Experimental investigation of PEM fuel cell aging under current cycling using segmented fuel cell

The effect of inhomogeneous humidification on the local aging of the membrane electrode assemblies (MEAs) was investigated using segmented fuel cells under current cycling conditions. The active area of the MEA was divided into 8 segments along the flow channels. The effects of the local humidification of the MEA were measured by the high frequency resistance (HFR). It was found that the HFR of the first segment was largest due to the lower humidification. The concentration of water and water vapor was higher downstream, the MEA was humidified, and consequently the value of HFR also decreased. The fuel cells were operated at 60% RH, cell temperature 60 °C for uniform cell performance, with current cycling between 700 and 70 mA/cm². The polarization curve showed less differentiation between each segment after 450 cycles and 150 hours of the current loading test. The MEA was exposed to cycling expansion and contraction during the drying and humidification processes, respectively. The results suggest that the membrane might experience mechanical degradation in downstream segments. In addition, the slope of the cyclic voltammetry (CV) curves increased from region 6 to region 8. The surface area of the catalyst also decreased due to the downstream water flooding, which might block the mass transfer and result in fuel starvation, carbon corrosion and catalyst degradation.     

Direct measurement of lateral current density distribution in a PEM fuel cell with a serpentine flow field

In a proton exchange membrane (PEM) fuel cell, local current density can vary drastically in the lateral direction across the land and channel areas. It is essential to know the lateral current density variations in order to optimize flow field design and fuel cell performance. Thus the objective of this work is to directly measure the lateral current density variations in a PEM fuel cell with a serpentine flow field. Five 1 mm-width partially-catalyzed membrane electrode assemblies (MEA), each corresponding to a different location from the center of the gas channel to the center of the land area are used in the experiments. Current densities for fuel cells with each of the partially-catalyzed MEAs are measured and the results provide the lateral current density distribution. The measurement results show that in the high cell voltage region, local current density is the highest under the center of the land area and decreases toward the center of the channel area; while in the low cell voltage region local current density is the highest under the center of the channel area and decreases toward the center of the land area. Besides, the effects of cathode flow rates on the lateral current density distribution have also been studied. Furthermore, comparisons have also been made by using air and oxygen in the cathode and it is found that when oxygen is used the local current density under the land is significantly enhanced, especially in the low cell voltage region.     

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.     

Effect on high frequency resistance behavior of proton exchange membrane fuel cell during storage process

The high frequency resistance (HFR) is used to characterize the water content in the proton exchange membrane fuel cell (PEMFC), and the change of cell HFR during the storage process after gas purge with different storage and purge conditions is studied in this paper. The repeatability of the experiment is verified firstly. Then, the changes of cell HFR with different storage conditions include cell temperature, environment temperature, initial cell HFR are analyzed. Finally, the effect of purge conditions in the preparation stage on the cell HFR is studied by change the purge flow rate, purge gas type, purge methods. The results show that the cell HFR is affected by both the storage conditions and the purge operation conditions. The time for the PEMFC to reach the maximum HFR increases with the decrease of the environment temperature and the increase of the initial cell temperature. The final stable HFR value decreases with the increases of the environment temperature.     

Prediction of local current distribution in polymer electrolyte membrane fuel cell with artificial neural network

In the development process of a fuel cell, understanding the local current distribution is essentially required to achieve better performance and durability. Therefore, many developers apply a segmented fuel cell to observe current distribution under various operating conditions. With the application, experimental data is collected. This study suggests a utilization method for this collected data to develop a local current prediction model. The details of this neural network-based prediction model are introduced, including the pretreatment of the data. In the pretreatment process, current residual values are used for better prediction performance. As a result, the model predicted local current values with a 2.98% error. With the model, the effects of pressure, temperature, cathode relative humidity, and cathode flow rate on local current distribution trends are analyzed. Since the non-uniform current distribution of a fuel cell often leads to low performances or fast local degradation, the optimal operating condition to achieve current uniformity is acquired with an additional model. This model is developed by switching inputs and outputs of the local current prediction model. With the model application, the uniform current distribution is achieved with a standard deviation of 0.039 A/cm² under the current load at 1 Acm^?2.     

Membrane resistance and current distribution measurements under various operating conditions in a polymer electrolyte fuel cell

The ability to make spatially resolved measurements in a fuel cell provides one of the most useful ways in which to monitor and optimise their performance. Localised membrane resistance and current density measurements for a single channel polymer electrolyte fuel cell are presented for a range of operating conditions. The current density distribution results are compared with an analytical model that exhibited generally good agreement across a broad range of operating conditions. However, under conditions of high air flow rate, an increase in current is observed along the channel which is not predicted by the model. Under such circumstances, localised electrochemical impedance measurements show a decrease in membrane resistance along the channel. This phenomenon is attributed to drying of the electrolyte at the start of the channel and is more pronounced with increasing operating temperature.     

Comprehensive influences measurement and analysis of power converter low frequency current ripple on PEM fuel cell

To deeply understand the influences of power converter's low frequency current ripple (LFCR) and harmonics on a proton exchange membrane fuel cell (PEMFC) in its power conditioning system (PCS), a comprehensive measurement and analysis of the influences of LFCR and harmonics on PEMFC's performance and durability is investigated in this paper. Based on an equivalent circuit model of PEMFC stack and a mechanism model for evaluating the LFCR effects on the PEMFC, this paper studies primarily and systematically the comprehensive influences of LFCR and harmonics on PEMFC performances and durability, such as (1) degrading the PEMFC performance, (2) shortening the lifetime of PEMFC, (3) reducing the stack output power, (4) lowing its availability efficiency, (5) producing more heat and raising the PEMFC temperature, (6) consuming more fuel, and (7) decreasing the fuel utilization. Finally, a Horizon 300 W PEMFC stack is implemented and tested.     

Effect of operating current dependent series resistance on the fill factor of a solar cell

The fill factor of a solar cell depends upon the series resistance, reverse saturation current, diode quality factor, operating current and voltage. Since the series resistance itself depends upon the operating current (or voltage), it makes the evaluation of fill factor very complicated. In this paper, we have evaluated the fill factor of a solar cell, taking into account operating current dependence of the series resistance.     

Durability improvement at high current density by graphene networks on PEM fuel cell

This study presents a novel structure of catalyst layers of membrane electrode assemblies (MEAs) by adding graphene to platinum on carbon black (Pt/C) to improve the durability at high current density operation (3 A cm⁻²). Graphene displays outstanding low electrical resistance and has the advantage of high electron mobility. It is also used in lithium ion batteries to improve electrical performance such as high rate charge/discharge capability and cycle-life stability. In this study, three MEAs are compared, and graphene is used as an excellent conductive additive in catalyst layers for better electrons transport at high current density operation. The MEA coated Pt/C mixed with 0.1 wt% graphene shows best durability for 0.3 V h⁻¹ which is almost 3.7 times better than that of without graphene additive (1.1 V h⁻¹). The graphene additive effectively extends the durability of the MEA. Furthermore, the MEAs are analyzed by AC impedance. The impedance arc of the MEA coated with Pt/C only is getting worse, but those two coated with graphene show similar and smaller impedance arcs after high current density operation for 80 h.     

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.     

Internal behavior of segmented fuel cell during cold start

In order to improve cold start capability and survivability of proton exchange membrane fuel cell (PEMFC), a fundamental understanding of its internal behavior is required. In this study, the cold start processes of a PEMFC with different operating conditions have been investigated, and the characteristics of current density and temperature distributions are studied through in-situ experiments with a printed circuit board (PCB). It is found that the start ability of PEMFC is strong at −3 and −5 °C, but weak at −7 and −10 °C. Also the self-start ability can be enhanced by decreasing the initial current load. Polarization curves show almost no degradation after successful cold start at −3 and −5 °C, while the PEMFC degrades a lot after failed cold start at low temperature like −10 °C. Also electrochemical impedance spectroscopy (EIS) shows a big degradation after galvanostatic mode cold starts. Local current density of segmented cell results shows that the highest current density is initially near the inlet region and then quickly moves downstream, reaching to the region near the middle eventually during the successful cold start process. However, during the failed cold start process, the highest current density is initially near the inlet region of the flow channels and quickly moves down stream, reaching the upper left corner region (A1) before shut down eventually. For both successful and failed cold starts, the highest temperature can be observed near the middle of the cell after the reaching of the highest current density.     

Current mapping of a proton exchange membrane fuel cell with a segmented current collector during the gas starvation and shutdown processes

Current distribution during the gas starvation and shutdown processes is investigated in a proton exchange membrane fuel cell with an active area of 184 cm². The cell features a segmented cathode current collector. The response characteristics of the segmented single cell under different degrees of hydrogen and air starvation are explored. The current responses of the segment cells at different positions under a dummy load in the shutdown process are reported for various operating conditions, such as different dummy loads, cell temperatures, and gas humidities under no back pressure. The results show that applying a dummy load during the cell shutdown process can quickly reduce the cell potential and thereby avoid the performance degradation caused by high potentials. The currents of all the segment cells decrease with time, but the rate of decrease varies with the segment cell positions. The rate for the segment cells near the gas outlet is much higher than that of the segment cells near the gas inlet. The current of the segment cells decreases much more quickly at a lower gas humidity and high temperature. This study provides insights in the development of mitigation strategies for the degradation caused by starvation and shutdown process.     

Effect of operating conditions on the performance of a direct methanol fuel cell with PtRuMo/CNTs as anode catalyst

PtRu/CNTs and PtRuMo/CNTs catalysts have been synthesized by microwave-assisted polyol process and used as the anode catalysts for a direct methanol fuel cell (DMFC). The catalysts were characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD) and X-ray photoelectron spectrometry (XPS). The effect of different anode catalysts, membrane electrode assembly (MEA) activation, methanol concentration, methanol flow rate, oxygen flow rate and cell temperature on the DMFC performance has been investigated. The results show that the PtRu or PtRuMo particles with face-centered cubic structure are uniformly distributed on CNTs, and the addition of Mo to PtRu/CNTs makes the binding energies of each Pt species shift to lower values. PtRuMo/CNTs is a promising anode catalyst for DMFCs, and the appropriate operating conditions of the DMFC with PtRuMo/CNTs as the anode catalyst are MEA activation for 10 h, 2.0–2.5 M methanol at the flow rate of 1.0–2.0 mL/min, and oxygen at the flow rate of 100–150 mL/min. The DMFC performance increases significantly with an increase in cell temperature.     

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.     

Optimizing the relative humidity to improve the stability of a proton exchange membrane by segmented fuel cell technology

The segmented fuel cell technology was applied to investigate the effects of the humidification conditions on the internal locally resolved performance and the stability of the fuel cell system. It was found at certain operating conditions, the time-dependent oscillation of current at potentio-static state appeared. The appearance of positive spikes of current indicated a temporary improved performance, while the negative current spikes indicated a temporary decreased performance. The periodic build-up and removal of liquid water in the cell caused unstable cell performance. Through the analyses of the evolution of the locally resolved current density distributions, the reasons for the positive or the negative spikes of current peaks with respect to a stationary value were found, which might be due to the drying-out of the membrane or the flooding of the membrane. The contour of the current density mapping differed to each other at the period of current peaks up or down, which might be due to different effect of the drying-out or flooding on the membrane. Through optimizing the relative humidity of anode (RHa) or cathode (RHc) of the fuel cell, the oscillation of the current disappeared and the performance of the cell became stable. RHc affects the performance of fuel cell much more obviously than RHa. The stability of the fuel cell system is also dependent on the imposed voltage. With the cell voltage decreased, the amplitude and the frequency of positive spikes of current increased.     

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 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.     

A parametric comparison of three fuel recirculation system in the closed loop fuel supply system of PEM fuel cell

Anodic fuel recirculation system has a significant role on the parasitic power of proton exchange membrane fuel cell (PEMFC). In this paper, different fuel supply systems for a PEMFC including a mechanical compressor, an ejector and an electrochemical pump are evaluated. Furthermore, the performances of ejector and electrochemical pump are studied at different operating conditions including operating temperature of 333 K–353 K, operating pressure of 2 bar–4 bar, relative humidity of 20%–100%, stack cells number from 150 to 400 and PEMFC active area of 0.03 m²–0.1 m². The results reveal that higher temperature of PEMFC leads to lower power consumption of the electrochemical pump, because activation over-potential of electrochemical pump decreases at higher temperatures. Moreover, higher operating temperature and pressure of PEMFC leads to higher stoichiometric ratio and hydrogen recirculation ratio because the motive flow energy in ejector enhances. In addition, the recirculation ratio and hydrogen stoichiometric ratio increase, almost linearly, with increase of anodic relative humidity. Utilization of mechanical compressor leads to lower system efficiency than other fuel recirculating devices due to more power consumption. Utilization of electrochemical pump in anodic recirculation system is a promising alternative to ejector due to lower noise level, better controllability and wide range of operating conditions.     

Study on the influence of segmented fuel cell by grooving method and its application in oxygen starvation diagnosis

The oxygen starvation in fuel cells is an important reason for the deterioration of durability. The segmented fuel cell is a method to study the gas distribution inside the fuel cell. In order to study the influence of the grooving method on segmented fuel cell and its application in oxygen starvation diagnosis, a five-serpentine-channel three-dimensional two-phase simulation model is established by FLUENT. Through steady-state simulation, the effect of grooving method on fuel cell performance is studied. The overall performance (polarization curve) of the fuel cell drops slightly, but the current density distribution on the anode graphite plate changes greatly due to the grooves. The “current concentration” phenomenon is proposed based on the current density distribution. Through dynamic simulation, the oxygen starvation under current load mode and voltage load mode is simulated, and the “starvation coefficient” is defined as an oxygen starvation diagnostic index. In the current load mode, the “starvation coefficient” never exceed 15%, because when the oxygen starvation is severe, the simulation cannot converge or even cannot maintain, which corresponds to the voltage reversal in reality. However, in the voltage load mode, the “starvation coefficient” can reach up to 100%. The conclusions have important guiding significance for the judgment of the internal reaction uniformity of the segmented fuel cell by grooving method and provide a theoretical basis for judging whether a fuel cell is out of oxygen by segmented fuel cell.     

Effect of anisotropic electrical resistivity of gas diffusion layers (GDLs) on current density and temperature distribution in a Polymer Electrolyte Membrane (PEM) fuel cell

A two-dimensional two-phase model is used to analyze the effects of anisotropic electrical resistivity on current density and temperature distribution in a PEM fuel cell. It is observed that a higher in-plane electrical resistivity of the gas diffusion layer (GDL) adversely affects the current density in the region adjacent to the gas channel and generates slightly higher current densities in the region adjacent to the current collector. Also, in case of GDLs with high anisotropic thermal conductivity, the maximum and minimum temperatures in a cathode catalyst layer depend on the average current density and not the local current density.