Carbon-Supported Platinum Electrocatalysts Probed in a Gas Diffusion Setup with Alkaline Environment: How Particle Size and Mesoscopic Environment Influence the Degradation Mechanism

Abstract

In recent years, extensive research has been performed concerning the stability of fuel cell catalysts in an acidic environment. By comparison, only few studies address the degradation mechanism(s) of fuel cell catalysts in alkaline media. In this work, we investigate the stability of four different types of Pt/C fuel cell catalysts upon applying accelerated degradation tests in a gas diffusion electrode (GDE) setup equipped with an anion exchange membrane. In contrast to previous investigations exposing the catalysts to a liquid electrolyte, the GDE setup provides a realistic three-phase boundary of the reactant gas, catalyst, and ionomer which enables reactant transport rates close to real fuel cells. Therefore, the GDE setup mimics the degradation of the catalyst under more realistic reaction conditions as compared to conventional electrochemical cells. Combining the determination of the loss in the electrochemically active surface area of the Pt/C catalysts via CO stripping measurements with the change in particle size distribution determined by small-angle X-ray scattering measurements, we demonstrate that (i) the degradation mechanism depends on the investigated Pt/C catalyst and might indeed be different from the one observed in conventional electrochemical cells, (ii) degradation is increased in an oxygen gas atmosphere (as compared to an inert atmosphere), and (iii) the observed degradation mechanism depends on the mesoscopic environment of the active phase. The measurements indicate an increased particle growth if small and large particles are immobilized next to each other on the same carbon support flakes as compared to a simple mix of two catalysts with small and large particles, respectively.

Elucidating Pt-Based Nanocomposite Catalysts for the Oxygen Reduction Reaction in Rotating Disk Electrode and Gas Diffusion Electrode Measurements

“…Copenhagen with a SAXSLab instrument as previously detailed 24,29,31 . The instrument is equipped with a 100XL + micro-focus sealed X-ray tube from Rigaku, producing a photon beam with a wavelength of 1.54 Å.…”

“…In addition to more realistic reaction conditions, the GDE investigations also allow an ex situ analysis of the used (degraded) catalyst layers with SAXS 24,29 . Comparing the particle size distributions derived from SAXS at BOT and EOT indicates that the improved stability and ORR activity of the Pt-Au/C nanocomposite can be assigned to a reduced Pt particle growth, see Figure 4 and Table S2.…”

Elucidating Pt-Based Nanocomposite Catalysts for the Oxygen Reduction Reaction in Rotating Disk Electrode and Gas Diffusion Electrode Measurements

“…Copenhagen with a SAXSLab instrument as previously detailed [24,29,31]. The instrument is equipped with a 100XL + micro-focus sealed X-ray tube from Rigaku, producing a photon beam with a wavelength of 1.54 Å.…”

“…In addition to more realistic reaction conditions, the GDE investigations also allow an ex situ analysis of the used (degraded) catalyst layers with SAXS [24,29]. Comparing the particle size distributions derived from SAXS at BOT and EOT indicates that the improved stability and ORR activity of the Pt-Au/C nanocomposite can be assigned to a reduced Pt particle growth, see Figure 4 and Table S2.…”

Stability and activity of platinum nanoparticles in the oxygen electroreduction reaction: is size or uniformity of primary importance?

“…The probability of agglomeration and coalescence of NPs during a stress test also decreases with an increase in the average distance between platinum NPs in catalysts, which results in an increase of their stability [14]. Apparently, a relatively high stability of small-sized systems is possible when the platinum NPs are similar in size and their distribution is uniform over the surface of the carbon support [35,[37][38][39][40]. For example, as it is shown in [40], the growth of the nanoparticles during the catalyst operation accelerates when large and small NPs are localized in the same regions of the support surface, and it slows down if the catalytic layer is formed from an ordinary mixture of two catalysts with small and large nanoparticles, respectively.…”