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Due to its zero emissions, high efficiency and low noise, proton exchange membrane fuel cell (PEMFC) is full of potential for the application of vehicle power source. Nonetheless, its lifespan and durability remain multiple obstacles to be solved before widespread commercialization. Frequent exposure to non-rated operating conditions could considerably accelerate the degradation of the PEMFC in various forms, thus reducing its durability. This paper first analyzes degradation mechanisms of PEMFCs under typical automotive operating conditions, including idling, startup-shutdown, dynamic loads, and cold start. The corresponding accelerated stress testing methods are also discussed. Then, as the impurities existed in the reaction gas source and generated from the degradation of the PEMFC itself may occur under all automotive conditions, the degradation mechanisms caused by impurity contamination are classified and reviewed in detail. After that, the techniques proposed by researchers to enhance the durability of PEMFCs are presented from four aspects: membrane electrode assembly materials, bipolar plates and flow fields design, stack assembly, and cell control strategies. The challenges in the field and the prospects for the future are summarized and analyzed at the end. The aim of this work is to provide guidelines for improving the durability of PEMFCs in vehicle applications.

Operation strategy of the integrated wind-hydrogen system is the key to ameliorate the negative impacts of the fluctuated wind power on the hydrogen production capacity and durability of electrolyzer. However, the desalination system supplying pure water to electrolyzers was neglected previously, with which the practical operation strategy of the hybrid wind-hydrogen-desalination system has yet to be investigated. In this work, we proposed a novel control strategy named step-by-step start for wind-hydrogen-desalination system consisting of wind turbine, proton exchange membrane water electrolyzer (PEMWE), battery and batch reverse osmosis (BRO) desalination system, which can produce hydrogen and fresh water. Real-time wind data from Shenzhen, China were used as input to the integrated system. The results demonstrate the wind-hydrogen-desalination system with the proposed strategy can improve the hydrogen production by 17.55 %, the energy efficiency by 17.68 % in August, and increase hydrogen production by 10.29 %, the energy utilization efficiency by 10.44 % in December compared with traditional strategies. At low wind speeds, the switching times of PEMWE for this strategy is higher than the other strategies, while at high wind speeds, the switching times of PEMWE is significantly decreased. This work provides insights into the practical operation strategy for wind-hydrogen-desalination system depending on the real-time wind speed.

As a promising carbon-neutral technology, the polymer electrolyte membrane fuel cell (PEMFC) is gaining considerable attention over the past decades. Many problems in PEMFC performance and durability can be ultimately ascribed to the flow-field design, which is a complex and systematic work owing to the inherent sophisticated nature of the PEMFC with multicomponent mass transportation and multiphysics field coupling. This paper presents a critical review of the state-of-the-art flow-field designs and an in-depth analysis of the key problems involved from a perspective of efficient mass transport within the PEMFC. In particular, flow-optimization principles are discussed specifically for the enhancement in reactant mass transfer, water management, optimized opening ratio, uniformity of flow distribution, and choice of appropriate numerical approaches assisting the flow-field design. The material formability and forming accuracy and their effects are also discussed for metallic bipolar plates. The objective of this review work is to present a comprehensive overview of the problems, progresses, and perspectives of the flow-field designs for bipolar plates in PEMFC and provide a general theoretical instruction for present and future relevant R&D activities that aim at high-performance, durable, and low-cost fuel cells.

A new system which combined cooling, heating and power (CCHP) and based on a solid oxide fuel cell (SOFC) with a LiBr absorption chiller and water tank is investigated. The influence of the working conditions of the SOFC on its steady-state and dynamic performances are studied. Meanwhile, the effects of working conditions of the stack and absorption chillers on multi-criteria assessments are also studied. The result shows that the operating temperature and current density can greatly affect the performance of the stack and system, which can also be influenced by the working conditions of the absorption chiller simultaneously. Increasing the operating temperature can greatly increase the electrical power, and the thermal power will be reduced correspondingly. Increasing the mass flows of the cooling, chilled and hot waters increase the cooling power and coefficient of performance (COP), thereby reducing the heating power.

The cathode channel of an air-cooled proton exchange membrane fuel cell (PEMFC) adopts a straight flow channel for air supply and heat dissipation. Considering the present cathode flow channel, the type of the anode flow channel has an impact on the water and heat management of air-cooled PEMFCs. In this study, we established three-dimensional (3D) multi-physics field models to investigate the performance of three types of anode flow channels: straight flow channel, improved straight flow channel, and serpentine flow channel. Among the three flow channel types under the same voltage condition, the straight flow channel exhibited the best temperature control ability, smallest cathode stoichiometric ratio at the peak power density output, and highest power density. Moreover, the temperature distribution of membrane electrode assembly in the straight flow channel was the most uniform. The coupling of the high cathode air speed and electro-osmotic drag (EOD) easily caused the water loss of anode and cathode membrane electrodes and had an adverse effect on the performance of the fuel cell. The straight flow channel demonstrated the optimal performance with the highest water content.

Fuel gas utilization and water management are particularly challenging integrated engineering problems in hydrogen–oxygen proton exchange membrane fuel cell (H2/O2 PEMFC) systems. Herein, a standardized process is adopted to evaluate the performance and investigate the degradation mechanisms of a PEMFC with dual exhaust gas recirculation. The purpose of incorporating recirculation subsystems in the fuel cell is to achieve a high fuel gas utilization rate and realize effective water management inside the stack, which consists of 3D‐printed ejectors and a customized recirculation pump. Evaluation of the electrochemical performance degradation and morphological characterization of the fuel cells under different operating strategies are performed after 50 h durability experiments. At a current density of 400 mA cm−2, the performance degradation rates of the stack decrease from 16.50% to 7.49% and 0.71% in the ejector and recirculation pump operation strategies, respectively. The results show that using exhaust gas recirculation devices (ejector/pump) in the fuel cell stack can help in effectively mitigating water flooding and chemical degradation of the membrane electrode assembly. The findings of the study provide a perspective on the exhaust gas recirculation behavior and contribute to the engineering application of H2/O2 PEMFCs.

Abstract Retrosynthesis is at the core of organic chemistry. Recently, the rapid growth of artificial intelligence (AI) has spurred a variety of novel machine learning approaches for data-driven synthesis planning. These methods learn complex patterns from reaction databases in order to predict, for a given product, sets of reactants that can be used to synthesise that product. However, their performance as measured by the top-N accuracy in matching published reaction precedents still leaves room for improvement. This work aims to enhance these models by learning to re-rank their reactant predictions. Specifically, we design and train an energy-based model to re-rank, for each product, the published reaction as the top suggestion and the remaining reactant predictions as lower-ranked. We show that re-ranking can improve one-step models significantly using the standard USPTO-50k benchmark dataset, such as RetroSim, a similarity-based method, from 35.7 to 51.8% top-1 accuracy and NeuralSym, a deep learning method, from 45.7 to 51.3%, and also that re-ranking the union of two models’ suggestions can lead to better performance than either alone. However, the state-of-the-art top-1 accuracy is not improved by this method. Graphical Abstract

Hydrogen has emerged as a new energy vector beyond its usual role as an industrial feedstock, primarily for the production of ammonia, methanol, and petroleum refining. In addition to environmental sustainability issues, energy-scarce developed countries, such as Japan and Korea, are also facing an energy security issue, and hydrogen or hydrogen carriers, such as ammonia and methylcyclohexane, seem to be options to address these long-term energy availability issues. China has been eagerly developing renewable energy and hydrogen infrastructure to meet their sustainability goals and the growing energy demand. In this review, we focus on hydrogen electrification through proton-exchange membrane fuel cells (PEMFCs), which are widely believed to be commercially suitable for automotive applications, particularly for vehicles requiring minimal hydrogen infrastructure support, such as fleets of taxies, buses, and logistic vehicles. This review covers all the key components of PEMFCs, thermal and water management, and related characterization techniques. A special consideration of PEMFCs in automotive applications is the highlight of this work, leading to the infrastructure development for hydrogen generation, storage, and transportation. Furthermore, national strategies toward the use of hydrogen are reviewed, thereby setting the rationale for the hydrogen economy.

The Self-adaptive control of the temperature can achieve the start of fuel cell at different operating temperatures, which is very important for the successful cold-start of the air-cooled PEMFC. The temperature distribution characteristics during the cold-start process were analyzed based on adaptive temperature recognition control in this paper. Preheating model and cold-start model were established and the optimal balance between the hot air flow rate and the temperature required to promote a uniform temperature distribution in the stack was explored in the preheating stage. Finally, the non-equilibrium mass transfer, as well as the temperature rise in the catalyst layer and gas diffusion layer with different current densities, were analyzed in the start-up stage. The results indicate that the air-cooled PEMFC stack can be successfully started up at -40 °C within 10 min by means of external gas heating. The current density and air velocity have significant impacts on the temperature of air-cooled PEMFC stack. Dynamic analysis of air-cooled PEMFCs and real-time monitoring are suitable for machine learning and self-adaptive control to set the operation parameters to achieve successful cold start. Optimize the matching of load current and cathode inlet speed to achieve thermal management in low temperature environment.

The proton exchange membrane fuel cell (PEMFC) demonstrates high commercial competitiveness due to its advantages: low operating temperature, high power/mass ratio, fast response, no emission, and low noise. Thermal and water management remains a challenging issue for ensuring the fuel cell’s performance at steady-state and dynamic conditions. The cathode moisture condensation uses a semiconductor cooler to effectively remove excess water from the PEMFCs and reduce the probability of flooding of the stack. The stack’s voltage uniformity is an essential factor that affects the performance and lifetime of PEMFC. This paper investigates the dynamic response characteristics of the voltage uniformity of a PEMFC stack under cathode moisture condensation conditions. The results show that the condensation temperature at 10°C during the steady-state or transient operation of the PEMFC can effectively optimize the stack’s performance. Compared to conventional PEMFC, applying the cathode moisture condensation technology to the PEMFC stack increases the stack voltage by 6% and decreases the voltage uniformity by up to 30%. This self-water-removal technology effectively improves the voltage uniformity of the stack, which then increase the stack durability.

Effective water management is particularly critical for fuel cells fed by hydrogen/oxygen. An ejector is an optimal device for the gas recirculation subsystem of a proton exchange membrane fuel cell (PEMFC) and is usually adopted for the auxiliary drainage of hydrogen/oxygen stacks. To explore the performance degradation of the fuel cells operating in dual ejector-based recirculation mode for both the anode and cathode, the dynamic characteristics of gas purging of the PEMFC was studied experimentally and the effects of the electrolyte and gas management strategy of the fuel cell on performance degradation were investigated in detail by using the measurement of polarization curves, electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV), and scanning electron microscopy (SEM) images of membrane electrode assembly (MEA) cross sections. The results indicated that the fuel cell with Nafion® 212 operating in the dual ejector-based recirculation mode has a better performance than that operating in the dead-ended mode, with total electrochemical surface area (ECSA) degradation rates of 10.61% and 17.02%, respectively. The ejector in the recirculation mode can accelerate the removal of liquid water from the fuel cell flow channel, avoiding water flooding and performance deterioration of fuel cells during long-term operation..

Carbon corrosion in catalysts is one of the main limitations of proton exchange membrane fuel cell (PEMFC) lifetime. Water flooding can accelerate the carbon corrosion rate in PEMFCs, especially at high current densities. Increasing the operation temperature can effectively mitigate water flooding and improve the water management performance of PEMFCs. However, elevated temperatures also increase the carbon corrosion reaction kinetics, accelerating the degradation of PEMFCs lifetime. To study the effects of temperature on carbon corrosion at high current densities, PEMFCs operating for 100 h at 65 °C and 90 °C under 1600 mA·cm−2 were investigated. The results showed that the electrochemical active surface area (ECSA) of the cathode catalyst layer decreased by 9.16% and 13.92% at 65 °C and 90 °C, respectively. The reduction rate of the catalyst layer in the nonflooding area increased as the operating temperature increased. However, the reduction rate decreased by 69.82% and 54.04% at 65 °C and 90 °C, respectively, at the outlet of the PEMFC. Therefore, water flooding is the key issue causing lifetime degradation, and water management is essential for PEMFCs that operate at high current densities, especially at the outlet of PEMFCs.

A capillary pumped loop (CPL) is a two-phase thermal control device applied in cooling electronic devices. A two-dimensional conjugate numerical model of a miniature flat-plate capillary evaporator is presented in order to describe liquid and vapor flow, heat transfer and phase change in the porous wick structure, liquid flow and heat transfer in the compensation cavity and heat transfer in the vapor grooves and the metallic wall. The entire evaporator is solved with the SIMPLE algorithm as a conjugate problem. The shape and location of the vapor-liquid interface inside the wick are calculated, and a side wall effect heat transfer limit is introduced to estimate the evaporator’s heat transport capability. The influence of various wall materials on the evaporator’s performance is discussed in detail. The results suggest that an evaporator with a combined wall is capable of dissipating high heat flux and stabilizing the temperature of electronic devices at a moderate temperature level.

The paper presents an analysis of the performance of a Proton Exchange Membrane (PEM) fuel cell, which is equipped with a flow field design featuring dual inlets and outlets, while operating under conditions of excess stoichiometry. These experiments were conducted using a fuel cell system connected to a station that allowed for the precise adjustment of gas flow rates. During the initial phase of experimentation, various proportions of excess oxygen were systematically applied, while maintaining constant hydrogen flow rates of 80 mL/min and 100 mL/min. Particularly noteworthy, for the case of a 100 mL/min hydrogen gas flow rate and the optimized excess oxygen proportion of 150%, further experiments were undertaken to ascertain the ideal humidification conditions. The outcomes of these experiments revealed that a hydrogen gas flow rate of 100 mL/min consistently outperformed the 80 mL/min flow rate in terms of fuel cell performance. Moreover, it was observed that the introduction of excess oxygen significantly improved performance, up to a 50% oxygen proportion for the 80 mL/min hydrogen flow rate and up to a 150% proportion for the 100 mL/min hydrogen flow rate. One intriguing observation pertained to the influence of humidification. Specifically, it was found that the utilization of a humidification temperature of 100°C, or the absence of humidification altogether, resulted in notably diminished power output. In contrast, intermediate humidification temperatures of 60°C, 70°C, 80°C, and 90°C consistently yielded identical maximum power points (MPP) when combined with a 150% excess oxygen supply and a hydrogen flow rate of 100 mL/min. The twin inlet-twin outlet flow field provides a slight advantage over the conventional serpentine flow field in the overall analysis. [ABSTRACT FROM AUTHOR]