The Ragone plot (RP) compares devices in energy and power density characteristics, helping engineers decide on a specific energy storage system for technological applications. RP would contrast the performance of several technologies together under similar boundary conditions. Unfortunately, that is not what is happening for electrochemical double‐layer capacitors (EDLCs) because conditions for calculating energy and power are not standardized in the literature, resulting in severe discrepancies in the obtained values. With this widespread misconception, RP may become an unreliable tool for characterizing EDLCs. An intervention in this vital issue now becomes necessary. To put action on that, a rational strategy for dealing with measurements needed for EDLCs and calculations to prepare a reliable RP is reported here. This manuscript presents a step‐by‐step procedure to provide a correct RP. Also, simple theoretical demonstrations of how to get each fundamental equation are presented. In general, this work contributes as a guide to obtain reliable RPs for EDLCs.
Comprehensive electrochemical and
operando Raman studies are performed
to investigate the electrochemical stability window (ESW) of supercapacitors
filled with normal (salt-in-water) and highly concentrated (water-in-salt,
WiSE) electrolytes. Impedance and chronoamperometric experiments are
employed and combined with cyclic voltammetry to correctly define
the ESW for a WiSE-based device. The total absence of water-splitting
resulted in phase angles close to −90° in the impedance
data. It is verified that a 17 m NaClO4 electrolyte avoids
the water-splitting up to 1.8 V. Furthermore, Raman studies under
dynamic and static polarization conditions corroborate the existence
of a solvent blocking interface (SBI), which inhibits the occurrence
of water-splitting. Also, the reversible nature of the charge-storage
process is assessed as a function of the applied voltage. At extreme
polarization, the SBI structure is disrupted, thus allowing the occurrence
of water-splitting and anionic (ClO4
–) intercalation between the graphene sheets.
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