Electrochemical characteristics and performance of anode-supported SOFCs fabricated using carbon microspheres as a pore-former

This paper evaluates the influence of carbon microspheres (CMSs) as an electrode pore-former on the fabrication and electrochemical properties of the anode-supported solid oxide fuel cells (SOFCs). The anode supports are fabricated by dry-pressing of CMS and NiO/YSZ (nickel-oxide/yttria-stabilized zirconia) composite powder, and the YSZ electrolyte layer is prepared by the electrophoretic deposition technique. The ohmic and polarization resistances for NiO/YSZ–YSZ half cells at different testing temperatures (650–850 °C) are analyzed by electrochemical impedance spectroscopy (EIS). The polarization ASR (area specific resistance) for the fabricated half cells increases from 0.583 Ω cm² to 3.047 Ω cm² when the temperature decreases from 850 °C to 650 °C. The electrochemical performance of single cells is measured at different temperatures (700–850 °C) and the results indicate that the cells fabricated using CMS as the pore-former exhibit much higher electrochemical performance than those without using CMS. A maximum power density of 207.7 mW cm⁻², 431.2 mW cm⁻², and 571.6 mW cm⁻² is recorded at 850 °C for the cells fabricated by adding 0 wt. %, 2.5 wt. % and 5 wt. % of CMS, respectively. The maximum fuel utilization efficiency is also found to increase from 26.5% for the cell prepared without CMS to 47.0% and 59.6% for the cells prepared with 2.5 wt. % and 5 wt. % of CMS, respectively. The increase in the electrochemical performance by adding CMS as pore-former to anode-supports is attributed to higher porosity and pore size of the electrode.     

GdBaCo2O5+x layered perovskite as an intermediate temperature solid oxide fuel cell cathode

GdBaCo₂O_5+x (GBCO) was evaluated as a cathode for intermediate-temperature solid oxide fuel cells. A porous layer of GBCO was deposited on an anode-supported fuel cell consisting of a 15 μm thick electrolyte of yttria-stabilized zirconia (YSZ) prepared by dense screen-printing and a Ni–YSZ cermet as an anode (Ni–YSZ/YSZ/GBCO). Values of power density of 150 mW cm⁻² at 700 °C and ca. 250 mW cm⁻² at 800 °C are reported for this standard configuration using 5% of H₂ in nitrogen as fuel. An intermediate porous layer of YSZ was introduced between the electrolyte and the cathode improving the performance of the cell. Values for power density of 300 mW cm⁻² at 700 °C and ca. 500 mW cm⁻² at 800 °C in this configuration were achieved.     

Comparative study on the performance of tubular and button cells with YSZ membrane fabricated by a refined particle suspension coating technique

Both tubular and button solid oxide fuel cells (SOFCs) with configuration NiO–YSZ/YSZ/PNSM–YSZ were assembled and compared in their performance. A refined particle suspension coating technique was used for preparing thin dense YSZ electrolyte layer on the two types of anode supports, and the thickness of YSZ membrane was controlled by the time of tubular anode dipped into YSZ suspension and the suspension volume dropped onto the button anode, respectively. Current–voltage tests and AC impedance measurements were carried out to characterize the performance and ohmic resistances in the two cells. Compared with tubular cell, higher peak power density values of 933 mW cm⁻² at 850 °C was achieved, which is 2.2 times higher than the value of tubular cell. AC impedance indicated that lower performance of tubular cell was restricted by the ohmic loss at the operating temperatures.     

Engineered anode structure for enhanced electrochemical performance of anode-supported planar solid oxide fuel cell

A novel approach of fabricating SOFC anode comprising graded compositions in constituent phases having layer wise microstructural variation is reported. Such anode encompasses conventional NiO–YSZ (40 vol% Ni) with higher porosity at the fuel inlet side and Ni–YSZ electroless cermet (28–32 vol% Ni) with less porosity toward the electrolyte. Microstructures and thicknesses of the bilayer anodes (BLA) are varied sequentially from 50 to 250 μm for better thermal compatibility and cell performance. Significant augmentation in performance (3.5 A cm⁻² at 800 °C, 0.7 V) is obtained with engineered trilayer anode (TLA) having conventional anode support in conjunction with layers of electroless cermet each of 50 μm having 28 and 32 vol% Ni. Engineered TLA accounts for substantial reduction both in cell polarization (ohmic ASR: 78 mΩ cm² versus 2835 mΩ cm²; cell impedance: 0.35 Ω cm² versus 0.9 Ω cm²) and degradation rate (76 μV h⁻¹ versus 219 μV h⁻¹) compared to cells fabricated with conventional cermet.     

Characterization of SmBaCoFeO5+δ−Ce0.9Gd0.1O1.95 composite cathodes for intermediate-temperature solid oxide fuel cells

The performance of SmBaCoFeO_5+δ (SBCF)–xCe_0.9Gd_0.1O_1.95 (GDC) (x = 0, 10, 30, 50, 60, wt%) composite cathodes has been investigated for their potential utilization in intermediate-temperature solid oxide fuel cells (IT-SOFCs). The powder X-ray diffraction (XRD), thermal expansion coefficient (TEC) and electrochemical property measurements are employed to study the materials. The XRD results prove that there is no serious reaction between SBCF and GDC oxides even at 1000 °C. The thermal expansion behavior shows that the TEC value of SBCF cathode decreases greatly with GDC addition. The addition of GDC to SBCF cathode further reduces the polarization resistance. The lowest polarization resistance of 0.036 Ω cm² is achieved at 800 °C for SBCF–50GDC composite cathode. An electrolyte-supported fuel cell is prepared using SBCF–50GDC as cathode and NiO–GDC (65:35 by weight) as anode. The cell generates good performance with the maximum power density of 691 mW cm⁻², 503 mW cm⁻² and 337 mW cm⁻² at 800 °C, 750 °C and 700 °C, respectively. Preliminary results indicate that SBCF–50GDC is especially promising as a cathode for IT-SOFCs.     

An anode-supported micro-tubular solid oxide fuel cell with redox stable composite cathode

A NiO–YSZ anode-supported hollow fiber solid oxide fuel cell (HF-SOFC) has been fabricated with redox stable (La_0.75Sr_0.25)_0.95Cr_0.5Mn_0.5O_3−δ–Sm_0.2Ce_0.8O_1.9–YSZ (LSCM–SDC–YSZ) composite cathode. The characterization of NiO–YSZ hollow fibers prepared by the phase inversion method is focused on the microstructure, porosity, bending strength and electrical conductivity. A thin YSZ electrolyte membrane (about 10 μm) can be prepared by a vacuum-assisted dip-coating process and is characterized in terms of microstructure and gas-tightness. The performance of the as-prepared HF-SOFC is investigated at 750–850 °C with humidified H₂ as fuel and ambient air as the oxidant. The peak power densities of 513, 408 and 278 mW cm⁻² can be obtained at 850, 800 and 750 °C, respectively, and the corresponding interfacial polarization resistances are 0.14, 0.29 and 0.59 Ω cm². The high performance at intermediate-to-high temperatures could be attributed to thin electrolyte and proper composite cathode with low interfacial polarization resistance. The low interfacial polarization resistance suggests potential applications of LSCM–SDC–YSZ composite oxides as the redox stable cathode. This investigation indicates that the redox stable LSCM–SDC–YSZ is a promising cathode material system for the next generation YSZ-based HF-SOFC. The results will be expected to open up a new phase of the research on the micro-tubular SOFCs.     

YSZ films fabricated by a spin smoothing technique and its application in solid oxide fuel cell

A dense single-layer YSZ film has been successfully fabricated by a spin smoothing method. Followed by a simplified slurry coating, an additional spin smoothing process was conducted to obtain a thinner and smoother film. By employment of high-viscosity slurry including high YSZ content, the film has a suitable thickness by a single coating cycle. With Sm_0.2Ce_0.8O_1.9 (SDC)-impregnated La_0.7Sr_0.3MnO₃ (LSM) cathode and porous NiO–YSZ anode, single solid oxide fuel cell (SOFC) based on an 8-μm-thick YSZ film was obtained. Open-circuit voltage (OCV) of the cell was 1.04 V at 800 °C, and maximum power densities were 676, 965 and 1420 mW cm⁻² at 700, 750 and 800 °C, respectively, using H₂ at a flow rate of 40 mL min⁻¹ as fuel and ambient air as oxidant. The power density could be increased to 1648 mW cm⁻² at 800 °C when the flow rate of H₂ was enhanced to 200 mL min⁻¹.     

Influence of anode's microstructure on electrochemical performance of solid oxide direct carbon fuel cells

The microstructure of anode has a significant influence on the whole electrochemical performance of solid oxide direct carbon fuel cells (SO-DCFCs). The tubular SO-DCFCs based on cathode supported solid oxide fuel cells was fabricated by dip-coating and co-sintering techniques. As the anode porosity mainly came from the pore former (graphite) in the dip-coating process, different contents of graphite were added into the anode slurries. When the graphite was 10.1% wt., the SO-DCFCs showed the best performance and stability. The peak power density reached 242 mW cm⁻² at 850 °C, with carbon black (located 5% Fe) as the fuel and air as the oxidant.     

An additional layer in an anode support for internal reforming of methane for solid oxide fuel cells

Anode supported solid oxide fuel cells (SOFCs) have been extensively investigated for their ease of fabrication, robustness, and high electrochemical performance. SOFCs offer a greater flexibility in fuel choice, such as methane, ethanol or hydrocarbon fuels, which may be supplied directly on the anode. In this study, SOFCs with an additional Ni–Fe layer on a Ni–YSZ support are fabricated with process variables and characterized for a methane fuel application. The addition of Ni–Fe onto the anode supports exhibits an increase in performance when methane fuel is supplied. SOFC with a Ni–Fe layer, sintered at 1000 °C and fabricated using a 20 wt% pore former, exhibits the highest value of 0.94 A cm⁻² and 0.85 A cm⁻² at 0.8 V with hydrogen and methane fuel, respectively. An impedance analysis reveals that SOFCs with an additional Ni–Fe layer has a lower charge transfer resistance than SOFCs without Ni–Fe layer. To obtain the higher fuel cell performance with methane fuel, the porosity and sintering temperature of an additional Ni–Fe layer need to be optimized.     

Applications of nano-composite materials for improving the performance of anode-supported electrolytes of SOFCs

In order to improve the performance of the anode-supported electrolyte of solid oxide fuel cells (SOFCs), the anode electrode is modified by inserting an anode functional layer of nano-composite powders between a Ni–YSZ electrode and YSZ electrolyte. The NiO–YSZ nano-composite powders are fabricated by coating nano-sized Ni and YSZ particles on the YSZ core particle by the Pechini process. The reduction of the polarization resistance of a single cell that is applied to the anode functional layer is attributed to the increasing reaction of three-phase boundaries (TPBs) within the layer and the micro-structured uniformity in the electrode. Two methods were used, namely tape-casting/dip-coating and tape-casting/co-firing, for studying the performance. It can be concluded that the cell with an anode functional layer thickness (15–20 μm) and a microstructure of NiO–YSZ nano-composite materials which was fabricated by the tape-casting/dip-coating method improved the output power (to 1.3 W cm⁻²) at 800 °C using hydrogen as fuel and air as an oxidant.     

Upgrading the performance of La2Mo2O9-based solid oxide fuel cell under single chamber conditions

Various anode-supported solid oxide fuel cells (SOFC), based on 10 mol% Dy-doped La₂Mo₂O₉ (LDM) electrolyte, are prepared analytically and operated under single chamber conditions to explore the connections between electrode and power performance. The cathode of tested SOFCs is compositionally graded with three composites of samarium strontium cobaltite and Gd-doped ceria (GDC) to relax the thermal stress, because of sizable thermal expansion differences above 400 °C. We focus the research attention on varying the anode pore structure and composition to promote the power performance in methane/air mixture at 700 °C. For the one-layer support of GDC+NiO+LDM anode, addition of 10 wt% graphite minimizes its mass transport resistance through creating 8–5 μm long and ∼1 μm wide slit-shaped pores. The graphite pore former raises the peak power value by 80 mW cm⁻². Adopting a more porous and active outer layer, the double-layer support further enhances the cell power. The peak power was first raised by 48 mW cm⁻², using an outer layer that was prepared with 63 wt% NiO. Dosing 3% Pd on this outer layer uplifts another 59 mW cm⁻². In this study, with an improved anode, the peak power value reaches 437 mW cm⁻².     

Microstructural factors of electrodes affecting the performance of anode-supported thin film yttria-stabilized zirconia electrolyte (∼1 μm) solid oxide fuel cells

The effects of the microstructural factors of electrodes, such as the porosity and pore size of anode supports and the thickness of cathodes, on the performance of an anode-supported thin film solid oxide fuel cell (TF-SOFC) are investigated. The performance of the TF-SOFC with a 1 μm-thick yttria-stabilized zirconia (YSZ) electrolyte is significantly improved by employing anode supports with increased porosity and pore size. The maximum power density of the TF-SOFCs increases from 370 mW cm⁻² to 624 mW cm⁻² and then to over 900 mW cm⁻² at 600 °C with increasing gas transport at the anode support. Thicker cathodes also improve cell performance by increasing the active reaction sites. The maximum power density of the cell increases from 624 mW cm⁻² to over 830 mW cm⁻² at 600 °C by changing the thickness of the lanthanum strontium cobaltite (LSC) cathode from 1 to 2-3 μm.     

Carbon-resistant Ni-YSZ/Cu–CeO2-YSZ dual-layer hollow fiber anode for micro tubular solid oxide fuel cell

A NiO-YSZ/porous YSZ dual-layer hollow fiber with an asymmetric structure was fabricated by a co-spinning-sintering method. A dense YSZ electrolyte film was prepared on NiO-YSZ layer by dip-coating method and calcined at 1450 °C; subsequently a porous cathode was dip-coated on the dense YSZ electrolyte film using LSM-YSZ (in the weight ratio 4:1) ink to fabricate a micro tubular solid oxide fuel cell (MT-SOFC). Cu–CeO₂ catalyst was impregnated into the porous YSZ layer to form the second anode composition. The power output of the MT-SOFC with Ni-YSZ/Cu–CeO₂-YSZ graded anode was up to 242 mW cm⁻² operated at 850 °C using CH₄ as fuel and air as oxidant. Little carbon deposition was observed on the double anode using methane as the fuel after 60 h' stable operation.     

Electrophoretic deposition on non-conducting substrates: The case of YSZ film on NiO–YSZ composite substrates for solid oxide fuel cell application

This paper report the results of our investigation on electrophoretic deposition (EPD) of YSZ particles from its suspension in acetylacetone onto a non-conducting NiO–YSZ substrate. In principle, it is not possible to carry out electrophoretic deposition on non-conducting substrates. In this case, the EPD of YSZ particles on a NiO–YSZ substrate was made possible through the use of an adequately porous substrate. The continuous pores in the substrates, when saturated with the solvent, helped in establishing a “conductive path” between the electrode and the particles in suspension. Deposition rate was found to increase with increasing substrate porosity up to a certain value. The higher the applied voltage, the faster the deposition. For a given applied voltage, there exists a threshold porosity value below which EPD becomes practically impossible. An SOFC constructed on bi-layers of NiO–YSZ/YSZ with YSZ layer thickness of 40 μm exhibited an open circuit voltage (OCV) of 0.97 V at 650 °C and peak power density of 263.8 mW cm⁻² at 850 °C when tested with H₂ as fuel and ambient air as oxidant.     

Microstructure tailoring of YSZ/Ni-YSZ dual-layer hollow fibers for micro-tubular solid oxide fuel cell application

YSZ/NiO-YSZ dual-layer hollow fibers with a thin YSZ top layer integrated on a porous NiO-YSZ (60:40 in weight) support, have been developed by one step method via a co-spinning-sintering process. Hydrogen reduction was performed to form YSZ/Ni-YSZ micro tube as the half solid oxide fuel cells (SOFCs). The microstructure of the dual-layer hollow fibers was tailored by adding ethanol as non-solvent in the initial mixture dopes for NiO-YSZ anode spinning. LSM cathode containing 20 wt%-YSZ was deposited on the electrolyte surface by dip-coating method to fabricate micro-tubular SOFCs. Experimental results indicate that the dual-layer hollow fibers from the anode dopes containing 15–20 wt% of ethanol possess the desired microstructure with optimized properties, such as the bending strength of 180 MPa, the porosity of 38–35% and the conductivity of 3000 S cm⁻¹ at room temperature. The micro-tubular SOFCs fabricated from such hollow fibers show a maximum power density up to 485 mW cm⁻² at 850 °C with 20 mL min⁻¹ of H₂ as fuel and 30 mL min⁻¹ air as oxidant, respectively.     

The effect of porosity gradient in a Nickel/Yttria Stabilized Zirconia anode for an anode-supported planar solid oxide fuel cell

In this paper, a graded Ni/YSZ cermet anode, an 8 mol.%YSZ electrolyte, and a lanthanum strontium manganite (LSM) cathode were used to fabricate a solid oxide fuel cell (SOFC) unit. An anode-supported cell was prepared using a tape casting technique followed by hot pressing lamination and a single step co-firing process, allowing for the creation of a thin layer of dense electrolyte on a porous anode support. To reduce activation and concentration overpotential in the unit cell, a porosity gradient was developed in the anode using different percentages of pore former to a number of different tape-slurries, followed by tape casting and lamination of the tapes. The unit cell demonstrated that a concentration distribution of porosity in the anode increases the power in the unit cell from 76 mW cm⁻² to 101 mW cm⁻² at 600 °C in humidified hydrogen. Although the results have not been optimized for good performance, the effect of the porosity gradient is quite apparent and has potential in developing superior anode systems.     

H2 and CH4 oxidation on Gd0.2Ce0.8O1.9 infiltrated SrMoO3–yttria-stabilized zirconia anode for solid oxide fuel cells

Strontium molybdate (SrMoO₃) as an electronic conductor was incorporated with yttria-stabilized zirconia (YSZ) to form an anode scaffold for solid oxide fuel cells. Gd_0.2Ce_0.8O_1.9 (GDC) nanoparticles were introduced by wet impregnation to complete the Ni-free GDC infiltrated SrMoO₃–YSZ anode fabrication. The effects of SrMoO₃ on the electrode conductivity and GDC infiltration on the catalytic activity were examined. A pronounced performance improvement was observed both on wet H₂ and CH₄ oxidation for the 56 wt.% GDC infiltrated SrMoO₃–YSZ. In particular, the polarization resistance decreased from 8 Ω cm² to 0.5 Ω cm² under wet H₂ (3% H₂O) at 800 °C with the introduction of GDC. Under wet CH₄ at 900 °C, a maximum power density of 160 mW cm⁻² was obtained and no carbon deposition was observed on the anode. It was found that the addition of H₂O in the anode caused an increase of electrode ohmic resistance and a decrease of open circuit voltage. Redox cycling stability was investigated and only a slight drop in cell performance was observed after 5 cycles. These results suggest that GDC infiltrated SrMoO₃–YSZ is a promising anode material for solid oxide fuel cells.     

Ultrafine amorphous Co–W–B alloy as the anode catalyst for a direct borohydride fuel cell

The ultrafine amorphous Co–W–B alloy has been synthesized by chemical reduction and used as anode catalyst in direct borohydride fuel cell. The results show that the maximum power output of the cell is 101 mW cm⁻² at 15 °C, and the essential power density of this material can be up to 350 mW cm⁻² at 15 °C and 500 mW cm⁻² at 60 °C, respectively. The cell has also a good durability, with no attenuation observed after one week of operation.     

Plasma sprayed metal supported YSZ/Ni–LSGM–LSCF ITSOFC with nanostructured anode

Intermediate temperature solid oxide fuel cells (ITSOFCs) supported by a porous Ni-substrate and based on Sr and Mg doped lanthanum gallate (LSGM) electrolyte, lanthanum strontium cobalt ferrite (LSCF) cathode and nanostructured yttria stabilized zirconia–nickel (YSZ/Ni) cermet anode have been fabricated successfully by atmospheric plasma spraying (APS). From ac impedance analysis, the sprayed YSZ/Ni cermet anode with a novel nanostructure and advantageous triple phase boundaries after hydrogen reduction has a low resistance. It shows a good electrocatalytic activity for hydrogen oxidation reactions. The sprayed LSGM electrolyte with ∼60 μm in thickness and ∼0.054 S cm⁻¹ conductivity at 800 °C shows a good gas tightness and gives an open circuit voltage (OCV) larger than 1 V. The sprayed LSCF cathode with ∼30 μm in thickness and ∼30% porosity has a minimum resistance after being heated at 1000 °C for 2 h. This cathode keeps right phase structure and good porous network microstructure for conducting electrons and negative oxygen ions. The APS sprayed cell after being heated at 1000 °C for 2 h has a minimum inherent resistance and achieves output power densities of ∼440 mW cm⁻² at 800 °C, ∼275 mW cm⁻² at 750 °C and ∼170 mW cm⁻² at 700 °C. Results from SEM, XRD, ac impedance analysis and I–V–P measurements are presented here.     

Investigation on microstructures of NiO–YSZ composite and Ni–YSZ cermet for SOFCs

NiO–YSZ composites and Ni–YSZ cermets were successfully performed for solid oxide fuel cell applications. These composites must have enough porosity and appropriate microstructure for transferring the fuel gases. In this study, ball-milling was used as a simple, cost-effective method for the purpose of mixing the raw materials. The homogeneity of NiO–YSZ composites was examined by Map mode of SEM. NiO–YSZ composites were reduced at the high temperature under the controlled atmosphere to fabricate Ni–YSZ cermet. Variations in the anode phases were investigated by XRD and microstructure and porosity of composites were observed by SEM. Effective parameters like temperatures and the amount of pore former were investigated on open porosity, bulk density, electrical conductivity as well as electrochemical impedance of NiO–YSZ composites and Ni–YSZ cermet. A thin layer of YSZ was deposited by EPD as an electrolyte on NiO–YSZ composites which had various amount of open porosity, to study its effect on the performance of semi-cells by electrochemical impedance.