semi-carbonization T / °C
4.1.5 Combining low activation reactant and semi-carbonization
4.1.5.2 Energy storage related properties of carbons synthesized with a KOH/carbon ratio of 2.5 and different semi-carbonization temperatures
149 Table 10: Nitrogen physisorption derived data of the carbon samples synthesized at different semi-carbonization temperatures and a KOH/carbon ratio of 2.5.
Sample SBET / m2·g−1
SDFT / m2·g−1
Vt / cm3·g−1
Vmic / cm3·g−1
C-900-2.5-T9 1449 1543 0.67 0.54
C-800-2.5-T9 1582 1675 0.75 0.59
C-700-2.5-T9 1892 1910 0.88 0.72
C-600-2.5-T9 1928 1945 0.90 0.73
C-400-2.5-T9 2205 1964 0.98 0.86
C-300-2.5-T9 1731 1860 0.92 0.63
SDFT (DFT surface area), Vt (total pore volume) and Vmic (micropore volume) ob-tained from QSDFT analysis. SBET (BET surface area).
In principle, conclusions from section 4.1.3 can be confirmed that a lower semi-carbonization temperature prior to the chemical activation leads to an enlargement of pores and thus also increases the surface area and pore volume of the carbons.
However, it has also been shown that a KOH/carbon ratio of 2.5 is not sufficient to generate larger micropores or mesopores in the carbon samples. As already de-scribed in 4.1.2, a larger ratio of at least 5 is required.
4.1.5.2 Energy storage related properties of carbons synthesized with a
150
energy, which limits the coulomb efficiency of the charge-discharge process at low scan rates.
0.0 0.5 1.0 1.5 2.0 2.5 -30
-20 -10 0 10 20
30 C-900-2.5-T9
specific capacitance / F·g-1
voltage / V
0.0 0.5 1.0 1.5 2.0 2.5 -200
-150 -100 -50 0 50 100 150
200 C-800-2.5-T9
specific capacitance / F·g-1
voltage / V
0.0 0.5 1.0 1.5 2.0 2.5 -200
-150 -100 -50 0 50 100 150
200 C-700-2.5-T9
specific capacitance / F·g-1
voltage / V
0.0 0.5 1.0 1.5 2.0 2.5 -100
-75 -50 -25 0 25 50 75
100 C-600-2.5-T9
specific capacitance / F·g-1
voltage / V
0.0 0.5 1.0 1.5 2.0 2.5 -200
-150 -100 -50 0 50 100 150
200 C-400-2.5-T9
specific capacitance / F·g-1
voltage / V
0.0 0.5 1.0 1.5 2.0 2.5 -150
-100 -50 0 50 100
150 C-300-2.5-T9
specific capacitance / F·g-1
voltage / V
Figure 91: Cyclic voltammograms of the activated carbons synthesized at different semi-carbonization temperatures and a KOH/carbon ratio of 2.5 at 10 mV·s−1 (black), 100 mV·s−1 (red), 500 mV·s−1 (blue), 1000 mV·s−1 (green), 1500 mV·s−1 (yellow), 2000 mV·s−1 (orange) and 2500 mV·s−1 (violet).
151 Results from galvanostatic charge-discharge tests are displayed in Figure 92 and the corresponding calculated discharge capacitance values are summarized in Table 11. Although, the semi-carbonization temperatures impact on the specific surface area is rather limited, as discussed in the previous section, the accessible surface areas show clear differences among the carbon samples. With decreasing semi-car-bonization temperature, down to 400 °C, the accessible surface area increases, thus confirming the enlargement of pores due to the low resistivity against the chemical activation of the semi-carbonized carbons. The accessible surface area of C-300-2.5-T9, is comparatively low with 873 m2·g−1, which may be due pore enlargement without the amount of pores increasing significantly. This reduces the accessible surface area compared to C-400-2.5-T9. The specific capacitance correlates clearly with the accessible surface area, with the exception of sample C-300-2.5-T9. The accessible surface area of samples C-300-2.5-T9 and C-700-2.5-T9 is almost iden-tical, but the capacitance differs by 20 F·g−1. The only significant difference be-tween the two samples is the larger micropore volume and micropore surface area of sample C-700-2.5-T9. Thus, it can be concluded micropores provide a greater proportion of the capacitance, as already indicated in the previous sections. It is known from the previous sections that pore sizes of at least 1.5 nm are required in carbon materials to ensure ion transport in the pores at high current densities. There-fore, the retention of capacitance values at higher current densities is consequently low for all carbon samples. Since the sample C-400-2.5-T9 has larger pores than the other carbon materials, the retention is also the highest with 68 %. The capaci-tance of C-900-2.5-T9 is extremely low despite a theoretical achievable surface area of 626 m2·g−1. The pore sizes are probably too small, so that a low current induces diffusion resistances large enough, preventing access of the electrolyte ions to the electrode surface.
152
0 20 40 60 80 100 120
0.0 0.5 1.0 1.5 2.0 2.5 3.0
voltage / V
time / s
C-900-2.5-T9 C-800-2.5-T9 C-700-2.5-T9 C-600-2.5-T9 C-400-2.5-T9 C-300-2.5-T9
0 5 10 15 20
0 25 50 75 100 125 150
specific capacitance / F·g-1
C-900-2.5-T9 C-600-2.5-T9 C-800-2.5-T9 C-400-2.5-T9 C-700-2.5-T9 C-300-2.5-T9
Id / A·g-1
Figure 92: Discharge curve at 1 A·g−1 for the carbon samples synthesized at dif-ferent semi-carbonization temperatures and a KOH/carbon ratio of 2.5 (left) and the specific capacitance as a function of discharge current density (right).
Table 11: Electrochemical properties of the EDLC prepared from activated carbons synthesized at different semi-carbonization temperatures and a KOH/carbon ratio in 1 M TEABF4/ACN.
Sample Sacc / m2·g−1
Vacc / cm3·g−1
CCV / F·g−1
CDC / F·g−1
C-900-2.5-T9 626 0.45 8.5 9.4
C-800-2.5-T9 736 0.52 88.0 82.7
C-700-2.5-T9 888 0.64 98.5 109.5
C-600-2.5-T9 992 0.66 101.3 118.2
C-400-2.5-T9 1112 0.79 107.9 123.0
C-300-2.5-T9 874 0.60 75.4 87.9
Sacc specific surface area for pores > 0.66 nm; Vacc pore volume for pores >
0.66 nm;CCV calculated from 10 mV·s−1; CDC calculated from 1 A·g−1.
The surface normalized capacitance values and pore volume normalized capaci-tance values are shown in Figure 93 and are summarized in Table 12. Compared to the gravimetric capacitance values, the surface capacitance of the carbons are in similar ranges except for sample C-900-2.5-T9, which display an exceptional case of extremely low capacitance in general. This applies both to capacitances normal-ized to the total surface area and to the values normalnormal-ized to the accessible surface area. There is only a slight tendency for the surface capacitance to increase with the
153 semi-carbonization temperature. This shows carbons with smaller pores use the sur-face more effectively for charge storage. However, the differences for these carbon samples are very small, as all materials are microporous and all have pores below 1.5 nm.
200 300 400 500 600 700 800 900 1000 0.00
0.03 0.06 0.09 0.12 0.15 0.18
total values
accessible values > 0.66 nm
Csurface / F·m-2
semi-carbonization T / °C
200 300 400 500 600 700 800 900 1000 0
40 80 120 160 200
C volume / F·cm-3
semi-carbonization T / °C
Figure 93: Surface area normalized capacitance (left) and pore volume normalized capacitance (right) as a function of the semi-carbonization temperature for carbons, synthesized at a KOH/carbon ratio of 2.5, including sample C-500-2.5-T9.
Table 12: Surface- and volumetric capacitance values for carbons synthesized at different semi-carbonization temperatures and a KOH/carbon ratio of 2.5.
Sample CS / F·m−2
CS>0.66 / F·cm−2
CV / F·cm−3
CV>0.66 / F·cm−3
C-900-2.5-T9 0.0061 0.015 14.0 20.7
C-800-2.5-T9 0.049 0.11 110.3 159.3
C-700-2.5-T9 0.057 0.12 124.3 170.7
C-600-2.5-T9 0.061 0.12 130.8 178.8
C-400-2.5-T9 0.063 0.11 124.8 155.5
C-300-2.5-T9 0.048 0.10 106.1 146.5
CS, CS>0.66 calculated from CDC and the respective surface areas, obtained from the DFT results; CV, CV>0.66 calculated form CDC and the respective pore volumes.
The results of the PEIS measurements are displayed in Figure 94. Nyquist plots for all carbon samples show low RESR values and short semicircles with comparable length. The high frequency response for all test devices is therefore similar. The
154
phase angle at 10 mHz are −71.6, −83.9, −86.0, −85.5, −86.0 and −81.0 for C-900-2.5-T9, C-800-C-900-2.5-T9, C-700-C-900-2.5-T9, C-600-C-900-2.5-T9, C-400-2.5-T9 and C-300-2.5-T9, respectively. With exception of sample 900-2.5-T9, all materials are able to ensure ion transport without diffusion limitations at low frequencies. This is re-markable because larger pores have been assumed to be necessary for ions transport, even at low frequencies and currents. As for 900-2.5-T9, the average pore sizes are too small to ensure resistance free ions transport, even at low frequencies and currents. Reevaluating the pore size distributions, pores below 1.2 nm are in-sufficient for enabling ion migration within the pore network. The frequency re-sponse of the normalized real capacitance further demonstrates the poor diffusion of electrolyte ions with in the electrode material 900-2.5-T9. Instead of a plateau in the low frequency region, a steep slope is observable indicating an instant loss of capacitance. Consequently, the time constant is higher than 100 s, deduced from the current response of the normalized imaginary capacitance. Interestingly, a maxi-mum can be observed at a high frequency of 10 mHz. This can be attributed to charge transfer reactions, which are either caused by the electrolyte degradation at the surface of the electrode or by the reaction of the current collector with the elec-trolyte. Since the time constant for the observed phenomena is rather low (0.1 s) the cause is a reaction at the electrode surface, because the ions at the high frequency are unable to penetrate through the material to the current collector in large quanti-ties. The other carbon samples show a plateau at high frequencies, followed by an exponential decay, resulting in frequency peaks at the currents’ response of the nor-malized real capacitance. Another exception is sample C-300-2.5-T9, where the real capacitance decreases by a smaller slope, which is reflected in the imaginary capac-itance by a very broad peak. Even at higher and lower frequencies around the peak, a relatively high capacitance loss is observable, which is attributed to the broader pore size distribution. Although, sample C-300-2.5-T9 shows a slightly larger con-tent of between 2 and 3 nm, the concon-tent of pores below 1 nm is higher as well, compared to the other carbon samples. These small micropores hinder the ion mi-gration in the pore framework, which is also displayed by a larger time relaxation constant. The time constants for the carbon samples are 5.13, 3.02, 2.91, 1.25 and 2.30 s for C-800-2.5-T9, C-700-2.5-T9, C-600-2.5-T9, C-400-2.5-T9 and C-300-2.5-T9, respectively. Therefore, the time relaxation constant decreases with de-creasing semi-carbonization temperature except for sample C-300-2.5-T9. This
155 could be attributed to the decreasing average pore size, although the differences in pore size for the discussed carbon samples are minor.
0 1 2 3 4 5
0 1 2 3 4 5
Z'' / ohm
C-900-2.5-T9 C-800-2.5-T9 C-700-2.5-T9 C-600-2.5-T9 C-400-2.5-T9 C-300-2.5-T9
Z' / ohm
0.01 0.1 1 10 100
0 -15 -30 -45 -60 -75 -90
phase angle / °
f / Hz
0.01 0.1 1 10 100
0.0 0.2 0.4 0.6 0.8
1.0 C-900-2.5-T9
C-800-2.5-T9 C-700-2.5-T9 C-600-2.5-T9 C-400-2.5-T9 C-300-2.5-T9
C'/ normalized
f / Hz
0.01 0.1 1 10 100
0.0 0.2 0.4 0.6 0.8 1.0
C''/ normalized
f / Hz
Figure 94: Nyquist plot of the activated carbons (top left), frequency response of the phase angle (top right), normalized real capacitance (bottom left) and normal-ized imaginary capacitance (bottom right).
The different pore sizes of the samples are difficult to assess and only minor assess-able from the total surface area, total pore volume and micropore volume, but can be seen in the effects on the electrochemical properties. The accessible surface area shows a clear dependence on the semi-carbonization temperature, and is decisive for the gravimetric capacitance. None of the materials showed a good retention of capacitance at high currents and frequencies, which can be attributed to the small pores. The previous sections have shown a large content of pores larger than 1.5 nm are required for a sufficiently good capacitance retention. For a good migration of the ions within the carbon particles at low currents and frequencies, however, the
156
maximum pore size of 1.2 nm was sufficient. This is a clear indication the solvate shells of the electrolyte ions are distorted or partially removed, as no significant formation of the electrochemical double layer would otherwise be possible.
Hydrogen storage
The gravimetric hydrogen uptake isotherms at 77 K of the carbons are shown in Figure 95. Here, too, the isotherms area fully reversible and have not reached satu-ration. The activated carbon samples show similar hydrogen uptakes in the pressure range between 0 and 0.1 bar, which is dedicated to the similar sizes of the mi-cropores. The uptakes at the pressure range at 1 bar, however, are different, which is due to the different surface areas.
The gravimetric uptake capacities are 2.10, 2.33, 2.51, 2.44, 2.67 and 2.43 wt% for 900-2.5-T9, C-800-2.5-T9, C-700-2.5-T9, C-600-2.5-T9, C-400-2.5-T9 and C-300-2.5-T9 respectively. This means no clear trend is recognizable. C-400-C-300-2.5-T9 has the highest uptake capacity, due to its high specific surface area, compared to the remaining samples. The capacity of C-500-2.5-T9 is similarly high and the capacity generally decreases with increasing semi-carbonization temperature. This can be traced back to the smaller surface areas (Figure 96). The specific surface area nor-malized capacity varies for all discussed carbons around the same value. The ca-pacity decreases marginally with increasing semi-carbonization temperature up to 500 °C, and then increases again. However, these values vary only slightly, as al-ready mentioned. This gives the reason to assume, for microporous carbons, the pores contribute evenly to hydrogen adsorption at 77 K and 1 bar.
4 Results and Discussion
157
0.0 0.2 0.4 0.6 0.8 1.0
0.0 0.2 0.4 0.6 0.8 1.0
0.0 0.5 1.0 1.5 2.0 2.5 3.0
H
2upt ak e / w t%
pressure / bar
C-900-2.5-T9 C-800-2.5-T9 C-700-2.5-T9 C-600-2.5-T9 C-400-2.5-T9 C-300-2.5-T9
Figure 95: Gravimetric hydrogen storage capacity at 77 K of activated carbons, synthesized at different semi-carbonization temperatures and a KOH/carbon ratio of 2.5.
200 300 400 500 600 700 800 900 1000
200 300 400 500 600 700 800 900 1000
0.0 0.5 1.0 1.5 2.0 2.5 3.0
H 2 uptake / wt %
semi-carbonization T / °C
200 300 400 500 600 700 800 900 1000
200 300 400 500 600 700 800 900 1000
0.0000 0.0004 0.0008 0.0012 0.0016 0.0020
H 2 perS DFT / wt% per m2 ·g-1
semi-carbonization T / °C
Figure 96: Gravimetric hydrogen uptake in dependence of the semi-carbonization temperature, including data of the sample C-2.5-500-T9 (left) and the specific sur-face area normalized uptake capacity in dependence of the semi-carbonization tem-perature (right). The KOH/carbon ratio was fixed at 2.5.
158
In this section, the conclusions from section 4.1.3 can be confirmed. The semi-car-bonization temperature prior to the KOH activation influences the pore sizes and thus the surface areas and pore volumes. Due to the higher organic content in the carbon after a low temperature treatment, the material is more susceptible to the etching processes during the KOH activation. On average, the pores are enlarged as the semi-carbonization temperature decreases. However, due to the low KOH/car-bon ratio, this effect is not pronounced, and the samples are all microporous with pores significantly smaller than 1.2 nm. Clearer differences can only be seen in the accessible surface area with regard to the electrolyte ion sizes. The gravimetric dou-ble layer capacitance is closely related to the accessidou-ble surface area, and no signif-icant differences in of the pore surface utilization could be determined. Capacitance retention at higher currents was not particularly good for any sample, suggesting micropores between 1.0 and 1.2 nm are not large enough to allow ion diffusion.
This seems reasonable as the TEA+ cation incl. solvent shell has a size of 1.3 nm and distortion or partial removal of this can only be operated with increased energy expenditure. This ultimately leads to a higher limitations of the diffusion kinetics, which ultimately leads to low capacitance values. At low currents and frequencies, the electrolyte ions are able to adsorb at the electrode surface, although nearly all samples in this section have few pores above 1.0 nm. This could be determined via PEIS. This means ions with distorted or partially removed solvent shell participate in the formation of the electrochemical double layer, which is in agreement with previous studies.[296] Generally, the hydrogen uptake capacities increase along with a decreasing semi-carbonization temperature which is ascribed to the simultane-ously increasing surface area.
159
500 1000 1500 2000 2500 3000
0.5 1.0
1.5 2.0
2.5 3.0
500 1000
15002000
25003000 40
80 120
160 200 0.02
0.04 0.06 0.08 0.10 0.12 0.14 100 50 200 150
0.5 1.0 1.5 2.0 2.5 3.0
H2 uptake / wt%
CV > 0.66 nm / F·cm-3
CS > 0.66 nm / F·m-2 CDC / F·g-1
Sacc / m2·g-1 Vtotal / cm3·g-1 SDFT / m2·g-1
C-900-2.5-T9 C-700-2.5-T9 C-400-2.5-T9 C-800-2.5-T9 C-600-2.5-T9 C-300-2.5-T9
Figure 97: Graphical summary of the carbon sample properties, synthesized at dif-ferent semi-carbonization temperatures and a KOH/carbon ratio of 2.5.