Ni(OH)2 Templated Synthesis of Ultrathin Ni3 S2 Nanosheets as Bifunctional Electrocatalyst for Overall Water Splitting

Interfacial Electronic Modulation of Mo5N6/Ni3S2 Heterojunction Array Boosts Electrocatalytic Alkaline Overall Water Splitting

Bifunctional electrocatalysts with excellent activity and durability are highly desirable for alkaline overall water splitting, yet remain a significant challenge. In this contribution, palm-like Mo5N6/Ni3S2 heterojunction arrays anchored in conductive Ni foam (denoted as Mo5N6-Ni3S2 HNPs/NF) are developed. Benefiting from the optimized electronic structure configuration, hierarchical branched structure and abundant heterogeneous interfaces, the as-synthesized Mo5N6-Ni3S2 HNPs/NF electrode exhibits remarkably stable bifunctional electrocatalytic activity in 1 m KOH solution. It only requires ultralow overpotentials of 59 and 190 mV to deliver a current density of 10 mA cm-2 for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in 1 m KOH solution, respectively. Importantly, the overall water splitting electrolyzer assembled by Mo5N6-Ni3S2 HNPs/NF exhibits an exceptionally low cell voltage (1.48 V@10 mA cm-2) and outstanding durability, surpassing most of the reported Ni-based bifunctional materials. Density functional theory (DFT) further confirms the heterostructure can optimize the Gibbs free energies of H and O-containing intermediates (OH, O, OOH) during HER and OER processes, thereby accelerating the catalytic kinetics of electrochemical water splitting. The findings provide a new design strategy toward low-cost and excellent catalysts for overall water splitting.

Iron, Tungsten Dual-Doped Nickel Sulfide as Efficient Bifunctional Catalyst for Overall Water Splitting

Developing low-cost and highly efficient bifunctional catalysts for both the oxygen evolution reaction (OER) and the hydrogen evolution reaction (HER) is a challenging problem in electrochemical overall water splitting. Here, iron, tungsten dual-doped nickel sulfide catalyst (Fe/W-Ni3S2) is synthesized on the nickel foam, and it exhibits excellent OER and HER performance. As a result, the water electrolyze based on Fe/W-Ni3S2 bifunctional catalyst illustrates 10 mA cm-2 at 1.69 V (without iR-compensation) and highly durable overall water splitting over 100 h tested under 500 mA cm-2. Experimental results and DFT calculations indicate that the synergistic interaction between Fe doping and Ni vacancy induced by W leaching during the in situ oxidation process can maximize exposed OER active sites on the reconstructed NiOOH species for accelerating OER kinetics, while the Fe/W dual-doping optimizes the electronic structure of Fe/W-Ni3S2 and the binding strength of intermediates for boosting HER. This study unlocks the different promoting mechanisms of incorporating Fe and W for boosting the OER and HER activity of Ni3S2 for water splitting, which provides significant guidance for designing high-performance bifunctional catalysts for overall water splitting.

Ru doped NiMoO4 nanoarray as a high-efficiency electrocatalyst for nitrite reduction to ammonia

The electrocatalytic reduction of nitrite to recyclable ammonia (NH3) is essential to maintain nitrogen balance and meet growing energy requirements. Herein, we report that Ru doped honeycomb NiMoO4 nanosheet with copious oxygen vacancies grown on nickel foam substrate has been prepared by a facile hydrothermal synthesis and immersion process, which can act as an efficient electrocatalyst for NH3 synthesis by reduction of nitrite. By optimizing the concentration of RuCl3 solution, 0.01Ru-NiMoO4/NF possesses excellent NO2-RR performance with NH3 yield of 20249.17 ± 637.42 μg h-1 cm-2 at -0.7 V and FE of 95.56 ± 0.72 % at -0.6 V. When assembled into a Zn-NO2- battery, it provides a remarkable level of power density of 13.89 mW cm-2, outperforming the performance of virtually all previous reports. The efficient adsorption and activation of NO2- over Ru-doped NiMoO4 with oxygen vacancy have been verified by density functional theory calculations, as well as the possible reaction pathway.

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.

Precise Atomic Structure Regulation of Single-Atom Platinum Catalysts toward Highly Efficient Hydrogen Evolution Reaction

Noble metal single-atom-catalysts (SACs) have demonstrated significant potential to improve atom utilization efficiency and catalytic activity for hydrogen evolution reaction (HER). However, challenges still remain in rationally modulating active sites and catalytic activities of SACs, which often results in sluggish kinetics and poor stability, especially in neutral/alkaline media. Herein, precise construction of Pt single atoms anchored on edge of 2D layered Ni(OH)2 (Pt-Ni(OH)2-E) is achieved utilizing in situ electrodeposition. Compared to the single-atom Pt catalysts anchored on the basal plane of Ni(OH)2 (Pt-Ni(OH)2-BP), the Pt-Ni(OH)2-E possesses superior electron affinity and high intrinsic catalytic activity, which favors the strong adsorption and rapid dissociation toward water molecules. As a result, the Pt-Ni(OH)2-E catalyst requires low overpotentials of 21 and 34 mV at 10 mA cm-2 in alkaline and neutral conditions, respectively. Specifically, it shows the high mass activity of 23.6 A mg-1 for Pt at the overpotential of 100 mV, outperforming the reported catalysts and commercial Pt/C. This work provides new insights into the rational design of active sites for preparing high-performance SACs.

Bifunctional Ni Foam Supported TiO2 @Ni3 S2 core@shell Nanorod Arrays for Boosting Electrocatalytic Biomass Upgrading and H2 Production Reactions

Replacing traditional oxygen evoltion reaction (OER) with biomass oxidation reaction (BOR) is an advantageous alternative choice to obtain green hydrogen energy from electrocatalytic water splitting. Herein, a novel of extremely homogeneous Ni3 S2 nanosheets covered TiO2 nanorod arrays are in situ growth on conductive Ni foam (Ni/TiO2 @Ni3 S2 ). The Ni/TiO2 @Ni3 S2 electrode exhibits excellent electrocatalytic activity and long-term stability for both BOR and hydrogen evolution reaction (HER). Especially, taking glucose as a typical biomass, the average hydrogen production rate of the HER-glucose oxidation reaction (GOR) two-electrode system reached 984.74 µmol h-1 , about 2.7 times higher than that of in a common HER//OER two-electrode water splitting system (365.50 µmol h-1 ). The calculated power energy saving efficiency of the GOR//HER system is about 13% less than that of the OER//HER system. Meanwhile, the corresponding selectivity of the value-added formic acid produced by GOR reaches about 80%. Moreover, the Ni/TiO2 @Ni3 S2 electrode also exhibits excellent electrocatalytic activity on a diverse range of typical biomass intermediates, such as urea, sucrose, fructose, furfuryl alcohol (FFA), 5-hydroxymethylfurfural (HMF), and alcohol (EtOH). These results show that Ni/TiO2 @Ni3 S2 has great potential in electrocatalysis, especially in replacing OER reaction with BOR reaction and promoting the sustainable development of hydrogen production.

Amorphous Nickel Hydroxide Shell on Ni8P3 Nanorods for Boosted Highly Stable Overall Water Splitting at High Current

Developing highly active, highly stable, and cheap electrocatalysts for water splitting is of great significance for hydrogen production. Herein, we report an amorphous Ni(OH)2-clothed transition Ni8P3 catalyst, in which the amorphous Ni(OH)2 shell provides catalytic active sites and serves as a proton conductive encapsulation layer to ensure efficient proton supply to the active Ni8P3 sites. As expected, the Ni8P3@Ni(OH)2 catalyst exhibits significant water decomposition performance at low and high current densities of 10, 100, and 1000 mA cm-2 at 1.45, 1.71, and 2.21 V, respectively, which is comparable to those of commercial electrocatalysts. In particular, the prepared Ni8P3@Ni(OH)2 electrodes possess exceptional long-term durability (200 h) at high current (over 1 A). The significantly improved water-splitting activity and durability in alkaline medium are expected to make them attractive catalyst materials to produce renewable chemical fuels.

A novel sacrificial solvent method to synthesize self-supporting Co9S8/Ni3S2 heterostructure catalyst for efficient oxygen evolution reaction

Synthesizing catalysts for efficient oxygen evolution reaction (OER) with lower cost and simpler design is of significant importance to achieve sustainable hydrogen production. In this work, we propose a novel "sacrificial solvent method" for the first time. Dicobalt octacarbonyl (Co2(CO)8), dimethyl sulfoxide (DMSO), and Ni foam (NF) were used as the raw materials in the solvothermal process. DMSO played the role of both the sacrificial solvent and the sulfur source. Through the self-consumption of DMSO, we finally obtained the Co9S8/Ni3S2 heterostructure supported on the NF (Co9S8/Ni3S2@NF) in one step. The Co9S8/Ni3S2@NF catalyst exhibited excellent OER activity in alkaline environment, with an overpotential of only 264 mV at a current density of 20 mA cm-2, a low Tafel slope of 68.28 mV dec-1 and maintained its current density after 20 h of constant potential testing. This work introduces a new method for synthesizing metal sulfide catalysts using DMSO as a sacrificial solvent. It provides broader opportunities for the development of more efficient and sustainable catalysts for energy conversion and storage.

Ultrafine Pt-based catalyst decorated with oxygenophilic Ni-sites accelerating alkaline H2O dissociation for efficient hydrogen evolution

Although Pt is a widely adopted commercial catalyst for the hydrogen evolution reaction (HER), its practical application is greatly limited by its prohibitive cost and high energy barrier for H2O dissociation in alkaline media. Herein, an ultrafine Pt-based catalyst decorated with oxygenophilic Ni-sites is rationally designed and successfully synthesized with Pt5(GS)10 (HGS = l-reduced glutathione) nanocluster precursor. The optimized Ni-decorated Pt catalyst (Ni-Pt-C-500) with ultrafine nanoparticles (about 1.6 nm) exhibits a low overpotential (14.0 mV) at 10 mA cm-2 and a mild Tafel slope of 20.8 mV dec-1 in the HER, which is superior to its undecorated counterpart (Pt-C-500), the commercial 20 wt% Pt/C catalyst and most of the previously reported Pt-based electrocatalysts. Experimental observations and theoretical calculations indicate that H2O could be spontaneously adsorbed to Ni-sites of the Ni-Pt-C-500 catalyst. Mechanistic studies reveal that Ni-sites promote HER by accelerating the kinetic of H2O cleavage and optimizing the electronic structure of Pt. This work paves a new avenue for designing other ultrafine hybrid electrocatalysts based on metal nanoclusters to enhance catalytic reaction kinetics.

Fabrication of Ultra-Durable and Flexible NiPx -Based Electrode toward High-Efficient Alkaline Seawater Splitting at Industrial Grade Current Density

Designing nonprecious metal-based electrocatalysts to yield sustainable hydrogen energy by large-scale seawater electrolysis is challenging to global emissions of carbon neutrality and carbon peaking. Herein, a series of highly efficient, economical, and robust Ni-P-based nanoballs grown on the flexible and anti-corrosive hydrophobic asbestos (NiP_x @HA) is synthesized by electroless plating at 25 °C toward alkaline simulated seawater splitting. On the basis of the strong chemical attachment between 2D layered substrate and nickel-rich components, robust hexagonal Ni₅ P₄ crystalline modification, and fast electron transfer capability, the overpotentials during hydrogen/oxygen evolution reaction (HER/OER) are 208 and 392 mV at 200 mA cm^-2 , and the chronopotentiometric measurement at 500 mA cm^-2 lasts for over 40 days. Additionally, the versatile strategy is broadly profitable for industrial applications and enables multi-elemental doping (iron/cobalt/molybdenum/boron/tungsten), flexible substrate employment (nickel foam/filter paper/hydrophilic cloth), and scalable synthesis (22 cm × 22 cm). Density functional theory (DFT) also reveals that the optimized performance is due to the fundamental effect of incorporating O-source into Ni₅ P₄ . Therefore, this work exhibits a complementary strategy in the construction of NiP_x -based electrodes and offers bright opportunities to produce scalable hydrogen effectively and stably in alkaline corrosive electrolytes.

Cauliflower-like NiFe alloys anchored on a flake iron nickel carbonate hydroxide heterostructure towards superior overall water and urea electrolysis

Exploring efficient, stable and multifunctional Earth-rich electrocatalysts is vital for hydrogen generation. Hence, an efficient heterostructure consisting of cauliflower-like NiFe alloys anchored on flake iron nickel carbonate hydroxide which is supported on carbon cloth (NiFe/NiFeCH/CC) was synthesized as a trifunctional electrocatalyst for efficient hydrogen production by overall water and urea splitting. While optimizing and regulating the ratio of Ni to Fe, benefiting from the special morphology and synergistic effect between the NiFe alloy and NiFeCH, the NiFe/NiFeCH/CC heterostructure exhibits outstanding oxygen evolution reaction (OER) performance with a low overpotential of 190 mV at 10 mA cm^-2 after a stability test for 150 h. Notably, when the NiFe/NiFeCH/CC heterostructure is used as both the anode and cathode simultaneously, it merely requires a cell voltage of 1.49 V for the overall water splitting and 1.39 V for urea electrolysis at 10 mA cm^-2 with excellent durability. Thus, this work not just provides the application of NiFe-based catalysts in overall water splitting, but also offers a viable method for the treatment of urea-rich wastewater.

Ultrathin Fe-ReS2 Nanosheets as Electrocatalysts for Accelerating Sulfur Reduction in Li-S Batteries

Lithium-sulfur batteries are promising next-generation energy storage systems with high theoretical specific capacity. Despite extensive research efforts, it is still challenging to rationally design electrocatalysts with fast kinetics and effective adsorption of polysulfides. Herein, Fe-doped ReS₂ (Fe-ReS₂) ultrathin nanosheets are prepared as an electrocatalyst to trap the intermediates and accelerate the sulfur reduction reaction kinetics. Density functional theory calculations combined with activation energies in the multistep sulfur reduction reaction reveal that the Fe-ReS₂ considerably reduces the activation energy and optimizes the optimum adsorption strength of polysulfides and catalytic activity. The Fe-ReS₂/S exhibits a highly reversible discharge capacity of 882.3 mA h g^-1 at 1 C. For 500 cycles, the capacity fade rate is 0.013% per cycle. Moreover, in situ Raman spectroscopy measurements further confirmed that both sulfur reduction and oxidation processes were significantly enhanced by Fe-ReS₂.

Synergistic catalysis of graphitic carbon nitride supported bimetallic sulfide nanostructures for efficient oxygen generation

Herein, a series of g-C₃N₄ supported bimetallic sulfide nanostructures (Ni₃S₂/MoS₂/ng-C₃N₄, n = 10, 20 and 30) was prepared by a hydrothermal method and subsequently a thermal annealing approach. Ni₃S₂/MoS₂/20g-C₃N₄ with controlled composition exhibits efficient OER activity with a low overpotential of 183 mV at 10 mA cm^-2, which outperforms the vast majority of sulfide OER electrocatalysts reported previously.

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 .

Rational Regulation of Crystalline/Amorphous Microprisms-Nanochannels Based on Molecular Sieve (VSB-5) for Electrochemical Overall Water Splitting

Rational regulation of the composition and structure of electrocatalysts is crucial to the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). Herein, a new electrocatalyst of nickel phosphate microprism (VSB/NiPO) is developed via a simple solvothermal reaction. The microprism is mainly composed of Versailles-Santa Barbara-5 (VSB-5, molecular sieve) with unique nanochannels, which contribute to accelerating mass transfer and exposing more active sites, thus displaying excellent HER activity. Subsequently, the crystallinity and electronic structure of the framework are modulated by incorporating Fe with the combination of calcination and impregnation. The nanochannels are converted to the amorphous arrangement, and the Ni centers are regulated to the higher valence. The resultant Fe-VSB/NiPO-500 exhibits a low OER overpotential of 227 mV at 50 mA cm^-2 . Interestingly, an integrated electrolyzer assembled by VSB/NiPO(-) and Fe-VSB/NiPO-500(+) performs well for overall water splitting, which requires only 1.487 V to achieve 10 mA cm^-2 , and remains stable at 100 mA cm^-2 over 100 h. This finding opens a new avenue for developing VSB-5 in the field of electrocatalysis.

Efficient Electrooxidation of 5-Hydroxymethylfurfural Using Co-Doped Ni3 S2 Catalyst: Promising for H2 Production under Industrial-Level Current Density

Replacing oxygen evolution reaction (OER) by electrooxidations of organic compounds has been considered as a promising approach to enhance the energy conversion efficiency of the electrolytic water splitting proces. Developing efficient electrocatalysts with low potentials and high current densities is crucial for the large-scale productions of H2 and other value-added chemicals. Herein, non-noble metal electrocatalysts Co-doped Ni3 S2 self-supported on a Ni foam (NF) substrate are prepared and used as catalysts for 5-hydroxymethylfurfural (HMF) oxidation reaction (HMFOR) under alkaline aqueous conditions. For HMFOR, the Co0.4 NiS@NF electode achieves an extremely low onset potential of 0.9 V versus reversible hydrogen electrode (RHE) and records a large current density of 497 mA cm-2 at 1.45 V versus RHE for HMFOR. During the HMFOR-assisted H2 production, the yield rates of 2,5-furandicarboxylic acid (FDCA) and H2 in a 10 mL electrolyte containing 10 × 10-3 M HMF are 330.4 µmol cm-2 h-1 and 1000 µmol cm-2 h-1 , respectively. The Co0.4 NiS@NF electrocatalyst displays a good cycling durability toward HMFOR and can be used for the electrooxidation of other biomass-derived chemicals. The findings present a facile route based on heteroatom doping to fabricate high-performance catalyses that can facilitate the industrial-level H2 production by coupling the conventional HER cathodic processes with HMFOR.

In Situ Synthesis of Morphology-Controlled MoOx/Fe1-xS Bifunctional Catalysts for High-Efficiency and Stable Alkaline Water Splitting

The advancement of a bifunctional electrocatalyst consisting of earth's rich elements and with high efficiency is the key to obtain hydrogen fuel by overall water splitting (OWS). Here, a facile...