KR20220065608A - Enhancement of Thermal Conductivity of Graphene Derivative-Magnesium Crystal Nanocomposite for High-Performance Solid-State Hydrogen Storage - Google Patents

Abstract

Provided is a graphene derivative-magnesium nanocrystal composite, which is the graphene derivative-magnesium nanocrystal composite wherein the content of a graphene derivative is less than 10 wt% based on a total weight of the composite. The graphene derivative-magnesium nanocrystal composite of one embodiment has excellent hydrogen storage capacity and thermal conductivity, thereby being able to be advantageously used as a hydrogen storage medium.

Description

Improvement of Thermal Conductivity of Graphene Derivative-Magnesium Crystal Nanocomposite for High-Performance Solid-State Hydrogen Storage

It relates to the improvement of thermal conductivity properties of graphene derivative-magnesium nanocrystal composites for high-performance solid hydrogen storage.

In recent years, as hydrogen has been spotlighted as a low-carbon fuel, research on an effective hydrogen storage method, in particular, a hydrogen storage system enabling rapid charging of a hydrogen vehicle is being conducted. Compared to the battery charging method of electric vehicles, the hydrogen fuel cell-based vehicle has a high output and is expected to have a short charging time, so it is particularly advantageous for commercial vehicles such as trucks and buses.

However, the current hydrogen storage system has a limited volumetric storage density, and it is difficult to store compressed gas due to safety concerns. In addition, when liquid hydrogen is used, a loss of potential efficiency of about 12.8% occurs compared to gaseous hydrogen.

Therefore, a method of storing hydrogen in a solid state such as metal hydride has a high storage capacity and high safety, so it is in the spotlight as an alternative to liquid hydrogen, and magnesium among metals is one of the promising hydrogen storage materials due to its high storage capacity and reversibility. .

However, magnesium hydride (MgH ₂ ) has a slow hydrogen absorption/release rate characteristic, and is thermodynamically stable, so there is a problem in that it requires a high temperature during hydrogenation/dehydrogenation and requires a long charging time.

In order to solve these drawbacks, research is being conducted on using a nanostructured magnesium composite using a carbon scaffold or a transition metal. In particular, in the case of a double carbon scaffold, it is possible to exhibit catalytic and nanoconfinement effects, and at the same time improve heat conduction in the hydrogen absorption/release process. For this reason, various carbon materials (CNT, Graphite) have been used to improve thermal conductivity. When simply added by a physical method, effective thermal conduction is difficult to achieve due to the collapse of the magnesium structure during the hydrogen absorption/release process.

In addition, although there have been studies that have analyzed the thermal conductivity of the composite by adding such a carbon material, it has only been limited to the analysis of the thermal conductivity effect by the carbon material present in the structure of the bulk magnesium composite. There are no studies analyzed.

As a material that can solve this problem, a graphene derivative-magnesium nanocrystal composite is spotlighted as a hydrogen storage medium. In this case, the magnesium particles exist in the nano size between the graphene layers, and may have structural stability during the hydrogen absorption/release process. However, although the heat transfer performance of the hydrogen storage material is very important for controlling the absorption/release reaction of hydrogen, studies on the heat transfer performance of the graphene derivative-magnesium nanocrystal composite have not been sufficiently conducted.

By controlling the content of the graphene derivative in the graphene derivative-magnesium nanocrystal composite, a graphene derivative-magnesium nanocrystal composite having excellent thermal conductivity is provided.

One embodiment provides a graphene derivative-magnesium nanocrystal composite, wherein the content of the graphene derivative is less than 10% by weight based on the total weight of the composite.

The effective thermal conductivity of the graphene derivative-magnesium nanocrystal composite under a helium atmosphere may be 0.3 W/m·K or more.

The effective thermal conductivity of the graphene derivative-magnesium nanocrystal composite under an argon atmosphere may be 0.1 W/m·K or more.

The graphene derivative-magnesium nanocrystal composite may have a porosity of 0.9 or more.

Based on the total weight of the graphene derivative-magnesium nanocrystal composite, the content of the graphene derivative may be 1 wt% to 8 wt%.

Based on the total weight of the graphene derivative-magnesium nanocrystal composite, the content of the graphene derivative may be 2 wt% to 6 wt%.

Based on the total weight of the graphene derivative-magnesium nanocrystal composite, the content of the graphene derivative may be 2% by weight.

The graphene derivative-magnesium nanocrystal composite may include multiple layers of a graphene derivative, and the magnesium nanocrystals may be sandwiched between multiple layers of the graphene derivative.

The ratio of the graphene derivative having a multilayer among the graphene derivatives may be 1% to 20%.

The hydrogen storage capacity of the graphene derivative-magnesium nanocrystal composite may be 6.5 wt% or more based on the total weight of the composite.

Another embodiment provides a hydrogen storage device including a graphene derivative-magnesium nanocrystal composite.

The graphene derivative-magnesium nanocrystal composite of one embodiment has excellent hydrogen storage capacity and thermal conductivity, and thus can be advantageously used as a hydrogen storage medium.

1 shows the effective thermal conductivity of the graphene derivative-magnesium nanocrystal composite according to Example 1 and Comparative Example 1, respectively, measured in helium and argon gas atmospheres and vacuum.
2 is a graph showing the effective thermal conductivity of the graphene derivative-magnesium nanocrystal composite according to Examples 1 to 3, respectively, measured in a helium and argon gas atmosphere, and in a vacuum.
3 shows a heat transfer 2D model for measuring the effective thermal conductivity of the graphene derivative-magnesium nanocrystal composite according to Examples 1 to 3;
4 is a graph showing the thermal conductivity of the composite according to the weight of the graphene derivative of the graphene derivative-magnesium nanocrystal composite.
5 is a graph showing the effective thermal conductivity according to the weight of the graphene derivative in the graphene derivative-magnesium nanocrystal composite stacked bed (packed bed).

Hereinafter, embodiments of the present invention will be described in detail. However, this is presented as an example, and the present invention is not limited thereto, and the present invention is only defined by the appended claims.

Existing metal hydrides have properties suitable as carbon substitutes, but their strong bonding enthalpy, long hydrogen diffusion distance, and high reactivity to oxygen act as obstacles to becoming a long-term target as an alternative energy. However, the recently developed graphene derivative-magnesium nanocrystal composite has improved the disadvantages of such metal hydrides, is gas-selective, protects the hydrogen storage alloy from moisture in the outside air, and the graphene derivative also has a catalytic effect. , has a relatively higher hydrogen storage density than conventional hydrogen storage alloys.

On the other hand, although the heat transfer performance of the hydrogen storage alloy is very important for controlling the absorption/release reaction of hydrogen, studies on the heat transfer performance of the graphene derivative-magnesium nanocrystal composite have not been sufficiently conducted.

The present inventors measured the effective thermal conductivity of the graphene derivative-magnesium according to the amount of graphene oxide added during the manufacturing process of the graphene derivative-magnesium nanocrystal composite to determine the thermal and structural properties of the graphene derivative-magnesium nanocrystal composite. The effect on hydrogen storage performance was analyzed. As a result, the graphene derivative-magnesium nanocrystal composite containing less than 10% by weight of the graphene derivative based on the total weight of the graphene derivative-magnesium nanocrystal composite may have excellent thermal conductivity and thus excellent hydrogen storage capacity. By confirming that there is, the present invention was completed.

Here, the term “effective thermal conductivity” refers to the thermal conductivity of a porous material composed of a graphene derivative-magnesium nanocrystal composite and a gas.

Accordingly, one embodiment provides a graphene derivative-magnesium nanocrystal composite, wherein the content of the graphene derivative is less than 10% by weight based on the total weight of the composite.

The graphene derivative may be reduced graphene oxide.

The content of the graphene derivative in the graphene derivative-magnesium nanocrystal composite is, based on the total weight of the composite, for example, 1 wt% or more, 2 wt% or more, 3 wt% or more, 4 wt% or more, 5 wt% or more, 6 wt% or more, 7 wt% or more, 8 wt% or more, or 9 wt% or more, and less than 10 wt%, 9 wt% or less, 8 wt% or less, 7 wt% or less, 6 wt% or less, 5 wt% or less, 4 wt% or less, 3 wt% or less, 2 wt% or less, or 1 wt% or less. In one embodiment, the content of the graphene derivative may be 1 wt%, 2 wt%, 3 wt%, 4 wt%, 5 wt%, 6 wt%, 7 wt%, 8 wt%, or 9 wt% there is. In the above weight range, the effective thermal conductivity of the graphene derivative-magnesium nanocrystal composite may be excellent, and thus the hydrogen storage capacity of the composite may be improved.

The graphene derivative-magnesium nanocrystal composite may have an effective thermal conductivity of 0.3 W/m·K or more in a helium atmosphere, and an effective thermal conductivity of 0.1 W/m·K or more in an argon atmosphere, which may have high effective thermal conductivity.

The effective thermal conductivity of the graphene derivative-magnesium nanocrystal composite may be affected by various factors in addition to the content of the graphene derivative in the composite. For example, the porosity of the composite, the structure of the composite, and multiple layers in the graphene derivative are those.

From this point of view, the porosity of the graphene derivative-magnesium nanocrystal composite may be 0.9 or more. For example, the porosity may be 0.9 or more, 0.91 or more, 0.92 or more, 0.93 or more, 0.94 or more, or 0.95 or more.

The graphene derivative of the graphene derivative-magnesium nanocrystal composite may have a multilayer structure, and the magnesium nanocrystals may be disposed between the layers of the multilayer structure. In this case, the ratio of the graphene derivative having a multilayer among the graphene derivatives may be 1% to 20%. As the graphene derivative having the multilayer is included in the above range, the effective thermal conductivity of the graphene derivative-magnesium nanocrystal composite including the same may be improved.

The hydrogen storage capacity of the graphene derivative-magnesium nanocrystal composite may be 6.5% by weight or more based on the total weight of the composite.

The graphene derivative-magnesium nanocrystal composite according to an embodiment can be easily prepared using a known method for preparing a graphene derivative-magnesium nanocrystal composite. For example, a graphene derivative is prepared by contacting graphene oxide with a reducing agent, and the graphene derivative and a magnesium precursor are co-reduced to prepare a graphene derivative-magnesium nanocrystal composite, It can be easily prepared by adjusting the content of the graphene derivative in the step of co-reducing the graphene derivative and the magnesium precursor so that the content of the pin derivative is less than 10% by weight based on the total weight of the composite.

As a magnesium precursor usable in the manufacturing method, bis(cyclopentadienyl) magnesium, magnesium chloride, di-n-butyl magnesium, or these a combination of, but not limited to.

The reducing agent and the reducing method used in the preparation method may also use a reducing agent and a reducing method known to those skilled in the art, and there is no particular limitation thereto.

Another embodiment provides a hydrogen storage device including the graphene derivative-magnesium nanocrystal composite according to the embodiment.

Hereinafter, through Examples, the preparation of the graphene derivative-magnesium nanocrystal composite according to an embodiment, and the thermal conductivity and graphene derivative-magnesium nanocrystal composite according to the content of reduced graphene oxide in the prepared composite, and the structural change of the magnesium nanocrystal composite etc. will be described in more detail. These examples are for illustrative purposes and are not intended to limit the present invention.

Example

Example 1

Preparation of Graphene Derivative-Magnesium Nanocrystal Composite

In an argon glove box, lithium 0.144 g (0.02 mol) and naphthalene 1.92 g (0.015 mol) were added to THF (96 mL), and then dissolved by stirring for 3 hours. Then, 1.848 g (0.012 mol) of Cp ₂ Mg was added to a graphene oxide solution (5 mg/10 ml THF) dispersed by sonication for 1.5 hours and stirred for 30 minutes. Then, the mixed solution was added to the lithium naphthalenide solution and stirred for 2 hours. After centrifuging the stirred solution at a speed of 10,000 rpm for 20 minutes, washing with THF was repeated to remove unreacted lithium naphthalenide. Then, it was completely dried under vacuum to obtain a graphene derivative-magnesium nanocrystal composite.

In the composite, reduced graphene oxide, which is a graphene derivative, was included in an amount of 2% by weight based on the total weight of the composite. At this time, the porosity of the composite was 92% to 94%.

Example 2

A composite was prepared in the same manner as in Example 1, except that the amount of graphene oxide input was 10 mg/20 ml THF to be included in 4 wt% based on the total weight of the composite. At this time, the porosity of the composite was 92% to 94%.

Example 3

A composite was prepared in the same manner as in Example 1, except that the amount of graphene oxide input was 20 mg/40 ml THF to be included in 8% by weight based on the total weight of the composite. At this time, the porosity of the composite was 92% to 94%.

Comparative Example 1

Without using reduced graphene oxide, a magnesium hydride composite having a diameter of 65 μm from H2 Energy was used. The porosity of the magnesium hydride was 58%.

evaluation example

Evaluation Example 1. Measurement of Effective Thermal Conductivity

The thermal conductivity of the composites of Examples 1 to 3 and Comparative Example 1 was measured through a guarded hot-plate method. The protective hot plate device consists of a high-temperature and low-temperature assembly and a sample holder positioned between the high-temperature assembly and the low-temperature assembly. The high temperature assembly includes a main plate and a guard plate, wherein the guard plate mitigates heat loss of the main plate. Graphene derivative-magnesium nanocomposite powder was placed in the sample holder to measure thermal conductivity, and a protective hot plate device was placed inside a vacuum chamber where the pressure was maintained at 3 mtorr or less to minimize heat loss of the plate of the high-temperature assembly.

The thermal conductivity was calculated on the assumption that one-dimensional axial heat was generated from the main plate-sample holder-low temperature assembly of the high temperature assembly according to Fourier's law.

The effective thermal conductivity (k _s ) was calculated according to Equation 1 below.

[Equation 1]

Q _s : amount of heat flowing from the main plate to the low temperature assembly

K)k _sc : thermal conductivity of the sample holder (0.15 W/m

K)

t _sc : thickness of the sample holder

t _sp : thickness of the sample

T- _m : temperature of main plate

T _c : temperature of the low temperature assembly

A _m : width of main plate

Effective thermal conductivity was measured in helium and argon gas atmosphere and vacuum, respectively, and the thermal conductivity of helium and argon was 0.15 W/m·K and 0.018 W/m·K, respectively.

The measurement results are shown in FIG. 1 . The effective thermal conductivity of the graphene derivative-magnesium nanocrystal composite of Example 1 was higher than that of the magnesium hydride composite of Comparative Example 1 in helium and argon gas atmospheres and vacuum. These results show that the reduced graphene oxide, that is, the graphene derivative containing less than 10% by weight based on the total weight of the graphene derivative-magnesium nanocrystal composite, is magnesium hydride that does not contain reduced graphene oxide. It shows that the effective thermal conductivity is superior to that of the composite.

In addition, the result of measuring the effective thermal conductivity according to the weight % of the reduced graphene oxide in the graphene derivative-magnesium nanocrystal composite is shown in FIG. 2 .

The difference in effective thermal conductivity increased as the thermal conductivity of the gas increased, and since the content of reduced graphene oxide in the graphene derivative-magnesium nanocrystal composite was low, in a vacuum where the particle structure was less affected, the graphene derivative-magnesium nanocrystal composite The change in effective thermal conductivity according to the content of reduced graphene oxide of the composite was small.

In addition, all of the graphene derivative-magnesium nanocrystal composites of Examples 1 to 3 exhibited excellent effective thermal conductivity of 0.3 W/m·K or more under a helium atmosphere, and excellent effective thermal conductivity of 0.1 W/m·K or more under an argon atmosphere. indicated. In addition, in the helium and argon atmosphere, the effective thermal conductivity of the graphene derivative-magnesium nanocrystal composite containing 2 wt% of reduced graphene oxide is the greatest, and the graphene derivative containing reduced graphene oxide of 4 wt% -Magnesium nanocrystal composite had the lowest effective thermal conductivity, and the graphene derivative containing reduced graphene oxide in 8 wt% - Magnesium nanocrystal composite contained 4 wt% reduced graphene oxide in effective thermal conductivity More increased, it was found that the content of the reduced graphene oxide is not proportional to the effective thermal conductivity of the graphene derivative-magnesium nanocrystal composite.

Evaluation Example 2. Evaluation of the effect of multilayer formation on thermal conductivity

How to measure

In order to evaluate the effect of multilayer formation on thermal conductivity, a 2D model was constructed as shown in FIG. 3 . To estimate the thermal conductivity of the graphene derivative-magnesium nanocrystal composite particles, parallel thermal resistance was constructed as shown in FIG. 3, and the number of magnesium nanoparticles in the graphene derivative-magnesium nanocrystal composite (i.e., Mg = 95) and The length (i.e., rGO = 1 μm) of the reduced graphene oxide (graphene derivative) layer was fixed. The reduced number of graphene oxide layers (rGO) is calculated as in Equation 2 below through the weight% of the reduced graphene oxide.

[Equation 2]

Here, ρrGO = 0.26 g/cm ³ [ACS materials], rGO = 0.9 nm (2.5 layers of rGO), and Mg = 15 nm [Cho et al., 2016]. The number of reduced graphene oxide layers (rGO) was calculated by Equation (1), and the average spacing ( _Mg ) between the magnesium nanocrystal particles was calculated according to Equation 3 below.

[Equation 3]

In addition, when the ratio of reduced graphene oxide having multiple layers among the total reduced graphene oxide in the graphene derivative-magnesium nanocrystal composite is α, the number of layers in which the magnesium nanocrystals are encapsulated is (1-α)n _rGO change to -1.

The thermal conductivity calculation result of the graphene derivative-magnesium nanocrystal composite is shown in FIG. 4 . γ indicated in FIG. 4 means the ratio of reduced graphene oxide having multiple layers α and graphene weight %. That is, for example, when the weight% of the reduced graphene oxide in the composite is 2% by weight, if γ is 2, it means that the ratio of graphene oxide having multiple layers is 4%. The ratio α of multilayered graphene oxide has a linear relationship with the reduced graphene oxide weight%, and the higher the ratio of the multilayer, the better the thermal conductivity value.

The result of calculating the effective thermal conductivity of the graphene derivative-magnesium nanocrystal composite packed bed is shown in FIG. 5 . Like the graphene derivative-magnesium nanocrystal composite, even when the composite is in the form of a stacked bed, the ratio α of reduced graphene oxide having multiple layers is linearly proportional to the weight% of reduced graphene oxide, and As the ratio of the layers increased, a higher effective thermal conductivity value was exhibited. These results indicate that the ratio α of the reduced graphene oxide having multiple layers is linearly proportional to the weight% of the reduced graphene oxide, and the effective thermal conductivity is excellent.

In the above, the present invention has been described through preferred embodiments as described above, but the present invention is not limited thereto and various modifications and variations are possible without departing from the concept and scope of the following claims. Those skilled in the art to which the invention pertains will readily understand.

Claims (11)

A graphene derivative-magnesium nanocrystal composite comprising:
Based on the total weight of the composite, the content of the graphene derivative is less than 10% by weight,
Graphene derivative-magnesium nanocrystal composite. The graphene derivative-magnesium nanocrystal composite of claim 1, wherein the graphene derivative-magnesium nanocrystal composite has an effective thermal conductivity of 0.3 W/m·K or more in a helium atmosphere. The graphene derivative-magnesium nanocrystal composite of claim 1, wherein the graphene derivative-magnesium nanocrystal composite has an effective thermal conductivity of 0.1 W/m·K or more under an argon atmosphere. In claim 1,
The graphene derivative-magnesium nanocrystal composite has a porosity of 0.9 or more, the graphene derivative-magnesium nanocrystal composite. In claim 1,
Based on the total weight of the graphene derivative-magnesium nanocrystal composite, the content of the graphene derivative is 1 wt% to 8 wt%, the graphene derivative-magnesium nanocrystal composite. In claim 1,
The graphene derivative-magnesium nanocrystal composite, wherein the content of the graphene derivative is 2 wt% to 6 wt%, based on the total weight of the graphene derivative-magnesium nanocrystal composite. In claim 1,
Based on the total weight of the graphene derivative-magnesium nanocrystal composite, the content of the graphene derivative is 2% by weight, the graphene derivative-magnesium nanocrystal composite. The method of claim 1,
The graphene derivative-magnesium nanocrystal composite comprises a graphene derivative having multiple layers, and the magnesium nanocrystals are sandwiched between the multiple layers of the graphene derivative. 9. The method of claim 8,
The ratio of the graphene derivative having multiple layers among the graphene derivative is 1% to 20%, the graphene derivative-magnesium nanocrystal composite. The method of claim 1,
The hydrogen storage capacity of the graphene derivative-magnesium nanocrystal composite is 6.5 wt% or more based on the total weight of the composite, the graphene derivative-magnesium nanocrystal composite. A hydrogen storage device comprising the graphene derivative-magnesium nanocrystal composite of any one of claims 1 to 10.

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