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

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

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.     

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.     

Hydrogen pumping effect induced by fuel starvation in a single cell of a PEM fuel cell stack at galvanostatic operation

In a proton exchange membrane fuel cell stack, a single cell is potentially subjected to voltage reversal under fuel starvation conditions, which is extremely harmful to its durability. In this work, we develop a two-dimensional computational model to investigate the current and potential distributions in a single cell under these voltage reversal conditions. It is found that most of hydrogen under these conditions is oxidized in a narrow region close to the fuel-inlet, and the anode area before hydrogen depletion can be characterized into an activation limited region and a mass-transport limited region. Meanwhile, an unexpected hydrogen evolution phenomenon is discovered in the cathode catalyst layer (CCL) adjacent to the fuel inlet, owing to the imbalance between the localized ultrahigh hydrogen oxidation current density in the anode and the lower limiting current density of oxygen reduction reaction in the adjacent CCL. Furthermore, the evolved hydrogen gas is also found to be oxidized nearby due to the steep variation of electrolyte potential in the CCL, indicating the coexistence of hydrogen evolution, hydrogen oxidation and oxygen reduction within the micron-scale thickness of CCL, which significantly adds to the complexity of the coupled phenomena in the voltage-reversal single cell.     

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.     

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.     

Structure failure of the sealing in the assembly process for proton exchange membrane fuel cells

Sealing stability in proton exchange membrane fuel cell (PEMFC) is critical to the performance and safety of stacks. However, sealing structure failure (SSF), which leads to the leakage of reactant gases, often occurs in the assembly process or start-up operation for PEMFCs. This study aims to investigate the effects of geometrical structure and material parameters of sealing components on the sealing structure failure. Slippage angle and slippage distance are adopted to evaluate the risk of SSF. Finite element (FE) simulations are conducted with consideration of the assembly process and start-up operation. Experiments are carried out to validate the accuracy of the FE model. Influences of parameters of gasket, membrane electrode assembly (MEA) frame, sealing groove shape of bipolar plate (BPP), and gas pressure are discussed in detail. Meanwhile, the risks of SSF for the stack by using metallic and graphite BPPs are compared. It is demonstrated that material properties and geometrical parameters of sealing components in PEMFC have great effects on SSF. The methodology developed is beneficial to the understanding of the SSF, and it can also be applied to guide the design of PEMFC stack assembly process to keep a good sealing reliability.     

Pumpless thermal management of water-cooled high-temperature proton exchange membrane fuel cells

Proton exchange membrane fuel cells (PEMFCs) have been considered for combined heat and power (CHP) applications, but cost reduction has remained an issue for commercialization. Among various types of PEMFC, the high-temperature (HT) PEMFC is gaining more attention due to the simplicity of the system, that will make the total system cost lower. A pumpless cooling concept is introduced to reduce the number of components of a HT PEMFC system even further and also decrease the parasitic power required for operating the system. In this concept, water is used as the coolant, and the buoyancy force caused by the density difference between vapour and liquid when operated above boiling temperate is utilized to circulate the coolant between the stack and the cooling device. In this study, the basic parameters required to design the cooling device are discussed, and the stable operation of the HT PEMFC stack in both the steady-state and during transient periods is demonstrated. It found that the pumpless cooling method provides more uniform temperature distribution within the stack, regardless of the direction of coolant flow.     

Investigation and analysis of proton exchange membrane fuel cell dynamic response characteristics on hydrogen consumption of fuel cell vehicle

The number of working points and response speed are two essential characteristics of proton exchange membrane fuel cell (PEMFC). The improper setting of the number of working points and response speed may reduce the life of PEMFC and increase the hydrogen consumption of the vehicle. This paper explores the impact of the response speed as well as the working points of the PEMFC on the hydrogen consumption in the real-system level. In this paper a dynamic model of the PEMFC system is established and verified by experiments. The model is able to reflect the dynamic response process of PEMFC under a series different number of working points and different response speed. Based on the proposed model, the influence of working points and the response speed of PEMFC on the hydrogen consumption in the vehicle under different driving cycles is analyzed and summarized, for the first time, in the open literature. The results highlight that the hydrogen consumption will decreases in both cases that with the increase of working point number and increase of response speed. However, the reduction range of hydrogen consumption trends to smaller and may reach to an optimal level considering the trade-off between the hydrogen saving and the other costs, for example the control cost. Also, with a more complex driving cycle, the working points and response speed have a greater the impact on the hydrogen consumption in the vehicle applications.     

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.     

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.     

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.     

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.     

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

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

Dynamic characteristics and mitigations of hydrogen starvations in proton exchange membrane fuel cells during start-ups

Hydrogen starvation during a start-up process in proton exchange membrane (PEM) fuel cells could result in drastic local current density variations, reverse cell voltage and irreversible cell damages. In this work, variations of local current densities and temperatures are measured in situ under both potentiostatic and galvanostatic modes. Experimental results show that when the cell starts up under potentiostatic mode with hydrogen starvation, current density undershoots occur in the downstream; while under the galvanostatic mode, local current density in the downstream almost drops to zero, but the current density near the outlet remains almost constant. The phenomenon of near constant current density near the outlet leads to a novel approach to alleviate hydrogen starvations - a hydrogen reservoir is added at the anode outlet. Experimental results show that the exit hydrogen reservoir can significantly reduce the zero current region and alleviate hydrogen starvations. A non-dimensional current-density variation coefficient is proposed to measure the magnitude of local current density changes during starvations. Experimental results show that the exit hydrogen reservoir can significantly reduce the current-density variations coefficient over the entire flow channel, indicating that adding an exit reservoir is an effective approach in mitigating hydrogen starvations.     

Microstructured membranes for improving transport resistances in proton exchange membrane fuel cells

Proton exchange membrane fuel cells (PEMFCs) have been identified as one of the most promising renewable energy system for use in automotive applications. However, due to the wide range of weather conditions around the world, the PEMFCs must be stable for operating under these variable conditions. One of the inefficiencies of PEMFCs in automotive applications is during vehicle warm-up, where the low hydration level within the PEMFC can lead to a low performance of the fuel cell. In this study, a proton exchange membrane (PEM) was prepared with regular, microstructured features tuned over a range of aspect ratios. These microstructured membranes were incorporated into MEAs and analyzed for their membrane, proton, and oxygen transport resistances. These fuel cells were tested under different conditions to simulate vehicle start-up, normal operating conditions, and hot operating conditions. It was determined that microstructured PEMs improved performance over planar PEMs under both the start-up and hot conditions. Despite the improved performance of the microstructured PEMs, a high hydrogen cross-over and short-circuit current were also observed for these samples. Adjusting the preparation techniques and tuning the dimensions of the microstructures may provide avenues for further optimization of PEMFC performance.     

Rapid cold start of proton exchange membrane fuel cells by the printed circuit board technology

Cold start from subzero temperature is one of the key barriers, which prevents proton exchange membrane fuel cell (PEMFC) from further commercialization. In this paper, we have applied the printed circuit board (PCB) technology to study the current density distributions of PEMFC and optimized the technology under rapid cold start. The results show that increasing the initial load, and the setup temperature can help to lower the cold start time and achieve rapid warm-up of PEMFC. The cell can be rapidly cold started for 10 s at −5 °C and 55 s at −10 °C under 0.2 V operation condition, but it failed at −15 °C and −20 °C. The inlet region and middle region produce half of the total current before the overall peak current density is reached, which is important for the successful cold start. Based on these characteristics, we optimized the rapid cold start strategy by co-operation of hot reactant gas and waste heat generation of PEMFC. It becomes possible to start up the PEMFC at temperatures down to −20 °C with about 20 min.     

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

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

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

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