Corrosion behaviour of AZ31 magnesium alloy in highly alkaline environments

Magnesium (Mg) and its alloys are known for their high chemical reactivity. This property often poses issues related to undesirable corrosion, or degradation of exposed surfaces. The chemical reactivity of Mg can be also exploited, and as a result Mg alloys often find use as anode materials for fuel cells. However, due to a long term immersion of the anodes in highly alkaline environments, the problem of corrosion remains and needs to be evaluated. Therefore, in this research, the corrosion behavior of a commercially available magnesium alloy AZ31 in 45 wt% potassium hydroxide (KOH), a common electrolyte for alkaline fuel cells, was studied. Immersion tests were performed for a total duration of 20 days to study the growth of corrosion products on the alloy’s surface. Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM) and X-ray Diffraction (XRD) were carried out to characterize the structure and chemistry of the corrosion products. Also, electrochemical studies were carried out to study the kinetics of corrosion of the AZ31 alloy. Finally, the effect of adding 2 wt% sodium silicate (Na₂SiO₃) to the KOH electrolyte in order to manipulate the corrosion rate was also examined. Tafel analysis confirmed that the corrosion potential of the AZ31 sample immersed in the Na₂SiO₃ + KOH solution reduced by 16% with respect to that of sample immersed in pure KOH. Although the AZ31 alloy contains only a trace amount of nickel, SEM-EDS characterization of the corrosion products revealed that they contained high levels of nickel, with XRD analysis confirming the presence of a nickel hydroxide layer. In the case of the sample immersed in Na₂SiO₃ + KOH electrolyte, an additional layer rich in silicates developed, and likely acted as a barrier for diffusion of ions from surface of the AZ31 sample to the electrolyte. EIS results of modeling the surface corrosion phenomena revealed that a modified Randle’s circuit represented the electrochemical processes occurring on the surface of the alloy. Warburg impedance for the sample immersed in Na₂SiO₃ + KOH was relatively high, suggesting a dissolution of ions from the surface into the highly alkaline KOH electrolyte.Graduate Studies, College of (Okanagan)Graduat

Upgrading of recycled carbon for energy storage applications

An indispensable component of many land-based transportation systems are tires. Tires are composed of materials resistant to both physical and chemical changes and do not readily degrade naturally in the environment. Particularly, mining tires are designed to operate in harsh environments and hence are highly resistant to degradation. However, despite their omnipresence, the recycling of mining tires has not been thoroughly researched, and without suitable end-of-life solutions, discarded mining tires pose dire ecological and economical challenges. In this research, chemical and heat treatment methods were employed to study the upgrading of carbon black obtained from a tire recycling operation. Chemical treatment was employed to remove inorganic impurities present in the char, while a heat treatment was required for the removal of volatiles (for example sulphur) which could not be removed by chemical treatments. Acids including HCl, HNO₃, H₂SO₄ were successful in removing Zn, Fe, and other metallic impurities, whereas NaOH and KOH were used to remove Si. Heat treatment experiments under N₂ proved to be successful in S removal. A combination of chemical treatment (5M HCl and 10M NaOH leaching), along with thermal treatment (950°C), was ideal for removing most impurities from the recovered carbon black (rCB) yielding a purity of >98 wt%. Activation experiments were performed on purified rCB using KOH and CO2. Activated carbon obtained from KOH treatment exhibited the highest surface area of 994 m²/g. Purified and activated rCB was tested for Li-ion battery applications. The activated rCB anode showed a maximum discharge capacity of 1853 mAh/g. However, the capacity drop for activated rCB cells was greater than 40% after the first cycle of charge, likely due to the formation of solid electrolyte interface and irreversible side reactions. In addition, rCB was also used as an additive material in lithium titanate oxide (LTO) battery anodes. rCB proved to be a stable additive to LTO anode with negligible irreversible capacity loss (<10%). Electrochemical testing of cells made with LTO electrodes revealed that activated rCB additives had better discharge capacity values compared to Super-P additive. Hence this study demonstrated that mining tire-derived rCB could be used as an additive material for commercial Li-ion battery applications.Applied Science, Faculty ofEngineering, School of (Okanagan)Graduat

Spark Plasma Sintering of Lithium Aluminum Germanium Phosphate Solid Electrolyte and its Electrochemical Properties

Sodium superionic conductor (NASICON)-type lithium aluminum germanium phosphate (LAGP) has attracted increasing attention as a solid electrolyte for all-solid-state lithium-ion batteries (ASSLIBs), due to the good ionic conductivity and highly stable interface with Li metal. However, it still remains challenging to achieve high density and good ionic conductivity in LAGP pellets by using conventional sintering methods, because they required high temperatures (>800 °C) and long sintering time (>6 h), which could cause the loss of lithium, the formation of impurity phases, and thus the reduction of ionic conductivity. Herein, we report the utilization of a spark plasma sintering (SPS) method to synthesize LAGP pellets with a density of 3.477 g cm-³, a relative high density up to 97.6%, and a good ionic conductivity of 3.29 × 10-⁴ S cm-¹. In contrast to the dry-pressing process followed with high-temperature annealing, the optimized SPS process only required a low operating temperature of 650 °C and short sintering time of 10 min. Despite the least energy and short time consumption, the SPS approach could still achieve LAGP pellets with high density, little voids and cracks, intimate grain–grain boundary, and high ionic conductivity. These advantages suggest the great potential of SPS as a fabrication technique for preparing solid electrolytes and composite electrodes used in ASSLIBs.Applied Science, Faculty ofEngineering, School of (Okanagan)ReviewedFacult