Hollow core-shell nanostructure supercapacitor electrodes: Gap matters

Hollow core-shell nanorods with a nanogap are designed and constructed with the assistance of atomic layer deposition (ALD) for energy storage applications. As a demonstration, CoO nanorods and NiO nanowalls are enclosed by a TiO₂ nanotube shell, forming the "wire in tube" and "wall in box" structures, respectively. A thin sacrificial layer of Al₂O₃ is deposited by ALD and removed eventually, forming a nanogap between the CoO core (or the NiO nanowall) and the TiO₂ shell. When they are tested as supercapacitor electrodes, an evident difference between the solid core-shell nanostructure and hollow ones can be found; for example, the hollow structure shows ∼2 to 4 times the capacitance compared to the solid wires. The electrochemical properties are also superior compared to the bare nanorods without the nanotube shell. The enhancement is ascribed to the conformal hollow design which provides enlarged specific surface areas and a shorter ion transport path. It is prospected that such a positive nanogap effect may also exist in other electrochemical cell electrodes such as lithium ion batteries and fuel cells.