The mechanism of water oxidation using transition metal-based heterogeneous electrocatalysts.

The water oxidation reaction, a crucial process for solar energy conversion, has garnered significant research attention. Achieving efficient energy conversion requires the development of cost-effective and durable water oxidation catalysts. To design effective catalysts, it is essential to have a fundamental understanding of the reaction mechanisms. This review presents a comprehensive overview of recent advancements in the understanding of the mechanisms of water oxidation using transition metal-based heterogeneous electrocatalysts, including Mn, Fe, Co, Ni, and Cu-based catalysts. It highlights the catalytic mechanisms of different transition metals and emphasizes the importance of monitoring of key intermediates to explore the reaction pathway. In addition, advanced techniques for physical characterization of water oxidation intermediates are also introduced, for the purpose of providing information for establishing reliable methodologies in water oxidation research. The study of transition metal-based water oxidation electrocatalysts is instrumental in providing novel insights into understanding both natural and artificial energy conversion processes.

Zeolitic imidazolate framework-90 loaded with methylprednisolone sodium succinate effectively reduces hypertrophic scar in vivo.

Hypertrophic scar (HS) is characterized by an abnormal fibroblast-myofibroblast transformation; non-apoptosis of fibroblasts; and redundant expression of TGF-ß1, VEGF, α-SMA, and collagen I/III. An HS affects patients' physical and psychological quality of life, leading to joint dysfunction and skin cancer. However, there is currently no satisfactory drug to treat this disorder. In this study, we constructed methylprednisolone sodium succinate (MPSS) encapsulated ZIF-90 (MPSS@ZIF-90) for the effective treatment of an HS. The encapsulation of MPSS in ZIF-90 can achieve the controllable drug release of MPSS and prolong its effective treatment time. MPSS@ZIF-90 enhanced the apoptosis of human hypertrophic scar fibroblasts and downregulated the overexpression of TGF-ß1, VEGF, α-SMA, and collagen I/III both in vitro and in vivo. The instant injection of MPSS@ZIF-90 effectively intervened with the formation of the HS after 28 days. On the contrary, MPSS@ZIF-90 greatly reduced the HS with two injections and 14 days of treatment after the HS was formed. This work provides evidence of effective intervention in the formation of an HS and the therapeutic effectiveness of MPSS@ZIF-90 with short treatment periods in vivo. It suggests that MPSS@ZIF-90 can be used as a biomedical option in the treatment of skin wounds and may reveal the potential molecular basis for promising future antifibrotic agents against scarring.

Hierarchically porous aggregates of Co-N-C nanoparticles for oxygen electrocatalysis.

Herein, a novel assembled Co-N-C (A-Co-N-C) material was reported for the first time by pyrolyzing zeolitic imidazolate framework-67 (ZIF-67) nanoparticle aggregates caused by the introduction of surfactant polystyrene sulfonic acid (PSS). The A-Co-N-C has a large surface area of 455 m2 g-1 with micropores (101 m2 g-1) and mesopores (354 m2 g-1). The A-Co-N-C exhibits good bifunctional catalytic oxygen reduction/evolution reaction (ORR/OER) and Zn-air battery activity. This work provides a simple but efficient strategy for constructing hierarchically porous aggregates of Co-N-C nanoparticles.

Electrocatalytic water oxidation with manganese phosphates.

As inspired by the Mn4CaO5 oxygen evolution center in nature, Mn-based electrocatalysts have received overwhelming attention for water oxidation. However, the understanding of the detailed reaction mechanism has been a long-standing problem. Herein, homologous KMnPO4 and KMnPO4•H2O with 4-coordinated and 6-coordinated Mn centers, respectively, are prepared. The two catalysts constitute an ideal platform to study the structure-performance correlation. The presence of Mn(III), Mn(IV), and Mn(V) intermediate species are identified during water oxidation. The Mn(V)=O species is demonstrated to be the substance for O-O bond formation. In KMnPO4•H2O, the Mn coordination structure did not change significantly during water oxidation. In KMnPO4, the Mn coordination structure changed from 4-coordinated [MnO4] to 5-coordinated [MnO5] motif, which displays a triangular biconical configuration. The structure flexibility of [MnO5] is thermodynamically favored in retaining Mn(III)-OH and generating Mn(V)=O. The Mn(V)=O species is at equilibrium with Mn(IV)=O, the concentration of which determines the intrinsic activity of water oxidation. This study provides a clear picture of water oxidation mechanism on Mn-based systems.

A twisted carbonaceous nanotube as the air-electrode for flexible Zn-Air batteries.

Exploring electrocatalysts with high-efficiency oxygen reduction reaction (ORR) is significant for practical applications of fuel cells and metal-air batteries. In this work, a twisted core@shell material has been prepared with helical polypyrrole nanotubes (HPPys) as the core and coordination polymers (CPs) as the shell. After the pyrolysis process, a dense twisted carbon layer was formed by the reaction of CP and HPPy at its interface under Ar. The derived twisted carbonaceous nanotube exhibits good performance in both electrocatalytic ORR and OER. When used as the air-electrode in a flexible Zn-air battery, the battery shows good performance and stability.

Anchoring Fe Species on the Highly Curved Surface of S and N Co-Doped Carbonaceous Nanosprings for Oxygen Electrocatalysis and a Flexible Zinc-Air Battery.

Oxygen reduction reaction (ORR) is of critical significance in the advancement of fuel cells and zinc-air batteries. The iron-nitrogen (Fe-Nx ) sites exhibited exceptional reactivity towards ORR. However, the task of designing and controlling the local structure of Fe species for high ORR activity and stability remains a challenge. Herein, we have achieved successful immobilization of Fe species onto the highly curved surface of S, N co-doped carbonaceous nanosprings (denoted as FeNS/Fe3 C@CNS). The induction of this twisted configuration within FeNS/Fe3 C@CNS arose from the assembly of chiral templates. For electrocatalytic ORR tests, FeNS/Fe3 C@CNS exhibits a half-wave potential (E1/2 ) of 0.91 V in alkaline medium and a E1/2 of 0.78 V in acidic medium. The Fe single atoms and Fe3 C nanoparticles are coexistent and play as active centers within FeNS/Fe3 C@CNS. The highly curved surface, coupled with S substitution in the coordination layer, served to reduce the energy barrier for ORR, thereby enhancing the intrinsic catalytic activity of the Fe single-atom sites. We also assembled a wearable flexible Zn-air battery using FeNS/Fe3 C@CNS as electrocatalysts. This work provides new insights into the construction of highly curved surfaces within carbon materials, offering high electrocatalytic efficacy and remarkable performance for flexible energy conversion devices.

Facet engineering of a two-dimensional metal-organic framework with uniquely oriented layered-structure for electrocatalytic oxygen reduction reaction.

The properties of metal-organic framework (MOF) nanocrystals are highly dependent on their sizes, morphologies, and exposed facets. Facet engineering of MOFs offers an efficient strategy to tailor the active sites and optimize the catalytic activity of both MOFs and their derivatives. In this study, we prepared 1D zeolitic imidazolate framework-nanorod (ZIF-NR) through facet engineering of the parental 2D ZIF-L. The introduction of cetyltrimethylammonium bromide (CTABr) surfactant into the synthesis solution hindered the crystal growth along the c-axis of leaf-like ZIF-L, resulting in the formation of 1D ZIF-NR. The derived Co nanoparticle encapsulated N doped carbon nanorod (denoted as Co-NCR) exhibited high activity and stability for electrocatalytic oxygen reduction reactions and Zn-air batteries. Facet engineering of a 2D MOF with a uniquely oriented layered structure demonstrates the possibility of designing novel electrocatalysts.

Co3 O4 Supported on ß-Mo2 C with Different Interfaces for Electrocatalytic Oxygen Evolution Reaction.

Invited for this month's cover is the group of Rui Cao at Shaanxi Normal University. The image shows the interface between Co3 O4 and ß-Mo2 C can be regulated to boost the electrocatalytic performance of water oxidation. The Research Article itself is available at 10.1002/cssc.202300709.

A layered CoSeO3 pre-catalyst for electrocatalytic water oxidation.

In electrocatalytic water oxidation, the surface reconstruction of electrocatalysts is a common issue due to the applied anodic potential. The study of the fundamentals of catalyst structure transformation and the relationship between structure and performance is important. Herein, we designed two cobalt selenites (CoSeO3 and CoSeO3·2H2O) with different structures for comparative studies. The cross channels in layered CoSeO3 provide space for easy surface reconstruction. The reasons are defined by a series of electrochemical studies, indicating a larger ion diffusion coefficient, more surface contacting OH- anions and faster charge transfer kinetics in CoSeO3. This work provided a paradigm for studying the influence of geometric structure on pre-catalyst reconstruction.

Co3 O4 Supported on ß-Mo2 C with Different Interfaces for Electrocatalytic Oxygen Evolution Reaction.

Interface engineering is an effective strategy for improving the activity of catalysts in electrocatalytic oxygen evolution reaction (OER). Herein, Co3 O4 supported on ß-Mo2 C with different interfaces were investigated for electrocatalytic OER. The morphological diversity of ß-Mo2 C supports allowed different Co3 O4 -Mo2 C interactions. Various techniques characterized the composition and microstructure of the interface in the composites. Due to the strong interaction between Co3 O4 nanoparticles and ß-Mo2 C nanobelts with opposing surface potentials, compact interface was observed between Co3 O4 active species and ß-Mo2 C nanobelt support. The compact interface enhanced the conductivity of the material and also regulated the interfacial electron redistribution of Mo and Co atoms, promoting the charge transfer process during OER. In addition, the surface loading of Co3 O4 can effectively improve the hydrophilicity of the surface. ß-Mo2 C has the capability in dissociating H2 O molecules. Thus, an example has been carefully demonstrated for interface engineering in electrocatalytic OER.

Helical Anatase Titanium Nanotubes through a Protected Crystallization Strategy for Enhanced Photocatalytic Performance.

Helical structure in catalysts has attracted attention and been recently investigated for various catalytic reactions. However, helical transition metal oxides suffer from uncontrollable crystallization processes at high temperatures when being transformed from an amorphous phase into a crystalline structure. Herein, we report a helical anatase TiO2 nanotube for the first time, which has been prepared using a protected crystallization strategy in the confined space of silica. A single chirality of helical TiO2 has been used to track the ordering of the twisted structure. The twisted structure in helical anatase TiO2 nanotube is maintained after a vigorous crystallization process. Helical anatase TiO2 nanotubes possess more accessible active sites and abundant defects of oxygen vacancy and Ti3+ species owing to the twisted structure. The obtained helical anatase TiO2 nanotube exhibits superior photocatalytic activity for hydrogen production without adding any co-catalysts. This work provides new insights into the role of helical structure in transition metal-based catalysts.

Two-dimensional hollow carbon skeleton decorated with ultrafine Co3O4 nanoparticles for enhanced lithium storage.

Transition metal oxides have shown high theoretical capacities as anode materials and have been considered as high potential materials to substitute graphite for composing new generations of lithium-ion batteries (LIBs). However, the considerable volume changes of transition metal oxide materials during practical processes have limited their applications. Herein, we report a simple approach to construct a two-dimensional (2D) hollow carbon skeleton decorated with ultrafine Co3O4 nanoparticles (Co3O4/C). This composite is derived from a leaf-like zeolitic imidazolate framework-L (ZIF-L (Co)) via etching coordination using tannic acid (TA). The Co3O4/C has a unique structure consisting of 2D carbon skeleton, ultrafine Co3O4 nanoparticle, and open channel, which can accelerate electron transport, alleviate volume change, and facilitate ion diffusion. Benefiting from these features, the LIBs assembled using Co3O4/C as anode material exhibits superior reversible cycle performance and impressive rate property. This study provides an efficient strategy for implementing transition metal oxide-based composites for energy storage applications.

A helical polypyrrole nanotube interwoven zeolitic imidazolate framework and its derivative as an oxygen electrocatalyst.

A helical polypyrrole nanotube interwoven zeolitic imidazolate framework (ZIF) has been prepared for the first time. After pyrolysis, the helical carbon could act as highly active sites, while the 3D-connected nanoarchitecture contributed to fast charge transfer. The derived carbon material exhibits high activity for the ORR and good performance for a Zn-air battery.

The Role of Surface Curvature in Electrocatalysts.

Excessive consumption of fossil fuels has caused unavoidable environmental problems. The development of renewable and clean alternatives is essential for the sustainable and green development of human society. Electrocatalysts are most important parts in these energy-related devices. Recently, scientists found that the surface curvature of electrocatalysts could play an important role for the improvement of catalytic performance and the optimization of intrinsic catalytic activity during electrocatalytic process. The role of surface curvature in electrocatalysts is still under investigating. In this minireview, we summarized the latest progress of electrocatalysts with different surface curvatures and their applications in energy-related applications. This review mainly involves the strategies for preparation of electrocatalysts with different surface curvatures, three typical electrocatalysts with different surface curvatures (curled surface, onion-like structure, and spiral structure), and the potential mechanisms that surface curvature in electrocatalysts affects activities.

On the completeness of three-dimensional electron diffraction data for structural analysis of metal-organic frameworks.

Three-dimensional electron diffraction (3DED) has been proven as an effective and accurate method for structure determination of nano-sized crystals. In the past decade, the crystal structures of various new complex metal-organic frameworks (MOFs) have been revealed by 3DED, which has been the key to understand their properties. However, due to the design of transmission electron microscopes (TEMs), one drawback of 3DED experiments is the limited tilt range of goniometers, which often leads to incomplete 3DED data, particularly when the crystal symmetry is low. This drawback can be overcome by high throughput data collection using continuous rotation electron diffraction (cRED), where data from a large number of crystals can be collected and merged. Here, we investigate the effects of improving completeness on structural analysis of MOFs. We use ZIF-EC1, a zeolitic imidazolate framework (ZIF), as an example. ZIF-EC1 crystallizes in a monoclinic system with a plate-like morphology. cRED data of ZIF-EC1 with different completeness and resolution were analyzed. The data completeness increased to 92.0% by merging ten datasets. Although the structures could be solved from individual datasets with a completeness as low as 44.5% and refined to a high precision (better than 0.04 Å), we demonstrate that a high data completeness could improve the structural model, especially on the electrostatic potential map. We further discuss the strategy adopted during data merging. We also show that ZIF-EC1 doped with cobalt can act as an efficient electrocatalyst for oxygen reduction reactions.

Space-confined construction of two-dimensional nitrogen-doped carbon with encapsulated bimetallic nanoparticles as oxygen electrocatalysts.

A space-confined strategy has been used to control the pyrolysis of two-dimensional (2D) NiCo-MOF@ZIF-L(Zn). A thin SiO2 layer as a confined space could avoid the destruction of the 2D morphology during pyrolysis and expose more active sites. The obtained NiCo-NC material exhibits high ORR and Zn-air battery performance.

High-Throughput Electron Diffraction Reveals a Hidden Novel Metal-Organic Framework for Electrocatalysis.

Metal-organic frameworks (MOFs) are known for their versatile combination of inorganic building units and organic linkers, which offers immense opportunities in a wide range of applications. However, many MOFs are typically synthesized as multiphasic polycrystalline powders, which are challenging for studies by X-ray diffraction. Therefore, developing new structural characterization techniques is highly desired in order to accelerate discoveries of new materials. Here, we report a high-throughput approach for structural analysis of MOF nano- and sub-microcrystals by three-dimensional electron diffraction (3DED). A new zeolitic-imidazolate framework (ZIF), denoted ZIF-EC1, was first discovered in a trace amount during the study of a known ZIF-CO3 -1 material by 3DED. The structures of both ZIFs were solved and refined using 3DED data. ZIF-EC1 has a dense 3D framework structure, which is built by linking mono- and bi-nuclear Zn clusters and 2-methylimidazolates (mIm- ). With a composition of Zn3 (mIm)5 (OH), ZIF-EC1 exhibits high N and Zn densities. We show that the N-doped carbon material derived from ZIF-EC1 is a promising electrocatalyst for oxygen reduction reaction (ORR). The discovery of this new MOF and its conversion to an efficient electrocatalyst highlights the power of 3DED in developing new materials and their applications.

Highly Curved Nanostructure-Coated Co, N-Doped Carbon Materials for Oxygen Electrocatalysis.

Nitrogen-doped graphene could catalyze the electrochemical reduction and evolution of oxygen, but unfortunately suffers from sluggish catalytic kinetics. Herein, for the first time, we report an onion-like carbon coated Co, N-doped carbon (OLC/Co-N-C) material, which possesses multilayers of highly curved nanostructures that form mesoporous architectures. These unique nanospheres are produced when surfactant micelles are introduced to synthesis precursors. Owing to the combined electronic effect and nanostructuring effect, our OLC/Co-N-C materials exhibit high bifunctional oxygen reduction/evolution reaction (ORR/OER) activity, showing a promising application in rechargeable Zn-air batteries. Experimental results are rationalized by theoretical calculations, showing that the curvature of graphitic carbon plays a vital role in promoting activities of meta-carbon atoms near graphitic N and ortho/meta carbon atoms close to pyridinic N.

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

Porphyrin-based frameworks, as specific kinds of metal-organic frameworks (MOFs) and covalent organic frameworks (COFs), have been widely used in energy-related conversion processes, including the oxygen reduction reaction (ORR), oxygen evolution reaction (OER) and CO2 reduction reaction (CO2RR), and also in energy-related storage technologies such as rechargeable Zn-air batteries. This review starts by summarizing typical crystal structures, molecular building blocks, and common synthetic procedures of various porphyrin-based frameworks used in energy-related technologies. Then, a brief introduction is provided and representative applications of porphyrin-based frameworks in ORR, OER, Zn-air batteries, and CO2RR are discussed. The performance comparison of these porphyrin-based frameworks in each field is also summarized and discussed, which pinpoints a clear structure-activity relationship. In addition to utilizing highly active porphyrin units for catalytic conversions, regulating the porous structures of porphyrin-based frameworks will enhance mass transfer and growing porphyrin-based frameworks on conductive supports will accelerate electron transfer, which will result in the improvement of the electrocatalytic performance. This review is therefore valuable for the rational design of more efficient porphyrin-based framework catalytic systems in energy-related conversion and storage technologies.

Metal-Organic-Framework-Supported Molecular Electrocatalysis for the Oxygen Reduction Reaction.

Synthesizing molecule@support hybrids is appealing to improve molecular electrocatalysis. We report herein metal-organic framework (MOF)-supported Co porphyrins for the oxygen reduction reaction (ORR) with improved activity and selectivity. Co porphyrins can be grafted on MOF surfaces through ligand exchange. A variety of porphyrin@MOF hybrids were made using this method. Grafted Co porphyrins showed boosted ORR activity with large (>70 mV) anodic shift of the half-wave potential compared to ungrafted porphyrins. By using active MOFs for peroxide reduction, the number of electrons transferred per O2 increased from 2.65 to 3.70, showing significantly improved selectivity for the 4e ORR. It is demonstrated that H2 O2 generated from O2 reduction at Co porphyrins is further reduced at MOF surfaces, leading to improved 4e ORR. As a practical demonstration, these hybrids were used as air electrode catalysts in Zn-air batteries, which exhibited equal performance to that with Pt-based materials.