燃料电池及其发展前景Fuel Cells and Their Prospects文献翻译.doc

文献翻译英文原文：Fuel Cells and Their ProspectsA fuel cell is an electrochemical conversion device. It produces electricity from fuel (on the anode side) and an oxidant (on the cathode side), which react in the presence of an electrolyte. The reactants flow into the cell, and the reaction products flow out of it, while the electrolyte remains within it. Fuel cells can operate virtually continuously as long as the necessary flows are maintained.Fuel cells are different from electrochemical cell batteries in that they consume reactant from an external source, which must be replenished-a thermodynamically open system. By contrast batteries store electrical energy chemically and hence represent a thermodynamically closed system.Many combinations of fuel and oxidant are possible. A hydrogen cell uses hydrogen as fuel and oxygen (usually from air) as oxidant. Other fuels include hydrocarbons and alcohols. Other oxidants include chlorine and chlorine dioxide. Fuel cell designA fuel cell works by catalysis, separating the component electrons and protons of the reactant fuel, and forcing the electrons to travel though a circuit, hence converting them to electrical power. The catalyst typically comprises a platinum group metal or alloy. Another catalytic process takes the electrons back in, combining them with the protons and oxidant to form waste products (typically simple compounds like water and carbon dioxide).A typical fuel cell produces a voltage from 0.6 V to 0.7 V at full rated load. Voltage decreases as current increases, due to several factors: Activation loss Ohmic loss (voltage drop due to resistance of the cell components and interconnects) Mass transport loss (depletion of reactants at catalyst sites under high loads, causing rapid loss of voltage) To deliver the desired amount of energy, the fuel cells can be combined in series and parallel circuits, where series yield higher voltage, and parallel allows a stronger current to be drawn. Such a design is called a fuel cell stack. Further, the cell surface area can be increased, to allow stronger current from each cell.Proton exchange fuel cellsIn the archetypal hydrogenoxygen proton exchange membrane fuel cell (PEMFC) design, a proton-conducting polymer membrane, (the electrolyte), separates the anode and cathode sides. This was called a solid polymer electrolyte fuel cell (SPEFC) in the early 1970s, before the proton exchange mechanism was well-understood. (Notice that polymer electrolyte membrane and proton exchange mechanism result in the same acronym.)On the anode side, hydrogen diffuses to the anode catalyst where it later dissociates into protons and electrons. These protons often react with oxidants causing them to become what is commonly referred to as multi-facilitated proton membranes (MFPM). The protons are conducted through the membrane to the cathode, but the electrons are forced to travel in an external circuit (supplying power) because the membrane is electrically insulating. On the cathode catalyst, oxygen molecules react with the electrons (which have traveled through the external circuit) and protons to form water in this example, the only waste product, either liquid or vapor.In addition to this pure hydrogen type, there are hydrocarbon fuels for fuel cells, including diesel, methanol (see: direct-methanol fuel cells and indirect methanol fuel cells) and chemical hydrides. The waste products with these types of fuel are carbon dioxide and water.The materials used in fuel cells differ by type. In a typical membrane electrode assembly (MEA), the electrodebipolar plates are usually made of metal, nickel or carbon nanotubes, and are coated with a catalyst (like platinum, nano iron powders or palladium) for higher efficiency. Carbon paper separates them from the electrolyte. The electrolyte could be ceramic or a membrane.Oxygen ion exchange fuel cellsIn a solid oxide fuel cell design, the anode and cathode are separated by an electrolyte that is conductive to oxygen ions but non-conductive to electrons. The electrolyte is typically made from zirconia doped with yttria.On the cathode side, oxygen catalytically reacts with a supply of electrons to become oxygen ions, which diffuse through the electrolyte to the anode side. On the anode side, the oxygen ions react with hydrogen to form water and free electrons. A load connected externally between the anode and cathode completes the electrical circuit.Fuel cell design issuesCostsIn 2002, typical cells had a catalyst content of US$1000 per-kilowatt of electric power output. In 2008 UTC Power has 400kw Fuel cells for $1,000,000 per 400kW installed costs. The goal is to reduce the cost in order to compete with current market technologies including gasoline internal combustion engines. Many companies are working on techniques to reduce cost in a variety of ways including reducing the amount of platinum needed in each individual cell. Ballard Power Systems have experiments with a catalyst enhanced with carbon silk which allows a 30% reduction (1 mg/cm2 to 0.7 mg/cm2) in platinum usage without reduction in performance.The production costs of the PEM (proton exchange membrane). The Nafion membrane currently costs 400/m. In 2005 Ballard Power Systems announced that its fuel cells will use Solupor, a porous polyethylene film patented by DSM.Water and air management (in PEMFC). In this type of fuel cell, the membrane must be hydrated, requiring water to be evaporated at precisely the same rate that it is produced. If water is evaporated too quickly, the membrane dries, resistance across it increases, and eventually it will crack, creating a gas short circuit where hydrogen and oxygen combine directly, generating heat that will damage the fuel cell. If the water is evaporated too slowly, the electrodes will flood, preventing the reactants from reaching the catalyst and stopping the reaction. Methods to manage water in cells are being developed like electroosmotic pumps focusing on flow control. Just as in a combustion engine, a steady ratio between the reactant and oxygen is necessary to keep the fuel cell operating efficiently.Temperature managementThe same temperature must be maintained throughout the cell in order to prevent destruction of the cell through thermal loading. This is particularly challenging as the 2H2 + O2 =2H2O reaction is highly exothermic, so a large quantity of heat is generated within the fuel cell.Durability, service life, and special requirements for some type of cells Stationary fuel cell applications typically require more than 40,000 hours of reliable operation at a temperature of -35C to40C, while automotive fuel cells require a 5,000 hour lifespan (the equivalent of 150,000 miles) under extreme temperatures. Automotive engines must also be able to start reliably at -30 C and have a high power to volume ratio (typically 2.5 kW per liter).HistoryThe principle of the fuel cell was discovered by German scientist Christian Friedrich Schnbein in 1838 and published in one of the scientific magazines of the time. Based on this work, the first fuel cell was demonstrated by Welsh scientist Sir William Robert Grove in the February 1839 edition of the Philosophical Magazine and Journal of Science, and later sketched, in 1842, in the same journal. The fuel cell he made used similar materials to todays phosphoric-acid fuel cell.In 1955, W. Thomas Grubb, a chemist working for the General Electric Company (GE), further modified the original fuel cell design by using a sulphonated polystyrene ion-exchange membrane as the electrolyte. Three years later another GE chemist, Leonard Niedrach, devised a way of depositing platinum onto the membrane, which served as catalyst for the necessary hydrogen oxidation and oxygen reduction reactions. This became known as the“Grubb-Niedrach fuel cell”. GE went on to develop this technology with NASA and McDonnell Aircraft, leading to its use during Project Gemini. This was the first commercial use of a fuel cell. It wasnt until 1959 that British engineer Francis Thomas Bacon successfully developed a 5 kW stationary fuel cell. In 1959, a team led by Harry Ihrig built a 15 kW fuel cell tractor for Allis-Chalmers which was demonstrated across the US at state fairs. This system used potassium hydroxide as the electrolyte and compressed hydrogen and oxygen as the reactants. Later in 1959, Bacon and his colleagues demonstrated a practical five-kilowatt unit capable of powering a welding machine. In the 1960s, Pratt and Whitney licensed Bacons U.S. patents for use in the U.S. space program to supply electricity and drinking water (hydrogen and oxygen being readily available from the spacecraft tanks).United Technologies Corporations UTC Power subsidiary was the first company to manufacture and commercialize a large, stationary fuel cell system for use as a co-generation power plant in hospitals, universities and large office buildings. UTC Power continues to market this fuel cell as the PureCell 200, a 200 kW system (although soon to be replaced by a 400 kW version, expected for sale in late 2009). UTC Power continues to be the sole supplier of fuel cells to NASA for use in space vehicles, having supplied the Apollo missions, and currently the Space Shuttle program, and is developing fuel cells for automobiles, buses, and cell phone towers; the company has demonstrated the first fuel cell capable of starting under freezing conditions with its proton exchange membrane automotive fuel cell.Fuel cell efficiencyThe efficiency of a fuel cell is dependent on the amount of power drawn from it. Drawing more power means drawing more current, which increases the losses in the fuel cell. As a general rule, the more power (current) drawn, the lower the efficiency. Most losses manifest themselves as a voltage drop in the cell, so the efficiency of a cell is almost proportional to its voltage. For this reason, it is common to show graphs of voltage versus current (so-called polarization curves) for fuel cells. A typical cell running at 0.7 V has an efficiency of about 50%, meaning that 50% of the energy content of the hydrogen is converted into electrical energy; the remaining 50% will be converted into heat. (Depending on the fuel cell system design, some fuel might leave the system unreacted, constituting an additional loss.)For a hydrogen cell operating at standard conditions with no reactant leaks, the efficiency is equal to the cell voltage divided by 1.48 V, based on the enthalpy, or heating value, of the reaction. For the same cell, the second law efficiency is equal to cell voltage divided by 1.23 V. (This voltage varies with fuel used, and quality and temperature of the cell.) The difference between these numbers represents the difference between the reactions enthalpy and Gibbs free energy. This difference always appears as heat, along with any losses in electrical conversion efficiency.Fuel cells do not operate on a thermal cycle. As such, they are not constrained, as combustion engines are, in the same way by thermodynamic limits, such as Carnot cycle efficiency. At times this is misrepresented by saying that fuel cells are exempt from the laws of thermodynamics, because most people think of thermodynamics in terms of combustion processes (enthalpy of formation). The laws of thermodynamics also hold for chemical processes (Gibbs free energy) like fuel cells, but the maximum theoretical efficiency is higher (83% efficient at 298K) than the Otto cycle thermal efficiency (60% for compression ratio of 10 and specific heat ratio of 1.4). Comparing limits imposed by thermodynamics is not a good predictor of practically achievable efficiencies. Also, if propulsion is the goal, electrical output of the fuel cell has to still be converted into mechanical power with the corresponding inefficiency. In reference to the exemption claim, the correct claim is that the limitations imposed by the second law of thermodynamics on the operation of fuel cells are much less severe than the limitations imposed on conventional energy conversion systems. Consequently, they can have very high efficiencies in converting chemical energy to electrical energy, especially when they are operated at low power density, and using pure hydrogen and oxygen as reactants.In practice, for a fuel cell operating on air (rather than bottled oxygen), losses due to the air supply system must also be taken into account. This refers to the pressurization of the air and dehumidifying it. This reduces the efficiency significantly and brings it near to that of a compression ignition engine. Furthermore fuel cell efficiency decreases as load increases.The tank-to-wheel efficiency of a fuel cell vehicle is about 45% at low loads and shows average values of about 36% when a driving cycle like the NEDC (New European Driving Cycle) is used as test procedure. The comparable NEDC value for a Diesel vehicle is 22%. In 2008 Honda released a car with fuel stack claiming a 60% tank-to-wheel efficiency.Fuel cells cannot store energy like a battery, but in some applications, such as stand-alone power plants based on discontinuous sources such as solar or wind power, they are combined with electrolyzers and storage systems to form an energy storage system. The overall efficiency (electricity to hydrogen and back to electricity) of such plants (known as round-trip efficiency) is between 30 and 50%, depending on conditions. While a much cheaper lead-acid battery might return about 90%, the electrolyzer/fuel cell system can store indefinite quantities of hydrogen, and is therefore better suited for long-term storage.参考译文：燃料电池及其发展前景燃料电池是一种电化学转换装置。它产生的电流来自于燃料（阳极侧）和氧化剂（阴极侧）在电解液作用下的化学反应。反应物（燃料）源源不断地流入电池，而反应产品（也就是电能）则从电池中流出，同时电解液依然保留在电池内部。只要保持必要的燃料供给,燃料电池几乎可以持续不断地产生电能。燃料电池是一种特殊的电化学电池，因为它们的反应消耗来源是从外部获得，所以必须加以补充，这是一个开放的热力学系统。相比之下，电池储存的是化学电能，因此代表的是一个封闭的热力学系统。有许多种燃料和氧化剂的组合都是可行的。氢燃料电池使用氢作为燃料，而用氧气（通常来自于空气）作为氧化剂。其它燃料包括碳氢化合物和醇类。其它氧化剂包括氯和二氧化氯。燃料电池设计 燃料电池是通过催化作用进行工作的，催化剂通常包括铂族金属或合金。在催化作用下将反应燃料的组成部分电子和质子分离，并迫使电子沿回路移动，从而将其转化为电流。另一种催化过程需要将电子与质子和氧化剂相结合，形成废物产品（通常是简单的化合物，像水和二氧化碳）。 一个典型的燃料电池在额定负载下所产生的电压从0.6伏至0.7伏不等。电压会随电流的增大而减小，主要取决于以下几个因素： （催化剂）活性的损失 欧姆损失（由于电池元件的自身电阻以及接触电阻引起的电压降） 大量传输损失（催化剂在高负荷下反应后枯竭,造成电压迅速降低）为了提供所需的大量能源，燃料电池可以串联或者并联使用，串联可以产生较高的电压而并联可以获得较大的电流。这种设计通常被称为燃料电池堆。此外，还可以通过增加电池的表面积来获得更为强大的电流。质子交换膜燃料电池在氢氧质子交换膜燃料电池（PEMFC）的原型中，一个质子导电聚合物膜（电解质），将燃料电池的阳极和阴极分开在两边。这就是在20世纪70年代初期质子交换原理还没有被广泛认识之前，被人们称为的“固体聚合物电解质燃料电池”（SPEFC）。（请注意，“聚合物电解质膜”和“质子交换机制”） 在阳极侧，氢扩散到阳极,催化剂分裂成质子和电子。这些质子常常会与氧化剂反应使之成为通常被人们称作的简易化质子膜（MFPM）。质子是通过交换膜向阴极移动的，但电子则被迫沿着外部电路穿行（提供外电流），因为质子膜是绝缘的。在阴极催化剂的作用下，氧分子与（已穿过外部电路返回的）电子和质子发生化学反应形成水。在这个反应模式中唯一的废物产品，要么是液体（水）要么是蒸汽。 除了这种纯粹的氢型燃料电池外，还有以碳氢作为燃料的燃料电池，包括柴油，甲醇（分直接甲醇燃料电池和间接甲醇燃料电池）和化学氢化物燃料电池。这些类型燃料电池的废料产品是二氧化碳和水。 不同类型的燃料电池使用的不同的反应材料。在一个典型的膜电极装置中，电极的两个极板通常都是采用金属制造的，镍或碳纳米管，并涂有催化剂（如铂，纳米铁粉或钯），从而使其具有更高的效率。复写纸将它们与电解质分开，电解质可以是陶瓷材料或者交换膜。氧离子交换燃料电池在固体氧化物燃料电池的设计中，阳极和阴极是由能够传导氧离子但是不能传导电子的电解质分隔开来。电解质通常是由参杂氧化钇的氧化锆材料组成。在阴极一侧，氧气与电子通过催化反应成为氧离子，它通过电解液扩散到阳极侧。在阳极一侧，氧离子与氢反应形成水和自由电子。于是连接在阳极和阴极之间的外接负载形成了电流的完整通路。燃料电池设计问题燃料电池的费用2002年，典型的燃料电池催化剂包含在电力输出中的费用约为1000美元每千瓦。在2008年美国联合技术公司安装400千瓦燃料电池的费用是100万美元。我们的目标是降低发电成本，以便于同当前市场上的常规发电方式比如汽油内燃机等竞争。许多公司正致力于提高技术，试图通过各种方式减少成本，包括减少铂在每个电池中的使用量。巴拉德动力系统曾经采用增强型碳丝催化剂做过实验，实验表明在不影响电池性能的情况下可减少30（1毫克/ CM2降至0.7毫克/ CM2）的铂金使用量。 PEM（质子交换膜）的生产成本费用。目前的Nafion膜费用400 /平方米。在2005年巴拉德动力系统宣布，该公司的燃料电池将使用Solupor膜，一种由DSM研制并拥有专利权的多孔聚乙烯薄膜。质子交换膜燃料电池中水和空气的管理。在此类型的燃料电池中，膜必须含水，水的蒸发速度要与该膜生产过程中的蒸发速度严格一致。如果交换膜中水的蒸发过快，膜就会太干燥，阻值增大，并最终裂缝，导致氢气和氧气直接结合形成天然气短路的现象，这样会产生大量的热量损坏燃料电池。如果水的蒸发速度太慢，电极将被淹没，从而阻止了反应物与催化剂的结合，化学反应停止。用电水泵流量控制的方法来管理燃料电池交换膜中的水是侧重点，正如在内燃机中保持反应物和氧气稳定的比例是非常重要的一样，从而保持燃料电池有效地运作。温度管理必须保持整个电池维持相同的温度，以防止热负荷对电池的破坏，这是特别具有挑战性的。2H2 + O2 =2H2O的反应会在燃料电池中产生大量的热，损坏燃料电池。特种类型的电池要求耐用性和使用寿命固定式燃料电池应该能够在35至40的温度下稳定运行超过4万小时，而汽车的燃料电池需要在极端温度下有5千小时的寿命（相当于行驶15万英里）。汽车发动机也必须能够可靠地运行在30温度下，并且具有较高的升功率（通常为2.5kw/升）。历史燃料电池的原理最初是由德国科学家Christian Friedrich Schnbein于1838年发表在当时的一本科学杂志上。在此基础上，由威尔士科学家威廉罗伯特格罗夫在1839年2月版的哲学杂志和科学期刊上首次论证了燃料电池，并于1842年在同一期刊上提出了设计原理图。他设计的燃料电池使用的材料类似于今天的磷酸燃料电池。1955年，在通用电气公司（GE）工作的化学工程师托马斯格拉布，进一步修改了原来的燃料电池设计方案，采用磺化聚苯乙烯离子交换膜作为电解质。三年后，另一位通用电气的化学工程师莱昂纳多涅德拉茨发明了一种方法，在膜上沉积铂作为氢与氧的氧化还原反应所必需的催化剂，这被称为格拉布-涅德拉茨燃料电池。通用电气公司继续与美国航空航天局和