*1.2. Conventional TPBs Extension in Electrodes*

In order to improve the stability and activity of the electrode/electrolyte interface, electrode modifications such as impregnation or infiltration can be regarded as an instrumental pathway. By infiltrating a layer of PrBaMn2O5+<sup>δ</sup> (PBM) into BaZr0.1Ce0.7Y0.1Yb0.1O3−<sup>δ</sup> (BZCYYb) electrode, a fuel cell substantially enhanced stability and performance at reduced temperatures [32].

The typical electrode TPBs extension microstructure of La0.75Sr0.25Cr0.5Mn0.5O3−<sup>δ</sup> (LSCrM)-impregnated anode has been manufactured via infiltrating 70% porous yttriastabilized zirconia (YSZ) matrixes with an LSCrM solution. The LSCrM is a principal electron conductive phase while the well-sintered YSZ porous anode supplies an ionic conduction route in every part of the electrode. Further, Silver (Ag) and Nickel (Ni) are complemented by nitrate impregnating methods for improving electronic conductivity and electrocatalytic activity. The various impregnated microstructures are shown in Figure 5 [33].

**Figure 5.** SEM micrographs of the cross-sections of: (**a**) pure YSZ anode backbone; (**b**) ~5 wt.% LSCrM-impregnated YSZ anode; (**c**) ~35 wt.% LSCrM-impregnated YSZ anode; (**d**,**e**) LSCrM/Ni/Ag (~32/6/2 wt.%) impregnated YSZ anode [33].

The interface polarization may be caused by a non-conducting phase at the interface resulting from the solid-state reaction between electrolyte and electrode. Control the interface reaction and extend the TPBs are both considering topics. A typical composite cathode is the adherence of meticulous SDC particles to the surface of crude BSCF cathode particles, which resulted from the mechanical admixture of Ba0.5Sr0.5Co0.8Fe0.2O3−<sup>δ</sup> (BSCF) + Sm0.2Ce0.8O1.9 (SDC) (70:30 in weight ratio). The phase reaction can contract merely at the interface between BSCF and SDC and insignificantly dominant in particles internal parts, thus, everywhere the whole BSCF cathode particles, individually, with the formation of a thin layer of novel (Ba, Sr, Sm, Ce) (Co, Fe)O3−<sup>δ</sup> perovskite phase at a firing temperature as has been shown in Figure 6. The fine SDC particles enclose the BSCF coarse particle and maintain a single phase of SDC and BSCF, individually. The inter-phase intimately adheres BSCF and SDC to reduce the interface polarization. Thus, the sintered BSCF + SDC electrode shows an area specific surface resistance reduction above the BSCF because of the amplified cathode surface area promoted by the meticulous SDC particles. An improved peak power density at 600 ◦C was achieved for a thin film of the electrolytic cell with the BSCF + SDC cathode fired from 1000 ◦C [34].

**Figure 6.** Schematic diagram for the sintering mechanism of BSCF + SDC composite cathode, redrawn from [34].

Other composite electrode examples are also presented as follows. The Ni-BZCY/SDC/ BSCF cell with an interfacial reaction that can be manipulated to form a secondary phase at anode Ni-BZCY and SDC with an electronic conductor to benefit cell performance and power output [35]. The secondary phases nickel aluminate spinel (NiAl2O4) and zirconium titanite (Zr5Ti7O24) formation by infiltrating a small amount of aluminum titanite (ALT, ~4 wt%) into the Ni-YSZ anode scaffold were found to suppress Ni coarsening and expand the electrode's TPBs [36,37].

Another TPBs extension method is an introduction of the anode functional layer (AFL) with fine microstructure at the anode/electrolyte interface to increase the TPB length and to restrain the activation polarization for hydrogen oxidation [38–40]. This kind of technique meets a trade-off between the enhancement of electrochemical performance due to the increasing TPB and the decrease in performance due to the increased gas diffusion resistance.
