Tunable metal hydroxide-organic frameworks for catalysing oxygen evolution

Nitrogen-doped carbon nanotube encapsulated CoCu bimetallic alloy particles to promote efficient oxygen evolution reaction via electronic structure regulation

The rational design of efficient and cost-effective transition metal alloy electrocatalysts represents a huge challenge for the oxygen evolution reaction (OER). Herein, the bimetallic CoCu-MOF is employed as a template to significantly enhance the catalytic activity through in situ pyrolysis into melamine-assisted nitrogen-doped carbon nanotube (NCNT) encapsulated metal alloy electrocatalyst (Co1Cu1@NCNT/CC). The Co1Cu1@NCNT/CC demonstrates superior OER activity in 1 M KOH. The overpotential is 263 mV at 10 mA cm-2. Meanwhile, the catalyst exhibits superior long-term stability. The experimental results and density functional theory (DFT) calculations reveal the bimetallic synergies regulate the electronic structure and strong electronic metal-support interaction (EMSI) of the catalysts, increase the electron transport efficiency and optimize the adsorption capacity of oxygen-containing intermediates, leading to a significant improvement in both the OER activity and stability.

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.

Oxide Heterostructure Engineering Drives Stable Lattice Oxygen Evolution for Highly Efficient and Robust Water Electrolysis

Achieving a highly active and stable oxygen evolution reaction (OER) is critical for the implementation of water electrolysis in green hydrogen production but remains challenging. Steering the OER pathway from an adsorbate evolution mechanism (AEM), where a metal site serves as the active site, to the lattice oxygen mechanism (LOM) has been found to enhance OER activity; however, it suffers from low stability. In this work, we propose to construct CuOx/Co3O4 heterointerface, which enables the realization of a stable LOM pathway. The lattice oxygen characteristics are modulated near the heterointerface, resulting in a shift in the reaction pathway from AEM to LOM. In situ X-ray Absorption Fine Structure results further reveal that the valence state of cobalt is stabilized during the OER process, which alleviates corrosion of cobalt and maintains LOM stability. Consequently, the obtained CuOx/Co3O4 exhibits outstanding activity and stability for overall water electrolysis in freshwater, natural seawater, and high-salt wastewater, with a low overpotential of 308 mV at 100 mA cm-2 and stable overall water electrolysis at 500 mA cm-2 for 100 h. Our work demonstrates interface engineering as an effective strategy to activate and stabilize lattice oxygen, advancing the design of high-performance electrocatalysts for energy and environmental applications.

In Situ Reconfigured Heterostructure Active Sites on Transition Metal Sulfides Heterojunction for Accelerated Water Oxidation

Transition metal sulfides (TMSs) are promising noble-metal-free electrocatalysts for electrochemical water splitting due to their distinctive physical and chemical properties, but they usually undergo complicated structure reconfiguration during the oxygen evolution reaction (OER). Precisely controlling the in situ reconfiguration of TMSs for in situ generation of high-activity real active sites still remains a great challenge. Herein, we propose to in situ reconfigure heterostructure active-sites on transition metal sulfides via heterojunction engineering and achieve high OER performances on (Ni,Fe)S2/MoS2 catalysts. The continuous leaching of Mo and S during electrooxidation induces the reconfiguration, and the strong electronic interaction of (Ni,Fe)S2 and MoS2 generates the special Ni(OH)2/NiOOH/FeOOH heterostructure sites via an asynchronous reconfiguration of Fe and Ni. The (Ni,Fe)S2/MoS2 heterostructure catalyst therefore exhibits excellent OER activity (a small overpotential of 228 mV at 100 mA cm-2) and a low voltage in an alkaline water electrolyzer (1.44 V at 10 mA cm-2), outperforming the homogeneous Mo-free NiFe sulfide catalysts with conventional reconfiguration of Ni-doped FeOOH. This work sheds light on the precise structures design under complicated electrochemical reconstruction and broadens the horizon of reconstruction chemistry to design low-cost and efficient electrocatalysts.

A split-type near-infrared photoelectrochemical and colorimetric dual-mode biosensor for the high-performance determination of HepG2 cells

Hepatocellular carcinoma (HCC) stands as a grave illness characterized by elevated death rates. Early identification plays a vital role in improving patient survival. Herein, a novel split-type dual-mode biosensor featuring with near-infrared photoelectronchemical (PEC) and colorimetric sensing characteristics was developed for the high-performance detection of HepG2 cells. Biotin labeled aptamer (Bio-Apt1) was immobilized onto 96-well plates functionalized with streptavidin to capture HepG2 cells through specific binding. HepG2 cells were then labeled with another aptamer (Apt-2) by recognizing GPC3 on the surface of HepG2 cells. Apt 2 could form DNA double strand (dsDNA-ALP) with ALP-labeled complementary DNA (cDNA-ALP). Subsequently, ALP was released to catalyze AAP to form ascorbic acid (AA), and AA reduced HAuCl4 to form gold nanoparticles (AuNPs). Then the mixture containing AuNPs was introduced onto the surface of Y-MOFs/GCE to enhance the photocurrent response. The change of photocurrent corresponding to the concentration of HepG2 cells can be used for the PEC determination. ALP can catalyze the hydrolysis of disodium phenyl phosphate to produce phenol, followed by a reaction with 4-aminoantipyrine and potassium ferricyanide, resulting in a quinone derivative for the colorimetric determination. The photoelectrochemical and colorimetric detection models show excellent selectivity and sensitivity in identifying HepG2 cells, exhibiting a linear reaction range from 1.0 × 102 to 1.0 × 106 cells mL-1 and a detection limit of 13 cells mL-1 and 51 cells mL-1, respectively. The dual-mode split type biosensor avoided direct damage to biomolecules from high-energy light, and the independent signal transduction enabled the acquisition of reliable results.

Electron delocalization-modulated hydroxyl binding for enhanced hydrogen evolution reaction activity

The introduction of foreign metals with a higher oxophilicity represents a promising strategy to promote water dissociation and in turn kinetics of alkaline hydrogen evolution reaction (HER). However, the further improvement of HER activity is limited by the unfavorable interaction of hydroxyl generated by the dissociation of water with active sites. Herein, we propose a strategy of alkaline earth metal cations-driven electron delocalization to elaborately tailor the binding of hydroxyl with the active sites. Taking FeNiMg-layered double hydroxides (FeNiMg-LDH) as a prototypical example, the combined operando spectroscopy analysis and theoretical calculations show that the introduction of Mg cations in solid- solution phase can create a local electronic field and delocalize the electron between Fe and adsorbed hydroxyl, resulting in an optimization of hydroxyl binding strength. Accordingly, FeNiMg-LDH lowers the overpotentials to deliver 10 mA cm-2 in alkaline electrolyte by 39 and 64 mV, compared to FeNi-LDH and Ni-LDH catalysts, respectively. This work sheds new light on the rational design of advanced HER electrocatalyst for alkaline water electrolysis.

Kg-Scale Synthesis of Ultrathin Single-Crystalline MOF/GO/MOF Sandwich Nanosheets with Elevated Electrochemical Performance

The scalable preparation of 2D ultrathin metal-organic framework (MOF) nanosheets remains a significant challenge due to low stripping efficiency and susceptibility to agglomeration. Herein, a facile strategy is developed for the synthesis of 2D ultrathin single-crystal MOF/graphene oxide (GO)/MOF (MGM) sandwich-like nanosheets based on the Ni/Co-BDC MOF (BDC = 1,4-terephthalic acid), with GO serving as a structure-directing agent. Impressively, 1 kg of MGM nanosheets can be obtained in a single batch with a metal-based yield of 98.73%. Furthermore, the universality of this strategy is implemented by the successful synthesis of three additional 2D ultrathin nanosheets: MGM7-ABDC, MGM7-FBDC, and MGM7-BPDC (metal = Ni, Co; ligands = 2-aminoterephthalic acid (2-ABDC), 2-fluoroterephthalic acid (2-FBDC), and 4,4'- biphenyl dicarboxylic acid (BPDC)). The as-prepared MGM7 nanosheets (Ni: Co ratio = 7:3) exhibit excellent electrochemical performance as the cathode for aqueous basic zinc battery (159.2 mA h g-1 at 1 A g-1), anode for sodium-ion battery (SIB, 593.0 mA h g-1 at 0.2 A g-1), and electrocatalyst for the oxygen evolution reaction (OER, 224 mV overpotential at 10 mA cm-2), significantly outperforming both bulk MOFs and conventional MOF nanosheets. This work enables the scalable synthesis of 2D MOF nanosheets with enhanced properties for multiple applications.

Heteroatom-Based Ligand Engineering of Metal Organic Frameworks for Efficient and Robust Electrochemical Water Oxidation

Metal-organic frameworks (MOFs) are promising catalysts for the electrochemical oxygen evolution reaction (OER) due to their high surface area, tunable pore structures, and abundant active sites. Ligand engineering is an important strategy to optimize their performance. Here, we report the synthesis of NiFe-MOFs based on three different ligands: 1,4-terephthalic acid (BDC), 2,4-thiophene dicarboxylic acid (TDC), and 2,5-furandicarboxylic acid (FDC), to investigate the effects of heteroatom-based aromatic rings on OER performance. It is revealed that by incorporating electronegative sulfur and oxygen atoms into the ligands, the electron density at the metal sites is reduced, leading to enhanced metal-oxygen covalency and improved charge transfer kinetics. The NiFe-FDC/NF catalyst demonstrates an overpotential of 189 mV at 10 mA⋅cm-2 and stable performance over 1300 hours at 1 A cm-2. In situ infrared spectroscopy reveal minimal structural reconstruction in NiFe-FDC/NF, contributing to its superior stability. The NiFe-FDC/NF were then subjected to 3600 hours of OER operation and it's metal elution was monitored. These findings offer a novel approach to ligand design for high-performance MOF-based OER catalysts, highlighting the potential of furan-based ligands for MOF ligand engineering.

Polyoxometalated metal-organic framework superstructure for stable water oxidation

Stable, nonprecious catalysts are vital for large-scale alkaline water electrolysis. Here, we report a grafted superstructure, MOF@POM, formed by self-assembling a metal-organic framework (MOF) with polyoxometalate (POM). In situ electrochemical transformation converts MOF into active metal (oxy)hydroxides to produce a catalyst with a low overpotential of 178 millivolts at 10 milliamperes per square centimeter in alkaline electrolyte. An anion exchange membrane water electrolyzer incorporating this catalyst achieves 3 amperes per square centimeter at 1.78 volts at 80°C and stable operation at 2 amperes per square centimeter for 5140 hours at room temperature. In situ electrochemical spectroscopy and theoretical studies reveal that the synergistic interactions between metal atoms create a fast electron-transfer channel from catalytic iron and cobalt sites, nickel, and tungsten in the polyoxometalate to the electrode, stabilizing the metal sites and preventing dissolution.

Real-Time Monitoring of Fe-Induced Stable γ-NiOOH in Binder-Free FeNi MOF Electrocatalysts for Enhanced Oxygen Evolution

Hydrogen energy is a promising renewable source, and metal-organic frameworks (MOFs) are considered potential electrocatalysts for water electrolysis due to their abundant active sites, high porosity, and large surface area. The synthesis of bimetallic iron-nickel-benzene-1,3,5-tricarboxylate/nickel foam (FeNi-BTC/NF) MOF is reported using a binder-free one-pot method by immersing nickel foam (NF) into a solution of benzene-1,3,5-tricarboxylic acid (BTC), N,N-dimethylformamide (DMF), and iron (Fe) salts. FeNi-BTC/NF exhibits a low overpotential of 276 mV at 100 mA cm- 2, a Tafel slope of 94 mV dec-1, and stability exceeding 120 h. The Fe-Ni interaction facilitates the formation of a stable gamma-nickel oxyhydroxide (γ-NiOOH) phase, preventing its reversion to nickel hydroxyide (Ni(OH)₂), which is crucial for improving oxygen evolution reaction (OER) performance. This phase transition, revealed via in situ Raman spectroelectrochemical analysis, enhances electrocatalytic activity. Additionally, high-valent Fe modulates the electronic structure of Ni, enabling FeNi-BTC/NF to transform into γ-NiOOH at higher potentials, with Fe and γ-NiOOH synergistically boosting OER efficiency. The findings offer insights into Fe/Ni atom interactions and phase transformations in FeNi-BTC/NF MOFs for enhanced water splitting.

Role of Site-Specific Iron in Fe-Doped Nickel Hydroxide Toward Water Oxidation Revealed by Spatially Resolved Imaging at the Single-Particle Level

Water electrolysis driven by renewable electricity is limited by the slow-kinetic oxygen evolution reaction (OER). NiFe-based hydroxides are considered promising non-noble electrocatalysts toward the OER but require profound insight into the role of site-specific iron incorporation. Herein, we determined the critical role of edge sites on single-crystalline NiFe-based hydroxide toward the OER using spatially resolved in situ single-particle imaging techniques. The potential-driven incorporation of Fe into the specific edge or plane sites was achieved on two-dimensional (2D) Ni layer double hydroxide (LDH) single crystals. The spatially resolved scanning electrochemical cell microscopy imaging illustrated that Fe-doped edge sites dominated the activity of the OER rather than Fe-doped plane sites. In situ Raman spectroscopy imaging of single particles was used to monitor the evolution of edge and plane sites, revealing that the incorporation of Fe impeded the oxidation of Ni. Moreover, spatially resolved 18O-isotope-labeling experiments demonstrated that Fe doping hindered the oxygen exchange between Ni LDH and the electrolyte, inducing the switch of partial active sites from Ni to Fe. Combined with theoretical calculations, the Fe-Obridge-Ni sites contributed to the enhanced OER activity on Ni LDH with Fe doping at the edge, whereas the Ohollow (NiNiFe) sites induced by the infiltration of Fe into the plane were detrimental to the OER performance. This work provides direct spectroscopic evidence for understanding the specific sites at the single-particle level and guides the rational design of optimal electrocatalysts.

Electrochemically Promoted Activation of Light Alkanes at Ambient Conditions

The electrochemical activation of light alkanes into value-added products represents a promising pathway for sustainable chemical synthesis and the storage of renewable energy. In this study, we introduce an electrochemically promoted system that employs copper plates as electrode and oxygen as oxidant, capable of converting ethane into ethylene and acetic acid with production rates of 6.9 and 6.2 µmol·cm-2 Cu·h-1, respectively, with a combined selectivity exceeding 92%, under ambient conditions. Additionally, this system can convert propane to propylene at a rate of 11.6 µmol·cm-2 Cu·h-1, with selectivity reaching up to 86%. A 10 h run with ethane demonstrates consistent production of ethylene and acetic acid, with a sustained selectivity above 96%, and achieves an acetic acid concentration of 19 mM. In situ spectroscopic analysis reveals the active surface and a critical reaction intermediate. Combining with partial pressure dependence study and density functional theory (DFT) calculations, we propose a potential reaction mechanism involving the competitive adsorption of oxygen and alkane producing an alkyl group as a key reaction step in the reaction process.

Inorganic-organic hybrid cobalt spinel oxides for catalyzing the oxygen evolution reaction

Fully triggering the deep-seated potential of traditional nanomaterials, such as the classic spinel family, is of paramount importance in the field of materials science, which is yet believed to heavily depend on advanced conceptual designs and synthetic strategies. Herein, a type of inorganic-organic hybrid spinel oxide is designed using a π-conjugated azobenzene single-tooth coordination method to overcome their stubborn problems of moderate activity and phase instability in electrocatalytic reactions. Taking spinel Co3O4 nanocubes as a pre-catalyst, after subtle etching of the cube surfaces, some oxygen atoms in the tetrahedral Co-O coordination field are replaced and selectively linked to weakly polar azo-extended π-conjugated units (π*-N=N-π*) via electrophilic carboxyl groups. The π-conjugation structure in Co3O4 suppresses the covalency competition between the tetrahedral and octahedral Co-O coordination fields, successfully preventing the phase transition during the electrocatalytic process and improving the electrocatalytic activity and durability. This study not only expands the spinel family but also provides useful guidelines for developing advanced functional materials.

Electronic Structure Tunable Metallosupramolecular Polymers as Bifunctional Electrocatalysts for Rechargeable Zn-Air Battery

Metallosupramolecular polymers (MSPs) have shown great potential in the area of oxygen electrocatalysis due to their tunable electronic structure, and predictable coordination environment. Further exploration of structure-performance relationships of oxygen electrocatalysts is crucial for designing highly efficient catalysts. Herein, a strategy is proposed to prepare MSP-based bifunctional oxygen electrocatalysts with different oxygen electrocatalytic preferences (Co-AQ and Co-AN) by adjusting the electronic structure of organic linkers. The electronic effects of organic linkers significantly influence the adsorbate evolution mechanism. Co-AQ, with an electron-withdrawing linker, demonstrated superior OER activity among the two with an overpotential of 280 mV at 10 mA cm-2 and 340 mV at 50 mA cm-2. In contrast, Co-AN, with an electron-donating linker, exhibited outstanding ORR activity with a large limiting current density of 6.14 mA cm-2. Furthermore, the Co-AQ-based Zn-air battery showed a high power density (135 mW cm-2) and excellent cycling stability of 100 h. This work presents a novel approach for adjusting bifunctional oxygen electrocatalysis performance and further reveals the structure-performance relationships of oxygen electrocatalysts.

Rational Design of a Quasi-Metal-Organic Framework by Ligand Engineering for Efficient Biomass Upgrading

The electrooxidation of 5-hydroxymethylfurfural (HMF) to 2,5-furandicarboxylic acid (FDCA), a monomer for degradable bioplastic, is a promising strategy for biomass upgrade and yet requires well-designed catalysts with high efficiency and selectivity. Taking advantage of the open metal sites of metal-organic frameworks (MOFs), quasi-MOFs represent viable catalysts, but the poor designability and unpredictable structures hinder their development. In this work, a Ni-based quasi-MOF was rationally designed and synthesized by controlled ligand engineering. Compared to the fully occupied metal clusters in the pristine MOFs, the accessible Ni sites in quasi-MOFs can efficiently convert HMF to FDCA with remarkable Faradaic efficiency (99.2%) and FDCA selectivity (98.3%). In situ characterizations and mechanistic analysis revealed that the open Ni sites created by partial ligand disconnection in quasi-MOFs are critical to the formation of high-valent active species and HMF oxidation. Moreover, serving as the anode in an integrated electrolysis system, such a quasi-MOF can not only reduce the cell voltage for hydrogen generation but also produce high-purity FDCA with good yield, offering a new opportunity for the simultaneous production of high value-added chemicals and sustainable hydrogen.

A zinc hydroxide-organic framework for electrochemical synthesis of H2O2

Zinc hydroxide-organic framework (Zn-HOF) nanosheets were synthesized by electrochemically triggered self-reconstruction of a double-ligand metal-organic framework (MOF) for the first time. This Zn-HOF catalyst exhibits high activity and selectivity towards the two-electron oxygen reduction reaction (2e-ORR), delivering a high H2O2 productivity of 3.95 mol gcat-1 h-1 at 0 V vs. RHE with a Faraday efficiency (FE) of approximately 96% in an alkaline electrolyte.

In situ formed CuSn alloy from multivariate metal-organic frameworks for tunable CO2 electroreduction

A molecular ligand separation method based on multivariate metal-organic frameworks (MOF) is developed to precisely regulate CuSn alloy for tuning the selectivity of HCOOH and CO in CO2 reduction. With this method, the agglomeration and heterogeneous nucleations of metals are effectively inhibited during the in situ electrochemical transformation of CuSn-MOFs into highly pure CuSn alloy. The low Sn content favors CO production, while the high Sn concentration facilitates HCOOH formation.

Unraveling the Role of Functional Groups of Terephthalate in Enhancing the Electrochemical Oxygen Evolution Reaction of Nickel-Organic Framework Nanoarrays

The platelike nickel-terephthalate-type metal-organic framework nanoarrays (Ni-BDC NAs) on carbon cloth are obtained by employing agaric-like Ni(OH)2 NAs as sacrificial templates. The microenvironment of Ni-BDC NAs is modulated by various neighboring functional groups (-NH2, -NO2, and -Br) on the carboxylate ligand, exerting minimal destructive effects on the structure and morphology of Ni-BDC NAs. The electrochemical oxygen evolution reaction (OER) of Ni-BDC-NH2 NAs, Ni-BDC-NO2 NAs, and Ni-BDC-Br NAs exhibited a significant enhancement compared to that of Ni-BDC NAs alone, as evidenced by both experimental and theoretical assessments. The presence of neighboring groups exerts a positive influence on the electronic coupling between Ni and O atoms, thereby facilitating the thermodynamically favorable formation of *O intermediates on Ni sites and accelerating the kinetics of the OER. The findings presented here provide valuable insights for the design and utilization of carboxylic acid molecules with functional group effects, enhancing the activity of the OER across diverse Ni centers.

Coupling Nitrate-to-Ammonia Conversion and Sulfion Oxidation Reaction Over Hierarchical Porous Spinel MFe2O4 (M═Ni, Co, Fe, Mn) in Wastewater

The construction of coupled electrolysis systems utilizing renewable energy sources for electrocatalytic nitrate reduction and sulfion oxidation reactions (NO3RR and SOR), is considered a promising approach for environmental remediation, ammonia production, and sulfur recovery. Here, a simple chemical dealloying method is reported to fabricate a hierarchical porous multi-metallic spinel MFe2O4 (M═Ni, Co, Fe, Mn) dual-functional electrocatalysts consisting of Mn-doped porous NiFe2O4/CoFe2O4 heterostructure networks and Ni/Co/Mn co-doped Fe3O4 nanosheet networks. The excellent NO3RR with high NH3 Faradaic efficiency of 95.2% at -0.80 V versus reversible hydrogen electrode (vs RHE) and NH3 yield rate of 608.9 µmol h-1 cm-2 at -1.60 V vs RHE, and impressive SOR performance (100 mA cm-2@0.98 V vs RHE) is achieved for MFe2O4. Key intermediates such as *NO, *NH2, and NH3 are identified in the NO3RR process by in situ Fourier transform infrared spectroscopy (in situ FTIR). The MFe2O4-assembled two-electrode coupling system (NO3RR||SOR) shows an ultra-low cell voltage of 1.14 V at 10 mA cm-2, much lower than the NO3RR||OER (oxygen evolution reaction, 10 mA cm-2@2.62 V), simultaneously achieving two expected targets of value-added ammonia generation and sulfur recovery, and also demonstrating high durability of 18 h. This work also demonstrates the great potential of spinel ferrite-based catalysts for environmental remediation.

Spin effects in regulating the adsorption characteristics of metal ions

Understanding the adsorption behavior of intermediates at interfaces is crucial for various heterogeneous systems, but less attention has been paid to metal species. This study investigates the manipulation of Co3+ spin states in ZnCo2O4 spinel oxides and establishes their impact on metal ion adsorption. Using electrochemical sensing as a metric, we reveal a quasi-linear relationship between the adsorption affinity of metal ions and the high-spin state fraction of Co3+ sites. Increasing the high-spin state of Co3+ shifts its d-band center downward relative to the Fermi level, thereby weakening metal ion adsorption and enhancing sensing performance. These findings demonstrate a spin-state-dependent mechanism for optimizing interactions with various metal species, including Cu2+, Cd2+, and Pb2+. This work provides new insights into the physicochemical determinants of metal ion adsorption, paving the way for advanced sensing technologies and beyond.

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.

Ligand-Tuning Metallic Sites in Molecular Complexes for Efficient Water Oxidation

Metal-organic hybrid catalysts with highly tunable single-sites are promising for oxygen-evolution reaction (OER), but molecular-scale understanding of underlying reaction mechanisms still remain elusive on these bulk materials. Herein, we report a direct construction of heterogenized molecular complexes stabilized on carbon substrates via coordinating Fe-Ni sites with four aromatic carboxylate ligands (FeNi-Lx). The ligands-tuning π-π stacking interaction between aromatic carboxylate ligands and carbon supports promote the oxidative charge accumulation on Fe-Ni sites via fast electron transferring, thus the optimized FeNi-Lx rendering a mass activity of 6680 A gFe/Ni -1 at 0.3 V overpotential. In situ characteristics and theoretical analysis demonstrate that the OH- nucleophilic attack on hypervalent iron sites induce the reconstruction of active Fe-O-Ni species, accompanying with fast valence increasing. Whereas, during OER, the unexpected valence reduction of Fe-O-Ni sites would be attributed to the oxygen-generating from OOH* intermediates. These findings would establish an essential understanding of the origin of active centers in molecular complexes catalysts for oxygen-evolution.

Roll-to-Roll Flash Joule Heating to Stabilize Electrocatalysts onto Meter-Scale Ni Foam for Advanced Water Splitting

The seamless integration of electrocatalysts onto the electrode is crucial for enhancing water electrolyzers, yet it is especially challenging when scaled up to large manufacturing. Despite thorough investigation, there are few reports that tackle this integration through roll-to-roll (R2R) methodology, a technique crucial for fulfilling industrial-scale demands. Here, we develop an R2R flash Joule heating (R2R-FJH) system to process catalytic electrodes with superior performance. The electrodes exhibited improved stability and activity, showcasing an exceptional performance within an alkaline water electrolysis (AWE) system. They achieved a low operation potential of 1.66 V at 0.5 A cm-2, coupled with outstanding durability over the operation of 800 h. We further demonstrated a prototype of a rolled-up water splitting apparatus, illustrating the efficiency of R2R-FJH electrodes in producing high-purity hydrogen through advanced water oxidation. Our study emphasized the practicality and scalability of the R2R-FJH strategy in the industrial manufacturing of high-performance electrodes for water electrolysis.

Origin of Enhanced Oxygen Evolution in Restructured Metal-Organic Frameworks for Anion Exchange Membrane Water Electrolysis

Metal-Organic Frameworks (MOFs), praised for structural flexibility and tunability, are prominent catalyst prototypes for exploring oxygen evolution reaction (OER). Yet, their intricate transformations under OER, especially in industrial high-current environments, pose significant challenges in accurately elucidating their structure-activity correlation. Here, we harnessed an electrooxidation process for controllable MOF reconstruction, discovering that Fe doping expedites Ni(Fe) MOF structural evolution, accompanied by the elongation of Ni-O bonds, monitored by in situ Raman and UV/Visible spectroscopy. Theoretical modeling further reveals that Fe doping and defect-induced tensile strain in the NiO6 octahedra augments the metal ds-O p hybridization, optimizing their adsorption behavior and augmenting OER activity. The reconstructed Ni(Fe) MOF, serving as the anode in anion exchange membrane water electrolysis, achieves a noteworthy current density of 3300 mA cm-2 at 2.2 V while maintaining equally stable operation 500 mA cm-2 for 300 h and 1000 mA cm-2 for 170 h. This undertaking elevates our comprehension of OER catalyst reconstruction, furnishing promising avenues for designing highly efficacious catalysts across electrochemical platforms.

Advances and Challenges in Interfacial Binding Forces for Electrocatalysts

As a practical chemical energy conversion technology, electrocatalysis could be used in fields of energy conversion and environmental protection. In recent years, significant research efforts have been devoted to the design and development of high-performance electrocatalysts because the rational design of catalysts is crucial for enhancing electrocatalytic performance. Creating electrocatalysts by forming interactions between different components at the interface is an important means of controlling and improving performance. Therefore, several common interfacial binding forces used for synthesizing electrocatalysts was systematically summarized in this review for the first time. The discussion revolves around the crucial roles these binding forces play in various electrocatalytic reaction processes. Various characterization techniques capable of proving the existence of these interfacial binding forces was also involved in the review. Finally, some prospects and challenges for designing and researching materials through the utilization of interfacial binding forces were presented.

High-Density Atomic Level Defect Engineering of 2D Fe-Based Metal-Organic Frameworks Boosts Oxygen and Hydrogen Evolution Reactions

Electrocatalysts based on metal-organic frameworks (MOFs) attracted significant attention for water splitting, while the transition between MOFs and metal oxyhydroxide poses a great challenge in identifying authentic active sites and long-term stability. Herein, we employ on-purpose defect engineering to create high-density atomic level defects on two-dimensional Fe-MOFs. The coordination number of Fe changes from 6 to 4.46, and over 28% of unsaturated Fe sites are formed in the optimized Fe-MOF. In situ characterizations of the most optimized Fe-MOF0.3 electrocatalyst during the oxygen evolution reaction (OER) process using Fourier transform infrared and Raman spectroscopy have revealed that some Fe unsaturated sites become oxidized with a concomitant dissociation of water molecules, causing generation of the crucial *OH intermediates and Fe oxyhydroxide. Moreover, the presence of Fe oxyhydroxide is compatible with the Volmer and Heyrovsky steps during the hydrogen evolution reaction (HER) process, which lower its energy barrier and accelerate the kinetics. As a result, the optimized Fe-MOF electrodes delivered remarkable OER (259 mV at 10 mA cm-2) and HER (36 mV at 10 mA cm-2) performance. Our study offers comprehensive understanding of the effect of phase transformation on the electrocatalytic process of MOF-based materials.

Synergistic Catalytic Sites in High-Entropy Metal Hydroxide Organic Framework for Oxygen Evolution Reaction

The integration of multiple elements in a high-entropy state is crucial in the design of high-performance, durable electrocatalysts. High-entropy metal hydroxide organic frameworks (HE-MHOFs) are synthesized under mild solvothermal conditions. This novel crystalline metal-organic framework (MOF) features a random, homogeneous distribution of cations within high-entropy hydroxide layers. HE-MHOF exhibits excellent electrocatalytic performance for the oxygen evolution reaction (OER), reaching a current density of 100 mA cm-2 at ≈1.64 VRHE, and demonstrates remarkable durability, maintaining a current density of 10 mA cm-2 for over 100 h. Notably, HE-MHOF outperforms precious metal-based electrocatalysts despite containing only ≈60% OER active metals. Ab initio calculations and operando X-ray absorption spectroscopy (XAS) demonstrate that the high-entropy catalyst contains active sites that facilitate a multifaceted OER mechanism. This study highlights the benefits of high-entropy MOFs in developing noble metal-free electrocatalysts, reducing reliance on precious metals, lowering metal loading (especially for Ni, Co, and Mn), and ultimately reducing costs for sustainable water electrolysis technologies.

Recent advances on metal-organic framework-based electrochemical sensors for determination of organic small molecules

Metal-organic frameworks (MOFs) are an extraordinarily versatile class of porous materials renowned for their intricate three-dimensional skeletal architectures and exceptional chemical properties. These extraordinary attributes have pushed MOFs into the vanguard of diverse disciplines such as microporous conduction, catalysis, separation, biomedical engineering, and electrochemical sensing. The focus of this review is to offer a comprehensive summary of recent advancements in designing MOF-based electrochemical sensors for detecting organic small molecules. offer a comprehensive survey of the recent progress in the methodologies adopted for the construction of MOF composites, covering template-assisted synthesis, Modification in synthesis, and post-synthesis modification. In addition, we discuss the practical application of MOF-based electrochemical sensors in the detection of organic small molecules. Our findings highlight the superior electrochemical sensing capabilities of these novel composites compared to those of their pristine counterparts. In conclusion, we provide a condensed perspective on the potential future trajectories in this domain, underscoring the impetus for continued enquiry and enhancement of MOF composite assemblies. With sustained investigation, the horizon appears bright for electrochemical sensing of small organic molecules and their myriad applications.

Corrosion-resistant NiFe anode towards kilowatt-scale alkaline seawater electrolysis

Development of large-scale alkaline seawater electrolysis requires robust and corrosion-resistant anodes. Here we propose engineering NiFe layered double hydroxide (LDH)-based anodes by incorporating a series of anions into the LDH interlayers. The most optimal NiFe LDH anode with intercalated phosphates demonstrates stable operation at a high current density of 1.0 A cm-2 for over 1000 hours in a 2 W-scale alkaline seawater electrolyzer (ASWE). Fundamental studies indicate that the basicity, indicated by pKa values, of the intercalated anions in NiFe LDH governs its oxygen evolution reaction activity and corrosion resistance. Highly basic anions (i.e., phosphates) securely anchor Fe sites and facilitate proton transfer to boost both durability and activity. Notably, we demonstrate the proof-of-concept for the NiFe anode in an industrial 1 kW-scale ASWE stack (1,081.2 cm2 anode area in total). This unit achieves a stable operating current density of 0.5 A cm-2 at about 2.0 V, twice that of the commercial alkaline pure water electrolyzer, contributing to an economically competitive hydrogen production cost of US$ 1.96 kgH2-1.

Metal-Hydroxide Organic Frameworks for Aqueous Nickel-Zinc Batteries

Hydroxides exhibit a high theoretical capacity for energy storage by ion release and are often intercalated with anions to enhance the ion migration kinetics. In this study, a series of metal-hydroxide organic frameworks (MHOFs) are synthesized by intercalating aromatic organic linkers into hydroxides using I-M/Ni(OH)2 (where M = Co2+, Cu2+, Mg2+, Fe2+). The coordination environment and layer spacing (1.09 nm) of I-M/Ni(OH)2 are explored by X-ray absorption fine structure and cryo-electron microscopy. The intercalation nanostructure improves the conductivity of the hydroxides and facilitates Zn2+ migration by increasing the interlayer spacing, while enhancing the rate capability and cycling stability. Consequently, the I-Co/Ni(OH)2 material exhibites a satisfactory specific capacity of 0.35 mAh cm-2 at 3 mA cm-2 and a high peak power density of 6.78 mW cm-2. This study offers a novel perspective on the design of intercalated hydroxide and provides new insights into high-performance nickel-zinc batteres.

Metal Hydroxide-Organic Framework Mediated Structural Reengineering Enables Efficient NiFe Interaction for Robust Water Oxidation

NiFe layered double hydroxide (NiFe LDH) derived oxyhydroxides are promising electrocatalysts for the alkaline oxygen evolution reaction (OER). However, NiFe LDH with a stable metal-oxygen-metal (M-O-M) structure suffers from inadequate NiFe interaction, leading to undesirable activity and stability. Herein, we develop a NiFe hydroxide-organic framework (NiFe HOF) via modification of NiFe LDH with an organic linker to break the structural constraint of M-O-M and thus boost the OER. NiFe HOF with reconfigurable metal sites facilitates structural reengineering under the OER condition to form abundant NiFe interaction and prolonged M-O bonds, stimulating lattice oxygen mechanism. Therefore, NiFe HOF shows a distinctly decreased overpotential at 50 mA cm-2, which is 68 mV lower than that of NiFe LDH. The anion exchange membrane electrolyzer using NiFe HOF as anode electrode displays ultralong stability exceeding 1050 h at 1 A cm-2 with a low attenuation of 0.16 mV h-1.

Operando-reconstructed polyatomic ion layers boost the activity and stability of industrial current-density water splitting

Metal-organic frameworks have garnered attention as highly efficient pre-electrocatalysts for the oxygen evolution reaction (OER). Current structure-activity relationships primarily rely on the assumption that the complete dissolution of organic ligands occurs during electrocatalysis. Herein, modeling based on NiFe Prussian blue analogs (NiFe-PBAs) show that cyanide ligands leach from the matrix and subsequently oxidize to corresponding inorganic ions (ammonium and carbonate) that re-adsorb onto the surface of NiFe OOH during the OER process. Interestingly, the surface-adsorbed inorganic ions induce the OER reaction of NiFe OOH to switch from the adsorbate evolution to the lattice-oxygen-mediated mechanism, thus contributing to the high activity. In addition, this reconstructed inorganic ion layer acting as a versatile protective layer can prevent the dissolution of metal sites to maintain contact between catalytic sites and reactive ions, thus breaking the activity-stability trade-off. Consequently, our constructed NiFe-PBAs exhibit excellent durability for 1250 h with an ultralow overpotential of 253 mV at 100 mA cm-2. The scale-up NiFe-PBAs operated with a low energy consumption of ∼4.18 kWh m-3 H2 in industrial water electrolysis equipment. The economic analysis of the entire life cycle demonstrates that this green hydrogen production is priced at US$2.59 [Formula: see text] , meeting global targets ( Collapse

Key Words

• Chemical recognition
• Industrial water electrolysis
• Oxygen evolution mechanism

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Affiliation(s)

Yingxia Zhao Guangdong Engineering Technology Research Center of Modern Fine Chemical Engineering, School of Chemical Engineering and Light Industry, Guangdong University of Technology, Guangzhou 510006, China; Analysis and Test Center Guangdong University of Technology, Guangdong University of Technology, Guangzhou 510006, China
Ying Wu Guangdong Engineering Technology Research Center of Modern Fine Chemical Engineering, School of Chemical Engineering and Light Industry, Guangdong University of Technology, Guangzhou 510006, China; Jieyang Branch of Chemistry and Chemical Engineering Guangdong Laboratory (Rongjiang Laboratory), Jieyang 515200, China; School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510641, China
Qunlei Wen State Key Laboratory of Materials Processing and Die & Mould Technology, and School of Materials Science and Engineering, Huazhong University of Science and Technology, Wuhan 430074, China
Danji Huang State Key Laboratory of Advanced Electromagnetic Engineering and Technology, and School of Electrical and Electronic Engineering, Huazhong University of Science and Technology, Wuhan 430074, China
Ruoou Yang State Key Laboratory of Materials Processing and Die & Mould Technology, and School of Materials Science and Engineering, Huazhong University of Science and Technology, Wuhan 430074, China
Haozhi Wang State Key Laboratory of Marine Resource Utilization in South China Sea, School of Materials Science and Engineering, Hainan University, Haikou 570228, China
Yingying Xu Guangdong Engineering Technology Research Center of Modern Fine Chemical Engineering, School of Chemical Engineering and Light Industry, Guangdong University of Technology, Guangzhou 510006, China
Ming Sun Guangdong Engineering Technology Research Center of Modern Fine Chemical Engineering, School of Chemical Engineering and Light Industry, Guangdong University of Technology, Guangzhou 510006, China; Jieyang Branch of Chemistry and Chemical Engineering Guangdong Laboratory (Rongjiang Laboratory), Jieyang 515200, China
Youwen Liu State Key Laboratory of Materials Processing and Die & Mould Technology, and School of Materials Science and Engineering, Huazhong University of Science and Technology, Wuhan 430074, China.
Jiakun Fang State Key Laboratory of Advanced Electromagnetic Engineering and Technology, and School of Electrical and Electronic Engineering, Huazhong University of Science and Technology, Wuhan 430074, China.
Tianyou Zhai State Key Laboratory of Materials Processing and Die & Mould Technology, and School of Materials Science and Engineering, Huazhong University of Science and Technology, Wuhan 430074, China
Lin Yu Guangdong Engineering Technology Research Center of Modern Fine Chemical Engineering, School of Chemical Engineering and Light Industry, Guangdong University of Technology, Guangzhou 510006, China; Jieyang Branch of Chemistry and Chemical Engineering Guangdong Laboratory (Rongjiang Laboratory), Jieyang 515200, China.

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Bioinspired Molecular Scalpel for Two-Dimensional Metal-Organic Nanosheet: Design Strategies and Recent Progress

Ultrathin two-dimensional (2D) metal-organic nanosheets (MONs) have attracted continued attention in the field of advanced functional materials. Their nanoscale thickness, high surface-to-volume ratio, and abundant accessible active sites, are superior advantages compared with their 3D bulk counterparts. Bioinspired molecular scalpel strategy is a promising method for the creation of 2D MONs, and may solve the current shortcomings of MONs synthesis. This review aims to provide a state-of-the-art overview of molecular scalpel strategies and share the results of current development to provide a better solution for MONs synthesis. Different types of molecular scalpel strategies have been systematically summarized. Both mechanisms, advantages and limitations of multiform molecular scalpel strategies have been discussed. Besides, the challenges to be overcome and the question to be solved are also introduced.

Nonalkaline Fabrication of Al-Based Metal-Organic Frameworks with Tailored Water Sorption Properties via Polymeric Hydroxy-Aluminum Basicity Modulation

Metal-organic frameworks (MOFs) are porous crystalline materials composed of metallic nodes and organic ligands, demonstrating increasing potential in water harvesting in arid and semiarid regions. This study presents a nonalkaline, water-based, and scalable synthesis strategy designed to adjust the water sorption properties of aluminum-based MOFs (Al-MOFs), specifically, AlFum and MOF-303, by modifying the basicity of the metal source, polymeric hydroxy-aluminum, as an alternative. Characterizations, including X-ray diffraction (XRD), scanning electron microscopy (SEM), and thermogravimetric analyses (TGA), confirmed the successful synthesis of Al-MOFs. The results revealed that high-basicity polymeric hydroxy-aluminum introduced additional mesoscopic intraparticle defects, interparticle voids, and hydrophilic surface sites to the primary microporous Al-MOFs. This led to an enhanced external surface area and uniformity in the particle size. Consequently, the water sorption performance of basicity-modulated Al-MOFs was significantly improved. Specifically, within the typical working humidity between 0.05 and 0.3, using polymeric hydroxy-aluminum of the highest basicity resulted in a 23% and 68% increase in water uptake for AlFum and MOF-303, respectively, achieving capacities of 0.43 and 0.37 g·g-1. Cyclic water adsorption-desorption tests further indicated the hydrolytic stability of prepared Al-MOFs. This study offers a novel approach to engineering MOF properties through metal source modulation, with important implications for applications in water harvesting and heat transfer.

Hollow Structure Derived Phosphide Nanosheets for Water Oxidation

Avoiding the stacking of active sites in catalyst structural design is a promising route for realizing active oxygen evolution reaction (OER). Herein, using a CoFe Prussian blue analoge cube with hollow structure (C-CoFe PBA) as a derived support, a highly effective Ni2P-FeP4-Co2P catalyst with a larger specific surface area is reported. Benefiting from the abundant active sites and fast charge transfer capability of the phosphide nanosheets, the Ni2P-FeP4-Co2P catalyst in 1 m KOH requires only overpotentials of 248 and 277 mV to reach current density of 10 and 50 mA cm-2 and outperforms the commercial catalyst RuO2 and most reported non-noble metal OER catalysts. In addition, the two-electrode system consisting of Ni2P-FeP4-Co2P and Pt/C is able to achieve a current density of 10 and 50 mA cm-2 at 1.529 and 1.65 V. This work provides more ideas and directions for synthesizing transition metal catalysts for efficient OER performance.

Unveiling the Enhancement of Electrocatalytic Oxygen Evolution Activity in Ru-Fe2O3/CoS Heterojunction Catalysts

The development of highly efficient electrocatalysts for the sluggish anodic oxygen evolution reaction (OER) is crucial to meet the practical demand for water splitting. In this study, an effective approach is proposed that simultaneously enhances interfacial interaction and catalytic activity by modifying Fe2O3/CoS heterojunction using Ru doping strategy to construct an efficient electrocatalytic oxygen evolution catalyst. The unique morphology of Ru doped Fe2O3 (Ru-Fe2O3) nanoring decorated by CoS nanoparticles ensures a large active surface area and a high number of active sites. The designed Ru-Fe2O3/CoS catalyst achieves a low OER overpotential (264 mV) at 10 mA cm-2 and demonstrates exceptional stability even at high current density of 100 mA cm-2, maintaining its performance for an impressive duration of 90 h. The catalytic performance of this Ru-Fe2O3/CoS catalyst surpasses that of other iron-based oxide catalysts and even outperforms the state-of-the-art RuO2. Density functional theory (DFT) calculation as well as experimental in situ characterization confirm that the introduction of Ru atoms can enhance the interfacial electron interaction, accelerating the electron transfer, and serve as highly active sites reducing the energy barrier for rate determination step. This work provides an efficient strategy to reveal the enhancement of electrocatalytic oxygen evolution activity of heterojunction catalysts by doping engineering.

Ce-4f as an electron-modulation reservoir weakening Fe-O bond to induce iron vacancies in CeFevNi hydroxide for enhancing oxygen evolution reaction

Designing novel rare-earth-transition metal composites is at the forefront of electrocatalyst research. However, the modulation of transition metal electronic structures by rare earths to induce vacancy defects and enhance electrochemical performance has rarely been reported. In this study, we systematically investigate the mechanism by which Ce-4f electron modulation weakens the Fe-O bond, thereby altering the electronic structure in CeFevNi hydroxide to improve oxygen evolution reaction (OER) performance. Theoretical calculations and experimental characterizations reveal that Ce-4f orbitals function as electron-modulation reservoirs, capable not only of retaining or donating electrons but also of influencing the material's electronic structure. Moreover, Ce-4f bands optimize the Fe lower Hubbard bands (LHB) and O-2p bands, leading to weakened Fe-O bonds and the formation of cationic vacancies. This change results in the upshift of the d-band center at the active sites, favoring the reaction energy barrier for oxygen intermediates in the OER process. The synthesized catalyst demonstrated an overpotential of 201 mV at 10 mA cm-2 and a lifetime exceeding 200 h at 100 mA cm-2 under alkaline conditions. This work offers a proof-of-concept for the application of the mechanism of rare earth-induced transition metal vacancy defects, providing a general guideline for the design and development of novel highly efficient catalysts.

Tailored interface engineering of Co3Fe7/Fe3C heterojunctions for enhancing oxygen reduction reaction in zinc-air batteries

The rational construction of highly active and robust non-precious metal oxygen reduction electrocatalysts is a vital factor to facilitate commercial applications of Zn-air batteries. In this study, a precise and stable heterostructure, comprised of a coupling of Co3Fe7 and Fe3C, was constructed through an interface engineering-induced strategy. The coordination polymerization of the resin with the bimetallic components was meticulously regulated to control the interfacial characteristics of the heterostructure. The synergistic interfacial effects of the heterostructure successfully facilitated electron coupling and rapid charge transfer. Consequently, the optimized CST-FeCo displayed superb oxygen reduction catalytic activity with a positive half-wave potential of 0.855 V vs. RHE. Furthermore, the CST-FeCo air electrode of the liquid zinc-air battery revealed a large specific capacity of 805.6 mAh gZn-1, corresponding to a remarkable peak power density of 162.7 mW cm-2, and a long charge/discharge cycle stability of 220 h, surpassing that of the commercial Pt/C catalyst.

Bimetal Oxides Anchored on Carbon Nanotubes/Nanosheets as High-Efficiency and Durable Bifunctional Oxygen Catalyst for Advanced Zn-Air Battery: Experiments and DFT Calculations

To meet increasing requirement for innovative energy storage and conversion technology, it is urgent to prepare effective, affordable, and long-term stable oxygen electrocatalysts to replace precious metal-based counterparts. Herein, a two-step pyrolysis strategy is developed for controlled synthesis of Fe2O3 and Mn3O4 anchored on carbon nanotubes/nanosheets (Fe2O3-Mn3O4-CNTs/NSs). The typical catalyst has a high half-wave potential (E1/2 = 0.87 V) for oxygen reduction reaction (ORR), accompanied with a smaller overpotential (η10 = 290 mV) for oxygen evolution reaction (OER), showing substantial improvement in the ORR and OER performances. As well, density functional theory calculations are performed to illustrate the catalytic mechanism, where the in situ generated Fe2O3 directly correlates to the reduced energy barrier, rather than Mn3O4. The Fe2O3-Mn3O4-CNTs/NSs-based Zn-air battery exhibits a high-power density (153 mW cm-2) and satisfyingly long durability (1650 charge/discharge cycles/550 h). This work provides a new reference for preparation of highly reversible oxygen conversion catalysts.

Spontaneous-Spin-Polarized 2D π-d Conjugated Frameworks Towards Enhanced Oxygen Evolution Kinetics

Alternative strategies to design sustainable-element-based electrocatalysts enhancing oxygen evolution reaction (OER) kinetics are demanded to develop affordable yet high-performance water-electrolyzers for green hydrogen production. Here, it is demonstrated that the spontaneous-spin-polarized 2D π-d conjugated framework comprising abundant elements of nickel and iron with a ratio of Ni:Fe = 1:4 with benzenehexathiol linker (BHT) can improve OER kinetics by its unique electronic property. Among the bimetallic NiFex:y-BHTs with various ratios with Ni:Fe = x:y, the NiFe1:4-BHT exhibits the highest OER activity. The NiFe1:4-BHT shows a specific current density of 140 A g-1 at the overpotential of 350 mV. This performance is one of the best activities among state-of-the-art non-precious OER electrocatalysts and even comparable to that of the platinum-group-metals of RuO2 and IrO2. The density functional theory calculations uncover that introducing Ni into the homometallic Fe-BHT (e.g., Ni:Fe = 0:1) can emerge a spontaneous-spin-polarized state. Thus, this material can achieve improved OER kinetics with spin-polarization which previously required external magnetic fields. This work shows that a rational design of 2D π-d conjugated frameworks can be a powerful strategy to synthesize promising electrocatalysts with abundant elements for a wide spectrum of next-generation energy devices.

Perovskite nanocomposites: synthesis, properties, and applications from renewable energy to optoelectronics

The oxide and halide perovskite materials with a ABX3 structure exhibit a number of excellent properties, including a high dielectric constant, electrochemical properties, a wide band gap, and a large absorption coefficient. These properties have led to a range of applications, including renewable energy and optoelectronics, where high-performance catalysts are needed. However, it is difficult for a single structure of perovskite alone to simultaneously fulfill the diverse needs of multiple applications, such as high performance and good stability at the same time. Consequently, perovskite nanocomposites have been developed to address the current limitations and enhance their functionality by combining perovskite with two or more materials to create complementary materials. This review paper categorizes perovskite nanocomposites according to their structural composition and outlines their synthesis methodologies, as well as their applications in various fields. These include fuel cells, electrochemical water splitting, CO2 mitigation, supercapacitors, and optoelectronic devices. Additionally, the review presents a summary of their research status, practical challenges, and future prospects in the fields of renewable energy and electronics.

Single-Crystal Structural Analysis of 2D Metal-Organic Frameworks and Covalent Organic Frameworks by Three-Dimensional Electron Diffraction

ConspectusIn the development of 2D metal-organic frameworks (MOFs) and 2D covalent organic frameworks (COFs), obtaining structural details at the atomic level is crucial to understanding their properties and related mechanisms in potential applications. However, since 2D-MOFs and COFs are composed of layered structures and often exhibit sheet-like morphologies, it is challenging to grow large crystals suitable for single-crystal X-ray diffraction (SCXRD). Therefore, ab initio structure determination, which refers to solving the structure directly from experimental data without using any prior knowledge or computational input, is extremely rare for 2D-MOFs and COFs. In contrast to SCXRD, three-dimensional electron diffraction (3DED) only requires crystals sized in tens or hundreds of nanometers, making it an ideal method for single-crystal analysis of 2D-MOFs and COFs and obtaining their fine structural details.In this Account, we describe our recent development of the 3DED method and its application in structure determination and property studies of 2D-MOFs and COFs. A key development is the establishment of a continuous 3DED data collection protocol. By collecting electron diffraction (ED) patterns continuously while performing crystal tilting, the electron dose applied to the target nanocrystal is greatly reduced. This allows the acquisition of high-resolution 3DED data from 2D-MOFs and COFs by minimizing their damage under the electron beam. We have also developed an approach to couple 3DED with real-space structure solution methods, i.e., simulated annealing (SA), for single-crystal structural analysis of materials that do not have high crystallinity. We successfully determined two 2D-COF structures by combining 3DED with SA.We provide several examples demonstrating the application of 3DED for the ab initio structure determination of 2D-MOFs and COFs, revealing not only their in-plane structures but also their stacking modes at the atomic level. Notably, the obtained structural details serve as the foundation for further understanding the properties of 2D-MOFs and COFs, such as their electronic band structures, charge mobilities, etc. Beyond structure determination, we describe our work on using 3DED as a high-throughput method for the discovery of new materials. Using this approach, we discovered a novel MOF that was present only in trace amounts within a multiphasic mixture. Through this discovery, we were able to tune the synthesis conditions to obtain its pure phase.We detail how 3DED can be used to probe different levels of molecular motions in MOFs through the analysis of anisotropic displacement parameters (ADPs). Additionally, we show that 3DED can provide accurate information about intermolecular weak interactions such as hydrogen bonding and van der Waals (vdW) interactions. Our studies demonstrate that 3DED is a valuable method for the structural analysis of 2D-MOFs and COFs. We envision that 3DED can accelerate research in these fields by providing unambiguous structural models at the atomic level.

Constructing Co4(SO4)4 Clusters within Metal-Organic Frameworks for Efficient Oxygen Electrocatalysis

Multinuclear metal clusters are ideal candidates to catalyze small molecule activation reactions involving the transfer of multiple electrons. However, synthesizing active metal clusters is a big challenge. Herein, on constructing an unparalleled Co4(SO4)4 cluster within porphyrin-based metal-organic frameworks (MOFs) and the electrocatalytic features of such Co4(SO4)4 clusters for the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) is reported. The reaction of CoII sulfate and metal complexes of tetrakis(4-pyridyl)porphyrin under solvothermal conditions afforded Co4-M-MOFs (M═Co, Cu, and Zn). Crystallographic studies revealed that these Co4-M-MOFs have the same framework structure, having the Co4(SO4)4 clusters connected by metalloporphyrin units through Co─Npyridyl bonds. In the Co4(SO4)4 cluster, the four CoII ions are chemically and symmetrically equivalent and are each coordinated with four sulfate O atoms to give a distorted cube-like structure. Electrocatalytic studies showed that these Co4-M-MOFs are all active for electrocatalytic OER and ORR. Importantly, by regulating the activity of the metalloporphyrin units, it is confirmed that the Co4(SO4)4 cluster is active for oxygen electrocatalysis. With the use of Co porphyrins as connecting units, Co4-Co-MOF displays the highest electrocatalytic activity in this series of MOFs by showing a 10 mA cm-2 OER current density at 357 mV overpotential and an ORR half-wave potential at 0.83 V versus reversible hydrogen electrode (RHE). Theoretical studies revealed the synergistic effect of two proximal Co atoms in the Co4(SO4)4 cluster in OER by facilitating the formation of O─O bonds. This work is of fundamental significance to present the construction of Co4(SO4)4 clusters in framework structures for oxygen electrocatalysis and to demonstrate the cooperation between two proximal Co atoms in such clusters during the O─O bond formation process.

Boosting oxygen evolution reaction by FeNi hydroxide-organic framework electrocatalyst toward alkaline water electrolyzer

The oxygen evolution reaction plays a vital role in modern energy conversion and storage, and developing cost-efficient oxygen evolution reaction catalysts with industrially relevant activity and durability is highly desired but still challenging. Here, we report an efficient and durable FeNi hydroxide organic framework nanosheet array catalyst that competently affords long-term oxygen evolution reaction at industrial-grade current densities in alkaline electrolyte. The desirable high-intensity performance is attributed to three aspects as follows. First, two-dimensional nanosheet porous arrays with maximum specific surface facilitate mass/charge transfer to accommodate high-current-density catalysis. Second, in situ derived FeNi hydroxide motifs offer bimetallic synergistic catalysis centers with high intrinsic activity. Third, carboxyl ligands alleviate metal oxidation favorable for charge tolerability against peroxidation dissolution under strong polarization. As a result, this catalyst requires an overpotential of only 280 mV to deliver high current density up to 1 A/cm2 with long durability over 1000 h. Moreover, an alkaline water electrolyzer with this catalyst alternative demonstrates an increased economic effectiveness compared to commercial levels at present.

Ceria-Optimized Oxygen-Species Exchange in Hierarchical Bimetallic Hydroxide for Electrocatalytic Water Oxidation

The utilization of rare earth elements to regulate the interaction between catalysts and oxygen-containing species holds promising prospects in the field of oxygen electrocatalysis. Through structural engineering and adsorption regulation, it is possible to achieve high-performance catalytic sites with a broken activity-stability tradeoff. Herein, this work fabricates a hierarchical CeO2/NiCo hydroxide for electrocatalytic oxygen evolution reaction (OER). This material exhibits superior overpotentials and enhanced stability. Multiple potential-dependent experiments reveal that CeO2 promotes oxygen-species exchange, especially OH- ions, between catalyst and environment, thereby optimizing the redox transformation of hydroxide and the adsorption of oxygen-containing intermediates during OER. This is attributed to the reduction in the adsorption energy barrier of Ni to *OH facilitated by CeO2, particularly the near-interfacial Ni sites. The less-damaging adsorbate evolution mechanism and the CeO2 hierarchical shell significantly enhance the structural robustness, leading to exceptional stability. Additionally, the observed "self-healing" phenomenon provides further substantiation for the accelerated oxygen exchange. This work provides a neat strategy for the synthesis of ceria-based complex hollow electrocatalysts, as well as an in-depth insight into the co-catalytic role of CeO2 in terms of oxygen transfer.

Directionally Exfoliated Ni/Co Hydroxide-Organic Framework Nanosheets for Enhanced Wearable Glucose Sensing

We report two-dimensional (2D) Ni/Co-based metal hydroxide-organic framework nanosheets (Ni/Co-MHOF NSs) for the construction of an efficient electrochemical nonenzymatic glucose sensor. The nanosheet architecture maximizes the exposure of coordinatively unsaturated metal sites, which enables a largely improved electrocatalytic performance toward the glucose oxidation reaction. The as-designed nonenzymatic sensor exhibits a high sensitivity of 235.71 μA·mM-1·cm-2 and a wide linear range of 1-3000 μM. The sensor presents excellent selectivity against several potential interferences and a short response time of 3.0 s. Of interest, a high-performance flexible sensor is developed by depositing the Ni/Co-MHOF NSs on screen-printed electrodes, which reveal decent bending stability. The designed glucose sensor patch can attach to the human body and realize noninvasive glucose monitoring in human sweat. This work may shed light on the application of novel MHOFs in the field of wearable electrochemical sensing.

Determining Structures of Layer-by-Layer Spin-Coated Zinc Dicarboxylate-Based Metal-Organic Thin Films

Thin films of crystalline solids with substantial free volume built from organic chromophores and metal secondary building units (SBUs) are promising for engineering new optoelectronic properties through control of interchromophore coupling. Zn-based SBUs are especially relevant in this case because they avoid quenching the chromophore's luminescence. We find that layer-by-layer spin-coating using Zn acetate dihydrate and benzene-1,4-dicarboxylic acid (H2BDC) and biphenyl-4,4'-dicarboxylic acid (H2BPDC) linkers readily produces crystalline thin films. However, analysis of the grazing-incidence wide-angle X-ray scattering (GIWAXS) data reveals the structures of these films vary significantly with the linker, and with the metal-to-linker molar ratio used for fabrication. Under equimolar conditions, H2BPDC creates a type of structure like that proposed for SURMOF-2, whereas H2BDC generates a different metal-hydroxide-organic framework. Large excess of Zn2+ ions causes the growth of layered zinc hydroxides, irrespective of the linker used. Density functional theory (DFT) calculations provide structural models with minimum total energy that are consistent with the experimentally observed diffractograms. In the broader sense, this work illustrates the importance in this field of careful structure determination, e. g., by utilizing GIWAXS and DFT simulations to determine the structure of the obtained crystalline metal-organic thin films, such that properties can be rationally engineered and explained.

Amorphous Engineering of Scalable Metal-Organic Framework-Derived Electrocatalyst for Highly Efficient Oxygen Evolution Reaction

The engineering of amorphous metal-organic frameworks (MOFs) offers potential opportunities for the construction of electrocatalysts for efficient oxygen evolution reaction (OER). Herein, highly efficient OER performance and durability in alkaline electrolyte are discovered for MOF-derived amorphous and porous electrocatalysts, which are synthesized in a brief procedure and can be facilely produced in scalable quantities. The structural inheritance of MOF amorphous catalysts is significant for the retention of catalytic sites and the diffusion of electrolytes, and the presence of Fe sites can change the electronic structure and effectively control the adsorption behavior of important intermediates, accelerating reaction kinetics. The obtained amorphous A-FeNi can be transformed from FeNi-MOF effortlessly and instantly, and it only needs low overpotentials of 152 and 232 mV at 10 and 100 mA cm-2 with a Tafel slope of 17 mV dec-1 in 1 m KOH for OER. Moreover, A-FeNi possesses high corrosion resistance and durability, therefore A-FeNi can work continually for at least 400 h at 100 mA cm-2. This work may pave a new avenue for the design of MOFs-related amorphous electrocatalyst.

In Situ Modulation of Oxygen Vacancies on 2D Metal Hydroxide Organic Frameworks for High-Efficiency Oxygen Evolution Reaction

The discovery of non-precious catalysts for replacing the precious metal of ruthenium in the oxygen evolution reaction (OER) represents a key step in reducing the cost of green hydrogen production. The 2D d-MHOFs, a new 2D materials with controllable oxygen vacancies formed by controlling the degree of coordination bridging between metal hydroxyl oxide and BDC ligands are synthesized at room temperature, exhibit excellent OER properties with low overpotentials of 207  mV at 10 mA cm-2. High-resolution transmission electron microscopy images and density functional theory calculations demonstrate that the introduction of oxygen vacancy sites leads to a lattice distortion and charge redistribution in the catalysts, enhancing the OER activity of 2D d-MHOFs comprehensively. Synchrotron radiation and in situ Raman/Fourier transform infrared spectroscopy indicate that part of oxygen defect sites on the surface of 2D d-MHOFs are prone to transition to highly active metal hydroxyl oxides during the OER process. This work provides a mild strategy for scalable preparation of 2D d-MHOFs nanosheets with controllable oxygen defects, reveals the relationship between oxygen vacancies and OER performance, and offers a profound insight into the basic process of structural transformation in the OER process.

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

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