Femtosecond laser-structured wastepaper as a biodegradable flexible current collector for supercapacitor applications

Flexible electronics have emerged as a transformative technological trend in recent years, driven by their versatile and compelling applications in display screens, wearable sensors, and implantable medical devices. To power up these devices, considerable research efforts have been directed towards developing advanced flexible supercapacitors, owing to their potential as a reliable and efficient energy storage solution. The performance of supercapacitors is highly dependent on the design of their integral components, including the current collector, which facilitates charge transfer by connecting the active material to the external terminals. Herein, we report a high-performance, low-cost, and biodegradable flexible current collector for supercapacitors, offering a sustainable solution for energy storage applications. Wastepaper was transformed into a current collector through the incorporation of Fe³⁺ ions into the wastepaper fibers via chemical crosslinking and performing femtosecond laser structuring in air or inside a dilute aqueous solution of FeCl₃. Among the different treatments, the current collector structured in air demonstrated the highest performance. This was attributed to its superior electrical conductivity, lower charge transfer resistance, and distinct superhydrophilicity, surpassing both the unstructured counterpart and the sample structured in the FeCl₃ solution. Furthermore, the femtosecond laser-induced surface structures significantly contributed to improved ion diffusion and charge storage capability by increasing the electrochemically active area. The calculated areal capacitance for a sample prepared in air at 5 mV s^-1 scan rate was 43.6 mF cm^-2. In addition, the current collector exhibited high areal-specific capacitance retention, which was 82% of its initial capacitance after 5000 galvanostatic charge-discharge (GCD) cycles. These results significantly outperform unstructured paper and highlight the pivotal role of laser-induced surface morphology in maximizing ion diffusion and charge storage. Our findings establish a simple yet powerful strategy for converting waste into a valuable component for sustainable energy storage applications.