Diffusion Limitations in the Porous Anodes of SOFCs

Williford, Rick E.; Chick, L.A.; Maupin, Gary D.; Simner, Steven P.; Stevenson, Jeffry W.

doi:10.1149/1.1586300

Cited by 181 publications

(156 citation statements)

References 38 publications

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“…In the absence of this information, effective transport properties of porous structures have been derived experimentally by means of diffusion cell experiments 16,17,[64][65][66][67][68][69][70] and electrochemical measurements. 15,19,46,71 …”

Section: Experimentally Derived Tortuositymentioning

confidence: 99%

Tortuosity in electrochemical devices: a review of calculation approaches

Tjaden

Brett

Shearing

2016

International Materials Reviews

211

188

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The tortuosity of a structure plays a vital role in the transport of mass and charge in electrochemical devices. Concentration polarisation losses at high current densities are caused by mass transport limitations and are thus a function of microstructural characteristics. As tortuosity is notoriously difficult to ascertain, a wide and diverse range of methods has been developed to extract the tortuosity of a structure. These methods differ significantly in terms of calculation approach and data preparation techniques. Here, we review tortuosity calculation procedures applied in the field of electrochemical devices to better understand the resulting values presented in the literature. Visible differences between calculation methods are observed, especially when using porosity-tortuosity relationships and when comparing geometric and flux based tortuosity calculation approaches.

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Section: Experimentally Derived Tortuositymentioning

confidence: 99%

Tortuosity in electrochemical devices: a review of calculation approaches

Tjaden

Brett

Shearing

2016

International Materials Reviews

211

188

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“…13,20 But the SM and DGM models have rarely been used before to analytically analyze impedance spectra of SOFC, although they have been used to describe gas diffusion in porous electrodes. By taking the derivative of the concentration overpotential with respect to current and evaluating it at a specified current, diffusion resistance (R b ) is obtained for all three models.…”

Section: Bulk Diffusion Impedance (R Bmentioning

confidence: 99%

Multicomponent Gas Diffusion in Porous Electrodes

Jiang

Poizeau

et al. 2015

J. Electrochem. Soc.

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Multicomponent gas transport is investigated with unprecedented precision by AC impedance analysis of porous YSZ anodesupported solid oxide fuel cells. A fuel gas mixture of H 2 -H 2 O-N 2 is fed to the anode, and impedance data are measured across the range of hydrogen partial pressure (10-100%) for open circuit conditions at three temperatures (800 • C, 850 • C and 900 • C) and for 300 mA applied current at 800 • C. For the first time, analytical formulae for the diffusion resistance (R b ) of three standard models of multicomponent gas transport (Fick, Stefan-Maxwell, and Dusty Gas) are derived and tested against the impedance data. The tortuosity is the only fitting parameter since all the diffusion coefficients are known. Only the Dusty Gas Model leads to a remarkable data collapse for over twenty experimental conditions, using a constant tortuosity consistent with permeability measurements and the Bruggeman relation. These results establish the accuracy of the Dusty Gas Model for multicomponent gas diffusion in porous media and confirm the efficacy of electrochemical impedance analysis to precisely determine transport mechanisms. The Solid Oxide Fuel Cell (SOFC) is currently the highesttemperature fuel cell in development and can be operated over a wide temperature range from 600• C-1000• C allowing a number of fuels to be used. To operate at such high temperatures, the electrolyte is a thin, nonporous solid ceramic membrane that is conductive to charge carrier, O 2− ions. The operating efficiency in generating electricity is among the highest of the fuel cells at about 60%.1 Furthermore, the high operating temperature allows cogeneration of high-pressure steam that can be used in many applications. Combining a hightemperature SOFC with a turbine into a hybrid fuel cell further increases the overall efficiency of generating electricity with a potential of an efficiency of more than 70%.1 Therefore, it is a very promising alternative energy source that could potentially be used for home heating or large scale electricity production in the future.Solid oxide fuel cell consists of a porous cathode, an electrolyte, a porous anode and interconnects. Two different types have been explored in the development of SOFC, the electrolyte supported cell and the electrode supported cell. In the former, electrolyte is the thickest and serves as the mechanical support for the whole cell. However, due to the high Ohmic resistance of the relatively thick electrolyte layer, the electrolyte supported design has been gradually replaced by the new electrode supported cells, in which one of the porous electrodes is the supporting structure. Moreover, since cathode supported cell usually gives higher resistance, and is much harder to fabricate due to the mismatched thermal expansion coefficient of cathode support and functional layer, the anode supported cell (ASC) is the most widely accepted design in current SOFC research.The solid oxide fuel cell is operated with fuel and oxidant being continuously fed from two sides of the ce...

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“…2,3,4 The electrochemistry model used was described by Chick et al, 5 calibrated 6,7 for application to planar stack simulations, and updated to provide an improved anode concentration polarization model. 8 The capabilities of these tools have also been expanded to incorporate steam-methane reformation for simulating on-cell reforming 9 and have been updated with a rate expression derived experimentally at PNNL:…”

Section: Methodsmentioning

confidence: 99%

Analysis of Percent On-Cell Reformation of Methane in SOFC Stacks: Thermal, Electrical and Stress Analysis

Recknagle¹,

Yokuda²,

Jarboe³

et al. 2006

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Executive SummaryThis report summarizes a parametric analysis performed to determine the effect of varying the percent on-cell reformation (OCR) of methane on the thermal, electrical, and mechanical performance for a generic, planar solid oxide fuel cell (SOFC) stack design. OCR of methane can be beneficial to an SOFC stack because the reaction (steam-methane reformation) is endothermic and can remove excess heat generated by the electrochemical reactions directly from the cell. The heat removed is proportional to the amount of methane reformed on the cell. Rapid reaction kinetics provided by the high-temperature SOFC operation and excess steam over the nickel-based anode catalyst ensure complete methane conversion. Thus, the thermal load varies with methane concentration entering the stack. The endotherm due to the fast reformation reaction can cause a temperature depression on the anode near the fuel inlet, resulting in large thermal stresses. This effect depends on factors that include inflowing methane concentration, local temperature, and stack geometry.The analysis assumed the fuel would be partially to fully pre-reformed in an external reformer such that the desired fuel compositions would be delivered to the stack, where the remaining percentage of the reformation reaction would be completed on-cell. Simulations were performed using an SOFC stack modeling tool developed at PNNL and validated for the prediction of fuel use, on-cell methane reforming, and distribution of temperature. The study was performed using three-dimensional stack model geometries. Cross-flow, co-flow, and counter-flow configuration stacks of 10x10-and 20x20-cm cell sizes were examined. Thermal performance was evaluated based on the predicted maximum temperature difference on the anode. Electrical performance was based on the predicted power output. Mechanical performance was based on the maximum principal stress on the anode. Fuel utilization was established at 75%. The effect of cathode air cooling was included in the study by examination of 30% and 15% air utilizations.The analysis showed for the counter-flow and cross-flow stacks of 10x10-cm size the stress and temperature difference would be minimized when between 40 and 50% of the reformation reaction occurred on the anode. Gross electrical power density was virtually unaffected by %OCR. For all stack configurations and sizes the inflow temperature increased with %OCR as the subsequent heat load decreased. Cooling provided by the cathode airflow associated with 30% air utilization was not substantially improved upon by 15% air use for the smaller (10x10-cm) stack size. The increased airflow associated with 15% air utilization was needed for cooling the larger (20x20-cm) stacks. The co-flow stack exhibited the largest benefit from the additional cathode air cooling and had the lowest anode stresses of the 20x20-cm stacks. For the conditions and particular generic stacks of this study, the results suggest 40 to 50% OCR should be considered for cross-flow and counter-flow stacks, ...

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Diffusion Limitations in the Porous Anodes of SOFCs

Cited by 181 publications

References 38 publications

Tortuosity in electrochemical devices: a review of calculation approaches

Tortuosity in electrochemical devices: a review of calculation approaches

Multicomponent Gas Diffusion in Porous Electrodes

Analysis of Percent On-Cell Reformation of Methane in SOFC Stacks: Thermal, Electrical and Stress Analysis

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