The type of fuel cell that the present invention concerns is disclosed in International patent applications PCT SE2007/050222 and in PCT SE2005/001514.
Fuel cells of this type typically consist of the following design features/functionalities:
1) A sealing functionality creating the anode gas chamber. This is accomplished by using an adhesive which thereby seals the Membrane Electrode Assembly (MEA) to an anode current collector foil.
2) A gas distribution functionality to distribute the hydrogen gas to different cells in a fuel cell device. This is accomplished by forming a support plate with gas channels for the hydrogen gas. The fuel cells are attached to the support plate by adhesive and/or clamping means. From the support plate there are holes leading to the anode gas chamber of each cell.
3) An electrical interconnect functionality which collects the current from one cell and distributes it to the adjacent cell, preferably with minimal resistance and in such a manner that uniform current density is obtained over the active area of the cells.
4) A clamping feature. By subjecting the fuel cell to a clamping force the internal resistance within the cell is decreased, i.e. contact resistances between different materials and specific resistances inside materials (e.g. by compressing the Gas Diffusion Layer (GDL) its fiber-fiber connections improves). Analogous to the electrical contact also the heat conductivity is improved by the clamping and thereby more heat can be dissipated from the reaction layers (i.e. the electrodes). The clamping feature is closely linked to the electrical interconnect functionality.
All these design features/functionalities applied together form a fuel cell device.
In the prior art devices according to the patent application cited above, the interconnect functionality of a multiple cell fuel cell device is obtained by an electrically conductive current collector foil which is leading from the anode GDL of one cell to the cathode GDL of an adjacent cell. At the GDL of the adjacent cell there is an electrical interface (a contact area) to the clamping means (e.g. a gold plated metal net) which distributes the current over the cathode GDL. The GDL is thus working as a compressible element, pushing the foil against the net, when the cell is being clamped together.
One disadvantage with this design is that the current collector foil must partially cover the cathode GDL of the adjacent cell (i.e. beneath the interconnect area between the foil and the net), thereby hindering the air access to that part of the GDL and the MEA lying under it.
Another disadvantage of this design is that the current collector foil can come in electrochemical contact with the cathode of the MEA of the adjacent cell. This can happen when the water produced in the electrochemical cathode reaction (2H++2e−+½O2→H2O) are forming an electrochemical electrolyte between the current collector foil and the cathode GDL and MEA. The electrochemical potential of the cathode causes ions (e.g. Cu-ions) to dissolve from the conductive foil and perhaps also from its adhesives. The ions are then transported into the MEA where they will poison both the catalysts and the proton conducting ionomer. The water produced in this way can also form a galvanic cell between the gold plated net and the conductive foil, thereby dissolving ions from the foil.
Life time experiments on prior art fuel cell devices designed according to FIG. 1 in the present application have shown that the performance is decreasing with more than 45% after 500 working hours. Post-test analysis on such cell membranes (with energy dispersive x-ray spectroscopy) has shown that the membrane contains Cu-ions which probably come from the conducting tape at the cathode side.