Chapter 3. Energy Conversion: Nanostructured Double Perovskite Cathode with Low
3.3. Results and Discussions
3.3.2. Characteristics of GBSCF–YSZ composite depending on sintering temperature
Figure 3-2 SEM images of GBSCF–YSZ composite cathode sintered at a) 600, b) 700, and c) 800 oC for 4 h in air (scale bars correspond to 2 mm). d–f) STEM HAADF images of GBSCF particles of the cathode.
To study the dependence of microstructural change on sintering temperature, scanning electron microscopy (SEM) images of GBSCF–YSZ composites sintered at different temperatures were recorded (Figure 3-2 a–c). Figure 3-2 d–f shows scanning transmission electron microscopy (STEM) high angle annular dark field (HAADF) images of particles on the cathodes shown in Figure 3-2 a–c. A cross-sectional view of the GBSCF–YSZ composite provides more information on the realistic structure of GBSCF particles infiltrated into the YSZ. For the GBSCF–YSZ composite, nanosized GBSCF particles cover the surface of the porous YSZ scaffold, showing an increase of particle size with increased sintering temperature. In Figure 3-2a, the microstructure of GBSCF–YSZ sintered at 600 oC can be described as not-fully-grown particles due to insufficient heat treatment. It is confirmed by Figure 3-2d that the porosity of the electrode seems to be too low to form long TPB although very fine pores can be observed. Therefore, the sintering temperature was set to 700 oC to obtain fully-formed particles. In the case of GBSCF–YSZ sintered at 700 oC, nanosized particles uniformly cover the YSZ scaffold with sufficient interconnections between the particles to transfer electrons and adequate porosity to form long TPBs (Figure 3-2 b and e). Also, the villus-like structure composed of nanoparticles improves the structural characteristics of the
GBSCF–YSZ composite sintered at 700 oC, including porosity, surface area, and interconnection.
The microstructure of GBSCF–YSZ sintered at 700 oC shows the structure of an infiltrated cathode, as proposed by many researchers.
The researchers suggested a model, demonstrating that the surface area strongly affects Rp.[8,31–33]
In the literature, it has been argued that a microstructure composed of small particles with a uniform distribution over YSZ is close to an ideal cathode microstructure because it is effective for maximizing the electrochemically active sites (TPB+2PB) while also providing sufficient porosity for effective gas diffusion. In this regard, the optimized microstructure of GBSCF–YSZ sintered at 700 oC is expected to improve the electrochemical properties of the cathode since the microstructure is similar to the ideal model explained above. Sintering at 800 oC induces a chemical reaction between GBSCF and YSZ, possibly due to excessive heat treatment, as confirmed by XRD results. The destruction of the villus-like structure and the agglomeration of particles sintered at 800 oC (Figure 3-2 c and f) decrease the electrochemically active surface area.
Schematic drawings of GBSCF–YSZ
Figure 3-3 Schematic drawings of the GBSCF–YSZ composite consisting of a–c) a villus-like structure with small particles (optimized sintering temperature) and d–f) destruction of the villus-like structure with large particles (excessive heat treatment).
Schematic drawings of the infiltrated GBSCF are shown in Figure 3-3 for better clarification of the microstructural changes occurring at various sintering temperatures. The GBSCF–YSZ composites are depicted as an array of YSZ rods growing from the dense YSZ backbone that are covered by GBSCF nanoparticles. In Figure 3-3 a–c, the villus-like structure composed of nanoparticles accelerates the diffusion of oxygen in the cathode.
The smaller size of the particles provides shorter oxygen ion conducting paths and corresponding faster oxygen ion diffusion from the active sites to the electrolyte. In contrast, the destruction of the villus-like structure and the agglomeration of particles prolong the path of oxygen gas diffusion from the atmosphere to active sites and lead to lower concentrations of oxygen gas around the active sites (Figure 3-3 d–f). Consequently, a diffusion of oxygen ions is limited in the cathode on account of the increased particle size.
Area specific resistance
AC impedance spectra was recorded using a symmetric GBSCF–YSZ/YSZ/GBSCF–YSZ cell to obtain the area specific resistance (ASR) for the GBSCF–YSZ composite. Figure S1 in the Supporting Information shows Nyquist plots measured at 650 oC and Arrhenius plots for various sintering temperatures. Electrochemical impedance spectroscopy is generally used to describe all resistances related with the electrode and electrolyte of a cell, involving the gas–cathode and cathode–electrolyte interfaces. From the spectra, the difference between the intercepts at the real axis of the Nyquist plots indicates the ASR, which is the non-ohmic resistance of the composite cathode. In the Nyquist plots of a symmetric cell, the ohmic resistance is eliminated, allowing direct comparison of ASRs of the cathodes sintered at various temperatures. Figure S1a reveals that the measured ASR values reach 0.048, 0.042, and 0.086 cm2 for GBSCF–YSZ sintered at 650, 700, and 750 oC, respectively. Figure S1b indicates that the GBSCF–YSZ sintered at 700 oC has the lowest value among the ASRs of as-sintered cathodes in the operating temperature range of 500–
600 oC. As expected and observed in Figure 3-2 and Figure 3-3, the lowest ASR value of the composite sintered at 700 oC can be mostly ascribed to microstructural improvement, including the highest surface area (3.83 m2 g-1, Figure S2), adequate porosity, and sufficient connections between nanoparticles. The GBSCF–YSZ composite sintered at 700 oC is, therefore, chosen for subsequent measurements and is hereafter denoted as GY700.