Composite reverse osmosis and nanofiltration membranes

Polymeric filtration membranes could be a cost-effective alternative to cation exchange membranes (CEMs) in electrolysis with a contained anolyte and saltwater catholyte because they size selectively hinder salt ion transport between compartments while facilitating proton and hydroxide transport. Optimizing membrane performance requires a better understanding of membrane properties that impact electrical resistances and ion retention. Twelve reverse osmosis (RO) membranes, one nanofiltration (NF) membrane, and one cellulose triacetate forward osmosis (FO) membrane were examined for their electrical resistances under conditions typically used for characterization of CEMs. Resistances measured at low current densities (0.07–0.3 mA cm−2) varied between different membranes by over an order of magnitude in 1 M NaCl at neutral pH, from 6.1 ± 0.1 Ω cm2 to 70 ± 30 Ω cm2. There was no significant correlation between membrane resistance and applied potential during saltwater electrolysis at 20 mA cm−2 (p = 0.44), or between membrane resistance and water permeability (p = 0.35). These results indicate that traditional CEM resistance characterization methods do not predict polymeric filtration membrane electrolysis performance because proton and hydroxide transport, which is important during electrolysis when large pH gradients develop, must be considered separately from salt ion and water molecule transport through size selective RO, NF, and FO membranes during water electrolysis.