Comparison between two methane reforming models applied to a quasi-two-dimensional planar solid oxide fuel cell model

Up to recently 2-D solid oxide fuel cell (SOFC) modelling efforts were based on global kinetic approaches for the methane steam reforming and water gas shift reactions (WGS) or thermodynamic equilibrium. Lately detailed models for elementary heterogeneous chemical kinetics of reforming (HCR) over Ni–YSZ anode became available in literature. Both approaches were employed in a quasi 2-D model of a planar high temperature electrolyte supported (ESC) SOFC and simulations were carried out for three different fuel gas compositions: pre-reformed natural gas (high CH₄ content), and two different biomass derived producer gases (low CH₄ content). The results show that the HCR predicts much slower reforming rates which leads to a more evenly distributed solid temperature but smaller power output and thus electrical efficiency. The two models result into predictions that differ greatly if high methane content fuels are used and for such cases the decision upon the modelling scheme to follow should be based on experimental investigations.     

Oxidation failure modes of anode-supported solid oxide fuel cells

Investigations on anode-supported solid oxide fuel cells (SOFCs) using Ni-based anode supports are presented aiming at understanding how much oxidation such a cell can tolerate before incurring irreversible mechanical damage. The cells were oxidised both directly in air and electrochemically. The different oxidation procedures performed exhibited different damage modes. For free-standing cells oxidised in air, the main damage mode was electrolyte cracking after oxidation of approximately 50% of the Ni in the substrate. However, cells oxidised electrochemically failed by substrate cracking after only ca. 5% of the Ni was oxidised, mainly due to the non-uniform nature of oxidation in the SOFC. Models of the stress generation and fracture processes were developed for interpretation of the results.     

Processing of high-performance anode-supported planar solid oxide fuel cell

Anode-supported planar single cells of dimensions 5 cm × 5 cm × 1.5 mm and 10 cm × 10 cm × 1.5 mm have been successfully fabricated using inexpensive and simple processing techniques. The process involves room temperature lamination of several porous layers of tapecast NiO–YSZ together with a dense layer of YSZ electrolyte followed by cofiring. The half-cell fabrication is optimized while the electrolyte thickness is lowered down from 40 to 10 μm. The fabricated single cells with screen printed La(Sr)MnO₃ (LSM)–YSZ as cathode active layer and LSM as current collector layer shows very high electrochemical performance although no separate active layer is used on the anode side of the fabricated cells. The single cells are tested with hydrogen on the anode and oxygen on the cathode sides. The current density and power density of a typical coupon cell of diameter 16 mm is found to be ≈1.7 A/cm² and ≈1.2 W/cm², respectively, at a cell voltage of 0.7 V measured at 800 °C. Area specific resistance (ASR) value, evaluated from the current–voltage plot, is as low as 0.205 Ω cm² at 800 °C. The performances of these cells are found to be almost size independent having excellent repeatability.     

Engineered anode structure for enhanced electrochemical performance of anode-supported planar solid oxide fuel cell

A novel approach of fabricating SOFC anode comprising graded compositions in constituent phases having layer wise microstructural variation is reported. Such anode encompasses conventional NiO–YSZ (40 vol% Ni) with higher porosity at the fuel inlet side and Ni–YSZ electroless cermet (28–32 vol% Ni) with less porosity toward the electrolyte. Microstructures and thicknesses of the bilayer anodes (BLA) are varied sequentially from 50 to 250 μm for better thermal compatibility and cell performance. Significant augmentation in performance (3.5 A cm⁻² at 800 °C, 0.7 V) is obtained with engineered trilayer anode (TLA) having conventional anode support in conjunction with layers of electroless cermet each of 50 μm having 28 and 32 vol% Ni. Engineered TLA accounts for substantial reduction both in cell polarization (ohmic ASR: 78 mΩ cm² versus 2835 mΩ cm²; cell impedance: 0.35 Ω cm² versus 0.9 Ω cm²) and degradation rate (76 μV h⁻¹ versus 219 μV h⁻¹) compared to cells fabricated with conventional cermet.     

Co-extruded dual-layer hollow fiber with different electrolyte structure for a high temperature micro-tubular solid oxide fuel cell

In this study, the phase inversion-based co-extrusion method was employed to fabricate a structural-improved electrolyte/anode dual-layer hollow fiber (HF) precursor, which was then co-sintered at 1450 °C. The electrolyte structures were thoroughly investigated by varying the loading of electrolyte material (i.e. Yttria-stabilized zirconia, YSZ) with differing particle sizes (i.e. micron, sub-micron, and nano-sized) during suspension preparation. The results showed that the most promising electrolyte layer with thin, dense, gas-tight, and defect-free properties was obtained by mixing 70% submicron-YSZ and 30% nano-YSZ in electrolyte suspension (E-0.7sub0.3nano). This electrolyte formulation co-extruded with a thick nickel-oxide-YSZ (NiO-YSZ) anode layer yielded the highest bending strength of 85 MPa, providing major mechanical strength to the HF. Besides that, the nitrogen permeability value at 2.87 × 10^?6 mol m^?2 s^?1 Pa^?1 suggested that the electrolyte was gas-tight, preventing fuel and oxidant transport. The fiber was then reduced to nickel (Ni)-cermet anode. It was developed to be a complete micro-tubular solid oxide fuel cell (MT-SOFC) by depositing the lanthanum strontium cobalt ferrite (LSCF)/YSZ cathode via brush painting on the dual-layer HF. The cell was fed with hydrogen gas and yielded an open-circuit voltage (OCV) as high as 1.06 V with maximum power density of 0.243 W cm^?2, at 875 °C. Based on this test, it was found that the electrolyte structural-modified dual-layer hollow fiber-based MT-SOFC using mixed particle sizes may result in a promising OCV. However, the relatively low value for power density may be due to a less porous anode; thus, improvements in the anode's structure are required in future research.     

Prediction of H2 leak rate in mica-based seals of planar solid oxide fuel cells

Mica-based materials, either laminated papers or layered single crystals, are popular in solid oxide fuel cell applications as sealing components. Their interface and bulk leak paths can be considered as slits with various heights, lengths and widths. A hydromechanics model was established based on the geometric assumptions to predict the influence of slit geometry and pressure difference of the seal on H₂ leak rate. The dependence of leak rate on slit geometry and the pressure difference as well as the effectiveness of compressive loading and compliant interface layer were discussed accordingly. The model's applicability is supported by reported experiments.     

Internal shorting and fuel loss of a low temperature solid oxide fuel cell with SDC electrolyte

A solid oxide fuel cell with Sm_0.2Ce_0.8O_1.9 (SDC) electrolyte of 10 μm in thickness and Ni–SDC anode of 15 μm in thickness on a 0.8 mm thick Ni–YSZ cermet substrate was fabricated by tape casting, screen printing and co-firing. A composite cathode, 75 wt.% Sm_0.5Sr_0.5CoO₃ (SSCo) + 25 wt.% SDC, approximately 50 μm in thickness, was printed on the co-fired half-cell, and sintered at 950 °C. The cell showed a high electrochemical performance at temperatures ranging from 500 to 650 °C. Peak power density of 545 mW cm⁻² at 600 °C was obtained. However, the cell exhibited severe internal shorting due to the mixed conductivity of the SDC electrolyte. Both the amount of water collected from the anode outlet and the open circuit voltage (OCV) indicated that the internal shorting current could reach 0.85 A cm⁻² or more at 600 °C. Zr content inclusions were found at the surface and in the cross-section of the SDC electrolyte, which could be one of the reasons for reduced OCV and oxygen ionic conductivity. Fuel loss due to internal shorting of the thin SDC electrolyte cell becomes a significant concern when it is used in applications requiring high fuel utilization and electrical efficiency.     

Intermediate temperature solid oxide fuel cell based on BaIn0.3Ti0.7O2.85 electrolyte

Electrolyte supported as well as anode supported single-cells based on BaIn_0.3Ti_0.7O_2.85 (BIT) electrolyte were developed. In these cells, Ni-BIT cermet was used as anode and La_0.8Sr_0.2MnO₃ as cathode. Electrolyte supported cells were fabricated by coating slurries of anode and cathode materials on the circular faces of sintered electrolyte discs. The maximum power (P_max) drawn was 15 mW cm⁻² at 30 mA cm⁻². Anode supported cells were fabricated by co-pressing and co-sintering anode and electrolyte powders. The thickness of electrolyte in anode supported cells was reduced to 80 μm and the area specific resistance decreased considerably. The value of P_max improved to ∼100 mW cm⁻².     

Computational model to predict thermal dynamics of planar solid oxide fuel cell stack during start-up process

Solid oxide fuel cell (SOFC) systems have been recognized as the most advanced power generation system with the highest thermal efficiency with a compatibility with wide variety of hydrocarbon fuels, synthetic gas from coal, hydrogen, etc. However, SOFC requires high temperature operation to achieve high ion conductivity of ceramic electrolyte, and thus SOFC should be heated up first before fuel is supplied into the stack. This paper presents computational model for thermal dynamics of planar SOFC stack during start-up process. SOFC stack should be heated up as quickly as possible from ambient temperature to above 700 °C, while minimizing net energy consumption and thermal gradient during the heat up process. Both cathode and anode channels divided by current-collecting ribs were modeled as one-dimensional flow channels with multiple control volumes and all the solid structures were discretized into finite volumes. Two methods for stack-heating were investigated; one is with hot air through cathode channels and the other with electric heating inside a furnace. For the simulation of stack-heating with hot air, transient continuity, flow momentum, and energy equation were applied for discretized control volumes along the flow channels, and energy equations were applied to all the solid structures with appropriate heat transfer model with surrounding solid structures and/or gas channels. All transient governing equations were solved using a time-marching technique to simulate temporal evolution of temperatures of membrane-electrode-assembly (MEA), ribs, interconnects, flow channels, and solid housing structure located inside the insulating chamber. For electrical heating, uniform heat flux was applied to the stack surface with appropriate numerical control algorithm to maintain the surface temperature to certain prescribed value. The developed computational model provides very effective simulation tool to optimize stack-heating process minimizing net heating energy and thermal gradient within the stack.     

Fabrication and performance evaluation of planar solid oxide fuel cell with large active reaction area

Anode-supported planar solid oxide fuel cells (SOFCs) with dimension from 5 × 5 cm² to 15 × 15 cm² have been successfully fabricated by tape casting, screen-printing and single-step co-firing technologies, which shows a potential way of cost-effective for mass production. The performance of the cells has been investigated at operating temperature between 650 °C and 750 °C. The typical cell with dimension of 10 × 10 cm² (active reaction area of 9 × 9 cm²) obtained open circuit voltage (OCV) of 1.15 V and power density of 770 mW/cm² at current density of 950 mA/cm² at 750 °C. The performance degradation of the cell is lower than 1.56%/1000 h. When external reformed methane gas was used as fuel, the cell showed no obvious performance decrease compared to that using pure hydrogen as fuel. The test results have demonstrated that the as-prepared large size cells have excellent performance and reliability, which is ready for SOFC stack assembly.     

Ink-jet printing of electrolyte and anode functional layer for solid oxide fuel cells

In this work, solid oxide fuel cells were fabricated by ink-jet printing. The cells were characterized in order to study the resulting microstructure and electrochemical performance. Scanning electron microscopy revealed a highly conformal 6–12 μm thick dense yttria-stabilized zirconia electrolyte layer, and a porous anode-interlayer. Open circuit voltages ranged from 0.95 to 1.06 V, and a maximum power density of 0.175 W cm⁻² was achieved at 750 °C. These results suggest that the ink-jet printing technique may be used to fabricate stable SOFC structures that are comparable to those fabricated by more conventional ceramics processing methods. This study also highlights the significance of overall cell microstructural impact on cell performance and stability.     

Ag current collector for honeycomb solid oxide fuel cells using LaGaO3-based oxide electrolyte

Honeycomb type solid oxide fuel cell (SOFC) using a Ag mesh as a current collector and La_0.9Sr_0.1Ga_0.8Mg_0.2O₃ (LSGM) as an electrolyte was studied for reducing production cost. When an Ag mesh was used as a current collector, the power density of the cell became lower than that of a cell using a Pt mesh due to the relatively worse contact caused by the lower calcination temperature, particularly in the case of the anode. The preparation method and the electrode structure were investigated for the purpose of increasing the power density of the cell using the Ag current collector. It was found that an interlayer of Ni–Sm_0.2Ce_0.8O_1.9 (1:9) between the NiFe–LSGM cermet anode and the LSGM electrolyte was effective for decreasing the pre-calcination temperature for anode fabrication. Much higher power densities of 300 mW cm⁻² and 140 mW cm⁻² at 1073 K and 973 K, respectively, were achieved by inserting an interlayer. These results for the power density of the cell using the Ag mesh current collector after the optimization of the electrode structure and the preparation procedure are close to those of the cell using the Pt mesh current collector cell presented in our previous work.     

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.     

Energy and carbon payback times for solid oxide fuel cell based domestic CHP

Fuel cells already provide heat and power to people’s homes with lower direct CO₂ emissions and fuel consumption than traditional methods. However, their whole life cycle, including manufacture and disposal, must be considered to verify that these environmental impacts are actually reduced and not merely shifted elsewhere. The total carbon footprint and energy payback times have been widely reported for other emerging microgeneration technologies, but have not previously been calculated for fuel cell systems.     

Performance of a novel type of electrolyte-supported solid oxide fuel cell with honeycomb structure

A novel design, alternative to the conventional electrolyte-supported solid oxide fuel cell (SOFC) is presented. In this new design, a honeycomb-electrolyte is fabricated from hexagonal cells, providing high mechanical strength to the whole structure and supporting the thin layer used as electrolyte of a SOFC. This new design allows a reduction of ∼70% of the electrolyte material and it renders modest performances over 320 mW cm⁻² but high volumetric power densities, i.e. 1.22 W cm⁻³ under pure CH₄ at 900 °C, with a high OCV of 1.13 V, using the standard Ni-YSZ cermet as anode, Pt as cathode material and air as the oxidant gas.     

Flash-sintering of Co2MnO4 spinel for solid oxide fuel cell applications

We show that cobalt manganese oxide (Co₂MnO₄) spinel can be sintered (without the application of external pressure) in a few seconds at about ∼325 °C by applying a DC electrical field of 12.5 V cm⁻¹, by a process known as flash-sintering. A transition from normal to flash-sintering occurs when the field is ≥7.5 V cm⁻¹. The flash sintering phenomenon has also been observed in yttria-stabilized zirconia (3YSZ). Together, the results for 3YSZ and Co₂MnO₄ point towards the generality of the process, since 3YSZ is an ionic conductor while the spinel is a predominantly electronic conductor. The Co₂MnO₄ spinels are used to protect metals, such as stainless steels, in solid oxide fuel cells. The low temperatures employed in flash sintering can obviate interfacial interdiffusion with the metal substrate; in nominal sintering these interfacial reactions can produce deleterious interfacial phases.     

Direct ceramic inkjet printing of yttria-stabilized zirconia electrolyte layers for anode-supported solid oxide fuel cells

Electromagnetic drop-on-demand direct ceramic inkjet printing (EM/DCIJP) was employed to fabricate dense yttria-stabilized zirconia (YSZ) electrolyte layers on a porous NiO-YSZ anode support from ceramic suspensions. Printing parameters including pressure, nozzle opening time and droplet overlapping were studied in order to optimize the surface quality of the YSZ coating. It was found that moderate overlapping and multiple coatings produce the desired membrane quality. A single fuel cell with a NiO-YSZ/YSZ (∼6 μm)/LSM + YSZ/LSM architecture was successfully prepared. The cell was tested using humidified hydrogen as the fuel and ambient air as the oxidant. The cell provided a power density of 170 mW cm⁻² at 800 °C. Scanning electron microscopy (SEM) revealed a highly coherent dense YSZ electrolyte layer with no open porosity. These results suggest that the EM/DCIJP inkjet printing technique can be successfully implemented to fabricate electrolyte coatings for SOFC thinner than 10 μm and comparable in quality to those fabricated by more conventional ceramic processing methods.     

Fracture process of nonstoichiometric oxide based solid oxide fuel cell under oxidizing/reducing gradient conditions

The influence of chemically induced expansion on the fracture damage of a nonstoichiometric oxide (ceria) based solid oxide fuel cell (SOFC) single cell laminate was investigated by using numerical stress analyses under oxidizing/reducing gradient condition. The single cell examined in this study was composed of electrolyte (Ce_0.8Sm_0.2O_2−δ), anode (Cermets of Ni-Ce_0.8Sm_0.2O_2−δ), and cathode (La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ), respectively. The finite element method (FEM) was employed to calculate the residual stress, thermal stresses, and chemically induced expansion stresses for the single cell. The residual and thermal stresses were calculated much smaller than the fracture strength of the individual components of the single cell. On the other hand, the chemically induced expansion stresses were shown to remarkably increase for the temperature range greater than 973 K and accounted their magnitude for primary part of the induced stress. It was shown from the FEM that the maximum circumferential stress induced in the single cell exceeded the fracture strength of the individual components at the onset of the fracture damage detect by acoustic emission (AE) method.     

B-site doping of lanthanum strontium titanate for solid oxide fuel cell anodes

In this study the properties of the compounds La_0.33Sr_0.67Ti_0.92X_0.08O_3+δ where X = Al³⁺, Ga³⁺, Feⁿ⁺, Mg²⁺, Mnⁿ⁺ and Sc³⁺, have been investigated in the search for alternative solid oxide fuel cell anodes. The choice of dopant controls the structure, redox properties, conductivity and electrocatalytic properties of the compound.