3D Characterization of Diffusivities and Its Impact on Mass Flux and Concentration Overpotential in SOFC Anode

Abstract

In recent years great effort has been taken to understand the effect of gas transport on the performance of electrochemical devices. This study aims to characterize the diffusion regimes and the possible inaccuracies of the mass transport calculation in Solid Oxide Fuel Cell (SOFC) anodes when a volume-averaged pore diameter is used. 3D pore size distribution is measured based on the extracted pore phase from an X-ray CT scan, which is further used for the calculation of a Knudsen number (K n ) map in the porous medium, followed by the voxel-based distribution of the effective diffusion coefficients for different fuel gases. Diffusion fluxes in a binary gas mixture using the lower boundary, upper boundary and average effective coefficients are compared, and the impact on overpotential is analyzed. The results show that pore diameters from tens to hundreds of nanometers result in a broad range of Knudsen number (1.1 ∼ 4.8 and 0.6 ∼ 3 for H 2 and CH 4 respectively), indicative of the transitional diffusion regime. The results highlight that for a porous material, such as an SOFC anode where Knudsen effects are non-negligible, using a volume-averaged pore size can overestimate the mass flux by ±200% compared to the actual value. The characteristic pore size should be chosen sensibly in order to improve the reliability of the mass transport and electrochemical performance evaluation. Mass transport can significantly limit the reaction rate and lead to concentration polarization in electrochemical devices such as fuel cells, electrolysers and oxygen transport membranes, especially under the conditions of high operating current density and fuel conversion ratios.1 Besides the material-specific transport properties of chemical species (e.g., diffusivity, viscosity, etc.), mass transport is mainly dependent on microstructural parameters of the electrode such as porosity, tortuosity and pore size. [2][3][4] In recent years great effort has been taken to understand the effect of gas transport on the performance of electrochemical devices in terms of electrochemical impedance, 5 concentration polarization loss, 4,6 and electrochemical simulation. 7-10Concentration polarization is caused by the consumption of the fuel gas resulting in a reduction of concentration at the anode/electrolyte interface. In solid oxide fuel cell (SOFC) anodes, concentration polarization is governed by the diffusion of fuel gas (e.g. H 2 ) from the gas inlet to the triple phase boundary. A faster diffusion rate can contribute to a larger partial pressure of the fuel gas within the pores and thus a larger limiting current density, which in turn mitigates the polarization loss. 11The effective diffusivity is closely related to the temperature, pressure, gas species as well as to the microstructure of the porous material.12 A considerable amount of work has been reported to measure the diffusivity, such as the experimental methods including diffusion cells, 13 gas chromatography, 14 thermo-gravimetric analysis, 15 as well as theoretical calculations ...

Developments in X-ray tomography characterization for electrochemical devices

“…Heenan et al reported the first three-phase segmentation of SOFC anode materials using lab-based X-ray nano-CT [159]. Since then a wealth of studies have been accomplished from the use of morphological computations on X-ray CT data to investigate the effects of Ni densification on the TPB density [160], to the investigation of phase tortuosity and its impact on mass flux [161,162] (Fig. 9).…”

Contradictory concepts in tortuosity determination in porous media in electrochemical devices

“…This might also explain the appreciable difference between the diffusion cell experiment tortuosity values and the image-based results shown in the next section. The effect of varying pore diameter on the diffusion behaviour has been recently analysed and published (Lu et al, 2017).…”

The Imaging Resolution and Knudsen Effect on the Mass Transport of Shale Gas Assisted by Multi-length Scale X-Ray Computed Tomography

“…Transport of gas molecules in porous media is mainly governed by two mechanisms: (1) continuum flow, in which the gas molecules interaction is dominant and is often modelled as a viscous effect in continuum physics and (2) the collisions between gas molecules and the wall, also known as molecular flow 18 . The predominant mechanism(s) in the transport regime will depend on the gas species, temperature, pressure and microstructure 19–21 . The Knudsen number K n , calculated as the ratio between the mean free path of the gas molecules and the pore size, is widely used to assess the flow regime in porous media: If K n < 0.01 (continuum regime), the flow is mainly governed by molecular diffusion and the Knudsen flow can be neglected; if K n > 10 (Knudsen regime), the gas is highly rarefied and effect of molecular flow outweighs the viscous flow in the continuum regime because of the frequent collisions between the molecules and the porous medium.…”

“…The wide distribution of the pore size causes two problems in the mass transport study: (1) it is not reliable to estimate the Knudsen-based diffusivity based on the averaged pore size, which could potentially over-estimate the gas flow due to the constriction effect 21,22 ; (2) Viscous flow fails in smaller pore spaces as the diffusion flow mechanisms associated with pore-wall interactions become dominant 23 , which leads to under-estimating the permeability. This means conventional continuum physics can no longer describe the flow field in shales 24 .…”