Chemistry: electrical current producing apparatus – product – and – Having magnetic field feature
Reexamination Certificate
2001-10-09
2004-05-18
Bell, Bruce F. (Department: 1746)
Chemistry: electrical current producing apparatus, product, and
Having magnetic field feature
C429S010000, C429S010000, C429S010000, C429S006000, C429S006000, C429S006000
Reexamination Certificate
active
06737183
ABSTRACT:
DESCRIPTION OF THE INVENTION
The invention relates to a fuel cell, and more precisely to a fuel cell using a polymeric membrane as the electrolyte.
Fuel cells are electrochemical generators of electric energy in the form of direct current; that is, they convert the free energy of reaction of a fuel (for example a gaseous mixture containing either hydrogen, or light alcohols such as methanol or ethanol) with an oxidant (for example air or oxygen) without its complete degradation to thermal energy, and therefore without being subject to the limitation of the Carnot cycle. In order to achieve the desired conversion of chemical to electrical energy, the fuel is oxidised at the cell anode, with the concurrent release of electrons and H
+
ions, while the oxidant is reduced at the cathode, where H
+
ions are consumed; the two poles of the generator must be separated by a suitable electrolyte, allowing a continuous flow of H
+
ions from the anode to the cathode, at the same time hindering the transfer of electrons from one pole to the other, thereby maximising the electrical potential difference thereof. This potential difference represents in fact the driving force of the process itself. The fuel cells are considered as an excellent alternative to the traditional systems of electric generation; especially in view of the extremely favourable environmental impact (absence of polluting emissions and noise, formation of water as the only by-product), they are used both in the field of stationary power generation of various sizes (electrical power stations, back-up power generators, etc.) as well as in the field of mobile applications (electric vehicle applications, generation of automotive energy or auxiliary energy for space, submarine and naval applications).
The polymeric membrane fuel cells offer, compared with other fuel cells, further advantages, due to their fast start-up and quick achievement of the optimum operation conditions, the high power density, the intrinsic reliability connected both to the lack of moving parts and to the absence of corrosion phenomena and severe thermal cycles; in fact, among all the fuel cells of the prior art, the polymer electrolyte fuel cells exhibit the overall lowest operating temperature (usually, 70-100° C.).
The polymeric electrolyte used for this purpose is an ion-exchange membrane, and more precisely a cation-exchange membrane, that is a chemically inert polymeric backbone, partially modified with functional groups capable of undergoing acid-base hydrolysis leading to a separation of electric charge; such hydrolysis consists more precisely in the release of positive ions (cations) and in the formation of fixed negative charges on the polymeric backbone. Porous electrodes are applied on the membrane surface, which allow the reactants to flow therethrough up to the membrane interface. A catalyst is applied on such interface on the electrode and/or on the membrane side, such as for example platinum black, which increases the relevant half-reaction rate of fuel oxidation or oxidant reduction. This arrangement provides also for the continuous flow of cations when a potential gradient is established between the two faces of the membrane and the external electric circuit is concurrently closed; being the H
+
ion the transported cation in this case, as previously mentioned, the potential difference generated upon feeding a species with a lower electrochemical potential at the anode, and a species with a higher electrochemical potential at the cathode, causes protonic conduction across the membrane and electron flow (i.e. electric current) across the external circuit to be established as soon as the latter is closed.
Protonic conduction is an essential condition for the operation of a fuel cell and is one of the decisive parameters to assess its efficiency. An insufficient protonic conduction causes a remarkable drop in the potential difference at the poles of the cell (cell voltage drop) once the electric circuit is closed on the external resistive load which exploits the produced electric output. This, in turn, causes an increased degradation of the energy of reaction to thermal energy and the consequent decrease of the fuel conversion efficiency.
Several cation-exchange membranes showing optimum protonic conduction characteristics are available on the market and are widely used in industrial fuel cells, such as for example those commercialised under the trademarks Nafion® by Dupont de Nemours, U.S.A., Gore Select® by Gore, U.S.A., Aciplex® by Asahi Chemicals, Japan. All these membranes are negatively affected by an intrinsic process limitation associated with their operation mechanism: a being the separation of electric charge which enables the protonic conduction set by a hydrolysis mechanism, such membranes develop their conductivity only in the presence of liquid water. Although the formation of water is an intrinsic consequence of the operation of a fuel cell, its extent results almost always insufficient to maintain the required hydration state of the membrane, especially when operating at a sufficiently high current density.
Operation at high current density involves a decrease in the investment cost for a given power output, but also a decrease in the energy efficiency as well as the generation of a higher amount of heat. The large amount of heat generated in a fuel cell operating a current density of practical use (for example between 150 and 1500 mA/cm
2
) must be efficiently removed to permit the thermal regulation of the system, not only in view of the limited thermal stability of the ion-exchange membrane, usually unfit for operation above 100° C., but also to limit as much as possible the evaporation of the product water and its consequent removal by the discharge of the inerts and unconverted reactants from the cell. Moreover, as the voltage of a single fuel cell is too small to allow a practical exploitation, said cells are usually connected in electrical series by means of bipolar connections and assembled in a filter-press arrangement feeding the reactants in parallel, as illustrated in U.S. Pat. No. 3,012,086. In such a fuel cell battery arrangement, usually called a stack, the problem of heat removal is amplified with respect to the case of a single cell, wherein it is possible to take advantage of the thermal convection through the external walls.
The above described drying-out of the ion-exchange membrane by removal of an excess amount of water with respect to the amount produced by the reaction is even more remarkable when the fuel cells are fed with gaseous reactants at low pressure. At an early stage of .development of this technology, the polymeric membrane fuel cells were operated under relative pressures of a few bars (from 2 to 10, and more commonly from 3 to 5), especially to increase the kinetics of the two half-reactions of fuel oxidation and oxidant reduction. With the later evolution of the technique, the improvements in the catalyst compositions and in the electrode manufacturing induced the fuel cell producers to design stacks capable of operating efficiently at lower pressures, aiming at the operation under hydrogen and air at nearly atmospheric pressure while maintaining a sufficient efficiency and current density as one of the most desirable goals, due to the relevant resulting impact in terms of overall efficiency of the electric generation system. While hydrogen, either pure or in a mixture, is often available at the pressure of a few bars, the compression of atmospheric air, which contains less than 20% of oxygen used as the oxidant in the cell, and more than 80% of inert, involves an extremely severe energy consumption. While the current level of development of the gas diffusion electrodes for fuel cells and of the catalysts therefor makes them already suitable for operating with reactants at substantially atmospheric pressure (aside from the slight overpressure needed to overcome the internal pressure drop of the device, in the range of a few tens of millibar), the fast drying-out
Brambilla Massimo
Fleba Gian Piero
Maggiore Antonio
Mazzucchelli Gabriele
Bell Bruce F.
Muserlian Lucas and Mercanti
Nuvera Fuel Cells Europe S.r.l.
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