JP2021014610A - Composition for hydrogen sensor and its manufacturing method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a novel composition which can be used as a hydrogen sensor. A composition comprising carbon fibers and a palladium layer deposited on the carbon fibers, wherein the palladium layer contains a mixture of palladium oxide and metallic palladium is provided. A method for producing a composition for a hydrogen sensor includes a method of performing sputtering in the presence of oxygen to deposit palladium oxide and metallic palladium on carbon fibers to form a palladium layer, and by sputtering. Provided is a method comprising a step of oxidizing a part of metallic palladium and converting it into palladium oxide to form a palladium layer by heating carbon fibers on which metallic palladium is deposited in the presence of oxygen. [Selection diagram] Fig. 2

Description

The present disclosure relates to a composition useful as a detector of a hydrogen sensor device. More specifically, the present disclosure relates to a composition for a hydrogen sensor based on palladium, which is a hydrogen storage metal, and a method for producing the same.

Hydrogen is attracting attention as an alternative energy source for fossil fuels, and is used, for example, in fuel cells for automobiles. Hydrogen is a colorless, transparent, odorless, explosive gas. Therefore, in order to disseminate hydrogen energy to society, a gas sensor for detecting hydrogen gas leaks is considered to be indispensable.

Hydrogen gas leak detection methods include a contact combustion method that detects heat generated by the reaction between hydrogen and platinum by thermoelectric conversion, and a contact combustion method that measures the electrical conductivity of the surface of metal oxides such as SnO ₂ and ZnO to obtain hydrogen. A semiconductor type that detects a change in electrical conductivity due to a reaction is known (Non-Patent Document 1). These conventional methods require a mechanism for converting a temperature change of the detection element into a voltage and heating the detection element to a high temperature. Hydrogen sensors with different structures and methods have their own strengths and weaknesses, and the development of various hydrogen sensors with different characteristics is being sought.

Hydrogen Energy System Vol. 30, No. 2 (2005), p. 35-40

The present disclosure provides a novel composition that can be used as a hydrogen sensor.

The inventors have combined palladium (Pd), which is known as a hydrogen storage metal, with carbon fiber, which has high thermal conductivity, to provide a hydrogen gas sensor that can operate at room temperature and can detect hydrogen gas leaks based only on temperature changes. I studied the possibility of providing it. As a result, they have discovered that an excellent hydrogen gas sensor can be obtained under specific conditions, and have completed the present invention.

The disclosure includes the following embodiments:
[1]
It contains carbon fibers and a palladium layer deposited on the carbon fibers.
The palladium layer contains a mixture of palladium oxide and metallic palladium.
Composition.
[2]
The amount ratio of the palladium oxide to the metallic palladium, which is determined based on the peak amount of the spectrum detected by X-ray photoelectron spectroscopy, is in the range of 100: 1 to 1:10, according to [1]. Composition.
[3]
The composition according to [1] or [2], wherein the palladium oxide is present in a larger amount than the metallic palladium based on the peak amount of the spectrum detected by X-ray photoelectron spectroscopy.
[4]
The composition according to any one of [1] to [3], wherein the carbon fiber is in the form of carbon paper.
[5]
A hydrogen sensor device containing the composition according to any one of [1] to [4].
[6]
[1] to include a step of forming the palladium layer by depositing palladium oxide and metallic palladium on the carbon fiber by performing sputtering in the presence of oxygen with the palladium metal as the target and the carbon fiber as the base material. The method for producing the composition according to any one of [4].
[7]
A step of performing sputtering using palladium metal as a target and carbon fiber as a base material to deposit metallic palladium on the carbon fiber, and
The carbon fiber on which the metallic palladium is deposited is heated in the presence of oxygen to oxidize a part of the metallic palladium and convert it into palladium oxide to form the palladium layer [1]. The method for producing the composition according to any one of [4].

FIG. 1 shows SEM images of carbon paper (CFP, left panel), which is a carbon fiber material, and carbon paper (Pd / CFP, right panel) in which palladium is deposited on the surface of carbon fibers by sputtering. Figure 2 shows the temperature change upon reaction with Pd / CFP and H ₂ gas having a different oxidation states. In either case, H ₂ gas was circulated in the measurement chamber in the time zone of measurement after the start 180 to 360 seconds (indicated by the vertical dashed line). FIG. 3 shows XPS spectra of Pd / CFP with different oxidation states. The peak appearing at the top in (b) is a composite peak of palladium oxide and metallic palladium, which is combined with the peak of palladium oxide (PdO, PdO ₂ ) (left) and the peak of metallic palladium (Pd) (right). The one separated into is shown on the lower side.

In the present disclosure, "~" indicating a numerical range means that the numerical values described before and after the numerical range are included as the lower limit value and the upper limit value, respectively. When a plurality of possible numerical ranges such as "A to B" and "C to D" are described separately, a numerical range in which one lower limit or upper limit is combined with the other upper limit or lower limit (for example, "A to D"). It is understood that "CB") is also possible. For example, the description "contains" element E may include both aspects that include not only element E but also other elements and aspects that do not include (ie, consist of) other elements of element E. To be solved.

In one embodiment, a composition comprising carbon fibers and a palladium layer deposited on the carbon fibers is provided. This composition is useful, for example, as a detector of a hydrogen sensor device.

Carbon fibers are usually obtained by heat-carbonizing an organic fiber precursor, and 90% by mass or more is composed of carbon (C) (JIS L 0204-2: 2010). The thickness of the single fiber of the carbon fiber is usually 1 to 100 μm, preferably 3 to 30 μm. The carbon fibers in this embodiment may be provided in a twisted, woven, entangled, or otherwise bonded state to each other, or a composite material combined with other materials. May be provided as part of. Carbon fiber paper (CFP) is a material in which a large number of carbon fibers are entangled to form a sheet-like form as a whole. For example, a material typically used as a gas diffusion layer in the field of fuel cells. Is. In the composition of the present embodiment, it is particularly preferable that the carbon fibers are provided in the form of carbon paper.

Materials composed of carbon fibers, such as carbon paper, can provide a large surface area and high air permeability, and also have the characteristics of good electrical conductivity, good thermal conductivity, and low heat capacity (specific heat). It was found that it can exhibit excellent performance as a base material for hydrogen sensors.

The deposition of the palladium layer on the carbon fibers can be preferably achieved by a vapor deposition method, especially a sputtering method, as will be understood based on the ordinary knowledge of those skilled in the art, but other methods may be applied. “On carbon fibers” as used herein means that palladium is deposited on the surface of individual carbon fibers, as shown in FIG. In the "palladium layer", it is preferable that palladium forms a continuous layer to cover the carbon fiber surface. However, it is not necessarily limited to that state, and there is also an embodiment of a palladium layer in which palladium is deposited as a discontinuous layer or a layer containing a discontinuous portion to expose the carbon fiber surface at least partially. possible. For example, the palladium layer may be present on the surface of the carbon fiber in an island shape.

When the carbon fibers are present in a planar form, such as carbon paper, the amount of palladium layer can be, for example, 0.005 to 1 mg per 1 cm ² of the flat surface, preferably 0.01 to 0.1 mg / 1 cm ² , more. It can preferably be 0.02 to 0.07 mg / 1 cm ² . The theoretical thickness of the palladium layer can be, for example, 1 to 500 nm, preferably 5 to 100 nm, and more preferably 20 to 70 nm. Here, the theoretical thickness is the theoretical volume of the palladium layer calculated based on the amount of the palladium layer (increased weight after sputtering) and the density of the palladium metal, divided by the area of the plane on which the palladium layer is deposited. It is the required thickness.

Alternatively, the amount of the palladium layer can be, for example, 0.05 to 10% by mass, preferably 0.1 to 5% by mass, more preferably 0.2 to 2% by mass, based on the amount of carbon fibers. It is preferably 0.3 to 1% by mass, and more preferably 0.3 to 1% by mass.

Importantly, the palladium layer in this embodiment contains both palladium oxide and metallic palladium, i.e., a mixture of palladium oxide (PdO _x , especially PdO and PdO ₂ ) and metallic palladium (Pd). As a result, it has been found that the heat generated when hydrogen is occluded becomes large, and the sensitivity as a hydrogen sensor can be improved.

In the present disclosure, the palladium layer containing a mixture of palladium oxide and metallic palladium may be referred to as a semi-oxidized Pd layer or simply semi-oxidized Pd. In the present disclosure, the term semi-oxidized is intended to describe the presence of both palladium oxide and non-oxidized (metal) palladium, both of which are necessarily present in a half-half amount ratio. Not necessarily.

The mixture of palladium oxide and metallic palladium in the palladium layer is a combination of the peak of metallic palladium and the peak of palladium oxide in the spectrum (Pd3d _5/2 ) detected by X-ray photoelectron spectroscopy (XPS). It can be confirmed by detecting a peak or a mixed peak. The peak of metallic palladium can be detected at a binding energy of 335.0 to 335.5 eV, and the peak of palladium oxide can be detected at 336.0 to 338.2 eV. Separating complex or mixed peaks detected in the XPS spectrum into individual constituent peaks is a routine procedure for those skilled in the art. For example, since it is known at which position the peak of metallic palladium and the peak of palladium oxide appear as described above, the measured peak is peak-fitted based on the known peak position and Gaussian distribution or Lorentz distribution. The separated peaks can be obtained by processing and obtaining the separated peak shape having the smallest error.

In the palladium layer of the present embodiment, the amount of palladium oxide determined based on the peak amount of the spectrum detected by X-ray photoelectron spectroscopy is preferably at least 1/10 of the amount of metallic palladium. The peak amount is an amount based on the area below the peak of the spectrum and can be determined by those skilled in the art based on ordinary knowledge.

More specifically, the quantitative ratio of palladium oxide to metallic palladium, which is determined based on the peak amount of the spectrum detected by X-ray photoelectron spectroscopy, is in the range of 100: 1 to 1:10. Preferably, this quantity ratio can be more preferably 50: 1 to 1: 5, still more preferably 10: 1 to 1: 1 and particularly preferably 5: 1 to 2: 1.

In the palladium layer, it is preferable that palladium oxide is present in a larger amount than metallic palladium based on the peak amount of the spectrum detected by X-ray photoelectron spectroscopy.

The above-mentioned composition is useful as a detection unit of a hydrogen sensor device. Accordingly, the present disclosure provides, in another aspect, a hydrogen sensor device comprising any of the compositions described above.

Palladium, which is a hydrogen storage metal, generates heat when it stores hydrogen. The principle of the hydrogen sensor in this embodiment is to detect or quantify hydrogen (H ₂ ) by measuring the temperature change at that time.

Hydrogen dissolves in the crystal lattice of palladium to form an intrusive hydride. The face-centered cubic lattice formed by palladium includes a 6-coordinating octahedral interstitial position (O-site) surrounded by 6 metal atoms and a tetrahedral interstitial position (T-site) surrounded by 4 metal atoms. ) Exists. In general, hydrogen atoms tend to enter the O-site when the atomic radius of the metal atom is small, and the T-site when the atomic radius is large. In the case of palladium, hydrogen enters the O site. Hydrogen enters palladium and stabilizes at an atomic composition ratio of H / Pd≈0.7. The reaction formula is shown in the following formula 1.
Pd + H ₂ ⇔ PdH _0.7 + Q kcal / mol (Equation 1)

In the above formula 1, Q is the heat of formation. Generally, the reaction in which a metal occludes hydrogen is an exothermic reaction. Hydrogen enters between the palladium lattices and becomes more stable, generating heat.

As a means for detecting or quantifying a temperature change due to the heat of reaction, any means known to those skilled in the art can be used, and a thermocouple is a preferable example. An example of a thermocouple is a K thermocouple using an alumel-chromel combination. A thermocouple is a junction of two different types of metals and detects a temperature difference by a voltage generated based on a thermoelectromotive force. When two types of metal conductors are electrically connected to form a closed circuit and a temperature difference is applied to both ends, a voltage is generated by moving electrons to the low temperature side due to the presence of a temperature gradient. This phenomenon is called the Seebeck effect. The magnitude of thermoelectromotive force is determined only by the temperature difference between both ends if the materials are uniform and the same combination. Using this principle, the temperature change due to the heat of reaction between palladium and hydrogen can be measured, and H ₂ in the measurement environment can be detected or quantified.

Therefore, the hydrogen sensor device of the present embodiment may include heat detecting means in addition to the above composition. Preferred examples of the heat detection means include, but are not limited to, thermocouples (particularly K thermocouples), resistance temperature detectors and radiation thermometers. The thermocouple or resistance temperature detector is usually physically contacted or connected to the composition. The hydrogen sensor device may further include a voltmeter or ohmmeter, respectively, especially if the heat sensing means is a thermocouple or resistance temperature detector. A voltmeter or ohmmeter is typically electrically contacted or connected to the heat detecting means and is configured to measure the voltage or resistance generated in the heat detecting means as the composition heats up. The radiation thermometer does not need to be in contact with the composition. The radiation thermometer is arranged adjacent to the composition and is configured to detect electromagnetic waves generated by the heat generation of the composition.

Next, a method for producing the above-mentioned composition will be described. For convenience of explanation, these compositions are hereinafter referred to as hydrogen sensor compositions, but the intention is not necessarily limited to the mode in which the composition is used as a hydrogen sensor.

The composition for a hydrogen sensor can be produced, for example, by performing sputtering (ion sputtering) in the presence of oxygen using a palladium metal as a target and a carbon fiber as a base material. By this operation, palladium oxide and metallic palladium can be deposited on the carbon fibers to form an appropriate palladium layer. Here, the metallic palladium means non-palladium oxide (Pd) as opposed to palladium oxide (PdO _x ).

The sputtering method itself has been established in the art and can be carried out by those skilled in the art as appropriate. Sputtering in the presence of oxygen is performed, for example, by performing sputtering in a state where the inside of the sputtering chamber is depressurized but a small amount of air is left, or in a state where the inside of the sputtering chamber is evacuated and then an oxygen-containing gas is introduced. be able to. The pressure at the time of depressurization can be, for example, 10 Pa or less.

The method for producing the composition for a hydrogen sensor according to another embodiment includes a step of depositing metallic palladium on the carbon fiber by performing sputtering using a carbon fiber as a base material with a palladium metal as a target, and carbon on which the metallic palladium is deposited. It comprises a step of oxidizing a part of metallic palladium on the carbon fiber and converting it into palladium oxide to form a palladium layer by an oxidation treatment including heating the fiber in the presence of oxygen. The palladium layer formed in this way may also have a state in which palladium oxide and metallic palladium are mixed.

Sputtering in this case can be suitably performed in an atmosphere of a rare gas (for example, argon) and / or a nitrogen gas. Oxygen may be included in the atmosphere, but it is more preferable to perform sputtering in a state where oxygen is removed because the degree of oxidation of palladium can be easily adjusted by the subsequent oxidation treatment.

The oxidation treatment can be preferably performed in air. The oxidation treatment may be performed in an atmosphere of a mixed gas containing oxygen (for example, argon + oxygen, nitrogen + oxygen, etc.). The heating temperature in the oxidation treatment can be, for example, 100 to 400 ° C, preferably 200 to 350 ° C. The heating temperature can be, for example, 0.5 to 60 minutes, preferably 1 to 30 minutes, or 5 minutes or less.

By performing the oxidation treatment at different temperatures and / or at different times and / or under different atmospheres, the degree of oxidation, i.e. the quantitative ratio of oxidized Pd to metal Pd, can be adjusted. Typically, by performing the oxidation treatment at a higher temperature and / or a longer time and / or a higher oxygen concentration, the degree of oxidation is increased and the ratio of the amount of oxidized Pd to the metal Pd is increased. Can be made to.

Experimental Carbon paper (CFP) made of carbon fiber (TGP-H-060, manufactured by Toray Industries, Inc.) was used as a substrate for sputtering. As a pretreatment, CFP was heat-treated in air at 350 ° C. for 30 minutes. Then, the CFP was cut out to a size of 3 cm × 1 cm and used in the following experiment.

Palladium (Pd) was deposited on the CFP surface by a sputtering method using a sputtering device SC-701MKII manufactured by Sanyu Electronics and a palladium target with a purity of 99.99% (4N), and Pd was deposited (supported) on the CFP (Pd). / CFP) was obtained. FIG. 1 shows scanning electron microscope (SEM) images of CFP (left panel) and Pd / CFP (right panel) used in this experiment.

More specifically, after removing air by evacuating the inside of the sputtering chamber according to a normal procedure, argon (Ar) is introduced and sputtering (discharge current: 5 mA) is performed for 5 minutes in an Ar atmosphere. Non-oxidized Pd / CFP (palladium is present in substantially all non-oxidized metal state (Pd)) was prepared. As another sample, oxidized Pd / CFP obtained by treating the non-oxidized Pd / CFP obtained in the same manner as above in air at 350 ° C. for 30 minutes or more (palladium exists in an oxidized state (PdO _x )). ) Was also prepared.

In addition, experiments were also conducted in which sputtering was performed under reduced atmospheric conditions, that is, in the presence of air. The sputtering time in this case was 10 minutes. As will be described later, in the Pd / CFP obtained under this condition, it was found that palladium in the non-metal oxide state (Pd) and palladium in the oxidized state (PdO _x ) were mixed, and therefore, this is described here. It is called semi-oxidized Pd / CFP.

Similar semi-oxidized Pd / CFPs perform the above oxidation treatments on non-oxidized Pd / CFP at different temperatures (eg lower temperatures) and / or different times (eg shorter times) and / or different. It could also be obtained by performing in an atmosphere (for example, a lower oxygen concentration), in which case the ratio of oxidized Pd to metal Pd could be easily adjusted.

The amount of palladium supported, that is, the amount of deposited palladium layer was determined as the weight change before and after sputtering. All Pd / CFP samples had substantially the same amount of palladium supported (about 0.15 mg per 3 cm ² CFP piece, 0.5-0.6 mass% of carbon fiber) and were deposited on the CFP. The theoretical thickness of the palladium layer was about 42 nm.

The chemical state of palladium was examined by X-ray photoelectron spectroscopy (XPS). The details of the XPS measurement conditions are as follows.
Equipment: Scanning X-ray photoelectron spectroscopy analyzer PHI X-tool (Albac Phi Co., Ltd.)
X-ray source: AlKα
X-ray condition: 200 μm 50 W
Analysis range: 204 μm
Take-Off Angle: 45 deg.
Neutralization gun conditions: 1.2 eV 20.0 μA
<Survey>
Range: 0-1200 eV
PassEnergy: 280 eV
StepSize: 1.0 eV
<Multi>
Range: O1s 523-543 eV
N1s 391-411 eV
Pd3d 330-350 eV
C1s 278-298 eV
Au4f 79-99 eV
PassEnergy: 112 eV
StepSize: 0.1 eV

Each of the prepared Pd / CFPs was placed in a chamber at room temperature, and the temperature change during the reaction with hydrogen gas under atmospheric pressure was measured using a K thermocouple. Specifically, nitrogen gas (N ₂ ) is allowed to flow in the chamber at 100 sccm for 3 minutes, then hydrogen gas (H ₂ ) is allowed to flow at 100 sccm for 3 minutes, and then nitrogen gas (N ₂ ) is allowed to flow at 100 sccm for 3 minutes. The mixture was allowed to flow for 3 minutes, and the measurement was performed for a total of 9 minutes.

Results and Discussion The CFP substrate itself before the sputtering treatment showed no hydrogen detection ability (not shown). Further, the non-oxidized Pd / CFP obtained by sputtering in an Ar atmosphere also showed almost no temperature change in the presence of hydrogen (FIG. 2 (a)). In contrast, in the oxidized Pd / CFP obtained by oxidizing the non-oxidized Pd / CFP in the air at 350 ° C. for 30 minutes, a temperature change of 0.3 ° C. was confirmed by the reaction with hydrogen. (Fig. 2 (c)). That is, hydrogen sensor activity was detected. On the other hand, in the semi-oxidized Pd / CFP obtained by sputtering in air, a significantly larger temperature change (1.5 ° C.) was obtained as compared with the other two conditions (Fig. 2 (b)).

The result of examining the chemical state of palladium in each Pd / CFP by XPS is shown in FIG. In the Pd / CFP obtained by sputtering in an Ar atmosphere, the palladium layer exists as a metal Pd in a almost completely non-oxidized state (FIG. 3A), and the Pd / CFP obtained by oxidizing the palladium layer is present. Then, in the almost completely oxide state (Fig. 3 (c)), and in the Pd / CFP obtained by sputtering in reduced pressure air, the palladium layer is in the state of a mixture of metal and oxide (Fig. 3 (b)). It was confirmed that there was. That is, it was found that the magnitude of the temperature change caused by the reaction of Pd / CFP with hydrogen changes depending on the oxidation state of Pd, and when metallic palladium and palladium oxide are mixed on the carbon fiber. It has been found that a more sensitive hydrogen sensor can be provided. Based on the peak amount of the spectrum detected by XPS, it was found that particularly high sensitivity was obtained when the amount of palladium oxide was larger than the amount of metallic palladium.

Treatment at 200 ° C. or 350 ° C. in air using different oxidation treatment conditions, for example, or at 350 ° C. in an atmosphere of Ar: O ₂ = 80:20 using a CVD apparatus. An experiment was also conducted in which various semi-oxidized Pd / CFPs were prepared by performing an oxidation treatment of non-oxidized Pd / CFPs. All semi-oxidized Pd / CFP samples showed hydrogen-dependent temperature changes, or hydrogen sensoring capabilities, significantly higher than non-oxidized Pd / CFP. The magnitude of the temperature change differs depending on the degree of the oxidation treatment. For example, the sample subjected to the oxidation treatment at 350 ° C. showed a larger temperature change than the sample subjected to the oxidation treatment at 200 ° C.

Palladium is reduced by the reducing power of hydrogen in the Pd / CFP composition after high concentration (100% in this example) of hydrogen gas is circulated and used for hydrogen sensor measurement as described above, and its XPS. The spectrum corresponds to that of non-oxidized Pd / CFP. Therefore, the following experiment was also conducted to investigate whether the oxidized state of palladium on the carbon fiber, which has a hydrogen sensor ability, can be reproduced even after repeated use. That is, the non-oxidized Pd / CFP obtained by sputtering in an Ar atmosphere was oxidized in air at 350 ° C. to prepare a semi-oxidized Pd / CFP sample, and the first hydrogen gas measurement was performed as described above. After that, the oxidation treatment was performed again under the same conditions, and then the second hydrogen gas measurement was performed. In this particular experiment, the temperature change obtained in the first measurement was 1 ° C. and the temperature change obtained in the second measurement was 0.6 ° C. It has been shown that the hydrogen sensor can be reused as a hydrogen sensor at least once after being exposed to a high concentration of hydrogen gas.

Claims (7)

It contains carbon fibers and a palladium layer deposited on the carbon fibers.
The palladium layer contains a mixture of palladium oxide and metallic palladium.
Composition. According to claim 1, the amount ratio of the palladium oxide to the metallic palladium, which is determined based on the peak amount of the spectrum detected by X-ray photoelectron spectroscopy, is in the range of 100: 1 to 1:10. The composition described. The composition according to claim 1 or 2, wherein the palladium oxide is present in a larger amount than the metallic palladium based on the peak amount of the spectrum detected by X-ray photoelectron spectroscopy. The composition according to any one of claims 1 to 3, wherein the carbon fiber is in the form of carbon paper. A hydrogen sensor device comprising the composition according to any one of claims 1 to 4. Claims 1 to 1, which include a step of forming the palladium layer by depositing palladium oxide and metallic palladium on the carbon fiber by performing sputtering in the presence of oxygen with the palladium metal as the target and the carbon fiber as the base material. The method for producing a composition according to any one of 4. A step of performing sputtering using palladium metal as a target and carbon fiber as a base material to deposit metallic palladium on the carbon fiber, and
Claim 1 includes a step of oxidizing a part of the metallic palladium and converting it into palladium oxide to form the palladium layer by heating the carbon fiber on which the metallic palladium is deposited in the presence of oxygen. The method for producing the composition according to any one of 4 to 4.