Energetics of a Zinc−Sulfur Fuel Cell

Energetics of a novel zinc−sulfur charge storage (generalized as Zn + S → ZnS) is explored to access the high (>1000 Ah/kg) charge capacity of sulfur. At 25 °C, the theoretical energy density of the complete Zn/S system is a high 572 Wh/kg, at E° = 1.04 V. From 273 to 373 K, the thermodynamic cell potential for 29 possible solution-phase reactions of the system were calculated including a variety of known coexisting aqueous polysulfide species. In this domain zinc sulfide products are thermodynamically preferred over zincate products, which can be advantageous compared to Al/S storage where aluminate is the favored discharge product. Initial experimental feasibility of a zinc−sulfur electrochemical system is explored in a zinc anode/aqueous polysulfide/CoS electrocatalytic cathode cell. Measured sustained cathode capacity approaches the theoretical 2 Faraday/sulfur limit. However, the anode tends to passivate due to the zinc sulfide discharge product, preventing sustained discharge. Passivation is overcome in potassium polysulfide solutions containing concentrated (>10 molal) KOH; this results in efficient, sustained 2 Faraday/Zn oxidation and high zinc−sulfur charge capacities.