Research ArticleMATERIALS SCIENCE

Critical role of intercalated water for electrocatalytically active nitrogen-doped graphitic systems

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Science Advances  18 Mar 2016:
Vol. 2, no. 3, e1501178
DOI: 10.1126/sciadv.1501178
  • Fig. 1 Synthesis of nitrogen-doped reduced GO (NrGO) catalysts.

    (A) Schematic of the process developed to make NrGO catalysts. Natural graphite (with a d spacing of 3.4 Å) is chemically functionalized and exfoliated using potassium permanganate and sulfuric acid (modified Hummers method) to produce a GO powder with a d spacing of ~1 nm. GO is then subjected to different solvent-rinsing treatments followed by vacuum drying to effectively remove unbound intercalated water as indicated by the decrease in d spacing (8.6 to 5.4 Å, depending on solvent used for rinsing). The resulting dried GO is doped with nitrogen (NH3 at 850°C), leading to the formation of NrGO catalysts with a final d spacing of 3.4 Å. Atoms: C (gray), Mn from KMnO4 (purple), N (blue), O (red), and H (white). (B and C) Electron micrographs [via scanning electron microscopy (SEM) and scanning transmission electron microscopy (STEM)] of (B) GO and (C) NrGO (ether) showing oxidized and graphitic domains. Brighter atoms correspond to Si atoms, a common graphene impurity (38).

  • Fig. 2 Structural and chemical characterization of solvent-treated GO.

    (A) XRD patterns show a steady decrease in d spacing (10.8 to 7.5 Å) and the emergence of a diffraction peak at 5.4 Å (KC8), as well as its associated higher-order lamellar peaks indicating long-range lamellar order, as a function of the solvent drying technique. The inset shows the KC8 structure. K (purple). a.u., arbitrary units. (B) Micro–FTIR (Fourier transform infrared) spectra demonstrating the presence of confined water (sharp –OH peak emerging at 3280 cm−1). (C) X-ray photoelectron spectroscopy (XPS) data showing an increase in graphitic carbon (284.6 eV) and a decrease in C═O (286.8 eV) content as a function of solvent rinsing.

  • Fig. 3 Theoretical models of solvent-treated GO structures.

    MD relaxed structures for water, water/ethanol, and water/ether. Brown spheres represent carbon atoms, red spheres represent oxygen atoms, and white spheres represent hydrogen atoms. Addition of solvent molecules (highlighted) in between each layer leads to a decrease in d spacing as observed from XRD data.

  • Fig. 4 Morphological and electrochemical characterization of NrGO catalysts.

    (A) SEM images of NrGO catalysts showing sheet-like morphology and micrometer-sized hole formation using solvent-treated GO samples. (B and C) Steady-state step voltammograms of the ORR for all electrocatalysts (B) and corresponding H2O2 yields (C). (D) Summary of E½ values and H2O2 yields at 0.4 V. Rotating ring-disc electrode (RRDE) conditions: catalyst loading, 0.6 mg/cm2; supporting electrolyte, 0.5 M H2SO4; rotation rate, 900 rpm; temperature, 25°C; potential steps of 30 mV per step; step duration, 30 s.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/2/3/e1501178/DC1

    Supplementary Results and Discussion

    Table S1. Table of Hansen solubility parameters of selected solvents.

    Table S2. Summarized XPS data showing differences in atomic ratio of elements for ether-treated and untreated catalysts.

    Table S3. Summarized XPS data showing differences in C and N speciation for ether-treated and untreated catalysts.

    Table S4. Summarized calculated BET (Brunauer-Emmett-Teller)–specific surface areas and pore volumes obtained from solvent-treated NrGO catalysts.

    Fig. S1. High-resolution STEM images of ether-treated GO and NrGO materials.

    Fig. S2. XRD spectra of starting, intermediate, and end products.

    Fig. S3. Thermogravimetric analysis of solvent-treated GO samples.

    Fig. S4. FTIR attenuated total reflectance spectra of water and ice.

    Fig. S5. High-resolution C 1s XPS spectra for treated GO precursors.

    Fig. S6. XRD data of ether-treated GO showing the presence of a lamellar KC8 structure at an intermediate temperature (400°C), which disappears at higher temperatures (850°C).

    Fig. S7. Koutecky-Levich and electrode loading mechanistic studies for ether-treated NrGO catalysts.

    Fig. S8. Tafel and accelerated durability studies for the ORR on NrGO electrocatalysts.

    Fig. S9. High-resolution XPS N 1s and Mn 2p spectra for ether-treated and untreated NrGO catalysts.

    Fig. S10. N2 adsorption isotherms for solvent-treated NrGO catalysts.

  • Supplementary Materials

    This PDF file includes:

    • Supplementary Results and Discussion
    • Table S1. Table of Hansen solubility parameters of selected solvents.
    • Table S2. Summarized XPS data showing differences in atomic ratio of elements for ether-treated and untreated catalysts.
    • Table S3. Summarized XPS data showing differences in C and N speciation for ether-treated and untreated catalysts.
    • Table S4. Summarized calculated BET (Brunauer-Emmett-Teller)–specific surface areas and pore volumes obtained from solvent-treated NrGO catalysts.
    • Fig. S1. High-resolution STEM images of ether-treated GO and NrGO materials.
    • Fig. S2. XRD spectra of starting, intermediate, and end products.
    • Fig. S3. Thermogravimetric analysis of solvent-treated GO samples.
    • Fig. S4. FTIR attenuated total reflectance spectra of water and ice.
    • Fig. S5. High-resolution C 1s XPS spectra for treated GO precursors.
    • Fig. S6. XRD data of ether-treated GO showing the presence of a lamellar KC8 structure at an intermediate temperature (400°C), which disappears at higher temperatures (850°C).
    • Fig. S7. Koutecky-Levich and electrode loading mechanistic studies for ether-treated NrGO catalysts.
    • Fig. S8. Tafel and accelerated durability studies for the ORR on NrGO electrocatalysts.
    • Fig. S9. High-resolution XPS N 1s and Mn 2p spectra for ether-treated and untreated NrGO catalysts.
    • Fig. S10. N2 adsorption isotherms for solvent-treated NrGO catalysts.

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