Boric acid templating synthesis of highly-dense yet ultramicroporous carbons for compact capacitive energy storage

Carbon-based supercapacitors have shown great promise for miniaturized electronics and electric vehicles, but are usually limited by their low volumetric performance, which is largely due to the inefficient utilization of carbon pores in charge storage. Herein, we develop a reliable and scalable boric acid templating technique to prepare boron and oxygen co-modified highly-dense yet ultramicroporous carbons (BUMCs). The carbons are featured with high density (up to 1.62 g cm^-3), large specific surface area (up to 1050 m² g^-1), narrow pore distribution (0.4-0.6 nm) and exquisite pore surface functionalities (mainly -BC₂O, -BCO₂, and -COH groups). Consequently, the carbons show exceptionally compact capacitive energy storage. The optimal BUMC-0.5 delivers an outstanding volumetric capacitance of 431 F cm^-3 and a high-rate capability in 1 M H₂SO₄. In particular, an ever-reported high volumetric energy density of 32.6 Wh L^-1 can be harvested in an aqueous symmetric supercapacitor. Our results demonstrate that the -BC₂O and -BCO₂ groups on the ultramicropore walls can facilitate the internal SO₄^2- ion transport, thus leading to an unprecedented high utilization efficiency of ultramicropores for charge storage. This work provides a new paradigm for construction and utilization of dense and ultramicroporous carbons for compact energy storage.