by ddx-k on 2008-12-4 at 15:44 ] |
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Nanowires may boost solar cell efficiency Nanowires may boost solar cell efficiency, engineers say Scanning electron microscope (SEM) image of n-type InP nanowire growth on indium tin oxide (ITO) taken at a 45 degree tilt with scale bar of 500 nanometers. Credit: UC San Diego University of California, San Diego electrical engineers have created experimental solar cells spiked with nanowires that could lead to highly efficient thin-film solar cells of the future. Indium phosphide (InP) nanowires can serve as electron superhighways that carry electrons kicked loose by photons of light directly to the device’s electron-attracting electrode – and this scenario could boost thin-film solar cell efficiency, according to research recently published in NanoLetters. The new design increases the number of electrons that make it from the light-absorbing polymer to an electrode. By reducing electron-hole recombination, the UC San Diego engineers have demonstrated a way to increases the efficiency with which sunlight can be converted to electricity in thin-film photovoltaics. Including nanowires in the experimental solar cell increased the “forward bias current” – which is a measure of electrical current – by six to seven orders of magnitude as compared to their polymer-only control device, the engineers found. The online journal NanoLetters published this new work on polymer/nanowire hybrid photovoltaics in February 2008. “If you provide electrons with a defined pathway to the electrode, you can reduce some of the inefficiencies that currently plague thin-film solar cells made from polymer mixtures. More efficient transport of electrons and holes – collectively known as carriers – is critical for creating more efficient solar cells,” said Clint Novotny the first author of the NanoLetters paper, and a recent electrical engineering Ph.D. from UC San Diego’s Jacobs School of Engineering. Novotny is now working on solar technologies at BAE Systems. Simplified Nanowire Growth The engineers devised a way to grow nanowires directly on the electrode. This advance allowed them to create the electron superhighways that deliver electrons from the polymer-nanowire interface directly to an electrode. “If nanowires are going to be used massively in photovoltaic devices, then the growth mechanism of nanowires on arbitrary metallic surfaces is an issue of great importance,” said co-author Paul Yu, a professor of electrical engineering at UC San Diego’s Jacobs School of Engineering. “We contributed one approach to growing nanowires directly on metal.” The UCSD electrical engineers grew their InP nanowires on the metal electrode –indium tin oxide (ITO) – and then covered the nanowire-electrode platform in the organic polymer, P3HT, also known as poly(3-hexylthiophene). The researchers say they were the first group to publish work demonstrating growth of nanowires directly on metal electrodes without using specially prepared substrates such as gold nanodrops. “Just a layer of metal can work. In this paper we used ITO, but you can use other metals, including aluminum,” said Paul Yu. Growing nanowires directly on untreated electrodes is an important step toward the goal of growing nanowires on cheap metal substrates that could serve as foundations for next-generation photovoltaics that conform to the curved surfaces like rooftops, cars or other supporting structures, the engineers say. “By growing nanowires directly on an untreated electrode surface, you can start thinking about incorporating millions or billions of nanowires in a single device. I think this is where the field is eventually going to end up,” said Novotny. “But I think we are at least a decade away from this becoming a mainstream technology.” Polymer Solar Cells and Nanowires Meet As in more traditional organic polymer thin-film solar cells, the polymer material in the experimental system absorbs photons of light. To convert this energy to electricity, each photon-absorbing electron must split apart from its hole companion at the interface of the polymer and the nanowire – a region known as the p-n junction. Once the electron and hole split, the electron travels down the nanowire – the electron superhighway – and merges seamlessly with the electron-capturing electrode. This rapid shuttling of electrons from the p-n junction to the electrode could serve to make future photovoltaic devices made with polymers more efficient. “In effect, we used nanowires to extend an electrode into the polymer material,” said co-author Edward Yu, a professor of electrical engineering at UCSD’s Jacobs School of Engineering. While the electrons travel down the nanowires in one direction, the holes travel along the nanowires in the opposite direction – until the nanowire dead ends. At this point, the holes are forced to travel through a thin polymer layer before reaching their electrode. Today’s thin-film polymer photovoltaics do not provide freed electrons with a direct path from the p-n junction to the electrode – a situation which increases recombination between holes and electrons and reduces efficiency in converting sunlight to electricity. In many of today’s polymer photovoltaics, interfaces between two different polymers serve as the p-n junction. Some experimental photovoltaic designs do include nanowires or carbon nanotubes, but these wires and tubes are not electrically connected to an electrode. Thus, they do not minimize electron-hole recombination by providing electrons with a direct path from the p-n junction to the electrode the way the new UCSD design does. Before these kinds of electron superhighways can be incorporated into photovoltaic devices, a series of technical hurdles must be addressed – including the issue of polymer degradation. “The polymers degrade quickly when exposed to air. Researchers around the world are working to improve the properties of organic polymers,” said Paul Yu. As it was a proof-of-concept project, the UCSD engineers did not measure how efficiently the device converted sunlight to electricity. This explains, in part, why the authors refer to the device in their NanoLetters paper as a “photodiode” rather than a “photovoltaic.” Having a more efficient method for getting electrons to their electrode means that researchers can make thin-film polymer solar cells that are a little bit thicker, and this could increase the amount of sunlight that the devices absorb. Source: University of California - San Diego May 14, 2008 |
65楼2008-11-13 00:41:20
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Platinum nanocrystals boost catalytic activity for fuel oxidation, hydrogen production (A) Low-magnification SEM image of a platinum tetrahexahedral nanocrystal and its geometrical model. (B) High-resolution transmission electron microscopy image recorded from a platinum tetrahexahedral nanocrystal to reveal surface atomic steps in the areas made of (210) and (310) sub-facets. Credit: Zhong Lin Wang A research team composed of electrochemists and materials scientists from two continents has produced a new form of the industrially-important metal platinum: 24-facet nanocrystals whose catalytic activity per unit area can be as much as four times higher than existing commercial platinum catalysts. The new platinum nanocrystals, whose "tetrahexahedral" structure had not previously been reported in the metal, could improve the efficiency of chemical processes such as those used to catalyze fuel oxidation and produce hydrogen for fuel cells. "If we are going to have a hydrogen economy, we will need better catalysts," said Zhong Lin Wang, a Regents Professor in the School of Materials Science and Engineering at the Georgia Institute of Technology. "This new shape for platinum catalyst nanoparticles greatly improves their activity. This work also demonstrates a new method for producing metallic nanocrystals with high-energy surfaces." The new nanocrystals, produced electrochemically from platinum nanospheres on a carbon substrate, remain stable at high temperatures. Their sizes can be controlled by varying the number of cycles of "square wave" electrical potential applied to them. "This electrochemical technique is vital to producing such tetrahexahedral platinum nanocrystals," said Shi-Gang Sun, an Eminent Professor in the College of Chemistry and Chemical Engineering at the Xiamen University in China. "The technique used to produce the new platinum nanostructures may also have applications to other catalytic metals." The research was supported by the Natural Science Foundation of China, Special Funds for Major State Basic Research Project of China and the U.S. National Science Foundation. Details will be reported in the May 4 issue of the journal Science. Platinum plays a vital role as a catalyst for many important reactions, used in industrial chemical processing, in motor vehicle catalytic converters that reduce exhaust pollution, in fuel cells and in sensors. Commercially available platinum nanocrystals – which exist as cubes, tetrahedra and octahedra – have what are termed "low-index" facets, characterized by the numbers {100} or {111}. Because of their higher catalytic activity, "high-index" surfaces would be preferable – but until now, platinum nanocrystals with such surfaces have never been synthesized – and therefore have not been available for industrial use. The nanocrystals produced by the U.S.-Chinese team have high energy surfaces that include numerous "dangling bonds" and "atomic steps" that facilitate chemical reactions. These structures, characterized by {210}, {730} or {520} facets, remain stable at high temperatures – up to 800 degrees Celsius in testing done so far. That stability will allow them to be recycled and re-used in catalytic reactions, Wang said. Though the process must still be fine-tuned, the researchers have learned to control the size of the particles by varying the processing conditions. They are able to control the size such that only 4.5 percent of the nanocrystals produced are larger or smaller than the target size. "In nanoparticle research, two things are important: size control and shape control," said Wang. "From a purity point of view, we have been able to obtain a high yield of nanocrystals whose shape was a real surprise." Depending on conditions, the new nanocrystals can be as much as four times more catalytically active per unit area than existing commercial catalysts. But since the new structures tested are more than 20 times larger than existing platinum catalysts, they require more of the metal – and hence are less active per unit weight. "We need to find a way to make these nanocrystals smaller while preserving the shape," Wang noted. "If we can reduce the size through better control of processing conditions, we will have a catalytic system that would allow production of hydrogen with greater efficiency." Production of the new crystals begins with polycrystalline platinum spheres about 750 nanometers in diameter that are electrodeposited onto a substrate of amorphous – also known as "glassy" – carbon. Placed in an electrochemical cell with ascorbic acid and sulfuric acid, the spheres are then subjected to "square wave" potential that alternates between positive and negative potentials at a rate of 10 to 20 Hertz. The electrochemical oxidation-reduction reaction converts the spheres to smaller nanocrystals over a period of time ranging from 10 to 60 minutes. The role of the carbon substrate isn't fully understood, but it somehow enhances the uniformity of the nanocrystals. "The key to producing this shape is to tune the voltage and the time period under which it is applied," Sun noted. "By changing the experimental conditions, we can control the size with a high level of uniformity." Scanning electron microscopy shows that the sizes average 81 nanometers in diameter, with the smallest just 20 nanometers. The microscopy also found that the structures were composed of single crystals with no dislocations. "Not only do we have a beautiful shape – which was observed for the first time in this research – but we also have a very valuable catalyst," Sun added. "And because these nanocrystals are stable, the shape is preserved after the catalytic reaction, which will allow us to use the same nanocrystals over and over again." Source: Georgia Institute of Technology |
2楼2008-11-12 23:48:30
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科技日报2007年6月27日讯 几年之前，包括东芝和NEC在内的一些日本公司宣称，他们将推出可供手机、笔记本电脑和音乐播放器使用的微型燃料电池。这种电池在需要充电时，只需往里加点酒精就行了，消费者似乎可以从此摆脱充电器的束缚，干电池时代也似乎就此宣告结束。 然而，此后则是一片寂静。直到今天，他们鼓吹的燃料电池在市场上也难觅其踪，究竟哪里出了问题？ 第一张大饼不好烙 其实这些公司也是有苦难言。他们没料到，小小的燃料电池竟然是个“难产儿”。 研究人员发现，电池内部的物质对于温度和湿度太敏感，而且容易产生一氧化碳这种有害气体。他们制造出的便携式燃料电池的最佳性能，也仅仅勉强和他们宣称要取代的锂电池相当，不具有市场竞争力。 目前，正在研制开发的燃料电池主要有两种：质子交换膜型（PEM）和固体氧化物型。两者都能使燃料和空气沿相反方向透过一层特殊的薄膜。 PEM的交换膜上涂有一层催化剂，这种催化剂能够将燃料（通常是氢原子）的电子夺走，氢原子于是成为带正电荷的氢离子，它可以透过薄膜，与另一侧来自空气中的氧结合形成水。被夺走的电荷则急于和氢离子复合，不停地向外迁移，这个过程连续下来就形成了电流。 实际上，PEM电池从上世纪60年代就已经开始使用，它可以在170℃下工作，这是一个还可以接受的温度。但PEM电池有一个致命弱点：它需要纯氢来产生电子，目前还没有人能找到一个经济可行的方法来做到这一点。这迫使许多开发PEM电池的厂商把目光转向甲醇。比较而言，甲醇当然是价廉易得，但它含能量较低，是一个弱的能量来源。 固体氧化物型的工作原理则恰好相反。空气中的氧原子渗透到膜的另一侧，和被夺走电子的氢原子及一氧化碳结合，形成水和二氧化碳。因为要在1370℃这样的高温下工作，固体氧化物电池一度被开发者放弃。但它可以使用密度更高的碳氢化合物（如丁烷）作燃料，因而能提供远高于PEM电池的动力。在液态下，丁烷的能量密度是7.4千瓦时/升，而甲醇只有4.4千瓦时/升，锂电池通常只能提供0.3千瓦时的电量。 MIT三精英联手再战 美国马萨诸塞州的理利普申公司最近宣布，他们制造出一种火柴盒大小的燃料电池，可以供笔记本电脑使用几天。这种燃料电池采用的是固体氧化物设计，以丁烷为燃料。 理利普申（Lilliputian，意思为小人儿）公司成立不过4年，公司的两位创始人撒穆尔•斯凯威兹和阿列克斯•弗远兹都来自麻省理工学院（MIT）。在上世纪90年代，他们曾经在MIT做过用顶针大小的反应器蚀刻硅晶元的实验，不过那只是纯理论性的研究。后来他们意识到，可以利用类似的微机械技术来建立微小的“转化器”，从酒精中分离出纯氢供燃料电池使用。于是他们离开MIT，创建了理利普申公司。 肯尼思•拉扎勒斯则是一位在MIT受训过的航空学工程师，他把自己的公司以3500万美元的价格卖掉后一直再想找点事干。2003年底，拉扎勒斯遇见了斯凯威兹和弗远兹，当时这哥儿俩正在只有一个房间的实验室里埋头苦干，他们对拉扎勒斯把热门技术变成商业产品的本领极为佩服，力邀他担任理利普申的 CEO。 三位来自MIT的精英一起开始研究微型PEM燃料电池，很快就遇到了现在仍困扰日本同行的难题：以酒精为燃料的电池不能产生足够的电量，因此难以和锂电池竞争。于是，他们决定在固体氧化物电池上下功夫。他们面临的最大挑战之一就是如何将陶瓷电解质和硅晶整合在一起，因为当受热时，陶瓷的膨胀速度是硅的 4倍。他们是如何解决这个难题的？这当然是商业秘密。不过他们承认使用了一个网状断裂结构来吸收压力，与人行道上一块块地砖之间的膨胀连接很相似。 另一个要克服的难题是，在一个微小的空间内如何保持巨大的热量？部分热量是有用的，它可以把丁烷分解成氢和一氧化碳，从而使电池中的反应持续进行。为此，他们设计了一个真空罩，装在整个装置的上部，就像玻璃球包裹着钨丝的电灯泡一样。 由于采用了独特的设计，电池摸上去才刚刚温热。拉扎勒斯说：“如果要想在电池行业的竞争中胜出，你必须在电池的能量密度上击败对手，而丁烷是已知的能量密度最高的燃料之一。而我们的电池克敌致胜的另一个法宝是把热量保存在里面，这简直是个设计上的奇迹。” 争雄市场看实力 在几年前就嚷嚷着要推出燃料电池的工业巨头最近又在发誓说，他们的电池就要大规模投产了。2005年，东芝公司展出了用于音乐播放器和其它设备的甲醇燃料电池，他们计划在明年推向市场。机械技术公司已经重新聘用吉列公司来开发甲醇燃料盒，并与三星公司达成协议来制造样机，希望在2008年就能推到市场上销售。 对此，理利普申公司早已成竹在胸。他们的燃料电池已经获得了国际民航组织和联合国危险材料运输管理部门的批准，今年年底可望获得美国运输部的批准。打火机一直被禁止带上飞机，因为燃料丁烷和打火石是在一个装置内。而理利普申的电池里的丁烷几乎不可能被点燃，除非你将电池砸碎，同时点燃一根火柴。 拉扎勒斯估计，他们公司的第一个产品将是专供笔记本电脑和智能手机使用的便携式充电电池，2美元一罐的丁烷燃料可以“充电”25次。虽然还没有宣称和哪个制造商达成大规模生产的协议，但拉扎勒斯对此非常乐观，因为迄今为止，他已经从阿特拉斯、美国硅谷顶级风险投资公司及其它风险投资基金那里筹集了 4000万美元。他们还有另一个优势，制造他们的燃料电池所需要的是已经具有20年历史的半导体制造设备，这些设备在亚洲各地都有。 今天，纳米技术飞速发展，我们没有理由怀疑，将来有一天燃料电池会变得像跳蚤一样大小，依靠这种电池供电的可注射传感器及各种微型医疗器械将出现在我们的生活中。 图为燃料电池芯片的内部构造。电池的能量来源———丁烷在燃料处理器中被转化成氢和一氧化碳，它们沿着在硅晶上蚀刻出的微管进入燃料电池阵列。在每一个阵列单元里，催化剂夺走燃料的电子，而空气中的氧则透过电解质薄膜带来补充电子，于是产生电流，副产物是水和二氧化碳。 来源: http://www.stdaily.com/gb/stdaily/2007-06/27/content_687488.htm |
3楼2008-11-12 23:49:49
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Portugal – June 28, 2007 – A new paper published in Journal of the American Ceramic Society proposes a new method of producing hydrogen for portable fuel cells. This new method negates the need for the complicated and expensive equipment currently used. With their ability to work steadily for 10-20 times the length of equivalently sized Lithium-ion batteries, portable fuel cells are ideal energy suppliers for devices such as computers, cell phones and hybrid vehicles. Significant amounts of hydrogen are needed to power these long-lived fuel cells, but producing the chemical has, until this point, been costly and difficult. Zhen-Yan Deng, lead author of the study, found that modified aluminum powder can be used to react with water to produce hydrogen at room temperature and under normal atmospheric pressure. The result is a cost-efficient method for powering fuel cells that will make their use a more practical and realistic option in many applications. Efforts to produce large amounts of hydrogen for portable devices have previously focused on other chemicals; however, compared to other hybrids, aluminum is cheaper and requires no other chemical in order to react with water. “This makes the modified aluminum powder a more economically viable material to generate hydrogen for the future use of portable fuel cells,” says Deng. Link: http://www.blackwellpublishing.com/press/pressitem.asp?ref=1308 |
4楼2008-11-12 23:50:14
