Cathode reaction mechanism of porous-structured Sm0.5Sr0.5CoO3−δ and Sm0.5Sr0.5CoO3−δ/Sm0.2Ce0.8O1.9 for solid oxide fuel cells

The cathode reaction mechanism of porous Sm_0.5Sr_0.5CoO_3−δ, a mixed ionic and electronic conductor (MIEC), is studied through a comparison with the composite cathode Sm_0.5Sr_0.5CoO_3−δ/Sm_0.2Ce_0.8O_1.9. First, the cathodic behaviour of porous Sm_0.5Sr_0.5CoO_3−δ and Sm_0.5Sr_0.5CoO_3−δ/Sm_0.2Ce_0.8O_1.9 are observed for micro-structure and impedance spectra according to Sm_0.2Ce_0.8O_1.9 addition, thermal cycling and long-term properties. The cathode reaction mechanism is discussed in terms of frequency response, activation energy, reaction order and electrode resistance for different oxygen partial pressures p(O₂) at various temperatures. Three elementary steps are considered to be involved in the cathodic reaction: (i) oxygen ion transfer at the cathode-electrolyte interface; (ii) oxygen ion conduction in the bulk cathode; (iii) gas phase diffusion of oxygen. A reaction model based on the empirical equivalent circuit is introduced and analyzed using the impedance spectra. The electrode resistance at high frequency (R_c,HF) in the impedance spectra represents reaction steps (i), due to its fast reaction rate. The electrode resistance at high frequency is independent of p(O₂) at a constant temperature because the semicircle of R_c,HF in the complex plane of the impedance spectra is held constant for different values of p(O₂). Reaction steps (ii) and (iii) are the dominant processes for a MIEC cathode, according to the analysis results. The proposed cathode reaction model and results for a solid oxide fuel cell (SOFC) well describe a MIEC cathode with high ionic conductivity, and assist the understanding of the MIEC cathode reaction mechanism.     

Sm0.5Sr0.5CoO3–Ce1.8Sm0.2O1.9 electrodes enhanced by Sm0.5Sr0.5CoO3 impregnation for proton conductor based solid oxide fuel cells

Sm_0.5Sr_0.5CoO_3−δ–Ce_0.8Sm_0.2O_2−δ (SSC–SDC) composites, which are often used as the cathodes for solid oxide fuel cells (SOFCs) with oxygen-ion conducting electrolytes, have been recently shown to be also applicable in SOFCs based on proton conductors such as BaZr_0.1Ce_0.7Y_0.2O_3−δ (BZCY). The electrochemical performances of blank SSC–SDC electrodes on BZCY electrolytes are substantially improved in this work by impregnating SSC nanoparticles additionally. When the loading increases, the interfacial polarization resistance of the symmetric cell decreases gradually at first, notably when it exceeds 14 wt.%, and to the lowest value at about 22 wt.%. Furthermore, impregnating SSC reduces the low-frequency-arc resistance that corresponds to the surface exchange step. In addition, impregnating SSC reduces the activation energy for oxygen reduction from 1.14 to 0.70 eV, thus resulting in significantly improvement on electrode performance at the reduced temperatures for SOFCs based on proton conductors.     

Investigation of Sm0.5Sr0.5CoO3−δ/Co3O4 composite cathode for intermediate-temperature solid oxide fuel cells

The electrochemical properties of an Sm_0.5Sr_0.5CoO_3−δ/Co₃O₄ (SSC/Co₃O₄) composite cathode were investigated as a function of the cathode-firing temperature, SSC/Co₃O₄ composition, oxygen partial pressure and CO₂ treatment. The results showed that the composite cathodes had an optimal microstructure at a firing temperature of about 1100 °C, while the optimum Co₃O₄ content in the composite cathode was about 40 wt.%. A single cell with this optimized C₄₀-1100 cathode exhibited a very low polarization resistance of 0.058 Ω cm², and yielded a maximum power density of 1092 mW cm⁻² with humidified hydrogen fuel and air oxidant at 600 °C. The maximum power density reached 1452 mW cm⁻² when pure oxygen was used as the oxidant for a cell with a C₃₀-1100 cathode operating at 600 °C due to the enhanced open-circuit voltage and accelerated oxygen surface-exchange rate. X-ray diffraction and thermogravimetric analyses, as well as the electrochemical properties of a CO₂-treated cathode, also implied promising applications of such highly efficient SSC/Co₃O₄ composite cathodes in single-chamber fuel cells with direct hydrocarbon fuels operating at temperatures below 500 °C.     

Preparation and characterization of Ba0.5Sr0.5Fe0.9Ni0.1O3−δ–Sm0.2Ce0.8O1.9 compose cathode for proton-conducting solid oxide fuel cells

A cobalt-free Ba_0.5Sr_0.5Fe_0.9Ni_0.1O_3−δ–Sm_0.2Ce_0.8O_1.9 (BSFN–SDC) composite was employed as a cathode for proton-conducting solid oxide fuel cells (H-SOFCs) using BaZr_0.1Ce_0.7Y_0.2O_3−δ (BZCY) as the electrolyte. The chemical compatibility between BSFN and SDC was evaluated. The XRD results showed that BSFN was chemically compatible with SDC after co-fired at 1100 °C for 5 h. The thermal expansion coefficient (TEC) of BSFN–SDC, which showed a reasonably reduced value (16.08 × 10⁻⁶ K⁻¹), was effectively decreased due to Ce_0.8Sm_0.2O_1.9 (SDC) added. A single cell of Ni–BZCY/Ni–BZCY/BZCY/BSFN–SDC with a 25-μm-thick BZCY electrolyte membrane exhibited excellent power densities as high as 361.8 mW cm⁻² at 700 °C with a low polarization resistance of 0.174 Ω cm². The excellent performance implied that the cobalt-free BSFN–SDC composite was a promising alternative cathode for H-SOFCs.     

Characterization of nanosized Ce0.8Sm0.2O1.9-infiltrated Sm0.5Sr0.5Co0.8Cu0.2O3−δ cathodes for solid oxide fuel cells

This study investigates the microstructure and electrochemical properties of Sm_0.5Sr_0.5Co_0.8Cu_0.2O_3−δ (SSC-Cu) cathode infiltrated with Ce_0.8Sm_0.2O_1.9 (SDC). The newly formed nanosized electrolyte material on the cathode surface, leading the increase in electrochemical performances is mainly attributed to the creation of electrolyte/cathode phase boundaries, which considerably increases the electrochemical sites for oxygen reduction reaction. Based on the experiment results, the 0.4 M SDC infiltration reveals the lowest cathode polarization resistance (R_P), the cathode polarization resistances (R_p) are 0.117, 0.033, and 0.011 Ω cm² at 650, 750, and 850 °C, and the highest peak power density, are 439, 659, and 532 mW cm⁻² at 600, 700, and 800 °C, respectively. The cathode performance in SOFCs can be significantly improved by infiltrating nanoparticles of SDC into an SSC-Cu porous backbone. This study reveals that the infiltration approach may apply in SOFCs to improve their electrochemical properties.     

Properties and performance of Ba0.5Sr0.5Co0.8Fe0.2O3−δ + Sm0.2Ce0.8O1.9 composite cathode

The properties and performance of Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ (BSCF) + Sm_0.2Ce_0.8O_1.9 (SDC) (70:30 in weight ratio) composite cathode for intermediate-temperature solid-oxide fuel cells were investigated. Mechanical mixing of BSCF with SDC resulted in the adhesion of fine SDC particles to the surface of coarse BSCF grains. XRD, SEM-EDX and O₂-TPD results demonstrated that the phase reaction between BSCF and SDC was negligible, constricted only at the BSCF and SDC interface, and throughout the entire cathode with the formation of new (Ba,Sr,Sm,Ce)(Co,Fe)O_3−δ perovskite phase at a firing temperature of 900, 1000, and ≥ 1050 °C, respectively. The BSCF + SDC electrode sintered at 1000 °C showed an area specific resistance of ∼0.064 Ω cm² at 600 °C, which is a slight improvement over the BSCF (0.099 Ω cm²) owing to the enlarged cathode surface area contributed from the fine SDC particles. A peak power density of 1050 and ∼382 mW cm⁻² was reached at 600 and 500 °C, respectively, for a thin-film electrolyte cell with the BSCF + SDC cathode fired from 1000 °C.     

Investigation of cobalt-free cathode material Sm0.5Sr0.5Fe0.8Cu0.2O3−δ for intermediate temperature solid oxide fuel cell

A cobalt-free cubic perovskite oxide Sm_0.5Sr_0.5Fe_0.8Cu_0.2O_3−δ (SSFCu) was investigated as a novel cathode for intermediate temperature solid oxide fuel cells (IT-SOFCs). The thermal expansion coefficient (TEC) of SSFCu was close to that of Sm_0.2Ce_0.8O_1.9(SDC) electrolyte and the electrical conductivity of SSFCu sample reached 72–82 S cm⁻¹ in the commonly operated temperatures of IT-SOFCs (400–600 °C). Symmetrical electrochemical cell with the configuration of SSFCu/SDC/SSFCu was applied for the impedance study and area specific resistance (ASR) of SSFCu cathode material was as low as 0.085 Ω cm² at 700 °C. Laboratory-sized tri-layer cells of NiO-SDC/SDC/SSFCu were operated from 450 to 700 °C with humidified hydrogen (∼3% H₂O) as fuel and the static air as oxidant. A maximum power density of 808 mW cm² was obtained at 700 °C for the single cell.     

Stable and easily sintered (Pr0.5Nd0.5)0.7Ca0.3CrO3−δ/Sm0.2Ce0.8O1.9 composite interconnect materials for IT-solid oxide fuel cells

(Pr_0.5Nd_0.5)_0.7Ca_0.3CrO_3−δ (PNCC) interconnect materials are stable in SOFC operating conditions. To improve the sintering ability and electrical conductivity of PNCC, a composite of PNCC/SDC interconnect materials was synthesized by a sol-gel auto-ignition process. The XRD results indicate that PNCC is chemically stable against reaction with SDC at high temperatures. Additionally, SDC-doped PNCC has enhanced sintering activity and electrical conductivity compared to PNCC alone. For 5 wt.% SDC, the relative density of the sample was 95.8% after sintering at 1400 °C, and the electrical conductivity of the sample reached 59.6 S cm⁻¹ in air and 4.48 S cm⁻¹ in H₂ at 650 °C. Both of these values are higher than those of pure PNCC. The average thermal expansion coefficient of the samples is close to that of other components of SOFCs. Our results indicate that PNCC/SDC composites are promising intermediate materials for IT-SOFCs.     

Hydrogen production aided by new (1−x)SrTi0.5Fe0.5O3–δ–xCe0.8(Sm0.8Sr0.2)0.2O2–δ (MIEC) composite membranes

In the present work, composite materials of the type (1–x)SrTi_0.5Fe_0.5O_3–δ–xCe_0.8(Sm_0.8Sr_0.2)_0.2O_2–δ (with х = 0, 0.25, 0.5, 0.75 and 1) are obtained by the two step solid state technique. Their transport properties are investigated in terms of their usage as mixed ionic-electronic conducting (MIEC) membrane materials for hydrogen production. It is found that, in reducing conditions the composites are characterized by mixed conductivity, which level is controlled by the electrical properties of the prevailing phase. Moreover, at 900 °C and pO₂ = 10⁻¹⁸ atm, total conductivity, ambipolar conductivity and oxygen permeability of composites dramatically grow (each of about 500%), when the fluorite component content x increases from 0 to 1. High-conducting and strengthened material 0.5SrTi_0.5Fe_0.5O_3–δ–0.5Ce_0.8(Sm_0.8Sr_0.2)_0.2O_2–δ is chosen for making tube shaped membranes using the tape rolling method, which are successfully applied for hydrogen production in laboratory scale. The hydrogen flux reached 0.176 ml cm⁻² min⁻¹ for x = 1, T = 900 °C and emf = 10 mV.     

Nanostructured palladium–La0.75Sr0.25Cr0.5Mn0.5O3/Y2O3–ZrO2 composite anodes for direct methane and ethanol solid oxide fuel cells

A palladium-impregnated La_0.75Sr_0.25Cr_0.5Mn_0.5O_3−δ/yttria-stabilized zirconia (LSCM/YSZ) composite anode is investigated for the direct utilization of methane and ethanol fuels in solid oxide fuel cells (SOFCs). Impregnation of Pd nanoparticles significantly enhances the electrocatalytic activity of LSCM/YSZ composite anodes for the methane and ethanol electrooxidation reaction. At 800 °C, the maximum power density is increased by two and eight times with methane and ethanol fuels, respectively, for a cell with the Pd-impregnated LSCM/YSZ composite anode, as compared with that using a pure LSCM/YSZ anode. No carbon deposition is observed during the reaction of methane and ethanol fuels on the Pd-impregnated LSCM/YSZ composite anode. The results show the promises of nanostructured Pd-impregnated LSCM/YSZ composites as effective anodes for direct methane and ethanol SOFCs.     

A cobalt-free Sm0.5Sr0.5FeO3−δ–BaZr0.1Ce0.7Y0.2O3−δ composite cathode for proton-conducting solid oxide fuel cells

A cobalt-free Sm_0.5Sr_0.5FeO_3−δ–BaZr_0.1Ce_0.7Y_0.2O_3−δ (SSF–BZCY) was developed as a composite cathode material for proton-conducting solid oxide fuel cells (H-SOFC) based on proton-conducting electrolyte of stable BZCY. The button cells of Ni-BZCY/BZCY/SSF–BZCY were fabricated and tested from 550 to 700 °C with humidified H₂ (～3% H₂O) as a fuel and ambient oxygen as oxidant. An open-circuit potential of 1.024 V, maximum power density of 341 mW cm⁻², and a low electrode polarization resistance of 0.1 Ω cm² were achieved at 700 °C. The experimental results indicated that the SSF–BZCY composite cathode is a good candidate for cathode material.     

Characterization of La0.5Sr0.5Co0.5Ti0.5O3−δ as symmetrical electrode material for intermediate-temperature solid-oxide fuel cells

La_0.5Sr_0.5Co_0.5Ti_0.5O_3−δ perovskite oxide has been prepared as polycrystalline powder, characterized and tested as cathode and anode material for solid-oxide fuel cells. The oxidized material is suggested to present mixed ionic-electronic conductivity (MIEC) from “in-situ” neutron powder diffraction (NPD) experiments, complemented with transport measurements; the presence of a sufficiently high oxygen deficiency, with large displacement factors for oxygen atoms suggest a large lability and mobility combined with a semiconductor-like behaviour with a maximum conductivity of 29 S cm⁻¹ at T = 850 °C. A complete reversibility towards reduction–oxidation processes has been observed, where the reduced Pm-3m perovskite with La_0.5Sr_0.5Co_0.5Ti_0.5O_2.64 composition has been obtained by topotactical oxygen removal without abrupt changes in the thermal expansion. The oxidized material shows good performance working as a cathode with LSGM electrolyte, yielding output power densities close to 500 mW/cm² at 850 °C. At intermediate temperatures (800 °C) it may be used as a cathode or as an anode, yielding power densities of 220 and 170 mW/cm², respectively. When used simultaneously as cathode and anode a maximum power density of 110 mW/cm² was obtained. Therefore, we propose the La_0.5Sr_0.5Co_0.5Ti_0.5O_3−δ composition as a promising candidate for symmetrical electrode in intermediate-temperature SOFC.     

Investigation of electrospun Ba0.5Sr0.5Co0.8Fe0.2O3−δ-Gd0.1Ce0.9O1.95 cathodes for enhanced interfacial adhesion

Fibrous Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3?δ-Gd_0.1Ce_0.9O_1.95 (BSCF-GDC) composite cathodes are fabricated by a facile electrospinning method. However, the electropun BSCF-GDC cathode shows poor adhesion to a GDC electrolyte because of the high shrinkage rate of the electrospun BSCF-GDC cathode during sintering. To solve this adhesion issue, mixed BSCF fiber-GDC powder cathode is investigated. As a result, mixed BSCF fiber-GDC powder cathode with an enhanced adhesion is successfully fabricated. This improvement can be attributed to the modified microstructure with the GDC powder that joins the BSCF fibers to the GDC electrolyte at the cathode and electrolyte interface. The polarization resistance of the mixed BSCF fiber-GDC powder cathode is 0.10 Ω cm², which is lower than 0.13 Ω cm² of conventional BSCF-GDC powder cathode at 700 °C. It is attributable to the improved oxygen gas and lattice oxygen diffusion, and the surface exchange of the mixed BSCF fiber-GDC powder cathode. The single cell with a mixed BSCF fiber-GDC powder cathode show 500 mW cm^?2 at 700 °C, which is 25% higher than conventional BSCF-GDC powder cathode.     

The synthesis and characterization of the Sm0.5Sr0.5Co1−xCuxO3−δ cathode by the glycine-nitrate process

In this study, a new oxygen-deficient cathode material, Sm_0.5Sr_0.5Co_1−xCu_xO_3−δ (SSCCu) was developed. It is expected to enhance the efficiency of intermediate-temperature solid oxide fuel cells (IT-SOFCs). The structure, conductivity and electrochemical performance of SSCCu were examined as a function of copper content. The structure of Sm_0.5Sr_0.5Co_0.9Cu_0.1O_3−δ and Sm_0.5Sr_0.5Co_0.8Cu_0.2O_3−δ samples was a single orthorhombic perovskite phase. Second phase SrCoO_2.8, however, formed in the Sm_0.5Sr_0.5Co_0.7Cu_0.3O_3−δ and Sm_0.5Sr_0.5Co_0.6Cu_0.4O_3−δ samples. The conductivity of the Sm_0.5Sr_0.5Co_0.7Cu_0.3O_3−δ cathode was higher than that of other samples. However, the Sm_0.5Sr_0.5Co_0.8Cu_0.2O_3−δ electrode exhibited the lowest overpotential of 25 mV at 400 mA cm⁻² and the lowest area special resistance of 0.2 Ω cm² at 700 °C.     

High performance Ni–Sm2O3 cermet anodes for intermediate-temperature solid oxide fuel cells

A novel anode consisting of Ni and Sm₂O₃ with negligible oxygen-ion conductivity was developed for intermediate-temperature solid oxide fuel cells (SOFCs). Its triple phase boundary length is pretty small compared with the conventional Ni-samaria doped ceria (SDC) anode, of which SDC is one of the electrolytes having high oxygen-ion conductivity. Even so, single cells with Ni–Sm₂O₃ anodes generated peak power density of 542 mW cm⁻² at 600 °C, comparable to, if not higher than those with the Ni–SDC anodes when the same cathodes and electrolytes were applied. In addition, Ni–Sm₂O₃ exhibited lower interfacial polarization resistance than Ni–SDC. The high electrochemical performance, which might be related to the high catalytic activity of Sm₂O₃ and the unique microstructures of the Ni–Sm₂O₃, suggests a viable alternative to the conventional anodes for SOFCs.     

High performance proton-conducting solid oxide fuel cells with a stable Sm0.5Sr0.5Co3−δ-Ce0.8Sm0.2O2−δ composite cathode

A Sm_0.5Sr_0.5CoO_3−δ-Ce_0.8Sm_0.2O_2−δ (SSC-SDC) composite is employed as a cathode for proton-conducting solid oxide fuel cells (H-SOFCs). BaZr_0.1Ce_0.7Y_0.2O_3−δ (BZCY) is used as the electrolyte, and the system exhibits a relatively high performance. An extremely low electrode polarization resistance of 0.066 Ω cm² is achieved at 700 °C. The maximum power densities are: 665, 504, 344, 214, and 118 mW cm⁻² at 700, 650, 600, 550, and 500 °C, respectively. Moreover, the SSC-SDC cathode shows an essentially stable performance for 25 h at 600 °C with a constant output voltage of 0.5 V. This excellent performance implies that SSC-SDC, which is a typical cathode material for SOFCs based on oxide ionic conductor, is also a promising alternative cathode for H-SOFCs.     

Intermediate-to-low temperature protonic ceramic membrane fuel cells with Ba0.5Sr0.5Co0.8Fe0.2O3-δ–BaZr0.1Ce0.7Y0.2O3-δ composite cathode

The perovskite-type Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3-δ–BaZr_0.1Ce_0.7Y_0.2O_3-δ (BSCF–BZCY) composite oxides were synthesized by a modified Pechini method and examined as a novel composite cathode for intermediate-to-low temperature protonic ceramic membrane fuel cells (ILT-PCMFCs). Thin proton-conducting BaZr_0.1Ce_0.7Y_0.2O_3-δ (BZCY) electrolyte and NiO–BaZr_0.1Ce_0.7Y_0.2O_3-δ (NiO–BZCY) anode functional layer were prepared over porous anode substrates composed of NiO–BaZr_0.1Ce_0.7Y_0.2O_3-δ by a one-step dry-pressing/co-firing process. A laboratory-sized quad-layer cell of NiO–BZCY/NiO–BZCY(∼50 μm)/BZCY(∼20 μm)/BSCF–BZCY(∼50 μm) was operated from 550 to 700 °C with humidified hydrogen (∼3% H₂O) as fuel and the static air as oxidant. A high open-circuit potential of 1.009 V, a maximum power density of 418 mW cm⁻², and a low polarization resistance of the electrodes of 0.10 Ω cm² was achieved at 700 °C. These investigations have indicated that proton-conducting BZCY electrolyte with BSCF perovskite cathode is a promising material system for the next generation solid oxide fuel cells (SOFCs).     

Nd0.5Sr0.5Fe0.8Cu0.2O3−δ–xSm0.2Ce0.8O1.9 cobalt-free composite cathodes for intermediate temperature solid oxide fuel cells

Cobalt-free composites Nd_0.5Sr_0.5Fe_0.8Cu_0.2O_3−δ (NSFCu)–xSm_0.2Ce_0.8O_1.9 (SDC) (x = 0–60 wt%) are investigated as IT-SOFC cathodes. The characteristic properties of cobalt-free composite cathodes comparing to cobalt-based composites are revealed. The DC conductivity and thermal expansion coefficient of the composite cathodes decrease with the content of SDC x, while the polarization resistance R_p shows the least value with addition of 40 wt% of SDC. The power density of the single cell with NSFCu-40% SDC composite cathode improved significantly compared with that of undoped NSFCu cathode, with peak values of 488, 623, 849 and 1052 mW cm⁻² at 600, 650, 700, and 750 °C, respectively. Moreover, the performance of the composite cathode is stable within testing period of 370 h at 700 °C, indicating that the NSFCu-40% SDC is an excellent cobalt-free composite cathode applied in IT-SOFC.     

Fabrication and characterization of Sm0.2Ce0.8O2−δ-Sm0.5Sr0.5CoO3−δ composite cathode for anode supported solid oxide fuel cell

The Sm_0.5Sr_0.5CoO_3−δ (SSC) with perovskite structure is synthesized by the glycine nitrate process (GNP). The phase evolution of SSC powder with different calcination temperatures is investigated by X-ray diffraction and thermogravimetric analyses. The XRD results show that the single perovskite phase of the SSC is completely formed above 1100 °C. The anode-supported single cell is constructed with a porous Ni-yttria-stabilized zirconia (YSZ) anode substrate, an airtight YSZ electrolyte, a Sm_0.2Ce_0.8O_2−δ (SDC) barrier layer, and a screen-printed SSC-SDC composite cathode. The SEM results show that the dense YSZ electrolyte layer exhibits the good interfacial contact with both the Ni-YSZ and the SDC barrier layer. The porous SSC-SDC cathode shows an excellent adhesion with the SDC barrier layer. For the performance test, the maximum power densities are 464, 351 and 243 mW cm⁻² at 800, 750 and 700 °C, respectively. According to the results of the electrochemical impedance spectroscopy (EIS), the charge-transfer resistances of the electrodes are 0.49 and 1.24 Ω cm², and the non charge-transfer resistances are 0.48 and 0.51 Ω cm² at 800 and 700 °C, respectively. The cathode material of SSC is compatible with the YSZ electrolyte via a delicate scheme employed in the fabrication process of unit cell.