O,N-Codoped 3D graphene hollow sphere derived from metal-organic frameworks as oxygen reduction reaction electrocatalysts for Zn-air batteries

ZIFs-Derived Hollow Nanostructures via a Strong/Weak Coetching Strategy for Long-Life Rechargeable Zn-Air Batteries

Recently, zeolitic imidazolate frameworks (ZIFs) composites have emerged as promising precursors for synthesizing hollow-structured N-doped carbon-based noble-metal materials with diverse structures and compositions. Here, a strong/weak competitive coordination strategy is presented for synthesizing high-performance electrocatalysts with hollow features. During the competitive coordination process, the cubic zeolitic-imidazole framework-8 (Cube-8)@ZIF-67 with core-shell structures are transformed into Cube-8@ZIF-67@PF/POM with yolk-shell nanostructures employing phosphomolybdic acid (POM) and potassium ferricyanide (PF) as the strong chelator and the weak chelator, respectively. After calcination, the hollow Mo/Fe/Co@NC catalyst exhibits superior performance in both oxygen evolution reaction (OER) and oxygen reduction reaction (ORR). Interestingly, the Mo/Fe/Co@NC catalyst exhibits efficient electrocatalytic performance for Zn-air batteries (ZABs), with a high power density (≈150 mW cm-2) and superior cycling life (≈500 h) compared to commercial platinum/carbon (Pt/C) and ruthenium dioxide (RuO2) mixture benchmarks catalysts. In addition, the density functional theory further proves that after the introduction of Mo and Fe atoms, the adsorption energy with the adsorption intermediates is weakened by adjusting the d-band center, thus weakening the reaction barrier and promoting the reaction kinetics of OER. Undoubtedly, this study presents novel insights into the fabrication of ZIFs-derived hollow structure bifunctional oxygen electrocatalysts for clean-energy diverse applications.

Unleashing the room temperature boronization: Blooming of Ni-ZIF nanobuds for efficient photo/electro catalysis of water

Water splitting provides an environmental-friendly and sustainable approach for generating hydrogen fuel. The inherent energetic barrier in two-core half reactions such as the Hydrogen Evolution Reaction (HER) and Oxygen Evolution Reaction (OER) leads to undesired increased overpotential and constrained reaction kinetics. These challenges pose significant challenges that demand innovative solutions to overcome. One of the efficient ways to address this issue is tailoring the morphology and crystal structure of metal-organic frameworks (MOF). Nickel Zeolite Imidazolate Framework (Ni-ZIF) is a popular MOF and it can be tailored using facile chemical methods to unleash a remarkable bifunctional electro/photo catalyst. This innovative solution holds the capability to address prevailing obstacles such as inadequate electrical conductivity and limited access to active metal centers due to the influence of organic ligands. Thereby, applying boronization to the Ni-ZIF under different duration, one can induce blooming of nanobuds under room temperature and modify oxygen vacancies in order to achieve higher reaction kinetics in electro/photo catalysis. It can be evidenced by the 24-h boronized Ni-ZIF (BNZ), exhibiting lower overpotentials as electrocatalyst (OER-396 mV & HER-174 mV @ 20 mA/cm2) in 1 M KOH electrolyte and augmented gas evolution rates when employed as a photocatalyst (Hydrogen-14.37 μmol g-1min-1 & Oxygen-7.40 μmol g-1min-1). The 24-h boronization is identified as the optimum stage of crystalline to amorphous transformation which provided crystalline/amorphous boundaries as portrayed by X-Ray diffraction (XRD) and High Resolution-Transmission Electron Microscopy (HR-TEM) analysis. The flower-like transformation of 24-BNZ, characterized by crystalline-amorphous boundaries initiates with partial disruption of Ni-N bonds and formation of Ni-B bonds as evident from X-ray Photoelectron Spectroscopy (XPS). Further, the 24-h BNZ exhibit bifunctional catalytic activities with pre-longed stability. Overall, this work presents a comprehensive study of the electrocatalytic and photocatalytic water splitting properties of the tailored Ni-ZIF material.

Precise Modulation of Carbon Activity Sites in Metal-Free Covalent Organic Frameworks for Enhanced Oxygen Reduction Electrocatalysis

Metal-free carbon-based materials have gained recognition as potential electrocatalysts for the oxygen reduction reaction (ORR) in new environmentally-friendly electrochemical energy conversion technologies. The presence of effective active centers is crucial for achieving productive ORR. In this study, we present the synthesis of two metal-free dibenzo[a,c]phenazine-based covalent organic frameworks (DBP-COFs), specifically JUC-650 and JUC-651, which serve as ORR electrocatalysts. Among them, JUC-650 demonstrates exceptional catalytic performance for ORR in alkaline electrolytes, exhibiting an onset potential of 0.90 V versus RHE and a half-wave potential of 0.72 V versus RHE. Consequently, JUC-650 stands out as one of the most outstanding metal-free COF-based ORR electrocatalysts report to date. Experimental investigations and density functional theory calculations confirm that modulation of the frameworks' electronic configuration allows for the reduction of adsorption energy at the Schiff-base carbon active sites, leading to more efficient ORR processes. Moreover, the DBP-COFs can be assembled as excellent air cathode catalysts for zinc-air batteries (ZAB), rivaling the performance of commercial Pt/C. This study provides valuable insights for the development of efficient metal-free organoelectrocatalysts through precise regulation of active site strategies.

3D TM-N-C Electrocatalysts with Dense Active Sites for the Membraneless Direct Methanol Fuel Cell and Zn-Air Batteries

Electrocatalysts with high cost-effectiveness for the oxygen reduction reaction (ORR) are essential for fuel cells (FC) and Zn-Air batteries (ZAB), which need highly active sites and suitable carbon substrates to accelerate the charge transfer kinetics. Herein, a simple and extensible method using ball milling and space-confinement pyrolysis is reported to prepare a series of transition metals and N-C catalysts (M-NLPC), which possess three-dimensional porous carbon substrates and dense active sites for efficient ORR. M-NLPC catalysts (especially Fe-NLPC) exhibit outstanding ORR activity with a half-wave potential (E_1/2, 0.88 V) in an alkaline medium, high stability, and strong methanol resistance. The M-N₄ sites are proven to be the active centers in M-NLPC by theoretical calculation, and methanol molecules are more likely to desorb than react on the Fe-N₄ sites, which is the origin of the inactivity for the methanol oxidation reaction (MOR). Furthermore, Fe-NLPC was applied to membraneless alkaline direct methanol FC (DMFC) in practice, exhibiting outstanding performance. Meanwhile, the Fe-NLPC-based ZAB also shows excellent electrochemical performance.

Design and synthesis of a 3D graphene-enhanced electrode for fast charge/ion transport for lithium storage

Design and fabrication of an N-graphene-enhanced anode with a hierarchical morphology and a unique electronic structure that exhibits superior electrochemical performances in LIBs.

N-Doped graphene supported on N-rGO nanosheets as metal-free oxygen reduction reaction electrocatalysts for Zn–air batteries

Metal-free electrocatalysts for the oxygen reduction reaction (ORR) are the desired materials for metal–air batteries.