Addition effects of erbia-stabilized bismuth oxide on ceria-based carbonate composite electrolytes for intermediate temperature-−solid oxide fuel cells

Highly conductive Er_0.2Bi_0.8O_1.5 (ESB) and rare-earth doped ceria solid oxide electrolytes (SOEs) at intermediate temperature (IT) continue to suffer disadvantages in terms of thermodynamic instability and significant electronic conduction, respectively, at low oxygen partial pressure for solid oxide fuel cell (SOFC) operations. It is therefore necessary to improve the low-temperature ionic conductivity in order to enhance the electrolytic domain of these materials and thereby mitigate cell efficiency dissipation by electronic conduction. In this work, an advanced multiphase carbonate composite material based on ceria has been developed to overcome this IT-SOE challenge. This advanced electrolyte is comprise of nanostructured neodymium-doped ceria (NDC) and 38 wt% (Li–0.5Na)₂CO₃ carbonate with a small amount of ESB phase. The addition of 2 wt% ESB in ceria-based materials decreases the grain boundary resistance of the SOEs in the IT range. Further, a small amount of highly conducting ESB phase in the NDC/[(Li–0.5Na)₂CO₃] composite electrolyte increases the overall conductivity of the composite SOEs. The NDC electrolyte containing 38 wt% carbonate shows the highest conductivity of 0.104 Scm⁻¹ at 600 °C, while the conductivity is increased to 0.165 Scm⁻¹ by the addition of 2 wt% ESB. In addition, the activation energy of the multiphase composite electrolytes (0.52 eV) is lower than that of the NDC/carbonates (0.65 eV) in the IT range. This is attributed to the effect of the physical properties of the NDC sample, induced by the light ESB doping, on the ionic conductivity, and this effect is closely associated with the grain boundary property. Furthermore, the interfacial effects of the multiphase materials also contribute to the improved conductivity of this advanced composite electrolyte.     

Nanocomposite electrode materials for low temperature solid oxide fuel cells using the ceria-carbonate composite electrolytes

Low temperature solid oxide fuel cell (LTSOFC, 300–600 °C) is one of the hot areas in recent fuel cell developments. In order to develop high performance LTSOFCs, compatible electrodes are highly demanded. We used NANOCOFC (nanocomposites for advanced fuel cell technology) approach to develop nanocomposite electrodes based on metal oxides Ni–Cu–Zn-oxide and samarium doped ceria (SDC). It was found that the materials consist of individual metal oxide and SDC phase, indicating the material as a composite with a homogenous distribution for all constituent components. Highly homogenous distribution of the particles enhanced the catalyst function for electrode applications in LTSOFC devices. We constructed the devices using the SDC-carbonate nanocomposite (NSDC) as the electrolyte and above as prepared composite as electrodes in a symmetrical configuration. We found that the prepared composite electrodes had good catalytic function for both H₂ and O₂, to prove its anode and cathode functions. Based on the material properties, the LTSOFC devices have reached a power output more than 730 mW cm⁻² at 550 °C.     

Developing cuprospinel CuFe2O4–ZnO semiconductor heterostructure as a proton conducting electrolyte for advanced fuel cells

The interfacial properties of electrolyte materials have a crucial impact on the ionic conductivity of solid batteries and solid oxide fuel cells. Here we construct cuprospinel CuFe₂O₄ (CFO)–ZnO composite as a functional electrolyte for fuel cell device. In an optimal composition of 0.3CFO-0.7ZnO electrolyte fuel cell, the maximum power output of 675 mW cm^?2 is obtained at 550 °C. The electrical properties and electrochemical performance are strongly dependent on the ratios between CFO and ZnO in CFO-ZnO composite. Notably, surprising fuel cell performance with high ionic conductivity is attained by constructing this p-type CFO composited with n-type ZnO. Proton conduction was further verified experimentally. The interfacial ionic conduction pathway between the two constituent phases plays a vital role to enhance the proton conductivity, and the bulk p-n heterojunction can block internal electronic pass. An excellent current and power densities of CFO-ZnO composite are observed along with a high conductivity of 0.35 S·cm-1 at 550 °C. This work opens a new perspective for the semiconductor materials that can widely be developed for electrolytes, based on their tunable band structure.     

Samarium doped ceria-(Li/Na)2CO3 composite electrolyte and its electrochemical properties in low temperature solid oxide fuel cell

A composite of samarium doped ceria (SDC) and a binary carbonate eutectic (52 mol% Li₂CO₃/48 mol% Na₂CO₃) is investigated with respect to its morphology, conductivity and fuel cell performances. The morphology study shows the composition could prevent SDC particles from agglomeration. The conductivity is measured under air, argon and hydrogen, respectively. A sharp increase in conductivity occurs under all the atmospheres, which relates to the superionic phase transition in the interface phases between SDC and carbonates. Single cells with the composite electrolyte are fabricated by a uniaxial die-press method using NiO/electrolyte as anode and lithiated NiO/electrolyte as cathode. The cell shows a maximum power density of 590 mW cm⁻² at 600 °C, using hydrogen as the fuel and air as the oxidant. Unlike that of cells based on pure oxygen ionic conductor or pure protonic conductor, the open circuit voltage of the SDC-carbonate based fuel cell decreases with an increase in water content of either anodic or cathodic inlet gas, indicating the electrolyte is a co-ionic (H⁺/O²⁻) conductor. The results also exhibit that oxygen ionic conductivity contributes to the major part of the whole conductivity under fuel cell circumstances.     

Ceria-carbonate composite for low temperature solid oxide fuel cell: Sintering aid and composite effect

In this study, the effect of carbonate content on microstructure, relative density, ionic conductivity and fuel cell performance of Ce_0.8Sm_0.2O_1.9-(Li/Na)₂CO₃ (SDC-carbonate, abbr. SCC) composites is systematically investigated. With the addition of carbonate, the nano-particles of ceria are well preserved after heat-treatment. The relative densities of SCC pellets increase as the carbonate content increases or sintering temperature rises. Especially, the relative density of SCC2 sintered at 900 °C is higher than that of pure SDC sintered at 1350 °C. Both the AC conductivity and DC oxygen ionic conductivity are visibly improved compared with the single phase SDC electrolyte. Among the composites, SDC-20 wt% (Li/Na)₂CO₃ (SCC20) presents high dispersion, relative small particle size, and the dense microstructure. The optimized microstructure brings the best ionic conductivity and fuel cell performance. It is hoped that the results can contribute the understanding of the role of carbonate in the composite materials and highlight their prospective application.     

Enhanced ionic conductivity in calcium doped ceria – Carbonate electrolyte: A composite effect

Recently, ceria-based nanocomposites, as a proton and oxygen ion conductor, has been developed as promising electrolyte candidates for low-temperature solid oxide fuel cells (LTSOFCs). Up to now, samarium doped ceria (SDC) was studied as a main oxide for nanocomposite electrolyte; while calcium doped ceria (CDC) is considered as a good alternative from both material performance and economical aspects. Yet the conduction behavior of CDC-based composite has not been reported. In the present study, calcium doped ceria was prepared by oxalate co-precipitation method, and used for the fabrication of CDC/Na₂CO₃ composite. The thermal decomposition process, structure and morphology of the samples were characterized by TGA, XRD, SEM, etc. The oxygen ion conductivity of single phase CDC sample was measured by electrochemical impedance spectroscopy (EIS), while the proton and oxygen ion conductivity of CDC/Na₂CO₃ nanocomposite sample were determined by four-probe d.c. measurements. The CDC/Na₂CO₃ samples show significantly enhanced overall ionic conductivity compared to that of single phase CDC samples, demonstrating pronounced composite effect. This confirms that the use of nanocomposite as electrolyte can effectively lower the operation temperature of SOFC due to improved ionic conductivity.     

Stability considerations of semiconductor ionic fuel cells from a LNCA (LiNi0.8Co0.15Al0.05O2-δ) and Sm-doped ceria (SDC; Ce0.8Sm0.2O2-δ) composite electrolyte

Recent advances in composite materials, especially semiconductor materials incorporating ionic conductor materials, have led to significant improvements in the performance of low-temperature fuel cells. In this paper, we present a semiconductor LNCA (LiNi_0.8Co_0.15Al_0.05O_2-δ) which is often used as electrode material and ionic Sm-doped ceria (SDC; Ce_0.8Sm_0.2O_2-δ) composite electrolyte, sandwiched between LNCA thin-layer electrodes in a configuration of Ni-LNCA/SDC-LNCA/LNCA-Ni. The incorporation of the semiconductor LNCA into the SDC electrolyte with optimized weight ratios resulted in a significant power improvement, from 345 mW cm^?2 with a pure SDC electrolyte to 995 mW cm^?2 with the ionic-semiconductor SDC-LNCA one where the corresponding ionic conductivity reaches 0.255 S cm^?1 at 550 °C. Interestingly, the coexistence of ionic and electron conduction in the SDC-LNCA membrane displayed not any electronic short-circuiting but enhanced the device power outputs. This study demonstrates a new fuel cell working principle and simplifies technologies of applying functional ionic-semiconductor membranes and symmetrical electrodes to replace conventional electrolyte and electrochemical technologies for a new generation of fuel cells, different from the conventional complex anode, electrolyte, and cathode configuration.     

Development of solid oxide fuel cell materials for intermediate-to-low temperature operation

The commercialization of solid oxide fuel cell (SOFC) needs the development of functional materials for intermediate-to-low temperature (400-700 °C, ILT) operation. Recently, we have successfully developed new electrolyte materials for ILT-SOFCs, including Ce_0.8Sm_0.2O_1.9 (SDC), BaCe_0.8Sm_0.2O_2.9 (BCSO) and SDC-carbonate composites. Compared with the state-of-the-art yttria-stabilized zirconia (YSZ), these materials exhibit much higher ionic conductivity at ILT range. Especially, SDC-carbonate composites show an ionic conductivity of 10⁻² to 1 Scm⁻¹ between 400 and 600 °C in fuel cell environment. Some new cathode materials were investigated for above electrolyte materials and showed promising performance. Alternative anode materials were developed to directly utilize alcohol fuels. A dry-pressing and co-firing process was employed to fabricate thin SDC and BCSO electrolyte membranes as well as thick SDC-carbonate composite electrolyte with acceptable density on anode substrate. Many efforts have also been made on fabrication of larger-size planar cells and exploitation of reliable sealing materials.     

Various synthesis methods of aliovalent-doped ceria and their electrical properties for intermediate temperature solid oxide electrolytes

This article investigates the relationship between ionic conductivity and various processing methods for aliovalent-doped, ceria solid solution particles, as an intermediate temperature-solid oxide electrolyte to explain the wide range of conductivity values that have been reported. The effects of doping material and content on the ionic conductivity are investigated comprehensively in the intermediate temperature range. The chemical routes such as coprecipitation, combustion, and hydrothermal methods are chosen for the synthesis of ceria-based nanopowders, including the conventional solid-state method. The ionic conductivity for the ceria-based electrolytes depends strongly on the lattice parameter (by dopant type and content), processing parameters (particle size, sintering temperature and microstructure), and operating temperature (defect formation and transport). Among other doped-ceria systems, the Nd_0.2Ce_0.8O_2−d electrolyte synthesized by the combustion method exhibits the highest ionic conductivity at 600 °C. Further, a novel composite Nd_0.2Ce_0.8O_2−d electrolyte consisting of a combination of powders (50:50) synthesized by coprecipitation and combustion is designed. This electrolyte demonstrates an ionic conductivity two to four times higher than that of any singly processed electrolytes.     

Nd2−xGdxZr2O7 electrolytes: Thermal expansion and effect of temperature and dopant concentration on ionic conductivity of oxygen

The effect of temperature and complex dopant composition on oxygen ion conductivity in solid oxide electrolyte fuel cells was investigated by atomistic molecular dynamics simulation. A new electrolyte (Nd_2−xGd_xZr₂O₇) was selected to study oxygen ion conductivity using three Gd compositions (x = 0.8, 1.0, and 1.2) in a wide range of temperature (T = 1273 K–1873 K). MSD results of cations showed these groups of electrolyte are stable at high operating temperature. The first composition (x = 0.8) had the highest ionic conductivity that was in good agreement with the experimental data. A simple effective model that works with configuration energy of the oxygen crossing plate was applied to explain the observed conductivity trend. The model illustrated the point as well. Increasing Gd concentration decreases existence probability of easy crossing plate. Radial distribution function analysis also confirmed results. Thermal expansion of the electrolyte has a major effect on the selecting of the electrolyte materials; thus, this important factor was also studied. Results showed the first composition had the greatest thermal expansion.     

Gadolinia-doped ceria mixed with alkali carbonates for SOFC applications: II – An electrochemical insight

Composite materials based on gadolinia-doped ceria (GDC) and alkali carbonates (Li₂CO₃-K₂CO₃ or Li₂CO₃-Na₂CO₃) are potential electrolytes for low temperature solid oxide fuel cell applications (LTSOFC). This paper completes a first one dedicated to the thermal, structural and morphological study of such compounds; it is fully focussed on their electrical/electrochemical properties in different conditions, temperature, composition and gaseous atmosphere (oxidative or reductive). The influence of the gaseous composition on the Arrhenius conductivity plots is evidenced, in particular under hydrogen atmosphere. Finally, electrical conductivity determined by impedance spectroscopy is presented as a function of time to highlight the stability of such composites over 6000 h. First results on single cells showed performance at 600 °C of 60 mW cm⁻².     

Green synthesis of faujasite-La0.6Sr0.4Co0.2Fe0.8O3-δ mineral nanocomposite membrane for low temperature advanced fuel cells

In this study, self-supporting mesoporous faujasite was prepared by a facile and environment-friendly process mainly using solid waste discharged coal fly ash. The obtained mesoporous faujasite with a large specific surface area (SSA) composited with La_0.6Sr_0.4Co_0.2Fe_0.8O_3-δ (LSCF) is used as an electrolyte membrane for the application of low-temperature solid oxide fuel cells (LTSOFC). With the increase of SSA of faujasite from 128.3 to 286.8 m² g^?1, the open circuit voltage of fuel cell based on faujasite-LSCF electrolyte raises from 0.90 to 0.98 V and the maximum power density also increases from 161 to 286 mW cm^?2 at 600 °C. It is found that the SSA parameter of faujasite plays an important role of tuning the electrical properties of the composite due to the strong correlation of LSCF nanoparticle dispersion degree on mesoporous faujasite. EIS results have confirmed that the faujasite-LSCF composite provides fast ionic channels for protons and inhibits electron long-range transfer between LSCF particles. This work shows the promising application of faujasite-LSCF composite synthesized by low-cost and environmental-friendly methods in the field of LTSOFC.     

CeO2 coated NaFeO2 proton-conducting electrolyte for solid oxide fuel cell

Nowadays, the low-temperature operation has become an inevitable trend for the development of SOFCs. Transition metal layered oxides are considered as promising electrolyte materials for low-temperature solid oxide fuel cells (LT-SOFCs). In this work, we report the CeO₂ coated NaFeO₂ as an electrolyte material for LT-SOFC. The study results revealed that the piling of CeO₂ significantly influenced the open-circuit voltage (OCV) as well as the power output of the fuel cells. In comparison with pure NaFeO₂, the denser structure of CeO₂ coated NaFeO₂ leads to higher OCV (1.06 V, 550 °C). The electrochemical impedance spectrum (EIS) fitted results showed that NaFeO₂–CeO₂ composites possessed higher ionic boundary conductivity. This is because that the hetero-interfaces between NaFeO₂ and CeO₂ provide fast ion conducting path. The high ionic conductivity of CeO₂ coated NaFeO₂ lead to admirable fuel cell power output of 727 mW cm^?2 at 550 °C.     

Studies on structural,morphological, and electrical properties of Ga3+ and Cu2+ co-doped ceria ceramics as solid electrolyte for IT-SOFCs

The present study highlights the effect of Ga³⁺ and Cu²⁺ co-doping on the crystal structure, surface morphology and ionic conductivity of ceria ceramics in the system Ce_0.8Ga_0.2-xCu_xO_2-δ for potential applications as the solid electrolyte material in the intermediate temperature solid oxide fuel cells (IT-SOFCs). Ultrafine Ce_0.8Ga_0.2-xCu_xO_2-δ (for x = 0, 0.05, 0.1, 0.15, and 0.2) nanopowders were prepared via glycine nitrate auto-combustion method. Phase identification, microstructural, and ionic conductivity of all the ceria ceramics were observed by powder XRD, SEM, TEM, and impedance analyses, respectively. Rietveld structural analysis using powder XRD pattern for all the co-doped systems confirms cubic fluorite type structure having Fm-3m space group, similar to cerium oxide. All these samples were found to have density above 85% after sintering at 1300 °C for 4 h. Raman spectra revealed the oxygen vacancies in all the compositions. Thermal analysis for change in weight and thermal expansion coefficient with temperature were performed by TGA and high temperature XRD measurements, respectively. Thermal expansion coefficient of the developed electrolytes matches with the commonly used electrode materials. The composition Ce_0.8Ga_0.05Cu_0·15O_1.825 was found to demonstrate the maximum ionic conductivity with the least activation energy among all the existing co-doped ceria ceramics. These features make it a promising candidate in the IT-SOFC as the electrolyte material.     

Electrical properties of Ni-doped Sm2O3 electrolyte

Nowadays, semiconductor ionic materials have drawn significant attention for developing new electrolytes in low temperature solid oxide fuel cells (LT-SOFCs). Here we investigate the effect of nickel doping on ionic conductivity of Sm₂O₃ as an electrolyte material for low temperature SOFCs. The amount of Ni ion doping has an intense effect on the electrochemical properties and power generation. An optimized composition of 10 mol% nickel doped samarium oxide (10NSO) as an electrolyte in the fuel cell has a high open circuit voltage (OCV) of 1.09 V and a notable power output of 1080 mW cm^?2 at 520 °C. Further investigation revealed that the 10NSO displays a superior ionic conduction up to 0.26 S cm^?1 at 520 °C. Moreover, the cell demonstrates high stability up to 80 h. The high electrochemical property and good stability recommend that the NSO is a favorable candidate for symmetrical SOFC electrolyte.     

Recent progress on solid oxide fuel cell: Lowering temperature and utilizing non-hydrogen fuels

A solid oxide fuel cell (SOFC) is a promising energy conversion device with high efficiency and low pollutant emission. The practical application of the conventional SOFCs is limited mainly because of their high operating temperature and the inconvenience brought by the H₂ fuel utilization. This work reviews the recent progress on intermediate temperature SOFCs especially with non-hydrogen fuels. Composite electrolyte consisting of a solid oxide ionic conducting phase and a molten carbonate phase exhibits sufficient ionic conductivity in the intermediate temperature range, i.e. 500–800 °C, and facilitates the simultaneous conduction of H⁺, O²⁻ and CO₃²⁻ ions. A single cell with the composite electrolyte shows a promising power density, 1700 mW cm⁻² at 650 °C with hydrogen as the fuel. The composite electrolyte has been also employed in a direct carbon fuel cell (DCFC), and the simultaneous conduction of O²⁻ and CO₃²⁻ in the electrolyte has been proposed. Recently, perovskite structured materials are found to have good resistance to coke formation as the anode of the direct hydrocarbon solid oxide fuel cell, and several carbon resistant perovskite anodes are employed in all-perovskite structured SOFCs, which exhibit excellent performance with CH₄ and methanol as the fuel.     

Li2TiO3–LaSrCoFeO3 semiconductor heterostructure for low temperature ceramic fuel cell electrolyte

Semiconductor-based electrolytes have significant advantages than conventional ionic electrolyte fuel cells, especially for high ionic conductivity and power outputs at low temperatures (<600 °C). This work reports a p-n heterojunction composite electrolyte developed by a p-type La_0.8Sr_0.2Co_0.8Fe_0.2O_3-δ (LSCF) and n-type Li₂TiO₃ (LTO). It achieved a power output of 350 mW cm^?2 at 550 °C using LSCF-LTO heterostructure as the electrolyte. On the other hand, pure LSCF and Li₂TiO₃ were made as the fuel cell electrolyte as well. The former resulted immediately a short circuiting problem and exhibited no device voltage because of high electron (hole) conductivity. While the Li₂TiO₃ can reach an open circuit voltage (OCV) but deliver too low power output, 37 mW cm^?2 at 550 °C. Scanning Electron Microscope (SEM) combined with High-Resolution Transmission Electron Microscope (HR-TEM) clearly proved the formation of heterogeneous interface. Also, Fourier Transform Infrared Spectroscopy (FTIR) was performed to demonstrate the functional group of the synthesized materials. The results demonstrate clearly the semiconductor heterostructure effect. By adjusting apriority composition of the n-type and p-type components, electronic conduction is well suppressed in the membrane electrolyte. Meanwhile, by constructing p-n heterostructure and build-in field, we have succeeded in high ionic conductivity, high current and power outputs for the low temperature fuel cells. The results are interesting in general that to construct a p-n heterostructure electrolyte can be an effective and common way in developing low temperature ceramic fuel cells.     

Role of carbonate phase in ceria–carbonate composite for low temperature solid oxide fuel cells: A review

Ceria–salt composites represent one type of promising electrolyte candidates for low temperature solid oxide fuel cells (LT‐SOFCs), in which ceria–carbonate attracts particular attention because of its impressive ionic conductivity and unique hybrid ionic conduction behavior compared with the commonly used single‐phase electrolyte materials. It has been demonstrated that the introduction of carbonate in these new ceria‐based composite materials initiates multi new functionalities over single‐phase oxide, which therefore needs a comprehensive understanding and review focus. In this review, the roles of carbonate in the ceria–carbonate composites and composite electrolyte‐based LT‐SOFCs are analyzed from the aspects of sintering aid, electrolyte densification reagent, electrolyte/electrode interfacial ‘glue’ and sources of super oxygen ionic and proton conduction, as well as the oxygen reduction reaction promoter for the first time. This summary remarks the significance of carbonate in the ceria–carbonate composites for low temperature, 300–600 °C, SOFCs and related highly efficient energy conversion applications. Copyright © 2016 John Wiley & Sons, Ltd.     

Semiconductor ionic Ce0.8Sm0.2O2-δ-Na2CO3 – LiCo0.225Cu0.075Ni0.7O 3-δ composite material as electrolyte for low temperature solid oxide fuel cells

Semiconductor ionic electrolytes have obtained much attention because of good ionic conductivity and electrochemical performance. Novel semiconductor ionic NSDC (Ce_0.8Sm_0.2O_2-δ-Na₂CO₃)-LCCN (LiCo_0·225Cu_0·075Ni_0·7O_3-δ) composite materials have been adopted as electrolyte membrane for the first time, in which symmetrical cell composed of NSDC-LCCN membrane is constructed with Ni_0·8Co_0·15Al_0·05LiO₂ (NCAl)-pasted Ni foam electrodes. An open circuit voltage (OCV) above 1 V and improved power density are obtained in the NSDC-LCCN cells, which confirms the functionality of the proposed semiconductor ionic materials. Meanwhile, X-ray diffractometer (XRD) and Scanning electron microscope (SEM) analyses identify the phase purity and homogenous nanocomposite morphology of all the NSDC-LCCN materials samples with various mass ratios. The performance illustrated by much more steady instead of transient state evaluation reveals that 3NSDC-LCCN composite electrolyte is most optimum, and the corresponding cell exhibits a considerable maximum power density of 598 mW cm⁻² at 550 °C, over five times of that of pure NSDC electrolyte cells. Short-term duration test of 3NSDC-LCCN cell at 550 °C shows that the cell could steadily operate up to ~9 h without obvious degradation at a remarkable current density of 469 mA cm⁻², which indicates that NSDC-LCCN composite electrolyte is a promising material for low temperature solid oxide fuel cells.     

Durability study of an intermediate temperature fuel cell based on an oxide–carbonate composite electrolyte

It was reported that ceria–carbonate composites are promising electrolyte materials for intermediate temperature fuel cells. The conductivity stability of composite electrolyte with co-doped ceria and binary carbonate was measured by AC impedance spectroscopy. At 550 °C, the conductivity dropped from 0.26 to 0.21 S cm⁻¹ in air during the measured 135 h. At a constant current density of 1 A cm⁻², the cell performance keeps decreasing at 550 °C, with a maximum power density change from 520 to 300 mW cm⁻². This is due to the increase of both series and electrode polarisation resistances. Obvious morphology change of the electrolyte nearby the cathode/electrolyte interface was observed by SEM. Both XRD and FT-IR investigations indicate that there are some interactions between the doped ceria and carbonates. Thermal analysis indicates that the oxide–carbonate composite is quite stable at 550 °C. The durability of this kind of fuel cell is not good during our experiments. A complete solid oxide-carbonate composite would be better choice for a stable fuel cell performance.