Pt Degradation Mechanism in Concentrated Sulfuric Acid Studied Using Rotating Ring−Disk Electrode and Electrochemical Quartz Crystal Microbalance

In this paper, we describe our electrochemical investigation of platinum degradation in concentrated sulfuric acid. The Pt electrode weight is found to decrease by an alternating potential cycle between <−0.5 V vs Ag 2 SO 4 and >+0.5 V vs Ag/Ag 2 SO 4 . No marked weight decrease occurs by excluding the former or latter region. Also, electrochemical reactions that participate in Pt dissolution and occur between the two potential regions were studied by a rotating ring−disk electrode (RRDE) method. The obtained voltammogram indicates (i) the electrodeposition of sulfur at <−0.5 V vs Ag/Ag 2 SO 4, (ii) the dissolution of sulfur at (+0.6)−(+0.8) V vs Ag/Ag 2 SO 4, (iii) the formation of a precursor to Pt dissolution at (+0.9)−(+1.2) V vs Ag/Ag 2 SO 4, and (iv) Pt dissolution at +0.5 V vs Ag/Ag 2 SO 4 . Then, (v) the dissolved Pt is reduced at −0.2 V vs Ag/Ag 2 SO 4 . Further investigation was conducted to elucidate the relationships between each reaction by employing an electrochemical quartz crystal microbalance (EQCM). Consequently, the amount of sulfur dissolution at (+0.6)−(+0.8) V vs Ag/Ag 2 SO 4 in step ii affects the amount of subsequent precursor formed that causes Pt dissolution in step iii. A large amount of precursor formed results in a large amount of Pt dissolving at around +0.5 V vs Ag/Ag 2 SO 4 in step iv. These results strongly indicate that the deposition and dissolution of sulfur affect the amount of precursor formed that causes Pt dissolution. After a 30 h potential sweep, which ensured the corrosion of Pt, no dissolved Pt was found in the electrolytic solution, but Pt deposited on the electrode as metallic Pt in particle form. These phenomena imply that the dissolved Pt in step iv then deposits in the neighborhood of the Pt surface in step v. The dependence of Pt degradation on the electrode potential was clearly explained by the results of RRDE and EQCM measurements.