Reversible Hydrogen and Pd Hydride Reference Electrodes with Electrochemically Supplied H₂ for High Temperature and Pressure Electrochemistry

A reversible hydrogen electrode (RHE) and a Palladium hydride (PdH) reference electrode are proposed with a convenient design for use at high temperatures and pressures (HTP). A constant electrochemically driven supply of H₂ on the RHE or PdH surface ensures preservation of the H₂ equilibrium potential and mixed (α + β)-phase in the PdH layer, respectively, thereby ensuring stable potentials at HTP conditions for prolonged periods of time. Furthermore, the potential of the PdH reference electrode is calibrated as a function of temperature and pressure with respect to the RHE. In contrast to earlier calibration attempts, we find that the potential of the PdH electrode in the mixed (α + β)-phase is independent of temperature and pressure, with a value of ∼55 mV vs the RHE at 25 °C, 1 bar, 45 wt% KOH. When the PdH is charged with enough hydrogen to assume the H-rich β-phase only, its potential follows that of the RHE, but shows a sluggish equilibration upon changing pressure or temperature, due to the slow diffusion of H in β-PdH.