No abstract

Reducing the cathode degradation in proton exchange membrane fuel cells during start-up and shut-down events (where the anode is filled with H 2 and air) is crucial for its widespread automotive implementation. The use of selective catalysts for the hydrogen oxidation reaction (HOR) that sparingly reduce oxygen on the anode could significantly reduce the carbon corrosion on the cathode. Herein, we report a novel system of carbon supported Pt/TiO x catalysts that combines the unique properties of a strong metal−support interaction (SMSI) with the known advantages of a carbon support. Highresolution transmission electron microscopy of the selective catalyst shows the encapsulation of the Pt nanoparticles (NPs) by a TiO x layer resulting from the SMSI. Rotating disk electrode experiments confirmed that Pt oxidation and oxygen reduction are hindered due to the TiO x layer. Furthermore, a high HOR activity that is retained even at high potentials proved the superior HOR selectivity of the catalyst.

Compared to Pt/C, the HOR activity of Pt-Ru alloys in alkaline electrolyte is exceptionally high. Nevertheless, it remains unknown whether this enhancement is due to a bifunctional mechanism involving Pt and Ru as active sites or an electronic effect of Ru on Pt. In this study, we distinguish between those fundamental differences using Ru@Pt core-shell nanoparticles as a model system. Ru@Pt catalysts were prepared from submonolayer to multilayer Pt coverage. The exposure of Ru on the surface of the catalyst was analyzed by cyclic voltammetry, showing that Ru is solely exposed on the surface of Ru@Pt particles with low Pt-coverage. The thickness of the Pt-shell was characterized by CO stripping in H 2 SO 4 , allowing to distinguish between single and bilayered Ru@Pt catalysts. Determining the HOR/HER activity of these catalysts in 0.1 M NaOH revealed that fully Pt-covered Ru is more active than partially covered Ru@Pt nanoparticles. Hence, the participation of Ru as active site in a bifunctional mechanism is of minor importance with respect to the HOR/HER activity compared to its influence on the electronic structure of Pt. Similar to Pt-Ru alloys, the most active Ru@Pt core-shell nanoparticles show a 4 to 5-fold enhancement of the surface-normalized HOR/HER activity compared to Pt/C.

The hydrogen oxidation and evolution reaction (HOR/HER) has been widely investigated due to its importance for a broad variety of applications especially in electrolysis and fuel cells. However, owing to its extremely fast kinetics, kinetic data can only be obtained with experimental setups that provide very fast mass transport, so that the effect of hydrogen partial pressure (pH2) and kinetic limitations at high overpotentials are not yet fully understood. Here we report detailed kinetic investigations on the temperature and pressure dependence of the HOR/HER kinetics on carbon supported platinum (Pt/C) using the PEMFC-based H2-pump approach. By using ultra-low platinum loadings, we could show that the apparent activation energy of the HOR/HER on platinum increases with increasing pH2, due to a diminishing effect of the hydrogen adsorption enthalpy with increasing coverage by adsorbed hydrogen atoms on the Pt surface. Consequently, the HOR/HER reaction order with respect to pH2 also depends on the temperature. We further observed that the HOR reaches a limiting current at high HOR overpotentials that showed a direct proportionality to pH2 and a pressure-independent activation energy. We ascribe this to a limitation of the hydrogen adsorption rate either by a rate limiting Tafel-reaction or mass transport limitations.

The loss of electrochemically active surface area (ECSA) in the cathode during load cycling remains a major durability issue for proton exchange membrane fuel cells (PEMFCs). Here, the degradation of low-loaded cathodes (0.1 mgPt cmMEA −2) was investigated by accelerated stress tests (ASTs) in H2/N2 configuration, varying the upper potential limit (UPL, 0.85-1.0 V) and the hold time (1, 2, or 8 s) of the square wave voltage cycling profiles. A full voltage loss analysis was performed at beginning-of-life and after 100, 300, 1k, 2k, 5k, 10k, 20k, 50k, 100k, 200k, and 500k cycles, determining: i) the roughness factor (rf) via CO-stripping; ii) the H2-crossover; iii) the cathode electrode’s proton conduction resistance; iv) the H2/O2 and H2/air performance; and, v) the O2 transport resistance. It was found that the ECSA/rf deteriorates linearly versus the logarithm of the number of cycles or time at UPL, with higher slopes for harsher ASTs. The individual voltage losses were found to be either unaffected by the aging (H2-crossover and proton conduction resistance) or depend exclusively on the cathode rf (mass/specific activity and O2 transport resistances), independent of the AST procedure. This results in a universal correlation between H2/air performance and rf during voltage cycling ASTs.