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

Metal-Free Graphene-Based Derivatives as Oxygen Reduction Reaction Electrocatalysts in Energy Conversion and Storage Systems: An Overview

Oxygen reduction reaction (ORR) is one of the most important reactions in electrochemical energy storage and conversion devices. To overcome the slow kinetics, minimize the overpotential, and make this reaction feasible, efficient, and stable, electrocatalysts are needed. Metal-free graphene-based systems are considered promising and cost-effective ORR catalysts with adjustable structures. This review is meant to give a rational overview of the graphene-based metal-free ORR electrocatalysts, illustrating the huge amount of related research developed particularly in the field of fuel cells and metal-air batteries, with particular attention to the synthesis procedures. The novelty of this review is that, beyond general aspects regarding the synthesis and characterization of graphene, above 90% of the various graphene (doped and undoped species, composites)-based ORR electrocatalysts have been reported, which represents an unprecedented thorough collection of both experimental and theoretical studies. Hundreds of references are included in the review; therefore, it can be considered as a vademecum in the field.

Recent Progress on Graphene-Based Derivatives for Enhanced Energy Storage Devices

2D materials have been widely utilized in efficient energy storage applications in recent years. In efforts to mitigate global warming concerns, the novel trends of using nonfossil fuel cells have drawn significant attention of scientific communities towards various 2D materials owing to their excellent physiochemical properties, preferably suitable for developing energy storage systems. Amongst the 2D materials, graphene and its derivatives, due to their tunable surface properties, have emerged as prominent 2D materials for energy storage. Graphene, leveraged with its highly reactive surface sites, can be tailored with myriads of functional groups to enhance its applicability as an energy storage material. This review focuses on the recent advancements in utilizing various dimensions of graphene, including 0D GQDs, 1D GNRs, 2D GO/rGO, and 3D architectures, along with vertical graphene and graphene paper for efficient energy storage devices. The review addresses the limitations of pristine graphene and highlights the benefits of functionalization and synergistic material combinations. This also discusses the challenges and future perspectives related to the commercialization and large-scale production of graphene-based energy storage devices.

Hierarchically Ordered Macro-/Mesoporous N,P-Codoped Carbon with Fe-Co Dual Sites for Efficient Electrocatalytic Oxygen Reduction

The development of high-performance, low-cost non-precious-metal electrocatalysts as alternatives to Pt-based catalysts for the oxygen reduction reaction (ORR) is crucial for promoting the commercial application of fuel cells and metal-air batteries. In this work, we report a novel type of ORR electrocatalyst with Fe and Co sites anchored on N,P-codoped hierarchically ordered macro-/mesoporous carbon (FeCo/NP-HOMMC) through a facile one-pot, controllable synthesis method with the aid of dual-templating technique. The FeCo/NP-HOMMC catalyst shows robust ORR performance under alkaline conditions with a half-wave potential (E1/2) of 0.90 V vs. reversible hydrogen electrode (RHE), significantly surpassing the commercial 20 wt% Pt/C catalyst, while also exhibiting remarkable long-term stability and great methanol tolerance. Control experiments reveal that the superior performance of the FeCo/NP-HOMMC catalyst for ORR benefits from the synergistic catalysis of highly dispersed Fe and Co dual active sites, and the advantages of the unique hierarchically ordered macro-/mesoporous structure, which can provide improved active site accessibility, excellent conductivity, and maximized mass/charge transport.

Manipulation of d-Orbital Electron Configurations in Nonplanar Fe-Based Electrocatalysts for Efficient Oxygen Reduction

Manipulation of the spin state holds great promise to improve the electrochemical activity of transition metal-based catalysts. However, the underlying relationship between the nonplanar metal coordination environment and spin states remains to be explored. Herein, we report the precise regulation of nonplanar Fe atomic d-orbital energy level into an irregular tetrahedral crystal field configuration by introducing P atoms. With the increase of P coordination number, the spin magnetic moment decreases linearly from 3.8 μB to 0.2 μB, and the high spin content decreases linearly from 31% to 5%. Significantly, a volcanic curve between the spin states of Fe-based catalysts (Fe-NxPy) and oxygen reduction reaction (ORR) activity has been unequivocally established based on the thermodynamic results. Thus, the Fe-N3P1 catalyst with a 19% medium spin state experimentally exhibits the optimal reaction activity with a high half-wave potential of 0.92 V. These findings indicate that regulating electron spin moments through coordination engineering is a promising catalyst design strategy, providing important insights into spin catalysis.

Deepening the Understanding of Carbon Active Sites for ORR Using Electrochemical and Spectrochemical Techniques

Defect-containing carbon nanotube materials were prepared by subjecting two commercial multiwalled carbon nanotubes (MWCNTs) of different purities to purification (HCl) and oxidative conditions (HNO3) and further heat treatment to remove surface oxygen groups. The as-prepared carbon materials were physicochemically characterized to observe changes in their properties after the different treatments. TEM microscopy shows morphological modifications in the MWCNTs after the treatments such as broken walls and carbon defects including topological defects. This leads to both higher surface areas and active sites. The carbon defects were analysed by Raman spectroscopy, but the active surface area (ASA) and the electrochemical active surface area (EASA) values showed that not all the defects are equally active for oxygen reduction reactions (ORRs). This suggests the importance of calculating either ASA or EASA in carbon materials with different structures to determine the activity of these defects. The as-prepared defect-containing multiwalled carbon nanotubes exhibit good catalytic performance due to the formation of carbon defects active for ORR such as edge sites and topological defects. Moreover, they exhibit good stability and methanol tolerances. The as-prepared MWCNTs sample with the highest purity is a promising defective carbon material for ORR because its activity is only related to high concentrations of active carbon defects including edge sites and topological defects.

Engineering functionalization and properties of graphene quantum dots (GQDs) with controllable synthesis for energy and display applications

Graphene quantum dots (GQDs), a new type of 0D nanomaterial, are composed of a graphene lattice with sp2 bonding carbon core and characterized by their abundant edges and wide surface area. This unique structure imparts excellent electrical properties and exceptional physicochemical adsorption capabilities to GQDs. Additionally, the reduction in dimensionality of graphene leads to an open band gap in GQDs, resulting in their unique optical properties. The functional groups and dopants in GQDs are key factors that allow the modulation of these characteristics. So, controlling the functionalization level of GQDs is crucial for understanding their characteristics and further application. This review provides an overview of the properties and structure of GQDs and summarizes recent developments in research that focus on their controllable synthesis, involving functional groups and doping. Additionally, we provide a comprehensive and focused explanation of how GQDs have been advantageously applied in recent years, particularly in the fields of energy storage devices and displays.

High Quantum Yield Amino Acid Carbon Quantum Dots with Unparalleled Refractive Index

Carbon quantum dots (CQDs) are one of the most promising types of fluorescent nanomaterials due to their exceptional water solubility, excellent optical properties, biocompatibility, chemical inertness, excellent refractive index, and photostability. Nitrogen-containing CQDs, which include amino acid based CQDs, are especially attractive due to their high quantum yield, thermal stability, and potential biomedical applications. Recent studies have attempted to improve the preparation of amino acid based CQDs. However, the highest quantum yield obtained for these dots was only 44%. Furthermore, the refractive indices of amino acid derived CQDs were not determined. Here, we systematically explored the performance of CQDs prepared from all 20 coded amino acids using modified hydrothermal techniques allowing more passivation layers on the surface of the dots to optimize their performance. Intriguingly, we obtained the highest refractive indices ever reported for any CQDs. The values differed among the amino acids, with the highest refractive indices found for positively charged amino acids including arginine-CQDs (∼2.1), histidine-CQDs (∼2.0), and lysine-CQDs (∼1.8). Furthermore, the arginine-CQDs reported here showed a nearly 2-fold increase in the quantum yield (∼86%) and a longer decay time (∼8.0 ns) compared to previous reports. In addition, we also demonstrated that all amino acid based CQD materials displayed excitation-dependent emission profiles (from UV to visible) and were photostable, water-soluble, noncytotoxic, and excellent for high contrast live cell imaging or bioimaging. These results indicate that amino acid based CQD materials are high-refractive-index materials applicable for optoelectronic devices, bioimaging, biosensing, and studying cellular organelles in vivo. This extraordinary RI may be highly useful for exploring cellular elements with different densities.

Modified graphene foam as a high-performance catalyst for oxygen reduction reaction

Gelatine and chitosan were used as natural precursors for nitrogen-doping of the graphene foam structure, creating specific types of active sites. The quantitative and qualitative content of nitrogen groups in the carbon structure was determined, which, under the influence of high temperature, were incorporated and transformed into forms of functional groups favorable for electrochemical application. Electrochemical studies proved that the form of pyridine-N, pyrrole-N, and quaternary-N groups have favorable electrochemical properties in the oxygen reduction reaction comparable to commercial platinum-based electrode materials. Using these materials as electrodes in metal-air batteries or fuel cells may eliminate the use of noble metal-based electrodes.

Advanced and Stable Metal-Free Electrocatalyst for Energy Storage and Conversion: The Structure-Effect Relationship of Heteroatoms in Carbon

Ever-developing energy device technologies require the exploration of advanced materials with multiple functions. Heteroatom-doped carbon has been attracting attention as an advanced electrocatalyst for zinc-air fuel cell applications. However, the efficient use of heteroatoms and the identification of active sites are still worth investigating. Herein, a tridoped carbon is designed in this work with multiple porosities and high specific surface area (980 m^-2 g^-1). The synergistic effects of nitrogen (N), phosphorus (P), and oxygen (O) in micromesoporous carbon on oxygen reduction reaction (ORR)/oxygen evolution reaction (OER) catalysis are first investigated comprehensively. Metal-free N-, P-, and O-codoped micromesoporous carbon (NPO-MC) exhibits attractive catalytic activity in zinc-air batteries and outperforms a number of other catalysts. Combined with a detailed study of N, P, and O dopants, four optimized doped carbon structures are employed. Meanwhile, density functional theory (DFT) calculations are made for the codoped species. The lowest free energy barrier for the ORR can be attributed to the pyridine nitrogen and N-P doping structures, which is an important reason for the remarkable performance of NPO-MC catalyst in electrocatalysis.

Engineering adjacent N, P and S active sites on hierarchical porous carbon nanoshells for superior oxygen reduction reaction and rechargeable Zn-air batteries

Heteroatom-doped carbon materials have been regarded as sustainable alternatives to the noble-metal catalysts for oxygen reduction reaction (ORR), while the catalytic performances still remain unsatisfactory. Herein, we develop a metal-free adjacent N, P and S-codoped hierarchical porous carbon nanoshells (NPS-HPCNs) through a novel layer-by-layer template coating method. The NPS-HPCNs is rationally fabricated by crosslinking of polyethyenemine (PEI) and phytic acid (PA) on nano-SiO₂ template surface and subsequently coating of viscous sulfur-bearing petroleum pitch, followed by pyrolysis and alkaline etching. Soft X-ray absorption near-edge spectroscopy (XANES) analysis and density functional theory (DFT) calculations prove the engineering of adjacent N, P and S atoms to generate synergistic and reinforced active sites for oxygen electrocatalysis. The NPS-HPCNs manifests excellent ORR activity with a half-wave potential (E_1/2) of 0.86 V, as well as promoted durability and methanol tolerance in alkaline medium. Remarkably, the NPS-HPCNs-based Zn-air battery delivers an open-circuit voltage of 1.479 V, a considerable peak power density of 206 mW cm^-2 and robust cycling stability (over 200 h), even exceeding the commercial Pt/C catalyst. This study offers fundamental insights into the construction and synergistic mechanism of adjacent heteroatoms on carbon substrate, providing advanced metal-free electrocatalysts for Zn-air batteries and other energy conversion and storage devices.

Research Progress on Graphite-Derived Materials for Electrocatalysis in Energy Conversion and Storage

High-performance electrocatalysts are critical to support emerging electrochemical energy storage and conversion technologies. Graphite-derived materials, including fullerenes, carbon nanotubes, and graphene, have been recognized as promising electrocatalysts and electrocatalyst supports for the oxygen reduction reaction (ORR), oxygen evolution reaction (OER), hydrogen evolution reaction (HER), and carbon dioxide reduction reaction (CO₂RR). Effective modification/functionalization of graphite-derived materials can promote higher electrocatalytic activity, stability, and durability. In this review, the mechanisms and evaluation parameters for the above-outlined electrochemical reactions are introduced first. Then, we emphasize the preparation methods for graphite-derived materials and modification strategies. We further highlight the importance of the structural changes of modified graphite-derived materials on electrocatalytic activity and stability. Finally, future directions and perspectives towards new and better graphite-derived materials are presented.

Low-Frequency Broadband Absorbing Coatings Based on MOFs: Design, Fabrication, Microstructure and Properties

Although most microwave absorbing materials (MAMs) have good absorption ability above 8 GHz, they perform poorly in the low-frequency range (1–8 GHz). Metal–organic frameworks (MOFs) derived carbon-based composites have been highly sought after in electromagnetic materials and functional devices, due to their high specific area, high porosity, high thermal stability, low reflection loss, and adjustable composition. In this review, we first introduce the three loss types of MAMs and argue that composite materials are effective ways to achieve broadband absorption. Secondly, the absorbing properties of traditional materials and MOF materials in the literature are compared, followed by a discussion of the promising strategies for designing MAMs with broadband absorption in low frequencies based on the recent progress. Finally, the main problems, fabrication methods, and applications are discussed for their future prospects.

Rational design of large flat nitrogen-doped graphene oxide quantum dots with green-luminescence suitable for biomedical applications

Rational synthesis and simple methodology for the purification of large (35-45 nm in lateral size) and flat (1.0-1.5 nm of height) nitrogen-doped graphene oxide quantum dots (GOQDs) are presented. The methodology allows robust metal-free and acid-free preparation of N-GOQDs with a yield of about 100% and includes hydrothermal treatment of graphene oxide with hydrogen peroxide and ammonia. It was demonstrated that macroscopic impurities can be separated from N-GOQD suspension by their coagulation with 0.9% NaCl solution. Redispersible in water and saline solutions, particles of N-GOQDs were characterized using tip-enhanced Raman spectroscopy (TERS), photoluminescent, XPS, and UV-VIS spectroscopies. The size and morphology of N-GOQDs were studied by dynamic light scattering, AFM, SEM, and TEM. The procedure proposed allows nitrogen-doped GOQDs to be obtained, having 60-51% of carbon, 34-45% of oxygen, and up to 7.2% of nitrogen. The N-GOQD particles obtained in two hours of synthesis contain only pyrrolic defects of the graphene core. The fraction of pyridine moieties grows with the time of synthesis, while the fraction of quaternary nitrogen declines. Application of TERS allows demonstration that the N-GOQDs consist of a graphene core with an average crystallite size of 9 nm and an average distance between nearest defects smaller than 3 nm. The cytotoxicity tests reveal high viability of the monkey epithelial kidney cells Vero in the presence of N-GOQDs in a concentration below 60 mg L-1. The N-GOQDs demonstrate green luminescence with an emission maximum at 505 nm and sedimentation stability in the cell culture medium.

Electronic and Potential Synergistic Effects of Surface-Doped P-O Species on Uniform Pd Nanospheres: Breaking the Linear Scaling Relationship toward Electrochemical Oxygen Reduction

Developing efficient oxygen reduction reaction (ORR) electrocatalysts is critical to fuel cells and metal-oxygen batteries, but also greatly hindered by the limited Pt resources and the long-standing linear scaling relationship (LSR). In this study, ∼6 nm and highly uniform Pd nanospheres (NSs) having surface-doped (SD) P-O species are synthesized and evenly anchored onto carbon blacks, which are further simply heat-treated (HT). Under alkaline conditions, Pd/^SDP-O NSs/C-HT exhibits respective 8.7 (4.3)- and 5.0 (5.5)-fold enhancements in noble-metal-mass- and area-specific activity (NM-MSA and ASA) compared with the commercial Pd/C (Pt/C). It also possesses an improved electrochemical stability. Besides, its acidic ASA and NM-MSA are 2.9 and 5.1 times those of the commercial Pd/C, respectively, and reach 65.4 and 51.5% of those of the commercial Pt/C. Moreover, it also shows nearly ideal 4-electron ORR pathways under both alkaline and acidic conditions. The detailed experimental and theoretical analyses reveal the following: (1) The electronic effect induced by the P-O species can downshift the surface d-band center to weaken the intermediate adsorptions, thus preserving more surface active sites. (2) More importantly, the potential hydrogen bond between the O atom in the P-O species and the H atom in the hydrogen-containing intermediates can in turn stabilize their adsorptions, thus breaking the ORR LSR toward more efficient ORRs and 4-electron pathways. This study develops a low-cost and high-performance ORR electrocatalyst and proposes a promising strategy for breaking the ORR LSR, which may be further applied in other electrocatalysis.

Phosphorus-Doped Graphene Electrocatalysts for Oxygen Reduction Reaction

Developing cheap and earth-abundant electrocatalysts with high activity and stability for oxygen reduction reactions (ORRs) is highly desired for the commercial implementation of fuel cells and metal-air batteries. Tremendous efforts have been made on doped-graphene catalysts. However, the progress of phosphorus-doped graphene (P-graphene) for ORRs has rarely been summarized until now. This review focuses on the recent development of P-graphene-based materials, including the various synthesis methods, ORR performance, and ORR mechanism. The applications of single phosphorus atom-doped graphene, phosphorus, nitrogen-codoped graphene (P, N-graphene), as well as phosphorus, multi-atoms codoped graphene (P, X-graphene) as catalysts, supporting materials, and coating materials for ORR are discussed thoroughly. Additionally, the current issues and perspectives for the development of P-graphene materials are proposed.

Synthesis of Doped/Hybrid Carbon Dots and Their Biomedical Application

Carbon dots (CDs) are a novel type of carbon-based nanomaterial that has gained considerable attention for their unique optical properties, including tunable fluorescence, stability against photobleaching and photoblinking, and strong fluorescence, which is attributed to a large number of organic functional groups (amino groups, hydroxyl, ketonic, ester, and carboxyl groups, etc.). In addition, they also demonstrate high stability and electron mobility. This article reviews the topic of doped CDs with organic and inorganic atoms and molecules. Such doping leads to their functionalization to obtain desired physical and chemical properties for biomedical applications. We have mainly highlighted modification techniques, including doping, polymer capping, surface functionalization, nanocomposite and core-shell structures, which are aimed at their applications to the biomedical field, such as bioimaging, bio-sensor applications, neuron tissue engineering, drug delivery and cancer therapy. Finally, we discuss the key challenges to be addressed, the future directions of research, and the possibilities of a complete hybrid format of CD-based materials.