Bottom-up preparation of hierarchically porous MOF-modified carbon sphere derivatives for efficient oxygen reduction

Interfacial Engineering of Heterogeneous Reactions for MOF-on-MOF Heterostructures

Metal-organic frameworks (MOFs), as a subclass of porous crystalline materials with unique structures and multifunctional properties, play a pivotal role in various research domains. In recent years, significant attention has been directed toward composite materials based on MOFs, particularly MOF-on-MOF heterostructures. Compared to individual MOF materials, MOF-on-MOF structures harness the distinctive attributes of two or more different MOFs, enabling synergistic effects and allowing for the tailored design of diverse multilayered architectures to expand their application scope. However, the rational design and facile synthesis of MOF-on-MOF composite materials are in principle challenging due to the structural diversity and the intricate interfaces. Hence, this review primarily focuses on elucidating the factors that influence their interfacial growth, with a specific emphasis on the interfacial engineering of heterogeneous reactions, in which MOF-on-MOF hybrids can be conveniently obtained by using pre-fabricated MOF precursors. These factors are categorized as internal and external elements, encompassing inorganic metals, organic ligands, lattice matching, nucleation kinetics, thermodynamics, etc. Meanwhile, these intriguing MOF-on-MOF materials offer a wide range of advantages in various application fields, such as adsorption, separation, catalysis, and energy-related applications. Finally, this review highlights current complexities and challenges while providing a forward-looking perspective on future research directions.

Noble-Metal-Free FeMn-N-C Catalyst for Efficient Oxygen Reduction Reaction in Both Alkaline and Acidic Media

The oxygen reduction reaction (ORR) is important cathodic reaction running in several electrochemical energy conversion devices. It is still difficult to develop non-precious nanocatalysts for ORR that have high activity and increased durability for practical application. Herein, bimetallic FeMn(mIm)-N-C composite incorporated with Fe and Mn via an encapsulation-ligand exchange technique is prepared and established as an efficient ORR catalyst. The results reveal that FeMn(mIm)-N-C shows outstanding ORR performance with E_1/2 of 0.861 V and 0.778 V in alkaline and acid solutions, along with robust durability. Additionally, the assembled Zn-Air batteries (ZAB) and proton exchange membrane fuel cells (PEMFCs) both have exceptional power densities and show promise for long-term stability compared to 20% Pt/C. The present work provides a useful strategy for designing and synthesizing a reliable low-cost and high-efficient electrocatalysts for energy conversion and storage.

Intelligent Chip-Controlled Smart Oxygen Electrodes for Constructing Rechargeable Zinc-Air Batteries with Excellent Energy Efficiency and Durability

High-performance rechargeable oxygen electrodes are key devices for realizing high-specific-energy batteries, including zinc-air and lithium-air batteries. However, these batteries have severe problems of premature decay in energy efficiency by serious corrosion, wide charge-discharge gap, and catalyst peeling off. Herein, we propose a "smart dual-oxygen electrode", which is composed of an intelligent switch control module + heterostructured Fe₁Ni₃-LDH/PNCNF OER catalysis electrode layer + ion conductive | electronic insulating membrane + Pt/C ORR catalysis electrode layer, where OER and ORR layers are automatically switched by the intelligent switch control module as required. This smart dual-oxygen electrode offers an ultralow energy efficiency decay rate of 0.0067% after 300 cycles during cycling, much lower than that of the commercial Pt/C electrode (1.82%). The assembled rechargeable zinc-air battery (RZAB) displays a super narrow voltage gap and achieves a high energy efficiency of 71.7%, far higher than that of the existing RZABs (about 50%). Therefore, this strategy provides a complete solution for designing various high-performance metal-air secondary batteries.

In-MOF-Derived Hierarchically Hollow Carbon Nanostraws for Advanced Zinc-Iodine Batteries

Hollow carbon materials are regarded as crucial support materials in catalysis and electrochemical energy storage on account of their unique porous structure and electrical properties. Herein, an indium-based organic framework of InOF-1 can be thermally carbonized under inert argon to form indium particles through the redox reaction between nanosized indium oxide and carbon matrix. In particular, a type of porous hollow carbon nanostraw (HCNS) is in situ obtained by combining the fusion and removal of indium within the decarboxylation process. The as-synthesized HCNS, which possesses more charge active sites, short and quick electron, and ion transport pathways, has become an excellent carrier for electrochemically active species such as iodine with its unique internal cavity and interconnected porous structure on the tube wall. Furthermore, the assembled zinc-iodine batteries (ZIBs) provide a high capacity of 234.1 mAh g^-1 at 1 A g^-1 , which ensures that the adsorption and dissolution of iodine species in the electrolyte reach a rapid equilibrium. The rate and cycle performance of the HCNS-based ZIBs are greatly improved, thereby exhibiting an excellent capacity retention rate. It shows a better electrochemical exchange capacity than typical unidirectional carbon nanotubes, making HCNS an ideal cathode material for a new generation of high-performance batteries.

Conversion of rice husk biomass into electrocatalyst for oxygen reduction reaction in Zn-air battery: Effect of self-doped Si on performance

An outstanding oxygen reduction reaction (ORR) electrocatalyst is firstly developed deriving from sustainable rice husk (RH) biomass. Benefiting from self-doped Si in RH, the higher proportion of pyridine N, graphite N and expecially Fe-N_x as well as thiophene S contents were produced in Si-Fe/S/N-RH₃ in comparison with those of Si-free Fe/S/N-RH_3. Consequently, the half-wave potential of 0.89 V and the onset potential of 0.96 V are achieved for Si-Fe/S/N-RH₃, outperforming the benchmark electrocatalyst Pt/C and other Fe-based electrocatalysts reported in alkaline media. Furthermore, it is found that the exisentence of self-doped Si can improve the graphitization degree of the catalyst, leading to the long-term stability (larger than 85% retention after 40000 s) and prominent methanol tolerance for Si-Fe/S/N-RH₃. In addition, Si-Fe/S/N-RH₃ shows a power density of 86.2 mW cm^-2 and excellent durability in Zn-air battery. The work highlights the potential to develop sustainable and cost-effective ORR electrocatalysts from waste biomass as the substitute for precious metal catalysts.

Phthalocyanine-induced iron active species in metal–organic framework-derived porous carbon for efficient alkaline zinc–air batteries

The FePc-encapsulated MOF can be thermally converted into a series of Fe-based active centers embedded into N-doped carbon nanomaterials. It shows good oxygen reduction activity in terms of mass activity, long-term durability and methanol tolerance.

Rational Design and Growth of MOF-on-MOF Heterostructures

Multiporous metal-organic frameworks (MOFs) have emerged as a subclass of highly crystalline inorganic-organic materials, which are endowed with high surface areas, tunable pores, and fascinating nanostructures. Heterostructured MOF-on-MOF composites are recently becoming a research hotspot in the field of chemistry and materials science, which focus on the assembly of two or more different homogeneous or heterogeneous MOFs with various structures and morphologies. Compared with one single MOF, the dual MOF-on-MOF composites exhibit unprecedented tunability, hierarchical nanostructure, synergistic effect, and enhanced performance. Due to the difference of inorganic metals and organic ligands, the lattice parameters in a, b, and c directions in the single crystal cells could bring about subtle or large structural difference. It will result in the composite material with distinct growth methods to obtain secondary MOF grown from the initial MOF. In this review, the authors wish to mainly outline the latest synthetic strategies of heterostructured MOF-on-MOFs and their derivatives, including ordered epitaxial growth, random epitaxial growth, etc., which show the tutorial guidelines for the further development of various MOF-on-MOFs.

Highly efficient zinc finger peptide detection with ZIF-8-modified micropipets

Zeolitic imidazolate framework-8 (ZIF-8)-modified micropipets can be an effective sensing platform for zinc finger peptides, the limit of detection of which reaches 10-2 μg ml-1. A series of techniques for detecting biomolecules are expected to emerge because of its simplicity, low cost, and universality by modifying other functional materials into the micropipets.

Enhanced electrocatalytic oxygen reduction reaction for Fe-N4-C by the incorporation of Co nanoparticles

Oxygen reduction reaction (ORR) catalytic activity can be improved by means of enhancing the synergy between transition metals. In this work, a novel porous Fe-N₄-C nanostructure containing uniformly dispersed Co nanoparticles (CoNPs) is prepared by an assisted thermal loading method. The as-prepared Co@Fe-N-C catalyst shows enhanced ORR activity with a half-wave potential (E_1/2) of 0.92 V vs. RHE, which is much higher than those of the direct pyrolysis CoNP-free sample Fe-N-C (E_1/2 = 0.85 V) and Pt/C (E_1/2 = 0.90 V) in alkaline media. It exhibits remarkable stability with only a 10 mV decrease in E_1/2 after 10 000 cycles and an outstanding long-term durability with 85% current remaining after 60 000 s. In acidic media, this catalyst exhibits catalytic activity with an E_1/2 of 0.79 V, comparable to Pt/C (E_1/2 = 0.82 V). X-ray absorption fine spectroscopy analysis revealed the presence of active centres of Fe-N₄. Density functional theory calculations confirmed the strong synergy between CoNPs and Fe-N₄ sites, providing a lower overpotential and beneficial electronic structure and a local coordination environment for the ORR. The incorporation of CoNPs on the surface of Fe-N₄-C nanomaterials plays a key role in enhancing the ORR catalytic activity and stability, providing a new route to prepare efficient Pt-free ORR catalysts.

Stable CuO with variable valence states cooperated with active Co2+ as catalyst/co-catalyst for oxygen reduction/methanol oxidation reactions

Catalysts/co-catalysts for cathodic oxygen reduction and anodic methanol oxidation reactions (ORR/MOR) play the major roles in promoting the commercialization of direct methanol fuel cells. Herein, bimetallic zeolite-imidazolate-frameworks (CoZn-ZIFs) is used as precursor to synthesize Co₃O₄@NPC/CuO composites as catalysts for ORR and Pt supports/co-catalysts for MOR. The ORR activity (E_1/2 = 0.83 V) and long-term stability (activity retention of 85.5% after 30,000 s) of Co₃O₄@NPC/CuO-400 (400 °C) dodecahedron are better than those of commercial Pt/C (10 wt%) in alkaline electrolytes. The surface CuO with variable valence states (Cu⁰ and Cu²⁺) can be used as both the active component for ORR and the protective layer for Co₃O₄ to enhance catalytic stability. Partial removal of CoO_x from carbon framework promotes the exposure of highly active sites (Co²⁺) on the Co₃O₄. For MOR, the mass activity of Pt-Co₃O₄@NPC/CuO-400 (5 wt%) (1947 mA mg_Pt^-1) is much higher than that of Pt/C (751 mA mg_Pt^-1), mainly attributing to that the Pt active sites are uniformly dispersed on Co₃O₄@NPC/CuO support. The strong interaction between Pt and CuO can reduce the bond strength of Pt-CO to enhance CO resistance. Co₃O₄ can activate H₂O molecules to provide sufficient OH^- species to promote MOR. This study provides a new idea for preparation of active ORR catalysts and MOR co-catalyst from bimetallic ZIFs.

A mass-producible integrative structure Pt alloy oxygen reduction catalyst synthesized with atomically dispersive metal-organic framework precursors

Oxygen reduction reaction (ORR) catalyst is one of the most significant influential factors for the application of proton exchange membrane fuel cells. This work introduces a mass-producible high-performance PtZn alloy integrative structure ORR catalyst, synthesized with the atomically dispersive metal-organic framework precursors. This PtZn catalyst displays excellent catalytic activity with the onset reduction potential of 1.0 V_RHE@ 0.16 mA cm^-2 (Reversible hydrogen electrode; RHE) and the half-wave potential of 0.934 V_RHE for the ORR catalysis. The calculated specific activity and mass activity at 0.9 V are 9.44 A m^-2 and 544 A g_Pt^-1, respectively, which are 5.62 times and 5.77 times as high as the commercial Pt/C. The mass activity is remarkably higher than the target put forward by the Department of Energy (DOE; 440 A g_Pt^-1). Furthermore, this PtZn catalyst also exhibits outstanding stability after the 10,000 potential cyclic degeneration test. The ORR current is much higher than Pt/C in the whole potential range not only before but also after the 10,000 potential cycles with identical Pt loading. This catalyst has a multifarious active-site catalytic structure with PtZn alloyed particles and atomically dispersive metal-N active sites on the N-doped graphited carbon matrix, exhibiting appealing ORR catalytic activity and sound stability for the application and scalable production of fuel cell catalysts.

Temperature-dependence on the optical properties of chitosan carbon dots in the solid state

We report the synthesis of chitosan-derived aminated carbon dots with dual fluorescence bands and their influence on the morphology, absorption and emission spectral profiles as well as on the band gap energy in relation to thermal treatment after synthesis. To unravel these changes, we performed spectroscopic measurements in the solid state on two stages at temperatures ranging from 303 to 453 K. For the first heating stage, the emission spectrum showed a 20 nm red shift and a new absorption band at 350 nm, possibly related to new bonds and/or nitrogenous molecular fractions. For the second heating stage in the same temperature range, no displacements in the emission spectrum were observed and both the energy gap and bandwidths for the two emission bands are practically constant, indicating a change nitrogen moiety exposed on the surface. Furthermore, through atomic force microscopy it was noted that the morphology and size of the CDs were not significantly affected by the increase in temperature. It is noteworthy that the values of the Huang-Rhys factor, respectively, 2.584 × 10-10 and 2.315 × 10-9 for band I and II emission after the second heating indicate a mechanism of weak electron-phonon interactions. This work may open a novel perspective for the development of new surface modulation strategies for carbon dots subjected to thermal treatment in the solid state.

Active site engineering of atomically dispersed transition metal-heteroatom-carbon catalysts for oxygen reduction

Owing to the advantage of atomic utilization, the single-atom catalyst has attracted much attention and been employed in multifarious catalytic reactions. Its definite site configuration is favorable for exploring the actual active centers and corresponding reaction mechanism. At the atomic scale, the tunable site configuration, from central metal atoms, coordinated heteroatoms, peripheral dopants, and feasible polymetallic centers to the synergetic intrinsic carbon defects, can effectively augment the intrinsic activity for oxygen reduction reaction (ORR). From a practical viewpoint, the propagation strategies of single-atom sites, the loading-activity relation and the structural retention during practical tests are crucial for the industrial applications. Furthermore, the activity contribution of multiple additional active centers including the active carbon sites and the pony-size well-wrapped metal species should be acknowledged. From the perspective mentioned above, this paper thoroughly analyses the consensuses, controversies, challenges and possible solutions based on the current research progress, thereby providing inspiration and guidance for the active center engineering of single-atom catalysts.

Fabrication of polyoxometalate-modified palladium–nickel/reduced graphene oxide alloy catalysts for enhanced oxygen reduction reaction activity

A novel nanocatalyst, polyoxometalate-modified palladium–nickel/reduced graphene oxide (Pd8Ni2/rGO-POM), is prepared and served as an effective ORR nanomaterial in alkaline media.