engleski

THE NATURE OF REDUCED GRAPHENE OXIDE CAPACITANCE FROM SCAN RATE DEPENDENCES: ADVANTAGES AND ISSUES

Pseudocapacitive behavior of electroactive materials is essential not only for higher specific energy/energy density of supercapacitors (compared to electrostatic capacitors), but also for baterries and electrocatalytically active materials[1]. In some cases, however, presence of active sites responsible for pseudocapacitance can have negative effects on other properties of the active material, responsible for the desired electrochemical behavior. For graphene-based materials this is reflected in the loss of conductivity upon introduction of different functional groups on graphene basal plane. Functional groups (as well as deffects and other „non-ideal“ graphene structural features) are essential for chemisorption and other (pseudo)faradaic processes responsible for pseudocapacitance, but their existence is followed by sp2 to sp3 change in hybridization of carbon atoms and consequent loss of conductivity due to disruption of delocalized π-electron system. Another phenomenon that can impede ion transport and consequently decrease the rate (power) performance of graphene-based material is restacking upon reduction, which leads to decrease in specific surface area and the emergence of microporosity. This lowers the electrolyte ions ability to reach all of the surface of graphene in short time intervals, which is essential for high-power demands for the application in capacitors. Understanding of these effects requires methods for the differentiation of pure electrostatic and faradaic processes, as well as in-pore diffusion limitations (which includes desolvation of ions to some extent), and estimation of their contributions to the total measured capacitance, which could be important in the design of the active material to achieve the best performance. Different approaches based on electrochemical response, such as dependence of voltammetric current on the scan rate, shape of CVs and electrochemical impedance spectra, step potentials spectroscopy and dependencies of C on the scan rate can be found in the literature. Some recent papers show that extrapolation of linear parts of the C = f (v-1/2) and C-1 = f (v1/2) dependencies can be used to evaluate cotributions of different processes to the overall capacitance[2-7]. Although this is simple and promising method for the evaluation of electrochemical behavior, at least for graphene- based materials, there are some issues that should be considered before its application. Here, use of the C - v dependecies is critically reviewed, with the experimental results given for different materials known for their pseudocapacitive behavior, but with the focus on the graphene-based materials. Reasons for unexpectedly low relative contributions of double-layer capacitance, even for the highly reduced graphene oxides with high surface areas, are discussed, as well as the choice of „linear part“ of C – v functions and discrepancies between extrapolated and experimentally observed values. Futhermore, correlations between capacitance retention at high scan rates and non- DL capacitance contributions, obtained from these functions, are discussed for different graphene materials.

reduced graphene oxide, capacitance contribution, C -v dependence

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