High-Performance Polyamide Thin-Film Nanocomposite Membranes Containing ZIF-8/CNT Hybrid Nanofillers for Reverse Osmosis Desalination

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

Gas Diffusion through Nanoporous Channels of Graphene Oxide and Reduced Graphene Oxide Membranes

Recently, graphene oxide (GO) has been investigated as a class of molecular filters for selective gas and ion transport. However, detailed transport mechanisms have been poorly understood thus far. Here, we report the gas transport behavior of noninterlocked GO and reduced GO (rGO) membranes, which contain nanoporous gas diffusion channels generated by the adjacent edges of GO and rGO sheets. Both membranes exhibited Knudsen gas diffusion behavior; however, the separation factors of these membranes exceeded the theoretical Knudsen separation factors for gas/CO2 selectivities of various gas mixtures owing to extremely low CO2 permeance. The unique transport features of the low CO2 permeance were explained by the blocking effect of CO2 adsorbed in the nanoporous diffusion channels because of the high CO2 affinity of the edges of GO and rGO sheets. Furthermore, the rGO lamellar structure generally shows impermeable interlayer spacing, indicating that the only gas diffusion channel is the nanopores created by neighboring the edges of the rGO sheets. Notably, both membranes maintained a higher H2/CO2 separation factor than the theoretical Knudsen selectivity, including the measurements of mixed-gas permeation experiments. This study provides insight that further GO modification may improve the gas separation performance suitable for specific separation processes

Measuring the Capacitance of Carbon in Ionic Liquids: From Graphite to Graphene

The physical electrochemistry of the carbon/ionic liquids interface underpins the processes occurring in a vast range of applications spanning electrochemical energy storage, iontronic devices, and lubrication. Elucidating the charge storage mechanisms at the carbon/electrolyte interface will lead to a better understanding of the operational principles of such systems. Herein, we probe the charge stored at the electrochemical double layer formed between model carbon systems, ranging from single-layer graphene to graphite and the ionic liquid 1-ethyl-3-methyl­imidazolium bis(trifluoro­methylsulfonyl)imide (EMIM-TFSI). The effect of the number of graphene layers on the overall capacitance of the interface is investigated. We demonstrate that in pure EMIM-TFSI and at moderate potential biases, the electronic properties of graphene and graphite govern the overall capacitance of the interface, while the electrolyte contribution to the latter is less significant. In mixtures of EMIM-TFSI with solvents of varying relative permittivity, the complex interplay between electrolyte ions and solvent molecules is shown to influence the charge stored at the interface, which under certain conditions overcomes the effects of relative permittivity. This work provides additional experimental insights into the continuously advancing topic of electrochemical double-layer structure at the interface between room temperature ionic liquids and carbon materials

Multilayered Graphene-Coated Metal Current Collectors with High Electrical Conductivity and Corrosion Resistivity for Flow-Electrode Capacitive Mixing

Flow-electrode capacitive mixing (F-CapMix) is a novel technology, harvesting electric energy from salinity gradient power. A continuously circulating flow-electrode enables constant electrical power generation without any intermittent step. Graphite is a widely used current collector because of its high electrical conductivity and corrosion resistivity to seawater for F-CapMix. However, insulating polyacrylate-based polymers should be employed for shaping the graphite body, resulting in reduced electrical conductivity, hindering efficient charge percolation in flow-electrodes. Metal current collectors are difficult to use because of their low corrosion resistance to seawater despite their good electrical conductivity. Here, we first report high electrical conductivity and corrosion-resistive multilayered graphene as a protective layer on metal current collectors in F-CapMix. The graphene is synthesized on metal current collectors through the chemical vapor deposition process. We study chemical and electrochemical aspects of protective layer-coated current collectors by controlling parameters for graphene growth on metal substrates. The multilayered graphene protective layer can not only prevent corrosion from seawater but also enable efficient charge percolation with high electrical conductivity. By employing multilayered graphene-coated Ni current collectors in the F-CapMix unit cell, an almost twice higher power density of 0.75 W·m–2 and current density of 22.3 A·m–2 could be achieved compared to those of graphite

Carbon Defect Characterization of Nitrogen-Doped Reduced Graphene Oxide Electrocatalysts for the Two-Electron Oxygen Reduction Reaction

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 13carbon 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