Embedding Ultrafine Metal Oxide Nanoparticles in Monolayered Metal-Organic Framework Nanosheets Enables Efficient Electrocatalytic Oxygen Evolution

Self-Assembled Conjugated Coordination Polymer Nanorings: Role of Morphology and Redox Sites for the Alkaline Electrocatalytic Oxygen Evolution Reaction

Electrocatalytic water splitting provides a sustainable method for storing intermittent energies, such as solar energy and wind, in the form of hydrogen fuel. However, the oxygen evolution reaction (OER), constituting the other half-cell reaction, is often considered the bottleneck in overall water splitting due to its slow kinetics. Therefore, it is crucial to develop efficient, cost-effective, and robust OER catalysts to enhance the water-splitting process. Transition-metal-based coordination polymers (CPs) serve as promising electrocatalysts due to their diverse chemical architectures paired with redox-active metal centers. Despite their potential, the rational use of CPs has faced obstacles including a lack of insights into their catalytic mechanisms, low conductivity, and morphology issues. Consequently, achieving success in this field requires the rational design of ligands and topological networks with the desired electronic structure. This study delves into the design and synthesis of three novel conjugated coordination polymers (CCPs) by leveraging the full conjugation of terpyridine-attached flexible tetraphenylethylene units as electron-rich linkers with various redox-active metal centers [Co(II), Ni(II), and Zn(II)]. The self-assembly process is tuned for each CCP, resulting in two distinct morphologies: nanosheets and nanorings. The electrocatalytic OER performance efficiency is then correlated with factors such as the nanostructure morphology and redox-active metal centers in alkaline electrolytes. Notably, among the three morphologies studied, nanorings for each CCP exhibit a superior OER activity. Co(II)-integrated CCPs demonstrate a higher activity between the redox-active metal centers. Specifically, the Co(II) nanoring morphology displays exceptional catalytic activity for OER, with a lower overpotential of 347 mV at a current density of 10 mA cm-2 and small Tafel slopes of 115 mV dec-1. The long-term durability is demonstrated for at least 24 h at 1.57 V vs RHE during water splitting. This is presumably the first proof that links the importance of nanostructure morphologies to redox-active metal centers in improving the OER activity, and it may have implications for other transdisciplinary energy-related applications.

Metal-Organic Framework (MOF)-Based Clean Energy Conversion: Recent Advances in Unlocking its Underlying Mechanisms

Carbon neutrality is an important goal for humanity . As an eco-friendly technology, electrocatalytic clean energy conversion technology has emerged in the 21st century. Currently, metal-organic framework (MOF)-based electrocatalysis, including oxygen reduction reaction (ORR), oxygen evolution reaction (OER), hydrogen evolution reaction (HER), hydrogen oxidation reaction (HOR), carbon dioxide reduction reaction (CO2RR), nitrogen reduction reaction (NRR), are the mainstream energy catalytic reactions, which are driven by electrocatalysis. In this paper, the current advanced characterizations for the analyses of MOF-based electrocatalytic energy reactions have been described in details, such as density function theory (DFT), machine learning, operando/in situ characterization, which provide in-depth analyses of the reaction mechanisms related to the above reactions reported in the past years. The practical applications that have been developed for some of the responses that are of application values, such as fuel cells, metal-air batteries, and water splitting have also been demonstrated. This paper aims to maximize the potential of MOF-based electrocatalysts in the field of energy catalysis, and to shed light on the development of current intense energy situations.

Rational Design of Organic Electrocatalysts for Hydrogen and Oxygen Electrocatalytic Applications

Efficient electrocatalysts are pivotal for advancing green energy conversion technologies. Organic electrocatalysts, as cost-effective alternatives to noble-metal benchmarks, have garnered attention. However, the understanding of the relationships between their properties and electrocatalytic activities remains ambiguous. Plenty of research articles regarding low-cost organic electrocatalysts started to gain momentum in 2010 and have been flourishing recently though, a review article for both entry-level and experienced researchers in this field is still lacking. This review underscores the urgent need to elucidate the structure-activity relationship and design suitable electrode structures, leveraging the unique features of organic electrocatalysts like controllability and compatibility for real-world applications. Organic electrocatalysts are classified into four groups: small molecules, oligomers, polymers, and frameworks, with specific structural and physicochemical properties serving as activity indicators. To unlock the full potential of organic electrocatalysts, five strategies are discussed: integrated structures, surface property modulation, membrane technologies, electrolyte affinity regulation, and addition of anticorrosion species, all aimed at enhancing charge efficiency, mass transfer, and long-term stability during electrocatalytic reactions. The review offers a comprehensive overview of the current state of organic electrocatalysts and their practical applications, bridging the understanding gap and paving the way for future developments of more efficient green energy conversion technologies.

Heterostructured Bimetallic MOF-on-MOF Architectures for Efficient Oxygen Evolution Reaction

Electron modulation presents a captivating approach to fabricate efficient electrocatalysts for the oxygen evolution reaction (OER), yet it remains a challenging undertaking. In this study, an effective strategy is proposed to regulate the electronic structure of metal-organic frameworks (MOFs) by the construction of MOF-on-MOF heterogeneous architectures. As a representative heterogeneous architectures, MOF-74 on MOF-274 hybrids are in situ prepared on 3D metal substrates (NiFe alloy foam (NFF)) via a two-step self-assembly method, resulting in MOF-(74 + 274)@NFF. Through a combination of spectroscopic and theory calculation, the successful modulation of the electronic property of MOF-(74 + 274)@NFF is unveiled. This modulation arises from the phase conjugation of the two MOFs and the synergistic effect of the multimetallic centers (Ni and Fe). Consequently, MOF-(74 + 274)@NFF exhibits excellent OER activity, displaying ultralow overpotentials of 198 and 223 mV at a current density of 10 mA cm-2 in the 1.0 and 0.1 M KOH solutions, respectively. This work paves the way for manipulating the electronic structure of electrocatalysts to enhance their catalytic activity.

Component regulation on ternary FeCoNi nano-bundles as efficient electrocatalysts for driving water oxidation

Metal organic frameworks (MOFs) are considered as promising electrocatalytic materials due to their tunable porosity, functional organic ligands, and large specific surface area for oxygen evolution reaction (OER). Recently, most reported electrocatalysts focus on the establishing heterogeneous structures by thermal treatments to improve OER performance. However, the thermal treatments are accompanied by the complex synthetic process and destruction of the MOFs structure. Therefore, improving the catalytic performance of pristine MOFs remains a challenge. Here, a series of trimetallic MaMbMc-MOFs (M represents metal element) were synthesized by one-pot method. Modulating the Co/Ni ratio not only adjusts the morphology of FeCoNi-MOFs, but also effectively optimizes the electronic structure. The composition-optimized FeCo0.5Ni2.5-MOF nano-bundles (FeCo0.5Ni2.5-NBs) only required a low overpotential of 273 mV to achieve the current density of 10 mA cm-2 in alkaline solution, with a Tafel slope of 51.1 mV dec-1, lower than other FeCoNi-MOFs and commercial RuO2 catalyst. The two-electrode couple FeCo0.5Ni2.5-NBs || Pt/C achieved the cell voltage of 1.55 V, delivering current density of 10 mA cm-2 for overall water splitting.

Nano-Scale Engineering of Heterojunction for Alkaline Water Electrolysis

Alkaline water electrolysis is promising for low-cost and scalable hydrogen production. Renewable energy-driven alkaline water electrolysis requires highly effective electrocatalysts for the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER). However, the most active electrocatalysts show orders of magnitude lower performance in alkaline electrolytes than that in acidic ones. To improve such catalysts, heterojunction engineering has been exploited as the most efficient strategy to overcome the activity limitations of the single component in the catalyst. In this review, the basic knowledge of alkaline water electrolysis and the catalytic mechanisms of heterojunctions are introduced. In the HER mechanisms, the ensemble effect emphasizes the multi-sites of different components to accelerate the various intermedium reactions, while the electronic effect refers to the d-band center theory associated with the adsorption and desorption energies of the intermediate products and catalyst. For the OER with multi-electron transfer, a scaling relation was established: the free energy difference between HOO* and HO* is 3.2 eV, which can be overcome by electrocatalysts with heterojunctions. The development of electrocatalysts with heterojunctions are summarized. Typically, Ni(OH)2/Pt, Ni/NiN3 and MoP/MoS2 are HER electrocatalysts, while Ir/Co(OH)2, NiFe(OH)x/FeS and Co9S8/Ni3S2 are OER ones. Last but not the least, the trend of future research is discussed, from an industry perspective, in terms of decreasing the number of noble metals, achieving more stable heterojunctions for longer service, adopting new craft technologies such as 3D printing and exploring revolutionary alternate alkaline water electrolysis.

Metal-Organic-Framework-Based Nanoarrays for Oxygen Evolution Electrocatalysis

The development of highly active and stable electrode materials for the oxygen evolution reaction (OER) is essential for the widespread application of electrochemical energy conversion systems. In recent years, various metal-organic frameworks (MOFs) with self-supporting array structures have been extensively studied because of their high porosity, abundant metal sites, and flexible and adjustable structures. This review provides an overview of the recent progress in the design, preparation, and applications of MOF-based nanoarrays for the OER, beginning with the introduction of the architectural advantages of the nanoarrays and the characteristics of MOFs. Subsequently, the design principles of robust and efficient MOF-based nanoarrays as OER electrodes are highlighted. Furthermore, detailed discussions focus on the composition, structure, and performance of pristine MOF nanoarrays (MOFNAs) and MOF-based composite nanoarrays. On the one hand, the effects of the two components of MOFs and several modification methods are discussed in detail for MOFNAs. On the other hand, the review emphasizes the use of MOF-based composite nanoarrays composed of MOFs and other nanomaterials, such as oxides, hydroxides, oxyhydroxides, chalcogenides, MOFs, and metal nanoparticles, to guide the rational design of efficient OER electrodes. Finally, perspectives on current challenges, opportunities, and future directions in this research field are provided.

Rapid Formation of Epitaxial Oxygen Evolution Reaction Catalysts on Dendrites with High Catalytic Activity and Stability

Oxygen evolution reaction is an essential but kinetically sluggish step in many energy storage and conversion processes and therefore is in pursuit of highly efficient and stable catalysts. Although nanosized transition-metal-based oxides/hydroxides exhibit high catalytic activity toward the oxygen evolution reaction (OER), many of them suffer from low stability at an anode current density in industrial scale. Herein, by combining a rapid epitaxial formation method with dynamic bubble-templated electrodeposition, we successfully developed single crystalline NiFeCu oxide catalysts with a hierarchical porous structure. It was found that the structure can facilitate fast electron transportation for the catalysts and retard the diffusion of the O atoms to the inner metallic current collector. The hierarchical pores inherited from the hydrogen bubble templates built ideal channels for the massive and rapid release of oxygen bubbles. As a consequence, the NiFeCu oxides catalyzed the OER more efficiently and steadily than the commercial RuO2 catalyst at an anode current density in industrial scale (300 mA/cm2). This work, by resolving the durability concerns for nanosized oxides, offers a series of highly efficient and stable catalysts for OER and a structure building strategy to boost the catalytic activity and stability for nonconductive catalysts.

Preparation of Trimetallic-Organic Framework Film Electrodes via Secondary Growth for Efficient Oxygen Evolution Reaction

Metal-organic frameworks (MOFs) are promising electrocatalysts for clean energy conversion systems. However, developing MOF-based electrodes with high performance toward oxygen evolution reaction (OER) is still challenging. In this work, a series of MOF film electrodes derived from Ni-btz were prepared by employing the secondary growth strategy under solvothermal conditions. Fe and Co ions were also incorporated into the Ni-btz framework to produce a trimetallic coupling effect to obtain enhanced OER activity. The as-prepared FeCoNi-btz/NF exhibited not only good stability but also excellent OER performance under alkaline conditions. Furthermore, the possible intermediates including metal oxides and metal oxyhydroxides were confirmed by X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM).

Rare-Earth-Modified Metal-Organic Frameworks and Derivatives for Photo/Electrocatalysis

Metal-organic frameworks (MOFs) and their derivatives have attracted much attention in the field of photo/electrocatalysis owing to their ultrahigh porosity, tunable properties, and superior coordination ability. Regulating the valence electronic structure and coordination environment of MOFs is an effective way to enhance their intrinsic catalytic performance. Rare earth (RE) elements with 4f orbital occupancy provide an opportunity to evoke electron rearrangement, accelerate charged carrier transport, and synergize the surface adsorption of catalysts. Therefore, the integration of RE with MOFs makes it possible to optimize their electronic structure and coordination environment, resulting in enhanced catalytic performance. In this review, progress in current research on the use of RE-modified MOFs and their derivatives for photo/electrocatalysis is summarized and discussed. First, the theoretical advantages of RE in MOF modification are introduced, with a focus on the roles of 4f orbital occupancy and RE ion organic coordination ligands. Then, the application of RE-modified MOFs and their derivatives in photo/electrocatalysis is systematically discussed. Finally, research challenges, future opportunities, and prospects for RE-MOFs are also discussed.

Multifunctional Aryl Sulfonium Decavanadates: Tuning the Photochromic and Heterogeneous Oxidative Desulfurization Catalytic Properties Using Salicylaldehyde-type Functional Moieties on Counterions

Multifunctional materials based on polyoxovanadates (POVs) have rarely been reported. Herein, we used aryl sulfonium counterions (ASCIs) bearing a salicylaldehyde-type functionality to tune the properties of decavanadate ([V10O28]6-)-based hybrids for their application in photochromism and heterogeneous oxidative desulfurization (ODS) catalysis. The counterions FHPDS ((3-formyl-4-hydroxyphenyl)dimethylsulfonium), DFHPDS ((3,5-diformyl-4-hydroxyphenyl)dimethylsulfonium), and EFPDS ((4-ethoxy-3-formylphenyl)dimethylsulfonium) were clubbed with the decavanadate cluster to generate the hybrids (FHPDS)4[H2V10O28](H2O)4 (HY1), (DFHPDS)4[H2V10O28](H2O)3 (HY2), and (EFPDS)4[H2V10O28](H2O)6 (HY3). The photochromic properties of these hybrids were tested under 365 nm irradiation, which showed a color change from yellow to green. Different hybrids exhibited different photocoloration half-life (t1/2) values in the range of 0.77-28.38 min, suggesting the dependence of the photocoloration properties upon functional groups on the counterions. The hybrid HY2, having a 2,6-diformyl phenol moiety on the ASCI, exhibited an impressive t1/2 of 0.77 min. UP to 70% reversibility of photocoloration was achieved for the best photochromic hybrid HY2 in 48 h at 70 °C under an oxygen atmosphere. Theoretical and experimental data suggested that some of these aryl sulfonium POVs follow a different e--h+ stabilization mechanism than traditional sulfonium POM hybrids. Further, the salicylaldehyde-type ASCIs control the solubility of the decavanadate hybrids, which enables their application as heterogeneous catalysts for the selective oxidation of various sulfides. The nature of the substituents on the ASCIs also affected their catalytic activities; the counterion that facilitates the reversible V4+/V5+ switching enhances the catalytic ODS efficiency of the hybrids. Using HY2 as the catalyst, up to 99% conversion and 96% selectivity toward sulfones were achieved in dibenzothiophene (DBT) oxidation. The present study suggests a new promising approach for controlling POVs' photoresponsive and catalytic properties by using ASCIs bearing salicylaldehyde-type functional moieties.

One-Step Electrodeposition of a Mesoporous Ni/Co-Imidazole-Based Bimetal-Organic Framework on Pyramid-like NiSb with Abundant Coupling Interfaces as an Ultra-Stable Heterostructural Electrocatalyst for Water Splitting

The exploration of highly efficient metal-organic framework (MOF)-based electrocatalysts is a research topic of high significance owing to their potential applications in sustainable and clean energy production. Herein, a mesoporous MOF containing Ni and Co nodes along with 2-methylimidazole (Hmim) ligands has been directly grown on the surface of the pyramid-like NiSb through a convenient cathodic electrodeposition strategy and evaluated as the catalyst for water splitting catalysis. Tailoring catalytically active sites through porous well-arranging architecture and the coupled interface offers a catalyst with exquisite performance that displays ultra-low Tafel constant of 33 and 42 mV dec^-1 toward the hydrogen evolution reaction and oxygen evolution reaction, sequentially, and also enhanced durability at high current densities over 150 h in a 1 M KOH medium. The success of the synthesized NiCo-MOF@NiSb@GB electrode is explained by the intimate contact between the NiCo-MOF and NiSb with well-tailored phase interfaces, the positive coupling effect between Ni and Co metal centers in the MOF, and the porous structure with abundant active sites toward electrocatalysis. Importantly, the present work provides a new technical reference for the electrochemical synthesis of heterostructural MOFs as a promising candidate for energy-related applications.

Improving the intrinsic activity of ultrathin 2D-2D heterostructures by bridge-bonded Ni-O-Ti ligands for efficient oxygen evolution

The integration of ultrathin two-dimensional (2D) semiconductors with other conductive 2D materials to form hybrid electrocatalysts with abundant heterointerfaces can enhance the electrocatalytic activity by facilitating interfacial charge transfer. However, the hybrid electrocatalysts with weak interfacial bonding have limited effect on the electrocatalytic performance because the intrinsic activity of interfacial sites cannot be altered by weak interfacial interactions. As a proof-of-concept, we design ultrathin 2D-2D heterostructures with bridge-bonded Ni-O-Ti ligands based on single-layered Ti₃C₂T_xMXene and metal hydroxides, and further reveal the structure-activity correlation between interfacial bonding and electrocatalytic oxygen evolution reaction by combining theoretical and experimental studies. Density functional theory calculations reveal the modulation of the electronic structure of interfacial metal sites after the formation of bridged interfacial Ni-O-Ti bonding. Compared with the hydrogen-bond-linked heterostructure, the ultrathin 2D-2D heterostructure with bridge-bonded Ni-O-Ti ligands shows enhanced intrinsic activity and stability towards electrocatalytic oxygen evolution with a very low overpotential of 205 mV at 10 mA cm^-2and the long-term durability. This work provides a new understanding and approach for the design and development of 2D hybrid catalysts with highly efficient electrocatalytic activity.

2D Metal-Organic Frameworks as Competent Electrocatalysts for Water Splitting

Hydrogen, a clean and flexible energy carrier, can be efficiently produced by electrocatalytic water splitting. To accelerate the sluggish hydrogen evolution reaction and oxygen evolution reaction kinetics in the splitting process, highly active electrocatalysts are essential for lowering the energy barriers, thereby improving the efficiency of overall water splitting. Combining the distinctive advantages of metal-organic frameworks (MOFs) with the physicochemical properties of 2D materials such as large surface area, tunable structure, accessible active sites, and enhanced conductivity, 2D MOFs have attracted intensive attention in the field of electrocatalysis. Different strategies, such as improving the conductivities of MOFs, reducing the thicknesses of MOF nanosheets, and integrating MOFs with conductive particles or substrates, are developed to promote the catalytic performances of pristine MOFs. This review summarizes the recent advances of pristine 2D MOF-based electrocatalysts for water electrolysis. In particular, their intrinsic electrocatalytic properties are detailly analyzed to reveal important roles of inherent MOF active centers, or other in situ generated active phases from MOFs responsible for the catalytic reactions. Finally, the challenges and development prospects of pristine 2D MOFs for the future applications in overall water splitting are discussed.

Modulating crystal and electronic structure of NiFe-MOFs by inorganic acid for highly efficient electrochemical water oxidation

Seeking new methods to modulate the structure of metal-organic frameworks (MOFs) for diverse applications, particularly for water splitting, is intensively urgent but challenging. Herein, a simple hydrothermal method employing HCl as the modulator is developed to synthesize a series of NiFe-MOF-n/NF. The amount of HCl modulator not only changes the elemental composition and crystal structure but also modulates the electronic structure of NiFe-MOF-n/NF, thus improving intrinsic activity. Owing to the synergetic interactions between Ni and Fe atoms, free-standing feature, the optimized NiFe-MOF-2/NF yields excellent OER activity with overpotentials of 209 and 260 mV at 10 and 100 mA cm^-2, respectively, a small Tafel slope of 36.4 mV dec^-1 and excellent OER stability for 24 h at 100 mA cm^-2 in 1 M KOH. This demonstrates that NiFe-MOF-2/NF are in situ converted into metal oxide/oxyhydroxide after OER, thereby serving as the real active sites. This study offers a feasible way to fabricate low-cost, efficient MOF-based electrocatalysts.

Self-Reconstructed Metal-Organic Framework Heterojunction for Switchable Oxygen Evolution Reaction

Designing metal-organic framework (MOF)-based catalysts with superior oxygen evolution reaction (OER) activity and robust durability simultaneously is highly required yet very challenging due to the limited intrinsic activity and their elusive evolution under harsh OER conditions. Herein, a steady self-reconstructed MOF heterojunction is constructed via redox electrochemistry and topology-guided strategy. Thanks to the inhibiting effect from hydrogen bonds of Ni-BDC-1 (BDC=1,4-benzenedicarboxylic acid), the obatained MOF heterojunction shows greatly improved OER activity with low overpotential of 225 mV at 10 mA cm^-2 , relative to the totally reconstructed Ni-BDC-3 (332 mV). Density function theory calculations reveal that the formed built-in electric field in the MOF heterojunction remarkably optimizes the ad/desorption free energy of active Ni sites. Moreover, such MOF heterojunction shows superior durability attributed to the shielding effect of the surface-evolved NiOOH coating.

MOFs and MOF-Derived Materials for Antibacterial Application

Bacterial infections pose a serious threat to people's health. Efforts are being made to develop antibacterial agents that can inhibit bacterial growth, prevent biofilm formation, and kill bacteria. In recent years, materials based on metal organic frameworks (MOFs) have attracted significant attention for various antibacterial applications due to their high specific surface area, high enzyme-like activity, and continuous release of metal ions. This paper reviews the recent progress of MOFs as antibacterial agents, focusing on preparation methods, fundamental antibacterial mechanisms, and strategies to enhance their antibacterial effects. Finally, several prospects related to MOFs for antibacterial application are proposed, aiming to provide possible research directions in this field.

Metal-Organic Framework Integrating Ionic Framework and Bimetallic Coupling Effect for Highly Efficient Oxygen Evolution Reaction

Metal-organic frameworks (MOFs) are recognized as promising electrocatalysts for the oxygen evolution reaction (OER) because of their permanent porosity and rich architectural diversity; however, ionic MOFs enabling fast ions exchange during OER are rarely explored. Here, an ionic MOF (Ni-btz) constructed with an azolate ligand is selected, and continuous 3D bimetallic MOF (NiFe-btz) films deriving from high-degree intergrowth of microsized MOFs particles are fabricated. The as-prepared NiFe-btz/NF-OH electrode exhibits excellent OER performance with a low overpotential of 239 mV at 10 mA cm^-2 under alkaline condition. The OER charge transfer process and bimetallic coupling effect in ionic NiFe-btz are probed by density functional theory calculations and confirmed via X-ray photoelectron spectroscopy and in situ Raman measurements. The partial density of states of NiFe-btz indicates that the main contribution for electron density around the Fermi level is from Cl ions clarifying the profitable impact of ionic MOF framework. This work systematically demonstrates the relationship of electronic structure and OER activity in ionic, bimetallic MOFs and expands the scope of 3D MOF films for efficient OER.

A Theoretical Investigation of Cu+, Ni2+ and Co2+-exchanged Zeolites for Hydrogen Storage

This study investigates the H 2 adsorption on Cu + , Ni 2+ and Co 2+ -exchanged SSZ-13 (CHA) and SSZ-39 (AEI) using periodic DFT computations. Most stable Cu + position was found to be the 6-membered-ring window for both zeolites. Similarly, for investigated Ni 2+ and Co 2+ loadings on 6-membered-ring windows, the third nearest neighbor Al positions, i.e., Al-O-Si-O-Si-O-Al coordination, was found to be the most stable position. H 2 adsorption was investigated for all the Cu + , Ni 2+ and Co 2+ centers. AEI and CHA resulted in similar H 2 -Cu interactions for the Al and B substituted structures. H 2 adsorption on Cu + located in the 8-membered-ring gave the highest adsorption energy for both frameworks. Replacing Al with B in the framework increased the electron back donation from Cu + (3d) orbitals to H 2 antibonding orbital (s H2 * ). The H 2 adsorption energies on the Ni 2+ and Co 2+ -exchanged zeolites were found to be between -15 and -44 kJ/mol. Higher energy values were observed on the AEI framework, especially when two Al atoms have the Al-O-Si-O-Al configuration. Lesser interaction of the d-orbitals in the case of the Co 2+ and Ni 2+ cations resulted in heat of H 2 adsorption close to optimum values required for H 2 storage on porous materials.

Oxygen-Plasma-Induced Hetero-Interface NiFe2O4/NiMoO4 Catalyst for Enhanced Electrochemical Oxygen Evolution

The electrolysis of water to produce hydrogen is an effective method for solving the rapid consumption of fossil fuel resources and the problem of global warming. The key to its success is to design an oxygen evolution reaction (OER) electrocatalyst with efficient conversion and reliable stability. Interface engineering is one of the most effective approaches for adjusting local electronic configurations. Adding other metal elements is also an effective way to enrich active sites and improve catalytic activity. Herein, high-valence iron in a heterogeneous interface of NiFe₂O₄/NiMoO₄ composite was obtained through oxygen plasma to achieve excellent electrocatalytic activity and stability. In particular, 270 mV of overpotential is required to reach a current density of 50 mA cm⁻², and the overpotential required to reach 500 mA cm⁻² is only 309 mV. The electron transfer effect for high-valence iron was determined by X-ray photoelectron spectroscopy (XPS). The fast and irreversible reconstruction and the true active species in the catalytic process were identified by in situ Raman, ex situ XPS, and ex situ transmission electron microscopy (TEM) measurements. This work provides a feasible design guideline to modify electronic structures, promote a metal to an active oxidation state, and thus develop an electrocatalyst with enhanced OER performance.

Ultrathin Ce-doped La2O3nanofilm electrocatalysts for efficient oxygen evolution reactions

It is still highly desired to develop efficient, resource-abundant and inexpensive electrocatalysts to improve the sluggish kinetics of oxygen evolution reaction (OER) in electrochemical water splitting systems. In this work, the large-area ultrathin (2.52 nm thick) Ce-doped La₂O₃nanofilms were developed via a facile and reliable ionic layer epitaxy method with different Ce content. The ultrathin Ce-doped La₂O₃nanofilm with optimum composition of La_1.22Ce_0.78O₃exhibited an excellent OER performance with a very low overpotential of 221 mV at 10 mA cm^-2and a small Tafel slope of 33.7 mV dec^-1. A remarkable high mass activity of 6263.2 A g^-1was also obtained from ultrathin La_1.22Ce_0.78O₃nanofilm at the overpotential of 221 mV. Such a high mass activity was three orders of magnitude higher than state-of-the-art commercial IrO₂powders (3.8 A g^-1) and more than 30 times higher than La₂O₃nanofilm (196.7 A g^-1) without Ce doping at the same overpotential. This high mass activity was even significantly higher than other recently reported typical OER catalysts. The substantial OER performance gain by the Ce doping was attributed to the improved conductivity and electrochemical active surface areas of nanofilms as a result of favorable tuning on the charge transfer and electronic structures. This work provides a promising approach to develop high-performance two-dimensional (2D) electrocatalysts by effective heteroatom doping strategy.

Emerging 2D Materials for Electrocatalytic Applications: Synthesis, Multifaceted Nanostructures, and Catalytic Center Design

Currently, the development of advanced 2D nanomaterials has become an interdisciplinary subject with extensive studies due to their extraordinary physicochemical performances. Beyond graphene, the emerging 2D-material-derived electrocatalysts (2D-ECs) have aroused great attention as one of the best candidates for heterogeneous electrocatalysis. The tunable physicochemical compositions and characteristics of 2D-ECs enable rational structural engineering at the molecular/atomic levels to meet the requirements of different catalytic applications. Due to the lack of instructive and comprehensive reviews, here, the most recent advances in the nanostructure and catalytic center design and the corresponding structure-function relationships of emerging 2D-ECs are systematically summarized. First, the synthetic pathways and state-of-the-art strategies in the multifaceted structural engineering and catalytic center design of 2D-ECs to promote their electrocatalytic activities, such as size and thickness, phase and strain engineering, heterojunctions, heteroatom doping, and defect engineering, are emphasized. Then, the representative applications of 2D-ECs in electrocatalytic fields are depicted and summarized in detail. Finally, the current breakthroughs and primary challenges are highlighted and future directions to guide the perspectives for developing 2D-ECs as highly efficient electrocatalytic nanoplatforms are clarified. This review provides a comprehensive understanding to engineer 2D-ECs and may inspire many novel attempts and new catalytic applications across broad fields.

Boosted Electrocatalysis of Bimetallic Sulfide Particles Incorporated in Fe/Co-based Metal-Organic Framework Ultrathin Nanosheets toward Oxygen Evolution Reaction

The development of inexpensive, high-performance, and long-lasting electrocatalysts toward oxygen evolution reaction (OER) proves crucial to enhance the efficiency of water splitting to obtain clean and sustainable energy. Herein, Fe/Co-based...

Carbonaceous Oxygen Evolution Reaction Catalysts: From Defect and Doping-Induced Activity over Hybrid Compounds to Ordered Framework Structures

Oxygen evolution reaction (OER) is expected to be of great importance for the future energy conversion and storage in form of hydrogen by water electrolysis. Besides the traditional noble-metal or transition metal oxide-based catalysts, carbonaceous electrocatalysts are of great interest due to their huge structural and compositional variety and unrestricted abundance. This review provides a summary of recent advances in the field of carbon-based OER catalysts ranging from "pure" or unintentionally doped carbon allotropes over heteroatom-doped carbonaceous materials and carbon/transition metal compounds to metal oxide composites where the role of carbon is mainly assigned to be a conductive support. Furthermore, the review discusses the recent developments in the field of ordered carbon framework structures (metal organic framework and covalent organic framework structures) that potentially allow a rational design of heteroatom-doped 3D porous structures with defined composition and spatial arrangement of doping atoms to deepen the understanding on the OER mechanism on carbonaceous structures in the future. Besides introducing the structural and compositional origin of electrochemical activity, the review discusses the mechanism of the catalytic activity of carbonaceous materials, their stability under OER conditions, and potential synergistic effects in combination with metal (or metal oxide) co-catalysts.

Templating Synthesis of Metal-Organic Framework Nanofiber Aerogels and Their Derived Hollow Porous Carbon Nanofibers for Energy Storage and Conversion

A kind of metal-organic framework (MOF) aerogels are synthesized by the self-assembly of uniform and monodisperse MOF nanofibers. Such MOF nanofiber aerogels as carbon precursors can effectively avoid the aggregation of nanofibers during calcination, resulting in the formation of well-dispersed hollow porous carbon nanofibers (HPCNs). Moreover, HPCNs with well-dispersion are investigated as sulfur host materials for Li-S batteries and electrocatalysts for cathode oxygen reduction reaction (ORR). On the one hand, HPCNs act as hosts for the encapsulation of sulfur into their hierarchical micro- and mesopores as well as hollow nanostructures. The obtained sulfur cathode exhibits excellent electrochemical features, good cycling stability and high coulombic efficiency. On the other hand, HPCNs exhibit better electrocatalytic activity than aggregated counterparts for ORR. Furthermore, a highly active single atom electrocatalyst can be prepared by the carbonization of bimetallic MOF nanofiber aerogels. The results indicate that well-dispersed HPCNs show enhanced electrochemical properties in contrast to their aggregated counterparts, suggesting that the dispersion situation of nanomaterials significantly influence their final performance. The present concept of employing MOF nanofiber aerogels as precursors will provide a new strategy to the design of MOF-derived nanomaterials with well-dispersion for their applications in energy storage and conversion.

Facile water oxidation by dinuclear mixed-valence CoIII/CoII complexes: the role of coordinated water

Rational design of a catalyst using earth abundant transition metals that can facilitate the smooth O-O bond formation is crucial for developing efficient water oxidation catalysts. The coordination environment around the metal ion of the catalyst plays a pivotal role in this context. We have chosen dinuclear mixed-valence Co^IIICo^II complexes of the general formula of [Co^IIICo^II(LH₂)₂(X)(H₂O)] (X = OAc or Cl) which bear a coordinated water molecule in the primary coordination sphere. We anticipated that the water molecule in the primary sphere can take part in the proton coupled electron transfer (PCET) mechanism which can accelerate the facile formation of the O-O bond under strong alkaline conditions (1 M NaOH). To understand the role of the coordinated water molecule we have generated an analogous complex, [Co^IIICo^II(LH₂)₂(o-vanillin)] (o-vanillin = 2-hydroxy-3-methoxybenzaldehyde), without coordinated water. Interestingly, we have found that the water coordinated complexes show better oxygen evolution reaction (OER) activity and stability.

Phyto-inspired Cu/Bi oxide-based nanocomposites: synthesis, characterization, and energy relevant investigation

A modified and sustainable approach is reported in this research for the synthesis of a spherical-shaped CuO–Bi₂O₃ electrode material for electrochemical studies. Aqueous extract derived from the plant Amaranthus viridis L. (Amaranthaceae) (AVL) was used as a reducing agent for morphological control of the synthesis of CuO–Bi₂O₃ nanocomposites. The modified nanomaterial revealed an average crystal size of 49 ± 2 nm, which matches very well with scanning electron microscopy (SEM) findings. Furthermore, the synthesized material was characterized using Fourier-transform infrared spectroscopy, field emission SEM and energy-dispersive spectroscopy. The optical band gap energy of 3.45 eV was calculated using a Tauc plot. Finally, the bioorganic framework-derived CuO–Bi₂O₃ electrode was tested for energy generating and storage applications and the results revealed a capacitance of 389 F g⁻¹ by cyclic voltammetry, with a maximum energy density of 12 W h kg⁻¹ and power density of 5 kW kg⁻¹. Hydrogen evolution reaction and oxygen evolution reaction studies showed good potential of CuO–Bi₂O₃ as an electrocatalyst for water splitting, with maximum efficiency of the electrode up to 16.5 hours.

Spherical-shaped CuO–Bi₂O₃ electrode material and its electrochemical studies.

Pseudo-Periodically Coupling NiO Lattice with CeO Lattice in Ultrathin Heteronanowire Arrays for Efficient Water Oxidation

Transition metal oxides (TMOs) have been under the spotlight as promising precatalysts for electrochemical oxygen evolution reaction (OER) in alkaline media. However, the slow and incomplete self-reconstruction from TMOs to (oxy)hydroxides as well as the formed (oxy)hydroxides with unmodified electronic structure gives rise to the inferior OER performance to the noble metal oxide ones. Herein, a unique dual metal oxides lattice coupling strategy is proposed to fabricate carbon cloth-supported ultrathin nanowires arrays, which are composed of pseudo-periodically welded NiO with CeO₂ nanocrystals (NiO/CeO₂ NW@CC). When served as an OER precatalyst in 1.0 m KOH, the NiO/CeO₂ NW@CC shows an ultralow overpotential of 330 mV at 50 mA cm^-2 , along with an impressive cycle durability of more than 3 days even at 50 mA cm^-2 , surpassing CC-supported NiO and commercial IrO₂ catalysts. The combined experimental and theoretical investigations unveil that the atomic coupling of CeO₂ can not only appreciably trigger the generation of oxygen vacancies and expedite phase transformation of NiO into active NiOOH, but also in situ create a chemical bond with the formed NiOOH and enable the electron injection, thus effectively inhibiting the aggregation of the accessible NiOOH nanodomains and optimizing their reaction free energy towards oxygen-containing intermediates.

Design Engineering, Synthesis Protocols, and Energy Applications of MOF-Derived Electrocatalysts

The core reactions for fuel cells, rechargeable metal-air batteries, and hydrogen fuel production are the oxygen reduction reaction (ORR), oxygen evolution reaction (OER), and hydrogen evolution reaction (HER), which are heavily dependent on the efficiency of electrocatalysts. Enormous attempts have previously been devoted in non-noble electrocatalysts born out of metal-organic frameworks (MOFs) for ORR, OER, and HER applications, due to the following advantageous reasons: (i) The significant porosity eases the electrolyte diffusion; (ii) the supreme catalyst-electrolyte contact area enhances the diffusion efficiency; and (iii) the electronic conductivity can be extensively increased owing to the unique construction block subunits for MOFs-derived electrocatalysis. Herein, the recent progress of MOFs-derived electrocatalysts including synthesis protocols, design engineering, DFT calculations roles, and energy applications is discussed and reviewed. It can be concluded that the elevated ORR, OER, and HER performances are attributed to an advantageously well-designed high-porosity structure, significant surface area, and plentiful active centers. Furthermore, the perspectives of MOF-derived electrocatalysts for the ORR, OER, and HER are presented.

Lattice Engineering to Simultaneously Control the Defect/Stacking Structures of Layered Double Hydroxide Nanosheets to Optimize Their Energy Functionalities

An effective lattice engineering method to simultaneously control the defect structure and the porosity of layered double hydroxides (LDHs) was developed by adjusting the elastic deformation and chemical interactions of the nanosheets during the restacking process. The enlargement of the intercalant size and the lowering of the charge density were effective in increasing the content of oxygen vacancies and enhancing the porosity of the stacked nanosheets via layer thinning. The defect-rich Co-Al-LDH-NO₃^- nanohybrid with a small stacking number exhibited excellent performance as an oxygen evolution electrocatalyst and supercapacitor electrode with a large specific capacitance of ∼2230 F g^-1 at 1 A g^-1, which is the largest capacitance of carbon-free LDH-based electrodes reported to date. Combined with the results of density functional theory calculations, the observed excellent correlations between the overpotential/capacitance and the defect content/stacking number highlight the importance of defect/stacking structures in optimizing the energy functionalities. This was attributed to enhanced orbital interactions with water/hydroxide at an increased number of defect sites. The present cost-effective lattice engineering process can therefore provide an economically feasible methodology to explore high-performance electrocatalyst/electrode materials.

Recent progress in pristine MOF-based catalysts for electrochemical hydrogen evolution, oxygen evolution and oxygen reduction

Among various kinds of materials that have been investigated as electrocatalysts for the hydrogen evolution reaction (HER), oxygen evolution reaction (OER) and oxygen reduction reaction (ORR), metal-organic frameworks (MOFs) has emerged as a promising material for electrocatalyzing these vital processes owing to their structural merits that integrate advantages of both homogeneous and heterogeneous catalysts; however there is still big room for their improvement in terms of inferior activity and poor conductivity, as well as the ambiguity of real active sites. In this review, advanced strategies with the aim of solving the activity and conductivity problems are summarized as microstructure engineering and conductivity improvement, respectively. The structural evolution of some MOFs and their real active species has also been discussed. Finally, perspectives on the development of MOF materials for HER, OER and ORR electrocatalysis are provided.

A high-efficiency oxygen evolution electrode material of a carbon material containing a NiCo bimetal

The preparation of highly efficient, stable, and low-cost electrocatalysts for the oxygen evolution reaction (OER) and the hydrogen evolution reaction (HER) is still a challenge for the development of new energy systems. In this work, a NiCo bimetal loaded on porous carbon (NiCo-C/NF) grown on nickel foam (NF) was obtained via the pyrolysis of a NiCo bimetal MOF (NiCo-MOF/NF) under a nitrogen atmosphere at 500 °C. Compared with NiCo-MOF/NF, NiCo-C/NF had a larger specific surface and uniform mesoporous structure. As an electrocatalyst in the OER, this new type of electrode operated with better stability in an alkaline solution (1.0 mol L⁻¹ KOH), the overpotential when the current density reached 10 mA cm⁻² was only 260 mV, and the electrode also exhibited long-term durability in a stability test for 10 h without significant changes. The excellent activity and stability toward the OER can be attributed to the synergistic effect of the NiCo bimetal and the abundant active sites exposed after the carbonization of NiCo-MOF, which compensated for the defect of the insufficient conductivity of the material and promoted the evolution of oxygen in the catalytic process.

The preparation of highly efficient, stable, and low-cost electrocatalysts for the oxygen evolution reaction (OER) and the hydrogen evolution reaction (HER) is still a challenge for the development of new energy systems.

Donor-Acceptor Couples of Metal and Metal Oxides with Enriched Ni3+ Active Sites for Oxygen Evolution

Exploiting precious-metal-free and high-activity oxygen evolution reaction (OER) electrocatalysts has been in great demands toward many energy storage and conversion processes, for example, carbon dioxide reduction, metal-air batteries, and water splitting. In this study, the simple solid-state method is employed for coupling Ni (electron donors) with lower-Fermi-level MoO₂ or WO_x (electron acceptors) into donor-acceptor ensembles with well-designed interfaces as robust electrocatalysts for OER. The resulting Ni/MoO₂ and Ni/WO_x electrocatalysts exhibit smaller overpotentials of 287 and 333 mV at 10 mA cm^-2 as well as smaller Tafel slopes of 51 and 65 mV/dec, respectively, with respect to the single Ni, MoO₂, WO_x, and even the benchmark RuO₂ in 1 M KOH. Specially, on account of a higher Fermi level of Ni in comparison with MoO₂ and WO_x, their strong electronic interaction results in directional interfacial electron transfer and increases the hole density over Ni, dramatically enriching the population of high-valence Ni³⁺ active sites and decreasing the Fermi level of Ni. The existence of Ni³⁺ can strengthen the chemisorption of OH^-, and the downshift of the Ni Fermi level can significantly expedite migration of electrons toward the surface of catalysts during OER, thus synergistically boosting the OER catalytic performance. Furthermore, the inner Ni/MoO₂ and Ni/WO_x heterostructures and the electrochemically induced surface layers of oxides/hydroxides collectively boost the OER kinetics. This study highlights the importance of designing highly efficient OER electrocatalysts with high-valence active species (Ni³⁺) and better matched energy levels induced by the work function difference through interfacial engineering.

Controlled growth of ultrafine metal nanoparticles mediated by solid supports

As a unique class of nanomaterials with a high surface-area-to-volume ratio and narrow size distribution, ultrafine metal nanoparticles (UMNPs) have shown exciting properties in many applications, particularly in the field of catalysis. Growing UMNPs in situ on solid supports enables precise control of the UMNP size, and the supports can effectively prevent the aggregation of UMNPs and maintain their high catalytic activity. In this review, we summarize the recent research progress in controlled growth of UMNPs using various solid supports and their applications in catalysis.

Designing MOF Nanoarchitectures for Electrochemical Water Splitting

Electrochemical water splitting has attracted significant attention as a key pathway for the development of renewable energy systems. Fabricating efficient electrocatalysts for these processes is intensely desired to reduce their overpotentials and facilitate practical applications. Recently, metal-organic framework (MOF) nanoarchitectures featuring ultrahigh surface areas, tunable nanostructures, and excellent porosities have emerged as promising materials for the development of highly active catalysts for electrochemical water splitting. Herein, the most pivotal advances in recent research on engineering MOF nanoarchitectures for efficient electrochemical water splitting are presented. First, the design of catalytic centers for MOF-based/derived electrocatalysts is summarized and compared from the aspects of chemical composition optimization and structural functionalization at the atomic and molecular levels. Subsequently, the fast-growing breakthroughs in catalytic activities, identification of highly active sites, and fundamental mechanisms are thoroughly discussed. Finally, a comprehensive commentary on the current primary challenges and future perspectives in water splitting and its commercialization for hydrogen production is provided. Hereby, new insights into the synthetic principles and electrocatalysis for designing MOF nanoarchitectures for the practical utilization of water splitting are offered, thus further promoting their future prosperity for a wide range of applications.

Metal-Organic-Framework-Based Materials for Antimicrobial Applications

To address the serious threat of bacterial infection to public health, great efforts have been devoted to the development of antimicrobial agents for inhibiting bacterial growth, preventing biofilm formation, and sterilization. Very recently, metal-organic frameworks (MOFs) have emerged as promising materials for various antimicrobial applications owing to their different functions including the controlled/stimulated decomposition of components with bactericidal activity, strong interactions with bacterial membranes, and formation of photogenerated reactive oxygen species (ROS) as well as high loading and sustained releasing capacities for other antimicrobial materials. This review focuses on recent advances in the design, synthesis, and antimicrobial applications of MOF-based materials, which are classified by their roles as component-releasing (metal ions, ligands, or both), photocatalytic, and chelation antimicrobial agents as well as carriers or/and synergistic antimicrobial agents of other functional materials (antibiotics, enzymes, metals/metal oxides, carbon materials, etc.). The constituents, fundamental antimicrobial mechanisms, and evaluation of antimicrobial activities of these materials are highlighted to present the design principles of efficient MOF-based antimicrobial materials. The prospects and challenges in this research field are proposed.

Lattice Matching Growth of Conductive Hierarchical Porous MOF/LDH Heteronanotube Arrays for Highly Efficient Water Oxidation

The conjugation of metal-organic frameworks (MOFs) into different multicomponent materials to precisely construct aligned heterostructures is fascinating but elusive owing to the disparate interfacial energy and nucleation kinetics. Herein, a promising lattice-matching growth strategy is demonstrated for conductive MOF/layered double hydroxide (cMOF/LDH) heteronanotube arrays with highly ordered hierarchical porous structures enabling an ultraefficient oxygen evolution reaction (OER). CoNiFe-LDH nanowires are used as interior template to engineer an interface by inlaying cMOF and matching two crystal lattice systems, thus conducting a graft growth of cMOF/LDH heterostructures along the LDH nanowire. A class of hierarchical porous cMOF/LDH heteronanotube arrays is produced through continuously regulating the transformation degree. The synergistic effects of the cMOF and LDH components significantly promote the chemical and electronic structures of the heteronanotube arrays and their electroactive surface area. Optimized heteronanotube arrays exhibit extraordinary OER activity with ultralow overpotentials of 216 and 227 mV to deliver current densities of 50 and 100 mA cm^-2 with a small Tafel slope of 34.1 mV dec^-1 , ranking it among the best MOF and non-noble-metal-based catalysts for OER. The robust performance under high current density and vigorous gas bubble conditions enable such hierarchical MOF/LDH heteronanotube arrays as promising materials for practical water electrolysis.

Co2P nanoparticle/multi-doped porous carbon nanosheets for the oxygen evolution reaction

Co₂P hybridized with multi-doped carbon nanoleaves is obtained via direct carbonization of ZIF-L/PEI/PA and show good electro-catalytic performance in OER.

Evolution of metal organic frameworks as electrocatalysts for water oxidation

The development of metal organic framework based water oxidation catalysts is discussed here in connection with various design strategies.