Metal-Organic Framework-Derived Fe-Doped Ni3Fe/NiFe2O4 Heteronanoparticle-Decorated Carbon Nanotube Network as a Highly Efficient and Durable Bifunctional Electrocatalyst

Surface-Modified Ruthenium Nanorods for an Ampere-Level Bifunctional Hydrogen Evolution Reaction/Oxygen Evolution Reaction Electrocatalyst

The practical applications of bifunctional ruthenium-based electrocatalysts with two active sites of Ru nanoparticles covered with RuO2 skins are limited. One reason is the presence of multiple equally distributed facets, some of which are inactive. In contrast, ruthenium nanorods with a high aspect ratio have multiple unequally distributed facets containing the dominance of active faces for efficient electrocatalysis. However, the synthesis of ruthenium nanorods has not been achieved due to difficulties in controlling the growth. Additionally, it is known that the adsorption capacity of intermediates can be impacted by the surface of the catalyst. Inspired by these backgrounds, the surface-modified (SM) ruthenium nanorods having a dominant active facet of hcp (100) through chemisorbed oxygen and OH groups (SMRu-NRs@NF) are rationally synthesized through the surfactant coordination method. SMRu-NRs@NF exhibits excellent hydrogen evolution in acid and alkaline solutions with an ultralow overpotential of 215 and 185 mV reaching 1000 mA cm-2, respectively. Moreover, it has also shown brilliant oxygen evolution electrocatalysis in alkaline solution with a low potential of 1.58 V to reach 1000 mA cm-2. It also exhibits high durability over 143 h for the evolution of oxygen and hydrogen at 1000 mA cm-2. Density functional theory studies confirmed that surface modification of a ruthenium nanorod with chemisorbed oxygen and OH groups can optimize the reaction energy barriers of hydrogen and oxygen intermediates. The surface-modified ruthenium nanorod strategy paves a path to develop the practical water splitting electrocatalyst.

Iron Active Center Coordination Reconstruction in Iron Carbide Modified on Porous Carbon for Superior Overall Water Splitting

In this work, a novel liquid nitrogen quenching strategy is engineered to fulfill iron active center coordination reconstruction within iron carbide (Fe3C) modified on biomass-derived nitrogen-doped porous carbon (NC) for initiating rapid hydrogen and oxygen evolution, where the chrysanthemum tea (elm seeds, corn leaves, and shaddock peel, etc.) is treated as biomass carbon source within Fe3C and NC. Moreover, the original thermodynamic stability is changed through the corresponding force generated by liquid nitrogen quenching and the phase transformation is induced with rich carbon vacancies with the increasing instantaneous temperature drop amplitude. Noteworthy, the optimizing intermediate absorption/desorption is achieved by new phases, Fe coordination, and carbon vacancies. The Fe3C/NC-550 (550 refers to quenching temperature) demonstrates outstanding overpotential for hydrogen evolution reaction (26.3 mV at -10 mA cm-2) and oxygen evolution reaction (281.4 mV at 10 mA cm-2), favorable overall water splitting activity (1.57 V at 10 mA cm-2). Density functional theory (DFT) calculations further confirm that liquid nitrogen quenching treatment can enhance the intrinsic electrocatalytic activity efficiently by optimizing the adsorption free energy of reaction intermediates. Overall, the above results authenticate that liquid nitrogen quenching strategy open up new possibilities for obtaining highly active electrocatalysts for the new generation of green energy conversion systems.

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

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

Electrocatalytic transformation of oxygen to hydroxyl radicals via three-electron pathway using nitrogen-doped carbon nanotube-encapsulated nickel nanocatalysts for effective organic decontamination

The selective electrochemical reduction of oxygen (O₂) via 3e^- pathway for the production of hydroxyl radicals (HO) is a promising alternative to conventional electro-Fenton process. Here, we developed a nitrogen-doped CNT-encapsulated Ni nanoparticle electrocatalyst (Ni@N-CNT) with high O₂ reduction selectivity for the generation of HO^•via 3e^- pathway. Exposed graphitized N on the CNT shell, and Ni nanoparticles encapsulated within the tip of the N-CNT, played a key role in the generation of H₂O₂ intermediate (*HOOH) via a 2e^- oxygen reduction reaction. Meanwhile, those encapsulated Ni nanoparticles at the tip of the N-CNT facilitated the sequential HO^• generation by directly decomposing the electrogenerated *H₂O₂ in a 1e^- reduction reaction on the N-CNT shell without inducing Fenton reaction. Improved bisphenol A (BPA) degradation efficiency were observed when compared with conventional batch system (97.5% vs 66.4%). Trials using Ni@N-CNT in a flow-through configuration demonstrated a complete removal of BPA within 30 min (k = 0.12 min^-1) with a limited energy consumption of 0.068 kW·h·g^-1 TOC.

Tailoring Metal-Organic Frameworks and Derived Materials for High-Performance Zinc-Air and Alkaline Batteries

Developing multifunctional materials from earth-abundant elements is urgently needed to satisfy the demand for sustainable energy. Herein, we demonstrate a facile approach for the preparation of a metal-organic framework (MOF)-derived Fe₂O₃/C, composited with N-doped reduced graphene oxide (MO-rGO). MO-rGO exhibits excellent bifunctional electrocatalytic activities toward the oxygen evolution reaction (η_j=10 = 273 mV) and the oxygen reduction reaction (half-wave potential = 0.77 V vs reversible hydrogen electrode) with a low ΔE_OER-ORR of 0.88 V in alkaline solutions. A Zn-air battery based on the MO-rGO cathode displays a high specific energy of over 903 W h kg_Zn^-1 (∼290 mW h cm^-2), an excellent power density of 148 mW cm^-2, and an open-circuit voltage of 1.430 V, outperforming the benchmark Pt/C + RuO₂ catalyst. We also hydrothermally synthesized a Ni-MOF that was partially transformed into a Ni-Co-layered double hydroxide (MOF-LDH). A MO-rGO||MOF-LDH alkaline battery exhibits a specific energy of 42.6 W h kg_total mass^-1 (106.5 μW h cm^-2) and an outstanding specific power of 9.8 kW kg_total mass^-1 (24.5 mW cm^-2). This work demonstrates the potential of MOFs and MOF-derived compounds for designing innovative multifunctional materials for catalysis, electrochemical energy storage, and beyond.

Dealloyed Porous NiFe2O4/NiO with Dual-Network Structure as High-Performance Anodes for Lithium-Ion Batteries

As high-capacity anode materials, spinel NiFe₂O₄ aroused extensive attention due to its natural abundance and safe working voltage. For widespread commercialization, some drawbacks, such as rapid capacity fading and poor reversibility due to large volume variation and inferior conductivity, urgently require amelioration. In this work, NiFe₂O₄/NiO composites with a dual-network structure were fabricated by a simple dealloying method. Benefiting from the dual-network structure and composed of nanosheet networks and ligament-pore networks, this material provides sufficient space for volume expansion and is able to boost the rapid transfer of electrons and Li ions. As a result, the material exhibits excellent electrochemical performance, retaining 756.9 mAh g^-1 at 200 mA g^-1 after cycling for 100 cycles and retaining 641.1 mAh g^-1 after 1000 cycles at 500 mA g^-1. This work provides a facile way to prepare a novel dual-network structured spinel oxide material, which can promote the development of oxide anodes and also dealloying techniques in broad fields.

Metal-Organic Framework-Derived Fe-Doped Ni3Se4/NiSe2 Heterostructure-Embedded Mesoporous Tubes for Boosting Oxygen Evolution Reaction

It is crucial but challenging to promote sluggish kinetics of oxygen evolution reaction (OER) for water splitting via finely tuning the hierarchical nanoarchitecture and electronic structure of the catalyst. To address such issues, herein we present iron-doped Ni₃Se₄/NiSe₂ heterostructure-embedded metal-organic framework-derived mesoporous tubes (Ni-MOF-Fe-Se-400) realized by an interfacial engineering strategy. Due to the hierarchical nanoarchitecture of conductive two-dimensional nanosheet-constructed MOF-derived mesoporous tubes, coupled with fine tuning of the electronic structure via Fe-doping and interactions between Ni₃Se₄/NiSe₂ heterostructures, the Ni-MOF-Fe-Se-400 catalyst delivers superior OER activity: it requires only a low overpotential of 242 mV to achieve 10 mA cm^-2 (E_j=10), surpassing the benchmark RuO₂ (E_j=10 = 286 mV) and displays exceptional durability in the chronoamperometric i-t test with a small current decay (6.2%) after 72 h. Furthermore, the water splitting system comprises a Ni-MOF-Fe-Se-400 anode and a Pt/C cathode requires a low cell voltage of 1.576 V to achieve E_j=10 with an excellent Faradic efficiency (∼100%), outperforming the RuO₂-Pt/C combination. This work presents a novel interfacial engineering strategy to finely adjust the morphology and electronic structure of the non-noble metal-based OER catalyst via a facile fabrication method.

Metal-Organic Framework-Derived Electrocatalysts Competent for the Conversion of Acrylonitrile to Adiponitrile

Electrochemical conversion of acrylonitrile (AN) to produce adiponitrile (ADN), the raw material for the production of Nylon 66, has become a crucial process owing to the increasing market demand of Nylon 66. Although the metallic Pb or Cd electrodes are commonly used for this reaction, the use of electrocatalysts or electrodes modified with catalysts has been barely investigated. In this study, nanoporous and electrically conductive metal-organic framework (MOF)-derived materials composed of Pb, PbO, and carbon are synthesized by carbonizing a Pb-based MOF through thermal treatments, and these MOF-derived materials are served as electrocatalysts for the electrosynthesis of ADN. The crystallinity, morphology, elemental composition, porosity, electrical conductivity, and electrochemically active surface area of each MOF-derived material are investigated. Mass-transport-corrected Tafel analysis is used to probe the enhanced kinetics for the electrochemical reduction of AN occurring at the electrode modified with the MOF-derived material. Electrolytic experiments at various applied potentials are conducted to quantify the production rate and Faradaic efficiency toward ADN, and the result shows that the MOF-derived materials can act as electrocatalysts to initiate the electrochemical reduction of AN to produce ADN at a reduced overpotential. The optimal MOF-derived electrocatalyst can achieve a Faradaic efficiency of 67% toward ADN at an applied potential of -0.85 V versus reversible hydrogen electrode─a much lower overpotential compared to that typically required for this reaction without the use of catalysts. Findings here shed light on the design and development of advanced electrocatalysts to boost the performances for the electrosynthesis of ADN.

Active Microstructure Transformation and Enhanced Stability of Iron Foam Derived from Industrial Water Oxidation

Tracking microstructure transformation under industrial conditions is significant and urgent for the development of oxygen evolution reaction (OER) catalysts. Herein, employing iron foam (IF) as an object, we closely monitor related morphologies and composition evolution under 300 mA cm^-2 at 40 °C (IF-40-t)/80 °C (IF-80-t) in 6 M KOH and find that the OER activity first increases and then decreases with the continuous generation of FeOOH. Moreover, the reasons for different tendencies of Tafel slope, double-layer capacitance, and impedance for IF-40-t/IF-80-t have been investigated thoroughly. In detail, the OER activity of IF-40-t is governed by electron and mass transport, while for IF-80-t, the dominating factor is electron transfer. Further, to improve the stability, guided by the above results, two versatile methods that do not sacrifice electron and mass transport have been proposed: surface coating and dynamic interface construction. The synchronous improvements of stability and activity are deeply revealed, which may provide inspiration for catalyst design for industrial applications.

Formation of carnation-like ZIF-9 nanostructure to achieve superior electrocatalytic oxygen evolution

Rational design and controllable synthesis of metal-organic frameworks nanosheets is critical for electrochemical catalysis. Herein, a carnation-like ZIF-9 nanostructure made of nanosheets is grown on nickel foam (ZIF-9/NF) by a simple one-step solvothermal method, the morphology evolution and the electrocatalytic oxygen evolution properties have been investigated by controlling the solvothermal time. The binder-free ZIF-9-d/NF (60 h, solvothermal time is 60 h) electrode delivers efficient electrocatalytic oxygen evolution reaction activity with low overpotentials of 312 and 337 mV at 50 and 100 mA cm^-2, respectively. Furthermore, ZIF-9-d/NF (60 h) exhibits excellent stability without obvious attenuation for at least 30 h at 200 mA cm^-2. The excellent performances can be attributed to the combination of the highly exposed active sites in the ZIF-9-d nanosheets, as well as the effective electron conduction and mass transfer. This work provides much possibilities for ZIF-9 nanosheets as binder-free electrode for electrocatalyst.

Nitrogen-doped biomass carbon fibers with surface encapsulated Co nanoparticles for electrocatalytic overall water-splitting

N-Doped biomass carbon fibers with surface encapsulated Co nanoparticles (Co/N-BCFs) are prepared by the in situ structure-directing effect of the Co-complex formed with 2,2-bipyridine. An electrolyzer equipped with a Co/N-BCFs electrode couple only needs a voltage of 1.31 V at 10 mA cm^-2 for overall water-splitting, which is better than that of an integrated RuO₂ and Pt/C couple.

Triple captured iron by defect abundant NiO for efficient water oxidation

Fe-doped NiO host in well-defined nanorod assembly (Fe-NiMoO4@NiO-30) with large surface area is designed to achieve the triple capture of Fe via adjustable surface reconstruction and impregnation to optimize OER...

CoTe2–NiTe2 heterojunction directly grown on CoNi alloy foam for efficient oxygen evolution reaction

One-step fabrication of a self-supported CoTe2–NiTe2 heterojunction electrocatalyst directly grown on CoNi foam for efficient and durable oxygen evolution reactions.

Promoting Electrocatalytic Oxygen Evolution of Ultrasmall NiFe (Hydr)oxide Nanoparticles by Graphene-Support Effects

Although the activity of electrocatalysts towards oxygen evolution reaction (OER) has achieved considerable improvement by modulating the intrinsic electron structure, the role of supports to OER performance, often being reduced to enhancing the conductivity, is not fully explored. In this paper, a proof-of-concept study based on a series of hybrids of nickel iron (hydr)oxide nanoparticles (NiFeO NPs) and carbon supports with different oxidation level compared the motivation of supports for OER activity. The key to implementation lay in anchoring and growing of NiFeO NPs on the various carbon supports by electrostatic assembly and subsequent in-situ reduction. A series of experiments indicated that the strong coupling of metal ions and graphene oxide (GO) contributed to the formation of ultrasmall NiFeO NPs (≈2 nm) and the firm interaction between NiFeO NPs and GO, which in turn resulted in exposing more metal atoms, modulating local electron structure of active sites, and accelerating the charge-transfer ability. The OER activity of optimal NiFeO NPs anchored on rGO (NiFeO NPs/rGO) was significantly elevated, achieving an overpotential as small as 201 mV at 10 mA cm^-2 and a low Tafel slope of 68 mV dec^-1 , as well as remarkable stability. Such exciting capacity for catalyzing OER prevailed over the vast majority of previously reported transition-metal electrocatalysts, even superior to numerous noble metal-containing catalysts. The electrolyzer employing NiFeO NPs/rGO and commercial Pt/C for anode and cathode could be powered by a solar cell for efficient alkaline seawater splitting. This work opens up a universal and scalable way for further advancing the intrinsic activity of energy-related materials.

Engineering Ultrafine NiFe-LDH into Self-Supporting Nanosheets: Separation-and-Reunion Strategy to Expose Additional Edge Sites for Oxygen Evolution

Here, a strategy is reported to prepare Ni-Fe layered double hydroxide (NiFe-LDH) with abundant exposed edge planes for enhanced oxygen evolution reaction (OER). The edge-to-edge assembly of ultrafine NiFe-LDH directed by graphite-like carbon is performed through a one-step hydrothermal process to form self-supporting nanosheet arrays (named NiFe-LDH/C), in which ascorbic acid is employed as the carbon precursor to control both the platelet size and the assembly mode of NiFe-LDH. Benefiting from the unique structural engineering, NiFe-LDH/C can not only achieve a fast surface reconstruction into the highly active γ-phase structure, but also exposes abundant active edge sites, thus leading to a superior OER performance with the overpotential as low as 234 mV at a current density of 50 mA cm^-2 . Furthermore, density functional theory (DFT) calculations reveal that the unsaturated Fe-sites and the bridge-sites connecting Ni and Fe atoms, which only exist on the edge planes of NiFe-LDH, are the main active centers responsible for promoting the intrinsic OER activity. This work provides a specific and valuable reference for the rational design of high-quality electrocatalysts through structural engineering for renewable energy applications.

Growth, Properties, and Applications of Branched Carbon Nanostructures

Nanomaterials featuring branched carbon nanotubes (b-CNTs), nanofibers (b-CNFs), or other types of carbon nanostructures (CNSs) are of great interest due to their outstanding mechanical and electronic properties. They are promising components of nanodevices for a wide variety of advanced applications spanning from batteries and fuel cells to conductive-tissue regeneration in medicine. In this concise review, we describe the methods to produce branched CNSs, with particular emphasis on the most widely used b-CNTs, the experimental and theoretical studies on their properties, and the wide range of demonstrated and proposed applications, highlighting the branching structural features that ultimately allow for enhanced performance relative to traditional, unbranched CNSs.

Construction of a Keggin heteropolyacid/Ni-MOF catalyst for esterification of fatty acids

This work reports the one-pot solvothermal synthesis of a Keggin heteropolyacid (phosphomolybdic acid, tungstophosphoric acid, or silicotungstic acid) immobilized on Ni-MOF composite catalysts for esterification of fatty acids, and the composites were further analyzed by XRD, FTIR, NH₃-TPD, SEM, TEM, N₂ adsorption/desorption, and XPS. Among the contrastive syntheses (i.e., HPW/Ni-MOF, HSiW/Ni-MOF, and HPMo/Ni-MOF), HPMo/Ni-MOF exhibits the most active catalyst toward fatty acids esterification, and the characterization results also revealed that HPMo/Ni-MOF has a strong acidity, large specific surface area, and appropriate average pore size. More significantly, this catalyst exhibits a good catalytic performance (86.1% conversion) during esterification under the optimized reaction conditions, and the HPMo/Ni-MOF catalyst can remain stable after the tenth cycle with a conversion of 73.5%. Intriguingly, the esterification reaction kinetics was studied, and the activation energy was found to be 64.6 kJ mol⁻¹. The results indicated that the esterification of fatty acids using the HPMo/Ni-MOF catalyst is a chemically controlled reaction.

The developed heteropolyacids immobilized on Ni-MOF catalysts have strong acidity and perform well in esterification.

Oriented and robust anchoring of Fe via anodic interfacial coordination assembly on ultrathin Co hydroxides for efficient water oxidation

The oriented distribution and strong bonding of Fe active sites in multiple metal hydroxides are crucial to modulate activity and stability for efficient oxygen evolution reaction (OER). However, the dispersion and inevitable dissolution of Fe species still need to be addressed through deliberate design. Here, trace amounts of Fe chelated with tannic acid (TA) are precisely anchored to ultrathin Co hydroxides (TF@Co(OH)₂-t) through a new anodic interfacial coordination assembly strategy: firstly, the ZIF-67@Co(OH)₂ precursor with ultrathin Co(OH)₂ nanosheets vertically grown on the shell, provides abundant active sites and sufficient anchoring regions for subsequent TA-Fe coating; secondly, the TA-Fe ligand network quickly and robustly coats the surface of the Co(OH)₂via positive potential-driven chronopotentiometry, yielding TF@Co(OH)₂-t with good dispersion and controllable Fe species. The TA-Fe network efficiently activates Co species and prevents the dissolution of Fe ions. Physical characterization and DFT simulations reveal that the optimized OER activity with 317 mV at 10 mA cm^-2 for TF@Co(OH)₂-500 can be attributed to the accelerated electron transfer, increased active sites, and the moderate fall in d-band center levels due to Fe integration. Moreover, prolonged stability is realized benefiting from the robust TA-Fe coating protecting the actives sites.

Self-assembled CoSe2-FeSe2 heteronanoparticles along the carbon nanotube network for boosted oxygen evolution reaction

Water electrolysis is a significant alternative technique to produce clean hydrogen fuel in order to replace environmentally destructive fossil fuel combustion. However, the sluggish oxygen evolution kinetics makes this process vulnerable as it requires relatively high overpotentials. Hence, significantly effective electrocatalysts are necessary to access the water-oxidation process at a low overpotential to make this process industrially viable. Therefore, in order to reduce the energy barrier, we developed bimetallic CoSe2-FeSe2 heteronanoparticles along the carbon nanotube network (CoSe2-FeSe2/CNT) via a facile selenization strategy. Due to the unique assembly of highly conductive nanoparticles along the CNT network, the CoSe2-FeSe2/CNT displays an exceptionally good oxygen evolution (OER) activity; it requires 248 mV overpotential to reach a current density of 10 mA cm-2 (η10) with an ultra-low Tafel slope of 36 mV dec-1 and displays an overpotential of 1.59 V (η10) in the full water-splitting catalysis with the commercial Pt/C cathode. The high OER activity of CoSe2-FeSe2/CNT over the monometallic CoSe2/CNT and FeSe2/CNT electrocatalysts approve the synergistic interactions. Therefore, the superior performance is possibly ascribed to the unique porous nanoarchitecture and the strong coupling interactions between CoSe2 and FeSe2 heteronanoparticles on the conductive network. This study introduces an innovative approach to rationally design and fabricate cost-effective and highly proficient electrocatalysts for boosted OER performance.

Borate Anion Dopant Inducing Oxygen Vacancies over Co3O4 Nanocages for Enhanced Oxygen Evolution

The rational design of cost effective and highly efficient oxygen evolution reaction (OER) catalysts plays an extremely important role in promoting the commercial applications of electrochemical water splitting. Herein we reported a sacrificial template strategy for the preparation of borate anion doped Co3O4@ZIF-67 nanocages assembled with nanosheets (B-Co3O4@ZIF-67) by hydrothermal boronation of zeolitic imidazolate framework-67 (ZIF-67). During the preparation process, two different kinds of borate anion sources were found to regulate the morphological structures by tuning the etching rate between ZIF precursors and the borate anion. Moreover, borate anion doping was also found to induce oxygen vacancy defects, which is beneficial for modulating the electronic structure and accelerating electron transport. Meanwhile, the resultant B-Co3O4@ZIF-67 nanocages possess a large specific surface area, which is beneficial for the mass transfer of the electrolyte and exposing more catalytic active sites. Benefiting from the advantages above, the resultant B-Co3O4@ZIF-67 nanocages exhibit impressive OER performance with a small overpotential of 334 mV, a current density of 10 mA cm−2, a small Tafel slope of 73.88 mV dec−1, as well as long-term durability in an alkaline electrolyte.