Hierarchical Porous Carbon-Carbon Dot Architecture as a High Energy Density Cathode for Lithium-Metal Capacitors

Article Hierarchical Porous Carbon-Carbon Dot Architecture as a High Energy Density Cathode for Lithium-Metal Capacitors Gayathry Ganesh 1,2, Gokul Raj Deivendran 3, Vaishak Sunil 1,2, Izan Izwan Misnon 1,2, Chun-Chen Yang 3,4 and Rajan Jose 1,2,3,* 1 Center for Advanced Intelligent Materials, Universiti Malaysia Pahang Al-Sultan Abdullah, Kuantan 26300, Malaysia 2 Faculty of Industrial Sciences and Technology, Universiti Malaysia Pahang Al-Sultan Abdullah, Kuantan 26300, Malaysia 3 Battery Research Center of Green Energy, Ming Chi University of Technology, New Taipei City 243303, Taiwan 4 Department of Chemical Engineering, Ming Chi University of Technology, New Taipei City 243303, Taiwan * Correspondence: [email protected] or [email protected] Received: 8 December 2024; Revised: 19 March 2025; Accepted: 24 March 2025; Published: 26 March 2025 Abstract: Hybrid devices such as lithium-metal capacitors (LMC) are in rising demand and can simultaneously meet the requirements of energy storage devices with superior specific energy and high specific power. LMCs combine a lithium anode with high specific energy and an activated carbon cathode with high specific power. Biomass-derived porous carbon (BC) is an ideal candidate as cathode material and stands out for its tuneable porosity, sustainability, and low cost. However, the inherent limitations of BC in delivering optimal electrochemical performance necessitate using additives with superior electronic conductivity. In this study, we introduce functionalized carbon quantum dots (f-CDs), synthesized from biomass, as an effective additive to enhance the performance of BC. The physicochemical and electrochemical figures of merit of BC integrated with 7 wt.% f-CDs (BC@f-CD) were systematically compared with BC modified with 0.4 wt.% single walled carbon nanotube (BC@s-CNT). Electrochemical evaluations revealed that BC@f-CD exhibited a superior specific capacitance of approximately 191 F·g−1 within a 2–4.3 V voltage window. The nano-sized dimensions and functional groups of f-CDs significantly improved performance, enabling a remarkable 111% increase in specific energy. Additionally, BC@f-CD demonstrated excellent cycling stability, retaining ~86% of its initial capacity after 5000 cycles, outperforming traditional lithium-metal batteries. This study underscores the potential of f-CDs as a cost-effective and efficient alternative additive to s-CNTs that can enhance the performance of LMCs, providing a sustainable solution for advanced energy storage applications.