*3.3. Psudo-Core-Shell Anode by Ultrasonic Spray Pyrolyzed Impregnation*

An La0.3Sr0.7TiO3 (LST) without Nb addition was prepared like SLTN in Section 2.1 and sintered at 1300 ◦C/6 h to form an approximately 500 μm thick porous substrate. The LSBC electrolyte with polyethylene glycol (PEG) as a chelating agent was prepared in an aqueous solution with a concentration of 0.01–0.1 M. via an ultrasonic nozzle that atomized nano-sized droplets of the LSBC solution by carrier gas into a hot zone. The precursor droplets were decomposed and deposited on a porous LST anode substrate at a temperature of 250–500 ◦C in the hot zone. An LSBC thin layer electrolyte was successfully deposited on and impregnated into porous LST anode support to form LST/iLSBC halfcells after co-firing at 1350 ◦C/6 h. This LSBC impregnation and droplets decomposition in porous LST created core-shell like structure, also termed as pseudo-core-shell or reverse impregnation structure. Continuous ultrasonic spray pyrolysis densified a 2.75 μm thin layer LSBC using 0.1 M precursor concentration, 450 ◦C substrate temperature, 2 mL/min solution flow rate and 25 mL spray volume [91].

Instead of anode function layer (AFL) [92], our ultrasonic spray pyrolyzed pseudocore-shell anode substrate improved contact and adhesion between the anode and the electrolyte. The dense electrolyte with a thickness of 2.75 μm, as shown in Figure 14a. The profile of the densified 2.75 μm-thick layer of LSBC (iLSBC) on the porous LST substrate is indicated by the EDS line-scanning analysis also shown in the Figure 14a. The top dense layer showed a clear Ce signal (red color) while a trace Ce signal was detected in the LST substrate region over 30 μm depth. The Ce impregnation into the porous LST anode effectively extended the TPBs from the interface between LSBC and LST into the porous LST inner structure. The LSBC impregnation into porous LST is also further identified by electron backscatter diffraction (EBSD) analyses [91].

Refer to LSTN-LSBC and BSNF-LSBC contained half-shell analyses, LST/iLSBC halfcell has approximately fitting to one ohmic resistance and two depressed impedances according to the RQ equivalent circuits, as shown in Figure 14c. This is equivalent to two parallel resistance (R)/constant phase element (CPE) circuits in series with an ohmic resistance of Re [75–81,91]. The AC impedances of LST/iLSBC half-cell decreases with the measured temperatures increase and diffusion polarization at low frequency decreases significantly as shown in Figure 14b,c. The ohmic resistance (Re) and diffusion impedance ReZ(d) significantly decreases and the decrease in ReZ(d) results in an increase in the equivalent capacitance. The impedances ReZ(i) and ReZ(d) of the fuel cell decrease indicated the extension of TPB sites due to electrolyte LSBC impregnation into the porous anode to become a pseudo-core-shell inner coverage. This results in more effective charge transfer and electrocatalytic activity due to longer TPB length and larger area.

**Figure 14.** Co-fired iLSBC/LST half-cell prepared by ultrasonic spray pyrolysis and measured at a temperature of 550–750 ◦C for (**a**) FESEM cross-section of iLSBC/LST with line scanning marking Ce (red color) and Ti (light blue color) elements, AC impedances among (**b**) 650–750 ◦C and (**c**) 550–600 ◦C and RQ fitting for 550 ◦C.

The porous microstructure of SOFC electrodes correlates with the TPBs, gas diffusion and concentration polarization. Adding an interlayer such as an AFL between the electrolyte and anode, and impregnating electrocatalytic particles into the porous anode effectively increased the TPBs and decreased the contact resistance between the electrolyte and the electrode [93–96]. However, the AFL and solution impregnation require additional processing and cost. Repeated impregnation by capillary action may decrease the efficiency of the reforming and electrocatalysis, possibly by hindering and even blocking the fuel gas path through the porous anode structure due to the serious agglomeration of impregnated particles [97]. The ultrasonic spray pyrolyzed thin layer electrolyte could impregnate into the porous anode during deposition to create intimate contact between the anode and electrolyte and act as a dense electrolyte on a highly porous anode support. The sound effects extended TPBs and adhesion of the thin layer electrolyte, enhancing the charge transfer and decreasing the gas diffusion resistances, as the evidence shows in Figure 14.

Our ultrasonic spray pyrolysis pseudo-core-shell deposition technique, using chelating solution precursor, does not require an additional loading press and low pressure or vacuum system. It is operated in an ambient atmosphere with easy adjustment of processing parameters, reduction of synthesis temperature and process cost, as well as

convenient operation. The Table 1 is our fabricated half-cell by ultrasonic spray pyrolysis in comparison with other reference anode supported full cells. In Table 1 [98–105], catalytic metals such as Ni or Ru are required in porous anodes for electrocatalytic activity and electronic conductivity enhancement for cells. The LSBC impregnated into LST from the LSBC electrolyte side and sequent formed dense LSBC thin layer electrolyte satisfying the electrocatalytic activity and conductivity requirements simultaneously. Our ultrasonic spray pyrolysis technique has a lower electrolyte preparation and phase formation temperature of 450 ◦C. The half-cell consisted of 2.75 μm thickness thin electrolyte with pseudo-core-shell anode could provide a peak power of 325 mW/cm<sup>2</sup> at 700 ◦C, which is comparable to other reference full cells' performance at 650 ◦C. The ultrasonic spray pyrolysis for pseudo-core-shell anode also extended TPBs, enhanced the electrolyte adhesion on porous anode, and deposited dense thin layer electrolyte, all in one process for a half-cell.

**Table 1.** Functional comparisons of anode-supported SOFCs with thin layer of ceria-based electrolyte operated in H2 (or humidified H2) as fuel and air as oxidant atmosphere (cited from the literatures and this work).


\* S.S.: porous stainless steel; SCSZ: 10ScSZ + 1 mol% CeO2 electrolyte.

In Table 1, Dogdibegovic et al. [98] used higher conductivity SCSZ electrolyte and backbones instead of YSZ, and conventional LSM cathode and SDC-Ni anode were replaced with Pr6O11 cathode and higher Ni content in anode. The power density of 1.56 Wcm−<sup>2</sup> was achieved at 700 ◦C. Such a result is the highest power density up to date using an H2-air system and almost approaches to the theoretical OCV value. However, the control of optimizing synthesis parameters is critical and must be careful concerning issues. For the comparison of similar electrolyte thickness (2.08 μm) using repeated spray pyrolysis process on Ni + YSZ anode [105] at 600 ◦C in Table 1, our future work will likely include spray pyrolysis equipment improvement, anode particle modification and full cell preparation for this anode supported half-cell to further enhance the electrochemical performance. In this overview, we demonstrate only half-cell with (pseudo-) core-shell electrode to achieve relative power density due to extending TPBS with simple, convenient, unexpansive and environmentally friendly technique. The full cells are under-working to promote the power density and study the core-shell interface electrocatalytic mechanisms. Our studies are further expected to apply to double ions (H+ and O2<sup>−</sup>) conducting low temperature fuel cells in the future.
