Previously, in the preparation of R-F derived carbon aerogels, the R/C ratios were in between 50 and 1500, and a catalyst such as sodium carbonate (NaCO3) was used [123]. It is because a high R/C ratio enables the formation of large colloidal particles with limited contacts among them, and a base catalyst promotes a wider pore size distributions [122,123]. However, the concentration of catalyst with such formula extremely low and it is rather difficult to initiate the condensation reaction. Therefore, we inferred that a lower R/C ratio of 5 or 10 might be beneficial because the concentrated precursors allow an intimate structure with relatively narrower pore size distribution. In addition, we selected an acidic catalyst because Al-Muhtaseb et al.
and Elkhatat et al. reported that in an acidic solution, the condensation reaction took less time to complete and finer interconnected mesopores were produced afterwards [122,123].
Representative SEM images for GA, GB, GC, and GD samples are presented in Fig. 6.1(a)-(d). Sample GA (shown in Fig. 6.1(a) and its inset) revealed uniform spherical carbons with diameter in 7-10 μm. These spherical carbons are in contact with each other in a string-of-pearls pattern. Since the sample was synthesized with relatively lower precursor concentrations, the condensation reaction took place slowly
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and resulted in larger primary particles. Similar results were observed by Scherdei et al. who observed the formation of spherical particles at lower precursor
concentrations [151]. In SEM image of sample GB shown in Fig. 6.1(b), once the precursor concentrations were increased (while the R/C ratio was kept at 5:1), many small irregularly-shaped carbon particles (50-500 nm in size) connecting to each other was observed. During sol-gel reaction, a higher precursor concentration often leads to faster condensation and stronger cross-linking among primary particles. As a result, sample GB exhibited a foam-like microstructure comprised of mostly smaller primary carbon particles compared to those of sample GA. Fig. 6.1(c) shows the morphology of sample GC, in which a reduced catalyst amount was used while the precursor concentration was identical to that of sample GB. Similar to that of Fig. 6.1(b), a foam-like microstructure with primary carbon particles in 50-100 nm size was observed. Moreover, a higher magnification picture, sample GC revealed larger internal pores with moderate sintering among primary carbon particles in sample GC.
We speculated that due to a reduced catalyst loading, there were fewer nucleation sites for the sol-gel reaction so that the resulting cross-linking was suppressed.
Morphology for sample GD is shown in Fig. 6.1(d). Since the difference between samples GC and GD was the treatment for CO2 activation, their morphologies are hence quite similar as the CO2 etching effect induces the micropores on the carbon surface and those micropores were not readily observed by SEM.
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Figure 6.1 SEM images for sample (a) GA, (b) GB, (b) GC, and (d) GD.
The pores of carbon ambient gels can be classified as microspores (<2 nm), mesopores (2-50 nm), and macrospores (>50 nm). Their relative amounts could be estimated by nitrogen adsorption and desorption isotherms as shown in Fig. 6.2.
Interestingly, samples GA, GB, GC, and GD exhibited a type I behavior, indicating the predominant presence of micropores. Relevant pore properties including BET surface area (SBET), micropore surface area (SMicro), external surface area (SExt), average pore diameter, micropore volume (VMicro), and total pore volume (VPore) are provided in Table 6.1. Despite their notable difference in SEM morphology, samples GA and GB exhibited similar Vmicro, implying that the R/C ratio and CO2 activation were responsible for micropore formation. Remarkably, at identical R/C ratio of 10, sample GC possessed four times larger Vmicro over that of sample GD. Moreover, the SExt for sample GC increased considerably as well. It is noted that the SExt represents the sum of macropores and mesopores. According to Fig. 6.1, the morphologies for sample GC and sample GD were quite similar, and hence their macropore volumes were likely to be close. Therefore, the large SExt for sample GC was attributed to the
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formation of abundant mesopores and micropores during CO2 activation. This was confirmed by the burn-off ratio listed in Table 6.1. With CO2 activation, the samples suffered from weight loss of 95-96%. In contrast, without CO2 activation, sample GD retained 11.7 % weight after pyrolysis. Our experimental results provide clear evidences for CO2 activation that effectively etches the carbon surface, resulting in the presence of excess micropores and mesopores. It is worthy to note that the surface area for sample GC was estimated at 3419 m2g-1, a value that is significantly larger than typical high surface area carbons like activated carbons or Black Pearl 2000 (BP2000). In previous studies of RF-derived samples, the largest surface area reported was 3125 m2g-1 from a carbon aerogel [152]. In our case, we were able to surpass that value by combining improved formula, solvent exchange, and CO2 activation simultaneously.
Table 6.1 Relevant parameters for the pore properties determined by nitrogen adsorption and desorption isotherms from GA, GB, GC, and GD, respectively.
GA GB GC GD
b SExt is the surface area including mesopores and macropores.
c SExt ratio = SExt/SBET.
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Relative pressure (PP
0-1)
Figure 6.2 N2 adsorption/desorption isotherms of sample GA (□), GB (●), GC (☆), and GD (╳).
To evaluate the bond status for our samples, Raman analysis was conducted and the results are displayed in Fig. 6.3. The Raman spectra revealed distinct peaks near 1340 cm-1 and 1580 cm-1, respectively. According to previous study [153], the first peak at 1340 cm-1 (D-band) is attributed to the sp3 hybridized bonding which is associated with disordered or amorphous carbon phase, while the graphitic component in sp2 arrangement accounts for the second peak at 1580 cm-1 (G-band). Hence, the degree of graphitization, or crystallinity for the carbon ambient gels, could be determined by the ratio of integrated from intensites D and G bands (ID/IG). For an ideal graphitic structure, the ID/IG ratio is expected to be zero as a singular IG peak is present. In our case, the ID/IG ratios were 1.93, 1.86, 2.05, and 1.78 for samples GA, GB, GC, and GD, respectively. These values suggested that the structures for the
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carbon ambient gels were poorly ordered with substantial numbers of defects.
Moreover, the ID/IG ratios for those samples undergoing CO2 activation were consistently larger than that of sample GD. This pattern confirmed that the CO2 activation was able to destroy sp2 bonding, exposing a larger surface upon pore formation.
1000 1200 1400 1600 1800
D-band G-band
In te n s it y ( a rb . u n it s )
Raman shift (cm
-1)
GA GB GC GD
Figure 6.3 Raman spectra for sample GA, GB, GC, and GD.
6.2 Electrochemical characterizations
Since binder was not used in the TCE for electrochemical analysis, the determination of intrinsic capacitive behaviors for the carbon ambient gels was
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feasible especially only a small amount of sample was necessary to fill the electrode cavity. Fig. 6.4 shows the representative CV profiles for sample GC at potential windows of 0-0.6, 0-0.8, and 0-1 V, respectively, along with the response from an empty TCE. As shown, the TCE itself revealed negligible currents in both forward and backward scans. In contrast, the carbon ambient gels exhibited quasi-rectangular responses typical of EDLCs [154]. At voltage approaching 1 V, there appeared moderate electrolysis which engendered an apparent current rise. Notably, the presence of functional groups on the carbon ambient gels was rather subdued as additional redox peaks were not observed in 0.2-0.6 V. It is known that the specific capacitance could be estimated from the CV profiles by the equation listed below,
E sample weight, CV potential window, and cathodic current density, respectively.
Table 6.2 lists the specific capacitance for the carbon ambient gels at various potential windows along with commercially available carbon blacks such as Vulcan XC72R (XC72R), BP2000, and Active Carbon (AC1100). The CS for those commercial carbon blacks were determined by a TCE in identical test conditions and their values were reported in our previous work [109].
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Table 6.2 Values of specific capacitance (Fg-1) calculated from CV profiles in various potential windows at 20 mVs-1 scan rate.
GA GB GC GD XC72a BP2000a AC1100a
0-0.6 V 55.8 98 168.4 31 16.7 78 75
0-0.8 V 69.3 115.2 183.4 33 18.7 94 80.8
0-1 V 84.5 150.4 209.9 38.3 25.7 130.3 88
a these values were reported in reference 109.
0.0 0.2 0.4 0.6 0.8 1.0
Figure 6.4 CV profiles at 20 mVs-1 from sample GC in potential windows of (a) 0-0.6 V, (b) 0-0.8 V, (c) 0-1 V, and (d) empty TCE of 0-1 V, respectively.
Among commercial carbon blacks, the BP2000 is known to possess a large surface area due to its fine particle size and consequently, its CS is considerably larger than that of XC72R (130.3 vs. 25.7 Fg-1). Remarkably, the carbon ambient gels of GB
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and GC exhibited even larger CS values of 150.4 and 209.9 Fg-1, respectively. These values, to our knowledge, are comparable to the CS s derived from other large-surface-area nanostructured carbons reported in previously [155-159]. For example, Weng et al. fabricated active carbons with a surface area of 2860 m2g-1 and obtained a CS of 130 Fg-1 [155]. Similar results were reported by Rufford et al. whose active carbons of 1788 and 2019 m2g-1 were synthesized and their CS were 300 and 368 Fg-1, respectively [156,157]. According to Table 3.1, sample GD was propared with the identical formulation to sample GC but the CO2 activation step was replaced by Ar heat treatment. Without the CO2 etching effect, sample GD possessed a substantially low Vmicro and as a result, its CS was merely 38.3 Fg-1.
It is known that the CV profiles for a double layer capacitor are affected by the scan rate imposed during CV measurements. The relation for a capacitive current from a double layer capacitor is as follows,
𝑖𝑐 = 𝑣𝐶d[1 − exp (−𝑡/𝑅s𝐶d)] (5-2)
where v is the scan rate, Rsis the electrolyte resistance, t is the time, and Cd is the capacitance (Fg-1). Hence, the current rises quickly and reaches a plateau in a short time for a true capacitor. In addition, an increasing scan rate allows a larger current plateau. Fig. 6.5 displays the representative CV profiles of sample GC at various scan rates in a potential window of 0-1 V. The CV profiles exhibited rectangular shapes at scan rates of 5 and 10 mVs-1, as expected for a typical double layer capacitor [160].
Apparently, once the scan rate exceeded to 20 mVs-1, the CV curves gradually distorted. This distortion was attributed to the compromised diffusion within the porous structure that rendered a progressively slower capacitance upon faster scan
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rate [129]. The CS is also a function of CV scan rate since at a sufficiently fast scan rate, some micropores become inaccessible due to diffusion limitation, and thus cease to contribute to the capacitive current. Hence, the largest CS is always recorded at the lowest scan rate where ions adsorption and desorption are allowed in available free surface and internal pores. Table 6.3 provides the CS at different scan rates for the carbon ambient gels and commercial carbon blacks. For the carbon ambient gels except sample GA, they exhibited capacitance retention characteristics similar to those of XC72, BP2000, and AC1100. We can thus reasonably assumed that the carbon ambient gels contained pores that were properly sized so access by ions was moderate at fast scan rate.
Table 6.3 Values of specific capacitance (Fg-1) calculated from CV profiles at various scan rates in potential window of 0-1 V.
mVs-1 GA GB GC GD XC72a BP2000a AC1100a
5 (a) 193.7 214.9 312.3 56.9 32.03 169.76 128.48 10 132.4 183.1 266.2 47.2 29.52 152.97 111.19
20 84.5 150.4 209.9 38.3 25.68 130.32 88
50 (b) 38.6 122.6 132.6 27.3 23.28 95.51 51.28 (b/a) (%) 19.9 57.1 42.5 48.8 72.68 56.26 39.91
a these values were reported in reference 109.
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Figure 6.5 CV profiles for sample GC in potential window of 0-1 V at scan rates of 5, 10, 20, and 50 mVs−1, respectively.
Fig. 6.6 provides the CRC responses at ±1 Ag-1 for the carbon ambient gels. It is known that the CS can be estimated by following equation [109],
𝐶𝑆 = 𝑤×| 𝑖𝑐d𝐸
d𝑡 |≈𝑤×△𝐸𝑄 (5-3) where d𝐸
d𝑡 is the slope for discharging curve. In addition, during current reversal, there is a sudden voltage drop (iR loss) whose magnitude is proportional to the electrical resistance of carbon ambient gel, TCE, and electrolyte. Values for the CS
and iR loss are listed in Table 6.4. Sample GD exhibited the lowest voltage drop, suggesting its smallest electrical resistance. This might be attributed to its intimate interconnected structure and surface integrity. On the other hand, sample GA
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exhibited the largest voltage drop which was likely caused by the finite contact areas among primary carbon particles that circumvented electron transports (see Fig. 6.1(a)).
In addition, all carbon ambient gels except GD possessed CS larger than 200 Fg-1, indicating that accessible surface area, as well as suitable microspores and mesopores structure were produced after CO2 activation.
0 100 200 300 400 500 600 700 800 0.0
0.2 0.4 0.6 0.8 1.0
P o te n ti a l (V )
Time (sec)
GA GB GC GD
Figure 6.6 CRC cruves at ±1Ag-1 in potential window of 0-1 V for sample GA, GB, GC, and GD, respectively.
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Table 6.4 Relevant parameters determined from the CRC measurements at ±1 Ag-1 in potential window of 0-1 V.
GA GB GC GD obtained by CV scans. Since the CRC method is conducted at a fixed potential window with a constant current density, it hence allows a sufficient time for ion adsorption and desorption. In contrast, the CS determined by CV scans is achieved by varying current so the kinetics for ion diffusion in the porous structure is limited. In principle, the capacitance obtained at slow scan rate during CV scans should to approach to of CRC method. In our measurements, indeed, the CS of CRC at 1 Ag-1 was indeed close to the CS of from the CV at a scan rate of 5 mVs-1.
To evaluate the life time performance for the carbon ambient gels, we carried out the cyclic CRC experiments between 0 and 1 V at ±1 Ag-1. The resulting capacitance variation over 3000 cycles are shown in Fig. 6.7, along with that of BP2000 for comparison purpose. The life time testing, it was found that the capacitances decreases smaller with increasing cycles, a fact that was possibly associated with detachment of individual carbon particles during prolonged cycling. We speculated that without binder, electrolyte inundation in the TCE cavity inevitably affects the packing density and local environments of carbon particles. As a result, the amount of
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carbon particles responsible for the capacitive responses decreases with time. This undesired effect was the worst in sample GA (with a capacity loss of 63%) since its SEM image indicated there is limited contact in between carbon particles and thus results in poor electrical conductivity and packing density. In contrast, sample GB exhibited the best capacitance retention with a moderate capacity loss of 13%. Sample GC showed a capacitance loss of 27% despite its IR loss was the smallest as listed in Table 6.4. Nevertheless, the value of GC was still significantly better than 41% of BP2000. Hence, we concluded that except sample GA, the carbon ambient gels retained reasonable EDLCs behaviors comparable or even better than that of BP2000.
0 500 1000 1500 2000 2500 3000 0.0
0.2 0.4 0.6 0.8 1.0
Cycles
N o rm al iz ed cap aci ty ( % )
Figure 6.7 Variation of specific capacitance for sample GA(□), GB(●), GC(☆), GD(╳), and BP2000 (▲) obtained from CRC measurements in potential window of 0-1 V at ±1Ag−1 for 3000 cycles.
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Impedance analysis is a powerful technique to examine the interfacial pheonmenon occuring in the ambient gels during capacitve responses. Fig. 6.8(a) shows the Nyquist plots for samples GA, GB, and GC over the frequency regime of 0.1-20 kHz. Apparently, the impedance spectra consist of a semi-circle at high frequncy and a Warburg diffusion at low frequency. The enlarged spectra at high frequency regime is shown in Fig. 6.8(b) and the equilivent circuit model for the fitting the impedance spectra is provided in Fig. 6.8(c). As shown, the equilivent circuit includes elements of Rs (ohmic resistance from electrolyte, carbon particles, and TCE), RCT (resistance from faradaic charge transfer reaction), CPEp (capacitance of constant phase element for faradaic charge transfer reaction), W (Warburg impedance), and CPEEDL (capacitance of constant phase element for electrochemical double layer). For both CPEp and CPEEDL, they can be derived from following equation [161]:
Z = 𝑌0(jω)I α (5-4)
where Z, Y0, j, ω, and α are impedance, capacitance of associated element, imaginary unit, angular frequency (ω = 2πf) of the AC signal, and a dimensionless parameter for fitting purpose (α = 0 for a pure resistor and α = 1 for an ideal parallel plate capacitor).
In practice, the α is between 0 and 1, and its exact value reflects the porous nature of the active material [162].
Table 6.5 presents the fitting results with relevant parameters clearly identified.
As listed, the Rs values were rather subdued confirming the conductive nature for the carbon ambient gels. The CPEp-Y0, W-Y0, and CPEEDL-Y0 indicate the capacitive components for CPEp, Warburg impedance, and CPEEDL, respectively. Among these
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samples, the q values for CPEp were consistently below 1, which was expected for a porous electrode. In addition, the q values of Warburg impedance for sample GB and GC were less than 1, a fact attributed to the limited diffusion in porous carbons. For sample GA, its q value of Warburg impedance was undetermined because the Warburg diffusion occurring at the low frequency regime was not clearly defined. It is possible that a lower frequency AC signal is necessary to clarity the diffusion effect.
As for the CPEEDL, interestingly, sample GC revealed a q value of 1, suggesting its response was close to an ideal capacitor.
Table 6.5 Parameters from fitting impedance spectra obtained at the open circuit voltage
GA GB GC
Rs (Ωg-1) 0.00115 0.00181 0.00151
CPEp Y0 (Fg-1) 1.113 0.228 0.281
q 0.553 0.707 0.642
RCT (Ωg-1) 0.0954 0.0339 0.0215
W
R (Ωg-1) - 0.0019204 0.00462
Y0 (Fg-1) - 0.69332 0.839
q - 0.741 0.417
CPEEDL
Y0 (Fg-1) 77.230 117.300 130.300
q 0.428 0.552 1.000
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0.00 0.01 0.02 0.03 0.04 0.05 0.000
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(c)
Figure 6.8 (a) Nyquist plots for sample GA, GB, and GC at the open circuit voltage in frequency range of 0.1-20 kHz, as well as (b) the enlarged spectra at high frequency regime. (c) is the equivalent circuit model used to fit the impedance spectra.
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