JP2007287393A - Contamination prevention method of fuel cell air electrode - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a contamination prevention method of a fuel cell air electrode capable of maintaining high power generation efficiency for a long period of time by effectively removing a contaminant in an atmosphere by a simple and compact facility. <P>SOLUTION: In supplying air A to the air electrode of a fuel cell 4, the air A is passed through an adsorbing removing layer in a contaminant removing device 2 formed by filling a contaminant adsorbent containing Mn oxide and/or Mn-Cu composite oxide and alkali metal ruthenate and/or alkaline earth metal ruthenate, and then is supplied to the air electrode. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a fuel cell that is used by supplying air in the atmosphere to an air electrode (cathode), and more particularly, to a technique for preventing contamination of the air electrode by a trace amount of contaminants such as NOx and SOx contained in the air. .

  In recent years, fuel cells have attracted attention as a technology that leads to improvement of the global environment. A fuel cell generates power by reacting hydrogen supplied to a fuel electrode (anode) with an oxygen-containing gas supplied to an air electrode (cathode), and air is usually used as the oxygen-containing gas. It is done.

In the case of a low temperature fuel cell such as a solid polymer fuel cell or a phosphoric acid fuel cell, the reaction needs to be promoted at a low temperature, and therefore a highly active catalyst is used as the electrode material. Currently, an electrode material in which a noble metal such as platinum is supported on the surface of a carbonaceous material is used. However, the atmosphere contains trace amounts of nitrogen oxides (NOx: NO and NO ₂ ), sulfur oxides (SOx), etc., and these contaminants accumulate on the electrode surface when introduced into the air electrode. This leads to a decrease in power generation efficiency.

  For this reason, attempts have been made to reduce pollutants by passing the air supplied to the air electrode through a filter filled with activated carbon.

  Further, on the outer surface of a metal oxide or carbonaceous material having an average pore diameter of 0.3 to 10 nm, a group consisting of Pt, Ru, Rh, Pd, Ag, Au, Os, Cu, Co, Ni, Fe, and Sn. An adsorbent having a coating layer formed of at least one active component selected from the group consisting of at least one selected metal, an oxide of the metal, an alloy of the metal, and an alloy of the metal and another metal. It is disclosed that contaminants in the air supplied to the air electrode are removed (see Patent Document 1).

Further, after oxidizing pollutants in the air using a catalyst containing at least one selected from the group consisting of Pd, Pt, Ru and Rh, ozone, or by heating the air, potassium permanganate is oxidized. A method of adsorbing and removing with alumina or the like carrying a material is disclosed (see Patent Document 2).
Japanese Patent Laying-Open No. 2005-74338 (claims, etc.) JP 2004-327429 A (claims, etc.)

  However, in the method using the activated carbon filter, the activated carbon has a very low removal performance with respect to NO that occupies the majority of NOx in the atmosphere. Therefore, a large amount of activated carbon is required, and the equipment becomes larger and the pressure loss of the filter increases. Therefore, there is a problem that a large blower is required and the equipment cost and running cost increase. In addition, the method using an adsorbent in which a coating layer of a noble metal or the like is formed on the outer surface of a metal oxide or carbonaceous material having fine pores has low activity against NO and has the same problem as the method using the activated carbon filter. Arise. In addition, in a method in which the catalyst is oxidized by using a catalyst containing Pd or ozone, or by heating the air or the like and then adsorbing and removing with alumina or the like carrying potassium permanganate or the like, the oxidation and adsorption removal are separate steps. However, there is a problem that the equipment becomes complicated and the equipment cost increases. Furthermore, when oxidizing by heating air, it is necessary to supply heat energy separately, and when oxidizing using ozone, it is necessary to supply ozone equivalent to the pollutant. The amount of water must be measured with high accuracy, leading to an increase in equipment costs. Furthermore, when ozone is supplied excessively, there is a problem that residual ozone deteriorates the electrode material.

  SUMMARY OF THE INVENTION An object of the present invention is to provide a method for preventing contamination of a fuel cell air electrode that can efficiently remove pollutants in the atmosphere with a simple and compact facility and maintain high power generation efficiency over a long period of time. .

  According to the first aspect of the present invention, when supplying air to the air electrode of the fuel cell, a pollutant adsorbent containing Mn oxide and / or Mn-Cu composite oxide and an alkali metal ruthenate is filled. A method for preventing contamination of a fuel cell air electrode is characterized in that air is passed through the adsorption removal layer and then supplied to the air electrode.

  The invention according to claim 2 is the method for preventing contamination of a fuel cell air electrode according to claim 1, wherein the contaminant adsorbent further contains an alkali metal compound other than the alkali metal ruthenate. is there.

  The invention according to claim 3 is to prevent contamination of the fuel cell air electrode according to claim 1 or 2, wherein a mass ratio of [Mn / (Mn + Cu)] in the Mn—Cu composite oxide is 0.15 or more. Is the method.

  Invention of Claim 4 is any one of Claims 1-3 in which the said contaminant adsorbent contains the said ruthenate 0.05-5.0 mass% in Ru conversion. The method for preventing contamination of a fuel cell air electrode as described in 1. above.

  The invention according to claim 5 is the fuel cell air electrode according to any one of claims 1 to 4, wherein the ruthenate of alkali metal is potassium ruthenate, sodium ruthenate, or a mixture thereof. This is a pollution prevention method.

  According to a sixth aspect of the present invention, while the fuel cell is not operated, the heated air is circulated through the adsorption removal layer, or the heated air is heated or heated while the adsorption removal layer is heated. The method for preventing contamination of a fuel cell air electrode according to any one of claims 1 to 5, wherein the contaminant adsorbent is regenerated by circulating air.

  The invention described in claim 7 is to replace the pollutant adsorbent periodically, regenerate the used pollutant adsorbent by heating, and use again. 2. The method for preventing contamination of a fuel cell air electrode according to claim 1.

  According to the present invention, Mn oxide and / or Mn-Cu composite oxide, which is excellent in adsorption performance of pollutants in the air, particularly NO difficult to remove with a normal adsorbent, alkali metal or alkaline earth By passing through an adsorption removal layer filled with a contaminant adsorbent containing a metal ruthenate, the contaminant can be directly adsorbed and removed without going through a step of oxidizing it once. Therefore, it is possible to efficiently remove pollutants in the air with simple and compact equipment, to prevent contamination of the air electrode of the fuel cell, to suppress the decrease in power generation efficiency, and to maintain high power generation efficiency over a long period of time. became.

  Hereinafter, embodiments of the present invention will be described in detail by dividing the process into a process for removing contaminants from air (contaminant removal process) and a process for regenerating adsorbent (adsorbent regeneration process).

[Contaminant removal process]
A fuel cell equipped with a pollutant removal apparatus according to the method of the present invention is shown in the flowchart of FIG. The air A sucked from the atmosphere by the blower 1 passes through the adsorption / removal layer in the pollutant removal device 2 and adsorbs and removes the pollutant, and then a humidifier is used to protect the electrolyte membrane of the fuel cell 4 from drying. 3 is humidified and introduced into the air electrode of the fuel cell 4.

  An example of the contaminant adsorbent (hereinafter also simply referred to as “adsorbent”) filled in the contaminant removal apparatus 2 is an example that contains Mn oxide and an alkali metal ruthenate as essential components. Will be described. This adsorbent can be produced by an impregnation method or a kneading method. For example, it can be obtained by a method in which powdered Mn oxide is impregnated with an aqueous solution of an alkali metal ruthenate and then dried.

The adsorbent removes contaminants such as NO _x , CO, O ₃ , and SO _x in the air A by adsorption.

For example, NO _x is removed without being oxidized in advance by the following mechanism. That is, the Mn oxide in the adsorbent has an ability to oxidize NO to NO ₂ at room temperature, but a part of NO ₂ is adsorbed because the ability to adsorb and hold NO ₂ is insufficient. However, most of them are released to the outlet side of the pollutant removing device 2, and this tendency becomes remarkable particularly when the humidity of the air is high. However, when an alkali metal ruthenate, which is a ruthenium compound, coexists with Mn oxide, the adsorption performance of NO ₂ is greatly improved.

This eliminates the need for an NO ₂ adsorbent (such as activated carbon or hydrophobic zeolite) that needs to be arranged downstream of the alkali metal ruthenate.

On the other hand, with respect to the action of removing Mn oxide from O _3, a certain degree of effect can be obtained without coexistence of an alkali metal ruthenate which is a ruthenium compound. That is, the O atom of the Mn oxide is donated to O ₃ and decomposed into O ₂ . Then, the Mn oxide donated with the O atoms returns to the original Mn oxide upon receiving supplementation of O atoms from O _{2 in} the air. This is because it is considered that the removal reaction of O ₃ is continued by repeating this.

From the above-mentioned effects, it seems that Mn oxide is unnecessary for adsorption removal of NO ₂ , but in reality, NO ₂ reacts with moisture to generate nitrate ions (NO ₃ ⁻ ). In this case, NO is generated (3NO ₂ + H ₂ O → 2HNO ₃ + NO), so that the coexistence of Mn oxide is effective for oxidizing the NO again.

Further, by using an alkali metal ruthenate which is an alkali metal compound, the effect of enhancing the durability of the remover against SO _x coexisting in the air is exhibited, and the effect of enhancing the SO _x removal performance is also obtained. . That, Mn oxides are also significant activity to oxidize SO ₂ to SO _3, SO ₃ produced by oxidation, sulfuric acid is produced by the reaction with moisture coexisting. When the alkali metal ruthenate, which is an alkali metal compound, is present in the remover of the present invention, sulfate is easily generated, so that sulfation of Mn oxide is suppressed, and SO ₃ is sulfate. to be removed as the alkali, the higher the adsorption performance of SO _x. On the other hand, when the alkali metal compound does not coexist, the Mn oxide is gradually sulfated by the SO ₃ , which causes a decrease in the catalytic activity of the Mn oxide. That is, the alkali metal ruthenate, which is an alkali metal compound, is extremely effective in suppressing the adsorption / removal of the adsorption / removal agent by SO _x that is often mixed in the air and improving the SO _x removal performance.

Based on the above-described mechanism, without oxidizing the pre pollutants, NO in NO _x is efficiently adsorbed and removed by Mn oxide of alkali metal ruthenate presence ruthenium compounds, NO ₂ However, CO, O ₃ , and SO _x are also efficiently decomposed or adsorbed and removed by the same adsorbent.

  As is apparent from the above, such an operational effect is exhibited effectively only when the Mn oxide and the alkali metal ruthenate are present (coexist) in the same removal agent. In the case where Mn oxide and alkali metal ruthenate are not allowed to coexist, such as when air is passed through a remover consisting of Mn oxide and then passed through a remover consisting of alkali metal ruthenate, If only one of the Mn oxide and the alkali metal ruthenate is not present, the above effect cannot be obtained.

  That is, the adsorbent in which Mn oxide and alkali metal ruthenate coexist exhibits excellent removal performance due to the synergistic effect of these substances.

  In order to effectively exhibit the coexistence effect with the ruthenium compound, it is recommended that the Mn oxide has an oxidation number in the range of 3.5 to 3.9, more preferably 3.5 to 3.8.

  Further, the content of ruthenate is preferably 0.05 to 5.0% by mass in terms of Ru in the ratio of the contaminant adsorbent. If the amount is less than 0.05% by mass, the removal performance improvement effect is small, and if 0.05% by mass, the effect is large. This is because the effect is not obtained and it is uneconomical. In view of both removal performance and economy, the more preferable content of ruthenate is in the range of 0.1 to 2.0% by mass in terms of Ru.

  The Mn oxide is preferably an oxide obtained by oxidizing a hydroxide, ash salt, sulfate, or the like of Mn in a liquid phase or a gas phase containing an oxidizing agent. By adopting such a method, the oxidation number of Mn can be easily increased to 3.5 or more, and the contaminant removal performance can be improved more effectively.

(Modification)
In the above embodiment, the Mn oxide is used. However, instead of or in addition to this, a Mn—Cu composite oxide may be used. Thereby, the removal activity far superior to the case of using the oxide of Mn alone is obtained.

  Here, the composite oxide is not a simple mixture of two kinds of metal oxides, but another kind of oxide in which a bond between both metals is formed through an O atom.

  The composite oxide can be typically produced by the following coprecipitation method. That is, ant potash is added to an aqueous solution in which two kinds of metal salts are dissolved, a precipitate in which hydroxides of both metals are mixed on a molecular scale is formed, and a composite oxide is obtained by oxidizing the precipitate. In this case, it is not sufficient to mechanically mix the hydroxide powder or slurry of both metals, and a large difference appears in the oxidation performance of NO.

The effect of the composite oxide is considered to be based on the following reason. That is, by making a complex oxide, O atoms in the oxide easily move, and it becomes easy to donate O atoms in the oxide and replenish O atoms from O _{2 in} the air. It is thought to improve.

  Here, the content of Mn in the complex oxide of Cu and Mn should be 0.15 or more, more preferably 0.45 or more in terms of Mn / (Cu + Mn) (mass ratio). Due to the insufficient amount of Mn, the adsorption performance is inferior to the oxide of Mn alone. However, if the mass ratio becomes too high, the combined effect with Cu becomes difficult to be exhibited effectively, and the adsorption performance tends to be lowered. Therefore, it is preferably suppressed to 0.95 or less.

Moreover, in the said embodiment, although alkali-metal ruthenate was used, you may use alkaline-earth metal ruthenate instead of or in addition to this. This is because even when an alkaline earth metal ruthenate that is an alkaline earth metal compound is used, SO _x removal performance can be obtained by the same effects as the alkali metal ruthenate that is an alkali metal compound. Further, in addition to the alkali metal ruthenate and / or the alkaline earth metal ruthenate, another alkali metal compound (that is, an alkali metal compound other than the alkali metal ruthenate) may be contained. The content of other alkali metal compounds, since SO _x removal performance can be surely obtained. As the alkali metal ruthenate, it is preferable to use potassium ruthenate, sodium ruthenate, or a mixture thereof. Examples of other alkali metal compounds include hydroxides, carbonates, and bicarbonates, and potassium is particularly preferable among the alkali metals constituting the alkali metal compounds.

[Adsorbent regeneration process]
Since the adsorption capacity of the adsorbent filled in the pollutant removing device 2 decreases as the adsorption capacity approaches saturation due to use, it is desirable that the adsorption capacity be restored periodically to restore the adsorption capacity. The contaminant adsorbed on the adsorbent is easily desorbed by heating the adsorbent. Therefore, heated air (heated air) may be circulated through the adsorption removal layer in the pollutant removal device 2 while the fuel cell is not operating, such as when the fuel cell is resting. Or you may make it distribute | circulate normal temperature air or heated air to an adsorption removal layer, heating the adsorption removal pipe | tube filled with adsorption agent from the outside. As a heat source for heated air and a heat source for heating the adsorption removal layer from the outside, exhaust gas sensible heat of a reformer (not shown) for producing hydrogen B for the fuel cell 4 or obtained by this reformer Sensible heat of high-temperature gas containing hydrogen or exhaust gas sensible heat of the fuel cell 4 can be utilized.

  Further, instead of regenerating at the site, the adsorbent may be periodically replaced, and the used adsorbent may be regenerated by heating and reusing at a factory or the like.

  In addition, it is preferable that the temperature of the adsorption removal layer at the time of reproduction shall be 60-250 degreeC. This is because if it is less than 60 ° C., it takes too much regeneration time, and if it exceeds 250 ° C., the adsorbent tends to be thermally deteriorated.

The air after regenerating the adsorption removal layer may be released to the atmosphere. However, if the regeneration frequency of the adsorption removal layer is low, the concentration of contaminants in the air after passing through the adsorption removal layer becomes high and cannot be released into the atmosphere as it is, so the regeneration frequency can be as much as possible within a range that is not excessively frequent. Higher is preferable.

  In order to confirm the effects of the present invention, the adsorbents of the following invention examples and comparative examples are used, and air containing pollutants (simulated air) is circulated through the adsorption removal layers filled with the respective adsorbents. An experiment was conducted to determine the removal rate of the substance.

[Adsorbent of Invention Example]
Mn: 53 mass% (hereinafter, mass% is referred to as%), Cu: 13%, Ru: 0.5%, and Mn oxide having an average Mn oxidation number of 3.73 were used.

[Adsorbent of Comparative Example]
Commercially available activated carbon (manufactured by Nippon Enviro Chemicals, trade name: G2X) was used.

[Experimental conditions]
The adsorbent of the invention examples and comparative examples were filled into the adsorption removal tube made water-cooling jacket and the heater is mounted stainless steel was evaluated NO _x removal performance under the following conditions and methods.

That is, “adsorbent filling amount: 10 g, simulated air: NO _x concentration 0.1 ppm (NO: 0.05 ppm, NO ₂ : 0.05 ppm), simulated air flow rate: 3 NL / min, adsorption removal temperature: 40 ° C., adsorption Simulated air was allowed to flow under the conditions of “removal humidity: 60% (relative humidity), adsorption removal time: 50 hours”, and NO concentration and NO _x concentration at the adsorption removal tube outlet were continuously measured with a chemiluminescence analyzer. Then, the NO and NO ₂ removal rates after 1 hour and 50 hours for each adsorbent were calculated by the following formula.

NO removal rate = (1-outlet NO concentration / inlet NO concentration) x 100 (%)
NO ₂ removal rate = (1−outlet NO ₂ concentration / inlet NO ₂ concentration) × 100 (%)

[Experimental result]
The experimental results are shown in Table 1. Compared with the case of using the adsorbent of the comparative example, by using the adsorbent of the invention example, not only NO ₂ but also NO is almost 100% removed. It was confirmed that high adsorption performance could be maintained for a long time.

It is a flowchart which shows the fuel cell provided with the contaminant removal apparatus which concerns on implementation of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Blower 2 ... Pollutant removal apparatus 3 ... Humidifier 4 ... Fuel cell A ... Air B ... Hydrogen

Claims (7)

  In supplying air to the air electrode of a fuel cell, air is supplied to an adsorption removal layer formed by filling a pollutant adsorbent containing Mn oxide and / or Mn-Cu composite oxide and ruthenate of alkali metal. A method for preventing contamination of a fuel cell air electrode, wherein the air electrode is supplied after passing through the air electrode.   The method for preventing contamination of a fuel cell air electrode according to claim 1, wherein the contaminant adsorbent further contains an alkali metal compound other than the alkali metal ruthenate.   The method for preventing contamination of a fuel cell air electrode according to claim 1 or 2, wherein a mass ratio of [Mn / (Mn + Cu)] in the Mn-Cu composite oxide is 0.15 or more.   The contamination of the fuel cell air electrode according to any one of claims 1 to 3, wherein the contaminant adsorbent contains 0.05 to 5.0 mass% of the ruthenate in terms of Ru. Prevention method.   The method for preventing contamination of a fuel cell air electrode according to any one of claims 1 to 4, wherein the alkali metal ruthenate is potassium ruthenate, sodium ruthenate, or a mixture thereof.   The pollutant is caused by circulating heated air through the adsorption / removal layer while the fuel cell is not operated, or by circulating unheated air or heated air while heating the adsorption / removal layer. The method for preventing contamination of a fuel cell air electrode according to any one of claims 1 to 5, wherein the adsorbent is regenerated.   The fuel cell air according to any one of claims 1 to 5, wherein the pollutant adsorbent is periodically replaced, and the used pollutant adsorbent is heated and regenerated for reuse. Pollution prevention method.

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