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In order to study the effects of temperature and humidity on the performance of proton exchange membrane fuel cell (PEMFC), a temperature and relative humidity-current (TRH-C) model was proposed. The model took the water flow and membrane water content of anode and cathode flow channels into consideration, and revealed the law of current density distribution and membrane water content distribution under the condition of current step change. According to the real flow channel configuration of the experimental single cell, a simplified PEMFC geometric model was established and the calculation grid was divided, and the TRH-C model was coupled to the multi-physical field calculation software COMSOL. A PEMFC test platform was established to test PEMFC under different temperature and humidity conditions. The operating temperatures of PEMFC were 65 °C and 70 °C, and the inlet humidities were 50% and 75%. The polarization curve, the membrane current density, and the membrane water content under different working conditions were analyzed and discussed. The results show that the TRH-C model can accurately predict the output performance of PEMFC. When the operating temperature is 70 °C, the inlet humidity is 50%, and the current density is 0. 018 A/cm², the relative errors of the TRH-C model of voltage and power density both reach the maximum value, which are 7.306% and 8.837%, respectively. The performance of PEMFC can be improved by increasing operating temperature and inlet air humidity. As the operating temperature increases, the distribution uniformity of membrane current density fluctuates and the mem-brane water content decreases. With the increase of inlet air humidity, the film current density distribution becomes more uniform and the film water content increases. This work provides a new theoretical method for optimizing inlet air strategy control and improving the output performance. [ABSTRACT FROM AUTHOR]

Carbon monoxide poisoning poses a significant challenge for proton exchange membrane (PEM) fuel cells operating with CO-containing hydrogen. One established solution involves air bleeding, a process that enhances the oxidation of carbon monoxide at the anode side of the fuel cell by blending a small quantity of air with the impure hydrogen before its introduction into the fuel cell. However, researchers have raised concerns regarding durability issues about catalyst sintering and membrane decomposition induced by air bleeding. The effects and underlying mechanisms of air bleeding on PEM fuel cells have yet to be comprehensively elucidated. To address this, this study conducts a detailed durability test to quantitatively evaluate the effects of air bleeding on the CO-containing hydrogen-fueled PEM fuel cells via the micro-current excitation method. The investigation substantiates that the PtRu/C catalysts significantly enhance both the performance and durability of fuel cells when air bleeding is employed, exhibiting different CO-tolerance and decay behaviors compared to the Pt/C anode catalyst. Furthermore, the evolution of MEA parameters indicates that the advantageous behaviors of PtRu/C catalysts can be attributed to their CO-tolerance capabilities, alleviated anodic catalyst sintering and loss, and decreased chemical carbon support corrosion and membrane decomposition through diminished hydrogen peroxide generation. This study contributes critical insights and empirical evidence for researchers focusing on CO-tolerant catalyst materials, the durability of PEM fuel cells, and the conversion and utilization of impure hydrogen energy derived from fossil fuels. • Auxiliary PtRu/C exhibits superior performance and durability simultaneously. • Quantitative durability analysis through changes in PEMFCs' MEA parameters. • Mitigated decay mechanisms involve less peroxide and catalyst sintering by PtRu/C. • Dual-catalyst layer anode is feasible for fuel cells utilizing impure hydrogen. [ABSTRACT FROM AUTHOR]

Newly fabricated proton exchange membrane fuel cells (PEMFCs) need an activation process to improve the initial performance. The long activation time leads to a high cost and a low production efficiency, thus it is significant to develop rapid and non-destructive activation methods. This review summaries possible activation mechanisms, compares and analyzes various activation methods, and afterwards, proposes the design principles for activation. Some criteria for evaluating activation completion are also provided as references. Finally, the influence of several activation methods on cell durability is overviewed from present available researches. In this review, hydrogen pumping, short circuit, and cathode starvation are considered as more effective methods versus traditional approaches. The performance improvement after activation is ascribed to the change in membrane morphology, the reduction of contamination, and the optimization of catalyst layers. More importantly, five factors including high temperature, sufficient water, change in current or voltage, reductive atmosphere, and valid combination of different methods are highlighted in designing rapid activation procedures. • Several common activation mechanisms are summarized. • Various activation procedures are further compared and analyzed. • Internal relationship among multiple activation methods is discussed. • Five principles of activation procedure design are proposed. • Influence of several activation methods on cell durability is reviewed. [ABSTRACT FROM AUTHOR]

Metallic bipolar plates (BPs) are extensively applied in fuel cell stacks to achieve an extremely high power density, and are thus suitable for vehicle applications. Corrosion of metallic BPs and contamination on membrane electrode assemblies have become key issues. Automotive operating conditions significantly accelerate BP corrosion and surface destruction. Accelerated stress tests, simulating dynamic load and startup-shutdown, are conducted to reveal the operation-induced corrosion mechanisms of typical SS316L BPs. The dynamic potential in the normal range (0.6–0.95 V) accelerates the local breakdown and damage of the passivation layer, leaving pits rich in Cr-species. The abundant Cr-species in the pits prevents further dissolution of Fe-speices and relieves local corrosion. The startup-shutdown condition extends the cathode potential to the trans -passivation and secondary passivation regions. Startup-shutdown drives BP into complex evolution stages of corrosion and passivation. Frequent alternation of potentials between passivation and trans -passivation regions accelerates alternant dissolution of outer Fe-species and Cr-species, thus causing global destruction represented by flocculent microstructure and interlinked pits. The ultrahigh cathode potential in startup-shutdown may induce an obvious surface film via strong secondary passivation. This paper can further guide the test protocols to evaluate the durability of metallic BP and condition optimization to avoid extreme corrosion damage. [ABSTRACT FROM AUTHOR]

The performance of proton exchange membrane fuel cells (PEMFCs) is directly affected by the nonlinear variations in water content. To study the variation in water content and its effect on PEMFC performance, the water condensation rate (WCR) model is established, which determines the proportional relationship between evaporation and condensation rates in terms of the switch function, and the two-phase flow evolution and pressure drop are considered as well. The WCR model is imported into Fluent software through a user-defined function for simulation, and the test system is established under different operating conditions. Then, the contours of H2O molar concentrations and polarization curves are analyzed and compared. The results show that the condensation rate value of the cathode channel is from 1.05 to 1.55 times higher than that of the anode channel. The WCR model can predict the variation in water content and improve the accuracy of the performance calculation by from 9% to 31%. The accuracy of the WCR model is especially improved, by 31%, at high current densities compared with the Fluent model when the inlet pressure is 30 kPa. [ABSTRACT FROM AUTHOR]

• Propose a combined activation procedure reducing time and gas consumption. • Validate effectiveness of the procedure at single-cell and stack level. • Reveal change of electrochemical factors and microstructure in activation. • Activation performance is related to hydration status and oxide reduction. Activation is a crucial procedure to improve the performance of newly fabricated fuel cell stacks to meet the power requirements in actual application scenarios. However, traditional activation procedures generally last for several hours or even days, which greatly increases the cost and limits the production efficiency. In response to this problem, this paper proposes a rapid activation procedure composed of hydrogen pumping (HP) and limiting-current activation with limited air (LCA). The influence of current density for HP and air supply for LCA on activation effect is studied and the best operating conditions are determined. The importance of water supply to the cathode is highlighted during HP. The influence of air supply for LCA on activation effect is limited, which contributes to gas saving. According to the experiments conducted on a single cell and a 10-cell stack, only 40 min are needed with this combined procedure, which is much shorter than that of the traditional step-current activation. Furthermore, nearly 75% of hydrogen is saved due to the short activation time and the control of gas supply, thus significantly reducing the cost during activation. Furthermore, to reveal the underlying mechanisms, membrane electrode assembly parameters before and after activation are obtained by micro-current excitation. A decrease in ohmic resistance and an increase in roughness factor of catalyst are observed, which contribute to performance improvement. According to the characterizations in the microstructure of membrane electrode assembly and platinum catalyst valence state, the electrolyte hydration and oxide reduction are determined as key factors and principles during activation. [ABSTRACT FROM AUTHOR]

• Relation of pressure drop current impedance water amount is studied originally. • The more current lowers, the closer pressure drop approaches ideal control line. • Give a new circuit model showing water amount at different layers. • This model is appropriate for large water amount at layers. • Resistances prove rationality of flooding detection based on pressure drop. The cathodic pressure drop is an effective indicator for detecting the water state in flow channels and gas diffusion layer of the proton exchange membrane fuel cell. Reducing current density is supposed to be the easiest way to cure the fuel cell from flooding. However, the relationship among pressure drop, current density and flooding has not been investigated, which is meaningful for preventing flooding, especially for the stationary power plant featuring constant condition and long-time operation. In this paper, the evolution of pressure drop, voltage, impedance, and water amount in terms of the resistance at different parts inside the fuel cell were analyzed quantitatively using a new equivalent circuit model, which could describe the water state more appropriately under large water amount. For one thing, when the flooding risk was diagnosed at different current densities, the cathodic resistance of catalyst layer was almost 238 mΩ·cm2, and the mass transfer resistance of the cathodic channels and gas diffusion layer was between 220 and 234 mΩ·cm2. It implied that pressure drop could be used to detect flooding. For another, the pressure drop could not return to the control line, but the more the current density decreased, the closer it approached to this line, which was verified by the impedance at different layers. In real application, the appropriate strategy of lowering current and diagnosis based on pressure drop could be selected for avoiding flooding in advance. [ABSTRACT FROM AUTHOR]

To examine the effects of the inlet temperature on a proton exchange membrane fuel cell (PEMFC), this study proposes a dynamic temperature (DT) model by considering the dynamic variation in temperature that influences the water content of the cell and its performance. A three-dimensional model featuring the geometry and nonconformal mesh of the PEMFC is first formulated. An experiment was then conducted featuring a 34-cm2 single-cell and test system. Following this, a simulation and another experiment were carried out at a working temperature of 333 K with 100% relative humidity (RH) at inlet temperatures of 316 K, 323 K, and 328 K (at 50%, 60%, and 80% RH, respectively). The contours, which included hydrogen in the channels and water in the membrane, were then described. The results show that the proposed DT model can more accurately predict the performance of the PEMFC than the FLUENT model. In particular, when the inlet temperature was 328 K (80% RH), the maximum relative difference between the DT model and FLUENT was 13.8% compared to the results of the experiment. The hydrogen content in the channels decreased and the water content in the membrane increased as the reaction continued. The proposed model provides a new means for studying the effects of dynamic temperature on the PEMFC. [ABSTRACT FROM AUTHOR]

Highlights • Propose impedance-based measuring method for zero-phase ohmic resistance. • Originally find increasing fluctuation of ohmic resistance as respond to flooding. • Analyze mass transfer in flooding process with improved equivalent circuit model. • Sensitivity of fix-low-frequency impedance to flooding changes with frequency. • Supply improved method for fault diagnosis benefiting reliability and durability. Abstract Water failures, which can be diagnosed with alternating-current impedance, is harmful to output properties and durability of the proton exchange membrane fuel cell. In this paper, an impedance-based measuring method of the zero-phase ohmic resistance is presented, which adjusts the measuring frequency to confine the impedance point within the ±3° sampling limitation area so as to guarantee the accuracy of 0.01 mΩ. The zero-phase ohmic resistance is sensitive to the membrane dehydration, and it decreases in the flooding process due to the gradual liquid water accumulation in the catalyst layer. The fluctuation of the zero-phase ohmic resistance increases in the flooding process, owing to the water state variation resulting from increasingly frequent water removal. A semi-empirical equivalent circuit model is used to analyze the change of mass transfer in the flooding process, and it is found that the diffusion resistance and the dissolution resistance of the reactant increase sharply. The fixed-low-frequency resistance is applicable in the flooding diagnosis, whose sensitivity changes with the chosen frequency. The highest sensitivity is corresponding to the peak value of the impedance magnitude. This paper provides adequate information about the water fault diagnosis, making sense to the fault-avoidance and the durability-promotion of the fuel cell. [ABSTRACT FROM AUTHOR]

Highlights • The hydrogen ejector is modeled coupled with anodic pressure drop characteristics. • Major geometric parameters are investigated and optimized in the operating range. • Effects of anodic operating pressure and water flooding are quantified. • Information is supplied for the parameter design of wide-operating-range ejector. Abstract The applicable operating range of the ejector is limited in the proton exchange membrane fuel cell system, although the ejector recirculates the unused hydrogen reliably without consuming any parasitic power. In this study, the ejector's Computational Fluid Dynamics model is established coupled with the stationary characteristic equation of the hydrogen ejector, which is derived by utilizing the anodic pressure drop formula. The model is capable of evaluating the entrainment performance in the overall operating range. The major geometric parameters are then optimized to promote the entrainment performance and extend the operating range, including the nozzle diameter (D n), the mixing tube diameter (D m), the mixing tube length (L m), and the primary nozzle exit position (NXP). It is found that the ejector hydrogen entrainment performance is sensitive to D m / D n and the optimal value range is 3–3.54. The entrainment ratio curve shows different changing tendencies along with D m / D n , separated by 3.54. The optimal L m / D m is confirmed, but the value increases with the primary flow rate. The hydrogen entrainment ratio decreases dramatically in the whole operating range when NXP is above the optimal value range. In addition, the effect of anodic operating pressure is investigated, and the performance reduction under lower pressure is mainly attributed to the higher water vapor content in the secondary flow. The adverse effect of anodic water flooding on the ejector performance is also quantified. This study supplies ways to extend the applicable operating range and helps the parameter design of wide-operating-range ejector. [ABSTRACT FROM AUTHOR]

In the rapid development of new energy technologies, fuel cells have exhibited their great potential for green, low-carbon applications. However, the deterioration of their performance after a certain time of use is considered as a serious handicap for their wide application. In this study, a micro-current working condition was found to be effective in recovering the performance of proton exchange membrane fuel cell. To investigate the mechanism behind, a series of experiments were conducted to study the fuel cell’s performance recovery and its membrane electrode assembly parameters, and membrane water content and proton conductivity were simulated by a fuel cell model. The results showed that fuel cell saw a performance recovery by 40% after 20 hours’ micro-current operation. The reduced hydrogen crossover and ohmic resistance, and the increased electrochemical active surface area should explain the recovered performance of the fuel cell. In addition, the higher, more uniform membrane water content and proton conductivity found in simulation results may also contribute to fuel cell’s performance recovery. This study proposes a new recovery mechanism for proton exchange membrane fuel cell and offers plausible explanations, which is a new addition to fuel cell theory and provides a theoretical basis for on-line performance recovery of fuel cells. [ABSTRACT FROM AUTHOR]

Hydrogen crossover current has a great influence on the performance and durability of proton exchange membrane (PEM) fuel cells. The common measuring method is linear sweep voltammetry (LSV). But some usual approximations, such as ignoring the influence of scan rates or short-circuit resistances, can lead to greater measurement deviations. Therefore, in this study to accurately measure hydrogen crossover current, LSV is improved by building a novel charging model based on fitted zero scan rate curves and on taking effects of short circuit into consideration. On the basis of this new model, galvanostatic charging method is improved by taking short circuit of PEMs into consideration and a mass spectrometry assisted with hydrogen pump is proposed with no need of calibration with standard gas. Hydrogen crossover current and short-circuit resistance of a 34 cm 2 single cell are measured by three improved methods, which are then compared with methods previously available. It is found that hydrogen crossover currents are reduced and more accurate than those obtained by previous methods, and values obtained by different improved methods are highly consistent with each other. So the proposed charging model is valid and can be used to optimize other electrochemical measurements of fuel cells. [ABSTRACT FROM AUTHOR]

Lifetime evaluation and prediction of proton exchange membrane fuel cells (PEMFCs) are essential for the lifetime extension and commercialization of fuel cells. Under the background that the accelerated stress test (AST) is difficult to evaluate the lifetime accurately, and the actual road test is time-consuming and costly, it is necessary to propose a quick lifetime prediction method of fuel cells. In this study, the current degradation law is analyzed based on the first-order kinetic model, and the lifetime prediction method based on the current degradation law is proposed. Moreover, the scale factor k is proposed to completely describe the fuel cell degradation due to complex degradation causes, and its calculation and selection methods are discussed. Finally, the accuracy of the proposed method is verified by the experimental results of single cells and fuel cell stacks, and the maximum relative error is less than 5% when the fuel cells conform to the first-order kinetic model. The lifetime prediction method of PEMFCs based on current degradation law improves the utilization of degradation information in current density, which is a feasible way for automotive fuel cell lifetime evaluation and prediction. • A current degradation law based on the first-order kinetic model is proposed. • A PEMFC lifetime prediction method using current degradation law is presented. • The selection method of scale factor k for lifetime prediction is discussed. • The lifetime prediction method is verified by experimental results. [ABSTRACT FROM AUTHOR]

In proton exchange membrane fuel cell (PEMFC), pressure drop at cathode can be used in water management. However, the equation to determine the cathode two-phase-flow pressure drop online and in real time has not been reported. This paper aims to develop a novel approach to calculate this pressure drop. The originalities are the fact that cathodic pressure drop actually experiences two jumps as it rises through two levels during flooding process and the proposal of spatial average water film to determine the pressure drop online. Firstly, the equation to calculate the pressure drop of cathode single-phase-flow, covering all operating conditions, is proposed and is verified at a 10 kW fuel cell stack. Secondly, we find that there exists a steady two-phase-flow pressure drop linked to an equivalent film flow in unit channel and put forward a novel approach to determine this pressure drop. Finally, water management strategy based on pressure drop is applied to a 34 cm 2 fuel cell and the voltage drop rate decreases by 35%, from 72 mV/h down to 47 mV/h, at a low cathode stoichiometric ratio 2.0 in long time operation, and the parasitic consumption is reduced by up to 50%. Hence, this strategy is shown to be effective in avoiding flooding, reducing air compressor consumption and extending the running time of single operation and the lifetime of fuel cell. This paper will contribute to the commercialization of fuel cells. [ABSTRACT FROM AUTHOR]

Buckets effect remains one of the most important factors limiting the fuel cell stack performance and lifetime. An experiment on a 16-cell stack with decreasing hydrogen concentration is conducted to investigate the effect of anode nitrogen concentration on stack performance and voltage consistency. The mean voltage decay rate of the stack is only 6% at the current density of 1.0 A cm−2, but the decay rate of the single cell with minimum voltage reaches up to 18%. The voltage standard deviation of the fuel cell stack increases by about 8 mV with the decrease in hydrogen concentration from 100% to 85%. Results show that the local voltage standard deviation is as high as 30 mV at 85% hydrogen concentration when the 16 cells are divided into four groups. The influence of local voltage consistency on the overall voltage consistency is further studied. The local voltage consistency can be served as a reliable indicator of anode purge strategy. Moreover, the uneven gas distribution amongst the cells in the stack can be analyzed and effectively detected, according to the local voltage consistency analysis with the experiment of anode nitrogen doping. • Influence of anode nitrogen concentration on voltage consistency is studied. • Local voltage consistency analysis method of the stack is proposed. • Influence of local voltage consistency on the stack is analyzed. • New anode purge strategy based on local voltage consistency is provided. • Local voltage consistency can be used to evaluate gas distribution of the stack. [ABSTRACT FROM AUTHOR]

[Display omitted] • Originally propose in-situ gas distribution characterization method for stacks. • Establish theoretical dual-chamber model decoupled by micro-current excitation. • Reveal distribution features and precisely recognize channel-choked fuel cell. • Perfectly verify characterization results by specially designed step-current test. • Fundamentally support breakthrough of power and durability of large-scale stacks. Uniform gas distribution contributes greatly to output performance, operating reliability, and durability of fuel cell stacks, which still lacks detection means and remains a great challenge. In this study, a non-invasive and in-situ characterization method of gas distribution is originally presented, based on the dual-chamber model that describes the mathematical relationship among gas flow, hydrogen crossover, and hydrogen transfer in the cathode side during electrochemical testing (N 2 /H 2). By means of micro-current excitation, the hydrogen crossover of every cell in a stack is synchronously identified and then the gas distribution can be decoupled. In a specially designed step-current test that aims for direct evidence, the voltage response characteristics to artificially induced air starvation coincide perfectly with the identified gas distribution. Typically, as the current density steps up to 175 mA·cm−2, the voltages of under-supplied cells, classified by the identified stoichiometric ratios of nearly-one and within 1.13–1.15, decline rapidly to around zero and abnormally to around only 0.5 V, respectively, while the sufficiently supplied cells remain normal. Based on the in-situ method, the artificially channel-choked fuel cell is precisely recognized, and different choke degrees can be distinguished. As identified at the corresponding current densities of 50, 100, and 500 mA·cm−2 and the average stoichiometric ratio of two, the gas flow rate of a certain cell only reduces by 22.8 %, 20.2 %, and 13.7 %, respectively, even with all the cathodic channels choked. The significant role of convection in the mass transfer is experimentally confirmed in fuel cells, and correspondingly, the mass transfer indicators are available by this electrochemical means. [ABSTRACT FROM AUTHOR]

• Originally build stack-level diagnosis framework to evaluate SOH and mass transfer. • In-situ diagnose MEA degradation and consistency deterioration in startup-shutdown. • Reveal evolution of CCL structure damage and mass transfer deterioration. • Determine hydrophobicity loss as primary factor of startup-shutdown-induced aging. • Contribute to stack-level durability investigation and lifespan extension. The durability of proton exchange membrane fuel cell stacks is a key limiting factor in transportation application due to severe conditions, where startup-shutdown and local fuel starvation most severely threaten the structure stability. In this paper, potentiostatic test at the average of 1.4 V is conducted to accelerate the carbon corrosion and structure damage in the cathode in a seven-cell commercial stack. A complete diagnosis framework based on micro-current excitation is established to systematically and in-situ evaluate the state-of-health of the cathode and the membrane, the mass transfer state, and the cell inconsistency in stack-level. Afterwards, the structure damage of cathode catalyst layer is characterized with scanning electron microscopy and energy disperse spectroscopy to provide direct evidences. In the early stage, the electrochemical surface area attenuates severely attributed to detachment and agglomeration of Pt particles, and the catalyst layer framework becomes more porous and hydrophilic, thus raising the double-layer capacitance. The weak catalyst layer structure tends to collapse starting from the lower layer close to the membrane, thereby leaving the dense lower layer and the porous upper layer in all cells. The damage mode is accurately manifested by the stepwise increase in the hydrogen transfer coefficient in the cathode that is identified in electrochemical testing. Meanwhile, the performance decay during operation exhibits typical characteristics of water flooding deterioration, as indicated by the three stages of steady voltage decay, drastic and periodic voltage fluctuation, and cell reversal. Therefore, the increase in water sensitivity and its dominant role in startup-shutdown-induced degradation are confirmed, which are also supported by the carbon-corrosion-induced ionomer agglomeration surrounding the enlarged pores in the upper layer. The enlarged pores are prone to be flooded, and the collapsed partial structure inherently limits the mass transfer. The aging process, evolution mechanisms of internal damage, and dominant factors during startup-shutdown are therefore identified and clearly revealed in stack-level. [ABSTRACT FROM AUTHOR]

Lifetime is a major bottleneck for the commercialization of fuel cells. A quick evaluating method for fuel cell lifetime can assist in immediately assessing technology progress, predicting lifetime in real-time, and extending durability. Among the existing methods, the lifetime prediction method based on working conditions is a cost-effective, time-saving, and realistic method. However, there are no methods and clear standards for extracting the time and frequency of working conditions in test protocols used in this method. In this study, the working condition spectrum extraction method is proposed and used to extract the working condition spectra of commonly used durability test protocols. The lifetime prediction of fuel cells operating under different specific test protocols based on the working condition spectrum extraction method is performed and verified to demonstrate its reliability. Furthermore, the proposed method is used to extract the working condition spectra and predict the lifetime of fuel cells in hybrid power systems with different energy management strategies (EMSs). The results show that the extraction method can provide an evaluation method for developing EMSs of fuel cell hybrid power systems from the lifetime perspective, which is useful for fuel cell durability research. • Propose a working condition spectra extraction method for lifetime evaluation. • Extract working condition spectrum of commonly used durability test protocols. • Verify the method by fuel cell lifetime prediction based on working conditions. • Lifetime of fuel cell in hybrid power systems of different EMSs is predicted. • A method for evaluating EMSs from the lifetime perspective is provided. [ABSTRACT FROM AUTHOR]

The pressure difference between the inlet and outlet of the reactant in fuel cells is called the pressure drop, which is related to the water amount inside the fuel cells. In recent years there have been many studies that used the pressure drop to detect the water content and diagnose water fault of proton exchange membrane fuel cells (PEMFCs). To our knowledge, there has not been a systematic review of these studies. In this paper, the effect variables of pressure drop are reviewed firstly. Then estimations of the theoretical pressure drop are reviewed mainly based on the following four aspects: Bernoulli’s equation, two-phase flow multiplier, Darcy’s law and artificial intelligence. Afterward, the water fault diagnosis based on the pressure drop using the following six indicators are reviewed: indicator of direct pressure drop, its deviation, frequency, multiplier, the ratio of pressure drop to flow rate and the flooding degree. In addition, the strategies of water fault recovery are also summarized. Finally the merits, demerits and application prospects of pressure drop-based water fault diagnosis are presented. [ABSTRACT FROM AUTHOR]

Lifetime and cost are two main factors that restrict the commercialization of Proton Exchange Membrane (PEM) fuel cells. This paper mainly studies the prediction and the evaluation methods of PEM fuel cell lifetime. A formula to predict the PEM fuel cell lifetime is presented. The formula is based on the vehicular operation records and the tested results in the lab. Also the difference between the vehicular operation condition and the test is taken into consideration. The formula realizes the PEM fuel cell lifetime rapid prediction. A PEM fuel cell residual life evaluation method is also presented. The evaluation method realizes online forecasting of the residual life through updating the environmental affecting factor and voltage degradation rate caused by the operating conditions. Furthermore, the PEM fuel cell economic lifetime is studied. The economic lifetime is the working lifetime which gains the lowest average cost. The synthesis of the lifetime and the cost provides a basis to confirm the best design lifetime. [ABSTRACT FROM AUTHOR]

• Identify ohmic resistance and update excitation model for precision upgrading. • Conduct systematic error analysis concerning decoupling algorithm and stack state. • Originally reveal condition sensitivity for identifying MEA inconsistency. • Validate adaptability to single excitation for a great advance in test efficiency. • Contribute to validity and reliability of MCE in stack-level MEA diagnosis. The inconsistency of membrane electrode assemblies (MEAs) greatly restricts the development of high-performance and long-lifetime fuel cell stacks. Micro-current excitation (MCE) method has promising prospects in consistency evaluation due to its capacity in MEA component diagnosis at the stack level. In this study, the ohmic resistance is identified with the initial voltage jump characteristic upon MCE and then introduced into the excitation-response model for precision upgrading. A detailed error analysis is then conducted concerning the decoupling algorithm, the simplified model, and the stack state, demonstrating the significance of error revision for degraded stacks. Based on the updated method, the condition sensitivity of inconsistency identification is further investigated, involving the operating temperature, the gas humidity, and the gas flow rate. It is essential to ensure the same hydration state of fuel cells for simultaneous component inconsistency diagnosis. Heating and minor gas humidification are sufficient and necessary. The MCE is proven to be very sensitive to the N 2 flow rate in the cathode. An extremely low N 2 flow rate enables a relatively uniform gas distribution and hydrogen adsorption saturation, and is thus recommended. Finally, the updated MCE is validated to adapt to a single excitation for inconsistency diagnosis, which means great progress in test efficiency. [ABSTRACT FROM AUTHOR]

Highlights: [•] Reviewed the PEMFC life degradation during the durability tests and its consequences. [•] Reviewed the water management problems: causes, consequence and mitigation methods. [•] Reviewed the reactant starvation issues: causes, consequence and mitigation methods. [•] Reviewed the effects of the operating parameters on PEMFC dynamic response. [•] The conclusion is a guidance for future research of prolong PEM fuel cell life time. [ABSTRACT FROM AUTHOR]

Abstract: To better measure and monitor the membrane electrode assembly (MEA) status in polymer electrolyte membrane fuel cell (PEMFC) stack, a galvanostatic charge method is improved. The electrochemical active surface area (EAS), double-layer capacitance, hydrogen crossover current, and cell ohmic resistance can be measured by this method. In this method, two or more constant currents are applied to the fuel cell stack, and the voltage response between two electrodes of each cell is recorded and analyzed. Tests on a two-cell stack which is supplied with hydrogen in anode and nitrogen in cathode are carried out, and the influences of temperature and relative humidity (RH) on the MEA parameters are investigated. Results show that with an increase of RH, both double-layer capacitance and EAS increase, while hydrogen crossover current and cell ohmic resistance decrease. With an increase of temperature, hydrogen crossover current increases, cell ohmic resistance decreases, and EAS and double-layer capacitance show little change. The galvanostatic charge method shows a convenient way to research cell consistency and MEA lifetime in a fuel cell stack. [Copyright &y& Elsevier]

• Establish complete fuel cell excitation-response model in integral form. • Originally propose analytic method of MEA parameters by micro-current excitation. • Validate accuracy, stability, robustness, and adaptability to general excitation. • Reveal evolution tendencies of MEA parameters along with operating conditions. • Practically applied in consistency evaluation of a seven-cell fuel cell stack. Consistency evaluation and degradation diagnosis of membrane electrode assemblies (MEAs) in a large-scale fuel cell stack remain critical problems despite the accelerated commercialization. In this paper, a novel analytic method of MEA parameters is proposed with high accuracy and stability, which does not require high sampling frequency and data filtering of voltage signals anymore. By means of micro-current excitation, regardless of whether it is galvanostatic or not, four key parameters of each MEA can be calculated simultaneously based on complete excitation model, including hydrogen crossover current density, electrochemical surface area (ECSA), double-layer capacitance, and short-circuit resistance. The universality of the method is demonstrated by the high consistency between galvanostatic and non-constant current excitation results. Effects of condition parameters, including operating temperature, relative humidity, and back pressure, on MEA parameters are further investigated. Low temperature, high relative humidity, and high back pressure make high ECSA. Elevated temperature and back pressure increase hydrogen crossover. Relative humidity is proved to determine the hydrogen crossover by affecting both anodic hydrogen partial pressure and membrane water content. Finally, the method is applied to evaluate the MEA consistency of a long-time-stored seven-cell stack and recognizes part of the membranes ineffective. This method shows a promising prospect in consistency-based MEA screening, aging evaluation of MEAs in a stack, and recombination of old and broken stacks. [ABSTRACT FROM AUTHOR]

Abstract: 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. [ABSTRACT FROM AUTHOR]

Abstract: After 500h accelerated lifetime test on an automotive PEM (proton exchange membrane) fuel cell stack, the performance of the cells at the stack''s foreside is the worst. The electrochemical surface area, internal resistance, particle size and hydrophobic nature of catalyst are measured based on the electrochemical methods, electronic-lens and dropped water test. The results show that the internal resistance increases to double and the average particle diameter increases to 3 times when electrochemically active area surface coefficient decreases to one-fifth. The open circle voltage of the worst cell is 0.7V and its maximum current density is 200mA/cm2. Especially, the performance of part at air-outlet is the worst, followed by the part at air-inlet, hydrogen-outlet, and hydrogen-inlet. The phenomenon mentioned above at the inlet of stack is the most serious among all the cells in the stack. [Copyright &y& Elsevier]

• Parameters in 9-state model, improved voltage model, and controller are identified. • Throttle factor of cathodic inlet is time-variant with motor voltage and its step. • Delay effect of motor voltage is noticeable and cannot be ignored. • Clear boundary estimation for super-twisting sliding mode controller is presented. The super-twisting sliding mode control (ST-SMC) is widely used in fuel cell system control due to the simple control law and strong robustness. However, it requires an accurate system model. Generally, literature usually reduces the system order and directly gives the empirical value for the system and controller parameters or conducts coefficient identification of the fuel cell voltage model, lacking the specific identification method for system physical parameters and controller parameters. In this paper, a relatively complete control-oriented nine-state fuel cell system model was established, including the model of compressor flow using the artificial neural network method, and the improved voltage model. Then, the data-driven method for key parameter identification was proposed, including the fuel cell throttle factor and motor voltage changing rate considering time delay. In addition, the parameter tuning method for controller design was proposed as well. These two methods are of originality. After the model validation in the perspective of steady and transient performance, the comparison was carried out between the ST-SMC and PID controller. It is found that the throttle factor of the cathodic fuel cell inlet and the delay effect in terms of changing rate of motor voltage impact the system model, where the throttle factor is time-variant, and the delay is noticeable which differs with the step magnitude of the motor voltage. The parameter tuning and boundary estimation of ST-SMC are very specific, owing to the process of treating flow rate as a state, not speed, and are convenient to be generalized. The better ability in anti-flooding exhibits the importance of parameter identification. Although the study is conducted in a low-pressure system, the method proposed in this paper is universal and could be applied to other fuel cell controls for better system efficiency and reliability. [ABSTRACT FROM AUTHOR]

Five prototype fuel cell hybrid–drive electric forklifts were built, and laboratory tests were used to confirm that they could function in a real warehouse environment. The electric forklifts were delivered to Far Eastern Group's Taoyuan, Taiwan A–mart supermarket warehouse for 200 h of operational testing. Each vehicle's hybrid–drive system included a 2.7 kW proton exchange membrane fuel cell stack and two auxiliary power sources, each composed of a lithium battery and supercapacitors in parallel. The components of the drive system could supply 7 kW of power. The system's potential advantages are low cost and high working flexibility. To collect sufficient data and determine whether this fuel cell hybrid–drive electric forklifts design could function in an actual warehouse environment, an initial test period of 200 h was considered. The vehicles were operated from 8:00–22:00 with 4–5 h of peak operating time. • The forklifts are transformed from fuel to fuel cells as power. • Fuel cells, lithium batteries and supercapacitors are used as active powers. • This research can avoid the disadvantages of fuel cells. • This paper has 200 h of test and provides detailed operation data. • A simple hydrogen economy calculation is carried out. [ABSTRACT FROM AUTHOR]

Compressor is the crucial device for the present fuel cell system, reflected by the compressor's selection, performance, cost, and control. The review papers on compressor usually focus on its category and the pros and cons in commercial use. This paper reexamines the necessity of the air compression of fuel cell system and systematically reviews the compressor progress and application from criteria for air compression to compressor control. The leading six parts of this paper answer, respectively, that which kind of stack is suitable for compression, which compressors are commonly used in China market, what features should the compressor have for the fuel cell application, which phase is the current compressor in and will be in, how to effectively control the compressor. Of them, the analyzing model for the criteria to conduct compression is firstly proposed because it is a generalized method and independent of experimental data. Using the thermal dynamics to analyze the phase and possible ceiling of the compressor is another novelty (DOE or any other organizations will not give this ceiling and analysis). Finally, the compressor control at the angle of system efficiency improvement is firstly reviewed, as well. This paper is beneficial for the researchers to know the overall state of the compressor and is of guidance for them to design a more efficient and commercial fuel cell system. • Generalized data-free analysis model for criteria to compress air is firstly given. • Analyzing the phase and development ceiling of the compressor. • Comparison of technique similarity between fuel cell and inner combustion engine. • Compressor control at the angle of system efficiency promotion is firstly reviewed. • Sliding mode control and extremum seeking control are promising for compressors. [ABSTRACT FROM AUTHOR]

The proton exchange membrane (PEM) fuel cell is an ideal automotive power source with great potential, owing to its high efficiency and zero emissions. However, the durability and life-span limit its large-scale application. Complex automotive operating conditions significantly accelerate fuel cell aging, and result in diverse degradation mechanisms that require comprehensive understanding. This review focuses on three harsh conditions of open-circuit/idling, dynamic load, and startup-shutdown. In-situ and ex-situ accelerated stress tests (ASTs) for the three conditions are summarized in terms of methodology, research objectives, and conditions of application. Reversible decay may arise during ASTs and lead to an over-estimation of the aging state, of which the causes and recovery procedures are emphasized. The degradation mechanisms are elaborated systematically according to parameter characteristics, microstructure, and aging reactions. First, increased gas permeation and a high cathode potential during open-circuit/idling combine to intensify generation of free radicals that cause membrane degradation. Pt degradation and migration are also accelerated, characterized by increased Pt particle growth and precipitation in the membrane. The debate regarding the effect of Pt precipitation on membrane degradation is resolved based on a literature review. Second, dynamic load brings about changes in the thermal/humidity state, altered reactant demand, and potential cycling, which lead to mechanical degradation, gas starvation, and Pt particle growth, respectively. To account for the accelerated particle growth, electrochemical Ostwald ripening and increased Pt dissolution are reviewed. Third, an air/hydrogen boundary appears in the anode under startup-shutdown condition and causes carbon corrosion in the local cathode via the reverse current mechanism. The cathode thereby suffers from severe and non-uniform structural damage and even structural collapse, accompanied by Pt agglomeration and detachment. Meanwhile, difficulties in mass transfer arise because of ionomer redistribution, decreased porosity, and carbon surface hydrophilization. In addition, cold start produces severe damage to component structures. This paper seeks to guide further investigation into improved fuel cell durability via mechanism analysis, condition optimization, control strategy development, structural design of the membrane electrode assembly, and component material development. [ABSTRACT FROM AUTHOR]

• A decrease of resistance due to clamping is a key factor of a cell performance boost. • Hydrogen pump increases the water content of membrane and active sites of catalyst. • The voltage of fuel cell at low current can be calculated by electrode parameters. • Rising ohmic resistance has the biggest effect on the early performance of fuel cell. • The catalysts deactivation is the primary factor of fuel cell aging after 150 cycles. The membrane electrode assembly (MEA) is one of the essential components of the polymer electrolyte membrane fuel cell. Its quality and state greatly influence the fuel cell performance, which can be characterized by MEA parameters, including ohmic resistance, hydrogen crossover current, double layer capacitance, and catalyst roughness factor. These parameters of a single cell or multiple cells in a stack can be detected simultaneously by the galvanostatic charging method. In this paper, this method was used to test two 34 cm2 single cells and one 300 cm2 four-cell stack during assembly, activation and aging, respectively, in order to investigate variations of MEA parameters and their effects on the fuel cell performance. With the assembly torque increasing, the decrease of ohmic resistance makes major contribution to the performance improvement of a fuel cell. With repeating hydrogen pump activation, the increase of active sites and the decrease of the ohmic resistance improve the fuel cell performance gradually. Simultaneously, the logarithmic relationship between the Tafel coefficients and the catalyst roughness factor is observed, based on which a functional expression for calculating the fuel cell performance only by MEA parameters is established. As for the accelerated aging test, the increase of ohmic resistance exerts a significant influence on the performance of a stack in the initial stage, but it only results in a 10% performance degradation after 200 aging cycles. Actually, the decrease of the catalyst roughness factor affects the stack performance primarily. [ABSTRACT FROM AUTHOR]

Abstract As an energy conversion device that converts hydrogen energy into electrical energy, the fuel cell is one of the most promising. Starvation is the main reasons that shortens the lifetime of Proton Exchange Membrane Fuel Cell (PEMFC) for vehicle usage. Therefore, combining the experimental and simulation results, as well as previous studies, based on the external characteristics of PEMFCs, the mechanism of judging the starvation in PEMFCs is explored. It provides a theoretical basis for judging the starvation using external characteristics. In addition, the starvation index is proposed to solve the problem that the starvation in the process of fuel cell loading, which could not be evaluated in the past. This factor can provide guidance for matching and controlling the fuel cell supply system, as well as optimizing the internal structural parameters of the fuel cell. [ABSTRACT FROM AUTHOR]

Reactant starvation is unfavorable to the durability and life extension of PEMFC engines. In this paper, a 25 cm2 segmented PEMFC is assembled to investigate starvation phenomenon, and the contour maps of current distribution are given to analyze the reactant starvation mechanism. The experimental results prove the high-quality homogeneity of current with triple-serpentine configuration even under starvation conditions. The air starvation experiments suggest the migration of lowest current region and the variation trend of standard deviation with the increasing of stoichiometry. The survival time increases with the anode stoichiometry, and the current density of anode downstream constantly increases with the reversal of cell, reflecting severe hydrogen starvation in these local areas. It is concluded ORR dominates the specific current distribution rather than any other electrochemical reactions. This study sheds the light of improving the lifespan and potential large-scale commercialization realization of PEMFC engines. [Display omitted] • Systematical analysis on reactant starvation of PEMFC was performed. • Triple-serpentine flow field exhibits excellent homogeneity of current density. • The lowest current area moves downstream with the increment of air supply. • Behavior of current density near the outlet closely relates to hydrogen supply. • Complexity of involved reactions affects the uniformity of current distribution. [ABSTRACT FROM AUTHOR]

• Originally establish excitation-discharge-combined electrochemical model for MEA. • Propose excitation-discharge-combined SOH analytical method with single excitation. • Accurately identify and compare MEA parameters between cathode and anode tests. • Validate precision and stability of MEA inconsistency diagnosis at stake level. • Verify irreversibility of partial reactions in high-potential region of excitation. Consistency evaluation and degradation diagnosis of membrane electrode assemblies (MEAs) in fuel cell stacks are critical issues for fuel cell lifetime extension and commercialization. This study aims to develop a high-efficiency, high-precision, and synchronous state-of-health (SOH) diagnosis methodology for MEAs at the stack level. The micro-current excitation (MCE) method based on multiple excitations is used to evaluate the MEA inconsistency of a 22-cell commercial stack, involving hydrogen crossover, short-circuit resistance, electrochemical surface area, and double-layer capacitance. High precision is demonstrated by the consistency in the membrane-related parameters and the difference in the electrode-related parameters between the cathode and anode tests. Herein, an efficient single excitation and natural discharging (SEND) analytical method based on the excitation-discharging-combined equivalent circuit model is originally proposed. The accuracy of SEND can be demonstrated by comparing its results with the standard results of the MCE, which fit the baseline y = x with high linearity and identical evaluation of inconsistency. Moreover, the stability of the SEND is proved by its independence from the excitation current, which is reflected by the presence of tiny standard deviations. In addition, the bifurcation feature in the integrated net charge curves of the excitation and natural discharging processes proves the irreversibility of partial oxidation reactions in the high-potential region of excitation. The SEND significantly increases the testing efficiency using only a single excitation. It has promising prospects for the rapid consistency-based MEA screening, degradation evaluation, and recombination of MEAs. [ABSTRACT FROM AUTHOR]

Because of its superior cold-start effect and lack of pollution, the proton exchange membrane fuel cell (PEMFC) engine has gained a lot of interest as the most common application type of hydrogen fuel cell engines. However, the difficulty in forecasting the remaining life and monitoring the operation state for a lengthy period limits its commercialization. As a result, PEMFC Prognostics and Health Management (PHM) is critical. A method based on bibliometrics is introduced into the research on this topic. In this work, the research in this field is divided into three stages. The characteristics of different stages are summarized through keywords, subject background, and other aspects. We can observe the transformation of the field from the idea based on analytical reduction to systematic theoretical control. Based on the results of bibliometric analysis, we have conducted a literature review, and we can see the typical research results and development direction of each topic clustering. • A bibliometric based approach is used to organize the structure of the review. • The hotspot and driving force of hydrogen fuel cell engine are visualized. • Development direction PHM in the PEMFC was analyzed. • Many fault causes and health management architecture are analyzed. [ABSTRACT FROM AUTHOR]

The geometry configuration of proton exchange membrane fuel cell (PEMFC) which is considered as a promising energy conversion device has great influence on PEMFC performance. In this paper, effect of channel-to-rib width ratio and relative humidity of reactant gas on the performance are compared based on two single PEMFCs. The EIS testing results below 50 A are given and analyzed. The results obtained from polarization curves, power density curves and EIS fitting results prove that: 1. Compared with cell 3:4, the anode high humidification has a greater addition to the performance of cell 1:1; 2. PEMFCs with different geometry configurations of flow field have their own suitable working condition ranges; 3. The charge transfer resistance is the dominating factor when current loading is below 2.0 A cm−2. • The performance of two single PEMFCs was evaluated and compared. • EIS fitting results are analyzed with equivalent circuit. • Geometric configuration plays different role under different loading amplitudes. • Charge transfer resistance dominates performance when loading is under 2 A cm−2. [ABSTRACT FROM AUTHOR]

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

Widely acknowledged by experts, the inconsistency between the cells of the proton exchange membrane fuel cell stack during operation is an important cause of the fuel cell life decay. Existing studies mainly focus on qualitative analysis of the effects of operating parameters on fuel cell stack consistency. However, there is currently almost no quantitative research on predicting the voltage consistency through operating parameters with machine learning methods. To solve this problem, a three-dimensional model of proton exchange membrane fuel cell stack with five single cells is established in this paper. The Computational Fluid Dynamic (CFD) method is used to provide the source data for prediction model. After predicting the voltage consistency with several machine learning methods and comparing the accuracy through simulation data, the integrated regression method based on Gradient Boosting Decision Tree (GBDT) gets the highest score (0.896) and is proposed for quickly predicting the consistency of cell voltage through operating parameters. After verifying the GBDT method with the experimental data from the fuel cell stack of SUNRISE POWER, in which the accuracy score is 0.910, the universality and accuracy of the method is confirmed. The influencing sensitivity of each operating parameter is evaluated and the current density has the greatest influence on the predicted value, which accounts for 0.40. The prediction of voltage consistency under different combination of operating parameters can guide the optimization of structural parameters in the process of the fuel cell design and operating parameters in the process of fuel cell control. • Predict voltage consistency through operating parameters with machine learning. • Gradient Boosting Decision Tree (GBDT) method gets the highest score. • The universality and accuracy of GBDT method are verified by experiments. • The influencing sensitivity of each parameter is evaluated. [ABSTRACT FROM AUTHOR]

Proton exchange membrane (PEM) fuel cell is the core equipment that can directly convert hydrogen energy into electricity. In the process of long-term operation, due to the aging of membrane electrode assembly and other components, the fuel cell performance gradually deteriorates. The voltage prediction of fuel cells is very important for performance and lifetime optimization. Long short-term memory neural network is one of the widely used prediction methods. Based on the prediction method of bidirectional long short-term memory neural network, the hyperparameters of the neural network model by Bayesian optimization algorithm is optimized to improve the accuracy of fuel cell performance degradation prediction. When the sampling time interval is 25 min and the training set is 45 %, the root mean square error and the average absolute percentage error of the prediction results is reduced to 6.3 mV and 0.1245 %, respectively. Moreover, by analyzing the influence of different sampling time intervals and training set proportion on the prediction results, a data set that takes into accounts both efficiency and accuracy is obtained. The proposed method based on Bayesian optimization can achieve more accurate performance degradation prediction. • A performance prediction method for fuel cells based on Bi-LSTM is proposed. • Bayesian algorithm was adopted to optimize the hyperparameters. • The effect of training set ratio on performance prediction is studied. • A reasonable frequency range of data sampling is proposed. [ABSTRACT FROM AUTHOR]

Load shedding could be a solution to water accumulation alleviation during the operation of proton exchange membrane fuel cell (PEMFC) engines. In this paper, multiple-step loading reduction strategies based on two laboratory-scale (25 cm2) single PEMFCs (assembled with triple-serpentine and single-serpentine flow field plates on the both electrodes and thus named after cell TS and cell SS respectively) are experimentally discussed and evaluated. The real-time pressure evolution and HFR are recorded, compared and analyzed to characterize water transfer phenomenon among the components. The investigation results suggest that: 1. From the perspective of output voltage and power after load reduction, the number of reduction step has little effect on the output performance; 2. Cell TS exhibits better homogeneous trend of pressure drop although with less water removal quantity comparing with that of cell SS, which is resulted from multi-channel configuration; 3. The cathode pressure drop of cell SS decreases obviously throughout the experiment, indicating the convinced water removal capability of single-serpentine configuration; 4. Constant increase of HFR reflects membrane dehydration when the loading reduces. This paper demonstrates the possibility of removing accumulated liquid water by reducing the load of PEMFCs. • Triple-serpentine configuration ensures a controllable fluctuation range of pressure. • Triple-serpentine results in little effect on water removal when the loading sheds. • Single-serpentine configuration alleviates water accumulation effectively. • Single-serpentine configuration leads to back diffusion during the shedding. • Membrane dehydration should be noticeable during the shedding process. [ABSTRACT FROM AUTHOR]

Water flooding is a crucial factor affecting the lifetime of a proton exchange membrane (PEM) fuel cell. In this paper, A laboratory-scale (25 cm2) single PEM fuel cell was tested to investigate the characteristics of fuel cell under a load shedding process with various air flow rates. The performance was assessed using output power and electrochemical impedance spectroscopy, while the cathode flooding was identified using voltage and cathode pressure drop. The results show that reducing the load within a specific range can lower the impedance and lessen floods. The results also demonstrate that there is an optimal inlet flow rate interval considering its effect on system efficiency, durability, and water flooding. In addition, a stepped load shedding pattern was proposed to solve the flooding problem of a fuel cell while taking the power demand into account. The effect of load shedding cycle and air inlet flow rate on flooding was evaluated by comparing the flooding severity coefficient and output power. The results prove that a longer current reduction cycle is more favorable in alleviating flooding problems. The optimal mode for the fuel cell in this paper was determined according to the flooding situation and power variation. Finally, the approach to obtaining the optimal load shedding pattern for other PEM fuel cells system was summarized. The flooding mitigation approach proposed in this paper can be utilized as a guide for developing strategies to extend the life of PEM fuel cells. [Display omitted] • Diagnosis of flooding using a combination of pressure drop and voltage is more accurate. • Load shedding mainly affects the concentration resistance and charge transfer resistance. • There is an optimal inlet flow rate interval considering its effect on system efficiency, water flooding and durability. • Long stepped load shedding cycle contributes to flooding mitigation under a heavy condition. • Purging with load shedding is a useful approach for water management under a heavy condition. [ABSTRACT FROM AUTHOR]

Highlights • A comprehensive review of researches on gas starvation issue of PEM fuel cell. • Include causes, severe consequences, diagnostic methods and mitigation measures. • Summarize the variable methods and compare their performance in different aspects. • Provide guidance to the diagnose methods, system control strategy and structure design. Abstract The short service life of fuel cell is a key problem that restricts the commercialization of fuel cell vehicles. Many scholars have found that gas starvation is one of the most important causes of the proton exchange membrane fuel cell lifetime decay, which leads to a series of severe consequences such as carbon support corrosion, cell reversal and output performance degradation. However, accurate diagnosis and effective mitigation of fuel cell gas starvation are not achieved currently. Gas starvation is a condition that the reaction gas of proton exchange membrane fuel cell working in the sub-stoichiometric state. In this paper, we will study the causes, severe consequences, diagnostic methods and mitigation measures of the gas starvation in proton exchange membrane fuel cells through previous literature review. This research is aim to provide guidance to the diagnose methods, to optimize the system control strategy and structure design and to contribute to the studies which are focus on prolong the proton exchange membrane fuel cell lifetime. [ABSTRACT FROM AUTHOR]

Highlights • Gas distribution quality evaluation based on statistical analysis firstly proposed. • Optimal working area is presented for stable cell performance under dynamic load. • Specific areas where gas starvation prone to occur during load change identified. • Provide basis for fuel cell optimal design, control and gas starvation diagnosis. Abstract The proton exchange membrane fuel cell is considered to be the best alternative to next generation power system due to its high efficiency without emission. It has been found that the lack of gas supply due to frequent load change and start-stop is a very crucial factor in fuel cell life decay and restricts its commercialization. Gas starvation is the state in which the reaction gas of proton exchange membrane fuel cell operates at a sub-stoichiometric level. The diagnosis of local gas starvation without additional expensive and intrusive sensors is still a tough problem not to be solved. In this paper, the area of gas starvation is calculated to evaluate the degree of gas starvation. A novel statistical analysis based method is applied as the index to measure the uniformity of the reaction gas concentration directly on the surface of electrode. The distribution quality of reaction gas under both steady-state and dynamic conditions was evaluated by simulating different operating conditions through the simulation model established. A suitable working area of the fuel cell under steady state is proposed to achieve more stable cell performance under load change conditions. The distribution quality of reaction gas concentration on the whole electrode surface under dynamic condition is evaluated to identify the specific areas where gas starvation is likely to occur. The results of this study will provide the basis for the optimal control and structural design of the fuel cell against dynamic gas starvation, and contribute to the long-life study of proton exchange membrane fuel cells. [ABSTRACT FROM AUTHOR]

Highlights • Intake gas uniformity of the dual-stack PEM fuel cell system was studied. • Porous media model was proposed to replace fuel cell stack pressure drop model. • The model was proved to be reasonable by mechanism. • Several features of intake gas was found through simulation. Abstract The Polymer Electrolyte Membrane Fuel Cell (PEMFC) system is considered as one of the most potential power plant on vehicle. However, the efficiency of the high power fuel cell stack has restricted the high power system which is developed for the full power fuel cell vehicle. The multi-stack system integrated by several fuel cell stacks is a solution aimed for high power level system. However, the inconsistency of the stacks flow distribution restricts the performance uniformity of each stacks and the efficiency of the multi-stack fuel cell system. In this paper, a dual-stack polymer electrolyte membrane fuel cell system was setup, and its intake manifold was established. A porous medium was applied to replace the complex flow field structure inside the fuel cells to simulate the pressure drop of the gas through the stack. According to the operating conditions of the vehicle, variable conditions are designed to carry out simulation experiments for the first time. The results show that there is not a significant effect of the changing load's range on the instantaneous uniformity. However, the initial value of the changing load has a great influence on the instantaneous uniformity. In addition, the settling time of the loading process is much shorter than that of the load shedding process. The equivalent model of porous media proposed provides a new method to model the fuel cell stack, which is very useful in the fuel cell pipe system design and system dynamic response analysis. The uniformity simulation result provides the directions for the fuel cell control strategy design and optimization, and the references for gas intake manifold design. [ABSTRACT FROM AUTHOR]

The lifetime of the proton exchange membrane fuel cell (PEMFC) is the main issue restricting its commercialization. During real vehicular applications, the fuel cell engine mainly experiences four dynamic conditions: load changing, start-stop, idling, and high power. Since the start-stop condition has a great impact on the lifetime of fuel cells, it is necessary to fully understand the degradation mechanism of PEMFC under this condition. This paper discusses the background and progresses in related research, analyses the gas distribution process inside the fuel cells during start-stop, and summarises the main mechanism and factors that lead to the degradation. Then, solutions in terms of material improvement and system control are listed. This review can provide a basis for solving the degradation problem in PEMFCs and improving the cell lifetime. [ABSTRACT FROM AUTHOR]

Proton exchange membrane (PEM) fuel cell is considered to be one of the promising alternatives to conventional engines in the future as on-board vehicle energy conversion device. In this paper, three single PEM fuel cells with different cathode flow field plates (FFPs), which vary from each other on the channel width along the cathode flow direction, are assembled. Polarization and power density curves, electrochemical impedance spectroscopy (EIS) fitting results are utilized to evaluate the output performance. Moreover, the economic characteristic of three single cells is calculated based on the assumption of parasitic pumping power generated by air compressor. Finally, the durability of three single cells is assessed based on comparison of electrochemical surface area (ECSA) obtained from analysis of cyclic voltammetry (CV) method. The comprehensive experimental results reveal that cell 0.8-1.0-1.2 exhibits better performance under high RH conditions and cell 1.0-1.0-1.0 is more favorable under a low RH condition. The mechanism behind the phenomenon of performance variation is analyzed and attributed to the switching of dominating factor from two-phase flow velocity to spatial availability degree under various conditions of RH. [Display omitted] • Characteristics of non-uniform cathode channels were investigated. • Narrow upstream configuration is preferred under high RH condition. • Cathode with a uniform flow plate is more favorable under low RH condition. • The dominating factor affecting comprehensive performance changes with RH values. [ABSTRACT FROM AUTHOR]

• A method for calculating the optimal interval of air stoichiometry is proposed. • Synthetically consider fuel cell power, working efficiency and gas starvation. • Calculate air stoichiometry optimal interval under different operating conditions. • Changing trends of optimal interval varying with operating conditions are analyzed. • Evaluate gas starvation by calculating the proportion of gas-starvation area. Cathode air stoichiometry is one of the crucial factors affecting the electrical performance and local gas starvation of proton exchange membrane fuel cells. A suitable air stoichiometric ratio can not only improve the generated power and working efficiency, but also effectively reduce or eliminate internal reactant gas starvation. At present, the optimal value of cathode air stoichiometry and its changing trends under different operating conditions remains to be further studied, and the optimization indicator in existing researches mainly tends to be maximum power, with rare attention to fuel cell degradation. This paper proposes a quantitative method for calculating the optimal interval of air stoichiometry, which considers synthetically the fuel cell generated power, working efficiency and internal gas starvation reduction, to make these three indicators reach the optimal balance. The local gas starvation inside is evaluated by proportion of gas-starvation area on fuel cell electrode surface. Based on the proposed methodology, a three-dimensional model of a five-channel serpentine flow field fuel cell is established in this paper to simulate and calculate the optimal interval of cathode air stoichiometry under different operational parameters and electrical load conditions. The changing trends of the optimal air stoichiometry with different operating conditions and the impacts of key operational parameters and loads on air stoichiometry optimal interval are also carefully analyzed. Among them, the operating pressure and current density have a significant influence on the optimal interval value. Increasing the working pressure can make the optimal air stoichiometry smaller, and the increase in current density results in a substantial raise of optimal air stoichiometry. The calculation method of cathode air stoichiometry optimal interval proposed in this paper can provide references for fuel cell air supply strategy research, and the conclusions can be beneficial for fuel cell performance optimization and long-lifetime design. [ABSTRACT FROM AUTHOR]

• Simulate impacts of operating parameters on PEMFC performance and gas distribution. • Influencing sensitivity of each operating parameter is quantitatively evaluated. • Analyze trends of impact and influencing sensitivity under various loads. • The optimal operating parameters combinations are summarized in this paper. Proton exchange membrane fuel cell has certain requirements on operating conditions. Suitable operating parameters can not only improve the output performance, but also prolong the life duration of fuel cells. Existing studies on the effects of operating parameters on fuel cell performance are often qualitative and lack quantitative indicator of influencing sensitivity. Moreover, there is currently almost no research on impacts of operating parameters on fuel cell internal reactant gas distribution, but local gas starvation caused by uneven gas distribution is one of the crucial reasons for fuel cell lifetime degradation. In this paper, a three-dimensional model of a three-channel serpentine flow field fuel cell is established to study the impacts of seven critical operating parameters on fuel cell output performance and internal gas concentration distribution under different load conditions. By calculating the influential proportion of each operating parameter, its influencing sensitivity on fuel cell performance is quantitatively evaluated. The changing trends of each operating parameter's impact and influencing sensitivity on the output voltage, the mean value and standard deviation of internal gas molar concentration distribution under different load conditions are also analyzed. Besides, the optimal operating parameters combinations at various loads have been summarized based on simulation and calculation results. The research content of this paper can make up for the current vacancy research on impacts of operating parameters on fuel cell internal gas distribution, the quantitative evaluation method of influential sensitivity proposed in this paper can be reference for the operational parameters studies, and the conclusions of this paper can provide more specific guidance for fuel cell performance improvement and control optimization. [ABSTRACT FROM AUTHOR]

• Fuel cell system-level modeling is achieved by Simulink and Fluent co-simulation. • Variable load processes in this paper are designed based on the DST protocol. • Simulate impacts of subsystem's response characteristics and operating parameters. • Analyze mechanism behind these impacts, combining internal distribution changes. • Can not only obtain response of subsystems, but also monitor internal distribution. The lifetime of vehicular proton exchange membrane fuel cell is one of the key factors restricting the commercialization of fuel cell vehicles. It's well recognized variable load conditions have the greatest impact on fuel cell degradation. Studying dynamic load characteristics is very crucial for fuel cell long-life design and optimal control. Since experiments are not easy to monitor fuel cell internal distribution, the dynamic response studying is commonly implemented in model simulation. The fuel cell system has complicated structures and large differences in length scale, to make up for the insufficient precision and limited research content in existing models, this paper uses an innovative modeling method, Simulink and Fluent co-simulation method to establish a fuel cell system-level model. It can obtain not only response characteristics of auxiliary subsystems and the system dynamic performance, but also the internal physical quantities distribution changes. Multiple simulations and comparisons are made to observe voltage dynamic response and internal concentration distribution. Impacts of subsystem's response characteristics and system's critical operational parameters and mechanism behind them are analyzed. The co-simulation method and obtained results in this paper can be used for future research of fuel cell system-level modeling and provide theoretical basis for dynamic capacity optimization. [ABSTRACT FROM AUTHOR]