JP6098871B2 - Catalyst and cathode for fuel cell - Google Patents

Description

  The present invention relates to a composite suitable for a fuel cell catalyst. More specifically, the present invention relates to a composite suitable for a catalyst for a polymer electrolyte fuel cell cathode having oxygen-deficient zirconium oxide and a predetermined coating, and a polymer electrolyte fuel cell cathode using the complex as a catalyst.

  A solid polymer fuel cell has a structure in which a proton conductive polymer electrolyte membrane is sandwiched between a negative electrode and a positive electrode, a fuel containing hydrogen, for example, an aqueous methanol solution, is supplied to the negative electrode, and air is supplied to the positive electrode. At the negative electrode, hydrogen contained in the fuel is oxidized, and at the positive electrode, oxygen is reduced, and electric energy is extracted outside. As the positive electrode, an electrode in which platinum is supported on carbon black is used. However, development of a positive electrode using an alternative material to expensive platinum is also underway.

  Patent Document 1 discloses an electrode catalyst for oxygen reduction using zirconium oxide or the like having oxygen deficiency as a main catalyst and gold as a promoter. Patent Document 2 discloses an electrode catalyst composed of ZrCN and ZrO2 and an oxygen reduction electrode for a positive electrode carrying the catalyst.

WO2006 / 019128 WO2009 / 060777

Y. Liu, A. Ishihara, S. Mitsushima, N. Kamiya, K. Ota; J. Electrochem. Soc. 154, B664-669 (2007). B. A. van Hassel and A. J. Burggraaf; Appl. Phys. A, 52, 410-417 (1991)

  However, a catalyst using zirconium oxide having oxygen vacancies and a catalyst made of ZrCN and ZrO2 are still insufficient in ability as a catalyst used for a positive electrode oxygen reduction electrode of a solid polymer fuel cell.

  Accordingly, an object of the present invention is to provide a composite suitable for an electrode catalyst having high catalytic performance suitable for an oxygen reduction electrode for a positive electrode of a polymer electrolyte fuel cell, and an oxygen reduction electrode for a positive electrode using the composite as a catalyst. Is to provide.

The present invention is as follows.
[1] Zirconium oxide having oxygen deficiency and having a sulfur or fluorocarbon coating.
[2] The composite according to [1], wherein a water contact angle on the surface of the composite is in the range of 70 to 120 °.
[3] The complex according to [1] or [2], wherein the zirconium oxide having an oxygen deficiency is represented by ZrO _2-x (x is in the range of 0.001 to 0.1).
[4] The composite according to any one of [1] to [3], wherein a covering amount of sulfur is in a range of 0.01 to 0.5 in terms of a molar ratio of sulfur to zirconium of zirconium oxide having oxygen deficiency.
[5] The composite according to any one of [1] to [3], wherein a coating amount of the fluorocarbon is in a range of 0.01 to 0.5 in terms of a molar ratio of fluorocarbon fluorine to zirconium of zirconium oxide having oxygen deficiency.
[6] The composite according to any one of [1] to [5], which is used as a catalyst for a polymer electrolyte fuel cell cathode.
[7] A cathode for a polymer electrolyte fuel cell, in which the composite according to any one of [1] to [5] is supported on an electrode carrier as a catalyst.

  ADVANTAGE OF THE INVENTION According to this invention, the composite_body | complex suitable for the catalyst for electrodes and the oxygen reduction electrode excellent in the oxygen reduction ability and stability in acidic electrolyte can be provided.

A synthesis apparatus used for synthesis of the active material is shown in FIG. Fig. 2 shows the emission spectrum when sulfur or PTFE is used as the doping material. FIG. 3 shows SEM images of the ZrO ₂ raw material, S—ZrO ₂ and CF—ZrO ₂ . The XPS spectrum is shown. FIG. 4 (a) shows the Zr3d narrow spectrum of the outermost surface of S-ZrO ₂ irradiated with microwaves for 70 seconds and the interior sputtered by 20 nm. FIG. 4 (b) shows the Zr3d narrow spectrum of the innermost surface sputtered by 20 nm and the outermost surface of CF-ZrO ₂ irradiated with plasma for 70 seconds. FIG. 4 (c) shows the Zr3d narrow spectrum of the outermost surface of ZrO ₂ baked at 1,200 ° C. in an N ₂ atmosphere and the interior sputtered by 20 nm. FIG. 5 shows the F1s narrow spectrum of the innermost surface sputtered by 20 nm and the outermost surface of CF-ZrO ₂ irradiated with plasma for 70 seconds. FIG. 6 (a) shows the relationship between the S plasma irradiation time and the change in the weight of ZrO ₂ . FIG. 6 (b) shows the relationship between the irradiation time of N and CN plasma and the change in the weight of ZrO ₂ . FIG. 7 (a-1) shows the relationship between the sulfur addition amount and the sulfur doping amount. FIG. 7 (a-2) shows the relationship between the amount of sulfur added and the water repellency of the S-ZrO ₂ electrode. FIG. 7 (b-1) shows the relationship between the amount of PTFE added and the amount of fluorine in the doped fluorocarbon. FIG. 7 (b-2) shows the relationship between the amount of PTFE added and the water repellency of the CF-ZrO ₂ electrode. Figure 8 shows cyclic voltammograms (CV) of 3, 6 and 9 cycles of ZrO ₂ , S-ZrO ₂ and CF-ZrO ₂ electrodes. Figure 9 shows the voltammograms of ZrO ₂ , S-ZrO ₂ and CF-ZrO ₂ electrodes. FIG. 10 shows the relationship between the doping amount of sulfur or PTFE and the current density i _O2 -i _N2 at 0.6 V vs. RHE.

<Complex>
The present invention relates to a composite having an oxygen deficiency and having a sulfur or fluorocarbon coating. Zirconium oxide having oxygen deficiency can be represented by ZrO _2-x (x is in the range of 0.001 to 0.1). The oxygen deficiency x is preferably in the range of 0.01 to 0.08 from the viewpoint of excellent oxygen reducing ability and stability in an acidic electrolyte when the composite of the present invention is used as a catalyst for a polymer electrolyte fuel cell cathode. More preferably, it is the range of 0.03-0.06. The amount of oxygen deficiency in zirconium oxide can be determined, for example, from the shift amount of Zr3d spectrum peaks (ZrO ₂ 3d5 / 2: 182.7 eV and ZrO ₂ 3d3 / 2: 184.7 eV) in the XPS spectrum of zirconium oxide to the lower energy side. Can do. Regarding this point, Non-Patent Documents 1 and 2 can be referred to.

  Zirconium oxide having oxygen vacancies in the composite of the present invention exhibits not only the outermost surface but also oxygen vacancies in the same amount as the outermost surface not only in the outermost surface but also in a 20 nm bulk. As described above, it is presumed that a catalyst excellent in oxygen reducing ability and stability in an acidic electrolyte can be provided because it uses zirconium oxide that exhibits high oxygen vacancies even in a bulk of 20 nm which is small from the outermost surface. .

  The composite has a sulfur or fluorocarbon coating on at least a part of the surface of zirconium oxide having oxygen deficiency. Both the sulfur coating and the fluorocarbon coating are physically supported on the surface of zirconium oxide having oxygen vacancies, and the sulfur atom and the zirconium atom of zirconium oxide, or the fluorine atom or carbon atom of the fluorocarbon, It does not have a bond with the zirconium atom of zirconium oxide. This point can also be confirmed by the XPS spectrum.

  In the case of a composite having a sulfur coating, the sulfur coating amount can range from 0.01 to 0.5 in terms of the molar ratio of zirconium to zirconium in the zirconium oxide having oxygen deficiency. When the composite of the present invention is used as a catalyst, the sulfur coating amount affects the oxygen reducing ability and stability in an acidic electrolyte. The oxygen reducing ability and stability in the acidic electrolyte can be appropriately selected in consideration of fluctuations depending on the amount of oxygen deficiency and sulfur coverage of zirconium oxide, and the state of sulfur coating. When the amount of oxygen deficiency x of zirconium oxide is 0.04, the molar ratio of zirconium oxide having oxygen deficiency to sulfur is preferably in the range of 0.015 to 0.035, and more preferably in the range of 0.02 to 0.03.

  In the case of a composite having a fluorocarbon coating, the coating amount of the fluorocarbon can be in the range of 0.01 to 0.5 in terms of the molar ratio of fluorocarbon fluorine to zirconium in zirconium oxide having oxygen deficiency. When the composite of the present invention is used as a catalyst, the coating amount of the fluorocarbon affects the oxygen reducing ability and stability in the acidic electrolyte. The oxygen reducing ability and stability in the acidic electrolyte can be appropriately selected in consideration of variations depending on the oxygen deficiency amount of zirconium oxide and the coating amount of the fluorocarbon, and the coating state of the fluorocarbon. When the oxygen deficiency x of zirconium oxide is 0.04, the molar ratio of fluorine of fluorocarbon to zirconium of zirconium oxide having oxygen deficiency is preferably in the range of 0.015 to 0.15, more preferably in the range of 0.02 to 0.12. .

  The covering state of sulfur or fluorocarbon on zirconium oxide having oxygen deficiency is not particularly limited. However, since the composite of the present invention forms a coating on the zirconium oxide powder in a stationary state using plasma as described later, the surface of the zirconium oxide powder is not uniformly coated and has a coating. In some cases, a portion and a portion having no coating coexist. However, the coating can be formed more uniformly by intermittently fluidizing the zirconium oxide powder in the plasma treatment.

  The composite of the present invention has a surface water contact angle in the range of 70 to 120 °, for example. The water contact angle on the surface varies depending on the oxygen deficiency amount of zirconium oxide, the coating amount of sulfur or fluorocarbon, and the covering state of sulfur or fluorocarbon. The water contact angle on the surface of the composite of the present invention is preferably in the range of 80 to 120 °, more preferably in the range of 90 to 120 °. The higher the water contact angle, the more excellent the oxygen reducing ability and stability in the acidic electrolyte.

  The composite of the present invention is used as a catalyst for a polymer electrolyte fuel cell cathode. When the composite of the present invention is used as an electrode catalyst for an oxygen reduction electrode, the oxygen reduction ability and stability in an acidic electrolyte are excellent.

  Although the manufacturing method of the composite_body | complex of this invention is explained in full detail in an Example, it can manufacture using a commercially available zirconium oxide, commercially available sulfur, and a commercially available fluorocarbon using a microwave resonance apparatus. Zirconium oxide, sulfur and fluorocarbon can all be powders. In particular, considering that the zirconium oxide is used as a catalyst for a polymer electrolyte fuel cell cathode, the average particle size can be, for example, in the range of 0.01 to 100 μm, preferably in the range of 0.1 to 10 μm. However, it is not intended to limit to this range.

  Production of a composite using a microwave resonator device is performed, for example, by placing zirconium oxide and sulfur or fluorocarbon between each layer of a three-layer carbon felt, depressurizing the inside of the device, and then microwave resonance. The apparatus is started, plasma is generated in the apparatus, and processing is performed for a predetermined time. The operating conditions of the microwave resonance device are not limited, but for example, the microwave can be 2.45 GHz and the output is 100 to 1000 W. The treatment time can be appropriately determined according to the operating conditions and the amount and type of the sample to be treated. For example, it can be in the range of 10 seconds to 10 minutes. However, it is not intended to be limited to this range. There is no particular limitation on the degree of pressure reduction in the apparatus during plasma generation, and conditions suitable for plasma generation can be selected as appropriate.

<Cathode for polymer electrolyte fuel cell>
The present invention includes a cathode for a polymer electrolyte fuel cell in which the composite of the present invention is supported on an electrode carrier as a catalyst.

  The electrode (cathode) can be manufactured, for example, as follows. First, the powder of the composite of the present invention is mixed with a known conductive agent and binder to form a paste, and this paste is applied to the surface of the electrode carrier and dried to produce an electrode. A solvent can also be used to make a paste. The conductive agent and the binder can be appropriately selected from those known for electrode production. As the conductive agent, carbon black such as ketjen black can be used. As the binder, polyvinylidene fluoride or the like can be used. The mixing ratio of the composite, the conductive agent and the binder is not particularly limited, and can be appropriately determined in consideration of the types of the composite, the conductive agent and the binder and the characteristics required for the electrode. For example, the mass ratio of the composite, conductive agent and binder is, for example, in the range of 0.1 to 10 in the conductive agent and in the range of 0.1 to 10 in the binder when the composite is 1. Can do. As the carrier, for example, a sheet of carbon or conductive oxide can be used.

  The electrode of the present invention is useful as a cathode for a polymer electrolyte fuel cell. Furthermore, the electrode of the present invention can be used as an electrode for a cathode of an electrochemical system using an acidic electrolyte such as water, an inorganic substance, an organic substance, or a fuel cell. The electrode of the present invention can also be used as an oxidant electrode when using an acidic electrolyte other than a polymer electrolyte fuel cell such as a phosphoric acid fuel cell.

  Hereinafter, the present invention will be described in detail based on examples. However, the present invention is not intended to be limited to the examples.

Experimental Method 1) Synthesis of Active Material The synthesis apparatus is shown in FIG. Between three pieces of disc-shaped carbon felt (diameter 3cm, thickness 0.5cm), 0.5g of zirconium oxide is placed on the upper part, and a predetermined amount of dope substance (sulfur or polytetrafluoroethylene (PTFE)) is placed on the lower part. It was placed in a hard glass container and placed in a multimode type microwave cavity resonator. After reducing the pressure to 0.001 MPa, 500 W, 2.45 GHz microwave was irradiated for a predetermined time. Zirconium oxide (S-ZrO ₂ or CF-ZrO ₂ ) doped with sulfur or PTFE was synthesized by the generated plasma.

The emission spectrum of the generated plasma was measured using a multichannel analyzer (PMA-11, Hamamatsu Photonics). The structure and bonding state of the synthesized S-ZrO _{2 and the} like were analyzed by X-ray photoelectron spectroscopy (PHI, Quantum 2000 type). The amount of doped sulfur was measured with an ICP emission spectrometer (manufactured by Thermo Fisher, iCAP6300). The water repellency was measured with an automatic contact angle meter (Kyowa Interface Science Co., Ltd., CA-VP).

2) Electrochemical measurement Ketjen black as a conductive agent and polyvinylidene fluoride as a binder in a synthesized S-ZrO ₂ etc. are mixed at a weight ratio of 10: 3: 1 and applied to carbon paper to produce an electrode. did. A three-electrode cell was used for electrochemical measurements. The prepared electrode was used for the working electrode, platinum was used for the counter electrode, a reversible hydrogen electrode was used for the reference electrode, and a 0.1 M sulfuric acid aqueous solution was used for the electrolyte. The stability of the electrode was evaluated by cyclic voltammetry at a temperature of 25 ° C. in a nitrogen atmosphere. The oxygen reduction catalytic ability was evaluated by linear sweep voltammetry in a nitrogen atmosphere at 25 ° C. and in an oxygen atmosphere.

Results and Discussion 1) Analysis of generated plasma Fig. 2 shows the emission spectrum when sulfur or PTFE is used as the doping material. When sulfur was added, an emission spectrum caused by sulfur was detected. When PTFE was added, an emission spectrum due to N or CN was detected. Plasma causing oxygen deficiency is S plasma in S-ZrO _2, the CF-ZrO ₂ was found to be N and CN plasma.

2) Structural analysis of the catalyst
FIG. 3 shows SEM images of the ZrO ₂ raw material, S—ZrO ₂ and CF—ZrO ₂ . In all cases, there was no significant change in morphology such as particle size.

In order to analyze the structure of ZrO ₂ treated with these plasmas, XPS spectra were measured.

FIG. 4 (a) shows the Zr3d narrow spectrum of the outermost surface of S-ZrO ₂ irradiated with microwaves for 70 seconds and the interior sputtered by 20 nm. These peaks of ZrO ₂ Zr3d spectrum _{(ZrO 2 3d5 / 2: 182.7eV} , ZrO 2 3d3 / 2: 184.7eV) both compared to shifted to a lower energy side.

FIG. 4 (b) shows the Zr3d narrow spectrum of the innermost surface sputtered by 20 nm and the outermost surface of CF-ZrO ₂ irradiated with plasma for 70 seconds. These peaks are also shifted to the lower energy side compared to the Zr3d spectrum.

FIG. 4 (c) shows the Zr3d narrow spectrum of the outermost surface of ZrO ₂ baked at 1,200 ° C. in an N ₂ atmosphere and the interior sputtered by 20 nm. The surface was shifted to the lower energy side compared to the Zr3d spectrum, but the bulk was not shifted.

The shift on the low energy side of the two peaks of the Zr3d spectrum is considered to be caused by oxygen deficiency (Non-patent Documents 1 and 2). From these results, it was found that the plasma-treated ZrO ₂ of S, N, or CN has oxygen deficient sites on the bulk side as compared with those fired in a reducing atmosphere.

FIG. 5 shows the F1s narrow spectrum of the innermost surface sputtered by 20 nm and the outermost surface of CF-ZrO ₂ irradiated with plasma for 70 seconds. The binding energy of this peak coincided with that of CF bond. This result suggests that a Zr—F bond was not generated and a fluorocarbon was formed. The molar ratio of F to Zr was about 0.1 for both surface and bulk.

In the S2p spectrum (not shown) of S-ZrO ₂ , the peak disappeared. This is thought to be due to the elimination of sulfur by X-ray irradiation, suggesting that Zr-S bonds are not generated.

FIG. 6 (a) shows the relationship between the S plasma irradiation time and the change in the weight of ZrO ₂ . The weight increased until the S plasma disappeared. This is thought to be due to sulfur doping.

FIG. 6 (b) shows the relationship between the irradiation time of N and CN plasma and the change in the weight of ZrO ₂ . It is speculated that the continuous minute weight loss is due to the loss of particles, but the intermittent weight loss from 20 seconds to 60 seconds of microwave irradiation is thought to be due to oxygen deficiency. Moreover, this result suggests that oxygen deficiency does not advance under microwave irradiation for 70 seconds or longer under these conditions. This oxygen deficiency is about 0.5%, and the composition of the product is assumed to be ZrO _1.96 . Combined with the results of the F1sXPS spectrum, the composition of the synthesized CF-ZrO ₂ can be estimated as ZrO _1.96 C _0.05 F _0.1 .

Even in the S plasma treatment, the weight loss after the disappearance of sulfur was about 0.5%. From this, it is presumed that oxygen vacancies similar to those in N and CN plasma treatment were generated. Thus, doped sulfur weight in the microwave irradiation time of about 70 seconds is presumed to be about 0.5 percent of ZrO _2, the composition of the S-ZrO ₂ it can be estimated as ZrO _1.96 S _0.02.

  Next, the influence of S or PTFE dope on water repellency was evaluated by measuring the contact angle of the electrode. The microwave irradiation time was set to 70 seconds at which the amount of oxygen deficiency was almost constant. In order to remove the influence of the substrate, the electrode used was an Al foil coated with a coater with a thickness of 30 μm. The amount of sulfur doped was measured by ICP emission analysis. The amount of fluorocarbon doped was measured by XPS.

FIG. 7 (a-1) shows the relationship between the sulfur addition amount and the sulfur doping amount. FIG. 7 (a-2) shows the relationship between the amount of sulfur added and the water repellency of the S-ZrO ₂ electrode. Even when the amount added was increased, the amount of dope did not increase in proportion thereto, and the water repellency tended to be constant at a contact angle of about 100 °.

FIG. 7 (b-1) shows the relationship between the amount of PTFE added and the amount of fluorine in the doped fluorocarbon, and FIG. 7 (b-2) shows the relationship between the amount of PTFE added and the water repellency of the CF—ZrO ₂ electrode. The amount of doped fluorine increased in proportion to the amount of PTFE added, and the water repellency also increased.

  These differences are because sulfur stabilizes itself in the plasma state, and diffuses from between the carbon felt to the entire glass container, whereas PTFE is vaporized by N or CN plasma, etc., and is cooled and precipitated in the carbon felt. Therefore, it is considered that it does not diffuse throughout the glass container.

3) Electrochemical properties Fig. 8 shows cyclic voltammograms (CV) of 3, 6 and 9 cycles of ZrO ₂ , S-ZrO ₂ and CF-ZrO ₂ electrodes. Both electrodes were stable with no change in cycle. This result suggests that there is no change in electrode stability due to sulfur or PTFE doping.

Figure 9 shows the voltammograms of ZrO ₂ , S-ZrO ₂ and CF-ZrO ₂ electrodes. The current density on the vertical axis represents the value obtained by subtracting the current density _iN2 in the nitrogen atmosphere from the current density _iO2 in the oxygen atmosphere, that is, the oxygen reduction catalytic activity. S-ZrO ₂ , CF-ZrO ₂ and ZrO ₂ showed high activity in this order, and S-ZrO ₂ showed remarkably high activity. In CF-ZrO ₂ , oxygen deficiency is generated by N or CN plasma, and water repellency is imparted by precipitation of vaporized PTFE. For this reason, there is a high possibility that the oxygen deficient sites are not water repellent. In contrast, S-ZrO ₂ is presumed to be water repellent in the vicinity of the oxygen deficient site because both oxygen deficiency and imparting water repellency are generated by S plasma. For this reason, it is presumed that S-ZrO ₂ showed remarkably higher oxygen reduction catalytic activity than CF-ZrO ₂ .

Next, the effect of sulfur or PTFE concentration on the oxygen reduction catalyst activity was evaluated. FIG. 10 shows the relationship between the doping amount and the current density i _O2 -i _N2 at 0.6 V vs. RHE. The catalytic activity of S-ZrO ₂ was maximum when S / Zr = 0.025, that is, the composition of ZrO _1.96 S _0.025 . This is presumably because the water repellency is insufficient when the sulfur doping amount is small, and the oxygen deficient sites are covered when the sulfur doping amount is large. The catalytic activity of CF-ZrO ₂ was not correlated with F / Zr. This is probably because the oxygen deficient sites generated by N or CN plasma do not match the sites repelled by PTFE.

  The present invention is useful in the field related to fuel cells.

Claims (7)

A composite having an oxygen deficiency and having a sulfur or fluorocarbon coating. 2. The composite according to claim 1, wherein a water contact angle on the surface of the composite is in a range of 70 to 120 °. 3. The composite according to claim 1, wherein the zirconium oxide having an oxygen vacancy is represented by ZrO _2-x (x is in the range of 0.001 to 0.1). The composite according to any one of claims 1 to 3, wherein a covering amount of sulfur is in a range of 0.01 to 0.5 in terms of a molar ratio of sulfur to zirconium of zirconium oxide having oxygen deficiency. The composite according to any one of claims 1 to 3, wherein a coating amount of the fluorocarbon is in a range of 0.01 to 0.5 in terms of a molar ratio of fluorine of fluorocarbon to zirconium of zirconium oxide having oxygen deficiency. 6. The composite according to any one of claims 1 to 5, which is used as a catalyst for a polymer electrolyte fuel cell cathode. 6. A cathode for a polymer electrolyte fuel cell, wherein the composite according to any one of claims 1 to 5 is supported on an electrode carrier as a catalyst.