Reliable unencapsulated Ag/Ag2S micro-reference electrodes for battery research and other applications in organic media

[Display omitted] •Fast, simple, reproducible method to fabricate Ag/Ag2S micro-reference electrode.•Reaction of Ag strip/wire with sulfur vapor at 105 °C.•No encapsulation needed: compact, small footprint.•Compatible with polar solvents and ionic liquids.•Repeatable and stable performance over years.•Optimal morphology corresponds to slightly porous Ag2S film of intermediate thickness. Reliable electrochemical measurements depend on the availability of robust reference electrodes (RE) with well-defined potentials. While many reliable REs are known, they are not applicable in certain demanding media such as ionic liquids, nor in small confined spaces. Here, we describe the fabrication of a simple yet robust Ag/Ag2S micro-reference electrode (μ-RE) where a micron-thick Ag2S layer is formed by isothermal reaction with sulfur vapor. Scanning electron microscopy, X-ray photoemission spectroscopy, and spectroscopic ellipsometry characterization reveals that the optimal morphology corresponds to a slightly porous Ag2S film. We demonstrate that the Ag/Ag2S μ-RE can be operated in and cycled through a wide variety of polar organic solvents, including common protic solvents (EtOH), aprotic solvents (ACN, DMSO, NMP, DMF) and ionic liquids (EMIM-TFSI, BMP-TFSI), with short equilibration time (tens of seconds) and little drift (