Nanofluidic transport through humic acid modified graphene oxide nanochannels

Abstract

The chemical similarity of graphene oxide (GO) and humic acid has been exploited to fine-tune the ionic and molecular transport properties of a lamellar GO membrane. Introduction of humic acid (in 10, 15 and 20%) is found to improve the nanofluidic transport characteristics, such as ionic mobility, molecular selectivity, diffusivity and permeability, of the GO membrane. Remarkably, the membrane prepared with 15% humic acid displayed superior proton mobility (μ_H = 1.04 × 10⁻⁴ cm² V⁻¹ s⁻¹), in-plane diffusivity (D = 4.8 × 10⁻⁶ cm² s⁻¹), and cross-plane permeability (P_L = 2.03 × 10⁻⁴ mm g cm⁻² s⁻¹ bar⁻¹) to the pure GO and other composite membranes. The favorable nanofluidic characteristics of the 15% membrane are attributed to the larger effective heights of the 2D nanochannels, derived from the onset point of the surface charge governed ionic conductivity of the membranes. The activation energy of proton transport (∼0.07 to 0.1 eV) confirmed the occurrence of a Grotthuss-like hopping mechanism in all the GO–HA membranes. Introduction of humic acid into two-dimensional GO channels also improved the solution stability and mechanical robustness of the pristine GO membrane. The lamellar GO–HA membranes were also found to be suitable for energy harvesting applications such as direct methanol fuel cells and reverse electrodialysis. Remarkably, even after 72 hours exposure to electrolyte solutions, open circuit potentials up to 0.05 V, 0.21 V, and 0.12 V were found for the 10, 15, and 20% membranes, respectively.