Numerical modeling and investigation of gas crossover effects in high temperature proton exchange membrane (PEM) fuel cells

A gas crossover model is developed for a high temperature proton exchange membrane fuel cell (HT-PEMFC) with a phosphoric acid-doped polybenzimidazole membrane. The model considers dissolution of reactants into electrolyte phase in the catalyst layers and subsequent crossover of reactant gases through the membrane. Furthermore, the model accounts for a mixed potential on the cathode side resulting from hydrogen crossover and hydrogen/oxygen catalytic combustion on the anode side due to oxygen crossover, which were overlooked in the HT-PEMFC modeling works in the literature. Numerical simulations are carried out to investigate the effects of gas crossover on HT-PEMFC performance by varying three critical parameters, i.e. operating current density, operating temperature and gas crossover diffusivity to approximate the membrane degradation. The numerical results indicate that the effect of gas crossover on HT-PEMFC performance is insignificant in a fresh membrane. However, as the membrane is degraded and hence gas crossover diffusivities are raised, the model predicts non-uniform reactant and current density distributions as well as lower cell performance. In addition, the thermal analysis demonstrates that the amount of heat generated due to hydrogen/oxygen catalytic combustion is not appreciable compared to total waste heat released during HT-PEMFC operations.     

Hydrogen purge and reactant feeding strategies in self-humidified PEM fuel cell systems

A 6 kW proton exchange membrane fuel cell system, operating in self-humidified conditions, was characterized in two anode operative modes: dead-end and flow through with exhaust recirculation. The anode sub-system was specifically designed in order to adjust the level of recycled anodic stream. The role of anode purge frequency, anode recirculation level and air stoichiometric ratio was analysed in the power range 1–5 kW. The aim of this study was to define management strategies to assure efficient and reliable cell performance during steady-state, warm-up and load variation phases. The results evidenced the combined effect of hydrogen purge, air flow rate impulse, and recycled anodic stream on individual cell performance recovery when unstable working conditions were detected during system start-up and load variations.     

Non-kinetic losses caused by electrochemical carbon corrosion in PEM fuel cells

This paper presented non-kinetic losses in PEM fuel cells under an accelerated stress test of catalyst support. A cathode with carbon-supported Pt catalyst was prepared and characterized during potential hold at 1.2 V vs. SHE in a PEM fuel cell. Irreversible losses caused by carbon corrosion were evaluated using a variety of electrochemical characterizations including cyclic voltammetry, linear sweep voltammetry, electrochemical impedance spectroscopy, and polarization technique. Ohmic losses at the cathode during potential hold were determined using its capacitive responses. Concentration losses in the PEM fuel cell were analyzed in terms of Tafel behavior and thin film/flooded-agglomerate dynamics.     

Three-dimensional numerical analysis of PEM fuel cells with straight and wave-like gas flow fields channels

Using a three-dimensional computational model, numerical simulations are performed to investigate the performance characteristics of proton exchange membrane fuel cells (PEMFCs) incorporating either a conventional straight gas flow channel or a novel wave-like channel. The simulations focus particularly on the effect of the wave-like surface on the gas flow characteristics, the temperature distribution, the electrochemical reaction efficiency and the electrical performance of the PEMFCs at operating temperatures ranging from 323 K to 343 K. The numerical results reveal that the wave-like surface enhances the transport of the reactant gases through the porous layer, improves the convective heat transfer effect, increases the gas flow velocity, and yields a more uniform temperature distribution. As a result, the efficiency of the catalytic reaction is significantly improved. Consequently, compared to a conventional PEMFC, the PEMFC with a wave-like channel yields a notably higher output voltage and power density.     

Prognostics of PEM fuel cell in a particle filtering framework

Proton Exchange Membrane Fuel Cells (PEMFC) suffer from a limited lifespan, which impedes their uses at a large scale. From this point of view, prognostics appears to be a promising activity since the estimation of the Remaining Useful Life (RUL) before a failure occurs allows deciding from mitigation actions at the right time when needed. Prognostics is however not a trivial task: 1) underlying degradation mechanisms cannot be easily measured and modeled, 2) health prediction must be performed with a long enough time horizon to allow reaction. The aim of this paper is to face these problems by proposing a prognostics framework that enables avoiding assumptions on the PEMFC behavior, while ensuring good accuracy on RUL estimates. Developments are based on a particle filtering approach that enables including non-observable states (degradation through) into physical models. RUL estimates are obtained by considering successive probability distributions of degrading states. The method is applied on 2 data sets, where 3 models of the voltage drop are tested to compare predictions. Results are obtained with an accuracy of 90 h around the real RUL value (for a 1000 h lifespan), clearly showing the significance of the proposed approach.     

Numerical studies on an air-breathing proton exchange membrane (PEM) fuel cell stack

Air-breathing proton exchange membrane (PEM) fuel cells provide for fully or partially passive operation and have gained much interest in the past decade, as part of the efforts to reduce the system complexity. This paper presents a detailed physics-based numerical analysis of the transport and electrochemical phenomena involved in the operation of a stack consisting of an array of vertically oriented air-breathing fuel cells. A comprehensive two-dimensional, nonisothermal, multi-component numerical model with pressurized hydrogen supply at the anode and natural convection air supply at the cathode is developed and validated with experimental data. Systematic parametric studies are performed to investigate the effects of cell dimensions, inter-cell spacing and the gap between the array and the substrate on the performance of the stack. Temperature and species distributions and flow patterns are presented to elucidate the coupled multiphysics phenomena. The analysis is used to determine optimum stack designs based on constraints on desired performance and overall stack size.     

Hydrogen crossover through perfluorosulfonic acid membranes with variable side chains and its influence in fuel cell lifetime

In this study, hydrogen crossover in long side chain Nafion 211 membrane and short side chain Aquivion membrane is studied under different conditions. It is found that both temperature and relative humidity significantly influence the hydrogen crossover in the polymer electrode membranes (PEMs). The difference in hydrogen crossover behavior between Nafion 211 membrane and Aquivion membrane is revealed. The influence of hydrogen crossover on the fuel cell lifetime is also investigated under open circuit voltage (OCV). It is proved hydrogen crossover in the PEM would lead to possible degradation of the PEM and the decrease of electro-chemical surface area in the catalyst of the single cell. Single cell assembled with Aquivion membrane shows slower OCV and ECSA decay compared to the Nafion 211 single cell. Our results suggest that the PEM fuel cell lifetime is closely related to the hydrogen crossover in the PEM. The current study also highlights the possibility of improving the fuel cell durability by rational design of the PEM morphology.     

Study on electrochemical and mass transfer coupling characteristics of proton exchange membrane (PEM) fuel cell based on a fin-like electrode surface

Proton exchange membrane (PEM) Fuel cells are widely used because of its environmental protection and high efficiency. In the present study, a novel fin-like structure of the electrode surface is investigated by establishing the theoretical model and numerical simulating. For this purpose, the influence of different fin spacing and pressure boost on the performance of the PEM fuel cell is analyzed by numerical simulation. Results show that increasing pressure of cathode or both side experiences greater performance improvement compared to the other cases, and the maximum values of power density during both conditions is founding for fin density 1/1, followed by fin density 1/12, then last the basic model. Furthermore, the analysis shows that increasing CL surface area combined with cathode pressurization is the best strategy for fuel cell performance optimization. The fin structure under the condition of cathode pressurization can effectively reduce the transmission resistance and over potential of the fuel cell by theoretical calculation, which is coinciding well with the simulation results.     

Study of sulfur dioxide crossover in proton exchange membrane fuel cells

As one of the most deleterious impurities to proton exchange membrane fuel cells (PEMFCs), sulfur dioxide (SO₂) in air can pass through the membrane from the cathode to the anode and poison the catalyst of the two electrodes. The phenomenon of SO₂ crossover is investigated electrochemically in this paper. The influences of SO₂ concentration, relative humidity, gas pressure and current density on SO₂ crossover are discussed. Experimental results reveal that the anode tends to be poisoned heavily with the increasing concentration of SO₂ in the cathode. The coverage of the anode catalyst by SO₂ permeating from the cathode enlarges with the decreasing relative humidity in the anode. The rate of SO₂ crossover from the anode to the cathode is promoted at high current density when SO₂ is directly introduced into the anode side instead of the cathode side, which can be ascribed to the electro-osmotic drag effect. Gas pressures show no obvious effects on SO₂ crossover. A co-permeation mechanism of SO₂ with water is deduced based on the overall analysis.     

Effects of temperature uncertainty on the performance of a degrading PEM fuel cell model

The performance of a fuel cell is subject to uncertainties on its operational and material parameters. Among operational parameters, temperature is one of the most influential factors. This work focuses on this parameter. A statistical analysis is developed on the output voltage of proton exchange membrane fuel cell models. The first model does not include any degradation, whereas the second one introduces a degradation rate on the cell active area. To complete the simulation work, a full factorial design is carried out and a statistical sensitivity analysis (ANOVA) is used to compute the effects and contributions of important parameters of the model on the output voltage.     

A novel approach on the determination of the minimal operating efficiency of a PEM fuel cell

In this article, a novel mathematical approach is proposed to determine the minimal proton exchange membrane fuel cell efficiency below which it is not recommended to operate the fuel cell. The objective of this proposal is to minimize the annual fuel cost and the electricity cost of a proton exchange membrane (PEM) fuel cell since both terms are efficiency dependent. A new concept developed in this article might be used as a valuable mathematical tool to determine the minimal efficiency required to operate a fuel cell in a reasonable fashion in order to make the fuel cell system technically and economically feasible. Two dimensionless mathematical criteria J₁ and J₂ were proposed for the annual fuel cost and electricity cost, respectively. A minimum fuel cell efficiency of was obtained with J₁ and J₂ values of 2.7 and 0.026, respectively.     

Detection of liquid water in the flow channels of PEM fuel cell using an optical sensor

An optical sensor was developed with the capability of detecting liquid water in the flow channels of a proton exchange membrane fuel cell (PEMFC) as well as simultaneously measuring temperature. This work is an extension of previous research in which an optical temperature sensor was developed for measuring the in situ temperature of PEM fuel cells based on the principles of phosphor thermometry. The optical sensor was installed in the cathode flow channel of a 5 cm² proton exchange membrane fuel cell. The fuel cell was tested under both dry and humid conditions. Liquid water formation in the flow channels was quantitatively measured from the experimental data. An observed time fraction value was estimated for characterizing flow channel flooding. The observed time fraction of liquid water in the flow channel was found to be closely related to the relative humidity of reactants and the operating current of the fuel cell.     

The effect of relative humidity of the cathode on the performance and the uniformity of PEM fuel cells

The effect of relative humidity of the cathode (RHC) on proton exchange membrane (PEM) fuel cells has been studied focusing on automotive operation. Computational fluid dynamics (CFD) simulations were performed on a 300-cm² serpentine flow-field configuration at various RHC levels. The dependency of current density, membrane water contents, net water flux on the performance and the uniformity was investigated. The uniformity of current density and temperature was evaluated by employing standard deviation. The water balance inside a fuel cell was examined by describing electro-osmotic drag and back diffusion. It was concluded that the RHC has strong effect on the cell performance and uniformity. The dry RHC showed low cell voltage and non-uniform distributions of current density and temperature, whereas high RHC presented increased cell performance and uniform distributions of current density and temperature with well-hydrated membrane electrode assembly (MEA). Also the local current density distribution was strongly dependent on the local membrane water contents distribution that has complex phenomena of water transport. The elimination of external humidifier is desirable for the automotive operation, but it could degrade cell performance and durability due to dehydration of the MEA. Therefore a proper humidification of the reactant is necessary.     

A Study of dynamic characteristics of PEM fuel cells by measuring local currents

Dynamic characteristics are important information in analyzing and understanding PEM fuel cell performances and detailed local dynamic information is extremely important in the understanding of fundamental mechanisms. In this study, the input parameters studied include humidification temperature of the reactant gases, operating temperature, and operating backpressure and the local dynamic responses are the local current densities. The fuel cell used is a PEM fuel cell with a single channel serpentine flow field and the local current densities are directly measured using the current density distribution measurement gasket and a multi-channel potentiostat. The experimental results show that very different local dynamic responses exist even though the response of the average current shows very little dynamics. Especially when the humidification temperature increases from a low value to moderate value, the responses of the local current density in the upstream are very different from those in the downstream.     

The effect of operating parameters on water transport in PEM fuel cells

Water content in the membrane and the presence of liquid water in the catalyst layers (CL) and the gas diffusion layers (GDL) play a very important role in the performance of a PEM fuel cell. To study water transport in a PEM fuel cell, a two‐phase flow mathematical model is developed. This model couples the continuity equation, momentum conservative equation, species conservative equation, and water transport equation in the membrane. The modeling results of fuel cell performances agree well with measured experimental results. Then this model is used to simulate water transport and current density distribution in the cathode of a PEM fuel cell. The effects of operating pressure, cell temperature, and humidification temperatures on the net water transfer through the membrane, liquid water saturation, and current density distribution are studied. © 2006 Wiley Periodicals, Inc. Heat Trans Asian Res, 35(2): 89–100, 2006; Published online in Wiley InterScience ( www.interscience. wiley.com ). DOI 10.1002/htj.20107     

Numerical analysis of effects of gas crossover through membrane pinholes in high-temperature proton exchange membrane fuel cells

Durability is a major issue in the widespread commercialization of proton exchange membrane fuel cells (PEMFCs). Various failure modes have been identified over their long runtime. These mainly originate from membrane and catalyst layer failures. One of the most common failure modes in PEMFCs is due to pinhole formation in the membrane and resultant reactant gas crossover through the membrane. Gas crossover induces several critical problems in PEMFCs, including severe reactant depletion in the downstream regions, mixed potential at the electrodes, and formation of local hot spots by hydrogen/oxygen catalytic reaction, which indicates that the cell performance decreases with increasing gas crossover. In this study, we numerically investigate the effects of gas crossover on the performance of a high-temperature PEMFC based on a phosphoric-acid-doped polybenzimidazole (PBI) membrane. In contrast to previous gas-crossover studies 1 and 2 in which uniform gas crossover throughout the entire membrane has been simply assumed, our focus is on examining the impacts of localized gas crossover due to membrane pinholes. Numerical simulations are carried out via arbitrarily assuming pinholes in the membrane. The simulation results clearly show that the presence of pinholes in the membrane significantly disrupts the species, current density, and temperature distributions. Our findings may improve the fundamental and detailed understanding of localized gas-crossover phenomena through the membrane pinholes and the influence of these phenomena on high-temperature PEMFC operation.     

PEM fuel cell supported distribution static compensator for power quality enhancement in three-phase four-wire distribution system

One of the major problems in electrical power system is the lack of quality of power due to the rapid growth of nonlinear load and unbalanced load utilization in three-phase four-wire distribution system. In this paper, PEM (Proton Exchange Membrane) fuel cell supported four-leg Distribution Static Compensator (DSTATCOM) is modelled to mitigate harmonics, neutral current and load balancing under nonlinear load and unbalanced load conditions in three-phase four-wire distribution system. The instantaneous reactive power (IRP) theory control algorithm is proposed for four-leg DSTATCOM. The Real coded Genetic Algorithm (RGA) optimized Proportional Integral (PI) controller and Adaptive Neuro Fuzzy Inference System (ANFIS) controller are used for regulating the DC link voltage of DSTATCOM. This paper also investigates the performance of ANFIS based DSTATCOM with conventional method. The proposed system is modelled and its performance is analyzed in MATLAB/SIMULINK.     

Numerical and experimental study of three-dimensional fluid flow in the bipolar plate of a PEM electrolysis cell

The bipolar plate is one of the key components in proton exchange membrane (PEM) electrolysis cell stacks for hydrogen production. Bipolar plates of electrolysis cells must be properly designed to distribute reactant (water) evenly, and efficient PEM electrolysis cell stacks will require optimized bipolar plates. Numerical simulations and experimental measurements of three-dimensional water flow were performed for the purpose of examining pressure and velocity distributions in the bipolar plate of a PEM electrolysis cell. For the studied flow range, the computed pressure drops agree very favorably with the measurements. Results show that pressure decreases from the inlet tube to the exit tube along the diagonal direction. Both velocity and temperature distributions are very non-uniform in the channels. A minimum of the peak values of mainstream velocity component in the channels develops in the center of the test plate. The maximum of these peak values appears in the channel near the exit tube. For the studied flow levels, these lines along which the mainstream velocity component is a peak in the channel almost overlay with each other, except that minor difference can be noticed in the channel near the exit tube.     

Analysis of PEM fuel cell experimental data using principal component analysis and multi linear regression

Polarisation curves performed at the Fuel Cell System Laboratory (FC LAB) at Belfort on a PEM fuel cell stack using a homemade fully instrumented test bench led to more than 100 variables depending on time. Visualising and analysing all the different test variables are complex. In this work, we show how the Principal Component Analysis (PCA) method helps to explore correlations between variables and similarities between measurements at a specific sampling time (individuals). To complete this method, an empirical model of the PEM fuel cell is proposed by linking the different input parameters to the cell voltage using Multiple Linear Regression.