Evaluation of Carbon Characteristics’ Influence on the Performance of Electrochemical Capacitor Systems Combining Double-Layer and Pseudocapacitive Electrode Materials

Abstract

Electrochemical capacitors are energy storage devices that are uniquely capable of high power and extraordinarily long cycle life. Carbon materials are typically used to store energy in electrochemical capacitors, but the low energy density of these materials limits the expanse of applications for which they can be used. Pseudocapacitive materials, such as manganese oxides, could improve the energy density of electrochemical capacitors but suffer from lower power and lower cycle life compared to their carbon counterparts. In recent years, there has been increasing interest in combining carbon and pseudocapacitive materials in order to benefit from the high power density and high energy density that these materials offer respectively.

The work presented in this thesis explores the electrochemistry of a number of different carbon materials, highlighting differences in the abundance of redox-active functional groups (which increase the carbon energy density) as well as differences in the carbons’ rate capabilities. Importantly, we show that electrochemical experiments in neutral-pH electrolyte cause an increase in redox-active surface functional groups on carbon materials, despite the fact that these species are not detectable in the neutral-pH electrolyte itself.

Additionally, this work identifies the origin of interfacial resistance in manganese oxide films as resistance between the manganese oxide and its substrate. This is an important discovery, as this resistance is commonly mis-identified in the literature as an intrinsic property of manganese oxide. Experiments exploring the performance of manganese oxide electrodeposited onto carbon do not demonstrate this resistance between manganese oxide and carbon substrates.

Experiments exploring the effect of carbon surface area and chemistry on the performance of electrodeposited manganese oxide found no significant impact for these carbon properties. However, electrodeposited manganese oxide is able to greatly improve the energy storage of carbon with only a small decrease in the rate capabilities. This is thought to perhaps be due to manganese oxide depositing on the outer surface of carbon, rather than down the pores where it will be impeded by resistance associated with ion movement through tight spaces.