Electrochemical Development and Characterization By SECM of 2D Carbon-Based Materials Architectured for Energy Storage and Conversion
Authors: Franklin Giovanna Formiga , Hassan Omar , Halim El-Mahdi , Taberna Pierre-Louis , Simon Patrice , Duclairoir Florence 
Published: July 2022

With the fastly-growing social and commercial demands for portable electronic devices and the need for developing environmentally friendly high-power energy sources, new non-critical, non-toxic materials are currently being widely studied [1]. Electrochemical Double Layer Capacitors, also termed as supercapacitors, store energy electrostatically by charge separation at the high-surface-area porous carbon / electrolyte interface, by charging the electrochemical double layer [2]. Carbon-based materials are the most widely used electrode materials for EDLCs because of their desirable physical-chemical properties and low cost. Out of the various carbon structures explored as electrode materials, graphene appears to be a promising candidate thanks to its key properties [3].
In this work, in-depth electrochemical characterizations of 2-Dimensional graphene-based materials was achieved using Scanning Electrochemical Microscopy (SECM) technique to study local electrochemical properties: ionic transport and charge transfer properties. 2D carbon materials with tuned interlayer distance were prepared by bridging reduced graphene oxide (rGO) sheets using hexanediamine electroactive molecules acting as pillars. Pillared and non-pillared porous graphene hydrogel (6GH and GHG respectively) composed were also synthetized and tested. Those measurements gave insights about the electroactivity and conductivity of the material’s surface. Also, the influence of the nature and lengths of the pillar on the electrochemical performance have been studied. Preliminary results using conventional electrochemical techniques show that the specific capacitance of raw GHG (170 F g-1) was reduced down to 70F g-1 when adding 6 carbons-hexanediamine pillars during its hydrothermal synthesis to prepare the 6GH (pillared graphene hydrogel). For the rGO, the addition of pillars with different lengths resulted in an increase of specific capacitance from 100 F g-1 to rGO to 140 F g-1 for rGO pillared with 7 atoms pillar diamine pillar (7RP) and 130 F g-1 for rGO pillared with 8 atoms diamine pillar (8RP) .
Then, SECM experiments in feedback mode allowed for a fast comparison of the change in local electrochemical activity for the different surfaces through redox reactions between the tip and the electrode surface for all developed materials. Fitting measured approach curves to simulated ones can be used to estimate rate coefficients, as suggested by Chang, Mirkin and Bard [5]. The effective rate coefficient (keff) values for the mediator regenerating surface reaction were estimated using Wittstock's method by fitting the approach curves recorded at different positions. A 10µm diameter platinum-disc microelectrode was the tip connected to the Bio-Logic SECM-150 bipotentiostat and responsible for the approach curve measurement. For the GHG, it was found a 2.3 10-2 cm s-1 effective constant, which is reduced to 1.6 1.8 10-2 cm s-1 for the 6GH. For the rGO, the keff has the value of 3.3 10-2 cm s-1. The next step is to define the effective constant for the pillared rGO 7RP and 8RP. This study contributed to define the key characteristics allowing for the use of electrodes for supercapacitors offering optimal performance in energy density and power density.
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