Oxygen Reduction at the Water|Oil|Electrode Interface Drives Tunable Transition Metal Hydroxide Electroprecipitation

Water microdroplets demonstrate unique and surprising chemical reactivity, representing a new frontier in physical chemistry. Multiphase microdroplet systems, where an aqueous microdroplet exists surrounded by a nonaqueous phase, are distinct in that the solubility of dioxygen (O₂) is generally much higher in the nonaqueous phase, allowing it to act as an almost endless supplier of O₂ to the aqueous phase. Nevertheless, this solubility difference is often ignored. Here, a sessile aqueous droplet containing fluorescent pH indicators or metal salts is placed on an electrode surrounded by 1,2-dichloroethane. By sufficiently biasing the electrode, the O₂ reduction reaction is driven, producing immediate pH gradients near the three-phase boundary, which are visualized in real-time via fluorescence microscopy and used to selectively drive transition metal electroprecipitation at the three-phase boundary. This work presents evidence of significant (and useful) pH gradients at three-phase boundaries and has implications across microdroplet reactivity and nanomaterial electrodeposition.