Polymer self-assembly guided heterogeneous structure engineering towards high-performance low-frequency electromagnetic wave absorption

Magnetic-dielectric synergy is currently regarded as among the most effective approaches to achieve low-frequency electromagnetic wave absorption (EMA). However, designing and fabricating EMA materials with tunable magnetic-dielectric balance towards high-performance low-frequency EMA remains challenging. Herein, a polymer self-assembly guided heterogeneous structure engineering strategy is proposed to fabricate hierarchical magnetic-dielectric nanocomposite. Polymer assemblies not only can be employed as intermediates to encapsulate metal-organic frameworks and load metal hydroxide, but also that they play a crucial role for the in-situ formation of polycrystalline FeCo/Co composite nanoparticles. As a result, the minimum reflection loss (RLmin) can reach -59.61 dB at 5.4 GHz (4.8 mm) with a 20 wt% filler loading, while the effective absorption bandwidth (EAB, RLmin ≤ -10 dB) is 2.16 GHz, exhibiting excellent low-frequency EMA performance. Systematic investigations demonstrate that hierarchical mesoporous carbon matrix that supports FeCo/Co composite nanoparticles is beneficial for optimizing impedance matching and increasing attenuation capacity. In general, this study opens up new prospects for developing magnetic-dielectric EMA materials using a polymer self-assembly guided heterogeneous structure engineering strategy, which may receive significant attention in future research.

Multilayered Molybdate Microflowers Fabricated by One-Pot Reaction for Efficient Water Splitting

The development of high-performance, low-cost and rapid-production bifunctional electrocatalysts towards overall water splitting still poses huge challenges. Herein, the authors utilize a facile hydrothermal method to synthesize a novel structure of Co-doped ammonium lanthanum molybdate on Ni foams (Co-ALMO@NF) as self-supported electrocatalysts. Owing to large active surfaces, lattice defect and conductive channel for rapid charge transport, Co-ALMO@NF exhibits good electrocatalytic performances which requires only 349/341 mV to achieve a high current density of 600 mA cm^-2 for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), respectively. Besides, a low cell voltage of 1.52 V is required to reach the current density of 10 mA cm^-2 in alkaline medium along with an excellent long-term stability for two-electrode configurations. Density functional theory calculations are performed to reveal the reaction mechanism on Co-ALMO@NF, which shows that the Mo site is the most favorable ones for HER, while the introduction of Co is beneficial to reduce the adsorption intensity on the surface of Co-ALMO@NF, thus accelerating OER process. This work highlighted the importance of the structural design for self-supporting electrocatalysts.

Sulfur Substitution and Defect Engineering in an Unfavored MnMoO4 Catalyst for Efficient Hydrogen Evolution under Visible Light

A novel and nonstoichiometric Mn_1-xMo(S,O)_4-y oxysulfide catalyst with oxygen vacancies and a partial Mo⁶⁺-to-Mo⁴⁺ transition after the substitution of sulfur was synthesized for an efficient photocatalytic hydrogen evolution reaction (PHER). With appropriate sulfur substitution, a MnMoO₄ semiconductor with a wide band gap was converted to Mn_1-xMo(S,O)_4-y with a narrow gap and a suitable band position for PHER. MnMo oxysulfide of 50 mg achieved a high PHER rate of 415.8 μmol/h under visible light, an apparent quantum efficiency (AQE) of 4.31% at 420 nm, and a solar-to-hydrogen (STH) conversion efficiency of 1.28%. Oxygen vacancies (V_O) surrounded by low coordination metal atoms act as active reaction sites, which strengthen water adsorption and activation. Here, we demonstrate that sulfur substitution of MnMoO₄ for lowering its wide band gap can not only disturb the strict periodicity of the lattice but also the valence states of Mn and Mo for enhancing PHER via material design.

Tracking the Role of Defect Types in Co3O4 Structural Evolution and Active Motifs during Oxygen Evolution Reaction

Dynamic reconstruction of catalyst active sites is particularly important for metal oxide-catalyzed oxygen evolution reaction (OER). However, the mechanism of how vacancy-induced reconstruction aids OER remains ambiguous. Here, we use Co₃O₄ with Co or O vacancies to uncover the effects of different defects in the reconstruction process and the active motifs relevant to alkaline OER. Combining in situ characterization and theoretical calculations, we found that cobalt oxides are converted to an amorphous [Co(OH)₆] intermediate state, and then the mismatched rates of *OH adsorption and deprotonation lead to irreversible catalyst reconstruction. The stronger *OH adsorption but weaker deprotonation induced by O defects provides the driving force for reconstruction, while Co defects favor dehydrogenation and reduce the reconstruction rate. Importantly, both O and Co defects trigger highly OER-active bridge Co sites in reconstructed catalysts, of which Co defects induce a short Co-Co distance (3.38 Å) under compressive lattice stress and show the best OER activity (η₁₀ of 262 mV), superior to reconstructed oxygen-defected Co₃O₄-V_O (η₁₀ of 300 mV) and defect-free Co₃O₄ (η₁₀ of 320 mV). This work highlights that engineering defect-dependent reconstruction may provide a rational route for electrocatalyst design in energy-related applications.

Coupling Transition Metal Compound with Single-Atom Site for Water Splitting Electrocatalysis

Single-atom site catalysts (SACs) provide an ideal platform to identify the active centers, explore the catalytic mechanism, and establish the structure-property relationships, and thus have attracted increasing interests for electrocatalytic energy conversion. Substantial endeavors have been devoted to the construction of carbon-supported SACs, and their progress have been comprehensively reviewed. Compared with carbon-supported SACs, transition metal compounds (TMCs)-supported SACs are still in their infancy in the field of electrocatalysis. However, they have also aroused ever-increasing attention for driving electrocatalytic water splitting, and emerged as an indispensable class of SACs in recent years, predominately owing to their inherently structural features, such as rich anchoring sites, surface defects, and lattice vacancy. Herein, in this review, we have systematically summarized the recent advances of a variety of TMC supported SACs toward electrocatalytic water splitting. The advanced characterization techniques and theoretical analyses for identifying and monitoring the atomic structure of SACs are firstly manifested. Subsequently, the anchoring and stabilization mechanisms for TMC supported SACs are also highlighted. Thereafter, the advances of TMC supported SACs for driving water electrolysis are systematically unraveled.

Defect-Induced Self-Poling in a W18O49/PVDF Piezoelectric Energy Harvester

W₁₈O₄₉ nanostructures, previously used for electrocatalysis, energy storage, electrochromic, and gas sensing applications, are incorporated in poly(vinylidene fluoride) (PVDF) in this work for mechanical energy-harvesting applications. X-ray diffraction spectroscopy (XRD), high-resolution transmission electron microscopy (HR-TEM), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy, differential scanning calorimetry (DSC), and the polarization-electric (P-E) field loop test prompts the addition of W₁₈O₄₉ nanorods in PVDF nucleates and stabilizes the piezoelectric polar γ-phase in the nanocomposite. Electrochemical experiments were employed for the first time to relate the event of the evolution of crystalline phases in PVDF to the transfer of electrons to the electrolyte from PVDF using the data from cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). High dielectric constant (ε') and low dielectric loss (ε″) values were obtained proportionately for different weight percentage additions of W₁₈O₄₉ nanorods in PVDF. DSC was employed to study the crystallization kinetics of γ-phase evolution. Piezoresponse force microscopy (PFM) was used to compare the piezoelectric responses from the PVDF nanocomposites. The W₁₈O₄₉/PVDF nanocomposite could generate a peak open circuit voltage of ∼6 V and a peak short circuit current of ∼700 nA. The W₁₈O₄₉/PVDF nanocomposite could light two commercial blue-light-emitting diodes (LEDs) with hand impulse imparting.

Hybrid Transition Metal (V, Fe, and Co) Oxide/Sulfide Catalyst for High-efficient Overall Water Splitting

Hydrogen is an ideal clean energy without any carbon emissions and producing hydrogen by water electrolysis is also eco-friendly and efficient. Herein we proposed a universal strategy of synthesizing transition...

Two-dimensional FeCo2O4 nanosheets with oxygen vacancies enable boosted oxygen evolution

Two dimensional FeCo2O4 nanosheets with abundant oxygen vacancies have been fabricated, which exhibit a superior OER performance compared to pristine FeCo2O4.

Engineering oxygen vacancies in CoO@Co3O4/C nanocomposites for enhanced electrochemical performances

Efficient electrocatalyst materials for several applications, including energy storage and conversion, have become vital for achieving technological progress. In this work, a CoO@Co₃O₄/C composite with abundant oxygen vacancies was successfully synthesized. The concentration of the oxygen vacancies was well controlled by changing the degree of vacuum during the heat treatment and was characterized by XPS and EPR. The existence of the porous structure arising from the cobalt oxide particles embedded in the carbon matrix provided an efficient charge and gas transmission path, significantly improving the performance of electrocatalytic oxygen evolution. Sufficient reactive sites were provided from both the oxygen vacancies and the heterogeneous interface. The mechanism of enhanced OER originating from the built-in electric field derived from oxygen vacancies was investigated. Consequently, the CoO@Co₃O₄/C composites offered an OER overpotential of 287 mV at a current density of 10 mA cm^-2 with good stability in 1 mol L^-1 KOH. In addition, combined with surface photovoltage (SPV), transient photovoltage (TPV), DFT, and in situ Raman spectroscopy, the effect of oxygen defects on the electron migration ability and transformation of the intermediate products were investigated to further understand the nature of catalytic activity in OER.

Push-Pull Electronic Effects in Surface-Active Sites Enhance Electrocatalytic Oxygen Evolution on Transition Metal Oxides

Sustainable electrocatalysis of the oxygen evolution reaction (OER) constitutes a major challenge for the realization of green fuels. Oxides based on Ni and Fe in alkaline media have been proposed to avoid using critical raw materials. However, their ill-defined structures under OER conditions make the identification of key descriptors difficult. Here, we have studied Fe-Ni-Zn spinel oxides, with a well-defined crystal structure, as a platform to obtain general understanding on the key contributions. The OER reaches maximum performance when: (i) Zn is present in the Spinel structure, (ii) very dense, equimolar 1 : 1 : 1 stoichiometry sites appear on the surface as they allow the formation of oxygen vacancies where Zn favors pushing the electronic density that is pulled by the octahedral Fe and tetrahedral Ni redox pair lowering the overpotential. Our work proves cooperative electronic effects on surface active sites as key to design optimum OER electrocatalysts.

Work Function Tuning in Hydrothermally Synthesized Vanadium-Doped MoO3 and Co3O4 Mesostructures for Energy Conversion Devices

The wide interest in developing green energy technologies stimulates the scientific community to seek, for devices, new substitute material platforms with a low environmental impact, ease of production and processing and long-term stability. The synthesis of metal oxide (MO) semiconductors fulfils these requirements and efforts are addressed towards optimizing their functional properties through the improvement of charge mobility or energy level alignment. Two MOs have rising perspectives for application in light harvesting devices, mainly for the role of charge selective layers but also as light absorbers, namely MoO3 (an electron blocking layer) and Co3O4 (a small band gap semiconductor). The need to achieve better charge transport has prompted us to explore strategies for the doping of MoO3 and Co3O4 with vanadium (V) ions that, when combined with oxygen in V2O5, produce a high work function MO. We report on subcritical hydrothermal synthesis of V-doped mesostructures of MoO3 and of Co3O4, in which a tight control of the doping is exerted by tuning the relative amounts of reactants. We accomplished a full analytical characterization of these V-doped MOs that unambiguously demonstrates the incorporation of the vanadium ions in the host material, as well as the effects on the optical properties and work function. We foresee a promising future use of these materials as charge selective materials in energy devices based on multilayer structures.

Oxygen-Deficient Cobalt-Based Oxides for Electrocatalytic Water Splitting

An apparent increased interest has been recently devoted towards the previously untrodden path for anionic point defect engineering of electrocatalytic surfaces. The role of vacancy engineering in improving photo- and electrocatalytic activities of transition metal oxides (TMOs) has been widely reported. In particular, oxygen vacancy modulation on electrocatalysts of cobalt-based TMOs has seen a fresh spike of research work due to the substantial improvements they have shown towards oxygen evolution reaction (OER) and hydrogen evolution reaction (HER). Oxygen vacancy engineering is an effective scheme to quintessentially tune the electronic structure and charge transport, generate secondary active surface phases, and modify the surface adsorption/desorption behavior of reaction intermediates during water splitting. Based on contemporary efforts for inducing oxygen vacancies in a variety of cobalt oxide types, this work addresses facile and environmentally benign synthesis strategies, characterization techniques, and detailed insight into the intrinsic mechanistic modulation of electrocatalysts. It is our foresight that appropriate utilization of the principles discussed herein will aid researchers in rationally designing novel materials that can outperform noble metal-based electrocatalysts. Ultimately, future electrocatalysis implementation for selective seawater splitting is believed to depend on regulating the surface chemistry of active and stable TMOs.

Metal-Organic Framework (MOF)-Derived Electron-Transfer Enhanced Homogeneous PdO-Rich Co3 O4 as a Highly Efficient Bifunctional Catalyst for Sodium Borohydride Hydrolysis and 4-Nitrophenol Reduction

Developing a bifunctional catalyst with low cost and high catalytic performance in NaBH₄ hydrolysis for H₂ generation and selective reduction of nitroaromatics will make a significant impact in the field of sustainable energy and water purification. Herein, a low-loading homogeneously dispersed Pd oxide-rich Co₃ O₄ polyhedral catalyst (PdO-Co₃ O₄ ) with concave structure is reported by using a metal-organic framework (MOF)-templated synthesis method. The results show that the PdO-Co₃ O₄ catalyst has an exceptional turnover frequency (3325.6 mol_H2  min^-1  mol_Pd ^-1 ), low activation energy (43.2 kJ mol^-1 ), and reasonable reusability in catalytic H₂ generation from NaBH₄ hydrolysis. Moreover, the optimized catalyst also shows excellent catalytic performance in the NaBH₄ selective reduction of 4-nitrophenol to 4-aminiphenol with a high first-order reaction rate of approximately 1.31 min^-1 . These excellent catalytic properties are mainly ascribed to the porous concave structure, monodispersed Pd oxide, as well as the unique synergy between PdO and Co₃ O₄ species, which result in a large specific surface area, high conductivity, and fast solute transport and gas emissions.

Nitrogen-Incorporated Cobalt Sulfide/Graphene Hybrid Catalysts for Overall Water Splitting

Water electrolysis is an advanced and sustainable energy conversion technology used to generate H₂ . However, the low efficiency of the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) hampers the overall water-splitting catalytic performance. Here, a hybrid catalyst was constructed from N-doped CoS₂ nanoparticles on N,S-co-doped graphene nanosheets (N-CoS₂ /G) using a facile method, and the catalyst exhibited excellent bifunctional activity. Introduction of N atoms not only promoted the adsorption of reaction intermediates, but also bridged the CoS₂ nanoparticles and graphene to improve electron transfer. Moreover, using thiourea as both N- and S-source ensured synthesis of much smaller-sized nanoparticles with more surface active sites. Surprisingly, the N-CoS₂ /G exhibited superior catalytic activity with a low overpotential of 260 mV for the OER and 109 mV for the HER at a current density of 10 mA cm^-2 . The assembled N-CoS₂ /G : N-CoS₂ /G electrolyzer substantially expedited overall water splitting with a voltage requirement of 1.58 V to reach 10 mA cm^-2 , which is superior to most reported Co-based bifunctional catalysts and other non-precious-metal catalysts. This work provides a new strategy towards advanced bifunctional catalysts for water electrolysis.

A nickel molybdenum oxide nanoarray as an efficient and stable electrocatalyst for overall water splitting

Experimental and DFT calculation results show that the presence of oxygen vacancies can decrease the adsorption energy of intermediates at active sites and facilitate their adsorption, thus improving the catalytic properties.

Interconnected porous nanoflakes of CoMo2S4 as an efficient bifunctional electrocatalyst for overall water electrolysis

Interconnected porous nanoflakes of the bimetallic CoMo₂S₄ are synthesized and investigated as bifunctional catalysts for highly efficient overall water electrolysis.

Confinement of fluorine anions in nickel-based catalysts for greatly enhancing oxygen evolution activity

A simple fluorine treatment was developed to confine abundant fluorine anions into Ni-based catalysts to greatly enhance OER catalytic activity.

Oxygen vacancies confined in nickel oxide nanoprism arrays for promoted electrocatalytic water splitting

Experimental and DFT calculation results show that the presence of oxygen vacancies can decrease the adsorption energy of intermediates at active sites and facilitate the adsorption of intermediates, thus improving the catalytic properties.

Intermetallic borides: structures, synthesis and applications in electrocatalysis

This review summarizes structural features and recent synthesis methods of structurally ordered intermetallic borides, and the theoretical–experimental advances in the emerging boride-catalyzed reactions.

Oxygen Vacancy-Rich In-Doped CoO/CoP Heterostructure as an Effective Air Cathode for Rechargeable Zn-Air Batteries

An efficient and low-cost electrocatalyst for reversible oxygen electrocatalysis is crucial for improving the performance of rechargeable metal-air batteries. Herein, a novel oxygen vacancy-rich 2D porous In-doped CoO/CoP heterostructure (In-CoO/CoP FNS) is designed and developed by a facile free radicals-induced strategy as an effective bifunctional electrocatalyst for rechargeable Zn-air batteries. The electron spin resonance and X-ray absorption near edge spectroscopy provide clear evidence that abundant oxygen vacancies are formed in the interface of In-CoO/CoP FNS. Owing to abundant oxygen vacancies, porous heterostructure, and multiple components, In-CoO/CoP FNS exhibits excellent oxygen reduction reaction activity with a positive half-wave potential of 0.81 V and superior oxygen evolution reaction activity with a low overpotential of 365 mV at 10 mA cm^-2 . Moreover, a home-made Zn-air battery with In-CoO/CoP FNS as an air cathode delivers a large power density of 139.4 mW cm^-2 , a high energy density of 938 Wh kg_Zn ^-1 , and can be steadily cycled over 130 h at 10 mA cm^-2 , demonstrating great application potential in rechargeable metal-air batteries.