JP5493085B2 - Cathode material - Google Patents

Description

  The present invention relates to a cathode material in which Pt particles are used as an electrode material and carbon particles are used as a conductive material, and more specifically, a cathode for a polymer fuel cell, a cathode for a direct methanol fuel cell, and a cathode for various sensors. The cathode material.

The Pt / conductive carbon cathode material is a typical cathode material used as a polymer fuel cell electrode material or a direct methanol fuel cell electrode material. Although this electrode shows a relatively high cathode activity, the oxygen reduction reaction is very slow in the first place, so there is a large loss due to overvoltage in the cathode reaction, such as a power loss of the fuel cell and a decrease in the response speed of the sensor. It has been thought to be the main factor.
On the cathode of the fuel cell, oxygen or oxygen contained in the air needs to be reductively decomposed to become oxygen ions or to proceed with an electrocatalytic reaction that easily reacts with protons to form water molecules. Conventionally, Pt / conductive carbon has been used as an electrode suitable for this oxygen reductive decomposition electrocatalytic reaction. However, the current density was low, and the oxygen reductive decomposition initiation potential was not sufficiently high.
However, in order to popularize fuel cells for home use, it is indispensable to create high-performance and inexpensive fuel cells. From that viewpoint, it has been desired to develop an electrocatalyst having higher cathode performance than conventional Pt / conductive carbon.

In order to solve this problem, as shown in Non-Patent Document 1, attempts are made to increase oxygen reductive decomposition activity by dispersing metallic iron (Fe) in a Pt / conductive carbon cathode material. As shown in Document 2, attempts have been made to support Pt particles on niobium oxide (NbO ₂ ). Although the PtFe / conductive carbon cathode material obtained in this way certainly shows a higher performance than the previous Pt / conductive carbon cathode material, when the Fe touches the acidic solution, the iron oxide (FeO or Since it changes to Fe ₂ O ₃ ), there is a problem that long-term stability cannot be expected due to the influence of the acidic solution flowing out from the inside of the polymer solid electrolyte, and an electrode carrying Pt particles on niobium oxide (NbO ₂ ) The catalyst is not practical because niobium of the support used is not the usual pentavalent niobium oxide but tetravalent niobium as the support, and a very expensive material needs to be used specially. Such a cathode material composed of Pt / second component / conductive carbon has not been put into practical use yet.

As described above, the conventional Pt / second component / conductive carbon-based cathode material has several problems such as electrocatalytic activity and cost. The present invention seeks to provide a cathode material free from such problems.

The cathode material of the invention 1 is characterized in that Pt particles are supported on the surface of ceria particles.
Invention 2 is characterized in that, in the cathode material of Invention 1, a Pt1 value of 5 vol% or more is present on the surface of the Pt particles.
Invention 3 is characterized in that in the cathode material of Invention 1 or Invention 2, a Pt2 value of 10 vol% or more is present on the surface of the Pt particles.
Invention 4 is characterized in that, in the cathode material of Inventions 1 to 3, even if Pt4 valence exists on the surface of the Pt particles, it is 10% by volume or less.
Invention 5 is characterized in that the specific surface area of Pt particles is 10 m ² / g or more (by CO stripping method) on the cathode materials of Inventions 1 to 4.
Invention 6 is characterized in that, in the cathode material of any one of Inventions 1 to 5, the specific surface area of the ceria particles is 20 m ² / g or more and 70 m ² / g or less (according to the BET method). .
Invention 7 is the cathode material according to any one of Inventions 1 to 6, wherein the general formula indicating the composition is _X Pt / _Y CeO ₂ / _Z carbon (Z = 100−XY), 0.01 ≦ X ≦ 70 and 20 ≦ Y ≦ 40.
Invention 8 is characterized in that, in the cathode material according to any one of Inventions 1 to 7, an active site is a Pt region diffused on CeO ₂ .
Invention 9 is the cathode material of Invention 8, wherein the active site has an area of 1 × 10 ² square nanometers or more and 4 × 10 ² square nanometers or less.
Invention 10 is characterized in that, in the cathode material of any one of Inventions 1 to 9, the Ce ³⁺ / Ce ⁴⁺ ratio is 1.0 or more.

As a result of continuing intensive studies in view of the above-mentioned problems of the prior art, the present inventors have found that when Pt is supported on ceria (CeO ₂ ), Pt is on the CeO ₂ surface at the interface between Pt and CeO _2. The interface layer formed by diffusion and Pt become active sites, and it is found that a cathode material having extremely high electrode activity can be produced at low cost by dispersing it on conductive carbon, and the present invention has been completed. It was.
Furthermore, the present invention forms a sufficient amount of Pt1 valence and ionized Pt, which has not been known to exist, at the Pt-CeO ₂ interface, and the Pt1 valence and Pt2 valence are the constituent components. Or a cathode material having an active site as a cluster. Preferably, a region (interface layer region) in which Pt diffuses and spreads on the CeO ₂ surface through the Pt 1 valence, Pt ₂ valence, and CeO ₂ interface is wide, and more preferable. Uses a Pt / CeO ₂ composite cathode material in which an area where Pt2 valence is 10 volume% or more and 40 volume% or less in the interface layer is an active site, thereby providing cathode characteristics that can be used for fuel cells and the like. It has succeeded in drastically improving and will be used in various sensors including fuel cells in the future. It is expected to greatly contribute to the long-term stability of performance. In particular, it is expected to greatly contribute to the miniaturization and high output of polymer fuel cells that have been attracting attention in recent years, and its utility value in the industry is extremely large and serious.

Scanning electron micrograph of CeO ₂ calcined powder X-ray diffraction pattern obtained from CeO ₂ calcined powder (showing fluorite structure single phase) Comparison of platinum (Pt) 4f spectra observed from PtCeO ₂ composite cathode and commercial Pt cathode surfaces Interface layer region observed using a high-resolution transmission electron microscope (acceleration voltage: 200 keV) Voltammetric oxygen reduction activity Determination of onset potential using Tafel plot Analysis results of cathode reaction electron number using Koutecky-Levic plot (comparison with platinum electrode)

In the cathode material of the present invention, Pt particles are supported on the surface of ceria particles.
The CeO ₂ surface, spread atomic state Pt particles by ^diffusion, Pt + ^{-Ce 3+} - oxygen defects and ^Pt 2+ ^{-Ce 3+} - are believed to form an unique nano hetero cluster of oxygen defects. It is thought that this nanoheterocluster becomes an active site and high cathode activity is born, but it is not clear.

The fact that the Pt1 valence is present on the surface of the Pt particles improves the current density is shown in the following experiment No. It became clear from 1. In particular, it has been clarified that the presence of 5% by volume or more, preferably 10% by volume or more, dramatically improves the current density.
In general, metal Pt is considered to be most effective for Pt, but in order to create an active site that exceeds the performance of existing Pt, it is desirable to create a heterocluster site containing ionized Pt. .
In the present invention, a monovalent which has not been confirmed in the past is allowed to exist at the interface between Pt and CeO ₂ (meaning that a heterocluster site containing Pt appears on the surface), thereby making it possible to obtain a conventional one using Pt. Compared to this, it is thought that the performance was dramatically improved.
Preferably, a region (interface layer region) in which Pt diffuses and spreads on the CeO ₂ surface through the interface between Pt1 and CeO ₂ is wide, and Pt ⁺ -Ce ³⁺ -oxygen defect clusters are also active sites in this interface layer region. Therefore, improvement in cathode performance can be expected.
More preferably, since a sufficient amount of clusters of Pt ²⁺ -Ce ³⁺ -oxygen defects are formed in the interface layer region, these clusters also function as active sites, so that improvement in cathode performance can be expected. .
Although the effect of this heterocluster has not yet been fully elucidated, as shown in this example, ionized Pt (Pt 1 valence or divalence) is stably stabilized, the Pt and CeO ₂ interface, and the CeO _2. In order to make it exist on the surface, it is more preferable that Ce trivalence is 50% or more with respect to the tetravalence of Ce appearing on the surface, so that the above range can be secured stably.
The Pt1 valence and the Pt2 valence can be quantitatively measured using ordinary photoelectron spectroscopy or high energy photoelectron spectroscopy using synchrotron radiation, and 72.1 eV in the Pt4f spectrum obtained by high electron spectroscopy. (Spin 7/2) and 75.3 eV (Spin 5/2) have peaks characteristic of Pt1 valence, and 72.9 eV (Spin 7/2) and 76.1 eV (Spin 5/2) have peaks unique to Pt2 valence. Can be easily confirmed. Note that the peak of Pt 4 valence spin 5/2 overlaps with other peaks and is difficult to evaluate quantitatively. Therefore, the existence ratio of the peak of Pt 4 valence (spin 7/2) near 78.1 eV is used. Thus, it is preferable to evaluate the Pt4 value of the surface.
When the amount of Pt1 value is less than 5% by volume, the volume% of cluster active sites in the total amount of Pt is too small, and a sufficient effect for improving the cathode activity cannot be obtained. On the other hand, it is preferable that the amount of Pt1 valence is large. However, since Pt1 valence is considered to exist mainly at the interface between Pt and CeO ₂ and in the very vicinity thereof, it is difficult to appear in large quantities. Since the effect of improving the cathode characteristics appears sufficiently if it is about, the amount of Pt1 valence is about 20% by volume.
The amount of Pt ₂ valence appears on the cathode more than Pt 1 valence because Pt divalent cations diffuse on the CeO ₂ surface through the interface between Pt and CeO ₂ , but below 10% by volume, Pt ²⁺ −Ce ^The amount of clusters composed of ^{3 + -oxygen} defects is reduced, and it is not preferable because it does not sufficiently contribute to the improvement of cathode performance. Even if it exceeds 40% by volume, the effect cannot be expected.
The amount of Pt tetravalent is very inactive. Therefore, when tetravalent Pt on the surface appears, the activity tends to decrease by that amount. It is preferable that it becomes 10 volume% or less.
The metal Pt (Pt0 valence) is obtained as a value obtained by adding all the Pt1, bivalent and tetravalent volume% and subtracting from 100.
Furthermore, the Pt and CeO ₂ interface and the cluster region extending in the vicinity thereof are preferably 1 × 10 ² square nanometers or more and 4 × 10 ² square nanometers or less. Below this range, the ratio of active sites consisting of clusters appearing on the cathode to the surface is too small, and improvement in cathode characteristics cannot be confirmed. On the other hand, the region where Pt diffuses to the CeO ₂ surface is The higher the temperature of the supporting treatment, the wider it is. However, the grain growth of Pt and CeO ₂ becomes conspicuous and the activity of the entire cathode decreases, so that the range exceeding 4 × 10 ² square nanometers can be achieved. Since it does not lead to an improvement in characteristics, it is sufficient if it is 4 × 10 ² square nanometers or less.
In the case where a cluster composed of ionized Pt and Ce as described above is formed on the electrode surface, the local charge is not neutralized only by the usual tetravalence of Ce, and as a result, This is not preferable because the cluster becomes unstable. In order for a cluster that improves the cathode activity to be stably formed on the surface, it is necessary to have more Ce ³⁺ than Ce ⁴⁺ and exist on the cathode surface. Accordingly, Ce ³⁺ / Ce ⁴⁺ needs to exceed 1.0. However, since Ce ³⁺ is originally unstable, this ratio is practically in the range of 1.0 to 2.0. It is preferable to produce a stable cathode material because the stability of the cluster containing ionized platinum is increased, and as a result, the stability of the cathode characteristics is also improved.

The specific surface area of the Pt particles on the cathode surface measured using the CO striping method is more than 20 m ² / g, preferably 25 m ² / g or more, more preferably from the following examples (Experiment No. 1 to No. 11). Is preferably 30 m ² / g or more.
The upper limit is 50 m ² / g or less, preferably 45 m ² / g or less, more preferably 40 m ² / g or less.
The larger the specific surface area value, the greater the activity. However, in order to make this value above the above range, a special processing environment such as a high vacuum environment or a pulsed laser is required. In this sense, the upper limit is the experiment No. described below. 2 is the upper limit value in the processing environment. Based on this embodiment, it is not denied that the specific surface area is increased by using a high vacuum environment or a pulse laser.
CeO ₂ particles are B. E. T method specific surface area of CeO ₂ particles as measured by (Brunauer-Emmett-Teller method) larger is desirable, particularly, ^{20 m} 2 / g or more, preferably at ^25m 2 / g or more and more preferably ^{30 m} 2 / It is desirable to set it as g or more.
When this specific surface area is small, the surface activity of CeO ₂ particles becomes low, and it becomes difficult to create a nanoheterocluster structure including Pt1 valence and Pt2 valence.
The upper limit is 70 m ² / g or less, preferably 75 m ² / g or less, more preferably 60 m ² / g or less.
CeO ₂ particles exceeding this range are mainly those having a mesopore structure. However, when such CeO ₂ particles are used, Pt particles fall into mesopores or are not on the surface but porous. A nano-heterocluster containing Pt1 and Pt2 valences will be formed inside the CeO ₂ particles having a body structure, and these clusters are difficult for oxygen molecules to reach deep inside the porous body from the electrode outermost surface. It tends not to improve electrode activity.

When the composition is expressed by a general formula, it becomes _X Pt / _Y CeO ₂ / _Z carbon (the unit of Z = 100−XY, X, Y, Z is wt%, and carbon is conductive carbon). Conductive carbon does not contribute to the cathode reaction activity at all in the first place, and is merely an additive for facilitating the flow of electrons, so when the value of Z becomes too low, The electrical resistance of the material itself becomes too large, which is not preferable. On the contrary, if it exists excessively, the amount of oxygen adsorbed on the carbon increases, and the oxygen that is the reactant is reduced to the active site, the nanoheterocluster site. Since it becomes difficult to sway, the cathode reaction activity is lowered, which is not preferable.
Accordingly, Z is desirably 10 wt% or more, preferably 15 wt% or more, more preferably 20 wt% or more, and the upper limit is 79 wt% or less, preferably 77 wt% or less, more preferably 75 wt% or less. Is desirable.
If the amount of Pt, which is the main component of the activity expression, is too small, the oxygen reductive decomposition reaction is not sufficiently performed, and as a result, the power generation characteristics of the fuel cell are not improved. In this sense, X is desirably 1 wt% or more, preferably 3 wt% or more, more preferably 5 wt% or more.
Further, when X exceeds 20 wt%, it is difficult to obtain an improvement in performance commensurate with the amount used. Therefore, from the viewpoint of more effective utilization of rare elements, 50 wt% or less, preferably 30 wt% or less, more preferably 20 wt%. The following is desirable.
Furthermore, CeO _{2 which} is indispensable for stabilizing the heterocluster structure, when it becomes too small, Pt1 valence and Pt2 valence are not sufficiently generated, and oxygen reductive decomposition reaction is not sufficiently performed. As a result, fuel cell This is not preferable because the power generation characteristics of the above are not improved. In this sense, Y is 20 wt% or more, preferably 23 wt% or more, more preferably 25 wt% or more.
Further, even if Y is excessive, CeO ₂ itself has poor conductivity, so that the resistance value of the entire cathode is increased and the cathode performance is deteriorated. Therefore, 40 wt% or less, preferably 38 wt% or less, more preferably 35 wt%. The following is preferable.

By producing a cathode material that satisfies the above conditions, the current density is high and the oxygen reduction reaction initiation potential is 095 V vs. 0, which is difficult with a conventional cathode. 1.2V vs. RHE. A Pt / CeO ₂ / conductive carbon-based nanoheterocathode material for polymer fuel cells, which is RHE and exhibits a four-electron reaction, is obtained.
Here, RHE adopted as a unit of potential means Reversible Hydrogen Electrode, and is a unit generally adopted as an index for quantitatively evaluating the potential.
The four-electron reaction is
O ₂ + 4H ⁺ + 4e ⁻ −> 2H ₂ O (1.2 V vs. RHE)
If the reaction shown by this reaction formula does not occur, it is not preferable as an electrode for a fuel cell or the like. Further, as can be seen from the above formula, since the theoretical potential is about 1.2 V (vs. RHE), an electrode with high cathode performance is required to exhibit an oxygen reduction reaction initiation potential close to this value.
Whether or not this four-electron reaction is shown can be easily determined by performing a Koutecky-Levich plot in which the current value is measured while rotating the electrode itself and changing the number of rotations when measuring the electrode performance. Can do.
This Koutecky-Levich plot will be described in detail. It is known that the following relationship is established between I _k as an activated current and ω as the rotation speed of the electrode.
I ⁻¹ = I _k ⁻¹ + B ⁻¹ ω ^−1/2
Here, B = 0.62 nFAD ₀₂ ^2/3 v ^{−1/6 CO} ₂
And calculated using the values of F, A, v, D ₀₂ , and _{CO 2} . F is the Faraday constant, A is the electrode area used for the measurement (in this experiment, 0.196 cm ² ), v is the kinematic viscosity coefficient (usually calculated using 1.07 × 10 ⁻² cm ² s ⁻¹ ), D ₀₂ is the oxygen diffusion coefficient (usually calculated using 2.16 × 10 ⁻⁵ cm ² s ⁻¹ ), and _{CO 2} is the concentration of the electrolyte solution (1.03 × 10 ⁻³ mol ^{−1 in} this experiment). Calculation).
When n in the formula for obtaining B shows a value close to 4, since it is known that it can be recognized as a 4-electron reaction, it can be easily confirmed from an experiment that it is a 4-electron reaction.
As described above, in the present invention, it is possible to propose a material useful as a cathode for a fuel cell or the like.

Next, a method for producing the cathode material will be described.
First, in order to produce nano-sized CeO ₂ particles, an aqueous cerium nitrate solution (concentration of 0.5 M or more and 3 M or less) is prepared.
The concentration of the raw material aqueous solution is an important factor for determining the crystal phase of the precipitate when the precipitate is obtained from the solution. The reason why it is important to produce this crystal phase will be described below.

By using the method of the present invention, between Pt and CeO _2, it is possible to form an extremely active, hetero cluster sites.
For that purpose, the surface activity of CeO ₂ itself needs to be high. It has been found that the surface activity of CeO ₂ has a very high surface activity when the precipitate is crystallized and the crystallized precipitate is composed of a plurality of crystals. Although the detailed reason is not clear, the crystallized precipitate has a self-form unique to its crystal phase, and the particles that form the surface in a state where crystals with different self-forms at the nano level are intricate are calcined. By doing so, the surface of the obtained CeO ₂ particles becomes a surface rich in undulations at the nano level, and it is considered that this surface works extremely effectively in creating the already-described heterocluster active site.
Therefore, it is preferable to perform the synthesis by searching for a condition in which the obtained precipitate becomes a crystallized state instead of an amorphous state.
When the concentration of the cerium nitrate aqueous solution is lower than the above range, the precipitate is in an amorphous state, and after calcination, sufficient crystallization does not occur, and an unreacted material in an amorphous state tends to remain, and CeO ₂ particles Since the surface activity of the CeO ₂ particles is lower than the above range, the aggregation of the precipitates becomes too large, and the surface activity of the CeO ₂ particles obtained after drying and firing is undesirably reduced.

The aqueous solution set in the above range is heated to a temperature of 50 ° C. or higher and 70 ° C. or lower, and an aqueous ammonium carbonate or ammonium hydrogen carbonate solution (both having a concentration of 0.5 mol / liter (M) to 9 mol / liter (M)) It was appropriate to stir at a stirring speed of 200 rpm to 800 rpm when dropping into the ceria precursor to prepare the ceria precursor. In this operation, as the precipitating agent, it is possible to use ammonium carbonate or an aqueous ammonium hydrogen carbonate solution (both having a concentration of 0.5 mol / liter (M) to 9 mol / liter (M)) to obtain CeO ₂ particles having high surface activity. Was appropriate to create.
The reason is that if the crystal phase contained in the precipitate is carbonate or hydroxide, very small needle-like crystals are likely to appear on the surface, and as a result, it is easy to synthesize a heterocluster with Pt. Because. The concentration of the precipitant aqueous solution is preferably within the above range.
Below this concentration range, the crystallization of the precipitated product does not proceed sufficiently, and the surface activity of the CeO ₂ particles obtained by firing after drying the precipitate does not increase, so it is necessary to increase the cathode activity. Such heterocluster active sites are not preferable because they cannot be sufficiently formed on the cathode surface. On the other hand, when the concentration range is exceeded, agglomeration of the precipitated product is increased, and no matter how much pulverization is performed after drying, the surface activity of CeO ₂ particles obtained after firing is not preferable.

Furthermore, these precipitant aqueous solutions are preferably heated to a temperature of 50 ° C. or higher and 70 ° C. or lower. If a solution having a temperature lower than this temperature range is used, crystallization of the precipitated product becomes insufficient, and after the precipitate is dried, the surface activity of the CeO ₂ particles obtained by firing is not increased, and the cathode activity is increased. This is not preferable because a hetero-cluster active site necessary for enhancement cannot be sufficiently formed. On the other hand, if it exceeds the above temperature range, the aggregation of the precipitated product becomes large, and no matter how much the pulverization treatment is carried out after drying, the surface activity of the CeO ₂ particles obtained after firing is not preferred.

In addition, it is appropriate that the degree of stirring of the aqueous solution in producing the ceria precursor is in the range of 200 rpm to 800 rpm. Below this stirring speed range, the crystallization of the precipitated product becomes insufficient, and the surface activity of the CeO ₂ particles obtained by calcination after drying the precipitate does not increase. This is not preferable because the heterocluster active site required in step 1 cannot be sufficiently formed on the cathode surface. On the other hand, even if the above stirring speed range is exceeded, only a reasonable effect can be expected, and if a too high stirring speed is employed, there is a concern about scattering of the solution, and therefore a stirring speed of about 800 rpm is sufficient.
The dropping speed of the cerium nitrate aqueous solution into the precipitant aqueous solution is preferably dropped at a rate of 0.1 ml / min to 1 ml / min. When dropping is performed at a rate exceeding the dropping rate range, the precipitation product is significantly aggregated, and no matter how much pulverization is performed after drying, the surface activity of the CeO ₂ particles obtained after firing is not preferred. . Moreover, even if a dropping speed lower than the above range is used, only a certain effect can be obtained. Therefore, in practice, it is sufficient to employ the lower limit value of the above dropping speed.

It is appropriate that the ripening after the completion of the dropping is carried out while maintaining the temperature for 24 hours or more and 48 hours or less. Below this ripening time range, crystallization does not proceed sufficiently, and after drying the precipitate, the surface activity of the CeO ₂ particles obtained by calcination does not increase, and the heterocluster activity required to increase the cathode activity It is not preferable because the site cannot be created sufficiently. On the other hand, when the above aging time range is exceeded, crystallization of the precipitated product is promoted, but the aggregation of the precipitated product becomes remarkable, and the surface activity of the CeO ₂ particles obtained after firing is no matter how much the pulverization treatment is performed after drying. Is not preferred because it does not increase.
The precipitated product thus obtained is washed with water for the purpose of removing a trace amount of impurities contained in the raw material, and then subjected to a drying treatment in a dry inert gas at around room temperature so as not to cause aggregation. It is preferable that the crystalline CeO ₂ nanopowder be prepared by calcining under a temperature of 300 ° C. to 500 ° C. under oxygen flow. Below the firing temperature range, the thermal decomposition of the CeO ₂ precursor does not proceed sufficiently, and all of the particles do not become fluorite-type CeO ₂ nanopowder, which is not preferable because the cathode activity does not increase.
On the other hand, when the calcination temperature is exceeded, although fluorite-type CeO ₂ particles are formed, grain growth becomes remarkable, and as a result, the cathode activity is lowered, which is not preferable. There is no particular restriction on the flow rate of oxygen gas during firing, but flowing a large amount of oxygen at a very high flow rate is problematic in terms of safety and reduced yield due to powder scattering. Therefore, it is sufficient to perform firing at a flow rate of 150 ml / min to 300 ml / min. In addition, it is not preferable to perform firing for an excessively long time, which leads to grain growth, and unreacted substances may remain even if the treatment is performed for a very short time, which may cause a decrease in cathode activity. Therefore, it is preferable to perform the baking process for 1 hour or more and about 4 hours.

CeO ₂ nano powder chloroplatinic acid _{(H 2 PtCl 6 · 6H 2} O) aqueous solution thus obtained (concentration 0.05 mol / liter (M) or 1 mol / liter (M) or less) and conductive carbon fine powder While creating a mixed to electrodes and, this time, the concentration of chloroplatinic acid _{(H 2 PtCl 6 · 6H 2} O) aqueous solution, 0.05 mol / liter (M) or 1 mol / liter (M) or less Preferably there is. Below this concentration range, the formation of active cluster heterocluster sites does not progress easily, and as a result, the cathode activity decreases, which is not preferable. On the other hand, when the above range is exceeded, Pt particles aggregate, and on the contrary, the heterocluster sites. Formation is inhibited, resulting in a decrease in cathode activity.

The conductive carbon fine powder used for making the cathode moves electrons generated at the heterocluster site formed by the reaction between Pt and CeO ₂ to the anode side through the electrolyte without staying on the electrode surface. Although it is necessary for the above, there is no particular limitation as long as sufficient conductivity can be ensured. Normally, if carbon black or glassy carbon is used, sufficient performance can be obtained to confirm the expected cathode performance. The carbon powder may be used because it exhibits the above.
After mixing these powders, the solvent used during mixing (usually distilled water) is evaporated and dried under an inert gas flow, and then calcined under a hydrogen flow at a temperature of 300 ° C to 500 ° C. Is appropriate. Below this temperature range, the thermal decomposition of H ₂ PtCl ₆ · 6H ₂ O becomes insufficient, and not only impurities remain on the electrode surface, but also a heterocluster unique to the present invention that is caused by the reaction of Pt and CeO _2. The formation of active sites is hindered, and as a result, the cathode characteristics are deteriorated. On the other hand, if the temperature range is exceeded, the aggregation of metal Pt caused by thermal decomposition of H ₂ PtCl ₆ · 6H ₂ O becomes more significant than the formation of heterocluster active sites. This is not preferable because the formation on the cathode surface of the site is inhibited.

The electrode performance is not limited only by the current value of the electrode, and it is also important that the potential at which the oxygen reduction reaction is started (referred to as onset potential) is high. This high potential means that the reduction reaction of oxygen adsorbed on the cathode occurs very easily. In addition, by using a cathode material having a high oxygen reduction reaction onset potential as an electrode for a fuel cell, the cathode polarization loss generated when the fuel cell is generated is reduced, and a large current density and output are extracted from the fuel cell. It becomes possible. Therefore, the oxygen reduction reaction onset potential must be sufficiently high.
As described above, by using the preparation method disclosed by the present invention, it becomes possible to obtain high cathode performance that cannot be obtained by a normal Pt electrode. Therefore, it is desirable to specify and implement this synthesis condition. This is extremely important for obtaining stable cathode characteristics.

  Next, examples will be described in order to clarify the contents of the present invention. However, the following experimental contents are disclosed as an aid for showing the present invention concretely and for easy understanding, and the contents of the present invention are not limited by these experiments.

Experiment No. 1
Composition so is _{20wt% Pt / 30wt% CeO 2} / 50wt% C, as a starting material, 1.0 mole / liter of cerium nitrate (99.99%) and 2.5 mol / l of an aqueous solution of ammonium carbonate (Purity 99.5%) was prepared, and the aqueous cerium nitrate solution was dropped into the aqueous ammonium carbonate solution heated to 55 ° C. at a rate of 0.5 milliliters per minute while stirring sufficiently at a stirring speed of 400 rpm. A precipitate was made. After completion of the dropwise addition of the cerium nitrate aqueous solution, aging was performed at a temperature of 55 ° C. for 24 hours.
The thus obtained precipitate was alternately washed with water and filtered three times, and then dried in dry nitrogen gas for 2 days to prepare a precursor powder. The precursor powder is then calcined at 400 ° C. for 2 hours under an oxygen flow (150 milliliters per minute) to produce crystalline ceria powder, which consists of a single crystalline phase of fluorite. This was confirmed by an X-ray diffraction test. (See Figure 1)
After that, this nano-CeO ₂ powder was put into a mixed solution of 0.1 mol / liter chloroplatinic acid aqueous solution and carbon black dispersed in distilled water in a glove box whose water content was controlled to 10 ppm or less. After dispersing and stirring with a magnetic stirrer for 2 hours, seal the container, transfer it to a glove box prepared for drying, and gently use a stirrer in a glove box for drying whose moisture content is controlled to 10 ppm or less. Dry at room temperature with stirring, transfer the resulting dry powder to a quartz U-shaped tube, remove the U-shaped tube from the glove box for drying, and load it into an electric furnace. Then, after connecting to the gas inlet pipe, firing at 400 degrees for 2 hours while flowing high-purity hydrogen gas at a flow rate of 150 ml / min. A powder was prepared. FIG. 2 shows the result of observation of an overview of the CeO ₂ powder in the cathode powder with a scanning electron microscope.
The specific surface area of the Pt particles that appeared on the cathode surface thus obtained was 25 m ² / g as a result of measurement using a CO striping method. E. The specific surface area of the CeO ₂ particles measured by the T method was 40 m ² / g.
Here, the procedure of the CO stripping method used for measuring the surface area of Pt appearing on the cathode surface is shown below.
The CO stripping method is a method in which CO is adsorbed on the surface of a Pt element existing on the cathode surface, the amount of adsorption is measured, and the Pt specific surface area of the cathode surface is known from the measured value. In order to perform this measurement, electrode powder is first mixed with ethanol to prepare a 2 gl ^-1 solution. After sufficiently stirring this solution, 5 microliters was collected using a micropipette, placed on the gold electrode, and the gold electrode on which this cathode material was placed was dried for 30 minutes in a dryer heated to 60 degrees. After that, this electrode was inserted into a 0.5 M sulfuric acid aqueous solution, and a potential of 0 V to 1.5 V (vs. RHE) was swept at 50 mV per second, and 0.05 to 0.4 V vs. The H ⁺ desorption peak appearing during RHE was quantified, and the charge used for H ⁺ desorption was calculated from the area. At this time, it is known that a charge of 210 microcoulombs is necessary for desorption of H ⁺ adsorbed on the electrode surface of 1 cm ² , so the charge obtained from the desorption peak area is 210 microcoulombs. The active site surface area of the cathode surface can be calculated by dividing by.
The specific surface area of Pt on the cathode surface was determined by dividing the value determined by this method by the amount of Pt used for preparing the cathode.
The particle diameters of Pt and CeO ₂ observed using a transmission electron microscope and an operation electron microscope were 5 nm and 30 nm, respectively.
Moreover, the valence of Pt on the surface of the produced electrode material active material was observed using photoelectron spectroscopy, and the ratio was quantified. As a result, all Pt (metal Pt, monovalent Pt, divalent Pt, and Pt tetravalent) were obtained. ), The Pt1 value was 8% by volume, and the Pt2 value was 40% by volume, while the tetravalent Pt was observed to be about a trace (7% by volume) (the platinum 4f spectrum measurement result in FIG. 3). Example). In order to clearly understand the presence of ionized platinum, FIG. 3 also shows the measurement result of the platinum 4f spectrum observed from the platinum electrode of a commercially available platinum electrode (manufactured by Johnson Matthey) (FIG. 3). (Figure below). From this comparison, it can be seen that as an unobserved data in the normal Pt cathode, an electronic state of platinum that is clearly ionized appears on the surface of the cathode material in the present invention.
The ratio of Ce trivalent and tetravalent was determined by calculating the ratio from the peak areas of Ce trivalent and Ce tetravalent appearing in the Ce3d signal using photoelectron spectroscopy as in the case of Pt. As a result, the Ce ³⁺ / Ce ⁴⁺ ratio was 1.5.
In addition, the region of Pt diffused on CeO ₂ is the region observed using the high-resolution electron microscope (acceleration voltage: 200 keV) shown in FIG. 4, and image analysis software (image analysis software, Mac-View Ver4, (Mountech Co. Ltd.), the area was calculated to be 130 nm ² .
The electrode active material thus obtained was rotated in a H ₂ SO ₄ aqueous solution with a concentration of 1 mol / liter while rotating the gold electrode at a speed of 2000 rpm while blowing oxygen on the electrode surface at a rate of 5 ml per minute. Cathodic catalyst activity was evaluated by voltammetry at a scan rate of 10 mV.
As a result, it was found that the oxygen reduction reaction becomes a curve as shown in FIG. 5 and exhibits high activity. For the onset potential of the oxygen reduction reaction, the Tafel plot shown in FIG. 6 is prepared based on the data obtained in FIG. 5, and this straight line is described as a current density value of 0.01 A / cm ^2. The point that intersected the axis was determined.
Although the current density is described as kinetic current density in FIG. 6, this is because the current density varies depending on the number of rotations of the rotating electrode. At a high rotational speed, the oxygen reduction reaction on the electrode surface is promoted, so that the current density tends to increase as the rotational speed of the electrode increases.
Therefore, as in this embodiment, the current density when the rotational speed is 2000 rpm is indicated in the figure as kinetic current density in order to clearly indicate that the current density is defined by the rotational speed of the electrode. And
Thereafter, unless otherwise specified, the current density indicates a value at the rotation speed of the electrode of 2000 rpm.
The potential estimated from FIG. 6 was as high as 0.98 V (vs. RHE).
At this time, the number of rotations of the electrode was changed from 1500 rpm to 3000 rpm in increments of 500 rpm, and the cathode reaction analysis using the Koutecky-Levich plot was simultaneously performed.
The Koutecky-Levich plot obtained by changing the number of rotations of the electrode at a constant potential is shown in FIG. 7. The n value obtained from the slope of each straight line in this figure is about 4, and is a four-electron reaction. Also confirmed.

Experiment No. 2
1.0 mol / liter cerium nitrate (purity 99.99%) and 2.5 mol / liter as starting materials so that the composition is 0.05 wt% Pt / 35 wt% CeO ₂ /64.95 wt% C Aqueous ammonium hydrogen carbonate solution (purity 99.5%) was prepared, and the aqueous cerium nitrate solution was added at 0.5 milliliters per minute while stirring sufficiently at an agitation speed of 800 rpm in an aqueous ammonium carbonate solution heated to 65 ° C. A precipitate was made by dropping at a rate. After completion of the dropwise addition of the cerium nitrate aqueous solution, aging was performed at a temperature of 65 ° C. for 48 hours.
The thus obtained precipitate was alternately washed with water and filtered three times, and then dried in dry nitrogen gas for 2 days to prepare a precursor powder. The precursor powder was subsequently calcined at 350 ° C. for 2 hours under an oxygen flow (150 milliliters per minute) to produce crystalline ceria powder. 1 was confirmed to be composed of a single crystal phase of fluorite by an X-ray diffraction test.
After that, this nano-CeO ₂ powder is supported in a glove box whose water content is controlled to 10 ppm or less so that a predetermined amount of Pt is supported using a 0.5 mol / liter chloroplatinic acid aqueous solution. Dilute chloroplatinic acid aqueous solution, prepare carbon black and disperse solution in distilled water, mix nano-CeO ₂ powder in it, stir with magnetic stirrer for 2 hours, seal container and dry The sample was transferred to a glove box prepared for use and dried at room temperature with gentle stirring using a stirrer in a glove box for drying whose moisture content was controlled to 10 ppm or less. 1 was fired at 450 ° C. for 2 hours while flowing high-purity hydrogen gas at a flow rate of 150 ml / min to prepare cathode powder.
The specific surface area of the Pt particles appearing on the cathode surface thus obtained was 40 m ² / g as a result of measurement using a CO striping method. E. The specific surface area of the CeO ₂ particles measured by the T method is 50 m ² / g,
The particle diameters of Pt and CeO ₂ observed using a transmission electron microscope and an operation electron microscope were 4 nm and 25 nm, respectively.
In addition, the valence of Pt on the surface of the produced electrode material active material was determined according to Experiment No. 1 As a result of measurement by the same method, Pt1 value was 6% by volume, Pt2 value was 28% by volume, and Pt4 value was 3% by volume for all Pt.
The ratio of Ce trivalent and Ce tetravalent was determined according to Experiment No. 1 As a result of analysis using the same method, the Ce ³⁺ / Ce ⁴⁺ ratio was 1.4.
In addition, the region of Pt diffused on CeO ₂ is shown in Experiment No. 1 Based on the same observation results, area calculation using image analysis software was 110 nm ² .
The electrode active material thus obtained was used as an experiment No. 1 When the cathode catalyst activity was evaluated by the same method, the onset potential of the oxygen reduction reaction was sufficiently high as 0.99 V (vs. RHE), and at the same time, Koutecky showing the electrode reaction mechanism. The n value obtained from the -Levich plot was about 4, and it was confirmed that the reaction was a four-electron reaction.

Experiment No. 3
Composition such that _{40wt% Pt / 25wt% CeO 2} / 45wt% C, as a starting material, 1.0 mole / liter of cerium nitrate (99.99%) and 7.5 mole / liter aqueous solution of ammonium carbonate (Purity 99.5%) was prepared, and the aqueous solution of cerium nitrate was dropped into the aqueous ammonium carbonate solution heated to 55 ° C. at a rate of 1 milliliter per minute while stirring sufficiently at a stirring speed of 400 rpm. Produced. After completion of the dropwise addition of the cerium nitrate aqueous solution, aging was performed at a temperature of 55 ° C. for 24 hours.
The thus obtained precipitate was alternately washed with water and filtered three times, and then dried in dry nitrogen gas for 2 days to prepare a precursor powder. The precursor powder was subsequently calcined at 300 ° C. for 2 hours under an oxygen flow (150 milliliters per minute) to produce a crystalline ceria powder. 1 was confirmed to be composed of a single crystal phase of fluorite by an X-ray diffraction test.
After that, this nano-CeO ₂ powder is platinum chloride so that a predetermined amount of Pt is supported using a 1 mol / liter chloroplatinic acid aqueous solution in a glove box whose water content is controlled to 10 ppm or less. The acid aqueous solution is diluted, and a solution dispersed in distilled water is prepared together with carbon black. After mixing the nano-CeO ₂ powder and stirring with a magnetic stirrer for 2 hours, the container is sealed and dried. The sample was transferred into the prepared glove box and dried at room temperature with gentle stirring using a stirrer in a glove box for drying whose moisture content was controlled to 10 ppm or less. 1 was fired at 300 ° C. for 2 hours while flowing high-purity hydrogen gas at a flow rate of 150 ml / min to prepare cathode powder.
The specific surface area of the Pt particles that appeared on the cathode surface thus obtained was 25 m ² / g as a result of measurement using a CO striping method. E. The specific surface area of the CeO ₂ particles measured by the T method was 60 m ² / g, and the particle diameters of Pt and CeO ₂ observed using a transmission electron microscope and an operation electron microscope were 5 nm and 25 nm, respectively. .
In addition, the valence of Pt on the surface of the produced electrode material active material was determined according to Experiment No. 1 As a result of measurement by the same method, Pt1 value was 19% by volume, Pt2 value was 37% by volume, and Pt4 value was 4% by volume for all Pt.
The ratio of Ce trivalent and Ce tetravalent was determined according to Experiment No. 1 As a result of analysis using the same method, the Ce ³⁺ / Ce ⁴⁺ ratio was 1.7.
In addition, the region of Pt diffused on CeO ₂ is shown in Experiment No. 1 Based on the same observation results, area calculation was performed using image analysis software, and the result was 160 nm ² .
The electrode active material thus obtained was used as an experiment No. 1 When the cathode catalyst activity was evaluated by the same method, the onset potential of the oxygen reduction reaction was sufficiently high as 1.05 V (vs. RHE), and at the same time, Koutecky showing the electrode reaction mechanism. The n value obtained from the -Levich plot was about 4, and it was confirmed that the reaction was a four-electron reaction.

Experiment No. 4
Composition such that _{20 wt% Pt / 25wt% CeO} 2 / 55wt% C, as a starting material, 1.0 mole / liter of cerium nitrate (99.99%) and 2.5 mol / l of ammonium carbonate An aqueous solution (purity 99.5%) was prepared, and the aqueous solution of cerium nitrate was dropped into an aqueous ammonium carbonate solution heated to 65 ° C. at a rate of 1 milliliter per minute while stirring sufficiently at a stirring speed of 600 rpm. Was made. After completion of the dropwise addition of the cerium nitrate aqueous solution, aging was performed at a temperature of 65 ° C. for 24 hours.
The thus obtained precipitate was alternately washed with water and filtered three times, and then dried in dry nitrogen gas for 2 days to prepare a precursor powder. The precursor powder was subsequently calcined at 400 ° C. for 2 hours under oxygen flow (150 milliliters per minute) to produce crystalline ceria powder. 1 was confirmed to be composed of a single crystal phase of fluorite by an X-ray diffraction test.
After that, this nano-CeO ₂ powder is platinum chloride so that a predetermined amount of Pt is supported using a 1 mol / liter chloroplatinic acid aqueous solution in a glove box whose water content is controlled to 10 ppm or less. The acid aqueous solution is diluted, and a solution dispersed in distilled water is prepared together with carbon black. After mixing the nano-CeO ₂ powder and stirring with a magnetic stirrer for 2 hours, the container is sealed and dried. The sample was transferred into the prepared glove box and dried at room temperature with gentle stirring using a stirrer in a glove box for drying whose moisture content was controlled to 10 ppm or less. 1 was fired at 400 ° C. for 2 hours while flowing high-purity hydrogen gas at a flow rate of 150 ml / min to prepare cathode powder.
The specific surface area of the Pt particles appearing on the cathode surface thus obtained was 35 m ² / g as a result of measurement using a CO striping method. E. The specific surface area of the CeO ₂ particles measured by the T method was 45 m ² / g, and the particle diameters of Pt and CeO ₂ observed using a transmission electron microscope and an operation electron microscope were 4 nm and 30 nm, respectively. .
In addition, the valence of Pt on the surface of the produced electrode material active material was determined according to Experiment No. 1 As a result of measurement by the same method, Pt1 value was 8% by volume, Pt2 value was 29% by volume, and Pt4 value was 3% by volume for all Pt.
The ratio of Ce trivalent and Ce tetravalent was determined according to Experiment No. 1 As a result of analysis using the same method, the Ce ³⁺ / Ce ⁴⁺ ratio was 1.4.
In addition, the region of Pt diffused on CeO ₂ is shown in Experiment No. 1 Based on the same observation results, area calculation was performed using image analysis software, and the result was 120 nm ² .
The electrode active material thus obtained was used as an experiment No. 1 When the cathode catalyst activity was evaluated by the same method, the onset potential of the oxygen reduction reaction was sufficiently high as 0.94 V (vs. RHE), and at the same time, Koutecky showing the electrode reaction mechanism. The n value obtained from the -Levich plot was about 4, and it was confirmed that the reaction was a four-electron reaction.

Experiment No. 5
Composition so is _{20wt% Pt / 30wt% CeO 2} / 50wt% C, as a starting material, 2.5 moles / liter cerium nitrate (99.99%) and 7.5 mole / liter ammonium bicarbonate An aqueous solution (purity 99.5%) was prepared, and the cerium nitrate aqueous solution was added dropwise at a rate of 0.1 milliliter per minute to the ammonium carbonate aqueous solution heated to 58 degrees with sufficient stirring at a stirring speed of 400 rpm. To prepare a precipitate. After completion of the dropwise addition of the cerium nitrate aqueous solution, aging was performed at a temperature of 58 ° C. for 24 hours.
The thus obtained precipitate was alternately washed with water and filtered three times, and then dried in dry nitrogen gas for 2 days to prepare a precursor powder. The precursor powder was subsequently calcined at 350 ° C. for 1 hour under oxygen flow (150 milliliters per minute) to produce crystalline ceria powder. 1 was confirmed to be composed of a single crystal phase of fluorite by an X-ray diffraction test.
After that, this nano-CeO ₂ powder is supported in a glove box whose water content is controlled to 10 ppm or less so that a predetermined amount of Pt is supported using 0.06 mol / liter chloroplatinic acid aqueous solution. Dilute chloroplatinic acid aqueous solution, prepare carbon black and disperse solution in distilled water, mix nano-CeO ₂ powder in it, stir with magnetic stirrer for 2 hours, seal container and dry The sample was transferred to a glove box prepared for use and dried at room temperature with gentle stirring using a stirrer in a glove box for drying whose moisture content was controlled to 10 ppm or less. 1 was fired at 350 ° C. for 2 hours while flowing high-purity hydrogen gas at a flow rate of 150 ml / min to prepare cathode powder.
The specific surface area of the Pt particles appearing on the cathode surface thus obtained was 28 m ² / g as a result of measurement using a CO striping method. E. The specific surface area of the CeO ₂ particles measured by the T method was 48 m ² / g, and the particle diameters of Pt and CeO ₂ observed using a transmission electron microscope and a manipulation electron microscope were 5 nm and 30 nm, respectively. .
In addition, the valence of Pt on the surface of the produced electrode material active material was determined according to Experiment No. 1 As a result of measurement by the same method, Pt1 value was 9% by volume, Pt2 value was 27% by volume, and Pt4 value was 3% by volume for all Pt.
The ratio of Ce trivalent and Ce tetravalent was determined according to Experiment No. 1 As a result of analysis using the same method, the Ce ³⁺ / Ce ⁴⁺ ratio was 1.4.
In addition, the region of Pt diffused on CeO ₂ is shown in Experiment No. 1 Based on the same observation results, area calculation using image analysis software was 109 nm ² .
The electrode active material thus obtained was used as an experiment No. 1 When the cathode catalyst activity was evaluated by the same method, the onset potential of the oxygen reduction reaction was sufficiently high as 0.96 V (vs. RHE), and at the same time, Koutecky showing the electrode reaction mechanism. The n value obtained from the -Levich plot was about 4, and it was confirmed that the reaction was a four-electron reaction.

Comparative example

Experiment No. 6
1.0 mol / liter cerium nitrate (purity 99.99%) and 2.5 mol / liter as starting materials so that the composition is 0.005 wt% Pt / 30 wt% CeO ₂ /69.995 wt% C Aqueous ammonium carbonate aqueous solution (purity 99.5%) was prepared, and the aqueous cerium nitrate solution was dropped into the aqueous ammonium carbonate solution heated to 65 degrees at a rate of 1 milliliter per minute while stirring sufficiently at a stirring speed of 600 rpm. To produce a precipitate. After completion of the dropwise addition of the cerium nitrate aqueous solution, aging was performed at a temperature of 65 ° C. for 24 hours.
The thus obtained precipitate was alternately washed with water and filtered three times, and then dried in dry nitrogen gas for 2 days to prepare a precursor powder. The precursor powder was subsequently calcined at 400 ° C. for 2 hours under oxygen flow (150 milliliters per minute) to produce crystalline ceria powder. 1 was confirmed to be composed of a single crystal phase of fluorite by an X-ray diffraction test.
After that, this nano-CeO ₂ powder is platinum chloride so that a predetermined amount of Pt is supported using a 1 mol / liter chloroplatinic acid aqueous solution in a glove box whose water content is controlled to 10 ppm or less. The acid aqueous solution is diluted, and a solution dispersed in distilled water is prepared together with carbon black. After mixing the nano-CeO ₂ powder and stirring with a magnetic stirrer for 2 hours, the container is sealed and dried. The sample was transferred into the prepared glove box and dried at room temperature with gentle stirring using a stirrer in a glove box for drying whose moisture content was controlled to 10 ppm or less. 1 was fired at 400 ° C. for 2 hours while flowing high-purity hydrogen gas at a flow rate of 150 ml / min to prepare cathode powder.
The specific surface area of the Pt particles appearing on the cathode surface thus obtained was 28 m ² / g as a result of measurement using a CO striping method. E. The specific surface area of the CeO ₂ particles measured by the T method was 45 m ² / g, and the particle diameters of Pt and CeO ₂ observed using a transmission electron microscope and an operation electron microscope were 5 nm and 30 nm, respectively. .
In addition, the valence of Pt on the surface of the produced electrode material active material was determined according to Experiment No. 1 As a result of measurement by the same method, Pt1 value was 2% by volume, Pt2 value was 7% by volume, and Pt4 value was 12% by volume for all Pt.
The ratio of Ce trivalent and Ce tetravalent was determined according to Experiment No. 1 As a result of analysis using the same method, the Ce ³⁺ / Ce ⁴⁺ ratio was 0.2.
In addition, the region of Pt diffused on CeO ₂ is shown in Experiment No. 1 Based on the same observation results, the area was calculated using image analysis software and found to be 10 nm ² .
The electrode active material thus obtained was used as an experiment No. 1 When the cathode catalyst activity was evaluated by the same method, the onset potential of the oxygen reduction reaction was not sufficiently high, 0.76 V (vs. RHE). At the same time, the n value obtained from the Koutecky-Levich plot showing the electrode reaction mechanism is about 2, indicating that it is a two-electron reaction (a reaction in which H ₂ O ₂ is generated) and is not a preferable electrode as a fuel cell electrode. It was.

Experiment No. 7
Composition so is _{20wt% Pt / 5wt% CeO 2} / 75wt% C, as a starting material, 1.0 mole / liter of cerium nitrate (99.99%) and 2.5 mol / l of an aqueous solution of ammonium carbonate (Purity 99.5%) was prepared, and the aqueous solution of cerium nitrate was dropped into the aqueous ammonium carbonate solution heated to 65 ° C. at a rate of 1 milliliter per minute while stirring sufficiently at a stirring speed of 600 rpm. Produced. After completion of the dropwise addition of the cerium nitrate aqueous solution, aging was performed at a temperature of 65 ° C. for 24 hours.
The thus obtained precipitate was alternately washed with water and filtered three times, and then dried in dry nitrogen gas for 2 days to prepare a precursor powder. The precursor powder was subsequently calcined at 400 ° C. for 2 hours under oxygen flow (150 milliliters per minute) to produce crystalline ceria powder. 1 was confirmed to be composed of a single crystal phase of fluorite by an X-ray diffraction test.
After that, this nano-CeO ₂ powder is platinum chloride so that a predetermined amount of Pt is supported using a 1 mol / liter chloroplatinic acid aqueous solution in a glove box whose water content is controlled to 10 ppm or less. The acid aqueous solution is diluted, and a solution dispersed in distilled water is prepared together with carbon black. After mixing the nano-CeO ₂ powder and stirring with a magnetic stirrer for 2 hours, the container is sealed and dried. The sample was transferred into the prepared glove box and dried at room temperature with gentle stirring using a stirrer in a glove box for drying whose moisture content was controlled to 10 ppm or less. 1 was fired at 700 ° C. for 2 hours while flowing high-purity hydrogen gas at a flow rate of 150 ml / min to prepare cathode powder.
The specific surface area of the Pt particles that appeared on the cathode surface thus obtained was 10 m ² / g as a result of measurement using a CO striping method. E. The specific surface area of the CeO ₂ particles measured by the T method was 45 m ² / g, and the particle diameters of Pt and CeO ₂ observed using a transmission electron microscope and a manipulation electron microscope were 19 nm and 30 nm, respectively. .
In addition, the valence of Pt on the surface of the produced electrode material active material was determined according to Experiment No. 1 As a result of measurement by the same method, Pt1 value was 3% by volume, Pt2 value was 8% by volume, and Pt4 value was 14% by volume for all Pt.
The ratio of Ce trivalent and Ce tetravalent was determined according to Experiment No. 1 As a result of analysis using the same method, the Ce ³⁺ / Ce ⁴⁺ ratio was 0.2.
In addition, the region of Pt diffused on CeO ₂ is shown in Experiment No. 1 Based on the same observation results, area calculation was performed using image analysis software, and the result was 13 nm ² .
The electrode active material thus obtained was used as an experiment No. 1 When the cathode catalyst activity was evaluated by the same method, the onset potential of the oxygen reduction reaction was not sufficiently high, 0.79 V (vs. RHE). At the same time, the n value obtained from the Koutecky-Levic plot showing the electrode reaction mechanism was about 4.

Experiment No. 8
Composition so is _{20wt% Pt / 30wt% CeO 2} / 50wt% C, as a starting material, 1.0 mole / liter of cerium nitrate (99.99%) and 2.5 mol / l of an aqueous solution of ammonium carbonate (Purity 99.5%) was prepared, and the aqueous solution of cerium nitrate was dropped into the aqueous ammonium carbonate solution heated to 90 ° C. at a rate of 1 milliliter per minute while stirring sufficiently at a stirring speed of 1000 rpm. Produced. After completion of the dropwise addition of the cerium nitrate aqueous solution, aging was performed at a temperature of 90 ° C. for 24 hours. However, since the rotational speed was fast, some of the solution was scattered and the yield was slightly reduced. In addition, with the formation of precipitates at a high temperature of 90 degrees, the generated precipitates grew greatly, and immediately after the stirring was stopped, sedimentation was confirmed in the precipitates.
The thus obtained precipitate was alternately washed with water and filtered three times, and then dried in dry nitrogen gas for 2 days to prepare a precursor powder. The precursor powder was subsequently calcined at 400 ° C. for 2 hours under oxygen flow (150 milliliters per minute) to produce crystalline ceria powder. 1 was confirmed to be composed of a single crystal phase of fluorite by an X-ray diffraction test.
After that, this nano-CeO ₂ powder is platinum chloride so that a predetermined amount of Pt is supported using a 1 mol / liter chloroplatinic acid aqueous solution in a glove box whose water content is controlled to 10 ppm or less. The acid aqueous solution is diluted, and a solution dispersed in distilled water is prepared together with carbon black. After mixing the nano-CeO ₂ powder and stirring with a magnetic stirrer for 2 hours, the container is sealed and dried. The sample was transferred into the prepared glove box and dried at room temperature with gentle stirring using a stirrer in a glove box for drying whose moisture content was controlled to 10 ppm or less. 1 was fired at 800 ° C. for 2 hours while flowing high-purity hydrogen gas at a flow rate of 150 ml / min to prepare cathode powder.
The specific surface area of the Pt particles that appeared on the cathode surface thus obtained was 8 m ² / g as a result of measurement using a CO striping method. E. The specific surface area of the CeO ₂ particles measured by the T method was 5 m ² / g, and the particle diameters of Pt and CeO ₂ observed using a transmission electron microscope and a manipulation electron microscope were 19 nm and 68 nm, respectively. .
In addition, the valence of Pt on the surface of the produced electrode material active material was determined according to Experiment No. 1 As a result of measurement by the same method, Pt1 value was 3% by volume, Pt2 value was 6% by volume, and Pt4 value was 15% by volume for all Pt.
The ratio of Ce trivalent and Ce tetravalent was determined according to Experiment No. 1 As a result of analysis using the same method, the Ce ³⁺ / Ce ⁴⁺ ratio was 0.3.
In addition, the region of Pt diffused on CeO ₂ is shown in Experiment No. 1 Based on the same observation results, the area was calculated using image analysis software and found to be 20 nm ² .
The electrode active material thus obtained was used as an experiment No. 1 When the cathode catalyst activity was evaluated by the same method, the onset potential of the oxygen reduction reaction was not sufficiently high at 0.72 V (vs. RHE). At the same time, the n value obtained from the Koutecky-Levic plot showing the electrode reaction mechanism was about 4.

Experiment No. 9
Composition so is _{20wt% Pt / 30wt% CeO 2} / 50wt% C, as a starting material, 10 mol / l of cerium nitrate (99.99%) and 12 mol / l of ammonium carbonate aqueous solution (purity 99. 5%) was prepared, and a cerium nitrate aqueous solution was dropped into an aqueous ammonium carbonate solution heated to 65 ° C. at a rate of 1 milliliter per minute while stirring sufficiently at a stirring speed of 600 rpm. After completion of the dropwise addition of the cerium nitrate aqueous solution, aging was performed at a temperature of 65 ° C. for 24 hours. The particle size of the produced precursor was very large, and immediately after the stirring operation was stopped, precipitation of the precipitated product was confirmed.
The thus obtained precipitate was alternately washed with water and filtered three times, and then dried in dry nitrogen gas for 2 days to prepare a precursor powder. The precursor powder was then calcined at 900 ° C. for 2 hours under an oxygen flow (150 milliliters per minute) to produce crystalline ceria powder. 1 was confirmed to be composed of a single crystal phase of fluorite by an X-ray diffraction test.
After that, this nano-CeO ₂ powder is platinum chloride so that a predetermined amount of Pt is supported using a 1 mol / liter chloroplatinic acid aqueous solution in a glove box whose water content is controlled to 10 ppm or less. The acid aqueous solution is diluted, and a solution dispersed in distilled water is prepared together with carbon black. After mixing the nano-CeO ₂ powder and stirring with a magnetic stirrer for 2 hours, the container is sealed and dried. The sample was transferred into the prepared glove box and dried at room temperature with gentle stirring using a stirrer in a glove box for drying whose moisture content was controlled to 10 ppm or less. 1 was fired at 400 ° C. for 2 hours while flowing high-purity hydrogen gas at a flow rate of 150 ml / min to prepare cathode powder.
The specific surface area of the Pt particles appearing on the cathode surface thus obtained was 28 m ² / g as a result of measurement using a CO striping method. E. The specific surface area of the CeO ₂ particles measured by the T method was 8 m ² / g, and the particle diameters of Pt and CeO ₂ observed using a transmission electron microscope and a manipulation electron microscope were 5 nm and 60 nm, respectively. .
In addition, the valence of Pt on the surface of the produced electrode material active material was determined according to Experiment No. 1 As a result of measurement by the same method, Pt1 value was 1% by volume, Pt2 value was 6% by volume, and Pt4 value was 16% by volume for all Pt.
The ratio of Ce trivalent and Ce tetravalent was determined according to Experiment No. 1 As a result of analysis using the same method, the Ce ³⁺ / Ce ⁴⁺ ratio was 0.3.
In addition, the region of Pt diffused on CeO ₂ is shown in Experiment No. 1 Based on the same observation results, the area was calculated using image analysis software and found to be 20 nm ² .
The electrode active material thus obtained was used as an experiment No. 1 When the cathode catalyst activity was evaluated by the same method, the onset potential of the oxygen reduction reaction was not sufficiently high at 0.72 V (vs. RHE). At the same time, the n value obtained from the Koutecky-Levic plot showing the electrode reaction mechanism was about 4.

Experiment No. 10
Composition so is _{20wt% Pt / 30wt% CeO 2} / 50wt% C, as a starting material, 1.0 mole / liter of cerium nitrate (99.99%) and 2.5 mol / l of an aqueous solution of ammonium carbonate (Purity 99.5%) was prepared, and the aqueous solution of cerium nitrate was dropped into the aqueous ammonium carbonate solution heated to 60 ° C. at a rate of 1 milliliter per minute with sufficient stirring at a stirring speed of 600 rpm. Produced. After completion of the dropwise addition of the cerium nitrate aqueous solution, aging was performed at a temperature of 60 ° C. for 20 minutes.
The thus obtained precipitate was alternately washed with water and filtered three times, and then dried in dry nitrogen gas for 2 days to prepare a precursor powder. The precursor powder was subsequently calcined at a temperature of 200 ° C. for 2 hours under an oxygen flow (150 milliliters per minute) to produce a crystalline ceria powder. No single crystal phase of fluorite similar to 1 could be confirmed, and peaks attributed to carbonates and hydroxides were confirmed by X-ray diffraction tests.
After that, this nano-CeO ₂ powder is platinum chloride so that a predetermined amount of Pt is supported using a 1 mol / liter chloroplatinic acid aqueous solution in a glove box whose water content is controlled to 10 ppm or less. The acid aqueous solution is diluted, and a solution dispersed in distilled water is prepared together with carbon black. After mixing the nano-CeO ₂ powder and stirring with a magnetic stirrer for 2 hours, the container is sealed and dried. The sample was transferred into the prepared glove box and dried at room temperature with gentle stirring using a stirrer in a glove box for drying whose moisture content was controlled to 10 ppm or less. 1 was fired at 900 ° C. for 2 hours while flowing high-purity hydrogen gas at a flow rate of 150 ml / min to prepare cathode powder.
The specific surface area of the Pt particles appearing on the cathode surface thus obtained was measured using a CO striping method, and as a result, 5 m ² / g, B.I. E. The specific surface area of the mixture containing CeO ₂ particles measured by the T method is 6 m ² / g, and the particle diameters of Pt and CeO ₂ observed using a transmission electron microscope and a manipulation electron microscope are 20 nm and 58 nm, respectively. Met.
In addition, the valence of Pt on the surface of the produced electrode material active material was determined according to Experiment No. 1 As a result of measurement by the same method, no peak of Pt1 value was observed for all Pt, Pt2 value was 5% by volume, and Pt4 value was 19% by volume.
The ratio of Ce trivalent and Ce tetravalent was determined according to Experiment No. 1 As a result of analysis using the same method, the Ce ³⁺ / Ce ⁴⁺ ratio was 0.3.
In addition, the region of Pt diffused on CeO ₂ is shown in Experiment No. 1 Based on the same observation results, area calculation was performed using image analysis software, and the result was 18 nm ² .
The electrode active material thus obtained was used as an experiment No. 1 When the cathode catalyst activity was evaluated by the same method, the onset potential of the oxygen reduction reaction was not sufficiently high at 0.73 V (vs. RHE). At the same time, the n value obtained from the Koutecky-Levic plot showing the electrode reaction mechanism was about 4.

Experiment No. 11;
Composition so is _{20wt% Pt / 30wt% CeO 2} / 50wt% C, as a starting material, 1.0 mole / liter of cerium nitrate (99.99%) and 2.5 mol / l of an aqueous solution of ammonium carbonate (Purity 99.5%) was prepared, and the aqueous solution of cerium nitrate was dropped into the aqueous ammonium carbonate solution heated to 60 ° C. at a rate of 1 milliliter per minute with sufficient stirring at a stirring speed of 600 rpm. Produced. After completion of the dropwise addition of the cerium nitrate aqueous solution, aging was performed at a temperature of 60 ° C. for 24 hours.
The thus obtained precipitate was alternately washed with water and filtered three times, and then dried in dry nitrogen gas for 2 days to prepare a precursor powder. The precursor powder was subsequently calcined at 400 ° C. for 2 hours under oxygen flow (150 milliliters per minute) to produce crystalline ceria powder. 1 was confirmed to be composed of a single crystal phase of fluorite by an X-ray diffraction test.
After that, this nano-CeO ₂ powder is platinum chloride so that a predetermined amount of Pt is supported using a 1 mol / liter chloroplatinic acid aqueous solution in a glove box whose water content is controlled to 10 ppm or less. The acid aqueous solution is diluted, and a solution dispersed in distilled water is prepared together with carbon black. After mixing the nano-CeO ₂ powder and stirring with a magnetic stirrer for 2 hours, the container is sealed and dried. The sample was transferred into the prepared glove box and dried at room temperature with gentle stirring using a stirrer in a glove box for drying whose moisture content was controlled to 10 ppm or less. 1 was fired at 200 ° C. for 20 minutes while flowing high-purity hydrogen gas at a flow rate of 150 ml / min to prepare cathode powder.
The specific surface area of the Pt particles appearing on the cathode surface thus obtained was 7 m ² / g as a result of measurement using a CO striping method. E. The specific surface area of the CeO ₂ particles measured by the T method was 35 m ² / g, and the particle diameters of Pt and CeO ₂ observed using a transmission electron microscope and an operation electron microscope were 20 nm and 30 nm, respectively. .
In addition, the valence of Pt on the surface of the produced electrode material active material was determined according to Experiment No. 1 As a result of measurement by the same method, Pt1 value was 1% by volume, Pt2 value was 3% by volume, and Pt4 value was 14% by volume for all Pt.
The ratio of Ce trivalent and Ce tetravalent was determined according to Experiment No. 1 As a result of analysis using the same method, the Ce ³⁺ / Ce ⁴⁺ ratio was 0.2.
In addition, the region of Pt diffused on CeO ₂ is shown in Experiment No. 1 Based on the same observation results, area calculation was performed using image analysis software, and the result was 15 nm ² .
The electrode active material thus obtained was used as an experiment No. 1 When the cathode catalyst activity was evaluated by the same method, the onset potential of the oxygen reduction reaction was not sufficiently high at 0.75 V (vs. RHE). At the same time, the n value obtained from the Koutecky-Levic plot showing the electrode reaction mechanism was about 4.

Summarizing the above experiments, the cathode material defined by the composition formula based on the general formula defined in the claims of the present invention, which has a monovalent and bivalent platinum surface area on the surface of the electrode active material. When the surface area of CeO ₂ and the area of the region where Pt diffuses on the CeO ₂ surface have specific values, it has been clarified that the cathode activity is extremely high compared to the range outside the range.

In recent years, carbon dioxide reduction has been called out as part of global warming countermeasures, and in order to meet increasing energy demand, development of high-power small-sized fuel cells for home use has been actively promoted. Research and development of high-performance cathode materials necessary to develop high power generation performance is indispensable for the development of such small household fuel cells.
Polymer fuel cells are now being used in many homes as household fuel cells, but they are actually used in apartment buildings and the like, and are large in size and have a short lifetime of about 3 years.
In order to reduce the size, it is necessary to improve the performance of the fuel cell and reduce the size of the fuel cell device. Loss due to a large overvoltage on the cathode greatly reduces the performance of the fuel cell device, while reducing the loss on the cathode greatly improves the performance of the fuel cell, and the fuel cell is installed in a housing complex. It is expected that it will open up the way to utilize it.
In addition, the life of the fuel cell is largely attributed to deterioration of the solid electrolyte membrane and electrode performance. Therefore, by achieving high performance of the cathode for fuel cells and extending the period of sufficiently ensuring the function as a power source used at home, it will extend the life of practical home fuel cells, The spread is expected to be further promoted.
The present invention provides a novel cathode material that exactly meets this need. By using a high-performance Pt / CeO ₂ / conductive carbon-based nanoheterosode material, the fuel cell can be reduced in size and practically used. It is expected to be used in the future because it will lead to a long life, and as a result, the spread of fuel cell devices to general households will be greatly promoted. In addition, the electrode material having excellent characteristics of the present invention is based on an extremely diversified and basic viewpoint, obtained new knowledge at the nano level, and has been successfully developed on that basis. Stable quality is guaranteed, and it is expected that it will be used and used as an excellent solid electrolyte not only in fuel cells but also in various technical fields. In particular, since it is a high-performance electrode with a small amount of platinum, its range of use is wide and it is expected to develop into the creation of new industries.

(Takao Toda, Hiroshi Igarashi, Hiroyuki Uchida, and Masahiro Watanabe, Journal of The Electrochemical Society, 146 (10), 3750-56). (K. Sasaki, L. Zhang, and RR Adzic, Physical Chemistry Chemical Physics, 10, 159-167 (2008).) C. Bock and B.B. Mac Dougall, ‘’ Novel method for the estimation of the electroactive Ptarea, ’“ J. Electrochem. Society, 150, E377-E383 (2003).

Claims (9)

A cathode material using Pt particles as an electrode material and carbon particles as a conductive material, the Pt particles being supported on the surface of the ceria particles , and having a Pt1 value of 5% by volume or more on the surface of the Pt particles. Cathode material characterized by The cathode material according to claim 1, wherein the Pt2 value is 10% by volume or more . 3. The cathode material according to claim 1, wherein the Pt tetravalent abundance ratio is 10% by volume or less and exhibits a four-electron reaction. 4 . 4. The cathode material according to claim 1, wherein the specific surface area of the Pt particles is 20 m ² / g or more (by CO stripping method) . 5. 5. The cathode material according to claim 1, wherein the specific surface area of the ceria particles is 20 m ² / g or more and 70 m ² / g or less (according to the BET method) . In the cathode material according to any one of claims 1 to 5, when the general formula indicating the composition is _X Pt / _Y CeO ₂ / _Z carbon (Z = 100−XY), 0.01 ≦ X ≦ A cathode material, wherein 70 and 20 ≦ Y ≦ 40 . The cathode material according to claim 1, wherein a Pt region diffused on CeO ₂ serves as an active site . The cathode material according to claim 7 , wherein the active site has an area of 1 × 10 ² square nanometers or more and 4 × 10 ² square nanometers or less . The cathode material according to any one of claims 1 to 8, wherein the Ce ³⁺ / Ce ⁴⁺ ratio is 1.0 or more and exhibits a four-electron reaction .