Heine A. Hansen

Universality in Oxygen Evolution Electrocatalysis on Oxide Surfaces

Trends in electrocatalytic activity of the oxygen evolution reaction (OER) are investigated on the basis of a large database of HO* and HOO* adsorption energies on oxide surfaces. The theoretical overpotential was calculated by applying standard density functional theory in combination with the computational standard hydrogen electrode (SHE) model. We showed that by the discovery of a universal scaling relation between the adsorption energies of HOO* vs HO*, it is possible to analyze the reaction free energy diagrams of all the oxides in a general way. This gave rise to an activity volcano that was the same for a wide variety of oxide catalyst materials and a universal descriptor for the oxygen evolution activity, which suggests a fundamental limitation on the maximum oxygen evolution activity of planar oxide catalysts.

Unifying the 2e– and 4e– Reduction of Oxygen on Metal Surfaces

Understanding trends in selectivity is of paramount importance for multi-electron electrochemical reactions. The goal of this work is to address the issue of 2e(-) versus 4e(-) reduction of oxygen on metal surfaces. Using a detailed thermodynamic analysis based on density functional theory calculations, we show that to a first approximation an activity descriptor, ΔGOH*, the free energy of adsorbed OH*, can be used to describe trends for the 2e(-) and 4e(-) reduction of oxygen. While the weak binding of OOH* on Au(111) makes it an unsuitable catalyst for the 4e(-) reduction, this weak binding is optimal for the 2e(-) reduction to H2O2. We find quite a remarkable agreement between the predictions of the model and experimental results spanning nearly 30 years.

Molybdenum Sulfides and Selenides as Possible Electrocatalysts for CO2 Reduction

Linear scaling relations between reaction intermediates pose a fundamental limitation to the CO2 reduction activity of transition‐metal catalysts. To design improved catalysts, we propose to break these scaling relations by binding key reaction intermediates to different sites. Using density functional theory, we demonstrate this principle in the active edge sites in MoS2, MoSe2, and Ni‐doped MoS2. These edges show the unique property of selectively binding COOH and CHO to bridging S or Se atoms and CO to the metal atom. DFT calculations suggest a significant improvement in CO2 reduction activity over the transition metals. Our results point to the broader application of the active edge sites of transition‐metal dichalcogenides in complex electrochemical processes.

Synthesis of thin film AuPd alloys and their investigation for electrocatalytic CO2 reduction

We synthesize and investigate AuPd alloys for the electrocatalytic reduction of CO2. Thin films of AuPd were synthesized using an electron-beam co-deposition method, which yields uniform, phase-pure metal alloys with composition control. Scanning electron microscope images show that the thin films are relatively uniform and flat in morphology. X-ray diffraction showed alloying and phase homogeneity within the AuPd thin films. Elemental mapping of Au and Pd with scanning transmission electron microscopy shows that AuPd thin films are uniform in composition on the nanometer scale. X-ray photoelectron spectroscopy characterization indicates that AuPd alloys are slightly Au-rich on the surface and follow a similar trend to the bulk composition as determined by Vegards Law. CO2 reduction activity and selectivity were investigated across the AuPd system. All AuPd alloys were found to be more active and selective for formate production than either of the pure metals, indicating that Au and Pd can act synergistically to yield new electrocatalytic properties.

Pt Skin Versus Pt Skeleton Structures of Pt3Sc as Electrocatalysts for Oxygen Reduction

In order for low temperature polymer electrolyte membrane fuel cells to become economically viable Pt catalyst loading must be significantly reduced. The cathode of the polymer electrolyte membrane fuel cell, where oxygen reduction takes place, is responsible for the main activity loss. The development of new materials for this reaction is essential in order to increase the overall effeciency of the fuel cell. Herein, we study the effect of ultra high vacuum annealing on the structure and activity of polycrystalline Pt3Sc. Upon annealing in ultra high vacuum a Pt overlayer is formed on the polycrystalline Pt3Sc. The reactivity of the overlayer is probed by temperature programmed desorption of CO. The onset of CO desorption is around 130 K lower on UHV annealed Pt3Sc than on Pt(111) and the temperature of the desorption peak maximum at saturation was ∼50 K lower on UHV annealed Pt3Sc, relative to Pt(111), consistent with the CO adsorption energies calculated using density functional theory calculations. Exposing the annealed Pt3Sc sample to 200xa0mbar O2 at room temperature results in ∼14xa0% Sc oxide as measured by X-ray photoelectron spectroscopy. Electrochemical testing of the oxygen reduction reaction shows the same activity as sputter cleaned Pt3Sc.

Electroreduction of Methanediol on Copper

We have used density functional theory calculations to study intermediates in the electroreduction of methanediol on copper. We find that methanediol, which is the hydrated form of formaldehyde, may be reduced to methanol with a limiting potential close to the experimental onset for reduction of aqueous formaldehyde.Graphical Abstract

Unifying Solution and Surface Electrochemistry: Limitations and Opportunities in Surface Electrocatalysis

The electrochemical transformations involving small molecules could play a key role in energy storage and utilization. In this work, we develop a link between surface and solution electrochemistry and this is accomplished by relating the correlated binding of hydrogenated species on metal surfaces to their formation energy in solution. This link provides a semi-quantitative treatment of thexa0thermodynamic landscape of multi-electron electrochemical reactions. Using this simple analysis, we identify some of the fundamental limits of oxygen, nitrogen and carbon electrochemistry. This analysis points to new avenues of electrocatalyst design and two such avenues are discussed in detail.

Volcano Relations for Oxidation of Hydrogen Halides over Rutile Oxide Surfaces

We investigate the heterogeneously catalysed oxidation of HX (X=Cl, Br and I) on the RuO2 (1u20091u20090) surface with DFT. We also solve a micro‐kinetic model of HX oxidation and compare oxidation activity at different coverages. We further establish linear energy relations for the reaction intermediates over a range of different rutile oxide surfaces. Based on the scaling relations, two descriptors are identified that describe the reactions uniquely. By combining scaling with the micro‐kinetic model, activity volcanoes for the three different oxidation reactions are derived. It is found that the commonly used RuO2 catalyst for HCl oxidation is closest to optimal for all three oxidation processes.