Oriented Transformation of Co‐LDH into 2D/3D ZIF‐67 to Achieve Co–N–C Hybrids for Efficient Overall Water Splitting

Construction of well‐defined metal–organic framework precursor is vital to derive highly efficient transition metal–carbon‐based electrocatalyst for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in water splitting. Herein, a novel strategy involving an in situ transformation of ultrathin cobalt layered double hydroxide into 2D cobalt zeolitic imidazolate framework (ZIF‐67) nanosheets grafted with 3D ZIF‐67 polyhedra supported on the surface of carbon cloth (2D/3D ZIF‐67@CC) precursor is proposed. After a low‐temperature pyrolysis, this precursor can be further converted into hybrid composites composed of ultrafine cobalt nanoparticles embedded within 2D N‐doped carbon nanosheets and 3D N‐doped hollow carbon polyhedra (Co@N‐CS/N‐HCP@CC). Experimental and density functional theory calculations results indicate that such composites have the advantages of a large number of accessible active sites, accelerated charge/mass transfer ability, the synergistic effect of components as well as an optimal water adsorption energy change. As a result, the obtained Co@N‐CS/N‐HCP@CC catalyst requires overpotentials of only 66 and 248 mV to reach a current density of 10 mA cm^?2 for HER and OER in 1.0 m KOH, respectively. Remarkably, it enables an alkali‐electrolyzer with a current density of 10 mA cm^?2 at a low cell voltage of 1.545 V, superior to that of the IrO₂@CC||Pt/C@CC couple (1.592 V).     

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‐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.     

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.     

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.     

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.     

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.     

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.     

Recent Advancement of p‐ and d‐Block Elements,Single Atoms,and Graphene‐Based Photoelectrochemical Electrodes for Water Splitting

Solar‐assisted photoelectrochemical (PEC) water splitting to produce hydrogen energy is considered the most promising solution for clean, green, and renewable sources of energy. For scaled production of hydrogen and oxygen, highly active, robust, and cost‐effective PEC electrodes are required. However, most of the available semiconductors as a PEC electrodes have poor light absorption, material degradation, charge separation, and transportability, which result in very low efficiency for photo‐water splitting. Generally, a promising photoelectrode is obtained when the surface of the semiconductor is modified/decorated with a suitable co‐catalyst because it increases the light absorbance spectrum and prevents electron–hole recombination during photoelectrode reactions. In this regard, numerous p‐ and d‐block elements, single atoms, and graphene‐based PEC electrodes have been widely used as semiconductor/co‐catalyst junctions to boost the performances of PEC overall water splitting. This review enumerates the recent progress and applications of p‐ and d‐block elements, single atoms, and graphene‐based PEC electrodes for water splitting. The focus is placed on fundamental mechanism, efficiency, cells design, and various aspects that contribute to the large‐scale prototype device. Finally, future perspectives, summary, challenges, and outlook for improving the activity of PEC photoelectrodes toward whole‐cell water splitting are addressed.     

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.     

Monolithic Photoassisted Water Splitting Device Using Anodized Ni‐Fe Oxygen Evolution Catalytic Substrate

Large‐scale industrial application of solar‐driven water splitting has called for the development of oxygen evolution reaction (OER) catalysts that deliver high catalytic activity and stability. Here it is shown that an efficient OER catalytic substrate can be developed by roll‐to‐roll fabrication of electrodeposited Ni‐Fe foils, followed by anodization. An amorphous oxyhydroxide layer directly formed on Ni‐Fe foils exhibits high catalytic activity toward water oxidation in 1 m KOH solution, which requires an overpotential of 0.251 V to reach current density of 10 mA cm^–2. The developed catalytic electrode shows the best OER activity among catalysts with film structure. The catalyst also shows prolonged stability at vigorous gas evolution condition for 36 h. To demonstrate the monolithic photoassisted water splitting device, an amorphous silicon solar cell is fabricated on Ni‐Fe catalytic substrate, resulting in lowering OER overpotential under light illumination. This monolithic device is the first demonstration that the OER catalytic substrates and the solar cells are integrated and can be easily applied for industrial scale solar‐driven water electrolysis.     

A Perovskite Nanorod as Bifunctional Electrocatalyst for Overall Water Splitting

The development of highly efficient and low‐cost electrocatalysts for oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) is paramount for water splitting associated with the storage of clean and renewable energy. Here, this study reports its findings in the development of a nanostructured perovskite oxide as OER/HER bifunctional electrocatalyst for overall water splitting. Prepared by a facile electrospinning method, SrNb_0.1Co_0.7Fe_0.2O_3–δ perovskite nanorods (SNCF‐NRs) display excellent OER and HER activity and stability in an alkaline solution, benefiting from the catalytic nature of perovskites and unique structural features. More importantly, the SNCF‐NR delivers a current density of 10 mA cm^?2 at a cell voltage of merely ≈1.68 V while maintaining remarkable durability when used as both anodic and cathodic catalysts in an alkaline water electrolyzer. The performance of this bifunctional perovskite material is among the best ever reported for overall water splitting, offering a cost‐effective alternative to noble metal based electrocatalysts.     

3D Graphene‐Based H2‐Production Photocatalyst and Electrocatalyst

Hydrogen (H₂) has been deemed as the most promising and valuable alternative to nonrenewable fossil fuels. Photocatalytic and electrocatalytic water splitting are considered to be the most efficient and environmentally friendly approaches for the sustainable H₂ evolution reaction (HER). Graphene with a 3D framework has been utilized for the HER due to its unique structure and properties, including its hierarchical network, large specific surface area, diverse pore distribution, outstanding light absorption ability, and excellent electrical conductivity. The large specific surface area and hierarchically porous structure of 3D graphene can not only maximize the exposure of active sites but also promote electron transfer and gas product diffusion. In addition, the free‐standing 3D graphene monolith is easily recycled compared with powder phase support, which can prevent the loss of active catalysts. By making full use of the aforementioned merits, 3D graphene‐based composite materials show great promise as high‐performance catalysts toward photocatalytic and electrocatalytic HER. In this review, recent advances in fabricating 3D graphene‐based composite materials and their applications in both photocatalytic and electrocatalytic HER are summarized and discussed. Furthermore, the current challenges and future vision associated with the design, fabrication, and integration of 3D graphene‐based composite materials toward HER are put forward.     

Construction of Defect‐Rich RhCu Nanotubes with Highly Active Rh3Cu1 Alloy Phase for Overall Water Splitting in All pH Values

The combination of precious metals with non‐noble metals is an effective way to develop highly efficient, stable, and low cost electrocatalysts for overall water splitting. Herein, RhCu nanotubes (NTs) with rich structural defects are successfully synthesized by a mixed‐solvent strategy, which display superior activity and excellent stability for both the hydrogen evolution reaction (HER) and oxygen evolution reaction in all pH values. In particular, it only needs 8, 12, and 57 mV to deliver the current density of 10 mA cm^?2 for HER in alkaline, acidic, and neutral conditions, respectively. Experiments combined with density functional theory (DFT) calculations reveal that the exposure of a suitable composition of a highly active Rh₃Cu₁ alloy phase through acid etching is the key to improve electrocatalytic performance, since it weakens the adsorption free energy of atomic oxygen and hydrogen, as well as facilitating the dissociation of water molecules. In addition, the structural defects can also boost the catalytic performance because the adsorption of reactants can be largely enhanced. The results provide a simple method to prepare alloy NTs as highly efficient electrocatalysts for overall water splitting in all pH values.