Dual Behaviour of Amorphous Carbon Released Electrochemically from Graphite

Abstract

Here we show that the main product released from graphite in the early stages of the anodic polarization is a carbon-based salt comprised of an amorphous carbonaceous material with delocalized negative charge, counterbalanced by sodium cations. Remarkably, this material shows a dual behaviour: it behaves as an electrolyte in aqueous medium, which has the ability to enhance the capacitive properties of the electrodesolution interface and in the solid state it can be used as an electrode material with meaningful capacitive properties.Although the electrochemical approach is one of the most successful synthetic methods for the top-down preparation of a wide spectrum of graphene particles with variable sizes and shapes, like sheets, [1][2][3][4][5] nanoribbons, [6] flakes [6][7][8] and quantum dots, [9][10][11] the nature of the species formed upon the electrochemical oxidation of graphite is still an unclear matter. A close reading of the literature reveals that there is no clear specification of the electrochemical conditions required to prepare selectively just one type of graphene-based material. Notwithstanding, it is known that the chemical composition of the electrolyte and water content of the medium have a crucial effect on the size of the carbon nanoparticles. [6,12,13] It is thus expected that a mixture of different graphene-based structures may be inevitably formed when the electrochemical synthesis is performed in aqueous medium, which explains the common lab practice of centrifugation and filtration after the electrochemical essays. Even though these operations are decisive to improve the selectivity of the production process, they inevitably mask the nature of the species formed. In order to overcome this constrain, the present work was initially motivated to clarify what type of carbon materials are released elec-trochemically from a graphite anode in the early stages of the electrode polarization. No separation process other than dialysis (to remove the electrolyte) was used. More importantly, this study lead us to the finding that the main type of carbon material generated at the anode shows unprecedent electrochemical properties when dissolved in aqueous medium: ability to behave as an electrolyte and to enhance the capacitive properties of the electrode-solution interface.The electrochemical oxidation of graphite was performed under experimental conditions adapted from the literature. [10,11] It was used a phosphate buffer solution (pH 7) and graphite rods as the anode and cathode electrodes. The anodic polarization was carried out under galvanostatic control. The solution removed from the anode compartment after one hour of electrode polarization was transparent and colourless and no visible signs of graphite expansion on the anode was observed. However, analysis of this solution by a Total Organic Carbon analyser (TOC) revealed a significant amount of carbon released from graphite during the galvanostatic polarization (typically 7-10 mg/L). Before further characterization, the anodic solution wa...

Hydrophilic Carbon Nanomaterials: Characterisation by Physical, Chemical, and Biological Assays

“…Actually, no nanoparticles were detected (Figure S1 in the Supporting Information) unless the AFM analysis was performed with a non‐fresh EHC solution (Figure F,G). We speculate that by using a fresh EHC solution, a smooth film is formed, similar to those found for EHC synthesised in phosphate buffer . With the non‐fresh EHC solution, the observation of round carbon domains, mostly <2 nm in diameter with a thickness <0.6 nm, suggested that they form by aggregation of carbon clusters over time.…”

“…Electrogenerated hydrophilic carbon (EHC) nanomaterials, recently developed in our group, might potentially emerge as materials that can overcome the above‐mentioned constraints . This type of nanomaterial, displaying outstanding solubility in water, is released from graphite in the early stages of a galvanostatic polarisation in a two‐compartment cell (Figure A), in a low‐cost, simple, green and fast procedure.…”

“…This type of nanomaterial, displaying outstanding solubility in water, is released from graphite in the early stages of a galvanostatic polarisation in a two‐compartment cell (Figure A), in a low‐cost, simple, green and fast procedure. The first EHC material was synthesised in a phosphate buffer, but other biocompatible electrolyte buffers were also tested, revealing that the redox properties of EHC nanomaterials can be tuned by the nature of the electrolyte . For example, EHC synthesised in phosphate buffer showed no relevant electron‐donating ability, whereas the nanomaterial generated in citrate buffer exhibited strong electron‐donating properties, typical of an antioxidant.…”

One‐Step Cathodic and Anodic Synthesis of Hydrophilic Carbon Nanomaterials

“…The 2D‐assembly may result from the self‐assembled aggregation of the sub‐units that compose the EHC due to strong intermolecular affinities. On the basis of our previous results, electrostatic interactions between oppositely charged species (sodium cations and negative charged carbon‐based nanomaterial) should be the main driving force to form the self‐assembled aggregations. Control experiments with Pt as the anode or cathode electrode allow confirming that EHC could be either generated at the cathode as well as at the anode of graphite (Figure d and 1e′).…”

“…This process promotes the removal of oxidation debris through the repulsion created between the negative charge of the deprotonated debris and the negative charge of the deprotonated surface functional groups of the graphene‐based material. Recently, a simple methodology to produce amorphous hydro‐soluble carbon nanomaterials in one single step was developed by our group based on an electrochemical approach . It was found that this raw material was the major product released at the anodic compartment in the early stages of the galvanostatic polarization using graphite electrodes.…”