DE2108847C3 - Method for storing at least one metal / oxygen cell before use - Google Patents

Description

40

The invention relates to a method for storing at least one metal / oxygen cell before use.

For most metal / oxygen cells, ζ. Β. Zinc / 4> Air cells, is caused by corrosion of the zinc anode (anode = negative Electrode) with the electrolyte, e.g. B. potassium hydroxide, hydrogen evolved. If the cell is not protected water in the electrolyte solution tends to evaporate and shorten the life of the cell. Furthermore, it is advisable to prevent carbon dioxide from entering the cell, as this is one Caused carbonation and decreased cell performance. It has now been found that Oxygen, if it can enter the cell through the air electrode (cathode = positive electrode), in which Electrolyte is dissolved and tends to oxidize the zinc anode, increasing the corrosion of the anode and thus continuing to reduce the useful power of the cell> o gladly.

In the case of galvanic primary elements, it is already known to design the envelope in such a way that gases can pass through but hydrogen is retained to protect the cell from internal gas pressure or from b5 To protect drying out (OE-PS 2 68 403, FR-PS 13 467, US-PS 27 29 693). The object of the invention is it. to create a process by which metal / Oxygen cells can be stored without the Cell life and performance are adversely affected.

According to the invention this is achieved in that the cell is placed in a tight container, the is designed so that on the one hand at least part of the container at least such an amount of hydrogen lets through that the container during the period of Storage is not damaged, but that on the other hand oxygen and water in one passage through the Containers are prevented.

Further features of the invention are the subject of the subclaims.

The invention is described below in conjunction with the drawing and on the basis of exemplary embodiments explained. It shows

F i g. 1 schematically shows a rigid container according to FIG Invention with a window,

Fig. 2 schematically shows a container with an outlet valve and

F i g. 3 schematically shows a bag as a cover for the metal / oxygen cell or cells.

In the embodiment according to FIG. 1 is a metal / oxygen cell or there are several such cells provided in a gas-tight container 2 which is provided with a palladium window 3, which the Permits passage of hydrogen from the interior of the container

In a further embodiment according to FIG. 2 is the metal / oxygen cell or cells in one gastight container 2 is provided, which is provided with a check valve 4, which optionally the Permits passage of hydrogen from the interior of the container.

In the embodiment of Figure 3, the cell or the cells are arranged in a bag 5, or they are enveloped with a material that allows the passage allows for sufficient hydrogen but prevents damage to the envelope or bag, and Protection against water evaporation and the entry of oxygen or carbon dioxide into the cell Offer. Such a material can consist of a bandage film, in particular a plastic film, or two special layers 5,6 made of polyethylene terephthalate and polyethylene. With such an arrangement the polyethylene terephthalate enables hydrogen to pass through without significant passage of oxygen, but does not prevent evaporation of the water. Polyethylene, on the other hand allows hydrogen to pass through and essentially prevents water evaporation, but does not sufficiently prevent the passage of oxygen if it is not used in a thickness which is practically impossible for normal operation.

The thicknesses and materials used depend on the degree of protection desired, and the the following description gives details of a practical example of a C-format cell.

Weight of zinc
External surface available for diffusion
Assumed loss of performance

15g

30 cm ²
10%
per year

It is considered feasible to reduce the rate of hydrogen evolution to an equivalent Reduce the value of 5% zinc loss per year. Therefore, the permissible zinc loss through acid

Substance oxidation of 5% per year is assumed, but it is 10% per year for hydrogen development assumed to have a factor of safety.

It is believed that the atmosphere outside the cell envelope is ambient air

(i.e. P ₀₂ = 0.21 atm and Pm = 0.79)

hydrogen

10% zinc per year = 0.046 equivalent per year.

1 equivalent of H ₂ = '/ ₂ mol, so the

permissible rate of hydrogen evolution = 0.023 χ 22.4 χ 10 ³ cm ³ per year
= 515 cm ³ per year.

It is assumed that the partial pressure of oxygen inside the cell is zero. In the cell, nitrogen is present in equilibrium with the air outside, ie Ps2 = 0.79 atm. So if the total pressure inside the cell is limited to 1 atm, the hydrogen partial pressure will be 0.21 atm and that is the maximum driving force available for diffusion

Therefore, the hydrogen diffusion rate should be> 515 cm ³ per year for 30 cm ² area and 0.21 atm partial pressure difference, ie

Water (as water vapor)

The permissible water loss is assumed to be 2.5 g / year, i.e. H.

2.5 10 ⁴ _, _, _ _x _ _g . _m day.

The requirement therefore applies: <2.3g · m- ² day- '(less than 16% = 2.0, see below).

Chemical water loss

From the reaction Zn + H ₂ O = H ₂ + ZnO. Chemical water loss is equivalent to 515 cm ³ • of H ₂ per year

cm ³ m ² day 'atm "

^YY 30 365 0.21
ie> 2240cm ³ m ~ ² day "" ¹ atm " ¹ .

(This assumes that the speed is constant over the year)

oxygen

5% zinc per year = 0.023 equivalent per year.
1 equivalent of oxygen = ¹ A mol, so must
be the oxygen diffusion rate

0 ^ 023

_{per year}

<128 cm ³ per year for 30 cm ² area and
0.21 atm partial pressure difference, ie

10 * II _{3 2} _,

⁸ * 1Ö * "365 * ÖJ \ ^{Cm mga}

<560cm ³ m ² day " ¹ atm" ¹ .

_ts ⁵¹⁵

- to X ~~ z ^ Γ X

= 0.41g.

This is around 16% of the permissible value per year.

Carbon dioxide

Electrolyte is 8 N - KOH, that is

8 equivalents / liter.
Volume in battery is 7 cm ³ , ie

5.6 χ 10- ² equivalent. It is assumed that 1% conversion into carbonate per year is permissible, ie

5.6 χ 10 * equivalents convertible per year. «Ι 2 KOH + CO ₂ = K ₂ CO ₃ + H ₂ O, ie

2.8 χ 10 ⁴ MoI of CO ₂ per year.

2.8 χ ΙΟ- ⁴ χ 22.4 χ 10 ³ cm ³ per year

Assume: 4Pco ₂ = 4 χ 10 ~ ⁴ atm. Permissible diffusion rate

2.8 χ IO " ⁴ x 22.4 χ 10 ¹ X 10 * ⁼ 365 χ 30x4 χ 10" ⁴ '

² day ^l atm

The requirement is <1.43 χ 10 ⁴ cm m- ² day- 'atm- ¹ .

Possible materials

The following numbers in Table 1 were determined for various plastic films. ;, All ratios apply to

0.025 mm thick films. The values for H ₂ , 0 ₂ and CO ₂ are in

cm ³ m- ² day- 'atm-' indicated. The values for H ₂ O are given in g · m ~ ² day- '.

Table 1 Polyethylene Polyethylene Polyvinylidene chloride Required Terephthalate Polyvinyl chloride Polyacrylonitrile 30000 1550 35 H ₂ <2240 8 500 90 14th O ₂ <560 45,000 250 75 CO ₂ <14,300 H ₂ O> 2.0 1.8 (25 C) 30% R.II. 10 (40 C) 30 (40 'C) 11 (40 C) 90-95% R.H.

The water ratios refer to the condition, relative humidity (R.H.) as indicated, and with completely dry air.

At 2O ⁰ C 30% RH are equivalent 5 mm Hg of water vapor pressure. This would be in equilibrium with 42.6 KOH at 20 ^° C.

For these ratios (i.e. 5 mm Hg partial pressure difference) applies

1 g m- ² day- '= 18800cm ³ m- ² day-' atm- ¹ .

In Table 2 below, the values given above for the thickness are in thousandths of a cm stated what about the required speed of hydrogen along with the flow rates the thicknesses of O2, CO2 and H2O allow c. (units as before.)

Table 2 Polyethylene Polyethylene
Terephllialal Polyvinylidene,
Polyvinyl chloride,
Polyacrylonitrile layer
Polyethylene
12.5X10 "'cm
Polyethylene
Tcrcphthalate
1.25X10 "'cm Required 13.4 0.69 0.016 Thick 2240 2240 2240 3100 H,> 2240 634 130 875 180 O, <560 3360 362 4700 500 CO ₂ <14,300 H ₂ O <2.0 0.13 0.36 30% R.H. 0.75 43.5 690 2.0 90-95% R.H.

It can be seen that χ 10- ³ cm thick polyethylene all the requirements except for O ₂ (but very near) meets 33.5 that 1.725 χ 10- ³ cm thick polyethylene terephthalate except for water in order, that the third substance (copolymer) fails on O2 and water and would have to be impractically thin in order to be able to compete with hydrogen.

The last column gives the calculated values for a layering of 12.5 10 ^-3 cm thick polyethylene and 1.25 χ 10 ^-3 cm thick polyethylene terephthalate, which indicates that this meets the requirements.

1 sheet of drawings

Claims (6)

Patent claims: 1. Method of storing at least one metal / oxygen cell prior to use, thereby characterized in that the cell is placed in a sealed container so formed is that on the one hand at least part of the container lets through at least such an amount of hydrogen, that the container will not be damaged during the period of storage, but that on the other hand Oxygen and water are prevented from passing through the container. 2. The method according to claim 1, characterized in that the metal / oxygen cell with an r> gastight container, a jacket or a bag is provided, which selectively a certain Lets amount of hydrogen through 3. The method according to claim 2, characterized in that the container has a rigid structure has a check valve for the passage of hydrogen to be dispensed from the container 4. The method according to claim 2, characterized in that the container with a palladium window is provided. 5. The method according to claim 2, characterized in that the sheath consists of a layered Foil consists of a layer of polyethylene and a layer of polyethylene terephthalate 6. The method according to claim 5, characterized in that when storing a C-format ZeDe the polyethylene layer has a thickness of about 12.5 χ IO- ³ to 32.5 χ 10 ^-3 cm and that the polyethylene Terephthalate does not have a thickness of about 1.25 10 ^{-3 cm.} S5