Kensaku Kodama

Activities and Stabilities of Au-Modified Stepped-Pt Single-Crystal Electrodes as Model Cathode Catalysts in Polymer Electrolyte Fuel Cells

The purpose of this study is to test the concept of protecting vulnerable sites on cathode catalysts in polymer electrolyte fuel cells. Pt single-crystal surfaces were modified by depositing Au atoms selectively on (100) step sites and their electrocatalytic activities for oxygen reduction reaction (ORR) and stabilities against potential cycles were examined. The ORR activities were raised by 70% by the Au modifications, and this rise in the activity was ascribed to enhanced local ORR activities on Pt(111) terraces by the surface Au atoms. The Au modifications also stabilized the Pt surfaces against potential cycles by protecting the low-coordinated (100) step sites from surface reorganizations. Thus, the surface modification by selective Au depositions on vulnerable sites is a promising method to enhance both the ORR activity and durability of the catalysts.

Kinetically induced irreversibility in electro-oxidation and reduction of Pt surface

A mean field kinetic model was developed for electrochemical oxidations and reductions of Pt(111) on the basis of density functional theory calculations, and the reaction mechanisms were analyzed. The model reasonably describes asymmetric shapes of cyclic voltammograms and small Tafel slopes of relevant redox reactions observed in experiments without assuming any unphysical forms of rate equations. Simulations using the model indicate that the oxidation of Pt(111) proceeds via an electrochemical oxidation from Pt to PtOH and a disproportionation reaction from PtOH to PtO and Pt, while its reduction proceeds via two electrochemical reductions from PtO to PtOH and from PtOH to Pt.

The “Particle Proximity Effect” in Three Dimensions: a Case Study on Vulcan XC 72R

AbstractThe “particle proximity effect” is a hypothesis claiming that Pt nanoparticles have higher ORR activity when they get closer to one another. In order to put this hypothesis under scrutiny, the “tool-box” approach was investigated in each process step by electron microscopy, ICP, and surface science methods. It is shown that particle size stability is brought about by a NaCl shell which can effectively be removed by washing with water. I.e., the “tool-box” synthesis with an additional washing step produces clean, closely spaced, and well-separated particles with interparticle distances necessary for the effect to occur. Despite this powerful synthesis route, a conclusive proof of the “proximity effect” could not be obtained. This is due to difficulties with catalyst film formation at higher platinum loadings on Vulcan XC 72R, suggesting that film deposition and drying methods have to be optimized for each catalyst loading separately and that a holistic approach is not very realistic. Graphical AbstractSometimes less is more: Oxygen reduction catalysts on Vulcan XC 72R with high Pt loading (> 10 %) show unfavorable film qualities on glassy carbon tips and cannot be evaluated for their kinetic current and specitic activities (extensive coffee ring formation) while for lower loadings reliable values can be obtained.

CO-Terminated Platinum Electrodeposition on Nb-Doped Bulk Rutile TiO2

AbstractNanoparticulate platinum on carbon as oxygen reduction electrocatalyst suffers from two major drawbacks, namely, low specific activity of the Pt particles and corrosion instability of the carbon support under hydrogen starvation conditions (reverse current degradation). Both issues can be tackled by Pt thin films on TiO2 support. Platinum films were synthesized on commercially available Nb-doped bulk TiO2 electrodes via electrodeposition from Ar- (bulk amounts) and CO-saturated solutions (monolayer amounts). The platinized electrodes were tested for ORR activity in a voltammetric fashion, and the field-free semiconductor properties were evaluated by a combination of X-ray photoelectron spectroscopy (XPS), spectroscopic ellipsometry (SE), photoelectron yield spectroscopy (PYS), electrochemical impedance spectroscopy (EIS), and bulk conductivity measurements. The deposition of bulk amounts of Pt leads to a situation where the catalytic activity is directly correlated to the conductivity of the electrode. Raising the majority carrier density in the Nb-doped TiO2, via proton intercalation in aqueous HClO4 or glycol, drastically increased the activity. In case of monolayer deposits (CO-terminated deposition), intercalation has practically no effect on activity. Thus, the hypothesis is formed that unfavorable electronic interaction of the TiO2 support with small amounts of platinum drastically reduces ORR activity, an effect which is not observed for monolayer amounts of Pt deposited on gold. Graphical AbstractToo Much Interaction: The oxygen reduction activity of bulk amounts of Pt, deposited on Nb-doped TiO2, can be significantly improved by raising the carrier density (via proton intercalation). Such treatment has no effect if monolayer amounts are deposited. This observation is attributed to unfavorable Pt/TiO2 interactions for minute amounts of Pt.

Acetylene-Treated Titania Nanotube Arrays (TNAs) as Support for Oxygen Reduction Reaction (ORR) Platinum Thin Film Catalysts

AbstractPlatinum layers show higher specific oxygen reduction reaction (ORR) activities than nanoparticles, and smooth monolayers of platinum on polycrystalline gold have been achieved by electrodeposition from CO-saturated solutions. Since Pt monolayers are interesting catalytic systems, this methodology was attempted on acetylene-treated titania nanotubes (TNAs) with high conductivity. However, the investigation of the as-treated TNAs found that probably nanotubes with an oxygen-containing graphitic overlayer were formed. It was observed that deposition after partial oxidative removal of the overlayer led to very low ORR activities while deposition on the intact overlayer gave rise to the highest activities obtained in our research so far. This is attributed to a “tie-layer” effect, in which the carbon layer screens the negative effects of the underlying TiO2 layer. The interesting effects of the graphitic overlayer on the ORR activity of the Pt deposits on acetylene-treated TNAs offer a strategy to mitigate the unfavorable interactions of the Pt/TiO2 interface. However, the carbon layer in this study was found not to be stable upon potential cycling. Graphical AbstractOnly Gold Lets Platinum be Platinum: Monolayer amounts of Pt, electrodeposited under CO termination, so far, only show specific activities comparable to polycrystalline platinum if deposited on polycrystalline gold. TiO2 modifications as supports facilitate higher activities only at enhanced conductivities, which usually sacrifice their stability.

CO-Terminated Pt/Au Codeposition on Titania Nanotube Arrays (TNAs)

AbstractForming platinum monolayers on less-expensive materials is an ingenious way to enhance the mass activities for oxygen reduction reaction (ORR) and decrease the amount of this scarce and precious metal in fuel cell technology. Previously, we observed low activities of Pt catalysts deposited directly on titania nanotube arrays (TNAs). Here, we propose a gold interlayer between Pt and TiO2. The synthesis is attempted by the codeposition of Pt and Au from CO-saturated solution. The activity of the Pt/Au codeposited catalyst is higher than in the case when Pt is deposited alone. This result suggests that Au is a promising way to enhance ORR activity. The activity of this catalyst, however, is still much lower than that of the Pt monolayer on bulk gold. This is explained by the negative effects of CO, disrupting the chance of monolayer formation by preadsorption to the TNA substrate. Graphical AbstractCO-terminated Pt/Au codeposition on titania nanotube arrays (TNAs)

The Native Oxide on Titanium Metal as a Conductive Model Substrate for Oxygen Reduction Reaction Studies

AbstractVery thin Pt layers on inexpensive substrates are promising oxygen reduction reaction (ORR) catalysts for polymer electrolyte fuel cells (PEFCs). TiOx is considered a suitable substrate but shows problems with conductivity, thus masking chemical effects by semiconductor effects (mismatch in energy states hindering electron transport). The native oxide on metallic Ti (TiOx/Ti) has been used as a novel and promising model substrate for ORR studies eliminating semiconductor effects. A high-coverage “particle” layer with high specific ORR activity was formed via electrodeposition from Ar-saturated solution. While high specific activities could be demonstrated, the concept could not be enhanced to high mass activities by limiting the Pt deposition amount. The approach to quench Pt deposition by introducing CO failed due to its adsorption to the TiOx/Ti substrate before metal deposition and thus the prevention of layer formation. A similar approach for the Pt/Au codeposition was also unsuccessful manifesting the TiOx/Ti-CO incompatibility even further. Graphical AbstractCO, blessing, and curse: Pt deposition from Ar-saturated solution leads to a “film”-like deposit with high specific ORR activity. In contrast, the corresponding CO-saturated solution leads to deposition termination but a smooth monolayer is not formed due to interaction of CO with the TiOx/Ti substrate and, consequently, very low ORR activity is obtained.