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.

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.

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.

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.

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.

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.