In a recent article published in the journal Nature Communications, researchers presented a novel ultraflexible energy harvesting-storage system that combines organic photovoltaics (OPVs) with zinc-ion batteries. The system aims to address the challenges associated with energy supply in wearable devices. The proposed system is designed to be highly efficient, safe, and adaptable, making it suitable for a range of applications in the field of wearable technology.
An Illustration detailing the structure and multi-layered configuration of the battery. B. Depiction of the synthesis of the thin hydrogel electrolyte film for the battery using a cold-lamination procedure. C, D SEM images showing the topography of the ultrathin hydrogel (C) unstrained and (D) strained to a bend radius of 60 µm. E Voltage-capacity profile of flexible Zn-MnO2 batteries, featuring hydrogel electrolyte thicknesses of 10 µm (red), 100 µm (blue), and 1 mm (cyan), charged by an external power supply. F Rate performance of the battery at 0.2 C, 1 C, and 5 C charging rates. G Stability of the battery over 200 consecutive charge-discharge cycles. Image Credit: https://www.nature.com/articles/s41467-024-50894-w
Background
The integration of energy harvesting and storage components is essential for creating self-sustaining wearable devices. Traditional energy sources like lithium-ion batteries often pose challenges, particularly in terms of flexibility and safety, which are critical for wearable applications. On the other hand, organic photovoltaic cells (OPVs) present a lightweight and flexible solution for energy harvesting, although their efficiency and stability have historically been concerns.
However, recent advancements in OPV technology have significantly improved power conversion efficiency (PCE) and operational longevity, making OPVs more suitable for wearable systems. Additionally, zinc-ion batteries have gained attention as a safer, more cost-effective, and environmentally friendly alternative to traditional lithium-ion batteries, further supporting the development of reliable and sustainable wearable devices.
The Current Study
In this study, the OPVs were engineered with a blend of donor and acceptor materials specifically optimized for high efficiency and flexibility. The donor material, a polymer known for its superior light absorption and charge transport properties, was paired with a non-fullerene acceptor that significantly boosts overall power conversion efficiency. The active layer, where the photovoltaic process occurs, was strategically placed between a transparent conductive oxide (TCO) layer and a metal electrode, ensuring efficient charge collection and overall device performance.
For the zinc-ion batteries, the electrodes were composed of a combination of carbon-based materials and zinc oxide, providing a robust and efficient platform for energy storage. The electrolyte used was a polyvinyl alcohol (PVA) and graphene oxide (GO) hydrogel, which offered excellent ionic conductivity and mechanical flexibility. This hydrogel allowed for a thinner battery design while still maintaining strong electrochemical performance, which is critical for wearable applications.
The PVA-GO hydrogel electrolyte was synthesized by dissolving PVA in deionized water and incorporating graphene oxide to enhance ionic conductivity. The resulting hydrogel was then cast onto the electrodes, forming a solid-state electrolyte that is both flexible and efficient, perfectly suited for the ultra flexible design of the zinc-ion batteries.
Various characterization techniques were employed to assess the performance of the OPVs. The power conversion efficiency (PCE) was measured under standard test conditions (STC) using a solar simulator with a calibrated photodiode. The current-voltage (I-V) characteristics were recorded to determine key parameters such as the open-circuit voltage (Voc), short-circuit current (Isc), and fill factor (FF), which are essential for evaluating the solar cells' efficiency.
For the zinc-ion batteries, electrochemical performance was rigorously evaluated using cyclic voltammetry (CV) and galvanostatic charge-discharge tests. These tests analyzed the charge-discharge profiles over multiple cycles and provided detailed insights into the specific capacity, energy density, and cycling stability. Additionally, the thickness of the battery components was measured using a micrometer, ensuring that they met the stringent requirements for ultra-flexible design, which is crucial for seamless integration into wearable devices.
Results and Discussion
The study demonstrated that the integrated energy harvesting and storage system achieved cutting-edge areal power outputs exceeding 10 mW cm–2. This performance is particularly impressive given the ultrathin design of the OPV modules, which measured just 4 μm in thickness. The combination of the high efficiency of the OPVs with the robust performance of the zinc-ion batteries resulted in a highly effective energy solution tailored for wearable applications.
One of the key highlights of the research was the use of a textile-based battery configuration, which significantly enhanced the system's flexibility and adaptability. This design allowed the energy storage component to conform to various shapes and surfaces, making it ideal for integration into a wide range of wearable devices. Ensuring high performance while maintaining user safety and comfort is crucial for the success of wearable technology, and this study's approach effectively addressed these needs.
The researchers also tackled the challenges involved in integrating energy harvesting and storage components. They emphasized that achieving a high voltage output across both modules is vital for effective system integration. To this end, the use of homo-tandem OPVs was proposed as a strategy to double the open-circuit voltage output, making it easier to charge both lithium-ion and sodium-ion batteries. This approach not only boosts the overall efficiency of the system but also broadens its applicability to different energy storage technologies, enhancing its versatility in various wearable and portable applications.
Conclusion
In conclusion, this system's innovative design and technical execution address the critical challenges of energy supply, efficiency, and safety in wearable applications. With a power conversion efficiency of up to 16.18 % and a highly flexible configuration, this integrated solution is likely to enhance the functionality and usability of wearable devices. The research underscores the importance of continued innovation in energy harvesting and storage technologies, particularly as the demand for flexible and efficient power systems grows.
Journal Reference
Saifi S., Xiao X., Cheng S. et al. (2024). An ultraflexible energy harvesting-storage system for wearable applications. Nature Communications 15, 6546. DOI: 10.1038/s41467-024-50894-w, https://www.nature.com/articles/s41467-024-50894-w