Intermediate Temperature Solid Oxide Fuel Cells Research Articles

A tutorial review on solid oxide fuel cells: fundamentals, materials, and applications

AbstractSolid oxide fuel cells (SOFCs) are recognized as a clean energy source that, unlike internal combustion engines, produces no CO2 during operation when H2 is used as a fuel. They use a highly efficient chemical-to-electrical energy conversion process to convert oxygen and hydrogen into electricity and water. They can provide smaller-scale power for transportation (e.g., cars, buses, and ships) and be scaled up to provide long-term energy for an electrical grid, making SOFCs a promising, clean alternative to hydrocarbon combustion. Conventional SOFCs faced challenges of high operating temperatures, high cost, and poor durability. Research into advanced cathode, anode, electrolyte, and interconnect materials is providing more insight into the ideal structural and chemical properties that enable the commercialization of highly stable and efficient intermediate temperature (IT) SOFCs. In this paper, we discuss the functional properties of the cathode, anode, electrolyte, and interconnectors for IT-SOFCs. The performance of SOFCs depends not only on the materials used but also on the optimization of operating conditions to maximize efficiency. The voltaic, thermodynamic, and fuel efficiency of SOFCs is presented.

Recent Advances in Barium Cobaltite-Based Perovskite Oxides as Cathodes for Intermediate-Temperature Solid Oxide Fuel Cells.

Solid oxide fuel cells (SOFCs) are considered as advanced energy conversion technologies due to the high efficiency, fuel flexibility, and all-solid structure. Nevertheless, their widespread applications are strongly hindered by the high operational temperatures, limited material selection choices, inferior long-term stability, and relatively high costs. Therefore, reducing operational temperatures of SOFCs to intermediate-temperature (IT, 500-800°C) range can remarkably promote the practical applications by enabling the use of low-cost materials and enhancing the cell stability. Nevertheless, the conventional cathodes for high-temperature SOFCs display inferior electrocatalytic activity for oxygen reduction reaction (ORR) at reduced temperatures. Barium cobaltite (BaCoO3-δ)-based perovskite oxides are regarded as promising cathodes for IT-SOFCs because of the high free lattice volume and large oxygen vacancy content. However, BaCoO3-δ-based perovskite oxides suffer from poor structural stability, inferior thermal compatibility, and insufficient ionic conductivity. Herein, an in-time review about the recent advances in BaCoO3-δ-based cathodes for IT-SOFCs is presented by emphasizing the material design strategies including functional/selectively doping, deficiency control, and (nano)composite construction to enhance the ORR activity/durability and thermal compatibility. Finally, the currently existed challenges and future research trends are presented. This review will provide valuable insights for the development of BaCoO3-δ-based electrocatalysts for various energy conversion/storage technologies.

Low temperature processing and characterization of novel ceria based electrolyte composition Ce0.8−xGd0.1Bi0.1TixO2−δ (x = 0.05,0.1) for intermediate temperature SOFC application

Low temperature processing and characterization of novel ceria based electrolyte composition Ce0.8−xGd0.1Bi0.1TixO2−δ (x = 0.05,0.1) for intermediate temperature SOFC application

Structural and Electrochemical Characterisation of NBZFO Cobalt-Free Cathode Material for Intermediate-Temperature Solid Oxide Fuel Cells: An Experimental Investigation

Compared to other energy-generating technologies and energy conversion devices, intermediate-temperature solid oxide fuel cells (IT-SOFCs) have gained significant attention from energy experts due to its high energy density, moderate operating temperature (600–800°C), low emissions and reliability. Enhancing the performance of IT-SOFCs requires suitable and excellent cathode materials. Thus, a perovskite-type Nd0.5Ba0.5Zr0.8Fe0.2O3+δ (NBZFO) material was synthesised via traditional solid-state reaction technique and analysed as a potential cathode material for IT-SOFCs. Analysis of X-ray diffraction data (XRD) revealed a single-phase perovskite material that crystallises in cubic space group (pm-3m). The thermal and electrochemical properties were analysed with the aid of thermogravimetric analysis (TGA) and electrochemical impedance spectroscopy (EIS). NBZFO has an electrical conductivity in air of 80 S cm−1 at 400°C and a polarisation resistance (Rp) of 0.106 Ω cm2 at 800°C. TGA reveals a slight loss in weight of about 0.58%, thereby suggesting a highly stable cathode material for IT-SOFC. Electrochemical investigation shows that NBZFO has good electronic and ionic conductivity and excellent oxygen stichometry. Further studies are required to understand the effects of varying B-site composition of the cathode material.

Comparative investigation of composition, microstructure and property influences on electrocatalysis of two representative cathodes: BaCo0.4Fe0.4Zr0.1Y0.1O3-δ versus Ba0.5Sr0.5Co0.8Fe0.2O3-δ

BaCo0.4Fe0.4Zr0.1Y0.1O3-δ (BCFZY) and Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF) perovskites are two representative high-performance cathode materials for intermediate-temperature solid oxide fuel cells (IT-SOFCs). Here, symmetric cell tests demonstrate that polarization resistance values of BCFZY cathode are only 0.21–0.29 times those of BSCF cathode at 740–660 °C, and 0.21–0.26 times those of BSCF cathode at the oxygen partial pressures of 1.00–0.10 atm. Distribution of relaxation times (DRT) is applied to detect individual electrode processes overlapped in impedance spectra, which finds that the dominant processes of oxygen dissociation, subsequent species transfer and reduction on BCFZY cathode are superior to those of BSCF cathode. By temperature-programmed desorption of oxygen (O2-TPD), thermal expansion coefficient (TEC) measurements, scanning electron micrograph (SEM) and energy dispersive spectra measurements, the strong oxygen combination ability with higher activation energy of oxygen desorption (Ed = 84 kJ/mol), peculiar surface state with evident nano-particle structure, more matchable TEC (19.0 × 10-6 K−1) and excellent interfacial connection with the electrolyte are observed on BCFZY cathode to account for its distinctive catalysis, as compared to BSCF cathode (Ed = 68 kJ/mol, TEC = 24.2 × 10-6 K−1).

Effect of Gadolinium Doping on the Structure of Ce1-xGdxO2-x/2 Solid Solutions Prepared by Ionic Gelation Approach

The current research aims to present the structural characterization of Gd-doped ceria powders and ceramics, investigating the structural evolution resulting from cerium substitution with Gd across the entire composition range from 0 to 100 mol.% Gd2O3. Ce1-xGdxO2-x/2 powders with varying Gd contents (0 ≤ x ≤ 1) were synthesized using the ionic gelation method followed by thermal annealing. The resulting powders were subjected to high-temperature treatment to obtain ceramics. Characterization methods included X-ray diffraction (XRD) to identify phase composition and confirm the formation of Ce1-xGdxO2-x/2 solid solutions, infrared spectroscopy (IR) and scanning electron microscopy (SEM) for structural and morphological studies, and X-ray photoelectron spectroscopy (XPS) to evaluate the electronic structure. Comparative analysis of Gd-doped calcined powders and sintered pellets revealed the impact of thermal treatment on the structural features of the resulting solid solutions, elucidating the influence of gadolinium substitution. The novelty of this research lies in demonstrating the successful preparation of Ce1-xGdxO2-x/2solid solutions via an alginate-mediated ion-exchange process and providing a detailed structural investigation over the entire range of dopant concentrations. This assessment highlights the feasibility for further research of these materials as suitable candidates for intermediate-temperature solid oxide fuel cells (IT-SOFCs) or catalyst applications. Doi: 10.28991/ESJ-2024-08-05-01 Full Text: PDF

Characterization of A-site doped PrBaCo2O5+δ perovskites as cathode materials for IT-SOFCs

Creating highly efficient and durable cathodes persists as a formidable task in the realm of intermediate temperature solid oxide fuel cells (IT-SOFCs). Hence, we present a double perovskite oxide, Pr0.6Sr0.4BaCo2O5+δ (PS0.4BC), and its electrocatalytic activity is thoroughly investigated. The XRD test is carried out and a notable transformation in the phase structure is observed upon the introduction of Sr2+ ions into the PBC matrix. That is, the phase structure of the material changes from double perovskite structure to single perovskite structure. Through the TG and TEC test that the introduction of Sr2+ ions into the lattice can create additional sites for oxygen vacancy formation and reduce TEC values. Further verify that Sr doping can enhance the concentration of oxygen vacancies of the material. The PS0.4BC cathode material exhibits remarkably low polarization resistance, achieving an impressive value of 0.027 Ω cm2 at an operating temperature of 800 °C. Oxygen partial pressure test shows that as the oxygen partial pressure decreases, there is a notable increase in impedance, particularly evident in the low frequency region of the Nyquist plots. This indicates that the oxygen content has obvious effects on the low-frequency processes. When the oxygen partial pressure exceeds 0.05 atm, the oxygen adsorption-dissociation and diffusion process is the rate-limiting step in the ORR. However, when the oxygen partial pressure is less than 0.05 atm, the formation of lattice oxygen through the combination of oxygen ions and oxygen vacancies processes is the rate-limiting step. Compared PBC cathode material, the more pronounced changes in the high frequency arc of the PS0.4BC cathode at the same oxygen content. It shows that Sr doping significantly changes the high frequency process. After doping, the oxygen vacancy concentration increases, which facilitates the occurrence of the charge transfer process at high frequency.

Enhanced protonic and oxygen-ionic conductivity in Ga-doped Nd2Ce2O7 electrolytes for intermediate-temperature solid oxide fuel cell

Developing mixed oxygen-ion and proton conductors with high ionic conductivity is crucial for advancing intermediate temperature-solid oxide fuel cell (IT-SOFC). In this study, a novel Nd2-xGaxCe2O7 (NGCO, x = 0, 0.05, 0.075, 0.10 and 0.15) ceramic is synthesized by the citric acid-nitrate sol-gel combustion process. The influence of Ga doping on crystal structure, oxygen vacancy, morphology, proton transport characteristic and electrochemical performance of this materials is systematically investigated. It is found that the NGCO exhibits the fluorite-type structure and belongs to space group Fm3¯m. NGCO shows remarkable chemical stability, resisting harmful effects from CO2, water, and wet hydrogen exposure. Adding a small amount of Ga results in high-density ceramics with enlarged grain size and reduced grain boundary density. Nd1.925Ga0.075Ce2O7 (7.5NGCO) displays outstanding conductivities, exceeding Nd2Ce2O7 (NCO) by 271.4% and 248.6% in dry air and wet hydrogen conditions. Further, an anode-supported structure fuel cell with 7.5NGCO electrolyte is constructed and evaluated. The peak power density (PPD) at 700 °C reaches 702 mW/cm2, a 96% enhancement over the fuel cells with NCO electrolyte. These findings indicate that Ga-doped NCO electrolyte holds promise as a proton-conducting electrolyte suitable for IT-SOFC.

High-speed laser cladding (Fe,Cu)-dopped Ni-Co protective coatings for SOFC MICs: From long-term oxidation behavior to mechanical stability

The long-term operation of intermediate temperature-solid oxide fuel cells (IT-SOFCs) could be significantly influenced by the stability of the interconnect itself, especially, its oxidation resistance under the cathode environment. To effectively prevent Cr-volatilization and improve the interconnector performance, application of highly dense and stable protective coatings over the interconnector is indispensable. In this work, Ni-Co-based alloy protective coatings doped with Fe and Cu were applied onto the 441 interconnector via a novel high-speed laser cladding technology. The microstructure evolution, phase composition, oxidation resistance, electrical conductivity and mechanical properties of the coated steels were investigated at different oxidation stages at 800 °C. It was found that the Ni33.8Co34Fe32.2-coated steel shows the best oxidation resistance and electrical properties. The oxidation rate (kp) and ASR of Ni33.8Co34Fe32.2-coated steel is respectively 4.44 × 10−13 g2 cm−4 s−1 and 25.55 mΩ cm2, significantly lower than the uncoated steel and Ni44.6Co44.7Cu10.7-coated steel at the end of 1000-h test cycle. The resultant conductivity of the Ni44.6Co44.7Cu10.7 and Ni33.8Co34Fe32.2-coated steels is approximately 7.8 and 4.1 S cm−1, respectively, which could meet the requirement for SOFC stacks. In addition, the nano-hardness of the Ni33.8Co34Fe32.2 and Ni44.6Co44.7Cu10.7-coated steels was found to be 8.3 GPa and 8.8 GPa, respectively, through nanoindentation testing. The improved surface hardness is attributed to grain refinement and increased oxide content on the coating surface. The results indicated that the Ni-Co-based alloy coatings effectively improved the long-term stability and mechanical property of the 441 interconnector under the SOFC cathode environment.

Exploring the defect formation and ionic migration in A2Zr2O7 (A = La, Ce, Nd, and Gd) Pyrochlore solid-state electrolytes

This study investigated the potential of A2Zr2O7 pyrochlore phase oxides (A = La, Ce, Nd, and Gd) as solid oxide fuel cell (SOFC) electrolyte materials. A2Zr2O7 ceramics were synthesized using the sol-gel process, and their disordered pyrochlore structure was confirmed by X-ray Diffraction and Rietveld refinement. Field Emission Scanning Electron Microscopy (FE-SEM) was used to examine the morphology of the materials, while X-Ray Photoelectron Spectroscopy (XPS) confirmed a high oxygen ion concentration. UV–Vis Diffuse Reflectance Spectroscopy (DRS) and density of states calculations consistently showed similar bandgap patterns in all zirconate pyrochlore. Density Functional Theory (DFT) simulations provided insights into the electronic structure and migration pathways. A two-probe conductivity study confirmed the relation between the ionic conductivity and ordering degree of pyrochlore. At 750 °C, LZO, CZO, and GZO exhibited ionic conductivity values of 5.44 × 10−3 S/cm, 3.00 × 10−3 S/cm, and 2.01 × 10−3 S/cm respectively, while NZO demonstrated a conductivity of 1.27 × 10−2 S/cm at 700 °C, which is higher than that of the other samples. This study favours the use of the sol-gel route for processing zirconate pyrochlore materials, highlighting its ability to achieve high densities at lower temperatures and exhibit enhanced ionic conductivity. A2Zr2O7 has emerged as a promising oxide-ion conducting solid electrolyte for intermediate temperature SOFC applications within the 600–750 °C temperature range.

The comparative effects of cobalt and/or iron ions as dopants into Ba(Zr, Y)O3-δ-based perovskite cathodes for intermediate-temperature SOFCs

The development of highly active cathode is crucial to promote the electrochemical performance of intermediate-temperature solid oxide fuel cells (IT-SOFCs). Here, it is reported the cobalt and/or iron ions as dopants into Ba(Zr, Y)O3-δ with the nominal chemical formulae of BaCo0.4Fe0.4Zr0.1Y0.1O3-δ (BCFZY), BaCo0.8Zr0.1Y0.1O3-δ (BCZY), and BaFe0.8Zr0.1Y0.1O3-δ (BFZY), which all form the main cubic perovskites in addition to a small amount of co-assembled hexagonal D-Ba(Co, Fe)O3-δ, D-BaCoO3-δ, and D-BaFeO3-δ under the same preparing condition. Among three samples, BCZY composite shows the highest capacity of O2 adsorption, lower thermal expansion coefficient (19.1×10-6 K-1), and highest electrical conductivity of 2.73 S cm-1 (750 °C). The symmetric cell with BCZY composite cathode exhibits excellent ORR activity with the lowest area specific resistance of 0.10 Ω cm2 at 700 °C, which is 0.71 and 0.67 times those of BCFZY composite and BFZY composite cathodes. Accordingly, the single cell with BCZY composite cathode shows the lowest RP of 0.16 Ω cm2 and the highest peak power density of 2.24 W cm2 at 700 °C. The distribution of relaxation times (DRT) analysis reveals that BCZY composite cathode shows the superior surficial oxygen exchange and charge transfer activity.

Sr0.90Ba0.10Co0.95Ti0.05O3-δ cathode as an improved electrode for IT-SOFCs

The novel perovskite Sr0.90Ba0.10Co0.95Ti0.05O3-δ has been designed as cathode material to be used in intermediate-temperature solid oxide fuel cells (IT-SOFC). To enlarge the unit-cell size and enhance oxygen diffusion, Sr atoms have been partially substituted by a 10 % of Ba. This new compound was characterized through X Ray Diffraction (XRD) unveiling a tetragonal phase at RT. An in situ Neutron Powder Diffraction (NPD) study between RT and 800 °C showed a transition to a cubic perovskite at 400 °C, which was also noticeable in the thermogravimetric analysis (TGA) and dilatometric analysis. Adequate chemical and mechanical compatibility were achieved with the components of the cell. Electrical conductivity peaked at 400 °C, with values over 200 S cm−1, where it changed its behavior from semiconductor-like to insulator. Finally, polarization resistances as low as 0.062 Ω cm−2 were obtained for the working temperature of the cell in a symmetric cell, associated with the excellent power densities of 564 and 752 mW cm−2 obtained at 850 °C using pure H2 as fuel for the conventional test fuel cell and in a Pd-infiltrated one.

Improved ionic conductivity of Ca-doped and Ca-La co-doped ceria electrolytes for intermediate temperature solid oxide fuel cells

Improved ionic conductivity of electrolytes is necessary for solid oxide fuel cells, meant for the development of efficient and clean energy devices. This study is focused on the enhancement of ionic conductivity in Ca-doped and Ca-La co-doped cerium oxide (ceria) electrolytes for intermediate-temperature solid oxide fuel cells (IT-SOFCs). The co-doped ceria electrolytes, LCDC20 (Ce0.8La0.1Ca0.1O2−δ), exhibited enhanced ionic conductivity of 1.06 ×10−1 S cm−1 at 1073 K, when compared to conventional yttria-stabilized zirconia (YSZ) electrolytes, and has lower activation energy of 1.02 eV. Also, these co-doped electrolytes showed decreased sintering temperatures, which amounts to cost-effective electrolyte materials for IT-SOFCs.

A critical investigation of the effect of silver migration at the silver | alumina scandia doped zirconia electrolyte interface in solid oxide fuel cell conditions

In this work, a critical evaluation of silver ion migration at the surface of Al0.04Sc0.06Zr0.9O1.95 (AlScZrO) electrolyte is explored in intermediate temperature solid oxide fuel cells (IT-SOFCs). The behaviour of silver is systematically studied at the operating conditions of silver as a cathode, anode, or sealant in IT-SOFCs. The results to those obtained at the surface of other electrolytes (gadolinia doped ceria, samaria doped ceria and yttria-stabilized zirconia) are also analysed. In the case of cathodic polarization, a thick silver deposit is formed around the electrode on the surface of the electrolyte, which may lead to the short-circuiting of the electrodes. While for the anodic polarization, oxygen bubbles are formed between the electrode and electrolyte, leading the delamination of electrode. Silver can also migrate from unpolarized parts of the cell such as sealants. Therefore, Issues related to the application of silver in IT-SOFCs should be concerned by the scientific community.

Effect of Mg-doping on electrochemical performance of PrBaFe2O5+δ cathode materials for solid oxide fuel cells

PrBaFe2O5+δ (PBF) is one of the promising cathode materials for intermediate-temperature solid oxide fuel cell (IT-SOFC) technology. However, as the operating temperature decreases, the electrochemical performance of this material deteriorates rapidly. To counter this, various doping strategies have been tested and reported in order to improve the electrochemical properties of this material at intermediate-temperatures. In this study, Mg-doping to partially substitute Fe of PBF was investigated. PrBaFe2–xMgxO5+δ (PBFMgx, x=0.1, 0.15, 0.2, 0.3) materials were successfully synthesized, and their electrochemical performance as IT-SOFC cathode was evaluated. It is shown that Mg-doping enhances the conductivity of PBF between 650–800 °C, impacts little on the area-specific resistance of oxygen reduction reaction at and above 700 °C, and, most significantly, improves the power density of the Ni-SDC/SDC/PBFMg0.15 single cell by 52% compared to the un-doped PBF. This enhanced electrochemical performance is attributed to the improvement in PBF conductivity by Mg-doping.

Enhancement of redox cycling stability of Ni/GDC cermets for intermediate-temperature SOFC anodes through Ge incorporation in the ceramic phase

In this study, Ge4+ was incorporated into the ceramic matrix of nickel-based anode materials, synthesizing anode materials NiO-Gd0.1Ce0.9-xGexO1.95 (x = 0.01, 0.04, 0.07) and NiO-Ce0.9Gd0.1O2–δ. Corresponding anode support samples were also prepared, designated as A-CGDC, A-C1Ge, A-C4Ge, and A-C7Ge. Additionally, single cell samples using the newly co-doped anode materials were prepared, identified as C-CGDC, C-C1Ge, C-C4Ge, and C-C7Ge. The research indicates that after doping the ceramic phase framework with Ge4+, these composite anode materials exhibit the potential to withstand more redox cycles than C-CGDC without catastrophic mechanical structural damage or degradation in electrochemical performance. Particularly, after incorporating 7mol% Ge4+ into the ceramic backbone, the issue of 86 % strain accumulation in the anode support was mitigated. Additionally, the ion doping ratio is closely linked to the stability of the single cell samples, with the A-C7Ge sample able to withstand up to 11 extreme recycling sessions at 700 °C without complete structural failure, improving structural stability by approximately 57 % compared to A-C1Ge. In terms of electrochemical output performance, the impedance of C-C1Ge prior to complete failure was nearly double that before oxidation-reduction, with a maximum power density reduction of about 26.47 %. As the co-doping concentration in the ceramic phase reached 7mol%, the impedance increase during single cell cycle testing was about four times smaller, with C-C7Ge 's impedance before mechanical failure only 37 % higher than before cycling, and a reduction in maximum power density of just 11.11 %. Moreover, during consecutive cycling tests, the average damage to the open-circuit voltage was only 0.02V, showing almost no negative impact from the cycling operations, and it maintained efficient output performance after multiple cycles. Therefore, the newly synthesized nickel-based anode materials, modified via co-doping methods within the ceramic phase, can be considered as potential composite anode materials for intermediate-temperature solid oxide fuel cells.

Prevention of morphological change for (Ba,Sr)(Co,Fe)O3 cathodes by incorporating (Ce,Gd)O2 for intermediate-temperature solid oxide fuel cells

Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF) generally exhibits superior cathode activity compared to La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF) for intermediate-temperature solid oxide fuel cells (IT-SOFCs). However, the chemical stability of BSCF is inferior to that of LSCF. When BSCF cathodes are sintered at a high temperature of 1050 °C, performance was compromised due to the formation of (Ba,Sr)ZrO3 between the scandia-stabilized zirconia (ScSZ) electrolyte and gadolinia-doped ceria (GDC) interlayer. The oxide ionic conductivity of (Ba,Sr)ZrO3 was low, decreasing the cathode performance. Furthermore, the BSCF grains enlarged, and cobalt oxide formed due to BSCF decomposition, decreasing the active specific surface area. However, by incorporating GDC into the BSCF cathode, the morphology remained unchanged and cobalt oxide formation was prevented, despite (Ba,Sr)ZrO3 still forming. The performance of the cell with the BSCF-GDC composite cathode surpassed that with the BSCF cathode due to decreased polarization resistance attributed to the oxygen surface exchange and diffusion processes in the cathode.

Improved electrochemical performance of high-entropy La0.8Sr0.2FeO3-based IT-SOFC cathode

It is widely accepted that solid oxide fuel cells (SOFCs) have the potential to be the most efficient and cost-effective system for the direct conversion of various fuels into electricity with low environmental pollution and high efficiency. Herein, we demonstrate a novel Fe-based high-entropy perovskite oxide as a cathode material for intermediate-temperature solid oxide fuel cells (IT-SOFCs). The high-entropy perovskite La0.2Pr0.2Sm0.2Nd0.2Sr0.2FeO3-δ (HE-LSF), composed of La, Pr, Nd, Sm, and Sr is prepared by incorporating five components into the A-site of La0.8Sr0.2FeO3-δ (LSF). Doping of Pr, Nd, Sm, and Sr into the A-site significantly enhances the redox stability of LSF and restrains the surface Sr-segregation in both oxidizing and reducing conditions. The electrode polarization resistance of the anode-supported single cell with HE-LSF cathode is significantly low at intermediate temperature. Meanwhile, at 800 °C, the power density is 1029.2 mW/cm2, and the polarization resistance (Rp) is 0.101 Ω cm2, while at 700 °C, the power density is 581.52 mW/cm2 with an Rp of 0.21 Ω cm2 in humidified H2. Further, the HE-LSF cathode presents good and long-term stability at a constant voltage of 0.7 V for 100 h at 700 °C. The excellent electrochemical performance and stability results exhibit that Fe-based high-entropy perovskite oxide is a new promising candidate and cathode material for SOFCs at intermediate temperatures.

Synthesis and characteristics of densified GDC-LSO composite as a new apatite-based electrolyte for intermediate-temperature solid oxide fuel cells (IT-SOFC)

Developing solid-state ionic conductors with desirable charge-transport efficiency and reasonable durability is a fundamental and long-lasting challenge for solid-oxide fuel cells (SOFCs). Ceria-based electrolytes, as one of the most common types of electrolytes for intermediate-temperature solid-oxide fuel cells (IT-SOFCs), offer a high surface-exchange coefficient and faster kinetics in the triple phase boundaries (TPBs); however, they suffer to some extent of electronic conductivity in the reducing atmosphere and gradual phase transitions. Here in this work, we have reported a novel co-doped IT-SOFC electrolyte composite combining the apatite structure of Lanthanum Silicate (LSO) with the fluorite structure of Gadolinium-doped Ceria (GDC), which showed a high ionic conductivity and a minimal electronic leak. The resulting GDC-LSO composite electrolyte also achieved an OCV (open-circuit voltage) of 1 V at 800 °C, implying a significant improvement in the OCV values reported for the typical ceria-based electrolytes (∼0.75 V). The sample was prepared using 40 wt% GDC and 60 wt% LSO (40GDC-60LSO) showed a maximum electrical conductivity of 25 mS cm−1 at 800 °C with good densification properties (>95 % relative density). The cell electrochemical performance measurement was conducted using a 3.0 vol% humified H2 stream at the anode side while the cathode side was exposed to the air at 800 °C. The interdiffusion of cations between La3+ in the LSO phase and Ce4+ and Gd3+ in the GDC phase was detected by XRD and EDX results after sintering samples at 1500 °C for 4 h using 0.5 wt% PVA or 0.5 wt% Ethyl cellulose (EC) as the binder. The proposed GDC-LSO composite could help better understand the influence of compositional constituents and processing variables on the densification and electrical properties of the electrolyte materials for the IT-SOFCs.

Reconstituting the microstructural properties and ionic conductivity of copper - doped yttria-stabilized zirconia via mechanochemical synthesis for intermediate-temperature solid oxide fuel cell applications

The development of highly conductive and thermally stable solid electrolytes is crucial for the next generation of intermediate temperature solid oxide fuel cells (IT-SOFCs). This study investigates the influence of copper (Cu) doping on the microstructure and ionic conductivity of 8 mol. % yttria-stabilized zirconia (8YSZ) when sintered at lower temperatures. The mechanochemical synthesized 1, 3, and 5 wt. % Cu-doped 8YSZ compacts were extensively characterized their properties through a suite of analytical techniques like X-ray diffraction, Raman, X-ray photoelectron, UV–Vis diffused reflectance, particle size analyzer, BET surface area, Emission Scanning Electron Microscopy-Energy Dispersive X-Ray, and AC impedance. The results revealed enhanced ionic conductivity and sinterability, at reduced temperature of 1373 K, with the 5 wt. % Cu-doped 8YSZ demonstrating the highest ionic conductivity of 4.18 × 10−3 S cm−1 at 1023 K, alongside reduced activation energy of 1.14 eV. The lower sintering temperatures and consequential benefits in performance metrics suggest Cu-doping as an effective strategy for optimizing 8YSZ-based electrolytes lowering SOFC manufacturing costs without compromising performance for IT-SOFCs.