Ultra‐Thin Hollow Carbon Nanospheres for Pseudocapacitive Sodium‐Ion Storage

Ultra‐thin hollow carbon nanospheres (UTH‐CNs) are fabricated for use as anodes of asymmetric sodium ion pseudocapacitors. The ∼3 nm thick amorphous carbon walls obtained from regenerated silk proteins as a template exhibit a well‐defined porous structure suitable for reversible sodium‐ion storage. The UTH‐CNs show remarkable electrochemical activity with sodium via a pseudocapacitive reaction, delivering a large reversible capacity as well as superior rate performance for more than 1000 cycles. The pseudocapacitors based on UTH‐CNs exhibit a capacitance of 186 F g −1 , a specific energy of 43 Wh kg −1 and a power density of 10 kW kg −1 . This represents the highest value yet reported for asymmetric sodium‐ion storage pseudocapacitors.