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La-doped ceria (LDC)/lanthanum silicate (LS) and Ga-doped ceria (GDC)/LS composites were prepared, and the influence of cation interdiffusion during co-firing on the electrical properties of composites was investigated. The electrical conductivity of GDC/LS was lower than that of LDC/LS despite the higher conductivity of GDC compared to LDC. The crystal phases of LDC/LS and GDC/LS were detected as fluorite-type cubic ceria (Fm 3 ‾ m) and a hexagonal lanthanum silicate (P63/m), respectively, by X-ray diffraction (XRD). From energy dispersive X-ray spectroscopy (EDS) measurements, it was found that the compositions of the ceria and silicate phases in LDC/LS were nearly stoichiometric after co-firing. In contrast, interdiffusion between Gd3+ in the ceria phase and La3+ in the silicate phase was recognized for the GDC/LS. The compositions of the ceria and silicate phases in GDC/LS after co-firing were estimated as Gd0.035La0.065Ce0.9O1.95 and La7.910Gd0.423(SiO4)6O0.5, respectively. The Gd3+ substitution for La3+ in the silicate phase in GDC/LS decreased the amounts of 4f site cations. Then, the lattice parameter of the silicate phase decreased. As a result, the decrease in electrical conductivity of GDC/LS is attributed to the decrease in the oxide anion at the 2a site, which is a conduction carrier. It was found from Raman spectroscopy that cation interdiffusion decreased the symmetry mode of the SiO4 tetrahedra for the silicate phase in GDC/LS and brought about increases in the activation energy of electrical conductivity for GDC/LS.

This paper presents high-performance Ni-YSZ tubular catalysts for CO2 methanation prepared by the extrusion molding. We fabricated tubular-shaped Ni-YSZ catalysts with various Ni contents (25–100 wt% NiO) and investigated the effect of Ni content on CO2 methanation performance under various temperatures and gas flow rates. Catalysts with Ni contents >75 wt% showed CH4 yields >91% above 270 °C with high CH4 selectivities (>99%). High CH4 yields were also observed under high GHSVs at 300 °C: 93% and 92% at 8700 and 17,500 h−1, respectively. Investigation of methanation with the catalysts revealed that CO2 methanation was accelerated by a localized hotspot at the reactor inlet arising from the interaction between reaction kinetics and heat generation. Using a numerical simulation, we considered the optimum arrangement of catalytic activity in the reactor to avoid hotspot generation and realize a stable high CO2 methanation performance. We can simultaneously achieve high CH4 production and prevent hotspot formation by properly arranging catalysts with different activities.

Perovskite oxides ABO3 (A = Sr, Ba, B = Ti, Zr, Hf) were prepared at around room temperature using the reaction between alkaline earth hydroxides and metal oxide hydrous gels. The products of the reaction crystallized without any calcination. High-symmetry perovskite oxides, such as cubic perovskite, were obtained at lower temperatures. The Madelung energy of each perovskite oxide was calculated from the crystal structure and the lattice parameter obtained by XRD measurement. The calculated Madelung energy change in the synthesis reaction showed a linear relationship with the synthesis temperature. A rise in the synthesis temperature increased the change of the Madelung energy. The results thus confirmed that the synthesis temperature depended on the symmetry of the perovskite oxide in this study. Drawing from these results, we concluded that the synthesis temperature could be estimated by considering the crystal structure and Madelung energy change in the synthesis reaction.

Gadolinia-doped ceria (GDC) and lanthanum silicate (LS) are expected to be promising materials for electrolytes of solid oxide fuel cells (SOFCs) because of their high ionic conductivities at intermediate temperatures. However, performance degradation of SOFCs is caused by current leakage through GDC and poor densification of LS. In the present study, LS was used as a blocking layer for preventing the current leakage of GDC electrolyte. Thermal shrinkage measurements and scanning electron microscopy (SEM) observation suggested that the addition of Bi2O3 in LS electrolyte (LSB) contributed to the decrease in the sintering temperature of the LS and improved densification of the GDC/LS bi-layer electrolyte. Consequently, the open-circuit voltage for the cell with GDC/LS and GDC/LSB bi-layer electrolytes increased effectively in comparison with that of the cell with GDC single-layer electrolyte. The electrical conductivity and fuel cell characteristics were compared among the cells with GDC, GDC/LS, and GDC/LSB electrolytes.

The present study investigates protonic ceramic fuel cells (PCFCs) with composite cathodes consisting of an electronic conductive material and a protonic conductive material. PCFCs have been expected as a future high-efficiency and high-performance power generation device. Electronic conductive perovskite-type oxides such as (La,Sr)(Co,Fe)O3, (La,Sr)CoO3, and (La,Ba)CoO3 (LBC), which are commonly used for the cathode material of conventional solid oxide fuel cells with oxide ion-conducting electrolytes, are also used for that of PCFCs. However, these perovskite-type oxides have almost no protonic conductivity, resulting in poor catalytic activity for cathode electrochemical reaction in PCFCs. Thus, to enhance cathode performance, composite cathodes with a proton-conducting oxide BaZr0.1Ce0.7Y0.1Yb0.1O3–δ (BZCYYb) are applied to anode-supported PCFCs. Results show that the composite cathodes effectively enhance the cell performance, and the 50 wt% BZCYYb composite LBC-based cathode exhibits the highest performance among the evaluated composite cathodes.

Although protonic ceramic fuel cells (PCFCs) have a great potential to realize higher energy-conversion efficiency compared with all other conventional devices, actual PCFCs have shown current leakage loss due to their intrinsic transport property with electron-hole conduction of proton-conducting electrolytes, resulting in a decrease in efficiency. The present study shows a new finding that open-circuit voltage (OCV), which is an indicator of current leakage loss, depends on the ratio of the electrode polarization resistance to the electrolyte ohmic resistance within a PCFC and experimentally demonstrates improvement in OCV by developing high-performance cathode and anode. Results show that our anode-supported PCFC with BaZr0.1Ce0.7Y0.1Yb0.1O3−δ electrolyte and high-performance electrodes achieves extremely high OCV, e.g., 1.112 V at 600 °C (≈ 98% of the theoretical OCV) and 1.132 V at 550 °C (≈ 99% of the theoretical OCV). Compared with recently reported OCVs for PCFCs, these measured OCVs are remarkabl...

Diffusion behavior of Ni during high temperature co-sintering was quantitatively investigated for anode-supported solid oxide fuel cells (SOFCs) that had BaZr 0.1 Ce 0.7 Y 0.1 Yb 0.1 O 3− δ (BZCYYb) proton-conducting electrolyte and NiO-BZCYYb anode. Although diffused Ni in such SOFCs effectively acts as a sintering aid to densify the BZCYYb electrolyte layer, it often negatively affects the electrolyte conductivity. In the present study, field emission electron probe microanalysis (with wavelength dispersive X-ray spectroscopy) clearly revealed that Ni diffused into the BZCYYb electrolyte layer, and that the amount of diffused Ni increased with increasing co-sintering temperature. In particular, relatively high Ni concentration within the electrolyte layer was observed near the electrolyte/anode interface, e.g., approximately 1.5 and 2.8 wt% at co-sintering temperature of 1300 and 1400 °C, respectively. Electrochemical measurements showed that, compared with the lower co-sintering temperatures (1300–1350 °C), the highest co-sintering temperature (1400 °C) led to the highest ohmic resistance because of lower electrolyte conductivity. These results suggest that high co-sintering temperature causes excessive Ni diffusion into the BZCYYb electrolyte layer, thus degrading the intrinsic electrolyte conductivity and consequently degrading the SOFC performance.

Internal partial oxidation reforming of butane and steam reforming of ethanol were investigated using microtubular solid oxide fuel cells (SOFCs) supported on nickel-gadolinia doped ceria (Ni-GDC) anodes for portable power sources in emergency situations and for mobilities, such as vehicles, robots and drones. At an oxygen/carbon (O/C) ratio of 1.0, which is a coking condition in the equilibrium, the Ni-GDC anode deteriorated for 28 h by internal partial oxidation of butane at 650 °C. However, power generation was also impossible after 8 h and 79 h at steam/carbon (S/C) = 1.0 and 1.5, respectively, by internal steam reforming of ethanol despite of no carbon deposition condition in the equilibrium at 650 °C. Power can be generated for more than 100 h at O/C = 1.5 in butane and at S/C = 2.0 in ethanol. For internal partial oxidation reforming of methane and steam reforming of ethanol in SOFCs, the O/C and S/C ratios are significantly important to prevent carbon deposition on the Ni-GDC anode.

A multi-layered electrolyte structure is proposed for proton-conducting solid oxide fuel cells (SOFCs) to achieve higher power density and higher open-circuit voltage (OCV). Although recent proton-conducting SOFCs have demonstrated high power density, their OCVs have been lower than that of conventional SOFCs with stabilized zirconia because proton-conducting oxides intrinsically have electron-hole conduction. The proposed electrolyte structure has a porous BaZr0.1Ce0.7Y0.1Yb0.1O3–δ (BZCYYb) layer deposited on a dense BZCYYb layer. This structure effectively improves both cathode polarization and ionic transport property, resulting in higher power density with higher OCV. Also, discussion based on an equivalent circuit model of proton-conducting SOFCs clearly reveals a mechanism that determines OCV, namely, higher ohmic resistance and lower electrode polarization resistance lead to higher OCV. Our results suggest that higher electrode performance is essential for proton-conducting SOFCs to achieve higher OCV, particularly in the case of anode-supported configurations with thin electrolyte.

The electrical conductivity of nickel (2 wt %)-doped BaZr0.1Ce0.7Y0.1Yb0.1O3−δ (BZCYYb) acceptor-doped perovskite oxide was evaluated under air and a 1% H2 atmosphere. The partial conductivity was calculated from the total conductivity and the transport number of each carrier (tH+, tO2–, and th+) obtained using the concentration cell method. Its correlation with the dissolution state of the protons in the oxide as studied by in situ diffuse reflection Fourier transform infrared spectroscopy is discussed. When the concentration of protons that dissolved in BZCYYb-Ni was high, the proton partial conductivity was also high. An increase in hole conductivity in the high-temperature region in an air atmosphere was observed, suggesting that dissociation of protons strongly correlates with such a dominant carrier change. The dissociation of protons should be determined by the stability of protons in the oxide by the interaction with the lattice oxygen, and it was suggested that the dissolution state of protons ca...

The dissociation behavior of protons incorporated in yttrium-doped barium zirconate (BZY20) was investigated via combination of in-situ diffuse reflectance Fourier transform infrared spectroscopy (FT-IR) and evolved gas analysis. The vicinities of dopant or host ions were considered as sites of proton dissolution. The temperature dependence of the dissociation of protons from each site was evaluated. Protons which were considered to have dissolved in the vicinity of dopant ions were easily removed and dissociated as water. On the other hand, those which dissolved in the vicinity of host ions were dissociated as hydrogen at high temperatures. Such dissociation behavior was also detected after using D2O saturated gas for pre-treatment. These results suggest that some protons which dissolved in the perovskite oxide could dissociate as hydrogen without forming oxygen vacancies. (131words)

A solid oxide fuel cell (SOFC) for high power density operation was developed with a microstructure-controlled cathode using a nano-composite powder of Sr-doped LaMnO 3 (LSM) and Y 2 O 3 -stabilized ZrO 2 (YSZ) synthesized by spray pyrolysis. The individual LSM-YSZ nano-composite particles, formed by crystalline and amorphous nano-size LSM and YSZ particles, showed spherical morphology with uniform particle size. The use of this powder for cathode material led to an extremely fine microstructure, in which all the LSM and YSZ grains (approximately 100–200 nm) were highly dispersed and formed their own network structures. This microstructure was due to the two phase electrode structure control using the powder, namely, nano-order level in each particle and micro-order level between particles. An anode-supported SOFC with the LSM-YSZ cathode using humidified H 2 as fuel and ambient air as oxidant exhibited high power densities, such as 1.29 W cm −2 under a voltage of 0.75 V and a maximum power density of 2.65 W cm −2 at 800 °C. Also, the SOFC could be stably operated for 250 h with no degradation, even at a high temperature of 800 °C.

Solid oxide electrolysis cells can theoretically achieve high energy-conversion efficiency, but current density must be further increased to improve the hydrogen production rate, which is essential to realize widespread application. Here, we report a structure technology for solid oxide electrolysis cells to achieve a current density higher than 3 A cm−2, which exceeds that of state-of-the-art electrolyzers. Bimodal-structured nanocomposite oxygen electrodes are developed where nanometer-scale Sm0.5Sr0.5CoO3−δ and Ce0.8Sm0.2O1.9 are highly dispersed and where submicrometer-scale particles form conductive networks with broad pore channels. Such structure is realized by fabricating the electrode structure from the raw powder material stage using spray pyrolysis. The solid oxide electrolysis cells with the nanocomposite electrodes exhibit high current density in steam electrolysis operation (e.g., at 1.3 V), reaching 3.13 A cm−2 at 750 °C and 4.08 A cm−2 at 800 °C, corresponding to a hydrogen production rate of 1.31 and 1.71 L h−1 cm−2 respectively., High-temperature solid oxide electrolysis cells are a promising technology for energy conversion, but higher current density is needed to increase efficiency. Here the authors design nanocomposite electrodes to improve electronic and ionic conductivity to achieve a high current density.

High power density operation is essential for cost reduction in solid oxide fuel cells (SOFCs), and thus the present work proposes a highly active Gd0.5Sr0.5CoO3−δ (GSC) and Ce0.8Gd0.2O1.9 (GDC) nanocomposite cathode for intermediate-temperature SOFCs. Its nanocomposite structure is formed by using GSC-GDC nanocomposite particles synthesized via spray pyrolysis, which produces uniform-sized secondary particles consisting of nano-sized GSC and GDC crystallites. The resulting cathode has an extensive electrochemically active triple-phase boundary and has electrically conductive networks formed with highly dispersed GSC and GDC crystallites. In addition, the GSC-GDC nanocomposite cathode exhibits a performance comparable to a high-performance Sm0.5Sr0.5CoO3−δ and Ce0.8Sm0.2O1.9 nanocomposite cathode. An anode-supported SOFC with the GSC-GDC nanocomposite cathode achieves remarkably high power densities at 0.75 V, i.e., 3.66, 2.60, 1.66, and 0.98 W cm−2 at 800, 750, 700, and 650 °C, respectively. These results suggest the proposed nanocomposite cathode yields a synergistic effect of a high catalytic activity of GSC-GDC and a fine nanocomposite structure, leading to high power density operation of an SOFC.

Nano-composite powders consisting of Sr-doped SmCoO 3 (SSC) and Sm-doped CeO 2 (SDC) were synthesized by spray pyrolysis using different cation concentration solutions, and the effect of this cation concentration on the electrochemical performance of electrodes fabricated using the powders was investigated. Regardless of the cation concentration, spherical particles consisting of nano-order fine SSC and SDC crystallites were successfully obtained. Although the crystal size of all the synthesized powders was almost the same, the particle size increased with increasing cation concentration. The microstructure of the cathodes fabricated using the nano-composite powders clearly depended on the particle size of the starting powders; the microstructure became coarser with increasing particle size and the original particle shapes partially remained in the cathode network. Also, electrochemical measurements for solid oxide fuel cells with these cathodes revealed that the electrode performance was related to the microstructure. The electrode polarization resistance decreased with decreasing particle size, and a relatively fine microstructure cathode exhibited lower electrode polarization resistance, resulting in a high performance of 1.52 W cm −2 at 700 °C. These results indicate that controlling the particle size, as well as controlling the nanostructure within the particles, is crucial to attain high performance electrodes.

This study shows the fabrication process of an anode-supported proton-conductive cell, then the electrochemical performances were investigated under SOFC mode in a wide temperature range from 222 to 486 °C. Ba(Ce0.7Zr0.1Y0.1Yb0.1)O3–δ (BCZYYb) was selected as an electrolyte material, and Ni–BCZYYb cermet and (La,Sr)(Co,Fe)O3–BCZYYb composite were selected as a fuel and an air electrodes materials, respectively. The cell showed open circuit voltages over 1.1 V and worked as a SOFC within the whole temperature range.

The nickel–zirconia cermet is widely used as an anode of solid oxide fuel cells (SOFCs). On the other hand, the nickel–ceria based anode indicates high electrochemical activity for hydrogen oxidation and hydrocarbon reforming. In this study, electrochemical and microstructural properties of microtubular SOFCs with Ni-based composite anodes containing yttria-stabilized zirconia (YSZ) and gadolinia-doped ceria (GDC) are investigated electrochemically using impedance spectroscopy (EIS) and microstructurally using focused ion beam-scanning electron microscopy (FIB-SEM). The solid solution of YSZ and GDC was easily formed after mechanical mixing and sintering at 1400 °C. The electrical conductivity and mechanical strength for the Ni–YSZGDC composite anodes are low relative to Ni–YSZ due to poor sinterability. The GDC-containing anodes show improved electrochemical activity for hydrogen oxidation, despite having lower three-phase boundary densities. Distribution of relaxation times (DRT) analysis of the EIS data shows that the concentration polarization is lower for the Ni–GDC anode, due to a higher measured pore volume. The maximum power density for the cell with the Ni–YSZGDC composite anode was higher than those with the Ni–YSZ and Ni–GDC anodes.

Barium zirconate have high chemical stability with CO2 comparison with barium cerate. Acceptor doped barium zirconate show high proton conductivity [1]. The proton conductivity of yttrium doped barium zirconate is 1×10-2 Scm-1 at 600 ℃ and it is attracted as the electrolyte of fuel cell and water vapor electrolysis cell. The fuel cell using proton conducting oxide was called to PCFC (proton ceramic fuel cell) and work at the intermediate temperature. PCFC is expected to be the energy conversion device in future because of high efficiency [2] and low production cost [3]. Generally, nickel is used as anode of PCFC. The anode overpotential of nickel is lower than that of the other metals. However, the nickels dissolve to the proton conducting oxide and possibly increase the ohmic resistance of electrolyte. Moreover, the nickel and the electrolyte possibly form the blocking layer such as the complex oxide. Although the proton conductivity of yttrium doped barium zirconate is highest in the zirconate-type proton conductor, BaY2NiO5 is formed between the nickel and the yttrium doped barium zirconate [4] and it might work as the blocking layer of proton. In this study, the reaction product between NiO and BaZr0.8 M 0.2O3-δ (M=Sc, In, Yb, Y, Gd) were confirmed by X-ray diffraction analysis. In order to clarify the NiO dissolution effect on the proton transport properties of BaZr0.8 M 0.2O3-δ (M=Sc, In, Lu, Yb, Y), the partial conductivity of the proton and the hole for BaZr0.8 M 0.2O3-δ (M=Sc, In, Lu, Yb, Y) with NiO were measured at temperature range of 200-600 ℃ by impedance analysis. The current efficiency on the polarization properties, then was examined for the fuel cell system using Acceptor-doped barium zirconate with NiO as electrolyte. Moreover, we investigated the effect of dopants on the ease of cell fabrication. The conventional ceramic tape-casting and firing process for fabricating laminated ceramic electronic devices was used, which is important for practical use. The power generation performance of planer cells fabricated by this process was also measured. The X-ray diffraction patterns of 0.4 mol% NiO doped BaZr0.8 M 0.2O3-δ (M= Sc, In, Yb, Y, Gd) were measured for the the samples as-sintered and annealed under 1%H2 at 873K. The peak of the NiO or Ni phase except the barium zircanate phase was observed for 0.4 mol% NiO doped BaZr0.8 M 0.2O3-δ (M= Sc, In, Yb). On the other hand, the BaM 2NiO5 phase was observed for M=Y, Gd. When the ionic radius of dopant is in excess of the size of Yb, the BaM 2NiO5 was formed as the reaction product between NiO and BaZr0.8 M 0.2O3-δ. The proton conductivity of the series of BaZr0.8 M 0.2O3-δ containing no NiO increased in the order of InM20 (M=Lu, Yb, Tm, Y) has an energy efficiency of more than 0.8 at 873 K. The proton conductivity decreased and the hole conductivity did not change for all series of BaZr0.8 M 0.2O3-δ due to NiO dissolution in BaZr0.8 M 0.2O3-δ. Therefore, proton transport number and the efficiency of the fuel cell decrease by addition to NiO. Considering the above, the ytterbium-doped barium zirconate did not form a complex oxide with NiO and might have high proton conductivity. The performance of PCFC using BZYb20 as electrolyte was measured. The maximum power density values at 600 and 700 °C were 0.50 and 0.70 Wcm−2 for the planar cells using BZYb20, respectively. Electrochemical impedance spectroscopy of the cell showed that the ohmic resistance was 0.28 Ωcm2 at 600 °C, which was approximately three times larger than the resistance value calculated using the bulk conductivity of BZYb20 and the electrolyte thickness and was in agreement with the resistance value of BZYb20 with NiO. [1] K.D. Kreuer et al., Solid State Ionics 145(2001)295-306. [2] Y. Matsuzaki et al., Scientific Reports, 5:12640(2015)1-10. [3] A. Doubois et al., ECS Transaction 78(1) (2017)1963-1972. [4] J. Tong et al., J. Mater. Chem. , 20(2010)6333.

Lanthanum silicate (LS) with 1–5 wt% Bi2O3 addition (LSB) was investigated for bi-layer solid oxide fuel cell (SOFC) electrolyte with gadolinia-doped ceria (GDC). Thermal shrinkage analyses suggested that adding more than 3 wt% Bi2O3 to LS approached to sintering behavior of GDC and Ni-GDC. Impedance measurement of GDC/LSB composites indicated that an additional resistive phase was formed in the grain boundary between GDC and LSB. Grain boundary resistance increased significantly with 5 wt% Bi2O3. This increase was inhibited in 1–3 wt% Bi2O3. Adding 3 wt% Bi2O3 to GDC/LSB is expected to densify electrolytes by co-sintering and limit the increase in resistance.

Distribution of relaxation times (DRT) analysis is a powerful assistant tool for electrochemical impedance spectroscopy (EIS) deconvolution. In the present work, electrode degradation is evaluated by estimation of polarization resistances using DRT analysis. Five or six DRT peaks are detected for anode-supported microtubular solid oxide fuel cells (SOFCs) at 600–650 °C. In a reduction-oxidation (redox) cycling test, the polarization resistance of charge transfer and ionic conduction processes in the anode increases due to the decrease in triple phase boundary (TPB) length. On the other hand, initial electrode degradation is also successfully evaluated using DRT analysis during a galvanostatic test. The rapid growth of nickel grains in the anode and the relatively slow growth of La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF) grains in the cathode are observed after the test, which corresponds with the increase in the polarization resistance of charge transfer and ionic conduction processes in the anode and that of oxygen surface exchange and diffusion processes in the cathode, respectively.

High power density solid oxide electrochemical cells were developed using nanostructure-controlled composite powder consisting of Sr-doped SmCoO 3 (SSC) and Sm-doped CeO 2 (SDC) for electrode material. The SSC-SDC nano-composite powder, which was synthesized by spray pyrolysis, had a narrow particle size distribution ( D 10 , D 50 , and D 90 of 0.59, 0.71, and 0.94 μm, respectively), and individual particles were spherical, composing of nano-size SSC and SDC fragments (approximately 10–15 nm). The application of the powder to a cathode for an anode-supported solid oxide fuel cell (SOFC) realized extremely fine cathode microstructure and excellent cell performance. The anode-supported SOFC with the SSC-SDC cathode achieved maximum power density of 3.65, 2.44, 1.43, and 0.76 W cm −2 at 800, 750, 700, and 650 °C, respectively, using humidified H 2 as fuel and air as oxidant. This result could be explained by the extended electrochemically active region in the cathode induced by controlling the structure of the starting powder at the nano-order level.

In the scope of electrochemical phenomena, concentration polarization at electrodes is theoretically inevitable, and lowering the concentration overpotential to improve the performance of electrochemical cells has been a continuing challenge. Electrodes with highly controlled microstructure, i.e., high porosity and uniform large pores are therefore essential to achieve high performance electrochemical cells. In this study, state-of-the-art technology for controlling the microstructure of electrodes has been developed for realizing high performance support electrodes of solid oxide fuel cells (SOFCs). The key is controlling the porosity and pore size distribution to improve gas diffusion, while maintaining the integrity of the electrolyte and the structural strength of actual sized electrode supports needed for the target application. Planar anode-supported SOFCs developed in this study realize 5 μm thick dense electrolyte (yttria-stabilized zirconia: YSZ) and the anode substrate (Ni-YSZ) of 53.6 vol.% porosity with a large median pore diameter of 0.911 μm. Electrochemical measurements reveal that the performance of the anode-supported SOFCs improves with increasing anode porosity. This Ni-YSZ anode minimizes the concentration polarization, resulting in a maximum power density of 3.09 W cm −2 at 800 °C using humidified hydrogen fuel without any electrode functional layers.

Perovskite (Ba,Sr)(Co,Fe)O3 (BSCF) cathodes are highly electrochemically active in the oxygen reduction process for intermediate-temperature solid oxide fuel cells (IT-SOFCs). However, the phase of perovskite zirconate is formed between the BSCF cathode and yttria-stabilized zirconia (YSZ) electrolyte, which increased the cathode polarization resistance. In the present work, the (Ba,Sr)ZrO3 formation was not prevented by the insertion of a porous gadolinia-doped ceria (GDC) interlayer between the cathode and the electrolyte, when the BSCF cathode was sintered at 1100 °C. Energy dispersive X-ray spectroscopy showed that the product of (Ba,Sr)ZrO3 formed between BSCF and YSZ was thicker than that of SrZrO3 formed between a conventional (La,Sr)(Co,Fe)O3 (LSCF) and YSZ. When a BSCF cathode was sintered at 900 °C, no (Ba,Sr)ZrO3 formation was observed between the cathode and the electrolyte. The maximum power density at 650 °C was improved from 7.5 to 740 mW/cm2 by decreasing the sintering temperature of BSCF cathode from 1100 °C to 900 °C. Distribution of relaxation times (DRT) analysis showed that the polarization resistance of the oxygen exchange process in the BSCF cathode sintered at 900 °C was lower than that in the LSCF cathode. The sintering temperature is important for obtaining high-performance BSCF cathodes for IT-SOFCs.

Zinc and barium phosphate glasses show good proton conductivity at intermediate temperature around 200 °C. Infrared spectra and 1 H magic angle spinning-nuclear magnetic resonance (MAS–NMR) spectra proposed that a 30 mol%ZnO-70 mol%P 2 O 5 glass melted at 800 °C has a large amount of ‘mobile’ protons. The proton conductivity at 250 °C was measured to be 1 × 10 −3 S/cm. A H 2 -air fuel cell using the ZnO–P 2 O 5 glass electrolyte of 1.8 mm in thickness showed the maximum power density of 1.2 mW/cm 2 at 200 °C.

Solid oxide fuel cells (SOFCs) can, in principle, directly use hydrocarbon fuels such as methane and liquefied petroleum gas (LPG). It was previously found that nickel-gadolinia doped ceria (Ni-GDC) anode has high durability under LPG utilization in SOFCs. However, the performance under direct LPG utilization was low compared to hydrogen fuel, because anode polarization resistance increased by internal reforming. Carbon was slightly deposited on the Ni-GDC anode after direct LPG utilization for 100 h at 650 oC. In this work, a catalytic partial oxidation reformer was developed for LPG utilization, and high performance was obtained using reformate gas as well as hydrogen without carbon deposition on the Ni-GDC anode for 100 h at 650 oC. Furthermore, microtubular SOFC stacks were developed using mass production technologies, and a proto-type 200 W micro power generator was demonstrated via NEDO project entitled "Technology Development for Promoting SOFC Commercialization".

We have been developing a tubular electrochemical cell composed of Ni-cermet support/YSZ/GDC/LSCF-GDC or Ni-cermet support/GDC/LSCF-GDC in order to synthesize energy-carrier chemicals (methane etc.) via co-electrolysis of H2O-CO2 mixed gas. In this report, we will show effects of (1) the cell fabrication processes, (2) the cell microstructure and (3) methanation ability of two types of SOECs at 400 ºC.

The electrical conductivity of nickel (2 wt %)-doped BaZr

When hydrocarbons are used as a fuel in solid oxide fuel cells (SOFCs), internal steam reforming increases the energy conversion efficiency and simplifies the system, including the balance-of-plant. However, conventional nickel–yttria stabilized zirconia (Ni–YSZ) anodes are prone to deterioration at high temperatures and high humidity. This paper focuses on effects in anode microstructure on performance and durability of microtubular SOFCs. The evaluations were conducted under high steam content and internal methane reforming conditions using Ni–YSZ anodes using acrylic resin and graphite pore formers. The initial cell performance was almost identical to that of SOFCs with anodes using acrylic resin and graphite pore formers in 40% H2–3% H2O at 700 °C. However, the anode using acrylic resin deteriorated rapidly in 40% H2–30% H2O over a period of 28 h. Furthermore, it generated almost no electric power by internal steam reforming of methane. The local oxidation of nickel particles was observed at the interface between the electrolyte and the deteriorated anodes. The anode using graphite pore former provided stable power generation in 40% H2–30% H2O, and was able to generate power in 10% CH4–30% H2O. The pore formers strongly affect fuel diffusivity in the SOFC anodes, which is an important factor in stable internal steam reforming of methane. Keywords: Solid oxide fuel cell (SOFC), Nickel–yttria stabilized zirconia (Ni–YSZ) anode, Internal steam reforming, Pore former, AC impedance

Micro flat-tube solid-oxide fuel cell (SOFC) modules consisting of 1 mm thick, 1.2 cm wide micro flat-tube SOFCs, gas manifold, and insulator have been fabricated and evaluated using simple gas heating apparatus. The cell consists of NiO – yttria stabilized zirconia (YSZ) as an anode (flat-tube support), scandia stabilized zirconia (ScSZ) as an electrolyte, gadolinia doped ceria (GDC) for an interlayer, and (La, Sr)(Fe, Co)O3 (LSCF) – GDC as a cathode, which has been fabricated using cost effective extrusion technique and dip-coating technique. The cell has been investigated between 600 and 650 °C operating temperature and showed the power density at 0.75 V of 0.19 and 0.385 W cm−2, respectively. Using the cell, a five and ten-series modules were assembled and stored in insulator with small gas heaters powered by a 24 V power source for start-up. The module successfully operated using hydrogen and methane fuel.

Effect of anode functional layer thickness on the performance of solid-oxide fuel cells (SOFCs) has been investigated in the intermediate temperatures of 600–650 °C. Three types of cells with different thickness (0, 4, 10 micron) of nanostructured anode functional layer (AFL) consisting of Ni-ScSZ (Scandia stabilized zirconia) are prepared. The SOFCs consist of Ni-3YSZ (3 mol% yttria stabilized zirconia) anode tube support with the AFL, ScSZ electrolyte, and LSCF (lanthanum strontium cobalt ferrite) and GDC (gadolinium doped ceria) mixture cathode. It is shown that the performance of the cell is improved as the thickness of the anode functional layer increases. Power densities of the cell with 10 micron thick AFL at 600 and 650 °C are shown to be 0.22 and 0.27 W/cm2 at 0.75 V, respectively. According to impedance spectroscopy, improvement of both ohmic and polarization resistances has been observed by increasing the thickness of the AFL, suggesting that the AFL also acts as a better contact layer between the electrolyte and the anode support, and the effectiveness of the AFL by optimizing the thickness.

Microtubular solid oxide fuel cells (SOFCs) with a diameter on the millimeter scale are suitable for small-scale applications such as micro combined heat/power systems and portable power sources. However, the use in reducing purge gas is not practical during start-up and shutdown in the small-scale applications. In this study, the redox tolerance was evaluated for nickel gadolinia-doped ceria (Ni–GDC) anode-supported microtubular SOFCs with an open-end at fuel outlet, when the hydrogen fuel supply was stopped 8 times for 10 min/cycle at 615 °C. The open circuit voltage was unchanged, and the degradation of less than 3% was confirmed after 8 redox cycles. The Ni–GDC anode-supported microtubular SOFCs have redox tolerance at 615 °C. Each electrode polarization resistance can be separated by the distribution of relaxation times (DRT) analysis from the measurement of AC impedance between the anode and the cathode. While the anode activation polarization (10–100 Hz) was clarified to increase because of the decrease in the TPB length, the anode concentration polarization (

Electrochemical ceramic cells such as solid-oxide fuel cells (SOFCs) are typically operated at 700–800°C in order to realize practical performances that, in turn, result in higher efficiencies compared to that of other types of electrochemical cells. High-temperature operation, on the other hand, leads to increased system cost and limits application. Thus, lowering the operating temperature is expected to solve such problems. This article shows the effectiveness of redesigning the cell structure for reduction of the operating temperature to 650°C or lower using conventional SOFC materials. A microtubular cell design is found to be one means of lowering the operating temperature of SOFCs. Such developments in fabrication technology are key to realizing high-performance cells with a thin electrolyte and controlled electrode microstructures.

Conductive Ag-precipitated glass powder was prepared by a heat reduction process of the glass/AgCl composite powder and its electrical conductivity was compared to conventional Ag-particle mixed glass powder. The Ag-precipitated glass powder with 30 wt.% dispersed Ag particles of diameter of 20 to 300 nm and the baked film made of the powder showed a good electrical conduction from 400 °C to 700 °C. On the other hand, the baked films made of the Ag-particle mixed glass powders of Ag particle size of 300 nm with 35 wt.% Ag content and that of size of 20 nm with 30 wt.% Ag content showed a poor electrical conduction. The electrical conduction of the film of Ag-precipitated glass powder resulted from a uniform dispersion of Ag particles. Since no continuous network of Ag particles was found in the microstructure observation of the glass powders, or films, the origin of the electrical conduction was considered to be due to a tunneling effect between the well-dispersed Ag particles.

The effect of operating temperature on durability was investigated for direct butane utilization in anode-supported microtubular solid oxide fuel cells (SOFCs). At 710 oC, the performance of the Ni-Gd-doped ceria (Ni-GDC) anode deteriorated rapidly for less than 2 h at a relatively low steam/carbon (S/C) ratio of 0.044 in butane, because a large amount of carbon was deposited on the anode by butane cracking. On the other hand, the electric power could be generated continuously for more than 24 h at 610 oC and S/C = 0.044 in butane for the cell using the Ni-GDC anode. Lowering the operating temperature realized high durability against carbon deposition on the Ni-GDC anode under the direct butane utilization. We manufactured and demonstrated a prototype portable SOFC system using a microtubular cell-stack. It can be heated up to 400 oC within 2 minutes by burning an LPG burner, and drive a USB device for 24 h continuously using a LPG cartridge (250 g; 3.2 kWh). This development has shown the potential of the microtubular SOFC system as portable power sources in disaster and emergency situations, for outdoor use, and for small-scale electric vehicles.

Phosphate glasses are promising materials for electrolytes of intermediate temperature fuel cells, because they have good proton conductivity at 150–250 °C. However, the effects of the glass composition and melting condition on proton conductivities are unclear yet. In this work, the structures of BaO–ZnO–P 2 O 5 glasses were investigated by magic angle spinning-nuclear magnetic resonance (MAS-NMR) and Raman spectroscopy, and the proton conductivities were measured by an AC impedance method. The proton conductivity of 30 mol%ZnO-70 mol%P 2 O 5 glass melted at 800 °C reached 1 × 10 −3 S/cm at 250 °C for. The proton transportation number of the ZnO–P 2 O 5 glass was almost unity, confirmed by a hydrogen concentration cell. The power density of 0.4 mW/cm 2 was obtained for a fuel cell using the ZnO–P 2 O 5 glass electrolyte at 250 °C. A branching phosphate structure was transformed into a middle phosphate structure by substituting BaO with ZnO, which caused an improvement in proton mobility.

The effects of anode microstructure on mechanical and electrochemical properties were investigated for anode-supported microtubular solid oxide fuel cells (SOFCs). The anode microstructures can be varied by the change in pore formers. For example, the acrylic resin pore former was burnt more rapidly at lower temperature than the graphite pore former during sintering. The acrylic resin pore former can introduce macropores with a diameter of several micrometers in nickel–yttria-stabilized zirconia (Ni–YSZ) anode. The walls of the macropores were packed with the nickel and YSZ particles. Although the Ni–YSZ anode microtube using the 10 wt% acrylic resin pore former was compatible with high porosity and mechanical strength, the maximum fuel utilization was limited to 72%. On the other hand, the graphite pore former can produce a relatively uniform distribution of micropores with a diameter of several hundred nanometers. The mechanical strength was reduced with a rise in porosity for the Ni–YSZ microtube using the graphite pore former in comparison with the acrylic resin. However, a high fuel utilization of 93% was realized for the microtubular SOFCs using the 10 wt% graphite pore former in spite of lower porosity than the acrylic resin. The selection of a pore former is important to obtain higher power generation efficiency for anode-supported microtubular SOFCs.

Carbon growth site on the surface of a solid oxide fuel cell (SOFC) comprising yttria stabilized zirconia (YSZ) and Ni was observed using transmission electron microscopy (TEM). TEM observations of the YSZ and Ni powder mixture, which was heated in a methane atmosphere, confirmed that graphite was grown on the Ni surface only, and not deposited on the YSZ surface or the interface between YSZ and Ni. Additionally, the grown graphite was confirmed to have a curled structure and to terminate growth at the Ni-YSZ interface. These phenomena were likely caused by differences in the carbon wettability against Ni or YSZ, as well as oxygen diffusion from YSZ to Ni.

Solid oxide cells with reversible mode would generate power during high peak demand, and produce chemical fuel with renewable energy such as wind power and solar power. In this study the reversible performance of an anode-supported proton-conductive cell was investigated under SOFC and SOEC modes. The cell can show the reversible behaviors in lower temperature range below 600°C.

Microtubular SOFCs with a diameter on the millimeter scale are suitable for portable power sources due to rapid start-up characteristics. The performance of conventional Ni-stabilized zirconia anodes deteriorated rapidly over a period of 3-4 h at 610 oC and a low steam/carbon (S/C) ratio of 0.044 in butane due to a large amount of carbon deposition. For the Ni-Gd doped ceria (Ni-GDC) anode, the electric power could be generated continuously for more than 24 h at 610 oC and S/C = 0.044 in butane. We manufactured and demonstrated a prototype portable SOFC system using a microtubular cell-stack. It can heat up to 400 oC within 2 min by burning an external LPG burner, and drive a USB device for 24 h continuously using a LPG cartridge (250 g; 3.2 kWh). This development has shown the potential of the microtubular SOFC system as portable power sources in disaster and emergency situations, for outdoor use, and for small-scale electric vehicles.

The performance of a conventional anode-supported microtubular SOFC using doped ceria as an electrolyte and Ni-based cermet as an anode is evaluated at low operating temperature between 294 and 542°C. An open-circuit voltage (OCV) of >0.9 V is obtained at all measured operating temperatures, and power generation is observed at temperatures as low as 294°C. The power density of the cell is 0.6 W/cm2 at 542°C operating temperature with 47% fuel utilization and is 5 mW/cm2 at 294°C operating temperature with an open-circuit voltage of 0.95 V. According to impedance spectroscopy, a greater influence of gas flow rate, on the cell performance, is observed at higher operating temperature.

The perovskite La 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3 –Ce 0.9 Gd 0.1 O 1.95 (LSCF–GDC) nanoparticulate cathode was applied for microtubular solid oxide fuel cells operated at intermediate temperatures. For the cell with the cathode sintered at 950 °C, maximum power densities of 0.26, 0.54 and 0.73 W cm −2 were obtained at 550, 600 and 650 °C, respectively. The ohmic resistance increased for the cathode sintered at 850 °C, and the polarization resistance increased for the cathode sintered at 1050 °C. The cathode polarization resistances of ionic conduction process in the LSCF bulk and adsorption/desorption process on the LSCF surface were estimated by the distribution of relaxation times analysis, which were only 0.066 and 0.065 Ω cm 2 at 600 °C for the cathode sintered at 950 °C. The grain size of the cathode was less than 100 nm, which resulted in high performance due to an overall decrease in cathode polarization resistance.