Numerous modified-carbon catalysts have been developed for the direct synthesis of hydrogen peroxide through electrochemical oxygen reduction. However, given the complex structure of most porous carbons and the poor oxygen reduction reaction (ORR) selectivity typically observed when they are used as catalysts, it is still unclear which carbon defects are responsible for the high two-electron ORR activity typically observed in these materials. Here, we study electrocatalytic peroxide formation activity of nitrogen-doped reduced graphene oxide (N-rGO) materials to relate carbon defects to electrocatalytic activity. To do so, we selected two N-rGO electrodes that selectively produce peroxide at all potentials studied (0.70− 0.10 V vs RHE) under alkaline conditions. Oxygen reduction studies, combined with material characterization, especially solid-state 13 carbon nuclear magnetic resonance coupled with magic angle spinning and cross-polarization, demonstrate that epoxy or ether groups in the N-rGO catalyst are likely associated with the active sites that form peroxide at the lowest overpotential in alkaline media.
Highly permselective and durable membrane materials have been sought for energy-efficient C 3 H 6 /C 3 H 8 separation. Mixed-matrix membranes (MMMs) comprising ap olymer matrix and metal-organic frameworks (MOFs) are promising candidates for this application;h owever,r ational matching of filler-matrix is challenging and their separation performances need to be further improved. Here,w ep ropose an ovel strategy of "defect engineering" in MOFs as an additional degree of freedom to design advanced MMMs. MMMs incorporated with defect-engineered MOFs exhibit exceptionally high C 3 H 6 permeability and maintained C 3 H 6 / C 3 H 8 selectivity,e specially with enhanced stability under industrial mixed-gas conditions.T he gas transport, sorption, and material characterizations reveal that the defect sites in MOFs provide the resulting MMMs with not only ultrafast diffusion pathwaysb ut also favorable C 3 H 6 sorption by forming complexation with unsaturated open metal sites, confirmed by in situ FT-IR studies.M ost importantly,t he concept is also valid for different polymer matrices and gas pairs,d emonstrating its versatile potential in other fields.
Incorporation of defects in metal-organic frameworks (MOFs) offers new opportunities for manipulating their microporosity and functionalities. The so-called "defect engineering" has great potential to tailor the mass transport properties in MOF/polymer mixed matrix membranes (MMMs) for challenging separation applications, for example, CO 2 capture. This study first investigates the impact of MOF defects on the membrane properties of the resultant MOF/polymer MMMs for CO 2 separation. Highly porous defect-engineered UiO-66 nanoparticles are successfully synthesized and incorporated into a CO 2 -philic crosslinked poly(ethylene glycol) diacrylate (PEGDA) matrix. A thorough joint experimental/simulation characterization reveals that defect-engineered UiO-66/PEGDA MMMs exhibit nearly identical filler-matrix interfacial properties regardless of the defect concentrations of their parental UiO-66 filler. In addition, nonequilibrium molecular dynamics simulations in tandem with gas transport studies disclose that the defects in MOFs provide the MMMs with ultrafast transport pathways mainly governed by diffusivity selectivity. Ultimately, MMMs containing the most defective UiO-66 show the most enhanced CO 2 /N 2 separation performance-CO 2 permeability = 470 Barrer (four times higher than pure PEGDA) and maintains CO 2 /N 2 selectivity = 41-which overcomes the trade-off limitation in pure polymers. The results emphasize that defect engineering in MOFs would mark a new milestone for the future development of optimized MMMs.
Thin-film nanocomposite (TFN) membranes incorporating nanofillers in ultrathin and selective polyamide layers have improved desalination performance in conventional reverse osmosis (RO) membranes. However, further enhancement of RO performance in TFN membranes using only a single nanofiller remains challenging due to difficulties in optimizing permselectivity, dispersibility, and chemical stability. To circumvent this limitation, we prepared hybrids of zeolitic imidazole framework-8 (ZIF-8) and carbon nanotubes (CNTs) to exploit the advantages of both filler phases for the development of high-performance TFN RO membranes. The synthesized ZIF-8/CNT hybrids showed continuous and well-distributed ZIF-8 nanocrystals grown on one-dimensional CNT templates. TFN membranes containing ZIF-8/CNT hybrids outperformed those prepared with a single phase both in RO performance and chlorine stability, attributed to a high aspect ratio and microporosity and the radical scavenging effect of oxygen functional groups in CNT templates. The results demonstrate that MOF/carbon hybrid nanofillers can contribute to the rational design of advanced TFN membranes for RO desalination.
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