To understand the formation process of the microstructure of the catalyst layer of a polymer electrolyte fuel cell (PEFC), we tried to observe the “real” structure of the catalyst ink by cryogenic scanning electron microscopy (cryo‐SEM). Catalyst inks with different water/alcohol compositions were successfully visualized, and they correlated well with the particle‐size distribution obtained by laser diffraction of the ink and the structures of the catalyst layers obtained by typical SEM. On the basis of other electrochemical characterization results, including current–voltage performance, oxygen reduction reaction kinetics, and mass‐transport properties, the microstructures of the catalyst inks and the catalyst layers were proposed. The proposed microstructures can explain the relationship between the catalyst materials and the performance of the cathode catalyst layer of the membrane electrode assembly through its formation and apparent properties. It was also found that the microstructure of the catalyst ink plays an important role in performance.
In this study, we experimentally demonstrate a class of lightweight acoustic metamaterial barriers that block low-frequency sound. The acoustic metamaterial barrier is composed of a thin rubber membrane coated over a stiff honeycomb plate. Our findings, combined with high-fidelity finite element simulations, demonstrate that the sound insulation performance of the acoustic metamaterial surpasses the mass law in three distinct frequency ranges: (a) the stiffness law dominates insulation up to 140 Hz, (b) degeneracy and destructive superposition of high-order natural modes dominate within the frequency range of 300–500 Hz, and (c) destructive interference between high-order resonance and membrane resonance dominates in the frequency range of 800–1200 Hz. Notably, our study highlights the potential of high-order shear vibration of the periodic structure for the resonant bending waves of the honeycomb cell that coincide with the wavelengths of longitudinal sound waves in air, thereby offering new design guidelines for lightweight acoustic metamaterial barriers. This study reports for the first time the coincidence of high-order and membrane resonance modes within the honeycomb cell by employing an accurate finite element model and experimental validation.
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