Heteroatom Doped Carbon Electrode Materials for Redox Flow Battery Application
The development and investigation of electrode materials for high electrocatalytic activity and surface area for redox reactions in all vanadium redox flow batteries (VRFBs) is studied in this dissertation. Electrochemical energy storage and especially redox flow batteries (RFBs) are favoured choice in many applications because of their flexibility, efficiency, and scalability. One of the challenges faced by the most developed RFB technology, the all-vanadium redox flow battery (VRFB), is the sluggish kinetics of the VO2+/VO2+ and V2+/V3+ redox couple at the typically used carbon electrodes. The objective of this project is to evaluate factors that are affecting the kinetics of the interfacial charge-transfer process, and to investigate the physiochemical treatment processes to functionalise the electrodes with heteroatoms such as nitrogen, to promote better performance. At the first part of this PhD work, the impact of extended charge-discharge cycling on carbon paper electrodes pre-treated using conventional heat treatment approach was investigated. Electrode degradation along with 70% decrease in charge – discharge capacity was observed after 100 charge – discharge cycles of a single cell vanadium redox flow battery operating at a current density of 80 mA cm?2 at room temperature (23?C). Electrochemical investigation reveals an increase in the activation overpotential at both the positive and negative electrodes originating from significant changes in the composition of oxygen functional groups at the electrode surface after degradation. In order to improve the electrocatalytic activity of carbon paper electrodes, a physiochemical process for nitrogen doping was developed. N-doped carbon paper have been shown to have superior electrocatalytic activity towards several redox reactions such as the [Fe(CN)6]3-/4- redox couple. The effect of pyrolytic pretreatments under different conditions on the performance of carbon paper were also studied to elucidate their electrocatalytic activity from a material physics perspective, using Raman spectroscopy. Although heating of carbon paper in air at around 500°C (a widely used method for activating carbon paper electrodes) increases the surface area by about 16-times compared to untreated and nitrogen-doped carbon paper, the latter exhibits superior electrocatalytic property for VO2+/VO2+, [Fe(CN)6]3-/4-, and the oxygen reduction reaction. The use of graphene material was explored with an aim of increasing the active surface area and electrocatalytic activity of the modified carbon paper electrodes. A novel scalable and high yield method for the graphene production using an electrochemical exfoliation pathway was developed. The electrochemical method involves two steps: (1) intercalation of aqueous ionic OH? species into the graphite at a constant current density of 30 mA cm?2, followed by (2) oxidation of these ionic species in (NH4)2SO4 (0.1M), and under a relatively high anodic voltage (10 V). The resultant graphene flakes were found to have high electrical conductivity of 44230 S m?1 compared to graphene produced using conventional hummers method, with a high graphene yield of about 93%, along with and a low oxidation level with a C:O ratio of about 15 (determined by XPS). The graphene modified carbon paper prepared by dip coating carbon paper electrodes in a graphene – water suspension, were evaluated for application in an all-vanadium redox flow battery and was found to improve the voltage efficiency by around 10% at a current density of 80 mA cm?2.
Carbon materials, Electrochemistry, Redox Flow Battery, Raman mapping, Doping
Singh, A. K. (2021). Heteroatom Doped Carbon Electrode Materials for Redox Flow Battery Application (Doctoral thesis, University of Calgary, Calgary, Canada). Retrieved from https://prism.ucalgary.ca.