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This experimental study examined the development of a lightweight aluminium-air cell. The novelty of this study was the characterisation of the behaviour of the lightweight aluminium-air cell, depending on the choice of aluminium anode and air-cathode. Development of the prototype aluminium-air cell involved three areas of study. For the first two high purity aluminium alloys, Al/0.5 Mg/0.07 Sn and Al/0.4 Mg/0.07 Sn/0.05 Ga, were evaluated as anodes in a half-cell in 4 mol dm-3 NaOH at 22 °C and 60 °C and compared against 99.999 %wt aluminium. Alloys were pre-treated by solution heat treating in a furnace at 600 °C for 8 hours followed by a water quench. Corrosion rates were quantified at open-circuit and under galvanostatic discharge via a hydrogen collection method. For the second method of study, the electrochemistry of a LaCaCo3/LaCaMnO3 air-electrode, known as KTH, supplied by a project collaborator, was compared against seven commercial gas diffusion electrodes. The catalysts on the commercial electrodes include Pt, MnOx, Ag2O and Co-based. Air-electrodes were compared structurally using scanning electron microscopy, mercury intrusion porosimetry and pycnometry. The KTH electrode outperformed the others with a limiting current density for oxygen reduction of −463 mA cm-2 at −0.49 V vs. Hg/HgO. In the final area of study three prototype aluminium-air cells were investigated with the third one being selected as the final design. Cell characterisation involved measurement of electrode potentials and cell temperature during variable loading and galvanostatic discharge experiments, and assessing their inter-relationship. The best performing prototype cell with a solution heat treated Al/Mg/Sn anode and KTH air-electrode had a peak power density of 174 mW cm-2 at 208 mA cm−2. Under galvanostatic discharge at 100 mA cm-2 over an hour duration, this cell had a specific power capability of 62 W kg-1 and a specific energy of 66 W h kg-1.

The use of NaBH4 for fuel cells offers a promising alternative to incumbent electrical power generation technologies. The predicted high energy density (9.3 kW h kg-1) of the direct borohydride fuel cell (DBFC) and its capacity to release 8e- per molecule converts it to a potential substitute for the H2/O2 system. Sodium borohydride, with 10.6 wt. % hydrogen content, can also generate H2 gas to be fed into a traditional H2/O2 fuel cell in an indirect borohydride fuel cell (IBFC). However, there are fundamental aspects of the DBFC and the IBFC that need to be addressed to achieve their optimal performance. The hydrolysis of borohydride is the key factor in both cases, being the undesired parallel reaction that occurs during the borohydride oxidation in the DBFC, and the main reaction taking place and to be promoted in the IBFC. The competition between BH4- oxidation and its hydrolysis depends on the electrode material, electrolyte composition and operation conditions, such as the temperature. In this work, two approaches to the use of NaBH4 for fuel cells are considered. Catalysts such as Pd-Ir alloy on microbifrous carbon, gold coated reticulated vitreous carbon (RVC), planar gold and dispersed nanoparticulate gold supported on carbon (Au/C), were tested for the direct borohydride oxidation while Pd-Ir alloy, Pt nanoparticles on carbon paper and Pd deposited on granular carbon (Pd/C) were evaluated to generate H2 from NaBH4 for use in an IBFC. The gold coated RVC electrodes demonstrated good activity towards the borohydride oxidation, increasing the kinetic rate constants and the current density with the thickness of the coating and the porosity grades. The Pd-Ir alloy was also catalytic towards the DBFC, with current densities between 100 and 200 mA cm-2 but with low H2 generation rates (< 0.1 cm3 min-1). As computational methods could play a prominent role in the design and characterisation of DBFCs, density functional theory (DFT) was used to investigate the reaction mechanism of borohydride oxidation at Pd-Ir surfaces. This work also studies the use of surfactants, including Triton X-100, Zonyl FSO, S-228M, sodium dodecyl sulphate and FC4430, during the direct oxidation of borohydride ions using a planar gold and Au/C electrodes. The addition of 0.001 wt. % Triton X-100 to the alkaline borohydride solution decreased the H2 generation by 23 % at the Au/C electrode, while the borohydride oxidation remained unaffected. In contrast, in the H2 generator, the Pd/C catalyst showed an excellent activity towards the borohydride hydrolysis, obtaining a maximum rate of 8 × 103 cm3 min-1 gmetal-1 during 120 minutes using 4 mol dm-3 NaBH4 in 350 cm3 distilled water and 15 g of catalyst. This is the highest H2 generation rate reported in a laboratory scale reactor using borohydride and a Pd base catalyst. Keywords: borohydride oxidation and decomposition, catalysis, DBFC, IBFC, hydrogen generation, hydrolysis inhibition, kinetic rate constant.

Redox flow batteries (RFBs) can be used to store energy on the large and medium scale (kW – MW), particularly in applications such as load levelling of electrical power supplies, power quality control application and facilitating renewable energy deployment. In this thesis, the development of a divided and undivided zinc-cerium redox flow battery from its fundamental chemistry in aqueous methanesulfonic acid has been described. This comprehensive investigation has focused on the selection of electrode materials, evaluation of zinc corrosion of the negative electrode, characterization of the redox flow battery and the cycling performance. Voltammetric studies of both the zinc and the cerium half-cell reactions have been carried out under various operating conditions and for electrolyte compositions. These studies suggested that the positive electrode reaction could limit the use of higher current densities. After testing a range of two- and three-dimensional positive electrode materials, only three-dimensional platinised titanium mesh and carbon felts were capable of discharge at 50 mA cm-2 with high charge ( > 70 %)and voltage ( > 60 %) efficiencies in an divided system based on the optimum electrolyte compositions obtained in the half-cell studies. In order to avoid the diffusion of protons across the membrane and to simplify the construction, an undivided, membraneless system was proposed. With specific design arrangement and carbon felt positive electrode, this system can operate at room temperature with a high energy efficiency (~ 75 %) instead of 60 oC as reported in the patented system in the literature. In order to facilitate zinc electrodeposition and prevent zinc corrosion, several electrolytic additives and corrosion inhibitors have been suggested. Further challenges and research directions are also discussed.