Reducing the operation temperature of a solid oxide fuel cell using a conventional nickel-based cermet anode on dimethyl ether fuel through internal partial oxidation

Dimethyl ether (DME)-oxygen mixture as the fuel of an anode-supported SOFC with a conventional nickel-cermet anode for operating at reduced temperatures is systematically investigated. The results of the catalytic tests indicate that sintered Ni-YSZ has high activity for DME partial oxidation, and DME conversion exceeds 90% at temperatures higher than 700 °C. Maximum methane selectivity is reached at 700 °C. Cell performance is observed between 600 and 800 °C. Peak power densities of approximately 400 and 1400 mW cm⁻² at 600 and 800 °C, respectively, are reached for the cell operating on DME-O₂ mixture. These values are comparable to those obtained using hydrogen as a fuel, and cell performance is reasonably stable at 700 °C for a test period of 340 min. SEM results demonstrate that the cell maintains good geometric integrity without any delimitation of respective layer after the stability test, and EDX results show that carbon deposition occurrs only at the outer surface of the anode. O₂-TPO analysis shows that carbon deposition over the Ni-YSZ operating on DME is greatly suppressed in the presence of oxygen. Internal partial oxidation may be a practical way to achieve high cell performance at intermediate-temperatures for SOFCs operating on DME fuel.     

Study on proton-conducting solid oxide fuel cells with a conventional nickel cermet anode operating on dimethyl ether

This study investigates dimethyl ether (DME) as a potential fuel for proton-conducting SOFCs with a conventional nickel cermet anode and a BaZr_0.4Ce_0.4Y_0.2O_3−δ (BZCY4) electrolyte. A catalytic test demonstrates that the sintered Ni + BZCY4 anode has an acceptable catalytic activity for the decomposition and steam reforming of DME with CO, CH₄ and CO₂ as the only gaseous carbon-containing products. An O₂-TPO analysis demonstrates the presence of a large amount of coke formation over the anode catalyst when operating on pure DME, which is effectively suppressed by introducing steam into the fuel gas. The selectivity towards CH₄ is also obviously reduced. Peak power densities of 252, 280 and 374 mW cm⁻² are achieved for the cells operating on pure DME, a DME + H₂O gas mixture (1:3) and hydrogen at 700 °C, respectively. After the test, the cell operating on pure DME is seriously cracked whereas the cell operating on DME + H₂O maintains its original integrity. A lower power output is obtained for the cell operating on DME + H₂O than on H₂ at low temperature, which is mainly due to the increased electrode polarization resistance. The selection of a better proton-conducting phase in the anode is critical to further increase the cell power output.     

Thermodynamic analysis of carbon formation boundary and reforming performance for steam reforming of dimethyl ether

Thermodynamic analysis of dimethyl ether steam reforming (DME SR) was investigated for carbon formation boundary, DME conversion, and hydrogen yield for fuel cell application. The equilibrium calculation employing Gibbs free minimization was performed to figure out the required steam-to-carbon ratio (S/C = 0–5) and reforming temperature (25–1000 °C) where coke formation was thermodynamically unfavorable. S/C, reforming temperature and product species strongly contributed to the coke formation and product composition. When chemical species DME, methanol, CO₂, CO, H₂, H₂O and coke were considered, complete conversion of DME and hydrogen yield above 78% without coke formation were achieved at the normal operating temperatures of molten carbonate fuel cell (600 °C) and solid oxide fuel cell (900 °C), when S/C was at or above 2.5. When CH₄ was favorable, production of coke and that of hydrogen were significantly suppressed.     

Further performance enhancement of a DME-fueled solid oxide fuel cell by applying anode functional catalyst

In order to increase the coking resistance of SOFCs operating on DME fuel, a Pt/Al₂O₃–Ni/MgO mixture catalyst was investigated for internal partial oxidation of DME. Catalytic test demonstrated the mixture catalyst has higher activity for DME partial oxidation and lower CH₄ selectivity than the individual Pt/Al₂O₃ and Ni/MgO catalysts. O₂-TPO analysis demonstrated that the mixture catalyst also had much higher coke resistance than sintered Ni-YSZ anode, especially at high O₂ to DME ratio. Raman spectroscopy of the carbon-deposited catalysts demonstrates that the graphitization degree of carbon is reduced with introducing O₂ into DME, and the carbon deposited on the mixture catalyst is almost in amorphous structure. Two operation modes of the mixture catalyst for indirect internal partial oxidation of DME, i.e, directly depositing on the anode surface and locating in the anode chamber were tried. The performance of the cells operating on DME fuel through both operation modes was studied by I–V polarization test and EIS characterization. The cells delivered attractive peak power density of around 750 mW cm⁻² by operating on DME-O₂ mixture gas, modestly lower than 1012–1065 mW cm⁻² operating on pure hydrogen fuel at 700 °C. The direct deposition of Pt/Al₂O₃–Ni/MgO onto anode surface to perform as a functional layer and a DME to O₂ ratio of 2:1 in the mixture gas is preferred to minimize coke formation and maximize power output for the cell to operate on DME fuel.     

Tolerance tests of H2S-laden biogas fuel on solid oxide fuel cells

Biogas is a variable mixture of methane, carbon dioxide and other gases. It is a renewable resource which comes from numerous sources of plant and animal matter. Ni-YSZ anode-supported solid oxide fuel cell (SOFC) can directly use clean synthesized biogas as fuel. However, trace impurities, such as H₂S, Cl₂ and F₂ in real biogas can cause degradation in cell performance. In this research, both uncoated and coated Ni-YSZ anode-supported cells were exposed to three different compositions of synthesized biogases (syn-biogas) with 20 ppm H₂S under a constant current load at 750-850 °C and their performance was evaluated periodically using standard electrochemical methods. Postmortem analysis of the SOFC anode was performed using X-ray diffraction (XRD), scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). The results show that H₂S causes severe electrochemical degradation of the cell when operating on biogas, leading to both complete electrochemical and mechanical failure. The Ni-CeO₂ coated cell showed excellent stability during CH₄ reforming and some tolerance to H₂S contamination.     

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.     

Catalytic activity of Ni-YSZ anodes in a single-chamber solid oxide fuel cell reactor

The importance of heterogeneous catalysis in single-chamber solid oxide fuel cells (SC-SOFC) is universally recognized, but little studied. This work presents a thorough investigation of the catalytic activity of three Ni-YSZ half-cells in a well-described single-chamber reactor. One in-house electrolyte-supported and two commercially available anode-supported half-cells composed of anodes with thicknesses ranging from 50 μm to 1.52 mm are investigated. They are exposed to methane and oxygen gas mixtures within CH₄:O₂ flow rate ratios (R_in) of 0.8-2.0 and furnace temperatures of 600-800 °C. The conversion of methane always results in the formation of syngas species (H₂ and CO). However, their yields vary considerably based on the individual anode, the operating temperature, and R_in. The SC-reactor design and the presence of hot-spots at the reactor entrance bring the methane and oxygen conversion rates well above the limit expected from experiments carried out with anode half-cells only. Major variations in the H₂/CO ratio are observed. In lowering the temperature from 800 °C to 600 °C, it spreads from well below to well above the stoichiometric value of 2.0 expected for the partial oxidation reaction. To optimize the SC-SOFC any further, the findings stress the need to undertake even more catalytic studies of its electrode materials under actual structure and morphology as well as final reactor configuration.     

Effect of various coal contaminants on the performance of solid oxide fuel cells: Part I. Accelerated testing

The contaminants that are potentially present in the coal-derived gas stream and their thermochemical nature are discussed. Accelerated testing was carried out on Ni-YSZ/YSZ/LSM solid oxide fuel cells (YSZ: yttria stabilized zirconia and LSM: lanthanum strontium manganese oxide) for eight main kind of contaminants: CH₃Cl, HCl, As, P, Zn, Hg, Cd and Sb at the temperature range of 750-850 °C. The As and P species, at 10 and 35 ppm, respectively, resulted in severe power density degradation at temperatures 800 °C and below. SEM and EDX analysis indicated that As attacked the Ni region of the anode surface and the Ni current collector, caused the break of the current collector and the eventual cell failure at 800 °C. The phosphorous containing species were found in the bulk of the anode, they were segregated and formed “grain boundary” like phases separating large Ni patches. These species are presumably nickel phosphide/phosphate and zirconia phosphate, which could break the Ni network for electron transport and inhibit the YSZ network for oxygen ion transport. The presence of 40 ppm CH₃Cl and 5 ppm Cd only affected the cell power density at above 800 °C and Cd caused significant performance loss. Whereas the presence of 9 ppm Zn, 7 ppm Hg and 8 ppm Sb only degraded the cell power density by less than 1% during the 100 h test in the temperature range of 750-850 °C.     

The effect of HCl in syngas on Ni-YSZ anode-supported solid oxide fuel cells

The Ni-YSZ cermet anode of the solid oxide fuel cell (SOFC) has excellent electrochemical performance in a clean blended synthetic coal syngas mixture. However, chloride, one of the major contaminants existing in coal-derived syngas, may poison the Ni-YSZ cermet and cause degradation in cell performance. Both hydrogen chloride (HCl) and chlorine (Cl₂) have been reported to attack the Ni in the anode when using electrolyte-supported SOFCs. In this paper, a commercial anode-supported SOFC was exposed to syngas with a concentration of 100 ppm HCl under a constant current load at 800 °C for 300 h and 850 °C for 100 h. The cell performance was evaluated periodically using electrochemical methods. A unique feature of this experiment is that the active central part of the anode was exposed directly to the fuel without an intervening current collector. Post-mortem analyses of the SOFC anode were performed using X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS). The results show that the 100 ppm concentration of HCl causes about 3% loss of performance for the Ni-YSZ anode-supported cell during the 400 h test. Permanent changes were noted in the surface microstructure of the nickel particles in the cell anode.     

Testing of a cathode fabricated by painting with a brush pen for anode-supported tubular solid oxide fuel cells

We have studied the properties of a cathode fabricated by painting with a brush pen for use with anode-supported tubular solid oxide fuel cells (SOFCs). The porous cathode connects well with the electrolyte. A preliminary examination of a single tubular cell, consisting of a Ni-YSZ anode support tube, a Ni-ScSZ anode functional layer, a ScSZ electrolyte film, and a LSM-ScSZ cathode fabricated by painting with a brush pen, has been carried out, and an improved performance is obtained. The ohmic resistance of the cathode side clearly decreases, falling to a value only 37% of that of the comparable cathode made by dip-coating at 850 °C. The single cell with the painted cathode generates a maximum power density of 405 mW cm⁻² at 850 °C, when operating with humidified hydrogen.     

Ethanol internal steam reforming in intermediate temperature solid oxide fuel cell

This study investigates the performance of a standard Ni-YSZ anode supported cell under ethanol steam reforming operating conditions. Therefore, the fuel cell was directly operated with a steam/ethanol mixture (3 to 1 molar). Other gas mixtures were also used for comparison to check the conversion of ethanol and of reformate gases (H₂, CO) in the fuel cell. The electrochemical properties of the fuel cell fed with four different fuel compositions were characterized between 710 and 860 °C by I-V and EIS measurements at OCV and under polarization. In order to elucidate the limiting processes, impedance spectra obtained with different gas compositions were compared using the derivative of the real part of the impedance with respect of the natural logarithm of the frequency.Results show that internal steam reforming of ethanol takes place significantly on Ni-YSZ anode only above 760 °C. Comparisons of results obtained with reformate gas showed that the electrochemical cell performance is dominated by the conversion of hydrogen. The conversion of CO also occurs either directly or indirectly through the water-gas shift reaction but has a significant impact on the electrochemical performance only above 760 °C.     

Ethane dehydrogenation over nano-Cr2O3 anode catalyst in proton ceramic fuel cell reactors to co-produce ethylene and electricity

Ethane and electrical power are co-generated in proton ceramic fuel cell reactors having Cr₂O₃ nanoparticles as anode catalyst, BaCe_0.8Y_0.15Nd_0.05O_3−δ (BCYN) perovskite oxide as proton conducting ceramic electrolyte, and Pt as cathode catalyst. Cr₂O₃ nanoparticles are synthesized by a combustion method. BaCe_0.8Y_0.15Nd_0.05O_3−δ (BCYN) perovskite oxides are obtained using a solid state reaction. The power density increases from 51 mW cm⁻² to 118 mW cm⁻² and the ethylene yield increases from about 8% to 31% when the operating temperature of the solid oxide fuel cell reactor increases from 650 °C to 750 °C. The fuel cell reactor and process are stable at 700 °C for at least 48 h. Cr₂O₃ anode catalyst exhibits much better coke resistance than Pt and Ni catalysts in ethane fuel atmosphere at 700 °C.     

Temperature and performance variations along single chamber solid oxide fuel cells

The catalytic activity of single chamber solid oxide fuel cells (SC-SOFCs) with respect to hydrocarbon fuels induces a major overheating of the fuel cell, temperature variations along its length, and changes in the original fuel/air composition mainly over the anode component. This paper assesses the temperature gradients and the variations in performance along electrolyte-supported Ni-YSZ/YSZ/LSM cells fed with methane gas. The investigations are performed in a useful range of CH₄/O₂ ratios between 1.0 and 2.0, in which the furnace temperature and flow rate of methane–air mixtures are held constant at 700 °C and 450 sccm, respectively. Electrochemical impedance spectroscopy (EIS) is used to sense the temperature at the location where smaller size cathodes are positioned on the opposite side of a full-size anode. Due to temperature increases, cells always perform better when the small cathodes are located at the inlet as well as at a CH₄/O₂ ratio of 1.0. With an increase in ratio, the results show the presence of artefacts due to the use of an active LSM material for the combustion of methane, and open-type gas distribution plates for the single chamber reactor.     

Direct methane solid oxide fuel cell working by gradual internal steam reforming: Analysis of operation

A solid oxide fuel cell was designed to be operated with pure hydrocarbons, without additive or carrier gas, in order to bring technological simplifications, cost reductions and to extend the fuel flexibility limits. The cell was built-up from a conventional cell (LSM/YSZ/Ni-YSZ), to which was added a Ir-CeO₂ catalyst layer at the anode side and an original current collecting system. The cell was first operated with steam in gradual internal reforming (GIR) conditions (R = [H₂O]/[CH₄] < 1) with carrier gas at the anode. The optimal operating parameters were determined in terms of flow rates, cell potential, and fuel utilisation. The cell was finally operated with pure dry methane at 900 °C at 0.6 V yielding current density of about 0.1 A cm⁻² at max power for 120 h. Small but abrupt deterioration of the performances was observed, but no carbon deposition. Electrical and chemical analysis of this degradation are provided.At total, the fuel cell was operated for more than 200 h in pure dry methane, demonstrating that gradual internal reforming actually occurred efficiently in the anode compartment, which make possible operation without reforming agent such as H₂O or CO₂ for other hydrocarbon fuels.     

Direct CH4 fuel cell using Sr2FeMoO6 as an anode material

A double-perovskite Sr₂FeMoO₆ (SFMO) has been synthesized with a combined citrate-EDTA complexing method. The material shows a double-perovskite structure after reduction in 5% H₂/Ar at 1100 °C for 20 h. A single fuel cell using this material as anode is constructed with the configuration of SFMO?La_0.8Sr_0.2Ga_0.83Mg_0.17O₃?Ba_0.5Sr_0.5Co_0.8Fe_0.2O₃. The cell exhibits a remarkable electrochemical activity in both H₂ and dry CH₄, respectively. With Oxygen as oxidant, the maximum power density is 863.7 mW cm⁻² with H₂ as the fuel and 604.8 mW cm⁻² with dry CH₄ as the fuel at 850 °C, respectively. SFMO has an almost linear thermal expansion coefficient from 30 to 900 °C and is very close to that of La_0.8Sr_0.2Ga_0.83Mg_0.17O₃. A durability test of the single cell indicates that SFMO is stable in dry CH₄ operation. Therefore SFMO can be recommended as a promising anode material for LaGaO₃-based solid oxide fuel cells operating with both H₂ and dry CH₄.     

Effect of nickel-phosphorus interactions on structural integrity of anode-supported solid oxide fuel cells

An integrated experimental/modeling approach was utilized to assess the structural integrity of Ni-yttria-stabilized zirconia (YSZ) porous anode supports during the solid oxide fuel cell (SOFC) operation on coal gas containing trace amounts of phosphorus impurities. Phosphorus was chosen as a typical impurity exhibiting strong interactions with the nickel followed by second phase formation. Tests were performed using Ni-YSZ anode-supported button cells exposed to 0.5-10 ppm of phosphine in synthetic coal gas at 700-800 °C. The extent of Ni-P interactions was determined by a post-test scanning electron microscopy (SEM) analysis. Severe damage to the anode support due to nickel phosphide phase formation and extensive crystal coalescence was revealed, resulting in electric percolation loss. The subsequent finite element stress analyses were conducted using the actual anode support microstructures to assist in degradation mechanism explanation. Volume expansion induced by the Ni phase alteration was found to produce high stress levels such that local failure of the Ni-YSZ anode became possible under the operating conditions.     

Testing and improving the redox stability of Ni-based solid oxide fuel cells

Despite active development, solid oxide fuel cells (SOFCs) based on Ni-YSZ anodes still suffer from thermomechanical instability under conditions where the anode side is exposed to oxidising conditions at high temperature. In the first part of the paper, structures and solutions, which could improve the redox stability of Ni-YSZ anode supported SOFC's in terms of dimensional and mechanical stability are reported. Porosity is identified as a major microstructural parameter linked to the dimensional and structural stability during redox cycling. The cumulative redox strain (CRS) after three isothermal redox cycles at 850 °C increases by a factor of more than 20 when the as-sintered porosity of the composites is reduced from 34 to 9%. The effect of reduction and redox cycling on the Ni-YSZ anode are discussed in light of electrochemical measurements using impedance spectroscopy on symmetric cells. When the symmetric cells are reduced and redox cycled isothermally at 850 °C, no major change in the serial or polarisation resistance of the cell and electrodes was measured. When the cells are, after the similar initial reduction treatment, redox cycled at 650 °C, the serial resistance remains almost unchanged but the polarisation resistance decreased by about 60%.     

Effect of PH3 poisoning on a Ni-YSZ anode-supported solid oxide fuel cell under various operating conditions

The Ni-YSZ anode-supported solid oxide fuel cell (SOFC) can generate electrical power by using coal-derived syngas as the fuel. However, trace contamination of phosphine (PH₃) in the syngas can cause irreversible degradation in cell performance. A series of tests at 10 ppm PH₃ in the fuel gas was carried out under a variety of operating conditions, viz, with/without electrochemical reaction in syngas and with/without H₂O in H₂ fuel at 750 °C, 800 °C and 850 °C. The poisoning effects were evaluated by both electrochemical methods and chemical analyses. The post-mortem analyses of the SOFC anode were performed by means of XRD, SEM/EDS, and XPS. The results show that the degradation rate is larger at the higher cell working temperature using syngas with PH₃ in a 200 h test though PH₃ is more reactive with Ni in the anode at lower working temperature and produces a secondary nickel phosphide (Ni_xP_y) phase. The dominant compositions of Ni_xP_y on the cell anode are Ni₅P₂ with the presence of H₂O, and Ni₁₂P₅ without the presence of H₂O. The production of Ni_xP_y can be generated on the cell anode using syngas or dry H₂ fuel with 10 ppm PH₃ contaminant. Further, the appearance of Ni_xP_y phases is independent of the electrochemical reactions in the cell.     

Crack severity in relation to non-homogeneous Ni oxidation in anode-supported solid oxide fuel cells

The full oxidation of Ni-YSZ anode-supported cells at high temperatures (>700 °C) is shown here to lead to much more severe degradation (larger quantity and wider cracks in the electrolyte) than at lower temperatures. This correlates with the linear mass gain/time profile observed in TGA experiments at high temperatures, indicative of diffusion controlled Ni oxidation and thus the presence of O₂ (and Ni/NiO) concentration gradients into the depth of the anode layer. At low partial pressures of O₂, the severity of cracking also increases. SEM studies of partially oxidized anode layers confirmed that Ni oxidation is non-homogeneous when carried out at either high temperatures or low pO₂, in which case the outer regions of the anode (near the anode/air interface) become almost fully oxidized, while the inner regions (near the electrolyte) remain metallic. Under these conditions, the continued volume expansion associated with NiO formation can then only occur towards the electrolyte, increasing the compressive stress inside the anode as the Ni continues to be oxidized, leading to electrolyte cracking and warping (convex to the electrolyte). To prevent severe degradation to the cell, efforts should therefore be made to avoid gradients in NiO/Ni content during oxygen exposure of Ni-YSZ anode-supported cells at high temperatures.     

Experimental investigation of 1 kW solid oxide fuel cell system with a natural gas reformer and an exhaust gas burner

An experimental investigation is performed to establish the optimal operating conditions of a porous media after-burner integrated with a 1 kW solid oxide fuel cell (SOFC) system fed by a natural gas reformer. The compositions of the anode off-gas and cathode off-gas emitted by the SOFC when operating with fuel utilizations in the range 0-0.6 are predicted using commercial GCTool software. The numerical results are then used to set the compositions of the anode off-gas and cathode off-gas in a series of experiments designed to clarify the effects of the fuel utilization, cathode off-gas temperature and excess air ratio on the temperature distribution within the after-burner. The experimental results show that the optimal after-burner operation is obtained when using an anode off-gas temperature of 650 °C, a cathode off-gas temperature of 390 °C, a flame barrier temperature of 700 °C, an excess air ratio of 2 and a fuel utilization of U_f = 0.6. It is shown that under these conditions, the after-burner can operate in a long-term, continuous fashion without the need for either cooling air or any additional fuel other than that provided by the anode off-gas.