Energy efficient, one-step microwave-solvothermal synthesis of a highly electro-catalytic thiospinel NiCo2S4/graphene nanohybrid as a novel sustainable counter electrode material for Pt-free dye-sensitized solar cells

Abstract

A sustainable rapid microwave-solvothermal (MW-ST) synthesis approach has been successfully demonstrated to develop thiospinel NiCo₂S₄ nanocrystals and their nanohybrids with graphene nanosheets (GNS) using transition metal-ions (Co²⁺, Ni²⁺) and thiourea as a sulfur precursor in the presence of graphite oxide (GO). The MW-ST method enables the nucleation and growth of cubic linnaeite-type thiospinel NiCo₂S₄ nanocrystals and simultaneous in situ ultra-fast polyol-reduction of GO to GNS and its hybridization within 15 min at 200 °C using ethylene glycol (EG) as the solvent. This process absolutely evades the use of toxic reducing agents or post solid-state sintering at elevated temperatures and subsequent sulphurization using toxic H₂S/Na₂S gases. The nanocrystallite aggregates (NCS-1)/GNS nanohybrid exhibited a remarkable electro-catalytic activity towards triiodide (I⁻/I₃⁻) reduction owing to improved electronic conductivity and synergistic effects at NCS-1 and GNS interfaces. The developed NCS-1/GNS nanohybrid as a novel counter electrode (CE) for Pt-free dye sensitized solar cells (DSSCs) demonstrated a high power conversion efficiency (PCE) of 7.98% in comparison to the conventional Pt CE of 8.01% under the same conditions. Hence, this work presents a scalable synthesis of an earth-abundant, thiospinel NiCo₂S₄/GNS nanohybrid CE via a facile single-step MW-ST process as a potential alternative to expensive Pt as a CE in DSSCs and other electrochemical clean energy systems.