Influence of the carbon precursor

Since all reaction parameters and their effects on the structure and porosity of the carbons have now been investigated in detail, the influence of the precursor com-position will be studied. For this purpose different ratios of phenol and resorcinol were used in polymer synthesis, and an additional pure resorcinol-formaldehyde resin was synthesized. The surface chemistry should be kept as similar as possible, which is the case with PF resins and RF resins. Phenol and resorcinol were used in the ratio 2/1, 1/1 and 1/2 and the samples are named 2/1, 1/1, C-PR-1/2 and C-RF. The ratio of formaldehyde and benzene alcohol remained the same.

The synthesis scheme is shown in Figure 113 and shows the semi-carbonization temperature was 500 °C, the activation temperature 900 °C and the KOH/carbon ratio 5/1. This applies to all samples in this section.

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Figure 113: Synthesis scheme for the preparation of HSAC from formaldehyde resins with different ratios of phenol and resorcinol. (1) Polymerization of the maldehyde resins under strong acidic conditions; (2), semi-carbonization of the for-maldehyde resin at 500 °C under argon atmosphere; (3) chemical activation of the semi-carbonized carbons at 900 °C and a KOH/carbon ratio of 5 under nitrogen atmosphere.

4.1.7.1 Characterization of carbons synthesized from different formaldehyde resins

During the polymer synthesis it could be observed that with increasing resorcinol content the polymers became more swellable towards the solvent ethanol. This is a known phenomenon of resorcinol resins due to the higher number of hydroxyl groups and the higher degree of chain crosslinking.^[300] Images of the dry polymers are shown in Figure 114. Due to the high swellability of resorcinol resins, the ma-terial contracts during the drying process due to capillary forces.^[301] Therefore, the polymer no longer appears as a reddish brown powder, like the phenol-formalde-hyde resin, but becomes darker and darker with increasing resorcinol content until the resorcinol resin appears as a hard black solid, as can be seen at sample C-RF.

The contraction during drying can also affect the pore sizes. This is why it is as-sumed the phenol-resorcinol ratio influences the porosity of the obtained carbons.

179 Figure 114: Images of the formaldehyde resins, polymerized with different ratios of phenol and resorcinol.

The macroscopic structure is investigated by SEM images of the carbons shown in Figure 115. The surface of sample PR-2/1 is smooth, in contrast to sample C-900-5-T9, which is based on a phenol-formaldehyde resin. There carbon surface is consisted of agglomerated spherical particles. Due to the higher proportion of oxy-gen functionalities on the carbon surface, increased carbon heteroatom bond cleav-age occur during carbonization. This is why the surface structure is subjected to greater stress and the spherical particles are no longer visible. However, the cavities, which have a diameter of approx. 5 to 20 µm, are still visible. The surface of sample C-PR-1/1 is similar, except the cavities there have a much smaller diameter and are only a maximum of 10 µm in size. This trend continues from sample C-PR-1/2 with increasing resorcinol content in the precursor to sample C-RF, which is based on a pure resorcinol resin. For C-RF, only very small cavities in the range of 1 µm can be detected with a smooth surface structure. This is due to the increasing proportion of resorcinol in the precursor resin. The above-mentioned higher amount of hy-droxyl groups increases the polarity of the polymer surface, and therefore increases the wettability of ethanol with the polymer surface. The ethanol accumulates in the cavities of the polymer and is removed during drying. This leads to a shrinkage process of the cavities, as the ethanol can only be removed if the adsorption forces on the surface and the capillary forces in the cavity spaces can be overcome. Even-tually, a shrinkage of the cavities can be observed. This effect is more pronounced with a higher degree of ethanol wetting on the polymer surface.

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Figure 115: SEM images of (a) PR-2/1, (b) PR-1/1, (c) PR-1/2 and (d) C-RF.

P-XRD patterns of the activated carbons are shown in Figure A.14. For all samples the broad reflection at 44° 2θ is visible, but no reflection at 22 2θ is visible. This shows the degree of graphitization of the samples must be very similar and it does not seem to depend on the precursor composition. The respective Raman spectra are shown in Figure A.15. The ID/IG ratios are 1.17, 1.09, 1.08 and 1.07 for C-PR-2/1, C-PR-1/1, C-PR-1/2 and C-PR, respectively. The ratios are in a very close range and only for sample C-PR-2/1 is a slightly higher ratio observable, indicating a slightly lower degree of graphitization. All in all, observations from Raman spec-troscopy support the observations from the P-XRD patterns; the degree of graphiti-zation seems mostly independent of the precursor composition. A different degree of graphitization could have been expected due to a possible stronger cross-linking of the polymer chains through the higher resorcinol content and through the more dense packing of these caused by contraction during the drying process. This is also indicated by the higher ID/IG ratio of sample C-PR-2/1. However, the degree of graphitization seems to be dominated by the semi-carbonization temperature and the activation conditions. This is not surprising since the graphitic arrangement of

181 carbon rings is a thermodynamically controlled process and therefore strongly tem-perature dependent. In addition, the activation conditions chosen for the carbons in this section have already been determined to be more structure-damaging, which can be deduced from section 4.1.2. These activation conditions therefore overcom-pensate a possible improved arrangement of graphite layers by a higher resorcinol content.

Results from thermogravimetry are shown in Figure 116. A complete combustion can also be detected for these carbon samples, meaning that all carbons are free of impurities. Interestingly, all the samples combust at the same temperature, around 500 °C, and all are also fully combusted around 650 °C. Furthermore, there are al-most no differences in the combustion curve. Differences in combustion tempera-ture are mainly caused by differences in porosity. This is a first indication that not only the degree of graphitization, but possibly also the porosity is independent of the precursor.

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