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A compact cell structure is highly desirable for next-generation high-power-density proton exchange membrane (PEM) fuel cell (>9.0 kW L−1). A novel integrated bipolar plate (BP) – gas diffusion layer (GDL) structure utilizing porous material is a promising configuration, which not only improves output power due to the reduced transport resistance but also reduces cell thickness. Herein, we propose a structured mesh material with gradient surface wettability for this cell structure, which is characterized by an ordered skeleton in the three-dimensional (3D) space with high structure design flexibility. It is experimentally found that the structured mesh material shows higher performance than serpentine and parallel flow fields. Meanwhile, 3D modeling work on an automobile fuel cell (cell area: 245.76 cm2) with the distribution zones of dot matrix structure for hydrogen, air and coolant flow, is conducted, and it is found that the structured mesh material effectively decreases the concentration and electric ohmic losses in comparison with the traditional "BP/flow field + GDL" structure, improving power density with uniform distribution characteristics. Moreover, the gradient wettability on the mesh surface helps promote the water detach the GDL and hold the residual liquid water in the top region, optimizing the water management. [ABSTRACT FROM AUTHOR]

Single‐atom Fe‐N‐C (denoted as Fe1‐N‐C) catalysts exhibit inadequate bifunctional activities to conquer the sluggish oxygen reduction and evolution reaction (ORR/OER), hindering their practical applications in rechargeable Zn‐air batteries (ZABs). Here, by employing Fe1‐N‐C hollow nanorods as ORR‐active support, OER‐active NiFe‐layered double hydroxide (NiFe‐LDH) nanodots are evenly decorated through a spatially confined process to form NiFe‐LDH/Fe1‐N‐C heterostructure hollow nanorods with abundant accessible catalytic sites. The NiFe‐LDH/Fe1‐N‐C heterostructure not only enhances the ORR activity of pristine Fe1‐N‐C but also realizes efficient bifunctional ORR/OER activity in one monolithic catalyst. Theoretical calculations reveal that introducing NiFe‐LDH nanodots results in donation of electrons to the Fe1‐N‐C matrix and thus lowers the Fe‐d band center of the Fe‐N4 sites, dramatically narrowing the energy barriers of the ORR rate‐limiting steps. As a result, NiFe‐LDH/Fe1‐N‐C nanorods deliver remarkable ORR activity with a half‐wave potential of 0.90 V versus reversible hydrogen electrode, surpassing bare Fe1‐N‐C and commercial Pt/C. Impressively, the integrated NiFe‐LDH/Fe1‐N‐C catalysts show outstanding bifunctional performance with a small overpotential gap of only 0.65 V. The liquid‐state ZABs with NiFe‐LDH/Fe1‐N‐C as an air‐cathode catalyst deliver a peak power density of 205 mW cm−2 and long‐term cycling stability of up to 400 h. [ABSTRACT FROM AUTHOR]

Next‐generation ultrahigh power density proton exchange membrane fuel cells rely not only on high‐performance membrane electrode assembly (MEA) but also on an optimal cell structure. To this end, this work comprehensively investigates the cell performance under various structures, and it is revealed that there is unexploited performance improvement in structure design because its positive effect enhancing gas supply is often inhibited by worse proton/electron conduction. Utilizing fine channel/rib or the porous flow field is feasible to eliminate the gas diffusion layer (GDL) and hence increase the power density significantly due to the decrease of cell thickness and gas/electron transfer resistances. The cell structure combining fine channel/rib, GDL elimination and double‐cell structure is believed to increase the power density from 4.4 to 6.52 kW L−1 with the existing MEA, showing nearly equal importance with the new MEA development in achieving the target of 9.0 kW L−1. [ABSTRACT FROM AUTHOR]

Highlights: The strong interaction between Ti3C2Sx and FeN4 species induces the central metal Fe(II) in FeN4 species with intermediate spin state transferred to high spin state, in which the latter is favorable to initiate the reduction of oxygen. This strong interaction induces a remarkable Fe 3d electron delocalization with d band center upshift, boosting oxygen-containing groups adsorption on FeN4 species and oxygen reduction reaction kinetics. The resulting FeN4–Ti3C2Sx with FeN4 moieties in high spin state exhibits high half-wave potential of 0.89 V vs. RHE and high limiting current density of 6.5 mA cm−2, enabling wearable zinc–air battery showing a good discharge performance with a maximum power density of 133.6 mW cm−2. Transition metal–nitrogen–carbon materials (M–N–Cs), particularly Fe–N–Cs, have been found to be electroactive for accelerating oxygen reduction reaction (ORR) kinetics. Although substantial efforts have been devoted to design Fe–N–Cs with increased active species content, surface area, and electronic conductivity, their performance is still far from satisfactory. Hitherto, there is limited research about regulation on the electronic spin states of Fe centers for Fe–N–Cs electrocatalysts to improve their catalytic performance. Here, we introduce Ti3C2 MXene with sulfur terminals to regulate the electronic configuration of FeN4 species and dramatically enhance catalytic activity toward ORR. The MXene with sulfur terminals induce the spin-state transition of FeN4 species and Fe 3d electron delocalization with d band center upshift, enabling the Fe(II) ions to bind oxygen in the end-on adsorption mode favorable to initiate the reduction of oxygen and boosting oxygen-containing groups adsorption on FeN4 species and ORR kinetics. The resulting FeN4–Ti3C2Sx exhibits comparable catalytic performance to those of commercial Pt-C. The developed wearable ZABs using FeN4–Ti3C2Sx also exhibit fast kinetics and excellent stability. This study confirms that regulation of the electronic structure of active species via coupling with their support can be a major contributor to enhance their catalytic activity. [ABSTRACT FROM AUTHOR]

A template-engaged multistep synthesis process is developed to fabricate ultrathin carbon nanosheets assembled hierarchical nanoboxes embedded with dense Mo-N 4 active sites, which exhibited excellent activity and superb stability for oxygen reduction reaction. [Display omitted] • Unique hierarchical SA-Mo-C nanoboxes were fabricated via a template-engaged multistep synthesis process. • Comprehensive characterizations including EXAFS reveal Mo-N 4 atomic sites were formed and densely dispersed in the SA-Mo-C nanoboxes. • SA-Mo-C nanoboxes exhibited better ORR performance compared to commercial Pt/C catalysts. Construction of single-atom catalysts (SACs) with maximally exposed active sites remains a challenging task mainly because of the lack of suitable host matrices. In this study, hierarchical N -doped carbon nanoboxes composed of ultrathin nanosheets with dispersed atomic Mo (denoted as hierarchical SA-Mo-C nanoboxes) were fabricated via a template-engaged multistep synthesis process. Comprehensive characterizations, including X-ray absorption fine structure analysis, reveal the formation of Mo-N 4 atomic sites uniformly anchored on the hierarchical carbon nanoboxes. The prepared catalysts offer structural and morphological advantages, including ultrathin nanosheet units, unique hollow structures and abundant active Mo-N 4 species, that result in excellent activity with a half-wave potential of 0.86 V vs. RHE and superb stability for the oxygen reduction reaction in 0.1 M KOH; thus, the catalysts are promising air–cathode catalysts for Zn-air batteries with a high peak power density of 157.6 mW cm−2. [ABSTRACT FROM AUTHOR]

Flow field design and optimization are critical for the proton exchange membrane (PEM) fuel cells to meet the requirement of ultra-high power density commercial applications. However, the multi-physics transport mechanism inside such large-scale PEM fuel cells has not been fully understood. In this study, we have developed a three-dimensional (3D) PEM fuel cell model. It is first validated against the experimental data obtained from a small-size cell. Thereafter, based on this model, the performance characteristics of a large fuel cell (cell area: 245.76 cm2) assembled with a partially narrow flow field are elucidated, while the full bipolar plate (BP) and cell morphology are also considered. It is found that the partially narrow flow field can not only significantly increase the net power density of PEM fuel cells, but also effectively improve the uniform distribution of species with the staggered partially narrow (SPN) zone arrangement between adjacent channels. While this flow field can improve the cell performance, the blindly increasing of the narrow zone numbers may play a negative impact on the net power density due to the dramatic increase in pressure drop. Finally, the partially narrow channel can indeed provide improved drainage capacity. • Liquid water cooling is considered in the large-scale 3D model of PEM fuel cell. • Comprehensive comparison between the partially narrow and parallel flow fields. • The partially narrow channel has the advantage of enhancing oxygen convection. • The drainage mechanism of the partially narrow channel is clearly presented. [ABSTRACT FROM AUTHOR]

• The full-morphology simulation of the large-scale automobile PEM fuel cell is proposed. • All components, the realistic morphology of distribution area and the coolant flow is fully considered. • The advantages and disadvantages and its influence on physical and chemical processes of double-cell structure are deeply analyzed. • Novel double-cell structure can significantly improve power density (almost 20%) under actual operating conditions. The structural design of PEM fuel cell is crucial to improving its power density, and the cell structure largely depends on the bipolar plate (BP). This study compares the conventional single-cell structure and double-cell structure through three-dimensional (3D) large-scale (cell area: 312 cm2) full-morphology simulation. As for the double-cell structure, the channels are arranged in a dislocation manner and there is one cooling flow field per two cells. For the structure of the BP, we also fully considered the realistic morphology of distribution area and the coolant flow. It is found that the heat dissipation effect of double-cell structure is worse but the performance is very close compared to single-cell structure. Moreover, its volumetric power density is significantly improved (∼ 20%) due to the reduction in height. It is also found that increasing the velocity of coolant in the double-cell structure increases the performance first and then decreases. [ABSTRACT FROM AUTHOR]

A comprehensive mathematical model is proposed to calculate the optimal leg length and cross-section area of TEG unit to maximize the peak output power. The model shows that for a TEG unit, there exists an optimal ratio of leg length and leg cross-section area corresponding to the maximum peak output power with a convective thermal boundary condition, and the optimal leg length and cross-section area can be further calculated based on the optimal ratio. The mathematical model is also validated in this paper, and the corresponding error is within a reasonable range. Moreover, the effects of the leg length and leg area on the peak output power, the peak output power density and the efficiency of TEG unit with different thermal boundary conditions are also discussed. This study will provide guidance for the structure design optimization of TEG unit. [ABSTRACT FROM AUTHOR]