In-Situ Analysis of Scan Rate Effect on Pt Dissolution Under Potential Cycling

Introduction

Degradation of a polymer electrolyte fuel cell (PEFC) caused by platinum (Pt) dissolution on the cathode remains a challenge for its durability in long-term applications. The cathode is repeatedly exposed to high potentials during ON-OFF cycles, which accelerates Pt dissolution. Sugawara et al.^[1] reported that Pt dissolution was affected by the scan rate at which potential cycling was performed. The ex-situ analysis by ICP-MS^[1]provided an average evaluation of the dissolution amount during one cycle or a given time period. It would be more helpful for specification of the dissolution mechanism in different potential regions, if the dissolution rate could be traced simultaneously with the dissolved species being identified. For this purpose, we employed a channel flow double electrode as an in-situ monitoring method, and investigated the dissolution behavior on a Pt electrode that underwent potential cycling at different scan rates.

Experimental

A channel flow double electrode (CFDE) consisted of a working electrode (WE) and a collector electrode (CE). The collection number (N) of the channel flow cell was 0.3, given the geometries shown in Figure 1.^[2] In a prior work,^[3] we proved that the submonolayer-level Pt dissolution on the WE under potential cycling could be detected on the CE. The current responses on the CE (collector current, I_C) could be used as a detection parameter, from which the dissolution rate on the WE can be calculated.

In this work, the WE was a thin film of Pt electroplated on a gold (Au) substrate. The solution was 0.5 M H₂SO₄ deaerated by argon gas. The average flow rate of the solution was 10 cm/s. The WE underwent potential cycling between 0.07 and 1.4 V (vs. SHE) at 1, 5, 20, 100, and 200 mV/s. A CE of Au was set at 0.3 V at which both Pt²⁺ and Pt⁴⁺ were reduced to Pt. Dissolution of Pt⁴⁺ was detected on a CE set at 0.7 V by reducing Pt⁴⁺ to Pt²⁺. All measurements were performed on a multi-channel potentiostat (PS-08, TOHO Technical Research, Japan) at room temperature.

Results and discussion

During a slow anodic scan (5 mV/s), both Pt²⁺ dissolution below 1.0 V and Pt⁴⁺ dissolution above 1.2 V are suppressed.  This decline of Pt dissolution is due to surface passivation by Pt oxide, similar to that in potentiostatic conditions.^[4] This result also suggests that surface passivation of Pt requires an excessive polarization such as a slow anodic scan. Interestingly, dissolution of Pt⁴⁺ is preponed as we slower the scan rate. Figure 2 shows the CVs of the WE (at 1 mV/s and 20 mV/s) and the I_C recorded during an anodic scan on a CE set at 0.7 V. When the WE is scanned at 1 mV/s, the I_C, representing detection of Pt⁴⁺, starts to increase at a lower potential. This shift of on-set potential of Pt⁴⁺ dissolution is only observed during a slow anodic scan. During a fast anodic scan (200 mV/s), the Pt⁴⁺ dissolution still starts around 1.2 V, though the dissolution rate tends to decrease. Thus, the place exchange between the O and Pt atoms^[5] seems necessary for Pt⁴⁺dissolution above 1.2 V.

However, the cathodic dissolution of Pt²⁺ from reduction of PtO₂ is enhanced during a fast cathodic scan. The reduction rate of PtO₂ is proportional to scan rate. As a result, more Pt²⁺ is produced when the PtO₂ is being reduced at a faster cathodic potential shift. Therefore, in an operating PEFC, where the potential sweep on the cathode may be faster than 200 mV/s, cathodic dissolution is expected to be more severe. Interestingly, the ratio between cathodic dissolution of Pt²⁺ and reduction of PtO₂in a cathodic scan decreases as we increase the scan rate.

Reference

1. Y. Sugawara, A.P. Yadav, A. Nishikata, T. Tsuru, ECS Trans., 16 (24), 117 (2009).
2. H. Matsuda, J. Electroanal. Chem., 16, 153 (1968).
3. Z. Wang, E. Tada, and A. Nishikata, J. Electrochem. Soc., 161(4), F380 (2014).
4. X.P. Wang, R. Kumar, D.J. Myers, Electrochem. Solid-State Lett., 9, A225 (2006).
5. G. Jerkiewicz, G. Vatankhah, J. Lessard, M.P. Soriaga, Y.S. Park, Electrochim. Acta., 49, 1451 (2004).