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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]

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]

• 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]

Hydrogen energy and polymer electrolyte membrane (PEM) fuel cells become concerned issues in recent years. Nevertheless, the construction of hydrogen refueling infrastructure and hydrogen storage and transportation constrains the commercial development of fuel cells. In this review, sources, production, storage, transportation, and purification methods of hydrogen are extensively reviewed and compared. The advantages of utilizing industrial by-product hydrogen and steam reforming gas in PEM fuel cell systems are analyzed. Using industrial wasted hydrogen can significantly reduce the cost of hydrogen. Also, it is indicated that the onboard hydrogen generation by steam methanol reforming can solve the difficulties of efficient storage and transportation of gaseous hydrogen, which means that methanol has great potential to be a convenient carrier of hydrogen. The effects of impurities contained in the reformate gas are generally introduced. After the methanol steam reforming and pretreatment purification processes, the reformate gas can be fed to PEM fuel cells. Thus, a fuel cell system integrated with onboard hydrogen production and impure hydrogen treatment subsystems is introduced, and key technologies therein for pretreatment purification and in-situ poisoning mitigation methods are reviewed. Finally, suggestions are proposed for further studies. Image 1 • H 2 production, storage, and transportation methods are introduced and compared. • Utilizing impure H 2 and fuel reforming reduce H 2 cost and solve pain points of H 2 storage and incomplete infrastructure. • Key technologies for impure H 2 utilization are reviewed in terms of PEM fuel cell system applications. • A PEM fuel cell system integrated with onboard SMR and PROX is promising to facilitate hydrogen energy development. • Further studies regarding material science and technology related to this field are suggested. [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]

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]

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]

Zinc-air batteries will be a promising candidate for storage energy and power supply due to their high specific energy, environmental compatibility, and economic availability. However, the problem of cycle life of rechargeable zinc-air battery remains unresolved mainly because of dendrite growth of electrodeposited zinc and performance degradation of air electrode. Here we show that rechargeable zinc-air battery with an optimized structure can stably run at large current densities, where air electrode is connected to the charging electrode through a stainless steel frame, and the effective area of charging electrode is larger than that of zinc electrode by way of a trapezoidal structure. This battery structure can control morphological change of zinc electrode and monitor dendrite growth without increasing the battery volume. The results demonstrate that the charge-discharge efficiency of rechargeable zinc-air battery can be improved by nickel foam as gas diffusion layer of air electrode, calcium oxide additive to the electrolyte, or a permanent magnet in parallel with the electrode. The lifetime of rechargeable zinc-air battery can be extended by electrolyte flow or battery structure optimization. These findings will be available for other metal-air batteries and electrolytic metal industry. [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]

Rechargeable zinc-air battery used for energy storage has a serious problem of charging capacity limited by oxygen bubble coalescence. Fast removal of oxygen bubbles adhered to the charging electrode surface is of great importance for improving the charging performance of the battery. Here we show that the law of oxygen bubble growth can be achieved by means of phase-field simulation, revealing two phenomena of bubble detachment and bubble coalescence located in the charging electrode on both sides. Hydrodynamic electrolyte and partial insulation structure of the charging electrode are investigated to solve the problem of oxygen bubble coalescence during charging. Two types of rechargeable zinc-air battery are developed on the basis of different tri-electrode configurations, demonstrating that the charging performance of the battery with electrolyte flow (Ⅰ) is better than that of the battery with the partially insulated electrode (Ⅱ), while the battery Ⅱ is superior to the battery Ⅰ in the discharging performance, cost and portability. The proposed solutions and results would be available for promoting commercial application of rechargeable zinc-air batteries or other metal-air batteries. [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]

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]

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]

Flooding experiments in various conditions are developed and the hydrogen pressure drop is investigated on a two-piece PEM fuel cell in this study. A two-level characteristic of hydrogen pressure drop is observed and analyzed in combination with water droplet accumulation in channels. Based on the characteristic, the flooding process can be divided into four continuous periods, which are the proper period, the humid period, the transitional period and the flooding period. The voltage shows the segmented tendency during these periods. Experimental results show that current and temperature have little influence on the growth rate of the two levels, while the effects of pressure and hydrogen stoichiometry are remarkable. The growth rate can be calculated through the channel dimensions and matches the experimental results well. Hydrogen purge is not a fundamental method to solve flooding. The end of the humid period should be the boundary before flooding. The moist section can be obtained in the beginning part of the humid period. In this section PEM fuel cell is neither flooding nor dehydration by adjusting the cell temperature, which is verified by two additional experiments. This water management is convenient and swift for PEM fuel cell applications and the fault diagnosis. [ABSTRACT FROM AUTHOR]

Highlights: [•] Outlined technological solutions for zinc–air battery’s cyclelife. [•] Summarized the latest development of zinc–air battery. [•] Analyzed failure of zinc electrode, air electrode. [•] Concluded materials, structure, and physicochemical characteristics. [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: 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]

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: The deducted equations about chemical species and temperature are presented to calculate hydrogen–oxygen ignition delay time. Steady-state assumptions for many of the intermediate species are introduced to derive a new simplified 3-step mechanism. The simplified 3-step mechanism for hydrogen–oxygen leads the steady-state assumptions to linear differential equations. The competition among the full mechanism containing 17-, 8- and the simplified 3-step mechanisms is carried out. The resulting closed form solutions describe the low-, the intermediate- and the high-temperature ignition regimes and obtain an “S-shaped curve”. Finally, the two parameters on ignition and extinction of the continuously stirred flow reactor are discussed in detail and the temperature error analysis is given. It reduces the computational costs and supplies theory and methods for understanding autoignition and explosion limits of hydrogen–oxygen mixtures in homogeneous systems. [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]

Flexible zinc‐air batteries (FZABs) have emerged as a promising alternative to lithium‐ion batteries in flexible electronic devices due to the advantages of excellent mechanical properties, high energy density, and notable safety. However, the unclear causes of performance degradation and failure mechanisms of FZABs significantly impede their commercialization. Therefore, extensive research is needed to fully reveal the factors and mechanisms responsible for the performance decline of FZABs. In this review, the failure mechanisms of FZABs' key components, including the Zn anode, solid electrolyte, catalyst air cathode, and electrolyte/electrode interface are analyzed and discussed. To promote further research and development of FZABs, a series of challenges and corresponding strategies are summarized and analyzed. Finally, the future development of FZABs is envisioned. This paper aims to comprehensively elucidate the failure mechanisms of FZABs, guide the development of high‐performance FZABs, and thus promote their commercialization. [ABSTRACT FROM AUTHOR]

Platinum catalysts are widely used in electrocatalytic water splitting for the hydrogen evolution reaction and in hydrogen fuel cells for the oxygen reduction. Nonetheless, the practical use of the noble metal platinum in commercial applications faces significant challenges due to its exorbitant cost and the intricate nature of its synthetic methods. In this work, nitrogen-doped carbon layers covering platinum particles (Pt@NCL) are uniformly distributed on carbon nanofiber (CNF) substrates to synthesize heterojunction catalysts (Pt@NCL-CNF). Multiple characterization methods reveal that the nanoscale ultra-thin carbon layer successfully activates platinum nanoparticles and creates a tremendous accumulation of valence electrons at the interface of the heterojunction catalysts. Furthermore, the greater the number of defects produced in the ultra-thin carbon layer of Pt@NCL-CNF during the reaction, the more active sites are exposed. Therefore, Pt@NCL- CNF exhibits much better hydrogen evolution reaction and oxygen reduction reaction performance in pH-universal electrolytes than the commercial carbon-supported platinum (Pt/C) catalyst. This study elucidates the reaction mechanism, highlighting the crucial role of the ultra-thin carbon layer within the catalyst, and also confirms the catalyst performance in device applications. The proposed method can provide a simple and feasible mass-produced approach for the preparation and application of high performance low platinum catalyst. A heterogeneous bifunctional catalyst (Pt@NCL-CNF) is engineered with a core-shell architecture, incorporating a Platinum core ensconced within an ultra-thin carbon shell, all mounted on supportive carbon nanofibers. [Display omitted] • Pt@NCL-CNF catalysts have better PH-universal HER/ORR performance than commercial Pt/C. • Illustrate the reaction mechanism for Pt@NCL-CNF heterojunction. • Reveal the interface electronic interaction and ultra-thin carbon surface formative defects. • Confirm the feasible route of ultra-thin carbon layer covering Pt-based catalysts. • Validation of the application potential of Pt@NCL-CNF in devices. [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]

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]

The estimation of lithium-ion battery capacity is of great importance in electric vehicle battery management system (BMS), its results will contribute to controlling the battery to have both excellent output performance and long life. However, there still lacks generalized approaches for different kinds of batteries with high estimating accuracy. Therefore, an experimental feature interval approach for LiFePO 4 (LFP) and LiNi x Co y Mn 1-x-y O 2 (NCM) capacity estimating is proposed in this paper. Firstly, two concepts of feature interval and remaining charge electricity (RCE) are defined, then partial charging electricity based on incremental capacity analysis is used to estimate capacity. According to the results, there is a strong linear relationship between RCE and capacity. We can obtain capacity directly through this linear function by calculating RCE from the feature interval to the end of charge. A satisfying estimation performance is verified by the results of another experiment data, where the accuracy is more than 98.5%. Moreover, it is found that this approach can be used to NCM battery by modifying the linear fitting weights. This proposed approach is verified in NASA dataset, with the estimating deviations less than 2.4%. Further, the proposed estimating approach may serve as a reference for batteries from other manufactures. • Concepts of feature interval and remaining-charge-electricity are proposed. • Remaining-charge-electricity exhibits a strong linear relation with capacity. • The experimental results show the deviations of estimation are less than 2.5%. [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]