Tandem Oppositely Charged Nanoclay Membranes with 2D Nanofluidic Channels for Osmotic Energy Conversion

2D nanoclay membranes are promising candidates for osmotic energy conversion owing to their high ion flux and ion selectivity. However, the fabrication of tandem stacks using 2D nanoclay membranes with opposite selectivity to simultaneously utilize both cations and anions for osmotic energy conversion remains a challenge. Here, negatively charged 2D montmorillonite (MMT) and positively charged 2D layered double hydroxide (LDH) membranes are used to fabricate a tandem stack for efficient osmotic conversion. These two kinds of nanoclay membranes are prepared by vacuum filtration-assisted assembly of liquid-exfoliated nanosheets and charged cellulose nanofibers. The LDH membrane exhibits an anion transference number of over 0.70, while the MMT membrane exhibits a cation transference number of over 0.80, and both of them demonstrate a surface-charge-governed ion transport property. A full-cell formed by one pair of 2D MMT and LDH membrane in series generates a maximum output power density of 1.14 W m^-2 under 50-fold salinity gradient of 0.01/0.5 m KCl without any contribution of redox potential difference, which is much higher than that of any individual membrane. A tandem stack composed of 17 cells achieves a high output voltage of 1.29 V, which can be used as a power source for a calculator.