Graphene-based “hybrid” aerogels with carbon nanotubes: Mesoporous network–functionality promoted defect density and electrochemical activity correlations

Abstract

Electrochemical activity of graphene and graphene-based “hybrid” nanomaterials is crucial for energy and water sustainability applications, which requires fine tuning over combined geometric and electronic structures. We demonstrate that precise control of defects, porosity, and topological interconnectedness, invoked in hydrothermally synthesized graphene aerogel integrated with multi-walled carbon nanotubes, promotes finely tuned morphology, structure, defect number density, hierarchical mesoporosity, and conductivity and enhances the electrochemical heterogeneous electron transfer rate (kET). We prepared a range of graphene-based “hybrid” scaffolds (or monolithic aerogels) and their nitrogenated equivalents with varying graphene–carbon nanotube compositions using two synthetic schemes known as approaches 1 and 2. This study allows us to correlate quantitatively between number defect density (via Raman spectroscopy; RS) and heterogeneous electron transfer rate (via scanning electrochemical microscopy). RS provided microscale structural characterization revealing localized lattice vibrations. The first- and second-order Raman bands were analyzed in terms of band position, intensity ratio, and integrated intensity determining structural disorder, in-plane cluster size, inter-defect distance, and number defect density. The role of oxygenated (carbonyl; C═O, carboxyl; —COOH) and nitrogenated (pryridinic-N and graphitic/pyrrolic-N) functionalities and bonding configurations besides mesoporosity is emphasized for understanding the role of surface chemistry in regionally improved physicochemical (electroactivity and catalytic) properties. The defect-induced increase in finite electronic density of states (DOS) near Fermi level calculated using density functional theory under hydration helped in establishing moderate defect density for enhanced heterogeneous electron transfer rate as a critical onset such that the carbon system is electroactive while maintaining integral sp2 C structural network. Moreover, the defect sites allow sufficient overlap between DOS for graphene-based aerogels and redox probe wavefunctions, which emphasizes the experimental correlation establishments.