Electron Paramagnetic Resonance Study of Carbon-based Materials and Their Applications in Electrochemical Energy Storage

Abstract

Carbon-based materials have been widely used in supercapacitors owing to their high conductivity and high specific surface area. The introduction of pseudo-capacitive parts in graphene frameworks, like nitrogen/oxygen hetero-elements and metal oxides, is an efficient route to increase capacitance due to the Faradic reaction. Here Electron Paramagnetic Resonance (EPR) spectroscopy, which is sensitive to unpaired electrons, is used to study carbon materials, commercialised activated carbon, graphene oxide, reduced graphene oxide and N-doped graphene. The capillary-based in situ EPR cell with the three electrode system is designed to investigate the electrochemical process during charging/discharging in aqueous electrolytes. The EPR study of solid-state carbon materials and their temperature-dependent study help to investigate the relationship between carbon structure and the spins. The sp3 structure (such as zigzag structure, defects) tends to give a narrow signal with Curie-Weiss type behaviour, which comes from localised carbon-centred spins. The sp2 structure tends to yield Pauli type behaviour with a broad signal, indicating the existence of π electrons on more extended aromatic structures. The time dependent EPR measurement demonstrates structured changes in various conditions. For example, the pH-dependence of purified graphene oxide clarified the decarboxylation process on exposure to solutions: the fragmentation of carbonaceous framework leads to continuously increased EPR signal in acidic solutions. The in situ EPR study finds the potential dependent narrow signal, which increases over potential then returns back when discharged, together with a g value over 2.0023. The increased narrow component in both graphene oxide and reduced graphene oxide with potential in KOH electrolyte is derived from the oxidised semiquinone radical species. A pH-dependent variation of the narrow signal clarifies the reason for the pH-dependent capacitance behaviour of oxygen containing graphene materials. The potential dependent of the broad signal reveals different electrochemical process, such as the adsorption of ions into the deep pores in activated carbon, or the improved capacitive property on defects of the basal plane of graphene materials.

Date of Award	31 Dec 2019
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Robert Dryfe (Supervisor) & Alistair Fielding (Supervisor)