An experimental study of ion diffusivity in graphene oxide membranes in single and binary salt solutions

Graphene oxide membranes (GOMs) are a promising material for next-generation membrane technologies. However, the mechanisms underlying their ion transport remain unclear, limiting the ability to optimise their design for specific applications. In this study, we systematically quantify ion transport by calculating the individual ion diffusion coefficients of GOMs equilibrated in LiCl, NaCl, KCl, and MgCl 2 and binary mixtures of the monovalent salts across varied external salt concentrations. We demonstrate that counter-ion diffusion coefficients generally remain unaffected by external salt concentration, while chloride co-ion diffusion coefficients increase with concentration up to ∼0.3 M before plateauing, primarily influenced by electrostatic interactions between the membrane fixed charge groups and ions, rather than membrane water content. Interestingly, the diffusion coefficients of Na + and Cl - ions in the GOMs are comparable to those in polymeric membranes, challenging previous reports of significantly enhanced ion transport in GOMs. Rather, our analysis reveals that ion permeability in GOMs is predominantly dictated by solubility effects rather than diffusion. In monovalent binary salt systems, counter-ion diffusion coefficients were generally lower than those of single-salt systems, while chloride diffusion coefficients remained comparable, reflecting the influence of ion interactions and membrane hydration on ion transport. We also find that the water permeability of the GOMs is low and does not show the promise of ultrafast water transport identified in early studies. • Ion diffusion coefficients determined through sorption, conductivity and permeation experiments • Counterion diffusivity independent of external salt concentration • Chloride co-ion diffusivity varies with water uptake and external salt concentration • Ion diffusivities are comparable to those in polymeric membranes • Counter-ion diffusivity lower in a binary salt mixture than in the equivalent single salt solution