An efficient ceramic-based anode for solid oxide fuel cells

A new ceramic-based multi-component material, containing one electronic conductor (Y-substituted SrTiO₃, SYT), one ionic conductor (YSZ) and a small amount (∼5 vol.%) of Ni catalyst, was investigated as an alternative anode material for solid oxide fuel cells (SOFCs). The ceramic framework SYT/YSZ shows good dimensional stability upon redox cycling and has an electrical conductivity of ∼10 S cm⁻¹ under typical anode conditions. Owing to the substantial electrocatalytic activity of the fine and well-dispersed Ni particles on the surface of the ceramic framework, the electrode polarization resistance of 5 vol.% Ni-impregnated SYT/YSZ anode reached 0.21 Ω cm² at 800 °C in wet Ar/5%H₂. Based on these results, a redox-stable anode-supported planar SOFC is expected using this anode material, thus offering great advantages over the current generation of Ni/YSZ-based SOFCs.     

Alternative anode materials for solid oxide fuel cells

The electrolyte of a solid oxide fuel cell (SOFC) is an O²⁻-ion conductor. The anode must oxidize the fuel with O²⁻ ions received from the electrolyte and it must deliver electrons of the fuel chemisorption reaction to a current collector. Cells operating on H₂ and CO generally use a porous Ni/electrolyte cermet that supports a thin, dense electrolyte. Ni acts as both the electronic conductor and the catalyst for splitting the H₂ bond; the oxidation of H₂ to H₂O occurs at the Ni/electrolyte/H₂ triple-phase boundary (TPB). The CO is oxidized at the oxide component of the cermet, which may be the electrolyte, yttria-stabilized zirconia, or a mixed oxide-ion/electron conductor (MIEC). The MIEC is commonly a Gd-doped ceria. The design and fabrication of these anodes are evaluated. Use of natural gas as the fuel requires another strategy, and MIECs are being explored for this application. The several constraints on these MIECs are outlined, and preliminary results of this on-going investigation are reviewed.     

Recent anode advances in solid oxide fuel cells

Solid oxide fuel cells (SOFCs) are electrochemical reactors that can directly convert the chemical energy of a fuel gas into electrical energy with high efficiency and in an environment-friendly way. The recent trends in the research of solid oxide fuel cells concern the use of available hydrocarbon fuels, such as natural gas. The most commonly used anode material Ni/YSZ cermet exhibits some disadvantages when hydrocarbons were used as fuels. Thus it is necessary to develop alternative anode materials which display mixed conductivity under fuel conditions. This article reviews the recent developments of anode in SOFCs with principal emphasis on the material aspects. In addition, the mechanism and kinetics of fuel oxidation reactions are also addressed. Various processes used for the cost-effective fabrication of anode have also been summarized. Finally, this review will be concluded with personal perspectives on the future research directions of this area.     

An additional layer in an anode support for internal reforming of methane for solid oxide fuel cells

Anode supported solid oxide fuel cells (SOFCs) have been extensively investigated for their ease of fabrication, robustness, and high electrochemical performance. SOFCs offer a greater flexibility in fuel choice, such as methane, ethanol or hydrocarbon fuels, which may be supplied directly on the anode. In this study, SOFCs with an additional Ni–Fe layer on a Ni–YSZ support are fabricated with process variables and characterized for a methane fuel application. The addition of Ni–Fe onto the anode supports exhibits an increase in performance when methane fuel is supplied. SOFC with a Ni–Fe layer, sintered at 1000 °C and fabricated using a 20 wt% pore former, exhibits the highest value of 0.94 A cm⁻² and 0.85 A cm⁻² at 0.8 V with hydrogen and methane fuel, respectively. An impedance analysis reveals that SOFCs with an additional Ni–Fe layer has a lower charge transfer resistance than SOFCs without Ni–Fe layer. To obtain the higher fuel cell performance with methane fuel, the porosity and sintering temperature of an additional Ni–Fe layer need to be optimized.     

Properties of bias-assisted sputtered gadolinia-doped ceria interlayers for solid oxide fuel cells

The growth and electrochemical properties of gadolinia-doped ceria (GDC) interlayers deposited by bias-assisted magnetron sputtering in solid oxide fuel cells have been investigated. Such interlayers act as diffusion barriers to protect the yttria-stabilized zirconia electrolyte, preventing possible degradation when mixed ionic-electronic conductor (La,Sr)(Co,Fe)O_3−δ is used as the cathode. The dependence of the applied bias during the sputtering deposition on both the interlayer microstructure and fuel cell performance has been studied in anode-supported single cells. The main experimental results showed that bias-assisted sputtering of GDC interlayers produced microstructures denser than those of unbiased depositions, which resulted in increased electrochemical properties of fuel cells.     

ALD CeO2-Coated Pt anode for thin-film solid oxide fuel cells

Surface modification of electrodes for realizing high electrochemical reactivity and thermal stability is an attractive strategy for high-performance low temperature solid oxide fuel cells (LT-SOFCs). Herein, the atomic-layer-deposited (ALD) CeO₂-coated Pt anode structure is fabricated and applied to anodized aluminum oxide (AAO)-based thin-film LT-SOFC. The effect of Pt anode morphology on the infiltration of ALD CeO₂ is elucidated. Anode kinetics are improved in the ALD CeO₂-coated porous Pt anode cell possibly due to the larger Pt–CeO₂ interface density, leading to a decrease in activation resistance by 86%. The maximum power density of the cell with the ALD CeO₂-coated porous Pt anode shows 478 mW/cm²; a dramatic improvement by a factor of two compared to the bare porous Pt anode.     

A novel electrolyte for intermediate solid oxide fuel cells

Scheelite-type, La_xCa_1−xMoO_4+δ electrolyte powders, are prepared by the sol-gel process. The crystal structure of the samples is determined by employing the technique of X-ray diffraction (XRD). According to XRD analysis, the continuous series of La_xCa_1−xMoO_4+δ (0 ≤ x ≤ 0.3) solid solutions have the structure of tetragonal scheelite. Their lattice parameters are greater than that of the original sample, and increase with increasing values of x in the La-substituted system. Results of sinterability and electrochemical testing reveal that the performances of La-doped calcium molybdate are superior to that of pure CaMoO₄. La_xCa_1−xMoO_4+δ ceramics demonstrate higher sinterability. The La_0.2Ca_0.8MoO_4+δ sample that achieved 96.5% of the theoretical density was obtained after being sintered at 1250 °C for 4 h. The conductivity increases with increasing lanthanum content, and a total conductivity of 7.3 × 10⁻³ S cm⁻¹ at 800 °C could be obtained in the La_0.2Ca_0.8MoO_4+δ compound sintered at 1250 °C for 4 h.     

Challenges in using perovskite-based anode materials for solid oxide fuel cells with various fuels: a review

Solid oxide fuel cells (SOFCs) produce electrical power with high efficiency via the electrochemical reaction of fuel with air. In addition to hydrogen, SOFCs are capable of utilizing various types of fuels, such as hydrocarbon-based fuels (e.g., natural gas and biogas), low-carbon-based fuels (e.g., methanol and ethanol) and carbon-free fuel (e.g., ammonia). However, conventional Ni-based anodes experience various challenges and limitations with these fuels. Thus, the discovery of alternative anodes or modification of existing anodes is crucial to address the challenges and limitations of anodes with various fuels. This review provides insight into challenges in selecting anode materials for operating in various fuel environments. Perovskite-based materials are the leading candidates for a range of fuels because of their high redox stability, satisfactory electric and/or ionic conductivity, high activity towards fuel decomposition and resistance towards carbon deposition and sulfur poisoning. Challenges in adapting perovskite-based anodes and strategies to improve their electrochemical performance with various fuels are thoroughly discussed in this review.     

High performance anode with dendritic porous structure for low temperature solid oxide fuel cells

A dendritic porous supported microstructure simultaneously creates small pore size and broad gas diffusion pathways in a solid oxide fuel cell anode membrane. This microstructure also achieves pore sizes that reduce with increasing depth within the membrane without increasing the structure tortuosity. Such a microstructure supplies high triple phase boundary density, fast gas diffusion and low polarization resistance. Here we characterise the performance of a porous anode with such a dendritic microstructure. The solid oxide fuel cell with this high performance anode achieved 0.92 W cm^?2 power density at 600 °C.     

Effect of the anode microstructure on the enhanced performance of solid oxide fuel cells

Anode microstructure has a vital effect on the performance of anode supported solid oxide fuel cells. High electrical conductivity, gas permeability and low polarization are the required features of anodes to achieve high power densities. The desired properties of the anodes were obtained by modifying their microstructural development using pyrolyzable pore former particles without introducing any functional layers and compositional modifications. The microstructures of fabricated anodes were characterized using scanning electron microscopy and mercury intrusion porosimetry techniques while their electrochemical properties were identified using impedance spectroscopy and voltammetric measurements. Detailed investigations demonstrated that the pore structure has a major impact on the electrical conductivity, polarization and gas permeability of the anodes. Through tailoring of conventional anode microstructures, a significantly high power density of 1.54 W/cm² was achieved at 800 °C using diluted hydrogen (10% H₂ in argon) as fuel.     

Advanced perovskite anodes for solid oxide fuel cells: A review

Solid oxide fuel cells (SOFCs) are at the frontline of clean energy generation technologies to convert chemical energy to electricity with high efficiency. In recent years, because of their fuel flexibility, multiple fuels are fed in anode, e.g., hydrogen, ammonia, hydrocarbons, solid carbon, etc.; in addition, these fuels are always mixed with a certain amount of H₂S. Perovskite is one of the most important classes of anode materials being investigated in laboratories, these materials to some extent are immune to coke formation and sulfur poisoning when using hydrocarbon fuels, and retain inherent stability upon reduction and oxidation cycling. In this review, recent developments in perovskite anode materials are summarized and future prospects are discussed.     

Self-rising synthesis of Ni-SDC cermets as anodes for solid oxide fuel cells

Ni-SDC cermets have been obtained using a self-rising approach by two different ways, one-step direct synthesis (OS) and ball milling the separately prepared NiO and SDC powders (BM). The results showed that self-rising approach was an efficient way for the synthesis of porous materials composed of evenly distributed uniform size nanocrystals. The as-synthesized powders have been applied as anodes for solid oxide fuel cells, whose electrochemical properties have been systematically studied. Cells with anodes from the BM method showed better performance compared with those of the OS method, achieving a maximum power density of 400 mW cm⁻² at 600 °C.     

Reactions of low and middle concentration dry methane over Ni/YSZ anode of solid oxide fuel cell

Directly using methane in solid oxide fuel cells (SOFC) requires the knowledge of the reaction of methane over the anode. The reactions of low and middle concentration dry methane were studied over the anode of solid oxide fuel cell with Ni/yttria-stabilized zirconia (YSZ) anode and YSZ electrolyte. The production rates of different types of gas at anode outlet were measured at different current density. Mass balance and relationships between production rates and reaction rates were used to analyze the chemical and electrochemical reactions that took place in parallel. When dry methane is in low concentration, methane decomposition and deposited carbon oxidation occurs at low current density with the overall reaction being partial oxidation of methane (POM). With increased current density, hydrogen oxidation and carbon monoxide oxidizing to carbon dioxide take place simultaneously, and the overall reaction becomes the direct oxidation of methane (DOM). When DOM occurs, a portion of methane participates the POM. However, the rate of POM decreases with increased current density. At medium methane concentration, only partial oxidation of methane takes place. Carbon deposition was found in all the tests across the concentration range investigated.     

Fabrication of solid oxide fuel cell anode electrode by spray pyrolysis

Large triple phase boundaries (TPBs) and high gas diffusion capability are critical in enhancing the performance of a solid oxide fuel cell (SOFC). In this study, ultrasonic spray pyrolysis has been investigated to assess its capability in controlling the anode microstructure. Deposition of porous anode film of nickel and Ce_0.9Gd_0.1O_1.95 on a dense 8 mol.% yttria stabilized zirconia (YSZ) substrate was carried out. First, an ultrasonic atomization model was utilized to predict the deposited particle size. The model accurately estimated the deposited particle size based on the feed solution condition. Second, effects of various process parameters, which included the precursor solution feed rate, precursor solution concentration and deposition temperature, on the TPB formation and porosity were investigated. The deposition temperature and precursor solution concentration were the most critical parameters that influenced the morphology, porosity and particle size of the anode electrode. Ultrasonic spray pyrolysis achieved homogeneous distribution of constitutive elements within the deposited particles and demonstrated capability to control the particle size and porosity in the range of 2-17 μm and 21-52%, respectively.     

A novel layered perovskite cathode for proton conducting solid oxide fuel cells

BaZr_0.1Ce_0.7Y_0.2O_3−δ (BZCY7) exhibits adequate proton conductivity as well as sufficient chemical and thermal stability over a wide range of SOFC operating conditions, while layered SmBa_0.5Sr_0.5Co₂O_5+δ (SBSC) perovskite demonstrates advanced electrochemical properties based on doped ceria electrolyte. This research fully takes advantage of these advanced properties and develops novel protonic ceramic membrane fuel cells (PCMFCs) of Ni-BZCY7|BZCY7|SBSC. The results show that the open-circuit potential of 1.015 V and maximum power density of 533 mW cm⁻² are achieved at 700 °C. With temperature increase, the total cell resistance decreases, among which electrolyte resistance becomes increasingly dominant over polarization resistance. The results also indicate that SBSC perovskite cathode is a good candidate for intermediate temperature PCMFC development, while the developed Ni-BZCY7|BZCY7|SBSC cell is a promising functional material system for next generation SOFCs.     

Characterization of a circular 80 mm anode supported solid oxide fuel cell (AS-SOFC) with anode support produced using high-pressure injection molding (HPIM)

The current study was oriented at analyzing the performance of an anode-supported solid oxide fuel cell produced using high-pressure injection molding. The cell with a total thickness of 550 μm was produced in the Ceramic Department (CEREL) of the Institute of Power Engineering in Poland and experimentally analyzed in the Energy Department (DENERG) of Politecnico di Torino in Italy. The high-pressure injection molding technique was applied to produce a 500 μm thick anode support NiO/8YSZ 66/34 wt% with porosity of 25 vol%. The screen printing method was used to print a 3 μm thick NiO anode contact layer, 7 μm thick NiO/8YSZ 50/50 wt% anode functional layer, 4 μm thick 8YSZ dense electrolyte, 1.5 μm thick Gd_0,1Ce_0,9O₂ barrier layer and a 30 μm thick La_0,6Sr_0,4Fe_0,8Co_0,2O_3–δ cathode with porosity 25 vol%.The experimental characterization was done at two temperature levels: 750 and 800 °C under fixed anodic and cathodic flow and compositions. The preliminary studies on the application of high-pressure injection molding are discussed together with the advantages of the technology. The performance of two generations of anode-supported cells is compared with data of reference cells with supports obtained using tape casting.     

Electrochemical performances of solid oxide fuel cells based on Y-substituted SrTiO3 ceramic anode materials

Yttrium-substituted SrTiO₃ has been considered as anode material of solid oxide fuel cells (SOFCs) substituting of the state-of-the-art Ni cermet anodes. Sr_0.895Y_0.07TiO_3−δ (SYT) shows good electrical conductivity, compatible thermal expansion with yttria-stabilized ZrO₂ (YSZ) electrolyte and reliable stability during reduction and oxidation (redox) cycles. Single cells based on SYT anode substrates were fabricated in the dimension of 50 mm × 50 mm. The cell performances were over 1.0 A cm⁻² at 0.7 V and 800 °C, which already reached the practical application level. Although Ti diffusion from SYT substrates to YSZ electrolytes was observed, it did not show apparent disadvantage to the cell performance. The cells survived 200 redox cycles without obvious OCV decrease and macroscopic damage, but performance decreased due to the electronic properties of the SYT material. The influence of water partial pressure on cell performance and coking tolerance of the cells are also discussed in this study.     

Anode supported solid oxide fuel cells (SOFC) by electrophoretic deposition

Solid oxide fuel cells (SOFC), with its ability to use hydrocarbon fuels and capability to offer highest efficiency, have attracted great attention in India in recent years as an alternative energy generation system for future. But a great deal of problems associated with SOFC is needed to be solved before it can find commercial application. The relatively high operating temperature of 800-1000 °C of SOFC imposes a stringent requirement on materials that significantly increases the cost of SOFC technology. Reducing the operating temperature of an SOFC to below 800 °C can reduce degradation of cell components, improve flexibility in cell design, and lower the material and manufacturing cost by the use of cheap and readily available materials such as ferritic stainless steel. The operating temperature can be reduced by two possible approaches: (i) developing alternative electrolyte materials with high ionic conductivity at lower temperature, and (ii) developing much thinner and denser electrolyte layer such that the ohmic losses are minimised.In this work we report the use of inexpensive Electrophoretic deposition (EPD) technique in making about 10 micron thin and dense YSZ electrolyte on NiO-YSZ substrate. The effect of different operating parameters such as applied voltage, deposition time etc have been optimised during deposition from YSZ suspension in acetylacetone. The YSZ/NiO-YSZ bi-layers were then co-sintered at 1450 °C for 5 h. The single SOFC cells were then fabricated by brush painting LSM:YSZ (50:50) paste on the electrolyte layer followed by sintering at 1200 °C for 2 h. The single SOFC cell when tested using H₂ as fuel and ambient air as oxidant exhibited an open circuit voltage (OCV) of 1.03 V and the peak power density of about 624 mW/cm² at 800 °C.     

Development of the anode pore structure and its effects on the performance of solid oxide fuel cells

Microstructural features, especially pore structure, has a substantial effect on the properties of the anode layer determining the electrochemical performance of the solid oxide fuel cells (SOFCs). Distinct anode pore structures were obtained by removal of various pyrolyzable pore formers (e.g. flake graphite, spheroidal graphite, spherical polymethyl methacrylate, random shaped sucrose, and spherical polystyrene particles). Determined processing parameters for the constituent layers allowed fabrication of Ni-YSZ anodes and complete multilayer fuel cells without macro defects (i.e. cracks, blisters and warpage). A systematic comparison was performed on the anode microstructures, as the fabricated fuel cells consisted of identical component layers (i.e. electrolyte, cathode and current collectors) supported by an anode layer with various pore structures. Voltammetric measurements and analyses of the corresponding impedance spectra on the developed fuel cells along with the investigations on the resultant microstructures using scanning electron microscopy and mercury intrusion porosimetry techniques led to the identification of the relationships between the anode pore structure and the electrochemical performance of the fuel cells. It was revealed that the anode pore structure has critical effects on the properties of the formed anode layers such as electrical conductivity, gas permeability and electrochemical polarization. The novel findings on the anode pore structure allowed increasing the power density of the fuel cells with identical components from 0.45 W/cm² to power densities over 1.75 W/cm² at 800 °C using diluted hydrogen (10% H₂ in Ar) as fuel.     

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.