Comprehension of the impact of electrolyte nature and concentration on Pt degradation is essential for the improvement of durability of catalyst layers (CLs), which are the heart of polymer electrolyte membrane fuel cells (PEMFCs). Electrochemical and chemical dissolution of polycrystalline Pt in aqueous CF 3 SO 3 H, H 2 SO 4 , and HClO 4 solutions of different concentrations (c = 0.1 and 0.5 M) upon potential switching and holding in the 0.60−1.20 V versus RHE range is analyzed using inductively coupled plasma mass spectroscopy. This potential range mimics the conditions encountered in operating PEMFCs. Trifluoromethanesulfonic acid (CF 3 SO 3 H) is employed because it is the smallest fluorinated sulfonic acid and can serve as a model molecule. Degradation of Pt in H 2 SO 4 and HClO 4 solutions is examined for comparative analysis. The results reveal that the electrolyte concentration has a significant impact on Pt electrochemical and chemical dissolution. The amount of dissolved Pt in 0.1 M solutions of CF 3 SO 3 H, H 2 SO 4 , and HClO 4 is practically the same and lower than that in analogous 0.5 M solutions. However, the amount of dissolved Pt in 0.5 M H 2 SO 4 solution is greater than that in 0.5 M solutions of CF 3 SO 3 H or HClO 4 . The influence of anion nature and pH on Pt dissolution is examined in 0.1 and 0.5 M HClO 4 solutions without and with 1.0 × 10 −2 M H 2 SO 4 addition. The results show that under these conditions the anion nature has no or negligible impact on Pt dissolution, but pH significantly affects the process. An analysis of potential versus pH diagrams (Pourbaix diagrams) for acid solutions of different pH values suggests that Pt degradation (with the formation of Pt 2+ (aq) and Pt 4+ (aq)) might proceed through both electrochemical and chemical pathways.

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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We report on the synthesis, characterization, and degradation behavior of spherical platinum nanoparticles (Pt-NPs). The Pt-NPs were synthesized with and without carbon-support using the "water-in-oil" microemulsion method. X-ray diffraction (XRD) was used to examine their average crystallite size, which was ca. 4.0 nm. The shape, size, and size distribution of the Pt-NPs were evaluated using transmission electron microscopy (TEM); the average size was ca. 4.0 nm, thus in agreement with the XRD data. The agreement between the XRD and TEM data indicates that the Pt-NPs were single crystallites in nature. Thermogravimetric analysis (TGA) measurements were carried out to evaluate the metal loading, which was close to the target value of 40 wt %. Cyclic voltammetry (CV) experiments were performed in 0.50 M aqueous H 2 SO 4 in the s = 1.00−50.0 mV s −1 potential scan rate to determine the specific surface area (A s ) of the catalysts and to assess the cleanliness of the system. The Pt surface oxide growth and reduction were successfully examined using in situ confocal Raman spectroscopy. The results allow monitoring the appearance and disappearance of crystallinity in the surface oxide layer. The stability of the catalyst was evaluated by recording 500 CV profiles in 0.50 M aqueous H 2 SO 4 solution in the 0.05 V ≤ E ≤ 1.55 V range at s = 50.0 mV s −1 . The corrosion behavior of Pt-NPs was studied using potentiodynamic polarization (PDP) measurements at s = 0.10 mV s −1 in the presence of different gaseous environments (N 2 (g), O 2 (g), or H 2 (g)). The nature of the dissolved gas has a profound impact on the stability/ corrosion behavior of the Pt-NPs. The Pt nanocatalysts are stable in the electrolyte saturated with H 2 (g), undergo slight corrosion in the electrolyte saturated with N 2 (g), and undergo significant corrosion in the electrolyte saturated with O 2 (g). The carbon support also undergoes corrosion and porosity changes. The corrosive degradation of the Pt-NPs and carbon support is pronounced the most in the case of the anodic PDP. Cyclic voltammetry measurements were employed to determine the loss of the electrochemically active surface area (A ecsa ) of the Pt-NPs prior to and after PDP measurements; the results correlate with the corrosion rates. The new and original results on the characterization and corrosive degradation of the Pt-NPs represent an important contribution that will benefit fuel cell science and technology.

Platinum in the form of nanoparticles is the key and most expensive component of polymer electrolyte membrane fuel cells, while trifluoromethanesulfonic acid (CF3SO3H) is the smallest fluorinated sulfonic acid. Nafion, which acts as both electrolyte and separator in fuel cells, contains -CF2SO3H groups. Consequently, research on the electrochemical behaviour of Pt in aqueous CF3SO3H solutions creates important background knowledge that can benefit fuel cell development. In this contribution, Pt electro-oxidation is studied in 0.1 M aqueous CF3SO3H as a function of the polarization potential (E(p), 1.10 ≤ E(p) ≤ 1.50 V), polarization time (t(p), 10(0) ≤ t(p) ≤ 10(4) s), and temperature (T, 278 ≤ T ≤ 333 K). The critical thicknesses (X1), which determines the applicability of oxide growth theories, is determined and related to the oxide thickness (d(ox)). Because X1 > d(ox) for the entire range of E(p), t(p), and T values, the formation of Pt surface oxide follows the interfacial place-exchange or the metal cation escape mechanism. The mechanism of Pt electro-oxidation is revised and expanded by taking into account possible interactions of cations, anions, and water molecules with Pt. A modified kinetic equation for the interfacial place exchange is proposed. The application of the interfacial place-exchange and metal cation escape mechanisms leads to an estimation of the Pt(δ+)-O(δ-) surface dipole (μ(PtO)), and the potential drop (V(ox)) and electric field (E(ox)) within the oxide. The Pt-anion interactions affect the oxidation kinetics by indirectly influencing the electric field within the double layer and the surface oxide.

The ion conductivity of a solid-state material is primarily a function of its chemical composition and crystal structure. However, interfaces can play an important role in the conduction process. The effects of interfaces on ionic properties can be controlled on the basis of "nanoionics". In this study, we demonstrate a nanoionics phenomenon observed in a blue platinum-containing perovskite. The proton conductivity changed reversibly in response to the precipitation of platinum nanoparticles and oxidation to form a solid solution. The results of XAFS measurements, TEM analysis, and electron holography provide evidence of the nanoionics phenomenon, and suggest a possible underlying mechanism for the conductivity change caused by the small amount of precipitated platinum nanoparticles in the perovskite oxide.

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