Flexible iontronics based on 2D nanofluidic material

nature communications Article https://doi.org/10.1038/s41467-022-32699-x Flexible iontronics based on 2D nanofluidic material Received: 6 May 2022 Di Wei 1 , Feiyao Yang 1 , Zhuoheng Jiang1 & Zhonglin Wang1,2 Accepted: 11 August 2022 1234567890():,; 1234567890():,; Check for updates Iontronics focuses on the interactions between electrons and ions, playing essential roles in most processes across physics, chemistry and life science. Osmotic power source as an example of iontronics, could transform ion gradient into electrical energy, however, it generates low power, sensitive to humidity and can’t operate under freezing point. Herein, based on 2D nanofluidic graphene oxide material, we demonstrate an ultrathin (∼10 µm) osmotic power source with voltage of 1.5 V, volumetric specific energy density of 6 mWh cm−3 and power density of 28 mW cm−3, achieving the highest values so far. Coupled with triboelectric nanogenerator, it could form a self-charged conformable triboiontronic device. Furthermore, the 3D aerogel scales up areal power density up to 1.3 mW cm−2 purely from ion gradient based on nanoconfined enhancement from graphene oxide that can operate under −40 °C and overcome humidity limitations, enabling to power the future implantable electronics in human-machine interface. In biological cellular membranes, ion-specific pores permit certain types of ions flow across the membrane driven by osmotic energy from ion gradient, responsible for nervous impulses, muscle contractions and physiological sensing. The osmotic energy could be generated based on either pressure-retarded osmosis (PRO) or reverse electrodialysis (RED)1, and the ion regulation component is the critical part for such power generation2. Iontronics couple the electron/ion charge transfers and exchange signals at the interface of electronic/ionic conductors, differentiating them from most electronics using just electrons and/or holes as the dominating charge carriers3–7. Bioinspired nanofluidic iontronics could have compatible signals with neurons to enable implantable iontronic devices or even neuronalcomputer interfaces3. Enhanced sensitivity of tactile sensor8 and pressure sensor9 could also be obtained by iontronic films. Unusual behavior of ion transport kinetics in channels narrower than the Debye length of electrolyte has been observed, the surface charges on the inner walls of nanofluidic channels repel ions of the same charge and attract counter ions, making them the dominating charge carriers10. Such unipolar ion transport can enhance ionic conductivity up to several orders of magnitude, which breaks the conventional continuum-based theory11. Recently, a variety of unusual ionic phenomena such as highly selective ion sieving12, ultrafast ion transport13, and anomalous increase of capacitance in nanopores14,15 were all observed due to the enhanced diffusion related to the strong ion-ion correlations under severe nanoconfinement16. Graphene oxide (GO) as 2D nanofluidic material with negative surface charge from carboxyl and hydroxyl groups etc. has shown special affinity to water17 and controllable ion transport properties18. It had also been reported to provide nanoconfined charging dynamic medium to be potentially used in many applications such as molecular sieving with ultrafast speed13 and voltage gating devices19 etc. The unipolar ion transport within 2D nanofluidic material and asymmetric charge distribution could be used to generate osmotic energy20,21. The asymmetric charge distribution could be introduced by wettability gradient22 as observed in the power generation from GO and reduced GO junctions under moisture23, as well as by induction of oppositely charged bilayer polyelectrolyte film that could generate a peak of 1.38 V at relatively humidity (RH) of 85%24. Osmotic energy originated from such charge gradient and regulated ion transport25 in nanoconfined structures has been observed by examples from nanostructured carbon materials26, graphene single microelectrode27, MoS2 nanopores28, boron nitride nanotube25, nanostructured silicon29, protein nanowires30 to membranes based on MoS231, cellulose32, silk33 and MXene/Kevlar nanofiber composites34 etc. Among above examples, 1 Beijing Institute of Nanoenergy and Nanosystems, Chinese Academy of Sciences, 101400 Beijing, People’s Republic of China. 2School of Materials Science e-mail: ; and Engineering, Georgia Institute of Technology, Atlanta, GA 30332, USA. Nature Communications | (2022)13:4965 1 Article nanofluidic channels with tailored ion transport dynamics could enable high-performance RED. However, their power densities are generally small, ranging from 10 µW cm−3 to 4 mW cm−3 in recent reports30,35. In addition, fabrication of osmotic power source based on nanofluidic materials typically relies on expensive deposition/lithography techniques or nanoporous templates with sophisticated growth and processing steps. Such fabrication methods usually have better defined geometries, but limited applications due to the cost and sophistication. Jiang et al.36 reviewed progress on enhanced ion regulation in nanofluidic devices for osmotic energy conversion. The essential challenges on their real world applications lie in the small energy and power densities, operation limitations on humidity and temperature, and feasibility of large-scale production. Here, we show a flexible, ultrathin and printable GO-based triboiontronics and osmotic energy power source based on the ion gradient and the fine-tuned interfacial electrochemical reactions. To maximize the ionic power, a modular design osmotic energy power source with current enhancement purely from ion gradient is made from GO aerogels and self-healing ionogels based on room temperature ionic liquids (RTILs). The GO-based osmotic energy device and triboiontronics developed in this paper could operate under harsh environment regardless of low humidity and subzero temperature. Results Planar ultrathin osmotic power source based on 2D nanofluidic material of GO Planar confinement in 2D nanofluidic material of GO expands translational degrees of freedom for ionic transport engendering unusual ion dynamics and ions transport much faster in the horizontal direction within 2D nanofluidic channels than in the vertical direction in the GO film14,15. Graphene nanopores37 were found to preferentially transporting K+ over its counter anions such as Cl- with selectivity ratios over 100 and hydrated K+ diffuses orders magnitude more quickly than most hydrated ions37 within the 2D nanofluidic channels. It was reported that potassium hydroxide (KOH) could partially remove the oxygen-containing groups of GO sheets through a series deoxygenation reaction leaving cations between graphene layers38,39. FTIR (Fourier-Transform Infrared Spectrometer) characterization and the related empirical structural formula of GO and rGO were shown in Supplementary Fig. 1. A reduction in the amount of hydroxyl and carboxyl groups hap (...truncated)