Well-defined N3 C1 -anchored Single-Metal-Sites for Oxygen Reduction Reaction.

N-, C-, O-, S-coordinated single-metal-sites (SMSs) have garnered significant attention due to the potential for significantly enhanced catalytic capabilities resulting from charge redistribution. However, significant challenges persist in the precise design of well-defined such SMSs, and the fundamental comprehension has long been impeded in case-by-case reports using carbon materials as investigation targets. In this work, the well-defined molecular catalysts with N3 C1 -anchored SMSs, i.e., N-confused metalloporphyrins (NCPor-Ms), are calculated for their catalytic oxygen reduction activity. Then, NCPor-Ms with corresponding N4 -anchored SMSs (metalloporphyrins, Por-Ms), are synthesized for catalytic activity evaluation. Among all, NCPor-Co reaches the top in established volcano plots. NCPor-Co also shows the highest half-wave potential of 0.83 V vs. RHE, which is much better than that of Por-Co (0.77 V vs. RHE). Electron-rich, low band gap and regulated d-band center contribute to the high activity of NCPor-Co. This study delves into the examination of well-defined asymmetric SMS molecular catalysts, encompassing both theoretical and experimental facets. It serves as a pioneering step towards enhancing the fundamental comprehension and facilitating the development of high-performance asymmetric SMS catalysts.

Isomeric Dual-Pore Two-Dimensional Covalent Organic Frameworks.

Two-dimensional (2D) covalent organic frameworks (COFs) with hierarchical porosity have been increasingly recognized as promising materials in various fields. Besides, the 2D COFs with kagome (kgm) topology can exhibit unique optoelectronic features and have extensive applications. However, rational synthesis of the COFs with kgm topology remains challenging because of competition with a square-lattice topology. Herein, we report two isomeric dual-pore 2D COFs with kgm topology using a novel geometric strategy to reduce the symmetry of their building blocks, which are four-armed naphthalene-based and azulene-based isomeric monomers. Owing to the large dipole moment of azulene, as-prepared azulene-based COF (COF-Az) possesses a considerably narrow band gap of down to 1.37 eV, which is much narrower than the naphthalene-based 2D COF (COF-Nap: 2.28 eV) and is the lowest band gap among reported imine-linked dual-pore 2D COFs. Moreover, COF-Az was used as electrode material in a gas sensor and exhibits high selectivity for NO2, including a high response rate (58.7%) to NO2 (10 ppm), fast recovery (72 s), up to 10 weeks of stability, and resistance to 80% relative humidity, which are superior to those of reported COF-based NO2 gas sensors. The calculation and in situ experimental results indicate that the large dipole moment of azulene boosts the sensitivity of the imine linkages. The usage of isomeric building blocks not only enables the synthesis of 2D COFs with isometric kgm topology but also provides an azulene-based 2D platform for studying the structure-property correlations of COFs.

Double-single-atom MoCu-embedded porous carbons boost the electrocatalytic N2 reduction reaction.

NH3 is an essential ingredient of chemical, fertilizer, and energy storage products. Industrial nitrogen fixation consumes an enormous amount of energy, which is counter to the concept of carbon neutrality, hence eNRR ought to be implemented as a clean alternative. Herein, we propose a double-single-atom MoCu-embedded porous carbon material derived from uio-66 (MoCu@C) by plasma-enhanced chemical vapor deposition (PECVD) to boost eNRR capabilities, with an NH3 yield rate of 52.4 µg h-1 gcat.-1 and a faradaic efficiency (FE) of 27.4%. Advanced XANES shows that the Mo active site receives electrons from Cu, modifies the electronic structure of the Mo active site and enhances N2 adsorption activation. The invention of rational MoCu double-single-atom materials and the utilization of effective eNRR approaches furnish the necessary building blocks for the fundamental study and practical application of Mo-based materials.

Single-Atom Anchored Curved Carbon Surface for Efficient CO2 Electro-Reduction with Nearly 100% CO Selectivity and Industrially-Relevant Current Density.

Although single metal atoms on porous carbons (PCs) are widely used in electrochemical CO2 reduction reaction, these systems have long relied on flat graphene-based models, which are far beyond reality because of abundant curved structures in PCs; the effect of curved surfaces has long been ignored. In addition, the selectivity generally decreases under high current density, which severely limits practical application. Herein, theoretical calculations reveal that a single-Ni-atom on a curved surface can simultaneously enhance the total density of states around Fermi level and decrease the energy barrier for *COOH formation, thereby enhancing catalytic activity. This work reports a rational molten salt approach for preparing PCs with ultra-high specific surface area of up to 2635 m2 g-1 . As determined by cutting-edge techniques, a single Ni atom on a curved carbon surface is obtained and used as a catalyst for electrochemical CO2 reduction. The CO selectivity reaches up to 99.8% under industrial-level current density of 400 mA cm-2 , outperforming state-of-the-art PC-based catalysts. This work not only offers a new method for the rational synthesis of single atom catalysts with strained geometry to host rich active sites, but also provides in-depth insights for the origin of catalytic activity of curved structure-enriched PC-based catalysts.

Organic-moiety-engineering on copper surface for carbon dioxide reduction.

Electrochemical conversion of carbon dioxide (CO2) into value-added products powered by sustainable electricity is considered as one of the most promising strategies for carbon neutrality. Among the products, hydrocarbons, especially ethylene and ethanol are the most desired species due to their wide industrial applications. Copper-based catalysts are currently the very limited option available for catalyzing the reduction of CO2 to multi-carbon products. How to enhance the selectivity and current density is the focus in both academia and industry. In recent years, some organic molecules, oligomers and polymers with well-defined structures have been applied and demonstrated to be effective on enhancing electrocatalytic activity of copper catalysts. However, the molecular/copper interaction and CO2 molecules' behavior at the hetero-interface remain unclear. In this review, we classify the different organic materials which have been applied in the field of electrochemical CO2 reduction. We focus on the regulation of local microenvironment on the copper surface by organic compounds, including surface hydrophobicity, local electric field, local pH, and coverage of intermediates etc. The relationship between local microenvironment and catalytic activity is specifically discussed. This review could provide guidance for the development of more organic/inorganic hybrid catalysts for further promoting CO2 reduction reaction.

Longitudinally Grafting of Graphene with Iron Phthalocyanine-based Porous Organic Polymer to Boost Oxygen Electroreduction.

Iron phthalocyanine-based polymers (PFePc) are attractive noble-metal-free candidates for catalyzing oxygen reduction reaction (ORR). However, the low site-exposure degree and poor electrical conductivity of bulk PFePc restricted their practical applications. Herein, laminar PFePc nanosheets covalently and longitudinally linked to graphene (3D-G-PFePc) was prepared. Such structural engineering qualifies 3D-G-PFePc with high site utilization and rapid mass transfer. Thence, 3D-G-PFePc demonstrates efficient ORR performance with a high specific activity of 69.31 µA cm-2 , a high mass activity of 81.88 A g-1 , and a high turnover frequency of 0.93 e s-1 site-1 at 0.90 V vs. reversible hydrogen electrode in O2 -saturated 0.1 M KOH, outperforming the lamellar PFePc wrapped graphene counterpart. Systematic electrochemical analyses integrating variable-frequency square wave voltammetry and in situ scanning electrochemical microscopy further underline the rapid kinetics of 3D-G-PFePc towards ORR.

Isometric Covalent Triazine Framework-Derived Porous Carbons as Metal-Free Electrocatalysts for the Oxygen Reduction Reaction.

Covalent triazine frameworks (CTFs) and their derivative N-doped carbons have attracted much attention for application in energy conversion and storage. However, previous studies have mainly focused on developing new building blocks and optimizing synthetic conditions. The use of isometric building blocks to control the porous structure and to fundamentally understand structure-property relationships have rarely been reported. In this work, two isometric building blocks are used to produce isometric CTFs with controllable pore geometries. The as-prepared CTF with nonplanar hexagonal rings demonstrates higher surface area, larger pore volume, and richer N content than the planar CTF. After pyrolysis, nonplanar porous CTF-derived N-doped carbons exhibit admirable catalytic activity for oxygen reduction in alkaline media (half-wave potential: 0.86 V; Tafel slope: 65 mV dec-1 ), owing to their larger pore volume and the abundance of pyridinic and graphitic N species. When assembled into a zinc-air battery, the as-made electrocatalysts show high capacities of up to 651 mAh g-1 and excellent durability.

CoN5 Sites Constructed by Anchoring Co Porphyrins on Vinylene-Linked Covalent Organic Frameworks for Electroreduction of Carbon Dioxide.

Developing effective electrocatalysts for CO2 reduction (CO2 RR) is of critical importance for producing carbon-neutral fuels. Covalent organic frameworks (COFs) are an ideal platform for constructing catalysts toward CO2 RR, because of their controllable skeletons and ordered pores. However, most of these COFs are synthesized from Co-porphyrins or phthalocyanines-based monomers, and the available building units and resulting catalytic centers in COFs are still limited. Herein, Co-N5 sites are first developed through anchoring Co porphyrins on an olefin-linked COF, where the Co active sites are uniformly distributed in the hexagonal networks. The strong electronic coupling between Co porphyrins and COF is disclosed by various characterizations such as X-ray absorption spectroscopy (XAS) and density functional theory calculation (DFT). Thanks to the CoN5 sites, the catalytic COF shows remarkable catalytic activity with Faraday efficiencies (FECO ) of 84.2-94.3% at applied potentials between -0.50 and -0.80 V (vs RHE), and achieves a turnover frequency of 4578 h-1 at -1.0 V. Moreover, the theoretical calculation further reveals that the CoN5 sites enable a decrease in the overpotential for the formation COOH*. This work provides a design strategy to employ COFs as scaffold for fabricating efficient CO2 electrocatalysts.

Covalent Triazine Frameworks and Porous Carbons: Perspective from an Azulene-Based Case.

Covalent triazine frameworks (CTFs) are among the most valuable frameworks owing to many fantastic properties. However, molten salt-involved preparation of CTFs at 400-600 °C causes debate on whether CTFs represent organic frameworks or carbon. Herein, new CTFs based on the 1,3-dicyanoazulene monomer (CTF-Azs) are synthesized using molten ZnCl2 at 400-600 °C. Chemical structure analysis reveals that the CTF-Az prepared at low temperature (400 °C) exhibits polymeric features, whereas those prepared at high temperatures (600 °C) exhibit typical carbon features. Even after being treated at even higher temperatures, the CTF-Azs retain their rich porosity, but the polymeric features vanish. Although structural de-conformation is a widely accepted outcome in polymer-to-carbon rearrangement processes, the study evaluates such processes in the context of CTF systems. A proof-of-concept study is performed, observing that the as-synthesized CTF-Azs exhibit promising performance as cathodes for Li- and K-ion batteries. Moreover, the as-prepared NPCs exhibit excellent catalytic oxygen reduction reaction (ORR) performance; hence, they can be used as air cathodes in Zn-air batteries. This study not only provides new building blocks for novel CTFs with controllable polymer/carbon features but also offers insights into the formation and structure transformation history of CTFs during thermal treatment.

Simultaneously Integrate Iron Single Atom and Nanocluster Triggered Tandem Effect for Boosting Oxygen Electroreduction.

Atomically nitrogen-coordinated iron atoms on carbon (FeNC) catalysts are emerging as attractive materials to substitute precious-metal-based catalysts for the oxygen reduction reaction (ORR). However, FeNC usually suffers from unsatisfactory performance due to the symmetrical charge distribution around the iron site. Elaborately regulating the microenvironment of the central Fe atom can substantially improve the catalytic activity of FeNC, which remains challenging. Herein, N/S co-doped porous carbons are rationally prepared and are verified with rich Fe-active sites, including atomically dispersed FeN4 and Fe nanoclusters (FeSA-FeNC@NSC), according to systematically synchrotron X-ray absorption spectroscopy analysis. Theoretical calculation verifies that the contiguous S atoms and Fe nanoclusters can break the symmetric electronic structure of FeN4 and synergistically optimize 3d orbitals of Fe centers, thus accelerating OO bond cleavage in OOH* for improving ORR activity. The FeSA-FeNC @NSC delivers an impressive ORR activity with half-wave-potential of 0.90 V, which exceeds that of state-of-the-art Pt/C (0.87 V). Furthermore, FeSA-FeNC @NSC-based Zn-air batteries deliver excellent power densities of 259.88 and 55.86 mW cm-2 in liquid and all-solid-state flexible configurations, respectively. This work presents an effective strategy to modulate the microenvironment of single atomic centers and boost the catalytic activity of single-atom catalysts by tandem effect.

Modulating intramolecular electron and proton transfer kinetics for promoting carbon dioxide conversion.

A novel pentagon-heptagon paired azulene group that possesses a large dipole moment is immobilized onto a porphyrin. The as-prepared azulene iron porphyrin exhibits a narrower bandgap and higher electrocatalytic CO2 reduction activity than the pristine iron porphyrin. The maximum CO faradaic efficiency reaches 99.9%, which is the state-of-the-art value among molecular catalysts.

N-confused porphyrin-based conjugated microporous polymers.

Metal porphyrins, which possess metal-N coordination centers, are important building blocks for the construction of porous organic materials with catalytic performance. However, most of the previous work has focused on controlling the metal elements instead of the metal-N coordinations. Here, Pt(II) N-confused porphyrin and Pt(II) porphyrin based conjugated microporous polymers were synthesized by Yamamoto coupling reaction. The structural and property differences of Pt-N3C and Pt-N4 were studied. Calculations demonstrate that the Pt-N3C-based porous polymer exhibits broader photoabsorption and narrower bandgap than conventional Pt-N4-based porous polymers.

Optimizing Microenvironment of Asymmetric N,S-Coordinated Single-Atom Fe via Axial Fifth Coordination toward Efficient Oxygen Electroreduction.

Single-atom catalysts (SACs) are attractive candidates for oxygen reduction reaction (ORR). The catalytic performances of SACs are mainly determined by the surrounding microenvironment of single metal sites. Microenvironment engineering of SACs and understanding of the structure-activity relationship is critical, which remains challenging. Herein, a self-sacrificing strategy is developed to synthesize asymmetric N,S-coordinated single-atom Fe with axial fifth hydroxy (OH) coordination (Fe-N3 S1 OH) embedded in N,S codoped porous carbon nanospheres (FeN/SC). Such unique penta-coordination microenvironment is determined by cutting-edge techonologies aiding of systematic simulations. The as-obtained FeN/SC exhibits superior catalytic ORR activity, and showcases a half-wave potential of 0.882 V surpassing the benchmark Pt/C. Moreover, theoretical calculations confirmed the axial OH in FeN3 S1 OH can optimize 3d orbitals of Fe center to strengthen O2 adsorption and enhance O2 activation on Fe site, thus reducing the ORR barrier and accelerating ORR dynamics. Furthermore, FeN/SC containing H2 O2 fuel cell performs a high peak power density of 512 mW cm-2 , and FeN/SC based Znair batteries show the peak power density of 203 and 49 mW cm-2 in liquid and flexible all-solid-state configurations, respectively. This study offers a new platform for fundamentally understand the axial fifth coordination in asymmetrical planar single-atom metal sites for electrocatalysis.

Asymmetric Push-Pull Type Co(II) Porphyrin for Enhanced Electrocatalytic CO2 Reduction Activity.

Molecular electrocatalysts for electrochemical carbon dioxide (CO2) reduction has received more attention both by scientists and engineers, owing to their well-defined structure and tunable electronic property. Metal complexes via coordination with many π-conjugated ligands exhibit the unique electrocatalytic CO2 reduction performance. The symmetric electronic structure of this metal complex may play an important role in the CO2 reduction. In this work, two novel dimethoxy substituted asymmetric and cross-symmetric Co(II) porphyrin (PorCo) have been prepared as the model electrocatalyst for CO2 reduction. Owing to the electron donor effect of methoxy group, the intramolecular charge transfer of these push-pull type molecules facilitates the electron mobility. As electrocatalysts at -0.7 V vs. reversible hydrogen electrode (RHE), asymmetric methoxy-substituted Co(II) porphyrin shows the higher CO2-to-CO Faradaic efficiency (FECO) of ~95 % and turnover frequency (TOF) of 2880 h-1 than those of control materials, due to its push-pull type electronic structure. The density functional theory (DFT) calculation further confirms that methoxy group could ready to decrease to energy level for formation *COOH, leading to high CO2 reduction performance. This work opens a novel path to the design of molecular catalysts for boosting electrocatalytic CO2 reduction.

Topological defect-containing Fe/N co-doped mesoporous carbon nanosheets as novel electrocatalysts for the oxygen reduction reaction and Zn-air batteries.

Developing effective electrocatalysts for the oxygen reduction reaction is of great significance for clean and renewable energy technologies, such as metal-air batteries and fuel cells. Defect engineering is the central focus of this field because the overall catalytic performance crucially depends on highly active defects. For the ORR, topological defects have been proven to have a positive effect. However, because preparation and characterization of such defects are difficult, a basic understanding of the relationship between topological defects and catalytic performance remains elusive. In this study, topological defect-containing Fe/N co-doped mesoporous carbon nanosheets were synthesized using azulene-based sandwich-like polymer nanosheets as the precursor. As electrocatalysts, such porous carbon nanosheets exhibited promising ORR activity, methanol tolerance ability, and stability with a half-wave potential of 841 mV under alkaline conditions, which is superior to those of most of the reported porous carbons. As the air cathode for Zn-air batteries, the catalyst exhibited a peak power density of 153 mW cm-2 and a specific capacity of 628 mA h g-1,which were higher than those of a Pt/C-based Zn-air battery. Density functional theory calculation further proved the positive effect of topological defects on the oxygen reduction activity. These results indicate that bottom-up topological defect engineering could be a new and promising strategy for developing high-performance electrocatalysts.

A Terpyridine-Fe2+-Based Coordination Polymer Film for On-Chip Micro-Supercapacitor with AC Line-Filtering Performance.

The preparation of redox-active, ultrathin polymer films as the electrode materials represents a major challenge for miniaturized flexible electronics. Herein, we demonstrated a liquid-liquid interfacial polymerization approach to a coordination polymer films with ultrathin thickness from tri(terpyridine)-based building block and iron atoms. The as-synthesized polymer films exhibit flexible properties, good redox-active and narrow bandgap. After directly transferred to silicon wafers, the on-chip micro-supercapacitors of TpPB-Fe-MSC achieved the high specific capacitances of 1.25 mF cm-2 at 50 mV s-1 and volumetric energy density of 5.8 mWh cm-3, which are superior to most of semiconductive polymer-based micro-supercapacitor (MSC) devices. In addition, as-fabricated on-chip MSCs exhibit typical alternating current (AC) line-filtering performance (-71.3° at 120 Hz) and a short resistance-capacitance (RC) time (0.06 ms) with the electrolytes of PVA/LiCl. This study provides a simple interfacial approach to redox-active polymer films for microsized energy storage devices.

A Novel Heterostructure Based on RuMo Nanoalloys and N-doped Carbon as an Efficient Electrocatalyst for the Hydrogen Evolution Reaction.

Heterostructures exhibit considerable potential in the field of energy conversion due to their excellent interfacial charge states in tuning the electronic properties of different components to promote catalytic activity. However, the rational preparation of heterostructures with highly active heterosurfaces remains a challenge because of the difficulty in component tuning, morphology control, and active site determination. Herein, a novel heterostructure based on a combination of RuMo nanoalloys and hexagonal N-doped carbon nanosheets is designed and synthesized. In this protocol, metal-containing anions and layered double hydroxides are employed to control the components and morphology of heterostructures, respectively. Accordingly, the as-made RuMo-nanoalloys-embedded hexagonal porous carbon nanosheets are promising for the hydrogen evolution reaction (HER), resulting in an extremely small overpotential (18 mV), an ultralow Tafel slope (25 mV dec-1 ), and a high turnover frequency (3.57 H2 s-1 ) in alkaline media, outperforming current Ru-based electrocatalysts. First-principle calculations based on typical 2D N-doped carbon/RuMo nanoalloys heterostructures demonstrate that introducing N and Mo atoms into C and Ru lattices, respectively, triggers electron accumulation/depletion regions at the heterosurface and consequently reduces the energy barrier for the HER. This work presents a convenient method for rational fabrication of carbon-metal heterostructures for highly efficient electrocatalysis.

Platinum Atoms and Nanoparticles Embedded Porous Carbons for Hydrogen Evolution Reaction.

Due to the growing demand for energy and imminent environmental issues, hydrogen energy has attracted widespread attention as an alternative to traditional fossil energy. Platinum (Pt) catalytic hydrogen evolution reaction (HER) is a promising technology to produce hydrogen because the consumed electricity can be generated from renewable energy. To overcome the high cost of Pt, one effective strategy is decreasing the Pt nanoparticle (NP) size from submicron to nano-scale or even down to single atom level for efficient interacting water molecules. Herein, atomically dispersed Pt and ultra-fine Pt NPs embedded porous carbons were prepared through the pyrolysis of Pt porphyrin-based conjugated microporous polymer. As-prepared electrocatalyst exhibit high HER activity with overpotential of down to 31 mV at 10 mA cm-2, and mass activity of up to 1.3 A mgPt-1 at overpotential of 100 mV, which is double of commercial Pt/C (0.66 A mgPt-1). Such promising performance can be ascribed to the synergistic effect of the atomically dispersed Pt and ultra-fine Pt NPs. This work provides a new strategy to prepare porous carbons with both atomically dispersed metal active sites and corresponding metal NPs for various electrocatalysis, such as oxygen reduction reaction, carbon dioxide reduction, etc.

Ionic Polyimide Derived Porous Carbon Nanosheets as High-Efficiency Oxygen Reduction Catalysts for Zn-Air Batteries.

Two-dimensional (2D) porous carbon nanosheets (2DPCs) have attracted great attention for their good porosity and long-distance conductivity. Factors such as templates, precursors, and carbonization-activation methods, directly determine their performance. However, rational design and preparation of porous carbon materials with controlled 2D morphology and heteroatom dopants remains a challenge. Therefore, an ionic polyimide with both sp2 - and sp3 -hybridized nitrogen atoms was prepared as a precursor for fabricating N-doped hexagonal porous carbon nanosheets through a hard-template approach. Because of the large surface area and efficient charge-mass transport, the resulting activated 2D porous carbon nanosheets (2DPCs-a) displayed promising electrocatalytic properties for oxygen reduction reaction (ORR) in alkaline and acidic media, such as ultralow half-wave potential (0.83 vs. 0.84 V of Pt/C) and superior limiting current density (5.42 vs. 5.14 mA cm-2 of Pt/C). As air cathodes in Zn-air batteries, the as-developed 2DPCs-a exhibited long stability and high capacity (up to 614 mA h g-1 ), which are both higher than those of commercial Pt/C. This work provides a convenient method for controllable and scalable 2DPCs fabrication as well as new opportunities to develop high-efficiency electrocatalysts for ORR and Zn-air batteries.

Charge Transfer Salt and Graphene Heterostructure-Based Micro-Supercapacitors with Alternating Current Line-Filtering Performance.

The rapid development of lightweight and wearable devices requires electronic circuits possessing compact, high-efficiency, and long lifetime in very limited space. Alternating current (AC) line filters are usually tools for manipulating the surplus AC ripples for the operation of most common electronic devices. So far, only aluminum electrolytic capacitors (AECs) can be utilized for this target. However, the bulky volume in the electronic circuits and limited capacitances have long hindered the development of miniaturized and flexible electronics. In this work, a facile laser-assisted fabrication approach toward an in-plane micro-supercapacitor for AC line filtering based on graphene and conventional charge transfer salt heterostructure is reported. Specifically, the devices reach a phase angle of 73.2° at 120 Hz, a specific capacitance of 151 µF cm-2 , and relaxation time constant of 0.32 ms at the characteristic frequency of 3056 Hz. Furthermore, the scan rate can reach up to 1000 V s-1 . Moreover, the flexibility and stability of the micro-supercapacitors are tested in gel electrolyte H2 SO4 /PVA, and the capacitance of micro-supercapacitors retain a stability over 98% after 10 000 cycles. Thus, such micro-supercapacitors with excellent electrochemical performance can be almost compared with the AECs and will be the next-generation capacitors for AC line filters.