Platinum as a non-perturbing reference electrode for half-cell diagnostics of instabilities in electrolysis for hydrogen production

• Oxygen reduction occurs at lower voltages than water electrolysis • Pt is stable and non-contaminating in pure water and non-chlorine-based electrolytes • Pt is unstable at cell voltages between ORR and HER in chlorine-based electrolytes • Pt acts as a diagnostic probe for sensing ion motion in the electrolyte To measure half-cell potentials, a reference electrode is required to establish the electrolyte potential. We show that a pseudo-reference electrode can be used to measure the electrolyte potential without perturbation provided that any charging of the electrode surface is stable so that calibrations can be made. We investigate experimentally the suitability of platinum (Pt) as a reference electrode. An electrolyzer was operated with Pt active electrodes in seven different electrolytes (pure water, 1M KOH, 1M NaOH, 1M H 2 SO 4 , 1M HCl, 1M NaCl, and 1M KCl). The Pt reference electrode was compared against a set of standard reference electrodes and found to act as the only reliable reference electrode in pure water. In all other electrolytes, the Pt reference electrode charged up when intercepting moving ions. The ion species and the characteristics of their motion is sensed by the Pt reference electrode, which diagnoses when reactions commence at the active electrodes and when they become rate-limiting. In all electrolytes, we observe a low voltage regime below an overall voltage of 0.40V where the anode has a larger half-cell potential than the cathode. Above the low voltage regime but below the minimum expected threshold for electrolysis, we observe an onset that we attribute to oxygen reduction reaction at the cathode. We demonstrate that a Luggin capillary shielding mitigates the charging and enables investigation of ohmic spatial dependence of potential in the electrolyte and double layer time dependence.