In this special section, we have gathered 5 papers with significant contributions in the field of fuel cell, which reflects the latest advances in developing and understanding the design and optimization of key materials and components of fuel cells. The papers, as listed below, cover the topics of carbon support evaluation, Pt-free cathode electrocatalyst, cathode catalyst layer design, local behaviours characterization of fuel cell and heat exchanger numerical analysis.

For current carbon-supported Pt catalysts in vehicle proton exchange membrane fuel cells (PEMFCs), the insufficient stability and durability of carbon supports are severe limitations under operating conditions. This paper adopts the accelerated stress test (AST) method to study the carbon corrosion of catalysts, which is significant to efficiently select the catalysts supports in fuel cells. Graphitized carbon blacks with various surface properties are heated under different conditions, followed by evaluation of their antioxidation capacity with the AST. It is shown that optimally graphitized carbon blacks demonstrate superior stability, retaining a constant quinone/hydroquinone (QH) transition peak potential for over 70,000 AST cycles. A Pt catalyst supported on the selected graphitized carbon exhibits excellent durability at both the rotating disk electrode (RDE) and membrane electrode assembly (MEA) levels. The final specific mass activity (MA) of the optimum catalyst is 47.87 mA/mgPt, which is 2.06 times that of commercial Pt/C (23.31 mA/mgPt) in the RDE tests. The final maximum power density of the optimum catalyst is 525.68 mW/cm2, which is 305.52 mW/cm2 higher than that of commercial Pt/C after undergoing the AST during the MEA measurements. These results prove that the rational surface features of carbon supports play a vital role in improving the overall fuel cell performance by realizing uniform dispersion of Pt nanoparticles, resisting corrosion, and reinforcing metal–support interactions.

Proton exchange membrane (PEM) fuel cells have gained increasing interest from academia and industry, due to its remarkable advantages including high efficiency, high energy density, high power density, and fast refueling, also because of the urgent demand for clean and renewable energy. One of the biggest challenges for PEM fuel cell technology is the high cost, attributed to the use of precious platinum group metals (PGM), e.g., Pt, particularly at cathodes where sluggish oxygen reduction reaction takes place. Two primary ways have been paved to address this cost challenge: one named low-loading PGM-based catalysts and another one is non-precious metal-based or PGM-free catalysts. Particularly for the PGM-free catalysts, tremendous efforts have been made to improve the performance and durability—milestones have been achieved in the corresponding PEM fuel cells. Even though the current status is still far from meeting the expectations. More efforts are thus required to further research and develop the desired PGM-free catalysts for cathodes in PEM fuel cells. Herein, this paper discusses the most recent progress of PGM-free catalysts and their applications in the practical membrane electrolyte assembly and PEM fuel cells. The most promising directions for future research and development are pointed out in terms of enhancing the intrinsic activity, reducing the degradation, as well as the study at the level of fuel cell stacks.

An advanced cathode design can improve the power performance and durability of proton exchange membrane fuel cells
(PEMFCs), thus reducing the stack cost of fuel cell vehicles (FCVs). Recent studies on highly active Pt alloy catalysts, shortside-chain polyfuorinated sulfonic acid (PFSA) ionomer and 3D-ordered electrodes have imparted PEMFCs with boosted
power density. To achieve the compacted stack target of 6 kW/L or above for the wide commercialization of FCVs, developing available cathodes for high-power-density operation is critical for the PEMFC. However, current developments still remain extremely challenging with respect to highly active and stable catalysts in practical operation, controlled distribution of ionomer on the catalyst surface for reducing catalyst poisoning and oxygen penetration losses and 3D (three-dimensional)- ordered catalyst layers with low Knudsen difusion losses of oxygen molecular. This review paper focuses on impacts of the cathode development on automotive fuel cell systems and concludes design directions to provide the greatest beneft.

When designing a cell stack and developing an operational strategy for proton exchange membrane fuel cell, it is critical to characterize the local current, water and heat. To measure distributions of current density, relative humidity and temperature for both anode and cathode simultaneously along the straight parallel flow channels, this paper uses a segmented tool based on the multilayered printed circuit board flow field plates with embedded sensors. In this study, two kinds of experimental operations of fuel cell reactants are carried out for comparison: the co-flow operation with identical gas flow direction of hydrogen and air and the counter-flow operation with opposite gas flow directions. The detected relative humidity (RH) distributions of both anode and cathode indicate that the asymmetry of RH distribution at two sides of the membrane in counter-flow operation is better at holding water inside the fuel cell compared with the co-flow operation. The in situ measured performance distributions show that segments around the middle of the fuel cell contribute the highest current in counter-flow operation, while for co-flow operation, the current peak locates near the outlet of reactants.

Fuel cell vehicles (FCVs) are facing severe heat dissipation challenges because fuel cell stacks are required to operate at a narrower temperature range and higher heat dissipation than those in the internal combustion engine. This study conducts a numerical analysis of a tube-strip heat exchanger applied in a high-performance FCV. The typical unit cell of the tube-strip heat exchanger is selected to numerically optimize the cell-level thermal performance of the heat exchanger. Effects of structural parameters and operational conditions are investigated. The optimal structure is obtained by focusing on the heat transfer rate and fan power at the air side, where the overall heat transfer rate of heat exchanger is determined by the effectiveness number of transfer unit method and the theoretical framework of volume averaging. The results show that the heat exchanger with rectangular fins exhibits a greater heat transfer rate than those with trapezoidal and triangular fins at an inlet air velocity of 4 m/s. Compared with the fin without a louver, the heat exchangers equipped with louvers parallel and vertical with the air flow achieve heat transfer rates of 33.1 and 42.8 kW, respectively, which increase by 2.0 kW (6.4%) and 11.7 kW (37.5%) in heat transfer rate. For high-power heat dissipation, the louvered heat exchanger with a fin pitch of 2 mm shows the best thermal performance given the same fan power.

An improved cubature Kalman filter (CKF) algorithm for estimating the state of charge of lithium-ion batteries is proposed. This improved algorithm implements the diagonalization decomposition of the covariance matrix and a strong tracking filter. First, a first-order RC equivalent circuit model is first established and verified, whose voltage estimation error is within 1.5%; this confirms that the model can be used to describe the characteristics of a battery. Then the calculation processes of the traditional and proposed CKF algorithms are compared. Subsequently, the improved CKF algorithm is applied to the state of charge estimation under the constant-current discharge and dynamic stress test conditions. The average errors for these two conditions are 0.76% and 1.2%, respectively, and the maximum absolute error is only 3.25%. The results indicate that the proposed method has higher filter stability and estimation accuracy than the extended Kalman filter (EKF), unscented Kalman filter (UKF) and traditional CKF algorithms. Finally, the convergence rates of the above four algorithms are compared, among which the proposed algorithm track the referenced values at the highest speed.

Efficient regenerative braking of electric vehicles (EVs) can enhance the efficiency of an energy storage system (ESS) and reduce the system cost. To ensure swift braking energy recovery, it is paramount to know the upper limit of the regenerative energy during braking. Therefore, this paper, based on 14 typical urban driving cycles, proposes the concept and principle of confidence interval of “probability event” and “likelihood energy” proportion of braking. The critical speeds of EVs for braking energy recovery are defined and studied through case studies. First, high-probability critical braking speed and high-energy critical braking speed are obtained, compared, and analyzed, according to statistical analysis and calculations of the braking randomness and likelihood energy in the urban driving cycles of EVs. Subsequently, a new optimized ESS concept is proposed under the frame of a battery/ultra-capacitor (UC) hybrid energy storage system (HESS) combined with two critical speeds. The battery/UC HESS with 9 UCs can achieve better regenerative braking performances and discharging performances, which indicates that a minimal amount of UCs can be used as auxiliary power source to optimize the ESS. After that, the efficiency regenerative braking model, including the longitudinal dynamics, motor, drivetrain, tire, and wheel slip models, is established. Finally, parameters optimization and performance verification of the optimized HESS are implemented and analyzed using a specific EV. Research results emphasize the significance of the critical speeds of EVs for regenerative braking.

Injection-induced rail pressure fluctuations are proven to cause nonuniform spray development. These fluctuations are also responsible for generating lower injection pressures, to the detriment of jet penetration length and break-up timing. Despite the vast literature dealing with such issues, several aspects of rail pressure fluctuations remain unclear. Additionally, the need for compliance with the emission legislation has shed light on the potential of alternative fuels, which represent a pathway for sustainable mobility. This scenario has motivated the present study dealing with the assessment of the time history of rail pressure correlated with fuel properties. Tests have been performed using a last-generation common rail injection equipment under various injection settings, employing diesel and 2-methylfuran-diesel blend. This paper describes the research activity and aims to provide new insights into the correlation of rail pressure fluctuations with fuel properties.

The in-wheel motor (IWM)-driven electric vehicles (EVs) attract increasing attention due to their advantages in dimensions and controllability. The majority of the current studies on IWM are carried out with the assumption of an ideal actuator, in which the coupling effects between the non-ideal IWM and vehicle are ignored. This paper uses the braking process as an example to investigate the longitudinal–vertical dynamics of IWM-driven EVs while considering the mechanical–electrical coupling effect. First, a nonlinear switched reluctance motor model is developed, and the unbalanced electric magnetic force (UEMF) induced by static and dynamic mixed eccentricity is analyzed. Then, the UEMF is decomposed into longitudinal and vertical directions and included in the longitudinal–vertical vehicle dynamics. The coupling dynamics are demonstrated under different vehicle braking scenarios; numerical simulations are carried out for various road grades, road friction, and vehicle velocities. A novel dynamics vibration absorbing system is adopted to improve the vehicle dynamics. Finally, the simulation results show that vehicle vertical dynamic performance is enhanced.