Coupling between ion transport and electronic properties in individual carbon nanotubes

Carbon nanomaterials exhibit unique electrokinetic phenomena due to rapid ion transport within the Debye layer, which have been exploited for energy conversion, membrane technology, and liquid lubrication. The electronic properties of solids have been found to influence water permeation and proton transport; however, their effect on ion transport has not been observed. Here, we present an experimental investigation of ion transport in individual double-walled carbon nanotubes (CNTs) of both semiconducting and metallic nature. Systematic measurements show that conductance, streaming current, and osmotic current are larger in semiconducting tubes than in metallic ones. Together with a complete theoretical framework, we found that such behavior is caused by the smaller liquid-solid friction with the same surface charge density for the semiconducting system. As fast ion transport is the key element for efficient energy conversion, in CNTs, the thermoelectric conversion efficiency with ions is two orders of magnitude larger than with electrons, showing the supremacy of ions to recover the waste heat.