Ambient Fast Synthesis and Active Sites Deciphering of Hierarchical Foam-Like Trimetal-Organic Framework Nanostructures as a Platform for Highly Efficient Oxygen Evolution Electrocatalysis

Anionic insertion-prompted corrosion resistance in a metal-organic framework anode for seawater oxidation

In this study, we incorporated tungstate ions into a metal-organic framework (MOF) structure for the purpose of seawater oxidation. The presence of tungstate ions on the surface results in a protective anionic layer that repels chloride ions and selectively enhances the adsorption of hydroxyl ions onto the catalytic sites.

Electrochemical Hydrogen Production Coupling with the Upgrading of Organic and Inorganic Chemicals

Electrocatalytic water splitting powered by renewable energy is a green and sustainable method for producing high-purity H2. However, in conventional water electrolysis, the anodic oxygen evolution reaction (OER) involves a four-electron transfer process with inherently sluggish kinetics, which severely limits the overall efficiency of water splitting. Recently, replacing OER with thermodynamically favorable oxidation reactions, coupled with the hydrogen evolution reaction, has garnered significant attention and achieved remarkable progress. This strategy not only offers a promising route for energy-saving H₂ production but also enables the simultaneous synthesis of high-value-added products or the removal of pollutants at the anode. Researchers successfully demonstrate the upgrading of numerous organic and inorganic alternatives through this approach. In this review, the latest advances in the coupling of electrocatalytic H2 production and the upgrading of organic and inorganic alternative chemicals are summarized. What's more, the optimization strategy of catalysts, structure-performance relationship, and catalytic mechanism of various reactions are well discussed in each part. Finally, the current challenges and future prospects in this field are outlined, aiming to inspire further innovative breakthroughs in this exciting area of research.

Recent Progress of Modulating Pristine Metal-Organic Frameworks for Oxygen Reaction Revolution

Highly efficient and stable oxygen evolution reaction (OER) electrocatalysts are essential for electrochemical water splitting. Among non-noble metal-based catalysts, metal-organic frameworks (MOFs) have recently emerged as particularly promising candidates due to their exceptional surface areas, hierarchical porous structures, and tunable morphologies and compositions. The rational regulation of the morphology and electronic structure of pristine MOFs is considered a critical pathway for enhancing active sites and structural stability, thereby significantly boosting the OER catalytic performance. This review systematically presents the recent advancements in modulating pristine MOFs via heterogeneous metal doping, ligand substitution, and hybrid composite construction. With particular emphasis on synthetic methods, modification mechanisms, and the OER properties of MOFs, we analyze the fundamental relationships between structural modifications and electrocatalytic performance. Through the systematic analysis of existing research achievements, this review provides a holistic assessment of current state-of-the-art developments, identifies critical challenges, and proposes future research directions with practical implications for OER electrocatalyst design.

Production of High-Purity 2,5-Furandicarboxylic Acid through Electrocatalytic Oxidation of 2,5-Bis(hydroxymethyl)furan over Reconstructed Ni2P/NF

The generation of high-valence surface active species (such as CoOOH and NiOOH) determines the crucial oxidation rates and stabilities in the electrocatalytic reactions. Herein, a unique Ni2P/NF catalyst is designed using a chemical vapor deposition method along with a rapid reconstruction (Ni2+ → Ni3+) rate, stably achieving ≈90% FDCA yields from BHMF electron-oxidation after 10 cycles at a formation rate of 218 μmolFDCA cm-2 h-1 (seven times higher than data in reported literature). The abundance of available electrons near the Ni-3d Fermi level, together with the reduced NiP bond strength and the lowest electronegativity of Ni2P, accelerates surface NiOOH species formation. In addition, the electrocatalytic oxidation of BHMF offers a more stable furan-based substrate, while also prolonging the residence time of the oxidative intermediate HMF. This mitigates humin formation, thereby enabling the synthesis of high-purity FDCA (>99%) at high concentrations (100 mmol L-1), making it a promising approach for efficient FDCA synthesis.

Ligand-rich oxygen evolution electrocatalysts reconstructed from metal-organic frameworks for anion-exchange membrane water electrolysis

Organic ligands in metal-organic frameworks (MOFs) play an indispensable role in the reconstruction and catalysis during the alkaline oxygen evolution reaction (OER). However, it is still a big challenge to maintain a high content of ligands in MOF-reconstructed OER electrocatalysts and to study the interaction between ligands and derived (oxy)hydroxides. Herein, a ligand-rich trimetallic amorphous electrocatalyst is fabricated through a two-step mechanochemical and electrochemical reconstruction strategy. Experimental and theoretical studies clearly reveal that the d-π interaction between delocalized π-electrons on the benzene ring of ligands and derived (oxy)hydroxides, can trigger the charge transfer from ligands to the active metal centers, thus optimizing the adsorption energy of the oxygen-containing intermediates and enhancing the OER performance. Moreover, an anion-exchange membrane water electrolyzer using such ligand-rich OER electrocatalyst can be operated steadily at 1.69 V and 55 °C under an industrial-level current density of 500 mA cm-2 for over 200 h. This work provides novel insights into the role of organic ligands in alkaline OER electrocatalysis, with the potential to facilitate the production of green hydrogen at industrial-level current densities.

Modern Catalytic Materials for the Oxygen Evolution Reaction

The oxygen evolution reaction (OER) has, in recent years, attracted great interest from scientists because of its prime role in a number of renewable energy technologies. It is one of the reactions that occurs during hydrogen production through water splitting, is used in rechargeable metal-air batteries, and plays a fundamental role in regenerative fuel cells. Therefore, there is an emerging need to develop new, active, stable, and cost-effective materials for OER. This review presents the latest research on various groups of materials, showing their potential to be used as OER electrocatalysts, as well as their shortcomings. Particular attention has been paid to metal-organic frameworks (MOFs) and their derivatives, as those materials offer coordinatively unsaturated sites, high density of transition metals, adjustable pore size, developed surface area, and the possibility to be modified and combined with other materials.

Tailoring Octahedron-Tetrahedron Synergism in Spinel Catalysts for Acidic Water Electrolysis

The instability issues of oxide-based electrocatalysts during the oxygen evolution reaction (OER) under acidic conditions, caused by the oxidation and dissolution of the catalysts along with the current-capacitance effect, constrain their application in proton exchange membrane water electrolysis (PEMWE). To address these challenges, we tailored the spinel structure of Co3O4 and exploited the synergism between the tetrahedron and octahedron sites by partially substituting Co with Ni and Ru (denoted as NiRuCoOx), respectively. Such a catalyst design creates a Ru-O-Ni electronic coupling effect, facilitating a direct dioxygen radical-coupled OER pathway. Density-functional theory (DFT) calculations and in situ Raman spectroscopy results confirm that Ru is the active site in the diatomic oxygen mechanism while Ni stabilizes lattice oxygen and the Ru-O bonding. The designed NiRuCoOx catalyst exhibits an exceptionally low overpotential of 166 mV to achieve a current density of 10 mA cm-2. Moreover, when serving as the anode in PEMWE, the NiRuCoOx requires 1.72 V to reach a current density of 3A cm-2, meeting the 2026 target set by the U.S. Department of Energy (DOE: 1.8 V@3A cm-2). The PEMWE device can operate stably for more than 1500 h with a significantly reduced performance decay rate of 0.025 mV h-1 compared to commercial RuO2 (2.13 mV h-1). This work provides an efficient method for tailoring the octahedron-tetrahedron sites of spinel Co3O4, which significantly improves the activity and stability of PEMWE.

Creating Dual Active Sites in Ru-doped FeMn-MOF-74 for Efficient Overall Water Splitting

The design of well-engineered bifunctional electrocatalysts is crucial for achieving durable and efficient performance in overall water splitting. In this study, Ru-doped FeMn-MOF-74 itself has Ru sites and generates FeMnOOH under catalytic conditions, forming dual active sites for overall water splitting. Density functional theory (DFT) calculations demonstrate that the Ru dopants exhibit optimized binding strength for H* and enhanced hydrogen evolution reaction (HER) performance. Moreover, the Mn sites within FeMnOOH lower the energy barrier for the rate-determining step (from O* to OOH*), serving as the active centre for oxygen evolution reaction (OER). The incorporation of Ru significantly improves the electron transfer properties of FeMn-MOF-74 and enhances its water adsorption capacity, synergistically boosting its bifunctional activity. This strategy of designing dual active sites provides new insights into the development of bifunctional metal-organic frameworks (MOFs) for efficient overall water splitting.

Engineering the Sandwich-Type Porphyrinic MOF-Ruthenium-Nickel Foam Electrode for Boosting Overall Water Splitting via Self-Reconstruction

The rational construction of a hierarchical noble metal-metal-organic frameworks (MOFs) structure is anticipated to yield enduring and highly efficient performance in alkaline electrocatalytic water splitting. Herein, a sandwich construction strategy is employed to enhance the stability, wherein active RutheniRu (Ru) nanosheets are incorporated onto nickel foam (NF) and subsequently covered with porphyrinic MOFs (PMOFs). In addition, activated PMOF-NiOOH-Ru20/NF-C/A electrodes are obtained by electrochemical self-reconstruction as cathode and anode, respectively. Density functional theory (DFT) calculations demonstrated that the resulting PMOF-NiOOH-Ru heterointerface effectively facilitated electron transfer, enhanced H2O adsorption capacity, and optimized ΔG values for *H and *O to *OOH. Consequently, PMOF-NiOOH-Ru20/NF-C/A exhibited low overpotentials for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), accompanied by minimal Ru leakage. Furthermore, stable overall water splitting can be achieved with a low voltage of 1.456 V@10 mA cm-2 for over 120 h. Even when operated in simulated seawater, the prepared electrodes demonstrated similar activity and stability. This study contributes to a deeper understanding of the regulation mechanism for the performance and stability of active sites in the electrocatalytic self-reconstruction process.

A photovoltaic-electrolysis system with high solar-to-hydrogen efficiency under practical current densities

The photovoltaic-alkaline water (PV-AW) electrolysis system offers an appealing approach for large-scale green hydrogen generation. However, current PV-AW systems suffer from low solar-to-hydrogen (STH) conversion efficiencies (e.g., <20%) at practical current densities (e.g., >100 mA cm-2), rendering the produced H2 not economical. Here, we designed and developed a highly efficient PV-AW system that mainly consists of a customized, state-of-the-art AW electrolyzer and concentrator photovoltaic (CPV) receiver. The highly efficient anodic oxygen evolving catalyst, consisting of an iron oxide/nickel (oxy)hydroxide (Fe2O3-NiOxHy) composite, enables the customized AW electrolyzer with unprecedented catalytic performance (e.g., 1 A cm-2 at 1.8 V and 0.37 kgH2/m-2 hour-1 at 48 kWh/kgH2). Benefiting from the superior water electrolysis performance, the integrated CPV-AW electrolyzer system reaches a very high STH efficiency of up to 29.1% (refer to 30.3% if the lead resistance losses are excluded) at large current densities, surpassing all previously reported PV-electrolysis systems.

S- and N-Co-Doped Carbon-Nanoplate-Encased Ni Nanoparticles Derived from Dual-Ligand-Assembled Ni-MOFs as Efficient Electrocatalysts for the Oxygen Evolution Reaction

To achieve the "double carbon" goal, it is urgent to reform the energy system. The oxygen evolution reaction (OER) is a vital semi-reaction for many new energy-storage and conversion devices. Metal nanoparticles embedded in heteroatom-doped carbon materials prepared by the pyrolyzing of metal-organic frameworks (MOFs) have been a key route to obtain high-performance electrochemical catalysts. Herein, a nanocatalyst embedding Ni nanoparticles into S- and N-co-doped carbon nanoplate (Ni NPs@SN-CNP) has been synthesized by pyrolysis of a Ni-MOF precursor. The prepared Ni NPs@SN-CNP exhibits superior oxygen evolution performance with an overpotential of 256 mV to attain 10 mA cm-2 and a low Tafel slope value of 95 mV dec-1. Moreover, a self-assembled overall-water-splitting cell with Ni NPs@SN-CNP/NF||Pt-C/NF achieves a low potential of 1.56 V at 10 mA cm-2 and a high cycling stability for at least 10 h. The improvement in this performance is benefit from its large surface area, unique morphology, and the nanostructure of the electrocatalyst. This study presents a novel and simple approach to designing high-performance OER catalysts.

In-situ synthesis to promote surface reconstruction of metal-organic frameworks for high-performance water/seawater oxidation

Metal-organic frameworks (MOFs) have gained tremendous notice for the application in alkaline water/seawater oxidation due to their tunable structures and abundant accessible metal sites. However, exploring cost-effective oxygen evolution reaction (OER) electrocatalysts with high catalytic activity and excellent stability remains a great challenge. In this work, a promising strategy is proposed to regulate the crystalline structures and electronic properties of NiFe-metal-organic frameworks (NiFe-MOFs) by altering the organic ligands. As a representative sample, NiFe-BDC (BDC: C8H6O4) synthesized on nickel foam (NF) shows extraordinary OER activity in alkaline condition, delivering ultralow overpotentials of 204, 234 and 273 mV at 10, 100, and 300 mA cm-2, respectively, with a small Tafel slope of 21.6 mV dec-1. Only a slight decrease is observed when operating in alkaline seawater. The potential attenuation is barely identified at 200 mA cm-2 over 200 h continuous test, indicating the remarkable stability and corrosion resistance. In-situ measurements indicate that initial Ni2+/Fe2+ goes through oxidation process into Ni3+/Fe3+ during OER, and eventually presents in the form of NiFeOOH/NiFe-BDC heterojunction. The unique self-reconstructed surface is responsible for the low reaction barrier and fast reaction kinetics. This work provides an effective strategy to develop efficient MOF-based electrocatalysts and an insightful view on the dynamic structural evolution during OER.

Enhancing Oxygen Evolution Reaction Performance of Metal-Organic Frameworks through Cathode Activation

Due to their abundant active sites and porous structures, metal-organic frameworks (MOFs) have garnered significant interest as oxygen evolution reaction (OER) electrocatalysts. Nevertheless, the development of MOF s-based electrocatalysts with efficient OER activity and excellent stability simultaneously still face challenges. Herein, a cathodic activation strategy was used to enhance the OER electrocatalytic performance of M-HHTP for the first time, where M refers to Ni, Cu, Co, Fe, while HHTP denotes 2, 3, 6, 7, 10, 11-hexahydroxytriphenylene. As a prototype, the activated Ni-HHTP (HA-Ni-HHTP) demonstrates outstanding OER performance, with an overpotential as low as 140 mV at 20 mA cm-2 and a small Tafel slope of 78.7 mV-1, surpassing commercial RuO2 and rivaling state-of-the-art MOFs-based electrocatalysts. Characterizations and density functional theory calculations reveal that the superior performance of HA-Ni-HHTP is primarily ascribed to changes in semiconductor type, contact angle, and oxygen vacancy content induced by cathodic activation. Electrochemical impedance spectroscopy analysis using the transmission line model confirms that cathodic activation accelerates charge transport, enhancing the OER process. Furthermore, the cathodic activation strategy holds promise for improving the water oxidation performance of other MOFs such as Fe-HHTP, Co-HHTP, and Cu-HHTP.

Electrochemical reconstruction of metal-organic gels into crystalline oxy-hydroxide heterostructures for efficient oxygen evolution electrocatalysis

Metal-organic gels (MOGs) are emerging soft materials with distinct metal active centers, multifunctional ligands and hierarchical porous structures, showing promising potential in the field of electrocatalysis. However, the reconfiguration of MOGs during the electrocatalytic process remains underexplored, with current studies in early developmental stages. To deeply investigate the application of MOG materials in electrocatalysis, the compositional transformations and structural changes under an electrochemical activation method were studied in detail, leading to high-performance OER pre-electrocatalysts. XRD and HRTEM results demonstrate the complete reconfiguration of amorphous Fe5Ni5-MOG into crystalline NiOOH/FeOOH heterostructures. The synergistic effect of the bimetallic center and the rich NiOOH-FeOOH interface in the reconstituted Re-Fe5Ni5-MOG exhibit excellent OER activity in alkaline electrolytes, with low overpotentials (205 mV at 10 mA cm-2) and a Tafel slope of 58 mV dec-1. In situ characterization during the electrocatalytic process reveals the gradual transformation of the metal center into metal hydroxyl oxides upon increasing the voltage to 1.55 V. DFT analysis suggests that in the Fe-Ni double site reaction pathway, active substances preferentially adsorb on the Fe site before the Ni sites.

Modulation of morphology and electronic structure of cobalt thiophenedicarboxylic coordination polymer via ligand exchange for high-performance oxygen evolution reaction and supercapacitor

Rationally designing metal organic frameworks (MOFs) as an ideal dual-function material for water electrolysis and supercapacitors is of great significance for energy storage and conversion. Herein, we successfully synthesized the nanoneedle-like structure CoNi-MOF by partially replacing 2, 5-thiophenedicarboxylic acid (TDA) with 1, 1'-Ferrocenedicarboxylate (Fc). The exchange of Fc ligand can modulate the morphology and electronic structure of CoNi-TDA, thus exposing the abundant active sites and improving the electrical conductivity. The as-prepared CoNi-TDA/0.2Fc exhibited a low overpotential of 236 mV at 10 mA cm-2 for oxygen evolution reaction (OER) and a low Tafel slope of 40.44 mV dec-1. Additionally, CoNi-TDA/0.2Fc demonstrated a notable specific capacitance of 1409 F g-1 at 1 A/g and excellent stability, maintaining a capacitance retention of 96.54 % after 20,000 cycles. The study proposes a new strategy to modulate the morphology and electronic structure of MOFs via the ligand exchange for high-performance energy storage and conversion device.

Zn-facilitated surface reconstruction of Ni-MOF for an enhanced oxygen evolution reaction

Facilitating the surface reconstruction of pre-catalysts has been considered an effective strategy for constructing low-cost and highly efficient OER electrocatalysts. Metal doping is a feasible way to activate the surface reconstruction, thus enhancing the OER performance. Herein, we report a facile hydrothermal method to synthesize a series of Zn-doped Ni-MOF on nickel foam (NiZn-MOF/NF) as promising pre-catalysts toward the oxygen evolution reaction (OER). The Zn leaching of NiZn-MOF/NF can promote the surface self-reconstruction of NiZn-MOF/NF into oxygen-vacancy-rich NiOOH after electrochemical activation. Benefiting from the optimized electronic structure, abundant defects, more accessible active sites, and enhanced electrical conductivity, the reconstructed metal oxyhydroxide hybrids exhibit better electrocatalytic activity than the catalysts transformed from Ni-MOF/NF without Zn doping. The optimized NiZn-MOF/NF-OH as an OER catalyst has an overpotential of 336 mV at 100 mA cm-2, and a Tafel slope of 65.9 mV dec-1, as well as stability over 12 h. This work reveals that Zn cation-doping/leaching induces the surface reconstruction of pre-catalysts for enhanced oxygen catalytic activity, which provides a new approach for the development of advanced electrocatalysts.

Laser-Induced Preparation of Anderson-Type Polyoxometalate-Derived Sulfide/Oxide Electrocatalysts for Electrochemical Water Oxidation

Developing cost-effective and high-active electrocatalysts is vital to enhance the electrocatalytic performance for oxygen evolution reaction (OER). However, traditional pyrolysis methods require complicated procedures, exact temperatures, and long reaction times, leading to high costs and low yields of electrocatalysts in potential industrial applications. Herein, a rapid and economic laser-induced preparation strategy is proposed to synthesize three bimetallic sulfide/oxide composites (MMoOS, M=Fe, Co, and Ni) on a nickel foam (NF) substrate. A focused CO2 laser with high energy is applied to decompose Anderson-type polyoxometalate (POM)-based precursors, enabling the creation of abundant heteropore and defective structures in the MMoOS composites that have multi-components of MS/Mo4O11/MoS2. Remarkably, owing to the structural interactions between the active species, FeMoOS shows superior electrocatalytic performance for OER in an alkaline medium, exhibiting a low overpotential of 240 mV at 50 mA cm-2, a small Tafel slope of 79 mV dec-1, and good durability for 80 h. Physical characterizations after OER imply that partially dissolved Mo-based species and new-formed NiO/NiOOH can effectively uncover abundant active sites, fasten charge transfer, and modify defective structures. This work provides a rapid laser-induced irradiation method for the synthesis of POM-derived nanocomposites as promoted electrocatalysts.

Interpretable Data-Driven Descriptors for Establishing the Structure-Activity Relationship of Metal-Organic Frameworks Toward Oxygen Evolution Reaction

The development of readily accessible and interpretable descriptors is pivotal yet challenging in the rational design of metal-organic framework (MOF) catalysts. This study presents a straightforward and physically interpretable activity descriptor for the oxygen evolution reaction (OER), derived from a dataset of bimetallic Ni-based MOFs. Through an artificial-intelligence (AI) data-mining subgroup discovery (SGD) approach, a combination of the d-band center and number of missing electrons in eg states of Ni, as well as the first ionization energy and number of electrons in eg states of the substituents, is revealed as a gene of a superior OER catalyst. The found descriptor, obtained from the AI analysis of a dataset of MOFs containing 3-5d transition metals and 13 organic linkers, has been demonstrated to facilitate in-depth understanding of structure-activity relationship at the molecular orbital level. The descriptor is validated experimentally for 11 Ni-based MOFs. Combining SGD with physical insights and experimental verification, our work offers a highly efficient approach for screening MOF-based OER catalysts, simultaneously providing comprehensive understanding of the catalytic mechanism.

Microwave-Assisted Ultrafast Synthesis of Bimetallic Nickel-Cobalt Metal-Organic Frameworks for Application in the Oxygen Evolution Reaction

Herein, a series of monometallic Ni-, Co- and Zn-MOFs and bimetallic NiCo-, NiZn- and CoZn-MOFs of formula M2(BDC)2DABCO and (M,M')2(BDC)2DABCO, respectively, (M, M'=metal) with the same pillar and layer linkers 1,4-diazabicyclo[2.2.2]octane (DABCO) and benzene-1,4-dicarboxylate (BDC) were prepared through a fast microwave-assisted thermal conversion synthesis method (MW) within only 12 min. In the bimetallic MOFs the ratio M:M' was 4 : 1. The mono- and bimetallic MOFs were selected to systematically explore the catalytic-activity of their derived metal oxide/hydroxides for the oxygen evolution reaction (OER). Among all tested bimetallic MOF-derived catalysts, the NiCoMOF exhibits superior catalytic activity for the OER with the lowest overpotentials of 301 mV and Tafel slopes of 42 mV dec-1 on a rotating disk glassy carbon electrode (RD-GCE) in 1 mol L-1 KOH electrolyte at a current density of 10 mA cm-2. In addition, NiCoMOF was insitu grown in just 25 min by the MW synthesis on the surface of nickel foam (NF) with, for example, a mass loading of 16.6 mgMOF/gNF, where overpotentials of 313 and 328 mV at current densities of 50 and 300 mA cm-2, respectively, were delivered and superior long-term stability for practical OER application. The low Tafel slope of 27 mV dec-1, as well as a low reaction resistance from electrochemical impedance spectroscopy (EIS) measurement (Rfar=2 Ω), confirm the excellent OER performance of this NiCoMOF/NF composite. During the electrocatalytic processes or even before upon KOH pre-treatment, the MOFs are transformed to the mixed-metal hydroxide phase α-/β-M(OH)2 which presents the active species in the reactions (turnover frequency TOF=0.252 s-1 at an overpotential of 320 mV). Compared to the TOF from β-M(OH)2 (0.002 s-1), our study demonstrates that a bimetallic MOF improves the electrocatalytic performance of the derived catalyst by giving an intimate and uniform mixture of the involved metals at the nanoscale.

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.

Structural Reconstruction of a Cobalt- and Ferrocene-Based Metal-Organic Framework during the Electrochemical Oxygen Evolution Reaction

Metal-organic frameworks (MOFs) are increasingly being investigated as electrocatalysts for the oxygen evolution reaction (OER) due to their unique modular structures that present a hybrid between molecular and heterogeneous catalysts, featuring well-defined active sites. However, many fundamental questions remain open regarding the electrochemical stability of MOFs, structural reconstruction of coordination sites, and the role of in situ-formed species. Here, we report the structural transformation of a surface-grown MOF containing cobalt nodes and 1,1'-ferrocenedicarboxylic acid linkers (denoted as CoFc-MOF) during the OER in alkaline electrolyte. Ex situ and in situ investigations of CoFc-MOF film suggest that the MOF acts as a precatalyst and undergoes a two-step restructuring process under operating conditions to generate a metal oxyhydroxide phase. The MOF-derived metal oxyhydroxide catalyst, supported on nickel foam electrodes, displays high activity toward the OER with an overpotential of 190 mV at a current density of 10 mA cm-2. While this study demonstrates the necessity of investigating structural evolution of MOFs during electrocatalysis, it also shows the potential of using MOFs as precursors in catalyst design.

A bimetallic NiFe MOF with ultra-thin two-dimensional nanosheet structure effectively accelerates oxygen evolution reaction

To address the shortage of fossil energy, the development of affordable and efficient non-precious metal catalysts for oxygen evolution reaction (OER) from electrocatalytic water splitting is still a crucial challenge. Herein, the bimetallic NiFe metal-organic frameworks (MOFs) are synthesized by hydrothermal and electro-deposition. Benefiting from the synergistic effect of Fe and Ni, the catalyst demonstrates extraordinary activity, which exhibits favorable OER catalytic activity in 1 M KOH solution with an overpotential of 206 mV at 10 mA cm-2. Meanwhile, the obtained NiFe-NDC presents promising stability in the 20 h test at 50 mA cm-2.

Modulating 3d Charge State via Halogen Ions in Neighboring Molecules of Metal-Organic Frameworks for Improving Water Oxidation

Modulating the coordination environment of the metal active center is an effective method to boost the catalytic performances of metal-organic frameworks (MOFs) for oxygen evolution reaction (OER). However, little attention has been paid to the halogen effects on the ligands engineering. Herein, a series of MOFs X─FeNi-MOFs (X = Br, Cl, and F) is constructed with different coordination microenvironments to optimize OER activity. Theoretical calculations reveal that with the increase in electronegativity of halogen ions in terephthalic acid molecular (TPA), the Bader charge of Ni atoms gets larger and the Ni-3d band center and O-2p bands move closer to the Fermi level. This indicates that an increase in ligand negativity of halogen ions in TPA can promote the adsorption ability of catalytic sites to oxygen-containing intermediates and reduce the activation barrier for OER. Experimental also demonstrates that F─FeNi-MOFs exhibit the highest catalytic activity with an ultralow overpotential of 218 mV at 10 mA cm-2, outperforming most otate-of-the-art Fe/Co/Ni-based MOFs catalysts, and the enhanced mass activity by seven times compared with that for the sample before ligands engineering. This work opens a new avenue for the realization of the modulation of NiFe─O bonding by halogen ion in TPA and improves the OER performance of MOFs.

Self-Reconstructed Metal-Organic Framework-Based Hybrid Electrocatalysts for Efficient Oxygen Evolution

Refining synthesis strategies for metal-organic framework (MOF)-based catalysts to improve their performance and stability in an oxygen evolution reaction (OER) is a big challenge. In this study, a series of nanostructured electrocatalysts were synthesized through a solvothermal method by growing MOFs and metal-triazolates (METs) on nickel foam (NF) substrates (named MET-M/NF, M = Fe, Co, Cu), and these electrocatalysts could be used directly as OER self-supporting electrodes. Among these electrocatalysts, MET-Fe/NF exhibited the best OER performance, requiring only an overpotential of 122 mV at a current density of 10 mA cm-2 and showing remarkable stability over 15 h. The experimental results uncovered that MET-Fe/NF underwent an in situ structural reconstruction, resulting in the formation of numerous iron/nickel (oxy)hydroxides with high OER activity. Furthermore, in a two-electrode water-splitting setup, MET-Fe/NF only required 1.463 V to achieve a current density of 10 mA cm-2. Highlighting its potential for practical applications. This work provides insight into the design and development of efficient MOF-based OER catalysts.

Constructing a Built-In Electric Field To Accelerate Water Dissociation for Efficient Alkaline Hydrogen Evolution

The alkaline hydrogen evolution reaction (HER) is intricately linked to the water dissociation kinetics. The quest for new strategies to accelerate this step is a pivotal aspect of enhancing the HER performance. Herein, we designed and synthesized a heterogeneous nickel phosphide/cobalt phosphide nanowire array grown on nickel foam (Ni2P/CoP/NF) to form a p-n junction structure. The built-in electric field (BEF) in the p-n junction optimizes the binding ability of hydrogen and hydroxyl intermediates, efficiently promoting water dissociation for the alkaline HER. Consequently, Ni2P/CoP/NF exhibits a lower overpotential of 58 and 118 mV at 30 and 100 mA cm-2, respectively, and high stability over 40 h at 300 mA cm-2 for the HER in 1 M KOH. Computational calculations combined with experiment results testify that the BEF presence in the p-n junction of Ni2P/CoP/NF effectively promotes water dissociation, regulates intermediate adsorption/desorption, and boosts electron transport. This study presents a rational design approach for high-performance heterogeneous electrocatalysts.

Encapsulate Co3O4 within ultrathin graphene sheets to enhance peroxymonosulfate activation by tuning surface electronic structures

Heterojunctions composed of cobalt-based materials and carbon materials have been recognized as the efficient catalysts for peroxymonosulfate (PMS) activation to generate reactive oxygen species for the removal of environmental contaminants. However, the role of carbon materials in promoting the heterojunction systems has not been fully understood. This study synthesized a heterojunction material of graphene sheets encapsulating Co3O4 (GCO-500) through the pyrolysis of cobalt MOF and applied it to activate PMS for the removal of lomefloxacin. The results showed a high removal rate of 93.59 % with a degradation rate of k1 = 0.0156 min-1. Co3O4 clusters was encapsulated within ultrathin graphene sheets (<2 nm). DFT calculations revealed that graphene layers improve the electron transfer ability of Co3O4 and increased the d-band center of Co3O4 (-1.61 eV) that promote the adsorption of PMS on GCO-500 (-1.32 eV). In the meanwhile, organic pollutant was enriched in graphene layers with high adsorption energy (-13.08 eV), which greatly enhanced the degradation efficiency of pharmaceuticals. This study provides an effective catalyst for PMS activation and sheds light on the fundamental electronic-level understanding of cobalt-based and carbon heterojunction catalysts in PMS activation.

Electrochemical Synthesis and Electrocatalytic Oxygen-Evolution Performance of Two-Dimensional NiCo-BPDC Materials

Metal-organic frameworks (MOFs) have been widely studied as electrocatalysts, and the research strategy to improve their electrocatalytic oxygen evolution reaction (OER) performance is to modify their structure. In this paper, two-dimensional bimetallic MOFs were constructed to improve electrocatalytic OER performance. Using a mild electrochemical method with Ni and Co as metal sources and 4, 4 '-biphenyl dicarboxylic acid (H2BPDC) as ligand, two-dimensional NiCo-BPDC was synthesized and then deposited on a carbon cloth electrode. The results show that NiCo-BPDC/CC possessed a low overpotential of 356 mV at a current density of 20 mA cm-2 with a small Tafel slope of 86 mV dec-1 in 1.0 M KOH solution. The two-dimensional NiCo-BPDC exhibits excellent electrocatalytic OER performance because the coordination of Ni and Co in the material and the interaction of the two-dimensional materials provide a large electrochemically active surface area and expose more metal active sites for OER, thus improving the reaction efficiency and indicating NiCo-BPDC as potential OER electrocatalyst.

The fractal geometry of polymeric materials surfaces: surface area and fractal length scales

Using three common polymeric materials (polypropylene (PP), polytetrafluoroethylene (PTFE) and polycaprolactone (PCL)), a standard oxygen-plasma treatment and atomic force microscopy (AFM), we performed a scaling analysis of the modified surfaces yielding effective Hurst exponents (H ≃ 0.77 ± 0.02 (PP), ≃0.75 ± 0.02 (PTFE), and ≃0.83 ± 0.02 (PCL)), for the one-dimensional profiles, corresponding to the transversal sections of the surface, by averaging over all possible profiles. The surface fractal dimensions are given by ds = 3 - H, corresponding to ds ≃ 2.23, 2.25, and 2.17, respectively. We present a simple method to obtain the surface area from the AFM images stored in a matrix of 512 × 512 pixels. We show that the considerable increase found in the surface areas of the treated samples w.r.t. to the non-treated ones (43% for PP, 85% for PTFE, and 25% for PCL, with errors of about 2.5% on samples of 2 µm × 2 µm) is consistent with the observed increase in the length scales of the fractal regime to determine H, typically by a factor of about 2, extending from a few to hundreds of nanometres. We stipulate that the intrinsic roughness already present in the original non-treated material surfaces may serve as 'fractal' seeds undergoing significant height fluctuations during plasma treatment, suggesting a pathway for the future development of advanced material interfaces with large surface areas at the nanoscale.

NiFe codoping-regulated amorphous/crystalline heterostructured Co-based hydroxides/tungstate with rich oxygen vacancies for efficient water oxidation catalysis

Oxygen evolution reaction (OER) is a crucial half-reaction in water splitting, generating hydrogen for sustainable development, but it is often subject to sluggish kinetics. Abundant transition metal-based OER electrocatalysts have been utilized to expedite the process. However, traditional amorphous catalysts suffer from low conductivity, while the activity of crystalline catalysts is also unsatisfactory. Herein, an amorphous/crystalline heterostructured Co-based hydroxide/tungstate was meticulously constructed and further tailored using a NiFe codoping method (NiFeCoW). Following NiFe codoping, the electronic structure had been modulated, subsequently altering the adsorption toward intermediates. From the electrochemical measurements, the NiFeCoW catalyst demonstrated superior electrocatalytic activity for OER in alkaline media, with a minimal overpotential of 297 mV at 10 mA cm-2 and a cell voltage of 1.57 V for water splitting. This study provides valuable guidance for regulating the amorphous/crystalline heterophase in catalysts through bimetallic modulating engineering.

Hierarchical Porous Bimetallic FeMn Metal-Organic Framework Gel for Efficient Activation of Peracetic Acid in Antibiotic Degradation

Effective techniques for eliminating antibiotics from water environments are in high demand. The peracetic acid (PAA)-based advanced oxidation process has recently drawn increasing attention for its effective antibiotic degrading capability. However, current applications of PAA-based techniques are limited and tend to have unsatisfactory performance. An additional catalyst for PAA activation could provide a promising solution to improve the performance of PAA. Bulky metal-organic framework gels (MOGs) stand out as ideal catalysts for PAA activation owing to their multiple advantages, including large surface areas, high porosity, and hierarchical pore systems. Herein, a bimetallic hierarchical porous structure, i.e., FeMn13BTC, was synthesized through a facile one-pot synthesis method and employed for PAA activation in ofloxacin (OFX) degradation. The optimized FeMn MOG/PAA system exhibited efficient catalytic performance, characterized by 81.85% OFX degradation achieved within 1 h owing to the specific hierarchical structure and synergistic effect between Fe and Mn ions, which greatly exceeded the performance of the only PAA-catalyzed system. Furthermore, the FeMn MOG/PAA system maintained >80% OFX degradation in natural water. Quenching experiments, electron spin resonance spectra, and model molecular degradation revealed that the primary reactive oxygen species responsible for the catalytic effect was R-O•, especially CH3C(=O)OO•, with minor contributions of •OH and 1O2. Overall, introduction of the MOG catalyst strategy for PAA activation achieved high antibiotic degradation performance, establishing a paradigm for the design of heterogeneous hierarchical systems to broaden the scope of catalyzed water treatment applications.

M-Ni-Co MOF (M=Zn, Fe, Mn) for high-performance supercapacitors by adjusting its morphology

Metal-organic frameworks (MOF) have been wildly synthesised and studied as electrode materials for supercapacitors, and bimetallic MOF of Ni and Co has been broadly studied to enhance both specific capacitance and stability of supercapacitors. Herein, a best performance (about 320 F/g) of Ni-Co bimetallic MOF was found in a uniform preparation condition by adjusting the ratio of Ni to Co. Then tiny third metal ion was introduced, and we found that the morphology of material has a significant change on the original basis. Furthermore, certain ions (Zn, Fe, Mn) introduced make a huge improvement in capacitance based on Ni-Co MOF of 320 F/g. The result shows that Zn-Ni-Co MOF, Fe-Ni-Co MOF and Mn-Ni-Co MOF perform specific capacitance of 1135 F/g, 870 F/g and 760F/g at 1 A/g, respectively. Meanwhile, the asymmetric supercapacitor (ASC) was constructed by Zn-Ni-Co MOF as positive electrode and active carbon (AC) as negative electrode. The Zn-Ni-Co MOF//AC ASC possesses a energy density of 58 Wh/kg at a power density of 775 W/kg. This research provides a new methods to regulate the morphology of MOF and a novel viewpoint for assembling high-performance, low-price, and eco-friendly green energy storage devices.

Metal-Organic Frameworks: Direct Synthesis by Organic Acid-Etching and Reconstruction Disclosure as Oxygen Evolution Electrocatalysts

Metal-organic frameworks (MOFs) have emerged as promising oxygen evolution reaction (OER) electrocatalysts. Chemically bonded MOFs on supports are desirable yet lacking in routine synthesis, as they may allow variable structural evolution and the underlying structure-activity relationship to be disclosed. Herein, direct MOF synthesis is achieved by an organic acid-etching strategy (AES). Using π-conjugated ferrocene (Fc) dicarboxylic acid as the etching agent and organic ligand, a series of MFc-MOF (M=Ni, Co, Fe, Zn) nanosheets are synthesized on the metal supports. The crystal structure is studied using X-ray diffraction and low-dose transmission electron microscopy, which is quasi-lattice-matched with that of the metal, enabling in situ MOF growth. Operando Raman and attenuated total reflectance Fourier transform infrared spectroscopy disclose that the NiFc-MOF features dynamic structural rebuilding during OER. The reconstructed one showing optimized electronic structures with an upshifted total d-band center, high M-O bonding state occupancy, and localized electrons on adsorbates indicated by density functional theory calculations, exhibits outstanding OER performance with a fairly low overpotential (130 mV at 10 mA cm-2 ) and good stability (144 h). The newly established approach for direct MOF synthesis and structural reconstruction disclosure stimulate the development of more prudent catalysts for advancing OER.

Precise Anchoring of Fe Sites by Regulating Crystallinity of Novel Binuclear Ni-MOF for Revealing Mechanism of Electrocatalytic Oxygen Evolution

Bimetallic metal-organic framework (BMOF) exhibits better electrocatalytic performance than mono-MOF, but deciphering the precise anchoring of foreign atoms and revealing the underlying mechanisms at the atomic level remains a major challenge. Herein, a novel binuclear NiFe-MOF with precise anchoring of Fe sites is synthesized. The low-crystallinity (LC)-NiFe0.33 -MOF exhibited abundant unsaturated active sites and demonstrated excellent electrocatalytic oxygen evolution reaction (OER) performance. It achieved an ultralow overpotential of 230 mV at 10 mA cm-2 and a Tafel slope of 41 mV dec-1 . Using a combination of modulating crystallinity, X-ray absorption spectroscopy, and theoretical calculations, the accurate metal sequence of BMOF and the synergistic effect of the active sites are identified, revealing that the adjacent active site plays a significant role in regulating the catalytic performance of the endmost active site. The proposed model of BMOF electrocatalysts facilitates the investigation of efficient OER electrocatalysts and the related catalytic mechanisms.

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.

Hollow Ru/RuO2 nanospheres with nanoparticulate shells for high performance electrocatalytic oxygen evolution reactions

This work shows that hollow Ru/RuO2 nanoparticles having nanoparticulate shells (HN-Ru/RuO2) can be prepared using hollow microporous organic polymers with Ru species (H-MOP-Ru) as precursors. Using silica spheres as templates, H-MOPs were prepared through the Sonogashira-Hagihara coupling of 1,3,5-triethynylbenzene with 2,3-ethoxymethylenedioxy-1,4-diiodobenzene. Acid hydrolysis of cyclic ethyl orthoformate protecting groups generated catechol moieties to form H-MOP-Cat. Then, H-MOP-Ru was obtained by incorporating Ru species into H-MOP-Cat. Heat-treatment of H-MOP-Ru under air induced the formation of HN-Ru/RuO2 with a diameter of 61 nm and shells consisting of 6-7 nm nanoparticles. Due to the hollow structure and nanoparticulate shells, HN-Ru/RuO2 showed a high surface area of 80 m2 g-1 and a pore volume of 0.18 cm3 g-1. The HN-Ru/RuO2 showed enhanced electrocatalytic performance for the oxygen evolution reaction (OER) with an overpotential of 295 mV @ 10 mA cm-2 and a Tafel slope of 46 mV dec-1 in alkaline electrolyte, compared with control RuO2 such as commercial Ru/RuO2 nanoparticles (A-Ru/RuO2) and home-made Ru/RuO2 nanoparticles (N-Ru/RuO2) prepared via the same synthetic procedure as HN-Ru/RuO2. While HN-Ru/RuO2 inevitably contained Pd originated from coupling catalysts, it showed superior performance to Ru/RuO2 nanoparticles with the same Pd content (N1-Ru/RuO2), indicating that the efficient electrocatalytic performance of HN-Ru/RuO2 is attributable to its hollow structure and nanoparticulate shells.

Recent Advancements in Electrochemical Hydrogen Production via Hybrid Water Splitting

As one of the most promising approaches to producing high-purity hydrogen (H2 ), electrochemical water splitting powered by the renewable energy sources such as solar, wind, and hydroelectric power has attracted considerable interest over the past decade. However, the water electrolysis process is seriously hampered by the sluggish electrode reaction kinetics, especially the four-electron oxygen evolution reaction at the anode side, which induces a high reaction overpotential. Currently, the emerging hybrid electrochemical water splitting strategy is proposed by integrating thermodynamically favorable electro-oxidation reactions with hydrogen evolution reaction at the cathode, providing a new opportunity for energy-efficient H2 production. To achieve highly efficient and cost-effective hybrid water splitting toward large-scale practical H2 production, much work has been continuously done to exploit the alternative anodic oxidation reactions and cutting-edge electrocatalysts. This review will focus on recent developments on electrochemical H2 production coupled with alternative oxidation reactions, including the choice of anodic substrates, the investigation on electrocatalytic materials, and the deep understanding of the underlying reaction mechanisms. Finally, some insights into the scientific challenges now standing in the way of future advancement of the hybrid water electrolysis technique are shared, in the hope of inspiring further innovative efforts in this rapidly growing field.

Oxygen-Bridged Cobalt-Chromium Atomic Pair in MOF-Derived Cobalt Phosphide Networks as Efficient Active Sites Enabling Synergistic Electrocatalytic Water Splitting in Alkaline Media

Electrochemical water splitting offers a most promising pathway for "green hydrogen" generation. Even so, it remains a struggle to improve the electrocatalytic performance of non-noble metal catalysts, especially bifunctional electrocatalysts. Herein, aiming to accelerate the hydrogen and oxygen evolution reactions, an oxygen-bridged cobalt-chromium (Co-O-Cr) dual-sites catalyst anchored on cobalt phosphide synthesized through MOF-mediation are proposed. By utilizing the filling characteristics of 3d orbitals and modulated local electronic structure of the catalytic active site, the well-designed catalyst requires only an external voltage of 1.53 V to deliver the current density of 20 mA cm-2 during the process of water splitting apart from the superb HER and OER activity with a low overpotential of 87 and 203 mV at a current density of 10 mA cm-2 , respectively. Moreover, density functional theory (DFT) calculations are utilized to unravel mechanistic investigations, including the accelerated adsorption and dissociation process of H2 O on the Co-O-Cr moiety surface, the down-shifted d-band center, a lowered energy barrier for the OER and so on. This work offers a design direction for optimizing catalytic activity toward energy conversion.

Interfacial Electron Regulation and Composition Evolution of NiFe/MoC Heteronanowire Arrays for Highly Stable Alkaline Seawater Oxidation

In alkaline seawater electrolysis, the oxygen evolution reaction (OER) is greatly suppressed by the occurrence of electrode corrosion due to the formation of hypochlorite. Herein, a catalyst consisting of MoC nanowires modified with NiFe alloy nanoparticles (NiFe/MoC) on nickel foam (NF) is prepared. The optimized catalyst can deliver a large current density of 500 mA cm-2 at a very low overpotential of 366 mV in alkaline seawater, respectively, outperforming commercial IrO2 . Remarkably, an electrolyzer assembled with NiFe/MoC/NF as the anode and NiMoN/NF as the cathode only requires 1.77 V to drive a current density of 500 mA cm-2 for alkaline seawater electrolysis, as well as excellent stability. Theory calculation indicates that the initial activity of NiFe/MoC is attributed to increased electrical conductivity and decreased energy barrier for OER due to the introduction of Fe. We find that the change of the catalyst in the composition occurred after the stability test; however, the reconstructed catalyst has an energy barrier close to that of the pristine one, which is responsible for its excellent long-term stability. Our findings provide an efficient way to construct high-performance OER catalysts for alkaline seawater splitting.

Ultrafine Pt Nanoparticles Anchored on 2D Metal-Organic Frameworks as Multifunctional Electrocatalysts for Water Electrolysis and Zinc-Air Batteries

Multifunctional electrocatalysts are crucial to cost-effective electrochemical energy conversion and storage systems requiring mutual enhancement of disparate reactions. Embedding noble metal nanoparticles in 2D metal-organic frameworks (MOFs) are proposed as an effective strategy, however, the hybrids usually suffer from poor electrochemical performance and electrical conductivity in operating conditions. Herein, ultrafine Pt nanoparticles strongly anchored on thiophenedicarboxylate acid based 2D Fe-MOF nanobelt arrays (Pt@Fe-MOF) are fabricated, allowing sufficient exposure of active sites with superior trifunctional electrocatalytic activity for hydrogen evolution, oxygen evolution, and oxygen reduction reactions. The interfacial Fe─O─Pt bonds can induce the charge redistribution of metal centers, leading to the optimization of adsorption energy for reaction intermediates, while the dispersibility of ultrafine Pt nanoparticles contributes to the high mass activity. When Pt@Fe-MOF is used as bifunctional catalysts for water-splitting, a low voltage of 1.65 V is required at 100 mA cm-2 with long-term stability for 20 h at temperatures (65 °C) relevant for industrial applications, outperforming commercial benchmarks. Furthermore, liquid Zn-air batteries with Pt@Fe-MOF in cathodes deliver high open-circuit voltages (1.397 V) and decent cycling stability, which motivates the fabrication of flexible quasisolid-state rechargeable Zn-air batteries with remarkable performance.

Terrace-Like 2D Hierarchically Porous Iron/Cobalt Metal-Organic Framework: Ambient Fast Synthesis and Efficient Oxygen Evolution Reaction Application

It is urgent to design a low-cost electrocatalyst with high activity to enhance the efficiency of oxygen evolution reaction (OER), which is limited by the slow four-electron transfer kinetics process. Nevertheless, traditional synthetic methods, including calcination and solvothermal, of the electrocatalysts are high-cost, low-yield, and energy-hogging, which limits their industrial application. Herein, an ambient fast synthetic method is developed to prepare terrace-like Fe/Co bimetal-organic framework (TFC-MOF) electrocatalyst materials in gram scale in 1 h. The method in this paper is designable based on coordination chemistry. Fe and Co ions can coordinate with the carboxyl groups on benzene-1,3,5-tricarboxylic acid (H3 BTC) to form a 2D-MOF structure. Structural characterizations, including SEM, TEM, and XRD are conducted to verify that the TFC-MOF is a terrace-like layered structure with uniform-sized mesoporous, which reduces the adsorption steric hindrance and facilitates the mass and electron transfer efficiency of OER. The TFC-MOF shows low overpotential, 255 mV at a current density of 10 mA cm-2 , and a low Tafel slope of 49.9 mV dec-1 , in an alkaline solution. This work provides a planar coordination strategy to synthesize 2D-MOF OER electrocatalyst on a large scale with low cost and low energy consumption, which will promote its practical OER applications.

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).

A Self-Templated Design Approach toward Multivariate Metal-Organic Frameworks for Enhanced Oxygen Evolution

Multivariate metal-organic framework (MOF) is an ideal electrocatalytic material due to the synergistic effect of multiple metal active sites. In this study, a series of ternary M-NiMOF (M = Co, Cu) through a simple self-templated strategy that the Co/Cu MOF isomorphically grows in situ on the surface of NiMOF is designed. Owing to the electron rearrange of adjacent metals, the ternary CoCu-NiMOFs demonstrate the improved intrinsic electrocatalytic activity. At optimized conditions, the ternary Co3 Cu-Ni2 MOFs nanosheets give the excellent oxygen evolution reaction (OER) performance of current density of 10 mA cm-2 at low overpotential of 288 mV with a Tafel slope of 87 mV dec-1 , which is superior to that of bimetallic nanosheet and ternary microflowers. The low free energy change of potential-determining step identifies that the OER process is favorable at Cu-Co concerted sites along with strong synergistic effect of Ni nodes. Partially oxidized metal sites also reduce the electron density, thus accelerating the OER catalytic rate. The self-templated strategy provides a universal tool to design multivariate MOF electrocatalysts for highly efficient energy transduction.

Self-Reconstructed Two-Dimensional Cluster-Based Co-Organic Layer Heterojunctions for Enhanced Oxygen Evolution Reactions

When it comes to an efficient catalytic oxygen evolution reaction (OER) in the production of renewable energy and chemicals, the construction of heterogeneous structures is crucial to break the linear scalar relationship of a single catalyst. This heterogeneous structure construction helps creatively achieve high activity and stability. However, the synthesis process of heterogeneous crystalline materials is often complex and challenging to capture and reproduce, which limits their application. Here, the dynamic process of structural changes in Co-MOFs in alkali was captured by in situ powder X-ray diffraction, FT-IR spectroscopy, and Raman spectroscopy, and several self-reconfigured MOF heterogeneous materials with different structures were stably isolated. The created β-Co(OH)2/Co-MOF heterojunction structure facilitates rapid mass-charge transfer and exposure of active sites, which significantly enhanced OER activity. Experimental results show that this heterogeneous structure achieves a low overpotential of 333 mV at 10 mA cm-2. The findings provide new insights and directions for the search for highly reactive cobalt-based MOFs for sustainable energy technologies.

Dense Crystalline/Amorphous Phosphides/Oxides Interfacial Sites for Enhanced Industrial-Level Large Current Density Seawater Oxidation

Designing high-efficiency and low-cost catalysts with high current densities for the oxygen evolution reaction (OER) is critical for commercial seawater electrolysis. Here, we present a heterophase synthetic strategy for constructing an electrocatalyst with dense heterogeneous interfacial sites among crystalline Ni2P, Fe2P, CeO2, and amorphous NiFeCe oxides on nickel foam (NF). The synergistic effect of high-density crystalline and amorphous heterogeneous interfaces effectively promotes the redistribution of the charge density and optimizes the adsorbed oxygen intermediates, lowering the energy barrier and promoting the O2 desorption, thus enhancing the OER performance. The obtained NiFeO-CeO2/NF catalyst exhibited outstanding OER catalytic activity, with low overpotentials of 338 and 408 mV required to attain high current densities of 500 and 1000 mA cm-2, respectively, in alkaline natural seawater electrolytes. The solar-driven seawater electrolysis system presents a record-setting and stable solar-to-hydrogen conversion efficiency of 20.10%. This work provides directives for developing highly effective and stable catalysts for large-scale clean energy production.

Arming Ru with Oxygen-Vacancy-Enriched RuO2 Sub-Nanometer Skin Activates Superior Bifunctionality for pH-Universal Overall Water Splitting

Water electrolysis has been expected to assimilate the renewable yet intermediate energy-derived electricity for green H₂ production. However, current benchmark anodic catalysts of Ir/Ru-based compounds suffer severely from poor dissolution resistance. Herein, an effective modification strategy is proposed by arming a sub-nanometer RuO₂ skin with abundant oxygen vacancies to the interconnected Ru clusters/carbon hybrid microsheet (denoted as Ru@V-RuO₂ /C HMS), which can not only inherit the high hydrogen evolution reaction (HER) activity of the Ru, but more importantly, activate the superior activity toward the oxygen evolution reaction (OER) in both acid and alkaline conditions. Outstandingly, it can achieve an ultralow overpotential of 176/201 mV for OER and 46/6 mV for the HER to reach 10 mA cm^-2 in acidic and alkaline solution, respectively. Inspiringly, the overall water splitting can be driven with an ultrasmall cell voltage of 1.467/1.437 V for 10 mA cm^-2 in 0.5 m H₂ SO₄ /1.0 m KOH, respectively. Density functional theory calculations reveal that armoring the oxygen-vacancy-enriched RuO₂ exoskeleton can cooperatively alter the interfacial electronic structure and make the adsorption behavior of hydrogen and oxygen intermediates much close to the ideal level, thus simultaneously speeding up the hydrogen evolution kinetics and decreasing the energy barrier of oxygen release.

Metal-organic framework-derived trimetallic oxides with dual sensing functions for ethanol

Metal-organic framework (MOF)-derived metal oxide semiconductors have recently received extensive attention in gas sensing applications due to their high porosity and three-dimensional architecture. Still, challenges remain for MOF-derived materials, including low-cost and facile synthetic methods, rational nanostructure design, and superior gas-sensing performances. Herein, a series of Fe-MIL-88B-derived trimetallic FeCoNi oxides (FCN-MOS) with a mesoporous structure were synthesized by a one-step hydrothermal reaction followed by calcination. The FCN-MOS system consists of three main phases: α-Fe₂O₃ (n-type), CoFe₂O₄, and NiFe₂O₄ (p-type), and the nanostructure and pore size can be controlled by altering the content of α-Fe₂O₃, CoFe₂O₄, and NiFe₂O₄. The sensors based on FCN-MOS exhibit a high response of 71.9, a good selectivity towards 100 ppm ethanol at 250 °C, and long-term stability up to 60 days. Additionally, the FCN-MOS-based sensors show a p-n transition gas sensing behavior with the alteration of the Fe/Co/Ni ratio.

Introducing non-bridging ligand in metal-organic framework-based electrocatalyst enabling reinforced oxygen evolution in seawater

Using seawater as the replacement of freshwater for electrolysis, with the integration of renewable energy, is deemed as an attractive manner to harvest green hydrogen. However, the complexity of seawater puts forward stricter requirement to the electrocatalyst to alleviate the chlorine electrochemistry and corrosion. Herein, a nanosheet array of NiFe-MOF@Ni₂P/Ni(OH)₂ is devised by partially substituting terephthalic acid (H₂BDC) ligand by ferrocenecarboxylic acid (FcCA). Tailoring the active site into an under-coordinated fashion affords NiFe-MOF@Ni₂P/Ni(OH)₂ excellent performance towards oxygen evolution reaction (OER), only requiring the overpotentials of 302 mV and 394 mV in alkaline seawater to drive the current densities of 100 and 1000 mA cm^-2, respectively. Moreover, the as-obtained electrocatalyst showed robust durability for operating more than 120 h at 500 mA cm^-2 under harsh condition (6 M KOH + 1.5 M NaCl, 60 ℃). Density functional theory (DFT) calculations confirmed that tuning the coordination environment of Ni in NiFe-MOF by incorporating the non-bridging FcCA ligands could boost the formation of more active catalytic sites, which can simultaneously enhance the electronic conductivity and accelerate OER kinetics. This work provides beneficial enlightenment of combining MOF-based electrocatalyst with direct electrolysis of seawater.

Dynamic Reconstructed RuO2 /NiFeOOH with Coherent Interface for Efficient Seawater Oxidation

Developing efficient oxygen evolution reaction (OER) electrocatalysts for seawater electrolysis is still a big challenge. Herein, a facile one-pot approach is reported to synthesize RuO₂ -incorporated NiFe-metal organic framework (RuO₂ /NiFe-MOF) with unique nanobrick-nanosheet heterostructure as precatalyst. Driven by electric field, the RuO₂ /NiFe-MOF dynamically reconstructs into RuO₂ nanoparticles-anchored NiFe oxy/hydroxide nanosheets (RuO₂ /NiFeOOH) with coherent interface, during which the dissolution and redeposition of RuO₂ are witnessed. Owing to the synergistic interaction between RuO₂ and NiFeOOH, the as-reconstructed RuO₂ /NiFeOOH exhibits outstanding alkaline OER activity with an ultralow overpotential of 187.6 mV at 10 mA cm^-2 and a small Tafel slope of 31.9 mV dec^-1 and excellent durability at high current densities of 840 and 1040 mA cm^-2 in 1 m potassium hydroxide (KOH). When evaluated for seawater oxidation, the RuO₂ /NiFeOOH only needs a low overpotential of 326.2 mV to achieve 500 mA cm^-2 and can continuously catalyze OER at 500 mA cm^-2 for 100 h with negligible activity degradation. Density function theory calculations reveal that the presence of strong interaction and enhanced charge transfer along the coherent interface between RuO₂ and NiFeOOH ensures improved OER activity and stability.