Enhanced nanofluidic transport in activated carbon nanoconduits

Abstract

Carbon emerged as a unique material in nanofluidics, with reports of fast water transport, molecular ion separation, efficient osmotic energy conversion. Many of these phenomena still await proper rationalization due to lack of fundamental understanding of nanoscale ionic transport, which can only be achieved in controlled environments. Here, we develop fabrication of 'activated' two dimensional carbon nanochannels. Comparing to nanoconduits with 'pristine' graphite walls, this enables investigation of nanoscale ionic transport with unprecedented details. We show that 'activated' carbon nanochannels outperforms pristine channels by orders of magnitude in terms of surface electrification, ionic conductance, streaming current, (epi-)osmotic currents. A detailed theoretical framework allows us to attribute the enhanced ionic transport across activated carbon nanochannels to an optimal combination of high surface charge and low friction. Further, this demonstrates the unique potential of activated carbon for energy harvesting from salinity gradients with single-pore power density across activated carbon nanochannels reaching hundreds of kW/m 2 , surpassing alternative nanomaterials.

Enhanced osmotic transport in individual double-walled carbon nanotube

“…These lead to the corresponding being A/m, A/m, and A/m respectively. It is worth noting that for KCl owing through DWCNT, is two orders higher than that of pristine graphene, and similar to that of activated carbon nanoconduits (32), showing extremely high mobility.…”

“…A variety of concentration ratios, C max /C min , are used while keeping the maximum concentration C max at 1 M. At each concentration ratio, we extract the channel conductance, G, by measuring the I -ΔV curve which gives . Following similar approach for the measurements of hBN nanotube (31,32) and activated carbon nanoconduits (31,32), the obtained current is corrected for the contribution that results from the Nernst potential, which originates from the difference in salt concentrations at the two electrodes, to get the osmotic current I osm (see SI for more details).…”

“…We use the values of surface charge density for pristine graphene (32), as the pH values of KCl solution are the same. For NaCl and LiCl solutions, due to the similar electronegativity of the cations and the same type of physisorbed ions on the carbon surface, it is reasonable to assume that the surface charge would remain.…”

“…Despite extensive studies on transport of ions and charges across individual CNT (1,12,13), little is known on the response of carbon nanotubes under osmotic forcings and the impact of the expected ultra-low friction at the water-solid interface on the salinity gradient generated transport. This is particular surprising considering the potential impact for salinity gradient energy conversion and storage which have been observed in other nanocon ned systems (28-30), such as hBN nanotube (31) and activated carbon nanoconduits (32).…”

Angstrofluidics: Walking to the Limit

“…However, let us note that the other component of the Posm, that is, Iosm is also sensitive to the dimensions of the channel (area and length) and the surface charge (see formula in the sidebar titled Osmotic Parameters). These would change the above discussed trends of dimensionality, for example, 2D slits with high surface charge showing much higher Posm than other systems (170).…”