Cobalt–Nickel Layered Double Hydroxides Modified on TiO2 Nanotube Arrays for Highly Efficient and Stable PEC Water Splitting

TiO₂‐based photoanodes have attracted extensive attention worldwide for photoelectrochemical (PEC) water splitting, but these materials still suffer from poor electron–hole separation and low photoconversion efficiency. Here, the high PEC water splitting activity and long‐term stability against photocorrosion of well‐aligned hierarchical TiO₂@CoNi‐layered double hydroxides nanotube arrays (TiO₂@CoNi‐LDHs NTAs) are reported. The typical TiO₂@CoNi‐LDHs NTAs exhibits enhancing photocurrent density of 4.4 mA cm^?2 at a potential of 1.23 V (vs reversible hydrogen electrode) under AM 1.5G simulated sunlight (100 mW cm^?2), 3.3 times higher than that of the pristine TiO₂ sample. Moreover, this hierarchical electrode displays excellent stability against photocorrosion with initial activity loss no more than 1.0% even after 10 h irradiation in Na₂SO₄ electrolyte solution (pH 6.8), much competitive to those reported TiO₂‐based photoelectrodes. This work may offer a combinatorial synthesis strategy for the preparation of hierarchical architectures with high PEC performances.     

Topotactically Transformed Polygonal Mesopores on Ternary Layered Double Hydroxides Exposing Under-Coordinated Metal Centers for Accelerated Water Dissociation

Layered double hydroxides (LDHs) have been recognized as potent electrocatalysts for oxygen evolution reaction (OER), but are lacking in hydrogen evolution reaction (HER) activities due to the sluggish kinetics of water dissociation in alkaline medium. Herein, aiming to simultaneously bolster the HER and OER kinetics, a metal–organic framework (MOF) mediated topotactic transformation tactic is deployed to fabricate holey ternary CoFeNi LDHs on nickel foam, exposing polygonal mesopores with atomistic edge steps and lattice defects. The optimized catalyst requires only an external voltage of 1.49 V to afford the water splitting current density of 10 mA cm⁻² apart from the superb electrolytic stability, far surpassing the benchmark Pt/C||RuO₂ couple. More importantly, mechanistic investigations utilizing advanced spectroscopies in conjunction with density function theory (DFT) understandings unravel while the synergetic effect among under-coordinated metal centers lowers the energy barrier of water dissociation, Fe-doping enables further modulating the d-band density of states (DOS) of Co and Ni in favor of intermediates binding, thereby promoting the intrinsic HER activity. Operando Raman studies reveal negligible structural change of the LDHs during the HER process, whereas for OER the active sites can quickly turn into oxyhydroxides in the presence of lattice defects and under-coordinated metal centers.     

Ultrathin Porous NiFeV Ternary Layer Hydroxide Nanosheets as a Highly Efficient Bifunctional Electrocatalyst for Overall Water Splitting

Herein, the hydrothermal synthesis of porous ultrathin ternary NiFeV layer double hydroxides (LDHs) nanosheets grown on Nickel foam (NF) substrate as a highly efficient electrode toward overall water splitting in alkaline media is reported. The lateral size of the nanosheets is about a few hundreds of nanometers with the thickness of ≈10 nm. Among all molar ratios investigated, the Ni_0.75Fe_0.125V_0.125‐LDHs/NF electrode depicts the optimized performance. It displays an excellent catalytic activity with a modest overpotential of 231 mV for the oxygen evolution reaction (OER) and 125 mV for the hydrogen evolution reaction (HER) in 1.0 m KOH electrolyte. Its exceptional activity is further shown in its small Tafel slope of 39.4 and 62.0 mV dec^?1 for OER and HER, respectively. More importantly, remarkable durability and stability are also observed. When used for overall water splitting, the Ni_0.75Fe_0.125V_0.125‐LDHs/NF electrodes require a voltage of only 1.591 V to reach 10 mA cm^?2 in alkaline solution. These outstanding performances are mainly attributed to the synergistic effect of the ternary metal system that boosts the intrinsic catalytic activity and active surface area. This work explores a promising way to achieve the optimal inexpensive Ni‐based hydroxide electrocatalyst for overall water splitting.     

Layered Perovskite Nanofibers via Electrospinning for Overall Water Splitting

The (111)‐layered perovskite materials Ba₅Ta₄O₁₅, Ba₅Ta₂Nb₂O₁₅ and Ba₅Nb₄O₁₅ are prepared with nanofiber morphology via electrospinning for the first time. The nanofibers are built up from small single crystals, with up to several micrometers length even after calcination. The formation mechanism is investigated in detail, revealing an intermediate formation of amorphous barium carbonate strengthening the nanofiber morphology for high temperature treatment. All nanofiber compounds are able to generate hydrogen without any co‐catalyst in photocatalytic reformation of methanol. After photodeposition of Rh‐Cr₂O₃ co‐catalysts, the nanofibers show better activity in overall water splitting compared to sol–gel‐derived powders.     

Heterostructured TiO2 Nanorod@Nanobowl Arrays for Efficient Photoelectrochemical Water Splitting

Heterostructured TiO₂ nanorod@nanobowl (NR@NB) arrays consisting of rutile TiO₂ nanorods grown on the inner surface of arrayed anatase TiO₂ nanobowls are designed and fabricated as a new type of photoanodes for photoelectrochemical (PEC) water splitting. The unique heterostructures with a hierarchical architecture are readily fabricated by interfacial nanosphere lithography followed by hydrothermal growth. Owing to the two‐dimensionally arrayed structure of anatase nanobowls and the nearly radial alignment of rutile nanorods, the TiO₂ NR@NB arrays provide multiple scattering centers and hence exhibit an enhanced light harvesting ability. Meanwhile, the large surface area of the NR@NB arrays enhances the contact with the electrolyte while the nanorods offer direct pathways for fast electron transfer. Moreover, the rutile/anatase phase junction in the NR@NB heterostructure improves charge separation because of the facilitated electron transfer. Accordingly, the PEC measurements of the TiO₂ NR@NB arrays on the fluoride‐doped tin oxide (FTO) substrate show significantly enhanced photocatalytic properties for water splitting. Under AM1.5G solar light irradiation, the unmodified TiO₂ NR@NB array photoelectrode yields a photocurrent density of 1.24 mA cm^–2 at 1.23 V with respect to the reversible hydrogen electrode, which is almost two times higher than that of the TiO₂ nanorods grown directly on the FTO substrate.     

High‐Yield Electrochemical Production of Large‐Sized and Thinly Layered NiPS3 Flakes for Overall Water Splitting

Achieving large‐sized and thinly layered 2D metal phosphorus trichalcogenides with high quality and yield has been an urgent quest due to extraordinary physical/chemical characteristics for multiple applications. Nevertheless, current preparation methodologies suffer from uncontrolled thicknesses, uneven morphologies and area distributions, long processing times, and inferior quality. Here, a sonication‐free and fast (in minutes) electrochemical cathodic exfoliation approach is reported that can prepare large‐sized (typically ≈150 µm²) and thinly layered (≈70% monolayer) NiPS₃ flakes with high crystallinity and pure phase structure with a yield ≈80%. During the electrochemical exfoliation process, the tetra‐n‐butylammonium salt with a large ionic diameter is decomposed into gaseous species after the intercalation and efficiently expands the tightly stratified bulk NiPS₃ crystals, as revealed by in situ and ex situ characterizations. Atomically thin NiPS₃ flakes can be obtained by slight manual shaking rather than sonication, which largely preserves in‐plane structural integrity with large size and minimum damage. The obtained high quality NiPS₃ offers a new and ideal model for overall water splitting due to its inherent fully exposed S and P atoms that are often the active sites for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). Consequently, the bifunctional NiPS₃ exhibits outstanding performance for overall water splitting.     

Vertical Growth of 2D Amorphous FePO4 Nanosheet on Ni Foam: Outer and Inner Structural Design for Superior Water Splitting

Rational design of highly efficient bifunctional electrocatalysts based on 3D transition‐metal‐based materials for oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) is of great importance for sustainable energy conversion processes. Herein, a novel strategy involving outer and inner structural engineering is developed for superior water splitting via in situ vertical growth of 2D amorphous FePO₄ nanosheets on Ni foam (Am FePO₄/NF). Careful experiments and density functional theory calculations show that the inner and outer structural engineering contributing to the synergistic effects of 2D morphology, amorphous structure, conductive substrate, and Ni?Fe mixed phosphate lead to superior electrocatalytic activity toward OER and HER. Furthermore, a two‐electrode electrolyzer assembled using Am FePO₄/NF as an electrocatalyst at both electrodes gives current densities of 10 and 100 mA cm^?2 at potentials of 1.54 and 1.72 V, respectively, which is comparable to the best bifunctional electrocatalyst reported in the literature. The strategies, introduced in the present work, may open new opportunities for the rational design of other 3D transition‐metal‐based electrocatalyst through an outer and inner structural control to strengthen the electrocatalytic performance.     

Porous Hybrid Network of Graphene and Metal Oxide Nanosheets as Useful Matrix for Improving the Electrode Performance of Layered Double Hydroxides

Mesoporous hybrid network of reduced graphene oxide (rG‐O) and layered MnO₂ nanosheets could act as an efficient immobilization matrix for improving the electrochemical activity of layered double hydroxide (LDH). The control of MnO₂/rG‐O ratio is crucial in optimizing the porous structure and electrical conductivity of the resulting hybrid structure. The immobilization of Co‐Al‐LDH on hybrid MnO₂/rG‐O network is more effective in enhancing its electrode activity compared with that of on pure rG‐O network. The Co‐Al‐LDH?rG‐O?MnO₂ nanohybrid deliveres a greater specific capacitance than does MnO₂‐free Co‐Al‐LDH?rG‐O nanohybrid. The beneficial effect of MnO₂ incorporation on the electrode performance of nanohybrid is more prominent for higher current density and faster scan rate, underscoring the significant enhancement of the electron transport of Co‐Al‐LDH?rG‐O. This is supported by electrochemical impedance spectroscopy. The present study clearly demonstrates the usefulness of the porously assembled hybrid network of graphene and metal oxide nanosheets as an effective platform for exploring efficient LDH‐based functional materials.     

FeS2/CoS2 Interface Nanosheets as Efficient Bifunctional Electrocatalyst for Overall Water Splitting

Electrochemical water splitting to produce hydrogen and oxygen, as an important reaction for renewable energy storage, needs highly efficient and stable catalysts. Herein, FeS₂/CoS₂ interface nanosheets (NSs) as efficient bifunctional electrocatalysts for overall water splitting are reported. The thickness and interface disordered structure with rich defects of FeS₂/CoS₂ NSs are confirmed by atomic force microscopy and high‐resolution transmission electron microscopy. Furthermore, extended X‐ray absorption fine structure spectroscopy clarifies that FeS₂/CoS₂ NSs with sulfur vacancies, which can further increase electrocatalytic performance. Benefiting from the interface nanosheets' structure with abundant defects, the FeS₂/CoS₂ NSs show remarkable hydrogen evolution reaction (HER) performance with a low overpotential of 78.2 mV at 10 mA cm⁻² and a superior stability for 80 h in 1.0 m KOH, and an overpotential of 302 mV at 100 mA cm⁻² for the oxygen evolution reaction (OER). More importantly, the FeS₂/CoS₂ NSs display excellent performance for overall water splitting with a voltage of 1.47 V to achieve current density of 10 mA cm⁻² and maintain the activity for at least 21 h. The present work highlights the importance of engineering interface nanosheets with rich defects based on transition metal dichalcogenides for boosting the HER and OER performance.     

3D Nitrogen‐Anion‐Decorated Nickel Sulfides for Highly Efficient Overall Water Splitting

Developing non‐noble‐metal electrocatalysts with high activity and low cost for both the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) is of paramount importance for improving the generation of H₂ fuel by electrocatalytic water‐splitting. This study puts forward a new N‐anion‐decorated Ni₃S₂ material synthesized by a simple one‐step calcination route, acting as a superior bifunctional electrocatalyst for the OER/HER for the first time. The introduction of N anions significantly modifies the morphology and electronic structure of Ni₃S₂, bringing high surface active sites exposure, enhanced electrical conductivity, optimal HER Gibbs free‐energy (ΔG_H*), and water adsorption energy change (ΔG_H2O*). Remarkably, the obtained N‐Ni₃S₂/NF 3D electrode exhibits extremely low overpotentials of 330 and 110 mV to reach a current density of 100 and 10 mA cm^?2 for the OER and HER in 1.0 m KOH, respectively. Moreover, an overall water‐splitting device comprising this electrode delivers a current density of 10 mA cm^?2 at a very low cell voltage of 1.48 V. Our finding introduces a new way to design advanced bifunctional catalysts for water splitting.     

Fe2+‐Doped Layered Double (Ni,Fe) Hydroxides as Efficient Electrocatalysts for Water Splitting and Self‐Powered Electrochemical Systems

Developing nonprecious electrocatalysts with superior activity and durability for electrochemical water splitting is of great interest but challenging due to the large overpotential required above the thermodynamic standard potential of water splitting (1.23 V). Here, in situ growth of Fe²⁺‐doped layered double (Ni, Fe) hydroxide (NiFe(II,III)‐LDH) on nickel foam with well‐defined hexagonal morphology and high crystallinity by a redox reaction between Fe³⁺ and nickel foam under hydrothermal conditions is reported. Benefiting from tuning the local atomic structure by self‐doping Fe²⁺, the NiFe(II,III)‐LDH catalyst with higher amounts of Fe²⁺ exhibits high activity toward oxygen evolution reaction (OER) as well as hydrogen evolution reaction (HER) activity. Moreover, the optimized NiFe(II,III)‐LDH catalyst for OER (O‐NiFe(II,III)‐LDH) and catalyst for HER (H‐NiFe(II,III)‐LDH) show overpotentials of 140 and 113 mV, respectively, at a current density of 10 mA cm^?2 in 1 m KOH aqueous electrolyte. Using the catalysts for overall water splitting in two‐electrode configuration, a low overpotential of just 1.54 V is required at a benchmark current density of 10 mA cm^?2. Furthermore, it is demonstrated that electrolysis of the water device can be drived by a self‐powered system through integrating a triboelectric nanogenerator and battery, showing a promising way to realize self‐powered electrochemical systems.     

Hierarchical Metal/Semiconductor Nanostructure for Efficient Water Splitting

A hierarchically patterned metal/semiconductor (gold nanoparticles/ZnO nanowires) nanostructure with maximized photon trapping effects is fabricated via interference lithography (IL) for plasmon enhanced photo‐electrochemical water splitting in the visible region of light. Compared with unpatterned (plain) gold nanoparticles‐coated ZnO NWs (Au NPs/ZnO NWs), the hierarchically patterned Au NPs/ZnO NWs hybrid structures demonstrate higher and wider absorption bands of light leading to increased surface enhanced Raman scattering due to the light trapping effects achieved by the combination of two different nanostructure dimensions; furthermore, pronounced plasmonic enhancement of water splitting is verified in the hierarchically patterned Au NPs/ZnO NWs structures in the visible region. The excellent performance of the hierarchically patterned Au NPs/ZnO NWs indicates that the combination of pre‐determined two different dimensions has great potential for application in solar energy conversion, light emitting diodes, as well as SERS substrates and photoelectrodes for water splitting.     

Multimetal Borides Nanochains as Efficient Electrocatalysts for Overall Water Splitting

The development of cost‐efficient, active, and stable electrode materials as bifunctional catalysts for electrochemical water splitting is crucial to high‐performance renewable energy storage and conversion devices. In this work, the synthesis of Co‐based multi‐metal borides nanochains with amorphous structure is reported for boosting the oxygen evolution (OER) and hydrogen evolution reactions (HER) by one‐pot NaBH₄ reduction of Co²⁺, Ni²⁺, and Fe²⁺ under ambient temperature. In all the investigated Co‐based metal borides, NiCoFeB nanochains show the excellent OER performance with a low overpotential of 284 mV at 10 mA cm^‐2 and Tafel slope of 46 mV dec^‐1, respectively, together with excellent catalytic stability, and robust HER performance with an overpotential of 345 mV at 10 mA cm^‐2. The density functional theory (DFT) calculations reveal that the excellent electrocatalytic performance is mainly attributed to optimal electronic structure by tuning the Co‐3d band activities by the incorporation of Ni and Fe for enhanced water splitting via the potentially existed Co⁰ state. Moreover, the electrolyzer using NiCoFeB nanochains as anode and cathode offers 10 mA cm^‐2 at a cell voltage of 1.81 V, comparable to commercial Pt/C // Ir/C, providing a simple method to design and explore highly efficient and cheap bifunctional electrocatalysts for overall water splitting.     

2D Metal Oxyhalide‐Derived Catalysts for Efficient CO2 Electroreduction

Electrochemical reduction of CO₂ is a compelling route to store renewable electricity in the form of carbon‐based fuels. Efficient electrochemical reduction of CO₂ requires catalysts that combine high activity, high selectivity, and low overpotential. Extensive surface reconstruction of metal catalysts under high productivity operating conditions (high current densities, reducing potentials, and variable pH) renders the realization of tailored catalysts that maximize the exposure of the most favorable facets, the number of active sites, and the oxidation state all the more challenging. Earth‐abundant transition metals such as tin, bismuth, and lead have been proven stable and product‐specific, but exhibit limited partial current densities. Here, a strategy that employs bismuth oxyhalides as a template from which 2D bismuth‐based catalysts are derived is reported. The BiOBr‐templated catalyst exhibits a preferential exposure of highly active Bi () facets. Thereby, the CO₂ reduction reaction selectivity is increased to over 90% Faradaic efficiency and simultaneously stable current densities of up to 200 mA cm^?2 are achieved—more than a twofold increase in the production of the energy‐storage liquid formic acid compared to previous best Bi catalysts.     

2D Superlattices for Efficient Energy Storage and Conversion

2D genuine unilamellar nanosheets, that are, the elementary building blocks of their layered parent crystals, have gained increasing attention, owing to their unique physical and chemical properties, and 2D features. In parallel with the great efforts to isolate these atomic-thin crystals, a unique strategy to integrate them into 2D vertically stacked heterostuctures has enabled many functional applications. In particular, such 2D heterostructures have recently exhibited numerous exciting electrochemical performances for energy storage and conversion, especially the molecular-scale heteroassembled superlattices using diverse 2D unilamellar nanosheets as building blocks. Herein, the research progress in scalable synthesis of 2D superlattices with an emphasis on a facile solution-phase flocculation method is summarized. A particular focus is brought to the advantages of these 2D superlattices in applications of supercapacitors, rechargeable batteries, and water-splitting catalysis. The challenges and perspectives on this promising field are also outlined.     

NiO as a Bifunctional Promoter for RuO2 toward Superior Overall Water Splitting

Conventional development of nanomaterials for efficient electrocatalysis is largely based on performance‐oriented trial‐and‐error/iterative approaches, while a rational design approach at the atomic/molecular level is yet to be found. Here, inspired by a fundamental understanding of the mechanism for both oxygen and hydrogen evolution half reactions (OER/HER), a unique strategy is presented to engineer RuO₂ for superior alkaline water electrolysis through coupling with NiO as an efficient bifunctional promoter. Benefitting from desired potential‐induced interfacial synergies, NiO‐derived NiOOH improves the oxygen binding energy of RuO₂ for enhanced OER, and NiO also promotes water dissociation for enhanced HER on RuO₂‐derived Ru. The resulting hybrid material exhibits remarkable bifunctional activities, affording 2.6 times higher OER activity than that of RuO₂ and an HER activity comparable to Pt/C. As a result, the simple system requires only 1.5 V to deliver 10 mA cm^?2 for overall alkaline water splitting, outperforming the benchmark PtC/NF||IrO₂/NF couple with high mass loading. Comprehensive electrochemical investigation reveals the unique and critical role of NiO on the optimized RuO₂/NiO interface for synergistically enhanced activities, which may be extended to broader (electro)catalytic systems.     

新型光解水制氢用半导体光催化材料的研究进展

综述了钛酸盐、钽酸盐和铌酸盐为主的层状钙钛矿型光催化材料,简述了层状钙钛矿型光催化剂光解水的机理,比较了各种层状钙钛矿型结构材料的催化活性,分析了影响活性的原因,并对层状钙钛矿型结构光催化材料的研究和应用提出了一些自己的观点.