JP4517398B2 - Electrolytes - Google Patents

Description

  The present invention relates to an electrolyte. More specifically, the present invention is used in various electrochemical devices such as fuel cells, water electrolysis apparatuses, hydrohalic acid electrolysis apparatuses, salt electrolysis apparatuses, oxygen and / or hydrogen concentrators, humidity sensors, and gas sensors. The present invention relates to an electrolyte suitable as an electrolyte membrane, an electrolyte in a catalyst layer, and the like.

  Generally, solid polymer electrolytes are used in various electrochemical devices such as fuel cells and water electrolysis devices. In this case, the solid polymer electrolyte is used in the form of a membrane electrode assembly (MEA) in which a membrane is formed and electrodes are bonded to both surfaces thereof. In the polymer electrolyte fuel cell, the electrode generally has a two-layer structure of a diffusion layer and a catalyst layer. The diffusion layer is for supplying reaction gas and electrons to the catalyst layer, and carbon fiber, carbon paper, or the like is used. The catalyst layer is a part that becomes a reaction field for electrode reaction, and is generally composed of a composite of an electrode catalyst and a solid polymer electrolyte.

The electrolyte membrane or the catalyst layer electrolyte that constitutes such an MEA includes a perfluorinated electrolyte excellent in oxidation resistance (an electrolyte that does not contain a C—H bond in the polymer chain. For example, Nafion (registered trademark, DuPont). ), Aciplex (registered trademark, manufactured by Asahi Kasei Co., Ltd.), Flemion (registered trademark, manufactured by Asahi Glass Co., Ltd.), etc.) are generally used.
In addition, perfluorinated electrolytes are excellent in oxidation resistance but are generally very expensive. Therefore, in order to reduce the cost of the solid polymer fuel cell, a hydrocarbon electrolyte (an electrolyte that includes a C—H bond and does not include a C—F bond in a polymer chain) or a partial fluorine electrolyte The use of (electrolytes containing both C—H bonds and C—F bonds in the polymer chain) has also been studied.

  However, in order to put the polymer electrolyte fuel cell into practical use as a vehicle-mounted power source, there remain problems to be solved. For example, hydrocarbon electrolytes are less expensive than perfluorinated electrolytes, but have a problem that they are easily degraded by peroxide radicals. On the other hand, perfluorinated electrolytes have better oxidation resistance than hydrocarbon based electrolytes. However, when the use conditions become severe, there is a problem that even perfluorinated electrolytes deteriorate.

  In order to solve this problem, various proposals have heretofore been made. For example, Patent Document 1 discloses a membrane electrode assembly in which a cerium-containing oxide is contained in a catalyst layer of an oxygen electrode. This document describes that by adding a cerium-containing oxide to the oxygen electrode, hydrogen peroxide generated at the oxygen electrode is oxidatively decomposed and deterioration of the electrolyte membrane due to radicals can be suppressed.

  Patent Document 2 discloses a solid polymer electrolyte in which an antioxidant is added to a fluorine-based electrolyte. In this document, when an antioxidant (particularly a phosphorus-based antioxidant) is added to a fluorine-based electrolyte, the generation of radicals is suppressed, the generated radicals are inactivated, and / or the peroxide is stable. It describes that the erosion of the fluorocarbon skeleton is suppressed because it decomposes into a new compound.

Further, although not an electrolyte, Patent Document 3 discloses that a coating liquid containing powdery metal-modified apatite and an inorganic coating liquid agent and having a metal-modified apatite content of 0.01 to 5 wt% is applied to a substrate. A method for forming a metal-modified apatite-containing film is disclosed. In this document, when a part of Ca of calcium hydroxyapatite (Ca ₁₀ (PO ₄ ) ₆ (OH) ₂ ) is replaced with Ti, it becomes a photocatalyst with high contact efficiency of the decomposition target, and in the coating liquid It is described that, when the metal-modified apatite is appropriately dispersed, sufficient antibacterial properties and antifouling properties can be obtained even when the content of the metal-modified apatite is low.

JP 2004-327074 A JP 2004-134294 A Japanese Patent Laid-Open No. 2003-334883

The methods disclosed in Patent Documents 1 and 2 are one of effective methods for suppressing deterioration of electrolyte membranes and electrolyte in the catalyst layer due to radicals. However, cerium-containing oxides and antioxidants have a relatively high solubility in water. Therefore, when used for a long time in an environment containing water, these are likely to elute out of the system. Moreover, depending on the kind of antioxidant, antioxidant itself may deteriorate with a peroxide radical. Therefore, a method for further improving the durability of the electrolyte membrane is desired.
Further, cerium-containing oxides and antioxidants generally do not exhibit proton conductivity. For this reason, if the amount of these additives added is increased in order to improve durability, proton conductivity may be reduced or the membrane may become brittle.

The problem to be solved by the present invention is to provide an electrolyte capable of maintaining durability against radicals over a long period of time.
Another object of the present invention is to provide an electrolyte having a high durability against radicals and having a relatively high proton conductivity at a low cost.

  In order to solve the above problems, an electrolyte according to the present invention is characterized in that it contains titania phosphate doped with Ca. In this case, the titania phosphate preferably has a phosphonic acid group in its skeleton.

  Ca-doped titania phosphate (particularly Ca-doped titania having a phosphonic acid group) exhibits relatively high proton conductivity. Further, when a part of Ti in titania phosphate is replaced with Ca, the hydrogen peroxide decomposition action is improved. Furthermore, Ca-doped titania phosphate has relatively low solubility in water and hot water. Therefore, if this is applied to, for example, an electrolyte membrane of a polymer electrolyte fuel cell or an electrolyte in a catalyst layer, the durability of the fuel cell can be improved.

Hereinafter, an embodiment of the present invention will be described in detail.
The electrolyte according to the present invention is characterized by including a metal oxide composed of phosphoric titania doped with Ca, that is, Ca-Ti-PO.
P contained in Ca-doped titania phosphate contributes to proton conduction. P is preferably introduced into the skeleton composed of Ti—O—Ti in the form of a phosphonic acid group (—PO (OH) ₂ ). P introduced in a form constituting a part of the skeleton hardly contributes to proton conduction, whereas the phosphonic acid group contributes to proton conduction. Moreover, since the phosphonic acid group also has a chelating supplement function for metal ions having the function of radically decomposing hydrogen peroxide, durability against hydrogen peroxide can be improved.
Further, Ca contained in the Ca-doped titania phosphate has an action of enhancing the function as a hydrogen peroxide decomposition catalyst possessed by the phosphate titania.

The proton conductivity and hydrogen peroxide decomposability of Ca-doped titania phosphate depend on its composition. Generally, as the Ca / P ratio (mass%) increases, proton conductivity and hydrogen peroxide decomposability improve. When Ca is added to titania phosphate, P bonds are not easily broken, and proton conductivity and hydrogen peroxide decomposability are improved. In order to obtain high characteristics, the Ca / P ratio is preferably 0.4 or more, more preferably 0.7 or more, still more preferably 0.8 or more, and still more preferably 0.9 or more.
On the other hand, when the Ca / P ratio is too large, the amount of P contained in the entire metal oxide is reduced, and conversely, proton conductivity and hydrogen peroxide decomposability are lowered. In order to obtain high characteristics, the Ca / P ratio is preferably 3.0 or less, more preferably 2.1 or less, still more preferably 1.8 or less, and still more preferably 1.5 or less.

In general, as the amount of substitution of Ca increases (that is, the Ti / Ca ratio (mass%) decreases), the hydrogen peroxide decomposability increases. In order to obtain high characteristics, the Ti / Ca ratio is preferably 0.70 or less, more preferably 0.65 or less, further preferably 0.50 or less, and further preferably 0.35 or less.
On the other hand, when the amount of substitution of Ca becomes too large, the hydrogen peroxide decomposability is reduced. In order to obtain high characteristics, the Ti / Ca ratio is preferably 0.05 or more, more preferably 0.11 or more, further preferably 0.13 or more, and further preferably 0.15 or more.

The specific surface area of the Ca-doped titania phosphate is an index indicating the size of the reaction field as a hydrogen peroxide decomposition catalyst and at the same time an index indicating the number of phosphonic acid groups that are conductive groups. In general, the higher the specific surface area, the more excellent the proton conductivity and the hydrogen peroxide decomposability. In order to obtain high characteristics, the specific surface area of the Ca-doped titania is preferably 30 m ² / g or more, more preferably 100 m ² / g or more, further preferably 150 m ² / g or more, more preferably 200 m ² / g. That's it.

The Ca-doped titania phosphate may be crystalline or amorphous. When the Ca-doped titania phosphate is crystalline, there are few nano-sized pores, so the water retention / water content decreases and the proton conductivity decreases. However, since the heat resistance is high, even when used at high temperatures, it exhibits a high hydrogen peroxide decomposition action.
On the other hand, when the Ca-doped titania is amorphous, the heat resistance is slightly lowered, but the water retention / water content is increased because there are many nano-sized pores. In addition, since the amount of conductive groups distributed in the surface layer inevitably increases, an electrolyte using water as a conductive medium exhibits high proton conductivity. Furthermore, since the reaction field with hydrogen peroxide increases, it shows a high hydrogen peroxide decomposition action.

Ca-doped titania phosphate is obtained as a powder when using the method described later. In addition, powdered Ca-doped titania can be added to the catalyst layer as it is. Furthermore, powdered Ca-doped titania can be combined with other components such as a binder (binder) to form a composite, which can be used as an electrolyte membrane or an electrolyte in a catalyst layer.
When the powder form is used as it is or as a composite, the particle shape is not particularly limited, and those having various shapes can be used according to the purpose. That is, the particle form may be any of spherical shape, plate shape, layer shape, fiber shape, columnar shape, tube shape, and the like.
Moreover, when using Ca dope titania as a composite, as other components, specifically,
(1) Elastomers such as SBS and PPS, triethoxysilyl, polyimide, polyamideimide, emulsion latex, resin such as liquid crystal polymer,
(2) Gels such as silica, titania hydroxide, ZnO, barium titanate, zirconia titanate, titanium,
(3) Polymer electrolytes such as various fluorine-based electrolytes and various hydrocarbon-based electrolytes represented by Nafion (registered trademark),
and so on.

Next, the manufacturing method of the electrolyte which concerns on this invention is demonstrated.
The electrolyte according to the present invention can be obtained by adding a second solution in which a P-containing compound is dissolved to a first solution in which a Ca-containing compound and a Ti-containing compound are dissolved. When a 2nd solution is added to a 1st solution, precipitation of the metal oxide comprised by Ca-Ti-PO can be obtained. At this time, it is not necessary to adjust the pH by adding an alkaline aqueous solution to the solution.

Specifically, calcium nitrate (Ca (NO ₃ ) ₂ ), calcium carbonate (CaCO ₃ ), calcium sulfate (CaSO ₄ ) and the like can be used as the Ca-containing compound. Specifically, titanium tetrachloride (TiCl ₄ ), titanium tetrasulfate (Ti (SO ₄ ) ₄ ), titanium sulfate (TiSO ₄ ), or the like can be used as the Ti-containing compound. Specifically, orthophosphoric acid (H ₃ PO ₄ ), phosphorus pentoxide (P ₂ O ₅ ), or the like can be used as the P-containing compound.
As the solvent of the first solution and the second solution, water is usually used, but a solvent other than water may be used as long as it can dissolve the Ca-containing compound, Ti-containing compound and P-containing compound.
The concentration of the Ca-containing compound and the Ti-containing compound contained in the first solution and the concentration of the P-containing compound contained in the second solution are appropriately selected so as to obtain the intended composition. .

When the obtained precipitate is repeatedly washed with ion-exchanged water until pH 6 and dried, Ca-doped titania having a desired composition is obtained.
Furthermore, when the obtained powder is molded and sintered into an appropriate shape, Ca-doped titania having a predetermined shape can be obtained. Moreover, the composite containing Ca dope titania is obtained if other components, such as resin, gel, and polymer electrolyte, are added to the obtained powder, and this is formed into a predetermined shape (for example, a film shape). Furthermore, a catalyst layer containing Ca-doped titania can be obtained by adding Ca-doped titania and, if necessary, a polymer electrolyte to the paste containing the catalyst, and applying this to a suitable substrate surface.

Next, the operation of the electrolyte according to the present invention will be described.
Phosphate salts such as titania phosphate (TiPO ₄ ), ceria phosphate (CePO ₄ ), calcium hydrogen phosphate (CaHPO ₄ ) all have a hydrogen peroxide decomposition action. In addition, since these phosphates all have a phosphonic acid group in the structure, they exhibit proton conductivity. However, the hydrogen peroxide decomposition ability of phosphate greatly varies depending on the phosphate composition.
Further, for example, ceria phosphate has a relatively high hydrogen peroxide decomposition ability, but is relatively easily decomposed in the presence of water containing hydrogen peroxide. Therefore, if ceria phosphate is continuously used in an environment containing water for a long time, P is eluted and it is difficult to maintain a high peroxide decomposition action for a long time. Moreover, the elution of P means the disappearance of the phosphonic acid group contributing to proton conduction, leading to a decrease in proton conductivity.

On the other hand, when a part of Ti of the titania phosphate is replaced with Ca, the hydrogen peroxide decomposition ability is improved as compared with the case of titania phosphate alone. Details of why this effect is available are unknown, but probably
(1) By combining Ti phosphate and Ca phosphate, the hydrogen peroxide decomposition action of both phases was enhanced by the interaction, and
(2) By doping Ca, it becomes easy to increase the specific surface area, and the reaction field of the hydrogen peroxide decomposition reaction is remarkably increased.
It is thought that.
Furthermore, when a part of Ti of titania phosphate is replaced with Ca, high hydrogen peroxide decomposition action and proton conductivity can be maintained over a long period of time. this is,
(1) Because the solubility of the Ca—Ti—PO compound itself is low, or
(2) Since the bond between the titania skeleton and P becomes more stable due to Ca doping,
This is probably because the elution of P is suppressed even when used for a long time in the presence of water.

Therefore, for example, if powdered Ca-doped titania is added to the electrolyte membrane or catalyst layer of a polymer electrolyte fuel cell, the electrolyte membrane is deteriorated by peroxide radicals without reducing the proton conductivity of the electrolyte membrane. Can be suppressed over a long period of time.
For example, when an electrolyte mainly composed of Ca-doped titania is applied to an electrolyte membrane or an electrolyte in a catalyst layer of a fuel cell, a fuel cell that can be operated at a high temperature exceeding 100 ° C. can be obtained.

Example 1
A predetermined amount of titanium tetrachloride and calcium nitrate were dissolved in water and stirred for several minutes to several tens of minutes or more. An aqueous solution containing a predetermined amount of orthophosphoric acid was added thereto to obtain a precipitate composed of Ca-doped titania phosphate (Ca—Ti—PO system oxide). The obtained precipitate was repeatedly washed with ion-exchanged water and heat-treated at 200 to 400 ° C. in the atmosphere.

(Comparative Examples 1 and 2)
Titania phosphate (TiPO ₄ ) was synthesized according to the same procedure as in Example 1 except that only titanium tetrachloride and orthophosphoric acid were used as starting materials (Comparative Example 1).
Further, ceria phosphate (CePO ₄ ) was synthesized according to the same procedure as in Example 1 except that only cerium nitrate and orthophosphoric acid were used as starting materials (Comparative Example 2).

  FIG. 1 shows the specific surface area of the synthesized sample. As can be seen from FIG. 1, the specific surface area of the sample doped with Ca in titania phosphate (Ca / P ratio = 0.91, 1.38) is significantly increased compared to titania phosphate and ceria phosphate. .

Next, the samples obtained in Example 1 and Comparative Examples 1 and 2 were evaluated for H ₂ O ₂ decomposability. That is, 0.1 g of the synthesized sample was added to 30 ml of 1 wt% H ₂ O ₂ solution, and ultrasonic mixing was performed for 1 minute. Subsequently, this was put into a polytetrafluoroethylene container and heat-treated at 100 ° C. × 1 hr. After the heat treatment, it was cooled with water and the remaining amount of H ₂ O ₂ was measured.
FIG. 2 shows the H ₂ O ₂ decomposition rate of each sample. From FIG.
(1) In the case where Ca is doped into titania phosphate, when the Ca / P ratio is optimized, the decomposition rate of H ₂ O ₂ is significantly improved.
(2) When the Ca / P ratio is 0.7 to 2.1, the decomposition rate of hydrogen peroxide is equal to or higher than that of TiPO ₄ .
(3) When the Ca / P ratio is 0.8 to 1.8, the hydrogen peroxide decomposition rate is equal to or higher than that of CePO ₄ , and
(4) It is understood that when the Ca / P ratio is 0.9 to 1.5, the hydrogen peroxide decomposition rate is approximately 80% or more.

FIG. 3 shows the relationship between the Ti / Ca ratio of Ca-doped titania phosphate and the hydrogen peroxide decomposition rate. From FIG.
(1) When the Ti / Ca ratio is 0.05 or higher, the hydrogen peroxide decomposition rate is equal to or higher than that of TiPO ₄ .
(2) When the Ti / Ca ratio is 0.11 to 0.64, the decomposition rate of hydrogen peroxide is equal to or higher than that of CePO ₄ .
(3) When the Ti / Ca ratio is 0.13 to 0.50, the hydrogen peroxide decomposition rate is approximately 70% or more, and
(4) It can be seen that when the Ti / Ca ratio is 0.15 to 0.35, the hydrogen peroxide decomposition rate is approximately 80% or more.

Furthermore, chemical stability evaluation was performed on the samples obtained in Example 1 and Comparative Examples 1 and 2. That is, 0.2 g of a sample was immersed in 20 ml of a 3 wt% H ₂ O ₂ solution heated to 98 ° C. for 1 to 7 hours. Next, the solution was filtered with a 0.1 μm filter, and the amount of P elution in the filtrate was measured using ICP emission spectrometry. Furthermore, weight loss was calculated from the measured amount of elution of P.
In the case of titania phosphate, the weight loss (corresponding to the P elution amount) after a 1 hour immersion test was 5 mass% or more. On the other hand, in the case of ceria phosphate, the weight loss after the immersion test for 1 hour was 32 mass%, and a larger weight change was observed than that of titania phosphate. On the other hand, in the case of Ca-doped titania phosphate, the weight loss after a 1-hour immersion test was 1 to 4 mass%.
FIG. 4 shows the relationship between the immersion time and the P elution amount in a 3 wt% aqueous hydrogen peroxide solution (98 ° C.) of Ca-doped titania and cerium phosphate having a Ca / P ratio of 2.2. From FIG. 4, it can be seen that a large amount of P was eluted from the initial stage of the immersion test for cerium phosphate, whereas the dissolution of P was suppressed for Ca-doped titania. This is thought to be because the bonding between the titania skeleton and P was further stabilized by doping Ca into titania phosphate.

  Although the embodiments of the present invention have been described in detail above, the present invention is not limited to the above embodiments, and various modifications can be made without departing from the scope of the present invention.

  The electrolyte according to the present invention includes electrolyte membranes and catalysts used in various electrochemical devices such as fuel cells, water electrolysis devices, hydrohalic acid electrolysis devices, salt electrolysis devices, oxygen and / or hydrogen concentrators, humidity sensors, and gas sensors. It can be used as a material constituting a layer or the like.

It is a figure which shows the specific surface area of the synthetic | combination Ca dope titania (Example 1), phosphate titania (comparative example 1), and cerium phosphate (comparative example 2). Synthesized Ca the doped acid titania (Example 1), titania phosphate (Comparative Example 1), and a diagram showing the H ₂ O ₂ decomposition ratio of cerium phosphate (Comparative Example 2). Is a diagram showing the relationship between the Ti / Ca ratio of Ca the doped acid titania and _H 2 _{O 2} decomposition rate. It is a figure which shows the relationship between the immersion time in 3 wt% hydrogen peroxide aqueous solution (98 degreeC) of Ca dope titania and Ca cerium phosphate whose Ca / P ratio is 2.2, and P elution amount.

Claims (8)

  An electrolyte containing Ca-doped titania phosphate.   The electrolyte according to claim 1, wherein the phosphonic acid group has a phosphonic acid group in the skeleton of the titania phosphate.   The electrolyte according to claim 1 or 2, wherein a Ca / P ratio (mass%) in the titania phosphate is 0.4 or more and 3.0 or less.   The electrolyte according to any one of claims 1 to 3, wherein a Ti / Ca ratio (mass%) in the titania phosphate is 0.05 or more. 5. The electrolyte according to claim 1, wherein the titania phosphate has a specific surface area of 30 m ² / g or more.   The electrolyte according to any one of claims 1 to 5, wherein the titania phosphate is amorphous.   The electrolyte according to any one of claims 1 to 5, wherein the titania phosphate is crystalline. The electrolyte according to any one of claims 1 to 7, further comprising a resin, a gel, and / or a polymer electrolyte.

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