Fuel crossover and internal current in polymer electrolyte membrane fuel cell from water visualization using X-ray radiography

The fuel crossover and internal current in a polymer electrolyte membrane fuel cell undergo a chemical reaction in the cell without power generation. These are the main phenomena for reduced cell voltage at low current density. This fuel crossover also degrades the fuel cell performance, efficiency, and durability. Thus, observation of these phenomena is important for understanding and developing a polymer electrolyte membrane fuel cell. Using X-ray radiography, the water distribution and membrane swelling, which indicate fuel crossover and internal current, in an operating polymer electrolyte membrane fuel cell under open-circuit conditions were examined. The X-ray images effectively demonstrated the transient changes of each phenomenon, which are related to the properties of each component and the operating conditions.     

Flow rate and humidification effects on a PEM fuel cell performance and operation

A new algorithm is presented to integrate component balances along polymer electrolyte membrane fuel cell (PEMFC) channels to obtain three-dimensional results from a detailed two-dimensional finite element model. The analysis studies the cell performance at various hydrogen flow rates, air flow rates and humidification levels. This analysis shows that hydrogen and air flow rates and their relative humidity are critical to current density, membrane dry-out, and electrode flooding. Uniform current densities along the channels are known to be critical for thermal management and fuel cell life. This approach, of integrating a detailed two-dimensional across-the-channel model, is a promising method for fuel cell design due to its low computational cost compared to three-dimensional computational fluid dynamics models, its applicability to a wide range of fuel cell designs, and its ease of extending to fuel cell stack models.     

Solid-phase temperature measurements in a HTPEM fuel cell

Segmented temperature measurements were performed to better understand the thermal behaviour and thermal interactions between the fluid-(gas)-phase and solid-phase temperature within a working high temperature polymer electrolyte membrane (HTPEM) fuel cell. Three types of flow-fields were studied, and the influence of temperature for no-load and load operating conditions was investigated. Tests were performed under various operating conditions, and the results demonstrate the utility of segmented temperature measurements. A significant difference in the temperature distribution was observed when the HTPEM fuel cell was operated with pure hydrogen and with hydrogen containing carbon monoxide. The findings may lead to improved HTPEM fuel cells and future middle temperature polymer electrolyte membrane (MTPEM) fuel cell designs.     

Liquid water visualization in PEM fuel cells: A review

Over the past few years, the importance of water management to the successful operation of polymer electrolyte membrane (PEM) fuel cells has stimulated an extensive research focus on liquid water transport and its effect on performance and durability. Empirical methods employed to investigate water transport in the fuel cell have the potential to provide useful feedback for developing empirical correlations and validating numerical models for fuel cell research and development. In this paper, a literature review is provided for the experimental techniques that have been applied to visualize liquid water in operating hydrogen PEM fuel cells and flow fields. The main hypotheses that have been proposed to describe liquid water transport in the gas diffusion layer (GDL) and current challenges will also be discussed.     

The impact of thermal conductivity and diffusion rates on water vapor transport through gas diffusion layers

Proper water management in a hydrogen-fueled polymer electrolyte membrane (PEM) fuel cell is critical for performance and durability. A mathematical model has been developed to elucidate the effect of thermal conductivity and water vapor diffusion coefficient in the gas diffusion layers (GDLs). The fraction of product water removed in the vapor phase through the GDL as a function of GDL properties/set of material and component parameters and operating conditions has been calculated. The current model enables identification of conditions wherein condensation occurs in each GDL component. The model predicts the temperature gradient across various components of a PEM fuel cell, providing insight into the overall mechanism of water transport in a given cell design. The water condensation conditions and transport mode in the GDL components depend on the combination of water vapor diffusion coefficients and thermal conductivities of the GDL components. Different types of GDLs and water transport scenarios are defined in this work, based on water condensation in the GDL and fraction of water that the GDL removes through the vapor phase, respectively.     

Water management fault diagnosis for proton-exchange membrane fuel cells based on deep learning methods

Water management failure is the most common fault in proton-exchange membrane fuel cells (PEMFCs), and it directly affects the durability and stability of fuel cells. This paper proposes a water fault diagnosis method based on 1DCNN-XGB. To promote its commercial applications, the cathode pressure drop, voltage, and current density were used as the characteristic diagnostic variables,which considered variations in the stack load and also facilitated hierarchical fault diagnosis. First, flooding and drying experiments under different current densities were simulated by changing the operating conditions of the stack, and the obtained experimental data were normalized to eliminate feature imbalances. Then, they were reconstructed into a 1D data set as the input of the model. Automatic feature extraction was performed by a 1DCNN, and the extracted feature maps were used as the input of the XGBoost classifier. Finally, the trained model was validated on the test set for fault diagnosis. The experimental results showed that the model accurately and efficiently distinguished normal and different degrees of two typical fault states (flooding and drying) of the stack with an overall accuracy of 98.10%. The comparative experiments revealed that the model was superior to the individual 1DCNN and XGBoost models, showing greater accuracy and better generalization ability.     

Investigation of the effects of the anisotropy of gas-diffusion layers on heat and water transport in polymer electrolyte fuel cells

The aim of this work is to study the effects of gas-diffusion layer (GDL) anisotropy and the spatial variation of contact resistance between GDLs and catalyst layers (CLs) on water and heat transfer in polymer electrolyte fuel cells (PEFCs). A three-dimensional, two-phase, numerical PEFC model is employed to capture the transport phenomena inside the cell. The model is applied to a two-dimensional cross-sectional PEFC geometry with regard to the in-plane and through-plane directions. A parametric study is carried out to explore the effects of key parameters, such as through-plane and in-plane GDL thermal conductivities, operating current densities, and electronic and thermal contact resistances. The simulation results clearly demonstrate that GDL anisotropy and the spatial variation of GDL/CL contact resistance have a strong impact on thermal and two-phase transport characteristics in a PEFC by significantly altering the temperature, water and membrane current density distributions, as well as overall cell performance. This study contributes to the identification of optimum water and thermal management strategies of a PEFC based on realistic anisotropic GDL and contact-resistance variation inside a cell.     

A hybrid model combining a support vector machine with an empirical equation for predicting polarization curves of PEM fuel cells

A hybrid model was proposed by combining a support vector machine (SVM) model with an empirical equation for more accurate prediction of the polarization curves of a PEM (polymer electrolyte membrane) fuel cell under various operating conditions. Operational data were obtained from designed experiments for a PEM fuel cell for training, testing, and validating the hybrid model, and a model training procedure was presented for determining the model coefficients and hyper-parameters of the hybrid model. The predictive performance of the hybrid model was compared with that of a SVM model. The SVM model showed somewhat poor performance, especially yielding large prediction errors in the high voltage ranges of the polarization curves as reported in the literature. In contrast, the hybrid model exhibited almost perfect matches between the predicted and measured polarization curves, resulting in significantly lower root-mean-square errors of 1.7–4.4 mV which correspond to only 14–21% of those obtained from the SVM model.     

Silicon carbide fiber-reinforced composite membrane for high-temperature and low-humidity polymer exchange membrane fuel cells

Currently, efforts are being made to commercialize a fuel cell system through research on fuel cell material enhancements. In particular, improvements in the membrane-electrode assembly, a key component of polymer electrolyte membrane (PEM) fuel cells, are essential to increase the performance of a fuel cell, in addition to accelerating its commercialization. Therefore, in this study, we used silicon carbide (SiC) fibers (web type) by electrospinning, which possess superior material, thermal, and chemical properties, as a structural material for the composite electrolyte membrane in the membrane-electrode assembly by impregnating it with the polymer electrolyte ionomer of short-side chain (SSC). In addition, we enhanced the ion-exchange capability of functionalized SiC fibers by introducing the hydroxyl (OH) group and phosphoric acid. The resulting functionalized composite electrolyte membrane exhibited a 70% better ion-exchange capability than the conventional cast electrolyte membrane and SiC webs composite electrolyte membranes was observed to excellent mechanical strength. We characterized and illustrative modeled the functionalized silicon carbide fibers, on the basis of which we further developed composite membrane. We then fabricated a unit cell of PEMFC based on this composite electrolyte membrane, and evaluated its single-cell performance, electrochemical properties, and accelerated voltage life-time durability test of operating 35 h according to the electro- and physic-chemical characteristics of the MEA under high-temperature and low humidity (120 °C/RH 40%).     

Sulfonated poly(arylene ether sulfone) nanocomposite electrolyte membrane for fuel cell applications: A review

Polymer electrolyte membrane (PEM) fuel cells are considered a promising technology for generating power with water as a byproduct. Recently, sulfonated poly(arylene ether sulfone) (SPAES) has emerged as a most suitable alternative for PEM applications because of its high proton conductivity, high CO tolerance, and low fuel crossover. However, the existing SPAES polymeric membrane materials have poor chemical reactivity, mechanical processability, and thermal usability. Thus, the effects of mixing inorganic nanomaterials with SPAES polymers on proton conductivity, power density, fuel crossover, thermal and chemical stability, and durability are discussed in this review. Further, the progress in preparation methods and fuel cell characteristics by the addition of silica, clay, heteropolyacids (HPA), and carbon nanotubes (CNTs) in polymer membrane materials for PEM applications is also discussed.     

Measurement of the water transport rate in a proton exchange membrane fuel cell and the influence of the gas diffusion layer

Water management in a PEM fuel cell significantly affects the fuel cell performance and durability. The gas diffusion layer (GDL) of a PEM fuel cell plays a critical role in the water management process. In this short communication, we report a simple method to measure the water transport rate across the GDL. Water rejection rates across a GDL at different cathode air-flow rates were measured. Based on the measurement results, the fuel cell operating conditions, such as current density, temperature, air stoichiometry and relative humidity, corresponding to membrane drying and flooding conditions were identified for the particular GDL used. This method can help researchers develop GDLs for a particular fuel cell design with specific operating conditions and optimize the operation conditions for the given PEM fuel cell components.     

Effects of an MPL on water and thermal management in a PEMFC

In this study, the effects of adding a microporous layer (MPL) as well as the impact of its physical properties on polymer electrolyte fuel cell (PEMFC) performance with serpentine flow channels were investigated. In addition, numerical simulations were performed to reveal the effect of relative humidity and operating temperature. It is indicated that adding an extra between the gas diffusion layer (GDL) and catalyst layer (CL), a discontinuity in the liquid saturation shows up at their interface because of differences in the wetting properties of the layers. In addition, results show that a higher MPL porosity causes the liquid water saturation to decrease and the cell performance is improved. A larger MPL thickness reduces the cell performance. The effects of MPL on temperature distribution and thermal transport of the membrane prove that the MPL in addition to being a water management layer also improves the thermal management of the PEMFC.     

Hierarchical fault diagnostic method for a polymer electrolyte fuel cell system

This study proposes a hierarchical method for on-line fault detection and diagnosis (FDD) of a stack and balance of plants (BoPs) in a polymer electrolyte fuel cell (PEFC) system. Because the fuel cell system consists of various subsystems with different characteristics, we have developed a multi-stage structure with subsystem-level FDD. In the first stage, faults were diagnosed at the subsystem level. In the next step, component-level faults were identified in the corresponding subsystem. The model-based approach in this study is composed of process estimation, residual generation, and FDD. Supervised machine learning methods were applied to train models for regression and fault classification. Residuals, the difference between analytic redundancies and measured results, were employed as fault indicators, i.e., residuals were used to detect faults and to generate fault patterns. Analytic redundancies were calculated using regression models. Several abrupt and performance degradation faults were considered. Because long-term performance degradations were difficult to introduce in the experimental system, the proposed method was evaluated using test data obtained by artificially decreasing the performance or sensor readings for a short period of time. This study focuses primarily on subsystem-level FDD and demonstrates one scenario of second level FDD. The experimental results verified the accuracy of the model-based approach and demonstrated that the proposed multi-stage hierarchical method effectively diagnosed faults in a PEFC system.     

Development of new energy management strategy for a household fuel cell/battery hybrid system

This work aims to construct an efficient and robust fuel cell/battery hybrid operating system for a household application. The ability to dispatch the power demands, sustain the state of charge (SOC) of battery, optimize the power consumption, and more importantly, ensure the durability as well as extend the lifetime of a fuel cell system is the basic requirements of the hybrid operating system. New power management strategy based on fuzzy logical combined state machine control is developed, and its effectiveness is compared with various strategies such as dynamic programming (DP), state machine control, and fuzzy logical control with simulation. Experimental results are also presented, except for DP because of difficulties in achieving real‐time implementation and much faster response to load variation. The given current from the energy management system (EMS) as a reference of the fuel cell output current is determined by filtering out various harmful signals. The new power management strategy is applied to a 1‐kW stationary fuel cell/battery hybrid system. Results show that the fuel cell hybrid system can run much smoothly with prolonged lifetime.     

Analysis of a Fuel Cell Durability Test Based on Design of Experiment Approach

The results of a durability test performed during 1000 h on a proton exchange or polymer electrolyte membrane (PEM) fuel cell are analyzed using some well-suited design of experiment techniques. The response surface methodology is adopted to model the performance degradation over ageing time from various load current-–fuel cell voltage curves recorded at regular time-spaced intervals and for various air utilization rates. The dual response surface approach is employed to determine the most convenient operating conditions for the cells (load current and air stoichiometry rate levels), leading to a tradeoff between elevated electrical efficiency and low voltage variability versus ageing time and cell positions in the stack. In addition, some electrochemical impedance spectra are used to provide some physical interpretations and to corroborate the observations made from the static measurements. The work shows notably the benefit of using some variable stoichiometry rates through ageing time to get high-power levels and stable performances as well.      

Thermal management issues in a PEMFC stack – A brief review of current status

Understanding the thermal effects is critical in optimizing the performance and durability of proton exchange membrane fuel cells (PEMFCs). A PEMFC produces a similar amount of waste heat to its electric power output and tolerates only a small deviation in temperature from its design point. The balance between the heat production and its removal determines the operating temperature of a PEMFC. These stringent thermal requirements present a significant heat transfer challenge. In this work, the fundamental heat transfer mechanisms at PEMFC component level (including polymer electrolyte, catalyst layers, gas diffusion media and bipolar plates) are briefly reviewed. The current status of PEMFC cooling technology is also reviewed and research needs are identified.     

Study of membrane electrode assemblies for direct methanol fuel cells

At DLR, membrane electrode assemblies (MEA) for direct methanol fuel cells (DMFC), are produced with the company’s own dry production technique. For improving this production technique, the MEAs in fuel cells are characterized electrochemically in fuel cell test facilities as well as physically by scanning electron microscopy (SEM).In order to measure the local current densities in polymer electrolyte membrane fuel cells, a method has been developed at DLR and tested in fuel cells supplied with hydrogen as fuel. For the DMFC, a measuring cell with 16 segments was built for examining MEAs with an overall active electrode area of 25 cm². With a sufficient resolution of location and time, simultaneous measurement of different local current densities in the cell can be carried out thus accelerating and improving operating parameter studies. This new tool is used at DLR for characterizing and developing improved MEAs and for examining the cell design (e.g. flow fields) and operating conditions of DMFC. In the measuring cell with its segments, the local mass conversion rates in the DMFC for liquid methanol–water mixtures are examined.     

Mechanical behavior of the membrane during the polymer electrolyte fuel cell operation

A computational modeling framework is developed to represent the transport phenomena, electrochemistry and the mechanical stresses in a polymer electrolyte fuel cell (PEFC). The model is able to predict the mechanical stresses developed in the polymer electrolyte due to hydration changes, and restriction of the membrane swelling as a result of these hydration changes in the PEFC assembly. Anisotropy in the mechanical properties of the gas diffusion layers is accounted in the stress calculations. It is seen that hydration variations during the PEFC operation can cause significant mechanical stresses. The effects of operating voltage and relative humidities of reactants are investigated. It is observed that high inlet humidities result in a better performance; however, it can potentially cause the polymer electrolyte membrane to go through plastic deformation irreversibly. Thermal stresses due to temperature variations are also calculated and compared with hygral stresses; and it is found that thermal stresses are not negligible but are typically a fraction of the hygral stresses in a typical PEFC operation.     

Real time power management strategy for fuel cell hybrid electric bus based on Lyapunov stability theorem

The fuel cell/battery durability and hybrid system stability are major considerations for the power management of fuel cell hybrid electric bus (FCHEB) operating on complicated driving conditions. In this paper, a real time nonlinear adaptive control (NAC) with stability analyze is formulated for power management of FCHEB. Firstly, the mathematical model of hybrid power system is analyzed, which is established for control-oriented design. Furthermore, the NAC-based strategy with quadratic Lyapunov function is set up to guarantee the stability of closed-loop power system, and the power split between fuel cell and battery is controlled with the durability consideration. Finally, two real-time power management strategies, state machine control (SMC) and fuzzy logic control (FLC), are implemented to evaluate the performance of NAC-based strategy, and the simulation results suggest that the guaranteed stability of NAC-based strategy can efficiently prolong fuel cell/battery lifespan and provide better fuel consumption economy for FCHEB.     

Modelling CO poisoning and O2 bleeding in a PEM fuel cell anode

Fuel gas containing carbon monoxide severely degrades the performance of a polymer electrolyte membrane (PEM) fuel cell. However, CO poisoning can be mitigated by introducing oxygen into the fuel (oxygen bleeding). A mathematical PEM fuel cell model is developed that simulates both CO poisoning and oxygen bleeding, and obtains excellent agreement with published, experimental data. Modelling efforts indicate that CO adsorption and desorption follow a Temkin model. Increasing operating pressure or temperature mitigates CO poisoning, while use of reformate fuel increases the severity of the poisoning effect. Although oxygen bleeding mitigates CO poisoning, an unrecoverable performance loss exists at high current densities due to competition for reaction sites between hydrogen adsorption and the heterogeneous catalysis of CO. Copyright © 2003 John Wiley & Sons, Ltd.