Exploitation of 2D Mn-MOF nanosheets for developing rapid, sensitive, and selective sensor for determination of Mn(II) ions in food and biological samples

Herein, for the first time, a sensitive potentiometric sensor exploiting ultrathin two-dimensional nanosheets of Mn metal-organic framework (2D Mn-MOF-NSs) was prepared to determine Mn(II) ion content with accuracy and precision. Furthermore, a comparative study between the 2D Mn-MOF-NSs-based sensor and the 3D Mn-MOF-based one has been established which proves the superiority of 2D Mn-MOF-NSs as a sensing material achieving a slope of 29.50 mV decade-1 within a wide linear range of 3.2 × 10-6 - 1.0 × 10-1 mol L-1. The 2D Mn-MOF-NSs-based sensor can be applied for measuring the Mn(II) ion content rapidly (3 s) without being affected by the sample pH within a range from 2.0 to 8.5. Additionally, the sensor demonstrates high selectivity towards Mn(II) ion compared to numerous other cations. To prove the broad and effective application of the proposed sensor in diverse sectors, it was applied successfully for the determination of Mn(II) ions content in different food samples in addition to biological sample. Notably, the results attained by the sensor align well with those of the inductively coupled plasma (ICP) technique. Therefore, this article presented the first Mn(II) ion selective sensor based on ultrathin nanosheets of 2D Mn-MOF as a unique sensing material which can be regarded as one of the few sensors currently available for monitoring Mn(II) levels in various food samples in addition to biological samples with a high reliability and sensitivity.

Bimetallic Fe(OH)x@Co0.8Fe0.2-MOF/NF composites as effective electrocatalysts for the production of 2,5-furandicarboxylic acid from 5-hydroxymethylfurfural

A wide range of catalytic techniques have been explored for the use of biomass components. For example, the electrocatalytic oxidation of 5-hydroxymethylfurfural (HMF) to 2,5-furandicarboxylic acid (FDCA) can be performed with excellent energy efficiency under safe operating conditions and with fine control of the production parameters. Metal-organic framework (MOF) catalysts with active metal centres have been prepared as electrocatalysts for the oxidation of HMF to FDCA. A Fe(OH)x@Co0.8Fe0.2-MOF/nickel foam (NF) was made via two steps: Co0.8Fe0.2-MOF/NF was synthesized by in situ solvothermal methods followed by the electrodeposition of Fe(OH)x. X-ray photoelectron spectroscopy (XPS) analysis confirmed the successful electrodeposition of Fe(OH)x on Co0.8Fe0.2-MOF/Ni. Fe(OH)x@Co0.8Fe0.2-MOF/NF demonstrated enhanced electrocatalytic activity for the oxidation of HMF in 1M KOH, requiring an overpotential of 236 mV and 263 mV versus RHE to achieve current densities of 50 and 100 mA cm-2, respectively, with an apparent Tafel slope of 92 mV. The electrochemically active surface area of the catalysts showed that Fe(OH)x incorporated samples possessed a higher number of active sites compared to Co0.8Fe0.2-MOF/Ni, enhancing efficiency and improving the yield of 5-Hydroxymethylfurfural oxidation reaction (HMFOR).

Electrocatalytic Conversion of Glucose into Renewable Formic Acid Using "Electron-Withdrawing" MoO3 Support under Mild Conditions

Electrocatalysis is a sustainable and effective approach to produce value-added chemical commodities from biomass, where highly effective catalyst is required. Since transition metal hydroxide is a feasible catalyst for electrochemical biomass conversion, rational optimization of its electrocatalytic activity is highly desired. Herein, electrocatalytic activity of glucose oxidation is significantly optimized by reducing the electron density at Ni active sites, which is achieved by depositing Ni(OH)2 at "electron-withdrawing" MoO3 support (Ni(OH)2MoO3-x). As results, the formation of active sites (NiOOH) and the adsorption of glucose are simultaneously facilitated in Ni(OH)2MoO3-x, which effectively converts glucose to formic acid (FA) with remarkable yield and Faraday efficiency (≈90.5 and 98%, respectively), far superior to conventional β-Ni(OH)2 catalyst (≈22.5 and 58.9%, respectively). In addition to a novel strategy for efficient FA production from glucose, this work offers valuable insights into the rational optimization of electrocatalytic oxidation of biomass-based substrates.

Cobalt-nickel composite nano-grass as an excellent electrode for urea oxidation

Urea-contaminated wastewater requires extensive energy for proper treatment before safe discharge to the surroundings. Direct urea fuel cells (DUFCs) could be utilized efficiently to treat urea-polluted water and generate electricity. The precious/expensive catalyst utilized at the electrodes is one of the main significant challenges to DUFC commercialization. In this study, a non-precious standalone electrode cobalt-nickel composites directly formed using a facile hydrothermal method on a highly porous conductive nickel foam (NF) surface. The developed electrode has an excellent nano-grass morphology and demonstrates outstanding activity towards urea electro-oxidation. Using a 0.33 M urea, the current density @ 0.5 V (vs. Ag/AgCl) in the case of the cobalt-nickel composite with the nano-grass electrode (Co/NF) is significantly higher than that obtained using the bare NF electrode. At the same conditions, the Co/NF electrode is successfully operated for a long term (24 h) with a slight degradation in the performance, with no effect on the surface morphology. The steady-state current generated after 24 hours of cell operation is twenty times that obtained using the bare NF. The perfect performance of the modified electrode is related to the synergetic effect between Ni and Co, excellent nano-grass morphology, and ease of charge transfer. The prepared materials on the surface of the NF have a high electrochemically active surface area of 44 cm2 that is significantly higher than that of bare NF.

Electrochemically embedded heterostructured Ni/NiS anchored onto carbon paper as bifunctional electrocatalysts for urea oxidation and hydrogen evolution reaction

Developing high-efficiency, cost-effective, and long-term stable nanostructured catalysts for electrocatalytic water splitting remains one of the most challenging aspects of hydrogen fuel production. Urea electrooxidation reaction (UOR) can produce hydrogen energy from nitrogen-rich wastewater, making it a more sustainable and cheaper source of hydrogen. In this study, we have developed Ni/NiS hybrid structures with cauliflower-like morphology on carbon paper electrodes through the application of dimethylsulfoxide solvents. These electrodes serve as highly efficient and long-lasting electrocatalysts for the hydrogen evolution reactions (HER) and UOR. In particular, the Ni/NiS cauliflower-like morphology is confirmed via X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). Furthermore, electrochemical characterization of the Ni/NiS@CP catalyst showed a 1.35 V onset potential versus RHE for the UOR in 1.0 M KOH and superior electrocatalytic performance compared to bare Ni@CP. Additionally, the Ni/NiS@CP catalyst also exhibits a low overpotential of 125 mV at 10 mA cm-2 for HER in 0.5 M H2SO4 with excellent durability, which is apparently lower than bare Ni@/CP (397 mV). Based on the results obtained, the synthesized Ni/NiS@CP catalyst may be a promising electrode candidate for handling urea-rich wastewater and generating hydrogen.

Synergistic Inclusion of Reaction Activator and Reaction Accelerator to Ni-MOF Toward Extra-Ordinary Performance of Urea Oxidation Reaction

Recently electrochemical urea oxidation reaction (UOR) has emerged as the technology of demand for commercialization of urea-based energy conversion. However, the nascent idea is limited by the energy burden of threshold voltage and the sluggish reaction kinetics involving a six-electron transfer mechanism. Herein, for the first time, the engineering of electrocatalysts are proposed with simultaneous inclusion of UOR activator and UOR accelerator. Nitrogen-doped carbon-decorated Ni-based Metal Organic Framework (MOF) has been synthesized as the base catalyst material. MoO2 and rGO with varied loading have been attached to the MOF to get the desired MoO2/Ni-MOF/rGO heterostructure incorporating defects and crystal strain within the materials. Investigations reveal that the invoked lattice strain and atomic defects promote plenteous Ni3+ active sites. The optimized sample demonstrates extraordinary performance of UOR having the potential value as low as 1.32 V versus RHE to reach the current density of 10 mA cm-2 and the tafel slope is only 31 mV dec-1 reflecting very fast reaction kinetics. Here MoO2 plays the role of UOR activator whereas optimized loading of rGO proliferates the reaction speed. This work, experimentally and theoretically, presents a new insight to enhance electrocatalytic urea oxidation reaction opening an avenue of urea-based energy-harvesting technology.

Recent advances in metal-catalysed oxidation reactions

Oxidation reactions are vital tools in synthetic organic chemistry. Oxidation of organic species such as alcohols, phenols, aldehydes and ketones provides synthetically valuable organic compounds, especially synthetic intermediates for several biologically active compounds. Some of these synthetic intermediates have shown their synthetic utility in the total synthesis of natural products. Several classical and modern synthetic approaches have been used to achieve these oxidation reactions. In this review article, various oxidation reactions achieved by metal catalysis are highlighted.

An Fe-doped Ni-based oxalate framework with a favorable electronic structure for electrocatalytic water and urea oxidation

An Fe-doped Ni-based oxalate framework, synthesized via a facile co-precipitation method, is applied as an excellent bi-functional electrocatalyst for water and urea oxidation reactions. The obtained framework achieved a large current density of 100 mA cm-2 at 1.497 V and 1.375 V (vs. RHE) for the OER and UOR, highlighting its potential for practical hydrogen production.

Two-dimensional metal-organic framework nanosheets coated solid-phase microextraction Arrow coupled with UPLC-Q-ToF-MS for the determination of three veterinary residues in milk and pork

This study presents a method utilizing solid-phase microextraction Arrow (SPME Arrow) combined with ultra-performance liquid chromatography-quadrupole time-of-flight mass spectrometry (UPLC-Q-TOF-MS) for the selective detection of three veterinary drugs-thiabendazole, sulfamethazine, and clenbuterol-in milk and pork. Two-dimensional metal-organic framework nanosheets (2D-MOFs) were employed as coating materials for the SPME Arrow. Three types of 2D-MOFs (Ni, Mn, and Co based) were synthesized and characterized using Fourier transform infrared spectroscopy, scanning electron microscopy, transmission electron microscopy, and a physical adsorption analyzer. The 2D-MOF coatings were fabricated using the electrospinning technique, with polyacrylonitrile (PAN) serving as the binder. Comparative analysis of the three 2D-MOF coatings revealed that 2D-Ni-MOF was the optimal coating material for the SPME Arrow. Optimization of the coating preparation conditions and SPME procedures included determining the optimal mass ratio of 2D-Ni-MOF to PAN, electrospinning time, and extraction and desorption parameters. Equilibrium extraction was achieved within 60 min, and desorption was completed within 30 min. Subsequently, the 2D-Ni-MOF-SPME Arrow-UPLC-Q-TOF-MS method was established and validated under optimal conditions, demonstrating high precision with inter-day precision ranging from 3.8 % to 9.5 % and intra-day precision ranging from 5.1 % to 11.5 %. The reusability study indicated that the extraction performance of the new SPME Arrow remained consistent after 90 adsorption-desorption cycles. The method exhibited linearity in milk and pork over the ranges of 0.002-5 μg L-1 and 0.01-5 μg L-1, respectively. The detection limits in milk and pork were 0.001-0.004 μg L-1 and 0.003-0.007 μg L-1, respectively. This method demonstrated excellent applicability for determining residues of the three veterinary drugs in milk and pork.

Interface engineering in Ni(OH)2/NiOOH heterojunction to enhance energy-efficient hydrogen production via urea electrolysis

Electrochemical urea electrolysis has merged as a promising alternative to conventional water splitting methods for hydrogen fuel production due to its cost-effectiveness and superior energy efficiency. The utilization of heterostructures has been proposed as a viable strategy to improve the efficiency of the urea oxidation reaction (UOR) by augmenting the quantity of active sites and optimizing the electronic structure. In this study, a Ni(OH)2/NiOOH heterojunction, referred to as H-Ni, was synthesized via a straightforward hydrothermal synthesis method. The notable performance of H-Ni in UOR is ascribed to the synergistic interaction between Ni(OH)2 and NiOOH, which constitute the principal components of the catalyst. Density functional theory (DFT) calculations reveal that the H-Ni composite is capable of modulating the d-band center, thereby enhancing the adsorption and desorption of reaction intermediates and decreasing the Gibbs free energy (ΔG) associated with the rate-determining step (RDS) of the UOR. Experimental results from catalytic performance tests indicate that the H-Ni-140 catalyst attains a current density of 10 mA·cm-2 in a 1.0 M KOH electrolyte containing 0.33 M urea at a relatively low potential of 1.341 V versus reversible hydrogen electrode (RHE), thereby highlighting its superior electrocatalytic performance. Furthermore, the catalyst requires only a cell voltage of 1.78 V to achieve a current density of 100 mA·cm-2, which is approximately 120 mV lower than that required for water electrolysis. This work presents a straightforward methodology for the cost-effective development of heterojunction catalysts.

Ceria-Graphene Oxide Nanocomposite for Electro-oxidation of Urea: An Experimental and Theoretical Investigation

This study explores the potential of ceria-graphene oxide (CeO2-GO) nanocomposites as efficient electrocatalysts for urea electro-oxidation (UOR). This work combines experimental and theoretical investigations and characterization techniques confirm the successful formation of the CeO2 embedded on graphene oxide sheets. UOR activity was found to be dependent on both OH- and urea concentrations. The optimal UOR performance was achieved in a 0.1 M urea and 1.0 M KOH solution, as evidenced by the low Tafel slope of 60 mV/dec and high turnover frequency (TOF) of 1.690 s-1. DFT calculations revealed that the CeO2-GO nanocomposite exhibited strong urea adsorption due to its favorable bond lengths (Ce-O: 2.58 Å, O-H: 1.77 Å) and high adsorption energy (-1.05 eV). These findings revealed that the CeO2-GO nanocomposites are promising as efficient and durable electrocatalysts for urea conversion to valuable products like nitrogen and hydrogen gas, with potential applications in clean energy generation and ammonia synthesis.

Synergistic etching of nickel foam by Fe3+ and Cl- ions to synthesize nickel-iron-layered double hydroxide nanolayers with abundant oxygen vacancies for superior urea oxidation

Urea electrolysis is an appealing topic for hydrogen production due to its ability to extract hydrogen at a lower potential. However, it is plagued by sluggish kinetics and noble-metal catalyst requirements. Herein, we developed nickel-iron-layered double hydroxide (NiFe-LDH) nanolayers with abundant oxygen vacancies (OV) via synergistically etching nickel foam with Fe3+ and Cl- ions, enabling the efficient conversion of urea into H2 and N2. The synthesized OV-NiFe-LDH exhibits a lower potential (1.30 vs. reversible hydrogen electrode, RHE) for achieving 10 mA cm-2 in the urea oxidation reaction (UOR), surpassing most recently reported Ni-based electrodes. OV provides favorable conductivity and a large surface area, which results in a 4.1-fold in electron transport and a 5.1-fold increase in catalyst reactive sites. Density Functional Theory (DFT) calculations indicate that OV can lower the adsorption energy of urea, and enhance the bonding strength of *CONHNH, giving rise to improved UOR. This study provides a viable path toward economical and efficient production of high-purity hydrogen.

Rational design of organic ligands for metal-organic frameworks as electrocatalysts for CO2 reduction

Electrocatalytic carbon dioxide (CO2) reduction to valuable chemical compounds is a sustainable technology with enormous potential to facilitate carbon neutrality by transforming intermittent energy sources into stable fuels. Among various electrocatalysts, metal-organic frameworks (MOFs) have garnered increasing attention for the electrochemical CO2 reduction reaction (CO2RR) owing to their structural diversity, large surface area, high porosity and tunable chemical properties. Ligands play a vital role in MOFs, which can regulate the electronic structure and chemical environment of metal centers of MOFs, thereby influencing the activity and selectivity of products. This feature article discusses the strategies for the rational design of ligands and their impact on the CO2RR performance of MOFs to establish a structure-performance relationship. Finally, critical challenges and potential opportunities for MOFs with different ligand types in the CO2RR are mentioned with the aim to inspire the targeted design of advanced MOF catalysts in the future to achieve efficient electrocatalytic CO2 conversion.

Metal-organic frameworks (MOFs) for hybrid water electrolysis: structure-property-performance correlation

Hybrid water electrolysis (HWE) is a promising pathway for the simultaneous production of high-value chemicals and clean H2 fuel. Unlike conventional electrochemical water splitting, which relies on the oxygen evolution reaction (OER), HWE involves the anodic oxidation reaction (AOR). The AORs facilitate the conversion of organic or inorganic compounds at the anode into valuable chemicals, while the cathode carries out the hydrogen evolution reaction (HER) to produce H2. Recent literature has witnessed a surge in papers investigating various AORs with organic and inorganic substrates using a series of transition metal-based catalysts. Over the past two decades, metal-organic frameworks (MOFs) have garnered significant attention for their exceptional performance in electrochemical water splitting. These catalysts possess distinct attributes such as highly porous architectures, customizable morphologies, open facets, high electrochemical surface areas, improved electron transport, and accessible catalytic sites. While MOFs have demonstrated efficiency in electrochemical water splitting, their application in hybrid water electrolysis has only recently been explored. In recent years, a series of articles have been published; yet there is no comprehensive article summarizing MOFs for hybrid water electrolysis. This article aims to fill this gap by delving into the recent progress in MOFs specifically tailored for hybrid water electrolysis. In this article, we systematically discuss the structure-property-performance relationships of various MOFs utilized in hybrid water electrolysis, supported by pioneering examples. We explore how the structure, morphology, and electronic properties of MOFs impact their performance in hybrid water electrolysis, with particular emphasis on value-added chemical generation, H2 production, potential improvement, conversion efficiency, selectivity, faradaic efficiency, and their potential for industrial-scale applications. Furthermore, we address future advancements and challenges in this field, providing insights into the prospects and challenges associated with the continued development and deployment of MOFs for hybrid water electrolysis.

Insights into the Understanding of the Nickel-Based Pre-Catalyst Effect on Urea Oxidation Reaction Activity

Nickel-based catalysts are regarded as the most excellent urea oxidation reaction (UOR) catalysts in alkaline media. Whatever kind of nickel-based catalysts is utilized to catalyze UOR, it is widely believed that the in situ-formed Ni3+ moieties are the true active sites and the as-utilized nickel-based catalysts just serve as pre-catalysts. Digging the pre-catalyst effect on the activity of Ni3+ moieties helps to better design nickel-based catalysts. Herein, five different anions of OH-, CO32-, SiO32-, MoO42-, and WO42- were used to bond with Ni2+ to fabricate the pre-catalysts β-Ni(OH)2, Ni-CO3, Ni-SiO3, Ni-MoO4, and Ni-WO4. It is found that the true active sites of the five as-fabricated catalysts are the same in situ-formed Ni3+ moieties and the five as-fabricated catalysts demonstrate different UOR activity. Although the as-synthesized five catalysts just serve as the pre-catalysts, they determine the quantity of active sites and activity per active site, thus determining the catalytic activity of the catalysts. Among the five catalysts, the amorphous nickel tungstate exhibits the most superior activity per active site and can catalyze UOR to reach 158.10 mA·cm-2 at 1.6 V, exceeding the majority of catalysts. This work makes for a deeper understanding of the pre-catalyst effect on UOR activity and helps to better design nickel-based UOR catalysts.

Open Active Sites in Ni-Based MOF with High Oxidation States for Electrooxidation of Benzyl Alcohol

The kinetics of electrocatalytic reactions are closely related to the number and intrinsic activity of the active sites. Open active sites offer easy access to the substrate and allow for efficient desorption and diffusion of reaction products without significant hindrance. Metal-organic frameworks (MOFs) with open active sites show great potential in this context. To increase the density of active sites, trimesic acid was utilized as a ligand to anchor more Ni sites and in situ construct the nickel foam-loaded Ni-based trimesic MOF electrocatalyst (Ni-TMA-MOF/NF). When tested as an electrocatalyst for benzyl alcohol oxidation, Ni-TMA-MOF/NF exhibited lower overpotential and superior durability compared to Ni foam-loaded Ni-based terephthalic MOF electrocatalyst (Ni-PTA-MOF/NF) and Ni(OH)2 nanosheet array (Ni(OH)2/NF). Ni-TMA-MOF/NF required only a low potential of 1.65 V to achieve a high current density of 400 mA cm-2. Even after 40000 s of electrocatalytic oxidation at 1.5 V, Ni-TMA-MOF/NF maintained a current density of 175 mA cm-2 with ∼68% retention, showing its potential for benzyl alcohol oxidation. Through a combination of experimental and theoretical investigations, it was found that Ni-TMA-MOF/NF displayed superior electrocatalytic activity due to an optimized electron structure with high-valence Ni species and a high density of active sites, enabling long-term stable operation at high current densities. This study provides a new perspective on the design of electrocatalysts for benzyl alcohol oxidation.

Engineering Internal and External Low-Coordination Atoms in Nickel-Organic Framework Nanoarrays to Promote the Electrochemical Oxygen Evolution Reaction

Monometallic nickel-organic frameworks based on a carboxylated ligand [2,6-naphthalenedicarboxylic acid (Ni-NDC)] have abundant and uniformly distributed single-atom Ni sites, enabling superior oxygen evolution reaction (OER) activity. In theory, most of the Ni atoms inside Ni-NDC microcrystals are coordinatively saturated except for the surface. Therefore, there are no accessible low-coordination atoms (LCAs) as electrocatalytic sites for the OER. One effective way is to expose more LCAs by preparing self-supporting Ni-NDC nanoarrays (Ni-NDC NAs) with hierarchical secondary structural units. Another effective method is to create more internal LCAs by removing partial ligands or coordination atoms attached to the Ni atoms. Herein, by combining the two strategies, we engineered LCAs in the interior and exterior of Ni-NDC to synergistically accelerate the OER. In brief, ultrathick "brick-like" Ni-NDC NAs were first prepared with dissolution and coordination effects of NDC on self-sacrificial templates of "agaric-like" nickel hydroxide nanoarrays [Ni(OH)2 NAs]. Subsequently, dual-coordinated NDC was partially replaced by monocoordinated 2-naphthoic acid (NA). The Ni-NDC NAs were further tailed into ultrathin "liner leaf-like" nanoneedle arrays (LCAs-Ni-NDC NAs). As a consequence, the LCAs-Ni-NDC NAs have more internal and external LCAs, which can deliver an OER performance that is superior to that of Ni-NDC NAs.

Water-Stable Pillared Three-Dimensional Zn-V Bimetal-Organic Framework for Promoted Electrocatalytic Urea Oxidation

Urea oxidation reaction (UOR) is one of the potential routes in which urea-rich wastewater is used as a source of energy for hydrogen production. Metal-organic frameworks (MOFs) have promising applications in electrocatalytic processes, although there are still challenges in identifying the MOFs' molecular regulation and obtaining practical catalytic systems. The current study sought to synthesize [Zn6(IDC)4(OH)2(Hprz)2]n (Zn-MOF) with three symmetrically independent Zn(II) cations connected via linear N-donor piperazine (Hprz), rigid planar imidazole-4,5-dicarboxylate (IDC3-), and -OH ligands, revealing the 3,4T1 topology. The optimized noble-metal-free Zn0.33V0.66-MOF/NF electrocatalysts show higher robustness and performance compared to those of the parent Zn monometallic MOF/NF electrode and other bimetallic MOFs with different Zn-V molar ratios. The low potential of 1.42 V (vs RHE) at 50 mA cm-2 in 1.0 M KOH with 0.33 M urea required by the developed Zn0.33V0.66-MOF electrode makes its application in the UOR more feasible. The availability of more exposed active sites, ion diffusion path, and higher conductivity result from the distinctive configuration of the synthesized electrocatalyst, which is highly stable and capable of synergistic effects, consequently enhancing the desired reaction. The current research contributes to introducing a practical, cost-effective, and sustainable solution to decompose urea-rich wastewater and produce hydrogen.

Ferric Oxide Nanocrystals-Embedded Co/Fe-MOF with Self-Tuned d-Band Centers for Boosting Urea-Assisted Overall Water Splitting

A novel semiconductive Co/Fe-MOF embedded with Fe2 O3 nanocrystals (Fe2 O3 @CoFe-MOF) is developed as a trifunctional electrocatalyst for the urea oxidation reaction (UOR), oxygen evolution reaction (OER), and hydrogen evolution reaction for enhancing the efficiency of the hydrogen production via the urea-assisted overall water splitting. Fe2 O3 @CoFe-TPyP-MOF comprises unsaturated metal-nitrogen coordination sites, affording enriched defects, self-tuned d-band centers, and efficient π-π interaction between different layers. Density functional theory calculation confirms that the adsorption of urea can be optimized at Fe2 O3 @CoFe-TPyP-MOF, realizing the efficient adsorption of intermediates and desorption of the final product of CO2 and N2 characterized by the in situ Fourier transform infrared spectroscopy. The two-electrode urea-assisted water splitting device-assembled with Fe2 O3 @CoFe-TPyP-MOF illustrates a low cell voltage of 1.41 V versus the reversible hydrogen electrode at the current density of 10 mA cm-2 , attaining the hydrogen production rate of 13.13 µmol min-1 in 1 m KOH with 0.33 m urea. The in situ electrochemical Raman spectra and other basic characterizations of the used electrocatalyst uncover that Fe2 O3 @CoFe-TPyP-MOF undergoes the reversible structural reconstruction after the UOR test, while it demonstrates the irreversible reconstruction after the OER measurement. This work redounds the progress of urea-assisted water spitting for hydrogen production.

Oxygen vacancy-rich Fe2(MoO4)3 combined with MWCNTs for electrochemical sensors of fentanyl and its analogs

Hundreds of thousands of people dying from the abuse of fentanyl and its analogs. Hence, the development of an efficient and highly accurate detection method is extremely relevant and challenging. Therefore, we proposed the introduction of oxygen defects into Fe2(MoO4)3 nanoparticles for improving the catalyst performance and combining it with multi-walled carbon nanotubes (MWCNTs) for electrochemical detection of fentanyl and its analogs. Oxygen vacancy-rich Fe2(MoO4)3 (called r-Fe2(MoO4)3) nanoparticles were successfully synthesized and characterized in detail by scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy-dispersive spectrometry (EDS), X-ray diffraction (XRD), Fourier transform infrared (FT-IR), Raman spectra, BET, X-ray photoelectron spectroscopy (XPS), and electron paramagnetic resonance (EPR) and investigated by comparison with oxygen vacancy-poor Fe2(MoO4)3 (called p-Fe2(MoO4)3). The obtained oxygen vacancy-rich Fe2(MoO4)3 was ultrasonically composited with MWCNTs for modification of glassy carbon electrodes (GCEs) used for the electrochemical detection of fentanyl and its analogs. The modified MWCNT-GCE showed ultrasensitivity to fentanyl, sufentanil, alfentanil, and acetylfentanyl with limits of detection (LOD) of 0.006 µmol·L-1, 0.008 µmol·L-1, 0.018 µmol·L-1, and 0.024 µmol·L-1, respectively, and could distinguish among the four drugs based on their peak voltages. Besides, the obtained r-Fe2(MoO4)3/MWCNT composite also exhibited high repeatability, selectivity, and stability. It showed satisfactory detection performance on real samples, with recoveries of 70.53 ~ 94.85% and 50.98 ~ 82.54% in serum and urine for the four drugs in a concentration range 0.2 ~ 1 µM, respectively. The experimental results confirm that the introduction of oxygen vacancies effectively improves the sensitivity of fentanyl electrochemical detection, and this work provides some inspiration for the development of catalytic materials for electrochemical sensors with higher sensitivity.

Urea catalytic oxidation for energy and environmental applications

Urea is one of the most essential reactive nitrogen species in the nitrogen cycle and plays an indispensable role in the water-energy-food nexus. However, untreated urea or urine wastewater causes severe environmental pollution and threatens human health. Electrocatalytic and photo(electro)catalytic urea oxidation technologies under mild conditions have become promising methods for energy recovery and environmental remediation. An in-depth understanding of the reaction mechanisms of the urea oxidation reaction (UOR) is important to design efficient electrocatalysts/photo(electro)catalysts for these technologies. This review provides a critical appraisal of the recent advances in the UOR by means of both electrocatalysis and photo(electro)catalysis, aiming to comprehensively assess this emerging field from fundamentals and materials, to practical applications. The emphasis of this review is on the design and development strategies for electrocatalysts/photo(electro)catalysts based on reaction pathways. Meanwhile, the UOR in natural urine is discussed, focusing on the influence of impurity ions. A particular emphasis is placed on the application of the UOR in energy and environmental fields, such as hydrogen production by urea electrolysis, urea fuel cells, and urea/urine wastewater remediation. Finally, future directions, prospects, and remaining challenges are discussed for this emerging research field. This critical review significantly increases the understanding of current progress in urea conversion and the development of a sustainable nitrogen economy.

Durable Pulse-Electrodeposited Ni-Fe-S Nanosheets Supported on a Ni-S Three Dimensional Pattern as Robust Bifunctional Electrocatalysts for Hydrogen Evolution and Urea Oxidation Reactions

This study aims to establish easy-to-fabricate and novel structures for the synthesis of highly active and enduring electrocatalysts for the hydrogen evolution reaction (HER) and urea oxidation reaction (UOR). Gradient electrodeposition and four different time regimes were utilized to synthesize Ni-S 3D patterns with the optimization of electrodeposition time. Pulse electrodeposition was employed for the synthesis of Ni-Fe-S nanosheets at three different frequencies and duty cycles to optimize the pulse electrodeposition parameters. The sample synthesized at 13 min of gradient electrodeposition with a 1 Hz frequency and 0.7 duty cycle for pulse electrodeposition demonstrated the best electrocatalytic performance. The optimized electrode further showed remarkable performance for HER and UOR reactions, requiring only 54 mV and 1.25 V to deliver 10 mA cm-2 for HER and UOR, respectively. Moreover, the overall cell voltage of the two-electrode system in 1 M KOH and 0.5 M urea was measured at 1.313 V, delivering 10 mA cm-2. Constructing Ni-Fe-S nanosheets on 3D Ni-S significantly increased the electrochemical surface area from 51 to 278 for the Ni-S and Ni-Fe-S layers. Tafel slopes were measured as 138 and 182 mV dec-1 for the HER and UOR for the Ni-S coating layer and 97 mV dec-1 for the HER and 131 mV dec-1 for the UOR for the optimal Ni-Fe-S nanosheets on Ni-S. Minimal changes in the potential were observed at 100 mA cm-2 in 50 h regarding the HER and UOR, signifying exceptional electrocatalytic stability. This study provides economically viable, highly active, and long-lasting electrocatalysts suitable for HER and UOR applications.

Rare earth oxide based electrocatalysts: synthesis, properties and applications

As an important strategic resource, rare earths (REs) constitute 17 elements in the periodic table, namely 15 lanthanides (Ln) (La-Lu, atomic numbers from 57 to 71), scandium (Sc, atomic number 21) and yttrium (Y, atomic number 39). In the field of catalysis, the localization and incomplete filling of 4f electrons endow REs with unique physical and chemical properties, including rich electronic energy level structures, variable coordination numbers, etc., making them have great potential in electrocatalysis. Among various RE catalytic materials, rare earth oxide (REO)-based electrocatalysts exhibit excellent performances in electrocatalytic reactions due to their simple preparation process and strong structural variability. At the same time, the electronic orbital structure of REs exhibits excellent electron transfer ability, which can reduce the band gap and energy barrier values of rate-determining steps, further accelerating the electron transfer in the electrocatalytic reaction process; however, there is a lack of systematic review of recent advances in REO-based electrocatalysis. This review systematically summarizes the synthesis, properties and applications of REO-based nanocatalysts and discusses their applications in electrocatalysis in detail. It includes the hydrogen evolution reaction (HER), oxygen evolution reaction (OER), hydrogen oxidation reaction (HOR), oxygen reduction reaction (ORR), carbon dioxide reduction reaction (CO2RR), methanol oxidation reaction (MOR), nitrogen reduction reaction (NRR) and other electrocatalytic reactions and further discusses the catalytic mechanism of REs in the above reactions. This review provides a timely and comprehensive summary of the current progress in the application of RE-based nanomaterials in electrocatalytic reactions and provides reasonable prospects for future electrocatalytic applications of REO-based materials.

Enhanced Hydroxyl Adsorption in Ultrathin NiO/Cr2 O3 In-Plane Heterostructures for Efficient Alkaline Methanol Oxidation Reaction

The exploration of advanced nickel-based electrocatalysts for alkaline methanol oxidation reaction (MOR) holds immense promise for value-added organic products coupled with hydrogen production, but still remain challenging. Herein, we construct ultrathin NiO/Cr2 O3 in-plane heterostructures to promote the alkaline MOR process. Experimental and theoretical studies reveal that NiO/Cr2 O3 in-plane heterostructures enable a favorable upshift of the d-band center and enhanced adsorption of hydroxyl species, leading to accelerated generation of active NiO(OH)ads species. Furthermore, ultrathin in-plane heterostructures endow the catalyst with good charge transfer ability and adsorption behavior of methanol molecules onto catalytic sites, contributing to the improvement of alkaline MOR kinetics. As a result, ultrathin NiO/Cr2 O3 in-plane heterostructures exhibit a remarkable MOR activity with a high current density of 221 mA cm-2 at 0.6 V vs Ag/AgCl, which is 7.1-fold larger than that of pure NiO nanosheets and comparable with other highly active catalysts reported so far. This work provides an effectual strategy to optimize the activity of nickel-based catalysts and highlights the dominate efficacy of ultrathin in-plane heterostructures in alkaline MOR.

Manipulation of Electron Spins with Oxygen Vacancy on Amorphous/Crystalline Composite-Type Catalyst

By substituting the oxygen evolution reaction (OER) with the anodic urea oxidation reaction (UOR), it not only reduces energy consumption for green hydrogen generation but also allows purification of urea-rich wastewater. Spin engineering of the d orbital and oxygen-containing adsorbates has been recognized as an effective pathway for enhancing the performance of electrocatalysts. In this work, we report the fabrication of a bifunctional electrocatalyst composed of amorphous RuO2-coated NiO ultrathin nanosheets (a-RuO2/NiO) with abundant amorphous/crystalline interfaces for hydrogen evolution reaction (HER) and UOR. Impressively, only 1.372 V of voltage is required to attain a current density of 10 mA cm-2 over a urea electrolyzer. The increased oxygen vacancies in a-RuO2/NiO by incorporation of amorphous RuO2 enhance the total magnetization and entail numerous spin-polarized electrons during the reaction, which speeds up the UOR reaction kinetics. The density functional theory study reveals that the amorphous/crystalline interfaces promote charge-carrier transfer, and the tailored d-band center endows the optimized adsorption of oxygen-generated intermediates. This kind of oxygen vacancy induced spin-polarized electrons toward boosting HER and UOR kinetics and provides a reliable reference for exploration of advanced electrocatalysts.

High-Valence Metal Doping Induced Lattice Expansion for M-FeNi LDH toward Enhanced Urea Oxidation Electrocatalytic Activities

The precise design of low-cost, efficient, and definite electrocatalysts is the key to sustainable renewable energy. The urea oxidation reaction (UOR) offers a promising alternative to the oxygen evolution reaction for energy-saving hydrogen generation. In this study, by tuning the lattice expansion, a series of M-FeNi layered double hydroxides (M-FeNi LDHs, M: Mo, Mn, V) with excellent UOR performance are synthesized. The hydrolytic transformation of Fe-MIL-88A is assisted by urea, Ni2+ and high-valence metals, to form a hollow M-FeNi LDH. Owing to the large atomic radius of the high-valence metal, lattice expansion is induced, and the electronic structure of the FeNi-LDH is regulated. Doping with high-valence metal is more favorable for the formation of the high-valence active species, NiOOH, for the UOR. Moreover, the hollow spindle structure promoted mass transport. Thus, the optimal Mo-FeNi LDH showed outstanding UOR electrocatalytic activity, with 1.32 V at 10 mA cm-2 . Remarkably, the Pt/C||Mo-FeNi LDH catalyst required a cell voltage of 1.38 V at 10 mA·cm-2 in urea-assisted water electrolysis. This study suggests a new direction for constructing nanostructures and modulating electronic structures, which is expected to ultimately lead to the development of a class of auxiliary electrocatalysts.

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

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

Tailoring the density of states of Ni(OH)2 with Ni0 towards solar urea wastewater splitting

Solar urea wastewater splitting is capable of producing hydrogen and degrading the urea pollutant simultaneously. Nickel hydroxide (Ni(OH)2) has been recognized as an effective cocatalyst for the urea oxidation reaction (UOR). But the lack of an efficient preparation method and a suitable Ni(OH)2 based cocatalyst limits the performances of solar urea wastewater splitting. Herein, a potential-cycling method is developed with a high-purity nickel plate serving as the counter electrode and nickel source in a three-electrode configuration. Spherical Ni0-doped Ni(OH)2 nanoparticles are successfully synthesized on the surface of TiO2 nanorod arrays. The photocurrent density of TiO2/Ni0:Ni(OH)2 can reach 0.56 mA cm-2 at 1.23 VRHE in 1 M NaOH and 0.33 M CO(NH2)2 mixed electrolyte under AM1.5G illumination, which is 1.75 and 1.93 times those of TiO2/Ni(OH)2 deposited using a normal potentiostatic method with nickel salt solution and pristine TiO2, respectively. Ni0 doping can significantly decrease the charge transfer resistance and provide a more favorable distribution of density of states of Ni(OH)2 for the UOR. Furthermore, Ni0:Ni(OH)2 decorated TiO2 photoanodes exhibit good photocurrent retention during 12 h continuous testing. This work expands the preparation technique of urea catalysts and the strategy for developing highly efficient nickel-based catalysts.

Accelerating structure reconstruction to form NiOOH in metal-organic frameworks (MOFs) for boosting the oxygen evolution reaction

Structural reconstruction of electrocatalysts to generate metal hydroxide/oxyhydroxide species is critical for an efficient oxygen evolution reaction (OER), but the controllable regulation of the reconstruction process still remains a challenge. Given the designable nature of metal-organic frameworks (MOFs), herein, we have reported a localized structure disordering strategy to accelerate the structural reconstruction of Ni-BDC to generate NiOOH for boosting the OER. The Ni-BDC nanosheets were modified by Fe3+ and urea to form cracks, which could promote the accessibility of the Ni sites by the electrolyte and thus promote the reconstruction to form NiOOH. In addition, the interaction between Ni2+ and Fe3+ allows the electron flow from Ni2+ to Fe3+, further enhancing the NiOOH generation. As a result, the optimized sample exhibits excellent OER activity with a small overpotential of 251 mV at 10 mA cm-2, which is superior to most of the MOF-based OER catalysts reported previously. This work provides a controllable strategy to regulate the structural reconstruction for promoting the OER, which could provide important guidance for the development of more efficient OER electrocatalysts.

A TiO2 nanorod and perylene diimide based inorganic/organic nanoheterostructure photoanode for photoelectrochemical urea oxidation

Visible light-driven photoelectrochemical (PEC) urea oxidation using inorganic/organic nano-heterostructure (NH) photoanodes is an attractive method for hydrogen (H2) production. In this article, inorganic/organic NHs (TiO2/PDIEH) consisting of a N,N-bis(2-ethylhexyl)perylene-3,4,9,10-tetracarboxylic diimide (PDIEH) thin layer over TiO2 nanorods (NRs) were fabricated for the PEC urea oxidation reaction (UOR). In these NHs, a PDIEH layer was anchored on TiO2 NR arrays using the spin-coating technique, which is beneficial for the uniform deposition of PDIEH on TiO2 NRs. Uniform deposition facilitated adequate interface contact between PDIEH and TiO2 NRs. TiO2/PDIEH NHs achieved a high current density of 1.1 mA cm-2 at 1.96 VRHE compared to TiO2 NRs. TiO2/PDIEH offers long-term stability under light illumination with 90.21% faradaic efficiency. TiO2/PDIEH exhibits a solar-to-hydrogen efficiency of 0.52%. This outcome opens up new opportunities for inorganic/organic NHs for high-performance PEC urea oxidation.

Interfacial Coupling of Graphene with Nickel Nanoparticles for Water Splitting and Urea Oxidation: A Spectroelectrochemical Investigation

Nickel nanoparticle and graphene interfaces of various stoichiometries were created through electrodeposition techniques. The catalytic behavior of the electrodeposited films was investigated through spectro-electrochemical methodologies. UV-vis absorbance spectra of the electrodeposited films are significantly different in the air and alkaline medium. Furthermore, UV-vis and Raman spectroscopy confirmed the coupling of Ni nanoparticles (Ni-NP) with the graphene framework, along with NiO and Ni(OH)2 . A combination of Raman and impedance spectroscopy revealed that the surface adsorption and charge transfer properties of the electrodeposited films are entirely dependent on the defects on graphene structure as well as distribution of Ni-NP on graphene. The electrodeposited films possess heterogeneous catalytic properties with a low overpotential of 50 mV (10 mA/cm-2 ) for hydrogen evolution reaction, as well as 601 mV and 391 mV (at 50 mA/cm-2 ) for the oxygen evolution reaction and urea oxidation reaction, respectively. In addition, eelectrodeposited samples show extraordinary overall water splitting performance by achieving a current density of 10 mA/cm2 at a very low applied potential of 1.38 V. This synergistic coupling of Ni and graphene renders the electrodeposited samples promising candidates as electrodes for overall water splitting in alkaline and urea-supplemented solutions.

Water-Stable Fluorous Metal-Organic Frameworks with Open Metal Sites and Amine Groups for Efficient Urea Electrocatalytic Oxidation

Urea oxidation reaction (UOR) is one of the promising alternative anodic reactions to water oxidation that has attracted extensive attention in green hydrogen production. The application of specifically designed electrocatalysts capable of declining energy consumption and environmental consequences is one of the major challenges in this field. Therefore, the goal is to achieve a resistant, low-cost, and environmentally friendly electrocatalyst. Herein, a water-stable fluorinated Cu(II) metalorganic framework (MOF) {[Cu2 (L)(H2 O)2 ]·(5DMF)(4H2 O)}n (Cu-FMOF-NH2 ; H4 L = 3,5-bis(2,4-dicarboxylic acid)-4-(trifluoromethyl)aniline) is developed utilizing an angular tetracarboxylic acid ligand that incorporates both trifluoromethyl (-CF3 ) and amine (-NH2 ) groups. The tailored structure of Cu-FMOF-NH2 where linkers are connected by fluoride bridges and surrounded by dicopper nodes reveals a 4,24T1 topology. When employed as electrocatalyst, Cu-FMOF-NH2 requires only 1.31 V versus reversible hydrogen electrode (RHE) to deliver 10 mA cm-2 current density in 1.0 m KOH with 0.33 m urea electrolyte and delivered an even higher current density (50 mA cm-2 ) at 1.47 V versus RHE. This performance is superior to several reported catalysts including commercial RuO2 catalyst with overpotential of 1.52 V versus RHE. This investigation opens new opportunities to develop and utilize pristine MOFs as a potential electrocatalyst for various catalytic reactions.

Anchoring Polydopamine on ZnCo2 O4 Nanowire To Facilitate Urea Water Electrolysis

To overcome the sluggishness of the oxygen evolution reaction (OER), the urea oxidation reaction was developed. In the case of OER application studies ZnCo2 O4 is an excellent electrocatalyst, towards the UOR has been performed with surface-grown polydopamine (PDA) with surface-grown polydopamine (PDA). ZnCo2 O4 @PDA is produced over the surface of nickel foam by a hydrothermal method followed by self-polymerization of dopamine hydrochloride. Dopamine hydrochloride was varied in solution to study the optimal growth of PDA necessary to enhance the electrochemical activity. Prepared ZnCo2 O4 @PDA was characterized by X-ray diffraction, electronic structural, and morphology/microstructure studies. With successful confirmation, the developed electrode material was applied to UOR and ZnCo2 O4 @PDA-1.5, delivering an excellent low overpotential of 80 mV at 20 mA cm-2 in the electrolyte mixture of 1 M potassium hydroxide+0.33 M urea. To support the excellent UOR activity, other electrochemical properties such as the Tafel slope, electrochemical surface active sites, and electrochemical impedance spectroscopy were also studied. Furthermore, a schematic illustration explaining the UOR mechanism is shown to allow a clear understanding of the obtained electrochemical activity. Finally, urea water electrolysis was carried out in a two-electrode symmetrical cell and compared with water electrolysis. This clearly showed the potential of the developed material for efficient electrochemical hydrogen production.

Surface Engineering over Metal-Organic Framework Nanoarray to Realize Boosted and Sustained Urea Oxidation

Facilitating C─N bond cleavage and promoting *COO desorption are essential yet challenging in urea oxidation reactions (UORs). Herein a novel interfacial coordination assembly protocol is established to modify the Co-phytate coordination complex on the Ni-based metal-organic framework (MOF) nanosheet array (CC/Ni-BDC@Co-PA) toward boosted and sustained UOR electrocatalysis. Comprehensive experimental and theoretical investigations unveil that surface Co-PA modification over Ni-BDC can manipulate the electronic state of Ni sites, and in situ evolved charge-redistributed surface can promote urea adsorption and the subsequent C─N bond cleavage. Impressively, Co-PA functionalization can impart a negatively charged catalyst surface with improved aerophobicity, not only weakening *COO adsorption and promoting CO2 departure, but also repelling CO3 2- approaching to deactivate Ni species, eventually alleviating CO2 poisoning and enhancing operational durability. Beyond that, improved hydrophilic and aerophobic characteristics would also contribute to better mass transfer kinetics. Consequently, CC/Ni-BDC@Co-PA exhibits prominent UOR performance with an ultralow potential of 1.300 V versus RHE to attain 10 mA cm-2 , a small Tafel slope of 45 mV dec-1 , and strong durability, comparable to the best Ni-based electrocatalysts documented thus far. This work affords a novel paradigm to construct MOF-based materials for promoted and sustained UOR catalysis through elegant surface engineering based on a metal-PA complex.

3D Hierarchical-Architectured Nanoarray Electrode for Boosted and Sustained Urea Electro-Oxidation

Exploring active and durable Ni-based materials with optimized electronic and architectural engineering to promote the urea oxidation reaction (UOR) is pivotal for the urea-related technologies. Herein a 3D self-supported hierarchical-architectured nanoarray electrode (CC/MnNi@NC) is proposed in which 1D N-doped carbon nanotubes (N-CNTs) with 0D MnNi nanoparticles (NPs) encapsulation are intertwined into 2D nanosheet aligned on the carbon cloth for prominently boosted and sustained UOR electrocatalysis. From combined experimental and theoretical investigations, Mn-alloying can regulate Ni electronic state with downshift of the d-band center, facilitating active Ni³⁺ species generation and prompting the rate-determining step (*COO intermediate desorption). Meanwhile, the micro/nano-hierarchical nanoarray configuration with N-CNTs encapsulating MnNi NPs can not only endow strong operational durability against metal corrosion/agglomeration and enrich the density of active sites, but also accelerate electron transfer, and more intriguingly, promote mass transfer as a result of desirable superhydrophilic and quasi-superaerophobic characteristics. Therefore, with such elegant integration of 0D, 1D and 2D motifs into 3D micro/nano-hierarchical architecture, the resulting CC/MnNi@NC can deliver admirable UOR performance, favorably comparable to the best-performing UOR electrocatalysts reported thus far. This work opens a fresh prospect in developing advanced electrocatalysts via electronic manipulation coupled with architectural engineering for various energy conversion technologies.

High-Density NiCu Bimetallic Phosphide Nanosheet Clusters Constructed by Cu-Induced Effect Boost Total Urea Hydrolysis for Hydrogen Production

The development of urea electrolysis technologies toward energy-saving hydrogen production can alleviate the environmental issues caused by urea-rich wastewater. In the current practices, the development of high-performance electrocatalysts in urea electrolysis remains critical. In this work, the NiCu-P/NF catalyst is prepared by anchoring Ni/Cu bimetallic phosphide nanosheets onto Ni foam (NF). In the experiments, the micron-sized elemental Cu polyhedron is first anchored on the surface of the NF substrate to provide more space for the growth of bimetallic nanosheets. Meanwhile, the Cu element adjusted the electron distribution within the composite and formed Ni/P orbital vacancies, which in turn accelerated the kinetic process. As a result, the optimal NiCu-P/NF sample exhibits excellent catalytic activity and cycling stability in a hybrid electrolysis system for the urea oxidation reaction (UOR) and hydrogen evolution reaction (HER). Further, the alkaline urea-containing electrolyzer is assembled with NiCu-P/NF as two electrodes reached a current density of 50 mA cm^-2 with a low driving potential of 1.422 V, which outperforms the typical commercial noble metal electrolyzer (RuO₂||Pt/C). Those findings suggest the feasibility of the substrate regulation strategy to increase the growth density of active species in preparation of an efficient bifunctional electrocatalyst for cracking the urea-containing wastewater.

A series of ultrasensitive electrocatalysts Fe-MOF/MWCNTs for fentanyl determination

Electrochemical determination of synthetic opioids such as fentanyl is meaningful but still challenging no matter from a social or academic perspective. Herein, we report a series of novel electrocatalysts based on Fe-containing metal-organic frameworks and multi-walled carbon nanotubes (Fe-MOF/MWCNTs). The obtained Fe-MOF/MWCNT electrode materials all show ultrasensitivity on fentanyl determination. In particular, MOF-235/MWCNTs even exhibit an ultra-low limit of detection (LOD) of 0.03 μM with a wide linear range from 0.1 to 50 μM. Besides, this series of Fe-MOF/MWCNTs also displays excellent repeatability, selectivity, and stability. Moreover, they show good performance in real sample detection and achieve good recovery of 95.47%-102.41% and 96.62%-103.15% in blood and urine samples, respectively. This high performance in fentanyl determination is mainly contributed by the Fenton-like process and the adsorption function of the Fe-MOF. Therefore, these novel Fe-MOF/MWCNTs are promising electrocatalysts for point-of-care device fabrication and also have potential applications in fentanyl rapid test technology.

Advanced hematite nanomaterials for newly emerging applications

Because of the combined merits of rich physicochemical properties, abundance, low toxicity, etc., hematite (α-Fe2O3), one of the most chemically stable compounds based on the transition metal element iron, is endowed with multifunctionalities and has steadily been a research hotspot for decades. Very recently, advanced α-Fe2O3 materials have also been developed for applications in some cutting-edge fields. To reflect this trend, the latest progress in developing α-Fe2O3 materials for newly emerging applications is reviewed with a particular focus on the relationship between composition/nanostructure-induced electronic structure modulation and practical performance. Moreover, perspectives on the critical challenges as well as opportunities for future development of diverse functionalities are also discussed. We believe that this timely review will not only stimulate further increasing interest in α-Fe2O3 materials but also provide a profound understanding and insight into the rational design of other materials based on transition metal elements for various applications.

Ni-Doped MnO2 Nanosheet Arrays for Efficient Urea Oxidation

Urea oxidation reaction (UOR), with a low thermodynamic potential, offers great promise for replacing anodic oxygen evolution reaction of electrolysis systems such as water splitting, carbon dioxide reduction, etc., thus reducing the overall energy consumption. To promote the sluggish kinetics of UOR, highly efficient electrocatalysts are required, and Ni-based materials have been widely investigated. However, most of these reported Ni-based catalysts suffer from large overpotentials, as they generally undergo self-oxidation to form NiOOH species at high potentials, which act as catalytically active sites for UOR. Herein, Ni-doped MnO₂ (Ni-MnO₂) nanosheet arrays were successfully prepared on nickel foam. The as-fabricated Ni-MnO₂ shows distinct UOR behavior with most of the previously reported Ni-based catalysts, as urea oxidation on Ni-MnO₂ proceeds before the formation of NiOOH. Notably, a low potential of 1.388 V vs reversible hydrogen electrode was required to achieve a high current density of 100 mA cm^-2 on Ni-MnO₂. It is suggested that both Ni doping and nanosheet array configuration are responsible for the high UOR activities on Ni-MnO₂. The introduction of Ni modifies the electronic structure of Mn atoms, and more Mn³⁺ species are generated in Ni-MnO₂, contributing to its outstanding UOR performance.

Accelerating Electrochemical Water Oxidation Activity by Tailoring Morphology and Electronic Structure of Nickel Organic Framework Nanoarrays with a Fe Etching Effect

Fe-mediated nickel organic framework nanoarrays (NiFe-MOFs NAs) on carbon cloth were successfully constructed from ultrathin nanosheets via an etching effect. This strategy also combined the dissolution and coordination effect of acidic ligand (2,6-naphthalenedicarboxylic acid, NDC) to a self-sacrificial template of Ni(OH)₂ NAs. Benefiting from the strong Fe etching effect, dense and thick brick-like Ni-NDC nanoplates were tailored into loose and ultrathin NiFe-NDC nanosheets with abundant squamous nanostructures, which were still tightly attached to carbon cloth. As a consequence, more coordinatively unsaturated metal sites (CUMSs) that served as active centers were exposed to accelerate oxygen production. Meanwhile, the electronic structure of active Ni centers was modulated by the incorporation of Fe atoms. The charge density redistribution between Ni and Fe ultimately optimized the energy barrier of the adsorption/desorption of oxygenated intermediates, promoting the kinetics for water oxidation.

Highly fluorescent nickel based metal organic framework for enhanced sensing of Fe3+ and Cr2O72- ions

Heavy metal contamination has sparked widespread concern among the populace. The significant issues necessitate the creation of high-performance fluorescent pigments that can identify harmful elements in water. The present study deals with metal organic framework [MOF] based on nickel [Ni-BDC MOF]. The Ni-BDC MOF was prepared by facile solvothermal method using nickel nitrate hexahydrate and terephthalic acid ligand as precursors. The MOF was characterized by various techniques in order to examine the crystal, morphological, structural, composition, thermal and optical properties. The detailed characterizations revealed that the synthesized Ni-BDC MOF are well-crystalline with high purity and possessing 3D rhombohedral microcrystals with rough surface. The MOF demonstrate good luminescence performance and excellent water stability. According to the Stern Volmer plot, the tests set up under optimized conditions demonstrate a linear correlation between the fluorescence intensity and concentration of both ions, i.e. Fe³⁺, and Cr₂O₇^2- ions. The linear range and detection limit for Fe³⁺ and Cr₂O₇^2- were found to be 0-1.4 nM and 0.159 nM, and 0-1 nM and 0.120 nM, respectively. The mechanisms for the selective detection of cations and anions were also explored. The recyclability for the prepared MOF was checked up to five cycles which showed excellent stability with just a slight reduction in efficiency. The constructed sensor was also used to assess the presence of Fe³⁺ and Cr₂O₇^2- ions in actual water samples. The results of the different experiments revealed that the prepared MOF is a good material for detecting Fe³⁺ and Cr₂O₇^2- ions.

Recent advances in the electrochemical applications of Ni-based metal organic frameworks (Ni-MOFs) and their derivatives

Nickel-based metal-organic skeletal materials (Ni-MOFs) are a new class of inorganic materials that have aroused attention of investigators during past couple of years. They offer advantages such as high specific surface area, structural diversity, tunable framework etc. This assorted class of materials exhibited catalytic activity and electrochemical properties and display wide range of applications in the fields of electrochemical sensing, electrical energy storage and electrocatalysis. In this context, the presented review focuses on strategies to improve the electrochemical performance and stability of Ni-MOFs through the optimization of synthesis conditions, the construction of composite materials, and the preparation of derivatives of precursors. The review also presents the applications of Ni-MOFs and their derivatives as electrochemical sensors, energy storage devices, and electrocatalysts. In addition, the challenges and further electrochemical development prospects of Ni-MOFs have been discussed.

Regulating the pyrolysis process of cation intercalated MnO2 nanomaterials for electrocatalytic urea oxidation performance

Exploring an efficient way to enhance electron/ion transport behavior of nanomaterials plays an important role in the study of energy storage & conversion. However, the evolution rules of lattice and electronic structure during the pyrolysis process of low-dimensional nanomaterials, which further regulate its electron/ion transport properties, have not been effectively elucidated. Here we study the pyrolysis process of cation intercalated MnO2 as a case for realizing optimized electron/ion transport behavior. In our case, thermogravimetry-mass spectrometry (TG-MS) was adopted for tracking the remaining products in pyrolysis and decomposition products, further finding out the evolution law of the manganese-oxygen polyhedron structure during the pyrolysis. Moreover, the internal relations between the crystal structure and the electronic structure during the pyrolysis process of low-dimensional manganese oxide are revealed by fine structure characterization. As expected, partially treated 2D MnO2 nanosheets with controlled pyrolysis displays ultrahigh UOR performance with the overpotential of 1.320 V vs. RHE at the current density of 10 mA cm-2, which is the best value among non-nickel-based materials. We anticipate that studying the mechanism of the pyrolysis process has important guiding significance for the development of high electron/ion transport devices.

H2O2 activation by two-dimensional metal-organic frameworks with different metal nodes for micropollutants degradation: Metal dependence of boosting reactive oxygen species generation

The existence of organic micropollutants (OPMs) in water poses a considerable threat to the environment. A centralized approach towards pollutants abatement has dominated over the recent decades wherein heterogeneous Fenton-like based advanced oxidation processes can be a promising technology. The application of engineered nanomaterials offers more opportunities to enhance their catalyst properties. This study synthesizes a series of ultrathin two-dimensional (2D) Metal-organic frameworks (MOFs) nanosheets with tunable metal clusters. The formation of reactive oxygen species (^•OH and ¹O₂) can be significantly boosted via transferring the adsorbed H₂O₂ onto the solid-liquid interface by systematically tuning the metal species. The Co-MOF nanosheets exhibited an ultrafast degradation kinetic for BPA with a rate of 2.23 min^-1 (4.98 times higher than that of the bulk MOF) and TOF (turnover frequency) value of 9.99 min^-1, which are observably greater than that of the existing materials reported to date. Density functional theory simulation and experimental results unravel the mechanism for ROS formation, which is strongly metal-depend. We further loaded the powder onto a flow-through poly (vinylidene fluoride) (PVDF) microfiltration membrane and observed that the representative OPMs could be rapidly degraded, indicating promising properties for practical application.