Andong Liu

Desalination by Membrane Distillation using Electrospun Polyamide Fiber Membranes with Surface Fluorination by Chemical Vapor Deposition

Fibrous membranes of poly(trimethyl hexamethylene terephthalamide) (PA6(3)T) were fabricated by electrospinning and rendered hydrophobic by applying a conformal coating of poly(1H,1H,2H,2H-perfluorodecyl acrylate) (PPFDA) using initiated chemical vapor deposition (iCVD). A set of iCVD-treated electrospun PA6(3)T fiber membranes with fiber diameters ranging from 0.25 to 1.8 μm were tested for desalination using the air gap membrane distillation configuration. Permeate fluxes of 2-11 kg/m2/h were observed for temperature differentials of 20-45 °C between the feed stream and condenser plate, with rejections in excess of 99.98%. The liquid entry pressure was observed to increase dramatically, from 15 to 373 kPa with reduction in fiber diameter. Contrary to expectation, for a given feed temperature the permeate flux was observed to increase for membranes of decreasing fiber diameter. The results for permeate flux and salt rejection show that it is possible to construct membranes for membrane distillation even from intrinsically hydrophilic materials after surface modification by iCVD and that the fiber diameter is shown to play an important role on the membrane distillation performance in terms of permeate flux, salt rejection, and liquid entry pressure.

CVD Polymers for Devices and Device Fabrication

Chemical vapor deposition (CVD) polymerization directly synthesizes organic thin films on a substrate from vapor phase reactants. Dielectric, semiconducting, electrically conducting, and ionically conducting CVD polymers have all been readily integrated into devices. The absence of solvent in the CVD process enables the growth of high‐purity layers and avoids the potential of dewetting phenomena, which lead to pinhole defects. By limiting contaminants and defects, ultrathin (<10 nm) CVD polymeric device layers have been fabricated in multiple laboratories. The CVD method is particularly suitable for synthesizing insoluble conductive polymers, layers with high densities of organic functional groups, and robust crosslinked networks. Additionally, CVD polymers are prized for the ability to conformally cover rough surfaces, like those of paper and textile substrates, as well as the complex geometries of micro‐ and nanostructured devices. By employing low processing temperatures, CVD polymerization avoids damaging substrates and underlying device layers. This report discusses the mechanisms of the major CVD polymerization techniques and the recent progress of their applications in devices and device fabrication, with emphasis on initiated CVD (iCVD) and oxidative CVD (oCVD) polymerization.

Monolithic Flexible Supercapacitors Integrated into Single Sheets of Paper and Membrane via Vapor Printing

A novel approach to fabricate supercapacitors (SCs) via vapor printing, specifically oxidative chemical vapor deposition (oCVD), is demonstrated. Compared to stacking multiple layers into a SC, this method enables the monolithic integration of all components into a single-sheet substrate, minimizing the inactive materials and eliminating the possibility of multilayer delamination. Electrodes comprised of pseudocapacitive material, poly(3,4-ethylenedioxythiophene) (PEDOT), are deposited into both sides of a sheet of flexible porous substrate. The film deposition and patterning are achieved in a single step. The oCVD PEDOT penetrates partially into the porous substrate from both surfaces, while leaving the interior of the substrate serving as a separator. Near the surface, the PEDOT coating conforms to the substrates structure without blocking the pores, resembling the substrates intrinsic morphology with high surface area. The porously structured PEDOT coating, paired with in situ ion gel electrolyte synthesis, gives enhanced electrode-electrolyte interfaces. The monolithic device demonstrates high volumetric capacitance (11.3 F cm-3 ), energy density (2.98 mWh cm-3 ), and power density (0.42 W cm-3 ). These outstanding performance metrics are attributed to the large loading of active materials, minimization of inactive materials, and good electrode-electrolyte interfaces. SC arrays can be printed on a single substrate without the use of wire interconnects.

Nanoscale, conformal polysiloxane thin film electrolytes for three-dimensional battery architectures

We report the development of nanoscale (10–40 nm), conformal thin film electrolytes realized by doping lithium ions (Li+) into poly-(tetravinyltetramethylcyclotetrasiloxane) (PV4D4) films, which were synthesized by initiated chemical vapor deposition (iCVD). This is the first time nanoscale films with siloxane ring moieties, which are excellent electrical insulators, have been demonstrated as room temperature ionic conductors. The films exhibit minimal changes in morphology and thickness during lithiation and are also demonstrated to be easily scalable over large areas. We show that the conformal nature of the iCVD polymerization process realizes complete coverage of nanostructured electrodes like nanowires by a uniform, continuous, and pinhole-free thin film, making the polysiloxane films attractive as a novel class of nanoscale electrolytes for the emerging field of three-dimensional (3D) batteries.

Recent progress on submicron gas-selective polymeric membranes

Polymeric membranes have been applied in industrial gas separations for decades. Competing technologies, such as cryogenic distillation and sorption processes, require the gases to be either condensed or thermally regenerated from the sorbents. In contrast, membrane gas separation does not involve phase transition, representing the potential for a more energy efficient and eco-friendly separation process. However, the overall energy consumption by membrane gas separation is highly dependent on the quality of the membrane employed for the separation process. With the goal of reducing the energy input needed for creating the transmembrane pressure difference, numerous bulk polymers have been investigated. However, less effort has been devoted to processing polymers into ultrathin membranes and investigating their gas permeation properties, which can be quite different from their bulk counterparts. This review summarizes recent advances in fabricating ultrathin gas-selective polymeric membranes. Several classes of ultrathin polymeric membranes are highlighted: microporous polymers, facilitated transport polymeric membranes, Langmuir–Blodgett (LB) films and Layer-by-Layer (LbL) deposited polyelectrolyte multilayers (PEMs), polyamides and other commercial polymers. The application of gas-selective polymeric membranes beyond gas separation is also included as a meaningful extension to this review.

iCVD Cyclic Polysiloxane and Polysilazane as Nanoscale Thin-Film Electrolyte: Synthesis and Properties.

A group of crosslinked cyclic siloxane (Si-O) and silazane (Si-N) polymers are synthesized via solvent-free initiated chemical vapor deposition (iCVD). Notably, this is the first report of cyclic polysilazanes synthesized via the gas-phase iCVD method. The deposited nanoscale thin films are thermally stable and chemically inert. By iCVD, they can uniformly and conformally cover nonplanar surfaces having complex geometry. Although polysiloxanes are traditionally utilized as dielectric materials and insulators, our research shows these cyclic organosilicon polymers can conduct lithium ions (Li(+) ) at room temperature. The conformal coating and the room temperature ionic conductivity make these cyclic organosilicon polymers attractive for use as thin-film electrolytes in solid-state batteries. Also, their synthesis process and properties have been systemically studied and discussed.