Template‐ and Metal‐Free Synthesis of Nitrogen‐Rich Nanoporous “Noble” Carbon Materials by Direct Pyrolysis of a Preorganized Hexaazatriphenylene Precursor

Vibrational dynamics and spectroscopy of water at porous g-C3N4 and C2N surfaces

Porous graphitic materials containing nitrogen are promising catalysts for photo(electro)chemical reactions, notably water splitting, but can also serve as "molecular sieves". Nitrogen increases the hydrophilicity of the graphite parent material, among other effects. A deeper understanding of how water interacts with C- and N-containing layered materials, if and which differences exist between materials with different N content and pore size, and what the role of water dynamics is - a prerequsite for catalysis and sieving - is largely absent, however. Vibrational spectroscopy can answer some of these questions. In this work, the vibrational dynamics and spectroscopy of deuterated water molecules (D2O) mimicking dense water layers at room temperature on the surfaces of two different C/N-based materials with different N content and pore size, namely graphitic C3N4 (g-C3N4) and C2N, are studied using ab initio molecular dynamics (AIMD). In particular, time-dependent vibrational sum frequency generation (TD-vSFG) spectra of the OD modes and also time-averaged vSFG spectra and OD frequency distributions are computed. This allows us to distinguish "free" (dangling) OD bonds from OD bonds that are bound in a H-bonded water network or at the surface - with subtle differences between the two surfaces and also to a pure water/air interface. It is found that the temporal decay of OD modes is very similar on both surfaces with a correlation time near 4 ps. In contrast, TD-vSFG spectra reveal that the interconversion time from "bonded" to "free" OD bonds is about 8 ps for water on C2N and thus twice as long as for g-C3N4, demonstrating a propensity of the former material to stabilize bonded OD bonds.

Tin (II) Chloride Salt Melts as Non-Innocent Solvents for the Synthesis of Low-Temperature Nanoporous Oxo-Carbons for Nitrate Electrochemical Hydrogenation

Carbonaceous electrocatalysts offer advantages over metal-based counterparts, being cost-effective, sustainable, and electrochemically stable. Their high surface area increases reaction kinetics, making them valuable for environmental applications involving contaminant removal. However, their rational synthesis is challenging due to the applied high temperatures and activation steps, leading to disordered materials with limited control over doping. Here, a new synthetic pathway using carbon oxide precursors and tin chloride as a p-block metal salt melt is presented. As a result, highly porous oxygen-rich carbon sheets (with a surface area of 1600 m2 g-1 ) are obtained at relatively low temperatures (400 °C). Mechanistic studies reveal that Sn(II) triggers reductive deoxygenation and concomitant condensation/cross-linking, facilitated by the Sn(II) → Sn(IV) transition. Due to their significant surface area and oxygen doping, these materials demonstrate exceptional electrocatalytic activity in the nitrate-to-ammonia conversion, with an ammonia yield rate of 221 mmol g-1 h-1 and a Faradic efficiency of 93%. These results surpass those of other carbon-based electrocatalysts. In situ Raman studies reveal that the reaction occurs through electrochemical hydrogenation, where active hydrogen is provided by water reduction. This work contributes to the development of carbonaceous electrocatalysts with enhanced performance for sustainable environmental applications.

Anhydride-Based Compound with Tunable Redox Properties as Advanced Organic Cathodes for High-Performance Aqueous Zinc-Ion Batteries

Organic materials with multiple active sites and flexible structural designs are becoming popular for use in aqueous zinc-ion batteries (AZIBs). However, their applicability is limited due to the low specific capacity and poor cycle stability originating from the introduction of inactive units and high solubility. Herein, three organic molecules with tunable redox properties were synthesized using anhydride (PMDA, 1,2,4,5-benzenetetracarboxylic anhydride-1,2-diaminoanthraquinone, NTCDA, 1,4,5,8-naphthalenetetracarboxylic dianhydride-1,2-diaminoanthraquinone, and PTCDA, 3,4,9,10-perylenetetracarboxylic dianhydride-1,2-diaminoanthraquinone, referred to as PM12, NT12, and PT12) in the solid-phase method. Density functional theory (DFT) simulations and experiments identified that NT12 exhibits superior electrochemical performance compared with PM12 and PT12 because of the low energy gap and large aromatic conjugated structure. They demonstrated specific capacities of 106.7, 192.9, and 124.9 mA h g-1 at 0.05 A g-1, respectively. Especially, NT12 displayed excellent initial specific capacity (85.4 mA h g-1 at 1 A g-1) and remarkable capacity retention (64.1% for 3000 cycles) due to dual active centers (C═N and C═O). The all-NT12 full-cell also had excellent performance (127.1 mA h g-1 under 1 A g-1 and 80.6% over 200 cycles). The organic compounds synthesized in this work have potential applications of AZIBs, highlighting the importance of molecular design to develop the next generation of advanced materials.

Green metrics in mechanochemistry

The development of new green methodologies and their broader adoption for promoting sustainable development in chemistry laboratories and industry play a significant role in society, due to the economic importance of chemistry and its widespread presence in everyday life. Therefore, a sustainable approach to chemistry contributes to the well-being of the worldwide population and complies with the United Nations Sustainable Development Goals (UN SDGs) and the European Green Deal. The review highlights how batch and continuous mechanochemical methods are an eco-friendly approach for organic synthesis, with a lower environmental footprint in most cases, compared to solution-based procedures. The assessment is objectively based on the use of green metrics (e.g., atom and real atom economy, E-factor, process mass intensity, material parameter recovery, Eco-scale, stoichiometric factor, etc.) and indicators (e.g. DOZN tool and life cycle assessment, LCA, studies) applied to organic transformations such as synthesis of the amide bond, carbamates, heterocycles, active pharmaceutical ingredients (APIs), porphyrins, porous organic polymers (POPs), metal- or acid-catalysed processes, multicomponent and condensation reactions, rearrangements, etc. The generalized absence of bulk solvents, the precise control over the stoichiometry (i.e., using agents in a stoichiometrically rather than in excess), and the more selective reactions enabling simplified work-up procedures are the distinctive factors, marking the superiority of mechanochemical processes over solution-based chemistry.

A simple and efficient process for the synthesis of 2D carbon nitrides and related materials

We describe here a new process for the synthesis of very high quality 2D Covalent Organic Frameworks (COFs), such a C2N and CN carbon nitrides. This process relies on the use of a metallic surface as both a reagent and a support for the coupling of small halogenated building blocks. The conditions of the assembly reaction are chosen so as to leave the inorganic salts by-products on the surface, to further confine the assembly reaction on the surface and increase the quality of the 2D layers. We found that under these conditions, the process directly returns few layers material. The structure/quality of these materials is demonstrated by extensive cross-characterizations at different scales, combining optical microscopy, Scanning Electron Microscopy (SEM)/Transmission Electron Microscopy (TEM) and Energy Dispersive Spectroscopy (EDS). The availability of such very large, high-quality layers of these materials opens interesting perspectives, for example in photochemistry and electronics (intrinsic transport properties, high gap substrate for graphene, etc...).

Heteroatom-doped noble carbon-tailored mixed matrix membranes with ultrapermeability for efficient CO2 separation

Membranes with ultrapermeability for CO2 are desired for future large-scale carbon capture projects, because of their excellent separative productivity and economic efficiency. Herein, we demonstrate that a membrane with ultrapermeability for CO2 can be constructed by combining N/O para-doped noble carbons, C2NxO1-x, with high-permeability polymer PIM-1. The optimal PIM-1/C2NxO1-x membranes exhibit superior CO2 permeability (22110 Barrer) with a CO2/N2 selectivity of 15.5, and an unprecedented CO2 permeability of 37272 Barrer can be obtained after a PEG activation treatment, far surpassing the 2008 upper bound. Both broad experiments and molecular dynamics simulations reveal that the numerous ordered polar channels of C2NxO1-x and their excellent compatibility with PIM-1 are responsible for the superior CO2 separation performance of the membrane. Although this is the first study on C2N-type gas separation membranes, the outstanding results indicate that noble carbon building blocks may pave a new avenue to advance high-performance CO2 separation membranes.

pH-Regulated Refinement of Pore Size in Carbon Spheres for Size-Sieving of Gaseous C8 , C6 and C3 Hydrocarbon Pairs

Selective separation of industrial important C8 , C6 and C3 hydrocarbon pairs by physisorbents can greatly reduce the energy intensity related to the currently used cryogenic distillation techniques. The achievement of size-sieving based on carbonaceous materials is desirable, but commonly hindered by the random structure of carbons often with a broad pore size distribution. Herein, a pH-regulated pre-condensation strategy was introduced to control the carbon pore architecture by the sp2 /sp3 hybridization of precursor. The lower pH value during pre-condensation of glucose facilitates the growth of aromatic nanodomains, rearrangement of stacked layers and a concomitant transition from sp3 -C to sp2 -C. The subsequent pyrolysis endows the pore size manipulated from 6.8 to 4.8 Å and narrowly distributed over a range of 0.2 Å. The refined pores enable effective size-sieving of C8 , C6 and C3 hydrocarbon pairs with high separation factor of 1.9 and 4.9 for C8 xylene (X) isomers para-X/meta-X and para-X/ortho-X, respectively, 5.1 for C6 alkane isomers n-hexane/3-methylpentane, and 22.0 for C3 H6 /C3 H8 . The excellent separation performance based-on size exclusion effect is validated by static adsorption isotherms and dynamic breakthrough experiments. This synthesis strategy provides a means of exploring advanced carbonaceous materials with controlled hybridized structure and pore sizes for challenging separation needs.

Hexaazatrinaphthalene-Based Covalent Triazine Framework-Supported Rhodium(III) Complex: A Recyclable Heterogeneous Catalyst for the Reductive Amination of Ketones to Primary Amines

The development of efficient and recyclable heterogeneous catalysts is an important topic. Herein, a rhodium(III) complex Cp*Rh@HATN-CTF was synthesized by the coordinative immobilization of [Cp*RhCl₂]₂ on a hexaazatrinaphthalene-based covalent triazine framework. In the presence of Cp*Rh@HATN-CTF (1 mo l% Rh), a series of primary amines could be obtained via the reductive amination of ketones in high yields. Moreover, catalytic activity of Cp*Rh@HATN-CTF is well maintained during six runs. The present catalytic system was also applied for the large scale preparation of a biologically active compound. It would facilitate the development of CTF-supported transition metal catalysts for sustainable chemistry.

Probing sub-5 Ångstrom micropores in carbon for precise light olefin/paraffin separation

Olefin/paraffin separation is an important but challenging and energy-intensive process in petrochemical industry. The realization of carbons with size-exclusion capability is highly desirable but rarely reported. Herein, we report polydopamine-derived carbons (PDA-Cx, where x refers to the pyrolysis temperature) with tailorable sub-5 Å micropore orifices together with larger microvoids by one-step pyrolysis. The sub-5 Å micropore orifices centered at 4.1-4.3 Å in PDA-C800 and 3.7-4.0 Å in PDA-C900 allow the entry of olefins while entirely excluding their paraffin counterparts, performing a precise cut-off to discriminate olefin/paraffin with sub-angstrom discrepancy. The larger voids enable high C2H4 and C3H6 capacities of 2.25 and 1.98 mmol g-1 under ambient conditions, respectively. Breakthrough experiments confirm that a one-step adsorption-desorption process can obtain high-purity olefins. Inelastic neutron scattering further reveals the host-guest interaction of adsorbed C2H4 and C3H6 molecules in PDA-Cx. This study opens an avenue to exploit the sub-5 Å micropores in carbon and their desirable size-exclusion effect.

Theoretical exploration of the structural, electronic and optical properties of g-C3N4/C3N heterostructures

Integration of graphene-like carbon nitride materials is essential for nanoelectronic applications. Using density-functional theory (DFT), we systematically investigate the structural, electronic and optical properties of a s-triazine-based g-C₃N₄/C₃N heterostructure under different modified conditions. The g-C₃N₄/C₃N van der Waals heterostructure (vdWH) formed has an indirect bandgap with type-II band alignment and the band structures can be tuned from type-II band alignment to type-I band alignment by applying biaxial strains and external electric fields (E_field). Compared to single transition metal (TM) atoms at g-C₃N₄/C₃N surfaces, the TM atoms anchored in the interlayer region exhibit more stability, and the corresponding bandgaps are changed from 0.19 eV to 0.61 eV. In addition, the g-C₃N₄/C₃N heterostructure has a strong absorption coefficient in the ultraviolet-visible light region along the x direction. It is found that compressive strain has a large influence on the absorption coefficient of the g-C₃N₄/C₃N system. With the increased compressive strain, the absorption spectra in the visible light region disappeared. Tensile strain has a slight effect on the absorption range, but causes a red shift of the absorption spectrum. In comparison, the light absorption coefficient of the g-C₃N₄/C₃N system remains almost unchanged under the E_field conditions. In summary, the formation of a s-triazine-based g-C₃N₄/C₃N heterostructure has shown potential for applications in nanoelectronic and optoelectronic devices.

Application of Thermal Response Measurements to Investigate Enhanced Water Adsorption Kinetics in Ball-Milled C2 N-Type Materials

Sorption-based water capture is an attractive solution to provide potable water in arid regions. Heteroatom-decorated microporous carbons with hydrophilic character are promising candidates for water adsorption at low humidity, but the strong affinity between the polar carbon pore walls and water molecules can hinder the water transport within the narrow pore system. To reduce the limitations of mass transfer, C2 N-type carbon materials obtained from the thermal condensation of a molecular hexaazatriphenylene-hexacarbonitrile (HAT-CN) precursor were treated mechanochemically via ball milling. Scanning electron microscopy as well as static light scattering reveal that large pristine C2 N-type particles were split up to a smaller size after ball milling, thus increasing the pore accessibility which consequently leads to faster occupation of the water vapor adsorption sites. The major aim of this work is to demonstrate the applicability of thermal response measurements to track these enhanced kinetics of water adsorption. The adsorption rate constant of a C2 N material condensed at 700 °C remarkably increased from 0.026 s-1 to 0.036 s-1 upon ball milling, while maintaining remarkably high water vapor capacity. This work confirms the advantages of small particle sizes in ultramicroporous materials on their vapor adsorption kinetics. It is demonstrated that thermal response measurements are a valuable and time-saving method to investigate water adsorption kinetics, capacities, and cycling stability.

Electrochemical Generation of Catalytically Active Edge Sites in C2 N-Type Carbon Materials for Artificial Nitrogen Fixation

The electrochemical nitrogen reduction reaction (NRR) to ammonia (NH₃ ) is a potentially carbon-neutral and decentralized supplement to the established Haber-Bosch process. Catalytic activation of the highly stable dinitrogen molecules remains a great challenge. Especially metal-free nitrogen-doped carbon catalysts do not often reach the desired selectivity and ammonia production rates due to their low concentration of NRR active sites and possible instability of heteroatoms under electrochemical potential, which can even contribute to false positive results. In this context, the electrochemical activation of nitrogen-doped carbon electrocatalysts is an attractive, but not yet established method to create NRR catalytic sites. Herein, a metal-free C₂ N material (HAT-700) is electrochemically etched prior to application in NRR to form active edge-sites originating from the removal of terminal nitrile groups. Resulting activated metal-free HAT-700-A shows remarkable catalytic activity in electrochemical nitrogen fixation with a maximum Faradaic efficiency of 11.4% and NH₃ yield of 5.86 µg mg^-1 _cat h^-1 . Experimental results and theoretical calculations are combined, and it is proposed that carbon radicals formed during activation together with adjacent pyridinic nitrogen atoms play a crucial role in nitrogen adsorption and activation. The results demonstrate the possibility to create catalytically active sites on purpose by etching labile functional groups prior to NRR.

Red edge effect and chromoselective photocatalysis with amorphous covalent triazine-based frameworks

Chromoselective photocatalysis offers an intriguing opportunity to enable a specific reaction pathway out of a potentially possible multiplicity for a given substrate by using a sensitizer that converts the energy of incident photon into the redox potential of the corresponding magnitude. Several sensitizers possessing different discrete redox potentials (high/low) upon excitation with photons of specific wavelength (short/long) have been reported. Herein, we report design of molecular structures of two-dimensional amorphous covalent triazine-based frameworks (CTFs) possessing intraband states close to the valence band with strong red edge effect (REE). REE enables generation of a continuum of excited sites characterized by their own redox potentials, with the magnitude proportional to the wavelength of incident photons. Separation of charge carriers in such materials depends strongly on the wavelength of incident light and is the primary parameter that defines efficacy of the materials in photocatalytic bromination of electron rich aromatic compounds. In dual Ni-photocatalysis, excitation of electrons from the intraband states to the conduction band of the CTF with 625 nm photons enables selective formation of C‒N cross-coupling products from arylhalides and pyrrolidine, while an undesirable dehalogenation process is completely suppressed.

Graphitic Aza-Fused π-Conjugated Networks: Construction, Engineering, and Task-Specific Applications

2D π-conjugated networks linked by aza-fused units represent a pivotal category of graphitic materials with stacked nanosheet architectures. Extensive efforts have been directed at their fabrication and application since the discovery of covalent triazine frameworks (CTFs). Besides the triazine cores, tricycloquinazoline and hexaazatriphenylene linkages are further introduced to tailor the structures and properties. Diverse related materials have been developed rapidly, and a thorough outlook is necessitated to unveil the structure-property-application relationships across multiple subcategories, which is pivotal to guide the design and fabrication toward enhanced task-specific performance. Herein, the structure types and development of related materials including CTFs, covalent quinazoline networks, and hexaazatriphenylene networks, are introduced. Advanced synthetic strategies coupled with characterization techniques provide powerful tools to engineer the properties and tune the associated behaviors in corresponding applications. Case studies in the areas of gas adsorption, membrane-based separation, thermo-/electro-/photocatalysis, and energy storage are then addressed, focusing on the correlation between structure/property engineering and optimization of the corresponding performance, particularly the preferred features and strategies in each specific field. In the last section, the underlying challenges and opportunities in construction and application of this emerging and promising material category are discussed.

Nitrogen-rich layered carbon for adsorption of typical volatile organic compounds and low-temperature thermal regeneration

Carbon-based adsorbents with a high adsorption capacity and low price have been widely used in the removal of volatile organic compounds (VOCs), but the poor gas selectivity and reusability limit their industrial applications. In this work, disc-like nitrogen-rich porous carbon materials (HAT-Xs) were synthesized to remove typical VOCs via adsorption. By controlling the synthesis temperature from 450 to 1000 °C, the C/N ratio of the HAT-Xs increased from 1.85 to 12.56. The HAT-650 synthesized at 650 °C with the high specific surface area of 305 m² g^-1 exhibits the highest adsorption capacity of 141 mg g^-1 for ethyl acetate (which is 3.2 times for that of activated carbon), and 39.4 mg g^-1 for n-hexane, 48.6 mg g^-1 for toluene. Kinetic studies indicated that the adsorption is physical adsorption and that the interior surface diffusion is the main rate-determining step during the adsorption progress, the interior surface diffusion rate of ethyl acetate on HAT-650 is 1.455 mg g^-1 min^-0.5. At the same time, the desorption and reuse tests show that HAT-650 has excellent reusability with low desorption and regeneration temperature of 120 °C, and high desorption efficiency of 95.2% and that it could be a promising ethyl acetate adsorbent for industrial applications.

Rational Design of Main Group Metal-Embedded Nitrogen-Doped Carbon Materials as Frustrated Lewis Pair Catalysts for CO2 Hydrogenation to Formic Acid

Developing efficient and inexpensive main group catalysts for CO₂ conversion and utilization has attracted increasing attention, as the conversion process would be both economical and environmentally benign. Here, based on the main group element Al, we designed several heterogeneous frustrated Lewis acid/base pair (FLP) catalysts and performed extensive first-principles calculations for the hydrogenation of CO₂. These catalysts, including Al@N-Gr-1, Al@N-Gr-2, and Al@C₂N, are composed of a single Al atom and two-dimensional (2D) N-doped carbon-based materials to form frustrated Al/C or Al/N Lewis acid/base pairs, which are all predicted to have high reactivity to absorb and activate hydrogen (H₂). Compared with Al@N-Gr-1, both Al@N-Gr-2 and Al@C₂N, especially Al@N-Gr-2, containing Al/N Lewis pairs exhibit better catalytic activity for CO₂ hydrogenation with lower activation energies. CO₂ hydrogenation on the three catalysts prefers to go through a three-step mechanism, i.e., the heterolytic dissociation of H₂, followed by the transfer of the hydride near Al to CO₂, and finally the activation of a second H₂ molecule. Other IIIA group element (B and Ga)-embedded N-Gr-2 materials (B@N-Gr-2 and Ga@N-Gr-2) were also explored and compared. Both Al@N-Gr-2 and Ga@N-Gr-2 show higher catalytic activity for CO₂ hydrogenation to HCOOH than B@N-Gr-2. However, the CO₂ hydrogenation path on Ga@N-Gr-2 tends to follow a two-step mechanism, including H₂ dissociation and subsequent hydrogen transfer. The present study provides a potential solution for CO₂ hydrogenation by designing novel and effective FLP catalysts based on main group elements.

Water on porous, nitrogen-containing layered carbon materials: The performance of computational model chemistries

Porous, layered materials containing sp2-hybridized carbon and nitrogen atoms, offer through their tunable properties, a versatile route towards tailormade catalysts for electrochemistry and photochemistry. A key molecule interacting with these...

Influence of Pore Architecture and Chemical Structure on the Sodium Storage in Nitrogen-Doped Hard Carbons

Hard carbon is the material of choice for sodium ion battery anodes. Capacities comparable to those of lithium/graphite can be reached, but the understanding of the underlying sodium storage mechanisms remains fragmentary. A two-step process is commonly observed, where sodium first adsorbs to polar sites of the carbon ("sloping region") and subsequently fills small voids in the material ("plateau region"). To study the impact of nitrogen functionalities and pore geometry on sodium storage, a systematic series of nitrogen-doped hard carbons is synthesized. The nitrogen content is found to contribute to sloping capacity by binding sodium ions at edges and defects, whereas higher plateau capacities are found for materials with less nitrogen content and more extensive graphene layers, suggesting the formation of 2D sodium structures stabilized by graphene-like pore walls. In fact, up to 84% of the plateau capacity is measured at potentials less than 0 V versus metallic Na, that is, quasimetallic sodium can be stabilized in such structure motifs. Finally, gas physisorption measurements are related to charge-discharge data to identify the energy storage relevant pore architectures. Interestingly, these are pores inaccessible to probe gases and electrolytes, suggesting a new view on such "closed pores" required for efficient sodium storage.

Solution-processed two-dimensional materials for next-generation photovoltaics

In the ever-increasing energy demand scenario, the development of novel photovoltaic (PV) technologies is considered to be one of the key solutions to fulfil the energy request. In this context, graphene and related two-dimensional (2D) materials (GRMs), including nonlayered 2D materials and 2D perovskites, as well as their hybrid systems, are emerging as promising candidates to drive innovation in PV technologies. The mechanical, thermal, and optoelectronic properties of GRMs can be exploited in different active components of solar cells to design next-generation devices. These components include front (transparent) and back conductive electrodes, charge transporting layers, and interconnecting/recombination layers, as well as photoactive layers. The production and processing of GRMs in the liquid phase, coupled with the ability to "on-demand" tune their optoelectronic properties exploiting wet-chemical functionalization, enable their effective integration in advanced PV devices through scalable, reliable, and inexpensive printing/coating processes. Herein, we review the progresses in the use of solution-processed 2D materials in organic solar cells, dye-sensitized solar cells, perovskite solar cells, quantum dot solar cells, and organic-inorganic hybrid solar cells, as well as in tandem systems. We first provide a brief introduction on the properties of 2D materials and their production methods by solution-processing routes. Then, we discuss the functionality of 2D materials for electrodes, photoactive layer components/additives, charge transporting layers, and interconnecting layers through figures of merit, which allow the performance of solar cells to be determined and compared with the state-of-the-art values. We finally outline the roadmap for the further exploitation of solution-processed 2D materials to boost the performance of PV devices.

Synthesis, Postsynthetic Modifications, and Applications of the First Quinoxaline-Based Covalent Organic Framework

We report a new synthetic protocol for preparing highly ordered two-dimensional nanoporous covalent organic frameworks (2D-COFs) based on a quinoxaline backbone. The quinoxaline framework represents a new type of COF that enables postsynthetic modification by placing two different chemical functionalities within the nanopores including layer-to-layer cross-linking. We also demonstrate that membranes fabricated using this new 2D-COF perform highly selective separations resulting in dramatic performance enhancement post cross-linking.

Two-Dimensional π-Conjugated Frameworks as a Model System to Unveil a Multielectron-Transfer-Based Energy Storage Mechanism

ConspectusAlthough electrochemical energy storage is commonplace in our society, further advancements in this technology are indispensable for the transition to a low-carbon society. Recent intensive research has expanded concepts in this field; however, finding one suitable material to obtain a high energy density accomplishing the criteria of next-generation batteries is still a conundrum. To solve this issue, material investigations based on big data combined with artificial intelligence are a present trend. On the contrary, this Account focuses on an alternative approach, i.e., fundamental research to shed light on key basic principles to design new electrode materials and new principles achieving multielectron transfer, which is a key to improve a specific capacity. In addition to the cation-redox mechanism, materials showing the multielectron-transfer mechanism based on cation-/anion-redox can enrich material choices with high theoretical energy densities. The challenge in this mechanism is that a rational design of electrode materials based on microscopic understanding of underlying electrode processes has not been fully achieved so far. This is a key bottleneck in machine-learning approaches as well because the reliability of outputs from an algorithm is dependent on the reliability of data from a corresponding microscopic electrode process. Therefore, uncovering fundamental mechanisms in electrochemical energy storage remains one of the primary goals for the present research. In our series of investigations, we developed concepts for replacing complex practical electrode materials, such as polyanion or Li-rich layered oxides, by simplified model systems based on two-dimensional (2D) π-conjugated frameworks, which are based on purely organic aromatic systems and metal-containing coordination polymers. These materials are relatively simple, but it is still possible to control their complexity of systems in order to mimic certain aspects of structure-property relations in practical electrode materials. In particular, recent studies have shown that we can tune electronic structures of 2D π-conjugated frameworks, which is a key feature to investigate electron-transfer mechanisms, along with the concept of the threefold correlation approach, i.e., the relations in chemical structures, electronic structures, and electrochemical reactions. In this Account, several model studies focusing on microscopic understandings of structure-electrochemical energy storage functions are presented in which we investigate how the structural periodicity and nature of the coordination environment affect their electronic properties and the electrochemical reactions. In particular, we investigate the effects of combinations of linkers and metal ions toward the mechanism of the electrochemical energy storage reaction. We identified few major factors determining the energy storage mechanism of 2D π-conjugated frameworks. Local configurations of coordinate covalent bonding and organic linkers interact with each other, and these effects provide unique electronic states. These electronic states are projections of intriguing electrochemical features in this materials system, such as cation/anion co-redox mechanism, anion-insertion mechanism, or inductive effect. This Account indicates that 2D π-conjugated frameworks can be applied as models to extract fundamental/microscopic principles in the complicated electrode processes, which is linked to practical electrode materials, such as oxides. Therefore, the approach shown here is a powerful tool to unveil microscopic electrochemical energy storage mechanisms, which is indispensable to advance clean energy technology and accelerate decarbonization.

"Giant" Nitrogen Uptake in Ionic Liquids Confined in Carbon Pores

Ionic liquids are well known for their high gas absorption capacity. It is shown that this is not a solvent constant, but can be enhanced by another factor of 10 by pore confinement, here of the ionic liquid (IL) 1-ethyl-3-methylimidazolium acetate (EmimOAc) in the pores of carbon materials. A matrix of four different carbon compounds with micro- and mesopores as well as with and without nitrogen doping is utilized to investigate the influence of the carbons structure on the nitrogen uptake in the pore-confined EmimOAc. In general, the absorption is most improved for IL in micropores and in nitrogen-doped carbon. This effect is so large that it is already seen in TGA and DSC experiments. Due to the low vapor pressure of the IL, standard volumetric sorption experiments can be used to quantify details of this effect. It is reasoned that it is the change of the molecular arrangement of the ions in the restricted space of the pores that creates additional free volume to host molecular nitrogen.

Ultrahigh water sorption on highly nitrogen doped carbonaceous materials derived from uric acid

HYPOTHESIS

Developing materials for thermally driven adsorption chillers and adsorption heat pumps is a growing research field due to the potential of these technologies to address up to 50% of the world's total energy demand. These materials must be abundant, easy to synthesize, hydrophilic, and low in cost. Bare carbon materials are hydrophobic and therefore usually not considered for these applications. However, by introducing heteroatoms and tuning their porosity, the hydrophilicity of carbonaceous networks can be increased significantly.

EXPERIMENTAL

Herein, a series of highly nitrogen doped carbonaceous materials (CNs) have been synthesized by submitting uric acid to heat treatment at different temperatures in the presence of an inorganic salt mix as solvent and pore template. The effect of the thermal treatment on the materials composition, pore network, and water sorption capability has been studied.

FINDINGS

At 800 °C, a nitrogen depleted carbonaceous material with a maximal water uptake of 1.38g_H2O g^-1 is obtained. Condensation at 750 °C creates an ultra-hydrophilic CN with a water uptake of 0.8 g_H2O g^-1 at already much lower partial pressures. While the maximum uptake is mainly ascribed to the mesopore volume of the material, the differences in hydrophilicity can be controlled by functionality.

Catalyst- and Solvent-Free Synthesis of a Chemically Stable Aza-Bridged Bis(phenanthroline) Macrocycle-Linked Covalent Organic Framework

Developing new linkage-based covalent organic frameworks (COFs) is one of the major topics in reticular chemistry. Electrically conductive COFs have enabled applications in energy storage and electrochemical catalysis, which are not feasible using insulating COFs. Despite significant advances, the construction of chemically stable conductive COFs by the formation of new linkages remains relatively unexplored and challenging. Here we report the solvent- and catalyst-free synthesis of a two-dimensional aza-bridged bis(phenanthroline) macrocycle-linked COF (ABBPM-COF) from the thermally induced poly-condensation of a tri-topic monomer and ammonia gas. The ABBPM-COF structure was elucidated using multiple techniques, including X-ray diffraction analysis combined with structural simulation, revealing its crystalline nature with an ABC stacking mode. Further experiments demonstrated its excellent chemical stability in acid/base solutions. Electrical-conductivity measurements showed that the insulating ABBPM-COF becomes a semiconducting material after exposure to iodine vapor.

Toward Efficient Synthesis of Porous All-Carbon-Based Nanocomposites for Enantiospecific Separation

Chiral separation and asymmetric synthesis and catalysis are crucial processes for obtaining enantiopure compounds, which are especially important in the pharmaceutical industry. The efficiency of the separation processes is readily increased by using porous materials as the active material can interact with a larger surface area. Silica, metal-organic frameworks, or chiral polymers are versatile porous materials that are established in chiral applications, but their instability under certain conditions in some cases requires the use of more stable porous materials such as carbons. In addition to their stability, porous carbon materials can be tailored for their ability to adsorb and catalytically activate different chemical compounds from the liquid and the gas phase. The difficulties imposed by the functionalization of carbons with chiral species were tackled in the past by carbonizing chiral ionic liquids (CILs) together with a template to create pores, which results in the entire body of a material that is built up from the precursor. To increase the atomic efficiency of ionic liquids for better economic utilization of CILs, the approach presented here is based on the formation of a composite between CIL-derived chiral carbon and a pristine carbon material obtained from carbohydrate precursors. Two novel enantioselective carbon composite materials are applied for the chiral recognition of molecules in the gas phase, as well as in solution. The enantiomeric ratio of the l-composite for phenylalanine from the solution was (L/D) = 8.4, and for 2-butanol from the gas phase, it was (S/R) = 1.3. The d-composite showed an opposite behavior, where the enantiomeric ratio for phenylalanine was (D/L) = 2.7, and for 2-butanol from the gas phase, it was (R/S) = 1.3.

The Functional Chameleon of Materials Chemistry-Combining Carbon Structures into All-Carbon Hybrid Nanomaterials with Intrinsic Porosity to Overcome the "Functionality-Conductivity-Dilemma" in Electrochemical Energy Storage and Electrocatalysis

Nanoporous carbon materials can cover a remarkably wide range of physicochemical properties. They are widely applied in electrochemical energy storage and electrocatalysis. As a matter of fact, all these applications combine a chemical process at the electrode-electrolyte interface with the transport (and possibly the transfer) of electrons. This leads to multiple requirements which can hardly be fulfilled by one and the same material. This "functionality-conductivity-dilemma" can be minimized when multiple carbon-based compounds are combined into porous all-carbon hybrid nanomaterials. This article is giving a broad and general perspective on this approach from the viewpoint of materials chemists. The problem and existing solutions are first summarized. This is followed by an overview of the most important design principles for such porous materials, a chapter discussing recent examples from different fields where the formation of comparable structures has already been successfully applied, and an outlook over the future development of this field that is foreseen.

Thermodynamically stable polymorphs of nitrogen-rich carbon nitrides: a C3N5 study

We have carried out an extensive search for stable polymorphs of carbon nitride with C₃N₅ stoichiometry using the minima hopping method. Contrary to the widely held opinion that stacked, planar, graphite-like structures are energetically the most stable carbon nitride polymorphs for various nitrogen contents, we find that this does not apply for nitrogen-rich materials owing to the high abundance of N-N bonds. In fact, our results disclose novel morphologies with moieties not previously considered for C₃N₅. We demonstrate that nitrogen-rich compounds crystallize in a large variety of different structures due to particular characteristics of their energy landscapes. The newly found low-energy structures of C₃N₅ have band gaps within good agreement with the values measured in experimental studies.

Insights into the sodiation mechanism of hard carbon-like materials from electrochemical impedance spectroscopy

To render the sodium ion battery (SIB) competitive among other technologies, the processes behind sodium storage in hard carbon anodes must be understood. For this purpose, electrochemical impedance spectroscopy (EIS) is usually undervalued, since fitting the spectra with equivalent circuit models requires an a priori knowledge about the system at hand. The analysis of the distribution of relaxation times (DRT) is an alternative, which refrains from fitting arbitrarily nested equivalent circuits. In this paper, the sodiation and desodiation of a hard carbon anode is studied by EIS at different states of charge (SOC). By reconstructing the DRT function, highly resolved information on the number and relative contribution of individual electrochemical processes is derived. During the sloping part of the sodiation curve, mass transport is found to be the most dominant source of resistance but rapidly diminishes when the plateau phase is reached. An equivalent circuit model qualitatively reproducing the experimental data of the sloping region was built upon the DRT results, which is particularly useful for future EIS studies on hard carbon SIB anodes. More importantly, this work contributes to establish EIS as a practical tool to directly study electrode processes without the bias of a previously assumed model.

C2N: A Class of Covalent Frameworks with Unique Properties

C2N is a unique member of the CnNm family (carbon nitrides), i.e., having a covalent structure that is ideally composed of carbon and nitrogen with only 33 mol% of nitrogen. C2N, with a stable composition, can easily be prepared using a number of precursors. Moreover, it is currently gaining extensive interest owing to its high polarity and good thermal and chemical stability, complementing carbon as well as classical carbon nitride (C3N4) in various applications, such as catalysis, environmental science, energy storage, and biotechnology. In this review, a comprehensive overview on C2N is provided; starting with its preparation methods, followed by a fundamental understanding of structure-property relationships, and finally introducing its application in gas sorption and separation technologies, as supercapacitor and battery electrodes, and in catalytic and biological processes. The review with an outlook on current research questions and future possibilities and extensions based on these material concepts is ended.

Boron Carbon Nitride Thin Films: From Disordered to Ordered Conjugated Ternary Materials

We present an innovative method for the synthesis of boron carbon nitride thin film materials in a simple furnace setup, using commonly available solid precursors and relatively low temperature compared to previous attempts. The as-prepared structural and optical properties of thin films are tuned via the precursor content, leading to a sp²-conjugated boron nitride–carbon nitride mixed material, instead of the commonly reported boron nitride–graphene phase segregation, with tunable optical properties such as band gap and fluorescence.

Electrochemical N2 Reduction to Ammonia Using Single Au/Fe Atoms Supported on Nitrogen-Doped Porous Carbon

The electrochemical nitrogen reduction reaction (NRR) to ammonia (NH₃) is a promising alternative route for an NH₃ synthesis at ambient conditions to the conventional high temperature and pressure Haber-Bosch process without the need for hydrogen gas. Single metal ions or atoms are attractive candidates for the catalytic activation of non-reactive nitrogen (N₂), and for future targeted improvement of NRR catalysts, it is of utmost importance to get detailed insights into structure-performance relationships and mechanisms of N₂ activation in such structures. Here, we report density functional theory studies on the NRR catalyzed by single Au and Fe atoms supported in graphitic C₂N materials. Our results show that the metal atoms present in the structure of C₂N are the reactive sites, which catalyze the aforesaid reaction by strong adsorption and activation of N₂. We further demonstrate that a lower onset electrode potential is required for Fe-C₂N than for Au-C₂N. Thus, Fe-C₂N is theoretically predicted to be a potentially better NRR catalyst at ambient conditions than Au-C₂N owing to the larger adsorption energy of N₂ molecules. Furthermore, we have experimentally shown that single sites of Au and Fe supported on nitrogen-doped porous carbon are indeed active NRR catalysts. However, in contrast to our theoretical results, the Au-based catalyst performed slightly better with a Faradaic efficiency (FE) of 10.1% than the Fe-based catalyst with an FE of 8.4% at -0.2 V vs. RHE. The DFT calculations suggest that this difference is due to the competitive hydrogen evolution reaction and higher desorption energy of ammonia.

Synthesis of a Porous C3N-Derived Framework with High Yield by Gallic Acid Cross-Linking Using Salt Melts

Porous carbon/nitrogen frameworks are an emerging class of noble organic materials with a wide range of potential applications. However, the design and controlled synthesis of those materials are still a challenge. Herein, we present the rational design of such a system with high microporosity, specific surface areas of up to 946 m² g^-1, and excellent condensation yields. The obtained noble frameworks were used for the delivery of larger organic molecules and changed the melting behavior of some daily drug molecules along their highly polarizable surfaces.

Micro-Blooming: Hierarchically Porous Nitrogen-Doped Carbon Flowers Derived from Metal-Organic Mesocrystals

Synthesis of 3D flower-like zinc-nitrilotriacetic acid (ZnNTA) mesocrystals and their conformal transformation to hierarchically porous N-doped carbon superstructures is reported. During the solvothermal reaction, 2D nanosheet primary building blocks undergo oriented attachment and mesoscale assembly forming stacked layers. The secondary nucleation and growth preferentially occurs at the edges and defects of the layers, leading to formation of 3D flower-like mesocrystals comprised of interconnected 2D micropetals. By simply varying the pyrolysis temperature (550-1000 °C) and the removal method of in the situ-generated Zn species, nonporous parent mesocrystals are transformed to hierarchically porous carbon flowers with controllable surface area (970-1605 m² g^-1 ), nitrogen content (3.4-14.1 at%), pore volume (0.95-2.19 cm³ g^-1 ), as well as pore diameter and structures. The carbon flowers prepared at 550 °C show high CO₂ /N₂ selectivity due to the high nitrogen content and the large fraction of (ultra)micropores, which can greatly increase the CO₂ affinity. The results show that the physicochemical properties of carbons are highly dependent on the thermal transformation and associated pore formation process, rather than directly inherited from parent precursors. The present strategy demonstrates metal-organic mesocrystals as a facile and versatile means toward 3D hierarchical carbon superstructures that are attractive for a number of potential applications.

Metal-Organic-Framework-Derived 2D Carbon Nanosheets for Localized Multiple Bacterial Eradication and Augmented Anti-infective Therapy

Recently emerging graphene-based 2D nanoplatforms with multiple therapeutic modalities provide enormous opportunities to combat pathogenic bacterial infections. However, because these materials suffer from complicated synthesis, massive dosage requirements, and abundant nonlocalized heat, much more simplified, tunable, and localized eradication approaches are urgently required. Herein, we report on the fabrication of the metal-organic-framework (MOF)-derived 2D carbon nanosheets (2D-CNs) with phase-to-size transformation and localized bacterial eradication capabilities for augmented anti-infective therapy. The MOF-derived, ZnO-doped carbon on graphene (ZnO@G) is first synthesized and then anchored with phase transformable thermally responsive brushes (TRB) by in situ polymerization to yield the TRB-ZnO@G. The TRB-ZnO@G exhibits flexible 2D nanostructures, high photothermal activities, sustained Zn²⁺ ions release, and ON-OFF switchable phase-to-size transformation abilities. Notably, the near-infrared-triggered formation of TRB-ZnO@G-bacteria aggregations enables localized massive Zn²⁺ ions penetration, physical cutting, and hyperthermia killing, which synergistically enhance the disruption of bacterial membranes and intracellular substances. The obtained novel 2D-CNs not only present robust and localized multiple bacterial eradication capabilities with nearly 100% bactericidal efficiency at low concentrations but also possess rapid and safe skin wound disinfection via a short-time photothermal treatment without damaging normal skin tissues or causing accumulative toxicities, thus presenting great potential for broad-spectrum eradication of pathogenic bacteria.

Distinct correlation between (CN2)x units and pores: a low-cost method for predesigned wide range control of micropore size of porous carbon

Flexible control of the micropore size of carbon was achieved in a cost-efficient way by predesigned quantitative correlation between the sizes of extendable (CN₂)_x nanosized modulators and the generated micropore sizes ranged from 0.2 to 2.3 nm, with effectiveness for boosting the electrochemical performance systematically.

Carbon nitride photocatalyzes regioselective aminium radical addition to the carbonyl bond and yields N-fused pyrroles

Addition of N-centered radicals to C=C bonds or insertion into C-H bonds is well represented in the literature. These reactions have a tremendous significance, because they afford polyfunctionalized organic molecules. Despite the tetrahydroisoquinoline (THIQ) moiety widely occurring in natural biologically active compounds, N-unsubstituted THIQs as a source of N-centered radicals are not studied. Herein, we report a photocatalytic reaction between tetrahydroisoquinoline and chalcones that gives N-fused pyrroles-1,3-disubstituted-5,6-dihydropyrrolo[2,1-a]isoquinolines (DHPIQ). The mechanism includes at least two photocatalytic events in one pot: (1) C-N bond formation; (2) C-C bond formation. In this process potassium poly(heptazine imide) is used as a visible light active heterogeneous and recyclable photocatalyst. Fifteen N-fused pyrroles are reported with 65-90% isolated yield. DHPIQs are characterized by UV-vis and fluorescence spectroscopy, while the fluorescence quantum efficiency of fluorinated DHPIQs reaches 24%.

Crucial Factors for the Application of Functional Nanoporous Carbon-Based Materials in Energy and Environmental Applications

This special issue of C—Journal of Carbon Research is dedicated to “Functional Nanoporous Carbon-Based Materials”. It contains contributions reporting on the synthesis of nanoporous carbons for the adsorption of proteins, their applications in electrochemical energy storage/conversion, and on the characterization/modification of their surface chemistry. Nanoporous carbon-based materials are widely researched, but at the same time, the field is still full of unutilized potential. The atomic construction of the carbon framework, pore sizes, pore geometries, presence of heteroatoms, particle size and shape, and many other “internal screws” are available; in the end, the high potential of carbon-based materials will only be fully explored if the interplay of these crucial factors is precisely controlled. This article is a summary of what we consider important for future targeted improvement of porous carbon nanomaterials for energy and environmental applications.