In particular, the graphene sandwich structure has been extensively studied as a container for well-defined 2D confined spaces. To understand chemical reactions in confined spaces, it is very important to create a well-defined 2D confined space. This paper discussed our approaches to understand the organic chemical reaction in the confined space of graphene sandwich and the use of h-BN for a proton exchange membrane.
The first part of the thesis concerns the use of graphene sandwich nanoreactors for the implementation of organic chemical reactions. Studies have observed phase transitions or hydrolysis of inorganic materials in the graphene sandwich structure, but organic chemical reactions in this structure have not been reported to date. To understand the reactions in confined spaces, it is important to study the organic chemical reactions in the graphene sandwich structure.
Research Background for Two-Dimensional (2D) Materials Membrane
Importance of confined space in chemistry
Graphene sandwich for confined reaction
Proton conductivity of h-BN
- Proton conductivity mechanism of h-BN
- Proton conductivity mechanism of h-BN in aqueous solutions
- Proton conductivity through exfoliated 2D crystal
- Chemical vapor deposition h-BN for proton exchange membranes in DMFCs
- Chemical vapor deposition h-BN for proton exchange membranes in battery
Research objectives and approaches
Graphene Sandwich for Confined Vessel
- Introduction
- Experimental process
- Growth of graphene
- Preparation of dopamine and oPD solutions
- Preparation of graphene sandwich vessel containing monomers and polymerization by
- Theoretical calculations
- Results and discussions
- Characterization: polymerization of dopamine in graphene sandwich vessel
- Crystallinity of polydopamine in graphene sandwich vessel
- Structural dependency of monomers in graphene sandwich vessel
- Theoretical calculations
- Effect of confined space for polymerization
- Proposed structure of polydopamine in graphene sandwich
- Improved sheet resistance and mechanical properties of G/polydopamine/G
- Optical property of 2D polydopamine
- Extraction of polydopamine from graphene sandwich
- Reference
UV-vis-NIR spectra before and after polymerization using f) opD and g) EDOT on sandwiched graphene. Diffraction patterns (insets) showed that both opD and PoPD showed ring patterns, indicating the polycrystalline structure. Diffraction patterns (insets) showed that PEDOT showed ring patterns, indicating the polycrystalline structure. e) IFFT image of PEDOT shows small size of crystalline domain.
Comparison of unclamped and clamped samples. a) XPS and b) Raman spectra of D/G and PD/G. PD/G is prepared by heating and XPS-attached polydopamine is successfully synthesized. Many five-membered rings were observed in single-layer polydopamine sheets. d-f) Proposed structure of polydopamine base on each figure a-c. Figure 2.3.12. a) Flexural stability test of G/G and G/PD/G. G/PD/G maintained plate resistance even at high voltage compared to G/G. The transmission of G/G and G/PD/G is 95.4% and 94.84%, respectively. b) Young's modulus of G/G and G/PD/G was measured using nanoindentation test. Reflection contrast spectra of G/G (black), PD/G (red, PD is amorphous), G/D/G (green) and G/PD/G (blue, PD is 2D polycrystalline polydopamine).
Introduction
In general, atomic thickness two-dimensional (2D) materials such as graphene and hexagonal boron nitride (h-BN) are used to form some of the thinnest membranes due to very low transport resistance and very high permeation flux.1-6 In particular, it has been demonstrated that the high proton conductivity of graphene and h-BN makes them particularly suitable for proton exchange membranes, both theoretically7,8 and experimentally9-12. So far, these reports on the high proton conductivity of h-BN have led to the evaluation of the suitability of these materials for use in direct methanol fuel cells (DMFCs), where h-BN used with a sandwich-like structure is formed by inserting a large area of graphene or h -BN monolayer between two commercial Nafion films. The DMFC performance is improved because the sandwich-like composite structure prevents methanol crossover.14,15 However, it is noted that the graphene or h-BN monolayer acts as an additional membrane for proton transport along with the Nafion film, which is a primary membrane.
Therefore, due to their inherent defects and grain boundaries, large-area CVD-grown 2D materials cannot be used independently as proton exchange membranes in the fuel cells. As a result, 2D materials find limited application in proton exchange membrane fuel cells (PEMFCs), even though these materials exhibit high proton conductivity and excellent chemical stability.16-20 To overcome this limitation, it is crucial to develop methods to fabricate defect-free, large-area, single crystal h-BN or graphene. Furthermore, multilayers of h-BN or graphene with an AA′ stacking order would be even more desirable if unexpected defects occur in these materials during their production and transfer.
Herein, we report the fabrication of membrane electrode assemblies (MEAs) using large-area single-layer graphene (1L-Gr) and h-BN (1L-BN), and three-layer h-BN (single-oriented 3L-BN with AA) ′ . stacking) for use as 2D material-based Nafion film-free proton exchange membranes in fuel cell systems. To simplify the handling of the atomic-scale-thick monolayer and trilayer membranes containing the MEAs, an interfacial bonding layer (IBL) was coated on these membranes. Since 1L-BN and 1L-Gr exhibit intrinsic defects and grain boundaries, the fuel cells with the 1L-BN and 1L-Gr membranes showed high H2 permeation current densities of 10.08 and 9.1 mA cm−2 at 0.4 V, respectively, indicating that gas crossing through the membranes has taken place.
However, the fuel cell with an AA′-stacked 3L-BN membrane showed a low H2 permeation current density at 2.69 mA cm−2, which is even lower than that obtained for a fuel cell with a commercial Nafion 211 film (3.7 mA cm) -2). Furthermore, the results of an accelerated stress test (AST) revealed that the chemical inertness of the AA′-stacked 3L-BN membrane greatly improved the long-term stability of the corresponding fuel cell. These results indicate that the AA′-stacked 3L-BN membrane effectively prevented gas crossover, which is one of the primary causes of radical formation during fuel cell operation.
Also, the AST results show that the AA′-stacked 3L-BN membrane can be used at low humidity (30% RH) and high temperature (90 oC).
Experimental process
- Growth of single-layer graphene
- Growth of single-layer h-BN
- Growth of trilayer h-BN
- Preparation of trilayer h-BN by repeated transfer method
- Characterization of trilayer h-BN with AA′ stacking order
- Fabrication process of fuel cells using 2D materials
- Evaluation of fuel cells..............................................................................................5 6
- Fabrication of fuel cells using 2D Materials
- Performances of fuel cell using single-layer materials
- Performances of fuel cell using only IBL layers
- Performances of fuel cell using trilayer h -BN; turbostratic stacking and AA′-
- Characterization of 3L-BN-transferred and AA′-stacked 3L-BN
- Measurement of proton conductivity of performances of fuel cell using trilayer h-BN;
- Direct methanol fuel cell application of h-BN membranes
- Accelerated stress test (AST) for fuel cell with 2D materials
- Comparison of before and after AST for fuel cell with 2D materials
Schematic illustration of fuel cells formed using 2D proton exchange membrane materials. a), Schematic illustration of the fuel cell formed using AA'-stacked 3L-BN for the proton exchange membrane. Cyclic voltammetry showed that the fuel cell with transferred 3L-BN and accumulated AA' is working well. The polarization data, power density, H2 current density and CV curves, and of the fuel cell based on 3L-BN stacked on AA' are shown in Figs.
With low power density, high OCV and low H2 permeation current density were obtained in the fuel cell with AA′-stacked 3L-BN. Schematic illustrations of proton exchange membranes in a) 1L-BN , b) 3L-BN made by repeated transfer, c) AA′-stacked 3L-BN. 20 µm x 20 µm size AFM images of AA′-stacked 3L-BN on sapphire substrate (a and b were measured at different positions), indicating wrinkle-free surface.
The membrane based on AA′-stacked 3L-BN ensured proton transport, and its value is lower than that based on 1L-BN. We confirmed whether AA′-stacked 3L-BN exhibits desirable performance as a proton exchange membrane in DMFCs. We measured the evolution of OCV over time (100 h) of fuel cell with 1L-BN, AA′-stacked 3L-BN and Nafion 211 at the harsh condition.
3L-BN and 1L-BN stacked with AA′ show excellent chemical stability (see red and blue arrows for degradation rate after 100 h AST) compared to Nafion 211 (see green arrow). In the case of AA′-stacked 3L-BN and 1L-BN fuel cells, marginal changes in OCV values after 100 h of AST. Although the differences in H2 permeability current densities of 1L-BN and AA′-stacked 3L-BN cells before and after AST were quite similar, their capacities were different.
It is noted that interfaces between the catalyst (both cathode and anode) and the AA′-stacked 3L-BN membrane are not damaged before and after AST. Also, there is no cracking and degradation in the AA′-stacked 3L-BN membrane after AST. The current density and power density of Nafion 211 decreased dramatically after AST (see green arrow) compared to AA′-stacked 3L-BN and 1L-BN (see red and blue arrows, respectively).
Pt NPs/h-BN hybrid system for cathode
Introduction
In 2018, I reported a hexagonal boron nitride (AA'-stacked 3L-BN) membrane stacked with three AA' layers as a proton exchange membrane: A fuel cell based on the AA'-stacked 3L-BN membrane exhibited excellent chemical and thermal stability compared to the commercial Nafion 211 membrane.1 However, the current density and power density of the fuel cell based on Nafion 211 were higher than those of the fuel cell based on stacked 3L-BN in AA'. For a long time, researchers have focused on the cathodic reaction of fuel cells, especially on the oxygen transport resistance in the cathodic system.2-4 The oxygen transport resistance and catalytic active sites constitute an important factor affecting the reaction of cathode.5,6 Minimizing the oxygen transport resistance while maintaining the active catalytic sites is important for increasing the current density and power density of the fuel cell based on 3L-BN stacked on AA'. Electrochemical and atomic layer deposition (ALD) was used to prepare Pt nanoparticles.7-9 I prepared thin and uniform Pt nanoparticles on SiO2 by ALD.
The Pt nanoparticles were transferred onto the AA′-stacked 3L-BN using a conventional transfer method, and I used the Pt nanoparticles on h-BN as the cathode of the fuel cell. I will present the preliminary results of this fuel cell system and plans for a new system.
Experimental process
- Growth of trilayer h-BN
- Preparation of Pt on substrate by electrochemical deposition
- Preparation of Pt on substrate by atomic layer deposition
- Fabrication of fuel cells using 2D materials with prepared Pt nanoparticles
- Evaluation of fuel cells
The final structure of the new Pt NPs/h-BN hybrid system for the fuel cell cathode is depicted in Fig. After that I will deal with the new structure of Pt NPs/h-BN hybrid system for the fuel cell cathode. White Pt nanoparticles can be observed in the SEM images, indicating that Pt nanoparticles have been successfully deposited on the graphene surface.
ICP elemental analysis showed that the mass per unit area of Pt NPs on SiO2 was 0.36 mg cm-2. Therefore, Pt NPs on SiO2 by ALD could be used as a fuel cell cathode. A fuel cell based on the Pt NPs/h-BN hybrid system was fabricated and evaluated using methods similar to those described in Chapter 3.
As a cathode material, annealed Pt nanoparticles were transferred to the AA′-stacked 3L-BN/IBL/anode to obtain a Pt nanoparticles/AA′-stacked 3L-BN/IBL/anode. Through SEM analysis, I compared SEM images obtained before and after the transfer of Pt nanoparticles. In particular, Pt nanoparticles have been successfully transferred using typical transfer methods involving poly(methyl methacrylate) (Fig. 4.3.6).
I determined the H2 permeability current density of the fuel cell based on the Pt NPs/h-BN hybrid system (Figure 4.3.7). In addition, I studied a system composed of Pt nanoparticles on h-BN to reduce the resistance cost of Pt and oxygen in cathodic reactions. Using the ALD method, I obtained uniform Pt nanoparticles with a high density of 10–15 nm in size, which is the target size.
The prepared Pt nanoparticles were suitable for activating the fuel cell based on the new Pt/h-BN hybrid system.
Results and disucssions
- Pt deposition on graphene by electrochemical deposition
- Pt deposition on SiO 2 by atomic layer deposition
- Characterization of Pt NPs
- Fabrication of fuel cell based on Pt NPs/h-BN hybrid system and evaluation
Conclusion and Future Work
Conclusion
Future work
Wrinkle-Free Epitaxial Growth of Unioriented Multilayer Hexagonal Boron Nitride on Sapphire” Nano Lett. Shin* “Conversion of Langmuir-Blodgett monolayers and bilayers of poly(amino acid) through polyimide to graphene” 2D Materials. Jang* “Enhancement of radio frequency transmission properties of graphene through control of carrier concentration in the direction of high frequency transmission lines” Adv.
