Scalable Synthesis and Electrocatalytic Performance of Highly Fluorinated Covalent Organic Frameworks for Oxygen Reduction

Structural Modulation of Covalent Organic Frameworks for Efficient Hydrogen Peroxide Electrocatalysis

The electrochemical production of hydrogen peroxide (H2O2) using metal-free catalysts has emerged as a viable and sustainable alternative to the conventional anthraquinone process. However, the precise architectural design of these electrocatalysts poses a significant challenge, requiring intricate structural engineering to optimize electron transfer during the oxygen reduction reaction (ORR). Herein, we introduce a novel design of covalent organic frameworks (COFs) that effectively shift the ORR from a four-electron to a more advantageous two-electron pathway. Notably, the JUC-660 COF, with strategically charge-modified benzyl moieties, achieved a continuous high H2O2 yield of over 1200 mmol g-1 h-1 for an impressive duration of over 85 hours in a flow cell setting, marking it as one of the most efficient metal-free and non-pyrolyzed H2O2 electrocatalysts reported to date. Theoretical computations alongside in situ infrared spectroscopy indicate that JUC-660 markedly diminishes the adsorption of the OOH* intermediate, thereby steering the ORR towards the desired pathway. Furthermore, the versatility of JUC-660 was demonstrated through its application in the electro-Fenton reaction, where it efficiently and rapidly removed aqueous contaminants. This work delineates a pioneering approach to altering the ORR pathway, ultimately paving the way for the development of highly effective metal-free H2O2 electrocatalysts.

Rational Design of Organic Electrocatalysts for Hydrogen and Oxygen Electrocatalytic Applications

Efficient electrocatalysts are pivotal for advancing green energy conversion technologies. Organic electrocatalysts, as cost-effective alternatives to noble-metal benchmarks, have garnered attention. However, the understanding of the relationships between their properties and electrocatalytic activities remains ambiguous. Plenty of research articles regarding low-cost organic electrocatalysts started to gain momentum in 2010 and have been flourishing recently though, a review article for both entry-level and experienced researchers in this field is still lacking. This review underscores the urgent need to elucidate the structure-activity relationship and design suitable electrode structures, leveraging the unique features of organic electrocatalysts like controllability and compatibility for real-world applications. Organic electrocatalysts are classified into four groups: small molecules, oligomers, polymers, and frameworks, with specific structural and physicochemical properties serving as activity indicators. To unlock the full potential of organic electrocatalysts, five strategies are discussed: integrated structures, surface property modulation, membrane technologies, electrolyte affinity regulation, and addition of anticorrosion species, all aimed at enhancing charge efficiency, mass transfer, and long-term stability during electrocatalytic reactions. The review offers a comprehensive overview of the current state of organic electrocatalysts and their practical applications, bridging the understanding gap and paving the way for future developments of more efficient green energy conversion technologies.

Anchoring covalent organic polymers on supports with tunable functional groups boosting the oxygen reduction performance under pH-universal conditions

Iron phthalocyanine (FePc) is an attractive nonprecious metal candidate for electrocatalytic oxygen reduction reaction (ORR). However, its low catalytic performance under acidic and neutral conditions limits its practical application. Herein, the FePc-based covalent organic polymers (COPFePc) polymerized in situ on the functionalized multiwalled carbon nanotubes (R-MWCNT) containing different electron-withdrawing or electron-donating groups (COPFePc/R-MWCNT, R = COOH, OH or NH2) were synthesized for ORR. Among them, COPFePc/COOH-MWCNT exhibited the best ORR performance under pH-universal conditions (acidic, neutral, and alkaline). Density-functional theory (DFT) calculations demonstrate that the electron-withdrawing or electron-donating effect of the functional groups in COPFePc/R-MWCNT causes charge redistribution of the active center Fe. The COOH functional group with an electron-withdrawing ability shifts the d-band center of Fe away from the Fermi energy level and reduces the binding strength of oxygen-containing intermediates, accelerating the ORR kinetics and optimizing the catalytic activity.

Exploring Advanced Oxygen Reduction Reaction Electrocatalysts: The Potential of Metal-Free and Non-Pyrolyzed Covalent Organic Frameworks

Oxygen reduction reaction (ORR) electrocatalysis is an area of increasing interest for the in-situ production of H2O2 or the development of energy-related devices such as hydrogen fuel cells. Although pyrolyzed catalysts still offer the best performances to date with reference to the organic-based catalysts, metal-free and non-pyrolyzed covalent organic frameworks (COFs) stands out as promising alternatives candidates due to their favourable characteristics such as crystallinity, porosity, and organic composition, allowing the study of structural-property relationships. Herein, we present the design principles and recent advances in COFs-based ORR electrocatalysts, demonstrating how composition influences the activity and electronic pathway of the oxygen reduction process.

Solvent Effects on Metal-free Covalent Organic Frameworks in Oxygen Reduction Reaction

Binding water molecules to polar sites in covalent organic frameworks (COFs) is inevitable, but the corresponding solvent effects in electrocatalytic process have been largely overlooked. Herein, we investigate the solvent effects on COFs for catalyzing the oxygen reduction reaction (ORR). Our designed COFs incorporated different kinds of nitrogen atoms (imine N, pyridine N, and phenazine N), enabling tunable interactions with water molecules. These interactions play a crucial role in modulating electronic states and altering the catalytic centers within the COFs. Among the synthesized COFs, the one with pyridine N atoms exhibits the highest activity, with characterized by a half-wave potential of 0.78 V and a mass activity of 0.32 A mg-1, which surpass those from other metal-free COFs. Theoretical calculations further reveal that the enhanced activity can be attributed to the stronger binding ability of *OOH intermediates to the carbon atoms adjacent to the pyridine N sites. This work sheds light on the significance of considering solvent effects on COFs in electrocatalytic systems, providing valuable insights into their design and optimization for improved performance.