Xianwen Mao

Electrospun carbon nanofiber webs with controlled density of states for sensor applications.

1 DOI: 10.1002/adma.((please add manuscript number)) Electrospun Carbon Nanofiber Webs with Controlled Density of States for Sensor Applications By Xianwen Mao, Fritz Simeon, Gregory C. Rutledge* and T. Alan Hatton* [*] Prof. T. Alan Hatton, Prof. Gregory C. Rutledge, Xianwen Mao, Dr. Fritz Simeon. Department of Chemical Engineering, Massachusetts Institute of Technology 77 Massachusetts Avenue, Cambridge Massachusetts, 02139, USA E-mail: tahatton@mit.edu, rutledge@mit.edu

A Review of Electrospun Carbon Fibers as Electrode Materials for Energy Storage

The applications of electrospun carbon fiber webs to the development of energy storages devices, including both supercapacitors and lithium ion batteries (LIB), are reviewed. Following a brief discussion of the fabrication process and characterization methods for ultrafine electrospun carbon fibers, recent advances in their performance as supercapacitors and LIBs anode materials are summarized. Optimization of the overall electrochemical properties of these materials through choice of thermal treatment conditions, incorporation of additional active components (such as carbon nanotubes, metal oxides, and catalysts), and generation of novel fibrous structures (such as core-shell, multi-channel or porous fibers) is highlighted. Further challenges related to improving the conductivity, surface area, and mechanical properties of the carbon nanofiber webs, as well as the scale-up ability of the fabrication technique, are discussed.

Continuous Flow Synthesis of Ketones from Carbon Dioxide and Organolithium or Grignard Reagents

We describe an efficient continuous flow synthesis of ketones from CO2 and organolithium or Grignard reagents that exhibits significant advantages over conventional batch conditions in suppressing undesired symmetric ketone and tertiary alcohol byproducts. We observed an unprecedented solvent-dependence of the organolithium reactivity, the key factor in governing selectivity during the flow process. A facile, telescoped three-step-one-flow process for the preparation of ketones in a modular fashion through the in-line generation of organometallic reagents is also established.

Ultra-Wide-Range Electrochemical Sensing Using Continuous Electrospun Carbon Nanofibers with High Densities of States

Carbon-based sensors for wide-range electrochemical detection of redox-active chemical and biological molecules were fabricated by the electrospinning of polyacrylonitrile fibers directly onto a polyacrylonitrile-coated substrate followed by carbonization at 1200 °C. The resulting electrospun carbon nanofibers (ECNFs) were firmly attached to the substrate with good mesh integrity and had high densities of electronic states (DOS), which was achieved without need for further modifications or the use of any additives. The mass of ECNFs deposited, and thus the electroactive surface area (ESA) of the sensor, was adjusted by varying the electrospinning deposition time, thereby enabling the systematic manipulation of the dynamic range of the sensor. A standard redox probe (Fe(CN)6(3-/4-)) was used to demonstrate that the ECNF sensor exhibits strong electrocatalytic activity without current saturation at high analyte concentrations. Dopamine was used as a model analyte to evaluate the sensor performance; we find that the ECNF device exhibits a dynamic range ∼10(5) greater than that of many existing carbon-based sensors. The ECNF sensors exhibited excellent sensitivity, selectivity, stability, and reproducibility for dopamine detection.

Polyvinylferrocene for Noncovalent Dispersion and Redox-Controlled Precipitation of Carbon Nanotubes in Nonaqueous Media

We report noncovalent dispersion of carbon nanotubes (CNTs) in organic liquids with extremely high loading (∼2 mg mL(-1)) using polyvinylferrocene (PVF). In contrast to common dispersants, PVF does not contain any conjugated structures or ionic moieties. PVF is also shown to be effective in controlling nanotube dispersion and reprecipitation because it exhibits redox-switchable affinity for solvents, while maintaining stable physical attachment to CNTs during redox transformation. This switchability provides a novel approach to creating CNT-functionalized surfaces. The material systems described here offer new opportunities for applications of CNTs in nonaqueous media, such as nanotube-polymer composites and organic liquid-based optical limiters, and expand the means of tailoring nanotube dispersion behavior via external stimuli, with potential applications in switching devices. The PVF/CNT hybrid system with enhanced redox response of ferrocene may also find applications in high-performance biosensors and pseudocapacitors.

Electrochemically responsive heterogeneous catalysis for controlling reaction kinetics.

We report a method to control reaction kinetics using electrochemically responsive heterogeneous catalysis (ERHC). An ERHC system should possess a hybrid structure composed of an electron-conducting porous framework coated with redox-switchable catalysts. In contrast to other types of responsive catalysis, ERHC combines all the following desired characteristics for a catalysis control strategy: continuous variation of reaction rates as a function of the magnitude of external stimulus, easy integration into fixed-bed flow reactors, and precise spatial and temporal control of the catalyst activity. Herein we first demonstrate a facile approach to fabricating a model ERHC system that consists of carbon microfibers with conformal redox polymer coating. Second, using a Michael reaction whose kinetics depends on the redox state of the redox polymer catalyst, we show that use of different electrochemical potentials permits continuous adjustment of the reaction rates. The dependence of the reaction rate on the electrochemical potential generally agrees with the Nernstian prediction, with minor discrepancies due to the multilayer nature of the polymer film. Additionally, we show that the ERHC system can be employed to manipulate the shape of the reactant concentration-time profile in a batch reactor through applying customized potential-time programs. Furthermore, we perform COMSOL simulation for an ERHC-integrated flow reactor, demonstrating highly flexible manipulation of reactant concentrations as a function of both location and time.

Microwave assisted synthesis of cyclic carbonates from olefins with sodium bicarbonates as the C1 source

An effective transformation of alkenes into cyclic carbonates has been achieved using NaHCO3 as the C1 source in acetone-water under microwave heating, with selectivities and yields significantly surpassing those obtained using conventional heating.

Energetically efficient electrochemically tunable affinity separation using multicomponent polymeric nanostructures for water treatment

We describe a water treatment strategy, electrochemically tunable affinity separation (ETAS), which, unlike other previously developed electrochemical processes, targets uncharged organic pollutants in water. Key to achieving ETAS resides in the development of multicomponent polymeric nanostructures that simultaneously exhibit the following characteristics: an oxidation-state dependent affinity towards neutral organics, high porosity for sufficient adsorption capacity, and high conductivity to permit electrical manipulation. A prototype ETAS adsorbent composed of nanostructured binary polymeric surfaces that can undergo an electrically-induced hydrophilic–hydrophobic transition can provide programmable control of capture and release of neutral organics in a cyclic fashion. A quantitative energetic analysis of ETAS offers insights into the tradeoff between energy cost and separation extent through manipulation of electrical swing conditions. We also introduce a generalizable materials design approach to improve the separation degree and energetic efficiency simultaneously, and identify the critical factors responsible for such enhancement via redox electrode simulations and theoretical calculations of electron transfer kinetics. The effect of operation mode and multistage configuration on ETAS performance is examined, highlighting the practicality of ETAS and providing useful guidelines for its operation at large scale. The ETAS approach is energetically efficient, environmentally friendly, broadly applicable to a wide range of organic contaminants of various molecular structures, hydrophobicity and functionality, and opens up new avenues for addressing the urgent, global challenge of water purification and wastewater management.