Electrochemical probing of the nanostructure of non-crystalline nitrogenated carbon materials for electrocatalysis

Abstract

Carbon materials are applied in a variety of electrochemical applications including fuel cells, electrolyzers, batteries, supercapacitors and biosensors. The doping of carbon materials with heteroatoms such as nitrogen has become a major area of research in recent years, particularly due to the applications of these materials in metal-free electrocatalysis. The electrocatalytic performance of nitrogenated carbon materials depends on a number of different factors including the bulk electronic properties, the presence of particular nitrogenous chemical moieties in the electrode surface and the nanostructure of the carbon scaffold. The design and preparation of new electrocatalysts for important processes such as the oxygen reduction reaction can be facilitated through fundamental investigations of the correlation between these factors and the electrochemical performance. This thesis presents structure-activity studies on model carbon electrode systems prepared from nitrogenated amorphous carbon films. Amorphous carbon is a non-crystalline or disordered form of carbon which can easily be prepared as topographically smooth and uniform films convenient for electrochemical characterisation. Properties such as the dopant concentration and carbon scaffold organisation may easily be tuned by altering the preparation conditions. Additionally, in practical electrocatalytic systems disordered carbon is the norm rather than the exception. This suggests that the effects of scaffold disorder in terms of parameters such as the defect density and degree of graphitization on electrocatalysis should also be investigated. This work first presents a characterisation of sputtered N-free and nitrogenated amorphous carbon electrodes using a combination of optical and spectroscopic techniques and electrochemical studies. Model systems with varied composition and nanostructure based on these electrodes after post deposition treatments are also presented. The response of these materials to different surface sensitive electrochemical probes including catechols and oxygen is correlated to the nitrogen site chemistry and the carbon nanostructure. Results suggest that the carbon scaffold organisation, particularly the size and packing of graphitic cluster domains, is as important as the presence of dopants or surface moieties in controlling the electrocatalytic response to these probes. Moreover, the use of voltammetric studies with surface sensitive redox probes emerges as an electrochemical surface characterisation technique for the probing of carbon nanostructure and organisation. The experimental data are complemented by computational studies of heteroatom doped and disordered carbon model surfaces with surface probes using density functional theory calculations.