2 research outputs found

    Efficient Graphene Production by Combined Bipolar Electrochemical Intercalation and High-Shear Exfoliation

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    In this study, we demonstrate that bipolar electrochemistry is a viable strategy for “wireless” electrochemical intercalation of graphite flakes and further large-scale production of high-quality graphene suspensions. Expansion of the graphite layers leads to a dramatic 20-fold increase in the yield of high-shear exfoliation. Large graphite flakes, which do not produce graphene upon high shear if left untreated, are exfoliated in a yield of 16.0 ± 0.2%. Successful graphene production was confirmed by Raman spectroscopy and scanning transmission electron microscopy, showing that the graphene flakes are 0.4–1.5 μm in size with the majority of flakes consisting of 4–6 graphene layers. Moreover, a low intensity of the D peak relative to the G peak as expressed by the <i>I</i><sub>D</sub>/<i>I</i><sub>G</sub> ratio in Raman spectroscopy along with high-resolution transmission electron microscopy images reveals that the graphene sheets are essentially undamaged by the electrochemical intercalation. Some impurities reside on the graphene after the electrochemical treatment, presumably because of oxidative polymerization of the solvent, as suggested by electron energy loss spectroscopy and X-ray photoelectron spectroscopy. In general, the bipolar electrochemical exfoliation method provides a pathway for intercalation on a wider range of graphite substrates and enhances the efficiency of the exfoliation. This method could potentially be combined with simultaneous electrochemical functionalization to provide graphene specifically designed for a given composite on a larger scale

    Selective CO<sub>2</sub> Reduction to CO in Water using Earth-Abundant Metal and Nitrogen-Doped Carbon Electrocatalysts

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    Earth-abundant transition metal (Fe, Co, or Ni) and nitrogen-doped porous carbon electrocatalysts (M-N-C, where M denotes the metal) were synthesized from cheap precursors via silica-templated pyrolysis. The effect of the material composition and structure (i.e., porosity, nitrogen doping, metal identity, and oxygen functionalization) on the activity for the electrochemical CO<sub>2</sub> reduction reaction (CO<sub>2</sub>RR) was investigated. The metal-free N-C exhibits a high selectivity but low activity for CO<sub>2</sub>RR. Incorporation of the Fe and Ni, but not Co, sites in the N-C material is able to significantly enhance the activity. The general selectivity order for CO<sub>2</sub>-to-CO conversion in water is found to be Ni > Fe ≫ Co with respect to the metal in M-N-C, while the activity follows Ni, Fe ≫ Co. Notably, the Ni-doped carbon exhibits a high selectivity with a faradaic efficiency of 93% for CO production. Tafel analysis shows a change of the rate-determining step as the metal overtakes the role of the nitrogen as the most active site. Recording the X-ray photoelectron spectra and extended X-ray absorption fine structure demonstrates that the metals are atomically dispersed in the carbon matrix, most likely coordinated to four nitrogen atoms and with carbon atoms serving as a second coordination shell. Presumably, the carbon atoms in the second coordination shell of the metal sites in M-N-C significantly affect the CO<sub>2</sub>RR activity because the opposite reactivity order is found for carbon supported metal meso-tetraphenylporphyrin complexes. From a better understanding of the relationship between the CO<sub>2</sub>RR activity and the material structure, it becomes possible to rationally design high-performance porous carbon electrocatalysts involving earth-abundant metals for CO<sub>2</sub> valorization