Synergistic Modulating Interlayer Space and Electron-Transfer of Covalent Organic Frameworks for Oxygen Reduction Reaction

Covalent organic frameworks (COFs) are an ideal template to construct high-efficiency catalysts for oxygen reduction reaction (ORR) due to their predictable properties. However, the closely parallel-stacking manner and lacking intramolecular electron transfer ability of COFs limit atomic utilization efficiency and intrinsic activity. Herein, COFs are constructed with large interlayer distances and enhanced electronic transfer ability by side-chain functionalization. Long chains with electron-donating features not only enlarge interlayer distance, but also narrow the bandgap. The resulting DPPS-COF displays higher electrochemical surface areas to provide more exposed active sites, despite <1/10 surface areas. DPPS-COF exhibits excellent electrocatalytic ORR activity with half-wave potential of 0.85 V, which is 30 and 60 mV positive than those of Pt/C and DPP-COF, and is the record among the reported COFs. DPPS-COF is employed as cathode electrocatalyst for zinc-air battery with a maximum power density of 185.2 mW cm-2, which is superior to Pt/C. Theoretical calculation further reveals that longer electronic-donating chains not only facilitate the formation of intermediate OOH* from O2, but also promote intermediates desorption , and thus leading to higher activity.

Effects of tuning the structural symmetry of cobalt porphyrin on electrocatalytic oxygen reduction reactions

Metalloporphyrins have attracted significant attention as highly promising alternatives to Pt-based electrocatalysts in the realm of oxygen reduction reactions (ORRs). While the structure of porphyrin is widely recognized as a pivotal factor influencing the ORR performance, the impact of molecular symmetry, which is one of the key properties of the molecular structure, has rarely been understood and its effects remain largely unexplored. Herein, we designed and synthesized two triphenylamine (TPA)-substituted cobalt porphyrins, the asymmetric aBz-TCoP and the symmetric Bz-2TCoP, which are doped onto carbon black to construct composite catalysts for ORRs. The electronic structures of both porphyrins are determined through density functional theory (DFT) calculations, and the morphology and electronic states of the composites are examined by spectroscopic techniques. A series of electrochemical measurements demonstrate the superior activity, selectivity and durability of Bz-2TCoP/C to aBz-TCoP/C in ORRs conducted in both acidic and alkaline electrolytes. The improved ORR properties of the symmetric porphyrin may stem from the steric properties rather than the electronic properties of the chemical structure. This work represents a preliminary study on the effects of porphyrin structural symmetry on electrocatalysis and provides a potential strategy for further structural modifications of metalloporphyrins, as non-noble metal electrocatalysts, to enhance the ORR performance.

Atomically Thin, Ionic-Covalent Organic Nanosheets for Stable, High-Performance Carbon Dioxide Electroreduction

The incorporation of charged functional groups is effective to modulate the activity of molecular complexes for the CO₂ reduction reaction (CO₂ RR), yet long-term heterogeneous electrolysis is often hampered by catalyst leaching. Herein, an electrocatalyst of atomically thin, cobalt-porphyrin-based, ionic-covalent organic nanosheets (CoTAP-iCONs) is synthesized via a post-synthetic modification strategy for high-performance CO₂ -to-CO conversion. The cationic quaternary ammonium groups not only enable the formation of monolayer nanosheets due to steric hindrance and electrostatic repulsion, but also facilitate the formation of a *COOH intermediate, as suggested by theoretical calculations. Consequently, CoTAP-iCONs exhibit higher CO₂ RR activity than other cobalt-porphyrin-based structures: an 870% and 480% improvement of CO current densities compared to the monomer and neutral nanosheets, respectively. Additionally, the iCONs structure can accommodate the cationic moieties. In a flow cell, CoTAP-iCONs attain a very small onset overpotential of 40 mV and a stable total current density of 212 mA cm^-2 with CO Faradaic efficiency of >95% at -0.6 V for 11 h. Further coupling the flow electrolyzer with commercial solar cells yields a solar-to-CO conversion efficiency of 13.89%. This work indicates that atom-thin, ionic nanosheets represent a promising structure for achieving both tailored activity and high stability.

An Initial Covalent Organic Polymer with Closed-F Edges Directly for Proton-Exchange-Membrane Fuel Cells

Covalent organic polymers (COPs) are a class of rising electrocatalysts for the oxygen reduction reaction (ORR) due to the atomically metrical control of the organic molecular components along with highly architectural robustness and thermodynamic stability even in acid or alkaline media. However, the direct application of pristine COPs as acidic ORR electrocatalysts, especially in device manner, e.g., in proton-exchange-membrane fuel cells (PEMFCs), remains a big challenge. Currently, the decoration toward electronic structures of active sites is considered a vital pathway to enhancing the acidic ORR activity of carbon-based electrocatalysts. Here, an initial F-decorated fully closed π-conjugated quasi-phthalocyanine COP (denoted as COP_BTC -F) is reported. The introduction of the closed-F edges stepwise drags more electrons from FeN₄ sites in COP_BTC -F into the catalyst margin, which weakens the occupied numbers of bonding orbitals between COP_BTC -F and OH* intermediates at the rate-determining step, exhibiting over five times intrinsic performance beyond the counterpart without F functionalities (termed as COP_BTC ). Significantly, the maximum power density utilizing COP_BTC -F as a cathode catalyst in PEMFCs is remarkably increased by an order of magnitude compared with COP_BTC , which is a stride forward among catalysts based on a pyrolysis-free conjugated-polymer network in device manner to date.

Porous organic polymers for electrocatalysis

Porous organic polymers (POPs) composed of organic building units linked via covalent bonds are a class of lightweight porous network materials with high surface areas, tuneable pores, and designable components and structures. Owing to their well-preserved characteristics in terms of structure and composition, POPs applied as electrocatalysts have shown promising activity and achieved considerable advances in numerous electrocatalytic reactions, including the hydrogen evolution reaction, oxygen evolution reaction, oxygen reduction reaction, CO₂ reduction reaction, N₂ reduction reaction, nitrate/nitrite reduction reaction, nitrobenzene reduction reaction, hydrogen oxidation reaction, and benzyl alcohol oxidation reaction. Herein, we present a systematic overview of recent advances in the applications of POPs in these electrocatalytic reactions. The synthesis strategies, specific active sites, and catalytic mechanisms of POPs are summarized in this review. The fundamental principles of some electrocatalytic reactions are also concluded. We further discuss the current challenges of and perspectives on POPs for electrocatalytic applications. Meanwhile, the possible future directions are highlighted to afford guidelines for the development of efficient POP electrocatalysts.

Covalent Organic Framework (COF)-Based Hybrids for Electrocatalysis: Recent Advances and Perspectives

Developing highly efficient electrocatalysts for renewable energy conversion and environment purification has long been a research priority in the past 15 years. Covalent organic frameworks (COFs) have emerged as a burgeoning family of organic materials internally connected by covalent bonds and have been explored as promising candidates in electrocatalysis. The reticular geometry of COFs can provide an excellent platform for precise incorporation of the active sites in the framework, and the fine-tuning hierarchical porous architectures can enable efficient accessibility of the active sites and mass transportation. Considerable advances are made in rational design and controllable fabrication of COF-based organic-inorganic hybrids, that containing organic frameworks and inorganic electroactive species to induce novel physicochemical properties, and take advantage of the synergistic effect for targeted electrocatalysis with the hybrid system. Branches of COF-based hybrids containing a diversity form of metals, metal compounds, as well as metal-free carbons have come to the fore as highly promising electrocatalysts. This review aims to provide a systematic and profound understanding of the design principles behind the COF-based hybrids for electrocatalysis applications. Particularly, the structure-activity relationship and the synergistic effects in the COF-based hybrid systems are discussed to shed some light on the future design of next-generation electrocatalysts.

Controllable Synthesis and Performance Modulation of 2D Covalent-Organic Frameworks

Covalent-organic frameworks (COFs) are especially interesting and unique as their highly ordered topological structures entirely built from plentiful π-conjugated units through covalent bonds. Arranging tailorable organic building blocks into periodically reticular skeleton bestows predictable lattices and various properties upon COFs in respect of topology diagrams, pore size, properties of channel wall interfaces, etc. Indeed, these peculiar features in terms of crystallinity, conjugation degree, and topology diagrams fundamentally decide the applications of COFs including heterogeneous catalysis, energy conversion, proton conduction, light emission, and optoelectronic devices. Additionally, this research field has attracted widespread attention and is of importance with a major breakthrough in recent year. However, this research field is running with the lack of summaries about tailorable construction of 2D COFs for targeted functionalities. This review first covers some crucial polymeric strategies of preparing COFs, containing boron ester condensation, amine-aldehyde condensation, Knoevenagel condensation, trimerization reaction, Suzuki CC coupling reaction, and hybrid polycondensation. Subsequently, a summary is made of some representative building blocks, and then underlines how the electronic and molecular structures of building blocks can strongly influence the functional performance of COFs. Finally, conclusion and perspectives on 2D COFs for further study are proposed.

Porphyrin-based frameworks for oxygen electrocatalysis and catalytic reduction of carbon dioxide

The recent progress made on porphyrin-based frameworks and their applications in energy-related conversion technologies (e.g., ORR, OER and CO₂RR) and storage technologies (e.g., Zn–air batteries).

Covalent organic frameworks (COFs) for electrochemical applications

Covalent organic frameworks are a class of extended crystalline organic materials that possess unique architectures with high surface areas and tuneable pore sizes. Since the first discovery of the topological frameworks in 2005, COFs have been applied as promising materials in diverse areas such as separation and purification, sensing or catalysis. Considering the need for renewable and clean energy production, many research efforts have recently focused on the application of porous materials for electrochemical energy storage and conversion. In this respect, considerable efforts have been devoted to the design and synthesis of COF-based materials for electrochemical applications, including electrodes and membranes for fuel cells, supercapacitors and batteries. This review article highlights the design principles and strategies for the synthesis of COFs with a special focus on their potential for electrochemical applications. Recently suggested hybrid COF materials or COFs with hierarchical porosity will be discussed, which can alleviate the most challenging drawback of COFs for these applications. Finally, the major challenges and future trends of COF materials in electrochemical applications are outlined.

Metalloporphyrinic metal-organic frameworks: Controlled synthesis for catalytic applications in environmental and biological media

Recently, as a new sub-family of porous coordination polymers (PCPs), porphyrinic-MOFs (Porph-MOFs) with biomimetic features have been developed using porphyrin macrocycles as ligands and/or pillared linkers. The control over the coordination of the porphyrin ligand and its derivatives however remains a challenge for engineering new tunable Porph-MOF frameworks by self-assembly methods. The key challenges exist in the following respects: (i) collapse of the large open pores of Porph-MOFs during synthesis, (ii) deactivation of unsaturated metal-sites (UMCs) by axial coordination, and (iii) the tendency of both coordinated moieties (at peripheral meso- and beta-carbon sites) and the N₄-pyridine core to coordinate with metal cations. In this respect, this review covers the advances in the design of Porph-MOFs relative to their counterpart covalent organic frameworks (Porph-COFs). The potential utility of custom-designed porphyrin/metalloporphyrins ligands is highlighted. Synthesis strategies of Porph-MOFs are also illustrated with modular design of hybrid guest@host composites (either Porph@MOFs or guest@Porph-MOFs) with exceptional topologies and stability. This review summarizes the synergistic benefits of coordinated porphyrin ligands and functional guest molecules in Porph-MOF composites for enhanced catalytic performance in various redox applications. This review shed lights on the engineering of new tunable hetero-metals open active sites within (metallo)porphyrin-MOFs as out-of-the-box platforms for enhanced catalytic processes in chemical and biological media.

Electrochemically active sites inside crystalline porous materials for energy storage and conversion

This review provides references for the preparation of electroactive CPMs via rational design and modulation of active sites and the space around them, and their application in electrochemical energy storage and conversion systems.

Bulk COFs and COF nanosheets for electrochemical energy storage and conversion

The current advances, structure-property relationship and future perspectives in covalent organic frameworks (COFs) and their nanosheets for electrochemical energy storage (EES) and conversion (EEC) are summarized.

Significantly improved electrocatalytic oxygen reduction by an asymmetrical Pacman dinuclear cobalt(ii) porphyrin-porphyrin dyad

Asymmetrical Pacman dinuclear Co bisporphyrin shows significantly improved activity and selectivity for catalytic reduction of O₂ to water in comparison with corresponding mononuclear Co porphyrins and symmetrical dinuclear Co bisporphyrins.

Pacman dinuclear Co^II triphenylporphyrin-tri(pentafluorophenyl)porphyrin 1 and dinuclear Co^II bis-tri(pentafluorophenyl)porphyrin 2, anchored at the two meso-positions of a benzene linker, are synthesized and examined as electrocatalysts for the oxygen reduction reaction (ORR). Both dinuclear Co bisporphyrins are more efficient and selective than corresponding mononuclear Co^II tetra(pentafluorophenyl)porphyrin 3 and Co^II tetraphenylporphyrin 4 for the four-electron electrocatalytic reduction of O₂ to water. Significantly, although the ORR selectivities of the two dinuclear Co bisporphyrins are similar to each other, 1 outperforms 2, in terms of larger catalytic ORR currents and lower overpotentials. Electrochemical studies showed different redox behaviors of the two Co sites of 1: the Co^III/Co^II reduction of the Co-TPP (TPP = triphenylporphyrin) site is well-behind that of the Co-TPFP (TPFP = tri(pentafluorophenyl)porphyrin) site by 440 mV. This difference indicated their different roles in the ORR: Co^II-TPFP is likely the O₂ binding and reduction site, while Co^III-TPP, which is generated by the oxidation of Co^II-TPP on electrodes, may function as a Lewis acid to assist the O₂ binding and activation. The positively charged Co^III-TPP will have through-space charge interactions with the negatively charged O₂-adduct unit, which will reduce the activation energy barrier for the ORR. This effect of Co-TPP closely resembles that of the Cu_B site of metalloenzyme cytochrome c oxidase (CcO), which catalyzes the biological reduction of O₂. This work represents a rare example of asymmetrical dinuclear metal catalysts, which can catalyze the 4e reduction of O₂ with high selectivity and significantly improved activity.

An Oxygen-Vacancy-Rich Semiconductor-Supported Bifunctional Catalyst for Efficient and Stable Zinc-Air Batteries

The highly oxidative operating conditions of rechargeable zinc-air batteries causes significant carbon-support corrosion of bifunctional oxygen electrocatalysts. Here, a new strategy for the catalyst support design focusing on oxygen vacancy (OV)-rich, low-bandgap semiconductor is proposed. The OVs promote the electrical conductivity of the oxide support, and at the same time offer a strong metal-support interaction (SMSI), which enables the catalysts to have small metal size, high catalytic activity, and high stability. The strategy is demonstrated by successfully synthesizing ultrafine Co-metal-decorated 3D ordered macroporous titanium oxynitride (3DOM-Co@TiO_x N_y ). The 3DOM-Co@TiO_x N_y catalyst exhibits comparable activities for oxygen reduction and evolution reactions, but much higher cycling stability than noble metals in alkaline conditions. The zinc-air battery using this catalyst delivers an excellent stability with less than 1% energy efficiency loss over 900 charge-discharge cycles at 20 mA cm^-2 . The high stability is attributed to the strong SMSI between Co and 3DOM-TiO_x N_y which is verified by density functional theory calculations. This work sheds light on using OV-rich semiconductors as a promising support to design efficient and durable nonprecious electrocatalysts.

Unraveling the relationship between the morphologies of metal–organic frameworks and the properties of their derived carbon materials

The effects of the morphologies of MOF precursors on the properties of their derived carbonaceous materials are systematically studied.

Optical activity of Co-porphyrin in the light of IR and Raman spectroscopy: A critical DFT investigation

A critical investigation on the structure, electronic properties and optical activities of a series of transition metal doped porphyrins (TMP; TM=Fe, Co, Ni) in the light of infrared and Raman spectroscopy is performed, under density functional formalism. The structure and electronic properties are studied in terms of ionization potential, electron affinity, chemical hardness (η), binding energies of the transition metals (BE_TM) etc. The origin of the optical activities, especially the visibly active cobalt porphyrin is addressed through critical study on their infrared and Raman spectra. The information availed from the spectral analysis will certainly ease their possible synthesis and useful applications in the sensor and optoelectronic domains.