Abstract
The ever-increasing demand for sustainable energy has led to the development of inexpensive energy storage devices. The electrode material is one of the most important parts of an energy storage device; the electrode material has a major impact on the device's price, sustainability, environmental friendliness, performance and lifetime.
Asphaltene, as a high carbon content by-product of the oil sands industry, currently has a higher supply than demand. Furthermore, asphaltene can be used to prepare low-cost carbon fibers (CFs) as the electrode material in energy storage devices. Electrochemical double layer supercapacitors (EDLCs), pseudocapacitors and the air-electrode in Zn-air batteries (ZABs) were prepared in this study by using asphaltene based CF.
Activated carbon fibers (ACFs) were prepared by chemically activating the CF derived from asphaltene produced in Alberta, Canada. ACFs were used to prepare stable, high performance and flexible EDLC and birnessite MnO2 type pseudocapacitors. CFs were also used as a conductive base layer for spinel type MnCo2O4, which is an efficient electrocatalyst for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). MnCo2O4 coated CFs were used to prepare stable and high efficiency homemade air electrodes for ZAB.
ACFs, that were used as the active material in an aqueous EDLC, had a specific surface area of 2290 m2 g-1 and total porosity (pore volume) of 1.27 cm3 g-1, which includes 0.88 cm3 g-1 of micropores (pore width < 2 nm) and 0.29 cm3 g-1 of mesopores (2 nm < pore width < 50 nm). The maximum specific capacitance (Cs) reached was 311 F g-1 at a specific current (is) of 0.04 A g-1; this was reduced to 248 F g-1 at a specific current of 1 A g-1. Capacitance retention of this EDLC was 91% after 10,000 cycles. This material was later used in an EDLC device with an ionic liquid electrolyte (EMIMBF4, 1-ethyl-3-methylimidazolium tetrafluoroborate); ionic liquid electrolytes provide wider voltage windows. As a result, a specific energy (Es) of 35.7 Wh kg-1 was achieved at a power density (Ps) of 525.4 W kg-1. These values are comparable to energy and power values delivered by some batteries.
Birnessite-type MnO2 (δ-MnO2) is a promising material for charge storage devices like pseudocapacitors (slower charge/discharge compared with EDLCs but faster charge/discharge compared with batteries). However, the insulating nature of δ-MnO2 limits its electrochemical performance. Because of the high performance and high conductivity of ACF used in EDLC devices, this material was chosen to fabricate ACF/δ-MnO2 composite electrodes to enhance capacitive performance of δ-MnO2. δ-MnO2 was coated onto ACFs through a hydrothermal process. The crystal structure of δ-MnO2 was subsequently thermally modified to reduce its crystallinity by introducing oxygen deficient defects. These defects acted as active sites to enhance electrolyte ion adsorption/desorption, which improved the capacitive performance. The maximum Cs reached for the composite electrode was 327 F g-1 at a specific current of 0.04 A g-1, which was significantly improved compared to the δ-MnO2 powder (not coated on ACF, 195 F g-1 at 0.04 A g-1). Capacitance retention for the composite electrode was 93% (initial capacitance of 298 F g-1 and final capacitance of 279 F g-1 at 1 A g-1), while the retention for the δ-MnO2 powder was 64% (from 154 F g-1 to 98 F g-1) after 10,000 cycles.
CFs were utilized to prepare homemade gas diffusion layers (GDLs) for use in air electrodes in ZABs. Air electrodes were prepared with CF carbonized at three different temperatures, i.e., 500 oC, 800 oC and 1500 oC. the ORR and OER activity of the homemade air-electrodes, as well as commercially purchased electrodes, were tested in 1 M KOH. All homemade electrodes showed much better OER activity than the purchased ones. ORR activity was similar for both commercial electrodes and homemade electrodes prepared with CF at 1500 oC (CF-1500). CF-1500 was coated with spinel type MnCo2O4 via a facile sonication procedure. MnCo2O4 coated CF-1500 had excellent catalytic activity towards both ORR and OER, outperforming the bench mark Pt-RuO2 catalyst. The cycling behavior of CF-100 was very stable with initial and final efficiencies of ~63% and ~58%, respectively, after 200 cycles (100 h) of charge and discharge at 10 mA cm-2.