Synergism of Geometric Construction and Electronic Regulation: 3D Se-(NiCo)Sx /(OH)x Nanosheets for Highly Efficient Overall Water Splitting

Fabricating a high-performance anode by coating a carbon layer on a yolk-shell bimetallic selenide microsphere for enhanced lithium storage

The rational synthesis of an electrode material with a highly active and stable architecture is very critical to achieving high-performance electrochemical energy storage. Herein, N-doped carbon restricting yolk-shell CoSe2/Ni3Se4 (CoSe2/Ni3Se4@NC) flower-like microspheres were successfully synthesized from solid CoNi-glycerate microspheres using a coating technology as an anode material for lithium-ion batteries (LIBs). The unique yolk-shell CoSe2/Ni3Se4@NC microspheres with hierarchical pores can increase the contact area with the electrolyte and provide enough transfer channels for the diffusion of Li+. The carbon layer on the surface of CoSe2/Ni3Se4@NC can not only improve the conductivity of the electrode but also provide the protective effect of active nanosheets during the process of synthesis, avoiding the overall structure collapse during the charge/discharge process of LIBs. Benefiting from the high conductivity, hollow structure, and elastic NC shell bestowed by the unique architecture, the yolk-shell CoSe2/Ni3Se4@NC anode shows excellent lithium storage performances, such as an excellent reversible specific capacity of 319 mA h g-1 at a current density of 1000 mA g-1 after 500 cycles and excellent cycling stability. This synthesis strategy provides a new way to optimize the lithium storage performance of transition metal compound electrode materials, which is helpful to the design of the next generation of high-performance LIBs.

Interface-Engineered NiFe/Ni-S Nanoparticles for Reliable Alkaline Oxygen Production at Industrial Current: A Sulfur Source Confinement Strategy

Using powder-based ink appears to be the most suitable candidate for commercializing the membrane electrode assembly (MEA), while research on the powder-based NPM catalyst for anion exchange membrane water electrolyzer (AEMWE) is currently insufficient, especially at high current density. Herein, a sulfur source (NiFe Layered double hydroxide adsorbedSO 4 2 - ${\mathrm{SO}}_4^{2 - }$ ) confinement strategy is developed to integrate Ni3S2 onto the surface of amorphous/crystalline NiFe alloy nanoparticles (denoted NiFe/Ni-S), achieving advanced control over the sulfidation process for the formation of metal sulfides. The constructed interface under the sulfur source confinement strategy generates abundant active sites that increase electron transport at the electrode-electrolyte interface and improve ability over an extended period at a high current density. Consequently, the constructed NiFe/Ni-S delivers an ultra-low overpotential of 239 mV at 10 mA cm-2 and 0.66 mAcm ECSA - 2 ${\mathrm{cm}}_{{\mathrm{ECSA}}}^{ - 2}$ under an overpotential of 300 mV. The AEMWE with NiFe/Ni-S anode exhibits a cell voltage of 1.664 V @ 0.5 A cm-2 and a 400 h stability at 1.0 A cm-2.

Recycling cobalt from spent lithium-ion batteries for designing the novel cobalt nitride followers: Towards efficient overall water splitting and advanced zinc-air batteries

Although cobalt nitride (CoN)-based nanomaterials have been widely designed as advanced oxygen evolution reaction (OER), hydrogen evolution reaction (HER), and oxygen reduction reaction (ORR) catalysts, the continuous consumption of lithium-ion batteries (LIBs) has led to a high price of cobalt metal. Therefore, in the future, recycling valuable Co elements from spent devices and boosting their service efficiency will inevitably promote the utilization of Co-based materials in water splitting and zinc-air batteries (ZABs). Herein, we realize the Co recycling from spent LIBs by a simple hydrometallurgy method. Under the assistance of hexamethylenetetramine and polystyrene spheres, after the hydrothermal and pyrolysis treatment in the NH3 atmosphere, the as-reclaimed cobalt oxalates were successfully transformed into novel three-dimensional (3D) CoN nanoflowers (denoted as CoN NFs). Benefiting from the unique 3D flower-like architectures, intrinsic high conductivity, large surface area, uniformly dispersed CoN nanoparticles, and the synergistic effect between Co3N and CoO phases, the 3D flower-like CoN NFs exhibited excellent OER catalytic activity. The performance was much better than commercial RuO2 in the 1.0 M KOH solution. Furthermore, the CoN NFs-based water splitting cell needed a voltage of 1.608 V to achieve the current density of 10 mA cm-2, which is even 16 mV smaller than that of Pt/C||RuO2 benchmark (1.624 V). Meanwhile, the CoN NFs-derived ZAB exhibited a high peak power density of 107.3 mW cm-2 (vs. 103.2 mW cm-2 of Pt/C-RuO2-based ZAB) and a low charge-discharge voltage gap (0.93 V vs. 1.43 V of Pt/C-RuO2-based ZAB). Due to the excellent structural and elemental stabilities, the corresponding water splitting cell and ZAB had outstanding durability. This work successfully explored an advanced industrial chain from recycling Co metal in spent devices to designing the high-efficiency HER/OER/ORR electrocatalysts for advanced water splitting devices and ZABs. This will further promote the value-added utilization of valuable Co metal in various energy storage or conversion devices.

Two Birds with One Stone: Contemporaneously Enhancing OER Catalytic Activity and Stability for Dual-Phase Medium-Entropy Metal Sulfides

Transition metal-based sulfides exhibit remarkable potential as electrocatalysts for oxygen evolution reaction (OER) due to the unique intrinsic structure and physicochemical characteristics. Nevertheless, currently available sulfide catalysts based on transition metals face a bottleneck in large-scale commercial applications owing to their unsatisfactory stability. Here, the first fabrication of (FeCoNiMn2 )S2 dual-phase medium-entropy metal sulfide (dp-MEMS) is successfully achieved, which demonstrated the expected optimization of stability in the OER process. Benefiting from the "cell wall" -like structure and the synergistic effect in medium-entropy systems, (FeCoNiMn2 )S2 dp-MEMS delivers an exceptionally low overpotential of 169 and 232 mV at current densities of 10 and 100 mA cm-2 , respectively. The enhancement mechanism of catalytic activity and stability is further validated by density functional theory (DFT) calculations. Additionally, the rechargeable Zn-air batteries integrated with FeCoNiMn2 )S2 dp-MEMS exhibit remarkable performance outperforming the commercial catalyst (Pt/C+RuO2 ). This work demonstrates that the dual-phase medium-entropy metal sulfide-based catalysts have the potential to provide a greater application value for OER and related energy conversion systems.

Tailoring the Growth and Morphology of Lithium Peroxide: Nickel Sulfide/Nickel Phosphate Nanotubes with Optimized Electronic Structure for Lithium-Oxygen Batteries

Heterogeneous crystalline-amorphous structures, with tunable electronic structures and morphology, hold immense promise as catalysts for lithium-oxygen batteries (LOBs). Herein, a nanotube network constructed by crystalline nickel sulfide/amorphous nickel phosphate (NiS/NiPO) heterostructure is prepared on Ni foam through the sulfurization of the precursor generated hydrothermally. Used as cathodes, the NiS/NiPO nanotubes with optimized electronic structure can induce the deposition of the highly porous and interconnected structure of Li2 O2 with rich Li2 O2 -electrolyte interfaces. Abundant active sites can be created on NiS/NiPO through the charge redistribution for the uniform nucleation and growth of Li2 O2 . Moreover, nanotube networks endow cathodes with efficient transport channels and sufficient space for the accommodation of Li2 O2 . A high discharge capacity of 27 003.6 mAh g-1 and a low charge overpotential of 0.58 V at 1000 mAh g-1 can be achieved at 200 mA g-1 . This work provides valuable insight into the unique role of the electronic structure and morphology of catalysts in the formation mechanisms of Li2 O2 and the performances of LOBs.

Plasma-Induced Formation of Pt Nanoparticles with Optimized Surface Oxidation States for Methanol Oxidation and Oxygen Reduction Reactions to Achieve High-Performance DMFCs

Plasma treatment and reduction are used to synthesize Pt nanoparticles (NPs) on nitrogen-doped carbon nanotubes (p-Pt/p-NCNT) with a low Pt content. In particular, the plasma treatment is used to treat the NCNT to give it with more surface defects, facilitating a better growth of the Pt NPs, while the plasma reduction produces the Pt NPs with a reduced fraction of the surface atoms at the high oxidation states, increasing the catalytic activities of the p-Pt@p-NCNT. Even at the low Pt content (7.8 wt.%), the p-Pt@p-NCNT shows superior catalytic activities and good stabilities for methanol oxidation reaction (MOR) and oxygen reduction reaction (ORR). The density functional theory (DFT) calculations indicate that the defects generated in the plasma treatment can help the growth of the Pt NPs on the NCNTs, leading to the stronger electronic coupling between Pt and NCNT and the increased stability of the catalyst. The plasma reduction can give the Pt NPs with optimized surface oxidation states, decreasing the energy barriers of the rate-determining steps for MOR and ORR. When used as the anode and cathode catalysts for the direct methanol fuel cells (DMFCs), the p-Pt@p-NCNT exhibits a higher maximum power density of 81.9 mW cm-2  at 80 °C and shows good durability.

Chelating Co-reduction Strategy for the Synthesis of High-Entropy Alloy Aerogels

Aerogels, as three-dimensional porous materials, have attracted much attention in almost every field owing to their unique structural properties. Designing high-entropy alloy aerogels (HEAAs) to quinary and above remains an enormous challenge due to the different reduction potentials and nucleation/growth kinetics of different constituent metals. Herein, a novel and universal chelating co-reduction strategy to prepare HEAAs at room temperature in the water phase is proposed. The addition of chelators (ethylenediaminetetraacetic acid tetrasodium salt, sodium citrate, salicylic acid, and 4,4'-bipyridine) with a certain strong coordination capacity can adjust the reduction potential of different metal components, which is the key to synthesize single-phase solid solution alloys successfully. The optimized AgRuPdAuPt HEAA can be an excellent electrocatalyst for hydrogen evolution reaction (HER) with an ultrasmall overpotential of 22 mV at 10 mA cm^-2 and excellent stability for 24 h in an alkaline solution. In situ Raman spectroscopy unveils the enhanced hydrogen evolution reaction mechanism of HEAAs. Overall, this work provides a novel chelating co-reduction strategy for the facile and versatile synthesis and design of advanced HEAAs and broadens the development and utilization of multi-elemental alloy electrocatalysts.

Reversible active bridging sulfur sites grafted on Ni3S2 nanobelt arrays for efficient hydrogen evolution reaction

Elaborate and rational design of cost-effective and high-efficiency non-noble metal electrocatalysts for pushing forward the sustainable hydrogen fuel production is of great significance. Herein, a novel VS₄ nanoparticle decorated Ni₃S₂ nanobelt array in-situ grown on nickel foam (VS₄/Ni₃S₂/NF NBs) was prepared by a self-templated synthesis strategy. Benefitting from the unique nanobelt array structure, abundant highly active bridge S₂^2- sites and strong electronic interaction between VS₄ and Ni₃S₂ on the heterointerface, the integrated VS₄/Ni₃S₂/NF NBs exhibited excellent electrocatalytic hydrogen evolution activity and robust stability. The density functional theory (DFT) further revealed the reversible conversion catalysis mechanism of bridging S₂^2- sites in VS₄/Ni₃S₂/NF NBs during HER process. Notably, bidentate bridging SS bonds as the predominant catalytically active centers can spontaneously open once H adsorbed its surface, leading to the aggregation of negative charges on S atoms and thus facilitating the generation of H* intermediates, and spontaneously close when H* desorption is going to form H₂. Our work provides fresh insights for developing potential polysulfides as high-performance hydrogen-evolving electrocatalysts for prospective clean energy production from water splitting.

High-entropy NiFeCoV disulfides for enhanced alkaline water/seawater electrolysis

Creating electrocatalysts with high activity and stability to meet the needs of highly effective seawater splitting is of great importance to achieve the goal of hydrogen production from abundant seawater source, which however is still challenging owing to sluggish oxygen evolution reaction (OER) dynamics and the existed competitive chloride evolution reaction. Herein, high-entropy (NiFeCoV)S₂ porous nanosheets are uniformly fabricated on Ni foam via a hydrothermal reaction process with a sequential sulfurization step for alkaline water/seawater electrolysis. The obtained rough and porous nanosheets provide large active surface area and exposed more active sites, which can facilitate mass transfer and are conducive to the improvement of the catalytic performance. Combined with the strong synergistic electron modulation effect of multi elements in (NiFeCoV)S₂, the as-fabricated catalyst exhibits low OER overpotentials of 220 and 299 mV at 100 mA cm^-2 in alkaline water and natural seawater, respectively. Besides, the catalyst can withstand a long-term durability test for more than 50 h without hypochlorite evolution, showing excellent corrosion resistance and OER selectivity. By employing the (NiFeCoV)S₂ as the electrocatalyst for both anode and cathode to construct an overall water/seawater splitting electrolyzer, the required cell voltages are only 1.69 and 1.77 V to reach 100 mA cm^-2 in alkaline water and natural seawater, respectively, showing a promising prospect towards the practical application for efficient water/seawater electrolysis.

Modular Flow Reactors for Valorization of Kraft Lignin and Low-Voltage Hydrogen Production

Recent studies have found that green hydrogen production and biomass utilization technologies can be combined to efficiently produce both hydrogen and value-added chemicals using biomass as an electron and proton source. However, the majority of them have been limited to proof-of-concept demonstrations based on batch systems. Here the authors report the design of modular flow systems for the continuous depolymerization and valorization of lignin and low-voltage hydrogen production. A redox-active phosphomolybdic acid is used as a catalyst to depolymerize lignin with the production of aromatic compounds and extraction of electrons for hydrogen production. Individual processes for lignin depolymerization, byproduct separation, and hydrogen production with catalyst reactivation are modularized and integrated to perform the entire process in the serial flow. Consequently, this work enabled a one-flow process from biomass conversion to hydrogen gas generation under a cyclic loop. In addition, the unique advantages of the fluidic system (i.e., effective mass and heat transfer) substantially improved the yield and efficiency, leading to hydrogen production at a higher current density (20.5 mA cm-2 ) at a lower voltage (1.5 V) without oxygen evolution. This sustainable eco-chemical platform envisages scalable co-production of valuable chemicals and green hydrogen for industrial purposes in an energy-saving and safe manner.

Deciphering the Space Charge Effect of the p-n Junction between Copper Sulfides and Molybdenum Selenides for Efficient Water Electrolysis in a Wide pH Range

Space charge transfer is crucial for an efficient electrocatalytic process, especially for narrow-band-gap metal sulfides/selenides. Herein, we designed and synthesized a core-shell structure which is an ultrathin MoSe₂ nanosheet coated CuS hollow nanoboxes (CuS@MoSe₂) to form an open p-n junction structure. The space charge effect in the p-n junction region will greatly improve electron mass transfer and conduction, and also have abundant active interfaces. It was used as a bifunctional electrocatalyst for water oxidation at a wide pH range. It exhibits a low overpotential of 49 mV for the HER and 236 mV for the OER at a current density of 10 mA·cm^-2 in acidic pH, 72 mV for the HER and 219 mV at 10 mA·cm^-2 for the OER in alkaline pH, and 62 mV for the HER and 230 mV at 10 mA·cm^-2 for the OER under neutral conditions. The experimental results and density functional theory calculations testify that the p-n junction in CuS@MoSe₂ designed and synthesized has a strong space charge region with a synergistic effect. The built-in field can boost the electron transport during the electrocatalytic process and can stabilize the charged active center of the p-n junction. This will be beneficial to improve the electrocatalytic performance. This work provides the understanding of semiconductor heterojunction applications and regulating the electronic structure of active sites.

Realizing Interfacial Electron/Hole Redistribution and Superhydrophilic Surface through Building Heterostructural 2 nm Co0.85 Se-NiSe Nanograins for Efficient Overall Water Splittings

Electrochemical overall water splitting using renewable energy input is highly desirable for large-scale green hydrogen generation, but it is still challenged due to the lack of low-cost, durable, and highly efficient electrocatalysts. Herein, 1D nanowires composed of numerous 2 nm Co_0.85 Se-NiSe nanograin heterojunctions as efficient precious metal-free bifunctional electrocatalyst are reported for both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in alkaline solution with the merits of high activity, durability, and low cost. The abundant microinterface among the ultrafine nanograins and the presence of lattice distortion around nanograin interface is found to create a superhydrophilic surface of the electrocatalyst, which significantly facilitate the fast diffusion of electrolytes and the release of the formed H₂ and O₂ from the catalyst surface. Furthermore, synergic effect between Co_0.85 Se and NiSe grain on adjusting the electronic structure is revealed, which enhances electron mobility for fast electron transport during the HER/OER process. Owing to these merits, the rationally designed Co_0.85 Se-NiSe heterostructures display efficient overall water splitting behavior with a low voltage of 1.54 V at 10 mA cm^-2 and remarkable long-term durability for the investigated period of 50 h.

Ultrafast self-heating synthesis of robust heterogeneous nanocarbides for high current density hydrogen evolution reaction

Designing cost-effective and high-efficiency catalysts to electrolyze water is an effective way of producing hydrogen. Practical applications require highly active and stable hydrogen evolution reaction catalysts working at high current densities (≥1000 mA cm-2). However, it is challenging to simultaneously enhance the catalytic activity and interface stability of these catalysts. Herein, we report a rapid, energy-saving, and self-heating method to synthesize high-efficiency Mo2C/MoC/carbon nanotube hydrogen evolution reaction catalysts by ultrafast heating and cooling. The experiments and density functional theory calculations reveal that numerous Mo2C/MoC hetero-interfaces offer abundant active sites with a moderate hydrogen adsorption free energy ΔGH* (0.02 eV), and strong chemical bonding between the Mo2C/MoC catalysts and carbon nanotube heater/electrode significantly enhances the mechanical stability owing to instantaneous high temperature. As a result, the Mo2C/MoC/carbon nanotube catalyst achieves low overpotentials of 233 and 255 mV at 1000 and 1500 mA cm-2 in 1 M KOH, respectively, and the overpotential shows only a slight change after working at 1000 mA cm-2 for 14 days, suggesting the excellent activity and stability of the high-current-density hydrogen evolution reaction catalyst. The promising activity, excellent stability, and high productivity of our catalyst can fulfil the demands of hydrogen production in various applications.

Construction of Synergistic Ni3 S2 -MoS2 Nanoheterojunctions on Ni Foam as Bifunctional Electrocatalyst for Hydrogen Evolution Integrated with Biomass Valorization

The intrinsic sluggish kinetics of the oxygen evolution reaction (OER) limit the improvement of hydrogen evolution reaction (HER) performance, and substituting the anodic oxidation of biomass materials is an alternative approach, given its lower oxidation potential and higher added value compared to those of OER. In this study, a Ni₃ S₂ -MoS₂ nanoheterojunction catalyst with strong electronic interactions is prepared. It exhibits high efficiency for both the HER and the electrooxidation of 5-hydroxymethylfurfural (HMF). In a two-electrode cell with Ni₃ S₂ -MoS₂ serving as both the anode and cathode, the potential is only 1.44 V at a current density of 10 mA cm^-2 , which is much lower than that of pure water splitting. Density functional theory calculations confirm that the strong chemisorption of H and HMF at the interface leads to outstanding electrocatalytic activity. The findings not only provide a strategy for developing efficient electrocatalysts, but also provide an approach for the continuous production of high value-added products and H₂ .

Research progress in improving the oxygen evolution reaction by adjusting the 3d electronic structure of transition metal catalysts

As a clean and renewable energy carrier, hydrogen (H₂) has become an attractive alternative to dwindling fossil fuels. The key to realizing hydrogen-based energy systems is to develop efficient and economical hydrogen production methods. The water electrolysis technique has the advantages of cleanliness, sustainability, and high efficiency, which can be applied to large-scale hydrogen production. However, the electrocatalytic oxygen evolution reaction (OER) at the anode plays a decisive role in the efficiency of hydrogen evolution during water splitting. Generally, noble metal catalysts (such as ruthenium and iridium) are considered to exhibit the best OER performance; however, they exhibit disadvantages such as high costs, limited reserves, and poor stability. Therefore, the research on highly efficient non-noble metal catalysts that can replace their noble metal counterparts has always been important. This review presents the recent advances in the preparation of high-performance OER electrocatalysts by regulating the electronic structure of 3d transition metals. First, we introduce the reaction mechanism of water splitting and the OER, which reveals the high requirement of the complex four-electron process of the OER. Second, the electron transfer mode and development progress of highly active transition metal electrocatalysts are used to summarize the research situation of transition metal OER catalysts in water splitting. Finally, the future development direction and challenges of transition metal catalysts are prospected based on the current research progress.

Reduction of Transition-Metal Columbite-Tantalite as a Highly Efficient Electrocatalyst for Water Splitting

We successfully report a liquid-liquid chemical reduction and hydrothermal synthesis of a highly stable columbite-tantalite electrocatalyst with remarkable hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) performance in acidic media. The reduced Fe_0.79Mn_0.21Nb_0.16Ta_0.84O₆ (CTr) electrocatalyst shows a low overpotential of 84.23 mV at 10 mA cm^-2 and 103.7 achieved at 20 mA cm^-2 current density in situ for the HER and OER, respectively. The electrocatalyst also exhibited low Tafel slopes of 104.97 mV/dec for the HER and 57.67 mV/dec for the OER, verifying their rapid catalytic kinetics. The electrolyzer maintained a cell voltage of 1.5 V and potential-time stability close to that of Pt/C and RuO₂. Complementary first-principles density functional theory calculations identify the Mn sites as most active sites on the Fe_0.75Mn_0.25Ta_1.875Nb_0.125O₆ (100) surface, predicting a moderate Gibbs free energy of hydrogen adsorption (ΔG_H* ≈ 0.08 eV) and a low overpotential of η = 0.47 V. The |ΔGMn_H*| = 0.08 eV on the Fe_0.75Mn_0.25Ta_1.875Nb_0.125O₆ (100) surface is similar to that of the well-known and highly efficient Pt catalyst (|ΔGPt_H*| ≈ 0.09 eV).

Dynamic Pt-OH-·H2O-Ag species mediate coupled electron and proton transfer for catalytic hydride reduction of 4-nitrophenol at the confined nanoscale interface

Generally, the catalytic transformation of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) at heterogeneous metal surfaces follows a Langmuir-Hinshelwood (L-H) mechanism when sodium borohydride (NaBH₄) is used as the sacrificial reductant. Herein, with Pt-Ag bimetallic nanoparticles confined in dendritic mesoporous silica nanospheres (DMSNs) as a model catalyst, we demonstrated that the conversion of 4-NP did not pass through the direct hydrogen transfer route with the hydride equivalents being supplied by borohydride via the bimolecular L-H mechanism, since Fourier transform infrared (FTIR) spectroscopy with the use of isotopically labeled reactants (NaBD₄ and D₂O) showed that the final product of 4-AP was composed of protons (or deuterons) that originated from the solvent water (or heavy water). Combined characterization by X-ray photoelectron spectroscopy (XPS), ¹H nuclear magnetic resonance (NMR) and the optical excitation and photoluminescence spectrum evidenced that the surface hydrous hydroxide complex bound to the metal surface (also called structural water molecules, SWs), due to the space overlap of p orbitals of two O atoms in SWs, could form an ensemble of dynamic interface transient states, which provided the alternative electron and proton transfer channels for selective transformation of 4-NP. The cationic Pt species in the Ag-Pt bimetallic catalyst mainly acts as a dynamic adsorption center to temporally anchor SWs and related reactants, and not as the active site for hydrogen activation.

Molecular-Scale Manipulation of Layer Sequence in Heteroassembled Nanosheet Films toward Oxygen Evolution Electrocatalysts

Flocculation or restacking of different kinds of two-dimensional (2D) nanosheets into heterostructure nanocomposites is of interest for the development of high-performance electrode materials and catalysts. However, lacking a molecular-scale control on the layer sequence hinders enhancement of electrochemical activity. Herein, we conducted electrostatic layer-by-layer (LbL) assembly, employing oxide nanosheets (e.g., MnO₂, RuO_2.1, reduced graphene oxide (rGO)) and layered double hydroxide (LDH) nanosheets (e.g., NiFe-based LDH) to explore a series of mono- and bilayer films with various combinations of nanosheets and sequences toward oxygen evolution reaction (OER). The highest OER activity was attained in bilayer films of electrically conductive RuO_2.1 nanosheets underlying catalytically active NiFe LDH nanosheets with mixed octahedral/tetrahedral coordination (NiFe LDH_Td/Oh). At an overpotential of 300 mV, the RuO_2.1/NiFe LDH_Td/Oh film exhibited an electrochemical surface area (ECSA) normalized current density of 2.51 mA cm^-2_ECSA and a mass activity of 3610 A g^-1, which was, respectively, 2 and 5 times higher than that of flocculated RuO_2.1/NiFe LDH_Td/Oh aggregates with a random appearance of a surface layer. First-principles density functional theory calculations and COMSOL Multiphysics simulations further revealed that the improved catalytic performance was ascribed to a substantial electronic coupling effect in the heterostructure, in which electrons are transferred from exposed NiFe LDH_Td/Oh nanosheets to underneath RuO_2.1. The study provides insight into the rational control and manipulation of redox-active surface layers and conductive underlying layers in heteroassembled nanosheet films at molecular-scale precision for efficient electrocatalysis.

Engineering Metallic Heterostructure Based on Ni3 N and 2M-MoS2 for Alkaline Water Electrolysis with Industry-Compatible Current Density and Stability

Alkaline water electrolysis is commercially desirable to realize large-scale hydrogen production. Although nonprecious catalysts exhibit high electrocatalytic activity at low current density (10-50 mA cm^-2 ), it is still challenging to achieve industrially required current density over 500 mA cm^-2  due to inefficient electron transport and competitive adsorption between hydroxyl and water. Herein, the authors design a novel metallic heterostructure based on nickel nitride and monoclinic molybdenum disulfide (Ni₃ N@2M-MoS₂ ) for extraordinary water electrolysis. The Ni₃ N@2M-MoS₂  composite with heterointerface provides two kinds of separated reaction sites to overcome the steric hindrance of competitive hydroxyl/water adsorption. The kinetically decoupled hydroxyl/water adsorption/dissociation and metallic conductivity of Ni₃ N@2M-MoS₂  enable hydrogen production from Ni₃ N and oxygen evolution from the heterointerface at large current density. The metallic heterostructure is proved to be imperative for the stabilization and activation of Ni₃ N@2M-MoS₂ , which can efficiently regulate the active electronic states of Ni/N atoms around the Fermi-level through the charge transfer between the active atoms of Ni₃ N and MoMo bonds of 2M-MoS₂  to boost overall water splitting. The Ni₃ N@2M-MoS₂  incorporated water electrolyzer requires ultralow cell voltage of 1.644 V@1000 mA cm^-2  with ≈100% retention over 300 h, far exceeding the commercial Pt/C║RuO₂ (2.41 V@1000 mA cm^-2 , 100 h, 58.2%).

Interfacial Microenvironment Modulation Enhancing Catalytic Kinetics of Binary Metal Sulfides Heterostructures for Advanced Water Splitting Electrocatalysts

Interfacial microenvironment modulation has been proven to be a promising route to fabricate highly efficient catalysts. In this work, the lattice defect-rich NiS₂ /MoS₂ nanoflakes (NMS NFs) electrocatalysts are successfully synthesized by a simple strategy. Benefiting from the abundant lattice defects and modulated interfacial microenvironment between NiS₂ and MoS₂ , the prepared NMS NFs show superior catalytic activity for water splitting. Particularly, the optimized NMS NFs (the molar ratio of Ni:Mo = 5:5) exhibit remarkable catalytic activity toward overall water splitting with a voltage of 1.60 V at 10 mA cm^-2 in alkaline media, which is lower than that of the noble-metal-based electrocatalysts (1.68 V at 10 mA cm^-2 ). The NMS NFs electrocatalysts also show exceptional long-term stability (>50 h) for overall water splitting. The density functional theory results demonstrate that the injection of NiS₂ into MoS₂ can greatly optimize the catalytic kinetics and reduce the energy barrier for hydrogen/oxygen evolution reactions. The work does not only offer a promising candidate for a highly efficient water splitting electrocatalyst but also highlights that interfacial microenvironment modulation is a potential strategy to optimize the catalytic kinetics.

Microwave-Assisted Auto-Combustion Synthesis of Binary/Ternary Co x Ni1-x Ferrite for Electrochemical Hydrogen and Oxygen Evolution

Enormous efforts have been dedicated to engineering low-cost and efficient electrocatalysts for both hydrogen evolution and oxygen evolution reactions (HER and OER, respectively). For this, the current contribution reports the successful synthesis of binary/ternary metal ferrites (Co x Ni1-x Ferrite; x = 0.0, 0.1, 0.3, 0.5, 0.7, and 1.0) by a simple one-step microwave technique and subsequently discusses its chemical and electrochemical properties. The X-ray diffraction analysis substantiated the phase purity of the as-obtained catalysts with various compositions. Additionally, the morphology of the nanoparticles was identified via transmission electron microscopy. Further, the vibrating sample magnetometer justified the ferromagnetic character of the as-prepared products. The electrochemical measurements revealed that the as-prepared materials required the overpotentials of 422-600 and 419-467 mV for HER and OER, respectively, to afford current densities of 10 mA cm-2. In the general sense, Ni cation substitution with Co influenced favorably toward both HER and OER. Among all synthesized electrocatalysts, Co0.9Ni0.1Ferrite displayed the highest performance in terms of OER in 1 M KOH solution, which is related to the synergistic effect of multiple parameters including the optimal substitution amount of Co, the highest Brunauer-Emmett-Teller surface area, the smallest particle size among all samples (26.71 nm), and the lowest charge transfer resistance. The successful synthesis of ternary ferrites carried out for the first time via a microwave-assisted auto-combustion route opens up a new path for their applications in renewable energy technologies.

Improved Interface Charge Transfer and Redistribution in CuO-CoOOH p-n Heterojunction Nanoarray Electrocatalyst for Enhanced Oxygen Evolution Reaction

Electron density modulation is of great importance in an attempt to achieve highly active electrocatalysts for the oxygen evolution reaction (OER). Here, the successful construction of CuO@CoOOH p-n heterojunction (i.e., p-type CuO and n-type CoOOH) nanoarray electrocatalyst through an in situ anodic oxidation of CuO@CoSx on copper foam is reported. The p-n heterojunction can remarkably modify the electronic properties of the space-charge region and facilitate the electron transfer. Moreover, in situ Raman study reveals the generation of SO4 2- from CoSx oxidation, and electron cloud density distribution and density functional theory calculation suggest that surface-adsorbed SO4 2- can facilitate the OER process by enhancing the adsorption of OH- . The positively charged CoOOH in the space-charge region can significantly enhance the OER activity. As a result, the CuO@CoOOH p-n heterojunction shows significantly enhanced OER performance with a low overpotential of 186 mV to afford a current density of 10 mA cm-2 . The successful preparation of a large scale (14 × 25 cm2 ) sample demonstrates the possibility of promoting the catalyst to industrial-scale production. This study offers new insights into the design and fabrication of non-noble metal-based p-n heterojunction electrocatalysts as effective catalytic materials for energy storage and conversion.

Zr-doped CoFe-layered double hydroxides for highly efficient seawater electrolysis

Efficient generation of hydrogen from electrocatalytic water-splitting is of great importance to realize the hydrogen economy. In that field, designing efficient and bifunctional electrocatalysts for oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) is critical for water splitting. With the increasing demands for bifunctional catalysts, a universal strategy in favor of these catalytic processes is particularly important. Herein, a variety of Zr-doped layered double hydroxide (LDH) with low-crystalline grown on nickel foam (NF) is designed to promote the bifunctional activities of electrocatalysts. It is found that the doping of Zr⁴⁺ into CoFe-LDH/NF can tune the electronic structure and also expose abundant catalytic active sites to enhance the electrocatalytic activities. In 1 M KOH, the as-prepared CoFeZr/NF exhibits superior OER and HER activities with low overpotentials of 233 and 159 mV at 10 mA cm^-2. When tested in alkaline simulated seawater electrolyte, CoFeZr/NF also shows high catalytic activities with almost no attenuation when compared with that in 1.0 M KOH. This work will provide a new way for the development of seawater electrolysis for large-scale hydrogen production.

Synergistically integrated Co9S8@NiFe-layered double hydroxide core-branch hierarchical architectures as efficient bifunctional electrocatalyst for water splitting

Efficient, low-cost, and robust electrocatalysts development for overall water splitting is highly desirable for renewable energy production yet still remains challenging. In this work, Co₉S₈ nanoneedles arrays are synergistically integrated with NiFe-layered double hydroxide (NiFe-LDH) nanosheets to form Co₉S₈@NiFe-LDH core-branch hierarchical architectures supported on nickel foam (Co₉S₈@NiFe-LDH HAs/NF). The Co₉S₈@NiFe-LDH HAs/NF exhibits high catalytic performances for oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) with overpotential of 190 and 145 mV at 10 mA cm^-2, respectively. The density functional theory calculations predict that the synergy between Co₉S₈ and NiFe-LDH contributes to the high catalytic performance by lowering the energy barrier of HER. When used as both anode and cathode electrocatalyst, it can deliver 10 mA cm^-2 at a low cell voltage of 1.585 V with excellent long-term durability. This work opens a new avenue toward the exploration of highly efficient and stable electrocatalyst for overall water splitting.

Advances in hydrogen production from electrocatalytic seawater splitting

As one of the most abundant resources on the Earth, seawater is not only a promising electrolyte for industrial hydrogen production through electrolysis, but also of great significance for the refining of edible salt. Despite the great potential for large-scale hydrogen production, the implementation of water electrolysis requires efficient and stable electrocatalysts that can maintain high activity for water splitting without chloride corrosion. Recent years have witnessed great achievements in the development of highly efficient electrocatalysts toward seawater splitting. Starting from the historical background to the most recent achievements, this review will provide insights into the current state, challenges, and future perspectives of hydrogen production through seawater electrolysis. In particular, the mechanisms of overall water splitting, key features of seawater electrolysis, noble-metal-free electrocatalysts for seawater electrolysis and the underlying mechanisms are also highlighted to provide guidance for fabricating more efficient electrocatalysts toward seawater splitting.

Fe(Co)OOH Dynamically Stable Interface Based on Self-Sacrificial Reconstruction for Long-Term Electrochemical Water Oxidation

FeOOH on the real catalytic interface for the oxygen evolution reaction (OER) is chemically unstable to dissolve in alkaline media. Herein, based on the perspective of the dynamically stable interface, we purposely design the well-dispersed nanorod arrays of CoMoO₄ as a host on activated iron foam (IF) to realize the optimal redeposition of FeOOH, constructing a self-sacrificial template rich in the FeOOH surface. Notably, at long-time oxidation potential, the precatalyst FeOOH-CoMoO₄ can realize MoO₄^2- dissolution and redeposition of Co oxyhydroxides on FeOOH host simultaneously, constructing a dynamically stable Fe(Co)OOH interface. The introduction of CoOOH improves conductivity and provides synergistic effect with FeOOH to lower the energy barrier for OER and maintain long-time stability, eventually exhibiting a low overpotential of 298 mV to reach the current density of 100 mA cm^-2 and high stability over 60 h. This work demonstrates the feasibility of manipulating metal dissolution-redeposition process for a dynamically stable interface.

Heterogeneous Ni3S2@FeNi2S4@NF nanosheet arrays directly used as high efficiency bifunctional electrocatalyst for water decomposition

Developing and designing bifunctional electrocatalysts are very important for the production of hydrogen from water electrolysis. The reasonable interface modulation can effectively lead to the optimization of electronic configuration through the interface electron transfer in the heterostructures and thus resulting in the enhanced efficiency. In this work, self-supported and heterogeneous interface-rich Ni₃S₂@FeNi₂S₄@NF electrocatalyst for overall water splitting was designed and prepared through a controllable step-wise hydrothermal process. Density functional theory calculations suggest that heterogeneous interface formed between Ni₃S₂ and FeNi₂S₄ can optimize the Gibbs free energy for H* adsorption (ΔG_H*). Benefiting from the open structure of the nanosheet arrays, the abundant heterogeneous interfaces in Ni₃S₂@FeNi₂S₄@NF composite, the positive synergistic effect between Ni₃S₂ and FeNi₂S₄, and the good conductivity of foamed nickel (NF) substrate, the optimized Ni₃S₂@FeNi₂S₄@NF nanoarray catalyst displayed excellent electrocatalytic activities, the overpotential is only 83 mV and 235 mV for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) at 10 mA cm^-2, respectively. Importantly, an alkaline electrolyser directly using the Ni₃S₂@FeNi₂S₄@NF as both the anode and cathode achieved an ultralow cell voltage of 1.46 V, accompanied by outstanding stability. The performance is better than that of most other transition-metal sulfides electrocatalysts. This work may provide a useful strategy for reasonably regulating heterogeneous interfaces to effectively improve the performance of materials, thus accelerating the practical application of transition-metal sulfides electrocatalysts for overall water splitting.

Iron Doped in the Subsurface of CuS Nanosheets by Interionic Redox: Highly Efficient Electrocatalysts toward the Oxygen Evolution Reaction

Modifying the electronic structure of electrocatalysts by metal doping is favorable to their electrocatalytic activity. Herein, by a facile one-pot redox process of Fe(III) and Cu(I), Fe(II) was successfully doped into the subsurface of CuS nanosheets (NSs) for the first time to obtain a novel electrocatalyst (Fe_sub-CuS NSs) that possesses not only subtle lattice defects but also an atomic-level coupled nanointerface, greatly enhancing the oxygen evolution reaction (OER) performances. Meanwhile, Fe(II) and Fe(III) coexisting in Fe_sub-CuS nanosheets are favorable to OER through valence regulation. As expected, by simultaneously controlling the abovementioned three factors to optimize Fe_sub-CuS nanosheets, they display a lower overpotential of 252 mV at a current density of 20 mA cm^-2 for OER, better than 389 mV for pristine CuS nanosheets. This discovery furnishes low-cost and efficient Cu-based electrocatalysts by metal doping. Density functional theory (DFT) calculations further verify that Fe_sub-CuS(100) is thermodynamically stable and is more active for OER. This research provides a strategy for the atomic-scale engineering of nanocatalysts and also sheds light on the design of novel and efficient electrocatalysts.

Self-Anti-Stacking 2D Metal Phosphide Loop-Sheet Heterostructures by Edge-Topological Regulation for Highly Efficient Water Oxidation

2D metal phosphide loop-sheet heterostructures are controllably synthesized by edge-topological regulation, where Ni₂ P nanosheets are edge-confined by the N-doped carbon loop, containing ultrafine NiFeP nanocrystals (denoted as NiFeP@NC/Ni₂ P). This loop-sheet feature with lifted-edges prevents the stacking of nanosheets and induces accessible open channels for catalytic site exposure and gas bubble release. Importantly, these NiFeP@NC/Ni₂ P hybrids exhibit a remarkable oxygen evolution activity with an overpotential of 223 mV at 20 mA cm^-2 and a Tafel slope of 46.1 mV dec^-1 , constituting the record-high performance among reported metal phosphide electrocatalysts. The NiFeP@NC/Ni₂ P hybrids are also employed as both anode and cathode to achieve an alkaline electrolyzer for overall water splitting, delivering a current density of 10 mA cm^-2 with a voltage of 1.57 V, comparable to that of the commercial Pt/C||RuO₂ couple (1.56 V). Moreover, a photovoltaic-electrolysis coupling system can as well be effectively established for robust overall water splitting. Evidently, this ingenious protocol would expand the toolbox for designing efficient 2D nanomaterials for practical applications.

(Ni,Co)Se@Ni(OH)2 heterojunction nanosheets as an efficient electrocatalyst for the hydrogen evolution reaction

A heterogeneous structure formed by coupling two or more phases can reinforce the activity of active sites and expedite electron transfer, which is conducive to boosting its electrocatalytic activity. Herein, we designed nickel foam supported (NiCo₂)Se@Ni(OH)₂ (NCS@NH) heterojunction nanosheets by a two-step method. First of all, the NiCo₂S₄@Ni(OH)₂ (NiCo₂S₄@NH) nanosheets coated on nickel foam were acquired via a hydrothermal method. In the selenization treatment that followed, NiCo₂S₄@NH was converted into NCS@NH heterogeneous nanosheets in which the selenide nanoparticles decorated on the surface of the Ni(OH)₂ nanosheets formed heterojunction interfaces, and the heterogeneous structure could accelerate electron transfer, thus improving the catalytic activity. The Ni(OH)₂ nanosheets can adequately contact the electrolyte and promote the decomposition of water. Meanwhile, the thickness of the Ni(OH)₂ nanosheets gradually decreases with the increase of Co doping (1.5-2.5 mmol), consequently affecting the HER properties. Notably, when the amount of Co salt added is 2 mmol, NCS@NH exhibited superior HER properties (with a voltage of 253 mV at 100 mA cm^-2) and excellent stability for 24 h.

Sulfur doped ruthenium nanoparticles as a highly efficient electrocatalyst for the hydrogen evolution reaction in alkaline media

Sulfur-doped ruthenium ultrafine nanoparticles is obtained via a simple solvothermal procedure, which shows excellent hydrogen evolution performance in alkaline media.

Metal–organic frameworks and their derivatives as electrocatalysts for the oxygen evolution reaction

This review summarizes the recent progress on MOFs and their derivatives used for OER electrocatalysis in terms of their morphology, composition and structure–performance relationship.

Regulating Crystal Structure and Atomic Arrangement in Single-Component Metal Oxides through Electrochemical Conversion for Efficient Overall Water Splitting

Single-component transition-metal oxide (TMO: FeO_x, NiO_x, or CoO_x) nanosheets grown on nickel foam (NF) were electrochemically optimized with Li ion (Na ion)-induced conversion reaction for bifunctional electrocatalysis. The optimum FeO_x/NF-Li electrocatalyst exhibits low overpotentials of 239 mV for hydrogen evolution reaction and 276 mV for oxygen evolution reaction at a current density of 100 mA cm^-2. A two-electrode water splitting cell using FeO_x/NF-Li as both anode and cathode requires only 1.60 V to achieve a current density of 10 mA cm^-2. The impressive water splitting performance of the FeO_x/NF-Li electrode is revealed to be attributed to Li-induced electrochemical conversion, which alters the crystal structure, creating more active sites for electrocatalytic reactions, as well as introduces O vacancies increasing the electron density and the intrinsic conductivity. More importantly, the atomic arrangement is regulated from tetrahedral Fe(Td) to octahedral Fe(Oh) coordination, which acts as catalytically active sites with reduced Gibbs free energy for the rate-determining steps. This electrochemical conversion reaction can be extended to other TMOs (i.e., NiO_x/NF and CoO_x/NF) for promoted electrocatalytic water splitting performances. This study provides an in-depth understanding on the nature of atomic and electronic structure evolution to promote the electrocatalytic activity.

Biodeposited Nano-CdS Drives the In Situ Growth of Highly Dispersed Sulfide Nanoparticles during Pyrolysis for Enhanced Oxygen Evolution Reaction

A novel, efficient, and stable graphene-based composite oxygen evolution reaction (OER) catalyst, BG@Ni/Ni₃S₂, was designed via high-specificity, low-cost biosynthesis and efficient electrostatic self-assembly. In the synthetic process, bacterial cells containing biodeposited CdS nanocrystals, graphene oxide (GO), and Ni²⁺ ions are assembled into a sandwich-type hybrid precursor. The nanosized sulfur source drives in situ sulfidation during pyrolysis, which induces the uniform formation and growth of Ni/Ni₃S₂ composite nanoparticles (NPs) on the graphene substrate. Benefiting from the high specific surface area and uniform distribution of NPs, the catalyst has a large number of exposed active sites and exhibits rapid mass transfer. In addition, the skeleton composed of a conductive carbon matrix and metallic Ni-Ni network ensures the excellent electron transfer during the OER, and the synergistic effect of Ni⁰ and Ni₃S₂ further optimizes the electronic structure and accelerates the OER kinetics. The dominant catalytic sites at the nanointerface between Ni⁰ and Ni₃S₂ provide favorable thermodynamic conditions for the adsorption of OER intermediates. As a result, BG@Ni/Ni₃S₂ exhibits efficient catalytic performance for the OER: the overpotential and Tafel slope are only 320 mV at 100 mA cm^-2 and 41 mV dec^-1, respectively. This work provides a novel understanding of the intrinsic activity of transition metal sulfide composites and a biological-based design for OER catalysts.

Trimetallic MOF-74 Films Grown on Ni Foam as Bifunctional Electrocatalysts for Overall Water Splitting

Developing earth-abundant and high-performance electrocatalysts for water splitting has long been a vital research in energy conversion field. Herein, we report the preparation of a series of trimetallic uniform Mn_x Fe_y Ni-MOF-74 films in-situ grown on nickel foam, which can be utilized as bifunctional electrocatalysts towards overall water splitting in alkaline media. The introduction of Mn can simultaneously regulate the morphology of MOF-74 to form uniform film and modulate electronic structure of Fe to form more Fe(II)-O-Fe(III) motifs, which is conducive to the exposure of active sites and stabilizing high-valent metal sites. The optimized Mn_0.52 Fe_0.71 Ni-MOF-74 film electrode showed excellent electrocatalytic performance, affording a current density of 10 mA ⋅ cm^-2 at an overpotential of 99 mV for HER and 100 mA ⋅ cm^-2 at an overpotential of 267 mV for OER, respectively. Assembled as an electrolyser, Mn_0.52 Fe_0.71 Ni-MOF-74 film electrode exhibited excellent performance towards overall water splitting, with ultralow overpotential of 245 and 462 mV to achieve current density of 10 and 100 mA ⋅ cm^-2 , respectively. This work provides a new view to develop multi-metal MOF-based electrocatalysts.

Bifunctional Single Atom Electrocatalysts: Coordination-Performance Correlations and Reaction Pathways

Single atom catalysts (SACs) are ideal model systems in catalysis research. Here we employ SACs to address the fundamental catalytic challenge of generating well-defined active metal centers to elucidate their interactions with coordinating atoms, which define their catalytic performance. We introduce a soft-landing molecular strategy for tailored SACs based on metal phthalocyanines (MPcs, M = Ni, Co, Fe) on graphene oxide (GO) layers to generate well-defined model targets for mechanistic studies. The formation of electronic channels through π-π conjugation with the graphene sheets enhances the MPc-GO performance in both oxygen evolution and reduction reactions (OER and ORR). Density functional theory (DFT) calculations unravel that the outstanding ORR activity of FePc-GO among the series is due to the high affinity of Fe atoms toward O₂ species. Operando X-ray absorption spectroscopy and DFT studies demonstrate that the OER performance of the catalysts relates to thermodynamic or kinetic control at low- or high-potential ranges, respectively. We furthermore provide evidence that the participation of ligating N and C atoms around the metal centers provides a wider selection of active OER sites for both NiPc-GO and CoPc-GO. Our strategy promotes the understanding of coordination-activity relationships of high-performance SACs and their optimization for different processes through tailored combinations of metal centers and suitable ligand environments.