Jakub Tymoczko

Finding optimal surface sites on heterogeneous catalysts by counting nearest neighbors.

Accounting for surface coordination The exploration of heterogeneous catalysts using first-principles calculations can be daunting because the large number of atoms and possible surface geometries. Calle-Vallejo et al. describe a simpler metric for assessing optimal reactivity: a weighted average of surface coordination that includes second-nearest neighbors (see the Perspective by Stephens et al.). The calculations identified three approaches for introducing cavity sites into the platinum(111) surface to improve its performance for the oxygen reduction reaction used in fuel cells. Science, this issue p. 185, see also p. 164 Detailed comparison of exposed atom coordination can guide the design of a metal surface for heterogeneous catalysis. [Also see Perspective by Stephens et al.] A good heterogeneous catalyst for a given chemical reaction very often has only one specific type of surface site that is catalytically active. Widespread methodologies such as Sabatier-type activity plots determine optimal adsorption energies to maximize catalytic activity, but these are difficult to use as guidelines to devise new catalysts. We introduce “coordination-activity plots” that predict the geometric structure of optimal active sites. The method is illustrated on the oxygen reduction reaction catalyzed by platinum. Sites with the same number of first-nearest neighbors as (111) terraces but with an increased number of second-nearest neighbors are predicted to have superior catalytic activity. We used this rationale to create highly active sites on platinum (111), without alloying and using three different affordable experimental methods.

Making the hydrogen evolution reaction in polymer electrolyte membrane electrolysers even faster

Although the hydrogen evolution reaction (HER) is one of the fastest electrocatalytic reactions, modern polymer electrolyte membrane (PEM) electrolysers require larger platinum loadings (∼0.5–1.0 mg cm−2) than those in PEM fuel cell anodes and cathodes altogether (∼0.5 mg cm−2). Thus, catalyst optimization would help in substantially reducing the costs for hydrogen production using this technology. Here we show that the activity of platinum(111) electrodes towards HER is significantly enhanced with just monolayer amounts of copper. Positioning copper atoms into the subsurface layer of platinum weakens the surface binding of adsorbed H-intermediates and provides a twofold activity increase, surpassing the highest specific HER activities reported for acidic media under similar conditions, to the best of our knowledge. These improvements are rationalized using a simple model based on structure-sensitive hydrogen adsorption at platinum and copper-modified platinum surfaces. This model also solves a long-lasting puzzle in electrocatalysis, namely why polycrystalline platinum electrodes are more active than platinum(111) for the HER.

Electrical potential-assisted DNA hybridization. How to mitigate electrostatics for surface DNA hybridization.

Surface-confined DNA hybridization reactions are sensitive to the number and identity of DNA capture probes and experimental conditions such as the nature and the ionic strength of the electrolyte solution. When the surface probe density is high or the concentration of bulk ions is much lower than the concentration of ions within the DNA layer, hybridization is significantly slowed down or does not proceed at all. However, high-density DNA monolayers are attractive for designing high-sensitivity DNA sensors. Thus, circumventing sluggish DNA hybridization on such interfaces allows a high surface concentration of target DNA and improved signal/noise ratio. We present potential-assisted hybridization as a strategy in which an external voltage is applied to the ssDNA-modified interface during the hybridization process. Results show that a significant enhancement of hybridization can be achieved using this approach.

Elucidation of the Oxygen Reduction Volcano in Alkaline Media using a Copper–Platinum(111) Alloy

The relationship between the binding of the reaction intermediates and oxygen reduction activity in alkaline media was experimentally explored. By introducing Cu into the 2nd surface layer of a Pt(111) single crystal, the surface reactivity was tuned. In both 0.1 m NaOH and 0.1 m KOH, the optimal catalyst should exhibit OH binding circa 0.1 eV weaker than Pt(111), via a Sabatier volcano; this observation suggests that the reaction is mediated via the same surface bound intermediates as in acid, in contrast to previous reports. In 0.1 m KOH, the alloy catalyst at the peak of the volcano exhibits a maximum activity of 101±8 mA cm-2 at 0.9 V vs. a reversible hydrogen electrode (RHE). This activity constitutes a circa 60-fold increase over Pt(111) in 0.1 m HClO4 .

Characterisation of non-uniform functional surfaces: towards linking basic surface properties with electrocatalytic activity

Functional materials, particularly heterogeneous catalysts, are often non-uniform at a microscopic level making their detailed characterisation extremely complex. This complexity inhibits the design and implementation of novel functional materials as such characterisation is a key to understanding interfaces for heterogeneous catalysis. We demonstrate that a combination of Scanning Kelvin Probe (SKP) and Scanning Electrochemical Microscopy (SECM) experiments made over the same sample surface using an integrated SKP–SECM system provides a powerful and robust tool to link basic surface properties with the observed electrocatalytic activity. As the SKP-response can be accurately assessed using modern quantum chemical approaches to benchmark analytical signals for different surface structures with varying compositions, application of an integrated SKP–SECM system can offer valuable insight into the origin of the observed electrocatalytic activity. As model objects, we used Pt(111)-like thin films modified with sub-monolayer and monolayer amounts of Cu atoms located at the electrode surface and in the sub-surface region. The exact position of the Cu atoms relative to the topmost Pt layer greatly affects basic surface properties and governs the electrocatalytic activity of the surface towards various reactions, i.e. the oxygen reduction reaction. SKP–SECM appeared to be a very sensitive tool to monitor those changes as a function of the spatial coordinates.