Manufacturing and characterization of metal-supported solid oxide fuel cells

A metal-supported solid oxide fuel cell design offers competitive advantages, for example reduced material costs and improved robustness. This paper reports the performance and stability of a recently developed metal-supported cell design, based on a novel cermet anode, on a 25 cm² (1 cm²/16 cm² active area) cell level. An electrochemical performance comparable to state-of-the-art anode-supported cells is demonstrated.Detailed electrochemical analysis allowed assignment of the overall polarization losses quantitatively to gas diffusion in the metal support, electrooxidation in the anode functional layer, oxygen reduction in the mixed ionic-electronic conducting cathode and an additional polarization process with a rather high relaxation frequency, which may be assigned to an insulating corrosion interlayer.The durability of the cells was investigated by means of galvanostatic operation for periods of up to 1000 h as well as the dynamic behavior, such as redox-, load- and thermal cycling tests.The galvanostatic stability tests indicated a fair, but significant degradation rate (∼5% decrease in cell voltage/1000 h at 650 °C and 0.25 A cm⁻²). Furthermore, the metal-supported cells underwent an endurance test of 100 redox cycles at 800 °C without severe degradation nor total failure.     

Effect of unsintered gadolinium-doped ceria buffer layer on performance of metal-supported solid oxide fuel cells using unsintered barium strontium cobalt ferrite cathode

In this study, a Gd_0.1Ce_0.9O_1.95 (GDC) buffer layer and a Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ (BSCF) cathode, fabricated without pre-sintering, are investigated (unsintered GDC and unsintered BSCF). The effect of the unsintered GDC buffer layer, including the thickness of the layer, on the performance of solid oxide fuel cells (SOFCs) using an unsintered BSCF cathode is studied. The maximum power density of the metal-supported SOFC using an unsintered BSCF cathode without a buffer layer is 0.81 W cm⁻², which is measured after 2 h of operation (97% H₂ and 3% H₂O at the anode and ambient air at the cathode), and it significantly decreases to 0.63 W cm⁻² after 50 h. At a relatively low temperature of 800 °C, SrZrO₃ and BaZrO₃, arising from interaction between BSCF and yttria-stabilized zirconia (YSZ), are detected after 50 h. Introducing a GDC interlayer between the cathode and electrolyte significantly increases the durability of the cell performance, supporting over 1000 h of cell usage with an unsintered GDC buffer layer. Comparable performance is obtained from the anode-supported cell when using an unsintered BSCF cathode with an unsintered GDC buffer layer (0.75 W cm⁻²) and sintered GDC buffer layer (0.82 W cm⁻²). When a sintered BSCF cathode is used, however, the performance increases to 1.23 W cm⁻². The adhesion between the BSCF cathode and the cell can be enhanced by an unsintered GDC buffer layer, but an increase in the layer thickness (1-6 μm) increases the area specific resistance (ASR) of the cell, and the overly thick buffer layer causes delamination of the BSCF cathode. Finally, the maximum power densities of the metal-supported SOFC using an unsintered BSCF cathode and unsintered GDC buffer layer are 0.78, 0.64, 0.45 and 0.31 W cm⁻² at 850, 800, 750 and 700 °C, respectively.     

A cost-effective process for fabrication of metal-supported solid oxide fuel cells

Metal-supported SOFC cells with Y₂O₃ stabilized ZrO₂ as the electrolyte were prepared by a low cost and simple process involving tape casting, screen printing and co-firing. The interfaces were well bonded after the reduction of NiO to Ni in the support and the anode. AC impedance was employed to estimate the cell polarizations under open circuit conditions. It was found that the electrode polarization resistance was high at low temperatures and became equivalent to the ohmic resistance at higher temperatures near 800°°C. The cell performance was evaluated with H₂ as the fuel and air as the oxidant, and maximum power density between 0.23 and 0.80  W/cm² was achieved in the temperature range of 650–800°C, which confirms the applicability of the cost-effective process in fabrication of metal-supported SOFC cells.     

Direct liquid methanol-fueled solid oxide fuel cell

Anode coking problem of solid oxide fuel cell (SOFC) when using hydrocarbon fuels has been the major barrier for the practice and commercialization of well-developed high performance SOFC. In this work, based on fuels consideration, we chose liquid methanol as the candidate fuel for SOFC with the configuration of NiO/SDC–SDC–SSC/SDC. For comparison, traditional fuels, hydrogen and ammonia, were tested. With methanol as fuel, the maximum power densities were 698, 430 and 223 mW cm⁻² at 650, 600 and 550 °C, respectively, which were higher than that with ammonia and lower than that of hydrogen. The electrochemical properties of the cells with the three fuels were investigated by AC impedance spectroscopy. The long-term stability of the cell with methanol, methane and ethanol were also studied at a constant output voltage of 0.5 V.     

Characterisation of electrical performance of anode supported micro-tubular solid oxide fuel cell with methane fuel

The reduction and operation of Ni–YSZ anode-supported tubular cells on methane fuel is described. Cells were reduced on pure methane from 650 °C to 850 °C, varying reduction time and methane flow rate. The effect on electrochemical performance with methane fuel was then investigated at 850 °C after which temperature-programmed oxidation (TPO) was employed to measure carbon deposition. Results showed that carbon deposition was minimized after certain reduction conditions. The conclusion was that 30 min reduction at 650 °C with 10 ml min⁻¹ methane reduction flow rate led to the highest current output over 1.2 A cm⁻² at 0.5 V when the cell operated at 850 °C between 10 ml min⁻¹ and 12.5 ml min⁻¹ methane running flow rate. From these results, it is evident that solid oxide fuel cell (SOFC) performance can be substantially improved by optimising preparation, reduction and operating conditions without the need for hydrogen.     

A high-performance ammonia-fueled SOFC based on a YSZ thin-film electrolyte

Ammonia is a possible candidate as the fuel for solid oxide fuel cells (SOFCs). In this work, an anode-supported SOFC based on yttrium-stabled zircite (YSZ) thin-film electrolyte was fabricated by a simple dry-pressing process. Directly fueled by commercial liquefied ammonia, the single cell was tested at temperatures from 650 to 850 °C. The maximum power densities were 299 and 526 mW cm⁻² at 750 and 850 °C, respectively, only slightly lower than that fueled by hydrogen. Analysis of open current voltages (OCVs) of the cell indicated the oxidation of ammonia within a SOFC is a two-stage process. Impedance spectra showed the cell fueled by ammonia had the same electrolyte resistances as that fueled by hydrogen, but a little larger interfacial polarization resistances. Further, the performances of the cell were essentially determined by the interfacial resistances under 750 °C.     

The interaction of biomass gasification syngas components with tar in a solid oxide fuel cell and operational conditions to mitigate carbon deposition on nickel-gadolinium doped ceria anodes

The combination of biomass gasification with solid oxide fuel cells (SOFCs) is gaining increasing interest as an efficient and environmentally benign method of producing electricity and heat. However, tars in the gas stream arising from the gasification of biomass material can deposit carbon on the SOFC anode, having detrimental effects to the life cycle and operational characteristics of the fuel cell. This work examines the impact of biomass gasification syngas components combined with benzene as a model tar, on carbon formation on Ni/CGO (gadolinium-doped ceria) SOFC anodes. Thermodynamic calculations suggest that SOFCs operating at temperatures > 750 °C are not susceptible to carbon deposition from a typical biomass gasification syngas containing 15 g m⁻³ benzene.However, intermediate temperature SOFCs operating at temperatures < 650 °C require threshold current densities well above what is technologically achievable to inhibit the effects of carbon deposition. SOFC anodes have been shown to withstand tar levels of 2-15 g m⁻³ benzene at 765 °C for 3 h at a current density of 300 mA cm⁻², with negligible impact on the electrochemical performance of the anode. Furthermore, no carbon could be detected on the anode at this current density when benzene levels were <5 g m⁻³.     

Electrochemical characteristics of solid oxide fuel cell cathodes prepared by infiltrating (La,Sr)MnO3 nanoparticles into yttria-stabilized bismuth oxide backbones

The electrochemical characteristics of the solid oxide fuel cell (SOFC) cathodes prepared by infiltration of (La_0.85Sr_0.15)_0.9MnO_3−δ (LSM) nanoparticles into porous Y_0.5Bi_1.5O₃ (YSB) backbones are investigated in terms of overpotential, interfacial polarization resistance, and single cell performance obtained with three-electrode cell, symmetrical cell, and single cell, respectively. X-ray diffraction confirms the formation of perovskite LSM by heating the infiltrated nitrates at 800 °C. The electrical conductivity of the electrode measured using Van der Pauw method is 1.67 S cm⁻¹, which is acceptable at the typical SOFC operating temperatures. The single cell with the LSM infiltrated YSB cathode generates maximum power densities of 0.23, 0.45, 0.78, and 1.13 W cm⁻² at 600, 650, 700, and 750 °C, respectively. The oxygen reduction mechanism on the cathode is studied by analyzing the impedance spectra obtained under various temperatures and oxygen partial pressures. The impedance spectra under various cathodic current densities are also measured to study the effect of cathodic polarization on the performance of the cathode.     

Solid oxide fuel cell with compositionally graded cathode functional layer deposited by pressure assisted dual-suspension spraying

In this work, the benefit of compositionally grading a cathode functional layer (CFL) for solid oxide fuel cells (SOFCs) is explored. Cells are prepared wherein either a standard cathode functional layer (SCFL) or a linearly compositionally graded cathode functional layer (CGCFL) is placed between the cell electrolyte and cathode current collecting regions. The electrochemical performance of these cells is compared with a SOFC cell containing no CFL. All cells are fabricated using a pressurized dual-suspension spraying system. Electrolytes, cathode functional layer, and cathode current collecting materials are deposited on a powder compacted anode support. SEM and EDAX area maps are taken to study the resulting micro-structures and to verify that the desired CFL profiles are produced. The EDAX area map verifies that a compositionally graded CFL and a SCFL are obtained. The cells are analyzed using impedance spectroscopy to evaluate the electrochemical performances of each cell. The open circuit voltage (OCV) and peak power densities of all three cells are 1.04 V with 80 mW cm⁻², 1.12 V with 108 mW cm⁻², and 1.08 V with 193 mW cm⁻² at 850 °C for the SCFL cell, the cell without a CFL, and the compositionally graded CFL cell respectively. The results show that this approach is a viable means for producing SOFC functional layers with unique composition and interfacial properties.     

High performance metal-supported solid oxide fuel cells fabricated by thermal spray

Metal-supported solid oxide fuel cells (SOFCs) have been fabricated and characterized in this work. The cells consist of porous NiO-SDC as anode, thin SDC as electrolyte, and SSCo as cathode on porous stainless steel substrate. The anode and electrolyte layers were consecutively deposited onto porous metal substrate by thermal spray, using standard industrial thermal spray equipment, operated in an open-air atmosphere. The cathode materials were applied to the as-sprayed half-cells by screen-printing and heat-treated at 800 °C for 2 h. The cell components and performance were examined by scanning electron microscopy (SEM), X-ray diffraction, leakage test, ac impedance and electrochemical polarization at temperatures between 500 and 700 °C. The half-inch button cells exhibit a maximum power density in excess of 0.50 W cm⁻² at 600 °C and 0.92 W cm⁻² at 700 °C operated with humidified hydrogen fuel, respectively. The half-inch button cell was run at 0.5 A cm⁻² at 603 °C for 100 h. The cell voltage decreased from 0.701 to 0.698 V, giving a cell degradation rate of 4.3% kh⁻¹. Impedance analysis indicated that the cell degradation included 4.5% contribution from ohmic loss and 1.4% contribution from electrode polarization. The 5 cm × 5 cm cells were also fabricated under the same conditions and showed a maximum power density of 0.26 W cm⁻² at 600 °C and 0.56 W cm⁻² at 700 °C with dry hydrogen as fuel, respectively. The impedance analysis showed that the ohmic resistance of the cells was the major polarization loss for all the cells, while both ohmic and electrode polarizations were significantly increased when the operating temperature decreased from 700 to 500 °C. This work demonstrated the feasibility for the fabrication of metal-supported SOFCs with relatively high performance using industrially available deposition techniques. Further optimization of the metal support, electrode materials and microstructure, and deposition process is ongoing.     

Low-temperature solid oxide fuel cells with novel La0.6Sr0.4Co0.8Cu0.2O3−δ perovskite cathode and functional graded anode

The perovskite La_0.6Sr_0.4Co_0.8Cu_0.2O_3−δ (LSCCu) oxide is synthesized by a modified Pechini method and examined as a novel cathode material for low-temperature solid oxide fuel cells (LT-SOFCs) based upon functional graded anode. The perovskite LSCCu exhibits excellent ionic and electronic conductivities in the intermediate-to-low-temperature range (400-800 °C). Thin Sm_0.2Ce_0.8O_1.9 (SDC) electrolyte and NiO-SDC anode functional layer are prepared over macroporous anode substrates composed of NiO-SDC by a one-step dry-pressing/co-firing process. A single cell with 20 μm thick SDC electrolyte on a porous anode support and LSCCu-SDC cathode shows peak power densities of only 583.2 mW cm⁻² at 650 °C and 309.4 mW cm⁻² for 550 °C. While a cell with 20 μm thick SDC electrolyte and an anode functional layer on the macroporous anode substrate shows peak power densities of 867.3 and 490.3 mW cm⁻² at 650 and 550 °C, respectively. The dramatic improvement of cell performance is attributed to the much improved anode microstructure that is confirmed by both SEM observation and impedance spectroscopy. The results indicate that LSCCu is a very promising cathode material for LT-SOFCs and the one-step dry-pressing/co-firing process is a suitable technique to fabricate high performance SOFCs.     

Infiltrated multiscale porous cathode for proton-conducting solid oxide fuel cells

A Sm_0.5Sr_0.5CoO_3−δ (SSC)-BaZr_0.1Ce_0.7Y_0.2O_3−δ (BZCY) composite cathode with multiscale porous structure was successfully fabricated through infiltration for proton-conducting solid oxide fuel cells (SOFCs). The multiscale porous SSC catalyst was coated on the BZCY cathode backbones. Single cells with such composite cathode demonstrated peak power densities of 0.289, 0.383, and 0.491 W cm⁻² at 600, 650, 700 °C, respectively. Cell polarization resistances were found to be as low as 0.388, 0.162, and 0.064 Ω cm² at 600, 650 and 700 °C, respectively. Compared with the infiltrated multiscale porous cathode, cells with screen-printed SSC-BZCY composite cathode showed much higher polarization resistance of 0.912 Ω cm² at 600 °C. This work has demonstrated a promising approach in fabricating high performance proton-conducting SOFCs.     

Electrode performance and analysis of reversible solid oxide fuel cells with proton conducting electrolyte of BaCe0.5Zr0.3Y0.2O3−δ

Reversible solid oxide fuel cells (R-SOFCs) are regarded as a promising solution to the discontinuity in electric energy, since they can generate electric powder as solid oxide fuel cells (SOFCs) at the time of electricity shortage, and store the electrical power as solid oxide electrolysis cells (SOECs) at the time of electricity over-plus. In this work, R-SOFCs with thin proton conducting electrolyte films of BaCe_0.5Zr_0.3Y_0.2O_3−δ were fabricated and their electro-performance was characterized with various reacting atmospheres. At 700 °C, the charging current (in SOFC mode) is 251 mA cm⁻² at 0.7 V, and the electrolysis current densities (in SOEC mode) reaches −830 mA cm⁻² at 1.5 V with 50% H₂O-air and H₂ as reacting gases, respectively. Their electrode performance was investigated by impedance spectra in discharging mode (SOFC mode), electrolysis mode (SOEC mode) and open circuit mode (OCV mode). The results show that impedance spectra have different shapes in all the three modes, implying different rate-limiting steps. In SOFC mode, the high frequency resistance (R_H) is 0.07 Ωcm² and low frequency resistances (R_L) are 0.37 Ωcm². While in SOEC mode, R_H is 0.15 Ωcm², twice of that in SOFC mode, and R_L is only 0.07 Ωcm², about 19% of that in SOFC mode. Moreover, the spectra under OCV conditions seems like a combination of those in SOEC mode and SOFC mode, since that R_H in OCV mode is about 0.13 Ωcm², close to R_H in SOEC mode, while R_L in OCV mode is 0.39 Ωcm², close to R_L in SOFC mode. The elementary steps for SOEC with proton conducting electrolyte were proposed to account for this phenomenon.     

Fabrication and characterization of metal-supported solid oxide fuel cells

Metal-supported solid oxide fuel cells (SOFCs) are an acceptable approach to solving the serious problems of SOFC technology, such as sealing and mechanical strength. In this work, commercial stainless-steel plates, STS430, are used as supporting bodies for a metal-supported SOFC in order to decrease the number of fabrication steps. The metal support for a single-cell has a diameter of 28 mm, a thickness of 1 mm, and a channel width of 0.4 mm. A thin ceramic layer, composed of yttria-stabilized zirconia (YSZ) and NiO/YSZ, is attached to the metal support by using a cermet adhesive. La_0.8Sr_0.2Co_0.4Mn_0.6O₃ perovskite oxide serves as the cathode material because of its low impedance on the YSZ electrolyte, according to half-cell tests. The maximum power density of the cell is 0.09 W cm⁻² at 800 °C. The effects of temperature, oxygen partial pressure, and current collection by pastes are investigated. The oxygen reduction reaction at the cathode dominates the overall cell performance, according to experimental and numerical analyses.     

Fabrication and evaluation of Ni-GDC composite anode prepared by aqueous-based tape casting method for low-temperature solid oxide fuel cell

Large-size, 8 cm × 8 cm, NiO-Gd_0.1Ce_0.9O_1.95 (Ni-GDC) composite anodes have been successfully fabricated by aqueous-based tape casting method for anode-supported solid oxide fuel cell (SOFC). The pre-sintered anode green tape was coated with a GDC electrolyte film by spray coating method and then co-sintered together to obtain electrolyte/anode bi-layer. The cathode, which is made of La_0.8Sr_0.2Co_0.2Fe_0.8O₃-GDC (LSCF-GDC) was screen printed onto the electrolyte film and sintered to form a complete anode-supported SOFC. The performance of the cell was evaluated on an in-house developed test station between 500 and 650 °C. Due to the limitation of the test station for large-cell testing, small-size samples with dimensions of 2.5 cm × 2.5 cm were cut out from the large-cell. For the single cell with humidified hydrogen as fuel and air as oxidant, the maximum power density achieved 909, 623, 335 and 168 mW cm⁻² at 650, 600, 550 and 500 °C, respectively. Impedance analysis confirmed that the performance of single cells below 600 °C was retarded primarily due to the slow interfacial reaction kinetics at reduced temperatures. Development of catalytically active electrode materials, especially the cathode material and improvement of the electrode microstructure are thus crucial for achieving a high performance low-temperature SOFC.     

Ba0.5Sr0.5Co0.8Fe0.2O3 − δ + LaCoO3 composite cathode for Sm0.2Ce0.8O1.9-electrolyte based intermediate-temperature solid-oxide fuel cells

A novel Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3 − δ + LaCoO₃ (BSCF + LC) composite oxide was investigated for the potential application as a cathode for intermediate-temperature solid-oxide fuel cells based on a Sm_0.2Ce_0.8O_1.9 (SDC) electrolyte. The LC oxide was added to BSCF cathode in order to improve its electrical conductivity. X-ray diffraction examination demonstrated that the solid-state reaction between LC and BSCF phases occurred at temperatures above 950 °C and formed the final product with the composition: La_0.316Ba_0.342Sr_0.342Co_0.863Fe_0.137O_3 − δ at 1100 °C. The inter-diffusion between BSCF and LC was identified by the environmental scanning electron microscopy and energy dispersive X-ray examination. The electrical conductivity of the BSCF + LC composite oxide increased with increasing calcination temperature, and reached a maximum value of ∼300 S cm⁻¹ at a calcination temperature of 1050 °C, while the electrical conductivity of the pure BSCF was only ∼40 S cm⁻¹. The improved conductivity resulted in attractive cathode performance. An area-specific resistance as low as 0.21 Ω cm² was achieved at 600 °C for the BSCF (70 vol.%) + LC (30 vol.%) composite cathode calcined at 950 °C for 5 h. Peak power densities as high as ∼700 mW cm⁻² at 650 °C and ∼525 mW cm⁻² at 600 °C were reached for the thin-film fuel cells with the optimized cathode composition and calcination temperatures.     

Carbon monoxide-fueled solid oxide fuel cell

This study explored CO as a primary fuel in anode-supported solid oxide fuel cells (SOFCs) of both tubular and planar geometries. Tubular single cells with active areas of 24 cm² generated power up to 16 W. Open circuit voltages for various CO/CO₂ mixture compositions agreed well with the expected values. In flowing dry CO, power densities up to 0.67 W cm⁻² were achieved at 1 A cm⁻² and 850 °C. This performance compared well with 0.74 W cm⁻² measured for pure H₂ in the same cell and under the same operating conditions. Performance stability of tubular cells was investigated by long-term testing in flowing CO during which no carbon deposition was observed. At a constant current of 9.96 A (or, 0.414 A cm⁻²) power output remained unchanged over 375 h of continuous operation at 850 °C. In addition, a 50-cell planar SOFC stack was operated at 800 °C on 95% CO (balance CO₂), which generated 1176 W of total power at a power density of 224 mW cm⁻². The results demonstrate that CO is a viable primary fuel for SOFCs.     

Synthesis,electrical and electrochemical properties of Ba0.5Sr0.5Zn0.2Fe0.8O3−δ perovskite oxide for IT-SOFC cathode

The Ba_0.5Sr_0.5Zn_0.2Fe_0.8O_3−δ (BSZF) complex oxide with cubic perovskite structure was synthesized and examined as a new cobalt-free cathode for intermediate-temperature solid oxide fuel cells (IT-SOFCs). The electrical conductivity was relatively low with a peak value of 9.4 S cm⁻¹ at about 590 °C, which was mainly caused by the high concentration of oxygen vacancy and the doping of bivalent zinc in B-sites. At 650 °C and under open circuit condition, symmetrical BSZF cathode on Sm-doped ceria (SDC) electrolyte showed polarization resistances (R_p) of 0.48 Ω cm² and 0.35 Ω cm² in air and oxygen, respectively. The dependence of R_p with oxygen partial pressure indicated that the rate-limiting step for oxygen reduction was oxygen adsorption/desorption kinetics. Using BSZF as the cathode, the wet hydrogen fueled Ni + SDC anode-supported single cell exhibited peak power densities of 392 mW cm⁻² and 626 mW cm⁻² at 650 °C when stationary air and oxygen flux were used as oxidants, respectively.     

Performance of a novel type of electrolyte-supported solid oxide fuel cell with honeycomb structure

A novel design, alternative to the conventional electrolyte-supported solid oxide fuel cell (SOFC) is presented. In this new design, a honeycomb-electrolyte is fabricated from hexagonal cells, providing high mechanical strength to the whole structure and supporting the thin layer used as electrolyte of a SOFC. This new design allows a reduction of ∼70% of the electrolyte material and it renders modest performances over 320 mW cm⁻² but high volumetric power densities, i.e. 1.22 W cm⁻³ under pure CH₄ at 900 °C, with a high OCV of 1.13 V, using the standard Ni-YSZ cermet as anode, Pt as cathode material and air as the oxidant gas.     

Solid oxide fuel cells with bi-layered electrolyte structure

In this work, we have developed solid oxide fuel cells with a bi-layered electrolyte of 2 μm SSZ and 4 μm SDC using tape casting, screen printing, and co-firing processes. The cell reached power densities of 0.54 W cm⁻² at 650 °C and 0.85 W cm⁻² at 700 °C, with open circuit voltage (OCV) values larger than 1.02 V. The electrical leaking between anode and cathode through an SDC electrolyte has been blocked in the bi-layered electrolyte structure. However, both the electrolyte resistance (R_el) and electrode polarization resistance (R_p,a+c) increased in comparison to cells with single-layered SDC electrolytes. The formation of a solid solution of (Ce, Zr)O_2−x during sintering process and the flaws in the bi-layered electrolyte structure seem to be the main causes for the increase in the R_el value (0.32 Ω cm²) at 650 °C, which is almost one order of magnitude higher than the calculated value.