Highly microporous carbon with nitrogen-doping derived from natural biowaste for high-performance flexible solid-state supercapacitor

The application of P-modified biochar in wastewater remediation: A state-of-the-art review

Phosphorus modified biochar (P-BC) is an effective adsorbent for wastewater remediation, which has attracted widespread attention due to its low cost, vast source, unique surface structure, and abundant functional groups. However, there is currently no comprehensive analysis and review of P-BC in wastewater remediation. In this study, a detailed introduction is given to the synthesis method of P-BC, as well as the effects of pyrolysis temperature and residence time on physical and chemical properties and adsorption performance of the material. Meanwhile, a comprehensive investigation and evaluation were conducted on the different biomass types and phosphorus sources used to synthesize P-BC. This article also systematically compared the adsorption efficiency differences between P-BC and raw biochar, and summarized the adsorption mechanism of P-BC in removing pollutants from wastewater. In addition, the effects of P-BC composite with other materials (element co-doping, polysaccharide stabilizers, microbial loading, etc.) on physical and chemical properties and pollutant adsorption capacity of the materials were investigated. Some emerging applications of P-BC were also introduced, including supercapacitors, CO2 adsorbents, carbon sequestration, soil heavy metal remediation, and soil fertility improvement. Finally, some valuable suggestions and prospects were proposed for the future research direction of P-BC to achieve the goal of multiple utilization.

In Situ Preparation of Three-Dimensional Porous Nickel Sulfide as a Battery-Type Supercapacitor

A one-step sulfurization method to fabricate Ni₃S₂ nanowires (Ni₃S₂ NWs) directly on a Ni foam (NF) was developed as a simple, low-cost synthesis method for use as a supercapacitor (SC), aimed at optimizing energy storage. Ni₃S₂ NWs have high specific capacity and are considered a promising electrode material for SCs; however, their poor electrical conductivity and low chemical stability limit their applications. In this study, highly hierarchical three-dimensional porous Ni₃S₂ NWs were grown directly on NF by a hydrothermal method. The feasibility of the use of Ni₃S₂/NF as a binder-free electrode for achieving high-performance SCs was examined. Ni₃S₂/NF exhibited a high specific capacity (255.3 mAh g^-1 at a current density of 3 A g^-1), good rate capability (2.9 times higher than that of the NiO/NF electrode), and competitive cycling performance (capacity retention of specific capacity of 72.17% after 5000 cycles at current density of 20 A g^-1). Owing to its simple synthesis process and excellent performance as an electrode material for SCs, the developed multipurpose Ni₃S₂ NWs electrode is expected to be a promising electrode for SC applications. Furthermore, the synthesis method of self-growing Ni₃S₂ NW electrodes on 3D NF via hydrothermal reactions could potentially be applied to the fabrication of SC electrodes using a variety of other transition metal compounds.

A Flexible Vertical-Section Wood/MXene Electrode with Excellent Performance Fabricated by Building a Highly Accessible Bonding Interface

Cross-section wood (CW) is generally used as a host for free-standing electrodes, as the abundant opened pores can provide large space for loading guest materials with high electrical conductivity and electrochemical activity. However, there is still a challenge for CW to be used in flexible supercapacitors (SCs) because of its low mechanical strength. Herein, as an alternative to CW, vertical-section wood (VW) with excellent mechanical strength and good flexibility is developed and used as a free-standing and flexible electrode by using Ti₃C₂T_x (MXene) with ultrahigh conductivity and good electrochemical activity as a guest material. In particular, the highly accessible bonding interface for Ti₃C₂T_x is first built by delignification on VW to generate abundant pores for continuously absorbing Ti₃C₂T_x and to expose cellulose with abundant oxygen-containing groups for stable combination with Ti₃C₂T_x. Then, cyclic pressing is used to form negative pressure to pump the Ti₃C₂T_x suspension into VW, combining with a preheating process to trigger layer-by-layer self-assembly of Ti₃C₂T_x nanosheets onto a wood cell wall by evaporating water in the suspension. As a result, the free-standing electrode has a large Ti₃C₂T_x loading mass proportion of 33 wt %, a high conductivity of 3.14 S cm^-1, and good flexibility with much higher mechanical strength of 15.1 MPa than 0.4 MPa of CW. The symmetric SC delivers a good specific capacitance of 805 mF cm^-2 at 0.5 mA cm^-2, a remarkably high rate capability of 84% to 10 mA cm^-2, and an energy density of 13.85 μW h cm^-2 at 87.5 μW cm^-2. Additionally, this SC shows a long lifespan of 90.5% after 10,000th charge and discharge cycles even at a constant bending angle of 90°, suggesting promising potential in flexible devices.

Activation of waste paper: Influence of varied chemical agents on product properties

Waste paper (WP) is rich in cellulose, which can be activated to produce porous carbon, bio-oil, and combustible gases. During chemical activation of WP, the use of varied chemical agents not only generates activated carbon of distinct pore structure but also bio-oil/gases of different property. In this study, the activation of WP with varied chemical agents was conducted. The distinct characteristics of activated carbon and also bio-oil/gases were correlated with the different nature of the used chemical agents. The results indicated that H₃PO₄ and ZnCl₂ catalyzed polymerization reactions for producing more bio-oil while less gases owing to their Brønsted and Lewis acidic sites. K₂C₂O₄ showed high activity for cracking/gasification reactions, forming bio-oil with higher abundance of organics with smaller π-conjugated structures. In addition, ZnCl₂ could create a very coarse porous structure with abundant macropores via destroying fiber structure in WP and promoting the growth of graphitic crystals. In comparison, K₂C₂O₄ hindered the aromatization and facilitated the formation of amorphous activated carbon. K₂C₂O₄ and ZnCl₂ were much more effective than H₃PO₄ for creating micropores and mesopores from WP, the derived activated carbon showed superior performances as the electrode of supercapacitor and adsorbent for adsorption of oxytetracycline from aqueous solution. In addition, K₂C₂O₄ as activating agent showed lower environmental impact than ZnCl₂ in terms of energy consumption, environmental pollution and the greenhouse effect.

Nest-Like MnO2 Nanowire/Hierarchical Porous Carbon Composite for High-Performance Supercapacitor from Oily Sludge

In the aim to go beyond the performance tradeoffs of classic electric double-layer capacitance and pseudo-capacitance, composites made out of carbon and pseudo-capacitive materials have been a hot-spot strategy. In this paper, a nest-like MnO2 nanowire/hierarchical porous carbon (HPC) composite (MPC) was successfully fabricated by a controllable in situ chemical co-precipitation method from oily sludge waste. Due to the advantages of high surface area and fast charge transfer for HPC as well as the large pseudo-capacitance for MnO2 nanowires, the as-prepared MPC has good capacitance performance with a specific capacitance of 437.9 F g-1 at 0.5 A g-1, favorable rate capability of 79.2% retention at 20 A g-1, and long-term cycle stability of 78.5% retention after 5000 cycles at 5 A g-1. Meanwhile, an asymmetric supercapacitor (ASC) was assembled using MPC as the cathode while HPC was the anode, which exhibits a superior energy density of 58.67 W h kg-1 at the corresponding power density of 498.8 W kg-1. These extraordinary electrochemical properties highlight the prospect of our waste-derived composites electrode material to replace conventional electrode materials for a high-performance supercapacitor.

A space-sacrificed pyrolysis strategy for boron-doped carbon spheres with high supercapacitor performance

Targeting the potential application of morphological carbon in electrode materials, a space-sacrificed pyrolysis strategy was applied for the preparation of boron-doped carbon spheres (B-CSs), using commercial triphenyl borate (TPB) as carbon and boron co-source. The unique structure of TPB play an important role in the sacrificed space, and has notable effect on the surface area of B-CSs. The as prepared B-CSs possess a high surface area and boron content with uniform boron atoms distribution and high surface polarity, which contributes to the improvement of pseudo-capacitance. The sizes, specific surface areas, and boron contents of B-CSs can be easily regulated by varying the experimental parameters. The optimal sample has a boron content of 1.38 at%, surface area of 560 m² g^-1 and specific capacitance of 235F g^-1. We can believe that this work would provide a flexible and extensible preparation technique of B-CSs for electrochemical applications.

KCl-assisted activation: Moringa oleifera branch-derived porous carbon for high performance supercapacitor

Porous carbons with a high specific surface area (2314–3470 m² g⁻¹) are prepared via a novel KCl-assisted activation strategy for high-performance supercapacitor.

Boosting electrochemical performance of activated carbon by tuning effective pores and synergistic effects of active species

Clean energy conversion/storage techniques have become increasingly significant because of the increasing energy consumption. Regarding practical applications like zinc-air batteries and supercapacitors, electrode materials are essential and often require both porous networks and active species to enhance their electrochemical performance. Nitrogen-doped porous carbon (NPC) is a kind of promising material, which provides efficient active sites and large surface areas for energy conversion/storage applications. However, rational modulation of properties for maximizing NPC performance is still a challenge. Herein, a promising NPC material derived from natural biomass is successfully synthesized by following a stepwise preparation method. Physisorption and X-ray photoelectron spectroscopy (XPS) analyses demonstrate both pore structures and nitrogen species of the NPC have been delicately tuned. The optimized sample NPC-800-m exhibits excellent performance in both oxygen reduction reaction (ORR) and three-electrode supercapacitor measurement. Moreover, the homemade zinc-air battery and symmetric supercapacitor assembled with NPC-800-m also display outstanding energy and power density as well as durable stability. Density functional theory (DFT) calculations further confirm the synergistic effects among graphitic, pyridinic and pyrrolic nitrogen. The existence of multispecies of nitrogen combined with the optimized pore structure is the key to the high electrochemical performance for NPC-800-m. This work not only provides feasible and green synthetic methodology but also offers original insights into the effective pores and the synergistic effects of different nitrogen species in the NPC materials.

Hydrothermal Carbonization as a Valuable Tool for Energy and Environmental Applications: A Review

Hydrothermal carbonization (HTC) represents an efficient and valuable pre-treatment technology to convert waste biomass into highly dense carbonaceous materials that could be used in a wide range of applications between energy, environment, soil improvement and nutrients recovery fields. HTC converts residual organic materials into a solid high energy dense material (hydrochar) and a liquid residue where the most volatile and oxygenated compounds (mainly furans and organic acids) concentrate during reaction. Pristine hydrochar is mainly used for direct combustion, to generate heat or electricity, but highly porous carbonaceous media for energy storage or for adsorption of pollutants applications can be also obtained through a further activation stage. HTC process can be used to enhance recovery of nutrients as nitrogen and phosphorous in particular and can be used as soil conditioner, to favor plant growth and mitigate desertification of soils. The present review proposes an outlook of the several possible applications of hydrochar produced from any sort of waste biomass sources. For each of the applications proposed, the main operative parameters that mostly affect the hydrochar properties and characteristics are highlighted, in order to match the needs for the specific application.

Ionic liquid-induced tunable N-doped mesoporous carbon spheres for supercapacitors

N-Doped mesoporous carbon spheres with tunable structures have been prepared by a feasible co-assembly under the induction of an ionic liquid for supercapacitors.

Dual potassium salt-assisted lyophilization of natural fibres for the high-yield synthesis of one-dimensional carbon microtubes for supercapacitors and the oxygen reduction reaction

Natural fibre-derived carbon microtubes exhibit excellent performances as supercapacitor electrodes and oxygen reduction electrocatalysts via dual-potassium-salt-assisted freeze-drying and post-nitrogen doping.

Dictyophora-derived N-doped porous carbon microspheres for high-performance supercapacitors

PCMS-T hierarchical porous structures were prepared from biomass dictyophora as electrodes for high-performance supercapacitors.