Extraordinary Structural Reconstruction of Nanolaminated Ta2FeC MAX Phase for Enhanced Oxygen Evolution Performance.

Renewable energy technologies, such as water splitting, heavily depend on the oxygen evolution reaction (OER). Nanolaminated ternary compounds, referred to as MAX phases, show great promise for creating efficient electrocatalysts for OER. However, their limited intrinsic oxidative resistance hinders the utilization of conductivity in Mn+1Xn layers, leading to reduced activity. In this study, a method is proposed to improve the poor inoxidizability of MAX phases by carefully adjusting the elemental composition between Mn+1Xn layers and single-atom-thick A layers. The resulting Ta2FeC catalyst demonstrates superior performance compared to conventional Fe/C-based catalysts with a remarkable record-low overpotential of 247 mV (@10 mA cm-2) and sustained activity for over 240 h. Notably, during OER processing, the single-atom-thick Fe layer undergoes self-reconstruction and enrichment from the interior of the Ta2FeC MAX phase toward its surface, forming a Ta2FeC@Ta2C@FeOOH heterostructure. Through density functional theory (DFT) calculations, this study has found that the incorporation of Ta2FeC@Ta2C not only enhances the conductivity of FeOOH but also reduces the covalency of Fe─O bonds, thus alleviating the oxidation of Fe3+ and O2-. This implies that the Ta2FeC@Ta2C@FeOOH heterostructure experiences less lattice oxygen loss during the OER process compared to pure FeOOH, leading to significantly improved stability. These results highlight promising avenues for further exploration of MAX phases by strategically engineering M- and A-site engineering through multi-metal substitution, to develop M2AX@M2X@AOOH-based catalysts for oxygen evolution.

Nanolaminated Ternary Transition Metal Carbide (MAX Phase)-Derived Core-Shell Structure Electrocatalysts for Hydrogen Evolution and Oxygen Evolution Reactions in Alkaline Electrolytes.

The development of abundant, cheap, and highly active catalysts for the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) is important for hydrogen production. Nanolaminate ternary transition metal carbides (MAX phases) and their derived two-dimensional transition metal carbides (MXenes) have attracted considerable interest for electrocatalyst applications. Herein, four new MAX@MXene core-shell structures (Ta2CoC@Ta2CTx, Ta2NiC@Ta2CTx, Nb2CoC@Nb2CTx, and Nb2NiC@Nb2CTx), in which the core region is Co/Ni-MAX phases while the edge region is MXenes, have been prepared. Under alkaline electrolyte conditions, the Ta2CoC@Ta2CTx core-shell structure showed an overpotential of 239 mV and excellent stability during the HER with MXenes as the active sites. For the OER, the Ta2CoC@Ta2CTx core-shell structure showed an overpotential of 373 mV and a small Tafel plot (56 mV dec-1), which maintained a bulk crystalline structure and generated Co-based oxyhydroxides that formed by surface reconstruction as active sites. Considering rich chemical compositions and structures of MAX phases, this work provides a new strategy for designing multifunctional electrocatalysts and also paves the way for further development of MAX phase-based materials for clean energy applications.

Hierarchical core-shell SiO2@PDA@BiOBr microspheres with enhanced visible-light-driven photocatalytic performance.

To explore catalysts combining highly accessible specific surface areas with low recombination of the photo-induced electron-hole pairs, a novel SiO2@PDA@BiOBr composite photocatalyst with a hierarchical core-shell structure was prepared by a facile solvothermal method. The catalyst shows a superior performance on photodegradation of Rhodamine B under visible light irradiation, especially for SiO2@PDA-2@BiOBr with the reactant kinetics constant (k = 0.0487 min-1). The enhanced photocatalytic performance of SiO2@PDA-2@BiOBr was ascribed to the decreased band-gap, higher surface area, and effectively photo-generated electron-hole pairs by the introduction of polydopamine (PDA). In addition, the photocatalytic degradation is initiated by ˙O2- derived from dye photosensitization and h+ from the BiOBr. Cyclic experiments also indicate that the SiO2@PDA-2@BiOBr is reusable during the photodegradation process. The hierarchical core-shell SiO2@PDA@BiOBr photocatalyst will provide a theoretical model for the development of physical chemistry and structural properties of BiOBr-based composites to enhance the photocatalytic performances.

Enhanced photocatalytic performance of BiOBr/NH2-MIL-125(Ti) composite for dye degradation under visible light.

Metal-organic frameworks (MOFs) are considered as suitable materials for various applications in the area of photocatalysis. On the other hand, 2D BiOBr materials are efficient for the photodegradation of organic dyes under visible light illumination. In this work, BiOBr/NH2-MIL-125(Ti) composite photocatalysts with different NH2-MIL-125(Ti) content were prepared by incorporating NH2-MIL-125(Ti) with BiOBr using a co-precipitation method. A series of characterizations confirmed the strong synergistic effect between BiOBr and NH2-MIL-125(Ti). In rhodamine B (RhB) degradation experiments, the composite photocatalyst with a mass percent of 7 wt% NH2-MIL-125(Ti) exhibited an improved photocatalytic activity compared to pristine BiOBr and NH2-MIL-125(Ti). Furthermore, the enhanced photocatalytic performance under visible light illumination could be attributed to the Ti3+-Ti4+ intervalence electron transfer and synergistic effect between NH2-MIL-125(Ti) and BiOBr, and also resulted in a separation efficiency of photo-generated electron-hole pairs during the photocatalytic reaction. This study can open up numerous opportunities for the development of various MOF-based visible light photocatalysts when combined with 2D bismuth oxyhalide materials for applications in environmental cleaning.

In situ growth of ZIF-8 nanocrystals on layered double hydroxide nanosheets for enhanced CO2 capture.

A hexagonal nanosheet LDH@ZIF-8 composite was fabricated by in situ growth of ZIF-8 on Zn-Al LDH without adding any zinc precursor, and exhibited a CO2 adsorption capacity of 1.0 mmol g(-1) at room temperature and 1 bar, which was significantly higher than that of pure Zn-Al LDH or ZIF-8, indicating a synergy between ZIF-8 and Zn-Al LDH.

MOF-derived hierarchical double-shelled NiO/ZnO hollow spheres for high-performance supercapacitors.

Nanorods-composed yolk-shell bimetallic-organic frameworks microspheres are successfully synthesized by a one-step solvothermal method in the absence of any template or surfactant. Furthermore, hierarchical double-shelled NiO/ZnO hollow spheres are obtained by calcination of the bimetallic organic frameworks in air. The NiO/ZnO hollow spheres, as supercapacitor electrodes, exhibit high capacitance of 497 F g(-1) at the current density of 1.3 A g(-1) and present a superior cycling stability. The superior electrochemical performance is believed to come from the unique double-shelled NiO/ZnO hollow structures, which offer free space to accommodate the volume change during the ion insertion and desertion processes, as well as provide rich electroactive sites for the electrochemical reactions.