An oxygen reduction reaction active and durable SOFC cathode/electrolyte interface achieved via a cost-effective spray-coating

One of the critical obstacles for commercialization of solid oxide fuel cells (SOFCs) technology is to develop efficient interfaces between cathode and electrolyte that enable high activity toward oxygen reduction reaction (ORR) while maintain long-term durability. Here, we report a cost-effective spray-coating process that applied in the building of an ORR active and durable cathode/electrolyte interface. When tested at 750 °C, such spray-coated cathodes show a typical interfacial polarization resistance of ~0.059 Ωcm², much lower than that of ~0.10 Ωcm² for screen-printed cathodes. Detailed distribution of relaxation time analyses of the impedance spectra over time indicates that the capability of mass transfer and surface exchange process in the spray-coated cathode/electrolyte interface has been enhanced and maintained in the testing periods of ~100 h. As a result, a Ni-based anode supported cell with thin electrolyte and spray-coated cathodes shows an excellent peak power density of 1.012 Wcm⁻², much higher than that of 0.712 Wcm⁻² for cells with screen-printed cathodes, when tested at 750 °C using wet H₂ as fuel and ambient air as oxidant. It is demonstrated that ORR activity and durability of the SOFC cathodes can be dramatically enhanced via a cost-effective spray-coating process.     

Enhancing the oxygen reduction activity of PrBaCo2O5+δ double perovskite cathode by tailoring the calcination temperatures

In this study, the oxygen reduction activity of PrBaCo₂O_5+δ (PBC) double perovskite is remarkably enhanced by rationally tuning the calcination temperatures of the cathode precursor for solid oxide fuel cells (SOFCs). Effects of the calcination temperatures on the phase structure, microstructure, surface area and oxygen reduction reaction (ORR) activity of PBC cathode is systematically investigated. The cathode with optimized calcination temperature (900 °C, PBC-900) shows excellent activity and stability for ORR at 600 °C in terms of area specific resistances (ASRs). A distinctive low ASR of 0.068 Ω cm² is obtained at 600 °C for PBC-900, which is 92.6%, 34.6% and 15.0% lower than PBC-800, PBC-1000 and PBC-1100, respectively. After operating for 250 h in air at 600 °C, the ASR value of PBC-900 is not significantly reduced. Furthermore, a single cell with PBC-900 cathode delivers attractive peak power density of 1.60 W cm⁻² at 600 °C. The present study suggests that the ORR activity of PBC cathode can be greatly boosted by rationally tailoring the calcination temperatures, which may bring new avenue for the design of highly active cathodes for SOFCs.     

Improvement of the internal reforming of metal-supported SOFC at low temperatures

Automotive Solid oxide fuel cells (SOFCs) require improvements in mechanical robustness, power generation at low temperatures, and system compactness. To address these issues, we attempt to improve the internal reformation of metal-supported SOFCs (MS-SOFCs) via catalyst infiltration. After introducing nickel/gadolinium-doped ceria (Ni/GDC) nanoparticles, power densities of 1.16 Wcm⁻² with hydrogen (3%H₂O) and 0.85 Wcm⁻² with methane (Steam-to-Carbon ratio, S/C = 1.0) are obtained at 600 °C, 0.7 V. This is the highest performance achieved in previous studies on MS-SOFCs. Internal reforming with various hydrocarbon is also demonstrated. In particular 0.64 Wcm⁻² at 600 °C, 0.7 V is obtained when the fuel is iso-octane. We develop a numerical model to separately analyze reforming and electrochemical reaction. Catalyst infiltration dramatically increases the number of active sites for steam reforming. In addition, ruthenium/gadolinium-doped ceria (Ru/GDC) should be suitable as a catalyst metal at low temperatures because of the lower activation energy of steam reforming.     

Bio-inspired honeycomb-shaped La0·5Sr0·5Fe0·9P0·1O3-δ as a high-performing cathode for proton-conducting SOFCs

A new honeycomb-shaped La_0·5Sr_0·5Fe_0·9P_0·1O_3-δ (LSFP) material has been proposed as a cathode for proton-conducting solid oxide fuel cells (H–SOFCs). Compared with conventional LSFP, the honeycomb-shaped does not change the crystal structure or the electronic structure of the material but offers a much higher surface area. The unique honeycomb structure allows the easier diffusion of air and H₂O evaporations in the cathode, thus benefiting the application of LSFP as a cathode for LSFP. The honeycomb LSFP cathode's fuel cell shows a peak power density of 1474 mW cm⁻² at 700 °C, which is higher than the conventional LSFP cell. In addition, the fuel cell performance is also the highest ever reported for H–SOFCs based on the Sr-doped LaFeO₃ (LSF) cathodes, making a new life for the first-generation LSF cathode for H–SOFCs. The distribution of relaxation times (DRT) analysis for the cell reveals that the honeycomb-shaped cathode improves the oxygen reduction reaction (ORR) at the cathode, thus improving the cathode kinetics.     

A La0.8Sr0.2MnO3/La0.6Sr0.4Co0.2Fe0.8O3−δ core–shell structured cathode by a rapid sintering process for solid oxide fuel cells

A La_0.8Sr_0.2MnO₃ (LSM)/La_0.6Sr_0.4Co_0.2Fe_0.8O_3?δ (LSCF) core–shell structured composite cathode of solid oxide fuel cells (SOFCs) has been fabricated by wet infiltration followed by a rapid sintering (RS) process. The RS is carried out by placing LSCF infiltrated LSM electrodes directly into a preheated furnace at 800 °C for 10 min and cooling down very quickly. The heating and cooling step takes about 20 s, substantially shorter than 10 h in the case of conventional sintering (CS) process. The results indicate the formation of a continuous and almost non-porous LSCF thin film on the LSM scaffold, forming a LSCF/LSM core–shell structure. Such RS-formed infiltrated LSCF–LSM cathodes show an electrode polarization resistance of 2.1 Ω cm² at 700 °C, substantially smaller than 88.2 Ω cm² of pristine LSM electrode. The core–shell structured LSCF–LSM electrodes also show good operating stability at 700 °C and 600 °C over 24–40 h.     

A novel dual phase BaCe0.5Fe0.5O3-δ cathode with high oxygen electrocatalysis activity for intermediate temperature solid oxide fuel cells

A novel iron-based perovskite BaCe_0.5Fe_0.5O_3-δ (BCF) powders were successfully fabricated and the phase composition, lattice structure, oxygen surface exchange coefficient and electrochemical performance were investigated. The ultrafine BCF powder with grain size of about 200 nm consisted of dual phase BaCe_0.15Fe_0.85O_3-δ and BaCe_0.85Fe_0.15O_3-δ. Electrical conductivity relaxation measurement illustrates the low conductivity activation energy and the high oxygen surface exchange kinetics with oxygen surface exchange coefficient of 3.8 × 10⁻⁵ cms⁻¹ at 600 °C. BCF cathode exhibits 1.04 Ωcm² on doped ceria electrolyte and remains stable in 400 h long term test at 600 °C. A single cell based on doped ceria electrolyte with BCF cathode shows a maximum power density of 228 mW cm⁻² at 650 °C. The preliminary results indicate that the dual phase BCF can be applied as cathode material for oxygen ion conductive solid oxide fuel cells.     

Preparation of 3D structure high performance Ba0·5Sr0·5Fe0·8Cu0·2O3-δ nanofiber SOFC cathode material by low-temperature calcination method

The electrospinning and sol-gel methods are used to prepare a Ba_0·5Sr_0·5Fe_0·8Cu_0·2O_3-δ (BSFC) cathode material with good chemical compatibility with the traditional electrolyte Ce_0.8Gd_0.2O_2?δ (GDC). Systematic experiments are performed to investigate the effect of sintering temperature on BSFC fibers microstructure, morphology and electrochemical properties. The results show that the three dimensional (3D) BSFC-F800 prepared by the low-temperature calcination method (800 °C) has a well-organized porous structure and larger specific surface area and porosity. In addition, the fibers are connected to each other to form a continuous electrode path, which provides an uninterrupted channel for charge transmission, and the low-temperature calcination can efficiently reduce the surface Sr segregation and increase ORR activity. At 700 °C, the 3D nanofiber cathode material BSFC-F800 has lower area specific resistance (ASR = 0.128 Ω cm²) and higher peak power density (PPD = 0.51 W cm^?2). The voltage decay rate in the 100 h long-term stability test is only 0.0342% h^?1.     

A novel Nb and Cu co-doped SrCoO3-δ cathode for intermediate temperature solid oxide fuel cells

The extensive explorations of potential cathode materials are prominently critical for the rapid development of high performance solid oxide fuel cells (SOFCs). Herein, we develop a novel Nb and Cu co-doped SrCoO_3-δ (SCNC) cathode base on solid state reaction, which exhibits decent compatibility with gadolinium doped cerium oxide (GDC) electrolyte. The SCNC is successfully stabilized with cubic structure at room temperature when incorporating of small amount of high valence Nb⁵⁺. Meanwhile, the oxygen vacancy concentration of SCNC is efficiently improved with the addition of Cu. The Nb and Cu co-doping also substantially promotes the electronic conductivity, achieving 550 S cm⁻¹ for the optical doped SrCo_0.85Nb_0.05Cu_0.10O_3-δ (SCNC10) at 400 °C. In addition, the polarization of SCNC is remarkably reduced, reaching as low as 0.021 Ω cm² for SCNC10 at 700 °C. The activation energy for reaction is also significantly lowered to 0.78 eV. The reaction order m is deduced to be about 0.30, implying that the rate determination step for SCNC10 is the charge transfer reaction. The peak power density of the single cell reaches 780 mW cm⁻² at 800 °C. All these outstanding performances demonstrate that SCNC is a promising cathode for SOFCs when operating at intermediate temperature (IT).     

Boosting electrocatalytic performance of Ruddlesden-Popper type Fe-based cathode catalysts by La and Ni co-doping for solid oxide fuel cells

Exploring advanced electrode materials with high electrochemical performance and sufficient durability is crucial to the commercialization of solid oxide fuel cells (SOFCs). Herein, a Ruddlesden-Popper Sr_2·9La_0·1Fe_1·9Ni_0·1O_7?δ (SLFN) oxide is systematically evaluated as efficient oxygen electrode material. La and Ni co-doping strategy demonstrates improved oxygen desorption ability and promoted electrochemical activity of pristine Sr₃Fe₂O_7?δ (SF) toward oxygen reduction react (ORR). Further, the ORR process of the SLFN electrode is probed by electrochemical impedance spectroscopy (EIS) and distribution of relaxation time (DRT) technique. The button cell with the SLFN cathode delivers a peak power density of 1.01 W cm^?2 at 700 °C, along with desirable stability over a period of 60 h. This study offers a feasible strategy for developing Ruddlesden-Popper type cathode candidates for SOFCs.     

Tuning oxygen vacancy of the Ba0·95La0·05FeO3−δ perovskite toward enhanced cathode activity for protonic ceramic fuel cells

Innovation of highly active cathode is of great significance to the development of protonic ceramic fuel cells (PCFCs). Herein, tailoring oxygen vacancies in Zn-doped Ba_0·95La_0·05FeO_3?δ (BLFZ) perovskite is proved to be beneficial for promoting the formation of proton defects. Hydration ability of the triple conducting BLFZ perovskites is confirmed by electrical conductivity relaxation (ECR). The results demonstrate that BLFZ exhibits a proton surface exchange coefficient of 1.34 × 10^?3 cm s^?1 at 600 °C, which greatly extends active sites from the electrolyte/cathode interface to the entire electrode. Mechanism and process elementary steps of the oxygen reduction reaction (ORR) of BLFZ-BaCe_0.7Zr_0·1Y_0.1Yb_0.1O_3?δ (BCZYYb) are detailedly studied. It is found that the rate-determining step of ORR is surface dissociative adsorption of oxygen on BLFZ-BCZYYb cathode. A maximum power density of 673 mW cm^?2 at 700 °C is achieved and BLFZ-BCZYYb based single-cell shows no obvious degradation at 600 °C for 200 h. The good performance is ascribed to the rapid proton diffusion of BLFZ-BCZYYb composite electrode by regulating the oxygen vacancies.     

Advanced electrocatalytic activity of praseodymium-deficient copper-based oxygen electrodes for solid oxide fuel cells

Herein praseodymium-deficient Pr_1.90−xCe_0.1CuO₄ oxides are evaluated as potential oxygen electrodes for solid oxide fuel cells (SOFCs). Introducing Pr-deficiency promotes the oxygen vacancy concentration, further improving electrocatalytic activity of the Pr_1.90−xCe_0.1CuO₄ electrodes towards oxygen reduction reaction (ORR). The Pr_1.75Ce_0·.1CuO₄ (P_1·75CC) component exhibits outstanding electrode performance, as supported by a polarization resistance as low as 0.025 Ω cm² and high peak power density of the single cell (1.11 W cm⁻²) at 700 °C. In addition, the rate-limiting steps for ORR kinetics are determined to be the charge transfer reaction and oxygen adsorption/diffusion process on the electrode surface. This work highlights an effective way for developing the cathode candidates with high electrocatalytic activity and superior stability.     

Facile synthesis of uniform LaSrCoO4 using amino acid-derived surfactant and its utilization as an excellent cathode material for intermediate temperature solid oxide fuel cell

In this paper, a uniform and defective LaSrCoO₄ was prepared by using amino acid derived surfactant (N-(2-hydroxylalkyl)-l-Phenylalanine). Owing to rich active groups of N-(2-hydroxylalkyl)-l-Phenylalanine, the resultant LaSrCoO₄ showed enhanced specific surface area and good electro-catalytic property. The porous K₂NiF₄-type LaSrCoO₄ was verified as a good cathode material for intermediate temperature solid oxide fuel cells (SOFCs). The LaSrCoO₄ electrode demonstrated good electrochemical performance, including good electrical conductivity (138.7 S cm⁻¹, in air at 800 °C), low polarization resistance R_p (0.108 Ω cm², symmetric cells in air at 800 °C) and high power densities (688 mW cm⁻² at 800 °C). The as-prepared LaSrCoO₄ shows a promising application as the cathode materials in the intermediate temperature SOFCs.     

An optimized synthesis route for high performance composite cathode based on a layered perovskite oxide of PrBa0.92Co2O6-δ with cationic deficiency

For Solid Oxide Fuel Cells (SOFCs) to become an economically attractive energy conversion technology, cathode materials with high catalytic activity over oxygen reduction reaction (ORR) and low cost are desired. In this work, a composite cathode material of PrBa_0.92Co₂O_6-δ-40 wt%Ce_0.8Sm_0.2O_1.9(OPCC) based on a layered perovskite oxide of PrBa_0.92Co₂O_6-δ with Ba²⁺-deficiency at A-sites has been successfully synthesized with a facile and effective one-pot sol-gel method, which was comparatively studied with the counterpart BMCC synthesized with the traditional ball-milling method and the single phase cathode of PrBa_0.92Co₂O_6-δ. Among the three cathodes, OPCC showed the lowest area specific resistances (ASRs) in both air and oxygen atmospheres, indicating the highest ORR catalytic activity. Such performance improvement for OPCC was closely related to its optimized microstructures obtained with the liquid-mixing one-pot synthesis method and existence of Ce_0.8Sm_0.2O_1.9 that has a high ionic conductivity. I-V and I-P curves were measured for the anode-supported single cells with the three cathodes, and the OPCC-based cell showed the highest peak power densities with typical value of 1011  mW cm⁻² at 750 °C in contrast to 783  mW cm⁻² for the BMCC-based cell and 574  mW cm⁻² for the PrBa_0.92Co₂O_6-δ-based cell respectively. The OPCC-based cell also showed a stable performance with no obvious degradation over 100 h at 700 °C.     

Multilayer tape casting of large-scale anode-supported thin-film electrolyte solid oxide fuel cells

Tape casting is conventionally used to prepare individual, relatively thick components (i.e., the anode or electrolyte supporting layer) for solid oxide fuel cells (SOFCs). In this research, a multilayer ceramic structure is prepared by sequentially tape casting ceramic slurries of different compositions onto a Mylar carrier followed by co-sintering at 1400 °C. The resulting half-cells contains a 300 μm thick NiO–yttria-stabilized zirconia (YSZ) anode support, a 20 μm NiO–YSZ anode functional layer, and an 8 μm YSZ electrolyte membrane. Complete SOFCs are obtained after applying a Gd_0.1Ce_0.9O₂ (GDC) barrier layer and a Sm_0.5Sr_0.5CoO₃ (SSC) -GDC cathode by using a wet-slurry spray method. The 50 mm × 50 mm SOFCs produce peak power densities of 337, 554, 772, and 923 mW/cm² at 600, 650, 700, and 750 °C, respectively, on hydrogen fuel. A short stack including four 100 mm × 150 mm cells is assembled and tested. Each stack repeat unit (one cell and one interconnect) generates around 28.5 W of electrical power at a 300 mA/cm² current density and 700 °C.     

Enhancing catalysis activity of La0.6Sr0.4Co0.8Fe0.2O3-δ cathode for solid oxide fuel cell by a facile and efficient impregnation process

The development of high performance electrocatalysts to promote oxygen reduction reaction (ORR) and to prolong the durability of cathodes of solid oxide fuel cell is essential at intermediate or low temperatures. Here, we report a facile and efficient spray impregnation strategy in enhancing catalytic activity of La_0.6Sr_0.4Co_0.8Fe_0.2O_3-δ (LSCF) due to the introduction of a multitude of homogeneous nanoparticles. With a highly active surface abundance in B-site cations, the modified LSCF cathode manifests an area-specific resistance (ASR) of ∼0.140 Ω cm², only a fifth of that for a pristine LSCF cathode (∼0.764 Ω cm²) at 600 °C, and anode-supported fuel cells with the decorated LSCF cathode show markedly improved peak power densities (∼0.94 W cm⁻² at 700 °C). Furthermore, the ORR kinetics of the modified LSCF cathode can be further enhanced by impregnating Ni(NO₃)₂·6H₂O and Co(NO₃)₂·6H₂O solution again. X-ray photoelectron spectroscopy analysis indicates that the homogeneous nanoparticles alter the distribution of Sr_surface and O_surface. It is found that ‘Co’ decoration can effectively alleviate the surface aggregation of Sr and ‘Co’ and ‘Ni’ decoration play a pivotal role in the reactivation of electrode surface.     

Investigation of structural,electrical and electrochemical properties of La0.6Sr0.4Fe0.8Mn0.2O3-δ as an intermediate temperature solid oxide fuel cell cathode

La_0.6Sr_0.4Fe_0.8Mn_0.2O₃ (LSFM) compound is synthesized by sol-gel method and evaluated as a cathode material for the intermediate temperature solid oxide fuel cell (IT-SOFC). X-ray diffraction (XRD) indicates that the LSFM has a rhombohedral structure with R-3c space group symmetry. The XRD patterns reveal very small amount of impurity phase in the LSFM and Y₂O₃-stabilized ZrO₂ (YSZ) mixture powders sintered at 600, 700, 800 and 850 °C for a week. The maximum electrical conductivity of LSFM is about 35.35 S cm⁻¹ at 783 °C in the air. The oxygen chemical diffusion coefficients, D_Chem, are increased from 1.39 × 10⁻⁶ up to 1.44 × 10⁻⁵ cm² s⁻¹. Besides, the oxygen surface exchange coefficients, k_Chem, are obtained to lie between 2.9 × 10⁻³ and 1.86 × 10⁻² cm s⁻¹ in a temperature range of 600–800 °C. The area-specific resistances (ASRs) of the LSFM symmetrical cell are 7.53, 1.53, 1.13, 0.46 and 0.31 Ω cm² at 600, 650, 700, 750 and 800 °C respectively, and related activation energy, E_a, is about 1.23 eV.     

Novel PrBa0.9Ca0.1Co2-xZnxO5+δ double-perovskite as an active cathode material for high-performance proton-conducting solid oxide fuel cells

Zn-doped PrBa_0.9Ca_0.1Co_2-xZn_xO_5+δ (PBCCZx) with x equaling 0, 0.05, 0.10, 0.15, and 0.2, designated as PBCCZ00, PBCCZ05, PBCCZ10, PBCCZ15, and PBCCZ20, respectively, are prepared and investigated in the aspects of crystal structure, thermal expansion behavior, defect chemistry, and electrochemical performance as the cathode of proton-conducting solid oxide fuel cells (H–SOFCs). With the increase of the content of doped Zn, the thermal expansion coefficient of PBCCZx remains unchanged essentially. But the bulk and surface concentrations of oxygen vacancy are increased, and the maximum power density (MPD) of the cells at 750 °C with PBCCZx-BaZr_0.1Ce_0.7Y_0.1Yb_0.1O_3-δ (BZCYYb) composite cathodes are improved from 335 (x = 0) to 876 (x = 0.15) mW cm⁻². With the PBCCZ15-BZCYYb cathode, the MPD of the cell increases with temperature from 299 mW cm⁻² at 600 °C to 876 mW cm⁻² at 750 °C with a reduction of cell polarization resistance from 0.68 to 0.04 Ω cm² accordingly.     

Optimisation of processing and microstructural parameters of LSM cathodes to improve the electrochemical performance of anode-supported SOFCs

To improve the electrochemical performance of LSM-based anode-supported single cells, a systematic approach was taken for optimising processing and materials parameters. Four parameters were investigated in more detail: (1) the LSM/YSZ mass ratio of the cathode functional layer, (2) the grain size of LSM powder for the cathode current collector layer, (3) the thickness of the cathode functional layer and the cathode current collector layer, and (4) the influence of calcination of YSZ powder used for the cathode functional layer.Results from electrochemical measurements performed between 700 and 900 °C with H₂ (3 vol.% H₂O) as fuel gas and air as the oxidant showed that the performance was the highest using an LSM/YSZ mass ratio of 50/50. A further increase of the electrochemical performance was obtained by increasing the grain size of the outer cathode current collector layer: the highest performance was achieved with non-ground LSM powder. In addition, it was found that the thickness of the cathode functional layer and cathode current collector layer also affects the electrochemical performance, whereas no obvious detrimental effects occurred with the different qualities of YSZ powder for the cathode functional layer. The highest performance, i.e. 1.50 ± 0.05 A cm⁻² at 800 °C and 700 mV, was obtained with a cathode functional layer, characterised by an LSM/YSZ mass ratio of 50/50, a d₉₀ of the LSM powder of 1.0 μm, non-calcined YSZ powder, and a thickness of about 30 μm, and a cathode current collector layer, characterised by d₉₀ of the LSM powder of 26.0 μm (non-ground), and a thickness of 50–60 μm. Also interesting to note is that the use of non-ground LSM for the cathode current collector layer and non-calcined YSZ powder for the cathode functional layer obviously simplifies the production route of this type of fuel cell.     

High-performance solid oxide fuel cell based on iron and tantalum co-doped BaCoO3-δ perovskite-type cathode and BaZr0.1Ce0.7Y0.1Yb0.1O3-δ electrolyte

Proton conducting solid oxide fuel cells (H–SOFCs) have attracted much interest for their various advantages. BaZr_0.1Ce_0.7Y_0.1Yb_0.1O_3-δ (BZCYYb) displays both good proton conductivity and stability among all of the barium zirconate-cerate oxides such as BaZr_0.1Ce_0.7Y_0.2O_3-δ (BZCY) with proton conducting. In this study, an Fe and Ta co-doped perovskite-type oxide with the composition of BaCo_0.7Fe_0.2Ta_0.1O_3-δ (BCFT) is synthesized by the solid state reaction (SSR) method and studied as a high-performance cathode for H–SOFCs operated between 650 and 800 °C. The BCFT is chemically compatible with BZCYYb electrolyte, even co-sintering at 1000 °C for 10 h. The activating energy (Ea) of electrical conductivity for the BCFT sample is only 0.038 eV. The BCFT demonstrates superior oxygen reduction reaction (ORR) kinetics than that of BaCo_0.7Fe_0.2Nb_0.1O_3-δ (BCFN), when the Nb is completely substituted by Ta. The Ea of the polarization resistance (Rp) for BCFT is 0.911 eV, which is 0.11 eV lower than that of BCFN. The peak power density (PPD) of the anode-supported H–SOFC using BCFT is 1.65 W cm⁻² and the Rp is 0.01 Ω cm² at 800 °C. Therefore, the BCFT is a promising cathode for H–SOFCs based on BZCYYb electrolyte.