Nanomaterials - 13

Abstract

Ammonium-ion supercapacitors have received significant attention as sustainable energy storage devices due to the high mobility of NH4+ ions in aqueous electrolytes, which is attributed to their smaller hydrated ionic radius. Moreover, compared to conventional metallic charge carriers such as Li+, Na+, or K+, ammonium ions offer safer operation, cost-effectiveness, faster electrode kinetics, and easier recycling. Currently, several host materials have been explored, with a focus on cathode materials such as MnO₂, V₂O₅, and Prussian blue analogues (PBAs) [1,2]. However, anode host materials have been sparsely explored. Developing high-performance anode materials is crucial to fully realize the enormous potential of asymmetric supercapacitors in the real world. In this work, we explored the self-assembled fullerene nanostructures as an innovative carbon host for ammonium-ion storage. Initially, we evaluated the self-assembled fullerene-based hierarchically porous hollow carbon spheres as an anode material for the storage of ammonium ions. We fabricated an asymmetric supercapacitor with δ-MnO₂ as a cathode in an aqueous 1 M (NH₄)₂SO₄ electrolyte [3,4]. Due to their ultra-high specific surface area (1800 m2 g-1) and hierarchical micro/mesoporous structures, the nanoporous hollow carbon spheres are highly favorable for NH₄ ion adsorption. Electrochemical studies revealed that the prepared hollow carbon spheres (HCS) anode achieved an excellent gravimetric capacitance of 125 F g-1 at a current density of 0.5 F g-1, demonstrating that the HCS can be a promising anode material for ammonium-ion storage.

Furthermore, we have extended the concept to one-dimensional fullerene nanorods-derived high-surface-area nanoporous carbon, anticipating enhanced electron transport and ion diffusion. We aim to integrate MnO₂ onto nanoporous carbon nanorods to leverage the high pseudocapacitance of MnO₂, complemented by the conductive properties of the carbon matrix. This structural evolution from spheres to rods is expected to offer a pathway to optimize the charge storage mechanism and advance the development of high-performance anode material for ammonium-ion storage.