DE1771084A1 - Methods and devices for maintaining the electrolyte concentration in fuel cell batteries - Google Patents

Description

Methods and devices for maintaining shark do.-; the electrolyte concentration in fuel cell batteries

The present invention relates to a method for erecting sharks tion of the electrolyte concentration in fuel elements as well as devices for its implementation.

In the electrochemical conversion of hydrogen and oxygen As is well known, water is produced in a fuel element leads to a dilution of the electrolyte and thereby reduces the efficiency of the fuel element. To discharge the reaction

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water from the electrolyte are therefore already a number of procedures have been developed. For example, in many cases the water is separated off by continuous evaporation of the water and subsequent condensation of the steam. Here, the evaporating water is separated from the circulating water Reaction gas from the porous electrode and excreted outside the fuel element in a condenser. The devices required for this are often used at the same time to remove the heat loss.

A disadvantage of this method and other methods is that that required for water removal and heat dissipation Auxiliary units, such as pumps and control devices, a considerable expenditure of energy is required, whereby the overall efficiency the fuel cell battery is degraded. this

is particularly noticeable in the case of low-power batteries .

Batteries with a low output are maintenance-free over long periods of time work. They are generally loaded with a low current density and work in the vicinity of the ambient temperature.

While the dissipation of the heat loss in fuel cell batteries of low power can take place via the relatively large surface area of the battery without the aid of additional cooling devices, the dissipation of the reaction water results in considerable amounts due to the energy consumption of the necessary fii 1 fsaggroga

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Trouble. This is due to the conversion of hydrogen and oxygen The amount of water formed in fuel elements is in batteries Smaller power, although small, but these small car ratings also lead to impermissible dilution during long operating times of the electrolyte and thus to a considerable drop in performance the battery.

The object of the present invention was therefore to provide a method and Devices for maintaining the electrolyte concentration in Fuel cell batteries, in particular fuel cell batteries to find small power, in which the energy losses of known methods are avoided.

The solution to the problem is that the fuel elements , each with at least one diffusion tube to a storage vessel are connected, the dissolved in the aqueous electrolyte and contains dissociating substances in the form of a saturated solution, in which these substances may be present as solid substances are, and that the electrolyte space. Each fuel element is provided with an overflow pipe from which the electrolyte diluted by the water of reaction drips continuously.

The invention is based on the knowledge that the concentration gradient that forms in the diffusion tube between the electrolyte space of the fuel element and the storage vessel is used to transport the substances contained in the storage vessel can be used. the

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As a result of the concentration gradient onset of diffusion of these substances into the electrolyte space of the fuel element has the effect that the electrolyte dilution that occurs when the current is drawn not taking place.

In the embodiment shown schematically in FIG In the invention, 1 denotes the storage vessel which contains the solid base denoted by 2 KOH. Located above the potassium hydroxide 2 the saturated KOH solution 3. The diffusion tube 4 leads from the storage vessel 1 to the electrolyte space 5 of the fuel element 6. The overflow pipe 8 leading to the collecting vessel 7 is connected to the electrolyte space 5. If multiple fuel elements are used in Connected in series, the tubes 8 of the individual elements can also be fed into a collecting line from which the diluted electrolyte drips off.

If now the fuel element shown in Fig. 1 with hydrogen and oxygen, the result is water of reaction, which dilutes and increases the volume of the electrolyte leads in the electrolyte compartment. Through the diffusion of KDH from the storage vessel, KOH is now continuously fed into the electrolyte space, so that the electrolyte concentration in the electrolyte compartment is almost constant is held. At the same time, diluted electrolyte is continuously pressed into the overflow vessel due to the increase in volume. In In the fuel element of FIG. 1, 2 m KOH was used as the electrolyte. At room temperature and 1 V, the element delivered a current

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of 0.1 A, which corresponds to a power of 0.1 W. Under these conditions, Ib cm of water is produced over an operating time of 3 months. In order to bring this amount of water to the KOH concentration of 2 m specified in the electrolyte, 8.2 g of KOH are required.

According to the equation F = q * D * ^ _- C

the transport F in the diffusion tube depends on the cross section q of the tube and the concentration gradient ^ ^c . On a slope;

dx ^%

between the saturation concentration of 12 m and the concentration in the element of 2 m, the KOH transport required to maintain the electrolyte concentration can for example

2 then be maintained when the cross-section of the pipe is 10 mm and the length is 1 cm.

The maintenance of the electrolyte concentration is further supported by the fact that the water of reaction in the present example only arises at the anode and is consumed at the cathode, i.e. due to the electrochemical conversion Convection.

The calculated amount of reaction water accumulates as 2 m KOH and drips continuously into the collecting vessel 7. According to the invention, each fuel element of the battery is provided with a storage container . In this way, electrolyte short-circuit currents are avoided in the case of fuel elements connected electrically in series. The electrolyte discharge pipes opening into a collecting pipe exist

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advantageously wholly or partly from a hydrophobic Material. This leads to the formation of leakage currents in network films the pipe ends prevented.

According to a further invention, the method for maintaining the electrolyte concentration can also be improved in that the storage container is made of a material whose temperature coefficient strongly deviates from that of the saturated solution. The diffusion process can therefore occur in the event of temperature fluctuations the resulting flow can also be influenced. By incorporating substances with particularly strongly deviating temperature coefficients, For example, gas, the electrolyte delivery increases significantly with increasing temperature. This effect can

especially when operating batteries, the strongly changing ambient temperatures are exposed to being exploited.

In the case of fuel elements that can still be operated with nominal load even with a very low electrolyte concentration, the waste eye can be expelled with a correspondingly lower concentration. For example, in a battery operated with hydrogen and oxygen, each individual element was loaded with 1.5 A, with a cell voltage of 0.85 V also being observed with a 0.1 m KOH solution. With a specification of 40 g of solid KOH in the storage vessel of each element, the battery ran for a period of 3 months without electrolyte circulation or renewal. According to FIG. 2, the concentration of the ejected waste eye adjusts to 0.1 m after about 40 days. This con-

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central ion did not change in the following months. During the period of operation mentioned, a total of 1.01 l of diluted KOH / element were emitted, which is roughly ^r -> $ less than the amount of water of reaction produced. This missing amount of water was removed with the purge gas of the fuel element.

The new process is particularly suitable for batteries that over long periods of time continuously, maintenance-free and with less Power should be operated. In the event of a longer power interruption, the KOH concentration in the fuel element increases sharply and becomes equal to concentration if the interruption is very long in the storage vessel, but the operational readiness is not impaired. A temporarily larger loss of electrolyte, related on the amount of electrolyte per kWh, but must be accepted in the case of longer interruptions in load.

If the fuel cell battery is more heavily loaded, the Accelerated diffusion process according to a further invention be that according to Pig. 3 the storage vessel 15 through a further diffusion tube 9 with the one leading out of the electrolyte space Tube 1'5 is connected, the * diffusion tubes 9 and 10 so arranged are that a flow in the direction of the arrows is set in motion. The indicated flow direction is the result from the higher temperature and the lower KOH concentration within the fuel element. The flow rate is also determined by the length and cross section of the diffusion tubes. In Pig. 3, 11 denotes a fuel element and 12 its

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Electrolyte room. H again provides a receptacle for the waste eye represent.

Finally it is emphasized that with the help of the new Do not process fuel cell batteries for long periods of time can only be operated without additional devices for water drainage and with the exclusion of leakage currents, but above in addition, the volume of the system can be reduced considerably.

4 claims
3 figures

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Claims (4)

PLA 6R / 1179 claims 1. Procedure for maintaining electrolyte concentration in fuels electric batteries, in particular fuel cell batteries low power, characterized in that from a storage vessel a saturated solution of the in the aqueous. Electrolyte dissolved and dissociating substances continuously over a Diffusion tube in the electrolyte space of the fuel element flows, and that the electrolyte diluted by the water of reaction formed is led out of the cell via an overflow pipe will. 2. Device for performing the method according to claim 1, characterized in that the fuel elements (6,12) each with at least one diffusion tube (4, -9 ″) are connected to a storage vessel (1.15) which contains the in the aqueous electrolyte contains dissolved and dissociating substances in the form of a saturated solution in which these substances may be present are present as a pest substance, and that the electrolyte space (5,12) of each fuel element is provided with an overflow pipe (8,13) from which the electrolyte diluted by the water of reaction drips off continuously. 3. Apparatus according to claim 2, characterized in that the overflow pipes of the fuel elements open into a collecting line, from which the diluted electrolyte drips continuously. 109887/0558 - 9 - By / Or BADORfOINAi. 4. Device according to claims 2 and "5, characterized in that the storage vessel (1 ^) is connected to the overflow pipe (13) by an additional diffusion pipe (4-Θ *), the di ffunionsrohr (1Θ" ^! Above) of the diffusion tube (J) -) . 109887/0558 - H) - By / Or BADORiQINAL