Synergistically enhanced activity of graphene quantum dots/graphene hydrogel composites: a novel all-carbon hybrid electrocatalyst for metal/air batteries

Review of carbon dot-hydrogel composite material as a future water-environmental regulator

As water pollution and scarcity pose severe threats to the sustainable progress of human society, it is important to develop a method or materials that can accurately and efficiently detect pollutants and purify aquatic environments or exploit marine resources. The compositing of photoluminescent and hydrophilic carbon dots (CDs) with hydrogels bearing three-dimensional networks to form CD-hydrogel composites to protect aquatic environments is a "win-win" strategy. Herein, the feasibility of the aforementioned method has been demonstrated. This paper reviews the recent progress of CD-hydrogel materials used in aquatic environments. First, the synthesis methods for these composites are discussed, and then, the composites are categorized according to different methods of combining the raw materials. Thereafter, the progress in research on CD-hydrogel materials in the field of water quality detection and purification is reviewed in terms of the application of the mechanisms. Finally, the current challenges and prospects of CD-hydrogel materials are described. These results are expected to provide insights into the development of CD-hydrogel composites for researchers in this field.

Carbon Dot-Based Hydrogels: Preparations, Properties, and Applications

Hydrogels have extremely high moisture content, which makes it very soft and excellently biocompatible. They have become an important soft material and have a wide range of applications in various fields such as biomedicine, bionic smart material, and electrochemistry. Carbon dot (CD)-based hydrogels are based on carbon dots (CDs) and auxiliary substances, forming a gel material with comprehensive properties of individual components. CDs embedding in hydrogels could not only solve their aggregation-caused quenching (ACQ) effect, but also manipulate the properties of hydrogels and even bring some novel properties, achieving a win-win situation. In this review, the preparation methods, formation mechanism, and properties of CD-based hydrogels, and their applications in biomedicine, sensing, adsorption, energy storage, and catalysis -are summarized. Finally, a brief discussion on future research directions of CD-based hydrogels will be given.

Review of research of nanocomposites based on graphene quantum dots

Graphene quantum dots (GQDs) belong to the vast and versatile family of carbon nanomaterials. Their unique position amongst versatile carbon nanoparticles (NPs) originates from the properties of quantum confinement and edge effects. GQDs are similar to conventional semiconductor QDs due to their tunable band gaps and high photoluminescence activity. However, GQDs have superior characteristics due to their excellent biocompatibility, low toxicity, good water dispersibility, large optical absorptivity, high fluorescence activity and photostability. These properties have generated significant interest in GQDs applications in various fields: nanosensor fabrication, drug delivery, photocatalysis, photovoltaics, and photodynamic therapy. Numerous GQD-based nanocomposites/nanohybrides have been synthesized and/or studied computationally. This review focuses on recent computational studies of various GQD-based nanocomposites/nanohybrides and systems which can be related to them.

Metal-Air Batteries—A Review

Metal–air batteries are a promising technology that could be used in several applications, from portable devices to large-scale energy storage applications. This work is a comprehensive review of the recent progress made in metal-air batteries MABs. It covers the theoretical considerations and mechanisms of MABs, electrochemical performance, and the progress made in the development of different structures of MABs. The operational concepts and recent developments in MABs are thoroughly discussed, with a particular focus on innovative materials design and cell structures. The classical research on traditional MABs was chosen and contrasted with metal–air flow systems, demonstrating the merits associated with the latter in terms of achieving higher energy density and efficiency, along with stability. Furthermore, the recent applications of MABs were discussed. Finally, a broad overview of challenges/opportunities and potential directions for commercializing this technology is carefully discussed. The primary focus of this investigation is to present a concise summary and to establish future directions in the development of MABs from traditional static to advanced flow technologies. A systematic analysis of this subject from a material and chemistry standpoint is presented as well.

N, P-Codoped Graphene Dots Supported on N-Doped 3D Graphene as Metal-Free Catalysts for Oxygen Reduction

Nitrogen and phosphorus-codoped graphene dots supported on nitrogen-doped three-dimensional graphene (N, P-GDs/N-3DG) have been synthesized by a facile freeze-annealing process. On the surface of the 3D interconnected porous structure, the N, P-GDs are uniformly dispersed. The as-prepared N, P-GDs/N-3DG material served as a metal-free catalyst for oxygen reduction reaction (ORR) in an alkaline medium and evaluated by a rotating ring-disk electrode. The N, P-GDs/N-3DG catalyst exhibits excellent ORR activity, which is comparable to that of the commercial Pt/C catalyst. Furthermore, it exhibits a higher tolerance to methanol and better stability than the Pt/C. This enhanced electrochemical catalytic performance can be ascribed to the presence of abundant functional groups and edge defects. This study indicates that P-N bonded structures play a vital role as the active sites in ORR.

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.

Charge Engineering of Mo2C@Defect-Rich N-Doped Carbon Nanosheets for Efficient Electrocatalytic H2 Evolution

Charge engineering of carbon materials with many defects shows great potential in electrocatalysis, and molybdenum carbide (Mo2C) is one of the noble-metal-free electrocatalysts with the most potential. Herein, we study the Mo2C on pyridinic nitrogen-doped defective carbon sheets (MoNCs) as catalysts for the hydrogen evolution reaction. Theoretical calculations imply that the introduction of Mo2C produces a graphene wave structure, which in some senses behaves like N doping to form localized charges. Being an active electrocatalyst, MoNCs demonstrate a Tafel slope as low as 60.6 mV dec-1 and high durability of up to 10 h in acidic media. Besides charge engineering, plentiful defects and hierarchical morphology also contribute to good performance. This work underlines the importance of charge engineering to boost catalytic performance.

Combined Electron and Structure Manipulation on Fe-Containing N-Doped Carbon Nanotubes To Boost Bifunctional Oxygen Electrocatalysis

It is a challenge to synthesize highly efficient nonprecious metal electrocatalysts with a well-defined nanostructure and rich active species. Herein, through electron engineering and structure manipulation simultaneously, we constructed Fe-embedded pyridinic-N-dominated carbon nanotubes (CNTs) on ordered mesoporous carbon, showing excellent oxygen reduction reaction activity (half-wave potential, 0.85 V) and an overpotential of 420 mV to achieve 10 mA cm^-2 for oxygen evolution reaction in alkaline media (potential difference, 0.80 V). Density functional theory calculation indicates those Fe@N₄ clusters improve charge transfer and further promote the electrocatalytic reactivity of the functionalized region in CNTs. Rechargeable Zn-air batteries were assembled, displaying robust charging-discharging cycling performance (over 90 h) with voltage gap of only 0.08 V, much lower than that of the Pt/C + Ir/C electrode (0.29 V). This work presents a highly active nonprecious metal-based bifunctional catalyst toward air electrode for energy conversion.

Edge Defect Engineering of Nitrogen-Doped Carbon for Oxygen Electrocatalysts in Zn-Air Batteries

Metal-free bifunctional oxygen electrocatalysts are extremely critical to the advanced energy conversion devices, such as high energy metal-air batteries. Effective tuning of edge defects and electronic density on carbon materials via simple methods is especially attractive. In this work, a facile alkali activation method has been proposed to prepare carbon with large specific surface area and optimized porosity. In addition, subsequent nitrogen-doping leads to high pyridinic-N and graphitic-N contents and abundant edge defects, further enhancing electrochemical activities. Theoretical modeling via first-principles calculations has been conducted to correlate the electrocatalytic activities with their fundamental chemical structure of N doping and edge defect engineering. The metal-free product (NKCNPs-900) shows a high half-wave potential of 0.79 V (ORR). Furthermore, the assembled Zn-air batteries display excellent performance among carbon-based metal-free oxygen electrocatalysts, such as large peak power density up to 131.4 mW cm^-2, energy density as high as 889.0 W h kg^-1 at 4.5 mA cm^-2, and remarkable discharge-charge cycles up to 575 times. Preliminarily, the rechargeable nonaqueous Li-air batteries were also investigated. Therefore, our work provides a low-cost, metal-free, and high-performance bifunctional carbon-based electrocatalyst for metal-air batteries.

Bioinspired fiber-like porous Cu/N/C electrocatalyst facilitating electron transportation toward oxygen reaction for metal-air batteries

Laccase is one of the most effective biocatalysts for oxygen reduction under physiological conditions. Its unique Cu- and N-based active sites with direct electrical pathways for electrons can guarantee rapid oxygen exchange and reduction. Inspired by this specific structure, we designed and fabricated porous fiber-like Cu/N/C based on MOFs as an ORR catalyst. Precision morphology control of fibers contributed to an increase in the electron transport rate compared with that of Cu/N/C nanoparticles, which could minimize interparticle ohmic contacts and provide direct electrical pathways along with sufficient active sites to catalyze oxygen reduction. The designed Cu/N/C catalyst exhibited similar electrochemical activity to that of a commercially available 20 wt% Pt/C ORR catalyst. Moreover, after being employed as the cathode of aluminium-air batteries, the catalyst outperformed a commercial 5 wt% Pt/C catalyst.

A Review of Carbon-Composited Materials as Air-Electrode Bifunctional Electrocatalysts for Metal–Air Batteries

Metal–air batteries (MABs), particularly rechargeable MABs, have gained renewed interests as a potential energy storage/conversion solution due to their high specific energy, low cost, and safety. The development of MABs has, however, been considerably hampered by its relatively low rate capability and its lack of efficient and stable air catalysts in which the former stems mainly from the sluggish kinetics of the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) and the latter stems from the corrosion/oxidation of carbon materials in the presence of oxygen and high electrode potentials. In this review, various carbon-composited bifunctional electrocatalysts are reviewed to summarize progresses in the enhancement of ORR/OER and durability induced by the synergistic effects between carbon and other component(s). Catalyst mechanisms of the reaction processes and associated performance enhancements as well as technical challenges hindering commercialization are also analyzed. To facilitate further research and development, several research directions for overcoming these challenges are also proposed.

Recent progress in two-dimensional inorganic quantum dots

This review critically summarizes recent progress in the categories, synthetic routes, properties, functionalization and applications of 2D materials-based quantum dots (QDs).

Fast Diffusion of O2 on Nitrogen-Doped Graphene to Enhance Oxygen Reduction and Its Application for High-Rate Zn-Air Batteries

N-doped graphene (NDG) was investigated for oxygen reduction reaction (ORR) and used as air-electrode catalyst for Zn-air batteries. Electrochemical results revealed a slightly lower kinetic activity but a much larger rate capability for the NDG than commercial 20% Pt/C catalyst. The maximum power density for a Zn-air cell with NDG air cathode reached up to 218 mW cm^-2, which is nearly 1.5 times that of its counterpart with the Pt/C (155 mW cm^-2). The equivalent diffusion coefficient (D_E) of oxygen from electrolyte solution to the reactive sites of NDG was evaluated as about 1.5 times the liquid-phase diffusion coefficient (D_L) of oxygen within bulk electrolyte solution. Combined with experiments and ab initio calculations, this seems counterintuitive reverse ORR of NDG versus Pt/C can be rationalized by a spontaneous adsorption and fast solid-state diffusion of O₂ on ultralarge graphene surface of NDG to enhance effective ORR on N-doped-catalytic-centers and to achieve high-rate performance for Zn-air batteries.

Free-Radical-Assisted Rapid Synthesis of Graphene Quantum Dots and Their Oxidizability Studies

This work reports a modified electrochemical method for rapid and large-scale preparing graphene quantum dots (GQDs) by introduction of active free radicals, which were produced by hydrogen peroxide or ultraviolet radiation. These free radicals can deepen the oxidized or reduced level of working electrode in electrochemical process and thus lead to GQDs with high concentration and small size, but different surface oxidized degree. The improved oxidation and reduction mechanism were analyzed in this work. Meanwhile, the optical properties and oxidizability of GQDs with different surface oxidized degree were investigated. It is found that these GQDs can be used as an oxidizing agent and their oxidizability is related to the degree being oxidized.

Metal coordination enhanced Ni–Co@N-doped porous carbon core–shell microsphere bi-functional electrocatalyst and its application in rechargeable zinc/air batteries

Primary and rechargeable zinc/air batteries could be the next generation of energy storage devices because of their high power density and safety.