Vladimir Tripkovic

Tuning the activity of Pt alloy electrocatalysts by means of the lanthanide contraction

A lanthanide boost for platinum High loadings of precious platinum are needed for automotive fuel cells, because the kinetics of the oxygen reduction reaction (ORR) are relatively slow. Escudero-Escribano et al. studied a series of platinum alloys with lanthanides and alkaline earth elements. When the surfaces were leached to leave pure platinum, they developed compressive strain that boosted the ORR activity—up to a factor of 6 for terbium. Enthalpy effects helped to stabilize these alloys under operating conditions. Science, this issue p. 73 Alloying platinum with lanthanide elements compresses its surface layer and boosts its oxygen reduction activity. The high platinum loadings required to compensate for the slow kinetics of the oxygen reduction reaction (ORR) impede the widespread uptake of low-temperature fuel cells in automotive vehicles. We have studied the ORR on eight platinum (Pt)–lanthanide and Pt-alkaline earth electrodes, Pt5M, where M is lanthanum, cerium, samarium, gadolinium, terbium, dysprosium, thulium, or calcium. The materials are among the most active polycrystalline Pt-based catalysts reported, presenting activity enhancement by a factor of 3 to 6 over Pt. The active phase consists of a Pt overlayer formed by acid leaching. The ORR activity versus the bulk lattice parameter follows a high peaked “volcano” relation. We demonstrate how the lanthanide contraction can be used to control strain effects and tune the activity, stability, and reactivity of these materials.

The Pt(111)/Electrolyte Interface under Oxygen Reduction Reaction Conditions: An Electrochemical Impedance Spectroscopy Study

The Pt(111)/electrolyte interface has been characterized during the oxygen reduction reaction (ORR) in 0.1 M HClO(4) using electrochemical impedance spectroscopy. The surface was studied within the potential region where adsorption of OH* and O* species occur without significant place exchange between the adsorbate and Pt surface atoms (0.45-1.15 V vs RHE). An equivalent electric circuit is proposed to model the Pt(111)/electrolyte interface under ORR conditions within the selected potential window. This equivalent circuit reflects three processes with different time constants, which occur simultaneously during the ORR at Pt(111). Density functional theory (DFT) calculations were used to correlate and interpret the results of the measurements. The calculations indicate that the coadsorption of ClO(4)* and Cl* with OH* is unlikely. Our analysis suggests that the two-dimensional (2D) structures formed in O(2)-free solution are also formed under ORR conditions.

Density functional theory based screening of ternary alkali-transition metal borohydrides: A computational material design project

We present a computational screening study of ternary metal borohydrides for reversible hydrogen storage based on density functional theory. We investigate the stability and decomposition of alloys containing 1 alkali metal atom, Li, Na, or K (M(1)); and 1 alkali, alkaline earth or 3d/4d transition metal atom (M(2)) plus two to five (BH(4))(-) groups, i.e., M(1)M(2)(BH(4))(2-5), using a number of model structures with trigonal, tetrahedral, octahedral, and free coordination of the metal borohydride complexes. Of the over 700 investigated structures, about 20 were predicted to form potentially stable alloys with promising decomposition energies. The M(1)(Al/Mn/Fe)(BH(4))(4), (Li/Na)Zn(BH(4))(3), and (Na/K)(Ni/Co)(BH(4))(3) alloys are found to be the most promising, followed by selected M(1)(Nb/Rh)(BH(4))(4) alloys.

pH in atomic scale simulations of electrochemical interfaces

Electrochemical reaction rates can strongly depend on pH, and there is increasing interest in electrocatalysis in alkaline solution. To date, no method has been devised to address pH in atomic scale simulations. We present a simple method to determine the atomic structure of the metal|solution interface at a given pH and electrode potential. Using Pt(111)|water as an example, we show the effect of pH on the interfacial structure, and discuss its impact on reaction energies and barriers. This method paves the way for ab initio studies of pH effects on the structure and electrocatalytic activity of electrochemical interfaces.

Metal Oxide‐Supported Platinum Overlayers as Proton‐Exchange Membrane Fuel Cell Cathodes

We investigated the activity and stability of n=(1, 2, 3) platinum layers supported on a number of rutile metal oxides (MO2; M=Ti, Sn, Ta, Nb, Hf and Zr). A suitable oxide support can alleviate the problem of carbon corrosion and platinum dissolution in Pt/C catalysts. Moreover, it can increase the activity of platinum if the interaction between the support and the metal is optimal. We found that both the activity and the stability depend on the number of platinum layers and, as expected, both converge toward platinum bulk values if the number of layers is increased. With use of a simple volcano curve for activity estimation, we found that the supported platinum layers could be active for the oxygen reduction reaction, with a few candidates possibly having an activity even greater than that of platinum. Furthermore, we established a correlation between stability and activity for supported platinum monolayers, which suggests that activity can be increased at the expense of stability and vice versa. Finally, the performance of the systems was evaluated against Pt(111) skins on Pt3X (X=Ni, Co, Fe, Ti, Sc and Y) alloys, which are the best catalysts known to date for the reaction.

Trends for Methane Oxidation at Solid Oxide Fuel Cell Conditions

First-principles calculations are used to predict a plausible reaction pathway for the methane oxidation reaction. In turn, this pathway is used to obtain trends in methane oxidation activity at solid oxide fuel cell SOFC anode materials. Reaction energetics and barriers for the elementary reaction steps on both the close-packed Ni111 and stepped Ni211 surfaces are presented. Quantum-mechanical calculations augmented with thermodynamic corrections allow appropriate treatment of the elevated temperatures in SOFCs. Linear scaling relationships are used to extrapolate the results from the Ni surfaces to other metals of interest. This allows the reactivity over the different metals to be understood in terms of two reactivity descriptors, namely, the carbon and oxygen adsorption energies. By combining a simple free-energy analysis with microkinetic modeling, activity landscapes of anode materials can be obtained in terms of these two descriptors. This not only simplifies the view of the oxidation process but it also gives insight into which reaction pathways are likely to be dominant over the different transition-metal anode materials. Most

Computational Screening of Doped α‐MnO2 Catalysts for the Oxygen Evolution Reaction

Minimizing energy and materials costs for driving the oxygen evolution reaction (OER) is paramount for the commercialization of water electrolysis cells and rechargeable metal-air batteries. Structural stability, catalytic activity, and electronic conductivity of pure and doped α-MnO2 for the OER are studied using density functional theory calculations. As model surfaces, we investigate the (110) and (100) facets, on which three possible active sites are identified: a coordination unsaturated, a bridge, and a bulk site. For pure and Cr-, Fe-, Co-, Ni-, Cu-, Zn-, Cd-, Mg-, Al-, Ga-, In-, Sc-, Ru-, Rh-, Ir-, Pd-, Pt-, Ti-, Zr-, Nb-, and Sn-doped α-MnO2 , the preferred valence at each site is imposed by adding/subtracting electron donors (hydrogen atoms) and electron acceptors (hydroxy groups). From a subset of stable dopants, Pd-doped α-MnO2 is identified as the best catalyst and the only material that can outperform pristine α-MnO2 . Different approaches to increase the bulk electron conductivity of semiconducting α-MnO2 are discussed.

Electro-Catalysis of Oxygen Reduction Reaction

This paper is a short review of the recent developments in understanding trends in electro-catalysis of the oxygen reduction reaction (ORR). Our focus is on atomic scale simulations at the density functional theory level. First, we investigate the models and the approximations that have been assumed, and thence we reach the conclusion that trends in electrocatalysis are well captured by only considering binding energies of reaction intermediates to the catalyst surface. We show, assuming a simple and very likely series of intermediates, existence- of a universal scaling relation common to all ORR catalysts, which defines the overpotential.