## 1 Introduction  
Solid oxide fuel cells (SOFC) have received much attention over the last decade due to advantages, such as a high efficiency, quietness, multiple fuels, and the inexpensive catalyst nature [1; 2; 3; 4]. One of the major challenges related to SOFCs is the short lifetime introduced by the high operating temperature. The decrease of the working temperature is an effective method to increase the lifespan of SOFC. Therefore, lower or intermediate temperature SOFC have been extensively studied in recent years [5; 6]. The electrochemical reactions in the SOFC electrode only take place at the so-called triple phase boundary (TPB), where oxygen ions, electrons, and gaseous species cohere. The reaction activity of the TPB sharply decreases with decreasing temperature. Therefore, it is significant for lower or intermediate temperature SOFC to have a long TPB. The TPB length is a critical geometric parameter of the electrode and dominates the SOFC and electrode performance [7; 8]. Meanwhile, the TPB itself is strictly associated with the electrode micromorphology [9; 10]. To further improve the electrode performance, many studies focused on deriving an optimal micromorphology from various fabricating processes, such as screen-printing [11; 12], freeze-casting/phase inversion [13; 14], co-precipitation [15], impregnation/infiltration [16; 17], and electrospinningning [18; 19]. Several types of electrodes with distinct microstructures have been successfully prepared. Conventional electrodes (CE), which are fabricated by sintering composite powders, have been widely and deeply studied both experimentally and theoretically. The TPB length of CE can be extracted from the detailed microstructure obtained, by focused ion beam-scanning electron microscopy (FIB-SEM) [20; 21] or X-ray diffraction [22]. Theoretical models for TPB calculation can be used to systematically study the impacts of the electrode properties, including the particle size, porosity, and component fraction on the TPB length. Among those models, the random sphere packing model has become popular [23; 24; 25]. However, to improve the electrode performance or lower the operation temperature, limitations of the CE, such as the incompatible thermal expansion coefficient of the anode (e.g., Ni and yttria-stabilized zirconia (YSZ)) and high polarization loss of the cathode [26; 27; 28], have to be overcome. To address those problems, a nanostructured electrode can be introduced to the SOFC, which can be achieved by impregnating nanoscale particles on the backbone surface of another phase [16; 29]. The nanostructure of the impregnated electrode (IE) leads to a tremendous enhancement of the TPB length and the impregnation phase volume fraction (e.g., Ni) is much lower than that of the CE [30]. From a phenomenological viewpoint, there are two distinct IE constituents: backbone and coated impregnation nanoparticles. These features constitute the basis of theoretical models because the IE is modeled in many cases as a sphere-packed backbone with a surface that is coated with nanospheres of another phase [26; 30]. Because the TPB length of the nanostructured electrode is directly related to the total backbone surface area [31], it is logical to explore new frameworks with increased surface areas to improve the electrode performance. The basic backbone formed by many one-dimensional nanostructures (e.g., nanofibers) possesses a high surface area and porosity [32], which implies a novel backbone structure. Because electrospinningning is a simple and effective method to produce uniform one-dimensional nanofibers with a high specific area and porosity, it has become a new state-of-the-art technology for the preparation of electrodes for SOFCs [33]. [PERSON] et al. [34] studied fibrous Ni-coated YSZ anodes, where the YSZ nanofibers were prepared via electrospinningning and then electrolessly plated with a layer of Ni. Their results suggested that the Ni loading is only 30 wt.%, that is, lower than that of CE.