Direct measurement of current density under the land and channel in a PEM fuel cell with serpentine flow fields

One of the most common types of flow field designs used in proton exchange membrane (PEM) fuel cells is the serpentine flow field. It is used for its simplicity of design, its effectiveness in distributing reactants and its water removal capabilities. The knowledge about where current density is higher, under the land or the channel, is critical for flow field design and optimization. Yet, no direct measurement data are available for serpentine flow fields. In this study a fuel cell with a single channel serpentine flow field is used to separately measure the current density under the land and channel, which is either catalyzed or insulated on the cathode. In this manner, a systematic study is conducted under a wide variety of conditions and a series of comparisons are made between land and channel current density. The results show that under most operating conditions, current density is higher under the land than that under the channel. However, at low voltage, a rapid drop off in current density occurs under the land due to concentration losses. The mechanisms for the direct measurement results and general guidelines for serpentine flow field design and optimizations are provided.     

Optimization of PEM fuel cell flow field via local current density measurement

The serpentine flow field is the leading type of flow field used today in proton exchange membrane (PEM) fuel cells and for this reason optimization of serpentine flow field design is extremely important. In this study, a unique technique developed in house is utilized to separately measure current density under the land and channel on a variety of serpentine flow field geometries. Each flow field is tested under a wide variety of operating conditions thereby providing guidance for the optimum design geometry. Experimental results show that generally flow fields with both thinner lands and thinner channels provide better overall performance. However, the optimal flow field designs are highly dependent on fuel cell operating parameters.     

Direct measurement of current density under land and two channels in PEM fuel cells with interdigitated flow fields

Interdigitated flow field is one of the commonly used designs in proton exchange membrane (PEM) fuel cells. The knowledge of how the current density differs under the inlet channel, the land and the outlet channel, is critical for flow field design and optimization. In this study, the current densities under the inlet channel, the land and the outlet channel in PEM fuel cell with an interdigitated flow field are separately measured using the technique of partially-catalyzed membrane electrode assemblies (MEAs). The experimental results show that the current density under the outlet channel is significantly lower than that under the inlet channel, and the current density under the land is higher than both channels at typical fuel cell operation voltages. Further experimental results show that the pattern of local current density remains the same with different cathode flow rates.     

Experimental and numerical studies of local current mapping on a PEM fuel cell

Local current distribution on a PEM fuel cell has been mapped experimentally by using a special-designed single cell fixture. It is composed of a composite cathodic flow-field plate, a membrane electrode assembly (MEA) and a stainless-steel anodic flow-field plate. An array of 16 individual conductive segments was distributed on the composite plate. A self-made MEA is in direct contact with the segmented current collectors. Regional-averaged current through each segment is determined by using the Hall-effect sensor. To ensure the data reliability, a comparison of polarization curves was made between the composite flow-field plate and the conventional flow-field plate. Then, the effects of flow-field patterns, dew points of the cathodic feedings and cathodic stoichiometrics on the local current distribution were examined. The transient variation of the local current distribution on the cathode under supersaturated conditions was further visualized to illustrate the flooding phenomena in different flow patterns. This technique developed by the present work has contributed to knowledge and understanding the local current distributions in a PEM fuel cell that is helpful in designing the fuel-cell components.     

Computational model of a PEM fuel cell with serpentine gas flow channels

A three-dimensional computational fluid dynamics model of a PEM fuel cell with serpentine flow field channels is presented in this paper. This comprehensive model accounts for the major transport phenomena in a PEM fuel cell: convective and diffusive heat and mass transfer, electrode kinetics, and potential fields. A unique feature of the model is the implementation of a voltage-to-current (VTC) algorithm that solves for the potential fields and allows for the computation of the local activation overpotential. The coupling of the local activation overpotential distribution and reactant concentration makes it possible to predict the local current density distribution more accurately. The simulation results reveal current distribution patterns that are significantly different from those obtained in studies assuming constant surface overpotential. Whereas the predicted distributions at high load show current density maxima under the gas channel area, low load simulations exhibit local current maxima under the collector plate land areas.     

Performance analysis of a new designed PEM fuel cell

In this paper, a new design for the flow channels is presented, and a parametric study of the proton exchange membrane (PEM) fuel cell is conducted in order to investigate the effect of the new flow channels, as well as different operating parameters, on the efficiency and energy output of the cell. Design parameters are selected based on studies presented in the literature to build a physical and practical model. With the new design of the flow channels, it is noticed that the cell efficiency increases from 33.8% to 47.7% if the temperature of the cell is increased. The power output of the cell increases from 2.6 to 282.5 W when the cell temperature and the current density are increased. Moreover, decrease in the efficiency of the cell ranges from 45.5% to 28.4% with the increase in the current density and membrane thickness. Based on the analytical model, design parameters were selected to manufacture a fuel cell that has a power output of 175 W and an efficiency of 35% running at 353 K and 3 bar, with an effective membrane area of 450 cm². Experiments are conducted to investigate the effect of newly designed flow channels on pressure distribution. It is found that when hydrogen is supplied from both inlets, pressure across the channels become symmetric and, therefore increasing the power output. This study reveals that, with the proper choice of design parameters, a PEM fuel cell is an attractive economical, efficient, and environmental solution when compared with conventional systems of power generation such as gas turbines. Copyright © 2011 John Wiley & Sons, Ltd.     

Combined neutron radiography and locally resolved current density measurements of operating PEM fuel cells

Neutron radiographic imaging is combined with locally resolved current density measurements to study the effects of local water content on the performance of the corresponding electrochemical active area in an operating PEM fuel cell. Liquid water agglomerates are detected, quantified and correlated with the activity of the respective area. At low currents, depletion of the reactant gas leads to a decreasing performance along the anodic flowfield channel. At high currents, an optimum humidification is reached in the central part of the fuel cell; close to the inlets respectively outlets, flooding and drying can be observed concurrently and cause a non-uniform current density distribution across the reactive area. The fast response of the local performance on water droplets migrating in the gas channel is tracked by short-term imaging taking place on a timescale of several seconds.     

An analytical analysis on the cross flow in a PEM fuel cell with serpentine flow channel

A serpentine flow channel can be considered as neighboring channels connected in series, and is one of the most common and practical channel layouts for polymer electrolyte membrane (PEM) fuel cells, as it ensures the removal of liquid water produced in a cell with good performance and acceptable parasitic load. During the reactant flows along the flow channel, it can also leak or cross directly to the neighboring channel via the porous gas diffusion layer (GDL) due to the high‐pressure gradient caused by the short distance. Such a cross flow leads to a larger effective flow area resulting in a substantially lower amount of pressure drop in an actual PEM fuel cell compared with the case without cross flow. In this study, an analytical solution is obtained for the cross flow in a PEM fuel cell with a serpentine flow channel based on the assumption that the velocity of cross flow is linearly distributed in the GDL between two successive U‐turns. The analytical solution predicts the amount of pressure drop and the average volume flow rate in the flow channel and the GDL. The solution is validated over a wide range of the thickness and permeability of the GDL by comparing the results with experimental measurements and 3‐D numerical simulations in literature. Excellent agreement is obtained for the permeability less than 10^?9 m², which covers the typical permeability values of the GDLs in actual PEM fuel cells. The solution presents an accurate and efficient estimation for cross flow providing a useful tool for the design and optimization of PEM fuel cells with serpentine flow channels. Copyright © 2010 John Wiley & Sons, Ltd.     

Effect of operating conditions on current density distribution and high frequency resistance in a segmented PEM fuel cell

Understanding losses in polymer electrolyte membrane fuel cells, in the form of ohmic and mass transport, is of great importance to their commercialization. In this study, we use a spatially resolved cell consisting of 49 segments to measure the local current density distribution and high frequency resistance (HFR). A parametric study is used to investigate the effects of cell voltage, inlet relative humidity and flow rate and configuration using a three-channel serpentine flow field. We found that as the cell voltage decreased, the current density increased, while the HFR decreased. However, at a low cell voltage of 200 mV, we found the HFR to be higher than that at 500 mV. This increase is attributed to the increased electro-osmotic drag. This trend is independent of the flow configuration. Further, we found that the effect of the inlet relative humidity on the HFR highly depends on the flow configurations. Finally, a sharp decrease in the current density at some specific bend segments was observed, which correlates with lower OCV values and higher HFR values at this position.     

Performance prediction of PEM fuel cell with wavy serpentine flow channel by using artificial neural network

Effects of serpentine flow channel having sinusoidal wave at the rib surface on performance of PEMFC having 25 cm² active area are investigated at different flow rates, three different amplitudes changing from 0.25 mm to 0.75 mm and three different cell operation temperatures. A proton exchange membrane fuel cell (PEMFC) is modeled for the prediction of the output current by using artificial neural network (ANN) that is utilized the aforementioned experimental parameters. Effect of hydrogen and air flow rate, the fuel cell temperature, amplitude of channel is tested. The results indicated that model C1 having lowest amplitude is enhanced maximum power output up to 20.15% as compared to indicated conventional serpentine channel (model C4) for 0.7 SLPM H₂ and 1.5 SLPM air and also model C1 has better performance than C2, C3 and C4 models. The maximum power output is augmented with increasing the cell temperature due to raising the fuel and oxidant diffusion ratio. Cell temperature, amplitude, H₂ and air flow rate and input voltage is used as input variables in train and test of the developing ANN model. MAPE of training and testing is determined as 2.89 and 2.059, respectively. Prediction results of developed ANN model including two hidden layer shows similar trend with experimental results. Developed ANN model can be used to both decrease the number of required experiments and find the optimum operation condition within the range of input parameters.     

Effect of sub-freezing temperatures on a PEM fuel cell performance, startup and fuel cell components

The cold-start behavior and the effect of sub-zero temperatures on fuel cell performance were studied using a 25-cm² proton exchange membrane fuel cell (PEMFC). The fuel cell system was housed in an environmental chamber that allowed the system to be subjected to temperatures ranging from sub-freezing to those encountered during normal operation. Fuel cell cold-start was investigated under a wide range of operating conditions. The cold-start measurements showed that the cell was capable of starting operation at −5 °C without irreversible performance loss when the cell was initially dry. The fuel cell was also able to operate at low environmental temperatures, down to −15 °C. However, irreversible performance losses were found if the cell cathode temperature fell below −5 °C during operation. Freezing of the water generated by fuel cell operation damaged fuel cell internal components. Several low temperature failure cases were investigated in PEM fuel cells that underwent sub-zero start and operation from −20 °C. Cell components were removed from the fuel cells and analyzed with scanning electron microscopy (SEM). Significant damage to the membrane electrode assembly (MEA) and backing layer was observed in these components after operation below −5 °C. Catalyst layer delamination from both the membrane and the gas diffusion layer (GDL) was observed, as were cracks in the membrane, leading to hydrogen crossover. The membrane surface became rough and cracked and pinhole formation was observed in the membrane after operation at sub-zero temperatures. Some minor damage was observed to the backing layer coating Teflon and binder structure due to ice formation during operation.     

Effects of leak current density and doping level on energetic,exergetic and ecological performance of a high-temperature PEM fuel cell

This paper presents a one-dimensional and semi-empirical model of a high-temperature PEM fuel cell (HT-PEMFC) to determine the performance characteristics through energy, exergy, and ecological analysis. The proposed model is compared with different experimental studies and supported by a few statistical approaches to prove its accuracy. As a result, the minimum and maximum R² values are determined to be 99.67% and 99.97%, respectively. In addition, the performance of the fuel cell is investigated under varying leakage current densities and doping levels. Accordingly, increasing the leak current density decreases the power density, net output voltage, energy efficiency, and exergy efficiency by 5.77%, 5.88%, 5.44%, and 5.48%, respectively, whereas increasing the doping level boosts these parameters by 23.07%, 11.76%, 30.25%, and 32.52%, respectively. In addition, increasing the leak density decreases all ecological functions. In contrast, raising the doping level increases the ecological parameters considerably and reduces the improvement potential.     

Numerical and experimental investigation of cascade type serpentine flow field of reactant gases for improving performance of PEM fuel cell

The distribution of reactant gases in polymer electrolyte membrane fuel cells (PEMFCs) plays a pivotal role in current density distribution, temperature distribution, and water concentration. Problems such as flooding or drying of the membrane are caused by the non-uniformity of the above mentioned parameters resulting in a reduced membrane electrode assembly (MEA) life time. In this study, a new cascade type serpentine flow field is introduced and the concept of design is explained. The simulation results are in good agreement with the literature. The optimal channel to rib ratio is obtained using simulation results. The results show that the proposed flow field produces a uniform current density and local stoichiometry as well as an improved water management. It is also determined that the two phase numerical method can estimate experimental results correctly.     

Tomographic diagnostics of current distributions in a fuel cell stack

A novel tomographic scheme for analysing the state of any single membrane electrode assembly (MEA) in a stack is suggested. Plates of very high conductivity placed between every fuel cell and slitted in an appropriate manner cause surface currents at well‐defined locations of the stack. We show that knowing these surface currents, information about anomalies of the currents in a MEA can be obtained using the methods of tomography. The results are mathematically not unique. However, when assuming plausible defect structures, one can exclude improbable deficiencies by applying a special form of simulated annealing. We present numerical calculations of typical examples demonstrating that the essential defects of the MEA in any single cell of the stack can be detected and their extent can be determined. Copyright © 2009 John Wiley & Sons, Ltd.     

Effects of humidification temperatures on local current characteristics in a PEM fuel cell

It is well known that water plays a very important role in the performance of proton exchange membrane (PEM) fuel cells. Non-uniform water content in the membrane leads to non-uniform ionic resistance, and non-uniform liquid water fraction in the porous electrode causes varied mass transfer resistances. The objective of this work is to study the effects of different anode and cathode humidification temperatures on local current densities of a PEM fuel cell with a co-flow serpentine flow field. The method used is the current distribution measurement gasket technique [H. Sun, G.S. Zhang, L.J. Guo, H. Liu, J. Power Sources 158 (2006) 326–332]. The experimental results show that both air and the hydrogen need to be humidified to ensure optimal cell performance, and too high or too low humidification temperature can cause severe non-uniform distribution of local current density. From the experimental results of local current density distributions, the local membrane hydration, the optimal humidification temperature, and if flooding occurs can be obtained. Such detailed local measurement results could be very valuable in fuel cell design and operation optimizations.     

The effect of flow distributors on the liquid water distribution and performance of a PEM fuel cell

Water management is one of the important factors which determine the performance of a Proton Exchange Membrane (PEM) fuel cell using hydrogen as fuel. For developing efficient water management systems, it is important to know the potential locations of formation and the nature of distribution of liquid water in the fuel cell. In the present study a PEM fuel cell with three different types of flow distributors are modeled and numerically simulated to find out the water formation and distribution characteristics. The model is validated by comparing the simulated polarization curve to experimental data. It is found that the type of flow distributor used plays a major role in determining the distribution of liquid water in the cell. A parallel flow distributor exhibits poor water removal capabilities whereas a serpentine flow distributor exhibits better water removal. A mixed flow distributor is found to give better water distribution characteristics compared to the parallel and serpentine distributors. Further the effect of liquid water formation and distribution on the species transport, temperature distribution and current generation are also investigated.     

An experimental and numerical investigation on the cross flow through gas diffusion layer in a PEM fuel cell with a serpentine flow channel

A serpentine flow channel is one of the most common and practical channel layouts for a polymer electrolyte membrane (PEM) fuel cell since it ensures the removal of water produced in a cell with acceptable parasitic load. During the reactant flows along the flow channel, it can also leak or cross to neighboring channel via the porous gas diffusion layer due to the high pressure gradient caused by the short distance. Such a cross flow leads to a larger effective flow area altering reactant flow in the flow channel so that the resultant pressure and flow distributions are substantially different from that without considering cross flow, even though this cross flow has largely been ignored in previous studies. In this work, a numerical and experimental study has been carried out to investigate the cross flow in a PEM fuel cell. Experimental measurements revealed that the pressure drop in a PEM fuel cell is significantly lower than that without cross flow. Three-dimensional numerical simulation has been performed for wide ranges of flow rate, permeability and thickness of gas diffusion layer to analyze the effects of those parameters on the resultant cross flow and the pressure drop of the reactant streams. Considerable amount of cross flow through gas diffusion layer has been found in flow simulation and its effect on pressure drop becomes more significant as the permeability and the thickness of gas diffusion layer are increased. The effects of this phenomenon are also crucial for effective water removal from the porous electrode structure and for estimating pumping energy requirement in a PEM fuel cell, it cannot be neglected for the analysis, simulation, design, operation and performance optimization of practical PEM fuel cells.     

Liquid water preferential accumulation in channels of PEM fuel cells with multiple serpentine flow fields

This work presents an experimental investigation on the preferential accumulation of liquid water in the channels of a multiple serpentine PEMFC with 50 cm² active area. Neutron imaging was used for visualizing the liquid water distribution during the cell operation for a wide range of operating conditions. Liquid water accumulation in the cathode channels was observed for most of the operating conditions, with a preferential accumulation in certain channels of the flow field. A statistical analysis was performed in order to determine the main characteristics of this accumulation (i.e. channel number and degree of accumulation). As cathode channels were positioned in vertical direction, it was found that gravity effects had an important influence in the accumulation, as well as the relative position of the channel with respect to the inlet and outlet locations. The gas flow direction had also a major impact on the water accumulation within the channels, with significantly more water accumulated in channels with upwards gas flow.     

Cold pre-compression of membrane electrode assembly for PEM fuel cells

Direct compression from the land structure of bipolar plate in a PEM fuel cell is considered as an important factor for the higher performance under the land than under the channel areas. Therefore the objective of this study is to determine if a cold pre-compression treatment on the whole membrane electrode assembly (MEA) area may have a significant positive effect on the overall performance of the cell. Five different levels of cold pre-compression have been applied and the experimental results show that the overall performance of the cell first increases with the level of compression to a maximum, and then decreases. These results clearly show that cold pre-compression of the MEA can significantly enhance the performance of the entire cell and there exists an optimal level of compression. Results of electrochemical impedance spectroscopy (EIS) show that the cold pre-compression results in a significant reduction in charge transfer resistance, especially in the high current density region. Further study by the cyclic voltammetry (CV) shows that the electro-chemical area (ECA) is changed with the different cold pre-compressed MEAs and there exists an optimal compression that results in the maximum ECA.     

Experimental investigation of current distribution in a direct methanol fuel cell with serpentine flow-fields under various operating conditions

The influences of various operating conditions on the current distribution of a direct methanol fuel cell with flow-fields of serpentine channels are investigated by means of a current-mapping method. The current densities generally deviate more from an even distribution when the cell temperature or flow rate of the cathode reactant is lower, or when the current loaded on the cell or the methanol concentration is higher. In addition, uneven current distributions decrease the cell performance. Relevant mass-transfer phenomena such as water flooding and methanol crossover are discussed. The characteristics of the channel configuration also affect the current density profiles. With a five-line serpentine channel, the current densities are lowered periodically where the flow direction is inverted due to the corner flow effect and the subsequent water accumulation. With a single serpentine channel, on the other hand, the current densities peak periodically where the flow direction is inverted due to enhanced air convection through the gas-diffusion layer.