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nanost-admin 2007-12-30 23:07

纳米技术对便携式实用燃料电池技术的影响

:nst [b][size=4]Nano enables practical, portable fuel cells,[/size][/b]$vuhf ]c

[color=Blue]A variety of approaches to nano- and micro-engineering have begun to yield portable fuel cells-with dramatic cost and size advantages-for niche markets[/color]

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A host of start-ups say that radical new approaches to the basic nanoscale process of running fuel through membrane-to separate out hydrogen electrons and generate electricity-look likely to finally, and significantly, reduce size and costs over the next several years.

Some of the initial commercial portable fuel cells improve conventional methanol polymer electrolyte membrane (PEM) fuel-cell technology with nanostructured catalytic layers and MEMS-based micro reformers. Other prototypes now being tested by the military and big consumer electronics companies use designed molecules that self-assemble into membranes with target properties and engineered membranes to passively control fluid flows, or have even replaced the membranes altogether with laminar flow boundaries in microchannels.D8x,j.px

[b]Initial products take various approaches[/b]
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Germany’s Smart Fuel Cell AG (SFC-[url=www.smartfuelcell.de]www.smartfuelcell.de[/url]) is shipping portable direct methanol fuel cells (under the name EFOY, or, Energy For You) in volume, reporting that more than 80% of its approximately $10 million in revenues in the first half of 2007 came from actually selling product, primarily rather hefty 25W to 65W fuel cells for off-grid power in recreational vehicles. Key to reducing size and costs to viable levels, notes CTO Jens Müller, was increasing the reactive surface area with a nanoparticle catalyst and a nanostructured three-dimensional membrane electrode assembly (MEA). “By careful design of the catalyst particles and electrode layers, MEAs have achieved three times the performance at less than one-fourth of the catalyst loadings compared to previous generations,” he says. That, plus optimizing the fuel, air, and water mixtures, enabled the ~15lb. units to deliver 600 to 1,600 watt hours/day, extending a typical ~eight-hour operating time of a lead acid battery to ~eight days, and attracting users willing to invest $3000 to $4000 for extended off-grid operation.
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[color=DimGray]PolyFuel manufactures its hydrocarbon membrane-which it says overcomes traditional fuel-cell limitations-by the roll.[/color]k1y9Q(Q$RY"q!Zl

Actual operation, however, is more economical. The company advertises that a 10 liter refill cartridge-essentially a plastic jug of pure methanol with safety features-supplies a motor home with independent power for several weeks for about $30. Several major European RV makers supply the fuel cells with their vehicles, and refill cartridges are available at about 600 outlets.z7QG8B4M,pF!z

Further development work for the military now has the fuel stack down to about the size of a book of matches and weight of the system down to 2.9 lbs., says company CEO Peter Podesser. He notes SFC has also been working on joint development with Korea’s LG Group on a universal power source for portable electronic gear.RT"T_4AU(qWxB

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[color=DimGray]Unit shipments of micro fuel cells providing either primary or auxiliary power to portable consumer devices such as notebook computers, mobile phones, portable audio/video, and digital cameras. Source: Frost & Sullivan[/color]
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Now starting production of portable reformed methanol fuel cells for the military and other off-grid users is the California-based UltraCell Corp. ([url=www.ultracellpower.com]www.ultracellpower.com[/url]), which opened a $74 million manufacturing plant in Ohio in September 2007. The company says the plant has started producing some components and should be making whole systems by year-end. UltraCell’s book-size 2.6 lb. 25W fuel cells reportedly deliver 600Wh/day and have passed standard military safety and reliability tests for standing up through heat, dust, shock, ice, and rain. Units have been delivered to soldiers for field training and testing.
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Key to the small size is a MEMS-based reformer on a chip, which first converts methanol to hydrogen to allow use of more-efficient hydrogen cell technology. Reactive heating elements and microchannels filled with catalyst particles are patterned on silicon. Methanol and water flowing through the channels are heated to 250°C to 300°C to react to produce hydrogen, and the specialty membrane tolerates impurities so the hydrogen doesn’t have to be cleaned up. Initial production is only in the hundreds and still using some handmade customized components. “But there’s nothing fundamentally costly about the technology,” says applications marketing manager Ted Prescop. He notes that costs could be reduced significantly by volume manufacture of the custom basic components like pumps and circuit boards. The company is reportedly also demonstrating to Motorola a recharging system for emergency-responder radios.7|zg`?4GZc)k(E
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[align=center][color=DimGray]PolyFuel says its small fuel-cell stack produces 56W peak power.[/color]-qy&]DD
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Also shipping small quantities of a lower power fuel cell is Medis Technologies Ltd. of New York ([url=www.medistechnologies.com]www.medistechnologies.com[/url]), which has distributed several thousand promotional 1W cigarette-pack-size units for recharging handheld electronics, based on a different technology. The 20Wh units are also available for purchase on MyTreo.net, where they’re listed among the site’s best-selling power products.

The 6.5oz. disposable unit, which retails for $20 to $30, is good for about 20 to 30 hours of recharging small devices over a three-month span after activation. Medis marketing VP Michelle Rush says the company is now producing volumes only in the thousands from its initial pilot line in Israel, but plans to start ramping its higher-volume automated line in Ireland by year-end, targeting the field service business market.

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[align=center][color=DimGray]UltraCell’s book-size, 2.6lb. 25W fuel cells promise 600Wh/day and have passed military safety and reliability tests.[/color]
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The company’s idiosyncratic approach uses sodium borohydride and alcohol for fuel, and novel nanoengineered catalysts. “There are no noble metals on the cathode, and almost none on the anode,” adds Rush. “It’s very simple: We’re able to sell it for $9 wholesale.” The initial model replaces the usual polymer membrane with an alkaline (KOH) liquid electrolyte and manages the flow of liquids through the cell passively, without pumps. Water produced and reused within the cell eventually builds up enough to dilute the fuel, however, and stop the reaction. A new version converts the fuel to a solid tablet and the electrolyte to a gel to reduce size and allow easy refilling.
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[b]Next generation looks at methanol[/b]'|OMj/J9mA;Kh Uu"[
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Most companies’ focus, however, is on methanol, though with new kinds of membranes and new approaches to manufacturing. The big Asian consumer electronics players are looking at using direct methanol for their portable fuel cells now targeted for commercial volumes in 2010-2011, says Sara Bradford, director of the energy and power system group at the market research firm Frost & Sullivan. “So many different fuels have really hampered commercialization,” she says, “but now direct methanol is becoming more of a clear leader.”

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[align=center][color=DimGray]MTI MicroFuel Cells’ Mobion chip company targets rechargers for cell phones and handhelds.[/color])[W|-q)k?Y1F.O
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Giving direct methanol a boost are new nano-engineered membranes specifically optimized for liquid methanol, to replace the traditional fluorocarbon polymers introduced in the 1960s for hydrogen gas. PolyFuel, of California ([url=www.polyfuel.com]www.polyfuel.com[/url]), says its hydrocarbon membrane prevents the common problem of excess methanol flowing through the PEM without giving up its hydrogen electrons to generate electricity, then reacting instead with oxygen from air on the other side to produce excess water that must be removed so it doesn’t drown out the cell. CEO Jim Balcom explains that his company designed molecules that self-assemble into a film with smaller nanoscale channels, which conduct across the hydrogen protons but restrict the flow of methanol. He notes that simply replacing the membrane cuts the methanol crossover, but re-engineering the fuel-cell system around the new membrane means more savings by reducing the water management systems. PolyFuel has demonstrated the technology in a fuel-cell stack approximately the size of a pack of cards (111cc) it says produces 56W peak power.

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[align=center][color=DimGray]Medis Technologies’ 6.5oz. disposable unit is good for about 20 to 30 hours of recharging small devices over a three-month period.[/color]
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PolyFuel claims 20 customers to date, including many of the large Asian consumer electronics suppliers and 10 of the 11 companies planning test marketing programs next year. The company has been working over the last several months with U.K.-based Johnson Matthey plc ([url=www.matthey.com]www.matthey.com[/url]), a major fuel-cell component supplier, to introduce the membrane into Johnson Matthey’s manufacturing process-and recently passed vendor qualification for commercial production. “No one has a system yet that can compete with a lithium ion battery,” says Balcom. “But based on our interactions with our customers, I think they will start test marketing products this year and next, for commercial sales starting thereafter.”
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New York’s MTI MicroFuel Cells Inc. ([url=www.mtimicrofuelcells.com]www.mtimicrofuelcells.com[/url]) recently delivered the second-generation prototype of its direct methanol fuel cell to Samsung for testing and reports it’s now in active discussions about moving forward with development of the injection molded passive unit. The company targets rechargers for cell phones and handhelds with 1W to 3W systems, with the current 200cc prototype a little bigger than a pack of cards, delivering 30Wh of power.gg&v8g;O)We6`

MTI argues its design brings down cost by eliminating the pumps and controls of the typical water-recirculation system to manage the flow passively, using a sandwich of nano-engineered hydrophilic and hydrophobic membrane layers that pull in methanol and push back water, essentially running the system by diffusion and differential pressures. “We’re only talking about drops of methanol, so there’s very little water,” notes George Relan, VP of corporate development. MTI also replaces the usual plates and screws that hold the cell stack together with a single injected molded plastic framework, designed for easy, low-cost manufacture. Relan says the company is working on manufacturing in 2008 and aims for commercial products in 2009. “This is the first time we’ve put out a date for a commercial product,” he notes. “And we’re fairly confident that we’ll meet it.”:\D:F4Uy!Zx1?

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[align=center][color=DimGray]Smart Fuel Cell AG’s EFOY 1600 fuel cell with methane cartridge installed in a vehicle.[/color]
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Another direct methanol alternative skips the polymer electrolyte membrane and instead relies on the properties of laminar flow, circulating the methanol and electrolyte in micro channels etched in flexible printed circuit substrate. INI Power Systems, of North Carolina ([url=www.inipower.com]www.inipower.com[/url]), says it delivered a beta 15W system to a military customer in August and signed a joint development agreement with a major Asian battery and laptop OEM to integrate its stacks into a consumer platform. It is also working on a telecommunications power supply with partner Advanced Power Systems, targeted for the first half of 2008. Anthony Atti, VP of business development, reports the military test unit is running more than nine hours on 200cc of neat methanol, and a 72-hour military mission would require total system and fuel weight of less than 9lb.

Fluids in micro channels exhibit unique laminar flow properties that eliminate turbulent mixing, so the methanol fuel and dilute sulfuric acid electrolyte remain segregated as they flow together through channels sandwiched between catalyst-coated electrodes. Protons diffuse from the anode through the liquid to the cathode. “Methanol crossover and cathode flooding are almost entirely mitigated,” says Atti. Manufacture looks to be inexpensive as well, as the micro channels are etched on flexible printed circuit board substrate using standard industry processes.
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[color=DimGray]Another approach is to replace the membrane with porous silicon. Neah Power Systems (Bothell, Wash.-[/color][url=www.neahpower.com]www.neahpower.com[/url][color=DimGray]) was just finishing up its first working prototype in late September, using standard semiconductor industry technology to make an etched micro porous silicon 3D reaction area.[/color]R3B\:R(D4RYOD

Progress continues as well with hydrogen fuel cells for higher-power mobile applications. Mobile fuel-cell pioneer Jadoo Power, of California ([url=www.jadoopower.com]www.jadoopower.com[/url]), has been selling higher-power cells for several years in commercial applications such as television cameras, emergency responder communications equipment, and heavy-duty military power sources, where the 100W 5lb. cells fueled by 2lb. canisters of metal hydride powder outperform lead acid batteries.
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But the consumer market will be a little harder, says CEO Lee Arikara. “People think it’s like the [current] computer industry where a new improved chip can be dropped into an existing PC to improve performance,” he says. “But the fuel-cell industry is more like the early PC industry when there were no standards. We’re still in the period of a lot of chaos before we settle in on which standard to adopt. But at the end of the day if we all keep insisting on our own standard, we won’t develop the market.” ]_g(|-^!Z|R,S

[b]How fuel cells work[/b]K6W`j3V%aeA
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A fuel cell will produce energy as long as it has a supply of fuel. It is made up of two electrodes (anode and cathode) surrounding an electrolyte. Hydrogen, or a fuel like methanol containing hydrogen, is fed into the anode, where the hydrogen atoms, encouraged by a catalyst, split into protons and electrons. The protons pass through a selective electrolyte membrane, while the electrons are shunted off to another path, creating a usable electric current. The protons and electrons rejoin at the cathode, where the hydrogen reacts with oxygen from the air to form water.\Ld9U*RKm'j
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Using pure hydrogen gas for fuel directly allows the simplest and most efficient fuel cell, but transporting and storing the hydrogen can be troublesome. Alternatively, the fuel cell can include a reformer to extract the hydrogen from most any hydrocarbon fuel, though that typically means a bulky system with high heat and considerable plumbing. Or a liquid fuel like methanol can be fed directly into the cell, where it produces hydrogen by chemical reaction, typically with water, requiring the addition of a circulation system for managing the flow of liquids.!np g4o5`"pfQmz

Adapted from “What is a fuel cell?” at Fuel Cells 2000 ([url=www.fuelcells.org]www.fuelcells.org[/url]).
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Small Times November, 2007
Author(s) : Paula Doe

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