Platinum Dissolution from Carbon Supported Nanoparticles

Platinum (Pt) dissolution concerns the instability of Pt-based cathode catalysts for polymer electrolyte fuel cells (PEFCs). Studies on unsupported Pt, polycrystalline plates or electroplated films, suggest that Pt dissolution relates to Pt oxide formation and reduction. Pt dissolution from the carbon supported nanoparticles, however, is rarely discussed. In this study, we investigated Pt dissolution using commercial carbon supported nanoparticles (Pt/C). We prepared Nafion-free inks by mixing Pt/C samples with H₂O and isopropyl alcohol (IPA). We evaluated the amount of Pt dissolution over potential cycling tests using inductively-coupled plasma-mass spectrometry (ICP-MS). We also scrutinized the Pt dissolution behaviors from the Pt/C in a single cycle using a channel flow double electrode (CFDE)¹.

The Pt/C samples we used were produced by Tanaka Kikinzoku Kogyo (TKK); the Pt particle size distributions before test were characterized by TEM. For each Pt/C sample, we mixed about 10 ~ 15 mg Pt/C with 3.8 ml H₂O and 1.2 ml IPA, and sonicated the mixture in an ice bath for 60 mins. We loaded an appropriate amount of ink on a glassy carbon (GC) substrate. The Pt/C electrode was then dried under an ambient atmosphere for 120 mins. The Pt/C electrode was cycled 30 or more times between 0.05 and 1.4 V (vs. SHE) in 0.5 M H₂SO₄ to remove organic adsorbents and to obtain a reproducible electrochemical surface area (ECSA). The ECSA was used for normalization of Pt dissolution.

First, we conducted 300 cycles of potentials cycling in 0.5 M H₂SO₄ in a stationary cell. The geometric surface area of the GC was ca. 0.25 cm² and we loaded about 2 µl ink on it. Two upper potential limits (E_U) of 1.0 and 1.4 V were used with a same lower potential limit (E_L) of 0.4 V. The testing solutions were analyzed by ICP-MS.

Second, we tested Pt dissolution from the Pt/Cs using a CFDE in argon-deaerated 0.5 M H₂SO₄. The CFDE consisted of a GC working electrode (WE) and a gold (Au) collector electrode (CE). The geometric surface area of the GC was ca. 0.05 cm² and we loaded about 0.6 µl ink on it. The CV of a 46wt% Pt/C obtained in the CFDE at 25^oC shows that the Pt/C is relatively stable without Nafion. Pt dissolution from the Pt/C-WE was monitored by the CE. The CE collected 30% of dissolved Ptⁿ⁺. We set the CE at 1.4 V for detecting Pt²⁺, and at 0.7 V for Pt⁴⁺. We also set the CE at 0.3 V for collection of both Pt species.

The ICP-MS data evaluate the overall amount of Pt dissolution and CFDE data provide details of the dissolution behaviors against potential. Using both data, we discuss the mechanism of Pt dissolutions from the Pt/Cs with respect to temperature, E_U, and Pt particle size.

Reference

1. Z. Wang, E. Tada, and A. Nishikata, J. Electrochem. Soc., 161 (4), F380 (2014).