CoP and Ni2P implanted in a hollow porous N-doped carbon polyhedron for pH universal hydrogen evolution reaction and alkaline overall water splitting

Highly-dispersed 2D NiFeP/CoP heterojunction trifunctional catalyst for efficient electrolysis of water and urea

The electrocatalytic production of "green hydrogen", such as through the electrolysis of water or urea has been vigorously advocated to alleviate the energy crisis. However, their electrode reactions including oxygen evolution reaction (OER), urea oxidation reaction (UOR), and hydrogen evolution reaction (HER) all suffer from sluggish kinetics, which urgently need catalysts to accelerate the processes. Herein, we design and prepare an OER/UOR/HER trifunctional catalyst by transforming the homemade CoO nanorod into a two-dimensional (2D) ultrathin heterojunction nickel-iron-cobalt hybrid phosphides nanosheet (NiFeP/CoP) via a hydrothermal-phosphorization method. Consequently, a strong electronic interaction was found among the Ni2P/FeP4/CoP heterogeneous interfaces, which regulates the electronic structure. Besides the high mass transfer property of 2D nanosheet, Ni2P/FeP4/CoP displays improved OER/UOR/HER performance. At 10 mA cm-2, the OER overpotential reaches 274 mV in 1.0 M KOH, and the potential of UOR is only 1.389 V in 1.0 M KOH and 0.33 M urea. More strikingly, the two-electrode systems for electrolysis water and urea-assisted electrolysis water assembled by NiFeP/CoP could maintain long-term stability for 35 h and 12 h, respectively. This work may help to pave the way for upcoming research horizons of multifunctional electrocatalysts.

Spherical cluster heterojunction engineering of NiFeP/g-C3N4 for efficient oxygen evolution reaction in alkaline solution

The construction of heterojunctions can reduce the energy barrier for the oxygen evolution reaction (OER), which is crucial for the design of efficient electrocatalysts. A novel OER electrocatalyst, composed of g-C3N4-supported NiFeP spherical nanoclusters, was successfully synthesized using a simple hydrothermal method and a gas-phase precipitation method. Benefiting from its unique spherical nanocluster structure and strong electronic interactions among Ni, Fe, and P, the catalyst exhibited outstanding performance under alkaline conditions, with an overpotential of only 232 mV at a current density of 10 mA cm-2 and a Tafel slope of 103 mV dec-1. Additionally, the electrical resistance of NiFeP/g-C3N4 (Rct = 5.1 Ω) was much lower than that of NiFeP (Rct = 10.8 Ω) and layered g-C3N4 (Rct = 44.8 Ω). The formation of a Schottky barrier heterojunction efficiently reduced electron transfer impedance during the OER process, accelerating the electron transfer from g-C3N4 to NiFeP, enhancing the carrier concentration, and thereby improving the OER activity. Moreover, The robust g-C3N4 chain-mail protects NiFeP from adverse reaction environments, maintaining a balance between catalytic activity and stability. Furthermore, ab initio molecular dynamics (AIMD) and density functional theory (DFT) were conducted to explore the thermal stability and internal electron transfer behavior of the cluster heterojunction structure. This study offers a broader design strategy for the development of transition metal phosphide (TMPs) materials in the oxygen evolution reaction.

Self-Assembled Preparation of Porous Nickel Phosphide Superparticles with Tunable Phase and Porosity for Efficient Hydrogen Evolution

Self-assembly of colloidal nanoparticles enables the easy building of assembly units into higher-order structures and the bottom-up preparation of functional materials. Nickel phosphides represent an important group of catalysts for hydrogen evolution reaction (HER) from water splitting. In this paper, the preparation of porous nickel phosphide superparticles and their HER efficiencies are reported. Ni and Ni2P nanoparticles are self-assembled into binary superparticles via an oil-in-water emulsion method. After annealing and acid etching, the as-prepared Ni-Ni2P binary superparticles change into porous nickel phosphide superparticles. The porosity and crystalline phase of the superparticles can be tuned by adjusting the ratio of Ni and Ni2P nanoparticles. The resulting porous superparticles are effective in driving HER under acidic conditions, and the modulation of porosity and phase further optimize the electrochemical performance. The prepared Ni3P porous superparticles not only possess a significantly enhanced specific surface area compared to solid Ni-Ni2P superparticles but also exhibit an excellent HER efficiency. The calculations based on the density functional theories show that the (110) crystal facet exhibits a relatively lower Gibbs free energy of hydrogen adsorption. This work provides a self-assembly approach for the construction of porous metal phosphide nanomaterials with tunable crystalline phase and porosity.

Modulating the electronic structures of cobalt-organic frameworks for efficient electrocatalytic oxygen evolution

The oxygen evolution reaction (OER) is a key process in various energy storage/generation technologies. Tuning the electronic structures of catalysts is an effective approach to improve the catalyst's activity. In this work, we synthesized Ce-doped cobalt-organic frameworks with benzene-1, 4-dicarboxylic acid (BDC) as the ligand as efficient OER electrocatalysts (denoted as Co3Ce1 BDC) with excellent stability and improved catalytic performance. The introduced Ce in Co3Ce1 BDC changes the surface configuration and tunes electronic structures of the active Co site, leading to enhanced interaction between intermediates and catalysts. Besides, the specific surface area, reaction kinetics, charge transfer efficiency, and turnover frequency are also improved in the presence of Ce. As a result, the Co3Ce1 BDC demonstrated excellent performance with a low overpotential of 285 mV at a current of 10 mA·cm-2, a preferable Tafel slope of 56.1 mV·dec-1, and an excellent durability in 1 M KOH, indicating the potential for practical applications in water splitting and other energy storage technologies wherein the OER plays a critical role. Comprehensive theoretical calculations and modeling further identified the key role of Ce in modulating the electronic structure and OER activity of cobalt-organic frameworks. Most importantly, this work provides a new strategy to the development of efficient cobalt-organic framework catalysts in OER-related applications.

Multistage interfacial engineering of 3D carbonaceous Ni2P nanospheres/nanoflowers derived from Ni-BTC metal-organic frameworks for overall water splitting

The regulation of the multi-dimensional interface plays an important role in optimizing the electron transport and gas mass transfer during catalysis, which is conducive to promoting the electrocatalytic process. Herein, a self-supporting electrode has been developed with the multistage interface within 3D Ni₂P@C nanospheres/nanoflowers arrays derived from metal-organic frameworks (MOFs) as template skeletons and precursors. The constructed nanosphere interface protrudes outward to optimize the contact with the electrolyte while the nanoflower lamellar connection promotes rapid electron transfer and exposes more active sites, and accelerates the gas diffusion with the abundant interspace channels. According to theoretical calculation, the synergistic effect between Ni₂P and C is conducive to the optimal adsorption and desorption of H*, thus contributing to the improvement of catalytic kinetics. With the optimized growth times assembled onto nickel foam substrates, the Ni₂P@C-12 h requires overpotentials of only 69 mV and 205 mV to drive the current density of 10 mA cm^-2 towards hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), respectively. And it reveals an ultralow cell voltage of 1.55 V at 10 mA cm^-2 to achieve overall water splitting (OWS). In addition, the stability of the Ni₂P@C/NF electrocatalyst emerges as prominent long-term stability, which is attributed to the carbonaceous nanosphere anchors on the substrate to minimize the possibility of oxidation of the catalyst surface. This strategy of in situ growth of MOF-derived phosphates provides a general idea for interfacial engineering modification of OWS electrode materials.

Revealing the Crystal Phase-Activity Relationship on NiRu Alloy Nanoparticles Encapsulated in N-Doped Carbon towards Efficient Hydrogen Evolution Reaction

Adjusting the crystal phase of a metal alloy is an important method to optimize catalytic performance. However, detailed understanding about the phase-property relationship for the hydrogen evolution reaction (HER) is still limited. In this work, the crystal phase-activity relationship of NiRu nanoparticles is studied employing N-doped carbon shell coated NiRu nanoparticles with different phase contents. It is found that the NiRu@NC (mix) with both face-centred cubic (fcc) and thermodynamically unstable hexagonal close-packed (hcp) phase NiRu give the best HER performance. Further activity studies demonstrate that hcp NiRu has better HER performance, and NiRu@NC (mix) with rich (∼70 %) hcp phase presented outstanding performance with an overpotential of only 27 mV @ 10 mA ⋅ cm^-2 . The high HER activity of NiRu@NC (mix) is attributed to the formation of hcp phase. This finding indicates that the regulation of crystal structure can provide a new strategy for optimizing HER activity.

Hollow Ni/NiO/C composite derived from metal-organic frameworks as a high-efficiency electrocatalyst for the hydrogen evolution reaction

Metal-organic frameworks (MOFs) constitute a class of crystalline porous materials employed in storage and energy conversion applications. MOFs possess characteristics that render them ideal in the preparation of electrocatalysts, and exhibit excellent performance for the hydrogen evolution reaction (HER). Herein, H-Ni/NiO/C catalysts were synthesized from a Ni-based MOF hollow structure via a two-step process involving carbonization and oxidation. Interestingly, the performance of the H-Ni/NiO/C catalyst was superior to those of H-Ni/C, H-NiO/C, and NH-Ni/NiO/C catalysts for the HER. Notably, H-Ni/NiO/C exhibited the best electrocatalytic activity for the HER, with a low overpotential of 87 mV for 10 mA cm^-2 and a Tafel slope of 91.7 mV dec^-1. The high performance is ascribed to the synergistic effect of the metal/metal oxide and hollow architecture, which is favorable for breaking the H-OH bond, forming hydrogen atoms, and enabling charge transport. These results indicate that the employed approach is promising for fabricating cost-effective catalysts for hydrogen production in alkaline media.

Development of Binder-Free Three-Dimensional Honeycomb-like Porous Ternary Layered Double Hydroxide-Embedded MXene Sheets for Bi-Functional Overall Water Splitting Reactions

In this study, a honeycomb-like porous-structured nickel-iron-cobalt layered double hydroxide/Ti3C2Tx (NiFeCo-LDH@MXene) composite was successfully fabricated on a three-dimensional nickel foam using a simple hydrothermal approach. Owing to their distinguishable characteristics, the fabricated honeycomb porous-structured NiFeCo-LDH@MXene composites exhibited outstanding bifunctional electrocatalytic activity for pair hydrogen and oxygen evolution reactions in alkaline medium. The developed NiFeCo-LDH@MXene electrocatalyst required low overpotentials of 130 and 34 mV to attain a current density of 10 mA cm-2 for OER and HER, respectively. Furthermore, an assembled NiFeCo-LDH@MXene‖NiFeCo-LDH@MXene device exhibited a cell voltage of 1.41 V for overall water splitting with a robust firmness for over 24 h to reach 10 mA cm-2 current density, signifying outstanding performance for water splitting reactions. These results demonstrated the promising potential of the designed 3D porous NiFeCo-LDH@MXene sheets as outstanding candidates to replace future green energy conversion devices.

Nitrogen Disturbance Awakening the Intrinsic Activity of Nickel Phosphide for Boosted Hydrogen Evolution Reaction

Nickel phosphide (Ni₂ P) has emerged as a promising candidate to substitute Pt-based catalysts for hydrogen evolution reaction (HER) due to the hydrogenase-like catalytic mechanism and concomitantly low cost. However, its catalytic activity is still not comparable to that of noble-metal-based catalysts, and innovative strategies are still urgently needed to further improve its performance. Herein, a self-supported N-doped Ni₂ P on Ni foam (N-Ni₂ P/NF) was rationally designed and fabricated through a facile NH₄ H₂ PO₂ -assisted gas-solid reaction process. As an HER catalyst in alkaline medium, the obtained N-Ni₂ P/NF revealed excellent electrocatalytic performance with a distinctly low overpotential of 50 mV at 10 mA cm^-2 , a small Tafel slope of 45 mV dec^-1 , and long-term stability for 25 h. In addition, the spectroscopic characterizations and density functional theory calculations confirmed that the incorporation of N regulated the original electronic structure of Ni₂ P, enhanced its intrinsic catalytic property, optimized the Gibbs free energy of reaction intermediates, and ultimately promoted the HER process. This work provides an atomic-level insight into the electronic structure modulation of metal phosphides and opens an avenue for developing advanced transition metal phosphides-based catalysts.

Synergistic coupling of heterostructured porous CoP nanosheets with P doped NiO for highly efficient overall alkaline water splitting

Exploring non-noble metal materials as bifunctional catalysts for water electrolysis is of great significance for the development and utilization of hydrogen energy. Herein, a flower branch-leaf shaped phosphide/oxide heterogeneous electrocatalyst located on Ni foam (CoP/P-NiO/NF) was developed through hydrothermal and phosphorization strategy. Benefiting from the strong ability to dissociate H₂O molecules on P-NiO and the suitable adsorption of intermediate H species on CoP, the optimal CoP/P-NiO/NF exhibited outstanding performance with low overpotentials of 52 mV at current density of 10 mA cm^-2, smaller Tafel slopes of 73.6 mV dec^-1 for hydrogen evolution reaction (HER). Meanwhile, CoP/P-NiO/NF indicated 265 mV at 100 mA cm^-2 with Tafel slope of 101.8 mV dec^-1 for oxygen evolution reaction (OER) due to the optimal redistribution of electrons among Ni²⁺, Co²⁺ and Co³⁺ for favorable adsorption/desorption of oxygen-intermediates. Both HER and OER shown robust stability during 32 h without decline. The corresponding two-electrode system for overall alkaline water splitting required a low voltage of 1.6 V at 100 mA cm^-2 with long stability (20 h) which is far lower than that on RuO₂-Pt/C and many other reported non-noble metal electrocatalysts. This work demonstrates that the synergistic effect and morphology engineering play vital roles in the enhanced electrocatalytic performance.

A dual-confinement strategy to construct cobalt-based phosphide nanoclusters within carbon nanofibers for bifunctional water splitting electrocatalysts

It is a challenging task to explore highly active and stable noble-metal-free bifunctional electrocatalysts for water splitting, both in hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). Herein, a new dual-confinement strategy for the fabrication of cobalt-base phosphide in the carbon nanofibers (CNFs) was proposed via electrospinning, followed by the corresponding pyrolysis. The ultrafine phosphides derived from the pore confinement of ZIF and space confinement of the polymer revealed abundant active sites and P defects. More importantly, by introducing a second metal element Ni or Cu, the electronic structure and synergistic effect were further enhanced, and the obtained bimetallic CoNiP_x-CNF electrocatalyst exhibited the remarkable performance for HER and OER, featuring the low η₁₀ values of 154 and 269 mV in 1.0 M KOH electrolyte, respectively. CoNiP_x-CNFs as a catalyst for both anode and cathode showed a current density of 10 mA cm^-2 at a voltage of 1.56 V, exceeding better stability, which is superior to most non-noble metal electrocatalysts reported in a previous research. The dual-confinement strategy is believed to provide an effective and simple approach for the synthesis of high-performance and cost-efficient bifunctional electrocatalysts for overall water splitting.

Boosting alkaline water splitting and the urea electrolysis kinetic process of a Co3O4 nanosheet by electronic structure modulation of F, P co-doping

Designing non-precious metal electrocatalysts for accelerated electron transfer and richer active site exposure is necessary and challenging to achieve the versatility of electrocatalysts. In this research, a self-grown nanosheet array electrocatalyst on nickel foam with high structural stability is first rationally designed through suitable anionic doping. The combined experimental and theoretical calculations reveal that the F-P-Co₃O₄/NF material optimizes the adsorption energy of hydrogen/water through electron coupling, and its nanosheet structure provides abundant active sites, accelerating the mass and electron transfer in the reaction process. It is worth noting that the as-developed F-P-Co₃O₄/NF materials exhibit outstanding catalytic activity for overpotentials of 192 and 110 mV at a current density of 10 mA cm^-2 for the oxygen evolution reaction and the hydrogen evolution reaction in 1 M KOH, respectively. More notably, an assembled F-P-Co₃O₄/NF//F-P-Co₃O₄/NF alkaline electrolytic cell requires only an ultra-low cell voltage of 1.53 V to achieve a current density of 10 mA cm^-2, which is one of the best activities reported so far. Furthermore, F-P-Co₃O₄/NF also shows excellent performance for urea electrolysis. Theoretical calculations show that the superior activity of the F-P-Co₃O₄/NF catalyst is attributed to the optimal electron configuration and the lower Gibbs free energy of hydrogen adsorption due to co-doping of P and F. The work provides an alternative solution for the preparation of electrocatalysts with high structural stability, high catalytic activity and multifunction for alkaline water splitting and urea electrolysis.

Critical Review, Recent Updates on Zeolitic Imidazolate Framework-67 (ZIF-67) and Its Derivatives for Electrochemical Water Splitting

Design and construction of low-cost electrocatalysts with high catalytic activity and long-term stability is a challenging task in the field of catalysis. Metal-organic frameworks (MOF) are promising candidates as precursor materials in the development of highly efficient electrocatalysts for energy conversion and storage applications. This review starts with a summary of basic concepts and key evaluation parameters involved in the electrochemical water-splitting reaction. Then, different synthesis approaches reported for the cobalt-based Zeolitic imidazolate framework (ZIF-67) and its derivatives are critically reviewed. Additionally, several strategies employed to enhance the electrocatalytic activity and stability of ZIF-67-based electrocatalysts are discussed in detail. The present review provides a succinct insight into the ZIF-67 and its derivatives (oxides, hydroxides, sulfides, selenides, phosphide, nitrides, telluride, heteroatom/metal-doped carbon, noble metal-supported ZIF-67 derivatives) reported for oxygen evolution reaction (OER), hydrogen evolution reaction (HER), and overall water splitting applications. Finally, this review concludes with the associated challenges and the perspectives on developing the best economic, durable electrocatalytic materials.

Interconnected magnetic carbon@NixCo1-xFe2O4 nanospheres with core-shell structure: An efficient and thin electromagnetic wave absorber

The applications of cobalt ferrite and nickel ferrite composite materials on electromagnetic (EM) wave absorption are the research hotspot currently. However, the systematical comparison study between these two ferrites composites have rarely been carried out. Thus, the EM wave absorption performance of interconnected carbon@Ni_xCo_1-xFe₂O₄ composites with core-shell structures were investigated comprehensively in this work. A series of magnetic nanospheres including NiFe₂O₄, cobalt-doped nickel ferrite, nickel-cobalt ferrite, nickel-doped cobalt ferrite and CoFe₂O₄ were synthesized firstly, and then uniformly encapsulation by carbon rendered the corresponding C@Ni_xCo_1-xFe₂O₄ composites nanospheres. Synthesis reactions involved for C@Ni_xCo_1-xFe₂O₄ formation were investigated in detail, and afterwards their magnetic behavior, EM wave absorption performance and absorbing mechanism were thoroughly explored and analyzed. Results show that when nickel is dominant element and cobalt is doping element (Ni_0.75Co_0.25Fe₂O₄), the composite nanosphere exhibits optimum EM wave absorption performance. When the sample thickness is just 1.9 mm, its RL_min value can reach -51 dB, and the corresponding EAB width is 3.3 GHz. The synthesized C@Ni_0.75Co_0.25Fe₂O₄ can be qualified as an efficient and thin electromagnetic wave absorber, which is mainly attributed to its special structure, fair electromagnetic matching and impedance matching.

Reactive template-engaged synthesis of Ni-doped Co3S4 hollow and porous nanospheres with optimal electronic modulation toward high-efficiency electrochemical oxygen evolution

Ni-doped Co3S4 hollow and porous nanoflowers are synthesized via a self-sacrificial reactive template-engaged strategy. The obtained sample with optimal electronic structure exhibits excellent oxygen evolution performance.

Amorphous NiCuFeP@Cu3P nanoarray for an efficient hydrogen evolution reaction

Transition metal phosphides are considered as ideal alternatives for noble metal catalysts for hydrogen evolution reactions. In this study, amorphous NiCuFeP nanosheets are uniformly coated on self-supporting Cu3P nanowire array...

Plant polyphenol-involved coordination assembly-derived Mo3Co3C/Mo2C/Co@NC with phase regulation and interface engineering for efficient hydrogen evolution reaction electrocatalysis

Heterostructured Mo3Co3C/Mo2C/Co@NC catalysts were obtained by the calcination of tannic acid ligand-assembled precursors, enabling enhanced HER electrocatalysis through phase and interface regulation.

Tuning the Electronic Structure of the CoP/Ni2P Nanostructure by Nitrogen Doping for an Efficient Hydrogen Evolution Reaction in Alkaline Media

As one of the most sustainable, efficient, and cleanest ways for hydrogen production, electrochemical water splitting relies heavily on cost-efficient and stable electrocatalysts. Herein, a self-supported and nitrogen-doped hybrid CoP/Ni₂P was synthesized through a simple two-step hydrothermal process followed by low-temperature phosphorization and nitridation (N-CoP/Ni₂P@NF). Both experimental and density functional theory calculation results suggest that nitrogen doping can tune the electrical structure of the CoP/Ni₂P heterostructure and thus optimize the free energy of adsorbed H on the surface of N-CoP/Ni₂P@NF and accelerate the electronic transport activity. The prepared N-CoP/Ni₂P@NF exhibits excellent electrocatalytic hydrogen evolution reaction (HER) performance, which merely requires an overpotential of -46 mV at -10 mA cm^-2 and shows a negligible decay after a long durability test for 72 h in alkaline (1.0 M KOH) media. Consequently, this work supplies a novel strategy with great potential for designing transition metal phosphate-based catalysts with high HER performance.

FeNiP nanoparticle/N,P dual-doped carbon composite as a trifunctional catalyst towards high-performance zinc-air batteries and overall water electrolysis

A composite catalyst with a novel construction of bimetallic phosphide FeNiP nanoparticles embedded in an N,P double-doped carbon matrix was prepared. It was demonstrated to be a trifunctional catalyst that can efficiently catalyze the oxygen reduction reaction (ORR), oxygen evolution reaction (OER) and hydrogen evolution reaction (HER). It was found that the introduction of oleylamine during the preparation can adjust the catalytic sites and finally lead to ideal catalytic performances. The obtained catalyst exhibited efficient ORR catalytic performance that surpassed the commercial Pt/C catalyst, with the OER performance comparable to that of RuO₂ as well as excellent HER performance. The ORR half-wave potential is 0.879 V (vs. RHE) in 0.1 M KOH solution, while the OER overpotential at a current density of 10 mA cm^-2 is only 280 mV in 1 M KOH solution. The potential gap between the ORR and OER was only 0.700 V in 0.1 M KOH solution. This trifunctional catalyst was further evaluated in energy devices including zinc-air batteries and water electrolysis. The liquid zinc-air battery assembly achieved a power density of 169 mW cm^-2 and stably undergoes charge-discharge cycles for 210 hours. The solid-state zinc-air battery achieved a power density of 70 mW cm^-2 and stably undergoes charge-discharge cycles for 40 hours. These performances surpassed the batteries assembled with a Pt/C-RuO₂ mixed catalyst. This work established a foundation of composite catalysts coupled with bimetallic phosphide and hybrid carbon substrates, which will promote the development of high-performance multifunctional catalysts and their application in energy devices.

Construction of 1D Heterostructure NiCo@C/ZnO Nanorod with Enhanced Microwave Absorption

Layered double hydroxides (LDHs) have a special structure and atom composition, which are expected to be an excellent electromagnetic wave (EMW) absorber. However, it is still a problem that obtaining excellent EMW-absorbing materials from LDHs. Herein, we designed heterostructure NiCo-LDHs@ZnO nanorod and then subsequent heat treating to derive NiCo@C/ZnO composites. Finally, with the synergy of excellent dielectric loss and magnetic loss, an outstanding absorption performance could be achieved with the reflection loss of - 60.97 dB at the matching thickness of 2.3 mm, and the widest absorption bandwidth of 6.08 GHz was realized at 2.0 mm. Moreover, this research work provides a reference for the development and utilization of LDHs materials in the field of microwave absorption materials and can also provide ideas for the design of layered structural absorbers.