Enhancement of electrochemical activity from modified graphenic materials for fuel cell and supercapacitor applications

Abstract

This thesis is focused on investigating the potential of two novel types of graphene nanomaterials for use as electrode coatings in the next generation of fuel cell and energy storage systems. In particular the development of nitrogenated graphene nanoplatelets (N-GPs) as metal-free oxygen reduction reaction (ORR) catalysts for cathodes in alkaline fuel cells is presented. Additionally hydrothermally reduced graphene oxide (rGO) as an ORR catalyst and power storage material have also been investigated.The grinding of graphite in ionic liquid for 30 minutes and 4 hours, respectively, was followed by pyrolysis at 600ᵒC in a 1:10 mixing ratio with urea (denoted as N-GP 0.5-hr 600ᵒC and N-GP 4-hr 600ᵒC, respectively). As confirmed by x-ray photoelectron spectroscopy (XPS), the longer the grinding duration, the higher the incorporation of nitrogen. This highest ORR cathodic current from the developed N-GP samples was from the N-GP 4-hr 600ᵒC sample, as confirmed during linear sweep voltammetry (LSV) and cyclic voltammetry (CV) analysis. The stability of the ORR current from the nitrogen doped GP samples is higher than platinum on carbon black as observed during chronoamperometry analysis.Graphene oxide (GO) hydrothermally reduced while suspended in 0.1M sulphuric acid (denoted as rGO H₂SO₄), provided a superior oxygen reduction efficiency to that of GO reduced in 0.33M hydrazine (denoted as rGO N₂H₄) as indicated from LSV measurements. The cathodic current from the ORR is seen to be similar for the rGO N₂H₄ and rGO H₂SO₄ samples, respectively. XPS analysis has indicated that an incorporation of sulphur promoted the oxygen reduction catalysis of the rGO H₂SO₄ sample. While the rGO N₂H₄ sample provides an increase in nitrogen along with a decrease in the oxygen content. This is indicated to promote the enhanced ORR current from the rGO N₂H₄ sample.CV analysis shows a pseudocapacitive response which is highest for the rGO N₂H₄ sample. While the rGO H₂SO₄ sample shows a slightly lower but similar pseudocapacitive current density. The restoration of the C-C sp² bonding and increased level of defects indicated from Raman spectroscopy, together with an indicated increase in porosity and specific surface area, promotes the pseudocapacitance at the rGO H₂SO₄ particle surface.

Thesis is embargoed until 30th November 2015

Date of Award	Nov 2013
Original language	English
Supervisor	Pagona Papakonstantinou (Supervisor)