Exploration of Graphene-like 2D Materials for Energy Management and Interface Enhancement Applications

Doctoral thesis, 2021

Ever since the discovery of graphene in 2004, graphene-like 2D materials and their derivatives have attracted extensive investigations because of their exceptional physical and chemical properties. At present, the study of graphene-like 2D materials is at a stage where most of their outstanding physical and chemical properties have been discovered, but the technology for incorporating them into practical commercial products is rarely revealed. For the potential practical industrial applications of graphene-like 2D materials, energy management and interface enhancement are two of the most promising areas. So far, the behavior of the commercialized graphene-like 2D material products is far from their theoretical performance and expectations as a result of defects and π-π agglomeration, etc. In this regard, there is plenty of research room at the bottom for exploring their practical industrial applications.

At present, surface modification is the most widely used strategy to cope with agglomerations. While to be widespread in market, developing low-cost, uniform, and high-quality preparation technology, and encountering the intrinsic agglomeration issues of graphene-like 2D materials are two of the main challenges. To focus on the above two issues, we developed the functionalization method for graphene-like 2D materials, including graphene and hexagonal boron nitride, and explored their potential industrial applications in energy management and interface enhancement. Further, mass production technology and industrial demonstration for graphene and hexagonal boron nitride were explored in some chapters. The main scientific conclusions and innovations of this thesis are listed as below:

At first, Chapter 2 presents the experimental research study on using graphene-like 2D materials for energy management, especially in heat dissipation. With the rapid development of microelectronics and 5G communications, efficient heat dissipation is severely demanded for future electronics. To improve heat dissipation efficiency of electronics, based on the ultrahigh thermal conductivity of graphene-like 2D materials, this chapter explored two experimental works, including lightweight and high-performance graphene enhanced heat pipe and hexagonal boron nitride enhanced thermally conductive and electrically insulation heat spreader.


(1) Graphene Enhanced Heat Pipe

In this work, a unique lightweight and high thermal performance graphene heat pipe were firstly designed and developed. At first, the inner structures of graphene enhanced heat pipe were optimized, including the wicker structures, the filling volume of working fluids and the preparation of high thermal conductivity graphene film. Compared to the conventional copper-based heat pipe, our graphene enhanced heat pipe improves the specific cooling capacity more than 3 times. Further, COMSOL Multiphysics was used to establish the cooling model for graphene enhanced heat pipe. And the equation for quantifying the contribution factor from container and phase change was established. Finally, a graphene/copper composite heat pipe was studied to further improve reliability and mechanical strength.

(2) Hexagonal Boron Nitride Enhanced Heat Spreader

In this work, a hexagonal boron nitride based heat spreader was prepared by electrospinning with polyvinylpyrrolidone. After electrospun, the hexagonal boron nitride nanosheets are aligned along the fiber, and thus increasing the thermal conductivity. At first, the exfoliation technology was investigated. The result shows that a mixture of water and isopropanol (Vwater:VIPA=1:3) shows the highest exfoliation efficiency. With the optimized hexagonal boron nitride particle geometry and loading, the in-plane thermal conductivity of hexagonal boron nitride based heat spreader reaches 22 W m-1 K-1, this value is comparable to most of the reported work. Particularly, such electrospinning process is constant and scalable, showing high potential for mass-production.

Chapter 3 still focuses on the application of utilizing graphene-like 2D materials for energy management but specifically in energy storage. Based on the ultrahigh electric mobility, large surface area, flexible, lightweight properties, graphene is an attractive option for energy storage. Therefore, graphene was investigated for electrical double layer capacitors and in-plane micro-supercapacitors in this chapter.

(1) Graphene Enhanced Electric Double-layer Capacitor

In this work, a scalable soft template strategy was developed to prepare graphene foam with high electrochemical performance as electrode for supercapacitors. The specific surface areas and wettability of graphene foam is tailored by doping. Further, density functional theory simulation reveals why increasing the polarity of graphene largely improves its wettability. Afterwards, the unique porous structure, low ohm resistance, and high electrical conductivity largely improve the electrochemical performance of graphene foam electrodes and thus achieve ultrahigh specific ca pa city (550 F g-1), cycling sta bility ( 96.1% ca pa city retention after 10 000 cycles at a high current density of 10 A g-1), and outstanding rate capability (308   F   g-1 a  t 100   A   g-1).

(2) Graphene Based In-plane Micro-supercapacitor

In this work, graphene assembled film was used to replace the conventional silicon wafer for fabricating flexible and high thermal performance micro-supercapacitors. The result shows that such replacement decreases the surface temperature of micro-supercapacitors by 4 °C, and the graphene based micro-supercapacitor present a similar electrochemical behavior with the referenced silicon based micro-supercapacitor. In addition, the graphene assembled film substrate can work as heat spreader for micro-supercapacitor, thus saving spaces and optimizing the following packaging procedures. This work paves the way for utilizing graphene assembled film in semiconductors.

Chapter 4 presents the application of using functional graphene-like 2D materials for interface enhancements due to their high Young’s module, large surface area, anti-friction, etc. Graphene-like 2D materials enhanced composites and bio-application are two of the main categories for the commercialization of interface enhancement. However, the graphene-like 2D materials suffer from π-π agglomeration, which leads to poor dispersibility in solvents and matrix. As a result, graphene-like 2D materials enhanced composites exhibit lower property than their theoretical expectations. At present, surface functionalization is the most effective strategy to encounter the π-π agglomeration. Therefore, this chapter explored the application of using functional graphene-like 2D materials in composites, including graphene enhanced water-borne epoxy coatings and hexagonal boron nitride enhanced cement repair materials.

(1) Graphene Enhanced Water-borne Epoxy Coating

Graphene was used to lower the coefficient of friction and extend the lifetime of the water-borne epoxy coating in this work. To improve the dispersibility and the compatibility with epoxy, p-hydroxybenzene diazonium salt was prepared to functional graphene. With the optimized geometry and loading, 30 times less coefficient of friction than graphene-free coatings were achieved. And the wear-out time is more than 2 times longer than the three commercial graphene oxide enhanced coatings. This result is confirmed by Applied Nanosurface AB, Sweden. Besides, mass production technology up to 300 g per batch was developed for the functional graphene. The geometry of graphene was optimized, and the result shows that with the same functional groups, the larger graphene sheets show higher tribological performance than their smaller encounters. Finally, this functionalization strategy was further developed to improve the dispersibility of carbon nanotubes too.

(2) Hexagonal Boron Nitride Enhanced Cement Repair Material

This work explored the application of using hexagonal boron nitride to enhance cement repair materials. To improve the dispersibility in cement repair materials and the adhesion with substrates, hexagonal boron nitride was functionalized by carboxymethyl cellulose. After functionalization, the surface zeta potential of hexagonal boron nitride decrease from -5.61 mV to -55.07 mV, and thus largely improves its dispersibility. Results show the incorporation of hexagonal boron nitride improve mechanical strength of cement repair materials by contributing to forming alite. Besides, for the repair material containing h-BN, most of the failure happened at the interface repair material/concrete, while the failure is mainly happening in the concrete for the sample containing FBN. Cooperated with a local cement company (Lanark AB), this work has demonstrated the commercial application as repair materials for walls.

Besides, we studied the functional graphene quantum dots for mRNA based drug delivery platform. After complexed with mRNA, the transfection efficiency of the graphene quantum dots based drug delivery platform is 25% with a formation concentration as 4000 ng mL-1. A comparable transfection efficiency could be achieved at much lower doses if the ratio between the carrier and the cargo is optimized. This graphene quantum dots based drug delivery platform exhibits excellent processability. This work describes a potentially strategy for prepare stable and effective mRNA delivery systems.