Development of functionally graded anode supports for microtubular solid oxide fuel cells by tape casting and isostatic pressing

This paper suggests an alternative method to manufacture functionally graded anode supports for microtubular solid oxide fuel cells by employing tape casting and isostatic pressing for the first time in the literature. In this regard, six different anode support strips with various pore former contents are produced by tape casting. Besides the anode supports made from uniform tapes, three-layered anode supports composed of various combination of these tapes are also fabricated by wrapping the corresponding tape(s) of the same total length on a metallic rod followed by isostatic pressing. Microtubular cells are then built on these anode supports by dip coating the other layers and evaluated by microstructural investigations and electrochemical performance tests performed under the same conditions. Porosity measurements of the homogeneous anode supports are also carried out. Microstructural examinations reveal that not only the homogeneous anode supports but porosity graded anode supports can be also successfully manufactured by the suggested method. Electrochemical tests indicate that the performance of the cells with a uniform anode support tends to increase with the anode support porosity up to ∼26% porosity then shows a decreasing trend. The highest maximum performance of 0.645 Wcm⁻² at 800 °C under 0.3 NLmin⁻¹ hydrogen and stationary air, on the other hand, is obtained from the cell with a porosity graded anode support.     

Fabrication of cathode supported solid oxide fuel cell by multi-layer tape casting and co-firing method

La_0.3Sr_0.7FeO_3-δ (LSF)/CeO₂ cathode supported Ce_0.8Sm_0.2O_2-δ (SDC) electrolyte was prepared by a simple multilayer tape casting and co-firing method. SDC electrolyte slurry and LSF/CeO₂ cathode slurry were optimized and the green bi-layer tapes were co-fired at different temperature. Phase characterizations and microstructures of electrolyte and cathode were studied by X-ray diffraction (XRD) and Scan Electronic Microscopy (SEM). No additional phase peak line was observed in electrolyte and cathode support when the sintering temperature lower was than 1400 °C. The electrolytes were extremely dense with the thickness of about 20 μm. The cathode support was porous with electrical conductivity of about 4.21 S/cm at 750 °C. With Ni/SDC as anode, Open Current Voltage and maximum power density reached 0.61 V and 233 mW cm⁻² at 750 °C, respectively.     

Fabrication and evaluation of anode and thin Y2O3-stabilized ZrO2 film by co-tape casting and co-firing technique

Co-tape casting and co-firing of supporting electrode and electrolyte layers could drastically increase productivity and reduce fabrication cost. In this study, Ni-YSZ anode supporting electrode and the YSZ electrolyte with the size of 6.5 cm × 6.5 cm have been successfully fabricated by co-tape casting and co-firing technique. The cell with 1.5 mm anode and 10 μm electrolyte is flat without warping, cracks or delaminations. The power density reaches 661, 856, 1085 mW cm⁻² at 0.7 V and 750, 800 and 850 °C, respectively. The EIS results demonstrate that the cathodic electrochemical resistance is 0.0680 Ω cm², about twice of the anode's which is 0.0359 Ω cm². SEM images show the dense YSZ film had a crack free of surface morphology. The anode and cathode layers are well-adhered to the YSZ electrolyte layer. The La_0.8 Sr_0.2 MnO_3−δ particles do not form a continuous network. Optimization of finer cathodic microstructure and anodic porosity are underway.     

Effect of anode thickness on impedance response of anode-supported solid oxide fuel cells

This study examines effects of the anode functional layer thickness on the performance of anode-supported solid oxide fuel cells (SOFCs). The SOFCs with different AFL thicknesses (8 μm, 19 μm, and 24 μm) exhibit similar power densities at the measured current density range (0–2 A cm⁻²), but show different impedance responses. Further investigation on the spectra using the CNLS fitting method based on DRT-based equivalent circuit model helps us pinpoint two electrochemical processes directly affected by the AFL thickness changes, the charge transfer reaction in the AFL as well as the diffusion-coupled charge transfer reaction in the AFL. The combined effects of these two electrochemical processes probably forged a minimal impact on the overall fuel cell performance by offsetting each other, which offers a reasonable explanation of the seemingly little influence of the AFL thickness on the SOFC performance.     

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

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

Fabrication and characterization of functionally-graded LSCF cathodes by tape casting

Functionally graded cathodes for solid oxide fuel cells are prepared using a tape casting process. The microstructures of the cathodes are gradually changed from a finer LSCF layer with smaller grains (to increase the number of active sites for oxygen reduction) to a coarser LSCF layer with larger grains and higher porosity (for efficient current collection and fast gas transportation). The microstructure and electrochemical properties of the porous electrodes are characterized using scanning electron microscopy and electrochemical impedance spectroscopy, respectively. The cathodic polarization resistance of test cells with functionally-graded LSCF cathodes fired at 1050 °C is reduced to 0.075 Ω cm² at 700 °C and 0.036 Ω cm² at 750 °C, demonstrating peak power densities of 371.5, 744.6, and 1075.3 mW/cm² at 700, 750, and 800 °C, respectively.     

Scalable fabrication process of thin-film solid oxide fuel cells with an anode functional layer design and a sputtered electrolyte

The fabrication process for anode-supported thin-film solid oxide fuel cells (SOFCs) was investigated by using scalable and cost-effective methods. The anode functional layer (AFL) was introduced on the surface of the substrate to stably deposit the thin-film electrolyte. In previous studies, the AFL has been generally designed to increase the catalytic activity; however, in this study, additional design parameters including the roughness and density were controlled to achieve a pinhole-free thin-film electrolyte and structural stability. Through the developed process, button and large-sized cells were fabricated, and the electrochemical performance evaluation showed potential power density and impedance values at relatively low operating temperature. Microstructural analyses showed that each layer of the AFL, electrolyte, and cathode was uniformly coated on the substrate. The thin-film electrolyte was densely deposited without cracks or pinholes. The electrochemical performance and microstructure confirmed that the developed thin-film SOFCs are reliable and reproducible without costly processes or materials.     

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.     

Fabrication and characterization of planar-type SOFC unit cells using the tape-casting/lamination/co-firing method

In this study, solid oxide fuel cells (SOFCs) consisting of a NiO-YSZ anode, a NiO/YSZ-YSZ functional layer, YSZ electrolyte and a (La_0.8Sr_0.2)MnO₃ + yttria-stabilized zirconia (LSM-YSZ) cathode were fabricated by tape-casting, lamination, and a co-firing process. NiO/YSZ-YSZ nano-composite powder was synthesized for the anode functional layer via the Pechini process in order to improve cell performance. After optimization of the slurries for the anode functional anode, electrolyte and cathode, all components were casted so as to fabricate the monolithic laminate. The co-firing temperature was optimized to minimize second phase formation between the (La_0.8Sr_0.2)MnO₃ (LSM) and yttria-stabilized zirconia (YSZ) and to increase the sinterability of the YSZ electrolyte. The YSZ electrolyte was fully sintered with the addition of 0.5 wt% CuO, and the second phases of La₂Zr₂O₇ and SrZrO₃ did not form at 1350 °C. Ni-YSZ anode-supported unit cells were fabricated by co-firing at 1250-1400 °C. The unit cells co-fired at 1250 °C, 1300 °C, 1325 °C, 1350 °C and 1400 °C had maximum power densities of 0.18, 0.18, 0.30, 0.46 and 0.036 W/cm², respectively, in humidified hydrogen (∼3% H₂O) and air at 800 °C.     

Experimental investigation on the effect of anode functional layer on the performance of anode supported micro-tubular SOFCs

In this study, anode supported micro-tubular solid oxide fuel cells (SOFCs) are fabricated by extrusion method and the effects of powder size, thickness and sintering temperature of the anode functional layer (AFL) on the electrochemical performance is experimentally investigated. For this purpose, four different commercial NiO powders are tested as initial powder for the fabrication of the anode functional layer. The thickness of AFL is also considered by varying the number of coatings. After deciding the optimum initial NiO powder size used in AFL and AFL thickness, the effect of pre-sintering temperature is examined. The performance tests are performed at an operating temperature of 800 °C under hydrogen and air. The microstructures of the samples are also investigated by a scanning electron microscope. The best peak power density is obtained as ～0.5 W/cm² from the cell having a single layer anode functional layer pre-sintered at 1250 °C prepared by NiO powders with 4 m²/g surface area.     

Investigation of the electrochemical active thickness of solid oxide fuel cell anode

Determination of the electrochemical active thickness (EAT) is of paramount importance for optimizing the solid oxide fuel cell (SOFC) electrode. However, very different EAT values are reported in the previous literatures. This paper aims to systematically study the EAT of SOFC anode numerically. An SOFC model coupling electrochemical reactions with transport of gas, electron and ion is developed. The microstructure features of the electrode are modeled based on the percolation theory and coordinate number theory. Parametric analysis is performed to examine the effects of various operating conditions and microstructures on EAT. Results indicate that EAT increases with decreasing exchange current density (or decreasing TPB length) and increasing effective ionic conductivity. In addition to the numerical simulations, theoretical analysis is conducted including various losses in the electrode, which clearly shows that the EAT highly depends on the ratio of concentration related activation loss R_act,con to ohmic loss R_ohmic. The theoretical analysis explains very well the different EATs reported in the literature and is different from the common understanding that the EAT is controlled mainly by the ionic conductivity of electrode.     

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.     

Effect of Al and Ce doping on the deformation upon sintering in sequential tape cast layers for solid oxide fuel cells

Water-based tape casting is an attractive production route for planar solid oxide fuel cells (SOFCs) due to its high productivity and reduced environmental issues. In this work planar anode supported SOFCs with thin electrolyte were prepared by water-based sequential tape casting and co-sintering. An in situ high temperature monitoring apparatus was assembled to allow the determination of free sintering shrinkage of thin green tape cast layers and to follow the curvature developed in multilayers during the entire sintering process.The instantaneous curvature developed upon co-sintering was studied as a function of the firing schedule and layer composition. It was found that by tailoring the electrode composition it is possible to reduce the shrinking rate difference between anode and electrolyte thus obtaining defect-free electrolyte, minimising the residual curvature of the half-cell and improving the electrochemical performances of the cell.     

Porous Ni/8YSZ anode of SOFC fabricated by the plasma sprayed method

In the present study, porous electrode coating of Ni/8YSZ on the interconnector material was made by the plasma-spraying. By introducing the pore former into the composite powder, the porous structure of SOFC anode will be obtained. By using the plasma spraying technique for SOFC fabrication, we can avoid the thermal failure between the components of SOFC which made from the traditional sintering method at high temperature. In this study, two kinds of composite powders in the granulate form were prepared, one with the nano carbon as a pore former and the other without the carbon. The results showed that the porous structure of SOFC anode could be achieved by the plasma spraying technique. The porosity of the anode made from the composite powder with pore former was 40%. Without pore former the porosity in the anode coating after hydrogen reduction was almost 30%. These results suggest that this method exhibits the potential to manufacture the porous ceramic/metal composite anode of SOFC to achieve the larger triple phase boundary for fuel oxidation.     

Improvement of output performance of solid oxide fuel cell by optimizing Ni/samaria-doped ceria anode functional layer

Anode functional layers (AFLs) were fabricated using slurry spin coating method on anode substrates to improve the performance of cells based on samaria-doped ceria (SDC) films. The effects of the chemical compositions of AFL and AFL thickness on the performance of solid oxide fuel cell anodes were investigated by studying their effect on the ohmic loss, electrode overpotential, and output performance of cells in different atmospheres. With humidified hydrogen used as fuel and oxygen as oxidant, the cell with an 8-μm-thick AFL (NiO:SDC = 6:4) exhibited excellent maximum power densities of 3.41, 2.89, 1.46 and 0.80 W cm⁻² at 650, 600, 550 and 500 °C, respectively.     

The properties and performance of micro-tubular (less than 2.0 mm O.D.) anode suported solid oxide fuel cell (SOFC)

Tubular solid oxide fuel cells (SOFCs) have many desirable advantages compared to other SOFC applications. Recently, micro-tubular SOFCs were studied to apply them into APU systems for future vehicles. In this study, electrochemical properties of the micro-tubular SOFCs (1.6 mm O.D.) have been characterized. Electrochemical analysis showed excellent performance with a maximum power density of 1.3 W/cm² at 550 °C. The impedance information gained at cell operating temperatures of 450, 500, and 550 °C showed individual cell ohmic resistances of 1.0, 0.6, and 0.2 Ω respectively. Within the operating temperature range of 450-550 °C, the ceria based micro-tubular SOFCs (cathode length: 8 mm) were found to have power densities ranging between 0.263 and 1.310 W/cm². The mechanical properties of the tubes were also analyzed through internal burst testing and monotonic compressive loading on a c-ring test specimen. The two testing techniques are compared and related, and maximum hoop stress values are reported for each of the fabrication parameters. This study showed feasible electrochemical properties and mechanical strength of micro-tubular SOFC for APU applications.     

Tape casting coupled with isostatic pressing as an alternative fabrication method for microtubular solid oxide fuel cells

A new production technique consisting mainly of a combination of tape casting and isostatic pressing to fabricate microtubular supports for solid oxide fuel cells is presented in this study. For this purpose, thin anode support layer is obtained by tape casting. The tape is then wrapped around a rod and subjected to isostatic pressing. The anode support microtube laminate is sintered after the removal of the rod. Microstructural observations show that the anode support with the suggested method is free of delamination and structural defect. Similar microtubular supports are also fabricated by conventional extrusion to compare the mechanical performance. Three point bending test results indicate that the anode supports with the suggested method provide higher mechanical strength due to improved compaction by isostatic pressing. Furthermore, similar microtubular cells are constructed on both anode supports for the electrochemical considerations. The results reveal that the cell, whose anode support is manufactured via tape casting and isostatic pressing, provides a reasonable electrochemical performance although no optimization is carried out in the fabrication steps. Therefore, the method recommended in this study is found to be an appropriate method for the fabrication of tubular/microtubular supports in solid oxide fuel cells or in similar areas.     

A versatile salt evaporation reactor system for SOFC operando studies on anode contamination and degradation with impedance spectroscopy

The dependence of the degradation kinetics in Ni-CGO (cerium-gadolinium oxide) solid oxide fuel cell (SOFC) anodes upon salt evaporation is demonstrated operando with a custom built versatile reactor system. The system is based on evaporation and subsequent condensation of low concentration salt vapor aerosol mixtures representative of salt vapors typically present in biomass gasification processes. Fast changes in the charge transfer and ohmic resistance are observed in the anodes fuelled with a gas mixture containing a high KCl vapor concentration. Rapid condensation of salt vapors into the porous anode and partial delamination of the anode from the electrolyte surface because of salt deposits inside the porous anode is observed. The flexibility to produce vapor-aerosol mixtures with different concentrations and particle size distributions is proved, and suitability of these aerosols for anode testing in long term fuel cell test is evaluated.     

CeO2-based thin-film electrolyte membranes for intermediate temperature SOFCs: Direct electrophoretic deposition on the supporting anode from additive-modified suspensions

In this work, direct electrophoretic deposition (EPD) of thin-film Ce_0.8Sm_0.2O_1.9 (SDC) electrolyte on porous nonconductive NiO–BaCe_0.8Sm_0.2O₃ (BCS) and NiO-SDC substrates is studied. To improve the electrolyte sintering, the suspensions for EPD have been modified by the addition of Co₃O₄, TiO₂, and Al₂O₃ oxides in the amount of 2, 2 and 5 mol. %, respectively. The high zeta potential values, necessary for the stable deposition, have been achieved by the introduction into the suspensions a nanosized SDC powder (10 wt %), obtained by laser evaporation. Dense composite electrolyte membranes up to 30 μm in thickness have been obtained after sintering at 1450 °C for 5 h. The influence of the sintering additives on the electrical properties of the films are studied. In SOFC mode, the effects of increasing the open circuit voltage (OCV) (1.06–0.92 V at the temperatures of 650–750 °C) are demonstrated as a consequence of the formation of a Ba-rich phase caused by the diffusion from the NiO-BCS substrate during sintering, which blocks the electron leakage current in the main SDC electrolyte. At 1450 °C, complete densification of Co-modified SDC films on the NiO-SDC anode substrate does not occur. Therefore, the sintering properties are influenced by both the Ba diffusion and the sintering additives.