Multicomponent Gas Diffusion in Porous Electrodes

Abstract

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...

Investigation of convective transport in the gas diffusion layer used in polymer electrolyte fuel cells

“…When Kn ∼ 1, as in the case described above, there is already significant effects due to interactions with the porous material, increasing with Kn [29]. A recent work by Fu and colleagues [30] shows the importance of a detailed treatment of diffusion using the dusty gas model, which incorporates the Maxwell-Stefan model of multicomponent diffusion, the Knudsen regime and pressure gradients. The author show that only by taking into account these different physical processes, a good qualitative comparison with experimental data can be achieved.…”

“…well bellow the value where molecular diffusion is expected to be significant, and slightly smaller than the one that can be calculated from [30]. Nevertheless, some tests were carried (see Section III of SI), using the Mixture-averaged model, with or without Knudsen diffusion, and the Maxwell-Stefan model, all coupled to the DB formulation.…”

Pressurized Operation of a Planar Solid Oxide Cell Stack

“…[25] Further, it is important to distinguish between Knudsen gas diffusion and binary gas diffusion in the fuel electrode support layer. A comprehensive analytic description involving the dusty gas model have recently been provided by Fu et al [34] Obviously, when the spectra are recorded with oxygen to the air-electrode binary gas diffusion doesn't contribute to the impedance of that electrode. When air is supplied to the air-electrode both binary gas diffusion and Knudsen gas diffusion will contribute to the impedance, although on hydrogen electrode supported single cells the contribution from air-electrode is expected to be small compared to the contribution from the fuel-electrode.…”

“…When air is supplied to the air-electrode both binary gas diffusion and Knudsen gas diffusion will contribute to the impedance, although on hydrogen electrode supported single cells the contribution from air-electrode is expected to be small compared to the contribution from the fuel-electrode. [18,34] On single cells, switching from air to oxygen have been shown to affect distribution of relaxation-time (DRT) deconvolutions of impedance spectra at around 10 Hz. [35] Due to the difference in geometry between single cells and stacks this could possibly be the same process that affects the stack impedance at a characteristic frequency around 1-2 Hz.…”

The application of hierarchical structures in energy devices: new insights into the design of solid oxide fuel cells with enhanced mass transport

“…Moreover, as the Knudsen number of the typical SOFC anodes falls within the range of transitional diffusion regime, 27 molecule/wall collisions and slippage effects become significant, but are often neglected in models using Fick's law and StefanMaxwell equations. 28,29 Direct simulation of the molecule/wall Fig. 1 Illustration of the workflow in this study.…”