• Energy Research

Abstract 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 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 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 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 (Dn), the mixing tube diameter (Dm), the mixing tube length (Lm), and the primary nozzle exit position (NXP). It is found that the ejector hydrogen entrainment performance is sensitive to Dm/Dn and the optimal value range is 3–3.54. The entrainment ratio curve shows different changing tendencies along with Dm/Dn, separated by 3.54. The optimal Lm/Dm 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 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 The cathodic pressure drop (PD) of proton exchange membrane fuel cell (PEMFC) can be used to conduct water management as it can perceive the water amount interior the fuel cell. Our previous research proposed the strategy to conduct this kind of management. In this paper the specific approaches, including regulating fuel cell temperature, inlet pressure, and inlet relative humidity (RH), were adopted to fulfill this strategy. The regulation was executed in case the real PD surpasses the theoretical control line. The electrochemical impedance spectroscopy (EIS) was tested to present the water amount. During more than three hours’ operation, the flooding did not occur anymore using the three management approaches. Of them, increasing the inlet pressure 25 kPa enabled voltage to recover 25 mV and made the performance not decline obviously. As a contrast, an 8 mV recovery was achieved via increasing temperature 2 K. The recovery range got much less if regulated inlet RH. In commercial fuel cell system, changing inlet pressure in a normal regulation range is a promising way to avoid flooding. This comprehensive approach based on PD can be directly used in the fuel cell system via sending commands to air compressor and air back pressure valve, without deploying extra devices.