Transient analysis for the cathode gas diffusion layer of PEM fuel cells

A one-dimensional, non-isothermal, two-phase transient model has been developed to study the transient behaviour of water transport in the cathode gas diffusion layer of PEM fuel cells. The effects of four parameters, namely the liquid water saturation at the interface of the gas diffusion layer and flow channels, the proportion of liquid water to all of the water at the interface of the cathode catalyst layer and the gas diffusion layer, the current density, and the contact or wetting angle, on the transient distribution of liquid water saturation in the cathode gas diffusion layer are investigated. Especially, the time needed for liquid water saturation to reach steady state and the liquid water saturation at the interface of the cathode catalyst layer and gas diffusion layer are plotted as functions of the above four parameters. The ranges of water vapour condensation and liquid water evaporation are identified across the thickness of the gas diffusion layer. In addition, the effects of the above four parameters on the steady state distributions of gas phase pressure, water vapour concentration, oxygen concentration and temperature are also presented. It is found that increasing any one of the first three parameters will increase the water saturation at the interface of the catalyst layer and gas diffusion layer, but decrease the time needed for the liquid water saturation to reach steady state. When the liquid water saturation at the interface of the gas diffusion layer and flow channels is high enough (≥0.1), the liquid water saturation at steady state is almost uniformly distributed across the thickness of the gas diffusion layer. It is also found that, under the given initial and boundary conditions in this paper, evaporation takes place within the gas diffusion layer close to the channel side and is the major process for water phase change at low current density (<2000 A m⁻²); condensation occurs close to the catalyst layer side within the gas diffusion layer and dominates the phase change at high current density (>5000 A m⁻²). For hydrophilic gas diffusion layers, both the time needed for liquid water saturation to reach steady state and the water saturation at the interface of the catalyst layer and gas diffusion layer will increase when the contact angle increases; but for hydrophobic gas diffusion layers, both of them decrease when the contact angle increases.     

The gas diffusion layer in polymer electrolyte membrane fuel cells: A process model of the two-phase flow

A two-phase flow process model for the gas diffusion layer (GDL) of a polymer electrolyte membrane fuel cell, considering also the cathode catalyst layer (CL), is presented. For this purpose, a systematic analysis of the factors affecting flooding and drying, including the liquid accumulation in the gas channel (CH), was performed using a one-dimensional reference model for the GDL and a compact channel model. The treatment proposed for the CH-GDL interface was compared with other boundary conditions in the literature. It was concluded that the liquid accumulation in the channel is determinant for estimating the steady state and transient GDL flooding, but that predicting the saturation level in the CL can help for determining operation policies for precluding flooding in the GDL-CL composite, in the absence of an adequate channel model. Bifurcation behavior, associated with the water phase change, was identified by means of the compact model.     

Phase change in the cathode side of a proton exchange membrane fuel cell

A three-dimensional steady state two-phase non-isothermal model which highly couples the water and thermal management has been developed to numerically investigate the spatial distribution of the interfacial mass transfer phase-change rate in the cathode side of a proton exchange membrane fuel cell (PEMFC). A non-equilibrium evaporation-condensation phase change rate was incorporated in the model which allowed supersaturation and undersaturation take place. The most significant effects of phase-change rate on liquid saturation and temperature distributions are highlighted. A parametric study was also carried out to investigate the effects of operating conditions; namely as the channel inlet humidity, cell operating temperature, and inlet mass flow rate on the phase-change rate. It was also found that liquid phase assumption for produced water in the cathode catalyst layer (CL) changed the local distribution of phase-change rate. The maximum evaporation rate zone (above the channel near the CL) coincided with the maximum temperature zone and resulted in lowering the liquid saturation level. Furthermore, reduction of the channel inlet humidity and an increase of the operation temperature and inlet mass flow rate increased the evaporation rate and allowed for dehydration process of the gas diffusion layer (GDL) to take place faster.     

Measurement of liquid water content in cathode gas diffusion electrode of polymer electrolyte fuel cell

Water management in cathode gas diffusion electrode (GDE) of polymer electrolyte fuel cell (PEFC) is essential for high performance operation, because liquid water condensed in porous gas diffusion layer (GDL) and catalyst layer (CL) blocks oxygen transport to active reaction sites. In this study, the average liquid water content inside the cathode GDE of a low-temperature PEFC is experimentally and quantitatively estimated by the weight measurement, and the relationship between the water accumulation rate in the cathode GDE and the cell voltage is investigated. The liquid water behavior at the cathode is also visualized using an optical diagnostic, and the effects of operating conditions and GDL structures on the water transport in the cathode GDE are discussed. It is found that the liquid water content in the cathode GDE increases remarkably after starting the fuel cell operation due to the water production at the CL. At a high current density, the cell voltage drops suddenly after starting the operation in spite of a low water content in the cathode GDE. When the GDL thickness is increased, much water accumulates near the cathode CL and the fuel cell shuts down immediately after the operation. In the final section of this paper, the structure of cathode GDL that has several grooves for water removal is proposed to prevent water flooding and improve fuel cell performance. This groove structure is effective to promote the removal of the liquid water accumulated near the active catalyst sites.     

Investigation of water accumulation and discharge behaviors with variation of current density in PEMFC by high-resolution soft X-ray radiography

The management of water is a challenging issue to achieving high power density, long-term operation, and increased robustness in PEMFCs. Development of in-situ diagnostic techniques to understand the dynamic behavior of liquid water is needed. In this study, visualization of liquid water across the membrane electrolyte assembly (MEA) of operating PEMFCs was performed by using high-resolution soft X-ray radiography, with which was possible to distinguish the catalyst layer (CL), polymer electrolyte membrane, and gas diffusion layer (GDL) of the MEA. Dynamic liquid water transport behavior in the cathode CL and GDL was observed at high spatial and temporal resolution (1 frame/s). At low current density, generation of liquid water was recognized under the rib, while at high current density, liquid water was observed both under the rib and the channel. Temporal behaviors of liquid water accumulation in GDL under the rib and discharge process of liquid water to under the channel were also visualized.     

Cold start analysis of polymer electrolyte membrane fuel cells

Cold start is critical to the commercialization of polymer electrolyte membrane fuel cell (PEMFC) for practical applications such as backup power and automotive applications. In this study, various numerically simulated PEMFC cold start processes are analyzed. The success of the cold start process depends on the competition between how fast the cell is heated up to the freezing point temperature and how fast ice is formed and built up in the pores of the cathode catalyst layer (CL) blocking oxygen transport to the reaction sites; the success of the cold start process thus depends on the product water (i) that is absorbed into the ionomer in the CL and membrane, (ii) that is taken away in vapour form by the gas flows (can be neglected), and (iii) that is frozen into ice in the CL pores. It is found that the membrane thickness and the ionomer volume fraction in the CL play pivotal roles in reducing the amount of ice formation. A thicker membrane leads to a larger water capacity but a slower water absorption process, and increasing the ionomer volume fraction in the CL enlarges the ionomer water capacity and enhances the membrane water absorption. Starting the cell under the potentiostatic condition is confirmed to be superior to the galvanostatic condition. Heating up the external surfaces and the inlet air enhances the temperature increment of the cell. However, the external heating methods have negligible improvement in reducing the amount of ice formation. Even though heating the inlet air is more effective in increasing the cell temperature than heating the outer surfaces, the heat capacity of the inlet air is low.     

Performance analysis of a PEM fuel cell cathode with multiple catalyst layers

The cathode catalyst layer (CL) of a PEM fuel cell (PEMFC) plays an important role in the performance of the cell because of the rate limiting mechanisms that take place in it. For enhancing the performance of a PEMFC, the use of multiple, ultra thin CLs instead of a single CL is considered in the present work. Since the concentration of oxygen decreases in a CL from the diffusion medium-CL interface towards the polymer membrane, the CL adjacent to the diffusion medium should be of higher porosity than the other CLs. Similarly, the CL adjacent to the polymer membrane should contain more ionomer than the other CLs. Furthermore, liquid water should be removed without causing significant mass transport and/or ohmic losses. Therefore, the design parameters of a CL can be varied spatially to minimize losses in a PEMFC. However, such a continuously graded CL is difficult to manufacture due to lack of commercially available techniques and associated costs. As an alternative, a combination of layers can be synthesized where each layer is manufactured with different design parameters. This approach provides the opportunity to optimize the design parameters of each layer. With this objective in mind, a detailed steady state model of a PEMFC cathode with multiple layers is developed. The model considers liquid water in all the layers. The catalyst layer microstructure is modeled as a network of spherical agglomerates. For improved water management, a thin micro-porous layer is considered between the gas diffusion layer (GDL) and the first catalyst layer. The performance curves for various combinations of the design parameters are shown and the results are analyzed. The results show that there exists an optimum combination of design parameters for each catalyst layer that can significantly improve the performance of a PEMFC.     

From microstructure to the development of water and major reaction sites inside the catalyst layer of the cathode of a proton exchange membrane fuel cell

Microstructures of various sizes and shapes are fabricated on the surface of the catalyst layer (CL) of the cathode of a PEMFC, adjacent to the micro porous layer (MPL). Three major experimental results are: (1) performance is improved by up to 60% and the percentage of the increase is the same as that of the increase in interface area of CL and MPL; (2) the cell suffers no significant performance loss when Pt loading of the cathode is reduced from 1 to 0.25 mg cm⁻² and; (3) transient responses in periodical linear sweep tests show an obvious performance “jump” for all the cathodes with microstructures when approaching steady state, but none for others. Based on observations, a proposal related to the development of water and, consequently, the major reaction sites in the CL is made: there is a general water “surface” inside the CL. Major electrochemical reactions occur above (on the MPL side) of this surface and within a limited height. The surface will “move” from the membrane toward the MPL as more water is produced. The vapor generation rate (current load) relative to the removal rate of the rest of the cell components will determine the steady state position of this water surface.     

Transport phenomena within the porous cathode for a proton exchange membrane fuel cell

A two-phase, one-dimensional steady model is developed to analyze the coupled phenomena of cathode flooding and mass-transport limiting for the porous cathode electrode of a proton exchange membrane fuel cell. In the model, the catalyst layer is treated not as an interface between the membrane and gas diffusion layer, but as a separate computational domain with finite thickness and pseudo-homogenous structure. Furthermore, the liquid water transport across the porous electrode is driven by the capillary force based on Darcy's law. And the gas transport is driven by the concentration gradient based on Fick's law. Additionally, through Tafel kinetics, the transport processes of gas and liquid water are coupled. From the numerical results, it is found that although the catalyst layer is thin, it is very crucial to better understand and more correctly predict the concurrent phenomena inside the electrode, particularly, the flooding phenomena. More importantly, the saturation jump at the interface of the gas diffusion layer and catalyst layers is captured, when the continuity of the capillary pressure is imposed on the interface. Elsewise, the results show further that the flooding phenomenon in the CL is much more serious than that in the GDL, which has a significant influence on the mass transport of the reactants. Moreover, the saturation level inside the cathode is determined, to a great extent, by the surface overpotential, the absolute permeability of the porous electrode, and the boundary value of saturation at the gas diffusion layer-gas channel interface. In order to prevent effectively flooding, it should remove firstly the liquid water accumulating inside the CL and keep the boundary value of liquid saturation as low as possible.     

Effects of inlet humidification on PEM fuel cell dynamic behaviors

The dynamic behaviors of a proton exchange membrane (PEM) fuel cell have been studied both experimentally and numerically. The objective of this paper is to investigate the effects of cathode inlet humidification on PEM fuel cell load change operations and the fuel cell performance during a simulated start‐up process. The PEM fuel cell was found to respond quickly and reproducibly to load changes. It was also found that an increase in the cathode inlet humidification significantly influences the start‐up performance of a PEM fuel cell. The cathode inlet relative humidity (RH) under 30% significantly dropped the cell dynamic performance. Extensive numerical simulations, with the transient processes of load jump and gradual changes considered, were performed to characterize dynamic responses of a singe‐channel PEM fuel cell under different inlet humidification levels. The results showed that the response time for a fuel cell to reach steady state depends on water accumulation in the membrane, which is consistent with the experimental results. Copyright © 2010 John Wiley & Sons, Ltd.     

An approach for determining the liquid water distribution in a liquid-feed direct methanol fuel cell

In determining the liquid water distribution in the anode (or the cathode) diffusion medium of a liquid-feed direct methanol fuel cell (DMFC) with a conventional two-phase mass transport model, a current-independent liquid saturation boundary condition at the interface between the anode flow channel and diffusion layer (DL) (or at the interface between the cathode flow channel and cathode DL) needs to be assumed. The numerical results resulting from such a boundary condition cannot realistically reveal the liquid distribution in the porous region, as the liquid saturation at the interface between the flow channel and DL varies with current density. In this work, we propose a simple theoretical approach that is combined with the in situ measured water-crossover flux in the DMFC to determine the liquid saturation in the anode catalyst layer (CL) and in the cathode CL. The determined liquid saturation in the anode CL (or in the cathode CL) can then be used as a known boundary condition to determine the water distribution in the anode DL (or in the cathode DL) with a two-phase mass transport model. The numerical results show that the water distribution becomes much more realistic than those predicted with the assumed boundary condition at the interface between the flow channel and DL.     

Non‐isothermal multi‐phase modeling of PEM fuel cell cathode

In this study, numerical simulation has been carried out for the heat transfer and temperature distribution in the cathode of polymer electrolyte membrane fuel cells along with the multi‐phase and multi‐species transport under the steady‐state condition. The commercial software, COMSOL Multiphysics, is used to solve the conservation equations for momentum, mass, species, charge and energy numerically. The conservation equations are applied to the solid, liquid and vapor phases in the bipolar plate and gas diffusion (GDL) and catalyst layers of a two‐dimensional cross section of the cathode. The catalyst layer is assumed to be a finite domain and the water production in the catalyst layer is considered to be in the liquid form. The temperature distribution in the cathode is simulated and then the effects of the relative humidity of the air stream, the permeability of the cathode and the flow channel shoulder to channel width ratio are investigated. It is shown that the highest temperature change, both in the in‐plane and across‐the‐plane directions, occurs in the GDL, while the highest temperature is reached in the catalyst layer. The distribution of temperature in the bipolar plate is shown to be relatively uniform due to the high thermal conductivity of the plate. A decrease in the inlet relative humidity of the air stream results in the decrease of the maximum temperature due to the absorption of heat during the evaporation of liquid water in the GDL and catalyst layer. The non‐uniformity of the temperature distribution, especially in the catalyst layer, is observed with the increase of the permeability of the cathode. Similarly, the decrease of the channel shoulder to channel width ratio leads to a non‐uniform distribution of temperature especially under the channel areas. Copyright © 2009 John Wiley & Sons, Ltd.     

Parametric study of the cathode and the role of liquid saturation on the performance of a polymer electrolyte membrane fuel cell—A numerical approach

A two-dimensional two-phase steady state model of the cathode of a polymer electrolyte membrane fuel cell (PEMFC) is developed using unsaturated flow theory (UFT). A gas flow field, a gas diffusion layer (GDL), a microporous layers (MPL), a finite catalyst layer (CL), and a polymer membrane constitute the model domain. The flow of liquid water in the cathode flow channel is assumed to take place in the form of a mist. The CL is modeled using flooded spherical agglomerate characterization. Liquid water is considered in all the porous layers. For liquid water transport in the membrane, electro-osmotic drag and back diffusion are considered to be the dominating mechanisms. The void fraction in the CL is expressed in terms of practically achievable design parameters such as platinum loading, Nafion loading, CL thickness, and fraction of platinum on carbon. A number of sensitivity studies are conducted with the developed model. The optimum operating temperature of the cell is found to be 80-85 °C. The optimum porosity of the GDL for this cell is in the range of 0.7-0.8. A study by varying the design parameters of the CL shows that the cell performs better with 0.3-0.35 mg cm⁻² of platinum and 25-30 wt% of ionomer loading at high current densities. The sensitivity study shows that a multi-variable optimization study can significantly improve the cell performance. Numerical simulations are performed to study the dependence of capillary pressure on liquid saturation using various correlations. The impact of the interface saturation on the cell performance is studied. Under certain operating conditions and for certain combination of materials in the GDL and CL, it is found that the presence of a MPL can deteriorate the performance especially at high current density.     

In situ investigation of water transport in an operating PEM fuel cell using neutron radiography: Part 2 – Transient water accumulation in an interdigitated cathode flow field

An interdigitated cathode flow field has been tested in situ with neutron radiography to measure the water transport through the porous gas diffusion layer in a PEM fuel cell. Constant current density to open circuit cycles were tested and the resulting liquid water accumulation and dissipation rates with in-plane water distributions are correlated to measured pressure differential between inlet and outlet gas streams. The effect of varying the reactant gas relative humidity on liquid water accumulation is also demonstrated. These results provide evidence that the reactant gas establishes a consistent in-plane transport path through the diffusion layer, leaving stagnant regions where liquid water accumulates. A simplified permeability model is presented and used to correlate the relative permeability to varying gas diffusion layer liquid water saturation levels.     

A novel cathode structure with double catalyst layers and low Pt loading for proton exchange membrane fuel cells

A novel cathode structure (NCS) was developed, which consisted of an inner and an outer catalyst layer (CL). It showed an improved platinum (Pt) utilization (above 50%), a lowered CL/gas diffusion layer interfacial resistance, and a decreased mass transport polarization compared with the traditional cathode structure (TCS). A hydrogen/air proton exchange membrane fuel cell employing NCS yielded an output power density up to 0.76 W cm⁻² with cathode Pt loading as low as 0.28 mg cm⁻². The enhanced performance of NCS is attributed to synergistic effect of the two catalyst supports in outer CL, which provides abundant pores to relieve water flooding and facilitates heat-induced proton conductor migration from the inner to outer CL, forming a hydrophilic proton conduction network. Moreover, the thin and compact inner CL can meet the demand of rich active sites and catalyst migration toward the regions nearest to the membrane under high current densities.     

Water management in alkaline anion exchange membrane fuel cell anode

Water management is an important issue for alkaline anion exchange membrane fuel cell (AAEMFC) due to its significant role in the energy conversion processes. In this study, a numerical model is developed to investigate the water transport in AAEMFC anode. The gas and liquid transport characteristics in the gas diffusion layer (GDL) and catalyst layer (CL) with different designs and under various operating conditions are discussed. The results show that the current density affects the liquid water distribution in anode most significantly, and the temperature is the second considerable factor. The stoichiometry ratio of the supplied reactant has insignificant effect on the liquid water transport in anode. The change of liquid water amount in anode with cathode relative humidity follows a similar trend with anode inlet relative humidity. Some numerical results are also explained with published experimental and modeling data with reasonable agreement.     

Effect of surface dynamic wettability in proton exchange membrane fuel cells

It is known that the static contact angle reflecting the “contact area” between liquid and solid is insufficient to represent the dynamic wettability of a solid surface, and another parameter called the sliding angle is needed to describe the relative easiness of liquid moving on a solid surface. However, sliding angle has been largely neglected in the previous studies for proton exchange membrane fuel cell (PEMFC). In this study, three-dimensional multiphase simulations are carried out for a PEMFC with single straight flow channels considering both the static contact angles and sliding angles of gas diffusion layer (GDL) and catalyst layer (CL). The results show that the liquid water volume fraction in cathode CL (CCL) and GDL (CGDL) can be increased by several times when the sliding angle is increased while the static contact angle is kept constant. This could have significant implication on the water management strategy due to the considerable changes in the water transport and removal processes. Since GDL is much thicker than CL, changing the surface dynamic wettability of GDL has more significant effect on liquid water transport than changing the surface dynamic wettability of CL.     

Multi-dimensional liquid water transport in the cathode of a PEM fuel cell with consideration of the micro-porous layer (MPL)

A multi-dimensional two-phase PEM fuel cell model, which is capable of handling the liquid water transport across different porous materials, including the catalyst layer (CL), the micro-porous layer (MPL), and the macro-porous gas diffusion medium (GDM), has been developed and applied in this paper for studying the liquid water transport phenomena with consideration of the MPL. Numerical simulations show that the liquid water saturation would maintain the highest value inside the catalyst layer while it possesses the lowest value inside the MPL, a trend consistent qualitatively with the high-resolution neutron imaging data. The present multi-dimensional model can clearly distinguish the different effects of the current-collecting land and the gas channel on the liquid water transport and distribution inside a PEM fuel cell, a feature lacking in the existing one-dimensional models. Numerical results indicate that the MPL would serve as a barrier for the liquid water transport on the cathode side of a PEM fuel cell.     

Cold start characteristics of proton exchange membrane fuel cells

In this study, the effects of the start-up temperature, load condition and flow arrangement on the cold start characteristics and performance of a proton exchange membrane fuel cell (PEMFC) are investigated through in-situ experiments with the simultaneous measurements of the current and temperature distributions. Rather than the commonly recognized cold start failure mode due to the ice blockage in cathode catalyst layer (CL), another failure mode due to the ice blockage in flow channel and gas diffusion layer (GDL) leading to significantly high pressure drop through cathode flow field is observed at a start-up temperature just below the lowest successful start-up temperature. Three ice formation mechanisms are proposed, corresponding to the ice formations in cathode CL, GDL and flow channel. The general distributions of current densities and temperatures during the constant current cold start processes are similar to the constant voltage cold start processes, except that the temperatures at the end of the constant current cold start processes are more evenly distributed over the active reaction area because of the increased heat generation rates. The cold start characteristics are mainly dominated by the cathode flow, and changing the flow arrangement has unimportant impact on the cold start performance.     

A two-dimensional steady state model including the effect of liquid water for a PEM fuel cell cathode

A two-dimensional steady state model for a PEM fuel cell cathode is described in this work. All the components in the cathode such as the gas manifold, diffusion layer, microporous layer and the catalyst layer are modeled. The effect of the liquid water is taken into account in every layer of the cathode. The model was developed and simulated using a combination of Maple and MATLAB. The combination provides a flexible framework for quickly developing models with various assumptions and different complexities. The cathode catalyst layer was modeled using both macrohomogeneous and spherical agglomerate characterizations. The model is validated using experimental data. During model validation, various assumptions are considered for transport within the porous layers of the cathode. Subsequently, the assumptions and characteristics that best predicts the experimental data are highlighted. The major conclusion of this work is that a model that includes liquid water in all the layers with a flooded spherical agglomerate characterization for the reaction layer best predicts the PEM fuel cell behavior in terms of an i–v characterization for a wide range of reactant flow rates. The utility of the steady state model for the optimization of the cathode catalyst layer design parameters is also described.