Effect of Surface Chemistry on Macrocycle and Conducting Polymer Based-Carbon Composites

Abstract

Redox-active carbon composite-based electrodes, especially nitrogen-based redox active materials, can provide high power and energy storage in electrochemical capacitors. These materials include porphyrin macrocycles which are found in nature and possess unique electronic and redox-active properties from their large π-conjugated systems [1], [2]. When compositing with carbon-based materials such as carbon nanotubes (CNTs), they have shown increased stability and long-term performance. Previous work using macrocyclic tetraphenyl porphyrin sulfonate (TPPS) in CNT-based composites demonstrated improved capacitive profiles, better interfacial kinetics, and charge retention introduced in capacitive electrodes [3], [4]. On the other hand, conducting polymers with π-conjugated backbones also enabled high charge storage when used in composites [5]. To further advance these capabilities, the effects and mechanisms of surface functionalities of carbon substrates need to be investigated. For example, the presence of carboxyl groups on CNTs has been shown to improve charge storage through favourable interfacial interactions in some conducting polymers [6], [7]. The challenge as highlighted in Figure 1 lies in understanding which redox-active species, e.g. macrocycle or conducting polymer, will favour certain surface functionalities based on the nature of their interactions. Thus, a systematic investigation to design and create desirable composites and interactions for high energy and power densities, and to extend to low-cost carbon sources including waste biomass-based activated carbons.In this work, we conducted a systematic study comparing the effect of several surface functionalities like carboxyl groups on TPPS macrocycles and on conducting polymer-based carbon composites to answer the question on why certain surface functionalities are favored for each species. Preliminary studies have shown presence of TPPS has increased the capacitive profiles, reaction kinetics and rate capabilities of CNT composites, exceeding the conducting polymer counterparts. But this capacitive increase can be further improved through surface modification. This study leveraged layer-by-layer deposition approaches to fabricate redox-active carbon composites, followed by electrochemical characterizations using cyclic voltammetry and electrochemical impedance spectroscopy. Surface morphology was studied using electron microscopy while surface functional groups were investigated using x-ray photoelectron spectroscopy. The findings from this study can be used to implement systematic surface modification for redox-active carbon composites to improve energy storage, towards a more sustainable future.