An Efficient Electron-Blocking Interlayer Induced by Metal Ion Diffusion

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Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

An Efficient Electron-Blocking Interlayer Induced by Metal Ion Diffusion for SOFC Based on Y‑Doped Ceria Electrolyte Jiafeng Cao, Yi Liu, Xianshan Huang, and Yuexia Ji* School of Mathematics and Physics, Anhui University of Technology, Maanshan 243032, P. R. China S Supporting Information *

ABSTRACT: To suppress the internal electronic leakage at ceria-based electrolyte, a novel electron-blocking layer consisting of doped BaCe0.8Y0.2O3−δ was fabricated in situ at the interface of Ba-containing anode and Y-doped ceria electrolyte. The anode-supported full cell based on Y0.2Ce0.8O1.9 (YDC20) electrolyte presents a remarkable peak power density of 814 mW/cm2 as well as an open-circuit voltage of 1.0 V at 650 °C, which are much higher than those of the cells with Gd0.1Ce0.9O1.95 (GDC10) electrolyte (453 mW/cm2 at 650 °C) and BaCe0.8Y0.2O3−δ|Y0.2Ce0.8O1.9 (BCY|YDC20) bilayered electrolyte (419 mW/cm2 at 650 °C). The efficient promotion of the electron-blocking interlayer with high oxygen ionic conductivity is considered as the main reason for the improved performance of YDC20-based solid oxide fuel cell. The composition and the microstructure of the electron-blocking interlayer are further analyzed by scanning electron microscope and transmission electron microscope characterizations. KEYWORDS: solid oxide fuel cells, internal electronic leakage, Y-doped ceria, electron-blocking layer, in situ reaction

1. INTRODUCTION Solid oxide fuel cell (SOFC) has been regarded as one of the most efficient and fuel flexible energy-conversion devices.1−4 Reducing the operating temperature (below 700 °C) of SOFC is an urgent issue to develop low-temperature solid oxide fuel cell (LT-SOFC), which can reduce the system cost, decrease the performance degradation rate, and substantially increase the lifetime of SOFC.5−7 Doped ceria (DCO) is a widely used solid electrolyte in the research of LT-SOFC. Gd3+- and Sm3+-doped ceria are the most popular electrolytes in the development of LT-SOFCs for their higher ionic conductivities than the other rare-earth ions-doped ceria.8,9 However, the partial internal electronic short circuit induced by the reduction of Ce4+ to Ce3+ under reducing atmosphere remains a significant challenge for the applications of ceria-based electrolytes in LT-SOFC devices.10,11 To eliminate the internal electronic short circuit through electrolytes mentioned above, many attempts have been reported, such as depositing Y0.1Zr0.9O2−δ (YSZ)12,13 or doped bismuth oxide14,15 on ceria electrolyte membrane. However, YSZ seems not to be a very proper choice considering its low ionic conductivity. Meanwhile, the partial internal short circuit still existed in the cell implanted by a doped bismuth oxide layer demonstrating limited improvement of the cell performance. Recently, the in situ fabrication of an electron-blocking layer between anode and electrolyte through Ba ions diffusion has been reported and considered as an efficient approach in suppressing the internal electronic leakage at ceria-based electrolytes.16,17 The new structure in the cell can sufficiently insulate the ceria electrolyte from the reducing fuel © XXXX American Chemical Society

and thus block off the internal electronic short circuit. Our previous research reported that a novel Ba-containing anode 0.6NiO−0.4BaZr0.45Ce0.45Gd0.1O3−δ (NiO−BZCG) was used to fabricate a thin electron-blocking interlayer in situ at the interface of anode and Gd0.1Ce0.9O1.95 (GDC10) electrolyte.18 The new functional interlayer can efficiently reduce the n-type electronic conductivity at GDC10 causing enhanced open cell voltages (OCVs). However, the performance of the cell supported by NiO−BZCG is inferior to most traditional ceria-based SOFCs. The functional interlayer can be considered as the main factor limiting the cell performance under working conditions. Therefore, improving the electron-blocking layer is an urgent issue to develop new strategy of eliminating the internal electronic short circuit at ceria-based electrolytes and to study the working mechanism of the interlayer. Until now, the research about the controllable design of the electron-blocking layer has not been frequently reported and not attracted sufficient attention. Therefore, the controllable construction of the interlayer between anode and electrolyte is valuable and important to the development of ceria-based SOFCs with high performance. In this research, 20 mol % Ydoped ceria was employed as the electrolyte to achieve the fabrication of doped BaCe0.8Y0.2O3−δ electron-blocking interlayer under high-temperature sintering. The micromorphology of this novel interlayer was characterized. The improvement of Received: December 13, 2017 Accepted: March 1, 2018 Published: March 1, 2018 A

DOI: 10.1021/acsami.7b18924 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces this functional layer to the performance of DCO-based SOFCs was further analyzed.

2. EXPERIMENTAL SECTION In this work, the samples for anode, electrolyte, and cathode were synthesized in one step by a citric acid−nitrate gel combustion process according to previous reports.18,19 C4H6BaO4 (99% purity), Zr(NO3)4· 5H2O (99% purity), Ce(NO3)3·6H2O (99% purity), Gd2O3 (99.95% purity), and Ni(NO3)2·6H2O (98% purity) raw materials from S i n o p h a r m w e r e u s e d i n t h e s y nt h e s i s o f 0 . 6 N i O − 0.4BaZr0.45Ce0.45Gd0.1O3−δ (NiO−BZCG) powder (NiO−BZCG; weight ratio, 6:4). Stoichiometric aforementioned raw powders were first dissolved in diluted nitric acid. Citric acid was used as a complexing agent, and the molar ratio of citric acid and metal ions at anode powder was 1.5. NH4OH was used to adjust the pH value at 7. The solution was under the conditions of pH 7 and stationary complexation for at least 8 h. Then, the solution was continuously heated under stirring until ignited to a flame and then burned off to be a black ash. The ash was then transferred into a furnace annealed at 1050 °C for 5 h to obtain pure NiO−BZCG anode powder. Starch (20 wt %) was added into the NiO−BZCG composite by sufficient ball milling for 24 h to form porosity at anode. Y0.2Ce0.8O1.9 (YDC20) was employed as the electrolyte and also synthesized by the same method with a calcining process at 600 °C for 5 h. For comparison, cells with Gd0.1Ce0.9O1.95 (GDC10) electrolyte and BaCe0.8Y0.2O3−δ|Y0.2Ce0.8O1.9 (BCY|YDC20) bilayered electrolyte were also studied in this issue. The electrolyte ash for comparison and Sm0.5Sr0.5CoO3−δ (SSC) and Ce0.8Sm0.2O3−δ (SDC) cathode ash were also synthesized by the same method. After combustion, the as-prepared GDC10 and BCY powders were heated at 600 °C for 5 h and 1000 °C for 3 h, respectively, to obtain pure phase. The SSC and SDC powders were mixed in a weight ratio of 7:3 and mixed thoroughly together with a 6 wt % ethylcellulose−terpineol binder to prepare the cathode slurry. Anode-supported half-cell was obtained by a co-pressing process with the same amount of electrolyte powder (the weight ratio of BCY/ YDC20 is 1:2 for the bilayered electrolyte) and co-fired at 1350 °C for 5 h to obtain half-cell on YDC20, GDC10, or BCY|YDC20 electrolytes of high density. Subsequently, SSC−SDC composite cathode slurry was painted onto the electrolytic side and co-sintered at 950 °C for 3 h in air to form full cell. To investigate the microstructural characteristics of the diffusion layer at the interface of anode and electrolyte, the sample from the diffusion layer was obtained through a co-firing process with YDC20 sintered between two pressed NiO−BZCG pellets at 1350 °C for 5 h. X-ray diffraction (XRD) patterns of the powders were analyzed by an X-ray diffractometer (Rigaku TTR-III) in the range of 20−80°. Microstructures of the samples were observed with a scanning electron microscope (JSM-6700F) and a high-resolution transmission electron microscope (JEM-2100F). Cell-testing measurements were performed using 3% H2O humidified hydrogen as fuel at a flow rate of 20 mL min−1, and the cathode was exposed to atmospheric air in a cell-testing system with the temperature changing from 650 to 500 °C. Silver paste was applied as a current collector over the cathodes, and silver wires were used as the conducting wires for fuel cells test. I−V curves of the cells were measured with a DC electronic load (ITech electronics model IT8511) based on the two-probe configuration. Electrochemical impedance spectra (EIS) were obtained under opencircuit conditions by a CHI604E impedance analyzer for the frequency-dependent observation from 100 kHz to 0.1 Hz. The polarization resistances of the cells under open-circuit conditions were determined from the recorded EIS results.

Figure 1. XRD patterns of NiO−BZCG, YDC20, and SSC−SDC powders.

YDC20 (PDF no. 75-0175), whereas the spectra of anode and cathode powders correspond to the phases of pure NiO− BZCG and SSC−SDC without any impurity, indicating successful syntheses of the aimed powders in this work. Scanning electron microscopy (SEM) detection was used to investigate the morphology and particle size of the YDC20 electrolyte sintered at 1350 °C for 5 h. As is presented in Figure 2, the YDC20 electrolyte with particles diameters in the range

Figure 2. SEM image of the electrolytic surface of NiO−BZCG| YDC20 half-cell after sintering at 1350 °C for 5 h.

of 1−5 μm possesses a smooth surface and high density without any pore, which is important to the sufficient separation of fuel and oxygen and beneficial for the energyconversion efficiency and stability of the cell. The morphology of the cross section of NiO−BZCG| YDC20|SSC−SDC full cell was investigated and is shown in Figure 3. Both the anode and cathode display a porous morphology and tightly attach on the facet of dense YDC20 electrolyte. It is clear that this porous construction is beneficial for the transport of the fuel, oxygen, and the reaction products of the cell under working conditions. SEM observation coupled with energy-dispersive X-ray spectroscopy (EDS) analysis was conducted to determine the elements distribution at the NiO−BZCG|YDC20 interface. An

3. RESULTS AND DISCUSSION Figure 1 provides the XRD spectra of the as-prepared powders for the fabrication of full cells. It is clear that through combustion preparations, well-crystallized samples with sharp diffraction peaks can be obtained. The diffraction peaks of the electrolyte powder are well matched with the cubic phase of B

DOI: 10.1021/acsami.7b18924 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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demonstrates that Ba ions have obviously diffused into the electrolyte in a micrometer scale (3.6 μm), forming a functional layer at the anode|electrolyte interface. Figure 5 displays the plots of cell voltage and power density versus current density for NiO−BZCG|YDC20|SSC−SDC,

Figure 3. SEM image of the cross section of NiO−BZCG|YDC20| SSC−SDC full cell.

SEM−EDS analysis of a representative interface of the sample is shown in Figure 4. The different positions of Ba (red curve) and Ni (green curve) peak edges along the interface can be regarded as an obvious illustration of the metal ion diffusion. Focusing on this interface, the peak edge of Ba signal can be found extending to the right-hand side of the Ni signal, which

Figure 5. (a) I−V and I−P curves of NiO−BZCG|YDC20|SSC−SDC full cell, (b) NiO−BZCG|GDC10|SSC−SDC,18 and (c) NiO−BZCG| BCY|YDC20|SSC−SDC at the same temperature range.

NiO−BZCG|GDC10|SSC−SDC, and NiO−BZCG|BCY| YDC20|SSC−SDC full cells. All of the as-prepared cells show higher OCVs compared to the cell supported by NiO−GDC anode (about 0.8 V)18 at the whole temperature range. In Figure 5a, the OCVs of the full cell with YDC20 electrolyte are observed to be 1.0, 1.011, and 1.02 V at 650, 600, and 550 °C, respectively, which are much higher than those of the ceriabased SOFCs with traditional anodes.16,18 The improved OCVs can be attributed to the sufficient suppression of internal short

Figure 4. SEM−EDS analysis of the interface between NiO−BZCG and YDC20 membrane for the half-cell sintered at 1350 °C for 5 h. C

DOI: 10.1021/acsami.7b18924 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Table 1. OCVs and Power Densities (PDs) of Recent Typical DCO-Based SOFCs with or without Electron-Blocking Layers anodes NiO−GDC NiO−GDC NiO−SDC NiO−SDC NiO−BZCYYb NiO−BZY NiO−BZCG NiO−SDC NiO−BZCG

electrolytes GDC10 (3 μm) GDC10 (20 μm) SDC15 (20 μm) SDC20 (18 μm) SDC (30 μm) SDC20 (15 μm) GDC10 (20 μm) BaZr0.1Ce0.7Y0.2O3−δ (11 μm)|Ce0.8Sm0.2 O2−δ (19 μm) YDC20 (17 μm)

cathodes LSCF SSC−GDC BSCF SSC−SDC LSCF SSC−SDC SSC−SDC SSC−SDC SSC−SDC

OCVs (V) 0.86 0.787 0.87 0.849 1.029 1.037 1.012 1.04

(600 (650 (600 (650 (650 (650 (650 (650

°C) °C) °C) °C) °C) °C) °C) °C)

1.0 (650 °C)

MPDs (mW/cm2) 992 694 1010 1011 640 638 453 267

(600 (650 (600 (650 (650 (650 (650 (650

°C) °C) °C) °C) °C) °C) °C) °C)

814 (650 °C)

PDs (mW/cm2/0.9 V) 0 0 0 0 200 239 101 ∼80

(600 (650 (600 (650 (650 (650 (650 (650

°C) °C) °C) °C) °C) °C) °C) °C)

206 (650 °C)

ref 22 23 2 9 17 24 18 25 this work

BaZr0.1Ce0.7Y0.2O3−δ|Ce0.8Sm0.2O2−δ electrolyte that works as an electron-blocking layer possesses a similar composition to the electron-blocking layer in this work.) The thick BaZr0.1Ce0.7Y0.2O3−δ layer (11 μm) and the insufficient contact surface between BaZr0.1Ce0.7Y0.2O3−δ and Ce0.8Sm0.2O2−δ are the main limiting factors that are disadvantageous for the enhancement of the cell performance. Through the in situ fabrication in this work, the electron-blocking layer can be formed in a micrometer scale (as shown in Figure 4), whereas the interface can be effectively improved, resulting in the enhanced MPDs. Figure 6 presents the electrochemical impedance spectra (EIS) of the cells on YDC20, GDC10, and BCY|YDC20 membranes measured from 650 to 550 °C under open-circuit conditions. The plots of these cells with different electrolytes can be simulated according to the equivalent circuit in the insets, which will be discussed in the following section. The high-frequency intercept of EIS corresponds to the Ohmic resistance (Ro) mainly representing the electrolyte resistance. The low-frequency intercept corresponds to the total resistance of the cell. The difference between the high-frequency and lowfrequency intercepts with the real axis is the total interfacial polarization resistance (Rp) of the cell, which is mainly contributed by the microstructure of the electrodes and the electrode|electrolyte interfaces. Accordingly, the improved cell in this work displays much higher Rp values than those of the cells without Ba-containing anodes19 at the measuring temperature points resulting from the formation of an electronblocking interlayer. These results are well consistent with the previous research works:26,27 the high Rp values of the improved cells can be ascribed to the in situ fabrication of the functional interlayer, which suppresses the leaking current through the electrolyte film and generally reduces the electrode reactions. To characterize the stability of the NiO−BZCG|YDC20| SSC−SDC cell, the power density during long-term testing under H2/air operation at a constant voltage of 0.7 V at 600 °C was obtained, as is provided in Figure 7. The power density of the improved cell remains extremely stable during the testing course of 50 h, which can be attributed to the good thermal expansion compatibility and chemical stability of the electronblocking layer in the cell induced by the diffusion of metal ions, including Ba, Ni, Zr, Gd, and so on. As is widely known, barium cerate that can react with H2O forming hydroxides is unstable with pH2O of 1 atm even below 400 °C.28,29 Commonly, Zrdoped barium cerate displays high improvement of the chemical stability in the presence of water or CO 2 vapor.30−32 The enhanced stability of the electron-blocking

circuit at YDC20 electrolyte. The maximum power densities (MPDs) of the cells with different electrolytes plotted in Figure 5a−c exhibit obvious difference under the same testing temperature. For the NiO−BZCG|YDC20|SSC−SDC cell (Figure 5a), the MPDs are 814, 632, and 449 mW/cm2 at 650, 600, and 550 °C, respectively, whereas those of the NiO− BZCG|GDC10|SSC−SDC cell are 453, 300, and 156 mW/cm2 at the same temperatures (Figure 5b). According to the abovetested OCVs and power densities, the electrolytes and the interlayers in the cells are the main reasons for the different performances under the same conditions. As is widely known, GDC10 electrolyte displays much higher oxygen ionic conductivity than YDC20,20,21 and GDC10 is commonly employed as the electrolytic candidate in ceria-based SOFCs. Thus, the superior power densities output for YDC20-based cell can be finally attributed to the different functional interlayer formed between the NiO−BZCG anode and the YDC20 electrolyte compared to the SOFC based on GDC10. In addition, it can be seen in Figure 5a that the OCVs of the cell on the bilayered electrolyte are relatively lower than those of the cell on YDC20 electrolyte. And the MPD values of the NiO−BZCG|YDC20|SSC−SDC cell are much superior to those of the NiO−BZCG|BCY|YDC20|SSC−SDC cell (419, 331, and 227 mW/cm2 at 650, 600, and 550 °C, respectively), demonstrating the advantage of the in situ fabrication in synthesizing thin electron-blocking interlayer between anode and electrolyte. Table 1 lists some common DCO-based SOFCs with or without electron-blocking layers. Compared to NiO−SDC and NiO−GDC anodes, Ba-containing anodes generally induce an obvious increase in the OCV value of the full cell. Traditionally, Gd or Sm dopants are commonly used in the research of ceriabased electrolytes. It is notable that the property of the cell with 20 mol % Y-doped ceria electrolyte in this work can be seen comparable to or even higher than many cells based on GDC or SDC electrolytes. Besides, the improved cell in this research shows a relatively higher power density of 206 mW/cm2 with a high working voltage of 0.9 V, whereas the cells with NiO− GDC or NiO−SDC anodes can hardly output any power density under the same voltage. Thus, it is notable that Y-doped ceria electrolyte may be more suitable in the ceria-based cell supported by Ba-containing anodes compared to the cells based on GDC or SDC electrolytes. Furthermore, the MPDs in this work are much higher than those of the fuel cell based on bilayered BaZr0.1Ce0.7Y0.2O3−δ (11 μm)|Ce0.8Sm0.2O2−δ (19 μm) electrolyte evaluated for the elimination of internal short circuit at SDC with peak power densities of only 267, 197, and 127 mW/cm2 at 650, 600, and 550 °C, respectively.25 (The BaZr0.1Ce0.7Y0.2O3−δ in the cell coupled with the bilayered D

DOI: 10.1021/acsami.7b18924 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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cell. (It can be seen in Figures S1 and S2 that the electronblocking interlayer possesses a high Zr concentration, which will be discussed in the following paragraphs.) These results show that the functional diffusion layer formed between NiO− BZCG and YDC20 is beneficial for the improvement of the stability of SOFC. To determine the chemical composition of the functional interlayer, the YDC20 fired between two NiO−BZCY pellets was observed by a high-resolution transmission electron microscope. Figure S1 shows the transmission electron microscopy (TEM) results in line-scan mode. It is obviously seen that strong signals of Ba, Ce, and Y elements can be detected. Besides, Zr, Ni, and Gd signals can also be observed and the distribution of these elements can be found uniform without obvious segregation. TEM surface scan of the sample was also carried out and is shown in Figure S2. The results in Figure S2 are well consistent with the TEM line-scan analyses, illustrating the same uniform distributions of the above ions and no obvious segregation. Accordingly, the metal-ions distributions in that electron-blocking interlayer are uniform, probably forming a new compound with a single phase. To characterize the phase of the electron-blocking layer, high-resolution transmission electron microscope analysis of the as-prepared grain from the interlayer was obtained, as is shown in Figure 8. It can be seen that the high-resolution image

Figure 6. (a) Impedance spectra plots of the cell on YDC20 membrane, (b) EIS results of NiO−BZCG|GDC10|SSC−SDC cell, and (c) EIS plots of the cell on BCY|YDC20 bilayered membrane. All of the above cells were measured under open-circuit conditions from 650 to 550 °C.

Figure 8. High-resolution TEM image of the grain in the electronblocking layer.

is highly consistent with the PDF card no. 82-2372 with the interplanar crystal spacing of 0.31 nm for (002) lattice plane, which can be typically identified as a doped BaCeO3 oxide. Thus, the functional interlayer in this work can be confirmed as a new BaCeO3-based oxide with perovskite structure induced by the in situ reaction between different diffused metal ions. Because of the presence of the chemical composition gradient at the interface of the NiO−BZCG anode and the YDC20 electrolyte, the metal ions (Ba, Zr, Ni, and Gd) at the anode can inevitably diffuse into the electrolyte under high-temperature sintering. According to the elements intensities in Figure S1, the in situ reaction between Ba ions and YDC20 can induce

Figure 7. Power density (mW/cm2) as a function of time (h) for NiO−BZCG|YDC20|SSC−SDC full cell operating at 600 °C under a working voltage of 0.7 V.

layer demonstrated that Zr ions sufficiently participate in the in situ reaction at the interface, increasing the stability of the full E

DOI: 10.1021/acsami.7b18924 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces the formation of BaCe0.8Y0.2O3−δ oxide. Other metal ions, Zr, Ni, and Gd, also participate in the in situ reaction, probably working as metal dopants. Thus, it can be deduced that Zr, Ni, and Gd co-doped BaCe0.8Y0.2O3−δ can be fabricated at the interface of NiO−BZCG and YDC20. Considering the interlayer with perovskite structure formed between anode and electrolyte, the oxygen ions should diffuse through the interlayer after they transfer from the electrolyte to react with the fuel at anode. And the resistance to oxygen ions diffusion through the interlayer is an important factor that cannot be negligible. Therefore, the equivalent circuit model,33 as is seen in the insets of Figure 6, should be used in this work considering the resistance to oxygen ions diffusion through the interdiffusion layer. The resistance to oxygen ions diffusion of the interlayer can be represented by a finite-diffusion Warburg impedance Zw. Ro represents the Ohmic resistance of bulk electrolyte, the Ohmic contact resistance at the anode and cathode sides. R1 and R2 are anodic polarization resistance and cathodic polarization resistance (Ω cm2), respectively. C1 and C2 represent the double-layer capacitances (F cm2) at the anode side and cathode side, respectively. Figure S3 provides the impedance fitting results of NiO−BZCG|YDC20|SSC− SDC cell based on the aforementioned equivalent circuit model (Figure 6). As clearly seen in Figure S3a−c, the simulated impedance data fit well with the measured impedance data at the operating temperatures from 650 to 550 °C. Subsequently, the impedance-fitted results of NiO−BZCG|GDC10|SSC− SDC and NiO−BZCG|BCY|YDC20|SSC−SDC were also obtained based on the same equivalent circuit model, as shown in Figures S4 and S5. All of the simulated results fit well with the measured impedance data except the result in Figure S5a, in which the fitted spots deviate more from the actual data due to the insufficient points from the EIS records for NiO− BZCG|BCY|YDC20|SSC−SDC cell induced by the partial decomposition of BCY under water vapor-containing atmospheres, which will inevitably result in recording discrete points at low frequencies.34,35 From the fitted results in Figures S3− S5, one interesting comparison of the Zw(R) (Ω cm2) values demonstrates a reasonable evolution trend for the oxygen ionic conduction abilities of the different interlayers between anode and electrolyte. For the NiO−BZCG|YDC20|SSC−SDC full cell, the values of Zw(R) are simulated to be 0.0473, 0.2492, and 0.6086 Ω cm2 at 650, 600, and 550 °C, whereas the corresponding values of the full cells based on GDC10 electrolytes are simulated to be 0.474, 0.9721, and 1.905 Ω cm2, respectively. In addition, the Zw(R) values of the cell on the BCY|YDC20 bilayered electrolyte are 0.3137, 0.8912, and 1.733 Ω cm2 at 650, 600, and 550 °C, respectively. Clearly, the cell with the in situ fabrication of doped BaCe0.8Y0.2O3−δ electron-blocking interlayer displays higher oxygen ionic conductivity than the cells with GDC10 or BCY|YDC20 electrolytes, which is beneficial for the oxygen ions diffusion through the interlayer and the improvement of cell performance. Figure 9 shows the schematic illustrations of the cells with different anodes and electrolytes. For the cell composed of the NiO−YDC20 anode and the YDC20 electrolyte, the electrolyte exposed to the reducing atmosphere inevitably shows reduction from Ce4+ to Ce3+, resulting in the partial internal electronic short circuit through the electrolyte, as shown in Figure 9a. On the basis of the reaction mechanism mentioned above, for the cell with GDC10 electrolyte supported by NiO−BZCG, the functional interlayer consisting of Zr, Ni co-doped Ba-

Figure 9. (a) Illumination of electronic conduction through NiO− YDC|YDC20 interface. (b) Mechanism of the in situ reaction induced by metal ion diffusion at the interface between NiO−BZCG and GDC10. (c) Mechanism of the in situ reaction at the interface of NiO−BZCG and YDC20. (d) Schematic representation of the electronic and oxygen ionic conduction at the interface under working conditions.

Ce0.9Gd0.1O3−δ can be formed and works as the electronblocking component between NiO−BZCG and GDC10 (Figure 9b). As is schematically illustrated in Figure 9c, under high-temperature sintering, the compacting attachment between NiO−BZCG and YDC20 is advantageous for the formation of a new composition due to the metal ion diffusion between electrolyte and anode. According to the in situ reaction, it is proposed that Zr, Ni, Gd co-doped BaCe0.8Y0.2O3−δ oxide can be formed in situ at the interface of anode and electrolyte. Commonly, doped BaCe0.8Y0.2O3−δ oxide is a mixed proton and oxide ion conductor under fuel cell conditions.36,37 The doped BaCe0.8Y0.2O3−δ oxide in the interlayer works as an oxide ionic conductor, but suppresses the electronic conduction through the electrolyte. The total conductivity of the interlayer can remarkably affect the performance of the full cell. Previous research works have provided that 20 mol % Y-doped BaCeO3 or BaCeO3-BaZrO3based electrolytes demonstrated higher conductivities than the electrolytes with 10 mol % Gd doped BaCeO3-BaZrO3-based oxides.36,37 It can be deduced that Zr, Ni, Gd co-doped BaCe0.8Y0.2O3−δ oxide presents a higher conductivity than Zr, Ni co-doped BaCe0.9Gd0.1O3−δ oxide. Therefore, the internal F

DOI: 10.1021/acsami.7b18924 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

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electronic leakage at Y-doped ceria electrolyte can be efficiently suppressed, whereas the total conductivity of the interlayer still maintains in a high level. Because of the higher conductivity of the improved interlayer, the cell based on the YDC20 electrolyte demonstrated higher MPDs than the cell with the GDC10 electrolyte.

4. CONCLUSIONS To eliminate the internal electronic leakage at ceria-based electrolyte and to improve the conductivity of the functional interlayer, NiO−BZCG anode-supported cell based on YDC20 electrolyte was fabricated. The corresponding results showed that the cell with YDC20 electrolyte exhibited higher performances than the cell based on GDC10 and BCY| YDC20 bilayered electrolyte. The main reason can be probably attributed to the different interlayer formed between anode and electrolyte under high-temperature sintering. Zr, Ni, Gd codoped BaCe0.8Y0.2O3−δ interlayer between NiO−BZCG and YDC20 demonstrated a higher total conductivity than Zr, Ni co-doped BaCe 0.9 Gd 0.1 O 3−δ (between NiO−BZCG and GDC10) and BCY interlayer fabricated by direct co-pressing process, which was beneficial for the improvement of cell performances. This research demonstrates that the construction of the electron-blocking layer with improved composition is an efficient strategy to improve the performance of ceria-based solid oxide fuel cells.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b18924. TEM and EDS images in line-scan mode of YDC20 sintered between two anode pellets; TEM−EDS elemental mappings of the sample; and impedance fitting results of different samples (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Jiafeng Cao: 0000-0002-7371-0383 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Science Foundation of China (Grant Nos. 51502004, 11404004, and 11474003). REFERENCES

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DOI: 10.1021/acsami.7b18924 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

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DOI: 10.1021/acsami.7b18924 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX