Fronts and stationary domains during electrochemical Hz oxidation on Pt: The impact of the position of the reference electrode on the spatiotemporal behaviour

We studied the local potential distribution in front of a rotating Pt ring-electrode during hydrogen oxidation in sulfuric acid for three different positions of the reference electrode. The potential distribution was measured with a potential micro-probe. The experiments were carried out in the bistable region of the system. For large and medium distances between the reference (RE) and the working electrode (WE), transitions between the two homogeneous states occurred through nucleation and growth of the globally stable state from the initially metastable one. The transition time was considerably longer for the intermediate distance between the RE and the WE. For the closest distance used, stationary patterns formed, consisting of two domains with high and low current densities, respectively. The difference in the double layer potential of the two domains amounted to more than 1 V. The low-current density domain was covered with Pt-O whereas in the high current density domain the electrode surface consisted of 'bare' Pt. The existence of these stationary non-equilibrium structures as well as the difference of the front behaviour for the other two electrode arrangements can be traced back to the existence of a negative global coupling for intermediate and close distances between the WE and the RE.