Nanoporous nickel thin film anode optimization for low-temperature solid oxide fuel cells

Thin film solid oxide fuel cells (TF–SOFCs) having anode–substrate nanostructure that was optimized for the low-temperature operation were fabricated. Nickel thin film anodes with four different anode thicknesses were deposited on anodic aluminum oxide templates, nanoporous substrates having two different pore sizes, by the sputtering method. Subsequently, a yttria-stabilized zirconia (YSZ) electrolyte and platinum cathode were deposited on them, which completed the entire fuel cell structure. The anode nanostructure of fuel cells in six combinations was analyzed by the cross-sectional view, surface microscopy method, and three-dimensional morphology observation. Those investigations enabled the anode nanostructure to be identified, such as the anode porosity and the roughness of the interface between anodes and electrolytes. Then, the six TF–SOFCs were electrochemically characterized in a 500 °C operating environment. The maximum power densities were obtained through the i–V–P curves, and the highest performance of 294.1 mW/cm² was measured in the cell having a combination of 200 nm–sized porous aluminum anodic oxide (AAO) and 1200 nm–thick Ni anode. This showed up to 20.1% improvement over the other cells. EIS analysis showed that the optimized ohmic and faradaic resistance originated from each part of the unique TF–SOFC structure.     

Dependence of polarization in anode-supported solid oxide fuel cells on various cell parameters

The performance of solid oxide fuel cells (SOFCs) is affected by various polarization losses, namely, ohmic polarization, activation polarization and concentration polarization. Under given operating conditions, these polarization losses are largely dependent on cell materials, electrode microstructures, and cell geometric parameters. Solid oxide fuel cells (SOFC) with yttria-stabilized zirconia (YSZ) electrolyte, Ni–YSZ anode support, Ni–YSZ anode interlayer, strontium doped lanthanum manganate (LSM)–YSZ cathode interlayer, and LSM current collector, were fabricated. The effect of various parameters on cell performance was evaluated. The parameters investigated were: (1) YSZ electrolyte thickness, (2) cathode interlayer thickness, (3) anode support thickness, and (4) anode support porosity. Cells were tested over a range of temperatures between 600 and 800 °C with hydrogen as fuel, and air as oxidant. Ohmic contribution was determined using the current interruption technique. The effect of these cell parameters on ohmic polarization and on cell performance was experimentally measured. Dependence of cell performance on various parameters was rationalized on the basis of a simple analytical model. Based on the results of the cell parameter study, a cell with optimized parameters was fabricated and tested. The corresponding maximum power density at 800 °C was ∼1.8 W cm⁻².     

A general electrolyte–electrode-assembly model for the performance characteristics of planar anode-supported solid oxide fuel cells

A general electrode–electrolyte-assembly (EEA) model has been developed, which is valid for different designs of solid oxide fuel cells (SOFCs) operating at different temperatures. In this study, it is applied to analyze the performance characteristics of planar anode-supported SOFCs. One of the novel features of the present model is its treatment of electrodes. An electrode in the present model is composed of two distinct layers referred to as the backing layer and the reaction zone layer. The other important feature of the present model is its flexibility in fuel, having taking into account the reforming and water–gas shift reactions in the anode. The coupled governing equations of species, charge and energy along with the constitutive equations in different layers of the cell are solved using finite volume method. The model can predict all forms of overpotentials and the predicted concentration overpotential is validated with measured data available in literature. It is found that in an anode-supported SOFC, the cathode overpotential is still the largest cell potential loss mechanism, followed by the anode overpotential at low current densities; however, the anode overpotential becomes dominant at high current densities. The cathode and electrolyte overpotentials are not negligible even though their thicknesses are negligible relative to the anode thickness. Even at low fuel utilizations, the anode concentration overpotential becomes significant when chemical reactions (reforming and water–gas shift) in the anode are not considered. A parametric study has also been carried out to examine the effect of various key operating and design parameters on the performance of an anode-supported planar SOFCs.     

Semiconductor electrolyte for low-operating-temperature solid oxide fuel cell: Li-doped ZnO

Semiconductors have been successfully demonstrated as the electrolytes for solid oxide fuel cells (SOFCs) in recent years. Many such semiconductors have shown their potentials as a competent ionic conductor for fuel cell operation, indicated by the appreciable ionic conduction and electrochemical performance. In the present study, we depart from traditional electrolyte concept to introduce a new semiconductor electrolyte, Li-doped ZnO to low-operating-temperature SOFCs. The used material was synthesized via a co-precipitation method and investigated from phase structure, morphology and UV–vis absorption perspectives. Utilizing Li-doped ZnO as electrolyte layer, we found the corresponding fuel cell exhibited a remarkable maximum power density of 443 mW cm^?2 along with open circuit voltage (OCV) of 1.07 V at 550 °C, and represented a lower-temperature operation feasibility with power outputs of 138–165 mW cm^?2 at 425–450 °C. Besides, high ionic conductivities of 0.028–0.087 S cm^?1 and low activation energy of 0.5 eV were also found in the synthesized Li-doped ZnO at 425–550 °C. Our investigation in terms of electrochemical impedance spectra (EIS) analysis manifested that Li-doped ZnO as the electrolyte layer boosted the electrode reactions of the device, which resulted in rather small polarization resistances and eventually realized good low-temperature performances. Further study based on the rectification characteristic of Ni/Li-doped ZnO contact verified the Schottky junction formation of Li-doped ZnO with anodic Ni, which can avoid the underlying electronic short-circuiting problem. These findings show a profound significance of using doped semiconductor for the future exploitation of SOFC electrolytes.     

Porous nickel-iron alloys as anode support for intermediate temperature solid oxide fuel cells: II. Cell performance and stability

Porous nickel–iron alloy supported solid oxide fuel cells (SOFCs) are fabricated through cost-effective ceramic process including tape casting, screen printing and co-sintering. The cell performance is characterized with humidified hydrogen as the fuel and flowing air as the oxidant. Effects of iron content on the cell performance and stability under redox and thermal cycle are investigated from the point of view of structural stability. Single cells supported by nickel and nickel–iron alloy (50 wt % iron) present relatively high discharge performance, and the maximum power density measured at 800 °C is 1.52 and 1.30 W cm^?2 respectively. Nickel supported SOFC shows better thermal stability between 200 and 750 °C due to its dimensional stable substrate under thermal cycles. Posttest analysis shows that a dense iron oxide layer formed on the surface of the nickel-iron alloy during the early stage of oxidation, which prevents the further oxidation of the substrate as well as the functional anode layer, and thus, making nickel-iron supported SOFC exhibits better redox stability at 750 °C. Adding 0.5 wt % magnesium oxide into the nickel-iron alloy (50 wt% iron) can inhibit the metal sintering and reduce the linear shrinkage, making the single cell exhibit promising thermal stability.     

A review on recent status and challenges of yttria stabilized zirconia modification to lowering the temperature of solid oxide fuel cells operation

Solid Oxide Fuel Cells (SOFCs) are an electrochemical energy converter that receives the world's attention as a power generation system of the future owing to its flexibility to consume various types of fuels, low emission of greenhouses gases, and having high efficiency reaching over 70%. A conventional SOFCs operates at high temperature, typically ranges between 800 to 1000°C. SOFCs use yttria-stabilized zirconia (YSZ) as the electrolyte, which exhibits excellent oxide ion conductivity in this temperature range. However, this temperature range poses an issue to SOFCs durability, as it leads to the degradation of the cell components. In addition, SOFCs application is limited and difficult to implement for the transportation sector and portable appliance. A viable solution is to lower the SOFCs operating temperature to intermediate (600 to 800°C) or low (<600°C) operating temperature. The benefit of this way, cell durability will improve, as well as other advantages such as facilitates handling, assembling, dismantling, cost reduction, and expanded the SOFCs application. Nonetheless, the key challenge for the issue is finding suitable electrolyte, as YSZ have lower ionic conductivity at low and intermediate temperature range. The aim of this paper is to review the status and challenges in the attempts made to modify YSZ electrolyte within the past decade. The resulting ionic conductivity, microstructure, and densification, mechanical and thermal properties of these 'new' electrolytes critically reviewed. The targeted conductivity of modification of YSZ electrolyte must be exceeded >0.1 S cm^–1 to enable high performance of SOFCs power generation systems to be realized for transportation and portable applications. Based on our knowledge, this paper is the first review which focused on the recent status and challenges of YSZ electrolyte towards lowering the operating temperature.     

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.     

Physical characterization and electrochemical performance of copper–iron–ceria–YSZ anode‐based SOFCs in H2 and methane fuels

Thermal instability and poor electrochemical activity of copper‐ceria‐YSZ anodes at the solid oxide fuel cells (SOFCs) operation temperature (>700 °C) necessitates the use of new strategy to improve the performance of respective anodes for direct hydrocarbon SOFCs. In the present study, iron is incorporated into copper–ceria–YSZ anodes in order to investigate the structural, morphological, and electrochemical properties by using various techniques such as X‐ray diffraction, elemental mapping, current–voltage testing, and electrochemical impedance spectroscopy. X‐ray diffraction shows that copper promotes the reduction of iron oxide, and formation of cubic phase of copper–iron metals is observed after reduction in H₂ at 800 °C. Elemental mapping shows better distribution of metal catalyst inside the pores of copper–ceria–YSZ anodes at 800 °C in the presence of iron. The maximum power densities of copper–ceria–YSZ anodes and copper–iron–ceria–YSZ anodes are observed to be 140 and 195 mW cm^?2 in H₂ fuel and 70 and 90 mW cm^?2 in CH₄ fuel at 800 °C. The maximum power density increases with the increase in Cu–Fe metal loading, temperature and with the addition of 1‐wt% Pd in copper–iron–ceria–YSZ anodes. The decrease in performance from 125 to 100 mW cm^?2 is observed during the exposure of CH₄ fuel for 46 h. Electrochemical impedance spectra show an increase in ohmic and total resistance of cell because of sintering and carbon formation, which affects the catalytic activity of anode lowering the performance of SOFC as suggested by post SEM analysis. Copyright © 2015 John Wiley & Sons, Ltd.     

Review: Enhancement of composite anode materials for low-temperature solid oxide fuels

Solid oxide fuel cell (SOFC) technology is attractive for its high-energy efficiency and expanded fuel flexibility. It is also more environmentally benign than conventional power generation systems. Recently, increasing attention has been paid to intermediate-to-low-temperature solid oxide fuel cells, which operating at 400–800 °C. Reducing its operating temperature can render SOFC more competitive with other types of fuel cells and portable energy storage system (EES) over a range of applications (eg: transportation, portable, stationary) and more conducive for commercialization. The high-performance composite anode requirements for low operating temperature (400–600 °C) demand microstructural and chemical stability, high electronic conductivity, and good electrochemical performance. The current high-temperature anode, Ni-YSZ (nickel-yttria stabilized zirconia) is generally reported with high interfacial resistance at reduced temperatures. This review highlights several potential composite anode materials (Ni-based and Ni-free) that have been developed for low-temperature SOFCs within the past 10 years. This literature survey shows that most of these anodes still exhibit relatively high polarization resistance. Focus is also given on reducing polarization resistance to maintain the cell power density. In literature, common approaches that have been adopted to enhance the performance of anodes are (i) selecting high-performance electrolyte, (ii) exploiting nanopowder properties, and (iii) adding noble metals as electrocatalysts.     

Electrospun yttria-stabilized zirconia nanofibers for low-temperature solid oxide fuel cells

We report a 3.5-fold improvement in the performance of solid oxide fuel cells (SOFCs) operating at 450 °C with the introduction of electrospun yttria-stabilized zirconia (YSZ) nanofiber interlayers between the electrolyte and cathode. YSZ nanofibers with diameters of 150–200 nm were uniformly deposited with various thicknesses on a single-crystal YSZ substrate. Electrochemical impedance spectroscopy analyses revealed a drastically reduced polarization resistance, which was mainly attributed to the high specific surface area and high porosity of the YSZ nanofiber interlayers. Our results demonstrate the possibility of using YSZ nanofibers for the development of high-performance SOFCs at low temperature.     

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.     

A cathode-supported solid oxide fuel cell prepared by the phase-inversion tape casting and impregnating method

Cathode-supported Solid Oxide Fuel Cells (SOFCs) have unique advantages of stability and operating life, but the commercialization process is limited by manufacturing cost and poor electrochemical performance. In this paper, a cathode-supported SOFC with 3YSZ-LSM95| porous 8YSZ| dense 8YSZ| porous 8YSZ sandwich structure was successfully fabricated by phase-inversion tape casting and co-sintering method. The cathode support demonstrated finger shaped macropore with high porosity. The long-term stability of symmetric cells with and without impregnated LSC nanoparticals was evaluated and no obvious degregadion were observed. The peak power densities of single cell reached 464, 209, 271 and 144 mW cm^?2 at 850, 800, 750 and 700 °C respectively when Ni nano-particles as the anode catalyst and LSC nano-particles as the cathode catalyst, showing a significant improvement in electrochemical performance compared with non-LSC cell. Additionally, the distribution of relaxation times (DRT) method was empoyed to analysis the polarization process at high-resolution, for better understanding the mechanism of electrochemical reaction of cells. The results indicated the impregnated LSC particles can increase the triple phase-boundaries (TPBs) for fast oxygen reduction reaction and improve the electrochemical performance. However, the optimization of anode and cathode are needed in the future work.     

Analysis of reformate syngas mixture fed solid oxide fuel cell through experimental and 0-D thermodynamic modeling studies

In this study, the performance assessment of a solid oxide fuel cell (SOFC) fed with a reformate syngas mixture and having anode off-gas recirculation is done in terms of energy and exergy analyses. In this regard, a zero-dimensional (0-D) mathematical model for SOFCs is developed. This model is validated by the results of the in-house experimental studies. In addition, parametric studies are carried out to assess the effect of operating parameters on fuel cell performance. The results show that the proposed model is very agreeable with experimental studies. The maximum error found in the validated model is 6.8% at the operating temperature of 800 °C. In addition, it is shown that the anode off-gas recirculation ratio does not have a significant effect on the performance of the SOFC at low current densities. Furthermore, the exergy destruction rate of SOFC increases by 23.2% under the high current density condition (i = 1.4 A/cm²) when the fuel utilization ratio increases from 0.75 to 0.95.     

Ag modified LSCF as cathode material for protonic conducting SOFCs

Abstract

BaCe_0·7Zr_0·1Y_0·2O_3?δ (BCZY) electrolyte and La_0·6Sr_0·4Co_0·2Fe_0·8O_3?δ (LSCF) cathode perovskite materials were synthesised using the citrate–nitrate combustion method. NiO–BCZY anode supported thin film sold oxide fuel cells (SOFCs) was fabricated using spin coating and co-sintering process. LSCF exhibited good chemical stability in H₂O containing atmosphere and chemical compatibility with BCZY proton conducting material. The electrochemical performance of the Ag–LSCF composite cathode was dependent on the Ag content. The thin film proton conducting SOFC displayed highest maximum power density of 563 mW cm^?2 at 700°C for 10 wt-%Ag–(LSCF+BCZY) composite cathode.     

Investigation of the effect of charge transfer coefficient (CTC) on the operating voltage of polymer electrolyte membrane (PEM) electrolyzer

Activation overpotential is a very important parameter in evaluating the performance of PEM electrolyzer, it is one of the major overpotentials that contribute towards overall operating voltage of the electrolyzer. One of the most significant parameters associated with this overvoltage is charge transfer coefficient (CTC). This study aims to contribute to this growing area of research by investigating the effects of operating temperature and CTC on the overall operating voltage. This study also provides an important opportunity to advance the understanding of the effect of different temperature and pressure on the CTC. The result of this study was successfully compared with experimental data. The simulation result shows that, within the temperature range of 10 °C–90 °C, the CTC values at the anode electrode ranges between 0.807 and 1.035 while at the cathode electrode, the variation is only within 0.202–0.259. It was observed that activation overvoltage decreases when the CTC increase from 0.1 to 2.0 both at anode and cathode electrodes. Interestingly it was observed that the CTC remains the same even at balanced and unbalanced pressure.     

Fabrication of a quasi-symmetrical solid oxide fuel cell using a modified tape casting/screen-printing/infiltrating combined technique

To develop the symmetrical electrode materials for solid oxide fuel cells (SOFCs) and to explore the facile cell fabrication technique are both meaningful and of great significance. Here a bi-functional hybrid material LaNi_0.82Fe_0.18O₃ (LNF)/NiO was synthesized by a one-pot citrate method and further used as the quasi-symmetrical electrode catalysts for solid oxide fuel cells (SOFCs). LNF and Ni (reduced NiO) functioned as the cathode/anode catalysts. The La_0.9Sr_0.1Ga_0.8Mg_0.2O₃ (LSGM) based asymmetrical tri-layered substrates were fabricated by a screen-printing assisted co-firing technique. The polarization resistances (Rp) of the infiltrated anode at 700, 650, 600 and 550 °C were only 0.08, 0.12, 0.18 and 0.3 Ω cm², respectively. Comparably, the Rp of the infiltrated cathode were much larger, e.g., 0.18, 0.35, 0.875 and 2.55 Ω cm² at 700, 650, 600 and 550 °C, respectively. Encouragingly, these cathode Rp values were largely reduced when discharge due to an activation process. The LNF and NiO reversibly formed and decomposed during the oxidation and reduction processes, suggesting that the LNF/NiO hybrid is a potential quasi-symmetrical SOFC electrode material. When using H₂ as fuel and air as oxidant, the maximum power densities of the single cell at 650, 600 and 550 °C were as high as 928, 580 and 329 mW cm^?2, respectively.     

Long-term stability of Ni–Sn porous metals for cathode current collector in solid oxide fuel cells

Ni–Sn porous metals with different concentrations of Sn were prepared as potential current collectors for solid oxide fuel cells (SOFCs). The weight increase of these species was evaluated after heat-treatment under elevated temperatures in air for thousands of hours to evaluate the long-term oxidation resistance. Ni–Sn porous metals with 5–14 wt% of Sn exhibited excellent oxidation resistance at 600 °C, although oxidation became significant above 700 °C. Intermetallic Ni₃Sn was formed at 600 °C due to phase transformation of the initially solid solutions of Sn in Ni in the porous metals. For the porous metal with 10 wt% of Sn, the oxidation rate constant at 600 °C in air was estimated to be 8.5 × 10^?14 g² cm^?4 s^?1 and the electrical resistivity at 600 °C was almost constant at approximately 0.02 Ω cm² up to an elapsed time of 1000 h. In addition, the gas diffusibility and the power-collecting ability of the porous metal were equivalent to those of a platinum mesh when applied in the cathode current collector of a SOFC operated at 600 °C. Ni–Sn porous metals with adequate contents of Sn are believed to be promising cathode current collector materials for SOFCs for operation at temperatures below 600 °C.     

Effects of humidity on Ba0.9Co0.7Fe0.2Nb0.1O3−δ cathode performance and durability of Solid Oxide Fuel Cells

Solid Oxide Fuel Cells (SOFCs) cathode often suffers from degradation resulting from different contaminations, such as water vapor from air, when operated under the realistic environments. In this work, we demonstrate an excellent water vapor tolerant Ba_0.9Co_0.7Fe_0.2Nb_0.1O_3?δ (BCFN) cathode for SOFCs. The concentration effects of humidity on BCFN cathode performance including oxygen reduction reaction (ORR) kinetics and durability have been studied. The X-ray diffraction measurement indicates that humidity has no observable effects on BCFN materials. The electrochemical performance change in BCFN cathode seems to be more sensitive to the humidity at a lower temperature such as 650 °C than that at 800 °C. A low polarization resistance of 0.069 Ω cm² at 650 °C is obtained in 3% water vapor, and then the polarization resistance increases with increase of water vapor content. Furthermore, the electrochemical impedance spectra indicate that BCFN cathode contaminated by higher humidity can be recovered by purging air again.     

SOFC stacks for mobile applications with excellent robustness towards thermal stresses

Solid oxide fuel cells (SOFC) are attractive power units for mobile applications, like auxiliary power units or range extenders, due to high electrical efficiencies, avoidance of noble metals, fuel flexibility ranging from hydrogen to hydrogen carriers such as ammonia, methanol or e-gas, and tolerance towards CO and other fuel impurities. Among challenges hindering more wide-spread use are the robustness under thermal cycling. The current study employs short stacks containing anode or metal supported SOFCs, which were subjected to thermal cycles in a furnace and under more realistic conditions without external furnace. Heating from 100 °C to operating temperature was accomplished by sending hot air through the cathode compartment and heating from bottom (and top) of the stack, reaching a fastest ramping time of ca. 1 h. The stacks remained intact under severe temperature gradients of at least 20 °C/cm for anode supported and 30 °C/cm for metal supported SOFCs.     

A direct carbon solid oxide fuel cell fueled with char from wheat straw

Direct carbon solid oxide fuel cell (DC‐SOFC) is a promising technology for electricity generation from biomass with high conversion efficiency and low pollution. Biochar derived from wheat straw is utilized as the fuel of a DC‐SOFC, with cermet of silver and gadolinium‐doped ceria as the material of both cathode and anode and yttrium stabilized zirconia as electrolyte. The output performance of a DC‐SOFC operated on pure wheat straw is 197 mW cm^?2 at 800°C and increases to 258 mW cm^?2 when 5% of Ca, as a catalyst of the Boudouard reaction, is loaded on the wheat straw char. Higher power and fuel conversion utilization are achieved by using Ca as the Boudouard reaction catalyst. X‐ray diffraction, scanning electron microscopy, energy‐dispersive spectrometer, and programmed‐temperature gravimetric experiment are applied to characterize the leaf char. It turns out that the wheat straw char is with porous structure and composed of C, K, Mg, Cl, Fe, and Ca elements. The effects of the Ca catalyst on the Boudouard reaction, the performance of the DC‐SOFCs operated on the wheat straw char, and the economic advantages of the wheat straw char are demonstrated and analyzed in detail.