Aqueous-S vs Organic-S Battery: Volmer-Step Involved Sulfur Reaction

Abstract

Aqueous-S batteries (ASBs) are emerging as promising energy storage technologies due to their high safety, low cost, and high theoretical energy density. However, the present understanding of sulfur evolution in water relies on experience derived from conventional organic electrolyte-based sulfur batteries (OSBs). The gap between ASB and OSB has impeded progress in advancing the rational design of sulfur catalysts in the aqueous phase. Herein, we reveal the unique interaction between H₂O and S species, which is fundamentally distinguishable from the organic counterparts. A series of spectroscopy analyses discloses that elemental sulfur is initially reduced to polysulfides (mainly S₄^2–), which subsequently react with H₂O to generate HS^–, involving both polysulfide conversion and the Volmer step of water dissociation. Combined electrochemical and computational analysis further proposes an aqueous-S catalyst selection metric based on simultaneous polysulfide adsorption and Volmer-step catalysis. As a proof of concept, we have successfully prioritized the Mo₂C-catalyzed ASBs with a superior rate capability of 1040 mAh g^–1 than the Fe₃C (693 mAh g^–1) and pure C (510 mAh g^–1) at a high current density of 5 A g^–1. This work provides insights into the aqueous-S charge storage mechanism and establishes a foundational catalyst research paradigm for advancing the following ASBs.