Monodisperse Mesoporous Carbon Nanoparticles from Polymer/Silica Self-Aggregates and Their Electrocatalytic Activities

Carbon nanosphere synthesis and applications for rechargeable batteries

Carbon nanospheres (CNSs) have attracted great interest in energy conversion and storage technologies due to their excellent chemical and thermal stability, high electrical conductivity and controllable size structure characteristics. In order to further improve the energy storage properties, many efforts have been made to design suitable nanocarbon spherical materials to improve electrochemical performance. In this overview, we summarize the recent research progress on CNSs, mainly focusing on the synthesis methods and their application as high-performance electrode materials in rechargeable batteries. As for the synthesis methods, hard template methods, soft template methods, the extension of the Stöber method, hydrothermal carbonization, aerosol-assisted synthesis are described in detail. In addition, the use of CNSs as electrodes in energy storage devices (mainly concentrated on lithium-ion batteries (LIBs)), sodium-ion batteries (SIBs) and potassium-ion batteries (PIBs) are also discussed in detail in this article. Finally, some perspectives on the future research and development of CNSs are provided.

Nanostructured Carbon Electrocatalysts for Energy Conversions

The growing energy demand worldwide has led to increased use of fossil fuels. This, in turn, is making fossil fuels dwindle faster and cause more negative environmental impacts. Thus, alternative, environmentally friendly energy sources such as fuel cells and electrolyzers are being developed. While significant progress has already been made in this area, such energy systems are still hard to scale up because of their noble metal catalysts. In this concept paper, first, various scalable nanocarbon-based electrocatalysts that are being synthesized for energy conversions in these energy systems are introduced. Next, notable heteroatom-doping and nanostructuring strategies that are applied to produce different nanostructured carbon materials with high electrocatalytic activities for energy conversions are discussed. The concepts used to develop such materials with different structures and large density of dopant-based catalytic functional groups in a sustainable way, and the challenges therein, are emphasized in the discussions. The discussions also include the importance of various analytical, theoretical, and computational methods to probe the relationships between the compositions, structures, dopants, and active catalytic sites in such materials. These studies, coupled with experimental studies, can further guide innovative synthetic routes to efficient nanostructured carbon electrocatalysts for practical, large-scale energy conversion applications.

A Blinking Mesoporous TiO2-x Composed of Nanosized Anatase with Unusually Long-Lived Trapped Charge Carriers

A mesoporous TiO_2-x material comprised of small, crystalline, vacancy-rich anatase nanoparticles (NPs) shows unique optical, thermal, and electronic properties. It is synthesized using polymer-derived mesoporous carbon (PDMC) as a template. The PDMC pores serve as physical barriers during the condensation and pyrolysis of a titania precursor, preventing the titania NPs from growing beyond 10 nm in size. Unlike most titania nanomaterials, during pyrolysis the NPs undergo no transition from the anatase to rutile phase and they become catalytically active reduced TiO_2-x . When exposed to a slow electron beam, the NPs exhibit a charge/discharge behavior, lighting up and fading away for an average period of 15 s for an extended period of time. The NPs also show a 50 nm red-shift in their UV/Vis absorption and long-lived charge carriers (electrons and holes) at room temperature in the dark, even long after UV irradiation. The NPs as photocatalysts show a good activity for CO₂ reduction.

Controllable Preparation of Monodisperse Mesoporous Silica from Microspheres to Microcapsules and Catalytic Loading of Au Nanoparticles

A unique structural transition from pomegranate-like monodisperse mesoporous silica microspheres (M-MSMs) with tunable mesopores to mesoporous silica microcapsules has been reported. The unique evolution occurred together with varying the cross-linking degrees (CLDs) of templates. Herein, using monodisperse sulfonated cross-linked polystyrene (S-CLPS) as templates, S-CLPS/SiO₂ composite microspheres were synthesized by the sol-gel method. Subsequently, the templates were removed by calcination to obtain the M-MSMs or microcapsules. The pore sizes of M-MSMs could be tailored from 3.2 to 7.4 nm by facilely varying the CLDs from 0.5 to 20%. Interestingly, mesoporous silica microcapsules were gradually formed when the CLDs were beyond 20%. Meanwhile, the specific surface area also could be adjusted by this strategy without hardly affecting the monodispersity, and the specific surface area increased to 391.9 m²/g. Significantly, Au@M-MSM was prepared by supporting Au nanoparticles (NPs) on M-MSM and used as nanocatalysts to reduce 4-nitrophenol (4-NP). The ultrathin shell and interconnected three-dimensional (3D) porous structure of M-MSMs can increase the mass transfer and protect the Au NPs from leakage, which reveals high recyclability and high conversion (>95%) after 10 regeneration-catalysis cycles. This approach provides a nanotechnology platform for the preparation of mesoporous silica materials with different microstructures, which will have enormous potential in practical applications involving different molecular sizes.

Ultrahigh yield synthesis of mesoporous carbon nanoparticles as a superior lubricant additive for polyethylene glycol

Mesoporous carbon nanoparticles (MCNPs) with an average particle size of 27.3 nm and a pore size of 3–5 nm were facilely synthesized in ultrahigh yield (91.7 wt%) and used as a high-performance lubricant additive for polyethylene glycol (PEG200).

Ammonia-controlled synthesis of monodispersed N-doped carbon nanoparticles

The presence of ammonia slowing down the acid-catalysed Schiff base formation as well as control the monodispersity through the separation of nucleation and growth stages.

Polymer-Based Synthetic Routes to Carbon-Based Metal-Free Catalysts

Carbons are increasingly important as possible alternatives to expensive metal catalysts owing to the wide range of chemical properties they can exhibit and the growing set of synthetic routes available to produce them. This progress report discusses the process of making catalytic carbons from polymeric precursors, focusing on mechanisms of carbonization and how the polymer structures and synthetic procedures affect the resulting carbons. In considering what is necessary to move laboratory catalytic carbons to industrial and commercial applications, the cost and complexity to produce them are a considerable challenge to overcome. Industrially produced carbons are typically made from biopolymers such as lignin while many of the catalytic carbons studied in literature are from synthetic polymers. Thus, studying polymer-derived carbons can provide insights into the carbonization process and the properties of catalytic carbons, which can subsequently be translated to improve biopolymer-derived carbons in an economical way. Aspects of polymer carbonization discussed include carbonization mechanisms, effects of crosslinkers, polymer microstructure, heteroatom control, and effects of nanostructuring.

Heteroatom-Doped Carbon Materials for Hydrazine Oxidation

The key in designing efficient direct liquid fuel cells (DLFCs), which can offer some solutions to society's grand challenges associated with sustainability and energy future, currently lies in the development of cost-effective electrocatalysts. Among the many types of fuel cells, direct hydrazine fuel cells (DHFCs) are of particular interest, especially due to their high theoretical cell voltages and clean emission. However, DHFCs currently use noble-metal-based electrocatalysts, and the scarcity and high cost of noble metals are hindering these fuel cells from finding large-scale practical applications. In order to replace noble-metal-based electrocatalysts with sustainable ones and help DHFCs become widely usable, great efforts are being made to develop stable heteroatom (e.g., B, N, O, P and S)-doped carbon electrocatalysts, the activities of which are comparable to, or better than, those of noble metals. Here, the recent research progress and the advancements made on the development of heteroatom-doped carbon materials, their general properties, their electrocatalytic activities toward the HzOR, and their dopant- and structure-related electrocatalytic properties for the HzOR are summarized. Perspectives on the different directions that the research endeavors in this field need to take in the future and the challenges associated with DHFCs are included.

Nitrogen-doped porous carbon sphere supported Pt nanoparticles for methanol and ethanol electro-oxidation in alkaline media

A simple method for the preparation of porous nitrogen-doped carbon spheres (MNCS) with honeycomb-like structures through the self-assembly of colloidal silica by using polyaniline as the nitrogen source and a carbon precursor is reported. The BET surface area calculated from N₂ adsorption/desorption measurement of the MNCS is up to 882 m² g⁻¹. The synthesized MNCS were employed as a support for Pt nanoparticles for the oxidation of methanol and ethanol in alkaline media. Compared to Pt supported on commercial Vulcan XC-72R carbon (Pt/C), Pt/MNCS exhibit higher electro-catalytic performance and greater stability, indicating that MNCS have potential application prospects as electro-catalyst supports for alcohol oxidation in alkaline media. This enhancement in performance is due to the honeycomb-like porous structure of the MNCS and the doping with nitrogen.

We synthesized honeycomb-like porous nitrogen-doped carbon spheres, which can be used as support of Pt for alcohol oxidation.

Monodisperse Ultrasmall Manganese-Doped Multimetallic Oxysulfide Nanoparticles as Highly Efficient Oxygen Reduction Electrocatalyst

The highly efficient and cheap non-Pt-based electrocatalysts such as transition-based catalysts prepared via facile methods for oxygen reduction reaction (ORR) are desirable for large-scale practical industry applications in energy conversion and storage systems. Herein, we report a straightforward top-down synthesis of monodisperse ultrasmall manganese-doped multimetallic (ZnGe) oxysulfide nanoparticles (NPs) as an efficient ORR electrocatalyst by simple ultrasonic treatment of the Mn-doped Zn-Ge-S chalcogenidometalate crystal precursors in H₂O/EtOH for only 1 h at room temperature. Thus obtained ultrasmall monodisperse Mn-doped oxysulfide NPs with ultralow Mn loading level (3.92 wt %) not only exhibit comparable onset and half-wave potential (0.92 and 0.86 V vs reversible hydrogen electrode, respectively) to the commercial 20 wt % Pt/C but also exceptionally high metal mass activity (189 mA/mg at 0.8 V) and good methanol tolerance. A combination of transmission electron microscopy, scanning electron microscopy, X-ray photoelectron spectroscopy, and electrochemical analysis demonstrated that the homogenous distribution of a large amount of Mn(III) on the surface of NPs mainly accounts for the high ORR activity. We believe that this simple synthesis of Mn-doped multimetallic (ZnGe) oxysulfide NPs derived from chalcogenidometalates will open a new route to explore the utilization of discrete-cluster-based chalcogenidometalates as novel non-Pt electrocatalysts for energy applications and provide a facile way to realize the effective reduction of the amount of catalyst while keeping desired catalytic performances.

Hollow Mesoporous Carbon Microparticles and Micromotors with Single Holes Templated by Colloidal Silica-Assisted Gas Bubbles

A simple, new synthetic method that produces hollow, mesoporous carbon microparticles, each with a single hole on its surface, is reported. The synthesis involves unique templates, which are composed of gaseous bubbles and colloidal silica, and poly(furfuryl alcohol) as a carbon precursor. The conditions that give these morphologically unique carbon microparticles are investigated, and the mechanisms that result in their unique structures are proposed. Notably, the amount of colloidal silica and the type of polymer are found to hugely dictate whether or not the synthesis results in hollow asymmetrical microparticles, each with a single hole. The potential application of the particles as self-propelled micromotors is demonstrated.

Heteroatom-Doped Carbon Materials for Electrocatalysis

Fuel cells, water electrolyzers, and metal-air batteries are important energy systems that have started to play some roles in our renewable energy landscapes. However, despite much research works carried out on them, they have not yet found large-scale applications, mainly due to the unavailability of sustainable catalysts that can catalyze the reactions employed in them. Currently, noble metal-based materials are the ones that are commonly used as catalysts in most commercial fuel cells, electrolyzers, and metal-air batteries. Hence, there has been considerable research efforts worldwide to find alternative noble metal-free and metal-free catalysts composed of inexpensive, earth-abundant elements for use in the catalytic reactions employed in these energy systems. In this concept paper, a brief introduction on catalysis in renewable energy systems, followed by the recent efforts to develop sustainable, heteroatom-doped carbon and non-noble metal-based electrocatalysts, the challenges to unravel their structure-catalytic activity relationships, and the authors' perspectives on these topics and materials, are discussed.

W-doped MoS2 nanosheets as a highly-efficient catalyst for hydrogen peroxide electroreduction in alkaline media

Novel W-doped MoS₂ electrocatalysts have been successfully fabricated through a facile one-pot solvothermal method and employed for the hydrogen peroxide reduction reaction (HPRR) in emerging alkaline H₂O₂-based fuel cells.