EC-STM Summary

The Pt-based anode catalyst in the Direct Methanol Fuel Cell (and related fuel cells using formic acid, etc.) needs improved fuel oxidation performance, and our research strategy has been to modify an initially pure Pt catalyst by spontaneous (electroless) deposition of another enhancing metal on the Pt surface. Although the catalysts actually used in fuel cells are high-surface area nanoparticles, more detailed information about the mechanism of enhancement can be gained by using Pt single crystal substrates, which have a well-defined geometry of surface atoms. The macroscopic investigation of single crystal surfaces covered by controlled amounts of admetal provides simulative information of the catalytic behavior of crystal faces of nanoscale fuel cell catalysts. The structure of the admetal deposits on such Pt(h k l) surfaces can be studied using Electrochemical Scanning Tunneling Microscopy (EC-STM). The effects of deposition time, deposition solution concentration, potential, electrolyte, and fuel oxidation on the structure of the admetal deposits determined from EC-STM images can then be correlated with the fuel oxidation performance as evaluated by electrochemical methods.

It was previously reported in this research group that Ru deposited on various well-ordered Pt surfaces by submonolayer amounts enhances the catalytic activity of Pt/Ru. Recent ex situ STM work with Pt(h k l)/Ru has shown that on all substrates, Ru exists as 2-5 nm islands for a single deposition and the maximum attainable surface coverage for all cases is less than 20%, with 10% or less having height greater than a single atomic layer. Depositing Ru on these surfaces multiple times allows an increase in surface coverage above the saturation limit of a single deposition, which increases the activity for fuel oxidation up to three depositions; more deposits reduce the rate of fuel oxidation. Recent studies have explored the effect of potential on these islands and whether the resulting structural changes enhance or suppress fuel oxidation.

Similar studies are also underway with Pt(h k l)/Os surfaces obtained from spontaneous deposition. The most promising catalysts for the DMFC contain both Ru and Os; hence a more detailed study of Pt/Os surfaces is merited to aid understanding of the more complicated ternary system. Ex situ STM images show that on Pt(h k l)/Os surfaces, the Os islands show a preference for step edges and defects in contrast to Ru on Pt(h k l)/Ru surfaces. The Os islands on Pt(111) and Pt(100) are smaller than Ru islands, 1-3 nm, whereas on Pt(110) the islands exist as large clusters on the steps. Single crystal electrochemistry is used to determined the fuel oxidation activity, which can then be correlated to the observed structure and coverage of Os on the Pt(h k l) surfaces. EC-STM is now being used to investigate the effect of potential on the Os island size and structure, and to determine the optimal Os coverage and island dispersion. The effect of the fuel oxidation itself on the islands will also be investigated.

Future work will investigate other catalytic admetals on Pt(h k l) as well as surfaces with more than one admetal present. As the search for catalytic materials continues, so increases the possibilities for fundamental study of the relevant model substrates.