A capsule of chemicals was kept on Batman's Utility Belt in his early career. The contents were capable of mixing with mustard gas and rendering the agent harmless. Batman made use of the vial to escape a trap set up by Sheldon Lenox in Sin Fang's Shop. Detective Comics #35: "The Case of the Ruby Idol" Though the capsule's contents are never specified, it's possible the agent was hydrogen peroxide.
Metabolites, Free Full-Text
Batman: Arkham Knight #12 - Neutralizing Agent [PC]
Chemical weapon - Wikipedia
Scarecrow's Fear Toxin, DC Database
Condiment King (Post-Crisis), VS Battles Wiki
Mustard Gas Neutralizing Agent, Batman Wiki
Ace Chemical Processing Plant, Batman Wiki
Scarecrow's Fear Toxin, DC Database
Mustard Gas Neutralizing Agent, Batman Wiki