Nanostructured materials on 3D nickel foam as electrocatalysts for water splitting

Ni2P/rGO/NF Nanosheets As a Bifunctional High-Performance Electrocatalyst for Water Splitting

The hydrogen generated via the water splitting method is restricted by the high level of theoretical potential exhibited by the anode. The work focuses on synthesizing a bifunctional catalyst with a high efficiency, that is, a nickel phosphide doped with the reduced graphene oxide nanosheets supported on the Ni foam (Ni₂P/rGO/NF), via the hydrothermal approach together with the calcination approach specific to the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER). The Raman, X-Ray Diffraction (XRD), X-ray Photoelectron Spectroscopy (XPS), Transmission Electron Microscope (TEM), Scanning Electron Microscopy (SEM), High-Resolution Transmission Electron Microscopy (HRTEM), as well as elemental mapping, are adopted to study the composition and morphology possessed by Ni₂P/rGO/NF. The electrochemical testing is performed by constructing a parallel two-electrode electrolyzer (Ni₂P/rGO/NF||Ni₂P/rGO/NF). Ni₂P/rGO/NF||Ni₂P/rGO/NF needs a voltage of only 1.676 V for driving 10 mA/cm², which is extremely close to Pt/C/NF||IrO₂/NF (1.502 V). It is possible to maintain the current density for no less than 30 hours. It can be demonstrated that Ni₂P/rGO/NF||Ni₂P/rGO/NF has commercial feasibility, relying on the strong activity and high stability.

Nickel Phosphide Electrocatalysts for Hydrogen Evolution Reaction

The production of hydrogen through electrochemical water splitting driven by clean energy becomes a sustainable route for utilization of hydrogen energy, while an efficient hydrogen evolution reaction (HER) electrocatalyst is required to achieve a high energy conversion efficiency. Nickel phosphides have been widely explored for electrocatalytic HER due to their unique electronic properties, efficient electrocatalytic performance, and a superior anti-corrosion feature. However, the HER activities of nickel phosphide electrocatalysts are still low for practical applications in electrolyzers, and further studies are necessary. Therefore, at the current stage, a specific comprehensive review is necessary to focus on the progresses of the nickel phosphide electrocatalysts. This review focuses on the developments of preparation approaches of nickel phosphides for HER, including a mechanism of HER, properties of nickel phosphides, and preparation and electrocatalytic HER performances of nickel phosphides. The progresses of the preparation and HER activities of the nickel phosphide electrocatalysts are mainly discussed by classification of the preparation method. The comparative surveys of their HER activities are made in terms of experimental metrics of overpotential at a certain current density and Tafel slope together with the preparation method. The remaining challenges and perspectives of the future development of nickel phosphide electrocatalysts for HER are also proposed.

Three-Dimensional Heterostructured NiCoP@NiMn-Layered Double Hydroxide Arrays Supported on Ni Foam as a Bifunctional Electrocatalyst for Overall Water Splitting

Herein, the rational design and preparation of three-dimensional heterostructured NiCoP@NiMn-layered double hydroxide arrays supported on Ni foam (NiCoP@NiMn LDH/NF) is reported as a new bifunctional water-splitting electrocatalyst with high performance. Prepared with facile hydrothermal reactions and phosphorization, the NiCoP@NiMn LDH/NF is simultaneously highly active toward oxygen evolution reaction (OER) (100, 300, and 600 mA cm^-2 at overpotentials of 293, 315, and 327 mV, respectively) and hydrogen evolution reaction (HER) (100, 200, and 300 mA cm^-2 at overpotentials of 116, 130, and 136 mV, respectively). Interestingly, with cell voltages of 1.519, 1.642, 1.671, and 1.687 V at 10, 100, 200, and 300 mA cm^-2, respectively, for overall water splitting, this electrocatalyst achieves 95.2% faradaic efficiency for OER, suggesting a relatively high contribution of water splitting in the apparent current in spite of the existence of partial catalyst oxidation. The heterostructure arrays supported on Ni foam have some advantages, acting as a bifunctional water-splitting electrocatalyst: (1) heterostructured NCoP@NiMn LDH combines the intrinsic properties of individual NiCoP (excellent activity for HER) and NiMn LDH (high activity for OER) via the effective interface engineering between the two phases; (2) the NiCoP core material serves as a fast electron transfer channel to enhance the electrode's electrical conductivity; and (3) Ni foam with a three-dimensional-network structure as a support is beneficial to exposing more active sites and ensures efficient gas bubble release and electron/mass transfer.

Comparative Study of the Structure, Composition, and Electrocatalytic Performance of Hydrogen Evolution in MoSx~2+δ/Mo and MoSx~3+δ Films Obtained by Pulsed Laser Deposition

Systematic and in-depth studies of the structure, composition, and efficiency of hydrogen evolution reactions (HERs) in MoS_x films, obtained by means of on- and off-axis pulsed laser deposition (PLD) from a MoS₂ target, have been performed. The use of on-axis PLD (a standard configuration of PLD) in a buffer of Ar gas, with an optimal pressure, has allowed for the formation of porous hybrid films that consist of Mo particles which support a thin MoS_x~2+δ (δ of ~0.7) film. The HER performance of MoS_x~2+δ/Mo films increases with increased loading and reaches the highest value at a loading of ~240 μg/cm². For off-axis PLD, the substrate was located along the axis of expansion of the laser plume and the film was formed via the deposition of the atomic component of the plume, which was scattered in Ar molecules. This made it possible to obtain homogeneous MoS_x~3+δ (δ~0.8–1.1) films. The HER performances of these films reached saturation at a loading value of ~163 μg/cm². The MoS_x~3+δ films possessed higher catalytic activities in terms of the turnover frequency of their HERs. However, to achieve the current density of 10 mA/cm², the lowest over voltages were −162 mV and −150 mV for the films obtained by off- and on-axis PLD, respectively. Measurements of electrochemical characteristics indicated that the differences in the achievable HER performances of these films could be caused by their unique morphological properties.

Recent advances in soft functional materials: preparation, functions and applications

Synthetic materials and biomaterials with elastic moduli lower than 10 MPa are generally considered as soft materials. Research studies on soft materials have been boosted due to their intriguing features such as light-weight, low modulus, stretchability, and a diverse range of functions including sensing, actuating, insulating and transporting. They are ideal materials for applications in smart textiles, flexible devices and wearable electronics. On the other hand, benefiting from the advances in materials science and chemistry, novel soft materials with tailored properties and functions could be prepared to fulfil the specific requirements. In this review, the current progress of soft materials, ranging from materials design, preparation and application are critically summarized based on three categories, namely gels, foams and elastomers. The chemical, physical and electrical properties and the applications are elaborated. This review aims to provide a comprehensive overview of soft materials to researchers in different disciplines.

Loading FeOOH on Ni(OH)2 hollow nanorods to obtain a three-dimensional sandwich catalyst with strong electron interactions for an efficient oxygen evolution reaction

Sustainable production of hydrogen by water splitting requires the exploration of highly efficient electrocatalysts from abundant non-precious metals on Earth. Ni(OH)2 hollow nanorod arrays were obtained on Ni foam by simple alkali etching, and FeOOH was electrodeposited on the walls of hollow nanorods to construct FeOOH@Ni(OH)2 sandwich hollow nanorod arrays, which help overcome the drawbacks of the poor conductivity and poor stability of FeOOH and boost the catalytic performance of the oxygen evolution reaction (OER) in comparison with the individual components. A fully contacted three-dimensional nanorod array structure provides many exposed catalytically active sites and promotes charge transfer during the electrochemical OER process. The presence of FeOOH can promote the formation of a more conductive catalytically active component, NiOOH, which improves the catalytic performance of Ni(OH)2. The electronic interaction and synergistic catalysis between nickel and iron enhances the electrochemical performance of the catalyst significantly. The optimized FeOOH@Ni(OH)2 sandwich hollow nanorod arrays show an outstanding OER activity with a small overpotential of 245 mV at 50 mA cm-2 and a low Tafel slope of 45 mV dec-1. The catalyst can maintain a substantially constant voltage over 40 h in 1.0 M KOH solution. Our work provides a new strategy to prepare Ni-Fe bimetallic materials as OER electrocatalysts.

Vertically stacked bilayer heterostructure CoFe2O4@Ni3S2 on a 3D nickel foam as a high-performance electrocatalyst for the oxygen evolution reaction

The as-obtained CoFe₂O₄@Ni₃S₂/NF can serve as an active and stable water oxidation catalyst under electrochemical reaction conditions.

Urea-assisted enhanced electrocatalytic activity of MoS2–Ni3S2 for overall water splitting

Urea-assisted enhanced electrocatalytic activity of MoS₂–Ni₃S₂ as a bifunctional electrocatalyst for overall water splitting.

Non-precious-metal catalysts for alkaline water electrolysis: operando characterizations, theoretical calculations, and recent advances

Advances of non-precious-metal catalysts for alkaline water electrolysis are reviewed, highlighting operando techniques and theoretical calculations in their development.

Facile synthesis of self-supported amorphous phosphorus-doped Ni(OH)2 composite anodes for efficient water oxidation

Self-supporting phosphorus-doped Ni(OH)₂ anodes were synthesized via a facile one-pot hydrothermal method. They are promising for real applications with low fabrication cost, high activity, long stability, and fast responses to current changes.

Neuron-like hierarchical manganese sulfide@Cu2S core/shell arrays on Ni foam as an advanced electrode for an asymmetric supercapacitor

Neuron-like hierarchical manganese sulfide@Cu₂S core/shell arrays on Ni foam as an advanced electrode for an asymmetric supercapacitor.

A flower-like CoS2/MoS2 heteronanosheet array as an active and stable electrocatalyst toward the hydrogen evolution reaction in alkaline media

CoS₂/MoS₂ heteronanosheet arrays (HNSAs) with vertically aligned flower-like architectures are fabricated through in situ topotactic sulfurization of CoMoO₄ nanosheet array (NSA) precursors on conductive Ni foam. CoMoO₄ NSAs are prepared by a self-template hydrothermal method without using any hard template and surfactant. Benefiting from a 3D flower-like architecture constituted by ultrathin nanosheets with abundant exposed heterointerfaces as highly active sites and predesigned void spaces, the as-synthesized CoS₂/MoS₂ HNSAs exhibit an excellent hydrogen evolution reaction (HER) performance with a low overpotential of 50 mV at 10 mA cm⁻², and a small Tafel slope of 76 mV dec⁻¹ in 1.0 M KOH, which outperforms most previously reported CoS₂ and MoS₂ based electrocatalysts with compositional or morphological similarity. This work demonstrates the great potential in developing high-efficiency and earth-abundant electrocatalysts for alkaline HER through heterointerface engineering and morphological design by utilizing transition metal molybdate as a promising platform.

CoS₂/MoS₂ heteronanosheet arrays with vertically aligned flower-like architecture are fabricated through in situ topotactic sulfurization of CoMoO₄ nanosheet arrays.

Recent Trends in Synthesis and Investigation of Nickel Phosphide Compound/Hybrid-Based Electrocatalysts Towards Hydrogen Generation from Water Electrocatalysis

Sustainable and high performance energy devices such as solar cells, fuel cells, metal-air batteries, as well as alternative energy conversion and storage systems have been considered as promising technologies to meet the ever-growing demands for clean energy. Hydrogen evolution reaction (HER) is a crucial process for cost-effective hydrogen production; however, functional electrocatalysts are potentially desirable to expedite reaction kinetics and supply high energy density. Thus, the development of inexpensive and catalytically active electrocatalysts is one of the most significant and challenging issues in the field of electrochemical energy storage and conversion. Realizing that advanced nanomaterials could engender many advantageous chemical and physical properties over a wide scale, tremendous efforts have been devoted to the preparation of earth-abundant transition metals as electrocatalysts for HER in both acidic and alkaline environments because of their low processing costs, reasonable catalytic activities, and chemical stability. Among all transition metal-based catalysts, nickel compounds are the most widely investigated, and have exhibited pioneering performances in various electrochemical reactions. Heterostructured nickel phosphide (Ni_xP_y) based compounds were introduced as promising candidates of a new category, which often display chemical and electronic characteristics that are distinct from those of non-precious metals counterparts, hence providing an opportunity to construct new catalysts with an improved activity and stability. As a result, the library of Ni_xP_y catalysts has been enriched very rapidly, with the possibility of fine-tuning their surface adsorption properties through synergistic coupling with nearby elements or dopants as the basis of future practical implementation. The current review distils recent advancements in Ni_xP_y compounds/hybrids and their application for HER, with a robust emphasis on breakthroughs in composition refinement. Future perspectives for modulating the HER activity of Ni_xP_y compounds/hybrids, and the challenges that need to be overcome before their practical use in sustainable hydrogen production are also discussed.

Template-Assisted Electrodeposition of Porous Fe-Ni-Co Nanowires with Vigorous Hydrogen Evolution

A novel method to fabricate porous Fe-Ni-Co nanowires directly by electrodepositing into polycarbonate membranes is reported when the electrolyte pH < 0.5. Hydrogen bubbles are used as a dynamic porous template created by operating in electrolytes with very low pH to drive the proton reduction reaction. The electrolyte pH was adjusted with sulfuric acid, and the added sulfate ions are thought to help reduce bubble coalescence, but not detachment at the electrode surface, to facilitate metal deposition within the nanopores. Porous nanowires were obtained when the electrolyte pH was less than 1.0. The average alloy composition was found to be pH sensitive, which shifted from an Fe-rich porous alloy to a Ni-rich porous alloy as the electrolyte pH decreased.

Anion-Modulated HER and OER Activities of 3D Ni-V-Based Interstitial Compound Heterojunctions for High-Efficiency and Stable Overall Water Splitting

Overall water splitting driven by a low voltage is crucial for practical H₂ evolution, but it is challenging. Herein, anion-modulation of 3D Ni-V-based transition metal interstitial compound (TMIC) heterojunctions supported on nickel foam (Ni₃ N-VN/NF and Ni₂ P-VP₂ /NF) as coupled hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) catalysts for efficient overall water splitting is demonstrated. The heterointerface in Ni₃ N-VN has a suitable H* absorption energy, being favorable for enhancing HER activity with onset overpotential (η_onset ) of zero and Tafel slope of 37 mV dec^-1 in 1 m KOH (close to that of Pt/C/NF). For the OER, the synergy of Ni₂ P-VP₂ with oxide species can give enhanced activity with η_onset of 220 mV and Tafel slope of 49 mV dec^-1 . The good activity is ascribed to heterointerface for activating the intermediates, good conductivity of TMICs for electron-transfer, and porous structure facilitation of mass-transport. Additionally, the minimal mutual influence of Ni₃ N-VN/NF and Ni₂ P-VP₂ /NF allows easy coupling for efficient overall water splitting with a low driving voltage (≥1.43 V), a voltage of 1.51 V at 10 mA cm^-2 , and remarkable durability for 100 h. It can be driven by a solar cell (1.5 V), indicating its potential to store intermittent energy.

3D Metallic Ti@Ni0.85 Se with Triple Hierarchy as High-Efficiency Electrocatalyst for Overall Water Splitting

In this study, Ti@Ni_0.85 Se electrodes with a triple hierarchy architecture were designed, and their applications in electrocatalytic water splitting were studied. The 3D electrode is comprised of three types of structures including the bottom square Ti mesh structure as the conductive substrate, a vertical and uniform Ni_0.85 Se nanosheet arrays structure in the intermediate section, and the topmost Ni_0.85 Se flower structure. This triple hierarchy architecture is binder-free, conductive, and has a particular feature of enlarged surface areas, exposing more active sites, promoting mass- and charge-transfer, and accelerating dissipation of gases generated during water electrolysis. Moreover, DFT calculations confirmed that the Ni_0.85 Se possesses metallic character, which further promotes the charge transfer of the electrocatalyst. Benefiting from this special structure and metallic character, the electrode displays a superior activity of 10 mA cm^-2 at 120 mV hydrogen evolution reaction overpotential and 30 mA cm^-2 at 270 mV oxygen evolution reaction overpotential. By using this electrode as a bifunctional electrocatalyst, an alkali electrolyzer affords a water splitting current of 10 mA cm^-2 at a cell voltage of 1.66 V.

Electrocatalytic Water Splitting through the Ni x S y Self-Grown Superstructures Obtained via a Wet Chemical Sulfurization Process

We report water-splitting application of chemically stable self-grown nickel sulfide (Ni _x S _y ) electrocatalysts of different nanostructures including rods, flakes, buds, petals, etc., synthesized by a hydrothermal method on a three-dimensional Ni foam (NiF) in the presence of different sulfur-ion precursors, e.g., thioacetamide, sodium thiosulfate, thiourea, and sodium sulfide. The S^2- ions are produced after decomposition from respective sulfur precursors, which, in general, react with oxidized Ni²⁺ ions from the NiF at optimized temperatures and pressures, forming the Ni _x S _y superstructures. These Ni _x S _y electrocatalysts are initially screened for their structure, morphology, phase purity, porosity, and binding energy by means of various sophisticated instrumentation technologies. The as-obtained Ni _x S _y electrocatalyst from sodium thiosulfate endows an overpotential of 200 mV. The oxygen evolution overpotential results of Ni _x S _y electrocatalysts are comparable or superior to those reported previously for other self-grown Ni _x S _y superstructure morphologies.

Insight into the Superior Electrocatalytic Performance of a Ternary Nickel Iron Poly-Phosphide Nanosheet Array: An X-ray Absorption Study

Although ternary components and doping with foreign atoms have been widely studied to enhance the electrocatalytic performance of transition metal phosphides, the underlying mechanism is not clear. Here, we fabricated ternary Ni-Fe-P nanosheets on carbon fiber paper as efficient electrodes and studied the local atomic and electronic structure alteration through X-ray absorption spectroscopy. The optimized ternary Ni-Fe-P nanosheet electrode exhibited superior hydrogen evolution activity and stability in 0.5 M H₂SO₄ with a low overpotential of 56 mV at 10 mA cm^-2. X-ray absorption spectroscopy studies revealed that with the Fe ion incorporation into the system, the Ni-P bonds elongated and few electrons transferred from Ni to P which resulted in a reduced oxidation state of Ni and reduced the interaction between the hydrogen atom and the catalyst surface. Our work not only demonstrates the future potential of high-performance electrocatalysts based on ternary Ni-Fe-P but also offers a promising method to explore the unique synergistic effect in ternary compounds.

Metallization of 3D Printed Polymers and Their Application as a Fully Functional Water-Splitting System

In this work, the plating of high-quality amorphous nickel-phosphorous coating with low resistivity of 0.45 µΩ m (298 K) on complex 3D printed polymeric structures with high uniformity is reported. Such a polymer metallization results in an effective conductivity of 4.7 × 104 S m-1. This process also allows flexible structures to maintain their flexibility along with the conductivity. Octet-truss structures with nickel-iron-(oxo) hydroxide nanosheets electrodeposited onto further displays excellent water-splitting performance as catalytic electrodes, i.e., in KOH (1 m, aq), a low oxygen evolution reaction (OER) overpotential of 197 mV at 10 mA cm-2 and Tafel slope of 51 mV dec-1. Using this light-weight electrode with high specific area, strength, and corrosion resistance properties, a fully functional water-splitting system is designed and fabricated through the concentric integration of 3D printed components. A dense polymeric mesh implemented is also demonstrated as an effective separator of hydrogen and oxygen bubbles in this system.

Integrating Catalysis of Methane Decomposition and Electrocatalytic Hydrogen Evolution with Ni/CeO2 for Improved Hydrogen Production Efficiency

Ni/CeO₂ enables either methane decomposition or water electrolysis for pure hydrogen production. Ni/CeO₂ , prepared by a sol-gel method with only one heat treatment step, was used to catalyze methane decomposition for the generation of H₂ . The solid byproduct, Ni/CeO₂ /carbon nanotube (CNT), was further employed as an electrocatalyst in the hydrogen evolution reaction (HER) for H₂ production. The Ni/CeO₂ catalyst exhibits excellent activity for methane decomposition because CeO₂ prevents carbon encapsulation of Ni nanoparticles during the preparation process and forms a special metal-support interface with Ni. The derived CNTs act as antenna to improve conductivity and promote the dispersion of agglomerated Ni/CeO₂ . In addition, they provide H₂ diffusion paths and prevent Ni/CeO₂ from peeling off the HER electrode. Although long-term methane decomposition reduces the HER activity of Ni/CeO₂ /CNTs (owing to degradation of the delicate Ni/CeO₂ interface), the tunable nature of the synthesis makes this an attractive sustainable approach to synthesize future high-performance materials.

Recent Advances in Metallic Glass Nanostructures: Synthesis Strategies and Electrocatalytic Applications

Recent advances in metallic glass nanostructures (MGNs) are reported, covering a wide array of synthesis strategies, computational discovery, and design solutions that provide insight into distinct electrocatalytic applications. A brief introduction to the development and unique features of MGNs with an overview of top-down and bottom-up synthesis strategies is presented. Specifically, the morphology and structural analysis of several examples applying MGNs as electrodes are highlighted. Subsequently, a comprehensive discussion of commonly employed kinetic parameters and their connection with the unique material structures of MGNs on individual electrocatalytic reactions is made, including the hydrogen evolution reaction, oxygen reduction reaction, and alcohol (methanol or ethanol) oxidation reaction. Finally, a summary of the challenges and perspective on the future research and development relevant to MGNs as electrocatalysts is provided.

Facile Synthesis of Amorphous Ternary Metal Borides-Reduced Graphene Oxide Hybrid with Superior Oxygen Evolution Activity

Metal borides represent an emerging family of advanced electrocatalyst for oxygen evolution reaction (OER). Herein, we present a fast and simple method of synthesizing iron-doped amorphous nickel boride on reduced graphene oxide (rGO) sheets. The hybrid exhibits outstanding OER performance and stability in prolonged OER operation. In 1.0 M KOH, only 230 mV is required to afford a current density of 15 mA cm^-2 with a small Tafel slope of 50 mV dec^-1. DFT calculations lead to a suggestion that the in situ formation of MO _xH _y during electrochemical activation acts as active sites for water oxidation. The superior OER activity of the as-prepared catalyst is attributed to (i) its unique amorphous structure to allow abundant active sites, (ii) synergistic effect of constituents, and (iii) strong coupling of active material and highly conductive rGO. This work not only provides new perspectives to design a highly effective material for OER but also opens a promising avenue to tailor the electrochemical properties of metal borides, which could be extended to other materials for energy storage and conversion technologies.

Designing graphene-wrapped NiCo2Se4 microspheres with petal-like FeS2 toward flexible asymmetric all-solid-state supercapacitors

An effective and accessible method is demonstrated for the construction of electrodes from graphene-wrapped NiCo₂Se₄ microspheres and petal-like FeS₂ with excellent durability for flexible all-solid-state asymmetric supercapacitors.

Facile synthesis of porous carbon/Ni12P5 composites for electrocatalytic hydrogen evolution

We demonstrate for the first time the facile synthesis of porous carbon/Ni₁₂P₅ (C/NiP-C) composites for electrocatalytic hydrogen evolution.

A hierarchically-assembled Fe–MoS2/Ni3S2/nickel foam electrocatalyst for efficient water splitting

Hierarchically-assembled Fe–MoS₂/Ni₃S₂/NF demonstrates excellent HER, OER and full water splitting catalytic performances in an alkaline electrolyte.

Dual tuning of nickel sulfide nanoflake array electrocatalyst through nitrogen doping and carbon coating for efficient and stable water splitting

Simultaneous carbon coating and nitrogen incorporation of a Ni₃S₂ nanoflake array electrocatalyst with enhanced activity and stability for water splitting.

Co9 S8 -Catalyzed Growth of Thin-Walled Graphite Microtubes for Robust, Efficient Overall Water Splitting

Co₉ S₈ crystals can catalyze the growth of thin-walled graphite microtubes (GMTs) through a catalytic chemical vapor deposition (CCVD) process using thiourea as the precursor. The growth of GMTs follows a tip-growth mechanism with tube diameters up to a few micrometer. The hollow interiors of the GMTs are filled with carbon nanotubes and wrinkled graphene layers, which form a unique nanotube/graphene-in-microtube structure. As-formed GMTs are N,S-codoped with lots of Co₉ S₈ nanoparticles encapsulated in their inner walls. These GMTs are room-temperature ferromagnets and can be loaded on Ni foams to work as binder-free electrocatalysts with low overpotential (310 mV at 50 mA cm^-2 for the oxygen evolution reaction (OER) and 284 mV at 50 mA cm^-2 for the hydrogen evolution reaction (HER)) and long-term durability (continuous work for 120 h without loss in performance). Our research proves that metal sulfides can catalyze the growth of graphite microtubes and as-formed GMTs may potentially be used as functional building blocks to construct new kinds of electrochemical devices for various energy-related applications.

Novel Cobalt-Doped Ni0.85Se Chalcogenides (Co xNi0.85- xSe) as High Active and Stable Electrocatalysts for Hydrogen Evolution Reaction in Electrolysis Water Splitting

In this paper, novel cobalt-doped Ni_0.85Se chalcogenides (Co _xNi_{0.85- x}Se, x = 0.05, 0.1, 0.2, 0.3, and 0.4) are successfully synthesized and studied as high active and stable electrocatalysts for hydrogen evolution reaction (HER) in electrolysis water splitting. The morphologies, structures, and composition of these as-prepared catalysts are characterized by X-ray diffraction, X-ray photoelectron spectroscopy, Raman spectroscopy, and transmission electron microscopy. The electrochemical tests, such as linear sweep voltammetry, cyclic voltammetry, electrochemical impedance spectroscopy, and chronoamperometry testing, are performed to evaluate these catalysts' HER catalytic performance including activity and stability. The results indicate that a suitable doping can result in synergetic effect for increasing the catalytic performance. Among different catalysts, Co_0.1Ni_0.75Se shows the highest HER performance. After introducing the reduced graphene oxide (rGO) into this catalyst as the support, the resulted Co_0.1Ni_0.75Se/rGO shows even better performance than unsupported Co_0.1Ni_0.75Se, which are confirmed by the reduction of HER overpotential of Co_0.1Ni_0.75Se/rGO to 103 mV compared to 153 mV of Co_0.1Ni_0.75Se at a current density of 10 mA/cm², and the smaller Tafel slope (43 mV/dec) and kinetic resistance (21.34 Ω) than those of Co_0.1Ni_0.75Se (47 mV/dec, 30.23 Ω). Furthermore, the large electrochemical active surface area and high conductivity of such a Co_0.1Ni_0.75Se/rGO catalyst, induced by rGO introduction, are confirmed to be responsible for the high HER performance.

Self-supported Pt nanoflakes-doped amorphous Ni(OH)2 on Ni foam composite electrode for efficient and stable methanol oxidation

Direct methanol fuel cells (DMFCs) are promising power sources for automobiles and portable electronic devices. Its commercialization depends on the anodes with high activity, low Pt content, and especially high stability towards methanol oxidation. Herein, a self-supported Pt nanoflakes and amorphous Ni(OH)₂ on nickel foam composite electrode (Pt-doped Ni(OH)₂, Pt content: 1.5 wt%) with rich defects was fabricated via a facile and low cost galvanic deposition method. This composite anode exhibits enhanced activity and stability for methanol oxidation in alkaline media, which mainly come from the synergistic effects between Pt nanoflakes and amorphous Ni(OH)₂ on Ni foam substrate and defect engineering. During a typical methanol oxidation process over Pt-doped Ni(OH)₂: Pt nanoflakes act as the active sites; amorphous Ni(OH)₂ promotes the poison removal; Ni foam provides high electric conductivity and large area; defects sites contribute to the enhanced activity and stability. This work suggests that this self-supported and defect-enriched Pt-doped Ni(OH)₂ composite catalyst is an alternative to commercial Pt-based electrocatalyst for low temperature DMFCs.

Oxygen-Doped Nickel Iron Phosphide Nanocube Arrays Grown on Ni Foam for Oxygen Evolution Electrocatalysis

A rationally designed oxygen evolution reaction (OER) catalyst with advanced structural and compositional superiority is highly desirable to optimize electrocatalytic performance. Prussian blue analogues (PBAs) with adjustable element compositions and accessible porous structures represent a promising precursor for the preparation of OER catalysts. Herein, oxygen-doped nickel iron phosphide nanocube arrays (Ni₂ P/(NiFe)₂ P(O) NAs) grown on Ni foam is rationally designed and fabricated from PBAs. The porous structure and the synergistic effect of Ni and Fe enable superior electrocatalytic performance and stability toward the OER in alkaline electrolytes. Density functional theory calculations reveal that Fe-incorporated Ni₂ P can generate new active sites on the Fe atoms, and the energy barriers of the intermediates and products are decreased efficiently in the presence of surface doped oxygen, both processes are crucial factors for enhanced catalytic performances. In 1 m KOH, the Ni₂ P/(NiFe)₂ P(O) NAs afford current densities of 10 and 800 mA cm^-2 at overpotentials of 150 and 530 mV, respectively, which outperform the commercial noble metal IrO₂ . Ni₂ P/(NiFe)₂ P(O) NAs also have long-term stability over 100 h at a high current density. The present approach may provide a new avenue for the controlled assembly of nanoarrays for energy storage and conversion applications.

Ultrathin Amorphous Iron-Nickel Boride Nanosheets for Highly Efficient Electrocatalytic Oxygen Production

A cost-effective and efficient electrocatalyst for the oxygen evolution reaction during the electrolysis of water is highly desired. In an effort to develop an economical material for replacing precious-metal-based catalysts, a novel and self-standing amorphous ultrathin nanosheet (NS) of bimetallic iron-nickel boride (Fe-Ni-B NSs) on Ni foam is presented, which displays a better oxygen-evolving activity compared to the precious-metal catalyst RuO₂ . In 1.0 m KOH electrolyte solution, it requires an overpotential of only 237 mV to reach a current density of 10 mA cm^-2 with a small Tafel slope of 38 mV dec^-1 and shows prominent long-term electrochemical stability. A synergistic effect between highly abundant catalytically active sites on the 3D porous substrate improved the electron transport arising from the presence of highly negative boron, and the high conductivity of the substrate results in an outstanding electrocatalytic activity. The advanced catalytic activity, facile electrode fabrication, and low costs make it a potential oxygen-evolving material, which may be extended to other energy-conversion and storage technologies.