Potassium-ion batteries are potential alternatives to lithium-ion batteries for large-scale energy storage considering the low cost and high abundance of potassium. However, it is challenging to obtain stable electrode materials capable of undergoing long-term potassiation/depotassiation due to the high accumulated stress associated with the huge volume variation of the electrode. Here, we simulate the von Mises stress distributions of four different carbon three-dimensional models under an isotropic initial stress by the finite element method and reveal the critical role of the structure of a hollow multihole bowl on the strain−relaxation behavior. In this regard, nitrogen/oxygen codoped carbon hollow multihole bowls (CHMBs) are synthesized via hydrothermal carbonization coupled with an emulsiontemplating strategy using biomass as the carbon source. Consistent with our simulation results, the CHMB anode remains stable for over 1000 cycles and delivers a high reversible capacity of 304 mAh g −1 at 0.1 A g −1 . In addition to the reduced stress accumulation, the good electrochemical performances are also attributed to the surface capacitive mechanism and the shortened electron/ion transport distance in CHMBs. In particular, the CHMB composite electrode has a volumetric specific capacity 56% higher than that of hollow spheres due to the high tapped density of the bowl-shaped particles.

COMMUNICATION (1 of 7)2D material is an excellent starting point for developing catalysts, size-selective membranes, and electronics and energy storage devices because of its atomic thickness and "nano-effect." [1] However, pores that serve as tunnels for target species are often preferred and need to be introduced in some cases. Graphene itself is an example as it is not permeable to gas species, metal ions, and liquid molecules. In order to realize their successful application in molecular sieves and energy storage, recently new graphene nanomesh and holey graphene are created to achieve excellent transport across the plane and ultimate access to the inner surfaces. [2] The common hole-making methods are postprocessing of graphene, such as ultraviolet-induced oxidative etching and lithography. [2c,d] The general chemical/mechanical exfoliation method for graphene is not applicable to synthesize porous graphene because high-crystalline peelable porous graphite cannot be produced.2D layered double hydroxide (LDH) is another important member of the 2D material family and has shown exciting perspectives in many fields, especially in electrocatalysis (e.g., the hydrogen and oxygen evolution reactions (HER and OER) in water splitting). [3] However, as opposed to graphene, the 2D hydroxide with in-plane pores has rarely been reported so far. If the common chemical-exfoliation strategy of 2D metal hydroxide were to be applied to synthesize holey 2D hydroxide, getting high-quality porous single-crystalline precursor would be the crucial prerequisite because most porous materials are amorphous or polycrystalline. [4] Fortunately, recently we have developed mesoporous single-crystalline Co(OH) 2 nanoplates via a copolymer-templating technique, which makes the synthesis of holey hydroxide 2D material possible. [5] OER is the oxidative half reaction of water splitting that involves the transfer of four protons and four electrons. [3d,6] Therefore, it is kinetically sluggish and always requires conspicuous overpotential. [3d,6,7] The precious-metal catalysts, e.g., IrO 2 and RuO 2 , despite performing well on the catalysis toward OER, are extremely scarce on the earth and also expensive, so earth-abundant transition-metal electrocatalysts including oxides/hydroxides and their derivatives have been developed recently. [8] Especially, some reports pointed that transition-metal LDH can be exfoliated into 2D nanosheets with large anisotropy to get extraordinarily improved catalytic performances, because Creating nanosized pores in 2D materials can increase the edge sites, improve the mass transfer, and contribute to different physical properties, which shows potential applications in many fields including filtration membranes, electronics and energy storage devices, and catalysts. An iconic member of this type of material is porous graphene. Herein, a unique 2D layered double hydroxide (LDH) nanomesh is designed and synthesized as a new class of 2D holey materials. It represents the first case of exfoliation method for p...

Maximizing the Pt utilization is important for the widescale implementation of Pt-based hydrogen evolution reaction (HER) electrocatalysts, owing to the scarcity of Pt. Here, three-component heterostructured HER catalysts with ultrahigh Pt mass activity in which hollow PtCu alloy nanospheres are supported on an array of WO 3 on Cu foam, are reported. It has been pointed out that the use of Pt counter electrode in a three-electrode configuration in evaluating catalysts' HER performances in acidic media carries the risk of contaminating the working electrode in previous reports. Here, the authors rationally utilize this "contaminating" to "activate" low-HER-activity materials, maximizing the Pt utilization. As a result, ultrahigh Pt mass activity is achieved, that is 1.35 and 10.86 A mg −1 Pt at overpotentials of 20 and 100 mV, respectively, 27 and 13 times higher than those of commercial Pt/C catalysts, outperforming some state-of-the-art Pt-single-atom catalysts. The hollow sphere structure and PtCu alloying increase the number and reactivity of active sites. Density function calculations and electrochemical experiments reveal that the synergy between WO 3 and Pt is also responsible for the high HER activity where the hydrogen spillover effect triggers the Volmer-Heyrovsky mechanism and promotes the rapid removal of H * from Pt to re-expose the active sites.

Bifunctional biomass-derived N, S dual-doped ladder-like porous carbon for supercapacitor and oxygen reduction reaction,

Hollow structures are often used to relieve the intrinsic strain on metal oxide electrodes in alkali-ion batteries. Nevertheless, one common drawback is that the large interior space leads to low volumetric energy density and inferior electric conductivity. Here, the von Mises stress distribution on a mesoporous hollow bowl (HB) is simulated via the finite element method, and the vital role of the porous HB structure on strain-relaxation behavior is confirmed. Then, N-doped-C coated mesoporous α-Fe 2 O 3 HBs are designed and synthesized using a multistep soft/hard-templating strategy. The material has several advantages: (i) there is space to accommodate strains without sacrificing volumetric energy density, unlike with hollow spheres; (ii) the mesoporous hollow structure shortens ion diffusion lengths and allows for high-rate induced lithiation reactivation; and (iii) the N-doped carbon nanolayer can enhance conductivity. As an anode in lithium-ion batteries, the material exhibits a very high reversible capacity of 1452 mAh g −1 at 0.1 A g −1 , excellent cycling stability of 1600 cycles (964 mAh g −1 at 2 A g −1 ), and outstanding rate performance (609 mAh g −1 at 8 A g −1 ). Notably, the volumetric specific capacity of composite electrode is 42% greater than that of hollow spheres. When used in potassium-ion batteries, the material also shows high capacity and cycle stability.