Structural characterization was conducted on the PM CSTs using SEM, observing pyrolysis-induced morphological transformations at the micro- and nanoscale. From the micrographs in Fig. 2, it can be seen that the common microstructural feature is nanoribbons, with a sometimes wrinkled appearance. They make up an extremely thin, secondary electron-transparent array of interconnected, branching ribbons with widths of about 10 μm, lengths of several tens of microns or longer and a thickness of about 20–100 nm. A key structural difference making 700CSTs (Fig. 2a,b) distinct from 900CSTs (Fig. 2c,d) and 1100CSTs (Fig. 2e,f) is the presence of round salt pockets. These salt pockets are thought to form during heating, as water escapes and naturally-present biological salts aggregate, both within the chitin-based organics and on the surface. At temperatures above 900 °C, these salt pockets disappear, having exceeded the melting points of most salts present. It should also be noted that, especially in Fig. 2a,e, the nanoribbons form a network. There is also layering and/or pseudo-layering, as evidenced in Fig. 2c, which shows a lateral view of the layered ribbon networks.

Figure 2 Low to high magnification SEM of PM CST anodes heat-treated at 700 °C (a,b), 900 °C (c,d) and 1100 °C (e,f) (scale bars for (a–f), respectively: 100 μm, 10 μm, 50 μm, 10 μm, 100 μm and 10 μm). Full size image

Observations were also made on higher-magnification SEM micrographs of the various PM CSTs. As mentioned, biological salts (primarily KCl) organize in pockets of varying sizes and begin to create additional void space at 700 °C. However, the melting point of KCl is 770 °C and above this temperature the salts escape the carbon structure more completely. As captured in Fig. 3a,b, salt deposits are still present at the surface of the nanoribbons, having a wide diameter distribution (although some pores are still formed). Beginning in Fig. 3c,d, no surface salts or salt pockets are present and instead there is a hierarchically porous texture. Pores observable from SEM have diameters between 26 nm or higher, down to an observable 6 nm, which is the lowest feature size observable from the SEM images. From this data, it is confirmed that this material becomes mesoporous upon pyrolysis, with pore sizes of 6–26 nm observable by SEM. Images in Fig. 3e,f, representing the 1100CSTs, display an even wider range of porosity than the 900CSTs, with macropores of up to 100 nm in diameter. To confirm that porosity exists throughout the bulk of the nanoribbons, rather than solely on the surface, cross-sections of the nanoribbons were captured under SEM, shown in Fig. 3d,f. In these images, the presence of pores is evident throughout the entire thickness of the nanoribbons.

Figure 3 Low to high magnification SEM showing the increasing degree of porosity of PM CST nanoribbons as temperature increases, including sample heat-treated at 700 °C (a,b), 900 °C (c,d) and 1100 °C (e,f) (Scale bars for (a–f), respectively: 1 μm, 200 nm, 500 nm, 100 nm, 1 μm and 100 nm). Full size image

Detailed transmission electron microscopy (TEM) analysis was also conducted on the 1100CSTs, since they were both the highest performing anode material and displayed the highest apparent porosity from SEM. The microstructure seen in the lower-magnification images (Fig. 4a,b) is consistent with what is seen in SEM, which is a thin, highly mesoporous nanoribbon architecture. The range of pore size is confirmed from Fig. 4b,d,f identifying macropores and mesopores. Remarkably, TEM analysis also confirms the presence of worm or channel-shaped micropores, as observed in Fig. 4f. The low thickness of the ribbon is observed from the transparency of the structure in Fig. 4a, wherein the lacey carbon from the TEM grid is visible from beneath the sample. The general shape of the meso- and macropores of the nanoribbons is circular, information which supports the idea that salt pocket formation and subsequent melting induces this type of void space generation. On the other hand, it is hypothesized that the worm-like micropores captured in Fig. 4f are generated through an activation-like process. N 2 adsorption/desorption isotherms show a long, flat plateau for adsorption and desorption and a significant hysteresis (a characteristic of mesoporous and/or microporous carbons). The BET surface area was measured at 19.6 m2/g, which is a discrepancy in relation to the isotherm and TEM data; this points the prevalence of “blind,” or inaccessible pores formed in the 1100CSTs, which then become accessed after deep cycling. Moreover, our leading hypothesis is that oxygenated organics (polysaccharides, oligosaccharides, amino acids, DNA), combined with the unusually high K concentration, may lead to carbonate formation and subsequent CO 2 generation; this is that basis of chemical activation, or a similar mechanism.

Figure 4 TEM of pristine PM 1100CST hierarchically porous nanoribbons, showing macroporosity (a,b), mesoporosity (c,d) and worm-like microporosity (e,f). Full size image

The phenomenon of small, biological salt pockets causing void spaces of various sizes, from the macroporous to microporous domains, leads us to propose that high PT-pyrolyzed PM CSTs are inherently a self-activating material, by a complex set of mechanisms. It is highly advantageous to have a material that can be naturally primed for optimal performance by simply applying heat-treatment. At PT of 1100 °C, macropores form, facilitating electrolyte infiltration and hence electrolyte interaction with high surface-area hard carbon (shown to be higher capacity than graphite, gravimetrically)7. Mesoporous carbons have been shown to perform with excellent stability over long-cycling in literature8, however, harsh chemical methods are required to achieve such performance. Hierarchically porous carbons, also achieved by many activation methods, tend to improve ion diffusion rates and expose additional active material for reversible capacity enhancement9,10.

Spectral data was obtained from the PM CSTs after PTs of 700–1100 °C to analyze elemental composition by point-ID energy dispersive x-ray spectroscopy (EDS) and phase information using x-ray diffractometry (XRD) of the free-standing anodes. From Fig. 5a, XRD patterns show the transition from large KCl peaks for the 700CSTs (blue), to the 1100CSTs (black) with no visible peaks of crystalline inorganics. The prevailing model for KOH-activation is shown in Eqn. (1) and (2). At temperatures above 400 °C Eqn. (1) occurs and above 700 °C Eqn. (2) occurs (gasification of CO 2 occurs throughout this process)10. Further, K 2 O from Eqn. (2) continues to be reduced by carbon to metallic K at above 700 °C. A physical means of activation then occurs, when metallic K diffuses into carbon, expanding the lattice10. Considering the classical chemical and physical models for KOH-activation, it is likely that the biological salts present in the PM provide suitable precursors for similar activation mechanisms. With salt deposits of various sizes composed of KCl and likely a host of carbonates and phosphates, the PM is an ideal self-activating carbon precursor for PTs above 900 °C.

Figure 5 Spectral data of the pristine, free-standing PM CSTs at various PTs, including XRD (a) and point-ID EDS (b). Full size image