Influence of Experimental Conditions on Reliability of Carbon Tolerance Studies on Ni/YSZ SOFC Anodes Operated with Methane

The effect of experimental parameters on carbon accumulation in the Ni/YSZ anode of SOFC operated at 1,073 K with CH₄, has been described in this paper. Experimental parameters including sealing of the cell to the cell holder, operating current, surface coverage by current collector paste on the surface of the anode, and the configuration of the current collector were evaluated in terms of carbon accumulation at the Ni/YSZ anode. The carbon accumulation was quantified using temperature‐programmed oxidation and cells were examined using scanning electron microscopy. The results suggested that variations in any of these experimental parameters could significantly increase or decrease the amount of carbon accumulation on Ni/YSZ anodes, and hence, the reliability of the carbon tolerance studies. In particular, the higher the air leakage rate, the less carbon that accumulated on equivalent anodes. The extent of surface coverage by current collector paste and the configuration of current collector also impacted the amount of carbon accumulation. Less carbon accumulated directly below and near the areas of current collector paste than on the anode areas directly exposed to CH₄ and far from the current collector paste. Additionally, variations in the fuel humidity and current levels also significantly influenced the carbon accumulation.     

Performance and Evaluation of Cu‐based Nano‐composite Anodes for Direct Utilisation of Hydrocarbon Fuels in SOFCs

The anodes for direct utilisation of hydrocarbon fuels have been developed by using Cu/Ceria‐based nano‐composite powders. The CuO/GDC/YSZ–YSZ or CuO/GDC‐GDC nano‐composite powders were synthesised by coating nano‐sized CuO and CeO₂ particles on the YSZ or GDC core particles selectively by the Pechini process. Their microstructures and electrical properties have been investigated with long‐term stability in reactive gases of dry methane and air. The anodes fabricated using Cu‐based nano‐composite anodes showed almost no carbon deposition until 500 h in dry CH₄ atmosphere. The type of an electrolyte‐supported single cell in conjunction with the Cu/Ceria‐based anode must be selected together for the low melting temperature of Cu/CuO. The GDC electrolyte supported unit cell with the Cu/GDC–GDC anode showed the maximum power density of 0.1 Wcm^–2 and long‐term stability for more than 500 h under electronic load of 0.05 Acm^–2 at 650 °C in dry methane atmosphere.     

Characterization of a Cu‐La0.75Sr0.25Cr0.5Mn0.5O3 ‐CeO2/La0.75Sr0.25Cr0.5Mn0.5O3‐YSZ/Ni‐ScSZ Three‐Layer Structure Anode in Thin Film Solid Oxide Fuel Cell Running on Methane Fuel

Anodes for Solid Oxide Fuel Cell that is capable of directly using hydrocarbon without external reforming have been of great interest recently. In this paper, a three‐layer structure anode running on methane is fabricated by tape casting and screen printing method. The slurry of catalyst layer Cu‐LSCM‐CeO₂ (with weight ratios of 1.5:7.0:1.5, 2.0:7.0:1.0, 2.15:7.0:0.85 and 2.25:7.0:0.75, weight ratios of Cu/CeO₂ is 1:1, 2:1, 2.5:1 and 3:1, respectively) is screen‐printed on LSCM‐YSZ support layer and Ni‐ScSZ active layer. Thus, LSCM‐YSZ/Ni‐ScSZ anodes with Cu‐LSCM‐CeO₂ catalyst layer (denoted as LSCM‐YSZ1010, LSCM‐YSZ2010, LSCM‐YSZ2510 and LSCM‐YSZ3010, respectively) are obtained. Single cells with three‐layer structure anode are also fabricated and measured, of which the maximum power density reaches 491 and 670 mW cm⁻² for the cell with LSCM‐YSZ2510 anode running on methane at 750 °C and 800 °C, respectively. No significant degradation in performance has been observed after 240h of cell testing when LSCM‐YSZ2510 anode is exposed to methane at 750 °C. Very little carbon deposition is detected on the anode, suggesting that carbon deposition is limited during cell operation. Consequently, Cu‐LSCM‐CeO₂ catalyst layer on the surface of LSCM‐YSZ support layer makes it possible to have good stability for long‐term operation in methane due to very little carbon deposition.     

Study of Carbon Deposition Behavior on Cu–Co/CeO2–YSZ Anodes for Direct Butane Solid Oxide Fuel Cells

Electro‐catalytic activity of Cu–Co/CeO₂–YSZ anodes towards oxidation of H₂ and n‐C₄H₁₀ fuels and carbon depositions are investigated using different Cu–Co loadings. Cu–Co/CeO₂–YSZ anode based SOFCs with YSZ as electrolyte and LSM/YSZ as cathode were prepared by tape casting and wet impregnation methods and performance was analyzed using I–V characteristics and impedance spectroscopy. The Cu–Co/CeO₂–YSZ anodes with Cu–Co loading of 10, 15, and 25 wt.% produced power density of 60, 197, and 400 mW cm^–2 in H₂ and 190, 225, and 275 mW cm^–2 in n‐C₄H₁₀ at 800 °C. The power density is increased with the increase in Cu–Co loading in Cu–Co/CeO₂–YSZ anodes. The electrochemical impedance spectra shows less ohmic and polarization resistance for 25 wt.% Cu–Co loading in comparison to 10 and 15 wt.% Cu–Co. Scanning electron microscopy and high resolution transmission electron microscopy shows that the carbon fibers formed are hollow in nature with 70 nm size, whereas, thermal gravimetric analysis and X‐ray diffraction points out that they are amorphous in nature. The performance degradation of Cu–Co/CeO₂–YSZ anodes in n‐C₄H₁₀ in 16 h is attributed to increasing amount of carbon deposition with time, which is contrary to our earlier observation in Cu‐Fe/CeO₂–YSZ anode.     

Electrochemical performance and microstructure characterization of nickel yttrium‐stabilized zirconia anode

A nickel and yttrium‐stabilized zirconia (Ni‐YSZ) composite is one of the most commonly used anode materials in solid oxide fuel cells (SOFCs). One of the drawbacks of the Ni‐YSZ anode is its susceptibility to deactivation due to the formation of carbonaceous species when hydrocarbons are used as fuel supplies. We therefore initiated an electrochemical study of the influence of methane (CH₄) on the performance of Ni‐YSZ anodes by examining the kinetics of the oxidation of CH₄ and H₂ over operating temperatures of 600–800°C. Anode performance deterioration was then correlated with the degree of carbonization observed on the anode using ex‐situ X‐ray powder diffraction and scanning electron microscopy techniques. Results showed that carbonaceous species led to a significant deactivation of Ni‐YSZ anode toward methane oxidation. © 2009 American Institute of Chemical Engineers AIChE J, 2010     

Evaluation of a Cu/YSZ and Ni/YSZ Bilayer Anode for the Direct Utilization of Methane in a Solid‐Oxide Fuel Cell

Carbon deposition is an issue when operating solid oxide fuel cells (SOFC) on fuels other than hydrogen, and so a variety of strategies have been used to prevent carbon accumulation on the anodes. In this paper, we describe a bilayer anode that contains a functional layer consisting of Ni/YSZ and a conduction layer consisting of Cu/YSZ. The anode‐supported button cells were fabricated using a uni‐axially pressing technique to produce the anode, followed by impregnation with Cu. The cells were tested at 1,023 K in dry CH₄ and their performance compared to that of a typical Ni/YSZ anode. The Cu does not catalyze the cracking of methane and as such less carbon deposits in the conduction layer resulting in anode stability for over 100 h. The limitation with using Cu in the anode is the temperature of operation.     

Hydrogen Production Via CH4 and CO Assisted Steam Electrolysis

Porous composite anodes consisting of a yttria-stabilized zirconia (YSZ) backbone that was impregnated with CeO₂ and various amounts of metallic components including Cu, Co and Pd were fabricated. The performance of these anodes was then tested in a solid oxide water electrolysis cell under conditions where the anode was exposed to the reducing gasses H₂, CH₄ and CO. The reducing gasses were used to decrease the electrochemical potential of the cell and increase overall efficiency. The results of this study show that Cu–CeO₂–YSZ anodes have low catalytic activity for the oxidation of CO and CH₄ and are not very effective in lowering the cell potential while operating in the reducing gas assisted mode. The addition of Co to the Cu–CeO₂–YSZ anode resulted in a modest increase in the catalytic activity and enhanced the thermal stability of the anode. A Pd–C–CeO₂–YSZ anode was found to have the highest catalytic activity of those tested and gave the largest reductions in the operating potential of the solid oxide electrolysis cell.     

Long‐term microstructural changes in solid oxide fuel cell anodes: 3D reconstruction

Microstructural changes in solid oxide fuel cell anodes after long‐term operation have been characterized by sequential sectioning with a focused ion beam, followed by scanning electron microscopy imaging and three‐dimensional reconstruction. The anodes were porous composites of Ni and Y₂O₃‐stabilized ZrO₂ (YSZ). The cells were operated at 800°C for 2, 4, and 8 kh, and at 925°C for 2 and 4 kh. For each specimen, the volume fraction, surface area, particle diameter, and tortuosity have been calculated for each phase (Ni, YSZ, and pores). The dependence of these microstructural parameters on the volume of sample analyzed was monitored; sufficiently large volumes were analyzed so as to eliminate any effect of sample volume. Gradients in volume fraction of Ni and porosity developed during fuel cell operation, with Ni fraction increasing, and pore fraction decreasing, at the electrolyte/anode interface. The magnitudes of these gradients increased with time.     

Biogas Catalytic Reforming Studies on Nickel‐Based Solid Oxide Fuel Cell Anodes

Heterogeneous catalysis studies were conducted on two crushed solid oxide fuel cell (SOFC) anodes in fixed‐bed reactors. The baseline anode was Ni/ScYSZ (Ni/scandia and yttria stabilized zirconia), the other was Ni/ScYSZ modified with Pd/doped ceria (Ni/ScYSZ/Pd‐CGO). Three main types of experiments were performed to study catalytic activity and effect of sulfur poisoning: (i) CH₄ and CO₂ dissociation; (ii) biogas (60% CH₄ and 40% CO₂) temperature‐programmed reactions (TPRxn); and (iii) steady‐state biogas reforming reactions followed by postmortem catalyst characterization by temperature‐programmed oxidation and time‐of‐flight secondary ion mass spectrometry. Results showed that Ni/ScYSZ/Pd‐CGO was more active for catalytic dissociation of CH₄ at 750 °C and subsequent reactivity of deposited carbonaceous species. Sulfur deactivated most catalytic reactions except CO₂ dissociation at 750 °C. The presence of Pd‐CGO helped to mitigate sulfur deactivation effect; e.g. lowering the onset temperature (up to 190 °C) for CH₄ conversion during temperature‐programmed reactions. Both Ni/ScYSZ and Ni/ScYSZ/Pd‐CGO anode catalysts were more active for dry reforming of biogas than they were for steam reforming. Deactivation of reforming activity by sulfur was much more severe under steam reforming conditions than dry reforming; a result of greater sulfur retention on the catalyst surface during steam reforming.     

Oxygen Dissociation and Interfacial Transfer Rate on Performance of SOFCs with Metal‐Added (LaSr)(CoFe)O3‐(Ce,Gd)O2 – δ Cathodes

A solid oxide fuel cell (SOFC) unit is constructed with Ni‐Ce_0.9Gd_0.1O_{2 – δ} (GDC) as the anode, yttria‐stabilised zirconia (YSZ) as the electrolyte and Pt, Ag or Cu‐added La_0.58Sr_0.4Co_0.2Fe_0.8O_{3 – δ} (LSCF)–GDC as the cathode. The current–voltage measurements are performed at 800 °C. Cu addition leads to best SOFC performance. LSCF–GDC–Cu is better than LSCF–GDC and much better than GDC as the material of the cathode interlayer. Cu content of 2 wt.‐% leads to best SOFC performance. A cathode functional layer calcined at 800 °C is better than that calcined at higher temperature. Metal addition increases the O₂ dissociation reactivity but results in an interfacial resistance for O transfer. A balance between the rates of O₂ dissociation and interfacial O transfer is needed for best SOFC performance.     

The Effect of H2S on the Performance of SOFCs using Methane Containing Fuel

In recent years, the interest for using biogas derived from biomass as fuel in solid oxide fuel cells (SOFCs) has increased. To maximise the biogas to electrical energy output, it is important to study the effects of the main biogas components (CH₄ and CO₂), minor ones and traces (e.g. H₂S) on performance and durability of the SOFC. Single anode‐supported SOFCs with Ni–Yttria‐Stabilised‐Zirconia (YSZ) anodes, YSZ electrolytes and lanthanum‐strontium‐manganite (LSM)–YSZ cathodes have been tested with a CH₄–H₂O–H₂ fuel mixture at open circuit voltage (OCV) and 1 A cm^–2 current load (850 °C). The cell performance was monitored with electric measurements and impedance spectroscopy. At OCV 2–24 ppm H₂S were added to the fuel in 24 h intervals. The reforming activity of the Ni‐containing anode decreased rapidly when H₂S was added to the fuel. This ultimately resulted in a lower production of fuel (H₂ and CO) from CH₄. Applying 1 A cm^–2 current load, a maximum concentration of 7 ppm H₂S was acceptable for a 24 h period.     

Effect of adding urea on performance of Cu/CeO2/yttria-stabilized zirconia anodes for solid oxide fuel cells prepared by impregnation method

Anode microstructure has a great influence on the cell performance. The addition of urea into impregnated solution has been proposed to tailor the distribution and/or morphology of Cu when fabricating the Cu-based anodes by impregnation method. While the previous reports demonstrated the single cell performance has not been improved in this route, in this paper, fuel cells with Cu/yttria-stabilized zirconia (YSZ) and Cu–CeO₂/YSZ anodes were fabricated and evaluated with improved outputs. The microstructure of Cu in anodes appeared significantly different after the addition of urea. The electronic conductivity obtained from the anodes impregnated with adding urea was twice as high as the ones without. Performance of fuel cells increases by 12% while operating on H₂ at 700 °C upon adding urea. Furthermore, the performance improvement was more prominent when such method was adopted in the fabrication of Cu–CeO₂/YSZ composite anodes. Cells with Cu–CeO₂/YSZ composite anodes operating in H₂ at 700 °C exhibited an increase of cell performance by 37%, from 337 to 462 mW cm⁻², by simply adding urea to the impregnated solution. And the performance enhancement for such fuel cells is also as high as 28% when using CH₄ as fuel.     

Redox Tolerance of Thin and Thick Ni/YSZ Anodes of Electrolyte‐Supported Single‐Chamber Solid Oxide Fuel Cells under Methane Oxidation Conditions

Redox tolerance of 50 and 500 μm thick Ni/YSZ (yttria‐stabilized zirconia) anodes supported on YSZ electrolytes were studied under single‐chamber solid oxide fuel cell conditions. Open circuit voltage, electrochemical impedance spectra, and discharge curves of the cells were measured under different methane/oxygen ratios at 700 °C. For the cell with the thin anode, a significant degradation accompanying oscillatory behaviors was observed, whereas the cell based on the thick anode was much more stable under the same conditions. In situ local anode resistance (R_s) results indicated that the Ni/NiO redox cycling was responsible for the oscillatory behaviors, and the cell degradation was primarily caused by the Ni reoxidation. Reoxidation of the thick anode took place at a low methane/oxygen ratio, but the anode can be recovered to its original state by switching to a methane‐rich environment. On the contrary, the thin anode was unable to be regenerated after the oxidation. Microstructure damage of the anode was attributed to its irreversible degradation.     

A comparison of La0.75Sr0.25Cr0.5Mn0.5O3−δ and Ni impregnated porous YSZ anodes fabricated in two different ways for SOFCs

In this paper, La_0.75Sr_0.25Cr_0.5Mn_0.5O_3−δ (LSCrM) and Ni impregnated porous yttria-stabilized zirconia (YSZ) anodes have been fabricated in two different ways. The testing results demonstrated the excellent performance of the anode made by infiltrating a mixture of LSCrM and Ni(NO₃)₂ solutions into porous YSZ matrix. After reduction of the anode with hydrogen, an inner nano-network structure with mixed ionic-electronic conducting path has been formed within and between these added particles. A single cell with the anode at 800 °C exhibited the maximum power densities of 1151 and 704 mW cm⁻² when dry H₂ and CH₄ were used as the fuels, respectively; under the same conditions, the cell performances for LSCrM and Ni impregnated YSZ anode separately were 810 and 508 mW cm⁻². A cavity model was proposed to simulate the impregnating process and the loading was calculated. No carbon deposition was detected in the anode, even with the presence of Ni, after operation in dry CH₄ for about 6 h under open-circuit condition.     

Study of a Single‐Chamber Solid Oxide Fuel Cell Microstack with V‐Shaped Congener‐Electrode‐Facing Configuration

Single‐chamber solid oxide fuel cell (SC‐SOFC) microstacks with V‐Shaped congener‐electrode‐facing configuration were fabricated and operated successfully in a box‐like stainless steel chamber. Two gas channels with small gas inlets were used to transport the fuel and oxygen to the anodes and cathodes, respectively. The temperature of an anode‐facing‐anode two‐cell stack was higher than that of a cathode‐facing‐cathode two‐cell stack during the test procedure. For a three‐cell stack, the cell in the middle region presented the highest power output. The open circuit voltage (OCV) and maximum power output of the three‐cell stack in a gas mixture of 100 sccm N₂, 120 sccm CH₄, and 80 sccm O₂ were 3.0 V and 413 mW, respectively.     

Modeling and Parametric Study of a Single Solid Oxide Fuel Cell by Finite Element Method

A 2D isothermal axisymmetric model of an anode‐supported solid oxide fuel cell has been developed. The model, which is based on finite element approach, comprises electronic and ionic charge balance, Butler–Volmer charge transfer kinetic, flow distribution and gas phase mass balance in both channels and porous electrodes. The model has been validated using available experimental data coming from a single anode‐supported cell comprising Ni–YSZ/YSZ/LSM–YSZ as anode, electrolyte and cathode, respectively. Hydrogen and steam were used as fuel inlet and air as an oxidant. The validation has been carried out at 1 atm, 1,500 ml min^–1 air flow rate and three different operating conditions of temperature and fuel flow rate: 1,073 K and 400 ml min^–1, 1,073 K and 500 ml min^–1, and 1,003 K and 400 ml min^–1. The polarization and power density versus current density curves show a good agreement with the experimental data. A parametric analysis has been carried out to highlight which parameters have main effect on the overall cell performance as measured by polarization curve, especially focusing on the influence of two geometrical characteristics, temperature and some effective material properties.     

Effects of Manganese Oxide Addition on Coking Behavior of Ni/YSZ Anodes for SOFCs

Solid oxide fuel cells with Ni‐MnO/yttria‐stabilized‐zirconia (YSZ) tricomposite anode supports were fabricated with different MnO concentrations, and the coking tolerances and catalytic activities were investigated in wet CH₄ atmosphere. Ni_0.9(MnO)_0.1/YSZ (10MnO) anode support cell exhibited a maximum power density of 210, 354, 505, and 620 mWcm⁻² at 700, 750, 800, and 850 °C, respectively, in H₂. Moreover, a maximum power density in wet CH₄ reaches 504 mWcm⁻² at 800 °C; while the Ni/YSZ cell showed poorer performances. The coking tolerance improved with an increase in their MnO content, and the 10MnO anode showed the highest tolerance. 10MnO exhibited stable performance for more than 40 h in wet CH₄ without undergoing deactivation. Furthermore, it showed negligible coke formation of 0.0045 g of coke per catalyst, during testing under steam reforming‐like conditions at a steam‐to‐carbon (S/C) ratio of 1. Outlet gas chromatography analysis indicated that MnO suppresses CH₄ cracking, while only minimally lowering the catalytic activity of steam reforming. Thus, it can be inferred that MnO promotes the adsorption of steam and oxygen on the reaction sites, owing to its high basicity and oxygen storage capacity. The increase in the local S/C and oxygen‐to‐carbon ratios suppresses CH₄ cracking and promotes coke gasification.     

Comparative Performance Analysis of Anode‐Supported Micro‐Tubular SOFCs with Different Current‐Collection Architectures

In this study, the performances of single micro‐tubular solid oxide fuel cells based on the NiO–YSZ/YSZ/LSM system with two different current‐collection architectures were compared. In the first case, a straight Ni wire was inserted within the hole of the cell before the electrochemical testing, and in the second case, a coil integrated‐current collector within the anode layer was already arranged for electrical connections during cell processing. The current produced in each case was collected from double terminal and the performance of the cells was estimated by electrochemical I–V characterization. The maximum power outputs generated in the cells with the integrated‐current collector and the common current‐collection architectures were of ∼200 and ∼55 mW cm^–2, respectively at 800 °C under a wet H₂ fuel flow.     

Planar Metal‐Supported SOFC with Novel Cermet Anode

Metal‐supported solid oxide fuel cells are expected to offer several potential advantages over conventional anode (Ni‐YSZ) supported cells. For example, increased resistance against mechanical and thermal stresses and a reduction in material costs. When Ni‐YSZ based anodes are used in metal supported SOFC, elements from the active anode layer may inter‐diffuse with the metallic support during sintering. This work illustrates how the inter‐diffusion problem can be circumvented by using an alternative anode design based on porous and electronically conducting layers, into which electrocatalytically active materials are infiltrated after sintering. The paper presents the electrochemical performance and durability of the novel planar metal‐supported SOFC design. The electrode performance on symmetrical cells has also been evaluated. The novel cell and anode design shows a promising performance and durability at a broad range of temperatures and is especially suitable for intermediate temperature operation at around 650 °C.     

Methane Oxidation Over M–8YSZ and M–CeO2/8YSZ (M = Ni, Cu, Co, Ag) Catalysts

The oxidation of methane has been studied by sequential flow reaction experiments over M–8YSZ and M–CeO₂/8YSZ (M=Ni, Cu, Co, Ag) catalysts as a function of CH₄/O₂ from 773 to 1073 K. Over Ni–8YSZ and Ni–CeO₂/8YSZ, methane pyrolysis is dominant leading to surface carbon formation at temperatures of 873 K and above. While the addition of ceria to Ni–8YSZ to produce Ni–CeO₂/8YSZ does not significantly affect the reaction kinetics, the activity of Cu–CeO₂/8YSZ, Co–CeO₂/8YSZ, and Ag–CeO₂/8YSZ are higher than their M–8YSZ counterparts. The activity of Co–CeO₂/8YSZ at high temperatures (973 K and above) is higher than Ni-8YSZ with selectivity towards partial methane oxidation and CO formation. Considering Ni-based catalysts are prone to deactivation due to surface carbon accumulation, Co–CeO₂/8YSZ, Cu–CeO₂/8YSZ, and Ag–CeO₂/8YSZ are possible alternative anode cermets for direct hydrocarbon oxidation solid-oxide fuel cells (SOFC).