Unraveling the Evolution of Dynamic Active Sites of LaNixFe1-xO3 Catalysts During OER

Perovskites have attracted tremendous attention as potential catalysts for the oxygen evolution reaction (OER). It is well-known that the introduction of Fe into rare earth perovskites such as LaNiO3 enhances the intrinsic OER activity. Despite numerous studies on structure-property relationships, the origin of the activity and the nature of the active species are still elusive and unclear. In this work, we study a series of LaNixFe1-xO3 perovskites using in situ electrochemical surface-enhanced Raman spectroscopy and electron energy loss spectroscopy to decipher the surface evolution and formation of active species during OER. While the origin of the activity arises from NiOOH species formed from the active Ni centers in LaNiO3, our work shows that Fe serves as the active center in LaNi0.5Fe0.5O3 and forms Fe-O-Ni and FeOOH species during OER. The OER activity of LaFeO3 originates from FeOOH species, which interact with the soluble Ni species in the electrolyte forming an active electrode-electrolyte interface with high-valent stable surface iron species (Fe4+) and thereby improving the performance. Our work provides deeper insights into the synergistic effects of Ni and Fe on the catalytic activity, which in turn provides new design principles for perovskite catalysts for the OER.

Roles of Heterojunction and Cu Vacancies in the Au@Cu2-xSe for the Enhancement of Electrochemical Nitrogen Reduction Performance

The utilization of hydrogen (H2) as a fuel source is hindered by the limited infrastructure and storage requirements. In contrast, ammonia (NH3) offers a promising solution as a hydrogen carrier due to its high energy density, liquid storage capacity, low cost, and sustainable manufacturing. NH3 has garnered significant attention as a key component in the development of next-generation refueling stations, aligning with the goal of a carbon-free economy. The electrochemical nitrogen reduction reaction (ENRR) enables the production of NH3 from nitrogen (N2) under ambient conditions. However, the low efficiency of the ENRR is limited by challenges such as the electron-stealing hydrogen evolution reaction (HER) and the breaking of the stable N2 triple bond. To address these limitations and enhance ENRR performance, we prepared Au@Cu2-xSe electrocatalysts with a core@shell structure using a seed-mediated growth method and a facile hot-injection method. The catalytic activity was evaluated using both an aqueous electrolyte of KOH solution and a nonaqueous electrolyte consisting of tetrahydrofuran (THF) solvent with lithium perchlorate and ethanol as proton donors. ENRR in both aqueous and nonaqueous electrolytes was facilitated by the synergistic interaction between Au and Cu2-xSe (copper selenide), forming an Ohmic junction between the metal and p-type semiconductor that effectively suppressed the HER. Furthermore, in nonaqueous conditions, the Cu vacancies in the Cu2-xSe layer of Au@Cu2-xSe promoted the formation of lithium nitride (Li3N), leading to improved NH3 production. The synergistic effect of Ohmic junctions and Cu vacancies in Au@Cu2-xSe led to significantly higher ammonia yield and faradaic efficiency (FE) in nonaqueous systems compared to those in aqueous conditions. The maximum NH3 yields were approximately 1.10 and 3.64 μg h-1 cm-2, with the corresponding FE of 2.24 and 67.52% for aqueous and nonaqueous electrolytes, respectively. This study demonstrates an attractive strategy for designing catalysts with increased ENRR activity by effectively engineering vacancies and heterojunctions in Cu-based electrocatalysts in both aqueous and nonaqueous media.

Recent Advances on Transition-Metal-Based Layered Double Hydroxides Nanosheets for Electrocatalytic Energy Conversion

Transition-metal-based layered double hydroxides (TM-LDHs) nanosheets are promising electrocatalysts in the renewable electrochemical energy conversion system, which are regarded as alternatives to noble metal-based materials. In this review, recent advances on effective and facile strategies to rationally design TM-LDHs nanosheets as electrocatalysts, such as increasing the number of active sties, improving the utilization of active sites (atomic-scale catalysts), modulating the electron configurations, and controlling the lattice facets, are summarized and compared. Then, the utilization of these fabricated TM-LDHs nanosheets for oxygen evolution reaction, hydrogen evolution reaction, urea oxidation reaction, nitrogen reduction reaction, small molecule oxidations, and biomass derivatives upgrading is articulated through systematically discussing the corresponding fundamental design principles and reaction mechanism. Finally, the existing challenges in increasing the density of catalytically active sites and future prospects of TM-LDHs nanosheets-based electrocatalysts in each application are also commented.

Phase Evolution on the Hydrogen Adsorption Kinetics of NiFe-Based Heterogeneous Catalysts for Efficient Water Electrolysis

Transition metal layered double hydroxides, especially nickel-iron layered double hydroxide (NiFe-LDH) shows significant advancement as efficient oxygen evolution reaction (OER) electrocatalyst but also plays a momentous role as a precursor for NiFe-based hydrogen evolution reaction (HER) catalysts. Herein, a simple strategy for developing Ni-Fe-derivative electrocatalysts via phase evolution of NiFe-LDH under controllable annealing temperatures in an argon atmosphere is reported. The optimized catalyst annealed at 340 ^o C (denoted NiO/FeNi₃ ) exhibits superior HER properties with an ultralow overpotential of 16 mV@10 mA cm^-2 . Density functional theory simulation and in situ Raman analyses reveal that the excellent HER properties of the NiO/FeNi₃ can be attributed to the strong electronic interaction at the interface of the metallic FeNi₃ and semiconducting NiO, which optimizes the H₂ O and H adsorption energies for efficient HER and OER catalytic processes. This work will provide rational insights into the subsequent development of related HER electrocatalysts and other corresponding compounds via LDH-based precursors.

Three-Dimensional Strawlike MoSe2-Ni(Fe)Se Electrocatalysts for Overall Water Splitting

The development of efficient and low-cost transition-metal electrocatalysts is of great significance for hydrogen production from water splitting. Herein, we synthesized three-dimensional strawlike MoSe₂-NiSe composed of microrods on nickel foam (NF) by a one-step hydrothermal reaction. The as-prepared MoSe₂-NiSe/NF exhibited effective hydrogen evolution reaction (HER) activity (low overpotential of 79 mV at 10 mA cm^-2 and stability of 21 h in 1 M KOH), benefiting from the large electrochemically active area provided by strawlike structures, proper Se content, and synergistic effect of active phases. The enhanced oxygen evolution reaction (OER) activity (the low overpotential of 217 mV at 10 mA cm^-2 and maintaining stability for 47 h in 1 M KOH) was further observed for Fe-doped MoSe₂-NiSe/NF (MoSe₂-NiFeSe/NF) prepared by facile soaking, which can be mainly ascribed to optimized active phases formed on the OER process after Fe doping. The two-electrode system (MoSe₂-NiSe/NF||MoSe₂-NiFeSe/NF) requires a low cell voltage of 1.54 V to obtain a current density of 10 mA cm^-2 in 1 M KOH, which provides an interesting idea for constructing an effective overall water splitting system.

Electroshock synthesis of a bifunctional nonprecious multi-element alloy for alkaline hydrogen oxidation and evolution

The design and production of active, durable, and nonprecious electrocatalysts toward alkaline hydrogen oxidation and evolution reactions (HOR/HER) are extremely appealing for the implementation of hydrogen economy, but remain challenging. Here, we report a facile electric shock synthesis of an efficient, stable, and inexpensive NiCoCuMoW multi-element alloy on Ni foam (NiCoCuMoW) as a bifunctional electrocatalyst for both HOR and HER. For the HOR, the current density of NiCoCuMoW could reach ∼11.2 mA cm-2 when the overpotential is 100 mV, higher than that for commercial Pt/C (∼7.2 mA cm-2) and control alloy samples with less elements, along with superior CO tolerance. Moreover, for the HER, the overpotential at 10 mA cm-2 for NiCoCuMoW is only 21 mV, along with a Tafel slope of low to 63.7 mV dec-1, rivaling the commercial Pt/C as well (35 mV and 109.7 mV dec-1). Density functional theory calculations indicate that alloying Ni, Co, Cu, Mo, and W can tune the electronic structure of individual metals and provide multiple active sites to optimize the hydrogen and hydroxyl intermediates adsorption, collaboratively resulting in enhanced electrocatalytic activity.

Interface-Coupling of NiFe-LDH on Exfoliated Black Phosphorus for the High-Performance Electrocatalytic Oxygen Evolution Reaction

Electrochemical oxygen evolution reaction (OER) always plays an important role in many electrochemical energy storage and conversion systems. Owing to the slow kinetics mainly brought from multiple proton-coupled electron transfer steps, the design and exploit low-cost, highly active, durable OER electrocatalysts are of significant importance. Although the black phosphorus (BP) shows good electrocatalytic OER performance, it still faces the problems of poor intrinsic activity and low stability due to its instability under ambient conditions. The NiFe-LDH was assembled onto the surfaces of exfoliated BP (EBP) nanoflakes to realize the interfacial coupling between them, achieving an effective improvement in electrocatalytic activity and stability. Benefitting from the interfacial P-O bonding, the NiFe-LDH@EBP hybrid shows high OER activity with a low overpotential of ∼240 mV@10 mA cm⁻² toward OER under alkaline conditions, as well as the good stability. Density functional theory (DFT) calculations proved that the interface-coupling of NiFe-LDH on BP promotes charge transfer kinetics and balances the adsorption/desorption of reaction intermediates, ultimately imparting excellent OER electrocatalytic activity.

Metal-Organic Framework-Derived Hollow CoSx Nanoarray Coupled with NiFe Layered Double Hydroxides as Efficient Bifunctional Electrocatalyst for Overall Water Splitting

For effective hydrogen production by water splitting, it is essential to develop earth-abundant, highly efficient, and durable electrocatalysts. Herein, the authors report a bifunctional electrocatalyst composed of hollow CoS_x and Ni-Fe based layered double hydroxide (NiFe LDH) nanosheets for efficient overall water splitting (OWS). The optimized heterostructure is obtained by the electrodeposition of NiFe LDH nanosheets on metal-organic framework-derived hollow CoS_x nanoarrays, which are supported on nickel foam (H-CoS_x @NiFe LDH/NF). The unique structure of the hybrid material not only provides ample active sites, but also facilitates electrolyte penetration and gas release during the reactions. Additionally, the strong coupling and synergy between the hydrogen evolution reaction (HER) active CoS_x and the oxygen evolution reaction (OER) active NiFe LDH gives rise to the excellent bifunctional properties. Consequently, H-CoS_x @NiFe LDH/NF exhibits remarkable HER and OER activities with overpotentials of 95 and 250 mV, respectively at 10 mA cm^-2 in 1.0 M KOH. Even at 1.0 A cm^-2 , the electrode requires small overpotentials of 375 mV (for HER) and 418 mV (for OER), respectively. An electrolyzer based on H-CoS_x @NiFe LDH/NF demonstrates a low cell voltage of 1.98 V at a current density of 300 mA cm^-2 and good durability for 100 h in OWS application.

Modulating the oxidation states in nickel–iron layered double hydroxides by natural cooling for enhanced oxygen evolution activity

Modulating the relationship of oxidation state–OER activity by natural cooling.

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.

NiFe Layered-Double-Hydroxide Nanosheet Arrays on Graphite Felt: A 3D Electrocatalyst for Highly Efficient Water Oxidation in Alkaline Media

It is of great importance to rationally design and develop earth-abundant nanocatalysts for high-efficiency water electrolysis. Herein, NiFe layered double hydroxide was in situ grown hydrothermally on a 3D graphite felt (NiFe LDH/GF) as a high-efficiency catalyst in facilitating the oxygen evolution reaction (OER). In 1.0 M KOH, NiFe LDH/GF requires a low overpotential of 214 mV to deliver a geometric current density of 50 mA cm^-2 (η_{50 mA cm^-2} = 214 mV), surpassing that NiFe LDH supported on a 2D graphite paper (NiFe LDH/GP; η_{50 mA cm^-2} = 301 mV). More importantly, NiFe LDH/GF shows good durability at 50 mA cm^-2 within 50 h of OER catalysis testing and delivers a faradaic efficiency of nearly 100% in the electrocatalysis of OER.

A three-dimensional flower-like NiCo-layered double hydroxide grown on nickel foam with an MXene coating for enhanced oxygen evolution reaction electrocatalysis

Electrolysis of water is currently one of the cleanest and most efficient ways to produce high-purity hydrogen. The oxygen evolution reaction (OER) at the anode of electrolysis is the key factor affecting the reaction efficiency, which involves the transfer of four electrons and can slow down the overall reaction process. In this work, using nickel foam coated with MXene (Ti3C2T x ) as the carrier, a three-dimensional flower-shaped layered double hydroxide (NiCo-LDH) is grown on Ti3C2T x by a hydrothermal method to fabricate a NiCo-LDH/Ti3C2T x /NF hybrid electrocatalyst for enhanced OER performance. The results reveal that the hybrid electrocatalyst has excellent OER activity in alkaline solution, in which a low overpotential of 223 mV and a small Tafel slope of 47.2 mV dec-1 can be achieved at a current density of 100 mA cm-2. The interface interaction and charge transfer between Ti3C2T x and NiCo-LDH can accelerate the electron transfer rate during the redox process and improve the catalytic activity of the overall reaction. This NiCo-LDH/Ti3C2T x /NF hybrid electrocatalyst may have important research significance and great application potential in catalytic electrolysis of water.

Co9S8 Nanosheet Coupled Cu2S Nanorod Heterostructure as Efficient Catalyst for Overall Water Splitting

Electrocatalytic water splitting is a promising technology for large-scale hydrogen production. However, it requires efficient catalysts to overcome the large overpotentials in the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER). Herein, we report a novel heterostructure catalyst Co₉S₈/Cu₂S on copper foam (Co₉S₈/Cu₂S/CF) with multistep impregnation and electrodeposition. Due to the strong interfacial interaction, the interfacial electrons transfer from Co sites to S sites, which promote the adsorption of oxygen-containing intermediates, water molecules, as well as the dissociation of water molecules. Therefore, the heterostructure catalyst exhibits low overpotentials of 195 mV for OER and 165 mV for HER at 10 mA cm^-2, respectively. Moreover, it only needs 1.6 V to realize water splitting at 10 mA cm^-2 in a two-electrode cell. This work provides an efficient method to tailor the surface electronic structure through specific morphological design and construct a heterostructure interface to achieve alkaline water splitting.

Interior and Exterior Decoration of Transition Metal Oxide Through Cu0/Cu+ Co-Doping Strategy for High-Performance Supercapacitor

Although CoO is a promising electrode material for supercapacitors due to its high theoretical capacitance, the practical applications still suffering from inferior electrochemical activity owing to its low electrical conductivity, poor structural stability and inefficient nanostructure. Herein, we report a novel Cu⁰/Cu⁺ co-doped CoO composite with adjustable metallic Cu⁰ and ion Cu⁺ via a facile strategy. Through interior (Cu⁺) and exterior (Cu⁰) decoration of CoO, the electrochemical performance of CoO electrode has been significantly improved due to both the beneficial flower-like nanostructure and the synergetic effect of Cu⁰/Cu⁺ co-doping, which results in a significantly enhanced specific capacitance (695 F g^-1 at 1 A g^-1) and high cyclic stability (93.4% retention over 10,000 cycles) than pristine CoO. Furthermore, this co-doping strategy is also applicable to other transition metal oxide (NiO) with enhanced electrochemical performance. In addition, an asymmetric hybrid supercapacitor was assembled using the Cu⁰/Cu⁺ co-doped CoO electrode and active carbon, which delivers a remarkable maximal energy density (35 Wh kg^-1), exceptional power density (16 kW kg^-1) and ultralong cycle life (91.5% retention over 10,000 cycles). Theoretical calculations further verify that the co-doping of Cu⁰/Cu⁺ can tune the electronic structure of CoO and improve the conductivity and electron transport. This study demonstrates a facile and favorable strategy to enhance the electrochemical performance of transition metal oxide electrode materials.

The rational design of Cu2-xSe@(Co,Cu)Se2 core-shell structures as bifunctional electrocatalysts for neutral-pH overall water splitting

Highly active and stable bifunctional electrocatalysts for H2 generation from neutral-pH water are desired, but difficult to achieve. The modification of the electronic and crystal structure of a material by element doping, morphology design and constructing a complex is a valid strategy for obtaining high-performance catalysts toward overall water splitting. In this study, a novel Cu2-xSe@(Co,Cu)Se2 core-shell structure with ultrathin (Co,Cu)Se2 nanosheets anchored as a shell on an internal Cu2-xSe core was fabricated, for the first time, by integrating the three above-mentioned modification methods. Benefiting from the synergistic effect between components and the unique structure, the Cu2-xSe@(Co,Cu)Se2 core-shell structure can serve as an efficient bifunctional electrocatalyst for both HERs and OERs in neutral-pH electrolytes with a current density of 10 mA cm-2 at the overpotentials of 106 mV and 396 mV, respectively. Additionally, the material just requires a cell voltage of 1.73 V to afford a current density of 10 mA cm-2 in a neutral two-electrode electrolyzer. Such performances significantly outperform control catalysts and analogues. Even more importantly, the original concept of coordinated regulation presented in this work can broaden our horizons in the design of new and highly efficient catalysts for neutral water splitting.

Nanostructured Iron Fluoride Derived from Fe-Based Metal-Organic Framework for Lithium Ion Battery Cathodes

A comprehensive strategy for the morphological control of octahedral and spindle Fe-based metal-organic frameworks (Fe-MOFs) via microwave-assisted adjustment is proposed in this research. Afterward, in situ copyrolysis under N₂ atmosphere contributes to the fabrication of two shape-maintained FeF₃·0.33H₂O nanostructures (named O-FeF₃·0.33H₂O and S-FeF₃·0.33H₂O, respectively) with confined hierarchical porosity and graphitized carbon skeleton. The lithium storage performances for the MOF-derived octahedral O-FeF₃·0.33H₂O and spindle S-FeF₃·0.33H₂O composites are investigated, and the prospective lithium storage mechanism is discussed. As a result, the main product of the porous O-FeF₃·0.33H₂O structure is found to be a promising cathode material for lithium ion batteries owing to its advantageous electrochemical capability. Even after being cycled over 1000 times at 2 C (1 C = 237 mAh g^-1), the capacity attenuation rate of the as-prepared O-FeF₃·0.33H₂O electrode is as low as 0.039% per cycle. The combination of proper octahedral morphology and highly graphitized carbon modification can not only enhance the conductivity of the cathode but also promote the diffusion of Li⁺ effectively. The remarkable performance of octahedral O-FeF₃·0.33H₂O can be confirmed by the Li-ion diffusion coefficient (D_Li⁺) calculation analysis and kinetics analysis of lithium storage behavior.

Interface engineering of oxygen-vacancy-rich NiCo2O4/NiCoP heterostructure as an efficient bifunctional electrocatalyst for overall water splitting

Inexpensive bifunctional electrocatalysts towards oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) is highly desirable from the perspective of energy conversion.