Contribution of Cations and Anions of Aqueous Electrolytes to the Charge Stored at the Electric Electrolyte/Electrode Interface of Carbon-Based Supercapacitors

Abstract

For their use in supercapacitors, aqueous electrolytes of acidic (H 2 SO 4 ), neutral (Na 2 SO 4 , K 2 SO 4 ) and basic (NaOH, KOH) nature are studied, using two microporous binder-free and self-standing carbon cloths as electrodes. The carbon cloths show similar porosities and specific surface areas, but different contents in surface oxygen groups. The working potential window and the specific capacitance associated with the cations and anions are measured. From these parameters, the charges stored by the cations and anions at the electric electrolyte/electrode interface are deduced. The charge stored by the cations is higher than that stored by the anions for the three types of electrolytes. The differences between cations and anions are higher for the acidic and basic electrolyte than for the neutral electrolytes, and also higher for the carbon cloth with the highest content in surface oxygen groups. The charge stored by the cations follows the sequence H 3 O + > Na + or K + from the basic electrolytes > Na + or K + from the neutral electrolytes. The charge stored by the anions follows the sequence SO 4 2-The results here reported provide a better understanding on the electric double layer of carbon-based supercapacitors. Those results are also of interest for asymmetric and hybrid supercapacitors.

A Procedure for Evaluating the Capacity Associated with Battery-Type Electrode and Supercapacitor-Type One in Composite Electrodes

“…The specific surface area of 1100 m 2 g −1 comes from the presence of big micropores (0.7-2 nm) and small mesopores (3-4 nm). The electrical conductivity, of 0.15 S cm −1 measured in the perpendicular direction of the CC, 53 is sufficient to perform the electrodepositions.…”

“…1000 m 2 g −1 ), which comes mainly from the presence of big micropores (0.7-2 nm). 53 The CC is also light and inexpensive. This substrate has been chosen for chemical depositions [58][59][60][61][62][63][64] and for electrodepositions 45,65 of active electrode materials.…”

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“…Furthermore, ionic liquids possess the following key advantages [ 53 , 54 , 55 ]: high thermal and electrochemical stability, non-inflammability, negligible volatility, a high operative voltage (above 3 V) even greater than that of organic electrolytes, and tunable physical and chemical properties.…”

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“…However, no faradic charge transfer behavior could be observed for OCN in 1 M H 2 SO 4 ( Figures 2D and S5A) and 0.5 M K 2 SO 4 electrolytes (Figures 2E and S5C). The asymmetrical rectangular shape in H 2 SO 4 electrolytes would be due to the distinct electroadsorption behaviors of cations and anions 98,99 and ion-sieving effects 100 of porous OCN. These results indicate that the proton (H + ), potassium cation (K + ), hydroxyl (OH À ), and sulfate anion (SO 4 2À ) in the electrolytes present accessible behaviors different to that of the OCN surface to accomplish the charging process.…”