Metal-free Materials Research Articles

Synthesis of Graphene Based Graphitic Carbon Nitride Hybrid Nanocomposite Photocatalysts for Hydrogen Generation: A Mini Review

The discovery of visible active photocatalysts for H2 evolution via water splitting is the most awaited and critical goal of many researchers in recent years. Novel polymeric graphitic carbon nitride (GCN/g-CN) has emerged as a versatile material which has attracted the scientific community and industrialist because of its distinctiveness and outstanding electronic properties. g-CN is a metal free semiconductor as well as non-toxic, biodegradable polymeric material with low band gap energy which makes it a promising candidate as a photocatalyst and its efficiency as a catalyst can be modified by forming a hybrid nanocomposite with other semiconducting materials. Reduced graphene oxide, another metal free 2D material is a very good choice for this purpose. This review is an outlook for the synthesis processes and various properties of both g-CN and graphene. Further, it gives the approaches attempted towards the modifications required and done towards the development of a metal-free nano-hybrid material which is cost-effective, eco-friendly, and highly active visible light catalyst for the water- splitting process.

Nitrogen-doped hollow carbon spheres as efficient metal-free peroxymonosulfate activator for degradation of organic pollutant

Developing effective metal-free catalytic materials via constructing copious surface-exposed active sites to promote peroxymonosulfate (PMS) activating for degradation of hazardous organics is highly desirable toward practical application. In this study, we report the utilization of N-functionalized hollow carbon spheres (N-HCS) as efficient PMS activator for Rhodamine B (RhB) degradation. Results indicate that N-doping of hollow carbon spheres can introduce an abundance of pyridinic N and CO sites, which serve as catalytically active sites for PMS activation toward RhB degradation. The fabricated catalyst exhibits excellent performances with a degradation efficiency of 100 % and a rate constant of 0.0566 min−1 for RhB degradation under optimal conditions. Besides, the developed N-HCS catalyst displays a good reusability, maintaining a high degradation efficiency after the 5th run. Quenching tests and electron spin resonance characterization results reveal the coexistence of radical and non-radical degradation pathways, and SO4− and 1O2 are demonstrated as the dominant reactive oxygen species. This architecture integrated surface microenvironment engineering strategy can be extended to regulate the degradation performance of carbon-based materials for PMS activation toward removing other organic pollutants.

Volatile guest molecule mediated strategy to convert covalent organic framework into nitrogen, sulfur-doped carbon as metal-free oxygen reduction electrocatalysts

Covalent organic framework (COF) derived metal-free carbon materials have emerged as promising electrocatalysts for the oxygen reduction reaction (ORR). Herein, a volatile guest molecule mediated-pyrolysis strategy was explored on a designed thiophene-rich and imine-linked COF. Through the modulation of guest mediators (iodine and sulfur), the properties of the as-obtained carbon materials can be well regulated. The optimized nitrogen and sulfur dual-doped carbon electrocatalyst demonstrates remarkable ORR activity with a half-wave potential of 0.87 V and impressive durability, with only an 8% current loss over 21 h. The corresponding assembled zinc-air battery has a comparable power density (60 mW cm−2) to that of the commercial Pt/C. It is proposed that the coexistence of the guest mediators iodine and sulfur in the channels of COFs could prevent the loss of N species. The enhanced N content and N/S ratio are assumed to be responsible for the ORR performance. This study puts forward a novel strategy to prepare COF-derived carbon materials mediated by volatile guest molecules, which may provide new insights into the development of metal-free ORR catalysts.

Mechanistic Insights into the Appel Reaction Mediated by a Poly-Phosphamide Material as a Heterogeneous Catalyst.

Due to notable thermochemical stability, polyphosphamides are often regarded as flame retardants, while molecular phosphamides can serve as versatile Lewis base to catalyze diverse organic transformations. Being chemically analogous to phosphine oxide, phosphamide can also be considered as a mediator for the phosphine-mediated reaction. Herein, an amorphous polymeric material consisting of phosphamide (-NH-P(O)) in the repeating unit (POP) has been prepared via condensation of tris(2-aminoethyl)amine (TREN) and phenyl phosphinic dichloride (PPDC). The POP is isolated as a metal-free and pure organic material which is made of a strong covalent bond and the phosphamide unit is deployed in the organic framework. The presence of phosphamide in the repeating unit of the isolated amorphous POP material can be confirmed by 31P CPMAS NMR, FTIR, and Raman studies. The core-level N 2p and P 2p X-ray photoelectron spectra are in accordance with the presence of tertiary amine nitrogen attached to carbon and secondary amine nitrogen attached to phosphorus. Elemental analyses have depicted approximately 19.7% of phosphorus content in the material, which is being utilized to study the catalytic Appel reaction with 76% conversion of alcohol to a corresponding halide and TON of 462. Quasi in situ Raman study has identified that amino phosphine formed via in situ reduction of the phosphamide unit of the POP catalyzes the halogenation of primary and secondary alcohols with wide substrate scope and functional group tolerance. Kinetic studies have established a first-order dependence with respect to alcohol, while deuterium labeling experiments emphasize that the deprotonation of alcohol is the rate-limiting step. High thermal stability of the material, scope of easy catalyst recyclability, and a cumulative TON of 1386 have led the POP as an emerging pure organic material to be explored further for other phosphine-mediated organocatalysis.

Guest molecule-directed conversion of covalent organic framework into carbon with synergistic high content N dopants and defects for efficient oxygen reduction

Creating intrinsic defects and N dopants in metal-free carbon matrix has proven to be an effective approach for enhancing their electroactivity toward oxygen reduction (ORR), though it is still a big challenge. Herein, metal-free carbon materials with tunable contents of defects and N dopants were prepared through the guest molecule-directed conversion of covalent organic framework (COFs). Due to the corrosion of guest molecules and subsequent substitution by ammonia (NH3) during the pyrolysis, the optimal carbon catalyst is provided with high content of N dopants (8.22 at%) and carbon defects (ID/IG, 1.41). It delivers remarkable ORR activity, with a half-wave potential (E1/2) of 0.863 V versus reversible hydrogen electrode (vs. RHE) and impressive durability, with a 10.9 % current loss over 40 h. Moreover, the optimal carbon catalyst-assembled Zn-air battery achieves a maximum power density of 107.1 mW cm−2, demonstrating its potential in practical devices. Theoretical calculations reveal that the synergistic effect between carbon defects and heteroatom N dopants is responsible for the enhancement of ORR activity. This work might inspire the development of highly efficient metal-free carbon catalysts for various energy conversion applications.

Comparison of the Surface Roughness of CAD/CAM Metal-Free Materials Used for Complete-Arch Implant-Supported Prostheses: An In Vitro Study.

The roughness of the intra-oral surfaces significantly influences the initial adhesion and the retention of microorganisms. The aim of this study was to analyze the surface texture of four different CAD-CAM materials (two high-performance polymers and two fifth-generation zirconia) used for complete-arch implant-supported prostheses (CAISPs), and to investigate the effect of artificial aging on their roughness. A total of 40 milled prostheses were divided into 4 groups (n = 10) according to their framework material, bio.HPP (B), bio.HPP Plus (BP), zirconia Luxor Z Frame (ZF), and Luxor Z True Nature (ZM). The areal surface roughness "Sa" and the maximum height "Sz" of each specimen was measured on the same site after laboratory fabrication (lab as-received specimen) and after thermocycling (5-55 °C, 10,000 cycles) by using a noncontact optical profilometer. Data were analyzed using SPSS version 28.0.1. One-way ANOVA with multiple comparison tests (p = 0.05) and repeated measures ANOVA were used. After thermocycling, all materials maintained "Sa" values at the laboratory as-received specimen level (p = 0.24). "Sz" increased only for the zirconia groups (p = 0.01). B-BP exhibited results equal/slightly better than ZM-ZF. This study provides more realistic surface texture values of new metal-free materials used in real anatomical CAISPs after the manufacturing and aging processes and establishes a detailed and reproducible measurement workflow.

Constructing an orderly electron transport channel on boron regulated biomass carbon fiber for selective ROS generation and water decontamination

Antibiotic pollution has raised widely attention due to the difficult biodegradation and lasting toxicity to public health, metal-free material based heterogeneous catalysis is a highly-promise and eco-friendly technology for organics elimination. Herein, boron doped biomass carbon fiber (B-CF) was synthesized to construct orderly electron transport channels for enhancing catalytic performance and deeply purifying organics polluted water. Integrating systematical quenching experiments and EPR detection, O2·- and 1O2 are found to be dominating reactive oxygen species (ROS) for norfloxacin (NOR) degradation rather than ∙OH or SO4∙-. Adsorption, catalytic degradation in pristine CF/peroxodisulfate (PDS) and B-CF/PDS systems, electrochemical tests, and theory calculations were compared and the results suggested B-CF surface can trigger intense electron transfer via simultaneous activating NOR and PDS, and electrons transferred from NOR to B-CF-PDS compound, resulting in selective and remarkably enhanced ROS generation. Moreover, it was found that B-CF exhibited surprising adsorption capacity for NOR (834.4 mg g−1), and it can also remove SO42- from the solution through electrostatic attraction. This B-CF/PDS system is efficient within a wide operation pH from 3 to 11 and exhibits long lasting activity (> 274 h maintaining over 80% efficiency). This study unveils the highly selective formation of O2-· and 1O2 and solves the short lifetime of catalysts in persulfate-based catalysis, which provides feasible technology for advanced water purification.

Cathode materials in microbial electrosynthesis systems for carbon dioxide reduction: recent progress and perspectives

Microbial electrosynthesis (MES) is an emerging technology that enables the synthesis of value-added chemicals from carbon dioxide (CO2) or inorganic carbon compounds by coupling renewable electricity to microbial metabolism. However, MES still faces challenges in achieving high production of value-added chemicals due to the limited extracellular electron transfer efficiency at the biotic-abiotic interfaces. To overcome this bottleneck, it is crucial to develop novel cathodes and modified materials. This review systematically summarizes recent advancements in cathode materials in the field of electrocatalyst-assisted and photocatalyst-assisted MES. The effects of various material types are further investigated by comparing metal-free and metal materials and photocatalyst materials of different semiconductor types. Additionally, the review introduces the maximum production rate of value-added chemicals and conversion efficiency achieved by these cathode materials while highlighting the advantages and disadvantages of different material types. To the best of our knowledge, in electrocatalyst-assisted systems, the maximum CH4 yield on graphene aerogel/polypyrrole cathode achieved 1,672 mmol m-2 d-1, and the maximum Faraday efficiency (FE) of CH4 reached up to 97.5% on graphite plate. Meanwhile, the maximum acetate yield achieved 1,330 g m-2 d-1 with CO2 conversion efficiency into acetate close to 100% on carbon nanotube cathodes. In photocatalyst-assisted systems, the maximum acetate yield could reach 0.51 g L-1 d-1 with the coulombic efficiency of 96% on the MnFe2O4/g-C3N4 photocathode. Finally, prospects for future development and practical applications of MES are discussed, offering theoretical guidance for the fabrication of cathode materials that can improve production efficiency and reduce energy input.

Cu doped crystalline carbon nitride with increased carrier migration efficiency for uranyl photoreduction

Graphitic carbon nitride (g-C3N4) has been widely used in photocatalytic removal of uranium pollutants due to its promising metal-free visible light-responsive semiconductor material. Herein, the heteroatom introduction decorated crystal g-C3N4 (Cu-CCN) photocatalysts were successfully synthesized through element doping and molten salt method. Photocatalytic activity experimental results showed that the Cu-CCN-50 exhibits high U(Ⅵ) removal efficient (nearly 100 % after 10 mins under irradiation) and durable cycle performance. The enhanced photocatalytic activity was primarily attributed to the higher specific surface area, red-shift in the absorption band and the high separation efficiency of light-induced electron-hole (e-/h+) pairs. Finally, the possible active species and mechanism based on the photocatalytic reduction results were proposed. Thus, the synergistic strategy of element doping and molten salt crystallization inspires new insights for visible light removal of U(Ⅵ) in toxic wastewater.

Thermal and Mechanical Properties of Nano-Carbon-Reinforced Polymeric Nanocomposites: A Review

Carbon nanomaterials are an emerging class of nano-reinforcements to substitute for metal-based nanomaterials in polymer matrices. These metal-free nano-reinforcement materials exhibit a high surface area, thermal stability, and a sustainable nature. Compared to conventional reinforcements, nano-carbon-reinforced polymer composites provide enhanced mechanical and thermal properties. While previous reviews summarized the functionality of nanocomposites, here, we focus on the thermomechanical properties of nano-carbon-reinforced nanocomposites. The role of carbon nanomaterials, including graphene, MXenes, carbon nanotubes, carbon black, carbon quantum dots, fullerene, and metal–organic frameworks, in polymer matrices for the enhancement of thermal and mechanical properties are discussed. Different from metal-based nanomaterials, carbon nanomaterials offer high specific strength, abundance, and sustainability, which are of considerable importance for commercial-scale applications.

Electrocatalytic degradation of p-nitrophenol on metal-free cathode: Superoxide radical (O2•−) production via molecular oxygen activation

Although metal-free electrodes in molecular oxygen-activated Fenton-like wastewater treatment technologies have been developed, the reactive oxygen species (ROS) generation mechanisms are still not sufficiently clear. As a typical example of refractory phenolic wastewater, p-nitrophenol (PNP) has been widely studied. This study demonstrated the critical role of superoxide radicals (O2•−) in PNP degradation by metal-free electrodes through electron spin resonance (ESR), ROS quenching, and density functional theory (DFT) tests. The most superior metal-free electrode exhibited a mass activity of approximately 133.5 h−1 gcatalyst−1. Experimental and theoretical studies revealed the mechanism of O2•− generation via oxygen activation, including one- and three-electron transfer pathways, and found that O2•− mainly attacked the nitro group of PNP to degrade and transform the pollutant. This study enhances the mechanistic understanding of metal-free materials in the electrochemical degradation of refractory pollutants.

2D Atomic Layers for CO2 Photoreduction.

Artificial photosynthesis can convert carbon dioxide into high value-added chemicals. However, due to the poor charge separation efficiency and CO2 activation ability, the conversion efficiency of photocatalytic CO2 reduction is greatly restricted. Ultrathin 2D photocatalyst emerges as an alternative to realize the higher CO2 reduction performance. In this review, the basic principle of CO2 photoreduction is introduced, and the types, advantages, and advances of 2D photocatalysts are reviewed in detail including metal oxides, metal chalcogenides, bismuth-based materials, MXene, metal-organic framework, and metal-free materials. Subsequently, the tactics for improving the performance of 2D photocatalysts are introduced in detail via the surface atomic configuration and electronic state tuning such as component tuning, crystal facet control, defect engineering, element doping, cocatalyst modification, polarization, and strain engineering. Finally, the concluding remarks and future development of 2D photocatalysts in CO2 reduction are prospected.

Advances of Zn Metal-Free "Rocking-Chair"-Type Zinc Ion Batteries: Recent Developments and Future Perspectives.

Aqueous zinc ion battery (AZIBs) has attracted the attention of many researchers because of its safety, economy, environmental protection, and high ionic conductivity of electrolytes. However, the battery greatly suffers from zinc dendrite produced by zinc metal anode leading to poor cycle life and even unsafe problems, which limit its further development for various important applications. It is known that the success of the commercialization of lithium-ion batteries (LIBs) is mainly due to replacement of lithium metal anode with graphite, which avoids the formation of Li dendrite. Therefore, it is an important step to develop aqueous zinc ion anode to replace conventional zinc metal one with zinc-metal free anode material. In this review, the working principle and development prospect of "rocking-chair" AZIBs are introduced. The research progress of different types of zinc metal-free anode materials and cathode materials in "rocking-chair" AZIBs is reviewed. Finally, the limitations and challenges of the Zn metal-free "rocking-chair" AZIBs as well as solutions are deeply discussed, aiming to provide new strategies for the development of advanced zinc-ion batteries.

Synthesis of well-defined ester-linked covalent organic polymer and its potential applications in C–H bond activation

Among all polymeric materials, the chemistry of covalent organic polymers has been the least covered in the field of photocatalysis due to the rare availability of design and synthetic protocols. In the present work, we have come up with an efficient design of a one-dimensional Covalent Organic polymer (COP) (S-COP) through the ester-linked co-polymerization of Erythrosine B with 2,2′-Dithiodibenzoic acid (2,2′-DDB). The COP was synthesized via the facile one-pot solvothermal method. The XRD data analysis favors high crystalline nanoscale polymeric sheet generation (S-COP, 90.5 % & 43.5 nm). The HOMO-LUMO calculation through an electrochemical study as well as Tauc's plot showed 2.1 eV band gap. In this order, S-COP, a metal-free polymeric material was observed as an effective photocatalyst in the solar-driven photocatalytic reaction of heteroarenes with aryl diazonium salts via direct C-H arylation (High yield (∼98 %), and high selectivity (∼98 %)).

Unveiling the dynamic active site of defective carbon-based electrocatalysts for hydrogen peroxide production

Active sites identification in metal-free carbon materials is crucial for developing practical electrocatalysts, but resolving precise configuration of active site remains a challenge because of the elusive dynamic structural evolution process during reactions. Here, we reveal the dynamic active site identification process of oxygen modified defective graphene. First, the defect density and types of oxygen groups were precisely manipulated on graphene, combined with electrocatalytic performance evaluation, revealing a previously overlooked positive correlation relationship between the defect density and the 2 e- oxygen reduction performance. An electrocatalytic-driven oxygen groups redistribution phenomenon was observed, which narrows the scope of potential configurations of the active site. The dynamic evolution processes are monitored via multiple in-situ technologies and theoretical spectra simulations, resolving the configuration of major active sites (carbonyl on pentagon defect) and key intermediates (*OOH), in-depth understanding the catalytic mechanism and providing a research paradigm for metal-free carbon materials.

CeO2-ZrO2-Sm2O3 anodes for intermediate temperature-solid oxide fuel cells

Ce0.9Zr0.1-xSmxO2-δ (0 ≤ x ≤ 0.1) mixed oxides were synthesized for the first time via the citrate-nitrate self-combustion method and, interfaced with 20 % Sm doped CeO2, were studied as metal-free anode materials for Intermediate Temperature- Solid Oxide Fuel Cells. The effect of Sm2O3 incorporation in the Ce0.9Zr0.1O2 structure on the electrocatalytic properties of the anodes under wet H2 and wet CH4 was investigated. The processes involved in the anode performance were identified analyzing the Electrochemical Impedance Spectroscopy results by combining Equivalent Circuit fitting and Distribution of Relaxation Time deconvolution methods. A new anode material with optimized composition (Ce0.9Zr0.04Sm0.06O2-δ), and extremely low polarization resistance without the need of a metallic phase was obtained (0.011 Ω.cm2 and 0.004 Ω.cm2 at 750 °C in 7 vol% H2 and 40 vol% H2, respectively; 0.14 Ω.cm2 and 0.09 Ω.cm2 at 750 °C in 7 vol% CH4. and 20 vol% CH4, respectively).

Supramolecular organic framework for recyclable heterogeneous photocatalysis of the reductive dechlorination of α-Chloroacetophenones and acetylation of amines with thioacetate

Due to the urgent demand for environmentally friendly synthetic methods, metal-free materials have gained increasing prominence in various photocatalytic organic transformations. The utilization of supramolecular organic frameworks (SOFs) as photocatalysts has gradually emerged in photo-induced organic reactions, which complements the widespread use of porous organic polymers and covalent organic frameworks. SOFs, which are constructed through noncovalent interactions, can function as heterogeneous catalysts due to their low solubility in typical organic solvents. The present study reports the application of a SOF for heterogeneous photocatalysis. A tetraphenylmethane-based monomer (TM) incorporating four 4,4′-(thiazolo[5,4-d]thiazole-2,5-diyl)bis(1-methyl-1-pyridinium) units (MPT) has been synthesized and co-assemble with cucurbit[8]uril (CB[8]) to afford a photoactive three-dimensional supramolecular organic framework (TM-SOF) through the encapsulation of two MPT units by CB[8]. Various methods have been employed to characterize the optical and electronic properties of TM-SOF and reveal its exceptional light-absorbing properties that facilitate photoredox catalysis by generating reactive radical species. As a heterogeneous photocatalyst, TM-SOF efficiently catalyzes photoinduced reductive dichlorination of α-chloroacetophenones and acetylation of amines with thioacetate with high recyclability. Mechanistic studies were further conducted to reveal the effect of the framework on the photocatalytic efficiency of the MPT unit.

Rational design of vitamin C/defective carbon van der Waals heterostructure for enhanced activity, durability and storage stability toward oxygen reduction reaction

Metal-free defective carbon materials with abundant active sites have been widely studied as low-cost and efficient oxygen reduction reaction (ORR) electrocatalysts in metal-air batteries. However, the active sites in defective carbon are easily subjected to serious oxidation or hydroxylation during ORR or storage, leading to rapid degradation of activity. Herein, we design a van der Waals heterostructure comprised of vitamin C (VC) and defective carbon (DC) to not only boost the activity but also enhance the durability and storage stability of the DC-VC electrocatalyst. The formation of VC van der Waals between DC and VC is demonstrated to be an effective strategy to protect the defect active sites from oxidation and hydroxylation degradation, thus significantly enhancing the electrochemical durability and storage anti-aging performance. Moreover, the DC-VC van der Waals can reduce the reaction energy barrier to facilitate the ORR. These findings are also confirmed by operando Fourier transform infrared spectroscopy and density functional theory calculations. It is necessary to mention that the preparation of this DC-VC electrocatalyst can be scaled up, and the ORR performance of the largely produced electrocatalyst is demonstrated to be very consistent. Furthermore, the DC-VC-based aluminum-air batteries display very competitive power density with good performance maintenance.

Precise Modulation of Carbon Activity Sites in Metal-Free Covalent Organic Frameworks for Enhanced Oxygen Reduction Electrocatalysis.

Metal-free carbon-based materials have gained recognition as potential electrocatalysts for the oxygen reduction reaction (ORR) in new environmentally-friendly electrochemical energy conversion technologies. The presence of effective active centers is crucial for achieving productive ORR. In this study, we present the synthesis of two metal-free dibenzo[a,c]phenazine-based covalent organic frameworks (DBP-COFs), specifically JUC-650 and JUC-651, which serve as ORR electrocatalysts. Among them, JUC-650 demonstrates exceptional catalytic performance for ORR in alkaline electrolytes, exhibiting an onset potential of 0.90V versus RHE and a half-wave potential of 0.72V versus RHE. Consequently, JUC-650 stands out as one of the most outstanding metal-free COF-based ORR electrocatalysts report to date. Experimental investigations and density functional theory calculations confirm that modulation of the frameworks' electronic configuration allows for the reduction of adsorption energy at the Schiff-base carbon active sites, leading to more efficient ORR processes. Moreover, the DBP-COFs can be assembled as excellent air cathode catalysts for zinc-air batteries (ZAB), rivaling the performance of commercial Pt/C. This study provides valuable insights for the development of efficient metal-free organoelectrocatalysts through precise regulation of active site strategies.

Synergistic activation of peroxymonosulfate by intrinsic defect and graphitic N of N, P co-doped carbon microspheres for BPA degradation

Doping nonmetallic heteroatoms into carbonaceous materials is a proven strategy for enhancing peroxymonosulfate (PMS) activation in the degradation of endocrine-disrupting compounds (EDCs). Understanding the role of heteroatom doping defects and intrinsic defects (topological, vacancies, or edge defects) on PMS activation is crucial for designing more efficient carbon catalysts. Herein, we synthesized nitrogen (N) and phosphorus (P) co-doped carbon microspheres (NPCs) through the pyrolysis of aminophosphonic acid resin (D418) precursors and utilized them to activate PMS for bisphenol A (BPA) degradation. The pyrolysis temperature was found to regulate the type and degree of defects, as well as the configuration of N and P. NPC-1000 (pyrolysis temperature at 1000 °C) exhibited the highest reaction rate constant (k) (1.20 × 10−1 min−1) with the lowest N and P doping defects and the highest intrinsic defect intensity resulting from the removal of N and P during pyrolysis. Density functional theory (DFT) calculations and correlation analysis between defect sites and k suggest that the adsorption of PMS was facilitated by graphitic N-coordinated C and intrinsic defect sites rather than P doping defect sites, resulting in the degradation of BPA through electron transfer and radical mechanisms, specifically superoxide radicals (O•–2). Excess halide anions led a significant increase of k with only a limited formation of trichloromethane (TCM) (disinfection byproducts) in presence of 100 mM Cl−. This research offers fundamental insights into constructing intrinsic defects through regulating heteroatom configuration, intending to enhance the catalytic activity of metal-free materials.