Nenad M. Markovic

Highly Crystalline Multimetallic Nanoframes with Three-Dimensional Electrocatalytic Surfaces

Giving Electrocatalysts an Edge Platinum (Pt) is an excellent catalyst for the oxygen-reduction reaction (ORR) in fuel cells and electrolyzers, but it is too expensive and scarce for widespread deployment, even when dispersed as Pt nanoparticles on carbon electrode supports (Pt/C). Alternatively, Chen et al. (p. 1339, published online 27 February; see the Perspective by Greer) made highly active ORR catalysts by dissolving away the interior of rhombic dodecahedral PtNi3 nanocrystals to leave Pt-rich Pt3Ni edges. These nanoframe catalysts are durable—remaining active after 10,000 rounds of voltage cycling—and are far more active than Pt/C. Highly active electrocatalysts are created by eroding away all but the edges of platinum-nickel nanocrystals. [Also see Perspective by Greer] Control of structure at the atomic level can precisely and effectively tune catalytic properties of materials, enabling enhancement in both activity and durability. We synthesized a highly active and durable class of electrocatalysts by exploiting the structural evolution of platinum-nickel (Pt-Ni) bimetallic nanocrystals. The starting material, crystalline PtNi3 polyhedra, transforms in solution by interior erosion into Pt3Ni nanoframes with surfaces that offer three-dimensional molecular accessibility. The edges of the Pt-rich PtNi3 polyhedra are maintained in the final Pt3Ni nanoframes. Both the interior and exterior catalytic surfaces of this open-framework structure are composed of the nanosegregated Pt-skin structure, which exhibits enhanced oxygen reduction reaction (ORR) activity. The Pt3Ni nanoframe catalysts achieved a factor of 36 enhancement in mass activity and a factor of 22 enhancement in specific activity, respectively, for this reaction (relative to state-of-the-art platinum-carbon catalysts) during prolonged exposure to reaction conditions.

Oxygen Reduction Reaction on Pt and Pt Bimetallic Surfaces: A Selective Review

In this review we selectively summarize recent progress, primarily from our laboratory, in the development of the oxygen reduction reaction (ORR) catalysis on well-defined surfaces. The focus is on two type of metallic surfaces: platinum single crystals and bimetallic surfaces based on platinum. The single crystal results provide insight into the effects of the platinum structure on the kinetics of the ORR, and create a fundamental link between the specific activity of Pt (rate per unit area) and particle size (for various particle shapes). The results show that the structure sensitive kinetics of the ORR arise primarily due to structure sensitive adsorption of anions. In the absence of specific adsorption, such as in Nafion polymer electrolyte, no particle size effect is expected. The knowledge of the electrocatalysis of the ORR on model bimetallic surfaces on Pt-Ni and Pt-Co bulk alloys was used to resolve the enhanced ORR kinetics on supported Pt-Ni and Pt-Co catalysts. Finally, we show that the ORR on platinum modified with pseudomorphic Pd metal film in alkaline solution is the best catalysts ever used in O2 reduction. For both bimetallic systems, we demonstrated that the ability to make a controlled and well characterized arrangement of two elements in the electrode surface region presage a new era of advances in the ORR electrocatalysis.

Enhancing Hydrogen Evolution Activity in Water Splitting by Tailoring Li+-Ni(OH)2-Pt Interfaces

Combining two different types of catalysts accelerated the hydrogen-generation step in water electrolysis. Improving the sluggish kinetics for the electrochemical reduction of water to molecular hydrogen in alkaline environments is one key to reducing the high overpotentials and associated energy losses in water-alkali and chlor-alkali electrolyzers. We found that a controlled arrangement of nanometer-scale Ni(OH)2 clusters on platinum electrode surfaces manifests a factor of 8 activity increase in catalyzing the hydrogen evolution reaction relative to state-of-the-art metal and metal-oxide catalysts. In a bifunctional effect, the edges of the Ni(OH)2 clusters promoted the dissociation of water and the production of hydrogen intermediates that then adsorbed on the nearby Pt surfaces and recombined into molecular hydrogen. The generation of these hydrogen intermediates could be further enhanced via Li+-induced destabilization of the HO–H bond, resulting in a factor of 10 total increase in activity.

Temperature‐Dependent Methanol Electro‐Oxidation on Well‐Characterized Pt‐Ru Alloys

The kinetics of methanol electro-oxidation on well-characterized Pt-Ru alloy surfaces were measured in sulfuric acid solution as a function of temperature. The alloy surfaces were prepared in ultrahigh vacuum with the surface composition determined by low energy ion scattering. It was found that the activity of Ru towards the dissociative adsorption of methanol is a strong function of temperature. This change in the adsorptive nature of the Ru sites with temperature produced a variation in the optimum surface composition with temperature. The optimum surface had an Ru content which increased with increasing temperature, from close to [approximately]10 atomic percent (a/o) Ru at 25 C to a value in the vicinity of [approximately]30 a/o at 60 C. The shift in optimum composition with temperature was attributed to a shift in the rate-determining step from methanol adsorption/dehydrogenation at low temperature to the surface reaction between the dehydrogenated intermediate and surface oxygen at high temperature. The apparent activation energies were consistent with this change in the rate-determining step.

Design and synthesis of bimetallic electrocatalyst with multilayered Pt-skin surfaces.

Advancement in heterogeneous catalysis relies on the capability of altering material structures at the nanoscale, and that is particularly important for the development of highly active electrocatalysts with uncompromised durability. Here, we report the design and synthesis of a Pt-bimetallic catalyst with multilayered Pt-skin surface, which shows superior electrocatalytic performance for the oxygen reduction reaction (ORR). This novel structure was first established on thin film extended surfaces with tailored composition profiles and then implemented in nanocatalysts by organic solution synthesis. Electrochemical studies for the ORR demonstrated that after prolonged exposure to reaction conditions, the Pt-bimetallic catalyst with multilayered Pt-skin surface exhibited an improvement factor of more than 1 order of magnitude in activity versus conventional Pt catalysts. The substantially enhanced catalytic activity and durability indicate great potential for improving the material properties by fine-tuning of the nanoscale architecture.

Electro-oxidation mechanisms of methanol and formic acid on Pt-Ru alloy surfaces

Abstract Voltammetry combined with single-potential alteration infrared spectroscopy (SPAIRS) were used to study the extent of adsorbed CO produced at Pt, Ru and Pt-Ru alloy electrodes during methanol and formic acid oxidation in acidic supporting electrolyte. The addition of even small atomic fractions of Ru to Pt surfaces caused a decrease in the quasi-steady-state level of CO on the surface for both reactions. This result is consistent with the bifunctional mechanism proposed previously: Ru sites nucleate oxygen containing species at ≈0.2-0.3 V lower potential than on the pure Pt surface; the adsorption of methanol occurs on Pt ensembles producing adsorbed CO; in the case of formic acid, adsorption is equally facile at Pt-Pt, Pt-Ru and Ru-Ru sites, with dehydration producing adsorbed CO; the further electro-oxidation of CO is catalyzed by oxygen-containing species nucleated onto nearby by Ru atoms. The improved efficiency of the alloy surfaces for oxidation of adsorbed CO at low potential shifts the rate limiting step to the adsorption step, which results in very low coverages of the surfaces by adsorbed CO.

Multimetallic Au/FePt3 Nanoparticles as Highly Durable Electrocatalyst

We report the design and synthesis of multimetallic Au/Pt-bimetallic nanoparticles as a highly durable electrocatalyst for the oxygen reduction reaction (ORR) in proton exchange membrane fuel cells. This system was first studied on well-defined Pt and FePt thin films deposited on a Au(111) surface, which has guided the development of novel synthetic routes toward shape-controlled Au nanoparticles coated with a Pt-bimetallic alloy. It has been demonstrated that these multimetallic Au/FePt(3) nanoparticles possess both the high catalytic activity of Pt-bimetallic alloys and the superior durability of the tailored morphology and composition profile, with mass-activity enhancement of more than 1 order of magnitude over Pt catalysts. The reported synergy between well-defined surfaces and nanoparticle synthesis offers a persuasive approach toward advanced functional nanomaterials.

Methanol electrooxidation on supported Pt and PtRu catalysts in acid and alkaline solutions

The kinetics of methanol oxidation on supported 47.5 wt.% Pt and 54 wt.% PtRu (with nominal Pt:Ru ratios of 2:3) catalysts are measured in 0.5 M H2SO4 and 0.1 NaOH at 295 and 333 K using thin-film rotating disk electrode (RDE) method. It was found that the activity of Pt and PtRu for methanol oxidation is a strong function of pH of solution and temperature. The kinetics are much higher in alkaline than in acid solution; at 333 K, a factor of 30 for Pt and a factor of 20 for Pt2Ru3 at 0.5 V. The pH effect is attributed to the pH competitive adsorption of oxygenated species with anions from supporting electrolytes. The activity of Pt and Pt2Ru3 catalysts at 333 K is higher (a factor of 5) than at 295 K. Irrespective of pH, only negligible differences in the kinetics are observed between Pt and on high Ru content Pt alloys, presumably owing to a slow rate of methanol dehydrogenation on the Ru-rich surface and insufficient number of Pt sites required for dissociative chemisorption of methanol.

On the reaction pathway for methanol and carbon monoxide electrooxidation on Pt-Sn alloy versus Pt-Ru alloy surfaces

It has been observed in this work and previous studies that Pt3Sn alloy surfaces are very effective catalysts for CO electrooxidation, but not for methanol electrooxidation. Since COads is postulated to be an intermediate in methanol electrooxidation on Pt alloy surfaces, the relative inactivity of Pt3Sn for methanol oxidation appears paradoxical. We present an explanation for this apparent contradiction in terms of a unique state of COads on this surface, which is not the same state of COads as occurs on either PtRu or pure Pt surfaces. It is also not a state of COads which is produced by methanol dehydrogenation. The state is unique in the sense that a significant fraction of COads is oxidized at a much lower (<400 mV) potential than the rest of the COads, a phenomenon that does not occur on any other Pt and Pt-alloy surfaces examined in the same way. This CO state is only formed at high coverages by direct adsorption from dissolved CO and is not formed by the dehydrogenation of methanol, since the multiple Pt atom sites needed to dehydrogenate methanol are blocked by COads at low coverage.

Surface Segregation Effects in Electrocatalysis: Kinetics of Oxygen Reduction Reaction on Polycrystalline Pt3Ni Alloy Surfaces**

Submitted to the Journal of Electroanalytical Chemistry, November 6, 2002 Surface Segregation Effects in Electrocatalysis: Kinetics of Oxygen Reduction Reaction on Polycrystalline Pt Ni Alloy Surfaces** V. Stamenkovic*, T.J. Schmidt - , P.N. Ross and N . M . Markovic Materials Sciences Division, Lawrence Berkeley National Laboratory University of California at Berkeley, CA 94720, USA Abstract Effects of surface segregation on the oxygen reduction reaction (ORR) have been studied on a polycrystalline Pt Ni alloy in acid electrolyte using ultra high vacuum (UHV) surface sensitive probes and the rotating ring disk electrode (RRDE) method. Preparation, modification and Depending on the characterization of alloy surfaces were done in ultra high vacuum (UHV). preparation method, two different surface compositions of the Pt Ni alloy are produced: a sputtered surface with 75 % Pt and an annealed surface (950 K ) with 100 % Pt. The latter surface is designated as the Pt-skin structure, and is a consequence of surface segregation, i.e., replacement of N i with Pt atoms in the first few atomic layers. Definitive surface compositions were established by low energy ion scattering spectroscopy (LEISS). The cyclic voltammetry of the Pt-skin surface as well as the pseudcapacitance in the hydrogen adsorption/desorption potential region is similar to a polycrystalline Pt electrode. Activities of ORR on Pt Ni alloy surfaces were compared to polycrystalline Pt in 0.1M H C l O electrolyte for the observed temperature range of 293 Pt Ni (75% Pt) > Pt with the maximum catalytic enhancement obtained for the Pt-skin being 4 times