Covalent Organic Framework with Multiple Redox Active Sites for High-Performance Aqueous Calcium Ion Batteries.

"… Despite the introduction of high-throughput and combinatorial methods that certainly can be useful in the process of catalysts optimization, it is recognized that the generation of fundamental knowledge at the molecular level is key for the development of new concepts and for reaching the final objective of solid catalysts by design …" Read more in the Editorial by Avelino Corma.

Covalent organic frameworks (COFs) have recently been emerged as promising anode candidates for aqueous ammonium ion batteries (AAIBs), but suitable COFs need to be developed and the energy storage mechanisms need to be further investigated. Here we report an anthraquinone carbonyl‐containing COF as anode for AAIBs, which exhibits a high specific capacity of 141 mAh g−1 at 0.1 A g−1 and good cycling stability with 90 % retention after 8000 cycles at 6 A g−1. We find that the anthraquinone carbonyls (denoted as C=O1) with superconjugated structure in the COF can form hydrogen bonds with NH4+ ions, thus serving as optimal sites and providing the major contribution to NH4+ ion storage, whereas another type of carbonyls (denoted as C=O2) in the same COF suffers from enol‐keto tautomerism, which prevents the formation of hydrogen bonds and is therefore unfavourable for NH4+ ion storage. This is further verified by the theoretical calculations where the hydrogen bonding rate of C=O1 groups is much faster than that of C=O2. This work not only provides insights into understanding the mechanism of NH4+ ion storage, but also offers new ideas for the design of advanced COF‐based materials for AAIBs.

Cystalysin is a C(beta)-S(gamma) lyase from the oral pathogen Treponema denticola catabolyzing L-cysteine to produce pyruvate, ammonia and H(2)S. With its ability to induce cell lysis, cystalysin represents a new class of pyridoxal 5'-phosphate (PLP)-dependent virulence factors. The crystal structure of cystalysin was solved at 1.9 A resolution and revealed a folding and quaternary arrangement similar to aminotransferases. Based on the active site architecture, a detailed catalytic mechanism is proposed for the catabolism of S-containing amino acid substrates yielding H(2)S and cysteine persulfide. Since no homologies were observed with known haemolysins the cytotoxicity of cystalysin is attributed to this chemical reaction. Analysis of the cystalysin-L-aminoethoxyvinylglycine (AVG) complex revealed a 'dead end' ketimine PLP derivative, resulting in a total loss of enzyme activity. Cystalysin represents an essential factor of adult periodontitis, therefore the structure of the cystalysin-AVG complex may provide the chemical basis for rational drug design.

Calcium ion batteries (CIBs) are promising for energy storage with volumetric capacity and reduction potential comparable to lithium, while richer in earth abundance. However, sluggish cation transport and unstable cycling performance primarily due to anode surface passivation remain vital challenges for realizing high‐performance CIBs. Herein, two kinds of redox covalent organic frameworks (PT‐COFs and PQ‐COFs) with different‐density carbonyl groups are prepared as quasi‐solid‐state electrolytes (QSSEs) to address those challenges. In particular, PT‐COFs exhibit ionic conductivity of 0.46 and 5.05 mS cm−1 at room temperature and 80 °C, respectively, and Ca2+ transference number of 0.532. Due to the efficient ionic conduction and intrinsic stability of PT‐COFs structure, the prepared full calcium ion cell with PT‐COFs demonstrates the highest reversible specific capacity of 155.9 mAh g−1 at 0.15 A g−1 (1 C), and stable cycle performance (capacity retention over 74.6% at 1 A g−1 after 1000 cycles). This work shows the effectiveness of the redox COFs and their promising potential as SSE for the development of high‐performance CIBs.

Electrochemical CO2 reduction is a potential approach to convert CO2 into valuable chemicals using electricity as feedstock. Abundant and affordable catalyst materials are needed to upscale this process in a sustainable manner. Nickel‐nitrogen‐doped carbon (Ni‐N‐C) is an efficient catalyst for CO2 reduction to CO, and the single‐site Ni−N x motif is believed to be the active site. However, critical metrics for its catalytic activity, such as active site density and intrinsic turnover frequency, so far lack systematic discussion. In this work, we prepared a set of covalent organic framework (COF)‐derived Ni‐N‐C catalysts, for which the Ni−N x content could be adjusted by the pyrolysis temperature. The combination of high‐angle annular dark‐field scanning transmission electron microscopy and extended X‐ray absorption fine structure evidenced the presence of Ni single‐sites, and quantitative X‐ray photoemission addressed the relation between active site density and turnover frequency.

This letter chooses DAE-BPy-CoPor [DAE, 1,2-bis(5'-formyl-2'-methylthien-3'-yl)cyclopentene; BPy, 2,2'-bipyridine-5,5'-dicarbaldehyde; and CoPor, two-dimensional cobalt porphyrin-based covalent organic framework] and Co-TAPP [10,15,20-tetrakis(4-aminophenyl)porphinatocobalt] as two model catalysts to investigate the effect of the hydrogen evolution reaction (HER) and oxygen reduction reaction (ORR) in the O2-tolerant CO2 electroreduction reaction (CO2RR). Both catalysts have been proven to have CO2RR activity at a potential range from -0.60 to -1.10 V when CO2 and O2 are co-feeding (Zhu, H.-J.; Si, D.-H.; Guo, H.; Chen, Z.; Cao, R.; Huang, Y.-B. Oxygen-tolerant CO2 electroreduction over covalent organic frameworks via photoswitching control oxygen passivation strategy. Nat. Commun. 2024, 15, 1479, DOI: 10.1038/s41467-024-45959-9). The calculated results by grand canonical density functional theory (GC-DFT) and microkinetic (MK) simulation show that the effect of HER could be categorized into two groups: (1) Positive *H formation energy: HER would have no effect on the turnover frequency (TOF) of the CO2RR. (2) Negative *H formation energy: HER would also have no effect to the TOF of CO2RR when the calculated limiting potential for HER is larger than that for CO2RR. Otherwise, all active sites would be covered by *H and poisoned before CO2RR can happen. The effect of ORR could be categorized into three types: (1) ORR would have no effect to the TOF of the CO2RR when ORR happened with the CO2RR at the same time. (2) ORR is suppressed; the active site is covered by an intermediate state (such as *OH); and the CO2RR is also suppressed because of the poisoned active site. (3) ORR is suppressed; the active site is not covered; and the TOF of the CO2RR is not affected.

We have described four superfamilies of enzymes where the members within each catalyze different overall reactions using broadly varied substrates. Within each superfamily, the different overall reactions are facilitated by a common mechanistic strategy that can be rationalized in the context of the structural scaffold. Although space limitations prevent their discussion, a number of additional superfamilies have been described in which these principles appear to obtain. Analyses of the relationships between structure and function in all of these superfamilies suggest two general conclusions regarding the evolution of new catalytic activities.

π-Conjugated electrochemically stable covalent quinazoline network for lithium ion storage

The effective utilization of clean energy and finding alternatives to fossil resources are highly important to ensure the sustainability of human society and are always among the major goals of both chemistry and material science research. Advanced electrochemical devices, such as fuel cells, water electrolysers and metal-air batteries, represent the most promising strategies for clean-energy utilization. In an electrochemical device, the redox reactions are spatially separated by a membrane, allowing direct extraction/transfer of electrons at an electrode-electrolyte interface, which leads to higher intrinsic energy conversion efficiencies, milder process conditions, easy product separation and excellent design features for coupling to renewable energy infrastructure. The performance of such electrochemical processes is fundamentally determined by the physicochemical properties of the electrochemical interfaces, encompassing both the electrocatalyst and the structure of the adjacent electrochemical double layer. Specifically, electrocatalysts play key roles in electrochemical reactions and often limit the performance of entire systems due to their insufficient activity, low durability or high cost. Ideally, the rate, efficiency, and selectivity of the above electrochemical reactions can be substantially improved by developing high-performance electrocatalyst. One of the central tasks for chemists and material scientists is to design and fabricate the high-efficient efficiency but low-cost electrocatalysts systems. The current promising electrochemical reactions mainly focus on the realization of the reversible conversion between chemical and electricity energy, e.g., the oxygen reduction reaction (ORR), oxygen evolution reaction (OER), hydrogen oxidation reaction (HOR), and hydrogen evolution reaction (HER). Coupling of the above electrochemical reactions provide a solid foundation for various essential electrochemical devices, such as direct hydrogen fuel cells (HOR + ORR); electrolysers (OER + HER); rechargeable zinc (Zn)-air battery (ORR + OER). Therefore, this thesis aims to design and synthesize high-performance electrocatalysts for HER, ORR and OER based on earth-abundant materials with proper hierarchical 2D or 3D nanostructures. Combined with the advanced characterization techniques and density functional theory (DFT) calculations, the relationship between the electrochemical activity and active sites of these earth-abundant electrocatalysts were detailedly explored and confirmed. Furthermore, to emphasize the hierarchical 2D or 3D nanostructures, the actual performance of these electrocatalysts was all evaluated in practical devices including Zn-air battery and proton exchange membrane fuel cell (PEMFC), specifically as follows: (1) The vast majority of the reported HER electrocatalysts performs poorly under alkaline conditions due to the sluggish water dissociation kinetics. In the first work, a hybridization catalyst construction concept is presented to dramatically enhance the alkaline HER activities of catalysts based on 2D transition metal dichalcogenides (TMDs) (MoS2 and WS2). A series of ultrathin 2D-hybrids are synthesized via facile controllable growth of 3d metal (Ni, Co, Fe, Mn) hydroxides on the monolayer 2D-TMD nanosheets. The resultant Ni(OH)2 and Co(OH)2 hybridized ultrathin MoS2 and WS2 nanosheet catalysts exhibit significantly enhanced alkaline HER activity and stability compared to their bare counterparts. The combined theoretical and experimental studies confirm that the formation of the heterostructured boundaries by suitable hybridization of the TMD and 3d metal hydroxides is responsible for the improved alkaline HER activities because of the enhanced water dissociation step and lowers the corresponding kinetic energy barrier by the hybridized 3d metal hydroxides. (2) Nitrogen-coordinated iron atoms on carbon matrix (Fe-N-C) materials are the most active Pt-group-metal-free ORR catalysts but still suffering their low stability and relatively lower activity compared to platinum-based materials. In the second work, Fe and Ni dual sites atomically dispersed in hierarchically ordered macroporous carbon support (Fe-Ni/N-HOMC) was designed and successfully prepared. Isolated atomic Fe- N4 and Ni-N4 active sites were confirmed via various characterizations. The ORR activity and stability of Fe-Ni/N-HOMC in both acid and alkaline electrolyte were much higher than commercial Pt/C and the mono-Fe doping counterpart, which was among the state-of-the-art ORR electrocatalysts. In addition, this 3D ordered interconnected macroporous structure with abundant mesopores and micropores could greatly increase the accessible ORR active site and also enhance the mass transport during the ORR process. When employed as cathodes for PEMFC, we found the excellent ORR activity of Fe-Ni/N-HOMC was completely translated to the cathode in the fuel cell. (3) High-performance bifunctional electrocatalysts with ORR and OER activity is the key to developing efficient rechargeable Zn-air batteries. In the third work, a high-performance bifunctional electrocatalysts for both OER and ORR were synthesized via further hybridizing as-prepared Fe-Ni/N-HOMC with NiFe layer double hydroxides (LDHs). Layered double hydroxides (LDHs) have been reported to be promising OER electrocatalysts with ultrahigh OER performances. The as-synthesized new composites exhibited almost the same ORR activity as Fe-Ni/N-HOMC, revealing that hybridization of NiFe-LDHs would not deteriorate the initial ORR activity. Moreover, the remarkable enhancement of OER activity was observed after the hybridization, which was attributed to the strong coupling of uniformly dispersed small NiFe-LDH nanoparticles with the carbon substrate. The prototype Zn-air battery was assembled using these new composites, which displayed the ultralow voltage gap and long-term stability. (4) Compared with Fe-N-C or Co-N-C based ORR electrocatalysts, the Cu-nitrogen-carbon composites were attracted little attention. However, the natural multicopper oxidases (MCOs) enzymes, such as laccase, can serve as efficient ORR catalyst with almost no overpotential. Inspired by their tris-copper centers in MCO, one novel Cu-nitrogen-carbon composite (Cu SAs/N-CS) with atomic Cu coordination sites were synthesized via the pyrolysis of the Cu-involved metal-organic-framework. The copper contents in Cu SAs/N-CS reaches as high as 3.17 wt.%, and the average distances of adjacent copper sites was around only 3.1 Å. Due to the synergetic effect of abundant single atomic copper active sites with closer distance and ultrathin carbon nanosheet structure, Cu SAs/N-CS exhibited superior ORR activity exceeding commercial Pt/C catalyst, methanol tolerance, and long-term stability in both alkaline and neutral electrolyte. In summary, four kinds of new composites were successfully designed and prepared as high-performance electrocatalysts for HER, ORR and OER. Multi-dimensional heterostructures, atomic metal coordination sites and 3D hierarchically porous structure were designed and observed, which contributed greatly to improve activities of these composites. This thesis suggests several new viewpoints in the design of electrocatalysts based on earth-abundant materials: (i) offering new strategies for the preparation of novel 2D and 3D heterostructures as electrocatalysts; (ii) expanding methods for the synthesis of atomic metal coordination sites and evaluating their activities for ORR; (iii) evaluating the practical performances of achieved electrocatalysts in proton exchange membrane fuel cell and Zn-air battery; (iv) attempting to explain reaction mechanisms of some electrocatalysts by DFT calculation.

Covalent organic frameworks (COFs) possessing a well-defined structure and abundant functional groups are prospective pseudocapacitive electrode materials. However, their intrinsic poor electrical conductivity and stacking problems usually impede the utilization of their active sites. Herein, we conduct an in situ growth of polyimide COFs (donated as NTDA COFs) enriched with carbonyl groups on multiwalled carbon nanotubes (MWCNTs). An impressive capacitance of 467 F g-1 at 1 A g-1 is achieved for the as-prepared NTDA/MWCNTs composite, significantly surpassing both the pure MWCNTs (60.3 F g-1) and NTDA COFs (284.4 F g-1). No decay of capacitance is observed after 10,000 cycles at 10 A g-1. The assembled device NTDA/MWCNTs//activated carbon reaches a high energy density of 17 Wh kg-1 at 750 W kg-1 while keeping superior charging/discharging stability of 89.5% after cycling for 19,000 times at 10 A g-1. In situ Fourier transform infrared (in situ FT-IR) tests together with the exploration of electrode kinetics show that the boosted capacitance of NTDA/MWCNTs is mainly donated by the redox reactions of carbonyl groups on NTDA COFs, which is largely activated by MWCNTs.

Covalent organic frameworks (COFs) hold great promise for rechargeable batteries. However, the synthesis of COFs with abundant active sites, excellent stability, and increased conductivity remains a challenge. Here, chemically stable fully sp2 carbon‐conjugated COFs (sp2c‐COFs) with multiple active sites are designed by the polymerization of benzo[1,2‐b:3,4‐b′:5,6‐b′′]trithiophene‐2,5,8‐tricarbaldehyde) (BTT) and s‐indacene‐1,3,5,7(2H,6H)‐tetrone (ICTO) (denoted as BTT‐ICTO). The morphology and structure of the COF are precisely regulated from “butterfly‐shaped” to “cable‐like” through an in situ controllable growth strategy, significantly promoting the exposure and utilization of active sites. When the unique “cable‐like” BTT‐ICTO@CNT is employed as lithium‐ion batteries (LIBs) cathode, it exhibits exceptional capacity (396 mAh g−1 at 0.1 A g−1 with 97.9 % active sites utilization rate), superb rate capacity (227 mAh g−1 at 5.0 A g−1), and excellent cycling performance (184 mAh g−1 over 8000 cycles at 2.0 A g−1 with 0.00365 % decay rate per cycle). The lithium storage mechanism of BTT‐ICTO is exhaustively revealed by in situ Fourier transform infrared, in situ Raman, and density functional theory calculations. This work provides in‐depth insights into fully sp2c‐COFs with multiple active sites for high‐performance LIBs.

Dermatan sulfate is a highly sulfated polysaccharide and has a variety of biological functions in development and disease. Iduronic acid domains in dermatan sulfate, which are formed by the action of two DS-epimerases, have a key role in mediating these functions. We have identified the catalytic site and three putative catalytic residues in DS-epimerase 1, His-205, Tyr-261, and His-450, by tertiary structure modeling and amino acid conservation to heparinase II. These residues were systematically mutated to alanine or more conserved residues, which resulted in complete loss of epimerase activity. Based on these data and the close relationship between lyase and epimerase reactions, we propose a model where His-450 functions as a general base abstracting the C5 proton from glucuronic acid. Subsequent cleavage of the glycosidic linkage by Tyr-261 generates a 4,5-unsaturated hexuronic intermediate, which is protonated at the C5 carbon by His-205 from the side of the sugar plane opposite to the side of previous proton abstraction. Concomitant recreation of the glycosidic linkage ends the reaction, generating iduronic acid. In addition, we show that proper N-glycosylation of DS-epimerase 1 is required for enzyme activity. This study represents the first description of the structural basis for epimerization by a glycosaminoglycan epimerase.

Introduction Nitrogen-doped carbon materials have been attracting great attention as one of the promising metal-free oxygen reduction reaction (ORR) catalysts due to the abundant resources, chemical stability, and high electronic conductivity, which could replace the high cost platinum-based materials limiting expansion of application fields of the fuel cells. Generally, nitrogen-doped carbons have been prepared by chemical vapor deposition with precursors containing nitrogen (pyridine or acetonitrile, etc.)[i], or by thermal/plasma treatment using graphitic carbons.[ii] The active site of nitrogen-doped carbons is yet controversial, though it has been suggested that incorporation of nitrogen atoms in the sp2 carbon network structures enhances ORR activity through active site formation by produced pyridinic N or graphitic N.1 Together with the nitrogen doped structures, the amount of the doped nitrogen atoms, surface area and degrees of graphitization can affect the ORR performance[iii] and control techniques of these factors are crucial to achieve the higher catalytic activity. Herein we fabricated the nitrogen-doped carbon from covalent organic frameworks (COFs) which have molecularly-ordered structures constructed by covalent linkage of organic building blocks. Due to the above unique structures and properties, COFs are promising for potential applications in gas storage, optoelectronic devices and catalysis.[iv] In this study, we focus on the structural features of COFs, which are versatile molecular design based on various combination of the building block molecules and the covalent bonding and highly porous structures. Thus we have been examining carbonization of COFs having hetero atoms such as N and B to create hetero atom doped carbon materials from the molecularly-ordered starting materials, COFs. Herein we synthesized two types of the nitrogen atom-containing COFs as the precursors. After carbonization, we found that the nitrogen-doped porous carbons were fabricated with high surface areas. The obtained materials would be useful to examine the effect of structural differences of the resulting nitrogen-doped structures on ORR activity to develop highly active carbon catalysts. Experiment Synthesis of COF1[v]Benzene-1,3,5-tricarboxylate (1, 24 mg) and phenylendiamine (2, 24 mg) were dissolved in dioxane (2.5 ml), respectively. After mixing the solutions, 3.0 M aqueous acetic acid (0.25 ml) was added. Two-day storage at 25 °C was found to generate a precipitate, which was collected by filtration, then washed with acetone. The obtained solid was dried overnight in vacuo at 80 °C to provide a yellow powder (COF1). Synthesis of ACOF-1 1 (24 mg) and hydrazine (3, 19 ml) were dissolved in dioxane (1.0 ml). Then 6.0 M tetrafluoroacetic acid (0.13 ml) was added. The solution was heated at 120 °C in a pressure resistant vessel yielding a precipitate, which was collected by filtration, then washed with acetone. The obtained solid was dried overnight in vacuo at 80 °C to provide a pale yellow powder (ACOF-1). Carbonization of COFs The obtained COF1 and ACOF-1 were heated at 800 °C for 3 h under N2 flow. Result and Discussion The XRD patterns of the obtained COFs show obvious diffraction peaks, which indicate crystalline structure formation. For porous structure investigation, N2 gas adsorption measurements were conducted. Based on the BET analysis, the surface area of COF1 and ACOF-1 were estimated to be 1528 m2/g and 1484 m2/g respectively. These results reveal that the prepared COFs have highly ordered structures with large surface areas. After carbonization of the both COFs, in Raman spectra, two peaks were observed at around 1300 cm-1 and 1590 cm-1, which are attributed to D band and G band of graphitic carbon structures. From deconvoluted XPS spectra of the N1s, three peaks were observed at 398, 400, and 402 eV ascribable to pyridinic, pyrrolic, and graphitic N. The results exhibit that the carbonized COFs have nitrogen-doped graphitic structures. The BET surface area of the carbonized COF1 and the carbonized ACOF-1 were calculated to be 538 m2/g and 1257 m2/g, respectively. Interestingly, the carbonized ACOF-1 showed a similar surface area to one before carbonization. This result indicates that COFs would be capable of producing porous carbon structures. The electrocatalytic properties of the carbonized ACOF-1 for ORR were investigated by cyclic voltammetry (CV) in 0.1M KOH saturated with N2 or O2 gas. Though no peak was observed in a N2 atmosphere condition, a significant reduction peak appeared at -0.198 V under an O2 atmosphere, which indicates ORR catalytic activity from the material. [i] M. Kawaguchi et al. Chem. Mater. 1996, 8, 1197. [ii] X. Wang et al. Science, 2009, 324, 768. [iii] C. H. Choi et al. ACS nano, 2 012, 6, 7084. [iv] A. P. Cote et al. Science, 2005, 310, 1166. [v] T. Shiraki et al. Chem. Lett. 2015, 44, 1488. Figure 1

ConspectusIn this Accounts article, we summarize our recent work on truncating conjugated two-dimensional framework materials down to a single pore, or a single macrocycle. Conjugated 2D architectures have emerged as one of the most synthetically adaptable motifs for coupling semiconductivity and porosity in metal-organic frameworks (MOFs) and covalent organic frameworks (COFs). However, despite their prevalence, 2D architectures have several limitations. In particular, the strong interlayer π-π stacking can limit both processability and the accessibility of internal active sites. We have found that simple macrocycles preserve key aspects of 2D framework structure and function, including porosity and out-of-plane electrical conductivity, while providing improved processability, surface tunability, and mass transport properties. In this article, we first describe our synthetic approach and general design considerations. Specifically, we show how ditopic analogues of the tritopic ligands commonly found in the synthesis of 2D MOFs and COFs can be used to achieve a diverse library of conjugated macrocycles that resemble fragments of semiconducting frameworks in both form and function. The length of the peripheral side chains, the size of the aromatic core, and the solubility of intermediates are all key variables in favoring selective macrocycle formation over undesired linear polymers and oligomers. Next, we highlight the unique advantages that macrocycles provide, including improved processability, atomically precise surface tunability, and greater active site accessibility. In particular, the identity of the peripheral side chains dramatically impacts both solubility and colloidal stability as well as crystal size and morphology. We further show how the solution processability and nanoscale dimensions of macrocycles can simplify electronic device fabrication and improve electrochemical performance. Finally, we end with a forward-looking discussion on how macrocycles offer a unique bridge between conjugated molecules and extended frameworks, enabling new application areas and fundamental science.

Thioesterase activity is typically required for the release of products from polyketide synthase enzymes, but no such enzyme has been characterized in deep-sea bacteria associated with the production of polyunsaturated fatty acids. In this work, we have expressed and purified the Orf6 thioesterase from Photobacterium profundum. Enzyme assays revealed that Orf6 has a higher specific activity toward long-chain fatty acyl-CoA substrates (palmitoyl-CoA and eicosapentaenoyl-CoA) than toward short-chain or aromatic acyl-CoA substrates. We determined a high resolution (1.05 Å) structure of Orf6 that reveals a hotdog hydrolase fold arranged as a dimer of dimers. The putative active site of this structure is occupied by additional electron density not accounted for by the protein sequence, consistent with the presence of an elongated compound. A second crystal structure (1.40 Å) was obtained from a crystal that was grown in the presence of Mg(2+), which reveals the presence of a binding site for divalent cations at a crystal contact. The Mg(2+)-bound structure shows localized conformational changes (root mean square deviation of 1.63 Å), and its active site is unoccupied, suggesting a mechanism to open the active site for substrate entry or product release. These findings reveal a new thioesterase enzyme with a preference for long-chain CoA substrates in a deep-sea bacterium whose potential range of applications includes bioremediation and the production of biofuels.