Bimetallic Cobalt‐Based Phosphide Zeolitic Imidazolate Framework: CoPx Phase‐Dependent Electrical Conductivity and Hydrogen Atom Adsorption Energy for Efficient Overall Water Splitting

Cobalt‐based bimetallic phosphide encapsulated in carbonized zeolitic imadazolate frameworks has been successfully synthesized and showed excellent activities toward both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). Density functional theory calculation and electrochemical measurements reveal that the electrical conductivity and electrochemical activity are closely associated with the Co₂P/CoP mixed phase behaviors upon Cu metal doping. This relationship is found to be the decisive factor for enhanced electrocatalytic performance. Moreover, the precise control of Cu content in Co‐host lattice effectively alters the Gibbs free energy for H* adsorption, which is favorable for facilitating reaction kinetics. Impressively, an optimized performance has been achieved with mild Cu doping in Cu_0.3Co_2.7P/nitrogen‐doped carbon (NC) which exhibits an ultralow overpotential of 0.19 V at 10 mA cm^–2 and satisfying stability for OER. Cu_0.3Co_2.7P/NC also shows excellent HER activity, affording a current density of 10 mA cm^–2 at a low overpotential of 0.22 V. In addition, a homemade electrolyzer with Cu_0.3Co_2.7P/NC paired electrodes shows 60% larger current density than Pt/RuO₂ couple at 1.74 V, along with negligible catalytic deactivation after 50 h operation. The manipulation of electronic structure by controlled incorporation of second metal sheds light on understanding and synthesizing bimetallic transition metal phosphides for electrolysis‐based energy conversion.     

Ultrafine Metal Nanoparticles/N‐Doped Porous Carbon Hybrids Coated on Carbon Fibers as Flexible and Binder‐Free Water Splitting Catalysts

By employing in situ reduction of metal precursor and metal‐assisted carbon etching process, this study achieves a series of ultrafine transition metal‐based nanoparticles (Ni–Fe, Ni–Mo) embedded in N‐doped carbon, which are found efficient catalysts for electrolytic water splitting. The as‐prepared hybrid materials demonstrate outstanding catalytic activities as non‐noble metal electrodes rendered by the synergistic effect of bimetal elements and N‐dopants, the improved electrical conductivity, and hydrophilism. Ni/Mo₂C@N‐doped porous carbon (NiMo‐polyvinylpyrrolidone (PVP)) and NiFe@N‐doped carbon (NiFe‐PVP) produce low overpotentials of 130 and 297 mV at a current density of 10 mA cm^?2 as catalysts for hydrogen evolution reaction and oxygen evolution reaction, respectively. In addition, these binder‐free electrodes show long‐term stability. Overall water splitting is also demonstrated based on the couple of NiMo‐PVP||NiFe‐PVP catalyzer. This represents a simple and effective synthesis method toward a new type of nanometal–carbon hybrid electrodes.     

High‐Performance Trifunctional Electrocatalysts Based on FeCo/Co2P Hybrid Nanoparticles for Zinc–Air Battery and Self‐Powered Overall Water Splitting

Currently, it is still a significant challenge to simultaneously boost various reactions by one electrocatalyst with high activity, excellent durability, as well as low cost. Herein, hybrid trifunctional electrocatalysts are explored via a facile one‐pot strategy toward an efficient oxygen reduction reaction (ORR), oxygen evolution reaction (OER), and hydrogen evolution reaction (HER). The catalysts are rationally designed to be composed by FeCo nanoparticles encapsuled in graphitic carbon films, Co₂P nanoparticles, and N,P‐codoped carbon nanofiber networks. The FeCo nanoparticles and the synergistic effect from Co₂P and FeCo nanoparticles make the dominant contributions to the ORR, OER, and HER activities, respectively. Their bifunctional activity parameter (?E) for ORR and OER is low to 0.77 V, which is much smaller than those of most nonprecious metal catalysts ever reported, and comparable with state‐of‐the‐art Pt/C and RuO₂ (0.78 V). Accordingly, the as‐assembled Zn–air battery exhibits a high power density of 154 mW cm^?2 with a low charge–discharge voltage gap of 0.83 V (at 10 mA cm^?2) and excellent stability. The as‐constructed overall water‐splitting cell achieves a current density of 10 mA cm^?2 (at 1.68 V), which is comparable to the best reported trifunctional catalysts.     

Interfacial Scaffolding Preparation of Hierarchical PBA‐Based Derivative Electrocatalysts for Efficient Water Splitting

The development of highly efficient and durable electrocatalysts is crucial for overall water splitting. Herein, the in situ scaffolding formation of 3D Prussian blue analogues (PBAs) on a variety of 2D or 1D metal hydroxides/oxides to fabricate hierarchical nanostructures is first demonstrated. Typically, cobalt hydroxide or oxide nanoarrays are used as the precursor and structural oriented template for the subsequent growth of 3D PBA nanocubes. The mechanism study reveals that the interfacial scaffolding process can be reversibly controlled via the in situ ion exchange process with adjusting coordination ions. Thus, the facile, versatile strategy can extend to successfully fabricate a variety of hierarchical PBA‐based nanostructures including on cobalt fluoride hydroxide, copper hydroxide, monometal or bimetal nickel–cobalt hydroxides, cobalt oxide, and manganese oxide nanosheets with structural tailor‐ability and chemical diversity. More interestingly, the metal nitride derivatives obtained via controlled calcination process exhibit good electrocatalytic activity for water splitting with low overpotentials, and remarkable durability for 1200 h, thanks to the superior intrinsic activity of bimetal nature and the scrupulous hierarchical structure. This versatile strategy provides a paradigm for rational design of PBA‐based functional nanomaterials, which is highly promising in energy conversion, storage, and electrocatalytic fields.     

Defect Engineering Promoted Ultrafine Ir Nanoparticle Growth and Sr Single-Atom Adsorption on TiO2 Nanowires to Achieve High-Performance Overall Water Splitting in Acidic Media

This work reports the use of the metal-support interaction strengthening through defect engineering and single atom adsorption to the supports to increase the catalytic activities of metals. Specifically, the plasma treated TiO₂ nanowires with the Ir nanoparticle growth and the Sr single atom adsorption (the Ir@Sr-p-TiO₂ NWs) are synthesized and demonstrated to be efficient catalysts for OER and HER. They only need overpotentials of 250 and 32 mV to drive 10 mA cm⁻² for OER and HER, respectively. Their OER and HER activities are much higher than the commercial IrO₂ and Pt/C. The high activities of the Ir@Sr-p-TiO₂ NWs mainly arise from the strengthened metal-support interactions between the Ir nanoparticles and the p-TiO₂ NWs, achieved by the plasma generated oxygen defects (Vo·) and the Sr adsorption on the p-TiO₂ NWs. Analysis and DFT calculations indicate that the Vo· and Sr adsorption can promote the charge transfer from the p-TiO₂ NWs to the Ir nanoparticles, optimizing the adsorptions of the OER and HER intermediates on the O- and H-covered Ir nanoparticles. Additionally, the strong metal-support interactions can increase the stabilities of the Ir NPs against the chemical corrosions, increasing the OER and HER durabilities of the Ir@Sr-p-TiO₂ NWs.     

Co–Ni‐Based Nanotubes/Nanosheets as Efficient Water Splitting Electrocatalysts

One promising approach to hydrogen energy utilization from full water splitting relies on the successful development of earth‐abundant, efficient, and stable electrocatalysts for oxygen evolution reaction (OER) and hydrogen evolution reaction (HER). Here, homologous Co–Ni‐based nanotube/nanosheet structures with tunable Co/Ni ratios, including hydroxides and nitrides, are grown on conductive substrates by a cation‐exchanging method to grow hydroxides, followed by anion exchanging to obtain corresponding nitrides. These hydroxide OER catalysts and nitride HER catalysts exhibit low overpotentials, small Tafel slopes, and high current densities, which are attributed to their large electrochemically reactive surface, 1D morphologies for charge conduction, and octahedral coordination states of metal ions for efficient catalytic activities. The homologous Co–Ni‐based nanotube hydroxides and nitrides suggest promising electrocatalysts for full water splitting with high efficiency, good stability, convenient fabrication, and low cost.     

High Efficiency Photocatalytic Water Splitting Using 2D α‐Fe2O3/g‐C3N4 Z‐Scheme Catalysts

Photocatalysis is the most promising method for achieving artificial photosynthesis, but a bottleneck is encountered in finding materials that could efficiently promote the water splitting reaction. The nontoxicity, low cost, and versatility of photocatalysts make them especially attractive for this application. This study demonstrates that small amounts of α‐Fe₂O₃ nanosheets can actively promote exfoliation of g‐C₃N₄, producing 2D hybrid that exhibits tight interfaces and an all‐solid‐state Z‐scheme junction. These nanostructured hybrids present a high H₂ evolution rate >3 × 10⁴ µmol g^‐1 h^‐1 and external quantum efficiency of 44.35% at λ = 420 nm, the highest value so far reported among the family of g‐C₃N₄ photocatalysts. Besides effectively suppressing the recombination of electron–hole pairs, this Z‐scheme junction also exhibits activity toward overall water splitting without any sacrificial donor. The proposed synthetic route for controlled production of 2D g‐C₃N₄‐based structures provides a scalable alternative toward the development of highly efficient and active photocatalysts.     

Interface Engineering of Hierarchical Branched Mo‐Doped Ni3S2/NixPy Hollow Heterostructure Nanorods for Efficient Overall Water Splitting

Rational design and construction of bifunctional electrocatalysts with excellent activity and durability is imperative for water splitting. Herein, a novel top‐down strategy to realize a hierarchical branched Mo‐doped sulfide/phosphide heterostructure (Mo‐Ni₃S₂/Ni_xP_y hollow nanorods), by partially phosphating Mo‐Ni₃S₂/NF flower clusters, is proposed. Benefitting from the optimized electronic structure configuration, hierarchical branched hollow nanorod structure, and abundant heterogeneous interfaces, the as‐obtained multisite Mo‐Ni₃S₂/Ni_xP_y/NF electrode has remarkable stability and bifunctional electrocatalytic activity in the hydrogen evolution reaction (HER)/oxygen evolution reaction (OER) in 1 m KOH solutions. It possesses an extremely low overpotential of 238 mV at the current density of 50 mA cm^?2 for OER. Importantly, when assembled as anode and cathode simultaneously, it merely requires an ultralow cell voltage of 1.46 V to achieve the current density of 10 mA cm^?2, with excellent durability for over 72 h, outperforming most of the reported Ni‐based bifunctional materials. Density functional theory results further confirm that the doped heterostructure can synergistically optimize Gibbs free energies of H and O‐containing intermediates (OH*, O*, and OOH*) during HER and OER processes, thus accelerating the catalytic kinetics of electrochemical water splitting. This work demonstrates the importance of the rational combination of metal doping and interface engineering for advanced catalytic materials.     

From Water Oxidation to Reduction: Homologous Ni–Co Based Nanowires as Complementary Water Splitting Electrocatalysts

A homologous Ni–Co based nanowire system, consisting of both nickel cobalt oxide and nickel cobalt sulfide nanowires, is developed for efficient, complementary water splitting. The spinel‐type nickel cobalt oxide (NiCo₂O₄) nanowires are hydrothermally synthesized and can serve as an excellent oxygen evolution reaction catalyst. Subsequent sulfurization of the NiCo₂O₄ nanowires leads to the formation of pyrite‐type nickel cobalt sulfide (Ni_0.33Co_0.67S₂) nanowires. Due to the 1D nanowire morphology and enhanced charge transport capability, the Ni_0.33Co_0.67S₂ nanowires function as an efficient, stable, and robust nonnoble metal electrocatalyst for hydrogen evolution reaction (HER), substantially exceeding CoS₂ or NiS₂ nanostructures synthesized under similar methods. The Ni_0.33Co_0.67S₂ nanowires exhibit low onset potential of ?65, ?39, and ?50 mV versus reversible hydrogen electrode, Tafel slopes of 44, 68, and 118 mV dec^?1 at acidic, neutral, and basic conditions, respectively, and excellent stability, comparable to the best reported non‐noble metal‐based HER catalysts. Furthermore, the homologous Ni_0.33Co_0.67S₂ nanowires and NiCo₂O₄ nanowires are assembled into an all‐nanowire based water splitting electrolyzer with a current density of 5 mA cm^?2 at a voltage as 1.65 V, thus suggesting a unique homologous, earth abundant material system for water splitting.     

Amorphous Cobalt Boride (Co2B) as a Highly Efficient Nonprecious Catalyst for Electrochemical Water Splitting: Oxygen and Hydrogen Evolution

It is demonstrated that amorphous cobalt boride (Co₂B) prepared by the chemical reduction of CoCl₂ using NaBH₄ is an exceptionally efficient electrocatalyst for the oxygen evolution reaction (OER) in alkaline electrolytes and is simultaneously active for catalyzing the hydrogen evolution reaction (HER). The catalyst achieves a current density of 10 mA cm^?2 at 1.61 V on an inert support and at 1.59 V when impregnated with nitrogen‐doped graphene. Stable performance is maintained at 10 mA cm^?2 for at least 60 h. The optimized catalyst, Co₂B annealed at 500 °C (Co₂B‐500) evolves oxygen more efficiently than RuO₂ and IrO₂, and its performance matches the best cobalt‐based catalysts reported to date. Co₂B is irreversibly oxidized at OER conditions to form a CoOOH surface layer. The active form of the catalyst is therefore represented as CoOOH/Co₂B. EXAFS observations indicate that boron induces lattice strain in the crystal structure of the metal, which potentially diminishes the thermodynamic and kinetic barrier of the hydroxylation reaction, formation of the OOH* intermediate, a key limiting step in the OER.     

Multifunctional Single‐Crystallized Carbonate Hydroxides as Highly Efficient Electrocatalyst for Full Water splitting

The controllable synthesis of single‐crystallized iron‐cobalt carbonate hydroxide nanosheets array on 3D conductive Ni foam (FCCH/NF) as a monolithic oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) bifunctional electrocatalyst for full water splitting is described. The results demonstrate that the incorporation of Fe can effectively tune the morphology, composition, electronic structure, and electrochemical active surface area of the electrocatalysts, thus greatly enhancing the intrinsic electrocatalytic activity. The optimal electrocatalyst (F_0.25C₁CH/NF) can deliver 10 and 1000 mA cm^?2 at very small overpotentials of 77 and 256 mV for HER and 228 and 308 mV for OER in 1.0 m KOH without significant interference from gas evolution. The F_0.25C₁CH‐based two‐electrode alkaline water electrolyzer only requires cell voltages of 1.45 and 1.52 V to achieve current densities of 10 and 500 mA cm^?2. The results demonstrate that such fascinating electrocatalytic activity can be ascribed to the increase in the catalytic active surface area, facilitated electron and mass transport properties, and the synergistic interactions because of the incorporation of Fe.     

A Large‐Scale Graphene–Bimetal Film Electrode with an Ultrahigh Mass Catalytic Activity for Durable Water Splitting

The practical industralization of water splitting needs high‐efficient and cost‐effective catalytic electrodes. A versatile and scalable solution‐processing method to prepare such a catalytic electrode with high flexibility and conductivity is introduced. This preparation method is applicable for a wide variety of metal species and takes graphene sheets as metal carriers and film‐forming agents, resulting in 100% utilization of raw materials. The obtained graphene–bimetal film has excellent comprehensive performance with high areal activity and superior turnover frequency at a low mass loading of 0.05 mg cm^?2, as well as a record‐high mass activity for oxygen or hydrogen evolution. The assembled two‐electrode configuration can be used in a practical full water splitting system, requiring a cell voltage of 1.58 or 1.50 V at 30 or 70 °C to afford a current density of 10 mA cm^?2; it also exhibits a long‐term durability over 200 h, superior to most of the reported systems for the same purpose. This work provides a new platform for large‐scale and high‐yield production of electrocatalysts and also uncovers the design principles of catalytic electrodes with high mass activity toward industralization.     

In Situ Grown Bimetallic MOF‐Based Composite as Highly Efficient Bifunctional Electrocatalyst for Overall Water Splitting with Ultrastability at High Current Densities

A newly designed water‐stable NH₂‐MIL‐88B(Fe₂Ni)‐metal–organic framework (MOF), in situ grown on the surface of a highly conducting 3D macroporous nickel foam (NF), termed NFN‐MOF/NF, is demonstrated to be a highly efficient bifunctional electrocatalyst for overall water splitting with ultrastability at high current densities. The NFN‐MOF/NF achieves ultralow overpotentials of 240 and 87 mV at current density of 10 mA cm^?2 for the oxygen evolution reaction and hydrogen evolution reaction, respectively, in 1 m KOH. For the overall water splitting, it requires only an ultralow cell voltage of 1.56 V to reach the current density of 10 mA cm^?2, outperforming the pairing of Pt/C on NF as the cathode and IrO₂ on NF as the anode at the same catalyst loading. The stability of the NFN‐MOF/NF catalyst is also outstanding, exhibiting only a minor chronopotentiometric decay of 7.8% at 500 mA cm^?2 after 30 h. The success of the present NFN‐MOF/NF catalyst is attributed to the abundant active centers, the bimetallic clusters {Fe₂Ni(µ₃‐O)(COO)₆(H₂O)₃}, in the MOF, the positive coupling effect between Ni and Fe metal ions in the MOF, and synergistic effect between the MOF and NF.