With the rapid growth and development of proton exchange membrane fuel cell (PEMFC) technology there has been an increasing demand for clean and sustainable global energy applications.While there are many device-level and infrastructure challenges still to be overcome before wide commercialization can be realized, increasing the PEMFC power density is a critical technical challenge, with ambitious goals proposed globally. For example, the short-term and long-term goals of the Japan New Energy and Industrial Technology Development Organization (NEDO) are 6 kW L -1 by 2030 and 9 kW L -1 by 2040, respectively. To this end, we propose technical development directions required for next-generation high power density PEMFCs. This perspective comprehensively embraces the latest advanced ideas for improvements in the membrane electrode assembly (MEA) and its components, bipolar plate (BP), integrated BP-MEA design, with regard to water and thermal management, and materials. The realization of these ideas is expected to be encompassed in next-generation PEMFCs with the aim of achieving a high power density.

Coding acoustic metasurfaces can combine simple logical bits to acquire sophisticated functions in wave control. The acoustic logical bits can achieve a phase difference of exactly π and a perfect match of the amplitudes for the transmitted waves. By programming the coding sequences, acoustic metasurfaces with various functions, including creating peculiar antenna patterns and waves focusing, have been demonstrated.

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We present a mid-IR highly wavelength-tunable broadband cross polarization conversion composed of a single patterned top layer with L-shaped graphene nanostructures, a dielectric spacer, and a gold plane layer. It can convert linearly polarized light to its cross polarization in the reflection mode. The polarization conversion can be dynamically tuned and realize a broadband effect by varying the Fermi energy without reoptimizing and refabricating the nanostructures. This offers a further step in developing the tunable polarizers and the polarization switchers.

Summary A three‐dimensional (3D) multi‐phase numerical model of proton exchange membrane fuel cell (PEMFC) is built. The catalyst layer (CL) spherical agglomerate model is used to replace traditional homogenous model, which can predict the concentration loss in PEMFC more accurately. Utilizing this multi‐phase model, the PEMFC with 3D fine mesh flow field is investigated at length, and the liquid water distribution in 3D flow field is qualitatively compared with the experimental image in previous literature. It is found that the 3D fine mesh flow field can improve the reactant gas supply from flow field to porous electrodes significantly and facilitate liquid water removal in PEMFC simultaneously. Therefore, it reduces the concentration loss of PEMFC effectively without increasing the pumping power loss thanks to the greatly increased mass transfer area between gas diffusion layer (GDL) and flow field and vertical flow design of hydrogen and air, which also make the reaction rate distribution in CL more uniform. However, the decreased contact area between GDL and bipolar plate in 3D flow field may decrease PEMFC performance at the current densities where ohmic loss is dominated, but its effect is insignificant.

or wavelength of incident light. However, the anomalous refraction effi ciency is closely related to the resonance. The highest anomalous refraction effi ciency will occur at the resonant wavelength, and decrease when the wavelength of incident light is away from it. The highest conversion effi ciency has to be tuned to different wavebands by carefully reoptimizing and resizing the geometric parameters of the structures. This lacks fl exibility for active control, which limits its uses in practice.One way to realize the active control of the anomalous refraction effi ciency may be integrating metasurfaces with permittivity-tunable materials. [17][18][19] By external stimulus such as electric and/or magnetic fi eld, voltage, or temperature, the optical response of the integrated metasurfaces can be actively controlled. Graphene, which is a single 2D plane of carbon atoms arranged in a honeycomb lattice, has been demonstrated to support surface plasmon polaritons. [ 20,21 ] As its conductivity can be dynamically controlled by electrostatic gating, it seems to be a good candidate for designing tunable devices and becomes a hot material in both physics and engineering. [22][23][24][25][26] Graphene-based metamaterials have been wildly demonstrated to achieve tunable devices such as absorbers, [ 27 ] antennas, [ 28,29 ] polarization converters, [30][31][32] and transformation optical devices. [ 33 ] Recently, metasurfaces based on 1D graphene nanoribbons have been demonstrated to manipulate wavefront of light. [ 34 ] Since only the width of 1D nanoribbon is adjustable, the conductivity of the nanoribbons needs to be individually adjusted to realize the phase range covering from 0 to 2π, which is diffi cult in practical applications. However, there are more adjustable structural parameters for 2D graphene nanostructures to satisfy the phase condition. The new degrees of freedom of graphene metasurfaces may facilitate arbitrary manipulation of light wavefront by uniform conductivity and will profoundly affect a wide range of photonic applications.Here, we propose a highly tunable broadband anomalous refraction composed of periodically patterned graphene nanocrosses for circularly polarized waves in the infrared regime. We demonstrate the applicability of the scheme to generalize anomalous refraction by investigating the effect at various incident angles and different wavelengths. More importantly, the anomalous conversion effi ciency can be dynamically tuned and remain as high in a broadband frequency range by varying the Metasurfaces, which are capable of generating structure and wavelength dependent phase shift, have emerged as promising means for controlling the wavefront of electromagnetic waves. Finding new ways to realize broadband frequency response as well as maintaining high conversion effi ciency still requires research efforts. For the design of plasmonic metasurfaces, graphene represents an attractive alternative to metals due to its strong fi eld confi nement and versatile tunability. Here, a novel metasurf...

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Weyl points (WPs), as the doubly degenerate points in three-dimensional momentum band structures, carry quantized topological charges and give rise to a variety of extraordinary properties, such as robust surface wave and chiral anomaly. Type-II Weyl semimetals, which have conical dispersions in Fermi surfaces and a strongly tilted dispersion with respect to type I, have recently been proposed in condensedmatter systems and photonics. Although the type-II WPs have been theoretically predicted in acoustics, the experimental realization in phononic crystals has not been reported so far. Here, we experimentally realize a type-II Weyl phononic crystal. We demonstrate the topological transitions observed at the WP frequencies and the topological surface acoustic waves between the Weyl frequencies. The experiment results are in good accordance with our theoretical analyses. Due to the violation of the Lorentz symmetry, the type-II WPs only exist in low energy systems. As the analog counterpart in classical waves, the phononic crystal brings a platform for the research of type-II WPs in macroscopic systems.