Reversible operation performance of microtubular solid oxide cells with a nickelate-based oxygen electrode

This paper describes the reversible operation of a highly efficient microtubular solid oxide cell (SOC) with a nickelate-based oxygen electrode. The fuel cell was composed of a microtubular support of nickel and yttria stabilized zirconia (Ni-YSZ), an YSZ dense electrolyte, and a double oxygen electrode formed by a first composite layer of praseodymium nickelate (PNO) and gadolinium-doped ceria (CGO) and a second one of PNO. A good performance of the cell was obtained at temperatures up to 800 °C for both fuel cell (SOFC) and electrolysis (SOEC) operation modes, specially promising in electrolysis mode. The current density in SOEC mode at 800 °C is about −980 mA cm⁻² at 1.2V with 50% steam. Current density versus voltage curves (j-V) present a linear behavior in the electrolysis mode, with a specific cell area resistance (ASR) of 0.32 Ω cm⁻². Durability experiments were carried out switching the voltage from 0.7V to 1.2V. No apparent degradation was observed in fuel cell mode and SOEC mode up to a period of about 100 h. However, after this period especially in electrolysis mode there is an accumulated degradation associated to nickel coarsening, as confirmed by SEM and EIS experiments. Those results confirm that nickelate based oxygen electrodes are excellent candidates for reversible SOCs.     

Development of LSM-based cathodes for solid oxide fuel cells based on YSZ films

In an attempt to achieve desirable cell performance, the effects of La_0.7Sr_0.3MnO₃ (LSM)-based cathodes on the anode-supported solid oxide fuel cells (SOFCs) were investigated in the present study. Three types of cathodes were fabricated on the anode-supported yttria-stabilized zirconia (YSZ) thin films to constitute several single cells, i.e., pure LSM cathode, LSM/YSZ composite by solid mixing, LSM/Sm_0.2Ce_0.8O_1.9 (SDC) composite by the ion-impregnation process. Among the three single cells, the highest cell output performance 1.25 W cm⁻² at 800 °C, was achieved by the cell using LSM/SDC cathode when the cathode was exposed to the stationary air. Whereas, the most considerable cell performance of 2.32 W cm⁻² was derived from the cell with LSM/YSZ cathode, using 100 ml min⁻¹ oxygen flow as the oxidant. At reduced temperatures down to 700 °C, the LSM/SDC cathode was the most suitable cathode for zirconia-based electrolyte SOFC in the present study. The variation in the cell performances was attributed to the mutual effects between the gas diffusing rate and three-phase boundary length of the cathode.     

Energy efficiency of a microtubular solid-oxide fuel cell

Energy efficiency of a single solid-oxide fuel cell (SOFC) has been studied in various experimental conditions and directly correlated to the character of the cell. A microtubular cell, which has the exact geometry for stack/module application, is used for this study. The energy efficiency simply improves by increasing the operating temperature, and reaches over 40% (lower heating value: LHV) at the operating temperature of over 700 °C in flowing 20% H₂-Ar fuel inside the tube. Impedance analysis has shown that the gas transport is the limiting factor for improving the energy efficiency at lower operating temperatures.     

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.     

Electrolyte degradation in anode supported microtubular yttria stabilized zirconia-based solid oxide steam electrolysis cells at high voltages of operation

Degradation of solid oxide electrolysis cells (SOECs) is probably the biggest concern in the field of high temperature steam electrolysis (HTSE). Anode supported, YSZ-based microtubular solid oxide fuel cells (SOFC) have been tested in fuel cell mode and also at high voltages (up to 2.8 V) under electrolysis mode. At high steam conversion rates the cell voltage tends to saturate. Our hypothesis is that this effect is caused by the electroreduction of the thin YSZ electrolyte which induces electronic conduction losses. YSZ reduction increases the cathode activity and reduces cathode overpotential. Operation of the cell in severe electrolyte reduction conditions induces irreversible damage at the YSZ electrolyte as observed in SEM experiments by the formation of voids at the grain boundaries of the dense YSZ electrolyte. Evidence of this damage was also given by the increase of the ohmic resistance measured by AC impedance. Signs of electrolyte degradation were also found by both EDX analysis and micro-Raman spectroscopy performed along a transverse-cross section of the cell. The observed oxygen electrode delamination is associated to the high oxygen partial pressures gradients that take place at the electrolyte/oxygen electrode interface.     

Fabrication and optimization of LSM infiltrated cathode electrode for anode supported microtubular solid oxide fuel cells

In this study, anode supported microtubular solid oxide fuel cells (SOFCs) with LSM (lanthanum strontium manganite) catalyst infiltrated LSM-YSZ (yttria stabilized zirconia) cathodes are developed to increase the density of triple/three phase boundaries (TPBs) in the cathode, thereby to improve the cell performance. For this purpose, two different porous YSZ layers are formed on the dense YSZ electrolyte, i.e., one is with co-sintering while the other one is not. Incorporation of LSM into these porous YSZ layers is achieved via dip coating of a sol-gel based infiltration solution. The effects of the fabrication method for porous YSZ, LSM solution dwelling time and the thickness of the porous YSZ layer on the cell performance are experimentally investigated and optimized in the given order. A reference cell having a conventional dip coated cathode prepared by mixing the commercial LSM and YSZ powders is also fabricated for comparison. The results show that among the cases considered, the highest peak power density of 0.828 W/cm² can be obtained from the cell, whose single dip coated porous electrolyte layer co-sintered with the dense electrolyte is impregnated with LSM for a dwelling time of 45 min. On the other hand, the peak power density of the reference cell is measured as only 0.558 W/cm². These results reveal that ～50% increase in the maximum cell performance compared to that of the reference cell can be achieved by LSM infiltration after the optimizations.     

Cu-YSZ cermet solid oxide fuel cell anode prepared by high-temperature sintering

Porous YSZ-Cu alloy cermet structures are prepared by sintering above the metal melting point in reducing atmosphere. Unexpectedly good wetting of the molten metal within the YSZ network is obtained, resulting in cermets with fine structure and excellent electronic conductivity. Anode-supported solid oxide fuel cells are prepared with YSZ-Cu alloy cermet as the anode. Addition of infiltrated ceria catalyst improved the initial performance. Maximum power density of about 275 mA cm⁻² and operation for about 110 h was achieved in the 700-800 °C range. After operation, AC impedance revealed that the high-frequency impedance was unchanged, whereas the low-frequency impedance increased. It was concluded that the Cu alloy network conductivity remains high, but catalyst stability needs improvement.     

Direct ceramic inkjet printing of yttria-stabilized zirconia electrolyte layers for anode-supported solid oxide fuel cells

Electromagnetic drop-on-demand direct ceramic inkjet printing (EM/DCIJP) was employed to fabricate dense yttria-stabilized zirconia (YSZ) electrolyte layers on a porous NiO-YSZ anode support from ceramic suspensions. Printing parameters including pressure, nozzle opening time and droplet overlapping were studied in order to optimize the surface quality of the YSZ coating. It was found that moderate overlapping and multiple coatings produce the desired membrane quality. A single fuel cell with a NiO-YSZ/YSZ (∼6 μm)/LSM + YSZ/LSM architecture was successfully prepared. The cell was tested using humidified hydrogen as the fuel and ambient air as the oxidant. The cell provided a power density of 170 mW cm⁻² at 800 °C. Scanning electron microscopy (SEM) revealed a highly coherent dense YSZ electrolyte layer with no open porosity. These results suggest that the EM/DCIJP inkjet printing technique can be successfully implemented to fabricate electrolyte coatings for SOFC thinner than 10 μm and comparable in quality to those fabricated by more conventional ceramic processing methods.     

Ink-jet printing of electrolyte and anode functional layer for solid oxide fuel cells

In this work, solid oxide fuel cells were fabricated by ink-jet printing. The cells were characterized in order to study the resulting microstructure and electrochemical performance. Scanning electron microscopy revealed a highly conformal 6–12 μm thick dense yttria-stabilized zirconia electrolyte layer, and a porous anode-interlayer. Open circuit voltages ranged from 0.95 to 1.06 V, and a maximum power density of 0.175 W cm⁻² was achieved at 750 °C. These results suggest that the ink-jet printing technique may be used to fabricate stable SOFC structures that are comparable to those fabricated by more conventional ceramics processing methods. This study also highlights the significance of overall cell microstructural impact on cell performance and stability.     

Microstructure and electrochemical characterization of solid oxide fuel cells fabricated by co-tape casting

A co-tape casting technique was applied to fabricate electrolyte/anode for solid oxide fuel cells. YSZ and NiO-YSZ powders are raw materials for electrolyte and anode, respectively. Through adjusting the Polyvinyl Butyral (PVB) amount in slurry, the co-sintering temperature for electrolyte/anode could be dropped. After being co-sintered at 1400 °C for 5 h, the half-cells with dense electrolytes and large three phase boundaries were obtained. The improved unit cell exhibited a maximum power density of 589 mW cm⁻² at 800 °C. At the voltage of 0.7 V, the current densities of the cell reached 667 mA cm⁻². When the electrolyte and the anode were cast within one step and sintered together at 1250 °C for 5 h and the thickness of electrolyte was controlled exactly at 20 μm, the open-circuit voltage (OCV) of the cell could reach 1.11 V at 800 °C and the maximum power densities were 739, 950 and 1222 mW cm⁻² at 750, 800 and 850 °C, respectively, with H₂ as the fuel under a flow rate of 50 sccm and the cathode exposed to the stationary air. Under the voltage of 0.7 V, the current densities of cell were 875, 1126 and 1501 mA cm⁻², respectively. These are attributed to the large anode three phase boundaries and uniform electrolyte obtained under the lower sintering temperature. The electrochemical characteristics of the cells were investigated and discussed.     

An ultra-fast fabrication technique for anode support solid oxide fuel cells by microwave

An effective and facile technique has been developed for high temperature anode-electrolyte co-sintering of anode support solid oxide fuel cells by using microwave activated sparking plasma. A high sintering temperature of 1600 °C can be achieved in a few minutes time by discharging effect. Anode support substrate pellet is uniaxially pressed, and then dip-coated with a 10 μm yttria stabilized zirconia electrolyte layer. After the microwave co-sintering, La_0.8Sr_0.2MnO_x cathode is screen-printed onto electrolyte and sintered by conventional thermal method. The cell has stably operated in 3% humidified hydrogen for more than 130 h.     

The effect of electrode infiltration on the performance of tubular solid oxide fuel cells under electrolysis and fuel cell modes

The electrochemical performance of two different anode supported tubular cells (50:50 wt% NiO:YSZ (yttria stabilized zirconia) or 34:66 vol.% Ni:YSZ) as the fuel electrode and YSZ as the electrolyte) under SOFC (solid oxide fuel cell) and SOEC (solid oxide electrolysis cell) modes were studied in this research. LSM (La_0.80Sr_0.20MnO_3−δ) was infiltrated into a thin porous YSZ layer to form the oxygen electrode of both cells and, in addition, SDC (Sm_0.2Ce_0.8O_1.9) was infiltrated into the fuel electrode of one of the cells. The microstructure of the infiltrated fuel cells showed a suitable distribution of fine LSM and SDC particles (50–100 nm) near the interface of electrodes and electrolyte and throughout the bulk of the electrodes. The results show that SDC infiltration not only enhances the electrochemical reaction in SOFC mode but improves the performance even more in SOEC mode. In addition, LSM infiltrated electrodes also boost the SOEC performance in comparison with standard LSM–YSZ composite electrodes, due to the well-dispersed LSM nanoparticles (favouring the electrochemical reactions) within the YSZ porous matrix.     

An all-in-one flourite-based symmetrical solid oxide fuel cell

A novel concept of solid oxide fuel cell (SOFC), the symmetrical SOFC, that uses simultaneously the same material as both anode and cathode has been investigated. Common materials typically used as anode components such as a combination of YSZ and CeO₂ plus a noble metal may be considered good candidates for such a configuration at relatively high temperatures (i.e. above 900 °C). These symmetrical electrodes exhibit enhanced electrochemical properties under both reducing and oxidising conditions, in part due to the catalytic properties of the noble metal used. In air the polarisation values are improved by a factor of four compared to electrodes without CeO₂, whereas under reducing conditions an improvement of two–three orders of magnitude has been observed. The best results correspond to cermets containing 50–60% of CeO₂.     

Performance of an anode support solid oxide fuel cell manufactured by microwave sintering

An effective and facile method has been developed to manufacture anode support solid oxide fuel cells in a multimode domestic microwave oven with selective susceptors. Anode support substrate pellets are prepared by an uniaxial pressing method, and then a thin YSZ electrolyte film is coated by a spray coating method. The electrolyte thickness is kept less than 10 m. The anode supported electrolyte is co-sintered being sandwiched by two spacers and two susceptors in the microwave oven. A cathode is then screen-printed onto the sintered dense electrolyte film and sintered again in the microwave oven with only one spacer and one susceptor. The whole solid oxide fuel cell is sintered at lower temperatures compared to conventional thermal sintering temperature. The performance of the present solid oxide fuel cell is measured in an intermediate temperature range of 650–800 °C. The maximum power densities of 0.09, 0.12, 0.2 and 0.26 W cm⁻² are obtained at operating temperatures of 650, 700, 750 and 800 °C, respectively.     

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.     

On the single chamber solid oxide fuel cells

Single chamber solid oxide fuel cells, SC-SOFCs, design and performance is discussed. It is shown that all of them operate on non-selective anodes. They operate on cathodes that become selective only under short residence times. As a result these cells are not functioning as true mixed reactant solid oxide fuel cells (MR-SOFC). The lack of selectivity is a serious draw back. True MR-SOFC can be constructed in ways that make them cheaper in fabrication, providing high power density, high fuel utilization and reduced explosion risk. The fact that SOFCs operate in a single cell is a necessary but not sufficient condition for the proper operation of MR-SOFCs.     

Ni infiltrated YSZ anode stabilization by inducing strong metal support interaction between nickel and titania in solid oxide fuel cell under accelerated testing

Sintering of Ni particles in Ni infiltrated porous YSZ anodes and decrease in triple phase boundary is the reason for performance loss in SOFC. In the present work, the idea of strong metal support interaction (SMSI) has been used to prevent the sintering of Ni particles by introducing TiO₂ as support with Ni catalyst. Electrical conductivity variation of porous YSZ matrix impregnated with Ni and Ni/TiO₂ have been investigated. Single button cells (anode supported) with and without TiO₂ impregnated Ni–YSZ anode were fabricated and characterized through current–voltage measurement at different loads. It is shown that the conductivity of porous Ni–YSZ anode and the performance of SOFC button cell with the same anode decreased with the increase in temperature and redox cycling at different time intervals. The power density of 12% Ni–YSZ anode was 116 mW/cm² and it increased to 180 mW/cm² for 12% Ni–4% TiO₂–YSZ based anodes at 800 °C. This increase was interpreted by strong attachment of Ni particles on TiO₂ preventing Ni coarsening during prolonged reduction in H₂ at 800 °C as observed by SEM. The power density increased with further increase in Ni loading and it reached to 400 mW/cm² for 16% Ni–4% TiO₂–YSZ based anodes. The performance increases with addition of TiO₂ support in Ni–YSZ based anodes corroborates with the impedance spectroscopy analyses.     

Low temperature anode-supported solid oxide fuel cells based on gadolinium doped ceria electrolytes

The utilization of anode-supported electrolytes is a useful strategy to increase the electrical properties of the solid oxide fuel cells, because it is possible to decrease considerably the thickness of the electrolytes. We have successfully prepared single-chamber fuel cells of gadolinium doped ceria electrolytes Ce_1−xGd_xO_2−y (CGO) supported on an anode formed by a cermet of NiO/CGO. Mixtures of precursor powders of NiO and gadolinium doped ceria with different particle sizes and compositions were analysed to obtain optimal bulk porous anodes to be used as anode-supported fuel cells. Doped ceria electrolytes were prepared by sol–gel related techniques. Then, ceria-based electrolytes were deposited by dip coating at different thicknesses (15–30 μm) using an ink prepared with nanometric powders of electrolytes dispersed in a liquid polymer. Cathodes of La_1−xSr_xCoO₃ (LSCO) were also prepared by sol–gel related techniques and were deposited on the electrolyte thick films. Finally, electrical properties were determined in a single-chamber reactor where propane, as fuel, was mixed with synthetic air below the direct combustion limit. Stable density currents were obtained in these experimental conditions. Flux rate values of the carrier gas and propane partial pressure were determinants for the optimization of the electrical properties of the fuel cells.     

Novel fabrication technique of hollow fibre support for micro-tubular solid oxide fuel cells

In this work, a cerium-gadolinium oxide (CGO)/nickel (Ni)-CGO hollow fibre (HF) for micro-tubular solid oxide fuel cells (SOFCs), which consists of a fully gas-tight outer electrolyte layer supported on a porous inner composite anode layer, has been developed via a novel single-step co-extrusion/co-sintering technique, followed by an easy reduction process. After depositing a multi-layers cathode layer and applying current collectors on both anode and cathode, a micro-tubular SOFC is developed with the maximum power densities of 440-1000 W m⁻² at 450-580 °C. Efforts have been made in enhancing the performance of the cell by reducing the co-sintering temperature and improving the cathode layer and current collection from inner (anode) wall. The improved cell produces maximum power densities of 3400-6800 W m⁻² at 550-600 °C, almost fivefold higher than the previous cell. Further improvement has been carried out by reducing thickness of the electrolyte layer. Uniform and defect-free outer electrolyte layer as thin as 10 μm can be achieved when the extrusion rate of the outer layer is controlled. The highest power output of 11,100 W m⁻² is obtained for the cell of 10 μm electrolyte layer at 600 °C. This result further highlights the potential of co-extrusion technique in producing high quality dual-layer HF support for micro-tubular SOFC.     

Effects of pore former type on mechanical and electrochemical performance of anode support microtubes in solid oxide fuel cells

The effects of pore formers added into the extrusion slurry of anode support microtubes on the mechanical and electrochemical performance of the microtubes are investigated in this study. For this purpose, several microtubular anode supports are fabricated by using various pore formers with different particle sizes. The effect of pore former content is also taken into consideration for a certain pore former type. The flexural strengths of the anode support microtubes are measured via three point bending tests and reliability analysis is performed. The porosities of the anode supports are also determined along with microstructural investigations. The results reveal that the flexural strength decreases with increasing the particle size of the pore former employed for a fixed pore former content and with increasing the pore former content for a certain pore former material considered. In addition, a number microtubular cells are fabricated based on the various microtubular anode supports and their electrochemical performances are evaluated via performance and impedance tests. The impedance results indicate that the cell performance is mainly restricted by the diffusion polarization. Among the pore former materials considered in this study, the highest cell performance for a certain pore former content of 20 vol% is measured from the cell prepared with graphite (325 mesh) pore former at all temperatures and hydrogen flowrates studied. The optimization studies display that the cell performance can be further improved by increasing the pore former content to 22.5 vol% for this pore former material.