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.     

Cold start characteristics of proton exchange membrane fuel cells

In this study, the effects of the start-up temperature, load condition and flow arrangement on the cold start characteristics and performance of a proton exchange membrane fuel cell (PEMFC) are investigated through in-situ experiments with the simultaneous measurements of the current and temperature distributions. Rather than the commonly recognized cold start failure mode due to the ice blockage in cathode catalyst layer (CL), another failure mode due to the ice blockage in flow channel and gas diffusion layer (GDL) leading to significantly high pressure drop through cathode flow field is observed at a start-up temperature just below the lowest successful start-up temperature. Three ice formation mechanisms are proposed, corresponding to the ice formations in cathode CL, GDL and flow channel. The general distributions of current densities and temperatures during the constant current cold start processes are similar to the constant voltage cold start processes, except that the temperatures at the end of the constant current cold start processes are more evenly distributed over the active reaction area because of the increased heat generation rates. The cold start characteristics are mainly dominated by the cathode flow, and changing the flow arrangement has unimportant impact on the cold start performance.     

Numerical simulation of thermal–hydraulic characteristics in a proton exchange membrane fuel cell

The thermal–hydraulic characteristics of a proton exchange membrane fuel cell (PEMFC) are numerically simulated by a simplified two‐phase, multi‐component flow model. This model consists of continuity, momentum, energy and concentration equations, and appropriate equations to consider the varying flow properties of the gas–liquid two‐phase region in a PEMFC. This gas–liquid two‐phase characteristic is not considered in most of the previous simulation works. The calculated thermal–hydraulic phenomena of a PEMFC are reasonably presented in this paper, which include the distributions of flow vector, temperature, oxygen concentration, liquid water saturation, and current density, etc. Coupled with the electrochemical reaction equations, current flow model can predict the cell voltage vs current density curves (i.e. performance curves), which are validated by the single‐cell tests. The predicted performance curves for a PEMFC agree well with the experimental data. In addition, the positive effect of temperature on the cell performance is also precisely captured by this model. The model presented herein is essentially developed from the thermal–hydraulic point of view and can be considered as a stepping‐stone towards a full complete PEMFC simulation model that can help the optima design for the PEMFC and the enhancement of cell efficiency. Copyright © 2003 John Wiley & Sons, Ltd.     

Development of non-perfluorinated hybrid materials for single-cell proton exchange membrane fuel cells

Development of low temperature fuel cells that operate under 100 °C are needed to reduce the costs, to design a class of hybrid membranes and to construct various structures of membrane-electrode-assembles (MEAs) for proton exchange membrane fuel cells (PEMFC). In this work, PVA/PMA/SiO₂ hybrid composite membranes were synthesized and their conductivities were determined by impedance measurements. We found a maximum conductivity value of 4.2 × 10⁻³ S/cm at 80 °C and 100% relative humidity (RH). A fuel cell test evaluation for various MEAs was conducted by the potentiodynamic analysis and the current density values were determined from the current–voltage (I–V) curves. A maximum current density of 635 mA/cm² was obtained at 80 °C and 100% RH. To the best of our knowledge, this is the first time that a high current density of PVA-based electrolytes for PEMFCs operating at low temperature is reported. The structural characters were examined using of XRD and FTIR methods, and thermal properties were studied using DSC and TGA techniques and the results were discussed (cf. supplementation). The present study revealed that the single cell performance depends mainly on the temperature, relative humidity and chemical compositions of the membranes.     

Polybenzimidazole containing ether units as electrolyte for high temperature proton exchange membrane fuel cells

A rapid method to synthesize poly[2,2′-(p-oxydiphenylene)-5,5′-benzimidazole] (OPBI) through a solution polycondensation under microwave irradiation is explored. Synthesis parameters affecting the molecular weight (M_w) of OPBI, including the mass ratio of solvent to P₂O₅, the monomer concentration, and reaction time, are optimized. The main characteristics of OPBI are studied, and the corresponding membrane is prepared through a solvent casting process. A series of sulfuric acid doped OPBI (H₂SO₄/OPBI) hybrid membranes with different acid doping levels (ADLs) are developed. The effects of H₂SO₄ on microstructure, ADL and electrochemical properties of these membranes are explored. Herein, the hybrid membrane shows high proton conductivity (190 mS cm⁻¹) at elevated temperature (160 °C) and anhydrous conditions, high ADL (18.73 mol of H₂SO₄ for OPBI per repeat unit, i.e., ADL = 18.73 mol PRU⁻¹) and excellent dimensional stability (40.3%). All these properties demonstrated that H₂SO₄/OPBI hybrid membrane can be used as an alternative membrane for high temperature proton exchange membrane fuel cells (HT-PEMFCs).     

Effects of platinum loading on the performance of proton exchange membrane fuel cells with high ionomer content in catalyst layers

The effect of Pt loading on the performance of proton exchange membrane fuel cells with atmospheric air feed was evaluated at various relative humidities. The membrane electrode assemblies (MEAs) were fabricated by decal methods with high Nafion ionomer content (30 and 40 wt.%). When the Pt loading was decreased, the performance of the MEAs with an ionomer content of 30 wt.% gradually decreased, mainly due to the insufficient active Pt surfaces with low proton conductivity. With a higher ionomer content of 40 wt.%, the activation overpotential was not significantly increased by the decrease in Pt loading, and the concentration overpotential could be largely reduced by decreasing the Pt loading to 0.25 mg/cm². When the Pt loading was further decreased to 0.15 mg/cm², even though the flooding became more severe, the cell performance at 0.6 V and intermediate relative humidity of 55% was about 71.6%, compared to the MEA with a high Pt loading of 0.35 mg/cm² (ionomer content: 30 wt.%). The cell performance could be further enhanced by decreasing the ionomer content in the anode to enhance the water back diffusion.     

Constructal flow distributor as a bipolar plate for proton exchange membrane fuel cells

A plate-type constructal flow distributor is implemented as a gas distributor for a proton exchange membrane fuel cell. A 3D complete model is simulated using CFD techniques. The fuel cell model includes the gas flow channels, the gas diffusion layers and the membrane-electrode assembly (MEA). The governing equations for the mass and momentum transfer are solved including the pertinent source terms due to the electrochemical reactions in the different zones of the fuel cell. Three constructal flow configurations were studied; each pattern is a fractal expansion of the original design, therefore, the only difference between them is the number of branches in the geometry. It was found that the number of branches is the key parameter in the performance of a fuel cell when using the constructal distributors as flow channels. The performance of the fuel cell is reported in I-V curves, power curves, and overpotential curves in order to determine which irreversibility is the main cause of energy losses. In terms of flow analysis, it was found that the constructal flow distributor presents a low pressure drop for a wide range of Reynolds number conditions at the inlet, as well as an excellent uniformity of flow distribution. Regardless of the outstanding hydrodynamic performance of the constructal distributors and the large current density values obtained, the implementation of these designs as flow patterns for PEMFCs need further optimization; first, the manufacturing of the plates have to be addressed in an efficient way; and secondly, the application in stacks will require an elaborate design to accomplish this task.     

质子交换膜燃料电池的稳态分析

通过建立质子交换膜燃料电池稳态模型,考察了电堆温度和反应压力对电堆性能的影响。仿真结果表明,升高电堆温度使得氢气分压和氧气分压下降,但氢气分压下降的更快;在电堆工作温度范围内,电堆温度升高,热动力电势、欧姆极化电势和活化极化电势均下降,但电堆总输出电压上升;提高阴极侧压力有利于提高热动力电势,同时使得活化极化电势降低,有利于电堆整体性能的改善;提高阳极侧压力对电堆性能改善影响不大。     

An adaptive RNA genetic algorithm for modeling of proton exchange membrane fuel cells

The accurate mathematical model is the key issue to simulation and design of the fuel cell power systems. Aiming at estimating the proton exchange membrane fuel cell (PEMFC) model parameters, an adaptive RNA genetic algorithm (ARNA-GA) which is inspired by the mechanism of biological RNA is proposed. The ARNA-GA uses the RNA strands to represent the potential solutions and new genetic operators are designed for improving the global searching ability. In order to maintain the population diversity and avoid premature convergence, on the basis of the dissimilarity coefficient, the adaptive genetic strategy that allows the algorithm dynamically select crossover operation or mutation operation to execute is proposed. Numerical experiments have been conducted on some benchmark functions with high dimensions. The results indicate that ARNA-GA has better search capability and a higher quality of solutions. Finally, the proposed approach has been applied for the parameter estimation of PEMFC model and the satisfactory results are reached.     

Membrane-hydration-state detection in proton exchange membrane fuel cells using improved ambient-condition-based dynamic model

Proton-exchange-membrane fuel cells (PEMFCs) are a popular source of alternative energy because of their operational reliability and compactness. This paper presents an improved model to represent the semi-empirical voltage of PEMFCs to overcome the limitations of existing models. The proposed model considers variations in ambient conditions, such as the ambient temperature and relative humidity, to obtain the accurate output voltage that corresponds to variations in dynamic and static loads. The proposed model was developed by conducting several experiments on the Horizon PEMFC system under normal, humid, and dry ambient conditions. Subsequently, the model parameters corresponding to each case were optimised using the quantum lightning search algorithm (QLSA). Parameters demonstrating significant variations with ambient conditions were finally represented as a function of the ambient temperature and relative humidity via statistical regression analysis. The voltage obtained using the modified model was verified by conducting experiments on both the Horizon and NEXA PEMFC systems by varying the ambient temperature and relative humidity with root mean square error (RMSE) less than 0.5. As observed, the results we obtained using the modified model closely approximated those obtained using PEMFCs under various operating conditions, and in both cases, the PEMFC voltage was observed to vary with the ambient and load conditions. The inherent advantages of the proposed PEMFC model include its ability to determine the membrane-water content and water pressure inside PEMFCs. The membrane-water content provides clear indications regarding the occurrence of drying and flooding faults. Under normal conditions, this membrane water content ranges from 11 to 7 for both the Horizon and NEXA PEMFC system. The simulation results suggested using the threshold membrane-water-content level as a possible indicator of fault occurrence under extreme ambient conditions. The limits of the said threshold were observed to be useful for fault diagnosis within PEMFC systems.     

Novel closed anode pressure-swing system for proton exchange membrane fuel cells

To overcome the low system efficiency and fuel efficiency in conventional recirculation systems and purging systems, a new anode closed pressure-swing system for proton exchange membrane fuel cells (PEMFCs) is proposed in this paper. Only two pressure regulators set at different pressures and a buffer tank are applied to produce pressure-swing inside the anode through the process of hydrogen feed and reaction, thereby achieving the anode-dead-end (ADE) operation (no purging). This system was successfully tested on a 20-cell open-cathode stack. The results indicate that the performance of the stack with the developed system is stable and efficient over 13,000 s, while its performance in ADE mode without purging deteriorates rapidly because of active area reduction. It can be concluded that the pressure swing system provides the following advantages: close to 100% hydrogen utilization; improved stack performance of around 12.5% for the power compare to the ADE mode with intermittent purging at 12 V load; improved humidification of the anode and membrane; and ease of implementation as there are no extra pumps.     

Development of shut-down process for a proton exchange membrane fuel cell

Several different shut-down procedures were carried out to reduce the degradation of membrane electrode assembly (MEA) in a proton exchange membrane fuel cell (PEMFC). The effects of close/open state of outlets of a single cell and application of a dummy load during the shut-down on the degradation of the MEA were investigated. Also, we elucidated the relationship between the thickness of the electrolyte membrane and the degradation of the MEA for different shut-down procedures. When a thin electrolyte membrane was used, the closer of outlets mitigated the degradation during on/off operation. For the thicker electrolyte membrane, the dummy load which eliminates residual hydrogen and oxygen in the electrodes should be applied to lower the degradation.     

Water removal characteristics of proton exchange membrane fuel cells using a dry gas purging method

Water removal from proton exchange membrane fuel cells (PEMFC) is of great importance to improve start-up ability and mitigate cell degradation when the fuel cell operates at subfreezing temperatures. In this study, we report water removal characteristics under various shut down conditions including a dry gas-purging step. In order to estimate the dehydration level of the electrolyte membrane, the high frequency resistance of the fuel cell stack was observed. Also, a novel method for measuring the amount of residual water in the fuel cell was developed to determine the amount of water removal. The method used the phase change of liquid water and was successfully applied to examine the water removal characteristics. Based on these works, the effects of several parameters such as purging time, flow rate of purging gas, operation current, and stack temperature on the amount of residual water were investigated.     

Circular genetic operators based RNA genetic algorithm for modeling proton exchange membrane fuel cells

Inspired by the biological RNA, a circular genetic operators based RNA genetic algorithm (cRNA-GA) is proposed to estimate the model parameters of the proton exchange membrane fuel cell (PEMFC). To maintain the population diversity and avoid premature convergence, we design the novel genetic operator of the double-loop crossover operator. To allow the algorithm to jump out of local optima, the adaptive mutation probabilities are presented and the stem-loop mutation operator is adopted with the other mutation operators. The simulated annealing method is also incorporated into the cRNA-GA to improve local search ability. Performance tests conducted on some typical benchmark functions have witnessed the validity of cRNA-GA. The cRNA-GA is also applied to estimate the parameters of the PEMFC model and the satisfactory results have shown its effectiveness.     

Dynamic characteristics of internal current during startups/shutdowns in proton exchange membrane fuel cells

Studying dynamic characteristics of proton exchange membrane fuel cells (PEMFCs) during startups/shutdowns is of great importance to proposing strategies to improve fuel cell performance and durability. In this study, internal current during startup and shutdown processes in PEMFC is investigated, and effects of gas supply/shutoff sequences and backpressure are analyzed by measuring local current densities and the cell voltage in situ. The experimental results show that when reactants were fed/shut off, internal current occurs and variation patterns of local current densities along the flow channel are different. During startups, local current densities in the downstream drop to negative values and internal current can be eliminated when air is first supplied into the cell. While during shutdowns, the results show that negative currents occur in the upstream, and if hydrogen is shut off first, all local current densities remain constant at zero, indicating the effectiveness of gas shutoff sequence in eliminating/mitigating internal current in PEM fuel cells. Further experimental results show that the magnitude of internal current increases with the pressure difference between the anode and the cathode.     

Study on the CCM breakdown voltage of proton exchange membrane fuel cells

Proton exchange membrane (PEM) short circuits are one of the failure forms of fuel cells. In this paper, the change in breakdown voltage (BV) after the preparation of PEMs into catalyst coated membranes (CCMs) is studied, and the impact of the catalyst layer (CL) and its composition on the BV of the CCM is analysed. The results show that the BV of the CCM is significantly lower than that of the uncoated PEM. The higher the platinum (Pt) loading of the coated CL is, the lower the BV. Further research finds that the BV of the single-side CL-coated CCM only decreases when the CL side is connected to the positive pole of the power supply, while it is comparable to that of the PEM when the CL side is connected to the negative pole. The experimental results demonstrate that the Pt and carbon particles in the CCM undergo electrochemical reactions during the breakdown process when the CL is connected to the positive pole, which eventually leads to thermal breakdown. Therefore, when the BV is chosen for detecting whether the CCM preparation process causes PEM damage, single-side CL-coated CCM should be adopted, and the CL should be connected to the negative pole of the power supply.     

Computational analysis of mixed potential effect in proton exchange membrane fuel cells

In a typical proton exchange membrane fuel cell (PEMFC), a gas crossover brings parasitic reaction, such as hydrogen and carbon oxidation at the cathode and oxygen reduction at the anode, which reduces open circuit potential (OCP) because of undesired potential mixing. Therefore, a two-dimensional computational fluid dynamics model was formulated to elucidate the variation of cell polarization, as the parameters affecting the mixed-potential effect change. The present model was validated by comparing the simulated cell polarization with experimentally measured cell polarization. The membrane electrode assembly was prepared by the decal transfer method, which gives uniform electrode formation. Model comparisons were also conducted to clearly state the significance of the fuel crossover and carbon oxidation reaction on OCP decrease. The results have shown that model prediction fits experimental data with an acceptable range of error, under two different relative humidity conditions of 50 and 100%. In addition, further investigations were conducted on (i) effect of gas permeation coefficient in membrane, (ii) effect of membrane thickness and (iii) effect of carbon oxidation and their influences on OCP and cell polarization are discussed.     

Numerical investigation on the performance of proton exchange membrane fuel cells with channel position variation

Such factors as mole fractions of species, water generation, and conductivity influence the performance of proton exchange membrane fuel cells (PEMFCs). The geometrical shape of the fuel cells also should be considered a factor in predicting the performance because this affects the species' reaction speed and distribution. Specifically, the position between the channel and rib is an important factor influencing PEMFC performance because the current density distribution is affected by the channel and rib position. Three main variables that decide the current density distribution are selected in the paper: species concentration, overpotentials, and membrane conductivity. These variables should be considered simultaneously in deciding the current density distribution with the given PEMFC cell voltage. In addition, the inlet relative humidity is another factor affecting current density distribution and membrane conductivity. In this paper, two channel‐to‐rib models, namely, channel‐to‐channel and the channel‐to‐rib, are considered for comparing the PEMFC performance. Thorough performance comparisons between these two models are presented to explain which is better under certain parameters. A three‐dimensional numerical PEMFC model is developed for obtaining the current density distribution. Water transfer mechanism because of electro osmotic drag and concentration diffusion also is presented to explain the PEMFC performance comparison between the two models. Copyright © 2012 John Wiley & Sons, Ltd.     

Ultra-low Pt loading for proton exchange membrane fuel cells by catalyst coating technique with ultrasonic spray coating machine

This paper reports use of an ultrasonic spray for producing ultra-low Pt load membrane electrode assemblies (MEAs) with the catalyst coated membrane (CCM) fabrication technique. Anode Pt loading optimization and rough cathode Pt loading were investigated in the first stage of this research. Accurate cathode Pt coating with catalyst ink using the ultrasonic spray method was investigated in the second stage. It was found that 0.272 mg_Pt/cm² showed the best observed performance for a 33 wt% Nafion CCM when it was ultrasonically spray coated with SGL 24BC, a Sigracet manufactured gas diffusion layer (GDL). Two different loadings (0.232 and 0.155 mg_Pt/cm²) exposed to 600 mA/cm² showed cathode power mass densities of 1.69 and 2.36 W/mg_Pt, respectively. This paper presents impressive cathode mass power density and high fuel cell performance using air as the oxidant and operated at ambient pressure.     

Carbon monoxide poisoning of proton exchange membrane fuel cells

Proton exchange membrane fuel cell (PEMFC) performance degrades when carbon monoxide (CO) is present in the fuel gas; this is referred to as CO poisoning. This paper investigates CO poisoning of PEMFCs by reviewing work on the electrochemistry of CO and hydrogen, the experimental performance of PEMFCs exhibiting CO poisoning, methods to mitigate CO poisoning and theoretical models of CO poisoning. It is found that CO poisons the anode reaction through preferentially adsorbing to the platinum surface and blocking active sites, and that the CO poisoning effect is slow and reversible. There exist three methods to mitigate the effect of CO poisoning: (i) the use of a platinum alloy catalyst, (ii) higher cell operating temperature and (iii) introduction of oxygen into the fuel gas flow. Of these three methods, the third is the most practical. There are several models available in the literature for the effect of CO poisoning on a PEMFC and from the modeling efforts, it is clear that small CO oxidation rates can result in much increased performance of the anode. However, none of the existing models have considered the effect of transport phenomena in a cell, nor the effect of oxygen crossover from the cathode, which may be a significant contributor to CO tolerance in a PEMFC. In addition, there is a lack of data for CO oxidation and adsorption at low temperatures, which is needed for detailed modeling of CO poisoning in PEMFCs. Copyright © 2001 John Wiley & Sons, Ltd.