Samaria-doped Ceria Electrolyte Research Articles

Fe-Doped SDC Solid Solution as an Electrolyte for Low-to-Intermediate-Temperature Solid Oxide Fuel Cells.

Ceria-based oxides, such as samaria-doped ceria (SDC), are potential electrolytes for low-to-intermediate-temperature solid oxide fuel cells (SOFCs). The sinterability of these materials can be improved by adding iron as the sintering aid. This work reveals that Fe is soluble in SDC, forming an Fe-doped SDC solid solution. It is found that the solubility is affected by the sintering temperature. Fe doping has obvious effects on electrolyte properties, including sintering characteristics, thermal expansion behaviors, and electrical conductivities in both air and hydrogen atmospheres. The conductivity obviously increases while the activation energy decreases by doping Fe. Compared with that of the bare SDC electrolyte, the performance of the single cell with the Fe-doped SDC is enhanced; for example, the peak power density is increased by 52.8% to 0.726 W cm-2 at 600 °C when humidified hydrogen is used as the fuel and ambient air is used as the oxidant. The single cell showed stable operation at 600 °C under a constant current density of 0.3 A cm-2 for 150 h. Therefore, the Fe-doped SDC solid solution shows promise as a potential electrolyte for low-to-intermediate-temperature SOFCs.

(Digital Presentation) Improvement of the Compatibility between the Current-Independent Constant Voltage Loss in Wagner’s Equation for SOFCs Using Sm-Doped Ceria Electrolytes and Jarzynski’s Equality

A solid oxide fuel cell (SOFC) converts chemical energy from a fuel gas, such as hydrogen or methane, to electrical energy. Yttria-stabilized zirconia (YSZ) films are often used as electrolytes, in which only oxygen ions are carriers. In such cases, the open-circuit voltage (OCV = 1.15 V at 1073 K) is equal to the Nernst voltage (Vth = 1.15 V at 1073 K).Samaria-doped ceria (SDC) has higher ionic conductivity than YSZ. Therefore, SDC films are possible electrolyte candidates. However, when SDC electrolytes are used in SOFCs, the OCV at 1073 K is only 0.80 V. The voltage loss has been explained by Wagner’s equation.Numerous subsequent models have been created based on Wagner’s equation. The current-voltage relationship can be calculated with the cathode and anode polarization voltage losses using the Riess model [1]. Furthermore, Duncan and Wachsman created a nonlinear model [2]. This model can also be used to explain the equilibration process. According to the Riess model, the OCV should decrease during electrode degradation. However, experimentally, the OCV does not change during electrode degradation [3]. The change in the equilibration of thick SDC electrolytes in response to a change in the anode gas has never been explained using the model of Duncan and Wachsman. Experimentally, when a very thick (6.6 mm) SDC electrolyte is used, the OCV can reach 0.80 V in only 5 minutes [4]. According to Weppner, the corresponding delay in the electron diffusion current should be more than 2080 minutes [5].We proposed a current-independent anode voltage loss (0.35 V = 1.15 V- 0.80 V) [6]. Since we disproved the existence of large leakage currents in SDC electrolytes, our explanations seem to disregard and disprove Wagner’s equation. However, we noted that the dismissal of the idea of large leakage currents in SDC electrolytes is only a side effect. When SDC electrolytes experience a large anode voltage loss (0.35 V), the leakage currents are very small. This phenomenon is called the “anode shielding effect” [7].The ionic activation energy (Ea) of SDC electrolytes is 0.7 eV (= 0.35 V ×2e). During ion hopping processes in SDC electrolytes, the work done by ions on the surrounding lattice structure is 0.7 eV. The ions should regain 0.7 eV after hopping. However, when there are many electrons in the hopping path, the hopping processes are very complex, and calculating the work loss is challenging [8].Jarzynski’s equality is very useful for calculating the Gibbs energy loss at the nanoscale. One ion hopping process in SDC electrolytes is at the nanoscale. However, the number of oxygen ions in SDC electrolytes is on the order of 1021, which is near Avogadro’s number. The number of oxygen ions that can participate in the hopping process is on the order of 1016 at 1073 K. Furthermore, the average frequency of the oxygen ions is 1013 Hz. Consequently, we are discussing an average Gibbs energy loss on the order of 1029 in one second. This means that our model is much more statistical as follows. Jarzynski’s equality ⊇ Our model (OCV = Vth -Ea /2e) (1) Fortunately, the canonical ensemble for the ion hopping process follows Boltzmann's distribution, which is an exponential distribution and determined only by the temperature of the environment. We try to improve the compatibility between the current-independent constant voltage loss in Wagner’s equation for SOFCs using SDC electrolytes and Jarzynski’s equality.

Solid Oxide Fuel Cells with Magnetron Sputtered Single-Layer SDC and Multilayer SDC/YSZ/SDC Electrolytes.

Samarium-doped ceria (SDC) is considered as an alternative electrolyte material for intermediate-temperature solid oxide fuel cells (IT-SOFCs) because its conductivity is higher than that of commonly used yttria-stabilized zirconia (YSZ). The paper compares the properties of anode-supported SOFCs with magnetron sputtered single-layer SDC and multilayer SDC/YSZ/SDC thin-film electrolyte, with the YSZ blocking layer 0.5, 1, and 1.5 μm thick. The thickness of the upper and lower SDC layers of the multilayer electrolyte are constant and amount to 3 and 1 μm, respectively. The thickness of single-layer SDC electrolyte is 5.5 μm. The SOFC performance is studied by measuring current-voltage characteristics and impedance spectra in the range of 500-800 °C. X-ray diffraction and scanning electron microscopy are used to investigate the structure of the deposited electrolyte and other fuel cell layers. SOFCs with the single-layer SDC electrolyte show the best performance at 650 °C. At this temperature, open circuit voltage and maximum power density are 0.8 V and 651 mW/cm2, respectively. The formation of the SDC electrolyte with the YSZ blocking layer improves the open circuit voltage up to 1.1 V and increases the maximum power density at the temperatures over 600 °C. It is shown that the optimal thickness of the YSZ blocking layer is 1 µm. The fuel cell with the multilayer SDC/YSZ/SDC electrolyte, with the layer thicknesses of 3/1/1 µm, has the maximum power density of 2263 and 1132 mW/cm2 at 800 and 650 °C, respectively.

Samarium Doped Ceria (SDC) Electrolyte Modification by Sintering Aids Addition to Reducing Sintering Temperature: A Review

The solid oxide fuel cell (SOFC) is a promising technology with specific characteristics for generating electricity by using hydrogen and oxidant as fuel. Typically, SOFC’s use Samarium doped Ceria (SDC) as an electrolyte material as the ionic conductivity of SDC was better at lower operating temperatures which are below than 700°C that making it a good option for low and moderate temperature applications for SOFC. However, SDC electrolytes are cannot be densified below 1500°C. If a densified ceria-based electrolyte can be prepared at lower temperatures it can be co-sintered with another electrode component. This simplifies the fabrication process and reduces the cost. Other than that, it can help with porous electrode microstructure control and avoiding phase diffusion and chemical interaction problems. As a result, decreasing the sintering temperature may be another step toward commercialising SOFC technology. The modification of electrolyte by adding sintering aid was found as an effective method to lowering the sintering temperature. This paper, therefore, focuses on reviewing the attempts made to modify SDC electrolyte by adding sintering aid (Li2O, CoO, CuO and FeO) in order to lowering sintering temperature. The studies related to temperature reduction, relative density, the microstructure of grains and conductivity of electrolyte was critically reviewed.

Effects of temperature on the electrical properties of samaria-doped ceria predicted with the statistical moment method

Samaria-doped ceria (SDC) is an important candidate electrolyte for use in solid oxide fuel cells, but the effect of lattice thermal vibrations on its electrical properties are unclear. In this study, the statistical moment method was employed to predict the temperature dependences of vacancy–dopant association, migration and activation energies, and ionic conductivity in four theoretical regimes comprising static, harmonic, semi-classical anharmonic, and quantum anharmonic regimes. The lattice thermal vibrations weakened vacancy–dopant associations and significantly inhibited vacancy hopping, and these effects were more pronounced as the temperature increased. The strongest vacancy–dopant associations and less convenient movement of oxygen vacancies could occur in the crystal lattice including anharmonic vibrations. Considerable enhancement of the ionic conductivity and a shift in the optimal dopant concentration toward higher values were found in the harmonic regime. Increasing the temperature also led to similar changes in the ionic conductivity. The maximum ion conductivity was governed by trapping and blocking effects, but also by lattice thermal vibrations. The results indicated the trivial influence of quantum effects in the anharmonic vibrations on the electrical properties. These findings provide a deeper understanding of the underlying atomistic mechanisms that determine the effects of temperature and lattice thermal vibrations on the electrical properties, and they may be beneficial for optimizing the ionic conductivity of SDC electrolytes.

(Digital Presentation) Additional Voltage Loss in Terms of Electromagnetic Potential Related to Jarzynski’s Equality Using Sm-Doped Ceria Electrolytes in Wagner’s Equation for SOFCs

A solid oxide fuel cell (SOFC) converts chemical energy from a fuel gas, such as hydrogen or methane, to electrical energy. Yttria-stabilized zirconia (YSZ) films are often used as electrolytes in which only oxygen ions are carriers. In such cases, the open circuit voltage (OCV = 1.15 V at 1073 K) is equal to the Nernst voltage (Vth = 1.15 V at 1073 K).Samaria-doped ceria (SDC) have higher ionic conductivity. Therefore, SDC films are possible electrolyte candidates. However, when SDC electrolytes are used in SOFCs, the OCV is only 0.80 V at 1073 K. The voltage loss has been explained by Wagner’s equation.Numerous subsequent models have been created based on Wagner’s equation. Using the Riess model, the current-voltage relationship can be calculated with the cathode and anode polarization voltage losses [1]. Furthermore, a nonlinear model was created by Duncan and Wachsman [2]. This model can also be used to explain the equilibrium process. According to the Riess model, the OCV should decrease during electrode degradation. However, experimentally, the OCV does not change during electrode degradation [3]. The change in the equilibration of thick SDC electrolytes in response to a change in the anode gas has never been explained using the model defined by Duncan and Wachsman. Experimentally, when a very thick (6.6 mm) SDC electrolyte is used, the OCV can reach 0.80 V in only 5 minutes [4]. According to Weppner, the corresponding delay in the electron diffusion current should be more than 2080 minutes [5].We proposed a current-independent anode voltage loss (0.35 V = 1.15 V- 0.80 V) [6]. Since we disproved the existence of large leakage currents in SDC electrolytes, our explanations seem to disregard and disprove Wagner’s equation. However, we noticed that the dismissal of this idea regarding large leakage currents in SDC electrolytes is nothing more than a side effect. When SDC electrolytes experience a large anode voltage loss (0.35 V), the leakage currents are very small. This is called the “anode shielding effect” [7].The ionic activation energy (Ea) of SDC electrolytes is 0.7 eV (= 0.35 V ×2e). During ion hopping processes in SDC electrolytes, the work done by ions on the surrounding lattice structure is 0.7 eV. The ions should regain the 0.7 eV after hopping. However, when there are many electrons in the hopping path, hopping processes are very complex, and calculating the loss of work is challenging. Jarzynski’s equality is a very useful method of calculating the loss of work only from the first equilibrium state to the second equilibrium state. Because the distribution of hopping ions always should be a canonical ensemble, the loss of work can be calculated. When there are enough electrons, the ions cannot regain the 0.7 eV, and voltage loss (0.35 V) occurs [8].To assist the above explanation, we use the electromagnetic potential to explain the voltage loss that occurs during ion hopping. We noticed that the potential of ions during hopping can be calculated from the Lorenz gauge condition.Reference:[1] I. Riess, J. Phys. Chem. Solids., 47(2), 129 (1986). doi: 10.1016/0022-3697(86)90121-6.[2] K. L. Duncan and E. D. Wachsman, J. Electrochem. Soc., 156, B1030 (2009). doi:10.1149/1.315851.[3] T. Miyashita, J. Mater. Sci., 41(10), 3183 (2006). doi: 10.1007/s10853-006-6371-8.[4] T. Miyashita, J. Electrochem. Soc., 164(11) 3190 (2017). doi: 10.1149/2.0251711jes[5] J. Liu and W. Weppner, Ionics, 5(1–2), 115 (1999). doi: 10.1007/BF02375914.[6] T. Miyashita, J. Mater. Sci., 40(22), 6027 (2005). doi: 10.1007/s10855-005-4560-2.[7] B. Dalslet, P. Blennow, P. V. Hendriksen, N. Bonanos, D. Lybye, and M. Mogensen, J. Solid State Electrochem., 10(8), 547 (2006). doi: 10.1007/s10008-006-0135-x.[8] T. Miyashita, Miyashita, ECSarXiv (2020) “Open-Circuit Voltage Anomalies in Yttria-Stabilized Zirconia and Samaria-Doped Ceria Bilayered Electrolytes”, https://ecsarxiv.org/xhn73/

Structural and Electrical Properties of Samarium-doped Ceria Electrolyte

Effect of dopant on structural and electrical properties of samarium-doped ceria (SDC) electrolyte was investigated using statistical moment method. Our results showed the lattice expansion due to doping and local distortion resulting from preferential distribution of oxygen vacancies around dopants. The ionic conductivity of SDC crystals is in more than three orders of magnitude larger than that of pure ceria. However, the presence of dopant in cation edge and the first nearest neighbor positions around migrating vacancy gives rise to the blocking and trapping effects on the vacancy transport. The maximum value of the ionic conductivity was found for the samples with the dopant concentration x = 0.15. These results can be compared to the one of the published data of boththe theoretical and experimental investigations.

Effect of fuel choice on conductivity and morphological properties of samarium doped ceria electrolytes for IT-SOFC.

The present investigation is emphasized on the effect of various combustion agents on the crystal properties, surface microstructure, and oxygen ion conductivity of 20% mole-Sm doped ceria (Ce0.80Sm0.20O1.90/SDC20) ceramics as solid electrolyte for IT-SOFCs. The most widely used combustion agents for engineering ceramic production as ethylene glycol, diethylene glycol, triethylene glycol, L-alanine, L-valine, glycine, citric acid monohydrate, urea, and EDTA-citric acid were compared in terms of SDC20 properties. X-ray diffraction (XRD), scanning electron microscopy (SEM), thermogravimetric analysis (TGA), and electrochemical impedance spectroscopy (EIS) were used to determine the microstructure properties, crystal structure and ionic conductivity of SDC20 powder. XRD pattern of the ceramics revealed the formation of single-phase fluorite structure. According to the results of electrochemical analysis, the maximum total ionic conductivity was observed in SDC20 electrolyte synthesized using triethylene glycol as the fuel among all the synthesized electrolytes (5.72 x 10–2 S.cm–1).

Impregnation of microporous SDC scaffold as stable solid oxide cell BSCF-based air electrode

Barium strontium cobaltite-ferrite (Ba1-xSrxCoyFe1-yO3-δ, BSCF) is a widely studied mixed ionic-electronic conductor material for air electrode in solid oxide cells (SOC). Despite having excellent features, due to fast oxygen surface exchange and oxygen bulk diffusion, it lacks long-term stability. Electrode/electrolyte thermal expansion coefficient (TEC) mismatch and structural instability at temperature lower than 900 °C are responsible for the increase of electrode polarization which becomes a crucial issue for the long-term stability.In this work, SOC stability was studied by adding a thin porous samarium-doped ceria (SDC) backbone on top of the dense SDC electrolyte. The porous SDC backbone was then infiltrated by precursor nitrates to obtain a Ba0.5Sr0.5Co0.8Fe3-δ composition. The SEM investigation showed a nano-sized BSCF-based layer covering the backbone structure. In addition, symmetrical cells were studied in the 400–700 °C temperature range and under anodic and cathodic polarization showing unexpected behavior associated to the electrode microstructure. The modified electrode synergistically enhanced ORR and OER by showing no oxygen vacancies clustering which induces a higher polarization resistance. Ageing procedure was performed for over 120 h at 600 °C under switched current load of ±0.2 A cm−2. The prepared system showed high stability coupled with remarkable electrocatalytic performance and good mechanical properties.

Additional Voltage Loss Considering Jarzynski’s Equality Using Sm-Doped Ceria Electrolytes in Wagner’s Equation for SOFCs

A solid oxide fuel cell (SOFC) directly converts chemical energy from a fuel gas, such as hydrogen or methane, to electrical energy. A solid oxide film is used as the electrolyte, in which the main carriers are oxygen ions. The use of high ion-conducting electrolyte materials, such as samaria-doped ceria (SDC), has been studied to lower the operating temperature. Using SDC electrolytes in solid oxide fuel cells, the open circuit voltage (OCV = 0.80 V at 1073 K) decreases to a smaller value than the Nernst voltage (Vth = 1.15 V at 1073 K), which is obtained using yttria stabilized zirconia (YSZ) electrolytes. This low value has been explained by Wagner’s equation.Based on Wagner’s equation, numerous excellent models have been created. The Riess model is a very famous method for calculating the current-voltage relationship [1]. Using the Riess model, the current-voltage relationship can be calculated with the electrode polarization voltage loss. Furthermore, a nonlinear model was created by Duncan and Wachsman [2]. Using the model described by Duncan and Wachsman, both the electrode polarization voltage loss and the equilibration process can be explained.Previously, we proposed that it is necessary to experimentally verify the existence of leakage currents using Sm-doped Ceria electrolytes in SOFCs. According to the Riess model, the OCV should change during electrode degradation. However, experimentally, the OCV does not change during electrode degradation [3]. Using the model defined by Duncan and Wachsman, the equilibration process that occurs in response to a change in the anode gas for thick Sm-doped ceria electrolytes has never been explained. Experimentally, using a very thick (6.6 mm) SDC electrolyte, the OCV can reach 0.80 V in only 5 minutes [4]. The corresponding electron diffusion current delay should be more than 2080 minutes [5].Next, we proposed an additional voltage loss (0.35 V = 1.15 V- 0.80 V) near the anode [6]. Furthermore, using Jarzynski’s equality, we attempted to explain this voltage loss [7]. The ionic activation energy (Ea) in SDC electrolytes is 0.7 eV. Considering the separation of the Boltzmann distribution during ion hopping by Ea (= 0.7 eV), according to Jarzynski’s equality, the distribution of hopping ions should be a canonical ensemble. When the electrical potential changed due to neutralization by free electrons during ion hopping, 0.7 eV was dissipated by the electrons. Oxygen ions have an electric charge (2e-), so the OCV is 0.8 V= 1.15 V - 0.7 eV/2e [7].Our explanations seem to disregard and disprove Wagner’s equation and other excellent subsequent theories. The reason for this contradiction is that we disproved the existence of large leakage currents in SDC electrolytes. However, we noticed that the dismissal of this idea regarding leakage currents in SDC electrolytes is nothing more than a side effect of introducing a large anode voltage loss (0.35 V). When there is a large voltage loss near the anode in an SDC electrolyte, the leakage currents should be very small. This is called the “anode shielding effect” [8].Our conclusion is that Wagner’s equation becomes more useful after considering the additional voltage loss based on Jarzynski’s equality. Furthermore, subsequent theories such as the Riess model and the Duncan and Wachsman model become more useful. Consequently, it is necessary to introduce new thermodynamics based on Jarzynski’s equality, an addition that will improve Wagner’s equation and subsequent theories.Reference:[1] I. Riess, J. Phys. Chem. Solids., 47(2), 129 (1986). doi: 10.1016/0022-3697(86)90121-6.[2] K. L. Duncan and E. D. Wachsman, J. Electrochem. Soc., 156, B1030 (2009). doi:10.1149/1.315851.[3] T. Miyashita, J. Mater. Sci., 41(10), 3183 (2006). doi: 10.1007/s10853-006-6371-8.[4] T. Miyashita, J. Electrochem. Soc., 164(11) 3190 (2017). doi: 10.1149/2.0251711jes[5] J. Liu and W. Weppner, Ionics, 5(1–2), 115 (1999). doi: 10.1007/BF02375914.[6] T. Miyashita, J. Mater. Sci., 40(22), 6027 (2005). doi: 10.1007/s10855-005-4560-2.[7] T. Miyashita, Miyashita, ECSarXiv (2020) “Open-Circuit Voltage Anomalies in Yttria-Stabilized Zirconia and Samaria-Doped Ceria Bilayered Electrolytes”, https://ecsarxiv.org/xhn73/ [8] B. Dalslet, P. Blennow, P. V. Hendriksen, N. Bonanos, D. Lybye, and M. Mogensen, J. Solid State Electrochem., 10(8), 547 (2006). doi: 10.1007/s10008-006-0135-x.

SrCo0.8Ti0.1Ta0.1O3-δ perovskite: A new highly active and durable cathode material for intermediate-temperature solid oxide fuel cells

Reducing the operating temperatures of solid oxide fuel cells (SOFCs) to the intermediate-temperature range (IT, 400–650 °C) can bring about several benefits including cost effectiveness, prolonged lifetime and flexible sealing. Nevertheless, the accompanying deterioration of cathodic activity for oxygen reduction reaction (ORR) introduces a large obstacle for commercial applications of IT-SOFCs. Herein, a new perovskite SrCo0.8Ti0.1Ta0.1O3-δ (SCTT) is developed by co-doping titanium and tantalum into the B-site of parent SrCoO3 oxide, which may tackle this problem. At 400–650 °C, SCTT shows high electrical conductivities (65–142 S cm−1), appropriate oxygen vacancy concentrations (0.23–0.27) and high bulk diffusion capability due to a synergy between the two dopants in SCTT. Consequently, SCTT exhibits a favorable ORR activity with an area-specific resistance of only 0.17 Ω cm2 at 500 °C on samaria-doped ceria electrolyte, and the corresponding cell generates a high peak power density (PPD) of 0.90 W cm−2 at 500 °C with negligible performance decay for 180 h. Additionally, SCTT performs well in protonic ceramic fuel cells, achieving a PPD of 0.78 W cm−2 at 650 °C and a high durability for ~176 h at 550 °C. This work provides a new promising cathode material that may accelerate the commercialization of IT-SOFC technology.

Synthesis and Electrochemical Studies of Novel Cobalt Free (Nd0.9La0.1)1.6Sr0.4 Ni0.75Cu0.25O3.8 (NLSNC4) Cathode Material for IT‐SOFCs

AbstractA novel cathode material of (Nd0.9La0.1)1.6Sr0.4Ni0.75Cu0.25O3.8 (NLSNC4), is synthesized by solid state reaction and is further investigated in combination with samarium doped ceria (SDC) as composite cathodes including 0, 10, 30, 50, and 70 of SDC electrolyte phase. The X‐ray diffraction analysis showed that NLSNC4 was produced in pure phase and was chemically compatible with SDC electrolyte. Electrical conductivities of single phase and composite cathode materials along with their electrochemical performances on SDC electrolytes in symmetric cells were measured by DC four‐probe method and electrochemical impedance spectroscopy (EIS), respectively. Results indicated that the maximum electrical conductivity is 135.56 S cm−1 at 350 °C, measured for single phase NLSNC4, sintered at 1,300 °C. Polarization resistance of pure NLSNC4 cathode was 2.71 Ω cm2 at 800 °C, and NLSNC4‐x wt.% SDC composite cathodes displayed Rp value of 0.35, 0.14, 0.12, and 0.32 Ω cm2 at 800 °C, for x = 10, 30, 50, and 70, respectively.

Synergistic effect of lithium and calcium for low temperature densification and grain boundary scavenging in samarium doped ceria electrolyte

A highly dense electrolyte with lower grain boundary resistance is desirable for improving the performance of solid oxide fuel cell. This paper studies the synergistic role of calcium and lithium on the densification and ionic conductivity of samarium doped ceria (SDC) electrolyte. Nanosized powders of compositions, CaxLiySm0.2Ce0.78O2-δ (CLSDC, x + y = 0.02) were prepared by citrate - nitrate combustion method and subsequently sintered at 900 °C. A single phase cubic fluorite structure of CeO2 was observed in X-ray diffraction analysis. The segregation of the co-dopants was identified from Raman spectroscopic analysis and FESEM-EDX analysis. The presence of lithium enhanced the densification at 900 °C while calcium improved the grain boundary scavenging. The synergistic effect of an increase in configurational entropy and density in 0.5 mol% Ca and 1.5 mol% Li (0.5CLSDC900) resulted in the highest conductivity of 5.3 × 10−3 S/cm at 800 °C with a scavenging factor of 20.2. Thus the optimum ratio of Li+ and Ca2+ plays a key role in suppressing the ordering of oxygen vacancy and improve the ionic conductivity for its use as an electrolyte for solid oxide fuel cells.

Influence of layer numbers on the structural and electrical performance of cobalt-free SrFe0.5Ti0.5O3-δ cathode for ntermediate-temperature solid oxide fuel cell application

The influence of layer numbers on the structural and electrical performance of SrFe0.5Ti0.5O3-δ cobalt-free cathode was studied. The SrFe0.5Ti0.5O3-δ cathode films fabricated using screen-printing technique with different layer numbers sintered at 1300 °C for 2 h were characterised using field-emission scanning electron microscopy (FESEM) for structural analysis and four-point van der Pauw method for direct current electrical conductivity (σDC). FESEM micrographs confirmed that the SrFe0.5Ti0.5O3-δ cobalt-free cathode films (fabricated with different layer numbers) adhered well on the samarium doped ceria electrolyte surface. The porous films were also uniform without crack formation. The thicknesses of the as-fabricated cathode films were 9.0 ± 0.5, 25.6 ± 1.0, 54 ± 0.6, 71.2 ± 1.4 and 92.2 ± 1.6 μm for layer numbers 1 (1×), 4, 7, 10 and 13 times (13×), respectively. The electrical performance of SrFe0.5Ti0.5O3-δ cobalt-free cathodes was reported within the operating temperature ranging from 550 °C to 800 °C as the targeted application was the intermediate temperature solid oxide fuel cell. The layer numbers (thickness) dependence of σDC suggested a mechanism of long electron pathway at the surface and through the films due to the increase in pores. While the sintering temperature is kept constant, increasing in the number of layers increased the pores accordingly. Hence, the lowest σDC value at 800 °C (2.45 S cm−1) is obtained for SrFe0.5Ti0.5O3-δ cathode films with high number of layers (13×). The highest σDC value (16.46 S cm−1) was recorded for a single layer (1×) SrFe0.5Ti0.5O3-δ cathode film. Although the conductivity value was still far from the desired theoretical conductivity of 100 S cm−1, this result was better than that of the literature that reported the same composition, thereby showing that the quality of cathode film was improved.

Open-Circuit Voltage Anomalies in Yttria-Stabilized Zirconia and Samaria-Doped Ceria Bilayered Electrolytes

The open-circuit voltage (OCV) in samaria-doped ceria (SDC) electrolytes has been explained by Wagner’s equation. According to this theory, even if there is no external current, two types of internal currents are created by electrons and oxygen ions. Due to the ohmic loss caused by the ionic internal short-circuit current, the OCV is 0.80 V, which is lower than the theoretical voltage of 1.15 V. Coating a thin electron-blocking layer of YSZ (yttria-stabilized zirconia) onto the SDC electrolyte is effective at stopping the electronic leakage current and improving the OCV. For cells with YSZ on the anode side, the OCV is 1.05 V-1.15 V. For cells with YSZ on the cathode side, the OCV is 0.81-0.87 V. Therefore, the electronic leakage currents have been thought to be not entirely stopped. YSZ films deposited on the anode side are very effective, but YSZ films deposited on the cathode side are clearly less effective. Thus, we proposed experiments to confirm whether electronic leakage currents can be blocked by the YSZ films deposited on the cathode side. A polished YSZ film (500 mm thickness) on the cathode side and a polished SDC film (970 mm) on the anode side were physically contacted. The OCV was measured to be 819 mV at 1073 K, 825 mV at 973 K and 874 mV at 873 K. Then, we noticed that the large voltage loss was not due to electronic leakage currents. However, there were objections in these experimental results. The surrounding air could enter the contact space between the YSZ film and the SDC film, which indicated that due to air entering the contact, an electronic leakage current could arise in SDC electrolytes. By measuring the transient process, we showed that the electronic leakage currents were stopped entirely. After changing the anode gas from N2 gas to H2 gas including 3% H2O, the OCV was changed from 85 mV to 755 mV with a short time constant (12 seconds) and from 755 mV to 819 mV with a long time constant (100 seconds) at 1073 K. The equilibrium time based on the electron diffusion coefficient equation should be longer than 49 minutes. Thus, the voltage loss (0.35 V) in SDC could not be explained by the electronic leakage current. Furthermore, a higher OCV (819 mV) was not due to stop the electronic leakage current. Figure 1

Divalent cations modified grain boundary scavenging in samarium doped ceria electrolyte for solid oxide fuel cells

Grain boundary conductivity of samarium doped ceria electrolyte (SDC) depends on the space charge potential and oxygen vacancy concentration existing at the grain boundary region. Here, we report the effect of co-doping 1 mol% divalent alkaline earth metals in samarium doped ceria (Ce0.78Sm0.2A0.01O1.89, [ASDC], A = Mg, Ca, Sr, Ba) on the grain boundary conductivity. The increase (Ca2+, Sr2+, Ba2+) or decrease (Mg2+) in lattice parameter in comparison to SDC (0.5445 nm) depends on the ionic radii of the co-dopant. The oxygen vacancy concentration of ASDC system, quantified from the Raman spectra, increased from 2.98 × 1021 cm−3 (SDC) to 3.18 × 1021 cm−3. The samples sintered at 1400 °C for 5 h, resulted in a dense electrolyte with a relative density of 98–99%. Calcium co-doped SDC (CSDC) showed the highest conductivity of 2.04 × 10−3 S/cm at 700 °C with activation energy of 0.90 eV. The oxygen vacancy profile at the space charge layer was found to be higher for CSDC while the potential reduced from 0.24 V (SDC) to 0.18 V. The observed lower space charge potential in co-doped SDC, contributed to the enhancement in oxygen vacancy concentration at the space charge layer and increased the grain boundary conductivity. Thus the present study highlights the correlation of space charge potential, oxygen vacancy concentration to grain boundary conductivity and provide a profound insights on the grain boundary understandings for improving the performance of the electrolyte for solid oxide fuel cell application.

Effect of bismuth oxide on crystallization and sealing behavior of barium borosilicate glass sealant for SOFCs

Barium borosilicate glasses (BaO–SiO2–B2O3) with the addition of 5–25 wt% bismuth oxide as additive (equivalent to 1.23–5.88 mol% of Bi2O3 in glasses) were prepared for using as a sealant for planar solid oxide fuel cells. The effect of Bi2O3 addition on thermal properties, structure, crystallization and joining interface of the BaO–SiO2–B2O3 glasses was investigated. IR spectra showed a significant effect of Bi2O3 addition in the BaO–SiO2–B2O3 glass structure, causing a progressive conversion of [BO3] to [BO4] unit in the glasses. The Bi2O3 addition made the BaO–SiO2–B2O3 glass network less rigid resulting in a decrease of a glass transition temperature and a softening point but still maintains the thermal expansion match between the Bi2O3-containing glasses and samaria-doped ceria solid electrolyte as well as an iron‑chromium metallic interconnect. 5-wt% Bi2O3 obviously decelerated the crystallization of the Bi2O3-containing glasses. Good adhesion of the 5-wt% Bi2O3-containing glass sealant with samaria-doped ceria electrolyte and iron-chromium metallic interconnect without any gaps was achieved at prolong test for 30 h at 800 °C.

Low temperature densification by lithium co-doping and its effect on ionic conductivity of samarium doped ceria electrolyte

Low temperature densification and improving the ionic conductivity of doped ceria electrolyte is important for the realization of efficient intermediate temperature solid oxide fuel cell system. Herein, we report the effect of lithium co-doping (1, 3, 5 and 7 mol%) in 20 mol% samarium doped ceria on the low temperature sinterability and conductivity. The synthesized nanoparticles by citrate-nitrate combustion method showed a decrease in lattice parameter and increase in oxygen vacancy with lithium content after calcination due to the substitution of Li+ into CeO2 lattice. Upon sintering at 900 °C, the density improved and reached a maximum value of 98.6% for 5% Li which exhibited a dense microstructure than at 7% Li. 5%Li co-doping exhibited the best conductivity of 3.65 × 10−04–1.81 × 10−3 S cm−1 in the operative temperature range of IT-SOFC (550–700 °C).Our results demonstrate the significance of lithium as co-dopant for efficient low temperature sintering as well as improving the electrolyte conductivity.

Effects of co-doped barium cerate additive on morphology, conductivity and electrochemical properties of samarium doped ceria electrolyte for intermediate temperature solid oxide fuel cells

The composite of samarium doped ceria (Sm0.2Ce0.8O2-δ, SDC) and co-doped barium cerate (BaZr0.1Ce0.7Y0.1Yb0.1O3-δ, BZCYYb) is prepared by mechanical mixing and investigated as electrolyte for intermediate temperature solid oxide fuel cells (IT-SOFCs). Coexistence of SDC and BZCYYb are observed for composite electrolyte by X-ray diffraction after sintering at 1500 °C for 5 h, while the slight deviation of the diffraction peak indicating the element diffusion between two phases. The scanning electron microscope and electron probe microanalyzer results demonstrate that small BZCYYb grains disperse uniformly around the grains of SDC, limiting the growth of SDC grains and decreasing the average grain size of composite electrolyte. Impedance spectroscopy measurement reveals that the grain boundary resistance can be significantly reduced by about an order of magnitude through adding 15–30 wt. % BZCYYb to SDC. Single cells based on the composite electrolyte are fabricated using nickel cermet (Ni-SDC) anode and perovskite (La0.6Sr0.4Co0.2Fe0.8O3-δ, LSCF) cathode.Relatively high open circuit voltage (OCV), much lower polarization resistance and encouraging high power density are obtained for cells with composite electrolyte compared to those with single SDC electrolyte. Among all of the samples, single cell based on 15 wt. % BZCYYb-85 wt. % SDC composite electrolyte exhibits the lowest total resistances of 0.641 Ω·cm2 and the highest peak power densities of 0.56 W·cm−2 at 600 °C.

Influence of Anode Functional Layers on Electrochemical Performance and Mechanical Strength in Microtubular Solid Oxide Fuel Cells Fabricated by Gel-Casting

Anode-supported microtubular solid oxide fuel cells (mT-SOFCs) using samaria-doped ceria (SDC) as electrolyte were fabricated, varying the composition and number of anode functional layers (AFLs), by combining the aqueous gel-casting and spray-coating techniques. Suitable aqueous slurry formulation of NiO–SDC was prepared using agarose as a gelling agent for gel-casting of tubular supports. Afterward, 40:60 and 50:50 NiO:SDC (wt %) as AFLs and SDC electrolyte were deposited by spray-coating and subsequently co-sintered. Finally, mT-SOFCs with 2.5 mm outer diameter and thicknesses of 380 μm support; 0, 12, and 24 μm AFLs; 15 μm electrolyte; and 30 μm cathode were obtained. The influence of AFLs on the performance and mechanical integrity was investigated for the three cells. For this purpose, electrochemical and mechanical tests at both macroscopic and micro-/nanometric scales (at the AFLs region) were determined by flexural strength and nanoindentation techniques, respectively. The results evidence that t...