Hierarchical Carbon Nanocages: Unlocking New Opportunities to Energy Applications

Conspectus Carbon nanocages (CNCs) are a new type of composite topological carbon materials composed of nanoscale hollow cavities and subnanoscale through-channels. “Putative” CNCs were first observed decades ago as impurities in preparing fullerene/nanotubes by arc discharge of graphite electrodes. Different from the “star” nanocarbons like 0D fullerene, 1D carbon nanotubes, 2D graphene, and graphdiyne, CNCs remained overlooked for decades due to a cognitive blind spot regarding the core structural characteristics. Early research focused only on their hollow inner cavities while neglecting the shell-embedded microchannels, leading to misclassification such as “hollow carbon nanospheres” and “hollow carbon shell”. This fundamental oversight, compounded by the challenge of synthesizing pure samples, prevented the discovery of their unique properties. Ten years ago, our group invented the in situ magnesium oxide templating method which enabled the synthesis of high-purity, hierarchical CNCs (hCNCs) with composition easily regulated by doping. In hCNCs, individual nanocage units arrange into an ordered hierarchical network, interconnecting macropores, mesopores, and micropores. Access to pure hCNC samples and a correct structural understanding revealed three unique intrinsic properties/functions vital for energy applications, i.e., the topological confinement effect, efficient mass-charge synergistic transport, and high-efficiency utilization of active species. These discoveries established CNCs as a distinct and promising branch of carbon nanomaterials. In this Account, we summarize the core concept and characteristics and recent progress in energy-related applications of hCNCs based on their intrinsic properties. We first trace the historical evolution from impurities of putative CNCs observed 30 years ago to high-performance materials today, establishing an experimentally validated definition of CNCs as “composite topological carbon materials composed of nanoscale hollow cavities and sub-nanoscale through-channels”. The recent development of a series of hCNC variants with tunable structural parameters and dopants has laid a solid material foundation for probing their properties and functions. Subsequently, we detail the latest advances in energy applications enabled by hCNCs’ unique attributes. A key feature is the topological confinement effect exhibited by both the subnanoscale through-channels (shell-embedded micropores) and the nanoscale hollow cavities (mesopores), which allows precise regulation of material functions via micro-nano composite engineering. For instance, shallow micropores serve as ideal sites for stabilizing single-/oligo-atom metal catalysts; inner cavities can confine catalytic active species to tailor microenvironments for specific reactions or encapsulate electrode materials to enable “loss-free pulverization” for advanced energy storage. Furthermore, the multilevel porous conductive network of hCNCs facilitates efficient mass-charge synergistic transport and maximizes the utilization of active species, thereby ensuring high-performance expression. Additionally, hCNCs possess excellent photothermal conversion capability, which provides a new robust platform for photothermal catalysis due to their ease of compounding with other materials and efficient performance expression aforementioned. Finally, we delineate existing challenges and future perspectives concerning pore structure regulation, expansion to new applications, deeper mechanistic understanding and theoretical guidance, and the scaling of industrial synthesis. Based on our group’s research, this Account provides a comprehensive overview of CNC materials, from fundamental concepts to functional mechanisms and broad applications, and aims to guide subsequent fundamental and applied research in this emerging field.