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 dynamics6,7. Graphene nanopores28 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 ions28 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 layers29,30. The mixture of GO with KOH was referred to as reduced GO (rGO) in our previous work and K+ was observed to migrate from rGO to GO when solid-state GO/rGO junction was formed at ambient humidity environment31. In this paper, rGO containing large amount of K+ was chosen as the cation reservoir in the iontronic device. The schematic of the planar osmotic power source and its mechanism is shown in Fig. 1a, GO ink was deposited on one side of the electrode and rGO ink was deposited on the other, overlapping to form a junction. To avoid interference reactions, gold (Au) electrode32 was used as charge collectors and K+ will transport from rGO through 2D nanofluidic channels of GO to the cathode side of Au electrode under humidity. The scanning electron microscopy (SEM) image (Fig. 1b) clearly shows the cross section of stacked layers of GO formed from the ink is similar to that of the GO paper made from the filtration8,33. Ionic conductivity as a function of salt concentration was measured through GO nano-channels (Supplementary Fig. 1a). The conductivity curves of the deposited GO layer shows characteristic properties of 2D nanofluidic channel network similar to those from GO paper3. SEM image in Fig. 1c shows that the cross section of rGO layer was different and some salt crystals seemed to be embedded inside the layered structure. The atom force microscopy (AFM) reveals different topographic morphology between GO (Supplementary Fig. 1b) and rGO (Supplementary Fig. 1c). The GO film had twisted flake-like topography, while the rGO film presented aggregates to form a more porous structure. The surface profile of the osmotic power source was shown in Supplementary Fig. 1d. The dotted white line in the photograph shows where the probe took place. The total thickness of the coating was about 10 µm, including the thickness of the charge collector.
The current-voltage (I-V) characteristics of the osmotic power sources based on 2D nanofluidic material exhibited strikingly nonlinear effects, which may be due to the asymmetric transport of K+ through the GO and rGO junction (GO/rGO). The open circuit voltages (VOC) and short circuit currents (ISC) could be obtained by reading the intercepts on the current and voltage axes by applying a sweeping voltage from 2 V to -2 V. VOC of the Au/GO/rGO/Au power source was about 1.2 V at room temperature under RH around 70% as shown in Supplementary Fig. 2a. Although the voltage of such osmotic power source was high, the current was low as it purely came from the ion gradient and there was no Faradaic reaction when using Au as charge collectors, however, addition of Room temperature ionic liquids (RTILs) was found to boost up the current31. RTILs are molten salts with a melting point close to or below room temperature34. Unlike organic electrolytes, RTIL will not evaporate away in the encapsulated power source and it can also significantly accelerate charging dynamics in nanopores35. Different RTILs, 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (BMIMTFSI), triethylsulfonium bis(trifluoromethylsulfonyl)imide (TESTFSI) and 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIMBF4) were tested, and they have increasing ionic conductivity at room temperature. TESTFSI was finally chosen for the development of anti-freezing osmotic power source since it has a relatively high ionic conductivity of 7.1 mS cm− 1 with a low melting point of -35.5°C36. Addition of RTIL enhanced the current but the VOC of Au/GO/RTIL/rGO/Au kept the same around 1.2 V (Supplementary Fig. 2a), thus the current increase from RTIL may come from the enhanced ionic conductivity instead of redox reactions. Supplementary Fig. 2b also did not show any redox peaks coming from addition of RTIL and only capacitive currents existed in the cyclic voltammogramme (CV). The multimeter measurement verified consistent voltage of around 1.2 V with and without RTIL in osmotic power source (Supplementary Fig. 2c). Inspired from RED, electrochemical redox reaction was introduced at the interface between GO and Au charge collector to increase the current. Saturated silver nitrate (AgNO3) aqueous solution was sprayed on top of Au at the cathode side before deposition of GO. The multimeter measurement showed voltage reached around 1.5 V (Supplementary Fig. 2d) unaffected by RTIL. The linear sweep voltammogramme in Supplementary Fig. 2e showed the VOC and ISC of Au/GO/RTIL/rGO/Au was 1.2 V and 1.4 µA respectively. In comparison, the VOC and ISC of Au/AgNO3/GO/RTIL/rGO/Au power source was 1.5 V and 11 µA respectively. ISC increased almost 10 times with addition of AgNO3, and the VOC of 1.5 V was consistent with that measured directly by the multimeter. The introduction of AgNO3 significantly enhanced the energy and power density of the osmotic power source. As Supplementary Fig. 2f showed, the VOC of the Au/AgNO3/GO/rGO/Au power source may consist of both the diffusion potential (Ediff) generated from ion gradient and the redox potential (Eredox) from electrochemical reactions. Eredox may come from the reduction of Ag+ on the cathode side between GO and Au interface, which is about 0.4 V as calculated in the Supplementary Discussion. Ediff comes from harvesting the Gibbs free energy existing in the ion gradient and it can be estimated by the Nernst equation. Due to the large salt intake and unimpeded water permeation, highly concentrated solutions that were close to saturation were formed within rGO, maintaining a large concentration gradient and enabling ultrafast ion permeation of hydrated K+ from rGO to GO driven by the gradient through 2D nanofluidic channels. The planar osmotic power source is in a solid-state form and it is impractical to calculation the concentration, however, the VOC of 1.2 V from Au/GO/rGO/Au power source came mainly from Ediff since there were no redox reactions in it. Thus the theoretical VOC of the Au/AgNO3/GO/rGO/Au power source is calculated to be 1.6 V that matches the measurement of 1.5 V. Furthermore, electrochemical impedance spectrum (EIS) confirmed the charge transfer resistance decreased significantly with addition of RTIL as shown in Supplementary Fig. 2g. To reduce the influence from the diffusion controlled current caused by mass transfer, slow scan rate of 0.1 mV s− 1 was used to reveal more information on the electron transfer process as the CV illustrated in Supplementary Fig. 2h. The device made of pure GO without ion gradient (Au/GO/RTIL/GO/Au) was used as benchmark, and it shows that whenever GO and rGO junction was formed (in case of Au/GO/RTIL/rGO/Au and Au/AgNO3/GO/RTIL/rGO/Au), strong ionic rectification-like curve appeared below 0.2 V. The largest ion current from the Au/AgNO3/GO/RTIL/rGO/Au may be due to the combination of ion gradient current from Ediff and the Faradaic current from Eredox. Inset of Supplementary Fig. 2h demonstrated an obvious peak that may be correlated to the Ag+/Ag redox potential. It seemed that 2D nanofludic channels enabled the confined electrochemical reactions at the interface between GO and Au charge collector. The image in Supplementary Fig. 3a, b also demonstrated the change in the AgNO3 coating underneath GO on the cathode side, from crystal salts of AgNO3 before discharge reduced to metallic silver particles after multi-cycles of discharge.
To evaluate the energy density of the ultrathin flexible Au/AgNO3/GO/RTIL/rGO/Au osmotic power source, it was discharge at 0.1 µA. The discharge profile in Fig. 1d shows the voltage decades with time and energy capacity is calculated to be 0.69 µWh as shown in Supplementary Fig. 3c. Although the capacity decreased within the first 5 charge–discharge cycles (Supplementary Fig. 3d), the results confirmed that the osmotic power source could be regenerated and its total weight is only 30 mg. Addition of AgNO3 enabled the partially reversible redox reactions, similar to the process in RED systems37. The directional ion migration was then converted to electron transportation through redox reactions at the electrode surface. RTIL helps to accelerate K+ transport in nanoconfined structures. The calculated maximum volumetric specific power density of 28 mW cm− 3 from I-V characteristics (Fig. 1e) is as high as supercapacitors and in good accordance with the measurement of output power as function of load resistance (Supplementary Fig. 3e). The maximum volumetric specific energy of the planar osmotic power source is 6 mWh cm− 3 that is comparable to lithium thin-film batteries38,39 as shown in the Ragone plot in Fig. 1f.
Conformable triboiontronic device. The ultrathin and printable osmotic power source could be readily integrated with energy harvesting triboelectric nanogenerator (TENG) to form a self-charging triboiontronic device. Iontronics are effective in modulating electrical properties through the electric double layer (EDL) assisted with ion migration. GO with abundant K+ can realize a faster migration of cations under electric field to form the EDL. Through EDL capacitive coupling, a triboiontronic energy harvesting and storage device was demonstrated. This device utilizes triboelectric potential and current originated from mechanical displacement to charge the osmotic power source. Due to the universal existence of the triboelectricity, TENG is promising for applications in scavenging small mechanical energy from human activities. In our TENG unit, polyimide (PI) tape was chosen as the triboelectric material, which also served as the encapsulation layer. Rectangle copper foils with dimensions of 2 cm × 2 cm by 50 µm thickness was attached on the PI tape as electrode. The total thickness of the TENG was less than 200 µm and it could be connected to the osmotic power source to form an ultrathin device that is conformal to human body (Fig. 2a). In order to elucidate the electrical output performances of the TENG unit, a linear motor was used to provide periodic contact-separation motion and a rabbit fur was used to be the contact material. With a rectifier in the integrated triboiontronic device as shown in Fig. 2b, the negative voltage and current can be reversed into positive as shown in Fig. 2c and 2d. The generated voltage and current of the TENG were tested under 3 different frequencies, and the maximum voltage and current generated is 45 V and 0.7 µA respectively at 3 Hz. The energy generated from such TENG was able to power a 1.5 V LED (Supplementary Video 1). The voltage profile of the integrated energy harvesting (TENG) and storage triboiontronic device was shown in Fig. 2e. When the osmotic power source was discharged to about 0.7 V, TENG can charge it back to 1.2 V in 200 s repeatedly. During sports, the motion frequency of human body may be even higher than 3 Hz, and larger energy generation could be expected. Such self-charging triboiontronic device can be directly pasted on the surface of the item, and capable of adapting to any amorphous curved surface of supporting objects in any irregular shape with its conformability and bendability.
Osmotic power source with nanoconfinement enhancement in 3D. To extend the nanoconfinement enhancement in 3D, we then further developed a nanoporous GO aerogel-based osmotic power source with self-healing ionogel electrolytes. Two GO aerogels with different ion concentrations were sandwiched between charge collectors and separated by an ionogel consisting of RTIL and polymer matrix. GO aerogel was fabricated through a self-assembled chemical reduction and freeze-drying process40 as shown in Fig. 3a. SEM images in Fig. 3b showed the ordered interconnected network within the GO aerogel. For the GO + KOH aerogel, GO hydrogel was soaked in KOH aqueous solutions before freeze-drying to introduce ion concentration difference. The uniform distribution and high concentration K+ were confirmed by the SEM images and energy dispersive X-Ray spectroscopy (EDS) results (Fig. 3c). The element composition analysis shows that the K+ concentration in GO + KOH aerogels could reach up to 17.3%, owing to the 3D architectures of the aerogel and the large salt intake of GO materials. To enhance the physical stability and control the size of nanopore, the GO aerogel was made by partially reduction under a mild condition in the presence of leucocyte ascorbic acid (LAA). GO aerogels with different GO:LAA ratio was fabricated to investigate the influence of the LAA concentration. As shown in Fig. 3d, Raman spectroscopy was employed to study the disorder degree of GO materials. The peak located near 1,350 cm− 1 is the disorder-induced band (D peak) and the peak close to 1,580 cm− 1 is the graphite band (G peak)30. The area ratio of two peaks (ID/IG) decreases from 2.15 to 1.55 with increasing LAA concentration, meaning the structure disorder of GO aerogels decreases with increasing reduction degree. This is consistent with the morphology characterized by SEM (Supplementary Fig. 4a), which shows increasing sheet stacking and densely packed interconnected GO network with increasing LAA concentration. The four-point resistance measurement shows that the resistivity of the aerogels decreases dramatically with increasing LAA concentration (Supplementary Table 1). The surface area of the porous GO aerogel and the pore distribution were characterized by nitrogen adsorption/desorption measurements as shown in Fig. 3e and 3f. The Brunauer-Emmett-Teller (BET) surface area decreased from 403.5 m2 g− 1 to 15.6 m2 g− 1 with increasing in LAA concentration, since the interaction between GO sheets increase with higher reduction degree and the restacking of the 2D sheets results in smaller surface area. In this case, charge is transported through the pore by the flux of ions that interact with the immobile negative surface charges from carboxyl groups. Osmotic energy conversion could be boosted by such 3D aerogel interface, when pore diameter of around 2 nm or less is in the order of Debye screening length like in the 2D nanofluidic channels. The Poisson-Nernst-Planck (PNP) model41 was used to simulate the osmotic potential from the ion gradient of the 2D nanofluidic material and it also showed that the appropriate pore size for membrane-scale osmotic energy conversion should be around 2 nm, matching our experimental results.
The output performance of the power sources with different GO aerogels was evaluated by the potentiostatic linear sweep voltammetry with RTIL electrolyte. The GO aerogel with pore size of 2.1 nm made by GO:LAA ratio of 1:0.5 (Fig. 3f) shows the maximum VOC of 0.679 V and ISC of 402 µA (Fig. 3g). Besides the optimized size effect in the nanoconfinement, this might be also due to the balance of lower internal resistance with increasing in LAA concentration and higher ion storage capacity with larger surface area (Supplementary Table 1). The thickness of the aerogels also influences the output performance of the power source. As shown in Fig. 3h, although the VOC remains the same for aerogels with same LAA concentration under different thickness of 30, 15 and 10 µm, the aerogels with the thickness of 15 µm has the highest ISC of 613 µA. The maximum output power density is calculated to be 22 mW cm− 3 (Fig. 3i), which is similar to that of the ultrathin planar osmotic power source (28 mW cm− 3) and 8 times higher than that of the ink-jet printed moisture-enabled power source (2.5 mW cm− 3) reported before31. The improved electrochemical performance might come from the large surface area of the 3D aerogel architecture and the superior ionic conductivity of ionogel electrolyte. The optimized LAA concentration and aerogel thickness were used in the following study. To study the effects of the K+ concentration, GO aerogels were soaked in KOH solutions with different concentrations during the preparing process. As shown in Supplementary Fig. 4b, with the increasing in KOH concentration, the surface of the aerogels showed increasing number of small particles, indicating larger salt intake. These GO + KOH aerogels were paired with GO aerogels without KOH to test the VOC of the power sources (Supplementary Fig. 4c). With increase in KOH concentration, the K+ content raises from 0–17.3% and the VOC increases from 0.01 V to 0.69 V. However, further increase in the KOH concentration could lead to crystallization and unevenly distributed cations, affecting the stability and reproducibility of the power sources. These results further suggest that the voltage of the power source mainly origins from the ion gradient between the two electrodes (Ediff), which is consistent with our previous results and other reports in power sources utilizing the ion gradient31,32. This also means that by selecting aerogels with different cation concentrations as building blocks, the voltage output could be customized based on the energy requirement. The directional ion migration needs to be converted to electron transportation to power external circuits. This electrochemical process is usually based on the chemical redox reactions or the physical adsorption of ions on the electrode surface. CV of such devices with and without RTIL in Supplementary Fig. 4d exhibit typical electric double layer capacitor (EDLC) behavior and no obvious redox peaks within the voltage window. RTIL accelerates ionic conductivity in the nanopores avoiding the humidity limitation for the osmotic power source and could even further overcome temperature limitations. Similar to the planar osmotic power source, the I-V characteristics show that the ISC increases with increasing ionic conductivity while the VOC remains almost the same (Supplementary Fig. 4e) It should be noted that ISC is much higher than that of the power sources without RTIL, suggesting that RTIL electrolytes could significantly improve the output performance of GO aerogel the power sources as well. The CV curves and galvanostatic charge-discharge results (Supplementary Fig. 5a, b) reconfirm EDLC behavior of the power source based on GO aerogel. To eliminate the effect of the charge collectors, Ag, Au, copper (Cu), and carbon were all tested as conductive substrates and the CV curves showed similar shapes (Supplementary Fig. 5c). It should be noted that the CV of the devices without RTIL shows capacitive behavior with various types of charge collectors, different from the previous planar osmotic power sources and moisture-based power sources with redox reactions31. This might be due to the large amount of ion adsorptions on porous aerogel surfaces which dominates the electrochemical process. VOC equals to Ediff in the osmotic power source made of nanoporous GO aerogels and the current purely comes from the ion gradient.
Self-healing ionogels. The challenge of application of RTIL in devices is related to its ‘liquid’ nature. The ionogel electrolyte consisting of RTIL (TESTFSI) with poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) was used here. It is reported that the cations in the ionic liquid and the PVDF-HFP polymer chain in the fluoro-elastomers could interact via ion-dipole interactions, giving rise to self-healing capability43,44. Such ionogel could be cut and then placed together, exhibiting fast self-healing capability in ambient conditions without external stimulations shown in Supplementary Fig. 6a. This self-healing ionogel with tunable ionic conductivity and antifreeze properties could improve the output performance and allow versatile design strategies of the power sources for portable devices. The ionic conductivity of the ionogels was also evaluated by the EIS (Supplementary Fig. 6b-c) and the conductivity increases by three orders of magnitude from 10− 6 S cm− 1 to 10− 3 S cm− 1 as the RTIL content raised from 30 wt.% to 70 wt.%. By tuning the mass ratio of the RTIL and polymer matrix, the ionic conductivity of the electrolyte could be customized on demands. The Tg of the ionogels with 70 wt.% RTIL obtained from the differential scanning calorimetry (DSC) test is -72°C, significantly lower than − 19.7°C for the pure PVDF-HFP polymer (Supplementary Fig. 6d). This could be attributed to the ion-dipole interaction of the RTIL and the polymer chain. The RTIL could act as a plasticizer weakening the interactions between host polymer chains, therefore increase the flexibility and processability of the ionogels45. The mobile polymer chains could also facilitate the self-healing and increase the ionic conductivity44. The ionic conductivity of the ionogels with 70 wt.% RTIL increases with the increasing temperature from − 40°C to 50°C (Supplementary Fig. 6e-f).
Lego-like design and scalable integration of the osmotic power source. A Lego-like design concept could be demonstrated (Fig. 4a), in which the output performance of the osmotic power sources could be tailored on demands by assembling building blocks with customizable properties. GO aerogels with adjustable cation concentrations are selected based on the voltage requirements and ionogels with programmable conductivities could be drop-casted onto aerogels affecting the current output. The component parts are stored separately before use, avoiding self-discharge. When needed, GO aerogel electrodes could be adhered together by ionogels through the self-healing process without external stimulation. As shown in Fig. 4b, the power source with optimized GO aerogels and ionogels with 70 wt.% RTIL has the VOC of 0.6 V and the ISC of 440 µA. The Lego-like design could enable the construction of custom-built power sources with pre-fabricated components and meet the energy requirements of various integrated electronic systems. By adopting the ionogels as electrolyte, the iontronic power source could operate in harsh environments like low humidity and subzero temperature. As shown in Fig. 4c, the assembled power source exhibit VOC of 0.54 V and the ISC of 57 µA at -10°C under nearly 0% relative humidity. With raising temperature, the ISC increases due to the enhanced ionic conductivity, while the VOC remains almost the same (Fig. 4d). In contrast, the moisture-based power source without RTIL electrolyte has low current output and could not work at temperature below zero (Supplementary Fig. 7), while this power source has much larger current output and could work even at the temperature of -40°C, greatly broadening the application conditions. By manipulating different components of such new type of power source, it offers a handy assembling process and abundant potential designs, for example, a simple Origami design of foldable power source was fabricated by connecting 20 devices in series on both sides of the film substrate (Fig. 4e and Supplementary Fig. 8a). A repeating series of device components on the flat substrate were stacked together forming a sandwich structure by taking advantage of the folding strategy. The tiny bulk of the folded osmotic power source from pure ion gradient has the VOC of approximate 10 V and can power not only electrochromic devices (Fig. 4f, Supplementary Fig. 8b and Supplementary Video 2) but also the liquid crystal display screen (Fig. 4g and Supplementary Fig. 8c), providing a versatile design for powering various electronic devices by osmotic energy from just ion gradient. Such Lego-like power source offers the areal power density of 1.3 mW cm− 2 at ambient environment that is significantly higher than 0.5 mW cm− 2, which has been flagged as the target for making salinity gradient power economically viable46.