JP4642342B2 - Proton conductor and fuel cell - Google Patents

Description

  The present invention relates to a proton conductor that exhibits good ionic conductivity even at an operating temperature of 100 ° C. or more and 300 ° C. or less without humidity or a relative humidity of 50% or less, and a fuel cell using the proton conductor.

An ion conductor in which ions move by applying a voltage is known. This ion conductor is widely used as an electrochemical device such as a battery or an electrochemical sensor.
For example, in a fuel cell, from the viewpoints of power generation efficiency, system efficiency, and long-term durability of components, it is satisfactory under an operating temperature of about 100 ° C. to 300 ° C. under non-humidified or low humidified operating conditions of 50% or less relative humidity. Proton conductors that have stable proton conductivity for a long period of time are desired. In the development of a conventional polymer electrolyte fuel cell, it has been studied in view of the above requirements, but perfluorocarbon sulfonic acid membranes have sufficient proton conduction at an operating temperature of 100 ° C. to 300 ° C. and a relative humidity of 50% or less. There was a drawback that it was not possible to obtain characteristics and output.
In addition, a material containing a proton conductivity-imparting agent (for example, see Patent Document 1), a material using a silica dispersion film (for example, see Patent Document 2), or a material using an inorganic mono-organic composite film ( For example, see Patent Document 3), using a phosphate-doped graft membrane (see, for example, Patent Document 4), or using an ionic liquid composite membrane (see, for example, Patent Document 5 and Patent Document 6). However, in any case, under the operating temperature of 100 ° C. or more and 300 ° C. or less and the usage environment of relative humidity 50% or less, sufficient pton conductivity cannot be stably exhibited for a long period of time.
In addition, in the phosphoric acid fuel cell, solid oxide fuel cell, and molten salt fuel cell, since the operating temperature greatly exceeds 300 ° C., there is a problem in the long-term stability of the constituent members, etc. It does not satisfy the requirements from. Therefore, in order to satisfy the above requirements, those using sol-gel porous glass (for example, see Patent Document 7), those using phosphate hydrogel (for example, see Patent Document 8), and the like are studied. However, proton conductivity and long-term stability are still not sufficient.
Japanese Patent Laid-Open No. 2001-035509 Japanese Patent Laid-Open No. 06-1111827 JP 2000-090946 A JP 2001-213987 A JP 2001-167629 A JP 2003-123791 A JP 2002-097272 A JP 2003-217339 A

From the viewpoint of power generation efficiency of fuel cells, system efficiency, and long-term durability of components, good proton conductivity at operating temperatures of about 100 ° C to 300 ° C under non-humidified or low humidified operating conditions of 50% or less relative humidity Although a proton conductor that stably stabilizes the long term is desired, it is difficult to achieve with a conventional technique, and sufficient performance has not been obtained yet.
The present invention has been made in order to solve the above-mentioned problems, and exhibits good proton conductivity stably for a long period of time at an operating temperature of about 100 ° C. to 300 ° C. under non-humidified conditions or operating conditions of 50% or less relative humidity. It is an object of the present invention to provide a proton conductor that can be used and a fuel cell using the same.

In order to solve the above problems, the proton conductor of the present invention is a proton conductor comprising (1) condensed phosphoric acid, (2) phosphate ion, (3) metal ion, and (4) proton-coordinating molecule. .
This proton conductor can exhibit good proton conductivity stably for a long period of time at an operating temperature of about 100 ° C. to 300 ° C. under the operating conditions of no humidification or a relative humidity of 50% or less.

In the proton conductor of the present invention, the ratio of the (1) condensed phosphate and (2) phosphate ion is in the range of 10:90 to 90:10 in terms of P ₂ O ₅ , and the (3) metal ion Is in the range of 10 to 90 mol% with respect to the number of moles of the (1) condensed phosphoric acid and (2) phosphate ion in terms of P ₂ O ₅ , and (1) the condensed phosphoric acid, (2 It is preferable that the ratio of the total weight of the phosphate ions and (3) metal ions to the weight of the (4) proton-coordinating molecule is in the range of 10:90 to 90:10.

Proton coordination molecules used in the proton conductor of the present invention include imidazole, imidazole derivatives, imidazolium salts, imidazolium derivatives, pyridine, pyridine derivatives, pyridinium salts, pyridinium derivative salts, tertiary alkyl ammonia, quaternary alkyl. Ammonium salts can be used.
In addition, the proton-coordinating molecule used in the present invention may be one obtained by adding water to the above substance.
When these proton coordination molecules are used, proton conductivity is stabilized at an operating temperature of about 100 ° C. to 300 ° C., and stability is maintained for a long time.

The fuel cell of the present invention is a fuel cell using the proton conductor of the present invention as an electrolyte membrane.
The fuel cell of the present invention using the proton conductor of the present invention as an electrolyte membrane exhibits good proton conductivity over a long period of time at 100 ° C. to 300 ° C. and a relative humidity of 50% or less. .

  According to the present invention, it is possible to obtain a proton conductor that stably exhibits good proton conductivity for a long period of time under an operating condition of no humidification or a relative humidity of 50% or less at an operating temperature of about 100 ° C. to 300 ° C. If this proton conductor is used as an electrolyte membrane, it is possible to obtain a fuel cell that stably exhibits good proton conductivity for a long period of time even in a high temperature use environment such as an automobile. Can be expanded.

The condensed phosphoric acid used in the present invention is a product obtained by dehydrating and condensing phosphoric acid, and there is no limitation on the degree of condensation and molecular weight. Condensed phosphoric acid is obtained by the usual method for producing phosphate glass.
The phosphate ion used in the present invention is a trivalent anion constituting normal phosphate.
The metal ion used in the present invention is a metal ion that can constitute a phosphate glass, and is an alkaline earth metal ion such as calcium ion or magnesium ion, or a metal ion such as zinc ion, copper ion, or iron ion. In the proton conductor of the present invention, it is sufficient that at least one kind of these metal ions is contained.

The proton-coordinating molecule used in the present invention is a molecule having an unshared electron pair capable of arranging a proton in the molecule, and more specifically, one or more nitrogen atoms and oxygen atoms in the molecular structure. It is a contained molecule. As an example, ammonia, pyridine, imidazole, oxazole, benzimidazole, benzoxazole, thiol and the like can be opened. The proton conductor of the present invention only needs to contain at least one of these proton coordination molecules. Among these proton coordination molecules, in particular, imidazole, imidazole derivatives, imidazolium salts, imidazolium derivative salts, pyridine, pyridine derivatives, pyridinium salts, pyridinium derivative salts, tertiary alkyl ammonia, quaternary alkyl ammonium salts are easy to handle. Excellent in terms of durability, performance and cost.
In addition, it is preferable to use water and a proton-coordinating molecule other than water as a proton-coordinating molecule from the viewpoint of moldability and cost, and particularly water and imidazole, imidazole derivatives, imidazolium salts, imidazolium derivatives. Combining one or more of salts, pyridine, pyridine derivatives, pyridinium salts, pyridinium derivative salts, tertiary alkyl ammonia, and quaternary alkyl ammonium salts is excellent from the viewpoints of handleability, durability, performance, and cost.

The proton conductor of the present invention is composed of a proton conductor composed of (1) condensed phosphate, (2) phosphate ion, (3) metal ion, and (4) proton-coordinating molecule. The preferred blending ratio of the components is as follows.
The (1) condensed phosphoric acid and (2) phosphoric acid ion have a ratio of (1) condensed phosphoric acid and (2) phosphoric acid ion in the range of 10:90 to 90:10 in terms of P ₂ O ₅ , preferably It is preferably in the range of 1: 4 to 4: 1. If there is more condensed phosphoric acid than this range, proton conductivity will be impaired, and if it is less, handling will be impaired.

The amount of metal ions, relative to the total mole number of the condensed phosphoric acid and phosphate ions in terms _of P ₂ O _5, ranges amount of 10 to 90 mol% of metal ions, preferably 20 to 80 mol% It is preferable to be in the range. When there are more metal ions than this range, proton conductivity is impaired, and when it is less, handling properties are impaired.

  Furthermore, the blending amount of the proton-coordinating molecule is such that the ratio of the total weight of the condensed phosphoric acid, phosphate ion and metal ion to the weight of the proton-coordinating molecule ranges from 10:90 to 90:10, preferably 1. : It is preferable that it exists in the range of 4-4: 1. If the blending amount of the proton-coordinating molecule is larger than this range, the proton conductivity is impaired, and if it is less, the long-term stability of the proton conductivity is impaired.

  The proton conductor of the present invention is composed of (1) condensed phosphate, (2) phosphate ion, (3) metal ion, and (4) proton-coordinating molecule, but its production method is particularly limited. In addition to being able to be obtained by mixing and stirring each component, it is preferable from the viewpoint of quality, cost, etc. to manufacture using two processes of condensation vitrification and gelation.

Next, the fuel cell of the present invention uses the proton conductor as an electrolyte membrane.
As is well known, a fuel cell has a structure in which an electrolyte membrane is sandwiched between a negative electrode (hydrogen electrode) and a positive electrode (oxygen electrode). An external circuit is connected to the negative electrode and the positive electrode via lead wires. On the negative electrode side, a cell having an inlet for introducing hydrogen gas (H ₂ ) and an outlet for discharging fuel gas is provided. On the positive electrode side, a cell having an inlet for introducing oxygen gas (O ₂ ) and an outlet for discharging (oxygen + water) is provided. In the fuel cell having such a configuration, hydrogen gas is supplied from the negative electrode side inlet and oxygen gas is supplied from the positive electrode side inlet to the respective outlets, and protons are moved between the two electrodes via the electrolyte membrane for discharging. is doing.
In the fuel cell of the present invention, the proton conductor of the present invention is used as an electrolyte membrane.

Preferred embodiments of the present invention will be described below based on examples.
The ion conductivity was measured by the following method.
The electrolyte membrane was sandwiched and fixed by a self-gold electrode having a diameter of 1 mm to obtain a measurement cell. This cell was conditioned for 24 hours in a thermostatic bath at 150 ° C., and thereafter impedance measurement was performed by an alternating current method. The measurement conditions at this time were a measurement frequency of 1 MHz to 0.1 Hz and a voltage amplitude of 50 mV. From the Cole-Cole plot of this measurement result, the value of Z ′ when Z ″ = 0 was taken as the membrane resistance, and the ionic conductivity was determined by calculation.

Example 1
23 g of normal phosphoric acid (purity 85%), 10 g of calcium carbonate, and 20 cc of water were weighed and mixed. This was dried at 100 ° C. for 24 hours, and then heated in an electric furnace at 1500 ° C. for 2 hours. After heating, the melt was cooled and solidified to obtain calcium phosphate glass. After this calcium phosphate glass was pulverized, an equivalent amount of an imidazole aqueous solution (20 wt%) was added as a proton-coordinating molecule to obtain a gel-like substance.
When this gel-like substance was analyzed by ³¹ PMAS-NMR, it was confirmed that the phosphoric acid molecules existed as a condensed phosphate structure and a normal phosphate ion structure with a difference in chemical shift. When this gelled substance was used as a proton conductor and the ionic conductivity was measured at 150 ° C., it was 7 × 10 ⁻² S / cm. Furthermore, when it hold | maintained in a 150 degreeC thermostat for 100 hours and the ionic conductivity was measured, the ionic conductivity was ^{7x10 <-2} > S / cm. The gel proton conductor was sandwiched between commercially available fuel cell electrodes (Electrochem) to form a membrane electrode assembly, and the fuel cell was constructed with hydrogen / air under conditions of 150 ° C. and no humidification. However, a terminal voltage of 0.66 V was obtained at a current density of 0.3 A / cm ² .

(Examples 2 to 10)
A proton conductor was obtained in the same manner as in Example 1 by adding the substances shown in Table 1 instead of imidazole as a proton-coordinating molecule. The ionic conductivity of this proton conductor was measured under the same temperature conditions as in Example 1. Moreover, the ion conductivity after hold | maintaining with a 150 degreeC thermostat for 100 hours was measured. Further, a fuel cell similar to that of Example 1 was constructed using this proton conductor, and the current density and the terminal voltage were measured. These results are summarized in Table 1.

(Comparative Example 1)
23 g of normal phosphoric acid (purity: 85%), 10 g of calcium carbonate, and 20 cc of water were weighed and mixed with a stem. This was dried at 100 ° C. for 24 hours, and then heated in an electric furnace at 1500 ° C. for 2 hours. After heating, the melt was cooled and solidified to obtain calcium phosphate glass. After the calcium phosphate glass was pulverized, an equal amount of water was added to obtain a gel-like substance. When this gel-like substance was analyzed by ³¹ PMAS-NMR, it was confirmed from the difference in chemical shift that the phosphoric acid element was present as a condensed phosphate structure and a normal phosphate ion structure. The ionic conductivity of this gel-like material as a proton conductor was 6 × 10 -3 ^{S / cm} was measured at 0.99 ° C.. Further, when the ionic conductivity was measured while being held in a thermostatic bath at 150 ° C. for 100 hours, it was found to be 2 × 10 ⁻⁶ S / cm, and it was revealed that there was a problem in the long-term stability of the ionic conductivity.
Further, a fuel cell similar to that of Example 1 was constructed using this proton conductor and the current density and the terminal voltage were measured. As a result, the terminal voltage was 0.24 V at a current density of 0.3 A / cm ² . It was.

According to the present invention, high ionic conductivity is obtained even in a high temperature use environment of 100 ° C. or more and about 300 ° C., and the ionic conductivity is stable over a long period of time.

Claims (4)

A proton conductor comprising (1) condensed phosphoric acid, (2) phosphate ion, (3) metal ion, and (4) proton-coordinating molecule ,
(3) The metal ion includes at least one selected from the group consisting of alkaline earth metal ions, zinc ions, copper ions and iron ions,
The (4) proton coordination molecule is any one of imidazole, imidazole derivative, imidazolium salt, imidazolium derivative, pyridine, pyridine derivative, pyridinium salt, pyridinium derivative salt, tertiary alkyl ammonia, and quaternary alkyl ammonium salt. A proton conductor characterized in that it is a kind . The ratio of (1) condensed phosphoric acid and (2) phosphate ion is in the range of 10:90 to 90:10 in terms of P ₂ O ₅ , and the amount of (3) metal ion in terms of P ₂ O ₅ It is in the range of 10 to 90 mol% with respect to the number of moles of (1) condensed phosphate and (2) phosphate ion, and (1) condensed phosphate, (2) phosphate ion, and (3) metal 2. The proton conductor according to claim 1, wherein the ratio of the total weight of ions to the weight of the (4) proton-coordinating molecule is in the range of 10:90 to 90:10. The proton conductor according to claim 1, wherein water is further added to the proton coordination molecule. A fuel cell using the proton conductor according to any one of claims 1 to 3 as an electrolyte.