DE19628888C1 - Fuel cell with increased mixing in the electrode pores and method for operating such a fuel cell with alternating operating pressure - Google Patents

Abstract

The claimed alternating working pressure with sufficiently high amplitude and frequency of pressure alternation markedly increases the average conversion of the medium on the electrocatalytic layer of the fuel cell because a stronger intermixture of medium and catalyst particles can be effected in the pores of the electrodes. It was found that the conversion rate of the methanol on the electrocatalytic particles does not follow a linear function, and so an increase in pressure does not entail a linearly proportional increase in conversion. Rather, the conversion of methanol increases with the pressure less than linearly. For example, when pressure undergoes a sinusoidal change of sufficiently high amplitude and frequency, it is possible to increase the average conversion of methanol compared with the conversion at the average working pressure due to the pressure changes. The invention makes use of this principle.

Description

The invention relates to a fuel cell, the elec trolytes, electrocatalysts on both sides and two po includes red electrodes, the performance data, at the same permanent operating parameters such as temperature, pressure and con concentrations of the media are optimized.

So far, fuel cells, such as the Di rect-methanol fuel cell (DMFC), under a constant Operating pressure. The disadvantage of this is that a constant operating pressure no movement and therefore Mixing of the media in or between the catalyst particles in the pores of the electrodes. A through Mixing increases the catalytic effectiveness of each one NEN catalyst particle and can either lead to a Ver reducing the need for catalyst or for sales or Performance increase of the fuel cell with otherwise the same Chen operating parameters such as concentration used, d. H. Amount per unit of time of medium, pressure and temperature, lead ren.

According to the prior art, the catalytic effectiveness only by increasing the concentration, temperature or pressure of the Medium are optimized; there is no mixing reached. With the future use of Brenn fabric cells, for example the DMFC, in the decentralized Energy supply and in electrical traction, however, it is desirable, so far only with higher prints, higher Temperature or higher concentrations achievable Lei performance data even at lower ones, for example only a little operating pressures above atmospheric pressure pass.

The object of the present invention is therefore to provide a burning to provide the cell in which the supply the catalyst particles with medium, i. H. Fuel and / or Oxidant, regardless of the absolute level of pressure that absolute level of temperature or concentration of medi um is optimized.

In the context of the invention it was found that the implementation the rate of methanol formation on the electrocatalyst particles does not follow a linear function, so that with pressure increase too there is no steadily increasing sales increase. Of the Rather, methanol sales decrease with pressure less than linear to. The reason for this was a lack, and with increasing pressure, the mixing of the Catalyst particles in the pores of the electrodes adopted. It could be shown accordingly that it is possible by a pressure that changes, for example, sinusoidally (with a sufficiently high amplitude and frequency) the middle one Sales of methanol compared to sales of (from the average operating pressure resulting in pressure changes increase. Finally, general knowledge of the invention is that an alternating operating pressure with a sufficiently high Amplitude and frequency of pressure alternation, the middle Sales of the medium in the electrocatalyst layer clearly increased because of an increased mixing of medium and Ka Talysatorpartchen takes place in the pores of the electrodes. This principle is used according to the invention.

The present invention relates to a fuel cell le, which comprises an electrolyte on both sides of Elek trocatalyst particles and two porous electrodes with Medium is washed around, is surrounded, provided in the means which are an increased mixing of electrocatalyst cause particles and medium in the pores of the electrodes.  

The present invention also relates to a ver drive to operate a fuel cell at which the altitude the operating pressure of the cell varies so that the pressure fluctuations in the system an increased mixing of elec trocatalyst particles and medium in the pores of the electrodes cause. Further advantageous embodiments of the invention result from the subclaims and from the description exercise.

According to one embodiment of the invention, pressure swans generated in the fuel cell. This pressure fluctuation gene can for example by piezoceramic membranes, Disks or plates that vibrate have arisen. It is possible that such disks, Membranes or plates in a short distance and approximately paral lel attached to the electrodes. So an uninhibited Flushing of the electrode with medium continues to be guaranteed is, the membrane, disc or plate made of piezokerami holes or other passages be. However, it is also possible to wash around unhindered to achieve that the membrane, disc or plate does not occupy the entire surface of the electrode in parallel, son who only refers to a sub-area, so that the Membrane, etc. the medium can flow freely. To this is caused by the vibrations of the - for example - piezoke ram membrane also sucked the medium and thus a ensures sufficient washing around the electrode.

As "electrolyte", "electrolyte layer or foil" the common electroly for the respective fuel cell types referred to in the example of the DMFC i.a. to proton-conducting, solid polymers such as Nafion (reg. Brand) or sulfonated polystyrene membranes.

As "electrocatalysts" or "electrocatalyst particles or particles "are generally the catalyst particles that used in the common fuel cells on the electrodes  are referred to. In the foreground according to the invention standing DMFC is anode side Platinum-ruthenium compound (preferred but not exclusive Lich) and cathode side around pure platinum, platinum on coals fabric or the like.

As "porous electrodes", "electrodes with pores" or "Current collectors" are usually materials on Koh lenstoffbasis, z. B. carbon fiber papers or fabrics puts. These are over the active catalyst layer with the Electrocatalyst particles attached and through them the media to the electrocatalyst particles and the Electrochemical conversion products such as electricity, heat and Drained water.

Both fuel and oxidant are used as "medium" records, the fuel in the case of DMFC methanol Methanol-water mixture or other fuel mixtures and the oxidant is air or pure oxygen. The invention however, as already mentioned, should not be tuned to DMFC be limited, but the exemplary explanation based on the DMFC only serves to clarify the invention put.

As "means that increased mixing of electrocata cause analyzer particles and medium in the pores of the electrodes ken ", means are preferred which produce pressure fluctuations gene, understood. However, other means, such as small stirring units or other means that the linear Disturb the flow with which the media wash around the electrodes, use according to the invention. For example, it is also conceivable the whole fuel cell stack, the whole "stack", to vibrate and thereby the invention to achieve increased mixing.

Means that produce pressure fluctuations are in turn before pulls, but not exclusively, membranes, plates or  Discs that vibrate to generate pressure can be. Versions have proven to be practical proven in which such disks etc. made of piezoceramic Material. However, there may be other means by which one alternating operating pressure can be generated, such as com pressors or pumps as "means that increased through mixture of electrocatalyst particles and medium in the bottom cause the electrodes "are used.

For example, "pressure fluctuations" become sinusoidal changing operating pressures, each being applied te amplitude of the pressure ensures that at all times an adequate supply of the porous electrodes and the Catalyst particles with medium is ensured and also, that too high a pressure, the pores of the electrodes or the Electrolyte damage would be avoided. The frequency and the amount (= amplitude) of the pressure fluctuations depends on the type the fuel cell variable and must for the respective Be drive system can be found and optimized.

The "operating pressure" is the mean pressure, with where the media flow around the electrodes of the fuel cell.

As a "short distance to the electrode" distances in the loading richly less centimeters, or on the order of millimeters and µm, understood.

The exact material of the electrocatalysts, electrodes or other components of the fuel cell should cover the scope not limit the invention because any common focal fabric cell, if their electrodes better with the invention mixed in the medium, shows a higher effectiveness and therefore provides higher performance data.

In the case of a piezoceramic piece used according to the invention be or membrane can do this on both sides with a metal be provided to a vibration with a  electrical alternating field with suitable frequency and amplitude de to enable. So that this alternating field and the metalli surfaces do not interfere with the operation of the fuel cell, is the piezoceramic disk, plate or membrane in turn with the excitation electrodes completely, except for contact surfaces or contact pins, with an insulating polymer layer envelop. The excitation electrodes are the above called metallization layers. The isolating bottom limer layer is then again with a layer of one, metal that is conductive but not corrosive in this medium or other material such as gold to cover. This would be an electrical contact between the Electrocatalyst or the carbon paper on the one hand and the Contact area of the electrodes or the bipolar plates of the Fuel cell stack on the other hand guaranteed.

The pressure fluctuations can over the entire electrode area che or only partially along the electrode surface probably be generated on the anode as well as on the cathode. Due to the pressure fluctuations or the deflection of the pie Ceramic ceramic membrane or disc is in the electrolyte and in the electrode or in the electrolyte between the Catalyst particles induced an additional movement that to an increased mixing of the electrolyte with the Me dium, for example consisting of a mixture of water and methanol. This is - in this example too remain - the conversion of the methanol at a given current density elevated. The consequence is u. a. that a lower methanol Concentration to achieve a certain performance enough, and with the humiliation of the original bet methanol concentration also less methanol due to the Polymer membrane reaches the cathode. This then becomes additionally also the aging of the cathode is reduced because the harmful rinsing rate of the cathode is reduced.

Claims (6)

1. Fuel cell with an electrolyte which is surrounded on both sides with medium-flushed electrocatalyst particles and two po rous electrodes, characterized in that the fuel cell has means for generating and maintaining frequency-variable fluctuations in the operating pressure, which results in an increased mixing of medium and electrocatalyst particles in the pores of the electrodes. 2. Fuel cell according to claim 1, wherein the means for Generation of pressure fluctuations in the operating pressure in the me dium, both or only one of the electrodes and the there electrocatalyst particles, washed around. 3. Fuel cell according to claim 2, wherein the means for Generation of pressure fluctuations, membranes, discs or Include plates. 4. The fuel cell according to claim 3, wherein the membranes, Disks or plates roughly parallel and at a short distance attached to the electrode (s). 5. Fuel cell according to one of the preceding claims 2 to 4, in which the means for generating the pressure fluctuations are made of piezoceramic material.   6. Method of operating a fuel cell Includes electrolytes on both sides of electrocatalyst particles and two porous electrodes, which are washed with medium are surrounded, at which the level of the operating pressure of the Cell is set variably, so that by frequency variations ble pressure fluctuations in the system an increased mixing of electrocatalyst particles and medium in the pores of the Electrodes is effected.