JP5167725B2 - Direct ethanol fuel cell - Google Patents

Description

  The present invention relates to a fuel cell using alcohol as a fuel, and more particularly to a solid polymer electrolyte fuel cell using alcohol directly as a fuel.

  A fuel cell is an electrochemical reaction device that has high energy conversion efficiency. In particular, a solid polymer electrolyte fuel cell (PEFC) using hydrogen fuel is a power source for a high-speed moving body such as an electric vehicle or a distributed power source. As expected. In general, in PEFC, a membrane electrode assembly (MEA) comprising two electrodes, a fuel electrode (anode) and an oxygen electrode (cathode), and a solid polymer electrolyte membrane (PEM) sandwiched between them is formed. A plurality of MEAs are stacked in units of power generation cells and stacked. In MEA, hydrogen is supplied to an anode, oxygen or air is supplied as an oxidant to a cathode, and an electromotive force is obtained by an oxidation reaction and a reduction reaction occurring at each electrode. In order to activate the electrochemical reaction in such an electrode, in most cases, a platinum catalyst is used for the electrode, and protons (hydrogen ions) generated at the anode pass through the solid polymer electrolyte membrane to the cathode. Electricity is generated by being conducted to. However, PEFC using hydrogen as a fuel has many problems to be solved for storage, transportation, supply, etc. of hydrogen fuel, especially supply of hydrogen fuel and its safety, improvement of power generation operation time using hydrogen fuel, etc. As long as the problem is not solved, it is generally recognized that it is not suitable for personal use aimed at power supplies for electronic devices such as small portable devices.

  Recently, a fuel cell that generates electricity by directly oxidizing an organic liquid fuel has attracted attention as a technique for solving these problems and realizing a small and light fuel cell. One form of the direct alcohol fuel cell (DAFC) is a fuel cell that performs an oxidation reaction by directly supplying alcohol as fuel to the anode with a power generation cell configuration similar to that of PEFC. It has attracted particular attention recently because it can be used. As a DAFC, a direct methanol fuel cell (DMFC) that uses methanol as a fuel is known, and it is smaller, lighter, and safer than PEFC and a fuel cell that uses hydrogen as fuel by reforming the fuel. It is excellent and is highly expected as a power source for portable electronic devices. In addition, direct ethanol fuel cells (DEFC) using ethanol as a fuel have safety and environmental advantages such as the use of plant-derived bioethanol, which is much greater than methanol, and recently researched and developed biobutanol is also fuel. As expected.

  Thus, although DAFC has many advantages, the anodic oxidation reaction of ethanol has a problem that the overvoltage is larger than that of methanol. That is, it is known that a platinum-based catalyst is suitable for anodic oxidation of alcohols such as methanol and ethanol, but a platinum-ruthenium catalyst is more suitable than a platinum catalyst (Non-patent Document 1). reference). However, even if a platinum-ruthenium catalyst is used for the anode, it is not sufficient for oxidation of methanol, and further insufficient for oxidation of other alcohols such as ethanol. For this reason, the development of a catalyst capable of obtaining output characteristics equivalent to or better than those of DMFC has been awaited as an anode catalyst for DEFC. Furthermore, many studies have been made on the mechanism of alcohol oxidation aimed at DAFC for the purpose of more effective catalysts (see Non-Patent Documents 2 to 5).

  However, an anode catalyst having practical performance, in particular, an anode catalyst that oxidizes alcohol having a large energy density such as ethanol having 2 carbon atoms or isopropyl alcohol (2-propanol) having 3 carbon atoms has not been realized. . This is due to the fact that the elucidation of the mechanism of the anodic oxidation reaction of alcohol is limited to a part, and the established theory has not been established. When an alcohol or an aqueous alcohol solution is used as a fuel, the anodic oxidation reaction has a complicated aspect that cannot be explained by a surface reaction between a PEFC catalyst and a solid-gas phase such as hydrogen. In this regard, representative knowledge is disclosed in Non-Patent Documents 4 and 5, but no theory has been established by those skilled in the art.

In DMFC, the reason that the anodic oxidation reaction of methanol is rate-limiting is that carbon monoxide generated by the anodic oxidation reaction of methanol is adsorbed and poisoned by a catalyst mainly composed of platinum. In order to solve such problems, development of various catalysts has been studied. Among them, the platinum-ruthenium catalyst is known to reduce poisoning by carbon monoxide, but in this case as well, the anodic oxidation reaction of methanol is rate limiting. It is generally considered that the anodic oxidation reaction of methanol using a platinum-ruthenium catalyst is as follows.
In DMFC, at the anode, the reaction formula CH ₃ OH + H ₂ O → 6H ⁺ + 6e ⁻ + CO ₂
In this case, a reaction formula CH ₃ OH + xPt → Pt _x CH ₂ OH + H ⁺ + e ⁻
CH ₂ OH + xPt → Pt _x CHOH + H ⁺ + e ⁻
CHOH + xPt → Pt _x COH + H ⁺ + e ⁻
CHOH + xPt → Pt _x CO + 2H ⁺ + 2e ⁻
Pt _x CHOH + PtOH → HCOOH + H ⁺ + e ⁻ + xPt
It is thought that an oxidation reaction represented by the above occurs.

At this time, COH (or HCO) and CO become poisonous species strongly adsorbed to platinum, and ruthenium is represented by the reaction formula Ru + H ₂ O → RuOH + H ⁺ + e−.
Pt _x COH + RuOH → CO ₂ + 2H ⁺ + 2e ⁻ + xPt + Ru
Pt _x CO ++ RuOH → CO ₂ + H ⁺ + e ⁻ + xPt + Ru
It is thought that it exerts the action represented by On the other hand, at the cathode, the reaction formula O ₂ + 4H ⁺ + 4e ⁻ → 2H ₂ O
It is considered that a reduction reaction represented by

Thus, even the anodic oxidation reaction of methanol having 1 carbon atom includes a complicated reaction, and therefore, the anodic oxidation reaction of other alcohols such as ethanol having 2 carbon atoms is considered to be further complicated. For example, as detailed in pages 800 to 802 of Non-Patent Document 3, since ethanol has a C—C bond, the reactant process is complicated and the intermediate product species are diverse. Briefly, the anodic oxidation reaction of ethanol is represented by the reaction formula C ₂ H ₅ OH + 3H ₂ O → 12H ⁺ + 12e ⁻ + 2CO _2.
It is more complicated than the anodic oxidation reaction of methanol. At this time, examples of poisoning species strongly adsorbed to platinum include —COH, CO, —COOH, —CH _3, etc., but dissociated species and intermediate generated species (for example, CH ₃₎ generated by an anodic oxidation reaction of ethanol. CHO, CH ₃ COOH, etc.).

In the anodic oxidation reaction of ethanol, as described above, intermediate products and by-products promote the complexity of the reaction system. According to Non-Patent Document 6, the anodic oxidation reaction of ethanol up to 40 to 50 ° C. is carried out by the reaction formula CH ₃ CH ₂ OH → [CH ₃ CH ₂ OH] ad → (R) ad → CO ₂ (complete oxidation reaction) Route)
CH ₃ CH ₂ OH → [CH ₃ CH ₂ OH] ad → CH ₃ CHO → CH ₃ COOH (incomplete oxidation reaction path)
It is supposed that the reaction pathway shown by follows advances in parallel. (R) ad means a poisoned species having 1 or 2 carbon atoms. Under normal conditions, it is difficult to oxidize (R) ad to carbon dioxide at many reaction sites (active sites) due to the strong adsorptive power of (R) ad, ethanol is oxidized to acetaldehyde, and acetaldehyde is acetic acid. It is oxidized to. In addition, since it is difficult to cut | disconnect the CC bond of acetic acid, the acetic acid which is an intermediate product remains without being oxidized, and it exists in a reaction system as a by-product. This is represented by the description of “oxidation of ethanol and acetic acid” in Table 6 of Patent Document 1.

At this time, if the temperature of the reaction system is increased, the anodic oxidation reaction proceeds due to an increase in the reaction rate and a decrease in poisoning sites that are easily poisoned by carbon monoxide, etc. Intermediate species also increase. For this reason, it is necessary to design a catalyst surface that selectively adsorbs and oxidizes reactive species that are oxidized to carbon dioxide, but it is very difficult. Acetaldehyde generated by anodic oxidation of ethanol is easily oxidized and transferred to acetic acid, but acetic acid that does not easily cleave the C—C bond is a by-product, which is resistance to reaction accompanying the movement and diffusion of substances, so-called concentration polarization. Appears as Specifically, FIG. 14 shows a comparison result of DAFC single-cell power generation reaction performed in the course of reaching the present invention using methanol and ethanol as fuel.
In the figure, the scale on the left is the generated voltage [unit: V], and the scale on the right is the output per unit area [unit: mW / cm ² ]. The output per unit area is obtained as the product of voltage and current per unit area. The arrows in the figure indicate which scale each curve corresponds to.
When ethanol is used, as compared with the case where methanol is used, when the current load region of 100 mA / cm ^{2 is} clearly exceeded, the voltage starts to drop rapidly. On the other hand, when methanol is used, the voltage drop is small even in the high current load region, and as a result, the highest power density is shown in the current load region near 400 mA / cm ² .

  As described above, when ethanol is used, the retention of reaction products (mainly by-products such as acetic acid) is considered to be an impediment to the diffusion and transfer of fuel. Loss of chemical reaction accompanying mass transfer, so-called concentration polarization Appears as As a technique for avoiding this problem, for example, wet oxidation (CWAO) using a catalyst disclosed in Non-Patent Document 7 can be cited. CWAO generally decomposes and removes acetic acid and the like using a catalyst and air (oxygen) in an aqueous environment of high temperature and high pressure (temperature of 200 ° C. or higher, pressure of several M to several tens of MPa or higher). It is. However, acetic acid is oxidized in a high-temperature and high-pressure aqueous environment of 200 ° C. or higher, and this oxidation reaction is not due to an electrochemical reaction using an electrode, and therefore does not contribute to the conversion of chemical energy to electric energy.

Special table 2006-501983 Souza et al, Journal of Electro analytical chemistry, 420 (1997), 17-20, Performance of a co-electrodeposited Pt-Ru electrode for the electro-oxidation of a ethanol studied by in situ FTIR spectro scopy Lamy et al, Journal of Power Sources, 105 (2002) 283-296, Recent advances in the development of direct alcohol fuel cells (DAFC) Lamy et al, Journal of Applied Electrochemistry, 31799-809 (2001), Electrocatalytic oxidation of aliphatic alcohols: Application to the direct alcohol fuel cell (DAFC) Zhu et al, Langmuir 2001, 17146-154, Attenuated Total Reflection-Fourier Transform Infrared Study of Methanol Oxidation on Sputtered Pt Film Electrode Wasmas et al, Journal of Electroanalytical Chemistry 461 (1999), 14-31, Methanol oxidation and direct methanol fuel cells: a selective review Fujiwara et al., Journal of Electroanalytical Chemistry, 472 (1999), 120-125, Ethanol oxidation on PtRu electrodes studied by differential electrochemical mass spectrometry Alec A. Klinghoffer et al., Catalysis Today, 40 (1998), 59-71, Catalytic wet oxidation of acetic acid using platinum on alumina monolith catalyst

  In view of the above-mentioned problems of the prior art, the present invention is an anode catalyst capable of improving the output characteristics of a fuel cell using alcohol containing two or more carbons as a fuel, particularly ethanol oxidation and acetic acid as a reaction product thereof. The present invention provides a catalyst and a method for operating a fuel cell that are effective for electrode oxidation.

The invention according to claim 1 is a fuel cell having a cell stack including at least one membrane electrode assembly in which an electrolyte membrane is sandwiched between a fuel electrode (anode) and an oxygen electrode (cathode). A direct ethanol fuel cell is characterized in that the operating temperature of the cell is divided into fuel cells having a temperature range of from normal temperature to 80 ° C. and at least two temperature ranges of 80 ° C. to 200 ° C.
The invention according to claim 2 is a fuel cell having a plurality of cell stacks each including at least one membrane electrode assembly in which an electrolyte membrane is sandwiched between a fuel electrode (anode) and an oxygen electrode (cathode). It is characterized by a direct ethanol fuel cell that is divided into a cell stack having a battery operating temperature ranging from room temperature to 80 ° C. and a fuel cell having at least two cell stacks with operating temperatures ranging from 80 ° C. to 200 ° C. .

According to a third aspect of the present invention, in the direct ethanol fuel cell according to the first or second aspect, at least one of the cell stacks in which the operating temperature is in the range of 80 ° C. to 200 ° C. is connected to the fuel electrode. An oxidation catalyst made of an alloy of platinum and molybdenum is used.
According to a fourth aspect of the present invention, in the direct ethanol fuel cell according to the first or second aspect, at least one of the cell stacks in which the operating temperature is in the range of 80 ° C. to 200 ° C. is connected to the fuel electrode. It is characterized by using an oxidation catalyst in which platinum and conductive tin oxide are combined.

According to a fifth aspect of the present invention, in the direct ethanol fuel cell according to the first or second aspect, at least one of the cell stacks in which the operating temperature ranges from 80 ° C. to 200 ° C. is provided in the fuel electrode. It is characterized by using an oxidation catalyst composed of platinum and tungsten trioxide.

  ADVANTAGE OF THE INVENTION According to this invention, the electrode catalyst which can improve the output characteristic of a direct type | mold ethanol fuel cell, and the efficient operating method of a fuel cell using the same can be provided.

FIG. 1 is a diagram showing a method for analyzing reactants and products in an electrode oxidation reaction.
In the figure, reference numeral 1 is a mass spectrometer, 2 is a Penning vacuum gauge, 3 is an ionization vacuum gauge, 4 is an orifice, 5 is a turbo molecular pump, 6 is a rotary pump, 7 is a bypass valve, 8 is a main valve, and 9 is a stop. Valves 10 are probes, and 11 is a test solution.
FIG. 2 is a graph showing the relationship between the concentration of reactants and products with respect to the charge amount of the electrode oxidation reaction. The figure (a) is a figure which shows the example by ethanol electrode oxidation by PtRu / C, and the figure (b) is a figure which respectively shows the example by ethanol electrode oxidation by PtSn / C.
The figure shows the relationship between the concentration of reactants and products with respect to the amount of charge in the electrode oxidation reaction.
Prior to describing the best mode for carrying out the present invention, knowledge of direct oxidation (by electrode oxidation) of ethanol obtained in the process leading to the present invention will be shown. FIG. 1 shows a method for analyzing a reaction product by ethanol electrode oxidation using a half cell (half battery), and FIG. 2 shows the result. FIG. 1 is a system devised by the inventor for analyzing dissolved components in a liquid. The inside of the mass spectrometer 1 is kept in a vacuum, and the test liquid 11 that has been subjected to electrode oxidation using an electrode having an electrode catalyst is sampled from the probe 10 into the apparatus by differential pressure (analysis path). PTFE (manufactured by Advantech Toyo Co., Ltd., pore size 0.5 μm) is attached to the tip of the probe and separates the liquid phase and the gas phase. In order to keep the inside of the mass spectrometer at a high vacuum, the sampling amount of the analysis path is limited by the orifice (fine hole) 4 and can be adjusted by the main valve 8 and the bypass valve 7. The bypass path is for exhausting the extra sampled portion and maintaining the inside of the mass spectrometer 1 at a high vacuum during the analysis. While monitoring the internal pressure of the apparatus with the Penning vacuum gauge 2 and the ionization vacuum gauge 3, the valves 7 and 8 are adjusted so that the internal pressure of the apparatus becomes a predetermined value. When the pressure is higher than the predetermined pressure, the bypass valve 7 is opened or the main valve is closed to reduce the pressure. The reverse is true when the pressure is lower than the predetermined pressure. The pressure when the sample is not introduced, that is, the background pressure, is set so that the valves 7 to 9 are fully closed, and the Penning vacuum gauge is 4.0 × 10 ⁻⁷ Torr or less. The ionization vacuum gauge is 5.0 × 10 ⁻⁷ Torr or less. To do. The pressure when introducing the sample gas is set to Penning vacuum gauge; 1.3 × 10 ⁻⁶ Torr, ionization vacuum gauge; 1.7 × 10 ⁻⁶ Torr.

For the electrode oxidation of ethanol, an aqueous solution (20 cc, ethanol; 0.02 mol) mixed with 1M ethanol and 1N sulfuric acid was used as the electrolyte. PtSn / C (manufactured by E-TEK, Pt ₂ Sn / C loading 40 wt%, 1.3 mg / cm ² -Pt) and PtRu / C (manufactured by Tanaka Kikinzoku, TEC61E54DM, catalyst element composition ratio Ru / Pt = 1 0.5, 1.3 mg / cm ² -Pt) was used as the working electrode. Also, a platinum wire was used for the counter electrode, and Ag / AgCl was used for the reference electrode. In the analysis of the components dissolved in the liquid, the electrolytic solution after electrode oxidation was used as a test solution. Ethanol electrode oxidation was performed at room temperature (23 ° C.) at 0.5 V vs. This was carried out by maintaining a constant potential of Ag / AgCl. For ethanol, the reaction products of electrode oxidation, acetaldehyde and acetic acid, measure the relationship between the dissolved concentration in the solution and the output (partial pressure value) by mass spectrometry in advance to obtain a calibration curve, and calculate the concentration. It was.

  Assuming that the ethanol electrooxidation is a two-electron reaction in which 100% acetaldehyde is calculated, a charge of about 4,000 C is obtained when all 0.02 mol of ethanol is reacted (2 × 0.02 mol × Faraday constant). ). Also, assuming that the 4-electron reaction is 100% acetic acid by the reaction of ethanol and water in electrode oxidation, about 8,000 C is obtained, and if it is assumed that the 12-electron reaction is 100% carbon dioxide, about 24,000 C is obtained. It is done. This time, the solution reacted at about 1,000 C increments was measured. The result is shown in FIG. Referring to FIG. 2, even when using PtSn / C or PtRu / C catalyst materials, the reaction temperature of about room temperature is far from the amount of charge obtained when ethanol is converted into carbon dioxide through complete oxidation. It can be easily seen that the amount of acetic acid produced gradually increases while repeating the incomplete oxidation reaction in which the electrode oxidation of ethanol becomes acetic acid via acetaldehyde.

  As described above, at the reaction temperature of about room temperature, it is difficult to efficiently promote electrode oxidation to carbon dioxide by ethanol electrode oxidation. Electrode oxidation of ethanol leads to acetic acid via an intermediate reaction that generates acetaldehyde, stops incomplete oxidation without being able to cleave the C—C bond of acetic acid, and the number of electrons that can be taken out is 12-electron reaction leading to carbon dioxide. Compared with energy efficiency, fuel utilization efficiency is remarkably lowered as well as energy efficiency. When used for power generation, a relatively large amount of products means that depending on the fuel supply state, mass transfer related to the reaction, that is, fuel diffusion, is not performed properly with respect to the distribution of fine reaction sites. This means that the ratio of reaction products cannot be increased efficiently, and the power generation characteristics cannot be maintained over time.

FIG. 3 is a diagram showing the concept of the present invention.
As shown in the previous FIG. 14, the above-mentioned influence appears greatly in a relatively large load current region. As a means to quickly realize a practical direct ethanol fuel cell (DEFC), development of a catalyst that promotes complete oxidation of ethanol at a low temperature of about 80 ° C., for example, a Pt-based metal alloy catalyst as shown in the prior art Although there is a method, the inventors have found a method for solving the above-mentioned problem by using a second catalyst that oxidizes acetic acid with high activity or selectively as shown in FIG. In the electrode reaction where the electrode oxidation of ethanol has progressed to acetic acid generation, acetic acid electrode oxidation is performed using a material exhibiting high activity in the reaction of acetic acid at a predetermined temperature as a second catalyst to obtain electrons, and carbon dioxide. Advance the reaction until. The material that promotes the reaction to electrodeacetate by ethanol oxidation and acetic acid by further oxidation may be an alloy catalyst (platinum-based alloy) based on Pt, and more preferably ethanol than the platinum-based alloy described above. Even if the electrode oxidation activity is inferior, the second catalyst which is excellent in selective reactivity with respect to the oxidation activity of acetic acid and is shown is desirable.

Next, the best mode for carrying out the present invention will be described with reference to the drawings.
FIG. 4 is a diagram showing the configuration of the MEA of the present invention.
In the figure, reference numeral 100 is a fuel cell, 101 is a cell stack, 102 is a fuel circulation part, 103 is an oxidant circulation part, 104 is a fuel container, 105 is a pump, 106 is a dilution mixing reservoir, 107 is a concentration sensor, and 108 is an ion. Reference numeral 109 denotes an air filter, 110 an air blower, 111 a condenser, 112 a cooling fan, 113 and 114 gas-liquid separators, 115 a heat exchanger, 116 an output regulator, and 117 a control system.
As the fuel of the fuel cell 100, ethanol fuel can be used. Moreover, air, oxygen, etc. can be used as an oxidizing agent. In addition, in order to forcibly supply the fuel and oxidant to the cell stack 101, the fuel cell 100 devises the flow of fuel and product water, and mixes high-concentration fuel and water in the dilution / mixing reservoir 106. However, a so-called passive type in which the fuel and oxidant are not forcibly supplied to the cell stack 101 is also possible. The cell stack 101 is provided with a fuel circulation part 102 and an oxidant circulation part 103. A high-concentration fuel is stored in the fuel container 104 and is supplied to the dilution / mixing reservoir 106 by the pump 105. Next, the high-concentration fuel is diluted with water, the concentration of the fuel is adjusted by the concentration sensor 107, and then forcibly supplied to the fuel circulation unit 102 through the ion filter 108. On the other hand, the oxidizing agent is forcibly supplied to the oxidizing agent circulation section 103 by the air blower 110 through the air filter 109.

FIG. 5 is a conceptual diagram showing the configuration of the cell stack.
In the figure, reference numeral 201 denotes an MEA, 202 denotes a casing, and 203 denotes a casing.
The cell stack 101 is configured as shown in FIG. The MEA 201 in which the electrolyte membrane is sandwiched between the anode and the cathode has a fuel channel 102a on the anode (fuel electrode) side, an oxidant channel 103a on the cathode (air electrode) side, and the fuel channel 102a. A housing 203 is formed on the housing 202 and the oxidant flow path 103a to form one power generation cell structure. The stack has the same configuration, and a large number of cells are stacked via a separator which is a casing having fuel and oxidant flow paths. At this time, the fuel flow part 102 and the oxidant flow part 103 that penetrate the cell stack 101 are connected to the fuel flow path 102a and the oxidant flow path 103a, respectively. Further, the cell stack 101 is heated to 140 ° C. to 200 ° C. by a thin flat heater (not shown). Therefore, an aqueous fuel containing an alcohol such as ethanol is vaporized to be introduced into the anode in a high temperature and high pressure aqueous environment.

As the electrolyte membrane constituting the MEA 201, a heat-resistant material is used, for example, a proton conductor using a non-aqueous proton carrier, or a high temperature response using polybenzimidazole (PBI) doped with phosphoric acid. And electrolyte membranes. As an example, the former is an electrolyte membrane using CDP / SiP ₂ O _{7 in} which cesium dihydrogen phosphate (CDP) and SiP ₂ O ₇ are combined as a solid acid salt complex electrolyte, and the latter is an aromatic sulfonated polymer ( There are electrolyte membranes in which PBI (Polybenzimidazole) is mixed with a SPEEK (Sulfonic Polyether Ether Ketone) membrane.

  For the anode of MEA201, multi-component alloy materials such as Pt-Ru, Pt-Ru-Sn, Pt-Ru-Mo, Pt-Ru-W are used as the Pt-based alloy catalyst described above as the electrode catalyst. An alloy of platinum Pt and molybdenum Mo, a composite of Pt and conductive metal oxide, and a composite of Pt and nonconductive oxide are preferable. These electrode catalysts are used by supporting them on fine carbon black particles (for example, Vulcan XV-72R manufactured by Cabot) or conductive fine particles. In addition, a cathode in which Pt or a Pt-based alloy is supported on carbon black or the like is used as an electrode catalyst. The entire cell stack 101 includes a member and a seal structure that can handle a pressure of several MPa, and can withstand a saturated water vapor pressure of about 0.5 MPa at 150 ° C., for example. As shown in the figure, pressure regulators are installed at the outlets of the fuel and the oxidant, and ordinary members can be used as members of the entire system even when the back pressure becomes excessive.

  In addition, the entire cell stack 101 is covered with a heat insulating material so that residual heat is not radiated to the outside so that mechanical parts and electronic parts are not affected. As the pressure regulator, for example, an ultra-small pressure regulator PRD-3 (US, manufactured by Bestic Engineering) having a maximum input rating of 3.5 MPa and a size of a coin and a weight of about 50 g was used. Exhaust matter containing unreacted fuel, water, carbon dioxide and other products discharged from the fuel circulation section 102 of the cell stack 101 is recovered as circulating water and fuel by the gas-liquid separator 114 and the heat exchanger 115. At the same time, it is discharged as exhaust gas. Exhaust matter containing products such as unreacted oxidant and water discharged from the oxidant circulation part 103 of the cell stack 101 is recovered as circulating water by the condenser 111, the cooling fan 112 and the gas-liquid separator 113. Is discharged as exhaust gas. The DC output of the cell stack 101 is controlled to a desired output voltage and output specifications by an output regulator (power conditioner) 116. The fuel cell 100 also requires a mechanism for starting the power generation of the cell stack 101 and maintaining the fuel cell power generation, that is, a so-called Balance of Plant (BOP). Specifically, the fuel cell 100 uses auxiliary equipment such as a pump 105, a concentration sensor 107, and an air blower 110 that are used to maintain power generation such as control of fuel concentration and supply amount, replenishment timing, and procedure. A control system 117 that performs various controls is provided.

FIG. 6 is a view showing an apparatus for measuring an electrode oxidation reaction.
In the figure, reference numeral 21 is a working electrode, 22 is a reference electrode, 23 is a counter electrode, 24 is a thermocouple, 25 is a protective tube, 26 is an electrolytic solution, 27 is a ceramic plate, and 28 is a container.
The catalyst material of the present invention can be evaluated in the form of the electrode oxidation activity of the anode electrode in the form of a half cell (half battery) without going through the form of MEA shown in FIG. That is, a catalyst material is formed on an electrochemically inactive substrate (for example, a gold substrate), electrode oxidation of alcohol or acetic acid is performed in the electrolytic solution in the electrolytic cell shown in FIG. 6, and the oxidation current is measured. The reaction activity can be evaluated. The electrolysis cell (electrochemical reaction apparatus) in the figure has an autoclave (pressure vessel) 11. The autoclave 11 is made of Hastelloy, and a container 28 made of quartz or Pyrex (registered trademark) is placed on a ceramic plate 27 inside. In the container 28, an electrolyte solution (solution containing an electrolyte) 26 and three electrodes, that is, a working electrode 21, a reference electrode 22, and a counter electrode 23 are arranged, and an electrochemical reaction is performed by a three-electrode method. The working electrode 21 is an electrode substrate or an electrode substrate having a catalyst formed on the surface thereof.

  The reference electrode 22 is an electrode that serves as a reference for the potential of the working electrode 21, and is also referred to as a reference electrode. In order to always check the reference potential under high temperature and high pressure, a pressure balanced external reference electrode is used. ing. The pressure balanced external reference electrode performs a predetermined temperature correction to determine the reference potential. Further, the counter electrode 23 uses a platinum wire or coil. As the electrolytic solution 26, for example, a 0.5 M sulfuric acid aqueous solution or a mixture of alcohol and the like can be used. The thermocouple 24 is a sensor that measures and controls the temperature of the electrolytic solution 26, and is protected by a Teflon (registered trademark) protective tube 25 so that the electrolytic solution 26 does not enter in order to prevent corrosion. Furthermore, the ceramic plate 27 has a role of adjusting and protecting the height of the container 28. Further, the autoclave 11 is provided with a reference electrode holder 11a for fixing the reference electrode 22, an electrode holder 11b for fixing the working electrode 21 and the counter electrode 23, and a pressure gauge 11c. In such an autoclave 11, electrode oxidation can be performed in an aqueous environment of 100 to 300 ° C. and 0.1 to 10 MPa using an electrolysis facility (not shown) typified by a potentiostat.

A sputtering method was used as the catalyst material. Specifically, conditions are set to a predetermined sputtering pressure using a sputtering apparatus (ULVAC CS-200S), and a catalyst metal or a metal oxide material placed on each sputtering cathode is formed on a gold substrate. The sputtering method was performed while rotating the substrate using Ar gas discharge. Samples sputtered on both sides of the gold substrate were punched out with a φ6 mm Thomson blade for all catalyst samples, φ0.3 mm × 35 mm gold wires were spot welded thereto, and gold-plated pin connectors were crimped to the tips of the gold wires to form working electrodes. .
Table 1 shows the list of catalyst materials prepared and the results of elemental analysis.

The compositional ratio of the elemental analysis result is analyzed using SEM-EDS, and the result indicates the atomic ratio (at%). The measurement mode of electrode oxidation measured using the apparatus shown in the figure is cyclic voltammetry (CV), the sweep rate is 50 mV / s, and the sweep potential width is Eobs: −0.10 to 1.20 V. However, since it was a measurement at high temperature, it was converted into _ESHE by the following formula.

E _SHE = Eobs + 0.2866-10 ⁻³ (T−t)
+ 1.745 × 10 ⁻⁷ (T−t) ²
−3.03 × 10 ⁻⁹ (T−t) ³
T: Autoclave internal temperature (° C), t: Room temperature (° C)

FIG. 7 is a diagram showing an example of the catalyst of the present invention. The figure (a) is a figure which shows the electrode oxidation of ethanol in electrode potential 0.5V (vs.SHE), and the figure (b) is a figure which shows the electrode oxidation of acetic acid.
The measurement environment is room temperature to 250 ° C., and the pressure is a saturated vapor pressure.
The gold substrate on which the catalyst is formed as described above is used as the working electrode 21 shown in FIG. 6, and electrochemical measurement using a half cell is performed. The horizontal axis represents the reciprocal of the absolute temperature (K) of electrode oxidation, and the vertical axis represents the oxidation current value. The scale at the top of the horizontal axis is a display in which the reciprocal of absolute temperature is converted to temperature (° C.).
Pt—Mo as an example of an alloy of platinum Pt and a metal element, Pt—SnO ₂ (conductive tin oxide) as an example of a composite of Pt and conductive metal oxide, and a composite of Pt and nonconductive oxide As an example, Pt—WO ₃ (tungsten trioxide) was prepared by a sputtering method and evaluated. From the result of FIG. 7, the oxidation current value of acetic acid is smaller by one digit or more than the oxidation current value of ethanol at around 130 ° C., but it is very close to that of ethanol at around 160 ° C. The oxidation current value is an index reflecting the reaction rate of the oxidation reaction. From this, it can be seen that the decomposition due to the oxidation of acetic acid is markedly improved compared to the temperature of room temperature to 100 ° C. or less with respect to the production of acetic acid by the oxidation of ethanol. Pt-Mo is good as the Pt-based alloy, and the composite of Pt and metal oxide is effective except for Pt-CuO (copper oxide).

FIG. 8 is a diagram showing electrode oxidation of ethanol / acetic acid of a Pt—SnO ₂ catalyst material.
FIG. 9 is a diagram showing electrode oxidation of ethanol / acetic acid of the Pt—Mo catalyst material.
In the same manner as in Example 1, a catalyst material was formed by a sputtering method, and the compounding ratio of the metal or metal oxide compounded with Pt was changed to compare with Pt alone (Comparative Example 1: Ref. 1). Table 2 shows the prepared catalyst materials. Similarly, the measured value of the oxidation current obtained by performing electrode oxidation up to a high temperature is shown. The same numbers as in Table 1 indicate the same examples.

As shown in FIG. 8, when Pt—SnO ₂ has no Pt, that is, SnO ₂ alone has no catalytic activity with respect to ethanol and acetic acid regardless of the reaction temperature. On the other hand, when the composition ratio of Pt is relatively larger than the ratio of Sn elements constituting SnO ₂ , the acetic acid oxidation rate is slow, which is disadvantageous. In the case of Pt alone, both ethanol and acetic acid have poor activity even when the reaction temperature is increased. From FIG. 9, it is desirable that the Pt—Mo alloy system has the same composition ratio of Pt and Mo. By the way, the electrode oxidation of ethanol is as follows when Pt alone is used.

FIG. 10 is a diagram showing the temperature dependence of electrode oxidation of ethanol by simple Pt.
As a comparative example, this figure shows CV (cyclic voltammetry) curves at temperatures of 30, 50, 75, 100, and 125 ° C. by changing the electrode oxidation of ethanol from room temperature to 130 ° C. It can be seen that the oxidation current changes when the potential (electrode potential) is changed at a constant rate (the direction in which the potential is increased and the direction in which the potential is decreased are taken as one set). As the temperature rises, the oxidation current of ethanol rapidly increases. From around 130 ° C, it exceeds the maximum measurement range (about 0.12A). It was. The change of ethanol oxidation current with temperature showed that Pt-Ru was larger than that of Pt alone, and that the catalyst material shown in the present invention such as a composite of Pt and metal oxide was larger than Pt-Ru. . Therefore, in the embodiment of the present invention, the electrode oxidation of ethanol is up to 130 ° C., and the temperature above this is compared with the catalyst material, and in the vicinity of 160 to 180 ° C. where the electrode oxidation of acetic acid is active, Comparison between acetic acid oxidation and ethanol oxidation was judged by extrapolating the current characteristics of ethanol oxidation up to 130 ° C.

FIG. 11 is a table summarizing the results of Example 1, Example 2, Comparative Example 1 (Pt alone), and Comparative Example 2 below.
Further, as a second comparative example, a comparison was made with the current value of methanol oxidation using Pt-Ru (represented as Ref. 2). Pt-Ru is an alloy catalyst suitable for electrooxidizing DMFC, that is, methanol. For ethanol oxidation, an oxidation current comparable to that of methanol oxidation by Pt—Ru can be obtained from room temperature to a high temperature range for all of Pt—Mo, Pt—WO ₃ and Pt—SnO ₂ . However, when this Pt-Ru is used for electrode oxidation of ethanol and acetic acid, the oxidation activity is low in the range from a low temperature to a high temperature range, and in the case of acetic acid, an oxidation reaction does not appear even at about 180 ° C. On the other hand, by combining Pt and a metal oxide, a value close to the activity level of ethanol oxidation can be obtained.

In order to compare the catalyst samples of Examples 1 and 2 and Pt-Ru, a comparison of the electrooxidation of Pt and Pt-Ru with methanol, ethanol and acetic acid was performed. The results are shown in Table 3. The electrode potential was fixed at 0.5 V (vs. SHE). When Pt—Ru is used, methanol oxidation at 100 ° C. shows the same reaction activity as Pt—Mo, Pt—WO ₃ , and Pt—SnO ₂ shown in Example 2, but ethanol oxidation is about 1/4 of that. there were. From this, the oxidation activity of ethanol is not as high as Pt—Mo, Pt—WO ₃ , and Pt—SnO ₂ at temperatures of 160 ° C. to 180 ° C., and electrode oxidation of ethanol is performed using Pt—Ru, and the generated acetic acid is The superiority of using Pt—Mo, Pt—WO ₃ , and Pt—SnO ₂ that can be efficiently and selectively oxidatively decomposed in this temperature range was confirmed. Further, Pt—Ru is less than 1/10 of the catalyst material shown in the present invention at a temperature of about 160 ° C. to 180 ° C. for oxidation of acetic acid. This gives a kinetic equilibrium between acetic acid production by ethanol oxidation and treatment by acetic acid oxidation. Therefore, efficient ethanol electrode oxidation becomes possible by using Pt—Ru and Pt—Mo, Pt—WO ₃ or Pt—SnO ₂ in combination.

FIG. 12 is a diagram for explaining the outline of the third embodiment of the present invention.
In the figure, reference numeral 210 is an electrochemical reaction device, 211 is an MEA, 212 is a fuel flow path, 213 is an oxidant flow path, 214 and 215 are housings, and 216 is a heat insulation part.
The fuel flow part 102 and the oxidant flow part 103 that penetrate the cell stack 101 shown in FIG. 4 are connected to the fuel flow path 102a and the oxidant flow path 103a in FIG. 5, respectively. Further, the cell stack 101 is a fuel cell in which the cell temperature is maintained at a temperature of about 80 ° C. As shown in FIG. 12, the configuration of this example is such that an electrochemical reaction device 210 that reacts at a saturated water vapor pressure of 160 to 180 ° C. in an aqueous environment of high temperature and high pressure is installed close to the cell stack 101. Yes. In the electrochemical reaction device 210, similarly to the cells of the cell stack 101, a fuel channel 212 is formed on the anode side of the MEA 211, an oxidant channel 213 is formed on the cathode side, and a casing 214 is formed on the fuel channel 212. The cell stack has a cell in which a casing 215 is formed on the oxidant channel 213, and these cells are stacked via the casing serving as a separator (stacked state). Is not shown).

  Further, a heat insulating portion 216 is formed outside the cell. At this time, the anode and cathode of the MEA function as a working electrode and a counter electrode, respectively. Note that a heat-resistant material is used as the electrolyte membrane constituting the MEA 211, and examples thereof include polybenzimidazole (PBI) doped with phosphoric acid as a proton conductor using a non-aqueous proton carrier. In addition, the electrochemical reaction device 210 is maintained at 160 to 180 ° C. by heating the heat insulating portion 216 using a thin film heater (not shown) to which a part of the output of the cell stack 101 is supplied. .

  In this way, the electrochemical reaction device 210 is kept at a high temperature and a high pressure while being insulated from the surroundings. In addition, after the carbon dioxide is removed by the gas-liquid separator (not shown), the waste containing the products such as unreacted fuel, water, and carbon dioxide discharged from the fuel circulation part 102 of the cell stack 101 It is supplied to the fuel flow path 212 of the chemical reaction device 210. In addition, the effluent containing products such as unreacted oxidant and water discharged from the oxidant flow part 103 of the cell stack 101 is supplied to the oxidant flow path 213 of the electrochemical reaction device 210. Further, the waste containing the products such as unreacted fuel, water and carbon dioxide discharged from the fuel flow path 212 is recovered as circulating water and fuel by the gas-liquid separator 114 and the heat exchanger 115, It is discharged as exhaust gas. In addition, the exhaust including the products such as the unreacted oxidant and water discharged from the oxidant flow path 213 is recovered as circulating water by the condenser 111, the cooling fan 112, and the gas-liquid separator 113, and the exhaust gas. As discharged.

As described above, the fuel and oxidant are supplied to the cell stack 101 to generate electric power, and the discharges discharged from the fuel flow passage 102 and the oxidant flow passage 103 are respectively supplied to the fuel flow passage of the electrochemical reaction device 210. 212 and the oxidant channel 213 are supplied to perform an electrochemical reaction. A pressure regulator for adjusting the back pressure is provided on the fuel and oxidant discharge side of the electrochemical reaction device 210. It is also possible to add the surplus output to the output of the entire fuel cell 100 by stacking the cells of the electrochemical reaction device 210 and devising the balance between the energy for heating the heat insulating portion 216 and the power generation output. For example, when ethanol is used as the fuel, the anode of the cell stack 101 in FIG. 4 uses Pt—Sn or Pt—Mo, which have high ethanol electrode oxidation reaction activity, as the main catalyst material, and reacts at 160 to 180 ° C. For the anode of the electrochemical reaction device 210 held at a temperature, a catalyst material that is excellent in acetic acid oxidation under high temperature such as Pt—Mo, Pt—WO ₃ , Pt—SnO ₂ is mainly used. Since the operating temperature is high, it is also effective for methanol and other alcohol fuels such as ethanol and isopropyl alcohol whose output could not be obtained at an operating temperature of 100 ° C. or lower.

FIG. 13 is a diagram showing an outline of the fourth embodiment of the present invention.
The stack 1 is a cell stack in which the cell temperature is maintained at a temperature of about 80 ° C. like the cell stack 101 of FIG. Similar to FIG. 12, the configuration of the present embodiment is similar to the method of supplying fuel and oxidant in a high temperature and high pressure aqueous environment where, for example, a stack 2 that reacts at a saturated water vapor pressure of 160 to 180 ° C. is connected to the stack 1. It is an aspect. Further, a stack 3 that reacts at a saturated water vapor pressure of 160 to 180 ° C. is connected downstream of the stack 2 with respect to the flow of the fuel and the oxidant. Similarly, a plurality of stacks to be reacted at a saturated water vapor pressure of 160 to 180 ° C. are connected, and a cascaded multi-stage connection configuration is adopted.

Stack 1 performs electrode oxidation using a catalyst material sufficient for the oxidation of ethanol, for example, an alloy-based catalyst such as Pt-Ru, and acetic acid is generated via an intermediate product such as acetaldehyde. A part of acetic acid that could not be oxidatively decomposed in this temperature range is discharged out of the system. The discharged fuel containing acetic acid is further supplied to the stack 2 maintained at 160 to 180 ° C. As can be seen from FIG. 2, when a Pt—Ru catalyst material is used, for example, when a charge amount of 2000 C is taken out, about 25% of the ethanol fuel is converted to acetic acid. When the stack 2 uses the same catalyst material Pt-Ru as that of the stack 1 and the temperature is similar, acetic acid is increased without being oxidatively decomposed. When trying to obtain a constant current load, the ethanol fuel concentration and the acetic acid concentration may be similar or reversed. Even if such a fuel is supplied, the stack 2 is kept at 160 to 180 ° C., and the catalyst material is a composite of Pt and metal oxide such as Pt—SnO ₂ of the present invention, thereby generating a reaction of acetic acid by ethanol oxidation. And the rate of oxidative degradation of acetic acid can be kept at the same level, or (orderly) not too far apart.

  For this reason, it is possible to cause the stack 2 to generate power with appropriate ethanol while oxidizing and decomposing acetic acid. According to the estimates of the inventors, the concentration of acetic acid, which was about 30% on the fuel discharge side of the stack 1 with respect to the ethanol concentration, can be further reduced by 23% by using the stack 2. A similar reduction rate was achieved with the stack 3 having the same configuration and operating temperature. Thus, by connecting the stacks having the same performance as the stack 2 in multiple stages, it is possible to obtain electrical energy from the electrochemical reaction while reducing the increase in acetic acid to the limit. In the configuration of this embodiment, the first stage fuel cell stack has a temperature of about 80 ° C., and normal fuel and oxidant supply means are used. In addition, the stack that is maintained at a relatively high temperature after stack 2 is insulated, and the pressure generated by the high temperature adjusts the back pressure. Therefore, the entire system is configured in such a way as to supply normal liquid fuel directly. does not change.

It is a figure which shows the analysis method of the reaction material and product in an electrode oxidation reaction. It is a figure which shows the relationship of the density | concentration of the reactant and the product with respect to the charge amount of the electrode oxidation reaction. It is a figure which shows the concept of this invention. It is a figure which shows the best embodiment of this invention. It is a figure which shows the structure of MEA of this invention. It is a figure which shows the apparatus which measures an electrode oxidation reaction. It is a figure which shows an Example about the catalyst of this invention. Ethanol Pt-SnO ₂ catalyst material is a diagram showing an electrode oxidation of acetic acid. It is a figure which shows the electrode oxidation of ethanol and acetic acid of a Pt-Mo catalyst material. It is a figure which shows the temperature dependence of ethanol electrode oxidation of Pt simple substance. It is a figure which shows the whole Example about the catalyst of this invention. It is a figure which shows the outline | summary of Example 3 of this invention. It is a figure which shows the outline | summary of Example 4 of this invention. It is a figure which shows the result of having compared the DAFC single cell power generation reaction.

Explanation of symbols

DESCRIPTION OF SYMBOLS 100 Fuel cell 101 Cell stack 102 Fuel distribution part 103 Oxidant distribution part 104 Fuel container 105 Pump 201 Membrane electrode assembly (MEA)
202 Case 203 Case 210 Electrochemical reaction device 211 MEA
212 Fuel channel 213 Oxidant channel

Claims (5)

  A fuel cell having a cell stack having at least one membrane electrode assembly in which an electrolyte membrane is sandwiched between a fuel electrode (anode) and an oxygen electrode (cathode), and the operating temperature of the fuel cell is from room temperature to 80 ° C And a direct ethanol fuel cell, wherein the fuel cell is divided into at least two temperature ranges of 80 ° C. to 200 ° C.   A fuel cell having a plurality of cell stacks having at least one membrane electrode assembly in which an electrolyte membrane is sandwiched between a fuel electrode (anode) and an oxygen electrode (cathode), and the operating temperature of the fuel cell is from room temperature to 80 ° C. A direct ethanol fuel cell, characterized in that it is divided into a fuel cell having at least two cell stacks having an operating temperature ranging from 80 ° C. to 200 ° C.   3. The direct ethanol fuel cell according to claim 1, wherein at least one of the cell stacks in which the operating temperature is in a range from 80 ° C. to 200 ° C. is an oxidation catalyst made of an alloy of platinum and molybdenum in the fuel electrode. A direct ethanol fuel cell characterized in that 3. The direct ethanol fuel cell according to claim 1, wherein at least one of the cell stacks in which the operating temperature is in the range of 80 ° C. to 200 ° C. is a composite of platinum and conductive tin oxide in the fuel electrode. A direct ethanol fuel cell using an oxidation catalyst. 3. The direct ethanol fuel cell according to claim 1 , wherein at least one of the cell stacks in which the operating temperature ranges from 80 ° C. to 200 ° C. is an oxidation in which platinum and tungsten trioxide are combined in the fuel electrode. A direct ethanol fuel cell characterized by using a catalyst .

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