Natural degradation and stimulated recovery of a proton exchange membrane fuel cell

In this paper, the stimulated recovery of a proton exchange membrane (PEM) fuel cells after natural degradation has been investigated. The performance degradation of a 63-cell PEM fuel cell stack over a storage interval of 40,000 h at temperature 24 °C and relative humidity 65% was analyzed by static and dynamical tests. The average cell voltage degradation rate was 309 μV h⁻¹, averaged over a range of currents. The performance was then partially recovered by application of a high frequency pulsing procedure after which the effective average degradation rate (from the commencement of storage to after the recovery) was approximately 170 μV h⁻¹. This indicates the existence of both recoverable and irrecoverable degradations in the fuel cell. Furthermore, the equivalent circuit model and membrane resistance were used to investigate the degradation mechanisms, suggesting that the natural degradation of the fuel cell is mainly caused by the increase of the resistance, which is most likely caused by membrane dehydration.     

Development of a proton exchange membrane fuel cell cogeneration system

A proton exchange membrane fuel cell (PEMFC) cogeneration system that provides high-quality electricity and hot water has been developed. A specially designed thermal management system together with a microcontroller embedded with appropriate control algorithm is integrated into a PEM fuel cell system. The thermal management system does not only control the fuel cell operation temperature but also recover the heat dissipated by FC stack. The dynamic behaviors of thermal and electrical characteristics are presented to verify the stability of the fuel cell cogeneration system. In addition, the reliability of the fuel cell cogeneration system is proved by one-day demonstration that deals with the daily power demand in a typical family. Finally, the effects of external loads on the efficiencies of the fuel cell cogeneration system are examined. Results reveal that the maximum system efficiency was as high as 81% when combining heat and power.     

Multivariable robust control of a proton exchange membrane fuel cell system

This paper applies multivariable robust control strategies to a proton exchange membrane fuel cell (PEMFC) system. From the system point of view, a PEMFC can be modeled as a two-input-two-output system, where the inputs are air and hydrogen flow rates and the outputs are cell voltage and current. By fixing the output resistance, we aimed to control the cell voltage output by regulating the air and hydrogen flow rates. Due to the nonlinear characteristics of this system, multivariable robust controllers were designed to provide robust performance and to reduce the hydrogen consumption of this system. The study was carried out in three parts. Firstly, the PEMFC system was modeled as multivariable transfer function matrices using identification techniques, with the un-modeled dynamics treated as system uncertainties and disturbances. Secondly, robust control algorithms were utilized to design multivariable H_∞ controllers to deal with system uncertainty and performance requirements. Finally, the designed robust controllers were implemented to control the air and hydrogen flow rates. From the experimental results, multivariable robust control is shown to provide steady output responses and significantly reduce hydrogen consumption.     

Transient power characteristic measurement of a proton exchange membrane fuel cell generator

An experimental study on the transient power characteristics of a fuel cell generator has been conducted. The generator is hybridized by a proton exchange membrane (PEM) as the main power source and a lithium-ion battery as the secondary power source. power-conditioning module consisting of a main bidirectional converter and an auxiliary converter has been designed to manage the hybrid power of the generator that copes with fast dynamics of variable loads. Sensors embedded in the generator have measured the electrical properties dynamically. It was found that the present power-conditioning scheme has well controlled the power flow between the fuel cell stack and the battery by regulating the power flow from or to the battery. In addition, the thermal management system using pulse width modulation (PWM) schemes could limit the operation temperature of the fuel cell generator in a designed range. Furthermore, the dynamics of electrical efficiency of the generator are found to be parallel with those of the net system power. Finally, the stability and reliability of the fuel cell generator is proven by the rational dynamic behaviors of thermal and electrical properties for over 30-h demonstration.     

The computer-aided analysis for the driving stability of a plug-in fuel cell vehicle using a proton exchange membrane fuel cell

In order to analyze the driving stability of a plug-in fuel cell vehicle (PFCV), a computer-aided simulator for PFCVs has been developed. PFCVs have been introduced around the world to achieve early commercialization of an eco-friendly and highly efficient fuel cell vehicle. The plug-in option, which allows the battery to be recharged from the electricity grid, enables a reduction in size of the fuel cell system (FCS) and an improvement of its durability. As such, the existing limitations of the fuel cell - such as its high cost, poor durability, and the insufficient hydrogen infrastructure – can be overcome. During the design phase of PFCV development, simulation-based driving stability test is necessary to determine the sizes of the electric engine of the FCS and the battery. The developed simulator is very useful for analyzing the driving stability of the PFCV with respect to the capacities of the FCS and battery. The simulation results are in fact very close to those obtained from a real system, since the estimation accuracy of PFCV component models used in this simulator, such as the fuel cell stack, battery, electric vehicle, and the other balance of plants (BOPs), are verified by the experiments, and the simulator uses the newly-proposed power distribution control logic and the pre-confirmed real driving schedule. Using these results, we can study which one will be the best in terms of driving stability.     

Behavior of a unit proton exchange membrane fuel cell in a stack under fuel starvation

Durability is an important issue in proton exchange membrane fuel cells (PEMFCs) currently. Fuel starvation could be one of the reasons for PEMFC degradation. In this research, the fuel starvation conditions of a unit cell in a stack are simulated experimentally. Cell voltage, current distribution and localized interfacial potentials are detected in situ to explore their behaviors under different hydrogen stoichiometries. Results show that the localized fuel starvation occurs in different sections at anode under different hydrogen stoichiometries when the given hydrogen is inadequate. This could be attributed to the “vacuum effect” that withdraws fuel from the manifold into anode. Behaviors of current distribution show that the current will redistribute and the position of the lowest current shifts close to the anode inlet with decreasing hydrogen stoichiometry, which indicates that the position of the localized fuel starvation would move towards the inlet of the cell. It is useful to understand the real position of the degradation of MEA.     

Dynamic modeling of a high-temperature proton exchange membrane fuel cell with a fuel processor

A dynamic model of a high-temperature proton exchange membrane fuel cell with a fuel processor is developed in this study. In the model, a fuel processing system, a fuel cell stack, and an exhaust gas burner are modeled and integrated. The model can predict the characteristics of the overall system and each component at the steady and transient states. Specifically, a unit fuel cell model is discretized in a simplified quasi-three-dimensional geometry; therefore, the model can rapidly predict the distribution of fuel cell characteristics. Various operating conditions such as the steam-to-carbon ratio, oxygen-to-carbon ratio, and autothermal reforming inlet temperature are varied and investigated in this study. In addition, the dynamic characteristics exhibited during the transient state are investigated, and an efficiency controller is developed and implemented in the model to maintain the electrical efficiency. The simulation results demonstrate that the steam-to-carbon ratio and the oxygen-to-carbon ratio affect the electrical and system efficiency and that controlling the fuel flow rate maintains the electrical efficiency in the transient state. The model may be a useful tool for investigating the characteristics of the overall system as well as for developing optimal control strategies for enhancing the system performance.     

Control of a heat-integrated proton exchange membrane fuel cell system with methanol reforming

This work presents a novel heat-integrated fuel cell stack system with methanol reforming. Its configuration is composed of fuel processing units (FPUs), proton exchange membrane (PEM) fuel cell stack, and heat exchangers (HEXs). Well mixed methanol and oxygen flows in contact with countercurrent flowing water dominates the production of hydrogen at the exit of FPUs and influences the stack temperature. The heat exchange connections can enhance the utilization of energy of FPUs. To ensure the stable steady-state operation, the model-free fuzzy incremental control scheme within the multi-loop feedback control framework is developed. Finally, the proposed system integration and control configuration are verified by closed-loop simulations.     

Dynamic modeling of a proton exchange membrane fuel cell system with a shell-and-tube gas-to-gas membrane humidifier

The proton exchange membrane fuel cell (PEMFC) system with a shell-and-tube gas-to-gas membrane humidifier is considered to be a promising PEMFC system because of its energy-efficient operation. However, because the relative humidity of the dry air flowing into the stack depends on the stack exhaust air, this system can be unstable during transients. To investigate the dynamic behavior of the PEMFC system, a system model composed of a lumped dynamic model of an air blower, a two-dimensional dynamic model of a shell-and-tube gas-to-gas membrane humidifier, and a one-dimensional dynamic model of a PEMFC system is developed. Because the water management during transient of the PEMFC system is one of the key challenges, the system model is simulated at the step change of current. The variations in the PEMFC system characteristics are captured. To confirm the superiority of the system model, it is compared with the PEMFC component model during transients.     

Study of the cell reversal process of large area proton exchange membrane fuel cells under fuel starvation

In this research, the fuel starvation phenomena in a single proton exchange membrane fuel cell (PEMFC) are investigated experimentally. The response characteristics of a single cell under the different degrees of fuel starvation are explored. The key parameters (cell voltage, current distribution, cathode and anode potentials, and local interfacial potentials between anode and membrane, etc.) are measured in situ with a specially constructed segmented fuel cell. Experimental results show that during the cell reversal process due to the fuel starvation, the current distribution is extremely uneven, the local high interfacial potential is suffered near the anode outlet, hydrogen and water are oxidized simultaneously in the different regions at the anode, and the carbon corrosion is proved to occur at the anode by analyzing the anode exhaust gas. When the fuel starvation becomes severer, the water electrolysis current gets larger, the local interfacial potential turns higher, and the carbon corrosion near the anode outlet gets more significant. The local interfacial potential near the anode outlet increases from ca. 1.8 to 2.6 V when the hydrogen stoichiometry decreases from 0.91 to 0.55. The producing rate of the carbon dioxide also increases from 18 to 20 ml min⁻¹.     

Analysis of operating parameters considering flow orientation for the performance of a proton exchange membrane fuel cell using the Taguchi method

This study has applied the L₁₈ 2 × 3⁷ orthogonal array of the Taguchi method to determine the optimal combination of six primary operating parameters (flow orientation, temperature of fuel cell, anode and cathode humidification temperatures, anode, and cathode stoichiometric flow ratios) of a PEM fuel cell. The optimal combination factor is co-flow, a cell temperature of 333 K, an anode humidification temperature of 353 K, a cathode humidification temperature of 333 K, a stoichiometric flow ratio for hydrogen of 2, and a stoichiometric flow ratio for oxygen of 3; and the amount of maximum power is 17.61 W. The results for the experiment indicate that flow orientation, temperature of fuel cell, and anode and cathode humidification temperatures are significant factors for affecting the performance. Furthermore, this study simulates the transport phenomenon and electrochemical reactions using a finite-element method at the optimal combination factor from the experimental results of Taguchi method.     

Covalent-ionically cross-linked polyetheretherketone proton exchange membrane for direct methanol fuel cell

In this paper, the proton exchange membrane prepared by covalent-ionically cross-linking water soluble sulfonated-sulfinated poly(oxa-p-phenylene-3,3-phthalido-p-phenylene-oxa-p-phenylene-oxy-phenylene) (SsPEEK-WC) is reported. Compared with covalent cross-linked PEEK-WC membrane, this covalent-ionically cross-linked PEEK-WC membrane exhibits extremely reduced water uptake and methanol permeability, but just slightly sacrificed proton conductivity. The proton conductivity of the covalent-ionically cross-linked PEEK-WC membrane reaches to 2.1 × 10⁻² S cm⁻¹ at room temperature and 4.1 × 10⁻² S cm⁻¹ at 80 °C. The methanol permeability is 1.3 × 10⁻⁷ cm² s⁻¹, 10 times lower than that of Nafion^® 117 membrane. The results suggest that the covalent-ionically cross-linked PEEK-WC membrane is a promising candidate for direct methanol fuel cell because of low methanol permeability and adequate proton conductivity.     

Behaviors of proton exchange membrane fuel cells under oxidant starvation

Durability is an important issue in proton exchange membrane fuel cells (PEMFCs) currently. Reactant starvation could be one of the reasons for PEMFC degradation. In this research, the oxidant starvation phenomena in a single cell are investigated. The local interfacial potential, current and temperature distribution are detected in situ with a specially constructed segmented cell. Experimental results show that during the cell reversal process due to oxidant starvation, the local interfacial potential in the oxidant inlet keeps positive while that of the middle and outlet regions become negative, which illustrates that oxygen and proton reduction reactions could occur simultaneously in different regions at the cathode. The current distribution would be more uneven with decreasing air stoichiometry before cell reversal. When cell reversal occurs, the current will redistribute and the current distribution tends more uniform. At the critical point of cell reversal, the most significant inhomogeneity in the current distribution can be observed. The temperature distribution in the cell is also monitored on-line. The local hot spot exists in the cell when cell reversal occurs. The study of the critical reversal air stoichiometry under different loads shows that the critical reversal air stoichiometry increases with the rising loads.     

Two dimensional dynamic modeling of a shell-and-tube water-to-gas membrane humidifier for proton exchange membrane fuel cell

Water management is a crucial factor in determining the performance of proton exchange membrane fuel cell (PEMFC) for automotive application. The shell-and-tube water-to-gas membrane humidifier is useful for humidifying the PEMFC due to its good performance. Shell-and-tube water-to-gas membrane humidifiers have liquid water on one side of the tube wall and a dry gas on the other. In order to investigate humidifier performance, a two-dimensional dynamic model of a shell-and-tube water-to-gas membrane humidifier is developed. The model is discretized into three control volumes – shell, tube and membrane – in the cross-sectional direction to resolve the temperature and species concentration of the humidifier. For validation, the dew point temperature of the simulation result is compared with that of experimental data and shows good agreement with only a slight difference. The distribution of humidification characteristics can be captured using the discretization along the air-flow direction. The humidification performance of two different flow configurations, counter and parallel, are compared under various operating conditions and geometric parameters. Finally, the dynamic response of the humidifier at the step-change of various air flow rates is investigated. These results suggest that the model can be used to optimize the inlet flow humidity of a PEMFC.     

Pore network modeling of cathode catalyst layer of proton exchange membrane fuel cell

In this paper, a pore network model is developed to investigate the coupled transport and reaction processes in the cathode catalyst layer (CCL) of proton exchange membrane fuel cell (PEMFC). The developed model is validated by comparing the predicted polarization curve with the experimental data, and the parametric studies are carried out to elucidate the effects of CCL design parameters. With the decrease of the CCL thickness and the Nafion content, the cell voltage reduces at the low current density but increases when the current density is higher. The cell performance is also improved by increasing the proton conductivity of the Nafion film in the CCL. As compared to the CCL of uniformly distributed Nafion, the CCL with the Nafion volume decreasing along the thickness direction exhibits better performance at the high current density.     

Cross-linked PEEK-WC proton exchange membrane for fuel cell

The low cost proton exchange membrane was prepared by cross-linking water soluble sulfonated-sulfinated poly(oxa-p-phenylene-3,3-phthalido-p-phenylene-oxa-p-phenylene-oxy-phenylene) (SsPEEK-WC). The prepared cross-linked membrane became insoluble in water, and exhibited high proton conductivity, 2.9 × 10⁻² S/cm at room temperature. The proton conductivity was comparable with that of Nafion^® 117 membrane (6.2 × 10⁻² S/cm). The methanol permeability of the cross-linked membrane was 1.6 × 10⁻⁷ cm²/s, much lower than that of Nafion^® 117 membrane.     

Synthesis methods of low-Pt-loading electrocatalysts for proton exchange membrane fuel cell systems

While the use of a high level of platinum (Pt) loading in proton exchange membrane fuel cells (PEMFCs) can amplify the trade off towards higher performance and longer lifespan for these PEMFCs, the development of PEMFC electrocatalysts with low-Pt-loadings and high-Pt-utilization is critical and the limited supply and high cost of the Pt used in PEMFC electrocatalysts necessitate a reduction in the Pt level. In order to make such electrocatalysts commercially feasible, cost-effective and innovative, catalyst synthesis methods are needed for Pt loading reduction and performance optimization. Since a Pt-deposited carbon nanotube (CNT) shows higher performance than a commercial Pt-deposited carbon black (CB) with reducing 60% Pt load per electrode area in PEMFCs, use of CNTs in preparing electrocatalysts becomes considerable. This paper reviews the literature on the synthesis methods of carbon-supported Pt electrocatalysts for PEMFC catalyst loading reduction through the improvement of catalyst utilization and activity. The features of electroless deposition (ED) method, deposition on sonochemically treated CNTs, polyol process, electrodeposition method, sputter-deposition technique, γ-irradiation method, microemulsion method, aerosol assisted deposition (AAD) method, Pechini method, supercritical deposition technique, hydrothermal method and colloid method are discussed and characteristics of each one are considered.     

Transient computation fluid dynamics modeling of a single proton exchange membrane fuel cell with serpentine channel

The proton exchange membrane fuel cell (PEMFC) has become a promising candidate for the power source of electrical vehicles because of its low pollution, low noise and especially fast startup and transient responses at low temperatures. A transient, three-dimensional, non-isothermal and single-phase mathematical model based on computation fluid dynamics has been developed to describe the transient process and the dynamic characteristics of a PEMFC with a serpentine fluid channel. The effects of water phase change and heat transfer, as well as electrochemical kinetics and multicomponent transport on the cell performance are taken into account simultaneously in this comprehensive model. The developed model was employed to simulate a single laboratory-scale PEMFC with an electrode area about 20 cm². The dynamic behavior of the characteristic parameters such as reactant concentration, pressure loss, temperature on the membrane surface of cathode side and current density during start-up process were computed and are discussed in detail. Furthermore, transient responses of the fuel cell characteristics during step changes and sinusoidal changes in the stoichiometric flow ratio of the cathode inlet stream, cathode inlet stream humidity and cell voltage are also studied and analyzed and interesting undershoot/overshoot behavior of some variables was found. It was also found that the startup and transient response time of a PEM fuel cell is of the order of a second, which is similar to the simulation results predicted by most models. The result is an important guide for the optimization of PEMFC designs and dynamic operation.     

Parametric analysis of the proton exchange membrane fuel cell performance using design of experiments

Proton exchange membrane fuel cell (PEMFC) performance depends on different fuel cell operating temperatures, humidification temperatures, operating pressures, flow rates, and various combinations of these parameters. This study employed the method of the design of experiments (DOE) to obtain the optimal combination of the six primary operating parameters (fuel cell operating temperatures, operating pressures, anode and cathode humidification temperatures, anode and cathode stoichiometric flow ratios). In the first stage, this study adopted a 2^k−2 fractional factorial design of the DOE to determine whether these factors have significant effects on a response and the interactions between various parameters. Second, the L₂₇(3¹³) orthogonal array of the Taguchi method is utilized to determine the optimal combination of factors for a fuel cell. Based on this study, the operating pressure, the operating temperature, and the interactions between operating temperature and operating pressure have a significant effect on the fuel cell performance. Among them, the operating pressure is the most important contributor. When the operating pressure increases, it should simultaneously lower the effects of other factors. While both the operating temperature and pressure increase simultaneously with that, the other factors are at appropriate conditions, it is possible to improve the fuel cell performance.