Abstract
This study aims at enlargement of operating voltage of carbon-based electrochemical capacitors (ECs) in organic-based electrolyte (1M TEABF4/PC), it relatively leads to improving specific energy, and especially demonstrates the unique electrochemical modification (EM) of ECs. There are three parts in this work; criteria of activated carbons (ACs), capacitance enhancement of ACs and asymmetric cell consist of ACs and carbon black (CB). The operating cell voltage is developed from 2.1 V, 2.6 V to 3 V by electrochemical technique in these three parts. In first part, the combination of potential-stressed floating test and EIS analyses precisely determines the working potential window between -1.9 and 0.2 V (against Ag/AgNO3), which is a reliable method to efficiently define the working potential window. In this potential range, ECs exhibits about 6% capacitance loss during the charge-discharge test for 10,000 cycles at 0.5 A g-1 between 1.9 and 2.1 V. Then, a novel EM to enhance the specific capacitance (CS) and operating voltage of ACs was demonstrated in second part. This EM has been optimized by repeating 5 times of 100 charge-discharge cycles between -1.9 and 0.5 V (against Ag/AgNO3) in the fresh electrolyte. Moreover, the EM was evaluated by a series of consecutive floating tests from 0 V to 0.5 V (against Ag/AgNO3) each for 3050 seconds (s) with 100 mV as interval. The surface morphology of ACs was thereafter examined by scanning electron microscope, reveals a film-like material which was formed onto the surface of ACs electrode during EM. Moreover, a film-like material has been identified to contain N, B, and F by X-ray photoelectron spectroscopic measurements. Besides, the characteristics of vibrational modes of ACs were investigated by in situ Raman spectroscopy, which shows the D band peak position of activated carbon present a blue-shift due to the surface structure change. After EM process, an asymmetric supercapacitor (ASC) consisting of the above modified-AC positive electrode and an as-received AC negative electrode, which shows excellent charge-discharge stability from the 1.9 % decay in the cell capacitance after 10,000-cycle stability test at 5 A g-1 between 0 and 2.6 V. In third part, the upper limit of the double-layer potential window for the interface between propylene carbonate and CB with low defects but relatively high specific surface area is effectively extended from 0.1 to 1 V (against Ag/AgNO3), compared with AC. Accordingly, commercial CB and AC are respectively employed as positive and negative electrode materials to construct an advanced ASC with cell voltage reaching 3 V, which is very stable in 10000-cycle charge-discharge tests at 1 A g-1 between 2.7 and 3 V.