Local Ordering Changes in Pt-Co Nanocatalyst Induced by Fuel Cell Working Conditions

We present a detailed investigation of the changes in the local structure and chemical disorder induced by controlled potential cycling in Pt3 +/-delta Co nanoparticles used as a catalyst in the proton exchange membrane fuel cell (PEMFC) technology. Various state-of-the art material science techniques were used to study the microscopic properties of those nanomaterials including ex-situ and in-situ X-ray absorption spectroscopy (XAS), X-ray diffraction (XRD), and transmission electron microscopy (TEM). XAS double-edge multiple-scattering structural refinements of the Pt-CAM_PartnerCost.pdfCo spectra were performed taking into account the reduction of coordination numbers and degeneracy of three-atom configurations resulting from the measured size distribution obtained by TEM and XRD. The effect of chemical disorder in the considered nanoalloy was also taken into consideration. The PEMFC performance appears to be related to specific changes of the microscopic structural properties of the nanocatalyst during the first operation hours, especially during a cell activation period. In operating PEMFCs a small amount of Co oxide, initially present in the nanoalloy (on the surface of particles), disappeared gradually. At the same time, interatomic Pt-Pt and Pt-Co distances were slightly longer for higher current densities, while distance variances (sigma(2)) tended to decrease. Co-Co distribution remained unchanged. By combining XRD and XAS data, we also found that after controlled potential cycling in PEMFC the stoichiometry of the considered alloy changed from the initial Pt3 +/-delta Co to Pt4 +/-delta Co. Comparison of XAS and XRD-extracted values of the Co atomic fraction indicated that mainly cobalt not fully alloyed with platinum dissolved, and this process occurred in the first degradation period. At the same time, no substantial aggregation processes and no change in the mean size of nanoparticles were observed for this alloy. Accelerated degradation test, lasting up to 150 h, showed structural and electrochemical catalyst stability. The observed increase of chemical and local structural order of the particle core alloy did not affect the ORR kinetics.