| Future developments in fuel cell technology require an improved and well characterized electrocatalyst to sustain optimal fuel cell performance under working conditions. Although many surface science techniques have been used to study the electrocatalyst surface, few provide detailed in situ information on the interactions of adsorbates, specifically the binding site preference of adsorbates on the electrode under an electrolyte. Such information is essential for formulating detailed atomic level reaction mechanisms in electrocatalysis.; In this work, X-Ray Absorption Spectroscopy (XAS) is utilized in situ to gain new insights into the electronic and chemical interactions of adsorbates on Pt, and PtM (M = Cr, Fe, Ni, and Co) based alloy clusters, supported on carbon. Extended X-Ray Absorption Fine Structure (EXAFS) was used to determine the geometrical properties including Pt-Pt and Pt-M coordination numbers and bond distances within the cluster. A new methodology was utilized, along with full-multiple scattering calculations and local density functional (LDF) results, to interpret the X-Ray Absorption Near Edge Structure (XANES). Comparison of theory with experimental data allowed unique spectral signatures to be determined for adsorbed H, OH, O, and anions such as bisulfate originating from the electrolyte solutions, in either atop, bridged, 3-fold fcc, and four-fold hcp absorption sites.; In summary, this work presents detailed new information on the fundamental properties of adsorption on Pt and Pt based alloy electrodes in an electrochemical cell. The experience gained from these studies then allowed application to operating phosphoric acid (PAFC) and state-of-the-art PEM fuel cells. The results reveal the presence of directly absorbed anions on the electrode surface, OH chemisorption increasing with the current densities, and even the presence of chemisorbed OOH on the electrodes in the PAFC. The techniques developed in this work should be applicable to the intelligent and directed design of new fuel cell electrode materials. |
