Graphitic carbon materials for energy and environment

Abstract

Over the years, carbon has become one of the most intensively investigated topics in both industry and academia and has wide ranging applications in energy conversion, energy storage, adsorption, sensing, photo-electrical water splitting, water purification and gas separation. Recently, carbon materials have been widely used for advancement of energy conversion and water remediation applications due to its unique physicochemical properties, surface chemistry, processability, mechanical stability and chemical resistance. Furthermore, carbon may form porous structures and be assembled in different shapes such as spheres, tubes, fibers, sheets and 3D structures providing a high degree of versatility for multiple applications. The low cost is advantageous in many catalytic applications for the replacement of expensive and less abundant metal catalysts such as platinum, which is heavily relied upon for fuel cells as an oxygen reduction catalyst. In this context, the Ph.D. project focussed on the synthesis of graphitic carbon composite materials with unique morphologies to the benefit of energy and environmental applications. The following four concepts were developed and explored in this thesis and summarised as: 1. Fabrication of a unique 3D- nitrogen doped carbon composite materials of N-doped carbon nanotubes and N-doped carbon spheres from bio source their application as oxygen reduction reaction (ORR) catalysts The synthesis of nitrogen doped carbon nanotubes (N-CNT) and N-doped carbon micro spheres (N-CMS) composites were demonstrated using low cost and eco-friendly bio source galactose, iron oxide nanoparticles (maghemite) and nitrogen precursor melamine. This unique integrated structure containing N-CNT and N-CMS showed enhanced ORR catalytic activity via predominantly four - electron kinetics (n = 3.55 - 3.64 in the potential range of 0.10 – 0.70 V (RHE)) with a low HO₂⁻ yield (22.44 – 16.96 % in the potential range of 0.10 – 0.70 V (RHE)). Furthermore, in the context of eliminating hazardous chemical usage and to utilise more green products as ORR catalysts, galactose containing naturally occurring apricot sap was used to synthesise a similar electro catalyst as described above using maghemite nanoparticles (N-APG-Fe) and a cobalt precursor (N-APG-Co). Both catalysts formed similar integrated structures comprise of N-CNT and N-CMS as mentioned above and showed excellent oxygen reduction properties with an electron transfer number 3.61 for both N-APG-Co and N-APG-Fe catalysts at 0.40 V (RHE) and low HO₂⁻ yield (> 20.00 %) for both catalysts. The presented synthetic concept opens doors for new approaches for the development of low cost non-hazardous hybrid catalysts using abundantly available bio sources and green products. 2. Investigation of different phases of low iron oxide catalysts as an alternative for Pt/C catalysts for ORR To explore the use of cheap, abundant and freely available iron oxide catalyst as a potential substituent for the expensive and scarce Pt catalysts, four different phases of iron oxide nanoparticles (magnetite, maghemite, hematite and goethite) were synthesised and systematically evaluated as oxygen reduction catalysts for ORR. The four different phases were separately synthesised and prudently dispersed in 3D-reduced graphene oxide aerogels without exposing them for any phase changes. These catalysts (rGO/Fe₃O₄, rGO/γ-Fe₂O₃, rGO/α-Fe₂O₃ and rGO/α-FeOOH) investigated as electro catalysts for oxygen reduction did not show significant enhancement for ORR compared to the standard Pt/C catalysts. Comparative study showed that rGO/Fe₃O₄ and rGO/γ-Fe₂O₃ catalysts with inverse spinel structures with magnetic and electron conduction properties showed significantly higher ORR activity compared to rGO/α-Fe₂O₃ and rGO/α-FeOOH with rhombohedral and orthorhombic structures, respectively. The outcome of these investigations revealed the need for the exploration of more spinel structure of different metal oxides to be investigated as low-cost substituent to the expensive Pt/C catalysts for ORR. 3. Synthesis of macro porous N-doped carbon catalysts using sulphonated aniline oligomers (SAO) and SAO/ phenol formaldehyde (PF) and SAO/reduced graphene oxide (rGO) composites for ORR Sulphonated aniline oligomers (SAO) with distinctive microstructures of flakes and rods were synthesised using aniline and oxidants; and used for the synthesis 3D N-doped composite combining phenol formaldehyde (PF) and reduced graphene oxide (rGO) pyrolysed with a nitrogen precursor (melamine). The electrochemical characterization confirmed that composites with higher concentration of pyridinic nitrogen species (42 At%) showed higher positive onset potential of 0.98 V and performed the ORR with four - electron transfer kinetics (n = 3.64) with a low yield of HO₂⁻ (19 %) at 0.50 V (RHE) compared to low concentration of pyridinic nitrogen (37 At%) and higher concentration of graphitic nitrogen (63 At%). Composites prepared with conductive graphene structures displayed higher current density of 7.89 mA/cm², which is more than 60 % of the standard Pt catalysts. This unique procedure demonstrates a new approach of synthesising macro porous carbon structures with potentially viable composites of carbon materials for many future catalytic applications. 4. Investigation of different organic coating materials to provide long term air stability to Zero-valent iron (ZVI) and evaluation of air stable materials dispersed in rGO as arsenic adsorbents Zero-valent iron (ZVI) nanoparticles have been extensively investigated for treatment of hazardous and toxic waste from contaminated sites and water remediation both these applications have been hindered due to their low air stability and tendency to agglomerate. To address this problem and stabilize the ZVIs nanoparticles, several coating materials and organic molecules with various functional groups (amine, thiol, hydroxyl and carboxyl) have been demonstrated and evaluated. The results show that the ZVI coated with organic molecules containg carboxyl groups (glycine) has unprecedented stability and shelf life (> 12 months) under atmospheric conditions. To solve the agglomeration problem, the glycine protected ZVI nanoparticles were dispersed in rGO solution to make an atmospherically stable and aggregation-free ZVI-rGO composites. The environmental remediation performance of the prepared composite was evaluated using arsenic (As) solutions and showed an outstanding adsorption efficiency (As(III) (400 mg/g) and As(V) (131 mg/g) over a range of pHs, making ZVI-rGO composites an ideal sorbent for the removal of arsenic in broad remediation applications.