simple-Stir Heterolayered MoS2/Graphene Nanosheets for Zn-Air Batteries

A facile, one-pot, top-down, protein-assisted, simply-stir method to produce aqueous suspensions of MoS2/graphene hybrid (MoS2/Gr) nanoplatelets of up to 14 mg mL-1 for applications in sustainable, next-generation batteries is reported here. Bovine serum albumin (BSA) served as the exfoliant to rapidly produce high concentrations of the hybrid by simply stirring on a desk top stirrer without any specialized equipment. The reactor is amenable for continuous flow, allowing for scale-up to produce large quantities, 24/7, with minimal human intervention. Raman spectroscopy, X-ray diffraction, ultraviolet-visible spectroscopy, and energy-dispersive X-ray spectroscopy indicated the coexistence of both MoS2 and graphene nanosheets in a majority of flakes. The Raman spectra indicated low defect, 3-4 layer graphene sheets with very low oxidation levels, as well as 4-8 layer MoS2 flakes. The exfoliation mechanism was studied as a function of the concentrations of bulk MoS2, graphite, protein, as well as the stirring speed, pH, and temperature. The reaction orders with respect to these parameters were determined. Temperature dependence data indicated a thermal energy barrier of 19.1 kJ/mol, but the rpm dependence gave an extremely low shear energy barrier of 22.2 nJ/mol. One unique insight gained here is that the lateral destacking of the platelets via translational excitation induced by stirring populates higher translational states in the direction along the solvent flow. Thus, stirring is very efficient in destacking the platelets at a low energy cost, a mechanism for efficient exfoliation of two-dimensional materials. The cyclic voltammograms of the MoS2/Gr hybrid with a 1:13 mole ratio showed enhanced oxygen reduction activities, nearly double that of Gr nanosheets or ∼25-fold that of MoS2 nanosheets, and vastly higher than those of both the bulk phases. The hybrid with a 1:13 mole ratio was used to fabricate flexible, functional, and bioabsorbable/biodegradable Zn-air batteries with high open circuit voltages (1.4 V) and high specific energy of up to 130 W h kg-1, establishing practical applications of the hybrids over the individual nanosheets