Fuel cell

Details Fuel cell Fuel cell

2000-04-26

2002-04-30

Kalafut, Stephen (Department: 1745)

Chemistry: electrical current producing apparatus, product, and

Having magnetic field feature

C429S010000, C429S006000, C429S010000

Reexamination Certificate

active

06379828

ABSTRACT:

The present invention is concerned with a fuel cell and in particular with a fuel cell which provides increased operating efficiency by maximising the available interfacial area between high and low density fluid phases or between gas-liquid-solid phases, across which phases heat and mass transfer and chemical reactions may take place. The present invention also provides for a novel method of improving the efficiency and performance of fuel cells by subjecting them to forced rotation.
Fuel cells are devices for utilizing the electrochemical conversion of the free-energy changes of a chemical reaction directly into electrical energy. By making use of gaseous or solid reactants (e.g. hydrogen, oxygen, or metallic powders) the anodic and cathodic reactants can be fed into their respective chambers where the electrochemical energy conversion proceeds. An electrolyte layer (often a liquid) is provided between the two electrodes of an electrochemical cell. At the anode, the half-cell reaction involving the anodic reagent yields electrons which are transported through an external circuit to the cathode where they are taken up in the half-cell reaction involving the cathodic reactant, usually oxygen. The circuit is completed by the transport of ions from one electrode to the other through the electrolyte, Current passing through the external electrical circuit provides electrical power and allows mechanical work to be done via, for example, an electric motor.
Unlike batteries which store electrical energy, fuel cells are energy producers which convert the energy of chemical reactions directly into electricity. They do so in an environmentally clean way, with no harmful pollutants such as those which arise from the normal burning of fuels in conventional combustion processes. Because fuel cells are not limited to the thermodynamic (Carnot) efficiencies of internal combustion engines (typically 40-50%), they offer much greater prospects for achieving high efficiencies (70-100%) and energy conversion rates. In order to attain these high efficiencies, however, new designs of compact fuel cells are required which can extract the electrochemical energy in a more effective manner. Achievable power output and performance is limited by the slow diffusion of ions and electrons at the reactant-electrolyte-electrode interface, especially in the case where the reactant is a gas.
Fuel cells are often classified according to their basic system configuration. The most common classifications include: phosphoric acid fuel cells (PAFCs); molten carbonate fuel cells (MCFCs); solid oxide fuel cells (SOFCs); proton exchange membrane fuel cells (PEMFCs); alkaline fuel cells (AFCs); and direct methanol fuel cells (DMFCs). In another classification, fuel cell types are grouped according to the fuel and oxidant consumed, e.g. hydrogen-oxygen (or air) fuel cells; organic compounds-oxygen (or air) fuel cells; carbon or carbon monoxide-oxygen (or air) fuel cells; nitrogenous compounds-oxygen (or air) fuel cells; and metal-oxygen (or air) fuel cells. Conventional fuel cells are typically composite quasi-static structures incorporating numerous individual electrochemical cells stacked in series and parallel to generate the required output voltage and current density. The present invention is particularly suited to the metal-oxygen type of fuel cell, and also the hydrogen-oxygen type of fuel cell, but is not exclusively limited to them.
One objective of the present invention is therefore to provide a fuel cell with increased working efficiency which alleviates to some extent problems associated with previously used fuel cells.
Accordingly, the present invention provides for a fuel cell comprising:
a chamber suitable for holding an electrolyte therein;
a mechanism which enables rotation of the electrolyte about an axis of rotation of the chamber;
one or more structures which define one or more inlets for introducing an oxidant and/or a fuel into the chamber, which one or more inlets are spaced from the axis of rotation of the chamber;
at least one electrode contactable with the electrolyte and the oxidant; and
at least one electrode contactable with the electrolyte and the fuel;
Fuel cells currently known in the art frequently employ a liquid electrolyte and a gaseous oxidant (e.g. air or oxygen) and/or fuel (e.g. hydrogen). The circulation of the gaseous oxidant and/or fuel through the electrolyte relies upon buoyancy-driven natural convection processes. In all buoyancy-driven natural convection processes on earth, the driving forces arise through the interaction of matter with the earth's gravitational field. The ‘rise velocity’, i.e. the natural velocity attained by the lighter phase, such as a bubble of gas, with respect to the heavier phase, usually a liquid, is therefore governed by the value of the local gravitational acceleration, which on earth is roughly constant at 9.81 m/s
2
. This rise velocity can be increased substantially by intensifying the local acceleration field using rotating frames such as centrifuges or other spinning devices.
In the fuel cell of the present invention, centrifugal forces acting on the electrolyte phase, together with inverse-centrifugal (centripetal) forces acting on the oxidant and/or fuel phases, act jointly to increase the overall circulation flow rate. When the induced inertial acceleration exceeds that of the normal local gravitational field, this externally-forced flow process may promote an increase in volumetric throughput of chemical substances in a reactor and hence improve the overall rate of chemical reaction. In ordinary ‘static’ fuel cells, the overall rate of chemical reaction and therefore the overall power density achievable from the device may be limited by the local gravitational field. In the fuel cell of the present invention the improved overall rate of chemical reaction will increase the power density achievable from the device.
Advantageously, the acceleration field induced in the fuel cell of the present invention as a result of rotation of the electrolyte about an axis of rotation of the chamber and the improved convection which results can also promote and enhance the internal circulation of the electrolyte fluid without the use of pumps. Under strong rotation, when the angular velocity is sufficiently high to induce a local linear acceleration substantially greater than the normal local gravitational field (approximately 9.81 m/s
2
), this flow process is referred to as ‘enhanced natural circulation’. The present invention thus advantageously utilises enhanced natural circulation to provide an increase in the power output of fuel cells, such as for example metal-oxygen (or air) fuel cells.
The increase in the bubble rise velocity also brings with it concomitant increases in the throughput of chemical substances which can substantially improve the yield from an electrochemical reaction process. This is particularly advantageous because, generally, the performance of conventional electrochemical reactors is limited by the maximum achievable flow rate of oxidant and/or fuel, which are usually gases, passing through a porous electrode in an electrolyte (usually a liquid), and the available interfacial area between the different phases across which heat and mass transfer and electrochemical reactions take place.
It will be appreciated by those skilled in the art that the rotation of the electrolyte about an axis of symmetry of said chamber may be achieved in a number of ways. However, preferably, the mechanism which enables rotation of said electrolyte comprises a mechanism which enables rotation of said chamber. Obviously, rotating the chamber containing the electrolyte automatically rotates the electrolyte itself. This may, for example, be achieved by means of an electric motor. Thus, the reactor may, in one embodiment, comprise a centrifuge or the like.
The chamber may, in another embodiment, be rotated by providing a plurality of guide vanes and/or impellers thereon, which cause said chamber to rotate upon introduction of the oxidant and/or fuel

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