Boosting the energy storage performance of aqueous NH₄⁺ symmetric supercapacitor based on the nanostructured molybdenum disulfide nanosheets

Nanostructured molybdenum disulfide (MoS₂-2H phase) is a well-known metal dichalcogenide and promising material for electrochemical energy storage due to its unique properties, including structural stability, tuneable bandgap, and dangling-bond free basal surface. However, electrochemical devices based on MoS₂ suffer from a short lifespan, low power density, and safety. Herein, we proposed 2H-MoS₂ nanosheets as a capacitive energy storage material for aqueous ammonium-ion supercapacitors (AASCs) to address the above-mentioned issues. MoS₂ nanosheets were grown on carbon cloth (MoS₂@CC) with a wet-chemical method as binder-free capacitive electrodes, and a symmetric supercapacitor (SSC) was assembled to evaluate its energy storage performance. According to our simulation results with density functional theory (DFT), the ammonium sulfate (NH₄)₂SO₄ aqueous solution was selected as the ammonium-ion electrolyte. The MoS₂@CC electrode exhibits outstanding NH₄⁺ ion storage by achieving a high capacitance of 1,010 Fg⁻¹ at 1 Ag⁻¹ and ultra-high rate performance (60% at 50-folded high current density) with 98% capacitance retention after 10,000 cycles. The pseudocapacitive charge storage process governs the NH₄⁺ intercalation/deintercalation mechanism through the bounding/breaking of H with S atoms. A symmetric AASC was fabricated using the MoS₂@CC and an aqueous (NH₄)₂SO₄ electrolyte to assess the practical performance. The assembled AASC can operate at a wide potential window (0.0–1.5 V) and demonstrates high charge storage performance (237 Fg⁻¹ at 1 Ag⁻¹ and 133 Fg⁻¹ at 50 Ag⁻¹) with extremely stable rate and cycling performance of ∼ 99% up to 50,000 cycles. This study would give an effective and a succinct technique for creating metal dichalcogenide with porous structure for advanced NH₄⁺ storage.