Characterization of carbons synthesized at different semi-carbonization temperatures

900 °C.

4.1.3.1 Characterization of carbons synthesized at different semi-carbonization temperatures

The SEM images of the prepared activated carbons are shown in Figure 54. It is visible C-800-5-T9, C-700-5-T9 and C-600-5-T9 possess a network structure, com-posed of loose agglomerated spherical carbon nanoparticles. The network structure matches with the previously discussed structure of C-500-1-T9 and C-500-2.5-T9 (see section 4.1.2.1). The loose agglomeration of the spherical carbon nanoparticles leads to the same formation of cavities in the network, which are 2 to 20 µm in diameter. The network structures of C-400-5-T9 and C-300-5-T9 differ signifi-cantly from the other HSAC in this chapter. The carbon surface is smooth without agglomerated spherical carbon nanoparticles, while they still possess cavities within the network with similar diameters from 5 to 20 µm. The agglomeration of the particles occurred during the activation process. When PF-resins are semi-car-bonized at lower temperatures, they become more susceptible towards the chemical activation in terms of chemical resistance. The reason is likely due to the higher content of heteroatoms, in this case oxygen, which weaken the otherwise chemical resistive carbon network, containing mainly carbon-carbon bonds. With a higher

100

semi-carbonization temperature treatment, more oxygen is removed from the car-bon network by forming carcar-bon dioxide or carcar-bon monoxide, leaving behind holes, or pores and a network mainly consisting of carbon with stable carbon-carbon bonds. ^[91,273] The results are a stronger altering of the carbon structure, leading to a larger amount of pores and larger pores in general, as discussed in section 4.1.2.

Apparently, this altering is also visible for macroscopic structure, as seen for sam-ples C-400-5-T9 and C-300-5-T9 and to some extent for sample C-500-5-T9 where the agglomerated particles are slightly larger than for carbon samples C-500-1-T9 and C-500-2.5-T9. Due to the lower semi-carbonization temperature, more oxygen heteroatoms were present in the carbon networks. During the chemical activation, not only oxygen atoms were removed due to the simple heat treatment, but also likely due to the reaction with KOH or activation side products because they are active reaction sides during the KOH activation. At the same time, the carbon atoms were removed via chemical reactions with KOH. That way, a more bonds of the substrate structure were broken, more pores were created, which caused a collapse of the macroscopic structure. While a simple temperature treatment removes more oxygen atoms from the carbon network, the structural damage of the macroscopic structure is not very pronounced. This changes when many oxygen atoms are pre-sent during a chemical activation, as carbon atoms are removed from the network at the same time, severely damaging the macroscopic structure.

101 Figure 54: SEM images of the activated carbons. (a) C-800-5-T9, (b) C-700-5-T9, (c) C-600-5-T9, (d) C-400-5-T9 and (e) C-300-5-T9.

The p-XRD patterns of the carbon samples are shown in Figure A.2. Similar to the previously discussed carbon samples, all carbons show weak and extremely broad reflections at 44 °2θ corresponding to (101) diffractions of a turbostratic carbon structure. Only C-800-5-T9 shows a weak diffraction at 20° 2θ likely corresponding to stacked graphene layers. The semi-carbonization of 800 °C was high enough to enable a stacking of graphene layers although only to a less extent. The Raman spectra are shown in Figure A.3. The calculated ID/IG ratios are 1.08, 1.15, 1.16, 1.22 and 1.26 for C-800-5-T9, C-700-5-T9, C-600-5-T9, C-400-5-T9 and C-300-5-T9 respectively. The intensity ratios are therefore increasing with decreasing semi-carbonization temperature indicating a lower degree of graphitization for the carbon samples, treated at a lower semi-carbonization temperature prior to the chemical activation. The low temperatures of 300 and 400 °C were not high enough to enable a stacking of graphene layers, which was only slightly observable for sample C-800-5-T9. Results from p-XRD and Raman scattering indicate the carbon samples are amorphous materials. This can be attributed to the low semi-carbonization tem-peratures, especially for samples C-400-5-T9 and C-300-5-T9 and the chemical ac-tivation with a high KOH/carbon ratio of 5, which causes structural damage to the

102

carbon network as seen in section 4.1.2.^[65] The results of the thermogravimetry for all carbon samples are shown in Figure A.4. In general, all carbons show a complete burn off leaving no residual mass. This indicates pure carbons, which are free of any impurities like inorganic salts, which could remain after the chemical activation process. The starting combustion temperatures for 800-5-T9, 700-5-T9 and C-600-5-T9 are quite similar at approximately 550 °C, while the starting combustion temperatures of C-400-5-T9 and C-300-5-T9 are higher at 575 and 600 °C respec-tively. This means that porous carbon samples with a possibly higher degree of porosity in terms of larger pore sizes and higher surface area, seem to be more ther-mal stable. Similar results have been observed in the previous section 4.1.2.1. The reason is the lower content of oxygen atoms in the carbon networks of the samples, treated at a lower semi-carbonization temperature. With a lower semi-carbonization temperature, the carbon networks possess a higher content of oxygen, which are active reaction sites for KOH and therefore are the reason, for the less rigid charac-ter of the carbons toward the chemical activation. However, the results in a lower content of oxygen in the carbon network for the samples, treated at a lower semi-carbonization temperature and thus leading to a higher thermal stability.

The corresponding isotherms of the nitrogen physisorption measurements are shown in Figure 55 and the nitrogen physisorption derived data are summarized in Table 4. The samples C-800-5-T9, C-700-5-T9 and C-600-5-T9 show a typical type I isotherm, indicating carbon materials with predominantly micropores. C-400-5-T9 and C-300-5-C-400-5-T9 show a slope at relative pressures from 0.1 to 0.4 and addition-ally hysteresis loops. Both phenomena indicate the presence of large micropores and mesopores. It seems the pore size increases, with decreasing semi-carboniza-tion temperature. The pore size distribusemi-carboniza-tions, calculated by the QSDFT method from the nitrogen desorption branch, shown in Figure 56, support this suggestion.

All carbon samples possess micropores smaller than 1 nm originating from the car-bonization process and the chemical removing of carbon atoms during the KOH activation. In addition to these small micropores, all carbons have larger pores, with C-400-5-T9 and 5-T9 having additional mesopores up to 5 nm, while C-300-5-T9 has slightly larger pores than C-400-C-300-5-T9. It is clear, the semi-carbonization temperature is in direct correlation with the pore size. As confirmed from section

103 4.1.2.1 the pore sizes of the carbon materials increase with a decreasing semi-car-bonization temperature while maintaining a constant KOH/carbon ratio and activa-tion temperature. This confirms that the carbon framework does not show high chemical resistance after a low temperature treatment in the form of a low semi-carbonization temperature. It is suspected a larger content of organic residues re-mains after a low temperature carbonization treatment, namely oxygen, which of-fers active reaction sites for the chemical activation reactant.^[65] This way, a greater amount of pores is created and more importantly, existing pores are enlarged. All in all, a lower semi-carbonization temperature leads to an increase in pore size.

However, the thermal stability is increased at the same time, due to the low content of oxygen for the final activated carbons.

0.0 0.2 0.4 0.6 0.8 1.0

0.0 0.2 0.4 0.6 0.8 1.0

0 400 800 1200 1600 2000

2400 C-800-5-T9

C-700-5-T9 C-600-5-T9 C-400-5-T9 C-300-5-T9

ads. vo l. / c m

3

·g

-1

relative pressure

Figure 55: Nitrogen adsorption-desorption isotherms (77 K) of the carbon materi-als synthesized at different semi-carbonization temperatures.

104

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0

0.0 0.6 1.2 1.8 2.4 3.0 3.6

4.2 C-800-5-T9

C-700-5-T9 C-600-5-T9 C-400-5-T9 C-300-5-T9

D v( d) / c m

3

·nm

-1

·g

-1

pore width / nm

Figure 56: Pore size distributions of the carbon materials synthesized at different semi-carbonization temperatures, calculated by the QSDFT method from nitrogen desorption branch assuming slit pores.

The specific surface area depends on the semi-carbonization temperature as it in-creases with decreasing semi-carbonization temperatures in the range of 800 to 400 °C. Accordingly, C-400-5-T9 hast the highest DFT surface area with

~ 2600 m²·g¹. A low semi-carbonization temperature not only results in a pore en-largement, but also increases the amount of pores, which leads to high specific sur-face areas. With a higher semi-carbonization temperature, the carbon framework becomes more rigid and resistive against the chemical activation and the enlarge-ment of pores is limited. The specific surface area of C-800-5-T9 is the lowest among the carbon samples discussed in this chapter, but still as high as 2045 m²·g⁻¹. This can be attributed to the high KOH/carbon ratio, which ensures new micropores are formed during activation. A similar correlation is observable for the total pore volume. C-300-5-T9 has an ultra-high pore volume of 2.82 cm³·g⁻¹ and the pore volume decreases with increasing semi-carbonization temperature in accordance with the decreasing pore sizes. The micropore volume increases with a decreasing semi-carbonization temperature up to 600 °C and then decreases again, with C-600-5-T9 having the highest micropores volume of 1.22 cm³·g⁻¹. This may lead to the conclusions, that a lower semi-carbonization temperature increases the amount of small pores in general. However, micropores are pores with the range up to 2 nm and the pores become larger with a lower semi-carbonization temperature. The pores are just large micropores, on the edge of mesopores. The micropore volume

105 in dependence of the semi-carbonization temperature reflects the transition of the carbon materials from being mainly microporous (600 °C) to having additional mesopores (400 °C) as previously observed. The results so far, emphasize the ob-servations and conclusions from section 4.1.2.1; the semi-carbonization tempera-ture has a major influence on the formation of pores and their sizes, which then determines the surface area and pore volume.

Table 4: Nitrogen physisorption derived data of the carbon samples synthesized at different semi-carbonization temperatures.

Sample SBET / m²·g⁻¹

SDFT / m²·g⁻¹

Vt / cm³·g⁻¹

Vmic / cm³·g⁻¹

C-800-5-T9 2259 2045 1.00 0.86

C-700-5-T9 2640 2220 1.20 1.02

C-600-5-T9 3045 2342 1.33 1.22

C-400-5-T9 3413 2606 2.45 1.00

C-300-5-T9 3367 2558 2.82 0.78

SDFT (DFT surface area), Vt (total pore volume) and Vmic (micropore volume) ob-tained from QSDFT analysis. SBET (BET surface area).

The micropore analysis was obtained from carbon dioxide physisorption measure-ments and the adsorption-desorption isotherms at 273 K are shown in Figure 57.

All isotherms are reversible adsorption desorption behavior and show an almost linear correlation of adsorbed volume with increasing relative pressure except for the samples C-800-5-T9 and C-700-5-T9, which show a larger increase at relative pressures up to 0.004. This is due to the larger presence of small micropores with the size around 0.335 nm. For the other carbon samples, this indicates evenly dis-tributed pores up to 1 nm in size. The pore size distribution calculated by the NLDFT method are shown in Figure 58. As described in the previous chapter the pore size distributions show multimodal pattern for all carbons due to the NLDFT artifact, which causes zero minima at 0.4 and 0.7 nm. Otherwise, the pore size dis-tributions are similar for the carbon samples. There are minor differences in the intensities at certain pore sizes, but they are difficult to interpret, since the NLDFT method provides results streaked with smaller artifacts. The analysis of the cumu-lative surface area and pore volume is more sophisticated.^[188,189]

106

0.000 0.005 0.010 0.015 0.020 0.025 0.030

0.000 0.005 0.010 0.015 0.020 0.025 0.030

0 40 80 120 160

200 C-800-5-T9

C-700-5-T9 C-600-5-T9 C-400-5-T9 C-300-5-T9

ads. vo l. / c m

3

·g

-1

relative pressure

Figure 57: Carbon dioxide adsorption-desorption isotherms (273 K) of the carbon materials synthesized at different semi-carbonization temperatures.

0.2 0.4 0.6 0.8 1.0

0.2 0.4 0.6 0.8 1.0

0.0 0.5 1.0 1.5 2.0

2.5 C-800-5-T9 C-700-5-T9 C-600-5-T9

Dv(d) / cm3 ·nm-1 ·g-1

pore width / nm

0.2 0.4 0.6 0.8 1.0

0.2 0.4 0.6 0.8 1.0

0.0 0.5 1.0 1.5 2.0

2.5 C-400-5-T9 C-300-5-T9

Dv(d) / cm3 ·nm-1 ·g-1

pore width / nm

Figure 58: Pore size distributions of the carbon materials synthesized at different semi-carbonization temperatures, calculated by the NLDFT method from carbon dioxide desorption branch assuming slit pores.

The cumulative specific surface area of the combined data of carbon dioxide and nitrogen physisorption is shown in Figure 59. The proportion of the surface gener-ated by pores smaller than 1 nm depends on the semi-carbonization temperature.

The surface area provided by pores up to 1 nm is 1250 m²·g⁻¹ for C-800-5-T9 and the surface area decreases with decreasing semi-carbonization temperature. The surface area, generated by pores up to 0.6 nm follows the same correlation. This

107 means, however, carbon samples synthesized at lower semi-carbonization temper-atures, have larger accessible surface areas with respect to the electrolyte ions used for the electrochemical characterization. The critical pore size is the size of the BF4–

ion with 0.67 nm and the accessible surface area describes the surface, promoted by pores larger than 0.67 nm. This means eventually, that the carbon samples, syn-thesized at lower semi-carbonization temperatures, not only possess a higher as-sessable surface area, but also likely a higher amount of pores between 0.67 and 1 nm, which are likely to be crucial for the electrochemical energy storage due to the partial or full removal of the solvent shell of electrolyte ions. The larger propor-tion of the total surface area for C-300-5-T9 and C-400-5-T9 compared to the other three carbon samples, is generated by pores between 1 and 3 nm. The cumulative pore volume for the activated carbon samples is shown in Figure 60 and confirms larger pores generate a higher total pore volume, leading to the ultra-high pore vol-ume of C-300-5-T9.

0.1 1 10

0.1 1 10

0 500 1000 1500 2000 2500

3000 C-800-5-T9

C-700-5-T9 C-600-5-T9 C-400-5-T9 C-300-5-T9

cu m ulat ive S

DFT

/ m

2

·g

-1

pore width / nm

Figure 59: Combined cumulative surface area derived from carbon dioxide and nitrogen physisorption data of the carbon samples synthesized at different semi-carbonization temperatures.

108

0.1 1 10

0.1 1 10

0.0 0.5 1.0 1.5 2.0 2.5

3.0 C-800-5-T9

C-700-5-T9 C-600-5-T9 C-400-5-T9 C-300-5-T9

cu m ulat ive V

DFT

/ c m

3

·g

-1

pore width / nm

Figure 60: Combined cumulative pore volume derived from carbon dioxide and nitrogen physisorption data of the carbon samples synthesized at different semi-carbonization temperatures.

Including the results of the previous section 4.1.2, it becomes clear the semi-bonization step is crucial in tailoring the porosity of the synthesized activated car-bon material. With high semi-carcar-bonization temperatures, the carcar-bon framework is more resistive against the chemical activation and the pore generation and enlarge-ment during the activation process is limited. Hence, the resulting carbons are pre-dominantly microporous and have low pore volumes and surface areas. The total and accessible surface area can be tailored as shown in Figure 61. While the total surface area seems saturated at a semi-carbonization temperature of 400 °C, the ac-cessible surface area is still increasing with decreasing semi-carbonization temper-ature. Whether or not the capacitance for EDLCs or the gravimetric hydrogen up-take capacity is pore size dependent, the surface area remains a key factor, as both applications need available activation sites for promoting high capacitance values and uptake capacities. A semi-carbonization temperature of 200 °C was not per-formed, since the burning process of starts above 200 °C and a semi-carbonization is therefore not meaningful.

109

0 200 400 600 800 1000

0 500 1000 1500 2000 2500 3000

total surface area accessible surface area

Im Dokument High Surface Area Nanoporous Carbons for Energy-Related Applications (Seite 111-121)