Shin, Juhun; (2022) Metal-free carbon-based oxygen bi-functional electrocatalysts for rechargeable metal-air battery applications. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

With rapid advancements in mobile systems and vehicles, there is an increasing expectation of performances for energy storage/conversion devices each year. New technologies such as metal-air batteries and fuel-cells have gained much attention as better substitutes of currently widely used batteries and fossil fuels however it is crucial to improve the sluggish oxygen kinetics occurring on the surface of the electrodes of these devices. Abstaining from using scarce noble-metal containing species, this thesis outlines syntheses and electrocatalytic performances of non-metal carbon-based materials as oxygen electrocatalysts for possible cost-effective cathode candidates in metal-air batteries. To explore a different synthesis approach of graphene-like carbon material, as to a harsh acid oxidation preparation method, oxygen electrocatalysts were obtained via high temperature (>700 °C) graphitisation of glucose and dicyandiamide. Pores created during polymerisation and the nitrogen species (pyridinic, pyrrolic, and graphitic) increased catalytically active sites on as prepared graphitic carbons. Annealing temperature was varied to study the effect of the ratio of N-dopant species, and the concentration of carbon defects as a function of the annealing temperature. Increasing the number of active sites whilst preserving electronic conductivity is challenging, but crucial to enhancing electrocatalysts’ performances. Heteroatom-doped ‘carbon dots’, quasi-spherical carbon particles with size of less than 20 nm, were enriched on the surface of graphene substrates to maximise catalytically available active sites. Embedding carbon dots (with many N and S dopant species) provide many catalytically active sites for each defect site compared to direct heteroatom doping. Dual heteroatom-doped carbon dots embedded graphene catalysts, NS-CD@gf_a900 exhibited significantly high catalytic performances of 7.71 mA cm-2 at 1600 rpm and only 0.91 V overpotential for oxygen bi-functional reactions; even close to noble-metal Pt/C and Ir/C counterparts (0.77 V) as a result of the effect of many available catalytic sites and the synergistic behaviour of the dopants to the electronic structures. An alternative to the traditional method of 2D carbon preparation via exfoliation and oxidation using harsh chemicals is via a bottom-up approach of forming carbon polymers. Pyrene, a polycyclic aromatic carbon with a four fused benzene-ring structure, was rationally selected as the building block for the formation of carbon substrate via a Friedel-Craft acylation mechanism. There was a significant increase in measured pore volume in the nano-range and throughout, compared to other carbon polymers and graphene samples, which can enhance the gas/ion diffusion mechanism. Nitrogen and sulphur doping with high temperature annealing at 900 °C resulted in a highly porous carbon substrate that exhibited comparable electrocatalytic performances to the Pt/C electrocatalyst.

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