Voltammetric Evidence of Proton Transport through the Sidewalls of Single-Walled Carbon Nanotubes

Understanding ion transport in solid materials is crucial in the design of electrochemical devices. Of particular interest in recent years is the study of ion transport across 2-dimensional, atomically thin crystals. In this contribution, we describe the use of a host–guest hybrid redox material based on polyoxometalates (POMs) encapsulated within the internal cavities of single-walled carbon nanotubes (SWNTs) as a model system for exploring ion transport across atomically thin structures. The nanotube sidewall creates a barrier between the redox-active molecules and bulk electrolytes, which can be probed by addressing the redox states of the POMs electrochemically. The electrochemical properties of the {POM}@SWNT system are strongly linked to the nature of the cation in the supporting electrolyte. While acidic electrolytes facilitate rapid, exhaustive, reversible electron transfer and stability during redox cycling, alkaline-salt electrolytes significantly limit redox switching of the encapsulated species. By "plugging" the {POM}@SWNT material with C60-fullerenes, we demonstrate that the primary mode of charge balancing is proton transport through the graphenic lattice of the SWNT sidewalls. Kinetic analysis reveals little kinetic isotope effect on the standard heterogeneous electron transfer rate constant, suggesting that ion transport through the sidewalls is not rate-limiting in our system. The unique capacity of protons and deuterons to travel through graphenic layers unlocks the redox chemistry of nanoconfined redox materials, with significant implications for the use of carbon-coated materials in applications ranging from electrocatalysis to energy storage and beyond.