Quantitative analysis on cold start process of a PEMFC stack with intake manifold

To systematically explore the low-temperature operating characteristics of polymer electrolyte membrane fuel cell (PEMFC) stack, a three-dimensional PEMFC stack model with intake manifold is developed in this study. The characteristics of different cold start modes in the stack are compared and analyzed. The distribution and transmission characteristics of water, ice, and heat in each cell of the stack are analyzed in detail. The location of water accumulation in each cell of the stack is also explored. Finally, finite difference sensitivity is calculated for the cumulated charge transfer density to quantify the effects of operating parameters on the cold start process at low temperature. And how these parameters affect the operation of the PEMFC stack at low temperature is investigated. The results show that inconsistency exists in stack operation due to the position particularity of the intermediate cell. Irreversible heat is the main heat source for the cold start of the stack, and the cathode catalyst layer is the main heat-generating component. The heat production proportion of cathode catalyst layer can reach 90%, which decreases with the increment of current density and the running time, especially for the edge cell. The initial ionomer water content is most sensitive to the cold start process of the stack, followed by the porosity of cathode catalyst layer. These parameters are sensitive to the cold start process mainly because of the change in volumetric exchange current density and oxygen concentration.     

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 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.     

Effects of operating and design parameters on PEFC cold start

During startup from subzero temperatures the water produced in a polymer electrolyte fuel cell (PEFC) forms ice/frost in the cathode catalyst layer (CL), blocking the oxygen transport and causing cell shutdown once all CL pores are plugged with ice. This paper describes an experimental study on the effects of operating and design parameters on PEFC cold-start capability. The amount of total product water in mg cm⁻² during startup is used as an index to quantify the cold-start capability. The newly developed isothermal cold-start protocol is used to explore the basic physics of cold start, and the effects of purge methods prior to cold start, startup temperature and current density, and the membrane thickness are shown. The experimental data also confirm the current density effect predicted earlier by a multiphase model of PEFC cold start.     

Multi-variable optimisation of PEMFC cathodes based on surrogate modelling

A two-dimensional, steady state, isothermal agglomerate model for cathode catalyst layer design is presented. The design parameters, platinum loading, platinum mass ratio, electrolyte volume fraction, thickness of catalyst layer and agglomerate radius, are optimised by a multiple surrogate model and their sensitivities are analysed by a Monte Carlo method based approach. Two optimisation strategies, maximising the current density at a fix cell voltage and during a specific range, are implemented for the optima prediction. The results show that the optimal catalyst composition depends on operating cell voltages. At high current densities, the performance is improved by reducing electrolyte volume fraction to 7.0% and increasing catalyst layer porosity to 52.9%. At low current densities, performance is improved by increasing electrolyte volume fraction to 50.0% and decreasing catalyst layer porosity to 12.0%. High platinum loading and small agglomerate radius improve current density at all cell voltages. The improvement in fuel cell performance is analysed in terms of the electrolyte coating thickness, agglomerate specific area, conductivity, overpotential, volumetric current density and oxygen mole fraction within the cathode catalyst layer. The optimisation results are also validated by the agglomerate model at different cell voltages to confirm the effectiveness of the proposed methodologies.     

A PEM fuel cell model for cold-start simulations

In this paper, a transient multiphase multi-dimensional PEM fuel cell model has been developed in the mixed-domain framework for elucidating the fundamental physics of fuel cell cold start. Cold-start operations of a PEM fuel cell at a subfreezing boundary temperature of −20 °C under both constant current and constant cell voltage conditions have been numerically examined. Numerical results indicate that the water vapor concentration inside the cathode gas channel affects ice formation in the cathode catalyst layer and thus the cold-start process of the fuel cell. This conclusion demonstrates that high gas flow rates in the cathode gas channel could increase fuel cell cold-start time and benefit the cold-start process. It is shown that the membrane plays a significant role during the cold-start process of a PEM fuel cell by absorbing the product water and becoming hydrated. The time evolutions of ice formation, current density and water content distributions during fuel cell cold-start processes have also been discussed in detail.     

Effect of membrane electrode assembly design on the cold start process of proton exchange membrane fuel cells

A transient multiphase model for cold start process is developed considering micro-porous layer (MPL), super-cooled water freezing mechanism and ice formation in cathode channel. The effect of MPL's hydrophobicity on the output performance and ice/water distribution is investigated under various startup temperatures, structural properties, membrane thicknesses and surrounding heat transfer coefficients. Under the maximum power startup mode, it is found that the hydrophobicity disparity of MPL has negligible influences when started from ?15 °C, but it strongly affects the overall performance when started from ?10 °C, especially after the cell survives the cold start. Decreasing the MPL's hydrophobicity leads to higher current density, meanwhile, it facilitates the super-cooled water's removal, which in turn reduces the ice formation in catalyst layer. However, excessive water accumulation happens if the generated water is hindered from getting into gas diffusion layer (GDL) due to the significant hydrophobicity gap. Weakening the GDL's hydrophobicity contributes to the water removal since the generated water is easier to diffuse out. A thinner membrane benefits the cold start owing to the reduction of ohmic loss and improvement of membrane hydration, and is more sensitive to the hydrophobicity of MPL. Ice formation in cathode channel is identified under various surrounding heat transfer coefficients.     

Numerical investigation of cold-start behavior of polymer electrolyte fuel cells in the presence of super-cooled water

In this study, a mathematical model has been developed to simulate the transient cold-start processes of polymer electrolyte fuel cells. The super-cooled water is assumed to exist within the cell. The non-equilibrium water transfer between the membrane and the catalyst layer is considered. The models of water freezing and ice melting in the catalyst layer and gas diffusion layer have been established. For the first time, the randomicity of the freezing process is captured by introducing a freezing probability function. Based on this model, the cold-start processes of a single polymer electrolyte fuel cell starting at various operating and initial conditions have been simulated numerically. The results indicate that the cold-start performance of the cell is determined by the water storage potential of the electrolyte in cathode catalyst layer. For each startup temperature and operating current load, there is a most appropriate initial membrane water content, which corresponds to the longest cell shutdown time. When the cold-start process is failed, the ice is mainly accumulated in the cathode catalyst layer. The ice distribution becomes more non-uniform as the cold-start temperature is lower.     

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.     

Cold start of proton exchange membrane fuel cell

In this review, “cold start” is defined as the startup of proton exchange membrane (PEM) fuel cells from subfreezing temperatures. Problems occurring during the cold start pose some of the remaining barriers to commercial applications of PEM fuel cells in transportation, stationary, auxiliary and portable systems. Fundamental studies of transport phenomena are critical to a better understanding of the mechanisms of cold start and offer ultimate solutions to resolving cold-start issues. In this review, experimental studies are discussed, focusing on output performance degradation, water and ice visualization, and component damages during a cold start. Analytical, numerical, and microscopic models and their results are also discussed. One of the emphases is on transport phenomena relevant to cold starts, including supercooling, phase change and transport of water in the membrane, catalyst layer, microporous layer, and gas diffusion layer. Another emphasis is placed on the strategies utilized to optimize cold-start processes for improved performance. The strategies include material designs of the components, cell/stack structures, and startup mode/load controls. It is shown that all of the effective strategies to mitigating cold-start problems derive from a basic understanding of the transport mechanisms during a cold start. It is also suggested that future models for this problem should place a great deal of attention in supercooling phenomena and water phase-change and transport in multilayer porous media. Lastly, more advanced experimental methods, such as real-time water/ice visualization and cryogenic microscopy, are needed to validate emerging theories and models.     

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.     

Microscopic Observations of Freezing Phenomena in PEM Fuel Cells at Cold Starts

In polymer electrolyte membrane (PEM) fuel cells, the generated water transfers from the catalyst layer to the gas channel through microchannels of different scales in a two phase flow. It is important to know details of the water transport phenomena to realize better cell performance, as the water causes flooding at high current density conditions and gives rise to startup problems at freezing temperatures. This article presents specifics of the ice formation characteristics in the catalyst layer and in the gas diffusion layer (GDL) with photos taken with an optical microscope and a cryo scanning electron microscope (cryo-SEM). The observation results show that cold starts at –10°C result in ice formation at the interface between the catalyst layer and the microporous layer (MPL) of the GDL, and that at –20°C most of the ice is formed in the catalyst layer. Water transport phenomena through the microporous layer and GDL are also a matter of interest, because the role of the MPL is not well understood from the water management angle. The article discusses the difference in the water distribution at the interface between the catalyst layer and the GDL arising from the presence of such a microporous layer.     

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.     

Effects of various operating and initial conditions on cold start performance of polymer electrolyte membrane fuel cells

Cold start is a challenging and important issue that hinders the commercialization of polymer electrolyte membrane fuel cell (PEMFC). In this study, a three-dimensional multiphase model has been developed to simulate the cold start processes in a PEMFC. Numerical simulations have been conducted for a single PEMFC starting at various operating and initial conditions, which are cell voltages, initial water contents and distributions, anode inlet relative humidity (RH), surrounding heat transfer coefficients, and cell temperatures. It is found that the heating-up time can be significantly reduced by decreasing the cell voltage and effective purge is critical for PEMFC cold start. The largest heating source at high cell voltages is the activational heat, and it becomes the ohmic heat at low cell voltages. The water freezing in the membrane is not observed when the cell is producing current due to the heat generation and the slow water diffusion into the membrane at subzero temperatures, and it is only observed after the cold start is failed, further confirming the importance of purge. Humidification of the supplied hydrogen has negligible effect on the cold start performance since only small amounts of water vapour can be taken by the gas streams at subzero temperatures. The surrounding heat transfer coefficients have significant influence on the heating-up time, indicating the importance of cell insulation or heating. The rate of cell heating up is reduced when the startup temperature is lowered due to the more sluggish electrochemical reaction kinetics.     

Effect of catalyst layer mesoscopic pore-morphology on cold start process of PEM fuel cells

Water transport is of paramount importance to the cold start of proton exchange membrane fuel cells (PEMFCs). Analysis of water transport in cathode catalyst layer (CCL) during cold start reveals the distinct characteristics from the normal temperature operation. This work studies the effect of CCL mesoscopic pore-morphology on PEMFC cold start. The CCL mesoscale morphology is characterized by two tortuosity factors of the ionomer network and pore structure, respectively. The simulation results demonstrate that the mesoscale morphology of CCL has a significant influence on the performance of PEMFC cold start. It was found that cold-starting of a cell with a CCL of less tortuous mesoscale morphology can succeed, whereas starting up a cell with a CCL of more tortuous mesoscale morphology may fail. The CCL of less tortuous pore structure reduces the water back diffusion resistance from the CCL to proton exchange membrane (PEM), thus enhancing the water storage in PEM, while reducing the tortuosity in ionomer network of CCL is found to enhance the water transport in and the water removal from CCL. For the sake of better cold start performance, novel preparation methods, which can create catalyst layers of larger size primary pores and less tortuous pore structure and ionomer network, are desirable.     

Investigation on sub-zero start-up of polymer electrolyte membrane fuel cell using un-assisted cold start strategy

The technical barriers for commercialization of polymer electrolyte membrane fuel cell (PEMFC) are the startup ability and survivability at sub-zero temperatures. Ice formation causes cold start fail and volume change damages the cell components leading to performance decay. Many strategies are used to assist successful cold start and to reduce the performance decay. But, unassisted cold start is very crucial and needs attention. Here, an experimental protocol is reported for successful unassisted cold start using low temperature gas purging at various temperatures (-5,-8,-10,-15, and -20 °C) as well as to recover temporary performance decay. The stability of the membrane electrode assembly is also studied in freeze/thaw and sequential cold start cycles. At temperature −10 °C, there is small performance decay after the 6th freeze/thaw cycle. However, the subsequent cold start cycle shows significant performance decay after the 6th cycle. Changes in microstructures and loss of hydrophobicity in the gas diffusion layer are attributed to the performance decay in both freeze/thaw and sequential cold start cycles. The effect of cold start temperature on the performance of a PEMFC in subsequent freeze/thaw cycles is also studied. It shows that depending upon the start-up temperature, the preferential ice formation can affect the performance decay characteristics.     

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.     

Investigation on cold start of polymer electrolyte membrane fuel cells with different cathode serpentine flow fields

The flow velocity and pressure distribution of the three cathode flow fields are simulated in this study. Larger pressure drop and more rapid flow rate reduce residual water, resulting in minimal ice formation during the cold start process. The simulation results show that the single variable cross section serpentine flow field has the largest pressure drop and the most rapid flow rate.The evolution of the temperature and the segment current density characteristics of three different cathode flow fields during cold start process is studied by printed circuit board technology. The results show that the 2 to 1 serpentine flow field has the best cold start performance and the best current density uniformity when cold start at constant voltage mode above −5 °C. However, the single variable cross section serpentine flow field has the best performance when cold start temperature is below −5 °C. Based on these results, cold start at −30 °C can be realized in 97s by using hot antifreeze liquid.     

Electrochemical impedance investigation of proton exchange membrane fuel cells experienced subzero temperature

Polarization losses of the fuel cells with different residual water amount frozen at subzero temperature were investigated by electrochemical impedance spectroscopy (EIS) taking into account the ohmic resistance, charge transfer process, and oxygen mass transport. The potential-dependent impedance before and after eight freeze/thaw cycles suggested that the ohmic resistance did not change, while the change of the charge transfer resistance greatly depended on the residual water amount. Among the four cells, the mass transport resistance of the cell with the largest water amount increased significantly even at the small current density region. According to the thin film-flooded agglomerate model, the interfacial charge transfer process and oxygen mass transport within the agglomerate and through the ionomer thin film in the catalyst layer both contributed to the high frequency impedance arc. From the analysis of the Tafel slopes, the mechanism of the oxygen reduction reaction (ORR) was the same after the cells experienced subzero temperature. The agglomerate diffusion changed a little in all cells and the thin film diffusion effect was obvious for the cell with the largest residual water amount. These results indicated that the slower oxygen diffusion within the catalyst layer (CL) was the main contributor for the evident performance loss after eight freeze/thaw cycles.     

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.