Direct-Write Printing Flexible Ion Channels with Graphene Oxide/Tannic Acid/Cellulose Nanofibers.

Flexible ion channels perform directional ion transport with high mechanical strength, which could bring many important intelligent applications. In this study, composite ink of graphene oxide (GO), tannic acid (TA), and cellulose nanofiber (CNF) is direct-write printed into a GO/TA/CNF porous structure (PS) with a freeze-dried process. With the negative charge of hydroxyl (-OH), carboxyl (-COOH), and phenolic hydroxyl (Ar-OH), the GO/TA/CNF PS of 3 cm length, 3 cm width, and 7 μm thickness realizes a maximum potential of 0.13 V and a maximum current density of 26.7 A/m2 in KCl solution with a 1000-fold salinity gradient. Meanwhile, an energy conversion efficiency of 36.98% and different cation transportation are realized. A hydrogen bond network of GO/TA/CNF PS is formed by intermolecular interaction among -OH and -COOH of GO, TA, and CNF, which could absorb external force and recover into the original structure through the elastic modulus of the hydrogen bond. With a stretching force of 152.7 MPa, a constant output current is exhibited with structural integrity. After 104 cycles bending at a 0.2 mm-1 curvature, output power density is only changed by 0.9%. Meanwhile, unchanged current is demonstrated after 35 h immersion in acidic or neutral solution, and changed current within 6% is obtained after 35 h immersion in alkaline solution. Furthermore, the flexible ion channel is used in wearable electronics as a wearable energy device or body fluid electrolyte monitor.