Hierarchical Cobalt Borate/MXenes Hybrid with Extraordinary Electrocatalytic Performance in Oxygen Evolution Reaction

Interface Charge Transfer of Heteroatom Boron Doping Cobalt and Cobalt Nitride for Boosting Water Oxidation

Designing high-performance transition-metal electrocatalysts with controlled active heterointerfacial sites for catalyzing the electrochemical oxygen evolution reaction (OER) is very desirable but remains a great challenge. Here, a facile strategy for the synthesis of transition-metal nitride-based interfacial electrocatalysts boron-doped cobalt/cobalt nitride (B-Co/Co2N) is demonstrated with optimal heterointerfaces between Co and Co2N electrocatalysts by introducing boron as a dopant to the former. Benefiting from the unique electronegativity of B, the obtained B-Co/Co2N electrocatalysts show excellent OER performance with overpotential inputs of as low as 262 and 310 mV for 10 and 100 mA cm-2, which are 1.4 and 6.6 times higher than those of Co/Co2N with the same potential input, respectively. The experimental and theoretical results demonstrate the role of the B dopant in inducing charge redistribution of Co active sites in the Co/Co2N interfacial region, which results in a downshift of the Co 3d band center, the optimal oxidation state of active sites for *OOH formation, and lower energy barriers. Furthermore, the assembled electrolyzer can steadily produce an industrial-grade current density of 1000 mA cm-2 at a cell voltage input of only 1.81 V for at least 100 h with a Faradaic efficiency near 100%. This study provides a promising strategy for heteroatom-doped interfacial electrocatalysts with high performance for energy and environmental applications.

Subtle 2D/2D MXene-Based Heterostructures for High-Performance Electrocatalytic Water Splitting

Developing efficient electrocatalysts is significant for the commercial application of electrocatalytic water splitting. 2D materials have presented great prospects in electrocatalysis for their high surface-to-volume ratio and tunable electronic properties. Particularly, MXene emerges as one of the most promising candidates for electrocatalysts, exhibiting unique advantages of hydrophilicity, outstanding conductivity, and exceptional stability. However, it suffers from lacking catalytic active sites, poor oxidation resistance, and easy stacking, leading to a significant suppression of the catalytic performance. Combining MXene with other 2D materials is an effective way to tackle the aforementioned drawbacks. In this review, the focus is on the accurate synthesis of 2D/2D MXene-based catalysts toward electrocatalytic water splitting. First, the mechanisms of electrocatalytic water splitting and the relative properties and preparation methods of MXenes are introduced to offer the basis for accurate synthesis of 2D/2D MXene-based catalysts. Then, the accurate synthesis methods for various categories of 2D/2D MXene-based catalysts, such as wet-chemical, phase-transformation, electrodeposition, etc., are systematically elaborated. Furthermore, in-depth investigations are conducted into the internal interactions and structure-performance relationship of 2D/2D MXene-based catalysts. Finally, the current challenges and future opportunities are proposed for the development of 2D/2D MXene-based catalysts, aiming to enlighten these promising nanomaterials for electrocatalytic water splitting.

Integrated MXene and metal oxide electrocatalysts for the oxygen evolution reaction: synthesis, mechanisms, and advances

Electrochemical water splitting is a promising approach to produce H2 through renewable electricity, but its energy efficiency is severely constrained by the kinetically slow anodic oxygen evolution reaction (OER), which uses about 90% of the electricity in the water-splitting process due to its multistep proton (H+)-coupled electron (e-) transfer process, high overpotential (η), and low energy efficiency. Therefore, the quest for efficient, sustainable, and cost-effective electrocatalysts for hydrogen production through water electrolysis has intensified, highlighting the potential of two-dimensional (2D) MXenes. MXenes have emerged as a promising class of materials characterized by excellent stability, hydrophilicity, and conductivity. However, challenges such as low oxidation resistance, facile stacking, and the absence of intrinsic catalytically active sites limit their performance. This review thoroughly explores various synthesis methods for MXenes and their integration with transition metal oxides (TMOs) to tackle the challenges and enhance catalytic activity. The review also delves into advanced strategies for structural tuning of MXenes and TMOs, such as termination engineering, heteroatom doping, defect engineering, and the formation of heterojunctions. The integration of MXenes with TMOs has addressed the current limitations of MXenes and significantly boosted OER activity. By considering these structural tuning parameters and limitation factors, researchers can gain insights into the design principles and optimization strategies for MXene- and TMO-integrated electrocatalysts. The review concludes with a summary of the key findings and an outlook on future research directions, emphasizing the unexplored potential and innovative approaches that could further advance the field of electrocatalytic water splitting.

Freestanding defective ammonium Vanadate@MXene hybrid films cathode for high performance aqueous zinc ion batteries

Aqueous zinc ion batteries (AZIBs) have gained extensive attention due to the numerous advantages of zinc, such as low redox potential, high abundance, low cost as well as high theoretical specific capacity. However, the development of AZIBs is still hampered due to the lack of suitable cathodes. In this work, the freestanding defective ammonium vanadate@MXene (d-NVO@MXene) hybrid film was synthesized by simple vacuum filtration strategy. Due to the presence of the hierarchical freestanding structure, outstanding MXene conductive networks and abundant oxygen vacancy (in the d-NVO nanoribbons), the d-NVO@MXene hybrid film can not only expose more active sites but also possess outstanding conductivity and kinetics of charge transfer/ion diffusion. When the d-NVO@MXene hybrid film was directly used as the cathode, it displayed a high specific capacity of 498 mAh/g at 0.5 A/g and superior cycling stability performance with near 100 % coulomb efficiency. Furthermore, the corresponding storage mechanism was elucidated by ex situ various characterizations. This work provides new ideas for the development of freestanding vanadium-based cathode materials for AZIBs.

Progress in the Synthesis Process and Electrocatalytic Application of MXene Materials

With their rich surface chemistry, high electrical conductivity, variable bandgap, and thermal stability, 2D materials have been developed for effective electrochemical energy conversion systems over the past decade. Due to the diversity brought about by the use of transition metals and C/N pairings, the 2D material MXene has found excellent applications in many fields. Among the various applications, many breakthroughs have been made in electrocatalytic applications. Nevertheless, related studies on topics such as the factors affecting the material properties and safer and greener preparation methods have not been reported in detail. Therefore, in this paper, we review the relevant preparation methods of MXene and the safer, more environmentally friendly preparation techniques in detail, and summarize the progress of research on MXene-based materials as highly efficient electrocatalysts in the electrocatalytic field of hydrogen precipitation reaction, nitrogen reduction reaction, oxygen precipitation reaction, oxygen reduction reaction, and carbon dioxide reduction reaction. We also discuss the technology related to MXene materials for hydrogen storage. The main challenges and opportunities for MXene-based materials, which constitute a platform for next-generation electrocatalysis in basic research and practical applications, are highlighted. This paper aims to promote the further development of MXenes and related materials for electrocatalytic applications.

Wide-temperature-range multispectral camouflage enabled by orientation-gradient co-optimized microwave blackbody metastructure coupled with conformal MXene coating

Cloaking against electromagnetic detection is a well-researched topic; yet achieving multispectral camouflage over a wide temperature range remains challenging. Herein, an orientation-gradient co-optimized graded Gyroid-shellular (GGS) SiOC-based metastructure with a conformal MXene coating (M@SiOC) is proposed to achieve wide-temperature-range microwave/infrared/visible-light-compatible camouflage. Firstly, the combination of coordinate transformation and genetic algorithm endows the GGS architecture with optimal orientation and gradient, allowing superior microwave blackbody-like behavior. Secondly, a microwave-transparent, low-infrared-emissivity MXene metasurface is constructed in situ to permit wide-temperature-range infrared camouflage. Finally, the outstanding spectral selectivity of MXene enables camouflage against 1.06 μm-lidar and visible-light detection. As a result, the as-fabricated [110]-oriented GGS M@SiOC metamaterials exhibit outstanding wide-temperature-range multispectral camouflage: (i) ultrabroadband microwave absorption exceeding 80% in the X-Ku band from room temperature (RT) to 500 °C with absorption above 86.0% (91.4% on average) at 500 °C; (ii) excellent long-wavelength infrared camouflage for object temperatures from RT to 450 °C, reaching an infrared signal intensity of 78.5% for objects at 450 °C; and (iii) camouflage against both 1.06 μm-lidar and dark environment. Compared with traditional hierarchical metamaterials necessitating complex micro/nano-fabrication processes, this work provides a novel pathway toward the realization of structurally integrated multispectral stealth components by combining flexible metastructure design and high-fidelity additive manufacturing.

Efficient BiVO4-Based Photoanode with Chemically Precipitated Hierarchical Cobalt Borate (Co-Bi) Oxygen Evolution Reaction Catalysts

The integration of cobalt borate OER catalysts with electrodeposited BiVO₄-based photoanodes through a simple drop casting technique was shown to provide an improvement of the photoelectrochemical performance of electrodes under simulated solar light. Catalysts were obtained by chemical precipitation mediated by NaBH₄ at room temperature. Scanning electron microscopy (SEM) investigation of precipitates showed a hierarchical structure with globular features covered in nanometric thin sheets providing a large active area, whereas X-ray diffraction (XRD) and Raman spectroscopy highlighted their amorphous structure. The photoelectrochemical behavior of samples was investigated by linear scan voltammetry (LSV) and electrochemical impedance spectroscopy (EIS) techniques. The amount of particles loaded onto BiVO₄ absorbers was optimized by variation of the drop cast volume. The enhancement of photocurrent generation by Co-B_i-decorated electrodes with respect to bare BiVO₄ was observed with an increase from 1.83 to 3.65 mA/cm² at 1.23 V vs RHE under AM 1.5 simulated solar light, corresponding to a charge transfer efficiency of 84.6%. The calculated maximum applied bias photon-to-current efficiency (ABPE) value for optimized samples was 1.5% at 0.5 V applied bias. Under constant illumination at 1.23 V vs RHE, a depletion of photoanode performances was observed within an hour, likely due to the detachment of the catalyst from the electrode surface.

The Rise of MXene: A Wonder 2D Material, from Its Synthesis and Properties to Its Versatile Applications-A Comprehensive Review

MXene, a new member of 2D material, unites the eminence of hydrophilicity, large surface groups, superb flexibility and excellent conductivity. Because of its prodigious characteristics, MXene has gained much approbation among researchers worldwide. MXene's noteworthy features, such as its electrical conductivity, structural property, magnetic behaviour, etc., manifest a broad spectrum of applications, including environment, catalytic, wireless communications, electromagnetic interference (EMI) shielding, drug delivery, wound dressing, bio-imaging, antimicrobial and biosensor. In this review article, an overview of the latest advancements in the applications of MXene has been reported. First, various synthesis strategies of MXene will be summarized, followed by the different structural, physical and chemical properties. The current advances in versatile applications have been discussed. The article aims to incorporate all the possible applications of MXene, making it a versatile material that juxtaposes it with other 2D materials. A separate section is dedicated to the bottlenecks for future developments and recommendations.

Recent advances in the synthesis and electrocatalytic application of MXene materials

MXenes are a class of two-dimensional materials with a graphene-like structure, which have excellent optical, biological, thermodynamic, electrical and magnetic properties. Due to the diversity resulting from the combination of transition metals and C/N, the MXene family has expanded to more than 30 members and been applied in many fields with broad application prospects. Among their applications, electrocatalytic applications have achieved many breakthroughs. Therefore, in this review, we summarize the reports on the preparation of MXenes and their application in electrocatalysis published in the last five years and describe the two main methods for the preparation of MXenes, i.e., bottom-up and top to bottom synthesis. Different methods may change the structure or surface termination of MXenes, and accordingly affect their electrocatalytic performance. Furthermore, we highlight the application of MXenes in the electrocatalytic hydrogen evolution reaction (HER), oxygen evolution reaction (OER), oxygen reduction reaction (ORR), carbon dioxide reduction reaction (CO₂RR), nitrogen reduction reaction (NRR), and multi-functionalization. It can be concluded that the electrocatalytic properties of MXenes can be modified by changing the type of functional groups or doping. Also, MXenes can be compounded with other materials to produce electronic coupling and improve the catalytic activity and stability of the resulting composites. In addition, Mo₂C and Ti₃C₂ are two types of MXene materials that have been widely studied in the field of electrocatalysis. At present, research on the synthesis of MXenes is focused on carbides, whereas research on nitrides is rare, and there are no synthesis methods meeting the requirements of green, safety, high efficiency and industrialization simultaneously. Therefore, it is very important to explore environmentally friendly industrial production routes and devote more research efforts to the synthesis of MXene nitrides.

Tunable Structured MXenes With Modulated Atomic Environments: A Powerful New Platform for Electrocatalytic Energy Conversion

Owing to their rich surface chemistry, high conductivity, tunable bandgap, and thermal stability, structured 2D transition-metal carbides, nitrides, and carbonitrides (MXenes) with modulated atomic environments have emerged as efficient electrochemical energy conversion systems in the past decade. Herein, the most recent advances in the engineering of tunable structured MXenes as a powerful new platform for electrocatalytic energy conversion are comprehensively summarized. First, the state-of-the-art synthetic and processing methods, tunable nanostructures, electronic properties, and modulation principles of engineering MXene-derived nanoarchitectures are focused on. The current breakthroughs in the design of catalytic centers, atomic environments, and the corresponding structure-performance correlations, including termination engineering, heteroatom doping, defect engineering, heterojunctions, and alloying, are discussed. Furthermore, representative electrocatalytic applications of structured MXenes in energy conversion systems are also summarized. Finally, the challenges in and prospects for constructing MXene-based electrocatalytic materials are also discussed. This review provides a leading-edge understanding of the engineering of various MXene-based electrocatalysts and offers theoretical and experimental guidance for prospective studies, thereby promoting the practical applications of tunable structured MXenes in electrocatalytic energy conversion systems.

Magnesium-regulated oxygen vacancies of cobalt-nickel layered double hydroxide nanosheets for ultrahigh performance asymmetric supercapacitors

Rational design of layered double hydroxide (LDH) electrodes is of great significance for high-performance supercapacitors (SCs). Herein, ultrathin cobalt-nickel-magnesium layered double hydroxide (CoNiMg-LDH) nanosheets with plentiful oxygen vacancies are synthesized via sacrificial magnesium-based replacement reaction at room temperature. Self-doping and mild reduction of magnesium can significantly increase the concentration of oxygen vacancies in CoNiMg-LDH, which promotes the electrochemical charge transfer efficiency and enhances the adsorption ability of electrolytes. Density functional theory (DFT) calculations also indicate that Mg²⁺ doping can decrease the formation energy of oxygen vacancies in CoNiMg-LDH nanosheets, which increases the concentration of oxygen vacancies. Thus, the assembled asymmetric supercapacitor CoNiMg-LDH//Actived Carbon accomplishes a superior capacity of ∼ 333 C g^-1 (208 F g^-1) at 1 A g^-1 and presents a gravimetric energy density of 73.9 Wh kg^-1 at 0.8 kW kg^-1. It presents only 13% capacity loss at 20 A g^-1 after 5000 cycles. This discovery emphasizes the positive role of magnesium in regulating oxygen vacancies to improve the performance of supercapacitors, which should be beneficial for extending the scope of superior SCs active materials.

MWCNT-modified MXene as cost-effective efficient bifunctional catalyst for overall water splitting

Utilization of cost-effective, bifunctional, and efficient electrocatalysts for complete water splitting is desirable for sustainable clean hydrogen energy.

Recent Advances in Transition Metal Dichalcogenide Cathode Materials for Aqueous Rechargeable Multivalent Metal-Ion Batteries

The generation of renewable energy is a promising solution to counter the rapid increase in energy consumption. Nevertheless, the availability of renewable resources (e.g., wind, solar, and tidal) is non-continuous and temporary in nature, posing new demands for the production of next-generation large-scale energy storage devices. Because of their low cost, highly abundant raw materials, high safety, and environmental friendliness, aqueous rechargeable multivalent metal-ion batteries (AMMIBs) have recently garnered immense attention. However, several challenges hamper the development of AMMIBs, including their narrow electrochemical stability, poor ion diffusion kinetics, and electrode instability. Transition metal dichalcogenides (TMDs) have been extensively investigated for applications in energy storage devices because of their distinct chemical and physical properties. The wide interlayer distance of layered TMDs is an appealing property for ion diffusion and intercalation. This review focuses on the most recent advances in TMDs as cathode materials for aqueous rechargeable batteries based on multivalent charge carriers (Zn2+, Mg2+, and Al3+). Through this review, the key aspects of TMD materials for high-performance AMMIBs are highlighted. Furthermore, additional suggestions and strategies for the development of improved TMDs are discussed to inspire new research directions.

Preparation of Hollow Cobalt-Iron Phosphides Nanospheres by Controllable Atom Migration for Enhanced Water Oxidation and Splitting

Transition metal phosphides (TMPs), especially the dual-metal TMPs, are highly active non-precious metal oxygen evolution reaction (OER) electrocatalysts. Herein, an interesting atom migration phenomenon induced by Kirkendall effect is reported for the preparation of cobalt-iron (Co-Fe) phosphides by the direct phosphorization of Co-Fe alloys. The compositions and distributions of the Co and Fe phosphides phases on the surfaces of the electrocatalysts can be readily controlled by Co_x Fe_y alloys precursors and the phosphorization process with interesting atom migration phenomenon. The optimized Co₇ Fe₃ phosphides exhibit a low overpotential of 225 mV at 10 mA cm^-2 in 1 m KOH alkaline media, with a small Tafel slope of 37.88 mV dec^-1 and excellent durability. It only requires a voltage of 1.56 V to drive the current density of 10 mA cm^-2 when used as both anode and cathode for overall water splitting. This work opens a new strategy to controllable preparation of dual-metal TMPs with designed phosphides active sites for enhanced OER and overall water splitting.

Rational Design of Two-Dimensional Transition Metal Carbide/Nitride (MXene) Hybrids and Nanocomposites for Catalytic Energy Storage and Conversion

Electro-, photo-, and photoelectrocatalysis play a critical role toward the realization of a sustainable energy economy. They facilitate numerous redox reactions in energy storage and conversion systems, enabling the production of chemical feedstock and clean fuels from abundant resources like water, carbon dioxide, and nitrogen. One major obstacle for their large-scale implementation is the scarcity of cost-effective, durable, and efficient catalysts. A family of two-dimensional transition metal carbides, nitrides, and carbonitrides (MXenes) has recently emerged as promising earth-abundant candidates for large-area catalytic energy storage and conversion due to their unique properties of hydrophilicity, high metallic conductivity, and ease of production by solution processing. To take full advantage of these desirable properties, MXenes have been combined with other materials to form MXene hybrids with significantly enhanced catalytic performances beyond the sum of their individual components. MXene hybridization tunes the electronic structure toward optimal binding of redox active species to improve intrinsic activity while increasing the density and accessibility of active sites. This review outlines recent strategies in the design of MXene hybrids for industrially relevant electrocatalytic, photocatalytic, and photoelectrocatalytic applications such as water splitting, metal-air/sulfur batteries, carbon dioxide reduction, and nitrogen reduction. By clarifying the roles of individual material components in the MXene hybrids, we provide design strategies to synergistically couple MXenes with associated materials for highly efficient and durable catalytic applications. We conclude by highlighting key gaps in the current understanding of MXene hybrids to guide future MXene hybrid designs in catalytic energy storage and conversion applications.

Two-Dimensional Earth-Abundant Transition Metal Oxides Nanomaterials: Synthesis and Application in Electrochemical Oxygen Evolution Reaction

Development of a universal synthetic strategy for two-dimensional (2D) Earth-abundant transition metal oxides nanomaterials is highly vital toward numerous electrochemical applications. Herein, a facile and general synthesis of highly ordered two-dimensional metal oxides nanomaterials includes Co₃O₄, NiO, CuO, and Fe₃O₄ nanosheets as an electrocatalyst for oxygen evolution reaction (OER) is demonstrated. Among the synthesized 2D transition metal oxides, the Co₃O₄ nanosheet exhibits smallest overpotential (η) of ∼384.0 mV at a current density of 10.0 mA cm^-2 and Tafel slope of ∼52.0 mV dec^-1, highest mass activity of ∼112.3 A g^-1 at the overpotential of ∼384.0 mV, and high turn over frequency (TOF) of 0.099 s^-1, which is relatively favorable with state-of-the-art RuO₂ catalyst. The present synthetic approach may unlock a brand new pathway to prepare shape-controlled Earth-abundant transition metal oxides nanomaterials for electrocatalytic OER.

Decorated nickel phosphide nanoparticles with nitrogen and phosphorus co-doped porous carbon for enhanced electrochemical water splitting

A novel free-standing electrode consisting of nickel phosphide (Ni₂P) nanoparticles on nitrogen and phosphorus co-doped porous carbon (NPC) are synthesized on carbon cloth (CC). Polyaniline (PANI) and nickel (Ni) are sequentially electro-deposited on the surface of CC, which are then transformed into NPC and Ni₂P by an in-situ carbonization-phosphorization combined process. The electrode surface is distributed with large amounts of uniform macropores, which could expose more active sites and enhance the interfacial exchange with the electrolyte. The Ni₂P@NPC@CC electrode delivers early overpotentials of 92 and 280 mV vs. Reversible Hydrogen Electrode (RHE) at 10 mA cm^-2 for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in alkaline condition, respectively. The electrolytic cell with Ni₂P@NPC@CC electrode both as anode and cathode can achieve 10 mA cm^-2 at a small bias of 1.54 V for the overall water splitting. Density functional theory (DFT) calculation indicates that combination with Ni₂P and NPC can decrease Gibbs free energy for H* adsorption (ΔG_H*) and increase charge density on the interface, thus could lead to the enhanced activity for water splitting. The free-standing and noble-metal free Ni₂P@NPC@CC electrode is stable, highly active and cost effective, thus have great potential for the hydrogen production.

Improving the performance of a titanium carbide MXene in supercapacitors by partial oxidation treatment

We developed a new strategy to controllably oxidize Ti₃C₂T_x by readily adjusting the loading of ammonium persulfate, in order to turn the –F terminal groups to –O terminal groups on the Ti₃C₂T_x MXene, contributing to high performance.

Computational Discovery and Design of MXenes for Energy Applications: Status, Successes, and Opportunities

MXenes (M_n+1X_n, e.g., Ti₃C₂) are the largest 2D material family developed in recent years. They exhibit significant potential in the energy sciences, particularly for energy storage. In this review, we summarize the progress of the computational work regarding the theoretical design of new MXene structures and predictions for energy applications including their fundamental, energy storage, and catalytic properties. We also outline how high-throughput computation, big data, and machine-learning techniques can help broaden the MXene family. Finally, we present some of the major remaining challenges and future research directions needed to mature this novel materials family.

Environmentally-Friendly Exfoliate and Active Site Self-Assembly: Thin 2D/2D Heterostructure Amorphous Nickel-Iron Alloy on 2D Materials for Efficient Oxygen Evolution Reaction

A class of 2D layered materials exhibits substantial potential for high-performance electrocatalysts due to high specific surface area, tunable electronic properties, and open 2D channels for fast ion transport. However, liquid-phase exfoliation always utilizes organic solvents that are harmful to the environment, and the active sites are limited to edge sites. Here, an environmentally friendly exfoliator in aqueous solution is presented without utilizing any toxic or hazardous substance and active site self-assembly on the inert base of 2D materials. Benefiting from thin 2D/2D heterostructure and strong interfacial coupling, the resultant highly disordered amorphous NiFe/2D materials (Ti3C2 MXene, graphene and MoS₂ ) thin nanosheets exhibit extraordinary electrocatalytic performance toward oxygen evolution reaction (OER) in alkaline media. DFT results further verify the experimental results. The study emphasizes a viable idea to probe efficient electrocatalysts by means of the synergistic effect of environmentally friendly exfoliator in aqueous solution and active site self-assembly on the inert base of 2D materials which forms the unique thin 2D/2D heterostructure in-suit. This new type of heterostructure opens up a novel avenue for the rational design of highly efficient 2D materials for electrocatalysis.