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.
It is common to find in the literature different values for the working voltage window (WVW) range for aqueous-based supercapacitors. In many cases, even with the best intentions of the widening the operating voltage window, the measured current using the cyclic voltammetry (CV) technique includes a significant contribution from the irreversible Faradaic reactions involved in the water-splitting process, masked by fast scan rates. Sometimes even using low scan rates is hard to determine precisely the correct WVW of the aqueous-based electrochemical capacitor. In this sense, we discuss here the best practices to determine the WVW for capacitive current in an absence of water splitting using complementary techniques such as CV, chronoamperometry (CA), and the electrochemical impedance spectroscopy (EIS). To accomplish this end, we prepare and present a model system composed of multiwalled carbon nanotubes buckypaper electrodes housed in the symmetric coin cell and soaked with an aqueous-based electrolyte. The system electrochemical characteristics are carefully evaluated during the progressive enlargement of the cell voltage window. The presence of residual Faradaic current is verified in the transients from the CA study, as well as the impedance changes revealed by EIS as a function of the applied voltage, is discussed. We verify that an apparent voltage window of 2.0 V determined using the CV technique is drastically decreased to 1.2 V after a close inspection of the CA findings used to discriminate the presence of a parasitic Faradaic process. Some orientations are presented to instigate the establishment in the literature of some good scientific practices concerned with the reliable characterization of supercapacitors.
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