Hierarchically Porous Carbon Rods Derived from Metal-Organic Frameworks for Aqueous Zinc-Ion Hybrid Capacitors

Aqueous zinc-ion hybrid capacitors (ZIHCs), as ideal candidates for high energy-power supply systems, are restricted by unsatisfied energy density and poor cycling durability for further applications. The construction of a surface-functionalized carbon cathode is an effective strategy for improving the performance of ZIHCs. Herein, a high-performance ZIHC is achieved using oxygen-rich hierarchically porous carbon rods (MDPC-X) prepared by the pyrolysis of a metal-organic framework (MOF) assisted by KOH activation. The MDPC-X samples displayed high electric double-layer capacitance (EDLC) and pseudocapacitance owing to their oxygen-rich surfaces, abundant electroactive sites, and short ions/electron transfer lengths. The surface oxygen functional groups for the reversible chemical adsorption/desorption of Zn2+ are identified using ex situ X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). Consequently, the as-assembled ZIHC exhibited a high capacity of 323.4 F g-1 (161.7 mA h g-1) at 0.5 A g-1 and a retention of 147 F g-1 (73.5 mA h g-1) at an ultrahigh current density of 50 A g-1, corresponding to high energy and power densities of 145.5 W h kg-1 and 45 kW kg-1, respectively. Furthermore, an excellent cycling life with 96.5% of capacity retention is also maintained after 10 000 cycles at 10 A g-1, demonstrating its promising potential for applications.

Phyllanthus emblica Seed-Derived Hierarchically Porous Carbon Materials for High-Performance Supercapacitor Applications

The electrical double-layer supercapacitance performance of the nanoporous carbons prepared from the Phyllanthus emblica (Amala) seed by chemical activation using the potassium hydroxide (KOH) activator is reported. KOH activation was carried out at different temperatures (700-1000 °C) under nitrogen gas atmosphere, and in a three-electrode cell set-up the electrochemical measurements were performed in an aqueous 1 M sulfuric acid (H₂SO₄) solution. Because of the hierarchical pore structures with well-defined micro- and mesopores, Phyllanthus emblica seed-derived carbon materials exhibit high specific surface areas in the range of 1360 to 1946 m² g^-1, and the total pore volumes range from 0.664 to 1.328 cm³ g^-1. The sample with the best surface area performed admirably as the supercapacitor electrode-material, achieving a high specific capacitance of 272 F g^-1 at 1 A g^-1. Furthermore, it sustained 60% capacitance at a high current density of 50 A g^-1, followed by a remarkably long cycle-life of 98% after 10,000 subsequent charging/discharging cycles, demonstrating the electrode's excellent rate-capability. These results show that the Phyllanthus emblica seed would have significant possibilities as a sustainable carbon-source for the preparing high-surface-area activated-carbons desired in high-energy-storage supercapacitors.

Nelumbo nucifera Seed-Derived Nitrogen-Doped Hierarchically Porous Carbons as Electrode Materials for High-Performance Supercapacitors

Biomass-derived activated carbon materials with hierarchically nanoporous structures containing nitrogen functionalities show excellent electrochemical performances and are explored extensively in energy storage and conversion applications. Here, we report the electrochemical supercapacitance performances of the nitrogen-doped activated carbon materials with an ultrahigh surface area prepared by the potassium hydroxide (KOH) activation of the Nelumbo nucifera (Lotus) seed in an aqueous electrolyte solution (1 M sulfuric acid: H2SO4) in a three-electrode cell. The specific surface areas and pore volumes of Lotus-seed-derived carbon materials carbonized at a different temperatures, from 600 to 1000 °C, are found in the range of 1059.6 to 2489.6 m2 g-1 and 0.819 to 2.384 cm3 g-1, respectively. The carbons are amorphous materials with a partial graphitic structure with a maximum of 3.28 atom% nitrogen content and possess hierarchically micro- and mesoporous structures. The supercapacitor electrode prepared from the best sample showed excellent electrical double-layer capacitor performance, and the electrode achieved a high specific capacitance of ca. 379.2 F g-1 at 1 A g-1 current density. Additionally, the electrode shows a high rate performance, sustaining 65.9% capacitance retention at a high current density of 50 A g-1, followed by an extraordinary long cycle life without any capacitance loss after 10,000 subsequent charging/discharging cycles. The electrochemical results demonstrate that Nelumbo nucifera seed-derived hierarchically porous carbon with nitrogen functionality would have a significant probability as an electrical double-layer capacitor electrode material for the high-performance supercapacitor applications.

Enhanced Energy Density for P-Doped Hierarchically Porous Carbon-Based Symmetric Supercapacitor with High Operation Potential in Aqueous H2SO4 Electrolyte

Phosphorus-doped hierarchically porous carbon (HPC) is prepared with the assistance of freeze-drying using colloid silica and phytic acid dipotassium salt as a hard template and phosphorus source, respectively. Intensive material characterizations show that the freeze-drying process can effectively promote the porosity of HPC. The specific surface area and P content for HPC can reach up to 892 m² g^-1 and 2.78 at%, respectively. Electrochemical measurements in aqueous KOH and H₂SO₄ electrolytes reveal that K⁺ of a smaller size can more easily penetrate the inner pores compared with SO₄^2-, while the developed microporosity in HPC is conducive to the penetration of SO₄^2-. Moreover, P-doping leads to a high operation potential of 1.5 V for an HPC-based symmetric supercapacitor, resulting in an enhanced energy density of 16.4 Wh kg^-1. Our work provides a feasible strategy to prepare P-doped HPC with a low dosage of phosphorus source and a guide to construct a pore structure suitable for aqueous H₂SO₄ electrolyte.

Vacancy Occupation-Driven Polymorphic Transformation in Cobalt Ditelluride for Boosted Oxygen Evolution Reaction

Transition-metal dichalcogenides (TMDs) hold great potential as an advanced electrocatalyst for oxygen evolution reaction (OER), but to date the activity of transition metal telluride catalysts are demonstrated to be poor for this reaction. In this study, we report the activation of CoTe₂ for OER by doping secondary anions into Te vacancies to trigger a structural transition from the hexagonal to the orthorhombic phase. The achieved orthorhombic CoTe₂ with partial vacancies occupied by P-doping exhibits an exceptional OER catalytic activity with an overpotential of only 241 mV at 10 mA cm^-2 and a robust stability more than 24 h. The combined experimental and theoretical studies suggest that the defective phase transformation is controllable and allows the synergism of vacancy, doping as well as the reconstructed crystallographic structure, ensuring more exposure of catalytic active sites, rapid charge transfer, and energetically favorable intermediates. This vacancy occupation-driven strategy of structural transformation can also be manipulated by S- and Se-doping, which may offer useful guidance for developing tellurides-based electrocatalyst for OER.

Hierarchically Porous Carbons Derived from Nonporous Coordination Polymers

Hierarchically porous carbons (HPCs) with multimodal pore systems exhibit great technological potentials, especially in the fields of heterogeneous catalysis, energy storage, and conversion. Here, we establish a simple and general approach to HPCs by carbonization of nonporous coordination polymers that are produced by mixing metal salts with polytopic ligands in alkaline aqueous solutions at room temperature. The proposed approach is applicable to a wide scope of ligand molecules (18 examples), thus affording the synthesized HPCs with high diversity in porosity, morphology, and composition. In particular, the prepared HPCs exhibit high specific surface areas (up to 2647 m² g^-1) and large pore volumes (up to 2.39 cm³ g^-1). The HPCs-supported atomically dispersed Fe-N_x catalysts show much-improved fuel cell cathode performance over the micropore-dominated carbon black-supported catalysts, demonstrating the structural superiority of the HPCs for enhancing the mass transport properties.

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.

Synthesis of Hierarchical Porous Carbon in Molten Salt and Its Application for Dye Adsorption

Hierarchical porous carbon was successfully synthesized from glucose in a molten salt at 800 °C for 2 h. It was amorphous and contained numerous oxygen containing functional groups on its surface. The porous carbon with 1.0 wt% Fe(NO₃)₃·9H₂O oxidizing agent showed the highest specific surface area of 1078 m²/g, and the largest pore volume of 0.636 cm³/g, among all of the samples. Raman and TEM results revealed that it had more defects and pores than other as-prepared carbon materials. The adsorption capacities of as-prepared porous carbon for methylene blue (MB) and methyl orange (MO) were 506.8 mg/g and 683.8 mg/g, respectively. The adsorption isotherms fit the Langmuir model and the adsorption kinetics followed the pseudo-second-order kinetic model.

Nitrogen/Oxygen Co-Doped Hierarchically Porous Carbon for High-Performance Potassium Storage

Although the insertion of potassium ions into graphite has been proven to be realistic, the electrochemical performance of potassium-ion batteries (PIBs) is not yet satisfactory. Therefore, more effort is required to improve the specific capabilities and achieve a long cycling life. The mild carbonization process in molten salt (NaCl-KCl) is used to synthesize nitrogen/oxygen co-doped hierarchically porous carbon (NOPC) for PIBs by using cyanobacteria as the carbon source. This exhibits highly reversible capacities and ultra-long cycling stability, retaining a capacity of 266 mA h g^-1 at 50 mA g^-1 (100 cycles) and presents a capacity of 104.3 mA h g^-1 at 1000 mA g^-1 (1000 cycles). Kinetics analysis reveals that the potassium ion (K⁺ ) storage of NOPC is controlled by a capacitive process, which plays a crucial role in the excellent rate performance and superior reversible ability. The high proportion of capacitive behavior can be ascribed to the hierarchically porous structure and improved conductivity resulting from nitrogen and oxygen doping. Furthermore, density functional theory (DFT) calculations theoretically validate the enhanced potassium storage effect of the as-obtained NOPC. More importantly, the route to NOPC from cyanobacteria in molten salt provides a green approach to the synthesis of porous carbon materials.

In Situ Generated Dual-Template Method for Fe/N/S Co-Doped Hierarchically Porous Honeycomb Carbon for High-Performance Oxygen Reduction

Heteroatoms doping is able to produce catalytic sites in carbon materials for oxygen reduction reaction (ORR); while hierarchically porous structure is necessary for efficient exposure and accessibility of the usually limited catalytic sites in such activated carbon catalysts. This work reports an in situ generated dual-template method to synthesize the Fe/N/S co-doped hierarchically porous carbon (FeNS/HPC), with NaCl crystallites formed during the precursor lyophilization process as the primary template to generate ∼500 nm macropores with ultrathin graphene-like carbon-layer walls, and Fe₃O₄ nanoparticles formed during the high-temperature carbonization process as the secondary template to produce mesopores on the walls of macropores. As well as the coexistence of graphitic-N, pyridinic-N, and thiophene-S which are beneficial to ORR, the as prepared FeNS/HPC possesses a highly graphitized and interconnected hierarchical porous structure, giving a specific surface area as high as 938 m² g^-1. As a consequence, it exhibits excellent four-electron oxygen reduction performance in both alkaline and acid electrolytes. The in situ generation and facile solution removal make the present template method a promising way for scale-up preparation of active porous carbon materials for various applications.

Hierarchically Porous Graphitic Carbon with Simultaneously High Surface Area and Colossal Pore Volume Engineered via Ice Templating

Developing hierarchical porous carbon (HPC) materials with competing textural characteristics such as surface area and pore volume in one material is difficult to accomplish, particularly for an atomically ordered graphitic carbon. Herein we describe a synthesis strategy to engineer tunable HPC materials across micro-, meso-, and macroporous length scales, allowing the fabrication of a graphitic HPC material (HPC-G) with both very high surface area (>2500 m²/g) and pore volume (>11 cm³/g), the combination of which has not been attained previously. The mesopore volume alone for these materials is up to 7.53 cm³/g, the highest ever reported, higher than even any porous carbon's total pore volume, which for our HPC-G material was >11 cm³/g. This HPC-G material was explored for use both as a supercapacitor electrode and for oil adsorption, two applications that require either high surface area or large pore volume, textural properties that are typically exclusive to one another. We accomplished these high textural characteristics by employing ice templating not only as a route for macroporous formation but as a synergistic vehicle that enabled the significant loading of the mesoporous hard template. This design scheme for HPC-G materials can be utilized in broad applications, including electrochemical systems such as batteries and supercapacitors, sorbents, and catalyst supports, particularly supports where a high degree of thermal stability is required.

Biomass-Derived Oxygen and Nitrogen Co-Doped Porous Carbon with Hierarchical Architecture as Sulfur Hosts for High-Performance Lithium/Sulfur Batteries

In this work, a facile strategy to synthesize oxygen and nitrogen co-doped porous carbon (ONPC) is reported by one-step pyrolysis of waste coffee grounds. As-prepared ONPC possesses highly rich micro/mesopores as well as abundant oxygen and nitrogen co-doping, which is applied to sulfur hosts as lithium/sulfur batteries’ appropriate cathodes. In battery testing, the sulfur/oxygen and nitrogen co-doped porous carbon (S/ONPC) composite materials reveal a high initial capacity of 1150 mAh·g⁻¹ as well as a reversible capacity of 613 mAh·g⁻¹ after the 100th cycle at 0.2 C. Furthermore, when current density increases to 1 C, a discharge capacity of 331 mAh·g⁻¹ is still attainable. Due to the hierarchical porous framework and oxygen/nitrogen co-doping, the S/ONPC composite exhibits a high utilization of sulfur and good electrochemical performance via the immobilization of the polysulfides through strong chemical binding.

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.

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.

Nitrogen-enriched hierarchically porous carbons prepared from polybenzoxazine for high-performance supercapacitors

Nitrogen-enriched hierarchically porous carbons (HPCs) were synthesized from a novel nitrile-functionalized benzoxazine based on benzoxazine chemistry using a soft-templating method and a potassium hydroxide (KOH) chemical activation method and used as electrode materials for supercapacitors. The textural and chemical properties could be easily tuned by adding a soft template and changing the activation temperature. The introduction of the soft-templating agent (surfactant F127) resulted in the formation of mesopores, which facilitated fast ionic diffusion and reduced the internal resistance. The micropores of HPCs were extensively developed by KOH activation to provide large electrochemical double-layer capacitance. As the activation temperature increased from 600 to 800 °C, the specific surface area of nitrogen-enriched carbons increased dramatically, micropores were enlarged, and more meso/macropores were developed, but the nitrogen and oxygen content decreased, which affected the electrochemical performance. The sample HPC-800 activated at 800 °C possesses a high specific surface area (1555.4 m(2) g(-1)), high oxygen (10.61 wt %) and nitrogen (3.64 wt %) contents, a hierarchical pore structure, a high graphitization degree, and good electrical conductivity. It shows great pseudocapacitance and the largest specific capacitance of 641.6 F g(-1) at a current density of 1 A g(-1) in a 6 mol L(-1) KOH aqueous electrolyte when measured in a three-electrode system. Furthermore, the HPC-800 electrode exhibits excellent rate capability (443.0 F g(-1) remained at 40 A g(-1)) and good cycling stability (94.3% capacitance retention over 5000 cycles).