Artificial Heterointerfaces Achieve Delicate Reaction Kinetics towards Hydrogen Evolution and Hydrazine Oxidation Catalysis

Molybdenum iron carbide-copper hybrid as efficient electrooxidation catalyst for oxygen evolution reaction and synthesis of cinnamaldehyde/benzalacetone

Oxygen evolution reaction (OER) is the efficiency limiting half-reaction in water electrolysis for green hydrogen production due to the 4-electron multistep process with sluggish kinetics. The electrooxidation of thermodynamically more favorable organics accompanied by CC coupling is a promising way to synthesize value-added chemicals instead of OER. Efficient catalyst is of paramount importance to fulfill such a goal. Herein, a molybdenum iron carbide-copper hybrid (Mo2C-FeCu) was designed as anodic catalyst, which demonstrated decent OER catalytic capability with low overpotential of 238 mV at response current density of 10 mA cm-2 and fine stability. More importantly, the Mo2C-FeCu enabled electrooxidation assisted aldol condensation of phenylcarbinol with α-H containing alcohol/ketone in weak alkali electrolyte to selective synthesize cinnamaldehyde/benzalacetone at reduced potential. The hydroxyl and superoxide intermediate radicals generated at high potential are deemed to be responsible for the electrooxidation of phenylcarbinol and aldol condensation reactions to afford cinnamaldehyde/benzalacetone. The current work showcases an electrochemical-chemical combined CC coupling reaction to prepare organic chemicals, we believe more widespread organics can be synthesized by tailored electrochemical reactions.

Interface Engineering of VOx/Ni/Ni3N Heterostructures for Electrochemical Urea-Assisted Hydrogen Production

Electrocatalytic hydrogen generation driven by renewable energy sources is severely limited by the slow oxygen evolution reaction (OER). Urea-assisted alkaline hydrogen production offers a perspective approach. However, the construction of efficient and robust anode catalysts is still challenging. Herein, an amorphous/crystalline VOx/Ni/Ni3N-heterostructured catalyst grown on carbon cloth was synthesized and used as a bifunctional electrocatalyst for the hydrogen evolution reaction (HER) and urea electrooxidation reaction (UOR). Benefiting from the electronic modification of intercomponents and abundant active sites, VOx/Ni/Ni3N exhibits an excellent electrochemical performance toward the HER and UOR. Theoretical calculations confirmed that the crystalline/amorphous VOx/Ni/Ni3N heterostructure has a suitable water dissociation energy and H* adsorption energy, thereby promoting the HER process. When the UOR and HER are integrated into an electrolytic device, VOx/Ni/Ni3N requires a potential of 1.40 V to achieve a current density of 10 mA cm-2.

Architecting Ni3Se4-NiSe2-Co3O4 Triple-Interface Heterostructure on MXene Nanosheets for Boosting Water Splitting by Electronic Modulation and Interface Effects

Strategically engineering electrocatalysts with optimized interfacial electronic architectures and accelerated reaction dynamics is pivotal for augmenting hydrogen generation via alkaline water electrolysis on an industrial scale. Herein, a novel triple-interface heterostructure Ni3Se4-NiSe2-Co3O4 nanoarrays are designed anchored on Ti3C2Tx MXene (Ni3Se4-NiSe2-Co3O4/MXene) with significant work function difference (ΔΦ) as bifunctional electrocatalysts for water electrolysis. Theoretical calculations combined with experiments uncover the pivotal role of the interface-induced electric field in steering charge redistribution, which in turn modulates the adsorption and desorption kinetics of reaction intermediates. Furthermore, the synergistic interaction between Ni3Se4-NiSe2-Co3O4 and Ti3C2Tx MXene nanosheets endows the hybrids with a large electrochemical surface area, abundantly active sites, and high conductivity. Thus, Ni3Se4-NiSe2-Co3O4/MXene manifests exceptional catalytic prowess for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). In addition, the Ni3Se4-NiSe2-Co3O4/MXene electrocatalyst in the water electrolyzer delivers excellent performance and maintains commendable stability beyond 100 h of electrocatalytic operation.

Simultaneous Interface Engineering and Phase Tuning of CeO2-Decorated Catalysts for Boosted Oxygen Evolution Reaction

Heterogeneous catalysts have attracted extensive attention among various emerging catalysts for their exceptional oxygen evolution reaction (OER) capabilities, outperforming their single-component counterparts. Nonetheless, the synthesis of heterogeneous materials with predictable, precise, and facile control remains a formidable challenge. Herein, a novel strategy involving the decoration of catalysts with CeO2 is introduced to concurrently engineer heterogeneous interfaces and adjust phase composition, thereby enhancing OER performance. Theoretical calculations suggest that the presence of ceria reduces the free energy barrier for the conversion of nitrides into metals. Supporting this, the experimental findings reveal that the incorporation of rare earth oxides enables the controlled phase transition from nitride into metal, with the proportion adjustable by varying the amount of added rare earth. Thanks to the role of CeO2 decoration in promoting the reaction kinetics and fostering the formation of the genuine active phase, the optimized Ni3FeN/Ni3Fe/CeO2-5% nanoparticles heterostructure catalyst exhibits outstanding OER activity, achieving an overpotential of just 249 mV at 10 mA cm-2. This approach offers fresh perspectives for the conception of highly efficient heterogeneous OER catalysts, contributing a strategic avenue for advanced catalytic design in the field of energy conversion.

Ruthenium Nanoclusters and Single Atoms on α-MoC/N-Doped Carbon Achieves Low-Input/Input-Free Hydrogen Evolution via Decoupled/Coupled Hydrazine Oxidation

The hydrazine oxidation-assisted H2 evolution method promises low-input and input-free hydrogen production. However, developing high-performance catalysts for hydrazine oxidation (HzOR) and hydrogen evolution (HER) is challenging. Here, we introduce a bifunctional electrocatalyst α-MoC/N-C/RuNSA, merging ruthenium (Ru) nanoclusters (NCs) and single atoms (SA) into cubic α-MoC nanoparticles-decorated N-doped carbon (α-MoC/N-C) nanowires, through electrodeposition. The composite showcases exceptional activity for both HzOR and HER, requiring -80 mV and -9 mV respectively to reach 10 mA cm-2. Theoretical and experimental insights confirm the importance of two Ru species for bifunctionality: NCs enhance the conductivity, and its coexistence with SA balances the H ad/desorption for HER and facilitates the initial dehydrogenation during the HzOR. In the overall hydrazine splitting (OHzS) system, α-MoC/N-C/RuNSA excels as both anode and cathode materials, achieving 10 mA cm-2 at just 64 mV. The zinc hydrazine (Zn-Hz) battery assembled with α-MoC/N-C/RuNSA cathode and Zn foil anode can exhibit 97.3 % energy efficiency, as well as temporary separation of hydrogen gas during the discharge process. Therefore, integrating Zn-Hz with OHzS system enables self-powered H2 evolution, even in hydrazine sewage. Overall, the amalgamation of NCs with SA achieves diverse catalytic activities for yielding multifold hydrogen gas through advanced cell-integrated-electrolyzer system.

Taking Advantage of Potential Coincidence Region: Insights into Gas Production Behavior in Advanced Self-Activated Hydrazine-Assisted Alkaline Seawater Electrolysis

Water electrolysis assisted by hydrazine has emerged as a prospective energy conversion method for achieving efficient hydrogen generation. Due to the potential coincidence region (PCR) between the hydrogen evolution reaction (HER) and the electro-oxidation of hydrazine, the hydrazine oxidation reaction (HzOR) offers distinct advantages in terms of strategy amalgamation, device architecture, and the broadening of application horizons. Herein, we report a bifunctional electrocatalyst of interfacial heterogeneous Fe2P/Co2P microspheres supported on Ni foam (FeCoP/NF). Benefiting from the strong interfacial coupling effect between Fe2P and Co2P and the three-dimensional microsphere structure, FeCoP/NF exhibits outstanding bifunctional electrocatalytic performance, achieving 10 mA cm-2 with low overpotentials of 10 and 203 mV for HER and HzOR, respectively. Utilizing FeCoP/NF for both electrodes in HzOR-assisted water electrolysis results in significantly reduced potentials of 820 mV for 1 A cm-2 in contrast to the electro-oxidation of alternative chemical substrates. The presence of a potential coincidence region makes the application of self-activated seawater electrolysis realistic. The gas production behavior at different current densities in this interesting hydrogen production system is discussed, and some rules that are distinguished from conventional water electrolysis are summarized. Furthermore, a new self-powered hydrogen production system with a direct hydrazine fuel cell, rechargeable Zn-hydrazine battery, and hydrazine-assisted seawater electrolysis is proposed, emphasizing the distinct benefits of HzOR and its potential role in electrochemical energy conversion technologies powered by renewable sources.

Activating Interfacial Electron Redistribution in Lattice-Matched Biphasic Ni3N-Co3N for Energy-Efficient Electrocatalytic Hydrogen Production via Coupled Hydrazine Degradation

The development of high-purity and high-energy-density green hydrogen through water electrolysis holds immense promise, but issues such as electrocatalyst costs and power consumption have hampered its practical application. In this study, we present a promising solution to these challenges through the use of a high-performance bifunctional electrocatalyst for energy-efficient hydrogen production via coupled hydrazine degradation. The biphasic metal nitrides with highly lattice-matched structures are deliberately constructed, forming an enhanced local electric field between the electron-rich Ni3N and electron-deficient Co3N. Additionally, Mn is introduced as an electric field engine to further activate electron redistribution. Our Mn@Ni3N-Co3N/NF bifunctional electrocatalyst achieves industrial-grade current densities of 500 mA cm-2 at 0.49 V without degradation, saving at least 53.3 % energy consumption compared to conventional alkaline water electrolysis. This work will stimulate the further development of metal nitride electrocatalysts and also provide new perspectives on low-cost hydrogen production and environmental protection.

Regulation of Sulfur Atoms in MoSx by Magneto-Electrodeposition for Hydrogen Evolution Reaction

Compared with crystalline molybdenum sulfide (MoS2) employed as an efficient hydrogen evolution reaction (HER) catalyst, amorphous MoSx exhibits better activity. To synthesize amorphous MoSx, electrodeposition serving as a convenient and time-saving method is successfully applied. However, the loading mass is hindered by limited mass transfer efficiency and the available active sites require further improvement. Herein, magneto-electrodeposition is developed to synthesize MoSx with magnetic fields up to 9 T to investigate the effects of a magnetic field in the electrodeposition processing, as well as the induced electrochemical performance. Owing to the magneto-hydrodynamic effect, the loading mass of MoSx is obviously increased, and the terminal S2- serving as the active site is enhanced. The optimized MoSx catalyst delivers outstanding HER performance, achieving an overpotential of 50 mV at a current density of 10 mA cm-2 and the corresponding Tafel slope of 59 mV dec-1. The introduction of a magnetic field during the electrodeposition process will provide a novel route to prepare amorphous MoSx with improved electrochemical performance.

Bifunctional Electrocatalysts for Overall and Hybrid Water Splitting

Electrocatalytic water splitting driven by renewable electricity has been recognized as a promising approach for green hydrogen production. Different from conventional strategies in developing electrocatalysts for the two half-reactions of water splitting (e.g., the hydrogen and oxygen evolution reactions, HER and OER) separately, there has been a growing interest in designing and developing bifunctional electrocatalysts, which are able to catalyze both the HER and OER. In addition, considering the high overpotentials required for OER while limited value of the produced oxygen, there is another rapidly growing interest in exploring alternative oxidation reactions to replace OER for hybrid water splitting toward energy-efficient hydrogen generation. This Review begins with an introduction on the fundamental aspects of water splitting, followed by a thorough discussion on various physicochemical characterization techniques that are frequently employed in probing the active sites, with an emphasis on the reconstruction of bifunctional electrocatalysts during redox electrolysis. The design, synthesis, and performance of diverse bifunctional electrocatalysts based on noble metals, nonprecious metals, and metal-free nanocarbons, for overall water splitting in acidic and alkaline electrolytes, are thoroughly summarized and compared. Next, their application toward hybrid water splitting is also presented, wherein the alternative anodic reactions include sacrificing agents oxidation, pollutants oxidative degradation, and organics oxidative upgrading. Finally, a concise statement on the current challenges and future opportunities of bifunctional electrocatalysts for both overall and hybrid water splitting is presented in the hope of guiding future endeavors in the quest for energy-efficient and sustainable green hydrogen production.

Regulating Local Atomic Environment around Vacancies for Efficient Hydrogen Evolution

Defect engineering is essential for the development of efficient electrocatalysts at the atomic level. While most work has focused on various vacancies as effective catalytic modulators, little attention has been paid to the relation between the local atomic environment of vacancies and catalytic activities. To face this challenge, we report a facile synthetic approach to manipulate the local atomic environments of vacancies in MoS2 with tunable Mo-to-S ratios. Our studies indicate that the MoS2 with more Mo terminated vacancies exhibits better hydrogen evolution reaction (HER) performance than MoS2 with S terminated vacancies and defect-free MoS2. The improved performance originates from the adjustable orbital orientation and distribution, which is beneficial for regulating H adsorption and eventually boosting the intrinsic per-site activity. This work uncovers the underlying essence of the local atomic environment of vacancies on catalysis and provides a significant extension of defect engineering for the rational design of transition metal dichalcogenides (TMDs) catalysts and beyond.

NiFeP nanosheets for efficient and durable hydrazine-assisted electrolytic hydrogen production

Hydrazine-assisted electrochemical water splitting is an important avenue toward low cost and sustainable hydrogen production, which can significantly reduce the voltage of electrochemical water splitting. Herein, we took a simple approach to fabricate NiFeP nanosheet arrays on nickel foam (NiFeP/NF), which exhibit superior electrocatalytic activity for the hydrogen evolution reaction (HER) and the hydrazine oxidation reaction (HzOR). Our investigations revealed that the excellent electrocatalytic activity of NiFeP/NF mainly arises from the bimetallic synergistic effect, abundant electrocatalytically active sites facilitated by the porous nanosheet morphology, high intrinsic conductivity of NiFeP/NF and strong NiFeP-NF adhesion. We assembled a hydrazine-boosted electrochemical water splitting cell using NiFeP/NF as a bifunctional catalyst for both electrodes, and the overall hydrazine splitting (OHzS) exhibits a considerably low overpotential (100 mV at 10 mA cm-2), and is stable for 40 h continuous electrolysis in a 1 M KOH + 0.5 M N2H4 electrolyte. When it is applied to hydrogen production by seawater electrolysis, its catalytic activity shows strong tolerance. This work provides a promising approach for low cost, high-efficiency and stable hydrogen production based on hydrazine-assisted electrolytic seawater splitting for future applications.

1D/3D Heterogeneous Assembling Body of Cobalt Nitrides for Highly Efficient Overall Hydrazine Splitting and Supercapacitors

Herein, the construction of a heterostructured 1D/3D CoN-Co2 N@NF (nickel foam) electrode used for thermodynamically favorable hydrazine oxidation reaction (HzOR), as an alternative to sluggish anodic oxygen evolution reaction (OER) in water splitting for hydrogen production, is reported. The electrode exhibits remarkable catalytic activities, with an onset potential of -0.11 V in HzOR and -71 mV for a current density of 10 mA cm-2 in hydrogen evolution reaction (HER). Consequently, an extraordinary low cell voltage of 53 mV is required to achieve 10 mA cm-2 for overall hydrazine splitting in a two-electrode system, demonstrating significant energy-saving advantages over conventional water splitting. The HzOR proceeds through the 4e- reaction pathway to release N2 while the 1e- pathway to emit NH3 is uncompetitive, as evidenced by differential electrochemical mass spectrometric measurements. The X-ray absorption spectroscopy, in situ Raman spectroscopy, and theoretical calculations identify cobalt nitrides rather than corresponding oxides/(oxy)hydroxides as catalytic species for HzOR and illustrate advantages of heterostructured CoN-Co2 N in optimizing adsorption energies of intermediates/reagents and promoting catalytic activities toward both HzOR and HER. The CoN-Co2 N@NF is also an excellent supercapacitive material, exhibiting an increased specific capacity (938 F g-1 at 1 A g-1 ) with excellent cycling stability (95.8%, 5000 cycles).

Copious Dislocations Defect in Amorphous/Crystalline/Amorphous Sandwiched Structure P-NiMoO4 Electrocatalyst toward Enhanced Hydrogen Evolution Reaction

The design and synthesis of efficient, inexpensive, and long-term stable heterostructured electrocatalysts with high-density dislocations for hydrogen evolution reaction in alkaline media and seawater are still a great challenge. An amorphous/crystalline/amorphous sandwiched structure with abundant dislocations were synthesized through thermal phosphidation strategies. The dislocations play an important role in the hydrogen evolution reactions. Copious dislocation defects, combined with cracks, and the synergistic interfacial effect between crystalline phase and amorphous phase regulate the electronic structure of electrocatalyst, provide more active sites, and thus endow the electrocatalysts with excellent catalytic activity under alkaline water and seawater. The overpotentials of P-NiMoO4 at 10 mA/cm2 in 1 M KOH aqueous solution and seawater are 45 and 75 mV, respectively. Additionally, the P-NiMoO4 electrocatalyst exhibits long-term stability over 100 h. This study provides a simple approach for synthesizing amorphous/crystalline/amorphous sandwiched non-noble-metal electrocatalysts with abundant dislocations for hydrogen evolution reaction.

Incorporating a built-in electric field into a NiFe LDH heterojunction for enhanced oxygen evolution and urea oxidation

Herein, a N-doped carbon-supported Co and NiFe LDH (Co-NC@NiFe LDH) array was developed, which demonstrated superior catalytic activities for both the OER and UOR in an alkaline medium. The intrinsic electron transfer is effectively regulated by the construction of a built-in electric field, which reduces the reaction energy barrier and consequently leads to a significant enhancement in electrocatalytic activity.

Recent Advancements in Electrochemical Hydrogen Production via Hybrid Water Splitting

As one of the most promising approaches to producing high-purity hydrogen (H2 ), electrochemical water splitting powered by the renewable energy sources such as solar, wind, and hydroelectric power has attracted considerable interest over the past decade. However, the water electrolysis process is seriously hampered by the sluggish electrode reaction kinetics, especially the four-electron oxygen evolution reaction at the anode side, which induces a high reaction overpotential. Currently, the emerging hybrid electrochemical water splitting strategy is proposed by integrating thermodynamically favorable electro-oxidation reactions with hydrogen evolution reaction at the cathode, providing a new opportunity for energy-efficient H2 production. To achieve highly efficient and cost-effective hybrid water splitting toward large-scale practical H2 production, much work has been continuously done to exploit the alternative anodic oxidation reactions and cutting-edge electrocatalysts. This review will focus on recent developments on electrochemical H2 production coupled with alternative oxidation reactions, including the choice of anodic substrates, the investigation on electrocatalytic materials, and the deep understanding of the underlying reaction mechanisms. Finally, some insights into the scientific challenges now standing in the way of future advancement of the hybrid water electrolysis technique are shared, in the hope of inspiring further innovative efforts in this rapidly growing field.

Self-Powered Hydrogen Production with Improved Energy Efficiency via Polysulfides Redox

In the pursuit of efficient solar-driven electrocatalytic water splitting for hydrogen production, the intrinsic challenges posed by the sluggish kinetics of anodic oxygen evolution and intermittent sunlight have prompted the need for innovative energy systems. Here, we introduce an approach by coupling the polysulfides oxidation reaction with the hydrogen evolution reaction for energy-saving H2 production, which could be powered by an aqueous zinc-polysulfides battery to construct a self-powered energy system. This unusual hybrid water electrolyzer achieves 300 mA cm-2 at a low cell voltage of 1.14 V, saving electricity consumption by 100.4% from 5.47 to 2.73 kWh per m3 H2 compared to traditional overall water splitting. Benefiting from the favorable reaction kinetics of polysulfides oxidation/reduction, the aqueous zinc-polysulfides battery exhibits an energy efficiency of approximately 89% at 1.0 mA cm-2. Specially, the zinc-polysulfide battery effectively stores intermittent solar energy as chemical energy during light reaction by solar cells. Under an unassisted light reaction, the batteries could release energy to drive H2 production through a hybrid water electrolyzer for uninterrupted hydrogen production. Therefore, the aim of simultaneously generating H2 and eliminating the restrictions of intermittent sunlight is realized by combining the merits of polysulfides redox, an aqueous metal-polysulfide battery, and solar cells. We believe that this concept and utilization of polysulfides redox will inspire further fascinating attempts for the development of sustainable energy via electrocatalytic reactions.

Constructing Fully-Active and Ultra-Active Sites in High-Entropy Alloy Nanoclusters for Hydrazine Oxidation-Assisted Electrolytic Hydrogen Production

The development of sufficiently high-efficiency systems and effective catalysts for electrocatalytic hydrogen production is of great significance but challenging. Here, high-entropy alloy nanoclusters (HEANCs) with full-active sites and super-active sites are innovatively constructed for hydrazine oxidation-assisted electrolytic hydrogen production. The HEANCs show an average size of only seven atomic layers (1.48 nm). As the catalysts for both hydrogen evolution reaction (HER) and hydrazine oxidation reaction, the HEANC/C exhibits the best-level performance among reported electrocatalysts. Especially, the HEANC/C achieves an ultrahigh mass activity of 12.85 A mg-1 noble metals at -0.07 V and overpotential of only 9.5 mV for 10 mA cm-2 for alkaline HER. Further, with HEANC/C as both anode and cathode catalysts, an overall hydrazine oxidation-assisted splitting (OHzS) electrolyzer shows a record mass activity of 250.2 mA mg-1 catalysts at 0.1 V and only requires working voltages of 0.025 and 0.181 V to reach 10 and 100 mA cm-2 , respectively, outperforming those of overall water-splitting system and other reported chemicals-assisted hydrogen production systems. Active site libraries including 72 sites on HEANC surface are originally constructed by theoretical calculations, revealing that all sites on HEANC surface are effective active sites for OHzS; especially some are super-active sites, endowing the best-level performance of HEANC/C.

Highly enhanced hydrazine oxidation on bifunctional Ni tailored by alloying for energy-efficient hydrogen production

The low-potential hydrazine oxidation reaction (HzOR) can replace the oxygen evolution reaction (OER) and thus assemble with the hydrogen evolution reaction (HER), consequently achieving energy-saving hydrogen (H2) production. Notably, developing sophisticated bifunctional electrocatalysts for HER and HzOR is a prerequisite for efficient H2 production. Alloying noble metals with eligible non-precious ones can increase affordability, catalytic activity, and stability, alongside rendering bifunctionality. Herein, RuNi alloy deposited onto carbon (RuNi/C) was directly prepared by a simple and highly practical co-reduction method, showing excellent performance for HER and HzOR. Interestingly, to achieve 10 mA cm-2, RuNi/C only required an ultralow potential of 24 mV for HER, on par with commercial 20 wt% platinum in carbon (Pt/C), and -65 mV for HzOR, surpassing most reported counterparts. Moreover, the two-electrode electrolyzer only required small operation voltages of 57.8 and 327 mV to drive 10 and 100 mA cm-2, respectively. Driven by a homemade hydrazine (N2H4) fuel cell and solar panel, appreciable H2 yields of 1.027 and 1.406 mmol h-1 were achieved, respectively, exhibiting the energy-saving advantages alongside robust practicability. Moreover, theoretical calculations revealed that alloying with Ru endows bifunctional Ni sites not only with a lower H2O dissociation barrier but also with more favorable H* adsorption alongside the reduced energy barrier between HzOR intermediates.

Iron-optimized oxygen vacancy concentration to strengthen the electrocatalytic ability of the urea oxidation reaction

Iron-modified Ni(OH)2/NiSe2 enhances oxygen vacancies, expanding the electrochemically active surface area, which exhibiting superior selectivity and stability in urea oxidation reaction, outperforming pristine Ni(OH)2@NiSe2. It also demonstrates superior catalytic performance in the oxidation reactions of other small molecules.

Hydrazine-Assisted Acidic Water Splitting Driven by Iridium Single Atoms

Water splitting, an efficient technology to produce purified hydrogen, normally requires high cell voltage (>1.5 V), which restricts the application of single atoms electrocatalyst in water oxidation due to the inferior stability, especially in acidic environment. Substitution of anodic oxygen evolution reaction (OER) with hydrazine oxidation reaction (HzOR) effectually reduces the overall voltage. In this work, the utilization of iridium single atom (Ir-SA/NC) as robust hydrogen evolution reaction (HER) and HzOR electrocatalyst in 0.5 m H2 SO4 electrolyte is reported. Mass activity of Ir-SA/NC is as high as 37.02 A mgIr -1 at overpotential of 50 mV in HER catalysis, boosted by 127-time than Pt/C. Besides, Ir-SA/NC requires only 0.39 V versus RHE to attain 10 mA cm-2 in HzOR catalysis, dramatically lower than OER (1.5 V versus RHE); importantly, a superior stability is achieved in HzOR. Moreover, the mass activity at 0.5 V versus RHE is enhanced by 83-fold than Pt/C. The in situ Raman spectroscopy investigation suggests the HzOR pathway follows *N2 H4 →*2NH2 →*2NH→2N→*N2 →N2 for Ir-SA/NC. The hydrazine assisted water splitting demands only 0.39 V to drive, 1.25 V lower than acidic water splitting.

Porous, Ultrathin PtAgBiTe Nanosheets for Direct Hydrazine Hydrate Fuel Cell Devices

Ultrathin 2D nanomaterials have attracted extensive attention due to their fascinating applications in sustainable and clean-energy-related devices, but obtaining ultrathin 2D multimetallic polycrystalline structures with large lateral dimensions remains a challenge. In this study, ultrathin 2D porous PtAgBiTe and PtBiTe polycrystalline nanosheets (PNSs) are obtained via a visible-light-photoinduced Bi2 Te3 -nanosheet-mediated route. The PtAgBiTe PNSs are assembled by sub-5 nm grains with widths beyond 700 nm. Strain and ligand effects originating from the porous, curly polycrystalline structure endow the PtAgBiTe PNSs with robust hydrazine hydrate oxidation reaction activity. Theoretical research demonstrates that the modified Pt activates the N-H bonds in N2 H4 during the reaction, and strong hybridization between Pt-5d and N-2p facilitates dehydrogenation while reducing energy consumption. The peak power densities of the PtAgBiTe PNSs in actual hydrazine-O2 /air fuel cell devices are boosted to 532.9/315.9 mW cm-2 , while those of the commercial Pt/C are 394.7/157.9 mW cm-2 . This work provides a strategy not only for preparing ultrathin multimetallic PNSs but also for finding promising electrocatalysts for actual hydrazine fuel cells.

A Strongly Coupled Ag(S)@NiO/Nickel Foam Electrode Induced by Laser Direct Writing for Hydrogen Evolution at Ultrahigh Current Densities with Long-Term Durability

Highly active, durable, and cost-effective electrodes for hydrogen evolution reaction (HER) at ultrahigh current densities (≥1 A cm-2 ) are extremely demanded for industrial high-rate hydrogen production, but challenging. Here, a robust strongly coupled Ag(S)@NiO/nickel foam (NF) electrode is reported. Taking advantage of millisecond laser direct writing in liquid nitrogen technique, lattice-matched and coherent interfaces are formed between Ag nanoparticles with stacking faults (denoted by Ag(S)) and NiO nanosheets, leading to strong interfacial electronic coupling, not only promoting H2 O adsorption and dissociation on Ni2+ but also enhancing H* adsorption on intrinsically inactive but most electrically conductive Ag. Strong chemical bonding is established at NiO/NF interface, guaranteeing rapid electron transfer and excellent mechanical durability under high-rate hydrogen evolution. The physicochemically stable electrode achieves record-low alkaline HER overpotential of 167 and 180 mV at 1 and 1.5 A cm-2 , respectively, along with negligible activity decay after 120 h test at ≈1.5 A cm-2 , surpassing reported non-platinum group metal electrocatalysts.

Nitrogen doped CoP on ammoniated black phosphorus nanosheets enabling highly efficient hydrogen evolution electrocatalysis

Developing a rational and cost-effective approach for designing highly-efficient and sustainable electrocatalysts is essential for clean and renewable hydrogen energy. Herein, we report nitrogen-doped CoP on two-dimensional ammoniated black phosphorus (BP) nanosheets (N-CoP/NH2-BP) as novel and highly-active heterostructure electrocatalysts for the hydrogen evolution reaction (HER). Using the reactive defects on the BP nanosheets as the original sites under NH3 gas, N-doped CoP nanocrystals were grown on the surface of the BP nanosheets that were functionalized with NH2 groups at their edge. The N-CoP/NH2-BP heterostructure exhibits low overpotentials of 90 and 246 mV at 10 and 200 mA cm-2, respectively, in an alkaline electrolyte. The excellent HER activity should be attributed to the synergistic effect between N-doped CoP and NH2-functionalized BP, in which NH2-BP, with its high electron mobility and hydrophilicity, accelerates the charge transfer and offers more active sites, moreover, N-doped CoP modulates the electronic structure of CoP for enhanced HER activity. This work not only provides a novel and effective electrocatalyst, but also opens up a straightforward strategy for the design of phosphorene-based electrocatalysts for highly efficient hydrogen evolution and beyond.

Recent advances in direct seawater splitting for producing hydrogen

Hydrogen production from electrocatalytic water splitting driven by renewable energy sources provides a promising path for energy sustainability. The current water electrolysis technologies mainly use fresh water as feedstock, which will further aggravate the shortage of water resources in the world. Seawater has an innate advantage in large-scale electrolysis hydrogen production because of its abundant reserves. However, direct seawater electrolysis without any pre-treatment faces serious challenges due to the electrode side reactions and corrosion issues caused by the complex compositions of seawater. In this review, we first discuss the basic principles of seawater electrolysis. Second, the recent progress in designing efficient direct seawater electrolysis systems is discussed in detail, including catalyst design, electrolyser assembly, membrane regulation, and electrolyte engineering. In addition, the challenges and future opportunities are highlighted for the development of seawater splitting technologies toward large-scale hydrogen production.

Microwave-Assisted Rational Designed CNT-Mn3 O4 /CoWO4 Hybrid Nanocomposites for High Performance Battery-Supercapacitor Hybrid Device

Extensive research interest in hybrid battery-supercapacitor (BSH) devices have led to the development of cathode materials with excellent comprehensive electrochemical properties. In this work, carbon nanotube (CNT)-Mn3 O4 /CoWO4 triple-segment hybrid electrode is synthesized by using a two-step microwave-assisted hydrothermal route. Systematic physical characterization revealed that, with the assistance of microwave, granular Mn3 O4 and spheroid-like CoWO4 with preferred orientation, and oxygen vacancies are stacked or arranged on CNTs skeletons to construct a rational designed hybrid nanocomposite with abundant heterointerfaces and interfacial chemical bonds. Electrochemical evaluations show that the synergistic cooperation in CNT-Mn3 O4 /CoWO4 resulted in an ultra-high specific capacity (1907.5 C g-1 /529.8 mA h g-1 at 1 A g-1 ), a wide operating voltage window (1.15 V), the satisfactory rate capability (capacity maintained at 1016.5 C g-1 /282.3 mA h g-1 at 15 A g-1 ), and excellent cycling stability (117.2% initial capacity retention after 13000 cycles at 15 A g-1 ). In addition, the assembled CNT-Mn3 O4 /CoWO4 //N doped porous carbon (NC) BSH device delivered a stable working voltage of 2.05 V and superior energy density of 67.5 Wh kg-1 at power density of 1025 W kg-1 , as well as excellent stability (92.2% capacity retained at 5 A g-1 for 12600 cycles). This work provides a new and feasible tactic to develop high-performance transition metal oxide-based cathodes for advanced BSH devices.

Heteroatom-Induced Accelerated Kinetics on Nickel Selenide for Highly Efficient Hydrazine-Assisted Water Splitting and Zn-Hydrazine Battery

Hydrazine-assisted water electrolysis is a promising energy conversion technology for highly efficient hydrogen production. Rational design of bifunctional electrocatalysts, which can simultaneously accelerate hydrogen evolution reaction (HER)/hydrazine oxidation reaction (HzOR) kinetics, is the key step. Herein, we demonstrate the development of ultrathin P/Fe co-doped NiSe2 nanosheets supported on modified Ni foam (P/Fe-NiSe2) synthesized through a facile electrodeposition process and subsequent heat treatment. Based on electrochemical measurements, characterizations, and density functional theory calculations, a favorable "2 + 2" reaction mechanism with a two-step HER process and a two-step HzOR step was fully proved and the specific effect of P doping on HzOR kinetics was investigated. P/Fe-NiSe2 thus yields an impressive electrocatalytic performance, delivering a high current density of 100 mA cm-2 with potentials of - 168 and 200 mV for HER and HzOR, respectively. Additionally, P/Fe-NiSe2 can work efficiently for hydrazine-assisted water electrolysis and Zn-Hydrazine (Zn-Hz) battery, making it promising for practical application.

Arming Ru with Oxygen-Vacancy-Enriched RuO2 Sub-Nanometer Skin Activates Superior Bifunctionality for pH-Universal Overall Water Splitting

Water electrolysis has been expected to assimilate the renewable yet intermediate energy-derived electricity for green H₂ production. However, current benchmark anodic catalysts of Ir/Ru-based compounds suffer severely from poor dissolution resistance. Herein, an effective modification strategy is proposed by arming a sub-nanometer RuO₂ skin with abundant oxygen vacancies to the interconnected Ru clusters/carbon hybrid microsheet (denoted as Ru@V-RuO₂ /C HMS), which can not only inherit the high hydrogen evolution reaction (HER) activity of the Ru, but more importantly, activate the superior activity toward the oxygen evolution reaction (OER) in both acid and alkaline conditions. Outstandingly, it can achieve an ultralow overpotential of 176/201 mV for OER and 46/6 mV for the HER to reach 10 mA cm^-2 in acidic and alkaline solution, respectively. Inspiringly, the overall water splitting can be driven with an ultrasmall cell voltage of 1.467/1.437 V for 10 mA cm^-2 in 0.5 m H₂ SO₄ /1.0 m KOH, respectively. Density functional theory calculations reveal that armoring the oxygen-vacancy-enriched RuO₂ exoskeleton can cooperatively alter the interfacial electronic structure and make the adsorption behavior of hydrogen and oxygen intermediates much close to the ideal level, thus simultaneously speeding up the hydrogen evolution kinetics and decreasing the energy barrier of oxygen release.

Robust and Highly Efficient Electrochemical Hydrogen Production from Hydrazine-Assisted Water Electrolysis Enabled by the Metal-Support Interaction of Ru/C Composites

Hydrazine oxidation-assisted water electrolysis provides a promising way for the energy-efficient electrochemical hydrogen (H₂) and synchronous decomposition of hydrazine-rich wastewater, but the development of highly active catalysts still remains a great challenge. Here, we demonstrate the robust and highly active Ru nanoparticles supported on the hollow N-doped carbon microtube (denoted as Ru NPs/H-NCMT) composite structure as HER and HzOR bifunctional electrocatalysts. Thanks to such unique hierarchical architectures, the as-synthesized Ru NPs/H-NCMTs exhibit prominent electrocatalytic activity in the alkaline condition, which needs a low overpotential of 29 mV at 10 mA cm^-2 for HER and an ultrasmall working potential of -0.06 V (vs RHE) to attain the same current density for HzOR. In addition, assembling a two-electrode hybrid electrolyzer using as-prepared Ru NPs/H-NCMT catalysts shows a small cell voltage of mere 0.108 V at 100 mA cm^-2, as well as the remarkable long-term stability. Density functional theory calculations further reveal that the Ru NPs serve as the active sites for both the HER and HzOR in the nanocomposite, which facilitates the adsorption of H atoms and hydrazine dehydrogenation kinetics, thus enhancing the performances of HER and HzOR. This work paves a novel avenue to develop efficient and stable electrocatalysts toward HER and HzOR that promises energy-saving hybrid water electrolysis electrochemical H₂ production.

Heterostructural Ni-Ni0.2 Mo0.8 N Interface Engineering Boosts Alkaline Hydrogen Electrocatalysis

Exploring efficient and low-cost bifunctional catalysts for hydrogen evolution reaction (HER) and hydrogen oxidation reaction (HOR) is highly desirable for the achievement of unitized regenerative fuel cells. Herein, a facile method to prepare hetero-interfacial Ni-Ni_0.2 Mo_0.8 N nanosheets with tailored d-band for efficient alkaline hydrogen electrocatalysis is presented. Mechanism studies indicate that interface engineering can downshift the d-band center of Ni-Ni_0.2 Mo_0.8 N nanosheets due to the electron transfer from Ni to Ni_0.2 Mo_0.8 N, which weakens the binding strength of reaction intermediates, thereby boosting the catalytic performance. Relative to pure Ni, Ni-Ni_0.2 Mo_0.8 N nanosheets show a lower overpotential of 83 mV at -10 mA cm^-2 and good stability during 2,000 cycles for HER. Meanwhile, Ni-Ni_0.2 Mo_0.8 N nanosheets exhibit an improved exchange current density for HOR with a 10.2-fold enhancement in comparison with that of pure Ni. This work provides valuable insight into the reasonable design of efficient energy-related electrocatalysts based on the tailoring of d-band center by interface engineering.

Active site recovery and N-N bond breakage during hydrazine oxidation boosting the electrochemical hydrogen production

Substituting hydrazine oxidation reaction for oxygen evolution reaction can result in greatly reduced energy consumption for hydrogen production, however, the mechanism and the electrochemical utilization rate of hydrazine oxidation reaction remain ambiguous. Herein, a bimetallic and hetero-structured phosphide catalyst has been fabricated to catalyze both hydrazine oxidation and hydrogen evolution reactions, and a new reaction path of nitrogen-nitrogen single bond breakage has been proposed and confirmed in hydrazine oxidation reaction. The high electro-catalytic performance is attributed to the instantaneous recovery of metal phosphide active site by hydrazine and the lowered energy barrier, which enable the constructed electrolyzer using bimetallic phosphide catalyst at both sides to reach 500 mA cm^-2 for hydrogen production at 0.498 V, and offer an enhanced hydrazine electrochemical utilization rate of 93%. Such an electrolyzer can be powered by a bimetallic phosphide anode-equipped direct hydrazine fuel cell, achieving self-powered hydrogen production at a rate of 19.6 mol h^-1 m^-2.

Interfacial engineering of Co5.47N/Mo5N6 nanosheets with rich active sites synergistically accelerates water dissociation kinetics for Pt-like hydrogen evolution

The development of highly efficient hydrogen evolution electrocatalysts with platinum-like activity requires precise control of active sites through interface engineering strategies. In this study, a heterostructured Co_5.47N/Mo₅N₆ catalyst (CoMoN_x) on carbon cloth (CC) was synthesized using a combination of dip-etching and vapor nitridation methods. The rough nanosheet surface of the catalyst with uniformly distributed elements exposes a large active surface area and provides abundant interface sites that serve as additional active sites. The CoMoN_x was found to exhibit exceptional hydrogen evolution reaction (HER) activity with a low overpotential of 44 mV at 10 mA cm^-2 and exceptional stability of 100 h in 1.0 M KOH. The CoMoN_x^(-)||RuO₂⁽⁺⁾ system requires only 1.81 V cell voltage to reach a current density of 200 mA cm^-2, surpassing the majority of previously reported electrolyzers. Density functional theory (DFT) calculations reveal that the strong synergy between Co_5.47N and Mo₅N₆ at the interface can significantly reduce the water dissociation energy barrier, thereby improving the kinetics of hydrogen evolution. Furthermore, the rough nanosheet architecture of the CoMoN_x catalyst with abundant interstitial spaces and multi-channels enhances charge transport and reaction intermediate transportation, synergistically improving the performance of the HER for water splitting.

Heteroatom Doped Amorphous/Crystalline Ruthenium Oxide Nanocages as a Remarkable Bifunctional Electrocatalyst for Overall Water Splitting

Developing robust and highly active bifunctional electrocatalysts for overall water splitting is critical for efficient sustainable energy conversion. Herein, heteroatom-doped amorphous/crystalline ruthenium oxide-based hollow nanocages (M-ZnRuO_x (MCo, Ni, Fe)) through delicate control of composition and structure is reported. Among as-synthesized M-ZnRuO_x nanocages, Co-ZnRuO_x nanocages deliver an ultralow overpotential of 17 mV at 10 mA cm^-2 and a small Tafel slope of 21.61 mV dec^-1 for hydrogen evolution reaction (HER), surpassing the commercial Pt/C catalyst, which benefits from the synergistic coupling effect between electron regulation induced by Co doping and amorphous/crystalline heterophase structure. Moreover, the incorporation of Co prevents Ru from over-oxidation under oxygen evolution reaction (OER) operation, realizing the leap from a monofunctional to multifunctional electrocatalyst and then Co-ZnRuO_x nanocages exhibit remarkable OER catalytic activity as well as overall water splitting performance. Combining theory calculations with spectroscopy analysis reveal that Co is not only the optimal active site, increasing the number of exposed active sites while also boosting the long-term durability of catalyst by modulating the electronic structure of Ru atoms. This work opens a considerable avenue to design highly active and durable Ru-based electrocatalysts.

Progress in Hydrogen Production Coupled with Electrochemical Oxidation of Small Molecules

The electrochemical oxidation of small molecules to generate value-added products has gained enormous interest in recent years because of the advantages of benign operation conditions, high conversion efficiency and selectivity, the absence of external oxidizing agents, and eco-friendliness. Coupling the electrochemical oxidation of small molecules to replace oxygen evolution reaction (OER) at the anode and the hydrogen evolution reaction (HER) at the cathode in an electrolyzer would simultaneously realize the generation of high-value chemicals or pollutant degradation and the highly efficient production of hydrogen. This Minireview presents an introduction on small-molecule choice and design strategies of electrocatalysts as well as recent breakthroughs achieved in the highly efficient production of hydrogen. Finally, challenges and future orientations are highlighted.

Decoupled Electrochemical Hydrazine "Splitting" via a Rechargeable Zn-Hydrazine Battery

Hydrogen generation via electrochemical splitting plays an important role to achieve hydrogen economy. However, the large-scale application is highly limited by high cost and low efficiency. Herein, a new type of rechargeable Zn-hydrazine (Zn-Hz) battery is proposed and realized by a bifunctional electrocatalyst based on two separate cathodic reactions of hydrogen evolution (discharge: 2H₂ O + 2e^-  → H₂  + 2OH^- ) and hydrazine oxidation (charge: 1 / 2   N 2 H 4 + 2 OH - → 1 / 2   N 2 + 2 H 2 O + 2 e - $1{\rm{/}}2\,{{\rm{N}}_2}{{\rm{H}}_4}{\bm{ + }}2{\rm{O}}{{\rm{H}}^{\bm{ - }}}{\bm{ \to }}1{\rm{/}}2\,{{\rm{N}}_2}{\bm{ + }}2{{\rm{H}}_2}{\rm{O}}{\bm{ + }}2{e^{\bm{ - }}}$ ). This Zn-Hz battery, driven by temporally decoupled electrochemical hydrazine splitting on the cathode during discharge and charge processes, can generate separated hydrogen without purification. When the highly active bifunctional cathode of 3D Mo₂ C/Ni@C/CS is paired with Zn foil, the Zn-Hz battery can achieve efficient hydrogen generation with a low energy input of less than 0.4 V (77.2 kJ mol^-1 ) and high energy efficiency of 96%. Remarkably, this battery exhibits outstanding long-term stability for 600 cycles (200 h), achieving continuous hydrogen production on demand, which presents great potential for practical application.

Stabilization of Ni0/NiII Heterojunctions inside Robust Porous Metal Silicate Materials for High-Performance Catalysis

Heterostructural nanomaterials demonstrate great potential to replace noble metal-based catalysts because heterojunctions could induce relocalization of electrons and facilitate the migration of electrons and charge carriers at the heterostructural boundary between electron-rich and electron-deficient metal sites; however, the instability of heterojunctions greatly hinders their practical applications. We report herein an effective strategy for the fabrication and stabilization of Ni⁰/Ni^II heterojunctions inside a porous metal silicate (PMS) material PMS-22 using a nickel coordination complex as the bifunctional template. The synergistic activity between metallic nickel and nickel silicate in PMS-22 highly boosts the catalytic activity in the hydrogenation of phenol, which could activate phenol at a very low temperature of 50 °C. Most importantly, PMS-22 demonstrates robust stability in catalysis, attributed to the strong interaction and charge transfer between metallic Ni and nickel silicate at the heterointerfaces inside the confined pores. Therefore, this work paves a new pathway to improve the stability and activity of heterostructural nanomaterials for catalytic applications.

Vacancy-Rich MXene-Immobilized Ni Single Atoms as a High-Performance Electrocatalyst for the Hydrazine Oxidation Reaction

Single-atom catalysts (SACs), on account of their outstanding catalytic potential, are currently emerging as high-performance materials in the field of heterogeneous catalysis. Constructing a strong interaction between the single atom and its supporting matrix plays a pivotal role. Herein, Ti₃ C₂ T_x -MXene-supported Ni SACs are reported by using a self-reduction strategy via the assistance of rich Ti vacancies on the Ti₃ C₂ T_x MXene surface, which act as the trap and anchor sites for individual Ni atoms. The constructed Ni SACs supported by the Ti₃ C₂ T_x MXene (Ni SACs/Ti₃ C₂ T_x ) show an ultralow onset potential of -0.03 V (vs reversible hydrogen electrode (RHE)) and an exceptional operational stability toward the hydrazine oxidation reaction (HzOR). Density functional theory calculations suggest a strong coupling of the Ni single atoms and their surrounding C atoms, which optimizes the electronic density of states, increasing the adsorption energy and decreasing the reaction activation energy, thus boosting the electrochemical activity. The results presented here will encourage a wider pursuit of 2D-materials-supported SACs designed by a vacancy-trapping strategy.

Design of a ratiometric near-infrared fluorescent probe with double excitation for hydrazine detection in vitro and in vivo

Hydrazine has a wide range of industrial applications, but it is also a toxic and explosive chemical substance, which brings potential risks to human health and environmental safety. Therefore, rapid and sensitive monitoring of hydrazine is of great importance in environmental sciences and biological systems. In this work, a new near-infrared (NIR) ratiometric fluorescent probe (Ac-HY) was designed to detect hydrazine under double excitation and emission mode. Ac-HY exhibited large stokes shift (130 nm), high selectivity and sensitivity to hydrazine detection. The applications of Ac-HY probe for detecting hydrazine in vapor and imaging hydrazine in lipid droplets and zebrafish. Therefore, Ac-HY can be used to monitor the distribution of exogenous hydrazine in vitro and in vivo.

Silver decorated nickel-cobalt (oxy)hydroxides fabricated via surface reconstruction engineering for boosted electrocatalytic oxygen evolution and urea oxidation

Electrochemical water splitting is considered to be a promising renewable hydrogen generation technology but is significantly limited by the kinetically sluggish oxygen evolution reaction (OER) at the anode. Herein, a silver nanoparticle decorated nickel-cobalt (oxy)hydroxide composite is fabricated on nickel foam (Ag@NiCo(OH)_x/NF) via electrodeposition followed by spontaneous redox reaction. Benefitting from the synergetic contributions of an amorphous/crystalline phase, abundant artificial heterointerfaces, and a 3D porous architecture, the as-designed Ag@NiCo(OH)_x/NF shows substantially enhanced electrocatalytic performance toward the OER and urea oxidation reaction. Impressively, in the urea-assisted alkaline electrolyzer (coupled with commercial Pt/C on NF as the cathode) for hydrogen production, a cell voltage of only 1.49 V is required to deliver a current density of 50 mA cm^-2, much lower than that of traditional water splitting (1.69 V). Importantly, this work represents a facile and feasible method to exploit efficient self-supported electrocatalysts toward overall water splitting and urea-rich wastewater purification.

CoP Nanoparticle Confined in P, N Co-Doped Porous Carbon Anchored on P-Doped Carbonized Wood Fibers with Tailored Electronic Structure for Efficient Urea Electro-Oxidation

Electronic structure optimization and architecture modulation are widely regarded as rational strategies to enhance the electrocatalysts catalytic performance. Herein, a hybridization of ZIF-67-derived CoP nanoparticles embedded in P, N co-doped carbon matrix (PNC) and anchored on P-doped carbonized wood fibers (PCWF) is constructed using a simple simultaneous phosphorization and carbonization strategy. Benefiting from the optimized surface/interface electronic structures, abundant exposed active sites, and outstanding conductivity, the CoP@PNC/PCWF can drive the urea oxidation reaction (UOR) with greater activity and better stability than most recently reported electrocatalysts, in which a potential as low as 1.32 V (vs reversible hydrogen electrode, RHE) is needed to reach 50 mA cm^-2 and shows excellent durability. Furthermore, for overall urea splitting, using the CoP@PNC/PCWF electrocatalyst as the anode and commercial Pt/C supported on nickel foam as the cathode, an ultralow cell voltage of 1.50 V (vs RHE) is expected to achieve the 50 mA cm^-2 and operate continuously for more than 50 h at 20 mA cm^-2 . The reported strategy may shed light on the use of renewable resources to design and synthesize high-performance non-Ni-based phosphides UOR electrocatalysts for energy-saving H₂ production.