# Development of Screen-Printable Nafion Dispersion for Electrochemical Sensor
## 3 Results
### Characterization of Nafion Membrane  
The presence of both hydrophilic sulfonic groups and hydrophilic PTFE backbone in Nafion has been reported to form certain zigzag structures with a low degree of crystallinity [28]. Crystallinity of Nafion is considered an important factor that affects the ion conductivity and mechanical strength. In fuel cell applications, a higher degree of crystallinity of Nafion is preferred to lower the hydrogen/methane crossover and enhance durability [28]. On the contrary, the ion mobility of Nafion is reduced with high crystallinity  
Figure 3: Yield stress analysis of modified Nafion inks by dynamic strain sweep. Black dotted line: storage modulus; red dotted line: loss modulus. Figure 2: Time-dependent viscosity (thixotropic property) of the modified Nafion ink measured by three-level flow method. and is not favored for electrochemical detection. The degree of crystallinity of modified Nafion as a function of curing temperature was characterized by X-ray diffraction. A lower angle scan was chosen due to the large lattice spacings in the sample. Commercial Nafion ink and modified Nafion inks were deposited onto zero background silicon wafers to minimize contribution of the substrate and allow for an accurate measurement. The commercial Nafion was cured at 55 \({}^{\circ}\)C. The modified Nafion inks were cured at different temperatures--55 \({}^{\circ}\)C, 90 \({}^{\circ}\)C, and 120 \({}^{\circ}\)C--to investigate the dependence of curing temperatures on the crystallinity. The degree of crystallinity (\(D\)) was calculated by the following equation:  
\[D=\frac{Crystalline\ Area}{Total\ Area} \tag{1}\]  
The results of X-ray diffraction are shown in Figure 4. It was found that the degree of crystallinity varies with the curing temperature of the Nafion. Increasing the curing temperature brings the crystallinity of the formulation more toward that of commercial Nafion. The overall intensity of the samples increases with the increase in the crystallinity of the Nafion. The modified Nafion ink cured at 120 \({}^{\circ}\)C has a diffraction peak at approximately 6.5\({}^{\circ}\) 20, which is not present in commercial Nafion or modified Nafion cured at other lower temperatures. The broad peak at approximately 38.5\({}^{\circ}\) 20 changes with the curing temperature significantly; interestingly, this peak is most intense when cured at 90 \({}^{\circ}\)C. The changes in the crystalline structure are a strong contributor to the properties of the cured Nafion and can be tuned accordingly. Overall, modified Nafion cured at 55 \({}^{\circ}\)C has the lowest degree of crystallinity (\(D\) = 32.3) with good film quality, while bubble issues were observed for membranes cured at the other two temperatures. Therefore, the curing temperature of modified Nafion was selected as 55 \({}^{\circ}\)C. The low degree of crystallinity indicates that the cured membrane is amorphous dominant with small, isolated crystal regions. Figure 5 shows the survey spectra of the commercial and modified Nafion. It is evident from the survey scans that, compared to the commercial Nafion material, the main difference is the modified Nafion material contains Na. Table 1 provides the elemental composition for each of two Nafion materials, showing comparable percent composition to that of the theoretical values [29]. Figure 4: XRD spectra of commercial Nafion and modified Nafion inks cured at different temperatures. The high-resolution scans of each of the two materials provides a more in-depth look at the chemical structure of the respective materials. Scrutinizing the high-resolution scans of the identified peaks from the survey scan of each of the two Nafion materials, starting with C1s at peak position of 292.0 eV, indicates the CF\({}_{2}\) backbone of the Nafion chemical structure, which is evident in both Nafion membranes.