Metallic Co 4 N Porous Nanowire Arrays Activated by Surface Oxidation as Electrocatalysts for the Oxygen Evolution Reaction

Decoupling Hydrogen and Oxygen Production in Acidic Water Electrolysis Using a Polytriphenylamine-Based Battery Electrode

Hydrogen production through water splitting is considered a promising approach for solar energy harvesting. However, the variable and intermittent nature of solar energy and the co-production of H₂ and O₂ significantly reduce the flexibility of this approach, increasing the costs of its use in practical applications. Herein, using the reversible n-type doping/de-doping reaction of the solid-state polytriphenylamine-based battery electrode, we decouple the H₂ and O₂ production in acid water electrolysis. In this architecture, the H₂ and O₂ production occur at different times, which eliminates the issue of gas mixing and adapts to the variable and intermittent nature of solar energy, facilitating the conversion of solar energy to hydrogen (STH). Furthermore, for the first time, we demonstrate a membrane-free solar water splitting through commercial photovoltaics and the decoupled acid water electrolysis, which potentially paves the way for a new approach for solar water splitting.

Co2P quantum dot embedded N, P dual-doped carbon self-supported electrodes with flexible and binder-free properties for efficient hydrogen evolution reactions

Transition metal phosphides (TMPs) are considered to be superb catalysts for water splitting. In this work, we introduce an efficient strategy to fabricate dicobalt phosphide (Co₂P) quantum dots embedded in N, P dual-doped carbon (Co₂P@NPC) on carbon cloth (Co₂P@NPC/CC) by in situ carbonization of cobalt ion induced phytic acid (PA) and polyaniline (PANI) macromolecule precursors. As a highly efficient self-supported electrode, it has a low onset overpotential (74 mV at 1 mA cm^-2) approaching that of the commercial Pt/C catalyst for the hydrogen evolution reaction (HER) in acidic media. Meanwhile, it also shows very low overpotentials of only 116 and 129 mV at 10 mA cm^-2 with robust stability in acidic and alkaline media, respectively.

Extraction of nickel from NiFe-LDH into Ni2P@NiFe hydroxide as a bifunctional electrocatalyst for efficient overall water splitting

The development of highly efficient, low-cost and stable electrocatalysts for overall water splitting is highly desirable for the storage of intermittent solar energy and wind energy sources. Herein, we show for the first time that nickel can be extracted from NiFe-layered double hydroxide (NiFe-LDH) to generate an Ni2P@FePO x heterostructure. The Ni2P@FePO x heterostructure was converted to an Ni2P@NiFe hydroxide heterostructure (P-NiFe) during water splitting, which displays high electrocatalytic performance for both the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in 1.0 M KOH solution, with an overpotential of 75 mV at 10 mA cm-2 for HER, and overpotentials of 205, 230 and 430 mV at 10, 100 and 1000 mA cm-2 for OER, respectively. Moreover, it could afford a stable current density of 10 mA cm-2 for overall water splitting at 1.51 V in 1.0 M KOH with long-term durability (100 h). This cell voltage is among the best reported values for bifunctional electrocatalysts. The results of theoretical calculations demonstrate that P-NiFe displays optimized adsorption energies for both HER and OER intermediates at the nickel active sites, thus dramatically enhancing its electrocatalytic activity.

A New Member of Electrocatalysts Based on Nickel Metaphosphate Nanocrystals for Efficient Water Oxidation

High-performance electrocatalysts are desired for electrochemical energy conversion, especially in the field of water splitting. Here, a new member of phosphate electrocatalysts, nickel metaphosphate (Ni₂ P₄ O₁₂ ) nanocrystals, is reported, exhibiting low overpotential of 270 mV to generate the current density of 10 mA cm^-2 and a superior catalytic durability of 100 h. It is worth noting that Ni₂ P₄ O₁₂ electrocatalyst has remarkable oxygen evolution performance operating in basic media. Further experimental and theoretical analyses demonstrate that N dopant boosts the catalytic performance of Ni₂ P₄ O₁₂ due to optimizing the surface electronic structure for better charge transfer and decreasing the adsorption energy for the oxygenic intermediates.

Brush-Like Cobalt Nitride Anchored Carbon Nanofiber Membrane: Current Collector-Catalyst Integrated Cathode for Long Cycle Li-O2 Batteries

To achieve a high reversibility and long cycle life for lithium-oxygen (Li-O₂) batteries, the irreversible formation of Li₂O₂, inevitable side reactions, and poor charge transport at the cathode interfaces should be overcome. Here, we report a rational design of air cathode using a cobalt nitride (Co₄N) functionalized carbon nanofiber (CNF) membrane as current collector-catalyst integrated air cathode. Brush-like Co₄N nanorods are uniformly anchored on conductive electrospun CNF papers via hydrothermal growth of Co(OH)F nanorods followed by nitridation step. Co₄N-decorated CNF (Co₄N/CNF) cathode exhibited excellent electrochemical performance with outstanding stability for over 177 cycles in Li-O₂ cells. During cycling, metallic Co₄N nanorods provide sufficient accessible reaction sites as well as facile electron transport pathway throughout the continuously networked CNF. Furthermore, thin oxide layer (<10 nm) formed on the surface of Co₄N nanorods promote reversible formation/decomposition of film-type Li₂O₂, leading to significant reduction in overpotential gap (∼1.23 V at 700 mAh g^-1). Moreover, pouch-type Li-air cells using Co₄N/CNF cathode stably operated in real air atmosphere even under 180° bending. The results demonstrate that the favorable formation/decomposition of reaction products and mediation of side reactions are hugely governed by the suitable surface chemistry and tailored structure of cathode materials, which are essential for real Li-air battery applications.

Boosting the oxygen evolution reaction performance of CoS2 microspheres by subtle ionic liquid modification

Owing to the hydrophobic environment and electrostatic affinity generated at the CoS₂/electrolyte interface, the OER performance of CoS₂ microspheres is boosted by IL modification.

Hierarchically structured CoN/Cu3N nanotube array supported on copper foam as an efficient bifunctional electrocatalyst for overall water splitting

Exploring nonprecious, efficient and stable electrocatalysts for overall water splitting is of great importance, but it is also challenging.

Efficient catalysts for oxygen evolution derived from cobalt-based alloy nanochains

One-dimensional nanomaterials are widely used in electrocatalysis owing to their high charge transfer efficiency.

Atomically Thin Mesoporous Co3 O4 Layers Strongly Coupled with N-rGO Nanosheets as High-Performance Bifunctional Catalysts for 1D Knittable Zinc-Air Batteries

Under development for next-generation wearable electronics are flexible, knittable, and wearable energy-storage devices with high energy density that can be integrated into textiles. Herein, knittable fiber-shaped zinc-air batteries with high volumetric energy density (36.1 mWh cm^-3 ) are fabricated via a facile and continuous method with low-cost materials. Furthermore, a high-yield method is developed to prepare the key component of the fiber-shaped zinc-air battery, i.e., a bifunctional catalyst composed of atomically thin layer-by-layer mesoporous Co₃ O₄ /nitrogen-doped reduced graphene oxide (N-rGO) nanosheets. Benefiting from the high surface area, mesoporous structure, and strong synergetic effect between the Co₃ O₄ and N-rGO nanosheets, the bifunctional catalyst exhibits high activity and superior durability for oxygen reduction and evolution reactions. Compared to a fiber-shaped zinc-air battery using state-of-the-art Pt/C + RuO₂ catalysts, the battery based on these Co₃ O₄ /N-rGO nanosheets demonstrates enhanced and stable electrochemical performance, even under severe deformation. Such batteries, for the first time, can be successfully knitted into clothes without short circuits under external forces and can power various electronic devices and even charge a cellphone.

A review of anion-regulated multi-anion transition metal compounds for oxygen evolution electrocatalysis

Recent advances in the anion regulation on multi-anion transition metal compounds as electrocatalysts for oxygen evolution reaction are reviewed.

Highly efficient oxygen evolution electrocatalysts prepared by using reduction-engraved ferrites on graphene oxide

A simple and efficient method was explored for synthesizing efficient ferrite-based OER electrocatalysts by using reduction-engraved ultrafine ferrite nanoparticles on a conductive GO support.

Electronic Modulation of Electrocatalytically Active Center of Cu7S4 Nanodisks by Cobalt-Doping for Highly Efficient Oxygen Evolution Reaction

Cu-based electrocatalysts have seldom been studied for water oxidation because of their inferior activity and poor stability regardless of their low cost and environmentally benign nature. Therefore, exploring an efficient way to improve the activity of Cu-based electrocatalysts is very important for their practical application. Modifying electronic structure of the electrocatalytically active center of electrocatalysts by metal doping to favor the electron transfer between catalyst active sites and electrode is an important approach to optimize hydrogen and oxygen species adsorption energy, thus leading to the enhanced intrinsic electrocatalytic activity. Herein, Co-doped Cu₇S₄ nanodisks were synthesized and investigated as highly efficient electrocatalyst for oxygen evolution reaction (OER) due to the optimized electronic structure of the active center. Density-functional theory (DFT) calculations reveal that Co-engineered Cu₇S₄ could accelerate electron transfer between Co and Cu sites, thus decrease the energy barriers of intermediates and products during OER, which are crucial for enhanced catalytic properties. As expected, Co-engineered Cu₇S₄ nanodisks exhibit a low overpotential of 270 mV to achieve current density of 10 mA cm^-2 as well as decreased Tafel slope and enhanced turnover frequencies as compared to bare Cu₇S₄. This discovery not only provides low-cost and efficient Cu-based electrocatalyst by Co doping, but also exhibits an in-depth insight into the mechanism of the enhanced OER properties.

Facile Spray-Pyrolysis Synthesis of Yolk-Shell Earth-Abundant Elemental Nickel-Iron-Based Nanohybrid Electrocatalysts for Full Water Splitting

The development of high-activity electrocatalysts for water splitting that comprise only inexpensive, earth-abundant elements is critical but remains a daunting challenge. In this work, yolk-shell Ni₃ Fe/Ni₃ FeN was prepared by a spray-pyrolysis technique, which could be scaleable. The yolk-shell Ni₃ Fe/Ni₃ FeN presents excellent catalytic activity for the oxygen evolution reaction (OER) and the hydrogen evolution reaction (HER) with overpotentials of 268 and 166 mV at 10 mA cm^-2 , respectively, and bears a prominent electrochemical durability. Overall water splitting with an electrolyzer containing the yolk-shell Ni₃ Fe/Ni₃ FeN as the cathode and anode only requires a cell voltage of 1.62 V to reach a current density of 10 mA cm^-2 . The present research not only introduces a new route for the synthesis of advanced functional electrocatalysts for overall water splitting but also sheds light on their potential commercial applications.

First-Row Transition Metal Based Catalysts for the Oxygen Evolution Reaction under Alkaline Conditions: Basic Principles and Recent Advances

Owing to its abundance, high gravimetric energy density, and environmental friendliness, hydrogen is a promising renewable energy to replace fossil fuels. One of the most prominent routes toward hydrogen acquisition is water splitting, which is currently bottlenecked by the sluggish kinetics of oxygen evolution reaction (OER). Numerous of electrocatalysts have been developed in the past decades to accelerate the OER process. Up to now, the first-row transition metal based compounds are in pole position under alkaline conditions, which have become subjects of extensive studies. Recently, significant advances in providing compelling catalytic performance as well as exploring their catalytic mechanisms have been achieved in this area. In this review, we summarized the fundamentals and recent progresses in first-row transition metal based OER catalysts, with special emphasis on the pathways of promoting catalytic performance by concrete strategies. New insight into material design, particularly the role of experimental approaches in the electrocatalytic performance and reaction mechanisms of OER are expected to be provided.

In Situ Coupling Strategy for the Preparation of FeCo Alloys and Co4 N Hybrid for Highly Efficient Oxygen Evolution

An in situ coupling approach is developed to create a new highly efficient and durable cobalt-based electrocatalyst for the oxygen evolution reaction (OER). Using a novel cyclotetramerization, a task-specific bimetallic phthalocyanine-based nanoporous organic framework is successfully built as a precursor for the carbonization synthesis of a nonprecious OER electrocatalyst. The resultant material exhibits an excellent OER activity with a low overpotential of 280 mV at a current density of 10 mA cm^-2 and high durability in an alkaline medium. This impressive result ranks among the best from known Co-based OER catalysts under the same conditions. The simultaneous installation of multiple diverse cobalt-based active sites, including FeCo alloys and Co₄ N nanoparticles, plays a critical role in achieving this promising OER performance. This innovative approach not only enables high-performance OER activity to be achieved but simultaneously provides a means to control the surface features, thereby tuning the catalytic property of the material.

Oxygen Vacancies Dominated NiS2 /CoS2 Interface Porous Nanowires for Portable Zn-Air Batteries Driven Water Splitting Devices

The development of highly active and stable oxygen evolution reaction (OER) electrocatalysts is crucial for improving the efficiency of water splitting and metal-air battery devices. Herein, an efficient strategy is demonstrated for making the oxygen vacancies dominated cobalt-nickel sulfide interface porous nanowires (NiS₂ /CoS₂ -O NWs) for boosting OER catalysis through in situ electrochemical reaction of NiS₂ /CoS₂ interface NWs. Because of the abundant oxygen vacancies and interface porous nanowires structure, they can catalyze the OER efficiently with a low overpotential of 235 mV at j = 10 mA cm^-2 and remarkable long-term stability in 1.0 m KOH. The home-made rechargeable portable Zn-air batteries by using NiS₂ /CoS₂ -O NWs as the air-cathode display a very high open-circuit voltage of 1.49 V, which can maintain for more than 30 h. Most importantly, a highly efficient self-driven water splitting device is designed with NiS₂ /CoS₂ -O NWs as both anode and cathode, powered by two-series-connected NiS₂ /CoS₂ -O NWs-based portable Zn-air batteries. The present work opens a new way for designing oxygen vacancies dominated interface nanowires as highly efficient multifunctional electrocatalysts for electrochemical reactions and renewable energy devices.

Hexagonal-Phase Cobalt Monophosphosulfide for Highly Efficient Overall Water Splitting

The rational design and synthesis of nonprecious, efficient, and stable electrocatalysts to replace precious noble metals are crucial to the future of hydrogen economy. Herein, a partial sulfurization/phosphorization strategy is proposed to synthesize a nonstoichiometric pyrrhotite-type cobalt monophosphosulfide material (Co_0.9S_0.58P_0.42) with a hexagonal close-packed phase for electrocatalytic water splitting. By regulating the degree of sulfurization, the P/S atomic ratio in the cobalt monophosphosulfide can be tuned to activate the Co³⁺/Co²⁺ couples. The synergy between the nonstoichiometric nature and the tunable P/S ratio results in the strengthened Co³⁺/Co²⁺ couples and tunable electronic structure and thus efficiently promotes the oxygen/hydrogen evolution reaction (OER/HER) processes toward overall water splitting. Especially for OER, the Co_0.9S_0.58P_0.42 material, featured with a uniform yolk-shell spherical morphology, shows a low overpotential of 266 mV at 10 mA cm^-2 (η₁₀) with a low Tafel slope of 48 mV dec^-1 as well as high stability, which is comparable to that of the reported promising OER electrocatalysts. Coupled with the high HER activity of Co_0.9S_0.58P_0.42, the overall water splitting is demonstrated with a low η₁₀ at 1.59 V and good stability. This study shows that phase engineering and composition control can be the elegant strategy to realize the Co³⁺/Co²⁺ couple activation and electronic structure tuning to promote the electrocatalytic process. The proposed strategy and approaches allow the rational design and synthesis of transition metal monophosphosulfides toward advanced electrochemical applications.

Ni nanoparticles@Ni-Mo nitride nanorod arrays: a novel 3D-network hierarchical structure for high areal capacitance hybrid supercapacitors

Because of the advanced nature of their high power density, fast charge/discharge time, excellent cycling stability, and safety, supercapacitors have attracted intensive attention for large-scale applications. Nevertheless, one of the obstacles for their further development is their low energy density caused by sluggish redox reaction kinetics, low electroactive electrode materials, and/or high internal resistance. Here, we develop a facile and simple nitridation process to successfully synthesize hierarchical Ni nanoparticle decorated Ni_0.2Mo_0.8N nanorod arrays on a nickel foam (Ni-Mo-N NRA/NF) from its NiMoO₄ precursor, which delivers a high areal capacity of 2446 mC cm^-2 at a current density of 2 mA cm^-2 and shows outstanding cycling stability. The superior performance of the Ni-Mo-N NRA/NF can be ascribed to the metallic conductive nature of the Ni-Mo nitride, the fast surface redox reactions for the electrolyte ions and electrode materials, and the low contacted resistance between the active materials and the current collectors. Furthermore, a hybrid supercapacitor (HSC) is assembled using the Ni-Mo-N NRA/NF as the positive electrode and reduced graphene oxide (RGO) as the negative electrode. The optimized HSC exhibits excellent electrochemical performance with a high energy density of 40.9 W h kg^-1 at a power density of 773 W kg^-1 and a retention of 80.1% specific capacitance after 6000 cycles. These results indicate that the Ni-Mo-N NRA/NF have a promising potential for use in high-performance supercapacitors.

Unique hybrid Ni2P/MoO2@MoS2 nanomaterials as bifunctional non-noble-metal electro-catalysts for water splitting

We successfully synthesized a novel electro-catalyst with a unique structure of Ni₂P nanoparticles decorating the surface of MoO₂@MoS₂ sub-microwires on titanium foil (denote as NiMoO-SP/Ti) via a facile temperature-programmed sulfuration-phosphorization from its nickel molybdate precursor. The metallic MoO₂ core facilitates electron transfer, and the interfaces between MoS₂ nanosheets and Ni₂P nanoparticles enhance catalytic activity both for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). Due to this unique structure, the obtained NiMoO-SP/Ti showed an enhanced OER performance in alkaline solution with a small Tafel slope of 85 mV dec^-1 and a low overpotential of 280 and 360 mV to achieve 10 and 100 mA cm^-2 in 1.0 M KOH, respectively. The catalyst also exhibited an excellent stability in 1.0 M KOH, with just 12 mV shift after electrolysis at 10 mA cm^-2 for 16 h and 27 mV shift after electrolysis at 20 mA cm^-2 for another 24 h. In addition, the NiMoO-SP/Ti also displayed high catalytic properties towards HER with a small Tafel slope of 77 mV dec^-1 and a low overpotential of 159 mV to obtain 10 mA cm^-2 in 1.0 M KOH. After electrolysis at -10 mA cm^-2 for 40 h, the overpotential increased by just 25 mV, which demonstrated its high stability for HER in 1.0 M KOH. This work provides an effective route to designing a high-performance catalyst with a favorable structure for the development of electro-catalysts for water splitting.

Magnetic γ'-Fe4 N/Fe3 C, χ-Fe5 C2 , and θ-Fe3 C by a Simple Route for Application as Electrochemical Catalysts

A new and simple method to fabricate magnetic Fe₄ N/Fe₃ C samples is reported. Meanwhile, pure phase iron carbides (θ-Fe₃ C and χ-Fe₅ C₂ ) were obtained by controlling experimental conditions. The structures, magnetic properties, and morphology of the samples were investigated according to the generalized analysis of X-ray diffraction, X-ray photoelectron spectroscopy, as well as transmission electron microscopy and vibrating sample magnetometry. The magnetic properties measurement revealed the remarkable magnetic properties of the samples at 2 and 300 K. The application of the prepared samples as catalysts for oxygen evolution reaction was also investigated in alkaline solution. This simple and convenient route provides a new path to fabricate other metal nitrides and carbides.

Mixed-Metal-Organic Framework Self-Template Synthesis of Porous Hybrid Oxyphosphides for Efficient Oxygen Evolution Reaction

Developing an efficient, stable yet cost-effective electrocatalyst is the key link along the path to hydrogen fuels produced by water splitting. The current bottleneck in the water electrolysis technology is the sluggish oxygen-evolving reaction (OER) which is also central to the rechargeable metal-air batteries. Herein, we report a promising mixed-metal-organic framework (MMOF) self-template strategy to synthesize CoFe hybrid oxyphosphides with highly porous morphology. Aided by the porous hybrid bulk structure beneficial to fast-ion diffusion to abundant highly active sites, the as-synthesized Co₃FeP_xO exhibited excellent electrocatalytic activity toward OER, with an overpotential of 291 mV at 10 mA cm^-2 and a low Tafel slope of 85 mV dec^-1. With the underpinnings of MMOF maintaining the structural rigidity and stability, the material also showed long life for OER without  discernible activity decay.

Enhancement of photoelectrochemical oxidation by an amorphous nickel boride catalyst on porous BiVO4

This paper describes an amorphous nickel boride (NiB) electrocatalyst loaded on porous bismuth vanadate (BiVO₄) with high activity for oxygen evolution in photoelectrochemical water oxidation. The NiB-decorated BiVO₄ photoanode exhibits an onset potential of 0.25 V versus the reversible hydrogen electrode (vs. RHE) and a photocurrent of 3.47 mA cm^-2 at 1.23 V vs. RHE under simulated 100 mW cm^-2 irradiation.

Mesoporous Nanosheet Networked Hybrids of Cobalt Oxide and Cobalt Phosphate for Efficient Electrochemical and Photoelectrochemical Oxygen Evolution

A novel mesoporous nanosheet networked hybrid comprising Co₃ O₄ and Co₃ (PO₄ )₂ is successfully synthesized using a facile and scalable method through calcinating the carbon, cobalt hydroxy carbonate, and cobalt phosphate composite precursor. Electron transfer from Co₃ O₄ to Co₃ (PO₄ )₂ , together with the special networked structure and the porous nature of the nanosheets enable the Co₃ (PO₄ )₂ -Co₃ O₄ hybrid to have a high oxygen evolution reaction (OER) activity and outstanding stability in alkaline electrolyte, e.g., an overpotential of 270 mV at current density of 10 mA cm^-2 , and a Tafel slope of 39 mV dec^-1 , which are superior to most non-noble metal-based OER electrocatalysts reported thus far and as well the commercial RuO₂ electrocatalyst. Furthermore, Co₃ (PO₄ )₂ -Co₃ O₄ hybrid is demonstrated to be used as an efficient cocatalyst to enhance the photoelectrochemical OER performance of BiVO₄ photoanode. A significantly increased photocurrent density of 3.0 mA cm^-2 at 1.23 V (vs reversible hydrogen electrode, RHE), and a potential reduction of 530 mV with respect to that of bare BiVO₄ at the photocurrent density of 0.5 mA cm^-2 are achieved. The electron transfer-induced enhancement of OER by a hybrid structure may pave the new routes for the design and synthesis of low-cost catalysts for electrochemical and photoelectrochemical oxygen evolution.

Architecting a Mesoporous N-Doped Graphitic Carbon Framework Encapsulating CoTe2 as an Efficient Oxygen Evolution Electrocatalyst

To improve the efficiency of cobalt-based catalysts for water electrolysis, tremendous efforts have been dedicated to tuning the composition, morphology, size, and structure of the materials. We report here a facile preparation of orthorhombic CoTe₂ nanocrystals embedded in an N-doped graphitic carbon matrix to form a 3D architecture with a size of ∼500 nm and abundant mesopores of ∼4 nm for the oxygen evolution reaction (OER). The hybrid electrocatalyst delivers a small overpotential of 300 mV at 10 mA cm^-2, which is much lower than that for pristine CoTe₂ powder. After cycling for 2000 cycles or driving continual OER for 20 h, only a slight loss is observed. The mesoporous 3D architecture and the strong interaction between N-doped graphitic carbon and CoTe₂ are responsible for the enhancement of the electrocatalytic performance.

Self-Supported Nickel Iron Layered Double Hydroxide-Nickel Selenide Electrocatalyst for Superior Water Splitting Activity

The design of efficient, low-cost, and stable electrocatalyst systems toward energy conversion is highly demanding for their practical use. Large scale electrolytic water splitting is considered as a promising strategy for clean and sustainable energy production. Herein, we report a self-supported NiFe layered double hydroxide (LDH)-NiSe electrocatalyst by stepwise surface-redox-etching of Ni foam (NF) through a hydrothermal process. The as-prepared NiFe LDH-NiSe/NF catalyst exhibits far better performance in alkaline water oxidation, proton reduction, and overall water splitting compared to NiSe_x/NF or NiFe LDH/NF. Only 240 mV overpotential is required to obtain a water oxidation current density of 100 mA cm^-2, whereas the same for the hydrogen evolution reaction is 276 mV in 1.0 M KOH. The synergistic effect from NiSe and NiFe LDH leads to the evolution of a highly efficient catalyst system for water splitting by achieving 10 mA cm^-2 current density at only 1.53 V in a two-electrode alkaline electrolyzer. In addition, the designed electrode produces stable performance for a long time even at higher current density to demonstrate its robustness and prospective as a real-life energy conversion system.

One Dimensional Silver-based Nanomaterials: Preparations and Electrochemical Applications

One dimensional (1D) silver-based nanomaterials have a great potential in various fields because of their high specific surface area, high electric conductivity, optoelectronic properties, mechanical flexibility and high electro-catalytic efficiency. In this Review, the preparations of 1D silver-based nanomaterials is classified by structure composed of simple silver nanowires/rods/belts/tubes, core-shells, and hybrids. The latest applications based on 1D silver nanomaterials and their composite materials are summarized systematically including electrochemical capacitors, lithium-ion/lithium-oxygen batteries, electrochemical sensors and electrochemical catalysis. The preparation process, tailored material properties and electrochemical applications are discussed.

Electrocatalysts Derived from Metal-Organic Frameworks for Oxygen Reduction and Evolution Reactions in Aqueous Media

Electrochemical energy conversion and storage devices such as fuel cells and metal-air batteries have been extensively studied in recent decades for their excellent conversion efficiency, high energy capacity, and low environmental impact. However, sluggish kinetics of the oxygen-related reactions at air cathodes, i.e., oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), are still worth improving. Noble metals such as platinum (Pt), iridium (Ir), ruthenium (Ru) and their oxides are considered as the benchmark ORR and OER electrocatalysts, but they are expensive and prone to be poisoned due to the fuel crossover effect, and may suffer from agglomeration and leaching after long-term usage. To mitigate these limits, it is highly desirable to design alternative ORR/OER electrocatalysts with prominent performance. Metal-organic frameworks (MOFs) are a class of porous crystalline materials consisting metal ions/clusters coordinated by organic ligands. Their crystalline structure, tunable pore size and high surface area afford them wide opportunities as catalytic materials. This Review covers MOF-derived ORR/OER catalysts in electrochemical energy conversion, with a focus on the different strategies of material design and preparation, such as composition control and nanostructure fabrication, to improve the activity and durability of MOF-derived electrocatalysts.

Nickel Diselenide Ultrathin Nanowires Decorated with Amorphous Nickel Oxide Nanoparticles for Enhanced Water Splitting Electrocatalysis

Well-designed hybrid materials based on noble metal-free elements have great potential to generate hydrogen (H₂ ) and oxygen (O₂ ) sustainably via overall water splitting for developing practical energy-related technologies. Herein, an accessible method is presented to synthesize nickel diselenide (NiSe₂ ) ultrathin nanowires decorated with amorphous nickel oxide nanoparticles (NiO_x NPs) as multifunctional electrocatalysts (NSWANs) for hydrogen and oxygen evolution reaction (HER and OER). The NSWANs exhibit quite low HER and OER overpotentials of 174 and 295 mV, respectively, holding the current density of 20 mA cm^-2 for 24 h continuous operations in alkaline media. Meanwhile, a cell voltage of 1.547 V at the current density of 10 mA cm^-2 for overall water splitting has been achieved by the NSWANs for the practical application, which could maintain fascinating activity of 20 mA cm^-2 for 72 h without degradation. The decorated NiO_x NPs not only prevent the NiSe₂ from further oxidation but also expose requisite active sites for electrocatalytic process. It is believed that this study may provide a valuable strategy to design high-efficiency electrocatalysts and expand the applications of selenide-based materials.

Co4N Nanowires: Noble-Metal-Free Peroxidase Mimetic with Excellent Salt- and Temperature-Resistant Abilities

Compared to natural enzymes, nanomaterial-based artificial enzymes have attracted immense attention because of their high stability and cost-effectiveness. In this study, cobalt nitride (Co₄N) as a noble-metal-free artificial enzyme exhibiting highly intrinsic peroxidase-like activity and good stability was reported. Kinetic studies revealed that the resultant Co₄N nanowires (NWs) exhibited a stronger affinity for 3,3',5,5'-tetramethylbenzidine (TMB) and H₂O₂ than HRP. Compared to Co₃O₄ NWs, Co₄N NWs exhibited highly improved catalytic activities, with H₂O₂ exhibiting an apparent K_m approximately 2 orders of magnitude less than that of Co₃O₄. In particular, the peroxidase-like activity of Co₄N was maintained well over a wide range of temperatures and ionic strength. A Co₄N-based method was further developed for the detection of glucose with good sensitivity and reliability. Because of advantages such as easy storage, cost-effectiveness, high sensitivity, and outstanding stability, Co₄N NWs demonstrate the potential for replacing noble-metal-based peroxidase mimetics in a wide range of promising applications.

Sugar Blowing-Induced Porous Cobalt Phosphide/Nitrogen-Doped Carbon Nanostructures with Enhanced Electrochemical Oxidation Performance toward Water and Other Small Molecules

Rational design of high active and robust nonprecious metal catalysts with excellent catalytic efficiency in oxygen evolution reaction (OER) is extremely vital for making the water splitting process more energy efficient and economical. Among these noble metal-free catalysts, transition-metal-based nanomaterials are considered as one of the most promising OER catalysts due to their relatively low-cost intrinsic activities, high abundance, and diversity in terms of structure and morphology. Herein, a facile sugar-blowing technique and low-temperature phosphorization are reported to generate 3D self-supported metal involved carbon nanostructures, which are termed as Co₂ P@Co/nitrogen-doped carbon (Co₂ P@Co/N-C). By capitalizing on the 3D porous nanostructures with high surface area, homogeneously dispersed active sites, the intimate interaction between active sites, and 3D N-doped carbon, the resultant Co₂ P@Co/N-C exhibits satisfying OER performance superior to CoO@Co/N-C, delivering 10 mA cm^-2 at overpotential of 0.32 V. It is worth noting that in contrast to the substantial current density loss of RuO₂ , Co₂ P@Co/N-C shows much enhanced catalytic activity during the stability test and a 1.8-fold increase in current density is observed after stability test. Furthermore, the obtained Co₂ P@Co/N-C can also be served as an excellent nonprecious metal catalyst for methanol and glucose electrooxidation in alkaline media, further extending their potential applications.

Ternary Porous Cobalt Phosphoselenide Nanosheets: An Efficient Electrocatalyst for Electrocatalytic and Photoelectrochemical Water Splitting

Exploring efficient and earth-abundant electrocatalysts is of great importance for electrocatalytic and photoelectrochemical hydrogen production. This study demonstrates a novel ternary electrocatalyst of porous cobalt phosphoselenide nanosheets prepared by a combined hydrogenation and phosphation strategy. Benefiting from the enhanced electric conductivity and large surface area, the ternary nanosheets supported on electrochemically exfoliated graphene electrodes exhibit excellent catalytic activity and durability toward hydrogen evolution in alkali, achieving current densities of 10 and 20 mA cm^-2 at overpotentials of 150 and 180 mV, respectively, outperforming those reported for transition metal dichalcogenides and first-row transition metal pyrites catalysts. Theoretical calculations reveal that the synergistic effects of Se vacancies and subsequent P displacements of Se atoms around the vacancies in the resulting cobalt phosphoselenide favorably change the electronic structure of cobalt selenide, assuring a rapid charge transfer and optimal energy barrier of hydrogen desorption, and thus promoting the proton kinetics. The overall-water-splitting with 10 mA cm^-2 at a low voltage of 1.64 V is achieved using the ternary electrode as both the anode and cathode, and the performance surpasses that of the Ir/C-Pt/C couple for sufficiently high overpotentials. Moreover, the integration of ternary nanosheets with macroporous silicon enables highly efficient solar-driven photoelectrochemical hydrogen production.

Engineering phase transformation of cobalt selenide in carbon cages and the phases' bifunctional electrocatalytic activity for water splitting

Using Co-based metal-organic frameworks as the precursor, we synthesized cobalt selenide (CoSe₂) nanoparticles imbedded in carbon cages. By simply controlling the annealing conditions, phase transformation of CoSe₂ from the orthorhombic phase to the cubic phase has been realized. Benefitting from the metallic character, the cubic phase CoSe₂ shows greatly enhanced electrocatalytic activity for both the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER). The as-prepared cubic phase CoSe₂ electrode possesses onset overpotentials of 43 and 200 mV, and Tafel slopes of 51 and 83 mV dec^-1 for HER and OER, respectively, which are remarkably superior to that of the orthorhombic phase CoSe₂ catalyst and comparable to those of commercial noble-metal catalysts. In addition, the cubic phase CoSe₂ electrode also demonstrates excellent stability after long-term operations. Our work not only provides a high performance catalyst for water splitting, but also introduces a new route to the design of a highly efficient catalyst by phase transformation.

Facilitating Active Species Generation by Amorphous NiFe-Bi Layer Formation on NiFe-LDH Nanoarray for Efficient Electrocatalytic Oxygen Evolution at Alkaline pH

Searching for a simple and fast strategy to effectively enhance the oxygen evolution reaction (OER) performance of non-noble-metal electrocatalysts in alkaline media remains a significant challenge. Herein, the OER activity of NiFe-LDH nanoarray on carbon cloth (NiFe-LDH/CC) in alkaline media is shown to be greatly boosted by an amorphous NiFe-Borate (NiFe-B_i ) layer formation on NiFe-layered double hydroxide (NiFe-LDH) surface. Such a NiFe-LDH@NiFe-B_i /CC catalyst electrode only needs an overpotential of 294 mV to drive 50 mA cm^-2 in 1.0 m KOH; 116 mV less than that needed by NiFe-LDH/CC. Notably, this electrode also demonstrates strong long-term electrochemical durability. The superior activity is ascribed to the pre-formed amorphous NiFe-B_i layer effectively promoting active species generation on the NiFe-LDH surface. This work opens up exciting new avenues for developing high-performance water-oxidation catalyst materials for application.

3D Nitrogen-Anion-Decorated Nickel Sulfides for Highly Efficient Overall Water Splitting

Developing non-noble-metal electrocatalysts with high activity and low cost for both the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) is of paramount importance for improving the generation of H₂ fuel by electrocatalytic water-splitting. This study puts forward a new N-anion-decorated Ni₃ S₂ material synthesized by a simple one-step calcination route, acting as a superior bifunctional electrocatalyst for the OER/HER for the first time. The introduction of N anions significantly modifies the morphology and electronic structure of Ni₃ S₂ , bringing high surface active sites exposure, enhanced electrical conductivity, optimal HER Gibbs free-energy (ΔG_H* ), and water adsorption energy change (ΔG_H2O* ). Remarkably, the obtained N-Ni₃ S₂ /NF 3D electrode exhibits extremely low overpotentials of 330 and 110 mV to reach a current density of 100 and 10 mA cm^-2 for the OER and HER in 1.0 m KOH, respectively. Moreover, an overall water-splitting device comprising this electrode delivers a current density of 10 mA cm^-2 at a very low cell voltage of 1.48 V. Our finding introduces a new way to design advanced bifunctional catalysts for water splitting.

Integrated Hierarchical Cobalt Sulfide/Nickel Selenide Hybrid Nanosheets as an Efficient Three-dimensional Electrode for Electrochemical and Photoelectrochemical Water Splitting

Developing highly active electrocatalysts for photoelectrochemical water splitting is critical to bring solar/electrical-to-hydrogen energy conversion processes into reality. Herein, we report a three-dimensional (3D) hybrid electrocatalyst that is constructed through in situ anchoring of Co₉S₈ nanosheets onto the surface of Ni₃Se₂ nanosheets vertically aligned on an electrochemically exfoliated graphene foil. Benefiting from the synergistic effects between Ni₃Se₂ and Co₉S₈, the highly conductive graphene support, and large surface area, the novel 3D hybrid electrode delivers superior electrocatalytic activity toward water reduction in alkaline media, featuring overpotentials of -0.17 and -0.23 V to achieve current densities of 20 and 50 mA cm^-2, respectively, demonstrating an electrocatalytic performance on the top of the Ni₃Se₂- and Co₉S₈-based electrocatalysts as reported in literature. Experimental investigations and theoretical calculations confirm that the remarkable activity of the obtained material results from the unique 3D hierarchical architecture and interface reconstruction between Ni₃Se₂ and Co₉S₈ through Ni-S bonding, which leads to charge redistribution and thus lowers the energy barrier of hydrogen desorption in the water splitting process. Further integration of the 3D hybrid electrode with a macroporous silicon photocathode enables highly active and sustainable sunlight-driven water splitting in both basic media and real river water. The overall water splitting with 10 mA cm^-2 at a low voltage of 1.62 V is achieved using our hybrid as both anode and cathode catalysts, which surpasses that of the Ir/C-Pt/C couple (1.60 V) for sufficiently high overpotentials.

Hierarchical Co(OH)F Superstructure Built by Low-Dimensional Substructures for Electrocatalytic Water Oxidation

The development of new materials/structures for efficient electrocatalytic water oxidation, which is a key reaction in realizing artificial photosynthesis, is an ongoing challenge. Herein, a Co(OH)F material as a new electrocatalyst for the oxygen evolution reaction (OER) is reported. The as-prepared 3D Co(OH)F microspheres are built by 2D nanoflake building blocks, which are further woven by 1D nanorod foundations. Weaving and building the substructures (1D nanorods and 2D nanoflakes) provides high structural void porosity with sufficient interior space in the resulting 3D material. The hierarchical structure of this Co(OH)F material combines the merits of all material dimensions in heterogeneous catalysis. The anisotropic low-dimensional (1D and 2D) substructures possess the advantages of a high surface-to-volume ratio and fast charge transport. The interconnectivity of the nanorods is also beneficial for charge transport. The high-dimensional (3D) architecture results in sufficient active sites per the projected electrode surface area and is favorable for efficient mass diffusion during catalysis. A low overpotential of 313 mV is required to drive an OER current density of 10 mA cm^-2 on a simple glassy carbon (GC) working electrode in a 1.0 m KOH aqueous solution.

Core-shell CoFe2O4@Co-Fe-Bi nanoarray: a surface-amorphization water oxidation catalyst operating at near-neutral pH

The exploration of high-performance and earth-abundant water oxidation catalysts operating under mild conditions is highly attractive and challenging. In this communication, core-shell CoFe₂O₄@Co-Fe-Bi nanoarray on carbon cloth (CoFe₂O₄@Co-Fe-Bi/CC) was successfully fabricated by in situ surface amorphization of CoFe₂O₄ nanoarray on CC (CoFe₂O₄/CC). As a 3D water oxidation electrode, CoFe₂O₄@Co-Fe-Bi/CC shows outstanding activity with an overpotential of 460 mV to drive a geometrical catalytic current density of 10 mA cm^-2 in 0.1 M potassium borate (pH 9.2). Notably, it also demonstrates superior long-term durability for at least 20 h with 96% Faradic efficiency. Density functional theory calculations indicate that the conversion from OOH* to O₂ is the rate-limiting step and the high water oxidation activity of CoFe₂O₄@Co-Fe-Bi/CC is associated with the lower free energy of 1.84 eV on a Co-Fe-Bi shell.