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Samuel, Edmund,Joshi, Bhavana,Jo, Hong Seok,Kim, Yong Il,Swihart, Mark T.,Yun, Je Moon,Kim, Kwang Ho,Yoon, Sam S. ELSEVIER SCIENCE 2017 JOURNAL OF ALLOYS AND COMPOUNDS Vol.728 No.-
<P><B>Abstract</B></P> <P>We demonstrate the fabrication of core-shell SnO<SUB> <I>x</I> </SUB>/carbon nanofiber (CNF) composite mats via single-nozzle one-step electrospinning for use as flexible freestanding electrodes in supercapacitors. The freestanding and flexible nature of the composites is essential for their use in lightweight, portable, and foldable electronic devices and eliminates the need for a separate current collector. We fully characterized the structural and morphological properties of the SnO<SUB> <I>x</I> </SUB>/CNF mats and optimized the SnO<SUB> <I>x</I> </SUB> to CNF precursor ratio. The optimized SnO<SUB> <I>x</I> </SUB>/CNF-based symmetric supercapacitor exhibited a capacitance of 289 F·g<SUP>−1</SUP> at a scan rate of 10 mV·s<SUP>−1</SUP>. Moreover, it retained more than 88% of its initial capacitance after 5000 cycles, highlighting the long-term stability of supercapacitors based on these SnO<SUB> <I>x</I> </SUB>/CNF mats.</P> <P><B>Highlights</B></P> <P> <UL> <LI> A core-shell-structured SnO<SUB>x</SUB>/CNF composite mat electrodes were synthesized by single-nozzle-electrospinning. </LI> <LI> The core-shell composite is highly flexible and freestanding. </LI> <LI> Capacitors using these electrodes had specific capacitance of up to 289 F·g<SUP>−1</SUP> at a scan rate of 10 mV·s<SUP>−1</SUP>. </LI> <LI> Specific capacitance at a current density of 100 mA·g<SUP>−1</SUP> reached 273 F·g<SUP>−1</SUP>. </LI> </UL> </P>
Samuel, Edmund,Joshi, Bhavana,Jo, Hong Seok,Kim, Yong Il,An, Seongpil,Swihart, Mark T.,Yun, Je Moon,Kim, Kwang Ho,Yoon, Sam S. Elsevier 2017 Chemical Engineering Journal Vol. No.
<P><B>Abstract</B></P> <P>We have produced flexible, freestanding, and light weight mats of FeO<I> <SUB>x</SUB> </I>-decorated carbon nanofibers (CNFs) and demonstrated their use in supercapacitors with high energy and power density and excellent long term capacitance retention. Highly flexible carbon-iron oxide nanofibers were synthesized by electrospinning a solution of polyacrylonitrile (PAN), polymethylmethacrylate (PMMA), and iron acetylacetonate (FeAcAc), followed by annealing to carbonize the PAN, pyrolyze the PMMA to produce pores, and convert FeAcAc to FeO nanoparticles. The morphology of the FeO<I> <SUB>x</SUB> </I>/CNF composite was determined by scanning and transmission electron microscopies, which showed that the embedded FeO<I> <SUB>x</SUB> </I> nanoparticles were well distributed in the CNF electrode. We employed cyclic voltammetry, galvanostatic charge/discharge measurements, and electrochemical impedance spectroscopy to evaluate the electrochemical performance of symmetric supercapacitors prepared from the FeO<I> <SUB>x</SUB> </I>/CNF composite. The supercapacitors exhibited high specific capacitance (427F·g<SUP>−1</SUP> at 10mV·s<SUP>−1</SUP> and 436F·g<SUP>−1</SUP> at 1A·g<SUP>−1</SUP> in the optimal case) and good stability, retaining 89% of their initial capacitance after 5000 cycles at a current density of 1A·g<SUP>−1</SUP>. The optimal device achieved an energy density of 167Wh·kg<SUP>−1</SUP> at a power density of 0.75kW·kg<SUP>−1</SUP>, and an energy density of 66Wh·kg<SUP>−1</SUP> at a power density of 7.5kW·kg<SUP>−1</SUP>. These combinations of energy and power densities can meet the needs of many emerging supercapacitor applications.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Highly flexible FeO<SUB>x</SUB>-carbon nanocomposite nanofibers were fabricated. </LI> <LI> Freestanding FeO<I> <SUB>x</SUB> </I>-CNF showed excellent retention (89%) after 5000 cycles at 1A·g<SUP>−1</SUP>. </LI> <LI> A FeO<SUB>x</SUB>/CNF-based supercapacitor provides 436F·g<SUP>−1</SUP> of capacitance at 1A·g<SUP>−1</SUP>. </LI> <LI> Excellent uniform decoration of CNF with FeO<SUB>x</SUB> was demonstrated. </LI> </UL> </P>
Samuel, Edmund,Jo, Hong Seok,Joshi, Bhavana,Park, Hyun Goo,Kim, Yong Il,An, Seongpil,Swihart, Mark T.,Yun, Je Moon,Kim, Kwang Ho,Yoon, Sam S. Elsevier 2017 APPLIED SURFACE SCIENCE - Vol.423 No.-
<P><B>Abstract</B></P> <P>We demonstrate the fabrication of a MnO<I> <SUB>x</SUB> </I>/carbamide carbon nanofiber (CCNF) composite consisting of MnO particles embedded in CCNFs as a highly flexible and freestanding electrode material for supercapacitors. A sacrificial polymer component, polymethylmethacrylate, included in the precursor solution, pyrolyzes during heating, resulting in pores in the fibers, some of which are filled by the MnO nanocrystals. Carbamide is added to control the size of the MnO<I> <SUB>x</SUB> </I> particles as well as to increase the carbon content of the composite and hence its conductivity. The X-ray diffraction and Raman spectra of the composite show that the MnO particles formed have low crystallinity. Transmission electron microscopy confirms that the MnO particles are distributed very uniformly over the CCNFs. Symmetric supercapacitors constructed using electrodes of this composite exhibit specific capacitances of 498F∙g<SUP>−1</SUP> at a scan rate of 10mV∙s<SUP>−1</SUP> and 271F∙g<SUP>−1</SUP> at a current density of 1A∙g<SUP>−1</SUP>. They also exhibit excellent long-term cycling performance, retaining 93% of their initial capacity after 5000 cycles of galvanostatic charging/discharging.</P> <P><B>Highlights</B></P> <P> <UL> <LI> We successfully fabricated a novel flexible MnO/CCNF composite. </LI> <LI> Flexible MnO/CCNF showed excellent retention (93%) after 5000 cycles at 1Ag<SUP>−1</SUP>. </LI> <LI> MnO/CCNF composite demonstrated specific capacitance of 498Fg<SUP>−1</SUP> at a scan rate of 10mVs<SUP>−1</SUP>. </LI> <LI> Dynamic MnO particle formation controlled by using carbamide. </LI> </UL> </P>
Experimental Splash Studies of Monodisperse Sprays Impacting Variously Shaped Surfaces
Yoon, Sam S.,Kim, Ho Y.,Lee, Dongjo,Kim, Namsoo,Jepsen, Richard A.,James, Scott C. Taylor Francis 2009 Drying technology Vol.27 No.2
<P> Despite numerous studies of the drop impact phenomena, studies of the fundamental mechanisms of how the splash corona and subsequent necking yield splashed droplets, not to mention characteristics of these splashed droplets, remain a subject of great interest. Here, we consider a simple question: After impact, what are the characteristics of splashed droplets? Spatial variations in the fraction of splashed liquid, Sauter mean diameter, and drop-size distribution for water and diesel impacting onto variously shaped rods are reported. Liquid drops of nearly uniform size are continuously injected onto a 2-mm-diameter aluminum cylindrical rod at velocities of up to 17 m/s. The impact face of the rod is flat with angles from &thgr; = 0 to 60° or it has a concave, convex, or conical shape. The experimental results indicate that diesel breaks up more easily than water due to its low surface tension. However, due to increased energy loss through viscous dissipation during drop collapse and spreading, dispersion of diesel drops upon and after impact is less energetic than that of water since diesel droplets do not travel as fast or as far as water droplets. During corona formation, stretching and necking of diesel drops before their snap-off are particularly evident due to diesel's high viscosity. Size distribution of splashed diesel droplets is more uniform than that of water near the impact region and water is more uniform further away.</P>
Yoon, Hyun,Na, Seung-Heon,Choi, Jae-Young,Latthe, Sanjay S.,Swihart, Mark T.,Al-Deyab, Salem S.,Yoon, Sam S. American Chemical Society 2014 Langmuir Vol.30 No.39
<P>We prepared a simple, low-cost membrane suitable for gravity-driven oil–water separation and water purification. Composite membranes with selective wettability were fabricated from a mixture of aqueous poly(diallyldimethylammonium chloride) solution, sodium perfluorooctanoate, and silica nanoparticles. Simply dip-coating a stainless steel mesh using this mixture produced the oil–water separator. The contact angles (CAs) of hexadecane and water on the prepared composite membranes were 95 ± 2° and 0°, respectively, showing the oleophobicity and superhydrophilicity of the membrane. In addition, a graphene plug was stacked below the membrane to remove water-soluble organics by adsorption. As a result, this multifunctional device not only separates hexadecane from water, but also purifies water by the permeation of the separated water through the graphene plug. Here, methylene blue (MB) was removed as a demonstration. Membranes were characterized by high-resolution scanning electron microscopy (HRSEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and Fourier transform infrared (FT-IR) spectroscopy to elucidate the origin of their selective wettability.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/langd5/2014/langd5.2014.30.issue-39/la5031526/production/images/medium/la-2014-031526_0015.gif'></P><P><A href='http://pubs.acs.org/doi/suppl/10.1021/la5031526'>ACS Electronic Supporting Info</A></P>
CuInSe<sub>2</sub>(CIS) Thin Film Solar Cells by Electrostatic Spray Deposition
Yoon, Hyun,Woo, Ji Hoon,Joshi, Bhavana,Ra, Young Min,Yoon, Sam S.,Kim, Ho Young,Ahn, Se Jin,Yun, Jae Ho,Gwak, Jihye,Yoon, KyungHoon,James, Scott C. The Electrochemical Society 2012 Journal of the Electrochemical Society Vol.159 No.4
Yoon, Hyun,Mali, Mukund G.,Choi, Jae Young,Kim, Min-woo,Choi, Sung Kyu,Park, Hyunwoong,Al-Deyab, Salem S.,Swihart, Mark T.,Yarin, Alexander L.,Yoon, Sam S. American Chemical Society 2015 Langmuir Vol.31 No.12
<P>We demonstrate, for the first time, electrostatically sprayed bismuth vanadate (BiVO<SUB>4</SUB>) thin films for photoelectrochemical water splitting. Characterization of these films by X-ray diffraction, Raman scattering, and high-resolution scanning electron microscopy analyses revealed the formation of nanotextured pillar-like structures of highly photoactive monoclinic scheelite BiVO<SUB>4</SUB>. Electrosprayed BiVO<SUB>4</SUB> nanostructured films yielded a photocurrent density of 1.30 and 1.95 mA/cm<SUP>2</SUP> for water and sulfite oxidation, respectively, under 100 mW/cm<SUP>2</SUP> illumination. The optimal film thickness was 3 μm, with an optimal postannealing temperature of 550 °C. The enhanced photocurrent is facilitated by formation of pillar-like structures in the deposit. We show through modeling that these structures result from the electrically-driven motion of submicron particles in the direction parallel to the substrate, as they approach the substrate, along with Brownian diffusion. At the same time, opposing thermophoretic forces slow their approach to the surface. The model of these processes proposed here is in good agreement with the experimental observations.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/langd5/2015/langd5.2015.31.issue-12/acs.langmuir.5b00486/production/images/medium/la-2015-00486d_0008.gif'></P><P><A href='http://pubs.acs.org/doi/suppl/10.1021/la5b00486'>ACS Electronic Supporting Info</A></P>