Figure 1a shows nitrogen adsorption-desorption isotherms of ACs prepared at different conditions. P and HTP exhibit type I isotherm that is indicative of the presence of microporosity. ZP and ZHTP prepared with the ZnCl 2 displayed a combination of both type II and type IV isotherms. In addition the hysteresis loops are more pronounced for both cases which suggest the formation of mesoporosity due to creation of additional pores and widening of small pores. Impregnation with ZnCl 2 results in degradation of the cellulosic material present in the raw CS and carbonization together with dehydration which leads to the charring and aromatization of the carbon skeleton that eventually results in the formation of meso- and micro- porous of ACs. The porosity data (Table 1) clearly shows the important role of hydrothermal treatment along with chemical activation in increasing mesoporosity. ZnCl 2 plays a vital role to enhance the pore formation process. First, the pore forming agent ZnCl 2 dehydrates the CS. Pyrolysis at high temperature leads to the dehydration of carbon and creation of the micropores. Later, these micropores are getting widened and subsequently converted to mesopores in the presence of inert atmosphere33,34. The hydrothermal treatment step provides a critical advantage that enhances mesopore formation. The environment of elevated temperature and pressure provided during hydrothermal treatment facilitates better mass transfer and diffusion of ZnCl 2 into the coconut shell which leads to more efficient chemical activation. As can be seen from Table 1, highest mesoporosity is obtained when the activation process includes use of ZnCl 2 as the activating agent in the hydrothermal environment. This enhancement is also aided by the formation of more oxygen functional groups during hydrothermal treatment; functional groups present in the hydrothermally treated CS is favorable for chemical activation and also requires lesser activation energy for the burn off to generate porosity compared to direct pyrolysis. Apart from the surface area, pore size distribution is an important criterion for supercapacitors. Figure 1b shows the pore size distributions of ACs. As expected, pore size distribution for the AC treated with ZnCl 2 falls in the mesoporous range. Both ZP and ZHTP exhibit two significant peaks at ~ 2.6 and ~ 3.4 nm which lie in the mesoporous range. In addition, mesopore content for ZHTP is ~ 60% compared to ~ 48% for ZP and ~ 20% for HTP. It is well known that presence of such mesopores will enhance the supercapacitance behavior of the carbonaceous electrode35. Therefore, enhanced electrochemical performance is expected for the chemically treated ACs in capacitance measurements. Figure 2 shows the FE-TEM images of ACs treated by different approaches. AC treated without ZnCl 2 (P and HTP) have smoother textures, whereas morphologies of ZP and ZHTP show the consequence of mesopore generation in the form of increased roughness in the texture. Generally, such high surface area AC exhibits highly disordered nature which is clearly supported from the Raman analysis (Figure S1, supporting information) and consistent with literature36.

Table 1 BET Surface area and mesoporosity of synthesized ACs at different conditions Full size table

Figure 1 (a) Nitrogen adsorption isotherms of activated carbons synthesized at different experimental conditions, (b) Pore size distribution of activated carbons using DFT (Density Functional theory) model. Full size image

Figure 2 FE-TEM images ACs synthesized at different conditions: (a) P, (b) ZP (c) HTP and (d) ZHTP. Scale bar of 20 nm is fixed for all the four images. Full size image

Single electrode performance of the carbonaceous material is very crucial to employ them in Li-HEC along with insertion type anode Li 4 Ti 5 O 12 . In the conventional two electrode symmetric supercapacitor configuration, the applied potential is equally split between the two electrodes, whereas the Li-HEC configuration contains two different electrodes which undergo two different storage mechanisms. Therefore, the applied voltage will split between the electrodes depending on the capacitance of each electrode. Hence, mass balance between the electrodes is necessary to achieve higher energy density while employing different kinds of materials which undergo two different energy storage mechanisms i.e. reversible Faradaic (Li-insertion type electrode) and reversible non-Faradaic process (supercapacitor component)37. Half-cell configuration (Li/AC) of ACs are tested between open circuit voltage (OCV) to decomposition potential of the conventional carbonate based electrolyte solution (~4.6 V vs. Li) at current density of 100 mA g−1. Moreover, such half-cell configuration is considered as three electrode or single electrode configuration to elucidate the supercapacitive properties of the individual component. During electrochemical reaction, the test electrode undergoes PF 6 − anion adsorption/de-sorption process, since metallic lithium acts as both counter and reference electrodes. Typical galvanostatic charge-discharge profiles are illustrated in Figure 3. A linear increase in potential with respect to time is evidenced irrespective of the samples tested. Such linear variation clearly suggests the perfect adsorption and de-sorption process of PF 6 − anions and subsequent double layer formation across the electrode/electrolyte interface28,37. The commercially available AC was also tested in half-cell configuration and compared along with CS derived AC under the same current rates. The half-cell delivered initial reversible capacities of 5, 44, 58, 71 and 33 mAh g−1 for P, HTP, ZP, ZHTP and CAC, respectively. The observed discharge capacities can be converted to specific capacitances according to the following relation that is valid for linear variation of voltage with time2.

where, I is applied current (A), Δt is discharge time (s), m weight of the active material (g) and ΔU is testing window (i.e. potential difference) of the aforementioned half-cell configuration (mV, 1600 mV). Initial specific discharge capacitance of ~ 11, ~ 99, ~ 132, ~ 159 and ~ 74 F g−1 are obtained for P, HTP, ZP, ZHTP and CAC, respectively. It is worth noting that increase in mesoporosity results the increase in specific capacitance and is consistent with the results of Simon and Gogosti35. Plot of specific discharge capacitance vs. cycle number is given in Figure 3b. Very stable specific capacitance profiles are observed during prolonged cycling for all samples, except for a small drop in the second cycle. Further, the observed capacitance value is linear with specific surface area which is also consistent with the literature28. Since increase in surface area and mesoporosity leads to the accommodation of more anions, the AC is able to provide higher specific capacitance. ZP and ZHTP delivered specific discharge capacitances that are almost two times higher than those obtained using CAC electrodes (Figure 3). Therefore, a similar enhancement in the performance is expected in Li-HEC configuration when coupled with spinel phase Li 4 Ti 5 O 12 anode. Furthermore, HTP also delivers higher reversible adsorption/de-sorption behavior than commercial AC powders. This clearly suggests that the tailoring of carbonaceous materials with appropriate pore sizes is very crucial to yield high performance materials. The enhancement of such AC are mainly because of phase purity (Table T1) and high surface area with mesopores, which are required for easy access of the electrolyte solution and thereby enables facile adsorption/de-sorption of PF 6 − anions especially at high current rates38. Based on the electrochemical performance of both AC cathodes derived from CS (Li/AC) and spinel phase Li 4 Ti 5 O 12 (Li/Li 4 Ti 5 O 12 , Figure S2, supporting information) in half-cell configuration under same current densities, the mass loading optimized and adjusted by the ratio of anode to cathode is 1:3.88, 1:2.91 and 1:2.41 for samples HTP, ZP and ZHTP, respectively. However, the performance of sample P is found to be very inferior compared to the rest of the ACs; therefore, the powder is not been tested in Li-HEC configuration.