# Lotus Root Type Nickel Oxide-Carbon Nanofibers: A Hybrid Supercapacitor Electrode Material
## 3 Results and Discussion  
The color of the nanofiber produced by electrospinning was initially white and later changed to yellow or brown through oxidation stabilization heat treatment, and the sample after final carbonization was black. The morphological characteristics of the final carbonization samples were observed using a field emission-scanning electron microscope (FE-SEM), and the corresponding images are presented in Figure 2. When viewed at a low magnification, the samples showed long nanofibers with diameters ranging from 700 nm to 1.5 \(\upmu\)m. Additionally, these fiber samples displayed numerous internal pores and noticeable waviness on the fiber surface. This suggests that in contrast to CNFs fabricated solely from PAN [23], the portion of fibers containing PS is removed during the heat treatment, as shown in Figure 2a. PS is removed as it decomposes, and during this process, it plays an important role in forming the lotus root shape and additionally forms the internal pores and external etched surface of the electrospun nanofiber. This removal process results in the formation of internal pores and surface curvature in the CNFs. Interestingly, Figure 2b-e, illustrating various Ni content levels and temperature differences during the third heat treatment process, did not exhibit significant morphological distinctions. However, Figure 2f, where carbon was entirely removed, leaving only NiO, exhibited intensified fiber destruction and surface curvature. Nonetheless, it was apparent that the fiber's morphology, including internal pores, was retained, similar to the CNF sample. These findings suggest that CNFs acted as a morphological template. Figure 3a displays the results of the TGA analysis, which was performed to assess the thermal decomposition characteristics and NiO content of LC, LCN series, and LN samples. To investigate the changes occurring during the heat treatment process, thermal gravimetric analysis (TGA) was conducted under the condition of a constant temperature increase of 6 \({}^{\circ}\)C per minute in an air atmosphere using a TGA 2050 (TA Instruments) instrument, with temperatures rising up to 800 \({}^{\circ}\)C. The NiO content was determined by comparing the remaining mass to the initial mass, considering the reduction in carbon and moisture loss due to thermal decomposition within the measured temperature range. The contents of NiO were determined to be 0, 35.6, 40.3, 45.5, 48.6, and 94.3 wt% for LC, LCN-35, LCN-40, LCN-45, LCN-48, and LN, respectively. In the case of LC samples without NiO, the thermal decomposition behavior indicated a change due to the moisture removal from the sample between 60 \({}^{\circ}\)C to 100 \({}^{\circ}\)C. Subsequently, carbon decomposition began at 450 \({}^{\circ}\)C, culminating in complete carbon removal at 750 \({}^{\circ}\)C. In contrast, the LCN series samples exhibited a steady mass decrease from around 450 \({}^{\circ}\)C until approximately 600 \({}^{\circ}\)C, signifying carbon decomposition. No further mass change was observed above 650 \({}^{\circ}\)C.  
Figure 3b shows the results of XRD analysis, confirming the crystalline properties of the samples. In the case of LC, the XRD pattern demonstrated an amorphous carbon, while LN, where most of the carbon had disappeared, exhibited a distinct NiO structure. For the LCN series, diffraction peaks associated with graphite and NiO were evident in all samples after undergoing three heat treatment processes. As the NiO content increased, carbon content decreased, and the peak at 26.02\({}^{\circ}\) corresponding to carbon gradually diminished. Notably, with the increase in NiO content from the LCN series to LN, the peaks related to NiO gradually intensified, relative to the proportion of NiO in the composite. Therefore, these XRD results confirmed the formation of a NiO-CNF composite in the LCN series samples. The specific surface area and pore properties of the prepared samples were analyzed using Brunauer, Emmett, and Teller (BET) and Barrett, Joyner, and Halenda (BJH) measurements.