A renaissance for alkaline fuel cells - Fuel Cell Markets

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INNOVATION A brief history of AFC applications An ELOFLUX fuel-cell cell stack from German developer Gaskatel. The ELOFLUX principle is based on flexible electrodes and a liquid electrolyte. The electrodes are extremely porous, with both hydrophobic and hydrophilic pores allowing the transport of gases and electrolyte, and with a thin separator between (approximately 0.3 mm). It’s within the US space programme that AFCs have enjoyed their greatest successes. In the 1960s and early 1970s, AFCs were used on nine Apollo Moon flights and three Sky Labs – a total of 54 missions. They have also been employed on the Space Shuttle since its first flight in 1975 and are still in use today. In terms of more down-to-earth applications, a number of isolated and diverse projects have relied on the AFC to deliver power from hydrogen. In the mid-1960s, for example, the US tractor maker Allis Chalmers pioneered the first “civilian” usage: a 15 kW power module for one of its vehicles. Also in the US, Union Carbide developed a number of initial uses, including a General Motors “Electrovan” powered by a 125 kW AFC in 1967. Three years later, one of Carbide’s inventors, Karl Kordesch, completed an Austin A-40 hybrid vehicle that relied on a 96 V, air-breathing AFC in combination with a leadacid battery. Kordesch operated the Austin A-40 for three years in city traffic, the fuel cell being shut-down and restarted as required by normal operation. In terms of European industrial activity, Elenco of Belgium completed a number of AFC systems during the 1970s and 80s, including a 52 kW system for the Belgian Geological Service. In 1994, the same group completed work on a 200 kW AFC system for a hybrid bus. The following year Elenco was closed, continuing operations as Zevco and ZeTek (both now defunct as well). These companies demonstrated an AFC-powered London taxi in 1999 and contributed to the completion of the Hydra AFC-powered boat, which was launched in Germany in the summer of 2000. Elsewhere in Europe, Siemens of Germany dedicated considerable effort to developing a fuel-cell system for submarines, work that was completed in the early 1990s. Back in the here and now there are several small companies and research institutes currently active in AFC technology. They include: electrodes and a liquid electrolyte. The electrodes are extremely porous, with both hydrophobic and hydrophilic pores allowing the transport of gases and electrolyte, and with a thin separator between (approximately 0.3 mm). In contrast with the other cell concepts, the electrolyte is transported through the separator and the electrodes in the direction perpendicular to the electrode plane. This AFC demonstrator from Hydrocell is equipped with an internal metal-hydride storage tank. The Finnish company is also developing portable AFC units to supply power to summer residences and lowenergy buildings. Because of the internal hydrogen storage, the device is sometimes called a “fuel-cell battery”, as it is as easy to use as a lead acid battery. ● Astris Energi (Mississauga, Ontario) develops systems (ranging in output power from a few hundred watts to several kilowatts) for light utility vehicles, portable applications and stationary systems targeting backup power and cogeneration. One such generator, the Model E8, is a 2.4 kW system that uses two 1.2 kW stacks; system efficiency is >50% and rated life is 2000 h. Astris has established an R&D laboratory and manufacturing facility in the Czech Republic. ● Deutsches Zentrum für Luft- und Raumfahrt (DLR) is a leading German aerospace organization. Its Institute of Technical Thermodynamics in Stuttgart has a long-running R&D programme on low-temperature fuel cells, including PEMFCs and AFCs. There are currently more than a dozen researchers working on various aspects of AFC technology, including catalysis, gas-diffusion electrodes, electrode production methodology, development of stacks, modelling and testing. ● Gaskatel (Kassel, Germany) works on cells, stacks and systems based on the ELOFLUX principle. The ELOFLUX cells are based on inexpensive materials – e.g. carbon and nickel – and deliver a power output up to 0.5 kW/l. ● Ovonic Fuel Cell Company (Rochester Hills, Michigan, US) is developing a metal-hydride fuel cell that uses alkaline electrolyte. Published benefits include instant start, intrinsic operation down to –20 ºC and operation without hydrogen fuel for several minutes. ● Oy Hydrocell (Finland) manufactures portable AFC systems for the supply of power to summer residences and low-energy buildings. The fuel cells are normally equipped with internal metal-hydride hydrogen storage (up to 40 dm 3 of hydrogen). There are two storage units: HC-MH200 is the size of a small cola can and contains 430 Wh (12 V DC, 36 Ah) of energy and the HC-MH1200 is the size of a small fire extinguisher and contains 2600 Wh (12 V DC, 220 Ah) of energy. ● ZAO Independent Power Technologies (Moscow, Russia) is a designer and manufacturer of AFC-based electrochemical generators. ● Falling-film cell: This format was developed by Hoechst, Germany, originally to increase the efficiency of chlor-alkali electrolysis. The electrolyte flows between the electrodes from top to bottom. Owing to the unhindered gravitational movement of the electrolyte in this way, the typical height-dependent hydrostatic-pressure distribution does not develop – so hydrostatic pressure in the free-falling column of liquid is the R EPRINTED WITH PERMISSION. COPYRIGHT I NSTITUTE OF P HYSICS AND IOP PUBLISHING 2006
1. Electrode aging potential vs. Hg/HgO (mV) at 100 mA/cm 2 100 50 0 –50 –100 –150 sample 76 (CO 2 contaminated gas) sample 75 sample 74 (CO 2 contaminated gas) sample 73 –200 0 1000 2000 3000 4000 time of operation of an Ag–GD electrode at 150 mA/cm 2 (h) Aging of a silver AFC cathode with either CO 2-containing oxygen (5%) or pure oxygen. The degradation in performance in both modes is comparable, which means that CO 2 neither enhances the degradation process nor does it induce a new detrimental effect. Studies on AFC anodes using nitrogen adsorption measurements, electron microscopy and X-ray photoelectron spectroscopy also show no deleterious effects. same at the inlet as at the outlet, and any place in between. The velocity of the “free fall” is limited only by the turbulent friction between the falling film and the electrode surface. Highly porous electrodes are used to separate the electrolyte and the gas phases, without the need for a dedicated separator. Cells in excess of 1 m 2 have been built using this format, with bipolar cell interconnections. Common to all AFC architectures are the selection of catalyst material(s) and the electrode production process. For example, Raney-nickel (anode) and a silver catalyst called SILFON (cathode) have been used for more than 20 years, though there are plenty of variations on this theme. One method of preparation of gas-diffusion electrodes is a rolling process in which catalysts and polytetrafluoroethylene (PTFE) undergo an initial reactive mixing process, typically using a knife-mill. This approach yields a bifunctional material system with hydrophilic and hydrophobic pores. The mixed material is subsequently rolled onto a metallic mesh that supports the electrodes, which are then assembled into cells. Debunking the myths There are all sorts of myths and misconceptions associated with AFCs. It is said that AFCs are expensive and useful only for applications in space; that they require very pure and costly gas feeds; that they cannot work using air; that they are poisoned by carbon dioxide; that the liquid electrolyte is a serious drawback; that carbon monoxide is a problem. So what is true and what is false? One thing at least is clear: each type of fuel-cell R EPRINTED WITH PERMISSION. COPYRIGHT I NSTITUTE OF P HYSICS AND IOP PUBLISHING 2006 INNOVATION chemistry has its individual advantages and disadvantages, but the disadvantages stated here for AFCs are grossly exaggerated or, often, simply incorrect. Let’s start with cost. AFCs yield savings because their catalysts need not be based on expensive noble metals – a result of the lower corrosivity of the basic environment relative to the acidic one. On top of this, the liquid electrolyte, usually an aqueous solution of potassium hydroxide, is cheap – just a few euros per kilogram. Taken together, the catalyst and electrolyte represent a considerable cost advantage in comparison with proton exchange membrane fuel cells (PEMFCs). However, AFCs are not just about cheaper components. One of the big pluses in terms of system design is the absence of process-gas humidification – a significant design headache in current-generation PEMFCs. Admittedly, some will see the liquid electrolyte as a disadvantage because AFCs need a circuit for it. In practice, though, a similar circuit is also needed in PEMFC systems, in this case for heat management of the stack. Effectively, the circulating electrolyte is a “self-healing” layer of liquid in which any defect (in the form of a bubble, for example) will not remain stationary. Consequently, failure modes (like hot-spots) common to PEMFCs do not exist. Clearly, the AFC electrolyte circuit poses some unique design/engineering challenges owing to its corrosive nature, though the dual purpose of the circuit (heat and water management) more than offsets those demands. Additional benefits of this architecture are the elimination of the water-management issues common to PEMFCs, which in turn ensures the AFC’s inherent freeze-start capability. The bogeyman: carbon dioxide There are some 350 ppm of CO 2 in the air that we breathe. What’s more, it is commonly accepted that CO 2 intolerance is the most pronounced disadvantage of air-breathing AFCs. But is this really the “show-stopper” that some would have us believe? Karl Kordesch, a pioneering battery and fuel-cell inventor at Union Carbide, US, noted as early as 1970 that the issue of CO 2 intolerance in AFCs appeared to be overstated. He found that the rate of absorption of CO 2 in the electrolyte was negligible at low concentrations and recommended using a simple dry scrubber with soda lime to counter any take-up. The same observations were later published by teams at Elenco in Belgium, Astris Energi in Canada and at the Royal Institute of Technology in Stockholm, Sweden. The CO 2 myth persists, however. To investigate the mechanisms of this CO 2 “poisoning”, a team of scientists at the Deutsches Zentrum für Luft und Raumfahrt (DLR) in Stuttgart, Germany, has carried out an in-depth study of the phenomenon 6 . More specifically, AFC gas-diffusion electrodes were operated in a half-cell configuration for up to several thousand hours with a constant load of 150 mA/cm 2 . The reactants were contaminated with 5% CO 2 – more than a hundred times the usual aerial concentration – and the experiments were performed with both anode and cathode. In parallel, the researchers carried out a series of control measurements by operating electrodes with pure reactants. Using techniques such as scanning electron microscopy and
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