Semiconductor-Ionic Nanocomposite La0.1Sr0.9MnO3−δ-Ce0.8Sm0.2O2−δ Functional Layer for High Performance Low Temperature SOFC

A novel composite was synthesized by mixing La0.1Sr0.9MnO3−δ (LSM) with Ce0.8Sm0.2O2−δ (SDC) for the functional layer of low temperature solid oxide fuel cell (LT-SOFC). Though LSM, a highly electronic conducting semiconductor, was used in the functional layer, the fuel cell device could reach OCVs higher than 1.0 V without short-circuit problem. A typical diode or rectification effect was observed when an external electric force was supplied on the device under fuel cell atmosphere, which indicated the existence of a junction that prevented the device from short-circuit problem. The optimum ratio of LSM:SDC = 1:2 was found for the LT-SOFC to reach the highest power density of 742 mW·cm−2 under 550 °C The electrochemical impedance spectroscopy data highlighted that introducing LSM into SDC electrolyte layer not only decreased charge-transfer resistances from 0.66 Ω·cm2 for SDC to 0.47–0.49 Ω·cm2 for LSM-SDC composite, but also decreased the activation energy of ionic conduction from 0.55 to 0.20 eV.


Introduction
Solid oxide fuel cells (SOFCs) research and development have been strongly driven towards low temperature (LT), <600 • C, operations. Thereby, the materials and technology associated with LT demand extensive research efforts. Among the three basic components of SOFC, the electrolyte is the core. Yttria-stabilized zirconia (YSZ) has dominated the SOFC R&D over many decades since it was first discovered by Nernst [1] in 1899. However, its sufficient O 2− conductivity,~0.1 S cm −1 , only takes place under high temperature (~1000 • C) [2,3], leading to high system costs and complexity, severe drawback on material durability, slow start-up rate, thus impeding the SOFCs commercial process. Therefore, the LTSOFC research and development have become worldwide, but to maintain the high efficiency and to design high ionic conductive electrolyte materials functioned at low temperatures, have been a critical challenge [4][5][6][7][8].
For developing new oxide-ion conducting materials aiming to replace YSZ as the electrolyte for LTSOFCs, the most common way is the structural design by doping to create structural defects, thus increasing ionic conductivity. Goodenough [9] proposed that the conductivity of oxide-ion conductors could be effectively enhanced by designing materials structure. For example, doping low examined by scanning electron microscope (FE-SEM, JSM7100F, Japan Electron Optics Laboratory Co., Ltd., Tokyo, Japan) equipped with energy-dispersive X-ray (EDX, Japan Electron Optics Laboratory Co., Ltd., Tokyo, Japan) spectrometer.

Electrochemical Measurement
The nickel foam-NCAL (Ni 0.8 Co 0.15 Al 0.05 LiO 2−δ , Tianjin B&M Science and Technology Joint-Stock Co., Ltd., Tianjin, China) electrodes were prepared by the brush coating-method [32,33]. After being pressed, the thickness of the Ni-NCAL electrode was about 0.4 mm. The Ni-NCAL (anode and cathode) and LSM-SDC (electrolyte layer) were dry-pressed (200-300 MPa pressure) together to obtain the SOFC device. There was no further firing procedures before testing the device to protect the hetero-interfaces between LSM and SDC [19,20]. Fuel cell thickness and effective areas were 0.7 mm and 0.64 cm 2 , respectively.
The electronic load (type IT8512C+, ITECH Electronic Co., Ltd., Nanjing, China) was used to evaluate the fuel cell performance (with air and H 2 as oxidant and fuel, respectively). Electrochemical impedance spectroscopy (EIS) measurement was conducted by Gamry Ref 3000 (Gamry Instruments, Philadelphia, PA, USA) under OCV (amplitude: 10 mV, frequency range: 1 MHz-0.1 Hz). An equivalent circuit mode LR Ω (R a Z a )((R b Z b )(R c Z c ) was used to fit the original EIS data by ZView (Copyright 1990(Copyright -2007 Scribner Associates, Inc., Southern Pines, NC, USA, written by Derek Johson). The electronic conductivities of LSM-SDC composites were measured by Keithley 2400 SourceMeter (Tektronix Corporation, Beaverton, WA, USA).

Results and Discussion
As shown in Figure 1, the characteristic peaks of LSM corresponded to a perovskite SrMnO 3 structure. The diffraction peaks of prepared SDC material were indexed to standard diffraction pattern for fluorite CeO 2 structure, indicating that Sm was doped into the crystal lattice of ceria. The major diffraction peaks of the original 1LSM-2SDC composite belonged to SDC. The characteristic peaks of LSM in the pattern of the original 1LSM-2SDC composite were not obvious due to the smaller amount of LSM in the composite. Compared with the 1LSM-2SDC composite material, there was no new phase in the XRD pattern of 1LSM-2SDC, which was heat-treated at 550 • C for 2 h under the air atmosphere, indicating that the composite is thermal stable under 550 • C. The average crystal sizes of SDC and LSM were estimated according to the Debye-Scherrer equation from the peaks at 28.3 • and 35.2 • , respectively. Both of them were nanoscale, 46 nm for SDC and 37 nm for LSM. The small radius of the materials was beneficial to increase the hetero-interfaces between LSM and SDC. After being heat-treated at 550 • C for 2 h, the average crystal size of SDC increased to about 48 nm, indicating that the SDC is stable under the fuel cell working temperature. Optics Laboratory Co., Ltd., Tokyo, Japan) equipped with energy-dispersive X-ray (EDX, Japan Electron Optics Laboratory Co., Ltd., Tokyo, Japan) spectrometer.

Electrochemical Measurement
The nickel foam-NCAL (Ni0.8Co0.15Al0.05LiO2−δ, Tianjin B&M Science and Technology Joint-Stock Co., Ltd., Tianjin, China) electrodes were prepared by the brush coating-method [32,33]. After being pressed, the thickness of the Ni-NCAL electrode was about 0.4 mm. The Ni-NCAL (anode and cathode) and LSM-SDC (electrolyte layer) were dry-pressed (200-300 MPa pressure) together to obtain the SOFC device. There was no further firing procedures before testing the device to protect the hetero-interfaces between LSM and SDC [19,20]. Fuel cell thickness and effective areas were 0.7 mm and 0.64 cm 2 , respectively. The electronic load (type IT8512C+, ITECH Electronic Co., Ltd., Nanjing, China) was used to evaluate the fuel cell performance (with air and H2 as oxidant and fuel, respectively). Electrochemical impedance spectroscopy (EIS) measurement was conducted by Gamry Ref 3000

Results and Discussion
As shown in Figure 1, the characteristic peaks of LSM corresponded to a perovskite SrMnO3 structure. The diffraction peaks of prepared SDC material were indexed to standard diffraction pattern for fluorite CeO2 structure, indicating that Sm was doped into the crystal lattice of ceria. The major diffraction peaks of the original 1LSM-2SDC composite belonged to SDC. The characteristic peaks of LSM in the pattern of the original 1LSM-2SDC composite were not obvious due to the smaller amount of LSM in the composite. Compared with the 1LSM-2SDC composite material, there was no new phase in the XRD pattern of 1LSM-2SDC, which was heat-treated at 550 °C for 2 h under the air atmosphere, indicating that the composite is thermal stable under 550 °C. The average crystal sizes of SDC and LSM were estimated according to the Debye-Scherrer equation from the peaks at 28.3° and 35.2°, respectively. Both of them were nanoscale, 46 nm for SDC and 37 nm for LSM. The small radius of the materials was beneficial to increase the hetero-interfaces between LSM and SDC. After being heat-treated at 550 °C for 2 h, the average crystal size of SDC increased to about 48 nm, indicating that the SDC is stable under the fuel cell working temperature.     Figure 3 revealed that the SOFC performances increased with LSM contents until the weight ratio of LSM to SDC was 1:2. At this weight ratio of 1:2, the Pmax (maximum power density) 742 mW cm −2 has been achieved at 550 °C. Further increasing the content of the LSM caused a decrease in the fuel cell performance. Compared with the pure SDC, the LSM-SDC composite possessed a larger amount of hetero-interfacial region/interface, which could provide fast pathways for ionic conducting [2,[14][15][16][17][18]. At the above optimum weight ratio, the ionic grain boundary conductivity was the best due to the highest hetero-interfacial area. The following characterizations will give further confirmation. LSM is a common SOFC cathode material with excellent electronic conductivity. It is interesting that no short circuit phenomenon occurred for the SOFC using the LSM-SDC composite layer to replace the conventional electrolyte layer with the pure ionic conductor. All of the OCVs were over than 1.0 V. It may be noticed that even in the case with pure doped ceria as the electrolyte for SOFCs,  Figure 3 revealed that the SOFC performances increased with LSM contents until the weight ratio of LSM to SDC was 1:2. At this weight ratio of 1:2, the P max (maximum power density) 742 mW cm −2 has been achieved at 550 • C. Further increasing the content of the LSM caused a decrease in the fuel cell performance. Compared with the pure SDC, the LSM-SDC composite possessed a larger amount of hetero-interfacial region/interface, which could provide fast pathways for ionic conducting [2,[14][15][16][17][18]. At the above optimum weight ratio, the ionic grain boundary conductivity was the best due to the highest hetero-interfacial area. The following characterizations will give further confirmation.   Figure 3 revealed that the SOFC performances increased with LSM contents until the weight ratio of LSM to SDC was 1:2. At this weight ratio of 1:2, the Pmax (maximum power density) 742 mW cm −2 has been achieved at 550 °C. Further increasing the content of the LSM caused a decrease in the fuel cell performance. Compared with the pure SDC, the LSM-SDC composite possessed a larger amount of hetero-interfacial region/interface, which could provide fast pathways for ionic conducting [2,[14][15][16][17][18]. At the above optimum weight ratio, the ionic grain boundary conductivity was the best due to the highest hetero-interfacial area. The following characterizations will give further confirmation. LSM is a common SOFC cathode material with excellent electronic conductivity. It is interesting that no short circuit phenomenon occurred for the SOFC using the LSM-SDC composite layer to replace the conventional electrolyte layer with the pure ionic conductor. All of the OCVs were over than 1.0 V. It may be noticed that even in the case with pure doped ceria as the electrolyte for SOFCs, LSM is a common SOFC cathode material with excellent electronic conductivity. It is interesting that no short circuit phenomenon occurred for the SOFC using the LSM-SDC composite layer to replace the conventional electrolyte layer with the pure ionic conductor. All of the OCVs were over than 1.0 V. It may be noticed that even in the case with pure doped ceria as the electrolyte for SOFCs, short circuiting problem with low OCVs (below 1.0 V, more often 0.9 V) has been commonly observed [11,34]. Therefore, both electronic and ionic conductivities were investigated for the LSM-SDC composite samples with various weight ratios. Ag paste was brushed onto both sides of the LSM-SDC functional layer in a configuration of Ag/LSM-SDC/Ag for electrical property measurements. The electronic conductivities of LSM-SDC composites were determined by potentiostatic polarization method with the constant voltage of 0.5 V. The test temperature and atmosphere were 550 • C and N 2 , respectively. In the beginning, both electrons and O 2− can make contributions to the current. But O 2− would be exhausted with time due to no ionic resource from the N 2 atmosphere, and electrons are then the only charge carriers in LSM-SDC composites when the current reach a steady state (I s ) and we can get the electronic resistance R e = 0.5/I s [35]. The total resistance of the LSM-SDC composites was measured by EIS under H 2 /air atmosphere. The EIS results were fitted through the equivalent circuit of LR Ω (R a Z a )((R b Z b )(R c Z c ), where L-inductance, R Ω -ohmic resistance, R a -ionic grain boundary transportation resistance, R b (R c )-charge-transfer resistance and Z-constant phase element. The total resistances were given by the following equation.
The ionic conductivity was calculated by subtracted electronic conductivity from total conductivity. The calculated data are listed in Figure 4, which revealed that the composite electronic conductivity increased with LSM content but still extremely small, indicating a low electronic conductivity owing to the discontinuity of the electronic pathway in the device. short circuiting problem with low OCVs (below 1.0 V, more often 0.9 V) has been commonly observed [11,34]. Therefore, both electronic and ionic conductivities were investigated for the LSM-SDC composite samples with various weight ratios. Ag paste was brushed onto both sides of the LSM-SDC functional layer in a configuration of Ag/LSM-SDC/Ag for electrical property measurements. The electronic conductivities of LSM-SDC composites were determined by potentiostatic polarization method with the constant voltage of 0.5 V. The test temperature and atmosphere were 550 °C and N2, respectively. In the beginning, both electrons and O 2− can make contributions to the current. But O 2− would be exhausted with time due to no ionic resource from the N2 atmosphere, and electrons are then the only charge carriers in LSM-SDC composites when the current reach a steady state (Is) and we can get the electronic resistance Re = 0.5/Is [35]. The total resistance of the LSM-SDC composites was measured by EIS under H2/air atmosphere. The EIS results were fitted through the equivalent circuit of LRΩ(RaZa)((RbZb)(RcZc), where L-inductance, RΩ-ohmic resistance, Ra-ionic grain boundary transportation resistance, Rb(Rc)-charge-transfer resistance and Z-constant phase element. The total resistances were given by the following equation.

Rt = RΩ + Ra
The ionic conductivity was calculated by subtracted electronic conductivity from total conductivity. The calculated data are listed in Figure 4, which revealed that the composite electronic conductivity increased with LSM content but still extremely small, indicating a low electronic conductivity owing to the discontinuity of the electronic pathway in the device. Linear scan voltage was supplied on the SOFC device with LSM-SDC (1:2, weight ratio) as the functional layer (Figure 5a). When the anode and cathode of the SOFC were both supplied with air, the I-V curve is linear, indicating that the fuel cell corresponds to a linear resistor under the air atmosphere; when the anode gas was changed into H2, the I-V curve becomes non-linear, indicating that there is a diode or rectification effect in the fuel cell device. As a kind of p-type semi-conductor, LSM, close to the anode side, can form a Schottky junction with metal Ni, production of NCAL reduction reaction [36,37]. The built-in electric field, generated by Schottky junction, not only drives the ionic transportation, but also prevents electrons transporting from the anode to cathode ( Figure  5b). Therefore, even though there is some electronic conduction in the LSM-SDC composite, it can be prevented by the electronic blocking layer in the device. Linear scan voltage was supplied on the SOFC device with LSM-SDC (1:2, weight ratio) as the functional layer (Figure 5a). When the anode and cathode of the SOFC were both supplied with air, the I-V curve is linear, indicating that the fuel cell corresponds to a linear resistor under the air atmosphere; when the anode gas was changed into H 2 , the I-V curve becomes non-linear, indicating that there is a diode or rectification effect in the fuel cell device. As a kind of p-type semi-conductor, LSM, close to the anode side, can form a Schottky junction with metal Ni, production of NCAL reduction reaction [36,37]. The built-in electric field, generated by Schottky junction, not only drives the ionic transportation, but also prevents electrons transporting from the anode to cathode (Figure 5b). Therefore, even though there is some electronic conduction in the LSM-SDC composite, it can be prevented by the electronic blocking layer in the device.  (2) where δ-pallet thickness (0.7 mm), A-pallet surface area (0.64 cm 2 ). It is noted that two samples exhibited a linear dependence between 550 and 450 °C. The activation energy can be calculated by the slope of the linear line, which is 0.20 eV for the LSM-SDC composite (1:2, weight ratio) and 0.55 eV for SDC, respectively. This indicates that the LSM-SDC composite possesses possible interfacial conduction with lower activation energy than that of the SDC via bulk conduction of pure SDC [32]. It is consistent with the fuel cell performance in Figure 3. To further investigate the ionic grain boundary transportation resistance of LSM-SDC composites, the EIS measurement was conducted, and the results were revealed in Figure 7a. The intercept at high frequency reflects ohmic resistance, whereas the diameters of the semicircles at medium and low frequency reflect the ionic grain boundary transportation resistance and charge-transfer resistance, respectively [37,38]. An equivalent circuit mode LRΩ(RaZa)((RbZb)(RcZc) (Figure 7b) is still employed to fit the EIS data. The fitting results (Table 1) showed that, compared with SDC (Ra = 0.084 Ω·cm 2 ), LSM-SDC composites possessed lower ionic grain boundary transportation resistances (Ra, 0.023-0.060 Ω·cm 2 ). At the hetero-interface, such as between perovskite LSM material and fluorite SDC material, atomic reconstruction generates high-mobility plane with a large amount of oxygen vacancies, yielding colossal values of the ionic conductivity [2].  Figure 6 shows Arrhenius plots of pure SDC and LSM-SDC (1:2, weight ratio) composite at the temperature from 450 • C to 550 • C. Conductivities were measured by the EIS method in the fuel cell atmosphere. The measured EIS at different temperatures were also fitted with the equivalent circuit of LR Ω (R a Z a )((R b Z b )(R c Z c ). The conductivity σ can be obtained by where δ-pallet thickness (0.7 mm), A-pallet surface area (0.64 cm 2 ). It is noted that two samples exhibited a linear dependence between 550 and 450 • C. The activation energy can be calculated by the slope of the linear line, which is 0.20 eV for the LSM-SDC composite (1:2, weight ratio) and 0.55 eV for SDC, respectively. This indicates that the LSM-SDC composite possesses possible interfacial conduction with lower activation energy than that of the SDC via bulk conduction of pure SDC [32]. It is consistent with the fuel cell performance in Figure 3.  Figure 6 shows Arrhenius plots of pure SDC and LSM-SDC (1:2, weight ratio) composite at the temperature from 450 °C to 550 °C. Conductivities were measured by the EIS method in the fuel cell atmosphere. The measured EIS at different temperatures were also fitted with the equivalent circuit of LRΩ(RaZa)((RbZb)(RcZc). The conductivity σ can be obtained by σ = δ/(A(RΩ + Ra)) (2) where δ-pallet thickness (0.7 mm), A-pallet surface area (0.64 cm 2 ). It is noted that two samples exhibited a linear dependence between 550 and 450 °C. The activation energy can be calculated by the slope of the linear line, which is 0.20 eV for the LSM-SDC composite (1:2, weight ratio) and 0.55 eV for SDC, respectively. This indicates that the LSM-SDC composite possesses possible interfacial conduction with lower activation energy than that of the SDC via bulk conduction of pure SDC [32]. It is consistent with the fuel cell performance in Figure 3. To further investigate the ionic grain boundary transportation resistance of LSM-SDC composites, the EIS measurement was conducted, and the results were revealed in Figure 7a. The intercept at high frequency reflects ohmic resistance, whereas the diameters of the semicircles at medium and low frequency reflect the ionic grain boundary transportation resistance and charge-transfer resistance, respectively [37,38]. An equivalent circuit mode LRΩ(RaZa)((RbZb)(RcZc) (Figure 7b) is still employed to fit the EIS data. The fitting results (Table 1) showed that, compared with SDC (Ra = 0.084 Ω·cm 2 ), LSM-SDC composites possessed lower ionic grain boundary transportation resistances (Ra, 0.023-0.060 Ω·cm 2 ). At the hetero-interface, such as between perovskite LSM material and fluorite SDC material, atomic reconstruction generates high-mobility plane with a large amount of oxygen vacancies, yielding colossal values of the ionic conductivity [2]. To further investigate the ionic grain boundary transportation resistance of LSM-SDC composites, the EIS measurement was conducted, and the results were revealed in Figure 7a. The intercept at high frequency reflects ohmic resistance, whereas the diameters of the semicircles at medium and low frequency reflect the ionic grain boundary transportation resistance and charge-transfer resistance, respectively [37,38]. An equivalent circuit mode LR Ω (R a Z a )((R b Z b )(R c Z c ) (Figure 7b) is still employed to fit the EIS data. The fitting results (Table 1) showed that, compared with SDC (R a = 0.084 Ω·cm 2 ), LSM-SDC composites possessed lower ionic grain boundary transportation resistances (R a , 0.023-0.060 Ω·cm 2 ). At the hetero-interface, such as between perovskite LSM material and fluorite SDC material, atomic reconstruction generates high-mobility plane with a large amount of oxygen vacancies, yielding colossal values of the ionic conductivity [2]. At the weight ratio of 1:2 (LSM:SDC), the R a of LSM-SDC composite was the lowest (0.023 Ω·cm 2 ), indicating that this proportion is beneficial to construct more hetero-interfaces. It is consistent with the Arrhenius plots ( Figure 6) and the fuel cell performance (Figure 3). At the weight ratio of 1:2 (LSM:SDC), the Ra of LSM-SDC composite was the lowest (0.023 Ω·cm 2 ), indicating that this proportion is beneficial to construct more hetero-interfaces. It is consistent with the Arrhenius plots ( Figure 6) and the fuel cell performance (Figure 3).

Conclusions
The semiconductor-ionic LSM-SDC composite materials have been synthesized and assembled the SOFCs successfully. The maximum power density of the assembled SOFC was improved from 437 mW cm −2 for SDC electrolyte to 742 mW cm −2 for LSM-SDC semiconductor-ionic membrane at 550 °C. The interfaces between the semiconductor LSM and ionic conductor SDC, as the ionic transport highways, significantly improved the ionic conductivity of the SOFCs. The Schottky junction, existing in LSM-SDC membrane device, avoided the electronic short circuiting problem. The influences of the composite weight ratio on the fuel cell performance were investigated to find the optimal proportion of the composite, LSM:SDC = 1:2. The successful application of LSM-SDC composite on the LT-SOFCs provides a new direction for the development of LT-SOFCs.

Conclusions
The semiconductor-ionic LSM-SDC composite materials have been synthesized and assembled the SOFCs successfully. The maximum power density of the assembled SOFC was improved from 437 mW cm −2 for SDC electrolyte to 742 mW cm −2 for LSM-SDC semiconductor-ionic membrane at 550 • C. The interfaces between the semiconductor LSM and ionic conductor SDC, as the ionic transport highways, significantly improved the ionic conductivity of the SOFCs. The Schottky junction, existing in LSM-SDC membrane device, avoided the electronic short circuiting problem. The influences of the composite weight ratio on the fuel cell performance were investigated to find the optimal proportion of the composite, LSM:SDC = 1:2. The successful application of LSM-SDC composite on the LT-SOFCs provides a new direction for the development of LT-SOFCs.

Conflicts of Interest:
The authors declare no conflict of interest.