Hierarchical NiMo Phosphide Nanosheets Strongly Anchored on Carbon Nanotubes as Robust Electrocatalysts for Overall Water Splitting

Tungsten doped FeCoP2 nanoparticles embedded into carbon for highly efficient oxygen evolution reaction

Designing active and stable electrocatalysts with economic efficiency for oxygen evolution reaction (OER) is essential for developing water splitting process at an industrial scale. Herein, we rationally designed a tungsten doped iron cobalt phosphide incorporated with carbon (Wx-FeCoP2/C), prepared by a mechanochemical approach. X-ray photoelectron spectroscopy (XPS) revealed that the doping of W led to an increasing of Co3+/Co2+ and Fe3+/Fe2+ molar ratios, which contributed to the enhanced OER performance. As a result, a current density of 10 mA cm-2 was achieved in 1 M KOH at an overpotential of 264 mV on the optimized W0.1-FeCoP2/C. Moreover, at high current density of 100 mA cm-2, the overpotential value was 310 mV, and the corresponding Tafel slope was measured to be 48.5 mV dec-1, placing it among the best phosphide-based catalysts for OER. This work is expected to enlighten the design strategy of highly efficient phosphide-based OER catalysts.

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.

Nickel Nanoparticles Protruding from Molybdenum Carbide Micropillars with Carbon Layer-Protected Biphasic 0D/1D Heterostructures for Efficient Water Splitting

It remains a tremendous challenge to achieve high-efficiency bifunctional electrocatalysts for both the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) for hydrogen production by water splitting. Herein, a novel hybrid of 0D nickel nanoparticles dispersed on the one-dimensional (1D) molybdenum carbide micropillars embedded in the carbon layers (Ni/Mo2C@C) was successfully prepared on nickel foam by a facile pyrolysis strategy. During the synthesis process, the nickel nanoparticles and molybdenum carbide were simultaneously generated under H2 and C2H2 mixed atmospheres and conformally encapsulated in the carbon layers. Benefiting from the distinctive 0D/1D heterostructure and the synergistic effect of the biphasic Mo2C and Ni together with the protective effect of the carbon layer, the reduced activation energy barriers and fast catalytic reaction kinetics can be achieved, resulting in a small overpotential of 96 mV for the HER and 266 mV for the OER at the current density of 10 mA cm-2 together with excellent durability in 1.0 M KOH electrolyte. In addition, using the developed Ni/Mo2C@C as both the cathode and anode, the constructed electrolyzer exhibits a small voltage of 1.55 V for the overall water splitting. The novel designed Ni/Mo2C@C may give inspiration for the development of efficient bifunctional catalysts with low-cost transition metal elements for water splitting.

Highly efficient sulfur-doped Ni3Fe electrocatalysts for overall water splitting: Rapid synthesis, mechanism and driven by sustainable energy

Designing efficient electrocatalysts and insight into their electrocatalytic mechanisms are significantly important for storing and converting the intermittent sustainable energy sources into clean hydrogen. In this study, we synthesize the bifunctional sulfur-doped Ni3Fe (NiFeS) electrocatalysts by a simple electrodeposition method only taking 30 s. After optimizing the components, it was found that the synthesized NiFeS electrocatalysts exhibit the excellent hydrogen and oxygen evolution reaction performances in 1.0 M potassium hydroxide solution. The results of experimental and theoretical calculations reveal that the introduced sulfur could optimize the electronic distribution, which make Ni electron-rich and Fe electron-deficient, thereby weakening the energy barriers of potential-determining steps, i.e. the absorption of H2O molecule on Ni sites for HER and formation of *OOH on Fe sites for OER, respectively. Besides, the NiFeS electrocatalysts are used as the bifunctional electrodes to water splitting, which only need 1.51 V to reach 10 mA·cm-2, and exhibits excellent durability and a >95% Faraday efficiency. Furthermore, the intermittent kinetic, wind and solar energies are used to power the assembled electrolyzer with NiFeS bi-electrodes to verify their great application potential. This work not only proved a deep insight into mechanism of the boosted electrocatalytic activities of NiFeS, but also the synthesized NiFeS electrocatalysts have great application prospect in the conversion of intermittent and sustainable energy sources into hydrogen by water electrocatalysis.

Experimental and Computational Insights into the Overall Water Splitting Reaction by the Fe-Co-Ni-P Electrocatalyst

Nonprecious transition-metal phosphides (TMPs) are versatile materials with tunable electronic and structural properties that could be promising as catalysts for energy conversion applications. Despite the facts, TMPs are not explored thoroughly to understand the chemistry behind their rich catalytic properties for the water splitting reaction. Herein, spiky ball-shaped monodispersed TMP nanoparticles composed of Fe, Co, and Ni are developed and used as efficient electrocatalysts for hydrogen and oxygen evolution reaction (HER, OER), and overall water splitting in alkaline medium; and their surface chemistry was explored to understand the reaction mechanism. The optimized Fe0.5CoNi0.5P catalyst shows attractive activities of HER and OER with low overpotentials and Tafel slopes, and with high mass activities, turnover frequencies, and exchange current densities. When applied to overall water splitting, the electrolyzer Fe0.5CoNi0.5P||Fe0.5CoNi0.5P cell can reach a 10 mA cm-2 current density at cell voltages of only 1.52 and 1.56 V in 1.0 M and 30 wt % KOH, respectively, much lower than those of commercial IrO2||Pt/C. The optimized electrolyzer with sizable numbers of chemically active sites exhibits superior durability up to 70 h and 5000 cycles in 1.0 M KOH and can attain a current density as high as 1000 mA cm-2, showing a class of efficient bifunctional electrocatalysis. Experimental and density functional theory-based mechanistic analyses reveal that surface reconstruction takes place in the presence of KOH to form the TMP precatalyst, which results in high coverage of oxygen active species for the OER with a low apparent activation energy (Ea) for conversion of *OOH to O2. These also evidenced the thermoneutral adsorption of H* for the efficient HER half-reaction.

A Review of Transition Metal Boride, Carbide, Pnictide, and Chalcogenide Water Oxidation Electrocatalysts

Transition metal borides, carbides, pnictides, and chalcogenides (X-ides) have emerged as a class of materials for the oxygen evolution reaction (OER). Because of their high earth abundance, electrical conductivity, and OER performance, these electrocatalysts have the potential to enable the practical application of green energy conversion and storage. Under OER potentials, X-ide electrocatalysts demonstrate various degrees of oxidation resistance due to their differences in chemical composition, crystal structure, and morphology. Depending on their resistance to oxidation, these catalysts will fall into one of three post-OER electrocatalyst categories: fully oxidized oxide/(oxy)hydroxide material, partially oxidized core@shell structure, and unoxidized material. In the past ten years (from 2013 to 2022), over 890 peer-reviewed research papers have focused on X-ide OER electrocatalysts. Previous review papers have provided limited conclusions and have omitted the significance of "catalytically active sites/species/phases" in X-ide OER electrocatalysts. In this review, a comprehensive summary of (i) experimental parameters (e.g., substrates, electrocatalyst loading amounts, geometric overpotentials, Tafel slopes, etc.) and (ii) electrochemical stability tests and post-analyses in X-ide OER electrocatalyst publications from 2013 to 2022 is provided. Both mono and polyanion X-ides are discussed and classified with respect to their material transformation during the OER. Special analytical techniques employed to study X-ide reconstruction are also evaluated. Additionally, future challenges and questions yet to be answered are provided in each section. This review aims to provide researchers with a toolkit to approach X-ide OER electrocatalyst research and to showcase necessary avenues for future investigation.

One-pot self-assembled bimetallic sulfide particle cluster-supported three-dimensional graphene aerogel as an efficient electrocatalyst for the oxygen evolution reaction

The preparation of an electrocatalyst for the oxygen evolution reaction (OER) with high catalytic activity, good long-term durability and rapid reaction kinetics through interface engineering is of great significance. Herein, we have developed a bimetallic sulfide particle cluster-supported three-dimensional graphene aerogel (FeNiS@GA), which serves as an efficient electrocatalyst for OER, by a one-step hydrothermal method. Profiting from the synergy of the FeNiS particle cluster with high capacitance and GA with its three-dimensional porous nanostructure, FeNiS@GA shows a high specific surface area, large pore volume, low contact resistance, and decreases the electron and ion transport routes. FeNiS@GA exhibits outstanding OER activity (when the current density is 50 mA cm-2, the overpotential is 341 mV), low Tafel slope (63.87 mV dec-1) and remarkable stability in alkaline solutions, outperforming FeNiS, NiS@GA, FeS@GA and RuO2. Due to its simple synthesis process and excellent electrocatalytic performance, FeNiS@GA shows great potential to replace noble metal-based catalysts in practical applications.

Self-supported Ni2P/NiMoP2 bimetallic phosphide with strong electronic interaction for efficient overall water splitting

Electronic regulation via interface engineering is recognized as an attractive strategy for boosting electrocatalytic activity. In this work, a self-supported heterostructure electrocatalyst is explored by a feasible hydrothermal-pyrolysis strategy, in which Ni₂P nanoparticles are anchored on NiMoP₂ nanosheet arrays grown on carbon cloth (Ni₂P/NiMoP₂/CC). Benefitting from the nanosheet array architecture and the synergy effect between the Ni₂P and NiMoP₂, the as-prepared Ni₂P/NiMoP₂/CC manifests highly efficient activity and stability toward both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). Density functional theory calculations further indicates that the heterointerface in Ni₂P/NiMoP₂/CC enable optimized interface electron structure and reduce the activation barriers for intermediates, improving the intrinsic electrocatalytic activity. Remarkably, the Ni₂P/NiMoP₂/CC||Ni₂P/NiMoP₂/CC electrolyzer affords 10 mA cm^-2 at a low voltage of 1.59 V, outperforming its monometallic phosphides counterparts and most of transition metal-based bifunctional electrocatalysts. The electrolyser was powered by a solar cell to produce H₂ and O₂ simultaneously, indicating its potential application in solar-to-hydrogen conversion.

Inverse 'intra-lattice' charge transfer in nickel-molybdenum dual electrocatalysts regulated by under-coordinating the molybdenum center

The prevalence of intermetallic charge transfer is a marvel for fine-tuning the electronic structure of active centers in electrocatalysts. Although Pauling electronegativity is the primary deciding factor for the direction of charge transfer, we report an unorthodox intra-lattice 'inverse' charge transfer from Mo to Ni in two systems, Ni₇₃Mo alloy electrodeposited on Cu nanowires and NiMo-hydroxide (Ni : Mo = 5 : 1) on Ni foam. The inverse charge transfer deciphered by X-ray absorption fine structure studies and X-ray photoelectron spectroscopy has been understood by the Bader charge and projected density of state analyses. The undercoordinated Mo-center pushes the Mo 4d-orbitals close to the Fermi energy in the valence band region while Ni 3d-orbitals lie in the conduction band. Since electrons are donated from the electron-rich Mo-center to the electron-poor Ni-center, the inverse charge transfer effect navigates the Mo-center to become positively charged and vice versa. The reverse charge distribution in Ni₇₃Mo accelerates the electrochemical hydrogen evolution reaction in alkaline and acidic media with 0.35 and 0.07 s^-1 turnover frequency at -33 ± 10 and -54 ± 8 mV versus the reversible hydrogen electrode, respectively. The corresponding mass activities are 10.5 ± 2 and 2.9 ± 0.3 A g^-1 at 100, and 54 mV overpotential, respectively. Anodic potential oxidizes the Ni-center of NiMo-hydroxide for alkaline water oxidation with 0.43 O₂ s^-1 turnover frequency at 290 mV overpotential. This extremely durable homologous couple achieves water and urea splitting with cell voltages of 1.48 ± 0.02 and 1.32 ± 0.02 V, respectively, at 10 mA cm^-2.

In Situ Growth of Self-Supporting MOFs-Derived Ni2P on Hierarchical Doped Carbon for Efficient Overall Water Splitting

The in situ growth of metal organic framework (MOF) derivatives on the surface of nickel foam is a novel type of promising self-supporting electrode catalyst. In this paper, this work reports for the first time the strategy of in situ growth of Ni-MOF, where the metal source is purely provided by a nickel foam (NF) substrate without any external metal ions. MOF-derived Ni2P/NPC structure is achieved by the subsequent phosphidation to yield Ni2P on porous N, P-doped carbon (NPC) backbone. Such strategy provides the as-synthesized Ni2P/NPC/NF electrocatalyst an extremely low interfacial steric resistance. Moreover, a unique three-dimensional hierarchical structure is achieved in Ni2P/NPC/NF, providing massive active sites, short ion diffusion path, and high electrical conductivity. Directly applied as the electrode, Ni2P/NPC/NF demonstrates excellent electrocatalytic performance towards both the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), with low overpotentials of only 58 mV and 208 mV to drive 10 mA cm−2, respectively, in 1 M KOH. Furthermore, Ni2P/NPC/NF acting as the overall water splitting electrodes can generate a current density of 10 mA cm−2 at an ultralow cell voltage of 1.53 V. This simple strategy paves the way for the construction of self-supporting transition metal-based electrocatalysts.

Bridging electrocatalyst and cocatalyst studies for solar hydrogen production via water splitting

Solar-driven water-splitting has been considered as a promising technology for large-scale generation of sustainable energy for succeeding generations. Recent intensive efforts have led to the discovery of advanced multi-element-compound water-splitting electrocatalysts with very small overpotentials in anticipation of their application to solar cell-assisted water electrolysis. Although photocatalytic and photoelectrochemical water-splitting systems are more attractive approaches for scaling up without much technical complexity and high investment costs, improving their efficiencies remains a huge challenge. Hybridizing photocatalysts or photoelectrodes with cocatalysts has been an effective scheme to enhance their overall solar energy conversion efficiencies. However, direct integration of highly-active electrocatalysts as cocatalysts introduces critical factors that require careful consideration. These additional requirements limit the design principle for cocatalysts compared with electrocatalysts, decelerating development of cocatalyst materials. This perspective first summarizes the recent advances in electrocatalyst materials and the effective strategies to assemble cocatalyst/photoactive semiconductor composites, and further discusses the core principles and tools that hold the key in designing advanced cocatalysts and generating a deeper understanding on how to further push the limits of water-splitting efficiency.

A Superior and Stable Electrocatalytic Oxygen Evolution Reaction by One-Dimensional FeCoP Colloidal Nanostructures

Transition metal phosphides (TMPs) are expected to be excellent electrocatalysts for oxygen evolution reaction (OER) because of their high stability, highly conducting metalloid nature, highly abundant constituting elements, and the ability to act as a precatalyst due to in situ surface-formed oxy-hydroxide species. Herein, a "one-pot" colloidal approach has been used to develop a rod-shaped one-dimensional non-noble metal FeCoP electrocatalyst, which exhibits an excellent OER activity with an exceptionally high current density of 950 mA cm^-2, a turnover frequency value of 7.43 s^-1, and a low Tafel slope value of 54 mV dec^-1. The FeCoP electrocatalyst affords OER ultralow overpotentials of 230 and 260 mV at current densities of 50 and 100 mA cm^-2, respectively, in 1.0 M KOH, and demonstrates a superior catalytic stability of 10,000 cycles and durability up to 60 h at 50 mA cm^-2. An insight into the superior and stable electrocatalytic OER performance by the FeCoP nanorods is obtained by extensive X-ray photoelectron spectroscopy, X-ray diffraction, Raman and infrared spectroscopy, and cyclic voltammetry analyses for a mechanistic study. This reveals that a high number of electrocatalytically active sites enhance the oxygen evolution and kinetics by offering metal ion sites for utilitarian in situ surface formation and adsorption of *O, *OH, and *OOH reactive species for OER catalysis.

Self-supported Ni3S2@Ni2P/MoS2 heterostructures on nickel foam for an outstanding oxygen evolution reaction and efficient overall water splitting

Hydrogen production by electrocatalytic water splitting is a pollution-free, energy-saving, and efficient method. The low efficiency of hydrogen production, high overpotentials and expensive noble-metal catalysts have limited the development of hydrogen production from electrocatalytic water splitting. Therefore, the exploration of bifunctional electrocatalysts for water overall splitting to produce hydrogen is of profound significance. Herein, Ni₃S₂@Ni₂P/MoS₂ heterostructure electrocatalysts were synthesized on Ni foam through an environmentally friendly hydrothermal method and low-temperature phosphating method. The synergistic effects between different components and the mutual substitution principle between sulfur atoms and phosphorus atoms greatly improve the OER performance of the electrocatalyst. It is also an effective strategy to optimize the adsorption energies of intermediates by the design of heterostructured catalysts composed of multiple substances. Ni₃S₂@Ni₂P/MoS₂ only requires a low overpotential (η₁₀) of 175 mV at a current density of 10 mA cm^-2 in 1.0 M KOH solution and the stable duration exceeds 40 h. In addition, this heterogeneous structure is assembled into an electrolytic cell for overall water splitting, which exhibits a low cell voltage of 1.61 volts and retains the robust stability over 30 h at 10 mA cm^-2. The Ni₃S₂@Ni₂P/MoS₂ heterostructure prepared in this research provides a strategy for exploring other heterostructured electrocatalysts with different components.

NiCoP with Dandelion-like Arrays Anchored on Nanowires for Electrocatalytic Overall Water Splitting

Although transition-metal-based phosphides as cost-effective catalysts have great potential for transforming water to hydrogen, their electrocatalytic property for industrial application is still limited. Herein, we focus on developing amorphous NiCoP with dandelion-like arrays anchored on nanowires through a universal strategy of hydrothermal and phosphorization. The hierarchical structure features in larger catalytic surface areas expedited reaction kinetics and improved structural stability. Benefiting from these merits, the NiCoP reaches 10 mA cm^-2 at an overpotential of mere 57 mV for a hydrogen evolution reaction in standard solution. Also, a profound activity for the generation of oxygen is along with it, which requires 276 mV to attain 10 mA cm^-2. Moreover, it demonstrates satisfying durability for both processes.

Hierarchical Nanostructured Co-Mo-B/CoMoO4-x Amorphous Composite for the Alkaline Hydrogen Evolution Reaction

Transition metal borides (TMBs) are a class of important but less well-explored electrocatalytic materials for water splitting. The lack of an advanced methodology to synthesize complex nanostructured TMBs with tunable surface properties is a major obstacle to the exploration of the full potential of TMBs for electrocatalytic applications. Here, we report the facile fabrication of a cobalt foam (CF)-supported hierarchical nanostructured Co-Mo-B/CoMoO_4-x composite using a hydrothermal method, followed by annealing and NaBH₄ reduction treatments. Our study found that NaBH₄ reduction of CoMoO₄ resulted in the concurrent formation of amorphous Co-Mo-B and an O-vacancy-rich CoMoO_4-x substrate, which cooperatively catalyzed the hydrogen evolution reaction (HER) in an alkaline electrolyte. The hierarchical nanoporous structure derived from the dehydration and partial reduction reactions of the CoMoO₄·nH₂O precursor could offer ample accessible active sites, as well as interconnected channels for rapid mass transfer. In addition, the in situ growth of electrically conductive Co-Mo-B nanoparticles on the defective structured CoMoO_4-x substrate imparted the electrocatalyst with good electrical conductivity. As a result, the Co-Mo-B/CoMoO_4-x/CF catalyst showed impressively high activity and outstanding stability for the alkaline HER, outperforming most reported TMB electrocatalysts. For instance, it required an overpotential of 55 mV to afford 10 mA·cm^-2 and showed a fluctuation of only ±8 mV in a 100 h constant-current test at 100 mA·cm^-2.

Ultraeven Mo-Doped CoP Nanocrystals as Bifunctional Electrocatalyst for Efficient Overall Water Splitting

To develop precious-metal-free bifunctional catalysts for overall water splitting, ultraeven Mo-doped CoP composites (Mo-CoP) have been fabricated by an in situ phosphorization protocol using CoMo-layered double hydroxide (CoMo-LDH) as the precursor. The ordered arrangement of cations in the CoMo-LDH could be easily phosphored and generate the ultraevenly dispersed Mo element within the CoP structure, resulting in the excellent bifunctional catalyst for overall water splitting. The hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) catalytic activities of the composites present an increase first and then a decreasing tendency with increased Mo doping content. Among all the Mo-doped CoP materials, the composite with a Mo/Co mole ratio of 1/2.3 presents the highest HER activity and stability in acidic conditions. At the current density of -10 mA·cm^-2 in 0.5 M H₂SO₄, the overpotential is only 116 mV. In addition, the composite also presents excellent HER and OER performance under alkaline conditions. The overpotential is 118 mV for HER and 317 mV for OER at 10 mA cm^-2 in 1 M KOH. It requires a cell voltage of 1.7 V to achieve a current density of 10 mA·cm^-2 and maintains a stable water-splitting current for at least 24 h, which is superior to most reported alkaline media. This simple and efficient synthetic approach could also be used for ultraeven doping between other transition metal ions.

Nanostructured metal phosphides: from controllable synthesis to sustainable catalysis

Metal phosphides (MPs) with unique and desirable physicochemical properties provide promising potential in practical applications, such as the catalysis, gas/humidity sensor, environmental remediation, and energy storage fields, especially for transition metal phosphides (TMPs) and MPs consisting of group IIIA and IVA metal elements. Most studies, however, on the synthesis of MP nanomaterials still face intractable challenges, encompassing the need for a more thorough understanding of the growth mechanism, strategies for large-scale synthesis of targeted high-quality MPs, and practical achievement of functional applications. This review aims at providing a comprehensive update on the controllable synthetic strategies for MPs from various metal sources. Additionally, different passivation strategies for engineering the structural and electronic properties of MP nanostructures are scrutinized. Then, we showcase the implementable applications of MP-based materials in emerging sustainable catalytic fields including electrocatalysis, photocatalysis, mild thermocatalysis, and related hybrid systems. Finally, we offer a rational perspective on future opportunities and remaining challenges for the development of MPs in the materials science and sustainable catalysis fields.

An FexNi4−xPy/N, P co-doped carbon nanotube composite as a bifunctional electrocatalyst for oxygen and hydrogen electrode reactions

The Fe_xNi_4−xP_y/N, P-CNT complex exhibits excellent bifunctional catalytic performance towards HER and OER due to the synergy of high electrical conductivity, multicomponent active sites, and anchoring effect of carbon nanotubes.

Hierarchical Highly Wrinkled Trimetallic NiFeCu Phosphide Nanosheets on Nanodendrite Ni3S2/Ni Foam as an Efficient Electrocatalyst for the Oxygen Evolution Reaction

The sluggish oxygen evolution reaction (OER) and costly noble-metal oxide catalysts hinder the vast usage of environmentally friendly water splitting for hydrogen production. Elemental doping by partial replacing of parent metal elements with elements of higher electronegativity is considered to be one of the most efficient strategies to promote the electrocatalytic OER performance. In this work, we synthesize an efficient hierarchical highly wrinkled NiFeCu phosphide nanosheet on nanodendrite Ni₃S₂/NiF substrates through partial replacement of Cu instead of Ni and Fe in NiFeP@Ni₃S₂/NiF by using a facile electrodeposition method. The NiFeCuP@Ni₃S₂/NiF electrocatalyst needs only 230, 282, and 351 mV to reach 10, 50, and 100 mA × cm^-2, respectively. Notably, this electrocatalyst shows one of the lowest OER overpotentials at 10 mA/cm^-2 for metal phosphides and endured the OER at 20 mA × cm^-2 for 18 h with negligible voltage elevation. The X-ray photoelectron spectroscopy (XPS), double-layer capacitance (C_dl) plots, and electrochemical impedance spectroscopy show that the partial Cu doping in NiFeP@Ni₃S₂/NiF not only can change the electron density around Ni and Fe but also can increase the electrochemically active surface area and conductivity of electrocatalysts.

Self-Supported FeP-CoMoP Hierarchical Nanostructures for Efficient Hydrogen Evolution

Fabricating highly efficient electrocatalysts for electrochemical hydrogen generation is a top priority to relief the global energy crisis and environmental contamination. Herein, a rational synthetic strategy is developed for constructing well-defined FeP-CoMoP hierarchical nanostructures (HNSs). In general terms, the self-supported Co nanorods (NRs) are grown on conductive carbon cloth and directly serve as a self-sacrificing template. After solvothermal treatment, Co NRs are converted into well-ordered Co-Mo nanotubes (NTs). Subsequently, the small-sized Fe oxyhydroxide nanorods arrays are hydrothermally grown on the surface of Co-Mo NTs to form Fe-Co-Mo HNSs, which are then converted into FeP-CoMoP HNSs through a facile phosphorization treatment. FeP-CoMoP HNSs display high activity for hydrogen evolution reaction (HER) with an ultralow cathodic overpotential of 33 mV at 10 mA cm^-2 and a Tafel slope of 51 mV dec^-1 . Moreover, FeP-CoMoP HNSs also possess an excellent electrochemical durability in alkaline media. First-principles density functional theory (DFT) calculations demonstrate that the remarkable HER activitiy of FeP-CoMoP HNSs originates from the synergistic effect between FeP and CoMoP.

TaS2, TaSe2, and Their Heterogeneous Films as Catalysts for the Hydrogen Evolution Reaction

Metallic two-dimensional transition-metal dichalcogenides (TMDs) of the group 5 metals are emerging as catalysts for an efficient hydrogen evolution reaction (HER). The HER activity of the group 5 TMDs originates from the unsaturated chalcogen edges and the highly active surface basal planes, whereas the HER activity of the widely studied group 6 TMDs originates solely from the chalcogen- or metal-unsaturated edges. However, the batch production of such nanomaterials and their scalable processing into high-performance electrocatalysts is still challenging. Herein, we report the liquid-phase exfoliation of the 2H-TaS₂ crystals by using 2-propanol to produce single/few-layer (1H/2H) flakes, which are afterward deposited as catalytic films. A thermal treatment-aided texturization of the catalytic films is used to increase their porosity, promoting the ion access to the basal planes of the flakes, as well as the number of catalytic edges of the flakes. The hybridization of the H-TaS₂ flakes and H-TaSe₂ flakes tunes the Gibbs free energy of the adsorbed atomic hydrogen onto the H-TaS₂ basal planes to the optimal thermo-neutral value. In 0.5 M H₂SO₄, the heterogeneous catalysts exhibit a low overpotential (versus RHE, reversible hydrogen electrode) at the cathodic current of 10 mA cm^–2 (η₁₀) of 120 mV and high mass activity of 314 A g^–1 at an overpotential of 200 mV. In 1 M KOH, they show a η₁₀ of 230 mV and a mass activity of 220 A g^–1 at an overpotential of 300 mV. Our results provide new insight into the usage of the metallic group 5 TMDs for the HER through scalable material preparation and electrode processing.

Ru nanoparticles encapsulated in ZIFs-derived porous N-doped hierarchical carbon nanofibers for enhanced hydrogen evolution reaction

Ru nanoparticles, encapsulated in ZIFs-derived porous N-doped hierarchical carbon nanofibers with excellent HER performance, were achieved.

NiFeP nanoflakes composite with CoP on carbon cloth as flexible and durable electrocatalyst for efficient overall water splitting

High-performance and earth-abundant NiFeP is an excellent bifunctional catalyst for water splitting in acidic and alkaline environments, and NiFeP nanoflakes on CoP layer composite with a conductive carbon cloth (CC) substrate as the trunk-leaf flexible structure (NiFeP/CoP/CC) is prepared by direct high-temperature phosphorization. Overpotentials of only 96.38 and 78.80 mV are required in hydrogen evolution reaction in 1 M KOH and 0.5 M H₂SO₄, respectively, to generate an electrocatalytic current density of 10 mA cm^-2. A small Tafel slope of 70.67 and 63.21 mV per decade are also observed from NiFeP/CoP/CC revealing a Volmer-Heyrovsky mechanism in both media. The electrocatalyst also delivers excellent oxygen evolution reaction performance in the alkaline environment and long-term electrochemical durability for at least 24 h in electrolytes over a wide pH range. A device is assembled with two identical flexible ultrathin NiFeP/CoP/CC as both the anode and cathode in 1 M KOH driven by a set of 1.6 V solar cells. During 32 h of electrolysis, the results show that the current of our electrodes maintains 80% performance at a constant voltage of 1.7 V for 32 h, and the NiFeP/CoP/CC anodes and cathodes have large potential in industrial alkaline water splitting.

Nitrogen-Doped Porous Carbon Nanosheets Strongly Coupled with Mo2C Nanoparticles for Efficient Electrocatalytic Hydrogen Evolution

Exploring earth-abundant and noble metal-free catalysts for water electrolysis is pivotal in renewable hydrogen production. Herein, a highly active electrocatalyst of nitrogen-doped porous carbon nanosheets coupled with Mo₂C nanoparticles (Mo₂C/NPC) was synthesized by a novel method with high BET surface area of 1380 m² g^-1 using KOH to activate carbon composite materials. The KOH plays a key role in etching out MoS₂ to produce Mo precursor; simultaneously, it corrodes carbon to form porous structure and produce reducing gas such as H₂ and CO. The resulting Mo₂C/NPC hybrid demonstrated superior HER activity in acid solution, with the overpotential of 166 mV at current density of 10 mA cm^-2, onset overpotential of 93 mV, Tafel slope of 68 mV dec^-1, and remarkable long-term cycling stability. The present strategy may provide a promising strategy to fabricate other metal carbide/carbon hybrids for energy conversion and storage.

Hierarchically heterostructured metal hydr(oxy)oxides for efficient overall water splitting

The design of highly efficient electrocatalysts containing non-precious metals is crucial for promoting overall water splitting in alkaline media. In particular, Janus catalysts simultaneously facilitating the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) are desirable. Herein, we fabricated a unique hierarchical heterostructure via growing Ni4W6O21(OH)2·4H2O (denoted as Ni-W-O) nanosheets on NiMoO4 rods, which was indispensable for regulating the morphology of the Ni-W-O structure. This heterostructure of Ni-W-O/NiMoO4 could be utilized as an electrocatalyst to realize superior activity for overall water splitting in 1.0 M KOH. It substantially promoted overall water splitting with 1.6 V at 30 mA cm-2, outperforming numerous bifunctional electrocatalysts under the same conditions. Notably, the remarkable stability for continuously splitting water endowed this hierarchical heterostructure with potential applications on a large scale. This work emphasizes the effectively controlled growth of heterostructured non-noble-metal catalysts for energy-conversion reaction.

Effect of Ion Diffusion in Cobalt Molybdenum Bimetallic Sulfide toward Electrocatalytic Water Splitting

The electrocatalyst comprising two different metal atoms is found suitable for overall water splitting in alkaline medium. Hydrothermal synthesis is an extensively used technique for the synthesis of various metal sulfides. Time-dependent diffusion of the constituting ions during hydrothermal synthesis can affect the crystal and electronic structure of the product, which in turn would modulate its electrocatalytic activity. Herein, cobalt molybdenum bimetallic sulfide was prepared via hydrothermal method after varying the duration of reaction. The change in crystal structure, amount of Co-S-Mo moiety, and electronic structure of the synthesized materials were thoroughly investigated using different analytical techniques. These changes modulated the charge transfer at the electrode-electrolyte interface, as evidenced by electrochemical impedance spectroscopy. The Tafel plots for the prepared materials were investigated considering a less explored approach and it was found that different materials facilitated different electrocatalytic pathways. The product obtained after 12 h reaction showed superior catalytic activity in comparison to the products obtained from 4, 8, and 16 h reaction, and it surpassed the overall water splitting activity of the RuO₂-Pt/C couple. This study demonstrated the ion diffusion within the bimetallic sulfide during hydrothermal synthesis and change in its electrocatalytic activity due to ion diffusion.

Tunable engineering hollow carbon nanomaterial served as an excellent catalyst for oxygen reduction reaction and hydrogen evolution reaction

Fe and N functionalized hollow carbon spheres (Fe/N-HCS) with hierarchically porous structure are constructed. Remarkably, it is discovered that the pyrolysis temperature effects the chemical composition intensively. At 800 °C, only graphitic-N and oxidized-N are formed for all as-prepared samples. The surface area and pores can be precisely tuned, the surface area of all Fe/N-HCS samples is more than 500  m² g^-1 benefiting from the porous hollow structure. Thus, the optimized Fe/N-HCS exhibits excellent oxygen reduction reaction performance in term of onset potential (1.00 V vs. RHE), half-wave potential (0.87 V vs. RHE), good stability as well as methanol tolerance for oxygen reduction reaction, even surpassing the Pt in alkaline condition and more competitive in acidic condition; Furthermore, the optimized Fe/N-HCS displays better hydrogen evolution reaction activity in acidic condition with onset overpotential of 40 mV and overpotential to deliver 10 mA cm^-2 at 170 mV, indicating better active. It is found that Fe/N-HCS improve the hydrogen evolution reaction activity after electrodeposition trace quantity of Pt, which shows 170 mV of overpotential to deliver 100 mA cm^-2. X-ray photoelectron spectroscopy result indicates the loading of Pt is roughly 0.11 at%, thus, the improved performance is basically due to the synergistic effect between Pt and Fe/N-HCS.

Activating hierarchically hortensia-like CoAl layered double hydroxides by alkaline etching and anion modulation strategies for the efficient oxygen evolution reaction

Hierarchically porous hortensia-like CoAl hydroxysulfide as an efficient electrocatalyst for the OER is designed and developed.

Ostwald Ripening Driven Exfoliation to Ultrathin Layered Double Hydroxides Nanosheets for Enhanced Oxygen Evolution Reaction

As a key half-reaction in water splitting, the oxygen evolution reaction (OER) process is kinetically sluggish. Layered double hydroxides (LDHs) are regarded as the highly promising electrocatalysts to promote the OER kinetics. However, the closely stacking layered structure of pristine bulk LDHs restricts the exposure of electrocatalytically active sites, and it remains a great challenge to find an efficient strategy to exfoliate the bulk LDHs into ultrathin and stable nanosheets with increased surface area and exposed active sites. Herein, a novel Ostwald ripening driven exfoliation (ORDE) of NiFe LDHs has been achieved in situ on the electrodes by spontaneously self-etching and redepositing via a simple hydrothermal treatment without the assistance of any exfoliating reagent or surfactant. The thermodynamically driven Ostwald ripening has been expanded to the exfoliation of two-dimensional layered materials for the first time. Compared with conventional exfoliation methods, this ORDE is a time-saving and green strategy that avoids the serious adsorption of surfactant molecules. The ORDE of NiFe LDHs is accomplished in situ on a Cu mesh electrode, which not only exhibits excellent electrical contact between LDHs catalyst and electrodes but also prevents the restacking of the exfoliated LDHs. As a result, the exfoliated ultrathin, clean, and vertically aligned NiFe nanosheets with much higher surface area and numerous exposed active edges and sites demonstrated significantly enhanced OER performances with low overpotential of 292 mV at 10 mA cm^-2 and long-term stability for more than 60 h, as well as remarkable flexibility. Additionally, bulk Ni(OH)₂ nanosheets on Ni foams have also been exfoliated by a similar mechanism, indicating this ORDE strategy can be widely extended to other 2D layered materials for novel applications.

Heterogeneous Co(OH)2 nanoplates/Co3O4 nanocubes enriched with oxygen vacancies enable efficient oxygen evolution reaction electrocatalysis

Heterogeneous Co(OH)₂ nanoplate/Co₃O₄ nanocube hybrids with rich oxygen vacancies have been constructed through a controllable approach. The high surface areas of such unique nanohybrids together with abundant oxygen vacancies provide more surface active sites, which can facilitate the charge transfer and boost the exchange of intermediates. Specifically, the resultant Co(OH)₂ nanoplate/Co₃O₄ nanocube hybrids display outstanding oxygen evolution reaction (OER) performances with a low overpotential of 281 mV at 10 mA cm^-2 and excellent durability after continuous CV of 3000 cycles, shedding light for large-scale applications in practical water splitting.

Self-supported nickel-cobalt nanowires as highly efficient and stable electrocatalysts for overall water splitting

The development and design of highly active and stable electrocatalysts based on cheap and Earth-abundant materials is critically important to enable water splitting as a desirable renewable energy source. Herein, we fulfill the significant electrochemical water splitting enhancement in both electrocatalytic activity and durability by constructing self-supported nickel-cobalt nanowire catalysts with abundant oxygen vacancies. Specifically, the rich oxygen vacancies can largely promote the oxygen evolution reaction (OER) activity of optimal Ni₁Co₁O₂ NWs with a relatively low overpotential of 248 mV to drive a current density of 10 mA cm^-2. More significantly, after the phosphorization of Ni₁Co₁O₂ NWs, the resultant Ni₁Co₁P NWs can also display excellent electrocatalytic hydrogen evolution reaction (HER) performances with an overpotential of only 101 mV to achieve a current density of 10 mA cm^-2. Furthermore, benefiting from the unique 1D nanowire structure, the synergistic effect, and the optimal Gibbs free energy for hydrogen evolution evolved from the phosphorization, the Ni₁Co₁O₂ NWs//Ni₁Co₁P NWs couple is thus highly active and stable for overall water electrolysis with a low voltage of 1.58 V at 10 mA cm^-2, showing extraordinary promise for practical overall water splitting electrolysis.