Carbon Derived from Soft Pyrolysis of a Covalent Organic Framework as a Support for Small-Sized RuO2 Showing Exceptionally Low Overpotential for Oxygen Evolution Reaction

Metal-organic framework derived vanadium oxide supported nanoporous carbon structure as a bifunctional electrocatalyst for potential application in metal air batteries

High-efficiency, sustainable, non-precious metal-based electrocatalysts with bifunctional catalytic activity for the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER) are essential for metal-air batteries. In this research, a bifunctional electrocatalyst is developed by synthesizing a novel nanoporous vanadium oxide/carbon composite (NVC-900) through pyrolysis of a highly efficient vanadium metal-organic framework, MIL-101 (V). The fabrication process was conveniently carried out by pyrolyzing the synthesized MIL-101 (V) at 900 °C, producing vanadium oxide nanoparticles embedded in the extensively distributed pores of the carbon network. The evenly distributed nanopores substantially improve the performance of the efficient electrocatalyst for both the oxygen reduction reaction and oxygen evolution reactions (ORR/OER) by increasing surface area and facilitating access to stable catalytic active sites. The unique structure was characterized by powder X-ray diffraction (XRD) and scanning electron microscopy (SEM). For oxygen reduction reaction (ORR), the electrocatalyst established a promising limiting current density (J _L) of 5.2 mA cm^-2 at 1600 rpm at an onset potential of 1.18 V and a half-wave potential of 0.82 V, and for OER, a current density of 10 mA cm^-2 was delivered at a potential of 1.48 V. In comparison to 10% Pt/C, the synthesized bifunctional electrocatalyst being almost equally active towards bifunctional activity, showed much better long-term cyclic stability. The one-step thermal pyrolysis strategy to synthesize the nanoporous functional material and the proposed electrocatalytic material's long-term bifunctional activity and durability make it an ideal fit for next-generation portable green metal-air batteries.

Conductive and Ultrastable Covalent Organic Framework/Carbon Hybrid as an Ideal Electrocatalytic Platform

Developing covalent organic frameworks (COFs) with good electrical conductivity is essential to widen their range of practical applications. Thermal annealing is known to be a facile approach for enhancing conductivity. However, at higher temperatures, most COFs undergo amorphization and/or thermal degradation because of the lack of linker rigidity and physicochemical stability. Here, we report the synthesis of a conductive benzoxazole-linked COF/carbon hybrid material (BCOF-600C) by simple thermal annealing. The fused-aromatic benzoxazole and biphenyl building units endow the resulting COF with excellent physicochemical stability against high temperatures and strong acids/bases. This allows heat treatment to further enhance electrical conductivity with minimal structural alteration. The robust crystalline structure with periodically incorporated nitrogen atoms allowed platinum (Pt) atoms to be atomically integrated into the channel walls of BCOF-600C. The resulting electrocatalyst with well-defined active sites exhibited superior catalytic performance toward hydrogen evolution in acidic media.

Covalent organic framework wrapped by graphene oxide as an efficient sulfur host for high performance lithium-sulfur batteries

The practical application of lithium-sulfur battery is seriously limited by the loss of active substances and the deterioration of cycle stability caused by the 'shuttle effect' of lithium polysulfides (LiPSs). In this work, graphene oxide (GO) coated covalent organic framework (COF) compound materials were synthesized as sulfur host material in spray-drying process. The polar groups on COF can efficiently adsorb LiPSs through lithiophilic interaction, which can reduce the 'shuttle effect' caused by soluble LiPSs. Besides, GO in the outer layer can wrap discrete sulfur to reduce the loss of active substances, which further improves the cycle stability of the cathode. The COF@GO/S cathode exhibits a high initial specific capacity of 848.4 mAh g^-1and retains a capacity of 601.1 mAh g^-1after 500 cycles at 1 C counting with a low capacity fading of 0.058% per cycle.

Confinement synthesis in porous molecule-based materials: a new opportunity for ultrafine nanostructures

A balance between activity and stability is greatly challenging in designing efficient metal nanoparticles (MNPs) for heterogeneous catalysis. Generally, reducing the size of MNPs to the atomic scale can provide high atom utilization, abundant active sites, and special electronic/band structures, for vastly enhancing their catalytic activity. Nevertheless, due to the dramatically increased surface free energy, such ultrafine nanostructures often suffer from severe aggregation and/or structural degradation during synthesis and catalysis, greatly weakening their reactivities, selectivities and stabilities. Porous molecule-based materials (PMMs), mainly including metal-organic frameworks (MOFs), covalent organic frameworks (COFs) and porous organic polymers (POPs) or cages (POCs), exhibit high specific surface areas, high porosity, and tunable molecular confined space, being promising carriers or precursors to construct ultrafine nanostructures. The confinement effects of their nano/sub-nanopores or specific binding sites can not only effectively limit the agglomeration and growth of MNPs during reduction or pyrolysis processes, but also stabilize the resultant ultrafine nanostructures and modulate their electronic structures and stereochemistry in catalysis. In this review, we highlight the latest advancements in the confinement synthesis in PMMs for constructing atomic-scale nanostructures, such as ultrafine MNPs, nanoclusters, and single atoms. Firstly, we illustrated the typical confinement methods for synthesis. Secondly, we discussed different confinement strategies, including PMM-confinement strategy and PMM-confinement pyrolysis strategy, for synthesizing ultrafine nanostructures. Finally, we put forward the challenges and new opportunities for further applications of confinement synthesis in PMMs.

Carbonaceous Oxygen Evolution Reaction Catalysts: From Defect and Doping-Induced Activity over Hybrid Compounds to Ordered Framework Structures

Oxygen evolution reaction (OER) is expected to be of great importance for the future energy conversion and storage in form of hydrogen by water electrolysis. Besides the traditional noble-metal or transition metal oxide-based catalysts, carbonaceous electrocatalysts are of great interest due to their huge structural and compositional variety and unrestricted abundance. This review provides a summary of recent advances in the field of carbon-based OER catalysts ranging from "pure" or unintentionally doped carbon allotropes over heteroatom-doped carbonaceous materials and carbon/transition metal compounds to metal oxide composites where the role of carbon is mainly assigned to be a conductive support. Furthermore, the review discusses the recent developments in the field of ordered carbon framework structures (metal organic framework and covalent organic framework structures) that potentially allow a rational design of heteroatom-doped 3D porous structures with defined composition and spatial arrangement of doping atoms to deepen the understanding on the OER mechanism on carbonaceous structures in the future. Besides introducing the structural and compositional origin of electrochemical activity, the review discusses the mechanism of the catalytic activity of carbonaceous materials, their stability under OER conditions, and potential synergistic effects in combination with metal (or metal oxide) co-catalysts.

Covalent Organic Framework (COF)-Based Hybrids for Electrocatalysis: Recent Advances and Perspectives

Developing highly efficient electrocatalysts for renewable energy conversion and environment purification has long been a research priority in the past 15 years. Covalent organic frameworks (COFs) have emerged as a burgeoning family of organic materials internally connected by covalent bonds and have been explored as promising candidates in electrocatalysis. The reticular geometry of COFs can provide an excellent platform for precise incorporation of the active sites in the framework, and the fine-tuning hierarchical porous architectures can enable efficient accessibility of the active sites and mass transportation. Considerable advances are made in rational design and controllable fabrication of COF-based organic-inorganic hybrids, that containing organic frameworks and inorganic electroactive species to induce novel physicochemical properties, and take advantage of the synergistic effect for targeted electrocatalysis with the hybrid system. Branches of COF-based hybrids containing a diversity form of metals, metal compounds, as well as metal-free carbons have come to the fore as highly promising electrocatalysts. This review aims to provide a systematic and profound understanding of the design principles behind the COF-based hybrids for electrocatalysis applications. Particularly, the structure-activity relationship and the synergistic effects in the COF-based hybrid systems are discussed to shed some light on the future design of next-generation electrocatalysts.

A-Site Cation-Ordering Layered Perovskite EuBa0.5Sr0.5Co2-x Fe x O5+δ as Highly Active and Durable Electrocatalysts for Oxygen Evolution Reaction

The developments of high-performance and tolerant catalysts may enable more sustainable energy in the future, especially toward water oxidation. Herein, we report A-site cation-ordering layered perovskite EuBa_0.5Sr_0.5Co_2-x Fe _x O_5+δ (EBSCFx) (x = 0.2-0.6) electrocatalysts. When evaluated for oxygen evolution reaction (OER) in alkaline media, EuBa_0.5Sr_0.5Co_1.6Fe_0.4O_5+δ (EBSCF0.4) exhibits the best catalytic activity among all of these catalysts, as evidenced by the lowest overpotential of 420 mV at a current density of 10 mA cm^-2. Notably, the catalytic activity of EBSCF0.4 is better than that of commercial IrO₂ at the overpotential >460 mV. Furthermore, the EBSCF0.4-20RuO₂ (involving 20 wt % RuO₂) composite catalyst is developed and gives an overpotential as low as 390 mV at 50 mA cm^-2, which is even superior to commercial RuO₂. For overall water splitting, an electrolysis voltage of merely 1.47 V is achieved at 10 mA cm^-2 in an electrolyzer employing EBSCF0.4-20RuO₂ as bifunctional catalysts, with exceptional durability for 24 h. Such a performance outperforms state-of-the-art IrO₂∥Pt/C and RuO₂∥Pt/C couples. According to density functional theory (DFT) calculations, the unique catalytic properties of EBSCF0.4 may benefit from highly active Fe sites with octahedral coordination, and the synergistic effects of Fe and Ru sites in the composite catalyst accelerate the electrochemical water oxidation.

Spray-Coated Thin-Film Ni-Oxide Nanoflakes as Single Electrocatalysts for Oxygen Evolution and Hydrogen Generation from Water Splitting

Electrochemical water splitting is a key process in many electrochemical energy conversion and storage phenomena. Simple synthesis methods to make highly porous and active nanostructured catalytic materials with large electroactive surface areas are very important to implement water-to-fuel conversion schemes. Herein, ultrafine, transparent thin-film nickel-oxide (NiO x ) nanoflakes are facilely synthesized following a simple spray-coating method from a solution-phase precursor. The NiO x nanoscale structures are grown on the FTO surface in the form of highly uniform smooth thin films. They are employed as promising bifunctional electrocatalysts for the overall water splitting process under alkaline conditions. During water oxidation catalysis, NiO x -SC/FTO initiates the oxygen evolution reaction (OER) at an overpotential η of just 250 mV while generating current decade at just 300 mV and demonstrates well-balanced kinetics toward OER. 10 mA cm-2 current density remains persistent for many hours of continuous electrolysis at just 1.53 VRHE illustrating high robustness of the system. The catalyst also showed substantial activity and durability toward the hydrogen evolution reaction (HER) under the same electrochemical conditions. Tafel slopes of just 57 and 89 mV dec-1 for OER and HER in 0.5 M aqueous KOH solution, respectively, showing high intrinsic kinetics for electrocatalysis. Having high electrochemical surface area and an optimum number of electrochemically active sites, these transparent NiO x thin films can be advantageously combined with photoelectrochemical devices for light-driven water-to-fuel conversion systems.

Bulk COFs and COF nanosheets for electrochemical energy storage and conversion

The current advances, structure-property relationship and future perspectives in covalent organic frameworks (COFs) and their nanosheets for electrochemical energy storage (EES) and conversion (EEC) are summarized.