Graphene synthesized by the reduction of graphene oxide (GO) features in a myriad of applications... more Graphene synthesized by the reduction of graphene oxide (GO) features in a myriad of applications ranging from sensors to batteries and catalysts to dye-sensitized solar cells. The exceptional physical and electrochemical properties of graphene originate from the presence of several residual functional groups and the non-stoichiometry in its structure. But, investigating the evolution of graphene from GO has been a daunting task. In this manuscript, simple electrochemical methods are reported to characterize GO subjected to thermal, electrochemical, and chemical reduction. The electrochemical features of these samples along with their FTIR spectra and XRD patterns help to identify the functional groups and provide compelling evidence for the transformation among them during the reduction of GO. The redox features of the voltammograms suggest the conversion of epoxides to carbonyl, carbonyl to carboxylic acid groups, and their subsequent removal with potential cycling. Thermal treatment of GO in the range of 80-150 C causes the conversion of some of the epoxides to carbonyls and removal of water content. At the same time, epoxides are more prevalent in chemically reduced GO. The double layer capacitanceone of the figure of merits that distinguishes graphene from other carbon allotropesgives an indication of the reduced graphene oxide content in the sample. Thus, electrochemical characterization sheds significant light onto the nature of oxygen moieties in non-heat-treated GO (n-HT-GO), thermally reduced GO (t-GO), chemically reduced GO (c-RGO) and electrochemically reduced GO (e-RGO), besides explaining the range of reported electrochemical capacitance.
Oxygen reduction reaction (ORR) in acidic media is investigated at various potentials in a thin-f... more Oxygen reduction reaction (ORR) in acidic media is investigated at various potentials in a thin-film rotating disk electrode (TF-RDE) configuration using electrochemical impedance spectroscopy (EIS). The ionomer-free and ionomer-containing thin-film catalyst layers are composed of Pt black and carbon-supported Pt catalysts of different metal loadings (5 and 20 wt%). The simplest EI spectrum consisting of an arc or a semicircle is obtained at high potentials with ionomer-free Pt catalyst layers. The most complex spectrum consisting of a high frequency (HF) arc and two semicircles is observed in the mixed diffusion-controlled region of the ionomer-containing catalyst layer with high loading of carbon-supported Pt. The nature of the EI spectrum is decided by the constituents of the thin-film catalyst layer and by the operating potential. The evolution of the EI spectra with ionomer and carbon contents is underlined. The effect of rotation rate (rpm) of the electrode on the impedance spectrum is also investigated. A series of equivalent circuits is required to completely describe the EI spectra of ORR. The kinetic parameters and the electrochemical surface area of the catalysts are derived from the impedance spectrum.
Graphene synthesized by the reduction of graphene oxide (GO) features in a myriad of applications... more Graphene synthesized by the reduction of graphene oxide (GO) features in a myriad of applications ranging from sensors to batteries and catalysts to dye-sensitized solar cells. The exceptional physical and electrochemical properties of graphene originate from the presence of several residual functional groups and the non-stoichiometry in its structure. But, investigating the evolution of graphene from GO has been a daunting task. In this manuscript, simple electrochemical methods are reported to characterize GO subjected to thermal, electrochemical, and chemical reduction. The electrochemical features of these samples along with their FTIR spectra and XRD patterns help to identify the functional groups and provide compelling evidence for the transformation among them during the reduction of GO. The redox features of the voltammograms suggest the conversion of epoxides to carbonyl, carbonyl to carboxylic acid groups, and their subsequent removal with potential cycling. Thermal treatment of GO in the range of 80-150 C causes the conversion of some of the epoxides to carbonyls and removal of water content. At the same time, epoxides are more prevalent in chemically reduced GO. The double layer capacitanceone of the figure of merits that distinguishes graphene from other carbon allotropesgives an indication of the reduced graphene oxide content in the sample. Thus, electrochemical characterization sheds significant light onto the nature of oxygen moieties in non-heat-treated GO (n-HT-GO), thermally reduced GO (t-GO), chemically reduced GO (c-RGO) and electrochemically reduced GO (e-RGO), besides explaining the range of reported electrochemical capacitance.
Oxygen reduction reaction (ORR) in acidic media is investigated at various potentials in a thin-f... more Oxygen reduction reaction (ORR) in acidic media is investigated at various potentials in a thin-film rotating disk electrode (TF-RDE) configuration using electrochemical impedance spectroscopy (EIS). The ionomer-free and ionomer-containing thin-film catalyst layers are composed of Pt black and carbon-supported Pt catalysts of different metal loadings (5 and 20 wt%). The simplest EI spectrum consisting of an arc or a semicircle is obtained at high potentials with ionomer-free Pt catalyst layers. The most complex spectrum consisting of a high frequency (HF) arc and two semicircles is observed in the mixed diffusion-controlled region of the ionomer-containing catalyst layer with high loading of carbon-supported Pt. The nature of the EI spectrum is decided by the constituents of the thin-film catalyst layer and by the operating potential. The evolution of the EI spectra with ionomer and carbon contents is underlined. The effect of rotation rate (rpm) of the electrode on the impedance spectrum is also investigated. A series of equivalent circuits is required to completely describe the EI spectra of ORR. The kinetic parameters and the electrochemical surface area of the catalysts are derived from the impedance spectrum.
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