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Proton exchange membrane (PEM) fuel cells require high-purity hydrogen. Carbon monoxide (CO) is a common impurity in hydrogen that significantly reduces the performance of PEM fuel cells. Currently, there are two main methods to eliminate the effect of CO: direct removal of CO and increasing the CO tolerance of PEM fuel cells. Each method has its advantages and disadvantages. It is important to consider the specific application when choosing between these methods. This review summarizes the mechanism of CO poisoning and its effect on the operation and durability of PEM fuel cells, as well as hydrogen purification and CO-tolerant techniques. Then analyzes the advantages and disadvantages of commonly used techniques and predicts future research directions for PEM fuel cells using impure hydrogen containing CO, aiming to expand the application scenarios, and promote the large-scale application of PEM fuel cells. • Mechanisms of CO poisoning on PEM fuel cells are summarized. • Impacts on potential oscillation and durability are reviewed. • Various hydrogen purifications techniques are compared. • CO-tolerant techniques for PEM fuel cells are discussed. • Futher directions for research on eliminating CO effect are presented. [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]

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]

• 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]

Highlights • The main cause of the voltage decay of zinc-air fuel cell at high current density is validated. • Isolate ZnO precipitations from the electrolyte solution. • Optimize the flow field to suppress anode passivation at high current density. • A zinc-air fuel cell system has achieved the highest electrolyte capacity of 1025 Ah L−1 to date. • The optimization strategy can be generalized to other metal-air fuel cells with aqueous electrolytes. Abstract Metal-air fuel cells are regarded as potential alternatives of power supply due to their high specific energy. However, the excessive accumulation of reaction products leads to performance degradation, low energy density, and short service life, hampering more widespread application. This study focuses on enormously increasing the fuel cell system energy density by electrolyte isolation and management. Filters are used to isolate metal oxide (including ZnO, MgO, and Al 2 O 3) from supersaturated electrolyte solutions in fuel cells. The filtration efficiency is close to 100%. The flow field is optimized to suppress the anode passivation. In zinc-air fuel cells (ZAFCs), the ratio of discharge capacity to electrolyte volume (electrolyte capacity) is up to 1025 Ah L−1, and the discharging voltage still remains stable. The zinc-air fuel cell system (ZAFCS) exhibits high energy density, high stability, and low cost, rendering this type of metal-air fuel cell a promising energy storage in electric vehicles. [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]

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]

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]

• Evaluation methods of fuel cell stack consistency are reviewed. • Influencing factors and reasons of fuel cell stack consistency are discussed. • Main detection methods of MEA parameters are summarized and compared. • Effect of MEA parameters on fuel cell performance and lifetime are analyzed. • Methods and directions for improving fuel cell stack consistency are proposed. The proton exchange membrane fuel cell (PEMFC) is an ideal energy conversion device. The performance of a fuel cell stack depends on the coordination of many aspects. Consistency is an important factor affecting the performance of fuel cell stacks. However, it is still not very clear how to evaluate and improve fuel cell stack consistency. In this review, the evaluation methods of fuel cell stack consistency are summarized, and various influencing factors of consistency are discussed, including operation conditions, operating mode, control strategy, structure design, component materials, and manufacturing processes. The membrane electrode assembly (MEA) is the core component of PEMFC, and the differences between MEAs also determine the voltage consistency of fuel cell stacks. The detection of MEA parameters is an important method to evaluate its quality, and the main detection methods of MEA parameters are reviewed and compared. Moreover, the influence of MEA parameters on the performance and lifetime of fuel cell stacks is systematically analyzed in this review. Finally, research orientations and methods for improving fuel cell stack consistency are proposed from MEA preparation, stack design, and operation optimization. [ABSTRACT FROM AUTHOR]

The zinc air fuel cell (ZAFC) is a promising candidate for electrical energy storage and electric vehicle propulsion. However, its limited durability has become a major obstacle for its successful commercialization. In this study, 2-cell stacks, 25 cm² cells and three-electrode half-cells are constructed to experimentally investigate the degradation characteristics of the air cathode. The results of electrochemical tests reveal that the peak power density for the 25 cm 2 cell with a new air cathode is 454 mW cm −2 , which is twice as the value of the used air cathode. The electrochemical impedance analysis shows that both the charge transfer resistance and the mass transfer resistance of the used air cathodes have increased, suggesting that the catalyst surface area and gas diffusion coefficient have decreased significantly. Additionally, the microstructure and morphology of the catalytic layer (CL) and gas diffusion layer (GDL) are characterized by scanning electron microscopes (SEM). SEM results confirm that the micropores in CL and GDL of the used air cathode are seriously clogged, and many catalyst particles are lost. Therefore, the performance degradation is mainly due to the clogging of micropores and loss of catalyst particles. Furthermore, hypotheses of degradation mechanism and mitigation strategies for GDL and CL are discussed briefly. [ABSTRACT FROM AUTHOR]

Morphology control is crucial both for zinc–air batteries and for zinc–air fuel cells during zinc regeneration. Zinc dendrite should be avoided in zinc–air batteries and zinc pellets are yearned to be formed for zinc–air fuel cells. This paper is mainly to analyze the mechanism of shape change and to control the zinc morphology during charge. A numerical three-dimensional model for zinc regeneration is established with COMSOL software on the basis of ionic transport theory and electrode reaction electrochemistry, and some experiments of zinc regeneration are carried out. The deposition process is qualitatively analyzed by the kinetics Monte Carlo method to study the morphological change from the electrocrystallization point of view. Morphological evolution of deposited zinc under different conditions of direct currents and pulse currents is also investigated by simulation. The simulation shows that parametric variables of the flowing electrolyte, the surface roughness and the structure of the electrode, the charging current and mode affect morphological evolution. The uniform morphology of deposited zinc is attained at low current, pulsating current or hydrodynamic electrolyte, and granular morphology is obtained by means of an electrode of discrete columnar structure in combination with high current and flowing electrolyte. [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: A zinc air fuel cell (ZAFC) stack with inexpensive manganese dioxide (MnO2) as the catalyst is designed, in which the circulation flowing potassium hydroxide (KOH) electrolyte carries the reaction product away and acts as a coolant. Experiments are carried out to investigate the characteristics of polarization, constant current discharge and dynamic response, as well as the factors affecting the performance and uniformity of individual cells in the stack. The results reveal that the peak power density can be as high as 435 mW cm−2 according to the area of the air cathode sheet, and the influence factors on cell performance and uniformity are cell locations, filled state of zinc pellets, contact resistance, flow rates of electrolyte and air. It is also shown that the time needed for voltages to reach steady state and that for current step-up or current step-down are both in milliseconds, indicating the ZAFC can be excellently applied to vehicles with rapid dynamic response demands. [Copyright &y& Elsevier]

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]

Abstract: Fuel cell vehicle commercialization and mass production are challenged by the durability of fuel cells and could be promoted by accelerated lifetime evaluating methods. In this paper, an arithmetic equation of fuel cell lifetime is presented, which is relating with load changing cycles, start–stop cycles, idling time, high power load condition and the air pollution factor. Basing on the practical data gathered from a fuel cell bus and the test results of a fuel cell stack in laboratory, the calculated lifetime fits the bus real running lifetime very well. It is shown that the automotive fuel cell lifetime mightily depends on driving cycles, and the potential lifetime in different operating mode can be effectively predicted by using this method with about 300h test time. The test results also show that the effect of start–stop cycling on fuel cell lifetime can be almost ignored if the stack open circuit voltage is dispelled quickly after fuel cell stops operating. It is worthwhile that from this quick lifetime-evaluating method we can find many possible directions to improve fuel cell durability. [Copyright &y& Elsevier]

Abstract: A one-dimensional mathematic computational fluid dynamics model of a proton exchange membrane (PEM) fuel cell is presented in this paper to simulate the transient behavior of hydrogen pressure in the flow field during a typical dynamic process—the purge process. This model accounts for the mechanism of pressure wave transmission in the channels by employing the characteristic line method. A unique parameter—pressure swing, which represents the top value of pressure variation at certain point in the channel during the purge process, is brought up and studied as well as the pressure drop. The pressure distribution along the channel and the pressure drop during the purge process for different operating pressures, lengths of purge time, stoichiometric ratios and current densities are studied. The results indicate that the distributed pressure, pressure drop and pressure swing all increase with the increment of operating pressure. With a high operating pressure a second-falling stage can be seen in the pressure drop profile while with a relatively low operating pressure, a homogeneous distribution of pressure swing can be attained. A long purge time will provide enough time to show the whole part of the pressure drop curve, while only a part of the curve can be attained if a short purge time is adopted, but a relatively uniform distribution of pressure swing will show up at the moment. Compared with the condition of stoichiometric ratio 1, the pressure drop curve decreases more sharply after the top value and the pressure swing displays a more uniform distribution when the ratio is set beyond 1. Different current densities have no apparent influence on the pressure drop and the pressure swing during this transient process. All the distribution rules of related parameters deducted from this study will be helpful for optimizing the purging strategies on vehicles. [Copyright &y& Elsevier]

Abstract: Research on hydrogen pressure characteristics was performed for a fuel cell stack to supply a rule of hydrogen pressure drop for flooding diagnostic systems. Some experiments on the hydrogen pressure drop in various operating pressure, temperature, flowrate and stack current conditions were carried out, and hydrodynamic calculation was managed to compare with the experiment results. Results show that the hydrogen pressure drop is strongly affected by liquid water content in the flow channel of fuel cells, and it is not in normal relation with flowrate when the stoichiometric ratio is inconstant. The total pressure drop can be calculated by a frictional pressure loss formula accurately, relating with mixture viscosity, stack temperature, operating pressure, stoichiometric ratio and stack current. The pressure drop characteristics will be useful for predicting liquid water flooding in fuel cell stacks before flow channels have been jammed as a diagnostic tool in electric control systems. [Copyright &y& Elsevier]

Abstract: Mathematical models were applied to predict the results of different strategies to improve automotive fuel cell driving systems. First, a fuel cell system was optimized on the base of a fuel cell stack model. Then calculations about the utilization of recovered energy from air exhaust steam were carried out. Two methods to combine the turbocharger with an electric compressor, namely in series and in parallel, were evaluated for a fuel cell system. Finally, research on the effect of removing the big power DC/DC converter, which is located between the fuel cell system and the driving motor, was conducted for a fuel cell driving system. The main results show that it is highly advantageous to connect the turbocharger with an electric compressor in series than in parallel; and that the fuel cell driving system without DC/DC converter before its motor could reach much higher performance characteristics, and even be so in lower power range while the cell voltage was designated to be lower. [Copyright &y& Elsevier]

This paper is to present a test platform for automotive fuel cell systems and report some test results on this platform. The test platform was developed based on a test bed of internal combustion engine with a dynamometer, the dynamometer acted as both a load and a measurement instrument. A fuel cell engine, a DC/DC converter and an induction traction drive motor with a DC/AC inverter were integrated to a system and were tested in the platform. Test results of one fuel cell system showed that the efficiency was 41% (LHV) while 50 kW of electrical power is produced in the engine; the cell current density was 400 mA/cm2 when 0.65 V of average cell voltage is obtained in the stacks; the maximum mechanical power of the fuel cell system was 41 kW, and the best specific fuel consumption was 102 g/kWh. This test platform is feasible for evaluating all components of fuel cell systems, such as stacks, parasitic powers, engines, DC/DC converters and traction drive motors; and in this platform it is convenient to uncover problems of electromagnetism compatibility in the fuel cell systems before being mounted into vehicles. [Copyright &y& Elsevier]

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]

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]

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]

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 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]

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]

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]

A series of novel and high-performing oxygen reduction reaction (ORR) catalysts based on N/S-Me-doped (Me = Fe, Co, Ni) hierarchical porous carbons (N/S-Fe-HPC, N/S-Co-HPC and N/S-Ni-HPC) have been synthesized by a green and cost-effective method combining a simple silicate templated two-step graphitization of the impregnated carbon. After optimization of the porous structures and enlargement of the BET specific surface area by combining the silicate template and the Fe-, Co- and Ni-metal sulfates, the obtained N/S-Me-HPC catalysts exhibit excellent ORR performance in both alkaline and acidic media. In particular, N/S-Fe-HPC shows superior catalytic ORR performance compared to commercially available 20% Pt/C in alkaline medium reflected by the onset potential of 0.98 V and half-wave potential of 0.86 V, and comparable ORR activity in acidic media with the onset potential of 0.84 V and half-wave potential of 0.73 V. In addition, the N/S-Fe-HPC catalyst exhibits excellent long-term stability and a high methanol tolerance. The outstanding electrochemical performance of such catalysts thereby makes it possible for these non-noble catalysts to replace the commercial Pt/C in both fuel cell and metal–air battery applications. [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]

Abstract: A voltage degradation model for the low-pressure proton exchange membrane fuel cell (PEMFC) stack used in a fuel cell bus is presented: (1) the oxygen concentration term was derived from the PEMFC output voltage equation, and the concept of oxygen concentration resistance coefficient was introduced; (2) a 5kW low-pressure PEMFC stack was used in this study. Two similar tests were carried out before and after the stack operating in the driving cycle for 640h. First, the ohmic losses under different temperatures were measured using the current interrupt method and formulized with linear fitting method. Then, the oxygen concentration term was studied by the experiments with different air stoichiometric ratios while keeping the other operating parameters unchanged. The oxygen concentration resistance coefficient was obtained from the difference of voltages for the PEMFC stack in different air stoichiometric ratios using the genetic optimization algorithm. Then, the activation loss was obtained based on the PEMFC output voltage, the ohmic loss, and the concentration loss. The degradation model of the stack was built finally by comparing the two test results; (3) the correlation of the model to the actual experimental data is good; (4) the overvoltage of the stack with aging was analyzed using this model. The analysis showed that the activation overvoltage dominated the stack loss with about 80% of the total losses, followed by the ohmic loss. The concentration loss almost does not change with aging in the driving cycle condition; (5) the comparison of the simulation with the actual data from the PEMFC bus running for 30,000km indicated that after 36,000km the rated power of the PEMFC bus must be reduced. [Copyright &y& Elsevier]

• 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]