Microporous carbon derived from pistachio shells as sulfur trap for room-temperature sodium–sulfur batteries

Abstract

The valorisation of agro-industrial residues as precursors for functional materials represents a sustainable and cost-effective approach for the development of advanced energy storage technologies. In this work, pistachio shells were used as a renewable biomass source to synthesise a hierarchical microporous carbon (denoted as PWC), which was activated using KOH as the porosity promoter. Textural characterisation revealed dual microporosity, with the coexistence of ultramicropores (<0.7 nm) and supermicropores (0.7–2 nm), thereby providing both a large accessible surface area and enhanced diffusion channels. This hierarchical microporous structure, combined with a homogeneous distribution of oxygen- and nitrogen-based surface functionalities, enabled efficient encapsulation of small sulfur molecules (S2-S4), while restricting the presence of large S8 molecules. When applied as a sulfur host in room-temperature sodium–sulfur (RT/Na-S) batteries, the PWC@S composite exhibited outstanding electrochemical performance, delivering specific capacities of up to 803 mAh·g−1 at a 1 C rate, with high coulombic efficiency and long-term cycling stability. These improvements were attributed to the effective suppression of the conversion of cyclo-S8 into soluble polysulfides, thereby mitigating the shuttle effect. Overall, this study demonstrates that pistachio shells are a suitable renewable feedstock for producing high-performance activated carbons with dual microporosity (ultra- and supermicropores), which is a key factor governing the electrochemical behaviour of RT/Na-S electrodes.