In this study, an analysis of the current distribution and oxygen diffusion in the Polymer Electrolyte Fuel Cell (PEFC) Cathode Catalyst Layer (CCL) has been carried out using Electrochemical ...Impedance Spectroscopy (EIS) measurements. Cathode EIS measurements obtained through a three-electrode configuration in the measurement system are compared with simulated EIS data from a previously validated numerical model, which subsequently allows the diagnostics of spatio-temporal electrochemical performance of the PEFC cathode. The results show that low frequency EIS measurements commonly related to mass transport limitations are attributed to the low oxygen equilibrium concentration in the CCL–Gas Diffusion Layer (GDL) interface and the low diffusivity of oxygen through the CCL. Once the electrochemical and diffusion mechanisms of the CCL are calculated from the EIS measurements, a further analysis of the current density and oxygen concentration distributions through the CCL thickness is carried out. The results show that high ionic resistance within the CCL electrolyte skews the current distribution towards the membrane interface. Therefore the same average current density has to be provided by few catalyst sites near the membrane. The increase in ionic resistance results in a poor catalyst utilization through the CCL thickness. The results also show that non-steady oxygen diffusion in the CCL allows equilibrium to be established between the equilibrium oxygen concentration supplied at the GDL boundary and the surface concentration of the oxygen within the CCL. Overall, the study newly demonstrates that the developed technique can be applied to estimate the factors that influence the nature of polarization curves and to reveal the effect of kinetic, ohmic and mass transport mechanisms on current distribution through the thickness of the CCL from experimental EIS measurements.
► Cathode impedance measurements gathered with a three-electrode configuration. ► Comparison between measured and simulated data using a validated impedance model. ► Electrochemical and diffusion mechanisms calculated from impedance measurements. ► Analysis of current density and oxygen distributions in the cathode catalyst layer. ► Effect of kinetic, ohmic and mass transport mechanisms on current distribution.
A lattice Boltzmann (LB) method is developed in this article in a combination with X-ray computed tomography to simulate fluid flow at pore scale in order to calculate the anisotropic permeability of ...porous media. The binary 3D structures of porous materials were acquired by X-ray computed tomography at a resolution of a few microns, and the reconstructed 3D porous structures were then combined with the LB model to calculate their permeability tensor based on the simulated velocity field at pore scale. The flow is driven by pressure gradients imposed in different directions. Two porous media, one gas diffusion porous layer used in fuel cells industry and glass beads, were simulated. For both media, we investigated the relationship between their anisotropic permeability and porosity. The results indicate that the LB model is efficient to simulate pore-scale flow in porous media, and capable of giving a good estimate of the anisotropic permeability for both media. The calculated permeability is in good agreement with the measured date; the relationship between the permeability and porosity for the two media is well described by the Kozeny–Carman equation. For the gas diffusion layer, the simulated results showed that its permeability in one direction could be one order of magnitude higher than those in other two directions. The simulation was based on the single-relaxation time LB model, and we showed that by properly choosing the relaxation time, it could give similar results to those obtained using the multiple-relaxation time (MRT) LB method, but with only one third of the computational costs of MRTLB model.
This research reports a feasibility study into multiscale polymer electrolyte fuel cell (PEFC) modeling through the simulation of macroscopic flow in the multilayered cell via one-dimensional (1D) ...electrochemical modeling, and the simulation of microscopic flow in the cathode gas diffusion layer (GDL) via three-dimensional (3D) single-phase multicomponent lattice Boltzmann (SPMC-LB) modeling. The heterogeneous porous geometry of the carbon-paper GDL is digitally reconstructed for the SPMC-LB model using X-ray computer microtomography. Boundary conditions at the channel and catalyst layer interfaces for the SPMC-LB simulations such as specie partial pressures and through-plane flowrates are determined using the validated 1D electrochemical model, which is based on the general transport equation (GTE) and volume-averaged structural properties of the GDL. The calculated pressure profiles from the two models are cross-validated to verify the SPMC-LB technique. The simulations reveal a maximum difference of 2.4% between the thickness-averaged pressures calculated by the two techniques, which is attributable to the actual heterogeneity of the porous GDL structure.
The Polymer Electrolyte Fuel Cell (PEFC) is well-poised to play a key role in the portfolio of future energy technologies for civil and military applications. Principally, the PEFC converts part of ...the chemical energy released during hydrogenoxidation and oxygen-reduction into electrical energy, generating water a bi-product. It is potentially a zero-emissions technology which can operate silently due to the absence of any moving parts, has quick start-up characteristics and can achieve high thermodynamic efficiency. In order to ensure that the PEFC emerges as a viable option for all applications, it is necessary to ensure that the technology is reliable, capable of delivering performance and cost-effective throughout its life-cycle. To achieve these objectives, a better fundamental understanding of the mechanisms of electrochemical transport in the PEFC is required than is presently available. The literature identifies that multi-component electrochemical transport within the PEFC plays a central role in fuel cell operation and longevity. Water transport is one of these. It is well-understood that excessive amounts of water within the porous electrodes of the cell can cause flooding, which impedes the supply of reactant gases. It is also well-understood that insufficient water can cause the polymer electrolyte membrane (PEM) to dehydrate, thereby reducing its proton conductivity. Both of these processes can undermine cell performance. Repetitive hydration cycles are also known to precipitate degradation mechanisms which can undermine reliability. However, the mechanisms of multi-component and potentially two-phase transport across the PEFC as a multi-layered assembly which includes the porous electrodes and the PEM are not understood as well: the mechanisms of contaminant transport, fuel crossover and liquid water infiltration particularly through the PEM are important examples. The modelling literature demonstrates that electrochemical transport in the PEFC is treated either through the use of dilute solution theory or concentrated solution theory. The modelling literature also demonstrates a wide spectrum in the application of modelling assumptions and the formulation of electrochemical equations to simulate transport in the different layers of the PEFC. This thesis describes research aimed at reconciling the different modelling approaches and philosophies in the literature by developing and applying a unified mechanistic electrochemical treatment to describe multi-component, two-phase transport across the layers of the PEFC. The approach adopted here is first to construct a multi-component zerodimensional model for multi-component input gases which is merged with a multilayer PEFC model to correctly predict the boundary conditions in the gas channels based on the cross-flow of components through the cell. The model is validated using data from the open literature and applied to understand contaminant crossover from anode to cathode. The second step is to develop a unified mechanistic electrochemical treatment to describe multi-component transport across the layers of the PEFC: the general transport equation. This is central to the contribution of this thesis. It is theoretically validated by deriving the key transport equations used in the benchmark fuel cell modelling literature. It is then implemented with the multi-component input model developed previously and validated using data from the open literature. The model is subsequently applied to understand fuel crossover characteristics in the cell. The third and final step is to further-develop the application of the general transport equation to account for two-phase transport across the layers of the PEFC. The resulting model is validated against three different sets of data from the open literature and subsequently applied to understand the effects of PEM thickness, anode gas humidification, cell compression and PEM structural reinforcement on liquid infiltration and two-phase transport across the PEM. It is demonstrated that the general transport equation developed in this thesis establishes a backbone understanding of the modelling and simulation of transport across the layers of the PEFC. The study successfully reconciles the different modelling philosophies in the fuel cell literature. The progressive validation and application of the general transport equation demonstrates the potential to enhance the scientific understanding of factors affecting PEFC performance and demonstrates its value as a tool for computationally-based cell design, optimisation and diagnostics.
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