A rational experimental approach to identify correctly the working voltage window of aqueous-based supercapacitors

Abstract

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.

Effect of electrolyte and carbon material on the electrochemical performance of high-voltage aqueous symmetric supercapacitors

“…It has been shown that electrochemical capacitors using electrodes with highly porous carbon materials operating in a neutral environment can achieve a wider electrochemical window due to the high cathodic overpotential of the hydrogen evolution reaction noticed at neutral pH [23]. Neutral aqueous electrolytes are also characterized by features that make them an attractive working environment for supercapacitors: environmentally friendly, inexpensive and noncorrosive [23][24][25].…”

New Insights on the Sodium Water-in-Salt Electrolyte and Carbon Electrode Interface from Electrochemistry and Operando Raman Studies

“…In this sense, the use of single-step chronoamperometry (SS-CA) and electrochemical impedance spectroscopy (EIS) techniques can be quite helpful to identify the ESW in SCs correctly. Nunes et al reported that very accurate findings showing excellent internal consistency regarding the different electrochemical techniques could permit the identification of the ESW unambiguously. Unfortunately, several authors have tried to establish the ESW in WiSEs considering only the CV findings at high scan rates and/or GCD using high gravimetric currents, which mask the irreversible faradaic (water-splitting) reaction.…”

“…Compared to other WiSEs, the highly concentrated NaClO 4 solutions have excellent solubility, low viscosity, and high conductivity. , From these considerations, the current work uses different NaClO 4 concentrations for applying the good practices recently proposed to identify the ESW in SCs correctly . Subsequently, operando Raman studies were performed for different NaClO 4 concentrations using dynamic (CV) and static voltage (SS-CA) conditions, as summarized in Figure .…”

“…In this sense, the use of single-step chronoamperometry (SS-CA) and electrochemical impedance spectroscopy (EIS) techniques can be quite helpful to identify the ESW in SCs correctly. Nunes et al 31 From these considerations, it was investigated in the present work the real ESW-values for the NaClO 4 -based WiSEs using a rational experimental approach comprising the combination of three different electrochemical techniques. Another recent development in the study of energy storage devices is based on the operando (in situ) experimental approach where an external radiation source impinges the electrode material during the static or dynamic polarization of the electrode/solution interface.…”

“…6,55 From these considerations, the current work uses different NaClO 4 concentrations for applying the good practices recently proposed to identify the ESW in SCs correctly. 31 Subsequently, operando Raman studies were performed for different NaClO 4 concentrations using dynamic (CV) and static voltage (SS-CA) conditions, as summarized in Figure 1. As seen, the advantages of using the Raman spectroscopy together with electrochemical methods are the following: (i) CV case, one might see the D-band shifts indicating a reversible anion insertion process in the host electrode material; (ii) CA case, it was verified to be an irreversible ionic intercalation process, i.e., a process affecting the E 2G and E 2G ′ bands.…”