Anion conducting material and cell

Abstract

The present invention aims to provide an anion conducting material having excellent anion conductivity and durability, which can be suitably used as a separator, an electrolyte, or an electrode protecting agent of an alkaline cell, for example. The present invention also aims to provide a cell including a cell component containing the anion conducting material. The present invention provides an anion conducting material containing a polymer and a compound containing at least one element selected from Groups 1 to 17 of the periodic table.

Claims

1. A cell component comprising an anion conducting material selectively permeable to ions in solution, the anion conducting material comprising: a polymer, and magnesium hydroxide, wherein the magnesium hydroxide is 3% by mass or more of the anion conducting material, and the cell component is at least one selected from the group consisting of a separator, a positive electrode, a negative electrode, and an electrolyte.

2. The cell component according to claim 1, wherein the polymer contains at least one selected from the group consisting of an aromatic group, a halogen atom, a carboxyl group, a carboxylate group, a hydroxyl group, an amino group, and an ether group, or the polymer is a hydrocarbon.

3. A cell comprising the cell component as defined in claim 1.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1 is a graph showing the results of an impedance test performed on an anion conducting material of Example 1.

(2) FIG. 2 is a graph showing the results of an impedance test performed on an anion conducting material of Comparative Example 1.

(3) FIG. 3 is a graph showing the results of a charge/discharge test in Example 2, indicating the charge curve of the 10th cycle and the discharge curve of the 10th cycle.

(4) FIG. 4 is a cross-sectional schematic view of a cell of Example 10.

(5) FIG. 5 is a graph showing the results of a charge/discharge test obtained as Example 12 and Comparative Example 2.

(6) FIG. 6 is a graph showing discharge rate characteristics in Example 13.

(7) FIG. 7 is a graph showing discharge rate characteristics in Comparative Example 3.

DESCRIPTION OF EMBODIMENTS

(8) The present invention is described in further detail below with reference to examples, but the present invention is not limited to these examples.

Example 1

(9) Zinc oxide (27.6 g), cerium(IV) oxide (2.4 g), ethanol (99.5%) (92.7 g), and water (92.7 g) were added and mixed in a ball mill. Then, the mixture was dried using an evaporator under reduced pressure at 100 C. for two hours, and further dried using a stationary-type vacuum dryer at 110 C. overnight. The dried solid was pulverized at 18000 rpm for 60 seconds using a pulverizer (X-TREME MX1200XTM available from WARING). The resulting solid (1.1 g), a solution of 12% polyvinylidene fluoride in N-methylpyrrolidone (2.0 g), and N-methylpyrrolidone (0.90 g) were placed in a glass vial and stirred overnight using a stirrer with a stir bar. The resulting slurry was applied to copper foil using an automatic coating device, and dried at 80 C. for 12 hours. The copper foil coated with the zinc mixture was pressed at 3 t, and then punched out using a punching device (diameter: 15.95 mm).

(10) The copper foil was peeled from the material to obtain a membranous compound. The membranous compound was immersed in a saturated solution of zinc oxide in an aqueous solution of 4 mol/L potassium hydroxide, and then placed on copper foil again. A zinc mixture electrode was thus obtained and used as a working electrode (zinc mixture weight: 0.98 mg) having an apparent area of 0.48 cm.sup.2.

(11) The saturated solution of zinc oxide in an aqueous solution of 4 mol/L potassium hydroxide was used as the electrolytic solution; a zinc plate was used as the counter electrode, and a zinc wire was used as the reference electrode. Then, a charge/discharge test was performed for 10 cycles using the three-electrode cell at a current of 0.64 mA/cm.sup.2 (charge and discharge times: one hour for each). In this manner, zinc oxide was removed, pores were formed, and an anion conducting material was produced.

(12) The anion conducting material was peeled from the copper foil, and placed on new copper foil again. Then, a charge/discharge test (charge and discharge times: one hour for each) was performed for 40 cycles under the same conditions. An impedance test was performed every 10 cycles. The results showed that the resistance did not change at all (FIG. 1). FIG. 1 is a graph showing the results of the impedance test performed on the anion conducting material of Example 1. Observation with a scanning electron microscope (SEM) confirmed that zinc dissolved and deposited (electrodeposited) and that there were no changes in the anion conducting material.

Comparative Example 1

(13) Zinc oxide (27.6 g), ethanol (99.5%) (92.7 g), and water (92.7 g) were added and mixed in a ball mill. Then, the mixture was dried using an evaporator under reduced pressure at 100 C. for two hours, and further dried using a stationary-type vacuum dryer at 110 C. overnight. The dried solid was pulverized at 18000 rpm for 60 seconds using a pulverizer (X-TREME MX1200XTM available from WARING). The resulting solid (1.1 g), a solution of 12% polyvinylidene fluoride in N-methylpyrrolidone (2.0 g), and N-methylpyrrolidone (0.90 g) were placed in a glass vial and stirred overnight using a stirrer with a stir bar. The resulting slurry was applied to copper foil using an automatic coating device, and dried at 80 C. for 12 hours. The copper foil coated with the zinc mixture was pressed at 3 t, and then punched out using a punching device (diameter: 15.95 mm).

(14) The copper foil was peeled from the material to obtain a membranous compound. The membranous compound was immersed in a saturated solution of zinc oxide in an aqueous solution of 4 mol/L potassium hydroxide, and then placed on copper foil again. A zinc mixture electrode was thus obtained and used as a working electrode (zinc mixture weight: 1.43 mg) having an apparent area of 0.48 cm.sup.2.

(15) The saturated solution of zinc oxide in an aqueous solution of 4 mol/L potassium hydroxide was used as the electrolytic solution; a zinc plate was used as the counter electrode, and a zinc wire was used as the reference electrode. Then, a charge/discharge test was performed for 10 cycles using the three-electrode cell at a current of 0.93 mA/cm.sup.2 (charge and discharge times: one hour for each). In this manner, zinc oxide was removed, pores were formed, and an anion conducting material was produced.

(16) The anion conducting material was peeled from the copper foil, and placed on new copper foil again. Then, a charge/discharge test (charge and discharge times: one hour for each) was performed for 40 cycles under the same conditions. An impedance test was performed every 10 cycles. The results showed an increase in the resistance (FIG. 2). FIG. 2 is a graph showing the results of the impedance test performed on the anion conducting material of Comparative Example 1. Observation with a scanning electron microscope (SEM) confirmed that zinc dissolved and deposited (electrodeposited) and that zinc species that deposited on the copper foil underwent passivation. Passivation is considered to be caused by shortage of hydroxide ions, i.e., lack of anion conductivity in the anion conducting material of Comparative Example 1.

Example 2

(17) An aqueous solution (11 mg) of 60% polytetrafluoroethylene and water were added to zinc oxide (149 mg) and mixed well with an agate mortar. The resulting zinc oxide paste was applied to a copper mesh (50 mesh) having a diameter of 14 mm and pressure-bonded at a pressure of 6 kN. Thus, an active material layer (A) was obtained. Separately, polytetrafluoroethylene (3.2 g) and water were added to hydrotalcite (2.5 g) and mixed well with an agate mortar. The resulting hydrotalcite paste was rolled to a thickness of 1 mm to obtain an anion conducting material. Then, the anion conducting material was punched out to a diameter of 14 mm to obtain an electrode protecting agent (B). Subsequently, the electrode protecting agent (B) was pressure-bonded to the active material layer (A) at a pressure of 6 kN. Thus, a zinc mixture electrode (C) containing an anion conducting material was obtained. This electrode (C) was used as the working electrode (zinc mixture weight: 79 mg) having an apparent area of 0.79 cm.sup.2.

(18) An air electrode with air holes (QSI-Nano manganese gas diffusion electrode available from TOMOE ENGINEERING CO., LTD.) was used as the counter electrode; a saturated solution of zinc oxide in an aqueous solution of 8 mol/L potassium hydroxide was used as the electrolytic solution. Then, a charge/discharge test was performed on the zinc-air storage cell (two-electrode cell) at a current of 5 mA (charge and discharge times: 20 minutes for each, cut off at 2.0 V and 0.5 V). FIG. 3 is a graph showing the results of the charge/discharge test in Example 2, indicating the charge curve of the 10th cycle and the discharge curve of the 10th cycle. The cell can be evaluated as being capable of stably performing charging and discharging. Observation with a field emission scanning electron microscope (FE-SEM) confirmed that at least 50 charge/discharge cycles can be stably performed and that components of the zinc electrode active material did not penetrate into the anion conducting layer.

Example 3

(19) Palladium chloride (0.1 g) and a trace amount of concentrated hydrochloric acid were dissolved in water (250 mL), and two pieces of stainless steel wire mesh (9 cm in height and 9 cm in width) were immersed therein for one hour for plating. Thus, an electrode was produced. Polytetrafluoroethylene (3.2 g), an aqueous solution of 2% sodium acrylate (0.3 g), and water were added to hydrotalcite (2.5 g), and mixed well with an agate mortar. The resulting hydrotalcite paste was rolled to a thickness of 2 mm to obtain an anion conducting material, and the anion conducting material was cut out to 10 cm in height and 10 cm in width. Then, the electrode was applied to both sides of the anion conducting material to obtain a cell. The anion conducting material was wetted with an aqueous solution of 1 mol/L potassium hydroxide. Subsequently, hydrogen was supplied to one electrode and oxygen was supplied to the other electrode to confirm generation of electricity by the cell as a fuel cell with an ammeter and voltmeter.

Example 4

(20) An experiment was performed as in Example 3 except that a mixture of hydrotalcite, cerium oxide, and polytetrafluoroethylene in a mass ratio of 4:1:6 was used as the anion conducting material. Operation of the fuel cell was confirmed.

Example 5

(21) An experiment was performed as in Example 3 except that a mixture of hydrotalcite, niobium oxide, and polytetrafluoroethylene in a mass ratio of 4:1:6 was used as the anion conducting material. Operation of the fuel cell was confirmed.

Example 6

(22) An experiment was performed as in Example 3 except that a mixture of cerium oxide and polytetrafluoroethylene in a mass ratio of 4:6 was used as the anion conducting material. Operation of the fuel cell was confirmed.

Example 7

(23) An experiment was performed as in Example 3 except that a mixture of ettringite and polytetrafluoroethylene in a mass ratio of 4:6 was used as the anion conducting material. Operation of the fuel cell was confirmed.

Example 8

(24) An experiment was performed as in Example 3 except that a mixture of hydrotalcite, ethylenimine, and polytetrafluoroethylene in a mass ratio of 4:0.5:6 was used as the anion conducting material. Operation of the fuel cell was confirmed.

Example 9

(25) An experiment was performed as in Example 3 except that a mixture of hydrotalcite, sodium polyacrylate, and polytetrafluoroethylene in a mass ratio of 4:0.2:6 was used as the anion conducting material. Operation of the fuel cell was confirmed.

Example 10

(26) For implementation of the present invention, a cell having a structure shown in FIG. 4 was formed and a charge/discharge cycle test was performed as described later. In Example 10, a zinc oxide active material layer (i.e., an anode active material) was pressure-bonded to a copper mesh current collector. The resulting product was covered with an anion conducting material to produce a zinc anode containing the anion conducting material. At this point, Zn(OH).sub.4.sup.2 ions can be effectively trapped in the anion conducting material of the anode to suppress diffusion of the ions. A mixture of hydrotalcite and polytetrafluoroethylene in amass ratio of 4:6 was used as an anion conducting material 10. Polytetrafluoroethylene is preferred because (1) it is an insulating material; (2) it allows powder of the anion conducting material to bind together; and (3) it has excellent physical strength.

(27) While disposing the thus-produced anion conducting material 10 on the zinc anode, a 2% aqueous solution of sodium polyacrylate was applied between the zinc anode and the anion conducting membrane to enhance adhesion.

(28) The zinc anode containing the anion conducting material produced above was used as the anode; a nickel electrode was used as the cathode; and the same electrode used as the cathode was charged to 50% and used as the reference electrode. A nonwoven fabric was disposed between the cathode and the anode, and a saturated solution of zinc oxide in an aqueous solution of 8 mol/L potassium hydroxide was used as the electrolytic solution. In this manner, a three-electrode cell was produced, and a charge/discharge cycle test was performed with an electrode area of 1.95 cm.sup.2 and at a current of 25 mA/cm.sup.2 (charge and discharge times: one hour for each). At this point, the coulombic efficiency did not decrease, and at least 200 charge/discharge cycles were stably performed.

Example 11

(29) The anion conducting material 10 was produced as in Example 10. Further, the anion conducting material was rolled with a nonwoven fabric to enhance its physical strength, and the resulting product was disposed on the zinc anode. The zinc electrode containing the anion conducting material produced above was used as the anode; a nickel electrode was used as the cathode; and the same electrode used as the cathode was charged to 50% and used as the reference electrode. A nonwoven fabric was disposed between the cathode and the anode, and an aqueous solution of 8 mol/L potassium hydroxide saturated with zinc oxide was used as the electrolytic solution. In this manner, a three-electrode cell was produced, and a charge/discharge cycle test was performed. The electrode area was 1.95 cm.sup.2 and the current was 25 mA/cm.sup.2 (charge and discharge times: one hour for each). At this point, the coulombic efficiency did not decrease, and at least 200 charge/discharge cycles were stably performed.

(30) The anion conducting membrane was rolled with a nonwoven fabric either on one side or both sides of the anion conducting membrane. A similar cycle life was observed in both cases. A similar cycle life was also observed in the case where an aqueous solution of 2% sodium polyacrylate was applied between the anion conducting membrane and the nonwoven fabric to enhance adhesion between the anion conducting membrane and the nonwoven fabric.

Example 12 and Comparative Example 2

(31) A zinc anode was produced as in Example 10.

(32) The zinc electrode containing the anion conducting material produced above was used as the anode; a nickel electrode was used as the cathode; and the same electrode used as the cathode was charged to 50% and used as the reference electrode. A nonwoven fabric was disposed between the cathode and the anode, and an aqueous solution of 8 mol/L potassium hydroxide saturated with zinc oxide was used as the electrolytic solution. In this manner, a three-electrode cell was produced, and a charge/discharge cycle test was performed. The electrode area was 1.95 cm.sup.2 and the current was 25 mA/cm.sup.2 (charge and discharge times: one hour for each). FIG. 5 shows potential for charge/discharge capacity as Example 12.

(33) Meanwhile, a three-electrode cell was produced into which two hydrophilic microporous membranes were inserted instead of the anion conducting membrane, and a comparative experiment was performed. As a result, an increase in ohmic loss was observed due to an increase in the number of resistance components. The potential for charge/discharge capacity of the cell produced as in Example 12 except that two hydrophilic microporous membranes were inserted instead the anion conducting membrane is shown as Comparative Example 2 in FIG. 5.

Example 13 and Comparative Example 3

(34) A zinc anode was produced as in Example 10.

(35) The zinc electrode containing the anion conducting material produced above was used as the anode; a nickel electrode was used as the cathode; and the same electrode used as the cathode was charged to 50% and used as the reference electrode. A nonwoven fabric was disposed between the cathode and the anode, and an aqueous solution of 8 mol/L potassium hydroxide saturated with zinc oxide was used as the electrolytic solution. In this manner, a three-electrode cell was produced, and a charge/discharge cycle test was performed. The electrode area was 1.95 cm.sup.2. Meanwhile, a three-electrode cell was produced into which two hydrophilic microporous membranes were inserted instead of the anion conducting membrane. To perform a comparative experiment on these structures for discharge rate characteristics, 5 charge/discharge cycles were performed for each of the following discharge rates: 0.25 C, 0.5 C, 0.75 C, 1 C, 2 C, and 5 C, while the charge rate was fixed at 25 mA/cm.sup.2 (0.25 C). FIG. 6 is a graph showing discharge rate characteristics of the three-electrode cell in which the anion conducting membrane was used (Example 13). FIG. 7 is a graph showing discharge rate characteristics of the three-electrode cell in which two hydrophilic microporous membranes were used (Comparative Example 3).

(36) As a result, there was almost no difference in the discharge rate characteristic between when the anion conducting membrane was used and when the hydrophilic microporous membranes were used; however, a decrease in the discharge capacity was observed upon the shift in the discharge rate due the membrane potential. Effects of the membrane potential were compared. As a result, the effect of the membrane potential was observed to be smaller when the anion conducting membrane was used (FIG. 6) than when the hydrophilic microporous membranes were used (FIG. 7).

(37) The electrodes of Examples 10 to 13 have the above-described structures. In the cells including the electrodes of Examples 10 to 13, anions such as hydroxide ions involved in cell reaction can easily pass through the anion conducting material covering the active material and/or the active material layer. The anion conducting material sufficiently contributes to excellent cell performance, sufficiently prevents diffusion of metal ions, and sufficiently suppresses short circuits caused by dendrites even after repeated charge/discharge cycles.

(38) The results of the examples revealed the following findings.

(39) The anion conducting material containing a polymer and a compound containing at least one element selected from Groups 1 to 17 of the periodic table demonstrated excellent anion conductivity and durability, and proved that it can be suitably used, for example, as a component such as a separator, an electrolyte, or an electrode protecting agent of a cell such as an alkaline cell.

(40) In Example 1, the polymer was a specific fluorine-based polymer, and the inorganic compound was a compound containing a specific element. Yet, in any case, the anion conducting material of the present invention has excellent anion conductivity and durability and can be suitably used, for example, as a separator of an alkaline cell, as long as the anion conducting material contains a polymer and an inorganic compound.

(41) In particular, as shown in Example 1, zinc dissolved and deposited (electrodeposited) and pores were formed in the anion conducting material. This advantageously improved the anion conductivity. For use in a fuel cell or an air cell, the anion conducting material without pores is considered to be preferred for minimizing crossover. For example, the anion conducting material without pores can be produced by not adding zinc oxide in advance. In the above examples, pores were formed by repeatedly performing a charge/discharge test. Yet, pores may be formed by methods other than the repeated charge/discharge test, as long as pores can be formed by dissolving or removing soluble particles such as zinc oxide. For example, in the case of the anion conducting material containing particles soluble in a basic solvent, pores may be formed by dissolving or removing the particles by washing the anion conducting material with a basic solvent.

(42) In Example 2, in the use of the anion conducting material containing a polymer and an inorganic compound as a protective agent for protecting the active material layer, the polymer was polytetrafluoroethylene, and the inorganic compound was hydrotalcite. Yet, the following effects can be achieved in any case when the anion conducting material (membrane) is formed on the electrode such as a zinc compound-containing electrode, as long as the membrane composition is controlled in such a manner that the anion conducting material at least contains a polymer and a layered double hydroxide as an inorganic compound: a change in the form of an active material such as a zinc electrode active material is sufficiently suppressed even after the passage of a current; and the anion conducting material is conductive and permeable to hydroxide ions but is only minimally conductive and permeable to zinc-containing ions such as [Zn(OH).sub.4].sup.2 so that the anion conducting material is conductive and permeable to only specific anions. Such an anion conducting material can also be suitably used as a cell component such as a separator and an electrolyte in addition to an electrode protecting agent.

(43) Further, in Example 3, in the use of the anion conducting material containing a polymer and an inorganic compound as an electrolyte of a cell, the polymer was polytetrafluoroethylene, and the inorganic compound was hydrotalcite. Yet, electricity can be generated in any case by the fuel cell that uses the anion conducting material as the electrolyte, as long as the anion conducting material contains a polymer and an inorganic compound. Such an anion conducting material can also be suitably used as a cell component such as a separator and an electrode protecting agent in addition to an electrolyte. The applicable cell is not limited to a fuel cell. The anion conducting material can be suitably used in various cells such as alkaline cells in addition to fuel cells.

(44) Thus, the results of the above examples show that the present invention is applicable to the entire technical scope of the present invention and can be used in various forms disclosed herein, and that the present invention achieves advantageous effects.

REFERENCE SIGNS LIST

(45) 10: anion conducting material 11, 21: current collector 13, 23: active material layer 30: separator