Vanadium Substitution Steering Reaction Kinetics Acceleration for Ni3N Nanosheets Endows Exceptionally Energy-Saving Hydrogen Evolution Coupled with Hydrazine Oxidation

Sub-3 nm 1 T-MoS2 self-supported nanosheets with fast ion transport and bubble release for membrane-free water electrolysis

Conventional water splitting is restricted by costly membrane electrode assemblies and a sluggish oxygen evolution reaction (OER) occurring at the anode. In light of this, the construction of a membrane-free water electrolysis system via a hydrazine-assisted seawater hydrogenation strategy surmounts both of these obstacles. Herein, we propose a new strategy to synthesize two-dimensional (2D) ultrathin porous 1 T-MoS2 (1 T-MoS2-10 %/CC), which consists of 2.3 nm triple-layer crystalline surface and has nano pores (2-4 nm). Due to the uniform and consistent nanosheet arrays composed of ultrathin large-layer-spacing nanosheets and abundant pores, the electrolyte and the 1 T-MoS2-10 %/CC can be fully contacted, showing superhydrophilicity, and increasing the bubble contact angle to release bubbles in time, showing superaerophobicity. Therefore, the special structure of 1 T-MoS2 shows advantageous for gas precipitation reaction-hydrogen evolution reaction (HER) and hydrazine oxidation reaction (HzOR), which require only 55 mV overpotential for HER and 7 mV (vs. RHE) working potential for HzOR to achieve a current density of 10 mA cm-2. In addition, we constructed a hybrid seawater membrane-free electrolysis system in which a current density of 100 mA cm-2 can be achieve with a cell voltage of only 0.27 V, which not only effectively replaces the high-energy-consuming OER for energy-saving hydrogen production, but also avoids the electrochemical reaction of chlorine evolution reaction (ClER) with the low cell voltage.

Enhanced Cooperative Generalized Compressive Strain and Electronic Structure Engineering in W-Ni3N for Efficient Hydrazine Oxidation Facilitating H2 Production

As promising bifunctional electrocatalysts, transition metal nitrides are expected to achieve an efficient hydrazine oxidation reaction (HzOR) by fine-tuning electronic structure via strain engineering, thereby facilitating hydrogen production. However, understanding the correlation between strain-induced atomic microenvironments and reactivity remains challenging. Herein, a generalized compressive strained W-Ni3N catalyst is developed to create a surface with enriched electronic states that optimize intermediate binding and activate both water and N2H4. Multi-dimensional characterizations reveal a nearly linear correlation between the hydrogen evolution reaction (HER) activity and the d-band center of W-Ni3N under strain state. Theoretically, compressive strain enhances the electron transfer capability at the surface, increasing donation into antibonding orbitals of adsorbed species, which accelerates the HER and HzOR. Leveraging both compressive strain and the modified electronic structure from W incorporation, the W-Ni3N catalysts demonstrate outstanding bifunctional performance, achieving overpotentials of 46 mV for HER at 10 mA cm-2 and 81 mV for HzOR at 100 mA cm-2. Furthermore, W-Ni3N catalyst achieves efficient overall hydrazine splitting at a low cell voltage of 0.185 V for 50 mA cm-2, maintaining stability for ≈450 h. This work provides new insights into the dual engineering of strain and electronic structure in the design of advanced catalysts.

Superhydrophilicity and superaerophobicity Ni/Ni3S4/1T-MoS2 for hydrazine-assisted seawater splitting

The overall hydrazine splitting (OHzS) is a promising strategy to achieve the efficient hydrogen production in seawater through replacing the slow kinetic oxygen evolution reaction (OER) and toxic chlorine evolution reaction (ClOR) by hydrazine oxidation (HzOR). We report an efficient bifunctional electrocatalyst of Ni/Ni3S4/1T-MoS2 on carbon cloth (Ni/Ni3S4/1T-MoS2/CC), which was formed from large layer spacing MoS2 and Ni3S4 with metal-Ni, and was applied as for both hydrogen evolution reaction (HER) and HzOR. The MoS2 had expanded interlayer spacing and showed 1T phase, with significantly improved conductivity and hydrophilicity, which promotes transfer process of reactants. Furthermore, the introduction of Ni/Ni3S4 on the 1T-MoS2 base surface leaded to superhydrophilic and superaerophobic properties, which makes it more conducive to the adsorption of H. The improvement of electrical conductivity induced the excellent HER and HzOR electrocatalytic properties of Ni/Ni3S4/1T-MoS2/CC, which showed an ultralow overpotential of 24 mV and working potential of 0 mV at a current density of 10 mA cm-2, respectively. Inspiringly, Ni/Ni3S4/1T-MoS2/CC also showed excellent performance in hydrazine-assisted alkaline seawater electrolysis, as well as solar panel powered electrolysis of seawater OHzS, therefore, exhibiting great potential for practical applications.

Vanadium-Doped Heterointerfaced Ni3N-MoOx Nanosheets with Optimized H and H2O Adsorption for Effective Alkaline Hydrogen Electrocatalysis

Nickel (Ni)-based materials represent a compelling avenue as platinum alternatives in the realm of alkaline hydrogen electrocatalysis. However, conventional nickel nitrides (Ni3N) have long been hindered by sluggish hydrogen evolution kinetics in alkaline environments, owing to inadequate adsorption strengths of both hydrogen and water molecules. Herein, a novel approach is presented involving the design of vanadium (V)-doped Ni3N/MoOx heterogeneous nanosheets (V-Ni3N@MoOx), engineered to achieve optimized adsorption strengths for hydrogen evolution and oxidation reactions (HER/HOR). Theoretical insights underscore the superior catalytic performance of this composite, attributed to a synergistic interplay between unique V doping and the heterointerfaced structure. This synergistic effect not only fine-tunes the electronic structure, establishing an optimal d band center to mitigate proton over-bonding, but also ameliorates the energy barrier through enhanced H2O dissociation capability. Consequently, V-Ni3N@MoOx manifests remarkable catalytic activities, evincing an overpotential of 56 mV at 10 mA cm-2 for HER and an exchange current density of 1.91 mA cm-2 for HOR in alkaline media. Notably, the stability assessment reveals the enduring performance of V-Ni3N@MoOx for HER/HOR, exhibiting no activity decay over extended operational durations. This study underscores the efficacy of heterogeneous interface modulation as a transformative strategy in designing Ni-based materials for alkaline hydrogen electrocatalysis.

Quantum-sized CoP nanodots with rich vacancies: Enhanced hydrazine oxidation, hydrazine-assisted water splitting, and Zn-hydrazine battery performance through interface modulation

Reducing the size of catalysts and tuning their electronic structure and interfacial properties are key to enhancing catalytic performance. Herein, a series of quantum-sized Co-based nanodot composites, including Co3O4/C, CoS2/C, CoN/C, and CoP/C, were synthesized using chemical vapor deposition (CVD) methods. By means of experimental measurement and theoretical calculation, CoP/C exhibited more robust electrochemical response than other Co-based compounds in electrochemical oxidation of N2H4 (HzOR) and hydrogen evolution reaction (HER). The catalytic activities of CoP/C can be further enhanced by introducing Co vacancies on the surface of CoP/C (labeled as Co1-xP/C). The results demonstrated that Co1-xP/C not only exhibited notable electrochemical responses at an ultra-low N2H4 concentration of 0.67 μM, showcasing its potential for ultra-sensitive N2H4 detection but also realized HzOR instead of the oxygen evolution reaction (OER) half-reaction, thereby lowering the overpotential to 2.0 mV at 10.0 mA cm-2. Finally, a Zn-hydrazine (Zn-Hz) battery was fabricated as a promising energy conversion device, showing the exceptional practical value of Co1-xP/C.

Heterointerface-Rich Ni3N/WO3 Hierarchical Nanoarrays for Efficient Glycerol Oxidation-Assisted Alkaline Hydrogen Evolution

Glycerol oxidation-assisted water electrolysis has emerged as a cost-effective way of co-producing green hydrogen and HCOOH. Still, preparing highly selective and stable nickel-based metal electrocatalysts remains a challenge. Herein, heterostructure Ni3N/WO3 nanosheet arrays of bifunctional catalysts with large specific surface areas loaded on nickel foam (denoted as Ni3N/WO3/NF) were synthesized. This catalyst was for glycerol oxidation reaction (GOR) and hydrogen evolution reaction (HER) with excellent catalytic performance, a voltage saving of 267 mV compared to oxygen evolution reaction (OER), and a HER overpotential of 104 mV at 100 mA cm-2. The cell voltage in the assembled GOR//HER hybrid electrolysis system reaches 100 mA cm-2 at 1.50 V, 296 mV lower than the potential required for overall water splitting. This work demonstrates that replacing GOR with OER using a cost-effective and highly active Ni-based bifunctional electrocatalyst can make hybrid water electrolysis an energy-efficient, sustainable, and green strategy for hydrogen production.

Enhancing the electronic structure of Ni-based electrocatalysts through N element substitution for the hydrogen evolution reaction

The weak orbital coupling between Ni3N and H2O, caused by its interstitial structure and attenuated Ni-N interaction, is attributed to the high unoccupied d orbital energy of Ni3N. Consequently, the kinetics for water dissociation in the HER are slow. In this study, we effectively lowered the energy state of vacant d orbitals in Ni3N, which resulted in an exceptionally efficient HER. The as-synthesized Ni3N catalyst demonstrates an overpotential of 135 mV when subjected to a current density of 10 mA cm-2. The refined structural characterization suggests that the introduction of oxygen results in a reduction in electron densities surrounding the Ni sites. Furthermore, DFT calculations provide additional evidence that the electrocatalyst of Ni3N generates a greater number of lowest unoccupied orbitals (LUMOs) and improved alignment, thereby enhancing the adsorption and splitting of water. The notion of orbital-regulated electronic levels on Ni sites introduces a distinctive methodology for the systematic development of catalysts used in hydrogen evolution and other applications.

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.

V-O Species-Doped Carbon Frameworks Loaded with Ru Nanoparticles as Highly Efficient and CO-Tolerant Catalysts for Alkaline Hydrogen Oxidation

Efficient and CO-tolerant catalysts for alkaline hydrogen oxidation (HOR) are vital to the commercial application of anion exchange membrane fuel cells (AEMFCs). Herein, a robust Ru-based catalyst (Ru/VOC) with ultrasmall Ru nanoparticles supported on carbon frameworks with atomically dispersed V-O species is prepared elaborately. The catalyst exhibits a remarkable mass activity of 3.44 mA μgPGM, which is 31.3 times that of Ru/C and even 4.7 times higher than that of Pt/C. Moreover, the Ru/VOC anode can achieve a peak power density (PPD) of 1.194 W cm-2, much superior to that of Ru/C anode and even better than that of Pt/C anode. In addition, the catalyst also exhibits superior stability and exceptional CO tolerance. Experimental results and density functional theory (DFT) calculations demonstrate that V-O species are ideal OH- adsorption sites, which allow Ru to release more sites for hydrogen adsorption. Furthermore, the electron transfer from Ru nanoparticles to the carbon substrate regulates the electronic structure of Ru, reducing the hydrogen binding energy (HBE) and the CO adsorption energy on Ru, thus boosting the alkaline HOR performance and CO tolerance of the catalyst. This is the first report that oxophilic single atoms distributed on carbon frameworks serve as OH- adsorption sites for efficient hydrogen oxidation, opening up new guidance for the elaborate design of high-activity catalysts for the alkaline HOR.

Unveiling the Structural Self-Reconstruction and Identifying the Reactive Center of a V, Fe Co-Doped Cobalt Precatalyst toward Enhanced Overall Water Splitting by Operando Raman Spectroscopy

The development of efficient and stable bifunctional electrocatalysts based on non-noble metals for water electrolysis is both urgent and challenging. However, unresolved issues remain regarding the challenge of identifying the active phase and gaining a comprehensive understanding of its surface reconstruction and functionality throughout the reaction process. In this study, we have combined doping and heterostructure construction by a one-step electrodeposition and a subsequent activation treatment to synthesize Fe, V co-doped Co3O4/Co(OH)2 and Co/Co(OH)2 heterointerfaces (referred to as A-Co60Fe1.1V). These heterointerfaces, composed of Co/Co(OH)2 and Co3O4/Co(OH)2, are proposed to facilitate charge transfer process during catalysis. X-ray photoelectron spectroscopy (XPS) analysis demonstrates that the introduction of V and Fe dopants increases the valence state of Co centers in Co3O4 and Co(OH)2. Further operando Raman spectroscopy reveals that Co(OH)2 and Co3O4 with the high-valence Co centers remain stable during the hydrogen evolution reaction (HER) process. These high-valence Co centers are believed to promote the crucial water dissociation step and therefore enhance the overall HER catalysis. On the other hand, during the oxygen evolution reaction (OER), Fe, V co-doping leads to an earlier formation of the active CoOOH species, while Fe doping can further help stabilize the more reactive β-CoOOH species instead of the less reactive γ-CoOOH. As a result, the A-Co60Fe1.1V catalyst exhibits significantly improved catalytic activity for both HER and OER that it requires low overpotentials of 51 and 250 mV, respectively, to attain a current density of 10 mA cm-2. Moreover, when utilized as both the cathode and anode in alkaline water electrolysis, the A-Co60Fe1.1V catalyst can operate at a mere 1.54 V voltage while maintaining 10 mA cm-2, surpassing the majority of non-noble metal catalysts. Remarkably, it also exhibits stability for at least 40 h at ∼100 mA cm-2.

Ensemble effects of high entropy alloy for boosting alkaline water splitting coupled with urea oxidation

FeCoNiMoRu/CNFs exhibits a small potential of 1.43 V vs. RHE (100 mA cm^-2) and superior stability for 90 h toward urea electro-oxidation (UOR). In situ electrochemical Raman results strongly demonstrate the ensemble effects of the various metal sites on improving the UOR activity by co-stabilizing the important intermediates. This work will open new directions in the application of high-entropy alloys for small molecule oxidation reactions.

Recent advances in transition metal nitrides for hydrogen electrocatalysis in alkaline media: From catalyst design to application

Hydrogen (H2) has been considered an ideal alternative energy source for solving energy supply security and greenhouse gas reduction. Although platinum group metal (PGM) catalysts have excellent performance in hydrogen electrocatalysis, their scarcity and high cost limit their industrial application. Therefore, it is necessary to develop low-cost and efficient non-PGM catalysts. Transition metal nitrides (TMNs) have attracted much attention because of their excellent catalytic performance in hydrogen electrochemistry, including hydrogen evolution reaction (HER)/hydrogen oxidation reaction (HOR). In this paper, we review and discuss the mechanism of HER/HOR in alkaline media. We compare and evaluate electrocatalytic performance for the HER/HOR TMN catalysts recently reported. Finally, we propose the prospects and research trends in sustainable alkaline hydrogen electrocatalysis.

Cerium-induced lattice disordering in Co-based nanocatalysts promoting the hydrazine electro-oxidation behavior

In this work, cerium-incorporated Co-based catalysts encapsulated in nitrogen-doped carbon were fabricated for the electrocatalytic hydrazine oxidation reaction (HzOR). The Ce incorporation could lead to the formation of surface oxide nanolayers with a disordered lattice, endowing the catalyst with enriched active sites and enhanced intrinsic activity for promoted HzOR.

Nickel-Cobalt Hydrogen Phosphate on Nickel Nitride Supported on Nickel Foam for Alkaline Seawater Electrolysis

Developing high-performance non-noble bifunctional catalysts is pivotal for large-scale seawater electrolysis but remains a challenge. Here we report a sandwichlike NiCo(HPO₄)₂@Ni₃N/NF (denoted by NiCoHPi@Ni₃N/NF) catalyst. Vertical Ni₃N nanosheet arrays are first grown and supported on nickel foam, and then a bimetallic NiCoHPi coating is decorated on Ni₃N nanosheets by one-step electrodeposition. The hierarchical sandwich like structure offers a large surface area and plenty of catalytic active sites, and the coupling of interconnected Ni₃N and NiCoHPi accelerates the electron transfer. Moreover, the surficial hydrogen phosphate ions contribute to a proper OH^- absorption capacity due to the Lewis acid-base reaction. As a result, the NiCoHPi@Ni₃N/NF catalyst exhibits good OER and HER activity, requiring overpotentials of 365 mV (for OER) and 174 mV (for HER) to deliver 100 mA cm^-2 in the alkaline simulated seawater electrolyte. When assembled the NiCoHPi@Ni₃N/NF catalyst as both the anode and cathode, it only needs 1.86 V to reach 100 mA cm^-2 in alkaline simulated seawater electrolyte. This work may inspire the design and exploration of self-supported hierarchical composite electrocatalysts for hydrogen production from the electrolysis of seawater.

Phase-Selective Synthesis of Ruthenium Phosphide in Hybrid Structure Enables Efficient Hybrid Water Electrolysis Under pH-Universal Conditions

Hydrazine-assisted hybrid water electrolysis is an energy-saving approach to produce high-purity hydrogen, whereas the development of pH-universal bifunctional catalysts encounters a grand challenge. Herein, a phase-selective synthesis of ruthenium phosphide compounds hybrid with carbon forming pancake-like particles (denoted as Ru_x P/C-PAN, x = 1 or 2) is presented. The obtained RuP/C-PAN exhibits the highest catalytic activity among the control samples, delivering ultralow cell voltages of 0.03, 0.27, and 0.65 V to drive 10 mA cm^-2 using hybrid water electrolysis corresponding to pH values of 14, 7, and 0, respectively. Theoretical calculation deciphers that the RuP phase displays optimized free energy for hydrogen adsorption and reduced energy barrier for hydrazine dehydrogenation. This work may not only open up a new avenue in exploring universally compatible catalyst to transcend the limitation on the pH value of electrolytes, but also push forward the development of an energy-saving hydrogen generation technique based on emerging hybrid water electrolysis.

NiOOH@Cobalt copper carbonate hydroxide nanorods as bifunctional electrocatalysts for highly efficient water and hydrazine oxidation

NiOOH@cobalt copper carbonate hydroxide nanorods demonstrate enhanced electrocatalytic activity toward hydrazine oxidation reaction (HzOR) and oxygen evolution reaction, enabling energy-saving hydrogen production with the assistance of the HzOR.

Environmentally benign general synthesis of nonconsecutive carbon-coated RuP2 porous microsheets as efficient bifunctional electrocatalysts under neutral conditions for energy-saving H2 production in hybrid water electrolysis

A nonconsecutive carbon-coated RuP2 porous microsheet (RuP2@InC-MS) with bifunctionality for HzOR and HER is realized. DFT calculations evidence that C is more thermoneutral for HER while Ru boosts the dehydrogenation kinetics during HzOR process.

Theoretical Insights into the Hydrogen Evolution Reaction on the Ni3N Electrocatalyst

Ni-based catalysts are attractive alternatives to noble metal electrocatalysts for the hydrogen evolution reaction (HER). Herein, we present a dispersion-corrected density functional theory (DFT-D3) insight into HER activity on the (111), (110), (001), and (100) surfaces of metallic nickel nitride (Ni3N). A combination of water and hydrogen adsorption was used to model the electrode interactions within the water splitting cell. Surface energies were used to characterise the stabilities of the Ni3N surfaces, along with adsorption energies to determine preferable sites for adsorbate interactions. The surface stability order was found to be (111) < (100) < (001) < (110), with calculated surface energies of 2.10, 2.27, 2.37, and 2.38 Jm−2, respectively. Water adsorption was found to be exothermic at all surfaces, and most favourable on the (111) surface, with Eads = −0.79 eV, followed closely by the (100), (110), and (001) surfaces at −0.66, −0.65, and −0.56 eV, respectively. The water splitting reaction was investigated at each surface to determine the rate determining Volmer step and the activation energies (Ea) for alkaline HER, which has thus far not been studied in detail for Ni3N. The Ea values for water splitting on the Ni3N surfaces were predicted in the order (001) < (111) < (110) < (100), which were 0.17, 0.73, 1.11, and 1.60 eV, respectively, overall showing the (001) surface to be most active for the Volmer step of water dissociation. Active hydrogen adsorption sites are also presented for acidic HER, evaluated through the ΔGH descriptor. The (110) surface was shown to have an extremely active Ni–N bridging site with ΔGH = −0.05 eV.