Functional Group-Dependent Supercapacitive and Aging Properties of Activated Carbon Electrodes in Organic Electrolyte

Abstract

The concentrations of surface functional groups on activated carbon (AC) are manipulated via heat treatment at various temperatures. The carboxyl (O−C = O) population clearly decreases at 600 °C, whereas the lactone (RO−C = O) and phenol (C−OH) populations decrease if the temperature exceeds 750 °C. Their effects on electrode capacitance, leakage current, and gas evolution are systematically investigated in 1 M tetraethylammonium tetrafluoroborate/propylene carbonate electrolyte. The assembled symmetric supercapacitors are also subjected to an aging test, where the cells are held at 2.5 V and 70 °C. The decreased functional group populations significantly reduce gassing and improve the cell durability; the mechanisms are explored using electrochemical impedance spectroscopy and postmortem SEM. Nevertheless, the AC surface area drops dramatically at 850 °C, resulting in a considerable reduction in capacitance. A rational control of heat-treatment temperature is critical for obtaining AC with balanced supercapacitor performance.

Supercapacitive Properties of Micropore‐ and Mesopore‐Rich Activated Carbon in Ionic‐Liquid Electrolytes with Various Constituent Ions

“…The V cell values, whicha re defined using symmetrical anodic and cathodic charges (less than 5% difference), are 3.5, 3.2, 3.0, 3.5, and 3. [11,31] shown in Figure 3. Table 2s ummarizes the C values measured in differente lectrolytes at various charge-discharge rates.…”

“…These V cell values are clearly higher than that (2.5 V) for the conventional organic electrolyte. The specific capacitance ( C ) of the AC micro electrodes can be calculated according to the galvanostatic charge–discharge curves, shown in Figure . Table summarizes the C values measured in different electrolytes at various charge–discharge rates.…”

“…As for the IL anion effects, with the [EMI][TFSI],[EMI]BF 4 ,and [EMI][FSI] as electrolyte the capacitances were 93, 108, and1 20 Fg À1 ,r espectively,a t0 .5 Ag À1 .A t 10 Ag À1 ,t he capacitances decreased to 10, 17, and 60 Fg À1 , correspondingt o1 1%,1 6%,a nd 50 %r etention, respectively, compared to those measured at 0.5 Ag À1 .T he capacitance reductionw ith increasing appliedc urrent density can be attributed to the kinetic limitation of ion adsorption/desorptiona nd migration in the AC pores. [15,17,31,32] TFSI À can withstand a higher anodic potential, [20] resulting in al arger V cell .H owever, its large size (see Table S2) occupies more space on the AC surface and thus decreases the charged ensity that can be stored. Moreover FSI À decomposed at al ower potential than BF 4 À and TFSI À .…”

Heteroatom‐Doped and Oxygen‐Functionalized Nanocarbons for High‐Performance Supercapacitors

“…In spite of the commonly acknowledged success, O-functionalized carbon structures have several shortcomings like distributed capacitance effect, higher potential drop, high leakage current, lower surface conductivity, hindered electrolyte ion migration to micropores, and electrolyte decomposition. [210,218,225,235,236] In addition, corrosive oxidation can severely damage the carbon basal planes, creates a high level of chemical and topological defects and eventually results in deteriorated supercapacitor performance. [229] As a result, poor rate performance and cycle life are frequently reported from the O-functionalized nanocarbon electrodes.…”

“…In addition, for example, edge-functionalized oxygen groups showed poor electrochemical stability and capacitive performance of the carbon nanostructures in the organic medium due to pore blockage of material and separator by the evolved gas. [235] Another acknowledged flaw of oxygenated carbon composites in the organic electrolyte is irreversible redox reactions between the organic electrolyte and oxygen. [275] However, one should beware of the recurring problem of organic electrolyte decomposition for N-doped nanocarbons due to the amine group through the nucleophilic attack.…”