Use of 1,10-Phenanthroline as an Additive for High-Performance

Article pubs.acs.org/JPCC

Use of 1,10-Phenanthroline as an Additive for High-Performance Supercapacitors A. Borenstein,*,† S. Hershkovitz,‡ A. Oz,‡ S. Luski,† Y. Tsur,‡ and D. Aurbach† †

Department of Chemistry, Bar Ilan University, Ramat Gan 5290002, Israel Department of Chemical Engineering, TechnionIsrael Institute of Technology, Haifa 3200003, Israel

‡

S Supporting Information *

ABSTRACT: In this study, we present the positive effect of 1,10phenanthroline as an electrolyte additive that is strongly adsorbed on activated carbon electrodes, thereby adding effective redox activity to their initially capacitive interactions with electrolyte solutions. We obtain a stable capacitance of 320 F/g for the negative electrode and 190 F/ gelectrode for full symmetric supercapacitor cells, operating up to 3.4 V in nonaqueous media, during many thousands of cycles. This corresponds to a specific capacity of 180 (mA h)/gelectrode. The high voltage and capacity of these systems can pave the way for developing high-energy-density pseudocapacitors that may be able to compete with battery systems. We explored the mechanisms of the electrode interactions using electrochemical tools, including impedance spectroscopy.

The energy stored in any capacitor is given by E = 0.5CV2, where C is the total capacitance of the cell and V is the voltage applied. Note that for, a symmetrical capacitor for which the electrode capacitance is C′, the specific energy based on the electrodes alone is E = 0.125C′V2. The voltage depends directly on the electrochemical window of the electrolyte solution. Three types of solution are possible for electrochemical capacitors: aqueous, organic, or ionic liquids. Aqueous solutions are favorable owing to their specific capacitance requirements due to good wetting, reasonably low viscosity, and excellent ion transport properties, but are limited by the narrow electrochemical window of water (1.2 V). Organic solutions usually enable less specific capacitance than aqueous solutions in contact with activated carbon electrodes, but may enable voltages of up to 3.5 V. In reality, the electrochemical window of supercapacitors with organic solutions is below 3 V. Intensive research has been conducted in recent years on ionic liquids (ILs) related to SCs, because their wide electrochemical window enables development of SCs with relatively high energy density (on scales related to SCs and PSCs). In turn, ILs suffer some crucial limitations such as low ionic conductivity, poor low-temperature performance, wetting problems, high cost, and the presence of impurities. In practice, it has been difficult to realize the high-voltage advantage of ILs in SCs, and many papers have reported limited voltages and cyclability when applying ILs for SCs.8,14,15 1,10-Phenanthroline (phen) is a heterocyclic organic compound that is soluble in many organic solvents. It is a

1. INTRODUCTION The global increase in energy consumption motivates the research and development of high-performance electrochemical energy storage systems. Supercapacitors (SCs) are energy storage devices for high-power and long-cycling applications. Unlike batteries, which use redox reactions to store energy, SCs work via electrostatic interactions. SCs have high power density and can be fully charged and discharged in seconds while exhibiting a very prolonged cycle life due to the fact that the electrostatic interactions do not degrade the electrodes, yet due to their operation via such interactions, SCs possess low energy density.1−4 Pseudocapacitors (PSCs) store energy by a combination of electrostatic and surface redox interactions. Their electrodes are prepared by coating their surfaces with active redox materials. These materials include transition-metal oxides, electronically conducting polymers, and organometallic moieties. PSCs sustain higher energy than SCs, in many cases at the expense of stability during prolonged cycling. In recent years, increasing attention has been devoted to the option of introducing redox species into the electrolyte solutions that can be adsorbed onto porous carbon electrodes, thereby enhancing their specific capacity in charge storage. In such systems, the electrodes themselves have initially only electrostatic properties in solution. Adsorption of redox moieties from the solution phase adds surface redox activity that can multiply their reversible charge storage capability (that is, their specific capacity). In such systems, however, it is important to fully understand the total influence of the redox additives to prevent detrimental side effects such as degradation of the solution and the electrodes, development of undesirable passivation phenomena, etc.5−13 © 2015 American Chemical Society

Received: March 10, 2015 Revised: April 27, 2015 Published: May 5, 2015 12165

DOI: 10.1021/acs.jpcc.5b02335 J. Phys. Chem. C 2015, 119, 12165−12173

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The Journal of Physical Chemistry C

material) on a 3.5 × 3.0 cm2 aluminum current collector were separated by the NKK separator and packed in a pouch cell. Each cell consisted of 0.4 mL of PC/1.8 (TEA)BF4 electrolyte solution (density 1.2 g/cm3). Some of the cells contain phenanthroline dissolved in the electrolyte solution. The overall mass of each total cell was less than 2 g. 2.2. Instruments. All electrochemical measurements were carried out using potentiostat/galvanostat computerized instruments from Bio-Logic Inc. Computerized multichannel battery analyzers from Arbin Inc. were used for prolonged cycling measurements (voltage vs time at constant current). The quasireference electrode used in this study was silver wire (SigmaAldrich), after determination of its stability compared to that of the standard Ag/AgCl reference electrode (Bio-Logic, France). We confirmed that it can be used as a stable pseudoreference electrode in these systems, by prolonged potentiometric measurements vs the saturated calomel electrode (SCE) in the same solution. The stable potential difference between the Ag wire and SCE was around 20 mV. Hence, the voltage scale in the charts related to voltammetric measurements was referred to the SCE. The gas adsorption measurements were carried out using the Autosorb-1 MP (Quantachrome, United States) system. The specific surface area (SSA) was calculated using the Brunauer− Emmett−Teller (BET) model.

redox-active material that can undergo three reversible redox steps. It was first used as an energy storage candidate in our laboratory as a ligand of ferroin ([Fe(phen)3]2+) and demonstrated high compatibility for that purpose.16 In this study, we use only the ligand phenanthroline, thereby gaining an increase in the gravimetric specific capacitance. The breakthrough made in this study is the demonstration of PSC electrodes working over a wide potential window, up to 3.4 V, with simple, widely used, safe, and cost-effective organic electrolyte solutions based on propylene carbonate (PC). Such a high voltage of operation means an impressive gain in energy density for PSC devices, based on the components described in this paper. In this paper, we move a step further, beyond the usual presentation of electrochemical performance, by the judicious use of electrochemical impedance spectroscopy (EIS) for a rigorous analysis of the electrochemical systems studied. The approach used herein for electrode analysis by EIS is described in detail in refs 17−19 and was further developed by Tsur et al.20 A full comprehensive explanation of the analysis of EIS data used herein is beyond the scope of this work, but is fully presented in the above references. The analysis of the EIS data that we present here is based on finding the distribution function of relaxation times (DFRT) of the measured sample according to ∞

Z(ω) = R pol

∫−∞

Γ(log(τ )) d(log(τ )) 1 + iωτ

3. RESULTS The porous character of the electrode materials must match well with the ions in the electrolyte solutions. The TEA cation’s diameter is reported as 0.45 nm.30 The size of the phenanthroline compound is 0.72 nm.31 We searched for a compatible electrode material that has large enough pores. Additionally, according to many papers, mesopores have a positive influence on fast ion diffusion. A commercial activated carbon from Energ2 was found to have a suitable hierarchical structure, with a relatively high concentration of mesoporosity and average pore size of 8 nm (see Figure S1 of the Supporting Information). Therefore, the electrode porosity is compatible with the ionic size, and good diffusion can be expected. We conducted various experiments to compare the electrochemical behavior of the commercial electrolyte solution PC/ (TEA)BF4 and the same solution with the phenanthroline additive. In all aspects, including high capacitance, voltage stability, cyclability, and impedance, the phen system demonstrates superior behavior compared to the reference system without phenanthroline. Figure 1 illustrates a typical cyclic voltammetry (CV) curve for the activated carbon electrodes used with 0.1 M phenanthroline in PC/1.8 M (TEA)BF4 electrolyte solution. We applied a potential window between −2.4 and +1.8 V vs SCE, with an overall voltage window of more than 4.2 V in these introductory experiments. The actual reference electrode used was a silver wire. The open-circuit voltage (OCV) was −0.2 V vs SCE, so that the working electrode could be oxidized up to 2 V and reduced down to −2.2 V from its OCV. The phenanthroline exhibits in these systems two sets of redox activities, as already reported:16 an oxidation interaction at 0.7 V vs SCE related to the reaction phen → [phen]1+ and three reduction interactions at −1.24, −1.6, and −2.1 V vs SCE related to the reactions phen → [phen]1− → [phen]2− → [phen]3−. For comparison, the CV curve of the same cell without phen additive is also shown in Figure 1. The difference

(1)

where Z(ω) is the impedance, ω the frequency, Rpol the total resistance, Γ the DFRT, and τ the relaxation time. To find the DFRT model that best fits the measured data, we analyze it using the Impedance Spectroscopy Genetic Program (ISGP),20−23 which finds a functional form of the DFRT. The outcome of this analysis is an analytical function of the distribution of relaxation times in the tested device, which provides convolution with the relevant kernel (see eq 1), with the best fit to the measured data. Hence, we can plot Γ vs frequency and see separately the elements that contribute to the impedance in the system on each frequency. A more detailed description of these methods, including the analysis of different impedance behaviors, can be found in the Supporting Information.24−29

2. MATERIALS AND METHODS The 1,10-phenanthroline compound used in this study was purchased from Sigma-Aldrich. The activated carbon (AC) powder was obtained from Energ2 (United States, ∼1500 m2/ g). Electric double layer capacitor (EDLC) nonwoven separators from NKK (Japan) were used. The electrolyte solutions containing 1.8 M tetraethylammonium tetrafluoroborate ((TEA)BF4) in PC were purchased from Honeywell. Poly(acrylic acid) (PAA; Sigma-Aldrich, United States) was used as a binder. The carbon black used in this study as a conducting agent was super P carbon from Timcal (Switzerland). 2.1. Cell Preparation. A slurry was made by mixing AC, carbon black, and binder (0.87:0.05:0.08, w/w/w), dissolved in N-methyl-2-pyrrolidone (NMP) solution. The slurry was pasted onto an aluminum current collector, dried, and rollmilled. The electrodes and separators were dried under vacuum prior to cell assembly. Two electrodes of 10 mg each (active 12166


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in the voltammetric response due to the presence of the additive is spectacular. The cells containing PC-based electrolyte solution demonstrated a stable capacitance of approximately 110 F/g in CV measurements (1 mV/s, Figure 2c). At high current density up to 3 V, we obtained 90 F/g (1 A/g, galvanostatic measurements, Figure 2a,c). The cells were cycled 3000 times with 6% capacitance loss. Following this, the voltage of the charge− discharge cycling was increased to 3.2 V for an additional 2000 cycles and finally to 3.4 V for an extra 3000 cycles. During the latter two steps, a change in the shape of the galvanostatic curve was observed, as well as a sharp decrease in capacitance (36%; see Figure 2a,f and Figure S2a of the Supporting Information). Similar experiments were conducted using a solution containing the phenanthroline additive (0.1 M). As expected, the capacitance obtained was higher, as a result of the additional faradic capacitance of the redox reaction. We obtained 190 F/ gelectrode at a slow scan rate (1 mV/s, Figure S2b) and current densities, while a capacitance of 110 F/g electrode was demonstrated at high current density in galvanostatic measurements (1 A/gelectrode, corresponding to 1 mA/cm2; see Figure 2b,d). It is important to note that this capacitance was obtained in a symmetric two-electrode cell, while in fact, phen does not respond symmetrically upon reduction and oxidation. Accord-

Figure 1. A typical CV curve of the system representing the redox potentials of phen in capacitors based on activated carbon electrodes. The cell consisted of PC/1.8 M (TEA)BF4 electrolyte solution with addition of 0.1 M phen (blue), two activated carbon electrodes, and silver wire as the quasi-reference electrode. The potential is expressed vs the potential of the saturated calomel electrode (SCE), on the basis of appropriate calibration. The scan rate was 5 mV/s. The CV curve of a similar system without the phen additive is also shown for comparison (dashed purple). Inset: 1,10-phenanthroline molecular structure.

Figure 2. Typical galvanostatic charge−discharge curves (current density 1 A/gelectrode) of two-electrode pouch cells (a) with the electrolyte solution PC/1.8 M (TEA)BF4 but without phenanthroline and (b) with the addition of 0.1 M phenanthroline. The voltage at each cycle is indicated in (f). (c) CV curve of the PC/(TEA)BF4 solution (purple) and of the same electrolyte solution with 0.1 M phenanthroline (blue). Scan rate 1 mV/s. (d) Typical galvanostatic charge−discharge curves in different current densities. (e) Ragone plot of the systems with and without the phen additive. (f) Prolonged cycling capability of the two types of cells presented in (a) and (b) (purple and blue curves, respectively). 12167


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current that decays very fast to negligible values. The effect of phen can be explained by its adsorption on the electrodes, which mitigates, by some sort of passivation, redox reactions of the electrolyte solution at the edges of its electrochemical window. This work marks a breakthrough in the development of pseudocapacitors. We demonstrate excellent performance in terms of high specific capacity, high voltage, and stability during a very prolonged cycling, yet the components are relatively simple and of low cost, based on commercial electrode materials and solutions. An important parameter is the effect of the phen concentration on the performance. A detailed optimization of parameters for further practical applications is beyond the scope of this paper. We have preliminary indications that the concentration of phen used herein, 0.1 M, may be close to optimum. The next step was a further study of these novel systems, based on impedance spectroscopy. The cells included two carbon electrodes, one of which served as both a counter and reference electrode. Such a configuration was easy to handle and provided fully reproducible and meaningful results for the electrode serving as the working electrode. EIS measurements were carried out with cells containing the electrolyte solutions with the phen additive, at zero polarization (0 V) and under constant polarization, 1 and 2 V. For comparison, impedance spectra were measured for a cell loaded with a phen-free solution at zero polarization. With these cells, the potentials are divided symmetrically between the electrodes; therefore, applying 0 and 1 V means a potential region in which phen has no redox activity (see Figure 1), while measuring the cell at 2 V means applying a potential in which phen is electrochemically active (Figure 1). Hence, we expect to see a similar response at 0 and 1 V, but quite a different response at 2 V. Typical impedance spectra, presented by Nyquist plots, measured at different potentials (0, 1, and 2 V vs a similar carbon electrode, serving as a reference and counter electrode) with cells containing electrolyte solutions with 0.1 M phen are presented in Figure 4. Comparison between impedance spectra measured with cells with and without phen is presented and discussed later in this paper. The impedance spectra thus obtained (Figure 4) can be divided into two regimes: The first part, related to the high frequencies (HF), exhibits a compressed semicircle. The second part, related to the low frequencies (LF), is characterized by a steep −Z″ vs Z′ curve which goes toward infinity as a straight almost vertical line (reflecting a nearly pure capacitive behavior). These two regimes are clearly marked in Figure 4. To perform a precise analysis of the EIS data, due to the large difference in the impedance magnitude and different behavior, each regime was analyzed separately. The HF impedance regime was analyzed in the conventional impedance plane (Zmode) according to eq 1, while the LF impedance regime was analyzed in the effective capacitance plane (C-mode). The impedance of the HF and the LF regimes and their corresponding DFRTs are shown in Figures 5 and 6, respectively. As explained and demonstrated in several key papers,17−21 the crude data provided by the impedance measurements, sets of Z″, Z′, and ω values, are translated to Γ, the DFRT that can be plotted as a function of frequency. This provides a presentation parallel to that of a Bode or Nyquist plot, but with a much better sensitivity to the time constants and processes that relate to charge transfer (redox interactions) and charge exchange by capacitive interactions. Γ

ing to the CV curves presented in Figure 1 (the three-electrode configuration), the capacitance of the carbon electrodes measured at low potentials (from OCV to −2.4 V) is approximately 320 F/g. Figure 2e presents a Ragone chart, calculated for symmetrical cells with electrodes and solutions as used herein. Energy density is plotted vs power density, on the basis of the cycling data obtained for the single electrodes. The effect of the presence of phen in the solutions is spectacular. It should be noted that the gravimetric energy and power density values presented in Figure 2e are related to the electrodes alone. Hence, taking into account all the components of full cells (e.g., the necessary solution volume that contains the active ions and additive), the specific energy and power density of practical supercapacitors based on the electrodes and solutions described herein should be at least 6 times lower than the values presented in Figure 2e, yet the expected practical energy density of supercapacitors based on the systems studied herein is relatively high, reaching values of the same order of magnitude as aqueous rechargeable batteries (e.g., lead−acid systems). We followed the exact procedure used for the cells without the phen additive. After cycling the system for 3000 cycles at a potential of 3 V, we increased the potential to 3.2 V for 2000 cycles and then to 3.4 V for the final 3000 cycles. The result, presented in Figure 2f, was significantly different as the capacitance remained stable at all applied potentials. A minor increase in capacitance upon cycling (3%) was obtained. Similar phenomena related to PSC electrodes have been reported by Guabicher et al.32 This increase in capacity upon cycling probably results from the gradual adsorption of more phen molecules, beyond their initial adsorption, which leads to high capacity, as cycling proceeds. To further confirm the stabilization contribution of the phen additive to these SC systems over a wide voltage span, chronoamperometric experiments were carried out, while a potential difference of 4 V was applied to the symmetric cells with bare aluminum foil electrodes (Figure 3). When the cells were loaded with a phen-free solution, a residual current of 0.4 mA was measured, indicating that side reactions occur in the cell upon such a high polarization. In contrast, as can be seen in Figure 3, when the cell contains a solution with phen, application of the same polarization results in a charging

Figure 3. Chronoamperometric measurements of symmetric cells with two aluminum foil electrodes, polarized to 4 V and containing PC/ (TEA)BF4 electrolyte solution without (purple curve) and with (blue curve) 0.5 M phen. The initial current response of the solution containing phen is shown in the inset. 12168


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capacitance of the system, according to eq 1. Since the DFRTs are normalized, the contribution of each peak to the impedance can be calculated by the peak area multiplied by the normalization factor. The corresponding areas of the peaks of the HF and LF regimes are shown in Figures 5c and 6c, respectively. The HF regime may be attributed to processes occurring in the bulk of the electrolyte solution. This assumption is based mainly on the clear difference between the responses with and without phen in this regime, as shown in Figure 7. For the electrolyte solution with the additive, the voltage changes cause changes in the charge concentration within the solution bulk, as verified by the CV measurements presented in Figure 1. The observed changes between 0 and 1 V are minor, as seen from the impedance spectra in Figure 4a, but at 2 V the impedance decreases significantly. This is in agreement with the CV measurements, since 0 and 1 V are potentials at which phen does not have a redox activity, while at 2 V phen is electrochemically active (corresponding to 1 V vs SCE, Figure 1, as explained above). The DFRTs obtained from the HF impedance spectra consist of three peaks. Peak 1 lies outside the measured impedance frequency range, and corresponds to a residual resistivity in series with the system. One can understand this by considering the residual resistivity as accompanied by a negligibly small capacitance, resulting in a very small time constant or very high frequency, out of the measured range.27 It is constant throughout all measurements, and therefore does not correspond to the electrolyte solution. The area of peak 2 does not change dramatically from 0 to 1 and 2 V. This peak may be attributed to ion transfer in the solution bulk which does not change when the voltage increases. Peak 3 corresponds to the charge carriers’ reaction (redox) occurring within the electrolyte solution bulk, and indeed changes significantly between 1 and 2 V.

Figure 4. Nyquist plots of the impedance spectra of a cell with the phen additive at three potentials, 0, 1, and 2 V, measured vs a similar carbon electrode serving as both a reference and counter electrode. The impedance spectra are divided into two regimes: high-frequency (HF), 1−150 kHz, and low-frequency (LF), 0.01 Hz to 1 kHz.

vs log ω (or log τ) plots usually show peaks at different frequency ranges, which reflect major sets of time constants. Integration of these peaks over their frequency ranges provide overall resistance or capacitance of the various sets of time constants that lead to the polarization resistance or the effective

Figure 5. (a) HF impedance spectra of a cell with the phen additive at three potentials. (b) DFRTs corresponding to (a). (c) Peak areas of the DFRTs in (b). Peak 1 is out of the measured range and represents series resistance. 12169


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Figure 6. (a) LF impedance spectra transformed to the C-mode of the cell with the phen additive at three potentials. (b) DFRTs corresponding to (a). (c) Peak areas of the DFRTs in (b).

Figure 7. (a) Comparison of the impedance spectra of a cell without and with the phenanthroline additive to the electrolyte and (b) corresponding DFRTs of the HF regime in (a). (c) Areas of the DFRTs in (b).

The LF regime is attributed to the capacitance response of the cells at the electrodes. The DFRTs obtained from the analysis in the C-mode consist of two peaks. The calculated capacitance and the corresponding DFRTs are illustrated in Figure 6. The area of peak 1 increases significantly at 2 V as compared to 0 and 1 V, so that the higher capacitance at this voltage is mostly related to peak 1. The capacitance increase observed at 2 V due to the redox reaction may be related to the higher concentration of the charge carriers that can be adsorbed on the surface of the electrodes to form the EDLC. It is also

possible that the higher charge on the phenanthroline molecules adsorbed to the electrodes results in a thinner double layer, due to the specific adsorption that brings the charged species in the solution phase into direct contact with the electrodes. This by itself increases the EDL capacitance in addition to the redox activity of the adsorbed phenanthroline. The area of peak 2 does not show any monotonous trend, but this should be further verified since its low-frequency limit is outside the measured bandwidth, impeding our ability to measure its contribution precisely. 12170


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The Journal of Physical Chemistry C The EIS data measured with cells filled with phenanthrolinefree electrolyte solution were compared to those obtained with cells of phenanthroline-containing electrolyte solution. A comparison of typical spectra and the analyzed data is presented in Figure 7. The HF regime shows much lower impedance for cells with phenanthroline-containing solution. The ISGP analysis reveals that the major contribution to the decrease in the system resistivity is related to bulk solution resistivity. Peak 2 changes dramatically, whereas peaks 1 and 3 remain the same. Peak 3 does not change, since at 0 V there is no redox reaction in the cell with phenanthroline-containing solution. The pronounced difference between the impedance of cells with and without phenanthroline at 0 V (i.e., no redox interactions of phenanthroline prevail) is striking. It indicates that phenanthroline adsorbs strongly to the activated carbon electrodes and affects pronouncedly their EDL behavior. We can speculate that the adsorption of the aromatic molecule affects the electronic structure of the electrode near the surface and thus reduces the resistance (as indeed reflected by the impedance spectroscopic measurements). In the LF regime (Figure S3 of Supporting Information), the area of peak 1 increases significantly when the solution contains phenanthroline. This indicates the higher capacitance achieved for this system. The area of peak 2 is decreased by a factor of 2, but this, again, should be taken tentatively due to bandwidth limitations as explained above. However, this behavior might be related to the fact that the phenanthroline molecules are competing with the electrolyte ions on adsorption sites.

impedance decrease was more pronounced at 2 V than at 1 V or when no voltage was applied to the system. We may understand these findings by quantifying the phenanthroline molecules that donate the faradic capacitance. It is hard to evaluate the theoretical capacitance of the redox molecules in the electrolyte solution since both the supporting ions of the electrolyte and the redox-active compounds compete for the same adsorption sites on the electrode surface. Assuming as a first approximation a Langmuir-type adsorption isotherm, the fractional surface coverage is proportional to the concentration of the adsorbent in the solution. However, if the concentration of the neutral redox compounds in the solution is too high, ion conductance of the electrolyte solution decreases dramatically. The fraction of sites occupied by electrolyte ions or redox-active molecules could be roughly approximated according to the capacitance difference of the electrodes in the solutions with and without the phenanthroline additive. To 90 F/gelectrode, one can calculate the number of phenanthrolinereduced molecules to be 0.8 mmol for 1 g of electrode (for 3electron reduction). The surface area of 1 g of activated carbon is ∼1500 m2/g, which, theoretically, could accumulate 40 mmol of phenanthroline. The relatively low fractional surface coverage can be explained by the fact that phenanthroline molecules are neutral, and are not attracted to the charge electrode as are the charged ions of the electrolyte. This insight leads logically to a possible description of this system as follows: Phenanthroline molecules in close proximity to the negative electrode are adsorbed on its surface and are reduced. Then, carrying the same charge as the electrode, they are desorbed to the electrolyte. This means that, at high voltage, the electrolyte solution is highly concentrated with charged molecules of the additive. It is possible then that the negatively charged phenanthroline diffuses toward the positive electrode and is adsorbed and reoxidized. This circular process creates a constant diffusion of charge carriers in the bulk of the electrolyte solution. This phenanthroline phenomenon shares the same concept of redox mediators that are used in dyesensitized solar cells (DSSCs)34 and in Li ion and Li−air batteries.35 The proposed mechanism is objectively supported by ISGP analysis of the impedance data. The HF regime, which is attributed to processes occurring at the electrolyte bulk, suggests that the charge carriers’ impact significantly increases at 2 V, the potential at which phenanthroline forgoes the redox reaction. This proves a notable presence of charged phenanthroline molecules flowing through the electrolyte bulk, as explained in our proposed mechanism. Furthermore, a comparison of the behavior of electrodes in the commercial electrolyte solution with the phenanthroline shows the dramatic influence of the presence of phenanthroline even in its noncharged state, which may be caused by dipole induction of the phenanthroline in solution. At the low-frequency regime of the impedance, again, the only component in the system that changes intensely from 1 to 2 V is the concentration of charge carriers on the electrode surface. According to the proposed mechanism, these charge carriers are not the nonactive electrolyte ions, the surface concentration of which should increase linearly with the voltage, but rather the activated phenanthroline molecules.

4. DISCUSSION It should be noted that this work is a first examination of the usefulness of the ISGP impedance analysis for the study of complicated supercapacitors, the specific capacity of which is enlarged and enhanced by adsorption of electrochemically active aromatic organic moieties. Therefor, impedance spectra were measured only at three potentials. The results presented herein seem to justify an extended use of EIS for the analysis of super- and pseudocapacitors. Further work is in progress. The results of this research can be summarized according to the three main effects of the presence of 1,10-phenanthroline in solution on the electrode electrolyte behavior: higher capacitance, higher voltage stability, and lower impedance. The capacitance increase was observed by both galvanostatic and potentiodynamic CV methods and was reconfirmed by the ISGP impedance analysis. The influence on the electrodes’ capacitance by the addition of 1,10-phenanthroline to a PCbased electrolyte solution is fairly predictable since EDLC interactions are known to hold about an order of magnitude less specific capacity than faradic reactions. However, stability at high voltage and the decrease of impedance are, to some extent, unexpected results and demand an appropriate explanation. The extension of the voltage range due to the presence of phenanthroline was evident by CV and chronoamperometric measurements as well as by prolonged charge−discharge measurements. In many cases, the low oxidation and high reduction potentials of the electrolyte solution limit the voltage of electrochemical systems. Extension of the electrochemical window is possible in some systems, as in lead−acid batteries or in aqueous alkaline electrolyte solutions.33 However, this expansion is related to high overvoltage for hydrogen evolution in the aqueous medium due to the nature of the electrode surface and cannot be the reason for our results. The

5. CONCLUSIONS We presented a redox-active organic additive to a commercial PC-based electrolyte solution, and the impact of its presence on 12171


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the electrochemical behavior of a symmetric super capacitor based on activated carbon electrodes. For a practical symmetric cell, we obtained 190 F/g (more than 170% improvement) along a voltage window of 3.4 V, without any capacitance fading after more than 8000 cycles. Taking into account the high capacitance and high operation voltage, such cells can reach energy densities which approach that of rechargeable batteries. Similar cells with a commercial electrolyte solution based on PC show pronounced capacity fading when cycled with an applied potential higher than 3 V. We also present herein the influence of the phenanthroline additive on the electrode impedance and propose that this effect is connected to charged phenanthroline molecules in the electrolyte solutions, which increase the charge carriers’ concentration. We present formation of the impedance data analysis using the ISGP method that supports this mechanism. Evidence for the redox reaction occurring within the electrolyte solution was found by analyzing the HF regime of the impedance spectra. This study should promote the use of aromatic compounds as redox-active materials that can be added to organic electrolyte solutions and remarkably improve the performances of super- and pseudocapacitors. Hence, this work encourages further studies of pseudocapacitors containing additives with redox activity that can bring such devices energy density closer to that of rechargeable batteries.

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ASSOCIATED CONTENT

S Supporting Information *

Description of the theoretical aspects of the study, electrode surface analysis, electrochemical measurements, and impedance spectroscopic measurements and analysis. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b02335.

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AUTHOR INFORMATION

Corresponding Author

*Phone: 972-3-5318832. Fax: 972-3-5317291. E-mail: arie. [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the Israel Science Foundation (ISF) in the framework of the Israel National Research Center for Electrochemical Propulsion (INREP).

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REFERENCES

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Use of 1,10-Phenanthroline as an Additive for High-Performance

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