Ruthenium Nanoparticles Confined in Covalent Organic Framework/Reduced Graphene Oxide As Electrocatalyst toward Hydrogen Evolution Reaction in Alkaline Media

Confining metal nanoparticles and nanoclusters in covalent organic frameworks for biosensing and biomedicine

Metal nanoscale particles, primarily including metal nanoparticles (MNPs) and nanoclusters (MNCs), have garnered substantial interests owing to their unique electronic configurations and distinct physicochemical properties. However, practical applications are frequently constrained by their limited stability and aggregation tendency. Covalent organic frameworks (COFs), featuring highly ordered periodic architectures, have emerged as ideal porous matrices for hosting metal nanoparticles. The resulting metal-embedded COFs synthesized through adsorption methods (M/COFs) or in-situ reduction (M@COFs) not only mitigate nanoparticle aggregation and enhance stability but also demonstrate synergistic effects that generate enhanced or novel functionalities, significantly broadening their application potential. This review firstly examines adsorption-based synthesis strategies for M/COFs through physical and chemical approaches. Subsequently, we analyze in-situ reduction methods for M@COFs, categorizing them by reduction pathways: deposition, impregnation-pyrolysis, and "one-step" synthesis. Special attention is given to an emerging pore wall engineering strategy within in-situ reduction approach. The biosensing and biomedical applications of metal-embedded COFs are systematically examined, highlighting their comparative advantages over conventional nanomaterials in sensing and antimicrobial applications. While metal-embedded COFs remain in their developmental infancy and face considerable challenges, the controlled synthesis of multifunctional variants promises transformative potential across biomedical domains.

Covalent Organic Frameworks and 2D Materials Hybrids: Synthesis Strategies, Properties Enhancements, and Future Directions

Covalent organic frameworks (COFs) are highly porous, thermally and chemically stable organic polymers. Their high porosity, crystallinity, and adjustable properties make them suitable for numerous applications. However, COFs encounter critical challenges, such as their difficult processability, self-stacking propensity, low electrical conductivity, pore blockage which limits their ionic conductivity, and high recombination rates of photoinduced electrons and holes. To overcome these issues, the hybridization of COFs with 2D materials (2DMs) has proven to be an effective strategy. 2DMs including graphene-like materials, transition metal dichalcogenides, and MXenes are particularly advantageous because of their unique physicochemical properties, such as exceptional electrical and optical characteristics, and mechanical resilience. Over the past decade, significant research efforts have been focused on hybrid 2DMs-COFs materials. These hybrids leverage the strengths of both materials, making them suitable for advanced applications. This Review highlights the latest advancements in 2DM-COF hybrids, examining the physicochemical strengths and weaknesses of the pristine materials, together with the synergistic benefits of their hybridization. Moreover, it emphasizes their most remarkable applications in chemical sensing, catalysis, energy storage, adsorption and filtration, and as anticorrosion agents. Finally, it discusses future challenges and opportunities in the development of 2DM-COFs for new disruptive technologies.

Application of Nanocomposites in Covalent Organic Framework-Based Electrocatalysts

In recent years, the development of high-performance electrocatalysts for energy conversion and environmental remediation has become a topic of great interest. Covalent organic frameworks (COFs), linked by covalent bonds, have emerged as promising materials in the field of electrocatalysis due to their well-defined structures, high specific surface areas, tunable pore structures, and excellent acid-base stability. However, the low conductivity of COF materials often limits their intrinsic electrocatalytic activity. To enhance the catalytic performance of COF-based catalysts, various nanomaterials are integrated into COFs to form composite catalysts. The stable and tunable porous structure of COFs provides an ideal platform for these nanomaterials, leading to improved electrocatalytic activity. Through rational design, COF-based composite electrocatalysts can achieve synergistic effects between nanomaterials and the COF carrier, enabling efficient targeted electrocatalysis. This review summarizes the applications of nanomaterial-incorporated COF-based catalysts in hydrogen evolution, oxygen evolution, oxygen reduction, carbon dioxide reduction, and nitrogen reduction. Additionally, it outlines design principles for COF-based composite electrocatalysis, focusing on structure-activity relationships and synergistic effects in COF composite nanomaterial electrocatalysts, as well as challenges and future perspectives for next-generation composite electrocatalysts.

Covalent Organic Framework/Graphene Hybrids: Synthesis, Properties, and Applications

To address current energy crises and environmental concerns, it is imperative to develop and design versatile porous materials ideal for water purification and energy storage. The advent of covalent organic frameworks (COFs), a revolutionary terrain of porous materials, is underscored by their superlative features such as divinable structure, adjustable aperture, and high specific surface area. However, issues like inferior electric conductivity, inaccessible active sites impede mass transfer and poor processability of bulky COFs restrict their wider application. As a herculean stride forward, COF/graphene hybrids amalgamate the strengths of their constituent components and have in consequence, enticed significant scientific intrigue. Herein, the current progress on the structure and properties of graphene-based materials and COFs are systematically outlined. Then, synthetic strategies for preparing COF/graphene hybrids, including one-pot synthesis, ex situ synthesis, and in situ growth, are comprehensively reviewed. Afterward, the pivotal attributes of COF/graphene hybrids are dissected in conjunction with their multifaceted applications spanning adsorption, separation, catalysis, sensing, and energy storage. Finally, this review is concluded by elucidating prevailing challenges and gesturing toward prospective strides within the realm of COF/graphene hybrids research.

Ruthenium Oxide Nanoparticles Immobilized on Ti3C2 MXene Nanosheets for Boosting Seawater Electrolysis

Seawater electrolysis represents a viable alternative for large-scale synthesis of hydrogen (H2), which is recognized as the most promising clean energy source, without relying on scarce fresh water. However, high energy cost and harmful chlorine chemistry in seawater limited its development. Herein, an effective catalyst based on a ruthenium nanoparticle-Ti3C2 MXene composite loaded on nickel foam (RuO2-Ti3C2/NF) with an open, fine, and homogeneous nanostructure was devised and synthesized by electrodeposition for high performance and stable overall seawater splitting. To drive a current density of 100 mA cm-2, the RuO2-Ti3C2/NF electrode required a small overpotential of 85 and 351 mV for HER and OER in 1 M KOH with only a slight increase in 1 M KOH seawater (156 and 378 mV for, respectively, HER and OER). An assembled RuO2-Ti3C2/NF-based two-electrode cell required an overpotential of only 1.84 V to acquire 100 mA cm-2 in 1 M KOH seawater and maintained its activity for over 25 h. This low cell voltage effectively prevented chlorine electrochemical evolution without anode protection. These promising results open up new avenues for the effective conversion of abundant seawater resources to hydrogen fuel.

In Situ Preparation of Polyamine-Derived Ru Cluster@N-Doped Porous Carbon Nanoplates for Hydrogen Evolution over Wide pH Ranges

Efficient and low-cost electrocatalysts for the hydrogen evolution reaction (HER) are required for producing hydrogen energy through water splitting. Carbon materials as HER catalyst supports are explored widely since the strong metal-support interactions are generally believed to be active and stable toward HER. Herein, we report N-doped porous carbon materials as novel substrates to stabilize the cluster metal sites through the Ru(III) polyamine complexes, which play an important role not only in efficient electron transfer but also in the increasing utilization of metallic active sites. Meanwhile, due to the strong metal-support interactions driven by Ru(III) polyamine complexes, the obtained Ru cluster with a mass loading of 3% on N-doped porous carbon nanoplates (Ru cluster@NCs) exhibits robust stability for HER at a constant voltage, proving to be a promising candidate catalyst for HER. Density functional theory calculations further indicate that the Gibbs free energy (ΔG) of adsorbed H* of Ru cluster@NCs is much closer to zero compared to Ru@(10%)NCs and Pt/C(20%), thus Ru cluster@NCs facilitate the HER process.

Electrochemical nitrate reduction to high-value ammonia on two-dimensional molybdenum carbide nanosheets for nitrate-containing wastewater upcycling

Electrochemical conversion of nitrate wastewater into high-value ammonia fertilizer has attracted extensive attention in wastewater treatment and resource recovery, but presents great challenges due to complicated reaction pathways and competing side reactions. Herein, we report a feasible method for the successful fabrication of Mo₂C nanosheets (Mo₂C NSs) as electrocatalyst for the electroreduction of nitrate to ammonia. Compared to Mo₂C nanoparticles, the Mo₂C NSs exhibited superior activity and selectivity in NH₃ electrosynthesis with an NH₃ yield rate of 25.2 mg·h^-1·mg^-1_cat. at -0.4 V and a Faradaic efficiency of 81.4 % at -0.3 V versus reversible hydrogen electrode. The X-ray diffraction and transmission electron microscopy characterization verifted the controllable conversion of 2D MoO₂ NSs into 2D Mo₂C NSs. In situ spectroscopic studies and on-line differential electrochemical mass spectrometry revealed the proposed reaction pathway of NO₃^- to NH₃ conversion, *NO₃^- → *NO₂^- → *NO→*NOH → *NH₂OH → *NH₃. Density functional theory calculations further verified the effective N-end NOH pathway with the conversion of *NH₂OH to *NH₂ as the rate-determining step requiring a low energy barrier of 0.58 eV. Importantly, the key hydrogenation of *NO to form *NOH species underwent a lower energy barrier of 0.39 eV compared with the formation of *ONH species (1.06 eV).

Covalent Triazine Frameworks Decorated with Pyridine-Type Carbonitride Moieties: Enhanced Photocatalytic Hydrogen Evolution by Improved Charge Separation

A simple procedure of calcination under an Ar atmosphere has been successfully applied to create a covalent triazine framework bearing pyridine-type carbonitride moieties (PCN@CTF). The appending of PCN on the CTF led to visible light absorption at up to 600 nm in the UV/Vis diffuse-reflectance spectra. Photoluminescence and electrochemical impedance spectroscopy have been applied to clarify how modification of the CTF with PCN enhanced the separation efficiency of photoexcited charge carriers. An optimized 1%PCN@CTF sample showed the highest photocatalytic hydrogen evolution reaction (HER) rate of 170.2 ± 2.3 μmol g−1·h−1, 3.9 times faster than that over the pristine CTF. The apparent quantum efficiency of the HER peaked at (7.57 ± 0.10)% at 490 nm. This representative 1% PCN@CTF sample maintained continuous function for at least 15 h. This work provides new guidance for modification with PCN materials as a means of obtaining high photocatalytic efficiency and sheds light on the effect of appended pyridine rings on a CTF.

Ruthenium Complex of sp2 Carbon-Conjugated Covalent Organic Frameworks as an Efficient Electrocatalyst for Hydrogen Evolution

It is still a great challenge to explore hydrogen evolution reaction (HER) electrocatalysts with both lower overpotential and higher stability in acidic electrolytes. In this work, an efficient HER catalyst, Ru@COF-1, is prepared by complexation of triazine-cored sp² carbon-conjugated covalent organic frameworks (COFs) with ruthenium ion. Ru@COF-1 possesses high crystallinity and porosity, which are beneficial for electrocatalysis. The large specific surface area and regular porous channels of Ru@COF-1 facilitate full contact between reactants and catalytic sites. The nitrogen atoms of triazines are protonated in the acidic media, which greatly improve the conductivity of Ru@COF-1. This synergistic effect makes the overpotential of Ru@COF-1 about 200 mV at 10 mA cm^-2 , which is lower than other reported COFs-based electrocatalysts. Moreover, Ru@COF-1 exhibits exceptionally electrocatalytic durability in the acidic electrolytes. It is particularly stable and remains highly active after 1000 cyclic voltammetry cycles. Density functional theory calculations demonstrate that tetracoordinated Ru-N₂ Cl₂ moieties are the major contributors to the outstanding HER performance. This work provides a new idea for developing protonated HER electrocatalysts in acidic media.