Use of a catalyst layer for anode-supported SOFCs running on ethanol fuel

Solid oxide fuel cells (SOFCs) operating directly on hydrocarbon fuels have attracted much attention in recent years. A two-layer structure anode running on ethanol was fabricated by tape casting and screen printing technology in this paper, the addition of a Cu–CeO₂ catalyst layer to the supported anode surface yielded much better performance in ethanol fuel. The effect that the synthesis conditions of the catalyst layer have on the performances of the composite anodes was investigated. Single cells with this anode were also fabricated, of which the maximum power density reached 566 mW cm⁻² at 800 °C operating on ethanol steam. Long-term performance of the anodes was presented by discharging as long as 80 h without carbon deposition.     

Combustion-synthesized Ru-Al2O3 composites as anode catalyst layer of a solid oxide fuel cell operating on methane

Ru-Al₂O₃ composites with varied Ru contents were synthesized by a glycine-nitrate combustion technique. Their potential application as anode catalyst functional layer of a solid-oxide fuel cell operating on methane fuel was investigated. Catalytic tests demonstrated the 3-7 wt.% Ru-Al₂O₃ composites had high catalytic activity for methane partial oxidation and CO₂/H₂O reforming reactions, while 1 wt.% Ru-Al₂O₃ had insufficient activity. The 3 wt.% Ru-Al₂O₃ catalyst also showed excellent operation stability and good thermal-mechanical compatibility with Ni-YSZ anode. H₂-TPR and TEM results indicated there was strong interaction between RuO_x and Al₂O₃ in the as-synthesized catalysts, which may account for the good catalytic stability of 3 wt.% Ru-Al₂O₃ catalyst. O₂-TPO results demonstrated Ru-Al₂O₃ also had excellent coking resistance. Furthermore, the carbon deposited over Ru-Al₂O₃ had lower graphitization degree than that deposited over Ni-Al₂O₃, suggesting the easier elimination of potential carbon deposited over the Ru-Al₂O₃ catalysts. A cell with 3 wt.% Ru-Al₂O₃ catalyst functional layer was prepared, wh-ich delivered peak power densities of 1006, 952 and 929 mW cm⁻² at 850 °C, operating on methane-O₂, methane-H₂O and methane-CO₂ gas mixtures, respectively, comparable to that operating on hydrogen fuel. It highly promised 3 wt.% Ru-Al₂O₃ as a coking resistant catalyst layer for solid-oxide fuel cells.     

Physically mixed LiLaNi-Al2O3 and copper as conductive anode catalysts in a solid oxide fuel cell for methane internal reforming and partial oxidation

Different concentrations of copper are added to LiLaNi-Al₂O₃ to improve the electronic conductivity property for application as the materials of the anode catalyst layer for solid oxide fuel cells operating on methane. Their catalytic activity for the methane partial oxidation, steam and CO₂ reforming reactions at 600-850 °C is systematically investigated. Among the three catalysts, the LiLaNi-Al₂O₃/Cu (50:50, by weight) catalyst presents the best catalytic activity. Thus, the catalytic stability, carbon deposition and surface conductivity of the LiLaNi-Al₂O₃/Cu catalyst are further studied in detail. O₂-TPO results indicate that the coking resistance of LiLaNi-Al₂O₃/Cu is satisfactory and comparable to that of LiLaNi-Al₂O₃. The surface conductivity tests demonstrate it is extremely improved for LiLaNi-Al₂O₃ catalyst due to the addition of 50 wt.% copper. A cell with LiLaNi-Al₂O₃/Cu (50:50) catalyst layer is operated on mixtures of methane-O₂, methane-H₂O and methane-CO₂, and peak power densities of 1081, 1036 and 988 mW cm⁻² are obtained at 850 °C, respectively, comparable to the cell with LiLaNi-Al₂O₃ catalyst layer. In summary, the results of the present study indicate that LiLaNi-Al₂O₃/Cu (50:50) catalysts are highly coking resistant and conductive catalyst layers for solid oxide fuel cells.     

Development of a Ni-Ce0.8Zr0.2O2 catalyst for solid oxide fuel cells operating on ethanol through internal reforming

Inexpensive 20 wt.% Ni-Ce_0.8Zr_0.2O₂ catalysts are synthesized by a glycine nitrate process (GNP) and an impregnation process (IMP). The catalytic activity for ethanol steam reforming (ESR) at 400-650 °C, catalytic stability and carbon deposition properties are investigated. Ni-Ce_0.8Zr_0.2O₂ (GNP) shows a higher catalytic performance than Ni-Ce_0.8Zr_0.2O₂ (IMP), especially at lower temperatures. It also presents a better coking resistance and a lower graphitization degree of the deposited carbon. The superior catalytic activity and coke resistance of Ni-Ce_0.8Zr_0.2O₂ (GNP) is attributed to the small particle size of the active metallic nickel phase and the strong interaction between the nickel and the Ce_0.8Zr_0.2O₂ support, as evidenced by the XRD and H₂-TPR. The Ni-Ce_0.8Zr_0.2O₂ (GNP) is further applied as an anode functional layer in solid oxide fuel cells operating on ethanol steam. The cell yields a peak power density of 536 mW cm⁻² at 700 °C when operating on EtOH-H₂O gas mixtures, which is only slightly lower than that of hydrogen fuel, whereas the cell without the functional layer failed for short-term operations. Ni-Ce_0.8Zr_0.2O₂ (GNP) is promising as an active and highly coking-resistant catalyst layer for solid-oxide fuel cells operating on ethanol steam fuel.     

Effect of Sm0.2Ce0.8O1.9 on the carbon coking in Ni-based anodes for solid oxide fuel cells running on methane fuel

To directly use hydrocarbon fuel without a reforming process, a new microstructure for Ni/Sm_0.2Ce_0.8O_2−δ (Ni/SDC) anodes, in which the Ni surface of the anode is covered with a porous Sm_0.2Ce_0.8O_2−δ thin film, was investigated as an alternative to conventional Ni/YSZ anodes. The porous SDC thin layer was coated on the pores of the anode using the sol–gel coating method. The cell performance was improved by 20%–25% with the Ni/SDC anode relative to the cell performance with the Ni/YSZ anode due to the high ionic conductivity of the Ni/SDC anode and the increase of electrochemical reaction sites. For the SDC-coated Ni/SDC anode operating with methane fuel, no significant degradation of the cell performance was observed after 180 h due to the surface modification with the SDC film on the Ni surface, which opposes the severe degradation of the cell performance that was observed for the Ni/YSZ anode, which results from carbon deposition by methane cracking. Carbon was hardly detected in the SDC-coated Ni/SDC anode due to the catalytic oxidation of the deposited carbon on the SDC film as well as the electrochemical oxidation of methane in the triple-phase-boundary.     

Effect of fabrication method on properties and performance of bimetallic Ni0.75Fe0.25 anode catalyst for solid oxide fuel cells

Ni/Fe alloy-based anodes have attracted much attention recently due to their potential for improving anodic activity and suppressing carbon deposition when operating on carbon-containing fuels. However, some inconsistent results about the iron alloying effect were reported in literature. In the present work, we systematically studied the influence of synthesis method on properties and cell performances of a Ni_0.75Fe_0.25 + SDC (60:40 v/o) alloy-ceramic anode for solid oxide fuel cells. Three different methods, i.e. physical mixing route (PMR), simultaneous glycine nitrate process/sol–gel route (S-GNP) and combined GNP sol–gel route (C-GNP), were used. Samples were analysed by X-ray diffraction, temperature-programmed reduction/oxidation, scanning electron microscopy and electrochemical impedance spectroscopy. It was revealed that the phase structure of anode components, chemical interaction between nickel and iron, and the electrode microstructure were strongly dependent on the synthesis method. The coking resistance was found to be more sensitive to anode phase structure and chemical binding between Ni and Fe phases, whereas the cell power output was mainly determined by the electrode microstructure. As a result, the iron content of the NiFe-based anode should be carefully controlled in different preparation methods to achieve high cell performances.     

A comprehensive evaluation of a Ni-Al2O3 catalyst as a functional layer of solid-oxide fuel cell anode

An inexpensive 7 wt.% Ni-Al₂O₃ composite is synthesized by a glycine-nitrate process and systematically investigated as anode catalyst layer of solid-oxide fuel cells operating on methane fuel by examining its catalytic activity towards methane partial oxidation, steam and CO₂ reforming at 600-850 °C, cell performance, mechanical performance, and carbon deposition properties. Ni-Al₂O₃ shows comparable catalytic activities to Ru-CeO₂ for the above three reactions. The cell with a Ni-Al₂O₃ catalyst layer delivers maximum peak power densities of 494 and 532 mW cm⁻² at 850 °C, operating on methane-H₂O and methane-CO₂ mixture gases, respectively, which are comparable to those operating on hydrogen. Ni-Al₂O₃ is found to have better mechanical performance than Ru-CeO₂. O₂-TPO demonstrates that Ni-Al₂O₃ does not inhibit the carbon formation under pure methane atmosphere, while the introduction of steam or CO₂ can effectively suppress coke formation. However, due to the low nickel content in the catalyst layer, the coke formation over the catalyst layer is actually not serious under real cell operation conditions. More importantly, Ni-Al₂O₃ effectively protects the anode layer by greatly suppressing carbon formation over the anode layer, especially near the anode-electrolyte interface. Ni-Al₂O₃ is highly promising as an anode functional layer for solid-oxide fuel cells.     

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.     

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

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

Co-sintering anode and Y2O3 stabilized ZrO2 thin electrolyte film for solid oxide fuel cell fabricated by co-tape casting

Fabricating a large-area unit cell is very important for the development of solid oxide fuel cell (SOFC) stack. In this study, details of sintering process of half cell with NiO-yttria stabilized zirconia (YSZ) anode-supported YSZ thin electrolyte film fabricated by co-tape casting have been discussed. The results demonstrates that the shrinkages and shrinking rates mismatches between the electrolyte and the anode can be controlled by the organic additive content in the anode slurry composition and heating rate. Low heating rate suppresses the cracks formation in the electrolyte films. A warp-free unit cell with size of 100 × 100 mm² and dense electrolyte has been successfully fabricated. A power of 22.2 W, with a power density of 0.27 W cm⁻² has been achieved at 0.7 V and 750 °C in O₂/humidified H₂ atmosphere. The area specific resistance of the cell is 1.20 Ω cm² at 0.7 V and 750 °C.     

Performance of La0.75Sr0.25Cr0.5Mn0.5O3−δ perovskite-structure anode material at lanthanum gallate electrolyte for IT-SOFC running on ethanol fuel

Perovskite-structure La_0.75Sr_0.25Cr_0.5Mn_0.5O_3−δ (LSCM) powders were prepared using a simple combustion process. Thermal analysis was carried out on the perovskite precursor to investigate the oxide-phase formation. The structural phase of the powders was determined by X-ray diffraction. These results showed that the decomposition of the precursors occurs in a two-step reaction and temperatures higher than 1100 °C are required for these decomposition reactions. For the electrochemical characterization, LSCM anode materials and (Pr_0.7Ca_0.3)_0.9MnO₃ (PCM) cathode materials were screen-printed on two sides of dense La_0.8Sr_0.2Ga_0.8Mg_0.2O₃ (LSGM) electrolyte layers prepared by tape casting with a thickness of about 600 μm, respectively. The morphology of the screen-printed La_0.75Sr_0.25Cr_0.5Mn_0.5O_3−δ perovskite thick films (65 μm) was investigated by field emission scanning electron microscope and showed a porous microstructure. In addition, fuel cell tests were carried out using humidified hydrogen or ethanol stream as fuel and oxygen as oxidant. The performance of the conventional electrolyte-supported cell LSCM/LSGM/PCM while operating on humidified hydrogen was modest with a maximum power density of 165, 99 and 62 mW cm⁻² at 850, 800 and 750 °C, respectively, the corresponding values for the cell while operating on ethanol stream was 160, 101 and 58 mW cm⁻², respectively. Cell stability tests indicate no significant degradation in performance has been observed after 60 h of cell testing when LSCM anode was exposed to ethanol steam at 750 °C, suggesting that carbon deposition was limited during cell operation.     

Effects of coal syngas major compositions on Ni/YSZ anode-supported solid oxide fuel cells

This paper presents a systematical evaluation of the effects of CO₂, H₂O, CO, N₂ and CH₄ in the coal syngas on the properties of typical Ni/YSZ anode-supported solid oxide fuel cells (SOFCs). The results show that CO₂, H₂O, CO, N₂ and CH₄ have complicated effects on the cell performance and the electrochemical impedance spectra (EIS) analysis reveals the addition of these gases influences electrode processes such as the oxygen ion exchange from YSZ to anode TPBs, the charge transfer at the anode TPBs, gas diffusion and conversion at the anode. Two kinds of mixture gases with different compositions are thus constituted and used as fuel for aging test on two cells at 750 °C. No degradation or carbon deposition is observed for the cell fueled with 40% H₂-20% CO-20% H₂O-20% CO₂ for 360 h while the cell fueled with 50% H₂-30% CO-10% H₂O-10% CO₂ exhibits an abrupt degradation after 50 h due to the severe carbon deposition.     

Sr2CoMoO6 anode for solid oxide fuel cell running on H2 and CH4 fuels

The double perovskite Sr₂CoMoO_6−δ was investigated as a candidate anode for a solid oxide fuel cell (SOFC). Thermogravimetric analysis (TGA) and powder X-ray diffraction (XRD) showed that the cation array is retained to 800 °C in H₂ atmosphere with the introduction of a limited concentration of oxide-ion vacancies. Stoichiometric Sr₂CoMoO₆ has an antiferromagnetic Néel temperature T_N ≈ 37 K, but after reduction in H₂ at 800 °C for 10 h, long-range magnetic order appears to set in above 300 K. In H₂, the electronic conductivity increases sharply with temperature in the interval 400 °C < T < 500 °C due to the onset of a loss of oxygen to make Sr₂CoMoO_6−δ a good mixed oxide-ion/electronic conductor (MIEC). With a 300-μm-thick La_0.8Sr_0.12Ga_0.83Mg_0.17O_2.815 (LSGM) as oxide-ion electrolyte and SrCo_0.8Fe_0.2O_3−δ as the cathode, the Sr₂CoMoO_6−δ anode gave a maximum power density of 1017 mW cm⁻² in H₂ and 634 mW cm⁻² in wet CH₄. A degradation of power in CH₄ was observed, which could be attributed to coke build up observed by energy dispersive spectroscopy (EDS).     

Preparation and performance of a Cu–CeO2–ScSZ composite anode for SOFCs running on ethanol fuel

Solid oxide fuel cells (SOFCs) that can operate directly on hydrocarbon fuels, without external reforming, have the potential of greatly speeding up the application of SOFCs for transportation and distributed-power supplies. In this paper, a dual tape casting method for fabricating an anode-supported thin-electrolyte (scandia stabilized zirconia (ScSZ)) film and SOFCs that are active for the oxidation of wet ethanol was presented. The fabrication method relies upon the inclusion of ammonium oxalate ((NH₄)₂C₂O₄·H₂O) pore formers in the anode green tape in order to produce a porous ScSZ matrix, which forms the anode after wet impregnation with aqueous solutions of Cu(NO₃)₂ and Ce(NO₃)₃, and firing. Anodes with different ratios of copper to ceria but with the same total loading were fabricated and measured. The performance characteristics for such cells were studied in both H₂ and C₂H₅OH + H₂O, for comparison, and the long-term performance of the cells in C₂H₅OH stream at 800 °C was also presented.     

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

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

The effect of current density on H2S-poisoning of nickel-based solid oxide fuel cell anodes

Sulphur-containing impurities can have a damaging impact on nickel-based SOFC anode performance even at sub-ppm concentrations, but the electrochemical mechanism of this interaction is not fully understood. In this work, three-electrode cells of Ni-Ce_0.9Gd_0.1O_1.95/YSZ/(La_0.8Sr_0.2)MnO_3−x have been used to obtain new electrochemical data on the sulphur poisoning behaviour of Ni-based SOFC anodes operating at different current densities in the temperature range 700-750 °C. The three-electrode arrangement enabled direct measurement of anode overpotential, with concurrent impedance measurement to provide detail into the electrochemical processes occurring at the anode during sulphur poisoning.The initial, stepwise degradation on exposure to 0.5 ppm H₂S caused an increase in anode polarization resistance, which was almost entirely recoverable on removal of H₂S. Operation at higher current density was found to result in a smaller increase in anode polarization resistance. It is proposed that this initial poisoning behaviour is caused by adsorbed sulphur inhibiting surface diffusion of H atoms to active sites.Exposure to 1 ppm and 3 ppm levels of H₂S led to an observed secondary degradation which was also recoverable on removal of sulphur. This degradation was caused by an increased ohmic resistance, and was more severe at higher temperatures. The authors discuss possible explanations for this behaviour.     

Electrochemical reduction of CO2 in solid oxide electrolysis cells

This paper describes results on the electrochemical reduction of carbon dioxide using the same device as the typical planar nickel-YSZ cermet electrode supported solid oxide fuel cells (H₂-CO₂, Ni-YSZ|YSZ|LSCF-GDC, LSCF, air). Operation in both the fuel cell and the electrolysis mode indicates that the electrodes could work reversibly for the charge transfer processes. An electrolysis current density of ≈1 A cm⁻² is observed at 800 °C and 1.3 V for an inlet mixtures of 25% H₂-75% CO₂. Mass spectra measurement suggests that the nickel-YSZ cermet electrode is highly effective for reduction of CO₂ to CO. Analysis of the gas transport in the porous electrode and the adsorption/desorption process over the nickel surface indicates that the cathodic reactions are probably dominated by the reduction of steam to hydrogen, whereas carbon monoxide is mainly produced via the reverse water gas shift reaction.     

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.     

A highly coking-resistant solid oxide fuel cell with a nickel doped ceria: Ce1−xNixO2−y reformation layer

A single phase mixed oxide ion-electron conducting electrochemical catalyst of Ce_1−xNi_xO_2−y is employed as an anode functional reformation layer for a coking-resistant solid oxide fuel cell (SOFC) based on oxide ion conducting electrolyte operated in methane and ethanol. The high catalytic activity of Ce_1−xNi_xO_2−y oxide for fuel reformation is demonstrated by the excellent cell performances in various fuels at relatively low temperatures (550–650 °C). The fast oxygen ions flux and formed steam at anode side are also found to be favorable for hydrocarbon reformation to promote the cell performance and long term stability. At 650 °C, maximum power densities of 415 and 271 mW cm⁻² are achieved in methane and ethanol respectively. The resistance against carbon deposition is significantly improved with stable voltage output in a long-term durability operation.     

Progress and challenges of cathode contact layer for solid oxide fuel cell

The contact layer is to provide and maintain a stable conductive path between interconnects and electrodes in a planar solid oxide fuel cell (SOFC) stack, which effects contact resistance and stack power loss. The materials used in the contact layer are well developed. This article focuses on cathode contact materials. The role and material requirements of the cathode contact layer in the SOFC stack are reviewed. The common cathode contact layer materials are listed, and their electrical properties as the cathode contact layer are discussed. Due to the evolution of the SOFC stack, the preparation technology and form of the cathode contact layer have changed. Finally, the future challenges of the cathode contact layer are presented.