Dual Doping Induced Interfacial Engineering of Fe 2 N/Fe 3 N Hybrids with Favorable d‐Band towards Efficient Overall Water Splitting

A comprehensive review on unleashing the power of hydrogen: revolutionizing energy systems for a sustainable future

Population growth and environmental degradation are major concerns for sustainable development worldwide. Hydrogen is a clean and eco-friendly alternative to fossil fuels, with a heating value almost three times higher than other fossil fuels. It also has a clean production process, which helps to reduce the emission of hazardous pollutants and save the environment. Among the various production methodologies described in this review, biochemical production of hydrogen is considered more suitable as it uses waste organic matter instead of fossil fuels. This technology not only produces clean energy but also helps to manage waste more efficiently. However, the production of hydrogen obtained from this method is currently more expensive due to its early stage of development. Nevertheless, various research projects are underway to develop this method on a commercial scale.

Fe3N Nanoparticle-Encapsulated N-Doped Carbon Nanotubes on Biomass-Derived Carbon Cloth as Self-Standing Electrocatalyst for Oxygen Reduction Reaction

The design and fabrication of low-cost catalysts for highly efficient oxygen reduction are of paramount importance for various renewable energy-related technologies, such as fuel cells and metal-air batteries. Herein, we report the synthesis of Fe3N nanoparticle-encapsulated N-doped carbon nanotubes on the surface of a flexible biomass-derived carbon cloth (Fe3N@CNTs/CC) via a simple one-step carbonization process. Taking advantage of its unique structure, Fe3N@CNTs/CC was employed as a self-standing electrocatalyst for oxygen reduction reaction (ORR) and possessed high activity as well as excellent long-term stability and methanol resistance in alkaline media. Remarkably, Fe3N@CNT/CC can directly play the role of both a gas diffusion layer and an electrocatalytic cathode in a zinc-air battery without additional means of catalyst loading, and it displays higher open-circuit voltage, power density, and specific capacity in comparison with a commercial Pt/C catalyst. This work is anticipated to inspire the design of cost-effective, easily prepared, and high-performance air electrodes for advanced electrochemical applications.

Tunable d-band center of a NiFeMo alloy with enlarged lattice strain enhancing the intrinsic catalytic activity for overall water-splitting

Developing efficient bifunctional electrocatalysts for the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) under alkaline conditions is prospective for reducing energy consumption during water electrolysis. In this work, we successfully synthesized nanocluster structure composites composed of NiFeMo alloys with controllable lattice strain by the electrodeposition method at room temperature. The unique structure of NiFeMo/SSM (stainless steel mesh) facilitates the exposure of abundant active sites and promotes mass transfer and gas exportation. The NiFeMo/SSM electrode exhibits a low overpotential of 86 mV at 10 mA cm^-2 for the HER and 318 mV at 50 mA cm^-2 for the OER, and the assembled device reveals a low voltage of 1.764 V at 50 mA cm^-2. Moreover, both the experimental results and theoretical calculations reveal that the dual doping of Mo and Fe can induce the tunable lattice strain of nickel, which in turn changes the d-band center and electronic interaction of the catalytically active site, and finally enhances the HER and OER catalytic activity. This work may provide more options for the design and preparation of bifunctional catalysts based on non-noble metals.

Pairing d-Band Center of Metal Sites with π-Orbital of Alkynes for Efficient Electrocatalytic Alkyne Semi-Hydrogenation

Electrocatalytic alkyne semi-hydrogenation has attracted ever-growing attention as a promising alternative to traditional thermocatalytic hydrogenation. However, the correlation between the structure of active sites and electrocatalytic performance still remains elusive. Herein, the energy difference (∆ε) between the d-band center of metal sites and π orbital of alkynes as a key descriptor for correlating the intrinsic electrocatalytic activity is reported. With two-dimensional conductive metal organic frameworks as the model electrocatalysts, theoretical and experimental investigations reveal that the decreased ∆ε induces the strengthened d-π orbitals interaction, which thus enhances acetylene π-adsorption and accelerates subsequent hydrogenation kinetics. As a result, Cu₃ (HITP)₂ featuring the smallest ∆ε (0.10 eV) delivers the highest turnover frequency of 0.36 s^-1 , which is about 124 times higher than 2.9 × 10^-3  s^-1 for Co₃ (HITP)₂ with the largest ∆ε of 2.71 eV. Meanwhile, Cu₃ (HITP)₂ presents a high ethylene partial current density of -124 mA cm^-2 and a large ethylene Faradaic efficiency of 99.3% at -0.9 V versus RHE. This work will spark the rapid exploration of high-activity alkyne semi-hydrogenation catalysts.

"Electron Complementation"-Induced Molybdenum Nitride/Co-Anchored Graphitic Carbon Nitride Porous Nanoparticles for Efficient Overall Water Splitting

Maximizing the usable space of electrocatalysts and fine-tuning the interface geometry as well as the electronic structure to facilitate hydrogen and oxygen evolution reactions (HER and OER) have always been the focus of research. Herein, a homogeneous porous nanoparticle construction strategy was proposed, in which molybdenum nitride (Mo₂N) particles were prepared by controlled heat treatment of the precursor nanoparticle induced by polyethylene glycol, and the Mo₂N/Co-C₃N₄ heterostructure with a pore size of about 1.13 nm was obtained by compounding Co-anchored graphitic carbon nitride. In particular, exploring the change of charge distribution at the interface based on the principle of "electron complementation" shows that under the regulation of nitrogen with high electronegativity, the affinity of active site Co to oxygenated species in the OER process and the adsorption as well as cleavage ability of HER reactants in the active site were effectively optimized. Thus, Mo₂N/Co-C₃N₄ not only inherits the functions of each component, but also provides an ideal heterogeneous interface for exhibiting impressive bifunctional activity, which only needs 100 and 210 mV to deliver 10 mA cm^-2 for the HER and OER, respectively. In addition, the Mo₂N/Co-C₃N₄ catalyst also demonstrates high overall water splitting stability with a slight current decrease after 95 h. Manipulating the electronic structure of multiple sites by constructing electronically complementary interfaces may provide another avenue to develop highly active catalysts for overall water splitting and other applications.

Tuning Electronic Structure and Composition of FeNi Nanoalloys for Enhanced Oxygen Evolution Electrocatalysis via a General Synthesis Strategy

Developing low-cost and efficient oxygen evolution electrocatalysts is key to decarbonization. A facile, surfactant-free, and gram-level biomass-assisted fast heating and cooling synthesis method is reported for synthesizing a series of carbon-encapsulated dense and uniform FeNi nanoalloys with a single-phase face-centered-cubic solid-solution crystalline structure and an average particle size of sub-5 nm. This method also enables precise control of both size and composition. Electrochemical measurements show that among Fe_x Ni_{(1- x )} nanoalloys, Fe_0.5 Ni_0.5 has the best performance. Density functional theory calculations support the experimental findings and reveal that the optimally positioned d-band center of O-covered Fe_0.5 Ni_0.5 renders a half-filled antibonding state, resulting in moderate binding energies of key reaction intermediates. By increasing the total metal content from 25 to 60 wt%, the 60% Fe_0.5 Ni_0.5 /40% C shows an extraordinarily low overpotential of 219 mV at 10 mA cm^-2 with a small Tafel slope of 23.2 mV dec^-1 for the oxygen evolution reaction, which are much lower than most other FeNi-based electrocatalysts and even the state-of-the-art RuO₂ . It also shows robust durability in an alkaline environment for at least 50 h. The gram-level fast heating and cooling synthesis method is extendable to a wide range of binary, ternary, quaternary nanoalloys, as well as quinary and denary high-entropy-alloy nanoparticles.

Cobalt Molybdenum Nitride-Based Nanosheets for Seawater Splitting

The development of cost-effective bifunctional catalysts for water electrolysis is both a crucial necessity and an exciting scientific challenge. Herein, a simple approach based on a metal-organic framework sacrificial template to preparing cobalt molybdenum nitride supported on nitrogen-doped carbon nanosheets is reported. The porous structure of produced composite enables fast reaction kinetics, enhanced stability, and high corrosion resistance in critical seawater conditions. The cobalt molybdenum nitride-based electrocatalyst is tested toward both oxygen evolution reaction and hydrogen evolution reaction half-reactions using the seawater electrolyte, providing excellent performances that are rationalized using density functional theory. Subsequently, the nitride composite is tested as a bifunctional catalyst for the overall splitting of KOH-treated seawater from the Mediterranean Sea. The assembled system requires overpotentials of just 1.70 V to achieve a current density of 100 mA cm^-2 in 1 M KOH seawater and continuously works for over 62 h. This work demonstrates the potential of transition-metal nitrides for seawater splitting and represents a step forward toward the cost-effective implementation of this technology.

Phase-Controlled Synthesis of Nickel-Iron Nitride Nanocrystals Armored with Amorphous N-Doped Carbon Nanoparticles Nanocubes for Enhanced Overall Water Splitting

Transition metal nitrides (TMNs) nanostructures possess distinctive electronic, optical, and catalytic properties, showing great promise to apply in clean energy, optoelectronics, and catalysis fields. Nonetheless, phase-regulation of NiFe-bimetallic nitrides nanocrystals or nanohybrid architectures confronts challenges and their electrocatalytic overall water splitting (OWS) performances are underexplored. Herein, novel pure-phase Ni_{2+ x} Fe_{2- x} N nanocrystals armored with amorphous N-doped carbon (NC) nanoparticles nanocubes (NPNCs) are obtained by controllable nitridation of NiFe-Prussian-blue analogues derived oxides/NC NPNCs under Ar/NH₃ atmosphere. Such Ni_{2+ x} Fe_{2- x} N/NC NPNCs possess mesoporous structures and show enhanced electrocatalytic activity in 1 m KOH electrolyte with the overpotential of 101 and 270 mV to attain 10 and 50 mA cm^-2 current toward hydrogen and oxygen evolution reactions, outperforming their counterparts (mixed-phase NiFe₂ O₄ /Ni₃ FeN/NC and NiFe oxides/NC NPNCs). Remarkably, utilizing them as bifunctional catalysts, the assembled Ni_{2+ x} Fe_{2- x} N/NC||Ni_{2+ x} Fe_{2- x} N/NC electrolyzer only needs 1.51 V cell voltage for driving OWS to approach 10 mA cm^-2 water-splitting current, exceeding their counterparts and the-state-of-art reported bifunctional catalysts-based devices, and Pt/C||IrO₂ couples. Additionally, the Ni_{2+ x} Fe_{2- x} N/NC||Ni_{2+ x} Fe_{2- x} N/NC manifests excellent durability for OWS. The findings presented here may spur the development of advanced TMNs nanostructures by combining phase, structure engineering, and hybridization strategies and stimulate their applications toward OWS or other clean energy fields.

Molybdenum doped induced amorphous phase in cobalt acid nickel for supercapacitor and oxygen evolution reaction

Reasonable structural design and metal-doping play significant roles in the optimization of electrochemical energy storage and conversion. Herein, in situ growth of Molybdenum-doped amorphous cobalt acid nickel nanoneedles on Ni foam (Mo-NiCo₂O₄/NF) has been successfully synthesized by a simple hydrothermal-annealing strategy. Benefiting from the unique hierarchical nanostructures and doping-optimized electronic structural configuration, the cross-link network structure of Mo-doped amorphous NiCo₂O₄ with large specific surface areas exhibit excellent supercapacitor performance and electrocatalytic activity. As expected, the optimized Mo-doped NiCo₂O₄ samples possess a specific capacitance of 3970 mF cm^-2 at 1 mA cm^-2 and remarkable rate performance. The assembled hybrid supercapacitor obtains a maximum energy density of 35 Wh kg^-1 (420 W kg^-1) and keeps a capacitance retention of 107% after 5000 cycles. As an electrocatalyst, Mo-NiCo₂O₄/NF shows a rapid self-reconstruction process during oxygen evolution reaction (OER) that produces rich oxygen vacancies and thus exhibits remarkable long-term stability. The nanocomposites exhibit small overpotential (280 mV at 10 mA cm^-2) and Tafel slope (43 mV dec^-1). These results strongly demonstrate that both local amorphous phase and porous hierarchical structure design from Mo dopant provide superiorities for the synthesis of efficient and stable multifunctional electrode materials for energy storage and conversion.

Metal nitride-based nanostructures for electrochemical and photocatalytic hydrogen production

The over-dependence on fossil fuels is one of the critical issues to be addressed for combating greenhouse gas emissions. Hydrogen, one of the promising alternatives to fossil fuels, is renewable, carbon-free, and non-polluting gas. The complete utilization of hydrogen in every sector ranging from small to large scale could hugely benefit in mitigating climate change. One of the key aspects of the hydrogen sector is its production via cost-effective and safe ways. Electrolysis and photocatalysis are well-known processes for hydrogen production and their efficiency relies on electrocatalysts, which are generally noble metals. The usage of noble metals as catalysts makes these processes costly and their scarcity is also a limiting factor. Metal nitrides and their porous counterparts have drawn considerable attention from researchers due to their good promise for hydrogen production. Their properties such as active metal centres, nitrogen functionalities, and porous features such as surface area, pore-volume, and tunable pore size could play an important role in electrochemical and photocatalytic hydrogen production. This review focuses on the recent developments in metal nitrides from their synthesis methods point of view. Much attention is given to the emergence of new synthesis techniques, methods, and processes of synthesizing the metal nitride nanostructures. The applications of electrochemical and photocatalytic hydrogen production are summarized. Overall, this review will provide useful information to researchers working in the field of metal nitrides and their application for hydrogen production.

Hierarchically structured nickel/molybdenum nitride heterojunctions as superior bifunctional electrodes for overall water splitting

A 3D hierarchical heterostructure of intermetallic compound heterojunctions is first rationally designed and presented as a highly-active bifunctional electrode for water splitting.

Coordination Polymer-Derived Fe3N Nanoparticles for Efficient Electrocatalytic Oxygen Evolution

Based on a coordination polymer, FeCl₂(4,4'-bpy) (4,4'-bpy = 4,4'-bipyridine) and the carbon nanotube (CNT)/NaCl dual template, Fe₃N nanoparticles (NPs) were synthesized via chemical thermolysis in the absence of an extra nitrogen source. The decomposition of 4,4'-bpy under high temperature produces thin carbon coating for Fe₃N NPs. Also, the CNT template anchors the Fe₃N NPs to avoid aggregation. The sample (denoted as Fe₃N-C N) exhibits excellent electrocatalytic oxygen evolution reaction (OER) behavior even with a small molar ratio of Fe₃N (Fe: 4.9 at. %), which can deliver a current density of 10 mA cm^-2 at an overpotential of 218 mV with a Tafel slope of 84 mV dec^-1 and long-term OER activity during 60 h electrolysis at 20 mA cm^-2. Furthermore, the sample after 20 h electrolysis, denoted as Post-Fe₃N-C N (20 h), displays enhanced OER activity with a smaller Tafel slope of 41 mV dec^-1 and overpotentials of 195 and 327 mV at 10 and 100 mA cm^-2, respectively, which is mainly due to the partial transformation of Fe₃N into FeOOH. The OER mechanism is investigated by density functional theory calculations, and it is found that the surface partial oxidation of Fe₃N leads to the effective OER electrolysis, which changes the electron density of the superficial atoms and induces the moderate adsorption for the intermediates.

Transferable Active Centers of Strongly Coupled MoS2@Sulfur and Molybdenum Co-doped g-C3N4 Heterostructure Electrocatalysts for Boosting Hydrogen Evolution Reaction in Both Acidic and Alkaline Media

Designing an excellent acidic and alkaline general-purpose hydrogen evolution electrocatalyst plays an important role in promoting the development of the energy field. Here, a feasible strategy is reported to use the strongly coupled MoS₂@sulfur and molybdenum co-doped g-C₃N₄ (MoS₂@Mo-S-C₃N₄) heterostructure with transferable active centers for catalytic reactions in acidic and alkaline media. Research studies have shown that the unsaturated S site at the edge of MoS₂ and the active N atom on the Mo-S-C₃N₄ substrate are, respectively, the active centers of acidic and alkaline hydrogen evolution reaction. Specifically, Mo-S-C₃N₄ is regarded as a synergistic catalyst for the active species MoS₂ in acidic hydrogen evolution, while MoS₂ acts as a co-catalyst when the alkaline active species are transferred to Mo-S-C₃N₄. The coordination of the electrons between the interfaces achieves a synergistic balance, which provides the optimal sites for the adsorption of the reactants. Such an electrocatalyst exhibits overpotentials of 193 and 290 mV at 10 mA cm^-2 in 0.5 M H₂SO₄ and 1 M KOH, respectively, which was better than numerous previous reports. This research provides an outstanding avenue to realize multifunctional electrocatalysts.

Bonding interface boosts the intrinsic activity and durability of NiSe@Fe2O3 heterogeneous electrocatalyst for water oxidation

The intrinsic activity and durability of oxygen evolution reaction (OER) electrocatalysts are mainly dominated by the surface and interface properties of active materials. Herein, a core-shell heterogeneous structure (NF/NiSe@Fe₂O₃) is fabricated via two-step hydrothermal method, which exhibits a low overpotential of 220 mV (or 282 mV) at 10 mA/cm² (or 200 mA/cm²), a small Tafel slope of 36.9 mV/dec, and long-term stability (~230 h) in 1 mol/L KOH for OER. X-ray photoelectron spectroscopy and X-ray absorption spectroscopy reveal the (oxy)hydroxide-rich surface and strong coupling interface between NiSe and Fe₂O₃ via the Fe-Se bond. Density functional theory calculation suggests that the d-band center and electronic state of NiSe@Fe₂O₃ heterojunction are well optimized due to the formation of Fe-Se bond, which is favorable for the enhanced OER activity because of the easy adsorption of oxygen-containing intermediates and desorption of O₂ in the OER process. In addition, the unique core-shell structure and robust bonding interface are responsible for the good stability for OER. This work provides fundamental insights on the bonding effect that determine the performance of OER electrocatalyst.

Transition metal nitrides for electrochemical energy applications

This review comprehensively summarizes the progress on the structural and electronic modulation of transition metal nitrides for electrochemical energy applications.

Polypyrrole Hollow Microspheres with Boosted Hydrophilic Properties for Enhanced Hydrogen Evolution Water Dissociation Kinetics

The water dissociation step (H₂O + M + e^- → M - H_ads + OH^-) is a crucial one toward achieving high-performance hydrogen evolution reaction (HER). The application of electronic conducting polymers (ECPs), such as polypyrrole (PPy), as the electrocatalyst for HER is rarely reported because of their poor adsorption energy per water molecule, which hinders the Volmer step. Herein, we strongly enrich PPy hollow microspheres (PPy-HMS) with attractive HER activity by enhancing their hydrophilic properties through hybridization with good water affinity SiO₂. The as-prepared PPy-coated SiO₂ (PPy@SiO₂-HMS) achieves a current density of 10 mA cm^-2 at -123 mV, which is lower than that of pristine PPy-HMS (-192 mV). Raman and X-ray photospectroscopy analyses reveal that the enhanced HER catalytic capability can be attributed to the strong electronic couplings between PPy and SiO₂, and this improves the adsorption energy per water molecule and in turn accelerates the water dissociation kinetics on PPy. This work highlights the potential application of low-cost ECPs as promising electrocatalysts for water electrolysis.

Separation of Metal-N4 Units in Metal-Organic Framework for Preparation of M-Nx/C Catalyst with Dense Metal Sites

Metal and nitrogen codoped carbon (M-N_x/C) materials with good metal dispersion in a carbon matrix have attracted great attention because they can efficiently catalyze various kinds of chemical/electrochemical reactions. However, the existing strategies for preparing M-N_x/C materials still face the challenge of metal site aggregation when the carbon skeleton in the precursor is substantially lost during pyrolysis. Herein, we propose a general strategy that the increase of metal site density and inhibition of metal aggregation could be realized by separating M-N₄ units in metal-organic frameworks (MOFs). The metal sites can be well separated and distributed in carbon materials during the pyrolysis of a metal-coordinated block copolymer, one of the specific MOFs. The strategy can be widely applicable for the synthesis of M-N_x/C materials with dense metal sites, such as Fe-, Mn-, Ni-, and Co-N_x/C materials, which are potentially used as the efficient catalysts for various kinds of reactions. Taking Fe-N_x/C as a model oxygen reduction reaction (ORR) electrocatalyst, it shows the ORR half-wave potentials of 0.90 and 0.81 V vs RHE in alkaline and acidic electrolytes, respectively.

Advanced non-noble materials in bifunctional catalysts for ORR and OER toward aqueous metal-air batteries

The catalyst in the oxygen electrode is the core component of the aqueous metal-air battery, which plays a vital role in the determination of the open circuit potential, energy density, and cycle life of the battery. For rechargeable aqueous metal-air batteries, the catalyst should have both good oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) catalytic performance. Compared with precious metal catalysts, non-precious metal materials have more advantages in terms of abundant resource reserves and low prices. Over the past few years, great efforts have been made in the development of non-precious metal bifunctional catalysts. This review selectively evaluates the advantages, disadvantages and development status of recent advanced materials including pure carbon materials, carbon-based metal materials and carbon-free materials as bifunctional oxygen catalysts. Preliminary improvement strategies are formulated to make up for the deficiency of each material. The development prospects and challenges facing bifunctional catalysts in the future are also discussed.

Ultrafine NiFe clusters anchored on N-doped carbon as bifunctional electrocatalysts for efficient water and urea oxidation

Hydrogen production through electrocatalysis is crucial in renewable energy technologies but significantly impeded by sluggish anodic reactions. Developing bifunctional anode noble-metal-free electrocatalysts towards oxygen evolution reaction (OER) and urea oxidation reaction (UOR) to boost cathodic hydrogen evolution reaction (HER) is promising but challenging to meet different reaction media and multiple applications for simultaneous clean energy production and pollution treatment. Herein, a facile one-pot thermal treatment strategy is presented to anchor NiFe nanoclusters (with a size of about 2 nm) on N-doped carbon as bifunctional electrocatalysts for both OER and UOR. Such an electrocatalyst can deliver a current density of 20 mA cm-2 with a low overpotential of 260 mV and a small Tafel slope of 42 mV dec-1 for OER, superior to the state-of-the-art Ru-based materials. Besides, this electrocatalyst also shows excellent activity for UOR with the need for just 1.37 V (vs. RHE) to attain a current density of 100 mA cm-2. In a two-electrode electrolyzer for both cathodic HER and anodic UOR, only a cell voltage of 1.50 V is required to drive a current density of 10 mA cm-2, which is 140 mV lower than that of overall water splitting electrolysis (1.64 V). The excellent electrooxidative performance can be attributed to the improved conductivity, abundant active sites and fast charge transfer and transport benefiting from the ultrafine structure of NiFe clusters and their synergistic effect with N-doped carbon.

Genuine Active Species Generated from Fe3 N Nanotube by Synergistic CoNi Doping for Boosted Oxygen Evolution Catalysis

The surface reconstruction of oxygen evolution reaction (OER) catalysts has been proven favorable for enhancing its catalytic activity. However, what is the active site and how to promote the active species generation remain unclear and are still under debate. Here, the in situ synthesis of CoNi incorporated Fe₃ N nanotubes (CoNi-Fe₃ N) on the iron foil through the anodization/electrodeposition/nitridation process for use of boosted OER catalysis is reported. The synergistic CoNi doping induces the lattice expansion and up shifts the d-band center of Fe₃ N, which enhances the adsorption of hydroxyl groups from electrolyte during the OER catalysis, facilitating the generation of active CoNi-FeOOH on the Fe₃ N nanotube surface. As a result of this OER-conditioned surface reconstruction, the optimized catalyst requires an overpotential of only 285 mV at a current density of 10 mA cm^-2 with a Tafel slope of 34 mV dec^-1 , outperforming commercial RuO₂ catalysts. Density functional theory (DFT) calculations further reveal that the Ni site in CoNi-FeOOH modulates the adsorption of OER intermediates and delivers a lower overpotential than those from Fe and Co sites, serving as the optimal active site for excellent OER performance.

Cobalt-based heterogeneous catalysts in an electrolyzer system for sustainable energy storage

Nowadays, the production of hydrogen and oxygen focuses on renewable energy techniques and sustainable energy storage. A substantial challenge is to extend low-cost electrocatalysts consisting of earth-abundant resources, prepared by straightforward approaches that display high intrinsic activity compared to noble metals. The expansion of bifunctional catalysts in alkaline electrolytes for both the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) has become very crucial in recent times. Herein, the recent progress in cobalt-based HER-OER electrocatalysts has been are brushed up and numerous bifunctional cobalt-based catalysts such as cobalt-oxides, phosphides, sulfides, selenides, nitrides, borides, carbides, perovskites, and MOF-based cobalt analogs have been investigated in detail. Specifically, much more attention has been paid to their structural variation, bifunctional activity, overpotential of the overall system, and stability. Cobalt-based catalysts with lower cell voltage, remarkable durability, and unique electronic structures, offer a new perspective in energy-related fields. In recent years, cobalt-based analogs with diagnostic facilities have been introduced due to their electronic structures, tunable d band structures, and tailorable active sites. This perspective also elucidates the present issues, promising ideas, and future forecasts for cobalt-based catalysts. The critical aspects of cobalt-based catalysts and the numerous opportunities, as discussed at the end, can possibly enrich the sustainable energy field.

Water Splitting: From Electrode to Green Energy System

Hydrogen (H2) production is a latent feasibility of renewable clean energy. The industrial H2 production is obtained from reforming of natural gas, which consumes a large amount of nonrenewable energy and simultaneously produces greenhouse gas carbon dioxide. Electrochemical water splitting is a promising approach for the H2 production, which is sustainable and pollution-free. Therefore, developing efficient and economic technologies for electrochemical water splitting has been an important goal for researchers around the world. The utilization of green energy systems to reduce overall energy consumption is more important for H2 production. Harvesting and converting energy from the environment by different green energy systems for water splitting can efficiently decrease the external power consumption. A variety of green energy systems for efficient producing H2, such as two-electrode electrolysis of water, water splitting driven by photoelectrode devices, solar cells, thermoelectric devices, triboelectric nanogenerator, pyroelectric device or electrochemical water-gas shift device, have been developed recently. In this review, some notable progress made in the different green energy cells for water splitting is discussed in detail. We hoped this review can guide people to pay more attention to the development of green energy system to generate pollution-free H2 energy, which will realize the whole process of H2 production with low cost, pollution-free and energy sustainability conversion.

Hydrogen Evolution Reaction-From Single Crystal to Single Atom Catalysts

Hydrogen evolution reaction (HER) is one of the most important reactions in electrochemistry. This is not only because it is the simplest way to produce high purity hydrogen and the fact that it is the side reaction in many other technologies. HER actually shaped current electrochemistry because it was in focus of active research for so many years (and it still is). The number of catalysts investigated for HER is immense, and it is not possible to overview them all. In fact, it seems that the complexity of the field overcomes the complexity of HER. The aim of this review is to point out some of the latest developments in HER catalysis, current directions and some of the missing links between a single crystal, nanosized supported catalysts and recently emerging, single-atom catalysts for HER.

Non-3d Metal Modulation of a 2D Ni-Co Heterostructure Array as Multifunctional Electrocatalyst for Portable Overall Water Splitting

Portable water splitting devices driven by rechargeable metal-air batteries or solar cells are promising, however, their scalable usages are still hindered by lack of suitable multifunctional electrocatalysts. Here, a highly efficient multifunctional electrocatalyst is demonstrated, i.e., 2D nanosheet array of Mo-doped NiCo₂ O₄ /Co_5.47 N heterostructure deposited on nickel foam (Mo-NiCo₂ O₄ /Co_5.47 N/NF). The successful doping of non-3d high-valence metal into a heterostructured nanosheet array, which is directly grown on a conductive substrate endows the resultant catalyst with balanced electronic structure, highly exposed active sites, and binder-free electrode architecture. As a result, the Mo-NiCo₂ O₄ /Co_5.47 N/NF exhibits remarkable catalytic activity toward the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER), affording high current densities of 50 mA cm^-2 at low overpotentials of 310 mV for OER, and 170 mV for HER, respectively. Moreover, a low voltage of 1.56 V is achieved for the Mo-NiCo₂ O₄ /Co_5.47 N/NF-based water splitting cell to reach 10 mA cm^-2 . More importantly, a portable overall water splitting device is demonstrated through the integration of a water-splitting cell and two Zn-air batteries (open-circuit voltage of 1.43 V), which are all fabricated based on Mo-NiCo₂ O₄ /Co_5.47 N/NF, demonstrating a low-cost way to generate fuel energy. This work offers an effective strategy to develop high-performance metal-doped heterostructured electrode.

A Co3O4/MnCO3 heterojunction on three-dimensional nickel foam for an enhanced oxygen evolution reaction

A Co₃O₄/MnCO₃ heterojunction on NF with a unique architecture exhibits prominent OER activity and stability that is superior to most Co₃O₄-based catalysts.

A high-performance bimetallic cobalt iron oxide catalyst for the oxygen evolution reaction

Herein, a facile solvothermal approach was designed to produce the CoFe₂O₄ nanospheres with unique porous structure. As an efficient electrocatalyst for OER, the CoFe₂O₄ nanospheres performed high performance.

Synergistic engineering of architecture and composition in NixCo1-xMoO4@CoMoO4 nanobrush arrays towards efficient overall water splitting electrocatalysis

Implementing the hierarchical structures of non-noble-metal-based electrocatalysts and modulating their composition can help accelerate surface reactions and fulfill the promise of renewable energy devices via water splitting. Herein, molybdenum-based compounds are constructed on activated nickel foam (act-NF) by a one-step hydrothermal growth. The product generated on the act-NF is Ni_xCo_1-xMoO₄@CoMoO₄, with a novel 3D hierarchical heterostructure, wherein the one-dimensional CoMoO₄ nanorods are hierarchically integrated with the two-dimensional Ni_xCo_1-xMoO₄ nanosheets (NCMO@CMO/act-NF). The formation of Ni_xCo_1-xMoO₄@CoMoO₄ attributes to the release and diffusion of Ni²⁺ from act-NF. Heterogeneous Ni_xCo_1-xMoO₄@CoMoO₄ has compositional differences, and synergistic interaction between cobalt and nickel results in the modulated electronic states. Meanwhile, the hierarchical structure facilitates the exposure of active sites. Combining these two advantages, NCMO@CMO/act-NF presents a low η₁₀ value of 61 and 180 mV in 1.0 M KOH for the HER and OER, respectively, and it shows a low cell voltage of 1.46 V for overall water splitting with robust stability. DFT calculations reveal that Ni doping leads to the charge depletion of Co, which further optimizes the d-band center of metal sites and tunes the adsorption of adsorbates to facilitate the water splitting reaction. Thus, a promising strategy of incorporating the nanostructure design with compositional modulation is presented to develop functional materials for energy conversion.