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Page 9 of 17 ACS Applied Materials & Interfaces The adsorption branch was fitted to the Brunauer‐Emmett‐ Teller (BET) isotherm equation between P/P0 = 0.02‐0.8 ac‐ cording to the standard consistency criteria for microporous materials63, yielding a monolayer capacity of 760 mLSTP g‐1 and a BET surface area of 3310 m2 g‐1 (see Figure S4). It should be noted that this value is significantly higher than that of a pris‐ tine double‐sided sheet of graphene (2650 m2 g‐1) owing to significant edge‐character of the ZTC structure. The mi‐ cropore volume of ZTC was found to be 1.41 mL g‐1, as deter‐ mined by Dubinin‐Radushkevich (DR) analysis (see Figure S5). Non‐local density functional theory (NLDFT) analysis of the adsorption branch reveals a narrow pore‐size distribution centered at a diameter of 1.19 nm (Figure 2c). The slightly longer overall pore‐to‐pore regularity of ~1.4 nm measured by XRD and TEM indicates that the pore walls in ZTC are atomically thin (<0.3 nm) and that all carbon surfaces are ex‐ posed on both sides. Together, these results are consistent with the open, double‐sided graphene‐fragment based molec‐ ular model of archetypical FAU‐ZTC.45 Electrochemical Oxidation of ZTC in KFSI Electrolyte. The electrochemical oxidation of ZTC in the presence of KFSI electrolyte was assessed in an analogous way as for recent in‐ vestigations of FSI‐ intercalation in graphite.64 The relevant electrochemical reactions proceed as follows (where the for‐ ward direction occurs during charging): viscosities that serve to significantly reduce ionic conductivity as well as long wetting times, and were therefore abandoned. The type of charge storage mechanism65 as well as the sta‐ bility window66 of electrochemical oxidation of ZTC were as‐ sessed by cyclic voltammetry (CV) measurements carried out in 4.8 M KFSI in EC/DMC (1:1 by weight). Early experiments suggested that the electrochemical oxidation behavior of pris‐ tine ZTC in the presence of KFSI was markedly different dur‐ ing the first several (typically six) electrochemical cycles, in‐ dicating some irreversibility in the electrochemical reactions undergone during the early stages of oxidation of the ZTC framework, and likely SEI formation at the surface. The spe‐ cific nature of these first six cycles (scan rate, voltage range, etc.) was found to play a significant role in the eventual re‐ versible electrochemical characteristics of the ZTC. Hence, a specific pretreatment regimen needed to be established that optimally prepared the ZTC for further electrochemical char‐ acterization and eventually for optimal electrode preparation (see Supporting Information for details). The optimal pre‐ treatment regimen was determined to consist of six galvanos‐ tatic cycles between 2.65‐4.7 V vs. K/K+ at 120 mA g‐1 after which all other electrochemical characterization was carried out. Successive CV measurements within incrementally widen‐ ing voltage windows were then used to determine the limits of reversible oxidation (FSI‐ insertion) and reduction (K+ in‐ sertion) of ZTC (with further details provided in the Support‐ ing Information). An overall “stability window” of ~1.0‐4.8 V vs. K/K+ was determined for ZTC (Figure S8); this is con‐ sistent with previous reports of the electrochemical stability of ZTC in other electrolytes (e.g., 1.2 to 4.7 V vs. Li/Li+ in 1 M LiPF6 in EC/DEC).39 However, it should be noted that there is a difference between the “stability window” and the optimal window of operation in a full‐cell KFSI DIB. Galvanostatic dis‐ charge and differential capacity measurements of ZTC within the full voltage range of 1.0‐4.7 V vs. K/K+ show a sharp de‐ cline in dQ/dV intensity below 2.65 V indicating the start of (undesirable) K+ ion insertion within ZTC (Figure S9). There‐ fore, the optimal window of operation of a KFSI DIB cell em‐ ploying ZTC as the active cathode material for the exclusive insertion/removal of FSI‐ ions was determined to be 2.65‐4.7 V vs. K/K+. Within this range, “cycled” ZTC shows highly re‐ versible behavior even at slow scan rates down to 0.1 mV s‐1 and a strictly capacitive mechanism of charge storage (Figure S10). ZTC as the Cathode in a KFSI DIB Cell. Electrochemical cells comprising pristine ZTC as the cathode (and a thin layer of potassium on stainless steel as the anode) in 4.8 M KFSI in EC/DMC (1:1 by weight) were prepared and galvanostatically cycled between 2.65‐4.7 V vs. K/K+ (see Figure 3). At the low‐ est current rate investigated (120 mA g‐1), the specific dis‐ charge capacity of ZTC was measured to be 139.5 mAh g‐1 dur‐ ing the 6th cycle, representing a ~40% increase over graph‐ ite64, at an average voltage of 3.49 V vs. K/K+. For comparison, identical KFSI DIB cells comprising several other porous car‐ bon cathode materials were also tested within the same volt‐ age window (Figure 3a). The reversible specific discharge ca‐ pacity of each cathode material was found to be linearly de‐ pendent on both the BET specific surface area and the DR spe‐ cific micropore volume of the bare porous carbon (see Figure S6). With regard to surface area, this corresponds to a packing Anode: K e ↔ K (1) Cathode: C FSI ↔ FSIC e (2) For graphite, it was found that the upper limit of FSI‐ incorpo‐ ration corresponds to a stage 1 intercalation compound with a composition of [FSI]C12 (i.e., = 12 in Equation 2) and the charge/discharge voltage plateau is at 4.7‐4.8 V vs. K/K+.64 The charging process ends when all of the K+ cations or FSI‐ anions are depleted in the electrolyte, or when the ZTC cath‐ ode reaches its maximum charge storage capacity. In this work, anticipating a similarly high voltage of operation as in graphite, a range of electrolyte compositions in several or‐ ganic solvents and mixtures thereof were investigated with the goal of determining a solution with an appropriate voltage stability and the highest possible concentration of KFSI. It should be noted that the KFSI solution serves as both an elec‐ trolyte (i.e., ion conduction medium) and as the source of elec‐ troactive species; the concentration of the electrolyte is a lim‐ iting factor in the energy density of the final cell (see Support‐ ing Information for details) and must be maximized. Prelimi‐ nary experimental results on the electrochemical perfor‐ mance of ZTC‐based KFSI DIBs with different electrolyte con‐ centrations also revealed that higher concentration was cor‐ related with higher charge storage capacity (Figure S7). Therefore, there is a two‐fold contribution of the electrolyte concentration to energy and power density due to the higher ZTC cathodic capacity contribution. Based on these consider‐ ations, all electrochemical experiments carried out in this work were performed using a high concentration electrolyte: 4.8 M KFSI in EC/DMC (1:1 by weight). It should be noted that the large volume expansion of the electrolyte solution upon dissolution of KFSI leads to a significantly lower actual con‐ centration than initially mixed (e.g., the true concentration of a nominally 10 M solution is only 4.84 M when measured by final volume). Higher concentrations of the KFSI salt in the EC/DMC mixture are possible, but were found to result in 4 ACS Paragon Plus EnvironmentPDF Image | Zeolite-Templated Carbon as the Cathode
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