Anode catalyst for polymer electrolyte fuel cells with excellent CO-resistant catalyst toxicity.

Abstract

The present invention provides an anode catalyst for polymer electrolyte fuel cells that exhibits superior resistance to CO catalyst poisoning and better durability compared to PtRu alloy catalysts. [Solution] The present invention relates to an anode catalyst for a polymer electrolyte fuel cell having catalyst particles for processing a fuel gas containing carbon monoxide (CO). The present invention is characterized by the application of catalyst particles composed of Pt and Ir. PtIr is a more suitable catalyst particle than PtRu, which has been known to have superior CO catalyst toxicity resistance. These catalyst particles include regions on their surface where four or more Ir atoms are adjacent and clustered.

Description

[Technical Field]

[0001] This invention relates to an anode catalyst for advancing the anode reaction in a polymer electrolyte fuel cell. In particular, it relates to an anode catalyst for a polymer electrolyte fuel cell with improved catalyst toxicity resistance to CO (carbon monoxide) contained in the fuel gas. [Background technology]

[0002] Solid polymer fuel cells are being increasingly commercialized as power sources for automobiles and homes. A solid polymer fuel cell primarily consists of a membrane / electrode assembly (MEA) comprising a hydrogen electrode (anode) supplied with a hydrogen-containing fuel gas, an air electrode (cathode) supplied with an oxidizing gas such as oxygen or air, and a solid polymer electrolyte membrane sandwiched between these electrodes. The anode and cathode are composed of catalysts in which catalyst particles made of precious metals such as Pt (platinum) are supported on a carrier such as carbon fine powder. In solid polymer fuel cells, the catalysts used in the anode and cathode are primarily required to exhibit catalytic activity to promote the reactions occurring at each electrode, but in addition, they are also required to have additional characteristics depending on the composition of the supplied raw materials (fuel gas and oxidizing gas) and the load during operation.

[0003] Here, the anode catalyst for polymer electrolyte fuel cell is involved in the oxidation reaction of hydrogen gas (HOR: H2 → 2H + +2e - Pt catalysts, which have high activity and are resistant to corrosion in polymer electrolyte fuel cells under acidic conditions, are generally used. In addition to this catalytic activity, resistance to CO (carbon monoxide) catalyst poisoning is required. The hydrogen supplied to the anode is sometimes a reformed gas obtained from gasoline or methanol. These reformed gases often contain trace amounts of CO from the synthesis process. The CO contained in the fuel gas adsorbs onto the catalyst particles and occupies the active sites, causing catalyst degradation (deactivation) over time. Therefore, the anode catalyst is required to be resistant to CO catalyst poisoning.

[0004] For anode catalysts used in polymer electrolyte fuel cells, catalysts using PtRu alloy, which is an alloy of Pt and Ru (ruthenium), as catalyst particles have been conventionally known as having excellent resistance to CO catalyst poisoning (Patent Documents 1, 2, etc.). Catalysts using Pt alloys such as PtRu alloy as catalyst particles can be manufactured by impregnating a support such as carbon fine powder with a solution containing Pt ions and Ru ions, followed by reduction treatment and heat treatment, thereby metallizing and alloying the Pt ions and Ru ions.

[0005] The improvement in CO catalyst toxicity resistance by alloying Pt with a dissimilar metal such as Ru (hereinafter, the dissimilar metal alloyed with Pt may be referred to as metal M) can be explained by two mechanisms: (i) the ligand effect and (ii) the bi-functional model.

[0006] (i) The Ligand effect is the effect caused by the change in electron density resulting from the difference in electronegativity between a metal M atom and a Pt atom when the two atoms are adjacent. When Ru, which has fewer electrons than Pt, alloys with Pt, the electronic state of the catalyst particle surface changes, making it easier for electrons to move, which leads to a decrease in CO adsorption capacity and an acceleration of the desorption reaction. In other words, alloying with Ru has the effect of reducing the frequency of CO adsorption on Pt and accelerating the release of the adsorbed state, thereby improving the catalyst particle's resistance to CO catalyst toxicity.

[0007] Furthermore, the Bi-functional model in (ii) is the mechanism by which Ru acts as a cocatalyst to remove CO adsorbed on Pt. In this mechanism, CO is adsorbed on Pt in (1) below (the CO adsorbed on Pt is hereafter referred to as CO ads (This is how it is written). On the other hand, the electrolysis (oxidation) of water in (2) causes the adsorption of OH onto Ru (the OH adsorbed onto Ru is referred to as OH below). ads (This is how it is written.) And the Pt-CO produced by these reactions ads and Ru-OH adsThrough the reaction with [substance], CO is desorbed as CO2. Such an effect of Ru is to improve the CO poisoning resistance of catalyst particles by releasing the state where CO is adsorbed on Pt even if it occurs. Pt + CO → Pt-CO ads (1) Ru + H2O → Ru-OH ads + H + + e - (2) Pt-CO ads + Ru-OH ads → Pt + Ru + CO2+ H + + e - (3)

Prior Art Documents

Patent Documents

[0008]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0009] As described above, the PtRu alloy catalyst is considered suitable as the anode catalyst for solid polymer fuel cells, but a catalyst with further improved CO poisoning resistance is required. In the reformed gas described above, it is difficult to supply a gas that does not contain any CO. In recent years when solid polymer fuel cells are becoming widely popular, further improvement in CO poisoning resistance is necessary for the long-term operation.

[0010] Also, it cannot be said that there are no problems with the PtRu alloy catalyst. According to the present inventors, when the operating conditions (cell potential) of a solid polymer fuel cell become high load conditions, Ru elution is observed in the PtRu alloy catalyst. The elution of Ru from the catalyst particles leads to a decrease in the CO tolerance of the catalyst particles. In addition, the Ru eluted from the catalyst may permeate through the polymer electrolyte membrane and precipitate on the catalyst in the counter electrode (cathode). Ru in the cathode becomes a poisoning factor for the cathode catalyst. Such high load conditions where Ru elution can occur are manifested during the startup and shutdown of the fuel cell and are not always present, but cannot be avoided. Therefore, for the widespread use of fuel cells, the development of a highly durable anode catalyst with less compositional change due to elution of alloy elements and the like is required.

[0011] The present invention has been made under the above background, and relates to an anode catalyst for a solid polymer fuel cell, and aims to present a catalyst that is superior in CO tolerance to the prior art PtRu alloy catalyst and is also superior in durability against elution of alloy metals and the like.

[0012] In solving such problems, the inventors of the present application assume the application of a Pt alloy catalyst. This is because catalyst particles mainly composed of Pt are optimal for the progress of the anode reaction of the fuel cell. And in specifying the composition of the Pt alloy catalyst excellent in CO tolerance, the present invention aims to integrate theoretical and computational science and experimental science.

Means for Solving the Problems

[0013] As described above, the present inventors examined from both theoretical and computational science and experimental science in order to clarify the composition of the Pt alloy catalyst excellent in CO tolerance. Based on these results, the present invention found that Ir (iridium) is the most suitable as the metal M alloyed with Pt for the Pt alloy constituting the catalyst particles. Then, the present invention was conceived by examining the preferred state of Ir in the catalyst particles made of a Pt alloy (PtIr alloy) and its production method.

[0014] That is, the present invention for solving the above problems is an anode catalyst for a solid polymer fuel cell having catalyst particles for treating a fuel gas containing carbon monoxide, wherein the catalyst particles are composed of Pt and Ir, and it is an anode catalyst for a solid polymer fuel cell.

[0015] The catalyst particles of the anode catalyst according to the present invention are preferably in a clustered state where a plurality of Ir atoms are adjacent on their surface. Specifically, it preferably contains a region where 4 or more Ir atoms are clustered.

[0016] The catalyst particles of the anode catalyst according to the present invention preferably have an atomic ratio of Pt to Ir (Pt:Ir) of 2:1 or more and 1:2 or less. Also, the average particle size of the catalyst particles is preferably 40 nm or more and 100 nm or less.

[0017] The anode catalyst for a solid polymer fuel cell according to the present invention can take a form in which the catalyst particles are supported on a carbon fine powder carrier. In this case, the loading density of the catalyst particles with respect to the whole catalyst is preferably 20% by mass or more and 70% by mass or less.

[0018] Furthermore, the present invention provides a power generation method using a solid polymer fuel cell. This power generation method includes a step of supplying a fuel gas to the anode of the solid polymer fuel cell. The fuel gas contains carbon monoxide, and the anode contains the anode catalyst according to the present invention described above. The concentration of the fuel gas containing carbon monoxide is not particularly limited, but the present invention is preferably applied to a fuel gas containing 0.5 ppm or more, more preferably 1 ppm or more of carbon monoxide. And the present invention is preferably used for a fuel gas containing 200 ppm or less, more preferably 100 ppm or less of carbon monoxide.

Advantages of the Invention

[0019] As described above, the present invention relates to an anode catalyst for polymer electrolyte fuel cells, which uses a PtIr alloy, formed by alloying Pt with Ir, as catalyst particles. The catalyst according to the present invention exhibits superior resistance to CO catalyst poisoning compared to PtRu alloy catalysts, which have conventionally been considered optimal. Furthermore, the catalyst according to the present invention suppresses the elution of constituent metals of the catalyst particles even under high load conditions, resulting in excellent durability. [Brief explanation of the drawing]

[0020] [Figure 1] Simulation results when metal M atoms (Ru, Rh, Pd, Ir) are substituted on the surface of catalyst particles made of Pt. [Figure 2] A model diagram of a Pt alloy catalyst for simulating the effect of improving CO-resistance to catalyst toxicity due to the Ligand effect. [Figure 3] A model diagram showing the state of CO and OH adsorbed on the surface of a Pt alloy catalyst, used to simulate the effect of improving CO-resistance to catalyst toxicity using a bifunctional model. [Figure 4] This figure shows the simulation results of the mechanism of the CO elimination reaction in a PtRu alloy catalyst. [Figure 5] This figure shows the simulation results of the CO elimination reaction mechanism in a PtIr alloy catalyst. [Figure 6] A diagram comparing the reaction barriers for CO elimination reactions in PtIr alloy catalysts and PtRu alloy catalysts. [Figure 7] This figure shows the simulation results when CO and OH are co-adsorbed onto PtIr alloy catalysts and PtRu alloy catalysts. [Figure 8] A model diagram for simulating the state of Ir atoms in PtIr alloy catalyst particles. [Figure 9] A model diagram for simulating the adsorption positions when CO and OH are co-adsorbed onto PtIr alloy catalysts and PtRu alloy catalysts. [Figure 10] Pourbet diagram of Ru and Ir. [Figure 11]This graph shows the evaluation results of CO-resistance catalyst toxicity (initial stage) when the Pt alloy catalysts of the examples and comparative examples are used as the anode electrode. [Figure 12] This graph shows the evaluation results of CO-resistance catalyst toxicity (durability) when the Pt alloy catalysts of the examples and comparative examples were used as the anode electrode. [Modes for carrying out the invention]

[0021] As described above, the Pt alloy catalyst for the anode of a polymer electrolyte fuel cell according to the present invention was studied from both the perspectives of (A) theoretical and computational science and (B) experimental science. Specifically, the theoretical and computational science-based study of the metal M alloyed with Pt involved three points: (A-1) selection of the metal species, (A-2) verification of the effects based on the two mechanisms for improving CO-resistant catalyst toxicity (Ligand effect, Bi-functional model) described above, and (A-3) the surface state of catalyst particles at the atomic level. Then, the results of these studies were verified, and the identification of a more specific catalyst configuration was investigated from the perspective of experimental science. The following description clarifies the aspects of the present invention conceived from these two approaches, (A) and (B).

[0022] A. The structure of the present invention as determined from theoretical and computational science. In this study, several noble metals other than Ru (Rh, Pd, Ir) that could be candidates for metal M were screened to determine which could exhibit a greater effect than Ru in improving CO catalyst toxicity resistance. For the identified metal M, the presence or absence of a ligand effect and CO catalyst toxicity improvement effect in the bi-functional model was then verified. Finally, the optimal catalyst metal state (surface state) was also confirmed.

[0023] In the theoretical and computational scientific studies conducted in this invention, simulations were performed using first-principles calculations based on density functional theory (DFT). First-principles calculations are computational methods based on density functional theory, which has shown that "the energy of the ground state of an interacting multi-electron system is determined by the electron density distribution." According to first-principles calculations, the electronic structure of a material can be discussed quantitatively without empirical parameters. Furthermore, in this simulation, it is preferable to use the generalized density gradient approximation (GGA) method, which is currently the most accurate method among first-principles calculations. Simulations using first-principles calculations based on density functionals enable evaluations that approximate experimental verification.

[0024] A-1. Screening of metals (M) that can be alloyed with Pt. To improve CO catalyst toxicity resistance by alloying a Pt catalyst with metal M, it is preferable to create a state in which CO preferentially adsorbs onto the alloyed metal M atoms rather than the Pt atoms. Therefore, as shown in Figure 1(a), we assume a state in which one Pt atom on the surface of a catalyst particle made of Pt is replaced by an atom of metal M. Metal M is Ru, Rh, Pd, or Ir. Then, as shown in Figure 1(b), we simulated the state in which CO molecules are adsorbed on Pt and the state in which CO molecules are adsorbed on metal M, and calculated the adsorption energy in each state. The CO adsorption energy on Ru, Rh, Pd, and Ir was calculated as follows, with the CO adsorption energy on Pt atoms as the reference (0 eV).

[0025] [Table 1]

[0026] Table 1 shows that the adsorption energy of CO to metal M is largest and most negative for Ir. In other words, by alloying Ir as metal M, CO preferentially adsorbs onto Ir atoms rather than Pt atoms, and this tendency is stronger than with other noble metals. Ir's CO adsorption capacity is better than Ru's. Therefore, Ir is considered the most promising metal M for achieving CO catalyst toxicity resistance superior to that of Ru.

[0027] A-2. Confirmation of the effect of improving CO catalyst toxicity resistance when metal M is Ir. Based on the screening results described above, we considered Ir as the metal M alloying with Pt and investigated the effect of improving CO catalyst toxicity resistance in comparison with Ru. In this investigation, we performed simulations on two mechanisms related to CO catalyst toxicity resistance (Ligand effect and Bi-functional model).

[0028] A-2-1. Simulation in the Ligand effect Figure 2 shows the models of the Pt alloy catalysts used in this study (Figure 2(a): PtRu alloy, Figure 2(b): PtIr alloy). The Ligand effect, a mechanism that affects CO-resistant catalyst toxicity, was investigated by simulating the local density of states on the catalyst particle surface in this model.

[0029] Referring to Figure 2, comparing the density of states of unalloyed Pt with the density of states of Pt alloys with Ru and Ir, Pt alloys (PtRu, PtIr) have a Fermi level (EE). F The electron density is high near (=0). From this, it can be inferred that in all cases, the Pt alloy is in a state where CO adsorption and desorption reactions are likely to occur.

[0030] And for PtRu and PtIr, the Fermi level (EE F Comparing the density of states at (=0), it is confirmed that PtIr has a larger density of states. A larger density of states indicates that electron transfer is more likely to occur, and that the CO elimination reaction is more effectively promoted. Therefore, from these simulation results, it was confirmed that Ir has a greater effect than Ru in improving the CO catalyst toxicity resistance of catalyst particles due to the Ligand effect.

[0031] A-2-2. Simulation in Bi-functional Models Next, we will explain the simulation results based on a bi-functional model regarding the effect of Pt alloy catalysts (PtRu, PtIr) on improving CO-resistance to catalyst toxicity.

[0032] In the bifunctional model, CO is adsorbed onto the catalyst particle surface, and OH is adsorbed by the electrolysis of water by metal M (Ru, Ir), and these adsorbed CO(CO) ads ) and OH(OH ads ) reacts with CO2 to produce CO2, which is then released from the catalyst particles (reaction equations (1) to (3) above). The reaction in this mechanism can be briefly described as follows. Catalysts with excellent resistance to CO catalyst poisoning will allow this reaction to proceed rapidly. CO ads + OH ads →CO2(3)'

[0033] First, we examine the adsorption state of each molecule when CO and OH coexist on the catalyst particle surface. Assuming the same model as in Figure 2 for the configuration of catalyst particles made of PtRu alloy and PtIr alloy, we simulated the adsorption energy when each CO and OH molecule is adsorbed on the Pt, Ru, and Ir atoms, as shown in Figure 3, and the results were as follows.

[0034] [Table 2]

[0035] Referring to Table 2, in both PtRu alloys and PtIr alloys, the adsorption energies of CO and OH onto the metal M(Ru,Ir) are negatively greater than the adsorption energies of CO and OH onto Pt. That is, in these Pt alloys, it is confirmed that the state in which both CO and OH are adsorbed onto the metal M(Ru,Ir) (co-adsorption state) is stable.

[0036] Table 2 also shows the relative adsorption energies of CO and OH on metal M(Ru,Ir) in PtRu alloys and PtIr alloys, with the adsorption energies of CO and OH on Pt set as the baseline (=0). Referring to these relative adsorption energies of CO and OH on metal M(Ru,Ir), in PtRu alloys, the OH adsorption energy is negatively larger than the CO adsorption energy. On the other hand, in PtIr alloys, conversely, the CO adsorption energy is negatively larger than the OH adsorption energy. From this, it can be inferred that CO adsorption occurs more easily in PtIr alloys than in PtRu alloys. This high CO adsorption capacity in PtIr alloys is consistent with the screening results described above.

[0037] Next, the mechanism of the CO desorption reaction (CO2 production reaction) described in (3)' above was simulated in a state where CO and OH molecules are co-adsorbed onto the metal M (Ru, Ir) of the Pt alloy catalyst. In this simulation, the state in which CO molecules and OH molecules are adsorbed onto the metal M atoms on the surface of the Pt alloy was set as the ground state (reaction coordinate = 0), and the reaction pathway from there to CO2 production was simulated to set the reaction coordinates, and the relative energy of each reaction coordinate was calculated.

[0038] The simulation results for the CO elimination reaction mechanism using a PtRu alloy catalyst are shown in Figures 4(a) and 4(b). From Figure 4(a), it was found that in this Pt alloy catalyst, the reaction pathway proceeds as follows: ground state where CO and OH are co-adsorbed to adjacent Ru atoms (reaction coordinate 0), rotation and movement of OH molecules (reaction coordinates 1-3), decomposition and dissociation of OH molecules (reaction coordinate 4), and bonding of CO molecules with O atoms and formation of CO2 molecules (reaction coordinates 5, 6). Referring to the calculation results of the relative energy at each reaction coordinate (Figure 4(b)), it can be seen that the decomposition and dissociation of OH molecules (reaction coordinate 4) is the transition state of the CO elimination reaction.

[0039] Next, the simulation results for the PtIr alloy catalyst are shown in Figures 5(a) and (b). The behavior of CO and OH in the PtIr alloy catalyst is basically the same as that of the PtRu alloy catalyst. That is, the reaction pathway proceeds from the co-adsorption state of CO and OH on the Ir atom (reaction coordinate 0), to the rotation and movement of OH molecules (reaction coordinates 1-2), the decomposition and dissociation of OH molecules (reaction coordinate 3), and the bonding of CO molecules with O atoms and the formation of CO2 molecules (reaction coordinates 4-6). Furthermore, according to the calculation results of the relative energy at each reaction coordinate (Figure 5(b)), the decomposition and dissociation of OH molecules (reaction coordinate 3) is the transition state of the CO elimination reaction.

[0040] Furthermore, the relative energy of the transition state in the CO elimination reaction in the PtIr alloy catalyst is 1.13 eV, which is lower than the relative energy of the PtRu alloy catalyst (1.75 eV). Since relative energy is the activation barrier for transitioning from the transition state to the next reaction pathway, it can be seen that the CO2 production reaction proceeds more easily in the PtIr alloy catalyst than in the PtRu alloy catalyst.

[0041] Figure 6 summarizes the calculation results of the relative energy at each reaction coordinate up to CO2 generation in the PtRu alloy catalyst and the PtIr alloy catalyst, as well as the calculation results of the relative energy after CO2 generation (reaction coordinate 6) and the desorption of CO2 from metal M. As mentioned above, the PtIr alloy catalyst has a lower reaction barrier for the CO2 generation reaction compared to the PtRu alloy catalyst. Furthermore, as shown in Figure 6, the PtIr alloy catalyst has a lower relative energy after CO2 desorption. Therefore, it can be said that CO2 generation and desorption (i.e., CO desorption) occur more easily in the PtIr alloy catalyst compared to the PtRu alloy catalyst. From the above, it can be confirmed that the PtIr alloy catalyst has excellent resistance to CO catalyst poisoning.

[0042] As described above, the reason why the CO elimination reaction proceeds more easily in the PtIr alloy catalyst compared to the PtRu alloy catalyst is presumed to be the difference in the charges of CO and OH when co-adsorbed on the metal M. Figure 7 shows the results of charge calculations (Bader charge analysis) of each molecule in the state where CO and OH are co-adsorbed (ground state in Figures 4 and 5). From Figure 7, it can be seen that in the PtIr alloy catalyst, the charge (negative charge) of CO and OH is larger than that of the PtRu alloy catalyst. It is thought that the charge of CO and OH adsorbed on Ir has a favorable effect on the generation of CO2.

[0043] Based on the simulation results considering the above bi-functional model, it was confirmed that Ir has a greater effect than Ru in improving the CO-tolerance of catalysts in catalyst particles.

[0044] A-3. Surface composition of catalyst particles (arrangement of Ir elements) Based on the above findings, we performed calculations to obtain a desirable structure for the PtIr alloy catalyst, which was shown to have higher CO-resistance to catalytic toxicity than conventional PtRu alloy catalysts. When Ir is alloyed with Pt, two states of Ir atoms are conceivable: one where individual Ir atoms are mixed among Pt atoms, and another where multiple Ir atoms are aggregated into clusters. Therefore, we assumed the model shown in Figure 8 as the dispersion state of Ir atoms on the catalyst particle surface and calculated the formation energy for each state. The results are shown below.

[0045] [Table 3]

[0046] Table 3 shows that catalyst particles made of PtIr alloy are stable when the Ir atoms are clustered on their surface. Therefore, it can be said that it is preferable for the Ir atoms to be clustered.

[0047] The reason why clustering of Ir atoms is preferable in PtIr alloy catalysts can be inferred from the fact that CO and OH co-adsorb in the CO elimination reaction (Bi-functional model). As mentioned above, in order for co-adsorbed CO and OH to effectively react with CO2 and be eliminated, it is preferable that these molecules are adsorbed in adjacent positions.

[0048] Here, we assume a model as shown in Figure 9 for the state when CO and OH are co-adsorbed on a PtIr alloy catalyst. In Figure 9, three adsorption states are assumed: a state where both CO and OH are adsorbed on Ir (MM), a state where CO is adsorbed on Pt and OH is adsorbed on Ir (Pt-M), and a state where CO is adsorbed on Ir and OH is adsorbed on Pt (M-Pt). The calculation results for the adsorption energy in these three adsorption states are as follows.

[0049] [Table 4]

[0050] Table 4 shows that the adsorption energy is lowest and most stable when both CO and OH are adsorbed on Ir (MM). To form this state where both CO and OH are adsorbed on Ir, it is preferable that the Ir atoms are clustered on the catalyst particle surface.

[0051] As explained above, in this invention, based on computational and theoretical science, we have identified Ir as the optimal metal M to alloy with Pt in a Pt alloy catalyst that exhibits excellent resistance to CO catalyst poisoning. Furthermore, it has been revealed that the PtIr alloy catalyst exhibits superior resistance to CO catalyst poisoning compared to the PtRu alloy catalyst, and that the Ir atoms are preferably clustered.

[0052] B. Specific configuration of the catalyst according to the present invention Next, we will describe the specific configuration of the anode catalyst for polymer electrolyte fuel cell according to the present invention, which can be determined based on experimental science.

[0053] As described above, the anode catalyst for polymer electrolyte fuel cell according to the present invention has catalyst particles composed of Pt and Ir. The reason why the metal M alloyed with Pt is specifically Ir is, as mentioned above, because Ir can impart to the Pt alloy catalyst CO toxicity resistance of Ru or higher.

[0054] Furthermore, the reason why Ir can be said to be useful as metal M is that Ir also contributes to improving the durability of the catalyst. Conventional PtRu alloys can experience Ru leaching under high-load operating conditions. One of the factors causing this Ru leaching is Ru ionization. This point will be explained using the Pourbet diagram (potential-pH diagram) in Figure 10. The high-load operating conditions mentioned above refer to the anode potential being higher than under normal operating conditions. On the other hand, each electrode of a polymer electrolyte fuel cell is basically in a strongly acidic environment. Therefore, in the region of high-load operating conditions (approximately 0.7V), trivalent Ru (Ru) leaching occurs in the PtRu alloy catalyst. 3+ This results in the substance leaching out from the PtRu alloy catalyst.

[0055] In contrast, referring to the Pourbet diagram for Ir, Ir does not have a region where it ionizes like Ru at the potential and pH conditions of a polymer electrolyte fuel cell. Therefore, in a PtIr alloy catalyst, even under high-load operating conditions, there is no ionization or elution of alloying elements, and the composition and structure can be maintained. Consequently, catalyst particles made of PtIr alloy are excellent in terms of both CO catalyst toxicity resistance and durability.

[0056] In this invention, the PtIr alloy constituting the catalyst particles preferably has an atomic ratio of Pt to Ir (Pt:Ir) of 2:1 or more and 1:2 or less. If the Ir content is low and the atomic ratio is less than 2:1, sufficient CO catalyst toxicity resistance cannot be obtained. Also, if the Ir content is high enough to exceed the atomic ratio of 1:2, the particle size becomes coarser, affecting CO catalyst toxicity resistance. More preferably, the atomic ratio of Pt to Ir is 1:1 or more and 1:2 or less.

[0057] Furthermore, as described above, the catalyst particles made of the PtIr alloy in the present invention preferably include regions on their surface in which Ir atoms are clustered. Clustering refers to a state in which two or more Ir atoms are adjacent to each other, preferably four or more Ir atoms are adjacent. The upper limit of the number of clustered Ir atoms is preferably 3300. If the number of clustered Ir atoms exceeds 3300, the Ir will be closer to a state in which it exists alone rather than as an alloy with Pt, and the CO-resistance catalyst toxicity improvement effect due to the Ligand effect of the alloy described above will decrease. In addition, the surface morphology of the PtIr alloy catalyst according to the present invention may be a form in which some sites on the surface of the Pt particles are replaced by clustered Ir atom groups. Alternatively, a so-called core-shell structure in which clustered Ir atom groups are bonded on the surface of the Pt particles may be adopted.

[0058] The average particle size of the catalyst particles made of PtIr alloy is preferably between 40 nm and 100 nm. This is because coarse catalyst particles exceeding 100 nm may reduce activity, and catalyst particles smaller than 40 nm will not clearly exhibit long-term activity persistence characteristics. More preferably, the average particle size of the catalyst particles is 80 nm or less. The particle size of the catalyst particles can be estimated by XRD analysis and can be calculated from the Scherrer equation (R = 0.9λ / Wcosθ (R: particle size (nm), λ: wavelength of X-ray (e.g., Cu Kα line is 0.154 nm), W: width at half maximum (degree), θ: angle of incidence (degree))) based on the peak full width at half maximum of the Pt alloy in the XRD profile.

[0059] The catalyst of the present invention is preferably in the form of a supported catalyst, supported on a suitable carrier. The carrier is a substrate in the catalyst particle formation process and a support material in the use process. In fuel cell applications, it also acts as a conductive material for the anode electrode. The carrier is preferably carbon fine powder, and preferably carbon fine powder having appropriate pores. The carbon fine powder has a specific surface area of ​​50 to 1500 m². 2 A product containing 1g is preferable.

[0060] When the catalyst according to the present invention is in the form of a supported catalyst, the supported density of the catalyst is preferably 20% by mass or more and 70% by mass or less. Supported density is the ratio of the mass of catalyst particles to the total mass of the catalyst including the support.

[0061] The catalyst manufacturing method according to the present invention basically applies the liquid-phase reduction method, which is a conventional catalyst manufacturing method. When manufacturing a PtIr alloy catalyst by the liquid-phase reduction method, metal salt solutions of each metal are brought into contact with a support to support Pt ions and Ir ions, and then each metal ion is metallized and alloyed by heat treatment.

[0062] When supporting Pt and Ir ions on a support, it is possible to form a PtIr alloy by simultaneously impregnating the support with solutions of each metal salt and performing a reduction treatment. However, in that case, it is difficult to produce a PtIr alloy catalyst with the preferred configuration described above. In other words, in the production of the PtIr alloy catalyst of the present invention, it is necessary to impregnate a Pt catalyst, on which only platinum has been previously supported on the support, with a metal salt of Ir, and then metallize the Ir and alloy it with Pt. The reason for additionally supporting Ir on the Pt catalyst to form a PtIr alloy is to promote the clustering of Ir atoms on the surface of the catalyst particles.

[0063] For Pt catalysts supporting Ir (Ir salts), commercially available Pt catalysts can be used, or the Pt catalyst may be prepared in advance by liquid-phase reduction. In the production of Pt catalysts by liquid-phase reduction, Pt salts such as dinitrodiammineplatin nitrate, chlorplatinate, potassium chlorplatinate, and hexaammineplatinate can be used. After impregnating a support with an aqueous solution of these Pt salts, the Pt catalyst can be obtained by performing a reduction treatment in which a reducing agent (alcohol, sodium borohydride, etc.) is brought into contact with the support.

[0064] In the process of supporting Ir on a Pt catalyst, the Pt catalyst is first impregnated with an Ir salt solution. Suitable Ir salt solutions include iridium chloride (IrCl3), iridium chloride salt (H2IrCl6), iridium nitrate (Ir(NO3)3), and iridium sulfate ((Ir2(SO4)3).

[0065] Then, after impregnating the Pt catalyst with an aqueous solution of Ir salt, when metallizing the Ir ions, the pH of the aqueous solution is first adjusted (neutralized) to convert the Ir ions into Ir hydroxides. The reason why treatment with a reducing agent is not performed, as in the production of the Pt catalyst, is that in the case of the catalyst of the present invention, the use of a reducing agent tends to generate coarse particles, which reduces the toxicity of the CO catalyst. pH adjustment at this time is preferably done by adding alkali, and it is preferable to set the pH range to 8.0 or higher and 11.0 or lower. It is also preferable to set the liquid temperature to 70°C or higher and 90°C or lower.

[0066] After forming Ir hydroxide on the Pt catalyst by the neutralization treatment described above, the Ir hydroxide is converted to metallic Ir by heat treatment, and alloyed with Pt to form catalyst particles made of a PtIr alloy. This heat treatment is preferably carried out in a reducing atmosphere (such as hydrogen) at a temperature of 800°C to 1500°C. [Examples]

[0067] Specific examples of the embodiments of the present invention described above will now be explained. In these examples, PtIr alloys with different Ir content (atomic ratio with Pt) were produced (Examples 1 to 4). In addition, as comparative examples (prior art), PtRu alloy catalysts were produced in which metal M was Ru and the atomic ratio of Ru was the same as in the examples (Comparative Examples 1 to 3).

[0068] [Manufacturing of PtIr alloy catalysts] A commercially available Pt catalyst (carrier: carbon powder, Pt loading density: 19.8% by mass) was prepared. 25 g of this Pt catalyst was weighed and placed in a separable flask, and then 4 L of pure water and an 8.612% by mass iridium chloride solution were added. The mixture was then stirred with a stirring blade to obtain a slurry.

[0069] The slurry described above was heated to 80°C, and then a 10% by mass aqueous sodium hydroxide solution was slowly added dropwise until the pH reached 8. The slurry was then held for 6 hours to support iridium hydroxide on the Pt catalyst. After 6 hours, the slurry was filtered and washed three times, and then dried at 60°C for more than 24 hours.

[0070] Next, the iridium hydroxide was reduced to metallic Ir and heat treatment was performed to form a PtIr alloy. This heat treatment was carried out by heating at 900°C for 30 minutes in 100% hydrogen gas. Through the above steps, a PtIr alloy catalyst was obtained. The composition (Ir concentration) of the PtIr alloy was adjusted by the amount of 8.612 mass% iridium chloride solution added. Here, 28.31 g (Example 1), 56.63 g (Example 2), 84.94 g (Example 3), and 113.26 g (Example 4) of iridium chloride solution were added to achieve Pt:Ir ratios of 2:1 (Example 1), 1:1 (Example 2), 1:1.5 (Example 3), and 1:2 (Example 4).

[0071] Furthermore, in this embodiment, as Example 5, a PtIr alloy catalyst was manufactured using a different manufacturing method than that used in Examples 1 to 4. The PtIr alloy catalyst of Example 5 was manufactured by adding the same carbon powder carrier as in Example 1 to a mixed solution of platinum chloride solution (platinum concentration 15.397% by mass) and the same iridium chloride solution as above, then adjusting the pH to neutralize the mixture to simultaneously form platinum and iridium hydroxides, and finally performing a reduction treatment by heat treatment. The PtIr alloy catalyst of Example 5 was manufactured with a Pt:Ir ratio of 1:1.5.

[0072] [Manufacturing of PtRu alloy catalysts] 25 g of the same Pt ​​catalyst as in the above example was weighed and placed in a separable flask. Then, 4 L of pure water and an 8.373% by mass ruthenium chloride solution were added. The mixture was then stirred with a stirring blade to obtain a slurry.

[0073] The slurry described above was heated to 93°C, then 200 mL of methanol was added dropwise as a reducing agent and stirred. The mixture was held for 6 hours to support Ru on the Pt catalyst. After the time had elapsed, the mixture was filtered and washed three times, and then dried at 60°C for more than 24 hours.

[0074] Next, a heat treatment was performed to alloy Pt and Ru. This heat treatment involved heating at 900°C for 30 minutes in 100% hydrogen gas. Through the above process, a PtRu alloy catalyst was obtained. The composition (Ru concentration) of the PtRu alloy was adjusted by the amount of 8.373 mass% ruthenic acid chloride solution added. Here, iridium acid chloride solution was added so that the Pt:Ru ratio was 2:1 (Comparative Example 1), 1:1 (Comparative Example 2), and 1:1.5 (Comparative Example 3).

[0075] [Evaluation of catalyst resistance to CO catalyst toxicity (initial properties)] The PtIr alloy catalysts of Examples 1 to 5 and the PtRu alloy catalysts of Comparative Examples 1 to 3, manufactured above, were evaluated for their CO-resistance to catalyst toxicity as anode catalysts for polymer electrolyte fuel cells. Here, anode electrodes were prepared from the Pt alloy catalysts of each example and comparative example to create single cells for evaluation, and the CO-resistance to catalyst toxicity of the anode electrodes in the initial operating state was evaluated.

[0076] A catalyst ink was prepared by mixing and stirring the catalyst with Nafion, alcohol, and water. This ink was then applied to carbon paper using a bar coater, and the solvent was removed by hot pressing to create the anode electrode. The electrode area of ​​the anode electrode was 25 cm². 2 The dimensions were set to (5cm x 5cm). Meanwhile, a cathode electrode was fabricated using the same method as above, with the Pt catalyst used in the PtIr alloy catalyst of Example 1. Then, a membrane / electrode assembly (MEA) was fabricated by sandwiching a proton-conducting polymer electrolyte membrane between the fabricated anode electrode and cathode electrode to produce a single cell.

[0077] The evaluation test involved electrolysis of the single cell prepared as described above, while flowing fuel gas (hydrogen) and oxidizer gas (oxygen) according to the following scheme consisting of steps 1 to 3. Step 1: While circulating 1 L / min of hydrogen to the anode and 1 L / min of oxygen to the cathode, maintain a current density of 3 A / cm². 2 Hold the cell for one hour. Measure the cell voltage after this one-hour period. Step 2: Flow hydrogen containing 1 ppm of CO at a rate of 1 L / min through the anode side and oxygen at a rate of 1 L / min through the cathode side, maintaining an OCV state for 15 minutes. Step 3: While circulating hydrogen containing 1 ppm of CO at a rate of 1 L / min on the anode side and oxygen at a rate of 1 L / min on the cathode side, maintain a current density of 3 A / cm². 2 Hold the cell for one hour. Measure the cell voltage after this one-hour period.

[0078] In the evaluation scheme described above, the cell potential when CO-containing fuel gas is passed through the anode electrode in step 3 to generate electricity will decrease compared to the cell potential measured in step 1. The CO toxicity resistance of the catalyst can be evaluated by assessing the potential difference (ΔV) at this time. Figure 11 shows the results of the above measurements performed on the PtIr alloy catalysts of Examples 1 to 5, the PtRu alloy catalysts of Comparative Examples 1 to 3, and the Pt catalyst used in the examples before Ir loading for reference.

[0079] For the Pt catalyst measured for reference with the examples and comparative examples, the potential difference ΔV was 205 mV. The conventional PtRu alloy catalyst, used as a comparative example, showed a significantly reduced potential difference ΔV compared to the Pt catalyst, indicating resistance to CO catalyst toxicity. Furthermore, the PtIr alloy catalyst of the examples exhibited even better resistance to CO catalyst toxicity than the PtRu alloy catalyst. When comparing the potential difference ΔV of catalysts with the same composition (metal M concentration), the PtIr alloy catalyst sometimes had a potential difference ΔV that was half that of the PtRu alloy catalyst. From this, it can be confirmed that the PtIr alloy catalyst according to the present invention has improved resistance to CO catalyst toxicity compared to the prior art.

[0080] Furthermore, regarding the composition (Ir concentration) of the PtIr alloy catalyst, it is preferable that the Pt:Ir ratio be 1:1 or higher. Also, even if the Pt:Ir ratio exceeds 1:1, there is no significant decrease in the potential difference ΔV.

[0081] Furthermore, regarding the PtIr alloy catalyst prepared using a different method in Example 5, although the Pt:Ir ratio is the same as in Example 3 (Pt:Ir = 1:1.5), the ΔV value is higher than in Example 3, indicating lower CO catalyst toxicity resistance. Moreover, the ΔV is higher than that of some comparative examples (PtRu alloy catalysts). It is considered that the PtIr catalyst prepared using the method in Example 5 does not exhibit sufficient CO catalyst toxicity resistance because the clustering of Ir atoms on the catalyst particle surface is insufficient.

[0082] [Evaluation of catalyst resistance to CO catalyst toxicity (durability test)] Next, durability tests were conducted to evaluate the durability of the anode electrode under high load and its resistance to CO catalyst toxicity. Using the same unit cell as above, electrolysis was performed while flowing fuel gas (hydrogen) and oxidizer gas (oxygen) according to the scheme consisting of steps 1 to 3 below. Step 1: While circulating 1 L / min of hydrogen / CO (1 ppm) on the anode side and 1 L / min of oxygen on the cathode side, maintain a current density of 3 A / cm². 2 Hold the cell for one hour. Measure the cell voltage after this one-hour period. Step 2: While 0.2 L / min of air was flowed through the anode electrode and 0.05 L / min of hydrogen was flowed through the cathode electrode, the potential was maintained at 0.8 V for 20 hours using a potentiostat. Step 3: While circulating 1 L / min of hydrogen / CO (1 ppm) on the anode side and 1 L / min of oxygen on the cathode side, maintain a current density of 3 A / cm². 2 Hold the cell for one hour. Measure the cell voltage after this one-hour period.

[0083] The scheme for the durability test described above is the same as the initial evaluation in steps 1 and 3, but the polarity is reversed in step 2. By reversing the polarity in step 2, the cathode reaction (ORR: 1 / 2O2 + 2H) occurs at the anode electrode. + +2e -→H2O) is produced, generating water. This acidic, watery environment and high cell potential can lead to catalyst degradation due to the leaching of alloying elements. In this evaluation test, the durability of the catalyst and its resistance to CO catalyst toxicity are evaluated from the change in cell voltage before degradation (step 1) and after degradation (step 3) due to polarity reversal in step 2.

[0084] This durability test was conducted on a Pt catalyst, a PtRu alloy catalyst (Comparative Example 3), and a PtIr alloy catalyst (Example 3). The results are shown in Figure 12. Comparing ΔV before and after the durability test in Figure 12, it was confirmed that the PtIr catalyst showed the lowest increase rate, clearly demonstrating its high durability. [Industrial applicability]

[0085] The present invention relates to an anode catalyst for polymer electrolyte fuel cells, specifically a PtIr alloy catalyst in which Ir is used as an alloying element to Pt, resulting in excellent resistance to CO catalyst toxicity. The PtIr alloy catalyst according to the present invention exhibits superior resistance to CO catalyst toxicity compared to PtRu alloy catalysts, which have been considered to have good resistance to CO catalyst toxicity. Furthermore, the PtIr alloy catalyst according to the present invention is also useful in addressing the problem of alloying element leaching under high-load conditions, which is a problem with PtRu alloy catalysts, and possesses durability under all operating conditions expected for polymer electrolyte fuel cells.

Claims

1. Anode catalyst for a polymer electrolyte fuel cell having catalyst particles for processing fuel gas containing carbon monoxide, The catalyst particles consist of Pt and Ir, with an atomic ratio of Pt to Ir (Pt:Ir) of 2:1 or more and 1:2 or less. The catalyst particles are characterized in that they include regions on their surface in which four or more Ir atoms are clustered adjacent to each other, making them an anode catalyst for polymer electrolyte fuel cells.

2. The anode catalyst for a polymer electrolyte fuel cell according to claim 1, wherein the catalyst particles are supported on a carbon fine powder carrier.

3. The anode catalyst for a polymer electrolyte fuel cell according to claim 1 or claim 2, wherein the loading density of the catalyst particles relative to the entire catalyst is 20% by mass or more and 70% by mass or less.

4. In a power generation method that includes the step of supplying fuel gas to the anode of a polymer electrolyte fuel cell, The fuel gas contains carbon monoxide, A method for generating electricity, characterized in that the anode comprises the anode catalyst described in claim 1 or claim 2.

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