Preparation and characterization of palladium-hydride-coated titanium as a reference electrode for the supercritical carbon dioxide emulsion electrochemical system

• The Pd/Ti electrode is prepared by electroplating from a simple aqueous medium.
• Hydrogen sorption/desorption behavior on Pd/Ti electrode in aqueous electrolytes has been investigated.
• The stable PdH potential formed on Pd/Ti can be used as a reference electrode in aqueous and sc-CO2 emulsion media.
• The temperature-dependent coefficient of the PdHx/Ti electrode is similar to that of an Hg/Hg2SO4 electrode.

This work develops a new reference electrode for the electrochemical system using an aqueous supercritical carbon dioxide (sc-CO2) emulsion solutions. Here, the Watts nickel bath is employed to be the standard testing electrolyte. A porous Pd-coated Ti (denoted as Pd/Ti) electrode is prepared by electroplating, which is subjected to the hydrogen sorption (including adsorption and absorption) reaction in 0.5 M H2SO4 for forming a palladium hydride-coated titanium (denoted as PdHx/Ti) electrode. The hydrogen sorption/desorption behavior on Pd/Ti is characterized by cyclic voltammetry and rest potential against time analyses. These results reveal that PdHx/Ti is a reliable reference electrode with a reproducible and stable PdHx potential (longer than 10 h) in normal aqueous electrolytes at room temperature. The equilibrium potential of this PdHx/Ti electrode is about 65 ± 15 mV (vs. RHE) in the Watts Ni bath. Although the hydrogen desorption rate is enhanced by elevating the solution temperature, all of the results confirm that the in-situ formed PdHx/Ti electrode can be practically utilized as a reference electrode in the Watts Ni bath containing the sc-CO2 emulsion medium.