Non-isothermal cold start of polymer electrolyte fuel cells

A multiphase, three-dimensional model has been developed to describe non-isothermal cold start of a polymer electrolyte fuel cell (PEFC) and to delineate intricate interactions between ice formation and heat generation during cold start. The effect of rising cell temperature is numerically explored by comparing a non-isothermal cold start with an isothermal one. It is found that more water is transported into the membrane and less ice formation occurs in the cathode catalyst layer (CL) in the presence of rising cell temperature. In addition, the more hydrated membrane and the rising cell temperature greatly lower the membrane resistance, thus giving rise to higher cell voltage. A lumped thermal analysis significantly over-estimates the overall thermal requirement of self-startup as a cell requires only a portion of its active area to reach the freezing point and be ice-free and operable. It is also found that pre-startup conditions have significant influence on cold start. Procedures to minimize residual water inside the cell prior to cold start, such as gas purge, are critically important. Finally, non-isothermal cold start becomes much easier from higher ambient temperatures.     

Two-dimensional analysis of PEM fuel cells

This study reports a two-dimensional numerical simulation of a steady, isothermal, fully humidified polymer electrolyte membrane (PEM) fuel cell, with particular attention to phenomena occurring in the catalyst layers. Conservation equations are developed for reactant species, electrons and protons, and the rate of electrochemical reactions is determined from the Butler–Volmer equation. Finite volume method is used along with the alternating direction implicit algorithm and tridiagonal solver. The results show that the cathode catalyst layer exhibits more pronounced changes in potential, reaction rate and current density generation than the anode catalyst layer counterparts, due to the large cathode activation overpotential and the relatively low diffusion coefficient of oxygen. It is shown that the catalyst layers are two-dimensional in nature, particularly in areas of low reactant concentrations. The two-dimensional distribution of the reactant concentration, current density distribution, and overpotential is determined, which suggests that multi-dimensional simulation is necessary to understand the transport and reaction processes occurring in a PEM fuel cell.     

Three-dimensional numerical simulation of straight channel PEM fuel cells

The need to model three-dimensional flow in polymer electrolyte membrane (PEM) fuel cells is discussed by developing an integrated flow and current density model to predict current density distributions in two dimensions on the membrane in a straight channel PEM fuel cell. The geometrical model includes diffusion layers on both the anode and cathode sides and the numerical model solves the same primary flow related variables in the main flow channel and the diffusion layer. A control volume approach is used and source terms for transport equations are presented to facilitate their incorporation in commercial flow solvers. Predictions reveal that the inclusion of a diffusion layer creates a lower and more uniform current density compared to cases without diffusion layers. The results also show that the membrane thickness and cell voltage have a significant effect on the axial distribution of the current density and net rate of water transport. The predictions of the water transport between cathode and anode across the width of the flow channel show the delicate balance of diffusion and electroosmosis and their effect on the current distribution along channel.     

Three-dimensional multiphase modeling of cold start processes in polymer electrolyte membrane fuel cells

Startup from subzero temperatures, referred to as “cold start”, has been one of the technical challenges hindering the commercialization of polymer electrolyte membrane fuel cell (PEMFC). In this study, a mathematical model has been developed to simulate the cold start processes in a PEMFC. The present three-dimensional multiphase model uniquely includes the water freezing in the membrane electrolyte, the non-equilibrium mass transfer between the water in the ionomer and the water (vapour, liquid and ice) in the pore region of the catalyst layer (CL), and the water freezing and melting in the CL and gas diffusion layer (GDL). Both the failed and successful cold start processes have been numerically investigated. Numerical results indicate that increasing the ionomer fraction in the cathode CL has more significant effects than increasing the thickness of the membrane layer in reducing the amount of ice formation, and the ohmic heat is the largest heating source at low cell voltages. It is observed that water freezes first in the cathode CL under the land, and ice melts first in the CLs under the flow channel, the melted water in the anode is also removed faster than in the cathode.     

Cold start of polymer electrolyte fuel cells: Three-stage startup characterization

In this paper, the electrochemical kinetics, oxygen transport and solid water formation within the cathode electrode of polymer electrolyte fuel cells (PEFCs) during cold start is investigated. We specifically evaluate the key parameters that govern the self-startup of PEFCs by considering a wide range of the relevant factors. These parameters include characteristic time scales of cell warm-up, ionomer hydration in the catalyst layer, ice build-up and melting, as well as the ratios of the time constants. Supporting experimental observation using neutron imaging and isothermal cold start experiment is discussed. Gas purge is found to facilitate the PEFC cold start but the improvement may be relatively small compared with other methods such as selecting suitable materials and modifying the cell design. We define a three-step electrode process for cold start and conduct a one-dimensional analysis, which enables the evaluation of the impact of ice volume fraction and temperature variations on the cell cold start performance. The ionic conductivity data of Nafion^® 117 membrane at subfreezing temperature, evaluated from experiment, is utilized to analyze the temperature dependence of the ohmic polarization during cold start.     

In-situ methods for the determination of current distributions in PEM fuel cells

This article describes three different techniques for the determination of the current density distribution in operating fuel cells and compares their relative benefits with respect to ease of implementation and information gain. Real-time current density distribution data under steady state as well as transient conditions are presented and it is shown that they can contribute to an improved understanding of water management and reactant distribution over the active fuel cell area. The importance of these factors for the optimisation of fuel cell performance is discussed.     

Evaluation of methods to increase the oxygen partial pressure in PEM fuel cells

This paper compares the performance of a hydrogen–air fuel cell system with the oxygen electrode operating under different conditions of pressure, stoichiometry and oxygen enrichment. This paper shows that the net power density can be improved using a pressurized or oxygen enrichment system when the oxygen electrode is limited by oxygen mass transfer. If the current density is determined by kinetics, then the ambient pressure system has a higher net power density at the same fuel efficiency.     

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.     

软起动的介绍及应用

从软起动的定义着手,指出了电机直接起动的要求及危害,介绍了软起动的工作原理、控制方式及技术特点,结合软起动的应用条件,通过计算验证了起动过程电压下降满足设计规范的要求,阐述了软起动的调试及同步电机软起的起动过程。总结列出了常见故障现象及处理方法,最后从运行的角度分析了应用效果及注意事项。     

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.     

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     

Simultaneous measurement of current and temperature distributions in a proton exchange membrane fuel cell during cold start processes

Cold start is critical to the commercialization of proton exchange membrane fuel cell (PEMFC) in automotive applications. Dynamic distributions of current and temperature in PEMFC during various cold start processes determine the cold start characteristics, and are required for the optimization of design and operational strategy. This study focuses on an investigation of the cold start characteristics of a PEMFC through the simultaneous measurements of current and temperature distributions. An analytical model for quick estimate of purging duration is also developed. During the failed cold start process, the highest current density is initially near the inlet region of the flow channels, then it moves downstream, reaching the outlet region eventually. Almost half of the cell current is produced in the inlet region before the cell current peaks, and the region around the middle of the cell has the best survivability. These two regions are therefore more important than other regions for successful cold start through design and operational strategy, such as reducing the ice formation and enhancing the heat generation in these two regions. The evolution of the overall current density distribution over time remains similar during the successful cold start process; the current density is the highest near the flow channel inlets and generally decreases along the flow direction. For both the failed and the successful cold start processes, the highest temperature is initially in the flow channel inlet region, and is then around the middle of the cell after the overall peak current density is reached. The ice melting and liquid formation during the successful cold start process have negligible influence on the general current and temperature distributions.     

Effect of cathode GDL characteristics on mass transport in PEM fuel cells

The effect of the content of the hydrophobic agent in the cathode gas diffusion layer (GDL) on the mass transport in the proton exchange membrane fuel cells (PEMFCs) was studied using mercury porosimetry, scanning electron microscopy, and electrochemical polarization techniques. The mercury intrusion data and SEM micrograph indicated that the hydrophobic agent alters the surface and bulk structure of the GDL, thereby controlling gas-phase void volume and liquid water transport. The electrochemical polarization curves were measured and quantitatively analyzed to determine the oxygen transport limitation both in the catalyst layer and the GDL. Evaluation of the parameter ζ, which represents the cathode GDL characteristics for liquid water transport, indicated that the optimized content of the hydrophobic agent and effective water management results from a trade-off between the hydrophobicity and the absolute permeability for faster water drainage.     

Sub-freezing endurance of PEM fuel cells with different catalyst-coated membranes

The sub-freezing endurance of proton exchange membrane (PEM) fuel cells with hydrophobic and hydrophilic catalyst-coated membranes (CCMs) was investigated. The polarization curves, electrochemical characteristics and physical structures of the CCMs were measured. The cells were frozen at −20 °C with saturated residual water after operating at 60 °C. After eight freeze/thaw cycles, no evident negative effect on the performance of the cell with a hydrophobic CCM was observed, while the cell with a hydrophilic CCM degraded severely. By analyzing the polarization curves, it was concluded that the mass transport limitation was the main reason for the performance loss of the hydrophilic cell. The electrochemical active surface area (ECA) results suggest that the hydrophobicity of the catalyst layer (CL) has an apparent impact on the residual water distribution of the membrane electrode assembly (MEA). A larger water content in the hydrophilic CL has a negative effect on the subzero endurance. From the polarization resistance obtained from electrochemical impedance spectroscopy (EIS) the origin of degradation was further clarified. Mercury intrusion porosimetry showed that the pore size of the hydrophilic catalyst layer changed significantly after freezing; the mean pore size increased from 5.68 to 6.71 nm. However, with a water removal method, namely, gas purging, it was possible to prevent degradation effectively.     

A high-throughput study of PtNiZr catalysts for application in PEM fuel cells

The effects of adding Zr to PtNi oxygen reduction reaction (ORR) electrocatalyst alloys were examined in a study aimed at probing the possibility of creating catalysts with enhanced resistance to corrosion in a PEM fuel cell environment. Samples consisting of pure Pt or PtNiZr alloys with a range of compositions (not exceeding 11 at.% Zr) were fabricated using co-sputter deposition. A high-throughput fabrication approach was used wherein 18 distinct thin film catalyst alloy samples with varying compositions were deposited onto a large-area substrate with individual Au current collector structures. A multi-channel pseudo-potentiostat allowed for the simultaneous quantitative study of catalytic activity for all 18 electrodes in a single test bath, a first for the study of ORR electrocatalysts. A properly stirred oxygenated 1 M H₂SO₄ electrolyte solution was used to provide each electrode with a steady-state flow of reactants during electrochemical evaluation. The onset potentials, absolute current density values, and Tafel analysis data obtained using this technique were compared with literature reports. The analyses showed that most PtNiZr alloys tested offered improvements over pure Pt, however those surfaces with a high mole fraction (>4 at.%) of Zr exhibited reduced activity that was roughly inversely correlated to the amount of Zr present. Film composition, morphology, and crystallographic properties were examined using X-ray energy dispersive spectroscopy (XEDS), X-ray photoelectron spectroscopy (XPS), SEM, and synchrotron X-ray diffraction. These data were then correlated with electrochemical data to elucidate the relationships between composition, structure, and relative performance for this ternary system.     

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.     

Characteristics of subzero startup and water/ice formation on the catalyst layer in a polymer electrolyte fuel cell

This work experimentally explores the fundamental characteristics of a polymer electrolyte fuel cell (PEFC) during subzero startup, which encompasses gas purge, cool down, startup from a subfreezing temperature, and finally warm up. In addition to the temperature, high-frequency resistance (HFR) and voltage measurements, direct observations of water or ice formation on the catalyst layer (CL) surface have been carried out for the key steps of cold start using carbon paper punched with microholes and a transparent cell fixture. It is found that purge time significantly influences water content of the membrane after purge and subsequently cold-start performance. Gas purge for less than 30 s appears to be insufficient, and that between 90 and 120 s is most useful. After gas purge, however, the cell HFR relaxation occurs for longer than 30 min due to water redistribution in the membrane-electrode assembly (MEA). Cold-start performance following gas purge and cool down strongly depends on the purge time and startup temperature. The cumulative product water measuring the isothermal cold-start performance increases dramatically with the startup temperature. The state of water on the CL surface has been studied during startup from ambient temperatures ranging from −20 to −1 °C. It is found that the freezing-point depression of water in the cathode CL is 1.0 ± 0.5 °C and its effect on PEFC cold start under automotive conditions is negligible.     

Improved performance of PEM fuel cell using carbon paper electrode prepared with CNT coated carbon fibers

Porous conducting carbon paper has been identified as the most suitable material to be used as a backing material for the fuel cell electrode. The surface of carbon fiber, the major constituent of the carbon paper was modified by: (1) removing the functional groups by heat cleaning process and (2) coating the non-functionalized carbon fiber with multi-walled carbon nanotubes (MWCNTs). This has a marked influence on the fiber–matrix interactions during later stages of processing of carbon paper that helped in controlling its various characteristic properties. Using the carbon paper formed with CNT coated carbon fiber as electrode, the maximum power density achieved from a unit fuel cell was found to be 783 mW/cm² as compared to 630 mW/cm² when the paper was formed with normal fiber.     

Measurement of single electrode potentials and impedances in hydrogen and direct methanol PEM fuel cells

A commercial proton exchange membrane fuel cell has been fitted with a simple dynamic hydrogen reference electrode (DHE). Single electrode potentials and impedances measured with hydrogen and methanol as the fuel have been critically evaluated. It has been shown that the anode overpotential and impedance can be very significant in hydrogen cells operated at ambient temperature, due to dehydration of the anode. The DHE provides a powerful way of monitoring the hydration state of the membrane and electrodes, so that operating conditions can be adjusted to optimise water management. Individual electrode potentials and impedances are even more important in methanol cells, and can be conveniently measured with the DHE.     

Unified mathematical modelling of steady-state and dynamic voltage-current characteristics for PEM fuel cells

In this study, a unified mathematical modelling technique for computing the steady-state and dynamic voltage-current (V-I) characteristics of PEM fuel cell stacks is developed. The proposed modelling method is based on the least squares technique and a set of electrochemical equations representing the PEM fuel cells. Three PEM fuel cell systems are considered for validating the proposed model. Furthermore, the authors investigated load current optimization by using the proposed method, in order to maximize the power output. Hence, this study provides a valuable approach for optimization of operating points of fuel cells and design of power conditioning units, simulators, and system controllers.