20 research outputs found
A proposed agglomerate model for oxygen reduction in the catalyst layer of proton exchange membrane fuel cells
Oxygen diffusion and reduction in the catalyst layer of PEM fuel cell is an important process in fuel cell modelling, but models able to link the reduction rate to catalyst-layer structure are lack; this paper makes such an effort. We first link the average reduction rate over the agglomerate within a catalyst layer to a probability that an oxygen molecule, which is initially on the agglomerate surface, will enter and remain in the agglomerate at any time in the absence of any electrochemical reaction. We then propose a method to directly calculate distribution function of this probability and apply it to two catalyst layers with contrasting structures. A formula is proposed to describe these calculated distribution functions, from which the agglomerate model is derived. The model has two parameters and both can be independently calculated from catalyst layer structures. We verify the model by first showing that it is an improvement and able to reproduce what the spherical model describes, and then testing it against the average oxygen reductions directly calculated from pore-scale simulations of oxygen diffusion and reaction in the two catalyst layers. The proposed model is simple, but significant as it links the average oxygen reduction to catalyst layer structures, and its two parameters can be directly calculated rather than by calibration
Method to improve catalyst layer model for modelling proton exchange membrane fuel cell
Correctly describing oxygen reduction within the cathode catalyst layer (CL) in modelling proton exchange membrane fuel cell is an important issue remaining unresolved. In this paper we show how to derive an agglomerate model for calculating oxygen reactions by describing dissolved oxygen in the agglomerates using two independent random processes. The first one is the probability that an oxygen molecule, which dissolves in the ionomer film on the agglomerate surface, moves into and then remains in the agglomerates; the second one is the probability of the molecule being consumed in reactions. The first probability depends on CL structure and can be directly calculated; the second one is derived by assuming that the oxygen reduction is first-order kinetic. It is found that the distribution functions of the first process can be fitted to a generalised gamma distribution function, which enables us to derive an analytical agglomerate model. We also expend the model to include oxygen dissolution in the ionomer film, and apply it to simulate cathode electrodes. The results reveal that the resistance to oxygen diffusion in ionomer film and agglomerate in modern CL is minor, and that the main potential loss is due to oxygen dissolution in the ionomer film
Modelling water intrusion and oxygen diffusion in a reconstructed microporous layer of PEM fuel cells
The hydrophobic microporous layer (MPL) in PEM fuel cell improves water management but reduces oxygen transport. We investigate these conflict impacts using nanotomography and pore-scale modelling. The binary image of a MPL is acquired using FIB/SEM tomography. The water produced at the cathode is assumed to condense in the catalyst layer (CL), and then builds up a pressure before moving into the MPL. Water distribution in the MPL is calculated from its pore geometry, and oxygen transport through it is simulated using pore-scale models considering both bulk and Knudsen diffusions. The simulated oxygen concentration and flux at all voxels are volumetrically averaged to calculate the effective diffusion coefficients. For water flow, we found that when the MPL is too hydrophobic, water is unable to move through it and must find alternative exits. For oxygen diffusion, we found that the interaction of the bulk and Knudsen diffusions at pore scale creates an extra resistance after the volumetric average, and that the conventional dusty model substantially overestimates the effective diffusion coefficient
Nanotomography based study of gas diffusion layers
Nano-computed tomography (nanoCT) was used for non-invasive 3D visualization and
characterization of porous gas diffusion layer (GDL) for polymer electrolyte membrane fuel cells
(PEMFC). The study was conducted using reconstruction of 3D images of a GDL of polymer
electrolyte fuel cell to determine the critical nanostructural parameters of the layer, such as porosity,
mean pore radii, structure model index and degrees of anisotropy. Furthermore, permeability of the
GDL was obtained through lattice Boltzmann numerical modeling
Flow properties of an intact MPL from nano-tomography and pore network modelling
Adding a hydrophobic micro-porous layer (MPL) between a gas diffusion layer (GDL) and a catalyst layer (CL) at the cathode of a PEM fuel cell was found capable of improving cell performance. However, how an MPL does this is not well-understood because current techniques are limited in measuring, observing and simulating multiphase pore fluid flow across the full range of pores that vary to a great extent in geometry, topology, surface morphology. In this work, we focused our investigation on estimating flow properties of an MPL volume to assess the limiting effect of strongly hydrophobic sub-micron pores on water transports. We adopted a nano-tomography and pore network flow modelling approach. A pore-structure model, purposely reconstructed from an intact MPL sample using Focused Ion Beam milling and Scanning Electron Microscope (FIB/SEM) previously, was used to extract a realistic pore network. A two-phase pore network flow model, developed recently for simulating the flow of gas, liquid or their mixture in both micrometre and nanometre pores, was applied to the pore network. We firstly tested the validity of the constructed pore network, and then calculated the properties: permeability for both water and selected gases, water entry pressure, and relative permeability. Knudsen diffusion was taken into consideration in calculations when appropriate. Our calculations showed that the water permeability was three orders of magnitude smaller than experimentally measured results reported in the literature, and when the water contact angle increased from 95° to 150°, the water-entry pressure increased from 2.5 MPa to 28 MPa. Thus our results revealed that for a strongly hydrophobic MPL that contains nanometre pores only it would behave like a buffer to water, and therefore the structural preferential paths in an MPL, such as cracks, are likely to be responsible for significant liquid water transport from the CL to the GDL that has been observed experimentally recently. We highlighted the needs for multi-scale modelling of the interplays of liquid water and gas transfer in MPLs that contain variable pores
Threshold fine-tuning and 3D characterisation of porous media using X-ray nanotomography
A common challenge in the X-ray nanotomography of porous media, such as fuel cell gas
diffusion layers (GDLs), is to binarize nanotomography greyscale images in order to differentiate
between solids and voids for structural characterisation and numerical flow analysis. In the process
threshold determination is critical. This paper presents a study on determination of and fine-tuning
threshold value based on comparison of material porosity and average fibre diameter obtained from
nanotomography images with porosity data from density experiments and average fibre diameter
achieved from scanning electron microscopy images respectively. The more accurate 3D
reconstructed model is then used to calculate pore size distribution and average pore size, while the
gas permeability of the representative 3D binary images are calculated using a single phase Lattice
Boltzmann (LB) model in the D3Q19 regime
Lattice Boltzmann simulation of water and gas flow in porous gas diffusion layers in fuel cells reconstructed from micro-tomography
The porous gas diffusion layers (GDLs) are key components in hydrogen fuel cells. During
their operation the cells produce water at the cathode, and to avoid flooding, the water has
to be removed out of the cells. How to manage the water is therefore an important issue in
fuel cell design. In this paper we investigated water flow in the GDLs using a combination
of the lattice Boltzmann method and X-ray computed tomography at the micron scale.
Water flow in the GDL depends on water–air surface tension and hydrophobicity. To
correctly represent the water–gas surface tension, the formations of water droplets in air
were simulated, and the water–gas surface tension was obtained by fitting the simulated
results to the Young–Laplace formula. The hydrophobicity is represented by the water–gasfabric
contact angle. For a given water–gas surface tension the value of the contact angle
was determined by simulating the formations of water droplets on a solid surface with
different hydrophobicity. We then applied the model to simulate water intrusion into
initially dry GDLs driven by a pressure gradient in attempts to understand the impact of
hydrophobicity on water distribution in the GDLs. The structures of the GDL were acquired
by X-ray micro-tomography at a resolution of 1.7 microns. The simulated results revealed
that with an increase in hydrophobicity, water transport in GDLs changes from piston-flow
to channelled flow
Influence of threshold variation on determining the properties of a polymer electrolyte fuel cell gas diffusion layer in X-ray nano-tomography
Morphological parameters of a 3D binary image of a porous carbon gas diffusion layer (GDL) for
polymer electrolyte fuel cells (PEFC) reconstructed using X-ray nano-tomography scanning have
been obtained, and influence of small alterations in the threshold value on the simulated flow
properties of the reconstructed GDL has been determined. A range of threshold values with 0.4%
increments on the greyscale map have been applied and the gas permeability of the binary images
have been calculated using a single-phase lattice Botlzmann model (LBM), which is based on the
treatment of nineteen velocities in the three dimensional domain (D3Q19). The porosity, degrees of
anisotropy and the mean pore radius have been calculated directly from segmented voxel
representation. A strong relationship between these parameters and threshold variation has been
established. These findings suggest that threshold selection can significantly affect some of the flow
properties and may strongly influence the computational simulation of micro and nano-scale flows in a
porous structure
The impacts of image resolution on permeability simulation of gas diffusion layer using lattice Boltzmann method
The effect of image resolution on gas permeability through the x-ray reconstructed carbon paper gas diffusion layer (GDL) of a polymer electrolyte fuel cell (PEFC) is examined in this paper. The 3D models of the GDL at six different resolutions are obtained by the x-ray tomography imaging technique. Each GDL image is then characterized its gas permeability through the lattice Boltzmann (LB) numerical method. The results suggest that the image resolution has a great impact on gas permeability in both principal and off-principal flow directions. The coarser resolutions can contribute to significant changes in the resulting permeability. However, it can reduce computational time to a great extent. The results also indicate that the GDL image at the resolution of 2.72 μm provides a good compromise between computational time and accuracy
Simulation of liquid water beakthrough in a nano-tomography reconstruction of a carbon paper gas diffusion layer
This study reports the feasibility of newly simulating liquid water intrusion into the porous gas diffusion
layer (GDL) of a polymer electrolyte fuel cell (PEFC) using X-ray nano-tomography and two-phase
lattice Boltzmann (LB) simulation. A digital 3D model of the GDL is reconstructed using X-ray nanotomography
while two-phase porous flow is simulated at two different levels of surface wettability by
applying a newly-developed numerical LB model. The results show liquid infiltration in a hydrophobic
GDL is comparatively lower (pore saturation of 0.11 to 0.90) than that for a hydrophilic GDL (pore
saturation of 0.36 to 0.96) over the liquid intrusion range of 1 kPa . 100 kPa. Visualisation of simulated
results in three dimensions reveal dissimilar liquid infiltration characteristics for the two levels of
wettability considered, yet also reveal a general breakthrough of liquid water at a pressure of 10 kPa
due to specific structural features of the GDL