Abstract
Conventional chemical oxidation routes for the production of graphene oxide (GO), such as the Hummers' method, suffer from environmental and safety issues due to their use of hazardous and explosive chemicals. These issues are addressed by electrochemical oxidation methods, but such approaches typically have a low yield due to inhomogeneous oxidation. Herein we report a two-step electrochemical intercalation and oxidation approach to produce GO on the large laboratory scale (tens of grams) comprising (1) forming a stage 1 graphite intercalation compound (GIC) in concentrated sulfuric acid and (2) oxidizing and exfoliating the stage 1 GIC in an aqueous solution of 0.1 M ammonium sulfate. This two-step approach leads to GO with a high yield (>70 wt %), good quality (>90%, monolayer), and reasonable oxygen content (17.7 at. %). Moreover, the as-produced GO can be subsequently deeply reduced (3.2 at. % oxygen; C/O ratio 30.2) to yield highly conductive (54 600 S m-1) reduced GO. Electrochemical capacitors based on the reduced GO showed an ultrahigh rate capability of up to 10 V s-1 due to this high conductivity.
Highlights
Graphene oxide (GO) is a two-dimensional, single-layer of carbon atoms that is covalently functionalized with oxygen containing groups (hydroxyl, epoxide, carboxylic, and carbonyl).[1,2] These oxygen functional groups allow GO to be processed in solution and give it the capability to act as a building block for the assembly of various macroscale graphene architectures (fiber, paper, hydrogel, aerogel, etc.).[3−5]upon reduction by either thermal or chemical means, the graphitic network can be restored, making reduced GO (rGO) that is electrically conductive.[3,6] These unique features of GO and its derivatives have led to research into applications including transparent conductive films,[7,8] electrochemical energy storage,[9−11] printed electronics,[12,13] water desalination,[14,15] and polymer composites.[16,17]Currently, the majority of the GO in the literature is produced by the chemical oxidation of graphite flakes, using either the Brodie,[18] Staudenmaier,[19] or Hummers’[20] methods.All these chemical oxidation approaches involve the use of strong acids and oxidants, despite their extensive optimization, leading to environmental and safety issues when scaled up.[21,22] The use of strong oxidants, such as potassium permanganate, complicates the removal of the metal ions from the as-made GO, but leads to permanent defects that cannot be restored upon reduction.[23,24]
Article min to allow the homogeneous formation of a stage 1 graphite intercalation compound (GIC) both at the surface and in the bulk of the graphite electrode
These results suggest the crucial role of low stage GICs (n ≤ 2) as intermediates in the formation of electrochemical GO (EGO), which is consistent with the formation of Graphene oxide (GO) from the stage 1 GIC in the H2SO4/KMnO4 system.[35]
Summary
Graphene oxide (GO) is a two-dimensional, single-layer of carbon atoms that is covalently functionalized with oxygen containing groups (hydroxyl, epoxide, carboxylic, and carbonyl).[1,2] These oxygen functional groups allow GO to be processed in solution and give it the capability to act as a building block for the assembly of various macroscale graphene architectures (fiber, paper, hydrogel, aerogel, etc.).[3−5]upon reduction by either thermal or chemical means, the graphitic network can be restored, making reduced GO (rGO) that is electrically conductive.[3,6] These unique features of GO and its derivatives have led to research into applications including transparent conductive films,[7,8] electrochemical energy storage,[9−11] printed electronics,[12,13] water desalination,[14,15] and polymer composites.[16,17]Currently, the majority of the GO in the literature is produced by the chemical oxidation of graphite flakes, using either the Brodie,[18] Staudenmaier,[19] or Hummers’[20] methods.All these chemical oxidation approaches involve the use of strong acids and oxidants, despite their extensive optimization, leading to environmental and safety issues when scaled up.[21,22] The use of strong oxidants, such as potassium permanganate, complicates the removal of the metal ions from the as-made GO, but leads to permanent defects that cannot be restored upon reduction.[23,24].
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