Transition metal-based layered double hydroxides for photo(electro)chemical water splitting: a mini review

Research progress on layered metal oxide electrocatalysts for an efficient oxygen evolution reaction

Hydrogen, highly valued for its pristine cleanliness and remarkable efficiency as an emerging energy source, is anticipated to ascend to a preeminent status within the forthcoming energy landscape. Electrocatalytic water splitting is considered a pivotal, eco-friendly, and sustainable strategy for hydrogen production. The substantial energy consumption stemming from oxygen evolution side reactions significantly impedes the commercial viability of water electrolysis. Consequently, the pursuit of a cost-effective and efficacious oxygen evolution reaction (OER) catalyst stands as an imperative strategy for realizing hydrogen production via water electrolysis. Layered metal oxides, owing to their robust anisotropic properties, versatile adjustability, and extensive surface area, have emerged as suitable candidates for OER catalysts. However, owing to the distinctive attributes of layered metal oxides, ongoing investigations into these materials are slightly fragmented, lacking universal consensus. This article comprehensively surveys the recent advancements in layered metal oxide-based OER catalysts, categorized into single metal oxides, alkali cobalt oxides, perovskites, and miscellaneous metal oxides. Initially, the main OER intermediate reaction steps of layered metal oxides are scrutinized. Subsequently, the design, mechanism, and application of several pivotal layered metal oxides in the OER are systematically delineated. Finally, a summary is provided, alongside the proposal of future research trajectories and challenges encountered by layered metal oxides, with the aspiration that this paper may serve as a valuable reference for scholars in the field.

Solar enhanced oxygen evolution reaction with transition metal telluride

The photo-enhanced electrocatalytic method of oxygen evolution reaction (OER) shows promise for enhancing the effectiveness of clear energy generation through water splitting by using renewable and sustainable source of energy. However, despite benefits of photoelectrocatalytic (PEC) water splitting, its uses are constrained by its low efficiency as a result of charge carrier recombination, a large overpotential, and sluggish reaction kinetics. Here, we illustrate that Nickel telluride (NiTe) synthesized by hydrothermal methods can function as an extremely effective photo-coupled electrochemical oxygen evolution reaction (POER) catalyst. In this study, NiTe was synthesized by hydrothermal method at 145°C within just an hour of reaction time. In dark conditions, the NiTe deposited on carbon cloth substrate shows a small oxygen evolution reaction overpotential (261 mV) at a current density of 10 mA cm-2, a reduced Tafel slope (65.4 mV dec-1), and negligible activity decay after 12 h of chronoamperometry. By virtue of its enhanced photo response, excellent light harvesting ability, and increased interfacial kinetics of charge separation, the NiTe electrode under simulated solar illumination displays exceptional photoelectrochemical performance exhibiting overpotential of 165 mV at current density of 10 mA cm-2, which is about 96 mV less than on dark conditions. In addition, Density Functional Theory investigations have been carried out on the NiTe surface, the results of which demonstrated a greater adsorption energy for intermediate -OH on the catalyst site. Since the -OH adsorption on the catalyst site correlates to catalyst activation, it indicates the facile electrocatalytic activity of NiTe owing to favorable catalyst activation. DFT calculations also revealed the facile charge density redistribution following intermediate -OH adsorption on the NiTe surface. This work demonstrates that arrays of NiTe elongated nanostructure are a promising option for both electrochemical and photoelectrocatalytic water oxidation and offers broad suggestions for developing effective PEC devices.

Particle-Based Photoelectrodes for PEC Water Splitting: Concepts and Perspectives

This comprehensive review delves into the intricacies of the photoelectrochemical (PEC) water splitting process, specifically focusing on the design, fabrication, and optimization of particle-based photoelectrodes for efficient green hydrogen production. These photoelectrodes, composed of semiconductor materials, potentially harness light energy and generate charge carriers, driving water oxidation and reduction reactions. The versatility of particle-based photoelectrodes as a platform for investigating and enhancing various semiconductor candidates is explored, particularly the emerging complex oxides with compelling charge transfer properties. However, the challenges presented by many factors influencing the performance and stability of these photoelectrodes, including particle size, shape, composition, morphology, surface modification, and electrode configuration, are highlighted. The review introduces the fundamental principles of semiconductor photoelectrodes for PEC water splitting, presents an exhaustive overview of different synthesis methods for semiconductor powders and their assembly into photoelectrodes, and discusses recent advances and challenges in photoelectrode material development. It concludes by offering promising strategies for improving photoelectrode performance and stability, such as the adoption of novel architectures and heterojunctions.

Heterostructured grafting of NiFe-layered double hydroxide@TiO2 for boosting photoelectrochemical cathodic protection

Accelerating the oxidation process at photoanode-electrolyte interfaces can prolong the lifetime of photoexcited electrons and improve the efficiency of photoelectrochemical cathodic protection (PECCP) systems without relying on hole scavengers. However, the systematic design of precisely structured heterostructures for efficient photoanodes remains challenging. Here we meticulously engineered a type-II heterostructure featuring precise spatial organization, wherein NiFe-layered double hydroxide nanosheets (NiFe-LDH NSs) were assembled onto annealed TiO2 nanorod arrays (ATNAs), demonstrating their effectiveness in achieving efficient PECCP. The interfacial electronic coupling and appropriate energy alignment between the NiFe-LDH NSs and ATNAs allowed rapid hole extraction from the ATNAs to the NiFe-LDH NSs. Furthermore, the uniform distribution of the NiFe-LDH NSs on top of ATNAs drastically reduced the overpotential of oxygen evolution reactions (OER) from 370 to 200 mV and Tafel slope from 162 to 56 mV dec-1, leading to significantly improved cathodic protection of 304 stainless steel (SS) under extended illumination and interesting post-illumination protection. In addition, with the increase of testing cycles, the as-prepared NiFe-LDH NSs@ATNAs demonstrated a progressively enhanced cathodic protection potential from 0.15 to 0.13 V vs. RHE over 50 cycles. These findings provide important guidelines for the design of future high-efficiency green metal protection through rational photoanode design.

Atomic Strain Wave-Featured LaRuIr Nanocrystals: Achieving Simultaneous Enhancement of Catalytic Activity and Stability toward Acidic Water Splitting

Rare earth microalloying nanocrystals have gotten widespread attention due to their unprecedented performances with customization-defected nanostructures, divided energy bands, and ensembled surface chemistry, regarded as a class of ideal electrocatalysts for oxygen evolution reaction (OER). Herein, a lanthanide microalloying strategy is proposed to fabricate strain wave-featured LaRuIr nanocrystals with oxide skin through a rapid crystal nucleation, using thermally assisted sodium borohydride reduction in aqueous solution at 60 °C. The atomic strain waves with alternating compressive and tensile strains, resulting from La-stabilized edge dislocations in form of Cottrell atmospheres. In 0.5 m H2SO4, the LaRuIr displays an overpotential of 184 mV at 10 mA cm-2, running at a steadily cell voltage for 60 h at 50 mA cm-2, eightfold enhancement of IrO2||Pt/C assemble in PEMWE. The coupled compressive and tensile profiles boost the OER kinetics via faster AEM and LOM pathways. Moreover, the tensile facilitates surface structure stabilization through dynamic refilling of lattice oxygen vacancies by the adsorbed oxyanions on La, Ru, and Ir sites, eventually achieving a long-term stability. This work contributes to developing advanced catalysts with unique strain to realize simultaneous improvement of activity and durability by breaking the so-called seesaw relationship between them during OER for water splitting.

Novel Heterostructure-Based CoFe and Cobalt Oxysulfide Nanocubes for Effective Bifunctional Electrocatalytic Water and Urea Oxidation

The development of effective oxygen evolution reaction (OER) and urea oxidation reaction (UOR) on heterostructure electrocatalysts with specific interfaces and characteristics provides a distinctive character. In this study, heterostructure nanocubes (NCs) comprising inner cobalt oxysulfide (CoOS) NCs and outer CoFe (CF) layered double hydroxide (LDH) are developed using a hydrothermal methodology. During the sulfidation process, the divalent sulfur ions (S2- ) are released from the breakdown of the sulfur source and react with the Co-precursors on the surface leading to the transformation of CoOH nanorods into CoOS nanocubes. Further, X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS) analyses reveal that the interactions at the interface of the CF@CoOS NCs significantly altered the electronic structure, thus enhancing the electrocatalytic performance. The optimal catalysts exhibited effective OER and UOR activities, the attained potentials are 1.51 and 1.36 V. This remarkable performance is attributable to the induction of electron transfer from the CoFe LDH to CoOS, which reduces the energy barrier of the intermediates for the OER and UOR. Furthermore, an alkaline water and urea two-cell electrolyzer assembled using CF@CoOS-2 NCs and Pt/C as the anode and cathode requires a cell voltage of 1.63 and 1.56 V along with a durability performance.

Efficient and durable MoFeNi hydroxide anode: Room temperature recrystallization regulated morphology-, valence- and crystallinity-dependent water oxidation performance

The formation of crystal-amorphous (c-a) interfaces by modulating the crystallinity of the material is a promising strategy for the oxygen evolution reaction (OER). Herein, a recrystallization growth at room temperature to regulate the crystallinity of catalysts is reported. The MoFeNi hydroxide precursor was synthesized by the solvothermal method, and then the crystallinity of the material was controlled by adjusting the concentration of Na2S in the immersion solution. These c-a heterogeneous interfaces significantly improved the OER activity of the catalysts while ensuring structural stability. The best catalyst exhibited a low overpotential of 195 mV to reach 10 mA cm-2 in 1 M KOH. It also showed good stability, operating stably at high current densities for 96 h without significant degradation. In addition, the anode of the two-electrode water splitting electrolyzer required only 1.46 V to reach 10 mA cm-2 and operated for a long time without significant degradation. This method will provide new insights and perspectives for developing efficient and stable OER catalysts.

Multiple-Strategy Design of MOF-Derived N, P Co-Doped MoS2 Electrocatalysts Toward Efficient Alkaline Hydrogen Evolution and Overall Water Splitting

The multiple strategy design is crucial for enhancing the efficiency of nonprecious electrocatalysts in hydrogen evolution reaction (HER). In this work, we successfully synthesized N, P-codoped MoS2 nanosheets as highly efficient catalysts by integrating doping effects and phase engineering using a porous metal-organic framework (MOF) template. The electrocatalysts exhibit excellent bifunctional activity and stability in alkaline media. The N, P codoping induces electron redistribution to enhance conductivity and promote the intrinsic activity of the electrocatalysts. It optimizes the H* adsorption free energy and the dissociative adsorption energy, resulting in significant enhancement of HER activity. Moreover, the porous MOF structure exposes a large number of electrochemically active sites and facilitates the diffusion of ions and gases, which improve charge transfer efficiency and structural stability. Specifically, at a current density of 10 mA cm-2, the overpotential of the HER is only 32 mV, with a Tafel slope of 47 mV dec-1 and Faradaic efficiency as high as 98.51% (at 100 mA cm-2). Only a 338 mV overpotential is required to achieve a current density of 50 mA cm-2 for oxygen evolution reaction (OER), and a potential of 1.49 V (at 10 mA cm-2) is sufficient to drive overall water splitting. Further experimental measurements and first-principles calculations evidence that the exceptional performance is primarily attributed to the dual functionality of N and P dopants, which not only activate additional S sites but also initialize the phase transition of 2H to 1T-MoS2 to facilitate the rapid charge transfer. Through in-depth exploration of the combined design of multiple strategies for efficient catalysts, our work paves a new way for the development of future efficient nonprecious metal catalysts.

Recent Advances in Co-Based Electrocatalysts for Hydrogen Evolution Reaction

Water splitting is a promising technique in the sustainable "green hydrogen" generation to meet energy demands of modern society. Its industrial application is heavily dependent on the development of novel catalysts with high performance and low cost for hydrogen evolution reaction (HER). As a typical non-precious metal, cobalt-based catalysts have gained tremendous attention in recent years and shown a great prospect of commercialization. However, the complexity of the composition and structure of newly-developed Co-based catalysts make it urgent to comprehensively retrospect and summarize their advance and design strategies. Hence, in this review, the reaction mechanism of HER is first introduced and the possible role of the Co component during electrocatalysis is discussed. Then, various design strategies that could effectively enhance the intrinsic activity are summarized, including surface vacancy engineering, heteroatom doping, phase engineering, facet regulation, heterostructure construction, and the support effect. The recent progress of the advanced Co-based HER electrocatalysts is discussed, emphasizing that the application of the above design strategies can significantly improve performance by regulating the electronic structure and optimizing the binding energy to the crucial intermediates. At last, the prospects and challenges of Co-based catalysts are shown according to the viewpoint from fundamental explorations to industrial applications.

Construction of Co2 P-Ni3 S2 /NF Heterogeneous Structural Hollow Nanowires as Bifunctional Electrocatalysts for Efficient Overall Water Splitting

Designing efficient and stable transition metal-based catalysts for electrocatalytic water splitting is vital for the development of hydrogen production. Herein, a facile synthetic strategy is developed to fabricate transition metal-based heterogeneous structural Co2 P-Ni3 S2 hollow nanowires supported on nickel foam (Co2 P-Ni3 S2 /NF). Owing to the multiple active sites provided by transition metal compounds, large surface area of the unique hollow nanowire morphology, and the synergistic effect of Co2 P-Ni3 S2 heterostructure interfaces, Co2 P-Ni3 S2 /NF requires ultralow overpotentials of 110, 164 mV for HER and 331.7, 358.3 mV for OER at large current densities of 100, 500 mA cm-2 in alkaline medium, respectively. Importantly, the two-electrode electrolyzer assembled by Co2 P-Ni3 S2 /NF displays a cell voltage of 1.54 V at 10 mA cm-2 and operates stably over 24 h at 100 mA cm-2 , which performs better than reported transition metal-based bifunctional electrocatalysts. This work presents a successful fabrication of transition metal-based bifunctional HER/OER electrocatalysts at large-current density and brings new inspiration for developing applicable energy conversion materials.

Enhanced Oxygen Evolution Reaction Performance on NiSx@Co3O4/Nickel Foam Electrocatalysts with Their Photothermal Property

Based on the principle of heterogeneous catalysis for water electrolysis, electrocatalysts with appropriate electronic structure and photothermal property are expected to drive the oxygen evolution reaction effectively. Herein, amorphous NiS_x-coupled nanourchin-like Co₃O₄ was prepared on nickel foam (NiS_x@Co₃O₄/NF) and investigated as a electrocatalyst for photothermal-assisted oxygen evolution reaction. The experimental investigations and simulant calculations jointly revealed NiS_x@Co₃O₄/NF to be of suitable electronic structure and high near-infrared photothermal conversion capability to achieve the oxygen evolution reaction advantageously both in thermodynamics and in kinetics. Relative to Co₃O₄/NF and NiS_x/NF, better oxygen evolution reaction activity, kinetics, and stability were achieved on NiS_x@Co₃O₄/NF in 1.0 M KOH owing to the NiS_x/Co₃O₄ synergetic effect. In addition, the oxygen evolution reaction performance of NiS_x@Co₃O₄/NF can be obviously enhanced under near-infrared light irradiation, since NiS_x@Co₃O₄ can absorb the near-infrared light to produce electric and thermal field. For the photothermal-mediated oxygen evolution reaction, the overpotential and Tafel slope of NiS_x@Co₃O₄/NF at 50 mA cm^-2 were reduced by 23 mV and 13 mV/dec, respectively. The present work provides an inspiring reference to design and develop photothermal-assisted water electrolysis using abundant solar energy.

Preparation of CoFe2O4-Doped TiO2 Nanofibers by Electrospinning and Annealing for Oxygen Electrocatalysis

In this paper, catalyst precursor fibers were prepared by a sol gel method combined with an electrospinning method using tetrabutyl titanate as a titanium source, cobalt acetylacetonate as a cobalt source, and iron acetylacetonate as an iron source. CoFe@TiO₂ nanofibers (NFs) with a bimetallic spinel structure were formed after thermal annealing, which have dual-functional catalytic activity. With the molar ratio of Co and Fe coming to 1:1, a typical spinel CoFe₂O₄ structure was generated in Co₁Fe₁@TiO₂ NFs. At a load of only 28.7 μg·cm^-2, Co₁Fe₁@TiO₂ NFs not only have a low overpotential (284 mV) and Tafel slope (54 mV·dec^-1) in the oxygen evolution reaction but also show a high initial potential (0.88 V) and limiting current density (6.40 mA·cm^-2) in the oxygen reduction reaction. Meanwhile, Co₁Fe₁@TiO₂ NFs exhibit good durability, cycle stability, and dual-function catalysis.

Linear Conjugated Coordination Polymers for Electrocatalytic Oxygen Evolution Reaction

Conjugated coordination polymers (CCPs) have attracted extensive attention for various applications related to energy storage and conversion in the past few years, despite that there are many CCPs with unclear chemical states and structures. Here, linear CCPs (LCCPs), with metal-O₄ active sites grown on carbon paper (CP) for oxygen evolution reaction (OER), are presented. The LCCPs with high crystallinity and simple structures exhibit the order of electrocatalytic activity of Co-O₄  > Ni-O₄  > Fe-O₄ in terms of the metal-O₄ centers. The Co-based LCCP shows higher OER performance (263 mV at 10 mA cm^-2 ) and better durability (90 h at 30 mA cm^-2 ) than commercial IrO₂ /CP. The structures and chemical states of LCCPs are carefully investigated, and density functional theory is used to reveal the mechanism of OER at the central metal site. This investigation into LCCPs provides new sights for a better understanding of CCPs and expands the applications of LCCPs with metal-O₄ sites.

Single-crystalline Mg-substituted Na4Mn3(PO4)2P2O7 nanoparticles as a high capacity and superior cycling cathode for sodium-ion batteries

Mn-based mixed phosphate Na₄Mn₃(PO₄)₂(P₂O₇) (NMPP) is a promising cathode for high-potential, low-cost and eco-friendly sodium-ion batteries. However, this material still faces some bottleneck issues in terms of low conductivity, disturbance of impure crystalline phase, micron-sized agglomerated particles and the Mn³⁺ Jahn-Teller effect. Herein, a Mg-substituted NMPP (NM_2.7Mg_0.3PP)@C composite was constructed via modified solution combustion and subsequent calcination treatment. The obtained NM_2.7Mg_0.3PP presents a highly pure phase and single-crystalline characteristics. It is noteworthy that the sample shows a smaller particle size of 100-300 nm due to the Mg²⁺ incorporation, and the prepared NM_2.7Mg_0.3PP@C cathode exhibits considerable discharge capacity (119 mA h g^-1), an improved rate capability and excellent long cycling stability of 1000 cycles. A series of measurements indicated that the Mg-substitution enhanced the electronic conductivity and ion diffusion rate, and effectively relieved the lattice distortion influenced by the multiphase transition from the Mn Jahn-Teller effect of the NM_2.7Mg_0.3PP@C cathode. In addition, NM_2.7Mg_0.3PP adopts an optimal 3Mg_0.1-Mn1-Mn2-Mn3 crystal structure based on density functional theory (DFT) calculations and refined X-ray diffractometry results. These findings provide new insight into the design of highly stabilized and high-conductivity polyanionic cathodes for sodium-ion batteries.

Interface engineering of Fe2P@CoMnP4 heterostructured nanoarrays for efficient and stable overall water splitting

Electrocatalytic water splitting to generate high-quality hydrogen is an attractive renewable energy storage technology; however, it is still far from becoming a real-world application. In this study, we developed an effective and stable nickel foam-supported Fe₂P@CoMnP₄ heterostructure electrocatalyst for overall water splitting. As expected, the as-obtained Fe₂P@CoMnP₄/NF electrocatalyst exhibits superb bifunctional catalytic activity and only requires extremely low overpotentials of 53 and 249 mV to achieve a current density of 10 mA cm^-2 for the hydrogen and oxygen evolution reactions, respectively. Moreover, a two-electrode electrolyzer assembled using Fe₂P@CoMnP₄/NF as electrodes operates at the low cell voltage of 1.54 V at 10 mA cm^-2, showing excellent long-term stability for 140 h. Theoretical calculations indicate that the surface electronic structure is effectively adjusted by the generated heterointerfaces between the Fe₂P and CoMnP₄ in a two-phase matrix, resulting in a Gibbs free energy of hydrogen adsorption close to zero and high intrinsic activity. This innovative strategy is a valuable route for producing low-cost high-performance bifunctional electrocatalysts for water splitting.

Morphology-Dependent Electrocatalytic Behavior of Cobalt Chromite toward the Oxygen Evolution Reaction in Acidic and Alkaline Medium

Exploiting an affordable, durable, and high-performance electrocatalyst for the oxygen evolution reaction (OER) under lower pH condition (acidic) is highly challengeable and much attractive toward the hydrogen-based energy technologies. A spinel CoCr₂O₄ is observed as a potential noble-metal-free candidate for OER in alkaline medium. The presence of Cr further leads to electronic structure modulation of Co₃O₄ and thereby greatly increases the corrosive resistance toward OER in acidic environment. Herein, a typical CoCr₂O₄ with three different morphologies was synthesized for the very first time and employed as an electrocatalyst for OER in alkaline (1 M KOH) and acidic (0.5 M H₂SO₄) medium. Moreover, different morphologies display a different intrinsic exposed active site and thereby display different electrocatalytic activities. Likewise, the CoCr₂O₄ Mic (synthesized by the microwave heating method) displays a higher catalytic activity toward OER and delivers a low overpotential of 293 and 290 mV to attain 10 mA/cm² current density and smaller Tafel slope values of 40 and 151 mV/dec, respectively, in alkaline and acidic environment than the synthesized CoCr₂O₄ Wet (wet-chemically synthesized) and CoCr₂O₄ Hyd (hydrothermally synthesized). Moreover, CoCr₂O₄ Mic exhibits a long-term durability of 24 h (1 M KOH) and 10.5 h (0.5 M H₂SO₄). The optimized Co-O bond energy in OER condition makes the CoCr₂O₄ Mic superior than the CoCr₂O₄ Hyd and CoCr₂O₄ Wet. Moreover, the substitution of Cr induces the electron delocalization around the Co active species and thereby, positive shifting of the redox potential leads to providing an optimal binding energy for OER intermediates. Also, interestingly, this work represents a catalytic activity trend by a simple experimental result without any complex theoretical calculation. The morphology-dependent electrocatalytic activity obtained in this work will provide a new strategy in the field of electrochemical conversion and energy storage application.

In situ synthesis of Ag/Ag2O-cellulose/chitosan nanocomposites via adjusting KOH concentration for improved photocatalytic and antibacterial applications

This work proposed a facile way to construct cellulose/chitosan-loaded Ag/Ag₂O nanocomposite films (ACC) from alkali/urea solution by increasing the content of alkali KOH in the solvent. The saturated alkali and hydroxyl groups of the cellulose and chitosan chains were accelerated to convert AgNO₃ to Ag⁰. Ag₂O served as nuclei to lower the energy barrier. The formation of Ag/Ag₂O nanoparticles (NPs) endowed the cellulose bio-reduced Ag composites with multifunction and stronger photocatalytic activity. Ag/Ag₂O NPs with the diameter of 139-360 nm were uniformly dispersed in the composite films, resulting in superior mechanical properties (64.6 MPa) and thermal stability. Almost 92 % of methyl orange was degraded under UV-irradiation within 40 min by ACC. After 3 runs of degradation, the photocatalytic abilities of ACC remained. Moreover, the films exhibited good antibacterial activities. The width of inhibition zones around ACC reached 9.2-12 mm and 8.6-10.4 mm for S. aureus and E. coli. The strategy provided a new avenue to construct multifunctional cellulose/chitosan materials for various applications, such as wastewater treatment, and electrocatalysis.

Layered Double Hydroxides for Photo(electro)catalytic Applications: A Mini Review

Chemical energy conversion strategies by photocatalysis and electrocatalysis are promising approaches to alleviating our energy shortages and environmental issues. Due to the 2D layer structure, adjustable composition, unique thermal decomposition and memory properties, abundant surface hydroxyl, and low cost, layered double hydroxides (LDHs) have attracted extensive attention in electrocatalysis, photocatalysis, and photoelectrocatalysis. This review summarizes the main structural characteristics of LDHs, including tunable composition, thermal decomposition and memory properties, delaminated layer, and surface hydroxyl. Next, the influences of the structural characteristics on the photo(electro)catalytic process are briefly introduced to understand the structure-performance correlations of LDHs materials. Recent progress and advances of LDHs in photocatalysis and photoelectrocatalysis applications are summarized. Finally, the challenges and future development of LDHs are prospected from the aspect of structural design and exploring structure-activity relationships in the photo(electro)catalysis applications.

Enhanced improvement of soda saline-alkali soil by in-situ formation of super-stable mineralization structure based on CaFe layered double hydroxide and its large-scale application

In-situ super-stable mineralization technology with mineralizers (CaSO₄, Fe₂(SO₄)₃) and attapulgite (ATP) clay were applied to improve soda saline-alkali soil. The addition of mineralizers and the existence of OH and CO₃^2- in soil resulted in the formation of CaFe-layered double hydroxide (CaFe-LDH) with super-stable mineralization structure (K_sp = 1.512 × 10^-61), which was confirmed by the characterization of physicochemical properties and density functional theory (DFT) calculation. The fixation of OH^- and CO₃^2- during the formation process of CaFe-LDH led to the transformation of the existing forms of OH^- and CO₃^2- in soil from free to stable state, resulting in the permanent decrease of soil pH and CO₃^2- concentration. The effect of ATP clay on the decrease of soluble Na ions in soil through electrostatic attraction and cation exchange was also indicated. Furthermore, mineralizers (1.2 t/ha CaSO₄ and 0.75 t/ha Fe₂(SO₄)₃) and ATP clay (1.2 t/ha) were applied to 1.33 ha soda saline-alkali land, and Rumex patientia L. was seeded meanwhile for the identification of improved performance. After five months of improvement, the physical and chemical properties of soil were improved that pH, electrical conductivity (EC), the concentration of CO₃^2- and soluble Na ions, and soil bulk density decreased significantly. In addition, the emergence rate of Rumex patientia L. increased from 0% to 98.3%. All above indicated that in-situ super-stable mineralization technology with the properties of high efficiency, long-term and cost-effective (234.88 $/ha) displays excellent potential in the improvement of soda saline-alkali soil.

Fabrication of cobaltous telluride and carbon composite as a promising carrier for boosting electro oxidation of ethylene glycol on palladium in alkaline medium

The development of highly active and earth-rich electrocatalysts remains a formidable challenge for the commercialization of fuel cells. Herein, a composite carrier composed of cobaltous telluride (CoTe) and carbon (C) has been designed for the first time to enhance the electrocatalytic performance of palladium (Pd) nanoparticles (NPs) for the electro-oxidation of ethylene glycol (EG). Remarkably, the mass activity for the as-prepared Pd/CoTe-C catalyst during the ethylene glycol oxidation reaction (EGOR) is found to reach up to 3917.3 mA mg^-1, which is 2.2 times higher than that of Pd/Co-C (1785.0 mA mg^-1) and 4.1 times greater than that of commercial Pd/C catalyst (962.4 mA mg^-1), exceeding that obtained for most Pd-based electrocatalysts reported thus far. In particular, the Pd/CoTe-C catalyst shows better electrochemical stability toward the EGOR than the Pd/Co-C and commercial Pd/C catalysts. Thus, the Pd/CoTe-C electrocatalyst is expected to exhibit broad application prospects in the field of fuel cells.

Tuning the Electronic Structure of a Ni-Vacancy-Enriched AuNi Spherical Nanoalloy via Electrochemical Etching for Water Oxidation Studies in Alkaline and Neutral Media

Internal Ni-vacancy-enriched spherical AuNi nanoalloys (AuNi_1-2-T) have been prepared via a noble electrochemical etching method. AuNi_1.5-T showed the highest oxygen evolution reaction (OER) activity compared to bare AuNi_1.5, and it demands only 239 mV overpotential, which was 134 mV lesser than the overpotential required by commercial RuO₂ at 10 mA cm^-2 current density in a 1 M KOH solution (pH = 14). The calculated turnover frequency (TOF) value for AuNi_1.5-T (0.0229 s^-1) was 11.74 times higher than that of AuNi_1.5 (0.00195 s^-1). Also, the electrochemically activated AuNi_1.5-T showed superior neutral water oxidation activity by demanding only 335 mV overpotential with a TOF value of 0.000135 s^-1 in a 1 M Na₂SO₄ solution (pH = 7) at 10 mA cm^-2. The long-term stability studies (over 60 h) reveal the excellent robustness of an electrochemically treated alloy system. Density functional theory based electronic structure calculations showed that in the case of AuNi and AuNi_1.5, Au d, Au s, and Ni d orbitals have significant contributions, whereas in the Ni-vacant systems, the density of states is mainly governed by d orbitals of Au and Ni. Also, the Ni-vacant system possesses a work function value of 4.96 eV, which is lower than that of the pristine system (5.27 eV) and thereby favored OH^- binding with an optimum adsorption energy. This result is in reasonable agreement with the experimental outcome of an accelerated OER in a vacancy-enriched Ni-rich AuNi alloy system. Also, mechanistic analysis reveals that the creation of a Ni vacancy can effectively alter the overall mechanism of the OER and thereby facilitate the same with a lower applied energy.

Enhanced photoelectrochemical performance of ZnO/NiFe-layered double hydroxide for water splitting: Experimental and photo-assisted density functional theory calculations

Hydrogen production technologies have attracted considerable attention with the increasing demand for renewable energy. Among them, the combined action of water electrolysis and solar energy has emerged. In this study, a hydrangea ZnO/NiFe-layered double hydroxide (LDH) heterojunction was synthesized using the two-step hydrothermal method. The resulting ZnO/NiFe-LDH improved the range and intensity of light response, thus meeting the requirement of electrocatalysis and photocatalysis in theory. Moreover, ZnO/NiFe-LDH demonstrated excellent activity in the electrochemical performance test in the presence of light. When used as a water splitting catalyst in a full cell, the cell voltage was 1.632 V, and Faradic efficiency was 99.1%. Moreover, from the in situ Raman and theoretical calculation results, it is possible to conclude that the synthesized ZnO/NiFe-LDH has the property of absorbing light energy, and the introduction of light energy can optimize the bandgap structure of the material and enhance the adsorption capacity of the system, thus significantly reducing the energy required for water splitting reaction. In sum, this study introduced a composition strategy for LDH heterojunction materials and presented a theoretical and experimental investigation of the light influence on the material structure and electrochemical reaction. Furthermore, it is believed that an important future direction of hydrogen production is photo-assisted water splitting.

Boosting oxygen evolution by nickel nitrate hydroxide with abundant grain boundaries via segregated high-valence molybdenum

High-valence metal doping and abundant grain boundaries (GBs) have been proved to be effective strategies to promote the oxygen evolution reaction (OER). However, the reasonable design of the two to facilitate OER collaboratively is challenging. Herein, a convenient and novel one-step molten salt decomposition strategy is proposed to fabricate segregated-Mo doped nickle nitrate hydroxide with substantial GBs on MoNi foam (Mo-NNOH@MNF). When processed in molten salt, the Mo species on the conductive substrate migrates unevenly to the surface of Mo-NNOH@MNF, which not only induces the formation of abundant GBs to modulate electronic structure, but also improves the intrinsic activity as high-valence dopants, synergistically elevating OER activity. As verification, the optimized Mo-NNOH@MNF-10h exhibits low overpotential of 150 mV at 10 mA cm^-2, which can be attributed to the reduced valence charge transition energy of Ni by high-valence Mo dopant, coupled with the fine-tuning of d-band center bond and corresponding local electron density by induced GBs and Mo doping, as DFT calculations revealed. Moreover, the intrinsic robustness and strong adhesion ensure the long-term stability of 6 h at 500 mA cm^-2. This work provides a promising molten salt decomposition approach to synthesize advanced materials with unique structures.

Operando Photo-Electrochemical Catalysts Synchrotron Studies

The attempts to develop efficient methods of solar energy conversion into chemical fuel are ongoing amid climate changes associated with global warming. Photo-electrocatalytic (PEC) water splitting and CO₂ reduction reactions show high potential to tackle this challenge. However, the development of economically feasible solutions of PEC solar energy conversion requires novel efficient and stable earth-abundant nanostructured materials. The latter are hardly available without detailed understanding of the local atomic and electronic structure dynamics and mechanisms of the processes occurring during chemical reactions on the catalyst–electrolyte interface. This review considers recent efforts to study photo-electrocatalytic reactions using in situ and operando synchrotron spectroscopies. Particular attention is paid to the operando reaction mechanisms, which were established using X-ray Absorption (XAS) and X-ray Photoelectron (XPS) Spectroscopies. Operando cells that are needed to perform such experiments on synchrotron are covered. Classical and modern theoretical approaches to extract structural information from X-ray Absorption Near-Edge Structure (XANES) spectra are discussed.

NiFe layered double hydroxide nanosheets array for high-efficiency electrocatalytic reduction of nitric oxide to ammonia

Here, we demonstrate that at ambient conditions, nickel-iron layered double hydroxide nanosheets array can achieve a promising NORR performance, delivering a maximal faradaic efficiency of 82% and a corresponding yield...

Hollow microspherical Bi2MoO6/Zn–Ti layered double hydroxide heterojunction for efficient visible-light photocatalytic degradation of organic contaminants

The construction of a Bi2MoO6/Zn–Ti layered double hydroxide heterojunction can effectively enhance the photocatalytic activity owing to efficient charge separation.

Layered Double Hydroxides as Reusable Catalysts for Cyclocondensation of Amidines and Aminoalcohols: Access to Multi-functionalized Oxazolines

An efficient catalytic protocol based on reusable MgAl-layered double hydroxides has been developed for the synthesis of multi-functionalized oxazolines via the cyclocondensation of amidines and aminoalcohols. The developed method has a broad substrate scope and excellent functional group tolerance and uses a reusable catalyst. The catalyst can be conveniently recycled by filtration and reused for at least five times without obvious deactivation. Additionally, the selective ortho C-H silylation of oxazolines was performed using Ru(II) as the catalyst and triethyl silane as the silylating reagent, which proved to be a convenient and practical method for the synthesis of versatile organosilyl-functionalized oxazolines with advantageous biological and physical properties.

A new strategy to construct cellulose-chitosan films supporting Ag/Ag2O/ZnO heterostructures for high photocatalytic and antibacterial performance

The industrial wastewater contaminants including dyes and bacteria have caused serious environmental pollutions. Herein, ternary Ag/Ag₂O/ZnO heterostructure decorating cellulose-chitosan films were constructed via in situ synthesis. Cellulose and chitosan dissolved in alkali/urea solvent and regenerated in ethylene glycol to form cellulose/chitosan nanofiber network, which was an ideal supporter for ZnO and Ag nanoparticles and beneficial for recycle usage. The hydroxyl groups of cellulose and chitosan chains exposed and were utilized for the synthesis of Ag particles, as well as ZnO nanoparticles by biomineralization. The Ag/Ag₂O/ZnO decorating cellulose/chitosan (AZ@CC) films exhibited excellent antibacterial activity against Staphylococcus aureus and Escherichia coli. The width of inhibition zones around AZ@CC films reached 10.0-19.6 mm and 12.4-15.0 mm for S. aureus and E. coli, respectively. Moreover, AZ@CC films exhibited good photocatalytic activity against methyl orange (MO), almost 97% degradation of methyl orange (MO) within 50 min was achieved with the assistance of AZ@CC film. Importantly, the nanocomposite films exhibited excellent tensile strength and thermal stability. This facile and eco-friendly approach provided a new route to utilize cellulose and chitosan advantages for constructing multifunctional materials.