Electrospinning of carbon nanofibers for investigating the behaviour of lead electrodeposits on the carbon surface

Over the years, the lead acid battery’s performance in modern vehicles has improved considerably to adapt to the new demands of vehicle electronics and start-stop applications. Although vehicles are valuable for commuting, their high volume of usage has resulted in an increase of fuel prices and emitted carbon dioxide (CO2). Due to the drastic global warming concerns, vehicle manufactures have been pressured into developing a variety of hybrid electric vehicles (HEV) that would reduce their fuel consumption and thereby reduced CO2 emission. The HEVs construction requires the implementation of advanced battery systems that operate under high rate partial state of charge (HRPSoC) conditions. During battery’s discharge, the Pb contained on the negative plate is oxidised to insulating PbSO4 crystals. When the battery is charged, the PbSO4 is reduced back to Pb. With prolonged use at HRPSoC, the battery undergoes an irreversible process wherein small PbSO4 crystallites dissolve and precipitate into large passivating PbSO4 crystals that accumulate as a dense layer on the surface of the negative plate. This layer restricts the diffusion of the H2SO4 electrolyte into the reaction sites for further reactions, resulting in the battery to fail prematurely. Research has shown that the inclusion of appropriate carbon materials on the negative active material (NAM) improves the performance of the batteries in HRPSoC cycling. This study looked at the making of some of the carbon-based additives by using relatively cheap materials such as polyvinyl alcohol (PVA) polymer with a sucrose blend in ratios of 75:25, 50:50 and 25:75, respectively. The blended materials were electrospun into PVA/Sucrose nanofibers. These were further stabilised by iodine and carbonised at 500 °C in a tube furnace. The properties of the carbon fibres were investigated by a number of analytical techniques such as TGA, DSC, XRD, BET, FTIR and SEM. The obtained carbon fibre surfaces were further electroplated with Pb in order to investigate the chemical bonding, the relationship between the carbon-based material interface and the Pb was investigated by SEM and CV techniques. The results of the study demonstrated that the varied ratios of the PVA/Sucrose nanofibers influenced the differences in their respective chemical and physical properties. Thermal analysis demonstrated that at approximately 500 °C, the final residue of the PVA/Sucrose nanofibers decreased as the sucrose content decreased. iii Carbonisation of the nanofibers was confirmed by the structural changes corresponding to the disappearance of the major bonds of PVA and sucrose materials, as well as by the appearance of C=C stretches. Carbonisation was also confirmed by the XRD diffractogram that demonstrated two broad amorphous peaks that corresponded to the planes of the graphite structure. The broadness of the peaks confirmed the amorphous nature of the carbon fibres. The morphological studies demonstrated that upon blending PVA with increasing quantities of sucrose, the uniformity of the fibres was altered into beaded fibrous structures with large diameters due to the increased viscosity of the electrospinning solution. The behaviour of the Pb deposits on the carbon surfaces was influenced by the high quantity of PVA present on the blend and the fibrous structure of the carbonised material. The Pb had high affinity towards the fibrous carbon derived from 75:25 as it deposited widely across the surface. Whereas, the absence of Pb deposits on the surface of the carbon derived from the 25:75 ratio was an indication that the high sucrose content and the bulky non-fibrous structure hindered the electrodeposition. The electrochemical study findings demonstrated that the bare Pb electrode had two distinct current peaks corresponding to the anodic (discharge) and cathodic (charge) reactions of Pb/PbSO4. Upon the inclusion of the electrospun carbon materials on the Pb electrode, the intensities of the anodic and cathodic peaks were reduced. Implying that none of the carbon materials enhanced the electrochemical redox reactions of the Pb/PbSO4 couple in the H2SO4 electrolyte