Simultaneous Suppression of Metal Corrosion and Electrolyte Decomposition by Graphene Oxide Protective Coating in Magnesium-Ion Batteries: Toward a 4-V-Wide Potential Window

Prabakar, S. J. Richard,Park, Chunguk,Ikhe, Amol Bhairuba,Sohn, Kee-Sun,Pyo, Myoungho American Chemical Society 2017 ACS APPLIED MATERIALS & INTERFACES Vol.9 No.50

<P>Despite remarkable developments in electrolyte systems over the past decades, magnesium-ion batteries still suffer from corrosion susceptibility and low anodic limits. Herein we describe how graphene oxide (GO), coated onto non-noble metals (Al, Cu, and stainless steel) via- electrophoretic deposition, can solve this problem. In all phenyl complex electrolytes, GO coating results in a significant suppression of corrosion and extends the anodic limits' (up to 4.0 V vs Mg/Mg2+) with no impact on reversible Mg plating/stripping reactions. The same effect of GO coating is also established in magnesium aluminum chloride complex electrolytes. This remarkable-improvement is associated with the electtostatic interaction between the ionic charges of electrolytes and the-surface-functional groups of GO: In addition, GO coating, does not aggravate the cathode performance of Mo6S8, which allows the use of non-noble metals as current collectors. We also discuss, the role of GO in. increasing anodic limits when it is hybridized with alpha-MnO2.</P>

Enhancement in High-Rate Performance of Graphite Anodes via Interface Modification Utilizing Ca(BF₄)₂ as an Electrolyte Additive in Lithium Ion Batteries

Prabakar, S. J. Richard,Sohn, Kee-Sun,Pyo, Myoungho The Electrochemical Society 2019 Journal of the Electrochemical Society Vol.166 No.4

<P>Herein, we report the use of inorganic calcium tetrafluoroborate (Ca(BF<SUB>4</SUB>)<SUB>2</SUB>) as an electrolyte additive to achieve a significant improvement in the reversible capacities of graphite anodes at high rates for lithium ion batteries (LIBs). Even in a low concentration (0.01 M), the addition of Ca(BF<SUB>4</SUB>)<SUB>2</SUB> distinctively modifies the composition of a solid-electrolyte-interface (SEI) film on a graphite surface. While maintaining solid-state diffusivity, the SEI film modified by Ca(BF<SUB>4</SUB>)<SUB>2</SUB> enables facile Li<SUP>+</SUP> transport, which leads to a noticeable decrease in over-potentials for Li<SUP>+</SUP> intercalation/de-intercalation and finally to a remarkable increase in reversible capacities at high rates. For example, the capacity enhancement by ca. 80% can be achieved by adding Ca(BF<SUB>4</SUB>)<SUB>2</SUB> at 0.3 A⋅g<SUP>−1</SUP> (346 vs. 192 mAhg<SUP>−1</SUP>). The Ca(BF<SUB>4</SUB>)<SUB>2</SUB> additive effect is not graphite-morphology dependent. This beneficial effect of Ca(BF<SUB>4</SUB>)<SUB>2</SUB> is ascribed to a synergistic combination of Ca<SUP>2+</SUP> cations and BF<SUB>4</SUB><SUP>−</SUP> anions. Ca<SUP>2+</SUP> salts with either different anionic species (Ca(TFSI)<SUB>2</SUB> and Ca(ClO<SUB>4</SUB>)<SUB>2</SUB>) or different cationic species (LiBF<SUB>4</SUB>) also show a certain improvement in capacities, but not as significant as that of Ca(BF<SUB>4</SUB>)<SUB>2</SUB>. We hope that the formation of a robust and more ionically conductive SEI layer via this simple addition of Ca(BF<SUB>4</SUB>)<SUB>2</SUB> will be useful in designing fast-charging LIBs.</P>

Graphite oxide as an efficient and robust support for Pt nanoparticles in electrocatalytic methanol oxidation

Prabakar, S.J.R.,Kim, Y.,Jeong, J.,Jeong, S.,Lah, M.S.,Pyo, M. Pergamon Press 2016 ELECTROCHIMICA ACTA Vol.188 No.-

Pt nanoparticles were encapsulated in graphite oxide (GTO) and used as electrocatalysts for methanol oxidation. When compared with Pt nanoparticles composited with graphene oxide (GO/Pt), Pt nanoparticles sandwiched between the graphene layers of a GTO framework (GTO/Pt) showed significantly improved electrocatalytic performance and stability for methanol oxidation. The simultaneous reduction of Pt<SUP>4+</SUP> and oxygen moieties of GTO during the synthesis of GTO/Pt resulted in a composite with moderately stacked graphene layers. This structure enhanced the kinetic feasibility of methanol oxidation with no decrease in the active surface area of Pt nanoparticles, and also increased the tolerance for the adsorption of surface-adsorbed intermediates. In addition, in contrast to the GO/Pt composite, the Pt nanoparticles in GTO/Pt did not aggregate during the electrochemical reaction, which was responsible for the superior long-term stability of the GTO/Pt. Thermal reduction of GTO/Pt (rGTO/Pt) resulted in rather inferior electrocatalytic responses due to a decrease in the accessible surface area of Pt nanoparticles in the densely stacked graphene layers. Therefore, other than a synthetic convenience (GTO does not require a mechanical exfoliation step.), this work suggests that the use of GTO rather than GO as a support could be a more efficient tactic in preparing graphene composites when the electrochemical reaction on the surface of impregnated nanoparticles is important.

Spontaneous Formation of Interwoven Porous Channels in Hard-Wood-Based Hard-Carbon for High-Performance Anodes in Potassium-Ion Batteries

Prabakar, S. J. Richard,Han, Su Cheol,Park, Chunguk,Bhairuba, Ikhe Amol,Reece, Michael J.,Sohn, Kee-Sun,Pyo, Myoungho Electrochemical Society 2017 Journal of the Electrochemical Society Vol.164 No.9

<P>For the first time we report that hard-wood (oak) can spontaneously create interconnected channels of mu m to nm in diameter during carbonization at an optimized temperature (1100 degrees C). These microstructural features have never been found in other hardcarbons without the use of additives. When compared with sucrose-based hard-carbon (SHC), oak-based hard-carbon (OHC) shows much higher charge-storage capability (ca. 223 vs. 112 mAh.g(-1) t 20 mA.g(-1)) and excellent stability (fading rate of 0.04 vs. 0.08% .cycle(-1)) as an anode in potassium-ion batteries. The high performance of OHC mainly results from interwoven nanoporous channels, which lead to facile charge transfer and fast K+-diffusion. (C)17 The Electrochemical Society.</P>

Graphene-Sandwiched LiNi_0.5Mn_1.5O₄ Cathode Composites for Enhanced High Voltage Performance in Li Ion Batteries

Prabakar, S. J. Richard,Hwang, Yun-Hwa,Lee, Bichna,Sohn, Kee-Sun,Pyo, Myoungho The Electrochemical Society 2013 Journal of the Electrochemical Society Vol.160 No.6

<P>Graphene was used to form efficient conducting networks and to suppress the formation of a solid-electrolyte interface layer, in a sandwiched composite with LiNi<SUB>0.5</SUB>Mn<SUB>1.5</SUB>O<SUB>4</SUB> (LNMO), which resulted in an improvement in rate capability and coulombic efficiency, respectively. The interaction between the LNMO particles and the residual oxygen functionalities on the basal plane of graphene in solution spontaneously produced LNMO particles that were sandwiched by the graphene. This configuration provided LNMO with less polarization due to higher electronic conductivity, which contributed to an increased rate capability. The graphene-sandwiched LNMO also enhanced a coulombic efficiency due to the suppression of continuous electrolyte decomposition, which eventually resulted in improved capacity retention. Unlike this graphene-sandwiched LNMO, a simple mixture of LNMO with graphene did not show noticeable improvements in rate capability due to a lack of efficient conducting network. A LiMn<SUB>2</SUB>O<SUB>4</SUB>/graphene composite prepared using the same procedure showed no improvement in coulombic efficiency and, thus, capacity retention was similar to graphene-free LiMn<SUB>2</SUB>O<SUB>4</SUB>, indicating that the graphene-sandwiching is an effective strategy for high-voltage cathode materials.</P>

SnO₂/Graphene Composites with Self‐Assembled Alternating Oxide and Amine Layers for High Li‐Storage and Excellent Stability

Prabakar, S. J. Richard,Hwang, Yun‐,Hwa,Bae, Eun‐,Gyoung,Shim, Sangdeok,Kim, Dongwook,Lah, Myoung Soo,Sohn, Kee‐,Sun,Pyo, Myoungho WILEY‐VCH Verlag 2013 Advanced Materials Vol.25 No.24

<P><B>An alternating stack (SG/GN) consisting of SnO<SUB>2</SUB>‐functionalized graphene oxide (SG) and amine‐functionalized GO (GN) is prepared in solution</B>. The thermally reduced SG/GN (r(SG/GN)) with decreased micro‐mesopores and completely eliminated macropores, results in a high reversible capacity and excellent capacity retention (872 mA h g<SUP>−1</SUP> after 200 cycles at 100 mA g<SUP>−1</SUP>) when compared to a composite without GN.</P>

SENSITIVIY OF A NANOCANTILEVER-BASED MASS SENSOR

K. PRABAKAR,J. JAYAPANDIAN,A. K. TYAGI,C. S. SUNDAR,BALDEV RAJ 성균관대학교(자연과학캠퍼스) 성균나노과학기술원 2010 NANO Vol.5 No.1

In the present work, mass and position sensitivity of a nanocantilever is simulated using finite element analysis. Changes in resonant frequency of various modes of a polysilicon cantilever, with a gold coating of different thickness, area, and spatial distribution is simulated. It is found that for a uniform increase in gold coating thickness, torsional mode gives an order higher mass sensitivity than vertical bending and lateral bending modes. In other cases, sensitivity is highly dependent on the position of the coating and is explained on the basis of flexural rigidity. Apart from this, simulations were performed for a point mass (gold cube) loading at various positions along the length and width axis of the cantilever. Further, for localized mass loading, cantilever was tailored for enhanced sensitivity. It is found that by introducing a step discontinuity in thickness of the cantilever, the mass sensitivity increases by two orders of magnitude and it depends on the position of step from free end, step thickness, and step width.

Enhanced Electrochemical Stability of Graphite Anodes via Adsorption of Reductively Polymerizable Dibromothiophene in Lithium Ion Batteries

Richard Prabakar, S. J.,Jeong, Jaehyang,Kwak, Joon Seop,Pyo, Myoungho The Electrochemical Society 2014 Journal of the Electrochemical Society Vol.161 No.6

<P>This is the first report showing the improvement of electrochemical stability of a graphite (Gt) anode in lithium ion batteries (LIBs) by using 2,5-dibromothiophene (DBT) as a surface-adsorbent. DBT adsorbed onto Gt underwent reductive electro-polymerization during the first discharge (Li<SUP>+</SUP> intercalation) to form a protective cap over catalytically active sites of Gt. Poly-DBT, thus formed, helped the formation of a compact and thin SEI layer during subsequent Li<SUP>+</SUP> ion intercalation, preventing the continuous formation of SEI layers. The cyclic performance of a half-cell with DBT-adsorbed Gt was compared with half-cells composed of various anode materials (i.e., mere Gt, pyrrole-adsorbed Gt, and thiophene-adsorbed Gt). The adsorption of small amounts of DBT on Gt improved the capacity retention of the half-cells by nearly 7.5% and decreased a fading rate to 0.09 mAh·g<SUP>−1</SUP>·cycle<SUP>−1</SUP>, relative to mere Gt which showed a fading rate of 0.17 mAh·g<SUP>−1</SUP>·cycle<SUP>−1</SUP>. The dissolution of DBT in electrolyte, however, had detrimental effects on cell performance, suggesting that surface-adsorption of DBT is more beneficial. The formation of a compact and thin SEI layer on DBT-adsorbed Gt was confirmed by surface-composition analysis and impedance spectroscopy.</P>

CoSn(OH)₆ hybridized with anionic and cationic graphenes as a new high-capacity anode for lithium ion batteries

Richard Prabakar, S.J.,Han, S.C.,Jeong, J.,Sohn, K.S.,Pyo, M. Elsevier Ltd 2017 Materials & Design Vol.118 No.-

<P>Herein is the first use of a CoSn(OH)(6)/graphene composites as a high-performance anode in lithium ion batteries (LIBs). CoSn(OH)(6) alone is found to be electrochemically active and to possess electrochemical properties that are superior to those displayed in its dehydrated form (CoSnO3). In contrast to the formation of 3Li(2)O in CoSnO3 during the first discharge (CoSnO3 + 6Lit(+) 6e(-) -> Co + Sn + 3Li(2)O), the production of 6 mol of LiOH in CoSn(OH)(6) (CoSn(OH)(6) + 6Li(+) + 6e(-) -> Co + Sn + 6LiOH) seems to provide nanometric Co/Sn particles with more efficient reversibility for the subsequent Li+-insertion/de-insertion via conversion/alloy reactions. The electrochemical performance of CoSn(OH)(6) is further improved when composited with graphenes. This is accomplished by electrostatically combining negatively charged Co2+/Sn2+-anchored graphene oxide (GO) with positively charged amine-functionalized graphene (GN) in a solution. The subsequent hydrothermal reaction produces CoSn(OH)(6) nanocubes that are tightly held by graphene sheets (GO/CS/GN). The GO/CS/GN delivers unprecedentedly high capacity and excellent cyclability (discharge capacities of 1475 mAh.g(-1) for the 100th charge/discharge (C/D) cycles at a 0.1 A.g(-1)). In contrast to the comparison materials (CoSnO3/graphene composites) the rate performance is also remarkable, delivering a capacity of 650 mAh.g(-1) at 2.0 at A.g(-1). (C) 2017 Published by Elsevier Ltd.</P>

Graphene oxide self-assembled with a cationic fullerene for high performance pseudo-capacitors

Senthilkumar, Krishnan,Prabakar, S. J. Richard,Park, Chunkuk,Jeong, Seok,Lah, Myoung Soo,Pyo, Myoungho The Royal Society of Chemistry 2016 Journal of Materials Chemistry A Vol.4 No.5

<▼1><P>Control of the microstructures of graphene oxide is realized by introducing a cationic fullerene, resulting in a high-performance pseudo-capacitor.</P></▼1><▼2><P>Control of the microstructures of graphene oxide (GO) is realized by introducing a cationic fullerene (CFU), resulting in a high-performance pseudo-capacitor. The strong electrostatic interaction between anionic GO and the CFU produces a self-assembled composite (GO/CFU), in which the CFU units intervene to form randomly stacked GO layers. The CFU acts as a spacer between GO layers, allowing a significant fraction of the oxygen-functional groups of GO to be redox-active. When tested as a pseudo-capacitor in 1.0 M H2SO4, the optimized GO/CFU composite delivers a capacitance of 357 F g<SUP>−1</SUP> at 0.4 A g<SUP>−1</SUP>, in contrast to 160 F g<SUP>−1</SUP> for GO alone, which is one of the greatest values reported for graphene composites with electro-inactive carbonaceous entities. The improvement in the capacitance by CFU incorporation is also evidenced at a high charge/discharge rate (285 and 137 F g<SUP>−1</SUP> at 5 A g<SUP>−1</SUP> for GO/CFU and GO, respectively). As a result, the GO/CFU composite delivers an energy density of 40 W h kg<SUP>−1</SUP> and a power density of 2793 W kg<SUP>−1</SUP> at 5 A g<SUP>−1</SUP>, in contrast to 19 W h kg<SUP>−1</SUP> and 2748 W kg<SUP>−1</SUP> for GO alone. During 5000 charge/discharge cycles at 5 A g<SUP>−1</SUP>, the capacitance of the GO/CFU composite increases slightly (4% increase in GO/CFU <I>vs.</I> 4% decrease in GO), which validates the effectiveness of a self-assembly strategy for high performance supercapacitor applications.</P></▼2>