CN1547012A - Measuring method for hydrogen storage quantity of nanometer hydrogen storage material - Google Patents

Abstract

The invention is a measuring method for hydrogen storing quantity of a kind of nano hydrogen storing device. The nano material size is small, the weight is light, thus there has many difficulties and errors when measuring the hydrogen storing quantity with current measuring material. The invention uses the computer film thickness monitoring system in the vacuum film plating, measure the hydrogen storing performance of the nano material generated on the quartz crystal plate. The quartz crystal oscillator in the system is based on the basic theory of quartz crystal microweighing (QCM), through measuring the change of the resonance frequency of sample needed to be tested in the generating process, and before and after the hydrogen charging. The invention has a high precision, good sensitivity.

Description

Method for measuring hydrogen storage capacity ofnano hydrogen storage material

Technical Field

The present invention belongs to a method for measuring hydrogen storage quantity of nano hydrogen storage material.

Background

Hydrogen energy is renewable ideal clean energy, and the utilization of hydrogen energy is a necessary trend in the field of energy development in this century. Hydrogen fuel cells have been regarded as a secondary energy source, but one of the biggest technical problems that have prevented the use of hydrogen fuel cells is hydrogen storage technology. For vehicular hydrogen storage systems, the U.S. department of energy (DOE) has proposed the goals of hydrogen storage capacity of not less than 6.5% by weight, 62kgH2/m3 by volume, and actual hydrogen storage capacity of greater than 3.1kg for vehicular hydrogen storage systems. A.C. Dillon et al (Dillon AC, Jones KM, Bekkedahl TA, et al [ J]. Nature 1997; 386: 377 379.) of national laboratory of renewable energy in 1997 have studied the hydrogen storage characteristics of single-walled carbon nanotubes by thermal desorption spectroscopy (TPD), and the one-dimensional nanomaterials become the research hotspot of hydrogen storage materials. As research progresses, a number of exciting and considerable figures are successively obtained, which are, however, subject to constant doubt and negation. The reason is that the nano material has small size and light weight, and the experimental results of different research groups are very different easily due to the difference of test means and conditions. Therefore, establishing a scientific, reasonable and accurate hydrogen storage test method is urgent!

The currently reported common methods for measuring hydrogen storage are mainly as follows: a volume method; gravimetric method; thermal desorption spectroscopy; an electrochemical method; drainage methods, and the like. The volume method requires a large amount of samples, and has high requirements on the air tightness of the system and the temperature constancy in the experimental process; the gravimetric method needs to avoid any non-hydrogen (such as water molecules and the like) adsorption of the sample, and the cost of the measuring device is high; the thermal desorption spectroscopy is to store hydrogen in a sample under certain hydrogen pressure and then transfer the sample into a high vacuum container to detect the concentration of released hydrogen, and has the defect that the released hydrogen in the process of transferring into high vacuum cannot be monitored; the electrode of the electrochemical method is difficult to manufacture, and the material which can be detected by the method is less; the drainage method obtains the hydrogen amount by measuring the volume of drainage, and has the defect that water molecules are easily combined with hydrogen molecules, so that the experimental data is higher.

Therefore, for the nano hydrogen storage material, no better hydrogen storage amount testing method exists at present.

Disclosure of Invention

The invention aims to obtain a method for measuring the hydrogen storage capacity of a nano hydrogen storage material, which has the advantages of accuracy and high sensitivity.

In the present invention, the hydrogen storage performance of the nanostructured hydrogen storage material is measured using a quartz crystal oscillation method.

The quartz crystal oscillation method is currently used in the vacuum coating process, and can monitor the deposition thickness of the film in real time. The method is not reported to be used for measuring the hydrogen storage performance of the material. According to the basic principle of QCM, a mass of material is deposited on a quartz wafer with a change in resonant frequency of:

$\mathrm{\Δf}=-\frac{2{f_0}^2\mathrm{\Δm}}{A\sqrt{\mathrm{\μ\ρ}}}=-\mathrm{C\Δm}---\left(1\right)$

in the formula f₀Is the initial resonant frequency of the quartz wafer; Δ m is the increment of mass; a is the surface area; μ is shear modulus (2.947X 10)¹¹gcm^-1s^-2) (ii) a ρ is the density of quartz (2.648 gcm)^-3). From the formula (1), C is a constant relating to the quartz crystal. Therefore, within a certain range, the change of the resonance frequency of the quartz crystal and the change of the mass of the substance adsorbed on the surface of the quartz crystal are linear changes. I.e. the decrease in the frequency of the wafer measured in the experiment corresponds to the mass of hydrogen adsorbed by the sample.

The method of the invention puts the sample to be measured into the vacuum chamber, and the resonance frequency is measured in real time by the computer control; pumping the vacuum chamber to a vacuum degree of 1-5 × 10^-3Pa, recording the change condition of the resonant frequencyof the sample in the air extraction process; then the inflation valve is opened to fill hydrogen, and the sample in the inflation process is recordedA change in resonant frequency; when the hydrogen gas is charged to one atmosphere, the hydrogen gas is kept for 4 to 8 hours until the resonance frequency is stabilized at a certain value.

The device of the invention is as follows: the sample chamber (1) is connected with the vacuum system (2) and is connected with the film thickness measuring instrument (quartz crystal oscillation device) and the computer system (3), and the hydrogen source (4) is connected between the sample chamber (1) and the vacuum system (2); the device utilizes a computer to measure the resonance frequency of a sample in real time, and the hydrogen storage amount of the material is measured according to the micro-scale principle of quartz crystals.

The specific test apparatus is shown in fig. 2.

The test method of the invention, namely the quartz crystal oscillation method, can measure the weight hydrogen storage amount of the nano material growing on the quartz crystal wafer, and the measurement mechanism is based on a quartz crystal micro scale (QCM). The result proves that the method for measuring the hydrogen storage capacity of the nano material is simple and convenient, the measurement is accurate, the sensitivity is high, and the repeatability is good. Is a new attempt to solve the problem of measuring the hydrogen storage capacity of the nano hydrogen storage material.

Drawings

Fig. 1 is a schematic view of a hydrogen storage amount measuring apparatus.

Figure 2 is a graph of the change in resonant frequency of a nanowire sample over time.

Fig. 3 is a graph of the change in resonant frequency of a blank wafer over time.

Figure 4 is a comparison of the hydrogen storage process for the sample and blank wafer.

FIG. 1 is a vacuum chamber; 2 is a vacuum system; 3, a film thickness measuring instrument and a computer system thereof; 4 is a source of hydrogen.

Detailed Description

The content of the invention is further illustrated by taking the measurement of the hydrogen storage capacity of the Ag (TCNQ) nanowire as an example.

Ag (TCNQ) nanowire sample preparation

The initial resonance frequency of the sample is f₁6001118Hz silver electrode quartz wafer. After a layer of Ag film with thickness of about 10nm is plated by vacuum evaporation method, its resonant frequency is changed into f₂6000624 Hz; then putting the sample and 7, 7, 8, 8-tetracyanoquinodimethane namely TCNQ together in a glass tube, obtaining an Ag (TCNQ) nanowire sample on a quartz wafer by adopting a vacuum saturated vapor reaction method, and measuring that the resonance frequency is changed into f₃＝5996682Hz。

2. Measurement of Hydrogen storage amount

The test apparatus used is shown in fig. 1. The testing steps are as follows:

(1) putting the sample prepared in the embodiment 1 into a small vacuum chamber, and starting a computer control system to measure the resonance frequency of the sample in real time;

(2) pumping the vacuum chamber until the vacuum degree reaches 5 × 10^-3Pa, recording the change condition of the resonant frequency of the sample in the air pumping process;

(3) opening an inflation valve, slowly inflating hydrogen, and recording the change condition of the resonant frequency of the sample in the inflation process;

(4) after the hydrogen pressure reached one atmosphere, it was maintained for 4 hours until the resonance frequency stabilized at a certain value, and this value was recorded.

In order to increase the reliability of the experimental results, the invention also tests the blank quartz wafer according to the above steps.

3. Computing

(1) Frequency change of Ag (TCNQ) nanowires

Under the condition that the silver electrode does not participate in the reaction, the change amount of the mass of the Ag (TCNQ) nanowire corresponding to the resonance frequency of the wafer is considered as follows: f. of₁-f₃4436 Hz. However, in experimental analysis, the silver electrode is found to participate in the reaction, so that the initial resonant frequency of the wafer is corrected in the calculation process. By

(0＜σ＜1) (2)

It can be calculated that there is a mass correspondence Δ f in the electrode_Ag1593 Hz. Therefore, the quality of Ag (TCNQ) nano-wire on the substrate actually corresponds to the change of the resonance frequency of the waferThe variables should be: Δ f_Ag(TCNQ)＝(f₁+Δf_Ag)-f₃＝4436+1593＝6029Hz。

(2) Hydrogen storage capacity by weight

The change of the resonance frequency of the quartz crystal oscillation sample of the Ag (TCNQ) nanowire and the blank quartz crystal oscillation piece along with time in the hydrogen absorption test process is shown in fig. 3 and 4. As can be seen from FIG. 3, the initial resonant frequency of the sample before the start of hydrogen gas filling is F₂5996688 Hz; after hydrogen absorption its resonant frequency becomes F₃5996606Hz, frequency variation Δ F_H2Is 82 Hz; while the frequency variation of the blank wafers during this process was only 10Hz respectively, a comparison of these is shown in fig. 5. Multiple experiments show that the results can be repeated, and the relative error of each experiment is 2%. It can be seen that the measurement system of the present invention is stable.

The principle of QCM is as follows:

$\frac{{\mathrm{\&Delta;<figure-callout\; id="2"\; label="F\; H"\; filenames="A20031010922400071.png"\; state="\{\{state\}\}">F</figure-callout>}}_{H_{}}}{{\mathrm{\&Delta;f}}_{\mathrm{Ag}\left(\mathrm{TCNQ}\right)}}=\frac{{\mathrm{\&Delta;m}}_{{\mathrm{<figure-callout\; id="2"\; label="H"\; filenames="A20031010922400071.png"\; state="\{\{state\}\}">H</figure-callout>}}_2}}{{\mathrm{\&Delta;m}}_{\mathrm{Ag}\left(\mathrm{TCNQ}\right)}}---\left(3\right)$

therefore, the weight hydrogen storage amount is:

$\mathrm{wt}\%=\frac{{\mathrm{\&Delta;<figure-callout\; id="2"\; label="F\; H"\; filenames="A20031010922400071.png"\; state="\{\{state\}\}">F</figure-callout>}}_{H_{}}}{{\mathrm{\&Delta;<figure-callout\; id="2"\; label="F\; H"\; filenames="A20031010922400071.png"\; state="\{\{state\}\}">F</figure-callout>}}_{H_{}}+{\mathrm{\&Delta;f}}_{\mathrm{AgTCNQ}}}\&times;100\%---\left(4\right)$

the weight hydrogen storage amount of the quartz crystal oscillation sample grown with Ag (TCNQ) nano-wires under normal temperature and pressure is 1.34 percent by substituting the experimental data into the formula (4).

Claims (2)

1.A method for measuring hydrogen storage capacity of a nano hydrogen storage material is a quartz crystal oscillation method. The method is characterized in that a sample to be measured is placed in a vacuum chamber, and the resonance frequency of the sample to be measured is measured in real time under the control of a computer; pumping the vacuum chamber to a vacuum degree of 1-5 × 10^-3Pa, recording the change condition of the resonant frequency of the sample in the air extraction process; then opening an inflation valve, filling hydrogen, and recording the change of the resonant frequency of the sample in the inflation process: when the hydrogen charge reaches an atmospheric pressure, it is maintained for 4-8 hours until the resonance frequency stabilizes at a certain value, according to:

$\mathrm{\&Delta;f}=-\frac{{{2f}_0}^2\mathrm{\&Delta;m}}{A\sqrt{\mathrm{\&mu;\&rho;}}}=-\mathrm{C\&Delta;m}---\left(1\right)$

and correction amount of resonance frequency:

Δf_{specific sample}＝(f₁+Δf_{Electrode for electrochemical cell})-f₃(2)

The principle of the quartz crystal micro-scale is as follows:

$\frac{{\mathrm{\&Delta;F}}_{H_2}}{\mathrm{\&Delta;}{f}_{\mathrm{Ag}\left(\mathrm{TCNQ}\right)}}=\frac{\mathrm{\&Delta;}{m}_{H_2}}{\mathrm{\&Delta;}{m}_{\mathrm{Ag}\left(\mathrm{TCNQ}\right)}}---\left(3\right)$

the weight hydrogen storage capacity of the nano hydrogen storage material can be obtained:

$\mathrm{wt}\%=\frac{{\mathrm{\&Delta;F}}_{H_2}}{{\mathrm{\&Delta;F}}_{H_2}+{\mathrm{\&Delta;f}}_{\mathrm{AgTCNQ}}}\&times;100\%---\left(4\right)$

2. the device for measuring the hydrogen storage capacity of the nano hydrogen storage material according to claim 1, wherein a sample chamber (1) is connected with a vacuum system (2) and is connected with a film thickness measuring instrument (quartz crystal oscillation device) and a computer system (3), and a hydrogen source (4) is connected between the sample chamber (1) and the vacuum system (2); the device utilizes a computer to measure the resonance frequency of a sample in real time, and the hydrogen storage amount of the material is measured according to the micro-scale principle of quartz crystals.

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