Structure-activity-stability relationships of Pt-Co alloy electrocatalysts in gas-diffusion electrode layers

摘要

We establish relationships between the atomic structure, composition, electrocatalytic activity, and electrochemical corrosion stability of carbon-supported Pt-Co alloy nanoparticles in electrode catalyst layers. These Pt-Co catalysts have received much attention for use as cathode layers in polymer electrolyte membrane fuel cells (PEMFCs) because of their favorable oxygen-reduction-reaction (ORR) activity and suspected corrosion stability. We reported an enhancement of activity of low-temperature Pt50Co50 of 3 times that of pure carbon supported Pt catalysts. The use of synchrotron X-ray diffraction has enabled structural characterization of the alloy nanoparticles both before and, importantly, after electrocatalysis under fuel cell like conditions. From this, a detailed picture of the relative activity and stability of Pt-Co alloy phases as a function of synthesis conditions has emerged. We have investigated the structure, composition, chemical ordering, and concentration of Pt-Co alloy phases in (i) a dry, freshly synthesized nanoparticle catalyst, (ii) the catalytic electrode layer in a proton-conducting polymer electrolyte before electrocatalytic activity, and (iii) the same electrode layer after electrocatalytic activity. We find that Pt50Co50 catalysts annealed at 600 degrees C consist of multiple phases: a chemically ordered face-centered tetragonal (fct) and two chemically disordered face-centered cubic (fcc) phases with differing stoichiometries. The Co-rich fcc phase suffers from corrosive Co loss during the preparation of conducting polymer electrode layers and, more significantly, during the ORR electrocatalysis. Most importantly, these fcc phases exhibit high catalytic activities for ORR (about 3x compared to a pure Pt electrocatalyst). Pt50Co50 catalysts annealed at 950 degrees C consist mainly of the fct Pt50Co50 phase. This phase shows favorable stability to corrosion in the conducting polymer electrode and during electrocatalysis, as the relative intensities of fcc(111)/fct(101) peak ratio remained consistently around 0.5 before and after preparation of conducting polymer electrode layers and before and after electrochemical measurements; however, it exhibits a lower catalytic ORR activity compared to the low-temperature fcc alloy phases (about 2.5x compared to a pure Pt electrocatalyst). Our results demonstrate the complexity in these multiphase materials with respect to catalyst activity and degradation. By understanding of the relationships between crystallographic phase, chemical ordering, composition, and the resulting electrochemical activity and corrosion stability of fuel cell catalysts within polymer-electrolyte/catalyst composites, we can move toward the rational design of active and durable catalyst materials for PEMFC electrodes.