Towards a determination of the active surface area of polycrystalline and nanoparticle electrodes by Cu upd and CO oxidation

Abstract

Methods for the determination of the surface area for Pt, Ru and Se modified Pt and Ru are compared, in view of their possible application for technical and nanoparticle electrodes. The hydrogen adsorption charge can hardly be used as a reliable measure for the surface area for Ru because it is paralleled by anion adsorption. The charge necessary for the oxidation of adsorbed CO also contains a large contribution due to anion or oxygen adsorption, which amounts to approx. 45% of the charge in the case of Ru. The mass spectrometrically determined amount of CO2 formed gives a more reliable measure for the surface area, provided that the maximum coverages are constant and independent of the particular surface. Values obtained in this way agree to within 20% with surface area values obtained from measuring the charge needed for the desorption of a complete monolayer of Cu upd on Pt(111) and polycrystalline Pt, polycrystalline Ru, submonolayers of Ru on polycrystalline Pt and on Pt(111) and for nanoparticle, carbon supported electrodes. Se modified Ru has recently found attention as a methanol tolerant cathode material for oxygen reduction. CO does not adsorb on Pt or Ru saturated by Se. For surfaces partially covered by Se, a comparison of the charge measured by cyclic voltammetry in the hydrogen region and of the mass spectrometrically determined amount of CO2 suggests that the latter can be used for a determination of the area not covered by Se. Cu upd, on the other hand, also takes place on surfaces completely covered by Se; the Cu desorption charge is independent of the Se coverage on Pt and Ru modified Pt as long as it does not exceed 70% of full coverage. In the presence of multilayers of Se, CuxSe is formed. On Se modified bulk Ru the amount of Cu upd decreases with increasing Se coverage, approaching only 105 μC m−2 for full Se coverage.