Catalytic Influence of Pt Monolayer Islands on the Hydrogen Electrochemistry of Ru(0001) Studied by Ultrahigh Vacuum Scanning Tunneling Microscopy and Cyclic Voltammetry

Enhanced electrochemical hydrogen oxidation reaction and suppressed hydrogen peroxide generation properties on Pt/Ir(111) bimetallic surfaces

Simultaneous accomplishment of high hydrogen oxidation reaction (HOR) activity and suppressed hydrogen peroxide (H₂O₂) generation is desired for anode catalysts of polymer electrolyte fuel cells. 0.3 monolayer-thick-Pt-deposited Ir(111) showed three-fold higher HOR activity than Pt(111) and suppressed H₂O₂ generation under the detection limit, providing insights for effective catalyst development.

Quantifying Confidence in DFT-Predicted Surface Pourbaix Diagrams of Transition-Metal Electrode-Electrolyte Interfaces

Density functional theory (DFT) calculations have been widely used to predict the activity of catalysts based on the free energies of reaction intermediates. The incorporation of the state of the catalyst surface under the electrochemical operating conditions while constructing the free-energy diagram is crucial, without which even trends in activity predictions could be imprecisely captured. Surface Pourbaix diagrams indicate the surface state as a function of the pH and the potential. In this work, we utilize error-estimation capabilities within the Bayesian ensemble error functional with van der Waals correlations exchange correlation functional as an ensemble approach to propagate the uncertainty associated with the adsorption energetics in the construction of Pourbaix diagrams. Within this approach, surface-transition phase boundaries are no longer sharp and are therefore associated with a finite width. We determine the surface phase diagram for several transition metals under reaction conditions and electrode potentials relevant for the oxygen reduction reaction. We observe that our surface phase predictions for most predominant species are in good agreement with cyclic voltammetry experiments and prior DFT studies. We use the OH* intermediate for comparing adsorption characteristics on Pt(111), Pt(100), Pd(111), Ir(111), Rh(111), and Ru(0001) since it has been shown to have a higher prediction efficiency relative to O*, and find the trend Ru > Rh > Ir > Pt > Pd for (111) metal facets, where Ru binds OH* the strongest. We robustly predict the likely surface phase as a function of reaction conditions by associating confidence values for quantifying the confidence in predictions within the Pourbaix diagram. We define a confidence quantifying metric, using which certain experimentally observed surface phases and peak assignments can be better rationalized. The probabilistic approach enables a more accurate determination of the surface structure and can readily be incorporated in computational studies for better understanding the catalyst surface under operating conditions.

Maximal Predictability Approach for Identifying the Right Descriptors for Electrocatalytic Reactions

Density functional theory (DFT) calculations are being routinely used to identify new material candidates that approach activity near fundamental limits imposed by thermodynamics or scaling relations. DFT calculations are associated with inherent uncertainty, which limits the ability to delineate materials (distinguishability) that possess high activity. Development of error-estimation capabilities in DFT has enabled uncertainty propagation through activity-prediction models. In this work, we demonstrate an approach to propagating uncertainty through thermodynamic activity models leading to a probability distribution of the computed activity and thereby its expectation value. A new metric, prediction efficiency, is defined, which provides a quantitative measure of the ability to distinguish activity of materials and can be used to identify the optimal descriptor(s) ΔG_opt. We demonstrate the framework for four important electrochemical reactions: hydrogen evolution, chlorine evolution, oxygen reduction and oxygen evolution. Future studies could utilize expected activity and prediction efficiency to significantly improve the prediction accuracy of highly active material candidates.

Bifunctional Interface of Au and Cu for Improved CO2 Electroreduction

Gold is known currently as the most active single-element electrocatalyst for CO2 electroreduction reaction to CO. In this work, we combine Au with a second metal element, Cu, to reduce the amount of precious metal content by increasing the surface-to-mass ratio and to achieve comparable activity to Au-based catalysts. In particular, we demonstrate that the introduction of a Au-Cu bifunctional "interface" is more beneficial than a simple and conventional homogeneous alloying of Au and Cu in stabilizing the key intermediate species, *COOH. The main advantages of the proposed metal-metal bifunctional interfacial catalyst over the bimetallic alloys include that (1) utilization of active materials is improved, and (2) intrinsic properties of metals are less affected in bifunctional catalysts than in alloys, which can then facilitate a rational bifunctional design. These results demonstrate for the first time the importance of metal-metal interfaces and morphology, rather than the simple mixing of the two metals homogeneously, for enhanced catalytic synergies.

Ambient synthesis of high-quality ruthenium nanowires and the morphology-dependent electrocatalytic performance of platinum-decorated ruthenium nanowires and nanoparticles in the methanol oxidation reaction

We report for the first time (a) the synthesis of elemental ruthenium nanowires (Ru NWs), (b) a method for modifying their surfaces with platinum (Pt), and (c) the morphology-dependent methanol oxidation reaction (MOR) performance of high-quality Pt-modified Ru NW electrocatalysts. The synthesis of our elemental Ru NWs has been accomplished utilizing a template-based method under ambient conditions. As-prepared Ru NWs are crystalline and elementally pure, maintain electrochemical properties analogous to elemental Ru, and can be generated with average diameters ranging from 44 to 280 nm. We rationally examine the morphology-dependent performance of the Ru NWs by comparison with commercial Ru nanoparticle (NP)/carbon (C) systems after decorating the surfaces of these structures with Pt. We have demonstrated that the deposition of Pt onto the Ru NWs (Pt~Ru NWs) results in a unique hierarchical structure, wherein the deposited Pt exists as discrete clusters on the surface. By contrast, we find that the Pt-decorated commercial Ru NP/C (Pt~Ru NP/C) results in the formation of an alloy-type NP. The Pt~Ru NPs (0.61 A/mg of Pt) possess nearly 2-fold higher Pt mass activity than analogous Pt~Ru NW electrocatalysts (0.36 A/mg of Pt). On the basis of a long-term durability test, it is apparent that both catalysts undergo significant declines in performance, potentially resulting from aggregation and ripening in the case of Pt~Ru NP/C and the effects of catalyst poisoning in the Pt~Ru NWs. At the conclusion of the test, both catalysts maintain comparable performance, despite a slightly enhanced performance in Pt~Ru NP/C. In addition, the measured mass-normalized MOR activity of the Pt~Ru NWs (0.36 A/mg of Pt) was significantly enhanced as compared with supported elemental Pt (Pt NP/C, 0.09 A/mg of Pt) and alloy-type PtRu (PtRu NP/C, 0.24 A/mg of Pt) NPs, both serving as commercial standards.

Interaction of CO and deuterium with bimetallic, monolayer Pt-island/film covered Ru(0001) surfaces

The adsorption properties of structurally well defined bimetallic Pt/Ru(0001) surfaces, consisting of a Ru(0001) substrate partly or fully covered by monolayer Pt islands or a monolayer Pt film, were studied by temperature programmed desorption (TPD) using CO and deuterium as probe molecules. Additionally, the adsorption of CO was investigated by infrared reflection absorption spectroscopy (IRAS). The presence of the pseudomorphic platinum islands or monolayer film leads to considerable modifications of the adsorption properties for both adsorbates, both on the Pt covered and, to a smaller extent, on the bare Ru part of the surfaces. In addition to distinct weakly bound adspecies, which are adsorbed on the monolayer Pt islands, we find unique contributions from island edge desorption, from spill-over processes during the desorption run, and a general down-shift of the peak related to desorption from Pt-free Ru(0001) areas with increasing Pt coverage. These effects, which we consider as characteristic for adsorption on bimetallic surfaces with large contiguous areas of the respective types, are discussed in detail.

Application of density functional theory to CO tolerance in fuel cells: a brief review

The large scale practical application of fuel cells in the hydrogen economy is possible only with a dramatic reduction of the cost and significant improvement of the electrocatalytic properties of the electrodes. This can be achieved through rational design of new materials, which requires an understanding of the microscopic mechanisms underlying electrocatalysis. We review briefly some applications of density functional theory (DFT) to this problem, with particular reference to the observed high CO tolerance of Pt-Ru-based anodes. These DFT-based calculations trace the changes in the surface electronic structure and the energy landscape induced by formation of Pt islets on facets of Ru nanoparticles which lead to the preferred diffusion of CO from Pt sites to Ru, where it exhibits a high rate of reaction with hydroxyls, which are generally present. We also consider the energetics of stabilization of the Pt islets on the Ru nanoparticles.

Tailoring the reactivity of bimetallic overlayer and surface alloy systems

Changing the composition and structure of a bimetallic surface system modifies its electronic properties and thus its catalytic activity. On the basis of density functional theory calculations, the electronic factors underlying the modified properties of bimetallic surfaces such as overlayer systems and, in particular, surface alloys will be discussed. It will be demonstrated that by mixing two metals a new metallic compound can result whose properties are not intermediate but beyond those of both constituents, so that for example by adding a relatively inert metal a more reactive surface can result. Besides the reactivity, the stability of the bimetallic systems will also be briefly discussed.

Electrochemical surface characterization and O2 reduction kinetics of Se surface-modified ru nanoparticle-based RuSe(y)/C catalysts

The electrochemical properties of Se surface-modified Ru/C catalysts (RuSey/C with y = 0 to 1) and their O2 reduction characteristics were determined in model studies under well-defined mass transport conditions, combining quantitative differential electrochemical mass spectrometry and double-disk electrode thin-layer flow-cell measurements. Surface characterization of the catalysts including the quantitative evaluation of the active surface area was performed by electrochemical/mass spectrometric (combined H-upd adsorption, preadsorbed CO monolayer oxidation, Cu-upd adsorption/stripping, and RuOx formation) methods. The suitability of these methods for the determination of the active surface area in the high and low Se coverage regime are discussed, and COad stripping is found to be the most relevant method for the present catalysts. The kinetic parameters for the ORR (activity and selectivity) under quasi-steady-state conditions and their variation with Se modification were evaluated in potentiostatic flow-cell measurements. Modification of Ru/C catalyst by Se improves the O2 reduction activity and reduces the tendency for H2O2 formation in the technically relevant potential region of 0.6-0.8 VRHE, but even for the best catalyst compositions a significant ( approximately 0.2 VRHE) overpotential for O2 reduction on the RuSey/C catalysts remains compared to that for the Pt/C catalyst, and we find H2O2 yields of at least 1% at typical cathode operation potentials. Consequences of the relatively high H2O2 yields for membrane/electrode stability in practical applications are discussed.