Amorphous Nanobelts for Efficient Electrocatalytic Ammonia Production

One-dimensional (1D) amorphous nanomaterials combine the advantages of high active site concentration of amorphous structure, high specific surface area and efficient charge transfer of 1D materials, so they present promising opportunities for catalysis. However, how to achievie the balance between the high orientation of 1D morphology and the isotropy of amorphous structure is a significant challenge, which severely obstructs the controllable preparation of 1D amorphous materials. Guided by the hard-soft acids-bases theory, here we develop a general strategy for preparing 1D amorphous nanomaterials through the precise modulation of bond strength between metal ions and organic ligands for a moderated fastness. The soft base dodecanethiol (DT) is multifunctionally served as both structure-regulating agent and morphology-directing agent. Compared with the borderline acids (e.g. Fe2+, Co2+, Ni2+) to construct amorphous structure, soft acid of Cu+ which produced crystalline nanobelts can still be amorphized by reducing the hardness of Cu ions through redox reaction to weak Cu-SR bond. Due to the combined advantages of amorphous structure and one-dimensional morphology, amorphous CuDT nanobelts exhibited excellent electrocatalytic activity in electrochemical nitrate reduction, outperformed most of the reported Cu-based catalysts. This work will effectively bridge the gap between traditional 1D crystalline nanomaterials synthesis and their amorphization preparation.

Reconstructing Hydrogen-Bond Network for Efficient Acidic Oxygen Evolution

Developing highly active oxygen evolution reaction (OER) catalysts in acidic conditions is a pressing demand for proton-exchange membrane water electrolysis. Manipulating proton character at the electrified interface, as the crux of all proton-coupled electrochemical reactions, is highly desirable but elusive. Herein we present a promising protocol, which reconstructs a connected hydrogen-bond network between the catalyst-electrolyte interface by coupling hydrophilic units to boost acidic OER activity. Modelling on N-doped-carbon-layer clothed Mn-doped-Co3O4 (Mn-Co3O4@CN), we unravel that the hydrogen-bond interaction between CN units and H2O molecule not only drags the free water to enrich the surface of Mn-Co3O4 but also serves as a channel to promote the dehydrogenation process. Meanwhile, the modulated local charge of the Co sites from CN units/Mn dopant lowers the OER barrier. Therefore, Mn-Co3O4@CN surpasses RuO2 at high current density (100 mA cm-2 @ ~538 mV).

Nickel selenide/g-C3N4 heterojunction photocatalyst promotes CC coupling for photocatalytic CO2 reduction to ethane

Photocatalytic CO2 reduction to generate high value-added and renewable chemicals is of great potential in facilitating the realization of closed-loop and carbon-neutral hydrogen economy. Stabilizing and accelerating the formation of COCO* intermediate is crucial to achieve high-selectivity ethane production. Herein, a novel 3D/2D NiSe2/g-C3N4 heterostructure that mesoscale hedgehog nickel selenide (NiSe2) grown on the ultrathin g-C3N4 nanosheets were synthesized via a successively high temperature calcination process and in-situ thermal injection method for the first time. The optimum 2.7 % NiSe2/g-C3N4 heterostructure achieved moderate C2H6 generation rate of 46.1 μmol·g-1·h-1 and selectivity of 97.5 % without any additional photosensitizers and sacrificial agents under light illumination. Based on the results of the theoretical calculations and experiments, the improvement of photocatalytic CO2 to C2H6 production and selectivity should be ascribed to the increased visible light absorption ability, unique 3D/2D heterostructures with promoted adsorption of CO2 molecules on the Ni active sites, the type II heterojunction with improved charge transfer dynamics and lowered interfacial transfer resistance, as well as the formation of COCO* key intermediate. This work provides an inspiration to construct efficient photocatalysts for the direct transformation of CO2 to multicarbon products (C2+).

CTAB-Assisted Synthesis of FeNi Alloy Nanoparticles: Effective and Stable Electrocatalysts for Urea Oxidation Reactions

Development of highly efficient electrocatalysts for treating urea-rich wastewater is an important problem in environmental management and energy production. In this work, an iron-nickel alloy (Fe-Ni alloy) was synthesized via soft-template cetyltrimethylammonium bromide (CTAB)-assisted precipitation using low-temperature calcination. The as-synthesized nanoalloy was characterized by X-ray diffraction (XRD), which revealed the formation of a face-centered cubic (FCC) structure of the Fe-Ni alloy; field emission-scanning electron microscopic (FE-SEM) analysis revealed the spherical shape of the Fe-Ni alloy; high-resolution transmission electron microscopy (HR-TEM) revealed the average size to be ∼33.09 nm; and X-ray photoelectron spectroscopy (XPS) showed the presence of Fe, Ni, C, and O components and their chemical composition and valence states in the Fe-Ni alloy. The electrochemical urea oxidation reaction (UOR) was investigated by conducting linear sweep voltammetry (LSV) tests on the synthesized electrocatalysts with different Ni/Fe ratios in alkaline electrolytes with urea. The potential required to reach a current density of 10 mA cm-2 is 1.27 V vs RHE, which demonstrates the higher electrochemical activity of the Fe-Ni alloy compared to other individual compounds. This could be due to CTAB which improved the structural stability and synergetic and electronic effects in the nanoscale. This study will further contribute to renewable energy generation technology with long-term energy sustainability and also opens up great potential for reducing water pollution.

Tetrahedral Bonding Structure (Ni3 -Se) Induced by Lattice-Distortion of Ni to Achieve High Catalytic Activity in Na-Se Battery

Fabrication of transition-metal catalytic materials is regarded as a promising strategy for developing high-performance sodium-selenium (Na-Se) batteries. However, more systematic explorations are further demanded to find out how their bonding interactions and electronic structures can affect the Na storage process. This study finds that lattice-distorted nickel (Ni) structure can form different bonding structures with Na2 Se4 , providing high activity to catalyze the electrochemical reactions in Na-Se batteries. Using this Ni structure to prepare electrode (Se@NiSe2 /Ni/CTs) can realize rapid charge transfer and high cycle stability of the battery. The electrode exhibits high storage performance of Na+ ; i.e., 345 mAh g⁻1 at 1 C after 400 cycles, and 286.4 mAh g⁻1 at 10 C in rate performance test. Further results reveal the existence of a regulated electronic structure with upshifts of the d-band center in the distorted Ni structure. This regulation changes the interaction between Ni and Na2 Se4 to form a Ni3 -Se tetrahedral bonding structure. This bonding structure can provide higher adsorption energy of Ni to Na2 Se4 to facilitate the redox reaction of Na2 Se4 during the electrochemical process. This study can inspire the design of bonding structure with high performance in conversion-reaction-based batteries.

Heteroatom-Induced Accelerated Kinetics on Nickel Selenide for Highly Efficient Hydrazine-Assisted Water Splitting and Zn-Hydrazine Battery

Hydrazine-assisted water electrolysis is a promising energy conversion technology for highly efficient hydrogen production. Rational design of bifunctional electrocatalysts, which can simultaneously accelerate hydrogen evolution reaction (HER)/hydrazine oxidation reaction (HzOR) kinetics, is the key step. Herein, we demonstrate the development of ultrathin P/Fe co-doped NiSe2 nanosheets supported on modified Ni foam (P/Fe-NiSe2) synthesized through a facile electrodeposition process and subsequent heat treatment. Based on electrochemical measurements, characterizations, and density functional theory calculations, a favorable "2 + 2" reaction mechanism with a two-step HER process and a two-step HzOR step was fully proved and the specific effect of P doping on HzOR kinetics was investigated. P/Fe-NiSe2 thus yields an impressive electrocatalytic performance, delivering a high current density of 100 mA cm-2 with potentials of - 168 and 200 mV for HER and HzOR, respectively. Additionally, P/Fe-NiSe2 can work efficiently for hydrazine-assisted water electrolysis and Zn-Hydrazine (Zn-Hz) battery, making it promising for practical application.

Region-Controlled Framework Interface Mediated Anion Exchange Chemical Transformation to Designed Metal Phosphosulfide Heteronanostructures

Postsynthetic chemical transformation provides a powerful platform for creating heteronanostructures (HNs) with well-defined materials and interfaces that generate synergy or enhancement. However, it remains a synthetic bottleneck for the precise construction of HNs with increased degrees of complexity and more elaborate functions in a predictable manner. Herein, we define a general transformative protocol for metal phosphosulfide HNs based on tunable hexagonal Cu_1.81S frameworks with corner-, edge- and face-controlled growth of Co₂P domains. The region-controlled Cu_1.81S-Co₂P framework interfaces can serve as "kinetic barriers" in mediating the direction and rate between P and S anion exchange reactions, thus leading to a family of morphology and phase designed Cu₃P_1-xS_x-Co₂P HNs with hollow (branched, dotted and crown), porous and core-shell architectures. This study reveals the internal transformation mechanism between metal sulfide and phosphide nanocrystals, and opens up a new way for the rational synthesis of metastable HNs that are otherwise inaccessible.

Mn-Incorporation-Induced Phase Transition in Bottom-Up Synthesized Colloidal Sub-1-nm Ni(OH)2 Nanosheets for Enhanced Oxygen Evolution Catalysis

Sub-1-nm structures are attractive for diverse applications owing to their unique properties compared to those of conventional nanomaterials. Transition-metal hydroxides are promising catalysts for oxygen evolution reaction (OER), yet there remains difficulty in directly fabricating these materials within the sub-1-nm regime, and the realization of their composition and phase tuning is even more challenging. Here we define a binary-soft-template-mediated colloidal synthesis of phase-selective Ni(OH)₂ ultrathin nanosheets (UNSs) with 0.9 nm thickness induced by Mn incorporation. The synergistic interplay between binary components of the soft template is crucial to their formation. The unsaturated coordination environment and favorable electronic structures of these UNSs, together with in situ phase transition and active site evolution confined by the ultrathin framework, enable efficient and robust OER electrocatalysis. They exhibit a low overpotential of 309 mV at 100 mA cm^-2 as well as remarkable long-term stability, representing one of the most high-performance noble-metal-free catalysts.

Transition Metal Non-Oxides as Electrocatalysts: Advantages and Challenges

The identification of hydrogen as green fuel in the near future has stirred global realization toward a sustainable outlook and thus boosted extensive research in the field of water electrolysis focusing on the hydrogen evolution reaction (HER), oxygen evolution reaction (OER), and oxygen reduction reaction (ORR). A huge class of compounds consisting of transition metal-based nitrides, carbides, chalcogenides, phosphides, and borides, which can be collectively termed transition metal non-oxides (TMNOs), has emerged recently as an efficient class of electrocatalysts in terms of performance and longevity when compared to transition metal oxides (TMOs). Moreover, the superiority of TMNOs over TMOs to effectively catalyze not only OERs but also HERs and ORRs renders bifunctionality and even trifunctionality in some cases and therefore can replace conventional noble metal electrocatalysts. In this review, the crystal structure and phases of different classes of nanostructured TMNOs are extensively discussed, focusing on recent advances in design strategies by various regulatory synthetic routes, and hence diversified properties of TMNOs are identified to serve as next-generation bi/trifunctional electrocatalysts. The challenges and future perspectives of materials in the field of energy conversion and storage aiding toward a better hydrogen economy are also discussed in this review.

1T-Phase molybdenum sulfide/cobalt oxide nanopillars hybrid nanostructure coupled with nitrogen-doped carbon thin-film as high efficiency electrocatalyst for oxygen evolution

High efficient and durable catalysts are always needed to lower the kinetic barriers as well as prolong the service life associated with oxygen evolution reaction (OER). Herein, a sequential synthetic strategy is considered to prepare a hierarchical nanostructure, in which each component can be configured to achieve their full potential so that endows the resulting nanocatalyst a good overall performance. In order to realize this, well-organized cobalt oxide (Co₃O₄) nanopillars are firstly grown onto ultrathin 1T-molybdenum sulfide (1T-MoS₂) to obtain high surface area electrocatalyst, providing electron transfer pathways and structural stability. After that, zeolitic imidazolate framework-67 (ZIF-67) derived carbonization film is further in situ deposited on the surface of nanopillars to generate plentiful active sites, thereby accelerating OER kinetics. Based on the combination of different components, the electron transfer capability, catalytic activity and durability are optimized and fully implemented. The obtained nanocatalyst (defined as 1T-MoS₂/Co₃O₄/CN) exhibits the superior OER catalytic ability with the overpotential of 202 mV and Tafel slope of 57 mV·dec^-1 at 10 mA·cm^-2 in 0.1 M KOH, and good durability with a minor chronoamperometric decay of 9.15 % after 60,000 s of polarization.

Fe–Ni–Co trimetallic oxide hierarchical nanospheres as high-performance bifunctional electrocatalysts for water electrolysis

Fe–Ni–Co spheres were used as bifunctional catalysts exhibit high total water decomposition activity. Only a cell voltage of 1.61 V was required to attain a current density of 10 mA cm−2.

Fe incorporation-induced electronic modification of Co-tannic acid complex nanoflowers for high-performance water oxidation

The exploration of high-efficiency, cost-effective and earth-abundant non-noble metal electrocatalysts toward oxygen evolution reaction (OER) is of vital importance for the advancement of renewable energy conversion technologies. Herein, we report...

Red Phosphorus: An Up-and-Coming Photocatalyst on the Horizon for Sustainable Energy Development and Environmental Remediation

Photocatalysis is a perennial solution that promises to resolve deep-rooted challenges related to environmental pollution and energy deficit through harvesting the inexhaustible and renewable solar energy. To date, a cornucopia of photocatalytic materials has been investigated with the research wave presently steered by the development of novel, affordable, and effective metal-free semiconductors with fascinating physicochemical and semiconducting characteristics. Coincidentally, the recently emerged red phosphorus (RP) semiconductor finds itself fitting perfectly into this category ascribed to its earth abundant, low-cost, and metal-free nature. More notably, the renowned red allotrope of the phosphorus family is spectacularly bestowed with strengthened optical absorption features, propitious electronic band configuration, and ease of functionalization and modification as well as high stability. Comprehensively detailing RP's roles and implications in photocatalysis, this review article will first include information on different RP allotropes and their chemical structures, followed by the meticulous scrutiny of their physicochemical and semiconducting properties such as electronic band structure, optical absorption features, and charge carrier dynamics. Besides that, state-of-the-art synthesis strategies for developing various RP allotropes and RP-based photocatalytic systems will also be outlined. In addition, modification or functionalization of RP with other semiconductors for promoting effective photocatalytic applications will be discussed to assess its versatility and feasibility as a high-performing photocatalytic system. Lastly, the challenges facing RP photocatalysts and future research directions will be included to propel the feasible development of RP-based systems with considerably augmented photocatalytic efficiency. This review article aspires to facilitate the rational development of multifunctional RP-based photocatalytic systems by widening the cognizance of rational engineering as well as to fine-tune the electronic, optical, and charge carrier properties of RP.

Double Active Sites in Co-Nx-C@Co Electrocatalysts for Simultaneous Production of Hydrogen and Carbon Monoxide

The hydrogen evolution reaction (HER) by electrocatalytic water splitting is a prospective and economical route. However, the approach is severely hindered by the sluggish anodic OER, poor reactivity of electrocatalysts, and low-value-added byproducts at the anode. Herein, formaldehyde was added as an anode sacrificial agent, and a bifunctional Co-N_x-C@Co catalyst containing abundant Co-N₄ sites and Co nanoparticles was successfully fabricated and evaluated as both a cathodic and an anodic material for the HER and formaldehyde selective oxidation reaction (FSOR), respectively. Co-N_x-C@Co displayed a remarkable electrocatalytic performance simultaneously for both HER and FSOR with high hydrogen (H₂) and carbon monoxide (CO) selectivity. Density functional theory calculations combined with experiments identified that Co-N₄ and Co nanoparticles were dominating active sites for CO and H₂ generation, respectively. The coupling tactic of FSOR at the anode not only expedites the reaction rate of HER but also offers a high-efficiency and energy-saving means for the generation of valuable H₂/CO syngas.

Ultralong NiSe nanowire anchored on graphene nanosheets for enhanced electrocatalytic performance of triiodide reduction

Like their higher-dimensional counterparts, nanowire structures possess desirable features for electrocatalysis applications. In this study, ultralong NiSe nanowires (of diameters 50-150 nm and length 20 μm) were successfully anchored onto graphene nanosheets (NiSe NW/RGO). The NiSe nanowires were coated with a thick (∼10 nm) disordered surface replete with active sites. Benefiting from the fast charge-transfer channels and plentiful electroactive sites on the NiSe nanowires, in synergy with the high electroactive surface and electrical conductivity of the graphene nanosheets, the optimized NiSe NW/RGO exhibited a remarkably higher electrocatalytic activity than NiSe nanowires and typical Pt counter-electrodes (CEs). NiSe NW/RGO also exhibited the low charge-transfer resistance of 1.64 Ω cm² and delivered a higher power conversion efficiency (PCE = 7.99%) than Pt CEs (PCE = 7.76%).

Dual-Doping and Synergism toward High-Performance Seawater Electrolysis

Hydrogen (H₂ ) production from direct seawater electrolysis is an economically appealing yet fundamentally and technically challenging approach to harvest clean energy. The current seawater electrolysis technology is significantly hindered by the poor stability and low selectivity of the oxygen evolution reaction (OER) due to the competition with chlorine evolution reaction in practical application. Herein, iron and phosphor dual-doped nickel selenide nanoporous films (Fe,P-NiSe₂ NFs) are rationally designed as bifunctional catalysts for high-efficiency direct seawater electrolysis. The doping of Fe cation increases the selectivity and Faraday efficiency (FE) of the OER. While the doping of P anions improves the electronic conductivity and prevents the dissolution of selenide by forming a passivation layer containing P-O species. The Fe-dopant is identified as the primary active site for the hydrogen evolution reaction, and meanwhile, stimulates the adjacent Ni atoms as active centers for the OER. The experimental analyses and theoretical calculations provide an insightful understanding of the roles of dual-dopants in boosting seawater electrolysis. As a result, a current density of 0.8 A cm^-2 is archived at 1.8 V with high OER selectivity and long-term stability for over 200 h, which surpasses the benchmarking platinum-group-metals-free electrolyzers.

Clean and Affordable Hydrogen Fuel from Alkaline Water Splitting: Past, Recent Progress, and Future Prospects

Hydrogen economy has emerged as a very promising alternative to the current hydrocarbon economy, which involves the process of harvesting renewable energy to split water into hydrogen and oxygen and then further utilization of clean hydrogen fuel. The production of hydrogen by water electrolysis is an essential prerequisite of the hydrogen economy with zero carbon emission. Among various water electrolysis technologies, alkaline water splitting has been commercialized for more than 100 years, representing the most mature and economic technology. Here, the historic development of water electrolysis is overviewed, and several critical electrochemical parameters are discussed. After that, advanced nonprecious metal electrocatalysts that emerged recently for negotiating the alkaline oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) are discussed, including transition metal oxides, (oxy)hydroxides, chalcogenides, phosphides, and nitrides for the OER, as well as transition metal alloys, chalcogenides, phosphides, and carbides for the HER. In this section, particular attention is paid to the catalyst synthesis, activity and stability challenges, performance improvement, and industry-relevant developments. Some recent works about scaled-up catalyst synthesis, novel electrode designs, and alkaline seawater electrolysis are also spotlighted. Finally, an outlook on future challenges and opportunities for alkaline water splitting is offered, and potential future directions are speculated.

Interfacial engineering of CeO2 on NiCoP nanoarrays for efficient electrocatalytic oxygen evolution

Transition metal phosphides (TMP)-based oxygen evolution reaction (OER) catalysts constructed by interface engineering strategy have a broad prospect due to their low cost and good performance. Herein, a novel CeO₂/NiCoP nanoarray with intimate phosphide (NiCoP)-oxide (CeO₂) interface was developed via in situ generation on nickel foam (NF). This structure is conducive to increasing active sites and accelerating charge transfer, and may be conducive to regulating electronic structure and adsorption energy. As expected, optimal 1.4-CeO₂/NiCoP/NF delivers a low overpotential of 249 mV at the current density of 10 mA cm^-2 with a Tafel slope of 77.2 mV dec^-1. CeO₂/NiCoP/NF boasts one of the best OER catalytic materials among recently reported phosphides (TMP)-based OER catalysts and composite catalysts involving CeO₂. This work provides an effective strategy for the construction of hetero-structure with CeO₂ with oxygen vacancies to improve the OER performance of phosphides.

Regioselective Construction of Chemically Transformed Phosphide-Metal Nanoheterostructures for Enhanced Hydrogen Evolution Catalysis

Engineering nanoheterostructures (NHs) plays a key role in exploring novel or enhanced physicochemical properties of nanocrystals. Despite previously reported synthetic methodologies, selective synthesis of NHs to achieve the anticipated composition and interface is still challenging. Herein, we presented a colloidal strategy for the regioselective construction of typical Ag-Co₂P NHs with precisely controlled location of Ag nanoparticles (NPs) on unique chemically transformed Co₂P nanorods (NRs) by simply changing the ratio of different surfactants. As a proof-of-concept study, the constructed heterointerface-dependent hydrogen evolution reaction (HER) catalysis was demonstrated. The multiple Ag NP-tipped Co₂P NRs exhibited the best HER performance, due to their more exposed active sites and the synergistic effect at the interfaces. Our results open up new avenues in rational design and fabrication of NHs with delicate control over the spatial distribution and interfaces between different components.

Porous CoSe2@N-doped carbon nanowires: an ultra-high stable and large-current-density oxygen evolution electrocatalyst

Nitrogen doped carbon functionalized CoSe₂ nanowires (CoSe₂@N-C NWs), which act as potential oxygen evolution reaction (OER) catalysts with a large current density and high stability have been reported. Owing to the collaborative optimization of electrical conductivity, free adsorption energy and binding strength of OER intermediates, the prepared CoSe₂@N-C NWs exhibit an enhanced 6.61-fold catalytic activity compared to the pristine CoSe₂ NW electrode in 1.0 M KOH solution at the overpotential of 340 mV.

Engineering Bimetallic NiFe-Based Hydroxides/Selenides Heterostructure Nanosheet Arrays for Highly-Efficient Oxygen Evolution Reaction

Developing cost-effective and high-efficiency electrocatalysts toward alkaline oxygen evolution reaction (OER) is crucial for water splitting. Amorphous bimetallic NiFe-based (oxy)hydroxides have excellent OER activity under alkaline media, but their poorly electrical conductivity impedes the further improvement of their catalytic performance. Herein, a bimetallic NiFe-based heterostructure electrocatalyst that is composed of amorphous NiFe(OH)_x and crystalline pyrite (Ni, Fe)Se₂ nanosheet arrays is designed and constructed. The catalyst exhibits an outstanding OER performance, only requiring low overpotentials of 180, 220, and 230 mV at the current density of 10, 100, and 300 mA cm^-2 and a low Tafel slope of 42 mV dec^-1 in 1 m KOH, which is among the state-of-the-art OER catalysts. Based on the experimental and theoretical results, the electronic coupling at the interface that leads to the increased electrical conductivity and the optimized adsorption free energies of the oxygen-contained intermediates plays a crucial role in enhancing the OER activities. This work focusing on improving the OER performance via engineering amorphous-crystalline bimetallic heterostructure may provide some inspiration for reasonably designing advanced electrocatalysts.

Metal–organic frameworks and their derivatives as electrocatalysts for the oxygen evolution reaction

This review summarizes the recent progress on MOFs and their derivatives used for OER electrocatalysis in terms of their morphology, composition and structure–performance relationship.

Controllable Synthesis of Fe-Doped NiCo2 O4 Nanobelts as Superior Catalysts for Oxygen Evolution Reaction

As one of the promising clean and renewable technologies, water splitting has been a hot topic, especially the half-reaction of oxygen evolution reaction (OER) due to its sluggish and complex kinetics. Hence, Fe-doped NiCo₂ O₄ nanobelts were designed and prepared as catalysts toward OER. By increasing the Fe amount, the catalytic performances of the as-synthesized products went up and then decreased. Profiting from the synergistic effect between Fe atom and NiCo₂ O₄ , all the Fe-NiCo₂ O₄ catalysts exhibited superior catalytic activities to the corresponding NiCo₂ O₄ . In addition, the characteristic nanobelt architecture facilitates the conduction of electrons and the exposure of active sites. With the optimal Fe content, the 9.1 % Fe-NiCo₂ O₄ yielded the smallest overpotential and Tafel slope among the catalysts, distinctly lower than that of RuO₂ .

Vacancy Occupation-Driven Polymorphic Transformation in Cobalt Ditelluride for Boosted Oxygen Evolution Reaction

Transition-metal dichalcogenides (TMDs) hold great potential as an advanced electrocatalyst for oxygen evolution reaction (OER), but to date the activity of transition metal telluride catalysts are demonstrated to be poor for this reaction. In this study, we report the activation of CoTe₂ for OER by doping secondary anions into Te vacancies to trigger a structural transition from the hexagonal to the orthorhombic phase. The achieved orthorhombic CoTe₂ with partial vacancies occupied by P-doping exhibits an exceptional OER catalytic activity with an overpotential of only 241 mV at 10 mA cm^-2 and a robust stability more than 24 h. The combined experimental and theoretical studies suggest that the defective phase transformation is controllable and allows the synergism of vacancy, doping as well as the reconstructed crystallographic structure, ensuring more exposure of catalytic active sites, rapid charge transfer, and energetically favorable intermediates. This vacancy occupation-driven strategy of structural transformation can also be manipulated by S- and Se-doping, which may offer useful guidance for developing tellurides-based electrocatalyst for OER.

Ti-Mesh supported porous CoS2 nanosheet self-interconnected networks with high oxidation states for efficient hydrogen production via urea electrolysis

The urea oxidation reaction (UOR) is an ideal alternative to the oxygen evolution reaction (OER) towards energy efficient hydrogen production. However developing Earth-abundant electrocatalysts for urea oxidation and hydrogen generation still remains a big challenge. Herein, porous CoS2 nanosheet self-interconnected networks with high oxidation states located on a Ti-mesh (P-CoS2/Ti) are synthesized and can act as a high activity catalyst for both the hydrogen evolution reaction (HER) and urea oxidation reaction (UOR). In this literature, we report a very interesting phenomenon that cobalt hydroxide with different chemical compositions and crystal structures can be synthesized by adjusting the concentration of NaOH during the etching process. Moreover, porous CoS2 nanosheets with different crystallite sizes can be synthesized by adjusting the sulfuration temperature. P-CoS2/Ti presents outstanding catalytic performance with an overpotential of 91 mV to deliver a current density of 10 mA cm-2 for the HER, and it gives an anode potential of 1.243 V vs. RHE at 10 mA cm-2 for the UOR. A two-electrode electrolyser is used to validate the catalyst performance, and the P-CoS2/Ti||P-CoS2/Ti electrode is capable of producing a current density of 10 mA cm-2 at a cell potential of only 1.375 V, demonstrating its potential feasibility in the practical application of efficient hydrogen production.

Pseudo-atomic-scale metals well-dispersed on nano-carbons as ultra-low metal loading oxygen-evolving electrocatalysts

Solving challenges for the scaling-up, high metal loadings and low turnover frequency (TOF, defined as mol O2 per mol metal per second), of FeNi catalysts in water electrolysis, we report the first discovery of pH tunable tannic acid single molecular layer formed on nano-sized carbons (NCs), which promotes the gram-production of pseudo-atomic-scale FeNi oxyhydroxide nanoclusters well-dispersed on NCs. It results in ultra-low metal loading (0.42 μg cm-2) and remarkably large TOF of 14.03 s-1 for the oxygen evolution reaction, which is three orders of magnitude higher than that of state-of-the-art FeNi catalysts. A "volcano"-shaped activity trend in specific activity and TOF was found to depend on the Fe content in FeNi oxyhydroxide. The micro-morphologies from the atomic-level exposure of active sites and surface spectra analyses confirm the model of synergism between Ni and Fe centers.

Two-Dimensional Earth-Abundant Transition Metal Oxides Nanomaterials: Synthesis and Application in Electrochemical Oxygen Evolution Reaction

Development of a universal synthetic strategy for two-dimensional (2D) Earth-abundant transition metal oxides nanomaterials is highly vital toward numerous electrochemical applications. Herein, a facile and general synthesis of highly ordered two-dimensional metal oxides nanomaterials includes Co₃O₄, NiO, CuO, and Fe₃O₄ nanosheets as an electrocatalyst for oxygen evolution reaction (OER) is demonstrated. Among the synthesized 2D transition metal oxides, the Co₃O₄ nanosheet exhibits smallest overpotential (η) of ∼384.0 mV at a current density of 10.0 mA cm^-2 and Tafel slope of ∼52.0 mV dec^-1, highest mass activity of ∼112.3 A g^-1 at the overpotential of ∼384.0 mV, and high turn over frequency (TOF) of 0.099 s^-1, which is relatively favorable with state-of-the-art RuO₂ catalyst. The present synthetic approach may unlock a brand new pathway to prepare shape-controlled Earth-abundant transition metal oxides nanomaterials for electrocatalytic OER.

Porous honeycomb-like NiSe2/red phosphorus heteroarchitectures for photocatalytic hydrogen production

Heterojunction construction of semiconductors with a matched bandgap can not only help promote visible light absorption but also restrain photoexcited charge carrier recombination and optimize the separation efficiency. Herein, a novel porous honeycomb-like NiSe₂/RP heterostructure is reported for the first time by in situ deposition of NiSe₂ nanoparticles on the surface of red phosphorus (RP). The optimized binary NiSe₂/RP composite showed superior photocatalytic H₂ evolution activity (1968.8 μmol g^-1 h^-1) from Na₂S/Na₂SO₃ solution under solar light illumination, which was 2.32, 1.90, 1.59 and 1.21 times that of pristine RP, NiSe₂, 5.3% FeS/RP and 8.1% NiS/RP, respectively. The formation process and function of various reactive oxygen species (˙OH, ˙O₂^- and H₂O₂), and the migration pathway of photocarriers are discussed in detail. Such a prominently improved photocatalytic performance could be ascribed to extended light absorption ability, massive reactive centers and lower interfacial transfer resistance, together with expedited charge separation, which arose from a successive two-electron/two-step reduction route. This study provides illuminating insights for the rational exploration and fabrication of potential photocatalytic systems with 0D/3D integrated nanoarchitecture and a multi-step electron transfer process for efficiently realizing solar energy capture and conversion.

Ultrathin Ni(0)-Embedded Ni(OH)2 Heterostructured Nanosheets with Enhanced Electrochemical Overall Water Splitting

The efficiency of splitting water into hydrogen and oxygen is highly dependent on the catalyst used. Herein, ultrathin Ni(0)-embedded Ni(OH)₂ heterostructured nanosheets, referred to as Ni/Ni(OH)₂ nanosheets, with superior water splitting activity are synthesized by a partial reduction strategy. This synthetic strategy confers the heterostructured Ni/Ni(OH)₂ nanosheets with abundant Ni(0)-Ni(II) active interfaces for hydrogen evolution reaction (HER) and Ni(II) defects as transitional active sites for oxygen evolution reaction (OER). The obtained Ni/Ni(OH)₂ nanosheets exhibit noble metal-like electrocatalytic activities toward overall water splitting in alkaline condition, to offer 10 mA cm^-2 in HER and OER, the required overpotentials are only 77 and 270 mV, respectively. Based on such an outstanding activity, a water splitting electrolysis cell using the Ni/Ni(OH)₂ nanosheets as the cathode and anode electrocatalysts has been successfully built. When the output voltage of the electrolytic cell is 1.59 V, a current density of 10 mA cm^-2 can be obtained. Moreover, the durability of Ni/Ni(OH)₂ nanosheets in the alkaline electrolyte is much better than that of noble metals. No obvious performance decay is observed after 20 h of catalysis. This facile strategy paves the way for designing highly active non-precious-metal catalyst to generate both hydrogen and oxygen by electrolyzing water at room temperature.

Iron-based nanoparticles and their potential toxicity: Focus on oxidative stress and apoptosis

Recently, there have been several studies indicating that iron-based nanomaterials may exhibit certain toxic properties. Compared to conventional iron and iron oxides, iron nanoparticles (FeNPs) have some unique physical and chemical traits which impact their absorption, biodistribution and elimination. Facilitated passage through biological barriers enables FeNPs to reach various tissues and cells, and interact with a variety of different compounds. Currently, most of the recent research is focused on the potential cytotoxicity of FeNPs, and its implications on cell viability and functions. Some studies suggested that, in certain cell types, FeNPs may increase levels of oxidative stress and induce generation of reactive oxygen species. Oxidative stress may be one of the most important mechanisms by which FeNPs exhibit cytotoxic effects. Some authors have also suggested that, in certain conditions, exposure to FeNPs, in combination with other factors, may lead to changes in intracellular signaling resulting in programmed cell death. In this short review, we focus on the recent research on potential cytotoxicity of iron-based nanomaterials, and the potential implications of this new knowledge in medicine, chemistry and biology.

Recent Advances in Non-Precious Transition Metal/Nitrogen-doped Carbon for Oxygen Reduction Electrocatalysts in PEMFCs

The proton exchange membrane fuel cells (PEMFCs) have been considered as promising future energy conversion devices, and have attracted immense scientific attention due to their high efficiency and environmental friendliness. Nevertheless, the practical application of PEMFCs has been seriously restricted by high cost, low earth abundance and the poor poisoning tolerance of the precious Pt-based oxygen reduction reaction (ORR) catalysts. Noble-metal-free transition metal/nitrogen-doped carbon (M–NxC) catalysts have been proven as one of the most promising substitutes for precious metal catalysts, due to their low costs and high catalytic performance. In this review, we summarize the development of M–NxC catalysts, including the previous non-pyrolyzed and pyrolyzed transition metal macrocyclic compounds, and recent developed M–NxC catalysts, among which the Fe–NxC and Co–NxC catalysts have gained our special attention. The possible catalytic active sites of M–NxC catalysts towards the ORR are also analyzed here. This review aims to provide some guidelines towards the design and structural regulation of non-precious M–NxC catalysts via identifying real active sites, and thus, enhancing their ORR electrocatalytic performance.

A core-shell structured CoMoO4•nH2O@Co1-xFexOOH nanocatalyst for electrochemical evolution of oxygen

Nickel-iron oxyhydroxide (Ni_1-xFe_xOOH) is well recognized as the best-performing oxygen evolution reaction (OER) catalyst in alkaline electrolytes, however its analogue cobalt-iron oxyhydroxide (Co_1-xFe_xOOH) is surprisingly less explored despite their structural similarity. Inspired by our recent study on high-performance HER catalyst using the nanostructured CoMoO₄•nH₂O precursor, herein, we report a facile synthesis of Co_1-xFe_xOOH catalyst derived from the same precursor and its excellent electrocatalytic properties towards the OER in alkaline electrolytes. A core-shell structured nanocatalyst consisting of disordered Co_1-xFe_xOOH layer over the surface of crystalline CoMoO₄•nH₂O nanosheets was synthesized using a simple hydrothermal method followed by anodic electrooxidation. Thus-prepared catalyst exhibited extraordinarily high and stable activity towards the OER in alkaline electrolyte, which outperformed most Co-based OER catalysts. Combined with the HER catalyst derived from the same CoMoO₄•nH₂O precursor as the cathode, we further developed and tested a simple water-splitting cell, which significantly surpasses the benchmarking IrO₂-Pt/C couple (1.63 V) and requires a voltage of only 1.517 V to afford 10 mA cm^-2 in 1.0 M KOH solution. Density functional theory calculations were conducted to gain insight into the Fe-doping induced improvement of OER activity.

A high-performance bimetallic cobalt iron oxide catalyst for the oxygen evolution reaction

Herein, a facile solvothermal approach was designed to produce the CoFe₂O₄ nanospheres with unique porous structure. As an efficient electrocatalyst for OER, the CoFe₂O₄ nanospheres performed high performance.

Water Oxidation Catalysts for Artificial Photosynthesis

Water oxidation is the primary reaction of both natural and artificial photosynthesis. Developing active and robust water oxidation catalysts (WOCs) is the key to constructing efficient artificial photosynthesis systems, but it is still facing enormous challenges in both fundamental and applied aspects. Here, the recent developments in molecular catalysts and heterogeneous nanoparticle catalysts are reviewed with special emphasis on biomimetic catalysts and the integration of WOCs into artificial photosystems. The highly efficient artificial photosynthesis depends largely on active WOCs integrated into light harvesting materials via rational interface engineering based on in-depth understanding of charge dynamics and the reaction mechanism.

Investigation of Binary Metal (Ni, Co) Selenite as Li-Ion Battery Anode Materials and Their Conversion Reaction Mechanism with Li Ions

Highly efficient anode materials with novel compositions for Li-ion batteries are actively being researched. Multicomponent metal selenite is a promising candidate, capable of improving their electrochemical performance through the formation of metal oxide and selenide heterostructure nanocrystals during the first cycle. Here, the binary nickel-cobalt selenite derived from Ni-Co Prussian blue analogs (PBA) is chosen as the first target material: the Ni-Co PBA are selenized and partially oxidized in sequence, yielding (NiCo)SeO₃ phase with a small amount of metal selenate. The conversion mechanism of (NiCo)SeO₃ for Li-ion storage is studied by cyclic voltammetry, in situ X-ray diffraction, ex situ X-ray photoelectron spectroscopy, in situ electrochemical impedance spectroscopy, and ex situ transmission electron microscopy. The reversible reaction mechanism of (NiCo)SeO₃ with the Li ions is described by the reaction: NiO + CoO + xSeO₂ + (1 - x)Se + (4x + 6)Li⁺ + (4x + 6)e^- ↔ Ni + Co + (2x + 2)Li₂ O + Li₂ Se. To enhance electrochemical properties, polydopamine-derived carbon is uniformly coated on (NiCo)SeO₃ , resulting in excellent cycling and rate performances for Li-ion storage. The discharge capacity of C-coated (NiCo)SeO₃ is 680 mAh g^-1 for the 1500th cycle when cycled at a current density of 5 A g^-1 .

Pristine versus Pyrolyzed Metal-Organic Framework-based Oxygen Evolution Electrocatalysts: Evaluation of Intrinsic Activity Using Electrochemical Impedance Spectroscopy

Metal-organic frameworks (MOFs) have emerged as outstanding electrocatalysts for water oxidation. Commonly, MOFs are utilized for electrocatalytic water oxidation either in pristine or pyrolyzed form. Yet, despite significant advancements in their catalytic performance, further improvement requires new insights on the parameters influencing MOF-based catalysts activity. Here, we have conducted a detailed comparison between the intrinsic electrocatalytic properties of pristine and pyrolyzed Ni-Fe-based MOFs. Interestingly, although pristine MOF exhibits the maximum overall electrocatalytic performance, apparent turnover frequency (TOF) values (intrinsic activity) of all pyrolyzed MOFs exceeded the one of pristine MOF. Moreover, an upper-limit estimation of TOF was extracted using electrochemical impedance spectroscopy (EIS), by excluding IR-drops linked with the electrochemical cell. By doing so, EIS-extracted TOF values were 10-times higher compared to the apparent TOFs. Accordingly, a great leap in performance should still be expected for these catalysts, by designing conductive MOF-platforms having larger pore-diameters to reduce mass-transport limitations.

Single atom tungsten doped ultrathin α-Ni(OH)2 for enhanced electrocatalytic water oxidation

Electrocatalytic water oxidation is a rate-determining step in the water splitting reaction. Here, we report one single atom W⁶⁺ doped Ni(OH)₂ nanosheet sample (w-Ni(OH)₂) with an outstanding oxygen evolution reaction (OER) performance that is, in a 1 M KOH medium, an overpotential of 237 mV is obtained reaching a current density of 10 mA/cm². Moreover, at high current density of 80 mA/cm², the overpotential value is 267 mV. The corresponding Tafel slope is measured to be 33 mV/dec. The d⁰ W⁶⁺ atom with a low spin-state has more outermost vacant orbitals, resulting in more water and OH⁻ groups being adsorbed on the exposed W sites of the Ni(OH)₂ nanosheet. Density functional theory (DFT) calculations confirm that the O radical and O-O coupling are both generated at the same site of W⁶⁺. This work demonstrates that W⁶⁺ doping can promote the electrocatalytic water oxidation activity of Ni(OH)₂ with the highest performance.

Electrocatalytic water splitting for hydrogen and oxygen generation provides an attractive path to obtain clean energy, but the half reaction of oxygen evolution remains the bottleneck for the progress. Here, the authors show single atom tungsten doped ultrathin α-Ni(OH)₂ exhibits enhanced performance in electrocatalytic water oxidation.