Revealing the influence of conversion-type Co3O4 dimensionality on cyclic and rate performance for lithium storage

Cobalt tetraoxide (Co₃O₄) is regarded as a promising anode material for Li-ion batteries owing to its high theoretical capacity (890 mAh g^-1), simple preparation, and controllable morphology. Nanoengineering has been proven to be an effective method for producing high-performance electrode materials. However, systematic research on the influence of material dimensionality on battery performance is lacking. Herein, we prepared Co₃O₄ with various dimensionalities (one-dimensional (1D) Co₃O₄ nanorod (NR), two-dimensional (2D) Co₃O₄ nanosheet (NS), three-dimensional (3D) Co₃O₄ nanocluster (NC), and 3D Co₃O₄ nanoflower (NF)) using a simple solvothermal heat treatment method, and their morphologies were controlled by varying the precipitator type and solvent composition. The 1D Co₃O₄ NR and 3D samples (3D Co₃O₄ NC and 3D Co₃O₄ NF) exhibited poor cyclic and rate performances, respectively, while the 2D Co₃O₄ NS exhibited the best electrochemical performance. The mechanism analysis revealed that the cyclic stability and rate performance of the Co₃O₄ nanostructures are closely related to their intrinsic stability and interfacial contact performance, respectively, and the 2D thin-sheet structure can achieve an optimal balance between the two, resulting in the best performance. This work presents a comprehensive study on the effect of dimensionality on the electrochemical performance of Co₃O₄ anodes, providing a new concept for the nanostructure design of conversion-type materials.

Surface fluorine preservation dependence of Ti3C2Tx MXene for high electrochemical properties in ionic liquid electrolytes

We identified the contribution of -CF₃ terminations to the Ti₃C₂T_x surface structure when ethanol and water were used as solvents during delamination through experimental and computational studies. Ethanol-treated -CF₃-terminated Ti₃C₂T_x achieves better prevention of nanoflake aggregation, hydrophobicity, and small size, enabling enhanced capacitive properties in ionic liquid compared to water-treated Ti₃C₂T_x in aqueous and ionic liquid electrolytes.

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.

Hierarchical MXene/transition metal oxide heterostructures for rechargeable batteries, capacitors, and capacitive deionization

2D MXenes have attracted considerable attention due to their high electronic conductivity, tunable metal compositions, functional termination groups, low ion diffusion barriers, and abundant active sites. However, MXenes suffer from sheet stacking and partial surface oxidation, limiting their energy storage and water treatment development. To solve these problems and enhance the performance of MXenes in practical applications, various hierarchical MXene/transition metal oxide (MXene/TMO) heterostructures are rationally designed and constructed. The hierarchical MXene/TMO heterostructures can not only prevent the stacking of MXene sheets and improve the electronic conductivity and buffer the volume change of TMOs during the electrochemical reaction process. The synergistic effect of conductive MXenes and active TMOs also makes MXene/TMO heterostructures promising electrode materials for energy storage and seawater desalination. This review mainly introduces and discusses the recent research progress in MXene/TMO heterostructures, focusing on their synthetic strategies, heterointerface engineering, and applications in rechargeable batteries, capacitors, and capacitive deionization (CDI). Finally, the key challenges and prospects for the future development of the MXene/TMO heterostructures in rechargeable batteries, capacitors, and CDI are proposed.

Urchin-like hierarchical ruthenium cobalt oxide nanosheets on Ti3C2Tx MXene as a binder-free bifunctional electrode for overall water splitting and supercapacitors

Synthesizing efficient electrode materials for water splitting and supercapacitors is essential for developing clean electrochemical energy conversion/storage devices. In the present work, we report the construction of a ruthenium cobalt oxide (RuCo₂O₄)/Ti₃C₂T_x MXene hybrid by electrophoretic deposition of Ti₃C₂T_x MXene on nickel foam (NF) followed by RuCo₂O₄ nanostructure growth through an electrodeposition process. Owing to the strong interactions between RuCo₂O₄ and Ti₃C₂T_x sheets, which are verified by density functional theory (DFT)-based simulations, RuCo₂O₄/Ti₃C₂T_x MXene@NF can serve as a bifunctional electrode for both water splitting and supercapacitor applications. This electrode exhibits outstanding electrocatalytic activity with low overpotentials of 170 and 68 mV at 100 A m^-2 toward the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER). The RuCo₂O₄/Ti₃C₂T_x MXene@NF-based alkaline water-splitting cell only requires 1.62 V to achieve a current density of 100 A m^-2, which is much better than that of RuO₂@NF and Pt/C@NF-assembled cells (1.75 V@100 A m^-2). The symmetric supercapacitor (SSC)-assembled electrode displays a high specific capacitance of 229 F g^-1 at 3 A g^-1. The experimental results, complemented with theoretical insights, provide an effective strategy to prepare multifunctional materials for electrocatalysis and energy storage applications.

Amorphous Co3O4 quantum dots hybridizing with 3D hexagonal CdS single crystals to construct a 0D/3D p-n heterojunction for a highly efficient photocatalytic H2 evolution

Herein, a novel amorphous monodisperse Co3O4 quantum dots/3D hexagonal CdS single crystals (0D/3D Co3O4 QDs/CdS) p-n heterojunction was constructed by a simple hydrothermal and electrostatic self-assembly method. The amorphous monodispersed Co3O4 QDs (≈4.5 nm) are uniformly and tightly attached to the surface of the hexagonal CdS single crystals. The sample, 0.5% CQDs/CdS exhibits outstanding hydrogen evolution activity of 17.5 mmol h-1 g-1 with a turnover number (TON) of 4214, up to 10.3 times higher than that of pure CdS. The enhanced photocatalytic activity can be attributed to the synergistic effect of the p-n heterostructure and the quantum confinement effect of Co3O4 QDs, which significantly promoted the separation efficiency of photo-generated electrons and holes. Additionally, the sulfur vacancy also can act as electron trappers to improve carrier separation and electron transfer. The photoelectrochemical and time-resolved fluorescence (TRPL) results further certify the effective spatial charge separation. This work gives an insight into the design of the 0D/3D Co3O4 QDs/CdS p-n heterostructure for a highly efficient photocatalysis.

Construction of two-dimensional bimetal (Fe-Ti) oxide/carbon/MXene architecture from titanium carbide MXene for ultrahigh-rate lithium-ion storage

The development of battery systems with high specific capacity and power density could fuel various energy-related applications from personal electronics to grid storage. (Fe_2.5Ti_0.5)_1.04O₄ possessing high theoretical specific capacity has been considered as a promising high rate anode material for lithium ion batteries due to the replacement of Fe³⁺ (0.64 Å) by Ti⁴⁺ (0.68 Å) with a larger radius to expand the interlayer space for ion intercalation. However, its extreme volume variation upon cycling as well as poor electrical conductivity hinder its further application. To tackle the above problems, in this work, we successfully synthesized two-dimensional (2D) (Fe_2.5Ti_0.5)_1.04O₄/C/MXene architecture derived from Ti₃C₂T_x MXene via solvo-hydrothermal, ultrasound hybridizing and high temperature annealing processes. The (Fe_2.5Ti_0.5)_1.04O₄/C/MXene shows a high discharge capacity of 757.2 mAh g^-1 after 800 cycles at a current density of 3 A g^-1 with excellent rate performance. The superior electrochemical performances are triggered primarily by the incorporation of carbon and MXene into (Fe_2.5Ti_0.5)_1.04O₄ moiety to construct a 2D layered structure, which can improve the ion diffusion and electron transport. In addition, the synergistic contributions from diffusion controlled and capacitive processes for (Fe_2.5Ti_0.5)_1.04O₄/C/MXene improve the ion diffusion rate and offer high specific capacity at high current density. The MXene-derived synthesis strategy in this work should be a promising pathway to synthesize other anode materials with 2D layered architecture for high performance lithium storage.

Self-Assembled Sandwich-like MXene-Derived Composites as Highly Efficient and Sustainable Catalysts for Wastewater Treatment

Photocatalysts play an increasingly important role in environmental remediation polluted by industrial wastewater. However, the preparation of adsorbents and catalysts with high activity by simple and easy methods is still a great challenge. Here, sandwich-like composite catalyst Cu₂O/TiO₂/Ti₃C₂ was prepared by an easily available solvent reduction measure for the highly efficient catalytic nitro compounds. In particular, sandwich-like composite catalyst Cu₂O/TiO₂/Ti₃C₂ exhibits excellent catalysis for 2-nitroaniline (2-NA) and 4-nitrophenol (4-NP), and its pseudo-first-order reaction rate constants (k) are 0.163 and 0.114 min^-1, respectively. Interestingly, even after eight consecutive cycles of catalytic experiments, the conversion rates of catalytic 2-NA and 4-NP are still greater than 95 and 92%, respectively, demonstrating that the obtained catalyst has excellent catalytic capability and a high reutilization rate. The excellent catalytic performances of Cu₂O/TiO₂/Ti₃C₂ can be attributed to the fact that Ti₃C₂ provides a greater reaction site for the formation of Cu₂O and reduces the aggregation during the formation of Cu₂O by in situ synthesis. Therefore, ternary composite catalyst Cu₂O/TiO₂/Ti₃C₂ prepared by solvent reduction not only supplies a technical method for the catalytic reaction of MXene-based material but also lays the foundation for the development of new photocatalysts.

Stabilizing Na metal anode with NaF interface on spent cathode carbon from aluminum electrolysis

We report the synthesis of spent cathode carbon (SCC) with a NaF interface from aluminum electrolysis, and its application as a Na metal anode host. The SCC anode exhibits superior ion conductivity and a high shear modulus. The natural NaF interface on the SCC anode can regulate Na+ transmission and inhibit dendrite growth. Furthermore, the anode can be used to turn waste into treasure through directly using spent cathodic carbon without any chemical processing. The green SCC electrode exhibits a higher flat voltage and better reversibility compared with purified cathode carbon without NaF.

Strongly Coupled NiCo2O4 Nanocrystal/MXene Hybrid through In Situ Ni/Co-F Bonds for Efficient Wearable Zn-Air Batteries

Recently, owing to the high energy density and excellent security, wearable Zn-air batteries (ZABs) have been known as one of the most prominent wearable energy storage devices. However, sluggish oxygen reaction kinetics of oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) in the air-breathe cathode seriously has limited further practical applications. In this work, we synthesize a NiCo₂O₄ nanocrystal/MXene hybrid with strong Ni/Co-F bonds. The prepared MXene-based hybrid composites show remarkable ORR and OER electrocatalytic activity, which results in the fabricated solid-state ZAB device to achieve an open-circuit voltage of 1.40 V, peak power density of 55.1 mW cm^-2, and energy efficiency of 66.1% at 1.0 mA cm^-2; to the best of our knowledge, this is the record performance among all reported flexible ZABs with MXene-based air cathodes and comparable with some noble metal catalysts. Moreover, even after cutting and suturing, our flexible solid-state ZAB devices are tailorable with high rate of performance.

Rational design of MXene@TiO2 nanoarray enabling dual lithium polysulfide chemisorption towards high-performance lithium-sulfur batteries

Lithium-sulfur (Li-S) batteries face a few vital issues, including poor conductivity, severe volume expansion/contraction, and especially the detrimental shuttle effect during the long-term electrochemical process. Herein, we designed a hierarchical MXene@TiO2 nanoarray via in situ solvothermal strategies followed by heat treatment. The MXene@TiO2 heterostructure achieves superior charge transfer and sulfur encapsulation. Based on the polar-polar and Lewis acid-base mechanism, the robust dual chemisorption capability to trap polysulfides can be synergistically realized through the intense polarity of TiO2 and the abundant acid metal sites of MXene. Hence, the MXene@TiO2 nanoarray as a sulfur host retains a substantially stable discharge capacity of 612.7 mA h g-1 after 500 cycles at a rate of 2 C, which represents a low fading rate of 0.058% per cycle.

Synergistically Coupling Black Phosphorus Quantum Dots with MnO2 Nanosheets for Efficient Electrochemical Nitrogen Reduction Under Ambient Conditions

The electrochemical nitrogen reduction reaction (NRR) is a promising strategy of nitrogen fixation into ammonia under ambient conditions. However, the development of electrochemical NRR is highly bottlenecked by the expensive noble metal catalysts. As a representative 2D nonmetallic material, black phosphorus (BP) has the valence electron structure similar to nitrogen, which can effectively adsorb the inactive nitrogen molecule and activate its triple bond. In addition, the relatively weak hydrogen adsorption can restrict the competitive and vigorous hydrogen evolution reaction. Herein, ultrafine BP quantum dots (QDs) are prepared via liquid-phase exfoliation and then assembled on catalytically active MnO₂ nanosheets through van der Waals interactions. The obtained BP QDs/MnO₂ catalyst demonstrates admirable synergetic effects in electrochemical NRR. The monodisperse BP QDs providing major activity manifest excellent ammonia production steadily with high selectivity, which benefits from the robust confinement of the BP QDs on the wrinkled MnO₂ nanosheets with decent activity. A high ammonia yield rate of 25.3 µg h^-1 mg_cat. ^-1 and faradic efficiency of 6.7% can be achieved at -0.5 V (vs RHE) in 0.1 m Na₂ SO₄ electrolyte, which are dramatically superior to either component. The isotopic labelling and other control tests further exclude the external contamination possibility and attest the genuine activity.

Sandwich-like Co3O4/MXene composites as high capacity electrodes for lithium-ion batteries

The synergistic effect of Co₃O₄ and Ti₃C₂T_x and the lithiation-induced refining architecture of Co₃O₄/Ti₃C₂T_x contribute to remarkable performance.

Fe-doped Co3O4 polycrystalline nanosheets as a binder-free bifunctional cathode for robust and efficient zinc–air batteries

Self-standing Fe-Co₃O₄@CC bifunctional electrocatalysts enabled high-performance Zn–air batteries with a power density of 268.6 mW cm⁻² and superior cycling stability.

Synthesis of amino-functionalized Ti3C2Tx MXene by alkalization-grafting modification for efficient lead adsorption

High-quality amino-functionalized Ti₃C₂T_x MXene (alk-MXene-NH₂) nanosheets were successfully synthesized by a facile alkalization-grafting modification for lead adsorption.

MXene Boosted CoNi-ZIF-67 as Highly Efficient Electrocatalysts for Oxygen Evolution

Oxygen evolution reaction (OER) is a pivotal step for many sustainable energy technologies, and exploring inexpensive and highly efficient electrocatalysts is one of the most crucial but challenging issues to overcome the sluggish kinetics and high overpotentials during OER. Among the numerous electrocatalysts, metal-organic frameworks (MOFs) have emerged as promising due to their high specific surface area, tunable porosity, and diversity of metal centers and functional groups. It is believed that combining MOFs with conductive nanostructures could significantly improve their catalytic activities. In this study, an MXene supported CoNi-ZIF-67 hybrid (CoNi-ZIF-67@Ti₃C₂T_x) was synthesized through the in-situ growth of bimetallic CoNi-ZIF-67 rhombic dodecahedrons on the Ti₃C₂T_x matrix via a coprecipitation reaction. It is revealed that the inclusion of the MXene matrix not only produces smaller CoNi-ZIF-67 particles, but also increases the average oxidation of Co/Ni elements, endowing the CoNi-ZIF-67@Ti₃C₂T_x as an excellent OER electrocatalyst. The effective synergy of the electrochemically active CoNi-ZIF-67 phase and highly conductive MXene support prompts the hybrid to process a superior OER catalytic activity with a low onset potential (275 mV vs. a reversible hydrogen electrode, RHE) and Tafel slope (65.1 mV∙dec⁻¹), much better than the IrO₂ catalysts and the pure CoNi-ZIF-67. This work may pave a new way for developing efficient non-precious metal catalyst materials.

Ti3C2Tx MXene nanosheet-confined Pt nanoparticles efficiently catalyze dye-sensitized photocatalytic hydrogen evolution reaction

In situ grown Pt NPs on Ti₃C₂T_x NSs show ∼30 times higher H₂ evolution activity than free Pt NPs and an AQY of 5.81% at 520 nm in an ErB–TEOA system.