Direct ammonia solid oxide fuel cell based on thin proton-conducting electrolyte

Thin proton-conducting electrolyte with composition BaCe_0.8Gd_0.2O_3−δ (BCGO) was prepared over substrates composed of Ce_0.8Gd_0.2O_1.9 (CGO)-Ni by the dry-pressing method. Solid oxide fuel cells (SOFCs) were fabricated with the structure Ni-CGO/BCGO/Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ (BSCFO)-CGO. The performance of a single cell was tested at 600 and 650 °C, with ammonia directly used as fuel. The open circuit voltages (OCVs) were 1.12 and 1.1 V at 600 and 650 °C, respectively. The higher OCV may be due to both the compaction of the BCGO electrolyte (no porosity) and complete decomposition of ammonia. The maximum power density was 147 mW cm⁻² at 600 °C. Comparisons of the cell with hydrogen as fuel indicate that ammonia can be treated as a substitute liquid fuel for SOFCs based on a proton-conducting solid electrolyte.     

Electrophoretically deposited thin film electrolyte for solid oxide fuel cell

Abstract

Thin films of 8 mol% yttria stabilised zirconia (YSZ) electrolyte have been deposited on non-conducting porous NiO–YSZ anode substrates using electrophoretic deposition (EPD) technique. Deposition of such oxide particulates on non-conducting substrates is made possible by placing a conducting steel plate on the reverse side of the presintered porous substrates. Thickness of the substrates, onto which the deposition has been carried out, varied in the range 0·5–2·0 mm. Dense and uniform YSZ thin films (thickness: 5–20 μm) are obtained after being cofired at 1400°C for 6 h. The thickness of the deposited films is seemed to be increased with increasing porous substrate thickness. Solid oxide fuel cell (SOFC) performance is measured at 800°C using coupon cells with various anode thicknesses. While a peak power density of 1·41 W cm^?2 for the cells with minimum anode thickness of 0·5 mm is achieved, the cell performance decreases with anode thickness.     

Performance of cone-shaped tubular anode-supported segmented-in-series solid oxide fuel cell stack fabricated by dip coating technique

A simple and feasible technique is developed successfully to fabricate the cone-shaped tubular segmented-in-series solid oxide fuel cell (SOFC) stack. The cone-shaped tubular anode substrates and yttria-stabilized zirconia (YSZ) electrolyte films are fabricated by dip coating technique. After sintering at 1400 °C for 4 h, a dense and crack-free YSZ film with a thickness of about 35.9 μm is successfully obtained. The single cell, NiO–YSZ/YSZ/LSM–YSZ, provides a maximum power density of 1.08 and 1.35 W cm⁻² at 800 and 850 °C, respectively, using moist hydrogen (75 ml/min) as fuel and ambient air as oxidant.A two-cell-stack based on the above-mentioned cone-shaped tubular anode-supported SOFC was assembled and tested. The maximum total power at 800 °C was about 3.7 W.     

Study of slurry spin coating technique parameters for the fabrication of anode-supported YSZ Films for SOFCs

A slurry spin coating method was developed to fabricate gas-tight anode-supported YSZ films for solid oxide fuel cells (SOFCs). Several technique parameters for slurry spin coating, such as the slurry viscosity, spinning speed, number of coating cycles, film thickness and their effects on YSZ electrolyte film were investigated. SEM results, open-circuit voltage (OCV) values and cell performance indicated that these parameters had crucial and obvious influences on YSZ film quality and fuel cell performance. Based on the optimized parameters, anode-supported YSZ films and several single fuel cells were successfully fabricated and tested. An OCV as high as 1.06 V was obtained at 800 °C and maximum power densities of 900, 1567, 2005 mW cm⁻² were achieved at 700, 750, 800 °C, respectively, using hydrogen as fuel and ambient air as oxidant.     

Short review on cobalt-free cathodes for solid oxide fuel cells

Cobalt-containing cathodes are known for their ability to operate under high-temperature applications in solid oxide fuel cells (SOFCs). Reducing the operation temperature into intermediate temperature-to-low temperature (IT-LT) zones may lead to a mismatch in the thermal expansion coefficient between the cathodes and the developed IT-LTSOFC electrolyte materials. Hence, cathode materials are adjusted to resolve this issue. Studies on IT-LTSOFC propose cobalt-free cathodes as an alternative way to produce high electrochemical performance cells for operation within the IT-LT range. Novel cobalt-free cathode powders are developed using perovskite structured materials, such as strontium ferrite oxide, as the main components together with dopants. This paper reviews various studies on cobalt-free cathode development, including the most important parameter in determining cathode performance, namely, the polarization resistance of SOFC cathodes.     

Improved performance of ammonia-fueled solid oxide fuel cell with SSZ thin film electrolyte and Ni-SSZ anode functional layer

Ammonia offers several advantages over hydrogen as an alternative fuel. However, using ammonia as a hydrogen source for fuel cells has not been received enough attention. In present paper, Scandia-stabilized Zirconia (SSZ) thin film electrolyte and Ni-SSZ anode functional layer were developed by tape casting in order to obtain high power output performance in ammonia, the results of a SOFC running on ammonia were described and its performance was compared with that when running on hydrogen. In order to improve the performance of the cell at higher temperatures, the anode was modified by iron through infiltration. A direct comparison of the performance of the modified cell running on either hydrogen or ammonia showed that the cell in ammonia generated slightly higher power densities at 700 and 750 °C. The performance in ammonia, using the anode catalyst, was comparable to that in hydrogen indicating ammonia could be treated as a promising alternative fuel by selecting an appropriate catalyst.     

Experimental optimization of the fabrication parameters for anode-supported micro-tubular solid oxide fuel cells

A systematic optimization of several parameters significant in the fabrication of anode-supported micro-tubular solid oxide fuel cell via extrusion and dip coating is presented in this study. Co-sintering temperature of anode-support and electrolyte, the vehicle type and solid powder content used in electrolyte dip-coating slurry, electrolyte submersion time, cathode sintering temperature, powder ratio in the cathode functional layer, submersion time for the cathode functional layer and, submersion time and coating number of the anode functional layer are studied in this respect and optimized in the given order according to the performance tests and microstructural analyses. The performance of the micro-tubular cell is significantly improved to 0.49 Wcm⁻² at 800 °C after the optimizations, while that of the base cell is only 0.136 Wcm⁻². 12-cell micro-tubular stack is also constructed with the optimized cells and the stack is tested. Each cell in the stack is found to show very close performance to the single-cell performance and the stack with a maximum power of ~26 W at an operating temperature of 800 °C is therefore evaluated to be successful.     

A novel direct carbon fuel cell by approach of tubular solid oxide fuel cells

A direct carbon fuel cell based on a conventional anode-supported tubular solid oxide fuel cell, which consisted of a NiO-YSZ anode support tube, a NiO-ScSZ anode functional layer, a ScSZ electrolyte film, and a LSM-ScSZ cathode, has been successfully achieved. It used the carbon black as fuel and oxygen as the oxidant, and a preliminary examination of the DCFC has been carried out. The cell generated an acceptable performance with the maximum power densities of 104, 75, and 47 mW cm⁻² at 850, 800, and 750 °C, respectively. These results demonstrate the feasibility for carbon directly converting to electricity in tubular solid oxide fuel cells.     

Development of the spinel powder reduction technique for solid oxide fuel cell interconnect coating

Mn_0.9Y_0.1Co₂O₄ spinel coatings are developed for solid oxide fuel cell (SOFC) alloy interconnects by a novel powder reduction technique. Material properties, electrical performance and long-term stability of the coatings are explored. The coating is about 9 μm in thickness and adheres well to the alloy substrate without any cracking or delamination. The area specific resistance (ASR) remains almost unchanged and is less than 3 mΩ cm² even though the coated alloy undergoes oxidation at 800 °C for 1017 h and ten thermal cycles from 800 °C to room temperature. The coated alloy presents excellent electrical performance and long-term stability. It exhibits a promising prospect for the practical application of SOFC alloy interconnect.     

Metallic interconnects for solid oxide fuel cell: A review on protective coating and deposition techniques

With the reduction of solid oxide fuel cells (SOFCs) operating temperature to the range of 600 °C–800 °C, metallic alloy with high oxidation resistance are used to replace traditional ceramic interconnects. Metallic interconnects is advantageous over ceramic interconnects; in terms of manufacturability, cost, mechanical strength, and electrical conductivity. To date, promising candidates for metallic interconnects are all Cr-containing alloys, which are susceptible to volatile Cr migration that causes cell degradation. As such, protective coatings have been developed to effectively inhibit Cr migration; as well as maintain excellent electrical conductivity and good oxidation resistance. This article reviews the progress and technical challenges in developing metallic interconnects; different types of protective coatings and deposition techniques for metallic interconnects for intermediate-temperature SOFC applications.     

A novel fabrication of yttria-stabilized-zirconia dense electrolyte for solid oxide fuel cells by 3D printing technique

Three-dimensional (3D) printing technique represents a revolutionary advancement in the manufacturing sector due to its unique capabilities to process the shape complexity. This work is focusing on dense 8 mol.% yttria-stabilized-zirconia (8YSZ) electrolyte fabrication via digital light processing (DLP)-stereolithography-based 3D printing technique. Multiple 8YSZ electrolyte green bodies are printed simultaneously in a batch using ceramic-resin suspension made of 30 vol% 8YSZ powder loading in a photo-curable resin. Together with an optimized debinding and sintering procedure, the 8YSZ green body changes into a dense electrolyte, and the density of the sintered electrolyte was measured as 99.96% by Archimedes' water displacement method. The symmetric cell fabricated of silver-Ce_0.8Gd_0.2O_1.9 (Ag-GDC) as cathode/anode and dense 8YSZ electrolyte printed by DLP-stereolithography delivers a high open circuit voltage of approximately 1.04 V and a peak power density up to 176 mW·cm⁻² at 850 °C by using hydrogen as the fuel and air as the oxidant. The electrochemical performance of the symmetric cell Ag-GDC|YSZ|Ag-GDC with 8YSZ electrolyte fabricated via DLP-stereolithography is comparable to that of the same cell with 8YSZ electrolyte fabricated by conventional dry pressing method. This 3D printing technique provides a novel method to prepare dense electrolytes for solid oxide fuel cell (SOFC) with good performance, suggesting a potential application for one-step fabrication of complex structure SOFC stack.     

Comparison of electrolyte fabrication techniques on the performance of anode supported solid oxide fuel cells

A comparison of three solid oxide electrolyte fabrication processes, namely dip coating, screen printing and tape casting, for planar anode supported solid oxide fuel cells (SOFCs) is presented in this study. The effect of sintering temperature (1325–1400 °C) is also examined. The anode and cathode layers of the anode-supported cells, on the other hand, are fabricated by tape casting and screen printing, respectively. The quality of the electrolytes is evaluated via performance measurements, impedance analyses and microstructural investigations of the cells. It is found that the density of the electrolyte increases with the sintering temperatures for all fabrication methods studied. The results also show that with the process and fabrication parameters considered in this study, both dip coating and screen printing do not yield a desired dense electrolyte structure as proven by open circuit potentials measured and SEM photos. The cells with tape cast electrolytes, on the other hand, provide the highest performances regardless of the electrolyte sintering and cell operating temperatures. The best peak performance of 0.924 W/cm² is obtained from the cell with tape cast electrolyte sintered at 1400 °C. SEM investigations and measured open circuit potentials reveal that almost fully dense electrolyte layer can be obtained with a tape cast electrolyte particularly sintered at temperatures higher than 1350 °C. Impedance analyses indicate that the main reason behind the significantly higher performances is due to not only increased electrolyte density but a decrease in the interface resistance of the anode functional and electrolyte layer is also responsible. This can be explained by the load applied during the lamination step in the fabrication of the tape cast electrolyte, providing better powder compaction and adhesion.     

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.     

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.     

An analytical model for solid oxide fuel cells with bi-layer electrolyte

A theoretical model for a solid oxide fuel cell (SOFC) with a bi-layer electrolyte is developed and analytical solutions of various important relationships, such as I–V relationship, distribution of oxygen partial pressure in the bi-layer electrolyte, leakage current density etc. are obtained. Based on the assumptions of constant ionic conductivity and reversible electrodes, the model takes into considerations of transports of both ions and electrons in the electrolyte. The modeling results are compared with both experimental data and results from other models in the literature and very good agreements are obtained.     

Sputtered MnCu metallic coating on ferritic stainless steel for solid oxide fuel cell interconnects application

MnCu (Mn:Cu = 1:1, atomic ratio) metallic coatings have been deposited by magnetron sputtering on bare and on 100 h pre-oxidized SUS 430 steel for planar solid oxide fuel cells interconnects application. After oxidation at 800 °C in air, the MnCu coating directly deposited on the bare steel has been thermally converted to (Mn,Cu)₃O₄ spinel with Fe, containing discrete CuO on the outer surface. Nevertheless, the converted (Mn,Cu)₃O₄/CuO layer from the MnCu coating deposited on the pre-oxidized steelis almost free of Fe. A double-layer oxide structure with a main (Mn,Cu)₃O₄ spinel layer atop a Cr-rich oxide layer has been developed on the bare and pre-oxidized steel samples with MnCu coatings after thermal exposure. The outer layer mainly consisted of (Mn,Cu)₃O₄ spinel has not only significantly suppressed Cr outward migration to the scale surface, but also effectively reduced the area specific resistance (ASR) of the scale. The sputtered MnCu metallic coating is a very promising candidate for steel interconnect coating material.     

Performance of anode-supported solid oxide fuel cell using novel ceria electrolyte

The potential of a novel co-doped ceria material Sm_0.075Nd_0.075Ce_0.85O_2−δ as an electrolyte was investigated under fuel cell operating conditions. Conventional colloidal processing was used to deposit a dense layer of Sm_0.075Nd_0.075Ce_0.85O_2−δ (thickness 10 μm) over a porous Ni-gadolinia doped ceria anode. The current-voltage performance of the cell was measured at intermediate temperatures with 90 cm³ min⁻¹ of air and wet hydrogen flowing on cathode and anode sides, respectively. At 650 °C, the maximum power density of the cell reached an exceptionally high value of 1.43 W cm⁻², with an area specific resistance of 0.105 Ω cm². Impedance measurements show that the power density decrease with decrease in temperature is mainly due to the increase in electrode resistance. The results confirm that Sm_0.075Nd_0.075Ce_0.85O_2−δ is a promising alternative electrolyte for intermediate temperature solid oxide fuel cells.     

Ni–Fe + SDC composite as anode material for intermediate temperature solid oxide fuel cell

Composite materials of Sm_0.2Ce_0.8O_1.9 (SDC) with various Ni–Fe alloys were synthesized and evaluated as the anode for intermediate temperature solid oxide fuel cell. The performance of single cells consisting of the Ni–Fe + SDC anode, SDC buffer layer, La_0.8Sr_0.2Ga_0.83Mg_0.17O_2.815 (LSGM) electrolyte, and SrCo_0.8Fe_0.2O_3 − δ (SCF) cathode were measured in the temperature range of 600–800 °C with wet H₂ as fuel. It was found that the anodic overpotentials of the different Fe–Ni compositions at 800 °C were in the following order: Ni_0.8Fe_0.2 < Ni_0.75Fe_0.25 < Ni < Ni_0.7Fe_0.3 < Ni_0.9Fe_0.1 < Ni_0.95Fe_0.05 < Ni_0.33Fe_0.67. The single cell with the Ni_0.8Fe_0.2 + SDC anode exhibited a maximum power density of 1.43 W cm⁻² at 800 °C and 0.62 W cm⁻² at 700 °C. The polarization resistance of the Ni_0.8Fe_0.2 + SDC anode was as low as 0.105 Ω cm² at 800 °C under open circuit condition. A stable performance with essentially negligible increase in anode overpotential was observed during about 160 h operation of the cell with the Ni_0.8Fe_0.2 + SDC anode at 800 °C with a fixed current density of 1875 mA cm⁻². The possible mechanism responsible for the improved electrochemical properties of the composite anodes with the Ni_0.8Fe_0.2 and Ni₀₇₅Fe_0.25 alloys was discussed.     

Fabrication of ceramic films for solid oxide fuel cells via slurry spin coating technique

Thin ceramic films of samaria-doped ceria (SDC) were deposited on green NiO–SDC substrate via a slurry spin coating technique followed by co-firing. The ceramic films as-prepared are homogenous and dense, without cracks and penetrating pinholes, as observed from cross-sectional SEM images. The thicknesses of the ceramic films for one coating run can be adjusted between 0.8 and 9 μm by altering the spin rate. The film thickness (h) is inversely proportional to the logarithm of the spin rate (Log(f)) in the range of 3000–10,000 rpm. The slurry spin coating procedure is largely a competition between the thinning and the drying. Half-cells with both 15 and 25 mm diameters were fabricated. In addition, YSZ electrolyte layer with a thickness of 15 μm was also deposited with a homogeneous and completely dense microstructure by two runs of slurry spin coating.     

Degradation of cathode current-collecting materials for anode-supported flat-tube solid oxide fuel cell

Different types of cathode current-collecting material for anode-supported flat-tube solid oxide fuel cells are fabricated and their electrochemical properties are characterized. Current collection for the cathode is achieved by winding Ag wire and by painting different conductive pastes of Ag–Pd, Pt, La_0.6Sr_0.4CoO₃ (LSCo), and La_0.6Sr_0.4Co_0.2Fe_0.8O₃ (LSCF) on the wire. Cell performance at the initial operation time is in the order of Pt > LSCo > LSCF > Ag–Pd. On the other hand, the performance degradation rate is in the order of LSCo < LSCF < Pt < Ag–Pd. LSCo paste as a cathode current-collector shows the most stable long-term performance of 0.8 V, 300 mA cm⁻² at 750 °C, even under a thermal cycle condition with heating and cooling rates of 150 °C h⁻¹. The performance degradation of the Ag–Pd and Pt pastes is caused by increased polarization resistance due to metal particle sintering. From these results, it is concluded that a cathode current-collector composed of wound silver wire with LSCo paste is useful for anode-supported flat-tube cells as it does not experience any significant degradation during a long operation time.