Jian Xue

Water Transport with Ultralow Friction through Partially Exfoliated g-C3N4 Nanosheet Membranes with Self-Supporting Spacers

Two-dimensional (2D) graphitic carbon nitride (g-C3 N4 ) nanosheets show brilliant application potential in numerous fields. Herein, a membrane with artificial nanopores and self-supporting spacers was fabricated by assembly of 2D g-C3 N4 nanosheets in a stack with elaborate structures. In water purification the g-C3 N4 membrane shows a better separation performance than commercial membranes. The g-C3 N4 membrane has a water permeance of 29 L m-2  h-1  bar-1 and a rejection rate of 87 % for 3 nm molecules with a membrane thickness of 160 nm. The artificial nanopores in the g-C3 N4 nanosheets and the spacers between the partially exfoliated g-C3 N4 nanosheets provide nanochannels for water transport while bigger molecules are retained. The self-supported nanochannels in the g-C3 N4 membrane are very stable and rigid enough to resist environmental challenges, such as changes to pH and pressure conditions. Permeation experiments and molecular dynamics simulations indicate that a novel nanofluidics phenomenon takes place, whereby water transport through the g-C3 N4 nanosheet membrane occurs with ultralow friction. The findings provide new understanding of fluidics in nanochannels and illuminate a fabrication method by which rigid nanochannels may be obtained for applications in complex or harsh environments.

A new CO2-resistant Ruddlesden–Popper oxide with superior oxygen transport: A-site deficient (Pr0.9La0.1)1.9(Ni0.74Cu0.21Ga0.05)O4+δ

A-site deficient (Pr0.9La0.1)1.9Ni0.74Cu0.21Ga0.05O4+d ((PL)1.9NCG), with the K2NiF4 structure, is found to exhibit higher oxygen transport rates compared with its cation-stoichiometric parent phase. A stable oxygen permeation flux of 4.6 10 7 mol cm 2 s 1 at 900 C at a membrane thickness of 0.6 mm is measured, using either helium or pure CO2 as sweep gas at a flow rate of 30 mL min 1. The oxygen flux is more than two times higher than that observed through A-site stoichiometric (PL)2.0NCG membranes operated under similar conditions. The high oxygen transport rates found for (PL)1.9NCG are attributed to highly mobile oxygen vacancies, compensating A-site deficiency. The high stability against carbonation gives (PL)1.9NCG potential for use, e.g., as a membrane in oxy-fuel combustion processes with CO2 capture.

A Dual-Phase Ceramic Membrane with Extremely High H2 Permeation Flux Prepared by Autoseparation of a Ceramic Precursor

A novel concept for the preparation of multiphase composite ceramics based on demixing of a single ceramic precursor has been developed and used for the synthesis of a dual-phase H2 -permeable ceramic membrane. The precursor BaCe0.5 Fe0.5 O3-δ decomposes on calcination at 1370 °C for 10 h into two thermodynamically stable oxides with perovskite structures: the cerium-rich oxide BaCe0.85 Fe0.15 O3-δ (BCF8515) and the iron-rich oxide BaCe0.15 Fe0.85 O3-δ (BCF1585), 50 mol % each. In the resulting dual-phase material, the orthorhombic perovskite BCF8515 acts as the main proton conductor and the cubic perovskite BCF1585 as the main electron conductor. The dual-phase membrane shows an extremely high H2 permeation flux of 0.76 mL min(-1)  cm(-2) at 950 °C with 1.0 mm thickness. This auto-demixing concept should be applicable to the synthesis of other ionic-electronic conducting ceramics.

Tuning the separation performance of hydrogen permeable membranes using an anion doping strategy

Overcoming the dilemma between hydrogen permeability and stability is critical for realizing the widespread application of mixed protonic–electronic conducting (MPEC) membranes. Herein, fluoride-anion doping is for the first time reported for tuning the separation performance of MPEC membranes. Lanthanum tungstate oxyfluoride membranes, La5.5W0.6Mo0.4O11.25−δFx (x = 0, 0.025, 0.05, 0.10, 0.20, 0.50), exhibit improved hydrogen permeability and enhanced stability compared to their parent oxides, achieving a maximum value of 0.20 mL min−1 cm−2 at x = 0.05. Moreover, the declining hydrogen permeability performance of lanthanum tungstate MPEC membranes during high-temperature operation was systematically analyzed and relative solutions are put forward. The anion-doping and stability-improving strategies might accelerate the development and future practical applications of MPEC membranes.

Paralyzed membrane: Current-driven synthesis of a metal-organic framework with sharpened propene/propane separation

ZIF-8 membranes with inborn-suppressed linker mobility sharpen molecular sieving with C3H6/C3H8 separation factor above 300. Metal-organic framework (MOF) membranes show great promise for propene/propane separation, yet a sharp molecular sieving has not been achieved due to their inherent linker mobility. Here, zeolitic imidazolate framework ZIF-8–type membranes with suppressed linker mobility are prepared by a fast current–driven synthesis (FCDS) strategy within 20 min, showing sharpened molecular sieving for propene/propane separation with a separation factor above 300. During membrane synthesis, the direct current promotes the metal ions and ligands to assemble into inborn-distorted and stiffer frameworks with ZIF-8_Cm (a newly discovered polymorph of ZIF-8) accounting for 60 to 70% of the membrane composition. Molecular dynamics simulations further verify that ZIF-8_Cm is superior to ZIF-8_I 4¯3m (the common cubic phase) for propene/propane separation. FCDS holds great potential to produce high-quality, ultrathin MOF membranes on a large scale.

Molybdenum Carbide Nanodots Enable Efficient Electrocatalytic Nitrogen Fixation under Ambient Conditions

Electrocatalytic nitrogen fixation is considered a promising approach to achieve NH3 production. However, due to the chemical inertness of nitrogen, it is necessary to develop efficient catalysts to facilitate the process of nitrogen reduction. Here, molybdenum carbide nanodots embedded in ultrathin carbon nanosheets (Mo2 C/C) are developed to serve as a catalyst candidate for highly efficient and robust N2 fixation through an electrocatalytic nitrogen reduction reaction (NRR). The as-synthesized Mo2 C/C nanosheets show excellent catalytic performance with a high NH3 yield rate (11.3 µg h-1 mg-1 Mo2C ) and Faradic efficiency (7.8%) for NRR under ambient conditions. More importantly, the isotopic experiments using 15 N2 as a nitrogen source confirm that the synthesized ammonia is derived from the direct supply of nitrogen. This result also demonstrates the possibility of high-efficiency nitrogen reduction even though accompanied with vigorous hydrogen evolution.