Fabrication of VOOH hollow spheres for ammonium-ion supercapacitors

[Display omitted] •The electrochemical performance of VOOH hollow spheres for NH4+ was evaluated for the first time.•The hollow structure of VOOH facilitates the insertion/extraction of NH4+.•Specific capacitance of 221F·g−1 at 0.1 A·g−1 in NH4Cl/PVA electrolyte was achieved.•Areal capacitance of VOOH//AC HSC device was 67 mF·cm−2 at 0.147 mA·cm−2.•This study can help fill the gap of low-valence vanadium oxides in NH4+ energy storage. The performance of vanadium hydroxide (VOOH) in ammonium-ion (NH4+) storage is currently unknown and attractive. Herein, low crystallinity VOOH hollow spheres are synthesized by a template-free hydrothermal method for investigating NH4+-storage properties. The hollow structure of VOOH, unlike the precursor V(OH)2NH2 solid spheres, promotes NH4+-insertion/extraction, resulting in improved NH4+-storage performance. The specific capacitance of VOOH hollow spheres is 221 F·g−1 (265 C·g−1) at 0.1 A·g−1. The quasi-solid-state VOOH//AC hybrid supercapacitor (HSC) device exhibits the capacitance of 67 mF·cm−2 at 0.147 mA·cm−2. The above results indicate that VOOH aims to become an excellent electrode material for NH4+-storage.