Non-crystalline nitrogenated carbon electrode materials: Bulk and surface doping methods for controlling electrochemical properties

Abstract

Carbon is ubiquitous as an electrode material and its electronic and capacitive properties and its surface chemistry are suitable for electrode materials for several applications such as fuel cells, capacitive storage devices and batteries. Carbon may be found as variety of allotropes and it can exhibit different properties and it represents a very attractive material for electrochemical applications especially as a material for energy storage and conversion. These carbon materials can be modified by doping or by employing physical/chemical treatment to achieve desirable electrochemical properties. The role of nitrogenation in carbons has gained huge attention in the materials science due to the electrocatalytic activity showed by nitrogen modified carbons in electrochemical reactions such as, oxygen reduction reaction (ORR) and the hydrogen evolution reaction (HER) for energy applications. This thesis presents the study on the effects of nitrogen doping on the electronic, chemical and structural properties of amorphous carbon electrodes and the role of nitrogenation in determining the electrochemical response of these materials. DC magnetron sputtering technique was used to prepare amorphous carbon and nitrogen doped amorphous carbon films. Various characterization techniques used in this work to characterize these electrode materials are also describes including x-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy, Raman spectroscopy, voltammetry, electrochemical impedance spectroscopy. Electronic and structural properties of these electrodes are directly correlated to the capacitive properties. Initial nitrogen incorporation significantly increases the capacitance of amorphous carbon electrode, resulting in an increase in metallic character. However, greater level of nitrogenation increases the disorder and it creates defects and localised N-sites.

This work also discusses nitrogen doped and nitrogen free graphitized amorphous carbon which are prepared though thermal annealing of sputtered amorphous carbon films. A combination of spectroscopic and electrochemical techniques was used to investigate the effect of selective N-site incorporation in the organization of the carbon scaffolds. Electrochemical capacitive properties of these nitrogenated electrodes is correlated with the organization of the carbon scaffolds due to selective N-site incorporation.

This work also investigated the effect of surface nitrogenation through RF plasma system on the electronic properties of the graphitised amorphous carbon. It is shown that total N/C content and the distribution of N-sites are not varying with plasma exposer time thus suggesting that the chemical composition of the carbon surface reaches steady state within ca. 5 min of exposure. Effect of nitrogenation results in a significant restructuring of the carbon scaffold and it increases defects and amorphization. Electrochemical characterization results show that nitrogenation increases the capacitive storage significantly. The ORR performance of these materials was also investigated and results suggest that nitrogenation improves the onset potential and reduce H2O2 yields.