Moisture-enabled self-charging and voltage stabilizing supercapacitor

The study addresses the challenge of frequent recharging and rapid self-discharge in supercapacitors used for portable and wearable electronics. While supercapacitors offer high power density and long cycle life, their practical use is limited by the need for frequent charging and inherent self-discharge. Integrating ambient energy harvesters with storage devices has been explored (solar, piezo/tribo-electric, thermoelectric), but these often rely on intermittent stimuli or specific conditions. Recently developed moist-electric generators (MEGs) can harvest ubiquitous atmospheric moisture to generate electricity continuously, offering an all-weather, 24-hour self-powering strategy. Motivated by this, the authors propose a flexible moisture-powered supercapacitor (mp-SC) that integrates a polyelectrolyte-based MEG with a graphene electrochemical capacitor (EC) to enable spontaneous self-charging from ambient moisture and persistent voltage stabilization by counteracting self-discharge.

Prior integrated self-charging systems combine supercapacitors with solar cells, piezo/tribo-electric generators, and thermoelectric devices across fibers, films, bulk, and textiles for applications in health monitoring, sensors, and wearables. However, these systems generally require external mechanical stimuli or specific environmental conditions. Moist-electric generators (MEGs) have emerged as a promising alternative, generating electricity via interaction with atmospheric water and enabling continuous power generation irrespective of lighting or motion. Reported MEG materials include graphene oxide and polyelectrolyte systems, with advances in output voltage approaching ~1.5 V and scalable configurations generating high voltages by serial integration. Nevertheless, MEGs alone typically suffer from high internal resistance and limited power density, constraining direct powering of electronics. This work leverages MEG–EC integration to overcome these limitations and suppress supercapacitor self-discharge.

Device architecture and fabrication: The mp-SC comprises two integrated parts: (1) a graphene-based interdigitated electrochemical capacitor (EC) and (2) a bilayer polyelectrolyte-based moist-electric generator (MEG). A graphene oxide (GO) dispersion is blade-coated onto a flexible PET substrate and dried to form an ~8 µm GO film. Interdigitated reduced GO (rGO) microelectrodes are patterned via direct-laser writing (355 nm, ~2.2 W), converting GO to porous, conductive rGO. A PVA/LiCl (5 M) gel electrolyte is sprayed through a mask onto the microelectrode region, which is then encapsulated with a screen-printed insulating epoxy resin (cured at 60 °C for 6 h). A conductive carbon paste current collector (pre-doped with ~1% carbon fiber) is screen-printed atop the epoxy to serve as the MEG bottom electrode and connected to one rGO microelectrode of the EC. A bilayer polyelectrolyte film (top: PDDA; bottom: PSS with 14.5 wt% PVA additive) is coated as the MEG active layer. Finally, a conductive carbon tape is adhered as the MEG top electrode and connected to the other rGO microelectrode of the EC, completing the internal MEG–EC integration.

Polyelectrolyte film preparation: A 10 wt% PVA solution is prepared and blended into a 30 wt% PSS solution to yield a PSS+PVA layer. PDDA (35 wt% in water) is spray-deposited (50 psi) onto the dried PSS+PVA film, with hot air applied during spraying to avoid dissolution and intermixing. The resulting bilayer thickness is ~100 µm with a PDDA:(PSS+PVA) thickness ratio of ~3:7. EDS mapping confirms Cl localization in PDDA and S in PSS. The PSS+PVA layer improves mechanical flexibility (failure strain ~4.5× higher than pure PSS). Dynamic vapor sorption shows up to 78.3% water uptake at 95% RH (25 °C).

Mechanism and modeling: Upon exposure to moisture, –SO3H, –OH, and –NCl groups facilitate water absorption and dissociation, producing mobile H+ and Cl− ions with gradient distributions across the bilayer. Kelvin probe measurements show surface potentials of −0.87 V (PSS) and +0.92 V (PDDA). Poisson–Nernst–Planck simulations describe ion diffusion-driven potential formation, predicting an open-circuit voltage ~1.07 V, consistent with experiments.

Characterization: The EC’s rGO morphology (SEM), reduced oxygen content (Raman, XPS), and conductivity improvements are confirmed. Electrochemical tests use a two-electrode configuration (CV, galvanostatic charge–discharge, EIS) with a 1 V window. Mechanical flexibility is assessed by bending. MEG and mp-SC voltages/currents are measured with a Keithley 2612 under controlled RH and temperature. Performance is evaluated over varying loads, RH (5–95%), temperature, and bending states. Arrays are fabricated via scalable laser processing, screen printing, and spraying, and connected in series to increase output voltage.

• EC part (rGO microelectrodes):

  • Exhibits ideal electric double-layer behavior (rectangular CVs; near-vertical low-frequency EIS). Areal capacitance ~2.21 mF cm−2 (from CV). Nearly symmetric galvanostatic profiles with ~100% Coulombic efficiency.
  • High robustness/flexibility: ~100% capacitance retention under 180° bending; 92.5% capacitance retention and 99% Coulombic efficiency after >120,000 cycles.
  • Laser reduction decreases oxygen content (O ~14.79% vs ~31.29% in GO) and yields porous, conductive rGO (conductivity increases from ~5.09×10^3 to ~2.73×10^3 S m−1 for microelectrodes).

• MEG part (polyelectrolyte bilayer):

  • Generates sustained voltage/current from atmospheric moisture: ~1.27 V and 2.7 µA cm−2 at 90% RH and 25 °C; still outputs ~0.65 V at 15% RH. Voltage increases with RH and remains stable under bending.
  • Water uptake up to 78.3% at 95% RH; mechanical enhancement from PVA blending (failure strain ~4.5× vs PSS only). PNP modeling predicts ~1.07 V OCV, aligning with measurements; Kelvin probe shows −0.87 V (PSS) and +0.92 V (PDDA) surface potentials.

• Integrated mp-SC (MEG + EC):

  • Moisture-enabled spontaneous self-charging to ~−0.9 V in ~380 s at 90% RH; distinctive charge curve (rapid rise then stabilization) reflects MEG-driven energy generation.
  • Greatly extended discharge: at 10 µA cm−2, discharge time ~12,985 s vs 459 s for EC alone, due to concurrent generation + storage.
  • Ultra-high areal capacitance: 138.3 mF cm−2 (orders of magnitude above many graphene and some pseudocapacitors). Areal capacitance vs current density presented; robust performance across RH and temperature ranges.
  • Power characteristics: maximum power density ~49.4 µW cm−2 at ~500 Ω load; combined device achieves power density ~1500× higher than the polyelectrolyte MEG alone due to EC synergy; output voltage increases while current decreases with load from 10 Ω to 1 GΩ.
  • Stability and mechanics: voltage maintenance of ~96.6% after 120 h open-circuit (self-discharge strongly suppressed vs EC alone, which drops to ~0.45 V within 10 min); durable moisture charging/discharging for >15 cycles at 90% RH; ~100% voltage retention after 1000 bending cycles at 180°.
  • Scalability and applications: Linear voltage scaling with series connection; 72 units in series deliver ~60 V in air (90% RH, 25 °C). Four-unit strings power a commercial temperature–humidity monitor immediately and again five days later; arrays power an electronic watch (in a wristband), a calculator, and a Bluetooth thermohygrometer/sensor transmitting data to a smartphone.

The mp-SC directly addresses the dual challenges of frequent recharging and rapid self-discharge in supercapacitors by continuously harvesting atmospheric moisture to generate a bias that counteracts voltage decay. The polyelectrolyte bilayer MEG establishes an ion concentration gradient-driven potential, while the integrated rGO-based EC stores charge via the electric double-layer mechanism. Their synergy enables simultaneous energy generation and storage, significantly extending discharge duration and stabilizing voltage over long times (96.6% retention at 120 h), which is not achievable with the EC alone. The device maintains operation across a broad RH and temperature window and under severe bending, underscoring suitability for flexible and wearable electronics. Furthermore, scalable fabrication and linear voltage increase via series integration allow powering of practical electronics without complex power management, highlighting relevance for distributed, maintenance-free IoT sensors and wearable devices. Overall, the findings demonstrate that coupling MEGs with ECs is an effective route to create self-powered, voltage-stabilized supercapacitors with enhanced power output relative to MEGs alone and suppressed self-discharge relative to standalone ECs.

This work presents a flexible, durable moisture-powered supercapacitor that self-charges from ambient humidity and stabilizes its voltage by integrating a PDDA/PSS(+PVA) polyelectrolyte MEG with an interdigitated rGO microelectrode EC. The device achieves ~−0.9 V self-charging in humid air, an ultra-high areal capacitance of 138.3 mF cm−2, a maximum power density of ~49.4 µW cm−2, and exceptional voltage maintenance (~96.6% over 120 h). It retains performance under extensive bending and scales to high voltages (60 V with 72 series-connected units), powering multiple commercial devices. These advances indicate a practical pathway toward self-powered, long-term stable energy storage for flexible and wearable electronics and distributed sensing.

Future research could focus on optimizing polyelectrolyte composition and architecture to enhance ion transport at low RH, reducing internal resistance to further boost power density, integrating energy management circuits for broader device compatibility, and assessing long-term environmental durability and encapsulation strategies for real-world deployment.

• Environmental dependence: Although functional down to ~15% RH, output voltage and current increase with RH; performance may degrade in persistently dry or hot/cold extremes not fully explored here.
• Power density and current: Despite improvement via EC integration, absolute current and power remain modest, often necessitating series/parallel arrays for many applications.
• Long-term durability: While 120 h voltage maintenance and 1000 bending cycles were demonstrated (and >120,000 EC cycles), multi-month operation, environmental cycling (humidity/temperature), and real-world wear/contamination effects were not reported.
• Materials and architecture scope: The study focuses on a specific PDDA/PSS(+PVA) bilayer and rGO EC; generality across other material systems and scaling of manufacturing yield were not detailed.