Efficient model selection for real-time adaptive cold start strategy of a fuel cell system on vehicular applications

The PEMFC maximum power is greatly influenced by subfreezing temperature and degradation phenomena. Therefore, a dependable model is required to estimate the power with respect to the variation of the operating conditions and state of health. Semi-empirical models are potent tools in this regard. Nonetheless, there is not much information about their cold environment reliability. This paper comprehensively compares the performance of some models (already tested in normal ambient temperature) in subfreezing condition to introduce the most reliable one for PEMFC cold start-up application. Firstly, seven models are compared regarding voltage losses and precision. Subsequently, the three most dependable ones are selected and experimentally compared at sub-zero temperature in terms of polarization curve estimation for three PEMFCs with different degradation levels. The results of this study indicate that the model introduced by Amphlett et al. has a superior performance compared to other ones regarding the characteristic's estimation in below-zero temperature.     

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.     

Effects of reactants/coolant non-uniform inflow on the cold start performance of PEMFC stack

The failure at equally distributing reactants among different channels within the stack leads to uneven reaction and gas concentration distribution in the catalyst layers, which consequently impacts the performance and durability of proton exchange membrane fuel cell stacks (PEMFCs). A three-dimensional, transient, non-isothermal cold start model for PEMFCs with parallel flow-field configuration and coolant circulation is developed in this work to investigate the effects of non-uniform distribution of reactants/coolant inflow rates on the cold start process. The results show that the effect of non-uniform inflow on ice formation amount is obvious and that on the distribution uniformity of current density is apparent over the cold start survival time. Additionally, the simulation predictions show that the non-uniform initial membrane water content distribution due to the purge procedure can significantly increase the rate of ice growth and deteriorate the uniformity of current density distribution in the membrane. It is found that high stoichiometry operating condition is favorable to cold startup, but may result in drying in the membrane at regions close to the channel inlet side. As non-uniform inflow rates issue is inevitable in actual PEMFC stack operation conditions, our results demonstrate that the initial membrane water content and cathode stoichiometry ratio need to be identified to moderate the effects of reactants/coolant inflow maldistribution and to maintain a stable cold start performance for the PEMFC stack.     

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.     

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.     

Experimental study on cold start performance of PEMFC based on parallel flow channels

In this study, experimental methods are used to study the effect of assembly torque and channel size on cold start performance. Through the analysis of pressure distribution, fuel cell voltage, cathode gas inlet and outlet pressure drop and high frequency impedance, it is considered that, under the experimental conditions of this study, the fuel cell assembly torque is 2 N m, which has better cold start performance than the assembly torque of 1 N m and 4 N m. In addition, Under the condition of 2 N m assembly torque, the effect of four different flow channel sizes on the pressure distribution and cold start performance of the fuel cell was studied. It is found that when the width of channel is 0.5 mm, the depth of channel is 0.3 mm, the cold start performance is best.     

Water management characteristics of electrospun micro-porous layer in PEMFC under normal temperature and cold start conditions

Micro-porous layer (MPL) is the key component in proton exchange membrane fuel cell for water management. Electrospinning technique, providing a novel nanofiber structure, is recently used to fabricate MPL which gave improved fuel cell performance at normal temperature operation. However, underlying causes are not well understood, and no attempt has been made to study its effects on the cold start performance. In this work, electrospun MPL using non-toxic solvent was fabricated, and water management characteristics under both normal temperature and cold start conditions were compared with commercial MPL using the same catalyst coated membrane (CCM). Electrospun MPL outperformed the commercial MPL at 70 °C under high relative humilities due to marked reduction in mass transport losses. Under cold start conditions, fuel cell with electrospun MPL generated electricity for a longer time, possibly due to better interfacial connection, which facilitated water removal from catalyst layer.     

Internal currents in response to a load change during fuel cell start-up

The transient response of proton exchange membrane fuel cells during start-up is an important issue for backup power systems. These require a very short start-up time which limits the use of batteries during a blackout. In this study the fuel cell was initially inerted with nitrogen at the cathode and thus the start-up procedures occurred in two stages: gas supply in open-circuit and load connection. The influence of the current time-profile, the cell voltage at the connection and the gas flow rates on the voltage variation were investigated using a segmented fuel cell permitting the measurements of the internal local currents. We found that the voltage during the filling of the cathode is not sufficient to determine which fraction of the cathode was filled with oxygen. In the case of a step change in current, the start-up time decreases as the voltage at the moment the cell is connected increases. In response to a ramp, the asymptotic power value is reached quickly.     

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.     

Effects of ionomer content and oxygen permeation of the catalyst layer on proton exchange membrane fuel cell cold start-up

We have verified the effectiveness of ionomer as a carrier of oxygen to improve cold start-up of proton exchange membrane fuel cells. Galvanostatic cold start was performed on proton exchange membrane fuel cells to evaluate the effect of ionomer content in the catalyst layer on the durability of power generation at −30 °C. Cell voltage and internal resistance were measured, and polarization analysis was conducted to evaluate the cell voltage reduction. Cold start-up durability improved significantly with higher ionomer content in the catalyst layer because of higher oxygen permeation of ice formed in the catalyst layer. These results enable robust design of membrane electrode assemblies for cold start-up.     

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.     

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.     

Effective purge method with addition of hydrogen on the cathode side for cold start in PEM fuel cell

The purge process is essential for successful cold start of fuel cell vehicles during winter, and it plays an important role in the removal of the residual water inside the fuel cell in a short time. In this study, a new purge method is introduced by adding a small amount of hydrogen to the cathode gas flow in order to increase the purge performance. The experimental results demonstrate that the hydrogen addition purge method is very effective in removing the residual water near the catalyst layer. The water removal is verified by measuring the resistance of the fuel cell, dew point temperature of the outlet purge gas, and weights of the membrane electrolyte assembly (MEA) and gas diffusion layer (GDL). In addition, the image of the GDL after the purge process is captured to show the advantage of the hydrogen addition purge method. Cold start experiments are also conducted after the optimal purge process. It is also found that the degradation of the catalyst layer is not serious after the hydrogen addition purge process.     

Maximum power cold start mode of proton exchange membrane fuel cell

Successful and fast cold start is important for proton exchange membrane (PEM) fuel cell in vehicular applications in addition to the desired maximum power in any case. In this study, the maximum power cold start mode is investigated in details and compared with other cold start modes based on a multiphase stack model. It is found that for the maximum power cold start mode, the current density is generally kept at high levels, and the performance improvement caused by the membrane hydration and temperature increment may not be observable. Therefore, before the melting point, the performance drops continuously. The maximum power cold start mode could better balance the heat generation and ice formation, leading to improved cold start survivability than that in the constant voltage and constant current modes, with a fast start-up generally guaranteed. Once the survivability can be ensured, the initial water content needs to be higher for fast cold start, suggesting that over purging should be avoided. The maximum power mode is suggested to be optimal for PEM fuel cell cold start based on the modeling results.     

The effects of the cathode flooding on the transient responses of a PEM fuel cell

This study investigates the effects of the flooding of the gas diffusion layer (GDL), as a result of liquid water accumulation, on the performance of a proton exchange membrane fuel cell (PEMFC). The transient profiles of the current generated by the cell are obtained using the numerical resolution of the transport equation for the oxygen molar concentration in the unsteady state. The dynamics of the system are captured through the reduction of the effective porosity of the GDL by the liquid water which accumulates in the void space of the GDL. The effects of the GDL porosity, GDL thickness and mass transfer at the GDL–gas channel interface on the evolution with time of the averaged current density are reported. The effects of the current collector rib on the evolution of the molar concentration of oxygen are also examined in detail.     

Experimental study on the endplate effect on the cold-start performance of an open-cathode air-cooled proton exchange membrane fuel cell stack

The temperature gradient inside an open-cathode air-cooled fuel cell is large because it uses air as its reaction and cooling media; moreover, the temperature of single cells near the endplates is low because of the high heat capacity of the endplate compared to single cells. Therefore, the cold start of open-cathode air-cooled fuel cells is difficult. In this work, the cold-start performance of an open-cathode air-cooled fuel cell stack, including the stack voltage, single-cell voltage and temperature distribution, are tested in a climatic chamber. The results show that the endplate effect has a significant adverse influence on the cold-start performance. Due to the existence of the endplate effect, the voltages of the single cells near the endplate decrease significantly. The stack can be successfully started at −5 °C without any external heating; however, when the temperature decreases below −10 °C, it cannot be started. At this time, if a certain power of endplate heating is adopted, successful cold-start can be achieved. However, if the temperature continues to decrease, the stack cannot be successfully started only through endplate heating because both the endplates and cold air affect the cold-start performance. Combining endplate and air heating may be a feasible cold-start method.     

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.     

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.