Spinel-structured tetragonal Mn3O4 nanocrystals as promising electrode for aqueous ammonium-ion storage

The development of sustainable and efficient energy storage systems based on abundant and environmentally friendly charge carriers is paramount to achieving global net-zero goals. Ammonium (NH 4 + )-ion-based systems present a promising non-metallic alternative owing to their atomic structure that enhances the kinetics, assisting charge storage. However, the identification of suitable host materials for reversible NH 4 ⁺ storage remains a significant challenge. Herein, we report the use of manganese oxide (Mn 3 O 4 ) as a novel electrode material for aqueous ammonium-ion storage. Tetragonal-shaped Mn 3 O 4 nanoparticles were synthesised directly on carbon cloth (Mn 3 O 4 @CC) using a controlled layer-by-layer assembly method. These electrodes exhibit an excellent specific capacity of 322.8 mAh/g at a current density of 0.5 A/g, with impressive rate capability and 77.7 mAh/g capacity retention over 3000 cycles. The charge storage kinetics analysed using ex-situ characterisations confirm the reversible insertion and extraction mechanism of the NH 4 + -ion in the Mn 3 O 4 structure. DFT calculations reveal the superior electronic conductivity and the interaction of the NH 4 + ion with Mn 3 O 4 , by which the material could achieve a high capacity. Furthermore, an ammonium-ion supercapacitor (AISC) was constructed using the Mn 3 O 4 @CC as the positive and activated carbon (AC) as the negative electrode material. The device delivered a maximum specific energy of 47.9 Wh/kg and a specific power of 8000 W/kg, with excellent cycling stability. This investigation highlights Mn 3 O 4 as a promising material for NH 4 ⁺ ion storage and paves the way for the exploration of other electrode materials synthesised using the layer-by-layer method for next-generation, environmentally friendly energy storage systems.