In situ assembled La_0.8Sr_0.2MnO₃ cathodes on a Y₂O₃–ZrO₂ electrolyte of solid oxide fuel cells – interface and electrochemical activity

Abstract

Formation of an intimate electrode/electrolyte interface is essential for solid oxide fuel cells (SOFCs). In this study, a comparative investigation has been undertaken to study the interface formation between a La_0.8Sr_0.2MnO₃ (LSM) cathode and Y₂O₃–ZrO₂ (YSZ) electrolyte by high temperature sintering and by cathodic polarization using EIS, SEM, AFM and HAADF-STEM techniques. The electrode/electrolyte interface formed by the conventional pre-sintering process is characterized by the formation of distinctive convex contact rings on the YSZ surface and such convex contact rings are due to the cation interdiffusion such as manganese species between LSM and YSZ. Similar to the thermally induced interface, the electrode/electrolyte interface can also be formed by electrochemical polarization for the in situ assembled LSM cathode on YSZ as well as on Gd₂O₃–CeO₂ (GDC) electrolytes without pre-sintering at high temperatures. The polarization induced interface has smaller contact marks due to the much finer grain size of the as-prepared LSM electrodes. Detailed electrochemical impedance studies indicate that both thermally and polarization induced LSM/YSZ interfaces show comparable electrocatalytic activity and behaviour for the oxygen reduction reaction with similar activation energies. The present study clearly demonstrates the formation of effective electrode/electrolyte interfaces in SOFCs under the influence of cathodic polarization without high temperature sintering steps.

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1 Introduction

Solid oxide fuel cells (SOFCs) are electrochemical conversion devices to generate electricity directly from the chemical energy of fuels with an inherently high efficiency. The most commonly used materials in SOFCs are Ni–yttria-stabilized zirconia (Ni–YSZ) based cermet anodes, YSZ electrolytes and strontium doped lanthanum manganite (LSM) cathodes. LSM has a high electrical conductivity and excellent electrocatalytic activity for the O₂ reduction reaction at high temperatures.^1–3 LSM is a predominantly electronic conductor with negligible ionic conductivity and thus the oxygen reduction reaction occurs at the three phase boundary (TPB) region where electrode, electrolyte and oxygen gas meet.^4–9 Thus the electrode/electrolyte interface plays a critical role in the electrochemical O₂ reduction reaction on LSM based cathodes.

The electrode/electrolyte interface in SOFCs is generally formed by pre-sintering at high temperatures, e.g., ∼1400 °C in the case of Ni–YSZ cermet anodes and ∼1150 °C for LSM cathodes.^1,10,11 In the case of LSM cathodes, it has been reported that the electrode/electrolyte interface is characterized by the formation of contact rings or islands on the YSZ electrolyte after the high temperature sintering process and the formation of the contact rings or islands are most likely related to the cation interdiffusion between LSM and YSZ.^4,12,13 On the other hand, the cathodic polarization has a significant effect on the electrode/electrolyte interface under fuel cell operation conditions. This is evident by the occurrence of microstructural changes at the interface, such as formation of micropores,^14–16 formation of a dense interfacial layer^17–19 and modification of the contact rings and marks.^4,20 For example, Nielsen and Jacobsen¹⁵ observed the transformation of dense LSM film into a porous structure after the linear potential sweep, and they proposed that the kinetics of microstructural change is enhanced by the loss of lattice oxygen of LSM by the cathodic polarization or at a lower oxygen partial pressure. The passage of polarization currents also affects the surface segregation and cation diffusion of the LSM electrode at the electrode/electrolyte interface.^21–25 Backhaus-Ricoult et al.²⁶ observed the rapid spread of manganese species from the TPBs at the electrode/electrolyte interface over the electrolyte surface under cathodic polarization using in situ photoelectron microscopy.

Most recently, we have shown that the electrode/electrolyte interface between LSM electrode and YSZ electrolyte can also be formed in situ by cathodic polarization without the high temperature pre-sintering steps, and the initial results indicate that the electrochemical performance of such a polarization induced interface is comparable to that of the pre-sintered electrodes.²⁷ In this paper, we carried out a detailed study on the electrochemical activity and microstructural change of the polarization induced electrode/electrolyte interface of the in situ assembled LSM electrode. The results show that the mechanism and kinetics of the O₂ reduction reaction are very similar on pre-sintered and in situ assembled LSM electrodes. The observed formation of an effective LSM/YSZ interface under the influence of cathodic polarization has significant implications for the fundamental understanding of the interface in SOFCs.

2 Experimental

Electrolyte pellets were prepared by pressing YSZ powder (8 mol% Y₂O₃ stabilized zirconia, Tosoh) and Gd_0.1Ce_0.9O_1.95 (GDC, AGC Seimi Chemical Co Ltd) powders, followed by sintering at 1450 °C for 5 h. The thickness of YSZ and GDC electrolytes was ∼1.0 mm. La_0.8Sr_0.2MnO₃ (LSM, Fuel Cell Materials) was mixed with ink vehicle in a weight ratio of 5 : 5 to form LSM ink. The ink was prepared on the YSZ and GDC electrolytes by slurry coating and dried at 85 °C in air for 2 h. Two types of LSM electrodes were prepared. The as-prepared LSM electrodes were directly tested without pre-sintering and were referred to as in situ assembled LSM. The as-prepared LSM electrodes were also sintered at 1150 °C for 2 h in air and were denoted as the pre-sintered LSM electrodes. The thickness of the LSM electrode coating was ∼20 μm and the electrode surface area was 0.5 cm².

Electrochemical measurements of the LSM electrodes were carried out in a three-electrode configuration, using a Gamry Reference 3000 Potentiostat. Pt paste (Pt ink, Gwent Group of Companies) was painted to the opposite of the electrolyte pellets as the counter electrode. A Pt ring reference electrode was attached to the edge of the electrolyte. Pt mesh was used as a current collector. Separate Pt wires were used as voltage and current probes. As there is no current passing through the Pt voltage probes, the Pt voltage probe would not contribute to the ohmic resistance measured. Electrochemical behaviour such as cathodic potential (E_Cathode) and electrode ohmic resistance (R_Ω) of LSM electrodes was generally studied at a cathodic current of 500 mA cm⁻², 800 °C in air. E_Cathode consists of ohmic losses (iR_Ω) and iR-free cathodic overpotential (η_Cathode), i.e., E_Cathode = iR_Ω + η_Cathode. R_Ω is the ohmic resistances between the working and Pt reference electrodes. Electrochemical impedance curves were measured under open circuit with the frequency range from 0.1 Hz to 100 kHz and the signal amplitude of 10 mV. R_Ω was obtained from the high frequency intercept, and is the sum of the ohmic resistance of the electrolyte, electrode and contact resistance at the electrode/electrolyte. The validity of R_Ω obtained from the high frequency intercept has been confirmed by the observations that in the case of LSM oxygen electrodes under electrolysis conditions, R_Ω of LSM oxygen electrode increases with the anodic polarization due to the gradual loss of electrode/electrolyte interface contact under the electrolysis operation conditions.^28–30 The overall electrode polarization resistance, R_E was estimated by the differences between the high and low frequency intercepts.

Microstructure of the electrodes was examined using scanning electron microscopy (SEM, Zeiss Neon 40EsB). Pre-sintered and in situ assembled LSM electrodes after the polarization tests were removed by hydrochloric acid treatment at room temperature for 24 h. After the removal of the LSM electrode coating, the electrolyte surface was examined with tapping mode atomic force microscopy (AFM) using a Dimension FastScan AFM (Bruker). A lamella sample was prepared on the YSZ electrolyte after the removal of a pre-sintered LSM electrode by an FEI Helios Nanolab G3 CX DualBeam focused ion beam (FIB)-SEM. The element distribution of the interface was analyzed by line scan, using high angle annular dark field scanning transmission electron microscopy (HAADF-STEM, FEI Titan G2 80-200 TEM/STEM with ChemiSTEM Technology) at 200 kV.

3 Results and discussion

3.1 Electrochemical performance of pre-sintered and in situ assembled LSM

Fig. 1 shows the polarization and impedance curves of the O₂ reduction reaction on a pre-sintered LSM electrode on YSZ electrolyte measured at 800 °C. The initial impedance responses of the O₂ reduction on the pre-sintered LSM electrode are characterized by a large depressed arc with the overall R_E of ∼45 Ω cm² and there is no change in the electrochemical activity of pre-sintered LSM electrodes, as indicated by the stable R_E under open circuit conditions for 40 h (Fig. 1a). With the cathodic current passage at 500 mA cm², both polarization potential (E_Cathode) and R_E decrease dramatically (Fig. 1b). For instance, after cathodic polarization at 500 mA cm² for 2 h, E_Cathode decreased from 1.42 V to 1.10 V and R_E decreased from 45 Ω cm² to 2.11 Ω cm². As shown in Fig. 1c, the reduction in the electrode impedance occurs primarily on the low frequencies. The significant reduction in the R_E values is clearly due to the activation effect of cathodic polarization on the electrochemical activity of pristine LSM electrodes, as shown in the literatures.^{16,21,31–34} The change of E_Cathode with the cathodic polarization passage is characterized by two regions with a rapid decrease in region I and a gradual decrease in region II (see the inset of Fig. 1b), a characteristic of the activation enhancement of cathodic polarization on LSM electrodes.^21,35 However, despite the substantial reduction in R_E, the activation energy of the LSM electrodes for the O₂ reduction reaction does not change significantly with the cathodic current passage. The activation energy for the O₂ reduction reaction on the pre-sintered LSM electrode is in the range of 170–199 kJ mol⁻¹ (Fig. 1d), which is close to the 170–185 kJ mol⁻¹ reported in the literature.^36–38

Fig. 1 Impedance and polarization curves of a pre-sintered LSM electrode on YSZ electrolyte measured at 800 °C. (a) Under open circuit for 0 and 40 h; (b) plots of E_Cathode, R_E and R_Ω as a function of cathodic polarization time at 500 mA cm⁻²; (c) impedance curves of the pre-sintered LSM after cathodic current passage at 500 mA cm⁻² for 0, 2 and 40 h; and (d) activation energy plots of the LSM cathode before and after cathodic current passage at 500 mA cm⁻² for 0, 2 and 40 h. Numbers in the impedance curves are frequency in Hz.

Impedance curves of the pre-sintered LSM electrode before and after polarization at 500 mA cm² for 40 h were fitted with an equivalent circuit, as shown in Fig. 2a. In the equivalent circuit, L, Q and R represent inductance, constant phase element and electrode polarization resistance, respectively. L is related to the high frequency inductance generally associated with the Pt leads and heating elements of the furnace of the test station. R_Ω is the ohmic resistance between the cathode and Pt reference electrode, and R_H and R_L correspond to the electrode polarization resistance associated with high and low frequency arcs, respectively. The activation energy plots of derived R_H and R_L show a linear dependence on the reciprocal of temperature and, from the slope of the curves, the E_a values were calculated to be 110 and 175 kJ mol⁻¹ for the reaction steps associated with high and low frequency arcs, respectively. It has been shown early that the high frequency process is related to oxygen ion migration from the LSM electrode to the YSZ electrolyte, while the low frequency electrode process is associated with the oxygen dissociation and/or surface diffusion processes.^36,39 On the other hand, R_Ω of the pre-sintered LSM is in the range of 1.77–1.88 Ω cm² (see the inset, Fig. 1c), essentially independent of cathodic polarization treatment as expected.

Fig. 2 (a) Equivalent circuit and (b) activation energy plots of the pre-sintered LSM cathode on YSZ electrolyte after cathodic current passage at 500 mA cm⁻² and 800 °C for 40 h.

Fig. 3 shows the polarization curves and impedance responses of an in situ assembled LSM electrode on YSZ electrolyte. Similar to that of the pre-sintered LSM electrode, E_Cathode and R_E also decrease substantially with the cathodic current passage at 500 mA cm⁻². After cathodic polarization at 500 mA cm⁻² for 2 h, E_Cathode and R_E is 1.39 V and 1.41 Ω cm², respectively, much smaller than the 2.03 V and 24.4 Ω cm² before the polarization (Fig. 3a). The E_Cathode curves are also characterized by changes in two regions (see the inset of Fig. 3a), similar to the case of the pre-sintered LSM. The decrease of R_E is mainly attributed to the decrease of impedance curves at low frequencies (Fig. 3b), also similar to the pre-sintered LSM electrode. The initial R_Ω was 2.50 Ω cm² and decreased to 2.27 and 2.13 Ω cm² after polarization for 2 and 40 h, respectively (Fig. 3a). Evidently, very different from the constant R_Ω of the pre-sintered LSM electrodes, R_Ω decreases with the cathodic polarization. As the ohmic resistance of the YSZ electrolyte is not expected to change, the decrease in R_Ω indicates the reduction of the contact resistance at the LSM/YSZ interface under the effect of the cathodic polarization. Nevertheless, the activation energy of the O₂ reduction reaction on the in situ assembled LSM electrode after the cathodic current passage is in the range of 150–171 kJ mol⁻¹ (Fig. 3c), close to that of pre-sintered LSM. The change of R_Ω and E_a of pre-sintered and in situ assembled LSM electrodes is given in Table 1.

Fig. 3 Impedance and polarization curves of an in situ assembled LSM electrode on YSZ electrolyte measured at 800 °C. (a) Plots of E_Cathode, R_E and R_Ω as a function of cathodic polarization time at 500 mA cm⁻²; (b) impedance curves of the in situ assembled LSM after cathodic current passage at 500 mA cm⁻² for 0, 2 and 40 h; and (c) activation energy plots of the LSM cathode before and after cathodic current passage at 500 mA cm⁻² for different periods. Numbers in (b) are frequencies in Hz.

Table 1 Electrode ohmic resistances (R_Ω, Ω cm²) and activation energies (E_a, kJ mol⁻¹) of pre-sintered and in situ assembled LSM electrodes as a function of polarization time at 500 mA cm⁻², 800 °C

Polarization time/h	Pre-sintered LSM on YSZ		In situ assembled LSM on YSZ		In situ assembled LSM on GDC
	RΩ	Ea	RΩ	Ea	RΩ	Ea
0	1.77	199	2.50	152	3.60	166
2	1.77	174	2.27	171	2.05	177
40	1.88	170	2.13	150	1.84	173

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	Ea (RH)	Ea (RL)	Ea (RH)	Ea (RL)	Ea (RH)	Ea (RL)
40	110	175	111	167	96	178

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The electrochemical activity of the in situ assembled LSM electrode was also studied on GDC electrolyte. Fig. 4 shows the polarization and impedance curves of an in situ assembled LSM electrode on GDC electrolyte. The initial E_Cathode and R_E before the polarization is 1.35 V and 18.4 Ω cm², respectively, and after cathodic polarization at 500 mA cm⁻² for 2 h, E_Cathode and R_E decreased to 1.13 V and 1.64 Ω cm², respectively. Similar to the per-sintered and in situ assembled LSM on YSZ electrolyte, the change in E_Cathode with polarization current passage is also characterized by two clearly separated regions (see the inset of Fig. 4a). The activation energy is in the range of 166–177 kJ mol⁻¹, close to that on the YSZ electrolyte. Similar to the in situ assembled LSM on YSZ electrolyte, the initial R_Ω was 3.60 Ω cm² and decreased significantly to 2.05 and 1.84 Ω cm² after polarization for 2 and 40 h, respectively (Fig. 4b), an indication of the formation of LSM electrode/GDC electrolyte interface induced by the cathodic polarization. The results indicate that the electrochemical behaviour of the in situ assembled LSM electrode on GDC electrolyte is very similar to that on YSZ electrolyte.

Fig. 4 Impedance and polarization curves of an in situ assembled LSM electrode on GDC electrolyte measured at 800 °C. (a) Plots of E_Cathode, R_E and R_Ω as a function of cathodic polarization time at 500 mA cm⁻²; (b) impedance curves of the in situ assembled LSM after cathodic current passage at 500 mA cm⁻² for 0, 2 and 40 h; and (c) activation energy plots of the LSM cathode before and after cathodic current passage at 500 mA cm⁻² for different periods. Numbers in (b) are frequencies in Hz.

The impedance curves of in situ assembled LSM electrodes were also fitted by the equivalent circuit and the results are shown in Fig. 5. After polarization at 500 mA cm⁻² for 40 h, the E_a values of R_H and R_L of in situ assembled LSM on YSZ are 111 and 167 kJ mol⁻¹ (Fig. 5a), respectively, which are very close to 110 and 175 kJ mol⁻¹ of the pre-sintered LSM electrode polarized under identical conditions (Fig. 2b). In the case of the in situ assembled LSM on GDC after polarization at 500 mA cm⁻² for 40 h, E_a values for R_H and R_L are 96 and 178 kJ mol⁻¹, respectively (Fig. 5b), also close to that on the YSZ electrolyte. This implies the in situ assembly approach does not alter the mechanism of O₂ reduction reaction on the LSM electrode.

Fig. 5 Activation energy plots of in situ assembled LSM cathodes on (a) YSZ electrolyte and (b) GDC electrolyte after cathodic current passage at 500 mA cm⁻² and 800 °C for 40 h.

3.2 Microstructure and interface of pre-sintered and in situ assembled LSM electrode

Fig. 6 shows the SEM images of the surface of pre-sintered and in situ assembled LSM cathodes. The pre-sintered LSM electrode is characterized by interconnected particles in the size of 0.5–1.6 μm (Fig. 6a). On the other hand, the in situ assembled LSM electrode has a loose structure with a mixture of small and large particles (Fig. 6b). The small particles are in the size range of 0.1–0.3 μm and the large particles are in the size range of 0.5–1.4 μm. The smaller particles of the in situ assembled LSM electrode could provide more contact points i.e., TPBs at the LSM/YSZ interface region for the oxygen reduction reaction. This appears to be supported by the similar R_E values of the in situ assembled LSM as compared to that of the pre-sintered one on YSZ electrolytes after polarization for 40 h (1.4 Ω cm² vs. 1.6 Ω cm²), despite the relatively poor contacts between LSM particles (see Fig. 6b). On the other hand, the close ohmic resistance of the in situ and pre-sintered LSM electrodes after polarization for 40 h (2.1 Ω cm² vs. 1.8 Ω cm²) implies that the loose structure of the in situ assembled LSM electrodes may have limited effect on the electron/charge transport across the electrode.

Fig. 6 SEM images of surface of (a) pre-sintered and (b) in situ assembled LSM electrodes.

Fig. 7 shows the AFM micrographs of the surface of the YSZ electrolyte in contact with a pre-sintered LSM electrode before and after cathodic polarization at 500 mA cm⁻² for 40 h. The LSM electrode coating was removed by HCl treatment. The pristine YSZ electrolyte without the electrode coating is clean (Fig. 7g and h). On the other hand, the morphology of the YSZ electrolyte surface in contact with LSM electrode changes significantly after sintering at 1150 °C. The YSZ electrolyte surface is characterized by the formation of convex rings with sharp edges or rims of 0.3–1.1 μm diameter (Fig. 7a and b). The diameter of the ring is 0.66 ± 0.17 μm on average, close to 0.5–1.6 μm of the particle size of the pre-sintered LSM electrode. The corresponding line scan of the rings shows that the height of the edges of the contact rings is 40 ± 10 nm (Fig. 7c). After cathodic polarization at 500 mA cm⁻² for 40 h, the morphology of the contact rings changes (Fig. 7d and e). The edges of the rings are widened to 160–300 nm and the height also increases to 83 ± 3 nm (Fig. 7f). The convex ring area became smaller as the result of the widening of the edge of the contact rings. As shown earlier,⁴ the contact rings are the TPB regions for the oxygen reduction reaction and the widening of the rings implies the increased TPBs for the reaction, consistent with the enhanced electrode activity under the cathodic polarization conditions.

Fig. 7 AFM images and height analysis of contact rings on YSZ electrolyte surface in contact with pre-sintered LSM electrodes. LSM electrode was removed by HCl treatment. (a–c) Before polarization and (d–f) after polarization at 800 °C, 500 mA cm⁻² for 40 h. AFM images of the surface of a pristine YSZ electrolyte are shown in (g) and (h).

The element distribution across the convex contact rings on the YSZ electrolyte surface was analyzed by HAADF-STEM and EDS, as shown in Fig. 8. The narrow bright stripe is a thin, conductive Pt layer sputtered on to the YSZ sample surface before the SEM analysis, as the YSZ is not electrically conductive. Above this are additional layers of Pt that have been added during the FIB preparation of the TEM sample. In the case of the rim of the convex contact ring, there appears to be a slight enrichment of La (and possibly Sr) on the surface of the YSZ (Fig. 8a). However, there is clear presence and accumulation of Mn cation within the edge of the contact ring area extending as far as 70–90 nm into the YSZ. In the same region, the intensity of Y is lower than that in the bulk, indicating that the Y deficiency is most likely due to the diffusion of Mn. The diffusion of Mn into YSZ is due to the limited solubility of Mn in YSZ, forming solid solution of Mn and YSZ.⁴⁰ In the case of the line scan along the center of the contact ring, Mn ions are slightly enriched on the electrolyte surface, similar to La, but the intensity of Mn is qualitatively much smaller than that within the rim region of the convex rings. The results indicate that the formation of convex rings is most likely due to the significant Mn diffusion from the LSM to the YSZ electrolyte particularly at the edge contact region between the LSM particles and YSZ electrolyte during the high temperature sintering steps of the electrode. It has been reported that the formation of contact marks or islands is correlated with the diffusion of excess Mn³⁺ into YSZ and interdiffusion of Zr⁴⁺ and Y³⁺ to the LSM electrode.^12,13

Fig. 8 Line scans of the contact convex ring on the YSZ electrolyte surface in contact with a pre-sintered LSM electrode before the polarization treatment. LSM electrode was removed by HCl treatment.

Fig. 9 shows the AFM images of the surface of YSZ and GDC electrolytes in contact with in situ assembled LSM electrodes after cathodic polarization at 500 mA cm⁻² for 40 h. On the YSZ electrolyte surface are contact marks in the size range of 40–210 nm with an average of 72 ± 31 nm (Fig. 9a and b), and the height is 12 ± 4 nm (Fig. 9c). The size of the contact marks is in good agreement with the 100–300 nm of the in situ assembled LSM electrode particles (Fig. 6b). There is also a formation of contact marks on the GDC electrolyte surface (Fig. 9d and e), as compared to the clean surface of pristine GDC electrolyte (Fig. 9g and h). The contact marks are in the range of 69 ± 23 nm in diameter and the height of the marks is ∼33 nm (Fig. 9f). The average size of contact marks on GDC electrolyte is similar to that on YSZ electrolyte surface, which confirms that the origin of these contact marks is due to the interaction between the LSM particles and electrolyte. The formation of contact marks indicates the formation of an intimate interface of the in situ assembled LSM electrode on YSZ and GDC electrolytes. This is consistent with the observed decrease of the electrode ohmic resistance, R_Ω of the in situ assembled LSM electrodes with the cathodic opalization treatment (see Table 1).

Fig. 9 AFM images and line scans of (a–c) YSZ electrolyte surface and (d–f) GDC electrolyte surface in contact with in situ assembled LSM electrodes after polarization at 800 °C, 500 mA cm⁻² for 40 h. LSM electrode coating was removed by HCl treatment. Pristine GDC electrolyte surface was shown in (g) and (h).

The results of this study clearly demonstrate that both sintering at high temperature, e.g., 1150 °C, and cathodic polarization at normal SOFC operation temperatures, e.g., 800 °C can induce the formation of an intimate electrode/electrolyte interface on the YSZ and GDC electrolytes. The thermally induced interface is characterized by the formation of distinct convex rings, while the polarization induced interface is indicated by the formation of numerous contact marks (see Fig. 10). However, despite the significant differences in the morphology or appearance of the interface, the reaction mechanism for the O₂ reduction reaction on the in situ assembled LSM electrode is almost identical to that of the pre-sintered LSM electrode. This indicates the polarization induced electrode/electrolyte interface is electrochemically identical to the thermally induced one, consistent with previous study.²⁷ The fundamental reason for the formation of the contact marks under the influence of cathodic polarization is not clear at this stage, but incorporation of oxygen ionic species from LSM electrode to YSZ electrolyte under the influence of cathodic polarization could induce the formation of the interface between LSM and YSZ, as evidenced by the significant change of the convex contact rings in the case of pre-sintered LSM and the presence of the contact marks in the case of in situ assembled LSM electrodes. Liu et al.⁴¹ adopted X-ray nanotomography to in situ reconstruct 3D microstructure of LSM electrode under cathodic polarization and observed oxygen vacancy formation on the LSM cathode. Thus, the incorporation of oxygen ions into the YSZ electrolyte could be the reason for the significant morphological change of the convex contact rings. On the other hand, manganese cations can be very mobile under the SOFC operation conditions, as indicated by the significant diffusion of Mn ions from the TPB region to the surface of YSZ electrolyte under cathodic polarization conditions.²⁶ The interdiffusion of Mn species into the YSZ electrolyte may also be possible under the cathodic polarization conditions.

Fig. 10 Scheme of the LSM electrode/YSZ electrolyte interface formation under the influence of (a) high temperature sintering, e.g., 1150 °C and (b) cathodic polarization at normal SOFC operation conditions, e.g., 800 °C.

4 Conclusions

Electrochemical activities and microstructure of pre-sintered and in situ assembled LSM electrode/YSZ electrolyte interfaces have been investigated under cathodic polarization conditions. In the case of the pre-sintered LSM electrode, sintering at high temperatures leads to cation interdiffusion and thereby formation of an electrode/electrolyte interface. On the other hand, an interface can be generated in situ for the LSM electrodes on YSZ and GDC electrolytes under the cathodic polarization conditions. The interface formation in the case of in situ assembled LSM is accompanied by the pronounced decrease of ohmic resistance, particularly in the first several hours of polarization. For example, the initial R_Ω of in situ assembled LSM electrode on GDC electrolyte was 3.60 Ω cm² and drastically reduced to 2.05 Ω cm² after polarization for 2 h. The polarization induced interface formation is most likely due to the enhanced oxygen ion migration processes under the influence of cathodic polarization, though the cation interdiffusion at TPBs under SOFC operation condition cannot be ruled out at this stage. The polarization induced interface does not alter the reaction mechanism of oxygen reduction on LSM. The activation energy for the reaction on in situ assembled LSM is 150–171 kJ mol⁻¹, close to 170–199 kJ mol⁻¹ for the reaction on pre-sintered LSM. This study provides an insight into in-depth understanding of electrode/electrolyte interface formation and may also have a profound technological implications on the design and development of new generation SOFC cathodes.

Acknowledgements

The project is supported by Curtin University Research Fellow Program, National Natural Science Foundation of China (U1134001) and Australian Research Council under Discovery Project scheme (project number: DP150102025 & DP150102044). The authors acknowledge the facilities, scientific and technical assistance of the Curtin University Microscopy & Microanalysis Facility and UWA Centre for Microscopy, Characterisation and Analysis, both of which are funded by the University, State and Commonwealth Governments, and the Department of Chemistry/Nanochemistry Research Institute, Curtin University.

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