Carbon-Based Metal-Free Catalysts for Energy Storage and Environmental Remediation

Carbon Dots-Decorated Carbon-Based Metal-Free Catalysts for Electrochemical Energy Storage

In the past ten years, carbon dots-decorated, carbon-based, metal-free catalysts (CDs-C-MFCs) have become the fastest-growing branch in the metal-free materials for energy storage field. However, the further development of CDs-C-MFCs needs to clear up the electronic transmission mechanism rather than primarily relying on trial-and-error approaches. This review presents systematically and comprehensively for the first time the latest advances of CDs-C-MFCs in supercapacitors and metal-air batteries. The structure-performance relationship of these materials is carefully discussed. It is indicated that carbon dots (CDs) can act as the electron-rich regions in CDs-C-MFCs owing to their unique properties, such as quantum confinement effects, abundant defects, countless functional groups, etc. More importantly, specific doping can effectively modify the charge/spin distribution and then facilitate electron transfer. In addition, present challenges and future prospects of the CDs-C-MFCs are also given.

Defect Engineering of Carbon-based Electrocatalysts for Rechargeable Zinc-air Batteries

Rechargeable zinc-air batteries (ZABs) are considered as one of the most promising electrochemical energy devices due to their various unique advantages. Oxygen electrocatalysis, involving the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), determines the overall performance of zinc-air batteries. Therefore, the development of highly efficient bifunctional ORR/OER catalysts is critical for the large-scale application of ZABs. Carbon-based nanomaterials have been widely reported to be efficient electrocatalysts toward both ORR and OER. The enhanced activity of these electrocatalysts are usually attributed to different doping defects, synergistic effects and even the intrinsic carbon defects. Herein, an overview of the defect engineering in carbon-based electrocatalysts for ORR and OER is provided. The different types of intrinsic carbon defects and strategies for the generation of other defects in carbon-based electrocatalysts are presented. The interaction of heteroatoms doped carbon and transition metals (TMs) is also explored. In the end, the existing challenges and future perspectives on defect engineering are discussed.

Advanced Heteroatom-Doped Porous Carbon Membranes Assisted by Poly(ionic liquid) Design and Engineering

Heteroatom-doped porous carbon membranes (HPCMMs) with a tailor-made pore architecture, chemical composition, atomic structural order, and surface state represent an exciting family of porous carbon materials for diverse potential applications in catalysis, water treatment, biofiltration, energy conversion/storage, and so forth. Conventional porous carbon membranes possess intrinsic structural integrity, interconnectivity, and chemical purity across the atomic-to-macro world and have been popularly incorporated into devices as separators or chemically inert conductive supports, circumventing otherwise the inevitable complicated processing and structure weakness of their fine powderous counterpart. Motivated by the distinguished heteroatom-doping effect that revolutionizes the chemical and physical nature of carbon materials, the HPCMM research surges very recently, and focuses not only on the eminent conductive supports or separators but also on electro(co)catalysts in energy devices. Synergy of the porous nature, incorporation of heteroatoms, and the membrane state creates a vivid profile pattern and new task-specific usage. It is also noteworthy that the inherent structural merits of HPCMMs plus a high electron conductivity imbue them as a reliable binder-free model electrode to derive the intrinsic structure-property relationship of porous carbons in electrochemical environments, excluding the complex and adverse factors in association with polymer binders in carbon powder-based electrodes. HPCMMs are of both intense academic interest and practical value because of their well-defined properties endowed by controllable structure and porosity at both atomic and macroscopic scales in a membrane form. The sole aim of this article is to bring this group of porous carbon materials to the forefront so their comprehensive properties and functions can be better understood to serve the carbon community to address pressing materials challenges in our society. In this Account, we highlight the latest discovery and proceedings of HPCMMs, particularly the advancements in how to tailor structures and properties of HPCMMs by rational structure design of porous polymer membranes as sacrificial template built up especially from heteroatom-rich poly(ionic liquid)s (PILs). We will also stress the carbonization craft and the state-of-the-art electrochemical applications for HPCMMs. Key factors and thoughts in heteroatom doping and porous systems in HPCMMs are discussed. A future perspective of the challenges and promising potential of HPCMMs is cast on the basis of these achievements.

Fe3O4@N-Doped Interconnected Hierarchical Porous Carbon and Its 3D Integrated Electrode for Oxygen Reduction in Acidic Media

The rational design of electrode structure with catalysts adequately utilized is of vital importance for future fuel cells. Herein, a novel 3D oriented wholly integrated electrode comprising core-shell Fe3O4@N-doped-C (Fe3O4@NC) nanoparticles embedded into N-doped ordered interconnected hierarchical porous carbon (denoted as Fe3O4@NC/NHPC) is developed for the oxygen reduction reaction (ORR). The as-prepared catalyst possesses novel structure and efficient active sites. In rotating disk electrode measurements, the Fe3O4@NC/NHPC exhibits almost identical ORR electrocatalytic activity, superior durability, and much better methanol tolerance compared with the commercial Pt/C in acidic media. To the authors' knowledge, this is among the best non-precious-metal ORR catalysts reported so far. Importantly, the Fe3O4@NC/NHPC is successfully in situ assembled onto carbon paper by the electrophoresis method to obtain a well-designed 3D-ordered electrode. With improved mass transfer and maximized active sites for ORR, the 3D-oriented wholly integrated electrode shows superior performance to the one fabricated by the traditional method.

Current Trends in the Application of Nanomaterials for the Removal of Emerging Micropollutants and Pathogens from Water

Water resources contamination has a worldwide impact and is a cause of global concern. The need for provision of clean water is becoming more and more demanding. Nanotechnology may support effective strategies for the treatment, use and reuse of water and the development of next-generation water supply systems. The excellent properties and effectiveness of nanomaterials make them particularly suitable for water/wastewater treatment. This review provides a comprehensive overview of the main categories of nanomaterials used in catalytic processes (carbon nanotubes/graphitic carbon nitride (CNT/g-C₃N₄) composites/graphene-based composites, metal oxides and composites, metal–organic framework and commercially available nanomaterials). These materials have found application in the removal of different categories of pollutants, including pharmaceutically active compounds, personal care products, organic micropollutants, as well as for the disinfection of bacterial, viral and protozoa microbial targets, in water and wastewater matrices. Apart from reviewing the characteristics and efficacy of the aforementioned nanoengineered materials for the removal of different pollutants, we have also recorded performance limitations issues (e.g., toxicity, operating conditions and reuse) for their practical application in water and wastewater treatment on large scale. Research efforts and continuous production are expected to support the development of eco-friendly, economic and efficient nanomaterials for real life applications in the near future.

Metal-Free Chemoselective Hydrogenation of Nitroarenes by N-Doped Carbon Nanotubes via In Situ Polymerization of Pyrrole

Chemoselective hydrogenation of nitroarenes to anilines is of great importance for the manufacture of pharmaceuticals, fine chemicals, and dyes. In this study, a series of metal-free N-doped carbon nanotubes (NCNTs) have been prepared by the carbonization of in situ polymerized pyrrole on CNTs. The concentration of pyrrole, pyrolysis temperature, and the outside diameter of CNTs were investigated to improve the catalytic performance. As characterized by Raman and X-ray photoelectron spectroscopy (XPS), the optimal catalyst (NCNTs-800) possessed a unique structure doped with the same content of pyrrolic N and graphitic N. The activity and selectivity of NCNTs-800 have been evaluated for the selective hydrogenation of substituted nitroarenes. Highly selective hydrogenation of the nitro group of 12 different substrates has been achieved on NCNTs-800, even in the presence of a fragile iodo group. The hydrogenation reaction on N-doped CNTs from polypyrrole involved a mixture of different hydrogen species including nonpolar H radicals. In addition, stability and recyclability of NCNTs-800 have been tested.

High-Performance, Long-Life, Rechargeable Li-CO2 Batteries based on a 3D Holey Graphene Cathode Implanted with Single Iron Atoms

A highly efficient cathode catalyst for rechargeable Li-CO₂ batteries is successfully synthesized by implanting single iron atoms into 3D porous carbon architectures, consisting of interconnected N,S-codoped holey graphene (HG) sheets. The unique porous 3D hierarchical architecture of the catalyst with a large surface area and sufficient space within the interconnected HG framework can not only facilitate electron transport and CO₂ /Li⁺ diffusion, but also allow for a high uptake of Li₂ CO₃ to ensure a high capacity. Consequently, the resultant rechargeable Li-CO₂ batteries exhibit a low potential gap of ≈1.17 V at 100 mA g^-1 and can be repeatedly charged and discharged for over 200 cycles with a cut-off capacity of 1000 mAh g^-1 at a high current density of 1 A g^-1 . Density functional theory calculations are performed and the observed appealing catalytic performance is correlated with the hierarchical structure of the carbon catalyst. This work provides an effective approach to the development of highly efficient cathode catalysts for metal-CO₂ batteries and beyond.

Sulfur Loading and Speciation Control the Hydrophobicity, Electron Transfer, Reactivity, and Selectivity of Sulfidized Nanoscale Zerovalent Iron

Sulfidized nanoscale zerovalent iron (SNZVI) is a promising material for groundwater remediation. However, the relationships between sulfur content and speciation and the properties of SNZVI materials are unknown, preventing rational design. Here, the effects of sulfur on the crystalline structure, hydrophobicity, sulfur speciation, corrosion potential, and electron transfer resistance are determined. Sulfur incorporation extended the nano-Fe⁰ BCC lattice parameter, reduced the Fe local vacancies, and lowered the resistance to electron transfer. Impacts of the main sulfur species (FeS and FeS₂ ) on hydrophobicity (water contact angles) are consistent with density functional theory calculations for these FeS_x phases. These properties well explain the reactivity and selectivity of SNZVI during the reductive dechlorination of trichloroethylene (TCE), a hydrophobic groundwater contaminant. Controlling the amount and speciation of sulfur in the SNZVI made it highly reactive (up to 0.41 L m^-2 d^-1 ) and selective for TCE degradation over water (up to 240 moles TCE per mole H₂ O), with an electron efficiency of up to 70%, and these values are 54-fold, 98-fold, and 160-fold higher than for NZVI, respectively. These findings can guide the rational design of robust SNZVI with properties tailored for specific application scenarios.

3D N,O-Codoped Egg-Box-Like Carbons with Tuned Channels for High Areal Capacitance Supercapacitors

Functional carbonaceous materials for supercapacitors (SCs) without using acid for post-treatment remain a substantial challenge. In this paper, we present a less harmful strategy for preparing three-dimensional (3D) N,O-codoped egg-box-like carbons (EBCs). The as-prepared EBCs with opened pores provide plentiful channels for ion fast transport, ensure the effective contact of EBCs electrodes and electrolytes, and enhance the electron conduction. The nitrogen and oxygen atoms doped in EBCs improve the surface wettability of EBC electrodes and provide the pseudocapacitance. Consequently, the EBCs display a prominent areal capacitance of 39.8 μF cm-2 (340 F g-1) at 0.106 mA cm-2 in 6 M KOH electrolyte. The EBC-based symmetric SC manifests a high areal capacitance to 27.6 μF cm-2 (236 F g-1) at 0.1075 mA cm-2, a good rate capability of 18.8 μF cm-2 (160 F g-1) at 215 mA cm-2 and a long-term cycle stability with only 1.9% decay after 50,000 cycles in aqueous electrolyte. Impressively, even in all-solid-state SC, EBC electrode shows a high areal capacitance of 25.0 μF cm-2 (214 F g-1) and energy density of 0.0233 mWh cm-2. This work provides an acid-free process to prepare electrode materials from industrial by-products for advanced energy storage devices.

The durability of carbon nanotubes in the selective reduction of nitrobenzene

Surface oxidized carbon nanotubes (oCNTs) were quite stable for the selective reduction of nitrobenzene, while notable deactivation was observed for the un-oxidized sample (rCNTs). The adsorption of N-containing compounds had a negligible effect, but the formation of a carboxyl group and anhydride was mainly responsible for the deactivation of rCNTs.

Copper/Carbon Hybrid Nanozyme: Tuning Catalytic Activity by the Copper State for Antibacterial Therapy

Metal-carbon hybrid materials have shown promise as potential enzyme mimetics for antibacterial therapy; however, the effects of metal states and corresponding antibacterial mechanisms are largely unknown. Here, two kinds of copper/carbon nanozymes were designed, with tuned copper states from Cu⁰ to Cu²⁺. Results revealed that the copper/carbon nanozymes exhibited copper state-dependent peroxidase-, catalase-, and superoxide dismutase-like activities. Furthermore, the antibacterial activities were also primarily determined by the copper state. The different antibacterial mechanisms of these two copper/carbon nanozymes were also proposed. For the CuO-modified copper/carbon nanozymes, the released Cu²⁺ caused membrane damage, lipid peroxidation, and DNA degradation of Gram-negative bacteria, whereas, for Cu-modified copper/carbon nanozymes, the generation of reactive oxygen species (ROS) via peroxidase-like catalytic reactions was the determining factor against both Gram-positive and Gram-negative bacteria. Lastly, we established two bacterially infected animal models, i.e., bacteria-infected enteritis and wound healing, to confirm the antibacterial ability of the copper/carbon nanozymes. Our findings provide a deeper understanding of metal state-dependent enzyme-like and antibacterial activities and highlight a new approach for designing novel and selective antibacterial therapies based on metal-carbon nanozymes.

1D Carbon-Based Nanocomposites for Electrochemical Energy Storage

Electrochemical energy storage (EES) devices have attracted immense research interests as an effective technology for utilizing renewable energy. 1D carbon-based nanostructures are recognized as highly promising materials for EES application, combining the advantages of functional 1D nanostructures and carbon nanomaterials. Here, the recent advances of 1D carbon-based nanomaterials for electrochemical storage devices are considered. First, the different categories of 1D carbon-based nanocomposites, namely, 1D carbon-embedded, carbon-coated, carbon-encapsulated, and carbon-supported nanostructures, and the different synthesis methods are described. Next, the practical applications and optimization effects in electrochemical energy storage devices including Li-ion batteries, Na-ion batteries, Li-S batteries, and supercapacitors are presented. After that, the advanced in situ detection techniques that can be used to investigate the fundamental mechanisms and predict optimization of 1D carbon-based nanocomposites are discussed. Finally, an outlook for the development trend of 1D carbon-based nanocomposites for EES is provided.