Moisture-enabled self-charging and voltage stabilizing supercapacitor

Miniaturized and flexible supercapacitors are in high demand for portable electronics due to their high power density, long lifespan, and safety. However, frequent charging and self-discharge significantly limit their convenience. Harvesting ambient energy offers a solution, enabling continuous self-charging and eliminating the need for frequent replacements or bulky external charging. While solar energy, mechanical movements, and thermal differences have been explored, these require specific environmental conditions. Recently developed moist-electric generators (MEGs) offer a promising alternative, utilizing atmospheric water for electricity generation in all-weather conditions. This work focuses on developing a highly integrated system combining an MEG with a supercapacitor to create a self-charging, long-term stable energy storage device independent of intermittent environmental conditions.

Existing research on self-charging energy storage devices has explored various energy harvesting methods integrated with supercapacitors. These include solar cells, piezoelectric/triboelectric generators, and thermoelectric devices. Self-charging integrated devices have been developed in various forms, including 1D fibers, 2D films, 3D structures, and textiles, finding applications in health monitoring bioelectronics, sensors, and wearable electronics. However, these often rely on external stimuli or specific geographic/climatic conditions. The use of MEGs for power generation is a relatively recent development, offering a sustainable and continuous power source.

The mp-SC is fabricated through a layer-by-layer integration process. First, a graphene oxide (GO) suspension is blade-coated onto a polyethylene terephthalate (PET) substrate. Direct laser writing creates interdigitated reduced GO (rGO) microelectrodes. A PVA/LiCl electrolyte and epoxy resin are then applied, forming the graphene-based interdigitated EC. A conductive carbon paste is coated and connected to one rGO microelectrode. A polyelectrolyte film (PDDA and PSS) is applied as the MEG, with a carbon tape top electrode connecting to the other rGO microelectrode. The electrochemical performance of the EC is characterized using cyclic voltammetry (CV), galvanostatic charge/discharge profiles, and electrochemical impedance spectroscopy (EIS). The power generation performance of the MEG is evaluated by measuring voltage and current density under varying relative humidity (RH) and bending conditions. The combined performance of the mp-SC is assessed through its self-charging capability, voltage maintenance, areal capacitance, power density, and cycling stability under different conditions. The large-scale integration of mp-SC units is demonstrated by connecting multiple units in series.

The fabricated mp-SC exhibits a self-charged voltage of -0.9 V in 90% RH air. It maintains 96.6% of its voltage for 120 hours, significantly exceeding the performance of conventional supercapacitors. The device shows a high areal capacitance of 138.3 mF cm² at a current density of 10 µA cm². The power output is enhanced through the synergy of electricity generation and energy storage, reaching 49.4 µW cm². The mp-SC demonstrates remarkable flexibility, with 100% voltage retention after 1000 cycles of 180° bending. A large-scale device with 72 units connected in series achieves a self-charged voltage of 60 V, sufficient to power various commercial electronics, including electronic watches, temperature and humidity meters, and calculators. The mp-SC shows high performance across a range of humidity levels and temperatures, demonstrating its robustness to various environmental conditions. The device outperforms many existing graphene supercapacitors and moisture power generators in terms of areal capacitance and power density.

The mp-SC successfully addresses the limitations of conventional supercapacitors by incorporating an integrated energy harvesting mechanism. The continuous power generation from the MEG counteracts the self-discharge effect of the EC, resulting in prolonged voltage maintenance and self-charging capabilities. The high areal capacitance and power density, coupled with flexibility and long-term stability, position the mp-SC as a highly promising energy storage solution for various applications. The successful integration of the MEG and EC demonstrates a viable pathway toward developing self-powered and long-lasting energy storage devices.

This study presents a novel flexible moisture-powered supercapacitor that effectively addresses the self-discharge challenges of conventional supercapacitors. The integration of a polyelectrolyte-based MEG and a graphene EC yields a device with high areal capacitance, significant power output, and exceptional voltage retention. The ability to power various electronic devices using atmospheric moisture highlights its potential for diverse applications in portable and wearable electronics. Future research could focus on optimizing the materials and device architecture to further enhance performance and explore new applications in the Internet of Things (IoT) and other fields.

While the mp-SC demonstrates significant advancement, some limitations exist. The performance is dependent on ambient humidity; lower humidity levels may reduce the power output. Further optimization of the MEG and EC materials could improve efficiency. Long-term reliability under extreme conditions (e.g., very high or low temperatures) requires further investigation. Scaling up the manufacturing process for mass production needs careful consideration.