Reversible Hydrogen and Pd Hydride Reference Electrodes with Electrochemically Supplied H-2 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 H-2 on the RHE or PdH surface ensures preservation of the H-2 equilibrium potential and mixed (alpha + beta)-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 (alpha + beta)-phase is independent of temperature and pressure, with a value of similar to 55 mV vs the RHE at 25 degrees C, 1 bar, 45 wt% KOH. When the PdH is charged with enough hydrogen to assume the H-rich beta-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 beta-PdH.

Automated summary

A reversible hydrogen electrode and a Palladium hydride reference electrode are proposed for stable potential measurements under high temperature and pressure conditions. The potential of the PdH electrode in the mixed (alpha + beta)-phase is found to be independent of temperature and pressure, offering the potential for long-term stable potential measurements.