High Performance Electrode and Catalyst Nano-materials for Energy Storage Devices: Supercapacitors, Pseudocapacitors and Zinc-air Batteries Made from Asphaltene Based Carbon Fibers
Creators: Abedi, Zahra
Publisher: name: University of Alberta Library
Year: 2023

<p>The demand for grid-scale energy storage becomes significant as the world continues to move towards renewable and sustainable energy storage technologies. Capacitors and batteries are energy storage devices used in almost all electronic devices. The first study in this work investigated the performance of asphaltene based carbon fibers (CF) as the electrode material for electrical double layer supercapacitors (EDLSs). Asphaltene based activated carbon fibers (ACF) were prepared via chemical activation using KOH pellets. The activation ratio (mass(KOH)/mass(CF)) was varied in the range of 1-4. N2 physisorption was employed to investigate the specific surface area (SSA) and porosity of the ACF-R samples (R = activation ratio). The best performing sample (ACF-3) in this work had a Brunauer–Emmett–Teller (BET) SSA of ~2300 m2 g-1 and a total pore volume of 1.27 cm3 g-1. The EDLS fabricated with ACF-3 had a specific capacitance of 205 F g-1 at 40 mA g-1. The focus of the second study was to use ACF as a conductive substrate for birnessite type MnO2 (δ-MnO2) as the active material for a pseudocapacitor. ACF-3 was chosen due to its high electrical conductivity and superior capacitance reported in the first study. δ-MnO2 was prepared through a hydrothermal method. Due to the poor performance of the as prepared material, an annealing step was employed in an oxygen-deficient environment (N2) to introduce oxygen-deficient defects and porosity into the microstructure. A temperature of 400 oC was the optimum annealing temperature (400-δ-MnO2). Sample 400-δ-MnO2 was then synthesized as a coating on ACF-3 (ACF-400-δ-MnO2). The pseudocapacitive performance of ACF-400-δ-MnO2 (328 F g-1 at 0.4 A g-1) was superior to that of 400-δ-MnO2 (195 F g-1 at 0.4 A g-1), demonstrating the improvement in the performance by using ACF-3 as the substrate.The third study dealt with zin-air batteries (ZABs) made with asphaltene based CFs. The main issues regarding rechargeable ZABs are low efficiency and unstable cycling behavior. Spinel type MnCo2O4 nanoparticles were coated on carbon fibers and carbonized at 1500 oC (not activated) to fabricate a homemade air electrode with electrocatalyst for ZABs. The cubic spinel phase was identified and confirmed by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). This electrode outperformed the benchmark Pt-RuO2 electrocatalysts with an efficiency of ~64% at 10 mA cm-2 and stable cycling behavior for at least 100 h (200 cycles). The electrolytes used in this work for half-cell and full-cell tests were aqueous, O2 saturated 1 M KOH and aqueous 6 M KOH + 0.25 M ZnO.The fourth study investigated the performance of the same homemade air electrode, developed in Chapter five of this thesis (MnCo2O4/CF), in an all solid-state ZAB. Alkaline poly(acrylic acid) (PAA) based gel polymer electrolytes (GPEs), with different crosslinker concentrations, were synthesized and studied. These electrolytes were examined via visual, rheological and electrochemical tests. Visual and rheological tests revealed that a crosslinker concentration of 20 mM did not result in a GPE that qualified as a solid, but was liquid. Electrochemical tests indicated that 30 mM of the crosslinker in the GPE (Hydrogel-30-mM) led to the best performing electrolyte. Full cell battery tests showed that the battery made with MnCo2O4/CF and Hydrogel-30-mM had very good performance with an initial efficiency of ~63% at 10 mA cm-2, which degraded by only 6.5% after 200 cycles. The maximum power density achieved by this battery was 240 mW cm-2. The last part of this work studied the low temperature battery performance of an all solid-state ZAB made with MnCo2O4/CF and PAA based GPE, that was developed in the previous study. The battery performance was studied in terms of charge/discharge voltage and efficiency, cycle life, power output and cell voltage in a temperature range of -45 oC to 21 oC. This battery successfully completed 200 cycles of charge/discharge at all temperatures, without failure. The current density used for the cycling test was 2 mA cm-2, which is at least twice the value reported in most of the literature. The power outputs at 0 and -45 oC were 75 and 12 mW cm-2, respectively.  </p>