METHOD FOR STARTING A FUEL CELL AND FUEL CELL SYSTEM

Abstract

A method for starting a fuel cell and to a fuel cell system which is configured to carry out the method. The fuel cell includes electrically conductive bipolar plates, arranged between which there is in each case a cathode, including a fluid-conducting cathode space, a membrane and an anode, including a fluid-conducting anode space. There is provision that the method includes the following steps in the specified order: purging the anode space with a fluid in order to expel fuel, impressing electricity on a unit composed of the cathode (2k)/membrane (1)/anode (2a) by applying a current and/or a voltage, wherein the fluid is applied to the anode space or continues to be so, switching off the electricity, and introducing a fuel into the anode space.

Claims

1-9. (canceled)

10. A method for starting a fuel cell, the fuel cell including electrically conductive bipolar plates, situated between which are in each case a cathode, including a fluid-conducting cathode chamber, a membrane and an anode, including a fluid-conducting anode chamber, the method comprising the following steps in the specified order: purging the anode chamber with a fluid in order to expel fuel; impressing electricity on the fuel cell by applying a current or a voltage, the anode chamber being or continuing to be acted upon by the fluid; switching off the electricity, and introducing a fuel into the anode chamber.

11. The method as recited in claim 10 wherein the fluid for purging the anode chamber is an inert gas.

12. The method as recited in claim 10 wherein the fluid for purging the anode chamber is nitrogen.

13. The method as recited in claim 10 wherein an intensity of the impressed electricity is controlled by an intensity of the applied voltage or of the applied current.

14. The method as recited in claim 10 wherein a voltage in the range of 0.5 V to 1.5 V is applied or induced.

15. The method as recited in claim 10 wherein a direction of the applied current flow is opposite a direction of the current flow during operation of the fuel cell.

16. The method as recited in claim 10 wherein a current flow of the current is induced for a duration in the range of 0.1 s to 50 s.

17. The method as recited in claim 10 wherein the fuel is introduced with a time delay after the electricity is switched off.

18. A fuel cell system comprising: a fuel cell including two electrically conductive bipolar plates, a cathode, including a fluid-conducting cathode chamber, a membrane and an anode, including a fluid-conducting anode chamber; a voltage source; and a control unit, the control unit configured to carry out the method steps as recited in claim 10.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0042] The present invention is explained below in exemplary embodiments with reference to the associated drawings.

[0043] FIG. 1 schematically shows a fuel cell,

[0044] FIG. 2 schematically shows a sectional view of an individual cell of a fuel cell stack,

[0045] FIGS. 3a through 3f show gas ratios in the anode chamber and the state of the anode at various points in time of the method according to the present invention in a preferred implementation, and

[0046] FIG. 4 shows a current-/voltage time diagram of the method according to the present invention in a preferred implementation.

DETAILED DESCRIPTION

[0047] FIG. 1 shows a highly schematic representation of a fuel cell (also referred to as a fuel cell stack). Fuel cell 100 includes a first end plate 6 and a second end plate 7. A plurality of stack elements stacked on top of one another, which include bipolar plates 5 and membrane-electrode assemblies 10, is situated between end plates 6, 7. Bipolar plates 5 are alternatingly stacked with membrane electrode assemblies 10. Membrane electrode assemblies 10 each include a membrane and electrodes connected on both sides of the membrane, namely, an anode and a cathode (not depicted). Sealing elements 8, which seal the anode chambers and cathode chambers to the outside in a gas-tight manner, are each situated between bipolar plates 5 and membrane electrode assembly 10. Fuel cell 100 is pressed between end plates 6 and 7 with the aid of ties 9, for example, tie rods or tensioning plates.

[0048] In FIG. 1, only the narrow sides of bipolar plates 5 and membrane electrode assemblies 10 are visible. The main surfaces of bipolar plates 5 and membrane electrode assemblies 10 abut one another. The representation in FIG. 1 is not dimensionally accurate in part. Typically, a thickness of an individual cell, made up of a bipolar plate 5 and a membrane electrode assembly 10, is a few mm, membrane electrode assembly 10 being by far the thinner component. In addition, the number of individual cells is normally significantly greater than is depicted.

[0049] A sectional view of an individual cell of a PEM fuel cell 100 is schematically depicted in FIG. 2.

[0050] Fuel cell 100 includes a membrane electrode assembly 10 as the core component, which includes a polymer electrolyte membrane 1, as well as one electrode each connected to the two flat sides of membrane 1, namely an anode 2a and a cathode 2k. Membrane 1 is a polymer electrolyte membrane, which is preferably capable of conducting cations, in particular protons (H.sup.+). Electrodes 2a, 2k include a catalytic material, for example, platinum, which is supported on an electrically conductive material, for example a carbon-based material.

[0051] A gas diffusion layer 3, which essentially has the function of uniformly distributing the supplied operating gases over the main surfaces of electrodes 2a, 2k and membrane 1, is connected to each of electrodes 2a, 2k.

[0052] A bipolar plate 5 is situated on the outsides of each gas diffusion layer 3. Bipolar plates 5 have the function of electrically connecting individual membrane electrode assemblies 10 of the individual cells in the stack to one another, of cooling the fuel cell stack and of supplying the operating gases to electrodes 2a, 2k. For the last-mentioned purpose, bipolar plates 5 (also called flow field plates) include flow fields 4a, 4k. Flow fields 4a, 4k, for example, include a plurality of flow channels situated in parallel to one another, which are incorporated into plates 5 in the form of grooves or slots. Each bipolar plate 5 normally includes an anode flow field 4a on its one side, which faces anode 2a and a cathode flow field 4k on its other side, which faces cathode 2k. In the present case, only one flow field 4a, 4k each is reproduced for each of the two bipolar plates 5 depicted, the section depicted extending along one flow channel, respectively. During normal operation of the fuel cell, a fuel, in particular, hydrogen (H.sub.2) is fed to anode flow field 4a, whereas an oxygen (O.sub.2)-containing operating medium, in particular, air, is fed to cathode flow field 4k.

[0053] The reactions taking place at anode 2a and cathode 2k are also depicted in FIG. 2. Accordingly, a catalytic oxidation of hydrogen H.sub.2 into protons H.sup.+ with release of electrons e.sup.− takes place at anode 2a. The protons migrate through proton-conducting membrane 1 and reach cathode 2k. There, the supplied oxygen O.sub.2 reacts with the protons to form water H.sub.2O, the oxygen being reduced and thus takes on electrons. The electrons of the anode reaction are fed to the cathodes via an external electric circuit not depicted herein.

[0054] The sequence of the method according to the present invention for starting a fuel cell is explained in a preferred implementation with reference to FIGS. 3 and 4. FIG. 3 schematically depicts the conditions in a flow channel of anode flow field 4a and of anode 2a at various points in time while the method according to the present invention is being carried out. The depiction of gas diffusion layer 3 was omitted in FIG. 3. FIG. 4 shows a current-/voltage time diagram (U(t)/l(t)) of a fuel cell during the method. The curve of a voltage (U) 20 (left ordinate) depicted in volts and the curve of a current (I) 21 (right ordinate) measured in mA are shown as a function of time (t) in minutes (abscissa).

[0055] At the start of the method at point in time t.sub.0, anode flow field 4a is filled completely with O.sub.2. As a result, the catalytic material is present in part in the form of platinum oxides PtO.sub.x (FIG. 3a). At point in time t.sub.0, the method is started by applying a purging fluid, in this case nitrogen N.sub.2, to the anode chamber. A nitrogen/oxygen front forms and in this phase, the nitrogen forces the oxygen out of anode flow field 4a (FIG. 3b). This first method phase labelled with I in FIG. 4 takes place over a purge duration of 0.5 min to 1.5 min, in particular, of 1 min to 1.25 min, in this case of 1.1 min to 1.2 min. At the end of this phase I, the entire anode flow field 4a is filled completely with nitrogen at point in time t.sub.1, whereas anode 4a continues to be present in part in the form of PtO.sub.x (FIG. 3c).

[0056] In a subsequent second method phase II, electricity is impressed on the fuel cell, in particular, on its bipolar plates for 18 s to 120 s, in particular, for 24 s to 36 s at point in time t.sub.1. This takes place current-controlled in the specific embodiment shown. A voltage 20 results from impressed current 21, which lags behind the current (see FIG. 4). This lagging behind becomes clear insofar as voltage 20, in contrast to induced current 21, shows a flat rise rather than an abrupt rise. In the specific embodiment depicted, the nitrogen supply is not interrupted even during the impression of the electricity. In this phase II, the electrochemical reduction of platinum oxides takes place as a result of induced current flow 21 and/or of resulting voltage 20. The observed lagging behind of the voltage is attributable to the ongoing reduction of the platinum oxides. Water forming during the reduction is discharged from the anode chamber by the uninterrupted purging with nitrogen. At the end of method phase II, the catalytic material of anode 2a is present in the form of platinum Pt at point in time t.sub.2 (FIG. 3d).

[0057] A third phase III begins at point in time t.sub.2 and is initiated by the interruption of current 21, by shutting off the current supply. Current 21 drops abruptly to 0 mA. Voltage 20 again lags behind the current so that measured voltage 20 in third phase III sinks, but does not reach the zero point. In method phase III, lasting approximately 2 s to 15 s, preferably 3 s to 10 s, in particular, 5 s to 7 s, the anode chamber continues to be continuously purged with nitrogen. The duration of this phase is preferably short in order to counteract the renewed formation of platinum oxides and platinum hydroxides as a result of oxygen diffusing in.

[0058] The supply of fuel, in this case hydrogen H.sub.2, begins at point in time t.sub.3, and with that fourth method phase IV. A hydrogen/nitrogen front forms and the hydrogen forces the nitrogen out of the anode chamber (FIG. 3e). The introduced hydrogen induces the actual fuel cell reaction and a potential difference forms across the membrane, since the cathode chamber is simultaneously supplied with oxygen, in particular, air. The electrochemical potential difference is measured as voltage 20, without a current 21 being applied. (FIG. 4).

[0059] At the end of the start process and during operation of the fuel cell, the entire anode chamber is filed with hydrogen (FIG. 3f).

LIST OF REFERENCE NUMERALS

[0060] 1 membrane [0061] 2 electrode [0062] 2a anode [0063] 2k cathode [0064] 3 gas diffusion layer—GDL [0065] 4a anode flow field [0066] 4k cathode flow field [0067] 5 bipolar plate—BPP [0068] 6 first end plate [0069] 7 second end plate [0070] 8 sealing element [0071] 9 tie [0072] 10 membrane electrode assembly—MEA [0073] 100 fuel cell [0074] 20 voltage [0075] 21 current