Supporting Information for Self-assembled Graphene Hydrogel via a One-Step Hydrothermal Process Yuxi Xu, Kaixuan Sheng, Chun Li, and Gaoquan Shi * Department of Chemistry, Tsinghua University, Beijing 100084, People s Republic of China E-mail: gshi@tsinghua.edu.cn 1. Characterization of GO by Atomic Force Microscopy (AFM) AFM images were taken out using a Nanoscope III MultiMode SPM (Digital Instruments) with an AS-12 ( E ) scanner operated in tapping mode in conjunction with a V-shaped tapping tip (Applied Nanostructures SPM model: ACTA). The images were taken at a scan rate of 2 Hz. Figure S1. AFM image of GO sheets deposited on a freshly cleaved mica surface. The heights of GO sheets range from 0.7 to 0.8 nm, indicating an individual layer. The lateral sizes of most GO sheets are about several micrometers.
2. Electrical conductivity measurements The electrical conductivities of SGHs were measured by two-probe method. A cylindrical SGH was sandwiched between two platinum foils and connected to a CHI440 potentiostat-galvanostat (CH Instruments Inc.) for conductivity measurements. 3. Characterization of SGH and GO by X-ray Photoelectron Spectroscopy (XPS) XPS analysis of freeze-dried SGH and GO was performed on a ESCALAB 250 photoelectron spectrometer (ThermoFisher Scientific USA) with Al Kα (1486.6 ev) as the X-ray source set at 150 W and a pass energy of 200 ev (survey scan) and 30 ev (high resolution scan) and a 500 µm beam size. Figure S2. The C 1s XPS spectra of (a) GO and (b-f) SGH1 5 prepared by hydrothermal reduction of 2 mg/ml GO at 180 o C for 1, 2, 4, 6 and 12 h, respectively. The C 1s XPS spectrum of GO (Figure S4a) indicates the presence of four types of
carbon bonds: C-C (284.8 ev), C-O (286.6 ev), C=O (288.0 ev), and O-C=O (289.2 ev). Although the C 1s XPS spectra of SGH1 5 (Figure S3, b-f) also exhibit the same peaks, the peak intensities of oxygenated groups are much weaker than those in the spectrum of GO. The significant decrease of the signals related to oxygen-containing functional groups reflects the reduction of GO to graphene by hydrothermal method. 4. Characterization of SGH by scanning electron microscopy (SEM) The freeze-dried SGHs were broken to small pieces with a tweezer to expose their interior surfaces. The interior morphologies of SGHs were analyzed by SEM (FEI Sirion 200, Japan). Figure S3. SEM images of SGHs before (a, b) and after (c, d) compression beyond its yield point. 5. Characterization of SGH and GO by X-ray Diffraction (XRD) The XRD patterns of freeze-dried SGH, GO and natural graphite were obtained by
using a D8 Advance (Bruker) X-ray diffractometer with Cu Kα radiation (λ = 1.5418 Å). 6. Rheological measurements The rheological behaviors of SGHs were investigated by a MCR 300 (Paar Physica) Rheometer using a 25-mm parallel-plate geometry at 25 o C. The gap distance between two plates was fixed to be 1 mm. Both steady and dynamic measurements were conducted. The steady flow behaviors were performed with a shear rate in the range of 0.01 10 1/s. Dynamic frequency sweep experiments were measured from 1 to 100 rad/s at a fixed oscillatory strain of 0.2%. The frequency sweep was then followed by measurement of rheological parameters as a function of strain amplitude. Temperature sweep experiments from 25 100 o C were done at a heating rate of 5 o C /min and the samples were covered by a thin layer of mineral oil of low viscosity to prevent evaporation of water. 7. Compressive mechanical measurements The compressive stress-strain measurements were performed by using an electronic universal testing machine (WDW 3020 Autograph, Changchun Xinke Co., China) with two test plates. The cylindrical SGHs were set on the lower plate and compressed by the upper plate connecting to a load cell. The strain ramp rate was controlled to be 1 mm/min for the measurements.
8. Electrochemical measurements of SGH-based supercapacitor. Two slices of SGH (each has a thickness of about 1 mm) were cut from the as-prepared cylindrical SGH sample. They were separated by a filtrate paper soaked with electrolyte (5 M KOH) and used as the supercapacitor electrode materials. Before electrochemical measurements, the SGH slices were immersed in 5 M KOH aqueous electrolyte for 1 h to exchange their interior water with electrolyte. Two Pt foils were used as the current collectors. All the components were assembled into a layered structure and sandwiched between two PTFE sheets. Cyclic voltammetry (CV) and galvanostatic charge/discharge experiments were performed on a CHI440 potentiostat-galvanostat (CH Instruments Inc.) in a two-electrode system to measure the specific capacitance of SGH-based supercapacitor. Capacitance values were calculated for the CV curves by diving the current by the voltage scan rate, C = I/(dV/dt). Capacitance determined by the galvanostatic charge/discharge was measured using C = I/(dV/dt) with dv/dt calculated from the slope of discharge curves. The specific capacitance reported is the capacitance for the carbon material of one electrode (specific capacitance = capacitance of single electrode/weight of dried SGH of single electrode), as per the normal convention.