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 BEEF-vdW 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 (ORR). 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 c-values to 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.Comment: 21 pages, 8 figures and Supporting Informatio

Computational Lattice-Gas Modeling of the Electrosorption of Small Molecules and Ions

We present two recent applications of lattice-gas modeling techniques to electrochemical adsorption on catalytically active metal substrates: urea on Pt(100) and (bi)sulfate on Rh(111). Both involve the specific adsorption of small molecules or ions on well-characterized single-crystal electrodes, and they provide a particularly good fit between the adsorbate geometry and the substrate structure. The close geometric fit facilitates the formation of ordered submonolayer adsorbate phases in a range of electrode potential positive of the range in which an adsorbed monolayer of hydrogen is stable. In both systems the ordered-phase region is separated from the adsorbed- hydrogen region by a phase transition, signified in cyclic voltammograms by a sharp current peak. Based on data from {\it in situ\/} radiochemical surface concentration measurements, cyclic voltammetry, and scanning tunneling micro- scopy, and {\it ex situ\/} Auger electron spectroscopy and low-energy electron diffraction, we have developed specific lattice-gas models for the two systems. These models were studied by group-theoretical ground-state calcu- lations and numerical Monte Carlo simulations, and effective lattice-gas inter- action parameters were determined so as to provide agreement with experiments.Comment: 17 pp. uuencoded postscript, FSU-SCRI-94C-9

Spectroelectrochemical study of carbon monoxide and ethanol oxidation on Pt/C, PtSn(3:1)/C and PtSn(1:1)/C catalysts

PtSn-based catalysts are one of the most active materials toward that contribute ethanol oxidation reaction (EOR). In order to gain a better understanding of the Sn influence on the carbon monoxide (principal catalyst poison) and ethanol oxidation reactions in acidic media, a systematic spectroelectrochemical study was carried out. With this end, carbon-supported PtSnx (x = 0, 1/3 and 1) materials were synthesized and employed as anodic catalysts for both reactions. In situ Fourier transform infrared spectroscopy (FTIRS) and differential electrochemical mass spectrometry (DEMS) indicate that Sn diminishes the amount of bridge bonded CO (COB) and greatly improves the CO tolerance of Pt-based catalysts. Regarding the effect of Sn loading on the EOR, it enhances the catalytic activity and decreases the onset potential. FTIRS and DEMS analysis indicate that the C-C bond scission occurs at low overpotentials and at the same potential values regardless of the Sn loading, although the amount of C-C bond breaking decreases with the rise of Sn in the catalytic material. Therefore, the elevated catalytic activity toward the EOR at PtSn-based electrodes is mainly associated with the improved CO tolerance and the incomplete oxidation of ethanol to form acetic acid and acetaldehyde species, causing the formation of a higher amount of both C2 products with the rise of Sn loading.This research was funded by Fundación Cajacanarias (project BIOGRAF) the Spanish Ministry of Economy and Competitiveness (MINECO) under projects CTQ2011-28913-C02 and ENE2014-52158-C02 (co-funded by FEDER). R.R. acknowledges the FPI program (MINECO) for financial support. We acknowledge support by the CSIC Open Access Publication Initiative through its Unit of Information Resources for Research (URICI

Underpotential deposition of Cu on Au(111) in sulfate-containing electrolytes: a theoretical and experimental study

We study the underpotential deposition of Cu on single-crystal Au(111) electrodes in sulfate-containing electrolytes by a combination of computational statistical-mechanics based lattice-gas modeling and experiments. The experimental methods are in situ cyclic voltammetry and coulometry and ex situ Auger electron spectroscopy and low-energy electron diffraction. The experimentally obtained voltammetric current and charge densities and adsorbate coverages are compared with the predictions of a two-component lattice-gas model for the coadsorption of Cu and sulfate. This model includes effective, lateral interactions out to fourth-nearest neighbors. Using group-theoretical ground-state calculations and Monte Carlo simulations, we estimate effective electrovalences and lateral adsorbate--adsorbate interactions so as to obtain overall agreement with experiments, including both our own and those of other groups. In agreement with earlier work, we find a mixed R3xR3 phase consisting of 2/3 monolayer Cu and 1/3 monolayer sulfate at intermediate electrode potentials, delimited by phase transitions at both higher and lower potentials. Our approach provides estimates of the effective electrovalences and lateral interaction energies, which cannot yet be calculated by first-principles methods.Comment: 36 pages, 14 Postscript figures are in uufiles for

Advances in catalysis for fuel cells

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Electrochemical Reduction of Perchlorate Ions

The electrochemical reduction of perchlorate ions is surveyed in the light of experimental results. The indications of the occurrence of perchlorate reduction in voltammetry, chronoamperometry, and in experiments applying the radiotracer method, the electrochemical bending beam technique, and impedance spectroscopy are presented. Some possible mechanisms of the complicated reduction processes are discussed. Environmental aspects and some methods for perchlorate removal and wastewater treatment are briefly summarized

Voltammetric properties of boron carbide electrodes

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