A Hierarchically Porous Carbon Fabric for Highly Sensitive Electrochemical Sensors

The hierarchically porous carbon fabrics with controlled conductivity and hydrophilicity have been fabricated by dual templating method of soft templates nested on hard templates. A non‐woven fabric coated with a solution of F127/resol has been carbonized for the synthesis of both macro‐porous structures of 10–15 µm in diameter having meso‐porous carbon structures of 4–6 nm, respectively. After carbonization treatment, not only conductivity is significantly improved, the hierarchically porous carbon also shows superhydrophilicity or water‐absorbing nature due to mild hydrophilic material and its dual scale roughness. The porous carbon becomes conductive with resistivity widely tuned from 5.4 × 10³ Ωm to 3.1 × 10^?3 Ωm by controlling the carbonization temperature. As the increased wettability for organic liquids could lead organic molecules deep into carbonized fabrics, the sensitivity of hierarchically porous carbon fabrics benefits the detection for methanol(CH₃OH) or hydrogen peroxide (H₂O₂). This new design concept of hierarchically porous structures having the multi‐functionality of high wettability and conductivity can be highly effective for electroanalytical sensors.

     

“Cubism” on the Nanoscale: From Squaric Acid to Porous Carbon Cubes

3D cube‐shaped composites and carbon microparticles with hierarchically porous structure are prepared by a facile template‐free synthesis route. Via the coordination of zinc acetate dihydrate and squaric acid, porous 3D cubic crystalline particles of zinc squarate can be obtained. These are easily transformed into the respective zinc oxide carbon composites under preservation of the macromorphology by heat treatment. Washing of the composite materials results in hierarchically porous carbons with high surface areas (1295 m² g^–1) and large pore volumes (1.5 cm³ g^?1) under full retention of the cube‐like architecture of the initial crystals. The materials are shown to be promising electrode materials for supercapacitor applications with a specific capacitance of 133 F g^?1 in H₂SO₄ at a scan rate of 5 mV s^?1, while 67% of this specific capacitance is retained, when increasing the scan rate to 200 mV s^?1.     

Biomass Derived N‐Doped Porous Carbon Supported Single Fe Atoms as Superior Electrocatalysts for Oxygen Reduction

Exploring sustainable and high‐performance electrocatalysts for the oxygen reduction reaction (ORR) is the crucial issue for the large‐scale application of fuel cell technology. A new strategy is demonstrated to utilize the biomass resource for the synthesis of N‐doped hierarchically porous carbon supported single‐atomic Fe (SA‐Fe/NHPC) electrocatalyst toward the ORR. Based on the confinement effect of porous carbon and high‐coordination natural iron source, SA‐Fe/NHPC, derived from the hemin‐adsorbed bio‐porphyra‐carbon by rapid heat‐treatment up to 800 °C, presents the atomic dispersion of Fe atoms in the N‐doped porous carbon. Compared with the molecular hemin and nanoparticle Fe samples, the as‐prepared SA‐Fe/NHPC exhibits a superior catalytic activity (E _1/2 = 0.87 V and J _k = 4.1 mA cm^?2, at 0.88 V), remarkable catalytic stability (≈1 mV negative shift of E _1/2, after 3000 potential cycles), and outstanding methanol‐tolerance, even much better than the state‐of‐the‐art Pt/C catalyst. The sustainable and effective strategy for utilizing biomass to achieve high‐performance single‐atom catalysts can also provide an opportunity for other catalytic applications in the atomic scale.     

Boosting Zn‐Ion Energy Storage Capability of Hierarchically Porous Carbon by Promoting Chemical Adsorption

The construction of advanced Zn‐ion hybrid supercapacitors (ZHSCs) with high energy density is promising but still challenging, especially at high current densities. In this work, a high‐energy and ultrastable aqueous ZHSC is demonstrated by introducing N dopants into a hierarchically porous carbon cathode for the purpose of enhancing its chemical adsorption of Zn ions. Experimental results and theoretical simulations reveal that N doping not only significantly facilitates the chemical adsorption process of Zn ions, but also greatly increases its conductivity, surface wettability, and active sites. Consequently, the as‐fabricated aqueous ZHSC based on this N‐doped porous carbon cathode displays an exceptionally high energy density of 107.3 Wh kg^?1 at a high current density of 4.2 A g^?1, a superb power density of 24.9 kW kg^?1, and an ultralong‐term lifespan (99.7% retention after 20 000 cycles), substantially superior to state‐of‐the‐art ZHSCs. Particularly, such a cathode also leads to a quasi‐solid‐state device with satisfactory energy storage performance, delivering a remarkable energy density of 91.8 Wh kg^?1. The boosted energy storage strategy by tuning the chemical adsorption capability is also applicable to other carbon materials.     

Molecular‐Level Design of Hierarchically Porous Carbons Codoped with Nitrogen and Phosphorus Capable of In Situ Self‐Activation for Sustainable Energy Systems

Hierarchically porous carbons are attracting tremendous attention in sustainable energy systems, such as lithium ion battery (LIB) and fuel cell, due to their excellent transport properties that arise from the high surface area and rich porosity. The state‐of‐the‐art approaches for synthesizing hierarchically porous carbons normally require chemical‐ and/or template‐assisted activation techniques, which is complicate, time consuming, and not feasible for large scale production. Here, a molecular‐level design principle toward large‐scale synthesis of nitrogen and phosphorus codoped hierarchically porous carbon (NPHPC) through an in situ self‐activation process is proposed. The material is fabricated based on the direct pyrolysis of a well‐designed polymer, melamine polyphosphate, which is capable of in situ self‐activation to generate large specific surface area (1479 m² g^?1) and hierarchical pores in the final NPHPC. As an anode material for LIB, NPHPC delivers a high reversible capacity of 1073 mAh g^?1 and an excellent cyclic stability for 300 cycles with negligible capacity decay. The peculiar structural properties and synergistic effect of N and P codopants also enable NPHPC a promising electrocatalyst for oxygen reduction reaction, a key cathodic reaction process of many energy conversion devices (for example, fuel cells and metal air batteries). Electrochemical measurements show NPHPC a comparable electrocatalytic performance to commercial Pt/C catalyst (onset potential of 0.88 V vs reversible hydrogen electrode in alkaline medium) with excellent stability (89.8% retention after 20 000 s continuous operation) and superior methanol tolerance.     

“Wiring” Fe‐Nx‐Embedded Porous Carbon Framework onto 1D Nanotubes for Efficient Oxygen Reduction Reaction in Alkaline and Acidic Media

This study presents a novel metal‐organic‐framework‐engaged synthesis route based on porous tellurium nanotubes as a sacrificial template for hierarchically porous 1D carbon nanotubes. Furthermore, an ultrathin Fe‐ion‐containing polydopamine layer has been introduced to generate highly effective FeN_xC active sites into the carbon framework and to induce a high degree of graphitization. The synergistic effects between the hierarchically porous 1D carbon structure and the embedded FeN_xC active sites in the carbon framework manifest in superior catalytic activity toward oxygen reduction reaction (ORR) compared to Pt/C catalyst in both alkaline and acidic media. A rechargeable zinc‐air battery assembled in a decoupled configuration with the nonprecious pCNT@Fe@GL/CNF ORR electrode and Ni‐Fe LDH/NiF oxygen evolution reaction (OER) electrode exhibits charge–discharge overpotentials similar to the counterparts of Pt/C ORR electrode and IrO₂ OER electrode.     

Micro‐Blooming: Hierarchically Porous Nitrogen‐Doped Carbon Flowers Derived from Metal‐Organic Mesocrystals

Synthesis of 3D flower‐like zinc‐nitrilotriacetic acid (ZnNTA) mesocrystals and their conformal transformation to hierarchically porous N‐doped carbon superstructures is reported. During the solvothermal reaction, 2D nanosheet primary building blocks undergo oriented attachment and mesoscale assembly forming stacked layers. The secondary nucleation and growth preferentially occurs at the edges and defects of the layers, leading to formation of 3D flower‐like mesocrystals comprised of interconnected 2D micropetals. By simply varying the pyrolysis temperature (550–1000 °C) and the removal method of in the situ‐generated Zn species, nonporous parent mesocrystals are transformed to hierarchically porous carbon flowers with controllable surface area (970–1605 m² g^?1), nitrogen content (3.4–14.1 at%), pore volume (0.95–2.19 cm³ g^?1), as well as pore diameter and structures. The carbon flowers prepared at 550 °C show high CO₂/N₂ selectivity due to the high nitrogen content and the large fraction of (ultra)micropores, which can greatly increase the CO₂ affinity. The results show that the physicochemical properties of carbons are highly dependent on the thermal transformation and associated pore formation process, rather than directly inherited from parent precursors. The present strategy demonstrates metal‐organic mesocrystals as a facile and versatile means toward 3D hierarchical carbon superstructures that are attractive for a number of potential applications.     

Heteroatom‐Containing Porous Carbons Derived from Ionic Liquid‐Doped Alkali Organic Salts for Supercapacitors

A simple strategy for the synthesis of heteroatom‐doped porous carbon materials (CMs) via using ionic liquid (IL)‐doped alkali organic salts as small molecular precursors is developed. Doping of alkali organic salts (such as sodium glutamate, sodium tartrate, and sodium citrate) with heteroatoms containing ILs (including 1‐butyl‐3‐methylimidazolium chlorine and 3‐butyl‐4‐methythiazolebromination) not only incorporates the heteroatoms into the carbon frameworks but also highly improves the carbonization yield, as compared with that of either alkali organic salts or ILs as precursors. The porous structure of CMs can be tuned by adjusting the feed ratio of ILs. The porous CMs derived from 1‐butyl‐3‐methylimidazolium chlorine‐doped sodium glutamate exhibit high charge storage capacity with a specific capacitance of 287 F g^?1 and good stability over 5000 cycles in 6 m KOH at a current density of 1 A g^?1 for supercapacitors. This strategy opens a simple and efficient method for the synthesis of heteroatom‐doped porous CMs.     

A Microribbon Hybrid Structure of CoOx‐MoC Encapsulated in N‐Doped Carbon Nanowire Derived from MOF as Efficient Oxygen Evolution Electrocatalysts

Developing highly efficient electrocatalysts for oxygen evolution is vital for renewable and sustainable energy production and storage. Herein, nitrogen‐doped carbon encapsulated CoOx‐MoC heterostructures are reported for the first time as high performance oxygen evolution electrocatalysts. The composition can be tuned by the addition of a Mo source to form a nanowire‐assembled hierarchically porous microstructure, which can enlarge the specific surface area, thus exposing more active sites, facilitating mass transport and charge transfer. Moreover, it is demonstrated that the formation of CoOx‐MoC heterostructures and the resulting synergistic effect between MoC and Co facilitate the reaction kinetics, leading to significantly improved oxygen evolution reaction (OER) activity with an onset overpotential of merely 290 mV, and a low overpotential of 330 mV to afford a current density of 10 mA cm^?2. The well‐constructed microarchitecture contributes to superior long term stability electrochemical behaviors. This work provides a facile strategy via composition tuning and structure optimization for the development of next‐generation nonprecious metal‐based OER electrocatalysts.     

Hierarchically porous carbon foams for electric double layer capacitors

The growing demand for portable electronic devices means that lightweight power sources are increasingly sought after. Electric double layer capacitors (EDLCs) are promising candidates for use in lightweight power sources due to their high power densities and outstanding charge/discharge cycling stabilities. Three-dimensional (3D) self-supporting carbon-based materials have been extensively studied for use in lightweight EDLCs. Yet, a major challenge for 3D carbon electrodes is the limited ion diffusion rate in their internal spaces. To address this limitation, hierarchically porous 3D structures that provide additional channels for internal ion diffusion have been proposed. Herein, we report a new chemical method for the synthesis of an ultralight (9.92 mg/cm³) 3D porous carbon foam (PCF) involving carbonization of a glutaraldehydecross-linked chitosan aerogel in the presence of potassium carbonate. Electron microscopy images reveal that the carbon foam is an interconnected network of carbon sheets containing uniformly dispersed macropores. In addition, Brunauer–Emmett–Teller measurements confirm the hierarchically porous structure. Electrochemical data show that the PCF electrode can achieve an outstanding gravimetric capacitance of 246.5 F/g at a current density of 0.5 A/g, and a remarkable capacity retention of 67.5% was observed when the current density was increased from 0.5 to 100 A/g. A quasi-solid-state symmetric supercapacitor was fabricated via assembly of two pieces of the new PCF and was found to deliver an ultra-high power density of 25 kW/kg at an energy density of 2.8 Wh/kg. This study demonstrates the synthesis of an ultralight and hierarchically porous carbon foam with high capacitive performance.

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Silk‐Derived 2D Porous Carbon Nanosheets with Atomically‐Dispersed Fe‐Nx‐C Sites for Highly Efficient Oxygen Reaction Catalysts

Controlled synthesis of highly efficient, stable, and cost‐effective oxygen reaction electrocatalysts with atomically‐dispersed Me–N_x–C active sites through an effective strategy is highly desired for high‐performance energy devices. Herein, based on regenerated silk fibroin dissolved in ferric chloride and zinc chloride aqueous solution, 2D porous carbon nanosheets with atomically‐dispersed Fe–N_x–C active sites and very large specific surface area (≈2105 m² g^?1) are prepared through a simple thermal treatment process. Owing to the 2D porous structure with large surface area and atomic dispersion of Fe–N_x–C active sites, the as‐prepared silk‐derived carbon nanosheets show superior electrochemical activity toward the oxygen reduction reaction with a half‐wave potential (E_1/2) of 0.853 V, remarkable stability with only 11 mV loss in E_1/2 after 30 000 cycles, as well as good catalytic activity toward the oxygen evolution reaction. This work provides a practical and effective approach for the synthesis of high‐performance oxygen reaction catalysts towards advanced energy materials.     

Scalable 2D Hierarchical Porous Carbon Nanosheets for Flexible Supercapacitors with Ultrahigh Energy Density

2D carbon nanomaterials such as graphene and its derivatives, have gained tremendous research interests in energy storage because of their high capacitance and chemical stability. However, scalable synthesis of ultrathin carbon nanosheets with well‐defined pore architectures remains a great challenge. Herein, the first synthesis of 2D hierarchical porous carbon nanosheets (2D‐HPCs) with rich nitrogen dopants is reported, which is prepared with high scalability through a rapid polymerization of a nitrogen‐containing thermoset and a subsequent one‐step pyrolysis and activation into 2D porous nanosheets. 2D‐HPCs, which are typically 1.5 nm thick and 1–3 µm wide, show a high surface area (2406 m² g^?1) and with hierarchical micro‐, meso‐, and macropores. This 2D and hierarchical porous structure leads to robust flexibility and good energy‐storage capability, being 139 Wh kg^?1 for a symmetric supercapacitor. Flexible supercapacitor devices fabricated by these 2D‐HPCs also present an ultrahigh volumetric energy density of 8.4 mWh cm^?3 at a power density of 24.9 mW cm^?3, which is retained at 80% even when the power density is increased by 20‐fold. The devices show very high electrochemical life (96% retention after 10000 charge/discharge cycles) and excellent mechanical flexibility.     

Synthesis of Olive‐Like Nitrogen–Doped Carbon with Embedded Ge Nanoparticles for Ultrahigh Stable Lithium Battery Anodes

The development of environment‐friendly and high‐performance carbon materials for energy applications has remained a great challenge. Here, a novel and facile method for synthesis of olive‐like nitrogen‐doped carbon embedded with germanium (Ge) nanoparticles using widespread and nontoxic dopamine as carbon and nitrogen precursors is demonstrated, especially by understanding the tendency of pure GeO₂ nanoparticles forming ellipsoidal aggregation, and the chelating reaction of the catechol structure in dopamine with metal ions. The as‐synthesized Ge/N‐C composites show an olive‐like porous carbon structure with a loading weight of as high as 68.5% Ge nanoparticles. A lithium ion battery using Ge/N‐C as the anode shows 1042 mAh g^?1 charge capacity after 2000 cycles (125 d) charge/discharge at C/2 (1C = 1600 mA g^?1) with a capacity maintaining efficiency of 99.6%, significantly exceeding those of the previously reported Ge/C‐based anode materials. This prominent cyclic charge/discharge performance of the Ge/N‐C anode is attributed to the well‐dispersed Ge nanoparticles in graphitic N‐doped carbon matrix, which facilitates high rates (0.5–15 C) of charge/discharge and increases the anode structure integrity. The synthesis strategy presented here may be a very promising approach to prepare a series of active nanoparticle–carbon hybrid materials with nitrogen doping for more and important applications.     

Aerosol‐Assisted Heteroassembly of Oxide Nanocrystals and Carbon Nanotubes into 3D Mesoporous Composites for High‐Rate Electrochemical Energy Storage

Nanostructured composites built from ordinary building units have attracted much attention because of their collective properties for critical applications. Herein, we have demonstrated the heteroassembly of carbon nanotubes and oxide nanocrystals using an aerosol spray method to prepare nanostructured mesoporous composites for electrochemical energy storage. The designed composite architectures show high conductivity and hierarchically structured mesopores, which achieve rapid electron and ion transport in electrodes. Therefore, as‐synthesized carbon nanotube/TiO₂ electrodes exhibit high rate performance through rapid Li⁺ intercalation, making them suitable for ultrafast energy storage devices. Moreover, the synthesis process provides a broadly applicable method to achieve the heteroassembly of vast low‐dimensional building blocks for many important applications.     

Sandwich Photothermal Membrane with Confined Hierarchical Carbon Cells Enabling High‐Efficiency Solar Steam Generation

Solar‐driven vaporization is a sustainable solution to water and energy scarcity. However, most of the present evaporators are still suffering from inefficient utilization of converted thermal energy. Herein, a universal sandwich membrane strategy is demonstrated by confining the hierarchical porous carbon cells in two energy barriers to obtain a high‐efficiency evaporator with a rapid water evaporation rate of 1.87 kg m^?2 h^?1 under 1 sun illumination, which is among the highest performance for carbon‐based and wood‐based evaporators. The significantly enhanced evaporation rate is mainly attributed to the inherently optimized porous evaporation mode derived from the hierarchical hollow structures of pollen carbon cells, and the synergistically regulated water transporting and thermal management performance of the sandwich membrane. Moreover, the constructed sandwich membrane also exhibits excellent self‐regenerating performance in simulated seawater and high salinity water. The developed device can maintain an average evaporation rate of 4.3 L m^?2 day^?1 in a 25 day consecutive outdoor test.     

Multifunctional Electroactive Heteroatom‐Doped Carbon Aerogels

The design and synthesis of highly active, durable, and cheap nanomaterials for various renewable energy storage and conversion applications is extremely desirable but remains challenging. Here, a green and efficient strategy to produce CoO_x nanoparticles and surface N‐co‐doped carbon aerogels (Co‐N‐CAs) is reported by multicomponent surface self‐assembly of commercially melamine sponge (CMS). In the methodology, the CMS simultaneously function as green N precursor for surface N doping and 3D support. The resulting Co‐N‐CAs exhibit 3D hierarchical, interconnected macro‐ and bimodal meso‐porosity (6.3 nm and <4 nm), high surface area (1383 m² g^?1), and highly dispersed, semi‐exposured CoO_x nanoparticles (diameter of 12.5 nm). The surface doping of N, semi‐exposured configuration of CoO_x nanoparticles and the penetrated complementary pores (<4 nm) in the carbon walls provide highly accessibility between electroactive components and electrolytes to improve reactivity. With their tailored architecture, the Co‐N‐CAs show superior electrocatalytic oxygen reduction (ORR) activities comparable to the commercially Pt/C catalysts, high specific capacitance (433 F g^?1), excellent lithium storage (938 mAh g^?1), and outstanding durability, making them very promising for advanced energy conversion and storage. In addition, the presented strategy can be extended to fabricate other metal oxide‐ and N‐ co‐doped carbon aerogels for diverse energy‐related applications.     

Promoted Glycerol Oxidation Reaction in an Interface‐Confined Hierarchically Structured Catalyst

Confined catalysis in a 2D system is of particular interest owing to the facet control of the catalysts and the anisotropic kinetics of reactants, which suppress side reactions and improve selectivity. Here, a 2D‐confined system consisting of intercalated Pt nanosheets within few‐layered graphene is demonstrated. The strong metal–substrate interaction between the Pt nanosheets and the graphene leads to the quasi‐2D growth of Pt with a unique (100)/(111)/(100) faceted structure, thus providing excellent catalytic activity and selectivity toward one‐carbon (C1) products for the glycerol oxidation reaction. A hierarchically porous graphene architecture, grown on carbon cloth, is used to fabricate the confined catalyst bed in order to enhance the mass‐diffusion limitation in interface‐confined reactions. Owing to its unique 3D porous structure, this graphene‐confined Pt catalyst exhibits an extraordinary mass activity of 2910 mA mg_Pt ^?1 together with a formate selectivity of 79% at 60 °C. This paves the way toward rational designs of heterogeneous catalysts for energy‐related applications.     

Tailoring Hierarchically Porous Nitrogen‐, Sulfur‐Codoped Carbon for High‐Performance Supercapacitors and Oxygen Reduction

Heteroatom‐doped carbon materials are intensively studied in supercapacitors and fuel cells, because of their great potential for sustainably bearing on the energy crisis and environmental pollution. Although enormous efforts are put in material perfection with a hierarchically porous microstructure, the simultaneous optimization of both porous structures and surface functionalities is hard to achieve due to inevitable concurrent dopant leaching effect and structural collapse under required high pyrolysis temperature. In this study, an in situ dehalogenation polymerization and activation protocol is introduced to synthesize nitrogen‐ and sulfur‐codoped carbon materials (NS‐PCMs) with hierarchical pore distribution and abundant surface doping, which endows them with good conductivity, abundant accessible active sites, and efficient mass transport. As a result, the as‐prepared carbon materials (NS‐a‐PCM‐1000) show an excellent mass specific capacitance of 461.5 F g^?1 at a current density of 0.1 A g^?1, long cycle life (>23 k, 10 A g^?1), and high device energy and power density (17.3 Wh kg^?1, 250 W kg^?1). Significantly, NS‐a‐PCM‐1000 also exhibits one of the highest oxygen reduction reaction activities (onset potential of 1.0 V vs reversible hydrogen electrode) in alkaline media among all reported metal‐free catalysts.     

Hierarchically Porous Fe2CoSe4 Binary‐Metal Selenide for Extraordinary Rate Performance and Durable Anode of Sodium‐Ion Batteries

Owing to high energy capacities, transition metal chalcogenides have drawn significant research attention as the promising electrode materials for sodium‐ion batteries (SIBs). However, limited cycle life and inferior rate capabilities still hinder their practical application. Improvement of the intrinsic conductivity by smart choice of elemental combination along with carbon coupling of the nanostructures may result in excellence of rate capability and prolonged cycling stability. Herein, a hierarchically porous binary transition metal selenide (Fe₂CoSe₄, termed as FCSe) nanomaterial with improved intrinsic conductivity was prepared through an exclusive methodology. The hierarchically porous structure, intimate nanoparticle–carbon matrix contact, and better intrinsic conductivity result in extraordinary electrochemical performance through their synergistic effect. The synthesized FCSe exhibits excellent rate capability (816.3 mA h g^?1 at 0.5 A g^?1 and 400.2 mA h g^?1 at 32 A g^?1), extended cycle life (350 mA h g^?1 even after 5000 cycles at 4 A g^?1), and adequately high energy capacity (614.5 mA h g^?1 at 1 A g^?1 after 100 cycles) as anode material for SIBs. When further combined with lab‐made Na₃V₂(PO₄)₃/C cathode in Na‐ion full cells, FCSe presents reasonably high and stable specific capacity.     

3D Mesoporous Graphene: CVD Self‐Assembly on Porous Oxide Templates and Applications in High‐Stable Li‐S Batteries

A nanostructured carbon with high specific surface area (SSA), tunable pore structure, superior electrical conductivity, mechanically robust framework, and high chemical stability is an important requirement for electrochemical energy storage. Porous graphene fabricated by chemical activation and liquid etching has a high surface area but very limited volume of electrochemically accessible mesopores. Herein, an effective strategy of in situ formation of hierarchically mesoporous oxide templates with small pores induced by Kirkendall diffusion and large pores attributed to evaporation of deliberately introduced volatile metal is proposed for chemical vapor deposition assembly of porous graphene frameworks (PGFs). The PGFs inherit the hierarchical mesoporous structure of the templates. A high SSA of 1448 m² g⁻¹, 91.6% of which is contributed by mesopores, and a mesopore volume of 2.40 cm³ g⁻¹ are attained for PGFs serving as reservoirs of ions or active materials in electrochemical energy storage applications. When the PGFs are applied in lithium‐sulfur batteries, a very high sulfur utilization of 71% and a very low fading rate of ≈0.04% per cycle after the second cycle are achieved at a current rate of 1.0 C. This work provides a general strategy for the rational construction of mesoporous structures induced by a volatile metal, with a view toward the design of hierarchical nanomaterials for advanced energy storage.