3.1. Impedance Study
The impedance plot of the electrolyte with various glycerol concentrations at room temperature is depicted in
Figure 3 where a spike and a semicircle is noted to exist at low and high frequencies, respectively [
32]. The semicircle arc is due to the conduction of charges in the bulk of the electrolyte that corresponds to the parallel combination of
Rb and a constant phase element (CPE1), while the spike represents the accumulation of charge in polarization process that represented by CPE2, which is also a feature of a diffusion mechanism [
33,
34]. It is clearly detected that the semicircle in
Figure 3 is getting smaller with the addition of glycerol, which attributed to prevalence of the resistive part in the electrolyte, hence causing the intercept of
Rb with
Zr axis to decrease [
35]. The impedance of CPE (
ZCPE) can be expressed as [
36,
37]:
where
C is the capacitance of CPE,
is the angular frequency and
p is associated with the deviation of the plot from the axis. For the plots that consist of semicircle and spike, the
Zr and
Zi of the equivalent circuits can be obtained via the following equations:
where
p2 and
p1 are the deviation of the spike from the horizontal axis and deviation semicircle from the vertical axis, respectively. The capacitances at high and low frequency are represented as
C1 and
C2, respectively. The determined
Rb value and calculated CPE values are tabulated in
Table 2.
Based on
Table 2, the value of CPE is higher at low frequency compared to high frequency. This phenomenon corresponds to the following equation:
where
d is the thickness of the polymer composite electrolyte while
A symbolizes the contact area.
εr and
εo stand for dielectric constant and vacuum’s permittivity. The value of
εr is high at lower frequency region which in turn giving a high value of
C [
38]. According to Guan et al. [
39], the divalent Mg
2+ ion has attracted towards the lone pair of oxygen atoms while the CH
3COO
− anion has delocalized negative charges, which can be stabilized by the resonance stabilization and makes it as a good leaving group. This phenomenon is beneficial for the ion dissociation hence, enhance the ionic mobility and conductivity [
40]. From the obtained
Rb values in
Table 2, the ionic conductivity of the electrolytes at room temperature can be calculated using the Equation (1) as listed in
Table 3.
The addition of glycerol into the electrolyte able to improve room temperature conductivity of the current system, which optimized at the highest value of 7.71 × 10
−6 S cm
−1 that possessed by the A3 electrolyte. This achievement can realize the electrolyte as a promising magnesium ion conductor for the electrochemical devices applications. The ionic conductivity in this work is influenced by several variables where their correlation can be expressed as:
where
µ,
n and
e are the concentration of the mobility of ions, number density of free ions and the charge of electron, respectively [
41]. The ionic conductivity increment of the electrolyte is caused by the increase in the number of
n and
µ and the reduction of potential barrier in the system, which correlated with the enhancement of the amorphous nature of the electrolyte. The glycerol plasticizer can dissociate more salts and disrupt hydrogen bonding between polymer chains [
25,
27]. Thus, this improve the overall amorphous phase of the prepared samples, which acts as a pathway for ion conduction [
25,
27]. Additionally, more free ions will be available for conduction. The impedance study is support this interpretation. Besides, the CH
3COO
− anion will penetrate into the polymer matrix and create an attractive force between plasticizer molecules and chain segments where these forces will reduce the cohesive attraction between polymer chains that caused the segmental mobility to increase hence further enhance the ionic conductivity [
42].
3.2. Dielectric and Electric Modulus Studies
To more understanding the polarization effects and the conductivity performance of the electrolyte system, dielectric analysis is carried out. The dielectric constant,
signifies the charge stored and the dielectric loss,
is the value of energy loss during the ions movement within the electrolytes [
43]. The values of dielectric loss and dielectric constant can be calculated using the following expressions where the vacuum capacitance,
Co is included.
The plots of both dielectric properties are shown in
Figure 4. The highest conducting electrolyte, A3 is observed to achieve the highest dielectric properties values at a lower frequency but then decreases as the frequency increased. This displays that the addition of glycerol may supports in enhancing the dielectric values and the number of free ions. The high values achieved from this dielectric study described the space charge effects as well as the electrode polarization that contributed by the accumulation of charge carriers [
44,
45]. This phenomenon explained that the electrolytes in this work obeyed the non-Debye behavior. However, the decrement of dielectric values at high frequency may due to the rapid periodic reversal of the electric field [
46]. Besides, the plots of dielectric properties also did not exhibit any peaks of dielectric relaxation, which means the system is dominantly due to the polymer relaxation segments in the ionic conductivity [
47]. The dielectric loss tangent (tan
δ) is plotted as in
Figure 5 to solve the relaxation processes.
The tan
δ is a ratio of energy dissipate to energy stored in a periodical field, which is also known as dissipation factor [
48]. The value of tan
δ can be calculated this equation:
Based on
Figure 5, the peaks observed are explained as the translational ion dynamics that related to the mobile ions conductivity relaxation which further explained the decrease in segmental motion within the polymer electrolyte that beneficial to support the transportation of ions [
49]. The broad
peaks in
Figure 5 also signifies that the relaxation process is following non-Debye behavior [
50]. Furthermore, the polarization suppression effect of the system can be analyzed through the electrical modulus as plotted in
Figure 6. The real part (
Mr) and imaginary part (
Mi) of electrical modulus can be determined using these relations:
At low frequencies,
Mr and
Mi approach to zero because the electrode polarization is dominant and no dispersion is observed [
31]. Long tail at low frequencies indicates the capacitive behavior. The highest conducting electrolyte will produce the lowest
Mr and
Mi values at a high frequency. The presence of peaks in both modulus plots explain that the electrolytes are good ionic conductors at higher frequencies [
51].
3.3. Transport Study
Based on Equation (6), two significant parameters,
n and
are important to influence as well as support the ionic conductivity of the electrolytes. According to Arof et al. [
52], these ionic transport parameters can be evaluated using the electrical impedance spectroscopy (EIS) approach by fitting the impedance curve in
Figure 3. This method expressed diffusion coefficient,
D using the Equation (12) for the Cole–Cole plots with semicircles and spikes.
The Equation (12) required the
, which represents the inverse of
at the minimum value of
Zi. Fadzallah et al. [
53] mentioned that
was taken from a constant value of log
in the transport study, while Yusof et al. [
54] mentioned in their report that
was taken at log
f > 5 where the values were almost constant. The
values can be determined by substituting Equation (12) into the following equation:
where
kB and
T are the Boltzmann constant and an absolute temperature, respectively.
Table 4 provides the values of
p2,
k2,
εr and
τ2 for the electrolytes in this work.
By using the Equations (6), (12) and (13), the transport parameter can be obtained as listed in
Table 5 for the electrolytes in this work. The trend of the transport parameters are harmonized with the ionic conductivity pattern where the addition of glycerol helped to increase the values and the highest conducting electrolyte possesses the highest transport parameters values. Pritam et al. [
55] also reported the similar trend where the increase in transport parameter values were possibly related to the low degree of crystallinity of an electrolyte. Thus, this would improve the segmental motion within the polymer chain, which therefore the electrolyte would achieve a high ionic conductivity [
55].
3.4. Transference Number Measurement (TNM)
To determine which charge carrier species is dominant within the electrolytes, a measurement called TNM was conducted in this work.
Figure 7 shows the plot of current versus time for the A3 electrolyte. The electrolyte is mostly due to ions if the ionic transference number,
tion value is larger than the transference number of electron,
te where these both values can be determined using the following expressions that initial current,
Ii and steady state current,
Iss are involved [
56].
Based on the polarization plot in
Figure 7, the
Ii is noticed to drastically decrease as the time increased. This is because the ionic species in the electrolyte is dropped at this stage and leading towards
Iss when the species is completely reduced [
57]. Besides, the decrement of ionic species is caused by the blockage of ions flows due to stainless steel electrodes that only allow the electron to move [
58]. The diffusion process is beneficial at
Iss to balance the mobile ions movements [
59]. In this study, the A3 electrolyte achieved a high
tion value which is 0.9936 while the
te is 0.0064. This result has signified that the electrolyte consists of more ions compared to electrons and also agreed with other studies [
30,
60].
3.6. Characterization of the EDLC
The cyclic voltammetry (CV) measurement is one of the crucial analyses to identify the charge storage and charge transfer behaviors in the fabricated EDLCs. This measurement was conducted using a pentiostat (Digi-IVY DY2300) (Neware, Shenzhen, China) at different scan rates,
v as shown in
Figure 9. The specific capacitance from CV analysis can be calculated via the following equation where the area of the CV curve,
I(V) dV and the mass of active materials,
m are involved [
65]. The voltage range for CV measurement was between 0.0 V (
V1) to 0.9 V (
V2).
The CV curves in
Figure 9 exhibit that as the scan rate increases, the shape of the curve changes from rectangular shape to a leaf-like shape. These changes are caused by the existence of the porosity of the carbon electrodes and the internal resistance presents during the measurement [
66]. The curves also depicted without any noticeable peaks that further explained the system did not undergo any reduction/oxidation reaction. The anions within the system will flow towards the positive electrode during the charging process where the positive electrode will repel the cations so that the cations will attracts to the negative electrode. The electric field during this process embraces the electrode and electrolyte to hold the electrons and ions, respectively [
67]. These processes are the explanation for the formation of double-layer charge to store the potential energy on the carbon electrodes surfaces [
68]. The calculated specific capacitance, Cs from CV curves analysis is listed in
Table 6 at various scan rates.
The low
Cs value at higher scan rate is because the energy loss is high at these scan rates due to the reduction of the amount of stored charges, which exhibits lower
Cs value [
69]. Further characterization of the fabricated EDLC involved the charge–discharge profile, as shown in
Figure 10, using the Galvanostatic technique by employing a Neware battery cycler. This profile is valuable to determine the cyclic durability of the system as well as their capacitive behavior. The capacitive behaviour in the EDLC is comfirmed when the discharge slope is near to a straight line or linear curve [
70]. As a comparison to the
Cs values from CV studies, the
Cs values also can be calculated from the profile in
Figure 10 by using the equation below and the obtained values are plotted in
Figure 11.
The calculated Cs value for the first cycle is 39.68 F/g, which is slightly higher than the values obtained from CV analysis. This difference is reliable because both analyses display the capacitive properties of the EDLC [
71]. At the 40th cycle, Cs value is increased to 50.34 F/g and then maintained at an average of 48.19 F/g until the system completed the 100 cycles. The achievement based on the satisfactory specific capacitance values obtained by the EDLC, it can be concluded that the electrolyte in this work is suitable enough to be applied in the electrochemical devices applications. Moreover, a tiny voltage drop,
Vd is noticed in the charge–discharge profile as shown in
Figure 10. The existence of
Vd can be attributed to the internal resistance within the system during the charge–discharge processes, which also called as equivalent series resistance (ESR). The determination of this parameter will involve the applied current,
i of 1.5 mA and can be expressed as:
The obtained ESR values for 100 cycles are illustrated in
Figure 12, where 189.33
is observed at the first cycle. Throughout the whole processes, the system is noticed to maintain its ESR at the range of 186.80 to 202.27
. This result shows that the EDLC has a small and constant internal resistance for 100 cycles which simplifies the electrostatic process between the ions and charged electrode [
72].
Figure 13 depicts the calculated energy and power densities of the EDLC for 100 cycles. The values are determined from the Equations (19) and (20), where the operating voltage value,
V, is included, which is at 0.9 V.
Based on
Figure 13, the EDLC in this work achieved the energy density of 4.46 Wh/kg at the first cycle. Then, the energy density is steadily increased to 5.66 Wh/kg (40th cycle), which remained around 5.42 Wh/kg until the cycle is completed. The pattern of energy density agrees with the specific capacitance trend in
Figure 11, which described that the required amount of energy for the charge carriers in each cycle of the charge–discharge process is almost equivalent in order to move towards the electrodes surfaces [
73]. On the other hand, the plot of power density in
Figure 13 shows a comparable trend as the ESR plot. The range of power density for the fabricated EDLC in this work is between 500.58 to 558.57 W/kg. The almost-consistent values throughout the 100 cycles clarifies that ELDC possesses the capacitive characteristics [
74]. This also means that the electrolyte consists of enormous amount of free ions, which contributed to the formation of a double layer [
75]. Lastly, the performance of EDLCs can be analyzed using the Columbic efficiency,
where the stability of each cycle can be revealed as plotted in
Figure 14. This parameter involves the time of discharging,
td as well as charging,
tc, which can be expressed using the following equation:
The fabricated EDLC has a low efficiency of 57.89% at the first cycle. However, the performance is found to enhance for the next cycle until the EDLC completed the 100 cycles where the efficiency values are consistent at an average of 92.88%. This result suggests that the system presents an excellent performance in electrochemical devices with good contact of electrolyte–electrode because the efficiency is above 90.0% [
76].