Waste-to-MoS₂ binder-free 2H-MoS₂ microflowers for ammonium-ion supercapacitors with operando mechanistic insights

Developing sustainable energy-storage devices is an increasingly urgent challenge, yet the conversion of recycled feedstocks into high-performance, binder-free electrodes operating in environmentally friendly electrolytes remains underexplored. Here, we synthesize MoS₂ nanostructures from Mo-based industrial waste via hydrothermal sulfurization and systematically investigate the effects of precursor-solution pH and hydrothermal temperature on morphology, structure, and electrochemical behaviour. Microscopy and spectroscopy confirm the formation of layered 2H-MoS₂ and show that higher temperature promotes microflower-like architectures and suppresses oxygen-containing residues, consistent with more complete sulfurization. The materials were directly evaluated as binder-free electrodes in 1 M (NH₄)₂SO₄, exhibiting quasi-rectangular voltammograms and surface-dominated charge storage. Increasing the hydrothermal temperature enhances the capacitance by ∼33% at 5 mV s⁻¹. By combining microflower-like MoS₂ with a carbon paper substrate, the specific capacitance reaches 1200 F g⁻¹ at 5 mV s⁻¹, with improved rate capability (300 F g⁻¹ at 10 A g⁻¹). A CR2032 proof-of-concept device assembled with a biochar counter electrode delivers 309 F g⁻¹ at 1 A g⁻¹ and an energy/power density of 52 Wh kg⁻¹ at 550 W kg⁻¹. Operando X-ray absorption spectroscopy and Raman measurements reveal intermediate structural/chemical changes during cycling, providing insight into the interfacial storage mechanism and degradation pathways. Overall, this work demonstrates a viable route to valorise Mo-based waste into high-performance binder-free MoS₂ electrodes for ammonium-ion supercapacitor devices.