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.     

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.     

Performance of proton exchange membrane fuel cells with microporous layer hydrophobized by polyphenylene sulfide at conventional temperature and cold start

Polyphenylene sulfide (PPS), a thermoplastic polymer with excellent chemical and thermal stability, has properties similar to those of polytetrafluoroethylene (PTFE) but is less expensive. In this study, the feasibility of using PPS to replace conventional PTFE as a hydrophobic agent for microporous layers (MPLs) in proton exchange membrane fuel cells (PEMFCs) is explored. First, PTFE-MPL and PPS-MPL with 30 wt% hydrophobic agent were prepared. The pore size, porosity, contact angle, and microstructure of the two MPL samples were measured and analyzed. Subsequently, PEMFCs with the two MPL samples were tested for their operational performance at a conventional temperature of 70 °C and cold-start capability at ?10 °C. The performances of PTFE-MPL and PPS-MPL at conventional temperatures were similar, but PPS-MPL showed obvious advantages in cold-start performance. In addition, the operating performance of PEMFC at the conventional temperature and cold-start capability at ?10 °C were investigated for PPS mass fractions of 10%, 20%, and 30% in MPL, and for PPS particle sizes of 10 and 30 μm. The results indicate that the optimal performance can be attained when the PPS particle size is 10 μm and the mass fraction is 20%.     

Characteristics of proton exchange membrane fuel cells cold start with silica in cathode catalyst layers

In this study, a novel strategy is reported to improve the cold start performance of proton exchange membrane (PEM) fuel cells at subzero temperatures. Hydrophilic nano-oxide such as SiO₂ is added into the catalyst layer (CL) of the cathode to increase its water storing capacity. To investigate the effect of nanosized SiO₂ addition, the catalyst coated membranes (CCMs) with 5 wt.% and without nanosized SiO₂ are fabricated. Although at normal operation conditions the cell performance with nanosized SiO₂ was not so good as that without SiO₂, cold start experiments at −8 °C showed that the former could start and run even at 100 mA cm⁻² for about 25 min and latter failed very shortly. Even at −10 °C, the addition of SiO₂ dramatically increased the running time before the cell voltage dropped to zero. These results further experimentally proved the cold start process was strongly related with the cathode water storage capacity. Also, the performance degradation during 8 cold start cycles was evaluated through polarization curves, cyclic voltammetry (CV) and electrochemical impedance spetra (EIS). Compared with the cell without SiO₂ addition, the cell with 5 wt.% SiO₂ indicated no obvious degradation on cell performance, electrochemical active surface area and charge transfer resistance after experiencing cold start cycles at −8 °C.     

Effect of different control strategies on rapid cold start-up of a 30-cell proton exchange membrane fuel cell stack

Many places experience extreme temperatures below −30 °C, which is a great challenge for the fuel cell vehicle (FCV). The aim of this study is to optimize the strategy to achieve rapid cold start-up of the 30-cell stack at different temperature conditions. The test shows that the stack rapidly starts within 30 s at an ambient temperature of −20 °C. Turning on the coolant at −25 °C show stability of the cell voltage at both ends due to the end-plate heating, however, voltage of intermediate cells fluctuates sharply, and successful start-up is completed after 60 s. The cold start strategy changes to load-voltage cooperative control mode when the ambient temperature reduced to −30 °C, the voltage of multiple cells in the middle of the stack fluctuate more drastic, and start-up takes 113 s. The performance and consistency of the stack did not decay after 20 cold start-up experiments, which indicates that our control strategies effectively avoided irreversible damage to the stack caused by freeze-thaw process.     

A review of low-temperature proton exchange membrane fuel cell degradation caused by repeated freezing start

Proton exchange membrane fuel cell (PEMFC) has advantages of zero emission, fast response and high-power density. There are still obstacles such as manufacturing cost, life span, infrastructure construction and subzero temperature star-up restricting commercialization of PEMFC. The low-temperature start-up is one of them that needs to be solved in the field of fuel cell vehicle. This paper presents research progresses involving PEMFC degradation caused by the low-temperature start-up. Degradation phenomena and mechanism under component-level caused by repeated freezing start, influencing factors and mitigation strategies are summarized and reviewed. Conclusions are made that frequent ice freezing and melting causes the membrane electrode assembly damaged irreversibly, the quality of cold start and low temperature influence the degradation strongly and purge after shutdown, better materials and optimal fuel cell structure design are helpful to reduce the impact of cold start on fuel cell performances. It is suggested that future work should be focused on optimizing strategies of the shutdown purge, promoting the quality of cold start, enhancing properties of the materials, improving internal structure design of stack and developing low-temperature attenuation models.     

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.     

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.     

Predicting current density distribution of proton exchange membrane fuel cells with different flow field designs

In a proton exchange membrane fuel cell (PEMFC), flow field design is an important factor that influences the distributions of current density and water accumulation. The segmented model developed in prior study is used to investigate the effect of flow field patterns on current density distribution. This model predicts the distributed characteristics of water content in the membrane, relative humidity in the flow channels, and water accumulation in the gas diffusion layers (GDLs).Three single cells with different flow field patterns are designed and fabricated. These three flow field designs are simulated using the segmented model and the predicted results are compared and validated by experimental data. This segmented model can be used to predict the effect of flow field patterns on water and current distributions before they are machined.     

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.     

Effects of chlorides on the performance of proton exchange membrane fuel cells

This paper investigates the effects of cathode gases containing chloride ions on the proton exchange membrane fuel cell (PEMFC) performance. Chloride solutions are vaporized using an ultrasonic oscillator and mixed with oxygen/air. The salt concentration of the mixed gas in the cathode is set by varying the concentration of the chloride solution. Five-hour tests show that an increase in the concentration of sodium chloride did not significantly affect the cell performance of the PEMFC. It is found that variations in the concentration of chloride do not show significant influence on the cell performance at low current density operating condition. However, for high current density operating conditions and high calcium chloride concentrations, the chloride ion appears to have a considerable effect on cell performance. Experimental results of 108-h tests indicate that the fuel cell operating with air containing calcium chloride has a performance decay rate of 3.446 mV h⁻¹ under the operating condition of current density at 1 A/cm². From the measurements of the I-V polarization curves, it appears that the presence of calcium chloride in the cathode fuel gas affects the cell performance more than sodium chloride does.     

Investigation of the effect of cathode stoichiometry of proton exchange membrane fuel cell using localized electrochemical impedance spectroscopy based on print circuit board

Proton Exchange Membrane Fuel Cell can have a large active area, and the working condition in different areas can be entirely different. Localized electrochemical impedance spectroscopy can directly observe the proton exchange membrane fuel cell internal reaction conditions. In this work, localized electrochemical impedance spectroscopy test system based on print circuit board is implemented in a 50 cm² multi-channel serpentine flow fields. The localized electrochemical impedance spectroscopy performances of different segments with different cathode stoichiometry (1.8, 2.3 and 2.8) at different current density (100  mA cm⁻², 500  mA cm⁻² and 900 mA cm⁻²) are studied. The result demonstrates that the fuel cell may suffer from local drying and flooding at the same time. To make full use of the potential of a fuel cell, a suitable cathode stoichiometry should be identified to control the drying of the inlet and the flooding of the outlet at the same time. It is shown that a cathode stoichiometry of 2.3 is close to the optimum cathode stoichiometry to keep the fuel cell in good consistency without gas waste. Besides, a current density distribution measurement is performed to verify the conclusions of this work.     

Comparison of current distributions in proton exchange membrane fuel cells with interdigitated and serpentine flow fields

Current distributions in a proton exchange membrane fuel cell (PEMFC) with interdigitated and serpentine flow fields under various operating conditions are measured and compared. The measurement results show that current distributions in PEMFC with interdigitated flow fields are more uniform than those observed in PEMFC with serpentine flow fields at low reactant gas flow rates. Current distributions in PEMFC with interdigitated flow fields are rather uniform under any operating conditions, even with very low gas flow rates, dry gas feeding or over-humidification of reactant gases. Measurement results also show that current distributions for both interdigitated and serpentine flow fields are significantly affected by reactant gas humidification, but their characteristics are different under various humidification conditions, and the results show that interdigitated flow fields have stronger water removal capability than serpentine flow fields. The optimum reactant gas humidification temperature for interdigitated flow fields is higher than that for serpentine flow fields. The performance for interdigitated flow fields is better with over-humidification of reactant gases but it is lower when air is dry or insufficiently humidified than that for serpentine flow fields.     

Experimental investigation on dynamic characteristics of proton exchange membrane fuel cells at subzero temperatures

Cold start and operation of a proton exchange membrane fuel cell (PEMFC) at the cold temperatures are crucial to the commercialization of it in the field of transportation. A 32 cm² two cell stack is prepared to conduct the experiments at subzero temperatures, including cold start processes and cell performance testing, aiming of the characteristics of the cell. The startup study under subfreezing temperatures is conducted by galvanostatic method at various operation conditions, i.e. ambient temperature (−3 and −5 °C), current density and anode stoichiometry. The results show that the voltage evolutions are proportional to the operating current densities under the former two conditions, but the relationship becomes the opposite at the last condition. It is also found that the time constant for the cell to reach steady status is no more than 100 s and highly depends on the startup mode. In addition, the performance of the cell is tested at the temperature of 0 °C and −3 °C. The comparison of pre-humidification and normal operations indicate that the initial water content of membrane affects the cell performance.     

Dynamic characteristics of local current densities and temperatures in proton exchange membrane fuel cells during reactant starvations

Reactant starvation during proton exchange membrane fuel cell (PEMFC) operation can cause serious irreversible damages. In order to study the detailed local characteristics of starvations, simultaneous measurements of the dynamic variation of local current densities and temperatures in an experimental PEMFC with single serpentine flow field have been performed during both air and hydrogen starvations. These studies have been performed under both current controlled and cell voltage controlled operations. It is found that under current controlled operations cell voltage can decrease very quickly during reactant starvation. Besides, even though the average current is kept constant, local current densities as well as local temperatures can change dramatically. Furthermore, the variation characteristics of local current density and temperature strongly depend on the locations along the flow channel. Local current densities and temperatures near the channel inlet can become very high, especially during hydrogen starvation, posing serious threats for the membrane and catalyst layers near the inlet. When operating in a constant voltage mode, no obvious damaging phenomena were observed except very low and unstable current densities and unstable temperatures near the channel outlet during hydrogen starvation. It is demonstrated that measuring local temperatures can be effective in exploring local dynamic performance of PEMFC and the thermal failure mechanism of MEA during reactants starvations.     

Coolant circuit modeling and temperature fuzzy control of proton exchange membrane fuel cells

Effective temperature management is necessary for the safe and efficient operation of proton exchange membrane fuel cells (PEMFC). Generally circulating coolant can be applied in removing the excess heat of the PEMFC whose electrical power exceeds 5 kW. So a coolant circuit modeling method and a temperature fuzzy control strategy are presented in the paper in order to keep the PEMFC within the ideal operation temperature range. Firstly, a coolant circuit mathematical model is developed, which includes a PEMFC thermal model, a water reservoir model, a water pump model, a bypass valve model, a heat exchanger model and a PEMFC electrochemical model. Secondly, the incremental fuzzy control with integrator technique is designed according to the established model and control experience rule. And the PEMFC temperature and circulating coolant inlet temperature are controlled by regulating the circulating coolant flux and bypass valve factor respectively. Finally, the established model and fuzzy controllers are simulated and analyzed in Matlab software, and the simulation results demonstrate that the incremental fuzzy controller with integrator can effectively control the PEMFC temperature and the inlet coolant temperature within their objective working ranges respectively. In addition, the modeling and control process are very concise, and they can be easily applied in various power classes PEMFC temperature control in real-time.     

Simultaneous measurement of current and temperature distributions in a proton exchange membrane fuel cell

Using a specially designed current distribution measurement gasket in anode and thin thermocouples between the catalyst layer and gas diffusion layer (GDL) in cathode, in-plane current and temperature distributions in a proton exchange membrane fuel cell (PEMFC) have been simultaneously measured. Such simultaneous measurements are realized in a commercially available experimental PEMFC. Experiments have been conducted under different air flow rates, different hydrogen flow rates and different operating voltages, and measurement results show that there is a very good correlation between local temperature rise and local current density. Such correlations can be explained and agree well with basic thermodynamic analysis. Measurement results also show that significant difference exists between the temperatures at cathode catalyst layer/GDL interface and that in the center of cathode endplate, which is often taken as the cell operating temperature. Compared with separate measurement of local current density or temperature, simultaneous measurements of both can reveal additional information on reaction irreversibility and various transport phenomena in fuel cells.     

The conception of in-plate adverse-flow flow field for a proton exchange membrane fuel cell

To improve species concentration and current density distribution uniformity of a proton exchange membrane (PEM) fuel cell, an in-plate adverse-flow (IPAF) flow field is developed. Its utility is conceptually examined through three-dimensional numerical simulation comparison between three typical fuel and air flow combinations out of those it can support. Under isothermal condition and constant velocity reactant feeding mode, as the simulation results indicate, there is no significant cell performance improvement by the new flow filed unless in mass transport limited region, while the species concentration and current density distribution uniformities are substantially improved. As data analysis supports, there are two mechanisms in the new flow field that are responsible for the distribution uniformity improvement: the along-channel offset effect and the across-rib transport effect, and their respective pure contributions to the improvement are well discerned.     

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.     

Nafion/PTFE/silicate membranes for high-temperature proton exchange membrane fuel cells

Fuel cell performance of membrane electrode assemblies (MEAs) prepared from poly(tetrafluoroethylene)/Nafion/silicate (PNS) membrane and Nafion-112 membrane were investigated. Due to the low conductivity of PTFE and silicate, PNS had a higher proton resistance than Nafion-112. However, in this work we show that PNS performs better than Nafion-112 for a high current density operation with a low inlet gas humidity. As the PEMFCs were operated at with 100% RH, the results showed the maximum power density (PD_max) of PNS was: at with both H₂ and O₂ flow rates of 300 ml/min, and at with H₂ flow rate of 360 ml/min and O₂ flow rate of 600 ml/min, which were much higher than the at of Nafion-112 with both H₂ and O₂ flow rates of 300 ml/min. The PD_max of PNS was: , , and at as the operating temperature and inlet gas humidity were set at with 67.7% RH, with 46.8% RH, and with 33.1% RH, respectively. However, no output power was detected for Nafion-112 MEA when the cell was operated at a temperature higher than and an inlet gas humidity lower than 67.7% RH. The high PEMFC performance of PNS at high current density and low humidity is attributed to the presence of silicate in the PNS membrane, which enhances water uptake and reduces electro-osmosis water loss at a high current density.