Effect of Electrolyte Composition on the Performance of Coal Char-Derived Carbon Supercapacitors

Abstract

Research regarding supercapacitors has vastly grown over the past decade. The advantages of supercapacitors include high power density, rapid charge/discharge capability, and a long cycle life. Current applications of supercapacitors are limited by the need to develop new, low-cost electrode materials for electric double layer capacitors and pseudocapacitors [1]. Coal char is the solid product of a low-temperature coal carbonization process, and is currently used primarily as an ultra-low-cost fuel for electric power generation. As such, coal char consists mainly of hard carbons and ash [2]. Upon ash removal, the hard carbons in coal char may serve as a promising, low-cost, supercapacitor electrode material. This research will present the use of inexpensive coal char as an electrode material for supercapacitors, with particular focus on the effects of various electrolyte compositions on the performance of coal-derived carbon supercapacitors.Electrolyte composition plays a large role in the electrochemical properties of a supercapacitor, affecting: ion interactions with electrode surface functional groups; electric double layer capacitance; thermal stability; and allowable voltage range. In this work, bituminous grade coal char from a Utah mine is pulverized, pyrolyzed, and washed in a base solution followed by an acid solution to remove residual ash. SEM imaging and BET surface area measurements are used to characterize the coal. The purified coal char is mixed into a slurry of coal char, Super P binder, and dimethylformamide, then applied on a stainless-steel plate to create supercapacitor electrodes. Three-electrode open-cell electrochemical measurements are used to characterize the supercapacitor performance of the coal char in varying electrolyte solutions using a Gamry Interface 1000E potentiostat. The different electrolytes explored include: aqueous acid 0.5 M H2SO4, aqueous base 6 M KOH, and aqueous neutral solutions 0.5 M Na2SO4, 4 M LiNO3, water-in-salt electrolyte using 12.5 M NaClO4, . Voltage range, specific capacitance, electrochemical impedance, and charge-discharge characteristics of the coal char in the different electrolytes are characterized by cyclic voltammetry, chronopotentiometry, and electrochemical impedance spectroscopy measurements. The results provide important insight into the optimal electrolyte composition for use with coal char supercapacitor electrodes, and contribute to current understanding of electrode-electrolyte interactions in carbon supercapacitors.This work is supported in-part by NSF Award #1742696.Reference1.Rakesh Kumar Gupta, Mukul Dubey, Parashu Kharel, Zhengrong Gu, Qi Hua Fan, Biochar activated by oxygen plasma for supercapacitors, Journal of Power Sources, Volume 274, 2015, Pages 1300-1305.2. H. H. Schobert, Coal; The Energy Source of the Past and Future, pg. 9, American Chemical Society, Washington (1987). Figure 1