Air cell electrode and process for preparing its catalyst

0001The present invention is concerned with fuel cells and, more particularly, with an air cell having an improved manganese oxide catalyst which provides excellent high rate discharge characteristics and good storage characteristics. The improved manganese oxide catalyst of the invention is particularly suitable for use in air-zinc button cells.

0002There have been attempts over many years to employ manganese oxide as an inexpensive catalyst for air cell use. For example, U.S. Patents 3948684,4121018, and 4269691 disclose such catalysts. According to these patents, various types of chemical manganese are subjected to chemical or heat treatment to precipitate Mn0₂, Mn₂0₃ and/or Mn₃0₄ on carbon or mix yMn0₂ with carbon and alkali to provide a catalyst.

0003As indicated in the U.S. Patents mentioned above, manganese oxides have catalyst activity and some of them have satisfactory discharge characteristics at a current not exceeding several mAlcm², but these oxides are not always completely satisfactory in their high-rate discharge characteristics at a current of more than 10 mA/cm² and in their discharge characteristics after storage. In the light of these drawbacks, there has been a demand for a manganese oxide having improved catalyst activity and improved storage stability.

0004The present invention is concerned with an improved catalyst for an alkaline air cell, the catalyst comprising a manganese oxide obtained by heat-treating y-manganese oxyhydroxide (y-MnOOH), particularly a manganese oxide obtained by heat-treating y-MnOOH at a temperature in the range 250 to 450°C whose X-ray diffraction pattern is mainly characteristic of Mn₅O₈.

0005According to the present invention, therefore, there is provided an air cell which contains a catalyst comprising a manganese oxide, characterized in that the manganese oxide has an empirical formula of MnO_., where 1.45≤x≤1.60, and gives an x-ray diffraction pattern (copper target) having peaks corresponding to lattice constants of 1.59, 1.90, 2.35, 2.48, 2.79, 3.09, 4.09 and 4.88.

0006The manganese oxyhydroxide y-MnOOH used in the present invention occurs as acicular crystals, as shown in Figure 1, the crystals having a major axis length a of not more than 10 µ, a minor axis length b of not more than 1 µ, and an aspect ratio alb of <10. This acicular crystalline configuration remains unchanged even after heat treatment.

0007Figure 2 shows the thermogravimetric weight-loss curves of y-MnOOH in various atmospheres. The inert atmospheres I of nitrogen, argon and helium gases and the air atmosphere II tend to show substantially the same course of weight change, but in the temperature region indicated by b in Figure 2, the weight changes in the inert atmospheres (I), i.e. nitrogen, argon and helium, are more than that in air (II).

0008With regard to the X-ray diffraction patterns of the resulting manganese oxides, the results are as follows. When the heat treatment is carried out in nitrogen, argon, or helium at a temperature in the range 250 to 300°C, Mn₅O₈ is produced in a substantial amount, but some y-MnOOH remains unreacted; a diffraction pattern characteristic of Mn₅O₈ and, to a lesser extent, of Mn₃0₄ is obtained at a temperature of 450°C, and a pattern characteristic of a-Mn₂0₃ is obtained at 500°C.

0009The heat treatment in air shows substantially the same tendency as that in nitrogen or argon, but Mn₅O₈ is predominant in the temperature range 250 to 300°C, where the presence of β-MnO₂ in addition to y-MnOOH is confirmed by reference to the ASTM Data File.

0010The studies conducted in the various atmospheres and temperature regions mentioned above showed that although y-MnOOH as such has only slight catalytic activity, a very high catalytic activity is obtained under the conditions which lead to an X-ray diffraction pattern characteristic of Mn₅O₈ and the storage stability of the product in alkaline electrolyte is also very high.

0011In the heat-treatment temperature region of 500°C and higher, there occurs a transition to a-Mn₂0₃ so that although the catalytic activity during initial discharge is satisfactory, the stability of this oxide in the electrolyte is somewhat inferior to that of Mn₅O₈.

0012Figure 3 shows a typical X_-ray diffraction pattern (copper target, carbon filter, voltage 40 kV, current 40 mA) of Mn₅O₈ obtained by heat-treating y-MnOOH in a nitrogen atmosphere at a temperature of 400°C for 2 hours. In Figure 3, the horizontal axis represents 20 and the vertical axis represents the peak intensity.

0013According to the ASTM Data File, the lattice constant (d) and diffraction peak intensity (l/l₁) values of Mn₅O₈ are as shown in Table 1. <tables id="tabl0001" num="0001"><img file="EP0140973B1_D0001.tif" /></tables>

0014The representative dvalues in the X-ray diffraction pattern of manganese oxide obtained in accordance with the present invention are shown in Table 2. <tables id="tabl0002" num="0002"><img file="EP0140973B1_D0002.tif" /></tables>

0015It is clear that the manganese oxide of the present invention shares many of the peak positions of Mn₅O₈ as given in the ASTM Data File.

0016However, since the relative intensities of diffraction peaks may vary according to crystal shape, the conditions of recording the X-ray diffraction pattern, and the heat-treating temperature, the relative intensity values do not necessarily correspond to the values in the ASTM Data File. Figure 4 shows the X-ray diffraction pattern of the manganese oxide obtained by heat treatment at 300°C.

0017Figure 5 shows the X-ray diffraction pattern of manganese oxide obtained by heat treatment at 450°C.

0018For the same reasons as mentioned above, diffraction peak positions may be displaced by ± about 0.02.

0019Chemical analysis of the MnO_x obtained by the heat treatment of y-MnOOH showed that the value x=1.6 corresponding to Mn₅O₈ is not necessarily obtained even when the heat treatment is conducted at a temperature of 400°C. Rather, the value of x is less than 1.6 in many cases.

0020The chemical analysis for the determination of x was carried out as follows. First, MnO_x was dissolved in known quantities of oxalic acid and sulphuric acid and the unreacted oxalic acid was titrated with potassium permanganate to determine the proportion of Mn0₂. The MnO,, was also dissolved in hydrochloric acid and titrated with EDTA to find the proportion of metallic manganese. From these data, the value of x in the MnO_x was calculated using the following equation:<maths id="math0001"><img file="EP0140973B1_D0003.tif" /></maths>where <ul id="ul0001" list-style="none"><li>a=_weight percent of Mn0₂</li><li>b=weight percent of Mn</li></ul>

0021The value of x for the MnO_x of the present invention as calculated by the above method was found to satisfy the condition 1.45≤x≤1.60 when the heat-treating temperature was 250°C or higher, regardless or the atmosphere in which the y-MnOOH was heated. This indicates that while the manganese oxide obtained by heat-treating y-MnOOH shows many of the characteristics of Mn₅O₈ according to the ASTM Data File, not only Mn₅O_a but other special manganese oxides close to Mn₅0₈ are formed as well.

0022As indicated earlier, the manganese oxide obtained by the method of the invention is in the form of acute acicular crystals and has the characteristics of Mn_sO_s in its X-ray diffraction pattern.

0023Using the manganese oxide of the invention, a catalyst electrode can be manufactured in the following manner.

0024The manganese oxide of the invention is mixed with activated carbon, carbon black, and an aqueous fluorine-containing resin dispersion to form a paste, which is then coated over a screen of stainless steel, silver, or similar material. The sheet obtained is then dried and heat-treated at a temperature of about 300°C in order to slightly sinter the fluorine-containing resin. In this case, the fluorine-containing resin acts as a binder and a repellent against alkaline electrolyte, etc. The activated carbon serves as a catalyst support, but since it is low in electrical conductivity, carbon black is also added. In some cases, carbon black alone can be used as a catalyst support instead of activated carbon. A catalyst electrode of wafer thickness, for example less than 0.2 mm, can be prepared in this way.

0025In order that the invention may be more fully understood, reference will be made in the following description to the accompanying drawings, in which: <ul id="ul0002" list-style="none"><li>Figure 1 is a transmission electron microphotograph showing the crystallographic structure of y-manganese oxyhydroxide according to this invention at a magnification of x10,000;</li><li>Figure 2 is a thermogravimetric weight-change curve for the y-manganese oxyhydroxide shown in Figure 1;</li><li>Figures 3, 4 and 5 show the X-ray diffraction patterns (copper target) of y-manganese oxyhydroxides obtained by heat-treatment at temperatures of 400°C, 300°C and 450°C, respectively;</li><li>Figure 6 is a cross-sectional view showing an air-zinc button cell according to the present invention;</li><li>Figure 7 shows the average retention voltages in continuous discharge at a temperature of 20°C and a load resistance of 130 Q for air-zinc button cells prepared using catalyst electrodes obtained by heat-treating y-manganese oxyhydroxide in an atmosphere of air (A), nitrogen (B) or argon (C) at various temperatures; and</li><li>Figure 8 shows the X-ray diffraction pattern of manganese oxide in accordance with the present invention after storage.</li></ul>

0026For a better understanding of the invention, the following examples are provided by way of illustration.

Example 1

002730 parts by weight of the manganese oxide obtained by heat-treating y-MnOOH was mixed with 20 parts by weight of activated carbon, 20 parts by weight of carbon black, 30 parts by weight (as non-volatile matter) of an aqueous dispersion of polyethylene tetrafluoride, and 100 parts by weight of distilled matter. The mixture was kneaded well and rolled to give a 0.5 mm thick sheet. The sheet was filled into a nickel screen with a wire diameter of 0.1 mm and a mesh size of 40 meshes, dried, and pressed to give a catalyst sheet having a thickness of 0.4 mm. The catalyst sheet was heat-treated at 300°C for 15 minutes to sinter the polyethylene tetrafluoride component for increased water repellency. The sintered catalyst sheet was then punched to give an air-zinc button cell catalyst electrode having a diameter of 11.0 mm. The diameter of the effective area of this catalyst electrode was 9.0 mm. The sheet electrode was laminated with a 0.2 mm-thick porous polyethylene tetrafluoride film of the same diameter to render the sheet gas-permeable, but liquid-impermeable. The catalyst electrode was then assembled as follows to provide an air button cell.

0028Referring to Figure 6, an air diffusion pad 3 was first set in position on the inside bottom of a positive electrode case 1 having air access holes 2 through the bottom, and the catalyst electrode 5 laminated with polyethylene tetrafluoride film 4 was set in position, followed by the insertion of a separator 6.

0029A metal-clad seal plate shaped in the form of a dish 7 was then assembled with a seal ring 8 of synthetic resin to provide a sealing assembly. This sealing assembly was filled with zinc powder as a negative electrode active material 9 and an alkaline electrolyte.

0030The sealing assembly containing the negative electrode was fitted with the above positive electrode case containing the positive electrode and other members, and the whole assembly was sealed tightly with a metal mould to give a leak-tight air-zinc button cell, as shown in Figure 6. The numeral 10 represents a seal paper for closing the air access holes 2 which is peeled off to expose the holes 2 when the button cell is used.

Example 2

0031y-MnOOH was heat-treated in air for about 3 hours at temperature increasing in increments of 50°C from 200 to 700°C to give a series of manganese oxides. Each of these manganese oxides (30 parts by weight) was then kneaded with 20 parts by weight of activated carbon, 20 parts by weight of carbon black, and 30 parts by weight (as non-volatile matter) of an aqueous dispersion of polyethylene tetrafluoride. The mixture was then filled into a metal screen with a wire diameter of 0.1 mm and a mesh size of 40 meshes to give a 0.5 mm thick catalyst sheet. The sheet was heat-treated at 300°C for 20 minutes to sinter the polyethylene tetrafluoride component for added water repellency and then punched to give a catalyst electrode with a diameter of 11.0 mm. The effective diameter of the electrode was 9.0 mm. The electrode was laminated with a 0.2 mm thick porous polyethylene tetrafluoride film to provide an air cell electrode.

0032Using this air cell electrode as a positive electrode, zinc as a negative electrode, and an aqueous solution of potassium hydroxide as an electrolyte, an air cell was made by placing the positive and negative electrodes in juxtaposition with a microporous resin separator between them to provide an air-zinc button cell with a diameter of 11.6 mm and a total height of 5.4 mm (IEC Standard Size R44), as shown in Figure 6. In the order of decreasing temperatures for the heat treatment of the y-MnOOH, these cells were designated A, through A_ii and the average discharge voltages at a temperature of 20°C and a load resistance of 130 Q were measured, both immediatley after manufacture and after storage at 60°C for 1 month. The results are indicated by the symbol O in Figure 7.

Example 3

0033y-MnOOH was heat-treated in nitrogen for about 3 hours at temperatures increasing in increments of 50°C from 200 to 700°C to give a number of manganese oxides; using these manganese oxides, air cells were made by the method described in Example 1. These cells were designated D, through D_l, and evaluated in the same way as those of Example 1. The results are indicated by the symbol A in Figure 7.

Example 4

0034y-MnOOH was heat-treated in argon by the method described in Examples 1 and 2 and air cells C, through C¹¹ were made using the resulting catalysts and evaluated. The results are indicated by the symbol 0 in Figure 7.

Control Example

0035As a control run, untreated y-MnOOH was mixed with activated carbon, carbon black, and an aqueous dispersion of polyethylene tetrafluoride in the same proportions as were used in Example 1 and the resulting catalyst was used to make an air cell by the method described in Example 1: the cell was designated D. Similarly, a control cell E was made using (3-Mn0₂ obtained by the pyrolysis of manganese nitrate by the method described above. The cells D and E were evaluated by the method described in Examples 1, 2 and 3. The results are also shown in Figure 7.

0036It can be seen from Figure 7 that regardless of whether the heat-treating atmosphere is air (0), nitrogen (A), or argon (V), the initial characteristics of the cells obtained are superior to those of the conventional cells D and E when the heat-treating temperatures are in the range 250 to 700°C, but after 1 month's storage at 60°C, the cells obtained at heat-treating temperatures in the range 250 to 450°C proved more satisfactory for all the atmospheres and, in particular, post-storage characteristics approximately the same as those of the initial characteristics were obtained when the heat-treating temperature was in the range 300 to 400°C. There was no difference between using nitrogen or argon atmospheres, and in both cases the retention voltage was higher than the initial level, and slightly higher than that obtained using air as the heat-treating atmosphere and a heat treatment temperature in the range 300 to 400°C.

0037The temperature region conducive to such superior characteristics is the region which gives rise to a manganese oxide having many peak positions in its X-ray diffraction pattern which agree with those of Mn₅O₈. It is therefore clear that those catalysts whose X-ray diffraction peaks correspond more or less in those of Mn_sO_s have satisfactory catalyst activity and are stable in alkaline electrolyte.

0038Furthermore, since the manganese oxide obtained by the heat treatment of the invention is in the form of acute needles, it is effectively brought into contact and interlaced with the activated carbon, carbon black, fluorine-containing resin, and metal screen which together form the catalyst electrode. The parameters of water repellency, hydrophilicity and electrical conductivity all operating in a single crystal domain appears to exert a favourable influence on the catalytic activity and storage stability of the cell.

0039Although the manganese oxide obtained by heat-treating v-MnOOH shows stable catalytic characteristics even after prolonged storage, its X-ray diffraction pattern after storage is not necessarily identical with the initial pattern, but may change as shown in Figure 8. This is because the manganese oxide is further stabilized in alkaline medium.

0040Thus, an air cell having a catalyst electrode employing the manganese oxide of the present invention has a high retention voltage at high-rate discharge and has excellent storage characteristics compared with cells employing conventional manganese oxide or manganese oxyhydroxide as the catalyst.