Modulated transition metal-oxygen covalency in the octahedral sites of CoFe layered double hydroxides with vanadium doping leading to highly efficient electrocatalysts

Lewis Acid Sites in Hollow Cobalt Phytate Micropolyhedra Promote the Electrocatalytic Water Oxidation

The acid-base microenvironment of the metal center is crucial for constructing advanced oxygen evolution reaction (OER) electrocatalysts. However, the correlation between acidic site and OER performance remains unclear for cobalt-based catalysts. Herein, Lewis acid sites in hollow cobalt phytate micropolyhedra (M-CoPA, M = Cu, Sr) were synthesized by a cation-exchange strategy, and their OER performances were studied systematically. Experimentally, Lewis acid Cu2+ sites with stronger Lewis acidity exhibited superior intrinsic activity and long-term stability in alkaline electrolytes. The spectroscopic and electrochemical studies show Lewis acid sites in hollow cobalt phytate micropolyhedra can modulate the electronic distribution of the adjacent cobalt center and further optimize the adsorption strength of oxygenated species. This study figures out the effect of Lewis acid sites on the OER kinetics and provides an effective way to develop high-efficiency electrocatalysts for energy conversion systems.

Tuning the local electronic structure of Co15V-ZIF through bimetallic synergies as a bifunctional electrocatalyst for overall water splitting

Co-based bimetallic zeolite imidazolate frameworks (ZIFs) have been shown as promising electrocatalysts for the oxygen evolution reaction, but their electronic structure's influence on the catalytic performance for overall water splitting still needs further investigation. In this study, Co15V-ZIF, structured as two-dimensional (2D) nanosheet arrays, are grown on nickel foam using one-step co-precipitation strategy. Owing to the synergistic effects of vanadium (V) and cobalt (Co) reasonably regulating the electronic structure, the synthesized bimetallic ZIFs demonstrate superior catalytic performance, which required the overpotentials of only 227 and 68 mV to achieve a current density of 10 mA cm-2 in 1 M KOH for oxygen evolution reaction (OER) and hydrogen evolution reaction (HER), respectively. Furthermore, the water electrolyzer assembled with bimetallic ZIF as cathode and anode exhibits the capability to achieve 10 mA cm-2 at a low cell voltage of 1.57 V. In situ Raman spectroscopy reveals that the introduction of V facilitates the formation of V-CoOOH, the real active site for OER, at lower applied potentials. Besides, it induces a local acidic environment on V-Co(OH)2, the real active sites, thereby enhancing the HER performance of the sample. Density Functional Theory (DFT) calculations further show that the synergistic effects of V and Co induce electron redistribution, thereby improving electrical conductivity, reducing the energy barrier for water dissociation and hydrogen adsorption, which promotes the formation of H3O+ and triggering H3O+-induced water reduction in alkaline media. This work provides new insight into tailoring electronic structures to rationally design highly efficient ZIF electrocatalysts.

Decoding the entropy-stabilized matrix of high-entropy layered double hydroxides: Harnessing strain dynamics for peroxymonosulfate activation and tetracycline degradation

The current understanding of the mechanism of high-entropy layered double hydroxide (LDH) on enhancing the efficiency of activating peroxymonosulfate (PMS) remains limited. This work reveals that a strong strain effect, driven by high entropy, modulates the structure of FeCoNiCuZn-LDH (HE-LDH) as evidenced by geometric phase analysis (GPA) and density functional theory (DFT) calculations. Compared to FeCoNiZn-LDH and FeCoNi-LDH with weaker strain effects, the high entropy-driven strain effect in HE-LDH shortens metal-oxygen-hydrogen (MOH) bond lengths, allows system to be in a constant steady state during catalysis, reduces the leaching of active M-OH sites, and enhances the adsorption capacity of these sites and the excess strain strength of the interfacial stretches the IO-O of the PMS, facilitates reactive oxygen species (·OH, SO4·-, 1O2 and O2·-) generation, and thereby improving the efficiency of PMS in degrading tetracycline (TC). Consequently, HE-LDH demonstrated a 90% TC degradation within 3 min, maintained over 92% TC removal across a wide pH range (3-11), and achieved over 90% degradation performance after 6 cycles. This study reports the first use of high-entropy LDH material as a non-homogeneous catalyst and provides insights into the extremely different catalytic behaviors of high entropy mechanisms for the activation of PMS.

V-Doping Strategy Induces the Construction of the CoFe-LDHs/NF Electrodes with Higher Conductivity to Achieve Higher Energy Density for Advanced Energy Storage Devices

Doping of metal ions shows promising potential in optimizing and modulating the electrical conductivity of layered double hydroxides (LDHs). However, there is still much room for improvement in common metal ions and conventional doping methods. In contrast to previous methodologies, a hollow triangular nanoflower structure of CoFeV-LDHs is devised, which is enriched with a greater number of oxygen vacancies. This resulted in a significant enhancement in the conductivity of the LDHs, leading to an increase in energy density following the appropriate doping of V. To investigate the impact of V-doping on the energy density of the LDHs, in situ XPS and in situ X-ray spectroscopy is employed. Regarding electrochemical performance, the CoFeV-LDHs/NF electrode with optimal doping ratio exhibited a specific capacitance of 881 F g-1 at a current density of 1 A g-1. The capacitance remained at 90.53% after 3000 cycles. In addition, the constructed battery-type supercapacitor CoFeV-LDHs/NF-2//AC exhibited an impressive energy density of 124.7 Wh kg-1 at a power density of 850 W kg-1 and capacitance remained almost unchanged at 95.2% after 3000 cycles. All the above demonstrates the great potential of V-doped LDHs and brings a new way for the subsequent research of LDHs.

Facile synthesis of bimetallic ACF/CC@FeOCl-Cu composite cathode for efficient degradation of sulfamethoxazole at neutral pH by a flow-through heterogeneous electro-Fenton process

Flow-through heterogeneous electro-Fenton (FHEF) process shows a broad prospect for refractory organic pollutants removal. However, maintaining a long-term service life of higher catalytic cathode is crucial for the development of cathode materials, especially for iron-functionalized cathode operated under harsh conditions. In this study, a novel bimetallic CC@FeOCl-Cu composite was synthesized through one-step calcination, coupled with a series of microstructure characterization methodology, including XRD, SEM-EDS, XPS, and FTIR. The superior catalytic activity of CC@FeOCl-Cu could be ascribed to Fe-Cu synergy and better dispersion of FeOCl nanosheets. With the optimal Cu:Fe ratio of 1:60, the bifunctional ACF/CC@FeOCl-Cu cathode was employed in FHEF process, exhibiting an outstanding performance for sulfamethoxazole (SMX) removal over a wide pH range (3.0-9.0). Comparison of experimental results indicated that the ACF/CC@FeOCl-Cu-FHEF process showed higher performance than ACF/CC@FeOCl-FHEF and homogeneous EF processes. The average SMX removal efficiency was 98% and TOC removal efficiency was more than 57% even after 10 cycles. Radical quenching experiments and electron spin resonance test confirmed that •OH was the primary active species. More •OH was generated in the ACF/CC@FeOCl-Cu-FHEF process because the doping of Cu could enhance catalytic activity of cathode. In addition, the satisfactory performance could be observed in the ACF/CC@FeOCl-Cu-FHEF process for the treatment of real landfill leachate, indicating its potential for practical application in wastewater treatment.

Degradation of emerging pollutants on bifunctional ZnFeV LDH@graphite felt cathode through prominent catalytic activity in heterogeneous electrocatalytic processes

The heterogeneous Electro-Fenton (EF) process is a promising wastewater treatment technology that can generate onsite H₂O₂, and operate in a wide pH range without generating a metal sludge. However, the heterogeneous EF process needs bifunctional cathode electrodes that can have high activity in 2e^- oxygen reduction reaction and H₂O₂ decomposition. Herein, ZnFeV layered double hydroxide (LDH), as a heterogeneous catalyst, was coated on the graphite felt (ZnFeV LDH@GF) cathode using the electrophoretic deposition method. ZnFeV LDH@GF cathode was able to generate 59.8 ± 5.9 mg L^-1 H₂O₂ in 90 min under a constant supply of O₂. EF process with ZnFeV LDH@GF cathode exhibited 89.8 ± 6.8% removal efficiency for pharmaceutical (ciprofloxacin) at neutral pH. Remarkably, the apparent reaction rate constant (k_app) of the ZnFeV LDH@GF-EF was 2.14 times that of the EF process with pristine GF. ZnFeV LDH coating increased the hydroxyl radical (^•OH) production of the EF process from 1.74 mM to 3.65 mM. The pathway of ^•OH production is thought to be a single electron transfer from redox couples of Fe²⁺/Fe³⁺ and [Formula: see text] to H₂O₂. After 10 reuse cycles, the ZnFeV LDH@GF cathode retained 90.2% of its efficiency. Eight intermediate compounds were identified by GC-MS including cyclic compounds and aliphatic compounds.

In Situ Anodic Oxidation Tuning of NiFeV Diselenide to the Core-Shell Heterojunction for Boosting Oxygen Evolution

Developing non-noble metal-based core-shell heterojunction electrocatalysts with high catalytic activity and long-lasting stability is crucial for the oxygen evolution reaction (OER). Here, we prepared novel core-shell Fe,V-NiSe₂@NiFe(OH)_x heterostructured nanoparticles on hydrophilic-treated carbon paper with high electronic transport and large surface area for accelerating the oxygen evolution rate via high-temperature selenization and electrochemical anodic oxidation procedures. Performance testing shows that Fe,V-NiSe₂@NiFe(OH)_x possesses the highest performance for OER compared to as-prepared diselenide core-derived heterojunctions, which only require an overpotential of 243 mV at 10 mA cm^-2 and a low Tafel slope of 91.6 mV decade^-1 under basic conditions. Furthermore, X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM) confirm the morphology and elementary stabilities of Fe,V-NiSe₂@NiFe(OH)_x after long-term chronopotentiometric testing. These advantages are largely because of the strong synergistic effect between the Fe,V-NiSe₂ core with high conductivity and the amorphous NiFe(OH)_x shell with enriched defects and vacancies. This study also presents a general approach to designing and synthesizing more active core-shell heterojunction electrocatalysts for OER.

Rational Regulation of Electronic Structure in Layered Double Hydroxide Via Vanadium Incorporation to Trigger Highly Selective CO2 Photoreduction to CH4

To realize excellent selectivity of CH₄ in CO₂ photoreduction (CO₂ PR) is highly desirable, yet which is challenging due to the limited active sites for CH₄ generation and severe electron-hole recombination on photocatalysts. Herein, based on the theoretically calculated effects of vanadium incorporation into the laminate of layered double hydroxides (LDHs), V into NiAl-LDH to synthesize a series of LDHs with various V contents is introduced. NiV-LDH is revealed to afford a high CH₄ selectivity (78.9%), and extremely low H₂ selectivity (only 0.4%) under λ > 400 nm irradiation. By further tuning the molar ratio of Ni to V, a CH₄ selectivity of as high as 90.1% is achieved on Ni₄ V-LDH, and H₂ is completely prohibited on Ni₂ V-LDH. Fine structural characterizations and comprehensive optical and electrochemical studies uncover V incorporation creates the lower-valence Ni species as active sites for generating CH₄ , and enhances the generation, separation, and transfer of photogenerated carriers.

Sacrificial W Facilitates Self-Reconstruction with Abundant Active Sites for Water Oxidation

Water oxidation is an important reaction for multiple renewable energy conversion and storage-related devices and technologies. High-performance and stable electrocatalysts for the oxygen evolution reaction (OER) are urgently required. Bimetallic (oxy)hydroxides have been widely used in alkaline OER as electrocatalysts, but their activity is still not satisfactory due to insufficient active sites. In this research, A unique and efficient approach of sacrificial W to prepare CoFe (oxy)hydroxides with abundant active species for OER is presented. Multiple ex situ and operando/in situ characterizations have validated the self-reconstruction of the as-prepared CoFeW sulfides to CoFe (oxy)hydroxides in alkaline OER with synchronous W etching. Experiments and theoretical calculations show that the sacrificial W in this process induces metal cation vacancies, which facilitates the in situ transformation of the intermediate metal hydroxide to CoFe-OOH with more high-valence Co(III), thus creating abundant active species for OER. The Co(III)-rich environment endows the in situ formed CoFe oxyhydroxide with high catalytic activity for OER on a simple flat glassy carbon electrode, outperforming those not treated by the sacrificial W procedure. This research demonstrates the influence of etching W on the electrocatalytic performance, and provides a low-cost means to improve the active sites of the in situ self-reconstructed bimetallic oxyhydroxides for OER.

Lewis acid Mg2+-doped cobalt phosphate nanosheets for enhanced electrocatalytic oxygen evolution reaction

Cobalt-based materials are considered to be promising electrocatalysts for oxygen evolution reaction (OER). However, their catalytic efficiencies are still limited by sluggish surface adsorption and high activation overpotentials. Herein, Lewis...

Ru doping induces the construction of a unique core-shell microflower self-supporting electrocatalyst for highly efficient overall water splitting

Since the large reaction energy barrier caused by multi-step electron transfer processes of the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) gravely restricts the practical application of electrocatalytic water splitting, it is urgent to develop a dual-functional electrocatalyst which can effectively reduce the reaction energy barrier and actually speed up the reaction. Herein, the Ru species are doped into the complex of magnetite and FeNi-layered double hydroxide by a one-step oil bath method, and a self-supporting binder-free bifunctional electrocatalyst was synthesized on the surface of iron foam (named Ru-Fe₃O₄@FeNi-LDH/IF). The unique 3D core-shell microflower structure of Ru-Fe₃O₄@FeNi-LDH/IF, the combination of active ingredient and conductive substrate, together with the doping of Ru may immensely provide a large number of active sites, adjust the electronic structure, accelerate electron transfer, and thus greatly improve the electrocatalytic activity and durability. It is worth mentioning that when Ru-Fe₃O₄@FeNi-LDH/IF is used as the anode and cathode for overall water splitting, only 1.52 V battery voltage can generate a current density of 10 mA cm^-2, and also maintain a prominent stability for at least 36 hours. This work provides a feasible strategy for heteroatom-doping LDH as a bifunctional electrocatalyst.

Unusual Role of Point Defects in Perovskite Nickelate Electrocatalysts

Low-cost transition-metal oxide is regarded as a promising electrocatalyst family for an oxygen evolution reaction (OER). The classic design principle for an oxide electrocatalyst believes that point defect engineering, such as oxygen vacancies (V_O^..) or heteroatom doping, offers the opportunities to manipulate the electronic structure of material toward optimal OER activity. Oppositely, in this work, we discover a counterintuitive phenomenon that both V_O^.. and an aliovalent dopant (i.e., proton (H⁺)) in perovskite nickelate (i.e., NdNiO₃ (NNO)) have a considerably detrimental effect on intrinsic OER performance. Detailed characterizations unveil that the introduction of these point defects leads to a decrease in the oxidative state of Ni and weakens Ni-O orbital hybridization, which triggers the local electron-electron correlation and a more insulating state. Evidenced by first-principles calculation using the density functional theory (DFT) method, the OER on nickelate electrocatalysts follows the lattice oxygen mechanism (LOM). The incorporation of point defect increases the energy barrier of transformation from OO*(V_O) to OH*(V_O) intermediates, which is regarded as the rate-determining step (RDS). This work offers a new and significant perspective of the role that lattice defects play in the OER process.

Boosting electrocatalytic oxygen evolution activity of bimetallic CoFe selenite by exposing specific crystal facets

It was of great significance to develop efficient and stable electrocatalysts for oxygen evolution reactions via simple and environmentally-friendly methods.

Tuning the properties of CoFe-layered double hydroxide by vanadium substitution for improved water splitting activity

Here, we demonstrate the enhanced water splitting activity of CoFe–LDH by vanadium substitution.

Manganese-based layered double hydroxide nanoparticles as highly efficient ozone decomposition catalysts with tunable valence state

Manganese oxides are well explored effective ozone decomposition catalysts, but the accumulation of oxygen trapped on their surfaces and high valence state restrict their catalyst efficiency. Herein, we report manganese based layered double hydroxide (LDH) catalysts with different average oxidation states (AOS) of Mn. MgMnAl-LDH catalysts show large specific surface area, abundant oxygen vacancies, stable structure and excellent catalytic ozone decomposition performance. The valence state of Mn can be tuned by adjusting the metallic element ratio in the LDH matrix, and a catalyst with AOS of only 2.3 is acquired. The impacts of the valence states of Mn on the catalytic ozone decomposition process were further studied by density functional theory (DFT) calculations. It is found that the Mn²⁺ facilitates the desorption of generated oxygen on the surface of LDHs, while Mn³⁺ and Mn⁴⁺ contribute to the dissociation of adsorbed ozone.

Study on the efficient removal of azo dyes by heterogeneous photo-Fenton process with 3D flower-like layered double hydroxide

As organic dyes are the main pollutants in water pollution, seeking effective removal solutions is urgent for humans and the environment. A novel environmentally friendly three-dimensional CoFe-LDHs (3D CoFe-LDHs) catalyst was synthesized by one-step hydrothermal method. Scanning electron microscopy, energy dispersive spectroscopy, Fourier transform infrared spectra, X-ray diffraction, X-ray photoelectron spectroscopy, Brunauer-Emmett-Teller technique as well as UV-Vis diffuse reflectance spectra were used to characterize the prepared samples. The experimental results revealed that 3D CoFe-LDHs exhibited a rapid decolorization of methyl orange and Rhodamine B by heterogeneous photo-Fenton process after reaching the adsorption equilibrium, and the final decolorization efficiency reached 91.18% and 93.56%, respectively. On the contrary, the decolorizing effect of 3D CoFe-LDHs on neutral blue was relatively weak. The initial concentrations of azo dyes, pH and H₂O₂ concentration affected the decolorization of dyes and the catalyst maintained excellent reusability and stability after reuse over five cycles. The quenching experiments found that •OH, •O₂ ^- and h⁺ were the main active substances and reaction mechanisms were further proposed. The study suggests that the synergistic effect of photocatalysis and Fenton oxidation process significantly improved the removal of azo dyes and the synthesized catalyst had potentially promising applications for difficult-to-biodegrade organic pollutants in wastewater.