Multistage coupling water-enabled electric generator with customizable energy output

The study addresses the challenge that current water-enabled electric generators (WEGs) typically harvest electricity from only a single form of water (liquid flow, moisture, or droplets) and then discharge the water after one-time use, wasting significant energy inherent in the hydrological cycle. Inspired by natural multistep water utilization (e.g., arid soil simultaneously experiencing liquid infiltration and moisture diffusion), the authors propose a multistage coupling WEG (mc-WEG) that synchronously exploits internal liquid flow and the concomitant moisture diffusion within one device. The purpose is to enhance energy harvesting efficiency and deliver customizable, application-ready electrical outputs through modular “flexible building blocks,” enabling practical powering of electronics under diverse environmental conditions.

Recent hydrovoltaic and WEG advances have demonstrated electricity generation from various water–material interactions: (i) moisture-enabled electric generators (MEGs) convert chemical potential gradients of water into electricity using materials such as graphene oxide; (ii) evaporation- or pressure-driven liquid flow in nanochannels produces streaming currents via electric double layers; and (iii) moving droplets on conductive nanostructures (e.g., carbon nanotubes, graphene) generate electricity. Despite these innovations, most devices are single-stage and discard water post-harvest, failing to capture subsequent energy available as moisture diffusion. The literature also highlights design strategies for ion-selective membranes, asymmetric structures for rectified ion transport, and environmental moisture harvesting, but integrated multistage coupling of liquid flow and moisture diffusion within a single scalable device remained underexplored prior to this work.

Device architecture: mc-WEG comprises (1) a water-flow-enabled electricity generation layer (wf-layer): CaCl2 asymmetrically loaded carbon fabric based on cotton fabric infiltrated with Ketjen black dispersion (providing carboxyl/hydroxyl groups for negative surface charge). Part of the fabric is soaked in 25 wt% CaCl2 and dried to create a hygroscopic, asymmetrically loaded region. Au electrodes are attached at the ends of the CaCl2-loaded and unloaded regions. (2) A hydrophilic, electrospun PAN nanofiber membrane (~60 μm) on porous Au mesh serving as a moisture diversion layer and bottom electrode for the md-layer. (3) A moisture-diffusion-enabled electricity generation layer (md-layer): an asymmetric stack of PVA-LiCl(c_h)/H-PSS/PVA-LiCl(c_l), where H-PSS is PSS doped with 0.05 M H2SO4 and PVA layers contain different LiCl concentrations (e.g., 0.5 M bottom, 0.01 M top). The md-layer is sandwiched between a porous Au mesh bottom electrode and a top Au foil electrode. Assembly: Layers are stacked in order (wf-layer/PAN/md-layer) and encapsulated with polyimide tape where needed. Working principles: Under humidity gradients, CaCl2 in the wf-layer absorbs moisture to form liquid that flows from the loaded to unloaded region along the negatively charged carbon fabric, generating streaming current via EDL charge imbalance (cations adsorb to Stern layer; anions lag in the diffuse layer). Concurrently, moisture evaporating from the wf-layer passes through the porous PAN into the md-layer. In the md-layer, initial moisture uptake at the bottom creates high ion concentration (dissociated Li+, Cl−, and H+; immobilized sulfonate anions in H-PSS). The asymmetric ion concentrations and cation-selective H-PSS promote directional Li+/H+ migration upward while suppressing Cl− back-diffusion, generating a potential. Water transport characterization: Fluorescein tracking under UV confirms liquid formation and flow on wf-layer; PAN shows moisture uptake and acts as both conduit and reservoir. PAN moisture permeability ~1284.98 g m−2 h−1; dehydrating capacity 179.91 g m−2 h−1 at 31% RH, 31.3 g m−2 h−1 at 90% RH. Water absorption of PAN on wf-layer stabilizes ~8.37% (unsaturated) vs ~35.91% in pure water. Electrical testing: wf-layer outputs characterized under asymmetric humidity (e.g., 90% RH at loaded region vs ~25% RH at unloaded region), varying CaCl2 load ratios, geometries, and resistance (via carbon black content), including folding/crimping effects. md-layer tested across RH (31–90%), areas (0.25–6 cm2), thicknesses, and series/parallel integration; diode-like I–V behavior and rectification assessed by applying biases and integrating charge to estimate rectification ratios. Integrated mc-WEGs (various configurations mc-WEG1–4) assembled to study modular “flexible building blocks,” connection schemes (series/parallel of internal layers), outdoor environmental robustness (Beijing, variable RH/temperature), capacitor charging, and powering of devices (atomized glass, table lamp, LED strip). Measurement instruments included Keithley 2400/2612b; standard protocols followed for permeability (GB/T 1037-2021) and water absorption (GB/T 21655.1-2008).

• Multistage coupling and performance: mc-WEG synchronously harvests electricity from internal liquid flow (wf-layer) and moisture diffusion (md-layer) within one device, achieving maximum output power density up to approximately -92 mW m−2 (≈ -11 W m−3), outperforming prior WEGs. - wf-layer performance: Under asymmetric humidity (∼90% RH vs 25% RH), a 3×6 cm wf-layer produced Voc ≈ -0.24 V and Isc ≈ -31 μA for >10,000 s. Increasing RH at the loaded region from 31% to 90% raised Voc from 0.08 to 0.24 V and Isc from 12.70 to 31.58 μA. Lower humidity at the unloaded side (25% RH) increased output relative to 90% RH due to enhanced water escape. CaCl2 load ratio increases (11–44 wt%) increased Voc (0.07→0.26 V). Geometry modulation (length/width) and folding/crimping notably increased current (e.g., 9×6 cm wf-layer Isc from 86.95 to 260.62 μA by crimping). Max wf-layer volumetric power density reached 8.86 W m−3 with stable power under load. - md-layer performance and mechanism: Asymmetric PVA-LiCl(0.5 M)/H-PSS/PVA-LiCl(0.01 M) exhibited diode-like I–V and strong ion rectification: maximum rectification ratio ≈382 (vs ≈15 without H-PSS and ≈221 for H-PSS alone), confirming upward cation-selective transport. A 1×1 cm md-layer delivered Voc ≈ -0.60 V and Isc ≈ -76 μA at 90% RH, stable for at least 10,000 s and over 20 absorption-desorption cycles. Isc increased with RH (12.08→76.32 μA, 31→90% RH) and with area (0.25→6 cm2 raised Isc 20.94→105.19 μA) while Voc remained ≈ -0.60 V. Maximum areal power density ~0.59 μW cm−2 at ~5.6 kΩ for 0.25×0.25 cm devices; performance preserved under bending (45°, 90°). - Integrated mc-WEG customization: mc-WEG1 (wf: 3×6 cm; md: 3×2 cm) produced ≈0.24 V/42.7 μA (wf) and ≈0.67 V/104.89 μA (md) simultaneously; internal connection modulated overall output: parallel 0.23 V/202.07 μA (max 320.94 mW m−3 at 3.9 kΩ), series 0.91 V/50.07 μA (133.08 mW m−3 at 20 kΩ). mc-WEG2 (two md 0.5×2 cm + one wf 1×6 cm in series) reached Voc 1.42 V and Isc 23.47 μA; outdoor test (Beijing, 3 days) showed Voc 1.18–1.93 V and Isc 7.18–25.50 μA across RH 33–84% and −18–34 °C. - Applications: Three mc-WEG3 in parallel charged a 47 mF capacitor to −2 V. Twenty-four mc-WEG3 in series (2 m length, folded to 8×6×4 cm3) provided Voc up to 36.8 V and Isc ≈ -12 μA, directly operating an intelligent atomized glass (13×6 cm). mc-WEG4 using folded wf-layers enabled 22 units in series to deliver 10.32 V and −280 μA, directly powering a table lamp and a 6-LED strip for >30 min without pre-charging. - Mechanical/environmental robustness: Devices maintained output after folding/crimping and under natural environmental variations, demonstrating flexibility and adaptability.

The mc-WEG addresses the inefficiency of single-stage WEGs by leveraging both stages of water utilization—liquid flow and subsequent moisture diffusion—in a single integrated architecture. The wf-layer’s asymmetric CaCl2 loading induces sustained internal liquid flow over negatively charged carbon fabric, while the md-layer’s asymmetric polyelectrolyte structure and cation-selective H-PSS convert moisture-driven ion gradients into directional cation transport and electricity. The PAN diversion layer effectively mediates mass transfer between stages with high permeability and dehydration capacity, ensuring continuous multistage operation. The modular “flexible building block” strategy enables fine-tuning of voltage/current through layer sizing, geometry (including folding/crimping), and electrical interconnection (series/parallel), allowing task-specific outputs for real devices. Demonstrations, including sustained operation of a table lamp and atomized glass without pre-charging, validate practical feasibility. Environmental tests confirm resilience to humidity and temperature fluctuations, supporting real-world applicability. Overall, the multistage coupling mechanism effectively taps additional energy otherwise lost after single-use in conventional WEGs, enhancing total harvested power and enabling scalable, customizable systems.

A multistage coupling water-enabled electric generator (mc-WEG) was developed that synchronously harvests electricity from internal liquid flow and moisture diffusion. Through asymmetric material design (wf-layer and md-layer) and a porous PAN diversion layer, the device achieved a maximum output power density near -92 mW m−2 (≈ -11 W m−3). The modular “flexible building blocks” approach—via size control, spatial optimization, and integration—allowed tailored outputs and scalable assemblies. Field robustness, mechanical flexibility, and the ability to directly power devices (e.g., a table lamp for >30 min; atomized glass) demonstrate practical potential. Future research could optimize ion-selective membranes, improve mass transport coupling, enhance long-term durability, and develop integrated packaging systems for diverse environments and Internet-of-Things applications.