Nitrogen-Rich Covalent Organic Polymer for Metal-Free Tandem Catalysis and Postmetalation-Actuated High-Performance Water Oxidation

Development of high-performing catalytic materials for selective and mild chemical transformations through adhering to the principles of sustainability remains a central focus in modern chemistry. Herein, we report the template-free assembly of a thermochemically robust covalent organic polymer (COP: 1) from 2,2'-bipyridine-5,5'-dicarbonyl dichloride and 2,4,6-tris(4-aminophenyl)triazine as [2 + 3] structural motifs. The two-dimensional (2D) layered architecture contains carboxamide functionality, delocalized π-cloud, and free pyridyl-N site-decked pores. Such trifunctionalization benefits this polymeric network exhibiting tandem alcohol oxidation-Knoevenagel condensation. In contrast to common metal-based catalysts, 1 represents a one of a kind metal-free alcohol oxidation reaction via extended π-cloud delocalization-mediated free radical pathway, as comprehensively supported from diverse control experiments. In addition to reasonable recyclability and broad substrate scope, the mild reaction condition underscores its applicability in benign synthesis of valuable product benzylidene malononitrile. Integration of 2,2'-bipyridyl units in this 2D COP favors anchoring non-noble metal ions to devise 1-M (M: Ni2+/ Co2+) that demonstrate outstanding electrochemical oxygen evolution reaction in alkaline media with high chronoamperometric stability. Electrochemical parameters of both 1-Co and 1-Ni outperform some benchmark, commercial, as well as a majority of contemporary OER catalysts. Specifically, the overpotential and Tafel slope (280 mV, 58 mV/dec) for 1-Ni is better than 1-Co (360 mV, 78 mV/dec) because of increased charge accumulation as well as a higher number of active sites compared to the former. In addition, the turnover frequency of 1-Ni is found to be 6 times higher than that of 1-Co and ranks among top-tier water oxidation catalysts. The results provide valuable insights in the field of metal-free tandem catalysis as well as promising electrochemical water splitting at the interface of task-specific functionality fuelling in polymeric organic networks.

MOFs-Based Materials with Confined Space: Opportunities and Challenges for Energy and Catalytic Conversion

Metal-Organic Frameworks (MOFs) are a very promising material in the fields of energy and catalysis due to their rich active sites, tunable pore size, structural adaptability, and high specific surface area. The concepts of "carbon peak" and "carbon neutrality" have opened up huge development opportunities in the fields of energy storage, energy conversion, and catalysis, and have made significant progress and breakthroughs. In recent years, people have shown great interest in the development of MOFs materials and their applications in the above research fields. This review introduces the design strategies and latest progress of MOFs are included based on their structures such as core-shell, yolk-shell, multi-shelled, sandwich structures, unique crystal surface exposures, and MOF-derived nanomaterials in detail. This work comprehensively and systematically reviews the applications of MOF-based materials in energy and catalysis and reviews the research progress of MOF materials for atmospheric water harvesting, seawater uranium extraction, and triboelectric nanogenerators. Finally, this review looks forward to the challenges and opportunities of controlling the synthesis of MOFs through low-cost, improved conductivity, high-temperature heat resistance, and integration with machine learning. This review provides useful references for promoting the application of MOFs-based materials in the aforementioned fields.

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.

Synthesis of three-dimensional (3D) hierarchically porous iron-nickel nanoparticles encapsulated in boron and nitrogen-codoped porous carbon nanosheets for accelerated water splitting

Designing two-dimensional (2D) porous carbon nanosheets is a promising strategy for enhancing the water-splitting activities of non-noble metal catalysts. In this study, we developed a novel method for synthesizing the novel three-dimensional (3D) hierarchically porous iron-nickel (FeNi) nanoparticles encapsulated in boron (B) and nitrogen (N)-codoped porous carbon nanosheets (denoted as FeNi@BNPCNS). Owing to the advantages of morphology and structure of B and N, 10.31 atom % of B/N active centers were successfully doped into the optimal FeNi@BNPCNS-800 nanosheets. FeNi@BNPCNS-800 exhibited better hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) electrocatalytic activities than control catalysts in an alkaline solution. However, the HER and OER electrocatalytic activities of FeNi@BNPCNS-800 were slightly lower than 20 wt% Pt/C and RuO2. The FeNi@BNPCNS-800||FeNi@BNPCNS-800 electrolyzer achieved 10 mA cm-2 at 1.514 V, which was 73 mV lower than that of 20 wt% Pt/C||RuO2 electrolyzer (1.587 V). The perfect 3D honeycomb-like architectures, abundant mesopores/defects, and abundant electrocatalytic active sites were attributed to the outstanding water-splitting performances of FeNi@BNPCNS-800 nanosheets. This study provides an efficient strategy for the large-scale, rapid, and low-cost fabrication of 2D porous carbon nanosheets without using any template, surfactant, or expensive raw material, thus presenting a simple approach to design advanced non-noble metal electrocatalysts for water splitting.

Synergistic modulation in a triphasic Ni5P4-Ni2P@Ni3S2 system manifests remarkable overall water splitting

The potential for water splitting electrocatalysts with high efficiency paves the way for a sustainable future in hydrogen energy. However, this task is challenging due to the sluggish kinetics of the oxygen evolution reaction (OER), which has a significant impact on the hydrogen evolution reaction (HER). Herein multi-heterointerface of Ni5P4-Ni2P@Ni3S2 was fabricated by a two-step synthesis procedure that consist the development of Ni5P4-Ni2P nanosheets over nickel foam followed by the electrodeposition of Ni3S2. The HR-TEM analysis shows that the Ni5P4-Ni2P@Ni3S2 nanosheets array provide numerous well-exposed diverse heterointerfaces. The electrochemical investigations conducted on the Ni5P4-Ni2P@Ni3S2 nanosheets for complete water splitting indicate that they possess an overpotential of 73 mV and 230 mV in HER and OER respectively, enabling them to generate a current density of 10 and 50 mA cm-2. The nanosheets also demonstrate Tafel slope values of 95 mV dec-1 and 83 mV dec-1 for HER and OER, respectively. The HER stability of the catalyst was conducted for 45 h using chronoamperometric technique under a current density of 20 mA cm-1, while the stability test for OER was carried out at current densities of 100 and 200 mA cm-1 for 100 h each. Furthermore, in the overall water splitting, the catalyst exhibits a cell voltage of 1.47 V@10 mA cm-2 and displayed a stability operation for 100 h at a current density of 150 mA cm-1.

Site-Memory-Triggered Reversible Acronym Encryption in a Nitrogen-Rich Pore-Partitioned MOF for Ultrasensitive Monitoring of Roxarsone and Dichloran over Multiple Platform

Stimuli-responsive emission color modulation in fluorescent metal-organic frameworks (MOFs) promises luminescence-ink-based security application, while task-specific functionality-engineered pores can aid fast-responsive, discriminative, and ultralow detection of harmful organo-aromatics in the aqueous phase. Considering practical applicability, a self-calibrated fluoro-switch between encrypted and decrypted states is best suited for antiforgery measures, whereas image-based monitoring of organo-toxins by repetitive and handy methods over multiple platforms endorses in-field sensory potential. Herein, we constructed a mixed-ligand based chemically stable and bilayered-pillar MOF from -NH2-hooked pyridyl linker and tricarboxylate ligand that embraces negatively charged [Cd3(μ2-OH)(COO)6] node and shows pore-space-partitioning by nitrogen-rich flanked organic struts. Owing to the presence of a self-calibrating triazolylamine moiety-grafted auxiliary linker, this anionic MOF delineates reversible and multicyclic fluoro-swapping between protonated-encrypted and deprotonated-decrypted domains in the alternative presence of acid and base. Such pH-triggered, site-specific luminescence variation is utilized to construct highly regenerative anticounterfeiting labels for vivid acronym encryption. The intense fluorescence signature of the material is further harnessed in extremely selective and quick responsive sensing of harmful feed additive roxarsone (ROX) and dichloran (DCNA) pesticide in highly recyclable fashion with significant quenching and nanomolar limits of detection (ROX: 52 ppb; DCNA: 26.8 ppb). Notably, the ultrasensitive fluoro-detection of both these organo-toxins is successfully demonstrated via a handy paper-strip method as well as on the vegetable surface for real-time monitoring. Comprehensive density functional theory studies validate the electron transfer mechanism through redistribution of molecular orbital energy levels by each of the targeted analytes in this electron-rich framework besides evidencing MOF-analyte supramolecular interactions.

Controllable fabrication of iron-nickel alloy embedded in nitrogen-doped carbon for oxygen evolution

Nickel-based electrocatalysts for the oxygen evolution reaction (OER) show the disadvantages of low activity and poor stability. In this paper, an FeNi alloy is wrapped by an amino-modified MOF-5-derived N-doped carbon layer to address these problems. Additionally, the improvement resulting from Fe doping of NiOOH catalysts is theoretically supported.

Coordination Unsaturation and Basic Site-Immobilized Nanochannel in a Chemorobust MOF for 3-Fold-Increased High-Temperature Selectivity and Fixation of CO2 under Mild Conditions with Nanomolar Recognition of Roxarsone

A multifaceted metal-organic framework (MOF) with task-specific site-engineered pores can promise high-temperature and moisture-tolerant capture and non-redox fixation of CO2 under mild conditions as well as ultrasensitive detection of carcinogenic contaminants in water. Herein, we report a pillar-bilayered MOF that holds a nanochannel with contrasting functionalities for both these sustainable applications with improved performance characteristics. The twofold entangled robust framework exhibits CO2 adsorption at elevated temperatures with considerable MOF-gas interaction. Interestingly, CO2 selectivity unveils nearly a 3-fold improvement upon the rise of temperature, affording a CO2/N2 value of 820 at 313 K, which outperforms many porous adsorbents. Additionally, breakthrough simulation establishes complete separation and attests the potential of this MOF in the separation of flue gas mixture. Importantly, minor CO2 loss during multiple capture-release cycles and under a relative humidity of 75% promise practical usability of the material. Density functional theory (DFT) not only portrays the atomistic level snapshots of temperature-triggered CO2 inclusion inside this microporous vessel alongside the role of diverse CO2-philic sites but also validates the basis of N2-phobicity of an azo-functionalized linker on such increased selectivity. The guest-free MOF further demonstrates non-redox and recyclable CO2 fixation with wide epoxide tolerance under solvent-free mild conditions and even works at atmospheric pressure and room temperature. The crucial roles of high-density acid-base sites in both adsorption and catalysis are supported by control experiments and by comparing the activity of an unfunctionalized MOF. The hydrolytic stability and strong luminescence signature benefit the framework in aqueous-phase selective and fast responsive detection of detrimental roxarsone (ROX) with high quenching (7.56 × 104 M-1) and very low sensitivity (68 nM). Apart from varying degrees of an energy-transfer mechanism, the fluorosensing of ROX is comprehensively supported by in-depth DFT studies that manifest alteration of MOF energy levels in the presence of organoarsenic compounds and depict MOF-analyte supramolecular interactions.

Controlled synthesis of ACo2O4 (A = Fe, Cu, Zn, Ni) as an environmentally friendly electrocatalyst for urea electrolysis

Water electrolysis is relatively an environmentally friendly hydrogen production technology, but due to the slow transfer of four electrons in the anodic oxidation reaction, it needs a theoretical voltage of up to 1.23 V. Therefore, in this experiment, a series of transition metal oxides, ACo₂O₄ (A = Fe, Cu, Zn, Ni), was synthesized on Ni foam current collectors by a hydrothermal and calcination method, and the material was applied in urea electrolysis to produce hydrogen. What is noteworthy is that the CuCo₂O₄ electrode has a unique flower-like nanoneedle structure, and has a larger electrochemical active area, more reactive active sites, and a faster charge transfer rate. In 1.0 M KOH and 0.5 M urea solution, CuCo₂O₄ provides a potential of only 1.268 V at a current density of 10 mA cm^-2 during the urea oxidation reaction (UOR), while in 1.0 M KOH solution, with the same current density, the oxygen evolution reaction (OER) is required to provide a potential of 1.53 V, indicating that the UOR can effectively replace the OER. Density functional theory calculations show that the CuCo₂O₄ material exhibits Gibbs free energy of the hydrogen closest to zero, thus promoting the electrochemistry performance of the electrode. In a cell composed of CuCo₂O₄//CuCo₂O₄, the current density of 10 mA cm^-2 can be achieved by providing a potential of only 1.509 V. This work offers a novel scheme for reducing energy consumption of the OER and improving catalytic performance of the UOR.

One-Step High-Temperature Electrodeposition of Fe-Based Films as Efficient Water Oxidation Catalysts

Electrolysis of water to produce hydrogen requires an efficient catalyst preferably made of cheap and abundant metal ions for the improved water oxidation reaction. An Fe-based film has been deposited in a single step by electrochemical deposition at temperatures higher than the room temperature. Until now, the electrodeposition of iron oxide has been carried out at 298 K or at lower temperatures under a controlled atmosphere to prohibit atmospheric oxidation of Fe²⁺ of the iron precursor. A metal inorganic complex, ferrocene, and non-aqueous electrolyte medium propylene carbonate have been used to achieve electrodeposition of iron oxide without the need of any inert or controlled atmosphere. At 298 K, the amorphous film was formed, whereas at 313 K and at higher temperatures, the hematite film was grown, as confirmed by X-ray diffraction. The transformation of iron of the ferrocene into a higher oxidation state under the experimental conditions used was further confirmed by X-ray photoelectron spectroscopy, ultraviolet-visible, and electron paramagnetic resonance spectroscopic methods. The films deposited at 313 K showed the best performance for water oxidation with remarkable long-term electrocatalytic stability and an impressive turnover frequency of 0.028 s^-1 which was 4.5 times higher than that of films deposited at 298 K (0.006 s^-1). The observed overpotential to achieve a current density of 10 mA cm^-2 was found to be 100 mV less for the film deposited at 313 K compared to room-temperature-derived films under similar experimental conditions. Furthermore, electrochemical impedance data revealed that films obtained at 313 K have the least charge transfer resistance (114 Ω) among all, supporting the most efficient electron transport in the film. To the best of our knowledge, this is the first-ever report where the crystalline iron-based film has been shown to be electrodeposited without any post-deposition additional treatment for alkaline oxygen evolution reaction application.

Design Strategy of Corrosion-Resistant Electrodes for Seawater Electrolysis

Electrocatalytic water splitting for hydrogen (H₂) production has attracted more and more attention in the context of energy shortages. The use of scarce pure water resources, such as electrolyte, not only increases the cost but also makes application difficult on a large scale. Compared to pure water electrolysis, seawater electrolysis is more competitive in terms of both resource acquisition and economic benefits; however, the complex ionic environment in seawater also brings great challenges to seawater electrolysis technology. Specifically, chloride oxidation-related corrosion and the deposition of insoluble solids on the surface of electrodes during seawater electrolysis make a significant difference to electrocatalytic performance. In response to this issue, design strategies have been proposed to improve the stability of electrodes. Herein, basic principles of seawater electrolysis are first discussed. Then, the design strategy for corrosion-resistant electrodes for seawater electrolysis is recommended. Finally, a development direction for seawater electrolysis in the industrialization process is proposed.

Multifunctional AuNPs@HRP@FeMOF immune scaffold with a fully automated saliva analyzer for oral cancer screening

Delayed diagnosis of cancer-causing death is a worldwide concern. General diagnosis methods are invasive, time-consuming, and operation complicated, which are not suitable for preliminary screening. To address these challenges, the sensing platform based on immune scaffold and fully automated saliva analyzer (FASA) was proposed for oral cancer screening for the first time by non-invasive detection of Cyfra21-1 in saliva. Through one-step synthesis method with unique covalent and electrostatic adsorption strategy, AuNPs@HRP@FeMOF immune scaffold features multiple functions including antibody carrier, catalytic activity, and signal amplification. Highly integrated FASA with the immune scaffold provides automatic testing to avoid false-positive results and reduce pretreatment time without any user intervention. Compared with the commercial analyzer, FASA has comparable performance for Cyfra21-1 detection with a detection range of 3.1-50.0 ng/mL and R² of 0.971, and superior features in full automation, high integration, time saving and low cost. Oral cancer patients could be distinguished accurately by the platform with an excellent correlation (R² of 0.904) and average RSD (5.578%) without sample dilution. The proposed platform provides an effective and promising tool for cancer screening in point-of-care applications, which can be further extended for biomarker detection in universal body fluids, disease screening, prognosis review and homecare monitoring.

Controlled synthesis of Crx-FeCo2P nanoarrays on nickel foam for overall urea splitting

Urea splitting is a highly promising technology for hydrogen production to cope with the fossil energy crisis, which requires the development of catalysts with high electrocatalytic activity. In this article, Cr_x-FeCo₂P/NF catalysts were synthesized by hydrothermal and low-temperature phosphorylation and used in the overall urea splitting process. Cr_0.15-FeCo₂P/NF and Cr_0.1-FeCo₂P/NF exhibited excellent urea oxidation reaction (UOR) activity (potential of 1.355 V at 100 mA cm^-2) and hydrogen evolution reaction (HER) activity (overpotential of 173 mV at 10 mA cm^-2) in 0.5 M urea solution containing 1 M KOH. In the assembled Cr_0.15-FeCo₂P/NF//Cr_0.1-FeCo₂P/NF electrolytic cell, only a small voltage of 1.50 V is needed to reach 10 mA cm^-2. Density functional theory (DFT) calculation results demonstrate that an appropriate amount of Cr doping accelerates the kinetic performance of hydrogen production as well as improving the metallic properties of the electrode.

Recent Progress of Advanced Conductive Metal-Organic Frameworks: Precise Synthesis, Electrochemical Energy Storage Applications, and Future Challenges

Metal-organic frameworks (MOFs) with diverse composition, tunable structure, and unique physicochemical properties have emerged as promising materials in various fields. The tunable pore structure, abundant active sites, and ultrahigh specific surface area can facilitate mass transport and provide outstanding capacity, making MOFs an ideal active material for electrochemical energy storage and conversion. However, the poor electrical conductivity of pristine MOFs severely limits their applications in electrochemistry. Developing conductive MOFs has proved to be an effective solution to this problem. This review focuses on the design and synthesis of conductive MOF composites with judiciously chosen conducting materials, pristine MOFs, and assembly methods, as well as the preparation of intrinsically conductive MOFs based on building 2D π-conjugated structures, introducing mixed-valence metal ions/redox-active ligands, designing π-π stacked pathways, and constructing infinite metal-sulfur chains (-M-S-)_∞ . Furthermore, recent progress and challenges of conductive MOFs for energy storage and conversion (supercapacitors, Li-ion batteries, Li-S batteries, and electrochemical water splitting) are summarized.

Coordination Effect-Promoted Durable Ni(OH)2 for Energy-Saving Hydrogen Evolution from Water/Methanol Co-Electrocatalysis

Electrocatalytic water splitting is a viable technique for generating hydrogen but is precluded from the sluggish kinetics of oxygen evolution reactions (OER). Small molecule oxidation reactions with lower working potentials, such as methanol oxidation reactions, are good alternatives to OER with faster kinetics. However, the typically employed Ni-based electrocatalysts have poor activity and stability. Herein, a novel three-dimensional (3D)-networking Mo-doped Ni(OH)₂ with ultralow Ni-Ni coordination is synthesized, which exhibits a high MOR activity of 100 mA cm^-2 at 1.39 V, delivering 28 mV dec^-1 for the Tafel slope. Meanwhile, hydrogen evolution with value-added formate co-generation is boosted with a current density of more than 500 mA cm^-2 at a cell voltage of 2.00 V for 50 h, showing excellent stability in an industrial alkaline concentration (6 M KOH). Mechanistic studies based on density functional theory and X-ray absorption spectroscopy showed that the improved performance is mainly attributed to the ultralow Ni-Ni coordination, 3D-networking structures and Mo dopants, which improve the catalytic activity, increase the active site density and strengthen the Ni(OH)₂ 3D-networking structures, respectively. This study paves a new way for designing electrocatalysts with enhanced activity and durability for industrial energy-saving hydrogen production.

Metal-organic framework derived bimetal oxide CuCoO2 as efficient electrocatalyst for the oxygen evolution reaction

Metal-organic framework (MOF) materials with tunable porous morphology, controlled crystalline structure, various compositions, and high specific surface area are widely used as precursors to synthesize electrocatalysts for water splitting, which is beneficial for improving their oxygen evolution reaction (OER) performance. Using ZIF-67 as a Co source and Cu-BTC as a Cu source, hexagonal MOF-derived CuCoO₂ (MOF-CCO) nanocrystals with the size of ∼288 nm were prepared through a one-step solvothermal method. The influence of the content of the precursor solvents (absolute ethanol and deionized water), reaction temperature, mass ratio of reactants, NaOH addition, and reactant concentration of precursors on the structure and morphology of the products was investigated. The optimal CuCoO₂ nanocrystals (MOF-CCO1) around 288 nm present the highest OER activity, such as a low overpotential of 364.7 mV at 10 mA cm^-2, a small Tafel slope of 64.1 mV dec^-1, and attractive durability in 1.0 M KOH solution. The XPS results showed that the higher catalytic efficiency of MOF-CCO1 nanocrystals could be due to the oxygen vacancies caused by lattice oxygen loss, the increase of OH^- content on the surface, and the synergistic effect of Cu²⁺/Cu⁺ and Co²⁺/Co³⁺ redox pairs. Finally, a possible OER mechanism for MOF-CCO nanocrystals of water splitting was proposed. This study provides a new approach for the preparation of delafossite nanomaterials and for the improvement of their OER performances.

Spray drying-assisted construction of hierarchically porous ZIF-8 for controlled release of doxorubicin

The intrinsic properties and structure of carrier materials, as well as the drug-loading method, are crucial to the fabrication of high-performance controlled drug release systems. Metal-organic frameworks (MOFs) have attracted great attention in drug delivery due to their rich variety and very precisely designable structures, but their inherent small pores limit their application towards large-size drug molecules. Herein, we report a facile and efficient approach for the construction of hierarchically porous ZIF-8 (HP-ZIF-8) by spray drying. The homogeneously distributed mesopores, which result from the interspaces in the closely arranged nanosized ZIF-8 (N-ZIF-8), can be tuned by adjusting the primary particle size. More importantly, a drug (doxorubicin (DOX), for example) can be simultaneously encapsulated during the fabrication process of HP-ZIF-8, achieving a high loading rate of 79% and an encapsulation efficiency of 79%. Furthermore, we demonstrate that the obtained DOX@HP-ZIF-8 is a pH-responsive system and the release can also be controlled by the mesopore size. Although HP-ZIF-8 shows obvious advantages in drug loading and release performance compared with N-ZIF-8 loaded with DOX by the same solvent adsorption approach, DOX@HP-ZIF-8 displays significantly increased loading capacity (more than 3 times) and the slowest release rate due to its drug-loading method. Their therapeutic efficacy on HeLa cells has also been proved. These findings have important implications for the construction of HP-MOFs as drug carriers and will also present a new platform for controlled drug release and biomedical applications.

A Superior and Stable Electrocatalytic Oxygen Evolution Reaction by One-Dimensional FeCoP Colloidal Nanostructures

Transition metal phosphides (TMPs) are expected to be excellent electrocatalysts for oxygen evolution reaction (OER) because of their high stability, highly conducting metalloid nature, highly abundant constituting elements, and the ability to act as a precatalyst due to in situ surface-formed oxy-hydroxide species. Herein, a "one-pot" colloidal approach has been used to develop a rod-shaped one-dimensional non-noble metal FeCoP electrocatalyst, which exhibits an excellent OER activity with an exceptionally high current density of 950 mA cm^-2, a turnover frequency value of 7.43 s^-1, and a low Tafel slope value of 54 mV dec^-1. The FeCoP electrocatalyst affords OER ultralow overpotentials of 230 and 260 mV at current densities of 50 and 100 mA cm^-2, respectively, in 1.0 M KOH, and demonstrates a superior catalytic stability of 10,000 cycles and durability up to 60 h at 50 mA cm^-2. An insight into the superior and stable electrocatalytic OER performance by the FeCoP nanorods is obtained by extensive X-ray photoelectron spectroscopy, X-ray diffraction, Raman and infrared spectroscopy, and cyclic voltammetry analyses for a mechanistic study. This reveals that a high number of electrocatalytically active sites enhance the oxygen evolution and kinetics by offering metal ion sites for utilitarian in situ surface formation and adsorption of *O, *OH, and *OOH reactive species for OER catalysis.

In situ growth of hierarchical bimetal-organic frameworks on nickel-iron foam as robust electrodes for the electrocatalytic oxygen evolution reaction

Evidence shows that self-supported electrocatalysts are crucial role to solving environmental and energy issues. In this study, self-supported 2D metal-organic framework (MOF) nanosheets grown in situ on nickel-iron foam (NFF) were prepared by a one-step solvothermal process. The hierarchical nanostructure possesses a high specific surface area and abundant metal sites, which are beneficial for electrocatalytic reactions. In the electrocatalytic oxygen evolution reaction (OER), the optimal NiFe(20Ni)-MOF/NFF can drive current densities of 10, 50 and 100 mA cm^-2 at small overpotentials of 226, 277 and 294 mV, respectively. According to the characterization results, the OER performance is improved by the synergistic action of bimetals and the generation of hydroxides/oxyhydroxides. This work provides new insights into fabricating self-supported MOF-based electrodes for water splitting that are simple and highly efficient.

Rapid “self-healing” behavior induced by chloride anions to renew Fe-Ni(oxy)hydroxide surface for long-term alkaline seawater electrolysis

Due to the surface adsorption and interlayer insertion behavior of chloride anions, Fe-Ni(oxy)hydroxide catalytic surface is easily destroyed, making it difficult to be used for long-term seawater electrolysis. Here, we...

A Ce-MOF as an alkaline phosphatase mimic: Ce-OH2 sites in catalytic dephosphorylation

For the first time, a metal–organic framework (Ce-MOF) bearing a Ce–OH2–Ce motif was used to mimic the active sites of alkaline phosphatase.

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...

Amorphous-crystalline FeNi2S4@NiFe-LDH nanograsses by molten salt as an industrially promising electrocatalyst for oxygen evolution

Inexpensive and accessible NiFe-based oxygen evolution reaction (OER) electrocatalyst are limited for practical industrial applications by its activity and stability under industrial conditions. Herein, FeNi2S4@NiFe-LDH heterostructure is constructed by molten...

One step synthesis of a bimetallic (Ni and Co) metal–organic framework for the efficient electrocatalytic oxidation of water and hydrazine

A series of metal–organic frameworks (MOFs) with varying Ni : Co ratios are synthesized by an easy one-step solvothermal method using trimesic acid as an organic linker.

Amorphous FeOOH nanoparticles decorated on defect-rich porous Ni MOF nanosheet based hierarchical architectures toward superior OER performance

Amorphous FeOOH nanoparticles decorated on defect-rich porous Ni MOF nanosheet based hierarchical architectures toward superior OER performance.

Ce-Substituted Spinel CuCo2O4 Quantum Dots with High Oxygen Vacancies and Greatly Improved Electrocatalytic Activity for Oxygen Evolution Reaction

Exploring effective electrocatalysts for oxygen evolution reaction (OER) is a crucial requirement of many energy storage and transformation systems, involving fuel cells, water electrolysis, and metal-air batteries. Transition-metal oxides (TMOs) have attracted much attention to OER catalysts because of their earth abundance, tunable electronic properties, and so forth. Defect engineering is a general and the most important strategy to tune the electronic structure and control size, and thus improve their intrinsic activities. Herein, OER performance on spinel CuCo₂O₄ was greatly enhanced through cation substitution and size reduction. Ce-substituted spinel CuCe_δCo_2-δO_x (δ = 0.45, 0.5 and 0.55) nanoparticles in the quantum dot scale (2-8 nm) were synthesized using a simple and facile phase-transfer coprecipitation strategy. The as-prepared samples were highly dispersed and have displayed a low overpotential of 294 mV at 10 mA·cm^-2 and a Tafel slope of 57.5 mV·dec^-1, which outperform commercial RuO₂ and the most high-performance analogous catalysts reported. The experimental and calculated results all confirm that Ce substitution with an appropriate content can produce rich oxygen vacancies, tune intermediate absorption, consequently lower the energy barrier of the determining step, and greatly enhance the OER activity of the catalysts. This work not only provides advanced OER catalysts but also opens a general avenue to understand the structure-activity relationship of pristine TMO catalysts deeply in the quantum dot scale and the rational design of more efficient OER catalysts.

Amorphous nickel tungstate films prepared by SILAR method for electrocatalytic oxygen evolution reaction

Development of electrocatalyst using facile way from non-noble metal compounds with high efficiency for effective water electrolysis is highly demanding for production of hydrogen energy. Nickel based electrocatalysts were currently developed for electrochemical water oxidation in alkaline pH. Herein, amorphous nickel tungstate (NiWO₄) was synthesized using the facile successive ionic layer adsorption and reaction method. The films were characterized by X-ray diffraction, Raman spectroscopy, Fourier transfer infrared spectroscopy, scanning electron microscopy, X-ray photoelectron spectroscopy, and transmission electron microscopy techniques. The electrochemical analysis showed 315 mV of overpotential at 100 mA cm^-2 with lowest Tafel slope of 32 mV dec^-1 for oxygen evolution reaction (OER) making films of NiWO₄ compatible towards electrocatalysis of water in alkaline media. The chronopotentiometry measurements at 100 mA cm^-2 over 24 h showed 97% retention of OER activity. The electrochemical active surface area (ECSA) of NW120 film was 25.5 cm^-2.

Promoting electrocatalytic overall water splitting by sulfur incorporation into CoFe-(oxy)hydroxide

The design and fabrication of highly cost-effective electrocatalysts with high activity, and stability to enhance the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) has been considered to be one of the most promising approaches toward overall water splitting. In this study, sulfur-incorporated cobalt-iron (oxy)hydroxide (S-(Co,Fe)OOH) nanosheets were directly grown on commercial iron foam via galvanic corrosion and hydrothermal methods. The incorporation of sulfur into (Co,Fe)OOH results in superior catalytic performance and high stability in both the HER and OER conducted in 1 M KOH. The incorporation of sulfur enhanced the electrocatalytic activity by modifying the electronic structure and chemical states of (Co,Fe)OOH. An alkaline water electrolyzer for overall water splitting was fabricated using a two-electrode configuration utilizing the S-(Co,Fe)OOH bifunctional electrocatalyst in both the HER and OER. The fabricated electrolyzer outperformed a precious metal-based electrolyzer using Pt/C as the HER electrocatalyst and IrO2 as the OER electrocatalyst, which are the benchmark catalysts. This electrolyzer provides a lower potential of 1.641 V at 10 mA cm-2 and maintains 98.4% of its performance after 50 h of durability testing. In addition, the S-(Co,Fe)OOH-based electrolyzer successfully generated hydrogen under natural illumination upon its combination with a commercial silicon solar cell and exhibited a solar to hydrogen (STH) efficiency of up to 13.0%. This study shows that S-(Co,Fe)OOH is a promising candidate for application in the future renewable energy industry due to its high cost-effectiveness, activity, and stability during overall water splitting. In addition, the combination of a commercial silicon solar cell with an alkaline water electrolyzer has great potential for the production of hydrogen.

Structural and Interfacial Engineering of Ni2P/Fe3O4 Porous Nanosheet Arrays for Efficient Oxygen Evolution Reaction

Rational design of transition-metal phosphide (TMPs)-based electrocatalysts can effectively promote oxygen evolution reaction (OER). Herein, the novel efficient Ni₂P/Fe₃O₄ porous nanosheets arrays supported on Ni foam (Ni₂P/Fe₃O₄/NF) as alkaline OER catalysts were synthesized using structural and interfacial engineering. The three-dimensional (3D) porous hierarchical structure of Ni₂P/Fe₃O₄/NF provides abundant active sites for OER and facilitates the electrolyte diffusion of ions and O₂ liberation. Furthermore, the strong interfacial coupling and synergistic effect between Ni₂P and Fe₃O₄ modify the electronic structure, resulting in the enhanced intrinsic activity. Consequently, the optimized Ni₂P/Fe₃O₄/NF exhibits excellent OER performance with low overpotentials of 213 and 240 mV at 60 and 100 mA cm^-2 in 1.0 M KOH, respectively, better than the RuO₂/NF and most Ni/Fe-based OER catalysts. Impressively, it can maintain its catalytic activity for at least 20 h at 60 mA cm^-2. In addition, the relationship between the structure and performance is fully elucidated by the experimental characterizations, indicating that the metal oxyhydroxides in situ generated on the surface of catalysts are responsible for the high OER activity.

NiFe Layered-Double-Hydroxide Nanosheet Arrays on Graphite Felt: A 3D Electrocatalyst for Highly Efficient Water Oxidation in Alkaline Media

It is of great importance to rationally design and develop earth-abundant nanocatalysts for high-efficiency water electrolysis. Herein, NiFe layered double hydroxide was in situ grown hydrothermally on a 3D graphite felt (NiFe LDH/GF) as a high-efficiency catalyst in facilitating the oxygen evolution reaction (OER). In 1.0 M KOH, NiFe LDH/GF requires a low overpotential of 214 mV to deliver a geometric current density of 50 mA cm^-2 (η_{50 mA cm^-2} = 214 mV), surpassing that NiFe LDH supported on a 2D graphite paper (NiFe LDH/GP; η_{50 mA cm^-2} = 301 mV). More importantly, NiFe LDH/GF shows good durability at 50 mA cm^-2 within 50 h of OER catalysis testing and delivers a faradaic efficiency of nearly 100% in the electrocatalysis of OER.

Fe, Ni-codoped W18O49 grown on nickel foam as a bifunctional electrocatalyst for boosted water splitting

Designing cost-effective bifunctional catalysts with high-performance and durability is of great significance for renewable energy systems. Herein, typical Fe, Ni-codoped W₁₈O₄₉/NF was prepared via a simple solvothermal method. The incorporation of Fe ions enhanced the electronic interaction and enlarged the electrochemically active surface area. The increased W⁴⁺ leads to a high proportion of unsaturated W[double bond, length as m-dash]O bonds, thus enhancing the adsorption capacity of water. The valence configuration of nickel (Ni) sites in such dual-cation doping is well adjusted, realizing a high proportion of trivalent Ni ions (Ni³⁺). Due to the orbital interactions, the Fe³⁺/Ni³⁺ ions and OER reaction intermediates exhibit strong orbital overlap. The positions of the valence band and conduction band are well modulated. As a result, the Fe, Ni-codoped W₁₈O₄₉/NF shows improved electrocatalytic activity, and achieves a low decomposition voltage of 1.58 V at 10 mA cm^-2 and retains long-time stability.

Hierarchical tube brush-like Co3S4@NiCo-LDH on Ni foam as a bifunctional electrocatalyst for overall water splitting

The Co3S4@NiCo-LDH/NF nanocomposite exhibits outstanding electrocatalytic performances toward both the HER and OER at high current densities, along with a remarkable durability.

Self-supported cobalt–nickel bimetallic telluride as an advanced catalyst for the oxygen evolution reaction

The Ni-doped Co@CoTe2 electrode exhibits an outstanding OER activity and excellent long-term stability and outperforms most of the well-studied Co-based dichalcogenides.

N-Doped carbon coated NiCo2O4 nanorods for efficient electrocatalytic oxygen evolution

Hybrid architectures composed of NiCo₂O₄ nanorods coated N-doped carbon (NiCo₂O₄@NC) are synthesized through the pyrolysis of Bi-metallic MOFs, which exhibit excellent electrochemical performance for oxygen evolution reaction.