To investigate the possible structural changes due to pretreatment and activation using various forms of CO2, the textural properties of samples are studied (see S3 for tabulated values). Micro-nanobubble-pretreated samples have typically exhibited fascinating BET specific surface areas as plotted in Fig. 3(a). The BET specific surface area was 1209 m2 g-1 after pretreatment and 3 hr of CO2 activation, which is 39.21 % higher than the surface area of ACF-3 (735 m2 g-1). Likewise, BACF-3 showed a 16.44 % increase in micropores when compared with the bare support (Fig. 3(b)). As the activation time using CO2 increases from 3 hr to 6 hr, the BET specific area of ACF-3 and BACF-3 increased 7.20 % and 13.33 %, respectively. CO2 is acknowledged as the most stable form of oxidized carbon compounds. Nevertheless, it reacts easily to form additional C-C and/or C-H bonds under a high-energy input. Given that, the use of various CO2 forms in both pretreatment and activation may lead to the highest surface characteristics (Fig. 3(d)). As observed in the yield curve (Fig. 3(a)), all samples showed the loss of their total mass due to the decomposition of unmatured polymers via a sandwich of CO2 treatment (Yoda et al. 2018). Although BACFs possessed higher surface characteristics than ACFs, the yield curves were 10.92 % and 11.52 % lower in 3 hr and 6 hr, respectively. The upward peaks in N2 adsorption-desorption isotherms suggest that all samples are of Type I, which is typical for microporous materials (Fig. 3(c)). The broadening of peaks shows an increasing adsorption trend and does not flatten at P/P0 between 0.8 ~ 1.0, which verifies the multilayer adsorption. Alongside the typically characterized adsorption force in microporous materials, the van der Waals force is given on the adsorption potential within micropores. This indicates that all samples have a large micropore region and a comparably small mesopore region. N2 volume adsorption-desorption isotherms are also in good agreement with the inert graph in Fig. 3(b).
The chemical state of elements
Introduction of micro-nanobubbles and post-treatment of CO2 (6 hr) seems to be the key factors to achieve superior surface characteristics. However, there are underlying uncertainties whether the samples are free of any impurities. Acknowledging that impurities are more likely to be attached to the samples with high BET specific surface, ACF-6 and BACF-6 were chosen to observe the chemical state of elements. Both samples reveal porous characteristics with no visible impurities (Figs. 4(a)-(b)). According to the average chemical proportion, C and O are considered to be major elements present in both samples (Fig. 4(c) and S4). The amount of C and O was 2.91 % and 2.93 % higher in ACF-6 and BACF-6, respectively. Evidently, C and O ratio difference is mainly due to the side chains of partial oxygen-containing functional groups (C-O, O-C=O, and C-C) introduced on the surface. Only N can be distinguished in the category of minor components (ACF-6: 1.65 % and BACF-6: 1.63 %). This is attributed to residual nitrogen from pitch. The wide scan spectrum displayed photoelectron lines in the order of C1s (approx. 288 eV) > O1s (530 eV) > N1s (400 eV). No characteristic peaks of any impurities were noticed, suggesting that micro-nanobubbles are solely the major culprit of high surface characteristics.
We merely hypothesized that dissolved micro-nanobubbles at the frontline of stabilization may chemically react with the surface and aid for pore-growth (Fig. 2(b)). However, it is still an arduous quest to theoretically understand the microchemical behavior of dissolved micro-nanobubbles without satisfactory reasons. Inspired by the two scenarios proposed with CO2 activation, we track O atoms under the inclusion of surrounding elements (Figs. 4(d)-(e)). On the one hand, dissolved micro-nanobubbles may release partial O atoms attached to the surface as the temperature increases. Partial O atoms could form additional CO2 with surrounding CO during activation, resulting in a higher mass of CO2 activation (Nabais et al. 2008). Another interesting approach lies on the partial O atoms, which are not attached with surrounding CO. Partial O atoms may directly compromise with C of crystallites and result in additional CO. The activation reaction could be promoted while the unmatured carbonaceous fiber consumes CO, which results in additional pore-growth (Lan et al. 2019). In summarization, dissolved micro-nanobubbles may not generate pores by themselves, but rather act as a swarm of catalyst at the beginning of stabilization. These chemical interactions made our samples bear great application potential in high-performance adsorbents or electrode materials.