Ceria-based Composite Electrolytes Research Articles

Synthesis and characterization of M-doped ceria-ternary carbonate composite electrolytes (M = erbium, lanthanum and strontium) for low-temperature solid oxide fuel cells

Three different series of doped ceria-based composite electrolyte consisting of M-doped ceria (M = Ce0.85Er0.15O2-δ (ErDC), Ce0.85La0.15O2-δ (LaDC) and Ce0.95Sr0.05O2-δ (SrDC)) and ternary carbonate (mixture of Li2CO3, Na2CO3 and K2CO3) were developed as functional solid electrolytes for low-temperature solid oxide fuel cells. X-ray diffraction, transmission electron microscopy and UV–visible spectroscopy were used to analyze the chemical stability, morphology, and bandgap, respectively, of the prepared composite powders. All composite electrolytes exhibited cubic fluorite structure as a primary phase and small peaks corresponding to (Li/Na/K)2CO3 as an amorphous phase. The bulk densities of the sintered composite electrolyte pellets were in the range of 3.34 g/cm3–3.45 g/cm3. The total conductivity of the composite electrolyte materials was evaluated by electrochemical impedance spectroscopy in air and hydrogen (90 vol% N2 + 10 vol% H2) in the temperature range of 350 ºC–650 °C. The SrDC-ternary carbonate composite electrolyte showed the highest conductivity of 0.092 Scm−1 and lowest activation energy at 650 °C in hydrogen. The formation of hydrated phase from chemical interaction between the carbonates and hydrogen gas and the presence of Sr2+ and cerium ions is useful in enhancing the proton conduction in the SrDC-ternary carbonate system.

Stability of metal oxides against Li/Na carbonates in composite electrolytes

Solid oxide–alkaline carbonate (Li2CO3 : Na2CO3, 1 : 1 molar ratio) composite electrolytes were prepared using different solid oxide matrices (TiO2, HfO2, Yb2O3, Y2O3, Dy2O3, Gd2O3 and La2O3), to cover a wide range of ceramic chemical characteristics. The chemical and microstructural stability of these oxides with the mixed carbonates were studied by powder X-ray diffraction, scanning electron microscopy, infrared and laser Raman spectroscopic techniques after reacting them at 690 °C for 1 h in air. The electrical performance of selected composites was evaluated using impedance spectroscopy, in air. Amongst the oxides hereby tested, TiO2 is found to be the most unstable in contact with the molten carbonates whereas Yb2O3 is quite stable. The corresponding composites have ionic conductivities (3.3 × 10−1 S cm−1 at 580 °C, in air) close to those reported for state-of-the-art ceria-based composite electrolytes. A draft equivalent circuit model underlines the transport in the carbonate phase and across the carbonate/oxide interfaces as the dominant contributions to the total conductivity of these composites. Yb2O3 + Li2CO3 : Na2CO3 composites show chemical stability at operating temperatures in the order of 690 °C, standing as a potential candidate for intermediate temperature applications.

Impact of ceramic matrix functionality on composite electrolytes performance

Ceria-based composite electrolytes (50 and 70vol%) including one mixture of Li and Na carbonates (1:1 molar ratio) were prepared using either pure CeO2 (modest conductor) or Gd-doped ceria (excellent oxide-ion conductor) as ceramic matrix. The composite electrolytes and their phase constituents were tested under pure CO2, air, and H2 diluted in N2, at temperatures ranging from 300 to 580°C, to study their electrical performance. Impedance spectroscopy measurements performed under these working conditions were complemented by microstructural characterization. Low temperature (below melting) impedance shows in a clear manner the relevance of the matrix functionality on the performance of these composites, but also reveals additional microstructural and even unusual composite effects. These data were used to test equivalent circuit models derived from the composite constituent properties, and to assess their potential to design composites for target functionalities at higher temperature.

Role of gas-phase composition on the performance of ceria-based composite electrolytes

Ceria-based composites (ranging from 30 to 70 vol%) including a mixture of Li and Na carbonates (1:2 M ratio, respectively) were sequentially exposed to pure CO2, O2 and a mixture of 10 vol% H2 diluted in N2, at temperatures ranging from 300 to 580 °C, to study the role of composite/gas interaction on electrical performance. Impedance spectroscopy measurements performed under these working conditions were complemented by microstructural and structural characterization. Changes in conductivity and electrode impedance when changing the atmosphere from CO2 to O2 and to H2 confirmed changes in concentration and/or in mobility of charge carriers and electroactive species. Novel species identified either by X-ray diffraction (hydrated carbonates, only present at the surface of the samples), or by infrared spectroscopy (hydrogen carbonate ions), might be related to the observed conductivity enhancement under H2.

La<SUB>0.3</SUB>Sr<SUB>0.2</SUB>Mn<SUB>0.1</SUB>Zn<SUB>0.4</SUB> Oxide-Sm<SUB>0.2</SUB>Ce<SUB>0.8</SUB>O<SUB>1.9</SUB> (LSMZ-SDC) Nanocomposite Cathode for Low Temperature SOFCs

Nanocomposite based cathode materials compatible for low temperature solid oxide fuel cells (LTSOFCs) are being developed. In pursuit of compatible cathode, this research aims to synthesis and investigation nanocomposite La0.3Sr0.2Mn0.1Zn0.4 oxide-Sm0.2Ce0.8O1.9 (LSMZ-SDC) based system. The material was synthesized through wet chemical method and investigated for oxide-ceria composite based electrolyte LTSOFCs. Electrical property was studied by AC electrochemical impedance spectroscopy (EIS). The microstructure, thermal properties, and elemental analysis of the samples were characterized by TGA/DSC, XRD, SEM, respectively. The AC conductivity of cathode was obtained for 2.4 Scm(-1) at 550 degrees C in air. This cathode is compatible with ceria-based composite electrolytes and has improved the stability of the material in SOFC cathode environment.

Fabrication and characterization of composite electrolyte for intermediate-temperature SOFC

In this study, a ceria-based composite electrolyte was investigated for intermediate-temperature solid oxide fuel cells (SOFCs) based on SDC–25 wt.% K 2CO 3. Sodium carbonate co-precipitation process by which SDC powder was adopted and sound cubic fluorite structure was formed after SDC powders were sintered at 750 °C for 3 h. The crystallite size of the particle was 21 nm in diameter as calculated from data obtained through X-ray diffraction. The conductivity of the composite electrolyte proposed in this study was much higher than that of pure SDC at the comparable temperature of 550–700 °C. The transition of the ionic conductivity occurred at 650 °C. Based on this type of composite electrolyte, single cell with the electrolyte thickness of 0.3 mm were fabricated using dry pressing, with nickel oxide adopted as anode and SSC as cathode. The single cell was then tested at 550–700 °C on home-made equipment in this study, using hydrogen/air. The maximum power density and open circuit voltage (OCV) achieved 600 mW cm −2 and 1.05 V at 700 °C, respectively.

Analysis and Modeling of Novel Low-Temperature SOFC With a Co-Ionic Conducting Ceria-Based Composite Electrolyte

In recent years, ceria-based composites (CBCs) have been developed as electrolytes for low-temperature solid oxide fuel cells. These materials exhibit extremely high ionic conductivities at 400–600°C. It has also been found that both oxide ion and proton can be conducted in the CBC electrolytes, which makes such co-ionic conducting fuel cell distinct from any other types of fuel cells. In this study, a model involving three charge carriers (oxide ion, proton, and electron) is developed to describe the fuel cell with CBC electrolytes. Various operating characteristics of the fuel cell with CBC electrolytes are investigated, compared to those of the fuel cell with doped ceria electrolytes. The results indicate that the CBC electrolyte behaves as a pure ionic conductor, the cell is more efficient, and a higher output is expected at low temperatures under the same pressure operation than that of the cell with doped ceria electrolytes.

Performance of fuel cells with proton-conducting ceria-based composite electrolyte and nickel-based electrodes

A ceria-based composite electrolyte with the composition of Ce 0.8Sm 0.2O 1.9 (SDC)–30 wt.% (2Li 2CO 3:1Na 2CO 3) is developed for intermediate temperature fuel cells (ITFCs). Two kinds of SDC powders are used to prepare the composite electrolytes, which are synthesized by oxalate coprecipitation process and glycine–nitrate process, respectively, and denoted as SDC(OCP) and SDC(GNP). Based on each composite electrolyte, two single cells with the electrolyte thickness of 0.3 and 0.5 mm are fabricated by dry-pressing technique, using nickel oxide as anode and lithiated nickel oxide as cathode, respectively. With H 2 as fuel and air as oxidant, all the four cells exhibit excellent performances at 400–600 °C, which can be attributed to the highly ionic conducting electrolyte and the compatible electrodes. The cell performance is influenced by the SDC morphology and the electrolyte thickness. More interestingly, such composite electrolytes are found to be proton conductors at intermediate temperature range for the first time since almost all water is observed at the cathode side during fuel cell operation for all cases. The unusual transport property, excellent cell performance and potential low cost make this kind of composite material a good candidate electrolyte for future cost-effective ITFCs.

Advanced Hybrid Ion Conducting Ceramic Composites and Applications in New Fuel Cell Generation

Our developments on ceramic composite conductors have experienced about 15 years from the oxyacid-salts oxide proton-based conductors, non-oxide containment salts, the ceria-based composite electrolytes, hybrid proton and oxygen ion conductors and nano-composites. A special emphasis is paid to new functional nano-composites based on hybrid proton and oxygen ion conductors that have demonstrated advanced properties and fuel cell applications, e.g., excellent ionic conductivity of 0.01 to 1 Scm-1 and performances of 200 - 1000 mWcm-2 for temperatures achieved for fuel cells between 400 and 700°C. Some proton and oxygen ion conducting mechanisms in the materials are reviewed and discussed. The hybrid ion conduction and dual electrode reactions and processes create a new generation fuel cell system.

Electrolysis studies based on ceria-based composites

Electrolysis behaviors have been investigated based on ceria-based composite electrolytes and fuel cells. The results showed that both proton and oxygen ion conduction exist in the ceria-based composite electrolytes resulting in high current outputs in both fuel cell and electrolysis operations. Corresponding to high current output in the electrolysis cell, a high hydrogen production can be expected. This work has first demonstrated the ceria-based composite electrolyte used for electrolysis process.

R&D for Low Temperature (300 to 600°C) SOFCs

A Swedish-Chinese research framework has been established targeting for low temperature solid oxide fuel cells (LTSOFCs, 300-650°C) based on ceria-based composites. The ceria-based composite electrolytes have excellent ionic conductivity of 0.01 to 1 Scm -1 at temperatures 300 to 650°C. For these electrolytes, new compatible electrode materials have been investigated, especially nickel and copper-based compounds. These electrolytes and electrodes were used to construct LTSOFCs which demonstrated a performance from 100 to 1000 mWcm -2 at temperatures between 400 and 650°C. This paper reports recent progress on LTSOFC R&D.

Novel intermediate temperature ceramic fuel cells with doped ceria-based composite electrolytes

A series of novel composite electrolytes, consisting of doped ceria (DCO) and salts were developed and examined. The salts included chlorides and carbonates. Their oxygen ion and/or proton conductivity at intermediate temperature (IT) range (500–700 °C) can be even 2–15 times higher than DCO. Furthermore, they also show higher ionic transference number than DCO. The long-term stability of fuel cells based on these composite electrolytes was also discussed.