We present a reversible cluster aggregation model for 2-D macromolecules represented by line segments in 2-D; and, we use it to describe the aggregation process of functionalized graphene particles in an aqueous SDS surfactant solution. The model produces clusters with similar sizes and structures as a function of SDS concentration in agreement with experiments and predicts the existence of a critical surfactant concentration (C-crit) beyond which thermodynamically stable graphene suspensions form. Around C-crit, particles form dense clusters rapidly and sediment. At C << C-crit, a contiguous ramified network of graphene gel forms which also densifies, but at a slower rate, and sediments with time. The deaggregation-reaggregation mechanism of our model captures the restructuring of the large aggregates towards a graphite-like structure for the low SDS concentrations. (C) 2017 American Institute of Chemical Engineers
We use molecular dynamics simulations in a constant potential ensemble to study the effects of solution composition on the electrochemical response of a double layer capacitor. We find that the capacitance first increases with ion concentration following its expected ideal solution behavior but decreases upon approaching a pure ionic liquid in agreement with recent experimental observations. The nonmonotonic behavior of the capacitance as a function of ion concentration results from the competition between the independent motion of solvated ions in the dilute regime and solvation fluctuations in the concentrated regime. Mirroring the capacitance, we find that the characteristic decay length of charge density correlations away from the electrode is also nonmonotonic. The correlation length first decreases with ion concentration as a result of better electrostatic screening but increases with ion concentration as a result of enhanced steric interactions. When charge fluctuations induced by correlated ion-solvent fluctuations are large relative to those induced by the pure ionic liquid, such capacitive behavior is expected to be generic.
The addition of dehydrated sucrose nano particles increases the gravimetric capacitance of electrochemical double-layer capacitor electrodes produced via the evaporative consolidation of graphene oxide-water-ionic liquid gels by more than two-fold. Dehydrated sucrose adsorbs onto graphene oxide and serves as a spacer, preventing the graphene oxide from restacking during solvent evaporation. Despite 61 wt % of the solids being electrochemically inactive dehydrated sucrose nanoparticles, the best electrodes achieved an energy density of similar to 13.3 Wh/kg, accounting for the total mass of all electrode components.
We demonstrate that functionalized graphene, rich with lattice defects but lean with oxygen sites, catalyzes the reduction of Co-III(bpy)(3) as well as platinum does, exhibiting a rate of heterogeneous electron transfer, k(0), of similar to 6 x 10(-3) cm/s. We show this rate to be an order of magnitude higher than on oxygen-site-rich graphene oxide, and over 2 orders of magnitude higher than on the basal plane of graphite (as a surrogate for pristine graphene). Furthermore, dye-sensitized solar. cells using defect-rich graphene monolayers perform similarly to those using platinum nanoparticles as the catalyst.
While graphene and other carbonaceous nanomaterials have shown promise in a variety of electrochemical applications, measurement of their intrinsic performance is often confounded with effects related to the complexities due to diffusion in a porous medium. To by-pass this limitation, we use effectively non-porous tiled monolayers of reduced graphene oxide as a model platform to study how rates of heterogeneous electron transfer evolve as a function of graphene structure/chemistry. A variety of electrochemical systems are investigated including the standard ferri/ferrocyanide redox probe, several common biomolecular redox systems as well as copper electrodeposition. We show that the rates of heterogeneous electron transfer can vary by as much as 3 orders of magnitude depending on the reduction or annealing conditions used and the redox system investigated. Performance changes are linked to graphene chemistry, and we show that the graphene oxide reduction procedure must be chosen judiciously to maximize the electrochemical performance for particular applications. (C) 2016 The Electrochemical Society. All rights reserved.
We use electrochemical impedance spectroscopy to measure the effect of diluting a hydrophobic room temperature ionic liquid with miscible organic solvents on the differential capacitance of the glassy carbon − electrolyte interface. We show that the minimum differential capacitance increases with dilution and reaches a maximum value at ionic liquid contents near 5 − 10 mol% (i.e., ∼ 1 M). We provide evidence that mixtures with 1,2-dichloroethane, a low- dielectric constant solvent, yield the largest gains in capacitance near the open circuit potential when compared against two traditional solvents, acetonitrile and propylene carbonate. To provide a fundamental basis for these observations, we use a coarse-grained model to relate structural variations at the double layer to the occurrence of the maximum. Our results reveal the potential for the enhancement of double-layer capacitance through dilution.
Infrared spectroscopy in combination with density functional theory calculations has been widely used to characterize the structure of graphene oxide and its reduced forms. Yet, the synergistic effects of different functional groups, lattice defects, and edges on the vibrational spectra are not well understood. Here, we report first-principles calculations of the infrared spectra of graphene oxide performed on realistic, thermally equilibrated, structural models that incorporate lattice vacancies and edges along with various oxygen-containing functional groups. Models including adsorbed water are examined as well. Our results show that lattice vacancies lead to important blue and red shifts in the OH stretching and bending bands, respectively, whereas the presence of adsorbed water leaves these shifts largely unaffected. We also find unique infrared features for edge carboxyls resulting from interactions with both nearby functional groups and the graphene lattice. Comparison of the computed vibrational properties to our experiments clarifies the origin of several observed features and provides evidence that defects and edges are essential for characterizing and interpreting the infrared spectrum of graphene oxide.
Graphene has been heralded as a promising electrode material for high energy and power density electrochemical supercapacitors. This is in spite of recent work confirming the low double-layer capacitance (CDL) of the graphene/electrolyte interface limited by graphene’s low quantum capacitance (CQ), an effect known for the basal plane of graphite for over four decades. Consistent with this limit, much of the supercapacitor research implies the use of pristine graphene but, in contrast, uses a functionalized and defective graphene formed through the reduction of graphene oxide, without clarifying why reduced graphene oxide is needed to achieve high capacitance. Herein, we show that an optimal level of functionalization and lattice disorder in reduced graphene oxide yields a 4-fold increase in CDL over that of pristine graphene, suggesting graphene-based materials can indeed be tailored to engineer electrodes with significantly higher gravimetric capacitance limits exceeding 450 F/g than what has been achieved (∼ 274 F/g) thus far, even in nonaqueous electrolytes capable of high voltage operation.
We have covalently grafted tetrazine derivatives to graphene oxide through nucleophilic substitution. Since the tetrazine unit is electroactive and nitrogen-rich, with a reduction potential sensitive to the type of substituent and degree of substitution, we used electrochemistry and X-ray photoelectron spectroscopy to demonstrate clear evidence for grafting through covalent bonding. Chemical modification was supported by Fourier transform infrared spectroscopy and thermal analysis. Tetrazines grafted onto graphene oxide displayed different mass losses compared to unmodified graphene and were more stable than the molecular precursors. Finally, a bridging tetrazine derivative was grafted between sheets of graphene oxide to demonstrate that the separation distance between sheets can be maintained while designing new graphene-based materials, including chemically bound, redox structures.
Graphene-TiO2 nanocomposites are a promising anode material for Li-ion batteries due to their good high-rate capacity, inherent safety, and mechanical and chemical robustness. However, despite a large number of scientific reports on the material, the mechanism of the enhanced high-rate Li+ storage capacity that results from the addition of graphene to TiO2 – typically attributed to improved electrical conductivity – is still not well understood. In this work, we focus on optimizing the processing of surfactant-templated graphene-TiO2 hybrid nanocomposites. Towards this end, we examine the influence of various processing parameters, in particular the surfactant-mediated colloidal dispersion of graphene, on the material properties and electrochemical performance of graphene-TiO2. We investigate the influence of electrode mass loading on Li+ storage capacity, focusing mainly on high-rate performance. Furthermore, we demonstrate an approach for estimating power loss during charge/discharge cycling, which offers a succinct method for characterizing the high-rate performance of Li-ion battery electrodes.
Battery technologies involving Li-S chemistries have been touted as one of the most promising next generation systems. The theoretical capacity of sulfur is nearly an order of magnitude higher than current Li-ion battery insertion cathodes and when coupled with a Li metal anode, Li-S batteries promise specific energies nearly five-fold higher. However, this assertion only holds if sulfur cathodes could be designed in the same manner as cathodes for Li-ion batteries. Here, the recent efforts to engineer high capacity, thick, sulfur-based cathodes are explored. Various works are compared in terms of capacity, areal mass loading, and fraction of conductive additive, which are the critical parameters dictating the potential for a device to achieve a specific energy higher than current Li-ion batteries (i.e., >200 Wh kg−1 ). While an inferior specific energy is projected in the majority of cases, several promising strategies have the potential to achieve >500 Wh kg−1 . The challenges associated with the limited cycle-life of these systems due to both the polysulfide shuttle phenomenon and the rapid degradation of the Li metal anode that is experienced at the current densities required to charge high specific energy batteries in a reasonable timeframe are also discussed.
Surfactants are widely used for dispersing graphene and functionalized graphene sheets (FGS) in colloidal suspensions, but there have been few studies of the structure of the dispersed graphene–surfactant complexes in suspension and of their time evolution. Here, we combine experimental study of efficiencies of ionic surfactants/polymers in suspending FGS in water with characterization using atomic force microscopy, small angle neutron scattering, and molecular simulations to probe the detailed structures of FGSs. The small angle scattering technique provides quantitative measurement of structure of graphene sheets in the solution. This study suggests that in both ionic and nonionic surfactants, the dispersion tends to degrade over time through detachment of the surfactant molecules and structural rearrangements. Ionic surfactants with strong interfacial binding and large molecular weight increase the dispersing power by over an order of magnitude.
Elastic instabilities, when properly implemented within soft, mechanical structures, can generate advanced functionality. In this work, we use the voltage-induced buckling of thin, flexible plates to pump fluids within a microfluidic channel. The soft electrodes that enable electrical actuation are compatible with fluids, and undergo large, reversible deformations. We quantified the onset of voltage-induced buckling, and measured the flow rate within the microchannel. This embeddable, flexible microfluidic pump will aid in the generation of new stand-alone microfluidic devices that require a tunable flow rate.
Graphene and ionic liquids are promising candidates for electrode materials and electrolytes, respectively, for modern energy storage devices such as supercapacitors. Understanding the interactions at the interfacial region between these materials is crucial for optimizing the overall performance and efficiency of supercapacitors. The interfacial region between graphene and an imidazolium-based ionic liquid is analyzed in a combined experimental and computational study. This dual approach reveals that the imidazolium-based cations mostly orient themselves parallel to the graphene surface due to pi-pi stacking interaction and form a primary interfacial layer, which is subsequently capped by a layer of anions from the ionic liquid. However, it also becomes apparent that the molecular interplay at the interfacial region is highly influenced by functional group defects on the graphene surface, in particular by hydroxyl groups. (C) 2012 Elsevier Ltd. All rights reserved.
Functionalized graphene sheets (FGSs) comprise a unique member of the carbon family, demonstrating excellent electrical conductivity and mechanical strength. However, the detailed chemical composition of this material is still unclear. Herein, we take advantage of the fluorination process to semiquantitatively probe the defects and functional groups on graphene surface. Functionalized graphene sheets are used as substrate for low-temperature (<150 °C) direct fluorination. The fluorine content has been modified to investigate the formation mechanism of different functional groups such as CF, CF2, OCF2 and (C=O)F during fluorination. The detailed structure and chemical bonds are simulated by density functional theory (DFT) and quantified experimentally by nuclear magnetic resonance (NMR). The electrochemical properties of fluorinated graphene are also discussed extending the use of graphene from fundamental research to practical applications.
We contrast the performance of monolayer electrodes and thin porous film electrodes of highly reduced functionalized graphene to demonstrate that the introduction of electrode porosity gives rise to strong apparent electrocatalytic effects resulting in vastly improved electrode selectivity. This is despite graphene showing no intrinsic advantage over glassy carbon electrodes when used as a monolayer. The simultaneous electrooxidation of ascorbic acid, dopamine and uric acid is used as an experimental model electrolyte system. Our results suggest that a large number of reports claiming the superior surface chemistry of carbon nanomaterials as the reason for outstanding electrochemical characteristics should be revisited considering electrode morphology as a significant contributor to the observed behavior. Our experimental results are supported by numerical simulations explaining the porosity-induced electrode selectivity by the dominance of pore depletion over diffusion-limited currents.
We report on the adsorption of sodium dodecyl sulfate (SDS) onto functionalized graphene sheets (FGSs) in an aqueous system, measured at broad SDS and FGS concentration ranges by conductometric surfactant titration. At dilute SDS concentrations (<12 mu M in bulk solution), there is evidence of a counterion exchange between hydronium ions (from the dissociation of acidic chemical functionalities on FGS) and sodium ions coadsorbing with dodecyl sulfate monomers onto FGSs. We find that, for FGS with a carbon-to-oxygen ratio of similar to 18, monolayer adsorption of SDS on FGS reaches full surface coverage by similar to 12 mu M SDS. Additionally, the critical surface aggregation concentration (csac) for surface micelle formation on FGS is measured to be similar to 1.5 mM SDS The transition from monolayer adsorption to surface micelle formation appears to occur at a similar SDS concentration on FGSs as on graphite, suggesting there is little difference in the surfactant adsorption behavior on both materials. We estimate that the FGS surface area available for SDS adsorption is similar to 600 m(2)/g which is significantly less than expected for FGSs in suspension and indicates the presence of regions on FGS on which SDS adsorption does not occur.
We have studied the processes leading to the cementation of colloidal particles during their autonomous assembly on corroding copper electrodes within a Cu-Au galvanic rnicroreactor. We determined the onset of particle immobilization through particle tracking, monitored the dissolution of copper as well as the deposition of insoluble products of the corrosion reactions in situ, and showed that particle immobilization initiated after reaction products (RPs) began to deposit on the electrode substrate. We further demonstrated that the time and the extent of RP precipitation and thus the strength of the particle-substrate bond could be tuned by varying the amount of copper in the system and the microreactor pH. The ability to cement colloidal particles at locations undergoing corrosion illustrates that the studied colloidal assembly approach holds potential for applications in dynamic material property adaptation.
The mechanisms leading to the deposition of colloidal particles in a copper-gold galvanic microreactor are investigated. Using in situ current density measurements and particle velocimetry, we establish correlations between the spatial arrangement and the geometry of the electrodes, current density distribution, and particle aggregation behavior. Ionic transport phenomena are responsible for the occurrence of strongly localized high current density at the edges and corners of the copper electrodes at large electrode separation, leading to a preferential aggregation of colloidal particles at the electrode edges. Preferential aggregation appears to be the result of a combination of electrophoretic effects and changes in bulk electrolyte flow patterns. We demonstrate that electrolyte flow is most likely driven by electrochemical potential gradients of reaction products formed during the inhomogeneous copper dissolution.