JP2010267417A - Solid polymer fuel cell - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a solid polymer fuel cell endowed with sufficient startability in a low temperature condition below a freezing point. <P>SOLUTION: In an oxidant gas catalyst layer 44 of an oxidant electrode 40, a ratio of ionomer 58 against carbon particles (catalyst carrier) 56 is set to be 0.45 vol.% or more, and since pores 62 formed in the oxidant gas catalyst layer are few and moreover small, water W generated through power generation under a low temperature condition below 0°C stays in a cluster of polymer electrolyte constituting ionomer until the temperature rises 0°C or higher by reaction heat of the solid polymer electrolyte fuel cell device so as to avoid freezing, and since oxygen as an oxidant is dissolved in an element constituting ionomer and in water contained in the ionomer and is conveyed to a catalyst 54, oxygen continues to be conveyed to the catalyst even if the water in the pores are frozen, and therefore, startability of the solid polymer fuel cell can be enhanced in the temperature below the freezing point. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a solid polymer fuel cell that obtains electric energy generated by an electrochemical reaction when electrodes are arranged on both sides of a solid polymer membrane functioning as an electrolyte membrane and fuel gas and oxidant gas are supplied respectively. In particular, the present invention relates to the structure and material of an electrode excellent in startability from a low temperature below freezing point.

  A fuel cell is a device that converts chemical energy of a fuel gas into electrical energy by causing an electrochemical reaction between a fuel gas containing hydrogen and an oxidant gas containing oxygen such as air. Although there are a plurality of types of fuel cells depending on the difference in electrolyte, in recent years, solid polymer fuel cells that operate at a relatively low temperature and obtain a high output have attracted attention.

  Such a polymer electrolyte fuel cell has a unit cell in which a solid polymer membrane functioning as an electrolyte layer is sandwiched between a fuel electrode (anode: hydrogen electrode) and an oxidant electrode (cathode: air electrode) from both sides. It is configured by stacking a plurality of fuel gas channels and oxidant gas channels through separators having front and back surfaces. When a fuel gas containing hydrogen is supplied to the fuel electrode and an oxidant gas containing oxygen is supplied to the oxidant electrode, the reactions shown by the following chemical reaction formulas occur at both electrodes.

(Anode reaction at the fuel electrode)
H ₂ → 2H ⁺ + 2e ⁻ (1)
(Cathode reaction at oxidant electrode)
_{^{2H + + (1/2) O 2}} + 2e - → H 2 O ··· (2)

As shown in the above formula (1), hydrogen is converted into protons (H ⁺ ) in the fuel electrode, and the proton (H ⁺ ) moves from the fuel electrode side to the oxidant electrode side through the electrolyte membrane with water. Then, as shown in the above formula (2), proton (H ⁺ ) reacts with oxygen at the oxidizer electrode to generate water. As the above electrolyte membrane, a perfluorocarbon sulfonic acid membrane (Nafion membrane manufactured by DuPont, USA), which is a proton exchange membrane, is known, and this membrane has a hydrogen ion exchange group in the molecule and hydrates in a saturated state. Therefore, it functions as an ion conductive polymer. Further, the fuel electrode and the oxidant electrode sandwiching the electrolyte membrane have a catalyst carrier containing a catalyst and a catalyst layer containing an ionomer (ion conducting polymer) on the electrolyte membrane side.

  Since the electromotive force of the unit cell is a low voltage of 1 V or less, a laminate in which several tens to hundreds of unit cells are usually stacked via a separator constitutes a fuel cell. When starting up under low-temperature conditions lower than 0 ° C., such as when used in outdoor installations in cold areas using city gas, LPG, etc. as fuel, in a fuel cell composed of such a laminate, Water generated by power generation clogs pores in the catalyst layer of the electrode by freezing, which causes problems in the supply of oxidant gas and diffusion of reaction gas, and impairs the startability of the polymer electrolyte fuel cell. there were.

  On the other hand, in order to improve the startability under a low temperature condition lower than 0 ° C., Patent Document 1 proposes to provide a cell with an ice crystal growth inhibitor such as antifreeze protein. Then, in order to quickly absorb the moisture generated on the oxidizer electrode side into the membrane, the cluster diameter containing moisture is gradually increased from the electrolyte membrane on the oxidizer electrode side to the electrolyte membrane on the fuel electrode side. It has been proposed to do.

JP 2007-134222 A JP 2008-047388 A

  By the way, in the conventional polymer electrolyte fuel cell of Patent Document 1, for example, there is a possibility that water generated in the oxidant catalyst layer provided on the electrolyte membrane side of the oxidant electrode may freeze at a stretch due to destruction due to overcooling. It was left. Further, in the conventional polymer electrolyte fuel cell of Patent Document 2, the control of the transfer of generated water between the ionomer of the catalyst layer and the electrolyte membrane, and the transfer of water in the electrolyte membrane is limited to the size of the cluster diameter. The problem of being difficult was left. For this reason, a polymer electrolyte fuel cell having sufficient startability from a low temperature state of below freezing point has not been obtained yet.

  The present invention has been made against the background of the above circumstances, and an object of the present invention is to provide a polymer electrolyte fuel cell having sufficient startability from a low temperature state below freezing point.

In particular, in the catalyst layer of the oxidant electrode, the catalyst carrier that supports the catalyst for promoting the reaction of the above formula (1) or (2) is coated with an ionomer, and ions (proton H ⁺ ) are transferred through the ionomer. In the past, pores were provided in the catalyst layer with a relatively large porosity. For this reason, as described above, water generated in a low temperature state lower than 0 ° C. freezes to close the pores, and the startability of the polymer electrolyte fuel cell is impaired. Contrary to this, the present inventor increased the ratio of the ionomer to the catalyst carrier contrary to the conventional idea described above, but surprisingly continued the power generation of the polymer electrolyte fuel cell in a low temperature state although the pores were reduced. I found the fact that time would be significantly longer. This is because most of the water generated by power generation at low temperatures below 0 ° C stays in the ionomer cluster, which is a polymer electrolyte, and freezing of the generated water is avoided and oxygen constitutes the ionomer as an oxidant. The element is dissolved in water contained in the element (fluorine) and ionomer and transported to the catalyst, so even if the water in the pores of the catalyst layer freezes, it is supported that oxygen continues to be supplied to the catalyst, The effect increased as the ratio of ionomer to catalyst support increased. The present invention has been made based on such knowledge.

  That is, the gist of the invention according to claim 1 is that: (a) a fuel electrode having a solid polymer electrolyte membrane on the side of the electrolyte membrane having a catalyst carrier containing a catalyst and a catalyst layer containing an ionomer; A solid polymer fuel cell in which a plurality of unit cells interposed between an oxidant electrode and a separator are interposed, and (b) the catalyst carrier in the catalyst layer of the oxidant electrode. The volume ratio of the ionomer is 0.45 or more.

  The gist of the invention according to claim 2 is that, in the invention according to claim 1, (c) the ionomer in the catalyst layer of the oxidant electrode is a fluorine-based polymer electrolyte containing fluorine atoms. It is characterized by.

  Further, the gist of the invention according to claim 3 is that, in the invention according to claim 1 or 2, (d) the catalyst carrier in the catalyst layer of the oxidant electrode is carbon particles; In the catalyst layer, the mass ratio of the ionomer to the catalyst support is 0.5 or more.

  According to the solid polymer fuel cell of the invention of claim 1, (a) a fuel having an electrolyte membrane made of a solid polymer, a catalyst carrier supporting a catalyst, and a catalyst layer containing an ionomer on the electrolyte membrane side A solid polymer fuel cell in which a plurality of unit cells interposed between an electrode and an oxidant electrode are stacked via a separator, and (b) the catalyst carrier in the catalyst layer of the oxidant electrode Since the volume ratio of the ionomer to is 0.45 or more, the pores of the catalyst layer are greatly reduced, and the pores are less likely to be blocked by freezing of water. Many are transported to the oxidant gas diffusion layer through the ionomer cluster, which is a polymer electrolyte, and the oxygen as the oxidant is dissolved in the water constituting the ionomer and water contained in the ionomer and transported to the catalyst. Be water in the pores of example catalyst layer freezing, the oxygen continues to be supplied to the catalyst, activation of the solid polymer electrolyte fuel cell in a low temperature state that below freezing is greatly enhanced.

  According to the polymer electrolyte fuel cell of the invention according to claim 2, (c) the ionomer in the catalyst layer of the oxidant electrode is a fluorinated polymer electrolyte containing a fluorine atom. As oxygen is dissolved in the water contained in the fluorine constituting the ionomer and transported to the catalyst, even if the water in the pores of the catalyst layer freezes, there is an advantage that oxygen is further supplied to the catalyst. .

  According to the solid polymer fuel cell of the invention of claim 3, (d) the catalyst carrier is carbon particles in the catalyst layer of the oxidant electrode, and (e) the catalyst carrier in the catalyst layer. Since the mass ratio of the ionomer to the body is 0.5 or more, the pores of the catalyst layer are significantly reduced and the pores are less likely to be blocked by water freezing, while the generated moisture Most of these are transported to the oxidant gas diffusion layer through the ionomer cluster, which is a polymer electrolyte, and oxygen as the oxidant is dissolved in the ionomer and water contained in the ionomer and transported to the catalyst. Therefore, even if the water in the pores of the catalyst layer freezes, oxygen continues to be supplied to the catalyst, and the startability of the polymer electrolyte fuel cell at a low temperature below the freezing point is greatly improved.

  Here, the oxidant electrode of the polymer electrolyte fuel cell includes a catalyst carrier and an ionomer in which the catalyst layer supports the catalyst, and the ionomer is 0.45 or more with respect to the catalyst carrier. The volume is increased so as to be a volume ratio, but the catalyst layer of the fuel electrode of the polymer electrolyte fuel cell also has a volume ratio of 0.45 or more with respect to the catalyst support. It may be increased.

  Preferably, the catalyst carrier for carrying the catalyst has a sufficiently larger diameter than the catalyst. That is, the catalyst has a sufficiently smaller diameter than the catalyst carrier that supports it.

  As the catalyst carrier for carrying the catalyst, conductive carbon particles are preferably used, but other substances such as titanium oxide and aluminum oxide may be used instead of or in addition thereto. These titanium oxide, aluminum oxide, and the like can be made conductive by having a small particle diameter of about several tens of nanometers.

The electrolyte layer is a substance excellent in corrosion resistance, heat resistance, and durability. For example, the electrolyte layer has a hydrogen ion exchange group in the molecule, and moves protons (H ⁺ ) by being saturated with water. A solid substance having a high ion conductivity and a function of separating fuel gas and oxidant gas, for example, a perfluorocarbon-based solid electrolyte in which a sulfone group is combined with a resin skeleton, and a sulfone group is introduced as a side chain For example, a fluorine-based polymer electrolyte typified by a perfluorosulfonic acid-based polymer electrolyte is used, such as a hydrocarbon-based solid electrolyte. Further, as the electrolyte layer, a hydrocarbon polymer electrolyte or the like may be used instead of the fluorine polymer electrolyte.

  Preferably, the fuel electrode (anode: hydrogen electrode) and the oxidant electrode (cathode: air electrode) are preferably composed of a catalyst carrier containing a substance having catalytic activity such as platinum and a catalyst layer containing an ionomer, and a reactive gas. In order to promote diffusion, the gas diffusion layer having high air permeability and high conductivity is laminated. However, another layer having conductivity and gas permeability may be further added. The gas diffusion layer is, for example, a cloth or a porous plate including carbon fiber or metal fiber, and has a function as a current collector and a function of supporting the catalyst layer. The catalyst layer is composed of a conductive material having active surface sites that promote the anode reaction and the cathode reaction, respectively. For example, platinum, a platinum alloy having a particle diameter of several nm by an impregnation method, a colloid method, an ion method, etc. , Non-platinum-based catalysts (for example, oxides and carbon alloys) are uniformly dispersed, carbon supports having a high specific surface area (carbon particles), carbon support materials having a fine structure such as carbon nanotubes and carbon nanohorns, and oxygen activity Macrocyclic organic compounds, those using metal oxides such as perovskite, and sulfides are used.

  The ionomer is used in the meaning of a polymer having ionic conductivity. For example, the ionomer is the same material as the electrolyte layer, for example, a fluorine-based polymer electrolyte typified by a perfluorosulfonic acid-based polymer electrolyte, or a hydrocarbon. It may be composed of a polymer electrolyte.

  In the polymer electrolyte fuel cell, since the electromotive force of a unit cell composed of a pair of electrolyte membranes and a pair of fuel electrode and oxidant electrode sandwiching the electrolyte membrane is a low voltage of 1 V or less, usually, a separator is interposed. Thus, it is composed of a laminate in which tens to hundreds of single cells are laminated. This separator is composed of a relatively thin substantially conductive plate, and supports the electrolyte layer and the fuel gas electrode and the oxidant gas electrode laminated so as to sandwich the electrolyte layer. The separator metal plate may be a single metal plate, a laminate of a plurality of thin metal plates, a high-density carbon plate, or a composite plate of carbon and resin. A fuel gas channel and an oxidant gas channel are formed on the front surface and the back surface of the electrode contact portion, which is a portion in contact with the electrodes of the separator, by plastic working. When the fuel electrode and the oxidant electrode are configured to participate locally in the cathode reaction and the anode reaction, the fuel gas flow path and the electrode contact portion of the separator at least in the portion that contacts the reaction portion An oxidant gas flow path is formed.

  In order to provide the whole separator or its electrode contact part with high conductivity in itself, it is preferable that at least the substrate has a highly conductive metal such as stainless steel, nickel-containing alloy, chromium-containing alloy, aluminum, It is comprised from at least 1 sort (s) of an aluminum containing alloy, copper, a copper containing alloy, titanium, and a titanium containing alloy.

  The electrode contact portion of the separator is preferably provided with a coating containing a conductive corrosion-resistant layer for enhancing the corrosion resistance of the surface, for example, at least one of gold, carbon, titanium oxide, and nickel oxide.

It is a block diagram explaining the structure of the polymer electrolyte fuel cell apparatus containing the polymer electrolyte fuel cell of one Example of this invention. It is sectional drawing explaining the laminated structure of the polymer electrolyte fuel cell of FIG. In the oxidant gas catalyst layer which is a part of the oxidant electrode constituting the unit cell of FIG. 2, the ionomer mass ratio (Iw / Cw) to the carbon particles functioning as the catalyst support is 0.5 or more. It is sectional drawing which illustrates this typically. It is a figure showing the chemical formula which shows the perfluorosulfonic acid type polymer electrolyte which comprises the electrolyte layer of the single battery of FIG. It is a figure which shows the electric power generation characteristic test in the 28 degreeC environment of the single cell which comprises the conventional polymer electrolyte fuel cell. It is a figure which shows the relationship between the output voltage which used the average current density of the conventional polymer electrolyte fuel cell as a parameter, and time to reach 28 degreeC in -30 degreeC atmosphere. The power generation characteristics in an atmosphere of −30 ° C. of three types of samples (single cells) in which the mass ratio (Iw / Cw) of the ionomer to the carbon particles functioning as a catalyst carrier is 0.5, 1 and 1.5. FIG. It is a chart which shows the value which estimated the power generation continuation time of the cell in average current density 0.08A / cm < ² > based on the power generation continuation time of the cell of FIG. In the Example of FIG. 1, it is a figure which shows the relationship between the pore radius in an ionomer, and the freezing point of the water | moisture content in an ionomer. For three types of samples (single cells) in which the ionomer mass ratio (Iw / Cw) to carbon particles functioning as a catalyst support is 0.5, 1 and 1.5, the porosity of the catalyst layer is determined by the mercury intrusion method. It is a figure which shows the estimation result of the freezing amount in the pores of the catalyst layer of the water produced | generated by the electrode reaction in the electric power generation test of FIG. Using propane and fluorinated propane as models of hydrocarbon polymers and fluorinated polymers that make up ionomers, the propane and fluorinated propane molecules were estimated by quantizing the binding energy between these molecules and oxygen. It is a chart which shows inter-energy (kJ / mol), respectively. It is a figure which shows the bond distance computed from the bond energy by the intermolecular force of the propane of FIG. 11, and (a) has shown propane, (b) has shown the propane fluoride, respectively. Configuration in which mass ratio (Iw / Cw) of ionomer to carbon particles functioning as catalyst support is less than 0.5 in the oxidant gas catalyst layer which is a part of the oxidant electrode constituting the unit cell of FIG. It is sectional drawing which illustrates this typically. In another Example of this invention, it is sectional drawing explaining the principal part structure of the laminated structure of a polymer electrolyte fuel cell.

  Hereinafter, an embodiment of the present invention will be described in detail with reference to the drawings. In the following embodiments, the drawings are simplified, and the dimensions and the like of each part are not necessarily drawn accurately.

  FIG. 1 is a diagram for explaining a main part of the configuration of the polymer electrolyte fuel cell device 10. In FIG. 1, a hydrogen gas storage device 12 storing hydrogen gas extracted from a fuel gas such as LPG or city gas, for example, supplies a fuel gas flow of a polymer electrolyte fuel cell 16 via a pressure reduction and flow rate control device 14. Hydrogen gas is supplied to the passage 18 as fuel gas. Further, the air pump 20 oxidizes the fuel cell 16 after the air sucked through the filter 22 is humidified by the humidity converter 29 as the oxidant gas through the pressure reduction and flow rate adjusting device 24 as necessary. The agent gas passage 26 is supplied. The fuel gas flow path 18 may be similarly supplied to the polymer electrolyte fuel cell 16 through the humidity converter 29. Excess hydrogen discharged from the polymer electrolyte fuel cell 16 through the fuel gas flow path 18 is recovered by the hydrogen recovery and circulation device 28 and recirculated to the pressure reduction and flow rate adjustment device 14 through the circulation gas flow path (not shown). . Excess air released from the polymer electrolyte fuel cell 16 is released into the atmosphere together with moisture generated in the fuel cell 16. Alternatively, excess air released from the polymer electrolyte fuel cell 16 is released to the atmosphere after moisture generated in the polymer electrolyte fuel cell 16 is collected by the humidity converter 29.

In the polymer electrolyte fuel cell 16, a plurality of unit cells 34, for example, about several tens to several hundreds are stacked via a conductive separator 28, and in the thickness direction with a predetermined fastening force from a fastening device (not shown). It has the laminated structure comprised by being pressed by. This is because, since the electromotive force of the unit cell 34 is a low voltage of 1 V or less, a high voltage output is obtained from the electrodes located at both ends in the stacking direction by connecting in series with the stacked structure. FIG. 2 is a cross-sectional view schematically showing main parts of the separator 28 and the unit cell 34. As shown in FIG. 2, the unit cell 34 includes a solid polymer electrolyte membrane 36, a fuel electrode 38 and an oxidant electrode 40 sandwiching the electrolyte membrane 36, and the electrolyte membrane 36 includes a fuel electrode 38, an oxidant electrode 40, and the like. It is comprised by laminating | stacking in the state intervened in close_contact | adherence between these. The electrolyte membrane 36 is made of an ion conductive solid polymer that allows permeation of protons (H ⁺ ) with water. For example, a perfluorocarbon sulfonic acid resin that is a proton exchange membrane (Nafion membrane manufactured by DuPont, USA) Product name)) is used.

  The separator 28 is composed of an airtight and conductive member having a thickness of about 0.1 mm to 3 mm such as a metal plate material such as a stainless steel plate or a carbon plate, and has a surface (one surface) and a back surface (other surface). The grooves constituting the straight fuel gas flow path 18 and the straight oxidant gas flow path 26 or the serpentine (meandering) grooves communicate in series over the entire electrode contact portion of the separator 28. It is formed to do. The separator 28 may be a laminate of a plurality of conductive members. Preferably, at least the electrode contact portion of the separator 28 is coated with a coating containing at least one of gold, carbon, titanium oxide, and nickel oxide in order to enhance corrosion resistance and electrical contact. An annular seal made of, for example, synthetic resin or synthetic rubber is formed so as to surround the fuel electrode 38 or the oxidizer electrode 40 in order to improve the airtightness or liquid tightness between the separator 28 and the electrolyte membrane 36. Each member 30 is interposed.

  As schematically shown in FIG. 3, the fuel electrode 38 and the oxidant electrode 40 are a fuel gas catalyst having catalytic activity for promoting the anode reaction of the formula (1) and the cathode reaction of the formula (2). The layer 42 and the oxidant gas catalyst layer 44, and the fuel gas catalyst layer 42 and the oxidant gas catalyst layer 44 are mechanically supported, and the diffusion of the gas is promoted to promote the fuel gas catalyst layer 42 and the oxidant gas catalyst. A fuel gas diffusion layer 46 and an oxidant gas diffusion layer 48 are provided for uniform contact with the layer 44 over the entire surface.

The fuel gas catalyst layer 42 and the oxidant gas catalyst layer 44 are catalyst carriers that carry a catalyst 54 such as a platinum catalyst, a platinum-based alloy catalyst containing platinum, and other non-platinum-based catalysts (for example, oxide or carbon). A functional carbon particle (powder) 56 and an ionomer 58 in which the carbon particles (powder) 56 are contained are provided. The carbon particles 56 functioning as a catalyst carrier have, for example, a particle diameter of several tens of nanometers or more, and are sufficiently larger in diameter than the catalyst 54 of, for example, several nanometers, and are in contact with each other to conduct electrons. In this state, the carbon fiber 60 and the electrolyte layer 36 are provided. The ionomer 58 is a polymer having ionic conductivity capable of conducting protons (H ⁺ ), and is composed of the same material as the electrolyte layer 36, for example, a fluorine-based polymer electrolyte typified by a perfluorosulfonic acid-based polymer electrolyte. May be. As such a substance, for example, Nafion from DuPont, Flemion from Asahi Glass, and Aciplex from Asahi Kasei can be used. The average molecular weight of Nafion 117 from DuPont is reported to be 250,000 [H.-G. Haubold, Th. Vad, H. Jungbluth, P. Hiller: Electrochimica Acts 46 (2001) 1559- 1563]. The chemical formulas of perfluorosulfonic acid polymer electrolytes obtained from the above three companies are shown in FIG.

  The fuel gas diffusion layer 46 and the oxidant gas diffusion layer 48 are composed of carbon paper made of carbon fiber 60, carbon cloth, carbon felt, and the like, so that they have a relatively high conductivity and high air permeability. The base material is made strong, and is formed by subjecting it to a hydrophilic treatment or a water repellent treatment. Thereby, the fuel gas diffusion layer 46 and the oxidant gas diffusion layer 48 have a function of supporting the fuel gas catalyst layer 42 and the oxidant gas catalyst layer 44, a function of uniformly diffusing the reaction gas, and a current collector. With functionality. The unit cell 34 formed by sandwiching the electrolyte membrane 36 between the fuel electrode 38 and the oxidant electrode 40 thus configured is sandwiched between a pair of separators 28 from both sides.

  In the polymer electrolyte fuel cell 16 of the present embodiment, as described above, the fuel electrode 38 and the oxidant electrode 40 are configured in the same manner, but at least in the oxidant gas catalyst layer 44 of the oxidant electrode 40, The ratio of the ionomer 58 to the carbon particles (powder) 56 functioning as a catalyst carrier is set to be 0.45 or more in volume ratio and 0.5 or more in mass (weight) ratio. When the weight and volume of the carbon particles 56 in the oxidizer electrode 40 are Cw and Cv, and the weight and volume of the ionomer 58 in the oxidizer electrode 40 are Iw and Iv, the mass ratio of the ionomer 58 to the carbon particles (powder) 56 is Iw / Cw, and the volume ratio is represented by Iv / Cv. The density of the carbon particles 56 is 1.8, the density of Nafion is 1.97, and the density of the carbon particles 56 / the density of Nafion = 0.91. Therefore, the above-mentioned “mass (weight) ratio is 0.5. The term “above” can be restated as “volume ratio of 0.45 or more”.

  In the polymer electrolyte fuel cell 16 configured as described above, in the oxidant gas catalyst layer 44 of the oxidant electrode 40, the ratio of the ionomer 58 to the carbon particles (powder) 56 functioning as a catalyst carrier is the volume ratio. 0.45 or more and the mass (weight) ratio is set to 0.5 or more, so that the number of pores 62 formed in the oxidant gas catalyst layer 44 is small and small as shown in FIG. Therefore, a part of the water W generated by the power generation under the low temperature condition of less than 0 ° C. constitutes the ionomer 58 until the temperature is raised to 0 ° C. or more by the reaction heat of the polymer electrolyte fuel cell device 10 which is a laminate. Freezing is avoided by staying in the polymer electrolyte cluster, and is transported to the oxidant gas diffusion layer 48 through the cluster, and oxygen as the oxidant constitutes the ionomer 58. Since it dissolves in the water W contained in the element and ionomer 58 and is transported to the catalyst 54, oxygen continues to be supplied to the catalyst 54 even if the water W in the pores 62 in the oxidant gas catalyst layer 44 is frozen. Therefore, the startability of the polymer electrolyte fuel cell 16 in a low temperature state below the freezing point is greatly improved. Further, since oxygen is dissolved in fluorine constituting the perfluorosulfonic acid polymer electrolyte containing fluorine that easily adsorbs oxygen and is transported to the catalyst 54 in the oxidant gas catalyst layer 44, even if the oxidant gas catalyst layer is used. Even if the pores 62 are blocked due to freezing in the pores 44, oxygen continues to be supplied to the catalyst 54. For this reason, for example, when starting under a low temperature condition of less than 0 ° C., power generation is continued without causing inconvenience due to the freezing. The arrows in FIG. 3 illustrate the route through which the oxygen is carried.

  Here, the US Department of Energy (DOE) has announced target startup times of 5 and 30 seconds for startup of fuel cells from 20 ° C. and −20 ° C. to 50% output. When converted from this value, the target activation time from −30 ° C. to 50% output is 45 seconds, and the target activation time from −40 ° C. to 50% output is 60 seconds.

FIG. 5 shows a power generation characteristic test of a conventional polymer electrolyte fuel cell conducted by the present inventors. In this test, the unit cell was installed in an environment of 28 ° C., the unit cell output voltage at startup was 0.6 V, data was collected at 2-second intervals, and the power generation characteristics of the unit cell were shown from the data. Yes. As shown in FIG. 5, it rose after 50 seconds and reached a steady power generation state within 2 seconds, and an output of 0.324 W / cm ² was obtained. This output corresponds to 54% of a preset maximum output of 0.6 W / cm ² .

In addition, the present inventors conducted thermal analysis using the average current density as a parameter for the conventional solid polymer fuel cell startup time from below freezing point, and the startup time from −30 ° C. to 28 ° C. And the output voltage of the unit cell was measured. FIG. 6 shows the relationship. In FIG. 6, when starting at an average current density of 0.04 A / cm ² and 0.08 A / cm ² in an atmosphere of −30 ° C., the start is instantaneous and an output of 50% or more of the maximum output is possible. In order to raise the temperature to 28 ° C., it is indicated that a start time of 90 seconds and 45 seconds is required, respectively.

Furthermore, the present inventors function as a catalyst carrier in the fuel gas catalyst layer 42 of the fuel electrode 38 and the oxidant gas catalyst layer 44 of the oxidant electrode 40 in the polymer electrolyte fuel cell of the above-described embodiment. The mass ratio (Iw / Cw) of the ionomer 58 to the carbon particles (powder) 56 is 0.5 (Cw: Iw = 1: 0.5), 1 (Cw: Iw = 1: 1), 1.5 (Cw : Iw = 1: 1.5) Three types of samples (single cells) were prepared, and the power generation test was carried out in a freezer below freezing (−30 ° C.) with an average current density of 0.04 A / cm ^2. Went. FIG. 7 shows the results of the power generation test. In FIG. 7, the power generation continuation time of the sample in which the mass ratio (Iw / Cw) of the ionomer 58 to the carbon particles (powder) 56 is 0.5 is 90 seconds, and the power generation continuation time of the sample in which the mass ratio is 1.0 is 360. Seconds, 1. The power generation continuation possible time of the sample which is 5 was 850 seconds. Thus, it is shown that the power generation continuation time becomes longer as the mass ratio of the ionomer 58 to the catalyst carrier increases.

This is because when the mass ratio (Iw / Cw) of the ionomer 58 is 0.5 or more, the ratio of the ionomer 58 increases. For example, as shown in FIG. Perfluorosulfonic acid system containing fluorine that is easy to adsorb oxygen while avoiding freezing because much of the water W generated by power generation below freezing point below ℃ stays in the cluster of the polymer electrolyte constituting the ionomer 58 Since oxygen is dissolved in fluorine constituting the polymer electrolyte and is transported to the catalyst 54 in the oxidant gas catalyst layer 44, the pores 62 are blocked due to freezing in the pores 62 of the oxidant gas catalyst layer 44. However, it is shown that oxygen continues to be supplied to the catalyst 54, and the effect becomes more significant as the mass ratio of the ionomer 58 to the catalyst carrier increases. Which indicates that. Figure 8 is based on the power generation continuation time of the unit cell in average current density 0.04 A / cm ² in FIG. 7, was estimated power continuation time of the unit cell in average current density 0.08A / cm ² The value is shown. From the values shown in the chart of FIG. 8, in order to start up in 45 seconds of the target startup time from the estimated −30 ° C. to 50% output announced by the US Department of Energy (DOE), an average current density of 0. a 08A / cm ^2, the mass ratio of the ionomer 58 to carbon particles (powder) 56 that functions as a catalyst carrier in the fuel gas catalytic layer 42 and the oxidant gas catalytic layer 44 (Iw / Cw) of 0.5 or more and It turns out that it is necessary to do.

  In FIG. 7, the power generation continuation time of 850 seconds in an environment of −30 ° C. indicates that the water generated by the electrode reaction is frozen in the pores 62 of the fuel gas catalyst layer 42 and the oxidant gas catalyst layer 44. Even if it freezes in the pores 62 of the fuel gas catalyst layer 42 and the oxidant gas catalyst layer 44 by being present in the form of liquid or quasi-liquid water in the electrolyte membrane 36 or the ionomer 58, This confirms that proton conduction was maintained and high diffusivity of oxygen in the ionomer 58 was maintained. FIG. 9 shows the freezing point depression characteristics of water in the electrolyte membrane 36 and the ionomer 58. FIG. 9 shows that the ionomer 58 contained in the electrolyte membrane 36, the fuel gas catalyst layer 42, and the oxidant gas catalyst layer 44 is present in a cluster having a diameter of several nanometers, that is, in a pore having a radius of several nanometers. This indicates that the water to be frozen is difficult to freeze even in a severe cold atmosphere such as −30 ° C., and is not shown in the ionomer 58 of the electrolyte membrane 36, the fuel gas catalyst layer 42, and the oxidant gas catalyst layer 44. However, it is estimated that such clusters or pores on the order of several nanometers are provided. In addition, when the amount of water generated by the continuous power generation exceeds the amount that can be retained in the electrolyte membrane 36 or the ionomer 58, the oxidant gas diffusion layer 48 having a larger pore volume as illustrated in FIG. Frozen.

  FIG. 10 shows that the mass (weight) ratio (Iw / Cw) of the weight Iw of the ionomer 58 to the weight Cw of the carbon particles 56 as the catalyst support is 0.5 (Cw: Iw = 1: 0.5), 1 ( From the results of measuring the porosity of the fuel gas catalyst layer 42 and the oxidant gas catalyst layer 44 with Cw: Iw = 1: 1) and 1.5 (Cw: Iw = 1: 1.5) by the mercury intrusion method. FIG. 8 shows the estimation result of the amount of freezing in the pores 62 of the oxidant gas catalyst layer 44 of water generated by the electrode reaction in the power generation test of FIG. 7 (accumulated amount of ice mol). As is apparent from FIG. 10, when the mass ratio (Iw / Cw) is 0.5, power generation is performed immediately after the pores 62 of the fuel gas catalyst layer 42 and the oxidant gas catalyst layer 44 are closed by freezing of water. Has stopped. When the mass ratio (Iw / Cw) is larger than 0.5, when it is 1.0, even if the water generated by the electrode reaction freezes in the pores 62 of the oxidant gas catalyst layer 44, power generation is possible. Even if the ionomer 58 is frozen in the pores 62 of the ionomer 58, oxygen is dissolved and diffused in the fluorine contained in the oxidant gas catalyst layer 44 and water contained in the ionomer 58 and continues to be supplied to the catalyst 54. I support that.

  Further, in order to support the oxygen permeability in the ionomer 58, propane and fluorinated propane were used as models of the fluorine-based polymer constituting the ionomer 58, and the binding energy between these molecules and oxygen was estimated by quantization calculation. did. FIG. 11 shows the intermolecular energy (kJ / mol) of propane and fluorinated propane, and FIGS. 12A and 12B show the bond distances calculated from the bond energy due to the intermolecular force of propane and fluorinated propane ( Å) is shown respectively. As shown in the chart of FIG. 11, propane and fluorinated propane have higher binding energy due to intermolecular force. And as shown in FIG. 12, the bond distance by intermolecular force is smaller in the fluorinated propane. This indicates that the fluorine-based polymer has a property of easily adsorbing and taking in oxygen, and it is presumed that the fluorine atom plays a role of carrying oxygen. For this reason, it is understood that the fluorine-based polymer has high oxygen solubility when water is added, and as a result, the oxygen permeation amount increases.

  Incidentally, FIG. 13 schematically shows a state in which the mass ratio (Iw / Cw) of the weight Iw of the ionomer 58 to the weight Cw of the carbon particles 56 as the catalyst support is less than 0.5. In this case, the ionomer 58 has a relatively small volume and a large number of pores 62 as compared with FIG. 3, so that the water W is frozen in the pores 62 below the freezing point, and the oxygen gas is ionomer. Since it is difficult to sufficiently supply the catalyst 54 supported on the carbon particles 56 through 58, the power generation sustainable time of the polymer electrolyte fuel cell 16 is shortened.

  Next, another embodiment of the present invention will be described. In the following description, parts common to those in the above-described embodiment are denoted by the same reference numerals and description thereof is omitted.

  FIG. 14 shows a main part of another embodiment of the polymer electrolyte fuel cell 16. In this embodiment, in order to suppress the temperature rise of the polymer electrolyte fuel cell 16 due to operation, a cooling plate 52 in which a coolant circulation groove 50 for circulating a refrigerant such as water is interposed at an appropriate position. It is different in that it is made.

  In addition, although not illustrated, the present invention is implemented with various modifications within a range not departing from the gist thereof.

16: polymer electrolyte fuel cell 34: single cell 36: electrolyte membrane 38: fuel electrode 40: oxidant electrode 44: oxidant gas catalyst layer (catalyst layer of oxidant electrode)
54: Catalyst 56: Carbon particles (catalyst carrier)
58: Ionomer

Claims (3)

A unit cell in which a solid polymer electrolyte membrane is interposed between a fuel electrode and an oxidizer electrode having a catalyst carrier containing a catalyst and a catalyst layer containing an ionomer on the electrolyte membrane side through a separator. A plurality of polymer electrolyte fuel cells stacked,
In the catalyst layer of the oxidant electrode, the volume ratio of the ionomer to the catalyst carrier is 0.45 or more.   2. The polymer electrolyte fuel cell according to claim 1, wherein the ionomer in the catalyst layer of the oxidant electrode is a fluorine-based polymer electrolyte containing a fluorine atom. In the catalyst layer of the oxidant electrode, the catalyst carrier is carbon particles,
3. The polymer electrolyte fuel cell according to claim 1, wherein a mass ratio of the ionomer to the catalyst support in the catalyst layer of the oxidant electrode is 0.5 or more.

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