Dong Hao

The optimization of operating points (OPs) is crucial for maximizing the energy efficiency of proton exchange membrane fuel cells (PEMFCs). However, existing strategies primarily focus on a single variable—the oxygen excess ratio (OER)—neglecting the potential of synergistic optimization with the pressure ratio (PR) and stack temperature. This limitation, coupled with the reliance on complex models or cumbersome cost functions, hinders further efficiency gains. This article presents a model-free, multivariate extremum seeking (ES) scheme that addresses these challenges by jointly optimizing OER, PR, and stack temperature. It further introduces a simplified cost function that removes the need for hydrogen consumption measurements and empirical weighting factors. The proposed multivariate ES framework is rigorously proven to achieve local exponential stability. Hardware-in-the-loop (HiL) experiments demonstrate that, after ES-based training under a series of constant-current loads, the optimized OPs scheduled as functions of the stack current achieve energy efficiency improvements ranging from 6.81% to 14.53% over two widely adopted techniques. Furthermore, the optimized OER and PR exhibit clear fitted correlations with the stack current, while the stack temperature converges near its initial value, revealing its secondary impact on parasitic power. The results confirm that the concurrent optimization of multiple OPs unlocks substantial efficiency reserves inaccessible to conventional single-variable approaches.

Bipolar plates are critical components in high-efficiency energy conversion devices such as electrolyzers, fuel cells, and flow batteries, and their durability directly affects the overall performance and lifespan of the system. Although graphite bipolar plates exhibit excellent electrical conductivity and corrosion resistance, their inherent brittleness and porous structure render them prone to thermal-stress-induced damage under dynamic temperature conditions. In this study, a self-designed thermal shock testing system was utilized to perform 16,000 cycles of accelerated aging tests on graphite bipolar plates, alternating between high-temperature (90 °C) and low-temperature (30 °C) water bath environments. Systematic analysis was conducted on the performance degradation behaviors under such thermal cycling conditions using multi-scale characterization techniques, including scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), electrical conductivity, contact angle, surface roughness, and corrosion current density analysis. The results demonstrate that the degradation in electrical conductivity, loss of hydrophobicity, and increased surface roughness were primarily attributed to thermal-stress-induced microcrack initiation and propagation, surface oxidation, and physical structural deterioration. Notably, the corrosion current density did not increase significantly after 16,000 thermal cycles, but slightly decreased in the later stage, indicating that the aging of graphite bipolar plates is dominated by physical fatigue damage, and the graphite matrix has excellent chemical stability. The novelty of this study lies in the construction of a thermal shock testing system under long-cycle conditions, revealing the synergistic mechanism of thermal cycle-induced performance degradation of graphite bipolar plates, which provides experimental evidence and theoretical guidance for the material selection, structural design, and protection strategies of highly durable bipolar plates.

The evolution of two key dynamic response parameters—undershoot voltage and stabilization time—was investigated throughout a 1000-h durability test on four distinct PEMFCs. Results demonstrate that cells with superior mass transport exhibited higher undershoot voltages, an effect amplified at higher current densities due to a more pronounced gas supply-demand imbalance. Notably, the undershoot voltage decreased during the initial cell activation phase. Conversely, stabilization time showed no clear correlation with load magnitude but increased systematically with performance degradation. These findings reveal a strong connection between dynamic response characteristics and performance decay, providing meaningful insights for the design, application, and health assessment of high-performance fuel cells.

This study conducted 1000-hour durability tests on four different single proton exchange membrane fuel cells (PEMFCs). The overall performance degradation of the cells was evaluated, and the evolution of two key dynamic response parameters—undershoot voltage and stabilization time—was analyzed during current density transients over time. The results showed that fuel cells with better mass transport properties consumed reactant gases more significantly in the upstream region during current transitions, leading to higher undershoot voltages. At higher current densities, the imbalance between gas supply and demand became more pronounced, further amplifying the undershoot. As the cell activation process progressed, partial internal performance improved, resulting in a decreasing trend in undershoot voltage. Stabilization time does not exhibit a clear correlation with load magnitude, but its increasing trend is closely related to the degradation of the cell's steady-state performance. These findings reveal a strong connection between dynamic response characteristics and performance decay, providing meaningful insights for the design, application, and health assessment of high-performance fuel cells.

Accurate extraction of polarization resistance is crucial in the application of proton exchange membrane fuel cells. It is generally assumed that the steady-state resistance obtained from the polarization curve model is equivalent to the AC impedance obtained from the electrochemical impedance spectroscopy (EIS) when the frequency approaches zero. However, due to the low-frequency stability and nonlinearity issues of the EIS method, this dynamic process leads to an additional rise in polarization resistance compared to the steady-state method. In this paper, a semi-empirical model and equivalent circuit models are developed to extract the steady-state and dynamic polarization resistances, respectively, while a static internal resistance correction method is proposed to represent the systematic error between the two. With the correction, the root mean square error of the steady-state resistance relative to the dynamic polarization resistance decreases from 26.12% to 7.42%, indicating that the weighted sum of the static internal resistance and the steady-state resistance can better correspond to the dynamic polarization resistance. The correction method can also simplify the EIS procedure by directly generating an estimate of the dynamic polarization resistance in the full current interval.