Electrochemical performance of polypyrrole derived porous activated carbon-based symmetric supercapacitors in various electrolytes

Abstract

The electrochemical performance of porous carbon prepared from the polymerization and carbonization of pyrrole is presented in this work. The produced carbon exhibited a high specific surface area and high mesopore volume that are desirable and beneficial for high capacitive performance. Symmetric supercapacitor devices fabricated from this carbon were tested in three different electrolytes (6 M KOH, 1 M NaNO₃, and 1 M Na₂SO₄). Higher capacitive performance (specific capacitance of 131 F g⁻¹) in the 1 M Na₂SO₄ medium was obtained compared to the other two electrolytes a with specific capacitance of 108 F g⁻¹ in 6 M KOH and 94 F g⁻¹ in 1 M NaNO₃ respectively. The difference observed in capacitance in the three electrolytes is linked to the individual properties of the electrolytes which include the conductivity and different ion solvation sizes. A potentiostatic floating test at the maximum voltage for 140 h was used to study the stability of the devices and from the experimental data, a 7% capacitance decrease was observed in the 6 M KOH electrolyte which is related to the corrosive atmosphere and oxidation of the positive electrode. A decrease of 18% in capacitance was observed in 1 M NaNO₃ with an increase in resistance and 1% capacitance decay was observed in 1 M Na₂SO₄ with no change in resistance value at the end of the floating test. These results suggest the good performance of the polypyrrole based activated carbon for symmetric supercapacitors in aqueous electrolytes in general with 1 M Na₂SO₄, in particular, showing excellent stability after floating.

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1. Introduction

Recently, three-dimensional (3D) porous carbons with a high specific surface area (SSA) are promising for electrochemical charge storage. This is because of the 3D architecture with diverse pore dimensions within the porous framework that could benefit charge storage in different electrolytes. According to the International Union of Pure and Applied Chemistry (IUPAC) classification, the pore sizes in porous materials are classified as follows: micropores (<2 nm), mesopores (2–50 nm) and macropores (>50 nm).¹ Thus, a material with a high SSA and optimal amount of micropores and mesopores would significantly pave the way and boost the charge storage capability, since the micropores are responsible for ion traps for energy storage whereas mesopores act as channels for ion transport pathways for power delivery and the macropores function as ion buffering reservoirs for the electrochemical process.^2,3 The electrochemical charge storage mechanism in carbon materials is based on Hermann von Helmholtz principles giving rise to double layer behaviour also known as the electric double layer (EDL).^4,5 Many forms of carbon allotropes have been tested as electrode materials for electrochemical capacitors (ECs), such as 3D graphene foam,⁶ activated carbon,⁷ carbon onions,⁸ carbides derived carbon⁹ as well as carbon nanotubes (CNTs).¹⁰ For electrode made from of these materials, the electrochemical performance (capacitance, energy storage, and efficiency) is often related to the pore structure, surface area, the pore size distribution, the surface functionalities and the type of electrolyte used.^11,12

Available electrochemical capacitors (ECs) use high SSA activated carbon (AC) as electrodes in organic and ionic liquids electrolyte with maximum operating voltage varying between 2.5 and 4 V.^13,14 These electrolytes are characterized by high operating voltage making them useful for high energy applications since the energy is directly proportional to the square of the voltage (E = 0.5CV²). However, there are many shortcomings linked with them namely; high viscosity, low ionic conductivity (high internal resistance), high flammability, high electrolyte leakage, hazardous to the environment and they also are quite expensive.^13,15 Compared with organic and ionic liquids, aqueous electrolytes have high ionic conductivity and high mobility of proton transport mechanism which are required to achieve low internal resistance for the development of high-performance ECs devices. Thus, ECs using aqueous electrolytes are attractive from industrial and commercial perspective and therefore many research activities are focussed on the use of aqueous electrolyte in order to extend their operating voltages beyond their thermodynamic limit restricted to 1.23 V to higher operating voltages.^16,17

Commonly used aqueous electrolyte are KOH (alkaline), H₂SO₄ (acidic) and Na₂SO₄ (neutral) which are usually compatible with many forms of carbon material and can provide high electrochemical performance (high energy and power densities). Amongst these electrolytes, neutral electrolytes have proven to be reliable substitutes to organic solutions as they have been shown to achieve higher operating cell voltage up to 1.9 V for symmetric carbon/carbon in aqueous 0.5 mol L⁻¹ Na₂SO₄ and 2.2 V for 1 mol L⁻¹ Li₂SO₄, respectively.^18,19 The high operating voltages were related to the high over-potential for di-hydrogen evolution at the negative electrode and to the small amount of corrosive atmosphere in neutral electrolyte as compared to, e.g., H₂SO₄ and KOH.¹⁸ Beside the voltage in aqueous electrolytes, the performance of porous carbon electrodes can significantly be improved through pseudo-faradic reactions of functional groups present on the carbon surfaces.²⁰ As an illustration, higher ECs (capacitance) were reported for asymmetric capacitor with nanoporous carbon electrodes operating in both KOH and H₂SO₄ aqueous media, and it was shown that oxygenated functionalities control the equilibrium potential and the potential window of the electrodes. Thus, it was revealed that by taking advantage of different redox reactions, it is possible to improve the electrochemical performance and the operating voltage of ECs in aqueous medium either by balancing the mass of the electrodes or by exploring different optimized carbons as positive and negative electrode.²¹

Thus, in view of the above, it is necessary to explore new porous carbon materials as electrodes for ECs, to study their electrochemical properties in different aqueous electrolytes and also to understand the influence of the functional groups on the stability of the ECs, taking into account the most accurate and reliable floating test for evaluation of the cycle lifetime stability of ECs. It is necessary to investigate whether or not the ECs could withstand such condition since floating is executed at high voltage for extremely long period of time. To our knowledge, few studies have been reported on floating test of ECs and their electrochemical behavior in different aqueous electrolytes. This paper presents the synthesis of low-cost porous graphitic carbon exhibiting a high SSA with a three dimensional interconnected porous architecture by annealing of polypyrrole (PPY). The performance of the produce carbon as EC electrode was investigated in three aqueous electrolytes namely; 6 M KOH, 1 M NaNO₃ and 1 M Na₂SO₄. Cycling of ECs is a property of major importance to analyse the lifetime of an EC device, thus, the main objective of this study was to investigate the long-term stability of porous PPY carbon ECs based on floating in this different aqueous electrolytes.

2. Experimental

2.1. Materials

Potassium hydroxide pellets, ethanol (99.9% purity) and hydrochloric acid were purchased from associated chemical enterprise (ACE) South Africa, pyrrole (reagent grade 98%), sodium nitrate (ACS reagent 99%) and sodium sulphate (ACS reagent 99%) were purchased from Sigma-Aldrich, South Africa. De-ionized water (18.2 MΩ cm Millipore Corp. Bedford, MA, USA) was used during the preparation processes. All chemical were used as received without any purification.

2.2. Preparation of polypyrrole

Polypyrrole (PPY) was prepared by dissolving 2.4 g FeCl₃ in 50 mL deionized water and stirred for 30 min followed by the slow addition of 0.5 mL of the pyrrole monomer with continuous stirring for 12 h. This procedure leads to the polymerization of pyrrole according to the following reaction

nC₄H₄NH + 2FeCl₃ → (C₄H₂NH)_n + 2FeCl₂ + 2HCl

After the reaction, the obtained product was washed continuously to remove the FeCl₂ and HCl from the sample. Afterwards the sample was dried in an electric oven at 80 °C for 24 h.

2.3. Preparation of porous carbon

The obtained PPY powder was chemically activated by mixing potassium hydroxide (KOH) pellets with the material in a 1 : 4 weight ratio.²² The mixture was then placed in a horizontal tube furnace ramped from room temperature to 800 °C at 5 °C minute⁻¹ under nitrogen flow and thermally annealed for 1 h. The activated carbon was then washed with 1 M HCl and deionized water to remove the residual salts until a neutral pH was achieved. The obtained sample was oven-dried at 80 °C for 24 h and denoted as activated carbon (AC).

2.4. Characterization

X-ray photoelectron spectroscopy (XPS) spectra were recorded on a Physical Kratos instrument AXIS SUPRA Ultra-DLD with Al Kα X-ray (hν = 1486.6 eV), Raman spectrum was collected by a WiTec-alpha 300R+ confocal Raman spectrometer (WiTec GmbH) with the laser power of 10 mW in order to minimize heating effects. The gas sorption analyses performed at −196 °C using a Micromeritics TriStar II 3020 (version 2.00) analyzer. All the samples were degassed for more than 12 h at 180 °C under high vacuum. The scanning electron micrographs were obtained using a Zeiss Ultra Plus 55 field emission scanning electron microscope (FE-SEM) operated at an accelerating voltage of 2.0 kV.

2.5. Electrochemical measurements

The positive and negative electrodes were fabricating by integrating 80 wt% of the as-prepared with 10 wt% carbon black and 10 wt% polyvinylidene fluoride (PVDF) as the electrical conductor and binder dispersed in N-methyl-2-pyrrolidone (NMP) solution to produce the paste, and then homogeneously pressed on nickel foam current collector with a diameter of 16 mm and dried at 60 °C in an electric oven for 24 h to ensure complete drying of the NMP. The electrochemical tests were performed in a two-electrode symmetric setup with each electrode having a mass loading of ∼2.0 mg separated by a 0.18 mm thick microfiber glass filter paper in CR2025-type coin cells, on a VMP300 Bio-Logic at room temperature with the following of electrochemical techniques; cyclic voltammetry (CV), constant current galvanostatic charge–discharge (CGCD) and electrochemical impedance spectroscopy (EIS). The EIS was recorded under open circuit potential in the frequency ranges 10⁵ to 0.01 Hz.

3. Results and discussion

3.1. Structural analysis

The characterization of the surface functional groups for the produced carbon (AC) was studied by XPS measurements. The AC was made into a pellet with a polytetrafluoroethylene (PTFE) binder, hence, the XPS spectra of the PTFE was also measured. The C 1s and O 1s spectra of AC and the PTFE binder are shown Fig. 1(a) and (b). The C 1s spectrum of the binder was deconvoluted into two peaks at 291.8 eV and 289.6 eV and are assigned to the CF₂ and CH₂ groups of PTFE, respectively. The C 1s spectrum of the AC + binder also contained these two peaks at 291.86 eV and 289.66 eV with additional peaks at 286.5 eV, 284.6 eV, and 281.5 eV ascribed to graphitic carbon (sp²) and carbon present in the form of carboxyl or ether groups. The O 1s spectrum for the AC was resolved in two peaks positioned at 532.0 eV and 533.2 eV and assigned to chemisorbed oxygen and ether groups in the form of (O–C) and (C–O), respectively.^23,24 The presence of these functional groups could improve the wettability of the electrodes in the electrochemical process due to the enhanced quantity of hydrophilic polar sites.

Fig. 1 XPS spectra of the deconvoluted (a) C 1s and (b) O 1s of the AC and PTFE binder respectively, (c) Raman spectra and (d) BET isotherm plot and PSD BJH (inset) of AC.

Fig. 1(c) shows the Raman spectrum of the AC with peaks at ∼1354 cm⁻¹, ∼1598 cm⁻¹ and a broad peak ranging between ∼2700 cm⁻¹ and 2900 cm⁻¹, corresponding to the D, G, and 2D-bands of graphitic material, respectively. The G-band is due to the in-plane vibration of sp² atoms in the carbon and is a doubly degenerate (TO and LO) phonon mode (E_2g symmetry) at the Brillouin zone center.²⁵ The D-band is ascribed to the breathing modes of sp² rings triggered by a dual resonance effect in the presence of defects.²⁶ The 2D-peak is the second order of the D-peak. The high intensity of the D-band is ascribed to the presence of large quantity of disorder in the AC. The intensity of the G-band to the intensity of the D-band I_D/I_G for the sample was ∼1.0, indicating a low degree of graphitic crystalline structure. The bands were further deconvoluted to analyse different vibration modes contribution to carbon materials using a Lorentzian curve fitting. The deconvoluted bands consist of D1 ascribed to probable graphene-sheet carbon atoms and the edge planes perpendicular to the graphene sheets of the bulk carbon materials,²⁷ the D2 is associated to lattice vibrations comparable to that of the G band but involving surface graphene sheets which are not directly intercalated between graphene sheets in the bulk of a carbon material,²⁸ The D3 band originates from the distribution of amorphous carbon in interstitial in the disturbed lattice of the carbon material²⁹ and the D4 band is the outcome of lattice vibrations corresponding to sp²–sp³ bonds.^30,31 Finally, the two peaks at ∼2700 cm⁻¹ and 2900 cm⁻¹ were assigned to the (2D) overtone and (G + D) combination, attributed to second-order bands, i.e. overtones and combinations of graphitic lattice vibration modes respectively.²⁸ The relative ratio of the deconvoluted bands (D1, D2, D3, D4, G, 2D + G + D) amounted to 44.3%, 13.4%, 5.5%, 16.2%, 16.7% and 3.9%, respectively.

The nitrogen (N₂) adsorption/desorption isotherm of the AC measured at −196 °C, shown in Fig. 1(d) was used to estimate the specific surface area (SSA) via the Brunauer–Emmett–Teller (BET) method. The BET reveals a high SSA value of 2230 m² g⁻¹ with a pore volume of 1.86 cm³ g⁻¹ as well as a type VI isotherm featuring hysteresis loop generated by the capillary condensation of the adsorbate in the micro- and mesopores of the carbon material. Besides the high SSA, the Barrett Joyner Halenda (BJH) pore size distribution (PSD) is shown in the inset and reveals mesopores within the range of 2 nm and 4 nm. It is worth stating that the BET system used in this work could not go to lower pressure which is necessary to see the pore size of the micropores present in the sample.

The formation of polypyrrole involves the polymerization of pyrrole (monomer) and iron chloride as an oxidizing agent leading to granular connected structure^32,33 as shown in Fig. 2(a) and (b). During activation and carbonization these structure get linked into three dimensional structure with agglomerates due to the presence of KOH as activation agent. At high temperature, above 600 °C, PPY will be completely converted in granular carbon^34,35 which will react with KOH, as follows: 6KOH + C ↔ 2K + 3H₂ + 2K₂CO₃. This will be followed by the decomposition of K₂CO₃ at such high temperature. The partial etching of the carbon precursor will result in porous PPY derived activated carbon. The microstructure and morphology of PPY derived carbon is presented in Fig. 2(c) low and high Fig. 2 (d) showing that the AC inherited the granular structure of the PPY and consists of uneven densely packed porous carbon structures with underlying disintegration; possessing a 3D porosity as discussed in the gas sorption analysis in Fig. 1(c). Taking into account this combination of porosity, high surface area and the microstructure, the AC is expected to benefit charge storage through easy transport and mobility of the ion through the mesoporous pathways to the electrochemical active sites which are fundamental for efficient electrochemical performance.

Fig. 2 FE-SEM images pure PPY sample at (a) low and (b) high magnifications and (c) low and (d) high magnifications of the AC.

3.2. Electrochemical analysis

Cyclic voltammetry (CV) is a well-known electrochemical technique for evaluating the operating cell voltage and performance of ECs, and the results obtained are presented in Fig. 3. The figure shows the CV of the AC symmetric cell at different scan rates in the three different electrolytes; (Fig. 3(a) 6 M KOH, Fig. 3(b) 1 M Na₂SO₄, and Fig. 3(c) 1 M NaNO₃), revealing distinctive symmetric rectangular shape typical for electrical double-layer charging behaviour. More rectangular CV observed in Fig. 3(b) and (c) reveals a better and rapid charge propagation when compared to Fig. 3(a) with a distorted quasi rectangular shape. However, the CV response in Fig. 3(b) exhibited a higher capacitive current than Fig. 3(a) and (c) signifying a higher electrochemical performance. The electrochemical performance in the aqueous electrolytes could be explained in terms of the ionic radii, radius of ionic hydrated sphere, the conductivity of the ions and the mobility of the ions in the electrolytes.³⁶ From literature, it has been shown that the hydrated ion size (3.31 Å for K⁺ and 3.58 Å for Na⁺), and ionic conductivity (73.5 S cm² mol⁻¹ for K⁺ compared to 50.11 S cm² mol⁻¹ for Na⁺), play a crucial role in electrochemical performance of carbon electrode materials.³⁶ Similarly, the ionic radius of the hydrated negatively charged anions contribute to the EDL behaviour via electroadsorption and the sizes are in the following order OH⁻ (3.00 Å) < NO₃⁻ (3.35 Å) < SO₄²⁻ (5.33 Å).³⁶ Hence the alkaline electrolyte is expected to give the best electrochemical performance taking into account the micro and mesoporous texture of the PPY carbon electrode that could easily accommodate the smaller size of K⁺ and the electroadsorption of the negatively charge anions (OH⁻), coupled with it better conductivity and ionic mobility. However, this is not the case in our experiment as we observed that higher current response was obtained in the neutral Na₂SO₄ electrolyte signifying better capacitive performance than in KOH and NaNO₃. The better electrochemical performance in the neutral Na₂SO₄ electrolyte is attributed to higher diffusion of SO₄²⁻ ion that is capable of existing in different hydrated solvated sulphate species such as n(SO₄²⁻(H₂O)_n) (where n can be 6 and 12) and it has been shown that these species have sulphate ion sizes of ∼10 Å, indicating that these ions can fit into the porosity of the AC electrodes with similar dimensions.³⁷ Fig. 3(d) compares the CV at a slow scan rate of 5 mV s⁻¹ due to the fact that at slower scan rates significant contribution from pseudo-capacitance can be detected. From the figure, it shows that the AC electrodes in all the three electrolytes are stable within the chosen potential window with predominantly EDLCs behavior.

Fig. 3 Cyclic voltammetry (CV) at various scan rates from 10–200 mV s⁻¹ of AC electrodes in the three electrolytes (a) 6 M KOH, (b) 1 M Na₂SO₄, (c) 1 M NaNO₃ and (d) CV comparison at 5 mV s⁻¹ in all the electrolytes.

The constant current galvanostatic charge–discharge (CGCD) curves of the symmetric cells are depicted in Fig. 4(a–c) for the three different electrolytes. The CGCD curves are triangular and proportional to each other, validating the pure electrostatic capacitive behaviours observed in the CV curves. Fig. 4(d) also compares the CGCD of all the electrolytes at a current density of 1 A g⁻¹, showing that the 1 M Na₂SO₄, with longer discharge time exhibit the highest specific capacitance. The capacitance and resistance of the whole symmetric cells with a total mass of 4 mg in each of the different electrolyte are estimated from the slope of the CGCD in Fig. 4(a–c) is presented in Fig. 5. The capacitance of the cell was calculated based on eqn (1).

Fig. 4 Constant current galvanostatic charge–discharges (CGCD) at different current densities from 0.5–5 A g⁻¹ (a) 6 M KOH, (b) 1 M Na₂SO₄ and (c) 1 M NaNO₃ and (d) CGCD comparison at 1 A g⁻¹ of AC electrodes in the three electrolytes.

Fig. 5 (a) specific capacitance and (b) the resistance (from voltage drop) as a function of the current densities from 0.5–5 A g⁻¹ in the three electrolytes.

The specific capacitance (F g⁻¹) for was calculated according to:^38,39

where I is the current (A), Δt is the discharge or charge duration (s), ΔV = V_o − V_IR-drop is the change in cell potential cell excluding the IR drop and m is the mass of carbon in the two electrodes (g). The resistance (Ω) was calculated from the difference of the voltage at the initial stage of the discharge from the CGCD curves at all the current densities using eqn (3).³⁹

Fig. 5(a) presents the capacitance change (taking into account the total mass of active material) per unit weight as a function of the output current density in the form of a bar chart for AC-based carbon electrodes in all the electrolytes used. For the symmetric cell in 1 M Na₂SO₄, the capacitance was 131 F g⁻¹ at 0.5 A g⁻¹ (2 mA), similarly, at the same current density in 1 M NaNO₃, and 6 M KOH, the capacitances were 94 F g⁻¹ and 102 F g⁻¹, respectively. For the 1 M Na₂SO₄, the capacitance was 122 F g⁻¹ at a current density of 5 A g⁻¹ (20 mA) showing a capacitance retention of 93% from the initial value. In the case of the other two electrolytes, capacitance retentions of 92% and 90% are observed at a similar current density in 1 M NaNO₃ and 6 M KOH, respectively, signifying superior rate and power capability of the electrodes in the different aqueous media. More specifically, compared to previous reports on symmetric supercapacitors from diverse material sources including oxides, the material presented here display moderately higher SSA value with good distribution of porosity which greatly benefited the electrochemical performance in aqueous electrolyte and are comparable to previous reports in literature. For example a specific capacitance of 135 F g⁻¹ was reported by Béguin et al.,⁴⁰ for a high voltage (1.6 V) symmetric carbon/carbon supercapacitor using a Na₂SO₄ aqueous solution with excellent cycle life. Similarly a specific capacitance of 129 F g⁻¹ at 0.5 A g⁻¹ was reported for porous carbons obtained from polymers such as polyvinylpyrrolidone (PVP) and polyvinyl alcohol (PVA) as carbon sources⁴¹ and, a specific capacitance of 52.66 F g⁻¹ at 0.625 A g⁻¹ was reported for a symmetric RuO₂/RuO₂ supercapacitor operating at 1.6 V in a neutral aqueous electrolyte.¹⁶ Mostly IR drop is inevitable at the beginning of discharge and is a reliable method to measure the equivalent series resistance (ESR) which influences the total power performance of EC device and is usually attributed to the ohmic resistance of electrolytes and the inner resistance of ion diffusion within the porous structure of the electrode material. The resistance change at different current densities is presented in Fig. 5(b), and in the case of the neutral electrolytes, namely 1 M Na₂SO₄ and 1 M NaNO₃, the resistance values obtained using eqn (3) were ∼1.8 Ω and ∼1.4 Ω, and ∼1.5 Ω in the alkaline electrolyte (6 M KOH) respectively. Indicating that increase in current density slightly increase the resistance of the devices with accepted margin of error.

The stability of ECs devices is one of the most vital properties for industrial or commercial applications. The traditional method of testing the stability of ECs devices is via the CGCD at a particular voltage for several thousands of cycles, after which the capacitance retention or coulombic efficiency can be deduced from the number of cycles to establish the cycle life of the ECs. A new more accurate and reliable analysis to test the long-term stability of EC electrodes devices has been proposed. This technique is based on potentiostatic floating or voltage-holding experiments. For example after every 10 h of aging at the maximum voltage, five CGCD sequence is followed and the cell full capacitance is estimated from the fifth discharge respectively. The floating and CGCD sequences were reiterated 14 times, i.e. a total floating time of 140 h.⁴² The resistance which is a measure of the conductivity of the material was also estimated from the IR drop at the initial discharge step. In this work, the stability of the symmetric cell in the three electrolytes was subjected to voltage-holding for 140 h at 2.5 A g⁻¹ and the performance is shown in Fig. 6(a). The cell capacitance performance in the 1 M Na₂SO₄ neutral electrolyte exhibited excellent stability showing 1% decay (i.e., retaining ∼99% of its original capacitance of 131 F g⁻¹) indicating that floating for a long period has no significant deterioration on the electrodes. The AC electrode in 1 M NaNO₃ displayed a drastic decrease in the specific capacitance with capacitance decay of 18% from the initial stage of the floating and a capacitance fall of 7% was observed in the alkaline electrolyte (6 M KOH). The capacitance decrease in the alkaline medium has been attributed to corrosive atmosphere and the drop in the 1 M NaNO₃ we attribute to the increase in oxygen-containing groups at the surface of the electrodes that leads to decrease in ionic conductivity hence lower capacitance.^43,44 Taking into account the recognized standards for the end of the lifetime for supercapacitors (i.e. 100% increase in the resistance),^42,45 it is reasonable to conclude that the aqueous electrolyte is suitable for the produced carbon material for ECs applications. The change in resistance with the floating time is shown Fig. 6(b) with 1 M Na₂SO₄ and 6 M KOH electrolytes exhibited almost constant resistance value with slight variations throughout the floating period. The 1 M NaNO₃ electrolyte was characterized by an increase in resistance associated with the capacitance loss observed in Fig. 6(a) attributed to the lower ionic conductivity when compared to the other two electrolytes.

Fig. 6 (a) specific capacitance and (b) the resistance (from voltage drop) as a function of the floating time at 2.5 A g⁻¹ in the three electrolytes.

The performance responses with frequency of supercapacitors in the three electrolytes was analysed using electrochemical impedance spectroscopy (EIS) to further elucidate the charging mechanisms at open circuit potential. The Nyquist plots shown in Fig. 7(a) are similar for the devices in the three electrolytes showing semicircle in the high frequency region ascribed to resistive elements and leakage processes occurring from the pseudo-capacitance contributions from functional groups of the electrode; a 45° slope line at mid-frequency region attributed to the distributed resistance–capacitance of the impedance and a vertical almost parallel to the y-axis at low-frequency region is distinctive of capacitive behavior. The intercept of the Nyquist plot on the axis at high frequency gives the equivalent series resistance (ESR), which comprises the intrinsic resistance of the electrode materials, the electrolyte, and the contact resistance between the interfaces of electrodes, electrolyte, and current collector.⁴⁶ The obtained ESR values are as follows 0.6 Ω (6 M KOH), 1.0 Ω (1 M NaNO₃) and 1.0 Ω (1 M Na₂SO₄), respectively as shown in the inset. The alkaline electrolyte exhibited the least ESR because of the higher ionic conductivity. The equivalent distributed resistance (EDR), consists of both the ESR and the ionic resistance of solution induced within the porous structures and is usually obtained by extrapolating the vertical intercept on the x-axis, often. From the Nyquist plots, the 6 M KOH and 1 M Na₂SO₄ devices shows similar EDR (5.3 Ω cm²) and (5.3 Ω cm²) with charge transfer resistance (R_ct = 0.5 Ω) and (R_ct = 1.0 Ω) respectively correlated with the higher ionic conductivities compared to the 1 M NaNO₃ electrolyte with EDR value 11.6 Ω cm² and R_ct = 1.6 Ω respectively. The increased EDR could be due to the mobility of hydrated ions in the inner pores, the reaction with oxygen containing groups at the surface and the frequency-dependent resistance associated with electrolyte penetration within the electrode porosity.^44,47 Fig. 7(b) presents the Bode plots, the phase angle for the electrode in 6 M KOH is −85.5°, −86° in 1 M Na₂SO₄ and −85° in 1 M NaNO₃ which is close to −90° for an ideal supercapacitor device proving the good capacitive performance of the symmetric AC supercapacitor in different electrolytes. Fig. 7(c) presents the real capacitance vs. frequency plot of the cells in the different electrolytes. The capacitance values obtained are comparable to that observed from the CGCD demonstrating excellent behaviour of the porous electrodes in aqueous electrolytes with a good charge propagation up to modest frequencies. Fig. 7(d) shows the imaginary part of the capacitance in the three electrolytes. The peak at the different frequencies of 0.51 Hz for 6 M KOH and 1 M Na₂SO₄, and 0.34 Hz for 1 M NaNO₃ were used to estimate the relaxation time τ₀, from τ₀ = 1/(2πf_max) where f_max is the characteristic relaxation frequency of the cell obtained at the phase angle of −45° corresponding to the time between the capacitive and resistive behavior of the supercapacitor electrode.³⁸ The value of τ₀ obtained are ∼1.96 s in 6 M KOH and 2.94 s in the neutral electrolytes respectively, indicating a measure of how fast the stored energy can efficiently be delivered. These results are in good agreement with CV and CGCD results in that they consistently show that the 1 M Na₂SO₄ exhibited the best electrochemical performance followed by the 6 M KOH, with the 1 M NaNO₃ showing the least performance.

Fig. 7 (a) EIS Nyquist plot (b) Bode plot (c) the real (C′) and (d) the imaginary (C′′) of the symmetric cell in the three different electrolytes.

4. Conclusion

The electrochemical performance of PPY derived carbon has been investigated in three different electrolytes namely; 6 M KOH, 1 M Na₂SO₄, and 1 M NaNO₃ using cyclic voltammetry, constant current charge–discharge, electrochemical impedance spectroscopy and stability of the symmetric cell based on floating test (aging) respectively. The symmetric cell in 1 M Na₂SO₄ consistently displayed superior electrochemical performance with a specific capacitance of 131 F g⁻¹ at current density of 0.5 A g⁻¹ compared to the other two aqueous electrolytes 6 M KOH, and 1 M NaNO₃ exhibiting specific capacitance of 108 F g⁻¹ and 94 F g⁻¹ at similar current density. The higher values obtained in the 1 M Na₂SO₄ electrolyte are due the negatively charged electro-adsorbed SO₄²⁻ ion that can exist in different hydrated solvated sulfate species compared to the OH⁻ and NO₃⁻ that are electro-adsorbed in the plain/normal form. Also the hydrated ion size that is similar to the porosity of the carbon material benefitted easy access to the pores within the electrodes resulting in better electrochemical performance. Nevertheless, taking into account the capacitance retention based on long-hour voltage-holding (VH) at the respective maximum voltages, a 7% decay in capacitance was observed in the alkaline electrolyte after VH for 140 h. The neutral electrolyte in particular the 1 M Na₂SO₄ exhibited excellent stability with capacitance loss of 1% and constant resistance throughout the 140 h. Overall, we can conclude that the PPY derived carbon showed good capacitive behavior in the 1 M Na₂SO₄ and 6 M KOH aqueous electrolytes with good capacitance retention, putting a possible new perception into this type of carbon material capacitive applications.

Acknowledgements

This work is based on research supported by the South African Research Chairs Initiative (SARChI) of the Department of Science and Technology and the National Research Foundation (NRF) of South Africa (Grant No. 97994). Any opinion, finding and conclusion or recommendation expressed in this material is that of the author(s) and the NRF does not accept any liability in this regard. A. Bello acknowledges the NRF through the SARChI Chair in Carbon Technology and Materials, and the University of Pretoria for Postdoctoral financial support.

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