Nanoparticle platinum-ruthenium adatom catalysts for direct methanol fuel cells. The study of a direct 2-propanol polymer electrolyte fuel cell

Pt-Ru systems are the most active, stable anode catalysts tested in direct methanol fuel cells. Investigations into the real activity and optimum surface composition of nanoparticle Pt-Ru catalysts toward methanol electrooxidation are difficult because there is a lack of proven, accurate methods to measure the number of active sites and the surface Pt/Ru ratio. In this study, Pt-Ru ad systems (Ruad is a ruthenium adatom) were employed to address these problems because controlled amounts of Ruad can be deposited onto Pt nanoparticles of known surface area to produce Pt-Ru ad nanoparticles of known surface areas and controlled surface compositions. Two non-electrochemical, self-limiting methods to deposit Ru adatoms onto nanoparticle Pt were devised and investigated. The organometallic precursor, Ru4(mu-H)4(CO)12, reacts with hydrogen over either blacked Pt gauze or Pt nanoparticles to deposit Ru adatoms and CO onto the Pt surface. The deposition is self-poisoned by the adsorbed CO, and it stops after ca. 0.05 surface equivalents (moles Ruad vs. moles surface Pt) of Ruad are deposited onto blacked Pt gauzes, and after ca. 0.10 surface equivalents of Ruad are deposited onto nanoparticle Pt black.; Aqueous RuCl3 reacts with hydrogen pre-adsorbed onto nanoparticle Pt black surfaces to deposit 0.18 surface equivalents of Ruad. The deposition was repeated several times, with each reaction depositing ca. 0.18 surface equivalents of more Ruad onto the Pt-Ruad nanoparticles. The resulting nanoparticle Pt-Ruad electrocatalysts with Ruad surface coverages ranging from 0.18 to 0.75 were studied as catalysts for methanol electrooxidation in 1.0 M H2SO 4 (3-electrode experiments), as well as in prototype direct methanol fuel cells. The optimum Ruad surface coverage is ca. 0.33 between 22°C and 60°C. It shifts to higher values as the temperature is increased. A NafionRTM-117 membrane fuel cell consisting of a Pt-Ru anode and a Pt black cathode was operated at 90°C using aqueous 2-propanol as fuel. The performance of the cell operating on 2-propanol was substantially higher than operating on methanol at current densities lower than ∼200 mA/cm2. This result shows that direct 2-propanol fuel cells are promising alternatives to direct methanol fuel cells.