Following the morphological characterization of the fabricated composite membrane on its top, bottom and transverse views, not only the confirmation of the homogeneous impregnation is shown, but also the trend as the weight percentage of h-BN increases. The result tells us the trend of decreasing proton conductivity as the weight percentage of h-BN increases, which means that more time is required to exfoliate multiple layers of exfoliated h-BN flakes impregnated in hPTFE matrix. Equation for the calculation of the proton conductivity (σ, S/cm), where T (cm) is the thickness of the membrane, G (S) is the conductance obtained by the current swing in the potentiostat measurement and A (cm2) is the surface area is of the membrane. measured membrane.
The conductance was obtained from the slope of the I-V curve (G=I/V, S), the measurement of which was tripled to ensure its reproducibility. In addition to their high proton conductivity, h-BN membranes are thermally stable up to 850℃, which is the strength of the 2D material as a proton exchange membrane at elevated temperatures compared to traditional polymer-based proton exchange membrane. Two main methods exist in the preparation of h-BN-based 2D materials: exfoliation of the 2D materials, both mechanically and chemically, and a chemical vapor deposition (CVD) method to form mono- or single-layer h -BN membrane to grow.
Speaking of the exfoliation method, many researchers have exerted their efforts to explore the proton conductivity of exfoliated h-BN-based 2D crystals. Among them, Hu et al reported. in 2014 proton conductivity of mechanically exfoliated monolayers of h-BN, graphene and MoS2.2. They measured proton current I over voltage V and obtained the conductance S = I/V. From this they calculated the area conductivity σ = S/A, where A is the area of the membrane exposed to current flow.
Furthermore, the conductivity of protons decreases significantly as the number of layers increases in all 2D materials tested, indicating that protons have a challenge in penetrating multiple layers of the membranes.
Proton conductivity mechanism of h-BN
Proton conductivity mechanism of h-BN in aqueous solutions
Research Purpose and Rationale in Membrane Fabrication
Therefore, a significant amount of research has been conducted on the use of h-BN in the field of 2D materials. Although CVD-grown monolayer h-BN exhibits the highest proton conductivity among h-BN-based 2D materials, careful consideration is needed to handle the material during use. This prompted some researchers to investigate h-BN based composite membranes by introducing other materials to form composite membranes and compensate for the poor processability.
Although much research has been done to investigate composites consisting of h-BN with Nafion or h-BN with PTFE, 11-15 little is known in the study of h-BN impregnation in PTFE to form the composites.
- Preparation of h-BN Dispersion
- Preparation of exfoliated h-BN
- Impregnation of h-BN dispersion into PTFE
- Membrane Characterization
After rolling, the other side of the hPTFE is also subjected to drop casting and rolling with the same applied pressure. This step ensures the impregnation of h-BN dispersion from top to bottom of the hPTFE to make a composite. After the coating method, the free-standing composite membranes were dried under room temperature for 30 min and then under 60 ℃ for an additional 30 min.
Milky, homogeneous and shrunken composite membranes were obtained which underwent pretreatment for membrane activation before use. Schematic illustrations of a) preparation of h-BN-based composite membrane, b) sliding glass frame holding hPTFE support, and c) real images of composite membrane fabricated by this preparation method. The surface and the cross-sectional morphology of h-BN in dispersion and in composite membranes were obtained from Scanning Electron Microscope (SEM, Nova Nano230).
For a purpose of measuring the dimension and the thickness of h-BN in dispersions and in composite membranes, a transmission electron microscope (TEM, Tecnai G2 F20 X-Twin) was performed. Sample preparation for the analysis of configuration and the thickness of h-BN in a dispersion was done by drop casting of h-BN dispersion diluted in deionized water by a factor of one hundred. In order to analyze the configuration of exfoliated h-BN flakes in composite membranes, ultra-microtome (RMC CR-X) was applied to obtain cross-sections of the samples.
Results and Discussions
- Morphological characterization of pristine h-BN flake
- Morphological characterization of h-BN dispersion
- Morphological characterization of exfoliated h-BN
- Morphological characterization on h-BN based composite membrane
The images show the increase in the amount of dispersed white flakes as the weight percentage of h-BN increases, indicating that the white flakes are exfoliated h-BN flakes. Although the identification of the white species as the h-BN was possible, it was difficult to identify the size and thickness of the exfoliated flakes just by looking at the top view images. This is because the edges of the h-BN flakes were blurred by the mixed Nafion solution that was also centrifuged during the sampling process.
Therefore, measurement of cross-sectional images of the spin-coated h-BN dispersion was required for the accurate measurement of the dimension and thickness of the exfoliated h-BN in Nafion. The cross-sectional image of the samples was obtained by spin-coating the dispersion on a SiO2. However, this was not to provide cross-sectional SEM images of the sample, as 5 wt% Nafion solution was not viscous enough to hold h-BN flakes on the substrates.
From the obtained images in Figure 2.3.4, we can clearly see that not only the amount of exfoliated h-BN, but also its size and thickness increases as the weight percentage of the h-BN increases. The attachment of the thoroughly impregnated h-BN composite membrane was one of the main research objectives in this study. The pores of the hPTFE were filled with h-BN flakes surrounded by polymeric structures of Nafion.
In the images, the pores of hPTFE supports were not observed, indicating that the impregnation was done from top to bottom of the hPTFE support. This claim is further strengthened when we look at the cross views of the composite membranes. Relatively low magnification SEM images in Figure 2.3.7g-i show the overall cross-sectional images of the composite membrane confirming the well-done impregnation by h-BN dispersions.
In the relatively high magnification SEM images in Figure 2.3.7k-m, we can see that the dimension and thickness of the horizontal white flakes as the wt % of saturated h-BN flakes increases. The horizontal flake shapes are due to the pressure applied by the Doctor Blade method in the impregnation process. On the other hand, we can conclude in the SEM cross-section analysis that the impregnation from the top to the bottom of the composites was confirmed and the tendency to increase the amount of flakes also appeared.
From the analysis, the thickness of the impregnated h-BN was obtained and compared based on the differences in weight. This result is confirmed from Figure 2.3.8g, which shows an increase in the average flake thickness.
Proton Conductivity Test on h-BN Based Composite Membrane
- Pretreatment of composite membrane for proton conductivity measurement
- Design and setup for proton conductivity measurements
For through-plane proton conductivity calculations, conductance studies of membranes were performed using potentiostat/galvanostat/FRA (ZIVE SP2, WonATech, Korea) and driven via a potentiostatic method under a DC current measurement to measure the membrane resistance over a range of mA at a scan rate of 1mA/sec. The proton conductivity (σ) was calculated using the equation,20 σ = GT/A where T is the thickness of the membrane, G is the conductivity obtained by current sweep in the potentiostat measurement and A is the area of the membrane. Here, the proton conductivity of N117 was tested to ensure that our system is safe and consistent with the values obtained from other literature.22,23 The conductance was obtained from the slope of the I-V curve (G=I/V, S) , of which the measurement was triplicated to ensure its reproducibility.
This shows the decreasing trend in not only the conductivity but also the conductivity as the wt% of h-BN increases. Based on the study of the thickness of the h-BN flakes present in the membranes from cross-sectional TEM results, we can rationalize that the increase in the thickness of the flakes negatively affects the conductance and sequentially on the conductivity. The result lets us think about approaches to reduce the thickness of the h-BN flakes more to improve the proton conductivity, which method can be found in such as the optimization of time in probe-type sonication of h-BN in Nafion dispersion mentioned early in this study.
In this work, h-BN-based composite membranes were fabricated on the grounds that h-BN is an excellent proton conducting material. Using Nafion solution, which has an amphiphilic character to interweave both hydrophobic h-BN and hydrophilic PTFE and is also known for good proton exchange properties, the size-reduced h-BN flakes were impregnated into hydrophilic PTFE support, which was selected in order to increase the hardness of the composite. Three different wt% of h-BN flakes were impregnated to produce three different composite membranes with different h-BN ratios, and they were morphologically examined by SEM and TEM measurement.
From the fabricated composite membranes, the conductivity was measured to give the proton conductivity using the equation σ = GT/A. The calculated proton conductivity of N117 ensured that our system is at the confidential level, and the value was used to compare the proton conductivity of the fabricated composite membranes. From the calculation and from the comparison, the decreasing trend of the conductivity against increasing wt % of h-BN in the composite membranes was observed, which was consistent with the morphological analysis of the h-BN dispersion and the composite membrane.
From this study, the trend was observed in the proton conductance with respect to the difference in the amount of exfoliated h-BN ratios in the composite membrane. However, this needs to be optimized to obtain an improved proton conductance value as a significant amount of h-BN flakes were not completely exfoliated after 1 hour of probe-type sonication. The relevant SEM images of a sample spin-coated with a 2-hour probe-type sonicated h-BN/Nafion dispersion confirmed a decrease in the size of the h-BN flakes compared to the sample with probe-type sonication 1 hour.
Results and Discussusions
- Proton conductivity calculation
- Analysis on proton conductivity of h-BN based composite membrane
Conclusion and Future Plan