Reliable auxiliary heating-free self-startup under low-temperature conditions is crucial for enhancing the environmental adaptability, reducing costs, and expanding the commercial application prospects of fuel cells. However, current research primarily focuses on single-cell levels, while studies on commercial high-power stacks, particularly regarding the influence of startup current loading strategies on the failure mechanisms of end-plate cells, remain insufficient. To address this, this study conducted multiple cold-start experiments under various ambient temperatures and combined them with one-dimensional model simulations to systematically analyze the effects of loading strategies on coolant temperature, stack voltage uniformity, and the performance of end-plate cells. Experimental and simulation results reveal that cold-start failure is primarily caused by the sudden voltage drop of end-plate cells due to the "end-plate effect" and ice blockage in the later stage. The study demonstrates that, compared to the step loading strategy, the linear loading strategy better balances the competition between heat generation and ice formation during startup, allowing system heat generation to gradually dominate. This assists end-plate cells in achieving temperature breakthrough and completing the startup process. Experimental results show that the linear loading strategy significantly improves startup performance, reducing startup time by 7 s and 2 s at -15 degrees C and - 20 degrees C, respectively, and successfully extending the auxiliary heating-free startup limit to -30 degrees C. Furthermore, this strategy effectively enhances stack voltage uniformity, with the voltage non-uniformity coefficient reduced by up to approximately 80.67%. This study provides an effective control method to enhance the all-climate operational capability of PEMFCs.

This study investigated variations in hydrogen concentration, high-frequency impedance, stack voltage, and typical monolithic voltage under different operating conditions based on the ultrasonic online monitoring device. The effects of hydrogen concentration and inlet stoichiometric ratio on performance heterogeneity were quantitatively analyzed. Additionally, the stack performance under different anode circulation modes was examined. The results indicated that elevated temperature promoted membrane dehydration and shrinkage, reducing the water phase volume and narrowing the transport channels, leading to a decrease in nitrogen crossover. Under dry membrane conditions, the stack becomes more sensitive to hydrogen concentration fluctuations, resulting in reduced output voltage stability. The monolithic voltage was attenuated slowly when hydrogen concentration fell below 60 %, with a significant sawtooth-type decay when concentration dropped below 45 %. The lowest monolithic voltage consistently occurred in cells near the inlet, where higher gas flow rates resulted in less diffusion of hydrogen. In the current density range of 0.1-0.3 A cm-2, the ejector experienced secondary flow back, while optimal performance was achieved at 0.9 A cm-2, with circulation rate of 52.3 %. System efficiency in parallel mode consistently exceeded that of single blower mode, with a maximum efficiency improvement of 1.02 % at a current density of 1.4 A cm-2.

To improve waste heat utilization and clarify the electricity–thermal–hydrogen coupling mechanism in integrated backup power systems, a 60 kW proton exchange membrane fuel cell (PEMFC)–solid-state hydrogen storage system (SSHSS) is developed. An electricity–thermal–hydrogen coordinated operation architecture and a coordinated energy management strategy are proposed and experimentally validated under startup, rated-load, peak-load, and reduced-pressure conditions. Furthermore, a comprehensive electricity–thermal–hydrogen evaluation framework is established to characterize multi-energy-flow coordination. The results demonstrate that the proposed strategy enables stable coordinated operation of the PEMFC, SSHSS, energy storage system, and thermal management system under all operating conditions. The system operates continuously for 120 min at 60 kW and 10 min at 90 kW, while maintaining stable operation under reduced-pressure conditions. The overall energy utilization ratio (EUR) remains above 50%, the hydrogen supply pressure stability index (HSPS) fluctuates by less than 10%, and the theoretical waste heat recovery ratio ( ) increases from approximately 20% during startup to 30% under steady operation. The results reveal that the integrated system exhibits demand-driven, waste heat-mediated, and multi-dimensional synergistic characteristics, with stable operation governed by the dynamic coordination of electrical, thermal, and hydrogen energy flows. The proposed framework provides a unified basis for the evaluation, operation, and design of PEMFC–SSHSS integrated backup power systems.

In high-power proton exchange membrane fuel cell (PEMFC) systems, insufficient hydrogen supply at the anode can induce hydrogen starvation, resulting in voltage non-uniformity, accelerated degradation, and reduced durability. However, the coupled effects of hydrogen concentration variation, nitrogen permeation, and hydrogen recirculation remain insufficiently understood at the stack level. This study experimentally investigates the output characteristics and voltage consistency of a high-power multi-cell PEMFC stack under varying hydrogen concentrations, current densities, and recirculation flow rates. By integrating electrochemical kinetics with thermodynamic analysis, the voltage decay mechanism induced by hydrogen dilution is quantitatively clarified. Results show that nitrogen permeation significantly reduces the anode hydrogen partial pressure and increases high-frequency resistance (HFR). As hydrogen concentration decreases, voltage consistency exhibits a two-stage deterioration behavior, with rapid degradation occurring below a critical concentration threshold that depends on recirculation intensity. Increasing the recirculation speed from 2500 rpm to 4500 rpm shifts the local starvation threshold from ~45% to ~17%. At 1.0 A cm−2, the hydrogen concentration difference between the anode inlet and outlet increases from 8.6% to 47%, indicating severe axial non-uniformity. Voltage loss analysis shows that reducing hydrogen concentration from 90% to 40% results in a total loss of ~46 mV, dominated by anode activation loss (54%) and thermodynamic loss (27%), while ohmic loss contributes 13%. These findings provide quantitative guidance for optimizing hydrogen supply strategies in high-power PEMFC systems.

Optimizing hydrogen supply control is critical to enhancing the efficiency and lifespan of fuel cell systems. Nitrogen permeation across the membrane dilutes hydrogen concentration and increases the risk of hydrogen starvation. However, the absence of real-time, cost-effective methods to monitor or estimate nitrogen concentration hinders efforts to optimize hydrogen utilization and mitigate hydrogen starvation. To address these challenges, this study establishes an anode pressure drop model incorporating key operational parameters, including nitrogen concentration. Then, a series of experiments under various operating conditions are conducted on a 130 kW full-scale fuel cell system to validate the model, with the ultrasonic sensor employed to measure the flow rate and gas concentration within the hydrogen recirculation loop. Finally, a nitrogen concentration estimation algorithm based on the model is proposed and experimentally verified. Results demonstrate that the mean absolute error of the estimated nitrogen concentration is around 1 vol% under steady-state and dynamic conditions. This work employs a mechanistic model based on the relationship between gas composition and viscosity to elucidate the coupled variation of anode pressure drop and nitrogen concentration. Compared with existing solutions, the proposed nitrogen concentration estimation algorithm features high accuracy, low cost, and robustness against stack degradation, and can be implemented in controllers for in-situ nitrogen concentration estimation. These advancements enable predictive hydrogen supply regulation, which is anticipated to improve the system's durability and efficiency.

Part of the performance loss of proton exchange membrane fuel cell (PEMFC) can be recovered in time by stopping or changing operating conditions, and is defined as reversible loss. Under the increasingly stringent lifespan requirements and the imperative to reduce PGM loading in fuel cells, accurate identification and recovery of reversible losses are key technologies to enhance the durability of current PEMFC stacks. However, there are few reports on the recovery characteristics of reversible losses in full-scale and high-power stacks. The differences in reversible loss recovery and consistency across various typical automotive operating conditions for high-power stacks have not been thoroughly investigated. In this paper, a 100 kW fuel cell stack was tested under four typical vehicle conditions: Idle, Rated, Dynamic and startup-shutdown (SUSD) conditions. The reversible loss recovery and consistency difference of the stack under different accelerated stress test (AST) conditions are investigated. The results indicate that both prolonged downtime and adjustments in operating parameters can facilitate the recovery of reversible losses, but altering operating temperatures and gas humidity proved to be more convenient and efficient. Under constant conditions such as Idle and Rated, the degradation and recovery of individual cells were relatively uniform. Under Dynamic and SUSD conditions, the recovery of cells was more significantly influenced by their positions, closely related to current and operating parameters. The findings of this study provide important references for the online recovery of reversible degradation in high-power fuel cell stacks.

Electrocaloric (EC) cooling technology, which offers advantages of high efficiency, stability, and miniaturization, faces challenges such as high driving electric fields and narrow operating temperature ranges. A synergistic “grain coarsening‐gradient compositing” strategy is proposed for sodium bismuth titanate (BNT)‐based ceramics with a simple compositional design. Through microstructural optimization, the composite ceramics achieve an average grain size of ≈4 µm, significantly reducing grain boundary density. Geometric phase analysis (GPA) reveals that Sr 2 ⁺ induces lattice distortion, thereby lowering the activation energy barrier of electric‐field‐driven switching. Phase‐field simulations further demonstrate that the parabolic compositional gradient structure effectively suppresses the interfacial energy barriers. These synergies collectively enable continuous multistage phase transitions under low electric fields. The dynamic coupling between long‐range ferroelectric domains and polarized ferroelectric clusters induces a peak EC strength of 0.41 K mm kV −1 , while maintaining EC strength >0.3 K mm kV −1 over a 52 °C operational temperature range. This work establishes a novel paradigm for developing low‐field, high‐efficiency EC materials, advancing their engineering applications.

High-power fuel cell systems represent a critical pathway toward large-scale hydrogen technology commercialization. These systems feature high-power stacks with larger active areas and more cells, which exacerbate issues such as local fuel starvation and voltage consistency deterioration, caused by nitrogen accumulation at the anode, flooding, and uneven gas flow distribution. While existing experimental studies on anode management have focused on low-power stacks, there remains a distinct lack of rapid, sensitive in-situ monitoring methods for internal state assessment of anode. To address these gaps, this paper introduces a novel ultrasonic-based sensor designed to monitor the concentration and flow rate of mixed gases simultaneously in the anode of the fuel cell system. Detailed observations and analyses were conducted on the variations in gas mixture concentration and flow rates within a 130 kW fuel cell system's hydrogen supply subsystem, examining their impacts on system output stability, hydrogen utilization rate, and energy conversion efficiency under various operational settings. The results reveal that increasing the nitrogen concentration from 1 % to 49 % has a minimal effect on the overall stack performance, as the average voltage decreases by only 1.53 %. However, the voltage fluctuation rate increases by 260 %, indicating a significant deterioration in voltage consistency. The voltage fluctuation rate proves to be an effective indicator for purge management in high-power systems. This study uniquely identifies a functional relationship between voltage fluctuation rate and hydrogen stoichiometric ratio. These insights will significantly contribute to advancing anode management strategies for high-power fuel cell systems in the future.

Nitrogen accumulation at the anode of proton exchange membrane fuel cell (PEMFC) systems necessitates periodic purging. However, excessive purging reduces system efficiency, while insufficient purging leads to hydrogen dilution and voltage inconsistency, particularly in high-power multi-cell stacks. This study investigates the influence of anode hydrogen concentration on stack performance and voltage uniformity under various operating conditions. Results reveal a two-stage voltage degradation behavior, with a clear turning point at 85 % hydrogen concentration under 231 A, beyond which the voltage decay rate accelerates sharply, reaching 15.6 % at 77 %. Cells near the hydrogen inlet exhibit the most pronounced decline. Moderate temperature elevation improves membrane hydration and gas diffusivity, enhancing voltage consistency, while overheating induces dehydration and performance loss. Adjusting blower speed and pressure differentials alleviates local hydrogen starvation but cannot fully address distribution imbalances. At 330 A, cell 209 shows an exponential increase in voltage decay as hydrogen concentration drops from 84 % to 80 %, indicating a heightened starvation risk. A compensatory relationship between hydrogen concentration and inlet stoichiometric ratio is established, enabling the definition of a boundary curve for purge control. This work provides a quantitative basis for optimizing hydrogen concentration thresholds and anode recirculation strategies, contributing to improved hydrogen utilization efficiency and supporting the broader goals of efficient, low-emission energy systems.

In proton exchange membrane fuel cells, partial performance loss accumulated during long-term operation can be recovered through optimized operation or shutdown, a process known as reversible degradation. However, most existing studies focus on small-scale single cells and laboratory-scale test conditions, without adequately considering the constraints inherent to practical system-level operation. Therefore, investigating the influence of practically adjustable shutdown parameters on recovery effectiveness is crucial. Moreover, large-scale stacks exhibit spatial heterogeneity in degradation and recovery, both across cell positions and within individual cells. This heterogeneity plays a key role in identifying reversible degradation and formulating recovery strategies. In this study, accelerated stress tests were conducted on a full-size short stack under New European Driving Cycle conditions. Experimental variables included shutdown temperature, operating temperature, and purging methods, to evaluate their effects on recovery. Changes in stack consistency and electrochemically active surface area before and after recovery were analyzed. Results indicate that moderate retention of condensed water promotes ionomer rehydration and performance recovery, while uneven water distribution leads to spatial differences in recovery. Inter-cell and in-plane inconsistencies increase with current density, with voltage deviations exceeding 40 mV between cells and 20 mV within cells at 594 A. The outlet region exhibited weaker recovery consistency and greater sensitivity to load fluctuations, with response amplification reaching approximately 300 %. Cooling measures during recovery improved both steady-state performance and dynamic response. This work provides important insights into the optimization of shutdown parameters and spatial performance variation in large PEMFC stacks, supporting the development of improved operational strategies to enhance durability and efficiency in practical applications.

Water management in fuel cell vehicles emerges as a critical bottleneck. The electrochemical impedance spectrum (EIS) is commonly used during the development of vehicular fuel cell systems to diagnose the internal state of the fuel cells. However, studies focusing on on-board impedance measurement devices for high-power vehicular fuel cell systems were limited. This research introduced a novel alternating current (AC) excitation module capable of delivering disturbance signals across a wide frequency range (1 1000 Hz) directly, addressing the need for accurate in-situ impedance measurement. The instruments integrated an auxiliary direct current to direct current (DC/DC) converter to modulate the AC excitation signals, enhancing the precision and adaptability of the impedance measurements under various operational conditions. The hardware circuit and calculation method for the impedance measurement module were detailed. Additionally, this paper analyzed the operating characteristics of the fuel cell's DC/DC converter in conjunction with the AC excitation module. Rigorous testing validated the self-developed devices, confirming their consistency with standard instruments. The effectiveness of the EIS measurement device underscores its potential to significantly enhance the reliability and efficiency of fuel cell electric vehicles by providing robust tools for performance monitoring and fault diagnosis.

The proton-exchange membrane fuel cell is a highly attractive clean energy technology for the future. However, differences in the consistency and uniformity of commercial-size fuel cell stacks significantly impact their health status and lifespan. This study is based on a commercial-size 10-cell stack. First, the characteristic frequency was determined by multipoint voltage and impedance sweeping experiments. Subsequently, the consistency and uniformity of the output characteristics of the stack at various loads under standard operating conditions were comprehensively analyzed using fixed-frequency multipoint impedance. Finally, a sensitivity analysis was conducted to quantitatively assess the effects of key parameters, including the air stoichiometry ratio and operating temperature, on the consistency and uniformity of voltage and high-frequency resistance. The inconsistency is more pronounced at high current densities, especially at the cathode inlet side compared to the outlet side. At low-current-density operation, the high-frequency resistance is greater at the cathode inlet side compared to the outlet side due to uneven gas distribution and more severe membrane drying at the inlet side. Increasing the air stoichiometry ratio enhances the consistency and uniformity of the stack. The effect of temperature on stack consistency is not significant in the appropriate temperature interval, but higher temperatures can reduce cell uniformity differences. The analytical approach in this study can provide guidance for stack consistency and uniformity studies. A commercial-size 10-cell proton exchange membrane fuel cell stack is characterized by multipoint voltage and impedance sweep experiments. The output characteristics of the stack at various loads under are analyzed using fixed-frequency multipoint impedance. A sensitivity analysis is conducted to assess the effects of key parameters.

Proton-exchange membrane fuel cells are widely utilized in transportation and stationary power generation applications due to their high energy conversion efficiency, substantial power density, and zero emissions. To address the requirements for internal status monitoring and hydrogen purge management in the hydrogen supply subsystem of automotive fuel cell systems, this paper proposes an innovative online monitoring scheme for real-time measurement of the anode exhaust mixture concentration from the fuel cell stack, utilizing an ultrasonic flowmeter. This scheme offers high measurement accuracy, short sampling intervals, simple configuration, and low cost. An acoustic analysis model of the anode exhaust gas was developed to systematically evaluate the influence of environmental parameters such as temperature, pressure, and humidity on the ultrasonic flowmeter's measurement. Additionally, an ultrasonic flowmeter prototype was constructed based on this acoustic model, and the accuracy of its flow rate and concentration measurements was validated using standard gases. The results indicate that, without a humidity sensor, the absolute error in hydrogen concentration can reach 7 vol%, while the absolute error in nitrogen concentration can exceed 11 vol%. Therefore, integrating additional sensors is essential for improving the accuracy of concentration calculations for the anode exhaust gas components. The relative error in flow measurement with the ultrasonic flowmeter prototype is consistently below 5%, and the absolute error in concentration measurements is less than 1%, reflecting the effectiveness and feasibility of the proposed method. Furthermore, this paper outlines a calculation method for converting sensor feedback into the mass flow rates of individual gas components in the mixture. This work offers a novel theoretical and practical framework for enhancing real-time internal status monitoring and hydrogen purge management in fuel cell systems, which is expected to significantly improve the reliability, stability, and fuel economy of automotive fuel cell systems in the future. An innovative online monitoring scheme for real-time measurement of the anode exhaust mixture concentration from a proton exchange membrane fuel cell stack utilizes an ultrasonic flowmeter. This scheme offers high measurement accuracy, short sampling intervals, simple configuration, and low cost.

Lead-free electrocaloric (EC) ferroelectrics are considered ideal for the next generation of environmentally friendly solid-state refrigeration materials. However, their inferior performance compared to lead-based materials significantly restricts their potential application. According to phase-field simulations, it is predicted that the pinning effect of a moderate number of defects can effectively enhance the reversible polarization response associated with the entropy change. Herein, sodium-bismuth titanate (BNT) ceramics with high spontaneous polarization are selected to construct B-site defects by introducing Li+ and Nb5+. Under an electric field of 6 kV mm(-1), ultrahigh EC temperature changes of Delta T-pos = 1.77 and Delta T-neg = 1.49 K are achieved at 65 degrees C by direct measurement (Delta T-neg > 1 K over 55-120 degrees C). Furthermore, Delta T-neg remains above 0.70 K in the temperature range from 25 to 130 degrees C, exhibiting immense potential for practical applications. This study offers a promising direction for optimizing the EC response in defect systems.

Inadequate water management undermines the reliability and durability of proton exchange membrane fuel cells (PEMFCs). Thus, it is necessary to identify the internal water state of the PEMFC accurately and control it within a reasonable range. The internal state of the PEMFC can be estimated online by simplifying the mechanism model. However, existing models neglect critical factors like water distribution across flow channels, gas diffusion layers, and catalyst layers, as well as the water content in the ionomer, liquid saturation, and vapor pressure within the membrane. Thus, in this work, a simplified mechanism model of PEM containing water content in ionomer, liquid water, and water vapor is established. Then, the influence of measurement noise and process noise set values on the performance of the observer is analyzed. The observer can exhibit the best performance when the noise variance is set as 10_ 4 and the process noise is set as 10_8 to match the actual noise variance. Finally, an internal state observer based on the model and the particle filter algorithm is developed. Based on the simulation, the internal water state trend of the PEMFC is analyzed, and the performance of the state observer based on voltage, high frequency resistance, and sensor signal fusion is compared. The results show that the observer based on sensor signal fusion is good at observing the water state.

The compressor can improve the performance of proton exchange membrane fuel cell (PEMFC) system, but excessive parasitic power can actually have a counterproductive effect. PEMFC system also faces the issue of plateau environmental adaptability. This paper experimentally verifies the energy recovery effect of the expander and explores the system performance in plateau environment. In plain environment, with a rated stack power of 162.2 kW, the application of the expander decreases the air compressor power from 18.7 kW to 13.2 kW, and increases the system efficiency from 44.5 % to 46.5 %. Based on simulation model, further research is conducted on plateau environment. PEMFC system can operate under rated conditions below an altitude of 3000 m. At an altitude of 0-3000 m, the compressor power increases from 18.1 kW to 24.9 kW, and the system efficiency reduces from 44.8 % to 42.5 %. By applying the expander, the air compressor power reduces to 16.8 kW, and system efficiency increases to 45.1 %. The working boundary of the compressor limits the operating load and altitude of PEMFC system. The expander can achieve better energy recovery effect in high current density and plateau environment. In future, the expander will be an indispensable part of high-power PEMFC system.

Replacing external humidifiers with self-humidification technology can simplify the structure of fuel cell systems and improve their cost-effectiveness. This paper analyzes a feasible method for achieving self-humidification at the system level, suggesting that membrane drying can be prevented by increasing the hydrogen circulation pump revolutions, reducing the air stoichiometric ratio, and controlling the stack temperature. A theoretical design for each subsystem of the 130 kW PEMFC-based self-humidifying fuel cell system was also proposed. The system was built and tested under steady-state conditions, achieving an efficiency of 86.7

The strategy of this study achieves a large electrocaloric response of ΔT > 1 K within a temperature span of 66 °C. The cooling performance exceeds 90% and 80% of the maximum for temperature spans of 44 °C and 67 °C, respectively.

Antiferroelectrics (AFEs) are ideal candidates in dielectric, electromechanical, and electrothermal applications. NaNbO3 (NN), as a lead-free antiferroelectric (AFE) material under extensive investigation, exhibits ferroelectric (FE)-like polarization-electric field (P-E) hysteresis loops, characterized by high remnant polarization and large hysteresis. Herein, the local defect structure design is proposed to achieve high energy storage (ES) density in NN-based AFE ceramics. The pinning effect of defect dipoles and the enhancement of local structural disorder stabilize the AFE phase and reduce hysteresis losses. Consequently, an excellent ES performance of large recoverable energy density (W-rec) of 9.70 J/cm(3) and high efficiency (eta) of 88.75 % is concurrently obtained in NN-based relaxor AFE ceramics, with outstanding charge-discharge performance (P-D similar to 413.2 MW/cm(3), W-D similar to 4.47 J/cm(3)), which are significantly improved compared to other reported values in lead-free ES ceramics. This indicates that NN-based ceramics are candidates for advanced ES capacitors and provides a feasible modulation approach for the development of new lead-free high-performance dielectric materials.