Ⅲ. Proton Conductivity Test on h-BN Based Composite Membrane
3.2 Experimental Section
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Ⅲ. Proton Conductivity Test on h-BN Based Composite
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electrochemical reaction which was applied in this setup was electrolysis of water molecule to generate protons. Protons were produced at Pt anode where oxygen gas evolves as a byproduct, pass through the membrane and meet Pt cathode where the protons meet with electrons to generate hydrogen gas. Each Pt plates were hold by stainless steel support and nuts and screws tightened the support to avoid any gaps between the sandwich. Removing the gaps ensured that the plates were not saturated by the gas molecules on the surface of electrodes and allowed to get linear I-V curves.
Figure 3.2.2b shows actual images of the experimental setup that are used for the proton conductivity test.
For through-plane proton conductivity calculations, investigations on the conductance of membranes were carried out using potentiostat/galvanostat/FRA (ZIVE SP2, WonATech, Korea) and operated via a potentiostatic method under a dc measurement to measure the membrane resistance over a range of -100 ~ 100 mA in a 1 mA/sec scan rate. The potentiostatic measurement was performed at room temperature, with no additional humidification process to the composite membranes but only kept them in 1 M HCl prior to use.
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Figure 3.2.1. a) Schematic showing an experimental design that I followed from the literature20 which measures through-plane conductivity of the membrane. b) Equation for proton conductivity (σ, S/cm) calculation, where T (cm) is the thickness of the membrane, G (S) is the conductance obtained by current sweep in the potentiostat measurement and A (cm2) is the area of the measured membrane.
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Figure 3.2.2. a) Schematic illustrations of proton measurement setup and b) actual images for the setup to measure proton conductivity of composite membranes.
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3.3. Results and Discussions
3.3.1. Proton conductivity calculation
Figure 3.2.2 illustrates the schematics (a) of proton measurement setup and the actual images (b) for the proton conductivity. The proton conductivity (σ) was calculated using the equation,20 σ = GT/A where T is the thickness of the membrane, G is the conductance obtained by current sweep in the potentiostat measurement and A is the area of the membrane.
Table 1 shows experimental conditions and results of the proton conductivity test. Here, proton conductivity of N117 was tested in order to make sure that our system is confident and in accordance with the obtained values from other literatures.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 make sure of its reproducibility. The representative I-V curves of each membrane is shown in Figure 3.3.1. The measured thickness of each membrane was from fully swollen membrane and was measured by Vernier calipers. Area of the membranes were fixed as 2 x 2 cm2. The calculations for the proton conductivity are as follows:
Nafion 117: 𝜎 =(12.52 𝑆)(0.025 𝑐𝑚)
4 𝑐𝑚2 = 𝟕𝟖. 𝟐𝟖 𝒎𝑺/𝒄𝒎 h-BN1/Nafion/hPTFE: 𝜎 =(11.13 𝑆)(0.021 𝑐𝑚)
4 𝑐𝑚2 = 𝟓𝟖. 𝟒𝟏 𝒎𝑺/𝒄𝒎 h-BN5/Nafion/hPTFE: 𝜎 =(7.95 𝑆)(0.020 𝑐𝑚)
4 𝑐𝑚2 = 𝟑𝟗. 𝟕𝟔 𝒎𝑺/𝒄𝒎 h-BN10/Nafion/hPTFE: 𝜎 =(6.81 𝑆)(0.018 𝑐𝑚)
4 𝑐𝑚2 = 𝟑𝟎. 𝟔𝟓 𝒎𝑺/𝒄𝒎
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Table 1. Experimental conditions and results of the proton conductivity test. The conductance was obtained from the slope of the I-V curve (G=I/V, S), of which the measurement was triplicated to make sure of its reproducibility. The measured thickness of each membrane was from fully swollen membrane and was measured by Vernier calipers. Area of the membranes were fixed as 2 x 2 cm2.
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Figure 3.3.1. The representative I-V curves of each membrane.
-10 -5 0 5 10
-100 -50 0 50 100
Current (mA)
Voltage (mV)
BN1/Nafion/PTFE BN5/Nafion/PTFE BN10/Nafion/PTFE N117
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3.3.2. Analysis on proton conductivity of h-BN based composite membrane
From the measurement, the obtained conductance of N117, h-BN1/Nafion/hPTFE, h- BN5/Nafion/hPTFE, and h-BN10/Nafion/hPTFE were 12 ± 0.4 S, 11 ± 0.2 S, 8 ± 0.2 S, and 7 ± 0.3 S, respectively, which led to the conductivity of 76 ± 2.3 mS/cm, 59 ± 1.3 mS/cm, 40 ± 1.0 mS/cm, and 31 ± 1.3 mS/cm, respectively. This shows the decreasing trend in not only the conductance but also in the conductivity as the wt % of h-BN increases. Based on the investigation on the thickness of the h- BN flakes existing in the membranes from cross-sectional TEM result, we can rationalize that the increase in the thickness of the flakes negatively affects to the conductance and sequentially to the conductivity. The result let us think of approaches to reduce thickness of the h-BN flakes more to enhance the proton conductivity, which method can be found in, such as, the optimization of time in probe-type sonication of the h-BN in Nafion dispersion mentioned early in this research.
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Ⅳ. Conclusion and Future Plan
4.1. Conclusion
In the present work, h-BN based composite membrane were fabricated based on the rationale that h- BN is an excellent proton conductive material. With aid of Nafion solution which has an amphiphilic nature to grab both hydrophobic h-BN and hydrophilic PTFE together, and is also known for good proton exchanging property, the size-reduced h-BN flakes were impregnated into hydrophilic PTFE support which was selected in order to enhance 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 investigated using SEM and TEM measurement. From the fabricated composite membranes, conductance was measured to give proton conductivity by using the equation σ = GT/A. The calculated proton conductivity of N117 ensured that our system is in confidential level, and the value was used to compare proton conductivity of fabricated composite membranes. From the calculation and from the comparison, decreasing trend in the conductivity versus increasing wt % of h-BN in composite membranes was observed, which was in accordance in the morphological analysis of the h-BN dispersion and of the composite membrane.
4.2. Future Plan
From this research, the trend was observed in the proton conductivity with regard to difference in the amount of exfoliated h-BN ratios in the composite membrane. This, however, needs to undergo an optimization to give enhanced proton conductivity value, since considerable amount of h-BN flakes were not fully exfoliated from 1 h probe-type sonication condition. A brief sonication time-dependent dimension size experiment was conducted by enhancing the time from 1 h to 2 h. The relevant SEM images from a sample spincoated by a 2 h probe-type sonicated h-BN/Nafion dispersion confirmed a decrease in the dimension of the h-BN flakes compared to the sample with 1 h probe-type sonication.
The time will, in turn, undergo an optimization to give h-BN based composite membrane with high conductivity which will be applied in a real fuel cell system.
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