Strategies for enhancing capacity and rate performance of two-dimensional material-based supercapacitors

With the profound transformation of the global energy landscape and the rapid advancement of portable electronic devices and electric vehicle industries, there is an increasingly urgent demand for high-performance energy storage devices. Among the available energy storage technologies, supercapacitors stand out due to their rapid charge/discharge capabilities, excellent cycling stability, and high power density, enabling reliable long-term operation as well as efficient energy conversion and storage. A fundamental challenge in contemporary energy storage research remains the enhancement of supercapacitor energy density while maintaining their inherent high power density capabilities. Two-dimensional (2D) materials have emerged as promising candidates for constructing high-performance supercapacitor electrodes. Materials such as graphene, transition metal nitrides and/or carbides (MXenes), and transition metal dichalcogenides possess unique layered structures with atomic thickness, exceptional surface areas, high theoretical capacities, and remarkable mechanical flexibility. These characteristics make them particularly suitable for developing next-generation energy storage devices. However, the inherent van der Waals interactions between nanosheets frequently result in restacking phenomena, significantly impeding ion transport and consequently limiting both practical capacity and rate performance. Thus, rational materials design and precise electrode architecture engineering are imperative for overcoming these performance limitations. This review first explores modification strategies for enhancing the electrochemical performance of 2D materials. Studies have shown that diverse modification approaches, including surface functionalization, defect engineering, and heterogeneous structure construction, can effectively increase active sites, enhance conductivity, and improve pseudocapacitive characteristics. These modifications lead to substantial improvements in both areal and volumetric capacitance of electrode materials. Notably, efforts to increase supercapacitor energy density typically necessitate higher active material mass loading, which inherently results in more complex and extended ion transport pathways within the electrode structure, thereby compromising rate performance. In addressing this challenge, we evaluate conventional methodologies for establishing ion transport channels in high mass loading electrodes, including template-based approaches, external field-induced assembly techniques, and three-dimensional (3D) printing processes. However, these traditional methods typically generate pore structures at the micrometer or sub-micrometer scale, making it challenging to simultaneously achieve optimal rate performance and volumetric capacitance. To concurrently optimize areal capacitance, volumetric capacitance, and rate performance, this review emphasizes recent innovative approaches for constructing nanoscale porous architectures. These include capillary force-driven densification, interlayer insertion strategies, surface etching techniques, and quantum dot methodologies. These advanced approaches aim to establish three-dimensional interconnected networks for efficient ion transport, thereby accelerating the development of miniaturized supercapacitor technologies that simultaneously achieve high energy density and high power density characteristics.