Folded nanocrystalline-stacked WO3 for efficient ammonium-ion storage

Ammonium-ion hybrid supercapacitors (AIHSs) hold great promises for high-rate energy storage, yet their performance is often restricted by sluggish ion transport and the structural instability of electrode materials. Here, we present a rapid and scalable electrodeposition strategy operated at an unusually high overpotential (−2.5 V) to fabricate amorphous tungsten oxide (a-WO3) with nanocrystallites irregularly stacked. The as-deposited a-WO3 also features abundant oxygen vacancies and surface oxygen-containing functional groups, which serve as additional NH4⁺ adsorption sites and enhance redox activity and ion diffusion kinetics. Benefiting from these synergistic effects, the a-WO3 electrode delivers a high areal capacitance of 2783 mF cm–2, excellent rate capability, and superior cycling stability. When coupled with a polyaniline (PANI) cathode, the resulting AIHS achieves an impressive energy density of 620 μWh cm–2. This work demonstrates a powerful strategy for engineering defect-rich amorphous nanomaterials toward next-generation ammonium-ion storage technologies.