The potential variation of Pt counter electrode (CE) in a three-electrode configuration is monitored as the potential of Pt working electrode (E WE ) follows a triangleshaped program in 0.5 M aqueous H 2 SO 4 . The spontaneously adopted CE potential (E CE ) is reported for different values of the ratio of geometric surface areas of WE and CE (A geom,WE /A geom,CE ). The E CE versus time (t) transients are non-linear and resemble charging/discharging curves. In the case of A geom,WE > A geom,CE , E CE adopts higher values than E WE , and vice versa. In the case of A geom,WE /A geom,CE = 10:1, the values of E CE are 0.3 -0.4 V higher than the highest values of E WE . The high E CE values give rise to the development of a thick surface oxide that undergoes subsequent dissolution. A novel three-compartment electrochemical cell is employed to examine simultaneously the dissolution of WE and CE, andto monitor their potentials; the amount of dissolved Pt is quantitatively analyzed using inductively coupled plasma mass spectrometry. The magnitude of the A geom,WE /A geom,CE ratio has a significant impact on the CE oxidation and dissolution. The oxidation and dissolution of CE depend on the lower potential limit of WE; the amount of surface oxide and the quantity of dissolved Pt significantly increase as the WE potential limit is reduced from 0.50 V to 0.05 V because CE adopts a high potential. The presence of dissolved O 2 also affects the dissolution of CE but to a lesser extent than the A geom,WE /A geom,CE ratio or the lower potential limit of WE. Field emission scanning electron microscopy analysis of the CE morphology following prolonged potential cycling in the presence of dissolved O 2 reveals a thick surface oxide that has dry mudlike structure. The slightly higher dissolution of CE under these conditions is attributed to physical detachment of some of the cracked surface oxide. This research advances the understanding of Pt dissolution with some of the new knowledge being applicable to fuel cells.

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