Role of Nitrogen Defects in Nitrogen-Doped Carbon for Catalyst Support and Electric Double-Layer Capacitor
Date of Completion
nitrogen-doped carbon, carbon, oxygen reduction, super-capacitors, ordered mesoporous carbon
William E Mustain
Christopher J Cornelius
Field of Study
Doctor of Philosophy
Electrochemical applications of carbon are wide and varied. They are used as electrodes for batteries, capacitors and proton exchange membrane fuel cells (PEMFCs). This is possible due to their excellent electrical conductivity, variable nanostructure and cost-effectiveness. Besides these properties, physicochemical properties such as microstructure and surface chemistry play a significant role in the performance of carbon as electrodes in these applications. In supercapacitors or ultracapacitors, surface oxygen functionalities can increase capacitance due to non-faradaic reactions between redox couples such as quinine and hydroquinone while in PEMFCs, surface functionalities can determine catalyst size distribution and dispersion. However, surface oxygen functionalities also facilitate carbon corrosion, which can spontaneously take place at as low as 0.2 V. This affects the long term performance of these electrochemical devices. Therefore, there has been considerable effort to improve surface chemistry of carbon using heteroatoms.
Nitrogen is at the forefront of this effort because It has been shown that by doping carbon with nitrogen, the capacitance can be increased and catalyst size distribution and dispersion can be improved. However the role of nitrogen is not well understood. To investigate the role of nitrogen-groups on capacitance and support properties of N-doped carbon, we chose Nitrogen-doped ordered mesoporous carbon (NOMC) casted on a SBA-15 template from polypyrrole precursors. Templating procedure was chosen because it allows precise control of pore structure which greatly affects the performance of carbon in electrochemical devices. SBA-15 was chosen because the carbon casted on it is an exact inverse replica. Polypyrrole precursor was chosen because of its high N/C ratio and forms graphitic carbon after pyrolysis.
In Chapter 1, background to physicochemical properties of carbon is given. In addition, a survey of current literature on N-doped carbon for supercapacitor and PEMFCs are provided. In PEMFCs, a detailed review of advanced carbon materials for Pt support for oxygen reduction reaction (ORR) is given. In Chapter 2, effect of N-doping on physical properties of carbon is discussed. The chapter discusses what kind of microstructural and surface modifications are expected and how they can effect its properties as a metal support and an electrode for supercapacitors. In Chapter 3, experimental methods to characterize physicochemical properties of carbon and investigate its electrochemical properties are described. In Chapter 4, the physical properties of NOMC are presented. The chapter gives experimental evidences of good ORR activity, very high capacitance and excellent stability of NOMC. In Chapter 5, the role of nitrogen and graphiticity were studied for capacitive effect. The pore structure of the NOMCs was kept constant while N-content and graphiticity was varied by the heat treatment. It is argued that the fundamental increase in capacitance of N-doped carbon is likely due to increase in electron density at the carbon surface. Chapter 6 and 7 deals with the role of NOMC as a Pt support for ORR. In Chapter 6, it was shown that the N-doped mesoporous carbon should have proper nanostructure to have better interaction with Pt that results in enhancement of Pt ORR activity. The proper nanostructure for NOMC in this work was disordered pore structure. In Chapter 7, effect of temperature-controlled N-content and graphiticity on Pt depsotion and its ORR activity was studied. It was established that high N-content and small Pt size (~ 2 nm) are required to achieve an enhancement in Pt-carbon interaction and ORR activity.
In summary, this work shows that both microstructure (graphiticity) and N-content have to be optimized for electrochemical applications of N-doped carbon. For supercapacitors, the role of nitrogen is to adjust edge plane/ basal plane ratio and electronic properties of carbon. As a catalyst support, nitrogen controls the Pt size and dispersion. Moreover, if the N-content is large and nanostructure amorphous, enhanced catalyst-support interaction could be achieved with small Pt nanoparticles which will increase Pt ORR activity.
Shrestha, Sujan, "Role of Nitrogen Defects in Nitrogen-Doped Carbon for Catalyst Support and Electric Double-Layer Capacitor" (2013). Doctoral Dissertations. 275.