High-aligned oppositely-charged nanocellulose/MXene aerogel membranes through synergy of directional freeze-casting and structural densification for osmotic-energy harvesting

Optimization of ion-transport paths can considerably improve ion-transport capabilities in charged nanochannels. Thus, the assembly of high-charge-density one- and two-dimensional nanomaterials into aligned nanochannels is necessary for high-performance osmotic-energy harvesting. Herein, we prepared cellulose/MXene aerogel membranes with opposite charges and aligned nanochannels via chemical modification on algal cellulose nanofibers and MXene nanosheets, unidirectional freeze casting, and structural densification to considerably enhance the ionic conductivity in low-concentration solutions (maximum conductivity of 2.88 × 10−3 S cm−1). In particular, the aligned membrane had an output power density of 2.97 and 4.89 times higher than membranes used for suction filtration and isotropic nanochannels, respectively, demonstrating the necessity for nanostructure control. In addition, due to the photothermal effect of MXene, the positively–negatively (P–N) charged unit produced a maximum output power density of 8.87 W m−2 under a concentration gradient of artificial seawater and river water and simulated sunlight. Moreover, the unit maintained 90.4% of its original output capacity after 168 h of operation. As a proof of concept, we created nine series of P–N units and successfully powered an electronic calculator with an output voltage of 1.85 V. This study reveals improvements in developing renewable materials with an aligned nanostructure for high-performance osmotic-energy conversion.