Single SOFC Research Articles

Numerical simulation of the performance of solid oxide fuel cell with different flow channel geometries

A three-dimensional numerical model based on the finite element method has been developed to evaluate the planar SOFC performance includinginterconnect, rectangular duct gas flow channels, gas diffusion electrodes and electrolyte layer. A single channel SOFC fuel cell is taken and electrochemical analysis is carried out on it. The results are compared with the available published experimental data obtained by other research group, and they are found in good agreement. In SOFC fuel cell the geometry of flow channel has a particular importance on properly distributing the reactants on the TPB region and thus has a significant impact on the performance. Therefore, in this paper, the geometry of the fuel cell channel is changed from rectangular to triangular and trapezoidal shapes so that the active area and other operating conditions of each cell are the same and the performances of the fuel cell are observed. The performance of the rectangular channel is better, while that of trapezoidal channel is poor compared to the rectangular channel design.

(Invited) Thermodynamic Theory of Durability of Soecs and SOFCs: The Role of Electrode Polarization

It is well-known that SOECs exhibit greater propensity for degradation than SOFCs. In a similar fashion, it is also known that batteries are more to degradation during charging than during discharging. SOFCs (power generation) and batteries can also degrade during discharge. This is related to cell imbalance. The greater the variability in the individual cell resistances in a stack or battery pack, the greater is the tendency for degradation. Onsager transport equations play a crucial role in degradation. A fundamental consideration is whether an electrochemical device is operating in galvanic mode or electrolytic mode. A battery during charging or SOEC generating hydrogen operates in electrolytic mode. A single battery cell during discharge or a single SOFC in power generation operates in galvanic mode. However, degradation in an SOFC stack or a battery pack during discharge initiates in an unbalanced cell, which actually operates in electrolytic mode. In any electrochemical device, transport coefficients must include not only transport through the electrolyte membrane but also transport across electrode/electrolyte interfaces and the corresponding polarization resistances. The main objective of this presentation is to quantitatively demonstrate the role of electrode polarizations in affecting durability. In addition, the resistance to electron transport across electrode/electrolyte interfaces will also be discussed. Some experimental results on degradation of cells with different characteristics will be presented.

Analysis of possibilities for carbon removal from porous anode of solid oxide fuel cells after different failure modes

This study focuses on the investigation of possibilities for carbon removal from the fuel electrode of anode supported solid oxide fuel cells (ASC-SOFCs) after different degradation modes. To design the conditions which generally lead the cell in the range of carbon depositions the performed thermodynamic calculations show that the SOFC operating temperature range seems to be appropriate for formation of elemental carbon in various types. Concerning this the loaded large planar single SOFCs are fed with synthetic diesel reformate thus simulating realistic operating conditions and enabling the formation and deposition of carbon on the anode side. A mixture of hydrogen/water vapor/nitrogen is used to remove the detected carbon depositions in a cell-protecting manner. For the purpose of this investigation several failure modes are induced after which determination the already defined regeneration strategy is applied. The cathode degradation is first induced and secondly the fuel supply is interrupted to induce re-oxidation of nickel (Ni) on the anode side. The undertaken investigations determine that carbon can be fully removed from the anode surface after nickel oxidation, while cathode degradation disables the complete cell regeneration.

Molybdenum doped Pr0.5Ba0.5MnO3−δ (Mo-PBMO) double perovskite as a potential solid oxide fuel cell anode material

A layered Mo doped Pr0.5Ba0.5MnO3−δ (Mo-PBMO) double perovskite oxide was prepared by a modified sol–gel method and the properties of the fabricated material are characterized by various technologies. The results of X-ray diffraction (XRD), H2-temperature programmed reduction (H2-TPR), NH3-temperature programmed desorption (NH3-TPD), and thermogravimetric analysis (TGA) demonstrate that the treatment in reducing atmosphere at high temperature lead to a significant phase transformation of the material to a single cubic phase as well as with the Mo in multiple oxidized states. Such character leads to the production of large amount of oxygen deficiency with facilitated oxygen diffusion. The electrochemical performance tests of half-cell and single cell SOFCs exhibit the promoted effect of Mo on catalytic activity for the oxidation of H2 and CH4, indicating that Mo-PBMO could serve as an anode material candidate for SOFCs.

Impedance Spectroscopy Studies of the Behavior of MoNi-CeO2 Anode in SOFC Using H2S-Containing Hydrogen as Fuel

Development of anode materials that can be operated on sulphur-containing fuels is recognized as an important challenge for SOFC development. Doped or undoped ceria oxides have been investigated in metal cermet anodes due to good performance, low material cost and more resistance to sulphur poisoning. Ni-CeO2/YSZ anodes showed excellent stability during CH4 reforming and good tolerance to H2S contamination [1]. In addition, molybdenum appeared in the composition of promising anode materials for running on hydrocarbons or H2S-containing H2 as fuels, such as double perovskites based on Sr2MgMoO6 [2] and mixed metal sulphides M-MoS2 (M=Fe, Co, Ni) [3]. Based on these results, combined application of Mo and Ni with CeO2 (MoNi-Ce) could increase the sulphur tolerance with an optimum performance in H2 and CH4. In a previously published manuscript [4], MoNi-Ce, was tested in a single cell as anode material under H2 and CH4, and the results revealed that this compound exhibited a high coke resistance and stability in pure CH4. In this work, the effect of H2S on the cell performance for a single SOFC cell of MoNi-Ce/LDC/LSGM/LSCF has been investigated under different concentrations of H2S at 750, 800 and 850 ºC using IV curves, impedance spectroscopy and load demands. Various levels of dry H2S/H2 (0-500 ppm) from a pressurized H2S/H2 bottle containing 500 ppm H2S were mixed with humidified H2 to keep a total constant fuel flow of 50 ml/min. As example, Fig. 1 displays IV curves and Nyquist plots measured at 850 °C. In general, it can be observed a small increase in the OCV values with an increasing H2S concentration, being much more significant for 500 ppm H2S that could be directly related to the different partial pressure of H2O (pH2O). In the case of 500 ppm H2S/H2 composition, the gas is feeding without humidification. As expected, the maximum current density and the maximum power density (MPD) decrease with increasing H2S concentration and the temperature reduction. At the 500 ppm H2S, the MPD decreased by 44% (from 257 mW/cm2 to 143 mW/cm2) at 750 ºC and by 26% (from 414 mW/cm2 to 306 mW/cm2) at 850 ºC, compared with than that obtained in humidified H2. This could be due to the adsorption of sulphur on the active sites. However, the cell performance remained stable under load demand, during 1h, in all H2S/H2 concentrations studied at three temperatures. To investigate the effect of H2S on the electrocatalytic activity of MoNi-Ce anode for the H2 oxidation reaction, the impedance data were fitted to an equivalent circuit, Ri(R1Q1)(R2Q2)(R3Q3) using Zview program. The Ri represents to the internal resistance of the cell and the (RQ) components correspond to the involved electrode processes. The polarization resistance (Rp) of the cell is defined as R1+R2+R3. The impedance spectra revealed three distinct rate limiting processes at high, medium and low frequencies. At each temperature the internal resistance remained constant while the polarization resistance increased with the amount of H2S. The analysis of impedance spectra shows that the high frequency arc is not affected by the H2S concentrations and could be attributed to the charge transfer processes. The intermediate frequency arc is the most sensitive to the anode atmosphere and appears to be associated with the sulphur poisoning processes. While the low frequency arc became significantly larger when dry fuel was used, then it could be related to the change in pH2O in the fuel. Upon the removal of H2S from the fuel stream during 1 h, the anode performance can be partially recovered, being more significant at higher temperature. Acknowledgement This work was supported by Spanish Ministry of Economic and Competitiveness (MAT2013-45043-P).

Thermochemical model and experimental validation of a tubular SOFC cell comprised in a 1 kWel stack designed for μCHP applications

Being aware of the needs for clean highly efficient micro combined heat and power (μCHP) systems for single and multifamily households, the Italian Ministry of Industry launched in 2009 the EFESO Project aiming to develop and operate four SOFC prototypes. An imperative part of the project foresaw computational modeling to optimize operating conditions of the power modules and pinpoint potential drawbacks in its design. This article deals with a 3-dimensional thermochemical model of a single SOFC tubular geometry cell comprised in a 1kWel stack operating under similar conditions to the characterized power module. An analysis is presented on the effects of current density distribution, temperature distribution in the cell and pressure drop in the air and fuel channels, being these the most critical variables when operating the SOFC-powered μCHP system. This model will serve as a platform to generate a model of the whole stack which will be further validated by means of experimental activities.

Anode supported single chamber solid oxide fuel cells operating in exhaust gases of thermal engine

This project deals with the development and the electrochemical characterization of anode supported single chamber SOFC in a simulated environment of thermal engine exhaust gas. In the present work, a gas mixture representative of exhaust conditions is selected. It is composed of hydrocarbons (HC: propane and propene), oxygen, carbon monoxide, carbon dioxide, hydrogen and water. Only oxygen content is varied leading to different gas mixtures characterized by three ratios R = HC/O2. Concerning the cell components, a cermet made of nickel and an electrolyte material, Ce0.9Gd0.1O1.95 (CGO) is used as anode and two cathode materials, La0.6Sr0.4Co0.2Fe0.8O3−δ (LSCF) and Pr2NiO4+δ (PNO), are evaluated. The prepared cells are investigated in the various gas mixtures for temperatures ranging from 450 °C to 600 °C. Ni-CGO/CGO/LSCF-CGO cell has delivered a maximum power density of 15 mW cm−2 at 500 °C with R = HC/O2 = 0.21, while lower power densities are obtained for the other ratios, R = 0.44 and R = 0.67. Afterwards, LSCF and PNO cathode materials are compared and LSCF is found to deliver the highest power densities. Finally, by improving the electrolyte microstructure, some cells presenting a maximum power density of 25 mW cm−2 at 550 °C are produced. Moreover, up to 17% of initial HC are eliminated in the gas mixture.

CFD model of a methane fuelled single cell SOFC stack for analysing the combined effects of macro/micro structural parameters

A fully coupled CFD model of a direct internal reforming single cell SOFC stack previously designed by Ceramic Fuel Cell Ltd (CFCL) has been developed. In this model, an innovative solution technique for accelerating finite volume treatment of the electrodes as two distinct layers, a diffusion layer and a catalyst layer, is taken to analyse the combined effects of the macro/microstructural parameters on distribution of fields and each of the reactions involved in the process. To assess the simulation results, the model is not only evaluated with the CFCL experimental data that was reported from the similar geometry, but it is also assessed for the effects of the reforming and water gas shift reactions. It is found that a 3D model is more representative of the global reforming reaction rate. Furthermore, distributions of the key parameters along different spatial domains disclose the complex interaction between the anode flow field design and microstructural parameters of the anode diffusion layer. In fact, an optimal set of the anode microstructure that promotes the reforming reaction rate will not automatically result in improved SOFC performance. The developed model is a powerful tool to study complex fuel cell related problems and to optimize fuel cells’ structure.

Reversible oxygen removal and uptake in the La2ZnMnO6 double perovskite: Performance in symmetrical SOFC cells

A polycrystalline oxygen-stoichiometric La2ZnMnO6 double-perovskite oxide has been prepared by soft-chemistry procedures, followed by annealing in air at 800 °C. A reduced specimen, with a La2ZnMnO6−δ composition, has been obtained by topotactical oxygen removal in an H2/N2 (5%/95%) flow at 600 °C. The structural characterization has been conducted from neutron powder diffraction (NPD) data, very sensitive to the contrast between Zn and Mn and the oxygen stoichiometry. Both perovskites (oxidized and reduced) crystallize in the monoclinic P21/n space group, exhibiting an antisite Zn/Mn disorder of about 15% and 14%, respectively. The partial reduction of Mn4+ to Mn3+ in the reduced phase is accompanied with the occurrence of oxygen vacancies, located at the axial octahedral sites. Thermogravimetric analysis (TGA) substantiates the oxygen stoichiometry and the stability range. Magnetic susceptibility measurements indicate an antiferromagnetic behaviour, confirming the presence of Mn3+ ions in the structure. The magnetic structure of the reduced phase, determined from NPD data at 3 K, shows an antiferromagnetic G-type coupling between Mn at 2c and 2d sites (promoted by the anti-site disorder); the ordered magnetic moment at Mn site is 0.789 μB at 3 K. Both phases display a semiconductor-like behaviour with a maximum conductivity of 0.052 S cm−1 for the reduced phase at 650 °C, due to the occurrence of Mn3+–Mn4+ mixed valence. Moreover, the measured thermal expansion coefficients perfectly match with the values usually displayed by SOFC electrolytes. The reversibility and versatility of the present compounds as catalysts for oxygen reduction (cathode) or fuel oxidation (anode) were tested in single SOFC cells yielding power density spanning from 120 to 155 W/cm2 using these perovskites as anode, cathode and symmetric electrodes for SOFC.

Effect of flow geometry on anode-supported single chamber SOFCs arrayed as V-shape

The flow geometry of a single chamber solid oxide fuel cell was studied in order to reduce the effect of cell distance in the anode-facing-cathode stack. V-shape and converse V-shape configuration micro stacks consisting of two anode-supported yttria-stabilized zirconia membrane fuel cells were fabricated. The flow geometry reduced the effect of cell distance and improved the uniformity of performance of single cells in both stacks. The converse V-shape configuration was beneficial for maintaining the heat and the gases, in particular at low flow rates; the performance of the converse V-shape stack was better than that of the V-shape stack.

Exergy Analysis of an Intermediate Temperature Solid Oxide Fuel Cell-Gas Turbine Hybrid System Fed with Ethanol

In the present work, an ethanol fed Solid Oxide Fuel Cell-Gas Turbine (SOFC-GT) system has been parametrically analyzed in terms of exergy and compared with a single SOFC system. The solid oxide fuel cell was fed with hydrogen produced from ethanol steam reforming. The hydrogen utilization factor values were kept between 0.7 and 1. The SOFC’s Current-Volt performance was considered in the range of 0.1–3 A/cm2 at 0.9–0.3 V, respectively, and at the intermediate operating temperatures of 550 and 600 °C, respectively. The curves used represent experimental results obtained from the available bibliography. Results indicated that for low current density values the single SOFC system prevails over the SOFC-GT hybrid system in terms of exergy efficiency, while at higher current density values the latter is more efficient. It was found that as the value of the utilization factor increases the SOFC system becomes more efficient than the SOFC-GT system over a wider range of current density values. It was also revealed that at high current density values the increase of SOFC operation temperature leads in both cases to higher system efficiency values.

Characterization of (Y 1-xCa x)BaCo 4-yZn yO 7 as cathodes for intermediate temperature solid oxide fuel cells

The effects of Ca and Zn substitution, respectively, for Y and Co in (Y 1-xCa x)BaCo 4-yZn yO 7 (0.25 ≤ x ≤ 0.75 and 1.0 ≤ y ≤ 1.75) on the structure, high-temperature phase stability, thermal expansion coefficient (TEC), and electrochemical performance for intermediate temperature solid oxide fuel cells (IT-SOFC) have been investigated. The (Y 1-xCa x)BaCo 4-yZn yO 7 oxides crystallize in a trigonal P31 c symmetry similar to YBaCo 4O 7. The substitution of Zn for Co improves the long-term phase stability at high temperatures, but at the expense of the electrochemical performance. In contrast, the substitution of Ca for Y is improves electrochemical performances, but deteriorates the long-term phase stability at high temperatures at high Ca contents ( x = 0.75 and 1.0). Among the various chemical compositions investigated in the (Y 1-xCa x)BaCo 4-yZn yO 7 system, the (Y 0.5Ca 0.5)BaCo 2.5Zn 1.5O 7 composition offers a combination of good electrochemical performance and low TEC, while maintaining the phase stability at 600–800 °C for 120 h. The (Y 0.5Ca 0.5)BaCo 2.5Zn 1.5O 7 + GDC (50 : 50 wt. %) composite cathodes exhibit a maximum power density of ∼ 450 mW cm −2 at 700 °C in anode-supported single SOFC.

Transient Operation Effects of SOFCs Driven with Tar Loaded Synthesis Gas

This paper describes recent results from the utilization of tar loaded synthetic wood gas in a planar single SOFC (Ni-GDC/YSZ/LSM) at OCV conditions. The distinct influence of naphthalene, toluene and xylene on direct steam reforming of methane is investigated. It is shown that the high reactivity of naphthalene causes a reduction of the conversion of other hydrocarbons such as methane which eventually impacts the OCV. While toluene and xylene are converted almost completely, a transient response time for naphthalene is observed until steady conversion is achieved. This transient behavior is detected not only by FID measurements but also by the OCV curve. This overlaying effect occurs during the use of tar loaded wood gas with fluctuating PGCs and tar concentrations and is finally discussed to estimate the operation characteristic for real wood gas.

Effects of the single chamber SOFC stack configuration on the performance of the single cells

A single chamber solid oxide fuel cell (SC-SOFC) micro-stack with three series-wound cells has been prepared and operated in methane–oxygen–nitrogen mixture. A maximum power output of 282 mW was generated at the furnace temperature of 800 °C. A mutual effect was found among the three cells. Cell 2 between Cell 1 and Cell 3 produced a maximum power density of 215 mWcm − 2 which was higher than other cells. The heat effect and flow geometry were the main factors in inducing the difference among the three single cells.

Performance evaluation of combined solid oxide fuel cells with different electrolytes

The performance of a hybrid system of solid oxide fuel cells with different electrolytes, i.e., an oxygen-ion conducting electrolyte (SOFC-O 2−) and a proton-conducting electrolyte (SOFC-H +) is evaluated in this study. Due to an internal reforming operation, SOFC-O 2− can produce electrical power as well as high-temperature exhaust gas containing remaining fuel, i.e., H 2 and CO that can be used for SOFC-H + operation. The remaining CO can further react with H 2O via water gas-shift reaction to produce more H 2 within SOFC-H + and thus, the possibility of carbon formation in SOFC-H + can be eliminated and overall system efficiency can be improved. The simulation results show that the performance of the SOFC-O 2−–SOFC-H + system provides a higher efficiency (54.11%) compared with the use of a single SOFC. Further, the SOFC hybrid system performance is investigated with respect to important operating conditions, such as temperature, pressure, degree of pre-reforming, inlet fuel velocity, and cell voltage.

Microfabrication of single chamber SOFC with co-planar electrodes via multi-step photoresist molding with thermosetting polymer

In this paper, we report a simple and novel microfabrication method, named photoresist molding with thermosetting polymer (PRM-TP) which is very efficient for fabricating multiple ceramic structures with sub-micron to several millimeter scaled dimension. PRM-TP is a novel hybrid process combining lithographic tools and customized ceramic slurry process which can utilize the advantages of micro-scale ceramic fabrication with well controlled microstructure. In order to demonstrate general usefulness of PRM-TP, we applied this process for the construction of single chamber solid oxide fuel cell (SC-SOFC) with co-planar electrodes. According to the cell test at 500 °C under single chamber operation condition with pre-mixed gas of methane and air, the open circuit voltage and power output of SC-SOFC reached 205 mV and 67 mW/cm 2, respectively.

Study on Nanocomposites Based on Carbonate @ Ceria

Nanocomposites based on the ceria-carbonate composite have demonstrated as electrolytes in development of successful 300-600 degrees C fuel cell technology. In this paper, the nanocomposite electrolyte based on carbonate@SDC (SDC: samarium doped ceria) was directly synthesized from the co-precipitation method and characterized by XRD, SEM, TEM, BET, etc. It was proved that the carbonate@SDC was a two-phase material with average particle size about 14.5 nm (S(BET)) and crystalline size (D(XRD)) ranged from 12 to 14 nm. When the carbonate@SDC electrolyte was used to fabricate single SOFC, the cell shows remarkable performance with maximum power density 1000-1180 mw/cm2 at low temperature (300-550 degrees C).

Feedback control of solid oxide fuel cell spatial temperature variation

A high performance feedback controller has been developed to minimize SOFC spatial temperature variation following significant load perturbations. For thermal management, spatial temperature variation along SOFC cannot be avoided. However, results indicate that feedback control can be used to manipulate the fuel cell air flow and inlet fuel cell air temperature to maintain a nearly constant SOFC electrode electrolyte assembly temperature profile. For example temperature variations of less than 5 K are obtained for load perturbations of ±25% from nominal. These results are obtained using a centralized control strategy to regulate a distributed temperature profile and manage actuator interactions. The controller is based on H-infinity synthesis using a physical based dynamic model of a single co-flow SOFC repeat cell. The model of the fuel cell spatial temperature response needed for control synthesis was linearized and reduced from nonlinear model of the fuel cell assembly. A single 11 state feedback linear system tested in the full nonlinear model was found to be effective and stable over a wide fuel cell operating envelope (0.82–0.6 V). Overall, simulation of the advanced controller resulted in small and smooth monotonic temperature response to rapid and large load perturbations. This indicates that future SOFC systems can be designed and controlled to have superb load following characteristic with less than previously expected thermal stresses.

Numerical Simulation of Intermediate-Temperature Disk Type Seal-less SOFC Using Pure Hydrogen Considering Air Back Diffusion Phenomenon

A one-dimensional simulation code for single cell intermediate-temperature disk-type seal-less SOFC is improved to include the air back diffusion phenomenon, which implies the diffusion of air into the fuel channel due to the mixing of the anode and cathode exhaust gases and the complete oxidation of hydrogen at the seal-less edge of the cell. The numerically obtained current density-single cell voltage (i -V) characteristic curves coincide well with the experimental ones. The effect of the air back diffusion phenomenon on the cell performance is investigated. It is revealed that the single cell voltage reduction due to the air back diffusion phenomenon is negligible under normal operating conditions. However, it is made clear that the reduction of single cell voltage caused by the air back diffusion phenomenon becomes significant when the cell is operated under low fuel utilization conditions.

Performance Measurements of a Single-cell Stack Using Various Designs of Flow Distributors for Planar SOFC

In 2008, Shy and his co-workers proposed that flow uniformity may play a role on the cell performance of a single SOFC stack based on 3D numerical models, in which four different designs of flow distributors with the same 12 rectangular flow channels divided by eleven ribs having different degrees of flow uniformity were studied. This work provides direct experimental measurements for the first time to show how exactly the cell performance of a single-cell stack would vary with a change in the degree of flow uniformity in these aforementioned flow distributors. It is found that by improving the degree of flow uniformity in flow distributors, the cell's peak power density of the single-cell stack can be increased up to 14%. Also discussed are the start-up procedure and effects of operating temperatures and air and hydrogen flow rates to cell performance.