Scan Rate Cyclic Voltammetry Research Articles

Effect of Anodic Deposition Parameters on Electrochemical Behavior and Microstructure of Mn-Ni Oxide As a Pseudocapacitive Electrode

Hybrid supercapacitors, or briefly pseudocapacitors, are an emerging class of energy storage devices, with the capability of providing both high power and energy density. In contrast to conventional electric double layer capacitors, which accumulate charge mostly through an electrostatic mechanism, pseudocapacitors utilize the pseudocapacitance arising from reversible Faradic reactions occurring at the electrode surface [1]. Although various metal oxides have been investigated as active material for pseudocapacitors, greater attention has been given to manganese oxides. In recent years, also Mn-based binary oxides have been studied for pseudocapacitance, in particular, mixed oxides comprising Ni or Co as the other component [2-4] In the present work, Mn-Ni oxide thin films were deposited potentiodynamically at scan rate of 100 mVs-1, pH=7 and room temperature, on a stainless steel substrate. Undoubtedly, a survey of the literature shows that a systematic study on the effect of operating parameters on the properties of the mixed oxide thin films is still lacking. Accordingly, this work was undertaken with the aim of exploring the processing space for these oxides studying in particular the effects of the electrolyte Ni to Mn molar ratio, the deposition peak potential and the deposited mass on chemical composition, microstructure and capacitance behavior of Mn-Ni oxide thin films. The binary oxide deposits were characterized by energy dispersive X-ray spectroscopy (EDX) and scanning electron microscopy (SEM). The electrochemical behavior of the Mn-Ni oxide electrodes was studied by cyclic voltammetry (CV) at 5, 20, 50 and 100 mVs-1 and electrochemical impedance spectroscopy (EIS) in 1 M Na2SO4electrolyte. Expectedly, with increasing the Ni to Mn molar ratio in solution (from 0.1 to 10), the molar fraction of Ni in the oxide film was found to increase, resulting, on the one hand, in smoothing of the surface morphology and, on the other hand, in a change of the specific capacitance through a maximum of 145 Fg-1 at approximately 12% at Ni fraction and CV scan rate of 20 mVs-1. With further increase of the Ni fraction in the oxide to about 19% at, the specific capacitance started to decrease gradually. Changing the peak potential in the potentiodynamic deposition from +1.2 to +1.6 V vs. Ag/AgCl, the CV curves revealed a box-like shape for all values of the peak potential, while the capacitance showed a decreasing trend. EIS results revealed that charge transfer resistance increased with increasing peak potential and the lowest Rct was found for sample prepared at peak potential of 1.2 V vs. Ag/AgCl. The mass load per unit area of the film was controlled by the number of cycles. Finally, specific capacitance of 196 Fg-1 was obtained at scan rate of 5 mVs-1 for the Mn-Ni oxide film deposited potentiodynamically (5 cycles, 100 mVs-1, pH=7 and RT) in solution with Ni to Mn molar ratio of 2. At the high scan rate of 100 mVs-1, the specific capacitance was 135 Fg-1 showing good retention of the capacitance at high rate.

An investigation into LiFePO4/C electrode by medium scan rate cyclic voltammetry

A LiFePO4/C sample was prepared via solid state reaction and characterized with X-ray powder diffraction, scanning electron microscopy and charge–discharge test. Conductive carbon and highly crystallized LiFePO4 were embedded in each other to form the as-prepared LiFePO4/C, which exhibited an excellent rate capability and capacity retention. The LiFePO4/C electrode reaction was investigated by the method of medium scan rate cyclic voltammetry (CV) under temperature variation. The limit values for the CV redox peak potentials of the LiFePO4/C electrode scanned at different rates were obtained by curve fitting. The reversibility of the LiFePO4/C electrode was studied and found to be both scan rate and temperature dependent. A higher temperature led to a higher critical CV scan rate for a reversible LiFePO4/C electrode. In the electrode process, a higher temperature resulted in a smoother Fe3+/Fe2+ redox reaction, better reversibility, lower Rcv, smaller charge transfer resistance and higher Li+ ion diffusion coefficient at the cathode of the LiFePO4/C.

Effects of transition-metal ions on the morphology and electrochemical properties of δ-MnO2 for supercapacitors

δ-MnO2 materials doped with transition-metal cations (Zn, Co, and Ag) were successfully synthesized using a hydrothermal technique. The structures and morphologies of the obtained oxides were analyzed using X-ray diffraction, scanning electron microscopy and Brunauer-Emmett-Teller measurements. Additionally, the electrochemical properties were evaluated through cyclic voltammetry, electrochemical impedance spectroscopy and galvanostatic cycling measurements. The results indicate that the pure and doped samples crystallize in the δ form with a layered structure and that the Mn/Zn, Mn/Co and Mn/Ag molar ratios are all approximately 1:0.09. Both the Zn-doped and pure MnO2 materials exhibit a petal-like morphology; however, the former has a higher specific surface area of up to 98.97m2 g−1. Furthermore, the Zn-doped MnO2 exhibits a near-rectangular cyclic voltammetry (CV) curve with broad quasi-reversible redox peaks and a specific capacitance of 182.9 F g−1 at a CV scan rate of 2 mV s−1. The Co-doped material exhibits a distinct spiny-fiber morphology, and the electrochemical performance of this material is significantly worse than that of pure MnO2. The average attenuation rate of the Ag-doped material is only 0.028% after 1000 cycles, which is lower than that of pure MnO2.

Mechanistic Studies of Electrode-Assisted Catalytic Oxidation by Flavinium and Acridinium Cations

Electrochemical behavior of flavinium (Et-Fl+) and acridinium (Acr+) cations is presented, in order to investigate their activity toward catalytic water oxidation. Cyclic voltammograms of Acr+ and Et-Fl+ in acetonitrile are qualitatively similar, with oxidation peaks at highly positive potentials, and these oxidation peaks depend strongly on the type of the working electrode being used. However, the two model compounds exhibit different behaviors in the presence of water: while Et-Fl+ facilitates electrocatalytic water oxidation through an electrode-assisted mechanism, water oxidation is not accelerated in the presence of Acr+. A comparative study of variable scan-rate cyclic voltammetry, concentration dependence, and spectroelectrochemical behavior of two model compounds suggest that Et-Fl+ and Acr+ exhibit different reaction pathways with the electrode surface. On the basis of the experimental results, a mechanism is proposed to account for the observed differences in electrocatalysis.

A Development of High Power Activated Carbon Using the KOH Activation of Soft Carbon Series Cokes

The process parameter in optimized KOH alkali activation of soft carbon series coke material in high purity was set with DOE experiments design. The activated carbon was produced by performing the activation process based on the set process parameters. The specific surface area was measured and pore size was analyzed by <TEX>$N_2$</TEX> absorption method for the produced activated carbon. The surface functional group was analyzed by Boehm method and metal impurities were analyzed by XRF method. The specific surface area was increased over 2,000 <TEX>$m^2/g$</TEX> as the mixing ratio of activation agent increased. The micro pores in <TEX>$5{\sim}15{\AA}$</TEX> and surface functional group under 0.4 meq/g were obtained. The contents of the metal impurity in activated carbon which is the factor for reducing the electrochemical characteristics was reduced less than 100 ppm through the cleansing process optimization. The electrochemical characteristics of activated carbon in 38.5 F/g and 26.6 F/cc were checked through the impedance measuring with cyclic voltammetry scan rate in 50~300 mV/s and frequency in 10 mHz ~100 kHz. The activated carbon was made in the optimized activation process conditions of activation time in 40 minutes, mixing ratio of activation agent in 4.5 : 1.0 and heat treatment temperature over <TEX>$650^{\circ}C$</TEX>.

Simultaneous electropolymerization and electro-click functionalization for highly versatile surface platforms.

Simple preparation methods of chemically versatile and highly functionalizable surfaces remain rare and present a challenging research objective. Here, we demonstrate a simultaneous electropolymerization and electro-click functionalization process (SEEC) for one-pot self-construction of aniline- and naphthalene-based functional polymer films where both polymerization and click functionalization are triggered by applying electrochemical stimuli. Cyclic voltammetry (CV) can be applied for the simultaneous oxidation of 4-azidoaniline and the reduction of Cu(II) ions, resulting in polymerization of the former, and the Cu(I)-catalyzed alkyne/azide cycloaddition ("click" chemistry). Properties of the films obtained can be tuned by varying their morphology, their chemically "clicked" content, or by postconstruction functionalization. To demonstrate this, the CV scan rates, component monomers, and "clicked" molecules were varied. Covalent postconstruction immobilization of horseradish peroxidase was also performed. Consequently, pseudocapacitance and enzyme activity were affected. SEEC provides surface scientists an easy access to a wide range of functionalization possibilities in several fields including sensors, fuel cells, photovoltaics, and biomaterials.

Formation and electrochemical characterization of anodic ZrO2–WO3 mixed oxide nanotubular arrays

ZrO2–WO3 mixed oxide nanotubes were synthesized by a simple electrochemical anodization route. The oxide nanotubes contained a mixture of metastable hexagonal WO3 and monoclinic (and orthorhombic) ZrO2 phases, as well as a mixed-oxide ZrW2O8 phase that showed a metastable tetragonal symmetry. Evaluation of photo-activity of the materials showed generation of photo-potentials of −85mV and −230mV in the as-anodized and annealed conditions. Because of the mismatch in the band edge positions of the WO3 and ZrO2 phases and the resultant relaxation of photo-generated charge carriers, no significant photo-current density could be observed. The arrays of oxide nanotubes are considered for electrochemical capacitor application because of their morphology-assisted fast charge/discharge kinetics and large surface area. Presence of a large concentration of charge defects (on the order of 1021cm−3) and the reported high proton conductivity of the ZrO2–WO3 mixed oxide rendered high capacitance, which decreased with an increase in the scan rate of cyclic voltammetry. The highest measured capacitance was 40.03mF/cm2 at a scan rate of 10mV/s and the lowest was 1.93 mF/cm2 at 1V/s in 1M sulfuric acid solution.

Amplification of the discharge current density of lithium-ion batteries with spinel phase Li(PtAu)0.02Mn1.98O4 nano-materials

In this study the synergistic and catalytic properties of a novel nano-composite cathode material of nominal composition Li(M)xMn2-xO4 (M=Pt-Au; x # 0.2) has been explored. Li(PtAu)xMn2-xO4 nanomaterial for use in lithium-ion batteries (LIB) was synthesized by incorporation of the Pt-Au (1:1) nanoparticles onto the spinel phase LiMn2O4. Ultra-low scan rate (0.01mV s−1) cyclic voltammetry of the cathode material in 1M LiPF6 (in 1:1 EC:DMC), showed four sets of redox peaks, which reflect the typical redox process of the active material in the spinel structure due to lithium intercalation and deintercalation. The Li/Li(PtAu)0.02Mn1.98O4 cell had less polarization as it effectively accommodates the structural transformation during Li+ ion charge and discharge. The Li(PtAu)0.02Mn1.98O4 cathode showed an increase in discharge currents densities with an exchange current density, i0, value of 2.8×10−4 A cm−2, which suggests increase in the rate of electron transfer compared to LiMn2O4 (1.8×10−4 A cm−2). Li(PtAu)0.02Mn1.98O4 exhibited excellent capacity retention upon extended cycling and can release 90 mAh g−1 at 10C with a capacity retention of 99% after 50 cycles. Faster charge transportation at high current rates proved to prevent the pronounced pile-up of Li+ ions and undesired Mn3+ ions on the surfaces. The electrochemical impedance spectroscopy (EIS) results showed a decrease in charge transfer resistance for LiMn2O4 after surface coverage with conductive PtAu NP's. For the lithium diffusion coefficient in Li(PtAu)0.02Mn1.98O4 thin film, its magnitude order is 10−11 cm2·s−1.

Nanofiber-Based Electrodes for High Power Supercapacitors

Fabrication of novel three-dimensional material architectures is essential for successful development of energy storage devices that allow high rate operation along with sufficient energy capacity. To this end, we report a facile methodology to fabricate hierarchically-porous nanofiber-based electrodes for electric double layer capacitors. In particular, blends of polyacrylonitrile and a sacrificial polymer at different compositions were electrospun into non-woven nanofiber mats with the fiber diameter in the range of 200–400 nm and the mat thickness of 150-200 µm. Electrospun nanofiber mats were then subjected to carbonization to obtain porous carbon nanofibers (CNFs) as polyacrylonitrile converted to carbon and the sacrificial polymer decomposed out creating intra-fiber pores resulting in high specific surface areas of upto 1600 m2/g. No additional chemical or physical activation process was used. Electrochemical measurements in aqueous electrolyte showed large specific capacitance of up to 210 F g−1 at a high cyclic voltammetry scan rate of 100 mV s−1. These materials retain 75% capacitance at a large current density of 20 A g−1 indicating excellent power handling capability. These porous CNFs provided an ideal three-dimensional, free standing (without any binders) electrode design with multi-levels of pore sizes; through-connected Macropores (~500nm-1 µm), created due to inter-fiber spacing in the nanofiber mat & Micro (d <2 nm) and/or Meso pores (2<d<50 nm) created in each nanofiber due to decomposition of sacrificial polymer.While the intra-fiber pores provided large specific surface areas and hence large capacity, the inter-fiber pores facilitated the permeation of electrolyte to the carbon surfaces to allow fast charge adsorption/desorption and enhanced power handling capability.

Electrochemical oxidation of isopropanol using a nickel foam electrode

In this study, a nickel foam electrode is used for the anodic conversion of isopropanol to acetone. The process is studied under well-defined experimental conditions including variation of the concentration of isopropanol, variation in the strength of the supporting electrolyte (aqueous KOH), different cyclic voltammetry scan rates, and different temperatures. The outcome of these experiments demonstrates that the isopropanol oxidation reaction is dependent on the presence of the β-NiOOH surface species, which is generated at potentials near the potential of the isopropanol oxidation reaction. However, the two processes do not occur at the exact same potential and their respective behaviors under the various experimental conditions can be understood independently. Nickel foams are successfully applied as the anode material for controlled-potential electrolysis of isopropanol to form acetone. The Ni foam anode sustains a current density of 2.6mAcm−2 for the electrolysis time of 600min without significant loss of activity. Isopropanol is converted to acetone at a rate of 5.6mM per hour.

Lithographically Patterned Thin Activated Carbon Films as a New Technology Platform for On-Chip Devices

Continuous, smooth, visibly defect-free, lithographically patterned activated carbon films (ACFs) are prepared on the surface of silicon wafers. Depending on the synthesis conditions, porous ACFs can either remain attached to the initial substrate or be separated and transferred to another dense or porous substrate of interest. Tuning the activation conditions allows one to change the surface area and porosity of the produced carbon films. Here we utilize the developed thin ACF technology to produce prototypes of functional electrical double-layer capacitor devices. The synthesized thin carbon film electrodes demonstrated very high capacitance in excess of 510 F g(-1) (>390 F cm(-3)) at a slow cyclic voltammetry scan rate of 1 mV s(-1) and in excess of 325 F g(-1) (>250 F cm(-3)) in charge-discharge tests at an ultrahigh current density of 45,000 mA g(-1). Good stability was demonstrated after 10,000 galvanostatic charge-discharge cycles. The high values of the specific and volumetric capacitances of the selected ACF electrodes as well as the capacity retention at high current densities demonstrated great potential of the proposed technology for the fabrication of various on-chip devices, such as micro-electrochemical capacitors.

Heck Coupling of Olefins to Mixed Methyl/Thienyl Monolayers on Si(111) Surfaces

The Heck reaction has been used to couple olefins to a Si(111) surface that was functionalized with a mixed monolayer comprised of methyl and thienyl groups. The coupling method maintained a conjugated linkage between the surface and the olefinic surface functionality, to allow for facile charge transfer from the silicon surface. While a Si(111) surface terminated only with thienyl groups displayed a surface recombination velocity, S, of 670 ± 190 cm s(-1), the mixed CH3/SC4H3-Si(111) surfaces with a coverage of θSC4H3 = 0.15 ± 0.02 displayed a substantially lower value of S = 27 ± 9 cm s(-1). Accordingly, CH3/SC4H3-Si(111) surfaces were brominated with N-bromosuccinimide, to produce mixed CH3/SC4H2Br-Si(111) surfaces with coverages of θBr-Si < 0.05. The resulting aryl halide surfaces were activated using [Pd(PPh3)4] as a catalyst. After activation, Pd(II) was selectively coordinated by oxidative addition to the surface-bound aryl halide. The olefinic substrates 4-fluorostyrene, vinylferrocene, and protoporphyrin IX dimethyl ester were then coupled (in dimethylformamide at 100 °C) to the Pd-containing functionalized Si surfaces. The porphyrin-modified surface was then metalated with Co, Cu, or Zn. The vinylferrocene-modified Si(111) surface showed a linear dependence of the peak current on scan rate in cyclic voltammetry, indicating that facile electron transfer had been maintained and providing evidence of a robust linkage between the Si surface and the tethered ferrocene. The final Heck-coupled surface exhibited S = 70 cm s(-1), indicating that high-quality surfaces could be produced by this multistep synthetic approach for tethering small molecules to silicon photoelectrodes.

Fabrication of porous carbon nanofibers with adjustable pore sizes as electrodes for supercapacitors

We report a facile method for obtaining extremely high surface area and uniformly porous carbon nanofibers for supercapacitors. Blends of polyacrylonitrile and sacrificial Nafion at different compositions have been electrospun into non-woven nanofiber mats with diameters in the range of 200–400 nm. Electrospun nanofiber mats are then subjected to carbonization to obtain porous carbon nanofibers (CNFs) as polyacrylonitrile converts to carbon and Nafion decomposes out creating intra-fiber pores. Resultant porous CNFs exhibit specific surface area of up to 1600 m2 g−1 with a large fraction of mesopores (2–4 nm). No additional chemical or physical activation process was used. We demonstrate the tunability of the pore sizes within CNFs by varying the amount of Nafion. The non-woven fiber mats of porous CNFs are studied as free-standing electrode materials for supercapacitors eliminating the need for polymeric binding agents. Electrochemical measurements showed large specific gravimetric and volumetric capacitances of up to 210 F g−1 and 60 F cm−3 in 1 M H2SO4 at a high cyclic voltammetry scan rate of 100 mV s−1 due to the large fraction of mesopores. These materials retain 75% performance at a large current density of 20 A g−1 indicating excellent power handling capability.

Interplay of Electron Hopping and Bounded Diffusion during Charge Transport in Redox Polymer Electrodes

Redox polymer electrodes (RPEs) have been prepared both by attachment of random copolymers of hydroxybutyl methacrylate and vinylferrocene (poly(HBMA-co-VF)) to carbon substrates by grafting either "to" or "from" the substrate surfaces, and by impregnation of porous carbon substrates with redox polymer gels of similar composition. An observed linear dependence of peak current on the square root of the applied voltage scan rate in cyclic voltammetry (CV) led to the conclusion that the rate controlling step in the redox process was the diffusive transfer of electrons through the redox polymer layer. The variation in the peak current with increasing concentration of the redox species in the polymer indicated that the electron transport transitioned from bounded diffusion to electron hopping. A modified form of the Blauch-Saveant equation for apparent diffusivity of electrons through a polymer film indicated that bounded diffusion was the dominant mechanism of electron transport in RPEs with un-cross-linked polymer chains at low concentrations of the redox species, but, as the concentration of the redox species increased, electron hopping became more dominant, and was the primary mode of electron diffusion above a certain concentration level of redox species. In the cross-linked polymer gels, bounded diffusion was limited because of the restricted mobility of the polymer chains. Electron hopping was the primary mode of electron diffusion in such systems at all concentrations of the redox species.

Transition metal alloy-modulated lithium manganese oxide nanosystem for energy storage in lithium-ion battery cathodes

This paper explores the synergistic and catalytic properties of a newly developed lithium ion battery (LIB) composite cathode of LiMn2O4 modified with bimetallic (Au–Fe) nanoparticle. Spinel phase LiMn2O4 was doped with bimetallic nanoparticles, LiMxMn2−xO4 (M=FeAu), with concomitant oxidation of the Mn3+ ions (responsible for LIB capacity loss) to Mn4+. This nano-composite architecture accommodates the structural transformation that occurs during Li+ ion charge and discharge. Ultra-low scan rate (0.01mVs−1) cyclic voltammetry of the pure LiMn2O4 cathode material in 1M LiPF6/electrolyte solution, showed two sets of redox peaks with a third observed at lower potentials for LiMxMn2−xO4. The FeAu incorporation increased the reaction rate upon reduction of LiMn2O4 as indicated by the enhanced reduction peak seen by cyclic voltammetry. Nyquist plots of the electrochemical impedance spectroscopy (EIS) results showed LiFeAuxMn2−xO4 having increased conductivity with lower resistance of charge. X-ray diffraction studies showed the LiMxMn2−xO4 material retained well-developed octahedral structures bounded by (111) planes. The material crystallite size was ∼10nm with clear lattice fringes having a separation value of 0.48nm which concurrently improved the diffusion rate of Li+. Solid-state NMR results showed the progressive increase in average nominal manganese oxidation state from +3.5 to +4 resulted in an increase in the super-transferred hyperfine field at the 7Li nucleus of the FeAu doped cathode material. The LiFeAuxMn2−xO4 material also showed improved cycleability, especially at high C rate. This improvement was due to the enhanced physical stability of LiMn2O4 and its improved electrical conductivity ascribed to the incorporated FeAu nanoparticles.

Radically Enhanced Molecular Switches

The mechanism governing the redox-stimulated switching behavior of a tristable [2]rotaxane consisting of a cyclobis(paraquat-p-phenylene) (CBPQT(4+)) ring encircling a dumbbell, containing tetrathiafulvalene (TTF) and 1,5-dioxynaphthalene (DNP) recognition units which are separated from each other along a polyether chain carrying 2,6-diisopropylphenyl stoppers by a 4,4'-bipyridinium (BIPY(2+)) unit, is described. The BIPY(2+) unit acts to increase the lifetime of the metastable state coconformation (MSCC) significantly by restricting the shuttling motion of the CBPQT(4+) ring to such an extent that the MSCC can be isolated in the solid state and is stable for weeks on end. As controls, the redox-induced mechanism of switching of two bistable [2]rotaxanes and one bistable [2]catenane composed of CBPQT(4+) rings encircling dumbbells or macrocyclic polyethers, respectively, that contain a BIPY(2+) unit with either a TTF or DNP unit, is investigated. Variable scan-rate cyclic voltammetry and digital simulations of the tristable and bistable [2]rotaxanes and [2]catenane reveal a mechanism which involves a bisradical state coconformation (BRCC) in which only one of the BIPY(•+) units in the CBPQT(2(•+)) ring is oxidized to the BIPY(2+) dication. This observation of the BRCC was further confirmed by theoretical calculations as well as by X-ray crystallography of the [2]catenane in its bisradical tetracationic redox state. It is evident that the incorporation of a kinetic barrier between the donor recognition units in the tristable [2]rotaxane can prolong the lifetime and stability of the MSCC, an observation which augurs well for the development of nonvolatile molecular flash memory devices.

Synthesis of hierarchically macro/meso/microporous carbon spheres and its application in fast rechargeable electric double layer capacitor

A porous carbon sphere (PCS), which exhibits hierarchical macro/meso/microporosity, is fabricated by nanocasting. The macropores within the PCS are penetrable, fully open and well interconnected, which is admirable for fast mass transfer applications. The carbon has been attempted as a novel electrode material for a fast rechargeable electrochemical double layer capacitor (EDLC). Though the total surface area is only 826 (±2)m2g−1 and a large quantity of micropores exist, the capacitance of the EDLC can reach 172 (± 3)Fg−1 regardless of the cyclic voltammetry scan rate. The macropores within the PCS acted as ion-buffering reservoirs in-situ, which facilitates the accessibility of the mesopores as well as the micropores within the PCS. As a result, unlike traditional microporous carbon material, the micropores in the PCS show no obvious negative effect to the performance of the EDLC. The PCS material is a promising candidate in fast mass transfer applications.

Ground-State Thermodynamics of Bistable Redox-Active Donor–Acceptor Mechanically Interlocked Molecules

Fashioned through billions of years of evolution, biological molecular machines, such as ATP synthase, myosin, and kinesin, use the intricate relative motions of their components to drive some of life's most essential processes. Having control over the motions in molecules is imperative for life to function, and many chemists have designed, synthesized, and investigated artificial molecular systems that also express controllable motions within molecules. Using bistable mechanically interlocked molecules (MIMs), based on donor-acceptor recognition motifs, we have sought to imitate the sophisticated nanoscale machines present in living systems. In this Account, we analyze the thermodynamic characteristics of a series of redox-switchable [2]rotaxanes and [2]catenanes. Control and understanding of the relative intramolecular movements of components in MIMs have been vital in the development of a variety of applications of these compounds ranging from molecular electronic devices to drug delivery systems. These bistable donor-acceptor MIMs undergo redox-activated switching between two isomeric states. Under ambient conditions, the dominant translational isomer, the ground-state coconformation (GSCC), is in equilibrium with the less favored translational isomer, the metastable-state coconformation (MSCC). By manipulating the redox state of the recognition site associated with the GSCC, we can stimulate the relative movements of the components in these bistable MIMs. The thermodynamic parameters of model host-guest complexes provide a good starting point to rationalize the ratio of GSCC to MSCC at equilibrium. The bistable [2]rotaxanes show a strong correlation between the relative free energies of model complexes and the ground-state distribution constants (K(GS)). This relationship does not always hold for bistable [2]catenanes, most likely because of the additional steric and electronic constraints present when the two rings are mechanically interlocked with each other. Measuring the ground-state distribution constants of bistable MIMs presents its own set of challenges. While it is possible, in principle, to determine these constants using NMR and UV-vis spectroscopies, these methods lack the sensitivity to permit the determination of ratios of translational isomers greater than 10:1 with sufficient accuracy and precision. A simple application of the Nernst equation, in combination with variable scan-rate cyclic voltammetry, however, allows the direct measurement of ground-state distribution constants across a wide range (K(GS) = 10-10(4)) of values.

Synthesis of core–shell Au@Pt nanoparticles supported on Vulcan XC-72 carbon and their electrocatalytic activities for methanol oxidation

Au@Pt core–shell nanoparticles were successfully synthesized by successive reduction of HAuCl4 and H2PtCl6 and were then assembled on Vulcan XC-72 carbon surface (noted as Au@Pt/C). The morphology and distribution of Au@Pt nanoparticles were characterized by UV–vis spectroscopy, transmission electron microscopy (TEM), and energy-dispersive X-ray spectroscopy (EDS) and the corresponding result is that core–shell like structure is observed. The Au@Pt/C catalysts with different Au/Pt atomic ratios were characterized by TEM, X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). The electrocatalytic activities of Au@Pt/C catalysts were investigated by cyclic voltammetry (CV) in 1.0M H2SO4+1.0M methanol aqueous solution. The effects of Au/Pt atomic ratio, CV scan rate and methanol concentration on the peak current of methanol oxidation and the long-term cycle stability were also discussed. The results revealed that compared with Au@Pt/C (1:4, 1:1, 2:1) catalysts, Au/C catalyst, and Pt/C catalyst, Au@Pt/C (1:2) catalyst exhibits higher electrocatalytic activity toward methanol oxidation in acidic media, and the peak current density of Au@Pt/C (1:2) catalyst is about 2.5 times as large as that of Pt/C catalyst without Au core. Further more, Au@Pt/C (1:2) catalyst shows good long-term cycle stability and 91.1% value of peak current of methanol oxidation remains after 200 cycles.

Electrochemically Induced Chemically Reversible Proton-Coupled Electron Transfer Reactions of Riboflavin (Vitamin B2)

The electrochemical behavior of the naturally occurring vitamin B(2), riboflavin (Fl(ox)), was examined in detail in dimethyl sulfoxide solutions using variable scan rate cyclic voltammetry (ν = 0.1 - 20 V s(-1)) and has been found to undergo a series of proton-coupled electron transfer reactions. At a scan rate of 0.1 V s(-1), riboflavin is initially reduced by one electron to form the radical anion (Fl(rad)(•-)) at E(0)(f) = -1.22 V versus Fc/Fc(+) (E(0)(f) = formal reduction potential and Fc = ferrocene). Fl(rad)(•-) undergoes a homogeneous proton transfer reaction with the starting material (Fl(ox)) to produce Fl(rad)H(•) and Fl(ox)(-), which are both able to undergo further reduction at the electrode surface to form Fl(red)H(-) (E(0)(f) = -1.05 V vs Fc/Fc(+)) and Fl(rad)(•2-) (E(0)(f) = -1.62 V vs Fc/Fc(+)), respectively. At faster voltammetric scan rates, the homogeneous reaction between Fl(rad)(•-) and Fl(ox) begins to be outrun, which leads to the detection of a voltammetric peak at more negative potentials associated with the one-electron reduction of Fl(rad)(•-) to form Fl(red)(2-) (E(0)(f) = -1.98 V vs Fc/Fc(+)). The variable scan rate voltammetric data were modeled quantitatively using digital simulation techniques based on an interconnecting "scheme of squares" mechanism, which enabled the four formal potentials as well as the equilibrium and rate constants associated with four homogeneous reactions to be determined. Extended time-scale controlled potential electrolysis (t > hours) and spectroscopic (EPR and in situ UV-vis) experiments confirmed that the chemical reactions were completely chemically reversible.