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Page 7 of 17 ACS Applied Materials & Interfaces materials that are both conductive and that can easily accom‐ modate large, polyatomic anions by adsorption on their inner surfaces as both high energy‐ and power‐density electrodes in DIBs. While some MOFs are redox‐active and/or electrically con‐ ductive, most are neither.42 Carbon‐based porous materials such as activated carbon, on the other hand, are known to ex‐ hibit very large surface areas and micropore volumes, akin to MOFs, and to exhibit modest electrical conductivity, while also remaining inexpensive to produce and only consisting of highly Earth‐abundant constituent elements. Indeed, activated car‐ bons serve as the standard electrode materials in electrochem‐ ical capacitors (i.e., supercapacitors) for these reasons, and ow‐ ing to their ability to be both oxidized and reduced. Capacitive charge storage increases as a function of surface area,43, 44 and the highest surface area carbon materials exhibit benchmark electrochemical capacitance in both aqueous and organic elec‐ trolytes.36 Zeolite‐templated carbon (ZTC) represents an extreme limit of ultra‐high surface area, carbon‐based framework materi‐ als. ZTC is synthesized within a crystalline microporous sili‐ cate (zeolite) template, exhibiting an ordered network of pores with an extremely narrow pore size distribution made up exclusively of micropores.39 The most well‐understood ZTC structure is the variant derived from faujasite (FAU‐ZTC) which features ~1.2 nm pores separated by curved graphene‐ like struts composed of disordered and irregular polycyclic carbon fragments.45 While the atomic‐level structure of FAU‐ ZTC is disordered, the pore‐to‐pore regularity (imparted by the zeolite template during synthesis) is very high, and the volumetric density of homogeneous micropores is higher than that of any other predominantly sp2‐hybridized carbon‐ based material. Importantly, the three‐dimensionally con‐ nected structure of ZTC is sufficiently conductive46 to permit its use as a bare (capacitive‐ or pseudocapacitive‐type) elec‐ trode material in electrochemical capacitors47‐54 (supercapac‐ itors) and batteries55, even at very high current rates48 and low temperatures56. These traits make ZTC an ideal material for hosting large, polyatomic anionic species via a capacitive storage mechanism on its inner surface. In this work, potassium bis(fluorosulfonyl)imide (KFSI) DIB cells based on ZTC along with several other bare porous car‐ bon materials were prepared and characterized as a proof‐of‐ principle, and to determine the dependence of the charge storage capacity (i.e., anion uptake) on the surface area and micropore volume. Bis(fluorosulfonyl)imide (FSI‐) was cho‐ sen as an optimal anion due to its demonstrated successful use in DIBs,41, 57 high oxidative stability,58 and solubility even in high concentrations in mixtures of ethylene carbonate (EC) and dimethyl carbonate (DMC), permitting high energy and power densities in full‐cell DIB prototypes. Potassium was chosen as a suitable counter ion owing to its high natural abundance and good electroplating/stripping characteristics under the conditions necessary for FSI‐ insertion into ZTC (imparting a similarly high cell voltage to lithium but being >100× more abundant in Earth’s upper crust). The full‐cell is thus comprised only of highly abundant elements (C, K, N, S, O, and F) and a schematic of the working principle of this KFSI DIB concept is shown in Figure 1, depicting FSI‐ insertion into ZTC during charging. Figure 1. Schematic Depiction of the Working Principle of a KFSI DIB Employing ZTC as the Active Cathode Material. (a) In the as‐ prepared cell, pristine (unoccupied) ZTC is dispersed onto a TiN‐coated stainless steel current collector and the electrolyte solution (KFSI in EC/DMC) is allowed to wet all components freely. (b) Upon charging of the cell, K+ cations combine with electrons at the anode and electroplating of metallic potassium occurs. Concomitantly, FSI‐ anions insert into the oxidized ZTC framework and [FSI]@ZTC is formed in the fully charged state. (c) Upon discharge, K+ cations are stripped from the anode back into solution and electrons travel to the cathode, recombining with the ZTC framework; most of the FSI‐ anions leave the ZTC and redissolve into the electrolyte, while some remain irreversibly adsorbed within the ZTC. 2 ACS Paragon Plus EnvironmentPDF Image | Zeolite-Templated Carbon as the Cathode
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