Solid oxide fuel cells (SOFCs) directly convert chemical energy stored in fuels to electrical energy ^[1]. The traditional SOFC has the components Ni-YSZ (YSZ = yttria stabilized zirconia) anode,YSZ electrolyte,and (La,Sr)MnO₃-YSZ cathode and operates in the 800-1000 °C range ^[2,3]. To reduce the operating temperature of SOFCs,La_1-xSr_xCo_yFe_1-yO_3-δ (LSCF) materials with a high electronic and ionic conductivity (MIEC) and excellent catalytic activity for oxygen reduction are extensively studied as the SOFC cathode ^[4]. However,unlike (La,Sr)MnO₃ materials,(La,Sr)(Co,Fe)O₃ suffers significant Sr surface segregation and easily reacts with YSZ to form non-conductive phases of SrZrO₃ and/or La₂Zr₂O₇ even at 700 °C ^[5]. This leads to very large interfacial losses when the LSCF cathode is directly prepared on the YSZ electrolyte. Much effort has been devoted to preventing the formation of the non-conductive phases,such as adding Gd-doped CeO₂ (GDC) as an interlayer to avoid direct contact between the cathode and YSZ electrolyte ^[6,7,8] or infiltrating the cathode material (such as La_1-xSr_xCo_yFe_1-yO_3-δ) into the YSZ scaffold and sintering at low temperature to avoid the unwanted interfacial reaction ^[9]. However,these methods add to the cost and complexity of the cell fabrication and introduce new interfacial losses between the GDC interlayer and YSZ electrolyte. Therefore,it is useful to develop a new method for the direct application of an LSCF cathode on the ZrO₂-based (such as YSZ) electrolyte.

The interface between different phases can be changed by adding a proper modifier. For example,Co₃O₄ has been studied as a sintering aid to modify the microstructure of a Co- containing perovskite cathode and the interface of the cathode/electrolyte ^[10]. TiO₂ has been widely used as a photocatalyst because of its high stability,non-toxicity,and relatively low cost ^[11,12]. In the field of SOFC glass sealing materials,TiO₂ has been used to tune the crystallization temperature and microstructure of glass materials,and TiO₂-modified Bi₂O₃- BaO-SiO₂-R_xO_y (R = K,Ca,etc.) shows a good match of the thermal properties and firm adherence to the electrolyte and connector ^[13]. TiO₂ has been added to a CeO₂-based electrolyte to reduce the sintering temperature and improve the grain boundary conduction ^[14]. The addition of TiO₂ into an LSCF can prevent the reaction of LSCF with YSZ by the interaction between LSCF and TiO₂ and modify the interface of the cathode/electrolyte.

In this study,nanosized TiO₂ particles were added into an LSCF cathode by an ultrasound-assisted blending process. The TiO₂-modified LSCF cathode was directly applied on the YSZ electrolyte. The interaction of the LSCF,TiO₂,and YSZ oxides were studied by X-ray diffraction (XRD). The effects of TiO₂ addition on cell performance were studied by electrochemical analysis and scanning electron microscopy (SEM).

Anode-supported single cells with a thin film YSZ (10 μm) were fabricated by the tape casting method. The anode- electrolyte bilayer assemblies were sintered at 1295 °C for 3 h. The homemade La_0.6Sr_0.4Co_0.2Fe_0.8O_3-δ (LSCF) powder was synthesized by a citric acid ammonium-assisted Pechini-type method ^[15] and sintered at 1050 °C for 3 h. The TiO₂-modified LSCF cathode was made by the ultrasound assisted process as follows. Commercial TiO₂ (Zhenjiang,China) nanoparticles were added into deionized water with a TiO₂ concentration of 0.25 mol/L. The LSCF powder was blended with various amounts of TiO₂ nanoparticles using an ultrasonic bath with a frequency of 25 kHz and a nominal power of 600 W for 30 min. The cathode slurry consisting of dried LSCF-TiO₂ composite oxides,organic binders,and solvent was coated on the YSZ electrolyte and sintered at 850 °C for 2 h. The thickness of the cathode was 35 μm.

The single cells were tested in the homemade electrochemical device. Humidified H₂ (3% H₂O) and pure O₂ flow were supplied as fuel and oxidant,respectively. The current-voltage curves and corresponding power density (I-V-P) of single cells were measured using the two-electrode four-wire method at 600-750 °C. Electrochemical impedance spectra were measured under open circuit condition using a Solartron 1260 frequency response analyzer with a Solartron 1287 electrochemical interface. The frequency ranged from 1 x 10⁶ to 0.08 Hz with an amplitude of 10 mV.

Powder XRD patterns were collected in the 2θ range of 20°-80° on a Rigaku D/Max-2500/PC X-ray diffractometer operated at 40 kV and 200 mA using Cu K_α (λ = 0.15406 nm) radiation. The average crystallite size of the cubic phase was calculated from the Scherrer equation,where the Scherrer constant (particle shape factor) was taken as 0.89. The SEM photographs of the TiO₂-modified LSCF cathode were taken with a JSM7800F microscope equipped with a field emission gun at 5 kV.

The interaction between LSCF and TiO₂ was investigated by powder reaction. LSCF powder and TiO₂ powder were mixed in a mass ratio of 4:1 and then calcined at various temperatures for 2 h. Figure 1 shows the XRD patterns of the LSCF-TiO₂ composite oxide calcined at 750-1050 °C. For the LSCF-TiO₂ composite oxide calcined at 750-800 °C,the characteristic peaks can be assigned to the perovskite LSCF phase and anatase phase of TiO₂. For the LSCF-TiO₂ composite oxide calcined at 850-1050 °C,the characteristic peaks of SrTi₂₁O₃₈ and CoFe₂O₄ started to appear in addition to the diffraction peaks of LSCF and TiO₂. These results suggested that the LSCF started to react with TiO₂ at 850 °C. The average size of nano-crystallite TiO₂,SrTi₂₁O₃₈,and CoFe₂O₄ from the LSCF-TiO₂ mixture that was sintered at 850 °C was calculated by the Scherrer equation to be 35,28,and 33 nm,respectively. In addition,SrTiO₃ can be formed from the reaction between LSCF and TiO₂. Surface strontium enrichment on the LSCF,in the form of SrO or Sr(OH)₂,is well known ^{[16,17,18,19,20]}. Sr-enriched LSCF can react with TiO₂ to SrTiO₃. However,the SrTiO₃ phase cannot be detected by XRD in the presence of the LSCF phase because of the same peak position of the perovskite LSCF phase and SrTiO₃ phase. In order to show SrTiO₃ formation,TiO₂ was blended with Sr(NO₃)₂ and calcined at 850 °C for 2 h. Figure 2 shows the XRD patterns of the TiO₂-Sr(NO₃)₂ composite with different molar ratio of Ti:Sr after calcination. For the molar ratios of Ti:Sr = 1:1,the diffraction peaks of the SrTiO₃ and Sr₂TiO₄ phases can be identified,indicating that SrTiO₃ can be easily formed by the reaction of TiO₂ with SrO. A similar conclusion was given for the reaction of Sr oxalate with TiO₂ at 800 °C ^[21]. When the molar ratio of Ti:Sr was increased to 4:1,Sr₂Ti₆O₁₃ phases were also identified besides the SrTiO₃ and Sr₂TiO₄ phases. These observations indicated that SrTiO₃ could be formed at 850 °C by the reaction of LSCF with TiO₂.

Fig. 1. XRD patterns of the LSCF-TiO2 composite oxide calcined at 750 (1),800 (2),850 (3),950 (4),and 1050 °C (5) for 2 h.

Fig. 2. XRD patterns of TiO2-Sr(NO3)2 composites with Ti:Sr molar ratios of 1:1 (1) and 1:4 (2) at 850 °C for 2 h.

The reaction of LSCF with YSZ was characterized by XRD. LSCF powder and YSZ powder were mixed at a mass ratio of 2:1 and then calcined at various temperatures for 2 h. Figure 3 shows the XRD patterns of the LSCF-YSZ composite oxides calcined at 800-900 °C for 2 h. When the LSCF-YSZ composite oxide was calcined at 800 °C,all the peaks can be assigned to the perovskite LSCF phase and fluorite YSZ phase. With an increase of the calcination temperature,besides the characteristic peaks of LSCF and YSZ,diffraction peaks of SrZrO₃ and CoFe₂O₄ were also found with the LSCF-YSZ composite oxide calcined at 850 and 900 °C,indicating that LSCF started to react with YSZ at 850 °C.

Fig. 3.XRD patterns of the LSCF-YSZ composite oxide calcined at 800 (1),850 (2),and 900 °C (3) for 2 h.

LSCF,TiO₂,and YSZ powder were mixed in a mass ratio of 4:1:2.5 and then calcined at various temperatures for 2 h. Figure 4 shows the XRD patterns of the LSCF-YSZ-TiO₂ composite oxide calcined at 800-900 °C for 2 h. When the LSCF-YSZ-TiO₂ composite oxide was calcined at 800 and 850°C,the peaks can be assigned to the perovskite LSCF,fluorite YSZ,anatase TiO₂,and SrTi₂₁O₃₈ phases,respectively,indicating that TiO₂ effectively prevented the formation of SrZrO₃ from the reaction between LSCF and YSZ. When the calcination temperature was increased to 900 °C,besides the peaks of the LSCF,YSZ,TiO₂ and SrTi₂₁O₃₈ phases,small peaks of SrZrO₃ were also found,but the intensity was obviously smaller than that of the SrZrO₃ from the LSCF-YSZ composite calcined at 900 °C. These results showed that TiO₂ could prevent or suppress the reaction between LSCF and YSZ to form the SrZrO₃ phase.

Fig. 4.XRD patterns of the LSCF-YSZ-TiO2 composite oxide with the mass ratio of 4:1:2.5 calcined at 800 (1),850 (2),and 900 °C (3) for 2 h.

Figure 5(a) shows the SEM micrograph of the LSCF cathode sintered at 850 °C for 2 h. The average size of the primary LSCF particles was 150 nm. The LSCF particles were well connected and constituted porous agglomerates (ca. 0.4 μm). Figure 5 (b-d) shows the SEM micrographs of the LSCF-0.25 wt% TiO₂,LSCF-0.5 wt% TiO₂,and LSCF-2 wt% TiO₂ cathodes sintered at 850 °C for 2 h. The commercial TiO₂ powder had irregular shapes on the nanoscale with grain sizes of 7-8 nm. The size of TiO₂ in the cathode was increased due to high tempeature sintering at 850 °C. Nanosized particles (30 nm) on the surface of LSCF were TiO₂ and/or the reaction product of TiO₂ with LSCF,as shown Fig. 1.

Fig. 5.SEM micrographs of the LSCF cathode (a) and LSCF-0.25 wt% TiO2 (b),LSCF-0.5 wt% TiO2 (c),and LSCF-2 wt% TiO2 (d) composite cathodes.

Figure 6 shows the current-voltage curves and corresponding power density of single cells with LSCF,LSCF-0.25 wt% TiO₂,LSCF-0.5 wt% TiO₂,LSCF-1 wt% TiO₂,and LSCF-2 wt% TiO₂ composite cathode at 600-750 °С. The current density of the cells with the LSCF cathode was 0.14 A/cm² at 0.7 V and 600 °С. The cell with the TiO₂-modified LSCF cathode showed better performance. The current density of the cells with LSCF-0.25 wt% TiO₂,LSCF-0.5 wt% TiO₂,and LSCF-1 wt% TiO₂ were 0.23,0.21,and 0.18 A/cm² at 0.7 V and 600 °С,respectively. The significant segregation of Sr on the surface of LSCF in the form of SrO or Sr(OH)₂ is well known ^{[16,17,18,19,20]}. Sr-enriched LSCF easily reacts with YSZ electrolyte to SrZrO₃ and/or La₂Zr₂O₇. As a result,the cell with LSCF on YSZ gave very poor performance. For the TiO₂-modified LSCF cathode,TiO₂ can react with Sr-enriched LSCF to give SrTiO₃ and Sr₂TiO₄,which prevents or suppresses the formation of the non-conductive SrZrO₃ layer at the cathode/electrolyte interface. Unlike SrZrO₃,SrTiO₃ and Sr₂TiO₄ improved the LSCF/YSZ interface due to good conductivity and improved cell performance ^[22]. With a further increase of TiO₂ amount up to 2 wt% in the cathode,the current density of the cells with LSCF-2 wt% TiO₂ was 0.12 A/cm² at 0.7 V and 600 °С,which was lower than that of the cell with the LSCF cathode. When the content of TiO₂ was too high,TiO₂ reacted with LSCF to SrTi₂₁O₃₈ as shown by the XRD results,which led to the drop of cell performance.

Fig. 6.Current-voltage curves and the corresponding power densities (I-V-P) of single cells with LSCF,LSCF-0.25 wt% TiO2,LSCF-0.5 wt% TiO2,LSCF-1 wt% TiO2,and LSCF-2 wt% TiO2 composite cathodes at 600-750 °С.

Figure 7 shows electrochemical impedance spectra (EIS) of the cells with LSCF,LSCF-0.25 wt% TiO₂,LSCF-0.5 wt% TiO₂,LSCF-1 wt% TiO₂,and LSCF-2 wt% TiO₂ cathodes at 600-750 °С. The ohmic resistance of cell showed very small differences,but the cell polarization resistance significantly varied with cathode composition. The polarization resistance (R_p) of the cells with the LSCF cathode was 9.5285 Ω cm² at 600 °C. The R_p of the cells with LSCF-0.25 wt% TiO₂,LSCF-0.5 wt% TiO₂,and LSCF-1 wt% TiO₂ were 5.1342,7.1998,and 7.8421 Ω cm²,respectively,which decreased by 44%,24%,and 18% compared with that of the cell with the LSCF cathode at 600 °C (Table 1). With a further increase of TiO₂ to 2 wt% in the cathode,the R_p of the cell with LSCF-2 wt% TiO₂ was 12.0989 Ω cm² at 600 °C,which was larger than that of the cell with the LSCF cathode.

Fig. 7.Electrochemical impedance spectra (EIS) of the cells with LSCF,LSCF-0.25 wt% TiO2,LSCF-0.5 wt% TiO2,LSCF-1 wt% TiO2,and LSCF-2 wt% TiO2 cathodes at 600-750 °С.

Table 1 Comparison of ohmic resistance and polarization resistance of the cell with various cathodes at 600-750 °С. Cathode composition Ohmic resistance (Ω cm2) Polarization resistance (Ω cm2) 750 °С 700 °С 650 °С 600 °С 750 °С 700 °С 650 °С 600 °С LSCF 0.2027 0.2172 0.2654 0.3743 0.9493 1.6820 3.5981 9.5285 LSCF-0.25 wt% TiO2 0.2051 0.2175 0.2620 0.3707 0.5917 0.9878 1.9817 5.1342 LSCF-0.5 wt% TiO2 0.1980 0.2076 0.2482 0.3532 0.7135 1.2284 2.6326 7.1998 LSCF-1 wt% TiO2 0.2062 0.2160 0.2632 0.3690 0.7747 1.3051 2.8435 7.8421 LSCF-2 wt% TiO2 0.2155 0.2342 0.2956 0.4059 1.1328 2.0406 4.3568 12.0989

Table 1 Comparison of ohmic resistance and polarization resistance of the cell with various cathodes at 600-750 °С.

The EIS results are also further shown in the form of Bode plots (Fig. 8). The impedance spectra of the cell with the LSCF and TiO₂-modified LSCF cathode consisted of three arcs: a high frequency arc (summit frequency f_summit > 1000Hz),intermediate frequency arc (f_summit = 100 Hz),and low frequency arc (f_summit = 2.5Hz),reflecting at least three rate determining steps in the electrochemical reaction. The electrochemical impedance spectra at 600 °C were fitted by an equivalent circuit as shown in Fig. 9,which consisted of a high frequency arc R_H,intermediate frequency arc R_I,and low frequency arc R_L. It can be seen that R_H dramatically decreased with TiO₂ addition,indicating that the corresponding process was accelerated by TiO₂. R_I increased with TiO₂ addition. R_L was unchanged with cathode composition. On the basis of the frequency of the impedance arcs and reported results ^[15,23,24],the corresponding processes of the cell with the LSCF cathode in the impedance spectra were identified. The high frequency arcs R_H was related to the incorporation of oxygen anions into the electrolyte across the cathode/electrolyte interface. The intermediate frequency arc R_I correlated with the dissociative adsorption of oxygen. The low frequency arc R_L showed very little change with temperature and oxygen partial pressure and was due to gas diffusion with a very low activation energy in the anode. TiO₂ mainly caused the change in the high frequency arc and affected the incorporation of oxygen anions into the electrolyte across the cathode/electrolyte interface. TiO₂ suppressed the formation of the inactive SrZrO₃ layer at the interface of the c athode/ electrolyte and decreased interfacial losses.

Fig. 8.Bode plots of the cells with LSCF,LSCF-0.25 wt% TiO2,LSCF-0.5 wt% TiO2,LSCF-1 wt% TiO2,and LSCF-2 wt% TiO2 composite cathodes at 600-750 °С.

Fig. 9.Resistances of the cells with LSCF,LSCF-0.25 wt% TiO2,LSCF-0.5 wt% TiO2,LSCF-1 wt% TiO2,and LSCF-2 wt% TiO2 cathodes at 600 °С based on the model circuit.

The stability of the TiO₂-modified LSCF cathode is very important for its application. Figure 10 shows the current density of the cell with the LSCF-0.25 wt% TiO₂ cathode as a function of time at a constant voltage of 0.8 V and 700 °C. It can be seen that the cell performance slightly degraded in the first 20 h of operation and then became stable over the next 60 h operation. The corresponding current density losses were 1.2 mA/h for the first 20 h operation and decreased to 0.2 mA/h over the next 60 h operation. Further work is needed to study the long term stability of the TiO₂-modified LSCF cathode for its practical application.

Fig. 10. Current density of the cell with the LSCF-0.25 wt% TiO2 cathode as a function of time at a constant voltage of 0.8 V and 700 °C.

A TiO₂-modified LSCF cathode was prepared directly on a YSZ electrolyte. TiO₂ prevented the reaction between LSCF and YSZ,which suppressed the formation of the non-conductive SrZrO₃ phase. The electrochemical performance of the TiO₂-modified LSCFcathode was better than that of the blank LSCF cathode. The electrochemical impedance spectra showed that there was accelerated incorporation of oxygen anion into the YSZ electrolyte on the TiO₂-modified LSCF cathode. The improvement was ascribed to the suppressed formation of the non-conductive SrZrO₃ layer at the cathode/electrolyte interface. This work provided new possibilities to apply highly active Co-containing perovskite cathodes such as (La,Sr)(Co,Fe)O₃ on a ZrO₂-based electrolyte by adding an appropriate amount of TiO₂.