Correction to: Scalable Water-Based Production of Highly Conductive 2D Nanosheets with Ultrahigh Volumetric Capacitance and Rate Capability (Advanced Energy Materials, (2018), 8, 18, (1800227), 10.1002/aenm.201800227)

Hyeonyeol Jeon, Jae Min Jeong, Heon Gyu Kang, Hyung Jin Kim, Jeyoung Park, Do Hyun Kim, Young Mee Jung, Sung Yeon Hwang, Young Kyu Han, Bong Gill Choi

Research output: Contribution to journalComment/debate

Abstract

Cyclic voltammogram (CV) and galvanostatic charge/discharge (GCD) measurements are experimental criteria for qualitatively evaluating the capacitive behavior of electrode materials or supercapacitor devices. The choice of formula for calculating capacitance and energy values is a source of a great deal of discussion and controversy in the supercapacitor community. Many researchers use linear equations derived from electrical double layer capacitors for pseudocapacitive or composite electrodes with non-ideal rectangular CV and non-linear GCD curves. However, in principle, integration equations could provide more accurate information on electrochemical performance than linear equations. In the original article, based on a GCD test using linear equations, gravimetric capacitance (Cg) and energy density (E, W h kg−1) were calculated using the following formulas: (Formula presented.) where I is the current density (A g–1), Δt is the discharge time (s), ΔU is the potential window (V), Ccell is the cell capacitance (F kg–1) and ΔV is the cell voltage (V).Linear equations were used to calculate the capacitance and energy density values of the electrode materials and supercapacitor devices in the original article. Here, data are now provided on the capacitance and energy density values using integral equations. Accordingly, the gravimetric and volumetric (Cvol) capacitances were calculated from the discharge curves of the GCD using the following formulas: (Formula presented.) where U is the potential window (V) with Ui and Uf being the initial and final values, respectively, I is the current density (A g–1), t is the discharge time (s), and ρ is the packing density (g cm–3) of the film electrode. Cvol was converted using the electrode density. The energy density of the supercapacitor device was calculated from the discharge curves of the GCD using the following formula: (Formula presented.) where I is the discharge current (A), V is the cell voltage (V) after iR drop, t is the discharge time (s), and m is the total mass (kg) of both electrodes. After recalculating the capacitance and energy values using integral equations, these values were compared with those in the original article (Table 1). Compared to the values reported in the original article, the recalculated values are somewhat lower, but the decreasing trends as the Gr loading increases are similar. Comparison of electrochemical performance of linear and integral equations based on GCD data (Table presented.) a)The values in the original article. The measurement methods (i.e., changing the techniques and parameters) used to evaluate the electrochemical performance of the electrode materials could also affect the reported results depending on whether CV or GCD techniques are used. In the original article, the specific capacitance values of the electrode materials were reported using the GCD technique only. Here, data on the volumetric capacitance and rate capability of the electrode materials using the CV technique are provided (Table 2). CV and GCD measurements were performed using a VersaSTAT 4 instrument (Princeton Applied Research) equipped with a power booster (specification of ± 20 A and ± 20 V). Although there is a small difference between the capacitance values of the CV and GCD methods, the decreasing tendency of the capacitances as the Gr loading increases are similar for both techniques. The rate capabilities of the electrode materials obtained from the GCD technique are also very close to those of the CV method. Comparison of the electrochemical performance of the GCD and CV techniques (Table presented.) A scalable water-based fluid dynamic exfoliation process was reported for preparing highly conductive metallic phase MoS2 and graphene sheets and then free-standing flexible hybrid films were fabricated for the high-volumetric performance of flexible supercapacitor devices. Additional studies on hybrid films were also conducted using organic electrolytes of 1-ethyl-3-methylimidazoluim tetrafluoroborate ([EMIM][BF4]) dissolved in acetonitrile (ACN) to increase the energy density. The energy density value of 564.91 W h kg−1 (1.14 W h cm–3), which was based on the organic electrolytes reported in the original article, resulted from an error in the calculation of the energy density using overcharged GCD curves. A cell voltage of 1 V was used for the aqueous electrolyte instead of 3.5 V for the energy density calculation of the organic electrolyte, thus leading to the incorrect energy density value in the original article. To address this error, the GCD of the supercapacitors was retested (Figure 5f) and the energy densities were recalculated using an integral equation instead of a linear equation (Figures g,h). The newly obtained CV and GCD data show stable cell voltages of ≈3.2 V. Based on these new results, the maximum energy density is 122.2 W h kg−1 at 1.4 kW kg−1 (246.9 mW h cm−3 at 2.97 W cm−3). (Figure presented.) f) The cyclic voltammetry of [EMIM][BF4]/ACN-SCs measured at a scan rate of 25 mV s−1 in the [EMIM][BF4]/ACN electrolyte (inset: the galvanostatic charge/discharge curves measured at 1, 50, and 100 A g−1). The Ragone plots of g) the volumetric and h) gravimetric energy and power densities of the PN-SCs and [EMIM][BF4]/ACN-SCs compared favorably with other previously reported SCs. The corrected Figure f–h are shown here: The authors apologize for these errors and any confusion they may have caused.

Original languageEnglish
Article number1802691
JournalAdvanced Energy Materials
Volume8
Issue number34
DOIs
StatePublished - 5 Dec 2018

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