JP2010092690A - Method of manufacturing electrocatalyst for fuel cell, and electrocatalyst for fuel cell - Google Patents

Abstract

A method for producing an electrode catalyst for a fuel cell, which can produce an electrode catalyst at low cost using coal.
A fuel cell electrode catalyst is manufactured as an electrode catalyst by nitriding coal with ammonia gas.
[Selection] Figure 1

Description

  The present invention relates to a method for producing a fuel cell electrode catalyst and a fuel cell electrode catalyst produced by the production method, and is particularly suitable for use in a polymer electrolyte fuel cell (PEFC).

In recent years, fuel cells have attracted attention as clean energy sources.
In particular, since the polymer electrolyte fuel cell (PEFC) has higher power generation efficiency and higher current density than other fuel cells, it can be reduced in size and weight at a low temperature and expected to be put to practical use.
A graphite-supported platinum-based noble metal catalyst is used as an electrode catalyst for a polymer electrolyte fuel cell.

However, platinum-based noble metal catalysts are extremely expensive, which is an obstacle to the spread of fuel cells.
In addition, when a reformed gas such as hydrocarbon or methanol is used as the fuel, the platinum-based noble metal catalyst reacts with CO (carbon monoxide) contained in the reformed gas and deactivates. is there.

  Therefore, in recent years, as various attempts to develop and research on platinum alternative catalysts, graphite having iron / cobalt or group 4/5 oxynitride or only graphite is nitrided to have a nitrogen-containing graphene structure. Research and development of electrode catalysts have been carried out (for example, see Non-Patent Documents 1 to 4).

  However, the graphite used in the method for producing an electrocatalyst having a nitrogen-containing graphene structure by nitriding graphite has a problem that it needs to be purified, and that graphite is very expensive.

  By the way, coal is a substance having a graphene structure, mainly composed of carbon, hydrogen, and oxygen and containing a small amount of various elements. Oxygen is included as phenol ether or the like, and hydrogen is bonded to the terminal bond or the terminal of the graphene structure. Coal contains nitrogen as pyridine and aniline, sulfur as sulfide and thiophene, a small amount of Fe, Ca, Mg, and Na in addition to the above three elements, and also contains a small amount of many elements. That is, although coal has a highly active graphene structure, it contains many elements, so there are many unclear points whether it can be used as an alternative to a fuel cell electrode catalyst. For this reason, it becomes possible to produce a high-performance electrode catalyst at a lower cost by using coal instead of purified graphite as a raw material of the electrode catalyst obtained by the above nitriding treatment. .

  In addition, this patent applicant discloses the following technical literature as a publication in which the literature well-known invention relevant to this invention was described.

Branko N. Popov et al., DOE Hydrogen Program, University of South Crolina, Center for Electrochemical engineering, (2007), p.1-21 J.-H. Kim et al., Electrochimica Acta, 52, (2007), p.2492-2497 Rajesh Bashyam & Piotr Zelenay, NATURE, Vol.443, (2006), p.63-66 Frederic Jaouen, Jean-PolDodelet, Electrochimica Acta, 52, (2007), p.5975-5984

  In view of the situation as described above, an object of the present invention is to provide a method for producing an electrode catalyst for a fuel cell, which can produce an electrode catalyst at low cost by using a large amount of coal existing in nature. Another object of the present invention is to provide an electrode catalyst excellent in various physical properties such as ORR characteristics and BET surface area required for an electrode catalyst for a fuel cell.

As a result of diligent research to solve the above problems, the present inventors have found that an electrode catalyst for a fuel cell produced by nitriding coal containing predetermined carbon with ammonia gas or the like has ORR characteristics, surface area, etc. The present inventors have found that it has excellent physical properties and have completed the present invention. The present invention is composed of the following technical matters. That is,
(1) A method for producing an electrode catalyst for a fuel cell,
A method for producing an electrode catalyst for a fuel cell, wherein an electrode catalyst is obtained by nitriding coal with ammonia gas.
(2) The method for producing an electrode catalyst for a fuel cell according to (1), wherein the coal is nanoparticulated by nitriding.
(3) The method for producing an electrode catalyst for a fuel cell according to (2), wherein coal having a carbon content (mass%) of 60% to 90% is used as the coal.
(4) A fuel cell electrode catalyst made of coal into which nitrogen has been introduced by nitriding with ammonia gas.
(5) The present invention relates to the fuel cell electrode catalyst according to (4), wherein the coal is nanoparticulated.
(6) The electrode catalyst for fuel cells according to (5), wherein the nanoparticle has a particle radius of 1 nm to 300 nm.

  According to the above-described method for producing a fuel cell electrode catalyst of the present invention, nitrogen atoms can be efficiently introduced into coal by ammonia gas, so that the ORR activity and BET surface area required for the fuel cell electrode catalyst are improved. Thus, it is possible to provide a catalyst excellent in various physical properties. The electrode catalyst obtained by nitriding in the method for producing a fuel cell electrode catalyst of the present invention contains a large amount of nitrogen atoms, and further contains elements such as Fe, Co, and Ca originally contained in coal. Therefore, an electrode catalyst having high activity can be produced.

  In addition, according to the fuel cell electrode catalyst of the present invention described above, since nitrogen is introduced by nitriding treatment with ammonia gas, the electrode catalyst obtained by nitriding treatment contains a lot of nitrogen atoms, Furthermore, it has high activity by containing elements such as Fe, Co, and Ca originally contained in coal.

  Furthermore, according to the method for producing a fuel cell electrode catalyst and the fuel cell electrode catalyst of the present invention, since coal is used as a raw material, an inexpensive fuel cell electrode catalyst can be realized.

Hereinafter, embodiments of the present invention will be described in detail.
The method for producing an electrode catalyst for a fuel cell according to the present invention is characterized in that an electrode catalyst is obtained by nitriding coal with ammonia gas. In the method for producing a fuel cell electrode catalyst of the present invention, the nitriding treatment refers to introducing nitrogen atoms into coal by raising the temperature in a gas atmosphere of a substance containing nitrogen atoms such as ammonia. The substance containing nitrogen atoms used in the nitriding treatment is not particularly limited as long as it is a substance containing nitrogen atoms. For example, in addition to ammonia, various substances such as pyridine and acetonitrile are exemplified. be able to. However, since acetonitrile generates cyan gas, it needs to be detoxified. Moreover, since nitrogen gas is inactive, it is not suitable for nitriding treatment.

  The treatment temperature in the nitriding treatment is preferably in the range of 900 to 1200K, more preferably in the range of 900 to 1100K. When the nitriding temperature is less than 900K, the nitriding of coal does not proceed sufficiently, and when the nitriding temperature exceeds 1200K, the catalytic activity of carbon becomes inactive, which is not preferable.

  Although various coal materials can be used as the raw material coal, more preferably, coal having a carbon content (mass%) of 60% to 90% is used. This range includes so-called bituminous coal, subbituminous coal, and lignite. The lignite includes those having a carbon content (mass%) of less than 60%. If a carbon content (mass%) of less than 60% is used, the nitriding treatment becomes inefficient because, for example, it takes a long time for the nitriding treatment due to the large amount of moisture. On the other hand, when the carbon content (% by mass) exceeds 90%, the content of elements other than carbon decreases, and the catalytic activity of the electrode catalyst may not be sufficiently obtained, which is not preferable.

As described above, coal as a raw material contains oxygen, nitrogen, sulfur, and a small amount of Fe, Ca, Mg, Na, and many other trace elements in addition to carbon and hydrogen. Examples of other trace elements include Co, Si, Al, As, Mn, and Cr.
By containing a transition metal element such as Fe or Co or an element such as Ca, the activity of the electrode catalyst can be improved.
It is possible to concentrate metal elements such as Fe by increasing the temperature during nitriding.

  Next, the fuel cell electrode catalyst produced by the above production method will be described. The fuel cell electrode catalyst produced by the method for producing a fuel cell electrode catalyst of the present invention, that is, the fuel cell electrode catalyst of the present invention is hereinafter referred to as a fuel cell electrode catalyst according to the present invention. To do.

  The fuel cell electrode catalyst according to the present invention is particularly suitable as an electrode catalyst for a polymer electrolyte fuel cell (PEFC).

In the polymer electrolyte fuel cell (PEFC), one electrode (anode or fuel electrode) for introducing hydrogen and an opposite electrode (cathode or air electrode) for introducing oxygen are arranged via an electrolyte. It is configured by connecting a lead wire or the like to each pole.
Since the polymer electrolyte fuel cell (PEFC) is configured as described above, it operates as follows.

First, the electrode into which hydrogen is introduced (anode or fuel electrode) is ionized by hydrogen and moves to the opposite electrode (cathode or air electrode) through the electrolyte. Through the current.
On the other hand, oxygen introduced into the opposite electrode (cathode or air electrode) reacts with electrons entering through an external conductor with hydrogen ions entering through the electrolyte, and is discharged as water.

The fuel cell electrode catalyst according to the present invention is suitable for use as a catalyst on the anode or fuel electrode side of the polymer electrolyte fuel cell (PEFC). The catalyst on the anode or fuel electrode side catalyzes in a separation reaction in which hydrogen molecules are separated into hydrogen ions and electrons.
The fuel cell electrode catalyst according to the present invention can be used not only as an anode or fuel electrode side catalyst but also as a cathode or air electrode side catalyst.

In order to use the fuel cell electrode catalyst according to the present invention for a fuel cell electrode, for example, a liquid in which the electrode catalyst is dispersed may be applied to an electrode (for example, an anode or a fuel electrode) and dried.
Also, a so-called MEA (membrane / electrode assembly) may be formed using an electrode catalyst.

Hereinafter, although the present invention is explained using an example, the present invention is not limited to these at all.
Actually, a carbon catalyst was produced by nitriding coal, and the characteristics of the obtained electrode catalyst (carbon catalyst) were examined.
Sub-bituminous coal, bituminous coal, and anthracite were used as coal types.

<Experiment 1> Difference in catalytic activity depending on coal treatment method Using subbituminous coal A, heat treatment, carbonization treatment, nitriding treatment with various gases, specifically nitrogen, methane / hydrogen, ammonia, carbon A catalyst was prepared.

Among these, in the case of nitriding treatment, ammonia gas was used to produce a carbon catalyst according to the flowchart shown in FIG.
As shown in FIG. 1, after a coal sample was ground with a ball mill, a carbon catalyst was produced by performing hydrochloric acid treatment or nitriding with grinding. Details of each step were as follows.

(Grinding)
About 20 ml of 2-propanol was added to about 3.0 g of the coal sample, and it was poured into an agate container containing an agate ball mill.
Then, this agate container was mounted on a planetary ball mill P-6 type (Fritche), and wet grinding was performed at 300 rpm for 30 minutes. Thus, by performing the wet grinding, the pulverized coal does not agglomerate and becomes a fine particle state.

(Hydrochloric acid treatment)
About 0.5 g of the ground coal sample was placed in a 15 ml sample tube, and a 5N HCl solution was added to make about 10 ml. This was stirred for 24 hours with ultrasonic waves and a tube vibrator.
Thereafter, washing with ion-exchanged water was performed until pH = 7, and drying was performed.

(Nitriding treatment)
About 0.15 g of coal sample treated with hydrochloric acid or as ground is charged into a fixed bed atmospheric pressure flow reactor.
And according to the temperature program shown in FIG. 2, the nitriding process was performed and the carbon catalyst was produced.
That is, while flowing ammonia (NH ₃ ) gas at a flow rate of 60 ml / min, as shown in FIG. Was held for 3 hours to perform nitriding treatment. After that, it was cooled to room temperature.

(Heat treatment only)
Using a nitrogen (N ₂ ) gas in place of the ammonia gas used in the nitriding treatment, the coal sample treated with hydrochloric acid is subjected to a heat treatment in accordance with the temperature program shown in FIG. Was made.

(Carbonization treatment)
A coal sample treated with hydrochloric acid using methane (CH ₄ ) gas and hydrogen (H ₂ ) gas in place of ammonia gas used in nitriding treatment, at the same gas flow rate as in nitriding treatment, according to the temperature program shown in FIG. A carbon catalyst was produced by subjecting to carbonization treatment.

<Experiment 2> Difference in catalytic activity by coal type Using coals of three coal types (sub-bituminous coal A, bituminous coal B, and anthracite coal C), after treating each coal type with hydrochloric acid, A carbon catalyst was prepared.
Hydrochloric acid treatment and nitriding treatment were performed in the same procedure as in Experiment 1.

<Measurement of characteristics>
Various characteristics of the produced carbon catalyst sample were measured as follows.

(Immobilization of catalyst to working electrode)
In order to perform the three-electrode measurement, a carbon catalyst was fixed to the working electrode of the three-electrode apparatus.
A schematic diagram of the working electrode used is shown in FIG.
The working electrode 20 is a rotating electrode HR-E2 manufactured by Hokuto Denko Corporation, and is a ring disk electrode having a cylindrical shape. The lower part of the working electrode 20 is constituted only by a carbon electrode (glassy carbon) 21. In the upper part of the working electrode 20, the carbon electrode 21 has a columnar shape that is thinner than the lower part, and is formed outside the carbon electrode 21 in a cylindrical shape in the order of an insulator 22, a platinum layer 23, and a holding material (plastic) 24. ing.
The disk portion 21A on the surface of the carbon electrode 21 becomes the sample surface.

First, a catalyst ink was prepared using a sample of the carbon catalyst obtained by each treatment.
A 35 vol% ethanol aqueous solution was used as the dispersion, and the carbon catalyst was dispersed in the dispersion to prepare a catalyst ink by adjusting the concentration to 1.4-cat / ml ethanol.
Next, 20 μl (catalyst amount: 28 μg) of catalyst ink was applied onto the disk portion 21A of the carbon electrode 21 using the working electrode 20 shown in FIG. At this time, care was taken not to apply the catalyst to the platinum layer 23 outside the insulator 22.
Further, 1.8 μl of 0.05 wt% Nafion (registered trademark; DuPont) solution was applied on the disk portion 21A of the carbon electrode 21.

(Three electrode measurement)
The ring disk electrode (electrode surface area: 0.07065 cm ² ) prepared by applying the catalyst ink as described above is used as a working electrode, and the counter electrode and the reference electrode are used in a three-electrode device (RRDE-1, manufactured by Nisatsu Kogyo Co., Ltd.). H ₂ SO ₄ with a concentration of 0.5 M was used as the electrolytic solution. A single junction Ag / AgCl was used for the reference electrode, a carbon electrode for the counter electrode, and HZ-5000 manufactured by Hokuto Denko Co., Ltd. for the measuring instrument.

(XPS measurement)
XPS measurement was performed using ESCA3200 (manufactured by Shimadzu Corporation). The sample was fixed to the sample stage with double-sided tape, placed in the sample introduction chamber, degassed for about 3 hours, and then placed in the measurement chamber to start measurement. The measurement was performed using an MgKα ray as an X-ray source and an X-ray output of 240 W.
In the analysis, the baseline was corrected by the Shirley method, and the atomic ratio of each element was obtained.

(BET measurement)
BET measurement (BET method specific surface area measurement) was performed by degassing at 200 ° C. for 2 hours as a pretreatment, and was measured by N ₂ adsorption.
OMISORP 100CX (manufactured by COULTER) was used as the measuring device.

<Results and discussion>
(Influence on ORR activity by treatment method)
The influence on the ORR activity by each treatment method of subbituminous coal A by ammonia gas, nitrogen gas, and methane / hydrogen gas in Experiment 1 was compared. The steady polarization curves of the samples subjected to the respective treatments are compared and shown in FIG. 4 (1) is CH ₄ / H ₂ (carbonization treatment), (2) is N ₂ (heat treatment only), (3) is NH ₃ (nitridation treatment), and (4) is a control. The product is a commercial product (E-TEK Co.) of 20% by mass Pt / C catalyst.

As can be seen from FIG. 4, the ORR activity improved in the order of heat treatment with nitrogen, carbonization with methane / hydrogen, and ammonia gas.
The values of the ORR start potential of each treatment are (1) CH ₄ / H ₂ (carbonization treatment) 0.20 V, (2) N ₂ (heat treatment only) 0.34 V, (3) NH ₃ (nitridation treatment) Was 0.62 V, and (4) a commercially available Pt / C catalyst (E-TEK) was 0.67 V.

Although only a low activity was obtained by heat treatment alone, this is considered to be because nitrogen gas was poor in reactivity and nitrogen atoms were not easily introduced into coal.
On the other hand, when nitriding with ammonia gas is performed, nitrogen atoms are sufficiently introduced into the coal due to the reactivity of ammonia, which seems to have contributed to a significant improvement in ORR activity.

  Here, Table 1 shows the atomic ratio of each element to carbon before and after nitriding treatment of subbituminous coal A obtained by XPS measurement. In Table 1, the case after nitriding is “nitriding coal A”.

From Table 1, it can be seen that the amount of nitrogen was significantly increased by the nitriding treatment.
In addition, the iron content was significantly increased by the nitriding treatment. This is probably because the metal was concentrated by the temperature rise and the content was increased.
Note that the weight change of the sample before and after the nitriding treatment was about 70%.

  In each of the pretreatment methods in Experiment 1, only the heat treatment and the carbonization treatment were observed to increase the content of the metal element.

Furthermore, from the results of the BET measurement, it was found that the surface area increased to 50 times the surface area before nitriding by nitriding treatment. That is, it is considered that the surface area of the coal is increased due to the formation of nanoparticles.
From this, it is considered that introduction of nitrogen into coal and nanoparticulation by ammonia treatment contributed to the improvement of the cathode catalyst activity.

(Comparison of ORR activity by charcoal type)
FIG. 5 shows comparison of ORR activities of carbon catalysts obtained by nitriding the three types of charcoal of Experiment 2 (subbituminous coal A, bituminous coal B, and anthracite coal C), and comparing the steady polarization curves of the respective samples. (1) in FIG. 5 is anthracite C, (2) is bituminous coal B, (3) is sub-bituminous coal A, and (4) is a commercial product of 20 mass% Pt / C catalyst of the control product ( E-TEK).

As can be seen from FIG. 5, the subbituminous coal A showed a significant improvement in activity by nitriding, but the oxygen reduction activity of the anthracite coal C was hardly observed. Regarding bituminous coal B, the ORR onset potential was close to that of subbituminous coal A, but the current density was lower than that of subbituminous coal A. This is considered to be caused by differences due to the respective coal types and inorganic inclusions in addition to the treatment method.
The values of the ORR start potential of each coal type are as follows: (1) Anthracite coal C is 0.02V, (2) Bituminous coal B is 0.58V, (3) Subbituminous coal A is 0.62V, (4) Pt / C catalyst The commercially available product (E-TEK) was 0.67V.

  Anthracite C has a composition that is significantly different from other coal types, and has a high carbon content, so the content of elements other than carbon is reduced, and the active species are less than other coals, so the activity is low. It is considered inferior.

(An endurance test)
Furthermore, about the electrode catalyst with high ORR activity, MEA (membrane electrode assembly) was comprised using the electrode catalyst, and the endurance test was done using the IV measuring apparatus (made by Okura Riken).
As a result, sufficient durability was obtained.

  The present invention is not limited to the above-described embodiments and examples, and various other configurations can be taken without departing from the gist of the present invention.

For example, the temperature profile in the nitriding process is not limited to the profile shown in FIG.
Various profiles are possible, such as a profile for raising the temperature by changing the temperature rise rate to two or more levels, and a profile for raising the temperature while gradually raising the temperature rise rate.
Further, the holding time after the temperature rise is not limited to 3 hours. An appropriate holding time may be set according to the coal type and composition of the coal to be used.

In an Example, it is a flowchart which shows the process which adjusted the carbon catalyst from the coal sample. It is the temperature profile used by the nitriding process of the Example. It is a figure which shows the structure of the working electrode of the three-electrode apparatus used in the Example. It is the figure which changed the pre-processing method of a coal sample and compared the stationary polarization curve. It is the figure which changed the charcoal type of a coal sample, and compared the stationary polarization curve.

Explanation of symbols

  20 working electrode, 21 carbon electrode (glassy carbon), 21A disc part

Claims (6)

A method for producing an electrode catalyst for a fuel cell, comprising:
A method for producing an electrode catalyst for a fuel cell, wherein an electrode catalyst is obtained by nitriding coal with ammonia gas.   The method for producing a fuel cell electrode catalyst according to claim 1, wherein the coal is nanoparticulated by nitriding.   The method for producing an electrode catalyst for a fuel cell according to claim 2, wherein coal having a carbon content (% by mass) of 60% to 90% is used as the coal. A fuel cell electrode catalyst made of coal into which nitrogen has been introduced by nitriding with ammonia gas.   The electrode catalyst for fuel cells according to claim 4, wherein the coal is nanoparticulated.   The electrode catalyst for a fuel cell according to claim 5, wherein the nanoparticle has a particle radius of 1 nm to 300 nm.

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