Development of in-plane models for the analysis of dead-ended and anode bleeding operation modes and the cell degradation with carbon corrosion

Low durability and high cost are main drawbacks against commercialization of the proton exchange membrane fuel cell (PEMFC). The anode-bleeding (AB) operation mode offers a very high hydrogen utilization and cost reduction by eliminating additional components for the recovery of hydrogen in the flow-through mode and avoids the carbon corrosion reaction (CCR), which causes degradation of the catalyst layer and occurs typically when the bleeding rate is set to zero in the dead-ended (DEA) mode. Three-dimensional (3D) models are necessary for the analysis of PEMFCs. However, computational cost of 3D models is extremely high because of the strong nonlinearity due to complex interactions in the cell. In this dissertation, a pseudo-three-dimensional (P3D) two-phase and non-isothermal model is developed to reduce the computational cost and predict the cell performance with high accuracy. Results demonstrate that the P3D model results compare very well with ones from 3D model and experimental data from the literature. The P3D model is used to investigate the effects of geometric and operation parameters on the cell performance under DEA and AB operation modes. Moreover, the bleeding rate is optimized to sustain a stable transient cell voltage without the CCR in the cathode catalyst layer (CCL) while hydrogen utilization is kept at more than 99%. Furthermore, effects of the anode flow field design on the cell performance under the AB operation mode are investigated. Lastly, the P3D model is used to study effects of cell degradation on transport properties of the CC

Design and modeling of a large proton exchange membrane fuel cell with high hydrogen utilization for automotive applications

Performance of proton exchange membrane fuel cell (PEMFC) depends on several factors, such as flow fields design, cooling technique, species transport, and water management. In order to enhance the performance of a high power (automotive) PEMFC, three-dimensional model of the anode flow field with ultra-low stoichiometric flow condition and without the effect of species transport, two-dimensional model of the anode flow field with species transport, and three-dimensional serpentine flow fields for the cathode and cooling domains are studied and optimized. In the anode models, widths of the channels and ribs and configurations of their headers are investigated to obtain a uniform flow and hydrogen concentration distribution through the channels. For the anode flow field, two approaches lead to different optimum designs, however, we prefer the one from the two-dimensional model with the mass transport. In the final design of the anode flow field, the hydrogen-depletion region ratio is less than 0.2%. In the cathode model, an unstructured search is used to obtain a design that has a pressure drop within 30% of the output power. In the cooling model, dimensions of the channels and ribs, and pressure difference between the inlet and outlet manifolds are investigated to find a uniform temperature distribution through the cooling plate with index of uniform temperature (IUT) less than 3 °C. Finally, a one-dimensional model of species and liquid water transport and distribution through the anode and cathode channels and their gas diffusion layers (GDLs) is studied. Results of this model agree reasonably with experimental data

Effects of PEM fuel cell degradation on the transport properties of the cathode catalyst layer

Durability is a major issue against the commercialization of proton exchange membrane fuel cells (PEMFC). Several mechanisms play an important role in the degradation of the cathode catalyst layer (CCL) by deteriorating the transport properties of reactants in the CCL mainly. A pseudo three-dimensional (P3D), two-phase, and non-isothermal model is used to study the effects of cell degradation on the transport properties of the CCL. Accuracy of the model is verified by comparing the polarization curves from the model with the experimental ones reported in the literature. The model is used to investigate the effects of CCL transport properties and agglomerate parameters on cell performance. Results demonstrate that the cell performance is improved for thinner ionomer film around agglomerates, smaller agglomerates, higher exchange current density, lower transport resistance and higher proton conductivity of the CCL. The transport parameters of the CCL are varied to fit the polarization curves to the experimental ones for an accelerated stress test. It is found that the transport resistances increase exponentially with the carbon loss in the CCL

Modeling of flow distribution in proton exchange membrane fuel cell

Analysis and design of flow fields for proton exchange membrane fuel cell (PEMFC) require coupled solution of the flow fields, gas transport and electrochemical reaction kinetics in the anode and the cathode. Computational cost prohibits the widespread use of three-dimensional models of the anode and cathode flow fields, gas diffusion layers (GDL), catalyst layers (CL) and the membrane for fluid flow and mass transport. On the other-hand, detailed cross-sectional two-dimensional models cannot resolve the effects of the anode and cathode flow field designs. Here, a two-dimensional in-plane model is developed for the resolution of the effects of anode and cathode flow channels and GDLs, catalyst layers are treated as thin-layers of reaction interfaces and the membrane is considered as a thinlayer that resist the transfer of species and the ionic current. Brinkman equations are used to model the in-plane flow distribution in the channels and the GDLs to account for the momentum transport in the channels and the porous GDLs. Fick’s law equations are used to model transport of gas species in the channels and GDLs by advection and diffusion mechanisms, and electrochemical reactions in the CL interfaces are modeled by Butler-Volmer equations. Complete features of the flow in the channels and inlet and outlet manifolds are included in the model using resistance relationships in the through-plane direction. The model is applied to a small cell having an active area of 1.3 cm2 and consisting of 8 parallel channels in the anode and a double serpentine in the cathode. Effects of the anode and cathode stoichiometric ratios on the cell performance and hydrogen utilization are investigated. Results demonstrate that for a sufficiently high cathode stoichiometric ratio enough, anode stoichiometric ratio can be lowered to unity to obtain very high hydrogen utilization and output power

Design and optimization of anode flow field of a large proton exchange membrane fuel cell for high hydrogen utilization

We developed a CFD model of the anode flow field of a large proton exchange membrane fuel cell that operates under the ultra-low stoichiometric (ULS) flow conditions which intend to improve the disadvantages of the dead-ended operation such as severe voltage transient and carbon corrosion. Very small exit velocity must be high enough to remove accumulated nitrogen, and must be low enough to retain hydrogen in the active area. Stokes equations are used to model the flow distribution in the flow field, Maxwell-Stefan equations are used to model the transport of the species, and a voltage model is developed to model the reactions kinetics. Uniformity of the distribution of hydrogen concentration is quantified as the normalized area of the region in which the hydrogen mole fraction remains above a certain level, such as 0.9. Geometry of the anode flow field is modified to obtain optimal configuration; the number of baffles at the inlet, width of the gaps between baffles, width of the side gaps, and length of the central baffle are used as design variables. In the final design, the hydrogen-depleted region is less than 0.2{\%} and the hydrogen utilization is above 99{\%}

Modeling and performance analysis of branched microfluidic fuel cells with high utilization

Microfluidic fuel cells (MFCs) are effective energy conversion devices in microscales due to their simplicity and high energy density as they are based on laminar co-flow of reactants in microchannels without a separator. However, fuel utilization at practical flow rates remains as one of the important challenges. In this study, a two-dimensional (2D) model is developed based on Brinkman equations for flow, Fick's law for mass transport and Butler-Volmer equations for reaction kinetics to study MFCs and address the effects of geometric and operation parameters on the performance and fuel utilization. Commercial finite-element software, COMSOL, is used to solve coupled equations and to analyze the performance of MFCs for different Peclet numbers, concentrations of reactants and geometric variables. According to simulation results, the 2D model compares very well with the three-dimensional model based on Navier-Stokes equations and with the experimental data reported in the literature. Moreover, the model is used for the analysis of the proposed branched-channel design of microfluidic cells that improves the fuel utilization and power output of the MFC

Effects of PEM fuel cell degradation on the transport properties of the cathode catalyst layer

Durability is a major issue against the commercialization of proton exchange membrane fuel cells (PEMFC). Several mechanisms play an important role in the degradation of the cathode catalyst layer (CCL) by deteriorating the transport properties of reactants in the CCL mainly. A pseudo three-dimensional (P3D), two-phase, and non-isothermal model is used to study the effects of cell degradation on the transport properties of the CCL. Accuracy of the model is verified by comparing the polarization curves from the model with the experimental ones reported in the literature. The model is used to investigate the effects of CCL transport properties and agglomerate parameters on cell performance. Results demonstrate that the cell performance is improved for thinner ionomer film around agglomerates, smaller agglomerates, higher exchange current density, lower transport resistance and higher proton conductivity of the CCL. The transport parameters of the CCL are varied to fit the polarization curves to the experimental ones for an accelerated stress test. It is found that the transport resistances increase exponentially with the carbon loss in the CCL

A pseudo three-dimensional, two-phase, non-isothermal model of proton exchange membrane fuel cell

Three-dimensional models of the proton exchange membrane fuel cell are necessary to study important issues such as water and thermal management and flow field design, but not practical due to computational cost of simulation of a full cell. Here, we present a pseudo three-dimensional model to mitigate the computational cost of three-dimensional models. The model includes in-plane transport equations in channels and gas diffusion layers explicitly, and through-plane transport is based on resistance relationships through the thin membrane and catalyst layers. Polarization curves and distributions of species for a small section of a fuel cell that consists of straight channels, gas diffusion layers, catalyst layers and the membrane are compared with the results from a full three-dimensional model of the same cell for the variations in the channel height, gas diffusion layer thickness, widths of the channels and ribs, operation temperature, and relative humidity of the reactant gases at the inlets. Overall, results from the pseudo three-dimensional model compare very well with the ones from the full three-dimensional model and experimental data