Out‐Of‐Plane Electrokinetics via Pumping Potential Achieved by 100 nm‐Thin Polyethylene Nanomembranes

Droplet‐induced electricity generation, as one of the most emerging environmental energy harvesting technologies, is extensively investigated for nearly a decade. Its interaction between ions (in droplets), electrons (in conductors, e.g., graphene), and charges (on the substrate surface) is the key to electricity generation. However, the indirect interaction between ions and charges due to the shielding effect from conductors, inevitably suppresses device performance and limits conductor selection. Herein, the above issues are addressed by proposing an out‐of‐plane electrokinetic effect based on a 100 nm‐thick negatively charged polyethylene nanomembrane, providing direct interactions between ions and charges, with the shielding effect of conductors eliminated. This new form of electrokinetics can induce a persistent potential for 6 h and a specific power of 177.2 nW µL−1 (highest droplet‐induced electrokinetics). With new device topologies and extensive conductor materials unlocked, this work provides a new concept and expanded scope for electrokinetic applications.


Introduction
Water is one of the most abundant resources on earth, and hydrovoltaic technology [1][2][3][4][5][6][7][8] is promising to address energy crises and industrial pollution issues. Among which the most delicate and microscopic form is droplet-induced electricity generation (DEG), [9][10][11][12][13][14][15][16][17][18] mainly via an electrokinetic mechanism to generate power based on diverse interactions between nanomaterials and water molecules. For the last decade, one of the main trends of DEG is to harvest environmental energy via DOI: 10.1002/admt.202300178 various methods, such as drawing, [18] flowing, [11] splashing, [10] etc. The induced voltage originates from the moving, establishing, or vanishing of the electric double layer (EDL). [18,19] Graphene/polymer composite membrane is the most typical DEG device form, [20][21][22][23] with the generation voltage regulated by the droplet velocity, [18] the ion species, [14,18] graphene layers, [10] and the substrates. [11] In principle, hydrovoltaics originate from the trilateral interactions [11,16,24,25] among ions (from the drops), electrons (from the conductors), and charges (of the substrate). It is governed mainly by two sequentially occurring interaction pairsthe one between the surficial substrate charges and the ions in the liquid, denoted as w i-c ; and the other between the ions and the electrons, denoted as w i-e . In detail, in the moving drops, counterions are first preferentially adsorbed out due to the attraction (w i-c , electrostatic force) from the substrate charges and keep the EDL ever evolving. Then the ions drag electrons (w i-e , Coulomb drag) in the conductors to move in accord with the macroscopic drop movements (Figure 1a). This electricity generation induced by in-plane ion movements above conductors, is termed in-plane electrokinetics (IPE, Figure 1a).
For most reported DEG, conductors need to be sandwiched by the liquid drops and the substrates, thus the ion-charge paring (w i-c ) is always separated by the conductors and unnecessarily suppressed (i.e the shielding effect), no matter how thin. Additionally, the conductors have almost no choice but only monolayer graphene, as the suppressed w i-c significantly further decays or even vanishes at elevated conductor thickness larger than the 2D range ( Figure 1b). To sum, the compromise between w i-c and w i-e results in the suppressing w i-c , and also the case of Hobson's choice when selecting the conductors. Recently the selection principle of DEG's conductors has been concluded as adequate electron conduction within a 2D-confined thickness. [2,16] Although with few alternatives reported, [13,17,[26][27][28][29] this principle severely constrains DEG's study and application scenarios.
Hereby a novel DEG paradigm was proposed to override the compromise between w i-c and w i-e , by geometrically reconfiguring ion movements, termed out-of-plane electrokinetics (OPE). First, we designed a porous ultrathin polyethylene membranesubstrate to establish direct w i-c between ions diffusing interiorly Figure 1. Comparison of in-plane versus out-of-plane electrokinetics. In-plane electrokinetics (IPE) using 2D-conductors a) and bulky conductors b) with no power generated due to shielding effects). Out-of-plane electrokinetics (OPE) of 2D-conductors c) and bulky conductors d).
inside the substrate and the negative charges of substrates ( Figure 1c). Then the preferentially adsorbed ions interact with the conductor's electrons to generate hydrovoltaics. Both intimate contacts of ion-charges and ion-electrons, annihilated the compromise between w i-c and w i-e . Thus, the long pending w i-c suppression issue in DEG could be solved (Figure 1d). Compared with IPE, the reduced charged-induced area of OPE could be compensated by this direct and adequate interaction of ions and charges. As a bonus, OPE considerably expands the available spectrum of material species and dimensions for conductors of electrokinetic power generation.

Results and Discussion
Based on our previous work, [30] surficial negative charging of ultrathin polyethylene (PE) membranes was confirmed, satisfying the basic criteria for DEG substrate. The zeta potential of the PE nanomembrane with a thickness of 109 nm was -81.2 mV ( Figure S1, Supporting Information) in 0.001 mol L −1 NaCl solution, which represented the EDL built on the surface of the PE nanomembrane. We then engineered them to be facilitated with the channel diameter designed to distribute around tens of nanometers. This channel configuration could drive microfluids to diffuse in Knudsen mode [31] with intensified interactions between ion and channel walls, enabling fine control of the fluid movement inside the membranes. Detailed information on the fabrication, morphology, thermodynamics, optical, and mechanical property of PE nanomembrane were introduced in Figures S2-S6 (Supporting Information).
Despite the newly unlocked multiple choices of DEG conductors, we still started with graphene as a control group. The CVD-grown monolayer graphene with the size of 5 × 5 cm 2 was transferred to the PE nanomembrane by polymer annealing and catalyst layer etching [32] (Figure S7, Supporting Information). And interestingly, the semi-freestanding monolayer graphene with centimeter square area was obtained (Figure 2a), as considerable share of it suspends in air self-standingly with maximum spanning length over 25 nm ( Figure S8c, Supporting Information). This was achieved by suspending the graphene above the PE ultrathin nanoporous structure (Figure 2b; Figure S9, Supporting Information), with the thickness of only 109 nm ( Figure  S10, Supporting Information). This graphene/PE ultrathin composite nanomembrane was denoted as GPE (the photograph can be seen in Figure S11a, Supporting Information). The quality of monolayer graphene after the transfer was characterized by Raman spectroscopy ( Figure S12, Supporting Information). Two Ag electrodes were constructed at the two ends of GPE nanomembrane and locally encapsulated with silicone to avoid direct contact with water ( Figure S11b, Supporting Information). The Cu wires were linked to the electrodes to detect the electrical signal ( Figure S11c, Supporting Information), and the resistance of the GPE device was 5.90 kΩ ( Figure S13, Supporting Information). Before studying OPE, IPE was first tested with the device, by flowing ionic droplets on the surface of the GPE ( Figure S14, Supporting Information), and the results were in line with existing reports. [18] Being different from the IPE which converts the momentum of external dynamic energy into electricity, the OPE mainly harnesses surface tension (chemical potential) to generate power. Specifically, OPE utilizes the spontaneous and continuous capillary micro-pumping effects of PE ultrathin nanomembranes to actuate the inner-membrane ionic microflow (Figure 2c left). E.g., by simply floating the GPE above the 0.6 m NaCl aqueous solution in the ambient environment without inputting any external work, a persistent voltage over 6 h was recorded as shown in Figure 2d. Power generation via this continuous micro-pumping phenomenon was coined as pumping potential, serving as the basis for the OPE in this work. Yet the modulation of purely spontaneous capillary pumping can be challenging due to numerous determining factors and subtle regulation effects. Thus, we artificially intensified this process by ethanol dripping to instantly provoke much more violent voltages to quantitatively investigate the OPE mechanism (Figure 2c right). Ethanol or other volatile solvent drops functioned as the primer to thoroughly purge the air in PE capillary channels, rapidly mixed with the aqueous solution, and then evacuated off during vaporizing to accelerate the micro-pumping. The results showed that micro-pumping at two bilateral symmetric locations (a & c) around the device center produced almost opposite numbered voltages. And the center-voltage (b) was seemingly neutralized by two counter potentials. It is the first time that a position dependence of DEG voltage was discovered, which suggests a unique mechanism of OPE compared to IPE. More details can be found in Figure S15 (Supporting Information) with the mechanism further explained below.
The relatively faint yet persistent spontaneous pumping potential could be treated as a background signal in this work. To validate this assumption, we switched off and on this background signal by simply taking up and releasing back of the floating GPE off/on the NaCl solution surface. The pumping potential quickly dropped to null when taking up GPE from the solution (Figure 3a). When releasing back to floating, the pumping potential peaked to ≈3 mV, followed by the restoration of spontaneous pumping potential of persistent 0-1 mV voltage. The peak voltages were attributed to the impulse momentum of releas-ing GPE. It has been found that the peak voltages were proportional to the volume of the primer dripping and thus the micropumping flux (Figure S16c, Supporting Information). As a result, the pumping potential elevated from 3.17 to 5.76 mV when primer dripping increased from 25 to 100 μL (Figure 3b). The signal duration increased linearly with the primer volume (Figure 3c). Thus, the peak voltage and background voltage were attributed to primer-induced intensified micro-pumping and the purely spontaneous micro-pumping, respectively. The former was the main research object to explore the mechanism of OPE.
And here, it was further assumed that each intensified micropumping site functions as an independent OPE power sourceone pumping, one power. We found the intensified pumping potential decayed from 2.12 to 0.90 mV as three 15 μL primer droplets dripped sequentially on the same site of the GPE nanomembrane (Figure 3d). If the primer droplets were dripped on different spots, three sharp peaks ranged from 2.39 to 2.15 mV were produced (Figure 3e). This proved that the intensification effects on exactly the same power source (pumping spot) by the primer decayed dropwise. While with the same usage of a primer, triggering new micro-pumping on different spots to establish multiple new individual power sources with higher efficiencies.
Thickness and porosity of PE nanomembrane were also identified as the factors determining pumping potential. As shown in Figure 3f, the voltage dropped from 8.47 to 0.88 mV as the thickness increased from 109 to 1028 nm ( Figure S10, Supporting Information) (150 μL primer dripping). And to shut down the pores, the PE nanomembrane was annealed at 140°C for 10 min before transferring the graphene (the morphology can be seen in Figure S17, Supporting Information). The transformation of the porous semi-crystalline structure of PE was based on the thermally induced recrystallization of polyethylene (Figure S4, Supporting Information), the detailed explanation can be found in Figure S17 (Supporting Information). The voltage decreases from 4.12 to 0.95 mV after the pores shutting down (Figure 3f, inset) (150 μL primer). Thus, modulating the thickness or porosity of the GPE nanomembrane can also regulate the intensified micro-pumping to adjust the pumping potential. To sum up, putting aside the background purely spontaneous pumping potential, the intensified pumping potential could be modulated by pumping flux, the number of intensified pumping sites, thickness, and porosity of PE substrate.
To uncover the mechanism of this out-of-plane electrokinetic effect, we further in-depth investigated the capillary micropumping (kinetic part), and the induction of electrons (electronic part). We first focused on the purely spontaneous pumping potential by simulating the capillary micro-pumping process in Figure 4a. The PE nanomembrane was simplified as a model with three layers of fiber bundles lapped together. The Red in the picture represents the water phase, along with the blue gas phase, and white solid PE phase. Under the capillary force, the water phase gradually spread to the solid phase and finally covered the solid surface at 1.045 μs. Therefore, trace ionic solutions could spread through the substrate via pores of the GPE nanomembrane even without primmer intensification (Figure 4d), resulting in the persistent voltage.
Then the mechanism of intensified pumping potential was studied. A series of four experiments were performed. The first experiment was designed to directly uncover the transmembrane movements of the capillary micro-pumping process. As shown in Figure 4b, the ionic solution was substituted by the red ink, and by spontaneous micro-pumping, it could not penetrate through PE nanomembrane to dye DI water due to the hydrophobicity of PE membrane (Figure 4b-i). However, when the primer was dripped into the water the red ink underneath the membrane was quickly pumped up to color the above water (Figure 4b-ii; Figure S18, Supporting Information). The second experiment was to test the intensified pumping potential without ionic solution. And no voltage was detected with NaCl solution removed ( Figure S19a, Supporting Information). These two sets of results elucidated more of the physical picture of the capillary micro-pumping. That is after dripping, the ethanol droplet spread on the surface of graphene (the contact angle on ethanol on PE was ≈20 , Figure S20, Supporting Information) and partially penetrated through the defects of graphene ( Figure S21, Supporting Information) into PE nanomembrane. When ethanol came into contact with the permeated NaCl solution, they quickly dissolve. Then the solution was forced to move upwards to accelerate the micro-pumping, and eventually generated the peak voltage. Then physical parameters of micro-pumping intensification were investigated. In the third experiment, the ethanol was replaced by acetone as the primer to alter the chemical potential configuration, the resulting intensified pumping potential decreased by 83.2% due to less miscibility ( Figure S22b, Supporting Information). In the fourth experiment, we found that the micro-pumping could not only be intensified by incorporating primer (chemical potential driven), but also could be achieved by external mechanical work. E.g., the intensified pumping potential could reach 3.53 mV by blowing air to the middle of the GPE surface (Figure 4c). And the macroscopic pressure could drive NaCl solution moving through substrates to induce electron migration of graphene. Therefore, distinct forms of forces including capillary pumping, miscibility process, and mechanical pressing functioned equivalently to initiate DEG based on GPE nanomembranes. The last two sets of experiments proved that the primary driving force of the micro-pumping could be modulated and even shifted, as long as the trans-membrane ion movement was maintained, which should be the core process for the pumping potential.
In Figure 4f, we verified the electron migration in graphene by fabricating four equally spaced electrodes on the GPE nanomembrane. The total voltage (V 1,4 ) was 1.05 mV, approximately the sum of all the individual voltages (≈0.93 mV) of three subsections on the GPE nanomembrane. It demonstrated that as a result of the accumulation of Na + , the electrons of graphene moved toward micro-pumping spots (w i-e ) (Figure 4e). This result was distinct from similar experiments of other DEG, such as waving potential and streaming potential. As the w i-e of OPE is not instantly induced by the blinking EDL fluctuating potentials, but by the enduring potential from the accumulated counterions. Thus, during electrokinetics in this work, the kinetics part and the electronic part got decoupled at a centimeter scale.
The electron migration has also been elucidated based on the 'one pumping, one power' hypothesis. As shown in Figure 4g, when two primer droplets (50 μL) were dripped simultaneously parallel to the inter-electrode line, the produced voltage (7.84 mV) was nearly double that induced by single primer droplet dripping (3.79 mV). By contrast, the voltage kept constant ( Figure  S23b, Supporting Information), while the current doubled (Figure 4h) when two primer droplets (50 μL) dripped perpendicular to the inter-electrode line. In the equivalent circuit, the above corresponded to the series and parallel power supply models, respectively, with each pumping spot equivalent to an independent power source.
Previously reported DEGs were all based on IPE where ions in-plane moved above substrates, such as drawing potential, flowing potential, or waving potential. Here, the ions of pumping potential uniquely travel through the substrate. In this way, e.g., Na + was dragged by Coulomb force and moved through negatively charged channels of GPE nanomembranes (w i-c ) during pumping, to induce the directional migration of the electron of graphene (w i-e ) (Figure 4e). Thus, the mechanism of OPE was distinguished in DEG by the through-plane ion moving direction perpendicular to electron migrating.
For traditional DEG devices based on graphene, the resistance inevitably reaches tens of kΩ due to the doping of impurities adsorbed on the surface, and the irreversible rupture during the transferring and handling of large-area monolayer graphene. This undoubtedly suppresses the electrokinetic conversion of the DEG, with the manifestation of only a few hundred nanoamps of current. The community long expects the utilization of other materials other than graphene, but is impeded by the shielding effect in IPE. As we discussed above, the OPE eliminates the shielding effect of the conductive material on the negative charge of the substrate, with a wide range of conductor utiliza- ii) a drop of 5 μL of primer was added to the DI water, and the water droplet instantly turned red. c) The intensified pumping potential induced by blowing on the surface of GPE nanomembrane thrice. Schematic illustration of GPE nanomembrane before d) and after e) intensified pumping triggered by primer dripping, followed by electrons dragging in graphene. f) Voltages of subsections in GPE nanomembrane device with evenly spaced electrodes induced by a 10 μL primer dripping between electrodes 1 and 2. The voltage and current induced by one (left peak) and two simultaneous (right peak) primers dripping parallel g) and perpendicular h) to electron movements (i.e, the straight line connecting two electrodes).
tion. We then employed three typical conductive materials such as Ag nanowires (AgNWs), carbon nanotubes (CNTs), and Pt nano-particles on the PE nanomembrane (the morphology can be seen in Figure S24, Supporting Information) to assemble the DEG devices. The conductor layer thickness was 77, 5286, and 301 nm, respectively ( Figure S25, Supporting Information). As shown in Figure 5a-c, the pumping potential could reach 0.27, 3.16, and 40.78 mV, respectively, and its sign depended on the position where the primer dropped on the membrane (the pumping current can be seen in Figure S26, Supporting Information). i.e., when primer droplet dripped on the left and right ends of the membrane, the signs of the voltages were opposite. It could be seen that the potential peaks of the three conductors other than graphene were hump-shaped and the signal duration signif- icantly increased, especially for Pt, which lasted 56 times longer than that of graphene. As graphene screened out most of the primer infiltration while for the three porous conductors, a considerable part of primer directly interacted with the ionic solution to reinforce micro-pumping in a persistent and intensified manner.
As shown in Figure 5d, compared with graphene, the improved conductivity of these three conductors produced much higher pumping currents and thus boosted electrokinetic conversion. In Figure 5e, we compared the output power of the OPE devices based on four types of conductors. A drop of 50 μL of primer could generate ≈407.6 nW of power for Pt, which was 357 times higher than that of graphene. In contrast with the specific power of previously-reported DEG devices typically below 10 nW μL −1 , the Pt exhibited a leading specific power of 177.2 nW μL −1 with a minimum ionic solution volume of 2.3 μL (Figure 5f). The above findings enabled stereo-configuration of OPE devices, paving the road to 3D design of environmental energy-harvesting system.

Conclusion
In summary, we reported a new paradigm of electrokinetic generation mechanism, termed out-of-plane electrokinetics (OPE). In this work, this exotic electrokinetics is manifested as the pumping potential utilizing the spontaneous capillary micro-pumping of a 100 nm ultrathin polyethylene nanomembrane. By redesigning the topological device structure (ion-electron-charge) in DEG, direct interaction between ions and charges was established. Thus, the chronic suppression issue of ion-charge interaction in DEG due to the shielding effects of conductors has been resolved. Multiple determining factors of this OPE were rigorously analyzed to provide a detailed explanation of mechanism. For the first time, DEG was enabled to choose bulky Ag or Pt as its conductors, with higher conductivity and practical applicability than monolayer graphene. This study provided more insightful perspectives on electrokinetics to help understand more of its physics. It may also inspire and expand the design and application of ambient energy harvesting technologies with more device forms and conductor materials unlocked for DEG.

Experimental Section
Fabrication of the PE Nanomembranes: The PE resins (GUR 1020) were purchased from Celanese with a viscosity average molecular weight of 1.1 × 10 7 g mol −1 . The PE dispersions were prepared by adding 0.96 g PE resin powders (accompanying contains 0.16 g AO 168 and 0.16 g AO 1010) to 31.04 g preheated and pre-stirred white petrolatum (Sonneborn) at 120°C. The prepared PE dispersions were poured into the internal mixer (XSS-300 torque rheometer), which preheated at 180°C and ran at 25 rpm for 25 min. The gel was processed into an ≈2 mm thick sheet with the size of 10 cm × 10 cm by a hot press at 180°C for 30 min. Then, the cooled gel sheet was stretched at 120°C to obtain a precursor membrane (with a draw ratio of 9 × 9) by a biaxial stretcher (Karo IV, Bruckner, Germany). The cut precursor membrane was further stretched at 140°C at a rate of 0.7% s −1 to obtain PE nanomembranes (with a final draw ratio of 9 × 9 × 2.73 × 2.73) with petrolatum. The petrolatum of PE nanomembranes was extracted by Soxhlet extraction with n-hexane for 12 h.
Fabrication of the GPE Devices: Monolayer graphene was purchased from Suzhou Tanfeng Tech, which was grown on copper foil by chemical vapor deposition. The transfer of the monolayer graphene is unique for our substrate: after dripping ethanol on the backside of the membrane and evaporating, the copper foil with a size of 5 × 5 cm was attached to a PE nanomembrane directly and annealed at 140°C for 10 min. For copper etching, the copper/graphene/PE membrane was floating on FeCl 3 solution for 3 h. The GPE nanomembrane was cleaned with deionized water several times. The electrodes with a length of 3.5 cm were fabricated with silver pulp and placed on the two ends of graphene. After connecting the copper wires to the electrodes, silicone elastomer was used to seal the electrodes to avoid contact with the ionic solution.
Fabrication of the Conductive PE Nanomembranes: The 10 g Ag nanowires solution (0.01 wt.%, with the solvent of isopropanol, 30 μm in length and 30 nm in diameter) was sprayed onto the PE nanomembrane and annealed at 140°C for 10 min. The 10 mg carbon nanotubes were added to 10 g isopropanol and dispersed by ultrasound for 30 min. This CNT solution was sprayed onto the PE nanomembrane and annealed at 140°C for 10 min. The Pt membrane was fabricated by sputtering platinum on the surface of the PE nanomembrane at 20 mA for 120 s.
Characterizations of the PE and GPE Nanomembrane: The surface morphology of the PE and GPE nanomembrane was obtained by Scanning electron microscope (Thermo Scientific Apreo 2 and JEOL JSM-7800F). The thickness was measured by an XP-200 stylus profilometer. The quality of monolayer graphene on the PE nanomembrane was identified by Raman spectroscopy (Horiba HR Evolution spectrometer) with an excitation wavelength of 532 nm. The water contact angle was measured by a Kruss DSA25 Contact Angle Analyzer. The zeta potential was tested by an Electric Solid Surface Analyzer (Anton Paar SurPASS 2).
Voltage and Current Measurement of the GPE Devices: The resistance of the GPE devices was measured by a multimeter. The voltage and current signals were measured by JYC 3100 comprehensive IV performance tester and recorded with 10 Hz.

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.