Platinum Electro-dissolution in Acidic Media upon Potential Cycling

Abstract

Electro-dissolution of Pt(poly) electrodes is examined under potential cycling conditions in relation to the applied lower and upper potential limits (EL, EU), the potential cycling range (ΔE), the number of potential cycles (n), the exposure time (texposure), and the electrolyte temperature (T); the amount of electro-dissolved Pt (mPt) present in the electrolyte is analyzed using inductively coupled plasma mass spectrometry. Monitoring the potential of working and counter electrodes (EWE, ECE) reveals that in many instances ECE is higher than EWE, indicating that a surface oxide also develops on CE and a considerable amount of electro-dissolved Pt originates from CE. Thus, in the case of research employing a two-compartment cell, the amount of electro-dissolved Pt corresponds to the species originating from both WE and CE. The application of a three-compartment cell, with a Nafion membrane used to separate the WE and CE compartments, allows the quantification of electro-dissolved Pt originating only from WE or CE. The amount of electro-dissolved Pt is much greater in the two-compartment cell than in the three-compartment one, when ECE covers a broad potential range that includes the regions of Pt oxide formation and reduction; this provides clear evidence that CE makes a major contribution to total amount of electro-dissolved Pt. The value of mPt depends on EL and EU that define ΔE. In the case of 0.10 ≤ ΔE ≤ 0.20 V, there is no electro-dissolution of Pt; in the case of ΔE = 0.30, there is slight electro-dissolution of Pt; and in the case of 0.40 ≤ ΔE ≤ 0.70 V, there is significant electro-dissolution of Pt. The value of mPt increases with the magnitude of ΔE. The greatest value of mPt is observed when ΔE = 0.50 V and when ΔE covers the potential range of Pt oxide formation and reduction, and the region of interfacial place exchange (1.10 ≤ E ≤ 1.20 V). Temperature variation has a slight impact on the electro-dissolution of Pt and for given ΔE and n the increase in T slightly decreases mPt. An analysis of the impact of s on the electro-dissolution of Pt reveals that the process is only slightly greater at s = 25 mV s–1 than at s = 50, 100, 200, or 500 mV s–1. For a given texposure, the value of mPt is greater for s = 500 mV s–1 than for lower values of s because a high value of s translates into a larger number of oxide formation-reduction events. The quantity of electro-dissolved Pt within 5,000 potential cycles varies from 0.12 to 4.91 monolayers (MLs) of Pt, depending on EL, EU, and ΔE. Eleven reactions can be used to explain anodic and cathodic electro-dissolution as well as chemical dissolution of Pt. Yet, it is impossible to explain the cathodic dissolution of Pt without making an arbitrary assumption that anodic polarization of Pt in the 0.85 ≤ E ≤ 1.40 V range generates PtO2, instead of PtO as reported in earlier literature.