Studies of the performance of PEM fuel cell cathodes with the catalyst layer directly applied on Nafion membranes

The performance of a proton exchange membrane fuel cell (PEMFC) with gas diffusion cathodes having the catalyst layer applied directly onto Nafion membranes is investigated with the aim at characterizing the effects of the Nafion content, the catalyst loading in the electrode and also of the membrane thickness and gases pressures. At high current densities the best fuel cell performance was found for the electrode with 0.35 mg Nafion cm⁻² (15 wt.%), while at low current densities the cell performance is better for higher Nafion contents. It is also observed that a decrease of the usual Pt loading in the catalyst layer from 0.4 to ca. 0.1 mg Pt cm⁻² is possible, without introducing serious problems to the fuel cell performance. A decrease of the membrane thickness favors the fuel cell performance at all ranges of current densities. When pure oxygen is supplied to the cathode and for the thinner membranes there is a positive effect of the increase of the O₂ pressure, which raises the fuel cell current densities to very high values (>4.0A cm⁻², for Nafion 112—50 μm). This trend is not apparent for thicker membranes, for which there is a negligible effect of pressure at high current densities. For H₂/air PEMFCs, the positive effect of pressure is seen even for thick membranes.     

Effects of hydrophobicity of the cathode catalyst layer on the performance of a PEM fuel cell

The effects of hydrophobicity of the cathode catalyst layer on the performance of a PEM fuel cell are studied. The surface contact angle is measured to understand the changes of the hydrophobicity of the cathode catalyst layer upon the addition of hydrophobic dimethyl silicone oil (DSO). The results show that the contact angle increases with the DSO loadings in the cathode catalyst layer ranging from 0 to 0.65 mg/cm². The subsequent electrochemical measurements of the fuel cells with various cathodes reveal that the addition of DSO in the cathode catalyst layer can effectively prevent the cathode flooding at high current density, thus leading to a much higher limiting current density and the maximum power density when compared to the fuel cell with a normal cathode. An optimal DSO loading in the cathode catalyst layer is found to be around 0.5 mg/cm² under the testing conditions in this work. The fuel cell with cathode loaded with 0.5 mg/cm² can reach the maximum power density of 356 mW/cm² in H₂/air (or 709 mW/cm² in H₂/O₂) at room temperature, which is around 2.5 times in H₂/air (or 1.8 times in H₂/O₂) of that with normal cathode. All of the results indicate that the hydrophobicity of the cathode catalyst layer plays a crucial role in the water management of the fuel cell. The possible function of the DSO on improved oxygen solubility for the oxygen starved cathode during flooding warrants some further investigation.     

Effect of hydrophobicity and pore geometry in cathode GDL on PEM fuel cell performance

The effect of hydrophobic and structural properties of a single/dual-layer cathode gas diffusion layer on mass transport in PEM fuel cells was studied using an analytical expression. The simulations indicated that liquid water transport at the cathode is controlled by the fraction of hydrophilic surface and the average pore diameter in the cathode gas diffusion layer. Deposition of a hydrophobic microporous layer reduces the average pore diameter in the macroporous substrate. It also increases the hydrophobic surface, which improves the mass transport of the reactant. The optimized hydrophobicity and pore geometry in a dual-layer cathode GDL leads to an effective water management, and enhances the oxygen diffusion kinetics.     

Modeling, simulation and experimental validation of a PEM fuel cell system

The aim of this work is the development and experimental validation of a detailed dynamic fuel cell model using the gPROMS modeling environment. The model is oriented towards optimization and control and it relies on material and energy balances as well as electrochemical equations including semi-empirical equations. For the experimental validation of the model a fully automated and integrated hydrogen fuel cell testing unit was used. The predictive power of the model has been compared with the data obtained during load change experiments. A sensitivity analysis has been employed to reveal the most critical empirical model parameters that should be estimated using a systematic estimation procedure. Model predictions are in good agreement with experimental data under a wide range of operating conditions.     

Reforming methanol to electricity in a high temperature PEM fuel cell

In this paper we demonstrate for the first time a compact power unit, where a methanol reforming catalyst is incorporated into the anode of a PEMFC. The proposed internal reforming methanol fuel cell (IRMFC) mainly comprises: (i) a H₃PO₄-imbibed polymer electrolyte based on aromatic polyethers bearing pyridine units, able to operate at 200 °C and (ii) a 200 °C active and with zero CO emissions Cu–Mn–O methanol reforming catalyst supported on copper foam. Methanol is being reformed inside the anode compartment of the fuel cell at 200 °C producing H₂, which is readily oxidized at the anode to produce electricity. The IRMFC showed promising electrochemical behavior and no signs of performance degradation for more than 72 h.     

A distributed dynamic model for chronoamperometry, chronopotentiometry and gas starvation studies in PEM fuel cell cathode

Electrochemical systems differ significantly from conventional chemical systems. The response of voltage to changes in current and that of current to changes in voltage is much faster compared to typical transients observed in transport variables. In this work, the transient characteristics of various transport and electrochemical phenomena are studied in the PEM fuel cell cathode using a dynamic model. Model-based chronoamperometry and chronopotentiometry studies are performed to investigate the interactions among the various phenomena and the limiting mechanisms under various operating modes. The dynamic response of current to changes in voltage under chronoamperometry and that of voltage to changes in current under chronopotentiometry are found to be significantly different. Moreover, it is also observed through simulations that the dynamics in the output variables are strongly influenced by the operating cell voltage. Results from chronoamperometry studies are used to highlight the problem of oxygen starvation, which is also reflected by the magnitude of oxygen excess ratio or stoichiometric ratio. Results from step tests in chronopotentiometry studies are used to study nonlinearities in the response of voltage to changes in inputs such as, current and air flow rate.     

Ethanol crossover and direct ethanol PEM fuel cell performance modeling and experimental validation

In the present work, a one-dimension, steady-state and single phase model is developed with the purpose of describing the mass transport within a PtRu/Nafion^®-115/Pt membrane-electrode assembly and the performance of a direct ethanol proton exchange membrane fuel cell (DE-PEMFC). The effect of the most important cell operating parameters on the ethanol crossover rate and the fuel cell performance is investigated. According to the results, in the case of low current density values and high concentrations of ethanol aqueous solutions, ethanol crossover could pose serious problems to the DEFC operation. Moreover, it was pointed out that the ethanol crossover rate dependence on the ethanol feed concentration is an almost linear function presenting a maximum at about . A further increase of the ethanol feed concentration leads to a steep decrease of ethanol crossover rate. This behavior could be attributed to the membrane swelling which is responsible for the membrane volume fraction decrement. It was also found that by the aid of the same model the performance of a direct ethanol PEM fuel cell over three different anode catalysts can be predicted. A relatively good agreement between theory and experimental results related to both ethanol crossover rates and direct ethanol fuel cell performance was found.     

质子交换膜燃料电池高稳定性低铂载量膜电极的研究进展

质子交换膜燃料电池由于高能量转化率、零污染、低温启动等优点在新能源领域备受关注,但其成本和耐久性仍是本领域的挑战性课题。本文首先回顾了近年来国内外研究者在降低燃料电池成本和提高其耐久性方面取得的成就,从催化剂制备技术、膜电极结构优化、耐久性提升三个方面介绍了近年来国内外在降低膜电极铂载量、提高膜电极功率密度和耐久性方面的发展趋势,通过构筑铂基合金、核壳结构和纳米结构等催化剂能有效地降低铂载量,从而降低燃料电池成本;通过构筑多孔结构催化层或气体扩散层可以改善膜电极的微结构,从而提高电池的功率密度;通过开发新型质子交换膜、更换催化剂载体等方法可以提高膜电极的耐久性。最后,本文针对目前研究进展阐述提高膜电极稳定性仍然是目前的研究难题,并对未来的研究方向进行了展望。     

Ice formation and distribution in the catalyst layer during freeze-start process - CRYO-SEM investigation

Ice distribution in the catalyst layer and gas distribution layer (GDL) of proton exchange membrane (PEM) fuel cells under isothermal constant voltage (ICV) operation at a subzero temperature was determined using a field emission scanning electron microscope with a cryogenic stage and sample preparation unit (CRYO-FESEM). The analysis method was designed to ensure that the entire experiment, from sample preparation to CRYO-FESEM characterization, are carried out under subzero (°C) conditions so that the water is always kept in a frozen state without thawing.Under a moderately wet shutdown, the porosity of the cathode catalyst layer decreased from an initial dry porosity of 65% to 15.9% for the frozen-only sample and to 8.2% for the sample which was operated at subzero temperatures (ICV). After the completion of the ICV, the catalyst surface was completely covered with ice and the gas was not able to reach the active sites and the reaction ceased. Two distinct regions with different porosities in the catalyst layer were observed at the half ICV state, which indicates that ice in the catalyst layer melted at the beginning of ICV operation.     

Ignition waves in a stirred PEM fuel cell

A mathematical model of slow transient behavior in an autohumidified stirred tank reactor (STR) polymer electrolyte membrane (PEM) fuel cell is developed. The key feature of the model is the positive feedback between current, water production, and membrane resistance which leads to two stable “ignited” states, corresponding to either a uniform current distribution or a partially ignited cell with localized current production. The switching between the two regimes is accompanied by hysteresis and transient behavior on the order of 2-4 h in a small cell. We compare the numerical results to experimental data gathered by [Benziger et al. 2005. Chemical Engineering Science 60 (6), 1743-1759] and show that the lateral diffusion of water within the ionomer membrane is a possible mechanism behind the hysteresis and slow transient behavior they observed.     

Modelling of performance of PEM fuel cells with conventional and interdigitated flow fields

A simple mathematical model is developed to investigate the superiority of the interdigitated flow field design over the conventional one, especially in terms of maximum power density. Darcy's equation for porous media and the standard diffusion equation with effective diffusivity are used in the gas diffuser, and a coupled boundary condition given by the Butler–Volmer equation is used at the catalyst layer interface. The performance of PEM fuel cells with a conventional flow field and an interdigitated flow field is studied with other appropriate boundary conditions. The theoretical results show that the limiting current density of a fuel cell with an interdigitated flow field is about three times the current density of a fuel cell with a conventional flow field. The results also demonstrate that the interdigitated flow field design can double the maximum power density of a PEM fuel cell. The modelling results compared well with experimental data in the literature.     

PEM fuel cell cathode contamination in the presence of cobalt ion (Co)

This paper reports the effects of Co²⁺ contamination on PEM fuel cell performance as a function of Co²⁺ concentration and operating temperature. A significant drop in fuel cell voltage occurred when Co²⁺ was injected into the cathode air stream, and Co²⁺ contamination became more severe with decreasing temperature. To investigate in detail the mechanism of Co²⁺ poisoning, AC impedance was monitored before and during Co²⁺ injection, revealing that both charge transfer and mass transport related processes deteriorated significantly in the presence of Co²⁺, whereas membrane conductivity decreased to a lesser extent. Surface cyclic voltammetry and contact angle measurements further revealed changes in physical properties, such as active Pt surface area and hydrophilicity, furthering our understanding of the contamination process.     

Numerical optimization of bipolar plates and gas diffusion layers for PEM fuel cells

This paper is devoted to the numerical optimization of the dimensions of channels and current transfer ribs of bipolar plates as well as the thickness and porosity of gas diffusion layers. A mathematical model of the transfer processes in a PEM fuel cell has been developed for this purpose. The results are compared with experimental data. Recommendations of the values of operating parameters and some design requirements to increase PEM fuel cell efficiency are suggested.This paper was originally Presented at the CHISA Congress, Prague, August 2004.An erratum to this article can be found at     

A novel design of distribution zone in scaled-up PEM fuel cells based on active control of transverse flow

The flow distribution is of significance to the fuel cell performance and durability, which has been studied from a theoretical and practical level in this work. The transverse-flow-control-based mechanism behind flow distribution processes is revealed. The core lies in the reasonable generation and distribution of transverse flow, which are the prerequisite and co-requisite for flow homogeneity. For the dual purpose, a novel design of combined-mesh-type distribution zone is proposed incorporating central horizontal meshes and lateral vertical meshes. The design philosophy and methodology are clarified. Under these guidelines, the novel distributor design is applied to different flow field plate geometries including the shorter distribution zone, higher expansion ratio, and scaled-up fuel cell. Through organized and detailed simulations, two key geometrical parameters (porosities of central and lateral meshes) are quantified and the superior effect on flow distribution is validated.     

Quantifying desorption and rearrangement rates of carbon monoxide on a PEM fuel cell electrode

A simple procedure to quantify the rates of carbon monoxide (CO) desorption from, and simultaneous rearrangement on, supported platinum fuel cell electrode (Pt on Vulcan XC-72R) is reported. The surface coverage of CO on Pt electrode in equilibrium with bulk CO was measured from the anodic peaks in the CO stripping voltammogram. The decline in these surface coverages due to desorption and rearrangement, once CO was replaced by N₂ in the gas phase was recorded and used in conjunction with a kinetic model to quantify the respective rates. Two distinct CO oxidation peaks observed in the voltammogram due to the oxidation of two distinct ad-species, namely weakly and strongly adsorbed CO ( and ), were baseline corrected and deconvoluted using a bimodal Gaussian distribution. Saturation surface coverage of decreased with increasing temperature, while the opposite was true for . Rearrangement from to was faster than the desorption rate of either CO species. The desorption rate of was at least an order of magnitude lower than that of molecules at all temperatures studied. The activation energies for desorption of and were estimated to be 24.08 and 27.99 kJ/mol, respectively. The activation energy for rearrangement from to was 35.23 kJ/mol and that from to was 27.55 kJ/mol.     

Oxygen mass transport limitations at the cathode of polymer electrolyte membrane fuel cells

Oxygen transport across the cathode gas diffusion layer (GDL) in polymer electrolyte membrane (PEM) fuel cells was examined by varying the O₂/N₂ ratio and by varying the area of the GDL extending laterally from the gas flow channel under the bipolar plate (under the land). As the cathode is depleted of oxygen, the current density becomes limited by oxygen transport across the GDL. Oxygen depletion from O₂/N₂ mixtures limits catalyst utilization, especially under the land.The local current density with air fed PEM fuel cells falls to practically zero at lateral distances under the land more than 3 times the GDL thickness; on the other hand, catalyst utilization was not limited when the fuel cell cathode was fed with 100% oxygen. The ratio of GDL thickness to the extent of the land is thus critical to the effective utilization of the catalyst in an air fed PEM fuel cell. © 2010 American Institute of Chemical Engineers AIChE J, 2011     

Measurement of through-plane effective thermal conductivity and contact resistance in PEM fuel cell diffusion media

An experimental study was performed to determine the through-plane thermal conductivity of various gas diffusion layer materials and thermal contact resistance between the gas diffusion layer (GDL) materials and an electrolytic iron surface as a function of compression load and PTFE content at 70 °C. The effective thermal conductivity of commercially available SpectraCarb untreated GDL was found to vary from 0.26 to 0.7 W/(m °C) as the compression load was increased from 0.7 to 13.8 bar. The contact resistance was reduced from 2.4×10⁻⁴ m²°C/W at 0.7 bar to 0.6×10⁻⁴ m²°C/W at 13.8 bar. The PTFE coating seemed to enhance the effective thermal conductivity at low compression loads and degrade effective thermal conductivity at higher compression loads. The presence of microporous layer and PTFE on SolviCore diffusion material reduced the effective thermal conductivity and increased thermal contact resistance as compared with the pure carbon fibers. The effective thermal conductivity was measured to be 0.25 W/(m °C) and 0.52 W/(m °C) at 70 °C, respectively at 0.7 and 13.8 bar for 30%-coated SolviCore GDL with microporous layer. The corresponding thermal contact resistance reduced from 3.6×10⁻⁴ m²°C/W at 0.7 bar to 0.9×10⁻⁴ m²°C/W at 13.8 bar. All GDL materials studied showed non-linear deformation under compression loads. The thermal properties characterized should be useful to help modelers accurately predict the temperature distribution in a fuel cell.     

PEM燃料电池中质子交换膜内水和质子的迁移特性

质子交换膜的水含量及水和质子的迁移对PEM燃料电池的性能具有重要影响.提出了一个稳态两相流数学模型,用以研究质子交换膜中的水迁移和水含量及其与质子传递阻力的关系.模型耦合了连续方程、动量守恒方程、物料守恒方程和水在质子交换膜中的传递方程.通过与实验数据对比,验证了模型的有效性.分析模拟结果发现,当电流密度相同时,沿气体流动方向,质子交换膜中水的电渗拉力系数、反扩散系数和水力渗透系数逐步增大,而水的净迁移系数逐步减小;同时,质子交换膜的含水量增加,质子传递阻力逐步下降;增大电池的操作压力,电渗拉力系数、反扩散系数、水力渗透系数、水净迁移系数和质子膜的含水量增加,而质子传递阻力下降,使燃料电池的性能得到了提高.     

The role of mechanically induced defects in carbon nanotubes to modify the properties of electrodes for PEM fuel cell

The characteristics and reactivity of two anodes based on Pt supported on carbon nanotubes (CNTs) without or with defects induced by ball-milling are studied by SEM, TEM, cyclic voltammetry (CV) and single-cell measurements using a flow of pure H₂ or containing 50 ppm CO. It is evidenced that the presence of defects influences several properties and not only the dispersion of Pt particles. Therefore, the performances cannot be correlated neither with the geometrical surface area of Pt particles, neither with the electrochemical active surface area determined from CV tests. The presence of defects, enhancing the amount of surface functional groups on CNT, influences various aspects: (i) the efficiency of three-phase boundary and thus the transport of protons to or from the active metal particles, (ii) the resistance of electron transfer and (iii) the tolerance of the catalyst to CO poisoning. The latter is attributed to carbon functional groups in close contact with very small Pt particles favoring the reactivation of Pt sites poisoned by CO.     

A thin-film/agglomerate model of a proton-exchange-membrane fuel cell cathode catalyst layer with consideration of solid-polymer-electrolyte distribution

A thin-film/agglomerate model for the cathode part of a proton-exchange-membrane fuel cell is developed. Parameter estimation is employed to determine the exchange current density in the catalyst layer, proton conductivity of the recast ionomer, and oxygen diffusivity in the solid polymer electrolyte. The effects of catalyst and polymer electrolyte loadings in the catalyst layer on the cell performance are demonstrated using this model. The influence of polymer electrolyte distribution in the catalyst layer is correlated with the oxygen diffusion and proton migration rates within the electrolyte. It is found that proton migration in the polymer electrolyte is the dominant factor for cell current density under normal operating conditions. A better cell performance is achieved by a concentrated polymer electrolyte near the catalyst layer/membrane interface.